New nickel ethylene oligomerization catalysts bearing bidentate P,N-phosphinopyridine ligands with different substituents a to phosphorus

Article abstract of DOI:10.1021/om034198i

Phosphinopyridine lignads were used for the determination of their corresponding paramagnetic Ni(II) complexes as catalyst precusors for the oligomerization of ethylene using ALEtCl₂ or methylalumoxane (MAO) as cocatalyst. X-ray diffraction was used for determining the planar coordination geometry about the metal of Ni complexes. It was found that for P,N ligands an increase in the degree of alkyl substitution at the carbon α to P tends to lead to higher activities and more selective formation of α-olefins. The results show that by using MAO as cocatalyst (800 equiv), selectivity to 1-butene within the C₄ fraction increased to 38% and 37% for differnt complexes.

Full text of DOI:10.1021/om034198i

Organometallics 2004, 23, 2625-2632
2625
New Nick el Eth ylen e Oligom er iza tion Ca ta lysts Bea r in g
Bid en ta te P ,N-P h osp h in op yr id in e Liga n d s w ith Differ en t
Su bstitu en ts r to P h osp h or u s†
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 25, 2003
The new phosphinopyridine ligands rac-2-[(diphenylphosphanyl)benzyl]pyridine (6), rac-
2-[1′-(diphenylphosphanyl)ethyl]pyridine (7), and 2-[1′-(diphenylphosphanyl)-1′-methyl]eth-
ylpyridine (8) with variable substitution at the carbon R to P were used for the synthesis of
the paramagnetic Ni(II) complexes [NiCl2(P,N)] 9-11, respectively. The complex 11‚0.5CH2-
Cl2 has been shown by X-ray diffraction to have an almost planar coordination geometry
about the metal, although the coordination sphere of all complexes in solution was determined
by the Evans method to be distorted tetrahedral. The Ni complexes provided activities for
the catalytic oligomerization of ethylene up to 58 100 mol C2H4/mol Ni‚h (11) in the presence
of only 6 equiv of AlEtCl2. The selectivity for C4 olefins reached 81% for 9 in the presence of
2 equiv of AlEtCl2, but the selectivity toward 1-butene was only 11-14%. In the presence of
400 or 800 equiv of methylalumoxane (MAO), complexes 9-11 yielded lower activities than
with AlEtCl2 as cocatalyst but higher selectivities for 1-butene. A turnover frequency of
22 800 mol C2H4/mol Ni‚h was observed for 11 in the presence of 800 equiv of MAO. The
selectivities were in the range 70-85% for the C4 olefins and in the range 33-38% for
1-butene within the C4 fraction. For these P,N ligands, increasing the degree of alkyl
substitution at the carbon R to P leads to higher activities for ethylene oligomerization and
more selective formation of R-olefins. The nature of the N-heterocycle influences the activity,
as shown by the turnover frequencies of 58 100 mol C2H4/mol Ni‚h for 11 in the presence of
6 equiv of AlEtCl2 and 45 900 mol C2H4/mol Ni‚h for the related phosphinooxazoline complex
16. Under these conditions, the selectivity to 1-butene within the C4 fraction was 11% for
11 and 20% for 16. When MAO was used as a cocatalyst (800 equiv), a similar trend was
observed for the activity, but the selectivity to 1-butene within the C4 fraction increased to
38% for 11 and 37% for 16.
In tr od u ction
phenomena are usually readily monitored by variable-
temperature 31P NMR spectroscopy.6 Phosphinopy-
ridines, phosphinoquinolines, and phosphinonaphthy-
ridines of type 1 are increasingly used in the synthesis
of heterobimetallic or polymetallic compounds.2,3,7,8
Introduction of chirality for asymmetric synthesis is
achieved with ligands of type 2.1,9-12 Another family of
phosphinopyridines with either C2 symmetry (3)13-16 or
Phosphinopyridine ligands find widespread applica-
tions in the coordination chemistry1-3 of transition
metals and in homogeneous catalysis1,4,5 owing to the
simultaneous presence of the ubiquitous pyridine and
phosphine donor groups. Their potential hemilabile
character represents an additional advantage for the
fine-tuning of catalytic processes, and such dynamic
* Corresponding author. Fax: +33-390 241 322. E-mail: braunst@
chimie.u-strasbg.fr.
(6) Braunstein, P.; Naud, F. Angew. Chem., Int. Ed. 2001, 40, 680-
699, and references therein.
† Dedicated to Prof. M. I. Bruce on the occasion of his 65th birthday,
with our warmest congratulations and best wishes.
(1) Espinet, P.; Hernando, R.; Iturbe, G.; Villafan˜e, F.; Orpen, A.
G.; Pascual, I. Eur. J . Inorg. Chem. 2000, 1031-1038.
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(8) Braunstein, P.; Durand, J .; Morise, X.; Tiripicchio, A.; Ugozzoli,
F. Organometallics 2000, 19, 444-450.
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nometallics 1997, 16, 1401-1409.
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2089-2095.
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Tetrahedron: Asymmetry 1994, 5, 299-302.
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10.1021/om034198i CCC: $27.50 © 2004 American Chemical Society
Publication on Web 04/23/2004

2626 Organometallics, Vol. 23, No. 11, 2004
Speiser et al.
axial chirality (4)17-20 has also been applied in asym-
metric catalysis.
of ongoing studies in our laboratories,26 we sought to
examine the influence of the ligand basicity in the new
series of phosphinopyridines 6-8 on the catalytic activ-
ity and selectivity of Ni(II) complexes for ethylene
oligomerization.
