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Chemical Bonding

Two-faced polar molecule grabs both cations and anions

All-cis hexafluorocyclohexane rivals crown ethers in cation binding capability, sets new precedent for anion binding

by Stephen K. Ritter
May 12, 2016 | APPEARED IN VOLUME 94, ISSUE 20

Credit: J. Am. Chem. Soc.
The fluorine face of all-cis hexafluorocyclohexane binds sodium ions, whereas its hydrogen face binds chloride ions.

Chemists have found that the all-cis isomer of hexafluorocyclohexane, a molecule that made headlines last year for being the most polar aliphatic compound known, is capable of binding both cations and anions. What’s more, the ionic binding is among the strongest ever measured.

All-cis hexafluorocyclohexane, synthesized by David O’Hagan and coworkers of the University of St. Andrews, is remarkable for having its six fluorine atoms aligned on one face of the cyclohexane ring and its six hydrogen atoms aligned on the opposite face. This arrangement endows the molecule with a large dipole moment that is surpassed only by some ionic compounds and lopsided aromatic compounds.

A team led by O’Hagan and Terrance B. McMahon of the University of Waterloo has now reported the molecule’s propensity to bind cations with its fluorine face and anions with its hydrogen face. The researchers used electrospray ionization to generate gas-phase mono- and dicoordinated sodium and chloride complexes of all-cis hexafluor­o­cyclohexane and mass spectrometry to separate the complexes. They further used infrared multiple photon dissociation spectroscopy and computational methods to assess the ion binding energy (J. Am. Chem. Soc. 2016, DOI: 10.1021/jacs.6b02856).

With the exception of crown ethers and the most basic amino acids, no molecular species have been found to exceed the compound’s sodium ion binding strength, the researchers note. And no neutral organic species are known with greater chloride ion binding strength, they say. The team suggests that one thing all-cis hexafluorocyclohexane’s prowess will enable it to do is deliver cations and anions in both polar and nonpolar media.

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Comments
Ebrahim H Ghazvini Zadeh (May 13, 2016 4:37 PM)
That is impressive! Simple but elegant. I can only think of new structures with larger circumference that can bind larger ions, both cations and anions.. the applications can go beyond the sensing, besides, the arrangement would be very interesting to study in an x-ray..
Motlatsi Phali (May 26, 2016 8:40 AM)
Impressive , this will definitely make the ion chromatography world exciting.
Mark Mascal (June 2, 2016 11:33 AM)
Does this solubilize NaCl in organic solvents? I don't see it in the paper.
Steve Ritter (June 10, 2016 1:29 PM)
The researchers say they have not looked at this...however the lattice energy of NaCl is very high ~180kcal mole-1 which has to be overcome, which might be a major barrier

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