Diazonium dications

Diazonium dications bearing two aryl rings have been observed by treating azoxybenzenes in HF-SbF5 [Eq. (4.140)]. The ortho and para protons are deshielded relative to the meta protons, which is indicative of charge delocalization into the aromatic rings. 

Diazonium dications have been studied extensively in both experimental and theoretical work. The parent system for diazonium dications... 

Topological analyses of the electron density functions of the cyclopropenium-diazonium dication and its 2,3-diamino derivative have been published by Glaser (1990). We will discuss them in Section 5.3. 

Figure 5-2 shows schematically the dependence of the relative concentration of the diazo equilibrium forms on the pH (for the diazoanhydride mentioned in this figure see Sec. 5.2). The relative concentrations of the two major equilibrium forms, the diazonium ion and the diazoate ion, decrease on the right and left sides, respectively, of the pH value corresponding to equal concentrations of these two forms ([ArNj] = [ArN20-]). The gradients correspond to a factor of 100 per pH unit, compared with only 10 per pH unit in the case of dibasic Bronsted acids. The equilibrium concentrations of the diazohydroxide and the diazoanhydride (except for very reactive diazonium ions such as the benzene-1,4-bisdiazonium dication mentioned above) are very small at all pH values, with a maximum at pH = pKm. 

Another interesting oxonium-based dication arises from p-methoxy-benzene-diazonium ion in FSCbH-SbFs solution (eq 81).80 Dication 237 was observed by low-temperature 13C NMR spectroscopy. Interestingly, the o-methoxybenzenediazonium ion does not form the oxonium dication 238 (eq 82),... 

Addition of a 3-4 m excess of HBF4 to a suspension of these cyclopro-penium-l-diazonium salts converted them in near quantitative yield to bright yellow salts, which analyzed correctly for the addition of HBF4. Normal BF and SbClg" vibrational modes in the IR spectra indicate a protonation of this small dication — a rather surprising result. The protonation site is open, however, for discussion. 

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