Preparation and Thermogravimetric and Antimicrobial Investigation of Cd (II) and Sn (II) Adducts of Mercaptopyridine, Amino Triazole Derivatives, and Mercaptothiazoline Organic Ligand Moieties

The solid adducts of SnCl2.(3amt).H2O, SnCl2.2(3amt).H2O, CdCl2.(3amt), CdCl2.2(3amt), SnCl2.(2mct).0.5H2O, SnCl2.2(2mct), CdCl2.(2mct), CdCl2.2(2mct).H2O, SnCl2.(2mcp).1.5H2O, >2.2(2mcp).4H2O, CdCl2.(2mcp), CdCl2.2(2mcp), SnCl2.(4amt).4H2O, SnCl2.2(4amt).1.5H2O, CdCl2.(4amt).H2O, and CdCl2.2(4amt) (where the 3amt, 4amt, 2mct, and 2mcp represent 3-amino-1,2,4-triazole, 4-amino-1,2,4-triazole, 2-mercaptothiazoline, and 2-mercaptopyridine simple organic chelates, respectively) were prepared using a solid-state route and investigated by CHN elemental analysis and infrared spectroscopy. Additionally, we investigated the thermogravimetric characterization and antimicrobial proprieties. It is verified that for 3amt and 4amt adducts, the coordination occurs through nitrogen atom. For 2mct compounds, the coordination occurs through nitrogen (Sn) or sulfur (Cd). For 2mcp adducts, both coordination sites nitrogen and sulfur are involved. By examination of TG curves, it is confirmed that for each hydrated compounds, the first mass loss step is linked with the release of water molecules followed by the release of ligand molecules and sublimation of the metal chloride. Furthermore, it is verified that, considering only the release of ligand molecules (3amp, 4amp, 2mct, or 2mcp), the cadmium adducts are always more stable than the correspondent tin adducts probably due to the formation of cross-linking bonds in these compounds. Finally, of these 16 adducts, 14 showed antimicrobial activities against different bacterial and fungal strains.


Introduction
In the past decades, the problem of multidrug-resistant microorganisms has reached an alarming level worldwide, and the synthesis of new antimicrobial compounds has become an urgent need to treat microbial infections. Organic compounds that include heterocyclic ring systems continue to attract significant interest due to their wide range of biological elements [1]. e nucleus 1,2,4-triazole is incorporated into a variety of important therapeutic agents, which mainly exhibit antimicrobial activities [1,2]. Among the various fivemembered heterocyclic systems, 1,2,4-triazoles and 1,3,4-thiadiazoles and their derivatives gain importance because they constitute the structural features of many bioactive compounds [2]. Triazole and thiadiazole rings are known to be included in the structure of various drugs [3,4]. From these classes of heterocyclic compounds, the synthesis of novel derivatives of 1,2,4-triazole-3-thionate and 2-amino-1,3,4thiazole has attracted great interest due to various biological properties such as antibacterial [5,6], antifungal [7], antituberculosis [8,9], interferon [10], antioxidant [11], antitumor [12], anti-inflammatory [13,14], and anticonvulsant [15]. e thermogravimetry analysis technique is employed to identify the acceptability level regarding the coordination nature in between the central metal ions and different kinds of interesting biomolecule chelates, such as amino acids [16], caffeine molecule [17], or chemical materials that have a biological behavior as ethylene-and propylene-urea as well as ethylene-thiourea [18]. Moreover, the thermogravimetric information shows very close relationships with the calorimetric data [19] and the spectral data [20]. e main goal of this article is to investigate the synthesis, thermal analyses, and antimicrobial data of the sixteen solid adducts for the Cd (II) and Sn (II) metal ions coordinated with the 3amt, 4amt, 2mct, or 2mcp organic molecules. e molecular structural formulas of 3amt, 4amt, 2mct, and 2mcp are displayed in Figure 1

Materials and Methods
All used reagents were purchased from Sigma-Aldrich and were utilized with no additional purification.
All solid Cd(II) and Sn(II) adducts were prepared by the solid-state pathway by grinding stoichiometric amounts of metal halides and organic moieties (3amt, 4amt, 2mct, and 2mcp) in a mortar for 70 minutes at room temperature (27°C). e prepared solid adducts were dried under vacuum at room temperature for 24 h. is solid-state pathway was successfully used to enhance coordination reactions [21][22][23][24] as an alternative to conventional synthesis in solution. e synthesis is performed at room temperature, and where no solvent is used, any unwanted reaction to the metal cation is avoided. e infrared spectra result considering both free organic ligands and sixteen solid adducts proved that there are no free ligand particles after the grinding process.
C, H, and N elemental analysis were performed using a Perkin-Elmer 2400 analyzer. Infrared spectra of the solid adducts as a powder in situ KBr discs were scanned using a Gengis II FTIR apparatus within the 4000-400 cm −1 range, with a resolution of 4 cm −1 . ermogravimetric diagrams under N 2 atmosphere were analyzed on the Shimadzu TG-50H apparatus with a heating rate of 15°C min −1 .
Tin(II) and cadmium(II) contents were determined by gravimetry by the direct ignition of the adducts at 600°C for 3 h till constant weight. e residue was then weighted in the forms of SnO and CdO, respectively. e Mohr method uses chromate ions as an indicator in the titration of chloride ions with a silver nitrate standard solution. After all the chloride precipitated as white silver chloride, the first excess of titrant results in the formation of a silver chromate precipitate, which signals the end point.
Preparation of standard AgNO 3 solution: 9.0 g of AgNO 3 was weighed out, transferred to a 500 mL volumetric flask, and made up to volume with distilled water. e resulting solution was approximately 0.1 M. is solution was standardized against NaCl. Reagent-grade NaCl was dried overnight and cooled to room temperature. 0.25 g portions of NaCl were weighed into Erlenmeyer flasks and dissolved in about 100 mL of distilled water. In order to adjust the pH of the solutions, small quantities of NaHCO 3 were added until effervescence ceased. About 2 mL of K 2 CrO 4 was added, and the solution was titrated to the first permanent appearance of red Ag 2 CrO 4 . e antimicrobial activity of all adducts were performed as previously explained in detail by Gaber et al. [21]. Escherichia coli and Pseudomonas aeruginosa were used as Gram-negative bacteria, whereas Bacillus subtilis and Staphylococcus aureus were used as Gram-positive bacteria. In addition, Aspergillus flavus and Candida albicans were used as fungal strains. Diameters of the inhibition zones around the hole were calculated [21]. Table 1 shows the data of the elemental analysis. ese results are like the proposed formulas. Additionally, the main infrared bands are displayed in Tables 2-5. Before a discussion on the infrared data, it is important to note that considering (3amt and 4amt) and (2mct and 2mcp), organic molecules have a rich electron donor sites through the lone pair of electrons presented on the nitrogen and sulfur atoms, respectively. Moreover, for the four chelates, there is more than one potential coordination site, which makes them able, at the very first moment, to act as cross-links.

