Chiral Cobalt(III) Complexes Based on (1R,2R)-Cyclohexanediamine and 4-Bromo-5-methylsalicylaldehyde: Synthesis, Modification, and Application in Catalysis and Enantioselective Sensors

Abstract

New chiral octahedral positively charged diastereomeric cobalt(III) complexes Λ(R,R)-1 and Δ(R,R)-1 with two asymmetric NNO-donor ligands (Schiff base of (1R,2R)-cyclohexanediamine and 4‑bromo-5-methylsalicylaldehyde) were synthesized. Single-crystal X-ray diffraction (XRD) analysis showed that the Δ(R,R)-1 complex is arranged in the crystal as a homochiral spiral with a “tube-in-tube” cavity, formed via hydrogen bonding between amino groups of the complex, water molecules, and the counterion. The bromo-containing complex Λ(R,R)-1 was further modified by a catalytic Suzuki–Miyaura cross-coupling reaction to give complexes Λ(R,R)-2 and Λ(R,R)-3 with introduced Ph and 4-CF₃C₆H₄ groups. The structure of complex Λ(R,R)-2 was confirmed by XRD. The catalytic activity of the synthesized complexes was studied in asymmetric phase-transfer alkylation and trimethylsilylcyanation reactions. Cobalt(III) complexes were also tested as chiral selectors for enantioselective voltammetric sensors, and the effect of complex structure on sensor characteristics toward the enantiomers of naproxen and thyroxine was evaluated.