Synthesis, characterization, density function theory and catalytic performances of palladium(II)–N-heterocyclic carbene complexes derived from benzimidazol-2-ylidenes

• Two Pd–NHC complexes are prepared by carbene transfer method.
• Catalytic olefin oxidations are studied.
• Functionalized Pd complex evinced moderate activity.
• All compounds are methodically characterized.

1-Benzyl-3-ethylbenzimidazolium iodide (1) and 1-benzyl-3-(2′-nitrilebenzyl)benzimidazolium bromide (2) were prepared by the reaction of 1-benzylbenzimidazole with ethyl iodide or 2-bromomethylbenzonitrile to act as N-heterocyclic carbene (NHC) precursors. Bis-NHC silver(I) complexes having halide (3a and 4a) as well as hexafluorophosphate (3b and 4b) counterions were yielded by the reaction of NHC precursors with silver(I) oxide. Subsequent reactions of the silver(I) halide/hexafluorophosphate complexes with [PdCl2(CNCH3)2] in methanol afforded the NHC palladium(II) complexes (5 and 6) via carbene transfer method. All synthesized compounds were fully characterized by analytical and spectrometric methods. Preliminary catalytic studies evinced that the nitrile-functionalized palladium(II)–NHC complex 6 is highly active in the oxidation of 1-octene as well as styrene in the presence of aqueous hydrogen peroxide as an oxidizing agent. Both the olefins were oxidized to their corresponding oxidized products with 45–52% conversion with moderate selectivity in the presence of NHC palladium complexes, which acted as oxidation catalysts. In order to investigate the suggested structure of the title complexes, density functional theory was used to find the stable structures of the palladium complexes and their isomers. Geometry parameters, molecular orbital energies, electronic energy, band gap, and vibrational frequencies were calculated.

Palladium–carbene complexes of benzimidazole based NHCs were prepared via transmetallation technique from corresponding silver–carbene complexes. Palladium–NHCs showed comparable activity in the oxidation of olefins viz., 1-octene and styrene.Figure optionsDownload as PowerPoint slide