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

Abstract

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 pre-programmed fashion. The ligands were synthesized by successive palladium-catalyzed coupling reactions. The complexation of H₃L¹ with zinc(II) and nickel(II) acetate afforded [L¹Zn₂(OAc)] and [(L¹)₂Ni₄](OAc)₂, respectively. Each of the ligand moieties in these complexes formed a one-turn single helix. The zinc(II) complex [L¹Zn₂(OAc)] underwent a helix compression–extension motion in solution. The complexation of the H₃L¹ ligand with iron(III) chloride gave a dinuclear complex [(HL¹)₂Fe₂Cl₂] with a non-helical dimeric structure. The longer ligand H₄L² afforded a trinuclear complex [L²Zn₃(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 [L²Co₃(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 Å, which was almost twice that of [L²Zn₃(OAc)₂] (4.0 Å).