A hyperbranched polysilane-based, borane cocatalyst for the metallocene-catalyzed polymerization of propylene

Article abstract of DOI:10.1021/ma035854t

The synthesis of a vinyl-terminated, hyperbranched polysilane and the additional hydroboration with bis(pentafluorophenyl) borohydride are reported. Quantitative transformation was proved by ¹H NMR and FT-IR analysis. The polymeric borane was t

Full text of DOI:10.1021/ma035854t

4
004
Macromolecules 2004, 37, 4004-4007
A Hyperbranched Polysilane-Based, Borane Cocatalyst for the
Metallocene-Catalyzed Polymerization of Propylene
†
‡
†
‡
Ma r tin Sch l o1 gl, Silk e Rieth m u eller , Ca r sten Tr oll, Ma r tin M o1 ller , a n d
,
†
Ber n h a r d Rieger *
Department of Materials and Catalysis, University of Ulm, D-89069 Ulm, Germany, and
Organic Chemistry III, University of Ulm, D-89069 Ulm, Germany
Received December 6, 2003; Revised Manuscript Received April 5, 2004
ABSTRACT: The synthesis of a vinyl-terminated, hyperbranched polysilane and the additional hydrobo-
ration with bis(pentafluorophenyl) borohydride are reported. Quantitative transformation was proved
1
by H NMR and FT-IR analysis. The polymeric borane was tested as cocatalyst in the metallocene-catalyzed
5
polymerization of propylene. The corresponding catalyst system comprising rac-[1-(9-η -fluorenyl)-2-(5,6-
5
cyclopenta-2-methyl-1-η -indenyl)ethane]zirconocene dichloride and the polymeric borane was activated
in situ in the presence of triisobutylaluminum (TIBA). Polymerizations were performed at temperatures
of 30 and 50 °C with a propylene concentration of 3 mol/L in toluene solution using various cocatalyst:
catalyst ratios (10:1, 50:1, and 100:1). This polymer-supported catalyst system produces polypropylenes
with excellent activities and good molecular weights. These results were compared with an additional
6 5 3
set of polymerization experiments using molecular dispersed B(C F ) as cocatalyst, resulting in
significantly reduced polymerization activities. This indicates a hindered catalyst mobility inside the
hyperbranched cocatalyst matrix, leading to a higher concentration of active species within the
hyperbranched nanostructure.
In tr od u ction
salts that would be present in the respective polyborate
activators. A major advantage of hyperbranched poly-
mers is the distribution of functional end groups through-
out their globular structure, in contrast to perfectly
Activation of metallocene catalysts for R-olefin poly-
merizations1,2 concentrates on the formation of the
active species via borane or borate cocatalysts introduc-
1
7,18
3
-5
branched dendrimers.
This should also lead to an
ing weakly or noncoordinating borate counterions.
Especially the application of borate cocatalysts enhances
the active site concentration and helps to improve
encapsulation of the active sites inside the polymeric
1
9
matrix, whereas catalysts immobilized with dendrim-
eric systems are exclusively located on the particle
molecular weights and therefore material properties
2
0
significantly.6-9 For commercialization, these homoge-
surface.
neous systems have to be adapted to existing industrial
processes, which require a control of the polymer
morphology toward an adjustable particle size. Con-
ventional supporting materials in commercial processes
Resu lts a n d Discu ssion
1
0
Syn t h esis of t h e P olysila n e Coca t a lyst . The
catalytic hydrosilylation reaction of the AB2 monomer
methyldiundecenylsilane 1 (Figure 1) affords a hyper-
branched polycarbosilane 2 in up to 99% yield using
bis(trioctylbenzylammonium) tetranitritoplatinate as
are silica or aluminum oxides that give excellent particle
morphologies with TiCl3-based ZN catalysts.1
1,12
How-
ever, these strategies have disadvantages, particularly
for the support of asymmetric, dual-side metallocenes
29
catalyst precursor. This polyaddition leads to hyper-
which produce high-performance thermoplastic polypro-
branched macromolecules with one terminal Si-H
pylene elastomers.2
8,30
The reason is based on the
group and (n + 1) vinyl end groups unreacted through-
influence of the supporting material surface on the
specific stereoerror formation mechanism of these new
polymerization catalysts. In this context it is obvious
to explore new anchoring processes on apolar organic
substrates that have less interaction with the catalyst
29
out the polymer structure (n ) degree of polymeriza-
tion). The resulting polycarbosilane showed an increased
chemical and thermal stability compared to that of the
monomer. The terminal vinyl groups were converted
into the boranes 4 by a direct and clean hydroboration
reaction with bis(pentafluorophenyl) borohydride 3,
which was generated by treatment of bromopentafluo-
robenzene-1 with n-BuLi and subsequent BCl3 addition
at -70 °C (Figure 2). Chloride/hydride exchange pro-
1
3-15
species.
