Formation of Strong Boron Lewis Acid Sites on Silica

Bis(1-methyl-ortho-carboranyl)borane
(HBMeoCb2) is a very strong
Lewis acid that
reacts with the isolated silanols present on silica partially dehydroxylated
at 700 °C (SiO2‑700) to form the well-defined
Lewis site MeoCb2B(OSi)
(1) and H2. 11B{1H}
magic-angle spinning (MAS) nuclear magnetic resonance (NMR) data of 1 are consistent with that of a three-coordinate boron site.
Contacting 1 with OPEt3 (triethylphosphine
oxide TEPO) and measuring 31P{1H} MAS NMR spectra
show that 1 preserves the strong Lewis acidity of HBMeoCb2. Hydride ion affinity and
fluoride ion affinity calculations using small molecules analogs of 1 also support the strong Lewis acidity of the boron sites
in this material. Reactions of 1 with Cp2Hf(13CH3)2 show that the Lewis sites are
capable of abstracting methide groups from Hf to form [Cp2Hf–13CH3][H313C–B(MeoCb2)OSi],
but with a low overall efficiency.