The synthesis and structure of the corresponding
Ni(II) complexes 9-11 as well as their catalytic proper-
ties in the oligomerization of ethylene using AlEtCl2 or
methylalumoxane (MAO) as cocatalyst will be discussed
here.
Despite the successful application of phosphinopy-
ridines in asymmetric transfer hydrogenation,13,16 asym-
metric hydroboration,19 allylic substitution,20 and car-
bonylation reactions21 including hydroformylation,22
only a few complexes containing such ligands have been
reported for their application in oligomerization or
polymerization reactions of R-olefins,23a,b and these
include 5a and 5b (Scheme 1).24
Sch em e 1. Str u ctu r a l Am biva len ce of th e
Com p lex [Ni(2-(d ip h en ylp h osp h in e)n icotin a te)-
m eth a llyl]BF 4 (5a ,b)
Resu lts a n d Discu ssion
1. Syn th esis of th e P h osp h in op yr id in e Liga n d s
6-8. The racemic phosphinopyridines 6 and 7 and
ligand 8 were prepared in good yields by one- and two-
step procedures (see Experimental Section). Ligand 6
was accessible by the reaction of 2-benzylpyridine 12
with n-BuLi at -78 °C and subsequent addition of
PPh2Cl (eq 1). The racemic ligand 2-ethyl-(1′-diphenyl-
phosphino)pyridine 7 was synthesized similarly by using
2-ethylpyridine 13 and borane-protected PPh2Cl as
reagents (eq 2).
In view of the academic and industrial importance of
olefin dimerization and oligomerization,25 and as part
(14) Sablong, R.; Newton, C.; Dierkes, P.; Osborn, J . A. Tetrahedron
Lett. 1996, 37, 4933-4936.
(15) Sablong, R.; Osborn, J . A. Tetrahedron Lett. 1996, 37, 4937-
4940.
(16) Zhang, X. The Penn State Research Foundation US 5 767 276,
1998.
(17) Alcock, N. W.; Hulmes, D. I.; Brown, J . M. J . Chem. Soc., Chem.
Commun. 1995, 395-397.
(18) Doucet, H.; Brown, J . M. Tetrahedron: Asymmetry 1997, 8,
3775-3784.
(19) Brown, J . M.; Hulmes, D. I.; Layzell, T. P. J . Chem. Soc., Chem.
Commun. 1993, 1673-1674.
(20) Brown, J . M.; Hulmes, D. I.; Guiry, P. J . Tetrahedron 1994,
50, 4493-4506.
(21) Drent, E.; Arnoldy, P.; Budzelaar, P. H. M. J . Organomet. Chem.
1993, 455, 247-253.
(22) Abu-Gnim, C.; Amer, I. J . Chem. Soc., Chem. Commun. 1994,
115-117.
(23) (a) Late Transition Metal Polymerization Catalysis; Rieger, B.,
Saunders Baugh, L., Kacker, S., Striegler, S., Eds.; Wiley-VCH:
Weinheim, 2003, and references cited. (b) Note added in proof:
a
relevant publication has appeared since acceptance of this paper:
Chen, H.-P.; Liu, Y.-H.; Peng, S.-M.; Liu, S.-T. Organometallics 2003,
22, 4893-4899.
(24) (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.
(25) (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.
Phosphine 8 could not be prepared by simple meta-
lation of 2-(isopropyl)pyridine and reaction of the meta-
(26) (a) Speiser, F. Ph.D. Thesis, Universite´ Louis Pasteur, 2002.
(b) Speiser, F.; Braunstein, P.; Saussine, L.; Welter, R. Organometallics
2004, 23, 2613. (c) Speiser, F.; Braunstein, P.; Saussine, L.; Welter,
R. Inorg. Chem. 2004, 43, 1649.

Nickel Ethylene Oligomerization Catalysts
Organometallics, Vol. 23, No. 11, 2004 2627
lated species with a chlorophosphine since only bases
such as potassium diisopropylamine (KDA), Lochmann
27b
Schlosser base (n-BuLi/KOt-Bu),27a or NaNH2/NH3
were able to deprotonate the isopropyl side chain. The
remaining alcoholate or amine readily reacts with the
chlorophosphine, and the desired phosphine 8 is ob-
tained in only 10-30% yields.
Another synthetic pathway was therefore explored,
and phosphinopyridine 8 was prepared by adding first
1 equiv of n-BuLi at room temperature to the borane-pro-
tected phosphine 14 and, subsequently, 1 equiv of CH3I.
After quantitative deprotection and workup, the gem-
disubstituted phosphinopyridine 8 was isolated in 74%
yield (eq 3). The racemic ligands 6 and 7 and the achiral
phosphinopyridine 8 show a resonance in the 31P NMR
spectrum at -1.0, 0.6, and 22.5 ppm, respectively.
F igu r e 1. Molecular structure of [Ni(2-{1′-(diphenylphos-
phanyl)-1′-methyl}ethylpyridine)Cl2] in 11‚0.5CH2Cl2.
