Diene-Containing Half-Sandwich MoIII Complexes as Ethylene Polymerization Catalysts: Experimental and Theoretical Studies

Abstract

Seventeen-electron compounds of MoIII having the general formula [(eta5-C5R5)Mo(eta4-diene)X2] (R = H, Me: dieney = butadiene, isoprene, or 2,3-dimethylbutadiene: X= Cl, CH3) are a new class of ethylene polymerization catalysts. The polyethylene obtained shows a bimodal distribution, the major weight fraction being characterized by very long (M around 10(6)) and highly linear polymer chains. The newly prepared pentamethylcyclopentadienyl (Cp*) derivatives are more active than the cyclopentadienyl (Cp) derivatives, but much less active than previously investigated niobiumIII compounds having the same stoichiometry. On the other hand, the turnover frequency of the active site leading to the high molecular weight chains is at least 10 times greater than that obtained with the corresponding Nb catalyst. The reason for the low activity is explained by a difficult activation process that is attributed to the low polarity and high strength of the Mo-alkyl bond. This is confirmed by a Mulliken charge analysis of density functional theory (DFT) geometry-optimized [CpM(eta4-C4H6)(CH3)2] (M = Nb, Mo) and by the calculation of the heterolytic bond dissociation energies. DFT calculations have also been carried out on the ethylene insertion coordinate for the [CpM(eta4-C4H6)(CH3)]+ model of the presumed active site. The results indicate an equivalent activation barrier to insertion for the Nb and Mo systems. Differences in optimized geometries for the reaction intermediates are attributed to the presence of the extra electron for the Mo system. This electron opposes the formation of M-H-C agostic interactions, while it strengthens the back-bonding M-ethylene interaction, but otherwise plays no active role in the polymer chain propagation mechanism. According to the calculations, the chain propagation for the Mo system occurs entirely on the spin doublet surface, the minimum energy crossover point with the quartet surface lying at a higher energy than the transition state for insertion on the doublet surface.

Advances in Non-Metallocene Olefin Polymerization Catalysis

“…Computational studies have been used to explore the nature of the oxidation state in catalysts derived from Cr VI bis(imido) complexes. , In more recent years, a significant amount of attention has been given to the development of homogeneous non-Cp systems. There have also been two reports of molecular catalysts for ethylene polymerization based on molybdenum, but both the bis(imido)Mo VI Cl 2 precatalyst and the Cp*(butadiene)Mo III Cl 2 system show only very low activities.…”

Diene complexes of early transition metals: ideas and progresses at Osaka University

“…Examination of this polymer with GPC gave an unusually sharp peak, indicating a living polymerization. R. Poli recently reported that diene half-metallocene molybdenum complexes became catalyst precursors for ethylene polymerization [78]. The mechanism of these catalyst systems was discussed on the basis of DFT calculations [78,79].…”

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