Observation of Inductive Effects That Cause a Change in the Rate-Determining Step for the Conversion of Rhenium Azides to Imido Complexes

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• 作者：Nicholas E. Travia ; Zhenggang Xu ; Jason M. Keith ; Elon A. Ison ; Phillip E. Fanwick ; Michael B. Hall ; Mahdi M. Abu-Omar
• 刊名：Inorganic Chemistry
• 出版年：2011
• 出版时间：October 17, 2011
• 年：2011
• 卷：50
• 期：20
• 页码：10505-10514
• 全文大小：1027K
• 年卷期：v.50,no.20(October 17, 2011)
• ISSN：1520-510X

文摘

The cationic oxorhenium(V) complex [Re(O)(hoz)₂(CH₃CN)][B(C₆F₅)₄] [1; Hhoz = 2-(2鈥?hydroxyphenyl)-2-oxazoline] reacts with aryl azides (N₃Ar) to give cationic cis-rhenium(VII) oxoimido complexes of the general formula [Re(O)(NAr)(hoz)₂][B(C₆F₅)₄] [2a鈥?b>2f; Ar = 4-methoxyphenyl, 4-methylphenyl, phenyl, 3-methoxyphenyl, 4-chlorophenyl, and 4-(trifluoromethyl)phenyl]. The kinetics of formation of 2 in CH₃CN are first-order in both azide (N₃Ar) and oxorhenium(V) complex 1, with second-order rate constants ranging from 3.5 脳 10^鈥? to 1.7 脳 10^鈥? M^鈥? s^鈥?. A strong inductive effect is observed for electron-withdrawing substituents, leading to a negative Hammett reaction constant 蟻 = 鈭?.3. However, electron-donating substituents on phenyl azide deviate significantly from this trend. Enthalpic barriers (螖H) determined by the Eyring鈥揚olanyi equation are in the range 14鈥?9 kcal mol^鈥? for all aryl azides studied. However, electron-donating 4-methoxyphenyl azide exhibits a large negative entropy of activation, 螖S = 鈭?1 cal mol^鈥? K^鈥?, which is in sharp contrast to the near zero 螖S observed for phenyl azide and 4-(trifluoromethyl)phenyl azide. The Hammett linear free-energy relationship and the activation parameters support a change in the mechanism between electron-withdrawing and electron-donating aryl azides. Density functional theory predicts that the aryl azides coordinate via N_伪 and extrude N₂ directly. For the electron-withdrawing substituents, N₂ extrusion is rate-determining, while for the electron-donating substituents, the rate-determining step becomes the initial attack of the azide. The barriers for these two steps are inverted in their order with respect to the Hammett 蟽 values; thus, the Hammett plot appears with a break in its slope.