Bezdek Máté J, Chirik Paul J
Department of Chemistry , Princeton University , Princeton , New Jersey 08544 , United States.
J Am Chem Soc. 2018 Oct 24;140(42):13817-13826. doi: 10.1021/jacs.8b08460. Epub 2018 Oct 15.
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 [(Tpy)(PPhMe)Mo(CH)][BArF] ([1-CH]; Tpy = 4'-Ph-2,2',6',2″-terpyridine, ArF = [CH-3,5-(CF)]) 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 [(Tpy)(PPhMe)Mo(NH)][BArF] ([1-NH]) to [1-CH] resulted in a net C-H bond-forming PCET reaction to yield the molybdenum ethyl [(Tpy)(PPhMe)Mo(CHCH)][BArF] ([1-CHCH]) and amido [(Tpy)(PPhMe)Mo(NH)][BArF] ([1-NH]) compounds. The reaction was reversed by addition of 2,4,6-tri tert-butylphenoxyl radical to [1-CHCH]. The solid-state structure of [1-CHCH] established a β-agostic ethyl ligand that is maintained in solution as judged by variable temperature H and C NMR experiments. A combination of variable-temperature NMR experiments and isotopic labeling studies were used to probe the dynamics of [1-CHCH] and established restricted β-agostic -CH rotation at low temperature (Δ G = 9.8 kcal mol at -86 °C) as well as ethyl isomerization by β-hydride elimination-olefin rotation-reinsertion (Δ H = 19.3 ± 0.6 kcal mol; Δ S = 3.4 ± 1.7 cal mol K). The β-(C-H) bond-dissociation free energy (BDFE) in [1-CHCH] was determined experimentally as 57 kcal mol (THF) supported by a DFT-computed value of 52 kcal/mol (gas phase). Comparison of p K and electrochemical data for the complexes [1-CH] and [1-NH] in combination with a deuterium kinetic isotope effect ( k/ k) of 3.5(2) at 23 °C support a PCET process involving initial electron transfer followed by protonation leading to the formation of [1-CHCH] and [1-NH] 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.
描述了通过质子耦合电子转移（PCET）实现的钼乙烯配合物和乙基配合物的相互转化，这是有机金属化学中一种不寻常的转化。合成了阳离子钼乙烯配合物[(Tpy)(PPhMe)Mo(CH)][BArF]（[1-CH]；Tpy = 4'-Ph-2,2',6',2″-三联吡啶，ArF = [CH-3,5-(CF)]），对其进行了结构表征，并通过光谱和计算方法相结合确定了其电子结构。整体电子结构最好描述为具有金属环丙烷和氧化还原中性三联吡啶配体的钼（III）配合物。向[1-CH]中加入非经典氨配合物[(Tpy)(PPhMe)Mo(NH)][BArF]（[1-NH]）导致净C-H键形成的PCET反应，生成钼乙基[(Tpy)(PPhMe)Mo(CHCH)][BArF]（[1-CHCH]）和酰胺基[(Tpy)(PPhMe)Mo(NH)][BArF]（[1-NH]）化合物。通过向[1-CHCH]中加入2,4,6-三叔丁基苯氧基自由基使反应逆转。[1-CHCH]的固态结构确定了一个β-亲铜乙基配体，通过变温H和C NMR实验判断其在溶液中得以保持。结合变温NMR实验和同位素标记研究来探究[1-CHCH]的动力学，并确定了在低温下（-86°C时ΔG = 9.8 kcal mol）受限的β-亲铜-CH旋转以及通过β-氢消除-烯烃旋转-再插入实现的乙基异构化（ΔH = 19.3 ± 0.6 kcal mol；ΔS = 3.4 ± 1.7 cal mol K）。通过实验确定了[1-CHCH]中β-(C-H)键的解离自由能（BDFE）为57 kcal mol（四氢呋喃），密度泛函理论（DFT）计算气相值为52 kcal/mol支持了该结果。配合物[1-CH]和[1-NH]的pK和电化学数据的比较，结合23°C时3.5(2)的氘动力学同位素效应（k/k），支持了一个涉及初始电子转移随后质子化导致形成[1-CHCH]和[1-NH]的PCET过程或协同途径。本文提供的数据为理解有机金属配合物与烯烃和烷基配体的PCET反应性提供了结构、热化学和机理基础。
