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)
Posted on 2018-11-16 - 19:19
Eight M(H2O)n(Z) salt hydrates
were characterized by single-crystal X-ray diffraction (Z2– = B12F122–): 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 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 pKa 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).
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Lacroix, Matthew
R.; Bukovsky, Eric V.; Lozinšek, Matic; Folsom, Travis C.; Newell, Brian S.; Liu, Yong; et al. (2018). 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). ACS Publications. Collection. https://doi.org/10.1021/acs.inorgchem.8b02786
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AUTHORS (8)
ML
Matthew
R. Lacroix
EB
Eric V. Bukovsky
ML
Matic Lozinšek
TF
Travis C. Folsom
BN
Brian S. Newell
YL
Yong Liu
DP
Dmitry V. Peryshkov
SS
Steven H. Strauss