Stability and Unimolecular Reactivity of Palladate(II) Complexes [LnPdR3](-) (L=Phosphine, R=Organyl, n=0 and 1)

The reduction of Pd-II precatalysts to catalytically active Pd-0 species is a key step in many palladium-mediated cross-coupling reactions. Besides phosphines, the stoichiometrically used organometallic reagents can afford this reduction, but do so in a poorly understood way. To elucidate the mechanism of this reaction, we have treated solutions of Pd(OAc)(2) and a phosphine ligand L in tetrahydrofuran with RMgCl (R=Ph, Bn, Bu) as well as other organometallic reagents. Analysis of these model systems by electrospray- ionization mass spectrometry found palladate(II) complexes [LnPdR3](-) (n=0 and 1), thus pointing to the occurrence of transmetallation reactions. Upon gas-phase fragmentation, the [LnPdR3](-) anions preferentially underwent a reductive elimination to yield Pd-0 species. The sequence of the transmetallation and reductive elimination, thus, constitutes a feasible mechanism for the reduction of the Pd(OAc)(2) precatalyst. Other species of interest observed include the Pd-IV complex [PdBn5](-), which did not fragment via a reductive elimination but lost BnH instead.