The interconversion of molybdenum ethylene and ethyl complexes by proton-coupled electron transfer (PCET) is described, an unusual transformation in organometallic chemistry. The cationic molybdenum ethylene complex [(PhTpy)(PPh2Me)2Mo(C2H4)][BArF24] ([1-C2H4]+; PhTpy = 4'-Ph-2,2',6',2″-terpyridine, ArF24 = [C6H3-3,5-(CF3)2]4) was synthesized, structurally characterized, and its electronic structure established by a combination of spectroscopic and computational methods. The overall electronic structure is best described as a molybdenum(III) complex with a metallacyclopropane and a redox neutral terpyridine ligand. Addition of the nonclassical ammine complex [(PhTpy)(PPh2Me)2Mo(NH3)][BArF24] ([1-NH3]+) to [1-C2H4]+ resulted in a net C-H bond-forming PCET reaction to yield the molybdenum ethyl [(PhTpy)(PPh2Me)2Mo(CH2CH3)][BArF24] ([1-CH2CH3]+) and amido [(PhTpy)(PPh2Me)2Mo(NH2)][BArF24] ([1-NH2]+) compounds. The reaction was reversed by addition of 2,4,6-tri tert-butylphenoxyl radical to [1-CH2CH3]+. The solid-state structure of [1-CH2CH3]+ established a β-agostic ethyl ligand that is maintained in solution as judged by variable temperature 1H and 13C NMR experiments. A combination of variable-temperature NMR experiments and isotopic labeling studies were used to probe the dynamics of [1-CH2CH3]+ and established restricted β-agostic -CH3 rotation at low temperature (Δ G‡ = 9.8 kcal mol-1 at -86 °C) as well as ethyl isomerization by β-hydride elimination-olefin rotation-reinsertion (Δ H‡ = 19.3 ± 0.6 kcal mol-1; Δ S‡ = 3.4 ± 1.7 cal mol-1 K-1). The β-(C-H) bond-dissociation free energy (BDFE) in [1-CH2CH3]+ was determined experimentally as 57 kcal mol-1 (THF) supported by a DFT-computed value of 52 kcal/mol-1 (gas phase). Comparison of p Ka and electrochemical data for the complexes [1-C2H4]+ and [1-NH3]+ in combination with a deuterium kinetic isotope effect ( kH/ kD) of 3.5(2) at 23 °C support a PCET process involving initial electron transfer followed by protonation leading to the formation of [1-CH2CH3]+ and [1-NH2]+ or a concerted pathway. The data presented herein provides a structural, thermochemical and mechanistic foundation for understanding the PCET reactivity of organometallic complexes with alkene and alkyl ligands.

中文翻译：

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质子耦合电子转移到钼乙烯配合物产生 -Agostic Ethyl：结构、动力学和机制

描述了通过质子耦合电子转移 (PCET) 进行的钼乙烯和乙基配合物的相互转化，这是有机金属化学中的一种不寻常的转变。阳离子钼乙烯络合物[(PhTpy)(PPh2Me)2Mo(C2H4)][BArF24]([1-C2H4]+；PhTpy = 4'-Ph-2,2',6',2"-三联吡啶，ArF24 = [C6H3-3,5-(CF3)2]4) 被合成，结构表征，并通过光谱和计算方法的组合建立其电子结构。整个电子结构最好描述为钼 (III) 与金属环丙烷和氧化还原中性三联吡啶配体的配合物。将非经典的氨络合物 [(PhTpy)(PPh2Me)2Mo(NH3)][BArF24] ([1-NH3]+) 添加到 [1-C2H4]+ 中，导致形成净 CH 键的 PCET 反应生成钼乙基 [(PhTpy)(PPh2Me)2Mo(CH2CH3)][BArF24] ([1-CH2CH3]+) 和酰胺基 [(PhTpy)(PPh2Me)2Mo(NH2)][BArF24] ([1-NH2]+) 化合物. 通过将 2,4,6-三叔丁基苯氧基自由基添加到 [1-CH2CH3]+ 来逆转反应。[1-CH2CH3]+ 的固态结构建立了一个 β-agostic 乙基配体，根据可变温度 1H 和 13C NMR 实验判断，该配体保持在溶液中。变温 NMR 实验和同位素标记研究相结合，用于探测 [1-CH2CH3]+ 的动力学，并在低温下建立受限的 β-agostic -CH3 旋转（Δ G‡ = 9. 8 kcal mol-1 a​​t -86 °C）以及通过 β-氢化物消除-烯烃旋转-重新插入进行乙基异构化（Δ H‡ = 19.3 ± 0.6 kcal mol-1；Δ S‡ = 3.4 ± 1.7 cal mol-1 K-1）。[1-CH2CH3]+ 中的 β-(CH) 键解离自由能 (BDFE) 通过实验确定为 57 kcal mol-1 (THF)，DFT 计算值为 52 kcal/mol-1（气相）。复合物 [1-C2H4]+ 和 [1-NH3]+ 的 p Ka 和电化学数据的比较以及 23 °C 下 3.5(2) 的氘动力学同位素效应 (kH/kD) 支持 PCET 过程，包括初始电子转移，随后质子化导致形成 [1-CH2CH3]+ 和 [1-NH2]+ 或协同途径。此处提供的数据提供了结构性、