title: Programmed multiple complexation for the creation of helical structures from acyclic phenol-bipyridine oligomer ligands.
abstract: Two new multidentate ligands, H3L(1) and H4L(2), possessing bipyridine-phenol repetitive units were designed so that the multi-metal complexation could produce a single-helical structure in a pre-programmed fashion. The ligands were synthesized by successive palladium-catalyzed coupling reactions. The complexation of H3L(1) with zinc(II) and nickel(II) acetate afforded [L(1)Zn2(OAc)] and [(L(1))2Ni4](OAc)2, respectively. Each of the ligand moieties in these complexes formed a one-turn single helix. The zinc(II) complex [L(1)Zn2(OAc)] underwent a helix compression-extension motion in solution. The complexation of the H3L(1) ligand with iron(III) chloride gave a dinuclear complex [(HL(1))2Fe2Cl2] with a non-helical dimeric structure. The longer ligand H4L(2) afforded a trinuclear complex [L(2)Zn3(OAc)2] with a 1.5-turn single-helical structure upon complexation with zinc(II) acetate. The reaction of the H4L(2) ligand with cobalt(II) acetate under aerobic conditions gave a mixed valence complex [L(2)Co3(OAc)3(OMe)], which had two trivalent and one divalent cobalt ions. The structural features of the trinuclear complexes significantly depended on the metals; [L(2)Co3(OAc)3(OMe)] had a helical pitch of 7.6 Å, which was almost twice that of [L(2)Zn3(OAc)2] (4.0 Å).
authors.affiliation: Faculty of Pure and Applied Sciences, University of Tsukuba, 1-1-1 Tennodai, Tsukuba, Ibaraki 305-8571, Japan. akine@chem.tsukuba.ac.jp nabesima@chem.tsukuba.ac.jp.
authors.forename: Shigehisa
authors.initials: S
authors.lastname: Akine
authors.forename: Hiroki
authors.initials: H
authors.lastname: Nagumo
authors.forename: Tatsuya
authors.initials: T
authors.lastname: Nabeshima