From two-dimensional trapezoid-like layer to three-dimensional porous indium-4,4′-biphenyldicarboxylate MOFs

Abstract

Three new indium coordination polymers formulated as In(phen)(BPDA)_1.5 (1), In(OH)(H₂O)(BPDA) (2), and In(BPDA)₂·Me₂NH₂·H₂O (3) (phen = 1,10-phenanthroline, C₁₂N₂H₈; H₂BPDA = 4,4′-biphenyldicarboxylic acid, C₁₄O₄H₁₀; Me₂NH = dimethylamine, C₂H₇N), have been synthesized under hydro(solvo)thermal conditions and characterized by infrared (IR), thermal gravimetric analysis (TGA) and luminescence studies. In the two-dimensional (2D) trapezoid-like layer of 1, three BPDA²⁻ anions and one phen group bind to a seven-coordinated In³⁺ ion with the distorted pentagonal bi-pyramid geometry. In 2, the extended –In–OH–In– chains are connected through BPDA²⁻ to form a 3D framework. Complex 3 possesses a two-fold interpenetrating structure, which consists of two independent infinite diamond-like frameworks with point symbol 6⁶. All three samples exhibit varied solid-state fluorescence: emissions at 468 nm (intense blue), 559 nm (greenish-yellow), and 577–645 nm (broad intense orange band with a maximum at 611 nm, λ_ex = 257 nm) for 1, 567 nm (weak greenish-yellow, λ_ex = 344 nm) for 2, and 480 nm (intense greenish-blue, λ_ex = 385 nm) for 3.

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Introduction

The design and synthesis of coordination polymers and novel metal–organic framework (MOF) complexes are under extensive investigation in recent years due to their fascinating topological structures and potential applications as functional solid materials.^1–3 The divalent transition metal coordination polymers, in particular, have received worldwide attention for their desirable topological structures and properties relevant to the fields of materials science, molecular adsorption, catalysis, and magnetism.^4–10 However, compared with the well-developed divalent transition metal coordination polymers, Group 13 metal complexes especially coordination polymers are rarely documented except for the aluminosilicate, gallium-phosphate and phosphite zeolites,^11–13 as well as some indium phosphates, phosphites, and aromatic carboxylate complexes.^14–21 Because Group 13 metals and divalent transition metals have obvious differences in electronic structures, it presents a great challenge for chemists to design and prepare new Group 13 metal complexes and coordination polymers with fascinating topological structures. Besides, Group 13 metal complexes have great potential as emitting materials for optoelectronic devices, and the most successful example is the discovery of tris(8-hydroxyquinolinolate)aluminum (Alq₃) fabricated as organic light-emitting diodes (OLEDs).^22,23 By employing appropriate organic ligands as linkers, In³⁺ metal cations exhibit excellent structural and coordinative flexibility as six-coordinated octahedra MO₆, seven-coordinated pentagonal bi-pyramids MO₇, and eight-coordinated polyhedra MO₈. Some indium complexes even adopt a high coordination number (e.g. 10) configuration.²⁴ The ligand 4,4′-biphenyldicarboxylic acid (H₂BDPA) has been employed to design and synthesize transition metal-organic frameworks with various structures that find potential applications as ion-exchange, adsorption, and luminescent materials.^25–28 In this work, we report the syntheses, structures, thermal properties and luminescent properties of three new indium coordination polymers formulated as In(phen)(BPDA)_1.5 (1), In(OH)(H₂O)(BPDA) (2), and In(BPDA)₂·Me₂NH₂·H₂O (3) (phen = 1,10-phenanthroline, C₁₂N₂H₈; H₂BPDA = 4,4′-biphenyldicarboxylic acid, C₁₄O₄H₁₀; Me₂NH = dimethylamine, C₂H₇N), in which H₂BDPA are employed as organic linkers.

Experimental

Materials and instrumentation

All materials and reagents were obtained commercially and used without further purification. IR spectra were recorded on a Nicolet Impact 410 FTIR spectrometer using KBr pellets. Elemental analyses (C, H, and N) were performed on a Perkin-Elmer 240c elemental analyzer. Metal In was determined by a Perkin-Elmer Optima 3300DV ICP spectrometer. TG analyses were carried out on a Diamond TG/DTA instrument (Perkin-Elmer) thermal analyzer (50–950 °C) in air at a scan rate of 10 °C min⁻¹. Luminescence spectra were measured on a Perkin-Elmer LS55 luminescence spectrometer at room temperature.

