ABILITY OF DFT CALCULATIONS TO CORRECTLY DESCRIBE REDOX POTENTIALS AND ELECTRON (DE)LOCALIZATION IN POLYOXOMETALATES

Abstract

Many interesting properties of polyoxometalates (POMs) are related to their ability to reversibly accept and release electrons. The correct computational description of metal electrons in POMs is central to fully understand the implications and mechanisms related to reduced structures. We performed DFT calculations on fully oxidized [ P 2 W 18 O 62]6- and α1/α2-[ P 2 MW 17 O 62]n-( M = V , Mo ) anions, and their one-electron reduced partners. Atomic spin populations of the reduced forms reveal that pure GGA functionals, which overdelocalize the electron density by nature, cannot correctly describe the extra electron especially when competition between the localized and delocalized situations is very close in energy, namely for α2-[ P 2 MoW 17 O 62]7-. On the other hand, hybrid functionals like B3LYP or M05 can localize the electron at the correct site. Within these "well-behaved" density functionals, only the B3LYP gives the correct ordering and relative reduction energies (REs) with respect to electrochemical measurements, although the absolute values are always less exothermic than expected. At present, this is the best density functional for computing redox properties of POMs.