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

Abstract

With the isolation of Cp^3tAl (<b>1</b>), the first monomeric Cp-based Al(i) species could be realized in a pure form <i>via</i> a three-step reaction sequence (salt elimination/adduct formation/adduct cleavage) starting from readily available AlBr₃. Due to its monomeric structure, reactions involving <b>1</b> were found to proceed more selectively, faster, and under milder conditions than for tetrameric (Cp*Al)₄. Thus, <b>1</b> readily formed simple Lewis acid-base adducts with <i>t</i>BuAlCl₂ (<b>6</b>) and AlBr₃ (<b>7</b>), reactions that before have always been interfered with by the presence of aluminum halide bonds. In addition, the 2 : 1 reaction of <b>1</b> with AlBr₃ enabled the realization of the very rare trialuminum adduct species <b>8</b>. <b>1</b> also reacted rapidly with N₂O and PhN₃ at room temperature to afford Al₃O₃ and Al₂N₂ heterocycles <b>9</b> and <b>10</b>, respectively. With the structural characterization of products <b>4</b> and <b>5</b>, the reaction of monovalent <b>1</b> with Cp^3tAlBr₂ (<b>2</b>) provided the first experimental evidence for the concept of valence isomerism between dialanes and their Al(i)/Al(iii) Lewis adducts.