Transition Metal Complexes with Antipyrine‐Derived Schiff Bases: Synthesis and Antibacterial Activity

The increase of death rate, associated with infectious diseases, is directly linked to the bacteria that have multiple resistance to antibiotics. The lack of efficient medical treat-ment is the main cause of this problem. The synthesis of new antibacterial agents, through various methods, is, for sure, an emergency medical issue. 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. This article presents the synthesis of several complexes with base Schiff ligands, derived from 4-aminoantipyrine and in vitro research of their antibacterial activities. The new compounds were tested for their in vitro antibacterial activity against Staphylococcus aureus var. Oxford 6538, Klebsiella pneumoniae ATCC 100131, Escherichia coli ATCC 10536, and Pseudomonas aeruginosa ATCC 9027 strains. Based on the “ in vitro ” studies, we can say that ten of the complexes synthetized can be successfully used instead of streptomycin, where there is resistance to this antibiotic.

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][28][29].
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.

Metal complexes with aminoantipyrine Schiff bases: structure and methods of synthesis
Complexes of Cu(II), Co(II), Ni(II), Zn(II), Mn(II), VO(II), and Fe(III) were prepared by direct reaction between Schiff base ligand and the corresponding metal salts.

Synthesis of the complexes with ligands HL 6-11 and various co-ligands
The metal complexes with ligands base Schiff HL 6-11 are obtained through three methods: Method 1. Previously, the complex combination with the Schiff base is obtained to which the coligand is added(α-picoline, β-picoline, γ-picoline, n-propylamine). After the complete precipitation, the solid compound is obtained that is filtered, washed with ether, and dried in the exicator ( Figure 5) [48].

Synthesis of the complexes with ligands HL 17-20
The metal complexes with Schiff base ligands HL [17][18][19][20] are obtained through treating a solution that contains the ligand dissolved in ethanol or acetonitrile with the solution of metal salt, in a molar ratio of L:M=1:1. The mixture is refluxed for 5-6h (Figures 12, 13) [61][62][63][64] or, in other cases, even 12h ( Figure 14) [45]. The precipitation begins immediately or after the concentration of the solution to a third of its volume, on a water bath. The precipitate is obtained which is filtered, washed with ether, and dried in vacuo.    The study methods used to describe the complexes were as follows: the basic chemical analysis, spectrometry IR, UV-VIS, EPR, the thermogravimetric analysis, the magnetic susceptibility, and the molar electric conductibility. The complexes synthetized were tested from the point of view of the antibacterian activity; the obtained results were presented in the respective papers.   Figure 9. Scheme of synthesis of complexes with ligand HL 12 .  Figure 10. Scheme of synthesis of complexes with ligand HL 10 . The study methods used to characterize the metal complexes were as follows: elemental analysis, the thermogravimetric analysis, IR, UV-Vis, EPR spectroscopy, the molar electric conductibility, the magnetic susceptibility, and the X-ray diffraction.

