Monomeric Cp(3t)Al(i): synthesis, reactivity, and the concept of valence isomerism

With the isolation of Cp(3t)Al (1), the first monomeric Cp-based Al(i) species could be realized in a pure form via a three-step reaction sequence (salt elimination/adduct formation/adduct cleavage) starting from readily available AlBr(3). Due to its monomeric structure, reactions involving 1 were found to proceed more selectively, faster, and under milder conditions than for tetrameric (Cp*Al)(4). Thus, 1 readily formed simple Lewis acid–base adducts with tBuAlCl(2) (6) and AlBr(3) (7), reactions that before have always been interfered with by the presence of aluminum halide bonds. In addition, the 2 : 1 reaction of 1 with AlBr(3) enabled the realization of the very rare trialuminum adduct species 8. 1 also reacted rapidly with N(2)O and PhN(3) at room temperature to afford Al(3)O(3) and Al(2)N(2) heterocycles 9 and 10, respectively. With the structural characterization of products 4 and 5, the reaction of monovalent 1 with Cp(3t)AlBr(2) (2) provided the first experimental evidence for the concept of valence isomerism between dialanes and their Al(i)/Al(iii) Lewis adducts.