Heterometallic Rings: Their Physics and use as Supramolecular Building Blocks

An enormous family of heterometallic rings has been made. The first were Cr7M rings where M=NiII, ZnII, MnII, and rings have been made with as many as fourteen metal centers in the cyclic structure. They are bridged externally by carboxylates, and internally by fluorides or a penta‐deprotonated polyol. The size of the rings is controlled through templates which have included a range of ammonium or imidazolium ions, alkali metals and coordination compounds. The rings can be functionalized to act as ligands, and incorporated into hybrid organic–inorganic rotaxanes and into molecules containing up to 200 metal centers. Physical studies reported include: magnetic measurements, inelastic neutron scattering (including single crystal measurements), electron paramagnetic resonance spectroscopy (including measurements of phase memory times), NMR spectroscopy (both solution and solid state), and polarized neutron diffraction. The rings are hence ideal for understanding magnetism in elegant exchange‐coupled systems. A playground for physics: Heterometallic rings featuring seven trivalent and a single divalent metal center were first reported in 2003. Since that time they have been shown to be ideal for studying the physics of anti‐ferromagnetically coupled nanomagnets. They have a rich chemistry, and can be incorporated in larger structures in at least three ways, including use of the inorganic rings as part of hybrid rotaxanes.