This paper reports the kinetics of the oxidation of 2,6-dimethylphenol (DMP) to get 3,3’,5,5’-tetra- methyl-4,4’-diphenoquinone (DPQ) using novel oxidative coupling complexes [(Pip)
4nCu
4X
4-(CO
3)
2] (n = 1 or 2, X = Cl or Br, Pip = piperidine). The new prepared tetranuclear complexes were characterized using cryoscopic measurements, electronic spectra, FTIR, EPR and cyclic voltammetry techniques. These complexes are catalytically active. The proposed mechanism of the catalytic oxidative coupling can be illustrated as a pre-equilibrium, K, between the catalyst and DMP to form a complex intermediate which is converted to activated complex through the rate determining step, k
2, to form the final product. The inverse of the observed rate constants k
obsd versus 1/[DMP]
2 gives a straight line with intercept. From the slope and the intercept, both K and k
2 are obtained. At different temperatures, thermodynamic and kinetic parameters are evaluated. It is worth to mention that, the dependence of k
obsd on [DMP]
2 indicates that the coordination number for every copper center in both n = 1 or 2 in [(Pip)
4nCu
4X4(CO
3)
2] is equal to six. Therefore, carbonato bridging centers in n = 1 acts as a tridentate ligand, while for n = 2 acts as a bidentate ligand.
The study of homogenous oxidative coupling of DMP using copper catalysts has attracted scientists for several years from the catalytic industry point of view as well as a model for tyrosinase enzyme. These copper catalysts showed activity towards phenol oxidation like tyrosinase [1] - [13] . In previous work it was reported that the oxo copper complexes showed a catalytic activity towards phenol oxidation [14] - [18] . The full 3D-molecular structure of tetranuclear copper (I) complex of [(Pip)CuI]4 was studied by Volker Schramm [19] and also the crystal structure of the copper (I) iodide-pyridine was previously reported [20] .
The aim of this work is to study the kinetics of oxidation of DMP to DPQ (Scheme 1) using the prepared [(Pip)4nCu4X4(CO3)2] complexes and the strategy is:
1) The use of well investigated tetranuclear carbonato-copper (II) complexes, [(Pip)4nCu4X4(CO3)2] as catalysts for oxidation of (DMP) to (DPQ).
2) To evaluate those initiators [(Pip)4nCu4X4(CO3)2] with [(Pip)4nCu4X4O2], the mechanism will deal with the first cycle under dinitrogen and the reaction followed by reduction of copper (II) to copper (I) at 740 nm or by DPQ formation at 431 nm.
3) The efficiency of the [(Pip)4nCu4X4(CO3)2] relative to the oxo analogues for oxidative coupling of (DMP) to (DPQ).
4) The impact of structural change in [(Pip)4nCu4X4(CO3)2] when n changes from 1 to 2, since the maximum coordination number of copper (II) is six.
2. Experimental2.1. Reagents
Piperidine, Pip (Aldrich) was used as received.10 cm column of Drierite was utilized to dry CO2 gas. The copper (I) halides as well as C6H5NO2, CH2Cl2, DMP and N2 gas were prepared for this work according to the procedures described in the literature [21] .
2.2. Instrumentation
The rates of oxidation of (DMP) by copper complexes [(Pip)4nCu4X4(CO3)2]; in C6H5NO2 were measured by 160 A uv-visible recording spectrophotometer Shimadzu in matched quartz cells. The monitoring wavelength was 740 nm. Activation parameters were elucidated by repeating the reactions at different temperatures (20˚C - 50˚C). All reactions and measurements are carried out at least three times under fixed conditions to give maximum error of ±4% in each reported rate constant.
2.3. Kinetic Measurements
The mechanism and kinetics of the catalytic oxidation of DMP (2.0 - 16.0) × 10−2 M using the copper [(Pip)4n- Cu4X4(CO3)2] complexes (1.0 × 10−3 M) in C6H5NO2 were investigated by uv-vis spectrophotometer at 740 nm. Activation parameters were elucidated by repeating the reactions at different temperatures (20˚C - 50˚C). All reactions and measurements are carried out at least three times under fixed conditions to give maximum error of ±4% in each reported rate constant.
3. Results and Discussion3.1. Stoichiometry and Products of Oxidation of Copper (I) Complexes by O<sub>2</sub> and CO<sub>2</sub>
The formation of tetranuclear complexes [(Pip)4nCu4X4]; n = 1 or 2, X = Cl, Br or I under N2 in C6H5NO2 or
CH2Cl2 as aprotic solvents were carried out as described in Equation (1) and Equation (2).
Scheme 1. Proposed catalytical cycle for homogenous oxidative coupling of phenols.
