Influence of the departing group on the electrophilic cleavage of silicon-carbon bonds adjacent to zirconocene dichloride. Preparation of electrophile-functionalized zirconocene dibromides

Article abstract of DOI:10.1021/om0005452

(EtMe₂SiC₅H₄)CpZrCl₂ (5) reacts with BBr₃ in 1, 2-dichloroethane (reflux, 24 h) to afford a 4:1 mixture of (EtMeBrSiC₅H₄)CpZrBr₂ (10) and (BrMe₂SiC₅H₄) CpZrBr₂ (11) in a nearly quantitative conversion. Similarly, (^tBuMe₂SiC₅H₄)CpZrCl₂ (6) reacts with BBr₃ to afford a 15:1 mixture of (^tBuMeBrSiC₅H₄)CpZrBr₂ (13) and 11. The product 11 is obtained independently by treating (Me₃SiC₅H₄)CpZrCl₂ (12) with BBr₃. In contrast, Si-Ph bonds are cleaved with complete selectivity in the presence of Si-Me groups. (PhMe₂SiC₅H₄)₂-ZrCl₂ (8) reacts with excess BCl₃ in dichloromethane (reflux, 15 h) to afford (ClMe₂SiC₅H₄)₂-ZrCl₂ (14) in 72% yield. (Ph₂ MeSiC₅H₄)₂ZrCl₂ (9) reacts with excess BBr₃ in 1, 2-dichloroethane (reflux, 15 h) to afford (Br₂MeSiC₅ H₄)₂ZrBr₂ (15) in 79% yield. Complexes 9 and 15 were analyzed by single-crystal X-ray diffraction. Crystalline 9 adopts a pseudo-C₂ conformation in which the face of the Ph group of one ligand shows a weak interaction with a C-H bond of the other ligand. Crystalline 15 also adopts a pseudo-C₂ conformation, in which the SiBr₂ groups are directed away from the ZrBr₂ group.

Full text of DOI:10.1021/om0005452

5404
Organometallics 2000, 19, 5404-5409
In flu en ce of th e Dep a r tin g Gr ou p on th e Electr op h ilic
Clea va ge of Silicon -Ca r bon Bon d s Ad ja cen t to
Zir con ocen e Dich lor id e. P r ep a r a tion of
Electr op h ile-F u n ction a lized Zir con ocen e Dibr om id es
Paul A. Deck,* Xu Cheng, Edward J . Stoebenau,1 Carla Slebodnick,
Damon R. Billodeaux,1 and Frank R. Fronczek
Department of Chemistry, Virginia Tech, Blacksburg, Virginia 24061-0212, and Department of
Chemistry, Louisiana State University, Baton Rouge, Louisiana 70803-1804
Received J une 23, 2000
(EtMe2SiC5H4)CpZrCl2 (5) reacts with BBr3 in 1,2-dichloroethane (reflux, 24 h) to afford
a 4:1 mixture of (EtMeBrSiC5H4)CpZrBr2 (10) and (BrMe2SiC5H4)CpZrBr2 (11) in a nearly
quantitative conversion. Similarly, (tBuMe2SiC5H4)CpZrCl2 (6) reacts with BBr3 to afford a
15:1 mixture of (tBuMeBrSiC5H4)CpZrBr2 (13) and 11. The product 11 is obtained
independently by treating (Me3SiC5H4)CpZrCl2 (12) with BBr3. In contrast, Si-Ph bonds
are cleaved with complete selectivity in the presence of Si-Me groups. (PhMe2SiC5H4)2-
ZrCl2 (8) reacts with excess BCl3 in dichloromethane (reflux, 15 h) to afford (ClMe2SiC5H4)2-
ZrCl2 (14) in 72% yield. (Ph2MeSiC5H4)2ZrCl2 (9) reacts with excess BBr3 in 1,2-dichloroethane
(reflux, 15 h) to afford (Br2MeSiC5H4)2ZrBr2 (15) in 79% yield. Complexes 9 and 15 were
analyzed by single-crystal X-ray diffraction. Crystalline 9 adopts a pseudo-C2 conformation
in which the face of the Ph group of one ligand shows a weak interaction with a C-H bond
of the other ligand. Crystalline 15 also adopts a pseudo-C2 conformation, in which the SiBr2
groups are directed away from the ZrBr2 group.
In tr od u ction
electrophilic “active species” toward poisoning.6 How-
ever, functional groups are useful for further structural
elaboration and for immobilizing precatalytic species on
inorganic supports.7
We showed earlier that trimethylsilyl substituents on
group 4 metallocene dichlorides are susceptible to a
single electrophilic bromodemethylation upon treatment
with boron tribromide.8 Bromodimethylsilylated met-
allocenes react with a wide range of nucleophiles such
as water,9 amines,10 and phenols,11 while neighboring
Group 4 metallocene complexes are best known for
their applications in olefin polymerization catalysis.2
Structural variation of the Cp ligands affords control
over several technologically important aspects of met-
allocene-catalyzed olefin polymerization.3 This control
motivates ongoing efforts in several laboratories to
conceive new metallocene structures and synthetic
methods. Much of this work attempts to define the steric
environment of the catalytic site, which influences the
stereoselectivity of R-olefin insertion.4 Fewer investiga-
tions have been directed toward the incorporation of
reactive functional groups onto the ancillary ligand
framework,5 largely because of the sensitivity of the
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J . G.; Na¨sman, J . H.; Seppa¨la¨, J . V. Macromolecules 1999, 32, 3547.
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P. J . Organomet. Chem. 1999, 572, 155. (f) Gomez, R.; Gomez-Sal, P.;
Manzanero, A.; Royo, P. Polyhedron 1998, 17, 1055. (g) Su¨nkel, K.;
Schubert, S.; Hofmann, J . J . Organomet. Chem. 1997, 541, 199. (h)
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Willuhn, M.; Psiorz, C.; Goodard, R. J . Chem. Soc., Chem. Commun.
