Tuning Secondary-Sphere\nLewis Acidity via Late-Stage\nModification of an Appended Borane

To develop synthetic strategies to construct ligands\ncontaining\nsecondary-sphere acids, we demonstrate that an appended borane of\nlow Lewis acidity (−BPin) can be upgraded to a strong Lewis\nacid (−BF<sub>2</sub>). Using a pyridine–pyrazole ligand\ncoordinated to Mo(CO)<sub>4</sub>, we show that a pendent −BPin\ngroup undergoes exhaustive fluorination to −BF<sub>3</sub>K,\na precursor to a highly acidic −BF<sub>2</sub> unit (acceptor\nnumber ∼15× greater than that of −BPin).