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.

Reactions of a Ga/P‐Based Frustrated Lewis Pair with HX(X= F – I), HeterocumulenesR–NCY(Y= O, S) and Chalcogens–Adduct Formation and Surprising Stability towards Protolysis

“…Spectroscopic shifts of phosphonium ions are known to depend on the counterions as a result of the formation of ion pairs. [16a], Similarly the t Bu groups showed a down field shift in the 1 H NMR spectra from δ = 1.05 ( 1 ) to δ = 1.34 to 1.46 (0.71 for 3 ) consistent with an increase of the coordination number at Ga from three to four. Compound 2a showed additionally the expected coupling between 19 F and 31 P ( 3 J PF = 54.5 Hz) and 19 F and 1 H [ 4 J FH (PH, C=CH) = 7.9, 3.5 Hz], respectively.…”

Photogeneration of carbocations: applications in physical organic chemistry and the design of suitable precursors

“…[38] A detailed investigation of ion pairing in solutions of benzyl and benzhydryl triphenylphosphonium salts was recently reported. [109] For the generation of highly Brønsted-acidic carbocations, the possibility of a proton transfer from the carbocation to the photo-leaving group in the solvent cage also has to be taken into account. Irradiation of PhC(CH 3 ) 2 -PPh 3 + in CH 2 Cl 2 , for example, presumably leads to intermediary cumyl cations PhC(CH 3 ) 2 + , which are rapidly deprotonated by PPh 3 before the photofragments can diffuse apart (Scheme 9).…”

“…[115] for acetic acid and estimated as described in Ref. [109] for the phosphonium salt) of carbocations by irradiation with 355-nm pulses from the third harmonic of the Nd/YAG laser thus required the development of suitable photo-leaving groups. Taking advantage of the fact that the UV-vis absorptions of pyridinium salts can be shifted toward higher wavelengths by bridged amino substituents, we were able to obtain carbocations by 355-nm irradiation of their adducts with 3,4,5-triamino-substituted pyridines (Scheme 15).…”

“…Tertiary phosphines are the most versatile among the different classes of photo-leaving groups investigated: [35][36][37][38][39][40][41][42][43][44][45][46][47][48][49][50][51][52] Quaternary phosphonium salts are easy to synthesize, [109] stable in solution, even in solvents with very high ionizing power such as hexafluoroisopropanol/water mixtures, [43] and provide excellent yields of carbocations even in solvents of moderate polarity such as CH 2 Cl 2 . [37][38][39][40] Tris(p-chlorophenyl)phosphine is one of the most efficient photo-leaving groups presently known for the generation of highly reactive carbocations, [38][39][40] whereas tributylphosphine is the photo-leaving group of choice for the photogeneration and kinetic investigation of highly stabilized carbocations with low Lewis acidity.…”

Formation constants in C–H hydrogen bonding. 4. Effects of cyano, nitro, and trifluoromethyl substituents in aromatic compounds

“…2 Benzhydryltriphenylphosphonium ions form H-bonds with Cl -and Br -, but not with BF 4 -nor SbF 6 -. 3 Certain substituted trioxanes form trifurcated C-K eq /H, or K eq divided by the number of equivalent H's, is also shown, whenever two or more such H's are present. We assume, and in some cases have shown, 8 that at sufficiently low concentrations of both donor and HMPA the extent of complexing of the second proton is negligible.…”