In the present study, Mn(II), Fe(II), Ni(II), and Cu(II) complexes of N-benzoyl -N′-2-thiophenethiocarbohydrazide (H2 BTTH) have been synthesized and characterized by elemental analysis, magnetic susceptibility measurements, infrared, NMR, electronic, and ESR spectral studies. The complexes were found to have compositions [Mn(H BTTH)2], [Ni(BTTH)(H2O)2], [Cu(BTTH)], and [Fe(H BTTH)2EtOH]. The antibacterial and antifungal properties of H2 BTTH and its metal complexes have been screened against several bacteria and fungi.

1. Introduction

The expansion of research in the coordination chemistry of nitrogen-sulphur donor ligands such as substituted thiosemicarbazides [1], thiosemicarbazones [24], and dithiocarbazates [5], during the recent years has been due to their remarkable antineoplastic activity against a variety of tumors [6] in addition to their applicable antifungal [7] and antibacterial [8] activities. Sulphur and nitrogen containing ligands and their transition metal complexes were also used as corrosion inhibitors [9, 10] and extreme pressure lubricant additives [11]. Keeping in view the above biological activity of the sulfur and nitrogen containing ligands, we planned to undertake the synthesis, characterization, antibacterial, and antifungal activity of N-benzoyl-N′-2-thiophenethiocarbohydrazide (H2 BTTH) (Figure 1) and its Mn(II), Fe(II), Ni(II), and Cu(II) complexes. This ligand is expected to form addition complexes without loss of protons, and deprotonated complexes by loss of one or both the hydrazinic protons.

2. Experimental

2.1. Starting Materials

All the chemicals used were of analytical grade. Ammonium polysulphide [12] and carboxymethyl-2-thiophenedithioate [13] were prepared by literature methods.

2.2. Preparation of N-benzoyl-N′-2-thiophenethiocarbohydrazide (H2 BTTH)

N-benzoyl-N′-2-thiophenethiocarbohydrazide (H2 BTTH) was prepared by mixing solutions of benzoic acid hydrazide (20 mmol) and carboxymethyl-2-thiophenedithioate (20 mmol) each dissolved separately in 50 mL of 0.5 N NaOH and allowing the mixture to stand at room temperature for 2 hrs. The product precipitated by adding dilute AcOH dropwise to the above ice-cold mixture, was filtered off, washed with H2O, dried, and recrystallized from EtOH.

2.3. Synthesis of [Mn(H BTTH)2]

The complex [Mn(H BTTH)2] was prepared by boiling together the methanolic solutions (25 mL) of Mn(OAc)2 · nH2O (1 mmol) and H2BTTH (2 mmol) for 1 h under reflux. The precipitated complexes were filtered, washed with methanol, and dried in vacuo.

2.4. Synthesis of [Ni (BTTH)(H2O)2] and [Cu (BTTH)]

The complexes [Ni (BTTH)(H2O)2] and [Cu (BTTH)] were synthesized by boiling together the methanolic solutions (25 mL) of the respective M(OAc)2 · 2H2O [M = Ni(II), Cu(II)] (1 mmol) and H2BTTH (1 mmol) for about 1 h under reflux. The precipitated complexes were filtered, washed with methanol and dried in vacuo.

2.5. [Fe(HBTTH)2(EtOH)]

[Fe(HBTTH)2(EtOH)] was prepared by boiling together an aqueous-methanolic solution (25 mL) of (NH4)2SO4·FeSO4·6H2O (1 mmol) and H2BTTH (2 mmol) in ethanol (25 mL) for about 1 h. The precipitated complex was filtered, washed with water, ethanol and dried in vacuo.

