Diverse coordination behaviors of the silyl-linked bis(amidinate) ligand [SiMe2{NC(Ph)N(Ph)}2]2- to zirconium center

Article abstract of DOI:10.1016/j.poly.2009.08.029

The reactions between the silyl-linked bis(amidinate) ligand [SiMe₂{NC(Ph)N(Ph)Li(thf)₂}₂] and ZrCl₄ in different stoichiometric chemistry, 2:1, 1:1, and 1:2, were explored. The resulting zirconium compounds, 1, 2, and 3, showed different molecular structural features which were confirmed by X-ray crystallographic analysis. The metal center in 1 was located in a highly saturated coordination environment with four amidinate moieties. Compound 2 exhibited an unprecedented linked tris(amidinate) N-C-N-Si-N-C-N-Si-N-C-N skeleton formed after the intermolecular rearrangement. In compound 3, a novel silyl-linked imino-amidinate ligand via the intramolecular rearrangement was involved. The mechanism for the formation of compounds 2 and 3 was proposed. Compound 2 was found to be a highly active catalyst for ethylene polymerization in the presence of methylalumoxane (MAO).

Full text of DOI:10.1016/j.poly.2009.08.029

Polyhedron 29 (2010) 262–269
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Polyhedron
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Diverse coordination behaviors of the silyl-linked bis(amidinate) ligand
[SiMe2{NC(Ph)N(Ph)}2]2À to zirconium center
*
*
Sheng-Di Bai , Hong-Bo Tong, Jian-Ping Guo, Mei-Su Zhou, Dian-Sheng Liu , Shi-Fang Yuan
Institute of Applied Chemistry, Shanxi University, Taiyuan, Shanxi 030006, PR China
a r t i c l e i n f o
a b s t r a c t
Article history:
Available online 15 September 2009
The reactions between the silyl-linked bis(amidinate) ligand [SiMe2{NC(Ph)N(Ph)Li(thf)2}2] and ZrCl4 in
different stoichiometric chemistry, 2:1, 1:1, and 1:2, were explored. The resulting zirconium compounds,
1, 2, and 3, showed different molecular structural features which were confirmed by X-ray crystallo-
graphic analysis. The metal center in 1 was located in a highly saturated coordination environment with
four amidinate moieties. Compound 2 exhibited an unprecedented linked tris(amidinate) N–C–N–Si–N–
C–N–Si–N–C–N skeleton formed after the intermolecular rearrangement. In compound 3, a novel silyl-
linked imino-amidinate ligand via the intramolecular rearrangement was involved. The mechanism for
the formation of compounds 2 and 3 was proposed. Compound 2 was found to be a highly active catalyst
for ethylene polymerization in the presence of methylalumoxane (MAO).
Keywords:
Zirconium
Silyl-linked
Bis(amidinate)
Tris(amidinate)
Rearrangement
Ó 2009 Elsevier Ltd. All rights reserved.
1. Introduction
mononuclear complexes analogous to the ‘‘ansa-metallocene”,
leading to intriguing geometric structures and properties. We next
Amidinate ligands are four-electron, monoanionic, and N-donor
bidentate chelates. They demonstrate a great diversity by variation
of substituents on the conjugated N–C–N backbone. Their steric
and electronic properties are easily tunable to meet the require-
ments of different metal centers. The coordination chemistry of
amidinates toward metals from across the periodic table was well
established in the past few decades, and amidinate complexes
proved to be productive [1,2]. They were found to be potential cat-
alysts for the oligomerization and polymerization of olefins, for
hydroamination, intramolecular hydroamination/cyclization, and
hydrosilylation [2–8]. Moreover, some of them, such as the
bis(amidinate) group IV metal complexes I, showed high perfor-
mance in the activation and reduction of dinitrogen or other mol-
ecules [4,7].
attempted to synthesize a series of compounds in the configuration
of type II. By comparing II with I (shown in Scheme 1), the different
properties resulting from bridging could be investigated. To our
surprise, the seemingly easy pathway from the silyl-linked
bis(amidinate) ligand [SiMe2{NC(Ph)N(Ph)Li(thf)2}2] and ZrCl4
could not afford the expected product as II. It was found that the
N–C–N–Si–N–C–N backbone of the ligand was both chelating and
labile toward the Zr center. Herein, we report the special intra-
and intermolecular rearrangements of the title linked bis(amidi-
nate) ligand.
