Cone Angles Quantify and Predict the Affinity and Reactivity of Anion Complexes between Trifluoroborates and Rigid Macrocycles

• Published in: Angewandte Chemie International Edition 2024-09, Vol.63 (39), p.e202409070-n/a
• Main Authors: Zhang, Zhao, Sheetz, Edward G., Pink, Maren, Yamamoto, Nobuyuki, Flood, Amar H.
• Format: Article
• Language: English
• Subjects: Affinity; Anion binding; Anion exchanging; Anions; Binding; Cone angle; Correlation; Cyanostar; Fluorides; Free energy; Hydrogen atoms; Lewis acid; Linear correlation; Reactivity
• ISSN: 1433-7851; 1521-3773; 1521-3773
• DOI: 10.1002/anie.202409070

Steric manipulation is a known concept in molecular recognition but there is currently no linear free energy relationship correlating sterics to the stability of receptor‐anion complexes nor to the reactivity of the bound anion. By analogy to Tolman cone angles in cation coordination chemistry, we explore how to define and correlate cone angles of organo‐trifluoroborates (R−BF3−) to the affinities observed for cyanostar‐anion binding. We extend the analogy to a rare investigation of the anion's reactivity and how it changes upon binding. The substituent on the anion is used to define the cone angle, θ. A series of 10 anions were studied including versions with ethynyl, ethylene, and ethyl substituents to tune steric bulk across the sp, sp2 and sp3 hybridized α‐carbons bearing 0, 1 and 2 hydrogen atoms. A linear relationship between affinity and cone angle is observed for anions bearing substituents larger than the −BF3− headgroup. This correlation predicted affinities of two new anions to within ±5 %. We explored how complexation affects the reactivity of fluoride exchange. The yield of fluoride transfer from R−BF3− to Lewis acid triphenylborane is correlated with cone angle. We predict that other rigid macrocycles, like commercially available bambusuril, could follow these trends. Cone angles of anions correlate with the affinity and reactivity of their complexes akin to Tolman cone angles seen in cation coordination. This behavior is exemplified with binding rigid organo‐trifluoroborate (R−BF3−) anions to rigid cyanostar macrocycles and expected to be general for other rigid pairs. Use of cone angles allow predictions, with affinities agreeing to within 5 %.