The inorganic chemistry of the cobalt corrinoids - an update

The inorganic chemistry of the cobalt corrinoids, derivatives of vitamin B₁₂, is reviewed, with particular emphasis on equilibrium constants for, and kinetics of, their axial ligand substitution reactions. The role the corrin ligand plays in controlling and modifying the properties of the metal ion is emphasised. Other aspects of the chemistry of these compounds, including their structure, corrinoid complexes with metals other than cobalt, the redox chemistry of the cobalt corrinoids and their chemical redox reactions, and their photochemistry are discussed. Their role as catalysts in non-biological reactions and aspects of their organometallic chemistry are briefly mentioned. Particular mention is made of the role that computational methods - and especially DFT calculations - have played in developing our understanding of the inorganic chemistry of these compounds. A brief overview of the biological chemistry of the B₁₂-dependent enzymes is also given for the reader's convenience.

The radical versus ionic mechanisms of reduced cobalamin inactivation by tert-butyl hydroperoxide and hydrogen peroxide in aqueous solution

It was demonstrated that antioxidants cannot protect reduced cobalamin against its modification by hydrogen peroxide.

Synthesis of vitamin B12 derivatives with sodium hydroxymethanesulfinate

The reaction of cyanocobalamin (CNCbl) with sodium hydroxymethanesulfinate (HMS) was studied over a wide range of pH (4–11) under aerobic conditions. CNCbl is destroyed in the presence of HMS in aqueous solution to form uncolored substances. The accumulation of stable yellow corrinoids (SYCs) preceded these changes at pH [Formula: see text] 8. The major stable yellow corrinoid is (15R)-Co[Formula: see text], Coß — dicyano-13-dehydro-15-hydro-l5-hydroxycob(III)alamin. The yield of this SYC is 25%, and the stability of this compound decreases significantly with increasing concentrations of HMS, pH and temperature.

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 K) for substitution of coordinated H(2)O 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, N-methylimidazole, and 4-methylpyridine) have been determined spectrophotometrically as a function of temperature. Log K values increase with the basicity of the ligand, but a strong compensation effect between ΔH and ΔS values causes a leveling effect. The aliphatic amines with a harder donor atom produce ΔH values that are more negative in their reactions with ACSYCbs than with ACCbs, while the softer, aromatic N donors produce more negative ΔH 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.