Axial ligand tuning of a nonheme iron(IV)-oxo unit for hydrogen atom abstraction
Axial ligand tuning of a nonheme iron(IV)-oxo unit for hydrogen atom abstraction
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DOI：
10.1073/pnas.0709471104
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发表时间：
2007-12-04
影响因子：
11.1
通讯作者：
Nam, Wonwoo
中科院分区：
文献类型：
--
作者：
Sastri, Chivukula V.;Lee, Jimin;Nam, Wonwoo
带有不同轴向配体的单核非血红素铁（IV）-氧代配合物的反应性，[Fe-IV（O）（TMC）（X）]（n+）[其中TMC为1，4，8，11-四甲基-1，4，8，11-四氮杂环十四烷，X为NCCH 3（1-NCCH 3），CF 3COO-（1-OOCCF 3）或N-3（-）（1-N-3）]和[Fe-IV（O）（TMCS）]（+）（1'-SR）（其中TMCS是1-巯基乙基-4，8，11-三甲基-1，4，8，11-四氮杂环十四烷），已经研究了关于氧转移到PPh 3和从苯酚O-H和烷基芳族C-H键的氢原子提取。这些反应性受到轴向配体身份的显着影响，但反应性趋势显着不同。在PPh 3的氧化反应中，反应顺序为1-NCCH 3> 1-OOCCF 3> 1-N3 > 1 '-SR，反映了铁（IV）-氧代单元的亲电性在CH 3CN被阴离子轴向配体取代后降低。令人惊讶的是，在烷基芳族C-Hand酚O-H键的氧化中反应性顺序颠倒，即，1 ′-SR > 1-N-3 > 1-OOCCF 3> 1-NCCH 3。此外，观察到铁（IV）-氧代物种在H原子提取反应中的反应性与它们的还原电位E-p，E-c之间存在良好的相关性，其中反应性最强的1 '-SR络合物表现出最低的电位。换句话说，轴向配体的给电子越多，铁（IV）-氧代物种在H原子夺取中的反应性就越强。量子力学计算表明，两个状态的反应模型适用于这一系列的配合物，其中三重基态和附近的五重激发态都有助于配合物的反应性。在H原子提取中的反转反应性顺序可以通过减少具有更多供电子轴向配体的三重态-五重态间隙来合理化，这增加了反应性更强的五重态的贡献并提高了整体反应性。
The reactivities of mononuclear nonheme iron(IV)-oxo complexes bearing different axial ligands, [Fe-IV(O)(TMC)(X)](n+) [where TMC is 1,4,8,11-tetramethyl-1,4,8,11-tetraazacyclotetradecane and X is NCCH3 (1-NCCH3), CF3COO- (1-OOCCF3) or N-3(-) (1-N-3)], and [Fe-IV(O)(TMCS)](+) (1'-SR) (where TMCS is 1-mercaptoethyl-4,8,11-trimethyl-1,4,8,11-tetraazacyclotetradecane), have been investigated with respect to oxo-transfer to PPh3 and hydrogen atom abstraction from phenol O-H and alkylaromatic C-H bonds. These reactivities were significantly affected by the identity of the axial ligands, but the reactivity trends differed markedly. In the oxidation of PPh3, the reactivity order of 1-NCCH3 > 1-OOCCF3 > 1-N3 > 1'-SR was observed, reflecting a decrease in the electrophilicity of iron(IV)-oxo unit upon replacement of CH3CN with an anionic axial ligand. Surprisingly, the reactivity order was inverted in the oxidation of alkylaromatic C-Hand phenol O-H bonds, i.e., 1'-SR > 1-N-3 > 1-OOCCF3 > 1-NCCH3. Furthermore, a good correlation was observed between the reactivities of iron(IV)-oxo species in H atom abstraction reactions and their reduction potentials, E-p,E-c, with the most reactive 1'-SR complex exhibiting the lowest potential. In other words, the more electron-donating the axial ligand is, the more reactive the iron(IV)-oxo species becomes in H atom abstraction. Quantum mechanical calculations show that a two-state reactivity model applies to this series of complexes, in which a triplet ground state and a nearby quintet excited-state both contribute to the reactivity of the complexes. The inverted reactivity order in H atom abstraction can be rationalized by a decreased triplet-quintet gap with the more electron-donating axial ligand, which increases the contribution of the much more reactive quintet state and enhances the overall reactivity.