title: 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 [H(3)5(3+)·Fe(2+)]·4(ClO(4)(-))·CF(3)COO(-) (V), [H(3)7(2+)·Fe(2+)]·2(SO(4)(2-)) (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·Fe(2+)] complex, the stability of which is comparable with the classical tris(2,2'-bipy) Fe(2+)-complex but it is a significant 3.7 logK units more stable than the non-preorganized tetrameric analog [6·Fe(2+)]. 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 [Zn(2+)<Co(2+)<Ni(2+)≈Fe(2+)<Cu(2+)] deviates significantly from that of typical tris(2,2'-bipy) complexes [Zn(2+)<Co(2+)<Cu(2+)≈Fe(2+)<Ni(2+)]. As a highlight, cyclophane 5 exhibits exceptionally high selectivity towards Cu(2+) ions but very weak binding for Co(2+).
authors.affiliation: Department of Chemistry, Nanoscience Center, University of Jyväskylä, PL 35, FIN-40014, Jyväskylä, Finland.
authors.forename: Kari
authors.initials: K
authors.lastname: Raatikainen
authors.forename: Juhani
authors.initials: J
authors.lastname: Huuskonen
authors.forename: Kari
authors.initials: K
authors.lastname: Rissanen
mesh_terms: D015082:2,2'-Dipyridyl / Q000737:chemistry*; D003300:Copper / Q000737:chemistry*; D007477:Ions / Q000737:chemistry; D008968:Molecular Conformation; D000077489:Piperazine; D010879:Piperazines / Q000737:chemistry*; D010880:Piperidines / Q000737:chemistry*; D013056:Spectrophotometry, Ultraviolet; D028561:Transition Elements / Q000737:chemistry*
chemical_list: D007477:Ions; D010879:Piperazines; D010880:Piperidines; D028561:Transition Elements; D000077489:Piperazine; D015082:2,2'-Dipyridyl; D003300:Copper