Complexes of 1,3-Diisobutyl Thiourea with Copper(I), Zinc(II) and Mercury(II): Their Antioxidant and Antibacterial Evaluation

The reaction of 1,3-Diisobutyl thiourea (Tu) with metal salts, {[CuX (X = Cl, I)], [ZnCl2] and [HgI2] in an appropriate stoichiometric ratio afforded the corresponding metal complexes [Tu2CuCl] (1), [Tu3CuI] (2), [Tu2ZnCl2] (3) and [Tu2HgI2] (4) in good yields. The FT-IR data show typically broad signals (3278–3288 cm−1) attributed to the involvement of NH bonds in extensive hydrogen bonding. The structures of complexes were proposed based on a spectroscopic data set. Compounds 1 and 2 were additionally characterized by single-crystal X-ray analysis. Complexes 1–4 were tested for their free radical scavenging efficiency using 2,2-diphenyl-1-picrylhydrazyl free radical (hereafter abbreviated as DPPH). The free radical scavenging activity was a function of decrease in the resultant absorption of DPPH solution after the mixing of an appropriate concentration of the respective complex. The activity of complexes was determined to be dose dependent and increased concentration of the complex resulted in improved antioxidant activity. Compound 1 was found to be the most efficient, with 79.9% free radical scavenging activity. Complexes were also tested for their efficiency against selected strains of bacteria (E. coli, S. flexneri, S. typhi, and P. aeruginosa) and the activities were compared to commercially available standard drug cephradine. Compound 1 was more active against P. aeruginosa (ZI 13.25), while compound 4 was found to be more active against E. coli (ZI 11.0), S. flexneri (ZI 11.2), and S. typhi (ZI 10.5).


Introduction
Thiourea derivatives make a versatile group of compounds, with applications in organic, coordination, and material chemistry [1,2]. These compounds act as organocatalysts in a variety of reactions which lead to outstanding products [3][4][5][6]. There are two potential functional groups in thiourea derivatives, the C=S and NH groups. These sites provide interesting coordination modes and make thiourea derivatives very attractive candidates for several applications. The NH function is mainly responsible for the establishment of H-bonding and is able to make these compounds usable as sensors [7,8]. The C=S function has also been effectively used for sensor applications of metal ions (cations) [9,10] and has a very rich coordination chemistry with late transition metals [11,12]. Since the discovery FT-IR (KBr) ν(cm −1 ) = 3281br, 2961s, 2872s, 1741s.

Determination of Antioxidant Potentials
DPPH is a stable free radical and is capable of utilizing its unpaired electron in a chemical interaction with any other species. The involvement of this unpaired radical with foreign species is a function of free radical scavenging of that species/compound. Compounds 1-4 were tested for their antioxidant activities against DPPH. A solution containing 0.039 g/100 mL of DPPH was prepared and used as a standard/control. Absorbance of this solution was measured at 517 nm under normal conditions of temperature. After mixing the solution of the respective compound with DPPH solution, a change in the resultant absorbance was observed. The decrease in maximum absorbance of the solution was taken as a function of the compound. Prior to the determination of absorbance, all solutions were incubated in the dark for 30 min at room temperature (23 ± 1 • C). The percent inhibition capacity of each complex (I)%, was calculated as below [36].

Antibacterial Screenings of Selected Compounds
The agar well diffusion assay was used to evaluate antimicrobial potentials of complexes 1-4 [37,38]. Sterile, nutrient agar was prepared and poured in Petri dishes. Bacterial cultures were evenly applied on the surface of the agar Petri dishes by sterile swab sticks. Wells of 6 mm diameter were bored (five per plate) with a sterile borer. An amount of 6.25 mg/mL of each compound was applied to each well. The commercially available antibiotic cephradine was used for comparing the efficiency of each complex. For the sake of accuracy, the same concentration of each complex and the standard was applied. The agar plates were covered with lids and were incubated at 37 ± 1 • C for 24 h in an oven. Growth inhibition was observed in each bore and the respective zones were measured manually. The diameter of the zones of inhibition is a measure of antimicrobial activity of the corresponding complex. The data presented for antibacterial activity are the average diameter of the zones of inhibition in mm.

