Nickel complexes with oxazoline-based P,N-chelate ligands: Synthesis, structures, and catalytic ethylene oligomerization behavior

Article abstract of DOI:10.1021/om034197q

The Ni(II) complexes were prepared from the phosphinooxiazoline ligands and were tested in the catalytic oligomerization of ethylene. The molecular structure of the ligand rac-2-[1-(diphenylphosphanyl)-ethyl]-4,4- dimethyl-4,5-dihydrooxazole was determined by X-ray diffraction. It was found that mononuclear Ni(II) complexes wtih bulky tris(pyrazolyl)borate ligands were active for ethylene oligomerization. The results show that nickel complexes yeiled a turnover frequency of 36 3000 mol of C₂H₄/((mol of Ni)h) in the presence of 14 equiv of ALEtCl₂.

Full text of DOI:10.1021/om034197q

Organometallics 2004, 23, 2613-2624
2613
Nick el Com p lexes w ith Oxa zolin e-Ba sed P ,N-Ch ela te
Liga n d s: Syn th esis, Str u ctu r es, a n d Ca ta lytic Eth ylen e
Oligom er iza tion Beh a vior †
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
Richard Welter
Laboratoire DECMET (UMR 7513 CNRS), Universite´ Louis Pasteur, 4 rue Blaise Pascal,
F-67070 Strasbourg Cedex, France
Received September 25, 2003
The phosphinooxazoline ligands 9-12 have been prepared and used to examine and
compare the catalytic properties of their corresponding Ni(II) complexes in ethylene
oligomerization. The molecular structure of rac-2-[1′-(diphenylphosphanyl)ethyl]-4,4-dim-
ethyl-4,5-dihydrooxazole (10) has been determined by X-ray diffraction. The paramagnetic
Ni(II) complexes [NiX(µ-X)(P,N)]2 (19, X ) Br, P,N ) 2-[(diphenylphosphanyl)methyl]-4,4-
dimethyl-4,5-dihydrooxazole (9)); 20, X ) Cl, P,N ) 9; 21, X ) Cl, P,N ) 10); 22, X ) Cl;
P,N ) 2-[(diphenylphosphanyl)methyl]-4-(R)-phenyl-4,5-dihydrooxazole (12)); 23, X ) Br,
P,N ) 12) and [NiCl2(P,N)] (24; P,N ) 2-[1′-(diphenylphosphanyl)-1′-methylethyl]-4,4-
dimethyl-4,5-dihydrooxazole (11)) were prepared in yields of 79-90%, and 19 and 24 have
been characterized by X-ray diffraction. The former is a dinuclear complex with distorted-
trigonal-bipyramidal geometry around the Ni(II) centers, whereas 24 is a mononuclear,
tetrahedral complex, which underlines the strong influence of a substituent on the carbon
atom R to P. The metal coordination sphere in complexes 19-24 was determined in solution
by the Evans method to be trigonal bipyramidal for 19-23 and tetrahedral for 24. Whereas
complexes 19-23 and [NiCl2(PCy3)2] were inactive for ethylene polymerization in the presence
of methylaluminoxane (MAO), the mononuclear complex 24 gave a TOF of 7900 mol of C2H4/
((mol of Ni) h) with a selectivity for 1-butene of 38%. Complexes 19, 20, and 22 were inactive
for the oligomerization of ethylene in the presence of NaBH4, but 19, 21, and 24 were active
with AlEtCl2 as a cocatalyst. Activities and selectivities were compared to those of [NiCl2-
(PCy3)2], a typical precatalyst used in the dimerization of R-olefins. Complex 19 yielded a
turnover frequency of 36 300 mol of C2H4/((mol of Ni) h) in the presence of 14 equiv of AlEtCl2.
In the presence of only 6 equiv of cocatalyst, the Ni complexes 21 and 24 showed TOF values
of 38 100 and 45 900 mol of C2H4/((mol of Ni) h), respectively, higher than that of 27 200
mol of C2H4/((mol of Ni) h) obtained with [NiCl2(PCy3)2].
In tr od u ction
hemilability, which represents an efficient way to
control the selectivity of catalytic processes.1
Unsymmetrical bidentate ligands with a nitrogen and
a phosphorus donor atom, referred to as P,N ligands in
the following, can chelate a metal center or bridge two
identical or different metal centers. Owing to their
bonding versatility and the relative ease with which the
electronic and steric properties of the donor atoms can
be modified, P,N ligands play an important role in the
coordination chemistry of transition metals and in
homogeneous catalysis.1-6 These heterofunctional sys-
tems often display unique dynamic features, such as
Despite their widespread application in asymmetric
catalysis,6 phosphinoimine ligands have only recently
attracted attention for the late-transition-metal-cata-
lyzed oligomerization, polymerization, and copolymer-
ization of ethylene.7-10 This includes the phosphi-
noimine ligands 1-4, which were developed by the
(1) (a) Braunstein, P.; Naud, F. Angew. Chem., Int. Ed. 2001, 40,
680. (b) Slone, C. S.; Weinberger, D. A.; Mirkin, C. A. Prog. Inorg.
Chem. 1999, 48, 233.
(2) Go´mez, M.; Muller, G.; Rocamora, M. Coord. Chem. Rev. 1999,
193-195, 769.
(3) Newkome, G. R. Chem. Rev. 1993, 93, 2067.
(4) Zhang, Z.-Z.; Cheng, H. Coord. Chem. Rev. 1996, 147, 1.
(5) Espinet, P.; Soulantica, K. Coord. Chem. Rev. 1999, 193-195,
499.
* To whom correspondence should be addressed. Fax: +33-390 241
322. E-mail: braunst@chimie.u-strasbg.fr.
† Dedicated to Prof. H. Werner on the occasion of his 70th birthday,
with our warmest congratulations for his scientific achievements and
best wishes.
(6) Helmchen, G.; Pfaltz, A. Acc. Chem. Res. 2000, 33, 336.
10.1021/om034197q CCC: $27.50 © 2004 American Chemical Society
Publication on Web 04/23/2004

2614 Organometallics, Vol. 23, No. 11, 2004
Speiser et al.
Sch em e 1
preparation of detergents, plasticizers, and fine chemi-
cals and as comonomers for the synthesis of linear low-
density polyethylene. In particular, the dimerization of
olefins catalyzed by transition-metal complexes is of
considerable industrial relevance.16c,d Following our
recent studies on the synthesis and application of the
Ni complexes 7 and 8 bearing anionic phosphinoamide
ligands17b,c and on SHOP-type P,O systems17a,c,18,19 for
the oligomerization of ethylene (Scheme 1), we were
interested in obtaining short-chain oligomers using Ni
complexes with other P,N donor ligands. The stronger
binding ability of the phosphine may lead to improved
thermal stability of the catalysts. Furthermore, the use
of nonenolizable imine donors should also be beneficial
to catalyst thermal stability.10c,f
Eastman Chemical Corporation (1),11 Shell (2),12 and
Rush and co-workers (3)13 with varying R groups on
either the phosphorus- or nitrogen-bound phenyl sub-
stituent. Asahi Industries published the similar system
4 with only one bulky substituent in the position ortho
to the imine nitrogen.14 The corresponding Ni and Pd
complexes were used for the catalytic polymerization of
ethylene.7,9-14 Other P,N systems used in the oligomer-
ization or polymerization of R-olefins include the anionic
ligands 5 and 6, developed by Symyx Technologies.15
The phosphinooxazolines 9-12 fulfill these require-
ments and were selected also because their stereoelec-
tronic properties can be modulated in a systematic
manner. The CH2 group in a position R to the phospho-
Catalytic olefin oligomerization represents a major in-
dustrial process, and suitable ligand design has led to
high-performance Ni catalysts.16 In particular, ethyl-
ene oligomerization providing R-olefins in the C6-C20
range is highly desirable, since they are used for the
rus atom is well suited for introducing substituents in
the vicinity of the P donor atom and thereby fine-tuning
its electronic properties (see 10 and 11). Furthermore,
the oxazoline heterocycle provides a rigid backbone with
the possibility of introducing chirality and steric hin-
(7) (a) Britovsek, G. J . P.; Gibson, V. C.; Wass, D. F. Angew. Chem.,
Int. Ed. 1999, 38, 428. (b) Ittel, S. D.; J ohnson, L. K.; Brookhart, M.
Chem. Rev. 2000, 100, 1169. (c) Mecking, S. Coord. Chem. Rev. 2000,
203, 325.
(8) Braunstein, P.; Fryzuk, M. D.; Dall, M. L.; Naud, F.; Rettig, S.
J .; Speiser, F. Dalton 2000, 1067.
(9) Late Transition Metal Polymerization Catalysis; Rieger, B.,
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S.-T. Organometallics 2002, 21, 3203. (b) Guan, Z.; Marshall, W. J .
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Wang, R.; Mecking, S.; Schro¨der, D. L. J . Organomet. Chem. 2002,
662, 150. (f) Tsukahara, T.; Kanno, T. J pn. Patent J P 2000128922,
2000. (g) Chen, J .-T.; Liu, S.-L.; Zhao, K.-Q. J . Chin. Chem. Soc. 2000,
47, 279.
(11) Killian, C. M.; McDevitt, J . P.; Mackenzie, P. B.; Moody, L. S.;
Ponasik, J . A. WO98/40420, 1998 (Eastman Chemical Company).
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1998 (Shell International Research).
(13) Rush, S.; Reinmuth, A.; Risse, W.; O’Brien, J .; Ferro, D. R.;
Tritto, I. J . Am. Chem. Soc. 1996, 118, 12230.
(16) (a) Parshall, G. W.; Ittel, S. D. In 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.
(17) (a) Braunstein, P.; Chauvin, Y.; Mercier, S.; Saussine, L.;
DeCian, A.; Fischer, J . J . Chem. Soc., Chem. Commun. 1994, 2203.
(b) Braunstein, P.; Pietsch, J .; Chauvin, Y.; Mercier, S.; Saussine, L.;
DeCian, A.; Fischer, J . J . Chem. Soc., Dalton Trans. 1996, 3571. (c)
Pietsch, J .; Braunstein, P.; Chauvin, Y. New J . Chem. 1998, 467.
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Chem., Int. Ed. Engl. 1978, 17, 466. (b) Keim, W.; Hoffmann, B.;
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Keim, W. Angew. Chem., Int. Ed. Engl. 1990, 29, 235. (d) Hirose, K.;
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Heinicke, J .; He, M.; Dal, A.; Klein, H. F.; Hetche, O.; Keim, W.; Flo¨rke,
U.; Haupt, H. J . Eur. J . Inorg. Chem. 2000, 431.
(14) Nobuyuki, U.; Katsuhiro, I. J pn. Patent J P 11158213, 1998
(Asahi Chem. Ind. Co. Ltd.).
(15) Lapointe, A. M.; Guram, A.; Powers, T.; J andileit, B.; Boussie,
T.; Lund, C. Symyx Technologies, Inc., WO 9946271, 1999.

