In this work, novel oxidative coupling complexes, [(Pip) 4nCu 4X 4(CO 3) 2] (n = 1 or 2, X = Cl or Br, Pip = piperidine), are synthesized from the reaction of well characterized Lewis base [(Pip) 4nCu 4X 4O 2] with carbon dioxide as a Lewis acid in CH 2Cl 2. These carbonato-derivatives are isolated as stable solids. They are easily soluble in aprotic solvents as CH 2Cl 2or phNO 2. Cryoscopic measurements support tetranuclear structure for all of them. Electronic spectra in the near infrared with high molecular absorptivity may be explained for tetranuclear cuban structure to fulfil 3 halo-ligands for each copper centre in [(Pip) 4nCu 4X 4(CO 3) 2]. The EPR spectra for [(Pip) 4nCu 4X 4(CO 3) 2] complexes are axial type of spectra (d x2-y2 G.S) suggesting elongated tetragonal distortion for all of them. Cyclic voltammograms for [(Pip) 4nCu 4X 4(CO 3) 2] are irreversible in character. These tetranuclear carbonato complexes show catalytical activity. They initiate the oxidation of 2,6-dimethylphenol (DMP) to 3,3’,5,5’-tetramethyl-4,4’-diphenoquinone (DPQ).
There has been a great worldwide interest in the preparation and characterization of a large number of copper complexes using elemental oxygen to imitate the active sites of certain copper enzyme models [
This work is designed to synthesize and characterize some novel µ-carbonato complexes, [(Pip)4nCu4X4 (CO3)2] (where: n = 1 or 2, X = Cl or Br, Pip = piperidine) from the reaction of tetranuclear-µ-oxo [(Pip)4n Cu4X4O2] complexes with CO2. In this work, both the basicity of oxo-centre and the non-linearity of Cu-O-Cu angle in [(Pip)4nCu4X4O2] allow the insertion of CO2 to form the corresponding carbonato complexes.
Pip (Aldrich), was used after vacuum distillation, (pKb = 2.8). Gaseous CO2, was dried by passage through a 10 cm column of Drierite. PhNO2, was distilled from P2O5, and kept over 4 Å molecular sieves (Kf = 7.0˚C/molal, d = 1.25). CH2Cl2 was washed with concentrated sulphuric acid, dried over Na2CO3, refluxed over P2O5, then distilled and stored over anhydrous Na2CO3. DMP was purified by sublimation, (m.p. 46˚C - 47˚C). Dinitrogen gas was deoxygenated by passage through a column of Alfa-DE-Ox solid catalyst and dried by passage through a 60 cm column of dehydrated silica gel and 30 cm column of (Calcium chloride and molecular sieves). Copper(I) halides were prepared as described in literature (CuCl and CuBr) [
UV-vis spectrophotometer model 160A (Shimadzu) was used to record the electronic spectra of the investigated complexes. FT-IR spectra of the free ligands and their complexes were performed as KBr discs using Perkin Elmer System 2000 FT-IR spectrophotometer. Calibration of wave numbers was made with a polystyrene film. EPR spectra for the investigated copper complexes were measured using a Radiopan varian spectrometer at 100.0000 KHz at different G modulation amplitude with rectangular TE 102 cavity and 100 KHz modulation field Resonance conditions were found at 9.7 GHz (X-band) at room temperature. The field was calibrated with a powder of diphenylpicrylhydrazyl (DPPH; g = 2.0037) [
A solution of Pip (2.5 mmole) in (30 ml) CH2Cl2 was flushed with pure N2 gas for 10 mins. The appropriate copper (I) halide (X = Cl orBr) (2.5 mmole) was then added under N2. The reaction mixture was stirred with a stream of N2.
[(Pip)4nCu4X4] solution in a deoxygenated CH2Cl2 was flushed with O2 and CO2 gases for about 10 min., then the solvent was removed by vacuum rotary evaporator leaving a solid of the dicarbonato complex, [(Pip)4nCu4 X4(CO3)2].
CH2Cl2 Solutions of [(Pip)4nCu4X4(CO3)2] complexes were added to various samples of 100 fold excess of DMP in CH2Cl2. O2 was then streamed through each solution for 20 min. DPQ was characterized at 431 nm by comparison with an authentic sample.
