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

作者：Raatikainen Kari; Huuskonen Juhani; Rissanen Kari^*

来源：Dalton Transactions, 2011, 40(21): 5706-5710.

DOI：10.1039/c1dt10406c

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 [H(3)5(3+)center dot Fe2+]center dot 4(ClO4-)center dot CF3COO- (V), [H(3)7(2+)center dot Fe2+]center dot 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 center dot 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 center dot 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+ approximate to Fe2+<Cu2+] deviates significantly from that of typical tris(2,2'-bipy) complexes [Zn2+<Co2+<Cu2+ approximate to Fe2+<Ni2+]. As a highlight, cyclophane 5 exhibits exceptionally high selectivity towards Cu2+ ions but very weak binding for Co2+.

• 出版日期2011