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

Eight M(HO) (Z) salt hydrates were characterized by single-crystal X-ray diffraction (Z = BF): 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(HO) cations and Z resulted in room-temperature Fourier transform infrared (FTIR) spectra having sharp ν(OH) bands, with full widths at half max of 10-30 cm, which are much more narrow than ν(OH) bands in room temperature FTIR spectra of most salt hydrates. Clearly resolved ν(OH/OD) and ν(OH/OD) bands with Δν(OH) as small as 17 cm and Δν(OD) as small as 11 cm were observed (Δν(OX) = ν(OX) - ν(OX)). The isomorphic hexahydrates ( R3̅) have two fac-(HO) sets of HO ligands and nearly octahedral coordination spheres. They exhibited four resolvable ν(OH) bands, one ν(OH)/ν(OH) pair for HO ligands with longer O(H)···F distances and one ν(OH)/ν(OH) pair for HO ligands with shorter O(H)···F distances. The ν(OH) bands for the three HO molecules with shorter, slightly stronger O(H)···F hydrogen bonds were broader, more intense, and red-shifted by ca. 25 cm relative to the bands for the three other HO 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 Mg > Co > Ni > Zn, essentially the same progression as the p K 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(HO)(Z), Sr(HO) (Z), and Ca(HO)(Z). Finally, the HO/DO exchange reaction Co(DO)(Z) → Co(HO)(Z) was ca. 50% complete in 1 h at 50 °C in N/17 Torr HO( g).