Synthesis, Characterization and Reactivity Studies of Mononuclear Nonheme High-Valent Iron-Imido Complexes

Abstract

[(TAML)FeV(NTs)]– (1)의 성공적인 합성은 iron-imido species에 대한 다양한 연구를 할 수 있는 기회가 되었다. 일-전자 산화제에 의한 1의 산화는 iron(V)-imido TAML cation radical complex인 [FeV(NTs)(TAML+•)] (2)을 얻을 수 있었으며, 여기서 2는 low-spin iron(V) ion (S = 1/2)과 일-전자 산화된 리간드(TAML+•)와의 antiferromagnetic coupling으로 인해 반자성(S = 0) 성질을 가짐을 알 수 있었다. 두번째로 1과 탄수 화물의 반응에서 수소 원자 전달 반응에 의하여 단핵 비헴성 철(IV)-아마이도 화합물인, [(TAML)FeIV(NHTs)]– (3)을 얻을 수 있었다. 여기서 3은 NHTs 그룹과 탄소 라디칼 사이의 질소 그룹 재결합 경로와 수소 원자 흡착 반응을 통해 3의 pKa와 BDE를 구할 수 있었다. 세번째로, [FeV(NTs)(TAML+•)] (2)와 [(TAML)FeV(NTs)]– (1), [(TAML)FeV(O)]– (4)의 다양한 형태의 반응을 통하여 반응성을 비교하였다. 즉 1, 2 그리고 4의 일-전자 환원 포텐셜 (Ered)은 각각 0.30 V, 0.86 V 및 1.04 V vs. SCE로 측정되었다. 또한 2의 λ(재결합 에너지) 값 (1.00 eV)은 1 (1.98 V)과 4 (2.25 eV)보다 매우 낮았으며 이는 1과 4가 금속 중심의 환원이 일어남과 대조적으로 2는 리간드 중심의 환원이 일어남으로 인하여 비롯된 것으로 사료된다. 이 연구는 처음으로 고-산화가 금속-옥소/이마이도 종들에서 리간드-중심과 금속-중심의 환원의 λ값이 매우 다름을 보여준 결과이다. 또한 산화반응에서 나이트렌(nitrene) 전달 반응(NT), 산소 원자 전달 반응 및 수소 원자 전달 반응을 통한 1, 2 및 4의 반응성을 비교할 수 있었다. 네번째로, 1에 산을 첨가 했을 때, 1의 불균둥화 반응에 의하여 2와 철(IV)-TAML 화합물인, [FeIV(TAML)] (5)가 생성됨을 확인할 수 있었다. 마지막으로 단핵 비헴성 철(III) 화합물인 [(TAML)FeIII]–은 아세톤 용매에서 아세트 산 존재하에 Me10Fc와 Me8Fc에 의해 산소 분자의 4-전자 환원 반응의 선택적 촉매임을 확인할 수 있었다. 그러므로 [(TAML)FeIII]–은 산소 분자 활성을 통하여 철(V)-옥소 종을 생성할 수 있으며, 산소 분자를 물 분자로 환원시킬 수 있는 좋은 촉매제임을 알 수 있었다.;In biomimetic studies, high-valent Fe=O complexes have been synthesized, characterized structurally and spectroscopically, and used to investigate various oxidative transformation reactions. However, their analogs, iron-imido, have been less clearly understood, although a number of iron(IV)-imido species were synthesized. Recently, the first example of a mononuclear nonheme iron(V)-imido complex with a tetraamido macrocyclic ligand (TAML), [(TAML)FeV(NTs)]– (1), was reported. The successfully synthetic [(TAML)FeV(NTs)]– provided me an excellent opportunity to explore the iron-imido species. Firstly, I attempted to achieve one-electron oxidation of 1 and found oxidation of 1 by one-electron oxidants afforded the production of an iron(V)-imido TAML cation radical complex, [FeV(NTs)(TAML+•)] (2); 2 is diamagnetic (S = 0), because of an antiferromagnetic coupling of a low-spin iron(V) ion (S = 1/2) with one-electron oxidized ligand (TAML+•). Comparison of the Fe–NTs bond lengths and the Fe–NTs stretching vibrations of 1 and 2 revealed that the Fe–NTs bond strength of 2 is slightly weaker than that of 1. 2 is a competent oxidant in C–H bond functionalization and nitrene transfer reactions. Next, when HAT reaction of 1 was carried out, a mononuclear nonheme iron(IV)-amido, [(TAML)FeIV(NHTs)]– (3), was formed. After 3 was fully characterized using various spectroscopic techniques along with DFT calculations, 3 was investigated in (1) the determination of the pKa of the Fe(IV)-NHTs, which is similar to the determination of the Fe(IV)-OH pKa in Compound II (Cpd II) of Cytochromes P450 (CYP450), (2) the reaction of the Fe(IV)-NHTs with a carbon-centered radical, which is similar to the oxygen rebound step of Cpd II Fe(IV)-OH and a carbon radical in the C-H hydroxylation by Compound I (Cpd I), and (3) the capability of the NHTs group in HAT reaction. This study reports the first successful synthesis of a high-valent iron(IV)-amido complex as an analogue of Cpd II of CYP450, the determination of the Fe(IV)-NHTs pKa, the nitrogen rebound process between the NHTs group and a carbon radical and the reactivity of the iron(IV)-imido complex in H-atom abstraction reaction. With [FeV(NTs)(TAML+•)] (2), [(TAML)FeV(NTs)]– (1) and [(TAML)FeV(O)]– (4) in hand, then the comparison of reactivity of these three intermediates towards electron-transfer and redox reactivity was