In recent literature an interesting approach
from Mager et al.16 is reported focusing on modified
dendrimers that comprise borate functionalities which
activate metallocene dialkyl species. However, the
perfect nature of such dendrimers requires a rather
difficult synthesis. We focus here on easily accessible
hyperbranched polysilanes,29 converting the remaining
vinyl end groups into perfluorinated phenylboranes by
a simple hydroboration step. This approach guarantees
the absence of any polymerization-inhibiting inorganic
31
ceeded via the addition of Me2Si(H)Cl at 0 °C. The
hydroboration reaction was monitored by FT-IR- and
1
H NMR spectroscopy which revealed complete conver-
sion of all vinyl end groups into borane functions.
5
Ca ta lyst Activa tion . The rac-[1-(9-η -fluorenyl)-2-
5
(
5,6-cylopenta-2-methyl-1-η -indenyl)ethane]zircono-
2
1
cene dichloride 5 (Figure 3) precursor was treated in
†
Department of Materials and Catalysis.
Organic Chemistry III.
situ with excess of triisobutylaluminum (TIBA, ratio:
‡
1
00:1) and subsequently reacted with the polymeric
*
Corresponding author: e-mail bernhard.rieger@chemie.uni-
ulm.de; fax +49 (0)731 5023039.
borane 4 to give the active catalyst species 6 in an
1
0.1021/ma035854t CCC: $27.50 © 2004 American Chemical Society
Published on Web 05/06/2004

Macromolecules, Vol. 37, No. 11, 2004
Polymeric Borane Cocatalyst 4005
F igu r e 1. Synthesis of poly(methyldiundecenylsilane) 2 from methyldiundecenylsilane 1 and subsequent hydroboration with
bis(pentafluorophenyl) borohydride 3 to the cocatalyst 4.
tions in homogeneous solution. The average period in
which each zirconocene molecule exists in its polymer-
ization active form is therefore enhanced by the poly-
meric cocatalysts, leading to a shift of the activation
equilibrium to 7 (Figure 4). This would afford the
observed higher activity values (Figure 4; “Asupport”)
relative to the situation in solution 8 (“Asolution”).26
Polypropylene Characteristics. The molecular weights
of the polypropylenes prepared in the present study
using B(C6F5)3 or the polymeric borane 4 as activators
F igu r e 2. Synthesis of bis(pentafluorophenyl) borohydride 3.
are generally higher compared to solution experiments
under MAO activation, probably due to hindered chain-
2
7
equilibrium. The results of the polymerization experi-
ments were additionally referred to runs in homoge-
neous solution using the respective zirconocene com-
pounds and TIBA/B(C6F5)3.22 This allows a comparison
of a hyperbranched, polymeric system and a homoge-
neously dissolved, molecular cocatalyst comprising a
similar electronic structure.
P r op ylen e P olym er iza tion Exp er im en ts. The po-
lymerizations were performed in an autoclave with
constant propylene concentrations of 3.00 mol/L at
temperatures of 30 °C (5 bar) and 50 °C (6.5 bar). The
cocatalyst:catalyst ratios were 10:1, 50:1, and 100:1.
Injection of the preactivated catalyst/cocatalyst solution
via a pressure buret started the polymerizations. Both
borane cocatalysts were suitable activators for the
transfer reactions to MAO species. A tendency for
higher molecular weights by using the polymeric activa-
tors relative to B(C6F5)3 can be identified. A maximum
Mw value of 270 000 was reached at a ratio of 100:1 and
Tp ) 30 °C (Table 1, Pborane 5). Broader polydispersi-
ties and significantly reduced molecular weights are
found for both polymeric (Table 1, Pborane 1) and
molecular dispersed B(C6F5)3 (Table 1, borane 1) activa-
tors at low borane/Zr ratios, indicating a slow activation
reaction. At higher ratios (50:1; 100:1) the polydisper-
sities are in the range expected for this catalyst family,
and the molecular weights reach constant maximum
values for both activating compounds.
