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

Abstract Reaction of 2,6-lutidine and 3-picoline with pd(CH 3 COO) 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, 1 H, 13 C[ 1 h]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 Δ E pa = +0.93 and +0.86 V vs Ag/AgCl. The complexes 1 and 2 with cocatalysts FeCl 3 or CuCl 2 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°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.