Synthesis, characterization, and polymerization of a neutral tantalacarborane sandwich complex derived from a pentaanionic exo-polyhedrally linked bis(C2B10-carborane) ligand

Article abstract of DOI:10.1021/om800516x

Tetramethyldisilazane-bridged closo-carborane compounds, [(closo-1-R-1,2-C₂B₁₀H₁₀)Me₂Si] ₂NH (R = Me (1), Ph (2)), and their tantalum complexes, [(closo-2-R-2,3-C₂B₁₀H₁₀

Full text of DOI:10.1021/om800516x

60
Organometallics 2009, 28, 60–64
Synthesis, Characterization, and Polymerization of a Neutral
Tantalacarborane Sandwich Complex Derived from a Pentaanionic
Exo-Polyhedrally Linked Bis(C2B10-carborane) Ligand
Zhu Yinghuai,*,† Lee Cjin Nong,† Li Chuan Zhao,† Effendi Widjaja,† Chong Siow Hwei,†
Wang Cun,† Jozel Tan,† Martin Van Meurs,† Narayan S. Hosmane,‡ and John A. Maguire§
Institute of Chemical and Engineering Sciences, 1, Pesek Road, Jurong Island, Singapore 627833,
Department of Chemistry and Biochemistry, Northern Illinois UniVersity, DeKalb, Illinois 60115-2862, and
Department of Chemistry, Southern Methodist UniVersity, Dallas, Texas 75275-0314
ReceiVed June 5, 2008
Tetramethyldisilazane-bridged closo-carborane compounds, [(closo-1-R-1,2-C2B10H10)Me2Si]2NH
(R ) Me (1), Ph (2)), and their tantalum complexes, [(closo-2-R-2,3-C2B10H10)Me2Si]2N]Ta(V) (R )
Me (3), Ph (4)), have been prepared and characterized. Compounds 3 and 4 were found to be active
catalysts for the polymerization of ethylene in the presence of the cocatalyst MAO to produce two
morphologically different polyethylenes with high molecular weight.
ligands coordinated to group 4 compounds (Figure 2a) are active
precatalysts for olefin polymerization.13,14 Herein we report the
syntheses of tetramethyldisilazane-bridged closo-carboranes, a
novel pentaanionic precursor, and its conversation to a neutral
tantalum(V) metallacarborane (Figure 2b). The tantalacarboranes
were found to be active precatalysts for olefin polymerization.
1. Introduction
It is well-known that metallocene complexes, such as
sandwich and half-sandwich group 4 compounds, play important
roles in ethylene polymerization, due to their ability to produce
stereospecific polymers.1-7 The design and synthesis of struc-
turally unique metallocenes are still attracting a great deal of
attention.6,7 Such complexes can be used as “single site”
catalysts for homo- or copolymerization of polar olefins, such
as styrene, methyl methacrylate (MMA), vinyl chloride, etc.,
to produce functionalized polymers.6,7 It is well recognized that
the nido-carborane dianions [R2C2B9H9]2-, [R2C2B4H4]2-, and
[R2C2B10H10]2- (R ) H, alkyl, or aryl groups) (Figure 1)8-10
are isolable with the cyclopentadienides, in that they have six
π electrons delocalized on the C2B3 or C2B4 open faces of the
dianions that can coordinate metals in an η5 or η6 fashion.
Because of this similarity and the high catalyst activity of
activated metallocenes, the design and synthesis of stable
carborane ligands for use in transition-metal complexes, with
the possibility of high catalytic activity, has been an ongoing
area of research.11-14 It was found that carboranyl trianionic
2. Experimental Section
2.1. General Methods. All operations were carried out under
an argon atmosphere using standard Schlenk techniques or in a
glovebox. The solvents diethyl ether, n-hexane, and tetrahydrofuran
(THF) were dried with sodium. All other reagents were purchased
from Sigma-Aldrich Pte. Ltd. or Katchem Spolo. s.r.o. and used
as received. 1,3-Dichloro-1,1,3,3-tetramethyldisilazane15 and Cl4-
16
TaN(SiMe3)2 were prepared according to literature methods.
