Metallocene | EPFL Graph Search

A metallocene is a compound typically consisting of two cyclopentadienyl anions (C5H5−, abbreviated Cp) bound to a metal center (M) in the oxidation state II, with the resulting general formula (C5H5)2M. Closely related to the metallocenes are the metallocene derivatives, e.g. titanocene dichloride or vanadocene dichloride. Certain metallocenes and their derivatives exhibit catalytic properties, although metallocenes are rarely used industrially. Cationic group 4 metallocene derivatives related to [Cp2ZrCH3]+ catalyze olefin polymerization. Some metallocenes consist of metal plus two cyclooctatetraenide anions (C8H82−, abbreviated cot2−), namely the lanthanocenes and the actinocenes (uranocene and others). Metallocenes are a subset of a broader class of compounds called sandwich compounds. In the structure shown at right, the two pentagons are the cyclopentadienyl anions with circles inside them indicating they are aromati

New polynuclear catalysts for atom-transfer radical addition reactions

Laurent Quebatte

Homo- and heterobimetallic complexes of the late transitions metals have been synthesized by metathesis reactions of the corresponding chloro-bridged dimers. These reactions have the advantage of being fast and giving rise to structurally defined products in quantitative yield. Complexes with three chloro-bridges of the general formula [LM(μ-Cl)3RuLm] (LM = (arene)Ru, CpIr, or CpRh; RuLm = RuCl(PPh3)2 or RuCp*) were investigated in detail and several of these were structurally characterized. Evidence is provided that triply bridged complexes of this kind can undergo fast exchange reactions. The complexes with the fragment {RuCl(PPh3)2} were found to catalyze atom-transfer radical addition (ATRA) reactions. Structurally related complexes with chelating 1,4-bis(diphenylphosphino)butane (dppb) or 1,4-bis(dicyclohexylphosphino)butane (dcypb) ligands instead of the two PPh3 ligands could likewise be generated. A total of 69 different combinations were prepared and tested in a combinatorial fashion for their ability to catalyze the ATRA of CCl4 to styrene. Two combinations were found to be remarkably active and the corresponding complexes [CpRh(μ-Cl)3RuCl(PPh3)2] and [{(tpc)Rh(μ-Cl)3Ru(dcypb)}2(μ-N2)] were identified and structurally characterized. They proved to be among the most active catalysts described so far. Other ATRA catalysts were investigated. The cationic complex [CpRu(PPh3)2-(CH3CN)]OTf was found to display a higher stability than the related neutral catalyst [Cp*RuCl(PPh3)2]. Total TONs of up to 890 for the addition of CHCl3 to styrene were obtained at a temperature of only 40°C. For the first time, ATRA reactions are reported using a mixture of [(arene)RuCl2]2 and PCy3 as the catalyst precursors. As a product of the reaction between [(C6H3iPr3)-RuCl2]2 and PCy3, the tetranuclear complex [{(C6H3iPr3)Ru(μ-Cl)3RuCl(PCy3)}2(μ-N2)] was isolated, which itself proved to be highly active. When the arene and the N2 ligands of the latter were exchanged for a cymene ligand and an ethylene ligand, respectively, the resulting binuclear complex [(cymene)Ru(μ-Cl)3RuCl(PCy3)(η2-C2H4)] was obtained. It proved to be even more efficient than the dinitrogen complex and allowed the Kharasch addition of CCl4 to 1-olefins to be carried out at a temperature as low as 0°C. The observed TOFs (1100 h-1 at 24°C and 1550 h-1 at 40°C) are comparable to those reported for the most active catalysts.

EPFL2006

Thermal and dynamic mechanical properties of blends of bitumen with metallocene catalyzed polyolefins

Jan-Anders Månson, John Christopher Plummer, Chiara Spadaro

A high penetration grade bitumen has been blended with up to 50 wt% of two different grades of metallocene catalyzed linear low density polyethylene (m-LLDPE) in order to investigate the potential of these and similar copolymers as a substitute for styrene butadiene styrene triblock copolymers in polymer-modified bitumens (PMB). A continuous polymer-rich phase was observed at m-LLDPE contents as low as 5-10 wt%, along with a significant decrease in the effective glass transition temperature of the PMBs with increasing polymer concentration, suggesting benefits for low temperature flexibility. The m-LLDPE-based PMBs also showed relatively low dynamic shear viscosities up to high polymer contents in the range of temperature and shear rate corresponding to typical PMB processing conditions. However, the presence of bitumen in the m-LLDPE-rich phase led to a significant reduction in the melting points of the m-LLDPE, and softening of the PMBs at temperatures as low as 40-50 degrees C, depending on the composition and the melting point of the pure polymer. PMBs based on the m-LLDPE with the higher melting point remained fully elastic in this temperature range, but at the expense of increased crystallinity and a higher glass transition temperature, which limit improvements in low temperature flexibility. On the other hand, the potentially broad composition and property windows associated with m-LLDPEs suggest considerable scope for the fine tuning of PMB properties by using combinations of different m-LLDPEs and/or other polyolefins as a means to optimize performance.

Springer US2011

New polynuclear catalysts for atom-transfer radical addition reactions

Laurent Quebatte

EPFL2006