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.