Elemental analyses were measured in a Euro EA elemental analyzer
and melting points determined by a Bu¨chi melting point analyzer.
1H, 13C, and 11B NMR were recorded on a Bruker Advance 400
MHz spectrometer. Chemical shifts were measured in ppm relative
to a TMS standard (1H, 400.2 MHz; 13C, 100.6 MHz; 11B, 128.4
MHz). Near-infrared (IR) spectra were measured using a Bio-Rad
spectrophotometer with KBr pellets. Far-infrared spectra were
measured using a Bruker FT-IR Vertex 70 spectrophotometer with
THF film. Molecular weights and molecular weight distributions
of the polymers were determined by means of gel-permeation
chromatography (GPC; Polymer Laboratories, PC-GPC-220) at 160
°C, using 1,2,4-trichlorobenzene stabilized with 0.0125% BHT as
eluent with a column set of 2xPLgel 10 µm MIXED-B (300 × 7.5
mm). The weight average molecular weight and polydispersity index
(Mw and Mw/Mn, respectively) were calculated on the basis of
polystyrene standards: flow rate 1.0 mL/min, injected volume 200
µL. The Scanning electron microscopy (SEM) images were obtained
* To whom correspondence should be addressed. Tel: +65 6796 3801
(3700). Fax: +65 6316 6182,. E-mail: zhu_yinghuai@ices.a-star.edu.sg.
† Institute of Chemical and Engineering Sciences.
‡ Northern Illinois University.
§ Southern Methodist University.
(1) Alt, H. G.; Koppl, A. Chem. ReV. 2000, 100, 1205–1221, and
references therein.
(2) Hopf, A.; Kaminsky, W. Catal. Commun. 2002, 3, 459–464.
(3) Corradini, P.; Guerra, G.; Cavallo, L. Acc. Chem. Res. 2004, 37,
231–241.
(4) Miller, S. A.; Bercaw, J. E. Organometallics 2006, 25, 3576–3592.
(5) Silanes, I.; Mercero, J. M.; Ugalde, J. M. Organometallics 2006,
25, 4483–4490.
(6) Luo, Y.; Baldamus, J.; Hou, Z. J. Am. Chem. Soc. 2004, 126, 13910–
13911.
(7) Jensen, T. R.; Yoon, S. C.; Dash, A. K.; Luo, L.; Marks, T. J. J. Am.
Chem. Soc. 2003, 125, 14482–14494.
(8) Hosmane, N. S.; Maguire, J. A.; Zhu, Y. Main Group Met. Chem.
2006, 5, 251–265, and references therein.
(9) Xie, Z. Acc. Chem. Res. 2003, 36, 1–9, and references therein.
(10) Chui, K.; Yang, Q.; Mak, T. C. W.; Xie, Z. Organometallics 2000,
19, 1391–1401.
(12) Zhu, Y.; Maguire, J. A.; Hosmane, N. S. Inorg. Chem. Commun.
2002, 5, 296–299.
(13) Zhu, Y.; Zhong, Y.; Keith, C.; Maguire, J. A.; Hosmane, N. S. J.
Organomet. Chem. 2005, 690, 2802–2808.
(14) Zhu, Y.; Shirley, L. P. S.; Fethi, C.; Keith, C.; Richard, A. K. J.
Organomet. Chem. 2005, 690, 6284–6291.
(15) Bacque, E.; Pillot, J.-P.; Birot, M.; Dunogues, J. J. Organomet.
Chem. 1994, 481, 167–172.
(11) Zhu, Y.; Vyakaranam, K.; Maguire, J. A.; Qiuintana, W.; Teixidor,
F.; Vinas, C.; Hosmane, N. S. Inorg. Chem. Commun. 2001, 4, 486–489.
