Isolation and Ring-Opening of New 1-Sila-3-metallacyclobutanes (η 5-C5H4Fe)(CO)2CH2SiR 2 Leading to A New Class of Organometallic Polymer

Article abstract of DOI:10.1021/ja030559+

Treatment of FpCH₂SiR₂Cl [Fp = (η⁵-C₅H₅)Fe(CO)₂] with lithium diisopropylamide leads to the formation of new metallacycle complexes. When R₂ = Me₂ and MeⁿBu, these ring-open to form a new class of organometallic polymer. When R₂ = ⁿBu₂, the metallacycle resists ring-opening and may be structurally characterized as the Ph₃P-substituted species. Copyright

Full text of DOI:10.1021/ja030559+

Published on Web 01/16/2004
Isolation and Ring-Opening of New 1-Sila-3-metallacyclobutanes
(η5-C5H4Fe)(CO)2CH2SiR2 Leading to A New Class of Organometallic Polymer
Hemant K. Sharma, Francisco Cervantes-Lee, and Keith H. Pannell*
Department of Chemistry, UniVersity of Texas at El Paso, El Paso, Texas 79968-0513
Received September 29, 2003; E-mail: kpannell@utep.edu
Scheme 1
Well-defined polymers containing transition metal atoms in the
backbone are attractive materials owing to their potential applica-
tions as catalysts,1 ion-exchange resins,2 polymeric support materi-
als,3 precursors for ceramic materials,4 conducting polymers,5,6 and
chemical sensors.6 Excellent examples of such materials are the
high-molecular-weight poly-silyleneferrocenylenes obtained from
the ring-opening of the appropriate strained [1]-sila-ferrocenophanes.5,6
There are reports on the synthesis and reactivity of 1-sila-3-
metallacyclobutanes (metal ) Zr,7 Nb,7 and Mo,7 and Ir8) that
undergo ring expansion upon reaction with paraformaldehyde,7c
isocyanides,7d and CO.7e Furthermore, a recent example of 1-sila-
2-platinacyclobutanes that exhibits a diphenylsilane insertion has
been reported.9 We now report the synthesis, isolation, characteriza-
tion, and unprecedented ring-opening polymerization of 1-sila-3-
metallacyclobutanes of the type (η5-C5H4)Fe(CO)2 (CH2SiR2) (3).
As originally reported by the Giering group, thermolysis of
molecular species were uniformly unsuccessful since removal of
FpSiR2CH2Cl [Fp ) (η5-C5H5)Fe(CO)2, R2 ) Me2 (1a)] at 80 °C
the solvent resulted in the formation of the ring-opened polymer
in benzene in sealed NMR tube results in clean and quantitative
4a.
formation of the isomeric iron complex containing a direct Fe-C
Progressive replacement of the methyl groups by n-butyl groups
bond, FpCH2SiR2Cl (2a).10 We find that similar chemistry occurs
increases the thermal stability of the new metallacycles, and the
with complexes in which R2 ) MenBu (1b), nBu2 (1c), and MePh
di-n-butyl complex 3c can be isolated as a pure red viscous oil in
(1d). The rearrangement of 1 to 2 is facile; even storing 1 at low
70% yield after column chromatography on silica gel. This simple
temperature (-5 °C) for a few days resulted in the formation of 2
substitution, dibutyl for dimethyl, results in a metallacycle 3c that
in 20-30% yield along with black decomposed material. NMR
is stable in air and has not, to date, exhibited any thermal, anionic,
spectroscopy and infrared spectra are useful to distinguish between
or transition-metal-catalyzed ring-opening polymerization. Photo-
the Fe-Si bond in 1 and the Fe-C bond in 2.11 The 29Si NMR
chemical treatment of a hexane solution of 3c in a quartz tube for
signal for the Fe-CH2Si, â-Si in 2 with respect to the Fe atom is
downfield shifted in the range of 6-8 ppm from the Fe-Si, R-Si
2 h in the presence of Ph3P resulted in the formation of the red
crystalline phosphine-substituted silametallacycle 5 in 41% yield,
thereby permitting a structural analysis of the new metallacycle
ring.
