Formation of Strong Boron Lewis Acid Sites on Silica

Bis(1-methyl-ortho-carboranyl)borane (HB(Me)oCb(2)) is a very strong Lewis acid that reacts with the isolated silanols present on silica partially dehydroxylated at 700 degrees C (SiO2-700) to form the well-defined Lewis site (Me)oCb(2)B(OSi equivalent to) (1) and H-2. B-11{H-1} 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 & boxH;PEt3 (triethylphosphine oxide TEPO) and measuring P-31{H-1} MAS NMR spectra show that 1 preserves the strong Lewis acidity of HB(Me)oCb(2). 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((CH3)-C-13)(2) show that the Lewis sites are capable of abstracting methide groups from Hf to form [Cp2Hf-(CH3)-C-13][(H3C)-C-13-B((Me)oCb(2))OSi equivalent to], but with a low overall efficiency.