Silylene-bridged dinuclear iron complexes [Cp(OC)2Fe]2SiX2 (X = H, F, Cl, Br, I). Synthesis, molecular structure, vibrational spectroscopy, and theoretical studies

Article abstract of DOI:10.1021/ic026138v

The μ₂-silylene-bridged iron complexes [Cp(OC)₂Fe]₂SiX₂ (X = F (2), Br (4), I (5)) have been prepared from the μ₂-SiH₂ functional precursor [Cp(OC)₂Fe]₂SiH₂ (1) by hydrogen/halogen exchange, using HBF₄, CBr₄, and CH₂I₂, respectively. The fluoro- and bromo-substituted derivatives 2 and 4 are converted upon UV irradiation to the carbonyl-and dihalosilylene-bridged dinuclear complexes [Cp(OC)Fe]₂(μ₂-CO)(μ₂-SiX₂) (X = F (6), Br (7)) via CO elimination. All new compounds have been characterized spectroscopically, and, in addition, the molecular structure of 2, 4, and the previously reported chloro derivative Cp(OC)₂Fe]₂SiCl₂ (3) has been determined by single-crystal X-ray diffraction methods. For 1-5, the Fourier transform infrared and Raman spectra have been recorded and discussed, together with density functional theory calculations, which support the experimental results of the structural and vibrational analysis. The computed geometries, harmonic vibrational wavenumbers, and their corresponding Raman scattering activities are in good agreement with the experimental data. A significant dependence of the CO and Fe-Si stretching modes on the X substituents of the μ₂-silylene bridge has been observed and discussed.

