Metallacrown
Metallacrowns are a unique class of macrocyclic compounds that consist of metal ions and solely or predominantly heteroatoms in the ring. Classically, metallacrowns contain an [M–N–O] repeat unit in the macrocycle. First discovered by Vincent L. Pecoraro and Myoung Soo Lah in 1989, metallacrowns are best described as inorganic analogues of crown ethers. To date, over 600 reports of metallacrown research have been published. Metallacrowns with sizes ranging from 12-MC-4 to 60-MC-20 have been synthesized.
In 2013, the project "Metallacrowns: Metallacrown-based innovative materials and supramolecular devices" has been funded by the European Union Research Executive Agency as a Marie Curie IRSES International Research Staff Exchange Scheme. This mobility project involves researchers from the Universities of Parma, Wrocław, Paris-Sud, Kyiv, and Michigan, and the CNRS in Orléans. This project has been funded by the European Community's Seventh Framework Programme (FP7/2007-2013) under grant agreement no. 611488.
Metallacrown nomenclature has been developed to mimic the nomenclature of crown ethers, which are named by the total number of atoms in the ring, followed by “C” for “crown,” and the number of oxygen atoms in the ring. For example, 12-crown-4 or 12-C-4 describes Figure 2a. When naming metallacrowns, a similar format is followed. However, the C becomes “MC” for “metallacrown” and the “MC” is followed by the ring metal, other heteroatom, and the ligand used to make the metallacrown. For example, metallacrown b in the figure above is named [12-MCFe(III)N(shi)-4], where “shi” is the ligand, salicylhydroxamic acid.
Metallacrowns form via self-assembly, i.e. by dissolving the ligand in a solvent followed by the desired metal salt. The first reported metallacrown was MnII(OAc)2(DMF)6[12-MCMn(III)N(shi)-4]. Metallacrowns can be prepared with a variety of metals in the ring and in a variety of ring sizes. In recent years, many other metallacrowns have been prepared, including 9-MC-3, 15-MC-5, and 18-MC-6. Ring size is controlled by a number of factors, such as the geometry of the ligand chelate ring, ring metal Jahn-Teller distortion, central metal size, steric effects, and stoichiometry. Common ring metals have included V(III), Mn(III), Fe(III), Ni(II) and Cu(II). Hydroxamic acids, such as salicylhydroxamic acid, and oximes are commonly utilized in metallacrown ligands.
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