Synthesis, crystal structure and catalytic activity of a novel zirconium complex bearing an unsymmetrical aliphatic benzamidinato ligand

Article abstract of DOI:10.1016/j.inoche.2012.12.002

A novel non-symmetric aliphatic benzamidinato zirconium complex, [{CyNC(Ph)(NSiMe₃)}₂ZrCl₂] (Cy = cyclohexyl) (1) has been readily prepared via the reaction of freshly prepared aliphatic lithium benzamidinate with zirconium tetrachloride. Complex 1 has been characterized by various spectroscopic techniques, elemental analysis, and single-crystal X-ray diffraction analysis. Complex 1 has a distorted pseudo-octahedral structure. Catalytic studies showed that the zirconium complex was active for ethylene polymerization with the activity of 6.66 × 10⁴ g PE/mol Zr h. The influences of Al/Zr molar ratios, ethylene pressures and reaction temperature on the activities were investigated.

Full text of DOI:10.1016/j.inoche.2012.12.002

Inorganic Chemistry Communications 29 (2013) 1–3
Contents lists available at SciVerse ScienceDirect
Inorganic Chemistry Communications
journal homepage: www.elsevier.com/locate/inoche
Synthesis, crystal structure and catalytic activity of a novel zirconium complex
bearing an unsymmetrical aliphatic benzamidinato ligand
a
b
a
Qiaokun Yang a, Meisu Zhou a, , Hongbo Tong , Donglong Guo , Diansheng Liu
⁎
a
Institute of Applied Chemistry, Shanxi University, Taiyuan 030006, People's Republic of China
College of Life Science, Shanxi University, Taiyuan 030006, People's Republic of China
b
a r t i c l e i n f o
a b s t r a c t
Article history:
A novel non-symmetric aliphatic benzamidinato zirconium complex, [{CyNC(Ph)(NSiMe3)}2ZrCl2] (Cy=
cyclohexyl) (1) has been readily prepared via the reaction of freshly prepared aliphatic lithium benzamidinate
with zirconium tetrachloride. Complex 1 has been characterized by various spectroscopic techniques, elemental
analysis, and single-crystal X-ray diffraction analysis. Complex 1 has a distorted pseudo-octahedral structure.
Catalytic studies showed that the zirconium complex was active for ethylene polymerization with the activity
of 6.66×104 g PE/mol Zr h. The influences of Al/Zr molar ratios, ethylene pressures and reaction temperature
on the activities were investigated.
Received 26 November 2012
Accepted 12 December 2012
Available online 21 December 2012
Keywords:
Aliphatic benzamidinato ligand
Bis(amidinate) zirconium complex
Synthesis
© 2012 Elsevier B.V. All rights reserved.
Structure
Polymerization of ethylene
Nitrogen-centered donor amidinate and guanidinate group 4 com-
plexes have been widely studied for the past decades, mainly due to
the growing interest in the development of catalysts for the polymeriza-
tion of olefins [1–7]. The amidinato and guanidinato ligands provide
tunable systems for exploring the effects of making rational modifica-
tions to both the steric and electronic properties [8]. To date, group 4
amidinate and guanidinate complexes have been shown to be promis-
ing for these purposes. However, aliphatic amidinato ligands and
group 4 metal complexes are rare [9]. In recent years, we have devel-
oped a series of guanidinato ligands. Their Zr and Hf complexes are use-
ful catalysts for the polymerization of ethylene [10,11]. Metal complexes
with silyl-linked bis(amidinate), silyl-linked amidinate-amidine, mixed
silyl-linked bis(amidinate) and cyclopentadienyl ligand also showed
good catalytic behaviors in the polymerization of olefins [12–15].
Other related zirconium complexes bearing [(N,N-dimethylamino)
dimethylsilyl]-2,6-diisopropylanilido ligand exhibited good activity
toward ethylene polymerization and gave polyethylenes with broad
molecular weight distributions [16]. Very recently, we reported our
ongoing investigation into the incorporation of aliphatic benzamidinato
ligand and related metal complexes [17]. Included in this paper are the
synthesis, characterization and catalytic behaviors toward ethylene of a
novel bis(amidinate) zirconium complex 1 (Eq. (1)).(1)
ð1Þ
The crystalline, colorless unsymmetrical aliphatic benzamidinato zir-
conium complex 1 was prepared from CyNH(SiMe3) (Cy=cyclohexyl)
after lithiation with n-BuLi and the concomitant reaction with
benzonitrile [17], and further metathesis reaction of the freshly pre-
pared solution of lithium amidinate with zirconium tetrachloride [18].
Complex 1 has been characterized by various spectroscopic techniques,
elemental analysis, and X-ray diffraction analysis [19]. In the IR spectra,
there is strong absorption of C_N stretch in the range of 1581 cm−1 for
1, which indicates the existence of the delocalized double bond of the
N\C\N linkage. The strong bands around 1448 cm−1 can be assigned
to the C\N single bond stretching [20]. The UV–vis spectrum of 1 in a
diluted tetrahydrofuran solution has maximum absorption at 230 nm,
being thought to arise from the delocalized system of N\C\N
backbone.
The monomeric 1 crystallizes in the monoclinic P21/c space group.
The metal center is in a distorted pseudooctahedral environment
consisting of the four nitrogen atoms of two bidentate amidinate
anions with cis chloride ligands completing the coordination sphere
(Fig. 1). The equatorial plane contains N(1), N(2), N(4) and Cl(2)
⁎
Corresponding author. Tel.: +86 351 7010722; fax: +86 351 7011688.
E-mail address: mszhou@sxu.edu.cn (M. Zhou).
1387-7003/$ – see front matter © 2012 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.inoche.2012.12.002

