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)H(2)O] where Hdmg = dimethylglyoximate, R = PhCH(2), Et and CF(3)CH(2), 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)H(2)O] for R = Et at pH 6.0, 25 degrees C and I= 0.1 M (NaClO(4)), 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 = PhCH(2) were found to be 755 and 691 M(-1) s(-1), respectively. The second order rate constants in the case of R = CF(3)CH(2) 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 > PhCH(2) > CF(3)CH(2). The activation parameters (DeltaH([not equal]), DeltaS([not equal]) and DeltaV([not equal])) 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 cm(3) mol(-1), respectively. These data suggest that an I(d) substitution mechanism operates where the azoles participate in the transition state.

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].…”

Coenzyme B12 Model Studies − Kinetics of Axial Ligation of Alkylcobalt Complexes Containing a Tridentate Amino‐Oximate Ligand

“…, respectively. [46] The sequence CF 3 CH 2 Ͻ Ph Ͻ CH 2 Ph is as expected and corresponds to an increase in the dissociative character of the process with increasing donor properties of R.…”

Kinetics and Mechanism of Axial Ligand Substitution of Alkyl Cobaloximes by Substituted Pyridines in Different Solvents

“…Analogously, the substitution rate constants of aquacobalamin by a variety of substituted pyridine ligands were found to be independent of their pK BHϩ values. [25] The second-order rate constants for substitution of H 2 O in trans-(CF 3 CH 2 )Co(Hdmg) 2 higher than that found for the corresponding reaction with cysteine, [34] and this was ascribed to metal-to-ligand π bonding. The range of rate constants for axial ligand binding to methylaquacobaloxime are 4.3Ϫ7.5 (RNH 2 ), 12.8Ϫ27.3 (RS Ϫ ), 49.6Ϫ55.2 (RSH), and 92Ϫ200 (X-py)…”