The nature of the axial ligand in high-valent iron-oxo heme enzyme intermediates and related synthetic catalysts is a critical structural element for controlling proton-coupled electron-transfer (PCET) reactivity of these species. Herein, we describe the generation and characterization of three new 6-coordinate, iron(IV)-oxo porphyrinoid-π-cation-radical complexes and report their PCET reactivity together with a previously published 5-coordinate analogue, Fe^IV(O)(TBP₈Cz^+•) (TBP₈Cz = octakis(p-tert-butylphenyl)corrolazinato^3–) (2) (Cho, K. A high-valent iron-oxo corrolazine activates C–H bonds via hydrogen-atom transfer. J. Am. Chem. Soc. 2012, 134, 7392–7399). The new complexes Fe^IV(O)(TBP₈Cz^+•)(L) (L = 1-methyl imidazole (1-MeIm) (4a), 4-dimethylaminopyridine (DMAP) (4b), cyanide (CN^–)(4c)) can be generated from either oxidation of the ferric precursors or by addition of L to the Compound-I (Cpd-I) analogue at low temperatures. These complexes were characterized by UV–vis, electron paramagnetic resonance (EPR), and Mössbauer spectroscopies, and cryospray ionization mass spectrometry (CSI-MS). Kinetic studies using 4-OMe-TEMPOH as a test substrate indicate that coordination of a sixth axial ligand dramatically lowers the PCET reactivity of the Cpd-I analogue (rates up to 7000 times slower). Extensive density functional theory (DFT) calculations together with the experimental data show that the trend in reactivity with the axial ligands does not correlate with the thermodynamic driving force for these reactions or the calculated strengths of the O–H bonds being formed in the Fe^IV(O–H) products, pointing to non-Bell–Evans–Polanyi behavior. However, the PCET reactivity does follow a trend with the bracketed reduction potential of Cpd-I analogues and calculated electron affinities. The combined data suggest a concerted mechanism (a concerted proton electron transfer (CPET)) and an asynchronous movement of the electron/proton pair in the transition state.

中文翻译：

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轴向连接阻碍合成化合物-I 类似物的质子耦合电子转移反应性

高价铁氧血红素酶中间体和相关合成催化剂中轴向配体的性质是控制这些物质的质子耦合电子转移（PCET）反应性的关键结构元素。在此，我们描述了三种新的 6 配位铁(IV)-氧代卟啉-π-阳离子-自由基配合物的生成和表征，并报告了它们的 PCET 反应性以及之前发表的 5 配位类似物 Fe ^IV (O)( TBP ₈ Cz ^+• ) (TBP ₈ Cz = 八基(对叔丁基苯基)咔啉^3– ) ( 2 ) (曹，K.高价铁氧科罗嗪通过氢原子转移激活 C-H 键。J. Am.化学。苏克。 2012 , 134 , 7392–7399）。新配合物 Fe ^IV (O)(TBP ₈ Cz ^+• )(L) (L = 1-甲基咪唑 (1-MeIm) ( 4a )、4-二甲基氨基吡啶 (DMAP) ( 4b )、氰化物 (CN ^– )( 4c ))可以通过三价铁前体的氧化或通过在低温下将L添加至化合物-I (Cpd-I)类似物而产生。这些复合物通过紫外可见分光光度计、电子顺磁共振（EPR）、穆斯堡尔光谱和冷冻喷雾电离质谱（CSI-MS）进行了表征。使用 4-OMe-TEMPOH 作为测试底物的动力学研究表明，第六个轴向配体的配位显着降低了Cpd-I 类似物的 PCET 反应性（速率减慢了 7000 倍）。扩展密度泛函理论 (DFT) 计算与实验数据表明，与轴向配体的反应性趋势与这些反应的热力学驱动力或 Fe ^IV中形成的 O-H 键的计算强度无关(O-H) 产品，指向非贝尔-埃文斯-波兰尼行为。然而，PCET 反应性确实遵循 Cpd-I 类似物括号内的还原电位和计算的电子亲和力的趋势。组合数据表明了协调机制（协调质子电子转移（CPET））和过渡态电子/质子对的异步运动。