Manifestations of Weak O–H···F Hydrogen Bonding in M(H₂O)_n(B₁₂F₁₂) Salt Hydrates: Unusually Sharp Fourier Transform Infrared ν(OH) Bands and Latent Porosity (M = Mg–Ba, Co, Ni, Zn)

Eight M­(H₂O)_n(Z) salt hydrates were characterized by single-crystal X-ray diffraction (Z^2– = B₁₂F₁₂^2–): M = Ca, Sr, n = 7; M = Mg, Co, Ni, Zn, n = 6; M = Ba, n = 4, 5. Weak O–H···F hydrogen bonding between the M­(H₂O)_n²⁺ cations and Z^2– resulted in room-temperature Fourier transform infrared (FTIR) spectra having sharp ν­(OH) bands, with full widths at half max of 10–30 cm^–1, which are much more narrow than ν­(OH) bands in room temperature FTIR spectra of most salt hydrates. Clearly resolved ν_asym(OH/OD) and ν_sym(OH/OD) bands with Δν­(OH) as small as 17 cm^–1 and Δν­(OD) as small as 11 cm^–1 were observed (Δν­(OX) = ν_asym(OX) – ν_sym(OX)). The isomorphic hexahydrates (R3̅) have two fac-(H₂O)₃ sets of H₂O ligands and nearly octahedral coordination spheres. They exhibited four resolvable ν­(OH) bands, one ν_asym(OH)/ν_sym(OH) pair for H₂O ligands with longer O­(H)···F distances and one ν_asym(OH)/ν_sym(OH) pair for H₂O ligands with shorter O­(H)···F distances. The ν­(OH) bands for the three H₂O molecules with shorter, slightly stronger O­(H)···F hydrogen bonds were broader, more intense, and red-shifted by ca. 25 cm^–1 relative to the bands for the three other H₂O molecules, the first time that such small differences in relatively weak O­(H)···F hydrogen bonds in the same crystalline hexahydrate have resulted in observable IR spectroscopic differences at room temperature. For the first time room temperature ν­(OH) values for salt hexahydrates showed the monotonic progression Mg²⁺ > Co²⁺ > Ni²⁺ > Zn²⁺, essentially the same progression as the pK_a values for these metal ions in aqueous solution. A further manifestation of the weak O–H···F hydrogen bonding in these hydrates is the latent porosity exhibited by Ba­(H₂O)_5,8(Z), Sr­(H₂O)_n,m(Z), and Ca­(H₂O)_4,6(Z). Finally, the H₂O/D₂O exchange reaction Co­(D₂O)₆(Z) → Co­(H₂O)₆(Z) was ca. 50% complete in 1 h at 50 °C in N₂/17 Torr H₂O­(g).