Kinetics and mechanism of the addition of azide ion to a carbonyl ligand of [Ru3(CO)12]: cluster activation

Abstract

Spectroscopic studies of the reaction of azide ion with the cluster [Ru₃(CO)₁₂] in acetone solvent confirmed the occurrence of process (i) under a CO atmosphere. Kinetic measurements of this rapid [Ru₃(CO)₁₂]+ N₃^–→[Ru₃(NCO)(CO)₁₁]^–+ N₂(i) reaction revealed the rate law, Rate =k[Ru][N₃^–], and the activation parameters ΔH^‡= 61.6 ± 3.4 kJ mol^–1 and ΔS^‡= 3.5 ± 11.8 J K^–1 mol^–1. These results are consistent with the addition of N₃^– to a CO ligand of [Ru₃(CO)₁₂] to form an intermediate under either steady-state or pre-equilibrium conditions, followed by a Curtius-type rearrangement (K₂) to give the isocyanato complex 1. In the absence of CO, the product 1 undergoes rapid loss of CO to give the bridged isocyanato complex [Ru₃(µ-NCO)(CO)₁₀]^–. Kinetic studies of reaction (i) in tetrahydrofuran (thf) or thf–MeOH (1 : 1), in contrast, showed no dependence of rate on azide-ion concentration. This is inconsistent with a steady-state mechanism, but may be rationalised by a pre-equilibrium mechanism provided the condition K₁[N₃^–] 1 is met, i.e. the pre-equilibrium constant K₁ is large under the kinetic conditions employed.