Full text of DOI:10.1021/ic026138v

Inorg. Chem. 2003, 42, 3274−3284
Silylene-Bridged Dinuclear Iron Complexes [Cp(OC)2Fe]2SiX (X ) H, F,
2
Cl, Br, I). Synthesis, Molecular Structure, Vibrational Spectroscopy, and
Theoretical Studies†
Matthias Vo1gler,‡ Ioana Pavel,§ Marco Hofmann,‡ Damien Moigno,§ Martin Nieger,#
,§
,‡
Wolfgang Kiefer,* and Wolfgang Malisch*
Institut fu¨r Anorganische Chemie der UniVersita¨t Wu¨rzburg, Institut fu¨r Physikalische Chemie der
UniVersita¨t Wu¨rzburg, Am Hubland, D-97074 Wu¨rzburg, Germany, and Institut fu¨r Anorganische
Chemie der UniVersita¨t Bonn, Gerhard-Domagk-Strausse 1, D-53121 Bonn, Germany
Received October 29, 2002
The µ2-silylene-bridged iron complexes [Cp(OC)2Fe]2SiX (X ) F (2), Br (4), I (5)) have been prepared from the
2
µ2-SiH functional precursor [Cp(OC)2Fe]2SiH (1) by hydrogen/halogen exchange, using HBF , CBr4, and CH I ,
2
2
4
2 2
respectively. The fluoro- and bromo-substituted derivatives 2 and 4 are converted upon UV irradiation to the carbonyl-
and dihalosilylene-bridged dinuclear complexes [Cp(OC)Fe]2(µ2-CO)(µ2-SiX ) (X) F(6), Br (7)) via CO elimination.
2
All new compounds have been characterized spectroscopically, and, in addition, the molecular structure of 2, 4,
and the previously reported chloro derivative [Cp(OC)2Fe]2SiCl2 (3) has been determined by single-crystal X-ray
diffraction methods. For 1−5, the Fourier transform infrared and Raman spectra have been recorded and discussed,
together with density functional theory calculations, which support the experimental results of the structural and
vibrational analysis. The computed geometries, harmonic vibrational wavenumbers, and their corresponding Raman
scattering activities are in good agreement with the experimental data. A significant dependence of the CO and
Fe−Si stretching modes on the X substituents of the µ2-silylene bridge has been observed and discussed.
Introduction
A central issue in understanding the mechanistic details
of these catalytic processes is the strength of the metal-
silicon bond. Some insight can be gained from structural data
that are available for many transition-metal silyl complexes.
For example, the bond lengths observed in H3Si-Co(CO)4
(2.381 Å)6 and F3Si-Co(CO)4 (2.226 Å)7 indicate that the
The chemistry of transition-metal silicon compounds has
been an active field of research ever since the early works
of Chalk and Harrod1 on the transition-metal-catalyzed
hydrosilylation of olefins. In the past decades, a huge number
of silyl complexes of almost all transition metals have been
synthesized and characterized.2 Especially, some palladium
and platinum derivatives are encountered as crucial inter-
mediates, not only in hydrosilylation reactions3 but also in
other catalytic processes (e.g., dehydrogenative coupling of
SiH moieties4) or in ring-opening methatesis polymerization
(ROMP) reactions of cyclic disilanes.5
(2) (a) Corey, J. Y.; Braddock-Wilking, J. Chem. ReV. 1999, 99, 175-
292. (b) Braunstein, P.; Knorr, M. J. Organomet. Chem. 1995, 500,
21-38. (c) Zybill, C.; Handwerker, H.; Friedrich, H. AdV. Organomet.
Chem. 1994, 36, 229-281. (d) Tilley, T. D. In Transition Metal Silyl
DeriVatiVes; Patai, S., Rappoport, Z., Eds.; Wiley: New York, 1991;
p 245. (e) Cundy, C. S.; Kingston, B. M.; Lappert, M. F. AdV.
Organomet. Chem. 1973, 11, 253-330. (f) Schubert, U. AdV.
Organomet. Chem. 1990, 30, 151-187. (g) Aylett, B. J. AdV. Inorg.
Chem. Radiochem. 1982, 25, 1-133. (h) Malisch, W.; Kuhn, M. Chem.
Ber. 1974, 107, 979-995.
(3) (a) Speier, J. L. AdV. Organomet. Chem. 1979, 407-447. (b) Ojima,
I. In The Chemistry of Organic Silicon Compounds; Patai, S.,
Rappoport, Z., Eds.; Wiley: Chichester, U.K., 1989; p 1479. (c) Chalk,
A. J.; Harrod, J. F. J. Am. Chem. Soc. 1965, 87, 16-21.
(4) (a) Yamashita, M.; Tanaka, M. Bull. Chem. Soc. Jpn. 1995, 68, 403-
419. (b) Tilley, T. D. Acc. Chem. Res. 1993, 26, 22-29. (c) Hengge,
E. In Organosilicon Chemistry; Auner, N., Weis, J., Eds.; Wiley-
VCH: Weinheim, Germany, 1994; p 275.
(5) (a) Uchimaru, Y.; Tanaka, Y.; Tanaka, M. Chem. Lett. 1995, 164. (b)
Recatto, C. A. Aldrichimica Acta 1995, 28, 85-92.
(6) Aylett, B. J. AdV. Inorg. Chem. Radiochem. 1968, 11, 249-307.
* Authors to whom correspondence should be addressed. E-mail address
(W.M.): Wolfgang.Malisch@mail.uni-wuerzburg.de. Fax: +49 931 888
4618.
† Part 59 of the series “Synthesis and Reactivity of Silicon Transition
Metal Complexes”. For part 58, see: Malisch, W.; Jehle, H.; Schumacher,
D.; Binnewies, M.; So¨ger, N. J. Organomet. Chem. 2003, 667, 35-41.
‡ Institut fu¨r Anorganische Chemie der Universita¨t Wu¨rzburg.
§ Institut fu¨r Physikalische Chemie.
# Institut fu¨r Anorganische Chemie der Universita¨t Bonn.
(1) (a) Chalk, A. J.; Harrod, J. F. J. Am. Chem. Soc. 1965, 87, 1133-
1135. (b) Chalk, A. J.; Harrod, J. F. J. Am. Chem. Soc. 1967, 89,
1640-1647.
3274 Inorganic Chemistry, Vol. 42, No. 10, 2003
10.1021/ic026138v CCC: $25.00 © 2003 American Chemical Society
Published on Web 04/16/2003

