Electrochemistry of a series of symmetric and asymmetric CpNiBr(NHC) complexes: Probing the electrochemical environment due to push-pull effects

摘要

The electrochemistry of nine Ni(II) N-heterocyclic carbene (NHC) complexes of general formula [CpNiBr(NHC)], bearing either a symmetric or asymmetric N-substituted alkyl/benzyl/phenethyl NHC ligand with Cp = cyclopentadienyl is described. Both metal and ligand-centred redox processes have been observed and systematically evaluated. The reversible to quasi-reversible Ni-II/Ni-III couple (0.062 V = E-0'= 0.127 V) is followed by the electrochemically and chemically irreversible ligand based oxidation peaks (E-pa similar to 0.37 V and similar to 0.5 V). The Ni-II/Ni-I and NHC ligand reduction processes were observed below -1.3 V. A comparison between the nine different Ni(II)-NHC complexes revealed that the electron-withdrawing NO2-containing NHC ligands exhibit the most positive metal-oxidation redox processes. A DFT study complements the experimental findings by establishing where the oxidation and reduction centres were based.