Synthesis, X-ray, and electronic structures of a new nickel dibromide complex. Activity in the regioselective catalyzed dimerization of ethylene into 1-butene

Article abstract of DOI:10.1021/ic701529a

Reaction of the bis-(3,4)-dimethylphosphole-Xanthene 1b with [NiBr ₂(DME)] afforded a new nickel(II) dibromide complex, 2. Both its color and its NMR behavior change with temperature and solvent due to changes in the spin state of the complex. This led us to study the complex spin states using DFT calculations. Furthermore, the activity of 2 in catalyzed ethylene dimerization was studied, revealing both high activity and selectivity toward the production of 1-butene.

Full text of DOI:10.1021/ic701529a

Inorg. Chem. 2007, 46, 10365−10371
Synthesis, X-Ray, and Electronic Structures of a New Nickel Dibromide
Complex. Activity in the Regioselective Catalyzed Dimerization of
Ethylene into 1-Butene
Guilhem Mora, Steven van Zutphen, Christian Klemps, Louis Ricard, Yves Jean,* and
Pascal Le Floch*
Laboratoire He´te´roe´le´ments et Coordination, Ecole Polytechnique, CNRS,
91128 Palaiseau Cedex, France
Received August 1, 2007
Reaction of the bis-(3,4)-dimethylphosphole-Xanthene 1b with [NiBr2(DME)] afforded a new nickel(II) dibromide
complex, 2. Both its color and its NMR behavior change with temperature and solvent due to changes in the spin
state of the complex. This led us to study the complex spin states using DFT calculations. Furthermore, the activity
of 2 in catalyzed ethylene dimerization was studied, revealing both high activity and selectivity toward the production
of 1-butene.
Scheme 1. Formula of Ligands 1a and 1b
Introduction
The incorporation of phospholes in ligands for transition
metal catalysts is an active area of research.1-3 Several recent
examples, including palladium, rhodium, and nickel com-
plexes of phosphole-containing ligands, have found applica-
tions in important catalytic transformations such as hydro-
formylation, amine allylation, carbon-carbon bond forming
reactions, or ethylene dimerization.4-13 We recently discov-
ered two phosphole-xanthene derivatives, DPP-Xantphos
(1a) and DMP-Xantphos (1b), featuring 2,5-diphenylphos-
phole and 3,4-dimethylphosphole moieties as pendant arms,
respectively (see Scheme 1).14 These ligands show unusual
reactivity toward palladium(II) allyl species, yielding a
dimeric palladium(0) complex with 1a and a dimeric
trinuclear species with 1b. Matt et al. studied ethylene
dimerization with calixarene ligands and found that the bite
angle of the ligand has an important effect on the catalyst’s
activity.15 This prompted us to study our ligands, which have
an unusually large bite angle, in this transformation. For this,
nickel is the metal of choice.16-19 We therefore decided to
investigate the coordination behavior of ligands 1a and 1b
* To whom correspondence should be addressed. E-mail: lefloch@
poly.polytechnique.fr (P.L.F.); yves.jean@polytechnique.edu (Y.J.). Fax:
+33-169334570 (P.L.F.). Phone: +33-169333990 (P.L.F.).
(1) Quin, L. D. Phosphorus-Carbon Heterocyclic Chemistry: The Rise
of a New Domain; Pergamon: Amsterdam, 2001.
(2) Mathey F. Phosphorus-Carbon Heterocyclic Chemistry: The Rise of
a New Domain, Pergamon: Amsterdam, 2001.
(3) van Zutphen, S.; Margarit, V. J.; Mora, G.; Le Floch, P. Tetrahedron
Lett. 2007, 48, 2857-2859.
(4) Thoumazet, C.; Grutzmacher, H.; Deschamps, B.; Ricard, L.; le Floch,
P. Eur. J. Inorg. Chem. 2006, 19, 3911-3922.
(5) Mora, G.; van Zutphen, S.; Thoumazet, C.; Le, Goff, X. F.; Ricard,
L.; Gru¨tzmacher, H.; Le, Floch, P. Organometallics 2006, 25, 5528-
5532.
(6) Thoumazet, C.; Ricard, L.; Grutzmacher, H.; le Floch, P. Chem.
Commun. 2005, 1592-1594.
(7) Thoumazet, C.; Melaimi, M.; Ricard, L.; Mathey, F.; le Floch, P.
Organometallics 2003, 22, 1580-1581.
(8) Melaimi, M.; Thoumazet, C.; Ricard, L.; le Floch, P. J. Organomet.
Chem. 2004, 689, 2988-2994.
(12) de Souza, R. F.; Bernardo-Gusmao, K.; Cunha, G. A.; Loup, C.; Leca,
F.; Re´au, R. J. Catal. 2004, 235-239.