Preparation of In(phen)(BPDA)_1.5 (1)

A mixture of In(NO₃)₃ (0.0603 g), H₂BPDA (0.0482 g), and phen (0.0394 g) with a molar ratio of 2 : 2 : 2 was mixed in H₂O (10 mL). After stirring for one hour in air, it was transferred into a 15 mL Teflon-lined stainless steel autoclave and heated at 160 °C for four days. The mixture was washed with distilled water and colorless prism crystals were filtered off and dried at room temperature (yield 54% based on In). The ICP and elemental analyses: Calcd. for C₃₃H₂₀InN₂O₆ (%): In, 17.55; C, 60.43; H, 3.05; N, 4.27. Found: In, 17.56; C, 60.45; H, 3.07; N, 4.30. IR data (KBr cm⁻¹): 3670(w), 3064(m), 1684(m), 1608(s), 1519(s), 1420(s), 1175(m), 1106(m), 1006(m), 867(s), 799(w), 769(s), 724(s), 689(s), 644(w), 531(w), 450(m).

Preparation of in(OH)(H₂O)(BPDA) (2)

A mixture of In(NO₃)₃ (0.0608 g), H₂BPDA (0.0969 g), and phen (0.0199 g) with a molar ratio of 2 : 4 : 1 was mixed in H₂O (10 mL). After stirring for one hour in air, it was transferred into a 15 mL Teflon-lined stainless steel autoclave and heated at 160 °C for three days. The mixture was washed with distilled water and colorless hexagonal block crystals were filtered off and dried at room temperature (yield 48% based on In). The ICP and elemental analyses: Calcd. for C₁₄H₁₁InO₆ (%): In, 29.48; C, 43.07; H, 2.82. Found: In, 29.49; C, 43.10; H, 2.85. IR data (KBr cm⁻¹): 3431(m), 1681(s), 1607(s), 1579(m), 1517(w), 1426(s), 1295(s), 1180(w), 1130(w), 1007(w), 932(w), 847(m), 804(w), 759(s), 691(w), 669(w), 556(w), 448(w).

Preparation of in(BPDA)₂·Me₂NH₂·H₂O (3)

A mixture of In(NO₃)₃ (0.0609 g), H₂BPDA (0.0968 g), and phen (0.0199 g) with a molar ratio of 2 : 4 : 1 was mixed in H₂O (5 mL) and N,N-dimethylformamide (DMF) (5 mL). After stirring for half an hour in air, it was transferred into a 15 mL Teflon-lined stainless steel autoclave and heated at 160 °C for three days. The mixture was washed with distilled water and colorless octahedral block crystals were filtered off and dried at room temperature (yield 51% based on In). The ICP and elemental analyses: Calcd. for C₃₀H₂₆InNO₉ (%): In, 17.41; C, 54.60; N, 2.12; H, 3.94. Found: In, 17.40; C, 54.63; N, 2.14; H, 3.92. IR data (KBr cm⁻¹): 3420(m), 3066(m), 1660(m), 1607(s), 1538(m), 1404(s), 1176(m), 1105(w), 1005(m), 872(s), 770(s), 691(m), 525(w), 444(m).

X-ray crystallography

Crystallographic data of complexes 1–3 were collected on a Bruker Apex II CCD diffractometer using graphite-monochromated Mo-Kα radiation (λ = 0.71073 Å) at 293 ± 2 K. Data processing was accomplished with the SAINT processing program.²⁹ The structures were solved by direct method and refined by full-matrix, least squares based on F² using the SHELXTL 5.1 software package.³⁰ The indium atoms were first located, and then the oxygen, carbon, and nitrogen atoms. The hydrogen atoms of the organic amine molecules were refined in calculated positions. All non-hydrogen atoms were refined anisotropically. Crystal data and details of data collection and refinement are given in Table 1. CCDC reference numbers: 832523 for 1, 832524 for 2, and 832525 for 3.†

