Synthesis and Characterization of trans-[Co((BA)2en)(amine)(CN)] and trans-K[Co((BA)2en)(CN)2] Complexes:Indication of Cobalt-cyanid π -Back Bonding Interaction

Abstract

The structure of vitamin B12 (cyano cobalamine) was determined in the laboratory of Hodgkin and co-workers in the mid-1960s. In the equatorial plane the central six coordinate cobalt (III) ion is surrounded by four pyrole nitrogen atoms of a corrin ligand. While one of the axial sites is occupied by a 5,6-dimethyl benzimidazol-α-Dribofuranose-3-phosphate, the second axial ligand position is occupied by a cyanide group. The vitamin can be converted in vivo to active alkylcobalamin coenzymes. These alkylcobalamin are know naturally occurring compounds with a biologically active carbon-metal bond. The macrocyclic tetradentate ligands with four nitrogen donor or Schiff base ligands (N2O2) which coordinated to four equatorial position octahedral structure of cobalt (III) center are considered as models for vitamin B12. 6-10 The effect of axial ligation (amine) on properties of cyano group with an organic monoanion ligand has played an important role in our knowledge of the properties of vitamin B12 and of model compounds. The coordination of the amine ligand in trans position can lead to cyano activation barrier of dissociation. A variety of physical methods such as infrared and NMR have been used to study of π-back bonding from the cobalt (III) metal center into cyano ligand. Infrared and NMR spectroscopy allows one to investigation of the effect amines on the cyano ligand π-back bonding in complexes. These results were indicative of the existence of dπ-pπ back bonding in cyano cobalt species (Scheme 1). The effect of trans axial ligands on the cyanide chemical shifts and stretching properties can influence the relative stabilization of I and II species by the axial ligand. In this paper, we report result of H NMR and IR study of a series of trans-(Schiff base)CN(amine)cobalt(III) complexes with different axial ligands (Scheme 2), which permits a more detailed analysis of these properties.