Helicity control of a polyaromatic coordination capsule through stereoselective CH–π interactions

Abstract

Although square-planar ML₄ units are essential building blocks for coordination cages and capsules, the non-covalent control of the chirality and helicity of the resultant nanostructures is quite difficult. Here we report the helicity control of an M₂L₄ polyaromatic capsule, formed from metal ions with square-planar coordination geometry and bent bispyridine ligands, through stereoselective CH–π interactions with monosaccharide derivatives. Thanks to host–guest CH–π multi-interactions, one molecule of various permethylated monosaccharides is quantitatively bound by the capsule in water (K_a up to >10⁸ M⁻¹). In the polyaromatic cavity, among them, the selective binding of a β-glucose derivative (>80 : 20 ratio) is demonstrated from a mixture of the α/β-glucoses, through the equatorial-selective recognition of the anomeric (C1) group. A similar stereoselective binding is accomplished from an α/β-galactose mixture. Interestingly, single equatorial/axial configurations on the bound monosaccharides can regulate the helical conformation of the capsule in water, confirmed by CD, NMR, and theoretical analyses. An intense capsule-based Cotton effect is exclusively observed upon encapsulation of the permethylated α-glucose (>20-fold enhancement as compared to the β-glucose derivative), via the induction of a single-handed host helicity to a large extent. Inverse capsule helicity is induced by the binding of a β-galactose derivative under the same conditions.