(16) Bernd, W.; Juergen, W.; Milius, Wolfgang, M. Z. Anorg. Allg.
Chem. 1998, 624, 98–102.
10.1021/om800516x CCC: $40.75
2009 American Chemical Society
Publication on Web 12/09/2008

A Neutral Tantalacarborane Sandwich Complex
Organometallics, Vol. 28, No. 1, 2009 61
Figure 1. nido-Carborane-derived ligands.
Figure 2. Carboranyl trianion (a), pentaanionic coordinated Ta(V) complex (b), and 5.
on a JSM-6700F field-emission microscope. The Raman scattering
spectra were measured at room temperature using a JY Horiba
LabRAM Raman microscope. The high-resolution MS was mea-
sured on a Thermo Finnigan MAT XP95 analyzer using the EI-
HR model (70 eV, source temperature 150 °C).
2.3. Synthesis of [(closo-1-Ph-1,2-C2B10H10)Me2Si]2NH (2). A
procedure similar to that used in the preparation of 1 was used to
prepare [(closo-1-Ph-1,2-C2B10H10)Me2Si]2NH (2). After purifica-
tion, complex 2 (mp 103-105 °C) was obtained in 42.1% yield
from the reaction of 2.21 g (10.03 mmol) of closo-1-Ph-1,2-
C2B10H11, 6.35 mL of n-BuLi (1.6 M in hexane), and HN(SiMe2Cl)2
(1.00 g, 4.94 mmol) in 10 mL of THF. Anal. Calcd for
C20H43B20NSi2 (2): C, 42.17; H, 7.60; N, 2.46. Found: C, 42.09;
2.2. Synthesis of [(closo-1-Me-1,2-C2B10H10)Me2Si]2NH (1).
A 1.59 g (10.05 mmol) sample of closo-1-Me-1,2-C2B10H11 was
dissolved in 30 mL of THF to produce a clear solution, which was
cooled to -78 °C. A 6.35 mL (10.16 mmol) amount of n-BuLi
solution (1.6 M in hexane) was slowly added to the THF solution
at -78 °C with syringe. After addition, the mixture was stirred at
-78 °C for 30 min before being warmed to room temperature
spontaneously and the reaction continued for 4 h. The solution was
then cooled to -10 °C and added dropwise to a solution of
HN(SiMe2Cl)2 (1.01 g, 4.99 mmol) in 10 mL of THF. After
addition, the solution was warmed to room temperature spontane-
ously for 2 h followed by heating to 80 °C for 10 h. The mixture
was then cooled to room temperature and concentrated to about 5
mL under reduced pressure. The resulting light brown residue was
purified by thin-layer chromatography (TLC; SiO2) and developed
with a mixed solvent of pentane and ethyl ether (10/1 v/v). The
main product was collected and dried under high vacuum to give
a colorless solid of 1: yield 1.14 g (2.56 mmol), 51.2% (mp 87-89
°C). Anal. Calcd for C10H39B20NSi2 (1): C, 26.94; H, 8.82; N, 3.14.
1
H, 7.64; N, 2.41. H NMR (THF-d8, ppm): δ 0.13 (s, 12H, 4 ×
Si-CH3); 1.20-3.61 (m, 20H, 2 × C2B10H10); 6.83-7.74 (m, 10H,
2 × Ccage-C6H5). 13C NMR (THF-d8, ppm): δ 1.05 (Si-CH3); 60.30
and 66.90 (Ccage); 126.84, 128.15, 130.53, and 133.42 (Ccage-C6H5).
