Synthesis and X-ray Structures of Elusive Imine/Oxime-Type Organocobalt B₁₂ Complexes. NMR Study Suggesting Steric Strain within the Axially Ligated Benzimidazole

Abstract

Organocobalt B12 model complexes of the imine/oxime-type containing R groups with good trans influence were prepared using the isolated CoI reagent (CO)CoI((DO)(DOH)pn). Complexes were of the type [LCo((DO)(DOH)pn)R]PF6 (or ClO4) (L = H2O, py, Me3Bzm for R = CH2OCH3; L = H2O, Me3Bzm for R = CH2SCH3). The −CH2SCH3 group is a new ligand for a B12 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, Me3Bzm for X = Cl; L = py, Me3Bzm for X = N3; L = H2O, py, Me3Bzm for R = CH2Cl, CH2NO2; L = py for R = CH2CN). X-ray crystal structures were determined for [Me3BzmCo((DO)(DOH)pn)CH2OCH3]PF6 (1), [pyCo((DO)(DOH)pn)CH2OCH3]PF6 (2), and [Me3BzmCo((DO)(DOH)pn)CH2Cl]PF6 (3). Combined with previous results, the structures of the new Me3Bzm 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 [Me3BzmCo((DO)(DOH)pn)R]PF6 [CH2CF3 < CH2Cl < CH3 < CH2CH3 < CH2OCH3] was similar to that of the well-studied cobaloximes LCo(DH)2R (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, 13C NMR chemical shifts for [Me3BzmCo((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 [Me3BzmCo((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 [Me3BzmCo((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 Me3Bzm 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 Me3Bzm ligand differs by 60−90° from the analogous plane in the imine/oxime model system.

Synthesis and Characterization of a Series of New Asymmetric Salphen Mono‐ and Binuclear Metal Complexes

“…The data of the 1 H NMR in DMSO-d 6 are given in experimental section. The 1 H NMR spectrum of Zn 2 (L 1 )(OAc) 2 shows characteristic peak of acetate (OAc) as singlet at about δ 1.777 and the result, 6H, of integration of peak area and chemical shifts is consistent with the data of ESI-MS (701.9). This indicates that the structure of the compound is the same as we presumed and is also in line with the literature.…”

Physicochemical studies and biological activity of mixed ligand complexes involving bivalent transition metals with a novel Schiff base and glycine as a representative amino acid

“…The synthesis and reactivity of cobalt complexes of Schiff base ligands have always attracted the attention of inorganic chemists [6]. For instance, the cobalt complexes of tetradentate Schiff base ligands have been extensively used to mimic cobalamin (B12) coenzymes [7][8][9][10], dioxygen carriers and oxygen activators [11,12]. The preparation of model complexes having similar spectroscopic features is perhaps the most important step to understand the structure and behavior of these biological systems.…”

Synthesis, characterization, solvatochromic behavior and crystal structures of complexes [CoIII(salophen)(thioacetamide)2]ClO4 and [CoIII(salophen)(thiobenzamide)2]ClO4

“…Quadridentate Schiff base cobalt complexes have been widely studied, particularly as homogeneous catalysts, oxygen carriers, and models of compounds with biological function [1][2][3][4][5][6][7][8][9][10][11][12]. It is known that variation of the size and nature of the ligand can be responsible for changes in the coordination geometry around central metal ions.…”