Phenoxido-bridged open-cubane ▫$[Cu_4(μ_3-OH)_2]$▫ and stepped-cubane ▫$[Cu_4(μ_3-OMe)_2]$▫ complexes from a semicarbazone Schiff base

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 Kcat 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).