Redox Reactivities of a Mononuclear Nonheme Chromium(III)-Superoxo Complex in the Absence and Presence of Lewis Acids

Abstract

Metal-superoxo species have been proposed as the foremost intermediate generated during dioxygen activation process by metalloenzymes that play a pivotal role in a variety of enzymatic oxidation reactions. Nevertheless, due to high thermal instability of the metal-superoxo species, their generation and isolation even in the cryogenic temperatures remains a challenge in the model system. In contrast, the thermal stability of a mononuclear nonheme CrIII-superoxo complex, [(Cl)(TMC)CrIII(O2)]+ (1, TMC = 1,4,8,11-tetramethyl-1,4,8,11-tetraazacyclotetradecane), which is generated at 263 K by simple dioxygen activation of its Cr(II)-precursor, enabled us to perform the reactivity studies of the CrIII-superoxo intermediate in different redox reactions. In this research work, first approach was made to investigate the hydride transfer reactions by a chromium(III)-superoxo complex with dihydronicotinamide adenine dinucleotide (NADH) analogues as hydride donors. In this study, the CrIII-superoxo complex, [(Cl)(TMC)CrIII(O2)]+ was found to act as a novel three-electron oxidant in the oxidation of NADH analogues. In the reactions of [(Cl)(TMC)CrIII(O2)]+ with NADH analogues, the formation of a CrIV-oxo complex, [(Cl)(TMC)CrIV(O)]+ was observed. The [(Cl)(TMC)CrIV(O)]+ was proposed to be generated via heterolytic O-O bond cleavage of a putative CrII-hydroperoxo complex, [(Cl)(TMC)CrII(OOH)], which is formed by hydride transfer from NADH analogues to [(Cl)(TMC)CrIII(O2)]+. The reactivities of hydride transfer form NADH analogues to hydride acceptor such as p-chloranil (Cl4Q) were also investigated under same reaction conditions. The comparison of the reactivity of NADH analogues with [(Cl)(TMC)CrIII(O2)]+ and Cl4Q suggested that oxidation of NADH analogues by [(Cl)(TMC)CrIII(O2)]+ proceeded by proton-coupled electron transfer with a very large tunneling effect (for example, with a kinetic isotope effect of 470 at 233 K), followed by rapid electron transfer. Secondly, I attempted to study the effect of proton on the reactivities of a chromium(III)-superoxo complex towards sulfoxidation and electron transfer reactions. In this investigation, remarkable acid catalysis was observed in oxygen atom transfer reactions by a mononuclear nonheme Cr(III)-superoxo complex, [(Cl)(TMC)CrIII(O2)]+, in the presence of triflic acid (HOTf). In the acid-catalyzed reactions, the reactivity of [(Cl)(TMC)CrIII(O2)]+ in the sulfoxidation of thioanisole (i.e., OAT reaction) was enhanced significantly, showing more than 104-fold acceleration in reaction rate. Electron transfer (ET) from one-electron donors to [(Cl)(TMC)CrIII(O2)]+ was also examined which occurred only in the presence of HOTf. The enhanced reactivity of [(Cl)(TMC)CrIII(O2)]+ by HOTf was well analyzed with the proton-coupled electron transfer (PCET) from one-electron donors, such as ferrocene, to [(Cl)(TMC)CrIII(O2)]+ in light of the Marcus theory of electron transfer. The present study reports for the first time the dramatic proton effect on the chemical properties of metal-superoxo species. These findings were further extended to investigate the effect of proton on the aromatic hydroxylation by Cr(III)-superoxo complex. The results demonstrated that the hydroxylation of aromatic compounds such as anthracene and its derivatives