(13) (a) Vogt, D. Applied Homogeneous Catalysis with Organometallic
Compounds; Wiley: Weinhein, 1996. (b) Chauvin, Y.; Olivier, H.
ibid.
(14) Mora, G; Deschamps, B; van Zutphen, S; Le Goff, X. F; Ricard, L;
Le Floch, P Organometallics 2007, 26, 1846-1855.
(15) Lejeune, M.; Se´meril, D.; Jeunesse, C.; Matt, D.; Peruch, F.; Lutz, P.
J.; Ricard, L. Chem. Eur. J. 2004, 10, 5354-5360.
(9) Cortes, J. G. L.; Ramon, O.; Vincendeau, S.; Serra, D.; Lamy, F.;
Daran, J-C.; Manoury, E.; Gouygou, M. Eur. J. Inorg. Chem. 2006,
24, 5148-5157.
(10) Holz, J.; Gensow, M. N.; Zayas, O.; Boerner, A. Curr. Org. Chem.
2007, 11, 61-106.
(11) Mourgues, S.; Serra, D.; Lamy, F.; Vinceandeau, S.; Daran, J.-C.;
Manoury, E.; Gouygou, M. Eur. J. Inorg. Chem. 2003, 2820-2826.
10.1021/ic701529a CCC: $37.00
Published on Web 10/26/2007
© 2007 American Chemical Society
Inorganic Chemistry, Vol. 46, No. 24, 2007 10365

Mora et al.
Scheme 2. Reaction of 1b with [NiBr2(DME)] to Form Complex 2
with NiBr2 and study the reactivity of these complexes in
ethylene dimerization. We here report the synthesis and
theoretical analysis of the spin states of the nickel(II) DMP-
Xantphos complex, as well as its activity in the ethylene
dimerization reaction.
Figure 1. View of one molecule of 2. Thermal ellipsoids are drawn at
the 30% probability level. Hydrogen atoms are omitted for clarity. Selected
bond distances (Å) and angles (deg): Ni-P(2), 2.192(1); Ni-P(1), 2.2035-
(5); Ni-Br(2), 2.3028(3); Ni-Br(1), 2.3265(3); Ni-O, 2.481(1); P(1)-
Ni-P(2), 158.74(2); P(2)-Ni-Br(2), 92.11(2); P(1)-Ni-Br(2), 93.58(2);
P(2)-Ni-Br(1), 88.71(2); P(1)-Ni-Br(1), 87.81(2); Br(2)-Ni-Br(1),
173.84(1); O-Ni-P(2), 80.24(3); O-Ni-P(1), 78.82(3); O-Ni-Br(2),
95.68(3); O-Ni-Br(1), 90.48(3).
Results and Discussion
a. Syntheses of Complexes 2, 5, and 6 and Their 31P
NMR Behavior. Synthesis of dibromide nickel complexes
was carried out by reacting the [NiBr2(DME)] (DME )
dimethoxyethane) complex with ligands 1a and 1b in
dichloromethane at room temperature. Whereas a clean and
rapid reaction took place with ligand 1b, yielding complex
2 (see Scheme 2), no reaction occurred with the tetraphenyl
ligand 1a. Whatever the experimental conditions used
(heating, prolonged reaction time, solvent ) tetrahydrofuran
(THF), acetonitrile, toluene), work up exclusively led to
recovery of the starting materials.
adopts a ML5 square-based pyramidal conformation. The two
phosphole moieties are coordinated in a mutual trans fashion
and the oxygen atom of the xanthene is pointing toward the
metal in the apical position. However, 2.481(1)Å for the
Ni-O bond appears to be too long to consider that real
bonding takes place. A similar coordination mode has been
observed before for [NiBr2(4,5-bis{di[(2-tert-butyl)phenyl]-
phosphonito}-9,9-dimethylxanthene].20 The observed bite
angle P1-Ni-P2 of complex 2 is 158.74(2)°, while the angle
Br1-Ni-Br2 is 173.84(1)°. Complex 2 must therefore be
regarded as a square-planar complex in which a very weak
interaction takes place with the oxygen atom. Note that, in
general, nickel complexes with coordinated ether moieties
show an average Ni-O bond length of 2.15 Å.21
Solvent effects on the electronic structure of 2 were
studied. The most significant result was obtained when
dissolving complex 2 in THF. At room temperature, the
mixture color was found to be unchanged (dark blue),
suggesting that no coordination of the solvent had taken
place. However, at low temperature (-80 °C), the solution
slowly turned to green, and contrary to what was previously
observed with CH2Cl2 solutions of 2, no 31P NMR signal
could be observed. This observation led us to propose that,
at low temperature, a new complex (3) with a coordinated
THF in the second apical position is formed. This process
was found to be fully reversible. Several examples of Ni(II)
in the presence or absence of coordinating solvent have been
studied before, and it was concluded that the decrease of
temperature in the presence of a coordinating solvent
transforms a square-planar singlet into an octahedral triplet.22-23
(Scheme 3). Unfortunately, despite many attempts, no
crystals of complex 3 could be obtained at low temperature.
