Department of Chemistry , Washington University in St. Louis , One Brookings Drive, Campus Box 1134 , St. Louis , Missouri 63130-4899 , United States.
Department of Chemistry and Center for Metals in Biocatalysis , University of Minnesota , 207 Pleasant Street SE , Minneapolis , Minnesota 55455 , United States.
J Am Chem Soc. 2019 Apr 3;141(13):5470-5480. doi: 10.1021/jacs.9b00466. Epub 2019 Mar 25.
The kinetics and mechanism(s) of the reactions of [K(Krypt)][LCuO] (Krypt = 4,7,13,16,21,24-hexaoxa-1,10-diazabicyclo[8.8.8]hexacosane, L = a bis(arylcarboxamido)pyridine ligand) with 2,2,6,6-tetramethylpiperdine- N-hydroxide (TEMPOH) and the para-substituted phenols ArOH (X = para substituent NO, CF, Cl, H, Me, Bu, OMe, or NMe) at low temperatures were studied. The reaction with TEMPOH occurs rapidly ( k = 35.4 ± 0.3 M s) by second-order kinetics to yield TEMPO and [LCuOOH] on the basis of electron paramagnetic resonance spectroscopy, the production of HO upon treatment with protic acid, and independent preparation from reaction of [NBu][LCuOH] with HO ( K = 0.022 ± 0.007 for the reverse reaction). The reactions with ArOH also follow second-order kinetics, and analysis of the variation of the k values as a function of phenol properties (Hammett σ parameter, O-H bond dissociation free energy, p K, E) revealed a change in mechanism across the series, from proton transfer/electron transfer for X = NO, CF, Cl to concerted-proton/electron transfer (or hydrogen-atom transfer) for X = OMe, NMe (data for X = H, Me, Bu are intermediate between the extremes). Thermodynamic analysis and comparisons to previous results for LCuOH, a different copper-oxygen intermediate with the same supporting ligand, and literature for other [CuO] complexes reveal significant differences in proton-coupled electron-transfer mechanisms that have implications for understanding oxidation catalysis by copper-containing enzymes and abiological catalysts.
[K(Krypt)][LCuO]（Krypt = 4,7,13,16,21,24-六氧-1,10-二氮杂环十八烷，L = 双（芳基羧酰胺基）吡啶配体）与 2,2,6,6-四甲基哌啶- N- 羟基（TEMPOH）和对取代苯酚 ArOH（X = 对位取代基 NO、CF、Cl、H、Me、Bu、OMe 或 NMe）在低温下的反应动力学和机理进行了研究。根据电子顺磁共振波谱、与质子酸反应生成 HO、以及独立制备自[NBu][LCuOH]与 HO 的反应（对于逆反应 K = 0.022 ± 0.007），TEMPOH 的反应以二级动力学快速进行（k = 35.4 ± 0.3 M s）。ArOH 的反应也遵循二级动力学，对 k 值随酚性质（Hammett σ参数、O-H 键离解自由能、p K、E）变化的分析表明，反应机理发生了变化，X = NO、CF、Cl 时为质子转移/电子转移，X = OMe、NMe 时为协同质子/电子转移（或氢原子转移）（X = H、Me、Bu 的数据介于两者之间）。热力学分析和与具有相同配体的不同铜-氧中间体 LCuOH 以及其他 [CuO] 配合物的文献的比较揭示了质子耦合电子转移机制的显著差异，这对理解含铜酶和非生物催化剂的氧化催化具有重要意义。
