Hydrated Metal Ion Salts of the Weakly Coordinating Fluoroanions PF6-, TiF62-, B12F122-, Ga(C2F5)4-, B(3,5-C6H3(CF3)2)4-, and Al(OC(CF3)3)4-. In Search of the Weakest HOH···F Hydrogen Bonds.

FTIR spectra of microcrystalline samples of 11 metal ion salt hydrates of a variety of weakly coordinating fluoroanions are reported. The compounds studied were Li(H2O)4(Al(OC(CF3)3)4), Li(H2O)(B(3,5-C6H3(CF3)2)4), Li(H2O)n(Ga(C2F5)4), Li(H2O)(PF6), Li2(H2O)2(TiF6), Li2(H2O)4(B12F12), Na(H2O)(PF6), Na2(H2O)2(B12F12), K2(H2O)2(B12F12), Rb2(H2O)2(B12F12), Cs2(H2O)(B12F12), and their partially or completely deuterated isotopologs and isotopomers. The O-D···F hydrogen bonds in Li(HOD)(H2O)3(Al(OC(CF3)3)4) (ν(OD) = 2706 cm-1), Li(HOD)(B(3,5-C6H3(CF3)2)4) (ν(OD) = 2705 cm-1), and Li(HOD)(H2O)n(Ga(C2F5)4) (ν(OD) = 2697 cm-1) rival HOD absorbed in polyvinylidene difluoride (ν(OD) = 2696 cm-1) and HOD···FCH3 in a frozen Ar matrix (ν(OD) = 2685 cm-1) for the weakest hydrogen bonds between a water molecule and an F atom in any compound. As weak as they are, minor differences in O-H···F hydrogen bonds in the same fluoroanion salt can be distinguished spectroscopically. Uncoupled HOD molecules in asymmetric F···HOD···F' hydrogen bonding environments in Rb+, Cs+, Mg2+, and Co2+ hydrates of B12F122- gave rise to two observable ν(OD) bands even though the two R(O···F) distances differ by only 0.010(4) Å (Mg2+), 0.033(2) Å (Co2+), 0.074(4) Å (Rb+), and 0.106(6) Å (Cs+). A plot of ν(OD) for hydrates with a single uncoupled HOD molecule per metal ion (e.g., Li(HOD)(H2O)3(Al(OC(CF3)3)4)) vs R(O···F) distance from single-crystal X-ray or neutron diffraction structures was prepared. The ν(OD) values range from 2305 to 2706 cm-1 and the R(O···F) distances range from 2.58 to 3.17 Å. The plot consists of 53 {ν(OD), R(O···F)} data points, 23 of which are new and have ν(OD) > 2600 cm-1, in contrast to a 1994 ν(OD) vs R(O···F) plot with 28 data points, none of which had ν(OD) > 2600 cm-1. There is a clear and significant difference between the new ν(OD) vs R(O···F) plot and a literature ν(OD) vs R(O···O) plot for hydrates containing O-D···O hydrogen bonds. For a given ν(OD) stretching frequency, the exponential regression curves show that R(O···F) is typically 0.1-0.2 Å shorter than R(O···O), in harmony with the lower basicity and smaller size of F atoms vs O atoms.

Room-Temperature FTIR Spectra of the Cyclic S4 (H2O)4 Cluster in Crystalline Li2(H2O)4(B12F12): Observation of B and E ν(OH) Bands and Coupling of Strong O-H···O and Weak O-H···F Vibrations.

