Self-assembly of Ni(ii) with a chiral ligand pair vs. mixture of the chiral ligand pair: structural features and recognition ability of Ni2L4 cages

Abstract

The self-assembly of NiCl₂ with a chiral bidentate ligand pair, (1R,2S)-(+)- and (1S,2R)-(−)-1-(nicotinamido)-2,3-dihydro-1H-inden-2-yl nicotinate (r,s-L and s,r-L) in a mixture of ethanol and dioxane, gives rise to stable crystals consisting of [2Cl@Ni₂Cl₂(s,r-L)₄(H₂O)₂]·4C₄H₈O₂·EtOH and [2Cl@Ni₂Cl₂(r,s-L)₄(H₂O)₂]·4C₄H₈O₂·EtOH chiral cages, respectively, with two encapsulated chloride anions in the cavities. The most interesting feature is that the self-assembly of NiCl₂ with the mixture of r,s-L and s,r-L (1 : 1–1 : 4) produces crystals of thermodynamically stable achiral cages, [2Cl·2H₂O@Ni₂Cl₂(s,r-L)₂(r,s-L)₂(H₂O)₂]·7C₄H₈O₂, in the molar ratio range. Furthermore, the [2Cl@Ni₂Cl₂(s,r-L)₄(H₂O)₂]·4C₄H₈O₂·EtOH and [2Cl@Ni₂Cl₂(r,s-L)₄(H₂O)₂]·4C₄H₈O₂·EtOH chiral crystals can recognize the pairs of L-,D-tryptophan and L-,D-cysteine via cyclic voltammetry (CV) signals, in contrast to the [2Cl·2H₂O@Ni₂Cl₂(s,r-L)₂(r,s-L)₂(H₂O)₂]·7C₄H₈O₂ achiral crystal.