Recent advances in carboxylate-based indium(III)–organic frameworks

Abstract

Indium-based metal–organic frameworks (In-MOFs) are gaining popularity as a research topic because of their distinct structural and practical advantages. Researchers understand that the indium atom has a comparatively big radius and is ecologically benign. Importantly, the In(III) cation has a distinct electronic configuration with a high cyclic p-block, and establishes strong coordination bonds with atoms such as O, N, and S in various Lewis acid environments. As a result, the stable In-MOFs with varied structural kinds and chemical properties deserve thorough exploration. The research progress must be properly evaluated to understand the future research trend of In-MOFs better. Furthermore, this analysis of In-MOFs will close gaps in the current literature, and a full assessment of such MOFs will immensely benefit future research. It should be noted that we are primarily interested in MOFs built using carboxylic acid ligands. Herein, we will discuss the preparation methodologies, structural features, and diverse applications of MOFs made up of indium(III) nodes and various linkages, as well as current and prospects.

---

1. Introduction

Metal–organic frameworks (MOFs) are a novel form of porous crystalline material that offers structural and functional tailorability.^1–3 They are commonly employed in adsorption separation,^4–6 chemical sensing,^7–9 catalysis,^10–12 and proton conduction.^13–15 Although MOFs have been thoroughly studied for many years, there are still significant obstacles in exploring their designability and application to the real world. Researchers have learned that almost all metal elements in the periodic table can be used as nodes to produce MOFs, leading to a wide range of MOF structures and applications, thanks to the rapid advancements in crystal engineering and synthesis technologies. Divalent metal cations were the most widely used cations in the construction of MOFs for a long time, followed by trivalent or tetravalent cations (Fe³⁺, V³⁺, Cr³⁺, Zr⁴⁺, Hf⁴⁺, etc.) and lanthanide cations (Dy³⁺, Tb³⁺, Eu³⁺ and so on).¹⁶ Recently, the third main-group metals have gained popularity as an attractive family due to the advantages of low toxicity, relative abundance of reserves on earth, low cost, and structural stability and functional diversity.¹⁷ Indium as a representative element in the third main group, firstly, compared with other trivalent metals (Cr³⁺, Al³⁺, Sc³⁺, Ga³⁺, etc.) constructed MOFs, often requires the addition of strong acids or competing ligands to regulate crystal formation due to their kinetic inertness when bonding with ligands. The larger radius of In³⁺ ions (0.94 Å) greatly boosts the kinetic instability of the coordination, promotes the ligand exchange rate, and raises the likelihood of achieving a single-crystal structure. Secondly, based on the HSAB principle, in the same coordination environment, high valence metals (hard acids) with high charge density often preferentially bind to O (hard bases) on the ligand, forming MOFs with strong coordination bonds. Therefore, In-MOFs usually have good stability.¹⁷

Based on their distinct structural advantages, indium-based MOFs (In-MOFs) have garnered a lot of attention in the development of diverse MOFs during the last two decades. As shown in Chart 1, from 2002 to 2024, the related reports on In-MOFs and their derivatives, while not significantly increasing, do show an upward trend, implying that there is still a large room for designing and preparing more In-MOFs to investigate their potential applications. Naturally, In-MOFs have similar general properties to other MOFs, such as high crystallinity, adjustable pore size/shape, permanent porosity, etc. More and more scientists are concentrating on the indium atom despite its relatively modest and dispersed distribution in the earth's crust¹⁸ since it is less poisonous than rare earth metals, which is more in line with the demands of green chemistry.^19,20 The specific reasons are as follows: firstly, in terms of electronic structure, the In³⁺ center has high valence chemical properties and a large ionic radius.^21–23 Furthermore, there is a valence electron on the p-orbital of metallic indium, while the valence layer electrons of transition metals are mainly concentrated on the s-orbital (such as Zn, Cd, Ni, Co and other metals) or d-orbital (such as Cu, Au, Ag and other metals). The high-energy p-orbital of the unsaturated In³⁺ cation makes it easier to acquire electrons in diverse Lewis acid reactions, which is conducive to the formation of coordination bonds during the reaction;^24–28 secondly, by altering its valence state, the low-toxicity p-block metal In(III) may create structures with exceptional coordination and high stability in both air and moisture.^29,30 These structures have found widespread application in the domains of chemical sensors,^31–33 catalysis,^34–36 adsorption, and separation.^37–39 That's because In-MOFs have the advantages of diverse structures with different dimensionalities, high stability, and easy preparation, which are potentially utilized in many fields. For example, In-MOFs are celebrated for their exceptional porosity, making them ideal candidates for gas storage and separation applications. Moreover, indium MOFs have begun to make strides in catalysis and photocatalysis applications. The other explorations of biomedical and sensing applications on In-MOFs have attracted much attention as well. However, while there have been individual studies summarizing In-MOFs, they are outdated and insufficiently structured. As a result, we would want to give a complete examination of the setup, structural properties, and applications of these MOFs.

Chart 1 The number of publications per year dedicated to In-MOFs from 2002 to 2024 (from Web of Science up to August 2024).

This review categorizes and analyzes In-MOFs based on the type of organic ligands. Given that the In-MOFs published thus far have primarily been assembled by carboxylic acid ligands,⁴⁰ we will discuss and comment on the MOFs constructed by such ligands in the previous eight years, focusing on synthesis methodologies, structural features, properties, applications, and so on. We believe that a complete analysis of the most recent advances in In-MOFs would be extremely beneficial for future study. Table 1 summarizes the reported In-MOFs in recent years.

Table 1 Summary of described In-MOFs bearing carboxylate ligands

MOF	Synthetic method	Structure	Property	Ref.
[Me2NH2][In(abtc)]·solvents (F29)	Solvothermal synthesis	3D	Sensing performance	21
[Me2NH2][In(bptc)]·solvents (F30)	Solvothermal synthesis	3D	Sensing performance	21
[In(BDC-NH 3 )(BDC-NH 2 )]·1.6DMF·1.9H 2 O	Solvothermal synthesis	3D	—	22
(TPP)^+[In(FcDCA)2]·5H2O	Hydrothermal synthesis	2D	Redox activity	25
(Hbpy)^+[In(FcDCA)(OX^2−)]·H2O	Hydrothermal synthesis	1D	Redox activity	25
(TPP)^+2[In2(FcDCA)3(OX^2−)]·5H2O	Hydrothermal synthesis	3D	Redox activity	25
In-DM-BP	Solvothermal synthesis	3D	Selective	26
In-DH-BP	Solvothermal synthesis	3D	Selective	26
In-HMe-BPA	Solvothermal synthesis	3D	Selective	26
In-TM-BF	Solvothermal synthesis	3D	Selective	26
[In(2-Hqlca)2Cl3·3H2O] (T12)	Solvothermal synthesis	3D	Iodine capture	30
[In(2-QLBM)(2-qlca)Cl2]2·CH3CN·2H2O] (T13)	Solvothermal synthesis	3D	Iodine capture	30
JOU-24	Solvothermal synthesis	2D	Sensing performance	33
MIL-68(In)	Solvothermal synthesis	3D	Catalyst	34
NUC-66	Solvothermal synthesis	3D	Catalyst	35
SNNU-120	Solvothermal synthesis	3D	Gas separation	39
SNNU-121	Solvothermal synthesis	3D	Gas separation	39
Na[In3(odpt)2(OH)2(H2O)2](H2O)4 (S1)	Hydrothermal synthesis	—	Dielectric behavior	41
{[In(btc)(H2O)2]·2H2O}n (S2)	Hydrothermal synthesis	—	Dielectric behavior	41
In 2 O 3 PHRs	Template method	—	Sensor	43
[In6(μ-OH)2L14(H2O)6]·(solv)x (AFMOF)	Hydrothermal synthesis	3D	Luminescence	44
{In2L2(μ2-O)(H2O)3}n (S3)	Solvothermal synthesis	3D	Photoluminescence	45
YCM-31	Solvothermal synthesis	2D	—	47
YCM-32	Solvothermal synthesis	3D	—	47
In/Gd-CBDA	Solvothermal synthesis	—	Gas adsorption	48
InPF-17	Conventional heating and microwave synthesis	2D	Catalyst	52
InPF-18	Conventional heating and microwave synthesis	3D	Catalyst	52
InPF-19	Conventional heating and microwave synthesis	3D	—	52
[In2(BTA)2μ-NO3·8H2O] (S4)	Solvothermal/oil bath/microwave-assisted synthesis	3D	Sorption properties	53
MIL-68	Oil bath synthesis	3D	—	56
MIL-68-NO 2	Oil bath synthesis	3D	—	56
MIL-68-NH 2	Oil bath synthesis	3D	—	56
HKUST-1	Electrochemical deposition	—	Microelectronic device	59
{[In2L32](Me2NH2)2(DMF)2(H2O)4.5}n (S6)	Solvothermal synthesis	3D	Photoluminescent	62
{[In2L42](Me2NH2)2(DMF)4(H2O)3}n (S7)	Solvothermal synthesis	3D	Photoluminescent	62
InPF-50	Solvothermal synthesis	2D	Catalysts	63
InPF-51	Solvothermal synthesis	2D	Catalysts	63
FJU-10	Solvothermal synthesis	3D	Proton conduction	64
[In(BTB)(2,2′-bipy)]·NMP (T8)	Solvothermal synthesis	2D	Fluorescence	65
[In(BTB)(2,2′-bipy)]·NMP·2H2O (T9)	Solvothermal synthesis	2D	Fluorescence	65
[In(BTB)(2,2′-bipy)]·NMP·0.5NMP (T10)	Solvothermal synthesis	2D	Fluorescence	65
CAU-43	Solvothermal synthesis	—	Redox activity	69
In-MIL-53-FcDC_a	Solvothermal synthesis	—	Redox activity	69
In-FcDC	Solvothermal synthesis	—	Redox activity	69
In-MIL-53-FcDC_b	Solvothermal synthesis	—	Redox activity	69
In(OH)CSA	Solvothermal synthesis	2D	—	70
In(OH)PDG	Solvothermal synthesis	2D	—	70
[(CH3)2NH2][In(EBDC)]·2DMF·2DMSO·H2O (T4)	Solvothermal synthesis	3D	Gas sorption	72
In-soc-MOF-1b	Solvothermal synthesis	—	Hydrogen storage	73
In-soc-MOF-1c	Solvothermal synthesis	—	Hydrogen storage	73
MOF-In1	Solvothermal synthesis	3D	Drug release	74
MOF-In2	Solvothermal synthesis	3D	Drug release	74
NU-50	Solvothermal synthesis	—	Sorption	77
UTSA-22	Solvothermal synthesis	3D	Storage gas	78
{[CH3NH3]-[In(TPTA)]·2(NMF)} (MOF 1)	Solvothermal synthesis	3D	Gas sorption	79
{[In2(TPTA)(OH)2]·2(H2O)·(DMF)} (MOF 2)	Solvothermal synthesis	3D	Gas sorption	79
[In(OH)bpydc]	Solvothermal synthesis	—	Fluorescence	81
InDCPN-Cl/Br/I	Solvothermal synthesis	3D	CO2 fixation	82
In3L^14·(CH3NH2CH3)3 (T14)	Solvothermal synthesis	3D	Selective	83
In2L^22·(CH3COO)2·(CH3)2NH2·(CH3)3NH·H2O (T15)	Solvothermal synthesis	3D	Selective	83
(Me2NH2^+){In(III)[Ni(C2S2(C6H4COO)2)2]}·3DMF·1.5H2O	—	3D	Electrocatalysis	88
InOF-25	Solvothermal synthesis	3D	Electrocatalysis	89
InOF-26	Solvothermal synthesis	3D	Electrocatalysis	89
USTC-8(In)	Solvothermal synthesis	3D	Photocatalysis	93
In-TPBD-20	Solvothermal synthesis	—	Photocatalysis	100
In-TPBD-50	Solvothermal synthesis	—	Photocatalysis	100
InPF-50	Solvothermal synthesis	2D	Catalytic reaction	106
InPF-51	Solvothermal synthesis	2D	Catalytic reaction	106
In12-GL	Solvothermal synthesis	3D	Catalytic reaction	107
InOF-1	Solvothermal synthesis	3D	Catalytic reaction	108
NNU-32	Solvothermal synthesis	—	Luminescence	110
V101	Solvothermal synthesis	3D	Luminescence	113
V102	Solvothermal synthesis	3D	Luminescence	113
[In(EBTC)][(CH3)2NH2](DMF)(H2O)5 (F9)	Solvothermal synthesis	3D	Luminescence	114
[(CH3)2NH2][In(bpdc)2]·(DMF)2 (F10)	Solvothermal synthesis	2D	Luminescence	115
[(CH3)2NH2]3[(In3Cl2)(bpdc)5]·(H2O)5(DMF)2.5 (F11)	Solvothermal synthesis	2D	Luminescence	115
BUT-172	Solvothermal synthesis	3D	Luminescence	116
BUT-173	Solvothermal synthesis	3D	Luminescence	116
ZJU-28	Solvothermal synthesis	—	Luminescence	118
InBTB-MOF	Solvothermal synthesis	—	Luminescence	120
JOU-25	Solvothermal synthesis	3D	Luminescence	121
BUT-205	Solvothermal synthesis	2D	Luminescence	122
HDU-1	Solvothermal synthesis		Luminescence	123
CPM-5	Solvothermal synthesis	—	Xe separation	127
CPM-6	Solvothermal synthesis	—	Xe separation	127
(Me2NH2)[In(SBA)2] (F15)	Solvothermal synthesis	—	Selective CO2	128
(Me2NH2)[In(SBA)(BDC)] (F16)	Solvothermal synthesis	2D	Selective CO2	128
(Me2NH2) [In(SBA)(BDC-NH2)] (F17)	Solvothermal synthesis	2D	Selective CO2	128
(NH4)3[In3Cl2(BPDC)5] (F18)	Solvothermal synthesis	2D	Selective CO2	128
[In3(μ3O)(L)1.5(H2O)3][NO3]·5H2O·EtOH·1.5DMF (F19)	Solvothermal synthesis	2D	Selective CO2	130
InOF-23	Solvothermal synthesis	1D	Gas adsorption	131
[EMIM][In(ABDC)2]·DEF·H2O (In-ABDC-MOF)	Solvothermal synthesis	3D	Gas adsorption	132
[In2(OH)2(TCPP)] (F20)	Solvothermal synthesis	—	NH3 capture	133
NUC-5	—	3D	Gas adsorption	135
In(aip) 2	Solvothermal synthesis	2D	Gas adsorption	136
FJI-H21	Solvothermal synthesis	3D	Adsorption dyes	139
JUC-210	Solvothermal synthesis	3D	Adsorption dyes	140
FJU-117	Solvothermal synthesis	3D	C2H2/CO2 separation	149
FJU-118	Solvothermal synthesis	3D	C2H2/CO2 separation	149
(Me2NH2)1.5[In1.5L92]·2DMF·2H2O (F28)	Solvothermal synthesis	3D	C2H2/CO2 separation	150
ZJNU-121	Solvothermal synthesis	3D	C2H2/CH4 separation	151
JOU-23	Solvothermal synthesis	3D	Sensing ions	155
MFM-300(In)	Solvothermal synthesis	3D	Sensing organic molecules	156
[In(BDC-NH2)(OH)]n (In1-NH2)	Solvothermal synthesis	3D	Sensing organic molecules	157
MROF-1	Solvothermal synthesis	3D	Proton conduction	164
PCMOF-17	Solvothermal synthesis	2D	Proton conduction	165
FJU-16	Solvothermal synthesis	3D	Proton conduction	166
FJU-17	Solvothermal synthesis	3D	Proton conduction	166
[(CH3)2NH2][In(m-TTFTB)] (F32)	Solvothermal synthesis	2D	Proton conduction	168
[(CH3)2NH2][In(TTFOC)] (F33)	Solvothermal synthesis	2D	Proton conduction	168
In-BQ	Solvothermal synthesis	2D	Proton conduction	169
[In2(OH)2(BPTC)]·6H2O (InOF-1)	Solvothermal synthesis	3D	Proton conduction	172
FJU-302	Solvothermal synthesis	—	Proton conduction	173
In-IPA	Solvothermal synthesis	—	Catalytic sites	175
MIL-68-In-NO 2	Solvothermal synthesis	3D	Proton conduction	178
MIL-68-In-Br	Solvothermal synthesis	3D	Proton conduction	178

---

2. Synthesis approaches of In-MOFs

According to the literature, the most common approach for In-MOF synthesis is hydrothermal/solvothermal. It also includes continuous flow synthesis, heating batch synthesis, ultrasonic-assisted batch synthesis, microwave approach, reflux technique, and electrochemical deposition, among other techniques. We'll go into more detail about each of these synthesis methods below.

2.1. Hydrothermal/solvothermal approaches

One of the most common methods for producing MOFs is using hydrothermal procedures. Metal salts and organic ligands, as well as other materials that react poorly or not at all at ambient temperature, are usually mixed evenly in H₂O before being sealed and heated in a reactor lined with PTFE. Because of this, the reactor is maintained for a considerable amount of time at a high enough temperature and pressure to enable the reactants to completely react before being cooled to produce single-crystal MOF products. Reactant concentration, temperature, reactant ratio, stirring duration, and reaction time are the primary parameters influencing the development of crystals. At the moment, this process for creating In-MOFs is more developed.

For example, in 2020, Lu et al. reported two In-MOFs, Na[In₃(odpt)₂(OH)₂(H₂O)₂](H₂O)₄ (S1) (odpt = 4,4′-oxydiphthalate) and {[In(btc)(H₂O)₂]·2H₂O}_n (S2) (btc = 1,2,3-benzenetricarboxylate), by hydrothermal method. They have different dielectric and bandgap properties due to the molecules’ varying degrees of polar solvation.⁴¹ By choosing In(NO₃)₃·xH₂O/InCl₃·4H₂O as the metal salts to react with 1,1′-ferrocenedicarboxylic acid (H₂FcDC) and oxalic acid (OX) linkages under hydrothermal conditions, Zhang and associates successfully prepared a range of ferrocene-based In-MOFs in 2023.²⁵

Nowadays, solvothermal synthesis is the most popular approach for producing In-MOF. Although the reaction solvents are different, solvent-thermal synthesis operates on the same principles as hydrothermal synthesis. In addition to water, solvothermal synthesis can use various solvents such as CH₃OH, CH₃CH₂OH, formic acid (HCOOH), and so on. Organic solvents are utilized to dissolve organic ligands because they are typically not easily soluble in water. One important observation is that organic solvents are occasionally used in coordination, which adds complexity to the relevant MOFs’ structures. In-MOFs are commonly synthesized using a variety of amide solvents, including N,N-dimethylformamide (DMF), N,N-dimethylacetamide (DMA), N-methylformamide (NMF), N,N-diethylformamide (DEF), and others.⁴⁰ They were selected as reaction solvents for the following reasons: first, amides can improve ligand solubility and slowly break down at high temperatures to create amine bases and carboxylic acids, causing self-assembly. Due to the unique host–guest nature of MOFs, second, various solvents could play as templates to affect self-assembly and result in the formation of desired structures.

It is quite uncommon to use amide by itself as a solvent for the synthesis of In-MOFs, even though amide is typically selected as the reaction solvent. To our knowledge, the use of DMF alone as a solvent has only been limited to the synthesis of the MIL-68 series up to now. For example, in 2016, M. Oh's group utilized In(NO₃)₃ to react with benzenedicarboxylic acid (H₂BDC), 2-bromoterephthalic acid (H₂BDC-Br), or naphthalene-1,4-dicarboxylic acid (H₂NDC), respectively, to prepare three In-MOFs, MIL-68, MIL-68-Br, and MOF-NDC ⁴² (Fig. 1), where the reaction time is about 80 minutes.

Fig. 1 Preparation of three In-MOFs based on aromatic carboxylate ligands. Reproduced from ref. 42, with permission from American Chemical Society, Copyright 2016.

In 2020, Ma and co-workers⁴³ successfully prepared MIL-68(In) by dissolving In(NO₃)₃ and H₂BDC in DMF and keeping it under 100 °C for 4 h. In comparison to the previous approach for synthesizing MIL-68,⁴² this synthesis protocol is time-consuming and unsuitable for high-volume production.

DMA can also be used as a reaction solvent for the synthesis of In-MOFs, and only one example has been successfully synthesized so far using only DMA as a solvent by Xue's group, in 2020.⁴⁴ They prepared a rare tritiated triple MOF, [In₆(μ-OH)₂L1₄(H₂O)₆]·(solv)_x (AFMOF) by dissolving In(NO₃)₃, oxalylbis(aza-nediyl)diisophthalic acid (H₄L1) and benzoic acid in a DMA solution at 105 °C for 1.5 d.

In-MOFs are often made by adding one or more additional solvents to amide solvents at the same time. For instance, in 2017,⁴⁵ Meng et al. adopted InCl₃ to react with the tetracarboxylic acid derivative of 1,1′-biphenol (H₄L2) in a combination of solvents including DMF, DMSO, THF, H₂O, and HOAc to create a new indium-based chiral metal–organic skeleton, {In₂L2(μ₂-O)(H₂O)₃}_n (S3). At 100 °C for 24 hours, it yielded up to 72%, a relatively high yield for In-MOF synthesis.

In 2019, Chen's team⁴⁶ synthesized NH₂-MIL-68 by reacting In(NO₃)₃ with NH₂-BDC in a mixed solvent of DMF and pyridine. It was then used to create a MOF@COF hybrid material by reacting with tris(4-formylphenyl)amine and tris(4-aminophenyl)amine (Fig. 2). The material combined the advantages of COF and MOF, overcoming MOF's low aqueous stability.

Fig. 2 Preparation process of MOF@COF hybrid material. Reproduced from ref. 46, with permission from Springer Nature, copyright 2019.

In the same year, Genna et al. added InCl₃ and 4,4′,4′′-benzene-1,3,5-triyl-tribenzoic acid (H₃BTB) to the spirocyclic ammonium cation spPPCl/spPPBr, respectively, and used a combination of DMF, dioxane, and water as the reaction solvent. Two MOFs, YCM-31 and YCM-32, backbone isomers of one another, were successfully synthesized.⁴⁷

By dissolving In(NO₃)₃·4H₂O, Gd(NO₃)₃·6H₂O, and [5,5′-(carbonylbis(azanediyl))-diisophthalic acid] (CBDA) in a mixed solvent of DMA, NMF, and H₂O in the presence of HNO₃, Li's group developed heterogeneous metallic MOF In/Gd-CBDA in 2020.⁴⁸ It was discovered that the kind and makeup of the solvent have a significant role in triggering the identification of carboxylic acid groups at various locations with various metal coordination sites.

Using DMF/dioxane solvent combinations, Genna's group⁴⁹ synthesized several In-MOFs in 2021, including MIL-68, QMOF-2, ATF-1, and ZJU-28. A series of methods was employed to comprehend the mutually beneficial relationship between dioxane and DMF. Dioxane functions as a ligand for direct coordination with In and is essential to the native solvent, decreasing the dielectric constant. Additionally, InCl₃(dioxane)₂(H₂O), a novel indium solvent complex polymer, was identified and demonstrated to be a productive source of In.

In conclusion, the current In-MOF synthesis relies heavily on the solvothermal approach, which, in addition to outstanding production efficiency, provides for regulated reaction steps that favor In-MOF crystallization.

2.2. Microwave approaches

The fundamental approach of microwave synthesis is to use microwave technology to swiftly absorb electromagnetic radiation from tiny molecules with unequal charge distribution, forcing them to rotate and collide rapidly.^50,51 This enables polar molecules to oscillate in reaction to changes in the external electric field, resulting in thermal effects that quickly raise the temperature of the reactants. It differs from traditional heating methods in that it consumes less energy, is extremely efficient, and delivers uniform heating.

For example, in 2017 Aguirre-Diaz's group prepared six novel In-MOFs by conventional solvothermal and microwave methods. The synthesis processes are shown in Fig. 3. When In(OAc)₃ reacts with 5-(4-carboxy-2-nitrophenoxy)isophthalic acid (H₃poph), 1,10-phenanthroline (1,10-phen), 1,7-phenanthroline (1,7-phen), 4,4′-bipy, or 2,2′-bipyridine (2,2′-bipy), respectively, to get six novel In-MOFs, [In₈(OH)₆(poph)₆(H₂O)₄]·3H₂O (InPF-16), [In(poph)(2,2′-bipy)]·3H₂O (InPF-17), [In₃(OH)₃(poph)₂(4,4′-bipy)]·4H₂O (InPF-18), [In₂(poph)₂(4,4′-bipy)₂]·3H₂O (InPF-19), [In(OH)(Hpoph)]·0.5(1,7-phen) (InPF-20), and [In(poph)(1,10-phen)]·4H₂O (InPF-21). Compared to the two synthesis processes, it can be seen that microwave synthesis only takes a few minutes, whereas solvothermal synthesis usually requires several days, indicating that microwave synthesis is a viable method for producing materials with a rapid reaction time and elevated yield.⁵²

Fig. 3 Preparation of In-MOFs by the reaction of H₃popha with In(OAc)₃. Reproduced from ref. 52, with permission from Wiley-VCH GmbH, Copyright 2016.

In 2023, Jangir's team⁵³ prepared one In-MOF, [In₂(BTA)₂μ-NO₃·8H₂O] (S4), by three methods: microwave synthesis, oil bath in a round bottom flask (RBF), and solvent heat in a vial using In(NO₃)₃ and benzene-1,2,4,5-tetracarboxylic acid (H₄BTA) (Fig. 4). All three procedures produced the predicted results, although the microwave-assisted method had a substantial benefit in terms of shorter reaction time. By reducing the 8-hour oil bath synthesis time to 30 minutes, the microwave approach confirmed its effectiveness and speed in the In-MOF synthesis.

Fig. 4 Synthesis of S4 in different conditions. Reproduced from ref. 53, with permission from Springer Nature, copyright 2023.

Furthermore, In-MIL-68 can be produced utilizing both microwave-assisted and conventional solvothermal approaches. Microwave-assisted solvothermal approach is more efficient than traditional solvothermal methods. The microwave-assisted technique is effective because of its unique heating mechanism, which ensures that the entire reaction mixture is heated evenly and effectively. Shorter synthesis durations resulted in improved kinetics for the process. These findings provide valuable information regarding the synthesis of In-MOFs as well as potential real-world applications for the development of novel synthetic methodologies.

2.3. Oil bath approaches

Oil is used as a heat bath ingredient in the heating process known as “oil bath synthesis”. The hot-bath approach involves immersing a container in a hot material and gradually raising its temperature to a predefined level. The temperature of the hot substance is then held for a predetermined amount of time before being gradually lowered to enhance the material's homogeneity and the container's thermal conduction properties. These techniques have been used to manufacture MOFs based on indium.^53–55

The particular MIL-68 series that were synthesized using these techniques are the most indicative of these. As exhibited in Fig. 5, Ji's group created MIL-68, MIL-68-X (X = NH₂ or NO₂) in 2018, by reacting In(NO₃)₃·xH₂O with H₂BDC, H₂BDC-NH₂, and H₂BDC-NO₂, respectively, in an oil bath reaction at 100 °C in less than one hour.⁵⁶ Furthermore, Wang et al.⁵⁷ synthesized the crude products by dissolving H₂BDC and indium nitrate hydrate in DMF solution, heating the mixture in an oil bath at 120 °C for 30 minutes, and then washing and purifying the mixture to produce MIL-68 with good crystallinity. While the reaction time was lowered, the yield was lower than with the conventional solvothermal technique.

