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Theoretical Study On The Activation Of Dioxygen By Fe Complexes And Related Oxidation Reaction Of Alkene

Posted on:2012-02-08Degree:DoctorType:Dissertation
Country:ChinaCandidate:Y SunFull Text:PDF
GTID:1111330371955336Subject:Chemistry
Abstract/Summary:PDF Full Text Request
Nowadays, environment-friendly techniques are very important for the development of chemical industry, because the problem of pollution is becoming worse and worse. Therefore, as one of the most important reaction, oxidation has attracted more and more attention. Green oxidation reaction is the target for both scientific research and chemical industry. The application of highly efficient catalyst and clean oxidant, such as air or dioxygen, is the key point of green oxidation. As we know, the ground state of dioxygen is triplet, which is inert and can not react with organic reactants. To use dioxygen efficiently, we need to use catalyst, which can activate dioxygen. Up to now, there are many kinds of catalysts, but which one is the best? It is impossible to carry out experiments to select the best catalyst. We know that, with the development of quantum chemical computation and computer, computational chemistry has developed significantly. Thus, we carried out some density functional theory (DFT) study on the interaction of catalyst and dioxygen as well as the related oxidation reaction process.Firstly, we explored the interaction between metalloporphyrin and dioxygen. In addition, we studied the effect of nitrogen-containing neutral axial ligands on the binding process of dioxygen (O2) to metalloporphyrins. There are two closely lying spin states for metalloporphyrins, namely, triplet and quintet. However, the ground state for oxy-metalloporphyrins is the open-shell singlet. DFT calculations indicate that back-donation to O2 is more accessible because of the axial ligand, facilitating the binding of O2. Moreover, the axial ligands lengthen the O-O bond of oxy-metalloporphyrins and make the bound O2 negatively charged with a spin population between that of 3O2 and O2-. Therefore, the coordination of the axial ligand makes O2 more active than free O2. triggering the catalytic oxidation processes. Finally, the larger the electronic density of the bound nitrogen atom (NL) of the axial ligand is, the more active the binding O2 becomes. Secondly, we researched the catalysis of iron-porphyrin and the effect of axial ligand on the catalysis. Density functional theory calculations reveal that iron(Ⅳ)-oxo porphyrin cation radical (CpdⅠ) models are indeed much more reactive than iron(Ⅳ)-oxo porphyrin (CpdⅡ) models in the case of 1,3-cyclohexdiene oxidation. The geometrical parameters and electronic configuration are quite different for CpdⅠand CpdⅡ. This is the origin of the reactivity difference. The axial ligand indeed affects the geometries of CpdⅠand CpdⅡand the oxidation process, even though it cannot switch the reactivity of CpdⅠand CpdⅡ. Interestingly, the axial ligand can switch the oxidation process from dehydrogenation to epoxidation by CpdⅠ, however, for CpdⅡ, it cannot switch the reaction process, but lessen the barrier difference of dehydrogenation and epoxidation, even though the decrease of barrier is not significant. The axial ligand effect is somewhat in relation to the Hammett substituent parameter (σp) of the substituent group in the axial ligand. The more negative theσp is, the longer the Fe=O bond of CpdⅠis, and the higher the reaction barrier is.Thirdly, we examined the influence of macrocyclic ligand on the interaction between iron and dioxygen. In an effort to examine the interaction between dioxygen and iron-macrocyclic complexes, and to know how this interaction was affected by those different macrocyclic ligands, the dioxygen binding with the iron-porphyrin, iron-phthalocyanine, iron-dibenzotetraaza[14]annulene, and iron-salen complexes are investigated by means of quantum chemical calculations utilizing density functional theory (DFT). Based on the analysis of factors influencing the corresponding dioxygen binding process, it showed that different macrocyclic ligands possess different O-O bond distances and different electronic configurations for the bound O2 and non-aromatic macrocyclic ligands favor the dioxygen activation. Furthermore, the smaller the energy gap between the HOMO of iron-macrocyclic complexes and the LUMO of dioxygen. the more active the bound O2 would become, with a longer O-O bond distance and a shorter Fe-O bond length.Fourthly, we investigated the catalysis of the active intermediates of iron-porphyrin, iron-phthalocyanine, iron-dibenzotetraaza[14]annulene, and iron-salen complexes. The intermediates have two different styles; one is similar to CpdⅠ, while the other is similar to CpdⅡ. Our calculation indicated that the intermediate of CpdⅡstyle was much less active than that of CpdⅠ. Moreover, the coordination of axial ligand can not switch the reactivity of these two kinds of intermediates. However, the axial ligand changed the steric selectivity of the oxidation of cyclohexene. The hydroxylation of C-H bond was favorable when there was no axial ligand, while the epoxidation of C=C bond was favorable with axial ligand. However, according to our results, it is impossible to predict which kind of macrocyclic ligand is better than the others.Lastly, because the reaction mechanism of metalloporphyrin is a little similar to that of NHPI, we studied the substituent effect on the catalysis of NHPI in the oxidation of cyclohexene, because the reaction mechanism of metalloporphyrin is a little similar to that of NHPI. The H-abstraction by pthalimide-N-oxyl radicals is an important step in oxidation reactions catalyzed by N-hydroxyphthalimide (NHPI). Herein, substituent electronic effects on the allylic H-abstraction process by phthalimide-N-oxyl radicals are evaluated by a systemically theoretical analysis in the case of cyclohexene. The catalyst with electron-withdrawing substituent possesses larger spin density on the oxygen atom of the N-O section, which results in a larger O-H bond dissociation energy (BDE) and smaller isotropic Fermi contact coupling constants of the nitrogen atom. The BDE of O-H bond plays a very important role, determining the H-abstraction activation energy. The isotropic Fermi contact coupling is closely related to the coupling constant of the EPR spectrogram. The conjugation effect plays an important part in the aryl substituent effect. According to the results above, not all ionic-compound-supported NHPIs are good catalysts. A cation-supported NHPI is better than an anion-supported NHPI. The present theoretical study reveals the relationship between the structure and the catalytic activity of NHPI and its analogues, complementary to the previous work on NHPI. and allows for a reasonable prediction of the catalysis efficiency of NHPI analogues.In summary, the results of our calculations are helpful for the explanation of some experiments in previous literature and also for the design of new catalysts.
Keywords/Search Tags:Oxidation of hydrocarbon, Macrocyclic metal complexes, Hydrogen attraction, Epoxidation, NHPI, DFT calculation
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