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

With the isolation of Cp^3tAl (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 AlBr₃. 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 tBuAlCl₂ (6) and AlBr₃ (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 AlBr₃ enabled the realization of the very rare trialuminum adduct species 8. 1 also reacted rapidly with N₂O and PhN₃ at room temperature to afford Al₃O₃ and Al₂N₂ heterocycles 9 and 10, respectively. With the structural characterization of products 4 and 5, the reaction of monovalent 1 with Cp^3tAlBr₂ (2) provided the first experimental evidence for the concept of valence isomerism between dialanes and their Al(i)/Al(iii) Lewis adducts.