Dynamic behaviour of SO2 and NO ligands in hexanuclear ruthenium clusters as probed by 1H NMR signals of a coexisting µ-allyl ligand

Abstract

A hexanuclear ruthenium carbide carbonyl cluster with SO ₂ and µ-η ³ -C ₃ H ₅ ligands, [Ru ₆ C(CO) ₁₄ (SO ₂ )(µ-η ³ -C ₃ H ₅ )] ^- shows fluxionality with ΔH ^‡ = 7.3 kcal mol ^-1 and ΔS ^‡ = -16.6 cal K ^-1 mol ^-1 as observed by variable-temperature ¹ H NMR spectroscopy of the syn- and anti-protons of the µ-η ³ -C ₃ H ₅ moiety. Likewise, fluxionality of a neutral cluster complex with NO and allyl groups, [Ru ₆ C(CO) ₁₄ (NO)(µ-η ³ -C ₃ H ₅ )], has been found to have ΔH ^‡ = 9.5 kcal mol ^-1 and ΔS ^‡ = -7.8 cal K ^-1 mol ^-1 . The 2-methoxycarbonylallyl analogue of [Ru ₆ C(CO) ₁₄ (NO)(µ-η ³ -C ₃ H ₄ CO ₂ Me)], exhibits similar fluxional behaviour with virtually identical parameters (ΔH ^‡ = 9.2 kcal mol ^-1 and ΔS ^‡ = -7.1 cal K ^-1 mol ^-1 ), while model analysis based on its single-crystal structure indicates that rotation of the η ³ -co-ordinated allyl group would have a very high steric barrier. The observed dynamic behaviour was thus concluded to arise from rapid migration of the SO ₂ or NO ligand over the metal atoms in the cluster core.