1
11B NMR (THF-d8, ppm): δ -2.86 (1B, JBH ) 144 Hz); -4.67
1
1
(1B, JBH ) 151 Hz); -8.99 (3B, JBH ) 160 Hz); -10.81 (3B,
1JBH ) 147 Hz); -12.75 (2B, 1JBH ) 152 Hz). 29Si NMR (THF-d8,
ppm): δ -113.75 (s). IR (KBr pellet, cm-1): ν 3374 (br, vs), 3068
(s, s), 2962 (s, s), 2582 (νBH, s, m), 1957 (s, w), 1637 (s, s), 1497
(s, m), 1448 (s, m), 1403 (s, m), 1261 (s, s), 1218 (s, m), 1196 (s,
m), 1162 (s, w), 1081 (s, s), 1021 (s, m), 957 (s, s), 857 (s, s), 803
(s, s), 755 (s, m), 730 (s, m), 691 (s, s), 562 (s, m), 455 (s, m).
2.4. Synthesis of [[(closo-2-Me-2,3-C2B10H10)Me2Si]2N]TaV
(3). In a glovebox, 1 (1.11 g, 2.49 mmol), 3.00 g (23.41 mmol) of
dry naphthalene and 0.30 g (13.05 mmol) of fresh-cut sodium were
suspended in 40 mL of dry THF, followed by addition of 1.60 mL
of n-BuLi (1.6 M in hexane, 2.56 mmol). The reaction mixture
was stirred at room temperature for 2 weeks. Tantalum(V) chloride
(TaCl5; 0.90 g, 2.51 mmol) was added, and the reaction was
continued for 1 week. After filtering, solvent and naphthalene were
removed under reduced pressure and a yellow-brown solid was
obtained. The crude product was then purified by recrystallization
from CH2Cl2/pentane to give 0.33 g of pale yellow solid [(closo-
2-Me-2,3-C2B10H10)Me2Si]2N]TaV (3) in 21.2% yield. Mp: 178-179
°C. Anal. Calcd for C10H38B20NSi2Ta (3): C, 19.19; H, 6.12; N,
2.24. Found: C, 19.09; H, 6.20; N, 2.13. MS: m/z 626.76 ([M +
H]+). 1H NMR (THF-d8, ppm): δ 0.18 (s (br), 12H, 4 × Si-CH3);
1.14-3.29 (m, 20H, 2 × C2B10H10); 1.94 (s, 6H, 2 × Ccage-CH3).
1
Found: C, 26.90; H, 8.80; N, 3.10. H NMR (THF-d8, ppm): δ
0.04 (s, 12H, 4 × Si-CH3); 1.13-3.20 (m, 20H, 2 × C2B10H10);
2.02 (s, 6H, 2 × Ccage-CH3). 13C NMR (THF-d8, ppm): δ 1.65
(Si-CH3); 26.65 (Ccage-CH3); 62.92 and 70.32 (Ccage). 11B NMR
(THF-d8, ppm): δ ) -2.39 (1B, 1JBH ) 148 Hz); -6.96 (1B, 1JBH
1
1
) 154 Hz); -9.47 (2B, JBH ) 155 Hz); -10.51 (2B, JBH ) 74
1
1
Hz); -11.29 (2B, JBH ) 122 Hz); -12.77 (2B, JBH ) 174 Hz).
29Si NMR (THF-d8, ppm): δ -107.93 (s). IR (KBr pellet, cm-1, vs
) very strong, s ) strong, m ) middle, w ) weak, br ) broad):
ν 3272 (br, vs), 2963 (s, s), 2521 (νBH, s, m), 2296 (s, w), 2229 (s,
w), 1642 (s, vs), 1509 (s, s), 1449 (s, s), 1413 (s, vs), 1315 (s, m),
1255 (s, w), 1180 (s, w), 1128 (s, w), 1068 (s, w), 938 (s, vs), 865
(s, w), 784 (s, m), 604 (br, s), 550 (s, m), 454 (s, m).

62 Organometallics, Vol. 28, No. 1, 2009
Yinghuai et al.
