Reactivity of the base-stabilized bis(silylene)iron complex (η5-C5H5)Fe(CO)(η2-SiMe 2-OtBu-SiMe2): Elevated temperature trapping of SiMe2 by R3EH (R = Me<su

Article abstract of DOI:10.1021/om000563j

The base-stabilized silylene complex (η⁵-C₅H₅)-Fe(CO)(η ²-SiMe₂-O^tBu-SiMe₂) is unreactive toward (Me₃-Si)₃EH (E = Si, Ge) under photochemical irradiation or

Full text of DOI:10.1021/om000563j

Organometallics 2001, 20, 7-9
7
Communications
Rea ctivity of th e Ba se-Sta bilized Bis(silylen e)ir on
Com p lex (η5-C5H5)F e(CO)(η2-SiMe2-OtBu -SiMe2): Eleva ted
Tem p er a tu r e Tr a p p in g of SiMe2 by R3EH (R ) Me3Si, E )
Si, Ge) a n d Elim in a tion of Me2(OtBu )SiSiMe2H by
n -Bu 3Sn H
Hemant K. Sharma and Keith H. Pannell*
Department of Chemistry, University of Texas at El Paso, El Paso, Texas 79968-0513
Received J une 30, 2000
Summary: The base-stabilized silylene complex (η5-C5H5)-
Sch em e 1
Fe(CO)(η2-SiMe2-OtBu-SiMe2) is unreactive toward (Me3-
Si)3EH (E ) Si, Ge) under photochemical irradiation
or at room temperature. However, at 80 °C it reacts,
presumably via the equilibrium concentration of base-
free complex (η5-C5H5)Fe(CO)(SiMe2OtBu)(dSiMe2), to
transfer the silylene group and form (Me3Si)3ESiMe2H.
Attempts to transfer the SiMe2 group to tributyltin
hydride led to formation of bis(tributylstannyl)iron
complexes.
The transition-metal silylene complexes LMdSiR2
have been proposed as transients in a number of metal-
mediated silylene group transfers,1 including metal-
catalyzed Si-Si bond formation.2 Although many metal
silylene complexes have been characterized,3 only scat-
tered reports on their direct reactivity are available.4
The Tilley group reported that the cationic ruthenium
silylene [(η5-C5Me5)(PMe3)2RudSiPh2]+ transfers the
silylene group to alcohols, ketones, and acetic acid,4b and
the related osmium complex [(η5-C5Me5)(PMe3)2Osd
SiMe2]+ reacts with benzyl chloride to form the (di-
methylchlorosilyl)osmium(III) derivative, a possible clue
to the mechanism of the copper-catalyzed Direct Pro-
cess.4c We report the elevated-temperature reaction
between the base-stabilized bis(silylene)iron complex
(η5-C5H5)Fe(CO)(η2-SiMe2-OtBu-SiMe2) (1b) and R3EH
(R ) Me3Si, E ) Si, Ge; R ) nBu, E ) Sn), which
demonstrates silylene transfer from such base-stabilized
complexes forming Si-Si and Si-Ge bonds.
The photolysis of (η5-C5H5)Fe(CO)2SiR2SiR3 results in
initial CO expulsion, followed by R-elimination to form
transient iron silyl silylene complexes that can isomerize
via a series of 1,3-alkyl/aryl migrations.5a Continued
irradiation results in silylene elimination or recombina-
tion isomerizations when the silicon chain exceeds 2,
e.g. R ) Me3Si(SiMe2)n (n ) 1, 2, etc.) in Scheme 1.5b,c
We have isolated the iron-silylene intermediates as
intramolecular arylCr(CO)2 species,5e and the Ogino
group has demonstrated that the presence of an alkoxy
group on silicon results in the formation of stable
alkoxy-stabilized bis(silylene)iron complexes, (η5-C5H5)-
Fe(CO)(η2-SiR2-OR′-SiR2).6 The latter complexes do not
undergo photochemical silylene elimination reactions,
and in general, they are very stable complexes. The
Ogino group also showed that the base-stabilized com-
plexes readily react with methanol4d and using VT NMR
(1) (a) Corey, J . Y.; Braddock-Wilking, J . Chem. Rev. 1999, 99, 175.
