Sterically modulated palladium(II)-N-heterocyclic carbene complexes for the catalytic oxidation of olefins: synthesis, crystal structure, characterization and DFT studies

Abstract

The synthesis of a series of sterically modulated palladium(II)-NHC complexes (9-11) of the general formula [Pd(NHC)(2)Cl-2] (NHC = 1-benzyl-3(2'-methyl)-propylbenzimidazol-2-ylidine, 1-benzyl-3 (2'-methyl)-butylbenzimidazol-2-ylidine and 1-benzyl-3-hexylbenzimidazol-2-ylidine) from their respective silver(I) counterparts (6-8) is presented. Two novel triazine-tethered Zwitterionic (benz) imidazolium salts, 4 and 5, were prepared and tested as NHC precursors for the preparation of silver(I) complexes. However, all our attempts to prepare silver(I) derivatives from salts 4 and 5 ended with negative results due to the low acidity of the C2 protons. The Zwitterionic derivative 4 was additionally characterized by the single crystal X-ray diffraction technique. The molecular structure of 4 evidenced pi-pi stacking interactions between the triazine rings of two crystallographically independent units. The palladium complexes 9-11 showed good catalytic activities in the epoxidation of 1-octene and styrene under homogeneous conditions with aqueous hydrogen peroxide as an oxidant. The effect of temperature and solvent on the epoxidation of the aforementioned olefins using complexes 9-11 was also explored. Density functional theory (DFT) was used to model the structures of the isomers of the palladium complexes. Geometry parameters, electronic energy, molecular orbital energies, band gap, vibrational frequencies and the cis-trans energy barrier were calculated.