The synthesis and x-ray structure of trans-NiCl2(1,3-bis (2,6-diisopropylphenyl)imidazol-2-ylidene)2; attempts to polymerize olefins utilizing a nickel(II) complex of a sterically demanding N-heterocyclic carbene

Article abstract of DOI:10.1016/S0022-328X(03)00458-3

The compound trans-NiCl₂(IPr)₂ (IPr=1,3-bis (2,6-diisopropylphenyl)imidazol-2-ylidene) is synthesized and characterized crystallographically. In combination with MAO, it is converted to an intermediate which catalyses the dimerization of ethylene but which not the polymerization of ethylene, propylene or 1-hexene.

Full text of DOI:10.1016/S0022-328X(03)00458-3

Journal of Organometallic Chemistry 683 (2003) 114Á119
/
www.elsevier.com/locate/jorganchem
The synthesis and X-ray structure of trans-NiCl2(1,3-bis(2,6-
diisopropylphenyl)imidazol-2-ylidene)2; attempts to polymerize
olefins utilizing a nickel(II) complex of a sterically demanding N-
heterocyclic carbene
Amanda L. MacKinnon, Michael C. Baird *
Department of Chemistry, Queen’s University, Kingston, Ont., Canada K7L 3N6
Received 7 March 2002; received in revised form 8 May 2002; accepted 8 May 2003
Abstract
The compound trans-NiCl2(IPr)2 (IPrꢀ
crystallographically. In combination with MAO, it is converted to an intermediate which catalyses the dimerization of ethylene
/1,3-bis(2,6-diisopropylphenyl)imidazol-2-ylidene) is synthesized and characterized
but which not the polymerization of ethylene, propylene or 1-hexene.
# 2003 Elsevier B.V. All rights reserved.
Keywords: Nickel; N-Heterocyclic carbene ligands; Olefin polymers
Recent years have seen a huge increase in interest in
the coordination chemistry of unusually stable N-
reaction of Ni(acac)2 with NHCs [6] and are found to
catalyse a variety of organic transformations, including
cross-coupling of aryl chlorides with Grignard reagents
[6a], amination of aryl chlorides [6b], reduction of aryl
halides [6c] and coupling of 1,3-dienes with aldehydes
[6d].
In view of the growing importance of nickel com-
plexes as olefin polymerization catalysts [7], we have
synthesized and investigated the possible utilization as
an olefin polymerization catalyst of the compound
NiCl2(1,3-bis(2,6-diisopropylphenyl)imidazol-2-yli-
heterocyclic carbenes (NHCs) of the type A (Rꢀbulky
/
alkyl, aryl groups) [1]. Not only are complexes of
virtually all transition metals known, but many of the
complexes have been shown to exhibit a wide variety of
interesting and useful catalytic properties [1,2].
dene)2, henceforth NiCl2(IPr)2 (IPrꢀB).
/
A relatively small number of NHC nickel compounds
has been reported, but these include nickel(II) com-
plexes of the types NiX2(NHC)2 (Xꢀ
/
Cl, Br, I, Me) [3],
[NiXL(NHC)2]ꢁ (Lꢀ
/
tertiary phosphine) [4a], [Ni(bi-
dentate chelating NHC)2]ꢁ [4a], h5-C5H5NiCl(NHC)
[4b] and h3-C3H5NiCl(NHC) [4c], and nickel(0) com-
plexes of the types Ni(NHC)2 [5a,b] Ni(CO)3(NHC) [5c]
and Ni(CO)2(NHC)2 [5c]. In addition, some as yet
incompletely characterized compounds are formed on
Only two nickel complexes of B are known, one a
somewhat ill-defined catalyst for the coupling of 1,3-
dienes with aldehydes [6d], the other being the above
mentioned h3-C3H5NiCl(IPr) [4c]. In contrast, many IPr
complexes of palladium [8] and ruthenium [9] are known
and exhibit a variety of very interesting catalytic
activities. Complexes of aluminum [10a], indium [10a],
cobalt [10b], rhodium [10c] and iridium [10d] are also
* Corresponding author. Tel.: ꢁ
6669.
