Poly(hydrogen Fluorides) with the Tetramethylammonium Cation: Preparation, Stability Ranges, Crystal Structures, [H n F n+1 ] − Anion Homology, Hydrogen Bonding F‐H…︁F [2]

The melting diagram of the system (CH 3 ) 4 NFHF was studied between 50 and 100 mole‐% HF and from −185°C to the respective liquidus temperatures (at most 162°C) by difference thermal analysis aided by temperature‐dependent X‐ray powder diffraction. The system was found to be quasi‐binary with the HF‐rich intermediary stable compounds (CH 3 ) 4 NF · 2 HF (melting point 110°C), (CH 3 ) 4 NF · 3 HF (20°C, decomposition), (CH 3 ) 4 NF · 5 HF (−76°C, decomposition), and (CH 3 ) 4 NF · 7 HF (−110°C, decomposition), most of which undergo solid‐solid phase transitions. Crystal structures were determined of the low‐temperature form of (CH 3 ) 4 NF · 2 HF (stable below 83°C, orthorhombic, space group Pbca, Z = 8 formula units per unit cell), the high‐temperature form of (CH 3 ) 4 NF · 3 HF (stable above −87°C, monoclinic, P2/c, Z = 4), and of (CH 3 ) 4 NF · 5 HF (tetragonal, I 4 , Z = 2). The structures are those of poly(hydrogen fluorides) (CH 3 ) 4 N[H n F n+1 ] with homologous anions [H 2 F 3 ] − , [H 3 F 4 ] − , and [H 5 F 6 ] − , respectively, formed by strong hydrogen bonding FH…F. The anion [H 5 F 6 ] − is the first one of this composition established by crystal structure analysis. Its structure can be written as [(FH) 2 FHF(HF) 2 ] − with four equivalent terminal hydrogen bonds of 248.4 pm and a very short central one of 226.6 pm (F…F distances) through a 4 point of the space group.