The intramolecular hydrogen bonded–halogen bond: a new strategy for preorganization and enhanced binding

Natural and synthetic molecules use weak noncovalent forces to preorganize structure and enable remarkable function. Herein, we introduce the intramolecular hydrogen bonded–halogen bond (HB–XB) as a novel method to preorganize halogen bonding (XBing) molecules, while generating a polarization-enhanced XB. Positioning a fluoroaniline between two iodopyridinium XB donors engendered intramolecular hydrogen bonding (HBing) to the electron-rich belt of both XB donors. NMR solution studies established the efficacy of the HB-XB. The receptor with HB–XBs (G2XB) displayed a nearly 9-fold increase in halide binding over control receptors. Gas-phase density functional theory conformational analysis indicated that the amine stabilizes the bidentate conformation. Furthermore, gas-phase interaction energies showed that the bidentate HB–XBs of G2XBme(2+) are more than 3.2 kcal mol(–1) stronger than the XBs in a control without the intramolecular HB. Additionally, crystal structures confirm that HB–XBs form tighter contacts with I(–) and Br(–) and produce receptors that are more planar. Collectively the results establish the intramolecular HB–XB as a tractable strategy to preorganize XB molecules and regulate XB strength.