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

Abstract

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)(2)B10H10 center dot xNH(3) (x = 1/2, 1 (alpha and beta)) and (NH4)(2)B12H12 center dot xNH(3) (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)(2)BnHn (n = 10 and 12) during heating. The proton conductivity of (NH4)(2)B(12)H(12 center dot)xNH(3) (x = 1 and 2) determined by electrochemical impedance spectroscopy is low but orders of magnitude higher than that of pristine (NH4)(2)B12H12. The thermal stability of the complex cation N2H7+ is high, up to 170 degrees C, which may provide new possible applications of these proton-rich materials.