Ni Complexes with Oxazoline-Based Ligands
Organometallics, Vol. 23, No. 11, 2004 2615
drance, as in the case of 12. In addition to the possible
variations at the P substituents, which we did not per-
form in the present work, changing the basicity and the
donor strength of the P,N ligand could have a significant
influence on complex stability and catalytic activity.
Resu lts a n d Discu ssion
1. Liga n d Syn th esis. The phosphinooxazoline ligand
9 was prepared from the oxazoline 13, itself obtained
by heating 2-amino-2-methylpropan-1-ol in glacial acetic
acid, followed by azeotropic distillation of the product
(eq 1).20 Lithiation of the oxazoline 13 in THF was
amounts of adventitious oxygen. Addition of 1 equiv of
n-BuLi to oxazoline 14 and 1 equiv of P(BH3)Ph2Cl at
-78 °C, followed by deprotection of 15, afforded ligand
10 as a slightly yellow oil, which solidifies upon stand-
ing, in 65% yield after column chromatography. The
borane-protected phosphine P(BH3)(n-Bu)Ph2 (16) was
formed as a byproduct and separated by column chro-
matography.
performed with 1.1 equiv of n-BuLi at -78 °C. The
solution must be kept at -78 °C in order to avoid
metalation on the heterocycle.21 After the oxazoline was
lithiated, 1.1 equiv of TMSCl was added, which allows
a clean reaction.8 In the final step, 1.1 equiv of PPh2Cl
was added and the solution was brought to room
temperature overnight (eq 1). Small amounts of oxidized
phosphinooxazoline were separated by column chroma-
tography over silica gel under a nitrogen atmosphere,
which afforded the product in 73% yield.
The ligand rac-2-[1′-(diphenyl-phosphanyl)ethyl]-4,4-
dimethyl-4,5-dihydrooxazole (10) was synthesized by
monolithiation of the PCH2 group of 9 with 1.0 equiv of
n-BuLi, followed by the addition of 1.0 equiv of CH3I
(eq 2). After workup, 10 was isolated in 83% yield as a
1
Ligand 10 was characterized by H, 13C, and 31P{1H}
NMR spectroscopy and X-ray diffraction. A view of the
crystal structure is shown in Figure 1, and selected bond
distances and angles are given in Table 1. Both enan-
tiomers of the racemate crystallize in the unit cell. The
P-C(6) bond is almost orthogonal to the plane of the
heterocycle, which minimizes the repulsion between the
lone pairs on the nitrogen and oxygen atoms and that
on phosphorus. A similar orientation was observed in
the X-ray structure of 2-[(diphenylphosphanyl)methyl]-
3,5,5-trimethyl-4,5-dihydro-3H-imidazol-1-ium bromide
(17).22 The C(1)-N bond length of 1.259(3) Å in 10 is in
accordance with a double bond23 and is shorter than the
corresponding bond distance of 1.335(4) Å in 17.22
For the synthesis of 2-[1′-(diphenylphosphanyl)-1′-
methylethyl]-4,4-dimethyl-4,5-dihydrooxazole (11), the
phosphinooxazoline 10 was first reacted with 1 equiv
of n-BuLi and then CH3I, yielding 11 as a yellow-white
solid (eq 4). Care has to be taken during the addition of
slightly yellow, transparent oil. An alternative and more
efficient synthesis of 10 was developed, using the
2-ethyl-4,4-dimethyl-4,5-dihydrooxazole 14 as precursor
(eq 3). This method reduces the number of reaction steps
and introduces the phosphine moiety in the final step,
which facilitates the isolation of a pure product, since
phosphinooxazolines are readily oxidized by small
(19) (a) Ostoja Starzewski, K. A.; Born, L. Organometallics 1992,
11, 2701. (b) Ostoja Starzewski, K. A.; Witte, J .; Reichert, K. H.;
Vasiliou, G. In Transition Metals and Organometallics as Catalysts
for Olefin Polymerization; Kaminsky, W., Sinn, H., Eds.; Springer:
Berlin, 1987; p 349.
(20) Meyers, A. I.; Temple, D. L.; Nolen, R. L.; Mihelich, E. D. J .
Org. Chem. 1974, 39, 2778.
(21) Gant, T. G.; Meyers, A. I. Tetrahedron 1994, 50, 2297.
(22) J alil, M. A.; Fujinami, S.; Senda, H.; Nishikawa, H. J . Chem.
Soc., Dalton Trans. 1999, 1655.
(23) CRC Handbook of Chemistry and Physics, 73rd ed.; CRC
Press: Boca Raton, FL, 1992-1993; pp 9-6/9-7.
CH3I, since an excess leads to quaternization of the
phosphorus atom. Ligand 11 was isolated after column
chromatography in 50% yield. This modest yield is due