2. Experimental
2.1. General considerations
In recent years, an increasing interest of metal complexes incor-
porated with the bianionic-linked bis(amidinate) ligands
[4f,4g,6h,7d,7h,9] arose. For example, a series of mononuclear
and binuclear lanthanide compounds were capable of catalyzing
the ring-opening polymerization of lactones and lactides efficiently
[9j–9m]. Besides those examples reported in the literatures, a spe-
cial class of linked bis(amidinate) ligands [SiMe2{NC(Ph)N(R)}2]2À
were developed in our laboratory [10]. They displayed a close con-
tact between the two amidinate moieties. These unique bianionic
ligands had the advantage of affording binuclear complexes and
All manipulations and reactions were performed under a puri-
fied nitrogen atmosphere using the standard Schlenk techniques
on a dual manifold Schlenk line. Tetrahydrofuran and diethyl ether
were distilled from sodium-benzophenone under nitrogen. Dichlo-
romethane was distilled from CaH2 under nitrogen. The ligand
transfer reagent was prepared according to the reported methods
[10]. 1H and 13C NMR spectra were recorded with a Bruker DRX-
300 spectrometer. Elemental analyses were performed with a
Vario EL-III instrument. Melting points were measured with a
Sanyo Gallenkamp Variable Heater.
* Corresponding authors. Tel./fax: +86 351 7011743 (S.-D. Bai).
E-mail addresses: sdbai@sxu.edu.cn (S.-D. Bai), dsliu@sxu.edu.cn (D.-S. Liu).
0277-5387/$ - see front matter Ó 2009 Elsevier Ltd. All rights reserved.
doi:10.1016/j.poly.2009.08.029

S.-D. Bai et al. / Polyhedron 29 (2010) 262–269
263
2.2.4. [{N(Ph)C(Ph)N}SiMe2{N(Ph)C(Ph)NH}]ZrCl2[N(Ph)C(Ph)NH}] (4)
ZrCl4 (0.49 g, 2.10 mmol) was slowly added into a solution of
[SiMe2{NC(Ph)N(Ph)Li(thf)2}2] (1.57 g, 2.10 mmol) in thf (ca.
30 mL) at À78 °C. The reaction mixture was stirred for half an hour
and PhCN (0.44 mL, 4.20 mmol) was added with a syringe at 0 °C. It
was warmed to room temperature and continued to stir overnight.
The resultant solution was dried in vacuum to remove all volatiles
and the residue was extracted with CH2Cl2 (25 mL). Concentrating
the filtrate to about 3 mL under reduced pressure and making it
stand gave colorless crystals of compound 4. Yield: 1.11 g (59%);
m.p. 124–125 °C. 1H NMR (300 MHz, CDCl3): d = 9.63, 9.46 (d,
NH), 7.92–6.67 (m, 30H; phenyl), 5.32 (s, CH2Cl2), 3.78 (s, OCH2
of thf), 1.88 (s, 3,4-2CH2 of thf), 0.66–À0.02 (m, 6H; SiMe2) ppm.
13C NMR (75 MHz, CDCl3): d = 181.5, 178.4, 167.8 (N–C–N),
149.1–120.4 (phenyl), 56.1, 28.3 (thf), 4.2, 2.9, 2.4 (SiMe2) ppm.
Anal. Calc. for C41H38Cl2N6SiZrÁ(CH2Cl2) (889.91): C, 56.68; H,
4.53; N, 9.44. Found: C, 56.88; H, 4.66; N, 9.28%.
Scheme 1.
