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Chiral hexa- and nona-methylene bridged bis(L-Leu-oxalamide) gelators. First oxalamide gels containing aggregates with chiral morphology

Chiral amino acid- and amino alcohol-oxalamides are well known as versatile and efficient gelators of various lipophilic and polar organic solvents and water. To further explore the capacity of the amino acid-oxalamide structural fragment as gelation generating motif the dioxalamide dimethyl ester 16Me, 19Me and dicarboxylic acid 26OH, 29OH derivatives containing flexible methylene bridges with odd (9 ; n = 7) and even (6 ; n = 4) number of methylene groups were prepared. Their self-assembly motifs and gelation properties were studied by a number of methods (FTIR, 1H-NMR, CD, TEM, DSC, XRPD, molecular modelling, MMFF94 and DFT). In contrast to the previously studied chiral bis(amino acid, or amino alcohol) oxalamide gelators where no chiral morphology was ever observed in the gels, the conformationally more flexible 16Me, 19Me, 26OH and 29OH endow gelators capable of forming diverse aggregates of achiral and chiral morphologies such as helical fibers, twisted tapes, nano-tubules, straight fibers and tapes in some cases coexisting in the same gel sample. It is shown that DSC determined gelation enthalpies could not be correlated with gelator and solvent clogP values. Spectroscopic results show that intermolecular hydrogen bonding between oxalamide units provides the major and self-assembly directing intermolecular interaction in the aggregates. Molecular modelling studies reveal that molecular flexibility of gelators due to the presence of the polymethylene bridges gives three conformations (zz, p1 and p2) close in energy which could form oxalamide hydrogen bonded layers. The aggregates of p1 and p2 conformations tend to twist due to steric repulsion between neighbouring iBu groups at chiral centres. XRPD results on 16Me and 19Me xerogels prove formation of p1 and p2 gel aggregates, respectively. The latter results explain the formation of gel aggregates with chiral morphology and also the simultaneous presence of aggregates of diverse morphology in the same gel system.

oxalamide gelators; chiral; self-assembly; gelation; chiral morphology

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