2616 Organometallics, Vol. 23, No. 11, 2004
Speiser et al.
or allylic alkylations,6,27 and we have shown that such
ligands can also be used as assembling ligands in bi-
metallic chemistry28a,b and that their deprotonated form
(phosphinoiminolate) leads to novel Pd(II) complexes28c
and Ru(II) catalysts for the transfer hydrogenation of
ketones.28d The phosphinooxazolines 9-12 show char-
acteristic signals in 31P{1H} NMR which are sensitive
to the degree of substitution in the position R to the
phosphorus moiety. The unsubstituted phosphines 9
and 12 give rise to singlets at δ -17.6 and -15.9,
respectively. A low-field shift to δ -2.3 ppm is observed
for the racemic phosphinooxazoline 10. This trend ex-
tends further to the tetramethylated phosphinooxazo-
line 11 with a resonance at δ 17.4. The assignment of
F igu r e 1. View of the molecular structure of 2-[1-(diphe-
nylphosphanyl)ethyl]-4,4-dimethyl-4,5-dihydrooxazole (10).
Ta ble 1. Selected Bon d Dista n ces (Å) a n d An gles
(d eg) in 10
1
the H NMR spectrum of 12 resulted from COSY and
C(1)-O
1.359(2)
1.259(3)
1.486(3)
C(6)-P
C(6)-C(7)
1.873(2)
1.531(3)
selective decoupling experiments, and the fact that both
C(1)-N
1
1
the H{31P} and H spectra were identical established
C(1)-C(6)
that the 2J (PH) coupling is zero. Also, the diastereotopic
PCH2 protons show a J (HH) coupling with the CHPh
C(1)-C(6)-P
C(1)-C(6)-C(7)
108.1(1)
112.3(2)
C(1)-C(6)-C(7)
N-C(1)-O
109.5(1)
118.2(2)
5
proton (see Experimental Section).
2. P r ep a r a tion of th e Ni(II) Com p lexes. The P,N
ligands were reacted with Ni(II) precursor complexes,
as indicated in Scheme 2. The reactions were clean and
rapid. After the reagents were stirred for 2 h at room
temperature in CH2Cl2, all NiX2 or [NiX2(DME)] (X )
Cl, Br) has dissolved, yielding a dark violet solution.
Solutions of the Ni(II) complexes were finally purified
by filtration through Celite, to separate unreacted
[NiCl2(DME)] or NiX2.
in part to the insufficient basicity of the reagent used
and to a difficult separation by column chromatography
between residual 10 and 11. However, when stronger
bases such as t-BuLi or the Lochmann-Schlosser base
were used, decomposition of the starting material or
formation of a product mixture was observed.20,24
To prepare the R-enantiopure ligand 12, D-phenylg-
lycinol, which is readily derived from D-phenylglycine
by reduction with NaBH4/I2 in THF in good yields (eq
5),25 was reacted with ethyliminoacetate hydrochloride
to give the oxazoline 18.26 The latter was then func-
1
Low-temperature H and 31P NMR spectra indicate
the paramagnetic nature of the Ni complexes. In all of
the cases, the proton resonances underwent dramatic
shifts to either lower or higher field compared to the free
ligand. Coordination of the phosphinooxazoline is eviden-
ced by a sharp absorption in the IR spectrum between
1650 and 1620 cm-1 for the ν(CdN) vibration, which is
significantly shifted compared to that in the free ligand
(1660-1680 cm-1).8 All complexes were characterized
in solution by the Evans method,29 and the magnetic
moments of complexes 19-23 were found in the range
3.21-3.90 µB, which corresponds to the presence of two
unpaired electrons in a trigonal-bipyramidal coordina-
tion sphere of the metal30-32 (see the X-ray structure
determination of 19 below). In rare cases, diamagnetism
has been observed for cationic trigonal-bipyramidal
Ni(II) complexes.32 The magnetic moment of 2.15 µB
tionalized to give 12, following the procedure used to
prepare 9 (eq 5). Ni(II) and Pd(II) complexes of chiral
phosphinooxazolines related to 12 have been success-
fully used as catalysts in asymmetric Grignard reactions
(26) Meyers, A. I.; Smith, R. K.; Whitten, C. E. J . Org. Chem. 1979,
44, 2250.
(27) Lloyd-J ones, G. C.; Butts, C. P. Tetrahedron 1998, 54, 901.
(28) (a) Braunstein, P.; Clerc, G.; Morise, X. New J . Chem. 2003,
27, 68. (b) Braunstein, P.; Clerc, G.; Morise, X.; Welter, R.; Mantovani,
G. Dalton 2003, 1601. (c) Apfelbacher, A.; Braunstein, P.; Brissieux,
L.; Welter, R. Dalton 2003, 1669. (d) Braunstein, P. Graiff, C.; Naud,
F.; Pfaltz, A.; Tiripicchio, A. Inorg. Chem. 2000, 39, 4468.
(29) (a) Evans, D. F. J . Chem. Soc. 1959, 2003. (b) Sur, S. K. J .
Magn. Reson. 1989, 82, 169.
(24) Meyers, A. I.; Smith, E. M.; Ao, M. S. J . Org. Chem. 1973, 38,
2129.
(25) McKennon, M. J .; Meyers, A. I.; Drauz, K.; Schwarm, M. J . Org.
Chem. 1993, 58, 3568.

Ni Complexes with Oxazoline-Based Ligands
Organometallics, Vol. 23, No. 11, 2004 2617
Sch em e 2
found in solution for the mononuclear complex 24 is
smaller than is usually observed for a tetrahedral coor-
dination geometry (3.5-4.0 µB30,31,33) (see the X-ray
structure determination of 24 below), but magnetic
properties are strongly dependent on the ligand field
and considerable fluctuations of the magnetic moments
are often observed, due to the occurrence of structural
equilibria in solution.30,34-36
Complexes 19 and 24 have been fully characterized
by X-ray diffraction. A view of the centrosymmetric
dinuclear complex 19 is shown in Figure 2, and selected
bond distances and angles are given in Table 2. The Ni
centers are in a distorted-trigonal-bipyramidal environ-
ment,37 and according to the Cambridge Structure Data-
base, 19 appears to be the first dinuclear complex of the
type [Ni(µ-X)X(P,N)]2 characterized by X-ray crystallo-
graphy. The angle P-Ni-N found for the coordinated
ligand is rather small, 80.83(7)°, compared to that in
related ligands whose bite angles range from 83.0(1)°38,39
to 84.4(3)°.22 The Ni-N distance (2.098(2) Å) and Ni-P
distance (2.3418(9) Å) are longer than in related com-
plexes, where they range from 1.918(4) to 1.937(4) Å and
from 2.176(1) to 2.180(1) Å, respectively.22,39
Complexes of the type [Ni(µ-X)X(P,N)]2 (X ) Cl) have
been reported by Nishikawa and co-workers,22 in which
the P,N ligand is 2-[(diphenylphosphanyl)methyl]-1-
methyl-1H-imidazole (25) (eq 6). The dinuclear nature
(30) Greenwood, N. N.; Earnshaw, A. Chemistry of the Elements;
Pergamon Press: Oxford, U.K., 1984; pp 1337-1349.
(31) Holleman, A. F.; Wiberg, E. Lehrbuch der Anorganischen
Chemie, 91st-100th ed.; Walter de Gruyter: Berlin, 1985; p 1152.
(32) Furlani, C. Coord. Chem. Rev. 1968, 3, 141.
(33) Venanzi, L. M. J . Chem. Soc. A 1958, 719.
(34) Hayter, R. G.; Humiec, F. S. Inorg. Chem. 1965, 4, 1701.
of complex 26 and the trigonal-bipyramidal coordination

2618 Organometallics, Vol. 23, No. 11, 2004
Speiser et al.
F igu r e 3. View of the molecular structure of [NiCl2{2-[1-
(diphenylphosphanyl)-1-methylethyl]-4,4-dimethyl-4,5-di-
hydrooxazole}] (24).
F igu r e 2. View of the molecular structure of [Ni(µ-Br)-
Br{2-[(diphenylphosphanyl)methyl]-4,4-dimethyl-4,5-dihy-
drooxazole}]2 (19).
Ta ble 3. Selected Bon d Dista n ces (Å) a n d An gles
(d eg) in 24
Ta ble 2. Selected Bon d Dista n ces (Å) a n d An gles
(d eg) in 19
Ni-P
2.307(1)
1.987(3)
2.191(1)
2.217(1)
P-C(13)
C(13)-C(16)
C(16)-N
1.882(4)
1.511(6)
1.269(5)
Ni-N
Ni-P
2.3418(9)
2.098(2)
2.4177(5)
2.5043(6)
Ni-Br(2)
P-C(1)
C(1)-C(2)
C(2)-N
2.6028(5)
1.843(3)
1.491(5)
1.282(4)
Ni-Cl(1)
Ni-Cl(2)
Ni-N
Ni-Br(1)
Ni-Br(2)
Cl(1)-Ni-Cl(2)
Cl(1)-Ni-P
Cl(1)-Ni-N
Cl(2)-Ni-P
Cl(2)-Ni-N
124.35(5)
120.40(2)
112.1(1)
103.25(5)
105.7(1)
P-Ni-N
82.7(1)
Ni-P-C(13)
P-C(13)-C(16)
Ni-N-C(16)
C(13)-C(16)-N
97.23(6)
104.1(3)
118.62(9)
125.4(3)
Br(1)-Ni-Br(2)
Br(1)-Ni-Br(2)
Br(1)-Ni-P
Br(1)-Ni-N
P-Ni-N
145.88(2)
Br(2)-Ni-P
Br(2)-Ni-N
Br(2)-Ni-P
Br(2)-Ni-N
100.34(3)
89.05(8)
101.53(2)
173.37(9)
89.30(2)
113.78(3)
95.42(7)
80.83(7)
and angles are given in Table 3). Except for the angles
around the metal, most structural parameters are
similar to those observed for the dinuclear complex 19.
The P,N bite angle is increased to 82.7(1)°, in compari-
son to 80.83(7)° for complex 19. According to the
Cambridge Structure Database, 24 is the first [NiX2-
(phosphinooxazoline)] complex to be characterized by
X-ray crystallography. Therefore, no comparative struc-
tural data are available.
geometry about the Ni centers were suggested on the
basis of solid-state electronic spectra, mass spectrom-
etry, and magnetic moment.22 Recently Laine and co-
workers have characterized binuclear Ni(II) complexes
of type 27 bearing an iminopyridine-type ligand (eq 7)
and used them in the polymerization of ethylene with
methylalumoxane (MAO) as cocatalyst.40,41
When the structures of 19 and 24 and the changes in
coordination geometries about the metal center solely
induced by substitution at the R-position to the phos-
phorus atom are considered, the versatility of the ligand
system appears remarkable. We shall examine below
its influence on the activity or selectivity of the com-
plexes in the catalytic oligomerization of ethylene.
In the course of the preparation of 21, the complex
[NiCl2{P(n-Bu)Ph2}2] was obtained unexpectedly, owing
to contamination of the phosphinooxazoline 10 with P(n-
Bu)Ph2 (eq 8). This complex was then prepared more
The Ni(II) center in complex 24 adopts a tetrahedral
coordination geometry (Figure 3; selected bond lengths
(35) Fro¨mmel, T.; Peters, W.; Wunderlich, H.; Kuchen, W. Angew.
Chem., Int. Ed. Engl. 1993, 32, 907.
(36) Dori, Z.; Gray, H. B. J . Am. Chem. Soc. 1966, 88, 1394.
(37) Riedel, E. Anorganische Chemie, 2nd ed.; Walter de Gruyter:
Berlin, 1990; pp 733-760.
(38) Morifuji, K.; Ikeda, S.; Saito, T.; Uchida, Y.; Misono, A. J . Am.
Chem. Soc. 1965, 87, 4652.
rationally by reaction of P(n-Bu)Ph2, isolated by column
chromatography, with [NiCl2(DME)] and was character-
ized by X-ray diffraction. The tetrahedral coordination
of the Ni center indicates that P(n-Bu)Ph2 behaves more
like an arylphosphine than an alkylphosphine (Figure
4, Table 4).
(39) Brag, K. L.; Butts, C. P.; Lloyd-J ones, G. C.; Murray, M. J .
Chem. Soc., Dalton Trans. 1998, 1421.
(40) Laine, T. V.; Lappalainen, K.; Liimatta, J .; Aitola, E.; Lo¨fgren,
B.; Leskela¨, M. Macromol. Rapid. Commun. 1999, 20, 487.
(41) Laine, T. V.; Piironen, U.; Lappalainen, K.; Klinga, M.; Aitola,
E.; Leskela¨, M. J . Organomet. Chem. 2000, 606, 112.