2.6. Instrumentation

Complexes were analyzed for their metal content, following a standard procedure [14] by decomposing the complexes with a mixture of HNO3 and HCl followed by H2SO4. Sulfur and chloride were determined as BaSO4 and AgCl, respectively. Carbon, hydrogen, and nitrogen were estimated on EA 1108 CHN Elemental Analyzer. Magnetic susceptibility measurements were made at room temperature on a Cahn-Faraday balance using Hg [Co (NCS)4] as calibrant. Electronic spectra were recorded on a CARY-2390 UV-Visible Spectrophotometer as Nujol mulls [15]. IR Spectra were recorded in the 4000–400 cm−1 region (KBr disc) on a JASCO FT/FR-5300 spectrophotometer. The 1H and 13C NMR spectra were obtained in DMSO-D6 on a JEOL FX-300 Q FT/NMR spectrometer using TMS as internal reference. ESR spectra were recorded on a X-band spectrometer model EPR-112 using DPPH as a marker. The electrical conductivity of the pressed pellets of the complexes was obtained by a conventional two-probe method in the 303–383 K range with contact made on the pellet surfaces using graphite paint.

2.7. Bactericidal Screening

The antibacterial activity of the ligand and the complexes was evaluated using the disc diffusion technique [16]. A stock solution of 2000 μg cm−3 was made by dissolving 2 mg cm−3 of each compound in DMSO and it was serially doubled diluted up to five dilutions, giving the concentrations of 1000, 250, 125, and 62.5 μg cm−3. Filter paper (Whattman no.42) discs (6 mm dia) were soaked in these solutions of different concentrations and placed on nutrient agar plates. The plates were then incubated for 24 hrs at 37°C. The inhibition zones around the discs were measured after 24 hrs. Co-trimoxazole was used as a standard drug in the form of disc, containing trimethoprim = 1.25 μg and sulfamethoxazole = 23.75 μg per disc. The zones of inhibition were found to be 20, 32, and 18 mm against Staph aureus, Escherichia coli, and Pseudomonas aeruginosa, respectively, in agreement with the sensitive zone reported in the literature.

2.8. Fungicidal Screening

The antifungal activity was evaluated by a drug dilution technique. The solution of the test compounds were prepared as described earlier to which sabouraud’s dextrose broth and slightly turbid suspension of fungus in normal saline (10 μL) were added and placed in an incubator for 48–72 hrs. A turbidity in the solution indicated the growth of fungus, which is represented as sign; however, a clear solution showed that there was no growth of fungus and is represented as + sign. The cases where compounds showed antifungal activity, no growth was observed in the solution. Amphotericin B was used as a standard drug.

3. Results and Discussion

All the complexes are insoluble in water, methanol, and ethanol but are soluble in polar organic solvents such as DMSO and DMF. It was determined by Job’s method that the complexes having 1 : 1 metal-ligand stoichiometry (Table 1) are formed by loss of two protons from the ligand, generating a conjugated system. Because of steric considerations, all the four potential sites cannot be attached to a single metal and, therefore, the ligand binds in a polymeric fashion. The following equations represent the formation of the ligand and the complexes:

3.1. Magnetic Moments and Electronic Spectra

The magnetic moments and electronic spectral data of the complexes are given in Tables 1 and 2, respectively. [Mn(H BTTH)2] shows a magnetic moment, 5.76 B.M. and exhibits a band at 16140 cm−1 assigned to the 64 transition (Figure 2) for the octahedral geometry [17]. The magnetic moment 5.31 B.M. and presence of bands at 11770 and 20840 cm−1 owing to the 5T25B1 and charge-transfer transitions, respectively, suggests a high-spin square pyramidal geometry for [Fe(HBTTH)2(EtOH)]. Ni(BTTH)(H2O)2] exhibits a magnetic moment of 3.06 B.M. and shows a band at 16140 cm−1 (Figure 2) attributed to the 3 (F)3 (F)() transition for a distorted octahedral geometry around Ni(II). [Cu(BTTH)] shows a magnetic moment of 1.98 B.M. indicating the presence of one unpaired electron. The complex shows a d-d band at 15635 cm−1 due to the envelope of the 22, 2, and 2Eg transitions, usually observed for square planar Cu(II) complexes [17].

