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Synthesis, characterization and biological properties of Co(II), Ni(II), Cu(II) and Zn(II) complexes with an SNO functionalized ligand
*Tel.: +970 82839314 ssaadeh2003@yahoo.com (Salman M. Saadeh)
-
Received: ,
Accepted: ,
This article was originally published by Elsevier and was migrated to Scientific Scholar after the change of Publisher.
Peer-review under responsibility of King Saud University.
Available online 8 October 2010
Abstract
Some new metal(II) complexes, ML2[M = Co, Ni, Cu and Zn], of 2-acetylthiophene benzoylhydrazone ligand (HL) containing a trifunctional SNO-donor system have been synthesized and characterized on the basis of physicochemical data by elemental analysis, magnetic moment, molar conductance, thermogravimetric and spectroscopic (electronic, IR, 1H NMR and 13C NMR) data. The ligand functions as monobasic SNO tridentates where the deprotonated enolic form is preferred in the coordination producing distorted octahedral complexes.
Keywords
Metal(II) complexes
Schiff bases
SNO-donor ligand
2-Acetylthiophene benzoylhydrazone
Thermal investigation
Biological activity
1 Introduction
Schiff base ligands are considered as “privileged ligands” (Singh et al., 2010) because they are easily prepared by condensation between aldehydes and primary amines. These ligands are able to coordinate many different metals and to stabilize them in various oxidation states (Abdel-Latif et al., 2007). The chemistry of transition metal complexes with Schiff bases has played an important role in the development of coordination chemistry as a whole. Multidentate Schiff bases have been widely used as ligands, because they can be easily attached to metal ions due to the formation of highly stable coordination compounds. These complexes show a large variety of catalytic (Youssef et al., 2009), biological (Patil et al., 2010), antifungal (Devi et al., 2004), anti tumor and anti HIV activities (Sheng et al., 2008). Furthermore, Schiff bases are utilized as starting material in the synthesis of industrial (Ando et al., 2004) and biological compounds such as β-lactams (Taggi et al., 2002). Furthermore, these compounds have been recently used in construction of poly vinyl chloride powder (PVC)-based membrane selective sensors (Mashhadizadeh et al., 2004) and as ionophore in metal ion-selective electrodes (Abu-Shawish et al., 2007). Metal complexes of S-, N-, and O-chelating ligands have attracted considerable attention because of their interesting physico-chemical properties, pronounced biological activities and their use as models for metalloenzyme active sites (Abu-Shawish et al., 2009). These results motivated making the present study that deals with the preparation and characterization of complexes of 2-acetylthiophene benzoylhydrazone (Fig. 1) containing an SNO donor system with some divalent metal ions, namely Co(II), Ni(II), Cu(II) and Zn(II) as well as their biological activity due to their potentially useful properties. The solid complexes have been synthesized and characterized by elemental analysis, conductivity, thermo gravimetric analysis (TG), IR, NMR and electronic spectra. These complexes were found to have similar properties to those found with an analogous NNO functionalized ligand (Patil et al., 2010) and an SNO donor ligand (Jang et al., 2005; Chohan, 2001).
Proposed structure of 2-acetylthiophene benzoyl hydrazone.
2 Experimental
2.1 Material and methods
2-Thiophenecarboxaldehyde and benzoylhydrazine were obtained from Merck. Reagent grade salts of all cations were purchased from Aldrich and used as received. Commercially available solvents were used as received. The infra red spectra for the materials were recorded on a Perkin–Elmer FT-IR spectrometer using KBr disc in the range of 4000–400 cm−1. Melting points were recorded on a Gallenkamp apparatus and are uncorrected. UV–Vis spectra of methanolic solutions of the complexes were recorded on a SHIMADZU UV–Vis spectrophotometer (UV-1601). Conductivity measurements were made on a PHYWE conductivity meter. The metal content of the complexes was determined using a Perkin–Elmer A Analyst-100. The complexes were decomposed and digested in concentrated sulfuric and/or nitric acid before determination of the metal ions in their aqueous solutions (by atomic absorption, spectrophotometric or titrimetric method) (Vogel, 1987). Microanalysis of C, H, N and S was performed at the Microanalytical Center – Cairo University – Egypt. 1H NMR and 13C NMR spectra were recorded on a Bruker 250-MHz spectrometer in DMSO-d6. Chemical shifts are in ppm relative to internal Me4Si. Magnetic measurements were done on the solid complexes using the Gouy method.