2.2. Synthesis of metal complexes
2.2.1. [SiMe2{NC(Ph)N(Ph)}2]2Zr (1)
A solution of [SiMe2{NC(Ph)N(Ph)Li(thf)2}2] (1.67 g, 2.24 mmol)
in thf (ca. 10 mL) was added into a stirred slurry of ZrCl4 (0.26 g,
1.12 mmol) in Et2O (ca. 15 mL) at 0 °C. The reaction mixture was
warmed to room temperature and it continued to stir for 12 h. It
was dried in vacuum to remove all volatiles and the residue was
extracted with CH2Cl2 (25 mL). The concentration of the filtrate un-
der reduced pressure resulted in compound 1 as yellow crystals.
Yield: 0.61 g (53%); m.p. 241–242 °C. 1H NMR (300 MHz, CDCl3):
d = 7.32–6.30 (m, 40H; phenyl), 5.32 (s, 1H; 0.5 CH2Cl2), 0.18–
À0.90 (m, 12H; SiMe2) ppm. 13C NMR (75 MHz, CDCl3): d = 191.6,
172.6 (N–C–N), 148.9–120.8 (phenyl), 53.5 (CH2Cl2), 2.1 (SiMe2)
ppm. Anal. Calc. for C56H52N8Si2ZrÁ(CH2Cl2)0.5 (1026.92): C, 66.08;
H, 5.20; N, 10.91. Found: C, 65.81; H, 5.20; N, 10.87%.
2.3. X-ray data collection and crystal structure determination
The single crystals of 1, 2, 3, and 4 suitable for X-ray diffraction
studies were obtained. X-ray diffraction data were collected with
graphite-monochromated Mo-Ka radiation (k = 0.71073 Å) on a
Bruker Smart Apex CCD diffractometer, equipped with an Oxford
Cryosystems CRYOSTREAM device. The collected frames were pro-
cessed with the proprietary software SAINT and an absorption cor-
rection was applied (SADABS) to the collected reflections [11,12].
The structures of these molecules were solved by direct methods
and expanded by standard difference Fourier syntheses using the
software SHELXTL [13]. Structure refinements were made on F2 using
the full-matrix least-squares technique. All non-hydrogen atoms
were refined with anisotropic displacement parameters. Hydrogen
atoms were placed in their idealized positions and allowed to ride
on the respective parent atoms. Pertinent crystallographic data and
other experimental details are summarized in Table 1.
2.2.2. [{N(Ph)C(Ph)N}SiMe2{NC(Ph)N(Ph)}SiMe2{NC(Ph)N(Ph)}]ZrCl2
(2)
A solution of [SiMe2{NC(Ph)N(Ph)Li(thf)2}2] (1.54 g, 2.05 mmol)
in thf (ca. 10 mL) was added into a stirred slurry of ZrCl4 (0.48 g,
2.05 mmol) in Et2O (ca. 20 mL) at À78 °C. The reaction mixture
was warmed to room temperature and it continued to stir for
12 h. It was dried in vacuum to remove all volatiles and the residue
was extracted with CH2Cl2 (25 mL). CH2Cl2 was removed and tolu-
ene was added. Concentrating the solution to about 3 mL and mak-
ing it stand gave colorless crystals of compound 2. Yield: 0.74 g
(38%); m.p. 255–257 °C. 1H NMR (300 MHz, CDCl3): d = 7.25–6.68
(m, 35H; phenyl), 2.35 (s, 3H; Me of toluene), 0.00, À0.06 (d,
12H; SiMe2) ppm. 13C NMR (75 MHz, CDCl3): d = 178.1, 174.4,
173.8 (N–C–N), 145.3–123.6 (phenyl), 26.3, 22.2 (Me of toluene),
2.5, 1.3 (SiMe2) ppm. Anal. Calc. for C43H42Cl2N6Si2ZrÁ(C7H8)
(953.26): C, 62.99; H, 5.29; N, 8.82. Found: C, 62.21; H, 5.17; N,
8.75%.
2.4. Ethylene polymerization
The polymerizations were performed in a 250 mL autoclave.
Compound 2 was dissolved in toluene and transferred into an auto-
clave with a syringe. The solution of methylalumoxane (MAO) in
toluene was added with a syringe for activation. While stirring,
the ethylene gas was filled into the reactor. After stirring for a def-
inite period of time, the reaction mixture was quenched with 5%
HCl/EtOH. The polymer was filtered, washed with ethanol, and
dried under vacuum.
