DOI: 10.1002/chem.201300642 Chiral Hexa- and Nonamethylene-Bridged Bis(l-Leu-oxalamide) Gelators: The First Oxalamide Gels Containing Aggregates with a Chiral Morphology Natas ˇa S ˇ ijakovic ´ Vujic ˇic ´, [a] Zoran Glasovac, [a] Niek Zweep, [b] Jan H. van Esch, [b, c] Marijana Vinkovic ´, [a] Jasminka Popovic ´, [a] and Mladen Z ˇ inic ´* [a] Introduction Through intensive research during the last decade, more than 1000 structurally different low-molecular-weight organ- ic molecules were shown to exhibit gelling ability toward various organic solvents and water. [1] Usually, such gels con- sists of a large amount of solvent and a very small amount of gelator. The solvent is entrapped within the 3D network of entangled nanosize fibrous gelator aggregates, which are formed by the unidirectional self-assembly of gelator mole- cules. Due to the weak noncovalent interactions that stabi- lize aggregates, most of the gels exhibit thermoreversible gel-to-sol transitions. In addition to heat, other internal or external stimuli such as pH, light, ultrasound, or a specific solute could be used to control gel-to-sol transitions. These possibilities for control make gels highly interesting as dy- namic nanostructured materials, which may be useful for many advanced applications. [2] Among the distinct structural families of organic molecules that exhibit gelation (such as amide, [3] urea, [4] cholesterol, [5] carbohydrate, [6] aromatic, [7] and organometallic [8] derivatives; chiral bis(aminoacid) or bis(aminoalcohol)), oxalamides [9] (Figure 1) showed versatile Abstract: Chiral amino acid- and amino alcohol-oxalamides are well- known as versatile and efficient gela- tors of various lipophilic and polar or- ganic solvents and water. To further ex- plore the capacity of the amino acid/ oxalamide structural fragment as a ge- lation-generating motif, the dioxala- mide dimethyl esters 1 6 Me and 1 9 Me, and dicarboxylic acid 2 6 OH/2 9 OH de- rivatives containing flexible methylene bridges with odd (9 ; n = 7) and even (6 ; n = 4) numbers of methylene groups were prepared. Their self-assembly motifs and gelation properties were studied by using a number of methods (FTIR, 1 H NMR spectroscopy, CD, TEM, DSC, XRPD, molecular model- ing, MMFF94, and DFT). In contrast to the previously studied chiral bis(a- mino acid or amino alcohol) oxalamide gelators, in which no chiral morphology was ever observed in the gels, the con- formationally more flexible 1 6 Me, 1 9 Me, 2 6 OH, and 2 9 OH provide gela- tors that are capable of forming diverse aggregates of achiral and chiral mor- phologies, such as helical fibers, twisted tapes, nanotubules, straight fibers, and tapes, in some cases coexisting in the same gel sample. It is shown that the differential scanning calorimetry (DSC)-determined gelation enthalpies could not be correlated with gelator and solvent clogP values. Spectroscopic results show that intermolecular hydro- gen-bonding between the oxalamide units provides the major and self-as- sembly directing intermolecular inter- action in the aggregates. Molecular modeling studies reveal that molecular flexibility of gelators due to the pres- ence of the polymethylene bridges gives three conformations (zz, p1, and p2) close in energy, which could form oxalamide hydrogen-bonded layers. The aggregates of the p1 and p2 con- formations tend to twist due to steric repulsion between neighboring iBu groups at chiral centers. The X-ray powder diffraction (XRPD) results of 1 6 Me and 1 9 Me xerogels prove the for- mation of p1 and p2 gel aggregates, re- spectively. The latter results explain the formation of gel aggregates with chiral morphology and also the simul- taneous presence of aggregates of di- verse morphology in the same gel system. Keywords: amino acids · chirality · gels · oxalamide gelators · self-as- sembly [a] Dr. N. S ˇ . Vujic ˇic ´, Dr. Z. Glasovac, Dr. M. Vinkovic ´, Dr. J. Popovic ´, Prof. M. Z ˇ inic ´ Department of Organic Chemistry and Biochemistry NMR Center and Division of Materials Physics Rudjer Bos ˇkovic ´ Institute, Bijenic ˇka 54 10002 Zagreb (Croatia) Fax: (+ 38) 51-46-80-195 E-mail: zinic@irb.hr [b] Dr. N. Zweep, Prof. J. H. vanEsch Laboratory of Organic Chemistry, Stratingh Institute University of Groningen Nijenborgh 4, Groningen (The Netherlands) [c] Prof. J. H. van Esch Present address: Department of Chemical Engineering Delft University of Technology Julianalaan 136, 2628 BL Delft (The Netherlands) Supporting information for this article (including equipment, proce- dures, and experimental conditions; temperature-dependent FTIR spectra of toluene gels, DSC spectra of cooling cycles of 2 6 OH, 1 6 Me, 2 9 OH, and 1 9 Me, NOESY spectra of 1 6 Me, 1 9 Me, 2 9 OH, and 2 6 OH gel samples, XRPD data, NMR spectra of 1a, 1b, 1 6 Me, 2 6 OH, 1c, and 2c) is available on the WWW under http://dx.doi.org/10.1002/ chem.201300642. Chem. Eur. J. 2013, 00,0–0 # 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim These are not the final page numbers! ÞÞ &1& FULL PAPER