2.2 Thermal analysis
The thermogravimetric analysis of the solid complexes was carried out from room temperature to 800 °C using a Shimadzu-50 thermal analyzer (Sheb, 2009).
2.3 Synthesis of the Schiff base ligand
2-Acetylthiophene benzoylhydrazone: The ligand was prepared from 2-acetyl thiophene and benzoylhydrazine similar to that described in the literature (Pelagatti et al., 2003). The product was characterized by its infrared and its 1H and 13C NMR spectra. HL C13H12N2OS, IR (cm−1): 3245 (s, NH), 1705 (m, NH+CN), 1635 (s, CH⚌N), 1020 (s, N–N) and 752–687 (C–S–C) where s: sharp; m: medium; br: broad. Anal. Calcd. C, 63.91; H, 4.9 5; N, 11.47, and S, 13.12%. Found: C, 63.59, H, 5.02; N, 13.15; S, 13.69, yield 84%, 1H NMR (DMSO-d6) spectra 10.6 (s, 1H, NH), 6.5–8.2 (m, 8H, Ar–H), 1.92 (s, 3H, CH3), 13C NMR: 187.2 (C⚌O), 152.1 (C⚌N), 139.2, 129.5, 127.0, 125.2, 123.6, 121.4, 112.7, 104.9 (Ar–C), 15.6 (CH3).
2.4 Preparation of metal complexes
To a well-stirred suspension of 2-acetylthiophene benzoylhydrazone (1.0 mmol, 0.244 g) in methanol (25 mL) was added the metal acetate, namely Co(OAc)2·4H2O, Ni(OAc)2·4H2O, copper(OAc)2·H2O and Zn(OAc)2·2H2O, (0.5 mmol) solid. The resulting mixture was stirred for 24 h during which a solid complex appeared. The solid was collected by filtration, washed with methanol, diethyl ether and dried in vacuo (Jang et al., 2005).
[CoL2] (1): tan solid. Yield: 0.178 g (65%). mp. 180–182 °C. Anal. Calcd. for C26H22N4O2S2Co: C, 56.62; H, 5.12; N, 10.18, S, 11.63. Found: C, 56.45; H, 5.07; N, 10.09; S: 11.69, ΛM (Mho cm2 mol−1): 0.23, and for chloroform solvent 0.36. UV–Vis. (nm, log ɛ): shoulder at 580 nm (3.48), 354 (4.34) and 260 (4.48). Significant infrared bands (cm−1): 1575, 1500, 1365, 1148, 1065, 1018, 894, 710, 527.
[NiL2] (2): green solid. Yield: 0.192 g (69%). mp. > 320 °C (dec.). Anal. Calcd. for C26H22N4O2S2Ni: C, 56.64; H, 5.10; N, 10.10; S, 11.60. Found: C, 56.75; H, 5.13; N, 10.18; S, 11.56. ΛM (Mho cm2 mol−1): 2.1, 7.26 and for DMF solvent 12.48, UV–Vis (nm, log ɛ): 400 (3.49), 318 (4.00) 254 (4.38). Significant infrared bands (cm−1): 1591, 1510, 1477, 1370, 1294, 1182, 1078, 1014, 904, 796, 752, 694, 457.