(b) Sharma, H. K.; Pannell, K. H. Chem. Rev. 1995, 95, 1351. (c) Tilley,
T. D. In The Silicon Heteroatom Bond; Patai, S., Rappoport, Z., Eds.;
Wiley: New York, 1991; Chapters 9 and 10.
(2) (a) Harrod, J .; Zeigler, T.; Tschinke, V. Organometallics 1990,
9, 897. (b) Yamashita, H.; Tanaka, M.; Goto, M. Organometallics 1992,
11, 3227. (c) Gavin, F.; Harrod, J . F.; Woo, H. G. Adv. Organomet.
Chem. 1998, 42, 363. (d) Tilley, T. D. Comments Inorg. Chem. 1990,
10, 37. (e) Hengge, E.; Weinberger, J . J . Organomet. Chem. 1993, 443,
167.
(3) (a) Straus, D. A.; Grumbine, S. D.; Tilley, T. D. J . Am. Chem.
Soc. 1990, 112, 7801. (b) Grumbine, S. D.; Tilley, T. D.; Arnold, F. P.;
Rheingold, A. L. J . Am. Chem. Soc. 1993, 115, 7884. (c) Denk, M.;
Hayashi, R. K.; West, R. J . Chem. Soc., Chem. Commun. 1994, 33. (d)
Grumbine, S. D.; Tilley, T. D. J . Am. Chem. Soc. 1994, 116, 6951. (e)
Mitchell, G. P.; Tilley, T. D. J . Am. Chem. Soc. 1997, 119, 11236. (f)
Mitchell, G. P.; Tilley, T. D. J . Am. Chem. Soc. 1998, 120, 7635. (g)
Peters, J . C.; Feldman, J . D.; Tilley, T. D. J . Am. Chem. Soc. 1999,
121, 9871. (h) Gerhhus, B.; Hitchcock, P. B.; Lappert, M. F.; Macie-
jewski, H. Organometallics 1998, 17, 5599.
(4) (a) Zybill, C.; Wilkinson, C. L.; Mu¨ller, G. Angew. Chem., Int.
Ed. Engl. 1989, 28, 203. (b) Zhang, C.; Grumbine, S. D.; Tilley, T. D.
Polyhedron 1991, 10, 1173. (c) Wamadi, P. W.; Glaser, P. B.; Tilley, T.
D. J . Am. Chem. Soc. 2000, 122, 972. (d) Ueno, K.; Seki, S.; Ogino, H.
Chem. Lett. 1993, 2159.
(5) (a) Pannell, K. H.; Cervantes, J .; Hernandez, C.; Cassias, J .;
Vincenti, S. P. Organometallics 1986, 5, 1056. (b) Pannell, K. H.; Wang,
L.-J .; Rozell, J . M. Organometallics 1989, 8, 550. (c) Hernandez, C.;
Sharma, H. K.; Pannell, K. H. J . Organomet. Chem. 1993, 462, 259.
(d) Pannell, K. H.; Brun, M.-C.; Sharma, H. K.; J ones, K.; Sharma, S.
Organometallics 1994, 13, 1075. (e) Pannell, K. H.; Sharma, H. K.;
Kapoor, R. N.; Cervantes-Lee, F. J . Am. Chem. Soc. 1997, 119, 9315.
(6) (a) Ueno, K.; Tobita, H.; Shimoi, M.; Ogino, H. J . Am. Chem.
Soc. 1988, 110, 4092. (b) Tobita, H.; Ueno, K.; Shimoi, M.; Ogino, H.
J . Am. Chem. Soc. 1990, 112, 3415. (c) Ueno, K.; Ogino, H. Bull. Chem.
Soc. J pn. 1995, 68, 1955.