Room-temperature-attenuated total reflection-Fourier transform infrared (FTIR) spectra of the structurally characterized crystalline lithium salt Li2(H2O)4(B12F12), which contains a cyclic S4 symmetry (H2O)4 cluster, and several partially or completely deuterated isotopologs, suggest the following conclusions: (i) B and E normal modes gave rise to two distinguishable ν(OHbound) bands, separated by 42 cm-1, two distinguishable ν(OHfree) bands, separated by 13 cm-1, and two distinguishable δ(HOH) bands, separated by 19 cm-1, in the FTIR spectrum of Li2(H2O)4(B12F12); (ii) B and E normal modes gave rise to two distinguishable ν(ODbound) bands separated by 20 cm-1 in the FTIR spectrum of Li2(D2O)4(B12F12); (iii) coupling between ν(OHbound) and ν(OHfree) normal modes or between ν(ODbound) and ν(ODfree) normal modes is weak, but finite, and resulted in shifts of 4-10 cm-1 for the respective bands in the spectra of Li2(H2O)4(B12F12) vs Li2(HOD)(D2O)3(B12F12) or in the spectra of Li2(D2O)4(B12F12) vs Li2(HOD)(H2O)3(B12F12); and (iv) a δ(DOD) band for an S4 (D2O)4 cluster, at ca. 1202 cm-1, was observed for the first time. In addition, FTIR spectra of the lithium salt containing cyclic [(HOD)(H2O)3] or [(HOD)(D2O)3] clusters are the first examples in which bands that are unambiguously assigned to both HO-D···O and DO-H···O hydrogen bonds in the same sample have been observed for a water tetramer.

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 (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 p Ka 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).

Latent Porosity in Alkali-Metal M2B12F12 Salts: Structures and Rapid Room-Temperature Hydration/Dehydration Cycles.

Structures of the alkali-metal hydrates Li2(H2O)4Z, LiK(H2O)4Z, Na2(H2O)3Z, and Rb2(H2O)2Z, unit cell parameters for Rb2Z and Rb2(H2O)2Z, and the density functional theory (DFT)-optimized structures of K2Z, K2(H2O)2Z, Rb2Z, Rb2(H2O)2Z, Cs2Z, and Cs2(H2O)Z are reported (Z2- = B12F122-) and compared with previously reported X-ray structures of Na2(H2O)0,4Z, K2(H2O)0,2,4Z, and Cs2(H2O)Z. Unusually rapid room-temperature hydration/dehydration cycles of several M2Z/M2(H2O)nZ salt hydrate pairs, which were studied by isothermal gravimetry, are also reported. Finely ground samples of K2Z, Rb2Z, and Cs2Z, which are not microporous, exhibited latent porosity by undergoing hydration at 24-25 °C in the presence of 18 Torr of H2O(g) to K2(H2O)2Z, Rb2(H2O)2Z, and Cs2(H2O)Z in 18, 40, and 16 min, respectively. These hydrates were dehydrated at 24-25 °C in dry N2 to the original anhydrous M2Z compounds in 61, 25, and 76 min, respectively (the exact times varied from sample to sample depending on the particle size). The hydrate Na2(H2O)2Z also exhibited latent porosity by undergoing multiple 90 min cycles of hydration to Na2(H2O)3Z and dehydration back to Na2(H2O)2Z at 23 °C. For the K2Z, Rb2Z, and Cs2Z transformations, the maximum rate of hydration (rhmax) decreased, and the absolute value of the maximum rate of dehydration (rdmax) increased, as T increased. For K2Z ↔ K2(H2O)2Z hydration/dehydration cycles with the same sample, the ratio rhmax/rdmax decreased 26 times over 8.6 °C, from 3.7 at 23.4 °C to 0.14 at 32.0 °C. For Rb2Z ↔ Rb2(H2O)2Z cycles, rhmax/rdmax decreased from 0.88 at 23 °C to 0.23 at 27 °C. For Cs2Z ↔ Cs2(H2O)Z cycles, rhmax/rdmax decreased 20 times over 8 °C, from 6.7 at 24 °C to 0.34 at 32 °C. In addition, the reversible substitution of D2O for H2O in fully hydrated Rb2(H2O)2Z in the presence of N2/16 Torr of D2O(g) was complete in only 60 min at 23 °C.

Lanthanide thioborates, an emerging class of nonlinear optical materials, efficiently synthesized using the boron-chalcogen mixture method.

The Boron-Chalcogen Mixture method was used to obtain single crystals of the previously extremely difficult to synthesize lanthanide orthothioborates to investigate their structures and their structurally connected optical behavior, such as second harmonic generation. Using a combined halide and polychalcogenide flux, the BCM method yielded single crystals of LnBS3 (Ln = La, Ce, Pr, Nd), which are isostructural and crystallize in the non-centrosymmetric space group, Pna21. Second harmonic generation measurements confirmed the expectation that LaBS3 would exhibit a strong SHG response, measured at 1.5 × KDP.