Exploring corrosion behavior, antimicrobial evaluation, molecular docking and DFT calculation of thiosemicarbazone ligand and its metal complexes

Abstract

In the current study, the execution of thiosemicarbazone ligand (HL) as a novel corrosion inhibitor for copper metal in 1 M HCl solution was evaluated through the electrochemical measurements which includes (open circuit potential (OCP) potentiodynamic polarization (PDP) and electrochemical impedance spectroscopy (EIS). The results confirmed that the ligand (HL) acted as a good corrosion inhibitor for copper metal in 1 M HCl solution; as it displayed high percentage of inhibition efficiency about 94.66% and 92.93% after PDP and EIS methods respectively; at its optimum concentration (1 × 10^-7 M). The morphology and surface constituents of the sample were examined before and after addition of the ligand (HL) by using the analysis (scanning electron microscope and an energy dispersive X-ray spectroscopy) which clarified the passivation effect of the ligand (HL) after formation of a protective layer of its adsorbed molecules on the surface of the copper sample. In addition, the metal complexes Ni (II), Co (II) and Cd (II) derived from thiosemicarbazone ligand (HL) were used in this study to shed light on some of their electrochemical properties. But based on their nature as they are insoluble in aqueous media the cyclic voltammetry method was used in this section. The results deducted from cyclic voltammetry technique showed that, the oxidation-reduction process of the ligand (HL) and its metal complexes Ni (II), Co (II) and Cd (II) under quasi-reversible system and the reaction occurred on the metal surface under diffusion control. In vitro, the antibacterial activity testing against S. aureus, S. pneumonia, E. coli and S. Typhimurium were performed for the ligand (HL) and its metal complexes Ni (II), Co (II) and Cd (II). The result showed that Co (II) and Cd (II), complexes exhibited the best antibacterial activity against S. pneumonia, S. Typhimurium and E. coli while, all the compounds did not show any antibacterial activity against S. aureus. To obtain a good relation that supports and explains the interactions between the molecules of the studied compounds and the metal surface and with the antibacterial activity; the theoretical study in detail was applied using density functional theory (DFT) and molecular docking. The parameters such as, energy level (ΔE), the highest HOMO (E_H), and the lowest occupied LUMO (E_L), molecular orbital and the binding energy are deducted and discussed. The main target investigated of this study is that the thiosemicarbazone ligand (HL) can be used as a new corrosion inhibitor for the metals and their alloys against the aggressive media. Also, from cyclic voltammetry technique which had been used for testing the metal complexes Ni (II), Co (II) and Cd (II) derived from the ligand (HL); all the details about the redox reactions of these compounds had been obtained. The importance of knowing oxidation and reduction reactions is due to their consideration as the main source of energy for the most biological process, energy productions, photosynthesis to immune responses and the synthesis and breakdown of biomolecules. Therefore, redox reactions are very important in our life.

Keywords: Antibacterial; Corrosion inhibitor; Cyclic voltammatery; DFT and docking study; Quasi-reversible; Thiosemicarbazone.

Fig. 1

The proposed chemical structure for metal complexes Ni (II), Co (II) and Cd (II).

Fig. 2

Open circuit potentials (OCP) as function of time for copper metal in 1.0 M HCl solution without and with different concentrations of the inhibitor (HL) recorded at 25 °C.

Fig. 3

Polarization curves of the copper metal in 1.0 M HCl solution containing various concentrations of the inhibitor (HL) recorded at 25 °C.

Fig. 4

(a) Nyquist (b) bode modulus and (c) bode phase angle plots of the copper metal in 1.0 M HCl solution containing different concentrations of the inhibitor (HL) at 25 °C.

Fig. 5

Equivalent circuit model used to fit the impedance data (EIS).

Fig. 6

Cyclic voltammograms of 0.005 M solution of the ligand (HL) and its Ni(II), Co(II) and Cd(II) complexes with 0.05 M solution of TBAP in DMSO at scan rate 50 mV s⁻¹.

Fig. 7

Cyclic voltammograms of the ligand HL (a) and its Ni (II) (b), Co (II) (c) and Cd (II) (d) complexes at different scan rate 50–500 mV s⁻¹.

Fig. 8

Relationship between the peak potential separation and the scan rate for the cyclic voltammograms of (a) Ni (II) (b) Co (II) and (c) Cd (II) complexes.

Fig. 9

Relationship between the anodic and the cathodic current peaks versus square root of the scan rate.

Fig. 10

Zone of inhibition of S. Typhimurium, E. coli and S. pneumoni for the inhibitor (HL) and its Ni (II), Co (II) and Cd (II) complexes.

Fig. 11

SEM micrographs of the copper metal (a) copper sample in the solution 1.0 M HCl without the inhibitor (HL) (b) copper sample in presence of the inhibitor (HL) in the solution 1.0 M HCl at 25 °C.

Fig. 12

EDX analysis of the copper metal (a) copper sample in the solution 1.0 M HCl without the inhibitor (HL) (b) copper sample in presence of the inhibitor (HL) in the solution 1.0 M HCl at 25 °C.

Fig. 13

Optimized molecular structure of HL. Ni (II), Co(II) and Cd(II) complexes.

Fig. 14

HOMO and LUMO frontier molecular orbitals along with their energies diagram for the ligand H₄L and their complexes calculated in vacuum at CAM-B3LYP/LAN2DZ.

Fig. 15

2D and 3D docking interactions of ligand (HL) and the Ni(II), Co(II) and Cd(II) complexes in the active pocket of ribosyl transferase (PDB ID: 3GEY) visualized with discovery studio software.