The structure of 5 has been confirmed by NMR spectroscopy
and by single-crystal X-ray crystallography (Figure 1). The structure
was solved using X-ray data collected at 173 K and still shows
significant disorder in one of the n-butyl groups. Structural analyses
in 1a-d. The two isomers 1 and 2 are also distinguishable by the
13C resonances of the CH2 group, ∼35-37 ppm in 1 vs ∼ -26.0
ppm for 2, and the carbonyl stretching frequencies for the Fp-Si
complexes are ∼15 cm-1 lower frequency than the corresponding
Fp-C complexes.12
Treatment of either 1 or 2 with lithium diisopropylamide (LDA)
results in the formation of 1-sila-3-metallacyclobutanes 3a-d
of several 1-metalla-3-silacyclobutanes are reported in the liter-
(Scheme 1). The formation of the metallacycles 3 from 2 presum-
ably results from an initial metalation of the cyclopentadienyl ring
followed by a ring-closing salt elimination reaction. For the
transformation of 1 f 3, we suggest that after the initial metalation
of the cyclopentadienyl ring a well-documented silyl group migra-
tion to the ring occurs,13 generating an iron-based anionic species
that performs the final intramolecular ring-closing salt elimination.
To date, we have not performed any detailed study to further
delineate this aspect of the research. The new silametallacycle 3a
could not be isolated and was characterized by NMR and infrared
spectroscopy. A special spectroscopic feature of the metallacycles
is the unusual high-field resonance at ∼ -53 ppm for the CH2
carbon in 13C NMR spectra. Complex 3a is reasonably stable in
solution for several hours; however, attempts to isolate it as a
ature;7e,14 however, 5 represents the first in which one of the C
atoms is part of a coordinated cyclopentadienyl ring. Thus, 5 may
be regarded as intermediate between the class of 1-metalla-3-
silacyclobutane metallacycles and the well-studied 1-sila-ferro-
cenophanes. These latter complexes exhibit significant ring strain
and a rich vein of ring-opened polymerization chemistry.5,6 The
ring Si-C bond distances of 5, 1.840(3) and 1.874(3) Å, are in the
range for such silacyclobutanes, while the C-Si-C angle of 94°
is relatively small for such angles, and the C-Fe-C angle of 80°
illustrates a degree of ring strain for this new structural arrangement.
Further evidence for a level of ring strain is the Cp(centroid)-
Fe-CH2 angle of 115.6° in 5, which is smaller than literature values
for related (η5-C5H5)Fe(CO)(PPh3)CH2R systems (R ) CH3,
118.7°;15a R ) CH2OCH3, 120.4;15b R ) CH2SCH2Ph, 121.6).15b
9
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J. AM. CHEM. SOC. 2004, 126, 1326-1327
10.1021/ja030559+ CCC: $27.50 © 2004 American Chemical Society

C O M M U N I C A T I O N S
National Laboratories, Albuquerque, NM, for molecular weight
determinations and DSC and TGA measurements.
Supporting Information Available: Experimental procedures for
the synthesis of 1-5 and NMR data (PDF). Full X-ray crystallographic
data for 5 (CIF). This material is available free of charge via the Internet
at http://pubs.acs.org.
References
(1) (a) Zeldin, M.; Wynne, K. J.; Allcock, H. R. Inorganic and Organometallic
Polymers; ACS Symposium Series 360; American Chemical Society:
Washington, DC, 1988. (b) Pittman, C. U., Jr.; Carraher, C. E.; Zelden,
M.; Sheats, J. E., Jr.; Culbertson, B. M. In Metal-Containing Polymeric
Systems; Sheats, J. E., Carraher, C. E., Jr., Pittman, C. U., Jr., Eds.; Plenum
Press: New York, 1996. (c) Kinborough, R. P.; Swager, T. M. Prog.
Inorg. Chem. 1999, 48, 123. (d) Allcock, H. R. AdV. Mater. 1994, 6, 106.
(e) Rosenblum, M. AdV. Mater. 1994, 2, 159. (f) Abd-El-Aziz, A. S., Ed.
Metal and Metalloid Containing Macromolecules; Wiley-VCH: Wein-
heim, Germany, 2003.
(2) Chemical Reactions on Polymers; Benham, J. L., Kinstle, J. F., Eds.; ACS
Symposium Series 364; American Chemical Society: Washington, DC,
1988.
(3) (a) Macomber, D. W.; Hart, W. P.; Rausch, M. D. In AdVances in
Organometallic Chemistry; Stone, F. G. A., West, R., Eds.; Academic
Press: New York, 1982; pp 1-55. (b) Tenhaeff, S. C.; Tyler, D. R.
Organometallics 1991, 10, 473.
(4) (a) MacLachlan, M. J.; Ginzburg, M.; Coombs, N.; Coyle, T. W.; Raju,
N. P.; Greedan, J. E.; Ozin, G. A..; Manners, I. Science 2000, 287, 1460.
(b) Yajima, S.; Hayashi, J.; Omori, M. Chem Lett. 1975, 931.
