Mechanistic Study of Electron-Transfer Reactions by Ground State and Photoexcited State of Nonheme Mononuclear Manganese-Oxo Complexes Binding with Lewis Acids

Abstract

고-산화가 금속-옥소 중간체는 물 산화 반응, 하이드록실화 반응, 옥시 할로겐화 반응 및 불포화 반응 등과 같은 중요한 생물학적 및 합성 산화 과정에 연계되어있다. 고-산화가 금속-옥소 중간체의 바닥 상태 반응성은 광범위하게 연구되어왔다. 대조적으로, 고-산화가 금속-옥소 중간체의 여기 상태 반응성에 대해서는 거의 알려져 있지 않다. 본 학위논문 연구에서 본 연구자는 비-헴성 망간(IV)-옥소 화합물의 바닥 상태 및 광 여기 상태에서의 전자 전달 반응을 연구하였다. 본 학위논문은 6가지 주제로 구성되어 있으며 다음과 같다: (1) 비-헴성 망간(IV)-옥소 화합물에의한 안트라센으로부터 안트라 퀴논으로의 다중-전자 산화 반응 연구, (2) 스칸듐(III) 트리플레이트 및 트리플릭 산의 존재 및 부재하의 비-헴성 망간 (IV)-옥소 화합물의 외부권 전자전달과 내부권 전자전달 경로의 비교 연구, (3) 스칸듐 이온이 결합된 비-헴성 망간(IV)-옥소 화합물의 오래 지속되는 광 여기 상태에 의한 벤젠의 하이드록실화 반응 연구, (4) 단핵 비-헴성 망간(IV)-옥소 화합물과 브로마이드 이온 (Br–)에 의한 벤젠 및 그 유도체의 위치-선택적 옥시 브롬화 반응 연구, (5) 스칸듐 나이트레이트가 결합된 비-헴성 망간(IV)-옥소 화합물의 오래 지속되는 광-여기 상태의 생성 및 전자-전달 반응성에 대한 연구 및 (6) 400 % 양자 효율을 갖는 트리플릭 산이 결합된 망간(IV)-옥소 화합물의 광-여기된 상태에 의한 톨루엔의 1-광자 4-전자 산화 반응에 대한 연구;High-valent metal-oxo intermediates have been implicated in a diverse array of important biological and synthetic oxidation processes such as water oxidation, hydroxylation, oxyhalogenation and desaturation. The ground state reactivity of high-valent metal-oxo intermediates has been studied extensively. In contrast, little is known about the excited state reactivity of high-valent metal-oxo intermediates. In this thesis research, I have studied electron-transfer reactions of both ground state and photoexcited state of a non-heme MnIV-oxo complex. This thesis consists of six different subjects: (i) multi-electron oxidation of anthracene to anthraquinone by non-heme manganese(IV)-oxo complexes; (ii) outer-sphere versus inner-sphere electron-transfer pathways of non-heme manganese(IV)-oxo complexes in the absence and presence of scandium triflate and triflic acid; (iii) hydroxylation of benzene by a long-lived photoexcited state of a non-heme Mn(IV)-oxo complex binding scandium ions; (iv) regioselective oxybromination of benzene and its derivatives by TBABr with a mononuclear non-heme Mn(IV)-oxo complex; (v) generation and electron-transfer reactivity of the long-lived photoexcited state of a manganese(IV)-oxo-scandium nitrate complex; (vi) one-photon four-electron oxidation of toluene by the photoexcited state of a triflic acid-bound Mn(IV)-oxo complex with 400% quantum efficiency. Firstly, six-electron oxidation of anthracene to anthraquinone has been studied using a non-heme MnIV-oxo complex, [(Bn-TPEN)MnIV(O)]2+, which proceeds through a rate-determining electron-transfer from anthracene to [(Bn-TPEN)MnIV(O)]2+, followed by subsequent fast oxidation reactions to give anthraquinone. Formation of anthracene radical cation was directly detected in the electron-transfer from anthracene to Sc3+ ion-bound MnIV(O) complex, [(Bn-TPEN)MnIV(O)]2+-(Sc(OTf)3)2, followed by subsequent further oxidation to yield anthraquinone. The driving force dependence of the rate constants of electron-transfer from the anthracene derivatives to [(Bn-TPEN)MnIV(O)]2+ and [(Bn-TPEN)MnIV(O)-(Sc(OTf)3)2]2+ was well-evaluated in light of the Marcus theory of electron-transfer. Secondly, epoxidation of styrene derivatives, sulfoxidation of thioanisole derivatives, and hydroxylation of toluene derivatives have been investigated by a non-heme manganese(IV)−oxo complex binding