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.