Triarylborane Lewis acids with indole or phenol group: B/H hybrid receptors for fluoride

• Novel boron/hydrogen bond (B/H) hybrid receptors were prepared.
• Fluoride binding affinity was enhanced by one order of magnitude.
• The dative B–F interaction was assisted by X–H…F hydrogen bonding (X = N, O).

Triarylboranes bearing an indole ((indol-7-yl)Mes2B, 1) or phenol ((phenol-2-yl)Mes2B, 2) group were prepared and characterized as a novel class of boron/hydrogen bond (B/H) hybrid receptors for fluoride. X-ray crystal structures revealed the presence of proximal X–H bond (X = N, O) to the trigonal boron center. UV/Vis absorption titrations of 1 and 2 with fluorides resulted in the corresponding fluoride adduct with high binding constants (4.0 × 107 M−1 in DMSO for 1 and 3.0 × 107 M−1 in THF for 2). Comparative fluoride binding experiments with the triarylboranes lacking an X–H bond showed the enhanced fluoride binding affinity of 1 and 2 with an increase of one order of magnitude. 1H NMR spectroscopic titrations confirmed that the dative B–F interaction was assisted by X–H…F hydrogen bonding.

Boron/hydrogen bond (B/H) hybrid receptors with indole or phenol group exhibited the enhanced fluoride binding affinity due to the additional X–H…F (X = N, O) hydrogen bonding that assists the dative B–F interaction.Figure optionsDownload as PowerPoint slide