All polypropylenes are isotactic materials bearing
variable [mmmm] pentad concentrations. The micro-
structures are identical to solution experiments and are
found to be independent of the nature of the activating
species. The occurring stereoerrors, identified by the
pentad distribution, are typical for variable tacticity PP
prepared with these asymmetric, dual-side catalysts
that afford increased [mmmm] pentad contents (30-
polymerization of propylene with 5/TIBA (Table 1).
Activities. The observed polymerization activities23
reveal a broad variation depending on temperature and
cocatalyst/catalyst ratio, independent of the particular
borane structure. Generally, increasing the temperature
from 30 to 50 °C leads to higher values, as expected.
An even stronger effect is caused by raising the cocata-
lyst:catalyst quotient, reflecting the reversibility of alkyl
group exchange between the Lewis acidic zirconocenium
5
3
0%) by raising the polymerization temperature from
0 to 50 °C.
2
8,30
9
Con clu sion s
ions and the borane agents. The activities reached at
borane:Zr ) 10:1 can be enhanced by a factor of about
Hyperbranched polyborane cocatalysts, like 4, are
proved to be excellent activating systems for our asym-
metric, “dual-side” metallocene catalysts leading to
homo-polypropylene elastomers. Both B(C6F5)3 and 4
result in polymers with increased molecular weight
1
0 by switching to a ratio of 100:1. The top activity
(
Table 1, Pborane 6) comes already close to values that
+
- 24
are typical for borate activators like Ph3C [B(C6F5)4] ,
which proceed irreversibly based on the formation of
covalent trityl-carbon bonds.25
24
relative to solution experiments with MAO. Signifi-
Interestingly, activation through the polymeric borane
leads in all cases to higher activities compared to that
cantly increased activities were found for the polymeric
activators relative to the molecular dispersed borane
cocatalyst. We attributed this effect to the particular
nature of the hyperbranched nanostructure of 4, leading
to a reduced catalyst mobility that enhances the con-
centration of active species. This is a major advantage
compared to perfect dendrimers (besides the easier
synthesis), which provide activating borate units only
on the outer particle sphere. In contrast to conventional
4
of B(C6F5)3 (Table 1), although the missing third electron-
withdrawing pentafluorophenyl group in 4 results in a
reduced Lewis acidity. This effect might find an expla-
nation by assuming a longer retention period of the
polymerization active zirconocenium ion 6 (Figure 3)
inside the hyperbranched matrix due to a hindered
freedom of movement compared to polymerization reac-

4
006 Schl o¨ gl et al.
Macromolecules, Vol. 37, No. 11, 2004
F igu r e 3. Catalyst activation.
6 5 3
F igu r e 4. Schematic activation processes with polymeric borane activator (heterogeneous) and B(C F ) activator (molecular
disperse).
Ta ble 1. P olym er iza tion Resu lts w ith 5/TIBA /B(C6F 5)3 (Bor a n es 1-6) a n d 5/TIBA/4 (P bor a n es 1-6)
run
[cat.]a
ratiob
tempc
pressured
timee
yieldf
Mwg
activityh
D
[mmmm]i
borane 1
borane 2
borane 3
borane 4
borane 5
borane 6
Pborane 1
Pborane 2
Pborane 3
Pborane 4
Pborane 5
Pborane 6
20
10
10
5
5
5
10
5
5
2.5
2.5
1.5
10:1
10:1
50:1
30
50
30
50
30
50
30
50
30
50
30
50
5
6.5
5
6.5
5
6.5
5
6.5
5
6.5
5
10
5
15
5
6
6
5
12
9
4
14
5
1.33
1.32
8.25
1.46
9.03
13.46
2.11
3.48
4.05
5.44
13.06
4.34
100 000
110 000
210 000
110 000
210 000
150 000
40 000
100
600
3.9
2.4
2.1
2.3
2.3
3.6
8.8
2.6
2.0
2.7
1.9
2.1
30.5
50.4
29.1
49.1
32.8
56.8
30.1
48.7
31.1
49.4
26.4
51.2
1000
1200
6000
9000
900
1200
1800
10400
7600
12100
50:1
100:1
100:1
10:1
10:1
50:1
50:1
100:1
100:1
90 000
250 000
120 000
270 000
130 000
6.5
a
µmol]. b Cocatalyst: catalyst. c [°C]. d [bar]. e [min]. f [g]. g Calibrated relatively against PP standards. h kg of PP/[molZr] × [C3] × h.
