Multiple C-H bond activation: Reactions of titanium-phosphinimide complexes with trimethylaluminum

Multiple C-H bond activation occurs upon reaction of phosphinimide complexe s of the form Cp'(R3PN)TiMe2 (Cp' = Cp, indenyl; R = i-Pr, Cy, Ph) with exc ess AlMe3, affording the carbide complexes Cp'Ti(mu (2)-Me)(mu (2)-NPR3)(mu (4)-C)(AlMe2)(3) or in some cases [CpTi(mu (2)-Me)(mu (2)-NPR3)(mu (5)-C)( AlMe2)(3). (AlMe3)]. These species contain four- and five-coordinate carbid e centers. VT-NMR studies established that such species exist in equilibriu m. The four-coordinate carbide complexes retain Lewis acidity at a planar t hree-coordinate Al center, as evidenced by the reaction with diethyl ether, THF, or PMe3. This affords species of the form [CpTi-(mu (2)-Me)(mu (2)-NP R3)(mu (4)-C)(AlMe2)(2)(AlMe2(L))(L = Et2O, THF, PMe3). The Lewis acidity i s also evidenced in the reaction of the carbide complexes with CpTi(NPR3)Me -2. In this case, labeling studies affirm methyl group exchange processes. The analogous reactions of Cp(R3PN)Ti-(CH2SiMe3)(2) or Cp*(R3PN)TiMe2 with AlMe2 afforded CpTi(mu (2)-Me)(mu (2)-NPR3)(mu (3)-CSiMe3)-(AlMe2)2 and Cp* Ti(mu (2)-Me)(mu (2)-NPR3)(mu (3)-CH)(AlMe2)(2), respectively. These observ ations confirm that steric congestion can impinge on the C-H activation pro cess. The nature of the above products of C-H bond activation was confirmed employing NMR, isotopic labeling, and crystallographic methods. The implic ations of these results with respect to C-H bond activation and polymerizat ion catalysis are considered.