Elucidating Molecular Iridium Water Oxidation Catalysts Using Metal–Organic Frameworks: A Comprehensive Structural, Catalytic, Spectroscopic, and Kinetic Study

Abstract

As a new class of porous, crystalline, molecular materials, metal-organic frameworks (MOFs) have shown great promise as recyclable and reusable single-site solid catalysts. Periodic order and site isolation of the catalytic struts in MOFs facilitate the studies of their activities and reaction mechanisms. Herein we report the construction of two highly stable MOFs (1 and 2) using elongated dicarboxylate bridging ligands derived from Cp*Ir(L)Cl complexes (L = dibenzoate-substituted 2,2'-bipyridine, bpy-dc, or dibenzoate-substituted 2-phenylpyridine, ppy-dc) and Zr(6)O(4)(OH)(4)(carboxylate)(12) cuboctahedral secondary building units (SBUs) and the elucidation of water oxidation pathways of the Cp*Ir(L)Cl catalysts using these MOFs. We carried out detailed kinetic studies of Ce(4+)-driven water oxidation reactions (WORs) catalyzed by the MOFs using UV-vis spectroscopy, phosphorescent oxygen detection, and gas chromatographic analysis. These results confirmed not only water oxidation activity of the MOFs but also indicated oxidative degradation of the Cp* rings during the WOR. The (bpy-dc)Ir(H(2)O)(2)XCl (X is likely a formate or acetate group) complex resulted from the oxidative degradation process was identified as a competent catalyst responsible for the water oxidation activity of 1. Further characterization of the MOFs recovered from WORs using X-ray photoelectron, diffuse-reflectance UV-vis absorption, luminescence, and infrared spectroscopies supported the identity of (bpy-dc)Ir(H(2)O)(2)XCl as an active water oxidation catalyst. Kinetics of MOF-catalyzed WORs were monitored by Ce(4+) consumptions and fitted with a reaction-diffusion model, revealing an intricate relationship between reaction and diffusion rates. Our work underscores the opportunity in using MOFs as well-defined single-site solid catalytic systems to reveal mechanistic details that are difficult to obtain for their homogeneous counterparts.