Selective Hydrogenation of Azobenzene to Hydrazobenzene via Proton-Coupled Electron Transfer from a Polyoxotungstate Cluster

In this report, we describe proton-coupled electron transfer (PCET) reactivity at the surface of the Keggin-type polyoxotungstate cluster nBu4N3PWVI12O40 (PW12) in acetonitrile. Bond dissociation free energies (BDFEs) of the O–H groups generated upon reduction of PW12 in the presence of acid are determined through the construction of a potential–pKa diagram. The surface O–H bonds are found to be weak (BDFE(O–H)avg < 48 kcal mol–1), comparable to the BDFE of H2. This is consistent with the observed formation of H2 upon addition of a suitably strong organic acid, H2NPh2+ (pKa MeCN = 5.98), to the reduced form of the cluster. The one-electron reduced form of PW12 is isolated and used in conjunction with acid to realize the stoichiometric semihydrogenation of azobenzene via PCET from the surface of the reduced cluster.