Table 1 Crystal data and structure refinement for complexes 1–3

Complexes	1	2	3
a R 1 = ∑‖Fo| − |Fc‖/∑|Fo|, wR2 = [∑[w(Fo^2 − Fc^2)^2]/∑[w(Fo^2)^2]]^1/2.
Empirical formula	C33H20InN2O6	C14H11InO6	C30H26InNO9
Fw	655.33	390.05	659.34
T/K	294(2)	294(2)	294(2)
λ/Å	0.71073	0.71073	0.71073
Crystal system	Monoclinic	Orthorhombic	Orthorhombic
Space group	P21/c	Pnma	Fddd
a/Å	11.805(2)	5.9716(12)	23.129(5)
b/Å	24.903(5)	28.516(6)	39.470(8)
c/Å	9.881(2)	7.5076(15)	40.610(8)
α (°)	90	90	90
β (°)	103.12(3)	90	90
γ (°)	90	90	90
V/Å^3	2828.8(10)	1278.4(4)	37073(13)
Z	4	4	32
μ/mm^−1	0.885	1.875	0.544
θ range (°)	3.03–27.46	3.46–27.48	3.11–25.00
R [I > 2σ(I)]^a	R 1 = 0.0405	R 1 = 0.0366	R 1 = 0.0696
	wR2 = 0.0783	wR2 = 0.1212	wR2 = 0.1404

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Results and discussion

Synthesis and characterization

Complexes 1, 2, and 3 have been successfully synthesized under hydro(solvo)thermal conditions. Complexes 1 and 2 were prepared using the same reactants but with different molar ratios and reactions. This suggests that the molar ratio is a critical factor in determining the final products. In addition, reaction time plays an important role in the growth of crystals. If the reaction time is too short, crystals are usually too small; if the reaction time is too long, amorphous powder is obtained instead of crystals. The synthetic method of complex 3 is similar to that of complex 2, the difference is that the solvent is changed from H₂O to a mixture of H₂O and DMF. In different solvents, the solubility of the synthesized mixture is different, and the result is that the crystal structure of complex 3 is completely different to that of 2. Though phen was not incorporated in the final products, complexes 2 and 3 cannot be obtained in a series of control experiments without using phen, indicating that phen plays important roles in the reaction process of complexes 2 and 3. The three complexes are all stable in the solid state in open air. Complexes 1, 2, and 3 were characterized by IR spectroscopy, elemental analyses and single-crystal X-ray diffraction analyses. The three complexes are not soluble in common solvents such as CH₂Cl₂, CH₃CN, DMF, or DMSO, and as a result, their ¹H NMR, UV-Vis and fluorescent spectroscopy in solutions could not be obtained.

The elemental analysis and ICP analysis results reveal that the components of the three complexes are in agreement with their formula. The IR spectra of the free ligand 4,4′-biphenyldicarboxylic acid (H₂BPDA) shows the characteristic band of the carboxyl group at 1705 cm⁻¹. When coordinated with In³⁺ metal cations, the absorption peak splits into two: 1608 and 1420 cm⁻¹ for 1, 1607 and 1426 cm⁻¹ for 2, and 1607 and 1404 cm⁻¹ for 3. These results correspond to asymmetric and symmetric stretching vibrations of the carboxyl groups, respectively. In the above three complexes, the stretching bands of the carboxyl groups shift toward lower frequencies and their intensity is significantly reduced. These phenomena confirm the coordinative interaction between the carboxyl O atoms and In³⁺ metal cations.

