Synthesis, Characterization and Reactivity of the Mononuclear Nonheme Mn(IV)-Oxo Intermediates and Mechanistic Insight in C-H Bond Activation by Metal(IV)-Oxo Complexes

Abstract

MnII 화합물로부터 산소공급자 물과 산화제 CeIV을 사용함으로써 처음으로 단핵 nonheme MnIVO 화합물이 생성되었다; MnIVO 화합물에서 산소공급자는 동위원소 표시된 물을 가지고 실험해봄으로 확실히 알 수 있다. 다양한 분광학적 방법들로 그 중간체들의 특성을 확인하였다. 또한 이 종이 Mn=O에서 양성자가 첨가됨을 효과적으로 이해하는데 DFT calculation이 사용되었다. 이러한 결과들은 MnIVO 화합물이 산화반응에서 좋은 산화제로 사용됨을 보여준다. 단핵 nonheme MnIVO 화합물은 MnII 화합물과 산화제인 PhIO가 반응하여 생성되고, 이는 다양한 분광학적 방법에 의해 특성을 확인 할 수 있다. MnIVO 화합물은 C-H bond activation이나, olefin, alcohol, sulfide, aromatic 화합물의 산화와 같은 산화반응에서 높은 반응성을 나타낸다. C-H bond activation에서, MnIVO 화합물은 cyclohexane에서의 C-H bond activation 만큼 강한 C-H bond activation을 보인다. nonheme MnIVO 화합물에서 일어나는 C-H bond activation은 oxygen-rebound 메커니즘을 통해 일어나지 않는다. 비슷하게, 합성된 nonheme FeIVO 화합물에서 일어나는 alkane의 C-H bond activation 또한 대체의 메커니즘을 따른다. Alkane으로부터 수소가 빠져서 형성되는 기질 라디칼의 해리는 산소 rebound나 불포화반응보다 더 유리하게 일어남을 이론적 계산으로 예측할 수 있다. 이러한 이론적 예측은 철과 nonheme FeIVO 화합물로 인해 C-H bond activation 되어 형성된 유기생성물을 분석한 실험적결과에 의해 확인되어진다. 따라서 nonheme FeIVO을 포함한 high-valent metal-oxo 종에 의한 C-H bond activation는 일반적인 hydrogen abstraction-oxygen rebound 메커니즘을 통해 일어남을 주의 깊게 보아야 한다.;A mononuclear nonheme MnIVO complex was generated for the first time from a MnII complex using water as an oxygen source and CeIV as an oxidant; the source of oxygen in the MnIVO complex was assigned unambiguously by carrying out isotopically labeled water experiments. The intermediate was characterized with various spectroscopic methods, and DFT calculations confirm that this species is indeed energetically accessible and too poorly basic to undergo protonation on the Mn=O moiety. Our findings also show that the MnIVO complex is a competent oxidant in oxidation reactions. A mononuclear nonheme MnIVO complex was also generated by reacting MnII complex with PhIO as an oxidant and characterized with various spectroscopic methods. The MnIVO complex shows high reactivity in oxidation reactions, such as C-H bond activation, the oxidation of olefin, alcohol, sulfide, and aromatic compounds, and N-dealkylation. In C-H bond activation, the MnIVO complex can activate C-H bonds as strong as those in cyclohexane. It is proposed that the C-H bond activation by the nonheme MnIVO complex does not occur via an oxygen-rebound mechanism. Similarly, C-H bond activation of alkanes by synthetic nonheme FeIVO complexes also follows an alternative mechanism. Theoretical calculations predict that dissociation of the substrate radical, which is formed via hydrogen abstraction from the alkane, is more favorable than the oxygen rebound or desaturation processes. This theoretical prediction is verified by experimental results that are obtained by analyzing iron and organic products formed in the C-H bond activation of substrates by nonheme FeIVO complexes. Thus, the general consensus that the C-H bond activation by high-valent metal-oxo species, including nonheme FeIVO, occurs via the conventional hydrogen abstraction-oxygen rebound mechanism should be viewed with caution.