1999, 1105. (j) Ashe, A. J .; Fang, X.-G.; Kampf, J . W. Organometallics
1999, 18, 2288. (k) Stelck, D. S.; Shapiro, P. J .; Basickes, N.; Rheingold,
A. L. Organometallics 1997, 16, 4546. (l) J utzi, P.; Redeker, T. Eur. J .
Inorg. Chem. 1998, 6, 663-74. (m) Alt, H. G.; J ung, M. J . Organomet.
Chem. 1998, 568, 127. (n) Shin, J . H.; Hascall, T.; Parkin, G.
Organometallics 1999, 18, 6.
(6) Catalysts have also been developed that are highly tolerant of
functional groups. For a lead reference, see: Younkin, T. R.; Conner,
E. F.; Henderson, J . I.; Friedrich, S. K.; Grubbs, R. H.; Bansleben, D.
A. Science 2000, 287, 460.
(7) Hlatky, G. G. Chem. Rev. 2000, 100, 1347. (b) Abbenhuis, H. C.
L. Angew. Chem., Int. Ed. 1999, 38, 1058.
(8) Deck, P. A.; Fisher, T. S.; Downey, J . S. Organometallics 1997,
16, 1193. (b) Lofthus, O. W.; Slebodnick, C.; Deck, P. A. Organome-
tallics 1999, 18, 3702.
(9) Ciruelos, S.; Cuenca, T.; Go´mez-Sal, P.; Manzanero, A.; Royo,
P. Organometallics 1995, 14, 177.
(1) Undergraduate research participants.
(2) Hlatky, G. G. Coord. Chem. Rev. 1999, 181, 243. (b) Britovsek,
G. J . P.; Gibson, V. C.; Wass, D. F. Angew. Chem., Int. Ed. 1999, 38,
429. (c) Locatelli, P. Trends Polym. Sci. 1996, 4, 326. (d) Bochmann,
M. Top. Catal. 1999, 7, 9. (e) Kaminsky, W. J . Chem. Soc., Dalton
Trans. 1998, 1413. (f) Soga, K.; Shiono, T. Prog. Polym. Sci. 1997, 22,
1503. (g) Machima, K.; Nakayama, Y.; Nakamura, A. Adv. Polym. Sci.
1997, 133, 1. (h) Bochmann, M. J . Chem. Soc., Dalton Trans. 1996,
225.
(3) Hoveyda, A. H.; Morken, J . P. Angew. Chem., Int. Ed. Engl. 1996,
35, 1262-1284. (b) Alt, H. G.; Ko¨ppl, A. Chem. Rev. 2000, 100, 1205.
(c) Brintzinger, H.-H., Fischer, D.; Mu¨lhaupt, R.; Rieger, B.; Waymouth,
R. M. Angew. Chem., Int. Ed. Engl. 1995, 34, 11443.
(4) Herzog, T. A.; Zubris, D. L.; Bercaw, J . E. J . Am. Chem. Soc.
1996, 118, 11988. (b) Veghini, D.; Henling, L. M.; Burkhardt, T. J .;
Bercaw, J . E. J . Am. Chem. Soc. 1999, 121, 564. (c) Ewen, J . A.
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10.1021/om0005452 CCC: $19.00 © 2000 American Chemical Society
Publication on Web 11/14/2000

Electrophile-Functionalized Zirconocene Dibromides
Organometallics, Vol. 19, No. 25, 2000 5405
(q, J ) 8.1 Hz, 2 H), 0.09 (s, 6 H). 13C NMR (THF-d8): δ 111.8
(CH), 108.7 (C), 106.9 (CH), 10.4 (CH3), 8.5 (CH2), -1.1 (CH3).
Satisfactory elemental analysis was not obtained. The 1H NMR
spectrum is provided in the Supporting Information (Figure
S1) as evidence of substantial purity.
M-Br bonds are comparatively inert. Recently, we also
showed that (trimethylstannyl)zirconocene dichloride
undergoes cleavage of two Sn-Me bonds upon treat-
ment with BBr3.12 Unfortunately, the corresponding
Sn-Br bonds are not as reactive as Si-Br bonds toward
nucleophiles, diminishing their further synthetic utility.
We therefore turned our attention toward understand-
ing the cleavage of Si-R bonds with BBr3 with the
ultimate goal of finding new methods to attach more
than one halogen to each silyl substituent, starting from
stable, readily prepared metallocene precursors. We now
report a study of the reactivity of Si-C bonds adjacent
to zirconocene dichloride using BBr3 as the electrophilic
bromodemethylation reagent. In particular, we show
that Si-Ph bonds are sufficiently more reactive to
enable an efficient synthesis of methyldibromosilyl-
functionalized group 4 metallocene complexes.
t
Syn th esis of Bu Me2SiC5H4Li (2). To a stirred solution
of NaCp (1.9 g, 21 mmol) in diethyl ether (50 mL) maintained
t
at 0 °C was added BuSiMe2OSO2CF3 (4.2 g, 16 mmol) in one
portion. After stirring for 2 h, the mixture was filtered, and
the filtrate was evaporated to afford 1.7 g (10 mmol, 59%) of
t
an orange oil tentatively identified as BuMe2SiC5H5 on the
basis of the assignment of 85% of the integratable 1H NMR
signal intensity in CDCl3: δ 6.9 to 6.5 (m, 4 H), 3.6 to 3.0 (m,
1 H), 1.0 to 0.8 (m, 9 H), 0.2 to -0.2, m, 6 H). The orange oil
was dissolved in hexanes (10 mL), filtered through Celite into
a 200 mL Schlenk flask, diluted with 100 mL of additional
n
hexanes, and treated with 10 mL of BuLi solution (1.4 M in
hexanes, 14 mmol). After 15 h, no precipitate had formed, so
10 mL of diethyl ether was added to the viscous solution. A
white precipitate formed after an additional 5 min of stirring.
