Nickel complexes bearing new P,N-phosphinopyridine ligands for the catalytic oligomerization of ethylene

Article abstract of DOI:10.1021/om034203i

The new phosphinopyridine ligands 2-[(diphenylphosphanyl)methyl]-6- phenylpyridine (5), 2-(2,6-dimethylphenyl)-6-[(diphenylphosphanyl)methyl] pyridine(6), 2-(2,6-diisopropylphenyl)6-[(diphenylphosphanyl)methyl]pyridine (7), and 2-[2-(diphenylphosphanyl)phenyl]-6-methylpyridine (8) have ben prepared in order to compare the influence of their stereoelectronic properties on the catalytic behavior of their respective mononuclear [NiCl₂(P,N)] complexes 9-12. These Ni(II) complexes are paramagnetic in solution, as determined by the Evans method. Whereas they were inactive for the oligomerization of ethylene in the presence of methylalumoxane (MAO), they provided activities up to 61 000 mol of C₂H₄/((mol of Ni) h) (11) in the presence of only 6 equiv of AlEtCl₂. The selectivities for C₄ olefins were as high as 92% (12 with 2 equiv of AlEtCl _a).

Full text of DOI:10.1021/om034203i

Organometallics 2004, 23, 2633-2640
2633
Nick el Com p lexes Bea r in g New P ,N-P h osp h in op yr id in e
Liga n d s for th e Ca ta lytic Oligom er iza tion of Eth ylen e
Fredy Speiser and Pierre Braunstein*
Laboratoire de Chimie de Coordination (UMR 7513 CNRS), Universite´ Louis Pasteur,
4 rue Blaise Pascal, F-67070 Strasbourg Ce´dex, France
Lucien Saussine
Institut Franc¸ais du Pe´trole, Direction Catalyse et Se´paration, CEDI Rene´ Navarre,
BP 3, F-69390 Vernaison, France
Received September 27, 2003
The new phosphinopyridine ligands 2-[(diphenylphosphanyl)methyl]-6-phenylpyridine (5),
2-(2,6-dimethylphenyl)-6-[(diphenylphosphanyl)methyl]pyridine (6), 2-(2,6-diisopropylphenyl)-
6-[(diphenylphosphanyl)methyl]pyridine (7), and 2-[2-(diphenylphosphanyl)phenyl]-6-me-
thylpyridine (8) have ben prepared in order to compare the influence of their stereoelectronic
properties on the catalytic behavior of their respective mononuclear [NiCl2(P,N)] complexes
9-12. These Ni(II) complexes are paramagnetic in solution, as determined by the Evans
method. Whereas they were inactive for the oligomerization of ethylene in the presence of
methylalumoxane (MAO), they provided activities up to 61 000 mol of C2H4/((mol of Ni) h)
(11) in the presence of only 6 equiv of AlEtCl2. The selectivities for C4 olefins were as high
as 92% (12 with 2 equiv of AlEtCl2).
In tr od u ction
in the presence of large quantities of methylalumoxane
(MAO).1 This triggered a renewed interest in this family
of ligands and led to the synthesis of Ni(II) and Pd(II)
complexes with, for example, unsymmetrical iminopy-
ridine (2),14-17 pyridinecarboxamido (3),18 and imino-
phosphine ligands (4),19,20 which have been used for the
Bidentate diimine ligands have played an increasing
role in coordination chemistry as well as in the transi-
tion-metal-catalyzed oligomerization and polymerization
of R-olefins.1-7 Following the early works of Balch and
Holm,8 Walther,9 and tom Dieck and co-workers,10-12
Brookhart and co-workers1b,13 discovered that [NiCl2-
(diimine)] complexes of type 1 provide excellent results
(2) (a) Helldo¨rfer, M.; Milius, W.; Alt, H. G. J . Mol. Catal. 2003,
197, 1-13. (b) Ceder, R. M.; Muller, G.; Ordinas, M.; Font-Bardia, M.;
Solans, X. Dalton 2003, 3052-3059.
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S.; Redshaw, C.; Solan, G. A.; White, A. J . P.; Williams, D. J . Chem.
Commun. 2001, 1639-1644.
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H.; Redshaw, C.; Solan, G. A.; White, A. J . P.; Williams, D. J . Eur. J .
Inorg. Chem. 2001, 431-437.
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Schumann, H.; Sieler, J . J . Organomet. Chem. 2001, 626, 243-259.
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for the conversion of R-olefins to oligomers and polymers
* To whom correspondence should be addressed. E-mail: braunst@
chimie.u-strasbg.fr. Fax: (+33) 390 241 322.
(1) (a) Late Transition Metal Polymerization Catalysis, Rieger, B.,
Saunders Baugh, L., Kacker, S., Striegler, S., Eds.; Wiley-VCH:
Weinheim, Germany, 2003. (b) Ittel, S. D.; J ohnson, L. K.; Brookhart,
M. Chem. Rev. 2000, 100, 1169-1203. (c) Britovsek, G. J . P.; Gibson,
V. C.; Wass, D. F. Angew. Chem., Int. Ed. 1999, 38, 428-447 and
references cited therein.
10.1021/om034203i CCC: $27.50 © 2004 American Chemical Society
Publication on Web 04/23/2004

2634 Organometallics, Vol. 23, No. 11, 2004
Speiser et al.
catalytic oligomerization or polymerization of ethylene
and illustrate the versatility of these systems.
found widespread use in the synthesis of annelated
pyridine or bipyridine systems,24,25,28 although they
imply the use of Pd catalysts, aryltin compounds, or
arylboronic acid esters which first have to be prepared
from the corresponding aryl Grignard compounds or
lithium reagents.24,25 Therefore, the Kumada coupling
reaction seemed to be the method of choice, since cheap
Ni(II) salts are used as catalysts in the presence of
phosphines. In contrast to the Stille or Suzuki couplings,
Grignard reagents can be used directly. In some cases,
the homocoupling reaction of the Grignard reagents
dominates, which prevents a universal use of the
Kumada reaction.26 In our case, however, hardly any
homocoupling products were observed (eqs 1-3).
