A strategy for polymerization catalyst design has been developed based on the steric and electronic analogy of bulky phosphinimides to cyclopentadienyl ligands. To this end, the family of complexes of the form (Cp†)TiCl2(NPR3) has been prepared and characterized. Alkyl and aryl derivatives of these species have also been synthesized, and a number have been evaluated for use as catalyst precursors in olefin polymerization. The polymerization of ethylene has been examined employing several types of cocatalyst activators. Trends and patterns in the structure−activity relationship are discussed, and the implications for catalyst design are evaluated.

Triple C−H bond activation in the reaction of [Cp(R3PN)Ti(Me)2] with AlMe3 affords Ti‐Al‐C aggregates. These species 1 and 2 exist in equilibrium with AlMe3, thus demonstrating a highly facile interconversion of carbide species with distorted tetrahedral and five‐coordinate, pseudo‐trigonal‐bipyramidal geometries, respectively. R=iPr.

The reaction of AlMe(3) with (t-Bu(3)PN)(2)TiMe(2) 1 proceeds via competitive reactions of metathesis and C-H activation leading ultimately to two Ti complexes: [(mu(2)-t-Bu(3)PN)Ti(mu-Me)(mu(4)-C)(AlMe(2))(2)](2) 2, [(t-Bu(3)PN)Ti(mu(2)-t-Bu(3)PN)(mu(3)-CH(2))(2)(AlMe(2))(2)(AlMe(3))] 3, and the byproduct (Me(2)Al)(2)(mu-CH(3))(mu-NP(t-Bu(3))) 4. X-ray structural data for 2 and 3 are reported. Compound 3 undergoes thermolysis to generate a new species [Ti(mu(2)-t-Bu(3)PN)(2)(mu(3)-CH(2))(mu(3)-CH)(AlMe(2))(3)] 5. Monitoring of the reaction of 1 with AlMe(3) by (31)P[(1)H] NMR spectroscopy revealed intermediates including (t-Bu(3)PN)TiMe(3) 6. Compound 6 was shown to react with AlMe(3) to give 2 exclusively. Kinetic studies revealed that the sequence of reactions from 6 to 2 involves an initial C-H activation that is a second-order reaction, dependent on the concentration of Ti and Al. The second-order rate constant k(1) was 3.9(5) x 10(-4) M(-1) s(-1) (DeltaH(#) = 63(2) kJ/mol, DeltaS(#) = -80(6) J/mol x K). The rate constants for the subsequent C-H activations leading to 2 were determined to be k(2) = 1.4(2) x 10(-3) s(-1) and k(3) = 7(1) x 10(-3) s(-1). Returning to the more complex reaction of 1, the rate constant for the ligand metathesis affording 4 and 6 was k(met) = 6.1(5) x 10(-5) s(-1) (DeltaH(#) = 37(3) kJ/mol, DeltaS(#) = -203(9) J/mol x K). The concurrent reaction of 1 leading to 3 was found to proceed with a rate constant of k(obs) of 6(1) x 10(-5) s(-1) (DeltaH(#) = 62(5) kJ/mol, DeltaS(#)= -118(17) J/mol x K). Using these kinetic data for these reactions, a stochastic kinetic model was used to compute the concentration profiles of the products and several intermediates with time for reactions using between 10 and 27 equivalents of AlMe(3). These models support the view that equilibrium between 1 x AlMe(3) and 1 x (AlMe(3))(2) accounts for varying product ratios with the concentration of AlMe(3). In a similar vein, similar equilibria account for the transient concentrations of 6 and an intermediate en route to 3. The implications of these reactions and kinetic and thermodynamic data for both C-H bond activation and deactivation pathways for Ti-phosphinimide olefin polymerization catalysts are considered and discussed.

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.

The redox chemistry of phosphinimide-containing group IV metal complexes has been investigated. Reaction of the simple phosphinimide species CpTi(NPR 3 )Cl 2 (R ) Me 1, i-Pr 2) with Mg affords complexes formulated as [CpTiCl(µ-NPR 3 )] 2 (R ) Me 3, i-Pr 4). In contrast, CpTi(NPt-Bu 3 )Cl 2 ( 5) is reduced by Mg to a putative Ti(II) species that can be intercepted by a variety of reagents including 2,3-dimethyl-1,3-butadiene, diphenylacetylene, phenylacetylene, bis(trimethylsilyl)acetylene, ethylene, and propylene to give monometallic metallacyclic complexes. In this fashion, the Ti(IV) metallacycles CpTi(NPt-Bu 3 )(CH 2 C(Me)-C(Me)CH 2 ), 6, CpTi(NPt-Bu 3 )(CPh) 4 , 7, CpTi(NPt-Bu 3 )(C(Ph)CHC(Ph)CH), 8, CpTi(NPt-Bu 3 )-(η 2 -C 2 (SiMe 3 ) 2 ), 9, CpTi(NPt-Bu 3 )(CH 2 ) 4 , 10, CpTi(NPt-Bu 3 )(CH 2 CHMe) 2 , 15, and CpTi(NPt-Bu 3 )(CH 2 ) 2 (CPh) 2 , 16, were prepared. Related intramolecular formation of metallacycle complexes was achieved upon reduction of Cp′Ti(t-Bu 2 (2-C 6 H 4 Ph)PN)Cl 2 (Cp′ ) Cp 18, Cp* 19). The products [Cp′Ti(NPtBu 2 (2-C 6 H 4 Ph)] (Cp′ ) Cp 20, Cp* 21) contained η 6 -interactions between Ti and the 2-phenyl substituent of the biphenyl unit. While Ti(II)-phosphinimide complexes have proven difficult to handle due to their reactivity, an unequivocal example of a Ti(II) species was obtained via reduction of Cp*Ti(NPt-Bu 3 )Cl 2 (11) with Mg in the presence of CO, affording the species Cp*Ti(NPt-Bu 3 )(CO) 2 ( 22). X-ray data for 4,