Fig. 5 Schematic representation for the construction of MIL-68, MIL-68-NO₂ and MIL-68-NH₂. Reproduced from ref. 56, with permission from American Chemical Society, Copyright 2018.

The oil bath approach has the advantage of making the response visible to the human eye, allowing for easy monitoring. This enables precise control of the reaction time and temperature as required.

2.4. Other approaches

Along with the many popular synthetic techniques listed above, some relatively specialized techniques can also be adapted to synthesize In-MOFs.⁵⁸ These techniques include electrochemical deposition,⁵⁹ heated batch synthesis (bs-th), ultrasound-assisted batch synthesis (bs-us), and unremitting flow synthesis (cf)⁶⁰ as depicted in Fig. 6.

Fig. 6 Three methods for synthesizing S5. Reproduced from ref. 60, with permission from American Chemical Society, Copyright 2016.

For instance, in 2016, Stock's group successfully prepared In-MOF [In₂(OH)₂(H₂TCPP)]·3DMF·4H₂O (S5) (H₆TCPP = 4-tetracarboxyphenylporphyrin) using three approaches: ultrasound-assisted, heated batch synthesis, and unremitting flow preparation, respectively. The starting materials used were H₆TCPP and In(NO₃)₃, and the solvents were DMF and H₂O. Using these diverse synthetic approaches, one may efficiently control the nucleation rate and crystal size. Furthermore, HKUST-1 on indium tin oxide (ITO) was effectively created in 2017 by Tang et al.⁵⁹ by a two-step electrochemical synthesis: electrodeposition of Cu on top of ITO, then electrooxidation of Cu/ITO to HKUST-1/ITO. Comparing this procedure to the usual method, it is easy to use, quick, gentle, and environmentally beneficial.

In 2020, Sinnwell et al.⁶¹ prepared sod-In MOF using solid-phase synthesis. The details are as follows: In(NO₃)₃·xH₂O, imidazole (HIm), and 4,5-imidazoledicarboxylic acid (H₃ImDC) were placed into the XRD reaction chamber in a stoichiometric ratio of 1 : 2 : 2. To avoid air contact with the powdered sample combination, add the DMF reservoir (40 μL) immediately after sampling. Then it was gradually heated to 40 °C, and continuous in situ XRD detection began. This MOF is worth a continuous reaction lasting about 23 hours.

A single-crystal of In-MOFs may only be formed after carefully selecting the solvent, taking into account its polarity, pH, and reaction temperature. These qualities complicate solvents’ involvement in synthesis since they might play as ligands, guests, templates, or all of the above. Moreover, acid modifiers (HF, HNO₃, HCl, HCOOH, CH₃COOH) must be included for the production of In-MOFs since they occasionally aid in the products’ crystallization.^48,62

For example, in 2016, Chen et al.⁶² successfully synthesized two unusual indium-organic backbones by reacting In(NO₃)₃ with 5,5′-(butane-1,4-diylbis(oxy))diisophthalic acid (H₄L3) and 5,5′-(1,4-phenylenebis(methylene))bis(oxy)diisophthalic acid (H₄L4) in a mixed solvent of concentrated HNO₃, DMF, and distilled water, respectively, namely, {[In₂L3₂](Me₂NH₂)₂(DMF)₂(H₂O)_4.5}_n (S6) and {[In₂L4₂](Me₂NH₂)₂(DMF)₄(H₂O)₃}_n (S7). It was discovered during the synthesizing process. Inorganic acid (HNO₃) has a favorable effect on crystal formation. When inorganic acid was introduced, colorless crystals formed; when HNO₃ was absent, only white turbidite formed. As a result, HNO₃ helps to induce crystal assembly and regulates solution pH. Furthermore, the reaction temperature is critical for indium MOF self-assembly. In-MOFs are typically produced in organic solutions (DMF, DMA) at 80 to 150 °C.

3. Structural features of In-MOFs

MOFs containing p-blocked In(III) cations, as opposed to d- or f-blocked metal ions, have numerous and distinct coordination geometries that promote the formation of a range of building blocks, such as chained {In(O or OH)}_n, trimeric {In₃O(CO₂)_n(OH₂)_n}, and monomeric {In(CO₂)_n}, with the trivalent In(III) ions typically exhibiting high coordination numbers. The great majority of In-MOFs that are currently created have three-dimensional (3D) structures. In(III) is usually bonded to O in the form of an 8- or 6-coordinated dodecahedron or 6-coordinated octahedron in its structures. It also exists as InO₆⁻ or In(CO₂)₄⁻ clusters, which assemble the unit by binding to carboxylate groups that are produced from organic ligands. Synthesized In-MOFs carry only negatively charged InO₆⁻ or In(CO₂)₄⁻ building units and typically exhibit an anionic backbone. Protonated [Me₂NH₂]⁺ ions derived from DMF are usually required to offset the negative charge of the backbone during MOF formation.^22,63 The trimeric [In₃O(COO)₆] cluster in ref. 64FJU-10, an indium cation cluster, is a secondary structural unit (SBU) that is still present. Six independent ligands connect the trimeric SBUs to form a cationic backbone with a 3D structure. Moreover, these three-dimensional constructions usually have large-diameter orifices with rhombic, triangular, hexagonal, and square channels.

In addition to 3D structures, In-MOFs have 2D or 1D structures. The majority of 2D MOFs consist of indium halide and bipyridine N-donor ligands, such as 4,4′-bipy and bpe.²⁷ In these 2D structures, indium is commonly connected to O by an 8-coordinated dodecahedron or a 7-coordinated monocapped trigonal prism. According to topological studies, they usually feature a planar structure with a zigzag chain composition and a (6,3) net hcb topology.⁶⁵ Further investigation into the newly synthesized In-MOFs revealed that they rarely have a 1D structure. Typically, triazine and metal halides create 1D MOFs. Furthermore, because of indium's flexibility and structural change, some unique organic ligands, such as 1,1′-ferrocene dicarboxylic acid (H2FcDC), can be combined to generate 1D, 2D, and 3D MOFs.

Furthermore, we discovered that the frameworks of In-MOFs are primarily related to the structural type of ligands, so this work categorizes ligands based on their structural properties to describe the structures of In-MOFs. The goal is to encourage additional researchers to focus on such MOFs by demonstrating the various architectures of In-MOFs. As a result, we largely characterize the relevant MOFs in this review using the major backbone classification of the utilized organic molecules.

3.1. In-MOFs constructed with aliphatic carboxylate ligands

There are very few examples of In-MOFs made with aliphatic carboxylic acids. Fig. 7 displays the chemical structures of the aliphatic carboxylic compounds.

Fig. 7 The aliphatic carboxylates discussed here.

Woschko and co-workers fabricated two metal–organic backbone single crystals, In-adc and In-fum, in 2023 via a solvothermal technique employing fumaric acid and acetylene dicarboxylic acid as organic ligands, and In(NO₃)₃·xH₂O as a starting material.⁶⁶ Among them, P4₁22 or P4₃22 crystallizes in enantiopure crystals (Fig. 8b), most likely racemic aggregates, while In-adc crystallizes in the chiral enantiomeric tetragonal space group. An In(III) cation, a μ-OH unit, and half of the adc²⁻ unit make up its asymmetric structural unit. Each In(III) ion takes on an octahedral coordination. Furthermore, throughout the infinite {InO₆} secondary building unit, each In³⁺ cation is coupled to two adjacent In³⁺ cations via four bridging carboxylate adc²⁻ units and two cis-bridged μ-OH groups. Similar to Fig. 8a, this connection results in a crystal structure that displays a square channel network in three dimensions. MIL-53 and In-fum share the same structure. Each indium atom is coordinated by four O atoms of four separate fumarate ligands and two O atoms of two independently linking OH units. The fundamental octahedral secondary SBU in In-fum is the resulting InO₆ building block. Fumarate linkers join the SBU's chains, creating a MOF structure with 1D rhombic channels.

Fig. 8 (a) 3D framework of In-adc; (b) right-handed 4₁ helix alongside the crystallographic 4₁ axis (brown) being colinear to the c-axis in In-adc. Reproduced from ref. 66, with permission from The Royal Society of Chemistry, Copyright 2023.

3.2. In-MOFs constructed with aromatic carboxylate ligands

Polycyclic and monocyclic ligands are the two general groups into which aromatic carboxylic acid ligands can be divided when choosing them for the building of In-MOFs. The common features of these ligands are that almost all of them contain one or more rings, and these rings are stacked with each other, which makes the structure of indium-based MOFs more stable. At the same time, these rings all contain one or more substituents that can be coordinated with indium in a variety of ways, thus forming a variety of structurally novel indium-based MOFs.

3.2.1. In-MOFs constructed with monocyclic carboxylate ligands. Nowadays, when choosing monocyclic ligands for the synthesis of In-MOFs, phenyl rings and other five-membered rings containing carboxylate units are the most adopted chemicals. Fig. 9 displays the monocyclic carboxylic acid ligands that are used in this review.

Fig. 9 Monocyclic carboxylic acid ligands are involved in this review.

In 2018, Chen's group systematically studied the structural transformation from MIL-68 to MIL-53 or QMOF-2 by changing the ratio of metal salts to organic ligands [M : L = 3 : 1, 1 : 1 and 1 : 3 or 1 : 6].⁶⁷ The selected metal salt is In(NO₃)₃·xH₂O, and the adopted ligands are H₂BDC, H₂BDC-NO₂, H₂BDC-Br or H₂BDC-NH₂. The results of the investigation demonstrated that the molar ratio of metal/ligand moles had a major impact on the creation of the products and that the structure altered as the number of ligands grew. In the presence of extra ligands, morphology changed from hexagonal needles to rhombic blocks. In³⁺ is hexa-coordinated with six O atoms for MIL-68 and MIL-53; two of these come from OH, and the other four are obtained from four distinct monodentate carboxylate ligands. On the other hand, eight O atoms, which come from four chelated bidentate carboxylate ligands, are octa-coordinated with In³⁺ in QMOF-2.

In 2017, Stock et al.²² reported three In-MOFs, [In(BDC-NH₃)(BDC-NH₂)] (T1), H[In(BDC)_0.57(BDC-NH₂)_0.43] (T2), and (DMA)₂[In₃(μ₃-O)(BDC-NH₂/NO₂)_4.5] (T3) using H₂BDC-NH₂, H₂BDC-NO₂, and 2-amino-5-nitro-terephthalic acid (H₂BDC-NH₂/NO₂). For T1, the structure contains the tetrahedral [In(-CO₂)₄]⁻ nodes. Each In(III) is linked with four other cations via organic ligands to build up a twisted quartz-like skeleton, which is interpenetrated by another skeleton, producing a 1D hexagonal channel (diameter: 4.4 Å). For T2, only BDC²⁻ and BDC-NH₂²⁻ anions were bound to In(III) cations. Therefore, BDC-NO₂²⁻ could be reduced in situ using In³⁺/DMF. For T3, H₂BDC-NH₂/NO₂ was used to form a bis-amino-functionalized MOF. The framework consists of InO₆ polyhedra joined by μ₃-O to form trinuclear clusters (Fig. 10a–d). The clusters were interconnected by tetragonal BDC-NH₂/NO₂²⁻ to constitute supertetrahedra (Fig. 10e). By confronting corners, the super-tetrahedron is shared throughout the construction of the complete framework. Structures with inaccessible cavities up to 10.4 Å in diameter are possible with this topology (Fig. 10e). Compared with the above literature reports,⁶⁷ although the same raw materials were used for the synthesis of some MOFs, the structures obtained were very different. A comparison of the two reveals that different temperatures and solvents were utilized in MOF synthesis, which could be the most important explanation for the structural differences.

Fig. 10 (a) Explanation of the crystal structure of T3 with trinuclear clusters. (b) A supertetrahedron (yellow bar) formed by a four-toothed BDC-NH₂/NO₂²⁻ ion connection. (c) By attaching the three-toothed BDC-NH₂/NO₂²⁻ ion to each face of the hypertetrahedron (green bar). (d and e) The clusters interconnected by tetragonal BDC-NH₂/NO₂²⁻. Reproduced from ref. 22, with permission from The Royal Society of Chemistry, Copyright 2017.

Genna's lab created the unique MOFs, ATF-1, YCM-21, YCM-22, and YCM-23, using 2,5-thiophenedicarboxylic acid (TDC) and indium trichloride, which are regulated by adding ammonium salts.⁶⁸ATF-1 is an anionic, 3D interpenetrating framework, in which a twisted tetrahedron with cis In–In–In angles of 83.7°, 97.7°, 98.7°, and 113.3° and trans angles of 131.0°, and 133.8°, respectively, makes up the In(III) center. The In(III) in YCM-21 was square planar with average cis In–In–In angles of about 90° and trans-angles of 180°. Both YCM-21-TEBA (triethylbutylammoniumbromide) and -TEA (triethylbutylammonium bromide) indicate cis In–In–In angles of 93.6° and 86.4°(TEA) and 97.0° and 83.0° (TEBA), respectively. YCM-22 bears a pseudotriangonal bipyramidal node and it coordinates with two TDC units and a Cl at the equatorial position, as well as two chlorides at the axial position. In addition, YCM-22 represents an unusual [InCl₃(K²-O₂CAr)₂]²⁻ node. It is hypothesized that sufficient chloride in the solution hinders the breakdown of the In–Cl bond, resulting in a five-coordinated In(III) core. YCM-23 is a novel 3D MOF bearing[In(μ-O₂CR)₂(μ-OH)]_∞ chains. These endless chains are joined together by carboxylic esters of TDC, with each carboxylic acid binding two In(III) atoms and each organic connection connecting two distinct chains.

Three ferrocene-based In-MOFs, CAU-43, In-MIL-53-FcDC_a, and In-FcDC were made by the solvothermal technique using H₂FcDC and In(III) salts.⁶⁹ The fundamental distinction between these MOFs is how their inorganic building units (IBU) are linked. The IBU in CAU-43's structure is made up of alternating cis–trans junctions connecting the corner-sharing arrangement of InO₆ octahedra, which creates a zigzag chain as a result of the joining pattern. For In-MIL-53-FcDC_a, the IBU is formed solely by trans-connected corner-sharing InO₆ octahedra. Four of the six coordinating O atoms in each InO₆ unit belong to the coordination carboxylate group of FcDC²⁻, whereas the O atoms of the shared corners come from the hydroxide group. This arrangement of the IBUs and linkers results in a windmill-type channel structure. For In-FcDC, the IBU is joined by a series of parallel-arranged, alternating trans–trans–cis corner-sharing InO₆ octahedra. Moreover, four of the six coordination O atoms in its octahedral coordination are generated from FcDC²⁻, with the remaining two coming from hydroxide groups. As a result, the linker and IBU are arranged to produce “dimers” of inorganic chains in a ladder-like framework devoid of pores.

Haddad et al. have synthesized two amide-functionalized In-MOFs, In(OH)CSA and In(OH-PDG),⁷⁰ employing flexible ligands, N-(4-carboxyphenyl)succinic acid (CSA) and N,N′-(1,4-phenylene dicarbonyl)diglycine (PDG), in 2017. The two MOFs contain {InO₄(OH)₂} octahedra forming trans-In(III)-OH chains (Fig. 11(a and b)) being joined across dicarboxylate ligands to build up a sheet (Fig. 11c).

Fig. 11 (a and b) Inorganic building units, the chain of {InO₄(OH)₂}, and the rectangular-shaped motif built by the L-shape CSA. (c) Sheet of In(OH)CSA. The red, white and gray atoms represent O, H, and In, respectively. Reproduced from ref. 70, with permission from The Royal Society of Chemistry, Copyright 2017.

One year later, MIL-68(In) was synthesized by Xing's group using a solvothermal technique. Unlike the previous literature,⁶⁷ which synthesized iso-configurational In-MOFs but did not go into detail about their structures, which are discussed below: an orthorhombic structure with the space group Cmcm is MIL-68(In).⁷¹ The kagome lattice in the ab-plane is formed by two chains of corner-sharing In-octahedra that are crystal-independent and joined by 1,4-H₂BDC ligands. The lattice has four triangular channels parallel to the c-axis and one hexagonal channel.

In short, it was discovered that all of the ligands had either carboxyl or hydroxyl groups and that the majority of the core indium atoms were coordinated to carboxyl or hydroxyl oxygens to form a wide range of configurations.

3.2.2. In-MOFs constructed with polycyclic carboxylate ligands. Apart from the previously mentioned selection of monocyclic carboxylic acid ligands, the most commonly utilized carboxylates in the fabrication of In-MOFs are those with multiple rings. Additionally, we distinguish between polycyclic carboxylic acid ligands and those with multiple benzene or heterocyclic rings.
3.2.2.1. In-MOFs constructed with polyphenyl carboxylate ligands. Polybenzene ring carboxylic acid ligands consist of multiple benzene rings and multiple carboxylic acid groups. These benzene rings contain a large number of π⋯π stacking interactions, which make the ring-to-ring contacts with each other tighter and more stable, while the large number of carboxylic acid groups contained in the benzene rings are coordinated with indium in various ways, resulting in the formation of structurally complex In-MOFs, and thus indium-based MOFs constructed from polybenzene-ring carboxylic acid ligands are potentially valuable to study. The chemical structures of the polybenzocyclic carboxylic acid ligands involved in this review are represented in Fig. 12.

Fig. 12 Polyphenyl carboxylic acid ligands involved in this review.

In 2016, Eddaoudi's group constructed four In-MOFs, [(CH₃)₂NH₂][In(EBDC)]·2DMF·2DMSO·H₂O (T4), [(CH₃)₂NH₂][In(EBDC)]·DMF (T5), [In₃O(EBDC)_1.5(H₂O)₃][NO₃]·2DMF·2(MeCN)·0.75(H₂O), (T6) [Me₂NH₂][In₃O(EBDC)_1.5·(H₂O)₃]₂[In(EBDC)]₃·8DMF·13(MeCN)·10(H₂O) (T7) using 5,5′-(1,2-ethynediyl)bis(1,3-benzenedicarboxylic acid) (H₄EBDC).⁷² The O atoms of 4 carboxylate units of 4 EBDC⁴⁻ anions are octa-coordinated to In(III) center in T4 (Fig. 13a), in which [In(O₂C)₄]⁻ and EBDC⁴⁻ could be simplified as 4-linked nodes, ultimately forming an anionic backbone with a pts topology (Fig. 13b and c), additionally containing two kinds of cross-channel along the c- and the b-axes (Fig. 13d and e), with diameters of ca. 6.655 × 5.567 Å and 5.567 × 5.567 Å, respectively. T5's asymmetric structural unit is made up of two EBDC⁴⁻ atoms and two In³⁺ atoms. A tetrahedron is formed when the In(III) is coordinated by four bidentate chelating carboxylic acids to create a 3D framework. For T6, its framework bears [In₃O(O₂C)₆(H₂O)₃]⁺ units. These trimers are linked across six individual ligands, yielding a 3D structure. The structure of T7 includes two distinct In³⁺ conformations: a monomeric [In(O₂C)₄]⁻ site and a trimeric [In₃O(O₂C)₆(H₂O)₃]⁺ cluster. Wherein, each H₄EBDC is connected by 2 monomeric In(III) atoms and 2 trimeric building units to form a new 3D ionic framework. Furthermore, T7 contains three types of channels, and the space-filling representation of the framework shows the channels along the [100], [110] and [111] directions, respectively.

Fig. 13 (a) [In(O₂C)₄]⁻ units linked by EBDC⁴⁻ to build up pts topology; (b and c) space-filling view of T4 demonstrating the channels along [010] and [001], respectively. (d and e) Ball-and-stick view of T4 with In(III) tetrahedral node. The green, red and gray atoms represent In, O, and C, respectively. Reproduced from ref. 72, with permission from American Chemical Society, Copyright 2016.

In 2016, three In-MOFs, [In(BTB)(2,2′-bipy)]·NMP (T8), [In(BTB)(2,2′-bipy)]·NMP·2H₂O (T9), and [In(BTB)(NMP)]·0.5NMP (T10) were created by Zhang et al.⁶⁵T8 has an asymmetric unit with an In³⁺ ion, a BTB³⁻ ligand, a coordinated 2,2′-bipy, and an uncoordinated NMP molecule. It has a 2D wave-layer structure. The three distinct BTB³⁻ ligands each contain five oxygen atoms, which are the source of the atoms coordinated to the seven-coordination environment that surrounds the In³⁺ ion, and from the coordinated 2,2′-bipy unit with two nitrogen atoms with a twisted single-capped trigonal prism geometry. An analysis of the topology shows that the In³⁺ cation is taken as a 3-linked node and BTB is simplified as a 3-linked joint. After this simplification, the framework belongs to a (6,3)-connected hcb topology. T9 shows a 2D interpenetrating wave-layer skeleton. The asymmetric unit contains an In³⁺ ion, a BTB³⁻ ligand, a coordinated 2,2′-bipy, an uncoordinated NMP molecule, and two uncoordinated water units. The coordination surrounding of In³⁺ is the same as that of coordinator T8 where each In³⁺ ion is linked to three carboxylic acid groups from three BTB ligands in the bc plane and each BTB ligand is attached to each of the three In³⁺ ions to form a 2D framework. For topological analysis, the 2D framework of T9 belongs to the 6,3-connected network with hcb topology. Harmony the orthorhombic Pbcn space group includes T10. An In³⁺ ion, a BTB³⁻ ligand, a coordinated NMP, and one-half of an uncoordinated NMP molecule are all present in the asymmetric unit. With one O atom of a coordination NMP unit and six O atoms of three distinct BTB³⁻ units, the In³⁺ ion is seven-coordinated (Fig. 14a). Moreover, a 2D framework (Fig. 14c and d) is formed by connecting each In³⁺ ion to three carboxylates from the BTB³⁻ ligand in the ab plane, and each BTB³⁻ ligand to three In³⁺ ions. This 6,3-connected network has the same hcb topology as T8 and T9 (Fig. 14b).

Fig. 14 (a) Complex surrounding of In³⁺ in T10. (b) 6,3-Connecting mode of T10. (c and d) 2D framework of T10. Reproduced from ref. 65, with permission from Chinese Chemical Society, Copyright 2016.

In the same year, Eddaoudi's group synthesized two homogeneous microcrystalline compounds, [In₃(C₁₆N₂O₈H₆)_1.5(Cl)(H₂O)₂]·(H₂O)_5.35 (In-soc-MOF-1b) (for Cl⁻) and [In₃(C₁₆N₂O₈H_6)1.5(Br)(H₂O)₂]·(H₂O)_6.25 (In-soc-MOF-1c) (for Br⁻) with cubic morphology.⁷³ Remarkably, the so-called modified inorganic trinuclear molecular building block (MBBs) were disclosed, where eight [In₃(μ₃-O)(X)(H₂O)₂(O₂C⁻)₆] units (X = Cl or Br) and twelve ABTC⁴⁻ anions define an anion-free cage.

One year later, [{H[In₃(TPO)₂(OH)₄]·2H₂O}_n] (MOF-In1) (TPO = tris(para-carboxylphenyl)phosphine oxide) and [In(TPO)]_n (MOF-In2), were made by Wang's team. The asymmetric unit of MOF-In1 has 1.5 In(III) ions, 0.5 neutral H₂O guest molecules, 1 TPO³⁻ anions, 2 OH⁻ anions, and 0.5 H₃O⁺ guest cations. In(III) adopts an octahedral 6-coordination mode with 4 independent TPO³⁻ anions and 2 OH⁻ connections, resulting in an 8-linked [In₃(CO₂)₆(OH)₄] units.⁷⁴ For the TPO³⁻ ligand, the two carboxylates are coordinated in a chelating fashion, connecting the two [In₃(CO₂)₆(OH)₄] units. Ligands that are usually connected in this way can be considered as 4-linked nodes with a quadrilateral surrounding. Thus, MOF-In1 shows a (4,8)-connected topological framework with a point symbol of (4¹⁰·6¹⁵·8³)(4⁵·6)₂. MOF-In2 is a 3D diamond-like skeleton bearing [InO(CO₂)₃] SBUs (Fig. 15c and d). A TPO³⁻ anion and an In(III) ion are present in the asymmetric unit. Four-connected [InO(CO₂)₃] SBUs are produced when In(III) ion is coupled to six O atoms of three carboxylate units and one O atom from the P=O unit (Fig. 15a). In the meantime, the four [InO(CO₂)₃] units are connected by the ligands, which results in four connecting nodes. The bridging TPO³⁻ ligands in SBUs-by-SBUs mode linkages connect the sheet of MOF-In2, resulting in a three-dimensional 2-fold diamond-like topology [(6⁶)] (Fig. 15b).

Fig. 15 (a) [InO(CO₂)₃] SBUs of MOF-In2; (b) layer of MOF-In2; (c and d) three-dimensional 2-fold diamond-like topology of MOF-In2. The yellow, red and blue atoms represent In, O, and C, respectively. Reproduced from ref. 74, with permission from American Chemical Society, Copyright 2017.

In 2017, Ren and co-workers⁷⁵ prepared {[In_2.5L5₂O(OH)_1.5(H₂O)₂]·DMF·MeCN·2H₂O} (T11) by a solvothermal technique using 2,2′,6,6′-tetramethoxy-4,4′-biphenyldicarboxylic acid (H₂L5). Its asymmetric unit therein contains 2.5 In³⁺ ions, 2 L5²⁻ ligands, 1 bridged μ₂-O atom, 1.5 bridged μ₂-OH⁻ anions, and 2 water units. All In(III) atoms adopt a six-coordination mode, forming a distorted octahedron. In1 is bound to four carboxylate O atoms of four L5²⁻ and two μ₂-OH⁻ ions, whereas In2 is bound to three L5²⁻, one μ₂-O atom, one μ₂-OH⁻ ion, and an H₂O unit. For In3, In³⁺ is connected to two carboxylate O atoms from two L5²⁻, two μ₂-O atoms, and two H₂O units. It is remarkable that, in contrast to earlier comparable findings, the In3 plays as the center to join two sets of In2–In1–In1–In2 chains as basic units from opposite places. These units are then further extended to constitute a 1D endless chain along the a-axis. Ultimately, a 3D framework is built by connecting these chains via L5²⁻ connections.

In 2018, utilizing solvothermal reactions of eight pre-designed concave tetracarboxylic acid linkers with In(III) salts, Zhang's group prepared four anionic In-MOFs with distinct topologies (dia, neb, lon, and pts) including In-DM-BP, In-DH-BP, In-TM-BP, In-HMe-BPA, In-TM-BF, In-DM-TP, In-DM-NTP, and In-DM-ATP.²⁶ The networks in all of these MOFs are constituted by 4-connected tetrahedral inorganic and 4-connected tetrahedral/square plane organic building blocks.

