Design, characterization, and different applications of naphthoquinone‐based Schiff base and its M(II/III) complexes with molecular modeling studies

Abstract

Due to the significant pharmacophoric role of 1,4‐naphthoquinone moiety, which is found in numerous biologically active natural products and well‐known chemotherapeutic drugs, including doxorubicin, shikonin, and alkannin, a newly prepared ligand ((E)‐1‐((2‐mercaptophenyl)imino)naphthalen‐2(1H)‐one) can be obtained from the condensation of 2‐aminothiophenol with diketone (1,2‐naphthoquinone) to form Schiff base (HL), which has been characterized using elemental analyses, FT‐IR, 1H‐NMR, and mass spectra. The inner ((La(III), Er(III), and Yb(III)) and outer ((Fe(III), Co(II), and Zn(II)) transitionmetal complexes have been prepared in 1:2 ratio and identified by several spectroscopic instruments. Different characterization tools have been used as elemental analyses, 1H‐NMR, molar conductivity, 13C‐NMR, FT‐IR, thermal decomposition (TG and DTG), UV‐vis, X‐ray diffraction, mass spectrometry, solid reflectance, and magnetic moment. All of them are conjugated to confirm the proposed structures of the metal complexes and are complementary. Also, molecular docking studies for the ligand and its Zn(II) and Yb(III) complexes (using MOE program) have been performed to support the experimental biological activity. Molecular docking studies recommended that (Yb(HL)2)Cl3·6H2O complex is more potent against Aspergillus niger fungus when docked with 1Y43 (aspergilloglutamic peptidase enzyme from A. niger) receptor, while Zn(II) complex recorded the strongest binding energy among the docked compounds against 2W3Z(the crystal structure of a Streptococcus mutans CE4 esterase) and 7AB4 (the crystal structure of the Escherichia coli toxin‐antitoxin system HipBST [HipT S59A]) proteins receptors, respectively. Experimental applications of the free ligand and its metal chelates as antimicrobial and antioxidant activities have been carried out. The former occurred against two fungal species (Aspergillus niger [ATCC:16404] and Candida albicans [ATCC:10231]), two G‐positive bacterial species (S. mutans [ATCC:25175] and Staphylococcus aureus [ATCC:13565]) and two G‐negative bacterial species (Klebsiella pneumonia [ATCC:10031] and E. coli [ATCC:10536]). The DPPH scavenging method has been used to measure the antioxidant activity of the tested compounds.