Understanding the Reactivity of Mn-Oxo Porphyrins for Substrate Hydroxylation: Theoretical Predictions and Experimental Evidence Reconciled.
Understanding the Reactivity of Mn-Oxo Porphyrins for Substrate Hydroxylation: Theoretical Predictions and Experimental Evidence Reconciled.
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DOI：
10.1021/acs.inorgchem.9b00476
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发表时间：
2019-05
影响因子：
4.6
通讯作者：
D. Ricciarelli;Q. Phung;L. Belpassi;J. Harvey;P. Belanzoni
中科院分区：
文献类型：
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作者：
D. Ricciarelli;Q. Phung;L. Belpassi;J. Harvey;P. Belanzoni
通过计算研究了锰氧卟啉 (MnOP) 底物羟基化机制，旨在更好地了解这些系统中的反应性。理论研究表明，Mn(V)OP 物质是非常活泼的中间体，具有以 Mn(V)OP 氧化剂中发生的低自旋/高自旋交叉为代表的热可及反应势垒，并且所选 Mn(V)OP 物质的动力学确实发现了高反应性。另一方面，MnOP 配合物在几种不同底物的羟基化反应中产生适度的产率，这意味着低速率常数和高反应势垒。解决这种不一致对于理解锰氧卟啉的反应活性和改善催化条件非常重要。在这项工作中，我们以 Mn(V)OP(H2O)+ 络合物的甲苯羟基化作为案例研究，以深入了解反应机理。甲苯的 H 抽象过程的最小能量交叉点 (MECP) 结果表明，具有热可及势垒的 Mn(V)OP(H2O)+ 物质从单重态到三重态自旋态的第一次交叉，随后是三重态配合物非常容易的 H 抽象。解决了以下问题：(i) 验证采用密度泛函理论 (BP86) 来描述 Mn(V)OP(H2O)+ 系统中单重态-三重态能隙与高精度 DMRG-CASPT2/CC 计算的水平，以及 (ii) 轴向配体（X = 无、Cl-、CH3CN、OH- 和 O2-）对 MnOP 反应性的影响（模拟不同的实验条件）。配体反式影响主要通过单重态-三重态能隙调制来控制反应性，卟啉皱褶畸变也对其进行微调。最后，提出了 H 抽象过程的逐步模型，该模型允许直接比较计算的吉布斯自由活化能势垒和实验测量的吉布斯自由活化能势垒（Zhang et al. J. Am. Chem. Soc. 2005, 127, 6573 - 6582）。催化产率低并不是因为 Mn(V) 的反应活性低。
The Mn-oxo porphyrin (MnOP) mechanism for substrate hydroxylation is computationally studied with the aim to better understand reactivity in these systems. Theoretical studies suggest Mn(V)OP species to be very reactive intermediates with thermally accessible reaction barriers represented by low-spin/high-spin-crossover occurring in the Mn(V)OP oxidant, and kinetics for selected Mn(V)OP species indeed find high reactivity. On the other hand, MnOP complexes lead to modest yields in hydroxylation reactions of several different substrates, implying low rate constants and high reaction barriers. The resolution of this inconsistency is very important to understand the reactivity of Mn-oxo porphyrins and to improve the catalytic conditions. In this work we use the toluene hydroxylation by the Mn(V)OP(H2O)+ complex as a case study to gain deep insight into the reaction mechanism. Minimum energy crossing point (MECP) results on the H-abstraction process from toluene indicate a first crossover from a singlet to a triplet spin state of the Mn(V)OP(H2O)+ species with a thermally accessible barrier, followed by a very facile H-abstraction by the triplet complex. Issues concerning (i) the validation of the level of the density functional theory employed (BP86) to describe the singlet-triplet energy gap in the Mn(V)OP(H2O)+ system versus highly accurate DMRG-CASPT2/CC calculations, and (ii) the influence of the axial ligand (X = none, Cl-, CH3CN, OH-, and O2-) on MnOP reactivity, which models the different experimental conditions, are addressed. The ligand trans influence mainly controls the reactivity through the singlet-triplet energy gap modulation, with the porphyrin ruffling distortion also finely tuning it. Finally, a stepwise model for the H-abstraction process is proposed which allows a direct comparison between the calculated and experimentally measured Gibbs free activation energy barriers ( Zhang et al. J. Am. Chem. Soc. 2005 , 127 , 6573 - 6582 ). The low yields in catalysis are shown not to be due to low reactivity of Mn(V).