Synthesis of Planar Metal Complexes and the Stacking Abilities of Naphthalenediol-Based Acyclic and Macrocyclic Salen-Type Ligands



A salen-type ligand, H4L1, having two 1,8-naphthalenediol moieties, was designed and synthesized to obtain planar metal complexes in which the naphthalene π surface was expected to increase the stacking ability. Spectroscopic and crystallographic investigations revealed that the uncomplexed H4L1 ligand existed exclusively in the keto/keto form and adopted a nonplanar structure. Upon complexation with divalent metal sources M (Ni, Cu, Zn, Pd), the H4L1 ligand gave the corresponding mononuclear complexes [H2L1M] (M = Ni, Cu, Pd) or [H2L1Zn(dmso)], in which the metal ion occupies the salen-type N2O2 coordination site. In particular, [H2L1Ni] and [H2L1Pd] adopted a highly planar structure, allowing the formation of well-organized stacking structures in the crystalline state as well as self-association in solution. The corresponding macrocyclic ligand, H4L2, was also designed to obtain a more organized stacking structure due to the higher π surface area. Although the [2+2] macrocyclization without a template did not efficiently give the ligand H4L2, the metal-templates of Ni2+ and Cu2+ significantly increased the [2+2] macrocyclization efficiency to produce the dinuclear [L2Ni2] and [L2Cu2] species. The [L2Ni2] molecule had a highly planar structure incorporating two nickel(II) ions in an almost ideal square-planar geometry. In the crystalline state, the [L2Ni2] molecules formed a parallel stacking structure with a brick-wall arrangement. The central O4 site of the [L2Ni2] complex was found to be capable of binding a third metal ion such as Ni2+, Cu2+ or Zn2+.