Ni Complexes with Oxazoline-Based Ligands
Organometallics, Vol. 23, No. 11, 2004 2619
center, as indicated by the 31P{1H} NMR singlets
observed between δ 26.1 and 26.6 ppm, but the exact
nature of this compound could not be determined.
The problems encountered in the alkylation reactions
of the complexes 19, 20, and 22 were no longer observed
with the Ni complex 21 (which has a methyl group in a
position R to P) when it was reacted with the Grignard
reagents CH3MgCl, PhCH2MgCl, and EtMgCl. Prelimi-
nary studies on the alkylation of 21 showed that the
reaction products were thermally unstable and sensitive
toward traces of air and moisture. In view of the
paramagnetic nature of the products and their high
sensitivity toward oxygen, water, and temperature, their
characterization proved to be extremely difficult and
therefore remains incomplete. In the catalytic experi-
ments (see below), alkylation was performed in situ.
4. Rea ction of Meta la ted a n d Neu tr a l P h osp h i-
n ooxa zolin e Liga n d s w ith tr a n s-[NiCl(P h )(P P h 3)2].
An alternative way to generate a Ni-carbon bond was
explored, and trans-[NiCl(Ph)(PPh3)2] was used as a
precursor for the preparation of Ni-aryl complexes,
which after chloride abstraction should afford cationic
Ni(II) complexes of potential interest in catalysis. trans-
[NiCl(Ph)(PPh3)2]46 was reacted with the ligands 9 and
12 under different reaction conditions. Neither at room
temperature nor at 80 °C in toluene could any reaction
be observed by 31P NMR spectroscopy. The complex
trans-[NiCl(Ph)(PPh3)2] was then reacted at room tem-
perature with 1 equiv of the metalated phosphinoox-
azoline 28 prepared in situ at room temperature (eq 9).
F igu r e 4. View of the molecular structure of [NiCl2{P(n-
Bu)Ph2}2].
Ta ble 4. Selected Bon d Dista n ces (Å) a n d An gles
(d eg) in [NiCl2{P (n -Bu )P h 2}2]
Ni-P(1)
Ni-P(2)
2.314(2)
2.316(2)
Ni-Cl(1)
Ni-Cl(2)
2.229(2)
2.214(1)
Cl(1)-Ni-Cl(2)
Cl(1)-Ni-P(1)
Cl(1)-Ni-P(2)
123.64(7)
97.96(5)
107.61(6)
Cl(2)-Ni-P(1)
Cl(2)-Ni-P(2)
P(1)-Ni-P (2)
114.22(5)
107.68(6)
103.87(6)
3. Alk yla tion of th e [Ni(µ-X)X(P ,N)]2 Com p lexes.
Ni alkyls are known to be active species in the catalytic
oligomerization and polymerization of ethylene, and
numerous studies have been reported on the role of
neutral or cationic Ni-alkyl complexes in these
reactions.7,9,42-44 The nickel complexes 19, 20, and 22
were treated with LiCH3 (2 or 4 equiv), Grignard
reagents (1, 2, or 4 equiv of CH3MgCl or PhCH2MgCl),
Sn(CH3)4 (1, 2, or 4 equiv), or Zn(CH3)2 (1, 2, or 4 equiv),
at different temperatures with various stoichiometries
of the alkylating reagent. No reaction was observed with
Sn(CH3)4. The different basicities of these alkylating
reagents need to be considered, since the PCH2 protons
exhibit enhanced acidity upon P coordination to the
metal and are therefore sensitive to basic reagents such
as CH3Li. Surprisingly, these reagents afforded products
with similar NMR and IR spectra. All reaction products
were only soluble in DMSO. Formation of a complex
resulting from deprotonation of the PCH2 group could
be generally ruled out, since deprotonation of coordi-
nated phosphinooxazolines leads to a shift of the ν(Cd
N) infrared absorption of about 100 cm-1 to smaller
wavenumbers.45 For all products the characteristic ν-
After the reaction mixture was stirred for 12 h and
workup, an impure product could be isolated which
contained PPh3 and the desired reaction product 29, as
indicated by the 31P NMR resonance at 25.5 ppm for
the deprotonated, coordinated phosphinooxazoline. This
reaction has not yet been optimized.
5. Rea ction Betw een Liga n d 9 a n d Ni(0) Com -
p lexes. Early results by Wilke et al.47 have shown that
Ni(0) complexes such as [Ni(COD)2] or Ni-allyl species
catalyze the oligomerization of ethylene. Therefore, we
envisaged the preparation of Ni(0) complexes containing
the phosphinooxazoline 9 from [Ni(COD)2]. After the
ligand and [Ni(COD)2] were mixed in toluene at room
(CdN) absorption was observed around 1660 cm-1 8,45
.
The phosphorus atom is still coordinated to the Ni
(42) J ohnson, L. K.; Killian, C. M.; Brookhart, M. J . Am. Chem. Soc.
1995, 117, 6414.
(43) Killian, C. M.; Tempel, D. J .; J ohnson, L. K.; Brookhart, M. J .
Am. Chem. Soc. 1996, 118, 11664.
(44) Killian, C. M.; J ohnson, L. K.; Brookhart, M. Organometallics
1997, 16, 2005.
(45) Braunstein, P.; Naud, F.; Graiff, C.; Tiripicchio, A. Chem.
Commun. 2000, 897.
(46) Hidai, M.; Kashiwagi, T.; Ikeuchi, T.; Uchida, Y. J . Organomet.
Chem. 1971, 30, 279.
(47) Wilke, G.; Bogdanovic, B.; Hardt, P.; Heimbach, P.; Keim, W.;
Kro¨ner, M.; Oberkirch, W.; Tanaka, K.; Steinbru¨cke, E.; Walter, D.;
Zimmermann, H. Angew. Chem. 1966, 78, 157.