Only few complexes containing phosphinopyridines
have been reported for their application in the oligo-
merization or polymerization of R-olefins,1,21,22 despite
the considerable academic and industrial importance of
these reactions.23 In view of the decisive influence of the
ligands and, in particular, of the steric hindrance of
their substituents in controlling oligomerization versus
polymerization, we decided to prepare Ni(II) complexes
bearing the new phosphinopyridines 5-8, which possess
the characteristics of a pyridine heterocycle with its
rigid and chemically inert structure and, at the same
time, allow easy variation of the steric hindrance around
the metal center. Modifying the ligand basicity also
provides an excellent tool to influence the catalytic
properties. Whereas in a previous study with other
phosphinopyridine ligands we have focused on substitu-
ent effects in the one-carbon connection between the
ortho pyridine carbon and the PPh2 group,22 we shall
be dealing here, with 5-8, with different substituents
in the ortho position of the pyridine ring. The catalytic
properties of the new Ni(II) halide complexes 9-12 have
been investigated with the objective of favoring ethylene
oligomerization vs polymerization while using the small-
est possible amount of added cocatalyst.
The Ni catalyst used for the Grignard coupling
reactions was based on the Ni(acac)2/P(t-Bu)3 catalyst
system,29 although the experimental procedure had to
be strongly modified for synthetic reasons (see Experi-
mental Section). After the Grignard reagent was added
at 0 °C to the catalyst/substrate mixture, the solution
was refluxed overnight in diethyl ether, which afforded
after workup the 2-aryl-6-methylpyridines 14-16 in
1
good yields. The H NMR spectrum of 16 contained two
doublets for the isopropyl CH3 protons. Assuming an
orientation of the aryl group perpendicular to the
pyridine ring, the isopropyl groups would be equivalent
and carry two inequivalent methyl groups. Consistently,
two singlets were observed in the 13C NMR spectrum
for these methyl groups. However, if the aryl and
pyridine groups are oriented as shown in eq 3, in the
mirror plane of the molecule, thus making the isopropyl
groups inequivalent but the methyl protons of each
isopropyl group equivalent, the spectral data would be
consistent with the rotation of the 2,6-bis(isopropyl)-
phenyl group around the aryl-pyridine bond being
blocked at room temperature. Only one singlet was
observed at room temperature for the o-CH3 protons of
the phenyl group in 15, which could be consistent with
a perpendicular orientation of the aryl and pyridine
Resu lts a n d Discu ssion
Syn th esis of th e Liga n d s. To prepare the phosphi-
nopyridines 5-8, aryl substituents have to be intro-
duced on the pyridine heterocycle 13, and typical
methods include Stille,24 Suzuki,25 and Kumada26,27
coupling reactions. Stille and Suzuki reactions have
(20) Nobuyuki, U.; Katsuhiro, I. J pn. Patent J P 11158213, 1998
(Asahi Chem Ind Co. Ltd.).
(21) (a) Bonnet, M. C.; Dahan, F.; Ecke, A.; Keim, W.; Schulz, R. P.;
Tkatchenko, I. J . Chem. Soc., Chem. Commun. 1994, 615-616. (b)
Ecke, A.; Keim, W.; Bonnet, M. C.; Tkatchenko, I.; Dahan, F. Orga-
nometallics 1995, 14, 5302-5307.
(22) Speiser, F.; Braunstein, P.; Saussine, L. Organometallics 2004,
23, 2625-2632.
(23) (a) Parshall, G. W.; Ittel, S. D. Homogeneous Catalysis: The
Applications and Chemistry of Catalysis by Soluble Transition Metal
Complexes; Wiley: New York, 1992; pp 68-72. (b) Vogt, D. In Applied
Homogeneous Catalysis with Organometallic Compounds; Cornils, B.,
Herrmann, W. A., Eds.; VCH: Weinheim, Germany, 1996; Vol. 1, pp
245-256. (c) Chauvin, Y.; Olivier, H. In Applied Homogeneous Ca-
talysis with Organometallic Compounds; Cornils, B., Herrmann, W.
A., Eds.; VCH: Weinheim, Germany, 1996; Vol. 1, pp 258-268. (d)
Small, B. L. Organometallics 2003, 22, 3178-3183.
(26) (a) Tamao, K.; Sumitani, K.; Kumada, M. J . Am. Chem. Soc.
1972, 94, 4374-4376. (b) Kumada, M. Pure Appl. Chem. 1980, 52, 669-
679.
(27) Corriu, R. J . P.; Masse, J . P. J . Chem. Soc., Chem. Commun.
1972, 144.
(28) Wolfe, J . P.; Buchwald, S. L. Angew. Chem., Int. Ed. 1999, 38,
2413-2416.
(29) Bo¨hm, V. P. W.; Weskamp, T.; Gsto¨ttmayr, C. W. K.; Herrmann,
W. A. Angew. Chem., Int. Ed. 2000, 9, 1602-1604.
(24) Stille, J . K. Angew. Chem., Int. Ed. Engl. 1986, 25, 508-524.
(25) Miyaura, N.; Suzuki, A. Chem. Rev. 1995, 95, 2457-2483.

Ni Complexes with P,N-Phosphinopyridine Ligands
Organometallics, Vol. 23, No. 11, 2004 2635
rings or with accidental equivalence of the o-Me groups
in a structure where both aryl and pyridine rings are
coplanar or there is easier rotation of the aryl group
about the C-C axis. On the basis of steric arguments,
the kinetic barrier for this rotation is anticipated to
increase in the order 14 < 15 < 16.
The first attempt to introduce the phosphine moiety
was made by metalation of the pyridine-bound methyl
group with an appropriate base and subsequent addition
of 1 equiv of PPh2Cl. This led to the formation of two
flate for 20-150 h in DMF in the presence of Pd(II) or
Ni(II) catalysts. Their synthesis allows the presence of
a large number of functional groups on the aromatic
cycles (eq 8), except at the 6-position of the pyridine ring.
Yields of ca. 50-70% were observed with the Pd catalyst
against 30-48% in the case of Ni. It remains to be
verified whether our synthetic pathway can be extended
to a large range of phosphinopyridines, but the ortho-
metalation route appears rather elegant and cost-
effective, since no palladium salts or aryl triflates are
required.
compounds, which could be identified as the desired
2-[(diphenylphosphanyl)methyl]-6-phenylpyridine (5) and
the ortho-metalation product 8 (eq 4). A similar ortho-
metalation reaction was observed by Zhang and co-
workers30 in the synthesis of the phosphine 17 (eq 5).
