1. Introduction

The synthesis of new antibacterial agents, through various methods, is, for sure, an emergency medical issue [1–3]. Schiff bases are important precursors for the synthesis of some bioactive compounds [4, 5]. Schiff bases have received considerable attention since the discovery of their antibacterial [6, 7], antifungal [8], anti-HIV [9, 10], anti-inflammatory [11], anticonvulsant [12, 13], antiviral [14], and anticancer properties [15–17]. The presence of the inimical grouping in these organic ligands plays an important part in manifesting these biological characteristics

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[18–20]. Schiff bases can be regarded as promising antimicrobial agents. For example, N-(salicylidene)-2-hydroxyaniline proved efficiency against Mycobacterium tuberculosis H37Rv, exhibiting an MIC value of 8μg/mL [21]. The 5-chloro-salicylaldehyde-Shiff base derivatives are efficient against Pseudomonas fluorescence (MIC=2.5–5.2μg/mL), Escherichia coli (MIC=1.6–5.7μg/mL), Bacillus subtilis (MIC=1.8μg/mL), and Staphylococcus aureus (MIC=1.6 and 3.1μg/mL), respectively, while the MIC values for the reference drug kanamycin against the same bacterial strains were 3.9μg/mL [22]. Some of the isatin-derived Schiff bases have shown antibacterial activity against Escherichia coli NCTC 10418 (MIC=2.4μg/mL), Vibrio cholerae non-01(MIC=0.3μg/mL), Enterococcus faecalis (MIC=1.2μg/mL), and Proteus shigelloides (MIC=4.9 μg/mL). The MIC values for the reference drug sulfamethoxazole against the same bacterial strain were in the range of 312–5000μg/mL. Therefore, these compounds were proven to be 1040-, 1040-, 4160-, and 1020-fold more potent than sulfamethoxazole [23, 24]. The studies run on the Schiff bases, derived from the isoniazid have allowed to identify a compound which has turned out to have a therapeutical effectiveness and safety, that is, 4000 times higher than that of isoniazid [25].

The morpholine-derived Schiff bases was effective against Staphylococcus aureus (MIC=20μg/mL), Micrococcus luteus (MIC=32μg/mL), Streptococcus epidermidis (MIC=17μg/mL), Bacillus cereus (MIC=21μg/mL), and Escherichia coli (MIC=16μg/mL).

Schiff bases with a 2,4-dichloro-5-fluorophenyl moiety completely inhibited the growth of Staphylococcus aureus, Escherichia coli, Pseudomonas aeruginosa, and Klebsiella pneumoniae. MIC values for these compounds varied from 6.3 to 12.5μg /mL, which are comparable to those obtained for the reference drug ciprofloxacin [26]. Lately, within the last couple of years, a special attention has been paid to the chemistry of the metal complexes of the Schiff bases. This is due to the chemical stability of the complexes as well as to the possibility of using them in the most varied fields. To a great extent, remarkable successes, in this field, have been obtained due to the various synthesis methods of the complexes. Recent research focuses more and more on the synthesis of complexes of the transitional metals with ligands of Schiff-base type, as a result of the biological properties which they have. In many cases, the conclusion has been that, through the coordination of the Schiff bases, to the metal ions, which are present in the biological systems, the biological activity of the respective Schiff base increases. A large number of Schiff bases and the corresponding metal complexes have proven antibacterian, antifungal, antitumor, and antileukemia activity [27–29].

Ever since it was synthesized [30], antipyrine (1-fenil-2,3-dimetil-5-pirazolona) has enjoyed a lot of attention due to its analgesic and antipyretic properties. The discovery of these properties has allowed for deeper research on antipyrine and its derivatives. Thus, 4-amino-2,3-dimethyl-1 phenyl-3-pyrazolin-5-one (4-aminoantipyrine) was discovered, a derivative with analgesic action, antipyretic, anti-inflammatory, antibacterian, and antineoplastic [31, 32]. The derivatives of 4 aminoantipyrine are used in the synthesis of azo-colorant, in analytical chemistry for spectrophotometric determination of metal ions [33], in pharmacology, as an effective antitumor [34], analgesic [35], antiviral [36], anti-inflammatory [37], anticancer [38], and antimicrobial drugs [39–42].

Lately, the research has been conducted in order to get metal complexes with a wide range of biological activities and with the lowest level of toxicity. In this work, the synthesis of some complexes with base Schiff ligands is presented, derived from 4-aminoantipyrine and in vitro research of their antibacterial activities.