2
Q. Yang et al. / Inorganic Chemistry Communications 29 (2013) 1–3
Table 1
Data for ethylene polymerization catalyzed by complex 1/cocat systema.
Entry
P/atm
T/°C
Al/Zr
Yield (mg)
Activityb
1
2
3
4
5
6
7
8
1
1
1
30
50
70
50
50
50
50
30
40
50
60
70
50
1000
1000
1000
500
1000
1500
2000
2500
2500
2500
2500
2500
3000
Trace
13.6
Trace
33.2
116
–
5.44×103
–
10
10
10
10
10
10
10
10
10
10
1.33×104
4.62×104
4.82×104
6.19×104
8.00×103
3.44×104
6.66×104
2.51×104
1.48×104
5.20×104
120
155
20.1
86.1
166
62.7
37.1
130
9
10
11
12
13
a
Polymerization conditions: 5 μmol of Zr, Cocat: MAO, 100 ml toluene, 0.5 h.
b
g mol−1 h−1
.
Fig. 1. Molecular structure of [{CyNC(Ph)N(SiMe3)}2ZrCl2] (Cy=cyclohexyl). Thermal
ellipsoids are plotted at 50% probability level. Hydrogen atoms are omitted for clarity.
Selected bond lengths (Å) and angles (deg): Zr(1)\N(1) 2.231(2), Zr(1)\N(2) 2.199(2),
Zr(1)\N(3) 2.242(2), Zr(1)\N(4) 2.194(2), Zr(1)\Cl(1) 2.4123(9), Zr(1)\Cl(2)
2.4124(9); N(1)\Zr(1)\N(2) 60.81(9), N(3)\Zr(1)\N(4) 60.72(8), Zr(1)\N(1)\C(7)
92.35(18), Zr(1)\N(3)\C(23) 92.17(17), Cl(1)\Zr(1)\Cl(2) 93.38(4).
show that 1 can catalyze the polymerization of ethylene [26] under
mild conditions (Table 1). When activated with MAO under 1 atm
of ethylene, complex 1 showed lower activity (5.44×103 g PE/mol
Zr h) (Entry 2), therefore, 10 atm pressure of ethylene was employed
in the remaining experiments, in which other reaction parameters
were changed.
At the temperature of 50 °C, with the increase of the Al/Zr molar
ratio from 500 to 3000 (Entries 4–7, 10, 13), complex 1 reached its
highest activity of 6.66×104 g PE/mol Zr h with an optimal Al/Zr
ratio of 2500. However, further increase of the temperature impaired
the activity of the catalyst (Entries 11–12).
Comparing with other six-coordination bis(amidinate) zirconium
dichloride complex such as [Me3SiNC(C6H5)NSiMe3]2ZrCl2 [24b], com-
plex 1 showed lower activity. However, both systems are substantially
lower than those of the analogous metallocene systems for the poly-
merization of ethylene (6×106 g PE (mol Zr)−1 h−1 atm−1) [27],
probably due to the smaller angle tilted to the back of the molecule
that prevented the approach of the olefin [24b].
atoms while the other two atoms (Cl(1) and N(3)) occupy the
axial position. The Zr atom lies on an approximate 2-fold axis which
bisects the Cl(1)\Zr(1)\Cl(2) angle [93.38(4)°] with two planar
bidentate amidinate anions. The four-membered chelating rings
N(1)\Zr(1)\N(2)\C(7) and N(3)\Zr(1)\N(4)\C(23) are pla-
nar with mean deviations of 0.0088 and 0.0093 Å, respectively.
The bond angle C(7)\Zr(1)\C(23) is 109.85 (9)°, much smaller
than those of [iPrNC[N(SiMe3)2]NiPr]2ZrCl2 [131.6(2)°] [21] and
Cp(centroid)\Zr\Cp(centroid) (134°) in Cp2ZrCl2 [22]. The bond
distances of Zr\Cl [2.4123(9), 2.4124(9)Å] in 1 are comparable
to those in other bis(amidinate) zirconium complexes such as
[CyNC(Me)NCy]2ZrCl2 [2.426(3)–2.436(3)Å] [23] and [Me3SiNC(C6H5)
NSiMe3]2ZrCl2 [2.401 Å] [24], but slightly shorter than those in Cp2ZrCl2
[2.44 Å]. The bond angle of Cl\Zr\Cl [93.38(4)°] in 1 is smaller than
the corresponding values in Cp2ZrCl2 [97.1°], much smaller than in
[Me3SiNC(C6H5)NSiMe3]2ZrCl2 [103.71°], but comparable to that in
[CyNC(Me)NCy]2ZrCl2 [93.1(1)°].
A further comparison of the structures of 1, bis(amidinate) zirco-
nium complex [CyNC(Me)NCy]2ZrCl2 (I) [23] and bis(guanidinate)
zirconium complex [CyNC(N\iPr2)NCy]2ZrCl2 (II) [25] featured C2
symmetric structure with a κ2-bonded ligand is made in Fig. 2. The
bond angle N\C\N of the planar four-membered chelating ring
N\C\N\Zr in 1 is slightly wider than those in I and II. The C\N
and N\Zr bond distances, and N\Zr\N bond angle in 1 are compara-
ble to those in I and II. The larger Cbridge\N\SiMe3 than Cbridge\N\Cy
angle and the smaller Zr\N\SiMe3 than Zr\N\Cy angle in 1 suggest
the steric interactions of the SiMe3 and Ph substituents.
In conclusion,
a new bis(amidinate) zirconium complex
[{CyNC(Ph)(NSiMe3)}2ZrCl2] (Cy=cyclohexyl) has been readily pre-
pared via the reaction of freshly prepared lithium amidinate with zirco-
nium tetrachloride. Structural studies confirm the C2 symmetric feature
of complex 1. Complex 1 represents a novel potential precursor of group
4 free of Cp ligands. Upon treatment with MAO, complex 1 showed
moderate activity of 6.66×104 g PE/mol Zr h in the polymerization of
ethylene.
Acknowledgments
The authors acknowledge the financial support of the Natural
Science Foundation of China (No. 20672070), Shanxi International
Education Exchange Association and Shanxi Scholarship Council of
China (No. 201012).
Polymerization reactions were carried out with complex 1 using
methylalumoxane (MAO) as a cocatalyst. The polymerization data
Fig. 2. Structural comparison of 1 and its four electron-donating analogs.

