Behaviour of [PdH(dppe)(2)]X (X = CF3SO3-, SbF6-, BF4-) as proton or hydride donor: Relevance to catalysis

The synthesis, characterization and properties of [PdH(dppe)(2)](+) CF(3)SO(3)(-.)0.125THF (1; dppe=1,2-bis(diphenylphosphanyl)ethane) and its SbF6- (1') and BF4- (1") analogues, the missing members of the [MH(dppe)(2)](+) X- (M = Ni, Pd, Pt) family, are described. The Pd hydrides are not stable in solution and can react as proton or hydride donors with formation of dihydrogen, [Pd(dppe)(2)](2+) and [Pd(dppe)(2)]. Complexes 1-1" react with carbocations and carbanions by transferring a hydride and a proton, respectively. Such H- or H+ transfer occurs also towards unsaturated compounds, for example, hydrogenation of a C=C double bond. Accordingly, 1 can hydrogenate methyl acrylate to methyl propionate. Complex 1" is an effective (hourly turnover frequency = 16) and very selective (100%) catalyst for the hydrogenation of cyclohexen-2-one to cyclohexanone with dihydrogen under mild conditions. Density functional calculations coupled with a dielectric continuum model were carried out to compute the energetics of the hydride/proton transfer reactions, which were used to rationalize some of the experimental findings. Theory provides strong support for the thermodynamic and kinetic viability of a tetracoordinate Pd complex as an intermediate in the reactions.