Anions hydrogen bond basicity

Some exceptions to this general observation were found halide and nitrite salts have unusually high hydrogen-bond basicities (as would be expected), while penta-cyanopropionide, picrate, triflate, and perfluorobezenesulfonate salts not only had unusually low hydrogen-bond basicities (also as would be expected) but also lower s values, perhaps due to the weakening of the Coulombic interactions by delocalization of the charge on the anions. 

At first glance the hydrogen bond basicity (3 is controlled solely by the anions, with basicity decreasing in the order Cl > [RS03] >[BF4] > [PF ] > [N03] > [SCN] . However, while the general trend is clear, this is not the order that one would have expected, and the cations are obviously playing a role. Again, this may be a consequence of competition for the basic site (anion) between the test solute and the acidic site (cation) of the ionic liquid. It is unfortunate that no study to date has used a common anion across all possible cations. 

The ionic liquids incorporating various anions associated with the same cation [BMIM] exhibited different basicity and dipolarity values. The effect of the cation on hydrogen bond basicity and dipolarity was quite small. Therefore, it was inferred that the anion has a greater influence than the cation on the overall hydrogen bond basicities of the ionic liquids. 

Protonation becomes a rapid reaction in protic solvents and in the presence of acids, as demonstrated for, e.g., -butyl acrylate in aqueous solution [207], methyl acrylate in EtOH [208], cinnamates in the presence of phenol in DMF [209], and benzaldehyde in ethanolic buffer solution [210]. Rate constants for protonation of aromatic radical anions (anthracene [211], naphthalene, 2-methoxynaphthalene, 2,3-dimethoxynaphthalene) by a number of proton donors including phenols, acetic acid, and benzoic acids in aprotic DMF were found to vary from 5.0 X 10 M- s-> (for anthracene, in the presence of p-chlorophenol) to 6.2 x lO s (for anthracene, in the presence of pentachlorophenol) [212]. For dimedone, PhOH, or PhC02H the rate of protonation depends on the hydrogen-bond basicity of the solvent and increases in the order DMSO < DMF MeCN [213],... 

As in other studies, the determining factor for hydrogen bond basicity was found to be the anion. However, the interaction of a given anion with the solute can be modulated somewhat using different cations. As opposed to the y °° study [74], that proved to be not sensitive enough to detect changes in the n-propanol interactions with the cation, the results obtained by NMR show that ethanol has the strongest... 

In summary, the literature data suggests that the hydrogen bond basicity is still the most reliable parameter to which reaction outcomes can be correlated. The effect of the cation is often not easily predicted, due to a delicate interplay specific to each anion and cation combination in the pure ionic liquid, which exhibits unique interactions when a solute is added. 

The factors ct, a and [3 are solvent-independent factors. The three values it, a and [3 can be obtained from UV-Vis absorption spectroscopy, the n value from the 7i —> 7t absorption of 4-nitroanisole or A,A-dimethyl-4-nitroaniline, the a and p values from pairs of homomorphic compounds like 4-nitroanisole and Reichardt s dye as well as 4-nitroaniline and AOV-diethyl-4-nitroaniline. While n reports the effect of the dipolarity and polarizability of the solvent, a appears to be largely controlled by the cation of the ionic liquid and like P is dependent on the hydrogen bond basicity of the solvent and is dominated by the nature of the anion. 

With respect to the influence of the anion and cation interactions, at first glance, the hydrogen bond basicity is controlled by the anions. However, this is a general trend because the hydrogen-bond acceptor ability of the ionic liquid also depends on the nature of cation. Thus, the overall ability of the IL to form a hydrogen bond with a molecule of solute comes from an antagonistic relationship between its constituent ions. This fact may be desalbed in terms of two competing equilibria. 

It becomes more interesting when the solvent properties are broken down into their component parts. Ionic liquids can act as hydrogen-bond acids and/or hydrogen-bond bases or neither. Generally, the hydrogen-bond basicity is determined by the anion and the hydrogen-bond acidity is determined by the cation. There is no obvious unique ionic effect to be seen in the available data. So far, no one has attempted to make an ionic liquid of any particular polarity, but this may be possible. 

There is, again, a great deal of synthetic flexibility in the P value. These have been shown to depend upon the gas-phase acidity of the conjugate acid of the anion [54]. Hence, Poole s sulfonates show very high values, whereas the anions used in conjunction with the [BMIM] cations show only moderate hydrogen bond basicity. 

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