Three new Schiff bases of N-substituted isatin , and = Schiff base of N-acetylisatin, N-benzylisatin, and N-benzoylisatin, respectively, and their metal complexes , , , , , , , , , , , , and , were reported. The complexes were characterized by elemental analyses, metal and chloride content, spectroscopic methods, magnetic moments, conductivity measurements, and thermal studies. Some of these compounds were tested as antibacterial and antifungal agents against Staphylococcus aureus, Proteus vulgaris, Candida albicans, and Aspergillus niger.

1. Introduction

Isatin (indole-2,3-dione) and its derivatives have shown a wide scale of biological activities such as antibacterial [13], antifungal [1, 35], anticonvulsant [2, 6], anti-HIV [7], anticancer [1, 2], antiviral [1], and enzyme inhibitors [2]. The Schiff bases (a) and (b) (Scheme 1) derived from isatin and its derivatives with different amines have been studied [1, 2, 6, 813]. The reaction of N-acetyl, N-benzoyl, and N-tosylisatin and their Schiff base derivatives (c) and (d) (Scheme 1) with ethanol, methanol, isopropyl alcohol, allyl alcohol, TsNH2, pyrrolidine, and water yield products resulting from nucleophilic attack at the C-2 carbonyl that leads to heterocyclic ring cleavage [8, 14]. The present work aims to study the synthesis and antibacterial activity of three new ligands derived from condensation of N-acetyl, N-benzyl, and N-benzoylisatin with the chelating agent dithiooxamide (ethanedithioamide or rubeanic acid) dto and their metal complexes. The Schiff bases of dithiooxamide and their complexes have received most of the attention because of the semiconductive, magnetic, spectroscopic, and thermal properties [1517] as well as being used as semiconductors antibacterial and antifungal agents [1820].


2. Experimental/Materials and Methods

All chemicals used were of analytical reagent grade (AR) except dto and ethanol which were purified prior to use [21]. FTIR spectra were recorded on SHIMADZU FTIR-8400S, Fourier Transform, Infrared spectrophotometer. The electronic spectra ( (200–1100) nm) in different solvents were recorded on Shimadzu (UV-Vis)-160 spectrophotometer. Elemental microanalyses were performed on Euro vector EA 3000 A. The metal contents of the complexes were determined by atomic absorption technique using Varian-AA775, Atomic Absorption Spectrophotomer. Mass spectra were recorded on Shimadzu QP 5050A. 1H NMR was performed by using Bruker Ultra Sheild 300 MHz NMR spectrophotometer. Thermal analyses (TG and DTG) were carried out by using Shimadzu Thermal Analyzer Type 50 H. Electrical conductivity measurements for complexes (10-3 M) in DMF and DMSO at room temperature were carried out by using Hunts Capacitors Trade Mark British made. Magnetic moments ( B.M) for the prepared complexes in the solid state at room temperature were measured by using Bruker Magnet B.M-6. The chloride content for complexes was determined by Mohr’s method. N-acetylisatin, N-benzylisatin, N-benzoylisatin, and PdCl2(phCN)2 were prepared by methods reported in literature [6, 2225].

3. Synthesis of Ligands

All attempts to prepare 1-(9a-Hydroxy-2,3-dithiooxo-1,2,3,9a-tetrahydro-1,4,9-triaza-fluoren-9-yl)-ethanone ( ) (Scheme 2) and 9-Benzyl-9a-hydroxy-9,9a-dihydro-1H-1,4,4-triaza-fluorene-2,3-dithione ( ) (Scheme 2) in solutions were unsuccessful; therefore solid reaction was carried out to prepare the two ligands.

