Abstract
A series of mono- and divalent fluorinated pyridine derivatives is investigated by electrospray ionization (tandem) mass spectrometry and quantum chemical calculations with respect to their capability to bind anions in the gas phase. The pyridine derivatives differ not only in valency, but also with regard to the degree of fluorination of the pyridine rings, the positions of the fluorine atoms, the rigidity of the spacers connecting the two pyridines in the divalent compounds, and the relative configuration. While the monovalent compounds did not form anion complexes, the divalent analogues exhibit anion binding even to weakly coordinating anions such as tetrafluoroborate. Three different tandem mass spectrometric experiments were applied to rank the gas-phase binding energies: (i) collision-induced dissociation (CID) experiments in a Fourier transform ion-cyclotron-resonance (FTICR) mass spectrometer on two different, simultaneously mass-selected complexes with different receptors, (ii) determination of the collision energy required to fragment 50 % of the mass-selected complexes in an ESI-QToF mass spectrometer, and (iii) CID of heterodimers formed from two different, competing pyridine receptors and indigo carmine, a dianion with two identical binding sites. All three experiments result in consistent binding energy ranking. This ranking reveals surprising features, which are not in agreement with binding through anion-π interactions. Density functional theory (DFT) calculations comparing different potential binding modes provide evidence that the ranking can instead nicely be explained, when C-H⋅⋅⋅anion interactions with the spacers are invoked. These results are supported by gas-phase IR spectroscopy and ion mobility-mass spectrometry (IM-MS) on a selected set of chloride pyridine complexes.
Highlights
Besides well-known and intensely studied non-covalent forces such as hydrogen bonding,[ 1 ] π-π stacking,[ 2 ] and cation-π interactions,[3][3] the non-covalent bonds between anions and π systems have attracted broad interest recently.[4]
Anion-π interactions are usually weak in solution due to the competing effects of counterions and solvent
As the determination of exact absolute binding energies in the gas phase is not a trivial task and requires specialized equipment,[15] we aim at establishing a ranking of relative binding forces of anions to the different pyridine derivatives discussed above
Summary
Besides well-known and intensely studied non-covalent forces such as hydrogen bonding,[ 1 ] π-π stacking,[ 2 ] and cation-π interactions,[3][3] the non-covalent bonds between anions and π systems have attracted broad interest recently.[4]. Four different binding motifs were experimentally observed: (i) the C-H···Xhydrogen bond, (ii) the mostly covalent Meisenheimer complex, (iii) the weakly covalent anion-donor-π-acceptor interaction and [b] Prof. (iv) the non-covalent anion-π interaction.[ 5 ] The anion-π interaction can be described as an attractive force between the anion and the positive π-acidic surface of an electron-deficient aromatic ring with permanent quadrupole moment. The anion is located above the center of the aromatic ring.[ 6 ] Hexafluorobenzene and triazine, for example, form complexes with anions – in contrast to benzene, which has a quadrupole moment with negative π face. While hexafluorobenzene binds a chloride ion above the aromatic ring, pentafluorobenzene realizes a C-H···Clinteraction as the π-acidity of the aromatic ring is reduced and the C-H bond is strongly polarized
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