Synthesis and molecular structures of neutral nickel complexes. Catalytic activity of (Benzamidinato)(acetylacetonato)nickel for the addition polymerization of norbornene, the oligomerization of ethylene, and the dimerization of propylene

Article abstract of DOI:10.1021/om0490379

The new neutral nickel complexes PhC(NSiMe₃)₂Ni(acac) (3), PhC(NSiMe₃)₂Ni(acac)-(TMEDA) (4), [PhC(NSiMe ₃)₂]₂Ni₂ (5), [PhC(NSiMe ₃)₂]₂Ni·Li₂Br ₂(TMEDA)₂ (6), (TMEDA)₂Ni (7), p-MePhC(NSiMe₃)₂Ni(acac) (8), p-MePhC(NSiMe ₃)₂Ni(acac)(TMEDA) (9), [p-MePhC-(NSiMe₃) ₂]₂Ni (10), NiMe₂Py₂ (11) and [p-MePhC(NSiMe₃)]₂Ni(Py) (14) have been synthesized and characterized. The solid-state molecular structures of all complexes have been confirmed by low-temperature X-ray diffraction analysis. The methyl complex p-MePhC(NSiMe₃)₂Ni(Me)(Py) (13) is not stable and rapidly decomposes, forming the complex [p-MePhC(NSiMe₃)]₂-Ni(Py) (14). Complex 3 activated with MAO has been shown to be an efficient catalytic system for the norbornene vinyl-type polymerization. The activity of the catalyst and the molecular weights of the resulting polynorbornenes were found to be dependent on the reaction time, the MAO/precatalyst ratio, and the reaction temperature. In addition, this catalytic system has been found to dimerize propylene to a mixture of hexenes with a high turnover frequency of η = 9040 h^-1 and oligomerize ethylene to either a mixture of C₄-C₆ or/and C₄-C₁₄ products, depending on the temperature and the solvent, extremely rapidly (η = 83 500 h^-1).

Full text of DOI:10.1021/om0490379

Organometallics 2005, 24, 2645-2659
2645
Synthesis and Molecular Structures of Neutral Nickel
Complexes. Catalytic Activity of
(Benzamidinato)(acetylacetonato)nickel for the Addition
Polymerization of Norbornene, the Oligomerization of
Ethylene, and the Dimerization of Propylene
Elza Nelkenbaum, Moshe Kapon, and Moris S. Eisen*
Department of Chemistry and Institute of Catalysis Science and Technology, Technion-Israel
Institute of Technology, Haifa, 32000 Israel
Received December 7, 2004
The new neutral nickel complexes PhC(NSiMe3)2Ni(acac) (3), PhC(NSiMe3)2Ni(acac)-
(TMEDA) (4), [PhC(NSiMe3)2]2Ni2 (5), [PhC(NSiMe3)2]2Ni‚Li2Br2(TMEDA)2 (6), (TMEDA)2Ni
(7), p-MePhC(NSiMe3)2Ni(acac) (8), p-MePhC(NSiMe3)2Ni(acac)(TMEDA) (9), [p-MePhC-
(NSiMe3)2]2Ni (10), NiMe2Py2 (11) and [p-MePhC(NSiMe3)]2Ni(Py) (14) have been synthesized
and characterized. The solid-state molecular structures of all complexes have been confirmed
by low-temperature X-ray diffraction analysis. The methyl complex p-MePhC(NSiMe3)2Ni-
(Me)(Py) (13) is not stable and rapidly decomposes, forming the complex [p-MePhC(NSiMe3)]2-
Ni(Py) (14). Complex 3 activated with MAO has been shown to be an efficient catalytic system
for the norbornene vinyl-type polymerization. The activity of the catalyst and the molecular
weights of the resulting polynorbornenes were found to be dependent on the reaction time,
the MAO/precatalyst ratio, and the reaction temperature. In addition, this catalytic system
has been found to dimerize propylene to a mixture of hexenes with a high turnover frequency
of η ) 9040 h-1 and oligomerize ethylene to either a mixture of C4-C6 or/and C4-C14 products,
depending on the temperature and the solvent, extremely rapidly (η ) 83 500 h-1).
Introduction
the synthesis of linear oligomers of ethylene in the C4-
C20 range.6 Higher molecular weight polymers were
obtained by using modified SHOP systems, which were
found to exhibit modest activities under special condi-
tions.7
While most “single-site” catalytic systems are based
on early transition metals and utilized extensively for
the coordinative polymerization of nonpolar R-olefins,
the use of these catalysts for the copolymerization of
functionalized monomers has been limited, due to their
highly oxophilic nature.1 During the past decade, olefin
polymerization/oligomerization catalysts promoted by
nickel complexes have received increasing interest.2,3
Their reduced oxophilicity and, hence, their enhanced
functional groups tolerance makes them very attractive
for the incorporation of polar monomers into a linear
polyolefin backbone, yielding polymers with unusual
microstructures.4 However, due to polymerization chain
termination via a â-hydrogen elimination process, they
were successfully applied for the production of dimers
or oligomers of ethylene.5
In the middle of the 1990s the Brookhart and Versi-
pol-DuPont groups introduced a new family of highly
active Ni(II) and Pd(II) square-planar cationic olefin
polymerization catalysts bonded to bulky aryl-substi-
tuted R-diimine (N-N) ligands. These catalysts, being
activated with methylalumoxane (MAO), produce a wide
range of linear to highly branched high-molecular-
weight polyethylenes, depending on the structure of the
ancillary ligation and the polymerization conditions.8
More recently, cationic nickel(II) R-diimine complexes
have been shown to copolymerize, at high temperatures
and low catalytic activities, ethylene and methyl acry-
late.9
In view of the high sensitivity of the cationic Ni
complexes toward polar substrates in comparison to that
of the neutral complexes, there has been a renewed
effort for the development of neutral (less electrophilic)
nickel-based polymerization catalysts, as they are ex-
pected to produce functionalized copolymers. With
regard to these neutral complexes a series of neutral
nickel(II) catalysts bearing bulky ortho-substituted
salicylaldimine ligands have been described recently by
Grubbs and co-workers. This new class of neutral
complexes operates as single-component catalysts for
the homopolymerization of ethylene, exhibiting good
One of the first catalytic systems investigated was a
neutral nickel-ylide complex containing a P-O chelate
ancillary ligation, which was developed by Keim and
employed in the Shell higher olefin process (SHOP) for
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2003.
10.1021/om0490379 CCC: $30.25 © 2005 American Chemical Society
Publication on Web 04/23/2005

