Rationalizing the Extent of the "Sergeants-and-Soldiers" Effect in Supramolecular Helical Catalysts: Effect of Copper Coordination.

Aggregation of supramolecular helices, for example through interdigitation of their alkyl side chains or through more directional supramolecular interactions, leads to hierarchical architectures with original structural and chiroptical properties. However, when a chiral monomer (the "sergeant") is introduced as a minor component in these assemblies composed of a majority of achiral monomers (the "soldiers"), it is not clear how the aggregation changes the ability of the sergeant to induce a preferential helicity to the polymer main chain (the so-called "sergeants-and-soldiers" effect). This study reports a detailed investigation of the influence of [Cu(OAc)2 ⋅H2 O] coordination on the structure and chiroptical properties of helical hydrogen-bonded co-assemblies composed of a catalytically-active benzene-1,3,5-tricarboxamide (BTA) monomer, acting as the "soldier", and an enantiopure BTA monomer derived from cyclohexylalanine, playing the role of the "sergeant". The copper actually significantly influences the extent of the "sergeants-and-soldiers" effect since it acts as a crosslink that induces some chiral defects in the supramolecular helices. These crosslinks appear to be preserved during the catalytic hydrosilylation of 4-nitroacetophenone. The aggregation of helices through the formation of copper crosslinks is reversible since homochiral single helices are exclusively formed in the case of sergeant-rich assemblies. The fact that both main chain and side chain aggregation affects the chiroptical properties of supramolecular helices must be considered in the design of elaborated chiral materials.

N-Substituted Benzene-1-Urea-3,5-Biscarboxamide (BUBA): Easily Accessible C2 -Symmetric Monomers for the Construction of Reversible and Chirally Amplified Helical Assemblies.

Non-C3 -symmetric supramolecular helices are gaining interest for the design of hierarchical assemblies, for the compartmentalisation or the self-assembly of polymer chains and for application in asymmetric catalysis. Herein, N-substituted benzene-1-urea-3,5-biscarboxamide (BUBA) monomers, which consist of one urea and two carbon-connected amide functions linked to an aromatic ring, are introduced as an easily accessible class of C2 -symmetric supramolecular synthons. In apolar solvents, BUBA monomers assemble into long helical assemblies by means of hydrogen-bonding and aromatic interactions, as assessed by several analytical techniques. To probe the influence of the urea function, BUBA and related benzene-1,3,5-tricarboxamide (BTA) helical polymers have been compared, in terms of their thermodynamics of formation, stability, reversibility and chiral amplification properties. Similar to BTA, BUBA monomers form long helices reversibly through a highly cooperative mechanism and the helicity of their assemblies is governed by chiral amplification effects. However, precise quantification of their properties reveals that BUBA monomers assemble in a more cooperative manner. Also, chiral amplification operates to a higher extent in BUBA helices, as probed by both sergeants-and-soldiers and majority-rules experiments. Compatibility between urea and amide functions also allows the formation of co-assemblies that incorporate both BUBA and BTA monomers. Importantly, a small amount of chiral BUBA monomers in these co-assemblies is sufficient to obtain single-handed helices; thus paving the way towards the development of functional supramolecular helices.

Tuning the nature and stability of self-assemblies formed by ester benzene 1,3,5-tricarboxamides: the crucial role played by the substituents.

As the benzene 1,3,5-tricarboxamide (BTA) moiety is commonly used as the central assembling unit for the construction of functionalized supramolecular architectures, strategies to tailor the nature and stability of BTA assemblies are needed. The assembly properties of a library of structurally simple BTAs derived from amino dodecyl esters (ester BTAs, 13 members) have been studied, either in the bulk or in cyclohexane solutions, by means of a series of analytical methods (NMR, DSC, POM, FT-IR, UV-Vis, CD, ITC, high-sensitivity DSC, SANS). Two types of hydrogen-bonded species have been identified and characterized: the expected amide-bonded helical rods (or stacks) that are structurally similar to those formed by BTAs with simple alkyl side chains (alkyl BTAs), and ester-bonded dimers in which the BTAs are connected by means of hydrogen bonds linking the amide N-H and the ester C[double bond, length as m-dash]O. MM/MD calculations coupled with simulations of CD spectra allow for the precise determination of the molecular arrangement and of the hydrogen bond pattern of these dimers. Our study points out the crucial influence of the substituent attached on the amino-ester α-carbon on the relative stability of the rod-like versus dimeric assemblies. By varying this substituent, one can precisely tune the nature of the dominant hydrogen-bonded species (stacks or dimers) in the neat compounds and in cyclohexane over a wide range of temperatures and concentrations. In the neat BTAs, stacks are stable up to 213 °C and dimers above 180 °C whilst in cyclohexane stacks form at c* > 3 × 10-5 M at 20 °C and dimers are stable up to 80 °C at 7 × 10-6 M. Ester BTAs that assemble into stacks form a liquid-crystalline phase and yield gels or viscous solutions in cyclohexane, demonstrating the importance of controlling the structure of these assemblies. Our systematic study of these structurally similar ester BTAs also allows for a better understanding of how a single atom or moiety can impact the nature and stability of BTA aggregates, which is of importance for the future development of functionalized BTA supramolecular polymers.