carried out. In this study, the ET properties of 1, 2 and 4 were clarified for the first time. The Ered of 1, 2 and 4 were calculated to be 0.30 V, 0.86 V and 1.04 V vs SCE, respectively. Based on the Marcus theory, The rate constants of electron-transfer (ET) of 1, 2 and 4 were analyzed to obtain the reorganization energies (λ) of their ET reactions. λ of 2 (1.00 eV) is greatly less than those of 1 (1.98 eV) and 4 (2.25 eV) resulting from the ligand-centered ET reduction of 2, but the metal-centered ET reduction of 1 and 4. This study at first time afforded a large difference in λ between the ligand-centered and the metal-centered ET reduction of M=O/NR species. In oxidation reactions, the reactivities of 1, 2 and 4 towards NT and OAT to thioanisole derivatives were compared; the rate constants determined in NT from 2 to thioanisole derivatives are close to those expected for ET from thioanisoles to 2, whereas the NT and OAT reactivities of 1 and 4 are larger than those expected for ET from thioanisoles to 1 and 4. The NT of 1 was much lower than that of 2, whereas the OAT reactivity of 4 was higher than the NT reactivity of 2. In the case of the H-atom abstraction reaction, for example, the hydrogen atom transfer (HAT) of CHD, the HAT rate constants of 1, 2 and 4 were larger than those expected for ET from CHD to 1, 2 and 4 as well. The reactivity toward H-atom abstration by 4 was ~100 times larger than those of 1 and 2, however, 1 and 2 exhibited a similar reactivity. Recently, it has been demonstrated that protons are able to increase significantly the oxidizing power of M=O species. In addition, in some cases, protons could be able to promote the disproportionation of an compound II to produce an iron(IV)-oxo porphyrin-cation radical and an iron(III) porphyrin. However, compared to the large number of studies toward the effect of acids on the reactions of Fe=O species, the effect of protons on the chemical properties of iron-imido complexes has never been explored previously. The successfully synthetic [(TAML)FeV(NTs)]– inspired me to investigate the effect of protons on the Fe(V)-imido complexes. Interestingly, 1 was disproportionated to 2 and an iron(IV) TAML complex, [FeIV(TAML)] (5), along with an organic product, NH2Ts, upon addition of proton sources (HOTf, HClO4, TFA). Mechanistic investigation demonstrated that a proton-mediated disproportionation of 1 took place to produce 2 and 3, followed by protonation of 3 to give 5 and NH2Ts. This study shows a proton-promoted disproportionation reaction of Fe=NTs species for the first time. Finally, a mononuclear nonheme iron(III) complex with a tetraamido macrocyclic ligand (TAML), [(TAML)FeIII]– (6) as a catalyst for reduction of dioxygen (O2) have been examined. It was found that 6 is a selective catalyst for four-electron reduction of O2 by decamethylferrocene (Me10Fc) or octamethylferrocene (Me8Fc) as a reductant in the presence of acetic acid in aerated acetone.This is the first time to show that a nonheme iron(III) complex catalyzes the four-electron reduction of O2 by one-electron reductants. The iron(V)-oxo complex, 2, was generated by the oxygenation of 1 with O2 via the production of triacetone triperoxide, acting as an autocatalyst that shortened the induction time for the generation of 2. Me10Fc and Me8Fc reduced 2 to 1 by two electrons in the presence of acetic acid to produce Me10Fc+ and Me8Fc+, respectively. Then, 1 was oxygenated by O2 to regenerate 2 via the formation of TATP. In the cases Fc, BrFc and Br2Fc, initial electron transfer (ET) from ferrocene derivatives to 2 occurred, however, no second proton-coupled electron transfer (PCET) from ferrocene derivatives to 2 occurred, resulting to no catalytic four-electron reduction of O2.