2646 Organometallics, Vol. 24, No. 11, 2005
Nelkenbaum et al.
functional group tolerance and remaining active in the
presence of polar solvents.10
1-hexene were obtained by the use of these complexes.11
Brookhart and co-workers have reported on highly
active neutral nickel polymerization catalysts that do
not require a cocatalyst, which are derived from the
2-anilinotropone12 and anilinoperinaphthenone 13 ligands,
forming five-membered chelate (N-O) rings with the
metal. The most remarkable finding with these neutral
catalysts is that the microstructure of the produced
polyethylenes is similar to that obtained by using
R-diimine Ni(II) complexes activated with MAO.
Gibson and co-workers have reported on a new nickel
catalysts containing bulky anthracenyl-derived (P-O)
chelating ligation. These catalysts were found capable
of copolymerizing ethylene with methyl methacrylate
(MMA), producing a functionalized polyethylene in
which the incorporated MMA group appears at the
chain ends. In addition, oligomers of propylene with
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To date, the search for new ligand structures repre-
sents a major route to designing novel R-olefin polym-
erization catalysts. A wide variety of neutral nickel
catalysts bearing different ancillary ligands such as
pyridinecarboxylate,14 phosphinosulfonamide,15 dia-
zene,16 â-diketiminate,17 amino-p-benzoqinone,18 (2-
pyridyl)benzamide,19 and 1-azaallyl,20 has been de-
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been also utilized for the polymerization of bicyclo[2.2.1]-
hept-2-ene (norbornene), yielding saturated polymers.
Nickel precursors containing the chelating acetylaceto-
nate,21 2-(diphenylamino)benzoate,22 pyrrolimine,23
salen,24 Schiff base,17c and (salicylideneiminato)25 ligands
comprise some of the systems that have been studied
for the homo- and/or copolymerization of norbornene.
Our previous developments in the use of benzamidi-
nate ancillary ligands with early transition metals as
active catalysts for the polymerization of R-olefins have
encouraged us to investigate the reactivity of the nickel
metal toward those ligations. Here we report on the
synthesis and structural X-ray diffraction studies on
seven new neutral nickel(II) complexes containing the
benzamidinate ligand, forming four- and five-membered
rings. The N,N′-bis(trimethylsilyl)benzamidinate and
N,N′-bis(trimethylsilyl)toluimidinate ligands were cho-
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Neutral Nickel Complexes
Organometallics, Vol. 24, No. 11, 2005 2647
sen for the reactivity study case.26 In addition, we
present preliminary results on the polymerization of
norbornene, the dimerization of propylene, and the
oligomerization of ethylene catalyzed by nickel(II) benz-
amidinate complexes.
norbornene (bicyclo[2.2.1]hept-2-ene) were purchased and used
as received (Aldrich). Et3NHCl (Aldrich) was sublimated under
vacuum at 200 °C.
The lithium benzamidinate complexes PhC(NSiMe3)2-
Li(TMEDA) (1a) and p-MePhC(NSiMe3)2Li(TMEDA) (1b),31
(TMEDA)Ni((acac)2 (2),32 (TMEDA)MgMe2,33 and (TMEDA)-
10d,32a
NiMe2
dures.
were prepared according to the literature proce-
Experimental Section
Synthesis of PhC(NSiMe3)2Ni(acac) (3) and PhC-
(NSiMe3)2Ni(acac)(TMEDA) (4). To a well-stirred solution
of complex 2 (3.20 g, 8,58 mmol) in 40 mL of toluene was added
3.32 g (8.58 mmol) of ligand 1a. A color change from turquoise
to a deep violet was observed. The resulting mixture was
stirred for 10 h at room temperature and filtered cold. Slow
cooling of this solution for 50 h at -40 °C provided a mixture
of fine ruby red 3 and emerald green 4 crystals. The mixture
was filtered through a frit and evaporated under vacuum.
Repeated washing of the crystal mixture with toluene allowed
the full removal of TMEDA from complex 4, forming complex
3 in 66% yield as the sole product. Anal. Calcd for C18H30N2O2-
Si2Ni (421.31): C, 51.31; H, 7.18; N, 6.65. Found: C, 51.52;
H, 7.41; N, 7.01. Mp: 75 °C. UV/vis (toluene): λmax 500 nm.
When the obtained mixture of complexes 3 and 4 was
washed with TMEDA, complex 4 was obtained in 63% yield.
Anal. Calcd for C24H46N4NiO2Si2 (537.51): C, 53.63; H, 8.63;
N, 10.42. Found: C, 53.52; H, 8.41; N, 10.01. Mp: 89 °C dec.
Synthesis of [PhC(NSiMe3)2]2Ni2(I) (5). A 1.2 g amount
(2.85 mmol) of complex 3 was dissolved in 30 mL of diethyl
ether, and 2.59 mL of a 1.1 M solution of methyllithium in
hexane (2.85 mmol) was added dropwise at -78 °C. The
reaction mixture was warmed to 0 °C and stirred for 3 h, and
the solvent was evaporated under vacuum. A 30 mL portion
of hexane was vacuum-transferred to the residue, and LiCl
was removed by filtration. Cooling the solution to -40 °C
overnight resulted in the formation of orange-brown crystals
of (benzamidinate)nickel dimer complex, which was found not
to be stable at room temperature and instead disproportion-
ated to metallic nickel and the bis[bis(trimethylsilyl)benz-
amidinate]nickel complex 6. The X-ray structure was possible
to determine because the decomposition of complex 5 was
rather slow at 220 K.
Synthesis of the Complex (TMEDA)2Ni(0) (7). A 0.25 g
portion (1.47 mmol) of (TMEDA)MgMe2 was added to a cooled
(-30 °C) solution of complex 3 (0.62 g, 1.47 mmol) in diethyl
ether. The resulting mixture was warmed to -10 °C and
stirred at this temperature for 4 h, and the solvent was
evaporated. A 20 mL portion of toluene was vacuum-trans-
ferred to the residue, and Mg(acac)(PhC(NTMS)2) was removed
by filtration. Cooling the solution to -40 °C for 1 week resulted
in the formation of brown crystals of (TMEDA)2Ni(0) in 43%
yield.
Synthesis of p-MePhC(NSiMe3)2Ni(acac) (8), p-MePhC-
(NSiMe3)2Ni(acac)(TMEDA) (9), and [p-MePhC(NSi-
Me3)2]2Ni (10). In a manner similar to that described for the
preparation of complexes 3 and 4, 2.33 g (6.24 mmol) of
complex 2 was dissolved in 40 mL of toluene and reacted with
2.50 g (6.24 mmol) of ligand 1b. The resulting mixture was
stirred at room temperature for 10 h, producing a dark violet
solution. After the solution was filtered and cooled to -50 °C
for 50 h, a mixture (2.5 g) of fine bright red (8), green (9), and
violet (10) crystals were obtained. The mixture of complexes
was filtered and dried under vacuum. Multiple fractional
General Methods. All manipulations were performed with
the exclusion of oxygen and moisture using flamed Schlenk-
type glassware on a dual-manifold Schlenk line or interfaced
to a high-vacuum (10-5 Torr) line. For storage of air-sensitive
materials, a nitrogen-filled Vacuum Atmospheres glovebox
with a medium-capacity recirculator (1-2 ppm O2) was used.
NMR spectra were recorded on Bruker AM-300 and AM-500
spectrometers. Chemical shifts for 1H NMR and 13C NMR were
referenced to internal solvent resonances and reported relative
to tetramethylsilane. The NMR experiments of the complexes
were conducted in Teflon valve sealed tubes after vacuum
transfer of the solvent in a high-vacuum line. The NMR
characterization of polynorbornenes was performed in deuter-
ated 1,1,2,2-tetrachloroethane at 130 °C. Molecular weights
of polymers were determined by the GPC method on a Waters-
Alliance 2000 instrument using 1,2,4-trichlorobenzene (HPLC)
as the mobile phase at 150 °C.
Low-temperature X-ray diffraction experiments were carried
out on a Nonius-KappaCCD diffractometer with graphite-
monochromatized Mo KR radiation. The crystals were placed
in dry and degassed Parathon-N (DuPont) oil in a glovebox.
Single crystals were mounted on the diffractometer under a
stream of cold N2 at 230 K. Cell refinements and data collection
and reduction were carried out with the Nonius software
package.27 The structure solution was carried out by the
SHELXS-9728 and SHELXSL-9729 software packages. The
ORTEP program incorporated in the TEXRAY structure
analysis package was used for molecular graphics.30
The electronic spectra were recorded on a HP 8452A diode
array spectrophotometer. GC analysis was conducted on a
Varian CP-3800 instrument with a DB-5 type capillary column
of 30 m length and 0.25 mm internal diameter. GS/MS analysis
was performed on a Finnigan MAT TSQ 70 mass spectrometer.
FTIR spectra were registered using a Bruker Vector 22
instrument.
Materials. Argon and nitrogen were purified by passage
through an MnO oxygen removal column and a Davison 4 Å
molecular sieves column. Analytically pure solvents were
distilled under argon from Na/K alloy (diethyl ether, hexane),
Na (toluene), P2O5 (dichloromethane), or BaO (pyridine). All
solvents for vacuum line manipulations were stored under
vacuum over Na/K alloy. Nitrile compounds (Aldrich) and
TMEDA (N,N,N′,N′-tetramethylethylenediamine) were de-
gassed and freshly distilled under argon. Methylalumoxane
(Witco) was prepared from a 30% suspension in toluene by
vacuum evaporation of the solvent at 25 °C/10-5 Torr.
MeLi‚LiBr, MeMgBr, Ni(acac)2, (DME)NiBr2, LiN(SiMe3)2, and
(26) (a) Wedler, M.; Recknagel, A.; Giljc, J. W.; Noltemeyer, M.;
Edelmann, F. T. J. Organomet. Chem. 1992, 426, 295. (b) Barker, J.;
Kilner, M. Coord. Chem. Rev. 1994, 133, 219. (c) Cotton, F. A.; Matonic,
J. H.; Murillo, C. A. Inorg. Chem. 1995, 35, 498. (d) Edelmann, F. T.
Coord. Chem. Rev. 1994, 137, 403 and references therein. (e) Volkis,
V.; Shmulinson, M.; Averbuj, C.; Lisovskii, A.; Edelmann, F. T.; Eisen,
M. S. Organometallics 1998, 17, 3155.
(27) KappaCCD Collect Program for data collection and HKL and
Scalepack and Denzo (Otwinowski and Minor, 2001) software package
for data reduction and cell refinement; Enraf-Nonius, Delft, The
Netherlands, 2001.
(28) Sheldrick, G. M. Acta Crystallogr. 1990, A46, 467.
(29) Sheldrick, G. M. SHELXL-97, Program for the Refinement of
Crystal Structures; University of Go¨ttingen, Go¨ttingen, Germany,
1997.
(30) ORTEP, TEXRAY Structure Analysis Package; Molecular
Structure Corp., 3200 Research Forest Drive, The Woodlands, TX
77381, 1999.
(31) (a) Dick, D. G.; Duchateau, R.; Edema, J. J. H.; Gambarotta,
S. Inorg. Chem. 1993, 32, 1959. (b) Volkis, V.; Nelkenbaum, E.;
Lisovskii, A.; Hasson, G.; Semiat, R.; Kapon, M.; Botoshansky, M.;
Eishen, Y.; Eisen, M. S. J. Am. Chem. Soc. 2003, 125, 2179.
(32) (a) Kaschube, W.; Porschke, K. R.; Wilke, G. J. Organomet.
Chem. 1988, 355, 525. (b) Zeller, A.; Herdtweck, E.; Strassner, Th.
Inorg. Chem. Commun. 2004, 7, 296.
(33) (a) Salinger, R.; Mosher, H. S. J. Am. Chem. Soc. 1964, 86, 1782.
(b) Coates, G. E.; Heslop, J. A. J. Chem. Soc. A 1966, 26.