Structural descriptions

Structure of In(phen)(BPDA)_1.5 (1). Single crystal X-ray diffraction analysis reveals that complex 1 crystallizes in a space groupP2₁/c. The central In³⁺ cation is coordinated by three BPDA²⁻ anions and one phen molecule, as shown in Fig. 1. Each In³⁺ cation is seven-coordinated with five carboxyl oxygen atoms (O(1), O(2), O(3), O(4), O(6B)) of three BPDA²⁻ anions and two nitrogen atoms (N(1), N(2)) from one phen molecule, forming a distorted InO₅N₂ pentagonal bi-pyramid. In the molecular structure of 1, the BPDA²⁻ ligands have two different coordination modes. The first type of BPDA²⁻ is coordinated to two In³⁺ cations through three carboxyl oxygen atoms (O(1), O(2), O(6)), in which one carboxyl arm adopts a bi-dentate coordination mode and the other adopts a mono-dentate mode. The second type of BPDA²⁻ links two In³⁺ cations through four carboxyl oxygen atoms, in which the two carboxyl arms both adopt a bi-dentate coordination mode. In the first coordination mode, the bond distances of In–O_COO– vary from 2.142(3) to 2.358(2) Å, of which the bond distance of In(1)–O(6) is the shortest. The bond distances of C–O_COO– of the bi-dentate coordinated carboxyl arms are 1.250(4) Å for C(13)–O(1) and 1.279(4) Å for C(13)–O(2). In the mono-dentate coordinated carboxyl arm, the C(26)–O(6) bond distance is 1.269(5) Å; it is coordinated to the central In³⁺ cation. The remaining C(26)–O(5) bond distance is slightly shorter with a value of 1.250(5) Å, and it is not coordinated to the central In³⁺ cation. In the second coordination mode, the bond distances of In–O_COO– are between 2.208(2) and 2.383(2) Å. The C–O_COO– bond distances are between 1.253(4) and 1.272(4) Å. The other phen molecule is coordinated to the central In³⁺ cation through two N atoms, with the bond distances of In(1)–N(1) = 2.384(3) Å, In(1)–N(2) = 2.332(3) Å, and the bond angle of N(1)–In(1)–N(2) = 69.86(10)°. Moreover, the bond angle of the mono-dentate carboxyl arm is 121.1(4)°, which is slightly larger than that of the bi-dentate carboxyl arm (about 119.6°). As shown in Fig. 2, the adjacent In³⁺ cations are connected through BPDA²⁻ ligands to form an infinite chain, in which the BPDA²⁻ ligands adopt the first coordination mode. The distance between the two In³⁺ centers is about 15.25 Å. The dihedral angle between the two benzene planes of the BPDA²⁻ ligand is about 27.35°. Besides, the neighboring chains are further linked through BPDA²⁻ ligands to form a 2D layered structure, in which the carboxyl ligands adopt the second coordination mode. In the 2D layered structure of complex 1, each In³⁺ cation is coordinated to three BPDA²⁻ ligands, and each BPDA²⁻ ligand links two In³⁺ cations. In this way, six In³⁺ centers and six BPDA²⁻ ligands connect to each other through carboxyl groups, forming a 12-membered grid with dimensions of about 23.7 Å × 33.2 Å (atom-to-atom distances, as shown in Fig. S1†). Fig. 3 shows the trapezoid-like (6,3) layer of complex 1, in which the In³⁺ centers are treated as 3-connected nodes. There exist aromatic π–π packing interactions between the neighboring layers in the structure of complex 1. The shortest distance between the adjacent benzene rings is about 3.39 Å, as shown in Fig. 4.

Fig. 1 The coordination environment around In³⁺ in complex 1 (50% thermal ellipsoids). Hydrogen atoms on the ligands are omitted for clarity. Symmetry codes: A: −x, −y, −z; B: x + 1, −y + 1/2, z − 1/2.

Fig. 2 The 2D layered structure of complex 1 along the c axis.

Fig. 3 A view of the (6,3) network of complex 1 along [100] (a), [010] (b), and [001] (c) directions.

Fig. 4 A view of the 2D trapezoid-like structure of complex 1 on the bc plane.

Structure of In(OH)(H₂O)(BPDA) (2). Single crystal X-ray diffraction analysis reveals that complex 2 crystallizes in the space groupPnma. As shown in Fig. 5, the molecular structure of 2 is composed of one In³⁺ cation, two BPDA²⁻ anions, two hydroxyl oxygen atoms, and one coordinated water molecule. Each central In³⁺ is seven-coordinated by four carboxyl oxygen atoms (O(1), O(2), O(1A), O(2A)) from two BPDA²⁻ anions, two hydroxyl oxygen atoms (O(3), O(3D)), and one coordinated water molecule (O(4)), forming a slightly distorted In(OH)₂(H₂O)O₄ pentagonal bi-pyramid geometry. The bond distances of In(1)–O_COO– are between 2.259(4) and 2.362(4) Å. Compared with the In(1)–O_COO– bond distances, the bond distances of the central In³⁺ cation and hydroxyl O atoms and coordinated water molecule are much shorter, which vary from 2.068(5) to 2.180(5) Å. The C–O_COO– bond distances of the bi-dentate coordinated carboxyl arms are 1.250(6) Å for C(1)–O(1) and 1.269(6) Å for C(1)–O(2). The bond angles of the bi-dentate coordinated carboxyl arm O(1)–C(1)–O(2) is 119.7(4)°, with the O(1)–In(1)–O(2) bond angle of 56.22(13)°. The bond angle of the vertexes and central In³⁺ O(3)–In(1)–O(4) is 171.3(2)°, which is close to 180°. As shown in Fig. 6a, every four BPDA²⁻ ligands and two hydroxyl groups link six In³⁺ cations to generate a In₆ ring with dimensions of about 5.88 × 28.78 Å (atom-to-atom distances). As shown in Fig. 6b, every two BPDA²⁻ ligands and four hydroxyl groups join six In³⁺ cations to form another In₆ ring (different from the above one) with dimensions of about 5.97 × 18.47 Å (atom-to-atom distances). On the one hand, the adjacent In³⁺ centers are connected through BPDA²⁻ anions to form a wavelike –In–BPDA–In–1D chain along the b axis, in which the shortest In–In distance is about 15.41 Å. On the other hand, the neighboring In³⁺ centers are linked by hydroxyl groups to form another sawtooth –In–OH–In–1D chain along the a axis. The neighboring parallel –In–BPDA–In– chains are joined through –In–OH–In– chains to form a 2D double-layer, which is further connected through –In–OH–In– chains to construct the 3D open-framework with 10-membered ring channels of complex 2 along the a axis (Fig. 7a). Molecular structure analysis reveals that complex 2 possesses 4-connected topological structure (Fig. 7b). Moreover, the In(1)–O(4) vectors point toward the 10-membered ring channels and form strong hydrogen bonds with the adjacent carboxyl O atoms (O(4)–H(4A)…O(2), 2.678(2) Å, x − 1/2, y, −z − 1/2). Moreover, the coordinated water molecules form weak hydrogen bonds with the adjacent hydroxyl O atoms (O(3)–H(3A)…O(2), 3.088(2) Å, x − 1/2, y, −z + 1/2; O(3)–H(3A)…O(2), 3.088(2) Å, x − 1/2, −y + 3/2, −z + 1/2).