The precipitate was collected on a filter, washed with 20 mL
of pentane, and dried under vacuum to afford 1.3 g (7.0 mmol,
44% based on tBuMe2SiOSO2CF3) of 2 as a white solid. 1H
NMR (THF-d8): δ 5.96 (m, 2 H), 5.88 (m, 2 H), 0.87 (s, 9 H),
0.12 (s, 6 H). 13C NMR (THF-d8): δ 112.8 (CH), 106.6 (CH),
missing ipso C of C5H4 group, 27.5 (CH3), 18.2 (C), -3.8 (CH3).
Satisfactory elemental analysis was not obtained. The 1H NMR
spectrum is provided in the Supporting Information (Figure
S2) as evidence of substantial purity.
Exp er im en ta l Section
Gen er a l P r oced u r es. Standard inert atmosphere tech-
niques were used for all manipulations. Swivel-frit vessels
sealed with Kalrez7 O-rings and interfaced to a high-vacuum
line were used for reactions involving boron trihalides. BCl3
t
(1.0 M solution in CH2Cl2), BBr3 (neat), EtMe2SiCl, BuMe2-
SiCl, PhMe2SiCl, Ph2MeSiCl, and tBuSiMe2OSO2CF3 were
used as received from Aldrich. NaCp was prepared from
freshly distilled cyclopentadiene and NaH. ZrCl4(THF)2 was
prepared according to Manzer’s procedure.13 (Phenyldimeth-
ylsilyl)cyclopentadiene and (methyldiphenylsilyl)cyclopenta-
diene were prepared according to Kira et al.14 CpZrCl3 was
used as received from Strem Chemicals; CpZrCl3(DME) was
prepared as reported by Lund and Livinghouse.15 (Me3SiC5H4)-
CpZrCl2 was prepared according to the method of Siedle et
al.16 NMR spectra were recorded on a J EOL Eclipse-500
instrument. Elemental analyses were performed by Desert
Analytics (Tucson, AZ) or Oneida Research Services (Whites-
boro, NY).
Syn th esis of P h Me2SiC5H4Li (3). To a solution of (phen-
yldimethylsilyl)cyclopentadiene14 (5.66 g, 28.2 mmol) in hex-
n
anes (250 mL) was added 20 mL of BuLi solution (1.6 M in
hexanes, 32 mmol) using a syringe. After stirring for 15 h at
25 °C, the white precipitate was collected by filtration, washed
with pentane, and dried under vacuum to afford 5.25 g (25.4
1
mmol, 90%) of a white powder. H NMR (THF-d8): δ 7.58 (m,
2 H), 7.20 (m, 3 H), 5.96 (m, 2 H), 5.87 (m, 2 H), 0.40 (s, 6 H).
13C NMR (THF-d8): δ 143.9 (C), 133.8 (CH), 127.3 (CH), 126.9
(CH), 111.69 (CH), 106.1 (CH), -0.9 (CH3). One (C) signal is
missing and may be under a (CH) signal. Satisfactory elemen-
tal analysis was not obtained. The 1H NMR spectrum is
provided in the Supporting Information (Figure S3) as evidence
of substantial purity.
Syn t h esis of E t Me2SiC5H 4Li (1). A solution of ethyldi-
methylsilyl chloride (4.91 g, 0.040 mol) and sodium cyclopen-
tadienide (3.50 g, 0.040 mol) in THF (100 mL) was stirred for
3 h at 25 °C and then under reflux for 1 h. After cooling, the
solvent was evaporated. Pentane (100 mL) and water (50 mL)
were added. The organic layer was separated, washed with
saturated aqueous sodium bicarbonate, dried over anhydrous
magnesium sulfate, and evaporated to afford 5.34 g (0.035 mol,
88%), tentatively assigned to (ethyldimethylsilyl)cyclopen-
Syn th esis of P h 2MeSiC5H4Li (4). To a solution of (meth-
yldiphenylsilyl)cyclopentadiene14 (4, 4.77 g, 18.2 mmol) in
hexanes (60 mL) was added 15 mL of nBuLi solution (1.6 M in
hexanes, 24 mmol) via syringe. After stirring for 15 h at 25
°C, the white precipitate was collected by filtration, washed
with pentane, and dried under vacuum to afford 3.95 g (14.7
1
mmol, 81%) of a white powder. H NMR (THF-d8): δ 7.55 (m,
1
tadiene by H NMR analysis in CDCl3: δ 6.60 (br, 2 H), 6.56
4 H), 7.20 (m, 6 H), 6.08 (m, 2 H), 5.98 (m, 2 H), 0.66 (s, 3 H).
13C NMR (THF-d8): δ 141.7 (C), 134.9 (CH), 127.7 (CH), 126.9
(CH), 112.9 (CH), 106.5 (CH), 103.5 (C), -2.5 (CH3). Satisfac-
tory elemental analysis was not obtained. The 1H NMR
spectrum is provided in the Supporting Information (Figure
S4) as evidence of substantial purity.
