Quinone-Based Metal Complexes Show Promise as Antibiotics and Rust Fighters

A team of chemists in Nigeria has created a family of metal compounds that can do two very different jobs at once: kill dangerous bacteria and fungi, and shield steel from the acid that eats it away. The work, published in the journal Discover Chemistry, centers on a naphthoquinone-derived Schiff base ligand and its complexes with six divalent metals: manganese, iron, cobalt, nickel, copper, and zinc. By binding the same organic chelator to different metal ions, the researchers produced a small library of coordination compounds whose properties shift dramatically depending on the metal at the center, offering a textbook demonstration of how coordination chemistry can tune both biological activity and industrial performance.

The starting material is built from 2-hydroxy-1,4-naphthoquinone, a quinone scaffold with a long history in medicinal chemistry, condensed with 2-amino-4-chlorophenol in refluxing ethanol for six hours. The condensation forms the azomethine linkage, the characteristic carbon-nitrogen double bond that defines Schiff bases and gives them their talent for grabbing metal ions. The resulting ligand, with the formula C16H10NO3Cl and a molecular weight of 299.70 grams per mole, emerged as a deep brown solid in a 66.5 percent yield after recrystallization from an ethanol-water mixture. The researchers then reacted half a gram of this ligand with weighed quantities of bivalent metal salts in a two-to-one ligand-to-metal molar ratio, producing the six homoleptic complexes in yields ranging from 53.0 percent for the copper compound to 72.7 percent for the iron analogue.

Characterization began with the most workhorse technique in coordination chemistry: infrared spectroscopy. The free ligand displayed an intense imine stretching band at 1645 wavenumbers, alongside a carbonyl stretch at 1667 wavenumbers, a phenolic O-H vibration, and a C-O stretch at 1304 wavenumbers. When the metals were introduced, that imine band slid down to the 1611 to 1632 wavenumber region, a shift that signals the azomethine nitrogen has donated its lone pair to the metal center. The C-O band also moved, implicating the phenolic oxygen as a second donor atom. Most convincing were entirely new bands appearing in the low-frequency region, between 508 and 587 wavenumbers for metal-nitrogen bonds and 475 to 493 wavenumbers for metal-oxygen bonds. Those vibrations simply do not exist in the free ligand, so their presence is direct evidence that new chemical bonds to the metal have formed.

Electronic spectroscopy filled in the picture of the geometry around each metal. The free ligand shows pi-to-pi-star transitions at 304 and 316 nanometers and n-to-pi-star transitions at 328, 334, and 394 nanometers, fingerprints of its aromatic rings and azomethine chromophore. Upon complexation these bands shifted, and the spectra acquired features diagnostic of octahedral coordination environments. The nickel, cobalt, iron, manganese, and copper complexes all displayed d-to-d transitions consistent with octahedral geometry, while the zinc complex, whose d10 configuration makes d-to-d transitions impossible, showed only ligand-centered bands. The team also observed ligand-to-metal charge transfer in the iron and copper compounds and metal-to-ligand charge transfer in the cobalt and nickel species, the latter reflecting the ligand’s ability to act as a pi-acceptor. Together with the infrared data, the spectra support proposed structures of the general form ML2(H2O)2.

Physical measurements added further confirmation. The free ligand melts at 216 to 218 degrees Celsius, but every complex melts higher, between 235 and 237 degrees and above 300 degrees in the case of the iron and cobalt compounds. Since a pure substance melts at a characteristic temperature, the fact that none of the complexes shared the ligand’s melting point rules out simple mixtures and points to genuinely new compounds with enhanced thermal stability, a consequence of the stable chelate rings and stronger metal-ligand interactions. Solubility testing told a complementary story: the ligand and all six complexes are insoluble in water but dissolve in coordinating organic solvents such as dimethylformamide and dimethyl sulfoxide. Chelation reduces polarity and increases lipophilicity, a shift that matters for biology because more lipophilic molecules cross microbial membranes more easily.

