Synthesis, structure and catalytic properties of a novel zirconium guanidinato complex [Zr{ArNC(NMe2)N(SiMe3)}(μ2-Cl)Cl2]2[Ar = 2,6-iPr2-C6H3]

Article abstract of DOI:10.1016/j.inoche.2007.08.001

A novel non-symmetric zirconium guanidinato complex, [{Zr{ArNC(NMe₂)N(SiMe₃)}(μ₂-Cl)Cl₂}₂] (Ar = 2,6-ⁱPr₂-C₆H₃), was synthesized and structurally characterized. Catalytic studies showed that the zirconium complex was active for ethylene polymerization with the activity of 4.98 × 10⁵ g PE/mol Zr h. The influences of cocatalysts, Al/Zr molar ratios and ethylene pressures on the activities were investigated.

Full text of DOI:10.1016/j.inoche.2007.08.001

Available online at www.sciencedirect.com
Inorganic Chemistry Communications 10 (2007) 1262–1264
www.elsevier.com/locate/inoche
Synthesis, structure and catalytic properties of a novel zirconium
guanidinato complex [Zr{ArNC(NMe2)N(SiMe3)}
(l2-Cl)Cl2]2[Ar = 2, 6-iPr2-C6H3]
a,
b
a
b
a,
*
Meisu Zhou *, Shu Zhang , Hongbo Tong , Wen-Hua Sun , Diansheng Liu
a
Institute of Applied Chemistry, Shanxi University, Taiyuan 030006, PR China
State Key Laboratory of Engineering Plastics and Center for Molecular Sciences, Institute of Chemistry,
Chinese Academy of Sciences, Beijing 100080, PR China
b
Received 22 May 2007; accepted 4 August 2007
Available online 10 August 2007
Abstract
A novel non-symmetric zirconium guanidinato complex, [{Zr{ArNC(NMe2)N(SiMe3)}(l2-Cl)Cl2}2] (Ar = 2,6-iPr2-C6H3), was syn-
thesized and structurally characterized. Catalytic studies showed that the zirconium complex was active for ethylene polymerization with
the activity of 4.98 · 105 g PE/mol Zr h. The influences of cocatalysts, Al/Zr molar ratios and ethylene pressures on the activities were
investigated.
ꢀ 2007 Elsevier B.V. All rights reserved.
Keywords: Zirconium complex; Guanidinate; Catalysis; Ethylene polymerization
Major advances have been achieved in the polymeriza-
tion of a-olefins by well-defined or single-site catalysis [1].
In searching for alternatives to cyclopentadienyl-based
ligands in early transition metal chemistry, bidentate N-
donor ligands have drawn increased attention in recent
years [2,3] and have been studied as active catalysts for
the polymerization of ethylene [4–6]. In this regard, we
were attracted to N-substituted guanidinato ligands with
a trigonal planar CN3 unit, as potential bulky supporting
ligands for group 4 metals. These ligands present a fine-
tuned system for exploring the effects of making rational
modifications to both the steric bulk and electronic proper-
ties [7], and play important roles in evaluating the activities
of their complexes at the stages of the proceeding catalytic
system in order to create the high active species. Our long
term aim is to gain access to a range of nitrogen-based
transition metal complexes as potential catalysts for ethyl-
ene polymerization. The focus was on developing diversi-
fied ligation modes with varied organic substituents and
introducing varied functions in the conjugated NCN or
NCCCN moieties. Herein, we report the synthesis, crystal
structure and catalytic properties of a new complex,
[{Zr{ArNC(NMe2)N(SiMe3)}(l2-Cl)Cl2}2] (Ar = 2,6-iPr2-
C6H3) (1) [Eq. (1)]
R
N
N
Cl
Zr
Cl
i) nBuLi, hexane, 0 °C
Cl
Zr
Cl
Cl
NHR
Me2N
NMe2
ii) Me2NCN, Et2O, -78 °C
iii) ZrCl4, -78 °C
N
Cl
N
R
R = SiMe3
1
ð1Þ
The crystalline, colorless zirconium N, N0 – disubstituted
guanidinate 1 was prepared from ArNH(SiMe3) (Ar =
2,6-iPr2-C6H3) after lithiation with n-BuLi and the con-
comitant reaction with dimethylcyanmide, and further
reaction with zirconium tetrachloride [8]. This involved
insertion of (CH3)2NCN into Li–N bond of Li[N(Si-
Me3)Ar] (Ar = 2,6-iPr2-C6H3) with a 1,3-SiMe3 N ! N0
*
Corresponding authors. Tel.: +86 351 7018091; fax: +86 351 7016048
(M. Zhou).
E-mail addresses: mszhou@sxu.edu.cn (M. Zhou), dsliu@sxu.edu.cn
(D. Liu).
