Nitrogen- and phosphorus-coordinated nickel(II) complexes as catalysts for the oligomerization of ethylene

Article abstract of DOI:10.1016/j.molcata.2004.11.015

Nitrogen- and phosphorus-coordinated nickel(II) complexes 1 and 2 are active precatalysts together with the cocatalyst MAO for the oligomerization of ethylene mainly favoring the production of butenes. The nickel(II) complex 2b reveals a good selectivity at higher ethylene pressure for the production of 1-butene.

Full text of DOI:10.1016/j.molcata.2004.11.015

Journal of Molecular Catalysis A: Chemical 229 (2005) 177–181
Nitrogen- and phosphorus-coordinated nickel(II) complexes
as catalysts for the oligomerization of ethylene
∗
¨
Martin E. Bluhm, Cristina Folli, Olaf Walter, Manfred Doring
Forschungszentrum Karlsruhe, Institute for Technical Chemistry (ITC-CPV), P.O. Box3640, 76021 Karlsruhe, Germany
Received 19 October 2004; received in revised form 18 November 2004; accepted 20 November 2004
Available online 4 January 2005
Abstract
Nitrogen- and phosphorus-coordinated nickel(II) complexes 1 and 2 are active precatalysts together with the cocatalyst MAO for the
oligomerization of ethylene mainly favoring the production of butenes. The nickel(II) complex 2b reveals a good selectivity at higher ethylene
pressure for the production of 1-butene.
© 2004 Elsevier B.V. All rights reserved.
Keywords: Dimerization; Oligomerization; Diimine complexes; Catalysts; Ethylene; Schiff base complexes
1. Introduction
alphabutol process (IFP) [35]. 2-Butene is also produced with
the Phillips Petroleum process by dimerization of ethylene
[35].
We have investigated new nitrogen- and phosphorus-
coordinated nickel(II) complexes as suitable catalysts for
oligomerization and dimerization reactions of ethylene. The
results of the catalytic ethylene oligomerization and dimer-
ization with the precatalysts 1 and 2 together with MAO as
cocatalyst will be presented.
The oligomerization of ethylene is one of the major in-
dustrial processes for the production of linear ␣-olefins [1–3].
Those oligomers in the range C6–C20, in particular 1-hexene,
are used as comonomers in the polymerization of ethylene
to give linear low-density polyethylene (LLDPE) or for the
preparation of detergents and synthetic lubricants. Catalysts
currently used in industry for the Shell Higher Olefin Process
(SHOP) contain Ni(II) complexes bearing bidentate monoan-
ionic ligands [4–15]. Furthermore, cationic Ni(II) ␣-diimine
complexes were intensively investigated [16–29], and espe-
cially Brookhart and co-workers have described differently
substituted ␣-diimine ligands to form stable nickel(II) com-
plexes which are highly effective ethylene oligomerization
or polymerization catalysts [17–25]. N,P-coordinated nickel
or palladium complexes were also described as active com-
pounds for the oligomerization and polymerization of ethy-
lene [30–34]. The dimerization of ethylene to butenes is used
industrially for generating powering components in fuel with
homogeneous titanium catalysts and special cocatalysts in the
2. Experimental
All manipulations were carried out under an argon at-
mosphere using standard Schlenk techniques. NMR spectra
were recorded on a Bruker spectrometer 250 MHz (1H) and
62.9 MHz (13C) at 293 K. Mass spectra were obtained us-
ing electron ionization (EI), electron spray ionization (ESI)
or field ionization (FI). Oligomer products were analyzed
by GC with a flame ionization detector, using a 50-m DB1
column, injector temperature 40 ◦C and the following tem-
perature program: 40 ◦C/5 min, 40–300 ◦C, 5 ◦C/10 min−1
.
The individual products were integrated, using n-tridecane
as internal standard.
Materials: MAO (10% solution in toluene) and all reac-
tants were purchased from Aldrich and used as received.
∗
Corresponding author. Tel.: +49 7247 82 2383; fax: +49 7247 82 2244.