PCN-167 was synthesized in 2019 by Zhou and colleagues adopting In(NO₃)₃·4H₂O to react with H₄SFTBA (4,4′,4′′,4′′′-(9,9′-spirobi[fluorene]-2,2′,7,7′-tetrayl)tetrabenzoic acid).⁷⁶ It is fabricated by deprotonated SFTBA⁴⁻ anions and [In₃O(RCOO)₆] units (Fig. 16a and b) being a cationic 3D network (Fig. 16c and d), and contains a cage-like SBU. This structure is composed of two adjacent triangular clusters along the c-axis, as well as three SFTBA⁴⁻ sections that connect the two clusters (Fig. 16e and h). Simplifying the SFTBA⁴⁻ units as planar 4-linked nodes and the trigonal units as 6-linked nodes, PCN-167 employs a 4,6-c stp network of {4⁴·6²}₃{4⁹·6⁶}₂.

Fig. 16 (a and b) The connection method of SFTBA⁴⁻ ligand and a [In₃O(RCOO)₆] cluster of PCN-167. (c and d) The cationic 3D network of PCN-167. (e and h) Side and top view of building units of PCN-167. (g) Definition of the parameters d₁, d₂, θ, and φ. Color scheme: gray, C; red, O; light blue, In. (f) Arrangement of Cage A in the framework. Reproduced from ref. 76, with permission from American Chemical Society, Copyright 2019.

A flexible and interpenetrating microporous In-MOF, NU-50,⁷⁷ was described by Farha's group in 2020. It was fabricated by employing a tetratopic pyrene-based compound, 1,3,6,8-tetrakis(p-benzoic acid)pyrene (H₄TBAPy) to react with indium(III) chloride under solvothermal conditions, and exhibited a 4,4-linked pts net. Half of the In(III) atoms and half of the H₄TBAPy ligand are present in its asymmetric unit. To generate mononuclear [In(COO)₄]⁻ nodes, In(III) is bound by eight O atoms of the carboxylate units. Every H₄TBAPy acts as a planar tetraconnected node, and every [In(COO)₄]⁻ monomer is a tetrahedral tetraconnected node. These two nodes work together to create a (4,4)-connected point-to-sociometric topology {4²·8⁴}.

Using 4,4′,4′′-((2,4,6-trimethylbenzene-1,3,5-triyl)tris(ethyne-2,1-diyl)) tribenzoic acid (H₃TTETA) as starting materials, Chen et al. produced UTSA-22 single crystals by solvothermal reaction in 2022.⁷⁸UTSA-22's asymmetric unit consists of a μ₂-OH⁻ group, 11/6 TTETA³⁻ ligands, and three In³⁺ atoms (Fig. 17b). Among these, four distinct TTETA³⁻ ligands and four carboxylate O atoms of the two μ₂-OH⁻ units in the apical position were coordinated to both In³⁺ atoms. The In–O and In–OH bond distances are between 2.050(4)–2.209(6) and 2.026(6)–2.094(6) Å, respectively. Two of the carboxylate groups in the TTETA³⁻ ligand are coordinated in monodentate coordination to two nearby In³⁺ atoms, while the remaining carboxylate group is coordinated in a monodentate form to the In³⁺ atom. The μ₂-OH⁻ groups and the uncoordination carboxylate O-atom (O12) can interact by H-bonding. Infinite rod-like SBUs are produced when the In³⁺ atoms connect to the carboxylate and μ₂-OH⁻ units in the following order: “In1–In2–In3–In2–In1” (Fig. 17a). TTETA³⁻ ligands bridge the SBUs to constitute a 3D structure (Fig. 17c and d).

Fig. 17 (a) One-dimensional rod-shaped SBUs. (b) Chemical structure of H₃TTETA. (c and d) A 3D framework of UTSA-22. The turquoise, red and black atoms represent In, O, and C, respectively. Reproduced from ref. 78, with permission from The Partner Organisations, Copyright 2022.

In 2022, Li's group⁷⁹ successfully constructed two new heterogeneous In-MOFs, {[CH₃NH₃]·[In(TPTA)]·2(NMF)} (MOF 1) and {[In₂(TPTA)(OH)₂]·2(H₂O)·(DMF)} (MOF 2) using tetracarboxylic acid, [1,1′:3′,1′′-terphenyl]-4,4′,4′′,6′-tetracarboxylic acid (H₄TPTA). An indium atom, an H₄TPTA ligand, and a [CH₃NH₃]⁺ equilibrium charge ion make up the asymmetric unit for MOF 1. In a double nodal state, each deprotonated ligand is coupled to four mononuclear indium, which reduces to a tetragonal form. Six carboxylate O atoms are joined to each mononuclear indium to form a tetrahedral shape. To summarize, the structure with a maximum hexagonal channel is created by connecting in the aforesaid method. {4²·8⁴} is the point symbol for this (4,4) connected network. Two In³⁺ ions, two μ₂-OH⁻ groups, and one H₄TPTA ligand comprise the asymmetric unit for MOF 2. Every In³⁺ ion consists of two μ₂-OH⁻ and six O atoms from four distinct ligands, forming an octahedral coordination geometry. Following the aforementioned connections, four connected planar square lattice layers are supported by the In³⁺ ions and the four ligands. These layers are subsequently contacted by oxygen in the bridging μ₂-OH⁻ with repeating units of In³⁺ ions to build an endlessly long zigzag {In-OH-In}_n chain that has a column-like appearance. One could think of this as a column-layer approach. When viewed as a whole, the layer forms a conventional 3D fsc architecture with many 1D open channels when combined with the columnar zigzag chain. It has both rhombic and rectangular channels based only on structure (Fig. 18a–f). H₄TPTA could be considered as a 4-linked node and the In-OH-In cluster as a 6-linked node when evaluated topologically. As a result, it is a part of the fsc topology of {4⁴·6¹⁰·8}{4⁴·6²}.

Fig. 18 (a–f) Coordination fashions of H₄TPTA and preparation procedure of MOF 1 and MOF 2 showing pts and fsc net topologies, respectively. Green, red and gray atoms represent In, O, and C, respectively. Reproduced from ref. 79, with permission from American Chemical Society, Copyright 2022.

In 2024, Liu and his colleagues successfully prepared four In-MOFs with different topologies, InOF-1 (nti), InOF-2 (unc), InOF-12 (pts) and InOF-13 (nou), based on the structural characteristics of 3,3′,5,5′-biphenyl tetracarboxylic acid (BPTC) (since the diphenyl group can rotate around the C–C single bond, resulting in a variety of dihedral angles of the ligands, which increases the diversity of MOF structures with metal coordination) and the coordination properties of In(III) cation.⁸⁰ Interestingly, InOF-2, InOF-12 and InOF-13 bearing mononuclear nodes demonstrated normal positive thermal expansion, while InOF-1 built by a one-dimensional chain of InO₆ octahedrons manifested prominent negative thermal expansion. The authors believed that the twisting of the biphenyl rings in BPTC and the spring-like geometric deformation of the inorganic helical chain work together to produce negative thermal expansion, as demonstrated by a thorough in situ crystal structure and lattice dynamic studies.

3.2.2.2. In-MOFs constructed with polyheterocyclic carboxylic ligands. In addition to the previously published polybenzene ring carboxylic acid ligands, polyheterocyclic carboxylic acid ligands have been employed in the preparation of In-MOFs due to their high coordination capabilities and several coordination modes. Fig. 19 displays the chemical structures that are the subject of this review.

Fig. 19 The multi heterocyclic carboxylic acid ligands involved in this review.

In 2016, Yan et al. presented a MOF, In(OH)bpydc, based on 2,2′-bipyridine-5,5′-dicarboxylic acid (H₂bpydc) under solvothermal conditions,⁸¹ which is isostructural with MOF-253. It is a 3D framework consisting of an infinite indium-centered octahedron, InO₆ linked to a bpydc²⁻ ligand containing 1D channels.

In 2016, Yang's group³⁰ solvothermally synthesized two In-MOFs, [In(2-Hqlca)₂Cl₃·3H₂O] (T12) and {[In(2-QLBM)(2-qlca)Cl₂]₂·CH₃CN·2H₂O} (T13) (2-Hqlca = 2-quinolinecarboxylic acid) by adopting 2-(quinoline-2-yl)-benzimidazole (2-QLBM). T12's asymmetric unit bears one In³⁺ atom, two 2-Hqlca units, three Cl⁻ ions and three water units. The In³⁺ is sited in a slightly distorted pentagonal bipyramid. The bond distances are 2.263(4)–2.422(4) Å for In–O and 2.457(1)–2.476(1) Å for In–Cl. An extensive H-bonded network exists in the structure and connects to the neighboring discrete monomers thus forming a 3D supramolecular structure. T13 denotes a rare (3,4)-linked 3D topology through a hydrogen-bonding interaction network. Its asymmetric unit is constituted by two In³⁺ cations, two 2-QLBM units, two 2-qlca units, four coordination Cl⁻ ions, two crystallization water units, and one free MeCN unit. Both In1 and In2 atoms display the same six-coordination surrounding, where each metal centre consists of two N atoms of 2-QLBM, one N atom and one carboxyl O atom of 2-qlca⁻, and two coordination Cl⁻ ions. The two mononuclear units built by the In1 cation are connected by C8–H8A⋯O1 and C16–H16A⋯O2 H-bonds to constitute a dimer. Additionally, there are intermolecular π⋯π interactions (3.672 Å) (Fig. 20a). These dimers linked each other to produce a layer in the ac plane (Fig. 20b and c). The 3D supramolecular structure is formed by extending these two 2D layers further, revealing a topological network connected in a (3,4) fashion with a point symbol of (6³)(6⁵·8).

Fig. 20 (a) Dimer built by π⋯π and C–H⋯O interactions in T13. (b and c) Chains formed by C–H⋯Cl interactions and sheets formed by chains and C–H⋯Cl interactions. Reproduced from ref. 30, with permission from The Partner Organisations, Copyright 2016.

In 2019, Verma's group chose 5-(3′,5′-dicarboxyphenyl)nicotinic acid (H₃DCPN) to prepare two kinds of unsaturated monomers, [In(OOC-)₂(-N-)X(H₂O)] and [In(OOC-)₂(-N-)X₂]⁻ (X = Cl, Br, and I). There are two uncommon pyridyl-N-involved 7-coordination conformations with distinct terminal substituents in two indium monomers. Two pairs of carboxylate oxygens, a pyridyl N in the channel wall plane, and two more vertically coordinated terminal atoms are coupled to each central In(III) atom of both SBUs. This kind of coordination finally leads to the formation of pentagonal bipyramidal geometry. Furthermore, among them, all contain tetrahedral In(CO₂)₄⁻ nodes. Consequently, the two monomers reacted with one trimeric cluster [In₃O(OOC-)₆(DMA)₃]⁺ to integrate a 3D In-MOF,⁸² which contains 1D nanotube channels (pore size: about 18 Å).

In the same year, Geng's group⁸³ prepared two In-MOFs, In₃L¹₄·(Me₂NH₂)₃ (T14) [H₃L¹ = 5-(3′,5′-dicarboxyphenyl)nicotinic acid] and In₂L²₂(CH₃COO)₂·Me₂NH₂·Me₃NH·H₂O (T15) [H₃L² = 3-(2,4-dicarboxyphenyl)-6-carboxypyridine]. The asymmetric structural unit of T14 is made up of two In³⁺ ions and an H₃L¹ ligand. In³⁺ is typically coupled to carboxylate O atoms; in the case of T14, 8 carboxylate O atoms are from four distinct L¹ units. In turn, each carboxylate uses a bidentate chelation mechanism to attach to the In³⁺ center. Furthermore, each L¹ is coordinated to three In³⁺ ions through three carboxylates, which produce a two-channel, three-dimensional framework (A and B). The asymmetric unit comprising an acetate molecule, one In³⁺ ion, and an L² ligand is present in T15. In a seven-coordination mode, the In³⁺ center is joined to six carboxylate O atoms and one pyridyl N atom. Moreover, every L² was bound to three In(III) cations forming a 2D layerlike structure.

In 2021, Gazit group presented a novel 4-fold interpenetrating In-MOF, TIF-1 (Fig. 21a),⁸⁴ by coupling a binary carboxylate ligand (H₂L²⁺(ClO₄⁻)₂) with a 4-conjugated In³⁺ node (Fig. 21b), in which each In³⁺ was linked to eight oxygen atoms. Coordination of the four ligands to In³⁺ results in a 4-linked 3D structure with an interpenetrating diameter topology. The inorganic In(III) building units could be seen as tetrahedral nodes, connected by ditopic organic ligands to constitute a 3-peridic dia framework. A more thorough examination demonstrates that the 3D network is made up of channels (28.6 × 31.5 Å) (Fig. 21c) resulting in the construction of a four-fold interpenetrating network of charged diameters (Fig. 21d).

Fig. 21 (a) Coordination mode of In³⁺ cation in TIF-1. (b) Photo of as-prepared crystal for TIF-1. (c) View of the 3D structure. (d) 4-Fold interpenetrated three-dimensional network. Reproduced from ref. 83, with permission from The Royal Society of Chemistry, Copyright 2021.

In the same year, Jeevananthan group reported a three-dimensional In-MOF, namely SRMIST-1 ⁸⁵ by using In(NO₃)₃·xH₂O and hexakis(4-carboxylatophenoxy)-cyclotriphosphazene (HCPCP), which consists of inorganic SBUs, {InO₇} formed by seven-coordinated indium(III) cations in a pentagonal bipyramidal geometry. HCPCP ligands were coupled by indium(III) ions via (3-c)₂(6-c) network of {4·8²}₂{4²·8¹³}.

This section describes the structures of In-MOFs made with aromatic carboxylic acid ligands. We discovered that the majority of the ligands chosen have carboxylic acid groups and coordinate with the majority of the central indium atoms to form new, intricate multidimensional structures. In particular, the MOFs constructed from polyheterocyclic aromatic carboxylic acid ligands are different from the aliphatic, monocyclic and polycyclic phenyl aromatic carboxylic acid ligands described in the previous section. The majority of heterocyclic ligands use N as the heteroatom, while some heterocycles contain S or O atoms, which can significantly boost coordination activity. In addition to the different functional units on heterocyclic ligands, their heteroatoms are excellent active sites for metal coordination. This also results in more complicated and diversified architectures for In-MOFs. Therefore, heterocyclic aromatic carboxylic acid ligands are unquestionably the greatest alternative for the future building of In-MOFs.

4. Applications of In-MOFs

Because of the complex interaction between indium and carboxylic acid ligands, In-MOFs can be produced in some configurations. These MOFs are widely employed in diverse applications, covering catalysis, luminescence, adsorption and separation, and electrochemistry, which are all covered in detail below.

4.1. Catalysis

Main-group metal ions, particularly those with large radii such as Sr²⁺, Ba²⁺, Ga³⁺, and In³⁺, have many coordination numbers, making them ideal candidates for Lewis acid catalysts. In(III) cation has a higher charge density, which makes it more likely to construct thermally and chemically stable MOFs. Such MOFs are suitable for Lewis acid chemical catalysis and photocatalysis in organic transformations. In this review, we categorize them into three groups based on their catalytic properties: electrocatalysis, photocatalysis, and catalyze the organic functional group transition reaction.

4.1.1. Electrocatalysis. There are relatively few examples of indium-based MOFs and their derivative materials used in electrocatalysis, which are mainly applied in the field of electrocatalytic CO₂ conversion and need to be further expanded and extended. In 2019, Han's group created MOF-derived In–Cu bimetallic oxides by reacting terephthalic acids with metal precursors with varying Cu/In molar ratios. They present the first attempts to develop porous In–Cu catalysts for CO₂ reduction using a simple MOF derivatization technique.⁸⁶ It was discovered that when the molar ratio of In/Cu was 0.92, In/CuO-0.92 was produced. With a total current density of up to 92.1% and 11.2 mA cm⁻², it was a very stable and effective catalyst for reducing CO₂ to CO in aqueous electrolytes. Extensive analysis has revealed that the catalyst's exceptional performance is primarily due to its strong CO₂ adsorption, mass diffusion-facilitating porous structure, increased electrochemical surface area, improved CO₂ adsorption, small charge transfer resistance, and cooperative relationship between In and Cu oxides.

In 2021, Han and colleagues reported that CO₂ can be electrically reduced to CO using atomic In-catalysts,⁸⁷ which were obtained by pyrolysis of In-MOFs and dicyandiamide, anchored to nitrogen-doped carbon (InA/NC). InA/NC was found to have excellent CO generation and selection properties in ionic liquid/MeCN hybrid electrolytes.

Ding's team, using [Ni(C₂S₂(C₆H₄COOH)₂)₂] and In(III) made a 3D MOF with high stability, (Me₂NH₂⁺){In(III)-[Ni(C₂S₂(C₆H₄COO)₂)₂]}·3DMF·1.5H₂O (F1) which has excellent performance in electrocatalytic CO₂ reduction.⁸⁸ By replicating the active [NiS₄] sites of formate dehydrogenase and CO-dehydrogenase, the conversion and Faraday efficiency (FE) were significantly improved, with an increase in FE_HCOO⁻ from 54.7% to 89.6% (at −1.3 V vs. RHE, j_HCOO⁻ = 36.0 mA cm⁻²). Further studies confirmed that [NiS₄] can act as a CO₂ binding site and an effective catalytic center.

In 2023, Qian's team⁸⁹ utilized one In-MOF (InOF-25/26) for conversion treatment to prepare phosphorus-doped multi-defect carbon nanoribbons for effective anchoring of iron phthalocyanine. This work demonstrates that phosphating the carbon surface, enhancing the tight binding with transition metal phthalocyanines, and causing charge transfer to boost electrocatalytic kinetics can all enhance ORR performance. Simultaneously, this synthesis approach provides a viable method for the creation of high-performance electrochemical catalysts through the combination of metal macrocyclic molecules and structurally changeable MOFs.

In 2023, Zhai's group⁹⁰ described a MOF namely InCo-ABDBC-HIN (HIN = isonicotinic) by modulating the metal type and replacing the ligand, which was tested and found to have a FE (FE_C1) of 81.5% and a current density of 21.20 mA cm⁻² for the C1 product at a potential of −2.2 V vs. Ag/Ag⁺. Theoretical calculations show that its high catalytic activity is mainly ascribed to the doping of Co atoms, Co substitution of In greatly transforms the electronic structure of the In group this offers abundant unpaired electrons for CO₂RR and improves the charge transfer efficiency. Meanwhile, the bimetallic InCo clusters altered the electronic structure to promote CO₂ adsorption and CO₂ activation, leading to excellent electrocatalytic CO₂RR activity.

Additionally, in 2023, Liu et al. converted 2D indium MOFs into elemental indium nanosheets by a straightforward electrochemical reduction approach, creating a high-performance In catalyst (In-NSs) for effective CO₂RR formate synthesis.⁹¹ Testing revealed that the rebuilt metal indium had a maximum partial current density of more than 360 mA cm⁻² and a high FE of 96.3% for formates. Nearly no degradation happened after 140 h of operation in a 1 M KOH solution, outperforming the majority of cutting-edge indium-based electrocatalysts.

Currently, In-MOFs realize electrocatalysis mainly through three ways: first, the conversion of In-MOFs into In atoms and the utilization of the Lewis acid sites of In atoms to realize electrocatalysis. Secondly, In-MOFs are doped with another metal during the preparation of In-MOFs, which improves the electrocatalytic ability by changing the structural features (e.g., favorable porous structure, increasing the electrochemical surface area). In addition, electrocatalysis can be achieved by introducing catalytic groups (e.g., NiS₄ groups or other groups).

4.1.2. Photocatalysis. In(III) as the d¹⁰ metal center has significant photoluminescence capabilities and has been shown to boost light absorption due to its low splitting energy. In 2017, Chen's group created a MIL-68(In)-NH₂/graphene oxide (GrO) composite material as a photocatalyst, resulting in the first visible light-driven photocatalytic degradation of amoxicillin (AMX) (Fig. 22d). MIL-68(In)-NH₂/GrO composite demonstrated considerably stronger photocatalytic activity than MIL-68(In)-NH₂. Irradiation with 0.6 g L⁻¹ of MIL-68(In)-NH₂/GrO at pH = 5 resulted in 93% and 80% degradation of AMX after 120 and 210 minutes, respectively.⁹² The addition of GrO boosts the visible light catalytic activity (Fig. 22a and b). The photocatalytic activity of the MIL-68(In)NH₂/GrO composite was enhanced by its high visible light utilization, excellent electron transport performance, and relatively high valence band potential and surface area. Importantly, MIL-68(In)-NH₂/GrO is very reusable and stable (Fig. 22c).

Fig. 22 (a and b) Photocatalytic properties of AMX at variable conditions. (c) Photodegradation efficiency MIL-68(In)-NH₂/GrO. (d) Photocatalytic properties of MIL-68(In)-NH₂/GrO for the degradation of AMX at different pH values. Reproduced from ref. 92, with permission from Elsevier B.V. Copyright 2016.

Jiang and co-workers (2018) used the In(OH)₃ precursor to control metal ion release and successfully prepared an unusual exoplanar porphyrinyl MOF with high stability,⁹³ which demonstrated unexpectedly high photocatalytic hydrogen production activity, far exceeding that of isomorphic MOF based on endoplanar porphyrin. It was revealed that when exposed to light, the In(III) ions at the outer porphyrins decreased and soon separated from the porphyrin ring, inhibiting reverse electron transport and impeding eh recombination. Further studies confirmed that USTC-8 species with different metal ions in the porphyrin core had dramatically varying photocatalytic hydrogen production capacities (Fig. 23c and d). USTC-8(In) had the maximum photocatalytic activity (341.3 μmol g⁻¹ h⁻¹) when cocatalyzed with H₂PtCl₆ under visible light irradiation (Fig. 23a and b). Because of their excellent stability, all of these MOFs maintain good skeleton integrity and crystallinity during photocatalysis. This is the first study on MOF photocatalysis based on the behavior of particular metal porphyrins.

Fig. 23 (a and b) Photocatalytic process of USTC-8. (c and d) Nyquist spectra of USTC-8(M) (M = Cu, In) in the absence or existence of light irradiation and UV–vis plots of USTC-8(In) suspension solution with and without light irradiation. Reproduced from ref. 93, with permission from American Chemical Society, Copyright 2018.

In the same year, MIL-68-In was used as a precursor by Han et al.⁹⁴ to successfully manufacture trapped hexagonal In₂S₃ nanorods with a hollow structure, which were then further vulcanized. First, under visible light irradiation, samples with varying vulcanization periods were used to assess the photodegradation capabilities of methyl orange (MO) dye. Next, to test the activity of photocatalytic removal of antibiotics, In₂S₃ was employed to degrade tetracycline hydrochloride (TC). Consequently, the resulting In₂S₃-8h manifested outstanding catalytic activity in the photodegradation of MO and TC.

In 2020, Lu et al. described a porphyrin-based In-MOF, In(H₂TCPP)_(1−n)[Fe(TCPP)(H₂O)]_(1−n)[DEA]_(1−n) (F2). Owing to the mutually beneficial relationship between the highly catalytic iron-active sites and the photoactive porphyrin,⁹⁵ the framework can be applied to high-efficiency visible light-induced CO₂ to CO conversion. In just one day, a high CO yield of 3469 μmol g⁻¹ with good CO selectivity (99.5%) was achieved. Experimental evidence has demonstrated that this catalytic activity is significantly greater than that of the iron-free pure In-MOF or its cobalt analog. It might be because the MOF matrix's porphyrin-loaded iron center functions as a useful active site for taking in electrons from photoexcited MOFs to facilitate CO₂ reduction.

In 2020, Zeng et al. created bimetallic In_αFe_1−α-based NH₂-MIL-68 photocatalysts by replacing part of the In with Fe.⁹⁶NH₂-MIL-68(In_αFe_1−α) outperformed NH₂-MIL-68(In) in charge separation and transport efficiency. Additionally, under visible light irradiation, it demonstrated improved photocatalytic activity for Cr(VI) reduction and TC-HCl oxidation (Fig. 24a and b).

Fig. 24 (a and b). Photocatalytic properties of NH₂-MIL-68(In_αFe_1−α) and NH₂-MIL-68(In). Reproduced from ref. 96, with permission from Elsevier B.V. Copyright 2020.

In 2021, Ni's group produced MOF-derived In₂O₃ nanorods, In₂O₃NS, and In₂O₃rods by using In-BDC MOFs as precursors before calcination.⁹⁷ They showed better performance than commercial In₂O₃ in the photocatalytic decomposition of perfluoroctanoic acid (PFOA). After UV irradiation, PFOA and intermediates were completely degraded by In₂O₃NS or rods within 8 h, whereas commercial In₂O₃ removed only 35% of PFOA (Fig. 25a). The relatively small contact angle of In-BDC-derived In₂O₃ compared to commercial In₂O₃ indicates its enhanced hydrophilicity (Fig. 25b). Because of the porous shape, the liquid medium can more easily enter the interior of the photocatalysts, producing a more hydrophilic surface. The hydrophilicity of PFOS increases its breakdown efficiency by facilitating its interaction with In₂O₃ photocatalysts.

Fig. 25 (a) Adsorption of PFOA onto In₂O₃ samples. (b) The contact angles of three In₂O₃ samples. Reproduced from ref. 97, with permission from The Royal Society of Chemistry, Copyright 2021.

In the same year, Yang et al. also created an effective visible-light-driven In₂O₃/CdZnS heterojunction photocatalyst the same year.⁵⁵ They did this by combining ultrafine CdZnS nanoparticles with spindle-shaped In₂O₃ mesoporous nanorods produced from NH₂-MIL-68. Under visible light irradiation, the catalysts manifested high photocatalytic activity without the need for any co-catalysts, and the rate of H₂ generation reached 1110 μmol g⁻¹ h⁻¹. They produced H₂ at a rate that was more than 185 times faster than pure In₂O₃ nanorods and faster than other In₂O₃-based photocatalysts that have been reported to date when exposed to visible light. This implies a bright future for them in sophisticated photocatalysts.

In addition, Dai's group designed a photocatalyst,⁹⁸ namely In₂O₃/ZnIn₂S₄, that can be used for photocatalytic hydrogen evolution (PHE) reaction by growing a thin layer of ZnIn₂S₄ on the surface of carbon-coated hollow tubular In₂O₃(C/HT-In₂O₃) derived from In-MOF. This work presents an optimal photoactivated catalyst for PHE reactions in water.

In 2022, Yi and colleagues chose In(III) to react with functional tetracarboxylic acids,³⁶ to construct an In-MOF (F3). The as-synthesized F3 could be further used as a photocatalyst for producing H₂ by H₂O decomposition. The test results show that F3 has a H production efficiency of 777.65 μmol g⁻¹ h⁻¹ (Fig. 26). The H₂ yield of F3 is greatly higher than that of some previous catalysts, USTC-8(In), etc.⁹³

Fig. 26 Photocatalytic H₂ production of five samples. Reproduced from ref. 36, with permission from American Chemical Society, Copyright 2022.