1387-7003/$ - see front matter ꢀ 2007 Elsevier B.V. All rights reserved.
doi:10.1016/j.inoche.2007.08.001

M. Zhou et al. / Inorganic Chemistry Communications 10 (2007) 1262–1264
1263
rearrangement. When ZrCl4 was added to the Et2O solu-
tion of the corresponding ligand at low temperature, the
color of the resulting solution changed from colorless to
pale-yellow with the increase of the temperature. The mix-
ture was stirred at room temperature for 24 h and gave a
clear solution. The volatiles were removed in vacuo and
the residue was extracted into dichloromethane. The ex-
tract was filtered. The filtrate was concentrated in vacuo
yielding single crystals of 1 for X-ray analysis. Complex 1
was very soluble in polar solvents such as CH2Cl2, and
slightly soluble in aliphatic hydrocarbons. 1 was not very
sensitive, no perceptible change was observed after several
hours when exposed to air.
It is noteworthy that the ortho-alkyl substituent on the
phenyl ring played an important role for the formation
of the desired zirconium complex. In the reaction of ZrCl4
and other less bulky analogous guanidinano ligands (L)
without alkyl substituent on the ortho-position of the phe-
nyl ring, L3MCl-type rather than LMCl3-type complexes
were formed with a seven-coordinate configuration around
the zirconium atom [9]. The result may be due to the adja-
cent steric interference between the ortho–isopropyl and the
R group (phenyl).
equatorial plane toward Cl(2). The four-membered chelat-
ing ring N(1)–Zr(1)–N(2)–C(13) is planar (mean deviation
0.01 A), where the bond angles N(1)–Zr(1)–N(2) and
N(1)–C(13)–N(2) are 62.30(18) and 110.2(5)ꢁ, respectively.
The ligand is nearly perpendicular to the N(1)–Cl(1)–
Cl(30)–Cl(3) plane with a dihedral angle of 94.3ꢁ.
˚
For the guanidine moiety, the bond lengths of N(1)–
C(13), N(2)–C(13) and N(3)–C(13) are 1.358(7), 1.352(7)
˚
and 1.346(7) A, respectively. Thus the CN3 fragment can
be rendered as a p-electron delocalized system. In addition,
the dihedral angle formed by the planar NMe2 function
and the Zr(1)–N(1)–C(13)–N(2) plane offers a means of
evaluating the possibility of p overlap between these two
moieties. The torsion angle between these two planes is
23.6ꢁ, which is much smaller than the value of 88.2ꢁ
observed in N-substituted zirconium guanidinate [(iPrNC-
[N(SiMe3)2]NiPr)2ZrCl2] [11], suggest that the steric inter-
action between the phenyl or trimethylsilyl functions and
the dimethylamido group is somewhat limited and is not
big enough to eliminate the p conjugation. In fact, the
sum of 359.9ꢁ for the three bond angles about N atom in
dimethylamido group is consistent with the sp2 hybridiza-
tion necessary for conjugation.
Single-crystal X-ray diffraction analysis [10] reveals that
1 crystallizes in the monoclinic P21/n space group. The
structure of 1 shows a centrosymmetric dinuclear zirco-
nium center (Fig. 1). Each zirconium atom is coordinated
by two nitrogen atoms (N(1) and N(2)) of the ligand, two
terminal chloride atoms (Cl(1) and Cl(2)) and two bridging
chloride atoms (Cl(3) and Cl(30)). The bond lengths of
Zr(1)–N(1), Zr(1)–N(2), Zr(1)–Cl(1), Zr(1)–Cl(2), Zr(1)–
Cl(3) and Zr(1)–Cl(30) are 2.131(5), 2.165(5), 2.382(2),
To demonstrate what factors, and how they may affect
the polymerization of ethylene, the activities of catalyst
precursor 1 were investigated at varied Al/Zr molar ratios,
cocatalysts and ethylene pressures. Table 1 summarized the
catalyst activities obtained for ethylene polymerization cat-
alyzed by complex 1.
The influences of various cocatalysts on the ethylene
activation were evaluated with the catalytic systems formed
from 1 in the presence of methylaluminoxane (MAO),
modified methylaluminoxane (MMAO) or Et2AlCl. The
catalytic system with methylaluminoxane (MAO) showed
higher catalytic activities for ethylene polymerization.
When activated with methylaluminoxane (MAO) under
1 atm of ethylene, complex 1 showed lower activity
(4.48 · 103 g PE/mol Zr h), therefore, 10 atm pressure of
ethylene was employed in the remaining experiments, in
which other reaction parameters were changed.
