A Bis-[C(trimethylsilyl)-N-arylimino]dimethylsilane, Me3Si(C= NAr)SiMe2(C=NAr)SiMe3 (Ar = 2,6-xylyl), as a New β-Diimine Ligand

Article abstract of DOI:10.1021/om700711r

A cobalt complex having bis-[C(trimethylsilyl)-N-arylimino]dimethylsilane, Me₃Si(C=NAr)SiMe₂(C=NAr)SiMe₃ (Ar = 2,6-xylyl), as a new β-diimine ligand was synthesized, and its structure was compared with those of α- and β-diimine cobalt complexes. Ethylene polymerization catalyzed by these cobalt complexes and their in situ generated nickel homologues was investigated.

Full text of DOI:10.1021/om700711r

Organometallics 2007, 26, 6055-6058
6055
Notes
A Bis-[C(trimethylsilyl)-N-arylimino]dimethylsilane,
Me3Si(CdNAr)SiMe2(CdNAr)SiMe3 (Ar ) 2,6-xylyl), as a
New â-Diimine Ligand|
Masao Tanabiki,‡,§ Yusuke Sunada,†,‡ and Hideo Nagashima*,†,‡
Institute for Materials Chemistry and Engineering and Graduate School of Engineering Sciences,
Kyushu UniVersity, Kasuga, Fukuoka 816-8580, and Yokkaichi Research Laboratory,
TOSOH Corporation, 1-8, Kasumi, Mie, 510-8540, Japan
ReceiVed July 17, 2007
Summary: A cobalt complex haVing bis-[C(trimethylsilyl)-N-
arylimino]dimethylsilane, Me3Si(CdNAr)SiMe2(CdNAr)SiMe3
(Ar ) 2,6-xylyl), as a new â-diimine ligand was synthesized,
and its structure was compared with those of R- and â-diimine
cobalt complexes. Ethylene polymerization catalyzed by these
cobalt complexes and their in situ generated nickel homologues
was inVestigated.
polyolefins (Brookhart’s catalysts).5 In contrast to the conven-
tional nickel catalysts,6 the R-diimine complexes produce
polyethylene with relatively high molecular weight, and the
polymer branch is controllable by the ligand design. Stimulated
by the success of R-diimine complexes, efforts have been
directed to seeking other transition-metal complexes for poly-
ethylene preparation.3,7 Studies on other diimine catalysts,
however, have been limited to â-diimine and o-phenylene-
diimines.8
Introduction
In 1988, Ito and co-workers reported palladium-catalyzed
insertion of isonitriles into the Si-Si bonds of trisilanes giving
oligo(silylimine) derivatives.9 The oligo(silylimine) derivatives,
Me2RSi(CdNAr){SiMe2(CdNAr)}nSiRMe2 (n ) 1,2,3,4; R )
Reactivity and selectivity of transition metal catalysts are
dependent on the electronic nature and steric environment of
the central metal, and the appropriate choice of the ligand is
important for catalyst design.1,2 Among nitrogen donor ligands,
which have recently received much attention in organometallic
chemistry and catalysis,3,4 special properties of R-diimine ligands
that have bulky aryl substituents on the nitrogen atoms in late
transition metal complexes have provided successful and
noteworthy examples of efficient catalysts for production of
t
Me, Bu, Ph; Ar ) 2,6-xylyl, 2,6-diisopropylphenyl), have a
structure analogous to â-diimines and potentially ligate with
transition metals. In this paper, we report the first synthesis of
a cobalt complex having Me3Si(CdNAr)SiMe2(CdNAr)SiMe3
(Ar ) 2,6-xylyl) (1a) as a bidentate nitrogen ligand. Its structure
is compared with those of known R- and â-diimine cobalt
complexes. The application of these cobalt complexes and their
in situ generated nickel homologues as a catalyst for ethylene
polymerization was investigated.
* To whom correspondence should be addressed. E-mail: nagasima@
cm.kyushu-u.ac.jp.
† Institute for Materials Chemistry and Engineering, Kyushu University.
‡ Graduate School of Engineering Sciences, Kyushu University.
§ TOSOH Corporation.
Results and Discussion
| This paper is dedicated to the memory of the late Professor Yoshihiko
Ito, Kyoto University.
