Synthesis of Triazole and Tetrazole-Functionalized Zr-Based Metal-Organic Frameworks Through Post-Synthetic Ligand Exchange

Post-synthetic ligand exchange (PSE) is a versatile and powerful tool for installing functional groups into metal-organic frameworks (MOFs). Exposing MOFs to solutions containing triazole- and tetrazole-functionalized ligands can incorporate these heterocyclic moieties into Zr-MOFs through PSE processes.

Our main research topic is functionalizing the porous metal-organic frameworks, MOFs, with interesting functional groups through various techniques. This study is focused on heterocycle installation in MOFs. PSE, post-synthetic ligand exchange, is the main technique in this work.

The ligand with target functional group could be installed in pre-synthesized MOFs through the solid-solution exchange process. Especially, some of the unstable ligands and ligands with coordinating groups could be efficiently incorporated into MOFs through the PSE techniques. Both triazole and tetrazole-functional groups have multiple coordinating nitrogen groups.

Therefore, there is competition for coordination between metal carboxylate and metal triazolate or tetrazolate. The incorporation ratio of target ligands is easily controlled in the PSE process, and multiple targets could be installed in a single MOF through this approach. Begin by measuring 23.3 milligrams of H2BDC-triazole and place it into the scintillation vial.

To dissolve the H2BDC ligand in the aqueous solution, transfer 1.0 milliliters of the 4%potassium hydroxide solution into the vial using a glass pipette. Next, sonicate the mixture until all solids completely dissolve. Add 1 molar hydrochloric acid solution into the dicarboxylate-containing vial and stir until a pH of seven is reached.

Add 33 milligrams of UiO-66 MOF to the scintillation vial. Then incubate the vial with the MOF and ligand at room temperature and 120 RPM in a shaker for 24 hours. Next, isolate the solid MOF from the mixture by centrifugation at 1, 166 g for five minutes at room temperature.

Add 10 milliliters of fresh methanol to the obtained solid MOF and shake the vial, creating a heterogeneous mixture to dissolve any remaining unexchanged BDC ligands. Isolate the solid once again by centrifuging the mixture at 1, 166 g for five minutes at room temperature and dry it. Transfer approximately 10 milligrams of the MOF solid exchanged with UiO-66-triazole to a powder X-ray diffraction sample holder.

Place the sample holder in the diffractometer and collect the powder X-ray diffraction pattern. To begin NMR characterization, transfer around 30 milligrams of the exchanged MOF solid to a fresh four-milliliter vial. Then move 400 microliters of DMSO-d6 to the MOF sample using a micro pipette.

Transfer 200 microliters of 4.14 molar ammonium fluoride deuterium oxide solution to a DMSO-d6 suspension of MOF powder using a micro pipette. Sonicate the heterogeneous mixture for 30 minutes until the MOF is dissolved in the dimethyl sulphoxide deuterium oxide mixed solvent after digestion. Once done, remove any remaining insoluble solids by filtering the solution through a polyvinylidene fluoride syringe filter while transferring it from the four-milliliter vial to a nuclear magnetic resonance tube.

To determine the success of the exchange, powder X-ray diffraction patterns were measured and compared to pristine UiO-66 MOF for crystallinity. The position of reflection peaks, relative intensity, and broadness in the powder X-ray diffraction patterns of exchanged UiO-66-triazole and UiO-66-tetrazole matched with pristine UiO-66 MOFs, indicating that the ligand exchange did not impact the framework structure. The exchange ratios were determined by comparing the integration of non-functionalized BDC and BDC-triazole or BDC-tetrazole.

The exchange ratios for triazole and tetrazole were found to be 33%and 30%respectively from post-synthetic ligand exchange analysis.