The preparation of the new µ-carbonato complexes [(Pip)4nCu4X4(CO3)2] was performed under N2 by the reaction of [(Pip)4nCu4X4] (n = 1 or 2) with O2 followed by fast reaction with CO2 as described in Equation (3) and Equation (4) and Scheme 1 [22] - [28] .
The µ-carbonato [(Pip)4nCu4X4(CO3)2] complexes are easily soluble in C6H5NO2 and CH2Cl2. The molar mass determination, analytical data, FTIR and electronic spectra (Figure 1) indicate that the formed complexes, [(Pip)4nCu4X4(CO3)2] are stable tetranuclear, similar to their copper (I) precursors (Scheme 2) [22] - [28] .
Scheme 2. Proposed molecular structures for [(Pip)4nCu4X4(CO3)2].
3.2. Kinetics of Oxidation of DMP to DPQ Using Novel [(Pip)<sub>4n</sub>Cu<sub>4</sub>X<sub>4</sub>(CO<sub>3</sub>)<sub>2</sub>] Complexes
The kinetics of homogeneous oxidative coupling of DMP to DPQ, Equation (5) are investigated at λmax = 740 nm under pseudo 1st order conditions, where the concentration of [(Pip)4nCu4X4(CO3)2] complex is 1.0 × 10−3 M while the excess DMP concentration changes from 2 × 10−2 to 16 × 10−2 M in C6H5NO2 under N2 atmosphere to
insure one cycle only, Figures 2-5. The first-order plots of with time t, where At is the absorbance
of [(Pip)4nCu4X4(CO3)2] at time t, were linear for at least 4 half-lives, Figures 6-9. A plot of the reciprocal of the observed pseudo first-order rate constant vs. at constant temperature gives a straight line, Figures 10-13, suggesting that 2 molecules of (DMP) are included in the rate determining step with 1 molecule of [(Pip)4nCu4X4(CO3)2]. Such results suggest a mechanism similar to that reported for oxo complexes [(Pip)4n- Cu4X4O2] (Scheme 1), Equations (6)-(10) [29] [30] . From the relation of vs., K and k2 are collected in Table 1. Thermodynamic and activation parameters associated with K and k2 respectively are shown in Figures 10-13, Table 1.
3.3. Thermodynamics of the Oxidation of DMP to DPQ
On changing X from Cl to Br in [(Pip)4nCu4X4(CO3)2]; n = 1 or 2, k2 and K are increased and both ΔH˚ and DS˚ are directed to a more favourable reaction (Table 1) indicating that reduction of copper (II) is involved in the
Absorbance-time data for the reaction of [(Pip)<sub>4</sub>Cu<sub>4</sub>Cl<sub>4</sub>(CO<sub>3</sub>)<sub>2</sub>], 1.0 × 10<sup>−3</sup> M with 2,6-dimethylphenol; (a, ´) 5.0 × 10<sup>−2</sup> M, (b, ・) 7.1 × 10<sup>−2</sup> M, (c, p) 9.1 × 10<sup>−2</sup> M, (d, ¨) 13.8 × 10<sup>−2</sup> M, in C<sub>6</sub>H<sub>5</sub>NO<sub>2</sub> at 25˚C. Monitoring wavelength is 740 nm
Absorbance-time data for the reaction of [(Pip)<sub>4</sub>Cu<sub>4</sub>Br<sub>4</sub>(CO<sub>3</sub>)<sub>2</sub>], 1.0 × 10<sup>−3</sup> M with 2,6-dimethylphenol; (a, ´) 2.0 × 10<sup>−2</sup> M, (b, ・) 3.8 × 10<sup>−2</sup> M, (c, p) 5.2 × 10<sup>−2</sup> M, (d, ¨) 8.0 × 10<sup>−2</sup> M, (e, ¾) 10.8 × 10<sup>−2</sup> M, in C<sub>6</sub>H<sub>5</sub>NO<sub>2</sub> at 22˚C. Monitoring wavelength is 740 nm
Absorbance-time data for the reaction of [(Pip)<sub>8</sub>Cu<sub>4</sub>Cl<sub>4</sub>(CO<sub>3</sub>)<sub>2</sub>], 1.0 × 10<sup>−3</sup> M with 2,6-dimethylphenol; (a, ´) 5.0 × 10<sup>−2</sup> M, (b, ・) 6.4 × 10<sup>−2</sup> M, (c, p) 9.6 × 10<sup>−2</sup> M, (d, ¨) 16.0 × 10<sup>−2</sup> M, in C<sub>6</sub>H<sub>5</sub>NO<sub>2</sub> at 25˚C. Monitoring wavelength is 740 nm
