Preparation and properties of sandwiched trinuclear palladium(II) complexes with tridentate phosphine and phosphine sulfide ligands

The central phosphino group of tripodal tetradentate tris[2-(diphenylphosphino)ethyl]phosphine (pp3) was selectively oxidized by the reaction with diethyl disulfide to give tridentate phosphine ligand pOp3. The terminal phosphino groups were reacted with sulfur to give pOp3 trisulfide (pOp3S3). Three palladium(II) ions were sandwiched in the two pOp3 and pOp3S3 ligands to form the trinuclear complexes with three trans(P) and trans(S) PdX2 (X = Cl, Br, I) moieties, respectively. The tripodal triphosphine, 1,1,1-tris(diphenylphosphinomethyl)ethane (i-p3), and its mono- and tri-sulfide, which have shorter carbon chains compared with pOp3, form the mononuclear dichloro palladium(II) complexes with cis(P) and cis(S) geometries. Difference in the catalytic activity for the C–C coupling reaction was discussed in connection with the coordinated groups and geometries of the complexes.