To introduce a diphenylphosphino substituent on the
o-methyl group of the pyridine heterocycle, the arylpi-
colines 14-16 were lithiated at -78 °C with 1 equiv of
t-BuLi. Slow addition of 1 equiv of PPh2Cl at -78 °C
afforded 5-7 in good yields (eq 9). These ligands display
The mixture of the phosphinopyridines 5 and 8 proved
difficult to separate by chromatographic methods or by
distillation. Therefore, the synthetic pathway was modi-
fied: 14 was reacted at room temperature with 1 equiv
of n-BuLi and the phosphine moiety was introduced by
reaction of the ortho-metalated pyridine with PPh2Cl
without cooling of the exothermic reaction. The ligand
8 was isolated in 60% yield (eq 6). When a similar
a singlet in the 31P{1H} NMR spectrum at δ -10.8, -9.7,
and -9.4, respectively. This trend is consistent with a
more electron donating R group increasing the electron
density at the phosphorus atom.
Syn th esis of th e Ni(II) Com p lexes. The Ni(II)
phosphinopyridine complexes 9-12 were prepared by
stirring equimolar amounts of [NiCl2(DME)] with the
corresponding ligand for 12 h in CH2Cl2 at 25 °C. After
workup, the Ni complexes were isolated in 70-80%
yields (eqs 10 and 11).
reaction was carried out using 2-phenylpyridine as
substrate, no orthometalation was observed (eq 7). This
suggests a directing ortho effect on the phenyl ring
reasonably due to facilitated chelation of Li+ by the more
basic nitrogen atom of 14.
Chan and co-workers31,32 prepared atropisomeric phos-
phinopyridines 18 by refluxing the corresponding tri-
(30) J iang, Y.; J iang, Q.; Zhu, G.; Zhang, X. Tetrahedron Lett. 1997,
38, 215-218. See also: Yamagishi, T.; Ohnuki, M.; Kiyooka, T.; Masui,
D.; Sato, K.; Yamaguchi, M. Tetrahedron: Asymmetry 2003, 14, 3275-
3279.

2636 Organometallics, Vol. 23, No. 11, 2004
Speiser et al.
All four complexes are paramagnetic in solution, as
determined by the Evans method,33,34 with magnetic
moments of 2.40 (9), 2.42 (10), 2.62 (11) and 1.82 µB (12),
respectively. The magnetic moment of Ni(II) complexes
depends strongly on the ligand field, and large varia-
tions can be observed.35-39 Therefore, the magnetic
moments of 9-12 should be compared to those of similar
complexes whose molecular structures are known. The
X-ray structure of [NiCl2{2-[1′-(diphenylphosphanyl)-1′-
methylethyl]-4,4-dimethyl-4,5-dihydrooxazole}] (19) has
been determined recently and showed a tetrahedral
coordination sphere for the metal center, and a magnetic
moment of 2.15 µB was experimentally determined in
solution.40Therefore, a distorted-tetrahedral coordina-
tion geometry of the metal is assumed for complexes
9-12 in solution. A comparison of the infrared spectra
of the phosphinopyridine ligands and their complexes
clearly shows that the nitrogen heterocycle is coordi-
nated to the metal center.41
Ca ta lytic P r op er ties of th e Com p lexes [NiCl2-
(p h osp h in op yr id in e)] (9-12) for Eth ylen e Oligo-
m er iza tion . Compounds 9-12 were tested for the
oligomerization of ethylene using the smallest possible
quantities of the cocatalysts AlEtCl2 and MAO. In the
presence of either 2 or 6 equiv of AlEtCl2 the complexes
showed good activities (Figure 1). When 2 equiv of
cocatalyst was used, turnover frequencies (TOF) were
47 300, 43 200, 44 400 and 22 100 mol of C2H4/((mol of
Ni) h), respectively (Table 1). Increasing the amount of
cocatalyst to 6 equiv raised the TOFs to 57 000, 57 200,
61 000 and 56 100 mol of C2H4/((mol of Ni) h), respec-
tively.
F igu r e 1. Activity of complexes 9-12 for the oligomer-
ization of ethylene using different quantities of AlEtCl2.
Conditions: T ) 30 °C, 10 bar of C2H4, 35 min, 4 × 10-2
mmol of Ni, 15 mL of toluene.
Only 2 equiv of AlEtCl2 was sufficient to lead to a high
selectivity for butenes: 82% (9), 83% (10), 73% (11) and
92% (12).
An increase in the amount of cocatalyst from 2 to 6
equiv tends, except for 11, to reduce the selectivities for
butenes to 71% (9), 78% (10), 77% (11), and 70% (12)
(Figure 2). The remaining products were ethylene
trimerization products (C6) and very small quantities
of C8 oligomers. The selectivities to 1-butene within the
C4 fraction of only 9-14% could be due to the ability of
complexes 9-12 to reversibly eliminate â-H after eth-
ylene insertion and reinsert the olefin with the opposite
regiochemistry and give 2-butene after chain transfer
or to lead to isomerization of 1-butene by a re-uptake
mechanism. The ability of Ni(II) complexes to isomerize
R-olefins is well-known.44 The kR values given in Table
1 correspond to the ratio hexenes (mol)/butenes (mol)
and not to the Schultz-Flory constant, since our
catalysts are mainly dimerization and trimerization
catalysts.
The considerable preference for ethylene dimerization
stands in interesting contrast to observations by
Brookhart and co-workers, who noted a wider distribu-
tion of oligomers in the oligomerization of ethylene and
propylene catalyzed by the square-planar Ni-diimine
complexes of type 20 with MAO, MMAO, MAO-IP, and
AlEtCl2 as cocatalysts. An increase of the steric hin-
In the presence of 400 or 800 equiv of MAO, only
decomposition of complexes 9-12 was observed. This
could be due to the presence of ca. 10% of AlMe3 in
commercial MAO.42 Other groups have also reported
that AlR3 impurities in AlClEt2 or pure trialkylalumi-
num compounds reduce Ni(II) catalysts to inactive Ni-
(0) species.43 Consistently, complexes 9-12 decomposed
in the presence of AlEt3 under ethylene pressure.
(31) Kwong, F. Y.; Chan, A. S. C.; Chan, K. S. Tetrahedron 2000,
56, 8893-8899.
(32) Kwong, F. Y.; Chan, K. S. Organometallics 2000, 19, 2058-
2060.