The excellent photocatalytic hydrogen production ability of 2D indium-based porphyrin-MOF cubic nanosheets (In-TCPP NS) was reported by Duan et al. in the same year.⁵⁸ Due to its greatly improved electron–hole separation capabilities, 2D In-TCPP NS demonstrated an activity improvement in photocatalytic hydrogen production assays of almost an order of magnitude when compared to 3D native In-TCPP MOF. Furthermore, in an aqueous solution, it demonstrated good chemical stability in the pH range of 2–11. The activity of 2D In-TCPP NS barely diminished even after 40 h of continuous photocatalytic testing, indicating its enormous potential for useful commercial applications.

In 2022, Yao et al. obtained MOF, MIL-68(In)-X (X = NH₂, OH, Br, NO₂, CH₃)⁹⁹ and investigated their visible light photocatalytic ammonia preparation performance. Among them, MIL-68(In)-NH₂ photocatalytic performance was superior to that of MIL-68(In), which exhibited an increased N₂ photofixation rate of 140.34 μmol g_cat⁻¹ h⁻¹ at 420 nm with the quantum efficiency of 5.69%. Furthermore, the adsorption energy of the N₂ molecule onto the –NH₂ group is seen to be larger (1.819 eV) than that of MIL-68(In) (1.342 eV) (Fig. 27a). Both the amine group content and the intermittent solvothermal reactor's size had an impact on the photocatalytic activity. Theory and experimentation indicate that amines modify the chemisorption of nitrogen molecules by acting as substituents and that there are two pathways via which nitrogen is converted to ammonia: the Mars–van Krevilen process and the ligand–metal charge-transfer mechanism (Fig. 27b–d).

Fig. 27 (a) Structures of N₂-adsorbed MIL-68(In) and MIL-68(In)-NH₂ and the adsorption energy. (b–d) A suggested mechanism for photocatalytic N₂ reduction over MIL-68(In)-NH₂. Reproduced from ref. 99, with permission from American Chemical Society, Copyright 2022.

Two MOFs, 2-fold interpenetrating In-TPBD-20, and 4-fold interpenetrating In-TPBD-50, were created by Duan's group in 2022.¹⁰⁰ The authors examined how the degree of MOF interpenetration affected the characteristics of light harvesting and O₂ activation. Adding the photoactive push–pull chromophore H₄TPBD to the interpenetrating In-MOFs not only improved light absorption and hole–electron separation, but also facilitated O₂ adsorption and activation. In addition, ROS measurements showed that both In-MOFs could produce O₂˙⁻ or ¹O₂, with In-TPBD-20 exhibiting a stronger ability to make O₂˙⁻ but a weaker ability to create ¹O₂ compared with In-TPBD-50. TD-DFT calculations further indicate that the oxygen adsorbed on In-TPBD-50 is easily converted by spin-triggered O₂˙⁻ to ¹O₂via a spin-up electron transfer process. Therefore, In-TPBD-20 possesses moderate light trapping ability, remarkable O₂˙⁻ generation rate, and high backbone stability, and it can be used as an efficient and stable multiphase photocatalyst for a variety of photocatalytic aerobic oxidation reactions.

Using indium MOF (In₃-TCA) as a template, Yang's group successfully created a novel robust MOF (Fe₂In-TCA) in 2023 by achieving single-crystal to single-crystal conversion via a post-synthesis approach.¹⁰¹Fe₂In-TCA has superior charge separation in addition to having a higher specific surface area, photocatalytic activity and migration efficiency. For instance, Fe₂In-TCA has shown strong photocatalytic activity in both peroxydisul fate (PDS) activation and ofloxacin (OFL) degradation, according to experimental investigations. Fe₂In-TCA could be utilized five times and eliminate 98% of OFL in 30 minutes under ideal test circumstances. As a result, it offers more alternatives as a fresh photocatalyst for removing OFL from wastewater. Fe₂In-TCA exhibited higher photocatalytic activity compared with In₃-TCA. It was confirmed that adding Fe³⁺ significantly hindered the complexation of photogenerated carriers in MOF. Fe₂In-TCA currently has the shortest fluorescence lifetime and the fastest photogenerated electron migration rate, making it ideal for speeding photocatalytic reactions.

In 2023, by growing Cu-MOF nanosheets in situ around three-dimensional In-MOF nanorods, Du et al. created a “dimensionally mixed” heterostructure on MOF (Cu-MOF@In-MOF Z-scheme heterojunction).¹⁰² When exposed to visible light, it functions as a potent photocatalyst that effectively removes Cr(VI) ions from wastewater systems. Its huge specific surface area (718.1 m² g⁻¹) and heightened light absorption properties in the visible light band, along with greatly improved electron transport and photocarrier separation capabilities, are the key reasons for this. The 2D/3D Cu-MOF@In-MOF Z-scheme heterojunction exhibited a high photocatalytic reduction efficiency (98%) for Cr(VI) under visible light irradiation (50 mg L⁻¹, pH = 5.4) for 1 h.

Hu et al.¹⁰³ designed a series of Zn–In–S photocatalysts (Zn-doped In₂S₃, ZnIn₂S₄, and ZnS/ZnIn₂S₄), using one-step vulcanization of Zn/In-MOFs. The prepared Zn–In–S with microtubule structure showed the large surface area (107.61–127.26 m² g⁻¹), porosity (0.25–0.52 cm³ g⁻¹), and tetracycline degradation rate (≥86%) within 120 minutes. Its extremely high photocatalytic degradation efficiency is mostly owing to the direct Z-type heterojunction generated between ZnIn₂S₄ and ZnS.

It is clear that the MIL-68 series, which are currently primarily used for photocatalysis, are more widely used as derived materials. This is because the indium MOFs-derived In₂O₃ can completely retain the MOFs’ high specific surface area and porous skeleton, supplying a great number of reaction sites for photocatalytic reactions and thus improving photocatalytic activity. Furthermore, the inherited or produced pores of MOF-derived In₂O₃ can act as guests or hosts for the in situ inclusion of other exotic semiconductor nanostructures, hence increasing photocatalytic activity via heterojunctions. Several studies have shown that the resulting heterojunctions have many reaction sites, high charge transfer efficiency, and consequently increased photocatalytic activity.

4.1.3. Organic functional group transformation reactions. Until now, examples of In-MOFs used in homogeneous catalysis are rare, and it is mainly used as heterogeneous catalysts to catalyze chemical reactions. In 2016, Phan and coworkers used synthetic MIL-68(In) as a reusable heterogeneous catalyst to successfully carry out a 3-component coupling reaction for benzaldehyde, phenylene, and morpholine. Propargyl amine is created by activating C–H bonds. MIL-68(In) acts as a catalyst, making the conversion to propargyl amine easy. The solvent had a substantial effect on the three-component coupling reaction, with toluene emerging as the preferred solvent for the conversion. The MIL-68(In) catalyst is noteworthy for its ability to be recovered and repurposed without significantly reducing its catalytic activity.³⁴

In 2017, Phan's group reported that MIL-68(In) can be used as a multiphase catalyst for the preparation of 2-nitro-3-arylimidazo [1,2-a] pyridines.¹⁰⁴ It was experimentally verified that the catalyst could be reused to produce 2-nitro-3-arylimidazo [1,2-a] pyridines without a significant decrease in catalytic efficiency. Importantly, homogeneous catalysts have never been utilized before for the synthesis of 2-nitro-3-arylimidazo [1,2-a] pyridines.

In 2019, Cui and coworkers successfully prepared three chiral porous indium-based metal–organic frameworks, [Me₂NH₂][In(ODDA)]·H₂O·2DMF (F4), [Me₂NH₂]₂[In(ODDP)₂]·2H₂O·DMF (F5), and [In₃(ODDM)₂]·[NO₃]·6.7H₂O·4DMF (F6) with different networks using three 1,1′-biphenyl-phosphate-derived tetracarboxylic acids,¹⁰⁵ which were doped by chiral phosphoric acid by spatially preventing them from coordinating. Doping chiral phosphoric acid into MOFs can significantly increase their Brønsted acidity and promote Brønsted acid-catalyzed asymmetric condensation, amine addition, and imine reduction. Modifying the 3,3′-substituent changes their enantioselectivity. DFT calculations show that the framework's limited milieu of catalytically active phosphoric acids is responsible for its high enantioselectivity. By pushing the boundaries of conventional solid-state Brønsted acid catalysts, this work opens the door to the development of a novel multiphase acid catalyst for the environmentally benign synthesis of fine molecules.

In 2019, MOFs, InPF-50 and InPF-51, were created by Reinares-Fisac et al., and it was demonstrated that these were efficient catalysts for the 4-component Ugi reaction.¹⁰⁶ The two MOFs contain Lewis basic sites, which aid in synchronizing substrate activation and boost reaction yield. The authors discovered that the presence of Lewis basic sites, along with an IBU that appears to be made up of a single indium atom, produces a highly active catalyst for the multicomponent Ugi reaction. They explain this high activity not only by the easy access to acidic and basic sites inside the MOF structure but also by their location.

Furthermore, Wang and colleagues discovered a discrete tetrahedral In-cage, In12-GL, with Lewis acidic sites from In(III) cations and uncoordinated O atoms from the μ₁-HCOO fraction.¹⁰⁷ All of these In(III) cations are visible and readily available to the reactive substrate on the surface of In12-GL, which provides an additional benefit in that it increases the potential Lewis acid active sites within the cage and eliminates the need for a unique activation procedure for catalytic reactions occurring on the cage surface. For instance, In12-GL demonstrates efficient multiphase catalysis in the Strecker reaction of aminonitriles and the cycloaddition of CO₂ with epoxides. Experimental research has shown that the catalytic efficiency of In12-GL is low for large-size substrates (2-(phenoxymethyl)oxirane), but more than doubles as the temperature is raised to 80 °C. Nevertheless, these catalytic effects are rather selective. The ligand and formate groups’ spatial limitations could be the cause of this size-selective catalysis.

In 2022, a team led by Jeong reported the development of an indium-based MOF (InOF-1) linked to combustion catalysis.¹⁰⁸ This MOF generates transient open metal sites in In(III) MOFs by taking advantage of the semi-stability of metal–carboxylate linkages. Without losing activity or crystallinity, the transient open metal sites catalyze the Strecker reaction across several cycles. Spectroscopic and computational approaches were used to confirm the formation of open metal sites caused by the brief breakage of In(III)-carboxylate bonds. Moreover, temperature can influence the number of transitory open metal sites, as well as the following catalytic properties.

Ekhwan et al.¹⁰⁹ produced MIL-68(In)-NHTr, an In-MOF catalyst, by attaching a triazole to the backbone of MIL-68(In)-NH₂. For the preparation of cyclic carbonates, the epoxide was synthesized in high yield and selectivity at solvent-free ambient conditions (CO₂ pressure of 1 bar, temperature of 50 °C, and reaction duration of 6 h). The catalytic performance of MIL-68(In)-NHTr is better than that of the original MIL-68(In)-NH₂, thanks to the synergistic acid–base impact of indium SBU and triazole. Moreover, following six consecutive catalytic cycle tests, there was no discernible decrease in MOF's crystallinity or catalytic efficiency, indicating that the newly synthesized catalyst is non-leachable.

In 2023, Zhang and colleagues reported a In-MOF of {[In₄(CPDD)₂(μ₃-OH)₂(DMF)(H₂O)₂]·2DMF·5H₂O}_n (NUC-66).³⁵ Because of its major composition of In³⁺ Lewis acid sites and N sites, Lewis alkaline μ₂-OH⁻ groups, and macropores microstructural properties, NUC-66a functions as a non-homogeneous phase catalyst that can significantly speed up the Knoevenagel condensation reaction of aldehydes with malononitrile (Fig. 28). These properties render NUC-66a highly catalytic for the cycloaddition reaction of epoxides with CO₂ under mild circumstances. Its strong chemical stability and wide surface area are noteworthy. Thus, their study shows that it is promising to construct stiff nanoporous cluster-based MOFs based on large radius-ratio, highly charged metal ions and use them in fields like catalysis.

Fig. 28 The reaction mechanism of catalytic cyclization reaction. Reproduced from ref. 35, with permission from The Royal Society of Chemistry, Copyright 2023.

In 2024, Jeevananthan group⁸⁵ synthesized a multiphase Lewis acid catalyst SRMIST-1, which exhibited excellent reactivity for the preparation of 2,3-dihydroquinazolin-4(1H)-ones and 3,4-dihydro-2H-1,2,4-benzothiadiazine-1,1-dioxides. Even at low catalyst loading (1–5 mol%), mild reaction conditions, choice of ethanol as the green solvent, and simple treatment, good reaction yields were still achieved. Furthermore, the catalyst may be recycled five times, indicating that it has a wide range of applications as a green recyclable catalyst.

In short, the catalytic activity of In-MOFs makes them potentially efficient and effective catalysts. As a result, the majority of In-MOFs are now used as photocatalysts. Meanwhile, applications in areas such as electrocatalytic CO₂ conversion and catalyzing the organic functional group conversion reaction have been widely reported.

4.2. Luminescence

Apart from their application as catalysts, In-MOFs have been thoroughly investigated for their intrinsic fluorescence. Sun et al.¹¹⁰ presented the results of a visible-light-responsive In-MOF, NNU-32, in 2016. The long-lived charge creation inside the framework is often realized under visible light irradiation by the ligand's production of free radicals. Experimental research on NNU-32 has shown that it can be employed as a photosensitizer for the atom transfer radical polymerization (ATRP) reactions triggered by visible light, which is useful for the production of polymers. NNU-32-mediated ATRP of methacrylate monomers occurs in a controllable manner, resulting in narrow molecular weight distributions and high-chain end-group retention. Kinetic studies indicate that the reaction is characterized by controlled free radical polymerization. Also, this work suggests that it is possible to incorporate photoactive MOF materials into a new polymer for photochemical reactions.

Ln-In-TATB (F7) and Ln-In-BTC (F8) are two Tb : Eu co-doped In-MOFs that Fu's group synthesized in the same year.¹¹¹ Additionally, the impacts of guest solvent units and ligands on the energy transfer process between Eu and Tb were examined. First, they looked into the luminescence characteristics of F7 and F8 (Fig. 29a). Both samples demonstrated the usual luminous characteristics of lanthanide ions with good control over the emission color (Fig. 29b). As opposed to the BTC ligand, the triplet state bridging ligand 4,4′,4′′-s-triazine-2,4,6-triyltribenzoate (TATB), which is positioned between the excited states of Tb and Eu, effectively facilitates the energy transfer between them. Polar guest solvent molecules affect energy transfer efficiency as well. Whereas for F8, the emission intensity ratio stays nearly constant with varying solvent polarity (Fig. 29d), the typical peaks of Tb : Eu emission intensity ratio declines with increasing solvent polarity (Fig. 29c). This demonstrates that although solvent polarity has an impact on the luminous characteristics of F7, it does not affect those of F8.

Fig. 29 (a) Luminescence spectra of F8 with variable content ratios of Tb : Eu. (b) Photo of F8 with variable content ratios of Tb : Eu under a UV light. (c) A plot of the raw content ratio vs. the real content ratio of Tb : Eu in F8. (d) The plot of the characteristic peak intensity ratio of Tb : Eu vs. the raw content ratio in F8. Reproduced from ref. 111, with permission from The Royal Society of Chemistry, Copyright 2016.

In 2017, Yan et al. prepared an In-MOF post-functionalized with Eu³⁺, namely In-MOF-Eu ¹¹² in which both the central ligand and Eu³⁺ have strong emission peaks at an excitation wavelength of 360 nm; therefore, this excitation wavelength can be chosen as a basis for further investigation of benzene homologues (BTEX) sensing luminescence. Furthermore, testing was done on the aromatic hydrocarbons in gas-phase and liquid-phase settings, respectively, and the findings demonstrated their ability to be differentiated. To make their practical employment easier, a fluorescent paper was made to immediately differentiate between liquid BTEX. The powdered material's fluorescence intensity was found to differ significantly from that of the solution when exposed to BTEX vapor. As a result, it can be applied to BTEX substances in both solution and vapor phases as a fluorescent probe. The different BTEX compounds emit different colors of light in two primary regions: green and blue when they are in a BTEX solution. Furthermore, it might be a novel luminous sensor for identifying benzene congeners in the environment as poisons and organic contaminants.

In 2018, two anionic In-based MOFs, namely V101 and V102,¹¹³ were reported by Zhao et al. The differences in the solvents used led to two different structures, 2-fold interpenetration and non-interpenetration, respectively. Firstly, the liquid luminescence of the two MOFs and the related free ligand H₄BCP in aqueous solution were determined at 25 °C. The emission of V101 and V102 was detected at 372 nm (λ_ex = 300 nm), which is in line with the emission of H₄BCP at 374 nm. Consequently, the π* → H₄BCP π-leap could be responsible for the luminescence of V101 and V102. Additional luminescence research revealed that V102 is capable of finding nitrofurazone (NZF) traces in water. V101, however, was unable to identify NZF. The various interpenetrating structures may be the cause of the significant variation in NZF luminescence detection. Importantly, V102 has high stability and recoverability in water as a luminescent probe.

In 2019, Ren's group synthesized a super-stable compound, [In(EBTC)][Me₂NH₂](DMF)(H₂O)₅ (F9).¹¹⁴ Under UV light, its crystal and powder sample emit a vivid blue color; in the daytime, they are colorless (Fig. 30a). The highest emission of F9 is centered at 406 nm (λ_ex = 335 nm) (Fig. 30b) being attributed to π* → π-electron leaps and luminescent quantum yield up to 61.4%. Interestingly, MOF materials are rarely found to produce pure blue light that is both steady and efficient. Even though there have been several reports of luminous MOFs, their applications are typically restricted by things like low quantum yield (QY) and infrequently perfect blue light emission, such as “The quantum yields of blue-emitting HSB-W1, ZnBDCA and PCN-94, respectively, were 31.1% and 53% and 76%, Nevertheless, the QY has not been given for ZJU-28, and only a QY of 60.72% has been indicated for the hybrid of the fluorescent dye 4-(p-dimethylaminostyryl)-1-methylpyridinium loaded into ZJU-28”. But in contrast to most previously reported luminous MOF materials, the QY found in F9 is substantially greater and emits pure blue light.

Fig. 30 (a) Luminescence plots of F9. Inset: photos of F9 under daylight (up) and UV light. (b) Time-resolved emission decay spectra of F9 under 406 nm. Reproduced from ref. 114, with permission from The Royal Society of Chemistry, Copyright 2019.

In 2019, Li's team prepared two In-based MOFs, [Me₂NH₂][In(bpdc)₂]·(DMF)₂ (F10) and [Me₂NH₂]₃[(In₃Cl₂)(bpdc)₅]·(H₂O)₅(DMF)_2.5 (F11),¹¹⁵ and found that neither of the compounds exhibited FL sensing behavior toward nitro-containing explosives, CS₂, or trace Fe³⁺ ions. However, F11 activated a sensitive FL to thiol-functionalized molecules after being stripped into many layers of FL material. Furthermore, a post-synthesis cation-exchange procedure could bind Eu³⁺ to F11, and the produced F11@Eu may be employed as a ratiometric FL probe that exhibits a discernible color shift with temperature. In addition to serving as the backbone of the blue emitting body, F11 can accept cationic red and green emitting dyes such as aprazine (AF) and 4-(p-dimethylaminostyryl)-1-methylpyridinium (DSM) to produce white emission.

In 2020, Li and colleagues synthesized two heterostructured In-MOFs, BUT-172 and BUT-173,¹¹⁶ which denoted similar fluorescence emission at 380 nm (λ_ex = 280 nm) to the relevant ligand, 5′-(4-carboxy-phenyl)-2′,4′,6′-trimethyl-[1,1′:3′,1′′-terphenyl]-4,4′′-dicarboxylic acid (H₃CTTA). So the fluorescence was ligand-derived luminescence. Moreover, the fluorescence detection property of BUT-172 to different antibiotics in H₂O was investigated. Three fluoroquinolone drugs were observed to exhibit noticeably bigger quenches than the other antibiotics, and the fluorescence intensity of the MOF samples dropped as soon as antibiotics were added. Following the addition of 300 μL of antibiotic (500 μM), the antibiotic norfloxacin (NOR) had the best quenching impact (91.8%) among such antibiotics. Antibiotics present in water can therefore be identified by variations in fluorescence intensity.

In addition, Li et al.¹¹⁷ described an anionic In-MOF (F12) that can photodegrade dyes via anionic and cationic dyes as photosensitizers. This MOF demonstrates remarkable quench response sensitivity to CrO₄²⁻, Cr₂O₇²⁻, and Pb²⁺, as well as outstanding selectivity. The detection limits in aqueous solution were 31.4, 1.2, and 0.52 ppb, respectively. They are the smallest values among the MOFs that have been reported. They also offered a speculative explanation for the fluorescence emission and quenching mechanism of F12. Furthermore, after seven successive detection cycles, it was discovered that F12 had the quantitative detection ability for Cr(VI), which provides a solid basis for its practical application.

In 2020, Wang et al. created CsPbX₃@ZJU-28 materials with dual-emission characteristics,¹¹⁸ which were achieved by growing CsPbX₃ quantum dots in situ within a mesoporous, blue-emitting In-MOF (ZJU-28). The emission spectra of CsPbBr₃@ZJU-28 vary depending on the wavelength of excitation used. The tiny peak is due to the exciton emission of the CsPbX₃ quantum dot, while the large blue emission is observed when CsPbBr₃@ZJU-28 is excited (254 nm). Interestingly, the excitation wavelength- and temperature-dependent fluorescence features of the CsPbX₃@ZJU-28 composite show promise for temperature monitoring and anti-counterfeiting applications. Furthermore, ZJU-28 matrix encapsulation significantly increases CsPbX₃ quantum dot stability, which is appealing for LED applications.

Furthermore, an anionic indium-based framework was constructed by Zhu's group, in which the inner cavity (F13) is occupied by [(CH₃)₂NH₂]⁺ ions.¹¹⁹ With plenty of Lewis-basic sites for securing Cu(II) and Fe(III) cations as well as unsaturated and uncoordinated carboxylate O atoms and free N atoms in benzimidazole, the polybenzene ring linker gave F13 exceptional photosensitivity. Among the very low values of all reported photochemical sensors for MOFs, this anionic framework notably displays excellent fluorescence quench response and high photoluminescence performance to the target analytes with high selectivity (Fig. 31a and b). The sensitivity and detection limits for Cu(II) and Fe(III) ions, respectively, are 3.88 and 4.2 ppb. In addition, even in the presence of other anions or cations, F13 can identify objects quickly and accurately. More importantly, detailed investigations of potential photoluminescence and fluorescence quench processes were conducted. The coordination interaction between Cu²⁺/Fe³⁺ cations (Lewis acidic sites) and imidazole N and carboxyl O atoms of the backbone is primarily responsible for the effective fluorescence quench.

Fig. 31 (a) Luminescence intensities at 371 nm for F13 after adding various cationic or anionic (b) analytes. Reproduced from ref. 119, with permission from Elsevier B.V. Copyright 2020.

In 2021, Huh's group prepared viologen@InBTB by encapsulating three different viologen cations in [Et₂NH₂]₃[In₃(BTB)₄]·10DEF·14H₂O (InBTB-MOF) using the cation exchange method. Three of the viologen cations were methyl viologen (MV²⁺), ethyl viologen (EV²⁺), and benzyl viologen (BV²⁺).¹²⁰ Both EV@InBTB and BV@InBTB exhibited photoluminescence (PL) emission quenches in the range between 280 and 500 nm. However, MV@InBTB shows good PL emission at 306.6 nm.

In 2022, Zhang and colleagues reported a luminescence In-based MOF (JOU-25).¹²¹ The similar fluorescence emission of free ligand, 4,4′,4′′-triphenylamine tricarboxylic acid (TCA) at 439 nm (λ_ex = 340 nm) and of JOU-25 at 443 nm (λ_ex = 340 nm), suggesting that the emission of JOU-25 originates from ligand-based fluorescence. JOU-25 acts as a fluorescence sensor for detecting Cu²⁺, Fe³⁺, and NB in water.

Li et al.¹²² presented a twofold interpenetrating indium-based metal organoskeleton (BUT-205) with high H₂O-stability. The room-temperature fluorescence emission of BUT-205 appeared at 404 nm (λ_ex = 300 nm), which is similar to the free ligand. Thus, the emission bands of BUT-205 might be due to ligand-based fluorescence (n–π* and π–π*). The authors further explored its application for sensing Fe³⁺ (Fig. 32a and b). The detection limit for Fe³⁺ ions was 1.3 μmol L⁻¹ and this feature remained after four cycles of testing, making it highly promising for practical applications.

Fig. 32 (a) UV-Vis plots of tested cations. (b) Anions in comparison with the excitation and emission plots of BUT-205. Reproduced from ref. 122, with permission from Chinese Chemical Society, Copyright 2022.

A stable anionic In-MOF, [(CH₃)₂NH₂]₂In₂[(TATAB)₄(DMF)₄](DMF)₄(H₂O)₄ (HDU-1) was fabricated by Kong's group two years later.¹²³ The cation-exchange technique was used to generate the fluorescent probe of Tb@HDU-1, which is a fluorescent probe that may be used to examine Fe(III) and Cu(II) with limits of detection of 1.52 and 8.91 μM, respectively. Also, Tb@HDU-1 was utilized as a ratiometric fluorescent probe for Cu(II) cations, while only a single-peak detection was performed for Fe(III) cations. This could be explained by variations in the competitive absorption and interactions between the metal ions and the backbone.

In conclusion, the luminescence of In-MOFs is primarily caused by the ligand or the introduction of rare-earth metals; however, luminescence research on In-MOFs is still required because their fluorescence characteristics can be employed to probe drugs and heavy metal ions in water, which is useful.

4.3. Adsorption and separation

Given the large amount of current research on adsorption/separation properties of In-based MOFs. These are further classified into three types: simple organics, dye molecules, and gas adsorption and separation. In the next sections, we will go into greater detail about them.

4.3.1. Adsorption and separation of gases. In 2016, Peralta et al.¹²⁴ first synthesized InOF-1 and then confined EtOH molecules within the micropores of InOF-1, successfully achieving enhanced CO₂ capture. A very strong hydrogen bonding interaction was observed between the μ₂-OH group and EtOH molecules. These combined EtOH units generate a “bottleneck effect”, which can more effectively accommodate CO₂ molecules and significantly enhance CO₂ capture.

In the same year, Li et al. chose H₄TCPBDA (H₄TCPBDA = N,N,N′,N′-tetrakis(4-carboxyphenyl)-biphenyl-4,4′-diamine) and InCl₃·4H₂O as the reactants to synthesize an In-MOF with a double interpenetrating microporous structure by the solvothermal method,¹²⁵i.e., In(TCPBDA)(MeNH₃)·6H₂O (F14). The flexibility of its structure makes it exhibit an irreversible dynamic response to N₂, Ar and CO₂ adsorption. Further studies show that this is due to the occurrence of conformational changes of the ligands and pronounced mobile leaps of the neighboring networks, leading to pore swelling upon gas adsorption. This study will motivate more researchers to conduct more studies on the response property of flexible MOFs.

