Electronically and sterically tuned trans labilization controls the substitution behaviour of cobaloximes

Abstract

The kinetics of axial water substitution by cysteine in six different cobaloximes, viz.trans-RCo(Hdmg)(2)H(2)O, where Hdmg = dimethylglyoximate, R = cyclo-C(5)H(9) (c-P), CH(3)CH(2) (Et), CH(3) (Me), C(6)H(5)CH(2) (Bz), C(6)H(5) (Ph) and CF(3)CH(2), were studied as a function of cysteine concentration, temperature and pressure. It was found that cysteine substitutes the coordinated H(2)O molecule trans to the alkyl group with second order rate constants that follow the order of reactivity: c-P > Et > Bz > Me > Ph > CF(3)CH(2). Rate and activation parameters (Deltan H(++), Delta S(++) and Delta V(++)) enable the formulation of a reaction mechanism that can account for the substitution behaviour of the investigated alkylcobaloximes. In particular, a gradual mechanistic changeover from I(d) to I is observed along the series of R groups from c-P to CF(3)CH(2).

Kinetic and thermodynamic studies on ligand substitution reactions and base-on/base-off equilibria of cyanoimidazolylcobamide, a vitamin B12 analog with an imidazole axial nucleoside

“…We have been interested in the application of high-pressure techniques to study the base-on/base-off equilibria and the axial ligand substitution processes both in the case of vitamin B 12 derivatives (alkylcobalamins) and model complexes to obtain conclusive information on the underlying reaction mechanisms. 29,[35][36][37][38][39][40][41][42][43][44] In this study, the mechanism of the reaction between CN(Im)Cbl and CN − was investigated to improve our understanding of the role of steric and basicity effects of the leaving group in controlling the mechanism. Imidazole is known to be an interesting ligand in bioinorganic chemistry for a variety of reasons, such as the presence of the imidazole group of histidine as a ligand in most of the known haemoproteins.…”

Trans and cis influences and effects in cobalamins and in their simple models

“…This is in agreement with a dissociative mechanism. However, evidence of a significant contribution of both steric and electronic properties of the alkyl group can be detected in the analysis of the second order rate constants, k, for the reaction (H 2 O) Co(DH) 2 R + L1→ (L1) Co(DH) 2 R+H 2 O in coordinating solvents, with R=Me, Et, CH 2 CF 3 , CH 2 Ph and L1= pyrazole, L1= 1,2,4-triazole [22] and, particularly, when L1 = cysteine [23]. An interchange mechanism has been proposed for these reactions [22].…”

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

“…For the displacement of water by cysteine in a series of cobaloximes with R = CF 3 CH 2 , Ph, Me, Bz, Et and cyclo-C 5 H 9 , the corresponding DV ‡ were found to be: 0 ± 1, +4.5 ± 0.5, +5.5 ± 0.5, +5.9 ± 0.4, +6.6 ± 0.4 and +8.9 ± 0.7 cm 3 mol -1 , suggesting that a gradual mechanistic change over from I to D along the series of R groups from CF 3 CH 2 to cyclo-C 5 H 9 occurs. 18 In addition, we found that the mechanism is also affected by the nature of the entering ligand. This results from a study of the reactions of trans-[(R)Co(Hdmg) 2 S] (where R = PhCH 2 or CF 3 CH 2 and S = H 2 O or MeOH) with 4-NH 2 Py, Py and 4-CNPy.…”