(33) Evans, D. F. J . Chem. Soc. 1959, 2003-2005.
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(35) Greenwood, N. N.; Earnshaw, A. Chemistry of the Elements;
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Chemie, 91st-100th ed.; Walter de Gruyter: Berlin, 1985; pp 1152-
1156.
(37) Dori, Z.; Gray, H. B. J . Am. Chem. Soc. 1966, 88, 1394-1398.
(38) Hayter, R. G.; Humiec, F. S. Inorg. Chem. 1965, 4, 1701-1706.
(39) Venanzi, L. M. J . Chem. Soc. (A) 1958, 719-724.
(40) Speiser, F.; Braunstein, P.; Saussine, L.; Welter, R. Organo-
metallics 2004, 23, 2613-2624.
(41) Pfeffer, M.; Braunstein, P.; Dehand, J . Spectrochim. Acta 1974,
30A, 331-340.
(42) Chen, E. Y.-X.; Marks, T. J . Chem. Rev. 2000, 100, 1391-1434.
(43) (a) Brown, S. J .; Masters, A. F. J . Organomet. Chem. 1989, 367,
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drance around the metal center caused by the ortho aryl
substituents led to the formation of long-chain oligo-
mers. The turnover frequencies were between 22 000

Ni Complexes with P,N-Phosphinopyridine Ligands
Organometallics, Vol. 23, No. 11, 2004 2637
Ta ble 1. Com p a r a tive Ca ta lytic Da ta for Com p lexes 9-12, 19, 23, a n d [NiCl2(P Cy3)2] in th e Oligom er iza tion
of Eth ylen e w ith AlEtCl2 a s Coca ta lysta
9
10
11
12
[NiCl2(PCy3)2]
19
23
amt of AlEtCl2 (equiv)
selectivity C4 (%)
selectivity C6 (%)
selectivity C8 (%)
selectivity C10 (%)
selectivity C12 (%)
6
2
6
2
83
18
6
2
6
2
92
8
6
6
6
2
88
12
71
29
1
82
18
1
78
21
1
77
22
1
73
26
1.5
70
28
2
54
40
1
65
32
2.5
86
14
0.5
3.5
productivity (g of C2H4/ 27 200 22 600 27 300 20 600 29 100 21 200 26 800 10 500 22 000 27 800 13 000
((g of Ni) h))
800
TOF (mol of C2H4/
((mol of Ni) h))
57 000 47 300 57 200 43 200 61 000 44 400 56 100 22 100 45 900 58 100 27 200
1600
R-olefin (C4) (%)
9
0.27
9
0.14
9
0.18
14
0.14
10
0.19
9
0.23
9
0.27
13
0.06
20
0.50
11
0.33
9
0.11
12
0.09
b
kR
a
b
Conditions: T ) 30 °C, 10 bar of C2H4, 35 min, 4 × 10-2 mmol of Ni complex, solvent 15 mL of toluene. kR) hexenes (mol)/butenes
(mol).
systems, where steric blocking of the axial positions of
the metal is crucial for chain length control,1,13,18b and
could indicate that our R groups were not bulky enough
to induce significant variations. The product distribution
observed with our catalysts may also be related to the
different metal coordination geometries, since our Ni(II)
complexes have a tetrahedral environment. This seems
to lead to catalysts that favor formation of internal
olefins, although this should not be generalized. Selec-
tivities to 1-butene within the C4 fraction similar to
those of our complexes were also observed with the
square-planar complex [NiCl2(PCy3)2], a typical R-olefin
dimerization catalyst, at variance with the results of
Bazan and co-workers.18 Our ligand system leads,
however, to a catalyst ca. 2.5 times more active than
that with PCy3. Table 1 contains comparative data,
including the five-membered-ring complex 19, which has
a tetrahedral geometry and contains a dimethyl-
substituted phosphinooxazoline ligand,40 and 23, which
F igu r e 2. Selectivity of complexes 9-12 for C4 oligomers
in mass percent. Conditions: T ) 30 °C, 10 bar of C2H4,
35 min, 4 × 10-2 mmol of Ni, 15 mL of toluene.
and 140 000 mol of C2H4/((mol of Ni) h) in the presence
of 200-400 equiv of cocatalyst, and a maximum selec-
tivity of 91% for R-olefin was observed.45
Recently Bazan et al. reported that ethylene oligo-
merization is favored over polymerization with pyridi-
necarboxamidato ligands that do not fully shield the
nickel center, as shown in 21. This was contrasted with
the situation in complex 22, where the ligand has almost
similar electronic but different steric properties.18
is almost planar in the solid state.22 Greater similarities
appear between the complexes with pyridine-containing
ligands than with the phosphinooxazoline. However, a
complete understanding and fine tuning of the param-
eters that lead to highly active and selective catalysts
for 1-butene are not yet available.
Con clu sion
The mononuclear Ni(II) complexes 9-12 have been
prepared from the new phosphinopyridine ligands 5-8
for their evaluation as catalyst precursors in ethylene
oligomerization. They are paramagnetic in solution, as
determined by the Evans method. Whereas in the
presence of MAO as a cocatalyst complexes 9-12 were
inactive, they provided activities up to 61 000 mol of
C2H4/((mol of Ni) h) (11) in the presence of only 6 equiv
of AlEtCl2. The selectivities for C4 oligomers were as
high as 92% (12 in the presence of 2 equiv of AlEtCl2).
Similar activities were obtained by other groups in the
No significant effect of the R group was noticed for
the catalysts 9-12. This is at variance with other
(44) (a) Birdwhistell, K. R.; Lanza, J . J . Chem. Educ. 1997, 74, 579.
(b) Since the submission of this paper, a relevant publication on
phosphinopyridine complexes has appeared: Chen, H.-P.; Liu, Y.-H.;
Peng, S.-M.; Liu, S.-T. Organometallics 2003, 22, 4893-4899.
(45) Svejda, S. A.; Brookhart, M. Organometallics 1999, 18, 65-74.

2638 Organometallics, Vol. 23, No. 11, 2004
Speiser et al.
