Spin crossover-induced colossal positive and negative thermal expansion in a nanoporous coordination framework material.

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 halogen-bonding bis-triazolium rotaxane for halide-selective anion recognition.

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.

An all-halogen bonding rotaxane for selective sensing of halides in aqueous media.

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.

Ion-pair recognition by a heteroditopic triazole-containing receptor.

A new heteroditopic calix[4]diquinone triazole containing receptor capable of recognising both cations and anions through Lewis base and C-H hydrogen-bonding modes, respectively, of the triazole motif has been prepared. This ion-pair receptor cooperatively binds halide/monovalent-cation combinations in an aqueous mixture, with selectivity trends being established by (1)H NMR and UV/Vis spectroscopy. Cation binding by the calix[4]diquinone oxygen and triazole nitrogen donors enhances the strength of the halide complexation at the isophthalamide recognition site of the receptor. Conversely, anions bound in the receptor's isophthalamide cavity enhance cation recognition. (1)H NMR investigations in solution suggest that the receptor's triazole motifs are capable of coordinating simultaneously to both cation and anion guest species. Solid-state X-ray crystallographic structural analysis of a variety of receptor ion-pair adducts further demonstrates the dual cation-anion binding role of the triazole group.