The robust, readily available cobalt(iii) trication [Co(NH2CHPhCHPhNH2)3]3+ is a progenitor of broadly applicable chirality and prochirality sensing agents

Abstract

When NMR spectra of chiral racemic organic molecules containing a Lewis basic functional group are recorded in the presence of air and water stable salts of the cobalt(iii) trication [Co((S,S)-NH2CHPhCHPhNH2)3]3+ (23+), separate signals are usually observed for the enantiomers (28 diverse examples, >12 functional groups). Several chiral molecules can be simultaneously analyzed, and enantiotopic groups in prochiral molecules differentiated (16 examples). Particularly effective are the mixed bis(halide)/tetraarylborate salts Λ-23+ 2X-BArf- (X = Cl, I; BArf = B(3,5-C6H3(CF3)2)4), which are applied in CD2Cl2 or CDCl3 at 1-100 mol% (avg 34 and 14 mol%). Job plots establish 1:1 binding for Λ-23+ 2Cl-BArf- and 1-phenylethyl acetate (4) or 1-phenylethanol (10), and ca. 1:2 binding with DMSO (CD2Cl2). Selected binding constants are determined, which range from 7.60-2.73 M-1 for the enantiomers of 10 to 28.1-22.6 M-1 for the enantiomers of 4. The NH moieties of the C2 faces of the trication are believed to hydrogen bond to the Lewis basic functional groups, as seen in the crystal structure of a hexakis(DMSO) solvate of Λ-23+ 3I-. These salts rank with the most broadly applicable chirality sensing agents discovered to date.