Issue 21, 2011

Discriminating octahedral transition metal ions: highly selective tripodal tris-(2,2′-bipyridine) functionalized piperazine cyclophane receptor for Cu2+ ions

Abstract

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+.

Graphical abstract: Discriminating octahedral transition metal ions: highly selective tripodal tris-(2,2′-bipyridine) functionalized piperazine cyclophane receptor for Cu2+ ions

Supplementary files

Article information

Article type
Paper
Submitted
24 Jan 2011
Accepted
18 Mar 2011
First published
20 Apr 2011

Dalton Trans., 2011,40, 5706-5710

Discriminating octahedral transition metal ions: highly selective tripodal tris-(2,2′-bipyridine) functionalized piperazine cyclophane receptor for Cu2+ ions

K. Raatikainen, J. Huuskonen and K. Rissanen, Dalton Trans., 2011, 40, 5706 DOI: 10.1039/C1DT10406C

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