title: Ion-pairing of phosphonium salts in solution: C-H⋅⋅⋅halogen and C-H⋅⋅⋅π hydrogen bonds.
abstract: The (1) H NMR chemical shifts of the C(α)H protons of arylmethyl triphenylphosphonium ions in CD2 Cl2 solution strongly depend on the counteranions X(-) . The values for the benzhydryl derivatives Ph2 CHPPh3 (+)  X(-) , for example, range from δH =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⋅⋅⋅Ph interactions between C(α)H and a phenyl group of the BPh4 (-) anion, which result in upfield NMR chemical shifts of the C(α)H protons. These interactions could also be observed in crystals of (p-CF3 -C6 H4 )CH2 PPh3 (+)  BPh4 (-) . 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 BPh4 (-) anions. The concentration dependence of the (1) H NMR chemical shifts allowed us to determine the dissociation constants of the phosphonium salts in CD2 Cl2 solution. The cation-anion interactions increase with the acidity of the C(α)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 CD2 Cl2 solution show strong red-shifts of the CH stretch bands. The CH stretch bands of the tetrafluoroborate salt PhCH2 PPh3 (+)  BF4 (-) in CD2 Cl2 , however, show a blue-shift compared to the corresponding BPh4 (-) salt.
authors.affiliation: Ludwig-Maximilians-Universität München, Butenandtstrasse 5-13, 81377 München (Germany), Fax: (+49) 89-2180-77398. Johannes.Ammer@cup.uni-muenchen.de.
authors.forename: Johannes
authors.initials: J
authors.lastname: Ammer
authors.forename: Christoph
authors.initials: C
authors.lastname: Nolte
authors.forename: Konstantin
authors.initials: K
authors.lastname: Karaghiosoff
authors.forename: Sebastian
authors.initials: S
authors.lastname: Thallmair
authors.forename: Peter
authors.initials: P
authors.lastname: Mayer
authors.forename: Regina
authors.initials: R
authors.lastname: de Vivie-Riedle
authors.forename: Herbert
authors.initials: H
authors.lastname: Mayr
keywords: NMR spectroscopy; crystal structure; hydrogen bonds; ion pairs; solution structure