(br, 2 H), 3.42 (br, 1 H), 0.95 (t, 3 H), 0.51 (q, 2 H), -0.06 (s,
6 H). This oil was transferred to a 500 mL Schlenk flask,
dissolved in 250 mL of hexanes, and treated with 25 mL of
nBuLi solution (1.6 M in hexanes, 0.040 mol). After stirring
for 15 h, the white precipitate was collected on a filter and
washed with pentane (2 × 50 mL) to afford 5.06 g (0.032 mol,
Syn th esis of (EtMe2SiC5H4)Cp Zr Cl2 (5). A mixture of
EtMe2SiC5H4Li (1, 0.414 g, 2.60 mmol), CpZrCl3(DME) (0.881
g, 2.50 mmol), and toluene (50 mL) was stirred at 25 °C for 12
h and then filtered. The filtrate was evaporated, and the
residue was recrystallized from hexanes/toluene to obtain
1
91%) of 1 as an air-sensitive white solid. H NMR (THF-d8):
δ 5.79 (m, 2 H), 5.72 (m, 2 H), 0.97 (t, J ) 8.1 Hz, 3 H), 0.58
(10) Alcalde, M. I.; Gomez-Sal M. P.; Royo, P. Organometallics 1999,
18, 546.
1
0.710 (1.89 mmol, 76%) of white crystals. H NMR (CD2Cl2):
(11) Deck, P. A.; Tobiesen, F. A. Unpublished results.
(12) Cheng, X.; Slebodnick, C.; Deck, P. A., Billodeaux, D. R.;
Fronczek, F. R. Inorg. Chem., in press. This paper shows that
trimethylstannyl (Me3Sn) groups attached to zirconocene dichloride
cleave two Me-Sn bonds when treated with BBr3, whereas one Me-
Sn bond is cleaved using BCl3, I2, or ICl.
δ 6.71 (m, 2 H), 6.56 (m, 2 H), 6.46 (s, 5 H), 0.92 (t, J ) 7.9
Hz, 2 H), 0.70 (q, J ) 7.9 Hz, 3 H), 0.28 (s, 6 H). 13C NMR
(CD2Cl2): δ 126.1 (C), 125.7 (CH), 117.6 (CH), 116.3 (CH), 8.7
(CH3), 7.38 (CH2), -2.74 (CH3). Anal. Calcd for C14H20Cl2-
SiZr: C, 44.42; H, 5.33. Found: C, 44.76; H, 5.58.
(13) Manzer, L. E. Inorg. Synth. 1987, 21, 135.
Syn th esis of (tBu Me2SiC5H4)Cp Zr Cl2 (10). A mixture of
tBuMe2SiCpLi (2, 651 mg, 3.50 mmol), CpZrCl3(DME) (1.20
g, 3.40 mmol), and toluene (50 mL) was stirred at 25 °C for 6
h, and then the solvent was evaporated. The residue was
(14) Kira, M.; Watanabe, M.; Sakurai, H. J . Am. Chem. Soc. 1977,
99, 7780.
(15) Lund, E. C.; Livinghouse, T. Organometallics 1990, 9, 2426.
(16) Siedle, A. R.; Lamanna, W. M.; Olofson, J . M.; Nerad, B. A.;
Newmark, R. A. ACS Symp. Ser. 1993, 517, 156.

5406 Organometallics, Vol. 19, No. 25, 2000
Deck et al.
recrystallized from hexanes/toluene to obtain 1.03 g (2.53
mmol, 74%) of colorless crystals in two crops. 1H NMR
(CDCl3): δ 6.70 (m, 2 H), 6.57 (m, 2 H), 6.46 (s, 5 H), 0.79 (s,
9 H), 0.34 (s, 6 H). 13C NMR (CDCl3): δ 125.2 (CH), 124.3 (C),
117.2 (CH), 116.0 (CH), 26.3 (CH3), 17.4 (C), -5.9 (CH3). Anal.
Calcd for C16H24Cl2SiZr: C, 47.27; H, 5.95. Found: C, 46.98;
H, 5.85.
Syn th esis of (Br 2MeSiC5H4)2Zr Br 2 (15). A solution of
(Ph2MeSiC5H4)2ZrCl2 (9, 0.34 g, 0.50 mmol) and BBr3 (2 mL)
in 1,2-dichloroethane (10 mL) was stirred under reflux for 15
h and then evaporated. Recrystallization of the dark solid
residue from hexanes/CH2Cl2 afforded 0.31 g (0.39 mmol, 79%)
of yellow-green needles. 1H NMR (CDCl3): δ 7.07 (m, 4 H),
6.73 (m, 4 H), 1.46 (s, 6 H). 13C NMR (CDCl3): δ 126.7 (CH),
Syn t h esis of (tBu Me2SiC5H 4)2Zr Cl2 (7). A mixture of
tBuMe2SiCpLi (2, 0.781 g, 4.20 mmol) and ZrCl4(THF)2 (792
mg, 2.1 mmol) in THF (50 mL) was stirred for 12 h at 25 °C.
After evaporating the solvent, the residue was recrystallized
from hexanes/toluene to afford 0.870 g (1.67 mmol, 80%) of
off-white crystals in two crops. 1H NMR (CDCl3): δ 6.61 (m, 4
H), 6.47 (m, 4 H), 0.73 (s, 18 H), 0.29 (s, 12 H). 13C NMR
(CDCl3): δ 125.8 (CH), 124.1 (C), 116.3 (CH), 26.3 (CH3), 17.4
(C), -5.9 (CH3). Anal. Calcd for C22H38Cl2Si2Zr: C, 50.73; H,
7.35. Found: C, 50.98; H, 7.25.
118.04 (CH), 118.00 (C), 10.2 (CH3). Anal. Calcd for C12H14
Br6Si2Zr: C, 18.36; H, 1.80. Found: C, 18.51; H, 1.65.
-
Rea ction of (EtMe2SiC5H4)Cp Zr Cl2 (5) w ith BBr 3. A
solution of 5 (0.378 g, 1.00 mmol) and BBr3 (2 mL) in
1,2-dichloroethane (15 mL) was stirred under reflux for 24 h.
After evaporating the volatile components, the dark residue
was extracted with 30 mL of hot hexanes, filtered, and
evaporated to give 0.460 g (about 85%) of a silver-gray solid.
