Titanium Complexes Bearing a Hemilabile Heteroscorpionate Ligand: Synthesis, Reactivity, and Olefin Polymerization Activity

A series of titanium complexes (bpzmp)TiR1R2R3 (bpzmp = (3,5-tBu2-2-phenoxo)bis(3,5-Me2-pyrazol-1-yl)methane; R1 = R2 = R3 = Cl (2); R1 = R2 = R3 = NMe2 (3); R1 = Cl; R2 =
R3 = NMe2 (4); R1 = R2 = Cl; R3= NMe2 (5); R1 = R2 = R3 = Me (6)) has been synthesized
and characterized by VT NMR spectroscopy and X-ray diffraction analysis (2, 3, 5). The
complexes 2−6 adopt in the solid state an octahedral structure in which the bpzmp ligand
is κ3-coordinated to the metal via the phenoxy group and the imino nitrogens of the two
pyrazolyl rings. The investigation of the solution structure of 3 by means of VT 1H NMR
spectroscopy revealed a fluxional behavior of the bpzmp ligand that produces an equilibrium
between the octahedral and tetrahedral form of the titanium complex, the latter resulting
from the η1-coordination of the ligand through exclusively the phenolate group. The
thermodynamic and kinetic parameters of this process were evaluated by VT NMR
spectroscopy. The selective replacement of chloride for dimethylamide in 4 and 5 shifts the
equilibrium toward the octahedral complex, which is the favorite configuration at room
temperature. Site exchange of the nonequivalent methyl groups in 6 was observed at room
temperature in the slow-regime 1H NMR time scale:  ΔH⧧ and ΔS⧧ values of 22.3 ± 1.1
kcal·mol-1 and −19.6 ± 3.7 cal mol-1 K-1 were respectively determined for the isomerization
process occurring with a rate constant of 3 s-1 and ΔG⧧ of 28.1 ± 0.1 kcal·mol-1 at 293 K.
Complexes 2 and 6 are active olefin polymerization catalysts after activation with MAO or
[Ph3C][B(C6F5)4]. Linear polyethylene and atactic polypropylene were obtained in the
polymerization experiments catalyzed by 6-[Ph3C][B(C6F5)4]. Highest polymerization activities
were found with the 2-MAO catalyst, where leaching of the ligand due to MAO excess was
suspected. Reaction of 6 with [Ph3C][B(C6F5)4] in the presence of THF readily produces the
ionic complex [(bpzmp)TiMe2(THF)][B(C6F5)4], proposed as a model of the active species in
this class of olefin polymerization catalysts.