Ion‐Pairing of Phosphonium Salts in Solution: CH⋅⋅⋅Halogen and CH⋅⋅⋅π Hydrogen Bonds. Issue 43 (6th September 2013)

• Record Type: Journal Article
• Authors: Ammer, Johannes; Nolte, Christoph; Karaghiosoff, Konstantin; Thallmair, Sebastian; Mayer, Peter; de Vivie‐Riedle, Regina; Mayr, Herbert
• Is Part Of: Chemistry. Volume 19:Issue 43(2013)
• Page Start: 14612
• Page End: 14630
• Publication Date: 2013-09-06
• Subjects: Chemistry -- Periodicals; 540
• DOI: 10.1002/chem.201204561
• Languages: English
• ISSNs: 0947-6539

Abstract

<abstract abstract-type="main" xml:lang="en"> <title>Abstract</title> <p>The <sup>1</sup>H NMR chemical shifts of the C(α)H protons of arylmethyl triphenylphosphonium ions in CD<sub>2</sub>Cl<sub>2</sub> solution strongly depend on the counteranions X<sup>−</sup>. The values for the benzhydryl derivatives Ph<sub>2</sub>CHPPh<sub>3</sub><sup>+</sup> X<sup>−</sup>, for example, range from <italic>δ</italic><sub>H</sub>=8.25 (X<sup>−</sup>=Cl<sup>−</sup>) over 6.23 (X<sup>−</sup>=BF<sub>4</sub><sup>−</sup>) to 5.72 ppm (X<sup>−</sup>=BPh<sub>4</sub><sup>−</sup>). Similar, albeit weaker, counterion‐induced shifts are observed for the <italic>ortho</italic>‐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<sup>−</sup> > Br<sup>−</sup> ≫ BF<sub>4</sub><sup>−</sup> > SbF<sub>6</sub><sup>−</sup>. For the less bulky derivatives PhCH<sub>2</sub>PPh<sub>3</sub><sup>+</sup> X<sup>−</sup>, we also find CH⋅⋅⋅Ph interactions between C(α)H and a phenyl group of the BPh<sub>4</sub><sup>−</sup> anion, which result in upfield NMR chemical shifts of the C(α)H protons. These interactions could also be observed in crystals of (<italic>p</italic>‐CF<sub>3</sub>‐C<sub>6</sub>H<sub>4</sub>)CH<sub>2</sub>PPh<sub>3</sub><sup>+</sup> BPh<sub>4</sub><sup>−</sup>. However, the dominant effects causing the counterion‐induced shifts in the NMR spectra are the CH⋅⋅⋅X<sup>−</sup> hydrogen bonds between the phosphonium ion and anions, in particular Cl<sup>−</sup> or Br<sup>−</sup>. This observation contradicts earlier interpretations which assigned these shifts predominantly to the ring current of the BPh<sub>4</sub><sup>−</sup> anions. The concentration dependence of the <sup>1</sup>H NMR chemical shifts allowed us to determine the dissociation constants of the phosphonium salts in CD<sub>2</sub>Cl<sub>2</sub> 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<sup>−</sup> 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<sup>−</sup> and Br<sup>−</sup> salts in CD<sub>2</sub>Cl<sub>2</sub> solution show strong red‐shifts of the CH stretch bands. The CH stretch bands of the tetrafluoroborate salt PhCH<sub>2</sub>PPh<sub>3</sub><sup>+</sup> BF<sub>4</sub><sup>−</sup> in CD<sub>2</sub>Cl<sub>2</sub>, however, show a blue‐shift compared to the corresponding BPh<sub>4</sub><sup>−</sup> salt.</p> </abstract>