The substituent-dependent base-treatment chemistry of (η5- C5H5)Fe(CO)2SiR2SiR2Cl: Formation of 1,2-disila-3-metallacyclobutanes, their ring-opened polymers, migrations and substitutions

Article abstract of DOI:10.1039/b410822a

We report the formation of disilametallacycles, high molecular weight polymers or simple substitution products from the reactions of 1-(cyclopentadienyliron dicarbonyl)-2-chlorodisilanes with bases, LDA or n-BuLi.

Full text of DOI:10.1039/b410822a

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The substituent-dependent base-treatment chemistry of
(g5-C5H5)Fe(CO)2SiR2SiR2Cl: formation of 1,2-disila-3-
metallacyclobutanes, their ring-opened polymers, migrations and
substitutions{
Hemant K. Sharma and Keith H. Pannell*
Department of Chemistry, University of Texas at El Paso, El Paso, TX, 79968-0513, USA.
E-mail: kpannell@utep.edu
Received (in Cambridge, UK) 15th July 2004, Accepted 2nd September 2004
First published as an Advance Article on the web 6th October 2004
We report the formation of disilametallacycles, high
synthesize the corresponding disilametallacyclobutane, resulted in
the isolation of no materials; however, repeating the reaction
and quenching with Ph3SnCl resulted in clean migration of
methoxydisilyl group from iron to the cyclopentadienyl ring, 5,
Scheme 1(iv).8 Given the utility of alkoxysilanes in Grignard
substitution reactions at silicon this is an unexpected result.
molecular weight polymers or simple substitution products
from the reactions of 1-(cyclopentadienyliron dicarbonyl)-2-
chlorodisilanes with bases, LDA or n-BuLi.
Organometallic polymers in which the transition metals are part
of the backbone offer useful chemical, electrochemical, optical
and magnetic properties.1,2 We recently reported that 1-sila-3-
metallacyclobutanes of the type [(g5-C5H4)Fe(CO)2CH2SiR2]
could be isolated, structurally characterized and ring-opened
to the corresponding high MW polymers by base-treatment of
(g5-C5H5)Fe(CO)2SiR2CH2Cl.3 Since 1,2-disila polymers that also
contain unsaturated or aromatic groups are photoactive and show
conducting properties when the polymers are doped with oxidizing
agents4–6 we have investigated, and now report, the chemistry of
the related disilyl systems, (g5-C5H5)Fe(CO)2SiR2SiR2Cl.
Base-treatment of FpSiMe2SiMe2Cl,7 1a, [Fp ~ (g5-C5H5)-
Fe(CO)2], with lithium diisopropylamide, LDA, in THF at 25 uC,
resulted in clean and quantitative formation of the polymer 2,
Scheme 1(i), as a light brown material that precipitated directly
from the THF solution. We observed no evidence for the forma-
tion of the supposed precursor 1,2-disila-3-ferracyclobutane,
(g5-C5H4)Fe(CO)2SiMe2SiMe2. However, similar base-treatment
of the 1,1,2,2-tetrabutyl and 1,1-dimethyl-2,2-dibutyl analogs,
FpSiBu2SiBu2Cl, 1b, and FpSiMe2SiBu2Cl, 1c, resulted only in
the isolation of the corresponding analytically pure 1,2-disila-3-
ferracycle, 3b and 3c, Scheme 1(ii). We suppose that these
complexes form via a base-mediated metal to cyclopentadienyl
ring silatropic shift, and the subsequently formed ferrate species
undergoes an intramolecular salt-elimination cyclization to form
the disilametallacycles.8 Gladysz et al. have reported the related
rhenium complex, (g5-C5H4)ReNO(PPh3)SiMe2SiMe2 synthesized
in an analogous reaction from (g5-C5H5)ReNO(PPh3)SiMe2-
SiMe2Cl and butyllithium.9 The 29Si NMR spectra of the metal-
lacycles 3b,c exhibit two signals for the a-Si and b-Si with respect to
the Fe atom at 239, 240 and 219, 215 ppm, respectively.
Upon changing the silicon-bonded substituents to 1,2-dimethyl-
1,2-dibutyl groups, FpSiMeBuSiMeBuCl, 1d, neither metallacycle
nor polymer formation was observed upon treatment with
LDA. However, treatment with n-BuLi resulted in the isolation
in moderate yield of the direct nucleophilic substitution product,
FpSiMeBuSiMeBu2, 4, Scheme 1(iii). NMR analysis of the crude
reaction products provided no evidence for formation of the
corresponding metallacycle. The nature of the substituents around
the Si–Si bond dramatically affects the outcome of the base-
mediated reactions.
Scheme 1
Polymer 2 is soluble in hot THF. Cross-polarization solid state
29Si NMR of the polymer 2 exhibits two narrow resonances at
22.0 ppm (Fe–Si) and at 28.2 ppm (C5H4–Si) indicating the
regular alternating arrangement of the building blocks in the
polymer backbone and the structure is essentially retained in
the solution as indicated by the solution NMR spectroscopy. Wide
angle laser light scattering analysis of 2 revealed molecular weight
Mw ~ 6400, Mn ~ 3200 with polydispersity of 2.0. Thin films can
be cast from the THF solution of the polymer. The morphology of
the polymer was investigated by WAXS; the scattering pattern
indicated distinct peaks at 13.3u and 14.2u which confirms that
the polymer was partially crystalline. Thermogravimetric analysis
(TGA) of the polymer under N2 atmosphere showed that the
polymer underwent two successive weight losses, 6% between 50–
75 uC and 16% weight loss between 200–300 uC, and about 50%
residue remained at 500 uC.
Of interest is the change of chemistry when the terminal chloro
group is replaced by a methoxy group. Treatment of FpSiMe2-
SiMe2OMe10 with LDA at low temperature in an attempt to
{ Electronic supplementary information (ESI) available: experimental
section. See http://www.rsc.org/suppdata/cc/b4/b410822a/
2 5 5 6
C h e m . C o m m u n . , 2 0 0 4 , 2 5 5 6 – 2 5 5 7
T h i s j o u r n a l i s ß T h e R o y a l S o c i e t y o f C h e m i s t r y 2 0 0 4

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Further studies on the properties of polymer 2 and synthesis and
reactivity of related group 14 metallacycles and polymers are
underway.
This research has been supported by the NIH-MARC program.
We thank Dr K. Rahimian from Sandia National Laboratories,
Albuquerque, NM for molecular weight determinations and TGA
measurements.
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C h e m . C o m m u n . , 2 0 0 4 , 2 5 5 6 – 2 5 5 7
2 5 5 7