3. Results and discussion
2.2.3. [{N(Ph)C(Ph)N}SiMe2{N(Ph)C(Ph)NH}]ZrCl3(thf) (3)
3.1. Analogous reactions about titanium
A solution of [SiMe2{NC(Ph)N(Ph)Li(thf)2}2] (0.96 g, 1.28 mmol)
in thf (ca. 10 mL) was added into a stirred slurry of ZrCl4 (0.60 g,
2.57 mmol) in Et2O (ca. 25 mL) at 0 °C. The reaction mixture was
warmed to room temperature and it continued to stir for 12 h. It
was dried in vacuum to remove all volatiles and the residue was
extracted with CH2Cl2 (25 mL). CH2Cl2 was removed and toluene
was added. Concentrating the solution to about 3 mL and making
it stand gave pale yellow crystals of compound 3. Yield: 0.78 g
(75%); m.p. 165–166 °C. 1H NMR (300 MHz, CDCl3): d = 9.58, 9.40
(d, 1H; NH), 7.52–6.63 (m, 25H; phenyl), 4.18 (br, 4H; OCH2 of
thf), 2.34 (s, 3H; Me of toluene), 1.58 (br, 4H; 3,4-2CH2 of thf),
0.11 (d, 6H; SiMe2) ppm. 13C NMR (75 MHz, CDCl3): d = 137.9–
123.2 (phenyl), 25.1, 21.4 (Me of toluene), À0.3 (SiMe2) ppm. Anal.
Calc. for C28H27Cl3N4SiZrÁ(C4H8O, C7H8) (809.43): C, 57.87; H, 5.35;
N, 6.92. Found: C, 57.53; H, 5.36; N, 6.89%.
The short distance between two amidinate moieties is the most
noted structural feature in this series of ligands. On the other hand,
the presence of the shared silyl-bridge brings the seven-membered
N–C–N–Si–N–C–N skeleton two labile N–Si bonds. N–Si bond
exhibited
a significant function in the reaction of [SiMe2
{NC(Ph)N(Ph)Li(thf)2}2] and TiCl4 (shown in Scheme 2) [14]. At-
tempts of preparing a titanium linked bis(amidinate) compound
were unsuccessful and the only isolated product was characterized
as an imido titanium species, suggesting that the original tetraden-
tate N–C–N–Si–N–C–N backbone was broken into small N–C–N
fragments at the silyl-bridge. In addition to the vulnerable site of
silyl-bridge, another factor of the ligand dissociation was supposed
to be the high Lewis acidity of TiCl4. More recently, the reaction be-
tween [SiMe2{NC(Ph)N(Ph)Li(thf)2}2] and TiCl3(C5H5) was investi-

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S.-D. Bai et al. / Polyhedron 29 (2010) 262–269
Table 1
Crystal data and refinement results for compounds 1, 2, 3, and 4.
Compound
1
2
3
4
Empirical formula
Molecular mass
Crystal system
Space group
a [Å]
C56.50H53ClN8Si2Zr
1026.92
C50H50Cl2N6Si2Zr
953.26
C39H43Cl3N4OSiZr
809.43
C42H40Cl4N6SiZr
889.91
triclinic
triclinic
triclinic
triclinic
ꢀ
ꢀ
ꢀ
ꢀ
P1
P1
P1
P1
10.7678(17)
10.9464(18)
21.668(4)
90.337(2)
90.605(2)
90.305(2)
2553.8(7)
2
13.8382(8)
14.0886(8)
14.2529(8)
87.8280(10)
67.4630(10)
74.3530(10)
2464.7(2)
2
11.658(2)
12.535(2)
14.725(3)
95.924(3)
105.488(3)
108.075(3)
1930.3(6)
2
12.7709(17)
13.5973(18)
14.5664(19)
88.071(2)
69.612(2)
68.647(2)
2195.6(5)
2
b [Å]
c [Å]
a
[°]
b [°]
c
[°]
V (Å3)
Z
F(0 0 0)
1066
1.335
0.361
988
1.284
0.420
836
1.393
0.560
912
1.346
0.557
Dcalcd [g cmÀ3
]
V(Mo K
a
) [mmÀ1
]
Reflections collected
Unique reflections
12 490
8808
16 708
11 681
8002
6649
9123
7590
Observed reflections
6772
1.033
6745
0.935
4223
0.870
6045
1.031
Goodness-of-fit (GOF) on F2
R1, wR2 [I > 2
r
(I)]
0.0601, 0.1391
0.0776, 0.1478
1.036 and À0.739
0.0530, 0.1170
0.1067, 0.1389
0.751 and À0.588
0.0513, 0.0998
0.0804, 0.1078
0.838 and À0.451
0.0445, 0.1300
0.0551, 0.1360
0.892 and À0.410
R1, wR2 (all data)
Largest difference in peak and hole [e ÅÀ3
]
Scheme 2.