General Chemistry and Spectroscopy
The proposed structures of compounds and the corresponding ligand (inserted) are shown in Scheme 1. The treatment of ligand (1,3-diisobutylthiourea) with copper(I) chloride in molar ratio 3:1, respectively, afforded Complex 1. The reaction of CuCl with a two-fold excess of the ligand was carried out under identical conditions and the same products were obtained. By changing the reaction medium from acetonitrile to MeOH, all attempts were unsuccessful in obtaining exclusively tetrahedral complex of geometry [CuL 3 Cl], [CuL 2 Cl 2 ], or [CuL 4 ]Cl, where L = thiourea ligand [39][40][41]. The ligand reacted with CuI in a molar ratio 3:1 and the proposed compound 2 was thus obtained. These observations reveal that the presence of halogen function can affect the coordination sphere around the metal ion. The reaction of the ligand with Zn and Hg metal ions was straightforward, as was reported for structurally analogous species [42].
The diameter of the zones of inhibition is a measure of antimicrobial activity of the corresponding complex. The data presented for antibacterial activity are the average diameter of the zones of inhibition in mm.

General Chemistry and Spectroscopy
The proposed structures of compounds and the corresponding ligand (inserted) are shown in Scheme 1. The treatment of ligand (1,3-diisobutylthiourea) with copper(I) chloride in molar ratio 3:1, respectively, afforded Complex 1. The reaction of CuCl with a twofold excess of the ligand was carried out under identical conditions and the same products were obtained. By changing the reaction medium from acetonitrile to MeOH, all attempts were unsuccessful in obtaining exclusively tetrahedral complex of geometry [CuL3Cl], [CuL2Cl2], or [CuL4]Cl, where L = thiourea ligand [39][40][41]. The ligand reacted with CuI in a molar ratio 3:1 and the proposed compound 2 was thus obtained. These observations reveal that the presence of halogen function can affect the coordination sphere around the metal ion. The reaction of the ligand with Zn and Hg metal ions was straightforward, as was reported for structurally analogous species [42]. Complexes 1-4 were obtained as crystalline material from their respective reaction mixtures and structures of complexes were deduced from a consistent set of spectroscopic data. The FT-IR spectra of all compounds contain typically broad signals (3278-3288 cm −1 ) that correspond to the involvement of NH bonds in extensive hydrogen bonding. NMR spectroscopy is a reliable technique in the structural elucidation of compounds and is of particular importance in coordination chemistry, where the coordination behavior of thiourea ligands can easily be determined to be an N or S donor. The 1 H-NMR of complex 3 and 4 show distinct typically broad signals at 4.01 and 3.47 ppm, respectively, which can be assigned unambiguously to NH group. These signals show considerable shift with respect to the free ligand (5.89 ppm, given in supporting information). The 13 C-NMR process is quite useful in this regard: the C=S signal appeared at 181.7 ppm in the free thiourea ligand and was found to be considerably shifted in complex 3 and 4 to 174.8 and 176.9 ppm, respectively. These data indicate the bond formation through the S atom, thus affording the proposed coordination behavior of the TU ligand [43]. The mass fragmentation pattern of compounds 1-3 was also studied using the EI-MS technique. The information retrieved from these data were not too informative because of the high-energy ionization technique. All the complexes gave a base peak at m/z 188, which corresponds to the ion of the thiourea ligand. A molecular ion peak for these complexes was not observed.

Structural Description of Complex 1
Ethyl fragments in the structure of compound 1 (shown in Figure 1) suffer from some sort of disorder which makes it very difficult to precisely solve and refine the structure. The diffraction data provide enough information which is sufficient for the establishment of connectivity. The reaction of the ligand with CuCl in molar ratio 3:1 exclusively afforded compound 1. The molecule is monoclinic, bearing space group Cc (further details pertaining to refinements and crystal structure solution are summarized in Table 1). The Complexes 1-4 were obtained as crystalline material from their respective reaction mixtures and structures of complexes were deduced from a consistent set of spectroscopic data. The FT-IR spectra of all compounds contain typically broad signals (3278-3288 cm −1 ) that correspond to the involvement of NH bonds in extensive hydrogen bonding. NMR spectroscopy is a reliable technique in the structural elucidation of compounds and is of particular importance in coordination chemistry, where the coordination behavior of thiourea ligands can easily be determined to be an N or S donor. The 1 H-NMR of complex 3 and 4 show distinct typically broad signals at 4.01 and 3.47 ppm, respectively, which can be assigned unambiguously to NH group. These signals show considerable shift with respect to the free ligand (5.89 ppm, given in supporting information). The 13 C-NMR process is quite useful in this regard: the C=S signal appeared at 181.7 ppm in the free thiourea ligand and was found to be considerably shifted in complex 3 and 4 to 174.8 and 176.9 ppm, respectively. These data indicate the bond formation through the S atom, thus affording the proposed coordination behavior of the TU ligand [43]. The mass fragmentation pattern of compounds 1-3 was also studied using the EI-MS technique. The information retrieved from these data were not too informative because of the high-energy ionization technique. All the complexes gave a base peak at m/z 188, which corresponds to the ion of the thiourea ligand. A molecular ion peak for these complexes was not observed.

