Self-Assembled Organometallic [12]Metallacrown-3 Complexes

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Abstract
The reaction of [(arene)RuCl2]2 (arene=C6H6, cymene, C6H3Et3, or C6Me6) or [Cp*RhCl2]2 with 3-hydroxy-2-pyridone in the presence of Cs2CO3 gives trinuclear metallamacrocyclic complexes. The self-assembly process was shown to be completely diastereoselective, and a racemic mixture of complexes with MRMRMR or MSMSMS (M=Ru, Rh) configuration was obtained. Plausible mononuclear intermediates of the formula [(arene)RuCl(C5H4NO2)] (arene=cymene, C6Me6) have been isolated and characterized. A structurally related trimer was synthesized by using [(cymene)RuCl2]2 and 3-acetamido-2-pyridone instead of 3-hydroxy-2-pyridone. The macrocycles were shown to be highly potent ionophores for Na+ and/or Li+ with negligible affinities for the larger cation K+. The selectivities of the receptors depend on the π-ligand present: whereas the (C6H6)Ru− and (cymene)Ru complexes bind both Li+ and Na+, the (C6Me6)Ru−, (C6H3Et3)Ru−, and Cp*Rh complexes bind exclusively Li+. For all receptors, the presence of alkali metal ions can be detected electrochemically: the peak potential is shifted by >300 mV toward anionic potential upon binding. This behavior was utilized to detect Li+ and Na+ colorimetrically. Single crystal X-ray analyses have been carried out on eight complexes, four of which are bound to an alkali metal halide ion pair. Structural parameters, which affect the affinity and selectivity are discussed. A computational study on [{MX}[12]crown-3] complexes (M=Li, Na; X=Cl, Br, I) was performed in order to compare relevant bond lengths and angles of the energy-minimized structures with those of the organometallic receptors.