Concerted Multiproton-Multielectron Transfer for the Reduction of O to HO with a Polyoxovanadate Cluster.

The concerted transfer of protons and electrons enables the activation of small-molecule substrates by bypassing energetically costly intermediates. Here, we present the synthesis and characterization of several hydrogenated forms of an organofunctionalized vanadium oxide assembly, [VO(TRIOL)], and their ability to facilitate the concerted transfer of protons and electrons to O. Electrochemical analysis reveals that the fully reduced cluster is capable of mediating 2e/2H transfer reactions from surface hydroxide ligands, with an average bond dissociation free energy (BDFE) of 61.6 kcal/mol. Complementary stoichiometric experiments with hydrogen-atom-accepting reagents of established bond strengths confirm that the electrochemically established BDFE predicts the 2H/2e transfer reactivity of the assembly. Finally, the reactivity of the reduced polyoxovanadate toward O reduction is summarized; our results indicate a stepwise reduction of the substrate, proceeding through HO en route to the formation of HO. Kinetic isotope effect experiments confirm the participation of hydrogen transfer in the rate-determining step of both the reduction of O and HO. This work constitutes the first example of hydrogen atom transfer for small-molecule activation with reduced polyoxometalates, where both electron and proton originate from the cluster.