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⁻BAr_f⁻ (X = Cl, I; BAr_f = 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⁻BAr_f⁻ 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.