Anion Receptors Based on Highly Fluorinated Aromatic Scaffolds

2014-06-19T00:00:00Z (GMT) by Neetha Mohan Cherumuttathu H. Suresh
Mono-, di-, and tri-pentafluorobenzyl-substituted hexafluorobenzene (HFB) scaffolds, <i>viz</i>., <b>R</b><sub><b>I</b></sub>, <b>R</b><sub><b>II</b></sub>, and <b>R</b><sub><b>III</b></sub> are proposed as promising receptors for molecules of chemical, biological, and environmental relevance, <i>viz</i>., N<sub>2</sub>, O<sub>3</sub>, H<sub>2</sub>O, H<sub>2</sub>O<sub>2</sub>, F<sup>–</sup>, Cl<sup>–</sup>, BF<sub>4</sub><sup>–</sup>, NO<sub>3</sub><sup>–</sup>, ClO<sup>–</sup>, ClO<sub>2</sub><sup>–</sup>, ClO<sub>3</sub><sup>–</sup>, ClO<sub>4</sub><sup>–</sup>, and SO<sub>4</sub><sup>2–</sup>. The receptor–guest complexes modeled using M06L/6-311++G­(d,p) DFT show a remarkable increase in the complexation energy (<i>E</i><sub>int</sub>) with an increase in the number of fluorinated aromatic moieties in the receptor. Electron density analysis shows that fluorinated aromatic moieties facilitate the formation of large number of lone pair−π interactions around the guest molecule. The lone pair strength of the guest molecules quantified in terms of the absolute minimum (<i>V</i><sub>min</sub>) of molecular electrostatic potential show that <i>E</i><sub>int</sub> strongly depends on the electron deficient nature of the receptor as well as strength of lone pairs in the guest molecule. Compared to HFB, <b>R</b><sub><b>I</b></sub> exhibits 1.1–2.5-fold, <b>R</b><sub><b>II</b></sub> shows 1.6–3.6-fold, and the bowl-shaped <b>R</b><sub><b>III</b></sub> gives 1.8–4.7-fold increase in the magnitude of <i>E</i><sub>int</sub>. For instance, in the cases of HFB···<b>F</b><sup><b>–</b></sup>, <b>R</b><sub><b>I</b></sub>···<b>F</b><sup><b>–</b></sup>, <b>R</b><sub><b>II</b></sub>···<b>F</b><sup><b>–</b></sup>, and <b>R</b><sub><b>III</b></sub>···<b>F</b><sup><b>–</b></sup> the <i>E</i><sub>int</sub> values are −21.1, −33.7, −38.1, and −50.5 kcal/mol, respectively. The results strongly suggest that tuning lone pair−π interaction provides a powerful strategy to design receptors for small molecules and anions.