2648 Organometallics, Vol. 24, No. 11, 2005
Nelkenbaum et al.
precipitation allows us to obtain in very low yields the
complexes for X-ray determination. Anal. Calcd for complex
8, C19H32N2NiO2Si2 (435.36): C, 52.37; H, 7.35; N, 6.43.
Found: C, 51.82; H, 7.46; N, 6.11. Anal. Calcd for complex 9,
General Procedure for the Dimerization of Propylene.
Experiments at higher pressures were performed in a 100 mL
stainless steel reactor. The reactor was charged inside a
glovebox with a certain amount of the catalytic precursor,
MAO, and a magnetic stirrer. The reactor was connected to a
high-vacuum line, evacuated under vacuum, and frozen at
liquid nitrogen temperature. A 25 mL portion of liquid pro-
pylene was vacuum-transferred to the frozen reactor. The
reactor was rapidly warmed to room temperature and the
solution stirred. After being stirred for a certain period, the
mixture was quenched by exhausting the unreacted propylene.
An aliquot of the reaction mixture was removed for GLC
analysis, whereas a set of 10 different hexenes was used as
internal reference.
Ethylene Oligomerization. The catalytic oligomerization
of ethylene activated by complex 3 and MAO was performed
in 100 mL stainless steal reactors. The reactor was charged
inside a glovebox with a certain amount of the catalytic
precursor, MAO, and a magnetic stirrer. The reactor was
connected to a high-vacuum line, and, after introduction of the
solvent (10 mL) under an argon flow, it was frozen at liquid
nitrogen temperature and evacuated under vacuum. The
reactor was then warmed to room temperature, pressurized
to 30 bar of ethylene pressure, and stirred for the desired
reaction time. The reaction temperature was maintained using
an external water-ice bath to control the reaction exotherm.
After 1 h the reaction was stopped by cooling the reactor to
-15 °C and depressurizing. The contents were transferred into
a cold tared 50 mL heavy duty Schlenk flask, sealed, and
weighed. The oligomers obtained were analyzed by GC/MS.
C
25H48N4NiO2Si2 (551.56): C, 52.21; H, 8.70; N, 10.15.
Found: C, 53.52; H, 8.43; N, 10.42. Anal. Calcd for complex
10, C28H50N4NiSi2 (613.78): C, 54.74; H, 8.15; N, 9.12. Found:
C, 54.25; H, 8.37; N, 8.93.
Synthesis of NiMe2Py2 (11). To a cooled (-70 °C) diethyl
ether solution (5 mL) of (TMEDA)NiMe2 prepared by the
reaction of (TMEDA)Ni(acac)2 (1.18 g, 3.16 mmol) with 0.54 g
(3.16 mmol) of (TMEDA)MgMe2 was added 5 mL of pyridine.
The reaction mixture was stirred at this temperature for 15
min and then for 4 h at 0 °C, producing a deep red solution.
After partial evaporation of toluene, the solution was cooled
to -40 °C overnight to obtain red-brown crystals, which were
characterized by low-temperature X-ray diffraction analysis.
NiMe2Py2 was found to be unstable and decomposes, yielding
black Ni(0) in solution within t1/2 ) 45-60 min at room
temperature.
Synthesis of p-MePhC(NSiMe3)(NHSiMe3) (12). A 0.86
g portion (6.24 mmol) of Et3NHCl was added to a solution of
the ligand 1b (2.5 g, 6.24 mmol) in 30 mL of toluene. The
mixture was stirred at room temperature for 20 h. The solution
was isolated via filtration through Celite, and the solvent was
evaporated under vacuum, producing a transparent yellow oil.
The product was washed with an additional 25 mL of toluene
and dried under vacuum overnight to obtain 1.2 g (81%) of
ligand 12 as a viscous oil. Anal. Calcd for C14H26N2Si2
(278.16): C, 60.43; H, 9.35; N, 10.07. Found: C, 60.89; H, 9.15;
N, 10.55. 1H NMR (C6D6, 300 MHz): δ 7.16 (d, 3J ) 8 Hz, 2H,
Ph), 6.91 (d, 3J ) 8 Hz, 2H, Ph), 4.25 (s, 1H, HN), 2.09 (s, 3H,
CH3Ph), 0.26 (s, 9H, Si(CH3)3), 0.16 (s, 9H, Si(CH3)3). 13C NMR
(C6D6, 75 MHz): δ 162.8 (NCdN), 138.3, 137.7, 128.0, 125.8,
(Ph), 20.3 (CH3Ph), 1.1 (Si(CH3)3), -1.0 (Si(CH3)3).
Synthesis of [p-MePhC(NSiMe3)]2Ni(Py) (14). (TMEDA)-
NiMe2 prepared by the reaction of (TMEDA)Ni(acac)2 (0.22 g,
0.59 mmol) with 0.10 g (0.59 mmol) of (TMEDA)MgMe2 was
dissolved in 10 mL of a cooled (-70 °C) solution mixture of
toluene and pyridine (3:1) and reacted with a toluene solution
(3 mL) of ligand 12 (0.145 g, 0.50 mmol). The resulting mixture
was warmed to -20 °C and stirred at this temperature for 15
min and then for 3 h at room temperature, giving a deep red
solution. Bubbling was observed. Initially, the reaction pro-
duces the expected methyl complex 13, which could be
observed by following the crude reaction in the NMR: 1H NMR
δ -0.25 (Ni-CH3); 13C NMR (C6D6) δ -8.0. However, after
repeated washing of the precipitated complex with cold toluene
and recrystallization at -30 °C for 2 weeks, green single
crystals of complex 14 have been obtained in 14% yield. The
molecular structure of complex 14 was determined by low-
temperature X-ray crystallography. Anal. Calcd for C33H55N5-
NiSi4 (692.86): C, 57.21; H, 8.00; N, 10.11. Found: C, 57.80;
H, 8.14; N, 10.45.
Results and Discussion
We begin the presentation of the results with the
synthesis of our starting materials followed by the
synthesis of the complexes and we end the presentation
by describing some of the catalytic reactions. The
ligands used in this study are the lithium benzamidi-
nates PhC(NSiMe3)2Li(TMEDA) (1a) and p-MePhC-
(NSiMe3)2Li(TMEDA) (1b).31 Since the use of Ni(acac)2
as the Ni(II) precursor showed very little reactivity in
our hands, we decided to use the more reactive (TMED-
A)Ni(acac)2 (2), following the Wilke procedure.32
Synthesis of the Nickel(II) N,N′-Bis(trimethyl-
silyl)benzamidinate Acetylacetonate Complexes 3
and 4. Reaction of complex 2 with an equimolar amount
of the ancillary ligand 1a in toluene at ambient tem-
perature afforded a mixture of the compounds PhC-
(NSiMe3)2Ni(acac) (3) PhC(NSiMe3)2Ni(acac)(TMEDA)
(4) in a moderate yield (75%; eq 1).
Norbornene Polymerization Procedure. The catalytic
polymerization of norbornene was studied using complex 3
activated with MAO. A 50 mL three-necked polymerization
bottle, equipped with a sidearm Schlenk tube for the addition
of solids and a stirrer, was charged inside a glovebox with a
certain amount of MAO and 0.5 g of norbornene. The bottle
was connected to a Schlenk vacuum line and, after introduction
of a toluene solution of the precatalyst under an argon flow
via a syringe and stirring of the catalytic mixture for 5 min,
the monomer was added in one portion from the sidearm. After
being stirred for a desired amount of time, the reaction mixture
was poured into acidified methanol (1:4) in order to quench
the polymerization. The resulting precipitated polymer was
separated by filtration, washed with water and acetone several
times, and finally dried under vacuum at 70 °C. The polynor-
bornene yield in percentage was calculated as the weight
fraction of converted monomer over the total monomer.
These two complexes were separated by fractional
precipitation from toluene and recrystallized to obtain

