Reticular chemistry guided precise construction of zirconium-pentacarboxylate frameworks with 5-connected Zr6 clusters

Zirconium-based metal–organic frameworks (Zr-MOFs) have been extensively studied due to their very rich structural chemistry. The combination of nearly unlimited carboxylic acid-based linkers and Zr6 clusters with multiple connectivities has led to diverse structures and specific properties of resultant Zr-MOFs. Herein, we demonstrate the successful use of reticular chemistry to construct two novel Zr-MOFs, HIAM-4040 and HIAM-4040-OH, with zfu topology. Based on a thorough structural analysis of (4,4)-connected lvt-type Zr-tetracarboxylate frameworks and a judicious linker design, we have obtained the first example of a Zr-pentacarboxylate framework featuring unprecedented 5-connected organic linkers and 5-connected Zr6 clusters. Compared with HIAM-4040, a larger Stokes shift is achieved in HIAM-4040-OH via hydroxyl group induced excited-state intramolecular proton transfer (ESIPT). HIAM-4040-OH exhibits high chemical and thermal stability and is used for HClO detection in aqueous solution with excellent sensitivity and selectivity.

As reported and shown in Fig. 1a and S1, † Zr-MOFs with 4connected Zr 6 clusters have been constructed, such as NU-1400 with an lvt underlying net using [1,1 0 :4 0 ,1 00 ]-terphenyl-3,3 00 ,5,5 00tetracarboxylic acid (H 4 TPTC) or similar linkers. 19,26In these Zr-MOFs, each linker is coordinated with four Zr 6 clusters and each Zr 6 cluster is connected with four linkers, leading to the formation of rhombic channels along the a axis.From the view of reticular chemistry, TPTCs are packed between two rows of 4connected Zr 6 clusters in NU-1400 viewed from the c axis (Fig. S1c †).It is thus highly possible to add another carboxylate group on the central benzene ring of TPTC to coordinate with the neighboring Zr 6 cluster and block the rhombic channels as depicted in Fig. 1b, in which the 5-connected Zr 6 cluster and 5connected pentacarboxylic acid will be generated.
The single crystal X-ray diffraction (sc-XRD) analysis at 193 K indicated that HIAM-4040 crystallizes in an orthorhombic crystal system with a P2 1 2 1 2 1 space group (Table S1 †).As expected, each Zr 6 O 8 cluster in HIAM-4040 is coordinated by ve fully deprotonated CBII linkers, three formate groups and four terminal H 2 O/OH − groups (Fig. 2d, S10 and S11 †), which was conrmed by the 1 H NMR spectrum of digested HIAM-4040 (Fig. S12 †).Each CBII is connected to ve Zr 6 O 8 clusters.As a result, HIAM-4040 possesses a rarely reported (5,5)-c zfu topology with the overall formula of Zr 6 O 4 (OH) 8 (H 2 O) 4 (-HCOO) 3 (CBII).To the best of our knowledge, HIAM-4040 is the rst example of Zr-MOFs constructed using a pentacarboxylatetype linker with 5-connected organic linkers and 5-connected Zr 6 clusters, which is different from the reported 5-connected Zr 6 cluster in NU-500, where one monodentate carboxylate ligand exists. 50,51A close structure analysis demonstrates that, similar to that in NU-1400, CBIIs are also packed between two rows of Zr 6 clusters as viewed along the a axis, where additional carboxylate groups are connected to the le/below or the right/ above the Zr 6 cluster (Fig. S10 †).As a result, the rhombic channels as observed in NU-1400 along the a axis are separated into two triangular-shaped channels and one rhombic-shaped channel along the b axis (Fig. S10, † middle).Aer removing the 4-(1H-imidazole-2-yl)benzoic acid group from the single crystal structure of HIAM-4040, the residual structure is almost the same as that of NU-1400 (Fig. S1 and S13 †).The length of the added moiety containing carboxyl group in H 5 CBII is about 8.33 Å, while the distances between the central benzene ring of the organic linker and the neighboring Zr 6 cluster are around 8.10 to 12.67 Å in NU-1400 under various conditions due to its exibility.Therefore, NU-1400 can accommodate another carboxylcontained moiety to generate the (5,5)-c underlying net.These results are consistent with our hypothesis that the 5-connected Zr 6 clusters can be generated using pentacarboxylic acids based on reticular chemistry guided structural analysis of (4,4)-c lvt type Zr-MOFs, which further conrms that reticular chemistry is a powerful strategy for designing and discovering MOFs with novel structures.
