Donor-free 9-aluminafluorenes: molecular structures and reactivity

Dalton Trans. 2024 Dec 17. doi: 10.1039/d4dt03148b. Online ahead of print.

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).