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.