Ion-Pairing of Phosphonium Salts in Solution: C-H···Halogen and C-H···p Hydrogen Bonds

摘要

The 1H NMR chemical shifts of the C(a)~-H protons of arylmethyl triphenylphosphonium ions in CD_2Cl_2 solution strongly depend on the counteranions X~-. The values for the benzhydryl derivatives Ph2CH-PPh3 + X~-, for example, range from dH=8.25 (X~-=Cl~-) over 6.23 (X~-=BF4~-) to 5.72 ppm (X~-=BPh_4~-). 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~- @ BF_4~- > SbF_6~-. For the less bulky derivatives PhCH_2~- PPh_3~+ X~-, we also find C-H···Ph interactions between C(a)~-H and a phenyl group of the BPh_4~- anion, which result in upfield NMR chemical shifts of the C(a)~-H protons. These interactions could also be observed in crystals of (p-CF_3-C_6H_4)CH_2~-PPh_3~+ BPh_4~-. However, the dominant effects causing the counterion-induced shifts in the NMR spectra are the C~-H···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 BPh_4~- anions. The concentration dependence of the 1H NMR chemical shifts allowed us to determine the dissociation constants of the phosphonium salts in CD_2Cl_2 solution. The cation- anion interactions increase with the acidity of the C(a)~-H protons and the basicity of the anion. The existence of C~-H···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 CD_2Cl_2 solution show strong red-shifts of the C~-H stretch bands. The C~-H stretch bands of the tetrafluoroborate salt PhCH_2~- PPh_3~+ BF_4~- in CD_2Cl_2, however, show a blue-shift compared to the corresponding BPh_4~- salt.