3.1. Schiff Base of N-Acetylisatin: 1-(9a-Hydroxy-2,3-dithiooxo-1,2,3,9a-tetrahydro-1,4,9-triaza-fluoren-9-yl)-ethanone ( )

A powdered mixture of N-acetylisatin (0.3092 g, 1.6 mmol) and dto (0.0983 g, 0.8 mmol) in a sealed Carius tube was heated in a stirred oil bath at 160–170°C for 2 hours. The melt colour was changed from orange to dark brown. After cooling to room temperature, the solid product was ground and dissolved in butanol, followed by precipitation with ether. A black precipitate was formed. The product was filtered off and washed several times with ether to remove the unreacted materials giving brown crystals. Yield (0.116 g, 48.76%), m.p (220°C decomp.). 1H NMR data (ppm), (CDCL3): 2.508 (3H,s,CH3); 3.34 (2H,s,OH and NH thioamide); 7.074–7.853 (4H, m, aromatic protons). MS(EI), m/z(%): 207(21), 161(10), 146(23), 133(9), 92(10), 78(59), 63(84), 44(100). Anal. for C12H9N3O2S2 Calcd. C, 49.48; H, 3.09; N, 14.43%; Found: C, 50.54; H, 3.22; N, 13.23%.

3.2. Schiff Base of N-Benzylisatin: 9-Benzyl-9a-hydroxy-9,9a-dihydro-1H-1,4,4-triaza-fluorene-2,3-dithione ( )

A powdered mixture of N-benzylisatin (0.829 g, 3.5 mmol) and dto (0.85 g, 7 mmol) was heated in a sealed Carius tube in an oil bath at 140°C for 10 hours. Colour of melt was changed from orange to dark brown. After cooling to room temperature, a solid mass was formed. The product was ground and purified several times in refluxing ethanol, filtered off, washed with hot ethanol followed by acetone and dried, giving dark brown crystals. Yield (0.2679 g, 22.3%), m.p ( 250°C). 1H NMR data (ppm), (DMSO): 3.45(2H, s, OH and NH thioamide); 5.1(2H, w, CH2 benzyl); 6.9–7.2(9H, m, aromatic protons). MS(EI), m/z(%): 156(11), 149(15), 127(13), 105(11), 78(100), 63(100). Anal. for C17H13N3OS2 Calcd.: C, 60.17; H, 3.83; N, 12.38%; Found: C, 61.10; H, 3.43; N, 12.98%.

3.3. Schiff Base of N-Benzoylisatin: N-[2-(3-oxo-5,6-dithioxo-3,4,5,6-tetrahydro-pyrazin-2-yl)-phenyl] benzamide ( )

Equimolar amounts of benzoylisatin (0.2 g, 0.79 mmol) and dto (0.0957 g, 0.79 mmol) in butanol (2 cm3) containing 4 drops of piperidine were heated under reflux with stirring for 5 hours during which the colour of solution was changed from orange to brown. The solution mixture was left to stand overnight and then cooled down to 0°C. Cold ether was added until a dark brown precipitate was formed. The product was filtered off, washed several times with acetone followed by ether. Yield (0.0857 g, 30.51%), m.p. (250°C decomp.), 1H NMR data (ppm), (DMSO): 4.902–5.101(1H, b, NH thioamide); 7.144–7.860(9H, m, aromatic protons); 10.124(1H, b, NH benzoyl moiety). MS(EI), m/z(%): 296.6(13), 267.5(7), 232.6(20), 195.6(15), 149.5(6), 104.4(27), 83.4(6). Anal. for C17H11N3O2S2 Calcd.: C, 57.79; H, 3.11; N, 11.89%; Found: C, 57.39; H, 3.54; N, 11.30%.

