Abstract
In this report, we describe proton-coupled electron transfer (PCET) reactivity at the surface of the Keggin-type polyoxotungstate cluster [(n)Bu(4)N](3)[PW(VI)(12)O(40)] (PW(12)) in acetonitrile. Bond dissociation free energies (BDFEs) of the O-H groups generated upon reduction of PW(12) in the presence of acid are determined through the construction of a potential-pK(a) diagram. The surface O-H bonds are found to be weak (BDFE(O-H)(avg) < 48 kcal mol(-1)), comparable to the BDFE of H(2). This is consistent with the observed formation of H(2) upon addition of a suitably strong organic acid, H(2)NPh(2)(+) (pK(a MeCN) = 5.98), to the reduced form of the cluster. The one-electron reduced form of PW(12) is isolated and used in conjunction with acid to realize the stoichiometric semihydrogenation of azobenzene via PCET from the surface of the reduced cluster.