[
i
Isotacticity [%].
supports, like silica or alumina, we find no influence of
this polymer-based supporting method on the PP mi-
crostructure. This lack of influence is considered as an
additional advantage of the here presented approach.
Thus, there is no hindrance of a solid support surface,
which in most cases affects the intrinsic stereoerror
formation mechanism of the particular metallocene
catalyst, to fine-tune the polymer microstructure and
hence the final polymer properties. It will be interesting
to find out whether this concept of hyperbranched
support materials, which is at the borderline between
homogeneous and heterogeneous catalysis, can also be
used to control particle-forming processes.
ization, benzene, and pentane were purchased from Merck and
dried via distillation over LiAlH
4
.
Mea su r em en ts. NMR spectra were recorded on a Bruker
AC 400 at ambient temperatures and referenced to TMS. FT-
IR measurements were performed on a Bruker IFS 113-V and
1
3
a Perkin-Elmer 1310 infrared spectrometer. PP analysis:
NMR spectra were recorded on a Bruker AMX 500 spectrom-
eter (C Cl , 90 °C, 125 MHz, 5 mm probe) in the inverse
C
2
D
2
4
gated decoupling mode with a 3 s pulse delay and a 45° pulse
to attain conditions close to the maximum signal-to-noise ratio.
Molecular weights and molecular weight distributions were
determined by gel permeation chromatography (GPC, Waters
2000, 140 °C in 1,2,4-trichlorobenzene) relative to polypropy-
lene standards.
Syn t h esis of Bis(p en t a flu or op h en yl) Bor oh yd r id e.
This compound was prepared by a modified literature proce-
dure.31 A three-necked round-bottom flask fitted with a reflux
condenser, a dropping funnel, and a stopper was charged with
15 mL of bromopentafluorobenzene in dry pentane and cooled
to -70 °C. 68.5 mL of n-BuLi (1.6 mol in hexane) was added
dropwise to the reaction mixture. After complete addition, the
resulting suspension was stirred for 2 h. The precipitate was
separated, washed several times with pentane, and dried. It
Exp er im en ta l Section
Gen er a l Rem a r k s. Methyldiundecenylsilane, the corre-
2
9
5
sponding polysilane, and rac-[1-(9-η -fluorenyl)-2-(5,6-cyclo-
5
30
penta-2-methyl-1-η -indenyl)ethane] zirconocene dichloride
were prepared and characterized by published procedures. All
synthetical work was done with standard Schlenk techniques
under an argon atmosphere. Triisobutylaluminum (1 M in
hexane) was purchased from Crompton and B(C
in hexane) from Nippon Shokubai. Bromopentafluorobenzene,
BCl , n-BuLi (1.6 mol in hexane), and Me Si(H)Cl were
purchased from Aldrich. Toluene p.A. for propylene polymer-
6
F
5
)
3
(0.5 mmol
was again dissolved in pentane and cooled to -70 °C. BCl
3
was added slowly and in order to remove all educts, and the
compound was washed with pentane. The white precipitate
was redissolved in 50 mL of freshly distilled dimethylchlorosi-
3
2

Macromolecules, Vol. 37, No. 11, 2004
Polymeric Borane Cocatalyst 4007
lane and stirred for an additional 4 h. The resulting product
was recrystallized and filtered.
Yield: 46% (white crystalline powder); purity 99%. H NMR
(7) Deck, P. A.; Marks, T. J . J . Am. Chem. Soc. 1995, 117, 6128.
(8) J ia, L.; Yang, X.; Stern, C. L.; Marks, T. J . Organometallics
1997, 16, 842.
1
1
9
(9) Chen, Y.-X. E.; Marks, T. J . Chem. Rev. 2000, 100, 1391-
(
C
6
D
6
6 6
, 400 MHz, δ in ppm): 4.2 (br). F NMR (C D , 400 MHz,
1
434.
δ in ppm): main component (80%): -134.5 (2F), -147.8 (1F),
(
(
(
10) Abbenhuis, H. C. L. Angew. Chem., Int. Ed. 1999, 38, 1058-
-
-
1
6
160.5 (2F); component (10%): -130.2 (2F), -143.0 (1F),
1
060.