Neutral Nickel Complexes
Organometallics, Vol. 24, No. 11, 2005 2649
Table 1. Crystal Data and Details of Data Collection for Complexes 3-5
3
4
5
empirical formula
formula wt
temp (K)
C
18H30N2NiO2Si2
C24H46N4NiO2Si2
537.54
C26H46N4Ni2Si4
644.45
421.33
230.0(1)
0.710 70
monoclinic, P21/c
230.0(1)
230.0(1)
wavelength (Å)
cryst syst, space group
unit cell dimens
a (Å)
0.710 70
monoclinic, P21/c
0.710 73
triclinic, P1h
15.8990(6)
6.8500(2)
21.3210(9)
90
96.5200(12)
90
10.8590(2)
13.9810(3)
20.4510(6)
90
101.0250(8)
90
11.3643(5)
12.0823(6)
13.7054(7)
106.974(2)
108.096(2)
91.455(2)
b (Å)
c (Å)
R (deg)
â (deg)
γ (deg)
V (Å3)
2307.01(15)
4
3047.56(12)
4
1696.84(14)
2
Z
Dcalcd (mg/m3)
1.213
1.172
1.261
µ (mm-1
F(000)
)
0.957
0.740
1.271
896
1160
684
cryst size (mm)
θ range for data collecn (deg)
limiting indices
0.08 × 0.12 × 0.30
1.92-25.05
0 e h e 18
0 e k e 8
-25 e l e 25
4071/4071
(R(int) ) 0.0000)
25.05; 99.8%
0.15 × 0.20 × 0.32
1.77-25.05
0 e h e 12
0 e k e 16
-24 e l e 23
5382/5382
(R(int) ) 0.0000)
25.05; 99.8%
full-matrix least squares on F2
5382/0/470
1.054
0.05 × 0.08 × 0.15
1.65-23.00
-12 e h e 12
-13 e k e 13
-15 e l e 14
8516/4715
(R(int) ) 0.0869)
23.00; 99.9%
no. of rflns collected/unique
completeness to θ
refinement method
no. of data/restraints/params
4071/0/259
1.026
R1 ) 0.0432,
wR2 ) 0.0932
R1 ) 0.0700,
wR2 )0.1012
0.275 and -0.221
4715/0/371
0.914
R1 ) 0.0561,
wR2 ) 0.0923
R1 ) 0.1279,
wR2 ) 0.1052
0.703 and -0.318
goodness of fit on F2
final R indices (I > 2σ(I))
R1 ) 0.0350,
wR2 ) 0.0805
R1 ) 0.0524,
wR2 ) 0.0851
0.495 and -0.296
R indices (all data)
largest diff peak and hole (e Å-3
)
single crystals, which were found suitable for X-ray
measurements. Repeated washing of the crystal mixture
with toluene allowed the full removal of the TMEDA,
forming complex 3 in 66% yield as the sole product, or
in contrast, the addition of TMEDA gave complex 4 as
the sole product in similar yield (63%). Attempts to
prepare these complexes from compound 2 and the
lithium (trimethylsilyl)benzamidinate ligand without
the coordinated TMEDA were unsuccessful.34
The crystal structure of complex 3 is presented in
Figure 1. Crystallographic data and structure refine-
ment details for the complex and selected bond lengths
and angles are listed in Table 1 and Table 4, respec-
tively. The low-temperature X-ray diffraction analysis
reveals that the ruby red complex 3 contains one η2
chelating benzamidinate and one η2-acetylacetonate
ligand around the nickel center, adopting a slightly
distorted square planar coordination geometry (the sum
of the angles around the Ni metal is 360.02°). The core
units of complex 3 comprise a four-membered Ni-N-
C-N ring and a six-membered Ni-O-C-C-C-O ring,
with identical Ni-N bond lengths and comparable
Ni-O bond lengths (see Table 4).
The four-membered ring is partially puckered, as
shown by the small dihedral angle N(5)-C(6)-N(13)-
Ni ) 3.7° and the angles O(24)-Ni-N(5) ) 166.78(10)°
and O(18)-Ni-N(13) ) 166.96(10)°. Interestingly, the
Ni-N distances in 3 are shorter than in complex 2 (by
∼0.27 Å) due to the lower density of charge at the metal
in the latter complex, which is involved in the chelating
of the TMEDA ligand, as compared to the former
complex, which has a higher positive charge, causing
Figure 1. Molecular structure of complex 3. Thermal
ellipsoids are given at the 50% probability level.
the shorter bond lengths. Similarly, the Ni-O bond
lengths, which are shorter in complex 3 than in complex
2 (by ∼0.2 Å), indicate the large difference between hard
and soft ligands around the cationic Ni(II) center,
inducing a more relaxed structure in the complex 2 and
a more compressed structure in complex 3.
Normally all square-planar complexes of nickel, which
are low spin, have no unpaired electrons and therefore
(34) Lisovskii, A.; Botoshansky. M.; Eisen, M. S. Dalton 2001, 1692.

2650 Organometallics, Vol. 24, No. 11, 2005
Nelkenbaum et al.
Table 2. Crystal Data and Details of the Data Collection for Complexes 6-9
6 (with Li2Br2(TMEDA)2)
7
8
9
empirical formula
formula wt
temp (K)
C
38H78Br2Li2N8NiSi4
C6H16N2Ni0.5
145.56
C
19H32N2NiO2Si2
C25H48N4NiO2Si2
991.85
435.36
551.56
230.0(2)
0.710 73
monoclinic, P21/n
230.0(2)
230.0(2)
0.710 73
orthorhombic, Pbca
230.0(2)
0.710 73
monoclinic, P21/c
wavelength (Å)
cryst syst, space group
unit cell dimens
a (Å)
0.710 73
monoclinic, C2/c
11.5880(2)
15.1020(3)
32.8200(8)
90
98.7610(8)
90
8.5850(3)
14.3240(6)
10.0170(4)
90
113.926(3)
90
17.6130(2)
12.8460(3)
20.9800(3)
90
90
90
14.9190(11)
10.9080(8)
20.112(2)
90
103.606(3)
90
b (Å)
c (Å)
R (deg)
â (deg)
γ (deg)
V (Å3)
5676.6(2)
4
1125.96(8)
8
4746.86(14)
8
3181.1(5)
4
Z
Dcalcd (mg/m3)
1.161
1.717
1.218
1.152
µ (mm-1
F(000)
)
1.864
1.707
0.932
0.711
2088
640
1856
1192
cryst size (mm)
θ range for data collecn (deg)
limiting indices
0.10 × 0.18 × 0.18
1.26-23.00
-12 e h e 12
-15 e k e 16
-36 e l e 35
13 399/7659
(R(int) ) 0.0796)
23.00; 96.9%
0.09 × 0.12 × 0.18
2.84-24.97
-9 e h e 9
-16 e k e 16
-11 e l e 10
2494/892
(R(int) ) 0.0548)
24.97; 90.7%
0.15 × 0.18 × 0.24
1.94-25.05
-20 e h e 20
-15 e k e 15
-24 e l e 24
7934/4200
(R(int) ) 0.0232)
25.05; 100.0%
0.06 × 0.24 × 0.28
1.40-25.04
-17 e h e 17
-11 e k e 12
-23 e l e 23
8874/5253
(R(int) ) 0.0737)
25.04; 93.7%
no. of rflns collected/unique
completeness to θ
refinement method
full-matrix least squares on F2
892/0/80
1.140
R1 ) 0.1050,
wR2 ) 0.3121
R1 ) 0.1579,
wR2 ) 0.3352
0.645 and -0.973
no. of data/restraints/params
7659/0/496
0.710
R1 ) 0.0415,
wR2 ) 0.0816
R1 ) 0.1646,
wR2 ) 0.1199
0.295 and -0.265
4200/0/279
0.934
R1 ) 0.0282,
wR2 ) 0.0775
R1 ) 0.0473,
wR2 )0.0801
0.181 and -0.239
5253/0/307
0.816
R1 ) 0.0647,
wR2 ) 0.1457
R1 ) 0.1430,
wR2 ) 0.1582
1.308 and -0.383
goodness of fit on F2
final R indices (I > 2σ(I))
R indices (all data)
largest diff peak and hole (e Å-3
)
Table 3. Crystal Data and Data Collection Details for Complexes 10, 11, and 14
10
11
14
empirical formula
formula wt
temp (K)
C
14H25N2Ni0.5Si2
C
12H16N2Ni
C33H55N5NiSi4
306.89
246.98
692.89
230.0(2)
0.710 73
monoclinic, C2/c
230.0(2)
0.710 73
monoclinic, C2/c
230.0(1)
0.710 73
monoclinic, P21/n
wavelength (Å)
cryst syst, space group
unit cell dimens
a (Å)
26.7070(12)
8.8830(6)
20.4120(12)
90
129.923(3)
90
9.809(2)
11.3360(2)
12.1220(2)
29.0420(5)
90
93.9240(8)
90
b (Å)
16.995(4)
c (Å)
15.197(3)
R (deg)
90
â (deg)
99.037(10)
γ (deg)
90
V (Å3)
3713.8(14)
8
2502.0(10)
8
3981.45(12)
4
Z
Dcalcd (mg/m3)
1.098
1.311
1.156
µ (mm-1
F(000)
)
0.672
1.521
0.635
1320
1040
1488
cryst size (mm)
θ range for data collecn (deg)
limiting indices
0.08 × 0.12 × 0.24
1.99-25.05
-31 e h e 31
-10 e k e 10
-24 e l e 24
5976/3289 (R(int) ) 0.0598)
0.06 × 0.33 × 0.33
2.40-25.04
0 e h e 11
0 e k e 19
-18 e l e 17
2142/2142 (R(int) ) 0.0000)
0.06 × 0.21 × 0.25
1.41-25.05
-13 e h e 13
-14 e k e 14
-34 e l e 34
13 235/7050 (R(int) ) 0.0405)
no. of rflns collected/unique
completeness to θ
refinement method
25.05; 99.6%
25.04; 96.4%
25.05; 99.9%
no. of data/restraints/params
full-matrix least squares on F2
2142/0/158
goodness of fit on F2
3289/0/172
0.901
R1 ) 0.0522, wR2 ) 0.1343
R1 ) 0.1314, wR2 ) 0.1482
0.336 and -0.189
7050/0/388
0.912
R1 ) 0.0344, wR2 ) 0.0818
R1 ) 0.0734, wR2 ) 0.1033
369 and -0.453
final R indices (I > 2σ(I))
R indices (all data)
1.067
R1 ) 0.0500, wR2 ) 0.1087
R1 ) 0.0976, wR2 )0.1214
0.246 and -0.257
largest diff peak and hole (e Å-3
)
show diamagnetic properties.35 However, 1H NMR
spectroscopy reveals that complex 3 is paramagnetic,
giving broad contact-shifted signals in the ranges of -15
to -4 and 60-90 ppm, indicating that a plausible
equilibrium between the square-planar and the expected
paramagnetic tetrahedral structure is operative in
solution.36
The crystal structure of complex 4 is presented in
Figure 2. Crystallographic data and structure refine-
ment details for the complex and selected bond lengths
(35) Shriver, D. F.; Atkins, P. W.; Langford, C. H. Inorganic
Chemistry, 2nd ed.; Oxford University Press: Oxford, U.K., 1998.

