Probing the nature of the Co(III) ion in cobalamins: The ligand substitution reactions of aquacyanocobester, aquacyano(10-nitro)cobester and aquacyano(10-amino)cobester

The substitution of axially coordinated H₂O in aquacyano-heptamethylcob(III)yriate (aquacyanocobester, [ACCbs]⁺) and its derivatives, aquacyano(10-nitro)cobester, [AC(10-NO₂)Cbs]⁺, and aquacyano(10-amino)cobester, [AC(10-NH₂)Cbs]⁺, in which the H at C10 of corrin is replaced by electron-withdrawing NO₂ and electron-donating NH₂, respectively, by a variety of anionic (N₃^-, NO₂^-, SCN^-, SO₃^2-, CN^-) and neutral (imidazole, methylamine, 4-N,N-dimethylaminopyridine (DMAP)) ligands was studied in 50% isopropanol. Density Functional Theory (BP86/TZVP) and QTAIM calculations indicate that Co becomes more positive and harder as the C10 substituent is varied from NH₂ to H to NO₂. [AC(10-NH₂)Cbs]⁺ and [ACCbs]⁺ have a similar affinity for virtually all the ligands but [AC(10-NO₂)Cbs]⁺ has a lower affinity for the soft anions CN^- and SO₃^2-, and DMAP and imidazole, a similar affinity for MeNH₂, and a much higher affinity for the harder anions SCN^-, N₃^- and NO₂^-. The nature of the metal center can be modified by the electronic structure of the equatorial ligand and is a primary discriminator of the metal's affinity for exogenous ligands. The second order rate constants k^II for substitution of H₂O by CN^- in the three cobesters are not very different at 25 °C because of a compensation effect between ΔH^‡ and ΔS^‡; if all three reactions had the same hypothetical ΔS^‡ values then the rate constants would vary approximately in the ratio 10⁹:10⁶:1 as the C10 substituent is changed from NH₂ through H to NO₂. The compensation effect therefore masks the very significant differences in the reactivity of the metal ion towards an entering ligand. Log K and k^II for the ligand substitution reactions of axially coordinated H₂O in the cobalt corrins can be modified by perturbing the electronic structure of the equatorial corrin ligand, illustrating the importance of cis effects in these compounds.