Ion-Pairing of Phosphonium Salts in Solution: C-H•••Halogen and C-H•••π Hydrogen Bonds

The H-1 NMR chemical shifts of the C(alpha)-H protons of arylmethyl triphenylphosphonium ions in CD2Cl2 solution strongly depend on the counteranions X-. The values for the benzhydryl derivatives Ph2CH-PPh3+X-, for example, range from delta(H)=8.25 (X- = Cl-) over 6.23 (X- = BF4-) to 5.72 ppm (X- = BPh4-). Similar, albeit weaker, counterion-induced shifts are observed for the ortho-protons of all aryl groups. Concentration-dependent NMR studies show that the large shifts result from the deshielding of the protons by the anions, which decreases in the order Cl- > Br- >> BF4- > SbF6-. For the less bulky derivatives PhCH2-PPh3+ X-, we also find C-H center dot center dot center dot Ph interactions between C(alpha)-H and a phenyl group of the BPh4- anion, which result in upfield NMR chemical shifts of the C(alpha)-H protons. These interactions could also be observed in crystals of (p-CF3-C6H4)CH2-PPh3+BPh4-. However, the dominant effects causing the counterion-induced shifts in the NMR spectra are the C-H center dot center dot center dot X- hydrogen bonds between the phosphonium ion and anions, in particular Cl- or Br-. This observation contradicts earlier interpretations which assigned these shifts predominantly to the ring current of the BPh4- anions. The concentration dependence of the H-1 NMR chemical shifts allowed us to determine the dissociation constants of the phosphonium salts in CD2Cl2 solution. The cation-anion interactions increase with the acidity of the C(alpha)-H protons and the basicity of the anion. The existence of C-H center dot center dot center dot X- hydrogen bonds between the cations and anions is confirmed by quantum chemical calculations of the ion pair structures, as well as by X-ray analyses of the crystals. The IR spectra of the Cl- and Br- salts in CD2Cl2 solution show strong red-shifts of the C-H stretch bands. The C-H stretch bands of the tetrafluoroborate salt PhCH2-PPh3+BF4- in CD2Cl2, however, show a blue-shift compared to the corresponding BPh4- salt.