E-mail addresses: martin.bluhm@itc-cpv.fzk.de (M.E. Bluhm),
manfred.doering@itc-cpv.fzk.de (M. Do¨ring).
1381-1169/$ – see front matter © 2004 Elsevier B.V. All rights reserved.
doi:10.1016/j.molcata.2004.11.015

178
M.E. Bluhm et al. / Journal of Molecular Catalysis A: Chemical 229 (2005) 177–181
2.1. Synthesis of ligands
2.2.3. Dimethyloxalic acid-bis(2-methylphenyl)-
imidoatenickel(II) dibromide (1b)
2.1.1. N-[(2-Chlorophenyl)methylene]-N-(1-pyridin-2-
ylethyl)amine (Ia)
A solution of dimethyl-N,Nꢀ-bis(2-methylphenyl)ethane
diimidoate (Ib) (1.10 g, 3.7 mmol) in 15 ml of DMF
was added dropwise to a solution of nickel(II) bromide
dimethoxyethane (1.15 g, 3.7 mmol) in 20 ml of DMF in a
Schlenk flask under argon while stirring. The mixture was
stirred for 5 h at room temperature. Then the solvent was re-
moved under vacuum and a pale blue powder was obtained.
The nickel complex was obtained in 95% yield and stored
under argon. ESI+-MS: m/z 515 [M+]. C18H20N2O2Br2Ni:
Calcd C 41.99, H 3.92, N 5.44; Found C 42.18, H 4.21, N
5.73.
A solution of 2-chlorobenzaldehyde (2.1 g, 15 mmol)
in 20 ml of methanol was added dropwise to a solution
of 1-pyridine-2-ylethanamine (1.8 g, 15 mmol) in 15 ml of
methanol at ambient temperature. The mixture was then
heated under reflux for 5 h. The solvent was removed un-
der reduced pressure, and a dark yellow oil was obtained
which was dried under vacuum. Yield: 3.2 g (13 mmol, 85%).
1H NMR (CDCl3) δ 8.28 (s, 1H, N CH), 8.50 (d, 1H, Pyr-
N CH), 8.11 (t, 1H, Ar H), 7.58 (t, 1H, Ar H), 7.47 (d,
1H, Ar H), 7.29–7.16 (m, 3H, Ar H), 7.07 (t, 1H, Ar H),
4.70 (q, 1H, CH3CH), 1.59 (d, 3H, CH3); EI+-MS: m/z = 245
[M+].
2.2.4. 2-[(Diisopropylphosphino)methyl]-
pyridinenickel(II) dibromide (2a)
2-[(Diisopropylphosphino)methyl]pyridine IIa [37]
(272 mg, 1.3 mmol) in 10 mL of DMF, NiBr2(dme) (400 mg,
1.3 mmol) in 40 mL of DMF. Compound 2a was obtained
as a purple red powder, yield 505 mg (1.18 mmol, 91%).
EI+-MS: m/z (%) 427 [M+, 10], 348 [M+ − Br, 100].
C12H20NPBr2Ni: Calcd C 33.69, H 4.71, N 3.27; Found C
33.97, H 4.95, N 3.03.
2.1.2. Dimethyloxalic acid-bis(2-methylphenyl)imidoate
(Ib) [36]
Oxalic acid-bis(2-methylphenyl)-bisimidoyldichloride
(3.04 g, 10 mmol) was suspended in 50 ml of methanol,
and then triethylamine (2.8 ml, 20 mmol) was added. The
mixture was heated under reflux as long as the product
was no longer visible through TLC (on SiO2, eluant
toluene/acetone = 10:1). After the methanol was evaporated
under reduced pressure, the residue was taken up in toluene.
After the precipitated HNEt3Cl was filtered, the toluene
was removed under reduced pressure. The residue was
crystallized from methanol. Yield: 1.99 g (6.7 mmol, 67%).