Ito and co-workers reported a conformational feature of their
oligo(silylimine) derivatives, in which two rotational isomers,
syn and anti, exist. The more stable isomer is the anti form,
and a significant rotational barrier prevents facile syn-anti
interconversion.9 Although the complexation of 1a requires the
conversion from the anti form to the syn form, the reaction of
1a with CoBr2 gave the corresponding cobalt complex of 1a in
reasonable yields. Thus, treatment of Me3Si(CdNAr)SiMe2(Cd
NAr)SiMe3 (Ar ) 2,6-xylyl) (1a) with CoBr2 in THF at room
temperature for 12 h resulted in a significant color change of
(1) (a) Collman, J. P.; Hegedus, L. S.; Norton, J. R.; Finke, R. G.
Principles and Application of Organotransition Metal Chemistry; University
Science Books: Mill Valley, CA, 1987. (b) Yamamoto, A. Organotransition
Metal Chemistry; Wiley: New York, 1986. (c) Crabtree, R. H. The
Organometallic Chemistry of Transition Metals, Wiley: New York, 1988.
(d) Elschenbroich, C.; Salzer, A. Organometallics, 2nd ed.; VCH: Wein-
heim, Germany, 1992; Organometallchemie; Teubner: Stuttgart, Germany,-
1988. (e) Parshall, G. W.; Ittel, S. D. Homogeneous Catalysis; Wiley: New
York, 1992.
(2) (a) Tolman, C. A. J. Am. Chem. Soc. 1970, 92, 2953-2956. (b) van
Leeuwen, P. W. N. M.; Kamer, P. C. J.; Reek, J. N. H.; Dierkes, P. Chem.
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1169-1203.
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Van Koten, G.; Vrieze, K. AdV. Organomet. Chem. 1982, 21, 151-239.
(c) Van Koten, G.; Jastrzebski, J. T. B. H.; Vrieze, K. J. Organomet. Chem.
1983, 250, 49-61. (d) Vrieze, K.; Van Koten, G. Inorg. Chem. Acta 1985,
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10.1021/om700711r CCC: $37.00 © 2007 American Chemical Society
Publication on Web 10/24/2007

6056 Organometallics, Vol. 26, No. 24, 2007
Notes
The cobalt center of 2a is bound to two nitrogen and two
bromine atoms with Co-N ) 2.039(2), 2.056(2) Å, and Co-
Br ) 2.361(2), 2.382(2) Å, assuming a tetrahedral coordination
geometry. The complex 2a has a six-membered 1,4-silacobal-
tacyclic structure, which adopts a boat conformation. The Co
and Si atoms are out of the plane defined by two CdN moieties
by 0.493 and 0.742 Å, respectively. The dihedral angles defined
by the plane that includes the two CdN moieties and a phenyl
plane connected to the nitrogen atoms are 79.93 and 84.28°.
The bond lengths of CdN(1.300(2), 1.299(2) Å) and C-Si-
(1.932(2), 1.939(2) Å) are comparable to those of the known
oligo(silylimine) derivatives.9
Similarly, the cobalt centers of 2b and 2c adopt a tetrahedral
coordination geometry with two nitrogen atoms and two Br
atoms, and the Co-N and Co-Br bond lengths are comparable
to those of the other related complexes.10 The cobalt atom of
2b is out of the plane defined by two imino groups by 0.380 Å,
while the five-membered cobaltacycle ring of 2c is almost planar
and the sum of the internal angles of the ring (539.9°) is
close to the theoretical value of 540°. Interesting structural
features of 2a∼c are seen in the angle θ and the distance d
defined in Figure 2; θ decreases in the order 2c (168.5 °) > 2b
(135.5 °) > 2a (118.4 °), whereas d decreases in the order 2c
(1.579 Å) > 2b (1.361 Å) > 2a (1.319 Å). The θ and d
determine the space volume available for coordination of the
other ligands to the cobalt center. In the cases of 2a∼c, larger
θ and d provide a larger space for the bromine coordination. In
other words, 2a is structurally similar to 2b in that both
complexes have a six-membered, nonplanar, boat-form confor-
mation, but 2a has a smaller coordination site for other ligands
than 2b or 2c. These structural features of 2a are derived from
the presence of two Si-C bonds in the cobaltacycle. The Si-
C(dNR) distances in 2a are longer than the corresponding
C-C(dNR) distances of 2b and 2c. The longer CdN bonds
are also the origin of the structural features of 2a. In the cobalt
complexes, the CdN bond lengths of 2a (1.300(2), 1.299(2)
Å) are longer than those of 2b and 2c (1.27∼1.28 Å).
Figure 1. Molecular structure of 2a showing 50% probability
ellipsoids. Hydrogen atoms are omitted for clarity. The ORTEP
drawings of 2b and 2c are depicted in the Supporting Information.
