The effect of {O,N}=X⋯M={Ti,Zr,Hf} interactions on the sensitivity of C–NO₂ trigger bonds in FOX-7: Approach based on the QTAIM/EDA-NOCV analysis

The local chemical reactivity of FOX-7 (1,1-diamino-2,2-nitroethylene, also known as DADNE from DiAminoDiNitroEthylene) was elucidated through a quantitative study of the electrostatic potential on the molecular surface, topological analysis based on Bader's theory, and the EDA-NOCV method. Unlike (O₂N)₂C[dbnd]C(NH₂)–H₂N⋯Cp₂MCH₃ ⁺ complexes, which exhibit both σ-donor and π-acceptor features, the situation is different concerning the (H₂N)₂C[dbnd]C(NO₂)–(O)NO⋯Cp₂MCH₃ ⁺ complexes, where both charge transfers correspond to the σ-donation. The two charge transfers reinforce each other, resulting in increased stability for (H₂N)₂C[dbnd]C(NO₂)–(O)NO⋯Cp₂ MCH₃ ⁺. This seems to strengthen the (H₂N)₂C[dbnd]C(NO₂)–(O)NO⋯M={Ti,Zr,Hf} bond, which may explain the high stability of (H₂N)₂C[dbnd]C(NO₂)–(O)NO⋯Cp₂MCH₃ ⁺ compared to (O₂N)₂C[dbnd]C(NH₂)-H₂N⋯Cp₂MCH₃ ⁺. Results from topological analysis revealed that the decreased sensitivity to decomposition of C–NO₂ bonds depends on the chemical nature of the interacting metal, and the best achievements are obtained for the Hf-based complex. Our results demonstrate that the interaction of M={Ti,Zr,Hf} with C–NO₂ is more favourable than that with C–NH₂, this specific action on the trigger bond may support the use of Metallocene Methyl Cations (MMC) as possible neutralisers.