Came across an interesting article on the JOC ASAP page today about arylnitrenium ions (NR2+, R = aryl). These lovely little guys have been shown to play roles in biological “DNA-damaging” reactions and the synthesis of polyaniline, and are gaining importance as intramolecular cyclization agents. The paper explores halogenated diphenylnitrenium ions, which are apparently much more stable than their nonhalogenated brethren. One of the main routes of arylnitrenium breakdown, cyclization to form carbazole, occurs negligibly for the di(4-halogenated) arylnitrenium ions. In addition they are much more resistant to nucleophilic attack than the diphenylnitrenium ion.
Both singlet and triplet nitrenium ions are possible, and the paper cites some interesting differences between ab initio calculations for the two states. The singlet is a more or less planar molecule with a C-N-C angle of 126 degrees, while the triplet exhibits a shallower C-N-C angle (150 degrees) and nearly perpendicular phenyl rings. In the singlet state, both electrons in nitrogen’s lone pair can be found in a relatively low-energy hybrid orbital, leaving nitrogen with an empty p orbital. The molecule is planar in order to maximize overlap between this empty p orbital and the adjacent aromatic pi systems. In the triplet on the other hand, one electron is in the hybrid orbital and the other is in nitrogen’s p orbital, which affects the C-N-C angle and leaves the orientation of the phenyl rings up to sterics. Not surprisingly, the singlet state in which both electrons are in hybrid orbitals is lower in energy than the triplet state.
For my few, proud readers, here is a link (I would write more but unfortunately I’m out of time): N,N-Di(4-halophenyl)nitrenium Ions: Nucleophilic Trapping, Aromatic Substitution, and Hydrogen Atom Transfer