Neutral Nickel Complexes
Organometallics, Vol. 24, No. 11, 2005 2651
Table 4. Comparison of the Bond Lengths (Å) and Angles (deg) for Complexes 3, 4, 8, and 9
3
4
8
9
Bond Lengths
Ni-N(5)
1.901(2)
1.906(2)
1.844(2)
1.850(2)
Ni-N(5)
2.0955(17)
Ni-N(1)
Ni-N(2)
Ni-O(1)
Ni-O(2)
1.8953(17)
1.9113(18)
1.8462(15)
1.8475(15)
Ni-N(1)
Ni-N(2)
Ni-O(1)
Ni-O(2)
Ni-N(3)
Ni-N(4)
2.101(4)
2.228(4)
2.029(4)
2.041(3)
2.188(5)
2.208(4)
Ni-N(13)
Ni-O(24)
Ni-O(18)
Ni-N(13)
Ni-O(18)
Ni-O(24)
Ni-N(30)
Ni-N(25)
2.2147(18)
2.0272(16)
2.0311(16)
2.1970(19)
2.2210(19)
Bond Angles
N(5)-Ni-N(13)
O(24)-Ni-N(5)
O(24)-Ni-O(18)
O(18)-Ni-N(5)
O(24)-Ni-N(13)
O(18)-Ni-N(13)
69.74(10)
166.78(10)
95.79(9)
N(5)-Ni-N(13)
O(24)-Ni-N(13)
63.56(7)
N(1)-Ni-N(2)
O(2)-Ni-N(1)
O(1)-Ni-O(2)
O(1)-Ni-N(1)
O(2)-Ni-N(2)
O(1)-Ni-N(2)
70.02(8)
168.37(8)
96.10(7)
95.53(7)
98.36(7)
165.54(7)
N(1)-Ni-N(2)
O(2)-Ni-N(2)
O(1)-Ni-O(2)
63.96(16)
161.24(15)
90.25(15)
161.00(6)
97.38(9)
O(18)-Ni-N(5)
93.90(7(
97.05(10)
166.96(10)
O(1)-Ni- N(2)
O(2)-Ni-N(1)
N(4)-Ni-N(2)
O(2)-Ni- N(4)
86.18(15)
97.96(16)
95.44(17)
90.82(17)
N(5)-Ni-N(30)
O(18)-Ni- N(25)
N(30)-Ni-N(25)
94.85(7)
88.99(7)
81.99(8)
Table 5. Comparison of the Bond Lengths (Å) and Angles (deg) for Complexes 10, 6, Ni(II),40 and 14
10 14
Ni(II)40
6
Bond Lengths
Ni-N(2)
1.990(3)
2.019(3)
1.990(3)
2.019(3)
Ni-N(5)
1.969(5)
2.045(6)
2.000(5)
2.004(5)
Ni-N(4)
2.009(3)
2.016(3)
2.007(3)
2.016(3)
Ni-N(4)
Ni-N(3)
Ni-N(2)
Ni-N(1)
2.050(2)
2.104(2)
2.063(2)
2.083(2)
Ni-N(1)
Ni-N(13)
Ni-N(30)
Ni-N(22)
Ni-N(3)
Ni-N(1)
Ni-N(2)
Ni-N(2)#1
Ni-N(1)#1
Bond Angles
N(2)-Ni-N(1)
67.65(13)
115.2(2)
N(5)-Ni-N(13)
N(30)-Ni-N(13)
N(5)-Ni-N(30)
N(22)-Ni-N(13)
N(5)-Ni-N(22)
C(23)-Ni-C(6)
68.6(2)
N(1)-Ni-N(2)
N(1)-Ni-N(3)
67.35(14)
120.8(2)
133.8(2)
120.7
N(2)-Ni-N(1)
N(4)-Ni-N(2)
N(2) -Ni-N(3)
N(4)-Ni-N(1)
N(1)-Ni-N(3)
65.59(9)
N(1)#1-Ni-N(1)
N(2)-Ni-N(1)#1
N(2)#1-Ni-N(1)
N(2)-Ni-N(2)#1
C(11)-Ni-C(11)#1
121.5(3)
137.1(2)
118.9(2)
148.0(3)
159.3(3)
105.53(9)
106.79(9)
115.68(9)
172.38(8)
125.36(14)
125.36(14)
158.7(2)
N(1)-Ni-N(4)
N(2)-Ni-N(3)
N(2)-Ni-N(4)
C(1)-Ni-C(14)
152.3
152.8(2)
159.4(2)
and angles, are listed in Tables 1 and 4, respectively.
The emerald green complex crystallizes with the metal
having a distorted-octahedral geometry. η2-Benzamidi-
nate, η2-acetylacetonate, and η2-TMEDA comprise the
three ligands around the metal. The axial position is
occupied by one oxygen and a nitrogen from the ben-
zamidinate ligand with a transaxial angle of O(24)-Ni-
N(13) ) 161.00(6)°, whereas the second oxygen atom
O(18), and the nitrogen atoms N(5), N(25), and N(30)
occupy the equatorial position with the sum of the
equatorial angles of 359.72°. The benzamidinate ligand
is more puckered (N(5)-C(6)-N(13)-Ni ) 10.0°) than
in complex 3, due to the steric hindrance between the
methyl groups of the trimethylsilyl moiety and the
acetylacetonate ligand. The bond lengths between the
metal center and the amidinate nitrogen atoms are
distinct, due to the difference in the static trans effects
between the axial oxygen and the equatorial amine of
the TMEDA ligand. Interestingly, this effect is less
pronounced among the nitrogen atoms of the coordina-
tive TMEDA ligands. Comparison of the M-N bond
lengths of the amidinate complexes 4 and 3 and the
M-N(TMEDA) bond lengths among complexes 4 and 2
shows that in complex 4 the M-N(amidinate) bonds are
longer than those in complex 3, whereas the M-N(T-
MEDA) bond lengths are alike. This result is in agree-
ment with regard to the role of the TMEDA, implicating
that its absence induces a more compressed structure
with shorter bond lengths. In addition, with regard to
the M-O bond lengths of the acac ligation, a similar
compression effect is observed. In complex 4 the M-O
bond lengths are longer than in complex 3 and similar
to those in complex 2.
Synthesis of the (µ2,µ2-N,N′-Bis(trimethylsilyl)-
benzamidinate)nickel(I) Dimer 5. In an attempt to
(36) (a) Hayter, R. G.; Humiec, F. S. Inorg. Chem. 1965, 4, 1701. (b)
Dori, Z.; Gray, H. B. J. Am. Chem. Soc. 1966, 88, 1394. (c) Fro¨mmel,
T.; Peters, W.; Wunderlich, H.; Kuchen, W. Angew. Chem., Int. Ed.
Engl. 1993, 32, 907. (d) La Mar, G. N., Horrocks, W. DeW., Holm, R.
H., Eds. NMR of Paramagnetic Molecules; Academic Press: New York
and London, 1973.
Figure 2. Crystal structure of complex 4. Thermal el-
lipsoids are given at the 50% probability level.

2652 Organometallics, Vol. 24, No. 11, 2005
Nelkenbaum et al.
replace the acetylacetonate ligand with an alkyl group,
we have reacted complex 3 with MeLi‚LiBr in an ether
solution at low temperature. Instead of a methyl benz-
amidinate nickel compound being produced, a reduced
orange (benzamidinate)nickel dimer, complex 5, was
obtained almost quantitatively (eq 2).
Å) is significantly shorter than in the analogous silver-
(I) (2.655 Å),37 gold(I) (2.644 Å),37 and copper(I) (2.424
Å)38 complexes and in the palladium amidinate complex
(µ-dpd)2[Pd(dpd)]2 (dpd ) N,N′-diphenylbenzamidinate)
(2.900 Å).39 When the angles are compared among the
different complexes, the angle N(5)-Ni-N(18) )
179.6(2)° in complex 5 is much larger, while the angle
N(5)-C(6)-N(13) ) 122.1(6)° is somewhat smaller, than
in the corresponding Ag(I) (170.10(9) and 125.3(2)°),
Au(I), (169.1(4) and 126.3(8)°), and Cu(I) (175.7(3) and
124.1°) complexes.
Complex 5 was found not to be stable in solution at
room temperature and to disproportionate to metallic
nickel and the tetrahedral bis[bis(trimethylsilyl)benz-
amidinate]nickel complex 6 (eq 3). Complex 6 was also
The low-temperature X-ray crystal structure of com-
plex 5 is presented in Figure 3. Crystallographic data
and structure refinement details for the complex are
given in Table 1. The core units of complex 5 are the
two five-membered Ni-N-C-N-Ni rings, which are
slightly tilted, to produce a “figure eight” shape (N(5)-
Ni(1)-Ni(2)-N(13) ) 13.5°). Both five-membered rings
are disposed in the same way with almost symmetrical
M-N and N-C bond lengths (Ni(1)-N(5) ) 1.874(5) Å,
Ni(1)-N(18) ) 1.876(5) Å, Ni(2)-N(30) ) 1.868(5) Å,
Ni(2)-N(13) ) 1.876(5) Å, N(5)-C(6) ) 1.328(8) Å,
C(6)-N(13) ) 1.336(7) Å, N(18)-C(23) ) 1.339(6) Å,
C(23)-N(30) ) 1.348(8) Å). By comparison to other late-
transition-metal binuclear benzamidinate-bridged com-
pounds, the Ni-Ni distance (Ni(1)-Ni(2) ) 2.2938(12)
synthesized following the reaction of equimolar amounts
oftheligand1aand(DME)NiBr2 (DME)1,2-dimethoxy-
ethane) in ether (eq 4).
Low-temperature X-ray analysis shows that complex
6 crystallizes as a molecular complex containing one
molecule of Li2Br2(TMEDA)2. The molecular structure
of complex 6 is presented in Figure 4a, whereas crystal-
lographic data and structure refinement details are
listed in Table 2 and selected bond lengths and angles
are presented in Table 5. The Ni-N bond lengths in
complex 6 were found to be not significantly different
from those in the reported bis(benzamidinate)nickel
complex (the reported values are also given in Table 5).40
The reported complex was prepared by the reaction of
(37) Fenske, D.; Baum, G.; Zinn, A.; Dehnicke, K. Z. Naturforsch.,
B 1990, 45, 1273.
(38) Maier, S.; Hiller, W.; Straehle, J.; Ergezinger, C.; Dehnicke, K.
Z. Naturforsch., B 1988, 43, 1628.
(39) Yao, C.-L.; He, L.-P.; Korp, J. D.; Bear, J. L. Inorg. Chem. 1988,
27, 4389.
Figure 3. Crystal structure of complex 5. Thermal el-
lipsoids are given at the 50% probability level.

