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Download fileThe Synthesis of a Corrole Analogue of Aquacobalamin (Vitamin B12a) and Its Ligand Substitution Reactions
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posted on 2014-05-05, 00:00 authored by Caitlin
F. Zipp, Joseph P. Michael, Manuel A. Fernandes, Sadhna Mathura, Christopher B. Perry, Isabelle Navizet, Penny
P. Govender, Helder M. MarquesThe
synthesis of a Co(III) corrole, [10-(2-[[4-(1H-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 B12a, H2OCbl+) 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/H2O solution, allowed to age for about 1 h, the predominant
species is the six-coordinate aqua species [H2O–DPTC-Co].
It is monomeric at least up to concentrations of 60 μM. The
coordinated H2O has a pKa =
9.76(6). Under the same conditions H2OCbl+ has
a pKa = 7.40(2). Equilibrium constants
for the substitution of coordinated H2O by exogenous ligands
are reported as log K values for neutral N-, P-,
and S-donor ligands, and CN–, NO2–, N3–, SCN–, I–, and Cys in 80:20 MeOH/H2O solution
at low ionic strength. The log K values for [H2O–DPTC-Co] correlate reasonably well with those for
H2OCbl+; 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 H2OCbl+ because of the sterically
hindered coordination site of the corrin. With few exceptions, [H2O–DPTC-Co] has a higher affinity for neutral ligands
than H2OCbl+, 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 H2OCbl+ 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.