/
1-613-533-2616; fax: ꢁ1-613-533-
/
E-mail address: bairdmc@chem.queensu.ca (M.C. Baird).
0022-328X/03/$ - see front matter # 2003 Elsevier B.V. All rights reserved.
doi:10.1016/S0022-328X(03)00458-3

A.L. MacKinnon, M.C. Baird / Journal of Organometallic Chemistry 683 (2003) 114Á
/
119
115
known and most exhibit interesting or unusual chem-
istry.
4H, m-C6H3), 7.25 (2H, p-C6H3); lit. (C6D6): d 1.13 (d,
J 9.2 Hz, 12H, CH(CH3)2), 1.23 (d, J 9.2 Hz, 12H,
CH(CH3)2), 2.91 (septet, J 9.2 Hz, 4H, CH(CH3)2), 6.57
(s, 4H, N(CH)2N), 7.11 (m, 4H, m-C6H3), 7.22 (2H, p-
C6H3) [9c]. 13C{1H}-NMR (C6D6): d 23.6 (CH(CH3)2),
24.7 CH(CH3)2), 28.7 CH(CH3)2), 121.6 (N(CH)2N),
123.6 (m-C6H3), 129.0 (p-C6H3), 139.0, 146.3 (ipso-
C6H3, o-C6H3), 220.7 (NCN); lit. (C6D6): d 23.6
(CH(CH3)2), 24.8 CH(CH3)2), 28.7 CH(CH3)2), 121.5
(N(CH)2N), 123.6 (m-C6H3), 129.0 (p-C6H3), 139.0
(ipso-C6H3), 146.2 (o-C6H3), 220.6 (NCN) [11]. The
assignments were confirmed by HMQC experiments.
A key feature of many olefin polymerization catalysts
based on square planar nickel compounds is the
presence of ligands which provide considerable steric
hindrance to access to the axial positions of the metal
[7]. Examples of such ligands are a-diimines such as
{2,6-C6H3(i-Pr)2)}Nꢀ
/
CMeCMeꢀN{2,6-C6H3(i-
/
Pr)2)}, which coordinate in such a way that the bulky
{2,6-C6H3(i-Pr)2)} groups shield quite effectively the
axial positions of the metal, as in C.
1.2. Synthesis of NiCl2(IPr)2 (C)
Typically 0.169 g NiCl2(PPh3)2 (Strem) (0.258 mmol)
was added in one portion to 200 mg of IPr (0.515 mmol)
in 15 ml THF. The reaction mixture immediately turned
red, and was stirred for 12 h. The solvent was then
removed under reduced pressure, and the resulting
residue was dissolved and eluted through a silica column
using 1:3 CH2Cl2-hexanes. The red band containing a
mixture of NiCl2(IPr)2 and PPh3 was collected, the
solvent was removed under reduced pressure and
residual PPh3 contaminant was removed via sublimation
at 70 8C under reduced pressure. The synthesis was
Chain transfer processes, which would result in
lowered molecular weights, are made less competitive
in these catalysts because the bulky ortho substituents
block axial approach of monomer, thus retarding either
the rate of displacement of polymer product or the rate
of direct b-hydrogen elimination to coordinated mono-
mer 7a. A compound such as NiCl2(IPr)2 thus seems
particularly attractive because here also because the
bulky ortho-isopropyl groups would be expected to
result in similar effects.
carried out several times, yielding pure product in yields
1
43%. H-NMR (toluene-d8): d 0.92 (d, J 6.7 Hz,
of 30Á
/
24H, CH(CH3)2), 1.15 (d, J 6.7 Hz, 24H, CH(CH3)2),
3.01 (septet, J 6.7 Hz, 8H, CH(CH3)2), 6.24 (s, 4H,
N(CH)2N), 7.15 (d, J 7.7 Hz, 8H, m-C6H3), 7.36 (t, J
7.7 Hz, 4H, p-C6H3). 13C{1H}-NMR (C6D6): d 23.3
(CH(CH3)2), 26.3 CH(CH3)2), 28.6 CH(CH3)2), 124.3
(m-C6H3 and N(CH)2N), 129.7 (p-C6H3), 137.3, 147.1
(ipso-C6H3, o-C6H3), 168.9 (NCN). The assignments
were confirmed by HMQC experiments.
