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; [NbV₅O₇(OCH₃)₁₂]) is reported. Addition of 5,10-dihydrophenazine to [NbV₅O₇(OCH₃)₁₂] results in formation of the 2 H⁺/e^- reduced assembly, [NbV₅O₆(OH₂)-(OCH₃)₁₂], via proton-coupled electron transfer. [NbV₅O₆(OH₂)-(OCH₃)₁₂] has a bond dissociation free energy (BDFE-(O-H)_avg) of 62.3 kcal mol^-1, resembling that of its homometallic congener, [V₆O₆(OH₂)-(OCH₃)₁₂] (BDFE-(O-H)_avg = 62.3 kcal mol^-1). Single-crystal X-ray diffraction reveals that [NbV₅O₆(OH₂)-(OCH₃)₁₂] exists as a mixture of two structural isomers, with the vanadium-aquo moiety formed in either the trans- or cis- 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 trans- isomer is achieved by decreasing the dielectric constant of the reaction solvent. Computational analysis predicts the preferential formation of trans-[NbV₅O₆(OH₂)-(OCH₃)₁₂] in solvents with low dielectric constants as a result of changes to the dispersed charge across the assembly.