Neutral Nickel Complexes
Organometallics, Vol. 24, No. 11, 2005 2653
Figure 5. ORTEP diagram of complex 7. Thermal el-
lipsoids are given at the 50% probability level.
Complex 7 has a symmetrical square-planar geom-
etry, and the low-temperature X-ray crystal structure
is presented in Figure 5. Crystallographic data and
structure refinement details for the complex are given
in Table 2. The four Ni-N bond lengths of the two
chelate units are equidistant (Ni(1)-N(2) ) 1.991(13)
Å, Ni(1)-N(1) ) 2.010(11) Å), and its square-planar
symmetry is corroborated by the sum of the angles
around the nickel center (360.00°) (N(2)-Ni(1)-N(1)#1
) 94.1(6)°, N(2)-Ni(1)-N(1) ) 85.9(6)°) and the tran-
soid angle N(2)-Ni(1)-N(2)#1 ) 179.999(1)°.
Synthesis of the Complexes (N,N′-Bis(trimeth-
ylsilyl)-p-toluimidinate)(acetylacetonate)nickel (8),
(N,N′-bis(trimethylsilyl)-p-toluimidinate)(acet-
ylacetonate)(tetramethylethylenediamine)nickel
(9), and Bis(µ2-N,N′-bis(trimethylsilyl)-p-toluim-
idinate)nickel (10). To study electronic effects on the
reactivity we have performed the reaction between the
ligand 1b, containing a p-methyl substituent in the
aromatic ring, and an equimolar amount of complex 2
in toluene at ambient temperature. The reaction pro-
duces a mixture of three compounds, which can be
separated in low yields by continuous fractional crystal-
lizations: p-MePhC(NSiMe3)2Ni(acac) (8), p-MePhC-
(NSiMe3)2Ni(acac)(TMEDA)(9),and[p-MePhC(NSiMe3)2]2-
Ni (10) (eq 6).
The solid-state molecular structures of complexes
8-10 were confirmed by single-crystal X-ray diffraction
studies. ORTEP diagrams are presented in Figures 6-8,
respectively, whereas crystallographic data and struc-
ture refinement details for the complexes are collected
in Tables 2 and 3. Selected bond lengths and angles are
collected in Tables 4 and 5. The X-ray analysis of the
bright red complex 8 shows that the nickel atom adopts
a slightly distorted square planar geometry chelated by
one acetylacetonate ligand and one slightly puckered
p-toluimidinate ligand (N(1)-Ni-N(2)-C(4) ) 1.2°).
Interestingly, in the comparison of complex 8 with the
aforementioned complex 3, there are no significant
differences observed for the first coordination sphere
around the nickel center and Ni-N and Ni-O bond
Figure 4. ORTEP diagrams of complex 6 (a) and the
cocrystallized Li2Br2(TMEDA)2 salt (b). Thermal ellipsoids
are given at the 50% probability level.
the (N,N′-bis((trimethylsilyl)benzamidinate)Mg(thf)2 com-
plex with NiBr2(thf), revealing a distorted-tetrahedral
environment around the Ni(II) ion. The bent vector
along the central ipso carbons C(23)-Ni-C(6) and the
angles N(5)-Ni-N(13) and N(30)-Ni-N(13) in complex
6 are similar to those exhibited in the reported Ni(II)
compound (C(1)-Ni-C(14), N(1)-Ni-N(2), and N(1)-
Ni-N(3), respectively), although there are some differ-
ences observed in their interligand angles (N(5)-Ni-
N(30) ) 137.1(2)° and N(5)-Ni-N(22) ) 148.0(3)° for
complex 6 and N(1)-Ni-N(4) ) 133.8(2)° and N(2)-
Ni-N(4) ) 152.3° for the reported complex40), which is
caused by the presence of Li2Br2(TMEDA)2. In the
cocrystallized Li2Br2(TMEDA)2 each lithium atom is
coordinated by two bridging bromine atoms and two
nitrogen atoms (Figure 4b).
Attempts to perform the methylation reaction of
complex 3 with (TMEDA)MgMe2 in an ether solution
at low temperature instead of with MeLi produced the
fully reduced complex (TMEDA)2Ni(0) (7) (eq 5).
(40) Walther, D.; Gebhardt, P.; Fischler, R.; Kreher, U.; Gorls, H.
Inorg. Chim. Acta, 1998, 281, 181.

2654 Organometallics, Vol. 24, No. 11, 2005
Nelkenbaum et al.
lengths and bond angles. This indicates that the exist-
ence of an electron-donating p-methyl group in the
benzamidinate ligation does not exert a noticeable
impact on the coordination structure.
O(1) and the N(1), N(3), and N(4) nitrogen atoms lie in
the equatorial positions, whereas the nitrogen atom N(2)
and oxygen atom O(2) occupy the distorted axial coor-
dination sites.
Similar to the relationship between complexes 8 and
3, the addition of the methyl substituent in complex 9
did not cause much difference as compared to complex
4. In a comparison of the reactions (eqs 1 and 6), it is
clear that the main effect of the p-methyl substituent
at the aromatic ring is the formation of the bis(bis-
(trimethylsilyl)toluimidinate)nickel complex 10 that is
similar to complex 6, which was obtained either via a
disproportionation pathway (eq 3) or via a reaction with
different starting materials (eq 4).
The crystal structure of the green complex 9 reveals
that the central nickel atom is connected similarly to
complex 4 with one p-toluimidinate ligand, one acetyl-
acetonate ligand, and one TMEDA molecule disposed
in a distorted-octahedral environment. The oxygen atom
The X-ray solid-state structure of complex 10 indi-
cates that the nickel is chelated by two p-toluimidinate
ligands in a distorted-tetrahedral geometry. The two
four-membered Ni-N-C-N rings are symmetrical with
both Ni-N and C(ipso)-N bond lengths being alike (Ni-
(1)-N(2) ) 1.990(3) Å, Ni(1)-N(1)#1 ) 2.019(3) Å,
N(1)-C(13) ) 1.327(5) Å, N(2)-C(11) ) 1.323(5) Å). The
Ni-N bond lengths in complex 10 are almost equidis-
tant and similar to those in the reported bis(benzamidi-
nate)nickel complex and in complex 6. When the geo-
metrical characteristics of the benzamidinate ligands for
complexes 10 and 6 and for the reported Ni(II) complex
are compared, interesting features can be observed. The
dissimilarity of the N-Ni-N angles (see Table 5) and
the differences in the bending along the central ipso
carbons are as follows: C(11)-Ni-C(11)#1 ) 152.8(2)°
Figure 6. ORTEP diagram of complex 8. Thermal el-
lipsoids are given at the 50% probability level.
Figure 7. ORTEP diagram of complex 9. Thermal el-
lipsoids are given at the 50% probability level.
Figure 8. ORTEP diagram of complex 10. Thermal
ellipsoids are given at the 50% probability level.

Neutral Nickel Complexes
Organometallics, Vol. 24, No. 11, 2005 2655
for complex 10, C(6)-Ni-C(23) ) 159.3(3)° for complex
6, and C(6)-Ni-C(23) ) 159.4(2)° for the reported
Ni(II) compound.40 The comparison indicates that the
existence of a p-methyl group in the benzamidinate
ligand affects the arrangement of the first coordination
ligand sphere around the nickel center in complex 10.
Synthesis of Bis(pyridine)dimethylnickel(II) (11).
In our attempts to prepare a stable nickel complex
containing a methyl group that can be activated for
catalytic purposes, we decided to prepare bis(pyridine)-
dimethylnickel(II) (11). This complex was obtained by
the reaction of (TMEDA)NiMe2, synthesized via a
procedure described in the literature,10d,32a with a 10-
fold excess of pyridine at low temperature (eq 7).
dimethyl plane. Complex 11 was found to be also not
very stable and decomposes, yielding black Ni(0) in
solution with t1/2 ) 45-60 min at room temperature.
As a solid, the complex is rather stable for a few weeks
at room temperature.
Selected bond lengths (Å) and angles (deg) for complex
11: Ni-C(11) ) 1.908(5), Ni-C(12) ) 1.918(5), Ni-N(1)
) 1.961(4), Ni-N(2) ) 1.976(4); C(11)-Ni-C(12) )
89.5(2), C(11)-Ni-N(1) ) 90.4(2), C(12)-Ni-N(1) )
178.4(2), C(11)-Ni-N(2) ) 178.3(2), C(12)-Ni-N(2) )
92.2(2), N(1)-Ni-N(2) ) 87.99(2)°.
Synthesis of Bis(µ2-N,N′-bis(trimethylsilyl)-p-
toluimidinate)(pyridine)nickel(II) (14). As described
above, the use of a benzamidinate or toluimidinate
lithium ligand induces the formation of a mixture of
compounds. In a different strategy to try to obtain an
alkylnickel complex, we have performed the protonolysis
of the lithium p-toluimidinate compound 1b with Et3-
NHCl in toluene at ambient temperature to yield the
neutral p-MePhC(NSiMe3)(NHSiMe3) (12) in 81% yield
(eq 8).
Interestingly, a similar pyridine-substituted complex
was prepared from the reaction of NiCl2Py4 with CH3-
MgX or MeLi,41 although its stability was found to be
low and its crystal structure was not determined.
Complex 11 was successfully crystallized from a
toluene solution to obtain red-brown single crystals
suitable for X-ray measurements. Complex 11 has a
nearly ideal square-planar geometry, and the low-
temperature X-ray crystal structure is presented in
Figure 9. Crystallographic data and structure refine-
The reaction of (TMEDA)NiMe2 with the neutral
ligand 12 in a solution mixture of toluene and pyridine
(3:1) initially provided the expected methyl complex 13,
which could be observed on following the crude reaction
1
by H NMR (δ -0.25, corresponding to Ni-CH3). The
signal disappears as a function of time, and the repeated
washing of the precipitated complex with cold toluene
Figure 9. ORTEP diagram of complex 11. Thermal
ellipsoids are given at the 50% probability level.
ment details for the complex are given in Table 3. The
nickel is surrounded by two pyridine molecules and two
methyl groups (C(11)-Ni-C(12) ) 89.5(2)°, C(11)-Ni-
N(1) ) 90.4(2)°). The pyridine ring is disposed with a
dihedral angle of 132.86° with regard to the nickel-
(41) Campora, J.; Conejo, M.; Mereiter, K.; Palma, P.; Perez, C.;
Reyes, M. L.; Ruiz, C. J. Organomet. Chem. 2003, 683, 220.