4. Preparation of Metal Complexes

(A)A solution mixture of the ligands and (0.01 mmol) (0.0029, 0.0033 g), respectively, with the metal salts CoCl2 6 , NiCl2 6 , and CuCl2 2 (0.01 mmol) and (0.02 mmol) (0.0058, 0.0067 g) of and , respectively, with the metal salts PdCl2(phCN)2 and K2PtCl6 (0.01 mmol), in DMF (C1), butanol (C2 and C3), or DMSO (C4–C10) was heated under reflux for four hours. Precipitation of complexes took place within 30 minutes, while those of was precipitated at the end of reflux time. The products were filtered, washed with hot ethanol and acetone, followed by ether and vacuum dried. Complexes of were prepared in the same manner using a mixture of (0.01 mmol, 0.0035 g) with the metal salts CoCl2 6 , NiCl2 6 (0.01 mmol), and (0.02 mmol, 0.007 g) of with PdCl2(phCN)2 (0.01 mmol). “ ”: colour(dark brown) Yield (26.24%). Anal. for (C24H18N6O4S4 Co2Cl3)Cl Calcd.: C, 34.21; H, 2.13; N, 9.97; S, 15.20%; Found: C, 34.32; H, 2.50; N, 9.42; S, 15.13%. M, 13.99(Calcd), 14.0(Found)%; Cl, 16.86(Calcd), 16.20(Found)%. “C2”: colour(dark brown). Yield (23.51% ). Anal. for [(C24H18N6O4S4 NiCl2)0.4(C4H10O)] Calcd.: C, 47.63; H, 5.75; N, 8.33%; Found: C, 48.32; H, 5.45; N, 8.83%. M, 5.82(Calcd), 5.72(Found)%. “C3”: colour(dark brown). Yield (23.35%). Anal. for [(C12H9N3O2S2CuCl ( ))Cl.0.5(C4H10O)] Calcd.: C, 34.96; H, 3.32; N, 8.74%; Found: C, 34.01; H, 3.72; N, 9.73%. M, 13.21(Calcd), 13.85(Found)%. Cl, 14.77(Calcd), 14.70(Found%). “C4”: colour(dark brown). Yield (35.18%). Anal. for [(C24H18N6O4S4PdCl)Cl] Calcd.: C, 37.94; H, 2.37; N, 11.06; S, 16.86%; Found: C, 38.40; H, 2.38; N, 11.15; S, 16.78%. M, 13.96(Calcd), 13.71(Found)%. Cl, 9.35(Calcd), 10.5(Found)%. “C5”: colour(brown). yield (19.08%). Anal. for [(C24H18N6O4S4PtCl2)Cl2 1.8(C2H6O) ] Calcd.: C, 32.47; H, 3.02; N, 8.23%; Found: C, 32.85; H, 3.27; N, 9.16%. M, 19.12(Calcd), 19.10(Found)%. “ ”: colour(dark brown). Yield (36.06%). Anal. for [(C17H13N3OS2CoCl)Cl 0.4(H2O) 0.3(C2H6SO)] Calcd.: C, 42.28; H, 3.12; N, 8.40; S, 12.81%; Found: C, 41.85; H, 2.72; N, 8.15; S, 12.00%. M, 11.79(Calcd), 12.11(Found)%; Cl, 14.21(Calcd), 14.58(Found)%. “C7”: colour(dark brown). Yield (45.30%). Anal. for [C17H13N3OS2NiCl2] Calcd.: C, 43.52; H, 2.77; N, 8.96%; Found: C, 44.20; H, 3.05; N, 9.24%. M, 12.52(Calcd), 12.23(Found)%; Cl, 15.14(Calcd), 15.47(Found)%. “C8”: colour(dark brown). Yield (45.39%). Anal. for [(C17H13N3OS2Cu)Cl2 )] Calcd.: C, 41.50; H, 3.05; N, 