New tripodal transition metal ion receptors, tris(5-ethoxycarbonyl-2,2′-bipyridine) and tris(5-carboxylate-2,2′-bipyridine) substituted 27-membered trimeric piperazine cyclophanes 5 and 7 as well as tetra(5-ethoxycarbonyl-2,2′-bipyridine) substituted 36-membered tetrameric piperazine cyclophane 6, have been prepared and their transition metal ion complexing properties studied in solution by UV-vis spectroscopy and in the solid state by single-crystal X-ray diffraction. The crystal structures of [H353+·Fe2+]·4(ClO4−)·CF3COO− (V), [H372+·Fe2+]·2(SO42−) (VII) and the reference complex [tris(5,5′-bis(ethoxycarbonyl)-2,2′-bipyridine)Fe(II) perchlorate] (I) showed that the robust piperazine cyclophane is an optimal platform in preorganizing the 2,2′-bipy moieties to form a very fixed octahedral coordination site. In an acidic water solution, the highly preorganized structure of 5 gives a [5·Fe2+] complex, the stability of which is comparable with the classical tris(2,2′-bipy) Fe2+-complex but it is a significant 3.7 logK units more stable than the non-preorganized tetrameric analog [6·Fe2+]. Detailed studies with other similar divalent octahedral transition metal cations showed that the restricted octahedral coordination in complexes of 5 results in an unusual selectivity. The selectivity order [Zn2+<Co2+<Ni2+≈Fe2+<Cu2+] deviates significantly from that of typical tris(2,2′-bipy) complexes [Zn2+<Co2+<Cu2+≈ Fe2+<Ni2+]. As a highlight, cyclophane 5 exhibits exceptionally high selectivity towards Cu2+ ions but very weak binding for Co2+.
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