A Very Long Cobalt to Nitrogen Bond in a Coenzyme B12 Model. Relevance to the Role of the 5,6-Dimethylbenzimidazole in C0-C Bond Cleavage in Coenzyme B12

Article abstract of DOI:10.1021/ja00358a012

The crystal and molecular structure of the complex trans-bis(dimethylglyoximato)isopropyl(2-aminopyridine)cobalt(III) is reported.The compound crystallizes in space group P2₁2₁2₁ with a = 27.83 (1) Angstroem, b = 8.564 (6) Angstroem, c = 8.198 (6) Angstroem, V = 1958.1 Angstroem³, D_m = 1.45 g cm^-3, Z = 4, and D_c = 1.45 g cm^-3.A total of 2740 reflections were measured.The structure was solved with conventional Patterson and Fourier methods.Block-diagonal least-squares refinement led to a final R value of 0.033.The complex is pseudooctahedral with the dimethylglyoximato ligands occupying the four equatorial positions and the isopropyl ligand occupying one of the axial positions.The geometry of these ligands and their relationship to Co are very similar to those of the known pyridine analogue.However, the bond from Co to the endocyclic N of 2-aminopyridine is unusually long, namely 2.194 (4) Angstroem.This long bond is believed to result from repulsion between the 2-amino group and the equatorial ligands.Another consequence of this interaction is the unsymmetrical nature of the Co-N-C angles, which are 129.7 (4) deg for the 2-carbon and 115.7 (3) deg for the 6-carbon.This angular distortion and the long Co-N bond are also found in coenzyme B₁₂.Rate studies with trans-alkylbis(dimethylglyoximato)(L)cobalt(III), in which 2-aminopyridine analogues (L) are exchanged by stronger ligands such as P(OCH3)3, can best be interpreted by the endocyclic binding mode of 2-aminopyridine in solution.In conjunction with literature on Co-C bond energies of related B₁₂ models, the potential influence of the 5,6-dimethylbenzimidazole ligand in Co-C bond cleavage in coenzyme B₁₂ dependent reactions is discussed.