Donor-free 9-aluminafluorenes: molecular structures and reactivity

Abstract

Aluminum-doped polycyclic aromatic hydrocarbons (PAHs) are underexplored despite the broad applications of boron-containing PAHs in areas such as catalysis and optoelectronics. We disclose the donor-free, sterically unprotected 9-methyl-9-aluminafluorene (Me-AlFlu; 2), synthesized by heating a 9,9-dimethyl-9-stannafluorene and AlMe₃ in hexanes. The compound is a dimer, (2)₂, with trans-positioned AlMe substituents in the solid state. In solution, (2)₂ shows a dynamic cis/trans-interconversion rather than a monomer-dimer equilibrium (Tol-d₈, RT). Lewis bases L cleave (2)₂ into monomeric adducts 2·L (L = OEt₂, thf, pyridine). Lewis acidic AlBr₃ transforms (2)₂ into a 2,2′-(Br₂Al)₂-1,1′-biphenyl (3), crystallographically characterized as dimeric (3)₂. (3)₂ is a synthetic equivalent for the elusive free Br-AlFlu: Treatment with donor molecules furnishes Br-AlFlu·L adducts (L = OEt₂, pyridine); the three-coordinate, monomeric aluminafluorene Mes*-AlFlu was prepared from (3)₂, Mes*Li, and a 2,2′-dilithio-1,1′-biphenyl in quantitative yield (Mes* = 2,4,6-(tBu)₃C₆H₂).