B12 Models with Highly Distorted N4 Equatorial Ligation and a Co-C-N Ring: Structural Assessment of the Steric Influence of Benzimidazole and Imidazole Axial Ligands.
by Luigi G Marzilli, Suzette M Polson,. Lory Hansen, Scott J Moore,. Patricia A Marzilli Inorg. Chem. 1997, 36(18), 3854-3860.


In human B12 enzymes, a histidyl imidazole is the lower axial ligand instead of the benzimidazole of the coenzymes. The differences in the binding interactions of these ligands have been examined using a novel class of organocobalt models, [LCo(N-CH2-chel)]X (I), with an unusually spacious lower coordination site created by a rare Co-C-N ring. The work compares two new analogs, 1 (L = 1,5,6-trimethylbenzimidazole = Me3Bzm) and 2 (L = N-methylimidazole = N-MeImd), with the simple analog, 3 (L = py).

Model for cobalamins

The three structures (X = PF6 for 1 and 2; X = ClO4 for 3) have similar geometrical parameters for the ring atoms (N(2), Co, C(12)). A pocket under the Co-C-N group is created by the raised position of N(2) above the plane of the other three equatorial N donors, the cis oxime N, N(1), the trans oxime N, N(4), and the cis imine N, N(3). A net upward bending is clearly shown by the sum of the four cis N-Co-N bond angles involving the L ligating atom, N(5). The sum is approximately 23° more in the new models than in related imine/oxime-type (I/O) models. The distortions around N(5) differ significantly for the three structures. The Co-N(5) bond of Me3Bzm complex 1 is tilted furthest away from the Co-C-N pocket, and the N(5)-Co-N(2) angle is 111°. The value of the N(5)-Co-N(4) angle (96°) is close to that of the related angle (95°) in the I/O model, [Me3BzmCo((DO)(DOH)pn)CH3]PF6. In contrast, the N(5)-Co-N(4) angle of the N-MeImd and the py complexes, 2 and 3, is larger than that in I/O complexes, suggesting that these ligands are small enough to move toward the pocket. These and other structural parameters suggest clear differences between the steric interactions of the equatorial ligand with the imidazole and with the benzimidazole ligands. These complexes have unusual 1H NMR properties, e.g. a large remote isotope effect on some CH signals after exchange of the oxime OH to OD. At pH 13, the N4C chelate of [H2OCo(N-CH2-chel)]+ reverts, in part, to the classical I/O N4 chelate, suggesting a stepwise mechanism involving C-N bond cleavage to form a Co-CH2OH intermediate, which then undergoes base-catalyzed Co-C bond cleavage