1H NMR (CDCl3): δ 6.86–6.92 (m, 6H), 6.54 (d, 2H), 3.83
(s, 6H), 1.24 (s, 6H); EI+-MS: m/z 296 [M+].
2.2.5. 2-(Diisopropyl)phosphinobenzyl-N,N-
dimethylaminenickel(II) dibromide (2b)
2-(Diisopropyl)phosphinobenzyl-N,N-dimethylamine
IIb [38] (326 mg, 1.3 mmol) in 10 mL of DMF, NiBr2(dme)
(400 mg, 1.3 mmol) in 40 mL of DMF. Compound 2a was
obtained as a blue powder, yield 568 mg (1.21 mmol, 93%).
EI+-MS: m/z (%) 391 [M+ − Br, 100]. C15H26NPBr2Ni:
Calcd C 38.35, H 5.58, N 2.98; Found C 38.47, H 5.77, N
3.15.
2.3. Oligomerization procedure
2.2. Synthesis of nickel(II) complexes
(a) Low-pressure tests. The precatalyst was dissolved in
30 mL of solvent in a Schlenk flask under argon. A com-
plete solution was obtained by leaving the flask in an ul-
trasonic bath for several minutes. A 150-mL glass reactor
was evacuated and then filled with argon. The precata-
lyst solution was added under argon into the reactor ves-
sel. Under stirring the cocatalyst MAO (0.6 mL, approx.
100 equiv. of a 10 mol.%MAOsolutionin toluene)and n-
tridecane standard solution were added under argon. For
several minutes ethylene was introduced while stream-
ing through the reactor to displace the argon. Then the
reactor was closed and pressurized to 3 bar with ethylene.
The reactor pressure maintained constant throughout the
oligomerization run by manually controlled addition of
ethylene. Runs were terminated by venting off volatiles
and extracting the solution with dilute hydrochloric acid
and water. Quantitative GC analysis of the organic layer
was performed immediately after the extraction.
2.2.1. General procedure for the synthesis of the nickel
complexes (1a–2b)
The appropriate ligand in dry DMF was added to a green
solution of NiBr2(dme) in dry DMF in a Schlenk flask under
argon. The mixture was stirred for 3 h at room temperature,
then the solvent was removed and finally the product was
dried in vacuum.
2.2.2. N-[(2-Chlorophenyl)methylene]-N-(1-pyridine-2-
ylethyl)-aminenickel(II) dibromide (1a)
N-[(2-Chlorophenyl)methylene]-N-(1-pyridine-2-yleth-
yl)-amine Ia (200 mg, 0.82 mmol), NiBr2(dme) (252 mg,
0.82 mmol), DMF 10 mL. Compound 1a was obtained
as a dark green solid, yield 261 mg (0.56 mmol, 69%).
ESI+-MS: m/z 463 [M+]; C14H13N2ClBr2Ni: Calcd
C 36.30, H 2.82, N 6.05; Found C 36.57, H 3.01,
N 6.14.
(b) High-pressure tests. Same procedure as described for
low-pressure tests. Instead of a glass reactor was used a

M.E. Bluhm et al. / Journal of Molecular Catalysis A: Chemical 229 (2005) 177–181
179
Table 2
150-mL stainless steel reactor with cooling mantle. After
Yield (%) of oligomers in the experiments 1–6
the reactor was closed, it was pressurized to 30 bar with
ethylene. The temperature of the reaction was controlled
by cooling the reactor vessel with water.
Fraction (%)
1
2
3
4
5
6
C4
C6
C8
9.3
89.0
6.5
3.8
0.2
78
17
4.6
0.4
–
–
–
–
–
–
73.0
23.0
3.5
0.5
0.1
–
–
–
–
–
73.5
18.7
4.0
1.3
0.7
0.5
0.4
0.3
–
80.8
13.7
3.6
1.2
0.4
0.1
0.05
0.03
–
–
–
–
16.7
16.2
11.5
9.5
8.4
6.4
5.2
4.8
4.5
4.1
C10
C12
C14
C16
C18
C20
C22
C24
C26
3. Results and discussion
0.15
0.13
0.1
0.07
–
–
–
Nitrogen- and phosphorus-substituted nickel(II) com-
plexes 1 and 2 were prepared to compare their catalytic abil-
ities as precatalysts for the oligomerization and dimerization
of ethylene together with the cocatalyst methylaluminoxane
(MAO) [39].
