Programmed multiple complexation for the creation of helical structures from acyclic phenol-bipyridine oligomer ligands

Two new multidentate ligands, H3L1 and H4L2, possessing bipyridine-phenol repetitive units were designed so that the multi-metal complexation could produce a single-helical structure in a preprogrammed fashion. The ligands were synthesized by successive palladium-catalyzed coupling reactions. The complexation of H3L1 with zinc(II) and nickel(II) acetate afforded [(LZn2)-Zn-1(OAc)] and [(L-1)(2)Ni-4](OAc)(2), respectively. Each of the ligand moieties in these complexes formed a one-turn single helix. The zinc(II) complex [(LZn2)-Zn-1(OAc)] underwent a helix compression-extension motion in solution. The complexation of the H3L1 ligand with iron(III) chloride gave a dinuclear complex [(HL1)(2)Fe2Cl2] with a non-helical dimeric structure. The longer ligand H4L2 afforded a trinuclear complex [(LZn3)-Zn-2(OAc)(2)] with a 1.5-turn single-helical structure upon complexation with zinc(II) acetate. The reaction of the H4L2 ligand with cobalt(II) acetate under aerobic conditions gave a mixed valence complex [(LCo3)-Co-2(OAc)(3)(OMe)], which had two trivalent and one divalent cobalt ions. The structural features of the trinuclear complexes significantly depended on the metals; [L2Co3(OAc)(3)(OMe)] had a helical pitch of 7.6 angstrom, which was almost twice that of [L2Zn3(OAc)(2)] (4.0 angstrom).