Supramolecular chemistry of 12-Metallacrown-3 complexes

12-Metallacrown-3 complexes were formed by self-assembly of organometallic half-sandwich complexes [(π-ligand)MCl2]2 (M = Ru, Rh, Ir) and 3-hydroxy-2-pyridone ligands in the presence of Cs2CO3 as base. Upon mixing of different macrocycles, reversible metal fragment exchange occured, resulting in the generation of small dynamic combinatorial libraries. Investigation of their adaptive behaviour revealed a strong bias for the amplification of hetero-assemblies, and especially of the member whose composition reflects the overall composition of the library. As a consequence, it is not necessarily the thermodynamically most stable member (e.g. the assembly with the highest affinity to a given target) that is amplified the most. Dynamic exchange processes of this kind have also been used to synthesise mixed-metal macrocycles by controlled desymmetrisation of sterically hindered 12-metallacrown-3 complexes. Using amino-substituted 3-hydroxy-2-pyridone ligands, monomeric O,O'-chelate complexes were formed in aqueous solution. Self-assembly into trimeric metallamacrocycles was observed upon addition of base and in phosphate buffer solution, as evidenced by NMR spectroscopy and single crystal X-ray analyses. The macrocycles were able to act as highly selective receptors for lithium ions. The binding constants depend on the nature of the halfsandwich complex, the ligand, and the pH. With a commercially available [(cymene)RuCl2]2 complex and 4-(N-methylpiperazine)-3-hydroxy-2-pyridone, a receptor with a Li+ binding constant of Ka = (5.8±1.0) × 104 M-1 and a Li+ to Na+ selectivity of 10000:1 has been obtained. The fact that the assembly process of the receptor is pH dependant has been used to detect the presence of lithium ions by a simple pH measurement. Furthermore, it was possible to transduce the binding of Li+ into a change of color by means of a chemical reaction with FeCl3. This allowed the detection of Li+ in the pharmacologically relevant concentration range of 0.5–1.5 mM by the 'naked eye'. Modifications of the bridging ligand allowed the design of a potential fluorescent PET sensor for Li+ ions in water. 7Li / 6Li isotope separation was achieved by complexation of LiCl with 12-metallacrown-3 complexes having sterically very demanding π-ligands. An enrichment of 7Li of 5.4 % was observed. Using ligands in which two 3-hydroxy-2-pyridone units were connected by a linker, hexanuclear structures of up to 2.8 nm have been obtained. These assemblies can be regarded as expanded triple-stranded helicates.

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