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.