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.