Insights into the Redox Landscape of Polyoxovanadate-Alkoxide Species

Polyoxovanadate-alkoxides are a class of polynuclear earth-abundant transition metal compounds with highly tunable catalytic, redox, and magnetic properties. These species are ideal electroactive molecular agents for environmentally friendly energy storage applications, such as non-aqueous redox flow batteries. As such, it is fundamental to understand structure-redox relationships in the multielectron redox profiles of polyoxovanadate-alkoxides species. Here, we studied the redox properties of monomeric vanadate-alkoxide species that control the formation of two distinct chemical spaces with mix-valent clusters and fully reduced cyclic species. The accuracy of various density functional theory approximations and basis sets choices was benchmarked against domain-based local pair natural orbital coupled cluster, specifically DLPNO-CCSD(T). To further confirm the broader applicability of density functional theory in higher nuclearity species, we studied the redox profile of [(VV6-nVIVnO6)(O)(OCH3)12]4-n, [(Nb=O)(VV5-nVIVnO5)(O)(OCH3)12]4-n, [(M-OCH3)(VV5-nVIVnO5)(O)(OCH3)12]4-n (M = Ti, Zr, or Hf), and [(Fe-Cl)(VV5-nVIVnO5)(O)(OCH3)12]3-n hexanuclear polyoxo vanadate-alkoxiode species. The calculated redox profiles of these species were in good agreement with experimental data with an average absolute error of 0.11 V. The coupling between vanadium centers in the reduced species was determined to be weak via second-order multireference complete active space calculations (CASSCF/CASPT2).