Synthesis, structure, and solution dynamics of pentamethylcyclopentadienyl nickel complexes bearing N-heterocyclic carbene ligands

Neutral pentamethylcyclopentadienyl nickel complexes of general formula [Ni(NHC)x(eta(5)-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, 1] were prepared from the reaction of pentamethylcyclopentadienyl acetylacetonate nickel(II) with I equiv of the corresponding imidazolium salt (NHC center dot HX). The new complexes [Ni(Me-NHC)I(eta(5)-C5Me5)]1a, [Ni(Mes-NHC)Cl(eta(5)-C5Me5)] 2, and [Ni(iPr-NHC)Cl(eta(5)-C5Me5)] 3 were obtained in moderate to good yields and were fully characterized by C-13 and H-1 NMR spectroscopy, and in the cases of la and 2 by single-crystal X-ray crystallography. The related cyclopentadienyl complex [Ni(Me-NHC)I(eta(5)-C5H5)] 1b was also synthesized and structurally characterized; its geometry and spectroscopic data are comparable to those of complex la. The variable-temperature (VT) H-1 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 U mol(-1) by VT NMR experiments.