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Abstract The objective of the present study is to investigate the reactions of two Schiff base ligands (L1 and HL2) with the metal hexacarbonyls M(CO)6 (M= Cr, Mo or W) and with some transition metal ions (Cu(II), Ni(II), Co(II), La(III) and Sm(III)). The two ligands (L1 and HL2) have been prepared by the reaction of pyridine-2-carboxaldehyde with an amine compound (2,6-diaminopyridine or 2-aminophenol) in 1:1 molar ratio. The ligands and metal complexes are characterised by elemental analysis, spectroscopic studies such as IR, UV–Vis, 1H NMR, fluorescence, mass, molar conductance and magnetic susceptibility measurements. Three complexes with molecular formulas [Cr(L1)3], 1, [MoO2(L1)2], 2 and [WO2(L1)2], 3 are isolated from the reaction of the metal hexacarbonyls with L1. These complexes give no 1H NMR signals indicating paramagnetic characterist- ics. The infrared spectrum of chromium complex, displays the characteristic bands of L1 with appropriate shifts due to complex formation. The effective magnetic moment of chromium complex, 1, is indicated to the spin-only moment of spectroscopic data, it can be concluded that the chromium complex has the tris configuration. This structural arrangement would give a chromium species with zero (d6) oxidation state. The infrared spectra of complexes 2 and 3 display two symmetric stretching bands at 974 and 938 cm-1 as well as two asymmetric stretching bands at 897 and 935cm-1 corresponding to the presence of two terminal cis M=O (M=Mo or W) bonds, respectively. The infrared spectra of the two complexes also displayed non-ligand bands due to M-N bonds confirming the coordination of the metal to L1 via azomethine and pyridyl nitrogen atoms. So, it is expected that the metal atom may exist in +4 formal oxidation state. Magnetic measurements of 2 and 3 indicate the presence of two unpaired electrons in the valence shell of the metals. The corresponding reactions with HL2 produced the complexes [Cr(HL2)2], 4, [Mo2(CO)4O2(HL2)2], 5 and [W(CO)4(HL2)], 6. The infrared spectrum of the chromium complex, 4, exhibits vibrational bands due to ν(OH), ν(C-O) and υ(C=N) bonds with proper shifts with respect to that of the free ligand. The 1H NMR spectrum of 4 displays a lower shift signal at 9.96 ppm indicating the coordination of the ligand HL2 to chromium atom through the oxygen of OH group without oxidation. Therefore, it can be concluded that phenolic oxygen, pyridyl nitrogen and azomethine nitrogen acting as a tridentate ligand. The IR spectrum of the complex 5 displays non-ligand two stretching bands corresponding to the presence of two terminal CO groups bonded to molybdenum in trans positions and also displays asymmetric and symmetric characteristic stretching bands due to the presence of two bridged CO groups. The two strong stretching bands at 925 and 903 cm-1 refer to the presence of two terminal Mo=O bonds in trans positions. Therefore, complex 5 could be considered as dinuclear with the two molybdenum atoms in distorted octahedral environment. Each molybdenum atom has +2 formal oxidation state with a low-spin d4 electronic configuration due to further splitting of the t2g orbitals in the low-symmetry complex. The IR spectrum of complex 6 shows a vibrational band due to ν(OH) frequency with the proper shift with respect to the HL2 ligand . The 1H NMR spectra of complex 6 confirm the presence of shifted OH group. These shifts proposed the coordination of tungsten to HL2 ligand through the oxygen of the hydroxyl group without oxidation. The IR spectrum of complex, 6, displays a pattern of four CO bands in the terminal metal carbonyl region. Therefore, it could be concluded that the complex is mono nuclear complex with the tungsten atom existing in a distorted octahedral environment. This structural arrangement would give a tungsten species with zero (d6) oxidation state. The solid complexes of the Schiff base ligand L1 are also prepared by the reaction with Cu2+, Ni2+, Co2+, La3+ and Sm3+forming the complexes [M(L1)Cl2(H2O)2] where M= Cu2+, Ni2+, Co2+, and the two complexes [La(L1)3](NO3)3.3H2O and [Sm(L1)(ClO4)3].3H2O. The ligand L1 acts here as a bidentate ligand where it is coordinated to the metal through its nitrogen atoms of the pyridine and azomethine group. The molar conductivity data for complexes indicate that the complexes have non electrolytic nature except La(III) complex. The IR spectra of these complexes show bands due to ν(-C=N) and δ(Py). Also, the IR spectrum of Sm(III) complex show that perchlorate groups are coordinated to the metal in a bidentate manner. The band at 1383 cm-1 in the spectrum of the La(III) complex indicats the existence of ionic nitrate group in the complex. The measured values of the magnetic moment of Cu(II), Ni(II) and Co(III) complexes have been found to be 1.98, 3.81 and 4.42 B.M., respectively, which are in the normal range observed for octahedral complexes. The diffused reflectance spectra of the Ni(II) and Co(II) complexes show three bands, while Cu(II) complex show one band due to d-d transitions. The mixed ligands complexes are prepared from the reactions of the metal ions Cu2+, Ni2+, Co2+or La3+ with the Schiff base HL2 in the presence of 2-aminopyridine (2-AP) to give the complexes: [M(HL2)(2-AP)Cl2].H2O where M= Cu2+, Ni2+, Co2+ ions in addition to the complex [La(HL2)(2-AP)(NO3)2].NO3. The molar conductivity data for 1mM solutions of all complexes suggest that they have non electrolytic nature except La(III) complex. The IR spectra of the complexes show characteristic bands due to OH, C–O, and C=N functional groups indicating the coordination of HL2 to the metal ions in a tridentate manner with NNO donor sites of the pyridine-N, nitrogen atom of the azomethine group and the oxygen atom of hydroxyl group. Magnetic moment and diffused reflectance spectra of Cu(II), Ni(II) and Co(II) complexes suggest an octahedral structure around metal ions. The fluorescence spectral results reveal that the fluorescence emission intensity of Schiff bases increases dramatically on complexation. Therefore, all synthesized compounds can potentially serve as photoactive materials as indicated from their characteristic fluorescence properties. Thermal studies indicate high thermal stability of the complexes. The activation thermodynamic parameters, such as activation energy, enthalpy, entropy and Gibbs free energy change of decomposition are calculated and discussed. The catalytic activities of the complexes towards hydrogen peroxide decomposition reaction are investigated. The ligands and their metal chelates have been screened for their antibacterial activities and the findings have been reported, explained and compared with some known antibiotics. |