Though the structure proposed is hypothetical, DFT calcula-
tions presented confirm this hypothesis.
The characterization of complex 2 was not straightforward.
Indeed, no 1H or 31P NMR signal is visible at room
temperature, suggesting a paramagnetic behavior. This
observation is consistent with d8 Ni(II) complexes adopting
a tetrahedral geometry. However, when the temperature was
decreased to -40 °C, the 31P NMR spectrum revealed the
presence of a very broad signal (ν1/2 ) 840 Hz) around 0
ppm, which then progressively sharpened upon further
cooling. At -80 °C, the 31P NMR spectrum exhibited a
singlet at 4.3 ppm (ν1/2 ) 98 Hz), indicative of a singlet
ground state. Moreover, during the same NMR experiment,
the solution of 2 in dichloromethane turned from dark blue
at room temperature to pale violet at low temperature (-80
°C), suggesting a change of the spin state. Note that this
phenomenon was found to be reversible. Unfortunately, no
1
clear H NMR data could be recorded even at low temper-
ature; only broad indistinct signals were observed.
Fortunately, suitable crystals for a X-ray structure deter-
mination were obtained by slow diffusion of a mixture of
hexanes into a dichloromethane solution of complex 2 at
room temperature. A view of one molecule of complex 2 is
presented in Figure 1, and the most relevant metric param-
eters are listed in the corresponding legend. Crystal data and
structural refinement details are presented in Table 1.
As can be seen, contrary to what might be expected from
31P NMR data recorded at room temperature, complex 2
(16) Speiser, F.; Braunstein, P.; Saussine, L. Acc. Chem. Res. 2005, 38,
784-793, and references herein.
(17) Sauthier, M.; Leca, F.; de Souza, R. F.; Bernardo-Gusmao, K.; Queiroz,
L. F. T.; Toupet, L.; Reau, R. New J. Chem. 2002, 26, 630-635.
(18) Weng, Z. Q.; Teo, S.; Koh, L. L.; Hor, T. S. A. Chem. Commun.
2006, 1319-1321.
(20) Van, der Vlugt, J. I.; Hewat, A. C.; Neto, S.; Sablong, R.; Mills, A.
M.; Lutz, M.; Spek, A. L.; Mu¨ller, C.; Vogt, D. AdV. Synth. Catal.
2004, 346, 993-1003.
(21) Allen, F. H.; Kennard, O. Chem. Des. Autom. News 1993, 8, 31.
(22) Ohtsu, H.; Tanaka, K. Inorg. Chem. 2004, 43, 3024-3030.
(23) Boiocchi, M.; Fabbrizzi, L.; Foti, F.; Vazquez, M. Dalton Trans. 2004,
2616-2620.
(19) Ajellal, N.; Kuhn, M. C. A.; Boff, A. D. G.; Horner, M.; Thomas, C.
M.; Carpentier, J.-F.; Casagrande, O. L. Organometallics 2006, 25,
1213-1216.
10366 Inorganic Chemistry, Vol. 46, No. 24, 2007

Ni(II) Dimethylphosphole-Xanthene
Table 1. Crystal Data and Structural Refinement Details for 2 and 5
compound
2
5
cryst size [mm3]
empirical formula
molecular mass
cryst syst
space group
a [Å]
0.26 × 0.16 × 0.10
C27H28Br2NiOP2
648.96
monoclinic
P21/c
10.5190(10)
18.0280(10)
13.7460(10)
90.00
92.8900(10)
90.00
0.18 × 0.08 × 0.04
C39H32Br2NiOP2,0.5(C7H8),0.5(CH2Cl2)
885.65
triclinic
P1h
9.504(1)
10.503(1)
19.600(1)
84.716(1)
89.498(1)
74.856(1)
1880.3(3)
2
b [Å]
c [Å]
R [deg]
â [deg]
γ [deg]
V [Å3]
2603.4(3)
4
1.656
Z
calcd density [g cm-3
]
1.564
2.831
abs coeff [cm-1
]
3.956
θ
max [deg]
F(000)
30.03
1304
26.02
896
index ranges
-14 e h e 11
-23 e k e 25
-18 e l e 19
24 900/7590
5621
-10 e h e 11
-12 e k e 12
-24 e l e 21
17 225/7367
4859
reflns collected/independent
reflns used
(Rint
)
0.0288
0.0414
abs corr
0.4261 min,
0.6931 max
304
0.6297 min,
0.8952 max
475
params refined
reflns/param
18
10
final R1a/wR2 [I > 2σ(I)]b
0.0287/0.0689
0.995
0.415(0.080)/-0.475(0.080)
0.0409/0.0921
0.975
0.902(0.090)/-1.025(0.090)
GOF on F2
diff. peak/hole [e Å-3
]
2
2
a R1 ) ∑|Fo| - |Fc||/∑|Fo|. b wR2 ) (∑w||Fo| - |Fc|| /∑w|Fo| )1/2
.