Ta ble 1. Selected Bon d Dista n ces (Å) a n d An gles
(d eg) in [Ni(2-{1′-(d ip h en ylp h osp h a n yl)-
1′-m eth yl}eth ylp yr id in e)Cl2]‚0.5CH2Cl2
(11‚0.5CH2Cl2)
P-Ni
2.129(1)
1.950(3)
2.167(1)
2.221(1)
1.866(3)
1.507(5)
1.359(4)
N-Ni
Ni-Cl(1)
Ni-Cl(2)
P-C(13)
C(13)-C(16)
C(16)-N
Cl(1)-Ni-Cl(2)
Cl(1)-Ni-P
Cl(1)-Ni-N
Cl(2)-Ni-P
Cl(2)-Ni-N
P-Ni-N
91.98(4)
88.03(4)
170.01(9)
178.47(4)
96.61(9)
83.52(9)
2. Com p lexes [NiCl2(p h osp h in op yr id in e)] 9-11.
Complexes 9-11 were prepared by reaction between
equimolar amounts of phosphine and [NiCl2(DME)] for
2 h in CH2Cl2. After workup, the Ni complexes were
isolated in 76-80% yields (Scheme 2).
three complexes should display in solution distorted
square planar or tetrahedral geometry.30,31 The solid-
state structure of 11‚0.5CH2Cl2 was determined by
X-ray diffraction, and a view of the structure is shown
in Figure 1 and selected bond distances and angles are
given in Table 1.
Sch em e 2. Syn th esis of th e Ni(II) Com p lexes 9-11
The coordination sphere around the Ni(II) center is
almost square planar since the maximum deviation out
of the mean plane passing through Ni, N, P, Cl(1), and
Cl(2) is 0.22(1) Å (for the N atom). This result is
somewhat unexpected in view of the paramagnetic
nature of this complex in solution, but of course solution
and solid-state structures need not be identical.30 Ac-
cording to the Cambridge Structure Database, [NiCl2-
(2-{1′-(diphenylphosphanyl)-1′-methyl}ethylpyridine)] (11)
appears to be the first square planar [NiX2(P,N)]-type
complex characterized by X-ray crystallography. No
comparative data are therefore available. Except for the
ligand bite angle (P-Ni-N), the bond angles around the
Ni atom, and the P-Ni distance, most structural
parameters are similar to those observed for the recently
described phosphinooxazoline complex 16.26b The bite
angle is increased to 83.59(2)° in comparison to 82.7-
(1)° in 16 and 80.83(7)° in 17.26b
Coordination of the nitrogen heterocycle to the metal
center can be easily verified by infrared spectroscopy
with the characteristic shift of the νC-C, νcycle, νγ(C-C), νR
sens, and νâ(C-H) vibrations.28 All three complexes proved
to be paramagnetic. Their magnetic moment was de-
termined by means of the Evans method29 and found
to be 1.37, 1.35, and 1.20 µB, respectively. Therefore all
(27) (a) Pasquinet, E.; Rocca, P.; Marsais, F.; Godard, A.; Que´guiner,
G. Tetrahedron 1998, 54, 8771-8782. (b) Brown, H. C.; Murphey, W.
A. J . Am. Chem. Soc. 1951, 73, 3308-3312.
(28) Pfeffer, M.; Braunstein, P.; Dehand, J . Spectrochim. Acta 1974,
30A, 331-340.
(29) (a) Evans, D. F. J . Chem. Soc. 1959, 2003-2005. (b) Sur, S. K.
J . Magn. Reson. 1989, 82, 169-173.

2628 Organometallics, Vol. 23, No. 11, 2004
Speiser et al.
Ta ble 2. Com p a r a tive Ca ta lytic Da ta for Com p lexes 9-11, 16,26b 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
9
10
10
11
11
16
16
[NiCl2(PCy3)2] [NiCl2(PCy3)2]
AlEtCl2 [equiv]
6
2
6
2
6
2
6
2
6
2
selectivity C4 [%]
selectivity C6 [%]
selectivity C8 [%]
selectivity C10 [%]
selectivity C12 [%]
72
26
2
81
18
1
66
30
3.5
0.3
75
23
2
65
32
2.5
54
40
1
2
3.5
67
28
3
2
1
86
14
0.5
88
12
0.1
productivity (g C2H4/g Ni‚h) 23 600 14 400 26 400 22 100 27 800 inactive 22 000 12 300 13 000
800
1600
12
TOF (mol C2H4/mol Ni‚h)
R-olefin (C4) [%]
49 400 30 100 55 200 46 100 58 100 inactive 45 900 25 400 27 200
11
0.24
14
0.15
11
0.31
14
0.20
11
0.33
20
0.50
25
0.28
9
0.11
b
kR
0.09
a
b
Conditions: T ) 30 °C, 10 bar C2H4, 35 min, 4 × 10-2 mmol Ni complex, solvent: 15 mL of toluene. kR ) hexenes [mol]/butenes
[mol].
F igu r e 3. Selectivity to 1-butene within the C4 fraction
of the complexes 9-11 using different quantities of AlEtCl2.
Conditions: T ) 30 °C, 10 bar C2H4, 35 min, 4 × 10-2 mmol
Ni, Ref: [NiCl2(PCy3)2].
F igu r e 2. Activity of the complexes 9-11 for the oligo-
merization of ethylene using different quantities of AlEtCl2.
