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), Ti-3-BPDC-Ir and Ti-3-BPDC-Ru, by doping [Ir(ppy)(2)(dcbpy)]Cl or [Ru(bpy)(2)(dcbpy)]Cl-2 (bpy = 2,2'-bipyridine, ppy = 2-phenylpyridine, dcbpy = 2,2'-bipyridine-5,5'-dicarboxylate) into the Ti-3-BPDC framework (BPDC = biphenyl-4,4'-dicarboxylate). Hierarchical assembly of photosensitizing ligands and Ti-3(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 Ti-3-BPDC-Ir and Ti-3-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 Ti-3(OH)(2) SBUs and explain the catalytic difference between the two MOFs. Density functional theory calculations reveal key steps of HER via protonation of Ti-III-OH to generate the Ti-III species with a vacant coordination site followed by protoncoupled electron transfer to afford the key Ti-IV-H intermediate.