Metal Complexes of Schiff Bases: Preparation, Characterization, and Biological Activity

Turk J Chem 30 (2006) , 361 – 382. c TÜBİTAK  Metal Complexes of Schiff Bases: Preparation, Characterization, and Biological Activity Gehad Geindy MOHAMED, Mohamed Mohamed OMAR∗, Ahmed Mohamed HINDY Cairo University, Department of Chemistry, Faculty of Science, Giza-EGYPT e-mail: Received 24.05.2005 Metal complexes of Schiff bases derived from 2-furancarboxaldehyde and o-phenylenediamine (L1 ), and 2-thiophenecarboxaldehyde and 2-aminothiophenol (HL2 ) are reported and characterized based on elemental analyses, IR 1 H NMR, solid reflectance, magnetic moment, molar conductance, and thermal analysis (TGA). The ligand dissociation, as well as the metal-ligand stability constants were calculated, pH-metrically, at 25 ◦ C and ionic strength μ = 0.1 (1 M NaCl). The complexes are found to have the formulae [M(L1 )(H2 O)2 ](Cl)n·yH2 O (where M = Fe(III), Ni(II), Cu(II); n = 2-3, y = 2-4); [M(L1 )](X)2 ·yH2 O (where M = Co(II), Zn(II), UO2 (II), X = Cl, AcO or NO3 , y = 1-3); [M(L2 )2 ]·yH2 O (where M = Co(II), Ni(II), Cu(II); X = Cl; y = 0-2 and Zn(II); X = AcO, y = 0); and [Fe(L2)2 ]Cl·2H2O and [UO2 (HL2 )2 ](NO3 )2 . The molar conductance data reveal that all the metal chelates of the L1 ligand, and Fe(III) and UO2 (II) chelates of HL2 are electrolytes, while Co(II), Ni(II), Cu(II), and Zn(II) chelates of HL2 are non-electrolytes. IR spectra show that L1 is coordinated to the metal ions in a tetradentate manner, with ONNO donor sites of azomethine–N and furan-O, whereas the HL2 ligand is coordinated to the metal ions in a terdentate manner with SNS donor sites of azomethine–N , thiophene-S, and thiol-S. From the magnetic and solid reflectance spectra, it is found that the geometrical structures of these complexes are octahedral and tetrahedral. The thermal behavior of these chelates shows that the hydrated complexes lose water molecules of hydration in the first step and is immediately followed by decomposition of the anions and ligand molecules in the subsequent steps. The activation thermodynamic parameters, such as E*, ΔH*, ΔS*, and ΔG*, are calculated from the DrTG curves using the CoatsRedfern method. The synthesized ligands, in comparison to their metal complexes, were also screened for their antibacterial activity against bacterial species, Escherichia coli, Pseudomonas aeruginosa, and Staphylococcus Pyogones, as well as fungi (Candida). The activity data show the metal complexes to be more potent antibacterials than the parent Schiff base ligand against one or more bacterial species. Key Words: 2-thiophenecarboxaldehyde, 2-furancarboxaldehyde, 2-aminothiophenol, o-phenylenediamine, transition metal complexes, stability constants, IR, 1 H NMR, conductance, solid reflectance, magnetic moment, thermal analysis, biological activity. ∗ Corresponding author 361 Metal Complexes of Schiff Bases:..., G. G. MOHAMED, et al., Introduction A large number of Schiff bases and their complexes have been studied for their interesting and important properties, e.g., their ability to reversibly bind oxygen1 , catalytic activity in hydrogenation of olefins2 and transfer of an amino group3 , photochromic properties4 , and complexing ability towards some toxic metals5 . The high affinity for the chelation of the Schiff bases towards the transition metal ions is utilized in preparing their solid complexes. Schiff base derived from the reaction of 2,5-thiophenedicarboxaldehyde and o-amino-benzenethiol gives 