Nickel ethylene oligomerization mechanism

It is largely accepted that the active species in ethylene oligomerization is a nickel hydride species like 8. The mechanism for the hydride formation is supported by the reactions depicted in eqs. (7)-(9). Complex 9 eliminates butadiene at low temperatures and becomes active at 40 °C. Insertion of ethylene and elimination of styrene from structure 4 at 70 °C causes the complex to become active, while the more strongly bound cyclopentadienyl ligand in structure 10 needs 130 °C [49]. The elimination products of these reactions could be detected by GLC. 

Scheme 1. Postulated mechanism for ethylene oligomerization via a P n 0-stabilized nickel hydride species pi, p2... Pn = propagation steps Ci, C2.. . e = elimination steps.

The mechanism of Ni-catalyzed ethylene oligomerization involves both nickel hydride and nickel alkyl species. The mechanism is known in the literature as the metal-hydride mechanism, Cossee-Arlman mechanism, or ethylene insertion - -hydride elimination mechanism and results in a Schulz-Flory distribution of the oligomerization products. The mechanism is depicted in Figure 6.16.4. Note that two other coordination sites at the nickel are occupied by one bidentate ligand or two monodentate ligands (see Section 2.4 for details) that have been omitted in Figure 6.16.4 for clarity. 

The mechanism of ethylene oligomerization by SHOP and related catalysts, such as phosphinophenolate complexes, has been the subject of intense investigation (see COMC (1982), Chapter 52, references cited by Heinicke et al. and Pietsch et air It is generally agreed that the actual catalytic species are nickel hydride complexes, generated by ethylene insertion into the Ni-aryl bond of the precursor followed by /3-H elimination reaction (Scheme 50). Styrene or styrene derivatives can be detected in the reaction medium as a product of this activation process. In the case of the salicylaldiminate and anilinotropolone catalysts, styrene elimination is not required,... 

A reasonable mechanism for the co-oligomerization of butadiene with ethylene on a naked-nickel catalyst is shown in Eq. (49). Interaction of an ethylene molecule with the bis(7r-allyl) C8 chain produces a C,0 chain, containing both an alkyl- and a 7r-allylnickel group (XLVI). Coupling of the alkyl bond with the terminal atom of a m-Tr-allyl group or the terminal... 

It is possible to oligomerize ethylene by nickel in the absence of a Lewis acid. Butenes are formed by heating nickelocene to 200°C in the presence of ethylene 1193]. In this process, homolytic decomposition of nickelocene produces excited nickel atoms, which catalyze dimerization. This is similar to heterogeneous catalysis. To explain the nickelocene-catalyzed dimerization of ethylene, Tsutsui [194] has proposed a three-step mechanism represented in the following reactions ... 

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