Anion effect on the cumene hydroperoxide decomposition in the presence of Cu(II) 1,10-phenanthrolinates
Graphical abstract
Introduction
Systems based on hydroperoxides (ROOH) and transition metals compounds are widespread initiators for the radical processes of organic compounds oxidation [[1], [2], [3], [4], [5]]. Hydroperoxides decomposition under the action of transition metals ions occurs in the catalytic cycle with interchange participation of the reduced and oxidized form of the metal ion according to the known redox scheme [[2], [3], [4], [5], [6]]:ROOH + Men+ → RO• + Me(n+1)+ + OH−ROOH + Me(n+1)+ → ROO• + Men++ H+
Transition metals salts decrease the activation barrier of the ROOH decomposition reaction and contribute to the radicals formation in the system at temperatures close to room temperature. A stage of the hydroperoxide interaction with the metal ion in the lower oxidation state (1) is characterized by a high reaction rate and a rather slow rate of the second stage is observed for reactions proceeding in accordance with the scheme (1–2) [1]. Therefore, in many cases, there is an undesirable stop of the oxidation reaction at low degrees of conversion. Metal reactivity can be regulated by using of compounds that capable to selective catalyst binding. The use of Cu(II) complex compounds [7,8] has proved to be successful in this direction. Results of current investigation and recent advances in the field of copper complexes catalyzed reactions are summarized in reviews [[2], [3], [4], [5]].
Complexes of Cu(II) with organic ligands have attracted the attention of researchers due to wide range of interesting properties and applications – medical applications [9,10], catalysis [[11], [12], [13]], sensoric properties [14,15], magnetism [16,17]. They are used to develop of new effective biomimetic DNA destructors [[18], [19], [20], [21], [22]]. 1,10-Phenanthroline (Phen) very often acts as a ligand and counterions are chloride-, acetate- or nitrate anion. Formation of the following types of complexes: [Cu(Phen)3]Cl2, [Cu(Phen)2Cl]Cl and [Cu(Phen)Cl2] depends on the initial ratio of the salt and 1,10-phenanthroline [23].
The classical redox scheme (equations (1), (2))) provides the catalysis of the hydroperoxide decomposition as a result of its interaction with a metal cation. Catalytic decomposition of hydroperoxides in the presence of transition metals ions includes the stage of complex formation (equations (3) and (4)) [6,24].
Complex formation can significantly change the cation reactivity, and then the nature of the anion can be decisive. For example, a different selectivity and reaction rate for the oxidation of 2-isopropylnaphthalene with molecular oxygen in the presence of halides and organic salts of Cu(II) were observed with change of the salt anion [25]. The rate of 2-isopropylnaphthalene hydroperoxide decomposition catalyzed by copper salts increases in the following row of anions: stearate < acetate < acetylacetonate < chloride. For manganese complexes with porphyrins of general structure XMnIII (TTP) (X = F−, Cl−, Br−, I−, OH−, CH3COO−) the catalytic activity of the complexes depends on the redox properties of the anions and the interaction of hydroperoxide with the anions is an alternative source of radicals in the system [26]. Thus, the investigations of anion effect on the catalytic decomposition of hydroperoxides in a presence of transition metals compounds is appropriate.
The purpose of this work is study of catalytic activity of complexes of 1,10-phenanthroline (Phen) with Cu(II) salts (CuX2, Х: NO3−, SO42−, CH3COO−, Cl−) in the cumene hydroperoxide decomposition reaction.
Section snippets
Experimental
Cumene hydroperoxide (ROOH) was purified according to the procedure given in Ref. [27]. Its purity (98.9%) was controlled by iodometry method as well as by NMR 1H spectroscopy. 1H NMR (400 MHz, CD3CN): δ = 1.52 (s, 3Н, -СН3), δ = 7.25–748 (m, 5Н, -С6Н5), 8.95 (s, 1Н, –COOH) ppm. Active oxygen (mas %): calculated – 21.0, found – 20.8.
CuCl2, (CH3COO)2Cu·H2O, CuSO4·5H2O, Cu(NO3)2·3H2O and 1,10-phenanthroline were Sigma-Aldrich reagents and used for initial solutions preparation without additional
The cumene hydroperoxide decomposition activated by CuCl2, CuCl2 in a presence 1,10- phenanthroline and complex of 1,10- phenanthroline with CuCl2
There is a significant increase in rate of the cumene hydroperoxide decomposition in the presence of CuCl2 - 1,10-phenanthroline complexes under experimental conditions. The typical kinetic curves of ROOH decomposition in the presence of CuCl2 (curve 1) and mixture of 1,10-phenanthroline and CuCl2 (curve 2) are presented on Fig. 1. The concentration of copper ions in the reaction mixtures was [CuCl2] = 4·10−3 mol dm−3. Almost full conversion of the hydroperoxide within 1.5 h was observed in a
Conclusions
Thus, Cu (II) 1,10-phenanthrolinates exhibit catalytic properties in the reaction of cumene hydroperoxide decomposition in aqueous-ethanol solution. It was shown that the reactivity of the CuX2 - 1,10-phenanthroline system depends on the nature of the salt anion. The reaction rate of cumene hydroperoxide decomposition in the presence of CuX2 1,10-phenanthrolinates increases in a row of anions: NO3– ≪< SO42– < Cl– < CH3COO–. The double activation effect is considered for the cumene hydroperoxide
Declaration of competing interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
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