Synthesis, crystal structure, and properties of triphenylguanidinium perrhenate hemihydrate

Triphenylguanidinium perrhenate hemihydrate, [(C₆H₅NH)₃C]ReO₄ · 0.5H₂O (I), is synthesized, and its crystal structure and some properties are studied. The colorless extended plate-like crystals of compound I are triclinic (space group $Pbar 1$ , Z = 4, 293 K, a = 9.8716(17), b = 14.093(2), c = 15.439(3)Å, α = 99.632(9)^○, β = 101.802(9)^○, γ = 95.361(10)^○). Compound I has no isostructural analogs, and the conformations of both crystallographically independent triphenylguanidinium cations differ by a higher symmetry (C _3h ) from those for cations of this type in all other structurally studied compounds. The following parameters are determined: the upper limit of the temperature stability of compound I (383 K), the melting point of anhydrous [(C₆H₅NH)₃C]ReO₄ (Ia) of 441 K, the enthalpy of dehydration of compound I (ΔH _dehydr ^{(383 K)} = 10.0(8) kJ/mol), and the enthalpy of melting of anhydrous Ia (ΔH _m ^{(441 K)} = 16.6(9) kJ/mol).