2656 Organometallics, Vol. 24, No. 11, 2005
Nelkenbaum et al.
Table 6. Data for the Polymerization of
Norbornene Promoted by the 3/MAO Systema
T, time,
activity × Mw
×
Mw/
entry Al:Ni °C
h
yield, %
10-3 c
10-3
378
Mn
1b
2b
3b
4
100 25 0.25
47
57
56
13
27
52
59
63
70
67
59
66
80
84
81
52
38
41
180
54
2.5
2.6
2.6
2.4
2.1
2.3
2.5
2.5
2.3
2.6
2.0
1.9
2.8
2.0
3.3
2.3
2.3
2.7
100 25
1
328
324
377
406
408
331
252
210
232
433
354
356
359
356
408
219
201
200 25 0.25
25 25 0.25
50 25 0.25
100 25 0.25
200 25 0.25
400 25 0.25
700 25 0.25
1000 25 0.25
100 25 0.5
210
49
5
100
200
220
240
260
250
110
62
6
7
8
9
10
11
12
13
14
15
16
17
18
100 25
100 25
100 25
1
2
38
13
6
0.25
100
0
310
200
150
150
100 25 0.25
100 40 0.25
100 70 0.25
Figure 10. ORTEP diagram of complex 14. Thermal
ellipsoids are given at the 50% probability level.
a Reaction conditions: [Ni] ) 1.06 × 10-3 M; [monomer]:[Ni] )
1000; 5 mL of toluene. b The activation of complex 3 with MAO in
and recrystallization at -30 °C for 2 weeks afforded
green single crystals of complex 14 (eq 9).
the presence of monomer. c In units of g of PNB (mol of Ni)-1 h-1
.
The molecular structure of 14 was confirmed by low-
temperature X-ray crystallography. Crystallographic
data and structure refinement details for the complex
and selected bond lengths and angles are given in
Tables 3 and 5, respectively. Figure 10 reveals that the
geometry of the nickel center is a distorted-trigonal
bipyramid with N(1) and N(3) at the axial positions
having the longer bond lengths to the metal (Ni(1)-N(1)
) 2.083(2) Å, Ni(1)-N(3) ) 2.104(2) Å) as compared to
the lengths for the other three nitrogen atoms forming
the equatorial plane. (The sum of the equatorial angles
is 359.45°; N(2)-Ni(1)-N(5) ) 151.16(9)°, N(4)-Ni(1)-
N(5) ) 102.76(9)°, N(4)-Ni(1)-N(2) ) 105.53(9)°.)
The two four-membered Ni-N-C-N rings are almost
symmetrical with both Ni-N and C(ipso)-N bond
lengths being alike (Ni(1)-N(4) ) 2.050(2) Å, Ni(1)-
N(2) ) 2.063(2) Å, N(1)-C(4) ) 1.330(3) Å, N(2)-C(4)
) 1.321(3) Å, N(3)-C(18) ) 1.333(3) Å, N(4)-C(18) )
1.328(3) Å). The angles N(2)-Ni(1)-N(1) ) 65.59(9)°
and N(4)-Ni(1)-N(3) ) 65.56(9)° are similar to those
exhibited by complexes 4 (63.56(7)°), 9 (63.96(16)°), 10
(67.65(13)°), and 6 (68.7(2)°), while somewhat smaller
than those exhibited in the square-planar complexes 3
(69.74(10)°) and 8 (70.02(8)°).
Interestingly, the pyridine distance to the metal
center is greater (Ni-N(5) ) 2.076(2) Å) as compared
to the pyridine-metal distance exhibited in complex 11
(1.976(4) Å), due to the steric hindrance imposed by the
two p-toluimidinate ligands and the difference in the
coordination numbers.
Catalytic Studies of Complex 3 Activated by
MAO. (a) Polymerization of Norbornene. Complex
3 has been studied as a catalytic precursor for the
polymerization of norbornene. Control experiments were
performed to identify the possible catalytic activity of
each one of the single components. Complex 3 was found
to be not an active catalyst for the polymerization of
norbornene in the absence of MAO in any of the studied
solvents (toluene and dichloromethane). Also, no poly-
mer formation was observed when MAO alone was used
in toluene. However, when MAO was used either with
dichloromethane or with hexane, it was found that MAO
is able to activate norbornene toward polynorbornenes,
giving 35% or 18% yield, respectively, over a period of
5 h. The NMR structures of the polymers obtained with
MAO in dichloromethane and hexane are totally differ-
ent, which is plausible because they are obtained
through a simple cationic mechanism. Thus, all the
polymerization studies were carried out in toluene. The
use of MAO as a cocatalyst leads to the formation of
aluminum salts that are absorbed into the polymer upon
quenching the reaction, and their full removal was
found to be rather difficult. Therefore, we decided to
minimize the MAO to precatalyst ratio while maintain-
ing the maximum catalytic activity. The activity of the
catalyst and the molecular weights of the resulting
polynorbornenes are affected by reaction conditions such
as reaction time, MAO to precatalyst ratio, and reaction
temperature. The polymerization results are presented
in Table 6, showing that complex 3 activated with MAO
is an excellent catalyst for the polymerization of nor-
bornene in toluene. Two different catalyst activation
processes were tested. In the first procedure, the pre-
catalyst and MAO were mixed, generating the active
sites before the addition of norbornene. The second
procedure was based on the activation of complex 3 with
MAO in the presence of the monomer. Table 6 shows
that the first procedure gave higher catalytic activities
(compare entries 1 and 6, 2 and 12, or 3 and 7).
Starting at a low MAO amount, up to a molar ratio
of Al:Ni ) 500, the polymer yields and the catalyst
activity rise with an increase in the MAO concentration
(compare entries 4-10). Interestingly, the reaction
profile is very similar to those obtained for (salicylaldi-
minato)nickel42 and Ni(acac)2
complexes being acti-
21b
vated with MAO (for (salicylaldiminato)nickel, 38% yield
at Al:Ni ) 500, [monomer]:[Ni] ) 5000, t ) 0.5 h; for
Ni(acac)2, 40% yield at Al:Ni ) 100, [monomer]:[Ni] )
1500, t ) 4 h).
The molecular weights of the polymers as a function
of the MAO amount were found to increase up to a
maximum where the Al:Ni ratio is ∼100 (Figure 11;
(42) Sun, W.-H.; Yang, H.; Li, Z.; Li, Y. Organometallics 2003, 22,
3678.

