Chiral recognition of mandelic acid and macrocyclic nickel(II) complexes

The reactions of a racemic four-coordinated macrocyclic nickel(II) complex [Ni(rac-L)](ClO4)(2) with l- and d-mandelic acid anion in acetonitrile/water gave six-coordinated enantiomers of [Ni(RR-L)(S-Man)]ClO4 (1) and [Ni(SS-L)(R-Man)]ClO4 (2) (L=5,5,7,12,12,14-hexamethyl-1,4,8,11-tetraazacyclotetradecane, Man=mandelic acid), respectively. The reaction of [Ni(rac-L)](ClO4)(2) with dl-Man(-) gave a conglomerate, in which the RR and SS enantiomers preferentially coordinate to l- and d-Man(-) respectively to give a racemic mixture of 1 and 2, and the chiral recognition occurs during the reaction, in which each crystal crystallizes into enantiopure. The reactions of [Ni(rac-L)](ClO4)(2) with dl-2-phenylpropionic acid and tropic acid anion gave complexes [Ni(rac-L)(dl-PPA)]ClO4 (3) (PPA=2-phenylpropionic acid) and [Ni(rac-L)(dl-Tro)]ClO4 (4) (Tro=tropic acid), respectively. Single-crystal X-ray diffraction analyses show that the Ni(II) ions display an octahedral coordination geometry by coordination with four nitrogen atoms of macrocyclic ligand in a folded configuration, plus two oxygen atoms of carboxyl and hydroxyl for 1 and 2, or two oxygen atoms of carboxyl for 3 and 4 in cis-position. Complexes 1 and 2 belong to a pair of enantiomers, which are constructed via hydrogen bonding linking of [Ni(RR-L)(S-Man)](+) and [Ni(SS-L)(R-Man)](+) monomers to form 1D hydrogen bonded zigzag chain, respectively. The homochiral natures of 1 and 2 are confirmed by the results of circular dichroism (CD) spectra measurements.