–
–
–
–
–
–
–
3.3
–
The imine nickel(II) complex 1a represents a typical class
of active neutral nickel(II) precursors for ethylene oligomer-
ization. Both the selectivity for formed oligomers and the
␣-selectivity are low, a broad spectrum of oligomers C4–C26
and small amounts of polymer being obtained (Table 1, entry
1). The distribution of oligomers obtained in the catalyzed
ethylene oligomerization with 1a follows Schulz–Flory rules
for oligomers C4, which can be characterized by the con-
stant α representing the probability of chain propagation
(α = rate of propagation/[(rate of propagation) + (rate of chain
transfer)] = (moles of Cn+2)/(moles of Cn)) [40–43]. Because
olefins with long chain lengths are obtained in the catalytic
oligomerization of ethylene with 1a, α has a high value of
0.8.
␣-Diimine nickel(II) complexes were already examined as
suitable catalysts for ethylene oligomerization [17]. Like the
imine precursor 1a, α-diimine nickel(II) complexes can pro-
duceoligomersintherangeC4–C26 [17]. Insteadofaglyoxal-
or ␣-diketo-diimine, the ␣-diimine nickel(II) complex 1b
contains the diimino ester of oxalic acid: the electron-
withdrawing methoxy groups influence the product distri-
bution of formed oligomers in the ethylene oligomerization
catalysis with 1b and MAO towards the production of olefins
with low chain lengths. Butenes are formed at 3 bar ethy-
lene pressure as main product in 89% yield (Table 1, entry
2). Higher ethylene pressure with a shorter reaction time has
nearly no effect on the activity of the catalyst and mostly
produces butenes and hexenes (Table 1, entry 3). For both
imine complexes 1a and 1b the ␣-selectivities of the formed
oligomers are low to moderate (Table 1, entries 1–3), derived
from a favored chain isomerization in the ␣-hydride elimina-
tion reinsertion step to give linear isomeric olefins [17,18,29].
The coordinating ligand IIa of the nickel complex 2a
was synthesized from 2-picoline and chloro-diisopropyl-
phosphane with n-butyllithium according to Ref. [37].
The 2-(Diisopropylphoshino-methyl)-pyridine (IIa) was
then reacted with (dme)NiBr2 to give the nickel(II) dibro-
mide complex 2a. A similar 6-aryl-substituted 2-(diaryl-
phosphinomethyl)-pyridine-nickel(II) dibromide complex
was recently investigated as precatalyst for the di- and
trimerization of ethylene [32]. 2a mostly produces butenes
and hexenes and shows a similar oligomer distribution
(Table 2, No. 4) compared with the 6-aryl-substituted
2-(diarylphosphinomethyl)-pyridine-nickel(II) dibromide
complex in [32] (70.5% butenes, 29.5% hexenes). 2a has a
low overall α-selectivity of 46% (Table 1, entry 4), similar
to the more bulky substituted nickel(II) complex in [32].
2a bears less bulky substituents and has a lower catalytic
activity (Table 1). The N,P nickel(II) complex 2b is less
reactive than 2a, but shows a much better selectivity for
the production of ␣-olefins. The α-value is low, so the
dimerization of ethylene is favored in the catalytic reaction
with 2b and MAO. A longer reaction time provides a
better yield, even at lower ethylene pressure, indicating a
substantial lifetime of the catalyst (Table 1, entries 5 and 6).
Table 1
Results of oligomerization of ethylene with catalysts 1 and 2
No.
