The Cation-dinitrogen Interaction in the "Benzyldiazonium Ion": Preferential Electrostatic Complex Formation and Dinitrogen Catalysis of Benzyl Cation Rotational Automerization

Diazonium ions are reactive intermediates in deamination reactions pertinent to chemical carcinogenesis. While methyldiazonium ion has been shown to exist as a short-lived intermediate in the gas phase and in solution, benzyldiazonium ions have never been observed, and the reaction intermediates in deaminations of benzyl systems remain a matter of debate. We therefore studied the benzylcation-dinitrogen interaction by ab initio methods; several important conclusions resulted. Analysis of the potential energy surface at the level MP4(sdtq)/6-31 G ^* / MP 2(full)/6-31 G* + ΔVZPE(MP 2(full)/6-31 G*) revealed that a classical "benzyldiazonium ion" does not exist. The interaction of N ₂ with benzyl cation M-1 results in an electrostatically bound complex 2C with a long C-N distance (2.935 Å) as the most stable structure. A covalently bound planar benzyldiazonium ion 2A with a "normal" C-N bond length (1.514 Å) is the transition-state structure for automerization of the electrostatic complex with concomitant rotation about the exocyclic C-C bond. The potential energy surface characteristics result from the highly efficient π-dative Ph → CH ₂ bonding in M-1 ; this is clearly demonstrated in its structure and that of its transition state for rotational automerization TS-1 by the very high activation barrier for rotation (47.6 kcalmol ^-1 !) and by the gradient vector fields of the total electron densities of conformers of 1 and 2 . The rotational barrier for 1 is reduced to 27.9 kcalmol ^-1 in the N ₂ complex 2 , and the potential energy surface characteristics of benzyldiazonium ion essentially facilitate the N ₂ -catalyzed rotational isomerization of benzyl cation. The benzyldiazonium ion system shows for the first time that the interaction of a donor molecule with a carbenium ion with a valence LUMO can lead to the formation of an electrostatic complex as opposed to dative bond formation. Dative σ-bond formation between N ₂ and the CH ₂ carbon of 1 is energetically not competitive with dative Ph → CH ₂ π;-bond formation.