Metallocycles and metallocages

Metallo-supramolecular assemblies, such as two-dimensional metallocycles and three-dimensional metallocages, have fascinated chemists for decades. Despite their (generally) facile synthesis by combining metal ions/nodes and ligands under thermodynamic control, assemblies of vast complexity can be generated from these simple components. In addition to their pleasing aesthetics, such assemblies can display interesting magnetic, emissive and guest-binding properties, leading to their exploitation in molecular recognition, catalysis, therapeutics, diagnostics and sensing, and other areas.

Examples of each of these are contained within this spotlight collection on Metallocycles and metallocages. Contributions to the field are included from authors in Europe (UK, Switzerland, Germany, France, Spain, Italy, the Netherlands), Oceania (Australia, New Zealand), North America and Asia (China, India, Korea, Japan, Russia), demonstrating the truly global nature of this area of chemistry.

A review by Xu and co-workers outlines the different design strategies used to access metallocyclic molecular necklaces – also known as radial[n]catenanes – assembled from the coordination of metal ions to (pseudo)rotaxane ligands (https://doi.org/10.1039/d2dt03594d).

Mishra and Chand have investigated the diastereoselective self-assembly of an unsymmetrical ligand into metallocycles (https://doi.org/10.1039/d2dt01571d). The process of self-assembly itself has been explored by Hiraoka and co-workers, who used an NMR-based approach to study the formation of a Pd₆L₁₂ octahedron (https://doi.org/10.1039/c8dt04931a).

Yin and co-workers demonstrated the incorporation of a metallacycle into a supramolecular polymer network for sensing acidic environments (https://doi.org/10.1039/d2dt02783f). A coordination polymer assembled from porphyrin ligands was reported by Konarev and co-workers and its magnetic properties studied (https://doi.org/10.1039/d2dt03170a). Interesting magnetic properties of discerete squares and cubes have been reported by Kamebuchi, Tadokoro and co-workers (https://doi.org/10.1039/d0dt04425c), with a team led by Li describing a novel spin-crossover active Fe₄L₆ tetrahedron (https://doi.org/10.1039/c9dt01947b).

One of the most exciting, and useful, properties of metallo-assemblies is their ability to act as hosts and encapsulate guest molecules. Han and co-workers have studied the host–guest chemistry of Rh(III) metallacages (https://doi.org/10.1039/c8dt05103h), whilst Zhang, Zhang and co-workers used a Ti₄L₆ tetrahedron to extract silver ions (https://doi.org/10.1039/d0dt03214j).

Guest-binding can actually result in alterations to the structure of the host architecture itself. Jung, Lee and co-workers investigated the structural flexibility of a Pd₂L₄ cage upon encapsulation of different anions (https://doi.org/10.1039/d0dt03005h), and guest molecules were shown by Blanco, Peinador, Quintela and co-workers to bias the dynamic equilibrium between metallocycles of different nuclearities (https://doi.org/10.1039/d2dt02955c).

When coupled to a measurable response, the host–guest chemistry of metallo-assemblies can be used for sensing. Mohan and Shanmugaraju were able to sense heparin in aqueous solvent using a metallocycle (https://doi.org/10.1039/d2dt03079a), and Sun and co-workers demonstrated selective sensing of thiophosphonate pesticides (https://doi.org/10.1039/c9dt00710e).

Extant interest in the use of metallocages as catalysts is exemplified by several works from this collection. He, Zhang and co-workers prepared a Co(II) metal–organic framework from a discrete Zr₄L₆ cage and demonstrated its capability in photocatalytic H₂ production (https://doi.org/10.1039/d2dt01976k). Jing, Duan and co-workers exploited photoinduced electron transfer between the electron-rich walls of a cubic metallocage and an encapsulated anthraquinone guest for photocatalytic reduction of a range of aryl chlorides (https://doi.org/10.1039/d2dt01166b). A metalloprism with cofacial Co(II) porphyrin ligands was shown by Cook and co-workers to be an effective electrocatalyst for the Oxygen Reduction Reaction (https://doi.org/10.1039/d2dt02724k), and bismetallacycles were used by Duan, Zhang and co-workers as photooxidation catalysts (https://doi.org/10.1039/d2dt02197h).

Debata and co-workers showed that a heterometallic Cu(I)/Pd(II) metallocycle could scavenge superoxide radials (https://doi.org/10.1039/d0dt00920b), whilst Zhou and co-workers reported a mixed-valence Cu(I)/Cu(II) metallocycle which, in the presence of TEMPO, was able to catalyse the oxidation of alcohols to aldehydes (https://doi.org/10.1039/d0dt01340d). Chao and co-workers achieved reduction of CO₂ to CO under visible light irradiation using a Fe₆L₆ metallocycle and photosensitiser (https://doi.org/10.1039/d1dt00692d). Diastereoselective photocatalytic formation of cyclobutanes using a metallocage was achieved by Liu, Cui and co-workers (https://doi.org/10.1039/d1dt00652e), and regioselective photodimerization of anthracenes was achieved by Han and co-workers using metallarectangles as scaffolds (https://doi.org/10.1039/d2dt01094a). Clever and co-workers demonstrated the ability of an interlocked metallo-catenane to act as a photosensitiser for the addition of oxygen to cyclohexadiene (https://doi.org/10.1039/d0dt01674h). Ward and co-workers have studied catalytic substrate hydrolysis at the exohedral surface of a coordination cube (https://doi.org/10.1039/d2dt01713j).

Interest has also grown in recent years regarding novel metallo-compounds as therapeutic agents. Deng, Sun and co-workers have reviewed the use of metallocages and metallocycles in biomedical applications, highlighting their use as imaging agents and therapeutic platforms (https://doi.org/10.1039/d2dt02766f).

Lisboa, Crowley and co-workers explored the host–guest chemistry and cytotoxicity of heterometallic PdPtL₄ cage systems (https://doi.org/10.1039/d2dt02720h). A trinuclear Pd(II) metallocycle was shown by Ghosh, Suntharalingam and co-workers to be cytotoxic towards both cancer and cancer stem cells (https://doi.org/10.1039/d0dt00006j). Therrien, Liagre and co-workers investigated metallo-prisms as photodynamic therapy agents for treating rheumatoid arthritis (https://doi.org/10.1039/d2dt00917j). The cellular uptake of fluorescent Pd₂L₄ cages was studied by Jones, Casini and co-workers (https://doi.org/10.1039/d2dt00337f).

This Spotlight Collection demonstrates the amazing breadth of ground-breaking research being undertaken in the area of metallocycles and metallocages all over the world. We would like to thank all those who have made this possible, including the authors, reviewers and the team at Dalton Transactions. Lastly, we would like to thank you, the reader, and hope you enjoy this collection.

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