Synthesis, molecular structure and catalytic activity of chiral benzamidinate nickel complexes

Article abstract of DOI:10.1016/j.jorganchem.2005.04.007

Two new nickel complexes containing the chiral benzamidinate ligation: [PhC(N-SiMe₃)(N′-myrtanyl)]₂Ni(py)₂ (3) and {[PhC(NH)(N′-myrtanyl)]₂Ni}₂ (6) have been synthesized and characterized. The solid-state molecular structures of these complexes have been determined by low-temperature X-ray diffraction analysis. Complex 3 was obtained via two different procedures. In complex 3, the metal adopts a nearly ideal octahedral environment, whereas in complex 6 the two divalent nickel metals are coordinated in a square-planar geometry, forming a dimer. Complex 3 activated with MAO has been found to oligomerize propylene producing a mixture of dimers, trimers and tetramers with a turnover frequency of 5200 h^-1, whereas complex 6 being activated with MAO oligomerizes ethylene to a mixture of dimers and trimers with a high turnover frequency of 15,400 h^-1. In addition, when activated with MAO both complexes showed a good activity for the vinyl-type polymerization of norbornene.

Full text of DOI:10.1016/j.jorganchem.2005.04.007

Journal of Organometallic Chemistry 690 (2005) 3154–3164
www.elsevier.com/locate/jorganchem
Synthesis, molecular structure and catalytic activity of chiral
benzamidinate nickel complexes
*
Elza Nelkenbaum, Moshe Kapon, Moris S. Eisen
Department of Chemistry and Institute of Catalysis Science and Technology, Technion – Israel Institute of Technology,
Kyriat Hatechnion, Haifa 32000, Israel
Received 27 January 2005; revised 3 April 2005; accepted 5 April 2005
Available online 26 May 2005
Abstract
Two new nickel complexes containing the chiral benzamidinate ligation: [PhC(N-SiMe3)(N0-myrtanyl)]2Ni(py)2 (3) and
{[PhC(NH)(N0-myrtanyl)]2Ni}2 (6) have been synthesized and characterized. The solid-state molecular structures of these complexes
have been determined by low-temperature X-ray diffraction analysis. Complex 3 was obtained via two different procedures. In com-
plex 3, the metal adopts a nearly ideal octahedral environment, whereas in complex 6 the two divalent nickel metals are coordinated
in a square-planar geometry, forming a dimer. Complex 3 activated with MAO has been found to oligomerize propylene producing
a mixture of dimers, trimers and tetramers with a turnover frequency of 5200 hꢀ1, whereas complex 6 being activated with MAO
oligomerizes ethylene to a mixture of dimers and trimers with a high turnover frequency of 15,400 hꢀ1. In addition, when activated
with MAO both complexes showed a good activity for the vinyl-type polymerization of norbornene.
ꢀ 2005 Elsevier B.V. All rights reserved.
Keywords: Chiral nickel complexes; Chiral benzamidinates; Polymerization; Ethylene oligomerization; Propylene oligomerization; Nickel; X-ray
structure
1. Introduction
olefins and their copolymerization with polar monomers
[1,2]. The well-known examples of neutral nickel com-
plexes are the SHOP-type catalysts containing monoan-
ionic (P–O) ligands for the oligomerization of ethylene to
linear a-olefins [5,6]. The modified SHOP-type catalysts
with bulky anthracenyl-derived (P–O) ligation were
found to be an efficient catalytic system for copolymeri-
zation of ethylene with methyl methacrylate (MMA),
producing a functionalized polyethylene in which the
incorporated MMA-group appears at the chain ends [7].
Most attention has been directed to neutral nickel
complexes with different anionic (N–O) ligations [8–
11]. The new class of complexes are based on bulky
ortho-substituted salycilaldimine ligands that operates
as single-component catalysts for the homo-polymeriza-
tion of ethylene exhibiting good functional group toler-
ance and remaining active in the presence of polar
solvents [12].
During the last decade, enormous advancements have
been made in the design and the synthesis of late-transi-
tion-metal complexes for the polymerization of olefins
[1,2]. Their reduced oxophilicity and, hence, their en-
hanced functional group tolerance allows to produce
functionalized linear polyolefin materials with unusual
microstructures and properties [3]. However, due to
polymerization chain termination via a b-hydrogen elim-
ination process, these complexes are mainly utilized for
the production of dimers or oligomers of ethylene [4].
Among the numerous late-transition-metal com-
plexes, the neutral nickel complexes are considered to
be very promising catalysts for the polymerization of a-
*
Corresponding author. Tel.: + 972 4829 2680; fax: +972 4829 5703.
E-mail address: chmoris@tx.technion.ac.il (M.S. Eisen).
