A Balancing Act: Stability versus Reactivity of Mn(O) Complexes.
A Balancing Act: Stability versus Reactivity of Mn(O) Complexes.
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DOI：
10.1021/acs.accounts.5b00273
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发表时间：
2015-10-20
影响因子：
18.3
通讯作者：
Goldberg DP
中科院分区：
文献类型：
--
作者：
Neu HM;Baglia RA;Goldberg DP
一大类血红素和非血红素金属酶利用O2或其衍生物（例如H2 O2）生成高价金属-氧代中间体，用于执行挑战性和选择性氧化。由于它们的反应性质，这些中间体通常是短暂的，非常难以表征。合成化学家已经试图用配体制备类似的金属-氧代配合物，所述配体赋予足够的稳定性以允许其表征和检查其固有的反应性。设计这些分子的挑战是在它们的稳定性和它们的反应性之间实现平衡，稳定性应该允许它们的原位表征或分离，反应性仍然可以参与有趣的化学转化。本文综述了近年来我们在制备和稳定高价锰-氧代卟啉配合物以及调节其在有机底物氧化反应中的活性方面所做的努力。通过在C-H底物存在下用可见光照射MnIII（TBP 8 Cz），可以使用分子氧来产生高价MnV（O）corolazine（MnVO（TBP 8 Cz））。MnV（O）络合物的定量形成伴随着苄基底物六亚甲基的选择性羟基化而发生。添加强H+供体将此光/O2/底物反应从化学计量转化为具有适度周转率的催化过程。H+的加入可能激活瞬时MnV（O）复合物以实现周转，而在没有H+的情况下，MnV（O）复合物是不反应的“死端”复合物。向MnV（O）络合物中添加阴离子供体也导致反应性增强，2电子氧原子转移（OAT）至硫醚底物的速率大幅增加。光谱表征（Mn K边X射线吸收和共振拉曼光谱）表明，阴离子供体（X−）与MnV离子结合形成六配位[MnV（O）（X）]−络合物。对位取代苯硫醚衍生物氧化的不寻常的“V形”Hammett图表明，六配位[MnV（O）（X）]−配合物可以作为亲电体或亲核体，这取决于底物的性质。MnV（O）corolazine的氧化导致原位生成MnV（O）π-自由基阳离子络合物[MnV（O）（TBP 8 Cz·+）]+，其在硫醚的氧化中表现出超过100倍的速率增加。在反磁性MnV（O）（TBP 8 Cz）中加入刘易斯酸（LA：ZnII，B（C6 F5）3），形成顺磁性价互变异构体MnIV（O）（TBP 8 Cz·+）：LA，通过NMR、EPR、UV-vis和高分辨CSI-MS表征其为第二个π-自由基阳离子络合物.相反，对于MnIV（O）（TBP 8 Cz·+）：LA络合物，观察到OAT的速率大幅降低。HAT的速率增强可能源于π-自由基阳离子络合物的更高氧化还原电位，而OAT所见的大速率降低可能来自MnIV（O）相对于MnV（O）络合物的亲电性降低。
A large class of heme and nonheme metalloenzymes utilize O2 or its derivatives (e.g. H2O2) to generate high-valent metal-oxo intermediates for performing challenging and selective oxidations. Due to their reactive nature, these intermediates are often short-lived and very difficult to characterize. Synthetic chemists have sought to prepare analogous metal-oxo complexes with ligands that impart enough stability to allow for their characterization and an examination of their inherent reactivity. The challenge in designing these molecules is to achieve a balance between their stability, which should allow for their in situ characterization or isolation, and their reactivity, in which they can still participate in interesting chemical transformations. This review focuses on our recent efforts to generate and stabilize high-valent manganese-oxo porphyrinoid complexes, and tune their reactivity in the oxidation of organic substrates. Dioxygen can be used to generate a high-valent MnV(O) corrolazine (MnVO(TBP8Cz)) by irradiation of MnIII(TBP8Cz) with visible light in the presence of a C–H substrate. Quantitative formation of the MnV(O) complex occurs with concomitant selective hydroxylation of the benzylic substrate hexamethylbenzene. Addition of a strong H+ donor converted this light/O2/substrate reaction from a stoichiometric to a catalytic process with modest turnovers. The addition of H+ likely activates a transient MnV(O) complex to achieve turnover, whereas in the absence of H+, the MnV(O) complex was an unreactive, “dead-end” complex. Addition of anionic donors to the MnV(O) complex also leads to enhanced reactivity, with a large increase in the rate of 2-electron oxygen-atom-transfer (OAT) to thioether substrates. Spectroscopic characterization (Mn K-edge X-ray absorption and resonance Raman spectroscopies) revealed that the anionic donors (X−) bind to the MnV ion to form six-coordinate [MnV(O)(X)]− complexes. An unusual “V-shaped” Hammett plot for the oxidation of para-substituted thioanisole derivatives suggested that six-coordinate [MnV(O)(X)]− complexes can act as both electrophiles or nucleophiles, depending on the nature of the substrate. Oxidation of the MnV(O) corrolazine resulted in the in situ generation of an MnV(O) π-radical cation complex, [MnV(O)(TBP8Cz•+)]+, which exhibited more than a 100-fold rate increase in the oxidation of thioethers. The addition of Lewis acids (LA: ZnII, B(C6F5)3) to the closed-shell, diamagnetic MnV(O)(TBP8Cz) stabilized a paramagnetic valence tautomer MnIV(O)(TBP8Cz•+):LA, which was characterized as a second π-radical cation complex by NMR, EPR, UV-vis, and high resolution CSI-MS. The MnIV(O)(TBP8Cz•+):LA complexes are able to abstract H• from phenols and exhibit a rate enhancement of up to ∼100-fold over the parent MnV(O) valence tautomer. In contrast, a large decrease in rate is observed for OAT for the MnIV(O)(TBP8Cz•+):LA complexes. The rate enhancement for HAT may derive from the higher redox potential for the π-radical cation complex, while the large rate decrease seen for OAT may come from a decrease in electrophilicity for an MnIV(O) versus MnV(O) complex.