Elemental analyses for NiCl2(IPr)2: Calc. C 71.53, H
8.00, N 6.18; Found: C 70.89, H 7.72 N 6.14%. Allowing
a red ether solution to evaporate slowly resulted in red
crystals of NiCl2(IPr)2 suitable for crystallographic
examination.
1. Experimental
All operations were performed under purified argon
using normal Schlenk techniques or an Mbraun glove
box. Solvents were purified by standard methods, and
1
distilled and degassed before use. All H- and 13C{1H}-
NMR spectra were recorded using Bruker Avance 300,
400 or 500 spectrometers, chemical shifts being refer-
enced to residual 1H and 13C solvent peaks. Gel
permeation chromatography experiments with the poly-
mers were carried out at room temperature (r.t.) using a
Waters Model 440 liquid chromatograph with THF as
eluant and polystyrene standards. Elemental analyses
were carried out by Canadian Microanalytical Services
Ltd, Delta, BC.
1.3. X-ray crystallography
A crystal of NiCl2(IPr)2 (red, block-shaped, size 0.4ꢂ
/
0.3ꢂ0.2 mm) was mounted and sealed on a glass fiber
/
with epoxy glue. Data collection was performed on a
Bruker SMART CCD 1000 X-ray diffractometer with
1.1. Synthesis of 1,3-bis(2,6-diisopropylphenyl)imidazol-
2-ylidene (IPr)
graphite-monochromated MoÁ
/
Ka radiation (lꢀ
0.71073 A), operating at 50 kV and 35 mA at 23 8C
over 2u ranges of 4.328Á57.268. No significant decay
/
˚
The ligand, IPr, was prepared in 90% yield via
deprotonation 1,3-bis(2,6-diisopropylphenyl)imidazo-
/
was observed during the data collection. Data were
processed on a Pentium PC using the Bruker AXS
WINDOWS NT SHELXTL software package (version 5.10)
[12a]. Neutral atom scattering factors were taken from
Cromer and Waber [12b]. The raw intensity data were
lium choride (IPr×
/
HCl) by KOBut [9c]. 1H-NMR
(toluene-d8): d 1.13 (d, J 7.0 Hz, 12H, CH(CH3)2),
1.15 (d, J 7.0 Hz, 12H, CH(CH3)2), 2.91 (septet, J 7.0
Hz, 4H, CH(CH3)2), 6.56 (s, 4H, N(CH)2N), 7.12 (m,

116
A.L. MacKinnon, M.C. Baird / Journal of Organometallic Chemistry 683 (2003) 114Á119
/
converted (including corrections for scan speed, back-
ground, and Lorentz and polarization effects) to struc-
ture amplitudes and their esd’s using the program
SAINT, which corrects for Lp and decay. Absorption
corrections were applied using program SADABS. The
crystal is orthorhombic space group P21212, based on
the systematic absences, E statistics and successful
refinement of the structure. The structure was solved
by direct methods. Full-matrix least-square refinements
minimizing the function aw(F2oꢃ
/
F2c)2 were applied to
the compound. All non-hydrogen atoms were refined
anisotropically. The positions for all hydrogen atoms
were calculated, and their contributions were included in
the structure factor calculations with isotropic thermal
parameters 1.2 times that of the attached carbon atoms
(1.5 times for methyl hydrogens).
