Molecular Packing of Group-10 Metal Complexes of a Thiophene-Fused Salophen Ligand Bearing a Wide π‍-‍Surface

Abstract

Naphthodithiophene-fused salophen ligands and their Zn(II), Ni(II), and Pd(II) complexes were synthesized to investigate the effect of π-surface extension on intermolecular self-assembly. Single-crystal X-ray diffraction revealed that the Group 10 complexes adopt highly planar molecular structures and form head-to-tail π-stacked dimers in the solid state, whereas the Zn(II) complex exhibits a distinct packing motif owing to its pentacoordinate geometry. Variable-concentration NMR and UV–vis absorption measurements demonstrated pronounced self-association of the Ni(II) and Pd(II) complexes in solution, affording dimerization equilibrium constants of 432 and 272 L mol⁻¹, respectively. In contrast, the corresponding salophen complexes lacking the naphthodithiophene unit showed only weak self-association, indicating that extension of the π-conjugated surface markedly enhances intermolecular π–π interactions. Furthermore, introduction of bulky tert-butyl substituents effectively suppresses dimer formation through steric hindrance. These results demonstrate that fusion of a naphthodithiophene unit provides an efficient molecular design strategy for controlling the self-assembly behavior of square-planar salophen metal complexes.