˚
2.386(2), 2.593(2), 2.613(2) A, respectively. The zirconium
atom adopts a distorted octahedral geometry, in which
the equatorial plane contains N(1), Cl(1), Cl(3) and Cl(30)
˚
atoms (mean deviation 0.0023 A) while the other two
atoms (Cl(2) and N(2)) occupy the axial positions. The
angle of N(2)–Zr(1)–Cl(2) is 147.82 (13)ꢁ (smaller than
˚
180ꢁ), and the zirconium atom locates 0.109 A above the
MAO has been known to play roles in initiating the
polymerization reactions, most likely by creating an empty
site on the catalyst for the insertion of an ethylene mono-
mer. Indeed, we observed that the amount of MAO used
in the complex 1-catalyzed ethylene polymerization reac-
tion was critical for the catalyst to exhibit high activity,
and the optimal Al/Zr ratio was 2000. The activity reached
as high as 4.98 · 105 g PE/mol Zr h with a 2000 Al/Zr
ratio. However, further increase of the Al/Zr ratio
impaired the activity of the catalyst. The explanation for
this observation is that, with the proper ratio of MAO to
the zirconium complex, the formation and stabilization of
the active species is achieved.
The properties of the resulted polymer were affected by
the Al/Zr molar ratio. With the increase of the Al/Zr molar
ratio from 1000 to 4000, the Tm of the resultant polyethyl-
ene changed from 133.6 to 136.7 ꢁC. Meanwhile, the
Fig. 1. Crystal structure of [{Zr{ArNC(NMe2)N(SiMe3)}(l2-Cl)Cl2}2].
Thermal ellipsoids are plotted at 50% probability level. Hydrogen atoms
are omitted for clarity.

1264
M. Zhou et al. / Inorganic Chemistry Communications 10 (2007) 1262–1264
Table 1
Ethylene polymerization catalyzed by complex 1/cocat system
Activity g molꢀ1 hꢀ1
Tm/ꢁCa
Mg (104)
Mw (104)
Mw/Mn
b
c
c
Entry
Cocat
P (atm)
T (ꢁC)
Al/Zr
Yield (mg)
1
2
3
4
5
6
7
8
9
MAO
MAO
MMAO
Et2AlCl
MAO
MAO
MAO
MAO
MAO
MAO
1
20
20
20
20
20
20
20
40
60
80
1000
1000
1000
500
2000
3000
4000
2000
2000
2000
11.2
538.4
228.1
–
1245
815.8
584.0
610
4.48 · 103
2.15 · 105
9.16 · 104
No
4.98 · 105
3.26 · 105
2.34 · 105
2.40 · 105
4.40 · 105
6.48 · 105
132.0
136.7
133.8
–
133.6
134.3
136.0
135.7
134.7
134.3
20.6
42.0
2.5
–
4.4
2.3
18.8
6.6
2.7
1.5
–
–
–
–
–
–
–
–
–
–
–
–
–
–
13
4.9
2.5
10
10
10
10
10
10
10
10
10
72.3
23.5
9.53
1100
1620
10
Conditions: 5 lmol catalyst, 30 ml toluene for 1 atm ethylene pressure, 100 ml toluene for 10 atm ethylene pressure, 0.5 h.
a
Determined by DSC.
b
Measured in decahydronaphthalene at 135 ꢁC using an Ubbelohde viscometer according to ½gꢁ ¼ 62 ꢂ 10ꢀ3M0g:7
.
c
Determined by GPC.
molecular weight of the polymers varied. Increasing the
References
reaction temperature from 20 to 80 ꢁC, the activity initially
decreased and then increased and the Tm of the resultant
polymer decreased from 136.7 to 134.3 ꢁC, which indicated
that the molecular weights of resultant polyethylenes
decreases along with elevated reaction temperature. Such
results are consistent to the molecular weights determined
by both viscosity and GPC (Entries 8–10). In addition,
NMR data showed the formation of linear polyethylenes
[12].
[1] (a) M. Bochmann, J. Chem. Soc., Dalton Trans. (1996) 255;
(b) J. Zhang, X. Wang, G.X. Jin, Coord. Chem. Rev. 250 (2006) 95;
(c) A.L. McKnight, R.M. Waymouth, Chem. Rev. 98 (1998) 2587;
(d) H.G. Alt, A. Ko¨ppl, Chem. Rev. 100 (2000) 1205.
[2] J.F. Li, S.P. Huang, L.H. Weng, D.S. Liu, J. Organomet. Chem. 691
(2006) 3003.
[3] C. Averbuj, E. Tish, M.S. Eisen, J. Am. Chem. Soc. 120 (1998) 8640.
[4] P.B. Hitchcock, M.F. Lappert, P.G. Merle, Dalton Trans. (2007) 585.
[5] M.S. Zhou, S.P. Huang, L.H. Weng, W.-H. Sun, D.S. Liu, J.
Organomet. Chem. 665 (2003) 237.
