H-Bonded anion-anion complex trapped in a squaramido-based receptor.

Herein we report the experimental observation (X-ray characterization) of an anion-anion complex (anion = hydrogen fumarate) stabilized by H-bonds that is trapped in a secondary squaramide receptor. High level ab initio calculations indicate that the anion-anion complex is thermodynamically unstable but kinetically stable with respect to the isolated anions.