(Self-)assembled news: recent highlights from the supramolecular chemistry literature (Quarter 4, 2023)

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• Highlight
• news
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• supramolecular chemistry

Ion pairing: a pervasive problem, even in water

It has long been known that even ‘non-coordinating’ cations and anions can form ion pairs in non-polar organic solvents and that this can complicate the analysis of association constants [1,2] (e.g. tetrabutylammonium chloride has a self-association constant of 63,000 M⁻¹ in dichloromethane) [3]. Generally, these effects become less pronounced in more polar solvents and tend to be ignored in water. However, in a detailed and thought-provoking study Pérez-Lorenzo, García-Río and colleagues recently demonstrated that ion pairing can be extremely important, even in water [4].

They studied the bromide salt of a highly cationic water-soluble pillar[5]arene macrocycle functionalised with 10 tetramethylammonium groups and used an ion selective electrode and fluorescent probe to study the concentration of free bromide anions. Their study showed that at millimolar concentrations, the dominant form of their macrocycle was associated with five bromide anions and only at concentrations of ~10⁻⁷ M⁻¹ did the ‘free’ 10⁺ form of the receptor dominate. They were also able to demonstrate that this significantly affected the measured association constants for binding of the toluenesulfonate anion to the host.

Discriminating similar nucleotides with a supramolecular sensor array

Nucleotides are a key biological building block and can act as biomarkers for disease. Discriminating between these under biologically relevant conditions is extremely difficult, as the nucleotides have very similar chemical structures and can be present as mono-, di- or triphosphates. Guo and co-workers have previously demonstrated that a sensor array comprised of calixarene macrocycles and fluorescent dyes can discriminate complex analytes using an indicator displacement assay [5]. Now, Hu, Xie, Guo and co-workers have reported a sensor library that can identify nucleotides [6].

Their library consists of four calix[n]arene macrocycles, all of which are functionalised with alkylammonium or guanidinium groups, and two fluorescent dyes that bind to these macrocycles. Binding of a nucleotide results in the dye being displaced, triggering a fluorescence response. By studying the response of the library of eight host pairs (four macrocycles each with two dyes) and using linear discriminant analysis, it was possible to achieve complete accuracy in identifying the nucleotides individually. High levels of accuracy were also achieved when measuring the concentration of one nucleotide in the presence of a relevant competing guest.

Hydrogen bonding halides separate noble gases

Porous solids assembled by hydrogen bonds were first reported in the 1990s [7,8] and have been the subject of intense research activity in the last decade or so. Two approaches are commonly used, either using one building block that recognises itself through hydrogen bonding [7] or using two charged components that form a porous salt [8]. Typically, charged groups with relatively well-defined hydrogen bonding geometries are used to allow for a degree of predictability in framework assembly (e.g. guanidinium/sulfonate or amidinium/carboxylate pairs). However, Wang, Ye and co-workers recently demonstrated that this is not a requirement, and even halide anions can be used to form porous materials [9].

They used the tetra-amine building block shown in Scheme 1 and crystallised it in the presence of small amounts of conc. HCl_(aq), HBr_(aq), HI_(aq), or mixtures thereof, to give five crystalline frameworks assembled through N–H∙∙∙X^– hydrogen bonds between the 4+ tetra-ammonium form of the organic compound and halide anions. The frameworks contain small channels, and in the case of the Cl^– and Br^– systems, demonstrated uptake of gases including CO₂, Xe and Kr (surface areas: 220–270 m² g⁻¹). Selective binding of Xe over Kr was observed and the framework used to separate mixtures of these gases. Computational studies suggested that this selectivity arose from favourable Xe∙∙∙halide interactions.

Scheme 1. Synthesis of one of the frameworks reported by Wang, Ye and co-workers, and its X-ray crystal structure.

In brief

Porous organometallic frameworks

Zhu and co-workers have reported metal–organic frameworks (MOFs) assembled through organometallic M–C bonds between copper(I) and deprotonated alkynes [10]. While these kinds of bonds are less labile than the M–O and M–N coordination bonds typically used in MOF synthesis, a moderately crystalline material could be obtained with only gentle heating. The framework has a surface area of approximately 640 m² g⁻¹ and takes up a range of gas molecules. Remarkably, despite being constructed from Cu–alkyne bonds, it can even reversibly take up water vapour.

Dozens of DOSYs

A recent paper by the Beuerle group conducted detailed DOSY NMR studies on a large group of organic cage molecules [11]. DOSY NMR is often a crucial technique for determining the structure of supramolecular cages, and this work provides a detailed guide to best practice. The researchers studied 15 of their own boronate-based cages as well as 15 literature cages and studied various effects including solvation and guest binding. Notably, some boronate cages appeared significantly larger in THF than in chlorinated solvents (15–25%), which the authors suggest may be due to solvent coordinating to the boron groups in the cage.

Extremely low symmetry cages

One of the criticisms of supramolecular self-assembly is that it inherently leads to high symmetry products. Over the past few years, there has been a concerted effort to produce low symmetry metal–organic cages using various tactics including sterics, geometric complementarity, and interactions between the ligands. Notably, earlier this year, the Clever group reported a Pd₂L₄ cage where each ligand was different [12]. Very recently, Preston and Evans reported the quantitative self-assembly of an unusual molecule consisting of a Pd₂L₄ cage with additional ‘ancillary’ metal–ligand tethers [13]. Within the Pd₂L₄ core of the molecule, each of the four ligands is not only different but also asymmetric, meaning that there are eight different donor environments around the two central Pd(II) ions.

Soapy bundles

The Guichard group has previously reported oligourea foldamers that assemble into hexameric bundles in water [14]. Now, work led by Collie and Guichard demonstrates that these bundles associate with a range of chemically diverse surfactants, including neutral, cationic and zwitterionic detergents [15]. Several X-ray crystal structure were obtained, which reveal that the surfactants associate with the outside of the bundles. These crystal structures, as well as solution circular dichroism experiments, suggest that the primary driving force for surfactant binding is hydrophobic effects.

Disclosure statement

No potential conflict of interest was reported by the author(s).