Conditions: T ) 30 °C, 10 bar C2H4, 35 min, 4 × 10-2 mmol
Ni, Ref: [NiCl2(PCy3)2].
was used, with TOF values of 49 400, 55 200, and 58 100,
respectively. The complexes remained active until the
catalytic reactions were quenched by addition of 10 mL
of ethanol, as indicated by monitoring the ethylene con-
sumption. The selectivities for the ethylene dimerization
products 1-butene and cis- and trans-2-butene varied
between 65% and 81% depending on the amount of co-
catalyst used. The moderate selectivity for 1-butene
obtained with complexes 9-11 is comparable to that
with [NiCl2(PCy3)2]. This may be due to the strong exo-
thermic character of the catalytic reactions, which favors
isomerization of the R-olefins (Figure 3).33 A larger
amount of cocatalyst is generally responsible for a shift
of the product distribution to longer chain oligomers,
as observed experimentally with other cocatalysts.33,34
In general, a considerable excess of MAO is used as
cocatalyst with Ni(II) or Pd(II) diimine complexes for
the oligomerization and polymerization of ethylene.34,35
When complexes 9-11 were tested in the presence of
400 or 800 equiv of MAO, the activities decreased
compared to those observed with AlEtCl2 as cocatalyst.
In the presence of 400 equiv of MAO, turnover frequen-
cies of 10 200, 10 000, and 7000 mol C2H4/mol Ni‚h were
observed for 9, 10, and 11, respectively (Table 3). When
800 equiv of MAO was used, an increase of the TOF to
13 600, 12 200, and 22 800 mol C2H4/mol Ni‚h, respec-
tively, was observed (Figure 4).
3. Catalytic Eth ylen e Oligom er ization . The [NiCl2-
(phosphinopyridine)] complexes 9-11 were tested for
the oligomerization of ethylene in order to elucidate the
influence of an increased substitution at the carbon
atom R to P on the activity and selectivity of the
catalysts. The cocatalysts used were methylalumoxane
(MAO) or AlEtCl2, and the complex [NiCl2(PCy3)2] was
taken as a reference compound since it is a typical
dimerization catalyst for R-olefins.32 For comparison, we
report in Table 2 the values found for this complex
under our conditions, whereas no activity was observed
for [NiCl2(PCy3)2] in the presence of either 400 or 800
equiv of MAO.
The catalytic properties of complexes 9-11 were eval-
uated in the presence of 2 and 6 equiv of AlEtCl2. When
2 equiv of cocatalyst was used, complexes 9 and 10
showed turnover frequencies (TOF) of 30 100 mol C2H4/
mol Ni‚h (9) and 46 100 mol C2H4/mol Ni‚h (10), where-
as complex 11 was inactive (Table 2). The reference
complex [NiCl2(PCy3)2] was hardly active in the pres-
ence of 2 equiv of cocatalyst but showed a TOF of 27 200
mol C2H4/mol Ni‚h when 6 equiv of AlEtCl2 was used
(Figure 2). Under the latter conditions, complexes 9-11
were also more active than when only 2 equiv of AlEtCl2
(30) Fro¨mmel, T.; Peters, W.; Wunderlich, H.; Kuchen, W. Angew.
Chem., Int. Ed. Engl. 1993, 32, 907-909.
(31) Hayter, R. G.; Humiec, F. S. Inorg. Chem. 1965, 4, 1701-1706.
(32) (a) Commereuc, D.; Chauvin, Y.; Le´ger, G.; Gaillard, J . Revue
de l′Institut Franc¸ais du Pe´trole 1982, 37, 639-649. (b) Knudsen, R.
D. Prepr.-Am. Chem. Soc., Div. Pet. Chem., Miami Beach Meeting;
September 10-15, 1989; pp 572-576.
(33) Skupinska, J . Chem. Rev. 1991, 91, 613-648.
(34) Ittel, S. D.; J ohnson, L. K.; Brookhart, M. Chem. Rev. 2000,
100, 1169-1203.
(35) Britovsek, G. J . P.; Gibson, V. C.; Wass, D. F. Angew. Chem.,
Int. Ed. 1999, 38, 428-447.

Nickel Ethylene Oligomerization Catalysts
Organometallics, Vol. 23, No. 11, 2004 2629
Ta ble 3. Com p a r a tive Ca ta lytic Da ta for Com p lexes 9-11 a n d 1626b in th e Oligom er iza tion of Eth ylen e
w ith MAO a s Coca ta lysta ,c
9
9
10
10
800
83
15
3
11
11
800
69
22
16
MAO [equiv]
selectivity C4 [%]
selectivity C6 [%]
selectivity C8 [%]
400
800
400
400
73
22
800
72
23
81
16
2
77
18
3
85
14
2
4
8
5
selectivity C10 [%]
productivity (g C2H4/g Ni‚h)
TOF (mol C2H4/mol Ni‚h)
R-olefin (C4) [%]
2
1
1
1
4850
10 200
34
6500
13 600
33
4800
10 000
38
5850
12 200
35
3300
7000
34
10 900
22 800
38
3800
7900
38
b
kR
0.14
0.16
0.11
0.12
0.20
0.22
0.21
a
b
Conditions: T ) 30 °C, 10 bar C2H4, 35 min, 4 × 10-2 mmol Ni complex, solvent: 20 mL of toluene. kR) hexenes [mol]/butenes
[mol]. c No C12 oligomers were detected.
ties of 11 with those of the related phosphinooxazoline
nickel(II) complex 16, of which the crystal structure
shows a distorted tetrahedral metal cordination in the
solid state.26b In the presence of 6 equiv of AlEtCl2, the
turnover frequency of 58 100 mol C2H4/mol Ni‚h found
for 11 is about 25% higher than that of 45 900 mol C2H4/
mol Ni‚h for 16 (Table 2).26b Both complexes show a
significant selectivity for the C4 olefins: 65% for 11 and
54% for 16. However, the selectivity to 1-butene within
the C4 fraction increases from to 11% for 11 to 20% for
16. Under these conditions, the phosphinopyridine
complex 11 is therefore a more active but less selective
catalyst for 1-butene than the phosphinooxazoline com-
plex 16. Note however that precatalyst 16 was already
active in the presence of only 2 equiv of AlEtCl2, in
contrast to 11.
