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

The melting diagram of the system (CH3)4NFHF 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 (CH3)4NF · 2 HF (melting point 110°C), (CH3)4NF · 3 HF (20°C, decomposition), (CH3)4NF · 5 HF (−76°C, decomposition), and (CH3)4NF · 7 HF (−110°C, decomposition), most of which undergo solid‐solid phase transitions. Crystal structures were determined of the low‐temperature form of (CH3)4NF · 2 HF (stable below 83°C, orthorhombic, space group Pbca, Z = 8 formula units per unit cell), the high‐temperature form of (CH3)4NF · 3 HF (stable above −87°C, monoclinic, P2/c, Z = 4), and of (CH3)4NF · 5 HF (tetragonal, I4, Z = 2). The structures are those of poly(hydrogen fluorides) (CH3)4N[HnFn+1] with homologous anions [H2F3]−, [H3F4]−, and [H5F6]−, respectively, formed by strong hydrogen bonding FH…︁F. The anion [H5F6]− is the first one of this composition established by crystal structure analysis. Its structure can be written as [(FH)2FHF(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.