Chart 1
Table 1. Selected Bond Lengths (Å) and Angles (deg) for
2a-2c
2a
2b
2c
ring size
6
6
5
θ
d
γ
l
δ
m
118.4
1.319
21.96
0.493
41.60
0.742
135.5
1.361
16.19
0.380
26.27
0.323
168.5
1.579
Bond Lengths
1.300(2)
1.299(2)
1.932(2)
1.939(2)
2.039(2)
2.056(2)
2.361(2)
2.382(2)
The longer CdN distances of oligo(silylimine) derivatives,
which are typically seen in the CdN distances in Me3Si[{Cd
N(2,6-xylyl)}SiMe2]2{CdN(2,6-xylyl)}SiMe3 (1.27∼1.32 Å),
are due to the σSi-C-π*CdN interaction.9 The σSi-C-π*CdN
interaction also contributes to reducing the force constant of
CdN bonds; IR absorption bands at 1536-1552 cm-1 seen in
Ito’s oligosilylimine derivatives (1a; 1549 cm-1) are signifi-
cantly lower than the νCdN of ordinary imines (1610-1680
CdN
1.277(5)
1.507(7)
2.020(3)
1.275(4)
1.285(4)
E-C
Co-N
Co-Br
2.052(2)
2.054(2)
2.356(6)
2.357(6)
2.378(2)
2.374(2)
Bond Angles
99.75(7)
109.39(9)
115.4(2)
114.5(2)
126.6(2)
127.8(2)
693.4
cm-1
: Me{CdN(2,6-xylyl)}CH2{CdN(2,6-xylyl)}Me (1b),
N-Co-N
C-E-C
95.3(2)
122.1(6)
123.2(4)
79.52(11)
1624 cm-1; Me{CdN(2,6-xylyl)}Me){C()N2,6-xylyl)}Me (1c),
1643 cm-1). These features are also seen in the cobalt
complexes; the νCdN absorptions decrease in the order 2b (1654
cm-1) > 2c (1643 cm-1) . 2a (1523 cm-1). Although another
special electronic property of 1a due to the σSi-C-π*CdN
interaction is the bathochromic shift of UV-vis absorptions,
there are few characteristic differences in UV-vis absorptions
of the cobalt complexes among 2a, 2b, and 2c.
E-C-N
C-N-Co
122.8(3)
709.4
114.6(2)
114.8(2)
539.9
Sum of the angle
in the metallacycle
the solution from blue to green. Removal of volatiles followed
by extraction with CH2Cl2 gave a solution containing the desired
Co(II) complex (2a) (Chart 1). Although the paramagnetism of
2a complicated the detailed structural analyses by NMR
spectroscopy, elemental analysis and X-ray crystallography
supported the formation of 2a. The molecular structure of 2a is
depicted in Figure 1, and selected bond lengths and angles are
summarized in Table 1. For comparison, structural parameters
of cobalt complexes of â-diimine and R-diimine, 2b, and 2c
synthesized are also listed in Table 1 (Details are described in
the Supporting Information).
Application of the bis-[C(trimethylsilyl)-N-arylimino]di-
methylsilane ligand to homogeneous catalysis was examined
in methyl aluminoxane (MAO)-assisted polymerization of
ethylene. Ethylene polymerization was carried out in a 100 mL
stainless steel autoclave fitted with a Teflon inner tube. MAO
was used as the activator. Under 0.8 MPa of ethylene, the
R-diimine cobalt complex 2c showed some activity to form
(10) (a) Laine, T. V.; Kinga, M.; Maaninen, A.; Aitola, E.; Leskela, M.
Acta Chem. Scand. 1999, 53, 968-973. (b) Tanabiki, M.; Tsuchiya, K.;
Motoyama, Y.; Nagashima, H. Chem. Commun. 2005, 3409-3411.

Notes
Organometallics, Vol. 26, No. 24, 2007 6057
Figure 2. Definition of the distances d, m, l, and the angles θ, δ, and γ.
Table 2. Polymerization of Ethylenea
apparatus. Elemental analyses were performed by a Perkin-Elmer
2400II/CHN analyzer. Melting temperature of polymer (Tm) was
measured by a differential scanning calorimeter (DSC). Analysis
of the polymer branch was performed by 13C NMR measured in
1,2-dichlorobenzene at 120 °C. Diimine ligands, 2,2,4,4,6,6-
hexamethyl-3,5-bis(2,6-dimethylphenylimino)-2,4,6-trisilaheptane-
(1a),9c 2,4-bis(2,6-dimethylphenylimino)pentane (1b),14 and 2,3-
bis(2,6-dimethylphenylimino)butane (1c) were synthesized by the
methods reported in the literature.
mol of catalyst yield
activity
Tm
°C
entry catalyst
µmol
g
g/(mmol‚h)
[η]
1
2
3
3c
3b
3a
5
100
100
36.62
0.70
8.54
7324
7
85.4
3.0 125.8
19.0 120.0
3.6 129.8
a Conditions: cocatalyst: MAO (200 equiv), ethylene 0.8 MPa, toluene
500 mL, 30 °C, 60 min.
