Ph₃P-BH₃中的路易斯酸-碱相互作用

Lewis acid-base theory defines acids as electron pair acceptors and bases as electron pair donors, while Brønsted-Lowry theory focuses on proton donation and acceptance. Lewis theory expands beyond aqueous solutions and encompasses metal ions and main-group compounds that cannot be explained by the Brønsted-Lowry model, making it more broadly applicable to inorganic chemistry.

Triphenylphosphine contains a lone pair of electrons on the phosphorus atom residing in an sp³ orbital. This lone pair can be donated to another molecule, which is the defining characteristic of a Lewis base. The sp³ hybridization at the phosphorous center creates a tetrahedral electronic geometry that stabilizes this electron pair for donation.

Borane is electron-deficient, containing only six valence electrons around the boron center instead of the typical octet. It has an empty p orbital with trigonal planar geometry and sp² hybridization. This empty orbital is ready to accept an electron pair from a Lewis base, which defines borane as a Lewis acid.

A coordinative covalent bond, also called a dative bond, forms between triphenylphosphine and borane. This bond is indicated with an arrow and results from the donation of the phosphorus lone pair into borane's empty p orbital. The formation of this bond changes borane's hybridization from sp² to sp³, creating a stable Lewis acid-base adduct.

Free triphenylphosphine shows a ³¹P NMR signal at -5.43 ppm, while the Ph₃P-BH₃ complex shows a downfield shift to 20.7 ppm. This deshielding of the phosphorus nucleus occurs because electron density is removed from the phosphorus center upon coordination to borane, confirming the formation of the Lewis acid-base adduct.

Close the pressure release valve and turn on the N₂ and vacuum pump before assembling the cold trap with dry ice/acetone. This minimizes the risk of O₂ condensation in the trap, which is explosive in the presence of organic solvents. Inspect all glassware for star cracks and wear appropriate personal protective equipment throughout the synthesis using the synthesis of a ti iii metallocene using schlenk line technique.

Frustrated Lewis pairs are Lewis acid-base combinations that cannot form a dative bond due to steric hindrance between the acid and base. Unlike typical Lewis acid-base adducts that form stable coordinative covalent bonds, frustrated Lewis pairs remain reactive and have found applications in developing new hydrogenation catalysts and other transformations.