0022-328X/$ - see front matter ꢀ 2005 Elsevier B.V. All rights reserved.
doi:10.1016/j.jorganchem.2005.04.007

E. Nelkenbaum et al. / Journal of Organometallic Chemistry 690 (2005) 3154–3164
3155
While the monoanionic (N–N) based ligands are
among the most widely investigated chelating system,
their nickel complexes are still extremely rare. Within
the (N–N) chelating ancillary ligation, a variety of bulky
b-diketiminato nickel complexes have received an
increasing attention [13]. In a recent communication,
nickel complexes containing six-membered anilido-
imine ligands, which combine the exceptional features
of b-diketiminate and salycilaldiminate ligations have
been described [14]. When activated with MAO (methyl-
alumoxane) these complexes show high activity for the
polymerization of norbornene, while low activity for
the oligomerization of ethylene, producing the mixture
of butenes and hexenes.
Our recent developments in the polymerization of a-
olefins using as catalysts early-transition metal com-
plexes containing the benzamidinate ancillary ligations
(N–N) encourage us to investigate the reactivity of the
nickel metal towards those ligations. Here, we reported
on the synthesis and structural X-ray diffraction studies
on two new neutral Ni(II) complexes containing a chiral
benzamidinate ligation forming four- and five-membered
rings and their activity in a number of catalytic processes.
SiMe3 as the sole product. In the absence of the tri-
ethylamine the reaction yields a quantitative conver-
sion up to 50% due to the formation of the chiral
salt R*NH3Cl. In both cases, no double silylation
product was formed, presumably due to bulkiness of
the myrtanyl moiety.
RꢁNH2 þ ClSiMe3 Et3N;hexane RꢁNðHÞSiMe
ƒƒƒƒƒ!
ð1Þ
3
ꢀEt3NHCl
Me3Si
Me
Me
N
N
BuLi/TMEDA
R*N(H)SiMe3
Li
+
CN
N
N
-C4H10
Me
Me
R*
H
CH2
R*=
1
ð2Þ
In the second reaction, the deprotonated chiral amine
reacts with benzonitrile with the concomitant 1,3-sigma-
tropic shift to obtain the lithium ligation. The reaction
of the previously described (TMEDA)Ni(acac)2 (acac =
acetylacetonate) (2) complex [16] with two equivalents
of the ligand 1, in toluene at ambient temperature, affor-
ded the compound [PhC(N-SiMe3)(N0-myrtanyl)]2Ni
(Eq. (3)).
2. Results and discussion
2.1. Synthesis of the complex bis-[(N-trimethylsilyl)(N0-
myrtanyl)benzamidinate]Ni(Py)2 (6)
Me3Si
R*
N
Me3Si
Me
Me
N
N
N
Li
Me
2
+ (TMEDA)Ni(acac)2
Ni
N
N
-2 Li(acac)
- 3 TMEDA
N
N
Me
R*
SiMe3
R*
1
2
ð3Þ
R*
N
Me3Si
R*
Me3Si
N
N
hexane :
N
N
N
Ni
Ni
N
N
N
N
N
SiMe3
R*
SiMe3
R*
3
The chiral ligand lithium PhC(N-SiMe3)(N0-myrta-
nyl)Li(TMEDA) (1) was obtained from the silylated chi-
ral amine according to a modified literature procedure
[15] in 82% yield (Eqs. (1) and (2)).
The formation of compound 1 consists of two con-
secutive reactions. In the first reaction an equimolar
amount of the stronger base triethylamine was added
in order to obtain the exchange product R*N(H)-
Attempts to crystallize the tetrahedral bis(benzamidi-
nato) nickel complex [PhC(N-SiMe3)(N0-myrtanyl)]2Ni
from a variety of solvents were unsuccessful. Only in
the presence of a strong coordinating polar solvent, pyr-
idine (hexane:pyridine = 1:1), it was possible to obtain
single green color crystals suitable for X-ray measure-
ments in 54% yield. In addition, in a second procedure,
complex 3 was also synthesized by the methatesis of

3156
E. Nelkenbaum et al. / Journal of Organometallic Chemistry 690 (2005) 3154–3164
(TMEDA)NiMe2 [12d,16] with the neutral ligation
PhC(NHSiMe3)(N0-myrtanyl) (4). The latter ligation
was prepared by the protonolysis of the ancillary ligand
1 with Et3NHCl in toluene at ambient temperature in
complex 3. The complex is precipitated from the reac-
tion mixture with toluene and recrystallized from a
supersaturated solution of toluene at ꢀ30 ꢁC for two
weeks (Eq. (5)).
Me3Si
Me3Si
toluene
N
N
NH
Me
3:1
Ni
+ (TMEDA)NiMe2
N
N
N
-CH4
5
R*
R*
2
4
4
-CH4
ð5Þ
R*
Me3Si
N
N
N
Ni
N
N
N
R*
SiMe3
3
92% yield (Eq. (4)). Interestingly, the reaction is quanti-
tatively regioselective since the protonolysis only insert
the proton at the silicon containing nitrogen group, as
expected for the silicon effect, stabilizing a better anion
at the a-position.