Absorbance-time data for the reaction of [(Pip)<sub>8</sub>Cu<sub>4</sub>Br<sub>4</sub>(CO<sub>3</sub>)<sub>2</sub>], 1.0 × 10<sup>−3</sup> M with 2,6-dimethylphenol; (a, ´) 2.5 × 10<sup>−2</sup> M, (b, ・) 5.4 × 10<sup>−2</sup> M, (c, p) 6.9 × 10<sup>−2</sup> M, (d, ¨) 9.4 × 10<sup>−2</sup> M, in C<sub>6</sub>H<sub>5</sub>NO<sub>2</sub> at 25˚C. Monitoring wavelength is 740 nm
First order plots for the reaction of [(Pip)<sub>4</sub>Cu<sub>4</sub>Cl<sub>4</sub>(CO<sub>3</sub>)<sub>2</sub>], 1.0 × 10<sup>−3</sup> M, with 2,6-dimethylphenol, 9.0 × 10<sup>−2</sup> M, in C<sub>6</sub>H<sub>5</sub>NO<sub>2</sub> at 24˚C (a, ¨), 32˚C (b, ¾), 38˚C (c, p) and 50˚C (d, ´). Monitoring wavelength is 740 nm
First order plots for the reaction of [(Pip)<sub>4</sub>Cu<sub>4</sub>Br<sub>4</sub>(CO<sub>3</sub>)<sub>2</sub>], 1.0 × 10<sup>−3</sup> M, with 2,6-dimethylphenol, 7.8 × 10<sup>−2</sup> M, in C<sub>6</sub>H<sub>5</sub>NO<sub>2</sub> at 22˚C (a, ¨), 28˚C (b, ¾), 36˚C (c, p) and 45˚C (d, ´). Monitoring wavelength is 740 nm
First order plots for the reaction of [(Pip)<sub>8</sub>Cu<sub>4</sub>Cl<sub>4</sub>(CO<sub>3</sub>)<sub>2</sub>], 1.0 × 10<sup>−3</sup> M, with 2,6-dimethylphenol, 9.6 × 10<sup>−2</sup> M, in C<sub>6</sub>H<sub>5</sub>NO<sub>2</sub> at 25˚C (a, ¨), 38˚C (b, ¾), 45˚C (c, p) and 50˚C (d, ´). Monitoring wavelength is 740 nm
First order plots for the reaction of [(Pip)<sub>8</sub>Cu<sub>4</sub>Br<sub>4</sub>(CO<sub>3</sub>)<sub>2</sub>], 1.0 × 10<sup>−3</sup> M, with 2,6-dimethylphenol, 9.4 × 10<sup>−</sup><sup>2</sup> M, in C<sub>6</sub>H<sub>5</sub>NO<sub>2</sub> at 20˚C (a, ¨), 25˚C (b, ¾), 40˚C (c, p) and 50˚C (d, ´). Monitoring wavelength is 740 nm
Dependence of the observed pseudo-first order rate constant (1/k<sub>obsd</sub>) on the {1/[DMP]<sup>2</sup>} for the reaction with [(Pip)<sub>4</sub>- Cu<sub>4</sub>Cl<sub>4</sub>(CO<sub>3</sub>)<sub>2</sub>], 1.0 × 10<sup>−3</sup> M, in C<sub>6</sub>H<sub>5</sub>NO<sub>2</sub> at 25˚C (a, ・), 32˚C (b, ¾), 38˚C (c, p) and 50˚C (d, ´). Monitoring wavelength is 740 nm
Dependence of the observed pseudo-first order rate constant (1/k<sub>obsd</sub>) on the {1/[DMP]<sup>2</sup>} for the reaction with [(Pip)<sub>4</sub>- Cu<sub>4</sub>Br<sub>4</sub>(CO<sub>3</sub>)<sub>2</sub>], 1.0 × 10<sup>−3</sup> M, in C<sub>6</sub>H<sub>5</sub>NO<sub>2</sub> at 22˚C (a, ・), 28˚C (b ¾), 36˚C (c, p) and 45˚C (d, ´). Monitoring wavelength is 740 nm
Dependence of the observed pseudo-first order rate constant (1/k<sub>obsd</sub>) on the {1/[DMP]<sup>2</sup>} for the reaction with [(Pip)<sub>8</sub>Cu<sub>4</sub>Cl<sub>4</sub>(CO<sub>3</sub>)<sub>2</sub>], 1.0 × 10<sup>−3</sup> M, in C<sub>6</sub>H<sub>5</sub>NO<sub>2</sub> at 25˚C (a, ・), 38˚C (b, ¾), 45˚C (c, p) and 50˚C (d, ´). Monitoring wavelength is 740 nm
Dependence of the observed pseudo-first order rate constant (1/k<sub>obsd</sub>) on the {1/[DMP]<sup>2</sup>} for the reaction with [(Pip)<sub>8</sub>Cu<sub>4</sub>Br<sub>4</sub>(CO<sub>3</sub>)<sub>2</sub>], 1.0 × 10<sup>−3</sup> M, in C<sub>6</sub>H<sub>5</sub>NO<sub>2</sub> at 20˚C (a, ・), 25˚C (b, ¾), 40˚C (c, p) and 50˚C (d, ´). Monitoring wavelength is 740 nm