Q. Yang et al. / Inorganic Chemistry Communications 29 (2013) 1–3
3
(%): C, 54.20; H, 7.11; N, 7.90. Found: C, 53.96; H, 7.01; N, 8.12. 1H NMR (C6D6):
δ 0.27 (s, 18 H, SiMe3), 0.94–0.98 (m, 4 H, Cy), 1.17–1.24 (m, 2 H, Cy), 1.28–1.33
(m, 2 H, Cy), 1.70 (s, 4 H, Cy), 2.01–2.05 (m, 4 H, Cy), 2.34–2.47 (m, 4 H, Cy), 3.27
(m,2 H, Cy), 7.08–7.25 (m, 10 H, Ph). 13C NMR (C6D6): δ 2.03 (SiMe3), 25.55,
25.71, 34.28, 34.54 (CH2/Cy), 58.78 (CH/Cy), 126.14 (m-CPh), 128.64 (p-CPh),
129.46 (o-CPh), 135.1 (Cipso-Ph), 180.84 (NCN). IR (KBr, cm−1): 3211, 3180,
3062, 2937, 2858, 1666, 1622, 1581, 1494, 1448, 1402, 1371, 1244, 933, 894, 837,
781, 750, 698. UV–vis (THF): λmax, nm=230.
Appendix A. Supplementary data
CCDC 907112 contains the supplementary crystallographic data
for 1. These data can be obtained free of charge via http://www.
ccdc.cam.ac.uk/conts/retrieving.html, or from the Cambridge Crystal-
lographic Data Center, 12 Union Road, Cambridge CB2 1EZ, UK; fax:
(+44) 1223-336-033; or e-mail: deposit@ccdc.cam.ac.uk.
[19] Crystal data for 1: C32H50Cl2N4Si2Zr, M=709.06, Monoclinic, space group P21/c,
T=200 K, a=18.4197(14) Å, b=9.1091(7) Å, c=22.0430(17) Å, β=96.142(2) °
V=3677.3(5) Å3, Z=4, Fooo=1488, GOF=1.068, ρcalcd.=1.281 gcm−3, crystal
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[18] Preparation of 1: PhCN (0.44 ml, 4.23 mmol) was added to a solution of CyN(Li)
SiMe3 (0.75 g, 4.23 mmol) in Et2O (30 cm3) at −78 °C. The resulting mixture was
warmed to ca. 25 °C and stirred overnight. The above solution was added to a
solution of ZrCl4 (0.49 g, 2.12 mmol) in Et2O (10 cm3) at −78 °C. The mixture
was warmed to ca. 25 °C and stirred for 24 h, then filtered. The filtrate was
concentrated in vacuo and stored at −25 °C for several days, yielding colorless
crystals of 1 (1.17 g, 77.8%). Mp 240–242 °C. Anal. calcd. for C32H50Cl2N4Si2Zr