The crystal structure of complex 3 is presented in
Fig. 1. Selected bond lengths and bond angles and crys-
tallographic data and structure refinement details are
listed in Tables 1 and 3, respectively. The low-tempera-
ture X-ray diffraction analysis reveals that the nickel
atom adopts a nearly ideal octahedral coordination
geometry chelated by two chiral g2-benzamidinate li-
gands and two pyridine molecules. The axial positions
are occupied by the two nitrogen atoms from the pyri-
dine moieties with a transaxial angle of N(3)–Ni(1)–
N(4) = 180.00(1)ꢁ, whereas the four nitrogen atoms from
the two benzamidinates form the equatorial plane with
the sum of equatorial angles of 360.24ꢁ (N(2)–Ni(1)–
SiMe3
SiMe3
N
N
N
NH
Et3NHCl
Li
-LiCl,-Et3N,
-TMEDA
N
N
R*
R*
*
H
1
CH2
R*=
N(1) = 63.69(9)ꢁ,
N(2)#1–Ni(1)–N(1)#1 = 63.69(9)ꢁ,
4
N(2)–Ni(1)–N(1)#1 = 116.43(9)ꢁ, N(2)#1–Ni(1)–N(1) =
116.43(9)ꢁ). The two four-membered rings are planar
with a dihedral angle of N(1)–C(4)–N(2) = 0.1ꢁ. The dis-
position of the benzamidinate ligation is not symmetrical
around the metal as observed by the corresponding bond
lengths Ni(1)–N(1) = 2.150(2) and Ni(1)–N(2) = 2.115(2)
ð4Þ
The reaction of (TMEDA)NiMe2 with the neutral li-
gand 4 in a solution mixture of toluene:pyridine (1:3)
initially produced the expected mono methyl complex
5, which can be observed by following the crude reac-
tion by 1H NMR (d = ꢀ0.32, corresponding to the
mono Ni–CH3 and the formation of methane). This
plausible assumption is due to the large paramagnetic
contact-shifted signals in the range from ꢀ15 to ꢀ4
and 60–90 ppm, which makes difficult to assure the
structure of the product, although paramagnetic Ni–
methyl complexes have been shown to exhibit a 1H
NMR around d = ꢀ0.32 [12d,16a]. As a function of
time, this signal disappears to yield the corresponding
˚
A. The same trend is observed by the distances within the
diazaallyl C-N bond lengths N(1)–C(4) = 1.350(4), N(2)–
˚
C(4) = 1.316(4) A, which is a result of the different hard-
ness of the two amidine nitrogens when substituted with
either a silyl or an alkyl group. Compared to other (N0-
myrtanyl)benzamidinate complexes, the Ni–N distances
are somewhat longer than in PhC(N-SiMe3)(N0-myrta-
˚
nyl)(THF)TiCl3 (2.030(9) and 2.070(9) A) [15b], while
slightly shorter than in [PhC(N-SiMe3)(N0-myrta-
˚
nyl)]3ZrCl–toluene (2.249(5) and 2.293(5) A) [15b], and
in [PhC(N-SiMe3)(N0-myrtanyl)]3ZrMe (2.217(13) and
˚
2.26(2) A) [15b]. By comparison to early-transition-

E. Nelkenbaum et al. / Journal of Organometallic Chemistry 690 (2005) 3154–3164
3157
C14
C13
C15
C12
C19
C26
C18
C11
C17
C7
C6
C25
C16
N4
N2
N1
C20
C8
C24
Ni1
N3
C5 C4
C2
C9
C10
C21
C23
Si1
C22
C3
C1
Fig. 1. ORTEP plot of the molecular structure of the complex 3. All hydrogens were removed for clarity and thermal ellipsoids are given at 50%
probability.
metals complexes the N(2)–C(4)–N(1) angle in complex
3 (115.1(2)ꢁ), is slightly larger than that in the complexes
of PhC(N-SiMe3)(N0-myrtanyl)(THF)TiCl3 (111.0(10)ꢁ),
[PhC(N-SiMe3)(N0-myrtanyl)]3ZrMe (112.0(2)ꢁ), simi-
lar to that in [PhC(N-SiMe3)(N0-myrtanyl)]3ZrCl–
toluene (114.6(5)ꢁ) [15b], while smaller than the angle
in other octahedral nickel complexes bearing the
N-silylated-benzamidinate moiety as in PhC(NSiMe3)2-
Ni(acac)-(TMEDA) (117.95(19)ꢁ) [17] and p-MePhC(N-
SiMe3)2-Ni(acac)(TMEDA) (118.1(5)ꢁ) [17].
Complex 6 was successfully crystallized from a tolu-
ene solution to obtain green color single crystals suitable
for X-ray measurements in 62% yield. The crystal struc-
ture of complex 6 is presented in Fig. 2. Selected bond
lengths and bond angles and crystallographic data and
structure refinement details are listed in Tables 2 and
3, respectively.
The low-temperature X-ray diffraction analysis shows
that complex 6 crystallizes with one molecule of toluene.
The most peculiar fact regarding complex 6 is the total
replacement of all trimethylsilyl groups by hydrogen
atoms during the course of reaction. A plausible mech-
anism for this replacement is suggested in Scheme 1.