-
1
-1
161.5 (2F). IR (Nujol, NaCl, λ in cm ): 1652, 1550, 1522,
11) Basell’s Spheripol process. For details see: Rieger, B.; M u¨ l-
406, 1394, 1379, 1315, 1282, 1143, 1112, 1018, 974, 766, 679,
haupt, R. Chimia 1995, 49, 486-491.
12) Hlatky, G. G. Chem. Rev. 2000, 100, 1347-1376.
(13) Barrett, A. G. M.; de Miguel, Y. R. Chem. Commun., 1998,
+
61, 623. GC-MS (CI): m/z 346 (M ).
Syn th esis of th e P olym er ic Coca ta lyst. A freshly pre-
pared solution of bis(pentafluorophenyl) borohydride in 5 mL
of toluene was added dropwise to a solution of poly(methyl-
diundecenylsilane) in 3 mL of toluene. The mixture was
treated in an ultrasonic bath for 10 min and stirred for an
additional hour. The product was washed two times with
2079.
(14) Roscoe, S. B.; Frechet, J . M. J .; Walzer, J . F.; Dias, A. J .
Science 1998, 280, 270-273.
15) Stork, M.; Klapper, M.; Muellen, K.; Gregorius, H.; Rief, U.
(
(
(
(
Macromol. Rapid Commun. 1999, 20, 210-213.
16) Mager, M.; Becke, S.; Windisch, H.; Denninger, U. Angew.
Chem., Int. Ed. 2001, 40, 1898-1902.
benzene to remove all educts. Yield: 98%; M ) 37 500 g/mol.
w
IR (Nujol, NaCl, λ- in cm ): 3698, 3551, 3496, 2926, 2655,
1
-1
17) Stiriba, S. E.; Kautz, H.; Frey, H. J . Am. Chem. Soc. 2002,
1
648, 1522, 1467, 1777, 1291, 1251, 1094, 975, 794.
1
24, 9698-9699.
The conversion was monitored via FT-IR by disappearance
18) Newkome, G. R.; Moorefield, C. N.; V o¨ gtle, F. Dendritic
Molecules: Concepts, Synthesis, Perspectives; VCH: Wein-
heim, 2001.
of the characteristic vinyl end groups, for instance the dC-H
-
1
stretching bond at 3077 cm , the CdC stretching resonance
-
1
-1
at 1641 cm , and the dC-H deformation bond at 992 cm -
.
(19) Slagt, M. Q.; Stiriba, S. E.; Klein Gebbink, R. J . M.; Kautz,
H.; Frey, H.; van Koten, G. Macromolecules 2002, 35, 5734-
5737.
1
The deformation frequency of the borohydride at 1550 cm
disappeared simultaneously. Also, H NMR showed no vinylic
1
(
20) Kreiter, R.; Kleij, A. W.; Klein Gebbink, R. J . M.; van Koten,
protons of the educt )CH
5.82 ppm) groups.
Ca ta lyst Activa tion . The activation process was performed
2
(4.97 ppm) groups and the -CH)
G. Top. Curr. Chem. 2001, 217, 163-197.
(
(
21) Additional polymerization experiments with different catalyst
structures and metal centers are underway and will be
published elsewere.
by the following procedure: The desired amount of dichloro-
zirconocene precursor was dissolved in 10 mL of dry toluene
under an argon atmosphere, and 100 equiv of TIBA was added.
(
22) For B(C6F5)3 as cocatalyst: Piers, W. E.; Chivers, T. Chem.
Soc. Rev. 1997, 26, 345-354 and references therein.
Addition of the adequate amount of B(C
6
F
5
)
3
or the polymeric
(23) The polymerizations (B:Zr ) 50:1 and 100:1) were terminated
after a relatively short time due to the very high activities
to avoid PP precipitation and diffusion processes which would
influence catalyst performance. Selected polymerization runs
borane 4 (0.5 mmol/mL in toluene) gave the active species.