Structural Description of Complex 1
Ethyl fragments in the structure of compound 1 (shown in Figure 1) suffer from some sort of disorder which makes it very difficult to precisely solve and refine the structure. The diffraction data provide enough information which is sufficient for the establishment of connectivity. The reaction of the ligand with CuCl in molar ratio 3:1 exclusively afforded compound 1. The molecule is monoclinic, bearing space group Cc (further details pertaining to refinements and crystal structure solution are summarized in Table 1). The geometry around the metal ion is trigonal planar. Two thiourea ligands are attached to the metal ion through the S atom, which is normal behavior of thiourea derivatives [28]. Bond angles around the metal ion are 115.76(13) • , (S1-Cu1-S2), 120.94(11) • , and 123.30 (12) • (S1-Cu1-C1 and S2-Cu1-Cl, respectively). The sum of all three angles around metal ion is 360 • , which clearly supports the trigonal environment. The Cu-S bond lengths with Crystals 2021, 11, 989 5 of 12 negligible difference, 2.213(4) and 2.216(3) Å, are shorter than the Cu-Cl bond (2.272(3) Å) and are within the expected limit [44] (Table 2). Coordination with the metal ion through S reduces the bond order between C and S; therefore, it results in C=S bond elongation as compared to the uncoordinated thiourea derivative and are comparable to structurally analogous compounds [28,30,45,46]. A close view of molecules of complex 1 in solid state reveals that chloro function is actively involved in intra-as well as intermolecular secondary interactions. The intramolecular separation distance between N4· · ·Cl is 3.268 Å, and the (intermolecular) Cl· · ·N3 distance is 3.268 Å ( Figure 2). The difference in bond lengths, C1-N3 1.306 (14) and C1-N4 1.349 (14) Å, can possibly be explained on the basis of intramolecular interactions wherein N atoms of one coordinated thiourea ligand are involved ( Figure 2). Each molecule offers an N3 of a coordinated ligand to participate in secondary interactions, which is the probable reason for slightly different C-N bond lengths.  (14) N3-C1-S1 120.5(9) N4-C1 1.349 (14) N4-C1-S1 N2-C1-S1 120.03 (16) (14) N3-C1-S1 120.5(9) N4-C1 1.349 (14) N4-C1-S1  Table 2. Two of the ethyl groups show disorder, which is the reason for relatively large displacement parameters and unusual bond distances (C21A-C51 1.08(3), C21A-C32 1.68(4) and C14-C35 1.29(3) Å).

Structural Description of Complex 2
The structure of complex 2 is depicted in Figure 3, together with selected bond lengths and angles. The structure solution and refinements were carried out as per details

Structural Description of Complex 2
The structure of complex 2 is depicted in Figure 3, together with selected bond lengths and angles. The structure solution and refinements were carried out as per details given in Table 1. The crystal structure is trigonal with space group P-3. The Cu + ion is surrounded by three thiourea ligands and a halogen (iodide), making the tetrahedral geometry with expected deviation owing to hetero-ligands [47,48]. The ligand is attached through the S atom, as normally expected for this class of compounds. The angles S1-Cu1-S1 and S1-Cu1-I1 are 100.32(2) and 117.55 (16), respectively, with the sum of all six angles amounting to 653.6 • , which supports distorted tetrahedral geometry around the metal ion [49]. These features are within the expected limit and are very close to structurally analogous complexes [50,51]. The M-S bond distance of 2.336(6) Å for complex 2 is within the expected limit for analogous thiourea complexes [41]. The C=S bond slightly elongates (1.713(2) Å) with respect to the uncoordinated ligand (1.698 Å) because of electronic flow towards the metal ion. The abovementioned behavior of the ligand causes an electron delocalization over the S-C-N fragment [1,52]. There are a number of secondary short-range interactions within the molecules of the complex, which can be useful for further studies and in predicting certain applications of the complex. The hydrogen of C2H and N2H of each ligand are involved in interactions with S atoms of neighboring molecules. Each S atom of the ligand is pseudo-four-coordinated, and the supramolecular structure of complex 2 extends in a 3D fashion. given in Table 1. The crystal structure is trigonal with space group P-3. The Cu + ion is surrounded by three thiourea ligands and a halogen (iodide), making the tetrahedral geometry with expected deviation owing to hetero-ligands [47,48]. The ligand is attached through the S atom, as normally expected for this class of compounds. The angles S1-Cu1-S1 and S1-Cu1-I1 are 100.32(2) and 117.55 (16), respectively, with the sum of all six angles amounting to 653.6°, which supports distorted tetrahedral geometry around the metal ion [49]. These features are within the expected limit and are very close to structurally analogous complexes [50,51]. The M-S bond distance of 2.336(6) Å for complex 2 is within the expected limit for analogous thiourea complexes [41]. The C=S bond slightly elongates (1.713(2) Å) with respect to the uncoordinated ligand (1.698 Å) because of electronic flow towards the metal ion. The abovementioned behavior of the ligand causes an electron delocalization over the S-C-N fragment [1,52]. There are a number of secondary short-range interactions within the molecules of the complex, which can be useful for further studies and in predicting certain applications of the complex. The hydrogen of C2H and N2H of each ligand are involved in interactions with S atoms of neighboring molecules. Each S atom of the ligand is pseudo-four-coordinated, and the supramolecular structure of complex 2 extends in a 3D fashion. Figure 3. Molecular structure of compound 2 with partial numbering scheme. Thermal ellipsoid drawn at 50% probability level, hydrogens are omitted for simplicity, summarized bond lengths (Å) and angles (°) are given in Table 2.

