Optimization of ethylene trimerization using catalysts based on TiCl3/half-sandwich ligands

Catalytic trimerization of ethylene using three titanium-based complexes [eta(5)-C9H6C(R)thienyl]TiCl3 with various types of bridges (R = cyclo-C5H10 (C1), cyclo-C4H8 (C2) and (CH3)(2) (C3)) has been successfully optimized and compared. First of all, three benzofulvene precursors, C9H6C(R), were synthesized. Then the corresponding indenyl-based ligands were obtained via the reaction of the precursors with thienyllithium. The final titanium-based catalysts display a distorted tetrahedral geometry, as expected for Ti(IV), with the ligand coordinated with a hemilabile behaviour. The structures of the compounds were confirmed on the basis of various analyses. The effect of catalyst concentration, ethylene pressure, reaction temperature and nature of the bridge as the significant factor affecting coordination and orientation of thienyl group relative to the metal centre on 1-hexene (1-C-6) productivity and selectivity was investigated. Results revealed that the bulky bridge groups such as cyclo-C5H10 and cyclo-C4H8 are appropriate for ethylene trimerization due to the closer coordination of sulfur atom with Ti, especially in cationic state, and catalyst C2 with cyclo-C4H8 bridge exhibits moderate productivity equal to 785 kg 1-C-6 (mol Ti)(-1) h(-1). According to the results, ethylene at a pressure of 10 bar, 50 degrees C and 1.5 mu mol of catalyst were selected as the best conditions for obtaining 1-C-6 with high productivity and selectivity. The presence of indenyl enhances the thermal stability of the catalysts and preserves their activity in higher temperatures such as 50 and 80 degrees C.