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) (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 (1)H NMR, (31)P NMR and UV/Vis spectroscopy and MALDI-TOF mass spectrometry. The unambiguous identification of the capping hydride was made from (2)H NMR spectroscopy by using DCO(2) (-) as starting material. The 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 HCO(2) (-) and Pd(3)(dppm)(3)(mu(3)-CO) and 2) a reductive elimination of CO(2). Two alternatives routes to the hydride complex were also examined : 1) hydride transfer from NaBH(4) to Pd(3)(dppm)(3)(mu(3)-CO) and 2) electrochemical reduction of Pd(3)(dppm)(3)(mu(3)-CO) to Pd(3)(dppm)(3)(mu(3)-CO) followed by an addition of one equivalent of H(+). Based on cyclic voltammetry, 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) is provided, corroborated by digital simulation of the experimental results. Geometry optimisations of the Pd(3)(H(2)PCH(2)PH(2))(3)(mu(3)-CO)(mu(3)-H) 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.