Convergence to final R1ꢀ
/
0.0436 and wR2ꢀ0.0780
/
by using 6434 independent reflections and 304 para-
meters were achieved [12c], with the largest residual
ꢃ3
˚
0.289 e A
peak and hole to be 0.349 and ꢃ
/
,
Fig. 2. Molecular structure of trans-NiCl2(IPr)2 with H atoms and
isopropyl groups removed and showing numbering scheme.
respectively. The full molecular structure is shown in
Fig. 1, a structure with H atoms and isopropyl groups
omitted but with important atom numberings included
in Fig. 2. Crystallographic data are given in Table 1,
bond lengths and angles in Table 2.
toluene containing a measured amount of 1-hexene or
saturated with ethylene or propylene; the molar ratio of
MAO:Ni was ꢀ100:1. Ethylene or propylene continued
/
to be bubbled through the reaction mixture while it was
stirred at room temperature.
1.4. General procedures for attempted polymerizations
Ethylene and propylene were purified by passing
through columns containing 4A molecular sieves, while
1-hexene was dried over 4A molecular sieves. As a
2. Results and discussion
general polymerization procedure, ꢀ0.8 ml of a solu-
tion of MAO in toluene (10% MAO by weight) was
added to a solution of 10 mg of NiCl2(IPr)2 in 10 ml
/
The original objective of this research was to synthe-
size a compound of the type NiCl2(NHC)2 and test it as
an olefin polymerization catalyst on activation with
MAO. For reasons of convenience, therefore, we
decided to begin with the known compound NiCl2(1,3-
dicyclohexylimidazol-2-ylidene)2 [3a]. Unfortunately, we
could not reproduce the synthesis of the required 1,3-
dicyclohexylimidazol-2-ylidene (A: Rꢀ
we next tried the procedure of Nolan and co-workers for
the synthesis of IPr (A: Rꢀisopropyl) [9c]. The ligand
/cyclohexyl), and
/
synthesis was now accomplished in excellent yield (see
also [13]). We next turned our attention to the synthesis
of NiCl2(IPr)2, obtaining this new compound via the
reaction of NiCl2(PPh3)2 with two equivalents of IPr in
reasonably good yields. We note that displacement of
the phosphine ligands of NiCl2(PPh3)2 by other NHCs is
reported to give yields varying between 0 and ꢀ80%,
/
although there is no obvious reason for the apparent
discrepancies [3a,b].
The compound NiCl2(IPr)2 is bright red and was
1
characterized satisfactorily by elemental analyses, H-
and 13C{1H}-NMR spectroscopy and X-ray crystal-
lography. The molecular structure is shown in Figs. 1
and 2; the crystal and structure refinement data are
Fig. 1. Molecular structure of trans-NiCl2(IPr)2.

A.L. MacKinnon, M.C. Baird / Journal of Organometallic Chemistry 683 (2003) 114Á
/
119
117
Table 1
Crystal data and structure refinement for trans-NiCl2(IPr)2
Table 2
˚
Comparison of Bond Lengths (A) and Bond Angles (8) of NiCl2(IPr)2
and of the two crystallographically independent molecules of
NiCl2(1,3-dicyclohexylimidazol-2-ylidene)2.3a
Empirical formula
C54H72Cl2N4Ni×(C2H5)2O
/
Formula weight
Temperature (K)
˚
Wavelength (A)
980.89
296(2)
NiCl2(IPr)2 NiCl2(1,3-dicyclohexylimidazol-2-
ylidene)2
0.71073
orthorhombic
P21212
Crystal system
Space group
Unit cell dimensions
Molecule A
Molecule B
Bond lengths
˚