8.54%; S, 13.02%; Found: C, 42.15; H, 2.74; N, 8.16; S, 12.94%. M, 12.91(Calcd), 13.12(Found)%; Cl, 14.44(Calcd), 14.55(Found)%. “C9”: colour(dark brown). Yield (43.38%). Anal. for [(C34H26N6O2S4Pd)Cl2)] Calcd.: C, 47.71; H, 3.04; N, 9.82%; S, 14.97%; Found: C, 47.75; H, 3.11; N, 9.69; S, 15.52%. M, 12.39(Calcd), 13.00(Found)%; Cl, 8.30(Calcd), 8.47(Found)%. “C10”: colour(dark brown). Yield (32.42%). Anal. for [((C17H13N3OS2)2.5PtCl)Cl3] Calcd.: C, 43.05; H, 2.74; N, 8.86%; S, 13.50%; Found: C, 43.84; H, 2.55; N, 9.17; S, 14.09%. M, 16.46(Calcd), 15.87(Found)%. “ ”: colour(brown). Yield (33.43%). Anal. for [(C17H11N3O2S2Co)Cl2 H2O] Calcd.: C, 40.72; H, 2.59; N, 8.38 %; Found: C, 40.05; H, 2.34; N, 7.82;%. M, 11.75(Calcd), 12.25(Found)%. Cl, 14.17(Calcd), 14.85(Found)%. “C12”: colour(brown). Yield (34.37%). Anal. for [(C34H22N6O4S4Ni)Cl2] Calcd.: C, 48.82; H, 2.63; N, 10.05%; Found: C, 49.62; H, 2.46; N, 9.41; %. M, 7.02(Calcd), 7.50(Found)%. Cl, 8.49(Calcd), 8.56(Found)%. “C13”: colour(brown). Yield (34.03%). Anal. for [(C34H22N6O4S4Pd)Cl2] Calcd.: C, 46.20; H, 2.46; N, 9.51; S, 14.49%; Found: C, 46.47; H, 2.66; N, 9.79; S, 14.78%. M, 12.00(Calcd), 11.50(Found)%. Cl, 8.04(Calcd), 8.63(Found)%.(B)To a solution mixture of N-acetyl, N-benzyl, or N-benzoylisatin (0.02 mmol) 0.0037, 0.0047, and 0.005 g, respectively, with dto (0.01 mmol) (0.0012 g), (0.04 mmol) (0.0048 g), and (0.02  mmol) (0.0024 g), respectively, in butanol was added a solution of CoCl2 6H2O (0.02 mmol) in butanol. The mixture was heated under reflux. Precipitation took place immediately. Heating was continued for 4 hours to achieve complet precipitation. The product was filtered, washed with hot butanol, followed by ethanol, acetone, ether, and vacuum dried. “ ”: colour(dark brown). Yield (25.15%). Anal. for (C24H18N6O4S4 Co2Cl3)Cl Calcd.: C, 34.21; H, 2.13; N, 9.97; S, 15.20%; Found: C, 34.15; H, 1.95; N, 10.31; S, 15.07%. M, 13.99(Calcd), 13.30(Found)%; Cl, 16.86(Calcd), 16.19(Found)%. “ ”: colour(dark brown). Yield (71.42%). Anal. for [(C17H13N3OS2CoCl) Cl 0.3(H2O) 0.1(C4H10O)] Calcd.: C, 43.34; H, 3.03; N, 8.71; S, 13.28%; Found: C, 43.89; H, 3.35; N, 8.54; S, 13.48%. M, 12.22(Calcd), 12.25(Found)%; Cl, 14.73(Calcd), 14.44(Found)%. “ ”: colour(brown). Yield (41.72%). Anal. for [(C17H11N3O2S2Co)Cl2 0.2H2O] Calcd.: C, 41.93; H, 2.34; N, 8.63%; Found: C, 42.56; H, 2.53; N, 8.73; %. M, 12.10(Calcd), 12.11(Found)%. Cl, 14.59(Calcd), 14.71(Found)%.

