Hydrogen bonded molecular assemblies in crystals and gels of chiral bis(amino alcohol) oxalamides

Abstract

The research of supramolecular organisation of organic molecules into nano-or micro-dimensional assemblies is of high relevance for design of new materials for versatile applications and for understanding of the organisation of matter in general. Macroscopic manifestation of self-aggregation can result in ordered gels which include entraped solvent molecules. It is recognised that gelation is based on self � assembly of gelators in solution, leading to supramolecular arrays of specific non-covalent interactions: hydrogen bonds, van der Waals interactions and metal coordination. It is a chalenge to predict gelation on the basis of the constitutional and conformational characteristics of an organic molecule and the properties of the liquid to be gelled. Recently we have shown that chiral bis(amino acid) oxalamides represent a rare group of gelators capable to gel water and various lipophilic organic solvents (Makarevic et al., Chem. Eur. J. 2001, 7, 3328), whereas our new results are related to chiral bis(leucinol)oxalamides exhibiting gelating properties. The crystal structures of both, SS- and rac-bis(leucinol)oxalamide reveal two-dimensional hydrogen bonded networks through oxalamide and alcohol functionalities. X-ray powder diffraction was used to identify an appearance of crystalline phases in gels and xerogels of enantiomeric and racemic samples. In all cases the monoclinic crystalline phase identical to the crystals of SS-enantiomer was found. The high gelating properties of racemic compound might be related by van der Waals interactions between enantiomeric fibres of oposite chirality, whereas molecular organisation in fibres is governed by hydrogen bonds.