Synthesis, Structure, and Solution Dynamics of Pentamethylcyclopentadienyl Nickel Complexes Bearing N-Heterocyclic Carbene Ligands

Abstract

Neutral pentamethylcyclopentadienyl nickel complexes of general formula [Ni(NHC)X(η5-C5Me5)] [NHC = 1,3-dimethylimidazol-2-ylidene (Me-NHC), 1,3-bis(2,4,6,-trimethylphenyl)imidazol-2-ylidene (Mes-NHC), 1,3-bis(2,6-diisopropylphenyl)imidazol-2-ylidene (iPr-NHC); X = Cl, I] were prepared from the reaction of pentamethylcyclopentadienyl acetylacetonate nickel(II) with 1 equiv of the corresponding imidazolium salt (NHC·HX). The new complexes [Ni(Me-NHC)I(η5-C5Me5)] 1a, [Ni(Mes-NHC)Cl(η5-C5Me5)] 2, and [Ni(iPr-NHC)Cl(η5-C5Me5)] 3 were obtained in moderate to good yields and were fully characterized by 13C and 1H NMR spectroscopy, and in the cases of 1a and 2 by single-crystal X-ray crystallography. The related cyclopentadienyl complex [Ni(Me-NHC)I(η5-C5H5)] 1b was also synthesized and structurally characterized; its geometry and spectroscopic data are comparable to those of complex 1a. The variable-temperature (VT) 1H NMR spectra of the sterically constrained complexes 2 and 3 are consistent with restricted rotation about the nickel−carbene carbon bond. The free energy of activation for the dynamic processes, in both cases, was determined to be on the order of 65−67 kJ mol−1 by VT NMR experiments.