The formation of complex 6 instead of complex 3 is
the result of using the solvent pyridine already at the
beginning of the reaction. The presence of pyridine re-
duces the chelating effect of the acetylacetonate ligation
(step 1), allowing the lithium benzamidinate to act as a
base removing the hydrogen from the methyl of the
2.2. Synthesis of the complex [tetra-(N-
myrtanyl)(benzamidinate)]Ni dimer (6)
Reaction of compound 2 with two equivalent of li-
gand 1 in a solution mixture of toluene:pyridine (2:3)
at ambient temperature afforded the unexpected com-
plex {[PhC(NH)(N0-myrtanyl)]2Ni}2 (6) (Eq. (6)).
Ph
N
NH
*R
Ph
NH
Me3Si
Me
Me
Me
: toluene
N
R*
Ni
N
N
N
Li
+ 2 (TMEDA)Ni(acac)2
Ni
4
ð6Þ
-4 Li(acac)
-2 TMEDA
N
N
N
R*
Me
HN
R*
*R
N
NH
Ph
Ph
1
2
6

3158
E. Nelkenbaum et al. / Journal of Organometallic Chemistry 690 (2005) 3154–3164
Table 1
Selected Bond Lengths [A] and Angles [deg] for complex 3
Table 2
Selected bond lengths (A) and bond angles (ꢁ) for complex 6
˚
˚
Ni(1)–N(2)
Ni(1)–N(1)
N(1)–C(4)
N(2)–C(4)
2.115(2)
2.150(2)
1.350(4)
1.316(4)
Ni(2)–N(2)
Ni(1)–N(4)
Ni(1)–N(6)
Ni(2)–Ni(8)
Ni(1)–N(1)
Ni(2)–N(3)
Ni(2)–N(5)
Ni(1)–N(7)
Ni(1)–Ni(2)
N(1)–C(1)
N(2)–C(1)
N(1)–C(4)
N(2)–C(4)
N(3)–C(18)
N(4)–C(18)
N(5)–C(35)
N(6)–C(35)
N(7)–C(52)
N(8)–C(52)
1.918(7)
1.900(7)
1.920(7)
1.894(7)
1.882(7)
1.904(6)
1.876(7)
1.894(7)
2.4484(13)
1.316(9)
1.351(10)
1.350(4)
1.316(4)
1.318(10)
1.349(9)
1.345(10)
1.323(10)
1.349(9)
1.310(10)
N(3)–Ni(1)–N(4)
N(2)#1–Ni(1)–N(1)#1
N(2)–Ni(1)–N(1)#1
N(2)–C(4)–N(1)
180.00(1)
63.69(9)
116.43(9)
115.1(2)
63.69(9)
116.43(9)
N(2)–Ni(1)–N(1)
N(2)#1–Ni(1)–N(1)
acetylacetonate ligation (step 2). The formed enolate
anion is highly nucleophilic towards the trimethylsilyl
moiety and will react again with the neutral benzamidi-
nate ligand forming the lithium benzamidinate without
the trimethylsilyl group (step 3). This ligand will again
react either with the new nickel complex containing
the silyl group or with another molecule of complex 2
forming ultimately complex 6.
Diffraction measurements on complex 6 reveal an
interesting ‘‘cross’’-like tetrameric structure with two
nickel metal centers adopting a slightly distorted square-
planar geometry. The sums ofthe angles aroundthe nickel
atoms are 359.6ꢁ (N(1)–Ni(1)–N(4) = 90.8(3)ꢁ, N(7)–
N(1)–Ni(1)–N(4)
N(7)–Ni(1)–N(4)
N(1)–Ni(1)–N(6)
N(7)–Ni(1)–N(6)
N(5)–Ni(2)–N(8)
N(8)–Ni(2)–N(3)
N(5)–Ni(2)–N(2)
N(3)–Ni(2)–N(2)
N(1)–C(1)–N(2)
N(8)–C(52)–N(7)
90.8(3)
89.7(3)
89.1(3)
90.0(3)
91.1(3)
89.7(3)
88.3(3)
90.6(3)
120.2(7)
120.2(8)
Ni(1)–N(4) = 89.7(3)ꢁ,
N(7)–Ni(1)–N(6) = 90.0(3)ꢁ) and 359.7ꢁ (N(5)–Ni(2)–
N(8) = 91.1(3)ꢁ, N(8)–Ni(2)–N(3) = 89.7(3)ꢁ, N(5)–
N(1)–Ni(1)–N(6) = 89.1(3)ꢁ,
C75
C70
C69
C74
C63
C64
C39
C73
C62
C66
C40
C67
C72
C71
C57
C13
C68
C37
C38
C65
C41
C58
C61
C17
C14
C36
C60
C9
N5
C56
C50
C12
C16
C15
C11
C53
C59
Ni2
C8
C35
C52
N8
N6
N7
N2
C7
C55
C10
C5
C54
C49
C48
C42
N3
N1
C6
C47
C46
Ni1
C1
C43
C2
C51
C44
C26
N4
C18
C19
C20
C3
C4
C25
C31
C45
C27
C21
C24
C22
C28
C32
C29
C23
C30
C33
C34
Fig. 2. ORTEP plot of the molecular structure of the complex 6. All hydrogens were removed for clarity and thermal ellipsoids are given at 50%
probability.