In 2017, Ibarra and colleagues reported the synthesis of InOF-1, followed by thermal activation of the as-prepared InOF-1. A small amount of residual DMF units were retained in InOF-1, forming the DMF@InOF-1. DMF@InOF-1 shows about 1.5 times higher CO₂ capture capacity than fully activated InOF-1.¹²⁶ This is due to the existence of residual DMF units, which build up a strong H-bonded interaction with the In₂(μ-OH) unit and DMF molecules. This hydrogen bond hinders DMF leaching and maintains CO₂ chelating activity even after 10 cycles.

Furthermore, three anionic porous MOFs (CPM-5, CPM-6, and Co²⁺-exchanged analog, Co²⁺-CPM-6) were prepared by Wang's group.¹²⁷Co²⁺-CPM-6 was demonstrated to have much higher Xe adsorption ability and Xe/Kr selectivity than CPM-5 and CPM-6. Small and polarised Co²⁺ ions can boost the pore size or accessible microporous volume and increase the electrical properties within the pore space inducing strong interactions with Xe while decreasing the affinity for Kr.

In 2018, Lin and colleagues synthesized four new layer MOFs,¹²⁸ ((CH₃)₂NH₂)[In(SBA)₂] (F15), ((CH₃)₂NH₂)[In(SBA)(BDC)] (F16) (H₂SBA = 4,4′-sulfonyldibenzoic acid), ((CH₃)₂NH₂)[In(SBA)(BDC-NH₂)] (F17), and (NH₄)₃[In₃Cl₂(BPDC)₅] (F18) (H₂BPDC = 4,4′-biphenyldicarboxylic acid). The Brunauer–Emmett–Teller (BET) surface area of F15 was 207 m² g⁻¹. The CO₂ adsorption at 1 atmospheric pressure was discovered to be 0.8 and 0.52 mmol g⁻¹ at 273 and 298 K, respectively. F15 was also able to adsorb CO₂ at liquid nitrogen temperature with a 0.32 wt% uptake of hydrogen adsorption.

Moreover, Hmadeh's team reported an In-MOF (AUBM-1) showing pts topology. Owing to its excellent chemical stability, it was used as an adsorbent for arsenic from H₂O, and its arsenate adsorption can reach 103.1 mg g⁻¹. The adsorption kinetics accorded with a quasi-second-order kinetic model, and thermodynamic investigations manifested that the adsorption of As was a heat-absorptive process.¹²⁹

In 2019, by using (5-(3,5-dicarboxybenzamido)-isophthalic acid) (H₄L6), Wang et al. reported an In-MOF, [In₃(μ₃-O)(L6)_1.5(H₂O)₃][NO₃]·5H₂O·CH₃CH₂OH·1.5DMF (F19).¹³⁰ Owing to the metal sites, and amide-functionalized pores in the framework, F19 exhibits a high CO₂ uptake rate (119.0 cm³ g⁻¹ at 1 bar and 0 °C). In addition, it exhibits superior adsorption selectivity for CO₂ (23.0 at 273 K) compared to N₂ adsorption. Importantly, the higher water stability of F19 makes it a promising material for water adsorption.

In 2019, Hong's team synthesized a flexible bilayer interpenetrating MOF, InOF-23.¹³¹ As shown in Fig. 33a, InOF-23 manifested an unusual adsorption behavior, which was divided into two steps. At first, the activated InOF-23 adsorbed only a small amount of N₂ at a low relative pressure (42.66 cm³ g⁻¹ at 0.31). When the P/P₀ was suddenly reduced to 0.2, the amount of N₂ adsorbed increased dramatically to 282.82 cm³ g⁻¹. This phenomenon suggests a dynamic structural transition from the narrow pore form (InOF-23-n) to the macroporous form (InOF-23-l) caused by N₂ adsorption. Meanwhile, to further investigate the dynamic adsorption behavior of InOF-23 its adsorption capacity for Ar was tested at 87 K (Fig. 33b). The Ar adsorption isotherm is similar to the N₂ adsorption isotherm. First, the amount of Ar adsorbed was 9.91 cm³ g⁻¹ until P/P₀ reached 0.85 when the adsorption fast reached 55.62 cm³ g⁻¹. In addition, they also investigated the CO₂ adsorption at 195 K. The CO₂ adsorption was only 9.82 cm³ g⁻¹ at P/P₀ = 0.3, while the CO₂ uptake could reach 54.92 cm³ g⁻¹ at 0.81 bar. In conclusion, InOF-23 exhibits unusual gas adsorption behaviors: at low temperatures, it shows a dynamic response to N₂, Ar, and CO₂ adsorption stimulated by different pressures, and at high temperatures, the InOF-23 exhibits superior CO₂ adsorption to N₂ and CH₄ adsorption, thus, InOF-23 has a broad application in the field of gas adsorption.

Fig. 33 (a) N₂-sorption/desorption isotherms of InOF-23 (77 K). (b) Ar-sorption isotherm of InOF-23 (87 K). Reproduced from ref. 131, with permission from Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim, Copyright 2019.

In 2019, Kim's group fabricated a 3D-In-ABDC-MOF using the ionic liquid, 1-ethyl-3-methylimidazolium tetrafluoroborate ([EMIM][BF₄]) reacting with [EMIM][In(ABDC)₂]·DEF·H₂O.¹³² The N₂ adsorption test (77 K) indicates that it is a microporous material. The presence of large-sized [EMIM]⁺ countercations resulted in a relatively low measured BET surface area. Notably, the presence of Lewis basic azo groups facilitates the enhancement of the interaction between the framework and CO₂ units through acid–base interactions, this gives it an excellent CO₂ adsorption capacity. Also, it demonstrated a good adsorption capacity with a moderately high isothermal heat of H₂ adsorption.

In the same year, Farha's team synthesized an In-MOF, [In₂(OH)₂(TCPP)] (F20).¹³³ Its BET surface area is 1610 m² g⁻¹. The NH₃ uptake of F20 at 1 bar was 9.41 and 7.83 mmol g⁻¹ in the first and second cycle determinations, respectively. This can be explained by the existence of a stronger interaction of NH₃ with Brønsted –OH sites in F20 in the first cycle. These results may contribute to the future development of improved MOF-based NH₃ adsorbents.

By utilizing 2,2′-(pyridine-2,6-diyl)diterephthalic acid (H₄L7) as an organic ligand, Hong's team synthesized a microporous MOF, [(CH₃)₂NH₂]₂[In₂L7₂]·3DMF·8H₂O (InOF-18)¹³⁴ in 2019. The saturation absorption at 1.0 bar was 396.61 cm³ g⁻¹, which belongs to the typical properties of microporous materials (Fig. 34a). Its Langmuir surface area (SBET) (1708 cm² g⁻¹) and BET (1214 cm² g⁻¹) were used. The MOF showed a wide range of micropores from 11.0 to 22.0 Å. Tests of their H₂ adsorption properties showed that the saturated H₂ uptake reached 179.77 cm³ g⁻¹ (77 K and 1.0 bar) (1.60 wt%) and 108.73 cm³ g⁻² (0.97 wt%) (87 K and 1.0 bar) (Fig. 34b and c), thus indicating that InOF-18 has a lofty H₂ adsorption capacity and a large H₂ binding affinity. Furthermore, it exhibits higher selectivity for CO₂ in comparison to N₂ and CH₄.

Fig. 34 (a) Nitrogen sorption curve of InOF-18 (77 K). (b) Hydrogen adsorption curves of InOF-18 (77 and 87 K). (c) Isosteric heats of hydrogen. Reproduced from ref. 134, with permission from American Chemical Society, Copyright 2019.

In 2020, Ma's group¹³⁵ designed a heterogeneous MOF {((CH₃)₂NH₂)[InTb₂(HTDP)₂]·3DMF·3H₂O}_n (NUC-5) with a unique nanocage-like framework using [In(CO₂)₄] and [Tb₂(CO₂)₈] units. Adsorption experiments on the activated NUC-5 showed that it exhibited type-I adsorption–desorption isotherm, with a calculated BET surface area of 1594 cm² g⁻¹ and a pore volume of 0.52 cm³ g⁻¹. Moreover, its maximum CO₂ uptake at 1 bar reached 69.07 and 43.88 cm³ g⁻¹ at 0 and 25 °C, respectively, which may be ascribed to the structural properties of NUC-5 (high porosity, abound Lewis and Brønsted acidic sites). Also, the MOF denoted the efficient and reversible iodine trapping performance, indicating that it is also a potential adsorbent for radioactive products.

In 2022, Wang's team reported a 2D stacked In-MOF, In(aip)₂.¹³⁶ Its CO₂ screening properties were systematically investigated. Static adsorption of a single gas shows that the MOF demonstrates a good uptake of CO₂ at room temperature (1.27 mmol g⁻¹ at 101 kPa), while no adsorption is found for N₂ and CH₄. The separation performance of CO₂ from CH₄ and N₂ was affirmed by real-time breakthrough determinations. This is due to the presence of an uninvolved coordination –NH₂ group in the ligand, which can play as a H-bond donor and Brønsted basic site to improve CO₂ affinity.

One year later, Zhai et al.⁹⁰ synthesized a bimetallic MOF, i.e., InCo-ABDBC-HIN. The introduction of Co(II) and functional groups was found to increase the specific surface area (SSA) by N₂ adsorption property tests. Then its CO₂ adsorption performance at low pressure was investigated. The maximum CO₂ uptake of In-ABDBC-HIN and InCo-ABDBC-HIN were 68.5 and 101.7 cm⁻³ g⁻¹, respectively, at a low pressure of 273 K. This suggests that the change in the electronic structure due to the inserted Co effectively contributes to the CO₂ gas uptake capacity.

In 2024, Testing Left et al.¹³⁷ synthesized two In-MOFs with high stability, namely (NH₂Me₂)₃[In₃(BTB)₄]·12DMA·4.5H₂O (In-MOF-1) and (NH₂Me₂)₉[In₉O₆(BTB)₈(H₂O)₄(DMSO)₄]·27DMSO·21H₂O (In-MOF-2). Among them, In-MOF-1 is a mononuclear structure, while In-MOF-2 exhibits a complex structure. Firstly, the N₂ adsorption properties of the activated samples were tested and it was found that the activated In-MOF-2 had a high BET surface area of 998.8 m²g⁻¹, while the BET of In-MOF-1 was only 6.88 m²g⁻¹. Then, the adsorption capacities of In-MOF-2 for CH₄, CO₂ and N₂ were determined. At 273 K (under 1 bar), its highest adsorption capacity for CO₂ is 14.8 wt%. The CH₄ uptakes are 1.60 wt% and the N₂ uptakes are 0.80 wt%. Thus, InMOF-2 exhibits highly selective CO₂ adsorption for CO₂/CH₄ and CO₂/N₂, respectively. The results were also verified by theoretical simulations.

In-MOFs are now mainly used for adsorptive separation of CO₂, H₂ and rare gases, among other things, due to their distinct structural properties, which include accessible open metal sites, a high ionic framework, and tiny channels. At the same time, compared with other adsorbents, the adsorption performance of In-MOFs is in an advantageous position; in particular, the adsorption performance of In-MOFs with a 2D structure is about one order of magnitude greater than that of other adsorbents.

4.3.2. Adsorption and separation of dyes. Organic dyes can cause great harm to our ecosystem and health, so it is urgent to remediate and treat them. In-MOFs as a powerful adsorbent have a variety of applications. In 2019, by altering the solvent, Wang et al. obtained two anionic indium–organic frameworks, [(CH₃)₂NH₂][In(L8)]·4H₂O·2DMF (F21) and [(CH₃)₂NH₂][In(L8)]·H₂O·3DMF (F22) by employing the ligand, (5-(3,5-dicarboxybenzamido)isophthalic acid) (H₄L8).¹³⁸ Due to size repulsion and preferential electron affinity, the cationic dye methylene blue (MB) is extremely selectively adsorbed onto both anionic skeletons. Apart from MB, F21 and F22 demonstrated the adsorption of RhB and rhodamine 6G (R6G), but an insert reaction to the cationic dye crystal violet (CV), a cationic dye. The findings also demonstrated that F21's adsorption capacity and kinetic constants were higher than those of F22 (Fig. 35a and b), highlighting the significance of porosity in the adsorption process. F21 can be utilized as a column chromatographic packing for the separation of dye molecules.

Fig. 35 (a and b) UV-vis plots of MB in CH₃OH at variable times during the adsorption with F21 and F22, respectively. Reproduced from ref. 138, with permission from The Royal Society of Chemistry, Copyright 2019.

In the same year, Hong's group¹³⁹ prepared an In-MOF, FJI-H21, which exhibits highly selective storage of C2–C3 hydrocarbons relative to CH₄ at ambient conditions due to its intrinsic microporosity. Additionally, it can absorb cationic dyes preferentially including MB, ethyl violet (EV), and cationic red (GTL), as well as anionic or neutral dyes. The second-order kinetic constant (k₂) of the dye MB reached 2.03 × 10⁻¹ g mg⁻¹ min⁻¹ when the crystal size was smaller than 10 μm. This increased the adsorption rate of cationic dyes by nearly 100 times compared to the kinetic constant obtained using blocked crystals, indicating that sample size has a significant impact on adsorption performance.

In 2021, Sun and colleagues, using spirobifluorene-based ligands synthesized an anionic In-MOF (JUC-20),¹⁴⁰ which has an anionic backbone with a bilayered interpenetrating pts framework for the separation of dyes, and can effectively adsorb MLB in DMF, but not the anionic dyes (Orange II and MO), and the neutral dyes, neutral red (NR) and Sudan I (SD I). When the solvent was altered to EtOH, JUC-210 could selectively adsorb MLB from the large cationic dyestuffs RhB, the anionic dyestuffs Orange II and MO, and the neutral dyestuffs SD I. It was evident from the data that solvent effects, size exclusion effects, and electrostatic interactions primarily impacted JUC-210's ability to selectively adsorb organic dyes.

In 2021, Chu group¹⁴¹ reported an In-MOF [(CH₃)₂NH₂]_1.5[In_1.5(CPTA)₂]·5.5NMF·6H₂O (F23) with a 3D anionic framework under solvothermal conditions using the asymmetric pyridinium carboxylate, 2-(2-carboxypyridin-4-yl)terephthalic acid (H₃CPTA). Its unique pore properties offer a proper environment for interfacial interactions between adsorbed units and pore matrix. Studies have confirmed that it exhibits good adsorption capacity for cationic dyes, with maximum adsorption capacities of up to 169.4, 170.8 and 202.8 mg g⁻¹ for MB, NR and MV (methyl violet), respectively.

In-MOFs for the adsorption of dyes mainly utilize an anionic framework with a microporous structure. Because it has a high void space with a proper pore size and is also based on ion exchange and size repulsion, it usually denotes highly selective adsorption of positively charged cationic dyes.

4.3.3. Adsorption and separation of simple organic substances. In-MOFs not only adsorb gases, dye molecules can also be used to adsorb small organic molecules. The adsorption of typical arsenic compounds on arsanilic acid (p-ASA) by [NH₂-MIL-68(In)] and MIL-68(In) was investigated by Liu et al. in 2018.¹⁴² From the results of the adsorption tests, it can be seen that the adsorption uptake, affinity, and adsorption rate of [NH₂-MIL-68(In)] were higher than those of MIL-68(In). Adsorption to p-ASA occurred spontaneously and exothermically, as predicted by the proposed second-order model. The adsorption mechanism is considered to entail a synergistic interplay of π–π and H-bond interactions.

In the same year, Zhang and co-workers¹⁴³ presented a new 3D In-MOF with a polar channel, H₃O[In₃(dcpy)₄(OH)₂]·3DMF·4H₂O (F24), through the use of the pyridinyl-modified dicarboxylate ligand, 3-(2′,5′-dicarboxyphenyl)pyridonic acid (H₂dcpy). It exhibits good selectivity for C₂H₂, C₂H₄, and CO₂, with maximum adsorption of the three gases reaching, respectively, 68, 51, and 61 cm³g⁻¹ at 273 K and 1 atm. This high selectivity may be attributed to the pore environment's high polarity, in which the anionic skeleton, uncoordination carboxylate O atoms, and pyridine N atoms generate an electric field that enhances the skeleton's interaction with the quadrupolar C₂H₂, C₂ H₄, and CO₂ units.

In 2019, Zhai and colleagues designed two isostructural In-MOFs,³⁹ [In(OH)(BDC)] (SNNU-120) and [In(OH)(OHBDC)] (SNNU-121). According to the ideal adsorbed solution theory (IAST) calculations, both SNNU-120 and SNNU-121 exhibit extremely good gas adsorption properties and substantial selectivity for CO₂ and C₂ hydrocarbon phases for CH₄. The alteration of the pore surface with –OH Lewis basic sites resulted in much greater CO₂ and light hydrocarbon adsorption for SNNU-121 compared to SNNU-120. The suitable porosity and functional units may improve the adsorption/separation performance of MOFs for gases.

In 2020, Chu group¹⁴⁴ gave a novel stable F-modified In-MOF, (Me₂NH₂)_1.5[In_1.5(FBDC)(BDC)]·2.5NMF·CH₃CN (F25) with a 3D cross-pore system by employing a mixed carboxylate ligand synthesis strategy. The adsorption of C₂H₂ for this MOF was studied at ambient temperatures up to 1 atm based on proper pore sizes and polar pore surfaces modified with free F atoms. The uptake was up to 53 cm³ g⁻¹ at 298 K. Its C₂H₂ uptake capacity is higher than that of MOF-5 (26 cm³ g⁻¹)¹⁴⁵ and ZIF-8 (25 cm³ g⁻¹),¹⁴⁶ and is comparable to that of previously described MOF BSF-1 (53 cm³ g⁻¹).¹⁴⁷

In the same year, Zhang et al. constructed two In-MOFs, [In₂(L1′′)(OH)₂]·2DMF·2H₂O (F26) and [(CH₃)₂NH₂][In(L1′′)]·2.5NMF·4H₂O (F27), with the help of the pyridinium tetracarboxylate compound, 2,5-bis(2′,5′-dicarboxyphenyl) pyridine (H₄L1′′),¹⁴⁸ in different solvent systems that exhibited a high adsorption capacity toward C₂H₂, C₂H₄, CO₂, and CH₄ all showed high adsorption capacity. The maximum adsorption amounts of F26 for the four gases can reach 125, 88, 79, and 24 cm³ g⁻¹ at 273 K (under 1 bar), while the sorption amounts of F27 are 85, 59, 59, and 12 cm³ g⁻¹, respectively. The gas adsorption behaviors indicate that they have excellent separation selectivity for both C₂H_x/CH₄ and CO₂/CH₄.

In 2022, Zhang et al. reported two In-MOFs, [(CH₃)₂NH₂]·[In(BDTA)] 5.5DMF·4H₂O (FJU-117) and [In(4Me-BDTA)·0.5H₂O]·6DMF·2H₂O (FJU-118).¹⁴⁹ In³⁺ in FJU-117 adopts a tetragonal coordination mode, whereas the In³⁺ ion in FJU-118 adopts seven coordination due to the spatial site-blocking effect of 4Me-BDTA. Accessible sites with partial residual charge in FJU-118 could offer electrostatic forces to fix the lattice H₂O. The electrostatic force-driven lattice H₂O connection stabilizes the FJU-118 structure. As shown in Fig. 36a, the BET and SBET of one of the activated samples, FJU-118, were 1860 and 2106 m² g⁻¹, respectively. The absorption uptakes of C₂H₂ and CO₂ at 296/273 K were 88.6/155.1 and 35.6/66.9 cm³ g⁻¹, respectively (Fig. 36b). Further studies showed that some of the remaining charges on the indium sites have electrostatic forces on the alkyne group of the C₂H₂ molecule for selective separation of C₂H₂/CO₂ mixtures (Fig. 36c and d).

Fig. 36 (a) N₂ adsorption isotherm of FJU-117a and FJU-118a (77 K). (b) CO₂, C₂H₆, C₂H₄ and C₂H₂ adsorption spectra of FJU-118a (296 K). (c and d) Separation spectra of FJU-118a for C₂H₂/CO₂/He gas mixtures and preferential adsorption sites for C₂H₂. Reproduced from ref. 149, with permission from The Royal Society of Chemistry, Copyright 2022.

By employing 5-(ethyloxamate)-isophthalic acid (H₂EtL9), an innovative porous In-MOF, ((CH₃)₂NH₂)_1.5[In_1.5L9₂]·2DMF·2H₂O (F28) was constructed by Wang's group in 2022. F28's structure includes two different types of pores that have been altered by amide groups. This allows F28 to exhibit good adsorption capacity for C₂ hydrocarbons and CO₂ at room temperature. The effective purification of CH₄ and C₂H₂ was made possible by F28's good adsorption capacity for C₂ hydrocarbons and CO₂.¹⁵⁰ Moreover, at mild conditions, F28 exhibits good conversion and size selectivity for the catalytic reaction of CO₂ and epoxides owing to the existence of strong Lewis acid In³⁺ and Brønsted acid Me₂NH₂⁺ sites.

In the same year, He and colleagues synthesized an organic coordination skeleton, ZJNU-121, with a rare bi-directional rod packing structure using a tailor-made assembly of bis-thiophene-functionalized tetracarboxylate ligands.¹⁵¹ Impressively, not only does it have unique structural features, but it also exhibits excellent adsorption capacity to capture C₂ hydrocarbons from CH₄ and CO₂ for C₂/C₁ and C₂/CO₂ separation. Wherein the ideal adsorption solution theory predicts adsorption selectivity in the ranges of 8.4–13.8 (C₂H_n/CH₄) and 2.3–3.4 (C₂H_n/CO₂) for equimolar mixtures under ambient conditions. The structural variety of rod MOFs is enhanced by this work, which also shows that it is a potent adsorbent with potential uses in the separation of different hydrocarbons.

In 2023, Wang et al.¹⁵² created a MOF (In-TATB) with 2,4,6-tris(4-carboxyphenyl)-1,3,5-triazine (H₃TATB) and In³⁺ cations. Two three-dimensional In-chains and TATB³⁻ units interpenetrate between the triazine rings through strong π⋯π interactions. Utilizing the limited ultramicroporous space (3.8 Å × 4.1 Å), it has a high enthalpy of adsorption for C₂H₂ (36.6 kJ mol⁻¹ at 298 K), thus exhibiting a good separation of C₂H₂/CO₂ mixtures.

To sum up, the adsorption of organic molecules, heavy metal ions, and dye molecules has received less attention than the adsorption of gases, which is the primary usage of In-MOFs at the moment. As a result, there is more room for growth and application of In-MOFs as adsorbents.

4.4. Chemical sensing

To make luminous In-MOFs, the indium(III) ion is commonly coupled with carboxylates with large conjugated double bonds (π-bonds), rigid planar structures, or electron-donating substituents. Photoluminescence is a key feature of In-MOFs, which are commonly employed in sensor probes to detect small biological units, gasses, metal cations and anions, and explosives.

4.4.1. Sensing ions. By selecting organic ligands with fluorescent properties and assembling them with In(III) to form In-MOFs with good fluorescent properties, an outstanding fluorescent sensor for selective probe of metal ions in water has been developed based on its good fluorescent properties. Back in 2017, Yang et al. reported a MOF, {[In(FDA)(HFDA)(H₂O)₄]·2H₂O} (In1).¹⁵³Ln³⁺@In1 was then produced by loading Dy³⁺ and Eu³⁺ cations into In1. Experimental test characterization showed that the Ln³⁺ ions can act as a complex center to assist the electron transfer from In1 to Dy³⁺ and Eu³⁺ cations. With the increase of Eu³⁺ ions in an aqueous solution, the biluminescence change of In1 provides a new visual colourimetric sensor for Eu³⁺ cations.

In 2019, Ding's team reported a MOF, AuNCs/MIL-68(In)-NH₂/Cys by doping the fluorescent gold nanoclusters (AuNCs) into MIL-68(In)-NH₂,¹⁵⁴ which acts as a fluorescent nanosensor to detect mercury ions (Hg²⁺). When utilized as a nanosensor, it emitted bright pink fluorescence, showing that AuNCs were uniformly dispersed on MIL-68(In)-NH₂. The sensor produced double fluorescence emission at 438 and 668 nm (λ_ex = 370 nm). In contrast, when Hg²⁺ was present, its fluorescent peak at 668 nm burst, while the peak at 438 nm changed (Fig. 37A and B). AuNCs/MIL-68(In)-NH₂/Cys demonstrated two linear ranges for detecting Hg²⁺: 20 pM to 0.2 μM and 0.2 μM to 60 μM (Fig. 37C and D). The sensor has been utilized to detect Hg²⁺ in actual H₂O samples.

Fig. 37 (A and B) Fluorescence emission plots of AuNCs/MIL-68(In)-NH₂/Cys in the presence of Hg²⁺ at variable concentration. (C and D) Linearity between FL_{668 nm}/FL_{438 nm} and Hg²⁺ concentration at different scales and concentration ranges. Reproduced from ref. 154, with permission from The Royal Society of Chemistry, Copyright 2019.

Lee and colleagues synthesized two microporous In-MOFs,²¹ [(CH₃)₂NH₂][In(abtc)]·solvents (F29) and [(CH₃)₂NH₂][In(bptc)]·solvents (F30). Owing to the anionic backbones and channel size, they could be used for the selective uptake and separation of cationic dyes and show reversible dye uptake and release capabilities. The luminescence intensity of F29 at 402 nm was decreased only when Fe(III) was added. F30 can be adopted as an outstanding dual-emission channel sensor for selective detection of Fe³⁺ ions.

In 2021, Zhang's team constructed a three-dimensional 2-fold interpenetrating MOF (JOU-23),¹⁵⁵ which denotes an anionic framework. JOU-23 can be a fluorescent sensor for sensing Fe(III) cations and nitroaromatic molecules. Among them, the detection limit for Fe³⁺ was as low as 2 × 10⁻⁶ M (K_SV = 59 755 M⁻¹), while JOU-23 can detect nitrobenzene (NB) with high sensitivity. Even when the concentration of NB reached only 4.175 × 10⁻⁵ M, the quenching efficiency still reached 86.4%.

In 2022, Zhang and co-workers synthesized an indium-based metal organoskeleton, [In(HTCA)(OH)]·2DMF·H₂O (JOU-24) with high stability using 4,4′,4′′-tricarboxytriphenylamine (H₃TCA),³³ which could be a fluorescent sensor for selective detection of Co(II) and NB in H₂O owing to its high stability and fluorescent properties. Where for Co²⁺ sensing, the high burst content was 24 500 with a low detection limit of 4.9 × 10⁻⁶ mol L⁻¹. For NB sensing, the burst content was 10 230.

In 2024, Zuo et al.¹³⁷ synthesized two In-MOFs, In-MOF-1 and In-MOF-2. In-MOF-1 has a mononuclear 3D skeleton, whereas In-MOF-2 has a 3D skeleton with three different multinuclear In-clusters. They exhibited excellent fluorescence sensing performance in the detection of Fe³⁺ cations with high sensitivity, excellent selectivity, and recoverability, which were attributed to the combined effects of competitive energy absorption and competitive energy adsorption and energy transfer, respectively.

In summary, these In-MOFs for fluorescent sensors usually contain organic ligands with fluorescent properties or rare earth elements with luminescent properties introduced through post-modification. Furthermore, most of these In-MOFs are anionic backbones.

