Oxygen atom transfer (OAT) reactions employing transition metal–oxo species have tremendous significance in homogeneous catalysis for industrial use. Understanding the structural and mechanistic aspects of OAT reactions using high-valent metal–oxo species is of great importance to fine-tune their reactivity. Herein we examine the reactivity of a non-heme high-valent oxo-manganese(IV) complex, [Mn^IVH₃buea(O)]⁻ towards a variety of substrates such as PPh₂Me, PPhMe₂, PCy₃, PPh₃, and PMe₃ using density functional theory as well as ab initio CASSCF/NEVPT2 methods. We have initially explored the structure and bonding of [Mn^IVH₃buea(O)]⁻ and its congener [Mn^IVH₃buea(S)]⁻. Our calculations affirm an S = 3/2 ground state of the catalyst with the S = 5/2 and S = 1/2 excited states predicted to be too high lying in energy to participate in the reaction mechanism. Our ab initio CASSCF/NEVPT2 calculations, however, reveal a strong multi-reference character for the ground S = 3/2 state with many low-lying quartets mixing significantly with the ground state. This opens up various reaction channels, and the admixed wave-function evolves during the reaction with the excited triplet dominating the ground state wave-function at the reactant complex. Our calculations predict the following pattern of reactivity, PCy₃ < PMe₃ < PPh₃ < PPhMe₂ < PPh₂Me for the OAT reaction with the Mn^IVO species which correlates well with the experimental observations. Detailed electronic structure analysis of the transitions states reveal that these substrates react via an unusual low-energy δ-type pathway where a spin-up electron from the substrate is transferred to the δ*_x²−y² orbital of the Mn^IVO facilitated by its multi-reference character. The unusual reactivity observed here has implications in understanding the reactivity of [Mn4Ca] species in photosystem II.

中文翻译：

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解释非血红素MnIV-氧代物种转移氧原子的机理：从头算和DFT探索。

采用过渡金属-氧代物种的氧原子转移（OAT）反应在工业应用的均相催化中具有重要意义。了解使用高价金属-氧代物种进行OAT反应的结构和机理方面对于微调其反应性非常重要。本文中，我们研究了非血红素高价氧-锰（IV）配合物[Mn ^IV H ₃ buea（O）] ^-对多种底物（如PPh ₂ Me，PPhMe ₂，PCy ₃，PPh）的反应性₃和PMe ₃使用密度泛函理论以及从头算CASSCF / NEVPT2方法。我们最初探讨了[Mn ^IV H ₃ buea（O）] ^-及其同系物[Mn ^IV H ₃ buea（S）] ^-的结构和键合。我们的计算证实了催化剂的S = 3/2基态，其中S = 5/2和S = 1/2激发态预测能量太高而无法参与反应机理。但是，我们的从头计算CASSCF / NEVPT2计算显示出地面S具有很强的多参考特征= 3/2状态，许多低洼的四重奏与基态显着混合。这就打开了各种反应通道，并且在反应过程中，混合的波函数随着被激发的三重态在反应物配合物上主导基态波函数而发展。我们的计算预测了以下反应性模式：PCy ₃ <PMe ₃ <PPh ₃ <PPhMe ₂ <PPh ₂ Me对于与Mn ^IV O物种进行OAT反应而言，与实验观察结果非常相关。对过渡态的详细电子结构分析表明，这些底物通过不寻常的低能δ型途径，其中自底物产生的自旋电子被转移至Mn ^IV O的δ* _{x ² - y ²}轨道，这是由于其多参考特性所致。此处观察到的异常反应性对理解光系统II中[Mn4Ca]物种的反应性具有影响。