Kinetic and mechanistic study on the reaction of alkylcobaloximes with azoles

Abstract

The kinetics of axial water substitution by azoles (pyrazole and 1,2,4-triazole) in three different cobaloximes, viz.trans-[Co(Hdmg)₂(R)H₂O] where Hdmg = dimethylglyoximate, R = PhCH₂, Et and CF₃CH₂, were studied as a function of azole concentration, temperature and pressure in aqueous solution. The second order rate constants for the substitution of water in trans-[Co(Hdmg)₂(R)H₂O] for R = Et at pH 6.0, 25 °C and I = 0.1 M (NaClO₄), were found to be 1309 and 1200 M⁻¹ s⁻¹ for pyrazole (Pz) and 1,2,4-triazole (Tz), respectively, and those obtained for R = PhCH₂ were found to be 755 and 691 M⁻¹ s⁻¹, respectively. The second order rate constants in the case of R = CF₃CH₂ were found to be 0.358 and 0.348 M⁻¹ s⁻¹ for Pz and Tz, respectively. The relative order of reactivity for the different alkyls being Et > PhCH₂ > CF₃CH₂. The activation parameters (ΔH^≠, ΔS^≠ and ΔV^≠) obtained for these reactions were found to be in the range of 65–87 kJ mol⁻¹, 24–47 J mol⁻¹ K⁻¹ and 2.5–7.7 cm³ mol⁻¹, respectively. These data suggest that an I_d substitution mechanism operates where the azoles participate in the transition state.