3
4J (H,H) not observed), 7.7 (dd, 2H, Ph Ho, J (H,H) ) 8.7 Hz,
4J (H,H) ) 1.7 Hz). 13C{1H} NMR (CDCl3): δ 24.0 (s, py-CH3),
103.7 (s, py C3), 107.9 (s, py C5), 113.3 (s, Ph Co), 115.0 (s, Ph
Cm), 122.7 (s, Ph Cp), 126.0 (s, py C4), 143.1 (s, py C6), 144.7
(s, py C2), 158.0 (s, Ph Cipso). Anal. Calcd for C12H11N: C, 85.17;
H, 6.55; N, 8.28. Found: C, 84.94; H, 6.23; N, 8.10.
dimerization of ethylene and propene, but only when
using between 200 and 400 equiv of cocatalyst.45 That
the kR value varies for a given catalyst as a function of
the nature or quantity of the cocatalyst suggests that
the C6 products are formed by both ethylene oligomer-
ization and incorporation of some of the 1-butene formed
(C4 + C2, consecutive reaction).
In general, the ligand basicity and the geometry of
the coordination sphere of the metal center have an
important influence on the catalytic properties. Never-
theless, a complete picture of the parameters that lead
to highly active and selective dimerization catalysts is
not yet available.
2-(2,6-Dim eth ylp h en yl)-6-m eth ylp yr id in e (15). To a
mixture of 1347 (6.25 g, 4.10 mL, 36 mmol), [Ni(acac)2] (0.131
g, 0.51 mmol), and P(t-Bu)3 (0.103 g, 0.130 mL, 0.51 mmol) in
diethyl ether (100 mL) in an ice bath was added (2,6-
dimethylphenyl)magnesium bromide (45 mmol of (2,6-dimeth-
ylphenyl)magnesium bromide in 100 mL of diethyl ether) over
a period of 10 min. After workup, as described for 14, an NMR-
pure brown oil was obtained, which was used without further
purification. If needed, the product may be distilled under
reduced pressure (5 mbar, 160 °C), affording a pale yellow
liquid (5.15 g, 26 mmol, 73%). 1H NMR (CDCl3): δ 2.05 (s,
6H, Ph o-(CH3)2), 2.61 (s, 3H, py-CH3), 7.02 (d, 1H, py H5,
3J (H,5H4) ) 7.8 Hz), 7.08 (d, 1H, py H3, 3J (H,H) ) 7.8 Hz),
7.14 (t, 1H, Ph Hp, 3J (H,H) ) 8.6 Hz), 7.17 (d, 2H, Ph Hm,
3J (H,H) ) 8.6 Hz), 7.63 (t, 1H, py H4, 3J (H4,H3,5) ) 7.8 Hz).
13C{1H} NMR (CDCl3): δ 20.5 (s, Ph o-(CH3)2), 24.3 (s, py-
CH3), 120.9 (s, py C3), 121.2 (s, py C5), 127.3 (s, Ph Cm), 127.7
(s, Ph Cp), 135.4 (s, Ph Co), 136.2 (s, py C4), 140.4 (s, Ph Cipso),
158.2 (s, py C2), 159.1 (s, py C6). Anal. Calcd for C14H15N: C,
85.24; H, 7.66; N, 7.10. Found: C, 85.52; H, 7.82; N, 6.98.
2-(2,6-Diisop r op ylp h en yl)-6-m eth ylp yr id in e (16). To a
mixture of 1347 (7.35 g, 4.90 mL, 43 mmol), [Ni(acac)2] (0.201
g, 0.784 mmol), and P(t-Bu)3 (0.157 g, 0.195 mL, 0.784 mmol)
in diethyl ether (100 mL) in an ice bath was added (2,6-
diisopropylphenyl)magnesium bromide (0.055 mol in 100 mL
of diethyl ether) over a period of 10 min. After workup, as
described for 14, a yellow-brown solid was obtained, which was
further purified by sublimation under reduced pressure (5
mbar, 190 °C), yielding a yellow solid (4.96 g, 19 mmol, 45%).
Exp er im en ta l Section
All solvents were dried and distilled using common tech-
niques, unless otherwise stated. All experiments were carried
out under an inert-gas atmosphere using standard Schlenk
techniques. NiCl2‚6H2O was dried by heating for 6 h at 160
°C under vacuum to give NiCl2. [NiX2(DME)] (X ) Cl, Br),46
2-bromo-6-methylpyridine,47 2,6-dimethyl-1-bromobenzene,48
and 2,6-diisopropyl-1-bromobenzene48 were prepared according
to the literature. All Grignard reagents were prepared by slow
addition of the corresponding aryl bromides to a suspension
of magnesium in diethyl ether (100 mL). The magnesium was
activated by adding some iodine crystals. Finally, the reaction
mixture was refluxed overnight and titrated against phenol-
phthalein before use. Other chemicals were commercially
available and used without further purification, unless oth-
1
erwise described. The H, 31P{1H}, and 13C{1H} NMR spectra
were recorded at 500.13 or 300.13, 121.5, and 76.0 MHz,
respectively, on FT Bruker AC300, Avance 300, and Avance
500 instruments. IR spectra in the range 4000-400 cm-1 were
recorded on a Bruker IFS66FT and a Perkin-Elmer 1600
Series FTIR. Gas chromatographic analyses were performed
on an Thermoquest GC8000 Top Series gas chromatograph
using an HP Pona column (50 m, 0.2 mm diameter, 0.5 µm
film thickness).
1H NMR (CDCl3): δ 1.15 (d, 6H, Ph(CH(CH3)(CH3))2, J (H,H)
3
) 6.9 Hz), 1.22 (d, 6H, Ph(CH(CH3)(CH3))2, 3J (H,H) ) 6.9 Hz),
2.52 (sept, 2H, Ph(CH(CH3)2)2, 3J (H,H) ) 6.9 Hz), 2.60 (s, 3H,
py-CH3), 7.06 (d, 1H, py H3, J (H,H) ) 7.8 Hz), 7.15 (d, 1H,
3
Ph Hp, 3J (H,H) ) 8.6 Hz), 7.46 (d, 1H, py H4, 3J (H,H) ) 7.8
Hz), 7.6 (dd, 2H, Ph Hm, 3J (H,H) ) 8.7 Hz, 4J (H,H) ) 1.7 Hz).