Catalystsa
Loading (␮mol)
Time (h)
p (bar)
T (◦C)
Yield (g)b
TON
TOF (h−1
)
Solvent
α
␣-Olefins (%)b
1
2
3
4
5
6
a
1a
1b
1b
2a
2b
2b
10
10
10
10
10
10
1
4.5
1
1.7
2
5
30
3
30
30
30
3
22–30
22
22–31
22–31
22–30
22
2.7c
<0.1
<0.1
4.4
0.2
0.4
9526
281
43
15792
758
1446
9526
62
43
9024
379
290
Toluene
Toluene
Toluene
Toluene
CH2Cl2
Toluene
0.80
0.26
0.24
0.24
0.35
0.36
33
58
40
46
77
50
All precatalysts were first dissolved in 30 mL of solvent in an ultrasonic bath and then activated with approx. 100 equiv. MAO (0.6 mL of a 10 mol% MAO
solution in toluene).
b
The yield and the 1-olefin content C4–C26 were determined by GC and a flame ionization detector using calibration curves with standard solutions.
Small amounts of polymer formed in the reaction.
c

180
M.E. Bluhm et al. / Journal of Molecular Catalysis A: Chemical 229 (2005) 177–181
Table 3
Crystallographic data
2b
Empirical formula
CCDC No.
C
15H26Br2NPNi
252695
469.87
293(2)
0.71073
Monoclinic
P21/n
fw (g mol−1
)
Temperature (K)
˚
Wavelength λ (A)
Crystal system
Space group
◦
˚
˚
˚
Unit cell dimensions
a = 10.116(3) A, α = 90
◦
b = 16.605(4) A, β = 94◦.06(3)
c = 11.064(3) A, γ = 90
3
˚
V (A )
1853.8(9)
Z
4
ρcalcd. (g cm−3
)
1.684
5.435
944
µ (mm−1
F(0 0 0)
Crystal size (mm3)
Theta range for data collection (◦)
Index ranges
)
Fig. 1. Structure of the precatalysts 1 and 2.
0.6 × 0.2 × 0.1
2.22–28.30
−13 ≤ h ≤ 13, −21 ≤ k ≤ 21,
−14 ≤ l ≤ 14
19385
Reflections collected
Independent reflections
Data/restraints/parameters
Goodness-of-fit on F2
Final R indices [I > 2σ(I)] a
R indices (all data)a
4516 [R(int) = 0.0536]
4516/0/192
1.001
R1 = 0.0293, wR2 = 0.0657
R1 = 0.0438, wR2 = 0.0698
ꢃꢃ
ꢁ
ꢄ
ꢀ
ꢂ
ꢁ
ꢁ
R1 =
||Fo| − |Fc|| / |Fo|,
ꢄ1/2
ꢂ
wR2 =
w(|Fo2 − Fc2|)2
/
a
ꢀ
ꢁ
w(Fo2)
, w = 1/[(σFo)2 + (aP)2]. The value of aP was obtained
from structure refinement.
Schulz–Flory α-values, which prove a low chain propagation
with preference for the production of 1-butenes and isomers.
˚
Fig. 2. Molecular structure of 2b. Selected bond distances (A) and angles
(◦): Ni1 N1 2.042(2), Ni1 P1 2.308(1), Ni1 Br1 2.363(1), Ni1 Br2
2.381(1), N1 C7 1.493(3), N1 Ni1–P1 96.61(6), C7 N1 Ni1 111.91(2),
Br1 Ni1 N1 112.47(6), Br1 Ni1 P1 101.73(3), Br2 Ni1 N1
103.74(6), Br2 Ni1 P1 123.98(2).
4.1. Crystal structure determination
The intensity data for the compounds were collected by
a Siemens Smart 1000 CCD diffractometer (compound 2b)
using graphite-monochromated Mo-K␣ radiation. The X-ray
analyses were performed with an irradiation time of 10 s per
frame collecting a full sphere of data. Data were corrected for
Lorentz and polarization effects, an experimental absorption
correction was performed with SADABS [45].
The structures were resolved by direct methods and re-
fined by full-matrix least-squares techniques against Fo2
(SHELXL-97 [46]). The hydrogen atoms were localized by
difference Fourier synthesis and refined isotropically. The
data are deposited in the Cambridge Crystallographic Data
Centre [47] (see Table 3).