[CuL2] (3): olive green solid. Yield: 0.150 g (54%). mp. 295–297 °C (dec.). Anal. Calcd. for C26H22N4O2S2Cu: C, 56.15; H, 5.07; N, 10.07; S, 11.53. Found: C, 56.01; H, 5.06; N, 10.12; S, 11.58. ΛM (Mho cm2 mol−1): 0.38. UV–Vis (nm, log ɛ): 370 (3.34), 354 (3.32) 260 (3.37). Significant infrared bands (cm−1): 1586, 1519,1378, 1301, 1171, 1067, 1019, 904, 792, 703, 435.
[ZnL2] (4): yellow solid. Yield: 0.168 g (60%). mp. 270–272 °C. Anal. Calcd. for C26H22N4O2S2Zn: C, 55.96; H, 5.06; N, 10.04; S, 11.49. Found: C, 56.11; H, 5.07; N, 10.16; S, 11.52. ΛM (Mho cm2 mol−1): 6.78. UV–Vis (nm, log ɛ): 328 (3.34) and 268.5 (3.15). Significant infrared bands (cm−1): 1581, 1491, 1425, 1372, 1305, 1167, 1068, 710, 454. 1H NMR (in CDCl3, ppm): 7.18–8.36 (br, 8H, ArH), 2.84 (s, 3H, CH3). NMR spectral data of ZnL2 1H NMR (ppm), 7.2–8.3 (br, ArH), 2.86 (s, 3H, CH3), 13C NMR: 169.14, 150.65, 136.0, 134.78, 134.29, 131.49, 130.23, 127.98, 125.8, 120.58, 40.33, 40.06, 39.77, 39.5, 39.22, 38.94, 38.67, 37.51, 37.2, 21.67.
2.5 Antibacterial studies
The newly synthesized complexes were tested for their antibacterial ad antifungal activity in vitro against bacterial strains such as Escherichia coli, Staphylococcus aureus and Pseudomonas employing the nutrient agar disc diffusion method at 1–10 mg/mL in N,N-dimethylformamide (DMF) by measuring the inhibition zone in millimeters (mm) (Saadeh et al., 2009).
3 Results and discussion
The reactions of appropriate metal(II) acetate with the Schiff base ligand derived from 2-acetylthiophene and benzoylhydrazone acting as tridentate ligand in methanol solution gave monomeric transition–metal(II) complexes, of MIIL2 type (M = Co, Ni, Cu, Zn and L = 2-acetylthiophene benzoylhydrazone), respectively. The formulations were in accordance with the data of elemental analysis and physicochemical measurements. The molar conductance of the complexes determined at a concentration of ca. 1 × 10−3 M in chloroform for cobalt and nickel complexes and in DMSO for copper and zinc complexes show small values in the range 1.0–6.78 Mho cm2 mol−1, respectively, indicating that the complexes are non-electrolytes. The complexes are soluble in chloroform or in dimethyl formamide, and dimethyl sulfoxide, but are not soluble in common organic solvents.
3.1 Spectral properties
The IR spectrum of the ligand has several prominent bands appearing at 3245, 1705 and 1635 cm−1 due to νN–H and C⚌O stretching modes, respectively. Comparison of the IR spectra of the Schiff base and its metal complexes, such as Fig. 2 the infrared spectrum of the zinc complex, indicates that the Schiff base is coordinated to the metal atom by three sites, thus suggesting that the ligand acts as a tridentate ligand. The band due to ν(C⚌O) is completely missing in the spectra of the complexes, suggesting (Patil et al., 1982) enolization of the Schiff base on complexation. This is also supported by the fact that no band for ν(OH) in the spectra of the ligand and also in the complexes is observed. Instead, a band due to ν(C–O) at about 1245 cm−1 was observed for all the complexes, which supports the observation of their enolization during coordination. This fact suggests that the Schiff base remains in the keto form in the solid state, but in solution both the keto and enol forms remain in equilibrium (Dey and Bandyopadhyay, 1991). Deprotonation occurs from the enol form on complexation.
The infrared spectrum of the zinc(II) complex.
