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)-N-boolean AND-C-boolean AND) PdX](+), 4c, 4d, 1,3-xylyl dicarbene complexes of the type [((CCC)-C-boolean AND-C-boolean AND) PdX], 5a, 5b, 5c and 1,2-xylyl dicarbene complexes [(CC) PdX2], 8, where carbene is 3-arylimidazol-2-ylidene, aryl = mesityl, 2,6-Pr-2(i) C6H3, X = Br or Cl, have been prepared by (i) transmetallation from the corresponding silver complexes, i. e. [(C-N-C) Ag-2 Cl-2], [((CNC)-N-boolean AND-C-boolean AND) 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-boolean AND(H)C-boolean AND(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.