Theoretical Study of C-H Bond Activation by Mononuclear and Dinuclear High-Valent Iron Complexes

Abstract

The C-H bond activation by high-valent iron complexes have been investigated into detail using density functional theory (DFT). The first part of the present PhD thesis concerns with the reaction mechanism of C-H bond hydroxylation by mononuclear iron(IV)oxo model complexes ([Fe^IV(O)(NH₃)₅]²⁺ (a), [Fe^IV(O)(OH)_(axial)(NH₃)₄]⁺ (b), [Fe^IV(O)(OH)_2(eq)(NH₃)₃] (c)). In addition to the classical σ-pathway for quintet state (⁵σ) and π-pathway (³π) for triplet state, two new reaction pathways, ⁵π and ⁵σ, have been established. This is the first time that all viable reaction pathways for the C-H bond hydroxylation by high-valent iron(IV)-oxo complex have been identified in the same system. The new triplet σ pathway (³σ) is too high in energy to be involved in C-H bond activation, but the reactivity of the quintet π channel (⁵π) is comparable or even higher than the triplet pathway. The existence of at least three energetically feasible pathways may offer, however, a new element of specificity control in C-H bond activation reactions by iron(IV)–oxo species.<br /> The second part of the thesis deals with the H-atom abstraction reactivity of six hypothetic iron–oxo (Fe(O)(NH₃)₄(OH)_axial) and iron–nitrido (Fe(N)(NH₃)₄(OH)_axial) model complexes. The iron oxidation state ranges from IV to VI. The calculations reveal that the iron-oxo species (<strong>1 - 3</strong>) and their nitrido analogues (<strong>4 - 6</strong>) feature dramatically different intrinsic reactivity towards C-H bonds. In the case of the iron-oxo series, the reactivity order of <strong>1 < 2 << 3</strong> was observed, reflecting an increase in the electrophilicity of iron-oxo complexes upon the increased iron oxidation state. Surprisingly, the iron-nitrido series is not as reactive as its oxo counterpart and the reactivity order was inverted in the oxidation of ethane C-H bonds, i.e., <strong>4 ≥ 5 > 6</strong>. All these results correlated well with the Bell-Evans-Polanyi principle in which a linear relationship between the energy barrier and the thermodynamic driving force was observed. Furthermore, the different properties of the iron–oxo and –nitrido complexes as well as the counterintuitive reactivity of these two series were understood by analyzing the thermodynamical nature of H-atom affinity, i.e. its electron and proton affinity component.<br /> The C-H bond activation by four high-valent diiron complexes, two with diamond core structure, (Fe^III(μ-oxo)₂Fe^IV, <strong>1</strong> and Fe^IV(μ-oxo)₂Fe^IV<strong>2</strong>) and two with open core structure (OH-Fe^IV-O-Fe^IV=O, <strong>3</strong> and OH-Fe^III-O-Fe^IV=O, <strong>4</strong>) was reported in the third part of this thesis. Our calculations show that, processing from <strong>1</strong> to <strong>4</strong>, the computed barriers decrease and follow the order <strong>1 < 2 < 3 < 4</strong>, in good agreement with the reactivity trend observed experimentally (Xue, G.; De Hont, R.; Münck, E.; Que, L. <em>Nature Chem.</em> <strong>2010</strong>, 2, 400–405.). Their reaction mechanisms fall essentially into two categories, hydrogen atom transfer in the case of <strong>1, 3 </strong>and<strong> 4</strong> and hydride transfer for ^IV. The different reactivity of complexes <strong>1 – 4</strong> can be well rationalized by the thermodynamic and kinetic considerations. First, the thermochemistry has successfully captured the essence of the hydride transfer reaction by <strong>2</strong> that, there is a thermodynamic preference of 27.5 kcal/mol for hydride transfer pathway than that for HAT. Second, the relative sluggish reactivity of diamond core complexes <strong>1 </strong>and<strong> 2</strong> can be attributed to the higher energetic penalty required for the structural arrangements upon redox processes than the open core ones <strong>3 </strong>and<strong> 4</strong>. Finally, the highest efficiency of HAT by complex <strong>4</strong> originates from the thermodynamic and kinetic preference. The strongest O–H bond formed during the oxidation process by <strong>4</strong> offers the largest thermodynamic driving force, and the lowest reorganization energy both for the diiron reagent and substrate makes <strong>4</strong> also favorable in kinetic aspect.<br /> The last part of the present thesis is about the hydrogen bond effect in modulating C-H bond activation. The C-H bond activation by two high-valent localized open core diiron complexes (<strong>1</strong>-OH_cis, OH–Fe^III–O–Fe^IV=O and <strong>1</strong>-F_trans, F–Fe^III–O–Fe^IV=O) have been explored using DFT. The computed geometry parameters of these two complexes show that <strong>1</strong>-OH_cis adopts a <em>cis</em> conformation in which an H–bond is formed between the terminal oxo and hydroxo group. However, a <em>trans</em> conformation is established for <strong>1</strong>-F_trans. Our detailed reactivity study demonstrates that <strong>1</strong>-F_trans displays even higher oxidation power than that of <strong>1</strong>-OH_cis, which is in good agreement with the experimental findings. Furthermore, our calculations revealed that the hydrogen bond between the oxo and hydroxo group in <strong>1</strong>-OH_cis does not significantly change the electrophilicity of the reactive Fe^IV=O unit. However, during the reaction of C-H bond oxidation, this hydrogen bond has to be partially broken. This leads to the slightly higher barrier for <strong>1</strong>-OH_cis relative to <strong>1</strong>-F_trans, which has similar open-core structure but no hydrogen bond.