4,4′-Diaminobiphenyl reacts with 2,4-pentanedione in absolute ethanol in a molar ratio 1 : 2 to form mainly the product of [1 + 2] condensation, 4,4′-(biphenyl-4,4′-diyldinitrilo)dipentan-2-one (H2L). The Schiff base was used as tetradentate chelating ligand to coordinate
Aromatic Schiff base compounds have significant importance in the fields of coordination chemistry and material sciences because they are potentially capable to form stable complexes with different metal ions [
Transition metal complexes with oxygen and nitrogen donor Schiff base ligands are of particular interest [
Recently, much research on macrocyclic complexes have been focused on species containing the first-row transition metal ions and tetradentate ligands [
In continuation of our investigation on the ligating properties of N2O2 Schiff base ligands [
All materials and solvents were analytical reagent grade. The compounds 4,4′-diaminobiphenyl and 2,4-pentanedione were commercial samples from Aldrich and purified by standard procedures. Their purity was determined by thin layer chromatography (TLC) or dry column “flash” chromatography (DCFC) (SiO2, Merck-Kieselgel 60 H). All metals were hydrated chlorides and used as received. Reactions of 4,4′-diaminobiphenyl with 2,4-pentanedione were carried out using ethyl alcohol-reactants in sealed Rotaflo-tapped Pyrex ampoules (50 cm3). Crude reaction product mixture was examined by TLC, and individual components were separated by filtration. Further purification, where necessary, was achieved by TLC or repeated DCFC.
Products were examined by IR and recorded with a Perkin-Elmer 1000 series FT-IR spectrophotometer using KBr disks. UV-vis spectra were obtained in DMF with a UNICAM UV-300 spectrophotometer. 1H NMR spectra of the ligand and the diamagnetic complexes were recorded on a Jeol GSX WB spectrometer instrument operating at 270 MHz, and the chemical shifts to low field of the reference are designated positive and given in ppm, using tetramethylsilane (TMS) as external reference. NMR samples were run as solutions in DMSO-D6. Mass spectra were recorded on a Bruker Daltonics Data Analysis 3.1 spectrometer using electron impact (EI) conditions. Elemental analysis was carried out with an EL III-ELEMENTAR. Melting points were determined with a Kofler bench and are uncorrected. Differential scanning calorimetry (DSC) diagrams were recorded in the 25–400°C range with a Mettler DSC
Electrochemical measurements were recorded on a Radiometer VOLTALAB 32 (DEA 332 type): the working electrode was a 2 mm diameter Pt rotating disk and the auxiliary electrode a Pt wire. A saturated calomel electrode was used as the reference electrode, and measurements were carried out at room temperature. DMF was used as solvent and the ionic strength maintained at 0.1 mol L−1, with Bu4NClO4 (TBAP) as supporting electrolyte. The concentrations of species were in the 2.5.10−3 to 5.10−3 mol L−1 range. The sweep speed was 100 mV s−1 unless otherwise indicated.
The Schiff base ligand (shown in Figure
Synthetic route of the Schiff base ligand H2L.
All the complexes were prepared using the same synthetic pathway described in literature [
Main analytical data for the ligand and its complexes in the solid state.
Compound | Color | Yield (%) | M.p. (°C) | M+ | Elemental analysis found (calcd.) (%) | ||
---|---|---|---|---|---|---|---|
C | H | N | |||||
H2L | Yellow | 83 | 134 | 348.2 | 75.68 (75.83) | 7.14 (6.94) | 7.75 (8.04) |
( |
Blue | 65 | >260 | 814.7 | 64.78 (64.86) | 6.21 (5.93) | 6.99 (6.87) |
( |
Yellow | 79 | >260 | 814.2 | 64.64 (64.90) | 6.16 (5.94) | 7.04 (6.88) |
|
Green | 60 | >260 | 608.1 | 43.72 (43.45) | 4.25 (3.97) | 4.75 (4.60) |
|
Yellow | 58 | >260 | — | 41.01 (41.18) | 4.30 (4.08) | 4.65 (4.36) |
The synthesized Schiff base H2L (Figure
The information about the (metal-ligand) ratio was obtained from mass spectrometry of the three complexes examined in the solid state which reveal that the complexes are binuclear compounds. The molecular weights of these products were determined using electron impact (EI) conditions which gave the parent ion peaks M+ at
Mass spectrometry data of H2L, ([Co(H2L)]4+)2, [(CoCl2)2(H2L)]4+, and ([Ni(H2L)]4+)2 in the solid state.
