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.