Ammonium-Ammonia Complexes, N₂H₇⁺, in Ammonium closo-Borate Ammines: Synthesis, Structure, and Properties

Metal closo-borates have recently received significant attention due to their potential applications as solid-state ionic conductors. Here, the synthesis, crystal structures, and properties of (NH₄)₂B₁₀H₁₀·xNH₃ (x = ¹/₂, 1 (α and β)) and (NH₄)₂B₁₂H₁₂·xNH₃ (x = 1 and 2) are reported. In situ synchrotron radiation powder X-ray diffraction allows for the investigation of structural changes as a function of temperature. The structures contain the complex cation N₂H₇⁺, which is rarely observed in solid materials, but can be important for proton conductivity. The structures are optimized by density functional theory (DFT) calculations to validate the structural models and provide detailed information about the hydrogen positions. Furthermore, the hydrogen dynamics of the complex cation N₂H₇⁺ are studied by molecular dynamics simulations, which reveals several events of a proton transfer within the N₂H₇⁺ units. The thermal properties are investigated by thermogravimetry and differential scanning calorimetry coupled with mass spectrometry, revealing that NH₃ is released stepwise, which results in the formation of (NH₄)₂BH (n = 10 and 12) during heating. The proton conductivity of (NH₄)₂B₁₂H₁₂·xNH₃ (x = 1 and 2) determined by electrochemical impedance spectroscopy is low but orders of magnitude higher than that of pristine (NH₄)₂B₁₂H₁₂. The thermal stability of the complex cation N₂H₇⁺ is high, up to 170 °C, which may provide new possible applications of these proton-rich materials.