New thermally stable cationic h3-allyl(1,4-diphospha-(1,3)-
butadiene)nickel complexes for ethylene polymerization†
Alex Ionkin* and William Marshall
DuPont Central Research & Development, Experimental Station, Wilmington, DE 19880-0328, USA.
E-mail: alex.s.ionkin@usa.dupont.com; Fax: (302) 695-8281
Received (in Corvallis, OR, USA) 13th November 2002, Accepted 17th January 2003
First published as an Advance Article on the web 17th February 2003
Nickel ethylene polymerization catalysts 1a,b bearing a P,P-
ligand with two sp2-hybridized phosphorus atoms have been
synthesized; their structure and polymerization studies have
been reported for the first time.
1,4-diphospha-1,3-butadiene ligand and formally bidentate p-
allyl ligand each occupying 2 sites. The quality of the crystals
and the resulting structure are poor and only serve to establish
the connectivity for this study.
From the low-resolution X-ray structure it appears that the
axis of the allyl group is slightly twisted, distorting the nickel
atoms’ square planar geometry. The aryl rings are almost
perpendicular to the plane of the central ring, although they are
shifted up and down from each other (Fig. 1).
The search for new homogenous ethylene polymerization
catalysts with late transition metals is rapidly moving into the
family of unusual sp2-hybridized phosphorus ligands. For
example, platinum and palladium complexes active in ethylene
polymerization with diphosphinidenecyclobutene ligands were
reported recently.1 1,4-Diphospha-1,3-butadienes are low-
coordinated di-phosphorus analogs of a-diimines, and are a key
feature in the discovery of new late transition metal ethylene
polymerization catalysts (trademarked the Versipol catalyst
system by DuPont).2 Low-coordinated phosphorus ligands3 can
participate in metal-to-phosphorus p-back-bonding, which
could result in more thermally stable catalyst systems then those
based on a-diimines.1
The general procedure for ethylene polymerization is as
follows. In a nitrogen-purged drybox, a 20 ml glass insert was
loaded with the nickel compound (0.001 mmol) and, optionally,
specified cocatalyst (e.g., B(C6F5)3). Next, the solvent (p-
xylene (10 ml) in most examples) was added to the glass insert.
The glass insert was then loaded in a pressure tube inside the
drybox. The pressure tube was then sealed, brought outside the
drybox, connected to the pressure reactor, placed under the
desired ethylene pressure (600 psig in all cases) and shaken
mechanically. After the stated reaction time (18 hours), the
ethylene pressure was released and the glass insert was removed
from the pressure tube. The polymer was precipitated by the
addition of MeOH ( ~ 20 mL). The polymer was then collected
on a frit and rinsed with MeOH. The polymer was transferred to
a pre-weighed vial and dried under vacuum overnight. Table 1
summarizes the results for ethylene polymerization catalyzed
by nickel complexes 1a,b.
As seen from sixth column of Table 1, the prepared ethylene
polymers have a low degree of branching compared with the a-
diimine based catalyst.2 Judging from DSC results (seventh
column), the polymers possess fairly good crystallinity. Re-
markably, the nickel catalysts 1a,b bearing the 1,4-diphospha-
1,3-butadiene ligand are thermally stable until 160 °C, which
could be tied to the ability of this ligand to participate in metal-
to-phosphorus p-back-bonding. Upon increasing the tem-
perature of the polymerization from 60 °C to 160 °C, common
trends with a-diimine based catalysts were observed: the
productivity and molecular weights (Mw) have a tendency to
decrease, but the degree of branching increases. The molecular
weights (Mw) of ethylene polymers in Table 1 prepared from
nickel catalysts 1a,b with sp2-hybridized phosphorus ligand
In this report, we describe the synthetic approach to,
3
structures, and catalytic properties of h -allyl(1,4-bis[2,4,6-
tri(tert-butyl)phenyl]-1,4-diphospha-(1,3)-butadiene)-
nickel(1+) tetrakis[3,5-bis(trifluoromethyl)phenyl]borates(12)
1a,b. The above complexes were obtained by the reaction of an
(E,E)-stereoisomer of 1,4-bis(2,4,6-tri(tert-butyl)phenyl)-
1,4-diphospha-1,3-butadiene4 2, p-allylnickel halide dimers
3a,b5 and sodium tetrakis[3,5-bis(trifluoromethyl)phenyl]bor-
ate 4 (Scheme 1). The complexes 1a and 1b were isolated as
green crystals in 95% and 42% yields after recrystallization
from methylene chloride. Both of them were characterized by
NMR spectroscopy and elemental analysis.6
The X-ray structure of 1a (Fig. 1)‡ shows the nickel atom has
4-coordinate square planar stereochemistry with the chelating
3
Fig. 1 An ORTEP drawing of the h -2-propenyl(1,4-bis[2,4,6-tri(tert-
Scheme 1
butyl)phenyl]-1,4-diphospha-(1,3)-butadiene)nickel(1+) moiety of 1a. The
borate anionic molecule, hydrogen atoms and the disordered allyl position
have been omitted for clarity.
† This is DuPont Contribution #08295.
710
CHEM. COMMUN., 2003, 710–711
This journal is © The Royal Society of Chemistry 2003
Ionkin, Alex
