The chemistry of vitamin B12. Part 17. The effect of steric distortion of the cobalt–carbon bond on the pK values and spectra of organocobalamins

Abstract

The spectra of the neutral and acid forms and the apparent pK values for protonation and displacement of the heterocyclic base have been determined for organocobalamins with various alkyl and cycloalkyl ligands possessing no functional group. Increasing the degree of substitution or distortion on either C_α(in the series methyl < ethyl < isopropyl and cyclo-propyl < -butyl < -pentyl ∼-hexyl) or C_β(ethyl ∼ n-propyl < isobutyl < neopentyl) leads to parallel changes in the spectra and in the pK values with no fundamental difference between primary and secondary groups (methyl cyclopropyl, ethyl cyclobutyl, isopropyl cyclo-pentyl and -hexyl) or between substitution on C_α and C_β(neopentyl cyclopentyl). It is shown that these effects must be steric, rather than electronic, in origin and are probably due to repulsion between the bonding electron pairs on C_α, on the one hand, and the lone pairs on Co and the equatorial N atoms on the other. It is suggested that the main variable, as seen by the Co ion and the rest of the complex, is the Co–C_α bond length.