Palladium(II) complexes of hemilabile NNS and NNSe iminophosphorane ligands: Synthesis, characterization, and reactivity

Four terdentate ligands combining (2-indolyl)iminophosphorane with a thioether or selenoether moiety, with general formula 2-C8H6N(Ph2P = NC6H4XR) [X = S, R = CH3 (2), C6H5 (3); X = Se, R = CH3 (4), C6H5 (5)], were synthesized and fully characterized, including the structure of ligand (2) by single-crystal X-ray crystallography. Treatment of ligands 2-5 with PdCl2(CH3CN)(2) produces non-symmetrical N,N,X-Pd(II) pincer complexes [PdCl{2-C8H5N(Ph2P = NC6H4XR-kappa(3)-N,N,X)}] [X = S, R = CH3 (6), C6H5 (7); X = Se, R = CH3 (8), C6H5 (9)]. All complexes were fully characterized spectroscopically and by single-crystal X-ray crystallography. In presence of NaPF6 or NaBF4, treatment of 6 with neutral ligands NEt3, Pyridine and PPh3 gives mononuclear cationic complexes 10, 11, 12 where Cl- is replaced by the corresponding ligand. The solid state structure of cationic complexes 10-12 show that kappa(3)-N,N,X pincer coordination is preserved. Reaction of 6 with diphosphane Ph2P(CH2)(2)PPh2 (dppe) in 1:1 ratio promotes liberation of the sulfur donor atom from Pd(II) and gives the mononuclear cationic complex 13 bearing the ligand in bidentate N,N chelation and the diphosphane in P,P chelation. Solid-state structure of 13 determined by X-ray crystallography confirms the labile character of the thioether group and reveals a strong interaction between the iminic nitrogen and sulfur atom, holding the thioether group very close to the metal center. Reversibility of the process is proven by reformation of the pincer structure upon treatment of 13 with O-2 and release of Ph2P(O)(CH2)(2)P(O)Ph-2 (dppeO(2)). When complexes 7-9 are treated with the same sequence of reactions, P-31 NMR monitoring indicates that pincer complexes with selenoether donors undergo the same reversible hemilability.