Table 1. Results of Ethylene Polymerization by 3a
Table 2. Results of Ethylene Polymerization by 3-5
polymera
Mw (103 mol)d
ethylene
wt % of
run temp (°C) pressure (atm) [Al]/[Ta] activityb fibrous PEc
cat.
activityb
Mw/Mn
4.7
1
2
3
4
5
6
7
25
25
25
50
50
50
50
5
10
10
5
10
10
10
100
200
1000
100
100
200
13
37
29
2.54
7.06
9.74
3
4
5
2470c
975
583.3
269
2.7
185 (47)e
47.9 (32.8)e
3.4 (1.8)e
18
8.53
a Results for 3) were obtained under polymerization conditions of
catalyst and cocatalyst: [Al]/[Ta]) ) 200, solvent toluene, T ) 50 °C,
Pethylene ) 10 atm, polymerization time 30 min. b Activity in units of kg
of PE/((mol of Ta) h atm). c Calculated from all polymer, including
fibrous and powdery polyethylene. d The molecular weight and
molecular weight distribution of the polymers produced from the
precipitate of toluene filtrate, powdery PE, were determined by means of
1500
2470
2055
13.22
21.72
26.44
1000
a Polymerization time ) 30 min. b Activity in units of kg of PE/((mol
of Ta) h atm). c wt % ) weight percent of fibrous polyethylene in
products.
13C NMR (THF-d8, ppm): δ 0.15 (Si-CH3); 24.24 (Ccage-CH3);
gel-permeation
chromatography
(GPC:
Polymer
Laboratories,
66.08 and 69.18 (Ccage). 11B NMR (THF-d8, ppm): δ -1.25 (1B,
PC-GPC-220) at 160 °C using 1,2,4-trichlorobenzene stabilized with
0.0125% BHT as eluent. e Literature results for 514 were obtained under
polymerization conditions of catalyst and cocatalyst: [Al]/[Zr]) ) 2000,
solvent toluene, T ) 50 °C, Pethylene ) 1.5 bar, polymerization time 2 h.
1
1
1JBH ) 136 Hz); -4.92 (1B, JBH ) 123 Hz); -9.19 (4B, JBH
)
1
68 Hz); -10.44 (4B, JBH ) 110 Hz). 29Si NMR (THF-d8, ppm):
δ -110.92 (s). IR (KBr pellet, cm-1): ν 3410 (br, vs), 2594 (νBH
,
s, m), 1634 (s, s), 1384 (s, w), 1261 (s, w), 1098 (br, w), 802 (s,
m), 623 (br, s), 417 (s, w).
Scheme 1. Synthesis of the Tetramethyldisilazane-Bridged
closo-Carboranes and Tantalum(V) Complexes
2.5. Synthesis of [[(closo-2-Ph-2,3-C2B10H10)Me2Si]2N]TaV
(4). A procedure similar to that used in the preparation of 3 was
used to prepare [[(closo-2-Ph-2,3-C2B10H10)Me2Si]2N]TaV (4). After
purification, complex 4 (mp 154-156 °C) was obtained in 31.7%
yield from the reaction of 2 (1.43 g, 2.50 mmol), 3.00 g (23.41
mmol) of naphthalene, 0.30 g (13.05 mmol) of sodium, 1.60 mL
of n-BuLi (1.6 M in hexane), and 0.90 g (2.51 mmol) of TaCl5 in
40 mL of THF. Anal. Calcd for C20H42B20NSi2Ta (4): C, 32.03; H,
5.65; N, 1.87. Found: C, 32.15; H, 5.64; N, 1.71. MS: m/z 749.90
1
([M]+). H NMR (THF-d8, ppm): δ 0.16 (s, 12H, 4 × Si-CH3);
1.27-3.70 (m, 20H, 2 × C2B10H10); 6.72-7.64 (m, 10H, 2 ×
Ccage-C6H5). 13C NMR (THF-d8, ppm): δ 0.18 (Si-CH3); 66.89
and 69.94 (Ccage); 126.05, 126.88, 128.27, and 129.24 (Ccage-C6H5).
