Anions Bind In A Twist

A novel family of metallohelicate hosts that bind anionic guests internally could potentially be developed as luminescent or redox sensors for anions, according to chemists at Trinity College Dublin.

The hosts, which consist of three helical strands, are [Fe₂L₃]⁴⁺ complexes formed by the coordination of three bis(amido-2,2'-bipyridine) ligands (L) to two Fe(II) atoms. They were designed and synthesized by postdoc Sandrine Goetz and senior lecturer in inorganic and supramolecular chemistry Paul E. Kruger (Dalton Trans., published online Jan. 3, dx.doi.org/10.1039/b514580e).

"We believe this to be the first report detailing the synthesis of metallohelicate complexes designed specifically to bind anions within an intrahelical cavity," Kruger says of the work. "The host complexes are fully soluble in water as their sulfate and perchlorate salts and competitively bind spherical chloride ions in preference to these tetrahedral anions [sulfate and perchlorate] in solution. The chloride ions are locked inside the strands and therefore protected from the environment. The systems are therefore more stable than previously reported metallohelicate host-guest systems that have external anion receptors."

The two iron atoms act as a template in the formation of the triple-helicate complex and introduce positive charges into the ligand scaffold that facilitate electrostatic binding with anions. Nuclear magnetic resonance spectroscopy reveals that, in solution, the host complexes exist as a mixture of helical and nonhelical isomers. Addition of anions, such as chloride, yielded a solution containing only helical isomers.

"We observed binding by electrospray mass spectrometry, which indicates that the anions are tightly bound, and by proton NMR in deuterated dimethyl sulfoxide by following the chemical shifts in the amide and bipyridyl proton resonances on addition of anions," Kruger explains.

The Dublin group is currently extending the work to incorporate other metal ions into the ligand systems.