3.2. IR Spectra

The important IR spectral bands and their assignments are given in Figure 3 and Table 3. The IR spectrum of H2BTTH shows bands at 3125 and 3100 cm−1 due to the presence of two NH groups. The bands at 1640, 1460, 1325, 1000, and 835 cm−1 are assigned to ν((C=O), thioamide I [β(NH) + ν(CN)], thioamide II [ν(CN) + β(NH)], ν(N–N) and ν(C=S), respectively. The spectra of [Mn(HBTTH)2] and [Fe(HBTTH)2(EtOH)] show only one peak at 3125 cm−1 due to ν(NH), suggesting loss of one hydrazinic proton via enolisation/thioenolisation. A strong band at 1640 cm−1 in the ligand due to ν(C=O) is found to be absent in these complexes, and in place of this a new band due to ν(N=C) of NCO appears, suggesting that enolic oxygen is involved in bonding. [Mn(HBTTH)2] shows a negative shift of 15 cm−1 in ν(C=S), suggesting an additional bonding through thione sulfur. Furthermore, the spectra of these complexes show a positive shift of 20 cm−1 in ν(N–N) indicating that one hydrazinic nitrogen is also involved in bonding. The presence of ν(NH) at 3180 cm−1 and ν(C=O) at 1625 cm−1 in [Cd(HBTTH)2] indicates loss of NH proton via thioenolisation. Further, the bands at 1460, 1325, and 1000 cm−1 due to thioamide I, II and ν(N–N) undergo positive shifts of 25, 15, and 20 cm−1, respectively, showing the involvement of thiolato sulfur [18] and one hydrazinic nitrogen in bonding.

The IR spectra of [M(BTTH)(H2O)2] (M = Co(II), Ni(II)) and [M(BTTH)] (M = Cu(II) and Zn(II)) show the absence of both the ν(NH), ν(C=O), and ν(C=S) bands and in place of these two new bands appear at 1580 and 745 cm−1, due to ν(N=C) of NCO and ν(C-S) modes, respectively, suggesting that both –NH–NH protons are lost via enolisation and thioenolisation and bonding of the resulting enolic oxygen and thiolato sulfur takes place with the metal ion. Further, the thioamide I, II and ν(N–N) bands at 1460, 1325, and 1000 in the free ligand undergo a positive shift of 33, 65, and 70 cm−1, respectively, in the spectra of the complexes [19] suggesting the involvement of these groups as bonding sites. These observations show the involvement of thiolato sulfur and both the hydrazinic nitrogens, in addition to the enolic oxygen in bonding. Thus, H2BTTH acts as a binegative tetradentate ligand in the 1 : 1 complexes.

3.3. NMR Spectra

The 1H NMR spectrum of H2BTTH (Figure 4, Table 4) in DMSO-d6 shows a signal at δ 9.88 and 9.24 ppm due to the presence of –NH–NH– protons which are lost on D2O exchange. The protons due to the thiophene ring appear at δ 7.12 (s,1H), 7.32 (d,1H), and 7.56 (s,1H) ppm and the benzene ring protons appear as a multiplet at at δ 6.64–6.92 (m,5H) ppm [20].

The NH signals are absent in the 1H NMR spectrum of [Ni(BTTH)(H2O)2], (Figure 4, Table 4) suggesting loss of both NH protons via enolization and thioenolization. The thiophene ring protons show three separate signals at δ 7.08 (q, 1H), 7.28 (d, 1H), and 7.52 (s, 1H) ppm and the benzene ring protons are observed at δ 6.61–6.88 (m,5H) ppm.

The 1H NMR spectrum of [Mn (H BTTH)2] shows separate signals for the thiophene ring protons at δ7.04 (s, 1H), 7.30 (d, 1H), and 7.51 (s, 1H) ppm and for benzene ring protons at δ 6.62–6.90 (m,5H) ppm. The two signals appearing at δ 9.42 and 4.16 ppm are due to the presence of –NH–C(O) and –N=C(SH) protons, respectively (Figure 4, Table 4). The latter is formed due to the thioenolization of the ligand. All the thiophene and benzene ring protons are observed nearly at the same position in the complex as compared to those of the H2 BTTH, suggesting non-involvement of the ring sulphur in bonding.

