Synthesis, structural characterization and catalytic carbonylation of nitrobenzene and amines by mononuclear palladium(II) complexes containing substituted pyridine ligands

Reaction of 2,6-lutidine and 3-picoline with pd(CH3COO)(2) in dry benzene gave diacetato-bis-2,6-lutidine palladium(II) (1) and diacetato bis-3-picoline palladium(II) (2) respectively. The complexes 1 and 2 were characterized by spectroscopic (IR, UV-Vis,H-1, C-13[H-1] NMR) and structural determination by single crystal X-ray methods. Both the complexes are nearly perfect square planar (trans) complexes, the coordination valencies being provided by one of the oxygens of two acetate ligands and the nitrogen of the pyridine. The structure of complex 1 is held by van der Waal's contacts only and the structure of complex 2 is held in space by intermolecular hydrogen bonding. The near identical bond length of Pd-O and Pd-N show that the acetate groups are labile, which may be a principle factor for the catalytic activity of the complexes. Cyclic voltametry studies exhibit an irreversible one electron oxidation with Delta E(pa) = +0.93 and +0.86 V vs Ag/AgCl. The complexes 1 and 2 with cocatalysts FeCl3 or CuCl2 were found to be active homogeneous catalysts in the carbonylation of nitrobenzene to phenylurethane and carbonylation of aromatic and aliphatic amines to diureas respectively in the temperature range 150-200 degrees C and 40 atm. CO, respectively. The catalytic studies indicated that complex 2 was more active than 1 in carbonylation of nitrobenzene and of amines under the reaction conditions studied. In the case of amines, the activities of complex 1 and 2 were decreased in the order n-butylamine > diethylamine > benzylaime > cyclohexylamine. However, both complexes were inactive in carbonylation of triethylamine.