11B NMR (THF-d8, ppm): δ -2.88 (1B, JBH ) 154 Hz); -4.76
1
of the carborane monoanions [closo-1-R-1,2-C2B10H10]- (R )
Me, Ph) led to the formation of the tetramethyldisilazane-bridged
dicarborane pentaanionic precursors [(closo-1-R-1,2-C2B10H10)-
Me2Si]2NH (R ) Me (1), Ph (2)) in yields of 51.2% and 42.1%
for 1 and 2, respectively (Scheme 1). The 1H, 13C, and 11B NMR
spectra of 1 and 2 show normal group resonances that are
consistent with substituted closo-C2B10H10 cages, as described
in Scheme 1.17 The IR spectra of 1 and 2 show the expected
absorption band of νBH at 2521 and 2582 cm-1, respectively.
Compounds 1 and 2 were converted to their pentaanions by
treatment with n-BuLi and Na in the presence of naphthalene
as electron-transfer reagent in THF.
1
1
(1B, JBH ) 153 Hz); -9.09 (2B, JBH ) 148 Hz); -10.99 (4B,
1JBH ) 175 Hz); -12.92 (2B, 1JBH ) 249 Hz). 29Si NMR (THF-d8,
ppm): δ -108.20 (s). IR (KBr pellet, cm-1): ν 3420 (br, s), 2950
(s, s), 2870 (s, s), 2521 (νBH, s, m), 1638 (s, s), 1489 (s, m), 1450
(s, m), 1375 (s, m), 1256 (s, s), 1097 (s, s), 1031 (s, m), 958 (s,
m), 803 (s, m), 698 (s, m), 609 (s, w).
2.6. Evaluation of Catalytic Activity. The polymerization of
ethylene, catalyzed by 3 (1.603 × 10-7 mol), was carried out for 30
min in 80 mL of toluene in the presence of the cocatalyst methyla-
luminoxane (MAO) in a Parr reactor. The argon pressure inside the
predried reactor was reduced by high vacuum, followed by applied
ethylene pressure. The reactor was adjusted to the optimized 50 °C
and 10 atm. The ratio [Al]/[Ta] was selected as 200. As seen in Table
1, these conditions afforded the maximum catalytic activity. After 30
min, the reaction was quenched with 20 mL of a 10% HCl solution of
MeOH. The toluene-insoluble polymers were collected by filtration
and washed with copious amounts of MeOH and hexane, followed
by drying under reduced pressure to produce 0.43 g of fibrous PE.
The toluene filtrate was then precipitated by the addition of 200 mL
of MeOH, collected by filtration, washed with MeOH (4 × 30 mL)
and hexane (2 × 30 mL), and dried at 60 °C to a constant weight to
give 1.55 g of powdery PE. A procedure similar to that described for
3 (above) was used to evaluate the ethylene polymerization using 4
as a catalyst. The results were essentially the same as found for 3,
except that the weight percent of the fibrous PE was 11.35 instead of
the value of 21.72 found for 3. Polymerization results are summarized
in Table 2.
The in situ synthesized “constrained geometry” pentaanionic
ligands were metalated with tantalum(V) chloride to form the
neutral carboranyl tantalum(V) complexes [(closo-2-R-2,3-
C2B10H10)Me2Si]2N]Ta(V) (R ) Me (3), Ph (4)) in 21.2% and
1
31.7% yields, respectively. The H and 13C NMR spectra of 3
and 4 show normal group resonances, as described in the
proposed structures. The near-IR spectra of 3 and 4 exhibit
typical B-H absorptions at about 2594 and 2527 cm-1. To
verify the formation of the Ta-N bond, Cl4TaN(SiMe3)2 was
synthesized16 and its Raman spectrum was used for comparison
(Figure 3). In the Raman spectrum of Cl4TaN(SiMe3)2, no
resonances in the region of 500-1200 cm-1, where peaks for
TaCl5 should be seen, were observed. However, both
Cl4TaN(SiMe3)2 and 3 exhibited complicated absorption patterns
in other regions. According to the literature,18 the strong
3. Results and Discussion
(17) (a) Gomez, F. A.; Hawthorne, M. F. J. Org. Chem. 1992, 57, 1384–
1390. (b) Xie, Z.; Wang, S.; Zhou, Z.-Y.; Mak, T. C. W. Organometallics
1999, 18, 1641–1652.
