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Synthesis, Structure and Catalytic Application of Mn, Fe and Mo Complexes with Schiff base and Oxazoline ligands and Theoretical Mechanistic Studies on the Role of Heme and non-Heme Iron(IV)-Oxo Species in Hydroxylation and Epoxidation Reactions

Tahsini , Laleh | 2009

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  1. Type of Document: Ph.D. Dissertation
  2. Language: Farsi
  3. Document No: 39828 (03)
  4. University: Sharif University of Technology
  5. Department: Chemistry
  6. Advisor(s): Bagherzadeh, Mojtaba
  7. Abstract:
  8. The first part of this thesis describes the synthesis and characterization of several Mn, Fe and Mo complexes containing tridentate Schiff base and oxazoline ligands and their reactivities as catalysts in the oxidation of organic compounds like olefins and sulfides, as well. The crystal structures of newly prepared cis-dioxo-bis[2-(2′-hydroxyphenyl)-oxazolinato]molybdenum(VI), cis-[MoO2(phox)2] and (N-hydroxyphenyl-salicylidenamine)(acetylacetonato)ethanol iron(III), [FeIII(N-OPh-sal)(acac)EtOH] complexes have been described by X-ray crystallography. Due to the difficulties in the preparation of suitable single crystals of novel (N-hydroxyphenyl-salicylidenamine)(2-(2′-hydroxyphenyl)oxazolinato)ethanol manganese(III), [MnIII(N-OPh-sal)(phox)EtOH] complex for X-ray crystallography, the nearest structure was predicted by density functional theory (DFT) calculations using Gaussian 03 package. Catalytic studies were performed to develop a simple oxidation system for various olefins and sulfides using two manganese(III)-tridentate Schiff base complexes, [MnIII(N-OPh-sal)(acac)EtOH] and [MnIII(N-OPh-sal)(phox)EtOH] and urea hydrogen peroxide (UHP) as terminal oxidant in short reaction times. High conversion yields up to 90% and 73% for olefins at 0 ºC were achieved with the first and second complex, respectively. Furthermore both complexes were highly active in the oxidation of organic sulfides at room temperature with conversions up to 96% and 88% as well as sulfoxide selectivities up to 73% and 85% using first and second complex, respectively. Also this study confirms the indispensible accelerating effect of strong π-donor axial ligands such as imidazole (ImH) on the oxidation reactions. The mononuclear mixed ligand complex of iron, [FeIII(N-OPh-sal)(acac)EtOH] was used as catalyst in the presence of UHP for the oxidation of various organic sulfides under mild conditions. This system was also found highly selective to sulfoxide in good yields. The electronic spectra of the catalytic system were applied to explore reactivity and stability of the catalyst during sulfide oxidation reactions and to examine the nature of active species, as well.
    Our studies on the catalytic properties of newly prepared complexes lead to the designing a highly efficient catalytic system for the epoxidation of various olefins by cis-[MoO2(phox)2] complex and tert-butylhydroperoxide (TBHP) as oxidizing agent. Using this system for the oxidation of aliphatic substrates at 80 ºC gives the epoxide as the sole product with yields up to 100% and turnover frequency up to 5000 h-1. The efficiency of the catalyst is strongly influenced by the nature of solvent, reaction time and temperature, and a significant increase in the epoxide yields is observed at higher temperatures and longer reaction times. This complex was also found to be an efficient and selective catalyst for the oxidation of various sulfides to sulfoxides with urea hydrogen peroxide (UHP) in excellent yields (100% for diallylsulfide) and short reaction times (20 min) at room temperature. The catalytic system shows complete chemoselectivity in the oxididation of substrates with more than one functional group like diallylsulfide and gives the corresponding sulfoxide without any over oxidation in double bond. To compare the catalytic activity of all applied catalysts in the mentioned oxidation reactions confirm that generally [MnIII(N-OPh-sal)(acac)EtOH] is more reactive than its iron analogue [FeIII(N-OPh-sal)(acac)EtOH] and other manganes(III) complex, [MnIII(N-OPh-sal)(phox)EtOH], as well. However the higher conversion yields and selectivities in the oxidation reactions catalysed by cis-[MoO2(phox)2] may confirm this complex as the most reactive catalyst for the oxidation of olefins and sulfides using hydrogen peroxide derivatives as oxidant. The second part of this thesis (Chapter 4) has been devoted to the theoretical mechanistic studies of hydroxylation and epoxidation reactions by heme and non-heme iron-oxo complexes. The whole work which was performed at University of Manchester during the visiting scholar contains two sections. The first part is related to Study of the Substrate Hydroxylation by High-Valent Iron(IV)-Oxo Models of Cytochrome P450. An Iron(IV)-oxo heme(+•) complex (Compound I, Cpd I) is the proposed active species of heme enzymes. In this work we present density functional theory (DFT) calculations on substrate hydroxylation by a Compound I mimic [FeIV=O(Por+•)Cl] and its one-electron reduced form [FeIV=O(Por)Cl]- or CpdII. Some recent experimental studies showed that [FeIV=O(Por)Cl]- is able to react with substrates via hydride transfer reactions, but theoretical studies on camphor hydroxylation by these two oxidants concluded that the Cpd II is a sluggish oxidant of hydroxylation reactions. To resolve the question why the one-electron reduced Compound I is an oxidant in one case and a sluggish oxidant in other cases, we have performed a DFT study on 10-methyl-9,10-dihydro acridine (AcrH2) hydroxylation by [FeIV=O(Por+•)Cl] and [FeIV=O(Por)Cl]-. The calculations presented in this work show that both CpdI and CpdII are plausible oxidants, but CpdI reacts via much lower reaction barriers. Moreover, it reacts via hydride transfer, while CpdII by hydrogen abstraction. The differences between hydride and hydrogen atom transfer reactions have been rationalized with thermodynamic cycles and shown to be the result of differences in electron abstraction abilities of the two oxidants. Thus, the calculations predict that [FeIV=O(Por)Cl]- is only able to hydroxylate weak C-H bonds, whereas [FeIV=O(Por+•)Cl] is more versatile. In the second part of the theoretical section DFT studies on the cis effects of ligands on oxoiron nonheme complexes have been performed. A detailed analysis of the electronic and oxidizing properties of [FeIV=O(TPA)L]+ with L= F-, Cl-, Br-, OH- and TFA- (trifluoroacetate) and TPA= tris-(2-pyridylmethyl)amine are presented. The calculations show that this complex have closely lying triplet and quintet spin states, but only the quintet spin state is reactive with substrates. The reaction of styrene with the reactant, that is, [FeIV=O(TPA)L]+ leads to an initial electron transfer from the substrate to the metal followed by a highly exothermic epoxidation mechanism. Systems that stabilize the quintet spin state reactant, therefore, are more efficient oxidants of epoxidation reactions than ones that stabilize triplet spin state. Thus, [FeIV=O(TPA)(OH)]+ is the most efficient oxidant with the lowest epoxidation barrier.
  9. Keywords:
  10. Axial Ligand ; Manganese Schiff Base ; Catalytic Epoxidation ; Manganese-Oxazoline Complexes ; Tertiary Butylhydroperoxide (TBHP) ; Density Functional Theory (DFT) ; Sulfide Oxidation ; Iron-Oxo Spesciese ; Hydroxylation

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