[CP*Cr(C6F5)(Me)(Py]) as a living chromium(III) catalyst for the "Aufbaureaktion"
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abstract
The reaction of [Cp*Cr(C 6F 5)(-Cl)] 2 (1) with 2 equiv of MeLi in THF yields the methyl-bridged Cr(III) dimer [Cp*Cr(C 6F 5)(-Me)] 2 (2). This dinuclear compound is very soluble in hydrocarbon solvent and has been isolated in low yield (6%). Compound 2 reacts with pyridine to afford [Cp*Cr(C 6F 5)(Me)(Py)] (3), which has been isolated in a 67% yield. Compound 3 is a 15-electron, coordinatively saturated chromium(III) species that has been characterized by NMR, magnetometry, and EA. The structures of 1, 2, and 3 have been determined by single-crystal X-ray diffraction. Compounds 1 and 2 exist in the form of centrosymmetrical dimers with bridging chloride and methyl ligands, respectively. Mononuclear compound 3 adopts the expected three-legged piano stool geometry. Compound 2 polymerizes ethylene in toluene under 1 atm of ethylene at room temperature in the absence of any activators. Compound 3 is not catalytically active by itself. Yet, the addition of excess AlEt 3 to a solution of 3 in toluene leads to a catalytic system that readily oligomerizes ethylene. Oligomerization experiments carried out with [3] = 10 -3 M and [AlEt 3] = 4.5 10 -2 M for 15 min lead to the production of ethylene oligomers with an activity of 221 kg mol Cr -1 h -1. As indicated by gas chromatography, the Poisson distribution formula accounts for the molecular weight distribution of the growing ethylene oligomers during this reaction, which is indicative of a living polymerization system. Experiments carried out at higher AlEt 3 concentrations ([3] = 10 -3 M and [AlEt 3] = 9 10 -2 M) lead to a lower activity (150 kg mol Cr -1 h -1) but still present the characteristic features of a living polymerization system. These results are interpreted on the basis of a catalytic cycle in which the chain grows at chromium and is transferred to aluminum via an alkyl-bridged chromium-aluminum bimetallic intermediate.