(18) Kravchenko, V. V.; Zaitseva, M. G.; Kopylov, V. M.; Petrov, K. I.
J. Struct. Chem. 1987, 27, 549–555.
3.1. Synthesis and Characterization of Tetramethyldisi-
lazane-Bridged closo-Carboranes and Derived Neutral Tan-
talum(V) Complexes. The reaction of 1,3-dichloro-1,1,3,3-
tetramethyldisilazane with a twofold excess of the lithium salt

A Neutral Tantalacarborane Sandwich Complex
Organometallics, Vol. 28, No. 1, 2009 63
Figure 5. Polyethylene (PE) from catalyst 3 with cocatalyst MAO:
(a) fibrous PE; (b) powdery PE.
Figure 3. Raman spectra of TaCl4(NSiMe3) (red) and 3 (blue).
Figure 6. SEM images of PE from 3: (a) fibrous PE; (b) powdery
PE.
As can be seen from Table 2, under these conditions, compound
4 exhibited a much lower activity (975 kg of PE/((mol of Ta)
h atm)). However, both are substantially higher than the value
of 185 kg of PE/((mol of Zr) h atm) found for the carboranyl
trianion, [nido-RR′C2B9H9]3-, coordinated zirconocarborane
closo-1-Zr(Cl)-2-Ph-3-(2′-σ(H)-N-cyclohexyl)-2,3-η5-C2B9H9 (5,
Figure 2),14 under the same conditions (see Table 2). Interest-
ingly, morphologically different polyethylenes, one fibrous and
the other powdery (Figure 5), have been produced from a one-
pot polymerization reaction with 3 and the cocatalyst MAO.
The fibrous polyethylene possesses a viscometric molecular
weight of 5.7 × 106, much higher than than that of the powdery
polyethylene (Table 1). The morphologically different PEs show
different Tm values (based on DSC analysis) of 135 and 127 °C
for fibrous polyethylene and powdery polyethylene, respectively.
Interestingly, they show dramatically different images in SEM
analysis (Figure 6). Microscale rods comprising nanoscale
particles are found for fibrous PE SEM images, which are not
seen in the powdery PE. The catalytic activities of 3 and 4
compare favorably with those of other reported η5-coordinated
nido-carboranyl metallocenes such as closo-1-Zr(Me)(η5-C5Me5)-
2,3-η5-C2B9H11 (activity 72 kg of PE/((mol of Zr) h atm),20
closo-1-TiCl2-3-(π-η2-N,N′-dimethylaminomethyl)-η5-
C2B9H11 (activity 85 kg of PE/((mol of Ti) h atm)21 and
σ-bonded closo-carboranyl metallocenes such as {[η5:σ-
Me2C(C9H6)(C2B10H10)]ZrCl(µ-Cl)1.5}2{Li(THF)2} (activity 2730
kg of PE/((mol of Zr) h atm).22 The mechanism for the neutral
carboranyltantalum(V)/MAO catalytic process is not known.
However, the results suggest that two pathways might occur,
one leading to the fibrous PE and the other to the powdery PE.
A tentative polymerization scheme is shown in Scheme 2. In
Figure 4. Far-IR spectra of TaCl5, TaCl4(NSiMe3), 1, 3, and 4.
absorptions at 1100 cm-1 (Cl4TaN(SiMe3)2) and 920 cm-1 (3)
may be assigned to νSi-N resonances. The peaks at 586 and 590
cm-1 found in 3 are tentatively attributed to the Ta-N
resonances; there are no other reports regarding the location of
the typical absorption of Ta-N bond in Raman analysis. The
other absorptions of 3 at 735, 756, and 777 cm-1 may be caused
by B-Ta, C-B, and Si-Ccage resonances.