Scheme 3. Thermochromic Equilibrium of Complex 2
Scheme 4. Synthesis of Complex 5
Figure 2. View of one molecule of 5. Thermal ellipsoids are drawn at
the 30% probability level. Hydrogen atoms are omitted for clarity. Selected
bond distances (Å) and angles (deg): Ni-P(2), 2.323(1); Ni-P(1), 2.333-
(1); Ni-Br(2), 2.342(1); Ni-Br(1), 2.347(1); Ni-O, 3.222(2); P(1)-Ni-
P(2),108.00(4); P(2)-Ni-Br(2), 103.93(3); P(1)-Ni-Br(2), 106.02(3);
P(2)-Ni-Br(1), 105.58(3); P(1)-Ni-Br(1), 102.88(3); Br(2)-Ni-Br(1),
129.20(3); P(2)-Ni-O, 58.58(4); P(1)-Ni-O, 58.24(4); Br(2)-Ni-O,
143.04(5); Br(1)-Ni-O, 87.76(4).
To complete our study on the electronic structure of these
systems, we synthesized [NiBr2(Xantphos)] (5) (Xantphos
) 9,9-dimethyl-4,5-bis(diphenylphosphino)xanthene, 4)
(Scheme 4) and [NiBr2(DMP)] (6) (DMP ) 1-phenyl-3,4-
dimethylphosphole). Complex 5 was prepared by adding 1
equiv of the Xantphos ligand (4) to a solution of [NiBr2-
(DME)] in dichloromethane. The solution turned green
instantaneously, indicative of the coordination taking place.
Complex 5 was found to be NMR silent due to it being
paramagnetic. Importantly, recording the 31P NMR spectra
at low temperature or replacement of CH2Cl2 by THF did
not induce the appearance of a signal. We therefore
concluded that the structure of 5 and its spin-state config-
uration were maintained whatever the temperature. The
structure proposed for 5 was confirmed by an X-ray crystal
analysis. Diffusion of toluene in a solution of 5 in dichlo-
romethane yielded crystals whose structure revealed a
tetrahedral geometry. A view of one molecule of 5 is
presented in Figure 2, and the most significant parameters
are listed in the corresponding legend. Crystal data and
structural refinement details are presented in Table 1.
In the case of DMP, the well-defined complex 6 resulting
from the coordination of two DMP units onto the NiBr2
fragment was readily prepared by reaction of 2 equiv of the
ligand with [NiBr2(DME)] in CH2Cl2 at room temperature.
The 31P NMR spectrum of 6 is characterized by a broad
Inorganic Chemistry, Vol. 46, No. 24, 2007 10367

Mora et al.
Scheme 5. Synthesis of Complex 6
signal at room temperature but showed two different signals
when the temperature was decreased (δ 18.1 ppm 2.2 times
more intense than δ 29.5 ppm, determined at -80 °C). These
results could be rationalized when 6 was structurally
characterized by monocrystal X-ray diffraction (see Sup-
porting Information) which showed two different structures
with the phospholes moieties syn or anti (see Scheme 5).
The hindered rotation of the phosphole ligand causes a broad
signal.
Less satisfactory results were obtained with the TPP ligand
(TPP ) 1,2,5-triphenylphosphole), which was found to be
reluctant toward coordination with the NiBr2 fragment, all
experiments yielding the starting materials whatever the
experimental conditions used (see Scheme 5).
b. Theoretical Study of Complexes 2, 3, and 5. All these
results prompted us to launch a theoretical study of the
DMP-xantphos and xantphos nickel dibromide complexes
2, 3, and 5. DFT calculations with the B3PW91 functional
were performed for both the singlet and the triplet states. In
these calculations, the four methyl groups of the phosphole
moieties (complexes 2 and 3) and the two methyl groups of
the xanthene backbone (complexes 2, 3, and 5) were replaced
by H atoms (see the Experimental Section for further details
regarding these calculations).
listed in the corresponding legend) was found to be in good
agreement with the X-ray structure. It can therefore be
described as a d8 ML5 square-based pyramidal complex with
an elongated apical bond (Ni-O ) 2.51 Å (exptl, 2.481(1)
Å)). The metal-ligand bond lengths are overestimated by
0.02-0.05 Å, and bond angles around the metal center are
reproduced within 2-3°. Optimization of the triplet state led
to a very different structure (see Figure 4, the more significant
metric parameters being listed in the corresponding legend).