E. Nelkenbaum et al. / Journal of Organometallic Chemistry 690 (2005) 3154–3164
3159
Table 3
Crystal data collection for complexes 3 and 6
Identification code
Complex 3
Complex 6
Empirical formula
Formula weight
T (K)
C50H72N6NiSi2
872.03
C75H100N8Ni2
1231.05
230.0(1)
230.0(1)
0.71073
˚
Wavelength (A)
0.71073
Orthorombic
P212121
Crystal system
Space group
Unit cell dimensions
Monoclinic
C2
˚
a (A)
16.7700(15)
14.2200(13)
11.64200(10)
12.67700(10)
˚
b (A)
˚
c (A)
11.9050(12)
90
45.6020(6)
90
a (ꢁ)
b (ꢁ)
c (ꢁ)
117.543(4)
90
2517.2(4)
90
90
3
˚
V (A )
6730.20(12)
4
1.215
Z
2
1.151
Dcalc (mg/m3)
Absorption coefficient (mmꢀ1
F(000)
Crystal size (mm)
)
0.471
940
0.608
2648
0.30 · 0.30 · 0.13
1.93–24.76
0.15 · 0.24 · 0.28
2.20–22.00
h Range for data collection (ꢁ)
Limiting indices
ꢀ19 6 h 6 19, ꢀ16 6 k 6 14, ꢀ13 6 l 6 13
3609/3609 (0.0000)
97.9% (h = 24.76ꢁ)
0.9413 and 0.8716
Full-matrix least-squares on F2
3609/1/306
ꢀ12 6 h 6 12, ꢀ13 6 k 6 13, ꢀ47 6 l 6 48
8072/4605 (0.0650)
Reflections collected/unique (Rint
Completeness to h
Maximum and minimum transmission
Refinement method
)
97.9% (h = 22.00ꢁ)
Full-matrix least-squares on F2
4605/0/749
0.967
Data/restraints/parameters
Goodness-of-fit on F2
0.887
Final R indices [I > 2r(I)]
R indices (all data)
R1 = 0.0378, wR2 = 0.0813
R1 = 0.0601, wR2 = 0.0849
0.014(15)
R1 = 0.0509, wR2 = 0.1139
R1 = 0.0869, wR2 = 0.1247
0.38(3)
Absolute structure parameter
Extinction coefficient
Largest difference in peak and hole (e A
0.0007(3)
0.350 and ꢀ0.447
ꢀ3
˚
)
0.196 and ꢀ0.214
˚
Ni(2)–N(2) = 88.3(3)ꢁ,
N(3)–Ni(2)–N(2) = 90.6(3)ꢁ).
1.349(9), N(8)–C(52) = 1.310(10) A). Comparing the
benzamidinate ligation with other nickel complex re-
veals a small difference between the N–C–N angles in
complex 6 (120.6ꢁ (average)), and those in (l2)2–(N,N0-
bis-(trimethylsilyl)benzamidinate Ni(I)) dimer (122.25ꢁ)
[17]. Interestingly, the Ni–Ni distance in complex 6
Each nickel atom is r-bonded to two myrtanyl-substi-
tuted and two H-substituted nitrogen atoms of the benz-
amidinate moiety. Interestingly, are the almost equal
Ni–N-myrtany bond lengths (Ni(2)–N(2) = 1.918(7),
Ni(1)–N(4) = 1.900(7), Ni(1)–N(6) = 1.920(7), Ni(2)–
˚
˚
Ni(8) = 1.894(7) A) and nearly equidistance Ni–N-
(2.4484(13) A) is longer than in N-silylated-benzamidi-
˚
nate nickel(I) complex (2.2990(12) A) [17].
hydrogen
bond
lengths
Ni(2)–N(3) = 1.904(6), Ni(2)–N(5) = 1.876(7), Ni(1)–
(Ni(1)–N(1) = 1.882(7),
˚
N(7) = 1.894(7) A). The bridging benzamidinates are dis-
2.3. Oligomerization of propylene by complex 3 activated
with MAO
posed in a trans fashion around the two nickel centers.
The core units of complex 6 comprised four five-
membered Ni–N–C–N–Ni rings that are not completely
planar as shown by the torsion angles N(1)–Ni(1)–
Complex 3 was tested as a pre-catalyst for the oligo-
merization of propylene after being activated by MAO
(Al:Ni = 200). Performing the oligomerization reaction
at liquid propylene (10 bar) and room temperature for
20 h without additional solvents leads to a mixture of di-
mers (70.5%), trimers (24.5%) and tetramers (5.0%) of
propylene with a turnover frequency of 5200 hꢀ1 (Eq.
(7)). Among the various dimerization products two mix-
tures of hexenes containing both primary dimerization
products (71.5%, trans-2-hexene, 2,3-dimethyl-1-butene,
cis-4-methyl-2-pentene) and isomerization products
(28.5%, trans-3-hexene, 2-methyl-2-pentene) were ob-
tained (Eq. (7)).