When 800 equiv of MAO was used as cocatalyst, the
turnover frequency of 22 800 mol C2H4/mol Ni‚h pro-
vided by complex 11 was considerably higher than that
of 7900 mol C2H4/mol Ni‚h found for 16 (Table 3).
Similar selectivities were obtained for the C4 olefins,
69% with 11 and 72% for 16 and, for 1-butene, 38% of
the C4 fraction with 11 and 16.26b
F igu r e 4. Activity of the complexes 9-11 for the oligo-
merization of ethylene using different quantities of MAO.
Conditions: T ) 30 °C, 10 bar C2H4, 35 min, 4 × 10-2 mmol
Ni, Ref: [NiCl2(PCy3)2].
The kR values given in Tables 2 and 3 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 fact that
this value varies for a given catalyst as a function of
the nature or quantity of cocatalysts used suggests that
some of the C6 production results from incorporation of
1-butene (consecutive reaction).
Con clu sion
F igu r e 5. Selectivity to 1-butene within the C4 fraction
of the complexes 9-11 using different quantities of MAO.
Conditions: T ) 30 °C, 10 bar C2H4, 35 min, 4 × 10-2 mmol
Ni.
The new phosphinopyridine ligands 6-8 have allowed
us to study the behavior of their corresponding para-
magnetic Ni(II) complexes 9-11 as catalyst precursors
for the oligomerization of ethylene using AlEtCl2 or
MAO as cocatalyst. The complex [NiCl2(2-{1′-(diphe-
nylphosphanyl)-1′-methyl}ethylpyridine)] (11) was shown
by X-ray diffraction to have an almost planar coordina-
tion geometry about the metal, although the coordina-
tion sphere of all complexes in solution was determined
by the Evans method to be distorted tetrahedral. The
Ni(II) complexes provided catalytic activities up to
58 100 mol C2H4/mol Ni‚h (11) in the presence of 6 equiv
of AlEtCl2. The selectivity for the C4 olefins was as high
as 81% (9 in the presence of only 2 equiv of AlEtCl2).
Similar activities have been obtained by other groups
in the dimerization of ethylene and propene, but they
required the presence of 200-400 equiv of this cocata-
The selectivities for R-olefins, which were low with
AlEtCl2 as cocatalyst, increased in the presence of either
400 or 800 equiv of MAO (Figure 5). Selectivities to
1-butene within the C4 fraction of 33, 35, and 38% were
observed in the presence of 800 equiv of MAO for 9, 10,
and 11, respectively, while the product distribution was
shifted toward C4 products, and up to 85% of ethylene
dimerization products were formed. Increased substitu-
tion at the carbon R to phosphorus leads, in the presence
of 800 equiv of MAO, to higher activities and better
selectivities for 1-butene in the oligomerization of eth-
ylene (Figures 2-5).
It is interesting to examine the influence of the nature
of the N-heterocycle by comparing the catalytic proper-

2630 Organometallics, Vol. 23, No. 11, 2004
Speiser et al.
δ: 55.4 (d, PhCPPh2, 1J (P,C) ) 12.3 Hz), 121.3 (s, py-C5), 123.6
(s, py-C4), 126.5 (s, py-C3), 126.6 (s, Ph-Cp), 128.0-128.5 (m,
PPh2), 128.6 (s, Ph-Cm, and Ph-Cm′), 129.1 (s, Ph-Co, 2J (P,C) )
8.3 Hz), 129.2 (s, Ph-Co′), 137.2 (s, Ph-Cipso), 149.3 (s, py-C6),
161.2 (s, py-C2). 31P{1H} NMR (CDCl3) δ: -1.0 (s). Anal. Calcd
for C24H20NP: C, 81.57; H, 5.70; N, 3.96. Found: C, 81.23; H,
5.40; N, 3.76.
lyst.36 In the presence of 400 or 800 equiv of MAO,
complexes 9-11 yielded lower activities but higher
selectivities for 1-butene. A turnover frequency of 22 800
mol C2H4/mol Ni‚h was observed for 11 in the presence
of 800 equiv of MAO. The selectivities for the C4 olefins
were in the range 70-85%, with a maximum selectivity
to 1-butene within the C4 fraction of 38% for 10 with
400 equiv of MAO or 11 with 800 equiv of MAO.