a (A)
Ni(1)Ã
Ni(1)Ã
Ni(1)Ã
Ni(1)Ã
N(1)Ã
N(1)Ã
N(1)Ã
N(2)Ã
N(2)Ã
N(2)Ã
C(2)Ã
Bond angles
C(1)ÃNi(1)Ã
C(1)ÃNi(1)Ã
C(1)#1ÃNi(1)Ã
C(1)ÃNi(1)ÃCl(2)
C(1)#1ÃNi(1)Ã
Cl(1)ÃNi(1)Ã
C(1)ÃN(1)Ã
C(1)ÃN(1)Ã
C(2)ÃN(1)Ã
C(1)ÃN(2)Ã
C(1)ÃN(2)Ã
C(3)ÃN(2)Ã
N(1)ÃC(1)Ã
N(1)ÃC(1)Ã
N(2)ÃC(1)Ã
C(3)ÃC(2)Ã
C(2)ÃC(3)Ã
/
C(1)
1.933(3)
1.933(3)
1.911(2)
1.911(2)
1.912(2)
1.912(2)
2.1858(8)
2.1858(8)
1.356(3)
1.382(3)
1.473(3)
1.356(3)
1.386(4)
1.472(3)
1.339(3)
12.924(5)
20.464(7)
10.598(4)
90
˚
/
C(1)#1
Cl(1)
C(2)
b (A)
˚
c (A)
/
2.1586(17) 2.1818(8)
2.1588(17) 2.1818(8)
/
a (8)
b (8)
g (8)
/
C(1)
C(2)
C(4)
C(1)
C(3)
C(16)
1.365(4)
1.367(4)
1.435(4)
1.366(4)
1.370(4)
1.440(4)
1.356(5)
1.354(3)
1.390(3)
1.475(3)
1.357(3)
1.387(4)
1.472(3)
1.333(5)
90
90
/
3
˚
/
V (A )
2802.8(17)
2
/
Z
Dcalc Mg mꢃ3
1.162
0.482
1056
/
Absorption coefficient (mmꢃ1
F(0 0 0)
)
/
/
C(3)
Crystal size (mm3)
0.4ꢂ
/
0.3ꢂ
28.63
/
0.2
/
/
C(1)#1 179.8(2)
180.00
180.00
Theta range for data collection 2.16Á
(8)
Index ranges
/
/
/
Cl(1)
90.11(11)
90.75(7)
89.25(7)
89.25(7)
90.75(7)
180.00
90.83(7)
89.17(7)
89.17(7)
90.83(7)
180.00
/
/
Cl(1) 90.11(11)
ꢃ
/
165
135l 5
19 803
6434 [Rint
28.63 (95.0%)
/
h 5
/
16, ꢃ
/
265
/
k 5
/
27, ꢃ
/
/
/
89.89(11)
/
/
12
/
/
Cl(2) 89.89(11)
Reflections collected
/
/
Cl(2)
180.0
Independent reflections
Completeness to u (8)
Absorption correction
Max and min transmission
Refinement method
ꢀ0.0659]
/
/
/
C(2)
111.3(3)
129.7(3)
118.8(3)
111.8(3)
128.9(3)
119.2(3)
103.4(3)
127.9(2)
128.7(2)
107.2(3)
106.2(3)
110.9(2)
124.6(2)
124.4(2)
110.6(2)
124.6(2)
124.8(2)
104.5(2)
127.81(17)
127.56(18)
106.7(3)
107.3(2)
110.8(2)
123.4(2)
125.5(2)
111.1(2)
124.1(2)
124.6(2)
104.3(2)
129.47(18)
126.12(17)
107.3(2)
106.4(2)
/
/
C(4)
Empirical (Bruker SADABS)
1.0000 and 0.7807
Full-matrix least-squares on F2
6434/2/304
/
/
C(4)
/
/
C(3)
/
/
C(16)
C(16)
N(2)
Ni(1)
Ni(1)
Data/restraints/parameters
Goodness-of-fit on F2
/
/
0.768
/
/
Final R indices [I ꢁ
R indices (all data)
Absolute structure parameter
Largest difference peak and hole 0.349 and ꢃ
/
2s(I)]
R1ꢀ
R1ꢀ
0.061(17)
/
0.0436, wR2ꢀ
/0.0780
/
/
/
0.1422, wR2ꢀ
/0.0921
/
/
/
/
N(1)
/0.289
ꢃ3
˚
/
/
N(2)
(e A
)
listed in Table 1, important bond lengths and angles in
Table 2. As can be seen, the compound assumes a trans
square planar structure very similar to those of the
previously reported trans-NiCl2(1,3-dicyclohexylimida-
zol-2-ylidene)2, which occurs in two crystallographically
independent forms [3a]. In these and in NiCl2(IPr)2, the
carbene ligands are mutually trans and at angles very
close to 908 to the halide ligands. It is interesting to note
not be identified. After exhaustive Soxhlet extraction by
benzene, followed by drying under reduced pressure,
elemental analyses of the compound resulted in the
following data: C 36.3, H 4.2, N 3.0, Cl 27.4. Assuming
the remaining 29.1% is nickel, then the data correspond
to no obviously reasonable stoichiometry. The lack of
NMR data because of the insolubility of the compound,
in addition to the observation that it is colorless
rather than the blue or green expected of nickel(II)
complexes, leave us with no reasonable suggestion for
structure.
