State Key Laboratory of Theoretical and Computational Chemistry, Institute of Theoretical Chemistry, Jilin University, Changchun, 130023, People's Republic of China.
Phys Chem Chem Phys. 2012 Oct 5;14(37):12863-74. doi: 10.1039/c2cp42423a.
Comprehensive density functional theory computations on substrate hydroxylation by a range of nonheme iron(IV)-oxo model systems Fe(IV)(O)(NH(3))(4)L (where L = CF(3)CO(2)(-), F(-), Cl(-), N(3)(-), NCS(-), NC(-), OH(-)) have been investigated to establish the effects of axial ligands with different degrees of electron donor ability on the reactivity of the distinct reaction channels. The results show that the electron-pushing capability of the axial ligand can exert a considerable influence on the different reaction channels. The σ-pathway reactivity decreases as the electron-donating ability of the axial ligand strengthens, while the π-pathway reactivity follows an opposite trend. Moreover, the apparently antielectrophilic trend observed for the energy gap between the triplet π- and quintet σ-channel (ΔG(T-Q)) stems from the fact that the reaction reactivity can be fine-controlled by the interplay between the exchange-stabilization benefiting from the (5)TS(H) relative to the (3)TS(H) by most nonheme enzymes and the destabilization effect of the orbital by the anionic axial ligand. When the former counteracts the latter, the quintet σ-pathway will be more effective than the other alternatives. Nevertheless, when the dramatic destabilization effect of the orbital by a strong binding axial σ-donor ligand like OH(-) counteracts but does not override the exchange-stabilization, the barrier in the quintet σ-pathway will remain identical to the triplet π-pathway barrier. Indeed, the axial ligand does not change the intrinsic reaction mechanism in its respective pathway; however, it can affect the energy barriers of different reaction channels for C-H activation. As such, the tuning of the reactivity of the different reaction channels can be realised by increasing/decreasing the electron pushing ability.
已对一系列非血红素铁（IV）-氧模型体系[Fe（IV）（O）（NH3）（4）L]（+）（其中 L = CF3CO2（-），F（-），Cl（-），N3（-），NCS（-），NC（-），OH（-））的基质羟化作用进行了全面的密度泛函理论计算，以确定具有不同电子给体能力的轴向配体对不同反应通道的反应性的影响。结果表明，轴向配体的电子推动能力会对不同的反应通道产生相当大的影响。随着轴向配体供电子能力的增强，σ-途径的反应性降低，而π-途径的反应性则呈现相反的趋势。此外，对于三重态π-和五重态σ-通道之间的能量间隙（ΔG（T-Q））观察到的明显反亲电性趋势，源自于反应活性可以通过大多数非血红素酶中有利于（5）TS（H）相对于（3）TS（H）的交换稳定化作用与阴离子轴向配体的轨道的去稳定化作用之间的相互作用来精细控制。当前者与后者相互作用时，五重态σ-途径将比其他途径更有效。然而，当强结合的轴向σ-给体配体（如 OH（-））对轨道的剧烈去稳定化作用抵消但不超过交换稳定化作用时，五重态σ-途径的势垒将保持与三重态π-途径势垒相同。实际上，轴向配体不会改变其各自途径中的固有反应机制，但是可以影响 C-H 活化的不同反应通道的能垒。因此，可以通过增加/减少电子推动能力来调节不同反应通道的反应性。
