1334722-48-3Relevant academic research and scientific papers
Observation of inductive effects that cause a change in the rate-determining step for the conversion of rhenium azides to imido complexes
Travia, Nicholas E.,Xu, Zhenggang,Keith, Jason M.,Ison, Elon A.,Fanwick, Phillip E.,Hall, Michael B.,Abu-Omar, Mahdi M.
, p. 10505 - 10514 (2011/11/29)
The cationic oxorhenium(V) complex [Re(O)(hoz)2(CH 3CN)][B(C6F5)4] [1; Hhoz = 2-(2′-hydroxyphenyl)-2-oxazoline] reacts with aryl azides (N 3Ar) to give cationic cis-rhenium(VII) oxoimido complexes of the general formula [Re(O)(NAr)(hoz)2][B(C6F5) 4] [2a-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 (N 3Ar) and oxorhenium(V) complex 1, with second-order rate constants ranging from 3.5 × 10-2 to 1.7 × 10-1 M -1 s-1. A strong inductive effect is observed for electron-withdrawing substituents, leading to a negative Hammett reaction constant ρ = -1.3. However, electron-donating substituents on phenyl azide deviate significantly from this trend. Enthalpic barriers (ΔH ?) determined by the Eyring-Polanyi equation are in the range 14-19 kcal mol-1 for all aryl azides studied. However, electron-donating 4-methoxyphenyl azide exhibits a large negative entropy of activation, ΔS? = -21 cal mol-1 K -1, which is in sharp contrast to the near zero ΔS ? 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 Nα 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 σ values; thus, the Hammett plot appears with a break in its slope.
