Mechanistic study of styrene aziridination by iron(iv) nitrides

A combined experimental and computational investigation was undertaken to investigate the mechanism of aziridination of styrene by the tris(carbene)borate iron(iv) nitride complex, PhB( BuIm)₃Fe[triple bond, length as m-dash]N. While mechanistic investigations suggest that aziridination occurs via a reversible, stepwise pathway, it was not possible to confirm the mechanism using only experimental techniques. Density functional theory calculations support a stepwise radical addition mechanism, but suggest that a low-lying triplet (S = 1) state provides the lowest energy path for C-N bond formation (24.6 kcal mol^-1) and not the singlet ground (S = 0) state. A second spin flip may take place in order to facilitate ring closure and the formation of the quintet (S = 2) aziridino product. A Hammett analysis shows that electron-withdrawing groups increase the rate of reaction σ _p (ρ = 1.2 ± 0.2). This finding is supported by the computational results, which show that the rate-determining step drops from 24.6 kcal mol^-1 to 18.3 kcal mol^-1 when (p-NO₂C₆H₄)CH[double bond, length as m-dash]CH₂ is used and slightly increases to 25.5 kcal mol^-1 using (p-NMe₂C₆H₄)CH[double bond, length as m-dash]CH₂ as the substrate.