Kinetics of Oxidation of 2,6-Dimethylphenol (DMP) Using Novel μ-Carbonato [(Pip)4nCu4X4(CO3)2] Complexes

This paper reports the kinetics of the oxidation of 2,6-dimethylphenol (DMP) to get 3,3’,5,5’-tetramethyl-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 kobsd 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 kobsd 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.


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) 4n Cu 4 X 4 (CO 3 ) 2 ] complexes and the strategy is: 1) The use of well investigated tetranuclear carbonato-copper (II) complexes, [(Pip) 4n Cu 4 X 4 (CO 3 ) 2 ] as catalysts for oxidation of (DMP) to (DPQ).
2) To evaluate those initiators [(Pip) 4n Cu 4 X 4 (CO 3 ) 2 ] with [(Pip) 4n Cu 4 X 4 O 2 ], 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.
4) The impact of structural change in [(Pip) 4n Cu 4 X 4 (CO 3 ) 2 ] when n changes from 1 to 2, since the maximum coordination number of copper (II) is six.

Reagents
Piperidine, Pip (Aldrich) was used as received.10 cm column of Drierite was utilized to dry CO 2 gas. The copper (I) halides as well as C 6 H 5 NO 2 , CH 2 Cl 2 , DMP and N 2 gas were prepared for this work according to the procedures described in the literature [21].

Instrumentation
The rates of oxidation of (DMP) by copper complexes [(Pip) 4n Cu 4 X 4 (CO 3 ) 2 ]; in C 6 H 5 NO 2 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.

Kinetic Measurements
The mechanism and kinetics of the catalytic oxidation of DMP (2.0 -16.0) × 10 −2 M using the copper [(Pip) 4n -Cu 4 X 4 (CO 3 ) 2 ] complexes (1.0 × 10 −3 M) in C 6 H 5 NO 2 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.

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)  1 DMP , K and k 2 are collected in Table 1. Thermodynamic and activation parameters associated with K and k 2 respectively are shown in Figures 10-13

Thermodynamics of the Oxidation of DMP to DPQ
On changing X from Cl to Br in [(Pip) 4n Cu 4 X 4 (CO 3 ) 2 ]; n = 1 or 2, k 2 and K are increased and both ΔH˚ and ∆S˚ are directed to a more favourable reaction ( Table 1) indicating that reduction of copper (II) is involved in the              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, k 2 is at least higher by about factor of 10 and K is higher by a factor of ∼50, while ∆H ≠ and ∆H˚ are more endothermic and also ∆S ≠ and ∆S˚ are getting more negative. This indicate that the oxo bridging centre let the catalyst, [(Pip) 8 Cu 4 Cl 4 O 2 ], more efficient to initiate the cycle than the less basic, more steric carbonato initiators [(Pip) 4n Cu 4 Cl 4 (CO 3 ) 2 ] in either structure a (n = 1) or b (n = 2).

Conclusion
Novel complexes of [(Pip) 4n Cu 4 X 4 (CO 3 ) 2 ] can be used as initiators for the oxidation of DMP to DPQ, Equation . 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.