Ni(2)–N(2) = 15.2ꢁ
and
N(4)–Ni(1)–Ni(2)–N(3) =
15.5ꢁ. The analogous deviation from planarity has been
observed for similar binuclear benzamidinate-bridged
nickel complexes, which is consisted of two five-mem-
bered rings (the torsion angle of 13.5ꢁ) [17].
There are similar differences observed in the N–C dis-
tances as presented above, indicating the different elec-
tronic delocalization within N–C–N framework
(N(1)–C(1) = 1.316(9), N(2)–C(1) = 1.351(10), N(3)–
C(18) = 1.318(10), N(4)–C(18) = 1.349(9), N(5)–C(35) =
1.345(10),
N(6)–C(35) = 1.323(10),
N(7)–C(52) =

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E. Nelkenbaum et al. / Journal of Organometallic Chemistry 690 (2005) 3154–3164
N
O
N
O
O
O
O
step 1
Ni
+
N
Ni
O
O
N
N
O
N
N
2
SiMe3
N
Li.TMEDA
step 2
N
R*
1
CH2
SiMe3
NH
N
N
O
OLi
+
Ni
O
N
N
O
N
R*
step3
CH2
N
H
N
O
OSiMe3
+
N
Ni
Li
O
N
O
N
N
R*
complex 6
Scheme 1. Proposed mechanism for SiMe3–H exchange in the ligand PhC(N-SiMe3)(N0-myrtanyl)Li(TMEDA).
3/MAO
different product distribution (42% of the primary
dimerization products and 58% of isomerization prod-
ucts) with a high turnover frequency of 9040 hꢀ1 [17].
C6H12
+
C9 H18
24.5 %
+
C12 H24
5.0 %
70.5 %
2.4. Oligomerization of ethylene with complex 6/MAO
ð7Þ
When activated with MAO complex 6 showed a high
activity toward the oligomerization of ethylene. Performing
the oligomerization reaction in toluene at ambient tempera-
ture and a constant ethylene pressure of 30 atm complex 6
yield a mixture of dimers (59%) and trimers (41%) with a high
turnover frequency of 16,400 hꢀ1 (Eq. (8)). A similar product
distribution (52% of butenes and 48% of hexenes) and turn-
over frequency of 17,100 hꢀ1 were observed for the pre-cata-
lyst PhC(NSiMe3)2Ni(acac) using toluene as the solvent [17].
+
C6H12
=
71.5 %
28.5 %
Interestingly, in comparison with complex 3 non-chi-
ral bis-N-silylated-benzamidinate nickel pre-catalyst
shows a large selectivity towards dimers (>98%) with

E. Nelkenbaum et al. / Journal of Organometallic Chemistry 690 (2005) 3154–3164
3161
@@ 6=MAO C H þ C H
ƒƒƒƒ!
2.5. Polymerization of norbornene with complex 3/MAO
ð8Þ
ware on a dual-manifold Schlenk line, or interfaced to a
high vacuum (10ꢀ5 Torr) line. For storage of air-sensi-
tive materials, a nitrogen-filled ‘‘Vacuum Atmospheres’’
glovebox with a medium capacity recirculator (1–2 ppm
O2) was used.
4
8
6
12
toluene
59%
41%
and complex 6/MAO
NMR spectra were recorded on Bruker AM-300 and
1
AM-500 spectrometers. Chemical shifts for H NMR
Complexes 3 and 6 were also tested as the catalytic
precursors for the addition polymerization of norborn-
ene (Eq. (9)).
and were referenced to internal solvent resonances and
reported relative to tetramethylsilane. Low-temperature
X-ray diffraction experiments were carried out on a
Nonius-KappaCCD diffractometer with graphite mono-
chromatized Mo Ka radiation. The crystals were placed
in dry and degassed Parathon-N (Du-Pont) oil in a
glovebox. Single crystals were mounted on the diffrac-
tometer under a stream of cold N2 at 230 K. Cell refine-
ments and data collection and reduction were carried
out with the Nonius software package [18]. The struc-
ture solution was carried out by the SHELXS-97 [19] and
SHELXSL-97 [20] software packages. The ORTEP pro-
gram incorporated in the TEXRAY structure analysis
package was used for molecular graphics [21].
3/MAO;6/MAO
ð9Þ
n
n
The preliminary results indicate that both complex 3
and complex 6 being activated with MAO exhibit a good
catalytic activity for the polymerization of norbornene
in toluene at ambient temperature (for complex 3 atactic
polynorbornene (PNB) yield = 42.4% for
1 h at
Al:Ni = 150, [monomer]:[Ni] = 1500; for complex 6
yield = 59.4% for 1 h at Al:Ni = 100, [monomer]:[Ni] =
1000). As compared to other nickel complexes activated
by MAO, complexes 3 and 6 were found to be less active
than PhC(NSiMe3)2Ni(acac) at the molar ratio
Al:Ni = 100 and 150 (for PhC(NSiMe3)2Ni(acac)
yield = 65.9% for 1 h) [17]. Interestingly, when the poly-
merization was performed at a MAO:Ni ratio of 200 the
activities of both complex 6 and PhC(NSiMe3)2Ni(acac)
were alike (yield = 72.0% for complex 6 and 70.0% for
PhC(NSiMe3)2Ni(acac)). The formation in all cases of
atactic polymers indicates the lack of ability to induce
stereospecific interactions presumably due to the large
distance of the chiral center to the metal center.
