Chloride-modulated insertion reactions of dimethylallene across the Pd-C bond in palladium methyl complexes bearing potentially terdentate pyridylthioether ligands

Palladium methyl complexes with potentially terdentate pyridylthioether (S-N-S(R) = 2,6-bis(R-thiomethyl)pyridine, R = Me, t-Bu, Ph; N-S-N = 2[(2-pyridylmethylthio)methyl]pyridine) ligands have been prepared and characterized. Both the bidentate chloride [Pd(Me)(S-N-S(R))]Cl and the terdentate chloride-free [Pd(Me)(S-N-S(R))](+) species are present in solution and display a substantially different reactivity toward allene insertion across the Pd-C bond. The structures of the complexes [Pd(Me)(S-N-S(t-Bu))]OTf and [Pd(Me)(S-N-S(t-Bu))]Cl were determined by X-ray diffraction. The chloride methyl substrates [Pd(Me)(S-N-S(R))]Cl display an enhanced reactivity in solution with respect to the allene insertion, and this reactivity was traced back to the distortion of the main coordination plane induced by the presence of an uncoordinated -CH2-S-R group in position 6 of the coordinating pyridine. The equilibrium position between the terdentate and the bidentate species can be modulated by addition of chloride ion, which therefore controls the overall reactivity of the system.