It thus appears for these P,N ligands that an increase
in the degree of alkyl substitution at the carbon R to P
tends to lead to higher activities and more selective
formation of R-olefins. The influence of the nature of
the N-heterocycle on the catalytic activity for ethylene
oligomerization was shown by comparing the turnover
frequencies of 58 100 mol C2H4/mol Ni‚h for 11 and of
45 900 mol C2H4/mol Ni‚h for the related phosphinoox-
azoline complex 1626b in the presence of 6 equiv of
AlEtCl2. This indicates a beneficial effect of an increased
basicity of the nitrogen donor atom. When MAO was
used as cocatalyst (800 equiv), a similar trend was
obtained, but the selectivities for 1-butene are similar
for 11 and 16. The limited selectivity for R-olefins may
result from (i) reversible â-H elimination after ethylene
insertion, followed by reinsertion with the opposite
regiochemistry and chain transfer to give 2-butene, or
(ii) a reuptake mechanism leading to isomerization of
1-butene.37a The known ability of Ni(II) complexes to
isomerize R-olefins37b has been recently observed in the
case of phosphino-pyridine chelates.23b
Syn th esis of r a c-2-[1′-(Dip h en ylp h osp h a n yl)eth yl]p y-
r id in e (7). A solution of 2-ethylpyridine 13 (3.37 g, 31 mmol,
3.57 mL) in THF (150 mL) was cooled to -78 °C, and 1 equiv
of n-BuLi (1.6 M solution in hexanes, 31 mmol, 19.38 mL) was
added and the solution was stirred for 1 h at -78 °C, yielding
a dark red solution. One equivalent of P(BH3)Ph2Cl (7.26 g,
31 mmol, 5.8 mL) was then added at dry ice temperature. The
solution was stirred for 1 h at -78 °C and warmed to room
temperature overnight. The reaction mixture was hydrolyzed
by addition of water (50 mL), and the organic layer was
separated. The aqueous phase was extracted twice with
dichloromethane (60 mL). The organic fractions were dried
over MgSO4, filtered, and taken to dryness, yielding the
protected phosphine 14 as a white solid. Yield: 6.70 g, 23
mmol, 76%. NMR data of 14: 1H NMR (CDCl3) δ: 0.5-2.0 (br,
3H, BH3), 1.59 (dd, 3H, CH3, 3J (H,H) ) 7.32 Hz, 3J (P,H) ) 3.6
Hz), 4.17 (dq, 1H, CH(PPh2), 3J (H,H) ) 7.32 Hz, 2J (P,H) )
7.8 Hz), 7.05 (m, 1H, py-H5), 7.29 (m, 1H, py-H3), 7.23-7.6
(m, 10H, PPh2), 7.85 (m, 1H, py-H4), 8.31 (m, 1H, py-H6). 31P-
{1H} NMR (CDCl3) δ: 25.4.
Quantitative deprotection of 14 (6.70 g, 0.023 mol) was
performed by stirring it for 14 h at room temperature in NHEt2
(50 mL). The volatiles were then evaporated, yielding phos-
phine 7 as a yellow oil. NMR data of 7: 1H NMR (CDCl3) δ:
1.49 (dd, 3H, CH3, 3J (H,H) ) 7.30 Hz, 3J (P,H) ) 15.9 Hz), 4.00
3
2
(dq, 1H, CHPPh2), J (H,H) ) 7.32 Hz, J (P,H) ) 7.6 Hz), 7.10
(m, 1H, py-H5), 7.30 (m, 1H, py-H3), 7.23-7.6 (m, 10H, PPh2),
7.85 (t, 1H py-H4, 3J (H,H) ) 6.2 Hz), 8.31 (d, 1H, py-H6,
3J (H6,H5) ) 5.9 Hz). 13C{1H} NMR (CDCl3) δ: 27.4 (s, PCCH3),
Exp er im en ta l Section
All solvents were dried and distilled using common tech-
niques unless otherwise stated. All manipulations were carried
out using Schlenk techniques. Anhydrous NiCl2 was obtained
by heating NiCl2‚6H2O for 6 h at 160 °C under vacuum. [NiX2-
(DME)] (X ) Cl, Br) was prepared according to the literature.38
Other chemicals were commercially available and used without
1
38.7 (d, J (P,C) ) 10.5 Hz, PCCH3), 119.2 (s, py-C5), 121.0 (s,
py-C3), 127.6 (m, PPh2), 133.3 (s, py-C4), 147.5 (s, py-C6), 165.5
(s, py-C2). 31P{1H} NMR (CDCl3) δ: 0.6 (s). Anal. Calcd for
C
19H18NP: C, 78.33; H, 6.23; N, 4.81. Found: C, 77.90; H, 5.89;
N, 4.55.
Syn th esis of 2-[1′-(Dip h en ylp h osp h a n yl)-1′-m eth yl]-
1
further purification unless otherwise stated. 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
eth ylp yr id in e (8). To a solution of the protected racemic
phosphine 14 (1.22 g, 4.18 mmol) in THF (50 mL) was added
1 equiv of n-BuLi (1.6 molar solution in hexanes, 2.6 mL, 4.18
mmol) at room temperature. The solution was stirred for 2 h,
then 1 equiv of CH3I (0.59 g, 0.25 mL, 4.18 mmol) was added
at room temperature and the reaction mixture was stirred
overnight, resulting in a color change from red to yellow. The
solution was hydrolyzed by the addition of H2O (30 mL). The
organic phase was separated, and the aqueous phase was
extracted twice with diethyl ether (40 mL). The organic
fractions were dried over MgSO4 and filtered with the help of
a cannula, and the solvent was evaporated under reduced
pressure, yielding the protected phosphine 15 as a white solid.
