Abstract
Herein, we report the synthesis and characterization of a variety of novel poly(hydrogen halide) halogenates (−I). The bifluoride ion, which is known to have the highest hydrogen bond energy of ≈160 kJ mol−1, is the most famous among many examples of [X(HX)n]− anions (X=F, Cl) known in the literature. In contrast, little is known about poly(hydrogen halide) halogenates containing two different halogens, ([X(HY)n]−). In this work we present the synthesis of anions of the type [X(HY)n]− (X=Br, I, ClO4; Y=Cl, Br, CN) stabilized by the [PPh4]+ and [PPN]+ cation. The obtained compounds have been characterized by single‐crystal X‐ray diffraction, Raman spectroscopy and quantum‐chemical calculations. In addition, the behavior of halide ions in hydrogen fluoride was investigated by using experimental and quantum‐chemical methods in order to gain knowledge on the acidity of hydrogen halides in HF.
Highlights
Polyhalides ([X(X2)n]À), which are prominent examples for halogen-bonded systems, show a large structural diversity as well as a variety of possible applications.[1]
For hydrogen bonding between halide ions and hydrogen halides the charge transfer contribution can be described as a donation of electron density from the lone pair of the halide ion into the s* orbital of the HX bond
The nearly linear geometry of the anion is in agreement with a strong hydrogen bond interaction. This observation is supported by quantum-chemical calculations (B3LYP(D3BJ)/def2-TZVPP (SCS-MP2/def2-TZVPP)) which show that the bromide HCN adduct has a similar stability as [BrHCl]À (Scheme 7) of which the hydrogen bond energy was experimentally determined to be 54 kJ molÀ1.[16]
Summary
Polyhalides ([X(X2)n]À), which are prominent examples for halogen-bonded systems, show a large structural diversity as well as a variety of possible applications.[1] A related class of compounds, the poly(hydrogen halide) halogenates (ÀI) ([X(HY)n]À), consists of a central halide (XÀ) that is coordinated to hydrogen halide molecules (HY) This combination of halide anions with hydrogen halides, which has a large positive charge on the hydrogen atom, yields compounds with quite strong hydrogen-bonding interactions. Free reaction energies were calculated for the formation of the most favorable geometry which is a tetrahedral structure for all [X(HCl)4]À anions (Figure 4) These calculations indicate a lower stability of the [I(HCl)4]À anion which might explain why it could not be isolated as a [PPh4]+ salt.
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