Goodwin Research Group

ABSTRACT Most high yielding low symmetry cages formed by metal–ligand coordination are assembled through strictly defined ligand and/or metal ion geometries which have been manipulated to form systems with complex architectures. In this work, we report the near-quantitative synthesis of a new S4-symmetry meso achiral [Fe4L4]8+ homoleptic face-capped tetrahedron from a symmetric and flexible tris-bidentate tritopic ligand. The symmetry of the tetrahedron is solvent mediated, with the meso cage forming near-quantitatively in acetonitrile, while nitromethane causes the partial isomerisation to the racemic homochiral tetrahedron. The formation of this low symmetry tetrahedron is driven by ligand flexibility, which we attribute to the meso isomer effectively minimising its solvent shell and the ligand strongly favouring negative stereochemical coupling between metal centres, compared to the homochiral cage, thus being the dominant cage when in acetonitrile.

Abstract Most metal organic cages are assembled through metal–ligand coordination, resulting in cages where the metal ions are part of the cage architecture, and thus have limited reactivity. There are only a handful of metal organic cages produced by metalation of a pre-synthesised organic cage. In this work, we show that hexa-cationic hydrazone cages coordinate a range of transition metal ions upon deprotonation to give cage complexes with metal ions oriented towards the cage cavity. Remarkably, a cage with ethyl solubilising groups gives the expected three-fold symmetric metallocage, cages with alkoxy solubilising groups give low-symmetry zinc metallocages, and a cage without a solubilising group switches between high and low symmetry conformations depending on solvent. These low symmetry arrangements persist on the NMR timescale at temperatures as high as 360 K and in the presence of a wide range of anions.

Selective anion recognition remains a key challenge in supramolecular chemistry: only a very small number of systems that can function in water are known, and these nearly always preferentially bind hydrophobic anions. In this work, we report three robust hexa-cationic cages that can be prepared on scales up to 14 g in two simple and high-yielding steps from commercially available materials. One of these cages displays unusually strong sulfate binding in water (Ka = 12,000 M–1), and demonstrates high selectivity for this anion over H2PO4–/HPO42– in DMSO/buffer mixtures. These results demonstrate that relatively large, three-dimensional supramolecular hosts can be prepared in high yields and on large scales, and can be highly potent receptors.