Effect of ligand spacer on metallosupramolecular architectures: From a dinuclear copper(II) double helicate to a tetranuclear copper(II) complex

Two new bis(N,O-bidentate) Schiff base ligands with asymmetric spacers between the donor sites have been synthesized. The asymmetric ligand H ₂L¹ self-assembles with copper(II) ions to give rise to a dinuclear copper(II) complex [Cu₂L¹₂] (1), while a more flexible ligand H₂L², having an additional methylene group in the spacer, forms a tetranuclear copper(II) cluster [Cu ₄L²₄] (2). Both complexes have been characterized by different analytical and spectroscopic techniques. Single-crystal X-ray diffraction study of 1 shows a dinuclear copper(II) double-stranded helicate with the two heterotopic ligands oriented head-to-head (HH), with a separation of 7.687(3) Å between the distorted tetrahedral copper(II) centers. The bis(bidentate) ligand strands wrap around the copper centers with a similar relative orientation (HH) to give rise to a double-helical structure. The intramolecular face-to-face π-π interaction stabilizes the double-helical structure of the molecule. In the tetranuclear complex 2, the four copper(II) centers, separated by 7.288(2)-8.239(2) Å, define a slightly distorted tetrahedron.