氏 名
Zhifengｼ ﾌ ｪ ﾝ Maﾏ所 属 理工学研究科 分子物質化学専攻 学 位 の 種 類 博士（理学）
学 位 記 番 号 理工博 第
334号 学位授与の日付 令和
2年
9月
30日
課程・論文の別 学位規則第４条第１項該当
学 位 論 文 題 名
Electronic Structure and Oxidation Reaction Mechanism of Oxoiron(IV) and Oxomanganese(V) Porphyrins: A DFT Study鉄
(IV)及びマンガン
(V)オキソポルフィリンの電子構造と酸化反応機 構：
DFTによる研究
(英文
)論 文 審 査 委 員 主査 教授 波田 雅彦 委員 准教授 中谷 直輝 委員 教授 野村 琴広
委員
教授 藤井 浩（奈良女子大学）
【論文の内容の要旨】
Porphyrins have been well-studied because of their role as biological ligands. The iron-bound and manganese-bound porphyrins are at the active site of many metalloproteins and perform a diverse range of chemistry such as oxygen transport and storage, electron transfer, and oxidation/oxygenation reactions using O2/H2O2 as the oxidant and in many cases the O-atom source. Examples of monooxygenation reactions catalyzed by heme enzymes include alkane hydroxylations and olefin epoxidations.
However, in these reactions, axial ligands and peripheral substitutions of the porphyrin ring, as well as the protein environment (e.g., charged amino acid residues Arg48 and Asp235 in cytochrome-c) greatly affect the distribution of products and oxidation reaction mechanism. To this end, it is necessary to perform theoretical investigations for insights into the original understanding of electronic structure and working mechanism, physical and chemical properties of oxoiron(IV) and oxomanganese(V) porphyrins.
Therefore, the aim of this thesis is to better understand and reveal the functional role of axial ligands and peripheral substituents, and the effect of protein environment using density functional theory (DFT) calculations. The important points are as follows.
The effects of peripheral fluorine atoms on epoxidations of ethylene by oxoiron(IV) porphyrin cation radical complex in the quartet and sextet spin states are
systematically investigated using the DFT method. By obtaining the energy diagrams and electron- and spin-density difference contour maps of the transition states and intermediate species, we confirm that the electron-withdrawing by peripheral fluorine atoms enhances the reactivity as the number of fluorine atoms increases, as is observed experimentally. The intersystem crossing (ISC) between the quartet and sextet spin states is discussed by means of the intrinsic reaction coordinate (IRC) method. We conclude that the rate-determining step is located at the first transition state (TS1) for the activation of C=C and Fe=O bonds, and the ground electronic state changes from quartet to sextet around the TS1.
Insights into environmental perturbation (an external electric field, EEF) are complicated but important in terms of experiments. Therefore, we study the effect of an EEF on olefin epoxidation by the Fe(IV)OCl–porphyrin complex using the DFT method.
The EEF along the electron flow greatly affects the potential energy profile, and thereby affects the reaction mechanism and stabilization of the species. The results show that a negative EEF catalyzes ethylene epoxidation, whereas a positive EEF inhibits the reaction. Moreover, an EEF can exchange the ground state with the low-lying excited states. Therefore, the potential energy profile along the epoxidation reaction is mainly modified by the electron transfer from ethylene to the Fe(IV)OCl–porphyrin complex.
A series of DFT studies on the epoxidation reactions of olefins by oxoiron(IV) porphyrin cation radical complexes are performed to elucidate the axial ligand effects on the electronic feature and reaction mechanism in detail. We analyzed the molecular orbitals, spin populations, and Mulliken charges along the IRC route. From the findings, we confirmed that the interaction between the axial ligand and the oxoiron(IV) porphyrin is strong and the initial changes in the electronic structures occur early during the reaction, which further enhances the reactivity toward olefin epoxidation.