4.4.2. Sensing organic molecules. Because In-MOFs often have good sensitivity, repeatability, and selectivity, they are useful as chemical sensors. Consequently, In-MOFs made with carboxylate ligands and In³⁺ have drawn increasing interest as sensors in practical research in recent years. In 2018, Salama's group synthesized the first pure MOF-based SO₂ sensor,¹⁵⁶MFM-300(In). It has a superior detection limit compared to other sensors at 25 °C, and it is capable of detecting SO₂ in the ppb range as low as 75 ppb, with a linear response from 75 to 1000 ppb (Fig. 38a), and a detection limit as low as 5 ppb. MFM-300(In) is backed by confirmation of its stability and endurance across the spectrum of determined concentrations (Fig. 38b and c). In addition, the unique and significant sensing selectivity of the prepared MFM-300(In) for SO₂ was confirmed by comparing the correlation signal strength of SO₂ with other evaluated gases/vapors detected (e.g. NO₂, CH₄, H₂ and others).

Fig. 38 (a) Detection of sulfur dioxide from 75 to 1000 ppb. (b and c) MFM-300(In) cyclic and persistent detection of SO₂ detection. Reproduced from ref. 156, with permission from The Royal Society of Chemistry, Copyright 2018.

In 2021, Yang's team presented an In-MOF, [In(BDC-NH₂)(OH)]_n (In1-NH₂)¹⁵⁷ showing a high sensitivity identification and short response time for heavy metal ions and ClO⁻ in aqueous solution. The LOD values are 0.11, 0.14, 0.64 and 1.15 μM for Fe³⁺, Cu²⁺, Pb²⁺, and ClO⁻, respectively. In addition, PBA-In1, which was produced by functionalizing In1-NH₂ with phenylboronic acid through the PSM strategy, denoted the effective sensing performance of H₂O₂.

In 2021, SNNU-153 and related sensors were reported by Zhai's group.¹⁵⁸ The synergistic effect of excited state intramolecular proton transfer signaling and the constrained MOF holes enables SNNU-153 to selectively sense hydrazine even in N-containing hydride analogues such as NH₃, NH₂OH and (CH₃)₂NNH₂.

In 2022, Manzar Sohail et al.¹⁵⁹ initially synthesized In-MOF [MIL-68(In)] using a solvothermal technique. They then inserted Ag⁺ by adding AgNO₃ continuously, and finally, they calcined the mixture for three hours at 650 °C to produce Ag@In₂O₃. Using Ag@In₂O₃ as a modified NF, a non-enzymatic glucose sensor was utilized to measure the concentration of glucose in alkaline media.

In the same year, Wang's team prepared MOFs-derived In₂O₃/Ti₃C₂T_xMXene composites by oil bath method and subsequent calcination treatment. It was found that In₂O₃/Ti₃C₂T_xMXene exhibited excellent gas sensing performance,¹⁶⁰ with a 60.6% response to 5 ppm NH₃ gas and a detection limit of 5 ppm at room temperature. Furthermore, the sensor indicated outstanding linear response, ultra-fast response/recovery, and high stability over a range (5–100 ppm).

Also in 2022, Shen and colleagues prepared porous hollow hexagonal columns of pure and Co-doped In₂O₃ using MIL-68(In). In₂O₃ inherits the morphology of MIL-68(In) with a porous hollow hexagonal column structure and homogeneous size.³² When doped with Co²⁺, the gas sensitivity of the material will be greatly affected. The sensor prepared from cobalt-doped and pyrolyzed samples at 500 °C showed good gas sensitivity to ethanol gas, with a sensitivity of 14.6 and response/recovery times of 21 and 107 s for 100 ppm EtOH gas at 210 °C. This excellent gas sensitivity may be attributed to the fact that it has the morphology of a hollow hexagonal column and has a porous structure for gas diffusion as well as for the enrichment of active sites with a high specific area. Additionally, the increase in oxygen vacancies due to Co²⁺ doping also contributes to the increase in gas sensitivity.

Using Pd@MIL-68(In) MOFs as precursors, Wang's group produced Pd@In₂O₃-2 in 2023 by a hydrothermal technique and a calcination reduction process.¹⁶¹ Pd@In₂O₃-2 demonstrated the best methane detection performance, according to performance tests conducted on it. Furthermore, the Pd@In₂O₃-2PHTs demonstrated outstanding anti-interference capabilities by tracking methane selectively in the presence of interfering gasses. As a result, this work shows that Pd@In₂O₃ is a good option for creating methane sensors because of its high specific surface area, porous microstructure, elevated oxygen vacancies, and palladium nanoparticle catalytic action.

Following a post-calcination procedure, Volanti et al. prepared In-MIL-68-derived single hollow phase In₂O₃ micro rods using the microwave-assisted solvothermal method (MAS).¹⁶² This nanorod material can be utilized as a microbial volatile organic compounds (MVOC) sensor and exhibits a very high sensitivity to 1-pentanol, Fig. 39a shows a detection limit of 0.5 ppm for 1-pentanol at 350 °C and the response increases with increasing concentration of 1-pentanol, with a response of 6–390 being achieved for 2–100 ppm (Fig. 39b), which suggests that the sensor is highly sensitive to 1-pentanol even at low concentrations. Finally, the sensor based on hollow In₂O₃ microrods denotes promise for detecting 1-pentanol.

Fig. 39 (a) Measurement of 1-pentenal by MVOC sensors in different temperature ranges. (b) Response of different MVOCs to different substances at 350 °C at 100 ppm. Reproduced from ref. 162, with permission from Elsevier Ltd, Copyright 2023.

In 2023, Yu and coworkers, synthesized MIL-68(In)@ZIF-8 precursors using oil bath and self-assembly methods and then obtained In₂O₃@ZnO S-type heterojunctions by heat treatment.¹⁶³ In₂O₃/ZnO-5, formed by loading a certain proportion of ZnO on In₂O₃, showed wonderful sensing performance for HCHO. It was confirmed through a series of experimental tests that the splendid gas-sensitive performance of In₂O₃/ZnO-5 can be ascribed to the derived bi-MOFs In₂O₃@ZnO S-type heterojunctions, which provide a larger specific surface area, thus boosting the amount of adsorbed O on the surface.

Currently, sensing-selective organic molecules typically include derivatives of MIL-68(In) or MOFs, and derivatives of In-MOFs have a considerable specific surface area, which could offer many reactive active sites. In addition, as another MOF is loaded onto the surface of the substrate MOF, a MOF heterojunction on the MOF is generated, which can greatly boost the electrons for the sensing reaction and thus improve the sensing performance.

4.5. Electrochemical properties

In addition to the previously described applications in catalysis, adsorption, and chemical sensing, In-based MOFs can also be used in electrochemistry. In 2016, Xiang's group,¹⁶⁴ reported an attractive MOF (MROF-1), which has interesting structural and topological characters with unique [In₆(thb)₆] metalloring nodes having the highest diameter of about 21 Å. In addition, owing to the abundant existence of (Me₂NH₂)⁺ cations and H₂O units in the 1D channel and the ease with which the hydrophilic (Me₂NH₂)⁺ dissociates from the proton in the presence of H₂O units. The proton conductivity (σ) of MROF-1 can attain 1.72 × 10⁻² S cm⁻¹ under 70 °C/97% relative humidity (RH) (Fig. 40a) being higher than that of many dimethylammonium-bearing compounds (Fig. 40b).

Fig. 40 (a) Nyquist spectra of MROF-1 under 97% RH. (b) Comparison of the σ values. Reproduced from ref. 164, with permission from The Royal Society of Chemistry, Copyright 2016.

One year later, Joarder et al. synthesized an In-sulfonated MOF, {[In(5-Hsip)₂(Me₂NH₂)]·DMF·(H₂O)_1.4}_n (PCMOF-17) (5-H₃sip = 5-sulfoisophthalic acid),¹⁶⁵ which contains H-bonded dimethylammonium cations and an anionic layered structure of water molecules, and the presence of all these groups facilitate proton transport. One of the impedance analyses of the powder showed the σ value >10⁻³ S cm⁻¹ at RT and 40% RH. Because of this MOF's modest RH stability, AC measurements were done on a single-crystal, revealing that the crystals’ major proton transport channel is linked to a continuous H-bonded array.

In 2018, Xiang's team prepared two anionic MOFs with different degrees of interpenetration,¹⁶⁶ [Me₂NH₂][In(PPTTA)]·2.5DMF·2H₂O (FJU-16) and [Me₂NH₂][In(PPTTA)]·4.5DMF·16H₂O (FJU-17) (H₄PPTTA = 4,4′,4′′,4′′′-(1,4-phenylenbis(pyridine-4,2,6-triyl))-tetrabenzoic acid). Owing to the abundant existence of [Me₂NH₂]⁺ and H₂O units in their pores, they attempted to examine their proton conductivity and found that, in the absence of additional humidity, FJU-17 showed a higher intrinsic proton conductivity compared to the 4-fold-interpenetrating FJU-16. Their study illustrates that proton conduction can be rationally improved by controlling the backbone interpenetration.

In 2019, Zhang's group reported a cationic MOF, {{[In₃O(PPTTA)_1.5(H₂O)₃](NO₃)}·(DMA)₃·(CH₃CN)₆·(H₂O)₃₀}_n (FJU-10),⁶⁴ and then encapsulated the dye, 8-hydroxy-1,3,6-stilbenesulfonic acid trisodium salt (HPTS), to get dye@FJU-10. Since HPTS is rich in –OH and –SO₃H, it serves as a good proton carrier. A study of their intrinsic proton conductance without additional RH found that the σ of FJU-10 was 2.05 × 10⁻⁴ S cm⁻¹ (30 °C). In contrast, the σ of dye@FJU-10 was 1.25 × 10⁻³ S cm⁻¹. The highest σ of dye@FJU-10 was 7.50 × 10⁻³ S cm⁻¹ (90 °C) being nearly 5 times higher than that of FJU-10, which suggests that encapsulated dyes in MOFs act as a key role in enhancing the σ.

An extremely thermally stable In-MOF, [In(EBTC)][Me₂NH₂](DMF)(H₂O)₅ (F31) was synthesized by Ren¹¹⁴ and co-workers. At 25 °C and 99% RH, it showed an outstanding σ of 3.49 × 10⁻³ S cm⁻¹. The hydrogen bonds that form between the carboxylate groups in the framework, the lattice H₂O units in the channels, and the [NH₂Me₂]⁺ cations are responsible for this occurrence. Proton transport is facilitated by the cooperation of the lattice. More opportunities for their practical uses in the future are provided by their strong hydrothermal stability and good electrochemical qualities.

Furthermore, Chu's group¹⁶⁷ obtained porous carbon microspheres (PCMSs) with 3D pomegranate-like structures by assembly of hollow highly-graphitized thin-walled nanoballs across directly carbonizing In-MOFs (CPM-5). When the sulfur addition was 70 wt%, the S/PCMSs cathode provided a large initial discharge uptake of 1239 mA h g⁻¹ at 0.2 C and exhibited an excellent high rate capacity of 742 mA h g⁻² at 4 C. Excellent cycle stability was achieved at both low and high rates. Among them, a capacity decay rate of 0.067% per cycle over 700 cycles at 0.5 C and a capacity decay rate of 0.014% per cycle over 500 cycles at 4 C were achieved.

In 2020, Zuo's group,¹⁶⁸ using [In(COO)₄]⁻ nodes and tetratopic tetrathiafulvalene (TTF)-based linkers, fabricated two two-dimensional proton-conducting MOFs, [Me₂NH₂][In(m-TTFTB)] (F32) and [Me₂NH₂][In(TTFOC)] (F33). (m-H₄TTFTB = 3,3′,3′′,3′′′-([2,2′-bi(1,3-dithiolylidene)]-4,4′,5,5′-tetrayl)tetrabenzoic acid). As denoted in Fig. 41a and b, F33 has a higher σ (1.30 × 10⁻² S cm⁻¹ at 303 K and 98% RH) than that of F32 (6.66 × 10⁻⁴ S cm⁻¹) due to the effective proton-conducting channel constituting of Me₂NH₂⁺, H₂O, and the uncoordination carboxylate group. In addition to its excellent proton conductivity, it also has an interfacial pseudocapacitance, realized through its unique out-of-plane proton channel and redox-switchable ligands. In addition, by calculating the activation energy (Fig. 41c and d), the proton/pseudocapacitive coupling mechanism provides a novel approach for building interesting structures using the ionic conductivity of MOFs.

Fig. 41 (a and b) Nyquist spectra of F32 and F33 at 303 K and various RHs. (c and d) The activation energy values of F32 and F33. Reproduced from ref. 168, with permission from Elsevier Inc, Copyright 2019.

In 2020, Zhang and co-workers synthesized¹⁶⁹ a MOF, ((CH₃)₂NH₂)((CH₃)₂NH)[In(mdhbqdc)₂] (In-BQ) (H₂mdhbqdc = dimethyl-3,6-dihydroxy-2,5-benzoquinone-1,4-dicarboxylicacid), which has a modified –OH unit anilicate ligand and dimethylamine cations. A large number of –OH units attached to the inner walls of the pores interacted with Me₂NH and/or Me₂NH₂⁺ by N–H⋯O H-bonds. The high concentration of protons in the porous framework and the dynamics of protonated amines readily lead to a moderate conductivity (2.10 × 10⁻⁴ S cm⁻¹) under 303 K and 95% RH and the E_a of 0.73 eV.

In 2020, Lu's team reported the synthesis of two In-MOFs,⁴¹S1 and S2. The conductivity was 6 and 7 μS cm⁻¹ for S1 and S2 at 300 K and 1 MHz, respectively. Despite their low conductivities, the conductivities at various temperatures and frequencies remained at the same order of magnitude.

In 2021, Zhai and colleagues created CPM-200/CNT@S by mixing indium-based MOF-CPM-200 with multi-walled carbon nanotubes (CNTs) and sulfur.¹⁷⁰ CNTs embedded in the framework of CPM-200 provided the compliant material with good conductive characteristics. CPM-200/CNT@S could be used as a cathode in Li-ion batteries, exhibiting high multiplicity performance and cycle stability, and has an initial discharge capacity of 0.1 C after 100 charge cycles and can reach a capacity of 1400 mA h g⁻¹. Coulombic efficiency is close to 100%. Finally, the unique composite structure of lithium–sulfur battery cathode materials can reduce the “shuttle effect” while increasing electrical conductivity.

By inserting (Me₂NH₂)⁺-encapsulated IN-based anionic metal–organic skeleton (FJU-17) as a “capsule” into HTM, Zhang's group created a bifunctional layer of HTM-FJU-17.¹⁷¹ By releasing (Me₂NH₂)⁺ ions, FJU-17 capsules will passivate organic cationic vacancies. In addition, its anionic skeleton can stabilize the positively charged oxidized HTM to enhance hole transport. Power conversion efficiency (PCE) rose from 18.32% to 20.34% in the perovskite solar cells (PSCs), which also showed decreased charge complexation. Furthermore, after 1000 hours, HTM-FJU-17's hole mobility rose from 3.11 × 10⁻⁴ to 1.05 × 10⁻³ cm² (V s)⁻¹ and ambient circumstances. Stable devices were produced with 90% retention of the original PCE. This study confirms the promising use of anionic MOFs/HTM-based bifunctional layers to create PSC devices.

In 2021, Zhang's group prepared a helical IN-MOF, [In₂(OH)₂(BPTC)]·6H₂O (InOF-1).¹⁷² Then, they proposed the elliptical substitution metal strategy, the elliptical substitution of In(III) by Ni(II), and succeeded in preparing the isostructural MOF, ([Ni₂(BPTC)(HCOOH)₂]·3H₂O) (NiOF-1). The walls of the parent compound InOF-1 with –OH groups cover the channel surface, resulting in a σ of InOF-1 being 7.86 × 10⁻³ S cm⁻¹ (328 K and 95% RH). Aliovalent-substituted and formic acid-modified NiOF-1 possesses stronger host–guest contacts and a richer H-bonding network, resulting in better σ of 3.41 × 10⁻² S cm⁻¹.

Also in 2021, they designed¹⁷³ a carbazole-based non-interpenetrating In-MOF (FJU-302), which has a stable layer structure and contains negatively charged [In(cdc)(bpdc)]⁻. The σ was measured without additional humidity from −30 °C to 70 °C (Fig. 42a and b). The maximum σ of FJU-302 was 6.47 × 10⁻⁴ and 5.88 × 10⁻⁷ S cm⁻¹ at 70 and −30 °C, respectively. E_a calculated from the antimetric measurements was 0.568 eV (Fig. 42c), which proves that the proton transport in FJU-302 follows a Vehicular mechanism.

Fig. 42 (a and b) Nyquist plots of FJU-302 at −30–70 °C without additional humidity. (c) The E_a of FJU-302. Reproduced from ref. 173, with permission from Chinese Chemical Society, Copyright 2021.

In 2022, Farbod and colleagues designed nanocomposites of MIL-68(In) interwoven with CNTs to make MIL-68(In)-CNTs material. Batteries made from MIL-68(In)-CNTs nanoplatforms achieved a substantial capacitance of 314 F g⁻¹ at a scan rate of 2 mV s⁻¹, 15.3 W h kg⁻¹ at a power density of 300 W kg⁻¹, and maintained 95% of the capacitance after 4000 charge/discharge cycles.¹⁷⁴MIL68(In)-CNTs material exhibits fast ion and electron transport. In this way, during charge and discharge, the porous cavities in this nanocomposite serve as electrolyte ion reservoirs.

Jiao et al. prepared In-IPA@rGO by hydrothermally coating three-dimensional In-IPA with rGO.¹⁷⁵ The rGO networks not only greatly enhanced the conductivity of In-IPA but also improved the kinetic reaction reduction and facilitated electron transport. The appreciable initial capacity of In-IPA@rGO 1672.3 mA h g⁻¹ at 0.2 C exhibited a reversible capacity of 898.7 mA h g⁻¹ over 100 cycles. This study confirms that the non-transition MOFs in collaboration with rGOs have a potential application in lithium–sulfur batteries.

Sun's group thermally carbonized InOF-1 under inert argon gas and then formed indium particles through redox reactions. Then, a porous hollow carbon nanostraw (HCNS) was achieved by fusing and removing indium in a decarboxylation process, which has more charge active sites, and short and fast electron and ion transfer channels.¹⁷⁶ In addition, the assembled zinc–iodine batteries (ZIBs) offer a high capacity of 234.1 mA h g⁻¹ at 1 A g⁻¹, and the multiplicity and cycling performance of the HCNS-based ZIBs are highly enhanced.

In 2023, Wen's team synthesized indium-MOFs with a hexagonal nanorod structure,¹⁷⁷ then pyrolyzed the In-MOFs at 450 °C to prepare In/C hexagonal nanorods, and introduced Gr nanosheets to tune the electromagnetic parameters of the In/C nanorods. As the ratio of In/C nanorods to Gr nanosheets is 4 : 1, a strong reflection loss of −43.7 dB at 14.0 GHz was obtained., which indicates that the In/C-Gr composites have significant applications.

In 2024, Li's group prepared five isostructural indium-based MOFs, MIL-68-In or MIL-68-In-X (X = NH₂, OH, Br or NO₂).¹⁷⁸ All five MOFs have high σ values with the optimal σ being 2.37 × 10⁻⁴, 1.70 × 10⁻³, 1.72 × 10⁻³, 4.11 × 10⁻⁴, and 4.47 × 10⁻⁴ S cm⁻¹ (100 °C and 98% RH), respectively.

In contrast to other domains where In-based MOFs are widely used, research into their usage in electrochemistry is still in its early stages. This is due to the strong affinity between the molecules and the backbone of In-MOFs, as well as their special structural stability and variation, and the fact that they form from tightly connected clusters. Its potential application in electrochemistry is thus quite promising. We believe that future studies into these MOFs will yield significant results for the development of innovative electrochemical materials.

5. Conclusions and perspectives

This review included the synthesis, design, and application of In-MOFs, as well as recent scientific breakthroughs. Based on this, a few common In-MOFs are illustrated, along with a brief discussion of their structures and properties. When it comes to MOFs in general, the In-MOF reports are only the tip of the iceberg. Despite indium's relatively low abundance and diffuse distribution in the earth's crust, an increasing number of In-based MOFs are being reported due to their low toxicity, environmental friendliness, and structural stability.

We think that the following issues should be taken into consideration in future research on In-MOFs:

(I) To begin, in the case of currently available In-MOFs, the functionalization of organic ligands may have a significant impact on the final structure. In the presence of various Lewis acids, In³⁺'s peculiar electronic configuration with high-cycle p-blocking enables it to expediently receive electrons and form strong coordination bonds. Making tight connections between O, C, N, S, and other atoms can result in exceptionally stable structures. Simple monocyclic or polycyclic carboxylic acids are still the most widely investigated organic ligands. Imidazole, triazole, and tetrazole are examples of organic ligands with multiple nitrogen atoms; similarly, organic ligands with multiple oxygen atoms in phosphoric acid and sulfonic acid groups require special attention.

(II) Second, the most used technique for synthesizing In-MOFs is still hydrothermal/solvothermal. Nevertheless, this process is usually labor-intensive and unsuitable for large-scale manufacturing. Instead, more should be done to prepare In-MOFs using the recently developed microwave, mechanochemical, and acoustochemical technologies.

(III) In conclusion, because of the diverse architectures and exceptional stability of In-MOFs, a significant deal of research has already been done on the subject. Since the research's applicability is now somewhat narrow, it should be further broadened. At the moment, the primary areas of applied research include luminescence, adsorption and separation, and catalysis. However, more study is required in the fields of heavy metal removal, chemical sensors, proton conductivity, drug delivery—particularly in cancer therapy—and these applications.

If these studies continue, we think there is a lot of promise for improving material design in general and creating new, stable In-MOFs with improved performance characteristics in particular.

Abbreviations

MOFs	Metal–organic frameworks
2D	Two-dimensional
3D	Three-dimensional
1D	One-dimensional
SBU	Structure building unit
σ	Proton conductivity
E a	Activation energy
RH	Relative humidity
DMA	N,N-Dimethylacetamide
DMF	N,N-Dimethylformamide
NMF	N-Methylformamide
DEF	N,N-Diethylformamide
MeOH	Methanol
EtOH	Ethanol
HCOOH	Formic acid
CH3COOH	Acetic acid
1,10-Phen	1,10-Phenanthroline
1,7-Phen	1,7-Phenanthroline
2,2′-bipy	2,2′-Bipyridine
4,4′-bipy	4,4′-Bipyridine
bpe	1,2-Di(4-pyridyl)ethylene
bpa	1,2-Di(4-pyridyl)ethane
H2FcDC	1,1′-Ferrocenedicarboxylic acid
OX	Oxalic acid
HTM	Hole transporting material
HCNS	Hollow carbon nanosheet
ZIBs	Zinc–iodine batteries
rGO	Graphene oxide
CNTs	Carbon nanotubes
MAS	Microwave-assisted solvothermal
ESIPT	Intramolecular proton transfer
TEA	Triethylamine
AuNCs	Fluorescent gold nanoclusters
IAST	Ideal adsorbed solution theory
FL	Fluorescence
NZF	Nitrofurazone
BTEX	Benzenehomologues
ATRP	Atom transfer radical polymerization
ORR	Oxygen reduction reaction
RhB	Rhodamine B
MO	Methyl orange
MB	Methylene blue
OFL	Ofloxacin
FE	Faraday efficiency
PFOA	Perfluoroctanoic acid
OOP	Out-of-plane
QY	Quantum yield
TC	Tetracycline hydrochloride
H4FBDC	2,5-Di(2′,5′-dicarboxylphenyl)-difluorobenzene
H2dcpy	3-(2′,5′-Dicarboxyphenyl)pyridonic acid
SSA	Specific surface area
ODDA	4,4′-((8r,11aR)-4,8-Bis(10-(4-carboxyphenyl)anthracen-9-yl)-6-hydoxy-1,11-dimethyl-6-oxidodibenzo[d,f][1,3,2]dioxaphosphepine-2,10-diyl)dibenzoic acid
ODDM	4′,4′′′-(2,10-Bis(4-carboxyphenyl)-6-hydoxy-1,11-dimethyl-6-oxidodibenzo[d,f][1,3,2]dioxaphosphepine-4,8-diyl)bis(1,1′-biphenyl)-4-carboxylic acid
H2BDC	Terephthalic acid
H2NDC	Naphthalene-1,4-dicarboxylic acid
H4L1	Oxalylbis(aza-nediyl)diisophthalic acid
H4L2	1,1′-Biphenol
H3BTB	4,4′,4′′-Benzene-1,3,5-triyl-tribenzoic acid
CBDA	[5,5′-(Carbonylbis(azanediyl))-diisophthalic acid]
tpt	2,4,6-Tris(4-pyridinyl)-1,3,5-triazine
H4BTA	Benzene-1,2,4,5-tetracarboxylic acid
H6TCPP	(4-Tetracarboxyphenylporphyrin)
H4L3	5,5′-(Butane-1,4-diylbis(oxy))diisophthalic acid
H4L4	5,5′-(1,4-Phenylenebis(methylene))bis(oxy)diisophthalic acid
TDC	2,5-Thiophenedicarboxylic acid
CSA	N-(4-Carboxyphenyl)succinic acid
PDG	N,N′-(1,4-Phenylene dicarbonyl)diglycine
H4EBDC	5,5′-(1,2-Ethynediyl)bis(1,3-benzenedicarboxylic acid)
H2L5	2,2′,6,6′-Tetramethoxy-4,4′-biphenyldicarboxylic acid
H4SFTBA	(4,4′,4′′,4′′′-(9,9′-Spirobi[fluorene]-2,2′,7,7′-tetrayl)tetrabenzoic acid)
H3popha	5-(4-Carboxy-2-nitrophenoxy) isophthalic acid
H4TBAPy	(1,3,6,8-Tetrakis(p-benzoic acid)pyrene)
H4ABTC	(E)-5,5′-(Diazene-1,2-diyl)diisophthalic acid
H4TPTA	([1,1′:3′,1′′-Terphenyl]-4,4′,4′′,6′-tetracarboxylic acid)
QLBM	(Quinoline-2-yl)-benzimidazole
H3DCPN	5-(3′,5′-Dicarboxyphenyl)nicotinic acid
H3L^1	5-(3′,5′-Dicarboxyphenyl)nicotinic acid
H3L^2	3-(2,4-Dicarboxyphenyl)-6-carboxypyridine
TPyP	5,10,15,20-Tetrakis(4-pyridyl)-21H,23H-porphyrin
H5CPDD	4, 4′-(4-(4-Carboxyphenyl)pyridine-2,6-diyl)diisophthalic acid
TATB	4,4′,4′′-s-Triazine-2,4,6-triyl tribenzoate
H4EBTC	1,1′-Ethynebenzene-3,3′,5,5′-tetracarboxylic acid
bpdcH2	4, 4′-Biphenyldicarboxylic acid
H3CTTA	5′-(4-Carboxy-phenyl)-2′,4′,6′-trimethyl-[1,1′:3′,1′′-terphenyl]-4,4′′-dicarboxylic acid
H3TATAB	(4,4′,4′′-s-Triazine-1,3,5-triyltri-p-amino benzoic acid)
H2SBA	4,4′-Sulfonyldibenzoic acid
([EMIM][BF4])	1-Ethyl-3-methylimidazolium tetrafluoroborate
HTDP	2,4,6-Tri(2,4-dicarboxyphenyl)pyridine
H2aip	5-Aminoisophthalic acid
H3sip	5-Sulfoisophthalic acid
H2mdhbqdc	Dimethyl-3,6-dihydroxy-2,5-benzoquinone-1,4-dicarboxylic acid
TTF	Tetratopic tetrathiafulvalene
Odpt	4,4′-Oxydiphthalate
H4PPTTA	4,4′,4′′,4′′′-(1,4-Phenylenbis(pyridine-4,2,6-triyl))-tetrabenzoic acid
H4bptc	3,3′,5,5′-Biphenyl tetracarboxylate
IPA	Isophthalic acid
TCA	4,4′,4′′-Triphenylamine tricarboxylic acid
DHBDC	2,5-Dihydroxyterephthalic acid
H2fdc	Furan-2,5-dicarboxylic acid
H2TDC	Thiophene dicarboxylic acid
H3BTC	Benzene-1,2,3-tricarboxylic acid
H4TCPBDA	N,N,N′,N′-Tetrakis(4-carboxyphenyl)-biphenyl-4,4′-diamine
H3CPTA	2-(2-Carboxypyridin-4-yl)terephthalic acid
H3TATB	2,4,6-Tris(4-carboxyphenyl)-1,3,5-triazine
ODDP	4,4′,4′′,4′′′-((11aR)-6-Hydoxy-1,11-dimethyl-6-oxidodibenzo[d,f][1,3,2]dioxaphosphepine-2,4,8,10-tetrayl)tetrabenzoic acid
HIN	Isonicotinic

Data availability

Data availability is not applicable to this article as no new data were created or analyzed in this study.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

The authors acknowledge the National Natural Science Foundation of China (grant 22071223).

