Solution-state Anion Binding Studies of Urea- and Amide-based Hydrogen-bonding Receptors for the Design of Supramolecular Assemblies

Abstract

This work investigates urea- and amide-based hydrogen-binding systems as small-molecule anion-binding receptors, as part of a project in the Gunnlaugsson group to develop complex molecular assemblies built around the urea–anion interaction. Chapter 1 introduces the concepts of anion binding, templating and directing, and discusses the utility of molecular assemblies, interlocked structures and urea-based anion-binding receptors, specifically in the context of recent advancements in supramolecular chemistry. An overview of anionreceptor clusters is provided, with a focus on helicates and barrels.

In Chapter 2, the anion binding behaviour of a series of novel electron-rich derivatives of the meta-phenylene bis(phenylurea) motif is analysed in detail through 1 H NMR titrations with the tetrabutylammonium salts of a range of anions. The meta-phenylene bis(phenylurea) motif is shown to bind anions such as dihydrogenphosphate, benzoate and acetate in 1:2 host–guest stoichiometries. No electronic effect due to the substitution patterns on the distal aromatic rings is observed. The adducts of these receptors with phosphate and acetate anions in the crystalline-phase is discussed, including a novel triple stranded phosphate-centred helicate. Part of Chapter 2 has been published in The Journal of Organic Chemistry (2018, 83, 10398–10408).

Chapter 3 investigates the trends in binding behaviour of the simple di(3amidophenyl)urea motif with varying aliphatic chain length. Two new receptors are developed as negative controls in this experiment. The titrations of the 3-, 6- and 11-carbon chain receptors with dihydrogenphosphate, benzoate and sulfate are discussed in detail, and in comparison to preliminary results which have been previously reported.

In Chapter 4, the synthesis and characterisation of two classes of H 6 –hexadentate receptor molecules is presented, which combine the structural elements of the receptors studied in Chapters 2 and 3. The attempted syntheses of macrocyclic derivatives of the ortho isomers is also discussed. The binding affinities of these molecules towards dihydrogenphosphate and benzoate anions is quantified and compared. The ortho isomers are shown to bind both anions in the expected 1:1 host–guest stoichiometry, while this is 1:2 for the meta isomer. A discussion on the complexation-induced shift of the latter is also featured.

The conclusions for this work are presented in Chapter 5, while Chapter 6 describes the experimental details of the work presented herein. The combined references for this thesis are found in Chapter 7. Additional details may be found in the appendices, including synthesis and characterisation (A1), titration and fitting data (A2) and publications (A3).