ansa-Cyclopentadienyl-Arene Tantalum Complexes: Structure and Reactivity of Neutral, Cationic, and Dicationic Derivatives

The cationic tantalum complex {[eta(6)-Ar-CMe2-eta(5)-C5H4]TaPr}[B(C6F5)(4)] (1; Ar = 3,5-Me2C6H3) serves as a starting material for a series of neutral, monocationic, and dicationic derivatives. The cationic hydride {[eta(6)-Ar-CMe2-eta(5)-C5H4]TaH}[B(C6F5)(4)] (2) that results from hydrogenolysis of 1 inserts the di- and trisubstituted olefins cyclopentene and 2-methyl-2-pentene; it reacts with styrene to give the 2,1-insertion product, for which the Ta-CH(Me)Ph group is bound in a sigma(3)-allylic fashion. A neutral complex is obtained from 1 by reaction with Br-, and a dicationic derivative is available by hydride abstraction from 2 using the Lewis acidic trityl cation. All compounds described here retain the unusual ansa-(eta(5)-cyclopentradienyl,eta(6)-arene) coordination mode of the ligand that stabilizes the formally Ta(III) center. X-ray structures and DFT calculations show that the metal-arene interaction contains a significant pi back-donation component (arene