The amide-II band[ν(NH+CN)] was split, displaced to higher frequency and reduced in intensity. A shift to higher frequency (5–10 cm−1) of the ν(N–N) band at about 1020 cm−1 and its splitting indicated (Yongxiang et al., 1989) coordination of the azomethine nitrogen. Moreover, the shift to lower frequency (5–10 cm−1) of the band due to the azomethine ν(CH⚌H) linkage at about 1635 cm−1 also indicated involvement of azomethine group in coordination.
The ν(C–S–C) undergoes a negative shift in the complexes, indicating the coordination of the ring sulfur to the metal. The coordination through the azomethine nitrogen and sulfur is further supported by the occurrence on few bands at 460 and 360 cm−1 in the spectra of the complex, which may be assigned to ν(N → M) and ν(S → M) vibrations, respectively (Turan et al., 2010). However, the IR spectrum of the complex (M–S) bands could not be observed because the infrared spectra of the complexes were studied in the range 4000–400 cm−1, which does not span the M–S stretching mode.
The NMR spectra of the free ligand and its zinc(II) complex (Fig. 3) have been recorded in DMSO-d6. NMR spectra of the free ligand support the conclusions derived from the IR spectra. The ligand exhibits aromatic and heteroaromatic proton signals at δ 6.5–8.2 ppm. The signal for the NH proton appeared at δ 10.9 ppm (s, br) in the spectra of the free Schiff base ligand; this signal is absent in the spectrum of its zinc(II) complex. This indicated that the Schiff base is coordinated to the metal ions in the enolic form by deprotonation.
The 1H-NMR spectrum of the zinc(II) complex.
In the Zn(II) complex, the aromatic and heteroaromatic proton signals appeared downfield, due to increased conjugation on coordination (Hong-Yun et al., 1992). 13C NMR spectra likewise showed similar diagnostic features (Pasto and Johnson, 1969) for the free ligand as well as for its complexes. Thienyl carbons were found in the range 104.5–123.2 ppm, aromatic carbons at 124.8–137.3 ppm and the azomethine carbon signal was found at 150.65 ppm. Similarly, the presence of the C⚌O signal at about 187.4 ppm in the spectrum of the ligand and the absence of this signal in the spectrum of the complex and the appearance of a signal at a lower value supported the evidence that the ligands act in an enolized form. However, 1H NMR spectrum for the complexes 1–3 was not helpful for identification of functional group due to paramagnetic electron configuration.
The cobalt(II) complex shows a magnetic moment value of 4.87 BM at room temperature. This high value of the magnetic moment and the stoichiometry suggest a coordination number of six for the central cobalt(II) ion and an octahedral geometry. The electronic spectrum of this complex is also consistent with its octahedral environment around the cobalt(II) ion. The spectra display two bands at 354 and 260 nm attributed to 4T1g (F) → 4T2g (F), 4T1g (F) → 4A2g (F) and 4T1g (F) → 4T1g (P) transitions, respectively, in a high-spin octahedral geometry (Chohan, 2001). The electronic spectra of the nickel(II) complex exhibit absorption bands at 400 and 318 and 254 nm, attributable to 3A2g → 3T2g (F), 3A2g → 3T1g (F) and 3A2g → 3T1g (P), transitions, respectively, in an octahedral geometry (Chandra, 1985). The calculated values of the ligand field parameters lie in the range reported for an octahedral structure. The value of the magnetic moment (3.2–3.7 BM) may be taken as additional evidence for its octahedral structure. The copper(II) complex exhibits magnetic moment of 1.71 BM, respectively, at room temperature. This value is quite close to the spin-allowed value expected for a S = 1/2 system and may be indicative of a distorted octahedral geometry around copper(II) ions. The copper(II) complex displays a broad band at 370 nm due to 2B1g → 2Eg and two bands at 354 and 260 nm assigned to d–d transitions and a charge transfer band, respectively, of a distorted octahedral environment (Hong-Yun et al., 1992). The diamagnetic zinc(II) complex did not show any d–d bands and its spectrum is dominated only by a charge transfer band. The charge transfer band at 328 nm was assigned to the transition 2Eg → 2T2g, possibly in an octahedral environment (Chohan, 2001). The band at 268 nm is likely to be an intraligand transition for it is observed for the free ligand. On the basis of the above observations, it is tentatively suggested that all of the complexes show an octahedral geometry (Fig. 4) in which the two ligands act as tridentates. These possibly accommodate themselves around the metal atom in such a way that a stable chelate ring is formed giving, in turn, stability to the formed metal complexes.
