The identification of all five isomers of [(η5-C5H5)2Mo2(CO)5(CNMe)] and the mechanisms of fluxionality †

Abstract

[(η5-C5H5)2Mo2(CO)5(CNMe)] has been re-examined by 1H and 13C NMR spectroscopy in (CD3)2NCDO and all five isomers have been identified. Activation energies have been determined for their interconversion and ligand scrambling. The lowest energy dynamic process is intramolecular ligand exchange via [(η5-C5H5)2Mo2(μ-CO)(μ-CNMe)(CO)4] which is more facile in the gauche-isomer with ΔG‡ = 11.7 ± 0.2 kcal mol−1 than in the trans-isomer with ΔG‡ = 12.2 ± 0.2 kcal mol−1. The next lowest energy dynamic process is intramolecular ligand exchange via [(η5-C5H5)2Mo2(μ-CO)2(CO)3(CNMe)] which is more facile in the gauche-isomer with ΔG‡ = 12.4 ± 0.2 and 12.6 ± 0.2 kcal mol−1 than in the trans-isomer with ΔG‡ = 14.5 ± 0.2 kcal mol−1. The highest energy dynamic process is rotation about the Mo–Mo bond which was only detected in the isomers with CNMe cis to the Mo–Mo bond where ΔG‡ = 14.4 ± 0.2 and 14.7 ± 0.2 kcal mol−1.