The oxygen center is no longer bound to the metal (Ni-O
) 3.33 Å), and a distorted tetrahedral arrangement was found
for the remaining ML4 unit, with P-Ni-P ) 111.3° and
Br-Ni-Br ) 143.6°. On geometrical grounds (comparison
with the X-ray structure), a singlet ground state is thus
predicted for complex 2. However, on energetic grounds,
the triplet state was found to be slightly more stable than
the singlet state, by 2.4 kcal mol-1. This result is consistent
neither with the X-ray structure nor with the NMR data at
low temperature. However, it must be considered with
caution since it is well known that in the DFT framework
the energy ordering of low-spin and high-spin states of a
given system may depend on the functional.24 The NMR data
at low-temperature prove that the singlet state is actually the
electronic ground state. On the other hand, calculations
suggest the existence of a low-lying triplet state, suggesting
a mixture of spin states in solution at room temperature. The
dark blue color could therefore be the result of the mixture
of the pale violet singlet and dark blue triplet states. It has
not been possible to crystallize complex 2 in its tetrahedral
geometry. Apparently this conformation is not favored in
the solid state.
The optimized geometry for the singlet state of complex
2 (see Figure 3, the more significant metric parameters being
DFT calculations were also carried out on complex 2 in
interaction with a THF solvent molecule (complex 3). The
theoretical structures are given in Figures 5 and 6, and the
most relevant metric parameters are listed in the correspond-
ing legends. Both structures can be described as distorted
octahedral d8 ML6 complexes. The triplet state is found to
be more stable than the singlet state by 12.1 kcal mol-1, a
large energy gap which rationalizes the absence of NMR
signal in THF even at low temperature. According to the d8
electronic configuration, two electrons are lying in the two
orbitals derived from the eg block of an ideal octahedron. In
going from the singlet to the triplet state, one electron of z2
(mainly Ni-O and Ni-OTHF antibonding) is transferred to
x2-y2 (mainly Ni-Br and Ni-P antibonding) so that Ni-O
and Ni-OTHF are reinforced whereas Ni-Br and Ni-P are
Figure 3. Singlet state of 2. Relevant bond distances (Å) and bond angles
(deg): Ni-P(2), 2.24; Ni-P(1), 2.24; Ni-Br(2), 2.33; Ni-Br(1), 2.35;
Ni-O, 2.51; P(1)-Ni-P(2), 156.8; Br(2)-Ni-Br(1), 176.1.
Figure 4. Triplet state of 2. Relevant bond distances (Å) and bond angles
(deg): Ni-P(2), 2.41; Ni-P(1), 2.41; Ni-Br(2), 2.35; Ni-Br(1), 2.37;
Ni-O, 3.33; P(1)-Ni-P(2), 111.3; Br(2)-Ni-Br(1), 143.6.
(24) Zein, S; Borshch, S. A.; Fleurat-Lessard, P; Casida, M. E.; Chermette,
H. J. Chem. Phys. 2007, 126, 014105.
10368 Inorganic Chemistry, Vol. 46, No. 24, 2007

Ni(II) Dimethylphosphole-Xanthene
Figure 8. Triplet state of complex 5. Relevant bond distances (Å) and
bond angles (deg): Ni-P(2), 2.41; Ni-P(1), 2.41; Ni-Br(2), 2.38;
Ni-Br(1), 2.39; Ni-O, 3.24; P(1)-Ni-P(2), 111.1; Br(2)-Ni-Br(1),
135.8.
Figure 5. Singlet state of complex 3. Relevant bond distances (Å) and
bond angles (deg): Ni-P(2), 2.25; Ni-P(1), 2.25; Ni-Br(2), 2.36;
Ni-Br(1), 2.37; Ni-O, 2.60; Ni-OTHF, 2.55; P(1)-Ni-P(2), 152.7;
Br(2)-Ni-Br(1), 173.9.
going from 2 to 5 can be rationalized by the presence of
sterically encumbering PPh2 ligands in 5 which favors a
pseudo-tetrahedral arrangement. It thus imposes a large
geometrical distortion for the singlet state while the geometry
of the triplet state is left almost unchanged. The destabiliza-
tion of the singlet state by steric interactions is also reflected
by the evolution of the singlet-triplet energy gap which
increases from 2.4 to 14.4 kcal mol-1. The triplet state is
now by far the most stable, a result consistent with the NMR
data.
c. Reactivity of Complexes 2, 5, and 6 in the Catalyzed
Oligomerization of Ethylene. The catalytic activities of
complexes 2, 5, and 6 were evaluated in the oligomerization
of ethylene to form R-olefins. All reactions were carried out
following a procedure which is detailed in the Experimental
Section. Reactions were conducted in toluene at 20 °C in
the presence of MAO (methylaluminoxane) as co-catalyst
(300 equiv) under an ethylene pressure of 30 bar. Reactions
were stopped after 30 min, and the amounts of butenes and
hexenes formed were evaluated by GC analysis. In order to
determine the effect of the ligand on catalyst selectivity and
activity, [NiBr2(DME)] was evaluated.
