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 OJIC  Vol.6 No.1 , January 2016
Synthesis and Characterization of Novel μ-Carbonato Tetranuclear Copper Complexes [(Pip)4nCu4X4(CO3)2] in Aprotic Media
Abstract: In this work, novel oxidative coupling complexes, [(Pip)4nCu4X4(CO3)2] (n = 1 or 2, X = Cl or Br, Pip = piperidine), are synthesized from the reaction of well characterized Lewis base [(Pip)4nCu4X4O2] with carbon dioxide as a Lewis acid in CH2Cl2. These carbonato-derivatives are isolated as stable solids. They are easily soluble in aprotic solvents as CH2Cl2or phNO2. 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)4nCu4X4(CO3)2]. The EPR spectra for [(Pip)4nCu4X4(CO3)2] complexes are axial type of spectra (dx2-y2 G.S) suggesting elongated tetragonal distortion for all of them. Cyclic voltammograms for [(Pip)4nCu4X4(CO3)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).

Received 22 November 2015; accepted 12 January 2016; published 15 January 2016

1. Introduction

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 [1] - [7] . Studies including the reactions of these models with dioxygen give good information for detecting the geometrical structure of the copper ion in protein [8] . Dioxygen is found to activate copper(I) complexes for synthesis of new oxidative coupling catalysts for phenols [9] - [11] . These catalysts show a great biological importance as they imitate the tyrosinase enzyme activity for phenol oxidation [9] - [11] . In these oxidation processes, copper(I) is oxidized to copper(II) and the molecular oxygen will be reduced to superoxo, peroxo, hydroxo or oxo species [12] - [16] . The oxo-type complexes are reported during the reaction of some tetranuclear Cu(I) complexes with O2 [13] - [16] . The 3-dimensional molecular geometry of tetranuclear [(Pip)CuI]4 is elucidated using X-ray in previous work [17] . The molecular (core) structure of [(Pip)CuI]4 is in fact, very closely identical to previous work for [LCuI]4; L = pyridine (Py) or N, N-diethylnicotinamide (DENC) [17] [18] .

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.

2. Experimental

2.1. Reagents

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) [19] .

2.2. Instrumentation

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) [20] . Cyclic voltammetery (CV) measurements were carried out using a bioanalytical system BAS-27 electrochemistry analyzer connected with BAS, X-Y recorder and in conjugation with a three electrodes cell fitted with a purged dinitrogen gas inlet and outlet. Three electrodes were a Beckman Pt working electrode at room temperature (5 mm diameter) and a Pt wire auxiliary electrode. All potentials of Cu-complexes (1.0 × 10−3 M) were determined using Ag/Ag+ as a reference electrode (1.0 × 10−3 M AgNO3 in a 0.1 M TBAP (tetrabutylammonium perchlorate) in CH2Cl2 under N2 gas at room temperature [21] . Molecular weight determination was performed via freezing point depression of nitrobenzene solution containing a known amount of solute using Eutechnics precision temperature, model 4600 thermometer [22] . The elemental analyses for Cu and X (Cl, Br) were estimated using the same protocols reported before [23] . CHNS analysis was obtained using LECO CHNS-932 Elemental Analyzer.

2.3. Synthesis of Complexes

2.3.1. Synthesis of [(Pip)4nCu4X4] (n = 1 or 2, X = Cl or Br)

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.

2.3.2. Synthesis of [(Pip)4nCu4X4(CO3)2] (n = 1 or 2, X = Cl or Br) Complexes

[(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].

2.4. Tests of Catalytic Activity

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.

3. Results and Discussion

3.1. Reaction of [(Pip)4nCu4X4] Complexes with O2 and CO2

[(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 [24] - [28] .

(1)

(2)

In this reaction, CO2 acted as a Lewis acid for the accessible basic µ-oxo copper(II) centers [24] - [28] . Solid [(Pip)4nCu4X4(CO3)2] products formed strong effervescence with dilute HCl to confirm the presence of carbonato moiety. The molar mass and analytical results for the prepared complexes are illustrated in Table 1. The molar mass determination confirmed that all [(Pip)4nCu4X4(CO3)2] complexes are stable tetranuclear species.

3.2. Infrared Spectra

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, Figure 1. For n = 2 complexes, these bands were broad or splitted which may be attributed to the fact that each Cu centre is surrounded by two Pip ligands, in which the hydrogen of one Pip ligand is free, while the other hydrogen in pip ligand is hydrogen bonded with basic centre existing in carbonato complexes as described before in similar reported cases for the oxo [(Pip)4nCu4X4O2] complexes [24] - [28] . The spectrum of Pip showed also two bands at 1652 cm−1 and 1542 cm−1 due to δNH, which became overlapped, broad and shifted to 1610 cm−1 on complexation, Figure 1 [24] - [28] . The carbonato bridge has characteristic vibrational bands, n3 at 1600 - 1500 cm−1 and at 1490 - 1350 cm−1, n2 (900 - 800) cm−1 and n4 (750 - 700) cm−1 [26] [27] . The increase in the intensity of n3 band at 1450 cm−1, for the [(Pip)4Cu4Cl4(CO3)2], Figure 1 confirmed the fact that the bridge is tridentate to fulfil the 6-coordinated Cu(II) centres, (Scheme 1(a)). While in the [(Pip)8Cu4Cl4(CO3)2], the bridge switches to bidentate ligand (Scheme 1(b)). From (900 - 800) cm−1, the carbonato bridge has n2 while Pip has three bands; 805, 830 and a very strong one at 863 cm−1. Therefore, the strong band at 863 cm−1 will be moved to 877 cm−1 for [(Pip)4Cu4Cl4(CO3)2] and its intensity at the same wave number, 877 cm−1, becomes weak by adding one extra Pip per each Cu(II) centre as in [(Pip)8Cu4Cl4(CO3)2], Figure 1, supporting the

Table 1. Analytical and cryoscopic data for [(Pip)4nCu4X4(CO3)2]; n = 1 or 2 and X = Cl or Br.

