Electronic Properties of the Axial Co−C and Co−S Bonds in B12 Systems − A Density Functional Study

Abstract

The numerous accurate structural data of cobalamins now available allows us to optimize the geometry of these systems, based on a simplified model by using density functional theory (DFT) calculations. This approach, which reproduces the trend of the experimental distances derived from EXAFS and X-ray crystal structures in the corrin macrocycle, permit us to interpret the electronic properties in the NB3−Co−X axial system. In particular, the results are analyzed for cobalamins containing a sulfur ligand which exhibits a “regular” trans influence, i.e., when the Co−S bond shortens, the trans Co−NB3 bond lengthens. This feature appears in contrast with an anomalous effect (“inverse” trans influence) postulated a few years ago by analyzing the structural data of several alkylcobaloximes, LCo(DH)2R, a simple B12 model, and attributed principally to the electronic properties of the alkyl group R. The present study on the NB3−Co−S fragment does not indicate that the “inverse” trans influence is a general rule in cobalamins. The accurate crystal structures of the [(SO3)Cbl](NH4) and [(thiourea)Cbl](PF6) cobalamins based on synchrotron diffraction data at 100 K are also reported for comparison with the theoretical study. The former crystal structure, including one co-crystallized glycerol molecule, presents “for the first time” an ordered hydrogen-bonding pattern for the solvent molecules.