10.1021/om000563j CCC: $20.00 © 2001 American Chemical Society
Publication on Web 12/01/2000

8
Organometallics, Vol. 20, No. 1, 2001
Communications
Sch em e 2
spectroscopy showed that at elevated temperatures
these complexes exist in equilibrium with the base-free
form, (η5-C5H5)Fe(CO)(SiR2OR)(dSiR2)6c (eq 1).
The presence of (Me3Si)3SiH during the photochemical
irradiation of (η5-C5H5)Fe(CO)2SiMe2SiMe2-OtBu (1a )
does not interfere with the direct formation and isolation
of the base-stabilized complex, and no further chemistry
is observed. This is in contrast to non-base-stabilized
analogues formed as transients upon photochemical
treatment of (η5-C5H5)Fe(CO)2SiR2SiR3, where the
silylene could be readily intercepted by (Me3Si)3SiH.5d
However, we reasoned that the equilibrium concentra-
tion of the base-free silylene was too small for a
significant reaction to occur and have repeated related
reactions at elevated temperatures, 80 °C, where sig-
nificant concentrations are present. After the room-
temperature photochemical irradiation of 1a in the
presence of (Me3Si)3SiH to form 1b, we stopped ir-
radiating, raised the temperature of the mixture, and
monitored the ensuing reaction by 1H, 13C, and 29Si
NMR spectroscopy.7 Such analysis demonstrated the
formation of (Me3Si)3Si-SiMe2H,8 (Me3Si)3Si-Si(SiMe3)2,9
Fp-Si(SiMe3)3,5b and Fp-SiMe2OtBu. We suggest that the
mechanism outlined in Scheme 2 satisfactorily accounts
for this product distribution, and it is relevant that the
addition of SiH to metal silylenes to form disilylmetal
complexes has been successfully modeled using density
functional theory.2d
Sch em e 3
The formation of high yields of (Me3Si)3SiSi(SiMe3)3
suggests that addition of the Si-H bond to the metal
silylene may involve a radical process, reminiscent of
benzyl chloride addition to the Os-silylene complex
noted above.4c Similar results were obtained using (Me3-
Si)3GeH as the trapping reagent. Both the bulky silane
and germane we used are useful reducing agents with
weak E-H bonds.11
(7) In a typical experiment, a 5-mm Pyrex NMR tube was charged
with 0.04 g (0.11 mmol) of FpSiMe2SiMe2OtBu6a and 0.081 g (0.32
mmol) of (Me3Si)3SiH in 1.0 mL of degassed C6D6 and sealed under
vacuum. Photolysis using a 450 W medium-pressure mercury lamp,
at a distance of 5 cm, was monitored by NMR spectroscopy. After 4 h
of irradiation, 1b was cleanly formed. No side products were noted.
Thermolysis of the product mixture was carried out at 80 °C in an
oven. The progress of the thermolysis reaction was monitored by NMR
spectroscopy. After 16 h, 29Si NMR spectroscopy showed the complete
consumption of 1b and the formation of (Me3Si)3SiSiMe2H,8a (Me3-
Si)3SiSi(SiMe3)3,9b FpSi(SiMe3)3,5b FpSiMe2OtBu7b along with unreacted
(Me3Si)3SiH and trace amounts of unidentified silicon-containing
compounds. GC/mass spectroscopic analysis of the reaction mixture
also confirmed the formation of (Me3Si)3Si-SiMe2H (17% yield) and
(Me3Si)3Si-Si(SiMe3)3 (33% yield), respectively. The solvent was
removed from the reaction mixture under vacuum, and 5 mL of hexane
was added. The solution was filtered and passed through a small
column (silica gel, 1 × 2 cm), and the resulting solution was evaporated
to dryness. The solid residue was recrystallized twice from hexanes to
yield (Me3Si)3SiSiMe2H (0.01 g, 10% yield), whose spectroscopic and
physical properties were identical with those reported.5d,8a Using the
same general procedure, (Me3Si)3GeSiMe2H8b was isolated in 15% yield
from the thermolysis reaction of 1b with (Me3Si)3GeH. Similar results
were obtained by starting with a pure sample of 1b rather than forming
it in situ. However, in this case the relative yield of FpSiMe2OtBu was
greatly reduced and is presumably formed by the liberation of CO from
general decomposition pathways of the complexes involved in the
chemistry. (b) Attempts to make this material by a direct route, e.g.
