Diazonium Ions. Topological Electron Density Analysis of Cyclopropeniumyldiazonium Dications and of their Stability toward Dediazoniation

Results are presented of topological analyses of the electron density functions of cyclopropeniumyldiazonium dication, 1, of its 2,3-diamino derivative, 2, and of their products 3 and 4, respectively, formed by dediazoniative ring-opening. The new CN-bonding type in 1 and 2, recently realized synthetically in derivatives of 2, is compared to prototypical aliphatic diazonium ions with regard to electronic structure and thermodynamic stability, factors that both are crucial for the appreciation of the mechanisms of deamination reactions of chemical and biochemical significance. Association of the dication with N2 involves density accumulation in the CN bonding region, occurs without major overall charge transfer, and leads to an electrostatically favorable quadrupolar charge distribution in the diazonium ion. The CN-bonding model recently proposed for aliphatic diazonium ions also applies to these dications. 1 is thermodynamically stable while the dediazoniation of 2 is exothermic but kinetically hindered. Our best estimates for the reaction energies of the dediazoniations 1 → 3 + N2 and 2 → 4 + N2, respectively, are 65.5 and -7.2 kcal/mol, respectively. We have found that, in general, the cation is destabilized and that N2 is stabilized upon CN-bond cleavage. Cations force N2 to form diazonium ions. The remarkable difference between the stabilities of 1 and 2 is primarily due to the larger destabilization of the open dication 3 compared to 4. Push-pull interactions between the diazo- and the overall electron-withdrawing amino-functions characterize the electronic structure of 2. CN-Bonding and the overall electronic structure of 2 are incompatible with the usual Lewis resonance notations. Instead of dismissing the Lewis notations, it is shown that the topological description can be reconciled with the Lewis notations if the resonance forms are interpreted in a way that appropriately reflects the atom populations and first moments. Implications of the model with regard to reactivity are discussed