New symmetrical binucleating ligands N,N-bis[2-hydroxy-5-methyl-3-(4-methyl-piperazinomethyl)benzyl]-alkylamines L1 and L2 and their copper(II) complexes [Cu2L(X)2]·2H2O, where X = CH3COO−, C6H5COO−, Cl−, and
Interest in the synthesis of new binucleating ligands with different donor atoms and flexibility [
All the chemicals and reagents used in this study were obtained from Merck and Sigma Aldrich. Tetra-N-butyl ammonium perchlorate was purchased from Fluka and recrystallised from ethanol-water mixture (caution: TBAP is potentially explosive; hence, care should be taken in the handling of this compound). HPLC grade dimethylformamide and acetonitrile were obtained from S.D. Fine chemicals.
Elemental analyses were carried out by using Carlo Erba model 1106 elemental analyser. Atomic absorption spectra were recorded on Varian AA-200 model atomic absorption spectrophotometer. 1H NMR (90 MHz) spectra were recorded in CDCl3 using a FX-90Q Fourier transform NMR spectrometer. 13C-NMR spectra (200 MHz) were recorded on a JEOL model 400 NMR spectrometer and the EI mass spectra were recorded on a JMS DX 303-HF mass spectrometer. IR spectra were recorded on a Hitachi 320 spectrophotometer on KBr disks in the range 250–4000 cm−1. Electronic spectra were recorded on a Hewlett-Packard 8452 A diode array spectrophotometer in the range 250–850 nm. Molar conductivity measurements were measured on an Elico digital conductivity bridge model M88 and conventional cell which was previously calibrated with an aqueous solution of KCl (0.1 N).
For cyclic voltammetry, a three-electrode system was used, in which the counter and working electrode were small platinum foils and saturated Ag/AgCl was the reference electrode. TBAP was used as the supporting electrolyte (0.1 M) and all the solutions were around 10−3 M in concentration. The cyclic voltammograms were obtained on an apparatus comprising a PAR model-173 potentiostat/galvanostat, model-175 universal programmer, model-176 coulometer, and a Perkin-Elmer Hitachi (057)
The ligands, L1 and L2, were prepared by following a two-step procedure. In the first step N,N-bis[2-hydroxy-5-methylbenzyl]methylamine (PC1) and N,N-bis[2-hydroxy-5-methylbenzyl]ethylamine (PC2) were prepared by reacting p-cresol, paraformaldehyde, and primary amines in 2 : 2 : 1 ratio in ethanol medium, by following the literature procedure [
Schematic diagram for the synthesis of ligands and complexes.
Paraformaldehyde (0.75 g, 0.025 mol) was taken in 150 mL of glacial acetic acid and allowed to stir for 4 hours. To this N-methylpiperazine (2.3 mL, 0.02 mol) was added and stirring was continued for 24 hours. The N,N-bis[2-hydroxy5-methylbenzyl]methylamine (2.71 g, 0.01 mol) was added and stirred for 48 hours. The resulting solution was kept at 75°C for about 3 hours. It was cooled and neutralised using saturated Na2CO3. A pasty product obtained was thoroughly washed using distilled water. Then it was extracted using dichloromethane (350 mL). The extracted was treated with activated charcoal and filtered. The filtrate was evaporated. A pale yellow oily substrate separated was recrystallised in benzene-ethanol mixture and dried under vacuum. Mp: 71°C. 13C-NMR-(CDCl3/200 MHz/TMS),
The above procedure was used for the preparation of this compound using N,N-bis[2-hydroxy-5-methylbenzyl]ethylamine, instead of N,N-bis[2-hydroxy-5-methylbenzyl]methylamine. Mp: 79°C. 13C-NMR-(CDCl3/200 MHz/TMS),
To a hot methanol-dichloromethane (2 : 1, 50 : 25 v/v) solution of the ligand L1 (1.0 g, 2 mmol) copper(II) perchlorate hexahydrate (1.48 g, 4 mmol) was added, and the mixture was refluxed on a water bath for 2 hours. Then sodium acetate trihydrate (2.7216 g, 2 mmol) dissolved in minimum amount of water was added and refluxed for one more hour. The resulting solution was filtered and kept at room temperature for few days. The green precipitate separated was filtered off, washed with cold methanol and a little diethyl ether, and dried under vacuum. Yield 1.15 g (70.6%). Anal. Calcd. for C31H50ClCu2 N5O10 (%): C, 45.7; H, 6.1; Cu, 15.6; N, 8.6. Found: C, 45.1; H, 6.0; Cu, 15.5; N, 8.4.
The above procedure was used for the preparation of this complex using ligand L2 (1.018 g, 2 mmol) instead of ligand L1. Green solid was obtained. Yield 1.17 g (70%). Anal. Calcd. for C32H52ClCu2N5O10 (%): C, 46.4; H, 6.3; Cu, 15.4; N, 8.4. Found: C, 45.9; H, 6.1; Cu 15.3; N, 8.2.