Compound |
|
Assignment | Abundance |
---|---|---|---|
H2L | 348.2 | M+ | 78 |
305.3 | (C20H21N2O)+ | 20 | |
248.2 | (C17H16N2)+ | 10 | |
| |||
( |
814.7 | M+ | 10 |
686.4 | (C37H34N3O3Co2)7+ | 27 | |
392.2 | (C21H21N2O2Co)5+ | 91 | |
241.1 | (C9H14N2O2Co)5+ | 17 | |
| |||
|
608.1 | M+ | 10 |
501.7 | (C22H24N2O2Co2Cl)5+ | 18 | |
380.1 | (C17H16NOCoCl2)3+ | 63 | |
233.1 | (C11H12NOCo)3+ | 84 | |
| |||
( |
814.2 | M+ | 13 |
599.3 | (C36H34N3O2Ni)5+ | 16 | |
393.1 | (C21H22N2O2Ni)5+ | 93 | |
316.1 | (C15H17N2O2Ni)5+ | 15 |
Mass spectra of H2L (a), ([Co(H2L)]4+)2 (b), [(CoCl2)2(H2L)]4+ (c), and ([Ni(H2L)]4+)2 (d).
A comparative study of the IR spectral data of the reported complexes with that of the uncomplexed ligand gives meaningful information regarding bonding sites of the ligand molecule with metal cations. In fact, the Schiff bases such as H2L contain a proton adjacent to the carbonyl group and consequently can undergo, in solution, an equilibrium between three tautomers as indicated in Figure
Ketoimine form of the Schiff base ligand H2L in solid state and its ketoamine or enolimine tautomer form when dissolved in organic solvent.
The main IR bands for the free ligand H2L recorded as KBr disks and its complexes in CH2Cl2 solution with their tentative assignments are given in Table
Main spectroscopic IR data for the ligand (solid state) and its complexes (CH2Cl2 solution). For UV-visible both in DMF solution.
Compound | Infrared (cm−1) | UV-vis (DMF solution) | ||||||
---|---|---|---|---|---|---|---|---|
|
|
|
|
|
|
|
| |
H2L | — | — | 1570 s | 1610 vs | — | — | — | 351 |
( |
3296 b | — | 1557 m | 1626 s | — | 516 w | 473 w | 299, 610, 678 |
( |
3293 m | — | 1551 m | 1619 s | — | 514 w | 437 w | 302 |
|
3289 m | — | 1550 m | 1625 s | — | 515 w | 435 w | 303, 608, 674 |
|
— | 3378 b | 1548 m | — | 1370 m | 514 w | 448 w | 300 |
vs: very strong; m: medium; b: broad; w: weak.
The IR spectra of [(CoCl2)2(H2L)]4+, ([Co(H2L)]4+)2 and ([Ni(H2L)]4+)2 complexes show the very strong band due to
Furthermore, the aliphatic and aromatic vibration protons are not greatly affected upon complexation. In the 3200–3500 cm−1 region, only the complex [(NiCl2)2L]2+·2H2O presents a broad band around 3378 cm−1, corresponding to the O–H stretching vibration band of water molecules. In all other spectra, the shape of the band is similar, indicating the absence of hydrogen bonding. Accordingly, the ligand acts as a dibasic tetradentate ligand coordinating to the Co2+ or Ni2+ ions and producing the 2 : 2 (metal-ligand) complexes via the ketooxygen and azomethine-nitrogen atoms. However, in case of the 2 : 1 compounds, the complexation to the Co2+ was via the ketooxygen and azomethine-nitrogen while to the Ni2+ was found to be via the enolatooxygen and azomethine-nitrogen atoms.
Electronic spectra of the ligand and its metal complexes have been measured in DMF, and the numerical data of the band maxima (
The ligand and the complexes exhibit intense bands in the high energy region in the 299–351 nm range (
The obtained values are of particular importance since they were highly dependent on the geometry of the molecule. It is known that the transitions from a square planar structure to a deformed tetrahedral one leads to a red shift of absorption in the electronic spectra [
Thus, the smaller value of the wavelength of the band corresponding to the transitions is resemblance between the geometry of the complex and that of square planar complex.
