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

Abstract

Multiple C−H bond activation occurs upon reaction of phosphinimide complexes of the form Cp‘(R3PN)TiMe2 (Cp‘ = Cp, indenyl; R = i-Pr, Cy, Ph) with excess AlMe3, affording the carbide complexes Cp‘Ti(μ2-Me)(μ2-NPR3)(μ4-C)(AlMe2)3 or in some cases [CpTi(μ2-Me)(μ2-NPR3)(μ5-C)(AlMe2)3·(AlMe3)]. 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 PMe3. This affords species of the form [CpTi(μ2-Me)(μ2-NPR3)(μ4-C)(AlMe2)2(AlMe2(L))] (L = Et2O, THF, PMe3). The Lewis acidity is also evidenced in the reaction of the carbide complexes with CpTi(NPR3)Me2. In this case, labeling studies affirm methyl group exchange processes. The analogous reactions of Cp(R3PN)Ti(CH2SiMe3)2 or Cp*(R3PN)TiMe2 with AlMe3 afforded CpTi(μ2-Me)(μ2-NPR3)(μ3-CSiMe3)(AlMe2)2 and Cp*Ti(μ2-Me)(μ2-NPR3)(μ3-CH)(AlMe2)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.