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(NH 3 )(en) 2 ]Br 2 ( 1 ) and Δ-(-) D - cis -[CoBr(NH 3 )(en) 2 ]Br 2 ( 2 ) (en=1,2-ethylenediamine) as well as the improved optical resolution of cis -[CoBr(NH 3 )(en) 2 ]Br 2 ·2H 2 O( 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(NH 3 )(en) 2 ]Br 2 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(NH 3 )(en) 2 ]Br 2 ·2H 2 O was achieved by the use of NH 4 ( d -BCS)(ammonium d -α-bromocamphor-π-sulfonate) or NH 4 ( l -BCS) while NH 4 ( dl -BCS) could only be resolved partially by chiral cis -[CoBr(NH 3 )(en) 2 ]Br 2 under the same conditions. It is deduced that there are notable differences in the chiral discrimination of the reciprocal resolution between cis -[CoBr(NH 3 )(en) 2 ]Br 2 and NH 4 BCS. In addition, a new “catalysis-crystal induction” mechanism in the preparation of the chiral Co(III) complex is put forward.