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)-phenyllactatonickel(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)-phenyllactatonickel(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.