Chiral resolution of ( S )- and ( R )-phenyllactic acid and a racemic macrocyclic nickel(II) complex: synthesis and crystal structures

The reactions of racemic four-coordinated macrocyclic nickel(II) complexes [Ni(rac-L)](ClO₄)₂ (L is 5,5,7,12,12,14-hexamethyl-1,4,8,11-tetraazacyclotetradecane) with (S)- and (R)-phenyllactic acid in acetonitrile/water gave the two six-coordinated enantiomers [(R,R)-5,5,7,12,12,14-hexamethyl-1,4,8,11-tetraazacyclotetradecane][(S)-phenyllactato]nickel(II) perchlorate, [Ni(C₉H₉O₃)(C₁₆H₃₆N₄)]ClO₄ or [Ni(S-Pla)(RR-L)]ClO₄ (Λ-1), and [(S,S)-5,5,7,12,12,14-hexamethyl-1,4,8,11-tetraazacyclotetradecane][(R)-phenyllactato]nickel(II) perchlorate, [Ni(R-Pla)(SS-L)]ClO₄ (Δ-1), respectively (Pla is phenyllactate). The reaction of [Ni(rac-L)](ClO₄)₂ with rac-Pla⁻ produced a conglomerate, in which the RR and SS enantiomers preferentially coordinated to S-Pla⁻ and R-Pla⁻, respectively, to give racemic Λ-1 and Δ-1. Chiral resolution during the reaction yielded enantiopure crystals. Single-crystal X-ray diffraction analyses showed that the NiII ions coordinated with the four N atoms of the macrocyclic ligand in a folded configuration and with two O atoms from the carboxylate and hydroxyl groups in complexes Λ-1 and Δ-1, to display octahedral coordination geometry. Compounds Λ-1 and Δ-1 are enantiomers that display hydrogen bonding of the [Ni(S-Pla)(RR-L)]⁺ and [Ni(R-Pla)(SS-L)]⁺ monomers to form one-dimensional zigzag chains. The homochiral natures of Λ-1 and Δ-1 were confirmed by circular dichroism spectral measurements.