Nb-Dopant Changes the Regioselectivity of Proton-Coupled Electron Transfer in a Polyoxovanadate-Alkoxide.

The synthesis of a niobium-(V) substituted polyoxovanadate-alkoxide (NbPOV-alkoxide; [NbVO(OCH)]) is reported. Addition of 5,10-dihydrophenazine to [NbVO(OCH)] results in formation of the 2 H/e reduced assembly, [NbVO(OH)-(OCH)], via proton-coupled electron transfer. [NbVO(OH)-(OCH)] has a bond dissociation free energy (BDFE-(O-H)) of 62.3 kcal mol, resembling that of its homometallic congener, [VO(OH)-(OCH)] (BDFE-(O-H) = 62.3 kcal mol). Single-crystal X-ray diffraction reveals that [NbVO(OH)-(OCH)] exists as a mixture of two structural isomers, with the vanadium-aquo moiety formed in either the - or - positions relative to the Nb-(V) dopant. The formation of two regioisomers is a departure from prior observations of H-atom uptake at the surface of heterometal-doped polyoxovanadate-alkoxides, and is credited to distortions in intercluster metal oxygen bond lengths. Improved selectivity for the - isomer is achieved by decreasing the dielectric constant of the reaction solvent. Computational analysis predicts the preferential formation of -[NbVO(OH)-(OCH)] in solvents with low dielectric constants as a result of changes to the dispersed charge across the assembly.