Similar built palladium iminophosphine complexes showed
a comparable reactivity in the ethylene oligomerization, but
they mainly form hexenes, octenes, and higher oligomers
[34]. The X-ray structure analysis of 2b shows a distorted
tetrahedral nickel(II) ion connected to its neighbouring
nitrogen- and phosphorus atoms in the ligand (Figs. 1 and 2).
Observed bond lengths are in the range of published data
[44].
All non-hydrogen atoms were refined anisotropically [46].
XPMA was used for visual analysis of the structure reported
[48], and Ortep [49] was used for structure representations.
4. Conclusions
In summary, the described nickel(II) imine and diimine
complexes 1 and the N,P-substituted nickel(II) complexes
2 reveal differences in their behavior as precatalysts for the
oligomerization of ethylene. 2a has a good activity with ethy-
lene in the presence of MAO as cocatalyst for the dimer-
ization of ethylene, while 2b mainly produces butenes with
an α-selectivity of 77%. The complexes 1b–2b show small
References
[1] D. Vogt, in: B. Cornils, W.A. Herrmann (Eds.), Applied Homo-
geneous Catalysis with Organometallic Compounds, vol. 1, VCH
Publishers, 1996, pp. 245–256.

M.E. Bluhm et al. / Journal of Molecular Catalysis A: Chemical 229 (2005) 177–181
181
[2] G.W. Parshall, S.D. Ittel, Homogeneous Catalysis: The Applications
and Chemistry of Catalysis by Soluble Transition Metal Complexes,
Wiley, New York, 1992, pp. 68–72.
[26] T. Schleis, T.P. Spaniol, J. Okuda, J. Heinemann, R. Mu¨lhaupt, J.
Organomet. Chem. 569 (1998) 159.
[27] M. Helldo¨rfer, J. Backhaus, W. Milius, H.G. Alt, J. Mol. Catal. A:
Chem. 193 (2003) 59.
[3] L. Skupinska, Chem. Rev. 91 (1991) 613.
[4] W. Keim, F.H. Kowaldt, R. Goddard, C. Kru¨ger, Angew. Chem., Int.
Ed. Engl. 17 (1978) 466.
[28] M. Helldo¨rfer, W. Milius, H.G. Alt, J. Mol. Catal. A: Chem. 197
(2003) 1.
[5] W. Keim, B. Hoffmann, R. Lodewick, M. Peuckert, G. Schmitt, J.
Mol. Catal. 6 (1979) 79.
[29] D.G. Musaev, R.D.J. Froese, K. Morokuma, Organometallics 17
(1998) 1850.
[6] M. Peuckert, W. Keim, Organometallics 2 (1983) 594.
[7] W. Keim, A. Behr, B. Limbacker, C. Kruger, Angew. Chem., Int.
Ed. Engl. 22 (1983) 503.
[8] W. Keim, A. Behr, G. Kraus, J. Organomet. Chem. 251 (1983) 377.
[9] M. Peuckert, W. Keim, J. Mol. Catal. 22 (1984) 289.
[10] W. Keim, Angew. Chem., Int. Ed. Engl. 29 (1990) 235.
[11] W. Keim, R.P.J. Schulz, J. Mol. Catal. 92 (1994) 21.
[12] W. Keim, R.S. Bauer, H.C. Chung, P. Glockner, US Patent 3,635,937
(1969).
[13] W. Keim, R.F. Mason, P. Glockner, US Patent 3,647,914 (1972).
[14] W. Keim, R.F. Mason, P. Glockner, Eur. Chem. News 16 (1977)
60.
[15] A. Behr, W. Keim, Arabian J. Sci. Eng. 10 (1985) 377.
[16] S.M. Pillai, M. Ravindranathan, S. Sivaram, Chem. Rev. 86 (1986)
353.
[17] C.M. Killian, L.K. Johnson, Organometallics 16 (1997) 2005.