4.2. Materials
Argon and nitrogen were purified by passage through
˚
a MnO oxygen removal column and a Davison 4 A
molecular sieves column. Analytically pure solvents
were distilled under argon from Na/K alloy (diethyl
ether, hexane), Na (toluene), P2O5 (dichloromethane),
or BaO (pyridine, triethylamine). All solvents for vac-
uum line manipulations were stored in a vacuum over
Na/K alloy. Nitrile compounds (Aldrich) and
N,N,N0,N0-tetramethylethylenediamine (TMEDA) were
degassed and freshly distilled under argon. Methylalu-
moxane (MAO) (Witco) was prepared from a 30% sus-
pension in toluene by vacuum evaporation of the
solvent at 25 ꢁC/10ꢀ5 Torr. (ꢀ)-cis-Myrtanylamine,
Me3SiCl, BuLi, norbornene (bicyclo-[2.2.1]hept-2-ene)
were purchased and used as received (Aldrich).
Et3NHCl (Aldrich) was sublimated under vacuum at
200 ꢁC. Bis-(acetylacetonate)nickel(II)–(TMEDA)Ni-
(acac)2 [16], (TMEDA)NiMe2 [12d,16] were prepared
via the literature procedures.
3. Conclusions
Two new chiral nickel complexes bearing the chelat-
ing benzamidinate ligation have been synthesized. Their
solid-state molecular structures have been determined
by low-temperature X-ray diffraction analysis. Complex
3 being activated with MAO oligomerizes propylene,
producing a mixture of dimers, trimers and tetramers.
Complex 6 activated with MAO has been found to oli-
gomerize ethylene to the mixture of dimers and trimers.
Moreover, both complex 3/MAO and complex 6/MAO
have been shown to be good catalytic systems for the
addition polymerization of norbornene.
4.3. Synthesis of ligand [PhC(N-SiMe3)
(N0-myrtanyl)]Li(TMEDA) (1)
4.3.1. Synthesis of Me3SiH(N-myrtanyl)
To a well-stirred solution of 5.65 g (33.1 mmol) (ꢀ)-
cis-myrtanylamine and 33.5 g (33.1 mmol) of freshly dis-
tilled Et3N in hexane (150 mL) 3.6 g (33.1 mmol) of
Me3SiCl was dropwise added over 1 h at room temper-
ature. The resulting mixture was stirred with reflux for
17 h. After filtration of the solid Et3NHCl and evapora-
tion of the solvent by vacuum, the product was distilled
under vacuum at 50–52 ꢁC producing 6.3 g (85%) of
4. Experimental
4.1. General procedure
All manipulations were performed with the exclusion
of oxygen and moisture using flamed Schlenk-type glass-

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E. Nelkenbaum et al. / Journal of Organometallic Chemistry 690 (2005) 3154–3164
transparent oil. For spectroscopic characterization and
for the second step of the preparation of ligand 1 see
[15b].
for C50H72N6NiSi2 (872.03): C, 68.80; H, 8.25; N,
9.63. Found: C, 69.40; H, 8.14; N, 10.45%.
Anal. Calc. for C20H31N2LiSi Æ 1/2TMEDA (392.2):
C, 70.39; H, 9.95; N, 10.71. Found: C, 69.61; H, 9.00;
N, 9.65%.
4.5.2. Method B
(TMEDA)NiMe2 prepared by the reaction of (TME-
DA)Ni(acac)2 (0.66 g, 1.77 mmol) with 0.30 g (1.77
mmol) of (TMEDA)MgMe2 was dissolved in 10 mL of
a cooled (ꢀ70 ꢁC) solution mixture of toluene:pyridine
(3:1) and reacted with a toluene solution (3 mL) of ligand
4 (0.42 g, 1.28 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 solu-
tion. Bubbling was observed. The reaction initially pro-
vided the expected methyl complex 5 which could be
observed by following the crude reaction in a NMR tube
1H NMR (d = ꢀ0.32, corresponding to the mono
Ni–CH3). However, after repeated washing of the precip-
itated complex with cold toluene and the re-crystalliza-
tion at ꢀ30 ꢁC for 2 weeks, single crystals with green
color of complex 3 have been obtained in 23% yield.
4.4. Synthesis of neutral ligand PhC(NHSiMe3)-
(N0-myrtanyl) (4)
1.03 g (7.47 mmol) of Et3NHCl was added to a solu-
tion of the ligand 1 (2.5 g, 7.47 mmol) in 40 mL of tol-
uene. The mixture was stirred at room temperature for
20 h. The solution was isolated via filtration through cel-
lite and the solvent was evaporated under vacuum pro-
ducing transparent yellow oil. The product was
washed with additional 25 mL of toluene and dried un-
der vacuum overnight to obtain 2.25 g (92%) of ligand 4
as a viscous oil. Anal. Calc. for C20H32N2Si (328.1): C,
73.15; H, 9.75; N, 8.53. Found: C, 72.80; H, 9.70; N,
8.87%.