by a mononuclear nonheme Cr(III)-superoxo complex, [(Cl)(TMC)CrIII(O2)]+, occurred in the presence HOTf via the rate-determining PCET from anthracene to [(Cl)(TMC)CrIII(O2)]+, followed by a fast further oxidation to give anthraquinone. The rate constants of electron transfer from anthracene derivatives to [(Cl)(TMC)CrIII(O2)]+ in the presence of HOTf are well analyzed in light of the Marcus theory of electron transfer. Finally, the effects of the Lewis acidity of the redox-inactive metal ions on the reactivities of a chromium(III)-superoxo complex towards electron-transfer reactions have been examined. It was found that the electron transfer (ET) from octamethylferrocene (Me8Fc) to a mononuclear nonheme Cr(III)-superoxo complex, [(Cl)(TMC)CrIII(O2)]+, occurred in the presence of redox-inactive metal ions (Mn+ = Ca2+, Mg2+, Y3+, Al3+, and Sc3+); however, in the absence of the redox-inactive metal ions, ET from Me8Fc to [(Cl)(TMC)CrIII(O2)]+ did not occur. The second-order rate constants (ket) of ET from Me8Fc to [(Cl)(TMC)CrIII(O2)]+ in the presence of a redox-inactive metal ion increased with increasing concentration of Mn+ ([Mn+]), exhibiting a second-order dependence on [Mn+]: ket = kMCET[Mn+]2, where kMCET is the fourth-order rate constant of metal ion-coupled electron transfer (MCET). This means that two Mn+ ions are bound to the one-electron reduced species of [(Cl)(TMC)CrIII(O2)]+. Such a binding of two Mn+ ions associated with the ET reduction of [(Cl)(TMC)CrIII(O2)]+ resulted in a 92 mV positive shift of the one-electron reduction potential of [(Cl)(TMC)CrIII(O2)]+ (Ered) with increasing log([Mn+]). The log kMCET values increased linearly with increasing the Lewis acidity of Mn+ (∆E), which was determined from the g values of O2•––Mn+ complexes. The driving force dependence of log ket of MCET from ferrocene derivatives to [(Cl)(TMC)CrIII(O2)]+ in the presence of Mn+ has been well evaluated in light of the Marcus theory of electron transfer.;금속-슈퍼옥소 종은 다양한 효소 산화 반응에서 중추적인 역할을 하는 금속 효소에 의해 산소분자 활성화 과정에서 생성된 가장 중간체로서 제안되었다. 그럼에도 불구하고, 금속-수퍼옥소 종의 높은 열적 불안정성으로 인해 극저온에서도 모델 시스템을 통한 생성 및 분리가 여전히 어려운 과제이다. 대조적으로, 263K에서 크롬(II)-전구체의 간단한 산소분자 활성화 반응에 의해 생성되는 단핵 비-헴성 크롬(III)-슈퍼옥소 화합물, [(Cl)(TMC)Cr(III)(O2)]+ (1),의 열적 안정성은 다양한 산화-환원 반응에서 크롬(III)-수퍼옥소 중간체의 반응성 연구를 수행할 수 있게 해주었다. 본 학위논문에서는 첫째, H– 공여체로서 디-하이드로 니코틴 아미드 아데닌 디-뉴클레오타이드 (NADH) 유사체와 크롬(III)-슈퍼 옥소 화합물에 의한 하이드라이드 (H–) 전달 반응에 대한 연구를 수행하였으며, 크롬(III)-슈퍼옥소 화합물은 NADH 유사체의 산화에서 3-전자 산화제로서 작용하는 것으로 규명하였다. 둘째, 슬폭시화 반응 및 전자-전달 반응에 대한 크롬(III)-수퍼옥소 화합물의 반응성에 대한 양성자의 영향을 연구하였다. 산-촉매 반응에서, thioanisole의 슬폭시화 반응 (즉, OAT 반응)에서 크롬(III)-슈퍼옥소 종 (1)의 반응성이 크게 향상되어 반응 속도에서 104 배 이상의 가속을 나타내었다. 한편 일-전자 공여체로부터 크롬(III)-슈퍼옥소 종 (1)로의 전자 전달 (ET) 반응을 조사하였으며, 단지, HOTf의 존재 하에서만 반응이 일어남을 확인할 수 있었다. 셋째, 위에서 얻은 결과를 확장하여 크롬(III)-슈퍼옥소 화합물에 의한 방향족 하이드록실화 반응에 대한 양성자의 효과를 조사하였다. 결과론적으로 단핵 비-헴성 크롬(III)-슈퍼옥소 화합물 (1)에 의한 안트라센 및 그의 유도체와 같은 방향족 화합물의 하이드록실화 반응은 안트라센에서 크롬(III)-슈퍼옥소 화합물 (1)로의 속도 결정 단계인 PCET를 통하여 반응이 진행되었으며, 빠른 추가적인 산화반응을 통하여 안트라 퀴논이 생성됨을 확인할 수 있었다. 마지막으로, 전자-전달 반응에서 크롬(III)-슈퍼옥소 착물의 반응성에 대한 레독스-비활성 금속 이온의 루이스 산도의 효과를 조사하였다. 레독스-비활성 금속 이온 (Mn+; Ca2+, Mg2+, Y3+, Al3+ 및 Sc3+)이 존재하지 않을 때 옥타메칠페로세인(Me8Fc)으로부터 크롬(III)-슈퍼옥소 화합물 (1)로의 전자-전달 반응은 일어나지 않았지만, 레독스-비활성 금속 이온이 존재할 때는 전자-전달 반응이 빠르게 진행되었다. 또한 레독스-비활성 금속 이온의 루이스 산도가 크면 클수록 전자-전달 반응 속도가 증가하는 것으로 확인할 수 있었다. Mn+의 존재 하에서 페로센 유도체로부터 크롬(III)-슈퍼옥소 화합물 (1)로의 MCET의 log ket의 추진력 의존성은 전자 이동의 Marcus이론을 통하여 잘 일치됨을 확인할 수 있었다.