Synthesis of the complexes with ligand HL 21
Ligand C 24 H 25 N 3 O 4 , ðHL 21 Þ Ethanolic solution of 3-formyl-6-methyl-chromone (1mmol) and 4-amino-2,3-dimethyl-1-phenyl-3-pyrazolin-5-one (1mmol) was stirred at room temperature, then refluxed for 2h, and kept at  Figure 11. Scheme of synthesis of complexes with ligands HL 15 and HL 16 .  Figure 13. Scheme of synthesis of complexes with ligand HL 18 .  4 C for 2days. The resulting precipitate of intense yellow color was filtered, washed with methanol, and dried. Yellow single crystals suitable for structure determination were obtained from methanolic solution upon slow evaporation at room temperature [65].
Complexes 1-3 and 5-9 were prepared by direct reaction between the ligand and the corresponding metal salts, while complex 4 was prepared by the metathetical displacement of the acetate ion, in Cu(OAc) 2 ÁH 2 O, by the thiocyanate ion [65] (Figures 15, 16).
To CuCl 2 Á2H 2 O (2mmol) dissolved in aqueous/ethanol solution (1:2v/v) was added ligand HL 21 (2mmol) dissolved in hot ethanol and refluxing for 2h. The green-brown precipitate, which separated on cooling, was filtered, washed with hot water, ethanol followed by ether, and dried in vacuo.
For the synthesis of complex 4, the acetate complex was first prepared and the acetate ion was then displaced by thiocyanate ion by using KSCN (2mmol). Dark-green solid.
Complex 5 was prepared similarly, using Cu(ClO 4 ) 2 Á6H 2 O (2mmol). The mixture was stirred at room temperature for 1h, when a dark-green precipitate appeared immediately.
For the synthesis of complex 13, the chloride complex was first prepared and chloride ion was then displaced by thiocyanate ion by using KSCN (2mmol). The green colored solid, which separated on cooling, were filtered, washed with hot water, ethanol followed by ether and dried in vacuo.
Complexes 22-28 were prepared by direct reaction between the ligand and the corresponding metal salts (Figures 21, 22) Complex 25 was prepared similarly, using Cu(ClO 4 ) 2 Á6H 2 O (2mmol). The mixture was stirred at reflux temperature for 5h. Brown solid.
For the synthesis of complex 27, the acetate complex was first prepared and the acetate ion was then displaced by thiocyanate ion by using KSCN (2mmol). Green solid.
Complex 28 was prepared in a similar fashion to complex 24, using VOSO 4 Á2H 2 O. Brown solid.

Antibacterial activity
The complexes and ligands HL [21][22][23][24] were tested for their in vitro antibacterial activity against de Staphylococcus aureus var. Oxford 6538, Klebsiella pneumoniae ATCC 100131, Escherichia coli ATCC 10536, and Pseudomonas aeruginosa ATCC 9027 strains using the paper disc diffusion method (for the qualitative determination) and the serial dilutions in liquid broth method (for determination of MIC) [66]. Streptomycin was used as internal standard.
The results of the antibacterial activity point out the fact that the activity of the Schiff bases HL 21-24 is more pronounced when it coordinates at the metal ion ( Table 1). In case of complexes 1, 6, 10, 12,15,18,19,20,23,25,26, and 28, there can be seen a visible increase in the antibacterial action.
Missing a clear action mechanism, in vitro, of the respective ligand of the complexes obtained on a microbian stem, there can be made the following stipulations: -the structure of the tested complexes seems to be the main element that influences the antibacterial activity. Thus, for complexes 1, 6, 10, 12,15,19,20,23,25,26, and 28, there has been determined an increased activity against all bacterial species, probably due to the presence of the monomeric form in DMSO solution and also due to the tetracoordination of the metal center.
-the presence of the anions with a large volume, in the outer coordination sphere of the complexes, can be deemed as another main element that can influence the antibacterial activity. The complexes 5, 6, 15, 19, 20, and 25 contain the groups ClO 4 and SO 4 2-, respectively, and prevent a visible increase in their activity against all species of bacteria used.
-if there is drawn a comparison between the coefficients of the molecular orbitals, computed on the basis of the transitions noticed in spectra UV-Vis and of the parameters g and A in spectra EPR [69] and the antibacterial activity, the conclusion is the fact that, for the complexes with the most pronounced activity, the values of δ 2 parameter to a weak covalent character of the link π out of the plan.

Conclusion
The investigations of the antibacterial screening, carried out for these new classes of compounds, reveal the fact that they present activity, especially toward the gram-positive bacteria, in comparison with the standard streptomycin. The increased antibacterial activity of the metal complexes can be accounted for by a cluster of reasons that refer to the chelation theory, nature of the ligand and of the metal ion, the geometry of the metal complexes, liposolubility, the presence of the co-ligands, and a series of sterical and pharmacokinetic factors. We can say that, ten of the complexes synthetized can be successfully used instead of streptomycin, where there is resistance to this antibiotic.