Mechanistic behaviour of alkylcobaloximes and imino-oxime complexes related to vitamin B12

Abstract

The ligand substitution reactions of complexes of the type trans-[(R)Co(Chel)S]^+/0 with L, where chel = (DO)(DOH)pn = 2,2′-(1,3-diaminopropanebis(2-methyl-3-butanone)oxime), R = CH₃, L = imidazole, pyrazole, 1,2,4-triazole and 1-methylimidazole, and S = water and MeOH, and chel = (Hdmg)₂ = bis(dimethylglyoximate), R = CH₂Cl, CH₂Br, and CH₂I, L = thiourea and pyridine, and S = water, were studied in detail as a function of temperature and pressure. The reported activation parameters (ΔH^‡, ΔS^‡ and ΔV^‡) support the operation of a dissociative interchange (I_d) mechanism. Complexes of the type trans-[RCo(Hdmg)₂L] (R = CH₂Cl, CH₂Br, and CH₂I; L = H₂O and Py) were fully optimized at the B3LYP/LANL2DZp level, and the structural data support the mechanistic assignment based on the reported activation parameters. For the reaction of trans-[(CH₃)₂Co((DO)(DOH)pn)] with acid, the activation parameters (ΔH^‡, ΔS^‡ and ΔV^‡) were found to be 37 ± 1 kJ mol⁻¹, −86 ± 3 J mol⁻¹K⁻¹ and −18.9 ± 0.7 cm³ mol⁻¹, respectively, and support a protonation mechanism.