Catalytic oxidation processes using functionalised 0,0'-bidentate ligand copper complexes
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Date
2013-12
Authors
Molokoane, Pule Petrus
Journal Title
Journal ISSN
Volume Title
Publisher
University of the Free State
Abstract
The aim of this study was to imitate the activity and behaviour of the enzyme catechol oxidase by employing simple copper nano molecular materials, and to investigate electronic and steric effects on this catalytic oxidation process. A range of O,O’
-bidentate ligands were systematically synthesized by imparting different electronic properties to the ligand systems. These ligands were then coordinated to copper(II) metal ions to form the corresponding complexes. A total of six ligands were characterized and studied extensively, namely: MM(naltol)H, ME(naltol)H,
MP(naltol)H, EM(naltol)H, EE(naltol)H and EP(naltol)H (3-hydroxy-1,2-dimethyl-4-pyridone, 1-ethyl-3-hydroxy-2-methyl-4-pyridone, 3-hydroxy-2-methyl-1-isopropyl-4-pyridone, 2-ethyl-3-hydroxy-1-methyl-4-pyridone, 1,2-diethyl-3-hydroxy-4-pyridone, 2-ethyl-3-hydroxy-1-isopropyl-4-pyridone). Only five crystal structures are reported: (MM(naltol)H, ME(naltol)H, EM(naltol)H, EE(naltol)H and EP(naltol)H). Six copper complexes were also synthesized and studied of which only two crystal structures were reported {[Cu(ME(naltol))2] and [Cu(EP(naltol))2]}, namely; [Cu(MM(naltol))2], [Cu(ME(naltol))2], [Cu(MP(naltol))2], [Cu(EM(naltol))2], [Cu(EE(naltol))
2] and [Cu(EP(naltol))2].
Structural data revealed that all the ligands were in the keto-enol tautomeric form in the solid state and in all the cases where a clear packing order was observed, weak hydrogen bonding is present. These interactions result in the formation of dimers, which stabilizes the structures. This data also indicated a C=O bond length increase with increasing electron donation in the synthesized O,O’-bidentate ligands systems. The synthesized copper complexes were planar with slight deviations from planarity and in both the complexes the copper atoms lie on inversion centers. These complexes exhibit strong intramolecular hydrogen interactions. The solution study results suggest that the complex with the least electron donating group on the ligand was the most effective catalyst; however, the same complex was coincidentally the most
sterically demanding complex in the study. As the catechol oxidase is a macro-molecule which is very sterically crowded, the data suggests that steric effects play an important role in the catalytic process and this was successfully demonstrated at a small-molecular level of detail via solution modelling experiments. The two proposed mechanisms favour the process equally and none is preferred over the other.
In the first mechanism the first step involves the coordination of 3,5-di-tert-butylcatechol to the copper which results in the subsequent loss of one of the coordinated ligands. In the second step the second 3,5-di-tert-butylcatechol coordinates to the copper with the subsequent loss of the second coordinated ligand. The third step involves the interaction with oxygen and the subsequent loss of one 3,5-di-tert-butylcatechol moiety. The fourth step is the rate determining formation of 3,5-di-tert-butylbenzoquinone, defined by the rate constants k3 and k-3, which also
generates the catalytic species. The second mechanism is similar to the first and includes a reversible equilibrium between a two coordinated catechol species and a one catechol coordinated species, as well as a direct coordination of a new catechol molecule on the naltol-copper(II) species. The one catechol coordinated species oxidises the catechol to the corresponding quinone and the copper center is reduced in the process. An interaction with molecular oxygen re-oxidises the metal center and generates the catalytic species, which yields
the product 3,5-di-tert-butylquinone. The rate constants in Mechanism 2 are defined by k8 and k-8. For the complexes [Cu(ME(naltol))2], [Cu(MP(naltol))2] and [Cu(EP(naltol))2] (in methanol at 25° C) the rate constants are kf (=k3 or k8) and
kr(=k-3or k-8): (3.4±0.6) x 10-4 M-1.s-1 and (3.6±0.4) x 10-6 M-1.s-1), (4.8±0.9) x 10
-4 M-1.s-1 and (3.2±0.6) x 10-6 M-1.s-1), (8.7±0.7) x 10-4 M-1.s-1 and (6.15±0.02) x 10
-6 M-1.s-1).
Description
Keywords
Dissertation (M.Sc. (Chemistry))--University of the Free State, 2013, Catalysis, Copper compounds -- Oxidation, Catechol oxidase, 3,5-di-tert-butylcatechol, 3,5-di-tert-butylquinone, O,O'-bidentate, Ligand, Sturctures, Keto-enol tautomer