1H NMR analysis showed two major products assigned to 10
and 11 in a ratio of 4:1. Data for 10: 1H NMR (CDCl3): δ 6.93
(m, 1 H), 6.91 (m, 1 H), 6.71 (m, 1 H), 6.59 (s, 5 H), 6.54 (m, 1
H), 1.22 (m, 2 H), 1.08 (m, 3 H), 0.88 (s, 3 H). The assignment
of 11 was corroborated by spectra obtained for the authentic
sample prepared from (Me3SiC5H4)CpZrCl2 and BBr3 (see
above). Recrystallization of crude 10 did not lead to an
improvement in its purity, so elemental analysis was not
attempted.
Syn th esis of (P h Me2SiCp )2Zr Cl2 (8). A solution of ZrCl4-
(THF)2 (3.38 g, 9.0 mmol) and PhMe2SiC5H4Li (3, 3.71 g, 18.0
mmol) in THF (200 mL) was stirred at 25 °C for 15 min, and
then the solvent was evaporated. The residue was recrystal-
lized from hexane/toluene to afford 3.57 g (6.4 mmol, 71%) of
white needles in two crops. 1H NMR (CDCl3): δ 7.48 (m, 4 H),
7.35 (m, 6 H), 6.55 (m, 4 H), 6.25 (m, 4 H), 0.57 (s, 12 H). 13C
NMR (CDCl3): δ 138.0 (C), 134.0 (CH), 129.4 (CH), 127.9 (CH),
126.1 (CH), 123.8 (C), 117.1 (CH), -1.8 (CH3). Anal. Calcd for
Rea ction of (tBu Me2SiC5H4)Cp Zr Cl2 (6) w ith BBr 3. A
solution of 6 (0.905 g, 2.22 mmol) and BBr3 (5 mL) in
1,2-dichloroethane (20 mL) was stirred under gentle reflux
(about 85 °C) for 24 h. After evaporating the volatile compo-
nents, the dark residue was extracted with 50 mL of hot
hexanes, filtered, and evaporated to give 1.15 g (about 92%)
C
26H30Cl2Si2Zr: C, 55.68; H, 5.39. Found: C, 55.71; H, 5.50.
Syn th esis of (P h 2MeSiCp )2Zr Cl2 (9). A suspension of
ZrCl4(THF)2 (1.13 g, 3.0 mmol) and Ph2MeSiC5H4Li (4, 1.70
g, 6.4 mmol) in toluene (60 mL) was stirred under reflux for 3
h. The resulting orange mixture was filtered, and the filtrate
was evaporated to afford an orange residue. Recrystallization
from toluene afforded 1.22 g (1.8 mmol, 60%) of yellow crystals
1
of a gray solid. H NMR analysis showed two major products
assigned to 13 and 11 in a ratio of about 15:1. Data for 13:
1H NMR (CDCl3): δ 7.11 (m, 1 H), 7.06 (m, 1 H), 6.86 (m, 1
H), 6.64 (s, 5 H), 6.33 (m, 1 H), 0.92 (s, 9 H), 0.84 (s, 3 H). 13C
NMR (CDCl3): δ 128.6 (CH), 126.1 (CH), 121.7 (CH), 116.8
(CH), 114.0 (CH), 25.4 (CH3), 20.5 (C), -1.4 (CH3); ipso C of
C5H4 group not observed. Recrystallization did not lead to
improvement in the purity, so elemental analysis was not
attempted.
Cr ysta llogr a p h ic Stu d ies. Crystals of 9 were grown by
preparing a concentrated solution in warm toluene and cooling
to 25 °C. Crystals of 15 were obtained by adding dichloro-
methane in small portions to a warm suspension of 15 in
hexanes until the complex dissolved and then allowing the
resulting solution to cool to 25 °C. Crystallographic data are
presented in Table 1. Complete data collection, solution, and
refinement data as well as complete tables of metric param-
eters are included in the Supporting Information.
1
in three crops. H NMR (CDCl3): δ 7.37 (m, 20 H), 6.49 (m, 4
H), 6.05 (m, 4 H), 0.89 (s, 6 H). 13C NMR (CDCl3): δ 136.0 (C),
135.2 (CH), 129.8 (CH), 128.0 (CH), 125.8 (CH), 120.7 (C),
119.5 (CH), -3.5 (CH3). Anal. Calcd for C36H34Cl2Si2Zr: C,
63.13; H, 5.00. Found: C, 63.45; H, 5.11.
Syn th esis of (Br Me2SiC5H4)Cp Zr Br 2 (11). A solution of
(Me3SiC5H4)CpZrCl2 (12, 540 mg, 1.5 mmol) and BBr3 (4.0 mL)
in 1,2-dichloroethane (50 mL) was stirred under reflux in a
sealed glass reaction vessel for 15 h. After cooling the reaction,
the volatile components were evaporated, and the product was
extracted with hot hexanes. Recrystallization from hexanes
afforded 0.580 g (1.1 mmol, 75%) of 11 in two crops of pale
1
green needles. H NMR (CDCl3): δ 6.90 (m, 2 H), 6.61 (m, 2
H), 6.57 (s, 5 H), 0.94 (s, 6 H). 13C NMR (CDCl3): δ 125.5 (CH),
121.0 (C), 116.8 (CH), 116.3 (CH), 4.3 (CH3). Anal. Calcd for
C
12H15Br3SiZr: C, 27.81; H, 2.92. Found: C, 28.03; H, 2.88.
