Similarities and Differences between Cobalamins and Cobaloximes. Accurate Structural Determination of Methylcobalamin and of LiCl- and KCl-Containing Cyanocobalamins by Synchrotron Radiation

The accurate crystal structure determinations of MeCbl (1), CNCbl·2LiCl (2), and CNCbl·KCl (3), based on
synchrotron diffraction data collected at 100 K and using high-quality single crystals, are reported. Refinements
gave R1 indices of 0.0834 (1), 0.0434 (2), and 0.0773 (3). The influence of the water of crystallization and ion
content on the crystal packing of these and other cobalamins (XCbl) is discussed, and a relationship between the
crystal packing and the corrin side chain conformations is presented. An analysis of the bond lengths within the
corrin moiety, based on 13 accurate structures with several X groups, shows that the trend of the C−C and C−N
distances can be interpreted in terms of electronic and steric factors. The variation in structural, NMR and IR
spectroscopic, and electrochemical properties are compared with those of cobaloximes, the B12 model, when X
is varied. This comparison indicates that the π-back-donation from metal to the CN axial ligand and the transmission
of the trans influence of the X ligand are more effective in cobalamins than in cobaloximes. These findings are
consistent with a significantly greater availability of electron charge on Co in cobalamins, and, hence, a
semiquantitative evaluation of the electronic difference between the cobalt centers in the two systems is allowed.