Fig. 5 The coordination environment around In³⁺ in complex 2 (50% thermal ellipsoids). Hydrogen atoms are omitted for clarity. Symmetry codes: A: x, −y + 3/2, z; B: −x + 1, −y + 1, −z; C: −x + 1, y + 1/2, −z; D: x − 1/2, −y + 3/2, −z + 1/2.

Fig. 6 Views of the two types of In₆ ring in complex 2.

Fig. 7 (a) A polyhedral view of the 3D open-framework of complex 2 along a axis; (b) The 4-connected topological structure of complex 2.

Structure of In(BPDA)₂·Me₂NH₂·H₂O (3). Single crystal X-ray diffraction analysis reveals that complex 3 crystallizes in the space groupFddd. The asymmetric unit of complex 3 contains two crystallographically independent In³⁺ cations (In(1) and In(2)), but they have the same chemical environments. As shown in Fig. 8, the central In(1)³⁺ cation is coordinated by eight carboxyl oxygen atoms (O(5), O(6), O(5A), O(6A), O(7B), O(8B), O(7C), O(8C)) of four BPDA²⁻ anions, forming a slightly distorted InO₈ polyhedron geometry. The bond distances of In(1)–O_COO– are between 2.234(3) and 2.321(3) Å. Compared with the In(1)–O_COO– bond distances, the bond distances of the In(2)–O_COO– are varying from 2.190(3) to 2.406(3) Å. The C–O_COO– bond distances of the bi-dentate coordinated carboxyl arms are between 1.200(6) and 1.286(5) Å. As shown in Fig. 9a, each In(1)³⁺ center joins the neighboring four In(1)³⁺ cations through four bridging ligands to form a tetrahedron geometry, and each BDPA²⁻ ligand links the adjacent two In(1)³⁺ centers through two carboxyl groups in bi-dentate chelating modes. The adjacent In(1)³⁺ centers are connected through the bridging BDPA²⁻ anions to form an infinite diamond-like framework with 6⁶ point symbol. In the same way, another infinite diamond-like framework is constructed from In(2)³⁺ cations and bridging organic ligands (Fig. 9b). Notably, though the chemical environments of the central In(1)³⁺ and In(2)³⁺ cations are the same, the major difference is the dihedral angle between the planes of the two benzene rings in one bridging organic ligand. The dihedral angle between the two benzene ring planes in one briding ligand is 5.27(27)° in the In(1) network, which is much smaller than that of 27.54(19)° in the In(2) network. Due to the strong distortion, the unique two-fold interpenetrating structure of complex 3 is generated, in which the metal centers provide the tetrahedral nodes, and the BDPA²⁻ ligands act as linear bridges between the metal centers. Every tetrahedral node is located at the center of an adamantane unit of the other framework, and the four connections to the nearest neighbor project through the four surrounding adamantine windows. Therefore, the two independent networks are interpenetrated, as shown in Fig. 9c. Moreover, protonated dimethylamine molecules are employed as charge compensation components, which are formed by the decomposition of the DMF molecules. The protonated dimethylamine and crystallizing water molecules are located in the channels of the network. As far as we know, the well-known interpenetrated structure has only been found in transition metal MOFs^31–34 and has not been reported in Group 13 metal frameworks.