(5) (a) Nguyen, P.; Go´mez-Elipe, P.; Manners, I. Chem. ReV. 1999, 99, 1515.
(b) Manners I. Angew. Chem., Int. Ed. Engl. 1996, 35, 1602. (c) Manners,
I. AdV. Organomet. Chem. 1995, 37, 131. (d) Resendes, R.; Nelson, J.
M.; Fischer, A.; Ja¨kle, F.; Bartole, A.; Lough, A. J.; Manners, I. J. Am.
Chem. Soc. 2001, 123, 2116 (e) Kulbaba, K.; Cheng, A.; Bartole, A.;
Greenberg, S.; Resendes, R.; Coombs, N.; Safa-Sefat, A.; Greedan, J. E.;
Stoever, H. D. H.; Ozin, G. A.; Manners, I. J. Am. Chem. Soc. 2002, 124,
12522 and references therein.
(6) (a) Nguyen, M. T.; Diaz, A. F.; Dementiev, V. V.; Pannell, K. H. Chem.
Mater. 1993, 5, 1389. (b) Pannell, K. H.; Dementiev, V. V.; Li, H.;
Cervantes-Lee, F.; Nguyen, M. T.; Diaz, A. F. Organometallics 1994,
13, 3644. (c) Kapoor, R. N.; Crawford, G.; Mahmoud, J.; Dementiev, V.
V.; Nguyen, M. T.; Diaz, A. F.; Pannell, K. H. Organometallics 1995,
14, 4944. (d) Castruita, M.; Cervantes-Lee, F.; Mahmoud, J. S.; Zhang,
Y.; Pannell, K. H. J. Organomet. Chem. 2001, 637-639, 664. (e) Espada,
L.; Pannell, K. H.; Papkov, V.; Leites, L.; Bukalov, S.; Suzdalev, I.;
Tanaka, M.; Hayashi, T. Organometallics 2002, 21, 3758. (f) Papkov, V.
S.; Gerasimov, M. V.; Dubovik, I. I.; Sharma, S.; Dementiev, V. V.;
Pannell, K. H. Macromolecules 2000, 33, 7107. (g) Espada, L.; Robillard,
J.; Shadaram, M.; Pannell, K. H. J. Inorg. Organomet. Polym. 2000, 10,
169.
Figure 1. Structure of 5, space group P21/n. Selected bond lengths (Å):
Fe-C6 ) 2.113(3), C6-Si ) 1.839(3), Si-C1 ) 1.871(3), Si-C26 )
1.867(3), Si-C30 ) 1.892(4). Selected bond angles (°): C26-Si-C30 )
114.7(2), C1-Si-C6 ) 94.06(11), Fe-C6-Si ) 93.24(10), Si-C1-Fe
) 92.45(11). The thermal ellipsoids are drawn at the 50% probability level.
An enlarged exocyclic C-Si-C angle of 114° is correspondingly
observed. Overall the ring is almost planar, with a dihedral angle
between the planes Fe1-Si1-C1 and Fe1-Si1-C6 of 5.9°.
The 13C NMR of 5 exhibited a doublet at -52.6 ppm (Jp-c
)
13.5 Hz) due to coupling between the C of the CH2 group and the
1
31P nucleus. The H NMR signals of the two diastereotopic CH2
hydrogens were observed as two ABX (X ) 31P) quartets due to
inequivalent coupling between the phosphorus nucleus and the two
3
3
H atoms (JAB ) 11.2 Hz, JAX ) 13.0 Hz, JBX ) 1.0 Hz). The
related acyclic compounds, (η5-C5H5)Fe(CO)2CH2SiR3, exhibit the
same pattern.16
A characteristic property of silacyclobutanes is their ability to
undergo ring-opened polymerization; however, none of the previ-
ously reported 1-metalla-3-silacyclobutanes exhibited such polym-
erization. Evaporation of the THF from 3a,b gave the corresponding
polymers 4a,b in an almost quantitative yield, thus more closely
resembling the 1-sila-ferrocenophanes. The thermal polymerization
can be conveniently monitored by 29Si NMR and 13C NMR
spectroscopy. In particular, the carbon resonance of the CH2 moiety
in for silametallacycles at ∼ -53.0 ppm shifts to a more conven-
tional value of -26.10 for 4a and -29.98 ppm for 4b. The
polymeric materials dissolve slowly but completely in THF,
suggesting that no appreciable cross-linking has taken place at this
stage. Multi-angle laser light scattering analysis of the polymer 4a
revealed a bimodal molecular weight distribution with a predomi-
nant high-molecular-weight fraction (Mw ) 1. 75 × 105, Mn ) 7.2
× 104). The molecular weight (Mw) for 4b was found to be relatively
low, 9500, with polydispersity of 6.3, suggesting that the introduc-
tion of the butyl group significantly retards the propagation of a
polymer chain. The polymers are amorphous, as determined by
WAXS.
