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 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 [(H2LM)-M-1] (M = Ni, Cu, Pd) or [(H2LZn)-Zn-1(dmso)], in which the metal ion occupies the salen-type N2O2 coordination site. In particular, [(H2LNi)-Ni-1] and [(H2LPd)-Pd-1] 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 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 [(LNi2)-Ni-2] and [(LCu2)-Cu-2] species. The [(LNi2)-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 [(LNi2)-Ni-2] molecules formed a parallel stacking structure with a brick-wall arrangement. The central O-4 site of the [(LNi2)-Ni-2] complex was found to be capable of binding a third metal ion such as Ni2+, Cu2+ or Zn2+.