Absolute Asymmetric Synthesis and Resolution Mechanism of Chiral cis-bromoamminebis(ethylenediamine)cobalt(III) Bromide

The absolute asymmetric synthesis of the title complexes Λ-(+)D-cis-[CoBr(NH3)(en)2]Br2(1) and Δ-(-)D-cis-[CoBr(NH3)(en)2]Br2(2) (en=1,2-ethylenediamine) as well as the improved optical resolution of cis-[CoBr(NH3)(en)2]Br2·2H2O(3) are described in detail. The products were characterized by means of elemental analysis, TG-DTA, optical rotation, UV-Vis, and CD spectra. The distribution of enantiomeric excess (ee) of cis-[CoBr(NH3)(en)2]Br2 in the absolute asymmetric synthesis was obtained by the measurements of their CD spectra. It was found that the complete resolution of rac-cis-[CoBr(NH3)(en)2]Br2·2H2O was achieved by the use of NH4(d-BCS)(ammonium d-α-bromocamphor-π-sulfonate) or NH4(l-BCS) while NH4(dl-BCS) could only be resolved partially by chiral cis-[CoBr(NH3)(en)2]Br2 under the same conditions. It is deduced that there are notable differences in the chiral discrimination of the reciprocal resolution between cis-[CoBr(NH3)(en)2]Br2 and NH4BCS. In addition, a new “catalysis-crystal induction” mechanism in the preparation of the chiral Co(III) complex is put forward.