Mono(aryloxido)Titanium(IV) Complexes and Their Application in the Selective Dimerization of Ethylene

We report on the synthesis of mono(aryloxido)titanium(IV) complexes of general formula {Ti[O(o-R)Ar]X(3)} with X = OiPr, ArO = 2-tert-butyl-4-methylphenoxy and R = CMe(3) (2a), CMe(2)Ph (2b) and CH(2)NMe(2) (2c). Attempts to reach pure mono(aryloxido) complexes when R = CH(2)NMe(CH(2)Ph) (2d) or CH(2)N(CH(2)Ph)(2) (2e) were unsuccessful. When R = CH(2)OMe, the analogous mononuclear complex was not obtained, and instead, a dinuclear complex [(2-tert-butyl-4-methyl-6-methoxymethylphenoxy) TiCl(OiPr)(mu(2)-OiPr)(2)TiCl-(OiPr)(2)] (3) was formed. Complexes 2b and 3 were characterized by single-crystal X-ray diffraction. The former contains a tetrahedrally coordinated Ti(IV) centre, whereas in the latter the aryloxido ligand behaves as a chelating-bridging ligand between the two, chemically very different metal centres that form two face-sharing octahedra. Different synthetic approaches starting from [Ti(OiPr)(4)] or [TiCl(OiPr)(3)] were evaluated and are discussed. The hemilabile behaviour of the aryloxido ligand resulting from reversible coordination of its side arm was studied by variable-temperature (1)H NMR spectroscopy for 2c (R = CH(2)NMe(2)). Complexes 2a-d were contacted with ethylene and AlEt(3) as cocatalyst. When activated with AlEt(3) (3 equiv.) at 20 bar and 60 degrees C, complex 2c exhibits interesting activity (2100 g/gTi/h) for the selective dimerization of ethylene to 1-butene (92% C(4)(=); 99(+)% C(4)(=1)). Noticeable differences in catalyst activity were observed when the R group was modified. ((C) Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2009)