Chelating and pincer dicarbene complexes of palladium; synthesis and structural studies

A series of pincer 2,6-pyridyl dicarbene complexes of the type [(C–N–C)PdX]+, 4a, 4b, 2,6-lutidinyl dicarbene complexes of the type [(CNC)PdX]+, 4c, 4d, 1,3-xylyl dicarbene complexes of the type [(CCC)PdX], 5a,5b,5c and 1,2-xylyl dicarbene complexes [(CC)PdX2], 8, where carbene is 3arylimidazol-2-ylidene, aryl = mesityl, 2,6-Pri2C6H3, X = Br or Cl, have been prepared by (i) transmetallation from the corresponding silver complexes, i.e.[(C–N–C)Ag2Cl2], [(CNC)Ag2Cl2] and [(CC)Ag2Cl2] to palladium, (ii) substitution of cod in (cod)PdX2 by the pincer(C–N–C)(iii) palladation of bis-imidazolium salts [C(H)C(H)C(H)] in the presence of base. The crystal structures show that the ligands act as chelates in 4–5, while in 8 as both chelate and bridging.