Activation of polymerization catalysts: Synthesis and characterization of novel dinuclear nickel(I) diimine complexes

Article abstract of DOI:10.1021/om060842s

The reaction of 1,4-bis(diisopropylphenyl)-aza-1,4-butadienenickel dibromide (1a) with stoichiometric amounts of phenyl Grignard or trimethylaluminum affords the purple Ni-(I) complexes 1b and 1c, respectively. Single-crystal X-ray diffraction reveals din

Full text of DOI:10.1021/om060842s

Organometallics 2007, 26, 751-754
751
Notes
Activation of Polymerization Catalysts: Synthesis and
Characterization of Novel Dinuclear Nickel(I) Diimine Complexes
Dieter Meinhard, Peter Reuter, and Bernhard Rieger*
Department of Inorganic Chemistry II, DiVision for Materials and Catalysis, UniVersity Ulm,
Albert-Einstein Allee 11, 89081 Ulm, Germany
ReceiVed September 14, 2006
Summary: The reaction of 1,4-bis(diisopropylphenyl)-aza-1,4-
butadienenickel dibromide (1a) with stoichiometric amounts of
phenyl Grignard or trimethylaluminum affords the purple Ni-
(I) complexes 1b and 1c, respectiVely. Single-crystal X-ray
diffraction reVeals dinuclear species in the solid state for both
compounds. UV/Vis spectroscopy supports this rare oxidation
state of nickel.
chain transfer reagent.4d The present contribution reports on the
formation of neutral, dinuclear, and hydrogen-sensitive Ni(I)
intermediates that are formed during the catalyst activation
process.
Activation of the neutral dibromo (1a) or monocationic
acetylacetonato species (2a,b) was performed by treatment with
aluminum alkyls, such as trimethyl aluminum (TMA) or MAO,
and was assumed to generate polymerization-active cationic Ni-
(II) alkyl complexes.5 Such compounds were characterized at
-78 °C by NMR spectroscopy in previous studies.6 Experiments
with 1a as catalyst precursor performed in our laboratories,
however, revealed intense purple colors after treatment with
aluminum alkyls, which is unusual for Ni(II) derivatives. In the
1970s tom Dieck reported on CH activation reactions of similar,
N-isopropyl-substituted NiBr2 complexes after reaction with
ortho-CH3-PhMgBr.7 Later on, Brookhart observed a color
change from dark blue R-diimine nickel(II) dipropyl solutions
to purple while heating to room temperature.4c These results
prompted us to react 1a with phenyl Grignard (Ni/Mg ) 1) to
study the alkylation process. Indeed, immediately after Grignard
addition to a brownish-orange diethyl ether suspension of 1a,
an intensive, deep purple color developed. An identical color
change was observed by application of either TMA or MAO
(Ni/Al ) 1:3). Dark purple, temperature- and water-stable, but
extremely oxygen-sensitive crystals were obtained from the
Grignard (1b) and the TMA (1c) reaction. Both dinuclear Ni(I)
complexes were isolated in up to 70% yield.8 This rare nickel
oxidation state was supported by UV/vis experiments performed
on the deeply colored toluene solutions of 1b,c (Table 1).9
One possible mechanism for the formation of such Ni(I)
compounds comprises the dialkylation of a (R-diimine)Ni(II)
complex. This initial step is followed by a rapid reductive
elimination producing a Ni(0) intermediate that is stabilized in
a comproportion reaction with remaining 1a. A similar sequence
With the work of Brookhart in the mid 1990s, late transition
metal catalysts such as 1a (Scheme 1) became available.1 After
activation, these catalysts produce branched products exclusively
from ethylene.2 This discovery caused considerable interest, as
it opened the chance to generate high-value products from cheap
monomer supplies.3 However, the predominantly4 used neutral
dibromide complexes are rapidly and quantitatively deactivated
in the presence of hydrogen, a reductant commonly used as a
* To whom all correspondence should be addressed. E-mail: bern-
hard.rieger@
uni-ulm.de.
(1) Brookhart, M.; Johnson, L. K.; Killian, C. M. J. Am. Chem. Soc.
1995, 117, 6414.
(2) (a) Gates, D. P.; Svejda, S. A.; Onate, E.; Killian, C. M.; Johnson,
L. K.; White, P. S.; Brookhart, M. Macromolecules 2000, 33, 2320. (b)
Brookhart, M.; Ittel, S. D.; Johnson, L. K. Chem. ReV. 2000, 100, 1169.
