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

The cationic oxorhenium(V) complex [Re(O)(hoz)2(CH3CN)][B(C6F5)4] [1; Hhoz = 2-(2′-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–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 × 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.