The procedure used for the preparation of these complexes is same as that of [Cu2L1 (OAc)]ClO4·2H2O. Here sodium benzoate (0.298 g, 2 mmol) was used instead of sodium acetate. The reaction was carried in methanol-dichloromethane mixture. Dark green solid colour was obtained Yield 1.32 g (75.42%) for (
To a hot methanolic solution (100 mL) of the ligand L1 (1.0 g, 2 mmol) copper(II) chloride dihydrate (3.409 g, 4 mmol) was added to dissolved distilled methanol (50 mL) and the mixture was refluxed on a water bath for 2 hours. The resulting yellowish green solution was evaporated at room temperature for several days, filtered off, washed with water, followed by little diethyl ether, and dried under vacuum. Yellowish green precipitate was obtained. Yield 0.95 g (65.5%). Anal. Calcd. for (
This complex was prepared by following the above procedure using ligand L2 (1.018 g, 2 mmol) instead of ligand L1. Green solid colour was obtained. Yield 1.05 g (70.9%). Anal. Calcd. for (
This complex was prepared by following the procedure adopted for the preparation of (
The above procedure was used, using ligand L2 (1.018 g, 2 mmol) instead of ligand L1. Dark green-coloured product formed was filtered, washed with water, and dried under a vacuum. Yield 1.15 g (66.4%). Anal. Calcd. for (
All these complexes (
The ligands (L1 and L2) were characterised by analytical method and mass spectral studies. In the 1H-NMR spectra the peaks for aromatic hydrogens appear around
Electronic spectra of all the complexes were studied in methanolic medium, and the data are summarised in Table
Electronic spectral data for the complexes (in methanol).
No. | Complexes |
|
LMCT | LLCT |
---|---|---|---|---|
| ||||
1 |
|
650 (184) | 388 (1270) | 288 (15 800) |
2 |
|
664 (297) | 436 (1330) | 288 (15 500) |
3 |
|
650 (143) | 390 (687) | 292 (10 900) |
4 |
|
645 (220) | 396 (1200) | 287 (15 200) |
5 |
|
650 (290) | 420 (1310) | 284 (14 100) |
6 |
|
638 (249) | 412 (540) | 286 (12 500) |
7 |
|
657 (208) | 405 (sh) | 287 (14 700) |
8 |
|
658 (192) | 410 (sh) | 285 (13 300) |
CT: charge transfer, Sh: shoulder.
The electrochemical behaviour of the complexes was studied by cyclic voltammetry and the electrochemical data such as cathodic peak potential (
Electrochemical data for the complexes (in dmfa).
No. | Complexes |
|
|
|
|
|
|
---|---|---|---|---|---|---|---|
1 |
|
−0.52 | −0.36 | −0.44 (160) | −0.94 | −0.70 | 0.82 (240) |
2 |
|
−0.58 | −0.42 | −0.50 (160) | −0.91 | −0.75 | −0.80 (160) |
3 |
|
−0.50 | −0.34 | −0.42 (160) | −0.98 | −0.74 | −0.86 (240) |
4 |
|
−0.48 | −0.32 | −0.40 (160) | −1.02 | −0.88 | −0.85 (140) |
5 |
|
−0.66 | −0.52 | −0.59 (140) | — | — | — |
6 |
|
−0.72 | −0.50 | −0.61 (220) | — | — | — |
7 |
|
−0.68 | −0.50 | −0.59 (180) | — | — | — |
8 |
|
−0.70 | −0.48 | −0.59 (220) | — | — | — |
supporting electrolyte: TBAP; concentration: complex (
Cyclic voltammogram for the complexes (a) [Cu2L2(OAc)]ClO4·2H2O. (
The coulometric studies of the chloro and perchlorate bridged complexes at a –0.85 V indicate that the complexes consumed two electrons per molecule (
A comparison of the reduction potentials of the complexes of a particular ligand indicates that the reduction potential is exogenous bridging donor dependent. Acetate and benzoate bridged complexes undergo two-step single electron transfer, whereas chloro and perchlorate complexes undergo single step two-electron transfer.
Room temperature as well as variable temperature (77–300 K) magnetic studies were performed for all the complexes, and the magnetic data are given in Table
ESR spectral and magnetic data for the complexesa.
No. | Complexes |
|
−2 J/cm−1 |
|
|
|
|
|
---|---|---|---|---|---|---|---|---|
|
| |||||||
1 |
|
1.56 | 26 | 2.20 | 0.003 | — | — | — |
2 |
|
1.52 | 38 | 2.20 | 0.003 | — | — | — |
3 |
|
1.45 | 53 | 2.20 | 0.001 | — | — | — |
4 |
|
1.50 | 48 | 2.20 | 0.002 | — | — | — |
5 |
|
1.72 | — | — | — | 165 | 2.17 | 2.05 |
6 |
|
1.73 | — | — | — | 170 | 2.23 | 2.10 |
7 |
|
1.71 | — | — | — | 160 | 2.20 | 2.10 |
8 |
|
1.72 | — | — | — | 160 | 2.22 | 2.09 |
Temperature dependence magnetic properties of the complex [Cu2L1(OAc)]ClO4·2H2O· (
The room temperature magnetic moment value of the complexes
Both room temperature and liquid nitrogen temperature ESR spectra of the complexes were recorded in DMF and the
ESR Spectra for the complexes (a) [Cu2L1(OAc)]ClO4·2H2O. (
Comparison of the coulometric, magnetic, and ESR spectral studies indicates that, in the complexes, the chemical behaviour varies depending upon the nature of the exogenous donor atoms. The electrochemical studies of these complexes indicate that all the complexes undergo quasi-reversible reduction at negative potential in the range of
Comparison of the chemical behaviour of the complexes of
The authors declare that there is no conflict of interests.
The authors thank the Department of Science and Technology (DST), Government of India, New Delhi, for financial support, Sanction no. SR/FT/CS-042/2008.