Equilibrium, Kinetic and Leaving Group Effect Studies on Ligand Substitution Reactions of a Simple Model for Coenzyme B12

Abstract

The ligand substitution reactions of trans-[Co III (en) 2 (Me)H 2 O] 2+ and trans-[Co III (en) 2 (Me)NH 3 ] 2+ were studied for CN − , SCN − , N 3 − , and NH 3 as entering nucleophiles. These nucleophiles displace the coordinated H 2 O and NH 3 molecules trans to the methyl group and form the sixcoordinate complex trans-[Co(en) 2 (Me)L]. The complexformation constant for the displacement of H 2 O by NH 3 was found to be 17.4 ± 1.1 M −1 at 20°C, and those for the displacement of NH 3 by SCN − and N 3 − were found to be 4.4 ± 1.5 and 3.1 ± 0.4 at 10°C, respectively. From temperature and pressure dependence studies, activation parameters (∆H ϶ , ∆S ϶ and ∆V ϶ ) for the reaction of trans-[Co III (en) 2 (Me)H 2 O] 2+

Equilibrium and Kinetic Studies on Ligand Substitution Reactions of Trans-[COIII(en)2(Me)H2O]2+: A Model for Coenzyme B12

“…The significantly positive values of ÁH 6 ¼ and ÁS 6 ¼ for the reverse reaction suggest a dissociatively activated transition state. Values of the activation enthalpy for the reaction with triazole, pyrazole and N-Ac-ImH (59 AE 2, 67 AE 6, and 72 AE 4 kJ mol À1 , respectively) are close to those obtained previously [16,17] for the reaction with CN À , imidazole and NH 3 (50 AE 4, 53 AE 2 and 62 AE 1 kJ mol À1 respectively). Values of activation entropy for the formation of trans-[Co(en) 2 (Me)azole] 2þ were found to be close to zero, compared to a significantly positive value for the back reaction.…”

“…This study presents kinetic and equilibrium data for the substitution of the axial H 2 O molecule in trans-[Co(en) 2 (Me)H 2 O] 2þ by azoles in comparison with previous work on the reactions of cyanide, imidazole and NH 3 [16,17]. The investigated reaction is shown in Eq.…”

“…Values of activation entropy for the formation of trans-[Co(en) 2 (Me)azole] 2þ were found to be close to zero, compared to a significantly positive value for the back reaction. ÁH 6 ¼ and ÁS 6 ¼ for the reaction of trans-[Co(en) 2 (Me)H 2 O] 2þ with pyrazole, triazole and N-Ac-ImH are in a good agreement with those obtained previously for CN À , NH 3 and Imidazole, where the mechanism was assigned as I d [16,17]. Thus it can be concluded that the mechanism for the reaction investigated here is an I d mechanism, where the azoles partially participate in the transition state.…”

“…Ligand substitution of trans-[Co III (en) 2 (Me)H 2 O] 2þ have been studied for cyanide, imidazole and NH 3 as entering nucleophiles [16,17]. These were found to displace the coordinated water molecule trans to the methyl group to form trans-[Co(en) 2 (Me)L].…”

“…However, those for the reaction trans-[Co III (en) 2 (Me)H 2 O] 2þ with NH 3 were found to be 62 kJ mol À1 , þ 3 J K À1 mol À1 and þ 5.7 cm 3 mol À1 , respectively. Based on these activation volumes, the reactions follow an I d mechanism in which the entering nucleophile is weakly bound in the transition state [16,17].…”