Cyclopalladation of meta-(Diphenylthiophosphoryloxy)benzaldimines: NCS and Unexpected NCO 5,6-Membered Pincer Palladium Complexes

Unsymmetrical NCS-pincer ligands of a new type, namely, m-(diphenylthiophosphoryloxy)benzaldimines 3 (1-[Ph2P(S)O]-3-[CH=NR]-C6H4, R = OMe (3a), Ph (3b), Bu-t (3c)), were obtained in two steps starting from commercially available 3-hydroxybenzaldehyde. These ligands easily underwent cyclopalladation at the C-2 position of the central benzene ring in the reaction with PdCl2(PhCN)(2) in benzene or benzene methanol solutions to afford the corresponding hybrid pincer complexes 4a-c with five- and six-membered fused metallacycles in moderate to good yields. The same reaction in dichloromethane followed by treatment with alcohol resulted in unexpected formation of the related NCO-palladacycles 5a,b, along with the above NCS-complexes. Complexes 5 present the products of formal oxidation of the P=S group in the starting ligand, which apparently proceeds in the metal ion coordination sphere. Realization of kappa(3)-NCS and kappa(3)-NCO coordination in 4a-c and 5a,b, respectively, was unambiguously confirmed by X-ray diffraction analysis as well as multinuclear (H-1, C-13, P-31) NMR, IR, and Raman spectroscopy. The NCS-pincer complexes 4a-c demonstrated excellent catalytic activity for the Suzuki cross-coupling reactions of aryl halides with phenylboronic acid.