Titanium Hydroxide Secondary Building Units in Metal-Organic Frameworks Catalyze Hydrogen Evolution under Visible Light.

Herein we report the design of two new titanium metal-organic frameworks (MOFs), Ti3-BPDC-Ir and Ti3-BPDC-Ru, by doping [Ir(ppy)2(dcbpy)]Cl or [Ru(bpy)2(dcbpy)]Cl2 (bpy = 2,2'-bipyridine, ppy = 2-phenylpyridine, dcbpy = 2,2'-bipyridine-5,5'-dicarboxylate) into the Ti3-BPDC framework (BPDC = biphenyl-4,4'-dicarboxylate). Hierarchical assembly of photosensitizing ligands and Ti3(OH)2 secondary building units (SBUs) facilitates multielectron transfer to drive photocatalytic hydrogen evolution (HER) under visible light with turnover numbers of 6632 and 786 for Ti3-BPDC-Ir and Ti3-BPDC-Ru, respectively. Photophysical and electrochemical studies establish the photocatalytic HER via reductive quenching of the excited photosensitizers followed by electron transfer from the reduced photosensitizers to Ti3(OH)2 SBUs and explain the catalytic difference between the two MOFs. Density functional theory calculations reveal key steps of HER via protonation of TiIII-OH to generate the TiIII species with a vacant coordination site followed by proton-coupled electron transfer to afford the key TiIV-H intermediate.