tion of 2d with nitrosyl tetrafluoroborate furnishes 3,5bis(dimethylamino)-4-phenylpyrazol~um tetrafluoroborate 9 in 72% yield as dark blue
On a preparative scale it was found that protonation of 2d with tetrafluoroboric acid and subsequent oxidation of the isolable pyrazolium salt 8, with dichlorodicyano-p-benzoquinone (DDQ), was more favorable.
The electronic spectrum of 9I"I is, as expected, similar to that of a structurally related cyclopentadienylium salt (6-dimethylamino-1,4-dimethyl-5,7-diphenyl-1,2,3,4-tetrahydrocyclopentapyrazine-6-ylium tetrafluoroborate:
A,,, = 624 nrnr3l). Also informative with respect to the electronic properties of the 4n system 9 is a comparison with the 6x system 8 : In the 'H-NMR spectrum of 9, two signals appear at 6=3.91 and 2.90, which are assigned to the protons of the dimethylamino groups. Upon increasing the temperature these signals broaden and the coalescence temperature is 121ยฐC. From this, an energy barrier of AG* = 19.45 kcal/mol is calculated for the rotation about the ring-NMe2 bond. The 'H-NMR spectrum of 8, on the other hand, shows only one signal for these protons, indicative of a rapid rotation of the dimethylamino groups.
Apparently-and as expected-the 4n system of 9 is a very much stronger electron acceptor than the 6n system of 8.