One-electron oxidation of ferrocenes by short-lived N-oxyl radicals. The role of structural effects on the intrinsic electron transfer reactivities
作者:Enrico Baciocchi、Massimo Bietti、Claudio D'Alfonso、Osvaldo Lanzalunga、Andrea Lapi、Michela Salamone
DOI:10.1039/c0ob01257b
日期:——
A kinetic study of the one electron oxidation of substituted ferrocenes (FcX: X = H, COPh, COMe, CO2Et, CONH2, CH2OH, Et, and Me2) by a series of N-oxyl radicals (succinimide-N-oxyl radical (SINO), maleimide-N-oxyl radical (MINO), 3-quinazolin-4-one-N-oxyl radical (QONO) and 3-benzotriazin-4-one-N-oxyl radical (BONO)), has been carried out in CH3CN. N-oxyl radicals were produced by hydrogen abstraction from the corresponding N-hydroxy derivatives by the cumyloxyl radical. With all systems, the rate constants exhibited a satisfactory fit to the Marcus equation allowing us to determine self-exchange reorganization energy values (λNO˙/NO−) which have been compared with those previously determined for the PINO/PINO− and BTNO/BTNO− couples. Even small modification of the structure of the N-oxyl radicals lead to significant variation of the λNO˙/NO− values. The λNO˙/NO− values increase in the order BONO < BTNO < QONO < PINO < SINO < MINO which do not parallel the order of the oxidation potentials. The higher λNO˙/NO− values found for the MINO and SINO radicals might be in accordance with a lower degree of spin delocalization in the radicals MINO and SINO and charge delocalization in the anions MINO− and SINO− due to the absence of an aromatic ring in their structure.
在一系列N-氧基自由基(琥珀酰亚胺-N-氧基自由基(SINO),马来酰亚胺-N-氧基自由基(MINO),3-喹唑啉-4-酮-N-氧基自由基(QONO)和3-苯并三嗪-4-酮-N-氧基自由基(BONO))的参与下,研究了取代二茂铁(FcX:X = H,COPh,COMe,CO2Et,CONH2,CH2OH,Et和Me2)的单电子氧化反应在CH3CN中的动力学。N-氧基自由基是通过樟脑氧基自由基从相应的N-羟基衍生物中抽取氢生成的。对于所有体系,速率常数与Marcus方程吻合良好,使我们能够确定自交换重排能量值(λNO˙/NO−),这些值已与先前为PINO/PINO−和BTNO/BTNO−体系确定的值进行了比较。即使N-氧基自由基的结构发生微小变化,也会导致λNO˙/NO−值的显著变化。λNO˙/NO−值的顺序为BONO < BTNO < QONO < PINO < SINO < MINO,这与氧化电位的顺序并不一致。对于MINO和SINO自由基,较高的λNO˙/NO−值可能与自由基MINO和SINO中较低的自旋离域度和阴离子MINO−和SINO−中的电荷离域度有关,这是由于它们结构中缺乏芳香环所致。