Generation, characterization, and electrochemical behavior of the palladium-hydride cluster [Pd3(dppm)3(mu3-CO)(mu3-H)]+ (dppm=Bis(diphenylphosphinomethane).

Addition of formate on the dicationic cluster [Pd-3(dPPM)(3)(mu-3- CO)](2+) (dppm=bis(diphenylphosphinomethane) affords quantitatively the hydride cluster [Pd-3(dppm)(3)(mu(3)-CO)(mu(3)-H](+). This new palladium-hydride cluster has been characterised by H-1 NMR, P-31 NMR and UV/Vis spectroscopy and MALDI-TOF mass spectrometry. The unambiguous identification of the capping hydride was made from H-2 NMR spectroscopy by using DCO2- as starting material. ne mechanism of the hydride complex formation was investigated by UV/Vis stopped-flow methods. The kinetic data are consistent with a two-step process involving: 1) host-guest interactions between HCO2- and [Pd-3(dppm)(3)(mu(3)-CO)](2+) and 2) a reductive elimination of CO2. Two alternatives routes to the hydride complex were also examined : 1) hydride transfer from NaBH4 to [Pd-3(dppm)(3)(mu(3)-CO)](2+) and 2) electrochemical reduction of [Pd-3(dppm)(3)(mu(3)-CO)](2+) to [Pd-3(dppm)(3)(mu(3)-CO)(0) followed by an addition of one equivalent of H+. Based on cyclic voltarnmetry, evidence for a dual mechanism (ECE and EEC; E =electrochemical (one-electron transfer), C=chemical (hydride dissociation)) for the two-electron reduction of [Pd-3(dppm)(3)(mu(3)-CO) (mu(3)-H)](+) to [Pd-3(dppm)(3)(mu(3)-CO)](0) is provided, corroborated by digital simulation of the experimental results. Geometry optimisations of the [Pd3(H2PCH2PH2)(3)(mu(3)-Co)(mu(3)-H)](n) model clusters were performed by using DFT at the B3 LYP level. Upon one-electron reductions, the Pd-Pd distance increases from a formal single bond (n = + 1), to partially bonding (n=0), to weak metal-metal interactions (n=-1), while the Pd-H bond length remains relatively the same.