In summary, a novel zirconium guanidinato complex,
[{Zr{ArNC(NMe2)N(SiMe3)}(l2-Cl)Cl2}2] (Ar = 2,6-iPr2-
C6H3), was synthesized and fully characterized. Upon
treatment with MAO, the complex showed moderate activ-
ity of 4.98 · 105 g (mol of cat.)ꢀ1 hꢀ1 in polymerization of
ethylene. Further detailed investigation, including varying
reaction conditions, modifying the coordinational ligand
and synthesizing hafnium and titanium metal complex ana-
logues, is under way.
[6] J.C. Florez, J.C.W. Chien, M.D. Rausch, Organometallics 14 (1995)
827.
[7] (a) P.J. Bailey, S. Pace, Coord. Chem. Rev. 214 (2001) 91;
(b) F.T. Edelmann, Coord. Chem. Rev. 137 (1994) 403.
[8] (CH3)2NCN (0.17 ml, 2.07 mmol) was added to a solution of (C6H3-
2,6-iPr2)N(Li)SiMe3 (0.53 g, 2.07 mmol) in Et2O (30 cm3) at ꢀ78 ꢁC.
The resulting mixture was warmed to ca. 25 ꢁC and stirred for 12 h.
ZrCl4 (0.48 g, 2.07 mmol) was added at ꢀ78 ꢁC. The resulting mixture
was warmed to ca. 25 ꢁC and stirred for 24 h. The volatiles were
removed in vacuo, and the residue was extracted with dichloromethane
and filtered. The filtrate was concentrated to give colorless crystals
(0.42 g, 39%). Anal. calcd. for C36H64Cl6N6Si2Zr2(%): C, 41.89; H,
6.25; N, 8.14. Found: C, 42.05; H, 6.11; N, 8.01. 1H NMR (CDCl3): d
0.05–0.48(m, 9H, SiMe3), d 1.13–1.23 (m, 12H, CH(CH3)2), d 2.5 (m,
6H, N(CH3)2), d 3.1 (s, CHMe2), d 7.0–7.2 (m, 3H, Ph). 13C NMR
(CDCl3): d 4.02, 5.03, 5.93 (t, SiMe3), d 25.2 (s, CH(CH3)2), d 28.5 (s,
CH(CH3)2), d 42.2 (s, N(CH3)2), d 126.6, 128.4, 129.3, 141.9, 145.7 (s,
Ph), d 158.5 (s, C(C5H5)), d 166 (s, NC(NMe2)N).
Acknowledgements
The authors acknowledge the financial support of the
Natural Science Foundation of China (No. 20672070,
20472046), the Foundation for the Returned Overseas Chi-
nese Scholars (2006, M.S. Zhou), the Natural Science
Foundation of Shanxi (2007011020) and Shanxi Key Lab
for Functional Molecules (No. 20053002).
[9] M.S. Zhou, H.B. Tong, X.H. Wei, D.S. Liu, J. Organomet. Chem.
(2007), in press, doi:10.1016/j.jorganchem.2007.07.051.
[10] Crystal data for 1: C36H64Cl6N6Si2Zr2, M = 1032.25, Monoclinic,
˚
˚
space group P21/n, T = 213 K, a = 10.041(6) A, b = 17.247(13) A,
3
˚
˚
c = 13.727(8) A, b = 91.99(2)ꢁ, V = 2376(3) A , Z = 2, F000 = 1064,
GOF = 1.067, qcalcd. = 1.443 g cmꢀ3, crystal size = 0.20 · 0.15 · 0.10
mm. Data were collected on a Bruker SMART APEX diffractometer/
CCD area detector, using mono-chromated Mo–Ka radiation,
Appendix A. Supplementary material
˚
k = 0.71073 A at 213 K. A total 9698 reflections were collected, of
CCDC 644379 contains the supplementary crystallo-
graphic 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 Crystallographic Data Centre, 12
Union Road, Cambridge CB2 1EZ, UK; fax: (+44) 1223-
336-033; or e-mail: deposit@ccdc.cam.ac.uk. Supplemen-
tary data associated with this article can be found, in the
online version, at doi:10.1016/j.inoche.2007. 08.001.
which 4169 unique reflections (3012 with I > 2r(I)) were for structure
elucidation. The final R1 was 0.691 for I > 2r(I) and 0.1035 for all
reflections. Absorption correction was performed using the multi-scan
method. The structures were solved by direct methods and refined by
full-matrix least squares on F2 using the SHELXTL-97 program
package.
[11] D. Wood, G.P.A. Yap, D.S. Richeson, Inorg. Chem. 38 (1999) 5788.
[12] W. Zuo, W.-H. Sun, S. Zhang, P. Hao, A. Shiga, J. Polym. Sci. A:
Polym. Chem. 45 (2007) 3415.