Kinetics of Oxidation of 2,6-Dimethylphenol (DMP) Using Novel μ-Carbonato [(Pip)_4nCu₄X₄(CO₃)₂] Complexes

Mohamed A. El-Sayed¹, Hoda A. Elwakeil¹, Ahmed H. Abdel Salam^1,2*, Hemmat A. Elbadawy¹

¹Chemistry Department, Faculty of Science, Alexandria University, Alexandria, Egypt.
²Chemistry Department, Faculty of Science, University of Jeddah, Jeddah, KSA.

DOI: 10.4236/ojic.2016.63014                

Abstract

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)_4nCu₄X₄-(CO₃)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, k₂, 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 k₂ 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)_4nCu₄X4(CO₃)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.

Keywords

Catalysts, Kinetics, Thermodynamic, Copper Complexes

Received 29 February 2016; accepted 19 July 2016; published 22 July 2016

1. Introduction

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. Experimental

2.1. 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 Discussion

3.1. Stoichiometry and Products of Oxidation of Copper (I) Complexes by O₂ and CO₂

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.

(1)

(2)

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] .

(3)

(4)

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] .

Figure 1. Electronic spectra of (a) [(Pip)₄Cu₄Cl₄(CO₃)₂], (b) [(Pip)₄- Cu₄Br₄(CO₃)₂], (c) [(Pip)₈Cu₄Cl₄(CO₃)₂], (d) [(Pip)₈Cu₄Br₄(CO₃)₂].

Scheme 2. Proposed molecular structures for [(Pip)_4nCu₄X₄(CO₃)₂].

3.2. Kinetics of Oxidation of DMP to DPQ Using Novel [(Pip)_4nCu₄X₄(CO₃)₂] Complexes

The kinetics of homogeneous oxidative coupling of DMP to DPQ, Equation (5) are investigated at λ_max = 740 nm under pseudo 1^st 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 A_t 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.

(6)

(7)

(8)

(9)

(10)

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

Figure 2. Absorbance-time data for the reaction of [(Pip)₄Cu₄Cl₄(CO₃)₂], 1.0 × 10⁻³ M with 2,6-dimethylphenol; (a, ´) 5.0 × 10⁻² M, (b, ・) 7.1 × 10⁻² M, (c, p) 9.1 × 10⁻² M, (d, ¨) 13.8 × 10⁻² M, in C₆H₅NO₂ at 25˚C. Monitoring wavelength is 740 nm.

Figure 3. Absorbance-time data for the reaction of [(Pip)₄Cu₄Br₄(CO₃)₂], 1.0 × 10⁻³ M with 2,6-dimethylphenol; (a, ´) 2.0 × 10⁻² M, (b, ・) 3.8 × 10⁻² M, (c, p) 5.2 × 10⁻² M, (d, ¨) 8.0 × 10⁻² M, (e, ¾) 10.8 × 10⁻² M, in C₆H₅NO₂ at 22˚C. Monitoring wavelength is 740 nm.

Figure 4. Absorbance-time data for the reaction of [(Pip)₈Cu₄Cl₄(CO₃)₂], 1.0 × 10⁻³ M with 2,6-dimethylphenol; (a, ´) 5.0 × 10⁻² M, (b, ・) 6.4 × 10⁻² M, (c, p) 9.6 × 10⁻² M, (d, ¨) 16.0 × 10⁻² M, in C₆H₅NO₂ at 25˚C. Monitoring wavelength is 740 nm.

Figure 5. Absorbance-time data for the reaction of [(Pip)₈Cu₄Br₄(CO₃)₂], 1.0 × 10⁻³ M with 2,6-dimethylphenol; (a, ´) 2.5 × 10⁻² M, (b, ・) 5.4 × 10⁻² M, (c, p) 6.9 × 10⁻² M, (d, ¨) 9.4 × 10⁻² M, in C₆H₅NO₂ at 25˚C. Monitoring wavelength is 740 nm.

Figure 6. First order plots for the reaction of [(Pip)₄Cu₄Cl₄(CO₃)₂], 1.0 × 10⁻³ M, with 2,6-dimethylphenol, 9.0 × 10⁻² M, in C₆H₅NO₂ at 24˚C (a, ¨), 32˚C (b, ¾), 38˚C (c, p) and 50˚C (d, ´). Monitoring wavelength is 740 nm.

Figure 7. First order plots for the reaction of [(Pip)₄Cu₄Br₄(CO₃)₂], 1.0 × 10⁻³ M, with 2,6-dimethylphenol, 7.8 × 10⁻² M, in C₆H₅NO₂ at 22˚C (a, ¨), 28˚C (b, ¾), 36˚C (c, p) and 45˚C (d, ´). Monitoring wavelength is 740 nm.

Figure 8. First order plots for the reaction of [(Pip)₈Cu₄Cl₄(CO₃)₂], 1.0 × 10⁻³ M, with 2,6-dimethylphenol, 9.6 × 10⁻² M, in C₆H₅NO₂ at 25˚C (a, ¨), 38˚C (b, ¾), 45˚C (c, p) and 50˚C (d, ´). Monitoring wavelength is 740 nm.

Figure 9. First order plots for the reaction of [(Pip)₈Cu₄Br₄(CO₃)₂], 1.0 × 10⁻³ M, with 2,6-dimethylphenol, 9.4 × 10⁻² M, in C₆H₅NO₂ at 20˚C (a, ¨), 25˚C (b, ¾), 40˚C (c, p) and 50˚C (d, ´). Monitoring wavelength is 740 nm.

Figure 10. Dependence of the observed pseudo-first order rate constant (1/k_obsd) on the {1/[DMP]²} for the reaction with [(Pip)₄- Cu₄Cl₄(CO₃)₂], 1.0 × 10⁻³ M, in C₆H₅NO₂ at 25˚C (a, ・), 32˚C (b, ¾), 38˚C (c, p) and 50˚C (d, ´). Monitoring wavelength is 740 nm.

Figure 11. Dependence of the observed pseudo-first order rate constant (1/k_obsd) on the {1/[DMP]²} for the reaction with [(Pip)₄- Cu₄Br₄(CO₃)₂], 1.0 × 10⁻³ M, in C₆H₅NO₂ at 22˚C (a, ・), 28˚C (b ¾), 36˚C (c, p) and 45˚C (d, ´). Monitoring wavelength is 740 nm.

Figure 12. Dependence of the observed pseudo-first order rate constant (1/k_obsd) on the {1/[DMP]²} for the reaction with [(Pip)₈Cu₄Cl₄(CO₃)₂], 1.0 × 10⁻³ M, in C₆H₅NO₂ at 25˚C (a, ・), 38˚C (b, ¾), 45˚C (c, p) and 50˚C (d, ´). Monitoring wavelength is 740 nm.

Figure 13. Dependence of the observed pseudo-first order rate constant (1/k_obsd) on the {1/[DMP]²} for the reaction with [(Pip)₈Cu₄Br₄(CO₃)₂], 1.0 × 10⁻³ M, in C₆H₅NO₂ at 20˚C (a, ・), 25˚C (b, ¾), 40˚C (c, p) and 50˚C (d, ´). Monitoring wavelength is 740 nm.

Table 1. Kinetic and thermodynamic parameters for oxidation of (DMP) by [(Pip)_4nCu₄X₄Y₂]; n = 1 or 2, X = Cl or Br and Y = O²⁻ or CO₃⁻², in C₆H₅NO₂ at 740 nm.

^*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.

NOTES

^*Corresponding author.

Conflicts of Interest

The authors declare no conflicts of interest.

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