Group 5 metal complexes with cyclooctatetraene: Formation and characterization of (butadiene)(cyclooctatetraene)(cyclopentadienyl)tantalum systems

The [(eta(4)-diene)(eta(5)-Cp-R)TaCl2] complexes 6a-e (diene = butadiene, i soprene or 2,3-dimethylbutadiene; Cp-R = pentamethylcyclopentadienyl, methy lcyclopentadienyl or cyclopentadienyl) react with cyclooctatetraene dianion (employed as [(cot)Li-2]) to yield the (eta(4)-diene)(eta(3)-cyclooctatetr aene)(eta(5)-Cp-R)tantalum complexes 7. The eta(3)-cot coordination was con firmed by an X-ray crystal structure analysis of the compound [(supine-s-ci s-eta(4)-C4H6)(eta(3)-cot)(eta(5)-Cp*)Ta] 7a. In solution, the cot ligand o f all the complexes 7 is fluxional. Two isomers [supine and prone orientati on of the (diene)CpTa subunit] were found for the (eta(4)-butadiene)(eta(3) -cot)CpTa system (7e/7e' = 95:5). Complex 7e cleanly adds the organometalli c Lewis acid B(C6F5)(3) to yield the betaine system [(eta(4)-C4H6){C8H8B(C6 F5)(3)}Cp*Ta] 8, which contains a substituted bicyclo[5.1.0]octadienyl liga nd.