Titanium Complexes Bearing a Hemilabile Heteroscorpionate Ligand: Synthesis, Reactivity, and Olefin Polymerization Activity Stefano Milione, † Valerio Bertolasi, ‡ Toma ´ s Cuenca, § and Alfonso Grassi* ,† Dipartimento di Chimica, Universita ` di Salerno, 84081 Baronissi, Italy, Centro di Strutturistica Diffrattometrica and Dipartimento di Chimica, Universita ` di Ferrara, 44100 Ferrara, Italy, and Departamento de Quı ´mica Inorga ´ nica, Universidad de Alcala ´ , Campus Universitario, Edificio de Farmacia, 28871 Alcala ´ de Henares, Spain Received January 31, 2005 A series of titanium complexes (bpzmp)TiR 1 R 2 R 3 (bpzmp ) (3,5- t Bu 2 -2-phenoxo)bis(3,5- Me 2 -pyrazol-1-yl)methane; R 1 ) R 2 ) R 3 ) Cl (2); R 1 ) R 2 ) R 3 ) NMe 2 (3); R 1 ) Cl; R 2 ) R 3 ) NMe 2 (4); R 1 ) R 2 ) Cl; R 3 ) NMe 2 (5); R 1 ) R 2 ) R 3 ) 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 κ 3 -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 1 H 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 η 1 -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 1 H NMR time scale: ΔH q and ΔS q 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 q 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 [Ph 3 C][B(C 6 F 5 ) 4 ]. Linear polyethylene and atactic polypropylene were obtained in the polymerization experiments catalyzed by 6-[Ph 3 C][B(C 6 F 5 ) 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 [Ph 3 C][B(C 6 F 5 ) 4 ] in the presence of THF readily produces the ionic complex [(bpzmp)TiMe 2 (THF)][B(C 6 F 5 ) 4 ], proposed as a model of the active species in this class of olefin polymerization catalysts. Introduction Olefin polymerization catalysis has been widely domi- nated in the past decades by group 4 metallocenes, which allow the synthesis of stereoregular poly-1-olefins with properties rivalling those of the polymers by classical heterogeneous Ziegler-Natta catalysts. 1 Re- cently research efforts have been produced to enlarge the typology of the ligands used in the coordination chemistry of the group 4 metals, opening the so-called postmetallocene era. 2 In this framework considerable attention has been paid to mono- and polydentate ligands bearing oxygen and nitrogen donors. These ligands are synthesized in a simple manner and modi- fied by well-established synthetic procedures, yielding group 4 metal complexes with tunable electronic proper- ties, flexible coordination geometry, and coordination number higher than four, typically found in metal- locenes. Examples of these ligands include bis-phenoxy- imine, 3 amidinate, 4 and pyrazolylborate. 5 Some recent reports highlighted the olefin polymerization perfor- mances of titanium complexes of the tris(pyrazolyl)- borate ligand carrying sterically demanding substitu- * To whom correspondence should be addressed. E-mail: agrassi@ unisa.it. † Universita ` di Salerno. ‡ Universita ` di Ferrara. § Universidad de Alcala ´. (1) (a) Bochmann, M. J. Chem. Soc., Dalton Trans. 1996, 255. (b) Brintzinger, H.-H.; Fischer, D.; Mu ¨ lhaupt, R.; Rieger, B.; Waymouth, R. M. Angew. Chem., Int. Ed. Engl. 1995, 34, 1143. (c) Marks, T. J. Acc. Chem. Res. 1992, 25, 57. (d) Coates, G. W. Chem. Rev. 2000, 100, 1223. (2) (a) Britovsek, G. J. P.; Gibson, V. C.; Wass, D. F. Angew. Chem., Int. Ed. 1999, 38, 428. (b) Gibson, V. C.; Spitzmesser, S. K. Chem. Rev. 2003, 103, 283. (3) (a) Matsui, S.; Tohi, Y.; Mitani, M.; Saito, J.; Makio, H.; Tanaka, H.; Nitabaru, M.; Nakano, T.; Fujita, T. Chem. Lett. 1999, 1065. 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