Dimerization of Ethylene by Palladium Complexes Containing Bidentate Trifluoroborate-Functionalized Phosphine Ligands

As an alternative to the widely reported phosphine-sulfonate ligand system, a series of potassium aryltrifluoroborate-functionalized phosphine ligands and zwitterionic phosphonium salts were prepared and structurally characterized. The phosphine ligands formed complexes of the general formula [kappa(2)-(P,F)(PdClMe)-Pd-R] (where R = Ph, 2-OMe-Ph) when reacted with PdClMe(COD); however, cleavage of the chloride ligand proved problematic. Reaction of the phosphonium salts with PdMe2(tmeda) yield complexes of the general type [kappa-(P)(PdMe)-Pd-R(tmeda)], which react with pyridine derivatives to displace tmeda. Manipulation of the steric bulk of the pyridine ligands affords some control over the coordination mode of the fluoroborate phosphine, yielding facile access to complexes of the general type [kappa(2)-(P,F)(PdMe)-Pd-R(lutidine)]. Investigations into the insertion chemistry of the palladium methyl moiety with simple small molecules revealed that the release of the lutidine ligand is slow and that insertion of ethylene occurs in a very slow manner; this is attributed to the relative electron deficiency of the aryltrifluoroborate moiety as compared to sulfonate. The palladium lutidine complexes slowly dimerize ethylene to a mixture of propene and butenes.