(3) In 2004 35 mio tons of PE-LD/PE-LLD and 25 mio tons of PE-HD
were consumed worldwide, and consumption is estimated to grow by 5%
p.a. at least until 2010. PlasticsEurope Deutschland, WG Statistics and
Market Research; cf. http://www.vke.de/de/infomaterial/download/.
(4) (a) Brookhart, M.; McCord, E. F.; McLain, S. J.; Nelson, L. T. J.;
Arthur, S. D.; Coughlin, E. B.; Ittel, S. D.; Johnson, L. K.; Tempel, D.;
Killian, C. M. Macromolecules 2001, 34, 362-371. (b) Brookhart M.;
Letherman M. D. Macromolecules 2001, 34, 2748-2750. (c) Brookhart,
M.; Leatherman, M. D.; Svejda, S. A.; Johnson, L. K. J. Am. Chem. Soc.
2003, 125, 3068-3081. (d) Alt, H. G.; Helldo¨rfer, M.; Backhaus, J.; Milius,
W. J. Mol. Cat. 2002, 193, 59-73. (d) de Souza, R. F.; Simon, L. C.;
Patel, H.; Soares, J. B. P. Macromol. Chem. Phys. 2001, 202 (17), 3237-
3247. (e) Brookhart, M.; McLain, S. J.; Feldman, J.; McCord, E. F.; Gardner,
K. H.; Teasley, M. F.; Coughlin, B. E.; Sweetman, J. K.; Johnson, L. K.
Macromolecules 1998, 31 (19), 6705-6707. (f) de Souza, R. F.; Simon, L.
C.; Mauler, R. S. J. Polym. Sci. Part A 1999, 37 (24), 4656-4663. (g)
Coates, G. W.; Hustad, P. D.; Reinartz, S. Angew. Chem. 2002, 114 (13),
2340-2361. (h) Galland, G. B.; da Silva, L. P.; Dias, M. L.; Crossetti, G.
L.; Ziglio, C. M.; Filgueiras; C. A. L. J. Polym. Sci. Part A 2004, 42, 2171-
2178. (i) Hlatky, G. G. Chem. ReV. 2000, 100, 1347-1376. (j) Junges, F.;
de Souza, R. F.; dos Santos, J. H. Z.; Casagrane Jr., O. L. Macromol. Mater.
Eng. 2004, 290 (1), 72-77. (k) Kunrath, F. A.; Mauler, R. S.; de Souza, R.
F.; Casagrande, O. L., Jr. Macromol. Chem. Phys. 2002, 203 (14), 2058-
2068. (l) Severn, J. R.; Chadwick, J. C.; Castelli, V. V. A. Macromolecules
2004, 37, 6258-6259. (m) de Souza, R. F.; Mauler, R. S.; Rochefort Neto
O. I. Macromol. Chem. Phys. 2001, 202 (17), 3432-3436. (n) Liu, H. R.;
Gomes, P. T.; Costa, S. I.; Duarte, M. T.; Branquinho, R.; Fernandes, A.C.;
Chien, J. C. W.; Singh, R. P.; Marques, M. M. J. Organomet. Chem. 2005,
690 (5), 1314-1323. (o) Camacho, D. H.; Guan, Z. Macromolecules 2005,
38 (7), 2544-2546. (p) Camacho, D. H.; Salo, E. V.; Ziller, J. W.; Guan
Z. Angew. Chem. 2004, 116 (14), 1857-1861.
(5) All attempts failed to achieve crystallizable NiBr2 complexes of
sterically bulky ligands, such as N,N′-bis(2,6-(4-tert-butylphenyl)phenyl)-
1,4-diazabutadiene. Therefore, the monoanionic Ni(acac) complexes were
applied. Moody, L. S.; Mackenzie, P. B.; Killian, C. M.; Lavoie, G. G.;
Ponasik, J. A., Jr.; Barrett, A. G.; Smith, T. W.; Pearson, J. C. WO 00/
50470, 2000.
(6) Svejda, S. A.; Johnson L. K.; Brookhart, M., J. Am. Chem. Soc. 1999,
121, 10634
(7) tom Dieck, H.; Svoboda, M. Chem. Ber. 1976, 109, 1657-1664.
(8) Recently, investigations on monomeric anilido-imine Ni(I) complexes
were published the following. (a) Wang, H.-Y.; Meng, X.; Jin, G.-X. Dalton
Trans. 2006, 2579 (b) Zhang, D.; Jin, G.-X.; Weng, L.-H.; Wang, F.
Organometallics 2004, 23 (13), 3270
(9) Due to the paramagnetic character of 1b,c NMR experiments were
unsuccessful.