5. Microbiological Test Methods

The two following methods were used to perform the antimicrobial tests.

5.1. Agar Diffusion Method

In this method the colonies of the selected bacteria, namely, Staphylococcus aureus (G+), Proteus vulgaris (G-), and the fungus Candida albicans were spread on the surface of solidified nutrient agar. Suitably separated 7 mm diameter holes were made in each agar plate. Each hole was injected with 0.1 mL of 150, 350, 650, and 1000 ppm of the studied compound in DMSO. The agar plates were incubated at 37°C for 24 hours. Diameters of growth inhibition zones were measured in mm depending on diameter and clarity.

5.2. Agar Dilution Method

In this method the antifungal activity of 250 ppm of some selected compounds in DMSO was screened against Aspergillus niger. 2.5 cm3 of 2000 ppm of tested solution was added to 20 cm3 of hot agar solution. The homogenized mixture was then poured into petridish and left to solidify. The Aspergillus colony (9 mm diameter) was fixed on the solidified agar, and the medium was incubated at 37°C for 8 days.

6. Results and Discussion

The IR spectra showed that the three ligands exhibited vibrational modes of of azomethine group [4, 6, 2628], ( , ), ( , ), , and of dto moiety [29, 30] (Table 1). Spectra of and showed vibrational bands related to stretching modes of OH groups [31, 32]. The position of the bands assigned to vibrations of the cyclic rings was dependent on their environment. of and were observed at lower frequencies compared with that of (Table 1) [27, 32]. The latter exhibited bands assigned to and of amide and lactam rings [6, 27, 31, 32]. The spectra of complexes with Co(II), Cu(II), and Pd(II) ions exhibited shift in and (azomethine) vibrations. The latter two complexes together with Ni(II) complex showed additional shifts in to lower frequencies while no significant changes were observed on vibrational modes of C=O group which rules out coordination with carbonyl oxygen. Shifts of thioamide bands (III and IV) were observed in the spectra of Cu(II) and Pt(IV) complexes and were attributed to coordination of metal ion with sulfur atom [33]. Metal complexes of showed bands assigned to and vibrations (Table 1). This may be attributed to cleavage of thioamide ring on complexation leading reappearance of and of both C-2 and NH2 of isatin and dto moieties, respectively. Shifts in (compared with of the free dto (3296, 3203 cm-1)) [34] to lower frequencies were observed in all spectra of complexes except that of Ni(II) which was shifted to higher frequency. Bands related to vibrations in spectra of both Ni(II) and Cu(II) complexes were shifted to higher frequencies while spectra of the other complexes showed shifts to lower frequencies. Additional shifts were observed in the bands assigned to (azomethine) in all complexes except that of Cu(II). The latter complex exhibited shift of band to lower frequency which refers to coordination of sulfur to Cu(II) ion [33]. The spectra of metal complexes exhibited shifts in vibrational modes of and band IV of thioamide group as a result of coordination with metal ions [33, 35]. Additional shift in position of bands assigned to was observed in the spectra of Co(II) and Ni(II) complexes. Shifts in the position of amide and of lactam ring were observed in the spectra of the Pd(II) complex as a result of coordination. Bands related to vibrational modes of lattice solvent, coordinated water were observed at 3500-3400 cm-1 [3638]. Bands appeared at lower frequencies were refered to M–O, M–N, M–S, and M–Cl stretching modes [3638]. Further data are collected in (Table 1).

Theelectronic spectra of , , and exhibited high-intensity multiple bands in DMF and DMSO at 36231–20000 cm-1. These bands were assigned to transition of conjugated system. exhibited additional low-intensity band which was assigned to transition. Changes in positions and profile (C8–C10) of bands were observed in the spectra of metal complexes. Bands related to the (CT) transition were observed as a shoulder on the ligand band in the spectra of C1, C3, C6, C7, C9, and C10 complexes (Table 2). The bands observed in the spectra of Co(II) complexes in the visible region were assigned to ( ), ( ), and ( ). The magnetic moment values of Co(II) complexes were in the range of (3.959–4.6 BM) (Table 2). This indicates tetrahedral geometry around Co(II) ions [3639] (Scheme 3). The Ni(II) complex C2 gave a greenish yellow colour in DMF indicating the exchange of weak ligand atoms with solvent molecules [4043]. The spectrum of this complex showed bands characteristic of octahedral Ni(II) complex [3638, 4043] (Table 2), while the other Ni(II) complexes (C7 and C12) showed tetrahedral geometries (Scheme 3).