References

1. G. Y. Ryu, S. J. An, S. Yu, K. J. Kim, H. Jae, D. Roh and W. S. Chi, Dual-sulfonated MOF/polysulfone composite membranes boosting performance for proton exchange membrane fuel cells, Eur. Polym. J., 2022, 180, 111601.
2. F. D. Wang, B. C. Wang, B. B. Hao, C. X. Zhang and Q. L. Wang, Designable guest-molecule encapsulation in metal-organic frameworks for proton conductivity, Chem. – Eur. J., 2022, 28, e202103732.
3. S.-L. Yang, G. Li, M.-Y. Guo, W.-S. Liu, R. Bu and E.-Q. Gao, Positive cooperative protonation of a metal–organic framework: pH-responsive fluorescence and proton conduction, J. Am. Chem. Soc., 2021, 143, 8838–8848.
4. D. Suttipat, S. Butcha, S. Assavapanumat, T. Maihom, B. Gupta, A. Perro, N. Sojic, A. Kuhn and C. Wattanakit, Chiral macroporous MOF surfaces for electroassisted enantioselective adsorption and separation, ACS Appl. Mater. Interfaces, 2020, 12, 36548–36557.
5. X. Dong, Q. Fan, W. Hao and Y. Chen, Adsorption and separation of hexane isomers in metal-organic frameworks (MOFs): A computational study, Comput. Theor. Chem., 2021, 1197, 113164.
6. C. Yang, J. Qi, A. Wang, J. Zha, C. Liu and S. Yao, Application of machine learning in MOFs for gas adsorption and separation, Mater. Res. Express, 2023, 10, 122001.
7. S. F. Hammad, I. A. Abdallah, A. Bedair, R. M. Abdelhameed, M. Locatelli and F. R. Mansour, Metal organic framework-derived carbon nanomaterials and MOF hybrids for chemical sensing, TrAC, Trends Anal. Chem., 2024, 170, 117425.
8. Y. Fei, K. Sun and L. Liu, Carbon-dots-referenced metal-organic frameworks for chemical sensing of tumor/mood biomarker 5-hydroxyindoleacetic acid in human urine: Covalent grafting blue-emitting carbon dots onto red-emitting MOF, Spectrochim. Acta, Part A, 2023, 290, 122244.
9. J. F. Olorunyomi, S. T. Geh, R. A. Caruso and C. M. Doherty, Metal–organic frameworks for chemical sensing devices, Mater. Horiz., 2021, 8, 2387–2419.
10. A. Ahmad, S. Khan, S. Tariq, R. Luque and F. Verpoort, Self-sacrifice MOFs for heterogeneous catalysis: Synthesis mechanisms and future perspectives, Mater. Today, 2022, 55, 137–169.
11. I. Ahmed, M. M. H. Mondol, M. J. Jung, G. H. Lee and S. H. Jhung, MOFs with bridging or terminal hydroxo ligands: Applications in adsorption, catalysis, and functionalization, Coord. Chem. Rev., 2023, 475, 214912.
12. K. Hemmer, M. Cokoja and R. A. Fischer, Exploitation of intrinsic confinement effects of MOFs in catalysis, ChemCatChem, 2021, 13, 1683–1691.
13. Y. Tian, G. Liang, T. Fan, J. Shang, S. Shang, Y. Ma, R. Matsuda, M. Liu, M. Wang, L. Li and S. Kitagawa, Chem. Mater., 2019, 31, 8494–8503.
14. K. Zhang, G.-H. Wen, S.-S. Bao, L.-Q. Wu and J.-G. Jia, Studying the proton conduction through the grain surface of UiO-66-NH₂, ACS Appl. Energy Mater., 2020, 3, 8198–8204.
15. E. O. Eren, N. Özkan and Y. Devrim, Preparation of polybenzimidazole/ZIF-8 and polybenzimidazole/UiO-66 composite membranes with enhanced proton conductivity, Int. J. Hydrogen Energy, 2022, 47, 19690–19701.
16. L. M. Aguirre-Díaz, D. Reinares-Fisac, M. Iglesias, E. Gutiérrez-Puebla, F. Gándara, N. Snejko and M. Á. Monge, Group 13th metal-organic frameworks and their role in heterogeneous catalysis, Coord. Chem. Rev., 2017, 335, 1–27.
17. W. Fan, K.-Y. Wang, C. Welton, L. Feng, X. Wang, X. Liu, Y. Li, Z. Kang, H.-C. Zhou, R. Wang and D. Sun, Aluminum metal–organic frameworks: From structures to applications, Coord. Chem. Rev., 2023, 489, 215175.
18. L.-L. Kang, M. Xue, Y.-Y. Liu, Y.-H. Yu, Y.-R. Liu and G. Li, Proton conductive metal–organic frameworks based on main-group metals, Coord. Chem. Rev., 2022, 452, 214301.
19. E. P. Gómez-Oliveira, D. Reinares-Fisac, L. M. Aguirre-Díaz, F. Esteban-Betegón, M. Pintado-Sierra, E. Gutiérrez-Puebla, M. Iglesias, M. Ángeles Monge and F. Gándara, Framework adaptability and concerted structural response in a bismuth metal-organic framework catalyst, Angew. Chem., Int. Ed., 2022, 61, e202209335.
20. Q.-X. Wang and G. Li, Bi(iii) MOFs: synthesesstructures and applications, Inorg. Chem. Front., 2021, 8, 572–589.
21. Y.-H. Luo, A. D. Xie, W.-C. Chen, D. Shen, D.-E. Zhang, Z.-W. Tong and C.-S. Lee, Multifunctional anionic indium–organic frameworks for organic dye separation, white-light emission and dual-emitting Fe³⁺ sensing, J. Mater. Chem. C, 2019, 7, 14897–14903.
22. M. Krüger, M. Albat, A. K. Inge and N. Stock, Investigation of the effect of polar functional groups on the crystal structures of indium MOFs, CrystEngComm, 2017, 19, 4622–4628.
23. C. Volkringer, T. Loiseau, N. Guillou, J. Marrot, G. Férey, M. Haouas, F. Taulelle, N. Audebrand and M. Latroche, The kagomé topology of the gallium and indium metal-organic framework types with a MIL-68 structure: synthesis, XRD, solid-State NMR characterizations, and hydrogen adsorption, Inorg. Chem., 2008, 47, 11901.
24. Y.-B. Tian, Q.-H. Li, Z. Wang, Z.-G. Gu and J. Zhang, Coordination-induced symmetry breaking on metal-porphyrinic framework thin films for enhanced nonlinear optical limiting, Nano Lett., 2023, 23, 3062–3069.
25. R. Zhang, B. Wang, F. Wang, S.-M. Chen and J. Zhang, Syntheses of ferrocene-functionalized indium-based metal–organic frameworks for third order nonlinear optical application, Inorg. Chem. Front., 2023, 10, 201–210.
26. V. Kumar Maka, P. Tamuly, S. Jindal and J. Narasimha Moorthy, Control of In-MOF topologies and tuning of porosity through ligand structure, functionality and interpenetration: Selective cationic dye exchange, Appl. Mater. Today, 2020, 19, 100613.
27. T. C. Schäfer, J. Becker, A. E. Sedykh and K. Müller-Buschbaum, 2D-Coordination polymers constituted from indium halides and dipyridyl N-donor ligands, Z. Anorg. Allg. Chem., 2021, 647, 1227–1233.
28. J. Qian, P. Yu, K. Su, Y. Dong, S. Huang and M. Hong, Crystal structure, morphology and sorption behaviour of porous indium-tetracarboxylate framework materials, CrystEngComm, 2015, 17, 8512–8518.
29. W. Dan, X. Liu, M. Deng, Y. Ling, Z. Chen and Y. Zhou, A highly stable indium phosphonocarboxylate framework as a multifunctional sensor for Cu²⁺ and methylviologen ions, Dalton Trans., 2015, 44, 3794–3800.
30. X. Du, R. Fan, J. Fan, L. Qiang, Y. Song, Y. Dong, K. Xing, P. Wang and Y. Yang, Self-assembly of two supramolecular indium(III) metal–organic frameworks for reversible iodine capture and large band gap change semiconductor behavior, Inorg. Chem. Front., 2016, 3, 1480–1490.
31. H. Zhang, J. Wu, W.-Y. Geng, Y.-H. Luo, Z.-J. Ding, Z.-X. Wang and D.-E. Zhang, An interpenetrated anionic In(III)-MOF for efficient adsorption/separation of organic dyes and selective sensing of Fe³⁺ ion and nitroaromatic compounds, J. Solid State Chem., 2021, 302, 122424.
32. P. Yong, S. Wang, X. Zhang, H. Pan and S. Shen, MOFs-derived Co-doped In₂O₃ hollow hexagonal cylinder for selective detection of ethanol, Chem. Phys. Lett., 2022, 795, 139517.
33. H. Zhang, Z.-X. Wang, Y.-H. Luo, F.-Y. Chen, C.-Y. Jia, X.-Q. Tan, Y.-Y. Zhang and D.-E. Zhang, Co²⁺ and nitrobenzene sensing using indium-based metal-organic framework, Polyhedron, 2022, 224, 116016.
34. H. N. K. Lam, N. B. Nguyen, G. H. Dang, T. Truong and N. T. S. Phan, Three-component coupling of aldehyde, alkyne, and amine via C–H bond activation using indium-based metal–organic framework MIL-68(In) as a recyclable heterogeneous catalyst, Catal. Lett., 2016, 146, 2087–2097.
35. H. Lv, L. Fan, T. Hu, C. Jiao and X. Zhang, A highly robust cluster-based indium(III)–organic framework with efficient catalytic activity in cycloaddition of CO₂ and Knoevenagel condensation, Dalton Trans., 2023, 52, 3420–3430.
36. C. Lu, D. Xiong, C. Chen, J. Wang, Y. Kong, T. Liu, S. Ying and F.-Y. Yi, Indium-based metal–organic framework for efficient photocatalytic hydrogen evolution, Inorg. Chem., 2022, 61, 2587–2594.
37. Y.-H. Li, S.-L. Wang, Y.-C. Su, B.-T. Ko, C.-Y. Tsai and C.-H. Lin, Microporous 2D indium metal–organic frameworks for selective CO₂ capture and their application in the catalytic CO₂-cycloaddition of epoxides, Dalton Trans., 2018, 47, 9474–9481.
38. Y.-Z. Shi, Q.-H. Hu, X. Gao, L. Zhang, R.-P. Liang and J.-D. Qiu, A flexible indium-based metal-organic framework with ultrahigh adsorption capacity for iodine removal from seawater, Sep. Purif. Technol., 2023, 312, 123366.
39. J. Lei, Y.-P. Li, Y.-Y. Xue, Y. Wang, S.-N. Li, Y.-C. Jiang, M.-C. Hu and Q.-G. Zhai, Design of robust rod-packing [In(OH)(BDC)] frameworks and their high CO₂/C₂-hydrocarbons over CH₄ separation performance, J. Solid State Chem., 2019, 279, 20936.
40. B. Zhang, W. Wang, B. Liu and L. Hou, Indium metal–organic frameworks based on pyridylcarboxylate ligands and their potential applications, Dalton Trans., 2021, 50, 5713–5723.
41. S. Kamal, K. R. Chiou, B. Sainbileg, A. I. Inamdar, M. Usman, A. Pathak, T.-T. Luo, J.-W. Chen, M. Hayashi, C.-H. Hung and K.-L. Lu, Thermally stable indium based metal–organic frameworks with high dielectric permittivity, J. Mater. Chem. C, 2020, 8, 9724–9733.
42. S. Choi, T. Kim, H. Ji, H. J. Lee and M. Oh, Isotropic and anisotropic growth of metal–organic framework (MOF) on MOF: logical inference on MOF structure based on growth behavior and morphological feature, J. Am. Chem. Soc., 2016, 138, 14434–14440.
43. J. Ma, H. Fan, X. Zheng, H. Wang, N. Zhao, M. Zhang, A. K. Yadav, W. Wang, W. Dong, S. Wang and J. Facile, Metal-organic frameworks-templated fabrication of hollow indium oxide microstructures for chlorine detection at low temperature, Hazard. Mater., 2020, 387, 122017.
44. R.-R. Xie, X.-A. Guo, Z.-H. Zhang and D.-X. Xue, An oxamide-functionalized ternary indium-organic framework, Inorg. Chem. Commun., 2020, 113, 107762.
45. C. L. Meng, Z. J. Li, Y. Liu, B. Z. Liu and Y. Cui, Synthesis, structure and characterization of a 3D chiral indium carboxylate metal-organic framework based on 1,1′-biphenol ligand, Chin. J. Struct. Chem., 2017, 12, 2086.
46. Z. Chen, C. Yu, J. Xi, S. Tang, T. Bao and J. Zhang, A hybrid material prepared by controlled growth of a covalent organic framework on amino-modified MIL-68 for pipette tip solid-phase extraction of sulfonamides prior to their determination by HPLC, Microchim. Acta, 2019, 186, 393.
47. S. E. Springer, J. J. Mihaly, N. Amirmokhtari, A. B. Crom, M. Zeller, J. I. Feldblyum and D. T. Genna, Framework isomerism in a series of btb-containing In-derived metal–organic frameworks, Cryst. Growth Des., 2019, 19, 3124–3129.
48. Q. Chen, Y. Ying, L. Wang, Z. Guo, Y. Zhou, D. Wang and C. Li, Z. A Heterometallic MOF based on monofunctional linker by “one-pot” solvothermal method for highly selective gas adsorption, Anorg. Allg. Chem., 2020, 646, 437–443.
49. C. J. Tatebe, S. Yusuf, M. K. Bellas, M. Zeller, C. Arntsen and D. T. Genna, On the role of dioxane in the synthesis of In-derived MOFs, Cryst. Growth Des., 2021, 21, 6840–6846.
50. A. K. Inge, M. Köppen, J. Su, M. Feyand, H. Xu, X. Zou, M. O'Keeffe and N. Stock, Unprecedented topological complexity in a metal–organic framework constructed from simple building units, J. Am. Chem. Soc., 2016, 138, 1970–1976.
51. M. Köppen, O. Beyer, S. Wuttke, U. Lüning and N. Stock, Synthesis, functionalisation and post-synthetic modification of bismuth metal–organic frameworks, Dalton Trans., 2017, 46, 8658–8663.
52. L. M. Aguirre-Díaz, M. Iglesias, N. Snejko, E. Gutiérrez-Puebla and M. Á. Monge, Synchronizing substrate activation rates in multicomponent reactions with metal-organic framework catalysts, Chem. – Eur. J., 2016, 22, 6654–6665.
53. K. Maru, S. Kalla, A. K. Ghosh and R. Jangir, Synthesis of microcrystalline indium(III)-MOF and adsorptive and selective removal of dyes, Res. Chem. Intermed., 2023, 50, 147–174.
54. S. Zhou, Q. Lu, M. Chen, B. Li, H. Wei, B. Zi, J. Zeng, Y. Zhang, J. Zhang, Z. Zhu and Q. Liu, Platinum-supported cerium-doped indium oxide for highly sensitive triethylamine gas sensing with good antihumidity, ACS Appl. Mater. Interfaces, 2020, 12, 42962–42970.
55. H. Yang, J. Tang, Y. Luo, X. Zhan, Z. Liang, L. Jiang, H. Hou and W. Yang, MOFs-derived fusiform In₂O₃ mesoporous nanorods anchored with ultrafine CdZnS nanoparticles for boosting visible-light photocatalytic hydrogen evolution, Small, 2021, 17, 2102307.
56. H. Ji, S. Lee, J. Park, T. Kim, S. Choi and M. Oh, Improvement in crystallinity and porosity of poorly crystalline metal–organic frameworks (MOFs) through their induced growth on a well-crystalline MOF template, Inorg. Chem., 2018, 57, 9048–9054.
57. J. Sun, Y. Wang, P. Song, Z. Yang and Q. Wang, Metal-organic framework-derived Cr-doped hollow In₂O₃ nanoboxes with excellent gas-sensing performance toward ammonia, J. Alloys Compd., 2021, 879, 160472.
58. Z. Zhang, Y. Wang, B. Niu, B. Liu, J. Li and W. Duan, Ultra-stable two-dimensional metal–organic frameworks for photocatalytic H₂ production, Nanoscale, 2022, 14, 7146–7150.
59. L.-L. Jiang, X. Zeng, M. Li, M.-Q. Wang, T.-Y. Su, X.-C. Tian and J. Tang, Rapid electrochemical synthesis of HKUST-1 on indium tin oxide, RSC Adv., 2017, 7, 9316–9320.
60. T. Rhauderwiek, S. Waitschat, S. Wuttke, H. Reinsch, T. Bein and N. Stock, Nanoscale synthesis of two porphyrin-based MOFs with gallium and indium, Inorg. Chem., 2016, 55, 5312–5319.
61. M. A. Sinnwell, Q. R. S. Miller, L. Palys, D. Barpaga, L. Liu, M. E. Bowden, Y. Han, S. Ghose, M. L. Sushko, H. T. Schaef, W. Xu, M. Nyman and P. K. Thallapally, Molecular intermediate in the directed formation of a zeolitic metal–organic framework, J. Am. Chem. Soc., 2020, 142, 17598–17606.
62. X.-L. Chen, Y. Pan and X.-Z. Song, Two novel anionic indium–tetracarboxylate frameworks: Syntheses, structures and photoluminescent properties, Polyhedron, 2016, 117, 513–517.
63. D. Reinares-Fisac, L. M. Aguirre-Díaz, M. Iglesias, E. Gutiérrez-Puebla, F. Gándara and M. Á. Monge, Anionic and neutral 2D indium metal–organic frameworks as catalysts for the Ugi one-pot multicomponent reaction, Dalton Trans., 2019, 48, 2988–2995.
64. L. Liu, Z. Yao, Y. Ye, C. Liu, Q. Lin, S. Chen, S. Xiang and Z. Zhang, Enhancement of intrinsic proton conductivity and aniline sensitivity by introducing dye molecules into the MOF channel, ACS Appl. Mater. Interfaces, 2019, 11, 16490–16495.
65. R. M. Zhang, M. H. Zhang, W. Wang, Y. W. Xu, Z. Y. Wen and Z. Y. Wang, Interpenetration of three 2D In-MOFs with (6, 3) topology: syntheses, structures and fluorescent properties, Chin. J. Struct. Chem., 2016, 35, 1722.
66. D. Woschko, S. Yilmaz, C. Jansen, A. Spieß, R. Oestreich, T. J. Matemb Ma Ntep and C. Janiak, Enhanced sorption in an indium-acetylenedicarboxylate metal–organic framework with unexpected chains of cis-μ-OH-connected {InO₆} octahedra, Dalton Trans., 2023, 52, 977–989.
67. L. Wu, W. Wang, R. Liu, G. Wu and H. Chen, Impact of the functionalization onto structure transformation and gas adsorption of MIL-68(In), R. Soc. Open Sci., 2018, 5, 181378.
68. J. J. Mihaly, M. Zeller and D. T. Genna, Ion-directed synthesis of indium-derived 2,5-thiophenedicarboxylate metal–organic frameworks: tuning framework dimensionality, Cryst. Growth Des., 2016, 16, 1550–1558.
69. J. Benecke, E. S. Grape, A. Fuß, S. Wöhlbrandt, T. A. Engesser, A. K. Inge, N. Stock and H. Reinsch, Polymorphous indium metal–organic frameworks based on a ferrocene linker: redox activity, porosity, and structural diversity, Inorg. Chem., 2020, 59, 9969–9978.
70. J. Haddad, G. F. S. Whitehead, A. P. Katsoulidis and M. J. Rosseinsky, In-MOFs based on amide functionalised flexible linkers, Faraday Discuss., 2017, 201, 327–335.
71. Z. Liu, Q. Li, H. Zhu, K. Lin, J. Deng, J. Chen and X. Xing, 3D negative thermal expansion in orthorhombic MIL-68(In), Chem. Commun., 2018, 54, 5712–5715.
72. B. Zheng, X. Sun, G. Li, A. J. Cairns, V. C. Kravtsov, Q. Huo, Y. Liu and M. Eddaoudi, Solvent-controlled assembly of ionic metal–organic frameworks based on indium and tetracarboxylate ligand: topology variety and gas sorption properties, Cryst. Growth Des., 2016, 16, 5554–5562.
73. A. J. Cairns, J. Eckert, L. Wojtas, M. Thommes, D. Wallacher, P. A. Georgiev, P. M. Forster, Y. Belmabkhout, J. Ollivier and M. Eddaoudi, Gaining insights on the H₂-sorbent interactions: robust soc-MOF platform as a case study, Chem. Mater., 2016, 28, 7353–7361.
74. X. Du, R. Fan, L. Qiang, K. Xing, H. Ye, X. Ran, Y. Song, P. Wang and Y. Yang, Controlled Zn²⁺-triggered drug release by preferred coordination of open active sites within functionalization indium metal organic frameworks, ACS Appl. Mater. Interfaces, 2017, 9, 28939–28948.
75. J. Li, Y. Fan and Y. Ren, Structures and characterization of four metal-organic frameworks constructed by 2,2′,6,6′-tetramethoxy-4,4′-biphenyldicarboxylic acid, Z. Anorg. Allg. Chem., 2017, 643, 612–618.
76. J. Pang, M. Wu, J.-S. Qin, C. Liu, C. T. Lollar, D. Yuan, M. Hong and H.-C. Zhou, Solvent-assisted, thermally triggered structural transformation in flexible mesoporous metal–organic frameworks, Chem. Mater., 2019, 31, 8787–8793.
77. Y. Chen, X. Zhang, H. Chen, R. J. Drout, Z. Chen, M. R. Mian, R. R. Maldonado, K. Ma, X. Wang, Q. Xia, Z. Li, T. Islamoglu, R. Q. Snurr and O. K. Farha, Tuning the atrazine binding sites in an indium-based flexible metal–organic framework, ACS Appl. Mater. Interfaces, 2020, 12, 44762–44768.
78. H. Cui, Y. Ye, R. Lin, Y. Shi, Z. A. Alothman, O. Alduhaish, B. Wang, J. Zhang and B. Chen, An indium-based microporous metal–organic framework with unique three-way rod-shaped secondary building units for efficient methane and hydrogen storage, Inorg. Chem. Front., 2022, 9, 6527–6533.
79. M. Feng, P. Zhou, J. Wang, X. Wang, D. Wang and C. Li, Two solvent-induced In(III)-based metal–organic frameworks with controllable topology performing high-efficiency separation of C₂H₂/CH₄ and CO₂/CH₄, Inorg. Chem., 2022, 61, 11057–11065.
80. Z. Liu, C. Xing, S. Wu, M. Ma and J. Tian, Biphenyl tetracarboxylic acid-based metal–organic frameworks: a case of topology-dependent thermal expansion, Mater. Horiz., 2024, 11, 3345–3351.
81. J.-X. Wu and B. Yan, Lanthanides post-functionalized indium metal–organic frameworks (MOFs) for luminescence tuning, polymer film preparation and near-UV white LED assembly, Dalton Trans., 2016, 45, 18585–18590.
82. Y. Yuan, J. Li, X. Sun, G. Li, Y. Liu, G. Verma and S. Ma, Indium–organic frameworks based on dual secondary building units featuring halogen-decorated channels for highly effective CO₂ fixation, Chem. Mater., 2019, 31, 1084–1091.
83. Y.-W. Peng, R.-J. Wu, M. Liu, S. Yao, A.-F. Geng and Z.-M. Zhang, Nitrogen coordination to dramatically enhance the stability of In-MOF for selectively capturing CO₂ from a CO₂/N₂ mixture, Cryst. Growth Des., 2019, 19, 1322–1328.
84. S. Pappuru, K. B. Idrees, Z. Chen, D. Shpasser and O. M. Gazit, A rare 4-fold interpenetrated metal–organic framework constructed from an anionic indium-based node and a cationic dicopper linker, Dalton Trans., 2021, 50, 6631–6636.
85. V. Jeevananthan, G. C. Senadi, K. Muthu, A. Arumugam and S. Shanmugan, Construction of indium(III)–organic framework based on a flexible cyclotriphosphazene-derived hexacarboxylate as a reusable green catalyst for the synthesis of bioactive aza-heterocycles, Inorg. Chem., 2024, 63, 5446–5463.
86. W. Guo, X. Sun, C. Chen, D. Yang, L. Lu, Y. Yang and B. Han, Metal–organic framework-derived indium–copper bimetallic oxide catalysts for selective aqueous electroreduction of CO₂, Green Chem., 2019, 21, 503–508.
87. W. Guo, X. Tan, J. Bi, L. Xu, D. Yang, C. Chen, Q. Zhu, J. Ma, A. Tayal, J. Ma, Y. Huang, X. Sun, S. Liu and B. Han, Atomic indium catalysts for switching CO₂ electroreduction products from formate to CO, J. Am. Chem. Soc., 2021, 143, 6877–6885.
88. Y. Zhou, S. Liu, Y. Gu, G.-H. Wen, J. Ma, J.-L. Zuo and M. Ding, In(III) metal–organic framework incorporated with enzyme-mimicking nickel bis(dithiolene) ligand for highly selective CO₂ electroreduction, J. Am. Chem. Soc., 2021, 143, 14071–14076.
89. Q. Huang, S. Xu, J. Liu, Y. Guo, D. Chen, Q. Sun, L. Zhang, H. Nie, Z. Yang and J. Qian, Fe phthalocyanine stabilized on phosphorous-doped multi-defective carbon nanoribbons as oxygen reduction electrocatalysts, Appl. Catal., B, 2023, 339, 123172.
90. S. Wang, Y. Wang, J.-M. Huo, W.-Y. Yuan, P. Zhang, S.-N. Li and Q.-G. Zhai, Multivariate indium–organic frameworks for highly efficient carbon dioxide capture and electrocatalytic conversion, Inorg. Chem. Front., 2023, 10, 158–167.
91. S.-H. Li, S. Hu, H. Liu, J. Liu, X. Kang, S. Ge, Z. Zhang, Q. Yu and B. Liu, Two-dimensional metal coordination polymer derived indium nanosheet for efficient carbon dioxide reduction to formate, ACS Nano, 2023, 17, 9338–9346.