[(Pip)4nCu4X4] complexes are oxidized by stoichiometric amount of O2 under N2 condition to form [(Pip)4n Cu4X4O2], Equation 1, followed by rapid reaction with CO2 in accordance with Equation 2 under N2 [
In this reaction, CO2 acted as a Lewis acid for the accessible basic µ-oxo copper(II) centers [
In the FTIR spectrum of the free Pip ligand, a peak appeared at 3445 cm−1 assigned as nNH which was shifted to 3281 cm−1 in the spectra of [(Pip)4nCu4X4(CO3)2] indicating the coordination of Cu-centres to piperidyl nitrogen,
Complex | Anal. % (Calc.) | Molar mass | ||||
---|---|---|---|---|---|---|
C | H | N | Cu | X | Ma | |
[(Pip)4Cu4Cl4(CO3)2] | 29.8 (30.8) | 5.3 (5.1) | 6.2 (6.4) | 29.2 (29.7) | 17.4 (16.6) | 890 ± 20 (856) |
[(Pip)4Cu4Br4(CO3)2] | 24.2 (25.5) | 4.4 (4.3) | 5.4 (5.4) | 24.9 (24.6) | 30.3 (30.9) | 1128 ± 20 (1033) |
[(Pip)8Cu4Cl4(CO3)2] | 39.0 (42.1) | 6.6 (7.3) | 9.2 (9.4) | 21.1 (21.2) | 12.2 (11.9) | 1190 ± 20 (1197) |
[(Pip)8Cu4Br4(CO3)2] | 34.0 (36.7) | 6.1 (6.4) | 7.8 (8.2) | 18.4 (18.5) | 23.0 (23.3) | 1390 ± 20 (1375) |
Scheme 1. Proposed molecular core structures for [(Pip)4nCu4X4(CO3)2].
change of carbonato bridge from structure a to structure b as in (Scheme 1) [
The electronic spectral data of [(Pip)4nCu4X4(CO3)2] complexes are presented in
The solid state EPR spectra of [(Pip)4nCu4X4(CO3)2],
Complex | EPR | Electronic spectra | |||
---|---|---|---|---|---|
A|| | g|| | g^ | l max., nm (e, M−1・cm−1) | ||
[(Pip)4Cu4Cl4(CO3)2] | 166.00 | 2.379 | 2.065 | 2.172 | 740 (680), 840 (675) |
[(Pip)8Cu4Cl4(CO3)2] | 102.13 | 2.390 | 2.075 | 2.180 | 740 (590), 840 (560) |
[(Pip)4Cu4Br4(CO3)2] | 162.16 | 2.440 | 2.077 | 2.198 | 740 (1130), 840 (1080) |
[(Pip)8Cu4Br4(CO3)2] | 53.00 | 2.387 | 2.090 | 2.187 | 740 (1000), 840 (950) |
The CV measurements,
CH2Cl2 solutions of [(Pip)4nCu4X4(CO3)2] complexes were added to various samples of a 100-fold excess of DMP in CH2Cl2. O2 was then streamed through each solution for 20 min. The DPQ which was characterized at
Complex | Cathodic peaks, Volt | Anodic peaks, Volt | |
---|---|---|---|
Ferrocene* | −0.25 | −0.05 | |
[(Pip)4Cu4Cl4(CO3)2] | −0.67 | −1.06 | |
[(Pip)8Cu4Cl4(CO3)2] | −0.70 | −1.13 | |
[(Pip)4Cu4Br4(CO3)2] | −0.85 | −1.16 | |
[(Pip)8Cu4Br4(CO3)2] | −0.81 | −1.13 |
*0.4 volt is the formal electrode potential of a reversible one-electron standard couple (Fc/Fc+) versus NHE [
431 nm by making comparison with an authentic sample (e = (5.05 ± 0.01) ´ 104 M−1・cm−1) [
After 3 days, the yield of (DPQ) formed was in the range of (55% ± 5%), the same yield was also observed for [(Pip)4Cu4Cl4O2] [
According the characterization data, novel complexes of [(Pip)4nCu4X4(CO3)2] can be used as oxidative coupling initiators for oxidation of DMP to DPQ, Scheme 2. It is worth to mention that the formation of [(Pip)4n Cu4X4(CO3)2] complexes can be explained on the basis that the angle of Cu-O-Cu in [(Pip)4nCu4X4O2] is
Scheme 2. Catalytical cycle for homogenous oxidative coupling of phenols by copper catalyst.
acute to a degree suitable to let oxo centre basic enough for catalytic activity and to permit CO2 insertion to produce the carbonato complexes. Cryoscopic measurements support tetranuclear structure for all of them.
Mohamed A.El-Sayed,Hoda A.Elwakeil,Ahmed H. AbdelSalam,Hemmat A.Elbadawy, (2016) Synthesis and Characterization of Novel μ-Carbonato Tetranuclear Copper Complexes [(Pip)4nCu4X4(CO3)2] in Aprotic Media. Open Journal of Inorganic Chemistry,06,66-75. doi: 10.4236/ojic.2016.61004