Comparison of 1H NMR spectral data of the ligand H2L and the diamagnetic Ni(II) complexes recorded in DMSO-D6 solution as indicated in Figure
1H NMR spectra of H2L (a), ([Ni(H2L)]4+)2 (b), and [(NiCl2)2L)]2+ (c) in DMSO-D6 solution and in the
The aromatic protons of the ligand (Figure
In case of the ([Ni(H2L)]4+)2 complex, no changes were observed in the 1H NMR spectrum (Figure
However, in case of the [(NiCl2)2L]2+·2H2O complex in solution the difference becomes more important since the signal which appeared in the free ligand, at
Apparently, the single peak which is located in the
In order to give more insight into the structure of the ligand and its complexes, the thermal studies of these compounds have been carried out using DSC technique where the diagrams of these products show significant differences between the ligand H2L and its complexes. The decomposition of all these species occurs at temperatures higher than 340°C and is generally followed by several exothermic peaks due to this phenomenon.
The DSC diagram of the ligand shows two endothermic peaks occurring at ca. 140 and 340°C. The first one either corresponds to the melting of the molecule or to the coordinated water, and the second one is followed by several exothermic peaks due to the chemical decomposition of the molecule.
The 2 : 1 and 2 : 2 (cobalt-ligand) complexes do not contain any hydrated or adsorbed water molecules. They are very thermally stable and show weak endothermic peaks at 360–390°C range. On the opposite side, the DSC diagram of the 2 : 1 (nickel-ligand) complex shows many peaks at the 110–130°C range and is due to loss of water molecules, while the 2 : 2 (nickel-ligand) compound shows a very weak endotherm at 190°C followed with a well-resolved endotherm at 380°C.
Depending on the IR, 1H NMR, and mass spectra, the suggested structures are given in Figures
Proposed structures for
The complexes in solutions
The complexes in the solid state
The cobalt(II) and nickel(II) complexes exhibit different structures. These species are binuclear complexes, and the coordination of the metal cation is linked through N(imino) and O(keto) atoms. The two cobalt(II) compounds present a distorted tetrahedral geometry while the two nickel(II) complexes exhibit a typical square planar surrounding [
The main electrochemical results of the examined ligand as well as its complexes are summarised in Table
Voltammetric results at room temperature (26 ± 1°C) in DMF, ionic strength 0.1 mol L−1 (TBAP), results in V versus SCE, sweep speed: 100 mVs−1,
Compound |
|
|
|
|
|
|
|
|
|
---|---|---|---|---|---|---|---|---|---|
H2L | — | +0.56 | +0.87 | +1.20 | −0.97 | +0.76 | — | 0.11 | 0.925 |
( |
−0.10 | +0.62 | +1.05 | +1.12 | −1.14 | +0.68 | +1.25 | 0.37 | 1.235 |
( |
−0.11 | +0.73 | +1.17 | +1.35 | −1.54 | +0.65 | +1.20 | 0.52 | 1.430 |
|
−0.10 | +0.59 | +0.92 | +1.31 | −1.17 | +0.68 | +1.17 | 0.24 | 1.040 |
|
+0.11 | +0.54 | +0.94 | +1.47 | −1.18 | +0.66 | +1.19 | 0.28 | 1.080 |
Representative cyclic voltammograms in DMF at room temperature (ionic strength: 0.1 mol L−1, Bu4NClO4,
The voltammetric response of the ligand H2L in the –1.8 to +1.8 V range (Figure
When the speed sweep is varying (i.e., 500, 300, 200, 100 mVs−1, resp.), the curve
The cyclic voltammograms of the four complexes exhibit oxidation and reduction metal centred processes, as for complexes previously published [
In case of nickel(II) complexes, these products in addition to the ligand peaks undergo reduction processes located at –1.54 V and –1.18 V (Figures
Furthermore, a linear dependence which was observed between
These results are in good agreement with the above proposed structures.
In summary, the Co(II) and Ni(II) complexes were synthesized using the binucleating tetradentate chelating agent formed by the condensation of 4,4′-diaminobiphenyl with 2,4-pentanedione. IR, 1H NMR, mass spectral data, and elemental analysis are used to confirm the structure of the Schiff base H2L. The mass spectra are also used to proof the stoichiometry and formulation of the complexes. Based on the UV-vis spectral data, a square planar geometry is assumed for Ni(II) complexes, while the Co(II) compounds present a distorted tetrahedral geometry. In addition, they are more easily oxidizable in the cyclic voltammetry and also show a quasireversible behavior. Finally, these products could be investigated in the near future in the fields of microbiology and electrocatalysis.
The authors are indebted to and thank the Pharmacy Department (U. F. A. Sétif) for the general facility projects and financial support.