| Abstract
| - In a novelclass of organocobalt B12 models,[LCo(N-CH2-CHEL)]X, with anN4C chelate and an unusually spacious lower coordinationsite created by a rare Co−C−N three-membered ring, differences inthe binding interactions of biorelevant imidazole and benzimidazoleligands have steric, not electronic, origins. Several new featuresof organocobalt chemistry were discovered, including the reversion ofthe N4C chelate to the classical N4 chelate anda new N5C chelate.
- In human B12 enzymes, a histidyl imidazole is thelower axial ligand instead of the benzimidazole of thecoenzymes.We have explored the differences in the binding interactions ofthese ligands using a novel class of organocobaltmodels, [LCo(N-CH2-CHEL)]X, with anunusually spacious lower coordination site created by a rareCo−C−Nring. We now compare two new analogs, 1 (L =1,5,6-trimethylbenzimidazole = Me3Bzm) and2 (L =N-methylimidazole = N-MeImd), with the firstanalog, 3 (L = pyridine = py). The three structures(X = PF6for 1 and 2; X = ClO4 for3) have similar geometrical parameters for the ring atoms(N(2), Co, C(12)). A pocketunder the Co−C−N group is created by the raised position of N(2)above the plane of the other three equatorialN donors, the cis oxime N, N(1), the trans oximeN, N(4), and the cis imine N, N(3). A net upwardbending isclearly shown by the sum of the four cis N−Co−N bondangles involving the L ligating atom, N(5). The sumis ∼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) bondof Me3Bzm complex 1 is tilted furthestawayfrom 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, theN(5)−Co−N(4) angle of the N-MeImd and the py complexes,2 and 3, is larger than that in I/Ocomplexes,suggesting that these L ligands are small enough to move toward thepocket. These and other structural parameterssuggest clear differences between the steric interactions of theequatorial ligand with the imidazole and with thebenzimidazole ligands. These complexes have unusual 1HNMR properties, e.g. a large remote isotope effect onsome 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 bondcleavageto form a Co−CH2OH intermediate, which thenundergoes base-catalyzed Co−C bond cleavage.
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