The origin of the remarkable stability of the 1H-3,5-dimethylpyrazole-4-diazonium cation: an X-ray crystallographic and MNDO theoretical investigation

Abstract

The unusual stability 1H-3,5-dimethylpyrazole-4-diazonium chloride has been investigated by a combination of X-ray crystallography and MNDO calculations. Crystals of the diazonium chloride are orthorhombic, space group Pnma, with four molecules in a unit cell of dimensions a= 7.964(1), b= 6.308(2), c= 15.060(3)Å. The structure was solved by direct methods and refined by full-matrix least-squares calculations to R= 0.032 for 539 reflections with I > 3σ(I) measured by diffractometer. The cation lies on a crystallographic mirror plane and the diazonium moiety is essentially linear [C(4)–N(7)–N(8) 177.5(4)°]. While the diazonium N–N distance [N(7)–N(8) 1.096(4)Å] is typical of that in arenediazonium cations, the C(4)–N(7) distance is short [1.351(4)Å] suggesting that the stability of the cation is due to a contribution from a diazo resonance form in which this bond acquires p_π–p_π character. The MNDO-optimised geometry of the cation agrees well with the experimental data. Calculated energy profiles for the dissociation of the heterocyclic diazonium cation and the benzenediazonium cation to the corresponding carbonium ion and N₂ show that the strength of the ipso-C–N₂ bond is the major distinguishing feature. The difference in bond strength arises from greater availability of π-electron density on the ipso carbon of the pyrazolyl cation and hence greater ipso-C→N₂π* back-donation. Both carbonium ions have large π density on the ipso-carbon but in the phenyl cation this is predominantly located in the lowest energy π orbital while in the pyrazolyl cation it is located in the HOMOs and hence available for interaction with the diazonium function.