Behaviour of [PdH(dppe) 2 ]X (X=CF 3 SO 3 − , SbF 6 − , BF 4 − ) as Proton or Hydride Donor: Relevance to Catalysis

The synthesis, characterization and properties of [PdH(dppe) 2 ] + CF 3 SO 3 − ⋅0.125 THF ( 1 ; dppe=1,2‐bis(diphenylphosphanyl)ethane) and its SbF 6 − ( 1′ ) and BF 4 − ( 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 CC 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.