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)

• Published in: Inorganic chemistry 2018-12, Vol.57 (23), p.14983-15000
• Main Authors: Lacroix, Matthew R, Bukovsky, Eric V, Lozinšek, Matic, Folsom, Travis C, Newell, Brian S, Liu, Yong, Peryshkov, Dmitry V, Strauss, Steven H
• Format: Article
• Language: English
• ISSN: 0020-1669; 1520-510X
• DOI: 10.1021/acs.inorgchem.8b02786

Eight M­(H2O) n (Z) salt hydrates were characterized by single-crystal X-ray diffraction (Z2– = 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) n 2+ cations and Z2– 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 νasym(OH/OD) and νsym(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) = νasym(OX) – νsym(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 νasym(OH)/νsym(OH) pair for H2O ligands with longer O­(H)···F distances and one νasym(OH)/νsym(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).