gated for comparison [15]. The ligand was found to undergo a rear-
rangement process from the linked bis(amidinate) configuration to
the linked imido-amidinate configuration though N–C–N–Si–N–C–
N backbone was maintained. Furthermore, [SiMe2{NC(Ph)N(Ph)-
Li(thf)2}2] was treated with TiCl2(C5H5)2 and it gave a titanium
compound with original ligand [16]. Comparing the analogous
reactions, the same ligand tended to be ‘‘stable” as the Lewis acid-
ity of titanium salts decreased with coordination of cyclopentadi-
enyl. Consequently, it was believed that the acid–base balance
between the metal salt and the linked bis(amidinate) ligand is a
crucial factor in the corresponding reactions.
angles, which are 57°, 65°, and 57°, respectively. It is interesting to
note that the two half cycles are perpendicular to each other and
their terminal atoms N(1), N(4), N(5), and N(8) are planar. The line
of Si(1)–Zr(1)–Si(2) acts as the vertical axis for that plane. The bulk
of this linked bis(amidinate) ligand is small, which could be in-
3.2. Synthesis of zirconium complexes and crystal structure
descriptions
The reactions between [SiMe2{NC(Ph)N(Ph)Li(thf)2}2] and ZrCl4
were supposed to be mild because the Lewis acidity of ZrCl4 is also
lower than that of TiCl4. Scheme 3 shows the synthetic routes for a
series of zirconium compounds. Lithium compound was treated
with half an equivalent of zirconium tetrachloride in tetrahydrofu-
ran. It gave compound 1 as yellow crystals after extraction with
CH2Cl2 and crystallization. The molecular structure of 1 is shown
in Fig. 1. The zirconium center is wrapped by two ligands with
eight Zr–N bonds in a tetradentate fashion, and the bond lengths
of Zr–N bonds are in the range of 2.25–2.37 Å, demonstrating the
half cycle configuration and affording three adjacent N–Zr–N bond
Scheme 3.

S.-D. Bai et al. / Polyhedron 29 (2010) 262–269
265
Fig. 1. ORTEP drawing (30% probability level) of the molecular structure of compound 1, and hydrogen atoms were omitted for clarity.
ferred through the crowded coordination environment of the metal
center.
three Zr–N bonds are in the range of 2.20–2.25 Å. Three chlorine
atoms are maintained. Atoms Cl(1), Zr(1), and Cl(2) are closely lin-
ear. This line is perpendicular to Zr(1)–Cl(3). In the presence of a
tetrahydrofuran molecule, the Zr adopts a similar coordination
configuration of pentagon bipyramid as that in compound 2. It
should be pointed out that only 20% excess of zirconium tetrachlo-
ride could also yield the same result as compound 3. Therefore, the
excessive zirconium salt beyond one equivalent was supposed to
be playing the catalytic function rather than the synthetic function.
Regarding an organometallic compound with good catalytic
properties, the corresponding metal center should be in low coor-
dination saturation and be electronically deficient. Obviously, com-
pound 1 could not meet the need. The model of II may be qualified.