[18] L.K. Johnson, C.M. Killian, M. Brookhart, J. Am. Chem. Soc. 117
(1995) 6414.
[19] S.A. Svejda, M. Brookhart, Organometallics 18 (1999) 65.
[20] D.P. Gates, S.A. Svejda, E. Onate, C.M. Killian, L.K. Johnson, P.S.
White, M. Brookhart, Macromolecules 33 (2000) 2320.
[21] S.A. Svejda, L.K. Johnson, M. Brookhart, J. Am. Chem. Soc. 121
(1999) 10634.
[30] G. Mu¨ller, M. Klinga, P. Osswald, M. Leskela¨, B. Rieger, Z. Natur-
forsch. 57b (2002) 803.
[31] O. Daugulis, M. Brookhart, Organometallics 21 (2002) 5926.
[32] H.-P. Chen, Y.-H. Liu, S.-M. Peng, S.-T. Liu, Organometallics 22
(2003) 4893.
[33] Z.B. Guan, W.J. Marshall, Organometallics 21 (2002) 3580.
[34] E.K. van den Beuken, W.J.J. Smeets, A.L. Spek, B.L. Feringa, Chem.
Commun. (1998) 223.
[35] K. Weissermel, H.-J. Arpe, Industrial Organic Chemistry, Wiley-
VCH Publishers, 1998, p. 74.
[36] J. Wuckelt, Dissertation Universita¨t Jena, 1998.
[37] A. Jansen, S. Pitter, Monat. Chem. 130 (1999) 783.
[38] M. Leibold, O. Walter, unpublished results.
[39] M. Bluhm, C. Folli, R. Beckert, M. Do¨ring, Book of abstracts, in:
The European Catalysis Forum, EuropaCat-VI, Innsbruck, Austria,
August 31–September 9, 2003, p. 3023.
[40] G.V. Schulz, Z. Phys. Chem. Abst. B 30 (1935) 379.
[41] G.V. Schulz, Z. Phys. Chem. Abst. B 43 (1939) 25.
[42] P.J. Flory, J. Am. Chem. Soc. 62 (1940) 1561.
[43] G. Henrici-Olive´, S. Olive´, Adv. Polym. Sci. 15 (1974) 1.
[44] A.G. Orpen, L. Brammer, F.H. Allen, O. Kennard, D.G. Watson, J.
Chem. Soc., Dalton Trans. (1989) S1.
[45] SADABS, Siemens area detector absorption correction programme,
Siemens, 1997.
[46] G.M. Sheldrick, SHELXL-97, University of Go¨ttingen, Germany,
1997.
[22] M. Brookhart, L.K. Johnson, C.M. Killian, E.F. McCord, S.J.
McLain, K.A. Kreutzer, S.D. Ittel, D.J. Tempel, Du Pont Co., US
Patent Appl. US 5,880,241 (1999).
[23] S.J. McLain, J. Feldman, E.F. McCord, K.H. Gardner, M.F. Teasley,
E.B. Coughlin, K.J. Sweetman, L.K. Johnson, M. Brookhart, Polym.
Mater. Sci. Eng. 76 (1997) 20.
[24] C.M. Killian, D.J. Tempel, L.K. Johnson, M. Brookhart, J. Am.
Chem. Soc. 118 (1996) 11664.
[47] CCDC 252695 (2b) contains the supplementary crystallographic
data for this paper. These data can be obtained free of charge via
www.ccdc.cam.ac.uk/conts/retrieving.html (or from the Cambridge
Crystallographic Data Centre, 12, Union Road, Cambridge CB21EZ,
UK; fax: (+44) 1223-336-033; or deposit@ccdc.cam.ac.uk).
[48] L. Zsolnai, XPMA, University of Heidelberg, Germany, 1997.
[49] I.J. Farrugia, J. Appl. Cryst. 30 (1997) 565.
[25] L.K. Johnson, M. Christopher, M. Brookhart, J. Am. Chem. Soc.
117 (1995) 254.