To further investigate the tunability of linkers and their induced structure and property diversity, two other pentacarboxylic acids, 5,  intramolecular proton transfer (ESIPT).H 5 CYBII is the extension of H 5 CBII by incorporating an additional phenyl ring to the central benzene ring to investigate the tolerance of linker length for constructing (5,5)-c zfu type Zr-MOFs.As a result, light yellow single crystals (HIAM-4040-OH) were formed using H 5 CHBII as the organic linker, which shows bright cyan emission (Fig. 2c) with the overall formula of Zr 6 O 4 (OH) 8 (H 2 O) 4 (-HCOO) 3 (CHBII) (Fig. S20 †).However, for H 5 CYBII, no crystals were obtained, which might be ascribed to the fact that the length of the added moiety is too large to be accommodated in the structure of NU-1400 to generate the (5,5)-c net.These results indicate that the formation of (5,5)-c zfu type Zr-MOFs has stringent requirements on the geometry and length of organic linkers.The sc-XRD analysis at 193 K revealed that HIAM-4040-OH also crystallizes in an orthorhombic crystal system but with a different space group, Pnma (Table S2

†).
The phase purity of HIAM-4040 and HIAM-4040-OH was conrmed by the matched powder X-ray diffraction (PXRD) patterns between the simulated (193 K) and experimental ones (298 K) (Fig. 3a), which also reveal the isoreticular nature of HIAM-4040 and HIAM-4040-OH.A small peak shi was observed in the PXRD pattern of the activated HIAM-4040-OH sample aer porosity analysis, indicative of its structural exibility.The chemical and thermal stability of these two MOFs were also tested.As depicted in Fig. 3a and S21, † the intact PXRD patterns demonstrate that the long-range orders of HIAM-4040 and HIAM-4040-OH were well retained aer treatment under various aqueous conditions, including soaking in water, in pH = 2 and 12 solutions at room temperature for 24 hours, respectively.Due to the relatively lower yield of HIAM-4040 compared with that of HIAM-4040-OH, the in situ temperature-dependent PXRD (TD-PXRD) and porosity measurements were conducted using HIAM-4040-OH.As depicted in Fig. 3a and S22, † the crystallinity of HIAM-4040-OH remained unchanged even upon heating to 873 K, which is consistent with the thermogravimetric analysis (Fig. S23 †).These results demonstrate that HIAM-4040 and HIAM-4040-OH have excellent chemical and thermal stability.To further conrm the framework stability, we obtained single crystal structures of HIAM-4040-OH aer various treatments, namely HIAM-4040-OH-293K, HIAM-4040-OH-323K, HIAM-4040-OH-EtOH (aer solvent exchange using EtOH), HIAM-4040-OH-pH2, and HIAM-4040-OH-pH12.As summarized in Tables S1 to S8, † the sc-XRD analyses not only conrm structural stability of HIAM-4040-OH upon heating and exposure to harsh chemical environments, but also indicate temperature-and solventdependent structural exibility (Fig. 3a, S22 and S24 †), which is similar to NU-1400. 26e attempted to analyze the permanent porosity of HIAM-4040-OH by N 2 and Ar sorption experiments; however negligible gas uptake was observed.We then carried out CO 2 sorption measurement at 195 K due to its smaller kinetic diameter compared with N 2 and Ar, which exhibited a type I adsorption prole (Fig. 3b).The corresponding BET surface area and total pore volume were estimated to be 308.2m 2 g −1 and 0.14 cm 3 g −1 , respectively, smaller than those of NU-1400, due to the introduction of 4-(1H-imidazole-2-yl)benzoic acid.The pore size distribution analysis indicated two types of pores with estimated sizes of 4.5 and 6.6 Å (Fig. S25 †).The experimental BET surface area and pore volume are much smaller than the calculated values of 2626.0 m 2 g −1 and 0.56 cm 3 g −1 , which demonstrates that only a small amount of porosity was accessible for HIAM-4040-OH.
The solid-state UV-vis absorption experiments revealed that the adsorption maxima are at 372 nm and 382 nm for HIAM-4040 and HIAM-4040-OH, respectively (Fig. 3c).The maximum emission peaks center at 436 nm and 487 nm for HIAM-4040 and HIAM-4040-OH, respectively.It should be noticed that a Stokes shi of 105 nm was observed for HIAM-4040-OH, which is 41 nm larger than that of HIAM-4040 (64 nm).These results indicate that the introduction of a hydroxyl group triggers ESIPT and leads to red-shi emission from the keto form of CHBII under excitation (Fig. S26 †), 52 which is the same as we have observed in the previous work. 47The photoluminescence quantum yields are 24.7% and 26.3% for HIAM-4040 and HIAM-4040-OH under 365 nm excitation, respectively.
