Donor-free 9-aluminafluorenes: molecular structures and reactivity.

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 AlMe in hexanes. The compound is a dimer, (2), with -positioned Al substituents in the solid state. In solution, (2) shows a dynamic /-interconversion rather than a monomer-dimer equilibrium (Tol-, RT). Lewis bases L cleave (2) into monomeric adducts 2·L (L = OEt, thf, pyridine). Lewis acidic AlBr transforms (2) into a 2,2'-(BrAl)-1,1'-biphenyl (3), crystallographically characterized as dimeric (3). (3) is a synthetic equivalent for the elusive free Br-AlFlu: Treatment with donor molecules furnishes Br-AlFlu·L adducts (L = OEt, pyridine); the three-coordinate, monomeric aluminafluorene Mes*-AlFlu was prepared from (3), Mes*Li, and a 2,2'-dilithio-1,1'-biphenyl in quantitative yield (Mes* = 2,4,6-(Bu)CH).