Formation of Strong Boron Lewis Acid Sites on Silica.

Bis(1-methyl--carboranyl)borane (HBCb) is a very strong Lewis acid that reacts with the isolated silanols present on silica partially dehydroxylated at 700 °C (SiO) to form the well-defined Lewis site CbB(OSi≡) () and H. B{H} magic-angle spinning (MAS) nuclear magnetic resonance (NMR) data of are consistent with that of a three-coordinate boron site. Contacting with O═PEt (triethylphosphine oxide TEPO) and measuring P{H} MAS NMR spectra show that preserves the strong Lewis acidity of HBCb. Hydride ion affinity and fluoride ion affinity calculations using small molecules analogs of also support the strong Lewis acidity of the boron sites in this material. Reactions of with CpHf(CH) show that the Lewis sites are capable of abstracting methide groups from Hf to form [CpHf-CH][HC-B(Cb)OSi≡], but with a low overall efficiency.