The polymers are stable to the atmosphere, and films can be
cast from THF solutions. Over a period of time their solubility is
decreased, presumably via cross-linking. The glass transition
temperature for 4a was detected to be 3 °C by DSC. Thermogravi-
metric analysis (TGA) of both 4a and 4b indicated that they undergo
∼45% weight loss between 100 and 200 °C and that a 25% residue
remains at 700 °C. Further study on the ring-opening reactions of
5 and related group 14 analogues of other transition metals are
underway, together with studies on the new polymeric species.
(7) (a) Tikkanen, W. R.; Liu, J. Z.; Egan, J. W.; Petersen, J. L. Organome-
tallics 1984, 3, 825. (b) Tikkanen, W. R.; Liu, J. Z.; Egan, J. W.; Petersen,
J. L. Organometallics 1984, 3, 1646. (c) Tikkanen, W. R.; Petersen, J. L.
Organometallics 1984, 3, 1651. (d) Berg, F. J.; Petersen, J. L. Organo-
metallics 1989, 8, 2461. (e) Petersen, J. L.; Egan, J. W. Organometallics
1987, 6, 2007.
(8) Andreucci, L.; Diversi, P.; Ingrosso, G.; Lucherini, A.; Marchetti, F.;
Adovasio, V.; Nardelli, M. J. Chem. Soc., Dalton Trans. 1986, 477.
(9) Tanabe, M.; Yamazawa, H.; Osakada, K. Organometallics 2001, 20, 4453.
(10) (a) Windus, C.; Sujishi, S.; Giering, W. P. J. Am. Chem. Soc. 1974, 96,
1951. (b) Windus, C.; Sujishi, S.; Giering, W. P. J. Organomet. Chem.
1975, 101, 279.
(11) Zhang, Y.; Cervantes-Lee, F.; Pannell, K. H. Organometallics 2002, 21,
5859.
(12) For spectroscopic data for 1-5, see Supporting Information.
(13) (a) Dean, W. K.; Graham, W. A. G. Inorg. Chem. 1977, 16, 1061. (b)
Berryhill, S. R. Clevenger, G. L.; Burdurlu, F. Y. Organometallics 1985,
4, 1509. (c) Thum, G.; Ries, W.; Greissinger, D.; Malisch, W. J.
Organomet. Chem. 1983, 253, C67. (d) Cervantes, J. Vincenti, S. P.;
Kapoor, R. N.; Pannell, K. H. Organometallics 1989, 8, 744. (e) Sharma,
S.; Cervantes, J.; Mata-Mata, L.; Brun, M.-C.; Cervantes-Lee, F.; Pannell,
K. H. Organometallics 1995, 14, 4269. (f) Crocco, G. L.; Young, C. S.;
Lee, K. E.; Gladysz, J. A. Organometallics 1988, 7, 2158.
(14) (a) Bruno, J. W.; Marks, T. J.; Day, V. V. J. Am. Chem. Soc. 1982, 104,
7357. (b) Behling, T.; Girolami, G. S.; Wilkinson, G.; Somerville, R. G.
J. Chem. Soc., Dalton Trans. 1984, 877. (c) Kabi-Satpathy, A.; Bajgur,
C. S.; Reddy, K. P.; Petersen, J. L. J. Organomet. Chem. 1989, 364, 105.
(d) Coughlin, E. B.; Henling, L. M.; Bercaw, J. E. Inorg. Chim. Acta
1996, 242, 205.
(15) (a) Davies, S. G.; Dordor-Hedgecock, I. M.; Sutton, K. H.; Whitttaker,
M. J. Organomet. Chem. 1987, 320, C19. (b) Blackburn, B. K.; Bromley,
L.; Davies, S. G.; Whitakker, M.; Jones, R. H. J. Chem. Soc., Perkin
Trans. 1989, 2, 1143.
(16) Pannell, K. H. Chem. Commun. 1969, 1346.
Acknowledgment. This research has been supported by the
NIH-MARC program. We also thank Dr. K. Rahimian from Sandia
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