13C{1H} NMR (CDCl3): δ 23.8 (s, Ph(CH(CH3)(CH3))2), 24.1
(s, Ph(CH(CH3)(CH3))2), 24.4 (s, py-CH3), 30.0 (s, Ph(CH-
(CH3)2)2), 120.8 (s, py C3), 121.7 (s, py C5), 122.3 (s, Ph Cm),
128.2 (s, Ph Cp), 135.7 (s, py C4), 138.3 (s, Ph Cipso), 146.2 (s,
Ph Co), 157.8 (s, py C2), 159.0 (s, py C6). Anal. Calcd for
Syn th esis of th e Liga n d s. We first describe the synthesis
of 14-16, which are needed for the preparation of ligands 5-8.
(2-Meth yl-6-ph en yl)pyr idin e (14). To a mixture of 2-bromo-
6-methylpyridine47 (13; 14.66 g, 9.77 mL, 85 mmol), [Ni(acac)2]
(0.400 g, 1.56 mmol), and P(t-Bu)3 (0.316 g, 0.388 mL, 1.56
mmol) in 250 mL of diethyl ether in an ice bath was added
phenylmagnesium bromide (110 mmol of PhMgBr in 100 mL
of diethyl ether) over a period of 10 min. After the solution
was refluxed overnight, the reaction mixture was hydrolyzed
by pouring it into 200 mL of diluted hydrochloric acid. The
solution was stirred for 15 min before the organic phase was
separated. The aqueous phase was extracted twice (50 mL)
with diethyl ether, and the ether was again discarded. The
aqueous phase was neutralized by slow addition of K2CO3 until
no further CO2 evolution was observed and a pale yellow
precipitate was formed. The basic solution was extracted three
times with CH2Cl2 (150 mL), and the organic phase was dried
over Na2SO4. Then the CH2Cl2 fraction was evaporated under
reduced pressure, yielding an NMR-pure brown oil, which was
used directly. If needed, the product can be distilled under
reduced pressure (5 mbar, 120 °C) to afford a pale yellow liquid
(9.141 g, 54 mmol, 64%). 1H NMR (CDCl3): δ 2.65 (s, 3H, py-
CH3), 7.09 (d, 1H, py H5, 3J (H5,H4) ) 7.6 Hz), 7.41 (dd, 1H, Ph
C
18H23N: C, 85.32; H, 9.15; N, 5.53. Found: C, 85.25; H, 9.10;
N, 5.42.
2-[(Diph en ylph osph an yl)m eth yl]-6-ph en ylpyr idin e (5).
A solution of 14 (3.18 g, 19 mmol) in THF (50 mL) was cooled
to -78 °C, and 1 equiv of t-BuLi (1.7 M solution in pentane,
11.18 mL, 19 mmol) was added. The reaction mixture was
stirred for 2 h at -78 °C before 1 equiv of PPh2Cl (3.40 g, 2.6
mL, 19 mmol) was added dropwise. The solution was further
stirred at -78 °C for 2 h before it was warmed to room
temperature overnight. The reaction mixture was hydrolyzed
with degassed water (20 mL), and the organic phase was
separated and dried over MgSO4. The solvent was evaporated
under reduced pressure, yielding the product as a yellow oil
(5.04 g, 14 mmol, 75%). 1H NMR (CDCl3): δ 3.79 (s, 2H, PCH2),
7.25 (m, 1H, py H4), 7.30 (m, 1H, Ph Hp), 7.30-7.60 (m, 10H,
PPh2), 7.40 (m, 2H, Ph Hm), 7.60 (s, 1H, py H3), 8.06 (d, 2H,
Ph Ho, 3J (H,H) ) 6.9 Hz). 13C{1H} NMR (CDCl3): δ 37.9 (d,
PCH2, 1J (P,C) ) 16.5 Hz), 117.6 (s, py C3), 127.0 (s, py C5),
127.2 (s, Ph Co), 127.6 (s, Ph Cp), 132.9 (s, Ph Cm), 128.1-
128.7 (m, PPh2), 136.9 (s, py C4), 139.5 (s, Ph Cipso), 156.7 (s,
py C2), 158.0 (s, py C6). 31P{1H} NMR (CDCl3): δ -10.8 (s).
Anal. Calcd for C24H20NP: C, 81.57; H, 5.70; N, 3.96. Found:
C, 81.30; H, 5.55; N, 3.85.
3
4
Hp, J (H,H) ) 8.6 Hz, J (H,H) ) 1.7 Hz), 7.46 (d, 1H, py H3,
3J (H3,H4) ) 7.8 Hz), 7.50 (d, 2H, Ph Hm, 3J (H,H) ) 8.6 Hz,
4J (H,H) not observed), 7.62 (d, 1H, py H4, J (H,4H3) ) 7.8 Hz,
3
(46) Cotton, F. A. Inorg. Synth. 1971, 13, 160-164.
(47) Wang, Z.; Reibenspies, J .; Motekaitis, R. J .; Martell, A. E. J .
Chem. Soc., Dalton Trans. 1995, 1511-1518.
(48) Schrock, R. R.; Wesolek, M.; Liu, A. H.; Wallace, K. C.; Dewan,
J . C. Inorg. Chem. 1988, 27, 2050-2054.
2-(2,6-Dim eth ylp h en yl)-6-[(d ip h en ylp h osp h a n yl)m eth -
yl]p yr id in e (6). A solution of 15 (2.61 g, 13 mmol) in THF

Ni Complexes with P,N-Phosphinopyridine Ligands
Organometallics, Vol. 23, No. 11, 2004 2639
(50 mL) was cooled to -78 °C, and 1 equiv of t-BuLi (1.7 M
solution in pentane, 7.6 mL, 13 mmol) was added. The reaction
mixture was stirred for 2 h at -78 °C before 1 equiv of PPh2Cl
(2.32 g, 1.76 mL, 13 mmol) was added dropwise. The solution
was further stirred at -78 °C for 2 h before it was warmed to
room temperature overnight. The reaction mixture was hy-
drolyzed with degassed water (20 mL), and the organic phase
was separated and dried over MgSO4. The solvent was evapor-
ated under reduced pressure, yielding the product as a yellow
to that detailed below for 11, a suspension of [NiCl2(DME)]
(0.506 g, 2.38 mmol) in CH2Cl2 (30 mL) was added to a solution
of the phosphinopyridine 5 (0.846 g, 2.38 mmol) in CH2Cl2 (15
mL) at room temperature, and the mixture was stirred for 12
h. After workup, the red-brown solid product was dried under
vacuum for 12 h (0.867 g, 1.79 mmol, 78%). IR (KBr; selected
pyridine vibrations): 1595 m (νCdC), 1160 m (νâ(C-H)), 998 s
and 1022 w (νcycle), 692 vs (νR sens) cm-1. Anal. Calcd for
C
24H20Cl2NNiP: C, 59.68; H, 4.17; N, 2.90. Found: C, 60.05,
1
oil (3.77 g, 9.88 mmol, 76%). H NMR (CDCl3): δ 1.96 (s, 6H,
H, 4.27; N, 2.48.