More importantly, the patterns of the electron transfer from olefin to oxoiron(IV) porphyrin were impacted by the axial ligand. The pattern of successive electron transfer from Fe–O to porphyrin and then from C=C to Fe–O for oxoiron(IV) porphyrin in case of fluorine and acetate axial ligands, whereas the pattern of electron transfer occurs from C=C to porphyrin for oxoiron(IV) porphyrin in case of chlorine and nitrate axial ligands during the epoxidation reaction of the olefins. We also determined the intersystem crossing between the quartet and sextet spin states occurring at the second transition state (TS2) by the analysis of the two-dimensional potential energy surface.
We have performed DFT calculations on the C=C epoxidation and C–H hydroxylation reactions of propene by a model of iron(IV)OCl-porphyrin cation radical complex with fluorine and methyl groups as the meso-substitution. In gas phase
calculations it is found that the meso-substituted fluorine enhances the reactivity. The environmental factor (i.e. solvent) affects the ratio of hydroxylation/epoxidation by the iron(IV)OCl-porphyrin with methyl group as the meso-substitution. According to an electronic feature analysis of the reactant complex and the transition state, the electron-withdrawing groups on meso-position stabilize the electron acceptor orbital of complex more than electron donation orbital of substrate, and decrease the energy gap between these orbitals, leading to lower the barrier. Additionally, the meso-substituted fluorine for pull effect makes hydroxylation favorable, whereas the meso-substituted methyl causes epoxidation preferable. The ability to oxidation reactivity of Fe(IV)OCl-porphyrin is largely ascribed to the amount of electron transfer affected by the electronegativity of meso-substituted groups from propene to Fe(IV)OCl-porphyrin.
We analyzed the intersystem crossing between quartet and sextet spin states using the potential energy surfaces (PESs), and the crossing seam between quartet and sextet PESs occurs around TS1.
Since the experimental data shows the low yield of alkane hydroxylation, and in the presence of plausible neutral and cationic intermediate species, the alkane hydroxylation reaction by neutral [PorMn(O)-L] (L=OH−, F−) and cationic [PorMn(O)-L]+ (L=H2O, Imidazole) complexes has been investigated by the DFT calculations to better understand the reaction mechanism. The oxoMn(V) porphyrin species lead to low yields of alkane hydroxylation, meaning high reaction barriers. In this part work, we find that the oxoMn(V) porphyrin species raise energy barrier, which results in the low yield of alkane hydroxylation. According to electronic structure analysis, it is found that in the C−H activation, the electron transfer from σCH of substrate to empty π*(Mn=O) orbitals occurs through a complicated initiation by interaction between σCH and rich-oxygen πpx(O,L)/πxz orbitals of catalyst. Moreover, the positive charge in the cationic complex stabilizes the acceptor orbital more than donor orbital, reducing the energy gap between these orbitals and, thus, lowering the reaction barrier.
OxoMn(V) porphyrin complex performs the competitive hydroxylation, desaturation and radical rearrangement using diagnostic substrate norcarane. Initial C−H cleavage proceeds through the two hydrogen abstraction steps from the two adjacent carbon on the norcarane, then selective reaction is performed to generate various products. Using the DFT calculations, we show that the hydroxylation and desaturation reactions are triggered by a rate-determining hydroxylation abstraction step, whereas the rate-determining step for the radical rearrangement is located in the rebound (TS2). We find that the endo-2 reaction is favorable over other reactions, which is consistent with experimental result. Furthermore, the competitive pathways for
norcarane oxidation depend on the van der Waals interaction and steric repulsion between norcarane and porphyrin-ring, as well as energy gaps between donor and acceptor orbitals. It predicts that the stereo-/regio-selectivity and competitiveness of norcarane oxidation are hardly sensitive to the zero-point energy and thermal free energy corrections. We have systematically studied the electronic structure and oxidation reaction mechanism of oxoiron(IV) and oxomanganese(V) porphyrins, thus I believe that these results are helpful to better understand the reaction process and to design the new the catalyst in the oxidation reactions.
