Coordinating and Non-Coordinating Anions

Iodide as an anion is a mild reducing agent and could easily be oxidised to iodine. In order to prevent this redox chemistry of the prepared salts the iodide should be exchange for non- redox active and non-coordinating anions. Considering that halide anions are mainly coordination anions it could be beneficial to exchange the iodine anion not only because of its redox activity. Non-coordinating anions like tetrafluoroborate or hexafluorophosphate enable better solubility in organic solvents for the following deprotonation step. Moreover, these complex anions are less nucleophilic and less basic compared to halides.

Anion exchange reactions are widely known in organic chemistry. Typically, a silver salt carrying the desired anion is reacted with the salt of interest because silver salts, especially silver halides have very low solubility products. Basic requirement is that the solubility product of silver and the compound’s anion is lower than the solubility product of the silver salt itself. In this particular case NaBF4, KPF6 and KOTf were used for the anion exchange for sustainability reasons as shown in Table 2-2.

Table 2-2: Comparison of purchase costs of different salts from Sigma-Aldrich.^[145]

Results and Discussion

The anion exchange reactions were carried out at room temperature in a biphasic solvent mixture in the ratio of 3:2 (DCM: water). The mixture was stirred for one day before the phases were separated. The organic phase was dried further with magnesium sulphate (MgSO4), then filtered and concentrated under reduced pressure. The compounds (IPrPh)BF4

(8), (IPrPh)PF6 (9) and (IPrPh)OTf (10) could be synthesised in good yields up to 86%.

Furthermore (IPrPh)Br (20) could be prepared, which will be explained in chapter 2.4. The reasonances of all compounds could be found with identical shifts, except for the backbone hydrogen atoms. The eight methyl groups of the isopropyl functionality could be found as two doublets at high field with a shift of 1.04 and 1.27 ppm. The four corresponding ipso protons show a discrete septet at 2.43 ppm. Furthermore, a doublet at 7.36 pm and a triplet at 7.61 ppm describe the meta and para positions of the Dipp ligand. The resonances of the phenyl group appear as three sets of multiplets. A doublet of doublets at 6.96 ppm, a triplett at 7.26 ppm and another triplet at 7.44 ppm complete the spectrum. The ¹H-NMR spectra show significant shifting for the backbone (C4/C5) proton singlet resonances towards higher field for all compounds. The bromine ligand shows a shift at 8.29 ppm compared to the iodine ligand at 8.07 ppm followed by the triflate- (7.97 ppm), tetrafluoroborate- (7.88 ppm) and hexafluorophosphate ligands (7.76 ppm).

Figure 2.11: Superimposed ¹H-NMR spectrums of the five (IPrPh)X compounds in CD2Cl2. X = Br^–, I^–,OTf^–, BF4–, PF6–.

The shifting of the backbone singlet resonance results from the different electronic influences of the counterions. Bromine and iodine are supposed to be coordinating anions which results in a low field shift. Triflate, tetrafluoroborate and hexafluorophosphate are known to be non-coordinating anions. Correlating the chemical shift of the backbone hydrogen atoms of these five compounds with the acidity of the hydrogen atoms it could be stated that the bromine compound is the most acidic and the hexafluorophosphate compound the least acidic.

We could show with ¹⁹F-¹H-2D heteronuclear Overhauser enhancement spectroscopy experiments (HOESY) that in all three cases coupling between the fluorine and the hydrogen atoms could be observed. This proves at least a partial coordination of the anion Furthermore,

Results and Discussion

1H- NMR experiments have been done using different concentrations of the sample to evaluate, if a coordination of the anion is present. In case of coordinating anions one should observe a high field shift going to lower concentrations due to the lower probability of interactions.

Figure 2.12: Exemplary ¹⁹F-¹H-HOESY spectrum of (IPrPh)PF6 in THF-d8.

Furthermore, diffusion-ordered spectroscopy (DOSY) experiments have been done to evaluate the coordination behaviour of those five anions to the ligand. It was assumed that a coordination of the anion would lead to larger diffusion coefficients and therefore higher molecular weights determined with the ECC-MW estimation method developed by R. Neufeld and pursued by S. Bachmann in our workgroup.^[146] External calibration curves (ECC) have been used to determine the molecular weight of the five salts. For the method to work an internal reference has to be added to the sample to get a normalised diffusion coefficient, which can be used to overcome the inherent errors when comparing DOSY experiments (diversities in temperature, concentration, viscosity, NMR-device properties, etc.). For the measurements the reference molecule adamantan was added to the samples.

Furthermore, the method includes different ECCs for the shape of the molecule. Compact spheres (CS), dissipated spheres and ellipsoids (DSE) and expanded discs (ED) could be distinguished, which led to lower standard deviations of the ECCs. If the shape of a molecule is not clear a merged calibration curve can be used to determine the molecular weight but with a higher standard deviation. The shape of the measured molecules was considered to be associated to the DSE shape following the classification suggested by Neufeld. One issue with

Results and Discussion

Table 2-3: Measured and calculated molecular weights and their standard deviations for the five salts using the ECC-DOSY method developed by Neufeld and Bachmann.

Compound MW

anion (IPrPh)OTf 570.49

22.61

632.12

35.85 465.30

with anion 7.13 2.90 614.30

without

anion (IPrPh)BF4 579.84

24.62

643.31

38.26 465.30

with anion 5.02 16.52 552.11

without

anion (IPrPh)PF₆ 571.70

22.87

633.57

36.16 465.30

with anion 6.38 3.75 610.65

Nevertheless, all results show a tendency towards coordinating aggregates even though most values are over interpreted. Following the results of Bachmann et al. a deviation of 14% could be tolerated for measurements in DCM. Those results fall into this given empirical error region. It is likely that all anions coordinate to a certain extent and molecular weights in between the values of the coordinated complex and the free cation are expected.

Another reason for the overestimated values could lie in DOSY experiment. ECCs possible inclusions of solvent in the cavities of the (IPrPh)I molecule or “free space” cannot be distinguished (Figure 2.13). The diffusion coefficients of molecules with or without these possible inclusions could ensure overestimated diffusion values, which again result in higher molecular weights. Unfortunately, it is not possible to prove the assumptions with this method until now.

Figure 2.13: Spacefill model of (IPrPh)I. Top view (left), side view (middle), bottom view (right).