Chirality and absolute helicity in a pair of enantiomeric amino acid derivatives and their complexes: structures, chiroptical, and photoluminescent properties

Abstract

A pair of enantiomeric amino acid derivatives [L-H₂cala (1L) and D-H₂cala (1D), H₂cala = N-(4-carboxylbenzyl)-alanine] were synthesized by a one step substitution reaction of enantiopure L-(or D-)alanine with 4-(bromomethyl)benzoic acid. Self-assembly of 1L and 1D with divalent Cu²⁺ and Cd²⁺ ions in water produced two pairs of enantiomers formulated as [Cu(L-cala)(H₂O)]_n·n(H₂O) (2L), [Cu(D-cala)(H₂O)]_n·n(H₂O) (2D), [Cd(L-Hcala)₂(H₂O)₂]_n·n(H₂O) (3L) and [Cd(D-Hcala)₂(H₂O)₂]_n·n(H₂O) (3D). X-ray diffraction analyses reveal that 1L and 1D are hydrogen-bonded two-dimensional (2D) supramolecules containing one-dimensional (1D) single-stranded supramolecular helices parallel to the crystallographic b-axis. 2L and 2D are 1D single-stranded coordination helices arranged along the direction of c-axis. They are further extended into three-dimensional (3D) supramolecular frameworks by inter-helical hydrogen bonds. 3L and 3D are 1D single-stranded coordination helices parallel to a-axis. Then three neighboring single-stranded coordination helices are further tied up by hydrogen bonds, forming 1D triple-stranded supramolecular helices extending along the same direction. Interestingly, 3L and 3D also show supramolecular helicities on the directions of b- and c-axes. The chiralities and absolute helicities of these complexes are controlled by the chiralities of ligands. In addition, the solid-state circular dichroism (CD) and photoluminescent properties of them were investigated.