A “Hemilabile” Palladium–Carbon Bond: Characterization and Its Implication in Catalysis

An unusual palladium–carbon bond was identified in the crystal structure of a Pd­(II) complex (VI) derived from 1-(2-diphenylphosphinophenyl)-2,5-dimethyl-1H-pyrrole (L4). Theoretical calculations indicate that the Pd–C bond is covalent in nature. The complex exhibits fluxional behavior in solution at ambient temperaturethe Pd binds alternately to C2 and C5 of pyrrole, generating equivalent, enantiomeric structures. Theory predicts that the transition state of this interconversion probably occurs through an unsaturated 14e complex, where the ligand binds the metal in a monodentate fashion. Comparison with related structures reported earlier has been made, and the possible implication of such unusual bonding in the context of catalysis of coupling reactions is discussed.