Kinetic and thermodynamic parameters for oxidation of (DMP) by [(Pip)<sub>4n</sub>Cu<sub>4</sub>X<sub>4</sub>Y<sub>2</sub>]; n = 1 or 2, X = Cl or Br and Y = O<sup>2−</sup> or CO<sub>3</sub><sup>−2</sup>, in C<sub>6</sub>H<sub>5</sub>NO<sub>2</sub> at 740 nm
[(Pip)4nCu4X4Y2] n, X, Y
Temp. ˚C
k2a,b
DH≠c
DS≠d
Ke
DH˚c
DS˚d
2, Cl, O*
18
0.170
16.5 ± 0.5
−116 ± 3
3440
13.4 ± 0.5
−86 ± 3
30
0.260
3800
36
0.440
8760
42
0.770
9100
1, Cl, CO3
25
0.015
11.2 ± 0.5
−30 ± 3
62
12.0 ± 0.5
−10 ± 3
32
0.018
78
38
0.040
104
50
0.062
380
1, Br, CO3
22
0.192
11.0 ± 0.5
−26 ± 3
430
4.0 ± 0.5
−32 ± 3
28
0.263
540
36
0.510
560
45
0.770
1040
2, Cl, CO3
25
0.067
12.0 ± 0.5
−23 ± 3
120
3.0 ± 0.5
−39 ± 3
38
0.167
176
45
0.213
196
50
0.263
250
2, Br, CO3
20
0.210
12.0 ± 0.5
−22 ± 3
323
0.4 ± 0.5
−46 ± 3
25
0.310
333
40
0.870
357
50
1.72
375
*Previously published data [29] [30] . aUnits are sec−1. bUncertainties for k2 ca. ± 5% sec−1. cUnits are Kcal∙mol−1. dUnits are cal deg−1∙mol−1. eUncertainties for K ca. ± 5%.
rate determining step as observed before for the oxo analogues [29] [30] . When the number of piperidine per each Cu changes from one to two, (i.e. for [(Pip)4nCu4X4(CO3)2]), the mode of coordination of the carbonato- moiety changes from structure a to structure b (scheme 2), such a change let DH˚ to decrease from 12 to 3 Kcal∙mol−1, when X = Cl and from 4.40 to 0.36 Kcal∙mol−1, when X = Br, while DS˚ are directed to more negative values for both halo ligands. Therefore, the efficiency of structure b, (bidentate carbonato), as an initiator is higher than structure a, (tridentate carbonato). On comparing the data for [(Pip)4nCu4Cl4(CO3)2]; n = 1 or 2, with [(Pip)8Cu4Cl4O2] (Table 1) where copper (II) centres in all of them are six coordinate, therefore the only difference is oxo versus carbonato, either structure a or b, (scheme 2). In case of oxo, k2 is at least higher by about factor of 10 and K is higher by a factor of ~50, while DH≠ and DH˚ are more endothermic and also DS≠ and DS˚ are getting more negative. This indicate that the oxo bridging centre let the catalyst, [(Pip)8Cu4Cl4O2], more efficient to initiate the cycle than the less basic, more steric carbonato initiators [(Pip)4nCu4Cl4(CO3)2] in either structure a (n = 1) or b (n = 2).
4. Conclusion
Novel complexes of [(Pip)4nCu4X4(CO3)2] can be used as initiators for the oxidation of DMP to DPQ, Equation (5). Formation of [(Pip)4nCu4X4(CO3)2] complexes suggested that, the Cu-O-Cu angle in [(Pip)4nCu4X4O2] is sharp to an extent enough to let oxo centre be sufficiently basic for catalytic activity and to ease CO2 insertion to give the carbonato complexes. On the basis of k2, K and their kinetic and thermodynamic parameters for the first catalytic cycle, the [(Pip)4nCu4X4(CO3)2] are less powerful initiators for oxidative coupling reactions when compared to [(Pip)4nCu4X4O2]. The above result was attributed to less basic, more steric carbonato moiety relative to the oxo analogue. However, the final yield of the overall catalytic cycles was about the same.
Cite this paper
Mohamed A. El-Sayed,Hoda A. Elwakeil,Ahmed H. Abdel Salam,Hemmat A. Elbadawy,1 1, (2016) Kinetics of Oxidation of 2,6-Dimethylphenol (DMP) Using Novel μ-Carbonato [(Pip)4nCu4X4(CO3)2] Complexes. Open Journal of Inorganic Chemistry,06,183-194. doi: 10.4236/ojic.2016.63014
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