Yield: 0.95 g, 3.11 mmol, 74%. NMR data of 15: 1H NMR
(CDCl3) δ: 0.5-2.0 (m, 3H, BH3), 1.75 (d, 6H, C(CH3)2, 3J (P,H)
) 15 Hz), 7.0 (t, 1H, py-H5, 3J (H,H) ) 7.1 Hz), 7.30 (d, 1H,
py-H3, 3J (H,H) ) 7.2 Hz), 7.1-7.6 (m, 10 H, PPh2), 7.62 (t,
1H, py-H4, 3J (H,H) ) 7.0 Hz), 8.45 (d, 1H, py-H6, 3J (H,H) )
7.0 Hz). 31P{1H} NMR (CDCl3) δ: 35.2.
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).
Syn th esis of r a c-2-[(Dip h en ylp h osp h a n yl)ben zyl]p y-
r id in e (6). To a solution of 2-benzylpyridine 12 (3.16 g, 18
mmol) in THF (40 mL) was added 1 equiv of n-BuLi (1.6 M
solution in hexanes, 18 mmol, 11.7 mL), and the reaction
mixture was stirred for 3 h at room temperature. The solution
was then cooled to 0 °C, and 1 equiv of PPh2Cl (4.12 g, 3.45
mL, 18 mmol) was added dropwise. The reaction mixture was
stirred overnight and then hydrolyzed by addition of degassed
water (40 mL). The organic phase was separated and the
aqueous phase was extracted twice with diethyl ether (40 mL).
The organic fractions were collected and dried over degassed
MgSO4, filtered, and taken to dryness, yielding the product
as a white solid. Yield: 4.77 g, 13 mmol, 75%. 1H NMR (CDCl3)
δ: 4.95 (d, 1H, PCH, 2J (P,H) ) 6.5 Hz), 7.01 (m, 1H, py-H5,
3J (H5,H4) ) 6.2 Hz), 7.15-7.50 (m, 15H, Ph and PPh2), 7.40
(m, 1H, py-H3), 7.50 (m, 1H, py-H4), 8.47 (dq, 1H, py-H6,
3J (H6,H5) ) 6.2 Hz, 4J (H6,H4) ) 1.1 Hz). 13C{1H} NMR (CDCl3)
Quantitative deprotection of 15 (0.95 g, 3.11 mmol) was
performed by stirring for 14 h at room temperature in NHEt2.
The volatiles were then evaporated, yielding the product 8 as
a white solid. NMR data of 8: 1H NMR (CDCl3) δ: 1.55 (d,
3
3
6H, C(CH3)2, J (P,H) ) 13.4 Hz), 7.0 (t, 1H, py-H5, J (H,H) )
7.1 Hz), 7.30 (d, 1H, py-H3, J (H,H) ) 7.2 Hz), 7.1-7.6 (m, 10
3
H, PPh2), 7.55 (t, 1H, py-H4, J (H,H) ) 7.0 Hz), 8.60 (d, 1H,
3
(36) Svejda, S. A.; Brookhart, M. Organometallics 1999, 18, 65-74.
(37) (a) We thank a referee for raising this question. (b) Birdwhistell,
K. R.; Lanza, J . J . Chem. Educ. 1997, 74, 579.
py-H6, 3J (H,H) ) 7.0 Hz). 13C{1H} NMR (CDCl3) δ: 25.4 (d,
2
1
C(CH3)2, J (P,C) ) 16.5 Hz), 40.7 (d, C(CH3)2, J (P,C) ) 18.2
(38) Cotton, F. A. Inorg. Synth. 1971, 13, 160-164.
Hz), 119.7 (s, py-C5), 120.8 (s, py-C3), 127.1 (m, PPh2), 133.3

Nickel Ethylene Oligomerization Catalysts
Organometallics, Vol. 23, No. 11, 2004 2631
(s, py-C4), 147.3 (s, py-C6), 165.0 (s, py-C2). 31P{1H} NMR
(CDCl3) δ: 22.5. Anal. Calcd for C20H20NP: C, 78.67; H, 6.60;
N, 4.59. Found: C, 78.70; H, 6.80; N, 4.75.
Ta ble 4. X-r a y Exp er im en ta l Da ta for
[Ni(2-{1′-(d ip h en ylp h osp h a n yl)-
1′-m eth yl}eth ylp yr id in e)Cl2]‚0.5CH2Cl2
(11‚0.5CH2Cl2)
Syn th esis of [Ni{r a c-2-[(d ip h en ylp h osp h a n yl)ben zyl]-
p yr id in e}Cl2] (9). To a suspension of [NiCl2(DME)] (0.600 g,
2.83 mmol) in CH2Cl2 (60 mL) was added a solution of 6 (1.00
g, 2.83 mmol) in CH2Cl2 (40 mL) at room temperature, and
the resulting solution was stirred for 2 h. The violet reaction
mixture was filtered through Celite in order to separate
unreacted [NiCl2(DME)]. The filtrate was evaporated under
reduced pressure, dissolved in CH2Cl2 (10 mL), and reprecipi-
tated with hexane (30 mL). The supernatant liquid was
removed with a cannula equipped with a filter cap. The solid
was dissolved in toluene (15 mL) and reprecipitated by the
addition of hexane. The olive green solid was filtered and dried
under vacuum for 12 h. Yield: 1.07 g, 2.22 mmol, 78%. IR
selected pyridine absorptions: 1601 s (νCdC), 1158m (νâ(C-H)),
1027w and 998s (νcycle), 692vs (νR sens) cm-1. Anal. Calcd for
formula
C20H20Cl2NNiP‚0.5CH2Cl2
477.41
fw
cryst syst
tetragonal
space group
a ) b, Å
P43212
9.209(5)
50.400(5)
c, Å
V, Å3
4274(3)
8
red
Z
color
cryst dimens, mm
D(calc), g cm-3
F(000)
0.15 × 0.12 × 0.10
1.484
1960
1.363
173
µ, mm-1
temp, K
wavelength, Å
radiation
0.71069
Mo KR graphite monochromated
C
24H20 Cl2NNiP: C, 59.68; H, 4.17; N, 2.90. Found: C, 61.10;
diffractometer
θ limits, deg
no. of data measd
no. of data with I > 2σ(I)
no. of variables
R
KappaCCD
2.25/33.13
6872
3980
240
0.0493
0.0951
0.98
H, 4.03; N, 2.75 (we have no explanation for the poor results
of the carbon analysis).