Figure 1. KBr disk or plates I.R. spectra for (a) Piperidine, (b) [(Pip)4Cu4Cl4 (CO3)2] and (c) [(Pip)8Cu4Cl4(CO3)2].

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) [24] - [28] .

3.3. Electronic Spectra

The electronic spectral data of [(Pip)4nCu4X4(CO3)2] complexes are presented in Table 2 and Figure 2. A splitted peak within (740 - 840) nm range for the studied complexes are observed indicating the presence of at least 3 halo ligands per each Cu(II) centre which indicates a tetranuclear cuban core structured complexes (Scheme 1) [24] - [28] . It is noticed that the values of e (M−1 cm−1) of the splitted peak in case of bromo-complexes are 1.7 times greater than those of the chloro-complexes for similar n. To maintain the coordination number 6 for Cu(II), the only possible change is the conversion of carbonato bridging ligand from tridentate as in structure a to bidentate as in structure b (Scheme 1). A previous work showed a similar spectral behavior for the comparable complexes, of which the electronic spectra were attributed to LMCT between a minimum of 3 halo ligands and a Cu(II) site [24] - [31] .

3.4. EPR Spectra

The solid state EPR spectra of [(Pip)4nCu4X4(CO3)2], Figure 3 and Table 2 show axial spectra with g|| > g^ > 2.04, confirming the dx2-y2 ground state for elongated tetragonal octahedral geometry [32] [33] .

Figure 2. Electronic spectra of (a) [(Pip)4Cu4Cl4(CO3)2], (b) [(Pip)4Cu4Br4(CO3)2], (c) [(Pip)8Cu4Cl4(CO3)2] (d) [(Pip)8Cu4Br4(CO3)2].

Table 2. EPR and electronic spectral data for [(Pip)4nCu4X4(CO3)2]; n = 1or2, X = Cl or Br, in CH2Cl2 at room temperature.

Figure 3. EPR spectra of (a) [(Pip)4Cu4Cl4(CO3)2], (b) [(Pip)8Cu4Cl4(CO3)2], (c) [(Pip)4Cu4Br4(CO3)2], (d) [(Pip)8Cu4Br4(CO3)2].

3.5. Redox Chemistry

The CV measurements, Table 3 and Figure 4 for [(Pip)4nCu4X4(CO3)2] complexes are conducted in CH2Cl2 made of 0.1 M TBAP using a Pt-working electrode with non-aqueous reference electrode (Ag/Ag+, 1.0 ´ 103 M AgNO3 in 0.1 M TBAP in CH2Cl2) and Pt-wire auxiliary electrode. The electrode potentials are measured against Ag/Ag+ as a reference electrode. The Ferrocene (Fc) internal standard was used against Ag/Ag+ under similar experimental conditions to correlate the electrode potentials to NHE. The formal electrode potential of a reversible one-electron standard Fc/Fc+ against NHE is 0.4 volt [34] [35] . The CV of [(Pip)4nCu4X4(CO3)2] complexes are irreversible in character and show two cathodic peaks as in Table 3. The above systems are quite similar to [(Pip)4nCu4X4O2] cyclic voltammograms in which the electrolysis at −1.3 volt, indicated four electrons to reduce four copper(II) [34] [35] . In [(Pip)4nCu4X4(CO3)2], the reduction occurs in two steps separated by about 0.35 volt, due to the morphology of the tetranuclear cuban structure on the electrode surface.

3.6. Test of Catalytic Activity

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

Figure 4. Cyclic voltammetry of 1.0 ´ 10−3 M of (a) [(Pip)4Cu4Cl4(CO3)2], (b) [(Pip)8Cu4Cl4(CO3)2], (c) [(Pip)4Cu4Br4(CO3)2], (d) [(Pip)8Cu4Br4(CO3)2] in (e) 0.1 M TBAP in CH2Cl2 solvent at Pt working electrode at room temperature, and scan rate 100 mV/s.

Table 3. Cyclic voltammetric data for 1.0 ´ 10−3 M [(Pip)4nCu4X4(CO3)2]; n = 1 or 2 and X = Cl or Br, at scan rate 100 mV/sec in 0.1 M TBAP in CH2Cl2 and at room temperature.

*0.4 volt is the formal electrode potential of a reversible one-electron standard couple (Fc/Fc+) versus NHE [21] .

431 nm by making comparison with an authentic sample (e = (5.05 ± 0.01) ´ 104 M1・cm1) [36] was the only product in all reactions, Scheme 2.

After 3 days, the yield of (DPQ) formed was in the range of (55% ± 5%), the same yield was also observed for [(Pip)4Cu4Cl4O2] [24] .

4. Conclusion

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.

NOTES

*Corresponding author.

Cite this paper: El-Sayed, M. , Elwakeil, H. , Salam, A. and Elbadawy, H. (2016) Synthesis and Characterization of Novel μ-Carbonato Tetranuclear Copper Complexes [(Pip)4nCu4X4(CO3)2] in Aprotic Media. Open Journal of Inorganic Chemistry, 6, 66-75. doi: 10.4236/ojic.2016.61004.
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