The 13C NMR spectrum of [H2 BTTH] (Figure 5, Table 5) shows eleven signals, of which two signals at δ 181 and 163 ppm are due to the C=S and C=O carbons, respectively. The chemical shifts for the benzene and thiophene ring carbons in H2 BTTH are (δ, ppm) C(2, 6) 151, 146, C(4) 136, C(3,5) 126, 122, C′(2) 150, C′(3) 116, C′(4) 112, and C′(5) 144.

The 13C NMR spectrum of [Ni(BTTH)(H2O)2] also shows eleven signals (Figure 5, Table 5). The chemical shifts for the ring carbons are: (δ, ppm) C(2,6) 150, 147, C(4) 134, C(3,5) 130, 122, C′(2) 150, C′(3) 116, C′(4) 112, and C′(5) 144. The signals at δ 188 and 171 ppm in [Ni(BTTH)(H2O)2] show downfield shift of 7 and 8 ppm, respectively, as compared to the ligand, suggesting the formation of C–S and C–O groups from the ligand on thioenolization and enolization, respectively.

The 13C NMR spectrum of [Mn (H BTTH)2] shows separate signals for all eleven carbons (Figure 5, Table 5). The chemical shifts for the ring carbons are: (δ, ppm) C(2,6) 152, 148 C(4) 135, C(3,5) 128, 123, C′(2) 150, C′(3) 117, C′(4) 112, and C′(5) 146. The signal at 181 ppm due to C=S carbons shows nearly the same chemical shifts as for the ligand, suggesting that the double bond character is retained in the complex. The signals at δ 174 ppm in the complex show downfield shift of 11 ppm as compared to the ligand, suggesting the the involvement of C=S in bonding.

3.4. ESR Spectra

The room temperature solid state ESR spectrum of [Cu(BTTH)] yields a broad signal with value of 2.02 characteristic of square planar geometry around Cu(II) (Figure 6).

3.5. Bactericidal Screening

The antibacterial activity of H2 BTTH (Table 6) and its complexes has been tested against S. aureus, E. coli, and P. aeruginosa. H2 BTTH, [Mn (H NTTH)2] and [Cu(BTTH)] show antibacterial activity starting from 62.5 to 2000 μg cm−3 against S. aureus and the activity increases with an increase in the concentration. The ligand is found to be active at a concentration of 1000 μg cm−3 against E. coli. [Mn (H NTTH)2] and [Cu(BTTH)] also show activity against E. coli at 500 and 250 μg cm−3, respectively. Only [Cu(BTTH)] has been found active against P. aeruginosa at 250 μg cm−3.

3.6. Fungicidal Screening

The antifungal screening results (Table 7) show that free H2 BTTH is active from 125 to 2000 μg cm−3 against C. kefri. However, [Mn (HBTTH)2] and [Cu(BTTH)] are found to be active even at a lower concentration of 62.5 μg cm−3 against C. kefri. H2 BTTH is active from 250 to 2000 μg cm−3 against C. albicans. However, [Mn (HBTTH)2] and [Cu(BTTH)] are found to be active even at a lower concentration of 62.5 μg cm−3 against C. albicans.

On the basis of physicochemical studies and the foregoing discussion, the proposed structures of the complexes is shown in Figure 7.

4. Conclusion

The magnetic and electronic spectral studies suggest square planar geometry for [Cu(BTTH)] and octahedral geometry for rest of the complexes. The infrared spectral studies of the 1 : 1 deprotonated complexes suggest bonding through enolic oxygen, thiolato sulfur, and both the hydrazinic nitrogens. Thus, H2BTTH acts as a binegative tetradentate ligand. The ESR spectrum of [Cu(BTTH)] shows that the unpaired electron is present in the orbital of Cu(II).


Financial assistance from Indian School of Mines, Dhanbad under FRS to M. Yadav is gratefully acknowledged.