Palladacycles with C,N-bidentate and N,C,N′-tridentate ligands: Structures, spectral study and catalytic methanolysis of P=S pesticides

Palladacycles 2-4 with C,N-bidentate and N,C,N′-tridentate ligands were prepared and characterized. The X-ray crystal structures of [2,6-bis(N,N-dimethylaminomethyl)phenyl-N,C¹,N′-](aqua) palladium(II) triflate (2), (N,N-dimethylaminobenzyl-C¹,N)(4- trifluoromethylpyridine)(aqua)palladium(II) triflate (3), and (N,N-dimethylaminobenzyl-C¹,N)(4-N,N-dimethylaminopyridine)(aqua) palladium(II) triflate (4), were determined. While 2 is much less active, 3 and 4 effectively catalyze the methanolysis of the P=S pesticides. The catalytic activities were higher with the trifluoromethylpyridine co-ligand as compared to palladacycles containing 4-N,N-dimethylaminopyridine and pyridine co-ligands. ¹H NMR spectra and the catalytic kinetic dependences on concentration and pH revealed that the active species was a palladacycle containing one methoxide and one pyridine in the coordination sphere. A plot of the catalytic activity vs. free [pyridine] indicated the participation of a common species. The proposed catalytic mechanism involves a pre-equilibrium binding of the P=S pesticide to palladium(II) center followed by dissociation of the pyridine and subsequent cleavage of the P-OAr unit through the intramolecular displacement on phosphorus by the adjacent Pd-coordinated methoxide.