Anion Recognition in the Gas Phase
Like calixarenes, simple resorcinarenes (Scheme 1, left) are known to bind cations, for example tetramethyl ammonium or tetraethyl ammonium. Some of them even form dimeric capsules when the appropriate templating cation is available as the guest inside the capsules cavity. With such guests, cation binding can be demonstrated not only by mass spectrometry in the gas phase, but also by NMR spectroscopy in solution, by crystal structure analysis in the solid state and by theoretical calculations in silico.
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Scheme 1. A resorcinarene (left) whose cone conformation is stabilized by a seam of hydrogen bonds and the corresponding cavitand (right). R and R2 usually correspond to solubilizing alkyl chains, R1 can be a variety of functional groups.
In a cooperation with the groups of Enrico Dalcanale, Arne Lützen, Kari Rissanen, and Jörg Grunenberg, it thus came as a great surprise when mass spectrometry provides evidence for anion binding to resorcinarene cavitands (Scheme 1, right), in which the cone-shaped conformation is fixed by four methylene groups each bridging the oxygen atoms of two adjacent resorcinol rings. Figure 1 shows the negative mode ESI-FTICR mass spectrum of a cavitand/NEt4+ PF6- mixture. While no tetraethyl ammonium complex is observed in the positive mode, a quite clean spectrum with an intense signal for the hexafluoro phosphate complex is observed.
Figure 1. Negative mode ESI-FTICR mass spectrum of a 1:1 mixture of
a cavitand and
NEt4+ PF6-. The formation of salt cluster signals is typical
for ESI mass spectra of samples containing salts.
The binding interaction must be substantial. If one offers sulfate as the guest, a dianionic sulfate complex is observed. Naked sulfate is intrinsically unstable in the gas phase and spontaneously undergoes electron autodetachment to the sulfate anion-radical. The cavitand must therefore provide sufficient binding energy to overcome the intrinsic instability. One can compare the situation to the solvation of sulfate with water molecules: A minimum of three water molecules forming six hydrogen bonds to the sulfate is necessary to stabilize the dianion.
A number of experiments and theory support the hypothesis that the anion is located on the concave side of the cavitand bowl bound by four C-H...anion interactions with the methylene hydrogens. Calculations of the compliance constants for this interaction result in the prediction of interactions strengths somewhat lower than a normal A-H...B hydrogen bond. Hexafluoro phosphate is predicted to bind with a binding energy of ca. 25 kJ/mol. The complex geometry is shown in Figure 2.
Figure 2. Calculated geometry of the cavitand/hexafluoro phosphate complex. For details, see the link to the article on the right.