2,5-bis(benzothiazolidin-2-yl)thiophene(I). Schiff base reacted as a neutral ligand with Pb(II) and a dianion with Cu(II), Ag(I), Cd(II), Pb(II), and Zn(II). The behavior of (I) with Hg(II), Ru(II), Pt(II), Rh(III), and Ni(II) involved the opening of the thiazoline rings of the ligand6. The condensation of o-aminothiophenol with 2-thiophenecarboxaldehyde yields 2-thiazolin-2-ylthiophene, rather than the expected Schiff base. However, upon reaction with metal ions, the thiazoline rearranged to the expected thiolate Schiff base. Complexes of Schiff base with Ni(II),Cu(II), Zn(II), Cd(II), Pb(II), Ag(I), and Pd(II) were isolated and characterized7 . MLL1 (M = Cu(II), Ni(II); HL = salicylideneamine; HL1 = 1-(2-hydroxyphenyl)ethyliden-eamine), ML1 L2 (HL2 = 2-hydroxy-1-naphthylmethyleneamine), and MLL2 reacted with 2-amino-benzenethiol to give Cu2 L32 (H2 L3 = N-(2-mercaptophenyl)salicylaldimine (SMAH), N-(2-mercaptophenyl)-2-hydroxy-1- naphthylmethyleneamine (NMAH), NiL3 (AMA) (AMAH = N-(2-mercaptophenyl)-1-(2-hydroxyphenyl)ethylideneamine) and Ni(SMA)(NMA). The complexes were characterized by IR, reflectance spectra, and TGA8 . Schiff base9 derived from the reaction of the aldehydes, 3-hydroxybenzaldehyde and 5-nitrosalicylaldehyde, with the amines, aniline and o-aminothiophenol, and their complexes with VO(II), Co(II), and Ni(II), were prepared and characterized by elemental analyses, magnetic measurement, and electronic absorption data. Complexes of Ni(II), Co(II), Cu(II), Zn(II), Pd(II), and Pb(II) with Schiff base derived from isatin with 2-aminothiophenol (HIATP) were synthesized and characterized by elemental analyses, molar conductance, magnetic moments, 1 H NMR, IR, and electronic spectra studies10 . Spectroscopic investigation of some thio-Schiff bases of 2-aminothiophenol with benzaldehyde derivatives has been described11 . Chemical shifts of the different types of protons in the NMR spectra of the prepared Schiff bases were also reported. Schiff bases derived from 5-nitrosalicylaldehyde and the amines, o- and p-aminophenols, o-aminothiophenol, and sulfanilic acid were prepared and their complexes were characterized by IR, electronic absorption, ESR spectra, and magnetic and conductance measurements12 . The complexes were tested for antibacterial activity against common pathogenic organisms and showed mild to moderate activity. Schiff bases derived from ethylene-2,2‘-(dioxydibenzenaldehyde) and 2-aminothiophenol and its complexes with Ni(II), Cu(II), and Cd(II) were synthesized and characterized by elemental analyses, IR, UV/VIS spectra, and conductance measurements13 . Synthesis, spectroscopic characterization, redox, and biological screening studies of some Schiff bases transition metal(II) complexes derived from salicylidene-4-aminoantipyrine and 2-aminophenol, 2aminothiophenol were studied by Raman et al.14. Schiff bases, 2,5-bis[formyl(2-hydroxyethylamine)]thiophene (H2 L) and 2,5-bis[formyl] (2-mercaptoani362 Metal Complexes of Schiff Bases:..., G. G. MOHAMED, et al., line)thiophene (H2 L‘) were prepared and their Cu(II) complexes were also prepared and characterized by elemental analyses, IR, 1 H NMR, and conductance and magnetic susceptibility15 . Experimental Materials and Reagents All chemicals used were of analytical reagent grade (AR) and of the highest purity available. They included 2-thiophenecarboxaldehyde (Sigma), 2-furancarboxaldehyde (Sigma), o-phenylenediamine (Aldrich), 2-aminothiophenol (Sigma), copper(II) acetate dihydrate (Prolabo), cobalt(II) and nickel(II) chloride hexa (...truncated)