Fig. 8 The coordination environment around In(1)³⁺ in complex 3 (50% thermal ellipsoids). Hydrogen atoms on the ligands are omitted for clarity. Symmetry codes: A: −x + 1/4, y, −z + 1/4; B: x − 1/4, y − 1/4, −z; C: −x + 1/2, y − 1/4, z + 1/4.

Fig. 9 A view of the diamond-like framework (a); two independent diamond-like frameworks (b); and the two-fold interpenetrating structure (c) of complex 3.

Thermal properties

Thermal gravimetric analyses (TGA) were carried out on complexes 1–3 to investigate their thermal stabilities (Fig. S2†). Complex 1 is thermally stable until around 400 °C. In the temperature range of 400–550 °C, a weight loss of about 79.50% is observed, which corresponds to the removal of phen and H₂BPDA (Calcd. 79.82%). The TGA curve of complex 2 shows the first weight loss of 1.83% up to 320 °C, which corresponds to the loss of part of the coordinated water molecules. The second weight loss of 18.16% occurs at around 320–435 °C, owing to the loss of coordinated water molecules and part of the H₂BPDA. Finally, a sharp weight loss of 45.50% is observed due to the removal of the rest of the H₂BPDA between 435 and 510 °C. Complex 3 decomposes gradually from 50 to 325 °C with a weight loss of 21.56%. After the removal of crystallizing water, dimethylamine, and part of H₂BPDA, the framework collapsed sharply at 325–950 °C due to the loss of the rest of the H₂BPDA, with a weight loss of 57.23%. Because the TG analyses were carried out in air, the three complexes finally transformed to the final residual of the In₂O₃ phase after the release of water and organic ligands (Calcd. 21.18% for 1, 35.58% for 2, and 21.05% for 3), which are in agreement with the experimental results (Found: 20.50% for 1, 34.51% for 2, and 21.21% for 3).

Luminescence properties

The luminescence properties of complexes 1, 2, and 3 are investigated in the solid state at room temperature (as shown in Fig. 10). Complex 1 displays intense blue emission at 468 nm, greenish-yellow emission at 559 nm, and an intense orange emission band (577–645 nm) maximum at 611 nm (λ_ex = 257 nm). Complex 2 exhibits an intense fluorescent emission at 414 nm and a weak greenish-yellow emission band at around 567 nm (λ_ex = 344 nm). An intense fluorescent emission band (395–406 nm) and an intense greenish-blue emission at 480 nm of complex 3 are observed (λ_ex = 385 nm). Free H₂BPDA ligand is known to exhibit a maximum emission at around 397 nm (λ_ex = 236 nm).³⁵ In comparison, the emission frequencies of the three complexes are obviously red-shifted. Among them, the biggest red-shift of complex 1 is observed, of which the luminescent band extends from blue to orange emission. The red-shift of emission energy is likely to be caused by the intermolecular interaction in the solid state that effectively decreases the energy gap. The fluorescent emission of the three complexes may be assigned to the charge transition of the ligand-to-metal In(III) center (LMCT).^2,16,17,19

Fig. 10 Emission spectra of complexes 1, 2, and 3 in the solid state.

Conclusion

In conclusion, three new main group In³⁺ coordination polymers have been hydro(solvo)thermally synthesized. In the 2D layer of complex 1, phen groups hinder the further connection of central metals and H₂BPDA ligands. Through adjusting the molar ratios of the metal sources and organic ligands and using different solvents, more open 3D In³⁺ coordination polymers of 2 and 3 are successfully prepared. TGA reveals that they are materials with good thermal stability. Compared with 2 and 3, complex 1 displays a broader luminescent band extending from the blue to orange region. The main reason is that the nice coplanar feature of the 2D layer structure in 1 can enhance the mobility of π electrons in organic aromatic rings, which are in favor of fluorescent emission. The good luminescent properties in the solid state reveal that they are a new class of luminescent main group metal materials with potential applications in optoelectronic devices. Further work to investigate the structural features and luminescent properties of main group metal MOFs related to organic carboxylic acids is in progress.

Acknowledgements

This work was supported by the National Science Foundation of China (Grant No. 20971031, 21071035 and 21171044), the China Postdoctoral Science Foundation Funded Project (No. 65204), and the key Natural Science Foundation of the Heilongjiang Provence, China (No. ZD201009).

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Footnote

† Electronic supplementary information (ESI) available. CCDC reference numbers 832523–832525. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/c1ce05841j

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