Syn th esis of [Ni{r a c-2-[1′-(d ip h en ylp h osp h a n yl)eth yl]-
p yr id in e}Cl2] (10). To a suspension of [NiCl2(DME)] (1.260
g, 5.96 mmol) in CH2Cl2 (60 mL) was added a solution of 7
(1.653 g, 5.96 mmol) in CH2Cl2 (40 mL) at room temperature,
and the resulting solution was stirred for 2 h. The violet
reaction mixture was filtered through Celite in order to
separate unreacted [NiCl2(DME)]. The filtrate was evaporated
under reduced pressure, dissolved in CH2Cl2 (10 mL), and
precipitated with hexane (30 mL). The supernatant liquid was
removed with a cannula equipped with a filter cap. The solid
was dissolved in the minimum volume of toluene (15 mL) and
precipitated by the addition of hexane. The olive green solid
was filtered and dried under vacuum for 12 h. Yield: 1.90 g,
4.53 mmol, 76%. IR selected pyridine absorptions: 1603s
Rw
GOF on F2
largest peak in final
diff map, e Å-3
0.799
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
collected from the reactor only 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
in 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.
Cr ysta l Str u ctu r e Deter m in a tion of 11‚0.5CH2Cl2. Dif-
fraction data were collected on a Kappa CCD diffractometer
using graphite-monochromated Mo KR radiation (λ ) 0.71073
Å). The relevant data are summarized in Table 4. Data were
collected using phi-scans, the structures were solved by direct
methods using the SHELX 97 software,39,40 and the refinement
was done 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 have also been deposited with the
Cambridge Crystallographic Data Centre, CCDC 227823.
Copies of this information may be obtained free of charge from
The Director, CCDC, 12 Union Road, Cambridge CB2 1EZ,
UK (fax, +44-1223-336033; e-mail, deposit@ccdc.cam.ac.uk;
web, http://www.ccdc.cam.ac.uk).
(νCdC), 1160m (νâ(C-H)), 1038w and 997s (νcycle), 693vs (νR
)
sens
cm-1. Anal. Calcd for C19H18Cl2NNiP: C, 54.21; H, 4.31; N,
3.33. Found: C, 55.00; H, 4.66; N, 3.30.
Syn th esis of [Ni(2-{1′-(diph en ylph osph an yl)-1′-m eth yl}-
eth ylp yr id in e)Cl2] (11). To a suspension of [NiCl2(DME)]
(0.222 g, 1.05 mmol) in CH2Cl2 (20 mL) was added a solution
of 8 (0.305 g, 1.05 mmol) in CH2Cl2 (10 mL) at room temper-
ature, and the resulting solution was stirred for 2 h. The violet
reaction mixture was filtered over Celite in order to separate
unreacted [NiCl2(DME)]. The filtrate was evaporated under
reduced pressure, dissolved in CH2Cl2 (5 mL), and precipitated
with hexane (20 mL). The supernatant liquid was removed
with a cannula equipped with a filter cap. The solid was
dissolved in toluene (10 mL) and precipitated by the addition
of hexane. The red-violet solid was filtered and dried under
vacuum for 12 h. Yield: 0.366 g, 0.842 mmol, 80%. IR (KBr)
selected pyridine absorptions: 1605s (νCdC), 1166m (νâ(C-H)),
1040w and 996s (νcycle), 694vs (νR sens) cm-1. Anal. Calcd for
C
20H20Cl2NNiP: C, 55.23; H, 4.63; N, 3.22. Found: C, 55.00;
H, 4.37; N, 3.08.
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 were 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 following equation:
(39) Kappa CCD Operation Manual; Nonius B.V.: Delft, The
Netherlands, 1997.
(40) Sheldrick, G. M. SHELXL97, Program for the refinement of
crystal structures; University of Go¨ttingen: Germany, 1997.
10 mL (Ni-solution) +
y mL (solvent) + z mL (cocatalyst solution) ) 15 mL

2632 Organometallics, Vol. 23, No. 11, 2004
Speiser et al.
Su p p or tin g In for m a tion Ava ila ble: Tables of atomic
coordinates, bond distances, angles, and anisotropic thermal
parameters and ORTEP plot for11‚0.5CH2Cl2; X-ray data in
CIF format are also available. This material is available free
of charge via the Internet at http://pubs.acs.org.
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 Ph.D. grant (F.S.) and
support. We are also grateful to Prof. R. Welter and the
Service de Diffraction des Rayons X (Institut de Chimie,
Strasbourg) for the determination of the crystal struc-
ture.
OM034198I