Results and Discussion
In case of 3amt adducts, the overall decrease observed for the symmetrical and asymmetrical N-H bands suggests that the NH 2 group is engaged with the coordination. Furthermore, positive shifts observed for the δ b bands reinforce this statement. It is worth noting that the observed shifts are more intense to Sn(II) adducts than to Cd(II), which is probably due to the higher acidity of Sn(II) (larger nuclear effective charge: 5.65 for Sn and 4.35 for Cd). It is verified that the symmetrical N-H bands are more sensitive to this acidity difference since a positive shift is observed for Cd(II) adducts, whereas a negative shift of this band is verified to Sn(II) adducts.
In case of 4amp approximation, the same general orientation is observed for asymmetric and symmetric N-H bands. is fact indicates that in this case, NH 2 is involved to a slight degree in the metallic coordination. is hypothesis is reinforced by the fact that for 4amp, the ringed breathing       bands exhibit a negative shift (compared to free chelates and synthesized solid adducts), whereas positive shifts are observed in 3amp. erefore, for 4amp adducts, the two "isolated" nitrogen atoms are the main coordination sites.
Suggested coordination modes for 3amp and 4amp molecules are shown schematically in Figure 2.
As explicatory examples, the infrared spectra of 3amt solid adducts are shown in Figure 3.     Table 6: TG data summary for Sn(II) and Cd(II) adducts with 3amt, 4amt, 2mcp, and 2mcp.

Adduct
Step  t i and t f are the initial and final temperatures of the thermal degradation process, respectively.  In case of 2mct adducts, positive shifts of the ]C � N band are observed for Sn(II) adducts, whereas negative shifts are verified to Cd(II) adducts. Such fact suggests a coordination through nitrogen to Sn(II) and a coordination through sulfur to Cd(II) in agreement with the fact that the nitrogen atom is a hard base and that Sn(II) is a harder acid than Cd(II).
For 2mcp adducts, the negative shifts observed for the ](C-S); C-SH and ](C�N) aromatic bands suggest that, in this case, both coordination sites N and S are involved in the coordination process for both cations considered. e data of thermogravimetric curves for the 16 solid adducts are demonstrated in Figure 4. e main TG data are elucidated in Table 6.
For each TG curve, the experimental mass losses (±5%) are similar to the proposed formulas. It is possible to verify that for all hydrated compounds, the first mass loss step is associated with the release of water molecules followed by the release of ligand molecules and sublimation of the metal chloride. Furthermore, it is verified that, considering only the release of ligand molecules (3amp, 4amp, 2mct, or 2mcp), the cadmium adducts are always more stable than the correspondent tin adducts. Since the infrared data suggest that the metal-to-ligand interaction is higher for tin adducts, this last result is an expected one, unless we take into account that the cadmium adducts generally polymerize [22][23][24][25][26][27] and so there is, probably for these compounds, the formation of crosslinking bonds, leading to more stable compounds, from a thermal point of view. e antimicrobial effect of the adducts was measured against a variety of microorganisms including bacteria and fungus (Table 7 and Figure 5). e no-growth zones around the hole indicated the inhibiting activity of the adducts on the microbe. ese were calculated and compared with the ampicillin as an antibacterial agent or amphotericin B as an antifungal agent.    )-where 3amt � 3amino-1,2,4-triazole; 4amt � 4-amino-1,2,4-triazole; 2mct � 2mercaptothiazoline; and 2mcp � 2-mercaptopyridine-were synthesized by a solid-state route and characterized by CHN elemental analysis and infrared spectroscopy. A thermogravimetric study was also performed. It is verified that, for all compounds, the monoadducts are the most stable ones. Such fact agrees with a higher ionic and covalent character of the metal-ligand bond for such compounds. From the result, it can be concluded that 14 of the 16 compounds have a good biological activity against these microorganisms.

Data Availability
e data used to support the findings of this study are included within the article.

Conflicts of Interest
e authors declare no conflicts of interest.