The electronic spectra and magnetic moments ( B.M) (Table 2) of these complexes were consistent with these assignment [3638, 4043]. Spectral data (B′, Dq/B′, 10Dq and β) (Table 2), for the Co(II) and Ni(II) complexes were calculated by applying band energies on Tanaba Saugano diagrams. The energy of for Co(II) complexes (C1, C6, C11) and Ni(II) complexes (C7, C12) and for Ni(II) complex C2 were also calculated from the diagrams. The spectrum of the Cu(II) complex C3 exhibited three bands (Table 2) attributed to the spin allowed transitions , and of Jahn Teller tetragonally distorted octahedral Cu(II) complexes [34]. The magnetic moment of the complex (2.36 B.M) indicated paramagnetic character with a high spin orbital coupling [4043]. The spectrum of Cu(II) complex C8 exhibited two bands (Table 2) which were assigned to , and . These bands were attributed to square planar Cu(II) complexes [44] (Scheme 3). Magnetic moment (   =  1.84 B.M) of the complex supported such conclusion [3638, 44]. The spectra of the diamagnetic Pd(II) complexes (C4, C9, and C13) showed two bands assigned to and and the additional band for C4. These bands are attributed to square planar Pd(II) complexes [3438, 4043]. The spectra of the diamagnetic Pt(IV) complexes exhibited two bands which were assigned to forbidden transitions and showing octahedral geometry around Pt(IV) ion [4043] (Scheme 3). The molar conductivities (Table 2) showed that electrolytic nature of the Pt(IV) complex (C10) was 1 : 3, Pt(IV), Cu(II), Pd(II), Co(II) and Ni(II) complexes (C5, C8, C9, C11, C12, and C13) 1 : 2, and Co(II), Cu(II), and Pd(II) complexes (C1, C3, C4, and C6) 1 : 1, while the Ni(II) complexes (C2 and C7) were nonelectrolyte [45]. From these observations, together with the results obtained from other analytical data, the sterochemical structures of the complexes were suggested (Scheme 3).

Thermogravimetric analyses (TG and DTG) have been studied at heating range of 50–800°C for the complexes (C1, C3, C4, and C7) under nitrogen atmosphere. The following results (Table 3) were explained according to analytical suggestions mentioned in literature [4648]. (i) Lattice water, free ions, and organic fragments that are not directly coordinated to the metal ions were found to leave the complex at earlier stages compared with coordinated fragments, (ii) The heating range (50–800°C) produced incomplete decomposition of metal complexes, and the final products were dependent on the type of metal ion and on (M-L) affinity [3638, 46, 49] which reflects the stability of complexes.

7. Biological Screening

The antibacterial activity for precursors, and , and some of their complexes was evaluated against Staphylococcus aureus (G+) and Proteus vulgaris (G-) using the agar diffusion method. Diameter (mm) of growth inhibition zones was measured after incubation for 24 hours at 37°C. The results showed that no antibacterial action was recorded by the studied compounds using concentration of 150, 350, and 650 ppm. Using 1000 ppm (Table 4), and its complexes were more active against Staphylococcus aureus, while and its complexes (except C13) were more active against Proteus vulgaris than the other studied compounds. The antifungal activity was evaluated against Candida albicans by the agar diffusion method and Aspergillus niger colony (9 mm diameter) by the agar dilution method using concentration of 250 ppm in DMSO. The results showed that and were inactive against Candida albicans; Co(II) (C11), Ni(II) (C12), and Pd(II) (C13) complexes were more active than the parent ligand ( ) while those of were inactive except Cu(II) complex (C3). , , and C4 which were inactive against Candida albicans showed moderate activity against Aspergillus niger which refer to the effective selectivity of specific inhibitor on the microorganisms.

8. Conclusions

(1)Condensation reaction of N-acetyl, N-benzyl, and N-benzoyl isatins with dto gave Schiff base ligands , as was confirmed by 1H, 13C NMR, and IR spectra.(2)The formation of the Schiff base ligand took place with ring cleavage at C-2 of the heterocyclic ring of the benzoylisatin. Whereas the formation of and took place without ring cleavage.(3)The presence of various donor atoms and the stereochemistry of the studied ligands enhanced different complexing behaviours and geometries using the studied metal ions.(4)The results of the physical properties and spectral analyses of cobalt complexes prepared by template reaction demonstrated the recommendation of for synthesis of metal complexes of the studied ligands, due to less time consuming and in general more yield of products.(5)The study of biological activity of the studied ligands and some of their metal complexes against bacteria and fungi showed selectivity nature of microorganism towards these compounds and indicated the possibility of using some of them as antibacterial and antifungal agents.