Metal complexes of (Z)-2-(pyrrolidin-2-ylidene)hydrazinecarbothioamide (L) with Cu(II), Co(II), and Ni(II) chlorides were tested against selected types of fungi and were found to have significant antifungal activities. The free-radical-scavenging ability of the metal complexes was determined by their interaction with the stable free radical 2,2′′-diphenyl-1-picrylhydrazyl, and all the compounds showed encouraging antioxidant activities. DFT calculations of the Cu complex were performed using molecular structures with optimized geometries. Molecular orbital calculations provide a detailed description of the orbitals, including spatial characteristics, nodal patterns, and the contributions of individual atoms. 1. Introduction Schiff bases have often been used as chelating ligands in the field of coordination chemistry, and their metal complexes have been of great interest to researchers for many years. It is well known that N and S atoms play a key role in the coordination of metals at the active sites of many metallobiomolecules [1]. The importance of metal ions in biological systems is well established. One of the most interesting features of metal-coordinated systems is the concerted spatial arrangement of the ligands around the metal ion. Among metal ions of biological importance, the Cu(II) ion involved in a large number of distorted complexes [2]. Over the past two decades, considerable attention has been paid to metal complexes of Schiff bases containing nitrogen and other donor atoms [3, 4]. Bioorganometallic chemistry is dedicated to the study of metallic complexes and their biological applications [5], including the design of new drugs that are more effective than those already known. The development of the field of bioinorganic chemistry has increased the interest in Schiff base complexes, because it has been recognized that many of these complexes may serve as models for biologically important species [6–9]. Antioxidants are extensively studied for their capacity to protect organisms and cells from damage induced by oxidative stress. Scientists in various disciplines have become more interested in new compounds, either synthesized or obtained from natural sources, that could provide active components to prevent or reduce the impact of oxidative stress on cells [10]. Thiosemicarbazones are well established as an important class of sulfur-donor Schiff base ligands that are particularly useful for transition metal ions. This is due to the remarkable biological activities observed for these compounds, which have been shown to be related to their
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