P r op en e P olym er iza tion Rea ction s. The polymerization
reactions were performed in a 0.5 L Buechi steel reactor at
constant pressure ((0.1 bar) and temperature ((1 °C). After
addition of 200 mL of toluene, subsequently, the polymeriza-
tion temperature was adjusted, the reactor was charged with
propene up to the desired partial pressure, and the preacti-
vated catalyst solution was injected into the autoclave via a
pressure buret. The monomer consumption was measured by
a calibrated gas flow meter (Bronkhorst F-111C-HA-33P), and
the pressure was kept constant during the entire polymeri-
zation period (Bronkhorst pressure controller P-602C-EA-33P).
Pressure, temperature, and consumption of propylene were
monitored and recorded online. The polymerization reactions
were stopped by injecting 1 mL of methanol. The reaction
mixture was poured into acidified methanol (500 mL), where
the polymer precipitated. The product was filtered, washed
with methanol, and dried in a vacuum at 50 °C overnight.
(
Table 1) were repeated several times to ensure reproduc-
ibility. The low activity values at 10:1 ratios reflect an, in
these cases, insufficient cocatalyst amount leading to â-hy-
dride abstraction and subsequent catalyst decomposition. To
obtain comparable data, the stated activities are maximum
values, which have been extrapolated to a theoretical polym-
erization time of 1 h.
(24) Rieger, B.; Troll, C.; Preuschen, J . Macromolecules 2002, 35,
5742-5743.
(
25) Chien, J . C. W.; Tsai, W. M.; Rausch, M. D. J . Am. Chem.
Soc. 1991, 113, 8570-8571.
(
26) An additional argument is based on a possible enhanced
propylene solubility inside a polymeric matrix compared to
the outer toluene phase. However, until now we have no
genuine experimental evidence that would prove this hy-
pothesis.
(27) NMR analysis of lower molecular weight PP samples gave
no indication for â-H-elimination reactions as major chain
transfer processes. However, the existence of isobutyl end
groups points toward chain transfer to MAO-aluminum as
the preferred reaction. Cf.: Kukral, K.; Lehmus, P.; Klinga,
M.; Leskel a¨ , M.; Rieger, B. Eur. J . Inorg. Chem. 2002, 1349-
Ack n ow led gm en t. The authors thank DFG Deut-
sche Forschungsgemeinschaft, SFB 569, for financial
support of this research.
1
356.
Refer en ces a n d Notes
(28) M u¨ ller, G.; Rieger, B. Prog. Polym. Sci. 2002, 27, 815-851.
(
29) For structure and solution behavior of the polyboranes/
polysilanes, cf.: (a) Drohmann, C.; M o¨ ller, M.; Gorbatsevich,
O. B.; Muzafarov, A. M. J . Polym. Sci., Part A1 2000, 38,
741. (b) Muzafarov, A. M.; Golly, M.; M o¨ ller; M. Macromol-
ecules 1995, 28, 8444. (c) Ponomarenko, S. A.; Rebrov, E. A.;
Boiko, N. I.; Muzafarov, A. M.; Shibaev, V. P. J . Polym. Sci.
1998, 1, 115. (d) Riethm u¨ ller, Ph.D. Thesis, Universitiy of
Ulm, 2003.
(
1) Brintzinger, H. H.; Fischer, D.; M u¨ lhaupt, R.; Rieger, B.;
Waymouth, R. M. Angew. Chem., Int. Ed. Engl. 1995, 34,
1
143.
(
(
(
(
(
2) Resconi, L.; Cavallo, L.; Fait, A.; Piemontesi, F. Chem. Rev.
2
000, 100, 1253-1345.
3) Chen, Y.-X. E.; Stern, C. L.; Yang, S.; Marks, T. J . J . Am.
Chem. Soc. 1996, 118, 12451.
4) Chen, Y.-X. E.; Stern, C. L.; Marks, T. J . J . Am. Chem. Soc.
(
30) Dietrich, U.; Hackmann, M.; Rieger, B.; Klinga, M.; Leskel a¨ ,
1
997, 119, 2582.
5) J ia, L.; Yang, X.; Ishihara, A.; Marks, T. J . Organometallics
995, 14, 3135.
M. J . Am. Chem. Soc. 1999, 121, 4348-4355.
(31) Parks, D. J .; Rupert, E.; Spence, H.; Warren, E. P. Angew.
Chem., Int. Ed. 1995, 7, 107.
1
6) Chen, Y.-X. E.; Metz, M. V.; Li, L.; Stern, C. L.; Marks, T. J .
J . Am. Chem. Soc. 1998, 120, 6287.
MA035854T