10.1021/om060842s CCC: $37.00 © 2007 American Chemical Society
Publication on Web 01/05/2007

752 Organometallics, Vol. 26, No. 3, 2007
Notes
Scheme 1. r-Diimine Nickel Complexes
Table 1. Experimental UV/Vis Signals for 1b,c and the
Polymerization-Inactive Species
(Figure 1, Figure 2). Both unit cells include two Ni(I) dimers
that possess a tetragonal coordination geometry.11 In case of
1b, the two Ni(I) fragments are stabilized by two µ2-Br ligands
(1b, Br1NiBr1* ≈ 94°, NiBr1 ) 2.42 Å, and NiBr1* ) 2.43
Å), whereas the application of the phenyl Grignard reagent
affords a trinuclear, dimetallic complex (1c), consisting of two
Ni(I)Br moieties, coordinated octahedrally to a Mg(II)Br2(thf)2
fragment (1c, Br1NiBr2 ≈ 97°, NiBr1 ) 2.41 Å, and NiBr2 )
2.42 Å). In 1b the Ni(I) atoms are bridged by the two µ2-bromo
ligands, affording a Ni-Ni distance of approximately 3.30 Å.
The Ni-Ni diagonal in 1c (7.09 Å) appears to be about twice
as long, due to the MgBr2thf2 spacer unit.10
λ in nm (ꢀ in L/(mol × cm))a
b
complex
λ1
λ2
λ3
1b
1c
2a + H2
300, 352sh
300, 348sh
308, 346sh
514sh, 556
502sh, 554
461sh, 495
740(1321)
750(1720)
621(4260)
a Main maxima are bold. b The intensive UV band is solvent invariant
and appears to be characteristic for the ligand.
Table 2. Crystal Data and Structure Refinement Parameters
of 1b and 1c
1b
1c
1b,c do not show any detectable polymerization activity
toward monomers, such as ethylene, 1-hexene, and cyclic olefins
(Scheme 2). Both Ni(I) complexes decomposed rapidly and
quantitatively by contact with hydrogen to give black Ni(0).12
If an excess of Al(III)-alkyl activator, however, is added, 1b,c
are converted into polymerization-active species again. We have
no genuine explanation for this surprising effect, but speculate
on a disproportion reaction of the Ni(I) dimers into Ni(0)13 and
polymerization-active (R-diimine)Ni(II) catalysts.
molecular
composition
cryst syst
space group
a, Å
b, Å
c, Å
R, deg
â, deg
C64H96Br4MgN4Ni2O2
C56H80Br2N4Ni2
triclinic
P2(1)/n
13.156(3)
16.357(3)
16.792(3)
90.00
triclinic
P2(1)/n
13.714(3)
14.086(3)
14.814(3)
90.00
103.48(3)
90.00
101.86
90.00
γ, deg
vol, Å3
3536.1(12)
1414.82
2
2783.0(10)
1086.48
2
The above-described dinuclear Ni(I) compounds just form
from dihalogeno-Ni complex precursors in the presence of low
Al/Ni ratios. Therefore, they will most presumably not play a
major role in solution polymerization experiments, which are
mostly run at high Al/Ni ratio. However, this might change in
experiments that apply supported catalysts for particle-forming
reactions, because they use much lower Al loadings. It might
well be that, especially in the presence of hydrogen, higher
activities could be found, because Ni(I) decomposed readily
under such conditions.
mol wt
Z
F(calcd), g/cm3
cryst size, mm3
recrystallization
solvent
1.329
1.297
0.30 × 0.40 × 0.30
0.40 × 0.30 × 0.30
diethyl ether
diethyl ether
refinement method
radiation used µ,
cm-1
2θ range, deg
hkl range
-18 e k e 18
-19 e l e 19
no. of collected reflns
no. of unique reflns
no. of obsd reflns
(I > 2σ(I))
goodness-of-fit
final R incides
(I > 2σ(I))
R indices (all data)
full-matrix least-squares on F2
Mo KR, 0.71073
Mo KR, 0.71073
1.76-24.15
-15 e h e 15
-17 e k e 17
-18 e l e 18
22 424
2.02-25.99
-15 e h e 16
Experimental Section
23 528
5287
3286
5461
2779
Toluene and diethylether (Merck, p.a.) were purified by distil-
lation over LiAlH4 and dichloromethane by distillation over CaH2.
TMA and MAO were purchased as 2 M solutions in toluene from
Crompton GmbH and NiII(acac)2 from Merck. Ethylene (Linde,
grade 3.0) and hydrogen (Linde, grade 5.0) were used as received.