Syn th esis of (Me3SiC5H4)Cp Zr Cl2 (12). A solution of
Resu lts
CpZrCl3 (1.3 g, 5.0 mmol) and Me3SiC5H4Li (0.73 g, 5.0 mmol)
in THF (100 mL) was stirred at 25 °C for 2 h, and then the
solvent was evaporated. The residue was extracted with
toluene (50 mL), filtered, and evaporated. The crude product
was recrystallized from hexanes (120 mL) to obtain 1.0 g (2.7
mmol, 55%) of colorless crystals. 1H NMR (CDCl3): δ 6.70 (m,
2 H), 6.52 (m, 2 H), 6.44 (s, 5 H), 0.23 (s, 9 H). 13C NMR
(CDCl3): δ 126.4 (C), 125.3 (CH), 117.1 (CH), 115.9 (CH), -0.1
(CH3). Anal. Calcd for C13H18Cl2SiZr: C, 42.82; H, 4.94.
Found: C, 42.45; H, 4.92.
Syn th esis of (ClMe2SiCp)2Zr Cl2 (14). A solution of (PhMe2-
SiC5H4)2ZrCl2 (8, 0.70 g, 1.2 mmol) and 50 mL of BCl3 solution
(1.0 M in CH2Cl2, 50 mmol) was stirred under reflux for 15 h
and then evaporated. Recrystallization of the residue from
hexanes afforded 0.43 g (0.90 mmol, 72%) of pink crystals in
two crops. 1H NMR (CDCl3): δ 6.81 (m, 4 H), 6.60 (m, 4 H),
0.74 (s, 12 H). 13C NMR (CDCl3): δ 126.1 (CH), 122.3 (C), 116.5
(CH), 3.1 (CH3). These spectra are consistent with those
obtained by Royo and co-workers for the same compound
prepared by a different method.9 Anal. Calcd for C14H20Cl4Si2-
Zr: C, 35.21; H, 4.22. Found: C, 35.35; H, 4.23.
Su bstr a te Syn th eses. Ligands 1-4 needed for this
study were prepared from NaCp and the corresponding
chlorosilane reagents (Scheme 1) followed by metalation
with nBuLi. These ligands are not presented here as
fully characterized compounds, as satisfactory elemental
analyses were not obtained. However, yields obtained
in their preparation are presented in the Experimental
Section, and NMR spectroscopic evidence for their bulk
purity is provided in the Supporting Information to
assist those who may wish to prepare these substances
by our methods. Further reaction of the lithiated ligands
(1-4) with CpZrCl3, CpZrCl3(DME), or ZrCl4(THF)2,
(Scheme 2) afforded the desired metallocene dihalide
substrates (5-9) in moderate yields.
Si-R/B-Br Meta th esis Selectivity Stu d ies. In the
reaction of the EtMe2Si-substituted metallocene (5) with
BBr3 in refluxing 1,2-dichloroethane (Scheme 3), the
crude product mixture showed a 4:1 mixture of 10 (Si-

Electrophile-Functionalized Zirconocene Dibromides
Organometallics, Vol. 19, No. 25, 2000 5407
Ta ble 1. Cr ysta llogr a p h ic Da ta
Sch em e 2
9
15
empirical formula
fw
C
36H34Cl2Si2Zr
C12H14Br6Si2Zr
785.03
684.93
diffractometer
Siemens P4
Enraf-Nonius
Kappa CCD
cryst dimens (mm)
cryst syst
a (Å)
b (Å)
c (Å)
R (deg)
â (deg)
γ (deg)
V (Å)3
0.35 × 0.35 × 0.35 0.10 × 0.10 × 0.12
triclinic
10.1109(14)
13.0604(18)
13.239(2)
80.538(12)
103.389(5)
71.456(10)
1634.4(4)
P1h (No. 2)
2
triclinic
6.8170(2)
14.6660(4)
22.1520(7)
105.3520(15)
91.5380(18)
90.4540(16)
2134.66(11)
P1h (No. 2)
4
space group
Z
D
calc (Mg m-3
)
1.392
2.443
abs coeff (mm-1
F000
)
0.60
11.8
704
1456
λ (Mo KR) (Å)
temp (K)
0.71073
293(2)
0.71073
100(2)
θ range for collection
no. of reflns colld
no. of indep reflns
abs corr method
1.60-29.00
1.4-30.00
9883
19 436
8610
ψ scans
12 401
multiscan
12 401/0/383
0.0400
0.0780
0.85
no. of data/restrts/params 8610/0/506
R [I > 2σ(I)]
0.0373
0.0785
0.86
Rw [I > 2σ(I)]
GoF on F2
largest diff peak and
0.292, -0.407
1.92, -0.91
hole (e Å-3
)
Sch em e 1
Sch em e 3
Me bond cleavage) and 11 (Si-Et bond cleavage). The
product mixture was not significantly enriched in either
component upon recrystallization from hexanes/toluene,
which prevented us from isolating 10 as a pure sub-
stance. Assignment of the products was carried out by
1H NMR spectroscopic of the product mixtures. The
minor product (11) retains the Cs symmetry of its
precursor (5), so only two resonances were observed for
BrMe2Si-substituted Cp ligand. The assignment of 11
was confirmed by comparison with an authentic sample
prepared from 12 (Scheme 3). The major product (10)
has C1 symmetry (stereogenic Si), and all four protons
of the EtMeBrSi-substituted Cp ligand are diaste-
reotopic.
3) was much more selective. The crude product con-
tained a 15:1 mixture of 13 (Si-Me bond cleavage) and
11 (Si-tBu bond cleavage), according to 1H NMR
spectroscopic analysis. Despite the small amount of 11
present, recrystallization of the crude product did not
1
lead to significantly higher purity of 13. The H NMR
spectrum of 13 showed the expected four signals for the
diastereotopic protons of the substituted Cp ligand. With
t
either the EtMe2Si-substituted or BuMe2Si-substituted
metallocenes (5 and 6), a second bromodealkylation did
not occur, even after long reaction times, in accord with
our previous studies of Me3Si-substituted group 4 met-
allocene complexes.8
The reaction of the tBuMe2Si-substituted metallocene
(6) with BBr3 in refluxing 1,2-dichloroethane (Scheme

5408 Organometallics, Vol. 19, No. 25, 2000
Deck et al.