[Fp]-Na+ + ClSiMe2OtBu, led to the desired product, bp 100-110 °C
(0.01 mm/Hg), contaminated by an uncharacterized material which,
to date, has defied separation. NMR (ppm, 300 MHz, C6D6): 1H, 4.16
(C5H5), 1.29 (tBu), 0.70 (SiMe2); 29Si, 56.0. Mass spectrum (m/z, % ion
current): FpSiMe2OtBu, molecular ion peak, M+ ) 308 (6%), 280 (M
- CO) (6%), 252 (M - 2 CO (5%), 235 (M - tBuO, 8%), 196 (M - C4H8,
18%), 180 (M - C4H8O, 6%), 131 (SiMe2OtBu, 7%), 121 (C5H5Fe, 8%),
56 (Fe, 4%).
We attempted to perform a similar trapping experi-
n
ment with the well-known organotin reducing agent
-
Bu3SnH. We monitored the reaction by 1H, 13C, and
119Sn NMR spectroscopy and did not observe the forma-
n
tion of the SiMe2 insertion product Bu3SnSiMe2H. The
observed reaction products and proposed reaction se-
quence is outlined in Scheme 3.
The hydridobis(stannyl)iron complex CpFe(CO)(Sn-
Bu3)2(H) (2) was isolated in 16% yield by column
chromatography.13,14 The other major product identified
by NMR spectroscopy, HSiMe2SiMe2-OtBu, was synthe-
sized independently to confirm its spectroscopic assign-
ments.15,16 It seems that, after addition of the SnH bond
to the FedSi linkage, the greater strength of the Fe-
Sn bond precludes reductive elimination of nBu3-
SnSiMe2H and favors elimination of the disilane. Fur-
(10) Thermolysis of 1b with Ph2MeSiH in the presence of catalytic
amounts of benzoyl peroxide was carried at 80 °C in an oven. NMR
spectroscopy shows the formation of FpSiPh2Me, FpSiMe2OtBu, and
unidentified silicon compound(s) with 29Si resonances at 28.2 and -4.3
ppm.
(11) (a) Chatagilialoglu, C. Acc. Chem. Res. 1992, 25, 188. (b) Brook,
M. A. In Silicon in Organic, Organometallic and Polymer Chemistry;
Wiley: New York, 1999; p 175.
(8) (a) Derouiche, Y.; Lickiss, P. D. J . Organomet. Chem. 1991, 407,
41. (b) Sharma, S.; Pannell, K. H. Organometallics 2000, 19, 1225.
(9) (a) Brook, A. G.; Abdesaken, F.; So¨llradl, H. J . Organomet. Chem.
1986, 299, 9. (b) Ishikawa, M.; Iyoda, J .; Ikeda, H.; Kotake, K.;
Hashimoto, T.; Kumada, K. J . Am. Chem. Soc. 1981, 103, 4845.
(12) (a) Dixon, C. R.; Netherton, M. R.; Baines, K. M. J . Am. Chem.
Soc. 1998, 120, 11049. (b) J ones, K. L.; Pannell, K. H. J . Am. Chem.
Soc. 1993, 115, 11336.

Communications
Organometallics, Vol. 20, No. 1, 2001
9
thermore, this result lends credence to the absence of
free silylene species generated by the reaction condi-
tions.
discrete FedSi complex to react with E-H bonds (E )
Si, Ge, Sn) entailing the ultimate formation of Si-Si
bonds, confirming a recent suggestion from less direct
evidence.8b The results also resolve the apparent dis-
tinction in reactivity between the base-stabilized iron
silylenes, (η5-C5H5)Fe(CO)(η2-SiR2-OR′-SiR2), and the
transient non-base-stabilized forms (η5-C5H5)Fe(CO)-
(SiR3)(dSiR2).