Trityltetrakis(pentafluorphenyl)borate14 and 1a1 were prepared
0.755
0.880
R1 ) 0.1088,
wR2 ) 0.0498
R1 ) 0.1303,
wR2 ) 0.1094
R1 ) 0.0814,
wR2 ) 0.0418
R1 ) 0.0892,
wR2 ) 0.0804
(11) Shao, Q.; Sun, H.; Shen, Q.; Zhang, Y. Appl. Organomet. Chem.
2004, 18, 289.
was already described by tom Dieck10 and finds further support
by the fact that monocationic 2a,b acac complexes do not give
such dinuclear Ni(I) compounds. Due to the absence of
stabilizing bromo ligands, treatment of 2a,b with Al(III)-alkyl
affords the precipitation of black Ni(0) (Scheme 2).
(12) Polymerization behavior of 1a was published earlier; see ref 1. The
experimental results for 2a,b will be presented in future works.
(13) Treatment of 1a and 2a with TMA in solution afforded the rapid
precipitation of Ni(0). We observed, however, dissolution of this black
precipitate and a “blue, soluble but polymerization-inactive species” formed
in the presence of Al(III) activators and ethylene monomer. All trials to
grow crystals suitable for X-ray analysis of the blue compound remained
unsuccessful until now. Therefore, we cannot give a structural suggestion
yet. UV/vis spectroscopy points toward the formation of a Ni(II)-dialkyl
species. For a similar observation of such a blue compound, see: Peruch,
F.; Cramail, H.; Deffieux, A. Macromolecules 1999, 32 (24), 7977-7983.
Suitable crystals of 1b,c were submitted to X-ray diffraction
analysis, which revealed the formation of dimeric Ni(I) species
(10) tom Dieck, H.; Svoboda, M.; Kopf, J. Z. Naturforsch. B: Chem.
Sci. 1978, 33b, 1381.

Notes
Organometallics, Vol. 26, No. 3, 2007 753
Scheme 2. Activation Pathways of 1a and 2a
according to published procedures. All reactions were carried out
under an argon atmosphere using standard Schlenk techniques.
Crystals were prepared in a glovebox (Braun).
1.533 g) were used. 1a was obtained as a purple-red crystalline
powder; yield 2.08 g (96%). H NMR (δ ) ppm, 400.13 MHz,
1
C2D2Cl4): δ 1.37 (d, 12H, CHCH3), 1.46 (d, 12H, CHCH3), 1.46
(s, 6H, CH3), 1.60 (s, 6H, CH3), 3.33 (sept, 4H, CHCH3), 5.37 (s,
1H, CH), 7.27 (d, 4Harom), 7.47 (t, 2Harom). 13C NMR (δ ) ppm,
100.61 MHz, C2D2Cl4): δ 20.19, 23.56, 23.88, 29.43, 102.37,
122.0-126.4 (br q, C-B), 124.65, 127.41, 127.99, 128.09, 130.36,
136.10, 136.26 (br d, 1JCF ) 238.58 Hz), 138.18 (br d, 1JCF ) 242.92
We performed all polymerization reactions in a 250 mL double-
walled jacketed flask connected to a external thermostat. Ethylene
and hydrogen were continuously fed to the running reaction through
calibrated liquid (Bronkhorst Liqui Flow Sensor L2) or gas flow
meters (Bronkhorst F-201C) at constant pressure. Continuous
hydrogen addition was performed by a master-slave control system
that allows dosing a defined amount of H2 with respect to ethylene
consumption (Bronkhorst E-7000). The system was set up to obtain
a constant mole fraction X [n mmol H2/n mol C2H4] that corresponds
to 0 to 75 mL of hydrogen per minute. UV/vis experiments were
performed using a Specord 50 (AnalytikJena) spectrometer with a
standard cuvette.
General Procedure for (ArNdC(CH3)-C(CH3)dNAr)Ni-
(acac)B(C6F5)4 (1b, Ar ) 2,6-C6H3(iPr)2; 2b, Ar ) 2,6-C6H3-
(C6H5)2). The corresponding R-diimine ligand (1.0 equiv) and
Ni(acac)2 (1.0 equiv) were dissolved in 15 mL of dry CH2Cl2.
Trityltetrakis(pentafluorphenyl)borate (1.0 equiv) in 12 mL of dry
CH2Cl2 was added slowly through a syringe. The resulting dark
red solution was stirred over night and afterward filtered on an
alumina column, using CH2Cl2 as eluent. The volume was reduced
in Vacuo to 15 mL, and n-pentane was added slowly to precipitate
the complex. The lightly colored supernatant was removed, and
the complex was taken up in 15 mL of CH2Cl2. The filtration
procedure was repeated four times.
