Formation of Strong Boron Lewis Acid Sites on Silica
Kavyasripriya K. Samudrala, Manjur O. Akram, Jason L. Dutton, Caleb D. Martin, Matthew P. Conley
Abstract
High Resolution Image Download MS PowerPoint Slide 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.