Neutral Nickel Complexes
Organometallics, Vol. 24, No. 11, 2005 2657
Figure 11. Dependence of the weight-average molecular
weight on the MAO to precatalyst ratio. Reaction condi-
tions: [Ni] ) 1.06 × 10-3 M; [monomer]:[Ni] ) 1000; 0.25
h, 25 °C.
Figure 13. Follow-up in the polymerization of norbornene
after subsequent introduction of a new portion of the
monomer after 0.25 h (b) and 0.5 h (c) and without addition
of the monomer (a). Reaction conditions: [Ni] ) 1.06 × 10-3
M; Al:Ni ) 100, 25 °C.
reaction rate at a constant level (Figure 13, compare
plots a and b). Addition of a new portion of the monomer
after 0.5 h also increases the yield of polynorbornenes,
but to a lesser extent, decreasing the reaction rate
(Figure 13, compare plots a and c). This result indicates
that the diffusion of the monomer through the formed
polymer to the active centers at a longer reaction time
may become the rate-determining stage. The formation
of the polymer as a function of time reveals that
polynorbornene starts to precipitate after 0.5 h, trapping
both monomer and catalyst. Therefore, if the addition
of new portions of the monomer is performed before the
precipitation, a linear rate correlation can be main-
tained.
When the reaction temperature is reduced from 70
to 0 °C, keeping Al:Ni ) 100 and a norbornene to
precatalyst ratio of 1000, this results in an augmenta-
tion of the polymer yield while a maximum crest is again
observed at 25 °C in the molecular weights of the
produced polynorbornenes (Table 6, entries 6 and 15-
18). This result indicates that higher temperatures
induce some deactivation of the active sites in addition
to more effective termination pathways.
Figure 12. First-order dependence of the monomer in the
polymerization of norbornene. Reaction conditions: [Ni] )
1.06 × 10-3 M; [monomer]:[Ni] ) 1000; Al:Ni ) 100; 25
°C.
Table 6, entries 5 and 6). Higher MAO to precatalyst
ratios result in the lowering of the Mw value of the
polymer, reaching a plateau around Mw ≈ 200 000.
Therefore, all the following studies were performed at
an MAO to precatalyst ratio of 100.
As shown in Table 6, the polymerization time consid-
erably affects the polymer yields, the molecular weights
of the polymers, and the catalytic activities (see entries
6 and 11-14). The polymer yield sharply increases in
the initial 0.5 h period, reaching a value of 59% (as a
comparison, for Ni(acac)2 70% yield is obtained at
Al:Ni ) 1000 over 0.5 h). Longer reaction times lead to
a modest increase in the yields (84% for 6 h) (for
Ni(acac)2 72% yield polynorbornene is produced at
Al:Ni ) 1000 over 5 h).21b
Interestingly, the formation of polynorbornene with
catalyst 3 follows a first-order dependence in monomer,
yielding k ) 3.04 × 10-4 s-1, as presented in Figure 12.
Since the activation of the precatalysts by the MAO
cocatalyst is a fast process and the chain termination
reaction must be the rate-determining step in the
polymerization reaction, the first-order dependence in
monomer indicates that the chain to olefin transfer
pathway is plausible as the main operative chain
termination mechanism. This result is corroborated by
the kinetic competition observed when the polymeriza-
tion is conducted with large concentrations of MAO,
inducing a reduction in the polymer molecular weights.
It is noteworthy that the consecutive introduction of
a new portion of the monomer after 0.25 h results in a
considerable increase in the yield of polynorbornene
after a subsequent reaction time of 0.25 h, keeping the
Analogous effects of the temperature on the polymer
yield and the activity have been shown for a bis(1-
aryliminomethylnaphthalen-2-oxy)nickel complex (a
2-fold increase in activity was observed by decreasing
the reaction temperature from 100 to 0 °C).43 A similar
tendency was also observed for a (salicylideneiminato)-
nickel(II) complex.25 In contrast to complex 3, an
increase in polymer yield and catalytic activity has been
observed by increasing the polymerization temperature
for (2-methyl-8-(diphenylphosphino)quinoline)nickel,44
whereas for Ni(acac)2 there is no effect on the polym-
erization temperature.21b
All polymers were characterized by 1H NMR, 13C
NMR, and FTIR spectroscopy, indicating the absence
of carbon-carbon double bonds that is typical for ROMP
polynorbornenes.45 In addition, no 1H NMR resonances
are observed for vinylic hydrogens between 5 and 6.7
(43) Chang, F.; Zhang, D.; Xu, G.; Yang, H.; Li, J.; Song, H.; Sun,
W.-H. J. Organomet. Chem. 2004, 689, 936.
(44) Yang, H.; Li, Z.; Sun, W.-H. J. Mol. Catal. A: Chem. 2003, 206,
23.
(45) Haselwander, T. F. A.; Heitz, W. Macromol. Rapid Commun.
1997, 18, 689.

2658 Organometallics, Vol. 24, No. 11, 2005
Nelkenbaum et al.
Figure 14. 13C NMR of polynorbornene produced by complex 3 with MAO in toluene.
Table 7. Data for the Oligomerization of Ethylene
Promoted by Complex 3 Activated by MAOa
ppm, and no absorptions were observed between 1620
and 1680 cm-1 in the IR spectra. The 13C NMR spectra
of polynorbornenes synthesized with 3/MAO catalytic
system present four groups of resonance at 29-32 ppm
for C5 and C6, 35-36 ppm for C7, 37-40 ppm for C1 and
C4, and 47-52 ppm for C2 and C3. The 13C NMR spectra
are similar to those reported for Ni(acac)2,21b pyr-
rolimine,23 and bis(benzimidazole)nickel catalyst,46 in-
dicating that the obtained polynorbornenes using the
catalytic system 3/MAO are vinyl-type addition poly-
mers (Figure 14).
amt, %
C8- C8- Schulz-Flory TOFc (η),
b
solvent T, C C4H8 C6H12 C12 C14
R
h-1
toluene 25
CH2Cl2 25
CH2Cl2 70
52
28
17
48
44
39
0.48
0.72
0.83
17 100
43 600
83 500
28
44
a Reaction conditions: [Ni] ) 1.2 × 10-3 M, Al:Ni ) 200, time
1 h, solvent 10 mL, 30 bar of ethylene. b C8-C12 ) hydrocarbons
with 8-12 carbons in the chain. c TOF ) turnover frequency (in
units of (mol of ethylene) (mol of catalyst)-1 h-1).
(b) Dimerization of Propylene. The reaction of
complex 3 with propylene and MAO (MAO to precata-
lyst ratio of 200) generates various dimerization prod-
ature and a constant ethylene pressure of 30 atm,
affords the extremely rapid production of a mixture of
dimers (52%) and trimers (48%) with a turnover fre-
quency of 17 050 h-1 (Table 7).
ucts with a remarkable turnover frequency of 9040 h-1
.
The reaction has been performed at liquid propylene (10
bar) and at room temperature without additional sol-
vents (eq 10).
The product distribution and the catalyst selectivity
in toluene have been found to be unaffected by the
reaction time (from 1 to 2 h). Interestingly, using
dichloromethane as a solvent under the same reaction
conditions leads to oligomers of ethylene in the C4-C12
range, which are distributed as 28% butenes, 44%
hexenes, and 28% of the mixture C8-C12 (Table 7). The
oligomerization follows a Schulz-Flory distribution48
that is characterized by the constant R. This constant
represents the probability for chain propagation as
displayed in eq 11. An oligomer distribution with a small
This complex shows a large selectivity toward dimers
(>98%), as determined by GC and GC-MS. Two mix-
tures of hexenes containing both primary dimerization
products (42%, trans-2-hexene, 2,3-dimethyl-1-butene,
cis-4-methyl-2-pentene) and isomerization products (58%,
trans-3-hexene, 2-methyl-2-pentene) were obtained. No
trimers, tetramers, or higher oligomers were detected
over a reaction period of 1 h; however, trace amounts
of trimers appear only at longer reaction periods.
Interestingly, a similar selectivity with a different
product distribution was observed for R-diimine nickel
complexes.47
rate of propagation
R )
)
rate of propagation + rate of chain transfer
moles of Cn+2
(11)
moles of Cn
R value (e. g. R < 0.5) is indicative of mainly dimers
and lower oligomers. A high value indicates higher
oligomers. As presented, mainly dimers are the oligo-
mers obtained.
(c) Oligomerization of Ethylene. Complex 3 was
also tested as a precatalyst for the oligomerization of
ethylene when activated with MAO (Al:Ni ) 200).
Performing the reaction in toluene, at ambient temper-
Performing the oligomerization reaction at a temper-
ature of 70 °C for 1 h leads to oligomers of ethylene in
the C4-C14 range having a different product distribu-
tion, with a larger amount of the higher oligomers (17%
(46) Patil, A. O.; Zushma, S.; Stibrany, R. T.; Rucker, S. P.; Wheeler,
L. M. J. Polym. Sci. A: Polym. Chem. 2003, 41, 2095.
(47) Svejda, S. A.; Brookhart, M. Organometallics 1999, 18, 65.
(48) (a) Flory, P. J. J. Am. Chem. Soc. 1940, 62, 1561. (b) Schulz,
G. V. Z. Phys. Chem., Abt. B 1935, 30, 379. (c) Schulz, G. V. Z. Phys.
Chem., Abt. B 1939, 43, 25.

Neutral Nickel Complexes
Organometallics, Vol. 24, No. 11, 2005 2659
of butenes, 39% of hexenes, and 44% of C8-C14) (Table
7). Interestingly, in the oligomerization of ethylene
under all the reaction conditions, all the products were
linear. No branched propenes, butenes, or pentenes were
observed. In addition, among the linear olefins, the ratio
of R-olefins to internal olefins is almost not affected by
the solvent (20:80, respectively). However, when the
reaction is carried out at higher temperatures (70 °C),
the terminal to internal alkene ratio changes to 12:88,
respectively, as expected due to the thermodynamic
stability of the more substituted alkenes.
nickel was obtained. Complex 3 activated with MAO has
been shown to be an efficient catalytic system for the
norbornene vinyl-type polymerization. In addition, this
catalytic system has been found to dimerize propylene
to a mixture of hexenes and oligomerize ethylene to
either C4-C6 or C4-C14 products, depending on the
temperature and the solvent used in the reaction.
Acknowledgment. This research was supported by
the German Israel Foundation under Contract I-621-
27.5/1999 and by the Fund for the Promotion of Re-
search at the Technion.
Conclusions
Supporting Information Available: Tables giving crys-
tallographic data for complexes 3-11 and 14; these data are
also available as CIF files. This material is available free of
charge via the Internet at http://pubs.acs.org.
Seven new nickel complexes bearing the chelating
benzamidinate ligand have been synthesized. Their
solid-state molecular structures have been determined
by low-temperature X-ray diffraction analysis. In addi-
tion, the solid-state structure of dimethylbis(pyridine)-
OM0490379