Static and dynamic study of metal salt hydrates of weakly-coordinating fluoroanions by vibrational spectroscopy, gravimetry, and an analysis of previously published x-ray structures
Date
2021
Authors
Lacroix, Matthew R., author
Strauss, Steven H., advisor
Chen, Eugene, committee member
Bandar, Jeff, committee member
Ridley, John, committee member
Journal Title
Journal ISSN
Volume Title
Abstract
Eighteen metal salt hydrates (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), Na(H2O)(PF6), Li2(H2O)4(B12F12), Na2(H2O)2(B12F12), K2(H2O)2(B12F12), Rb2(H2O)2(B12F12), Cs2(H2O)(B12F12), Mg(H2O)6(B12F12), Ca(H2O)n(B12F12), Sr(H2O)n(B12F12), Ba(H2O)n(B12F12), Co(H2O)6(B12F12), Ni(H2O)6(B12F12), Zn(H2O)6(B12F12), and Li2(H2O)2(TiF6)) containing weakly coordinating anions were analyzed using room temperature ATR-FTIR spectroscopy. The goal was to investigate the relative strengths of water–anion hydrogen bonds in the solid-state. In all but one case, these hydrogen bonds take the form of O–H···F hydrogen bonds. The one exception is in the salt Li2(H2O)4(B12F12) where there are both O–H···F and O–H···O hydrogen bonds present. Based on the magnitude of the redshift of the ν(OH) band(s) a qualitative scale for the comparison of the relative hydrogen bond strength is constructed. Included in this scale are additional metal salt hydrates taken from the literature. This spectroscopic study has produced some of the only room temperature spectra for water participating in hydrogen bonding in the solid-state where the νasym(OH) and νsym(OH) bands are individually resolvable. The weak nature of the O–H···F hydrogen bonds allows for resolution of ν(OH) bands only 5 cm−1 apart in some cases. The two metal salt hydrates (Li(H2O)4(Al(OC(CF3)3)4) and Li(H2O)(B(3,5-C6H3(CF3)2)4) are shown to possess the weakest O–H···F hydrogen bonds observed in the solid state at room temperature. The salt Li2(H2O)4(B12F12) contains a cyclic (H2O)4 water cluster, also known as the R4 cluster, is presented, and discussed in the context of the FTIR spectrum of water clusters. Due to the nature of the weak O–H···F hydrogen bonding between the cluster and the surrounding anions the E and B fundamental vibrations for the cluster were able to be determined. The peak-to-peak separation, and relative intensities of these two bands are consistent with computational results from the literature. This is the first time that the R4 water cluster has been successfully studied via FTIR spectroscopy without the presence of other clusters leading to ambiguity in the results. Finally, direct observation of the effect of cation acidity on the relative strength of water–anion hydrogen bonding has been directly observed for the first time in the metal hexahydrate salts M(H2O)6(B12F12) (M = Mg, Co, Ni, Zn). These results, along with the correlation curves constructed in this work, show that it is not possible to assign relative hydrogen bond strength based on O–H···X bond length, nor is it possible to accurately approximate O–H···X bond length based on degree of ν(OH) redshift. Instead, it is shown that the relative basicity of the anion is the primary factor governing the relative hydrogen bond strength, and thus the degree of redshifting experienced by the ν(OH) band(s). The cation acidity also is shown to have a lesser, but observable, effect on the relative strength of O–H···X hydrogen bond. In addition to broadening our fundamental understanding of hydrogen bonding in the solid state, this work also shows that FTIR spectroscopy can be a useful tool for rapidly assigning relative basicity of new weakly coordinating anions without need the for complex protonation experiments.
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Subject
hydratation
infrared spectroscopy
weakly-coordinating anions
hydrogen bonding
fluorine chemistry
salt hydrates