Supramolecular Aggregates of Defined Stereochemical Scaffolds: Aggregation/Deaggregation in Schiff-Base Zinc(II) Complexes Derived from Enantiopure trans 1,2-Diaminocyclohexane

This contribution explores, through detailed 1H NMR, DOSY NMR, optical absorption, and circular dichroism spectroscopic studies, the aggregation properties in solution of noncoordinating solvents of some new ZnII Schiff-base complexes,(R)-1, (S)-1, and (R)-2, derived from the chiral trans-1,2-diaminocyclohexane. It is found that chloroform solutions of 1 arecharacterized by the presence of three species, the predominance of which consists of large oligomeric aggregates. For chloroformsolutions of 1, upon heating or standing, all species are irreversibly converted into a dimer, 1C, which is very stable and hardly disaggregable. Analysis of 1H NMR, UV/vis, and CD spectroscopic data and chemical evidence allow proposing a double helicateZn2L2 structure with a tetrahedral coordination around the ZnII ions for 1C, as a consequence of the defined stereochemistry ofthe trans-1,2-diaminocyclohexane chelate bridge. This represents a different, uncommon aggregation mode in ZnII complexes of tetradentate Schiff-bases.