Transition Metal Compounds of Pyridine-Amide-Functionalized Carbene Ligands: Synthesis, Structure, and Electrocatalytic Properties in Proton Reduction

摘要

Three pyridyl-amide substituted (benz)imidazolium salts H(2)L1Cl, H(2)L2Cl and H(2)L3Cl were synthesized and successfully employed as ligand precursors for the syntheses of novel nickel(II) and cobalt(III) complexes. The compounds H(2)L1Cl and H(2)L2Cl are precursors to tetradentate ligands and differ in the nature of the N-heterocyclic carbene (NHC) functionality, being imidazole-based and benzimidazole-based, respectively. The ligand precursor H(2)L3Cl resembles H(2)L1Cl, but with one of the pyridyl groups replaced with a benzyl group, thus yielding a potential tridentate ligand. The nickel(II) compounds [Ni(L1)]Cl and [Ni(L2)]PF6 were obtained, bearing tetradentate ligands comprising an amidate and two pyridine nitrogen donor atoms and an (NHC) carbon donor. Single crystal X-ray crystallography revealed that the nickel ions in both compounds are in slightly distorted square-planar geometries. Reactions of cobalt salts with the ligands H(2)L1Cl and H(2)L3Cl resulted in the cobalt(III) compounds [Co(L1)(2)]Cl and [Co(L3)(2)]PF6; the cobalt ions in both complexes are in octahedral geometries, bound by two tridentate ligands in a meridional binding mode, with two dangling pyridine and benzyl groups, respectively. The four compounds show electrocatalytic activity in proton reduction in dimethylformamide solutions in presence of acetic acid; their activity is compared using cyclic voltammetry and quantified with gas chromatography.