Chiral resolution of l- and d-alanine and a racemic macrocyclic nickel(II) complex: synthesis and crystal structures

The reactions of a racemic four-coordinate Ni(II) complex Ni(rac-L)](ClO₄)₂ with l- and d-alanine in acetonitrile/water gave two six-coordinate enantiomers formulated as Ni(RR-L)(l-Ala)](ClO₄)·2CH₃CN (1) and Ni(SS-L)(d-Ala)](ClO4) (2) (L = 5,5,7,12,12,14-hexamethyl-1,4,8,11-tetraazacyclo-tetradecane, Ala^? = alanine anion), respectively. Evaporation from the remaining solutions gave two four-coordinate enantiomers characterized as Ni(SS-L)](ClO₄)₂ (S-3) and Ni(RR-L)](ClO₄)₂ (R-3), respectively. Single-crystal X-ray diffraction analyses of complexes 1 and 2 revealed that the Ni(II) atom has a distorted octahedral coordination geometry, being coordinated by four nitrogen atoms of L in a folded configuration, plus one carboxylate oxygen atom and one nitrogen atom of l- or d-Ala^? in mutually cis-positions. Complexes 1 and 2 are supramolecular stereoisomers, constructed via hydrogen bonding between Ni(RR-L)(l-Ala)]⁺ or Ni(SS-L)(d-Ala)]⁺ monomers to form 1D hydrogen-bonded zigzag chains. The homochiral natures of complexes 1 and 2 have been confirmed by CD spectroscopy.