Multiple C−H Bond Activation: Reactions of Titanium−Phosphinimide Complexes with Trimethylaluminum

Multiple C−H bond activation occurs upon reaction of phosphinimide complexes of the form Cp‘(R 3 PN)TiMe 2 (Cp‘ = Cp, indenyl; R = i- Pr, Cy, Ph) with excess AlMe 3, affording the carbide complexes Cp‘Ti(μ 2 -Me)(μ 2 -NPR 3 )(μ 4 -C)(AlMe 2 ) 3 or in some cases [CpTi(μ 2 -Me)(μ 2 -NPR 3 )(μ 5 -C)(AlMe 2 ) 3 ·(AlMe 3 )]. These species contain four- and five-coordinate carbide centers. VT-NMR studies established that such species exist in equilibrium. The four-coordinate carbide complexes retain Lewis acidity at a planar three-coordinate Al center, as evidenced by the reaction with diethyl ether, THF, or PMe 3 . This affords species of the form [CpTi(μ 2 -Me)(μ 2 -NPR 3 )(μ 4 -C)(AlMe 2 ) 2 (AlMe 2 (L))] (L = Et 2 O, THF, PMe 3 ). The Lewis acidity is also evidenced in the reaction of the carbide complexes with CpTi(NPR 3 )Me 2 . In this case, labeling studies affirm methyl group exchange processes. The analogous reactions of Cp(R 3 PN)Ti(CH 2 SiMe 3 ) 2 or Cp*(R 3 PN)TiMe 2 with AlMe 3 afforded CpTi(μ 2 -Me)(μ 2 -NPR 3 )(μ 3 -CSiMe 3 )(AlMe 2 ) 2 and Cp*Ti(μ 2 -Me)(μ 2 -NPR 3 )(μ 3 -CH)(AlMe 2 ) 2, respectively. These observations confirm that steric congestion can impinge on the C−H activation process. The nature of the above products of C−H bond activation was confirmed employing NMR, isotopic labeling, and crystallographic methods. The implications of these results with respect to C−H bond activation and polymerization catalysis are considered.