92. C. Yang, X. You, J. Cheng, H. Zheng and Y. Chen, A novel visible-light-driven In-based MOF/graphene oxide composite photocatalyst with enhanced photocatalytic activity toward the degradation of amoxicillin, Appl. Catal., B, 2017, 200, 673–680.
93. F. Leng, H. Liu, M. Ding, Q.-P. Lin and H.-L. Jiang, Boosting photocatalytic hydrogen production of porphyrinic MOFs: the metal location in metalloporphyrin matters, ACS Catal., 2018, 8, 4583–4590.
94. Y. Fang, S.-R. Zhu, M.-K. Wu, W.-N. Zhao and L. Han, MOF-derived In₂S₃ nanorods for photocatalytic removal of dye and antibiotics, J. Solid State Chem., 2018, 266, 205–209.
95. S.-S. Wang, H.-H. Huang, M. Liu, S. Yao, S. Guo, J.-W. Wang, Z.-M. Zhang and T.-B. Lu, Encapsulation of single iron sites in a metal–porphyrin framework for high-performance photocatalytic CO₂ reduction, Inorg. Chem., 2020, 59, 6301–6307.
96. S. Wang, F. Meng, X. Sun, M. Bao, J. Ren, S. Yu, Z. Zhang, J. Ke and L. Zeng, Bimetallic Fe/In metal-organic frameworks boosting charge transfer for enhancing pollutant degradation in wastewater, Appl. Surf. Sci., 2020, 528, 147053.
97. X. Liu, B. Xu, X. Duan, Q. Hao, W. Wei, S. Wang and B.-J. Ni, Facile preparation of hydrophilic In₂O₃ nanospheres and rods with improved performances for photocatalytic degradation of PFOA, Environ. Sci.: Nano, 2021, 8, 1010–1018.
98. Q. Zhang, J. Zhang, X. Wang, L. Li, Y.-F. Li and W.-L. Dai, In–N–In sites boosting interfacial charge transfer in carbon-coated hollow tubular In₂O₃/ZnIn₂S₄ heterostructure derived from In-MOF for enhanced photocatalytic hydrogen evolution, ACS Catal., 2021, 11, 6276–6289.
99. Q. Sun, Y. Zhu, X. Zhong, M. Jiang, Y. Fan and J. Yao, Tuning photoactive MIL-68(In) by functionalized ligands for boosting visible-light nitrogen fixation, ACS Appl. Mater. Interfaces, 2022, 14, 53904–53915.
100. L. Hou, X. Jing, H. Huang and C. Duan, Interpenetrating dye-functionalized indium–organic frameworks for photooxidative cyanation and oxidative cyclization, J. Mater. Chem. A, 2022, 10, 24320–24330.
101. H. Zhang, F.-Y. Chen, Z.-Y. Liu, Y.-H. Luo, Z.-L. Zhou, X.-M. Jia, X. Wang and Y.-Q. Yang, Photocatalytic activation of peroxodisulfate using iron-containing MOFs synthesized by single-crystal-to-single-crystal transformation for ofloxacin degradation, New J. Chem., 2023, 47, 21081–21090.
102. J.-Y. Tian, W.-C. Lv, A.-S. Shen, Y. Ma, M. Wang, S. Zhang, X.-L. Liu, Z. Zhang and M. Du, Construction of the copper metal-organic framework (MOF)-on-indium MOF Z-scheme heterojunction for efficiently photocatalytic reduction of Cr(VI), Sep. Purif. Technol., 2023, 327, 124903.
103. X. Li, X. Guo, J. Niu, J. Lin, J. Cheng, Y. Hu, Y. Chen and W. Wen, Constructing self-assembled microtubular structured Zn–In–S and modulation mechanism towards efficient photocatalytic degradation of tetracycline, J. Cleaner Prod., 2023, 385, 135685.
104. P. T. M. Ha, T. N. Lieu, S. H. Doan, T. T. B. Phan, T. T. Nguyen, T. Truong and N. T. S. Phan, Indium-based metal–organic frameworks as catalysts: synthesis of 2-nitro-3-arylimidazo[1,2-a]pyridines via oxidative amination under air using MIL-68(In) as an effective heterogeneous catalyst, RSC Adv., 2017, 7, 23073–23082.
105. X. Chen, H. Jiang, X. Li, B. Hou, W. Gong, X. Wu, X. Han, F. Zheng, Y. Liu, J. Jiang and Y. Cui, Chiral phosphoric acids in metal–organic frameworks with enhanced acidity and tunable catalytic selectivity, Angew. Chem., Int. Ed., 2019, 58, 14748–14757.
106. D. Reinares-Fisac, L. M. Aguirre-Díaz, M. Iglesias, E. Gutiérrez-Puebla, F. Gándara and M. Á. Monge, Anionic and neutral 2D indium metal–organic frameworks as catalysts for the Ugi one-pot multicomponent reaction, Dalton Trans., 2019, 48, 2988–2995.
107. G.-M. Liang, P. Xiong, K. Azam, Q.-L. Ni, J.-Q. Zeng, L.-C. Gui and X.-J. Wang, A discrete tetrahedral indium cage as an efficient heterogeneous catalyst for the fixation of CO₂ and the strecker reaction of ketones, Inorg. Chem., 2020, 59, 1653–1659.
108. R. A. Peralta, M. T. Huxley, P. Lyu, M. L. Díaz-Ramírez, S. H. Park, J. L. Obeso, C. Leyva, C. Y. Heo, S. Jang, J. H. Kwak, G. Maurin, I. A. Ibarra and N. C. Jeong, Engineering catalysis within a saturated In(III)-based MOF possessing dynamic ligand–metal bonding, ACS Appl. Mater. Interfaces, 2022, 15, 1410–1417.
109. A. Helal, M. H. Zahir, A. Albadrani and M. M. Ekhwan, Triazole appended metal–organic framework for CO₂ fixation as cyclic carbonates under solvent-free ambient conditions, Catal. Lett., 2022, 153, 2883–2891.
110. X. Li, D. Chen, Y. Liu, Z. Yu, Q. Xia, H. Xing and W. Sun, Anthracene-based indium metal–organic framework as a promising photosensitizer for visible-light-induced atom transfer radical polymerization, CrystEngComm, 2016, 18, 3696–3702.
111. W. Yan, L. Wang, K. Yangxiao, Z. Fu and T. Wu, Effects of ligand and guest solvent molecules on the luminescence properties of Tb:Eu-codoped indium-based MOFs, Dalton Trans., 2016, 45, 4518–4521.
112. J. X. Wu and B. Yan, A Europium ion post-functionalized indium metal–organic framework hybrid system for fluorescence detection of aromatics, Analyst, 2017, 142, 4633–4637.
113. S.-L. Hou, J. Dong, X.-L. Jiang, Z.-H. Jiao, C.-M. Wang and B. Zhao, Interpenetration-dependent luminescent probe in indium-organic frameworks for selectively detecting nitrofurazone in water, Anal. Chem., 2018, 90, 1516–1519.
114. L. Zhai, J.-W. Yu, J. Zhang, W.-W. Zhang, L. Wang and X.-M. Ren, High quantum yield pure blue emission and fast proton conduction from an indium–metal–organic framework, Dalton Trans., 2019, 48, 12088–12095.
115. Z.-F. Wu, B. Tan, E. Velasco, H. Wang, N.-N. Shen, Y.-J. Gao, X. Zhang, K. Zhu, G.-Y. Zhang, Y.-Y. Liu, X.-Z. Hei, X.-Y. Huang and J. Li, Fluorescent In based MOFs showing “turn on” luminescence towards thiols and acting as a ratiometric fluorescence thermometer, J. Mater. Chem. C, 2019, 7, 3049–3055.
116. W.-B. Zhong, R.-X. Li, J. Lv, T. He, M.-M. Xu, B. Wang, L.-H. Xie and J.-R. Li, Two isomeric In(iii)-MOFs: unexpected stability difference and selective fluorescence detection of fluoroquinolone antibiotics in water, Inorg. Chem. Front., 2020, 7, 1161–1171.
117. Q. Li, B.-B. Guan, W. Zhu, T.-H. Liu, L.-H. Chen, Y. Wang and D.-X. Xue, Highly selective and sensitive dual-fluorescent probe for cationic Pb²⁺ and anionic Cr₂O₇²⁻, CrO₄²⁻ contaminants via a powerful indium−organic framework, J. Solid State Chem., 2020, 291, 121672.
118. J. Ren, X. Zhou and Y. Wang, Dual-emitting CsPbX₃@ZJU-28 (X = Cl, Br, I) composites with enhanced stability and unique optical properties for multifunctional applications, Chem. Eng. J., 2020, 391, 123622.
119. B.-B. Guan, Q. Li, Y.-T. Xu, L.-H. Chen, Z. Wu, Z.-L. Fan and W. Zhu, Highly selective and sensitive detection towards cationic Cu²⁺ and Fe³⁺ contaminants via an In-MOF based dual-responsive fluorescence probe, Inorg. Chem. Commun., 2020, 122, 108273.
120. S. Yoon, H. C. Kim, Y. Kim and S. Huh, Photophysical properties and electrochromism of viologen encapsulated viologen@InBTB metal–organic framework, Bull. Korean Chem. Soc., 2020, 42, 326–332.
121. H. Zhang, Z.-J. Ding, Y.-H. Luo, W.-Y. Geng, Z.-X. Wang and D.-E. Zhang, Assembly of a rod indium–organic framework with fluorescence properties for selective sensing of Cu²⁺, Fe³⁺ and nitroaromatics in water, CrystEngComm, 2022, 24, 667–673.
122. Y. L. Zhao, Y. B. Xie, L. Wang, L. H. Xie, X. Zhang and J. R. Li, An interpenetrated anionic In metal-organic framework for selective sensing of Fe³⁺ in water, Chin. J. Inorg. Chem., 2022, 38, 1824.
123. B. Wang, W. Zheng, J. Chen, Y. Wang, X. Duan, S. Ma, Z. Kong and T. Xia, A Tb³⁺ ion encapsulated anionic indium-organic framework as logical probe for distinguishing quenching Fe³⁺ and Cu²⁺ ions, Spectrochim. Acta, Part A, 2024, 304, 123388.
124. R. A. Peralta, A. Campos-Reales-Pineda, H. Pfeiffer, J. R. Álvarez, J. A. Zárate, J. Balmaseda, E. González-Zamora, A. Martínez, D. Martínez-Otero, V. Jancik and I. A. Ibarra, CO₂ capture enhancement in InOF-1 via the bottleneck effect of confined ethanol, Chem. Commun., 2016, 52, 10273–10276.
125. X. Li, X. Chen, F. Jiang, L. Chen, S. Lu, Q. Chen, M. Wu, D. Yuan and M. Hong, The dynamic response of a flexible indium based metal–organic framework to gas sorption, Chem. Commun., 2016, 52, 2277–2280.
126. E. Sánchez-González, E. González-Zamora, D. Martínez-Otero, V. Jancik and I. A. Ibarra, Bottleneck Effect of N,N-dimethylformamide in InOF-1: increasing CO₂ capture in porous coordination polymers, Inorg. Chem., 2017, 56, 5863–5872.
127. B. Y. Liu, Y. J. Gong, X. N. Wu, Q. Liu, W. Li, S. S. Xiong, S. Hu and X. L. Wang, Enhanced xenon adsorption and separation with an anionic indium–organic framework by ion exchange with Co²⁺, RSC Adv., 2017, 7, 55012–55019.
128. Y.-H. Li, S.-L. Wang, Y.-C. Su, B.-T. Ko, C.-Y. Tsai and C.-H. Lin, Microporous 2D indium metal–organic frameworks for selective CO₂ capture and their application in the catalytic CO₂-cycloaddition of epoxides, Dalton Trans., 2018, 47, 9474–9481.
129. H. Atallah, M. Elcheikh Mahmoud, F. M. Ali, A. Lough and M. Hmadeh, A highly stable indium based metal organic framework for efficient arsenic removal from water, Dalton Trans., 2018, 47, 799–806.
130. H.-Y. Liu, G.-M. Gao, F.-L. Bao, Y.-H. Wei and H.-Y. Wang, Enhanced water stability and selective carbon dioxide adsorption of a soc-MOF with amide-functionalized linkers, Polyhedron, 2019, 160, 207–212.
131. Z. Cao, L. Chen, S. Li, M. Yu, Z. Li, K. Zhou, C. Liu, F. Jiang and M. Hong, A flexible two-fold interpenetrated indium MOF exhibiting dynamic response to gas adsorption and high-sensitivity detection of nitroaromatic explosives, Chem. – Asian J., 2019, 14, 3597–3602.
132. I.-H. Choi, S. B. Yoon, S.-Y. Jang, S. Huh, S.-J. Kim and Y. Kim, Gas sorption properties of a new three-dimensional In-ABDC MOF with a diamond Net, Front. Mater., 2019, 6, 7.
133. S. Moribe, Z. Chen, S. Alayoglu, Z. H. Syed, T. Islamoglu and O. K. Farha, Ammonia capture within isoreticular metal–organic frameworks with rod secondary building units, ACS Mater. Lett., 2019, 1, 476–480.
134. Z. Cao, L. Chen, F. Jiang, K. Zhou, M. Yu, T. Jing, S. Li, Z. Li and M. Hong, Incorporating three chiral channels into an In-MOF for excellent gas absorption and preliminary Cu²⁺ ion detection, Cryst. Growth Des., 2019, 19, 3860–3868.
135. H. Chen, L. Fan, X. Zhang and L. Ma, Nanocage-based InIII{TbIII}₂-organic framework featuring lotus-shaped channels for highly efficient CO₂ fixation and I₂ capture, ACS Appl. Mater. Interfaces, 2020, 12, 27803–27811.
136. Y.-M. Gu, H.-F. Qi, S. Qadir, T.-J. Sun, R. Wei, S.-S. Zhao, X.-W. Liu, Z. Lai and S.-D. Wang, A two-dimensional stacked metal-organic framework for ultra highly-efficient CO₂ sieving, Chem. Eng. J., 2022, 449, 137768.
137. C. Zuo, Q. Li, M. Dai and C. Fan, Solvent-regulated formation of metal/metal-oxo Nodes in two indium metal-organic Frameworks: syntheses, structures, selective gas adsorption and fluorescence sensor properties, Chem. – Eur. J., 2024, e202402437.
138. H. Liu, G. Gao, J. Liu, F. Bao, Y. Wei and H. Wang, Amide-functionalized ionic indium–organic frameworks for efficient separation of organic dyes based on diverse adsorption interactions, CrystEngComm, 2019, 21, 2576–2584.
139. P. Huang, C. Chen, M. Wu, F. Jiang and M. Hong, An indium–organic framework for the efficient storage of light hydrocarbons and selective removal of organic dyes, Dalton Trans., 2019, 48, 5527–5533.
140. X. Shi, Y. Zu, S. Jiang and F. Sun, An anionic indium–organic framework with spirobifluorene-based ligand for selective adsorption of organic dyes, Inorg. Chem., 2021, 60, 1571–1578.
141. Q. Chu, B. Zhang, Z. Yang, H. Zhou, H. Mu, W. Zhang, B. Liu and Y.-Y. Wang, Stable indium pyridylcarboxylate framework with highly selective adsorption of cationic dyes and effective nitenpyram detection, Inorg. Chem., 2021, 60, 5232–5239.
142. Y. Lv, R. Zhang, S. Zeng, K. Liu, S. Huang, Y. Liu, P. Xu, C. Lin, Y. Cheng and M. Liu, Removal of p-arsanilic acid by an amino-functionalized indium-based metal–organic framework: Adsorption behavior and synergetic mechanism, Chem. Eng. J., 2018, 339, 359–368.
143. B. Zhang, S.-H. Zhang, B. Liu, K.-F. Yue, L. Hou and Y.-Y. Wang, Stable indium-pyridylcarboxylate framework: selective gas capture and sensing of Fe³⁺ ion in water, Inorg. Chem., 2018, 57, 15262–15269.
144. Q. Chu, B. Zhang, H. Zhou, B. Liu, L. Hou and Y.-Y. Wang, Effective C₂H₂ separation and nitrofurazone detection in a stable indium–organic framework, Inorg. Chem., 2020, 59, 2853–2860.
145. Z. Chen, S. Xiang, H. D. Arman, P. Li, S. Tidrow, D. Zhao and B. Chen, A microporous metal–organic framework with immobilized –OH functional groups within the pore surfaces for selective gas sorption, Eur. J. Inorg. Chem., 2010, 2010, 3745–3749.
146. S. H. Xiang, W. Zhou, Y. Liu and B. L. Chen, Exceptionally high acetylene uptake in a microporous metal-organic framework with open metal sites, J. Am. Chem. Soc., 2009, 131, 12419.
147. Y. Zhang, L. Yang, L. Wang, S. Duttwyler and H. Xing, A microporous metal-organic framework supramolecularly assembled from a cuII dodecaborate cluster complex for selective gas separation, Angew. Chem., Int. Ed., 2019, 58, 8145–8150.
148. B. Zhang, P.-Y. Guo, L.-N. Ma, B. Liu, L. Hou and Y.-Y. Wang, Two robust In(III)-based metal–organic frameworks with higher gas separation, efficient carbon dioxide conversion, and rapid detection of antibiotics, Inorg. Chem., 2020, 59, 5231–5239.
149. Q. Song, Y. Yang, F. Yuan, S. Zhu, J. Wang, S. Xiang and Z. Zhang, Electrostatic force-driven lattice water bridging to stabilize a partially charged indium MOF for efficient separation of C₂H₂/CO₂ mixtures, J. Mater. Chem. A, 2022, 10, 9363–9369.
150. L.-N. Ma, L. Zhang, W.-F. Zhang, Z.-H. Wang, L. Hou and Y.-Y. Wang, Amide-functionalized In-MOF for effective hydrocarbon separation and CO₂ catalytic fixation, Inorg. Chem., 2022, 61, 2679–2685.
151. Y. Lv, X. Wang, C. Jian, L. Yue, S. Lin, T. Zhang, B. Li, D.-L. Chen and Y. He, A two-way rod-packing indium-based nanoporous metal–organic framework for effective C₂/C₁ and C₂/CO₂ separation, ACS Appl. Nano Mater., 2022, 5, 18654–18663.
152. W. Wang, W. Yuan, C. Kong, Y. Yang, S. Xi, X. Liu and B. Liu, An ultramicroporous multi-walled metal–organic framework for efficient C₂H₂/CO₂ separation under humid conditions, J. Mater. Chem. A, 2023, 11, 25605–25611.
153. X. Du, R. Fan, L. Qiang, P. Wang, Y. Song, K. Xing, X. Zheng and Y. Yang, Encapsulation and sensitization of Ln³⁺ within indium metal–organic frameworks for ratiometric Eu³⁺ sensing and linear dependence of white-light emission, Cryst. Growth Des., 2017, 17, 2746–2756.
154. X.-J. Wu, F. Kong, C.-Q. Zhao and S.-N. Ding, Ratiometric fluorescent nanosensors for ultra-sensitive detection of mercury ions based on AuNCs/MOFs, Analyst, 2019, 144, 2523–2530.
155. H. Zhang, J. Wu, W.-Y. Geng, Y.-H. Luo, Z.-J. Ding, Z.-X. Wang and D.-E. Zhang, An interpenetrated anionic In(III)-MOF for efficient adsorption/separation of organic dyes and selective sensing of Fe³⁺ ion and nitroaromatic compounds, J. Solid State Chem., 2021, 302, 122424.
156. V. Chernikova, O. Yassine, O. Shekhah, M. Eddaoudi and K. N. Salama, Highly sensitive and selective SO₂ MOF sensor: the integration of MFM-300 MOF as a sensitive layer on a capacitive interdigitated electrode, J. Mater. Chem. A, 2018, 6, 5550–5554.
157. X. Jiang, R. Fan, X. Zhou, K. Zhu, T. Sun, X. Zheng, K. Xing, W. Chen and Y. Yang, Mixed functionalization strategy on indium-organic framework for multiple ion detection and H₂O₂ turn-on sensing, Dalton Trans., 2021, 50, 7554–7562.
158. J. Lei, B. Wang, Y.-P. Li, W.-J. Ji, K. Wang, H. Qi, P.-T. Chou, M.-M. Zhang, H. Bian and Q.-G. Zhai, A new molecular recognition concept: multiple hydrogen bonds and their optically triggered proton transfer in confined metal–organic frameworks for superior sensing element, ACS Appl. Mater. Interfaces, 2021, 13, 22457–22465.
159. D. Arif, Z. Hussain, A. D. Abbasi and M. Sohail, Ag functionalized In₂O₃ derived from MIL-68(In) as an efficient electrochemical glucose sensor, Front. Chem., 2022, 10, 10.
160. M. Liu, J. Wang, P. Song, J. Ji and Q. Wang, Metal-organic frameworks-derived In₂O₃ microtubes/Ti₃C₂T_x MXene composites for NH₃ detection at room temperature, Sens. Actuators, B, 2022, 361, 131755.
161. X. Zuo, Z. Yang, J. Kong, Z. Han, J. Zhang, X. Meng, S. Hao, L. Wu, S. Wu, J. Liu, Z. Wang and F. Wang, Imbedding Pd nanoparticles into porous In₂O₃ structure for enhanced low-concentration methane sensing, Sensors, 2023, 23, 1163.
162. B. S. de Sá, T. M. Perfecto, D. S. de Oliveira and D. P. Volanti, Microwave-assisted solvothermal synthesis of In-MIL-68 derived hollow In₂O₃ microrods for enhanced 1-pentanol sensing performance, Mater. Today Commun., 2023, 37, 107174.
163. Y. Peng, B. Cheng, L. Zhang, J. Liu and J. Yu, In₂O₃/ZnO S-scheme heterojunction nanocomposite hollow microtubes with highly sensitive response to formaldehyde, Sens. Actuators, B, 2023, 385, 133700.
164. Y.-H. Han, Y. Ye, C. Tian, Z. Zhang, S.-W. Du and S. Xiang, High proton conductivity in an unprecedented anionic metalloring organic framework (MROF) containing novel metalloring clusters with the largest diameter, J. Mater. Chem. A, 2016, 4, 18742–18746.
165. B. Joarder, J.-B. Lin, Z. Romero and G. K. H. Shimizu, Single crystal proton conduction study of a metal organic framework of modest water stability, J. Am. Chem. Soc., 2017, 139, 7176–7179.
166. L. Liu, Z. Yao, Y. Ye, Q. Lin, S. Chen, Z. Zhang and S. Xiang, Enhanced intrinsic proton conductivity of metal–organic frameworks by tuning the degree of interpenetration, Cryst. Growth Des., 2018, 18, 3724–3728.
167. S. Liu, T. Zhao, X. Tan, L. Guo, J. Wu, X. Kang, H. Wang, L. Sun and W. Chu, 3D pomegranate-like structures of porous carbon microspheres self-assembled by hollow thin-walled highly-graphitized nanoballs as sulfur immobilizers for Li–S batteries, Nano Energy, 2019, 63, 103894.
168. J. Su, W. He, X.-M. Li, L. Sun, H.-Y. Wang, Y.-Q. Lan, M. Ding and J.-L. Zuo, High electrical conductivity in a 2D MOF with intrinsic superprotonic conduction and interfacial pseudo-capacitance, Matter, 2020, 2, 711–722.
169. H. Gao, Y.-B. He, J.-J. Hou, Q.-G. Zhai and X.-M. Zhang, Enhanced proton conductivity by aliovalent substitution of cadmium for indium in dimethylaminium-templated metal anilicates, ACS Appl. Mater. Interfaces, 2020, 12, 41605–41612.
170. G. Han, T. Deng, X. Jiao, X. Men, J. Wang, Y. Wang and Q. Zhai, Indium-based MOFs and carbon nanotube embedded efficient cathodes for high-performance lithium-sulfur batteries, Ionics, 2021, 27, 5115–5125.
171. J. Zhang, S. Guo, M. Zhu, C. Li, J. Chen, L. Liu, S. Xiang and Z. Zhang, Simultaneous defect passivation and hole mobility enhancement of perovskite solar cells by incorporating anionic metal-organic framework into hole transport materials, Chem. Eng. J., 2021, 408, 127328.
172. H. Gao, Y.-B. He, J.-J. Hou and X.-M. Zhang, In situ aliovalent nickle substitution and acidic modification of nanowalls promoted proton conductivity in InOF with 1D helical channel, ACS Appl. Mater. Interfaces, 2021, 13, 38289–38295.
173. S. S. Guo, L. L. Huang, Y. X. Ye, L. Z. Liu, Z. Z. Yao, S. C. Xiang, J. D. Zhang and Z. J. Zhang, Carbazole based anionic MOF for proton conductivity, Chin. J. Struct. Chem., 2021, 40, 60.
174. F. Farbod, M. Mazloum-Ardakani, H. R. Naderi, A. Mirvakili, M. Wang, D. V. Shinde, S. Dante, P. Salimi, S. Lauciello and M. Prato, Indium based metal-organic framework/carbon nanotubes composite as a template for In₂O₃ porous hexagonal prisms/carbon nanotubes hybrid structure and their application as promising super-capacitive electrodes, J. Energy Storage, 2022, 51, 104238.
175. X. Jiao, T. Deng, X. Men and Y. Zuo, Indium metal-organic framework with catalytic sites coated conductive graphene for high-performance lithium-sulfur batteries, Ceram. Int., 2022, 48, 16754–16763.
176. L. Chai, X. Wang, Y. Hu, X. Li, S. Huang, J. Pan, J. Qian and X. Sun, In-MOF-derived hierarchically hollow carbon nanostraws for advanced zinc-iodine batteries, Adv. Sci., 2022, 9, 1–9.
177. R. Zhang, C. Mu, B. Wang, J. Xiang, K. Zhai, T. Xue and F. Wen, Composites of In/C hexagonal nanorods and graphene nanosheets for high-performance electromagnetic wave absorption, Int. J. Miner., Metall. Mater., 2023, 30, 485–493.
178. Y.-J. Song, Y.-L. Sang, K.-Y. Xu, H.-L. Hu, Q.-Q. Zhu and G. Li, Ligand-functionalized MIL-68-type indium(III) metal–organic frameworks with prominent intrinsic proton conductivity, Inorg. Chem., 2024, 63, 4233–4248.

---