Ph(CH3)2), 3.72 (s, 2H, PCH2), 7.05 (m, 1H, py H5), 7.10-7.15
(m, 3H, Ph Hm, Ph Hp), 7.30-7.50 (m, 10H, PPh2), 7.45 (m,
Syn t h esis of {2-(2,6-Dim et h ylp h en yl)-6-[(d ip h en yl-
p h osp h a n yl)m eth yl]p yr id in e}n ick el Dich lor id e (10). Us-
ing a procedure similar to that detailed below for 11, a
suspension of [NiCl2(DME)] (0.287 g, 1.35 mmol) in CH2Cl2
(30 mL) was added to a solution of the phosphinopyridine 6
(1.35 mmol, 0.515 g) in CH2Cl2 (10 mL) at room temperature,
and the mixture was stirred for 12 h. After workup, the green-
brown solid product was dried under vacuum for 12 h (1.26 g,
1.04 mmol, 77%). IR (KBr; selected pyridine vibrations): 1596
m (νCdC), 1161 m (νâ(C-H)), 997 s and 1024 w (νcycle), 692 vs (νR
sens) cm-1. Anal. Calcd for C26H24Cl2NNiP: C, 61.11; H, 4.73;
N, 2.74. Found: C, 61.13, H, 4.51; N, 2.70.
3
1H, py H3) 7.60 (t, 1H, py H4, J (H4,H3,5) ) 7.71 Hz). 13C{1H}
1
NMR (CDCl3): δ 24.9 (s, Ph(CH3)2), 39.4 (d, PCH2, J (P,C) )
16.5 Hz), 121.7 (s, py C3), 121.9 (s, py C5), 127.9 (s, Ph Co),
128.1 (s, Ph Cp), 128.7-129.0 (m, PPh2), 133.2 (s, Ph Cm), 136.8
(s, py C4) 141.0 (s, Ph Cipso), 158.3 (s, py C2), 159.8 (s, py C6).
31P{1H} NMR (CDCl3): δ -9.7 (s). Anal. Calcd for C26H24NP:
C, 81.87, H, 6.34; N, 3.67. Found: C, 81.50; H, 6.22; N, 3.45.
2-(2,6-Diisop r op ylp h en yl)-6-[(d ip h en ylp h osp h a n yl)-
m eth yl]p yr id in e (7). A solution of 16 (1.61 g, 6.35 mmol) in
THF (40 mL) was cooled to -78 °C, and 1 equiv of t-BuLi (1.7
M solution in pentane, 3.73 mL, 6.35 mmol) was added. The
reaction mixture was stirred for 2 h at -78 °C before 1 equiv
of PPh2Cl (1.40 g, 1.10 mL, 6.35 mol) was added dropwise. The
solution was further stirred at -78 °C for 2 h before it was
warmed to room temperature overnight. The reaction mixture
was hydrolyzed with degassed water (20 mL), and the organic
phase was separated and dried over MgSO4. The solvent was
evaporated under reduced pressure, yielding the product as a
Syn th esis of {2-(2,6-Diisop r op ylp h en yl)-6-[(d ip h en yl-
p h osp h a n yl)m eth yl]p yr id in e}n ick el Dich lor id e (11). To
a suspension of [NiCl2(DME)] (0.592 g, 2.78 mmol) in CH2Cl2
(30 mL) was added a solution of the phosphinopyridine 7 (1.218
g, 2.78 mmol) in CH2Cl2 (20 mL) at room temperature, and
the reaction mixture was stirred for 12 h and then filtered
through Celite in order to separate unreacted [NiCl2(DME)].
The brown filtrate was evaporated under reduced pressure,
dissolved in CH2Cl2 (10 mL), and reprecipitated with hexane
(20 mL). The supernatant liquid was filtered with a cannula
equipped with a filter cap. The solid was partially dissolved
in toluene (15 mL) and precipitated by addition of hexane. The
green-brown solid was dried under vacuum for 12 h (1.26 g,
2.22 mmol, 80%). IR (KBr; selected pyridine vibrations): 1599
m (νCdC), 1166 m (νâ(C-H)), 996 vs and 1026 w (νcycle), 695 vs
(νR sens) cm-1. Anal. Calcd for C30H32Cl2NNiP: C, 63.53; H, 5.69;
N, 2.47. Found: C, 63.10; H, 5.71; N, 2.79.
1
yellow oil (1.99 g, 4.57 mmol, 72%). H NMR (CDCl3): δ 1.17
(d, 6H, Ph(CH(CH3)(CH3))2, 3J (H,H) ) 6.8 Hz), 1.22 (d, 6H,
Ph(CH(CH3)(CH3))2, 3J (H,H) ) 6.8 Hz), 2.49 (sept. 2H,
3
CH(CH3)2, J (H,H) ) 6.8 Hz), 3.70 (s, 2H, PCH2), 7.0 (d, 1H,
py H5, 3J (H5,H4) ) 7.65 Hz), 7.20 (d, 2H, Ph Hm, 3J (H,H) )
8.55 Hz), 7.30 (m, 1H, Ph Hp), 7.30-7.40 (m, 10H, PPh2), 7.42
(m, 1H, py H3), 7.72 (t, 1H, py H,4 3J (H4,H5) ) 7.65 Hz). 13C{1H}
NMR (CDCl3): δ 24.5 (s, Ph(CH(CH3)(CH3))2), 24.2 (s, Ph(CH-
1
(CH3)(CH3))2), 30.6 (s, Ph(CH(CH3)2)2), 39.4 (d, PCH2, J (P,C)
) 16.1 Hz), 121.4 (s, py C3), 122.7 (s, py C5), 123.0 (s, Ph Cm),
128.7-129.0 (m, PPh2, Ph Co, Ph Cp), 133.0 (s, py C4), 136.0
(s, Ph Cipso), 152.3 (s, py C2), 156.8 (s, py C6). 31P{1H} NMR
(CDCl3): δ -9.4 (s). Anal. Calcd for C30H32NP: C, 82.35; H,
7.37; N, 3.20. Found: C, 82.30; H, 7.32; N, 3.20.