1H NMR (C6D6, 300 MHz): d = 7.19 (m, 5H, Ph),
3.48 (s, 1H N–H), 3.22 (m, 2H, N–CH2CH), 2.40 (m,
2H, N–CH2CHCHC(CH3)), 1.93 (m, 5H, C–
CH3 + CHCH2CH2), 1.54 (m, 1H, CH2CHC(CH3)2),
1.21 (m, 5H, C–CH3 + CH2CH2CH), 1.11 (m, 1H,
CHCHCH), 0.76 (m, 1H, CHCHCH), 0.4 (s, 9H,
Si(CH3)3). 13C NMR (C6D6, 75 MHz): d = 161.0
(N–C@N), 130.4, 129.7, 129.0, 128.2 (Ph), 58.8 (HN–
CH2CH), 45.8 (CH2CH(CH2)2), 43.5 (CHCHC-
(CH3)2 + CH2CHC(CH3)2), 40.3 (C(CH3)2), 35.4
(CHCH2CH), 29.7 (CH3), 28.2 (CH2CH2CH), 24.8
(CH3), 21.7 (CHCH2CH2), 1.8 (SiCH3).
4.6. Synthesis of [tetra-(N-myrtanyl)(benzamidinate)]-
Ni dimer (6)
To a well-stirred solution of 0.48 g (1.29 mmol) of
complex 2 in toluene:pyridine mixture (25 mL, 2:3) 1.0
g (2.58 mmol) of ligand 1 was added. A color change
from turquoise to a deep brown was immediately ob-
served. The resulting mixture was stirred for 15 h at
room temperature, filtrated and the solvents were evap-
orated under vacuum. The product was washed with
additional 25 mL of toluene and after toluene evapora-
tion, 15 mL of hexane was transferred under vacuum to
the residue. Slow cooling of this solution to ꢀ40 ꢁC for
two weeks provided the green color crystals of complex
6 in 62% yield.
4.5. Synthesis of the complex [PhC(N-SiMe3)(N0-
myrtanyl)]2Ni(Py2) (3)
Anal. Calc. for C75H100N8Ni2 (1231.05): C, 73.11; H,
8.12; N, 9.09. Found: C, 73.40; H, 8.14; N, 9.45%.
4.5.1. Method A
To a well-stirred solution of compound 2 (1.12 g 3.0
mmol) in 30 mL of diethyl ether 2.0 g (6.0 mmol) of li-
gand 1 was added. A color change from turquoise to a
brown was observed. The resulting mixture was stirred
for 10 h at room temperature and filtrated cold. The sol-
vent was evaporated under vacuum producing brown
color viscous oil. The product was washed with addi-
tional 25 mL of toluene and dried under vacuum. At-
tempts to crystallize this compound from any solvent
were unsuccessful. After addition of a solution mixture
of hexane:pyridine (20 mL, 1:1) to a nickel compound
the solution was stirred for 1 h. Slow cooling of this
solution at ꢀ40 ꢁC for two weeks provided the green
color crystals of complex 3 (54%), which was isolated
by filtration and drying under vacuum. Complex 3 is
paramagnetic, giving broad contact-shifted signals in
the range from ꢀ15 to ꢀ4 and 60–90 ppm. Anal. Calc.
4.7. General procedure for the dimerization of propylene
Experiments at higher pressures were performed in a
100 mL stainless steal 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, pumped-down
and frozen at liquid nitrogen temperature. 25 mL of li-
quid propylene was transferred under vacuum 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 ten different hexenes was used as inter-
nal reference.

E. Nelkenbaum et al. / Journal of Organometallic Chemistry 690 (2005) 3154–3164
3163
4.8. General procedure for ethylene oligomerization
associated with this article can be found, in the online
version at doi:10.1016/j.jorganchem.2005.04.007.
The catalytic oligomerization of ethylene activated
by complex 6 and MAO was performed in 100 mL
stainless steal reactors. The reactor was charged inside
a glovebox with a certain amount of the catalytic pre-
cursor, MAO and a magnetic stirrer. The reactor was
connected to a high vacuum line, and, after introduc-
tion of the solvent (10 mL) under an argon flow, it
was frozen at liquid nitrogen temperature and evacu-
ated under vacuum. The reactor was then warmed
to room temperature, pressurized to 30 bar ethylene
pressure and stirred for the desired reaction time.
The reaction temperature was maintained using an
external water-ice bath to control the reaction exo-
therm. After 1 h the reaction was stopped by cooling
the reactor to ꢀ15 ꢁC and depressurizing. The con-
tents were transferred into a cold tarred 50 mL heavy
duty glass Schlenk flask, sealed and weighed. The olig-
omers obtained were analyzed by GC/MS.
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