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