Phenoxido-bridged open-cubane [Cu4(μ3-OH)2] and stepped-cubane [Cu4(μ3-OMe)2] complexes from a semicarbazone Schiff base: synthesis, anion-coordination-tunable structures, biomimicking functions, DNA binding, magnetic behavior and theoretical support

Abstract

The bridging nature of in situ solvent-generated hydroxido (HO⁻) and methoxido (MeO⁻) groups has been utilized to obtain tetranuclear open-cubane and stepped-cubane [Cu₄] complexes: [Cu₄(L)₂(H₂O)₂(μ₃-OH)₂](ClO₄)₄·3H₂O (1) and [Cu₄(L)₂(μ-NO₃)₂(μ₃-OMe)₂](NO₃)₂·H₂O (2) (HL = 2,6-bis-{(semicarbazidoimino)}-4-methylphenol). The ligand HL, possessing two metal ion-capturing bay regions, immediately seizes two copper(II) ions in solution to form Cu₂L species, with their remaining coordination sites loosely occupied by H₂O or NO₃⁻ groups, ultimately leading to the formation of complexes 1 and 2. The treatment of HL with Cu(ClO₄)₂·6H₂O and Cu(NO₃)₂·3H₂O metal ion salts is responsible for the in situ generation of the supporting HO⁻ and MeO⁻ linkers to sustain two different topologies from the same MeOH–H₂O reaction medium. In 2, the bridging coordination of the nitrate anion to two copper(II) centers guides the approach of individual Cu₂L units for Cu₄ cluster formation, whereas for 1, the approach of the Cu₂L units is different due to the coordination of labile H₂O molecules to the copper(II) centers. These complexes have been characterized by X-ray crystallography, and their magnetic properties have been studied. Both the complexes exhibit interesting catalytic oxidation behavior for mimicking the enzyme behaviours of catecholase oxidase and phenoxazinone synthase, using model substrate 3,5-DTBCH₂ and AP with the K_cat values 12.08, 1.845 and 0.101, 1.522 h⁻¹, respectively. In vitro ct-DNA interaction studies of complexes 1 and 2 revealed electrostatic binding in the groove region of DNA. Variable-temperature magnetic studies provided a J value of approximately −180 cm⁻¹ for 1 and −190 cm⁻¹ for 2, which were supported by DFT-based calculations with various functionals (PBE0, B3LYP, CAM-B3LYP, and ωr²SCAN).