Formation of Strong Boron Lewis Acid Sites on Silica.

Bis(1-methyl- ortho -carboranyl)borane (HB Me o Cb 2 ) 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 Me o Cb 2 B(OSi≡) ( 1 ) and H 2 . 11 B{ 1 H} 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═PEt 3 (triethylphosphine oxide TEPO) and measuring 31 P{ 1 H} MAS NMR spectra show that 1 preserves the strong Lewis acidity of HB Me o Cb 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 Cp 2 Hf( 13 CH 3 ) 2 show that the Lewis sites are capable of abstracting methide groups from Hf to form [Cp 2 Hf- 13 CH 3 ][H 3 13 C-B( Me o Cb 2 )OSi≡], but with a low overall efficiency.