that the metalÁligand bond distances of trans-
/
NiCl2(1,3-dicyclohexylimidazol-2-ylidene)2 are slightly
longer than those of trans-NiCl2(IPr)2, although it is
not clear whether this reflects electronic or steric
differences. In addition, as anticipated, the planes of
the carbene ligands are perpendicular to those of the
metal centers and hence provide apparently significant
steric hindrance to the axial sites as may be necessary for
compounds of this type to behave as olefin polymeriza-
tion catalysts (see above).
2.1. Attempted polymerizations utilizing NiCl2(IPr)2
Polymerizations of e.g. ethylene by a-diimine com-
plexes such as C generally require the intermediacy of
cationic alkyl complexes of the type D, with a vacant site
cis to the alkyl group and to which the monomer can
coordinate [7].
As indicated above, the yields of NiCl2(IPr)2 isolated
were routinely less than 50%. The major product in all
cases was an insoluble, off-white compound which could

118
A.L. MacKinnon, M.C. Baird / Journal of Organometallic Chemistry 683 (2003) 114Á119
/
Under argon, 9.6 mg NiCl2(IPr)2 (1.05ꢂ
/
10ꢃ5 mol)
and 4.5 mg MAO (7.75ꢂ
/
10ꢃ5 mol of ‘MeAlO’) were
placed in an NMR tube to which 0.5 ml toluene-d8 was
added. Although the resulting solution turned blue and
a small amount of blue solid slowly formed, a 1H-NMR
spectrum exhibited only the resonances of NiCl2(IPr)2
and MAO. The blue solid was isolated and dried, but a
1H-NMR spectrum in CD2Cl2 was uninformative.
In a complementary experiment, 5.1 mg NiCl2(IPr)2
While simple substitution of the chloride ligands on
(0.56ꢂ
10ꢃ5 mol) were dissolved in 0.5 ml toluene-d8
/
the trans structure of NiCl2(IPr)2 does not naturally
lend itself to the necessary cis isomer of a presumed
intermediate such as [Ni(Me)(IPr)2]ꢁ, we anticipated
that dimethylation of NiCl2(IPr)2 would result because
of the high trans influence of the methyl ligand [14] in
formation of the cis isomer which should form the
desired cationic catalyst on treatment with a co-catalyst
such as B(C6F5)3 [15]. To our surprise, however, several
attempts to substitute the chloride ligands of NiCl2(IPr)2
using methyl lithium failed; only NiCl2(IPr)2 was
recovered. This rather surprising finding may be a result
of the steric hindrance to axial attack, as discussed
above. Certainly nickel complexes of cis chelating
NHCs do react with methyl lithium to form unstable
methyl complexes [3c].
