Copper based nanomaterials for oxidation catalysis

[摘要] English: The aim of this study was the synthesis simple O,O-bidentate ligand copper complexes of the type [Cu(Memal)2], [Cu(Etmal)2], [Cu(Trop)2] and [Cu(TropBr3)2], where Memal = 2- methyl-3-hydroxy-4-pyrone, Etmal = 2-ethyl-3-hydroxy-4-pyrone, Trop = Tropolonato and TropBr3 = tri-bromotropolonato and to do an evaluation of the oxidation catalysis of 3,5-ditert- butylcatechol to the 3,5-di-tert-butyl-1,2-benzoquinone and study the kinetics associated therewith. The ligands were selected to provide a systematic range of electronic and steric variation at the Cu(II) metal centre. Characterisation of the complexes was done by infrared spectroscopy (IR) and elemental analysis, as well as an X-ray crystallographic structure determination of selected compounds i.e., [Cu(TropBr3)2(DMSO)2] and 3,5-di-tert-butylcatechol. [Cu(TropBr3)2(DMSO)2] crystallised in the monoclinic space group C2/c with Z = 4 in the unit cell dimensions a = 18.002(7) Å, b = 20.351(7) Å, c = 7.578(3) Å, = 108.331(2) ° and the data were collected at -173 °C. Least-squares refinement led to a value of the conventional R1 index of 0.0320 and Rw = 0.0571 for 2873 reflections having I > 2 (I). The copper ion is coordinated by four oxygen atoms from two tri-bromotropolonato ligands. In addition there are two oxygen atoms from the two dimethylsulphoxide molecules coordinated to the copper centre in the axial positions, ensuring an overall octahedral geometry. The Cu-O(DMSO) bond lengths in the axial positions are significantly longer than the Cu-O bonds in the equatorial positions due to Jahn-Teller distortions. The 3,5-di-tert-butylcatechol crystallised in the triclinic space group P1 with Z = 4 in the unit cell dimensions a = 9.101(5) Å, b = 11.394(5) Å, c = 13.203(5) Å, = 83.653(5) °, = 79.601(5) °, = 85.091(5) °; and the data were collected at -173 °C. Leastsquares refinement led to a value of the conventional R1 index of 0.0124 and Rw = 0.3548 for 5675 reflections having I > 2 (I). The tert-butyl group on the 5 position of the phenyl ring crystallises in a 45:55 % statistically disordered position. A kinetic investigation was conducted to study the catalytic oxidation of 3,5-di-tertbutylcatechol to 3,5-di-tert-butyl-1,2-benzoquinone by the copper complexes listed above. The reaction was studied by two techniques namely, UV-Vis and GC in order to characterise the final product. All four complexes have the ability to oxidize 3,5-di-tert-butylcatechol, but there is a significant difference in the rate of oxidation between the complexes. [Cu(Memal)2] and [CuEtmal)2] have comparable rates and it is much faster than the rate of [Cu(Trop)2] and [Cu(TropBr3)2]. Different solvents also have an effect on the rate of the reaction: in acetonitrile the reaction only proceed to 14% completeness, but in methanol a 52% conversion is achieved in 24 hr under the same conditions. The rate of the reaction exhibits a limiting relationship with respect to molecular oxygen concentration. The catalytic mechanism that best describes the kinetics consists of five reversible steps. The first steps involves the coordination of 3,5-di-tert-butylcatechol to the copper followed by a second 3,5-di-tert-butylcatechol coordinating to the copper in the subsequent step. The third step involves the interaction with oxygen and the loss of one 3,5-di-tert-butylcatechol. A limiting value for the rate constant of the copper complex with molecular oxygen, k2 = 3.4(3) s-1 for [Cu(Memal)2] was determined. The fourth step is the rate determining formation of 3,5-di-tert-butyl-1,2 benzoquinone, defined by rate constants k3 and k-3, for [Cu(Memal)] is determined to be 3.7(6) x 10-5 M-1 s-1 and 2(5) x10-6 s-1. The fifth step completes the catalytic cycle and regenerates the starting complex i.e. the coordination of the Memal to the copper. The rates of the reaction seem to be both sterically and electronically dependent. TON and TOF values of 70.4 and 2.93 h-1 were observed for [Cu(Memal)2].

[发布机构] University of the Free State