CONTENTS 1. Introduction to Halogen Bonding 7119 1.1. Nature of the Halogen Bond 7119 1.2. Scope of the Review 7120 2. Computational and Theoretical Investigations of Halogen Bonding 7120 2.1.Quantum Mechanics Methods 7120 2.2. σ-Hole Model of Halogen Bonding 7120 2.3. Other Contributions to the Nature of Halogen Bonding 7122 2.4. Recent Examples of Computationally Investigated Halogen-Bonded Complexes 7123 2.4.1. XB to Neutral Species 7123 2.4.2. XB to Anions 7126 2.4.3. XB in Protein−Ligand Complexes 7127 2.4.4. Electron-Transfer Processes Affected by XB Interactions 7127 2.5. Classical Force Field Calculations 7127 2.6. Conclusions and Outlook 7129 3. Gas-Phase Studies of Halogen-Bonding Interactions 7130 4. Halogen Bonding in the Solid State 7131 4.1. Introduction to Crystal Engineering and Functional Materials 7131 4.2. Fundamentals 7132 4.3. Halogen-Bonding Hierarchy 7134 4.3.1. Ranking Halogen-Bond Donors 7134 4.3.2. HB/XB Complementarity/Competition 7136 4.3.3. Predicting XBs 7138 4.4. Control of Solid-State Supramolecular Architectures 7138 4.4.1. Polymorphism 7138 4.4.2. Stoichiometry 7139 4.4.3. Tautomeric Control 7140 4.4.4. XBs Involving Metals and Metal-Bound XBs 7140 4.4.5. XB with Anions in the Solid State 4.5. Solid-State Architectures 4.6.

The synthesis and anion binding properties of the first rotaxane host system to bind and sense anions purely through halogen bonding, is described. Through a combination of polarized iodotriazole and iodotriazolium halogen bond donors, a three-dimensional cavity is created for anion binding. This rotaxane incorporates a luminescent rhenium(I) bipyridyl metal sensor motif within the macrocycle component, thus enabling optical study of the anion binding properties. The rotaxane topology was confirmed by single-crystal X-ray structural analysis, demonstrating halogen bonding between the electrophilic iodine atoms and chloride anions. In 50% H2O/CH3CN solvent mixtures the rotaxane host exhibits strong binding affinity and selectivity for chloride, bromide, and iodide over a range of oxoanions.

External control over the mechanical function of materials is paramount in the development of nanoscale machines. Yet, exploiting changes in atomic behaviour to produce controlled scalable motion is a formidable challenge. Here, we present an ultra-flexible coordination framework material in which a cooperative electronic transition induces an extreme abrupt change in the crystal lattice conformation. This arises due to a change in the preferred coordination character of Fe(II) sites at different spin states, generating scissor-type flexing of the crystal lattice. Diluting the framework with transition-inactive Ni(II) sites disrupts long-range communication of spin state through the lattice, producing a more gradual transition and continuous lattice movement, thus generating colossal positive and negative linear thermal expansion behaviour, with coefficients of thermal expansion an order of magnitude greater than previously reported. This study has wider implications in the development of advanced responsive structures, demonstrating electronic control over mechanical motion.

A systematic study on the anion-binding properties of acyclic halogen- and hydrogen-bonding bis-triazolium carbazole receptors is described. The halide-binding potency of halogen-bonding bis-iodotriazolium carbazole receptors was found to be far superior to their hydrogen-bonding bis-triazolium-based analogues. This led to the synthesis of a mixed halogen- and hydrogen-bonding rotaxane host containing a bis-iodotriazolium carbazole axle component. The rotaxane's anion recognition properties, determined by (1)H NMR titration experiments in a competitive aqueous solvent mixture, demonstrated the preorganised halogen-bonding interlocked host cavity to be halide-selective, with a strong binding affinity for bromide.

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