Preparation of 2a, 2b, and 2c. In a typical example, CoBr2
(0.689 g, 3.15 mmol) and 1a (1.534 g, 3.29 mmol) were dissolved
in THF (40 mL). The mixture was stirred at room temperature for
12 h during which time the color of the solution changed from
blue to green. The solvent was evaporated and the residue extracted
with CH2Cl2. Removal of volatiles followed by washing with
hexane gave desired Co(II) complex (2a) as a green powder (1.14
g, 92%). Single crystals of 2a were grown from CH2Cl2/pentane;
mp 118 °C. Anal. Calcd for C26H42Br2CoN2Si3: C, 45.55; H, 6.17;
N, 4.09. Found: C, 45.42; H, 6.15; N, 4.19%. IR 1523 cm-1
(νCdN). UV-vis (THF; λmax, nm; ꢀ, M-1 cm-1) 684 (381), 596 (sh)
(163).
Using similar procedures, 2b and 2c were synthesized. 2b: mp
256 °C. Anal. Calcd for C21H26Br2CoN2: C, 48.03; H, 4.99; N,
5.33. Found: C, 47.90; H, 4.96; N, 5.26%. IR 1654 cm-1(νCdN).
UV-vis (THF; λmax, nm; ꢀ, M-1 cm-1) 687 (268), 615 (sh) (159).
2c: mp >290 °C. Anal. Calcd for C20H24Br2CoN2: C, 46.99; H,
4.73; N, 5.48. Found: C, 47.33; H, 4.94; N, 5.10%. IR 1643
cm-1(νCdN). UV-vis (THF; λmax, nm; ꢀ, M-1 cm-1) 684 (328),
594 (sh) (130).
Ethylene Polymerization. In a 100 mL stainless steel autoclave
fitted with a Teflon inner-tube was placed the precatalyst 2a-c
(100 µmol) and toluene (43 mL). A toluene solution of MAO
(PMAO-S available from TOSOH-FINECHEM Corp. 2.85 M in
toluene) was added via syringe (7 mL, 200 equiv for precursor
2a-c), and the mixture was stirred for 1 h at room temperature.
Then ethylene (0.8 MPa) was introduced, and the mixture was
stirred at room temperature for 40 min. Unreacted ethylene was
released, and methanol (5 mL) was added to stop the polymeriza-
tion. Polyethylene was precipitated when the reaction mixture was
poured into a methanol solution of HCl. The polymer was collected
by filtration and dried in vacuo.
X-ray Diffraction Experiments. Single crystals of all complexes
were grown from CH2Cl2/pentane. X-ray crystallography was
performed on a Rigaku Saturn CCD area detector with graphite
monochromated Mo KR radiation (λ ) 0.71070A). The data were
collected at 123(2) K using ω scan in the θ range of 3.0 e θ e
27.5° (2a), 3.0 e θ e 27.5° (2b), 3.1 e θ e 27.5° (2c). The data
obtained were processed using Crystal-Clear (Rigaku) on a Pentium
computer and were corrected for Lorentz and polarization effects.
The structures were solved by direct methods15 for 2a, 2b, and 2c
polyethylene (2.0 g/(mmolCo‚h)). Under the same conditions,
the â-diimine complex and the silylimine complex did not give
polyethylene. Although attempts to isolate the bis-[C(trimethyl-
silyl)-N-arylimino]dimethylsilane complex of nickel were un-
successful because of the instability of the product, the nickel
homologue of 2a was generated in situ by treatment of NiBr2-
(dme) with 1a. As summarized in Table 2, the R-diimine nickel
catalyst 3c exhibited high catalytic activity (over 7 kg/(mmolNi‚
h)), whereas the â-diimine catalyst 3b showed almost no activity.