In addition, the far-IR spectra of compounds 3 and 4 were
compared to those of Cl4TaN(SiMe3)2, TaCl5, and 1 (Figure 4).
As can be seen from the figure, compounds 3, 4, and
Cl4TaN(SiMe3)2 show strong absorptions at 397, 416, and 400
cm-1, respectively, which are consistent with reported νTa-N
absorptions.19 On the other hand, neither TaCl5 nor 1 shows
such absorptions (see Figure 4). These results clearly indicate
the presence of a Ta-N bond in 3 and 4.
3.2. Catalytic Evaluation of Carboranyl Pentaanionic
Coordinated Tantalum(V) Complexes. In order to evaluate
the catalytic activities of the novel neutral tantalum(V) com-
plexes, compounds 3 and 4 have been investigated as precata-
lysts for ethylene polymerization in the presence of the
cocatalyst MAO. Different conditions of reaction temperature,
ethylene pressure, and the [Al]/[Ta] ratio have been examined
for compound 3, and the results are given in Table 1. It was
found that a temperature of 50 °C with [Al]/[Ta] ) 200 and
Pethylene ) 10 atm gave the highest catalytic activity (2470 kg
of PE/((mol of Ta) h atm)); these conditions were selected for
the comparison of the catalytic activities of 3 and 4 (Table 2).
(20) Crowther, D. J.; Baenziger, N. C.; Jordan, R. F. J. Am. Chem. Soc.
1991, 113, 1455–1457.
(21) Kim, D.-H.; Won, J. H.; Kim, S.-J.; Ko, J.; Kim, S. H.; Cho, S.;
Kang, S. O. Organometallics 2001, 20, 4298–4300.
(19) Bradley, D. C.; Hursthouse, M. B.; Malik, K. M. A.; Nielson, A. J.;
Vuru, G. B. C. J. Chem. Soc., Dalton Trans. 1984, 1069–1072.

64 Organometallics, Vol. 28, No. 1, 2009
Scheme 2. Proposed Ethylene Polymerization by 3 and 4
Yinghuai et al.
keeping with other MAO-assisted schemes, the precursor 3 or
4 could react with MAO, leading to a metal-ligand bond
rupture; either a N-Ta or a (η6-C2B4)-Ta bond could be broken
to form catalytically active species (I, II) upon coordination
with the ethylene monomer. For individual processes, the single-
site polymerization mechanism of the conventional Ziegler-
Natta catalysts may be operable.2-4 At this point it is impossible
to predict which metal-ligand bond is broken or whether the
two possible paths could lead to morphologically different forms
of PE. More detailed mechanistic studies are currently underway
in our laboratory, using FT-IR and NMR spectroscopy.
Si]2N]Ta(V) (R ) Me, Ph) were found to be active catalysts
for the polymerization of ethylene in the presence of cocatalyst,
MAO, to produce morphologically different polyethylenes in a
single reaction.
Acknowledgment. We gratefully acknowledge financial
support by the Institute of Chemical and Engineering
Sciences (ICES) in Singapore, through grants from the Robert
A. Welch Foundation (No. N-1322 to J.A.M.) and the
National Science Foundation (No. CHE-0601023 to N.S.H.),
and through the second research prize from the Alexander
von Humboldt Foundation (to N.S.H.).
4. Conclusion
OM800516X
The novel carborane-based pentaanionic precursors [(closo-
1-R-1,2-C2B10H10)Me2Si]2NH (R ) Me, Ph) and their derived
neutral tantalum(V) complexes have been synthesized and
characterized. The complexes [[(closo-2-R-2,3-C2B10H10)Me2-
(22) Wang, H.; Wang, Y.; Li, H.-W.; Xie, Z. Organometallics 2001,
20, 5110–5118.