1
Hz), 140.77, 146.79, 148.14 (br d, JCF ) 233.51 Hz), 168.03,
187.59. MS (MALDI) m/z: 562.4 ([M+ - borate], 100). Anal.
Calcd for C57H47BF20N2NiO2 (1241.5): C 55.15, H 3.82, N 4.73.
Found: C 55.19, H 3.76, N 4.70.
Analytical Data for 2b. R-Diimine ligand (1.56 mmol, 0.84 g),
Ni(acac)2 (1.56 mmol, 0.40 g), and [C(C6H5)3][B(C6F5)4] (1.56
mmol, 1.44 g) were used. Complex 2b was obtained as a dark red
1
glassy powder; yield 2.04 g (95%). H NMR (δ in ppm; 400.13
MHz, C2D2Cl4): δ 1.44 (s, 6H, CH3), 1.60 (s, 6H, CH3), 5.34 (s,
1H, CH), 7.20-7.35 (m, 14Harom), 7.45-7.55 (m, 12Harom). 13C
NMR (δ in ppm; 100.61 MHz,C2D2Cl4): δ 20.37, 24.35, 101.89,
122.5-125.5 (br q, C-B), 128.64, 128.77, 129.27,129.35, 130.94,
1
136.26 (br d, JCF ) 250.29 Hz), 136.28, 136.59, 138.18 (br d,
1JCF ) 248.09 Hz), 138.69, 148.18 (br d, 1JCF ) 240.00 Hz), 176.17,
186.48. MS (MALDI) m/z: 697.4 ([M+ - borate], 100).
Synthesis of (ArNdC(CH3)-C(CH3)dNAr)NiBr)2MgBr2thf2
(1b) (Ar ) 2,6-C6H3(iPr)2). The R-diimine Ni(II) complex 1a (630
mg, 1 mmol) was suspended in Et2O (20 mL). The reaction was
cooled to -15 °C. The phenyl Grignard reagent (Merck), diluted
in Et2O (15 mL), was added slowly. The suspension changed color
Analytical Data for 1b. R-Diimine (1.66 mmol, 0.671 g), Ni-
(acac)2 (1.66 mmol, 0.427 g), and [CPh3][B(C6F5)4] (1.66 mmol,
Figure 1. ORTEP-imaged complex 1b. The atoms are drawn as
Figure 2. ORTEP-imaged complex 1c. The atoms are drawn as
50% thermal ellipsoids. H atoms are excluded for clarity.
50% thermal ellipsoids. H atoms are excluded for clarity.

754 Organometallics, Vol. 26, No. 3, 2007
Notes
spontaneously from orange to dark violet. After complete addition
the solution was stirred further for 30 min. The organic phase was
washed with water (20 mL). After separation the aqueous phase
was extracted with Et2O (5 mL). The combined organic phases were
evaporated in Vacuo to give 990 mg of 1b (70% of theory). Crystals
of 1b suitable for X-ray diffraction analysis were obtained by
recrystallization in Et2O.
Synthesis of (ArNdC(CH3)-C(CH3)dNAr)2Ni2Br2 (1c) (Ar
) 2,6-C6H3(iPr)2). The R-diimine Ni(II) complex 1a (630 mg, 1
mmol) was suspended in Et2O (20 mL). The reaction was cooled
to -78 °C. Then the TMA toluene solution (1.5 mL of a 2 M
solution), diluted in Et2O (15 mL), was added slowly (30 min) to
the solution. After complete addition, the reaction was brought to
room temperature and stirred for 2 h. The organic phase was washed
with water (20 mL). After separation the aqueous phase was
extracted with Et2O (5 mL). The combined organic phases were
evaporated to dryness to give 706 mg of 1c (65% of theory).
Crystals of 1c suitable for X-ray diffraction were obtained by
recrystallization from the corresponding ether solution.
CCDC 627203 and 627204 contain the supplementary crystal-
lographic data for this paper. These data can be obtained free of
charge from The Cambridge Crystallographic Data Centre via
www.ccdc.cam.ac.uk/data_request/cif.
Acknowledgment. The authors thank E.I. DuPont de Nem-
ours & Co. for financial support.
Supporting Information Available: Crystallographic informa-
tion files (CIFs). This material is available free of charge via the
Internet at http://pups.acs.org.
(14) Rausch, M. D.; Chien, J. C. W.; Tsai, W. M. J. Am. Chem. Soc.
1991, 113, 8570.
OM060842S