Ta ble 2. Metr ic P a r a m eter s for Cr ysta llin e 9
Bond Distances (Å)
Cp(1)a-Zr
Cp(2)b-Zr
Zr-Cl(1)
2.210(2)
Si(1)-C(21)
Si(1)-C(31)
Si(2)-C(6)
Si(2)-C(12)
Si(2)-C(41)
Si(2)-C(51)
1.881(3)
1.872(2)
1.876(2)
1.858(4)
1.873(3)
1.869(2)
2.224(2)
2.4253(9)
2.4337(8)
1.880(2)
1.849(4)
Zr-Cl(2)
Si(1)-C(1)
Si(1)-C(11)
Bond Angles (deg)
Cp(1)a-Zr-Cp(2)b
Cl(1)-Zr-Cl(2)
C(1)-Si(1)-C(11)
C(1)-Si(1)-C(21)
C(1)-Si(1)-C(31)
130.2(2)
95.90(3) C(6)-Si(2)-C(12)
111.3(1)
104.5(1)
110.0(2)
C(21)-Si(1)-C(31) 109.1(1)
113.5(2)
104.1(1)
107.4(1)
C(6)-Si(2)-C(41)
C(6)-Si(2)-C(51)
C(12)-Si(2)-C(41) 109.5(2)
C(12)-Si(2)-C(51) 111.5(2)
C(41)-Si(2)-C(51) 110.6(1)
C(11)-Si(1)-C(21) 110.8(2)
C(11)-Si(1)-C(31) 110.9(2)
Torsional Angles (deg)
C(2)-C(1)-Si(1)-C(11) 177.5 C(7)-C(6)-Si(2)-C(12) 158.3
a
b
Cp(1) is the centroid of C(1), C(2), C(3), C(4), and C(5). Cp(2)
is the centroid of C(6), C(7), C(8), C(9), and C(10).
F igu r e 1. Thermal ellipsoid plot of the molecular structure
of 9 shown at 50% probability. Hydrogen atoms (except H8)
are omitted for clarity. The dashed line is drawn from H(8)
to the C(31)-C(36) ring centroid to indicate a “hydrogen
bonding” interaction.
Sch em e 4
F igu r e 2. Thermal ellipsoid plot of the molecular structure
of 15 (one of two nearly identical molecules in the asym-
metric unit) shown at 50% probability. Hydrogen atoms are
omitted for clarity.
presumably to avoid intramolecular repulsions of the
bulky substituents. Both Ph2MeSi substituents are
oriented so that the methyl groups are directed into the
ZrBr2 hemisphere, with C(5)-C(1)-Si(1)-C(11) and
C(10)-C(6)-Si(2)-C(12) torsion angles of 17.1° and
42.7°, respectively. Crude semiempirical (PM3) model-
ing of 9 using Spartan Pro software (Wavefunction, Inc.)
suggests that the preferences for particular Cp-Si
conformations are relatively weak. Local conformational
energy minima with either CH3 or Ph directed toward
the dichloride “wedge” region were found to be within
1 kcal mol-1 of one another. A noteworthy feature of
the structure of 9 is the presence of an edge-to-face Cp-
to-phenyl “hydrogen bonding” contact (presented as a
dashed line in Figure 1). The nearly identical H-to-
centroid (2.613 Å) and H-to-least-squares-plane (2.602
Å) distances show that the hydrogen atom (H8) is well-
centered on the face of the C31-C36 ring.
Si-P h /B-X Meta th esis Stu d ies. When the bis-
(phenyldimethylsilyl)-substituted zirconocene dichloride
(8) reacted with excess BCl3 in refluxing dichlo-
romethane, rapid conversion to bis(chlorodimethylsilyl)-
zirconocene dichloride (14) was observed (Scheme 4).
When the corresponding bis(methyldiphenylsilyl)-sub-
stituted complex (9) was treated with excess BCl3 under
similar conditions, a complex mixture of products was
observed, suggesting that some cleavage of a second
phenyl group occurred. Treatment of 9 with an excess
of the more reactive BBr3 in refluxing 1,2-dichloroethane
afforded the desired 1,1′-bis(methyldibromosilyl)zircono-
cene dibromide (15) in 79% yield.
Str u ctu r al Resu lts. Crystalline 1,1′-bis(methyldiphe-
nylsilyl)zirconocene dichloride (9) was analyzed by X-ray
diffraction. A thermal ellipsoid plot of the molecular
structure is presented in Figure 1, and selected metric
parameters are presented in Table 2. The complex
adopts an approximately C2-symmetric conformation,
A thermal ellipsoid plot of the molecular structure of
15 is shown in Figure 2, with accompanying metric data
in Table 3. This complex also adopts an approximate
C2-symmetric conformation strikingly similar to that
found in its immediate synthetic precursor (9), with
C(5)-C(1)-Si(1)-C(11) and C(7)-C(6)-Si(2)-C(12) tor-
sion angles of 11.5° and 19.2°, respectively. In the case

Electrophile-Functionalized Zirconocene Dibromides
Organometallics, Vol. 19, No. 25, 2000 5409
Ta ble 3. Metr ic P a r a m eter s for Cr ysta llin e 15a
by using the phenyl substituent to accommodate most
of the positive charge as in electrophilic aromatic
substitution (17).
