Cyclotribenzoin Esters Bind Anions via Aromatic and Aliphatic C–H Bonds

Abstract

We report anion binding by cyclotribenzoin ester macrocycles. Three cyclotribenzoin esters were readily prepared via benzoin condensation followed by esterification: two derived from isophthalaldehyde and the third from 2,7-diformylnaphthalene. All macrocycles were isolated as the all-cis diastereomers with the three aromatic panels at ∼90° angles to one another, and these structures were confirmed by single-crystal X-ray diffraction. Cyclotribenzoin esters are capable of binding anions─using exclusively [C–H···anion] interactions─because of their cone-shaped geometry, which convergently positions aromatic and aliphatic C–H donors facing the anionic guest. The enhanced acidity of those C–H bonds is brought about by the adjacent ketone and ester groups, and it helps anion binding. Both phenylene- and naphthylene-based macrocycles bind anions with a 1:1 stoichiometry, showing the highest affinity for chloride. The smaller phenylene-based receptor selectively binds only anions that fit its narrow pocket─chloride > bromide > iodide > nitrate. The larger naphthalene-based receptor also accommodates perchlorate, thiocyanate, phosphate, and hexafluorophosphate and exhibits higher affinities for bromide, iodide, and nitrate on account of its larger, more polarizable top rim and three additional aromatic C–H functionalities. The solid-state structure of cyclotribenzoin pivaloate with Cl– reveals [C–H···Cl–] distances between 2.6 and 3.3 Å.