Syn t h esis of {2-[2-(Dip h en ylp h osp h a n yl)p h en yl]-6-
m eth ylp yr id in e}n ick el Dich lor id e (12). To a suspension
of [NiCl2(DME)] (1.77 g, 8.30 mmol) in CH2Cl2 (100 mL) was
added a solution of the phosphinopyridine 8 (2.945 g, 8.30
mmol) in CH2Cl2 (60 mL) at room temperature, and the
reaction mixture was stirred for 2 h. The red solution
was filtered through Celite in order to separate unreacted
[NiCl2(DME)]. The filtrate was evaporated under reduced
pressure, and the solid was dissolved in CH2Cl2 (50 mL) and
reprecipitated with hexane (150 mL). The supernatant solution
was filtered with a cannula equipped with a filter cap. The
solid was partially dissolved in toluene (70 mL) and precipi-
tated by the addition of hexane. The brick red solid was dried
under vacuum for 12 h (2.72 g, 5.79 mmol, 70%). IR (KBr;
selected pyridine vibrations): 1565 m (νCdC), 1188 m (νâ(C-H)),
998 vs and 1026 w (νcycle), 688 vs (νR sens) cm-1. Anal. Calcd for
2-[2-(Dip h en ylp h osp h a n yl)p h en yl]-6-m eth ylp yr id in e
(8). To a solution of 14 (2.439 g, 14 mmol) in THF (50 mL)
was added 1 equiv of n-BuLi (1.6 M solution in hexanes, 9.0
mL, 14 mmol) at room temperature, and the reaction mixture
was stirred for 2 h. One equivalent of PPh2Cl (3.18 g, 2.6 mL,
14 mmol) was then added dropwise over 5 min. This exother-
mic reaction brought the temperature to about 60 °C. The
solution was stirred at room temperature overnight before it
was hydrolyzed with degassed water (30 mL). The organic
phase was separated with a cannula, and the aqueous phase
was extracted twice with Et2O (40 mL). The organic phases
were collected and dried over MgSO4. After filtration, all
volatiles were removed under reduced pressure, yielding the
reaction product as a white powder (2.95 g, 8.3 mmol, 60%).
1H NMR (CDCl3): δ 2.67 (s, 3H, py-CH3), 7.10 (d, 1H, py H5,
3J (H5,H4) ) 7.0 Hz), 7.24-7.6 (m, 10 H, PPh2), 7.43 (d, 1H, py
C
24H20Cl2NNiP: C, 59.68; H, 4.17; N, 2.90. Found: C, 59.48;
H, 4.18; N, 2.67.
Oligom er iza tion of Eth ylen e. All catalytic reactions were
carried out in a magnetically stirred (900 rpm) 100 mL
stainless steel autoclave. The interior of the autoclave was
protected from corrosion by a protective coating. All catalytic
tests were started at 30 °C, and no cooling of the reactor was
done during the reaction. In all catalytic experiments with
MAO and AlEtCl2, 4.0 × 10-2 mmol of the Ni complex was
used. The necessary amount of complex for six catalytic runs
was dissolved in 60 mL of toluene. For each catalysis, 10 mL
of this solution was injected into the reactor. Depending on
the amount of cocatalyst added, between 0 and 5 mL of solvent
was added so that the total volume of all solutions was 15 mL.
This can be summarized by the equation
3
3
H3, J (H3,H4) ) 7.0 Hz), 7.41 (dd, 1H, py H4, J (H4,H3,5) ) 7.0
3
Hz), 7.46-7.70 (m, 3H, Ph Hm,o), 8.01 (dd, 1H, Ph Ho, J (H,H)
) 6.75 Hz, J (P,H) ) 1.74 Hz). 13C{1H} NMR (CDCl3): δ 23.7
4
(s, py-CH3), 118.7 (s, py C3,5), 125.9 (s, Ph Co,p), 127.5-128.6
(m, Ph Cm, PPh2 Cm,p), 130.2 (d, PPh2 Co), 131.1 (d, PPh2 C,
1J (P,C) ) 16.6 Hz), 131.3 (s, py C4), 151.6 (s, Ph Cipso), 156.9
(s, py C2,6). 31P{1H} NMR (CDCl3): δ -5.2 (s). Anal. Calcd for
C
24H20NP: C, 81.57; H, 5.70; N, 3.96. Found: C, 81.85; H, 6.00;
N, 4.00.
Syn th esis of {2-[(Diph en ylph osph an yl)m eth yl]-6-ph en -
ylp yr id in e}n ick el Dich lor id e (9). Using a procedure similar

2640 Organometallics, Vol. 23, No. 11, 2004
Speiser et al.
ated, for GC analysis. The products in the reactor were
hydrolyzed in situ by addition of ethanol (10 mL), transferred
into a Schlenk flask, and separated from the metal complexes
by trap-to-trap distillation (120 °C, 20 Torr). All volatiles were
evaporated (120 °C, 20 Torr static pressure) and recovered in
a second flask previously immersed in liquid nitrogen in order
to avoid any loss of product. For gas chromatographic analyses,
1-heptene was used as internal reference.
10 mL (Ni solution) +
y mL (solvent) + z mL (cocatalyst solution) ) 15 mL
When MAO was used as cocatalyst, the total volume was
increased to 20 mL. After injection of the catalyst solution
under a constant low flow of ethylene, the reactor was brought
to working pressure and continuously fed with ethylene, using
a reserve bottle placed on a balance to allow continuous
monitoring of the ethylene uptake. The temperature increase
observed resulted solely from the exothermicity of the reaction.
The oligomerization products and remaining ethylene were
only collected from the reactor at the end of the catalytic
experiment. At the end of each test, the reactor was cooled to
10 °C before the gaseous phase was transferred into a 10 L
polyethylene tank filled with water. An aliquot of this gaseous
phase was transferred into a Schlenk flask, previously evacu-
Ack n ow led gm en t. We thank the CNRS and the
Ministe`re de la Recherche (Paris, France) for support
and the Institut Franc¸ais du Pe´trole for a Ph.D. grant
(F.S.) and financial support.
OM034203I