Free Radical Scavenging Activities of Complexes 1-4
The inhibitory effects of thiourea complexes 1-4 were studied using DPPH as the free radical. For these complexes, which include S-coordinated 1,3-diisobutyl thiourea molecules, high antioxidant activity was discovered. The absorption intensity of DPPH was reduced by the addition of complexes 1-4, in a dose-dependent manner ranging from 00-100 ppm. The decrease in absorbance of the resultant solution was quite regular, as expected for an antioxidant reagent. The percent RSA (radical scavenging activity) of compounds was at its lowest at a lower concentration and was at its maximum at a 100 ppm concentration of each complex, in a dose-dependent manner. A comparison of the activity of 1-4 at 100 ppm (maximum concentration) was found to be 79.9%, 19.5%, 29.3%, and 19.2%, respectively, which indicates that compound 1 is more potent as compared to 2-4. Data pertaining to the free radical scavenging efficiency of complexes are summarized in Table 3 and the same are graphically described in Figure 4. The complex reveals a reasonable R 2 value of 0.9453, shown in Figure 5 (see Figures S1-S6 for complexes 2, 3 and 4) (see Supplementary Materials). Among these four complexes, 1 is a far better antioxidant and the activity can be observed by the naked eye due to the change in the color of the solution after mixing the appropriate concentration with DPPH ( Figure 6). The activity of compounds 2-4 were the least, and color change was difficult to be observed with the naked Figure 3. Molecular structure of compound 2 with partial numbering scheme. Thermal ellipsoid drawn at 50% probability level, hydrogens are omitted for simplicity, summarized bond lengths (Å) and angles ( • ) are given in Table 2.

Free Radical Scavenging Activities of Complexes 1-4
The inhibitory effects of thiourea complexes 1-4 were studied using DPPH as the free radical. For these complexes, which include S-coordinated 1,3-diisobutyl thiourea molecules, high antioxidant activity was discovered. The absorption intensity of DPPH was reduced by the addition of complexes 1-4, in a dose-dependent manner ranging from 00-100 ppm. The decrease in absorbance of the resultant solution was quite regular, as expected for an antioxidant reagent. The percent RSA (radical scavenging activity) of compounds was at its lowest at a lower concentration and was at its maximum at a 100 ppm concentration of each complex, in a dose-dependent manner. A comparison of the activity of 1-4 at 100 ppm (maximum concentration) was found to be 79.9%, 19.5%, 29.3%, and 19.2%, respectively, which indicates that compound 1 is more potent as compared to 2-4. Data pertaining to the free radical scavenging efficiency of complexes are summarized in Table 3 and the same are graphically described in Figure 4. The complex reveals a reasonable R 2 value of 0.9453, shown in Figure 5 (see Figures S1-S6 for complexes 2, 3 and  4) (see Supplementary Materials). Among these four complexes, 1 is a far better antioxidant and the activity can be observed by the naked eye due to the change in the color of the solution after mixing the appropriate concentration with DPPH ( Figure 6). The activity of compounds 2-4 were the least, and color change was difficult to be observed with the naked eye. The activity of compound 1 was better than the Cu complex stabilized by phosphine ligands, as has been reported recently [25]. eye. The activity of compound 1 was better than the Cu complex stabilized by phosphine ligands, as has been reported recently [25].    eye. The activity of compound 1 was better than the Cu complex stabilized by phosphine ligands, as has been reported recently [25].

