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 PPri2-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.