Figure 6. Triplet state of complex 3. Relevant bond distances (Å) and
bond angles (deg): Ni-P(2), 2.39; Ni-P(1), 2.39; Ni-Br(2), 2.52;
Ni-Br(1), 2.52; Ni-O, 2.35; Ni-OTHF, 2.12; P(1)-Ni-P(2), 159.2;
Br(2)-Ni-Br(1), 166.7.
Results of the catalytic experiments are reported in Table
2. Nickel-based systems bearing bidentate P,15 N,17 and P,N12
ligands have previously been evaluated in ethylene oligo-
merization. Sauthier et al. reported activities of up to 135 000
mol(C2H4) × mol-1[Ni] × h-1 with a 1,2-diiminophospho-
ranyl nickel catalyst; however, the R-selectivity in the butenes
fraction is as low as 17% and a significant portion (11%) of
higher olefins (>C6) are produced.17 A series of 2-py-
ridylphosphole nickel complexes were tested by de Souza
et al. in the ethylene oligomerization reaction and activities
of up to 56 150 mol(C2H4) × mol-1[Ni] × h-1 were observed
with a tight C4-centered product distribution and R-selectivi-
ties of 2-80% within the butenes fraction.12 Taking the
activity and selectivity of [NiBr2(DME)] as a reference point,
it can be observed that addition of TPP or DMP only slightly
changes the activity but reduces the selectivity toward
1-butene from 85% to 30-40%. Even though complex 6
exhibits a good selectivity toward the formation of 1-butene
(97% in 1-C4), its activity is substantially decreased com-
pared to [NiBr2(DME)]. The [NiBr2(Xantphos)] complex, 5,
shows an activity and selectivity pattern almost identical to
[NiBr2(DME)] which suggests that decoordination of the
ligand may occur during catalysis. As mentioned above, no
Figure 7. Singlet state of complex 5. Relevant bond distances (Å) and
bond angles (deg): Ni-P(2), 2.26; Ni-P(1), 2.26; Ni-Br(2), 2.31;
Ni-Br(1), 2.37; Ni-O, 2.66; P(1)-Ni-P(2), 149.4; Br(2)-Ni-Br(1),
156.4.
weakened (the orbitals are presented in the Supporting
Information).
Finally, geometry optimizations were performed for both
the singlet and the triplet state of the [NiBr2(Xantphos)]
complex, 5. The theoretical structures are presented in
Figures 7 and 8, and the most relevant metric parameters
are listed in the corresponding legends. For the triplet state,
there is little change with respect to the related [NiBr2(DMP-
Xantphos)] complex, 2, a pseudo-tetrahedral arrangement
being found for both complexes. In contrast, a large geo-
metrical change occurs in the singlet state. The near square-
planar arrangement of the NiBr2P2 unit in 2 (Br-Ni-Br )
176.1° and P-Ni-P ) 156.8°) evolves toward a pseudo-
tetrahedral geometry in 5 (Br-Ni-Br ) 156.4° and P-Ni-P
) 149.4°), leading to a further lengthening of the Ni-O bond
(2.66 instead of 2.61 Å). These geometrical evolutions in
Inorganic Chemistry, Vol. 46, No. 24, 2007 10369

Mora et al.
Table 2. Catalytic Results in Ethylene Oligomerization
catalysta
% C4
%1-C4
% C6
%1-C6
% C8
%1-C8
TOF mol(C2H4) × mol-1 [Ni] × h-1
2
5
6
97
96
93
93
92
94
94
90
88
97
40
34
66
85
3
4
7
7
8
6
6
56
34
26
20
15
21
29
Tr.
Tr.
Tr.
Tr.
Tr.
Tr.
Tr.
-
-
-
-
-
-
-
43 000
14 000
900
11 000
15 000
1000
[NiBr2(DME)] + DMPb
[NiBr2(DME)] + TPPb
[NiBr2(DME)] + 1ab
[NiBr2(DME)]
15 000
a Conditions: T ) 20 °C, 30 bar C2H4, 0.5 h, 8 µmol Ni complex, 300 equiv of MAO, solvent: toluene (20 mL) (Tr. ) traces). b 1.7 mol equiv of ligand
per mole of [NiBr2(DME)].
observed. The solvent was removed in vacuo, and the residue
triturated with petroleum ether (2 × 3 mL) to yield 97.8 mg (0.151
mmol, 93%) of [NiBr2(DMP-Xantphos)] as a blue powder.
31P{1H} NMR (CH2Cl2, -80 °C): δ 4.3 (s); Anal. Calcd for
C27H28Br2NiOP2: C, 49.97; H, 4.35. Found: C, 49.51; H, 4.32.
coordination of 1a to [NiBr2(DME)] was observed. Catalytic
testing of an in situ prepared mixture of [NiBr2(DME)] and
1a gave a C4/C6 ratio identical to the one obtained with
[NiBr2(DME)] only, but the overall activity decreased
dramatically and selectivity toward 1-butene was reduced
to 66%. Finally, more encouraging results were obtained with
complex 2, which exhibits almost three-fold better activity
than [NiBr2(DME)] along with a considerably better selectiv-
ity toward 1-butene (90% in 1-C4). It should be noted that
formation of neither oligomeric products of higher molecular
mass (>C8) nor polymeric material were observed in any
of these catalytic experiments.
