N−C Cleavage in Pincer PNP Complexes of Palladium

Several new N-methylated diarylamine-based PNP pincer ligands have been prepared. The synthesis of these ligands is modular and allows incorporation of a variety of substituents that change the solubility and the stereoelectronic properties of the ligand as well as allow for the introduction of a sensitive 19F NMR spectroscopic probe. The reactions of PN(Me)P ligands with PdX2 (X = Cl, OAc) initially proceed with formation of an adduct, (PN(Me)P)PdX2, that may exist in either the neutral or the ionic forms. These adducts are unreactive in the case of PPh2-bearing ligands, but with the more donating PPri 2-bearing ligands, the adduct evolves into square planar (PNP)PdX with irreversible loss of MeX. Thus, the feasibility of cleavage of an unstrained N−C bond by PdII is demonstrated. The N−C cleavage is accelerated by decreasing the solvent polarity. The mechanism may involve either N−C oxidative addition or a nucleophilic attack (external or internal) of X- on the Me group of the N-bound PN(Me)P ligand.