2620 Organometallics, Vol. 23, No. 11, 2004
Speiser et al.
Ta ble 5. Ca ta lytic Resu lts for th e Din u clea r Com p lexes 19 a n d 21 a n d th e Mon on u clea r Com p lexes 24 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
19
21
24
NiCl2(PCy3)2
amt of AlEtCl2 (equiv)
selectivity C4 (%)
selectivity C6 (%)
6
100
12
62
36
2
14
56
41
2
6
2
67
28
3
6
54
40
1
2
88
12
6
64
33
3
86
14
0.5
selectivity C8 (%)
selectivity C10 (%)
2
2
2
selectivity C12 (%)
1
3.5
productivity (g of C2H4/((g of Ni) h))
TOF (mol of C2H4/((mol of Ni) h))
R-olefin (C4) (%)
traces
traces
16 300
33 800
3
17 500
36 300
3
18 400
38 100
13
12 300
25 400
25
22 000
45 900
20
800
1600
12
13 000
27 200
9
b
kR
0.39
0.49
0.34
0.28
0.50
0.09
0.11
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].
temperature, a color change from yellow to dark red was
h). Some catalysts produced oligomers ranging from C6
to C24 with a high selectivity for R-olefins (94%).44
range of Ni(II) six-membered chelate complexes bearing
the phosphinoimine ligands 1-4 have been reported for
the polymerization of ethylene, and the active species
were generated in situ upon addition of a large amount
of MAO (100-350 equiv).11-14
We have evaluated complexes 19-22 and 24 for the
oligomerization of ethylene with the aim of producing
short-chain oligomers in the presence, if possible, of only
small quantities of cocatalyst. Different activators and
experimental conditions have been used to improve the
productivity and selectivity of the catalysts. The com-
plexes were treated with different quantities of AlEtCl2
or MAO as cocatalysts. To evaluate the transferability
of the catalyst/cocatalyst system developed by Keim and
co-workers18 to the Ni(II) complexes 19, 20, and 22,
NaBH4 was used as activator for the oligomerization of
ethylene. The catalytic reactions have been carried out
at either 65 or 85 °C, but these complexes were almost
inactive. Only small amounts of C4 oligomers could be
detected by GC analysis.
observed, followed by rapid formation of a black pre-
cipitate (zerovalent nickel). The phosphinooxazoline 9
proved unable to stabilize a Ni(0) complex. Because of
the lability of the COD ligand, a Ni complex with more
strongly bound ligands was chosen and [Ni(PPh3)4] was
therefore employed for the synthesis of [Ni(P,N)(PPh3)2]-
type complexes. Ligand 9 was reacted either at room
temperature in toluene with [Ni(PPh3)4] or at 80 °C. In
neither case could any reaction be observed. Only
uncoordinated PPh3 and unreacted 9 could be detected
by 31P NMR spectroscopy. This may indicate that two
PPh3 ligands have better stabilizing properties toward
Ni(0) than the bidentate phosphinooxazoline. When 9
was reacted with [Ni(PPh3)4] in the presence of PhCl in
toluene with the hope of forming a substituted Ni aryl
complex, only formation of trans-[NiCl(Ph)(PPh3)2] could
A
1
be identified by H and 31P NMR and IR spectroscopy.
6. Ca ta lytic Oligom er iza tion of Eth ylen e Usin g
Com p lexes 19-24. Ni complexes have long been
studied as catalysts for the oligomerization and polym-
erization of ethylene.7-9,48 In particular, the groups of
Keim18 and Ostoja Starzewski19 have used P,O chelate
complexes which afford highly linear R-olefins (C6-
C20).49 The chain length distribution strongly depends
on the steric properties of the substituents on the
phosphorus atom of the chelate ligand. With increasing
steric hindrance, longer chain lengths were obtained.
Various modifications have been developed around these
[NiX2(P,O)]-type catalysts (X ) Cl), such as a NaBH4
(1-2 equiv)/1,4-butanediol system.49,50 The diol allows
an easy separation of the oligomerization products by
either evaporation or distillation. In the IFP Dimersol
process, Ni(II) salts are applied in combination with
halogenoalkylaluminum compounds as activators (Al:
Ni > 10) in a nonpolar solvent and butenes and
dimethylbutenes are formed, depending on the R-olefin.51a
Brookhart and co-workers7,42-44 used a large excess of
methylaluminoxane (MAO; 300-1000 equiv) with the
[NiX2(diimine)] complexes, which led to ethylene po-
lymerization and oligomerization catalysts with turn-
over frequencies up to 3 × 106 mol of C2H4/((mol of Ni)
Therefore, Lewis acidic cocatalysts such as AlEtCl2,
AlEt3, and methylaluminoxane (MAO) were chosen.
AlRCl2 compounds are used in the Dimersol process,
where in situ formation of a Ni-alkyl complex leads to
the active Ni-hydride species after â-elimination.51b The
[Ni(µ-Cl)Cl(phosphinooxazoline)]2 dinuclear complexes
19 and 21 and the mononuclear complex 24 were tested
in the presence of different quantities of AlEtCl2.
Preliminary results with 22 have shown that introduc-
tion of steric hindrance on the oxazoline heterocycle did
not have any significant influence on either activity or
selectivity. Therefore, only the results with complexes
19, 21, and 24 will be detailed below. To compare the
activities and selectivities in the oligomerization of
ethylene using AlEtCl2 as cocatalyst, [NiCl2(PCy3)2] was
chosen as a reference, because it is a typical catalyst
for the Ni-catalyzed dimerization of R-olefins.51 All
selectivities reported in the following refer to the total
amount of products formed in each catalytic test.
The dinuclear compound 19 was inactive when less
than 6 equiv of cocatalyst was added. However, turnover
frequencies of 33 800 and 36 300 mol of C2H4/((mol of
Ni) h) were obtained in the presence of 12 and 14 equiv
of cocatalyst, respectively (Table 5). Complex [NiCl2-
(PCy3)2] is less active with small amounts of AlEtCl2.
Dinuclear 21 and mononuclear 24 were inactive in the
presence of 1.3 equiv of AlEtCl2. However, when 2 equiv
(48) (a) Klabunde, U.; Ittel, S. D. J . Mol. Catal. 1987, 41, 123. (b)
Klabunde, U.; Mu¨hlhaupt, R.; Herskowitz, T.; J anowicz, A. H.; Cala-
brese, J .; Ittel, S. D. J . Polym. Sci., A: Polym. Chem. 1987, 25, 1989.
(49) Kemp, R. A. Phosphorus, Sulfur Silicon Relat. Elem. 1994, 87,
83.
(50) Lutz, E. F. U.S. Patent, 4,528,416, 1985 (Shell Oil Company).
(51) (a) Commereuc, D.; Chauvin, Y.; Le´ger, G.; Gaillard, J . Rev.
Inst. Fr. Pet. 1982, 37, 639. (b) Knudsen, R. D. Prepr.-Am. Chem. Soc.,
Div. Pet. Chem. 1989, 572-576.

Ni Complexes with Oxazoline-Based Ligands
Organometallics, Vol. 23, No. 11, 2004 2621
F igu r e 5. Catalytic activities of the complexes 19, 21, and
24 in the oligomerization of ethylene using AlEtCl2 as
activator ([NiCl2(PCy3)2] reference). Conditions: T ) 30 °C,
10 bar of C2H4, 35 min, 4 × 10-2 mmol of Ni, 15 mL of
toluene.
F igu r e 6. Selectivities for 1-butene within the C4 fraction
of the complexes 19, 21, and 24 ([NiCl2(PCy3)2] reference).
Conditions: T ) 30 °C, 10 bar of C2H4, 35 min, 4 × 10-2
mmol of Ni, 15 mL of toluene.
was used, a turnover frequency (TOF) of 25 400 mol of
C2H4/((mol of Ni) h) was observed for 24. The turnover
frequencies could be increased to 38 100 (21) and 45 900
mol of C2H4/((mol of Ni) h) (24) upon addition of 6 equiv
of cocatalyst (Table 5 and Figure 5). These results are
much better than those obtained with [NiCl2(PCy3)2]
under similar conditions.
Ta ble 6. Ca ta lytic Resu lts for 24 in th e
Oligom er iza tion of Eth ylen e w ith MAO a s
Coca ta lysta
amt of MAO (equiv)
selectivity C4 (%)
800
72
selectivity C6 (%)
23
selectivity C8 (%)
5
The catalytic results with 19, 21, and 24 indicate
increased activity on going from ligand 9 < 10 < 11,
which parallels the degree of substitution at the carbon
R to phosphorus and therefore the ligand basicity (see
also 31P chemical shifts; Figure 5). The main products
formed during the oligomerization reactions were C4 and
C6 oligomers. When 2 or 6 equiv of AlEtCl2 was used,
the mononuclear Ni complex 24 yielded 67% or 54% of
C4 and 28% or 40% of C6 oligomers, respectively. Only
small quantities of octenes and decenes were observed.
Precatalysts 19 and 21 provided almost the same
product distribution. An increase in the amount of
cocatalyst displaced the product distribution toward the
formation of C6 oligomers.
Since butenes proved to be the main products in the
oligomerization of ethylene using precatalysts 19, 21,
and 24, we focused on 1-butene formation, knowing that
nickel complexes tend to isomerize R-olefins.51,52a The
selectivity for 1-butene within the C4 fraction (Figure
6) follows a trend similar to that for the turnover
frequencies of the complexes (Figure 5). An increase in
the basicity of the P,N chelate leads to a better selectiv-
ity for 1-butene within the C4 fraction: 9% ([NiCl2-
(PCy3)]) < 13% (21) < 20% (24) (in the presence of 6
equiv of AlEtCl2).
selectivity C10 (%)
selectivity C12 (%)
1
-
3800
7900
38
productivity (g of C2H4/((g of Ni) h))
TOF (mol of C2H4/((mol of Ni) h))
R-olefin (C4) (%)
b
kR
0.21
Conditions: T ) 30 °C, 10 bar of C2H4, 35 min, 4 × 10-2 mmol
a
b
of Ni complex, solvent 20 mL of toluene. kR ) hexenes [mol]/
butenes [mol].
AlEtCl2, it leads to a higher selectivity for 1-butene
within the C4 fraction.
The kR values given in the Tables 5 and 6 correspond
to the ratio hexenes [mol]/butenes [mol] and not to the
Schultz-Flory constant, since our catalysts are mainly
dimerization and trimerization catalysts.
Con clu sion
The Ni(II) complexes prepared from the phosphinoox-
azoline ligands 9-12 have been evaluated in the
catalytic oligomerization of ethylene. The molecular
structure of the ligand rac-2-[1′-(diphenylphosphanyl)-
ethyl]-4,4-dimethyl-4,5-dihydrooxazole (10) has been
determined by X-ray diffraction. The Ni(II) phosphi-
nooxazoline complexes 19-24 have been prepared in
79-90% yields, and 19 and 24 have also been charac-
terized by X-ray diffraction. The former is a dinuclear
complex with distorted-trigonal-bipyramidal Ni(II) cen-
ters, whereas 24 is a mononuclear, tetrahedral complex,
which underlines the strong influence of a substituent
on the carbon atom R to P. The coordination geometry
of the Ni(II) center in complexes 19-24 in solution was
determined by the Evans method to be trigonal bipy-
ramidal in 19-23 and tetrahedral in 24.
When MAO was used as a cocatalyst in the oligomer-
ization of ethylene, complexes 19, 21, and [NiCl2(PCy3)2]
decomposed, whereas the mononuclear complex 24
showed a TOF of 7900 mol of C2H4/((mol of Ni) h) (Table
6). Dimerization (72%) and trimerization (23%) of eth-
ylene dominated, and a selectivity for 1-butene within
the C4 fraction of 38% was observed. It thus appears
that, although MAO is a less suitable cocatalyst than
The precatalysts 19, 21, and 22 were inactive for the
oligomerization of ethylene in the presence of NaBH4.
However, with AlEtCl2 as a cocatalyst, complexes 19,
21, and 24 were active and activities and selectivities
(52) (a) See for exemple: Birdwhistell, K. R.; Lanza, J . J . Chem.
Educ. 1997, 74, 579. (b) Chen, H.-P.; Liu, Y.-H.; Peng, S.-M.; Liu, S.-
T. Organometallics 2003, 22, 4893. (c) Kunrath, F. A.; de Souza, R. F.;
Casagrande, O. L., J r.; Brooks, N. R.; Young, V. G., J r. Organometallics
2003, 22, 4739.