Due to strong emission and excellent chemical stability, especially in aqueous solution, HIAM-4040-OH was used to detect hypochlorous acid (HClO), one of the reactive oxygen species (ROS).HClO and other ROS and reactive nitrogen species (RNS) play an essential role in biology, [53][54][55][56] where the production of excess HClO is related to various diseases.Therefore, it is of great interest to monitor the concentration of HClO under aqueous conditions.The HClO concentrationdependent emission spectra were thus measured by gradual addition of HClO into the aqueous suspension of HIAM-4040-OH.As shown in Fig. 4a, the emission at 490 nm of HIAM-4040-OH gradually decreased with an increasing concentration of HClO.This result is consistent with the decreased emission lifetime of HIAM-4040-OH aer addition of HClO (Fig. 4b), which decreased from 2.00 ns to 1.58 ns and 1.24 ns when 300 and 600 mM HClO was added.The emission intensity of HIAM-4040-OH showed a linear correlation coefficient of 0.998 toward the concentration of HClO in the range of 0 to  0.6 mM with a calculated detection limitation of 1.57 mM, which is comparable with those of the reported MOF-based materials for HClO detection (Table S9 †).A similar emission quenching behavior was also observed for HIAM-4040 aer adding HClO (Fig. S27 †).HIAM-4040-OH also exhibited high selectivity towards HClO compared with other RNS and ROS, including t-BuOOH, ONOO − , NO 2 − , NO 3 − , 1 O 2 , H 2 O 2 and $OH (Fig. 4c).Moreover, the well matched PXRD patterns of the simulated HIAM-4040-OH and as-synthesized HIAM-4040-OH, aer grinding and detection of HClO indicate the excellent stability of HIAM-4040-OH (Fig. S28 †).These results demonstrate that HIAM-4040-OH can be used for highly sensitive and selective detection of HClO in aqueous solutions.
To understand the turn-off sensing mechanism of HIAM-4040-OH toward HClO, several control experiments were conducted.(i) The similar emission quenching behavior of HIAM-4040 and HIAM-4040-OH upon the addition of HClO indicates that the OH group on the linker skeleton of HIAM-4040-OH is not the reaction site for the turn-off mechanism (Fig. S27 †).(ii) Almost no change was observed for the emission of HIAM-4040-OH when HCl was added, indicating that the emission quenching is not from the acidic nature of HClO (Fig. S29 †).(iii) A slight emission decrease was recorded when HClO was added into a benzothiadiazole-based luminescent MOF solution, HIAM-4011, 57 which demonstrates that the N atom on the benzothiadiazole is silent toward HClO (Fig. S30 †).(iv) The UVvis absorption range of HClO is less than 350 nm, ruling out the energy transfer induced emission quenching between HIAM-4040-OH and HClO (Fig. S31 †).According to the aforementioned experiments, we conclude that the reason for the turn-off emission is the reaction between HClO and the NH group on the imidazole ring.As a strong oxidizing agent, the NH group will be oxidized to N aer adding HClO to HIAM-4040-OH solution, where the electrons will rearrange to generate a carbon radical, which will quench the emission of HIAM-4040-OH.To conrm our hypothesis, the Raman spectra of HIAM-4040-OH before and aer adding HClO were measured.As shown in Fig. 4d, the typical Raman peaks at 998, 1295 and 1617 cm −1 were attributed to the trigonal bending, C-N and C]C stretching vibrations of linkers in HIAM-4040-OH.Aer addition of HClO, a new Raman peak originating from C-N-C appeared at 940 cm −1 and was attributed to the oxidation of N-H on the imidazole ring, while no peak at 940 cm −1 was observed when H 2 O 2 was added, which further conrms that only HClO has this specic interaction with HIAM-4040-OH.

Conclusion
In conclusion, we report the successful application of reticular chemistry to construct pentacarboxylate-based Zr-MOFs, HIAM-4040 and HIAM-4040-OH, inspired by (4,4)-c lvt type Zr-MOFs.To the best of our knowledge, this is the rst Zrpentacarboxylate framework and the rst study disclosing a 5connected organic linker and 5-connected Zr 6 node.By introducing a hydroxyl group into the linker skeleton of HIAM-4040, an isoreticular structure, HIAM-4040-OH, is generated with a larger Stokes shi via ESIPT.Due to its excellent chemical and thermal stability, HIAM-4040-OH was utilized for HClO detection in an aqueous solution with high sensitivity and selectivity.This work points to a new avenue for rationally designing and constructing Zr-MOFs with unique structures guided by reticular chemistry.