Ligand-Mediated Proton-Coupled Electron Injection into Reactive Cores of Soluble Macroanion-Like Complexes of...

Although widely utilized as reactive components of devices and functional materials, the harnessing of metal-oxide nanocrystals (NCs) as ligand-tunable reaction centers of soluble catalysts remains an ongoing challenge. We now report that readily modifiable, oxidatively inert polyoxometalate (POM) ligands amplify and rationally control rates of photocatalytic H₂ evolution by anatase-TiO₂ NCs. This was achieved using a series of four POM-derived oxo-donor ligands, [(Xⁿ⁺W₁₁O₃₉)Ti^IV-O^-]^(10-n)-, Xⁿ⁺ = Al³⁺, Si⁴⁺, P⁵⁺, and [(P₂W₁₇O₆₁)Ti^IV-O^-]^8- (written here as oxo-anion donors) coordinated via bridging-oxo linkages to Ti atoms at the surfaces of 7 ± 1.6 nm anatase-TiO₂ NCs, wherein the four respective macroanion-like complexes are each ligated, on average, by 90 ± 15 POM anions. Under UV-vis light irradiation in water (10% MeOH), rates of H₂ evolution are a linear function of the quantum yield for photocatalytic oxidation of MeOH by the Ti-substituted POM ligands. This rate-limiting ligand reduction is followed by rapid visible-light driven proton-coupled electron injection into TiO₂, which proceeds until rates of H₂ evolution achieve parity with those of POM reduction, resulting finally in a dynamic steady state. Here, the NC core of the most reactive, [(P⁵⁺W₁₁O₃₉)Ti^IV-O-]^5--ligated complex, 3, is populated by 440 electrons and 440 protons, representing an effective H⁺ concentration of 10 M (per nm³ of TiO₂). The findings show that metal-oxide NCs can function as the reactive centers of ligand-tunable catalysts and, more generally, provide fundamental mechanistic understanding that establishes new structure-reactivity relationships for the design of rationally tunable modular components of functional materials.