Therefore, the ligand transfer reagent was treated with one equiv-
alent of zirconium tetrachloride. However, the attempt to obtain
compound as II was unsuccessful in this way. After being worked
up in toluene, it gave a colorless crystalline compound 2. The X-
ray crystallography determination revealed 2 as an interesting
structure (shown in Fig. 2). An unprecedented linked tris(amidi-
nate) ligand was formed, and it coordinated to Zr. The original
N–C–N–Si–N–C–N backbone is prolonged, resulting in an 11-mem-
bered chain N–C–N–Si–N–C–N–Si–N–C–N. It bends around the
metal center nearly to be a cycle and provides five nitrogen atoms
as donor sites. The two open ends of the chain bind the metal cen-
ter with an angle of 95.04(9)°. Two terminal amidinate moieties
coordinate to Zr in a chelating mode. The related bite angles are
over 57°, being 2° bigger than the corresponding values in com-
pound 1. The linked tris(amidinate) ligand and the two remaining
chlorine atoms form a pentagon bipyramid environment for Zr.
Regarding the ease for the linked bis(amidinate) ligands to sup-
port the binuclear species, the ligand transfer reagent was treated
with two equivalents of zirconium tetrachloride. In this case, a pale
yellow crystalline compound 3 was obtained. The structure of 3
(shown in Fig. 3) demonstrates a mononuclear species. Though
the ligand around the Zr has the same N–C–N–Si–N–C–N backbone
as the lithium precursor, 3 has been converted as a linked imino-
amidinate complex. On both sides of the silyl-bridge, two N–C–N
moieties exhibit different coordination styles to the metal center:
one is ionic chelating and the other is neutral mono-dentate. The
3.3. Rearrangement reactions and proposed mechanism
As is apparent from the above-mentioned reactions, different
stoichiometries of the ligand to metal salt lead to different prod-
ucts. Moreover, it is noteworthy that compounds 2 and 3 are not
coordinated with the starting linked-bis(amidinate) ligand and
they are not consistent with the stoichiometries of their parent
reactants. It is convincing that rearrangement reactions occur.
The proposed mechanism for the formation of 2 and 3 is outlined
in Scheme 4: (1) It forms the ‘‘normal” compound, a transition
state a in the same configuration as II after combination of the par-
ent reactants. (2) The linked bis(amidinate) species a transforms
into the linked imido-amidinate species b, which is another transi-
tion state (the intramolecular rearrangement process is induced by
the active metal center and has been reported in previous work
[15]). (3) The ligand in b is polarized and the terminal imido nitro-
gen is more nucleophilic in the absence of hindrance of the phenyl;
then intermolecular rearrangement is initiated with nucleophilic
attack of the imido nitrogen toward the bridging silicon atom of
another molecule, which leads to the breakage of N–Zr and N–Si
bonds, formation of N–Zr and N–Si bonds as well as displacement
of N–C double bonds. (4) It gives compound 2 and c (c is presumed

266
S.-D. Bai et al. / Polyhedron 29 (2010) 262–269
Fig. 2. ORTEP drawing (30% probability level) of the molecular structure of compound 2, and hydrogen atoms were omitted for clarity.
Fig. 3. ORTEP drawing (30% probability level) of the molecular structure of compound 3, and hydrogen atoms were omitted for clarity.

S.-D. Bai et al. / Polyhedron 29 (2010) 262–269
267
Scheme 4.
according to titanium species in scheme 2). (5) In case ZrCl4 is
excessive, b will strongly incline to be protonated at the imino
nitrogen to form compound 3 instead of intermolecular attacking
because a high Lewis acidic environment is provided.
tetrachloride were combined. Half an hour later, two equivalents
of benzonitrile was added. It was proposed that the imido nitrogen
atom could attack the benzonitrile instead of another molecule. On
the other hand, benzonitrile could increase the Lewis basicity of
the system as neutral ligand coordinating to Zr. However, the ob-
tained compound named 4 was not the expected product. Com-
pound 4 displays the pentagon bipyramid configuration as 2 and
3. Its molecular structure is illustrated in Fig. 4 and is nearly similar
to that of 2, except that it lacks a silyl-bridge. The tridentate ligand
in 4 is identical to that in 3. Additionally, the separate N–C–N moi-
ety in 4 is related to the titanium compound shown in Scheme 2,
and both are substantial proofs for the proposed byproduct c.