The antimicrobial screening is where the compounds revealed their most striking behavior. Using agar well diffusion on Mueller-Hinton and Sabouraud dextrose media, the team tested the ligand and all six complexes against six bacterial strains and three fungal species, with streptomycin and miconazole as positive controls. Against Staphylococcus aureus, the manganese complex produced a mean inhibition zone of 21.0 millimeters, edging out even streptomycin at 19.0 millimeters, while the copper complex matched the antibiotic at 19.5 millimeters. The cobalt complex was the top performer against Bacillus species at 18.0 millimeters, again matching streptomycin. On the fungal side, the iron complex led against Aspergillus niger with a 14.0 millimeter zone, followed by zinc at 12.5 millimeters, and the iron and zinc compounds also ranked highest against Aspergillus flavus. Notably, Pseudomonas aeruginosa resisted every complex, underscoring the selective, strain-dependent nature of the activity.

The pattern of activity does not map neatly onto the Irving-Williams series, the classical ordering of divalent first-row transition metal complex stability that peaks at copper. The researchers argue that stability alone cannot explain the results; kinetic lability, redox behavior, membrane permeability, and specific interactions with microbial biomolecules all contribute. Copper and iron complexes are known to engage in redox cycling and generate reactive oxygen species inside cells, a plausible mechanism for their strong antibacterial effects, while zinc may disrupt fungal metalloenzymes. Nickel complexes were the weakest across the board, consistent with earlier reports that nickel Schiff base complexes often suffer from poor cellular uptake. The pronounced activity against Gram-positive bacteria such as Staphylococcus and Bacillus fits the idea that lipophilic chelates penetrate the exposed peptidoglycan layer of Gram-positive cell walls more readily than the outer membrane of Gram-negative organisms.

The second act of the study is industrial rather than biomedical. Corrosion of mild steel in hydrochloric acid is a costly problem in industries that use acid pickling and cleaning, and organic molecules rich in heteroatoms and conjugated pi-systems are classic candidates for inhibiting it. Using the weight-loss method, the team immersed polished steel coupons in 1 molar hydrochloric acid with inhibitor concentrations of 100, 300, and 500 parts per million, measuring mass loss after one, three, six, twelve, and twenty-four hours at temperatures from 303 to 373 kelvin. Inhibition efficiency climbed with concentration, reaching a maximum of 69 percent at 500 parts per million after twelve hours at 323 kelvin, where the corrosion rate fell to 0.459. The mechanism is adsorption: the ligand’s nitrogen, oxygen, and chlorine atoms and its aromatic pi-electrons form donor-acceptor and electrostatic interactions with the steel surface, building a protective film that blocks both anodic metal dissolution and cathodic hydrogen evolution.

Time and temperature, however, impose limits. Efficiency dropped after twenty-four hours at 323 kelvin, falling to 56 percent at the highest concentration, which the authors attribute to partial desorption of inhibitor molecules and gradual weakening of the protective film under prolonged acid attack. At the highest temperature studied, 373 kelvin, efficiency remained concentration-dependent but more modest, rising from 25 percent at 100 parts per million to 54 percent at 500 parts per million after one hour. These trends suggest the ligand works best as a short-duration, high-concentration inhibitor, and that formulation strategies to strengthen film adhesion could extend its practical utility. The authors are candid about the study’s limitations and recommend follow-up work including elemental analysis, advanced spectroscopic characterization such as NMR and metal-content assays, mechanistic biological studies, electrochemical corrosion analysis, and computational modeling. Even so, the message is clear: a single naphthoquinone-based chelator, when paired with the right metal, can rival a frontline antibiotic against Staphylococcus aureus while simultaneously protecting steel in one of chemistry’s harshest environments, a reminder that the periodic table remains the most versatile design toolkit available to materials and drug developers alike.

Subject of Research: Synthesis and characterization of divalent metal complexes of a naphthoquinone-derived Schiff base ligand with antimicrobial and corrosion inhibition properties

Article Title: Synthesis, characterization, antimicrobial, and corrosion inhibition studies of divalent complexes of 2-((5-chloro-2-hydroxyphenyl) amino) naphthalene-1,4-dione chelator

Article References: Synthesis, characterization, antimicrobial, and corrosion inhibition studies of divalent complexes of 2-((5-chloro-2-hydroxyphenyl) amino) naphthalene-1,4-dione chelator. (n.d.). https://doi.org/10.1007/s44371-026-00836-w

Image Credits: AI Generated

DOI: 10.1007/s44371-026-00836-w

Keywords: Schiff base, naphthoquinone, metal complexes, coordination chemistry, antimicrobial activity, antifungal, corrosion inhibition, mild steel, octahedral geometry, FTIR spectroscopy, copper complex, chelation