Organocobaloximes, RCo(dmgH)₂Py, have extensively been used to mimic the vitamin B₁₂ coenzyme. The solution studies in alkylcobalamins and in organocobaloximes suggest that the Co–C bond length is responsive to both steric and electronic effects in organocobalt (III) compounds.

The spectral and structural properties of cobaloximes and NMR, in particular, have been extensively used. In majority of the complexes, the dmgH (Me) signal appears as a sharp singlet at »2.0 ppm in the ¹H NMR spectra indicating the chemical equivalence of all four methyl groups. Non-equivalence of the dmgH in 1:1 ratio, however, has been observed in a few cases, when either of the axial ligands is asymmetric.

The variable temperature NMR study in 2-substituted benzyl cobaloximes will be discussed. The cobalt bound CH₂ becomes diastereotopic and the dmgH methyl protons show non-equivalence (1:1) at sub-zero temperature. The non-equivalence is quite striking since there is no chiral group attached to the cobalt. The interaction of the axial with the equatorial ligand causes hindered rotation of the Co–C and /or C–Ph bond and leads to nonequivalence. This is partly due to the weak interactions between the benzyl group and the dioxime ring current and is supported by the crystal structures of benzyl cobaloximes.

Related References: B. D. Gupta et al

Organometallics, 26 (2007) 658; Organometallics, 25 (2006) 92; Organometallics, 24 (2005) 4454; Organometallics, 24 (2005) 1501; Organometallics, 23 (2004) 2069