How can multielectron transfer be realized? A case study with keggin-type polyoxometalates in acetonitrile.

Theoretical consideration and computational simulation have been performed on the voltammetric properties of Keggin polyoxometalates (POMs), and the conversion from successive one-electron transfer in unacidified media to four-electron transfer (through two-electron transfer) in acidified media has been discussed. Perfect simulation of the cyclic voltammograms of POMs could be achieved using the standard formal potentials and the protonation constants, systematically evaluated by the equations, in which “simple (intrinsic)” and “synergistic (extrinsic)” electron-withdrawing effects of the μ4-oxygen were taken into consideration. In the proposed model, the formal potential of the one-electron redox waves for the ith reduction step is presented by Ei°(z0, s) = Ei** + 0.51(z0 – i + 1) + 1.067s (i = 1, 2, 3, 4; E1** = E2** = 0.577 V; E3** = E4** = 0.377 V), where z0 is the initial ionic charge of a Keggin POM and s is the mean bond valence of the μ4-O–W bonds in the POM. The values of Ei**s are related to the energy levels of the two lowest unoccupied molecular orbitals (LUMOs) of a hypothetical Keggin POM with null charge and null bond valence. Then it was revealed that the LUMOs have small on-site repulsion, which may be an important factor that makes multielectron transfer feasible. These findings would give a big clue in developing novel redox materials exhibiting multielectron transfers.