Ligand Substitution Behavior of a Simple Model for Coenzyme B₁₂

Abstract

The ligand substitution reactions of trans-[CoIII(en)2(Me)H2O]2+, a simple model for coenzyme B12, were studied for cyanide and imidazole as entering nucleophiles. It was found that these nucleophiles displace the coordinated water molecule trans to the methyl group and form the six-coordinate complex trans-[Co(en)2(Me)L]. The complex-formation constants for cyanide and imidazole were found to be (8.3 +/- 0.7) x 10(4) and 24.5 +/- 2.2 M-1 at 10 and 12 degrees C, respectively. The second-order rate constants for the substitution of water were found to be (3.3 +/- 0.1) x 10(3) and 198 +/- 13 M-1 s-1 at 25 degrees C for cyanide and imidazole, respectively. From temperature and pressure dependence studies, the activation parameters delta H++, delta S++, and delta V++ for the reaction of trans-[CoIII(en)2(Me)H2O]2+ with cyanide were found to be 50 +/- 4 kJ mol-1, 0 +/- 16 J K-1 mol-1, and +7.0 +/- 0.6 cm3 mol-1, respectively, compared to 53 +/- 2 kJ mol-1, -22 +/- 7 J K-1 mol-1, and +4.7 +/- 0.1 cm3 mol-1 for the reaction with imidazole. On the basis of reported activation volumes, these reactions follow a dissociative mechanism in which the entering nucleophile could be weakly bound in the transition state.

Synthesis, characterization, and thermodynamics of tertiary phosphine cobalt(III) Schiff-base complexes

“…Some extensive works have been done on these models (4)(5)(6)(7)(8). Recently the kinetics of ligand replacement reactions of the axial H 2 O in the trans-[Co(en) 2 (H 2 O)(Me)] 2+ complex have been studied by Hamza et al (9). The equilibrium of the adduct formation of [Co(Salen)X] + (X = tertiary phosphines) with NO 2 has been studied.…”

The Thermodynamics of Tertiary Phosphine Cobalt (III) Schiff Base Complexes

“…Extensive work has been done on these models. [4][5][6][7][8][9] Recently the kinetic of ligand replacement reactions of the axial H 2 O in the trans -[Co(en) 2 (H 2 O)(Me)] 2+ complex have been studied by Hamza et al 10 The equilibrium of the adduct formation of [Co (Salen) X] + , (X= tertiary phosphines) with NO 2 has been studied. It was shown that the equilibrium constants decrease according to the following trend: PPh 3 < PEtPh 2 < PBu 3 .…”

Equilibria and kinetics for pH-dependent axial ligation of ethylester and methylester(aquo)cobaloximes with aromatic and aliphatic N-donor ligands and a molecular mechanistic study of the Co-C bond

“…27 Compared to both cobalamins and other model systems, cobaloximes have stronger Co-C bonds and shorter Co-L (L = pyridine or substituted pyridines) bonds. 28,29 van Eldik et al 30,31 studied the ligand-substitution reactions of trans-[Co(en) 2 Me (H 2 O)] +2 , a simple model for coenzyme B 12 , with cyanide and imidazole as incoming ligands where they found that these ligands displace the coordinated water molecule trans to the methyl group forming the six coordinate complex. Hence, it is important to study ligand substitution reactions trans to the axial alkyl ligand in coenzyme B 12 and related model complexes.…”