Selective Oligomerization of Ethylene By Nitrogen Bridged Diphosphine Nickel(II) Complexes

Selective Oligomerization of Ethylene By Nitrogen
Bridged Diphosphine Nickel(II) Complexes

Sivaramakrishna Akella¹, Lenin Kumar V¹, Varun Kaushal² and Anubhav Mishra²

(1) Chemistry, VIT
University, Vellore, India,

(2) Chemical
Engineering, VIT University, Vellore, India

Abstract

Traditional
linear alpha olefin (LAO) production processes typically produce a broad and relatively
inflexible distribution of linear alpha olefins. It
is very important for industry to produce valuable linear alpha olefins
selectively without co-producing other low-value by-products. Selective
processes like production of 1-hexene and 1-octene are the targets. In this
connection, numerous catalysts for ethylene trimerization to 1-hexene have been
reported, and a process is now practiced commercially. Recently, the first catalysts
for the selective tetramerization of ethylene to 1-octene were reported using chromium
based catalysts. The active catalyst is typically generated by a combination of
a chromium source with a diphosphine ligand in the presence of an aluminoxane activator.
Mechanistic studies on these catalyst systems indicate that the 1-octeneis liberated
from a metallcyclononane catalytic intermediate, formed by an extension of the metallacyclic
mechanism previously proven for ethylene trimerization reactions. Herein, we
report the catalytic ethylene oligomerization reactions to produce LAOs using phosphine
based Ni complexes (Eq.1).

Eq.1:
General scheme for ethylene oligomerization reaction

Results indicate
that formation of 1-hexene and 1-octene with nickel complexes are quite
feasible. Here the temperature can control the product distribution, while
Pd(II) complexes prefer to yield more dimerization to 1-butene. This could be
due to the better stability of nickelacycloalkanes and they will be allowed to
insert further ethylene molecules. Palladacycloalaknes can undergo relatively faster
β-hydride elimination to yield 1-butene. The bimetallic catalyst showed a
controlled distribution of C6 and C8 products based on the reaction conditions.
By fine-tuning the ligands as well as reaction conditions it is possible to
achieve the selective trimerization and tetramerization. The product
distribution strongly depends on the nature of remote substituent ‘R’, pressure,
temperature and solvent employed.