International Conference on Supramolecular Science and Technology (ICSS&T2004) location:Prague (Czech Republic) date:5-9 September 2004
Metallacrowns are a new class of macrocyclic compounds, where the metal is an integral part of the macrocycle ring1. These macrocycles have structural similarities with the crown ethers but show a larger structural and chemical variety because the hydroxamic acid, the ring metal and the central metal can be varied. The building block of a metallacrown is a hydroxamic acid. A hydroxamic acid contains the functional group C(O)NHOH. Metallacrowns can be synthesized in two steps, the first step involving the synthesis of the ligand and the second the self-assembly of the ligand in presence of the metal ions. Varieties of complexes with copper(II) as ring metal and lanthanide(III) ions as cavity ions have been synthesized. Characterization methods include mass spectrometry (MS), nuclear magnetic resonance (NMR)and X-ray diffraction. A first class of ligand we used are hydroxamic acids derived from alpha amino acids. Because alpha amino hydroxamic acids (except glycine hydroxamic acid) are chiral, the resulting metallacrowns will also be chiral. Circular dichroism (CD) was used to study these metallacrowns. A second class of metallacrowns with derivatives of lactic hydroxamic acid are currently also under investigation although they have not been isolated yet. The lanthanide(III) ion in a 15-metallacrown-5 is coordinated by five oxygen atoms from the ring. Because lanthanide(III) ions have a high coordination number, oxygen atoms of additional ligands as nitrate or water molecules can also coordinate them. These water molecules play an important role in the relaxation efficiency of the metallacrown. Compounds that show good relaxation are used in medicine as contrast-agents (CA) for magnetic resonance imaging (MRI)2.