under argon. Saturation of the resulting pink solution
with ethylene resulted in neither a colour change nor a
shift in the resonances of ethylene (d 5.35) or Ni-
Cl2(IPr)2. Subsequent addition of 0.2 ml of 0.495 M
solution of MAO (9.9ꢂ
/
10ꢃ5 mol of ‘MeAlO’) resulted
1
in a yellow solution, the H-NMR spectrum of which
exhibited resonances of ethylene, NiCl2(IPr)2 and 1-
butene at d 0.90 (t, Me), 1.93 (m, CH2), 4.92, 5.79 (m,
CH2Ä
/
CHÃ
/
). Integration after 1 and 2 h showed that the
1:1 and ꢀ2:1,
ratio of 1-butene:NiCl2(IPr)2 was ꢀ
/
/
respectively. After 24 h, the spectrum also exhibited
the resonances of cis- and trans-2-butene, but partial
decomposition of the catalyst had resulted in a rather
complicated NMR spectrum and useful integrations
could not be obtained. However, the total amount of
butenes present certainly exceeded the amount of
NiCl2(IPr)2 in solution, and since NiCl2(IPr)2 seems to
be essentially unreactive with MeLi and MAO, it seems
likely that the actual catalyst is some other species,
present in very low concentrations. Thus the resultant
ethylene dimerization was catalytic.
We therefore attempted to use NiCl2(IPr)2 as an olefin
polymerization catalyst using MAO [15] as an activator.
As a general procedure, ꢀ0.8 ml of a solution of MAO
/
in toluene (10% MAO by weight) was added to a
solution of 10 mg of NiCl2(IPr)2 in 10 ml toluene
saturated with ethylene or propylene; the molar ratio of
MAO:Ni was ꢀ100:1. Ethylene or propylene continued
/
Catalytic ethylene dimerization by nickel compounds
has been known for many years [16], and played a
special role in the original discovery of Ziegler catalysts
16a. While this work was in progress Cavell and co-
workers [3d] reported similar olefin dimerizations cata-
lyzed by nickel(II) complexes of NHCs in ionic liquid
solvents. From our results, however, it is clear that an
ionic solvent medium is not necessary for catalysis by
NHC complexes of nickel(II). It is, however, not clear
why NiCl2(IPr)2 is a poor catalyst for ethylene oligo-
merization or polymerization. Given its inertness with
respect to alkylation reactions, it seems possible that the
2,6-diisopropylphenyl groups of the IPr ligands are too
sterically demanding.
to be bubbled through the reaction mixture while it was
stirred at room temperature. The procedure for 1-hexene
was similar except that 0.9 ml of 1-hexene was added to
the solution prior to the addition of MAO. In the case of
ethylene, the reaction mixture turned yellow on the
addition of MAO and a small amount of yellow
precipitate appeared. In contrast, the propylene and 1-
hexene reaction mixtures both turned bright blue, and a
small amount of a dark precipitate formed. In all cases,
any reactions were stopped after 30 min by the addition
of 5 ml of a methanolÁHCl solution. At this time the
/
reaction mixtures became colourless. Work-up in all
cases involved removal of volatiles under reduced
pressure followed by extraction of the solid residue
with hexanes. The resulting hexanes solutions were then
filtered through silica and solvent was removed from the
eluants under reduced pressure. In no case was any
polymeric material obtained!
3. Supplemental material
The reaction(s) with MAO were studied further by
NMR spectroscopy. Dry, solvent free MAO was ob-
tained by removing the toluene from the commercial,
10% solution under reduced pressure. A part of the
resulting white solid was used to make a stock solution
of 0.495 M MAO in toluene-d8.
Crystallographic data for trans-NiCl2(IPr)2 have been
deposited with the Cambridge Crystallographic Data
Centre. Copies of these data may be obtained free of
charge from The Director, CCDC, 12 Union Road,
Cambridge CB2 1EZ, UK (Fax: ꢁ
/
44-1223-336033; e-
mail: deposit@ccdc.cam.ac.uk or www: http://

A.L. MacKinnon, M.C. Baird / Journal of Organometallic Chemistry 683 (2003) 114Á
/
119
119
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21 (2002) 1554;
www.ccdc.cam.ac.uk). The deposition number is CCDC
205515.
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Acknowledgements
We thank Queen’s University and the Natural
Sciences and Engineering Research Council for financial
support, S.P. Nolan, N. Burford and C. Crudden for
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