Discrete metallosupramolecular systems are often macrocyclic or cage-like architectures with an accessible internal cavity. Guest molecules can reside within these cavities and much of the interest in these systems is derived from these fascinating host-guest interactions. A range of potential applications stem from the ability of these metallosupramolecular architectures to encapsulate guests. These applications include catalysis or acting as molecular reaction flasks, the molecular scavenging of pollutants, storage of reactive species, and drug delivery. Multicavity metallosupramolecular architectures combine the ability of large hollow assemblies to bind multiple guests concurrently with the binding specificity associated with small cages. A variety of different approaches to generating separate compartments within a single metallosupramolecular assembly have emerged. These include interpenetrated cages, cages with polytopic ligands that have a long backbone, and molecules that have two or more clefts. This review examines these approaches, and highlights key contributions to the field.

Two new ferrocene-containing [Pd2(LFc)4]4+(X−)4 (where X− = BF4− or SbF6−) self-assembled cages were synthesised from the known, rotationally flexible, 1,1′-bis(3-pyridylethynyl)ferrocene ligand (LFc). The cages were shown to bind neutral and anionic guest molecules and displayed reversible redox activity.

The need for effective CO capture systems remains high, and due to their tunability, metallosupramolecular architectures are an attractive option for gas sorption. While the use of extended metal organic frameworks for gas adsorption has been extensively explored, the exploitation of discrete metallocage architectures to bind gases remains in its infancy. Herein the solid state gas adsorption properties of a series of [Pd (L) ] lantern shaped coordination cages (L = variants of 2,6-bis(pyridin-3-ylethynyl)pyridine), which had solvent accessible internal cavities suitable for gas binding, have been investigated. The cages showed little interaction with dinitrogen gas but were able to take up CO . The best performing cage reversibly sorbed 1.4 mol CO per mol cage at 298 K, and 2.3 mol CO per mol cage at 258 K (1 bar). The enthalpy of binding was calculated to be 25-35 kJ mol , across the number of equivalents bound, while DFT calculations on the CO binding in the cage gave ΔE for the cage-CO interaction of 23-28 kJ mol , across the same range. DFT modelling suggested that the binding mode is a hydrogen bond between the carbonyl oxygen of CO and the internally directed hydrogen atoms of the cage.

The cavities of metallosupramolecular cages can be used to mimic the central spaces of naturally occurring proteins and bind a wide variety of molecular guests. A range of potential applications have arisen from this capacity for host-guest chemistry. However, to truly harness the opportunities thus afforded, methodologies to controllably allow the release and reuptake of guests from the cavities of metallosupramolecular cages are required. Methods to accomplish this have centered upon reversibly altering the character of either the guest or host. This minireview outlines the current approaches used to carry out the binding and release of guests from metallosupramolecular hosts using important examples from the field.

New bis-quinoline (Lq) and bis-isoquinoline-based (Liq) ligands have been synthesized, along with their respective homoleptic [Pd2(Lq or Liq)4]4+ cages (Cq and Ciq). The ligands and cages were characterized by 1H, 13C and diffusion ordered (DOSY) NMR spectroscopies, high resolution electrospray ionization mass spectrometry (HR-ESIMS) and in the case of the bis-quinoline cage, X-ray crystallography. The crystal structure of the Cq architecture showed that the [Pd2(Lq)4]4+ cage formed a twisted meso isomer where the [Pd(quinoline)4]2+ units at either end of the cage architecture adopt the opposite twists (left and right handed). Conversely, Density Functional Theory (DFT) calculations on the Ciq cage architecture indicated that a lantern shaped conformation, similar to what has been observed before for related [Pd2(Ltripy)4]4+ systems (where Ltripy = 2,6-bis(pyridin-3-ylethynyl)pyridine), was generated. The different cage conformations manifest different properties for the isomeric cages. The Ciq cage is able to bind, weakly in acetonitrile, the anticancer drug cisplatin whereas the Cq architecture shows no interaction with the guest under the same conditions. The kinetic robustness of the two cages in the presence of Cl− nucleophiles was also different. The Ciq cage was completely decomposed into free Liq and [Pd(Cl)4]2− within 1 h. However, the Cq cage was more long lived and was only fully decomposed after 7 h. The new ligands (Liq and Lq) and the Pd(II) cage architectures (Ciq and Cq) were assessed for their cytotoxic properties against two cancerous cell lines (A549 lung cancer and MDA-MB-231 breast cancer) and one non-cancerous cell line (HDFa skin cells). It was found that Lq and Cq were both reasonably cytotoxic (IC50S ≈ 0.5 μM) against A549, while Ciq was slightly less active (IC50 = 7.4 μM). Liq was not soluble enough to allow the IC50 to be determined against either of the two cancerous cell lines. However, none of the molecules showed any selectivity for the cancer cells, as they were all found to have similar cytotoxicities against HDFa skin cells (IC50 values ranged from 2.6 to 3.0 μM).

Self-assembled metallosupramolecular architectures have become an increasingly popular area of inorganic chemistry. These systems show a range interesting biological, electronic and photophysical properties. Additionally, they display extensive host-guest chemistry that could potentially be exploited for drug delivery and catalysis. To fully realise these types of applications the ability to generate more functionalised metallosupramolecular architectures is required. In this perspective review we examine the exploitation of 1,2,3-triazole ligands, generated using the Cu(i)-catalysed 1,3-cycloaddition of organic azides with terminal alkynes (the CuAAC "click" reaction), for the assembly of discrete functional metallosupramolecular architectures. These "click" ligands have been used to generate metallomacrocycles, cages and helicates. Some of the architectures have shown promise as anti-cancer and anti-bacterial agents while others have been exploited for small molecule activation and catalysis.

Abstract:A small family of [Co 2 (L pytrz ) 3 ] 6+ cylinders was synthesised from bis(bidentate) 2-pyridyl-1,2,3-triazole "click" ligands (L pytrz ) through an "assembly-followed-by-oxidation" method. The cylinders were characterised using 1 H, 13 C, and DOSY NMR, IR, and UV-Vis spectroscopies, along with electrospray ionisation mass spectrometry (ESMS). Stability studies were conducted in dimethyl sulfoxide (DMSO) and D 2 O. In contrast to similar, previously studied, [Fe 2 (L pytrz ) 3 ] 4+ helicates the more kinetically inert [Co 2 (L pytrz ) 3 ] 6+ systems proved stable (over a period of days) when exposed to DMSO and were even more stable in D 2 O. The triply stranded [Co 2 (L pytrz ) 3 ] 6+ systems and the corresponding "free" ligands were tested for antimicrobial activity in vitro against both Gram-positive (Staphylococcus aureus) and Gram-negative (Escherichia coli) microorganisms. Agar-based disk diffusion and Mueller-Hinton broth micro-dilution assays showed that the [Co 2 (L pytrz ) 3 ] 6+ cylinders were not active against either strain of bacteria. It is presumed that a high charge of the [Co 2 (L pytrz ) 3 ] 6+ cylinders is preventing them from crossing the bacterial cell membranes, rendering the compounds biologically inactive.

One of the most appealing features of [Pd2L4]4+ cages is their well-defined cavities, giving binding affinity for specific guests. If seeking to bind larger and more complex guests, an attractive...