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

Anhydrous H[BH2 (CN)2 ] crystallizes from acidic aqueous solutions of the dicyanodihydridoborate anion. The formation of H[BH2 (CN)2 ] 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)3 ]- (X=H, F) gives the expected oxonium salts (H3 O)[BX(CN)3 ] while (H3 O)[BF2 (CN)2 ]/H[BF2 (CN)2 ] is unstable. H[BH2 (CN)2 ] forms chains via N-H⋅⋅⋅N bonds in the solid state and melts at 54 °C. Solutions of H[BH2 (CN)2 ] in the room-temperature ionic liquid [EMIm][BH2 (CN)2 ] contain the [(NC)H2 BCN-H⋅⋅⋅NCBH2 (CN)]- anion and are unusually stable, which enabled the study of selected spectroscopic and physical properties. [(NC)H2 BCN-H⋅⋅⋅NCBH2 (CN)]- slowly gives H2 and [(NC)H2 BCN-BH(CN)2 ]- . The latter compound is a source of the free Lewis acid BH(CN)2 , as shown by the generation of [BHF(CN)2 ]- and BH(CN)2 ⋅py.