Co²⁺ and Ni²⁺ Complexes of the New (E)-N′-(2-hydroxy-5-nitrobenzylidene) morpholine-4-carbothiohydrazide Chelating Agent: Synthesis, Spectral, Thermal, and Corrosion Inhibition Investigations

In the current study, the new (E)-N′-(2-hydroxy-5-nitrobenzylidene)morpholine-4-carbothiohydrazide (H₂L) ligand and its [Co(HL)₂] (1) and [Ni(HL)Cl]·1.5H₂O (2) complexes have been produced and analyzed using analytical and spectral tools as elemental microanalyses, conductivity measurements, ¹H- and ¹³C-NMR, FT-IR, UV–vis, XRD, SEM, and thermogravimetric and differential thermogravimetric (TG-DTG) analyses. The FT-IR vibrational data indicated that the ligand loses its phenolic proton and acts as a monobasic ONS tridentate anion in both of the complexes. UV–vis measurements, as well as magnetic data, proposed that the Co²⁺ complex (1) has octahedral geometry, whereas the Ni²⁺ complex (2) exists in a tetrahedral shape. Powder XRD patterns for the investigated compounds show that the ligand and its tetra-coordinated Ni²⁺ complex (2) have a crystalline nature with crystallite sizes of 96.68 and 170.29 nm, respectively, whereas, hexa-coordinated Co²⁺ complex (1) is amorphous. Furthermore, the thermal pyrolysis of the organic ligand and its metal chelates were presented. Also, the Coats–Redfern and Horowitz–Metzger models were employed to estimate the activation thermodynamic parameters for different pyrolytic stages of the two complexes. Furthermore, the efficacy of the (H₂L) ligand and its cobalt and nickel complexes as inhibitors of corrosion for carbon steel in 0.5 M H₂SO₄ solutions was examined through electrochemical (PDP and EIS) and theoretical (MC simulations and DFT) analyses. The information gathered from the PDP analysis demonstrated that the (H₂L) ligand and its Co²⁺ and Ni²⁺ complexes function as mixed-type inhibitors. Additionally, findings from EIS studies confirm the development of a protective coating over carbon steel, thereby offering a shield against corrosion. The conclusions of both studies corroborate the effectiveness of the organic (H₂L) and its Co²⁺ and Ni²⁺ complexes as corrosion inhibitors.