Formation of Strong Boron Lewis Acid Sites on Silica

Bis(1-methyl-ortho-carboranyl)borane (HB^MeoCb₂) is a very strong Lewis acid that reacts with the isolated silanols present on silica partially dehydroxylated at 700 °C (SiO_2‑700) 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 HB^MeoCb₂. 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.