Tetra(μ₃-hydroxo) bridged copper(II) tetranuclear cubane complexes: Synthesis, crystal structure, and DNA binding studies

The tridentate Schiff base H₂L was synthesized by the condensation of equimolar amount of 1-amino-2-propanol and salicylaldehyde. The reaction of H₂L with an equimolar amount of Cu(CH₃COO)₂·H₂O in methanol leads to the formation of the tetranuclear Cu₄L₄, 1. However, reaction of equimolar amount of H₂L, copper(II) acetate, and 2,4,6-trimethylaniline in methanol forms a mixture of products which includes a discrete mononuclear complex Cu(L')₂, 2m (where HL' is a bidentate ligand), in addition to the tetranuclear Cu₄L₄ species, 2c. In both tetranuclear cubane species, the tridentate H₂L is both a chelating and a bridging ligand, after deprotonation of the enolic and the phenolic OH. The copper(II) centers are five-coordinate with a [N, O₄] donor set from the ligands. The coordination geometry about each copper is distorted square pyramidal with one nitrogen and two oxygen from one ligand and two oxygen from adjacent ligands in the next unit of the cubane. In mononuclear 2m, the ligand is bidentate and the coordination geometry around copper(II) is square planar. The absorption spectra strongly suggest that tetranuclear 1 interacts with CT-DNA.