ELECTRONIC-STRUCTURE OF THE COENZYME VITAMIN-B(12) AND RELATED SYSTEMS .1. CO(DH)2(L)(R) COMPOUNDS (DH = DIMETHYLGLYOXIME, L = NH3, PY, 2-NH2PY, 5,6-DIMETHYLBENZIMIDAZOLE, R=CH3, I-C3H7, 5'-DEOXYADENOSYL) AS MODEL SYSTEMS FOR THE VITAMIN-B(12) COENZYME

In this study, part 1 of a series examining the electronic structure o f the coenzyme vitamin B12 and related derivatives, we present a study of the electronic structure of Co(DH)2(L)(R) (DH = dimethylglyoxime; L = NH3, py, 2-NH2py, 5,6-dimethylbenzimidazole; R = CH3, i-C3H7, 5'-d eoxyadenosyl) as model systems for the vitamin B12 coenzyme. We have o ptimized 12 DH derivatives with varying degrees of steric bulkiness an d sigma donor capability of the axial ligands. Our goal was to determi ne which factors may contribute to structural changes in the equatoria l ligand and in the Co-axial bonds and to validate the use of the mode l system in understanding the function of coenzyme B12. In the model s ystems, we were able to demonstrate the effects upon the Co-axial bond s as a result of (a) puckering of the DH ligand, (b) trans-steric inte ractions, and (c) trans-electronic influence. By constraining the DH l igand to initially remain planar and then allowing full relaxation of the equatorial ligand, we were able to determine how puckering of the equatorial ligand alone affected the Co-axial bonds. We found that upo n relaxation, the nitrogens of the glyoxime rings bend predominantly t oward the nitrogen-bound axial ligand, while the carbons of the glyoxi me rings could be displaced in either direction depending on the steri c bulkiness of the axial ligands. The Co was displaced toward the alky l group, where the amount of displacement depended upon the steric bul kiness of the axial ligands. Puckering of the equatorial ligand alone did not cause an elongation of the Co-C bond, even though there was a lengthening of the Co-N(ax) bond upon distortion of the DH ligand. To examine the trans-steric influence, we substituted the axial groups wi th bulky substituents. Bulky axial ligands induced conformation change s in the equatorial ligand with a slight elongation of the Co-alkyl bo nd. The Co-C(ax) bond length increased with an increase in the bulkine ss of R as well as L. The longest Co-C(ax) and Co-N(ax) bonds were fou nd in R = adenosyl and L = 2-NH2py. To examine the trans-electronic in fluence upon the axial bond, we varied the basicity of L and the sigma donor character of R. There was no structural evidence that the Co-al kyl bond weakened as a result of a decrease in the basicity of L. Comp arison of overlap populations in the alkyl derivatives with planar equ atorial ligands and fixed average Co-C and C-N(ax) bond lengths indica ted little variation in the electronic contributions from the alkyl gr oups, even though they possessed different sigma-donating strengths. A lthough the DH derivatives have provided useful information for modeli ng the alkyl-cobalt bond in the coenzyme, our results indicate that th ere is very little structural changes in the equatorial ligand that si gnificantly influence the alkyl-cobalt bond.