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 [ 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-p K 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 + (p K 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.