The supramolecular chemistry of anions

This themed collection of articles brings together aspects of modern supramolecular chemistry related to anionic species. This field has grown from the pioneering work of Shriver and Biallas¹ and Simmons and Park² in the late 1960s on Lewis acid-based anion hosts and ammonium-based cage compounds, respectively, through the 1970s and early 1980s with work from Graf and Lehn on cryptand-based hosts for anions³ and Schmidtchen on quaternary ammonium based anion receptors⁴, through to the 1990s with work on neutral hydrogen bond donor hosts.^5,6 More recent developments include receptors that employ CH hydrogen bonding interactions to bind anions⁷ and hydrophobic hosts that bind large, charge diffuse anions selectively in water.⁸ There are now a plethora of different receptors employing a range of non-covalent interactions to snare their anionic guests. These compounds have been applied in many applications including sensing, extraction, transport, assembly and catalysis.⁹

This collection of articles covers the development of new receptors and the applications of anionophores in anion sensing and transport. Interestingly, the themes of hydrophobicity and affinity for phosphates, particularly in relation to anion transport, recur in papers throughout this collection.

Bowman-James and co-workers (DOI: 10.1039/D1OB01605A) have studied a series of monotopic and ditopic 2,6-pyridine dicarboxamides and 2,3,5,6-pyrazine tetracarboxamides with a range of hydrophilic to hydrophobic components. Additional OH⋯A⁻ hydrogen bonding interactions may in these cases enhance the affinity of the hydrophilic members of this series. Coote, White and co-workers (DOI: 10.1039/D1OB00282A) have also explored OH⋯A⁻ hydrogen bonding, in this case charge assisted, in a series of receptors based on pyridinium and quinolinium scaffolds. These simple compounds containing one or two hydrogen bond donors can form surprisingly stable complexes with chloride in acetonitrile. Pfeffer and co-workers have studied the binding affinity of 4-amido- and 4-amino-1,8-naphthalimides functionalised with a pendant (thio)urea, observing that the enhanced acidity of the amide versus amino NH⋯A⁻ hydrogen bond is outweighed by the diminished conformational flexibility (DOI: 10.1039/D1OB01664D).

A different approach to the construction of anion receptors has been taken by Mezei and co-workers (DOI: 10.1039/D1OB01318A), who have employed rigidified nanojars to produce receptors that are selective for carbonate and sulfate. Maeda and co-workers (DOI: 10.1039/D1OB01094H) have continued their work on assemblies. Dipyrrolyldiketone skeletons have been modified with carboxylate groups to form supramolecular polymers. By changing the position of the carboxylate group, liquid crystals and gels could be formed.

Tay and Beer (DOI: 10.1039/D1OB00601K) have reviewed the use of macrocycles and mechanically interlocked hosts as optical sensors for anions, whilst Ballester and coworkers have investigated the role that the solvent plays in binding anions and ion pairs with an interlocked rotaxane host, finding that the solvent influences both the binding affinity and complex stoichiometry (DOI: 10.1039/D1OB01845K).

With a focus on anion sensors, Kubik and co-workers (DOI: 10.1039/D1OB00341K) report the use of gold nanoparticles surface functionalized by zinc(II)-dipicolylamine units as selective colorimetric sensors for AMP over ADP, ATP and inorganic phosphate, whilst Butler and co-workers have used a cationic europium(III) probe as a fluorescent anion sensor, exploiting the different fluorescent responses of the europium(III) probe to two structurally similar anions to monitor the activity of heparan sulfotransferase in real time (DOI: 10.1039/D1OB02071D).

Moving on to the area of supramolecular chemistry in lipid bilayers and transmembrane anion transport, Busschaert and co-workers (DOI: 10.1039/D1OB00263E) have reported the synthesis of a urea functionalized crown ether that can bind phosphatidylethanolamine and facilitate its flip–flop across lipid bilayers. The receptor also displays bactericidal activity against Gram-positive Bacillus cereus.

Félix, Valkenier and co-workers (DOI: 10.1039/D1OB01279G) have reported the effect of adding a second thiourea binding site to hydrazone-based anion transporters. The addition of the second binding site hinders the chloride transport ability of the receptor by interacting strongly with the phospholipid head groups. Gale and co-workers (DOI: 10.1039/D1OB01545A) have reported a series of anion transporters based on an acridinone scaffold. These compounds also have a high affinity for phospholipid head groups, resulting in predominant HCl co-transport. The high phospholipid affinity prevents the free receptors diffusing across the lipid bilayer, resulting in either deprotonated receptors or fatty acid carboxylate transport in addition to chloride transport and, hence, overall HCl co-transport.

Duarte, Langton and co-workers (DOI: 10.1039/D1OB01457A) have reported azo-benzene based bis-squaramides which only transport chloride when the receptor adopts the Z-conformation. The switching of the azobenzene unit has been tuned by halogenation to be switched by red light, potentially allowing switching in biological tissue. Moving from anion to ion pair transport, Wang and co-workers (DOI: 10.1039/D1OB01617B) employ anion-π binding sites in a heterotopic receptor capable of transporting KCl across lipid bilayers.

The papers in this collection demonstrate that the development of new receptors goes hand in hand with the application of these systems in sensing and in lipid bilayer anion transport. We would like to thank all the authors for contributing their work to this collection.

KAJ and PAG acknowledge and pay respect to the Gadigal People of the Eora Nation, the traditional owners of the land on which we research, teach, and collaborate at the University of Sydney.

Philip A. Gale

Katrina (Kate) A. Jolliffe

References

1. D. F. Shriver and M. J. Biallas, J. Am. Chem. Soc., 1967, 89, 1078.
2. C. H. Park and H. E. Simmons, J. Am. Chem. Soc., 1968, 90, 2431.
3. E. Graf and J.-M. Lehn, J. Am. Chem. Soc., 1975, 97, 5022; E. Graf and J.-M. Lehn, J. Am. Chem. Soc., 1976, 98, 6403.
4. F. P. Schmidtchen, Angew. Chem., Int. Ed. Engl., 1977, 16, 720; F. P. Schmidtchen, Chem. Ber., 1980, 113, 864; F. P. Schmidtchen, Chem. Ber., 1981, 114, 597; F. P. Schmidtchen and G. Muller, J. Chem. Soc., Chem. Commun., 1984, 1115.
5. P. A. Gale, J. L. Sessler, V. Král and V. Lynch, J. Am. Chem. Soc., 1996, 118, 5140–5141.
6. K. Kavallieratos, S. R. de Gala, D. J. Austin and R. H. Crabtree, J. Am. Chem. Soc., 1997, 119, 2325.
7. Y. Liu, W. Zhao, C. H. Chen and A. H. Flood, Science, 2019, 365, 159–161.
8. V. Havel, M. Babiak and V. Sindelar, Chem. – Eur. J., 2017, 23, 8963–8968.
9. N. Busschaert, C. Caltagirone, W. Van Rossom and P. A. Gale, Chem. Rev., 2015, 115, 8038–8155.

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