2622 Organometallics, Vol. 23, No. 11, 2004
Speiser et al.
P r ep a r a tion of r a c-2-[1′-(Dip h en ylp h osp h a n yl)eth yl]-
4,4-d im eth yl-4,5-d ih yd r ooxa zole (10). A solution of 9 (4.86
g, 0.016 mol) in THF (150 mL) was cooled to -78 °C, and 1.0
equiv of n-BuLi was added (1.6 M solution in hexane, 10.00
mL, 0.016 mol). After the solution was stirred for 1 h at -78
°C, 1.0 equiv of CH3I (0.93 mL, 0.016 mol) was added dropwise
at -78 °C. The reaction mixture was stirred for 2 h at -78 °C
and warmed to room temperature overnight. The solution was
hydrolyzed with degassed water (30 mL) and extracted with
diethyl ether (3 × 30 mL). Finally the organic phase was
separated and dried over degassed MgSO4. After evaporation
of the diethyl ether, the product was isolated as a slightly
yellow oil. Yield: 4.23 g, 0.013 mol, 83%.
have been compared to those of [NiCl2(PCy3)2], a typical
precatalyst used in the dimerization of R-olefins. Com-
plex 19 yielded a turnover frequency of 36 300 mol of
C2H4/((mol of Ni) h) in the presence of 14 equiv of
cocatalyst. In the presence of only 6 equiv of AlEtCl2,
the Ni complexes 21 and 24 showed TOF values of
38 100 and 45 900, respectively, higher than that of
27 200 mol of C2H4/((mol of Ni) h) obtained with [NiCl2-
(PCy3)2]. The dinuclear complexes 19 and 21 led to
selectivities for 1-butene within the C4 fraction lower
than the mononuclear complex 24. In the presence of
MAO, the complexes 19-23 and [NiCl2(PCy3)2] were
inactive, whereas 24 showed a TOF of 7900 mol of C2H4/
((mol of Ni) h) and a selectivity for 1-butene within the
C4 fraction of 38%. The fact that the kR value varies for
a given catalyst as a function of the nature or quantity
of cocatalysts used suggests that some incorporation of
the butene formed occurs during chain growth (consecu-
tive reaction). We have not yet studied the reasons for
the limited selectivity for R-olefins which could result
from (i) reversible â-H elimination after ethylene inser-
tion, followed by reinsertion with the opposite regio-
chemistry and chain transfer to give 2-butene, or
(ii) a re-uptake mechanism leading to isomerization of
1-butene. The ability of Ni(II) complexes to isomerize
R-olefins is well-known52a and has again been recently
evidenced in the case of phosphinopyridine chelates.52b
It has recently been reported that mononuclear Ni-
(II) complexes with bulky tris(pyrazolyl)borate ligands
were active for ethylene oligomerization, in contrast to
the chloride-bridged dinuclear complex.52c In our case,
the catalytic results obtained with the dinuclear com-
plexes could be due to their specific ligand sphere or to
a likely cocatalyst-induced dissociation in solution. The
observation that the dinuclear complexes require more
than 2 equiv of cocatalyst to display significant activity,
in contrast to 24, could be related to the need to use
this Lewis acid to break the halide bridges before
generating the active species.
Alternatively, 10 can be prepared by adding 1 equiv of
n-BuLi (1.6 M solution in hexane, 76.71 mL, 0.12 mol) to a
cold solution of 14 (15.00 g, 0.12 mol) in THF (200 mL) at -78
°C and stirring for 2 h at -78 °C. After addition of 1.1 equiv
of P(BH3)Ph2Cl (29.12 g, 0.132 mol), the solution was stirred
for 2 h at -78 °C and then slowly warmed to room temperature
and stirred overnight. The reaction mixture was hydrolyzed
with degassed water (100 mL). The organic phase was sepa-
rated with a cannula, and the aqueous phase was extracted
with diethyl ether (2 × 50 mL). The organic fractions were
pooled and dried over degassed MgSO4 and filtered through a
cannula equipped with a filter cap. The solvent was evaporated
under reduced pressure, yielding the borane-protected ligand
15 as a slightly yellow oil. Deprotection by stirring in degassed
diethylamine (120 mL) for 10 h at room temperature afforded
the phosphinooxazoline 10. All volatiles were evaporated, and
the product mixture was separated by column chromatography
(length of the stationary phase 60 cm, diameter 8 cm) over
silica gel using hexane (5% diethylamine). Yield: 24.38 g, 0.078
mol, 65%. IR (CH2Cl2): 1655 cm-1 (ν(CdN)). 1H NMR (CDCl3)
δ: 0.97 (s, 3H, C(CH3)2), 1.11 (s, 3H, C(CH3)2), 1.36 (dd, 3H,
CHCH3, 3J (H,H) ) 7.0 Hz, 3J (P,H) ) 14 Hz), 3.38 (dq, 1 H,
CHCH3, 3J (H,H) ) 7.0 Hz, 2J (P,H) ) 4.9 Hz), 3.71 and 3.73
2
(AB spin system, 2H, OCH2, J (H,H) ) 8 Hz), 7.26-7.36 (m,
10H, PPh2). 13C{1H} NMR (CDCl3): δ 27.9 (d, PC(CH3), 1J (P,C)
) 23.0 Hz), 28.0 (s, NC(CH3)2), 32.5 (d, 2J (P,C) ) 10.5 Hz, PC-
(CH3)), 66.6 (s, NC(CH3)2), 78.3 (s, OCH2), 127.9-136.5 (m,
2
PPh2), 166.3 (d, J (P,C) ) 4.1 Hz, CN). 31P{1H} NMR (CDCl3)
δ: -1.0 (s). Anal. Calcd for C19H22NOP: C, 73.29; H, 7.12; N,
4.50. Found: C, 73.50; H, 7.30; N, 4.73.
Syn th esis of 2-[1′-(Dip h en ylp h osp h a n yl)-1′-m eth yleth -
yl]-4,4-d im eth yl-4,5-d ih yd r ooxa zole (11). A solution of 10
(4.43 g, 0.014 mol) in THF (100 mL) was cooled to -78 °C,
and 1.1 equiv of n-BuLi was added (1.6 M solution in hexanes,
9.8 mL, 0.016 mol). The color changed from yellow to dark red.
The reaction mixture was stirred for 90 min before 1.1 equiv
of degassed CH3I (0.97 mL, 0.016 mol) was added. The solution
was further stirred for 4 h at -78 °C before it was warmed to
room temperature overnight. The reaction was quenched by
addition of degassed water (20 mL), and the hydrolyzed
reaction mixture was extracted with Et2O (2 × 30 mL). The
organic phase was separated by a cannula, dried over degassed
MgSO4, and filtered via a cannula equipped with a filter cap,
and the solvent was evaporated under vacuum, giving the
product as a yellow-white solid. Yield: 2.27 g, 50%. IR (CH2-
Cl2): 1663 cm-1 (ν(CdN)). 1H NMR (CDCl3): δ 1.21 (s, 6H,
NC(CH3)2), 1.39 (d, 6H, 3J (P,H) ) 12.6 Hz, PC(CH3)2), 3.71 (s,
2H, OCH2), 7.32-7.62 (m, 10 H, PPh2). 13C{1H} NMR
(CDCl3): δ 25.4 (d, 2J (P,C) ) 10.0 Hz, PC(CH3)2), 28.4 (s, NC-
(CH3)2), 36.4 (s, PC(CH3)2, 1J (P,C) ) 22.0 Hz), 66.8 (s,
NC(CH3)2), 79.3 (s, OCH2), 128.1 (d, 3J (P,C) ) 4.0 Hz, m-aryl),
128.9 (s, p-aryl), 134.6 (d, o-aryl, 2J (P,C) ) 16.7 Hz), 135.7 (d,
1J (P,C) ) 11.2 Hz, ipso-aryl), 169.9 (s, OCN). 31P{1H} NMR
(CDCl3): δ 17.4 (s). Anal. Calcd for C20H24NOP: C, 73.83; H,
7.43; N, 4.30. Found: C, 74.00; H, 7.55; N, 4.41.
Exp er im en ta l Section
Gen er a l Con sid er a tion s. All solvents were dried and
distilled using common techniques unless otherwise stated. All
manipulations were carried out using Schlenk techniques.
NiCl2‚6H2O was dried by heating for 6 h at 160 °C under
vacuum to give anhydrous NiCl2. The complexes [NiX2(DME)]
(X ) Cl, Br),53a [Ni(PPh3)4],53b,54 and trans-[NiCl(Ph)(PPh3)2]46
and 2,4,4′-trimethyloxazoline (13),20 2-[(diphenylphosphanyl)-
methyl]-4,4-dimethyl-4,5-dihydrooxazole (9),8 2-ethyl-4,4-dim-
ethyl-4,5-dihydrooxazole,20 D-phenylglycinol,25 and 2-methyl-
4-phenyl-4,5-dihydrooxazole (18)26 were prepared according to
literature procedures. Other chemicals were commercially
available and used without further purification unless other-
1
wise stated. The H, 31P{1H}, and 13C{1H} NMR spectra were
recorded at 500.13 or 300.13, 121.5, and 76.0 MHz, respec-
tively, 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 a HP Pona column (50 m, 0.2 mm diameter, 0.5 µm film
thickness).
P r ep a r a tion of 2-[(Dip h en ylp h osp h a n yl)m eth yl]-4-(R)-
p h en yl-4,5-d ih yd r ooxa zole (12). To a solution of n-BuLi
(0.014 mol, 8.6 mL) cooled to -78 °C was added a solution of
(53) (a) Cotton, F. A. Inorg. Synth. 1971, 13, 160. (b) Cotton, F. A.
Inorg. Synth. 1971, 13, 124.
(54) MacDiarmid, A. G. Inorg. Synth. 1977, 17, 120.

