Induced Circular Dichroism in Phosphine Gold(I) Aryl Acetylide Urea Complexes through Hydrogen-Bonded Chiral Co-Assemblies.

Phosphine gold(I) aryl acetylide complexes equipped with a central bis(urea) moiety form 1D hydrogen-bonded polymeric assemblies in solution that do not display any optical activity. Chiral co-assemblies are formed by simple addition of an enantiopure (metal-free) complementary monomer. Although exhibiting an intrinsically achiral linear geometry, the gold(I) aryl acetylide fragment is located in the chiral environment displayed by the hydrogen-bonded co-assemblies, as demonstrated by induced circular dichroism (ICD).

Orthohalogen substituents dramatically enhance hydrogen bonding of aromatic ureas in solution.

The phenylurea moiety is a ubiquitous synthon in supramolecular chemistry. Here we report that the introduction of chlorine or bromine atoms in the ortho positions to the urea unit is a simple and very efficient way to improve its intermolecular hydrogen bond (HB) donor character. This effect was demonstrated in solution both in the context of self-association of bis-ureas and hydrogen bonding of mono-ureas to strong HB acceptors.

Direct probing of the free-energy penalty for helix reversals and chiral mismatches in chiral supramolecular polymers.

The amplification of chirality, where a small imbalance in a chiral constituent is propagated into a strong optical purity, can occur in the spontaneous formation of helical 1-D stacks of molecules stabilized by hydrogen bonding, also known as supramolecular polymers. We have extended a statistical model by van Gestel et al. describing the highly nonlinear relationship between supramolecular helicity and enantiomeric excess for mixtures of enantiomers (the majority-rules effect) and quantitatively account for how this affects the thermodynamic stability of the assemblies. Our method allows for a direct comparison with experimental data, providing an unambiguous determination of the key parameters of the model (i.e., the mismatch and the helix reversal penalties). We demonstrate the successful application of this model to calorimetry data for bis-urea-based helical nanotubes, showing that reversals in the handedness of these nanotubes are not all that rare even though the helix reversal penalty is fairly large. By contrast, the mismatch penalty we obtain is small, implying that a large proportion of enantiomers are present in tube fractions not of their preferred handedness.

Hydrogen bonded supramolecular polymers in moderately polar solvents.

Hydrogen bonded assemblies are usually decomposed by polar organic solvents. However, we have succeeded in preparing a strongly associated supramolecular polymer which forms viscous solutions in competitive solvents such as tetrahydrofuran.

Formation of inclusion complexes between 1,1'-dialkyl-3,3'-(1,4-phenylene)bisimidazolium dibromide salts and cucurbit[7]uril.

N,N'-Dialkyl-(1,4-phenylene)bisimidazolium salts form inclusion complexes with cucurbit[7]uril (CB[7]) with high association constants. The stoichiometry of the complexes depends on the alkyl chains and on the relative concentration of the imidazolium salt and CB[7]. The binding interactions of CB[7] with these bisimidazolium salts were studied experimentally by (1)H NMR, high resolution mass spectroscopy, and UV spectroscopy, and theoretically by semiempirical molecular modeling. Nonlinear regression analysis was used to calculate or estimate the binding parameters for the supramolecular inclusion complexes present in solution. The detailed study of the binding association of these imidazolium salts may allow us to understand the exact role of each recognition site in these salts and to assemble higher supramolecular complexes.

Revealing molecular self-assembly and geometry of non-covalent halogen bonding by solid-state NMR spectroscopy.

We report a new spectroscopic fingerprint of intermolecular contacts in halogen bond-driven self-assembling aggregates and a precise determination of intermolecular NI distances in microcrystalline samples.