triflic acid, [(N4Py)-MnIV(O)]2+−(HOTf)2 (1-(H+)2), and scandium triflate, [(N4Py)MnIV(O)]2+−(Sc(OTf)3)2 (1-(Sc3+)2), which proceed via outer-sphere electron-transfer (OSET) pathways exhibiting a singly unified driving force dependence, enabling one to predict absolute values of the second-order rate constants of these three types of substrate oxidations by the manganese(IV)−oxo complex, using the Marcus theory of electron transfer. When [(N4Py)MnIV(O)]2+ (1) was replaced by [(N4Py)FeIV(O)]2+ (2), OSET pathways were changed to inner-sphere electron-transfer (ISET) pathways, which is clarified based on the difference in the Lewis basicity of the oxo moieties in 1 and 2. In the third section of this thesis, generation of photoexcited state of a non-heme manganese(IV)−oxo complex was demonstrated followed by study of photo-induced electron-transfer reactivities using nanosecond laser excitation. Photoexcitation of [(Bn-TPEN)MnIV(O)]2+−(Sc-(OTf)3)2 in a solvent mixture of trifluoroethanol and acetonitrile (v/v = 1:1) resulted in formation of the long-lived photoexcited state which has an absorption band at max = 640 nm and lifetime of 6.4 s, which is capable to hydroxylate benzene to phenol. The photohydroxylation of benzene by [(Bn-TPEN)MnIV(O)]2+−(Sc(OTf)3)2 was made possible by ET from benzene to the long-lived 2E excited state of [(Bn-TPEN)MnIV(O)]2+−(Sc(OTf)3)2 to produce a benzene radical cation, which reacted with water as revealed by laser-induced transient absorption measurements. The regioselective oxybromination of methoxy-substituted benzenes by ground state of [(Bn-TPEN)MnIV(O)]2+−(Sc(OTf)3)2, in the presence of tetrabutylammonium bromide have been studied as described in Chapter IV. It was observed that under photoirradiation, the regioselective oxybromination of benzene by [(Bn-TPEN)MnIV(O)]2+−(Sc(OTf)3)2 can be achieved via electron-transfer from benzene to the photoexcited state of [(Bn-TPEN)MnIV(O)]2+−(Sc(OTf)3)2, although no reaction occurs between benzene and the ground state of [(Bn-TPEN)MnIV(O)]2+−(Sc(OTf)3)2 in the dark. Photoexcitation of a manganese(IV)-oxo-scandium nitrate complex ([(Bn-TPEN)MnIV(O)]2+–Sc(NO3)3) in a solvent mixture of trifluoroethanol and acetonitrile (v/v = 1:1) resulted in generation of the long-lived photoexcited state, which was detected by nanosecond laser transient absorption measurements as described in Chappter V. The transient absorption maximum (max) of the 2E excited state of [(Bn-TPEN)MnIV(O)]2+–Sc(NO3)3 is observed at 620 nm with lifetimes of 7.1 s. In the last part of this thesis (Chapter VI), the photoexcitation of a triflic acid-bound MnIV-oxo complex (1: [(N4Py)MnIV(O)]2+–(HOTf)2) in a deaerated solvent mixture of trifluoroethanol and acetonitrile (v/v = 1:1) using nanosecond laser transient absorption measurements has been discussed. The formation of the long-lived photoexcited state, which can oxidize toluene to produce the four-electron oxidized product (benzaldehyde) selectively was observed. The formation of benzyl alcohol was observed in the initial reaction, which was followed by the further reaction of benzyl alcohol with ground state of 1 to produce benzaldehyde as the final product. Such a one-photon four-electron oxidation of toluene by ([(N4Py)MnIV(O)–(HOTf)2]2+ afforded 400% quantum efficiency.