Structure and Reactivity of the β-Agostic [<i>ansa</i>-Cp-arene]Ta(<i>ⁿ</i>Pr) Cation: Ambivalent Behavior Induced by Intramolecular Arene Coordination

Cationic tantalum complexes of the type {η6-Ar-CMe2-η5-C5H4]TaR}+ (Ar = 3,5-dimethylphenyl, R = nPr, H) were prepared. In these complexes, the pendant arene group on the cyclopentadienyl ancillary ligand is coordinated intramolecularly to the metal center. Structural and spectroscopic studies revealed that the arene moiety is partly reduced by the low-valent metal center, leading to ambivalent Ta(III)−Ta(V) behavior. The compounds show significant differences in structure and reactivity to the isoelectronic neutral tantalocene derivatives; for example, the nPr complex has a β-agostic alkyl ground state structure, and the system is able to catalyze the hydrogenation of 1-alkenes; the metal center is also able undergo oxidative addition reactions. The use of an intramolecularly coordinating arene group presents a strategy to add stability of low and intermediate valencies of early transition metals without the use of classic strong π-acceptor ligands.