Probing the Nature of the Co(III) Ion in Corrins: Comparison of Reactions of Aquacyanocobyrinic Acid Heptamethyl Ester and Aquacyano-Stable Yellow Cobyrinic Acid Hexamethyl Ester with Neutral N‑Donor Ligands

Equilibrium constants (log <i>K</i>) for substitution of coordinated H₂O in aquacyanocobyrinic acid heptamethyl ester (aquacyanocobester, ACCbs) and aquacyano-stable yellow cobyrinic acid hexamethyl ester (aquacyano-stable yellow cobester, ACSYCbs), in which oxidation of the C5 carbon of the corrin interrupts the normal delocalized system of corrins, by neutral N-donor ligands (ammonia, ethanolamine, 2-methoxyethylamine, <i>N</i>-methylimidazole, and 4-methylpyridine) have been determined spectrophotometrically as a function of temperature. Log <i>K</i> values increase with the basicity of the ligand, but a strong compensation effect between Δ<i>H</i> and Δ<i>S</i> values causes a leveling effect. The aliphatic amines with a harder donor atom produce Δ<i>H</i> values that are more negative in their reactions with ACSYCbs than with ACCbs, while the softer, aromatic N donors produce more negative Δ<i>H</i> values with ACCbs than with ACSYCbs. Molecular modeling (DFT, M06L/SVP, and a quantum theory of atoms in molecules analysis of the electron density) shows that complexes of the aliphatic amines with SYCbs produce shorter and stronger Co–N bonds with less ionic character than the Co–N bonds of these ligands with the cobester. Conversely, the Co–N bond to the aromatic N donors is shorter, stronger, and somewhat less ionic in the complexes of the cobester than in those of the SYCbs. Therefore, the distinction between the harder Co­(III) in ACSYCbs and softer Co­(III) in ACCbs, reported previously for anionic ligands, is maintained for neutral N-donor ligands