Protonation versus Oxonium Salt Formation: Basicity and Stability Tuning of Cyanoborate Anions.

Anhydrous H[BH (CN) ] crystallizes from acidic aqueous solutions of the dicyanodihydridoborate anion. The formation of H[BH (CN) ] is surprising as the protonation of nitriles requires strongly acidic and anhydrous conditions but it can be rationalized based on theoretical data. In contrast, [BX(CN) ] (X=H, F) gives the expected oxonium salts (H O)[BX(CN) ] while (H O)[BF (CN) ]/H[BF (CN) ] is unstable. H[BH (CN) ] forms chains via N-H⋅⋅⋅N bonds in the solid state and melts at 54 °C. Solutions of H[BH (CN) ] in the room-temperature ionic liquid [EMIm][BH (CN) ] contain the [(NC)H BCN-H⋅⋅⋅NCBH (CN)] anion and are unusually stable, which enabled the study of selected spectroscopic and physical properties. [(NC)H BCN-H⋅⋅⋅NCBH (CN)] slowly gives H and [(NC)H BCN-BH(CN) ] . The latter compound is a source of the free Lewis acid BH(CN) , as shown by the generation of [BHF(CN) ] and BH(CN) ⋅py.