Crystal Structures of Tris(tert-butyl)boron, -aluminum, -gallium, and -indium: Nonplanarity of the AlC3Skeleton and Evidence of Inter- and Intramolecular “Agostic” or Hyperconjugative Interactions

Abstract

X-ray crystal structures determined at low temperatures (150−220 K) for the tris(tert-butyl) derivatives of boron, aluminum, gallium, and indium (M) reveal essentially monomeric molecular units throughout with consistently longer M−C bonds than in the corresponding monomeric trimethyl derivatives. Comparison of the three structures shows a significant strengthening of intermolecular M···β-CH3 binding in the order M = B ∼ Ga < In < Al, resulting in a distinctly nonplanar MC3 skeleton with the M atom displaced 0.25 A above the plane of the quaternary C atoms in the case where M = Al. Tilting of the tert-butyl groups about the M−C bonds, which is concerted when M = B or Al, appears to reflect the influence of intramolecular hyperconjugation or “agostic” bonding.