Ni Complexes with Oxazoline-Based Ligands
Organometallics, Vol. 23, No. 11, 2004 2623
the oxazoline 18 (2.00 g, 0.012 mol) in THF (20 mL) over a
period of 10 min. After the solution was stirred for 1 h at -78
°C, 1 equiv of TMSCl (1.68 mL, 1.44 g, 0.012 mol) was added
dropwise over a period of 10 min. The reaction mixture was
stirred for 90 min at -78 °C before 1 equiv of PPh2Cl (2.64 g,
0.12 mol) was reacted with the silylated oxazoline. The solution
was further stirred for 90 min at -78 °C and slowly warmed
to room temperature overnight. All volatiles were evaporated
under reduced pressure to give a yellow oil, which was
extracted with toluene. This solution was filtered and the
solvent evaporated under reduced pressure, leaving an oil that
solidifies in the refrigerator. Alternatively, purification can be
performed by column chromatography over silica gel (pentane/
ethyl acetate 3:1, 5% diethylamine). The product was isolated
as a yellow solid. Yield: 3.23 g, 0.009 mol, 78%. Rf (pentane:
rated, and the resulting green solid was washed with hexane
(2 × 15 mL) in order to separate remaining 10. Finally, the
product was dried under vacuum for 12 h, giving the nickel
complex 21 as a green-gray powder. Yield: 7.10 g, 4.05 mmol,
90%. IR (KBr): 1624 cm-1 (ν(CdN)). Magnetic moment in
solution (Evans method): 3.21 µB. Since NMR measurements
were not possible, no attempt was made to identify or separate
the diastereomers expected from the use of a racemic ligand.
Anal. Calcd for C38H44Cl4N2Ni2O2P2: C, 51.75; H, 5.03; N, 3.18.
Found: C, 50.43; H, 4.57; N, 2.89 (we have no explanation for
these poor results).
P r ep a r a tion of [Ni(µ-X)X{2′-[(d ip h en ylp h osp h a n yl)-
m eth yl]-4-(R)-p h en yl-4,5-d ih yd r ooxa zole}]2 (22, X ) Cl;
23, X ) Br ). Solid, anhydrous NiBr2 (0.190 g, 0.86 mmol) or
[NiCl2(DME)] (0.342 g, 0.86 mmol) was added to a solution of
12 (0.300 g, 0.86 mmol) in CH2Cl2 (30 mL). The suspension
was stirred for 2 h at room temperature. The reaction progress
was monitored by the dissolution of the Ni(II) precursors, and
the color changed from yellow to violet. After the solvent was
evaporated, the dark violet solid was dissolved in toluene (15
mL) in order to eliminate the more soluble fraction (which
could contain residual ligand). Addition of hexane (30 mL)
precipitated a solid, and the supernatant liquid was separated
with the help of a cannula. To separate any unreacted nickel
salt, the solid was dissolved in CH2Cl2 (30 mL) and the solution
filtered through Celite. The solvent was evaporated, and the
violet product was dried under vacuum. Yield of 22: 0.424 g,
0.376 mmol, 87% based on [NiCl2(DME)]; IR (KBr): 1637 cm-1
(ν(CdN)). Magnetic moment in solution (Evans method) 3.79
µB. Anal. Calcd for 22, C44H40Cl4N2Ni2O2P2: C, 55.63; H, 4.24;
N, 2.95. Found: C, 57.35; H, 5.38; N, 2.19.
1
ethyl acetate 3:1): 0.53. H NMR (CDCl3; assignments based
on selective decoupling experiments and comparison between
1H and 1H{31P} spectra): δΑ 3.23 and δΒ 3.30 (ABM (M ) H)
spin system, 2H, 2J (H,H) ) 14.1 Hz, 5J (HA,H) ) 1.2 Hz,
2
3
5J (HB,H) ) 0.7 Hz, PCHAHB), 4.00 (t, 1H, J (H,H) ) J (H,H)
2
3
) 8.3 Hz, OCHH), 4.58 (dd, 1H, J (H,H) ) 8.3 Hz, J (H,H) )
3
10.1 Hz, OCHH), 5.12 (overlaping dd, 1H, J (H,H) ) 8.3 and
10.1 Hz, CHPh), 6.9-7.7 (m, 15H, aryl). 13C{1H} NMR
1
(CDCl3): δ 28.4 (δ, J (P,C) ) 19.2 Hz, PCH2), 69.7 (s, CHPh),
75.1 (s, OCH2), 165.7 (d, 2J (P,C) ) 7.4 Hz, CdN), 125-138
(aromatic). 31P{1H} NMR (CDCl3): δ -15.9 (s).
P r ep a r a tion of [Ni(µ-Br )Br {2-[(d ip h en ylp h osp h a n yl)-
m eth yl]-4,4-d im eth yl-4,5-d ih yd r ooxa zole}]2 (19). Ligand
9 (0.220 g, 0.74 mmol) was added to a solution of [NiBr2(DME)]
(0.55 g, 1.76 mmol) in CH2Cl2 (40 mL). After the mixture was
stirred for 2 h at room temperature, the dark violet solution
was separated from the remaining nickel salt by cannula. The
solvent was evaporated under reduced pressure. The crude
product was dissolved in toluene (20 mL) and reprecipitated
by addition of hexane (50 mL). The supernatant organic phase
(in which the free ligand is well soluble) was filtered off by
cannula, and the solid product 19 was dried under vacuum.
Alternatively, this product can be prepared by heating [NiBr2-
(DME)] (0.228 g, 0.74 mmol) or NiBr2 (0.10 g, 0.46 mmol) and
9 (0.220 g, 0.74 mmol or 0.136 g, 0.46 mmol for NiBr2) in
degassed ethanol or n-butanol (30 mL) for 45 min until all the
metal salt had dissolved. Workup of the product 19 was as
described above. Yield: 0.200 g, 85%. IR (KBr): 1642 cm-1
(ν(CdN)). Magnetic moment in solution (Evans method): 3.82
µB. Anal. Calcd for C36H40Br4N2Ni2O2P2: C, 41.91; H, 3.91; N,
2.72. Found: C, 42.3; H, 4.28; N, 2.78.
P r ep a r a tion of [Ni(µ-Cl)Cl{2′-[(d ip h en ylp h osp h a n yl)-
m eth yl]-4,4-d im eth yl-4,5-d ih yd r ooxa zole}]2 (20). Solid an-
hydrous NiCl2 (0.194 g, 1.51 mmol) was added to a solution of
9 (0.450 g, 1.51 mmol) in CH2Cl2 (20 mL). The suspension was
stirred for 2 h until all NiCl2 had dissolved. The organic phase
was evaporated under reduced pressure, yielding a brown
solid. To eliminate the more soluble fraction, the crude product
was partially dissolved in toluene (15 mL) and reprecipitated
by addition of hexane (30 mL) and, after decantation and
removal of the solvent, the solid was dissolved in CH2Cl2 (30
mL) and the solution filtered over Celite in order to remove
unreacted NiCl2. The product was dried under vacuum for 24
h, giving a light brown solid. Yield: 0.510 g, 1.19 mmol, 79%.
IR (KBr): 1635 cm-1 (ν(CdN)). Magnetic moment in solution
(Evans method): 3.90 µB. Anal. Calcd for C36H40Cl4N2-
Ni2O2P2: C, 50.64 H, 4.72; N, 3.28. Found: C, 50.35, H, 4.48,
N, 3.41.
Yield of 23: 0.447 g, 0.397 mmol, 92% based on NiBr2. IR
(KBr): 1635 cm-1 (ν(CdN)). Magnetic moment in solution
(Evans method): 3.89 µB. Anal. Calcd for C44H40Br4N2-
Ni2O2P2: C, 46.86; H, 3.58; N, 2.48. Found: C, 46.45; H, 3.28;
N, 2.19.
Syn t h esis of [NiCl2{2′-[1′-(d ip h en ylp h osp h a n yl)-1′-
m eth yleth yl]-4,4-d im eth yl-4,5-d ih yd r ooxa zole}] (24). To
a solution of 11 (0.214 g, 0.658 mmol) in degassed ethanol (20
mL) was added 0.9 equiv of [NiCl2(DME)] (0.129 g, 0.592
mmol), and the solution was stirred for 24 h at room temper-
ature. After 1 h a color change from yellow-green to violet was
observed. The solvent was evaporated under reduced pressure.
The violet residue was dissolved in CH2Cl2 (15 mL), this
solution was filtered through Celite, and the Celite was washed
with CH2Cl2 (10 mL). The filtrate was evaporated to dryness,
giving the product as a violet solid which was dried in vacuo
overnight. Yield: 0.210 g, 0.462 mmol, 78%. IR (CH2Cl2): 1627
cm-1 (ν(CdN)). Anal. Calcd for C20H24Cl2NNiOP: C, 52.80; H,
5.32; N, 3.08. Found: C, 52.70; H, 5.22; N, 2.99.
Syn th esis of [NiCl2{P (n -Bu )P h 2}2]. To a solution of P(n-
Bu)Ph2 (0.458 g, 1.89 mmol) in CH2Cl2 (20 mL) was added 0.9
equiv of [NiCl2(DME)] (0.373 g, 1.70 mmol), and the solution
was stirred for 2 h at room temperature. After 5 min, a color
change from yellow-green to violet was observed. The solvent
was evaporated under reduced pressure. The violet residue
was dissolved in CH2Cl2 (15 mL), this solution was filtered
through Celite, and the Celite was washed with CH2Cl2 (10
mL). The filtrate was evaporated, giving the product as a violet
solid, which was dried in vacuo overnight. Yield: 0.924 g, 1.51
mmol, 88%. Anal. Calcd for C32H38Cl2NiP2: C, 62.58; H, 6.24.
Found: C, 62.90; H, 6.30.
Rea ction of Lith ia ted 12 w ith tr a n s-[NiCl(P h )(P P h 3)2].
A solution of 12 (0.645 g, 1.86 mmol) in THF (30 mL) was
cooled to -78 °C. After 15 min, 1.1 equiv of n-BuLi (1.6 molar
solution in hexane, 1.28 mL, 2.05 mmol) was added and the
reaction mixture was stirred for 30 min. A solution of trans-
[NiCl(Ph)(PPh3)2] (1.29 g, 2.05 mmol) in toluene (30 mL) was
transferred to the solution of the deprotonated ligand 28. The
solution was stirred for 12 h at room temperature. The dark
red reaction mixture was filtered over Celite, and the solvent
P r ep a r a tion of [Ni(µ-Cl)Cl{2′-[1′-(d ip h en ylp h osp h a -
n yl)eth yl]-4,4-d im eth yl-4,5-d ih yd r ooxa zole}]2 (21). To a
solution of 10 (2.81 g, 9 mmol) in CH2Cl2 (40 mL) was added
1.2 equiv of [NiCl2(DME)] (2.36 g, 11 mmol). While the reaction
mixture was stirred for 2 h at room temperature, a color
change from orange to dark green was observed. The reaction
mixture was filtered through Celite in order to remove
unreacted [NiCl2(DME)]. The organic phase was again evapo-

