Selective ethylene oligomerization: Recent advances in chromium catalysis and mechanistic investigations

Selective Ethylene Oligomerization with Chromium Complexes Bearing Pyridine–Phosphine Ligands: Influence of Ligand Structure on Catalytic Behavior

“…Given the absence of oligomeric products in the Cr(acac) 3 /1/MAO/DEAC system, the in situ generated active species is expected to differ from the one resulting from CrCl 3 (THF) 3 /1/MAO, although both combinations contain chlorine. Using another chlorine-free compound, Cr(EH) 3 (chromium 2-ethylhexanoate), as the chromiun precusor resulted in similar selectivity as that obtained for the Cr(acac) 3 /1/MAO/DEAC system but with a much lower ethylene polymerization activity.…”

“…Complexation of 1 with CrCl 3 (THF) 3 was conducted in dichloromethane, and the ligation was indicated by an instant color change from purple to blue upon mixing in dichloromethane. The resulting complex 1a was isolated as a blue powder by removing the solvent in vacuo.…”

“…Since the catalytic behavior of complex 1a and a mixture of CrCl 3 (THF) 3 plus ligand 1 in a 1:1 ratio were found to be identical, extensive oligomerization runs were conducted with in situ generated catalysts (Table 1). Upon activation, CrCl 3 (THF) 3 /1 afforded fairly good activity, which is comparable to the highest activity reported in a similar chromium-catalyzed selective ethylene oligomerization system (1322 g/mmol Cr/h, 80°C, 40 bar of ethylene).…”

“…Their corresponding chromium complexes ([PyN(Me)PR 2 ]CrCl 3 THF; R = o-tolyl (2a), isopropyl (3a)) were obtained from the reaction of 2 and 3 with CrCl 3 (THF) 3 in THF. In the case of 2a, crystals suitable for single crystal Xray structure determination were grown by slow vapor diffusion of petroleum ether into a THF solution.…”

“…1−6 The majority of the currently existing ethylene trimerization and tetramerization systems are based on chromium complexes bearing a wide variety of ancillary ligands. 1,3,4 As a result of ready interconversion between mono-, di-, and trivalent oxidation states of chromium, the same precursor might promote state-dependent selective or nonselective ethylene oligomerization as well as ethylene polymerization as a function of the reaction conditions (i.e., cocatalyst, solvent, temperature). This versatility in catalytic performance makes chromium catalysts excellent candidates for investigating factors that influence catalytic behavior.…”