An interesting question has arisen. What is the driving force for
intra- and intermolecular rearrangement? It is believed that the
combination of the following factors, including Lewis basicity of
the ligand, Lewis acidity of the Zr, and lability of the N-Si bond,
forces the transformation process. For example, when treating
the ligand and ZrCl4 in the ratio of 2:1 to prepare compound 1,
the Lewis basicity of the ligand is predominant and the metal cen-
ter is tightly wrapped, losing the catalyzing ability. On the con-
trary, when the ratio of the ligand to ZrCl4 is not more than 1:1,
the Lewis acidity is not ‘‘neutralized” completely. The electron defi-
cient Zr will ‘‘catalyze” the breakage of the N–Si bond and rear-
rangement of the ligand to give b. The degree of Lewis acidity in
the reaction mixture will determine the intermolecular rearrange-
ment reaction or addition reaction for b. Consequently, compounds
2 and 3 were obtained, respectively.
As for the transition state b, its linked imido-amidinate ligand
contains an active imido nitrogen atom. In order to trap this tran-
sition compound, the reaction route shown in Scheme 5 was con-
trived. One equivalent of ligand and one equivalent of zirconium
Scheme 5.

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S.-D. Bai et al. / Polyhedron 29 (2010) 262–269
Fig. 4. ORTEP drawing (30% probability level) of the molecular structure of compound 4, and hydrogen atoms were omitted for clarity.
Table 3
The formation of 4 especially for the separate N–C–N moiety is not
clear and is under investigation. Nevertheless, it reflects the high
accordance with 2 and 3, obeying the rule of different conditions
leading to different products. The selected structural data of this
series of zirconium compounds are listed in Table 2.
Ethylene polymerization by the 2/MAO system.a
Entry
T [°C]
P [atm]
Productivityb
1
2
3
4
5
30
50
70
30
50
1
1
1
10
10
no
trace
trace
438
1043
3.4. Catalytic activity for ethylene polymerization of compound 2
a
Reaction conditions: 5 lmol catalyst, MAO/catalyst = 1000, time = 30 min, tol-
uene solvent (100 mL).
The catalytic activity of compound 2 for ethylene polymeriza-
tion was examined under a variety of conditions. The preliminary
results are listed in Table 3. It shows that the activity of compound
2 is highly dependent on pressure. At normal pressure, compound
2 was found to be an inactive catalyst for ethylene polymerization
b
kg(PE) molÀ1(Zr) hÀ1
.
at 30 °C. Only trace amount of polymer formation was observed
even when heating the reaction system to 70 °C. The reaction
Table 2
Selected bond lengths (Å) and bond angles (°) for compounds 1, 2, 3, and 4.
