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From these results, a mechanism can be proposed that includes an initial oxide(·–) ion transfer from Fe IV O group to NO to form nitrite, followed by an oxygen atom transfer from a second equivalent of [Fe IV O(N4Py)] 2+ to the nitrite intermediate to form nitrate. This second step chemistry was confirmed by independently studying the reaction of [Fe IV O(N4Py)] 2+ with nitrite to form nitrate. There is also a biological inorganic chemistry in which metal nitrosyl species are oxidized to form innocuous nitrite or nitrate. In this context, the oxidation of the synthetic nitrosyl complex [Fe(tmc)(NO)] 2+ has been studied, which also produced [Fe II (tmc)(ONO)] + . The molecular structure of [Fe II (tmc)(ONO)] + determined by X-ray crystallography indicates a bidentate binding mode of the nitrito ligand via both oxygen atoms. The oxidation results are consistent with a net oxide(·- ) ion transfer mechanism forming [Fe II (tmc)(NO 2)] +, followed by a subsequent linkage isomerization. For comparison purposes, several related, independently synthesized [Fe II (tmc)X] + complexes (X = NO 2 -, NO 3 -, AcO - ) have been characterized by spectroscopic techniques, X-ray crystallography and differential pulse and cyclic voltammetry. A final investigation involved studying the reactivity of a series of [Fe IV O(tmc)X] + (X = CF 3 SO 3 -, CF 3 CO 2 -, AcO - ) complexes toward organic substrates by oxygen atom transfer and hydrogen atom abstraction to construct a reactivity trend depending on the strength of the axial ligand X.
Abstract.
Experiments and computational chemistry have been used to explore the identity of the active Fe/ZSM-5 surface oxygen which participates in redox reactions and the possible kinetics pathways involving it. The experiments entail monitoring the mass of the catalyst during reactions using a Tapered Element Oscillating Microbalance (TEOM), measuring kinetics in a tubular reactor as well as in the TEOM, and characterizing the catalysts using Fourier Transform Infrared (FTIR) spectroscopy, X-ray Diffraction (XRD) and Temperature Programmed Reduction (TPR). Computer studies involve kinetics modeling using microkinetic modeling theory, and probing the structures and thermochemistry of possible catalytic intermediate species using Density Functional Theory (DFT). In-situ gravimetry results show that the redox capacity of Fe/ZSM-5 using H 2 /O 2 probe is high (0.6 ~1.25), additionally two peaks are observed in H2-TPR. This indicates that following reduction, some of the iron may exist in an oxidation state lower than +2. However, for the redox reactions studied herein, the working state of the catalyst is fully oxidized and corresponds to ferric cations. The catalyst mass was monitored while making step changes in the gas phase composition at N 2 O decomposition conditions. The results suggest that NOx surface species do not form in N 2 O/He, and if they form in N 2 O/NO/He, their surface concentration is exceedingly small. A single mechanistic framework was proposed and used to model a variety of redox reactions; the microkinetic model is consistent with experimental observations of conversion and catalyst mass change as well as DFT calculations. Water Gas Shift (WGS) can not proceed over Fe/ZSM-5 up to 500°C. Unlike NO, trace CO and H 2 don't have a promotional effect beyond stoichiometric reaction when introduced at trace levels into N 2 O decomposition. Similarly, trace NO does not promote CO/H 2 oxidation. It appears that NO produces nitrite/nitrate intermediates and creates a fast pathway for O 2 desorption. The latter species' coverage is under 1% both in modeling as well as experimental results. However, the modeling was successful only if the surface bond energy of oxygen on the active site is different, when generated from N 2 O decomposition than when generated from oxygen. Density functional calculations were performed on the surface species involved. The computational chemistry shows that when this site is reduced, one of the terminal hydroxyl groups moves into a bridging position. In this way there exists a pool of 1.5 oxygen atoms per iron cation which participate in the N 2 O decomposition reaction whereas the redox capacity of the catalyst is only 0.5 oxygen atoms per iron cation.
In nature, flavoproteins (FMN and FAD) are known to catalyze several chemical transformations which play a vital role in the growth, development, and survival of organisms. They are involved in one-electron and two-electron transfer reactions, photo-induced electron transfer reactions, dehydrogenase reactions, oxidative atom transfer reactions and also rare non-redox reactions. Their enhanced stability and ability to turn over in presence of dioxygen has inspired synthetic chemists, including our group, to perform biomimetic transformations within a range of function of natural flavoproteins. In chapter 1, both intramolecular and intermolecular dehydrogenative coupling between the alpha carbon of tertiary amines and various nitrogen, phosphorus, and carbon-based nucleophilesare reported. This study signifies the flavin dependent oxidase type chemistry promoted by synthetic flavins, rendering the catalytic construction of some sophisticated heterocycles through an atom economical and aerobic approach. Mechanistic studies with different radical probes suggest the involvement of radical intermediates in the reaction cycle. Moreover, intramolecular kinetic isotope studies performed reveal possibility of Hydrogen atom abstraction being rate determining step. In chapter 2, a non-redox type of chemistry is disclosed. A subclass of riboflavin mimics was found to catalyze C-C bond formation by activating small molecules in a new manner. This approach was successfully applied to synthesize various industrially important dyes and chemical reagents. Additionally, the relationship discovered between molecular structure and catalytic function of riboflavin mimics in these new chemical reactions revealed a plausible explanation for the function of natural riboflavin-dependent hydroxynitrilase enzymes in biological system. Mechanistic studies using nuclear magnetic resonance (NMR) spectroscopy, UV-vis spectroscopy and electron paramagnetic resonance (EPR) spectroscopy showed a possible frustrated lewis-pair (FLP) type of interaction between aldehydes and flavin mimics. In chapter 3, studies on benzimidazole synthesis by iron catalysts will be discussed. 1,2-disubstituted benzimidazoles serve as important class of molecules in several area of chemistry including drug discovery, catalysis, etc. Our investigation in this area with redox active iron catalysts revealed N,N'-disubstituted-ortho-phenylenediamine substrates being superior to N,N disubstituted-ortho-phenylenediamines in generating 1,2-disubstituted benzimidazoles. Extensive UV-vis spectroscopy studies and kinetic studies have been performed in addition to EPR spectroscopy to understand the nature of mechanism. Both Lewis acid property and redox active property of iron trichloride are thought to play a significant role in catalysis. Smooth complex formation between N,N'-disubstituted-ortho-phenylenediamine substrates and iron catalyst provides the driving force for the electron transfer process to form productive iminiumintermediate.A simple method for chemo-selective oxidation of isoindolines to isoindolinones was also studied in chapter 4. This method utilizes no catalyst, no additive, mild condition and is highlighted as just solvent mediated transformation. Mechanistic investigation shows hydrogen atom abstraction process leading to isoindole intermediates which further binds to oxygen to give desired isoindolinones products.