2624 Organometallics, Vol. 23, No. 11, 2004
Ta ble 7. X-r a y Exp er im en ta l Da ta
Speiser et al.
10
19H22NOP
311.37
orthorhombic
Pca21
19
24
[NiCl2{P(n-Bu)Ph2}2]
formula
fw
cryst syst
space group
C
C36H40Br4N2Ni2O2P2
1031.74
monoclinic
P21/n
C
20H24Cl2NNiOP
C32H38Cl2NiP2
614.18
orthorhombic
Pna21
455.01
orthorhombic
Pna21
a, Å
b, Å
c, Å
10.7839(3)
9.4726(4)
17.2463(8)
11.7810(9)
12.447(1)
13.2950(8)
95.859(4)
1939.4(4)
2
17.9824(2)
8.0761(8)
14.753(1)
16.152(1)
19.663(1)
9.794(1)
â, deg
V, Å3
1761.7(2)
4
2142.5(3)
4
3110.4(4)
4
Z
color
colorless
0.20 × 0.20 × 0.16
1.17
1.17
173
violet
violet
blue
cryst dimens (mm)
0.20 × 0.14 × 0.12
0.14 × 0.10 × 0.06
0.15 × 0.12 × 0.10
F(calcd), g cm-3
1.77
1.41
1.312
µ(Mo KR), mm-1
5.208
173
1.238
294
0.918
293
T, K
θ limits (deg)
no. of data measd
2.5-27.47
4053
2.5-27.50
14942
2.5-27.49
4920
2.07-30.08
8843
no. of data, I > 3σ(I)
no. of variables
2662
198
3073
217
3044
234
6162
334
R
Rw
GOF
0.035
0.043
1.009
0.249
0.030
0.038
1.025
0.762
0.035
0.046
1.052
0.553
0.0714
0.1769
1.043
largest peak in final diff map (e Å-3
)
0.997
was evaporated under reduced pressure. The orange-red solid
collected after cooling was washed with hexane (50 mL) and
dried under vacuum for 24 h. 31P{1H} NMR (CDCl3; selected
data): δ 25.5 (s, CHPPh2).
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. The catalytic results
are presented in Tables 5 and 6.
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 reaction. In all catalytic experiments with MAO
and AlEtCl2, 4.0 × 10-2 mmol of 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
Cr ysta l Str u ctu r e Deter m in a tion s of 10, 19, 24, a n d
[NiCl2{P (n -Bu )P h 2}2]. Diffraction data were collected on a
Kappa CCD diffractometer using graphite-monochromated Mo
KR radiation (λ ) 0.710 73 Å). The relevant data are sum-
marized in Table 7. Data were collected using ψ scans, the
structures were solved by direct methods using the SHELX
97 software,55,56 and the refinement was by full-matrix least
squares on F2. No absorption correction was used. All non-
hydrogen atoms were refined anisotropically with H atoms
introduced as fixed contributors (dC-H ) 0.95 Å, U11 ) 0.04).
Full data collection parameters and structural data are
available as Supporting Information. Crystallographic data for
all structures in this paper have been deposited with the
Cambridge Crystallographic Data Centre, CCDC Nos. 227819-
227822. Copies of this information may be obtained free of
charge from The Director, CCDC, 12 Union Road, Cambridge
CB2 1EZ, U.K. (fax, +44-1223-336033; e-mail, deposit@
ccdc.cam.ac.uk; web, http://www.ccdc.cam.ac.uk).
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 transferring the gaseous phase into a 10 L
polyethylene tank filled with water. An aliquot of this gaseous
phase was transferred into a Schlenk flask, previously evacu-
ated, for GC analysis. The products in the reactor were
hydrolyzed in situ by addition of ethanol (10 mL), transferred
to a Schlenk flask, and separated from the metal complexes
Ack n ow led gm en t. We thank the CNRS and the
Ministe`re de la Recherche (Paris) for support and the
Institut Franc¸ais du Pe´trole for a PhD grant (F.S.) and
support. We are also grateful to Dr. A. DeCian, Mrs. N.
Kyritsakis, and the Service de diffraction des rayons X,
(Institut de Chimie, Strasbourg, France) for X-ray
structure determinations and to Dr. N. Oberbeckmann
and R. Patacini for the NMR spectra of 12.
Su p p or tin g In for m a tion Ava ila ble: Tables of atomic
coordinates, bond distances and angles, and anisotropic ther-
mal parameters and ORTEP views for 10, 19, 24 and [NiCl2-
{P(n-Bu)Ph2}2]; X-ray data as CIF files are also available. This
material is available free of charge via the Internet at
http://pubs.acs.org.
(55) Kappa CCD Operation Manual; Nonius BV: Delft, The Neth-
erlands, 1997.
(56) Sheldrick, G. M. SHELXL97, Program for the Refinement of
Crystal Structures; University of Gottingen, Gottingen, Germany,
1997.
OM034197Q