SYNTHESIS AND X-RAY STRUCTURES OF ELUSIVE IMINE/OXIME-TYPE ORGANOCOBALT B-12 COMPLEXES - NMR STUDY SUGGESTING STERIC STRAIN WITHIN THE AXIALLY LIGATED BENZIMIDAZOLE

摘要

Organocobalt B-12 model complexes of the imine/oxime-type containing R groups with good trans influence were prepared using the isolated Co-I reagent (CO)Co-I((DO)(DOH)pn). Complexes were of the type [LCo((DO)(DOH)pn)R]PF6 (or ClO4) (L = H2O, py, Me(3)Bzm for R = CH2OCH3; L = H2O, Me(3)Bzm for R = CH2SCH3). The -CH2SCH3 group is a new ligand for a B-12 model system. Different synthetic routes were used to prepare other new complexes of the type [LCo((DO)(DOH)pn)R(or X)]PF6 (or ClO4) (L = H2O, N-MeImd, Me(3)Bzm for X = Cl; L = py, Me(3)Bzm for X = N-3; L = H2O, py, Me(3)Bzm for R = CH2Cl, CH2NO2; L = py for R = CH2CN). X-ray crystal structures were determined for [Me(3)BzmCo((DO)(DOH)pn)CH2OCH3]PF6 (1), [pyCo((DO)(DOH)pn)CH2OCH3]PF6 (2), and [Me(3)BzmCo((DO)(DOH)pn)CH2Cl]PF6 (3). Combined with previous results, the structures of the new Me(3)Bzm complexes create a series of five imine/oxime structures. The trend of axial Co-N bond lengthening with increasing electron-donating ability of the R group found for [Me(3)BzmCo((DO)(DOH)pn)R]PF6 [CH2CF3 < CH2Cl < CH3 < CH2CH3 < CH2OCH3] was similar to that of the well-studied cobaloximes LCo(DH)(2)R (DH = monoanion of dimethylglyoxime). The axial Co-N and Co-C bonds of the imine/oxime-type complexes were slightly longer compared to the respective bonds in the corresponding cobaloximes. Such a slight dependence of Co-C bond length on the nature of the equatorial ligand has not been established previously. For some carbons, C-13 NMR chemical shifts for [Me(3)BzmCo((DO)(DOH)pn)R(or X)]ClO4 in CDCl3 correlated linearly with the electronic parameter (EP), a spectroscopic measure of the electronic trans influence. The shifts for [Me(3)BzmCo((DO)(DOH)pn)CH2SCH3]ClO4 allowed us to calculate the EP value for the -CH2SCH3 group. The value indicates a trans influence for -CH2SCH3 similar to that of -CH3. Shifts for some types of carbons in [Me(3)BzmCo((DO)(DOH)pn)R(or X)]ClO4 did not correlate well. This lack of correlation can be attributed, in part, to steric strain in the Me(3)Bzm induced by non-bonded repulsions of this axial ligand with the equatorial ligand. The influence of such steric strain is not so evident in the shift trends of the corresponding carbons in cobaloxime analogs. This result suggests less ligand repulsion in the cobaloximes since the orientation of the plane of the Me(3)Bzm ligand differs by 60-90 degrees from the analogous plane in the imine/oxime model system. [References: 48]