Donor-free 9,10-dihydro-9,10-dialuminaanthracenes

Abstract

Despite their promising potential, e.g., as ditopic, cooperatively binding Lewis acids, 9,10-dihydro-9,10-dialuminaanthracenes (DAA-R₂; R: terminal Al-bonded substituent) have remained unexplored for long due to the challenges in synthesizing the ligand-free species. We demonstrate that DAA-Me₂ is accessible via the reaction of 1,2-(Me₃Sn)₂C₆H₄ with AlMe₃, producing volatile SnMe₄ as the sole byproduct. In non-coordinating solvents and in the solid state, DAA-Me₂ exists as a dimer (DAA-Me₂)₂. Treatment of (DAA-Me₂)₂ with 4 equiv. AlBr₃ cleaves the dimer, leads to quantitative Me/Br exchange, and forms the double AlBr₃ adduct DAA-Br₂·(AlBr₃)₂. Removal of AlBr₃ with 2,2′-bipyridine gives free DAA-Br₂, which also dimerizes in the absence of bases to form (DAA-Br₂)₂. (DAA-Me₂)₂ and (DAA-Br₂)₂ readily react with mono- (e.g., pyridine) or ditopic Lewis bases (e.g., potassium pyrazolide) to afford trans-diadducts or triptycene-type frameworks. Upon addition of [nBu₄N]Br, DAA-Br₂·(AlBr₃)₂ undergoes selective cleavage of Al–C bonds to produce the Br⁻ chelate complex of 1,2-(Br₂Al)₂C₆H₄, a valuable synthon for 1,2-dideprotonated benzenes.