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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 121°C. 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.

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