TY - JOUR
T1 - Effect of external electric fields on the C-H bond activation reactivity of nonheme iron-oxo reagents
AU - Hirao, Hajime
AU - Chen, Hui
AU - Carvajal, Maria Angels
AU - Wang, Yong
AU - Shaik, Sason
PY - 2008/3/19
Y1 - 2008/3/19
N2 - The effect of external electric fields (EFs) on the reactivity of nonheme iron(IV)-oxo species toward alkanes is investigated computationally using density functional theory. It is shown that an external EF changes the energy landscape of the process and thereby impacts the mechanisms, rates, and selectivities of the reactions, in a manner dependent on the nature of the iron(IV)-oxo/alkane pair. When the iron-oxo species is a good electron acceptor, like N4PyFeO2+, and the alkane is a good electron donor, like toluene, the application of the EF changes the mechanism from hydrogen abstraction to electron transfer. With cyclohexane, which is a poorer electron donor than toluene, the EF promotes hydride transfer and generates a carbocation. However, in the reaction between a poorer electron acceptor TMC(SR)FeO+ and cyclohexane, the EF preserves the hydrogen abstraction/rebound mechanism but improves its features by lowering the barriers for both the C-H activation and rebound steps; larger effects were observed for the quintet-state reaction. In all cases, the EF effect obeys a selection rule; the largest effects are observed when the EF vector is aligned with the Fe=O axis (z) and directed along the molecular dipole. As such, an EF aligned in the direction of the electron flow from substrate to the iron-oxo center lowers the reaction barrier and affects both the reactivity and selectivity of the molecular catalysts.
AB - The effect of external electric fields (EFs) on the reactivity of nonheme iron(IV)-oxo species toward alkanes is investigated computationally using density functional theory. It is shown that an external EF changes the energy landscape of the process and thereby impacts the mechanisms, rates, and selectivities of the reactions, in a manner dependent on the nature of the iron(IV)-oxo/alkane pair. When the iron-oxo species is a good electron acceptor, like N4PyFeO2+, and the alkane is a good electron donor, like toluene, the application of the EF changes the mechanism from hydrogen abstraction to electron transfer. With cyclohexane, which is a poorer electron donor than toluene, the EF promotes hydride transfer and generates a carbocation. However, in the reaction between a poorer electron acceptor TMC(SR)FeO+ and cyclohexane, the EF preserves the hydrogen abstraction/rebound mechanism but improves its features by lowering the barriers for both the C-H activation and rebound steps; larger effects were observed for the quintet-state reaction. In all cases, the EF effect obeys a selection rule; the largest effects are observed when the EF vector is aligned with the Fe=O axis (z) and directed along the molecular dipole. As such, an EF aligned in the direction of the electron flow from substrate to the iron-oxo center lowers the reaction barrier and affects both the reactivity and selectivity of the molecular catalysts.
UR - http://www.scopus.com/inward/record.url?scp=41449114606&partnerID=8YFLogxK
U2 - 10.1021/ja070903t
DO - 10.1021/ja070903t
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AN - SCOPUS:41449114606
SN - 0002-7863
VL - 130
SP - 3319
EP - 3327
JO - Journal of the American Chemical Society
JF - Journal of the American Chemical Society
IS - 11
ER -