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.