To date we have not been able to trap SiMe2 by the
reaction of 1b with unhindered silanes, i.e., Ph2MeSiH
and Et3SiH, in the presence or absence of radical
initiators such as benzoyl peroxide.10 This has prece-
dents, since both we and the Baines group have used
(Me3Si)3SiH as a successful trapping agent for silylenes/
germylenes in circumstances where the use of Et3SiH
failed.5d,12
Ack n ow led gm en t. This research has been sup-
ported by the NIH-MARC program.
The results presented illustrate the capacity of a
OM000563J
(13) A 5 mm Pyrex NMR tube was charged with 0.086 g (0.25 mmol)
of 1b and 0.22 g (0.75 mmol) of nBu3SnH in 1.0 mL of degassed C6D6
and sealed under vacuum. Thermolysis of the mixture was carried out
at 80 °C in an oven. The progress of the reaction was monitored by
NMR spectroscopy. After 2 h, 29Si NMR spectroscopy showed the
resonances at 3.4 and -41.6 ppm due to the formation of 1-tert-butoxy-
2-hydrido-1,1,2,2-tetramethyldisilane, HSiMe2SiMe2OtBu, and another
resonance at -4.9 ppm due to HSiMe2OtBu. The 119Sn NMR showed a
resonance at 132.6 ppm due to the formation of (η5-C5H5)Fe(CO)(nBu3-
Sn)2H (2), along with another small resonance at -83.8 ppm due to
Bu3SnSnBu3. After 10 h of thermolysis 1b was completely consumed
and the volatile materials were removed at 50 °C under vacuum. The
residue was extracted from 10 mL of hexanes, filtered, and passed
through a small column (silica gel, 1 × 3 cm). The orange band was
eluted with hexanes, and upon evaporation of the solvent 0.03 g (16%
yield) of 2 was obtained. Complex 2 was independently synthesized
from the photochemical reaction of FpSnnBu3 with nBu3SnH in a sealed
NMR tube. All the spectroscopic data are in agreement with the
reported data.14.
(15) (a) Tobita, H.; Wada, H.; Ueno, K.; Ogino, H. Organometallics
1994, 13, 2545. (b) Ueno, K.; Masuko, A.; Ogino, H. Organometallics
1997, 16, 5023.
(16) tert-Butoxyhydridodisilane, HSiMe2SiMe2OtBu, was synthesized
by modification of the reported method.15 A 5 mm Pyrex NMR tube
15b
was charged with 0.61 g (3.2 mmol) of HSiMe2SiMe2NEt2 and 0.5 g
(6.75 mmol) of BuOH in 1 mL of C6D6 at 0 °C. The NMR tube was
t
sealed under vacuum and heated at 80 °C for 2 h in an oven. After 2
h, 29Si NMR spectroscopy showed the disappearance of resonances at
-2.8 and -42.4 ppm due to the aminohydridodisilane and the
appearance of two new resonances at 3.7 and -41.3 ppm due to the
formation of tert-butoxyhydridodisilane, HSiMe2SiMe2OtBu. Distilla-
tion through a small column at 98-100 °C gave the title compound
(0.15 g, 36% yield). 1H NMR (ppm, 300 MHz, C6D6): 0.17 (d, 6H, J )
t
4.5 Hz, SiMe2), 0.29 (s, 6H, SiMe2), 1.20 (s, 9H, Bu), 3.95 (m, 1H, J )
4.5 Hz, Si-H). 13C NMR (ppm, 75.5 MHz, C6D6): -6.35 (SiMe2), 2.66
(SiMe2), 32.20 (CMe3), 72.49 (CMe3). 29Si NMR (59.6 MHz, C6D6): 3.7,
-41.3 (SiMe2H). IR (hexane, cm-1): 2095.6 (Si-H).
(14) Akita, M.; Oku, T.; Tanaka, M.; Moro-oka, Y. Organometallics
1991, 10, 3080.