Hydrated Metal Ion Salts of the Weakly Coordinating Fluoroanions PF₆^–, TiF₆^2–, B₁₂F₁₂^2–, 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

Abstract

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 B12F12 2– 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.

Hydrogen Bond Acceptor Propensity of Different Fluorine Atom Types: An Analysis of Experimentally and Computationally Derived Parameters

“…In the molecule KEFMIU (Figure 8 top) a HB with a water molecule trapped during the crystallization process is observed, providing crystallographic evidence that a shielded fluorine atom can make a HB complex with water. Spectroscopic evidences for the formation of HB between water and fluorine atom have recently been reported [18,50,51] . As previously discussed, this has important implications, regarding the solubility and permeability of the molecules bearing these fluorinated motifs [18] .…”

“…Spectroscopic evidences for the formation of HB between water and fluorine atom have recently been reported. [18,50,51] As previously discussed, this has important implications, regarding the solubility and permeability of the molecules bearing these fluorinated motifs. [18] In the molecule BACMIE an intermolecular HB is formed between one of the four fluorine atoms and the OH of a nearby molecule.…”