Electrophilic aromatic nitrosation. Isolation and X-ray crystallography ofthe metastable NO+ complex with nitrosoarene

Isolation of the unstable 1:1 complex of 4- nitrosoanisole with NO+PF6- all ows its precise X-ray structural characterization. The charge-transfer crys tal is formed via strong N ... N coordination [the distance of 1.938(5) Ang strom corresponding to a sigma-bond order of similar to 0.2] in the mean pl ane of the planar 4-nitrosoanisole donor. Thorough analysis of its molecula r geometry in terms of valence resonance and MO schemes reveals a strong ch arge polarization with a local negative charge localized on the nitroso gro up and a local positive charge distributed over the adjacent p-methoxybenzy l moiety. Such a charge distribution accommodates the well-known passivatio n of nitrosoarenes to multiple nitrosation and explains the ease of demethy lation of the complex. Comparison of a variety of nitroso- and nitroarene s tructures has shown that the nitrosoarene experiences a much stronger quino idal distortion of the aromatic ring as compared with the latter. This indi cates a stronger electron-withdrawing effect of the nitroso group relative to that of the nitro group. The weakened aromatic resonance in the nitrosoa renes could be responsible for the observed slower rate and the measurable isotope effect in electrophilic nitrosation as opposed to nitration.