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

A salen‐type ligand, H 4 L 1 , 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 H 4 L 1 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 H 4 L 1 ligand gave the corresponding mononuclear complexes [H 2 L 1 M] (M = Ni, Cu, Pd) or [H 2 L 1 Zn(dmso)], in which the metal ion occupies the salen‐type N 2 O 2 coordination site. In particular, [H 2 L 1 Ni] and [H 2 L 1 Pd] 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, H 4 L 2 , 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 H 4 L 2 , the metal‐templates of Ni 2+ and Cu 2+ significantly increased the [2+2] macrocyclization efficiency to produce the dinuclear [L 2 Ni 2 ] and [L 2 Cu 2 ] species. The [L 2 Ni 2 ] molecule had a highly planar structure incorporating two nickel(II) ions in an almost ideal square‐planar geometry. In the crystalline state, the [L 2 Ni 2 ] molecules formed a parallel stacking structure with a brick‐wall arrangement. The central O 4 site of the [L 2 Ni 2 ] complex was found to be capable of binding a third metal ion such as Ni 2+ , Cu 2+ or Zn 2+ .