Allenediazonium ions and their protonation chemistry: a DFT study

The parent allenediazonium monocation H2C[double bond]C[double bond]CH(N2+) and ten of its substituted derivatives XYC[double bond]C[double bond]C(Z)N2+ (with F, CF3, Me, OMe, and Me2N as substituents) were studied by DFT at the B3LYP/6-31++G** level. Except for the Me2N-substituted derivative that forms a monocation-N2 complex, structurally intact allenediazonium ions were obtained as minima in all cases. Protonation studies at various sites were performed on allenediazonium cations, and relative energies of the resulting minima were used to identify the energetically most favored dications. In the majority of cases, protonation at the central carbon of the allenic moiety (C2) is most favored, forming delocalized allyl cation-N2+ species. The same dication structure is formed via initial C3-protonation, followed by a formal hydride shift, in cases where a carbocation-stabilizing group is placed at C3. When a CF3 group is placed at C3, initial protonation at C1 resulted in a 1,3-fluorine shift, to generate a fluoroallyl cation linked to a CH2N2+ moiety. Structural features in the allenediazonium monocations and their protonated dications were examined, taking into account their geometrical features, computed charges, and the GIAO NMR shifts.