These are in accord with the results reported in the literature.8a
Interestingly, the silylimine nickel catalyst 3a is moderately
active. The order of activity, a complex having a five membered
ring (3c) > complexes having a six-membered ring (3a and
3b), suggests a primary factor of the active nickel catalyst to
be rigidity of the structure11 and a space for coordination of the
polymer chain and ethylene, which is related to θ and d (vide
supra). Although detailed investigations of the facets of
coordination chemistry and homogeneous catalysis are required
to clarify the ligand effect of bis-[C(trimethylsilyl)-N-arylimino]-
dimethylsilanes, the present results clearly demonstrate that this
is a new class of the bidentate nitrogen ligands.12,13
Experimental
General. Manipulation of air and moisture sensitive organome-
tallic compounds was carried out under a dry argon atmosphere
using standard Schlenk-tube techniques associated with a high-
vacuum line. All solvents were distilled over appropriate drying
reagents prior to use (THF, hexane; Ph2CO/Na, CH2Cl2; CaH2,
acetone; MS 4A). 1H, 13C NMR spectra were recorded on a JEOL
Lambda 600 or a Lambda 400 spectrometer at ambient temperature
1
unless otherwise noted. H, 13C NMR chemical shifts (δ values)
were given in ppm relative to the solvent signal (1H, 13C). Melting
points were measured on a Yanaco SMP3 micromelting point
(11) Keim, W.; Schulz, R. P. J. Mol. Catal. 1994, 92, 21-33.
(12) In the literature, the low activity of 3b is explained by abstraction
of a methylene proton by MAO during the polymerization, which gives
inactive Ni(nacnac) complexes (see ref 13). The bis-[C(trimethylsilyl)-N-
arylimino]dimethylsilane complex 2a has no proton which can react with
MAO.
(13) (a) Cope-Eatough, E. K.; Mair, F. S.; Pritchard, R. G.; Warren, J.
E.; Woods, R. J. Polyhedron 2003, 22, 1447-1454. (b) Radzewich, C. E.;
Guzei, L. A.; Jordan. R. F. J. Am. Chem. Soc. 1999, 121, 8673-8674.
(14) Budzelaar, P. H. M.; Moonen, N. N. P.; de Gelder, R.; Smits, J. M.
M.; Gal, A. W. Eur. J. Inorg. Chem. 2000, 753-769.
(15) (a) Sheldrick, G.M. SHELX97; Universitat Go¨ttingen, Go¨ttingen,
Germany, 1997. (b) Altomare, A.; Burla, M. C.; Camalli, M.; Cascarano,
G.; Guagliardi, A.; Moliterni, A.; Polidori, G.; Spagna, R. SIR97. J. Appl.
Cryst. 1999, 32, 115-119.

6058 Organometallics, Vol. 26, No. 24, 2007
Notes
and expanded using Fourier techniques.16 The non-hydrogen atoms
were refined anisotropically except for the solvent atoms (THF for
2b and acetone for 2c). Hydrogen atoms were refined using the
riding model. The final cycle of full-matrix least-squares refinement
on F2 was based on 7297 observed reflections and 349 variable
parameters for 2a, 3050 observed reflections and 155 variable
parameters for 2b, and 5127 observed reflections and 266 variable
parameters for 2c. Neutral atom scattering factors were taken from
Cromer and Waber.17 All calculations were performed using the
CrystalStructure18,19 crystallographic software package. Details of
final refinement as well as the bond lengths and angles are
summarized in the Supporting Information, and the numbering
scheme employed is also shown in the Supporting Information, and
were drawn with ORTEP at 50% probability ellipsoid.
Acknowledgment. We appreciate the financial support
provided by Grants-In-Aid for Scientific Research from the
Ministry of Education, Culture, Sports, and Technology of
Japan.
Supporting Information Available: The molecular structures
of 2a, 2b, and 2c and details of crystallographic studies (2a, 2b,
2c). This material is available free of charge via the Internet at
http://pubs.acs.org.
(16) Beurskens, P. T.; Admiraal, G.; Beurskens, G.; Bosman, W. P.; de
Gelder, R.; Israel, R.; Smits, J. M. M. The DIRDIF-99 Program System;
Technical Report of the Crystallography Laboratory; University of
Nijmegen: Nijmegen, The Netherlands, 1999.
OM700711R
(17) Cromer, D. T.; Waber, J. T. International Tables for X-ray
Crystallography; Kynoch Press: Birmingham, U.K., 1974; Vol. 4.
(18) CrystalStructure 3.8.0: Crystal Structure Analysis Package; Rigaku
and Rigaku/MSC: The Woodlands, TX, 2000-2006.
(19) Carruthers, J. R.; Rollett, J. S.; Betteridge, P. W.; Kinna, D.; Pearce,
L.; Larsen, A.; Gabe, E. CRYSTALS; Chemical Crystallography Labora-
tory: Oxford, U.K., 1999; Issue 11.