Bond Distances (Å)
Cp(1)b-Zr
Cp(2)c-Zr
Zr-Br(1)
2.205(3)
2.206(3)
2.6018(8)
2.6013(8)
1.841(6)
1.831(6)
Si(1)-Br(3)
Si(1)-Br(4)
Si(2)-C(6)
Si(2)-C(12)
Si(2)-Br(5)
Si(2)-Br(6)
2.218(2)
2.223(2)
1.841(6)
1.823(6)
2.219(2)
2.231(2)
Zr-Br(2)
Si(1)-C(1)
Si(1)-C(11)
Bond Angles (deg)
129.7(2) Br(3)-Si(1)-Br(4) 105.2(6)
Cp(1)b-Zr-Cp(2)c
Br(1)-Zr-Br(2)
C(1)-Si(1)-C(11)
C(1)-Si(1)-Br(3)
C(1)-Si(1)-Br(4)
95.31(2) C(6)-Si(2)-C(12)
116.4(3)
110.1(2)
104.1(2)
114.1(3)
112.2(2)
105.2(2)
C(6)-Si(2)-Br(5)
C(6)-Si(2)-Br(6)
C(12)-Si(2)-Br(5) 111.5(2)
C(12)-Si(2)-Br(6) 108.6(2)
Br(5)-Si(2)-Br(5) 105.2(6)
C(11)-Si(1)-Br(3) 110.5(2)
C(11)-Si(1)-Br(4) 109.1(2)
Torsional Angles (deg)
C(2)-C(1)-Si(1)-C(11) 1.9(4) C(7)-C(6)-Si(2)-C(12) 19.2(6)
a
Data are shown for one of two nearly identical molecules in
the asymmetric unit. b Cp(1) is the centroid of C(1), C(2), C(3), C(4),
and C(5). c Cp(2) is the centroid of C(6), C(7), C(8), C(9), and C(10).
of 15, however, one can rationalize the apparent prefer-
ence for Cp-Si conformations; the electrostatic dipole
of the ZrBr2 group (negative end toward the viewer in
Figure 2) opposes the dipoles of the SiBr2 groups
(negative end away from the viewer in Figure 2),
minimizing the overall polarity of the complex. Exami-
nation of several packing diagrams revealed numerous
weak Cp-H‚‚‚Br-Zr and Cp-H‚‚‚Br-Si contacts rang-
ing in distance from 2.85 to 3.20 Å. These packing forces
could also be responsible for the observed Cp-Si con-
formations. A PM3 modeling study provided no further
insight into these Cp-Si conformations.
Con text of Th ese Stu d ies. An alternative method
has been developed by Royo and co-workers, in which
ClMe2Si and Cl2MeSi groups were attached to the ligand
prior to metal complexation.9,19
Our Si-R and Si-Ph metathesis approaches intro-
duce the reactive Si-X groups after the formation of the
metallocene. The availability of these two complemen-
tary routes is advantageous to the synthetic chemist
designing new metallocene complexes.
Con clu sion s
Although interesting from a mechanistic point of view,
the selectivity observed for Si-Me vs Si-Et cleavage
using BBr3 does not appear to be useful synthetically.
The higher selectivity of Si-Me vs Si-tBu cleavage may
enable us to prepare metallocenes such as 18, which
should form both rac and meso diastereomers. Studies
directed toward the preparation of 18 are underway.
Phenylated silyl substituents (PhMe2Si and Ph2MeSi)
are readily introduced into cyclopentadienyl ligands,
and these substituents are stable under the conditions
required to prepare group 4 metallocenes. After the
assembly of the metallocene, the phenyl groups can be
easily cleaved using BCl3 or BBr3 to afford metallocene
complexes having highly electrophilic ClMe2Si or Br2-
MeSi substituents. We are presently exploring the
reactivity of methyldibromosilyl-substituted metal-
locenes (15) toward nucleophiles and the immobilization
of these functionalized metallocenes on inorganic sup-
ports.
Discu ssion
Im p lica tion s of Obser ved Exch a n ge Selectivi-
ties. The presence of two methyl groups and one ethyl
group in 5 favors Si-Me cleavage by a 2-fold statistical
factor. The observed ratio of 10:11 (4:1) means that
intrinsic structural features contribute an additional
2-fold factor to the overall observed selectivity. An
electrophilic pathway proceeding via a transition state
(or intermediate) in which silicon bears a partial positive
charge (16) could rationalize this selectivity,17 and the
simplest argument is that the methylene carbon of the
ethyl group is sterically more encumbered than the
methyl carbons. The drawback of 16 as a proposed
transition state is that one would not expect to observe
Si-tBu cleavage products, because Si-tBu cleavage
would require direct approach of the boron atom to the
quaternary carbon atom. Yet, Si-tBu cleavage products
account for about 7% of the isolated product mixture.
In contrast, the Si-Ph bond was previously known to
undergo rapid metathesis with B-X bonds to afford
arylboron dihalides in high yields under relatively mild
conditions.18 The relative facility of Si-Ph cleavage in
complexes 8 and 9 was expected to be much greater than
Si-alkyl cleavage. This difference can be rationalized
Ack n ow led gm en t. We thank H.-Y. Kang for the
syntheses of 11 and 12. Funding was provided by the
National Science Foundation (CHE-9875446) and Re-
search Corporation (CS0567).
Su p p or tin g In for m a tion Ava ila ble: Tables of crystal-
lographic data for complexes 9 (Tables S1-S8) and 15 (Tables
S9-S16). 1H NMR spectra of 1-4 (Figures S1-S4). This
material is available free of charge via the Internet at
http://pubs.acs.org.
(17) Several different permutations of axial and equatorial ligands
could be envisioned in the putative metathesis transition state. The
permutation selected for diagram 20 was arbitrary, except that we
assumed that either the entering halide or leaving hydrocarbyl group
would be axial.
OM0005452
(18) Gross, U.; Kaufmann, D. Chem. Ber. 1987, 120, 991-4. (b)
Kaufmann, D. Chem. Ber. 1987, 120, 901-5. (c) Kaufmann, D. Chem.
Ber. 1987, 120, 853-4.
(19) Royo, B.; Royo, P.; Cadenas, L. M. J . Organomet. Chem. 1998,
551, 293.