Stepwise Acetylene Insertion and Ammonia Activation at a Digallene and Diindene

Sequential [2 + 2] cycloaddition reactions between acetylene and the digallene and diindene compounds (ETer)₂ (E = Ga, In; Ter = 2,6-Dipp₂-C₆H₃; Dipp = 2,6-diisopropylphenyl) are described. Careful control of the reaction conditions leads to selective formation of four- and six-membered rings with 2π E₂C₂ and 4π E₂C₄ cores, respectively. A structural analysis of the heterocycles by single crystal X-ray diffraction suggests limited electronic delocalization within the rings, which is borne out in their reactivity. For example, the six-membered cyclohexadiene analogues exhibit Lewis-acidic behavior and can form stable, isolable adducts with ammonia. Upon heating, these adducts transform into the corresponding bimetallic triel amides with concomitant generation of ethene.