In summary, a new nickel (II) dibromide complex (2) has
been synthesized. Its high-spin and low-spin states pertain
to levels of comparable energy and either the presence of a
coordinating solvent or the change of temperature can favor
one state with respect to the other. Moreover, 2 appeared to
be particularly active and selective in the catalyzed dimer-
ization of ethylene into 1-butene.
Synthesis of Complex 5 [NiBr2Xantphos]. [NiBr2(DME)] (50
mg, 0.162 mmol) was added to a solution of 94.0 mg (0.162 mmol)
of Xantphos in 5 mL of dichloromethane. An immediate color
change to green was observed. The reaction mixture was stirred
for 12 h, the solvent was removed in vacuo, and the residue
triturated with petroleum ether (2 × 3 mL) to yield 107 mg (0.134
mmol, 83%) of [NiBr2(Xantphos)] as a green powder. Due to the
paramagnetic nature of the complex, no characterization by NMR
was possible. Anal. Calcd for C39H32Br2NiOP2: C, 58.76; H, 4.05.
Found: C, 58.65; H, 4.08.
Synthesis of Complex 6 [NiBr2DMP2]. [NiBr2(DME)] (41 mg,
0.1333 mmol) was added to a solution of 50 µL (0.266 mmol) of
3,4-dimethyl-1-phenyl-1H-phosphole (DMP) in 5 mL of dichlo-
romethane. An immediate color change from transparent to brown
was observed. The reaction mixture was stirred for 1 h, the solvent
was removed in vacuo, and the residue triturated with petroleum
ether (2 × 3 mL) to yield 64 mg (0.108 mmol, 81%) of [NiBr2-
(DMP)2] as a brown powder. 31P{1H} NMR (CH2Cl2, -80 °C): δ
18.1 (s), 29.5 (s) for the two conformations (see Supporting
Information for X-ray structures). Anal. Calcd for C24H26Br2NiP2:
C, 48.45; H, 4.41. Found: C, 48.61; H, 4.46.
Experimental Section
Synthesis. All reactions were routinely performed under an inert
atmosphere of argon or nitrogen using Schlenk and glovebox
techniques and dry deoxygenated solvents. Dry hexanes and THF
were obtained by distillation from Na/benzophenone. Dry dichlo-
romethane was distilled on P2O5, and dry toluene on metallic Na.
Nuclear magnetic resonance spectra were recorded on a Bruker AC-
300 SY spectrometer operating at 121.5 MHz for 31P. 31P chemical
shifts are relative to a 85% H3PO4 external reference. The following
abbreviation is used: s, singlet. 1a, 1b,14 [NiBr2(DME)],25 4,26 1,2,5-
triphenylphosphole (TPP),27 and 1-phenyl-3,4-dimethylphosphole
(DMP)28 were prepared according to literature procedures. All other
reagents and chemicals were obtained commercially and used as
received. Elemental analyses were performed by the “Service
d’analyse du CNRS”, at Gif sur Yvette, France. The GC yields
were determined on a PERICHROM 2100 gas chromatograph
equipped with a HP PONA column (50 m × 202 µm, 0.5 mm film).
Synthesis of Complex 2 [NiBr2DMP-Xantphos]. To a solution
of [NiBr2(DME)] (DME ) dimethoxyethane) (50 mg, 0.162 mmol)
in 5 mL of dichloromethane was added 69.7 mg (0.162 mmol) of
DMP-Xantphos. An immediate color change to dark blue was
General Procedure for the Catalyzed Dimerization of Eth-
ylene. All catalytic reactions were carried out in a magnetically
stirred 120 mL stainless steel autoclave, equipped with a pressure
gauge and needle valves for injections. The interior of the autoclave
was protected from corrosion by a Teflon protective coating and a
glass liner. A typical reaction was performed by introducing in the
reactor under nitrogen atmosphere the nickel complex (8 µmol)
and 20 mL of toluene. After injection of the MAO solution (300
equiv), the reactor was immediately brought to the desired working
pressure and continuously fed by ethylene using a reserve bottle.
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M. A.; Cheeseman, J. R.; Montgomery, J. A., Jr.; Vreven, T.; Kudin,
K. N.; Burant, J. C.; Millam, J. M.; Iyengar, S. S.; Tomasi, J.; Barone,
V.; Mennucci, B.; Cossi, M.; Scalmani, G.; Rega, N.; Petersson, G.
