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

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 (Delta H-double dagger, Delta S-double dagger and Delta V-double dagger) support the operation of a dissociative interchange (I-d) mechanism. Complexes of the type trans-[RCo(Hdmg)(2)L] (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)(2)Co((DO)(DOH)pn)] with acid, the activation parameters (Delta H-double dagger, Delta S-double dagger and Delta V-double dagger) were found to be 37 +/- 1 kJ mol(-1), -86 +/- 3 J mol(-1) K-1 and -18.9 +/- 0.7 cm(3) mol(-1), respectively, and support a protonation mechanism.