Proposed structure of the metal(II) complexes.
The results obtained in this study justify the structure proposed to MIIL2 complexes. These results are the same as those found for the crystallographically determined structure and spectroscopically confirmed by Jang et al. (2005).
The thermal behavior of Co(II), Ni(II), Cu(II) and Zn(II) complexes has been studied as a function of temperature. The thermal behavior of all the complexes is almost same. The thermal decomposition of Co(C13H11N2OS)2, Ni(C13H11N2OS)2, Cu(C13H11N2OS)2 and Zn(C13H11N2OS)2 takes place in two major steps as indicated by DTG peaks around 272–350 °C and 418–550 °C. These show the mass loss of the Schiff base moiety. That is the metal complexes decompose gradually with the formation of metal oxide above 600 °C. The nature of proposed chemical change with the temperature range and the percentage of metal oxide are presented in Table 1.
Sample
Stage
DTAmax (K)
TGA temp.
rangeMass loss (%)
Evolved moiety
Exper.
Calcd.
CoL2
I
303.9
275–340
41.3
44.2
C13H12N2OS
II
424.4
340–480
42.7
44.2
C13H12N2OS
NiL2
I
351
280–380
44.5
44.1
C13H12N2OS
II
419
360–550
45.7
44.1
C13H12N2OS
CuL2
I
273
225–375
44.8
43.7
C13H12N2OS
II
443
375–500
42.5
43.7
C13H12N2OS
ZnL2
I
346
275–420
42.4
42.4
C13H12N2OS
II
507, 551
420–575
43.3
42.4
C13H12N2OS
3.2 Antibacterial properties
The title Schiff bases and their metal(II) chelates were evaluated for their antibacterial activity against the standard bacterial strains of (a) E. coli, (b) S. aureus and (c) Pseudomonas aeruginosa. The compounds were tested at a concentration of 30 μg/0.01 mL in DMF solution, using the paper disc diffusion method as earlier reported (Chohan and Kausar, 2000). The inhibition zones are the clear ones around the discs, which were measured in mm.
The ligand and its MII complexes are inactive on the standard bacterial strains used in this study. In contrast, N-(2-thienylbenzylmethyl) benzoylhydrazone and its metal(II) complexes showed marked antimicrobial activity (Chohan, 2001) despite the apparent similarity in these ligands and in their analogous complexes. This appears to be a case of the structure–function relationship commonly observed in biological effect of chemical compounds. The compounds presented herein contain a methyl group in place of hydrogen as compared to those reported (Chohan, 2001). This exchange apparently made these compounds inactive. The activity of any compound is a complex combination of steric, electronic and pharmacokinetic factors.
4 Conclusions
A number of new metal(II) complexes, MIIL2 (M = Co, Ni, Cu and Zn) with the trifunctional SNO-donor atoms have been synthesized and characterized on the basis of elemental analysis, magnetic moments, molar conductances, thermogravimetric and spectroscopic (electronic, IR, 1H NMR and 13C NMR) data. The ligands behave as monobasic SNO tridentate, bonding to the metal(II) ion through the acetylthiophene ring sulfur, azomethine nitrogen, and benzoyl oxygen to form a distorted octahedral structure where the deprotonated form is preferred in the coordination. The resulting data indicate that the complexes are in accordance with the above formulation.
Acknowledgements
The author would like to acknowledge Dr. Hazem Abu Shawish Al-Aqsa University – Gaza – Palestine, for fruitful discussions. In addition, thanks are extended to the staff of the Medical Technology Department at the Islamic University for testing the antimicrobial activity of the compounds.
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