A.; Nakatsuji, H.; Hada, M.; Ehara, M.; Toyota, K.; Fukuda, R.;
Hasegawa, J.; Ishida, M.; Nakajima, T.; Honda, Y.; Kitao, O.; Nakai,
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Bakken, V.; Adamo, C.; Jaramillo, J.; Gomperts, R.; Stratmann, R.
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Dannenberg, J. J.; Zakrzewski, V. G.; Dapprich, S.; Daniels, A. D.;
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Cioslowski, J.; Stefanov, B. B.; Liu, G.; Liashenko, A.; Piskorz, P.;
Komaromi, I.; Martin, R. L.; Fox, D. J.; Keith, T.; Al-Laham, M. A.;
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10370 Inorganic Chemistry, Vol. 46, No. 24, 2007

Ni(II) Dimethylphosphole-Xanthene
The reaction was stopped by closing the ethylene supply and cooling
down the system to -70 °C. After the pressure in the reactor
decreased to atmospheric pressure, the reaction was quenched by
adding 1 mL of methanol. n-Heptane, used as internal standard,
was also introduced and the mixture was analyzed by quantitative
GC, calibrated with standard samples (except in the case of butenes
for which the calibration was based upon the response factor of
n-pentane).
Computational Details. Calculations were performed with the
GAUSSIAN 03 series of programs.29 DFT30 was applied with the
B3PW91 functional.31,32 A quasi-relativistic effective core potential
operator was used to represent the 10 innermost electrons of the
nickel atom.33 The basis set for the metal was that associated with
the pseudo potential, with a standard double-ú LANL2DZ contrac-
tion33 completed by a set of polarization f functions (exponent )
1.472),34 and the standard 6-31G* basis set was used for all the
other atoms (C, N, O, P, and H). The stationary points were
characterized as minima by full vibration frequencies calculations.
X-ray Crystallography for complexes 2, 5 and 6. Dark blue
needles of 2 crystallized by slow diffusion of hexanes into a
saturated dichloromethane solution of the complex. Green blocks
of 5 crystallized by slow diffusion of toluene into a saturated
dichloromethane solution of the complex. Brown blocks of 6 were
obtained by diffusion of hexanes into a saturated dichloromethane
solution of the complex. Data were collected on a Nonius Kappa
CCD diffractometer using a Mo KR (λ ) 0.71073 Å) X-ray source
and a graphite monochromator at 150 K. An additional experiment
was conducted at room temperature for crystals of 2 but gave the
same structure. Experimental details are described in Table 1. The
crystal structures were solved using SIR 9735 and SHELXL97.36
ORTEP drawings were made using ORTEP III for Windows.37
Crystallographic data can be obtained free of charge at www.
ccdc.cam.ac.uk/conts/retrieving.html [or from the Cambridge Crys-
tallographic Data Centre, 12 Union Road, Cambridge CB21EZ, UK;
fax: (int) +44-1223/336-033 ; e-mail: deposit@ccdc.cam.ac.uk]
with the deposition numbers CCDC 649846-649848, respectively,
for 2, 5, and 6.
Acknowledgment. The CNRS, the Ecole Polytechnique,
the Institut Franc¸ais du Pe´trole, and the IDRIS (for computer
time, project no. 171616) are thanked for supporting this
work.
Supporting Information Available: Computational details,
computed Cartesian coordinates, thermochemistry, energies, three
lower frequencies of all theoretical structures, CIF files and tables
giving crystallographic data for 2 and 5 (including atomic coordi-
nates, bond lengths and angles, and anisotropic displacement
parameters), and X-ray structure of 6 and its crystallographic data.
This material is available free of charge via the Internet at
http://pubs.acs.org.
(30) (a) Ziegler, T. Chem. ReV. 1991, 91, 651-667. (b) Parr, R. G.; Yang,
W. DFT; Oxford University Press: Oxford, 1989.
(31) Becke, A. D. J. J. Phys. Chem. 1993, 98, 5648-5662.
(32) Perdew, J. P.; Wang, Y. Phys. ReV. B 1992, 45, 13244-13249.
(33) Hay, P. J.; Wadt, W. R. J. Chem. Phys. 1985, 82, 299-310.
(34) Ehlers, A.; Bo¨hme, M.; Dapprich, S.; Gobbi, A.; Ho¨llwarth, A.; Jonas,
V.; Ko¨hler, K. F.; Stegmann, R.; Veldkamp, A.; Frenking, G. Chem.
Phys. Lett. 1993, 208, 111-114.
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Appl. Cryst. 1999, 32, 115-119.
IC701529A
(36) Sheldrick, G. M. SHELXL-97; Universita¨t Go¨ttingen: Go¨ttingen,
Germany, 1997.
(37) Farrugia, L. J. ORTEP-3; Department of Chemistry: University of
Glasgow, 2001.
Inorganic Chemistry, Vol. 46, No. 24, 2007 10371