1
2
3
4
Zr(1)–N(1)
Zr(1)–N(2)
Zr(1)–N(3)
Zr(1)–N(4)
N(1)–C(7)
N(2)–C(7)
Si(1)–N(2)
N(2)–Zr(1)–N(1)
N(3)–Zr(1)–N(2)
N(4)–Zr(1)–N(1)
N(5)–Zr(1)–N(1)
N(8)–Zr(1)–N(1)
N(4)–Zr(1)–N(5)
N(8)–Zr(1)–N(4)
N(3)–Zr(1)–N(4)
N(1)–C(7)–N(2)
N(3)–Si(1)–N(2)
2.370(3)
2.273(3)
2.253(3)
2.342(3)
1.333(5)
1.333(5)
1.717(3)
57.08(11)
65.00(11)
175.67(11)
89.10(11)
90.38(11)
88.88(11)
91.93(11)
57.23(11)
112.8(3)
90.64(15)
Zr(1)–N(1)
Zr(1)–N(2)
Zr(1)–N(3)
Zr(1)–Cl(1)
N(1)–C(7)
N(2)–C(7)
Si(2)–N(4)
Si(2)–N(5)
N(2)–Zr(1)–N(1)
N(3)–Zr(1)–N(2)
N(5)–Zr(1)–N(6)
Cl(2)–Zr(1)–Cl(1)
N(6)–Zr(1)–N(1)
N(1)–C(7)–N(2)
N(3)–Si(1)–N(2)
N(3)–C(16)–N(4)
N(5)–Si(2)–N(4)
2.316(3)
2.172(3)
2.336(3)
2.4847(11)
1.326(4)
1.336(4)
1.797(3)
1.694(3)
59.08(10)
66.99(10)
59.56(10)
171.66(4)
95.04(9)
112.7(3)
91.34(13)
122.2(3)
103.62(13)
Zr(1)–N(1)
Zr(1)–N(2)
Zr(1)–N(4)
Zr(1)–O(1)
Zr(1)–Cl(1)
N(1)–C(7)
N(2)–C(7)
Si(1)–N(2)
2.255(3)
2.203(3)
2.246(3)
2.284(3)
2.4776(12)
1.312(5)
1.331(5)
1.715(3)
1.790(3)
59.09(12)
76.15(12)
178.98(5)
90.95(5)
89.96(5)
91.70(9)
112.6(4)
103.26(16)
Zr(1)–N(1)
Zr(1)–N(2)
Zr(1)–N(4)
Zr(1)–Cl(1)
N(1)–C(7)
N(2)–C(7)
Si(1)–N(2)
Si(1)–N(3)
N(1)–Zr(1)–N(2)
N(4)–Zr(1)–N(2)
N(4)–Zr(1)–N(5)
N(6)–Zr(1)–N(1)
Cl(2)–Zr(1)–Cl(1)
N(1)–Zr(1)–Cl(1)
N(6)–Zr(1)–Cl(1)
N(1)–C(7)–N(2)
N(3)–Si(1)–N(2)
2.223(3)
2.225(2)
2.266(2)
2.4857(9)
1.321(4)
1.335(4)
1.710(3)
1.800(3)
59.39(9)
79.60(9)
79.90(9)
81.99(10)
174.30(3)
91.37(8)
93.62(9)
112.1(3)
104.34(12)
Si(1)–N(3)
N(2)–Zr(1)–N(1)
N(4)–Zr(1)–N(2)
Cl(2)–Zr(1)–Cl(1)
Cl(1)–Zr(1)–Cl(3)
Cl(2)–Zr(1)–Cl(3)
N(1)–Zr(1)–Cl(1)
N(1)–C(7)–N(2)
N(2)–Si(1)–N(3)

S.-D. Bai et al. / Polyhedron 29 (2010) 262–269
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269
was then performed at 10 atm. It resulted in an abrupt increase of
the polymerization activity, which reached 438 kg(PE) molÀ1(Zr)
hÀ1 at 30 °C. The amount of polymer was far doubled at 50 °C,
which suggested that compound 2 was also a temperature-pro-
moted catalyst.
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Four zirconium compounds derived from a linked bis(amidi-
nate) ligand were obtained. These compounds are in high coordi-
nation state to meet the need of Zr. They were found to be
closely related to the Lewis acidic/basic environment resulting
from the reactants. In the cases of the linked bis(amidinate) ligand
being deficient, rearrangement reactions will take place to
compensate the coordination number of metal center. The trans-
formations could happen because of the ligand containing labile
N–Si bond. The group IV metal complexes with this series of
linked bis(amidinate) ligands are potential catalysts for olefin
polymerization.
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Supplementary data
CCDC Nos. 623283, 623284, 623285, and 623286 contain the
supplementary crystallographic data for 1, 2, 3, and 4. These data
can be obtained free of charge via http://www.ccdc.cam.ac.uk/con-
ts/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.
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We are grateful for the financial support from the Natural Sci-
ence Foundation of China (20702029 to S.-D. Bai, 20872084 to J.-
P. Guo and 20772074 to D.-S. Liu) and the Natural Science Founda-
tion of Shanxi Province (2008011024 to S.-D. Bai).
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