The resolution of cis-di(isothiocyanato)-bis(ethylenediamine) chromium(III) and cobalt(III) thiocyanate and the circular dichroism spectrum and absolute configuration of the (+) 436 and (−) 436 cations

Abstract

Racemic cis-[ M(en) 2(NCS) 2]NCS ( M = Cr, Co) have been resolved using potassium antimonyl-(+)-tartrate [K(+)SbOT] and ammonium α-bromo-(+)-camphorsulfonate [NH 4(+)BCS] ( M = Cr only). In each case, both diastereoisomers have been isolated. The less soluble diastereoisomers with (+)SbOT ( M = Cr, Co) are the (−) 436 forms and are isomorphous, while the less soluble (+)BCS diastereoisomer ( M = Cr) is the (+) 436 form. The separation of the optically active cations from the resolving gegen-ion has been achieved using both cation and anion ion-exchange resins. Circular dichroism (CD) spectra of the (+) 436- and (−) 436- cis-Cr(en) 2(NCS) 2 + cations show three bands under the first absorption maxima (485 nm), whereas the analogous cis-Co( AA) 2(NCS) 2 + cations ( AA = en, tmd) have only two bands. Nevertheless, the CD spectra are sufficiently similar, that the absolute configurations of Δ (or L) and Λ (or D) can be assigned to the (+) 436- and (−) 436 isomers of cis-Cr(en) 2-(NCS) 2 +, respectively, as the absolute configuration of the (+) 436- cis-Co( AA) 2(NCS) 2 + cation is known to be Δ (or L).