Synthesis and Characterization of New Schiff Bases Derived from N (1)-Substituted Isatin with Dithiooxamide and Their Co(II), Ni(II), Cu(II), Pd(II), and Pt(IV) Complexes

Three new Schiff bases of N-substituted isatin LI, LII, and LIII = Schiff base of N-acetylisatin, N-benzylisatin, and N-benzoylisatin, respectively, and their metal complexes C1a,b = [Co2(LI)2Cl3]Cl, C2 = [Ni(LI)2Cl2]0.4BuOH, C3 = [CuLICl(H2O)]Cl ⋅ 0.5BuOH, C4 = [Pd(LI)2Cl]Cl, C5 = [Pt(L1)2Cl2]Cl2 ⋅ 1.8EtOH.H2O, C6a = [CoLIICl]Cl ⋅ 0.4H2O ⋅ 0.3DMSO, C6b = [CoLIICl]Cl ⋅ 0.3H2O ⋅ 0.1BuOH, C7 = [NiLIICl2], C8 = [CuLII]Cl2 ⋅ H2O , C9 = [Pd(LII)2]Cl2, C10 = [Pt(LII)2.5Cl]Cl3, C11a = [Co(LIII)]C12 ⋅ H2O, C11b = [Co(LIII)]Cl2 ⋅ 0.2H2O, and C12 = [Ni(LIII)2]Cl2, C13 = [Ni(LIII)2]Cl2 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.


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 (λ(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. 1

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

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.

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 cm 3 of 2000 ppm of tested solution was added to 20 cm 3 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.

Results and Discussion
The IR spectra showed that the three ligands exhibited vibrational modes of ν C=N of azomethine group [4,6,[26][27][28], (ν C-N , δ NH ), (ν C-N , ν C-S ), ν C-S , and ν C=S of dto moiety [29,30] (Table 1). Spectra of L I and L II showed vibrational bands related to stretching modes of OH groups [31,32]. The position of the bands assigned to ν NH vibrations of the cyclic rings was dependent on their environment. ν NH of L II and L III were observed at lower frequencies compared with that of L I (Table 1) [27,32]. The latter exhibited bands assigned to ν C=O and ν NH of amide and lactam rings [6,27,31,32]. The spectra of L I complexes with Co(II), Cu(II), and Pd(II) ions exhibited shift in ν OH and ν C=N (azomethine) vibrations. The latter two complexes together with Ni(II) complex showed additional shifts in ν NH 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 L II showed bands assigned to ν C=O and ν NH2 vibrations (Table 1). This may be attributed to cleavage of thioamide ring on complexation leading reappearance of ν C=O and ν NH2 of both C-2 and NH 2 of isatin and dto moieties, respectively. Shifts in ν NH2 (compared with ν NH2 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 ν C=O 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 ν C=N (azomethine) in all complexes except that of Cu(II). The latter complex exhibited shift of ν C=S band to lower frequency which refers to coordination of sulfur to Cu(II) ion [33]. The spectra of L III metal complexes exhibited shifts in vibrational modes of ν C=O and band IV of thioamide group as a result of coordination with metal ions [33,35]. Additional shift in position of bands assigned to ν C=N was observed in the spectra of Co(II) and Ni(II) complexes. Shifts in the position of ν NH amide and ν C=O 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 [36][37][38]. Bands appeared at lower frequencies were refered to M-O, M-N, M-S, and M-Cl stretching modes [36][37][38]. Further data are collected in (Table 1).

Biological Screening
The antibacterial activity for precursors, L I and L III , 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), L I and its complexes were more active against Staphylococcus aureus, while L III and its complexes (except C 13 ) 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 L I and L III were inactive against Candida albicans; Co(II) (C 11 ), Ni(II) (C 12 ), and Pd(II) (C 13 ) complexes were more active than the parent ligand (L III ) while those of L I were inactive except Cu(II) complex (C 3 ). L I , L III , and C 4 which were inactive against Candida albicans showed moderate activity against Aspergillus niger which refer to the effective selectivity of specific inhibitor on the microorganisms.

Conclusions
(1) Condensation reaction of N-acetyl, N-benzyl, and N-benzoyl isatins with dto gave Schiff base ligands L I -L III , as was confirmed by 1 H, 13 C NMR, and IR spectra.
(2) The formation of the Schiff base ligand L III took place with ring cleavage at C-2 of the heterocyclic ring of the benzoylisatin. Whereas the formation of L I and L II 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.