Schiff bases represent a class of biologically active and structurally diverse organic compounds that may be involved in copper(II) coordination as N,O-donor ligands. A redox-active biometal, copper is known to play a significant role in cellular metabolism and accounts for a wide range of biological activities exhibited by its compounds, namely antibacterial, antiproliferative, antifungal, antiinflammatory, etc. 4,6-Di-tert-butyl-2,3-dihydroxybenzaldehyde-derived Schiff bases and their Cu(II) complexes have been synthesized and characterized by means of FT-IR, UV-Vis, NMR and ESR spectroscopy, mass-spectrometry, elemental, XRD and thermogravimetric analysis, electrochemical measurements, quantum chemical calculations and molecular docking studies. The compounds were tested in vitro for antibacterial activity on Gram-positive and Gram-negative bacteria, bovine serum albumin and haemoglobin binding, as well as for hemolytic activity and for antioxidant activity in DPPH and ABTS assays. The synthesized ligands and their copper(II) complexes have been found to possess low cell toxicity and exhibit antioxidant activity higher than that of Trolox. Furthermore, several copper(II) complexes have demonstrated antibacterial activity comparable to those of tetracycline and streptomycin. The results obtained have been corroborated by molecular docking studies and interpreted in terms of the effects of copper(II) coordination by the organic ligand on radical-scavenging and bactericidal properties, protein affinity and toxicity of the latter.

Novel Ag(I) complexes (2a–2c) with phenolic Schiff bases were synthesized using 4,6-di-tert-butyl-3-(((5-mercapto-1,3,4-thiadiazol-2-yl)imino)methyl)benzene-1,2-diol (1a), 4,6-di-tert-butyl-3-(((4-mercaptophe­nyl­)­imino)­methyl)benzene-1,2-diol (1b), and 4,6-di-tert-butyl-3-(((3-mercaptophenyl)imino)methyl)­benzene-1,2-diol (1c). They were examined by elemental analysis, FT-IR, UV-Vis, 1H-NMR spectroscopy, XRD, cyclic voltammetry, conductivity measurements, and biological methods. The complexes are characterized by distorted geometry of the coordination cores AgN2S2 (2c), AgNS (2b) and AgS2 (2a). These stable complexes were not typified by the intramolecular redox reaction in organic solvents resulting in the formation of silver nanoparticles (AgNPs). Antibacterial activity of 1a–1c and 2a–2c was evaluated in comparison with AgNPs and commonly used antibiotics. All the complexes were more active than the ligands against the bacteria tested (14), but they were less active than AgNPs and commonly used antibiotics. Both 1a–1c and their complexes 2a–2c exhibited the capability for the bovine heart Fe(III)-Cyt c reduction. The ligands 1b and 1c were characterized by the highest reduction rate among the compounds under study, and they showed a higher reducing ability (determined by cyclic voltammetry) as compared with that of their Ag(I) complexes 2b and 2c.

With radical scavenging and reduction potential assays, cyclic voltammetry and density functional theory calculations, we have revealed structure–activity relationships that challenge the conventional understanding of antioxidant mechanisms.

The fundamental importance of copper as a redox-active metal essential to the functioning of several metabolic enzymes provides a wide range of its biological activity pathways. Copper(II) coordination compounds are known to exhibit potent antiproliferative, antibacterial, nuclease, anti-inflammatory and antimycobacterial activities. Hydrazones are organic ligands commonly used for complexation with copper(II) that possess antibacterial, antiviral and antifungal properties. Copper–ligand interaction might facilitate charge delocalization and increase net hydrophobicity of the system, resulting in its enhanced pharmacological activity. Coordination compounds of Cu(II) with 4,6-di-tert-butyl-2,3-dihydroxybenzaldehyde derived hydrazone, nitrofurantoin and ftivazide have been synthesized, characterized by means of elemental and XRD analysis, FT-IR, UV-Vis and NMR spectroscopy and tested for antibacterial activity in vitro on Gram-positive and Gram-negative bacteria.

Biological activity of hydrazones and thiasemicarbazones is well-described: thiosemicarbazide moiety is present in the structure of an antibacterial drug thioacetazone, hydrazone derivatives of isoniazid demonstrate a high activity against Mycobacterium spp., whereas 5-nitrofuran-2-carbaldehyde-derived hydrazones have proven effective in the treatment of urinary tract infections. Such compounds are also known to readily form Cu(II) complexes with a high antimicrobial activity attributed to ROS generation and subsequent caspase-dependent apoptotic cell death. Antifungal properties of thiosemicarbazone ligands and their Cu(II) complexes have been linked to alteration of ergosterol biosynthesis and membrane damage. 4,6-di-tert-butyl-2,3-dihydroxybenzaldehyde-derived hydrazones, thiosemicarbazones, and their Cu(II) complexes have been synthesized and characterized by NMR, ESR, FT-IR, UV-visible absorption spectroscopy, mass spectrometry, as well as elemental, XRD powder diffraction and thermogravimetric analysis studies. The obtained binuclear Cu(II) complexes have been found to possess the coordination polyhedron of [Cu₂L₂], HL being the formula of the corresponding ligand. Microbiological screening of the synthesized compounds against Gram-positive (Bacillus subtilis, Staphylococcus saprophyticus) and Gram-negative (Escherichia coli, Pseudomonas putida) bacteria has demonstrated a markedly enhanced antibacterial activity of hydrazone and thiosemicarbazone derivatives upon their complexation with the Cu(II) ion. Interestingly, antifungal activity of the 4,6-di-tert-butyl-2,3-dihydroxybenzaldehyde-derived hydrazones and thiosemicarbazones against Phytophthora spp. has been found to slightly decrease upon Cu(II) complexation. Due to tautomerism and metal chelation via anionic or neutral forms, biological activity of hydrazone and thiosemicarbazone derivatives can be controllably tuned by the formation of Cu(II) complexes of various coordination cores and nuclearities, rendering them promising basic structures for further modification.