Oligomerisation of Ethylene by Bis(imino)pyridyliron and -cobalt Complexes

Abstract

A series of bis(imino)pyridyliron and -cobalt complexes [[2,6-(CR=NAr)2C5H3N]MX2] (R=H, Me; M=Fe, Co; X=Cl, Br) 8-16 containing imino-aryl rings (Ar) with at least one small ortho substituent, as well as Ar=biphenyl and Ar=naphthyl, has been synthesised. Crystallographic analyses of complexes 9 (Ar = 2,3-dimethylphenyl), 13 and 14 (Ar= biphenyl; X= Cl or Br, respectively) reveal a distorted trigonal-bipyramidal geometry in the solid state. These complexes, in combination with methyl aluminoxane (MAO), are active catalysts for the oligomerisation of ethylene, yielding >99% linear alpha-olefin mixtures that follow a Schulz-Flory distribution. Iron ketimine (R = Me) precatalysts give the highest activities and a greater alpha-value than their aldimine (R = H) analogues. Cobalt precatalysts follow a similar trend, though their activities are almost two orders of magnitude lower than those of the corresponding iron catalysts. Ethylene pressure studies on cobalt precatalyst 15 reveal a first-order dependence on ethylene for both the rate of propagation and the rate of chain transfer, and a pressure independence of the alpha value.

Distinguishing Chain Growth Mechanisms in Metal-catalyzed Olefin Oligomerization and Polymerization Systems: C2H4/C2D4 Co-oligomerization/Polymerization Experiments Using Chromium, Iron, and Cobalt Catalysts

“…Chain transfer to aluminum is a common termination mechanism for catalyst 2 in the presence of MAO co-catalyst. 33 For catalysts 1, 3 and 4, a linear relationship between catalyst productivity and ethylene pressure has been observed and the α values, which describe the shape of the distribution, 30 are invariant with ethylene concentration, which indicates that both the rate of propagation (k p ) and the rate of termination (k t ) are first order with respect to ethylene concentration. Although chain transfer to aluminum is more commonly associated with a Cossee-type mechanism, it can also occur in a metallacyclic mechanism, as is shown here for catalyst 2.…”

“…18,30,31,33 The catalysts 1, 3 and 4 result in a Schultz-Flory distribution of linear α-olefins (LAO), while catalyst 2 gives a slightly different product distribution due to an additional, but minor, chain transfer to aluminum process. Chain transfer to aluminum is a common termination mechanism for catalyst 2 in the presence of MAO co-catalyst.…”

“…[29][30][31] Chain growth with these catalysts is believed to occur via a Cossee-type mechanism whereby k p ≈ k t . Experimental evidence for the involvement of this mechanism was initially obtained from C 2 H 4 /C 2 D 4 co-oligomerization experiments using a bis(imino)pyrdine cobalt(I) chloride catalyst similar to 3, in combination with MAO.…”

Simple Preparation of Halogen-Substituted <i>α</i>-Diimine Nickel Complexes Immobilized into Clay Interlayer as Catalysts for Ethylene Oligo-/Polymerization

“…The steric crowding at the axial sites is critical for suppressing the associative chain transfer process with the ethylene monomer at the axial site and results in the formation of a high molecular-weight polymer [12] [13] [14]. Thus, the products prepared by the L1 and L3/Ni 2+ -mica possessing relatively bulky ortho-substituents were composed Scheme 1.…”

“…For example, orthosubstituents of the aryl rings on the imino groups play an important role in controlling the molecular weight of the products. In general, the molecular weight of polyethylene depends on the rate ratio of the chain transfer process relative to the chain propagation process [12] [13] [14]. The ortho-substituents on the aryl rings positioned at the axial sites for the triangle consisting of N-Ni-N in the complex can block the chain transfer reaction, such as β-hydrogen transfer to a monomer, because the rate of the chain transfer mainly depended on the steric bulk of the substituents [10].…”