The Synthesis of a Corrole Analogue of Aquacobalamin (Vitamin B<sub>12a</sub>) and Its Ligand Substitution Reactions

The synthesis of a Co­(III) corrole, [10-(2-[[4-(1<i>H</i>-imidazol-1-ylmethyl)­benzoyl]­amino]­phenyl)-5,15-diphenylcorrolato]­cobalt­(III), DPTC-Co, bearing a tail motif terminating in an imidazole ligand that coordinates Co­(III), is described. The corrole therefore places Co­(III) in a similar environment to that in aquacobalamin (vitamin B<sub>12a</sub>, H<sub>2</sub>OCbl<sup>+</sup>) but with a different equatorial ligand. In coordinating solvents, DPTC-Co is a mixture of five- and six-coordinate species, with a solvent molecule occupying the axial coordination site trans to the proximal imidazole ligand. In an 80:20 MeOH/H<sub>2</sub>O solution, allowed to age for about 1 h, the predominant species is the six-coordinate aqua species [H<sub>2</sub>O–DPTC-Co]. It is monomeric at least up to concentrations of 60 μM. The coordinated H<sub>2</sub>O has a p<i>K</i><sub>a</sub> = 9.76(6). Under the same conditions H<sub>2</sub>OCbl<sup>+</sup> has a p<i>K</i><sub>a</sub> = 7.40(2). Equilibrium constants for the substitution of coordinated H<sub>2</sub>O by exogenous ligands are reported as log <i>K</i> values for neutral N-, P-, and S-donor ligands, and CN<sup>–</sup>, NO<sub>2</sub><sup>–</sup>, N<sub>3</sub><sup>–</sup>, SCN<sup>–</sup>, I<sup>–</sup>, and Cys in 80:20 MeOH/H<sub>2</sub>O solution at low ionic strength. The log <i>K</i> values for [H<sub>2</sub>O–DPTC-Co] correlate reasonably well with those for H<sub>2</sub>OCbl<sup>+</sup>; therefore, Co­(III) displays a similar behavior toward these ligands irrespective of whether the equatorial ligand is a corrole or a corrin. Pyridine is an exception; it is poorly coordinated by H<sub>2</sub>OCbl<sup>+</sup> because of the sterically hindered coordination site of the corrin. With few exceptions, [H<sub>2</sub>O–DPTC-Co] has a higher affinity for neutral ligands than H<sub>2</sub>OCbl<sup>+</sup>, but the converse is true for anionic ligands. Density functional theory (DFT) models (BP86/TZVP) show that the Co–ligand bonds tend to be longer in corrin than in corrole complexes, explaining the higher affinity of the latter for neutral ligands. It is argued that the residual charge at the metal center (+2 in corrin, 0 in corrole) increases the affinity of H<sub>2</sub>OCbl<sup>+</sup> for anionic ligands through an electrostatic attraction. The topological properties of the electron density in the DFT-modeled compounds are used to explore the nature of the bonding between the metal and the ligands.