Doping Metal–Organic Frameworks for Water Oxidation, Carbon Dioxide Reduction, and Organic Photocatalysis

Abstract

Catalytically competent Ir, Re, and Ru complexes H₂L₁–H₂L₆ with dicarboxylic acid functionalities were incorporated into a highly stable and porous Zr₆O₄(OH)₄(bpdc)₆ (UiO-67, bpdc = para-biphenyldicarboxylic acid) framework using a mix-and-match synthetic strategy. The matching ligand lengths between bpdc and L₁–L₆ ligands allowed the construction of highly crystalline UiO-67 frameworks (metal–organic frameworks (MOFs) 1–6) that were doped with L₁–L₆ ligands. MOFs 1–6 were isostructural to the parent UiO-67 framework as shown by powder X-ray diffraction (PXRD) and exhibited high surface areas ranging from 1092 to 1497 m²/g. MOFs 1–6 were stable in air up to 400 °C and active catalysts in a range of reactions that are relevant to solar energy utilization. MOFs 1–3 containing [Cp*Ir^III(dcppy)Cl] (H₂L₁), [Cp*Ir^III(dcbpy)Cl]Cl (H₂L₂), and [Ir^III(dcppy)₂(H₂O)₂]OTf (H₂L₃) (where Cp* is pentamethylcyclopentadienyl, dcppy is 2-phenylpyridine-5,4′-dicarboxylic acid, and dcbpy is 2,2′-bipyridine-5,5′-dicarboxylic acid) were effective water oxidation catalysts (WOCs), with turnover frequencies (TOFs) of up to 4.8 h^–1. The [Re^I(CO)₃(dcbpy)Cl] (H₂L₄) derivatized MOF 4 served as an active catalyst for photocatalytic CO₂ reduction with a total turnover number (TON) of 10.9, three times higher than that of the homogeneous complex H₂L₄. MOFs 5 and 6 contained phosphorescent [Ir^III(ppy)₂(dcbpy)]Cl (H₂L₅) and [Ru^II(bpy)₂(dcbpy)]Cl₂ (H₂L₆) (where ppy is 2-phenylpyridine and bpy is 2,2′-bipyridine) and were used in three photocatalytic organic transformations (aza-Henry reaction, aerobic amine coupling, and aerobic oxidation of thioanisole) with very high activities. The inactivity of the parent UiO-67 framework and the reaction supernatants in catalytic water oxidation, CO₂ reduction, and organic transformations indicate both the molecular origin and heterogeneous nature of these catalytic processes. The stability of the doped UiO-67 catalysts under catalytic conditions was also demonstrated by comparing PXRD patterns before and after catalysis. This work illustrates the potential of combining molecular catalysts and MOF structures in developing highly active heterogeneous catalysts for solar energy utilization.