Liquefaction of Solid-State BH3NH3 by Gaseous NH3

This paper reports for the first time that under ammonia atmosphere, ammonia borane (AB) reversibly absorbs up to at least 6 equiv of NH3, forming liquid AB(NH3)(n) (n = 1-6) complexes at 0 degrees C. Reasonable structures for AB(NH3)(n) were identified via density functional theory calculations, which indicate that the strong classical hydrogen bond formed between the lone pair of NH3 and the -NH3 of AB is the driving force for the absorption of ammonia by AB. By use of the van't Hoff equation, the enthalpy change (Delta H) for AB to absorb one NH3 was determined to be -2.24 kcal/mol, which is in good agreement with the theoretical calculations. Other organic amines were screened to further confirm the role of the N lone pair; only 1,4-diazabicyclo [2.2.2]octane (DABCO) formed a stable adduct, which X-ray structural analysis showed was the DABCO-BH3 species. Finally, Raman spectra of AB(NH3)(n) were collected, and its unique spectral features are also discussed.