Antibacterial Activity
The agar well diffusion method was used to assess the antimicrobial activity of complexes 1-4. Escherichia coli, Shigella flexneri, Pseudomonas aeruginosa, and Salmonella typhi were selected for this study and complexes 1-4 were studied against these strains. The antibacterial evaluation of these complexes reveal that they exhibit moderate activity in comparison to cephradine. The overall inhibition range of 6.2-13.25 mm (Table 4) was observed for these complexes. Compound 1 shows comparatively better activity against E. coli and P. aeruginosa, comparable with standard drugs, and compound 4 is most active against three strains: E. coli, S. typhi and S. flexneri. Compounds 2 and 3 were found to be poor antibacterial agents among these four complexes. The activity of all complexes and the standard is graphically represented in Figure 7. Table 4. Antimicrobial activities of compounds 1-4, ZI were measured in mm.

Antibacterial Activity
The agar well diffusion method was used to assess the antimicrobial activity of complexes 1-4. Escherichia coli, Shigella flexneri, Pseudomonas aeruginosa, and Salmonella typhi were selected for this study and complexes 1-4 were studied against these strains. The antibacterial evaluation of these complexes reveal that they exhibit moderate activity in comparison to cephradine. The overall inhibition range of 6.2-13.25 mm (Table 4) was observed for these complexes. Compound 1 shows comparatively better activity against E. coli and P. aeruginosa, comparable with standard drugs, and compound 4 is most active against three strains: E. coli, S. typhi and S. flexneri. Compounds 2 and 3 were found to be poor antibacterial agents among these four complexes. The activity of all complexes and the standard is graphically represented in Figure 7.  Figure 6. Naked eye detection of DPPH radical scavenging activity of compound 1. Sample flask to the right-hand side contains free DPPH, and concentration of compound 1 increases in the preceding flasks from right to left.

Antibacterial Activity
The agar well diffusion method was used to assess the antimicrobial activity of complexes 1-4. Escherichia coli, Shigella flexneri, Pseudomonas aeruginosa, and Salmonella typhi were selected for this study and complexes 1-4 were studied against these strains. The antibacterial evaluation of these complexes reveal that they exhibit moderate activity in comparison to cephradine. The overall inhibition range of 6.2-13.25 mm (Table 4) was observed for these complexes. Compound 1 shows comparatively better activity against E. coli and P. aeruginosa, comparable with standard drugs, and compound 4 is most active against three strains: E. coli, S. typhi and S. flexneri. Compounds 2 and 3 were found to be poor antibacterial agents among these four complexes. The activity of all complexes and the standard is graphically represented in Figure 7.

Conclusions
Four new heteroleptic transition metal complexes (1-4) were successfully obtained by reacting 1,3-diisobutyl thiourea with CuCl, CuI, ZnCl 2 , and HgI 2 , respectively. Despite the same reaction conditions, reaction with CuCl and CuI afforded bis-ligated and tris-ligated copper halide complexes, respectively. Compounds 1 and 2 were also characterized by single-crystal X-ray analysis. The Zn and Hg metals were included in the study with the intent to explore the coordination behavior of the ligand, and the proposed geometries were obtained. All complexes were tested for their free radical scavenging ability and antimicrobial potentials; the data reveal that compound 1 is an excellent antioxidant and the activity could be observed with the naked eye. All compounds showed antioxidant potentials in a dose-dependent manner. In comparison to the standard, compound 1 was active against E. coli and P. aeruginosa, while compound 4 was a potent antibacterial agent against E. coli, S. typhi and S. flexneri. Future work regarding functionalization can explore potentials of thiourea complexes as antioxidant and antibacterial reagents.  Figure S1. Absorption spectra of DPPH in the absence of compound 2 (Top spectra) and presence of increasing concentration of the compound (20, 40, 60, 80 & 100 ppm). Arrow shows the change in absorption as a function of activity with respect to increasing concentration of compound 2. Figure S2. Plot of % Inhibition versus concentration of compound 2 for radical scavenging activity. Figure S3. Absorption spectra of pure DPPH, the absorbance decreased when compounds 3 was added, in a dose dependent manner. Figure S4. Percent inhibition versus concentration of compound 3 for radical scavenging activity. Figure S5. Absorption spectra of free radical (DPPH) in the absence (Top spectra) and presence of increased concentration of the compound 4 (20, 40, 60, 80 & 100 ppm). Arrow show the change in spectra on increasing concentration of compound. Figure S6. Plot of % Inhibition versus concentration of compound 4 for radical scavenging activity.