Bis(Imido) Tungsten Complexes: Efficient Precatalysts for the Homogeneous Dimerization of Ethylene

The ethylene dimerization performance of a range of bis(imido)tungsten precatalysts, after activation with aluminum cocatalysts, has been assessed. The best-performing system achieved an activity of 409？410 (mol C2H4)(mol W)？1h–1 [79.6 wt % 1-butene] with a 1-butene selectivity of 82.4 wt % [290？770 (mol C2H4)(mol W)？1h–1 activity]. In addition to selected previously reported variants, the symmetrical bis(imido)tungsten(VI) complexes [WCl2(NPh)2(tmeda)] (3) and [WCl2(NR)2(dme)] [R = 2,4,6-trifluorophenyl, 7; 2,4,6-triphenylphenyl, 8; 2,4,6-tris(trifluoromethyl)phenyl, 9], were prepared. A series of unsymmetrical bis(imido)tungsten(VI) complexes [WCl2(NDipp)(NR)(dme)] (R = iPr, 11; tBu, 12; 2,4,6-trimethylphenyl, 13; 2,4,6-trichlorophenyl, 14; 2,4,6-trifluorophenyl, 15; 2,6-dinitrophenyl, 16; 2,4,6-triphenylphenyl, 17) were also synthesized using an improved version of a previous preparative route that eases purification. The molecular structures of complexes 5–13 and 15–17 were determined by X-ray crystallographic analysis. Catalysis employing complexes 1–17 unveils a complicated relationship between steric and electronic factors that affects activity and catalyst lifetime; however, selectivity is dominated by electronic considerations. Higher reaction temperatures were shown to favor 1-butene selectivity and higher activity, while replacement of EtAlCl2 with MeAlCl2 as co-initiator was found to be detrimental. Tungsten(IV) species were demonstrated to be unlikely as the active species during catalysis. Studies of catalysis runs undertaken to varying productivities found that during the nascent phase of reaction a notably lower activity is observed, while the selectivity favors C8+ formation during this period, most likely due to activation processes still occurring. The rate of formation of 1-butene was shown to increase with productivity, as a result of the greater solubility of ethylene in the 1-butene generated compared with the chlorobenzene solvent. The formation of C6 byproducts was shown to be independent of 1-butene concentrations, consistent with a mechanism whereby 1-butene is only incorporated into secondary products when it reacts prior to dissociation from the tungsten center