Distinct m and p helical complexes of H2O and metal ions NiII, CuII, and ZnII with enantiomerically pure chiral bis(pyrrol-2-ylmethyleneamine)cyclohexane ligands:: Crystal structures and circular dichroism properties

The enantiomerically pure bis-bidentate ligands of bis(pyrrol-2-ylmethyleneamine)cyclohexane [H-2(L-R,L-S)] are easily synthesized from condensation of the pure R,R and S,S enantiomers of the 1,2-diaminecyclohexane spacer with 2 equiv of pyrrole-2-carbaldehyde. The coordination of [H-2(L-R,L-S)] with a H2O molecule and metal ions Ni-II, Cu-II, and Zn-II gives rise to distinct helical structures and crystal packing motifs: homochiral and enantiopure infinite single-helical polymeric chains of [(H-2(L-R,L-S)center dot H2O)(n)] via hydrogen bonds, mononuclear single helices of [Ni-II(L-R,L-S)] and [Cu-II(L-R,L-S)], and a double-stranded dinuclear helicate of [Zn-2(II)(L-R,L-S)(2)], respectively. The helical structures for all metal complexes in the solid state still remain in the solution. Remarkably, chiral ligands of [H-2(L-R)] and [H-2(L-S)] predetermine the chirality of the helices and helicates, i.e., P left-handedness and M right-handedness, respectively. The structural changes of these complexes induced by different coordinators are also characterized by circular dichroism (CD) and absorption spectra in both the solid state and solution. Analysis of CD spectra, with aids of absolute determination of single-crystal X-ray diffraction structures, reveals both intraligand and interligand chromophore couplings. For the potential applications of these complexes, other experiments such as magnetism, photoluminescence, and nonlinear optical properties have also been investigated.