Characterization and Reactivity of Bis(μ-oxo)dimanganese(III,IV) and Metal Ion-Bound Manganese(IV)-Oxo Complexes

Abstract

Oxygen-coordinating metal intermediates, such as metal-oxo, -hydroperoxo, -peroxo, and .superoxo species, are involved as key intermediates in many biological and catalytic processes. In addition to the mononuclear nonheme metal-oxygen species, dinuclear nonheme metal-oxygen species are also involved as key intermediates in enzymatic reactions. In the first part, we report an amphoteric reactivity of mixed-valent bis(μ-oxo)dimanganese(III,IV) complex acting as an electrophile as well as nucleophilie. A mixed-valent bis(μ-oxo dimanganese(III,IV) complex,[(dpaq)MnIII(O)2MnIV(dpaq)]+ (dpaq = 2-[bis(pyridine-2-ylmethyl)]-amino-N-quinolin-8-yl-acetamidate), was prepared by reacting a hydroxomanganese(III) complex, [(dpaq)MnIII(OH)]+, with hydrogen peroxide in the presence of trimethylamine and well characterized by UV-vis, EPR and CSI-MS techniques. The electrophilic and nucleophilic reactivities of [(dpaq)MnIII(O)2MnIV(dpaq)]+ were investigated in the oxidation of 2,6-di-tert-butylphenol derivatives and aldehydes, respectively. We found that [(dpaq)MnIII(O)2MnIV(dpaq)]+ exhibited an amphoteric reactivity in both electrophilic and nucleophilic oxidative reactions. In the second part, we investigated the remarkable effect of metal ions to inhibit the production of the bis-μ-oxo dimanganese(III,IV) complex in the reaction with PhIO and to produce the metal ionbound manganese(IV)-oxo complex. The reaction of [(dpaq)MnIII(OH)]+ with PhIO was identified as the dinuclear bis-μ-oxo complex [(dpaq)MnIII(O)2MnIV(dpaq)]+. In the presence of metal triflate, a metal ion-bound manganese(IV)-oxo compelx,[(dpaq)MnIV(O)]+-Mn+(OTf)n (Mn+ = Sc3+, Lu3+, and Zn2+), was generated by reacting [(dpaq)MnIII(OH)]+ with PhIO. The binding of Sc3+ to the oxo moiety of [(dpaq)MnIV(O)]+ was confirmed by various spectroscopic techniques, such as UV-vis, ESI-MS, EPR and EXAFS/XANES. The [(dpaq)MnIV(O)]+-Sc(OTf)3 exhibited a much higher reactivity towards substrates as compared with [(dpaq)MnIII(O)2MnIV(dpaq)]+, because of the much more positive one-electron reduction potential of [(dpaq)MnIV(O)]+-Sc(OTf)3 as compared with [(dpaq)MnIII(O)2MnIV(dpaq)]+. This study also shows the way to produce metal ion-bound MnIV(O) complexes, such as [(dpaq)MnIV(O)]+-Mn+(OTf)n (Mn+ = Sc3+, Lu3+, and Zn2+), the reactivity of which is controlled by the Lewis acidity of metal ion. The reactivity of the MnIV(O) complex are markedly influenced by binding of metal ions to a MnIV(O) moiety in oxidation reactions. The reactivity order of [(dpaq)MnIV(O)]+-Mn+(OTf)n in the oxidation of thioanisole (i.e., oxygen atom transfer (OAT)) was MnIV(O)-Sc3+ > MnIV(O)-Lu3+ > MnIV(O)-Zn2+, whereas that in C-H bond activation reaction of alkyl aromatic substrates (i.e., hydrogen atom transfer(HAT)) was MnIV(O)-Sc3+ < MnIV(O)-Lu3+ < MnIV(O)-Zn2+.;촉매효소의 활성 자리인 금속-산소 결합체(metal-superoxo, metal-peroxo, metal-hydroperoxo, 그리고 metal-oxo)는 산소 활성화의 촉매 순환과정에서 발견되는 핵심적인 중간체이다. 생체 모방 생무기화학 분야에서는 이러한 촉매 효소의 활성화 자리를 모방한 여러 가지 모델 화합물의 반응성 및 중간체의 구조, 그리고 반응 메커니즘을 규명하고자 많은 연구가 진행되고 있다. 1장에서는 다섯 자리 N5 리간드(pentadentate N5ligand)가 배위된 hydroxomanganese(III) 화합물을 TEA, H2O2와 반응시켜 bis(μ-oxo)dimanganese(III,IV) 종을 생성시키고 다양한 분광학적 기기로 물리-화학적 특성을 확인하였다. 생성된 bis(μ-oxo)dimanganese(III,IV) 종이 친전자성, 친핵성 반응 모두에서 활성 산화제의 역할을 하는 것을 관찰하였다. 2장에서는 다양한 Lewis 산이 결합된 새로운 금속-산소 중간체를 생성하여 Lewis 산이 망간-산소 화합물의 반응성에 미치는 영향에 관하여 연구하였다. 다양한 분광학적 기기를 통해 Sc3+, Lu3+, Zn2+이 결합한 망간-산소 중간체 각각의 분광학적 특성들을 확인하였고, 여러가지 기질들을 사용하여 금속-산소 중간체에 결합한 금속이온의 Lewis acidity에 따라 반응속도가 달라지는 것을 확인하였다. 우선 산소 원자 전달반응으로 thioanisole기질을 사용하여 Sc3+, Lu3+, Zn2+이 결합한 망간-산소 중간체의 sulfoxidation 반응을 관찰한 결과, Lewis acidity가 증가함에 따라 반응속도가 현저하게 증가함을 확인하였다. 반면 C-H 결합 활성 반응의 경우에는 Lewis acidity가 증가할수록 반응속도가 감소함을 관찰하였으며, 이는 Lewis acidity가 증가할수록 산소와 금속이온 간의 결합력이 증가하며, 보다 큰 입체장애를 야기함에 기인한다.