Silylene-Bridged Dinuclear Iron Complexes
presence of electronegative substituents on the Si atom can
greatly increase the strength of the metal-silicon bond. The
M-Si bond distances in many structurally characterized silyl
complexes LnM-SiR3 (M ) Mn, Fe, Co, Rh, Pt) were found
to be up to 0.28 Å shorter than those that would be predicted
on the basis of the covalent atom radii.2e These short bond
distances are generally attributed to a π-type back-bonding
from the d-orbitals of the transition metal into the antibonding
σ*(Si-R) orbitals of the silyl group sSiR3.8 Consequently,
the M-Si bonds in silyl complexes of d0 metals (TiIV, ZrIV)
show no significant shortening and closely match the
predicted covalent values.9 In the Mo¨ssbauer and IR spectral
investigations of Pannell et al. on Cp(OC)2Fe-R (where R
represents a large range of alkyl and silyl groups), the
increase in s-electron density at the metal center for the silyl
compounds has been attributed to a superior σ-donation from
the silyl group, as compared with the alkyl group, rather than
to significant iron-silicon π-type back-bonding.10 Important
π-type back-bonding effects were only observed for the
Cp(OC)2Fe-SiMe3-nPhn complexes. However, valence photo-
electron spectra that were recorded by Lichtenberger and Rai-
Chaudhuri for the complexes Cp(OC)2Fe-SiR3 (R ) Me,
Cl) have confirmed that silyl substituents can act as effective
π-acceptors.11 In a more recent structural and theoretical
study on a series of osmium complexes (Ph3P)2(OC)(Cl)-
Os-SiR3 (R ) F, Cl, OH, Me), Roper et al. examined the
ionic and covalent contributions to the metal-silicon bond
in detail.8a
useful tools for obtaining information about the strength of
the bonds in a molecule, even though the direct assignment
of observed IR and Raman bands of comparatively complex
molecules is fraught with problems. Theoretical calculations
can certainly assist in obtaining a deeper understanding of
vibrational spectra of larger organometallic arrangements,
such as transition-metal silyl complexes. In particular, recent
developments in density functional theory (DFT) have shown
that DFT may become a powerful computational alternative
to the conventional quantum chemical methods, because DFT
methods are far less computationally demanding and take
account of the effects of electron correlation.13,14 Methods
based on DFT were successfully used recently to predict the
structural properties and the Raman vibrational modes of new
transition-metal complexes accurately.15,16 Thus, to complete
the previous characterization of [Cp(OC)2Fe]2SiH2 (1)12b,i and
to obtain a better understanding of the influence of a wide
range of SiX2 groups on the nature of the Fe-Si and CO
bonds, Fourier transform infrared and Raman (FT-Raman)
spectra of complexes 1-5 were recorded for the first time
and discussed in combination with the results of our DFT
calculations.
Experimental Section
General. All manipulations were performed in an inert atmo-
sphere of purified and dried nitrogen, using standard Schlenk
techniques. Solvents were appropriately dried and purified, using
standard procedures. NMR spectra were recorded at room temper-
ature on a JEOL model JNM-LA 300 spectrometer. All chemical
shifts are in parts per million (ppm) and have been referenced to
solvent signals (1H and 13C) or to the external standards H3PO4
(31P) and trimethylsilane (TMS) (29Si). IR spectra were recorded
using a Perkin-Elmer model 283 spectrophotometer. Samples were
prepared as solutions in a NaCl cell. [Cp(OC)2Fe]2SiH2 (1)12b and
[Cp(OC)2Fe]2SiCl2 (3)12c were obtained according to literature
procedures.
We have now prepared five homologous complexes of the
general formula [Cp(OC)2Fe]2SiX2 (X ) H (1), F (2), Cl
(3), Br (4), I (5)) with the original intention of using these
compounds as precursors for the synthesis of the bis-
12a
(metallo)silanediol [Cp(OC)2Fe]2Si(OH)2 or the bis(met-
allo)silylene [Cp(OC)2Fe]2Si. Furthermore, the availability
of this series of compounds gave us the opportunity to study,
in detail, the bonding situation in bismetalated silanes, which
are among a class of compounds for which a growing number
of representatives has been prepared in recent years.12 In this
sense, the experimental research and development has
demonstrated vibrational spectroscopy to be one of the most
[Cp(OC)2Fe]2SiF2 (2). A solution of [Cp(OC)2Fe]2SiH2 (1) (280
mg, 0.73 mmol) in benzene (15 mL) was treated at 5 °C with a
solution of HBF4 (128 mg, 1.46 mmol) in diethyl ether (10 mL).
While the solution was stirred, gas evolution (BF3, H2) was
observed, and the gas was removed from the reaction vessel in a
continuous stream of nitrogen. After 1 h, the solution was filtered
through a pad of Celite and condensed in vacuo to 1 mL. Upon the
addition of n-pentane (5 mL), a light-yellow solid of 2 precipitated,
which was filtered from the solution, washed twice with n-pentane
(each 3 mL), and dried in vacuo. Yield: 150 mg (50%). Light-
yellow solid; mp: 178 °C. 1H NMR (300.4 MHz, C6D6): δ ) 4.28
ppm (s, H5C5). 13C{1H} NMR (75.5 MHz, C6D6): δ ) 84.1 (s,
C5H5), 214.5 ppm (s, CO). 19F{1H} NMR (376.5 MHz, C6D6): δ
(7) Archer, N. J.; Haszeldine, R. N.; Parish, R. V. Chem. Commun. 1971,
524-525.
(8) (a) Hu¨bler, K.; Hunt, P. A.; Maddock, S. M.; Rickard, C. E. F.; Roper,
W. R.; Salter, D. M.; Schwerdtfeger, P.; Wright, L. J. Organometallics
1997, 16, 5076-5083. (b) Lemke, F. R.; Galat, K. J.; Youngs, W. J.
Organometallics 1999, 18, 1419-1429. (c) Freeman, S. T. N.; Lofton,
L. L.; Lemke, F. R. Organometallics 2002, 21, 4776-4784.
(9) Muir, K. W. J. Chem. Soc. A 1971, 2663-2666.
(10) Pannell, K. H.; Wu, C. C.; Long G. J. J. Organomet. Chem. 1980,
186, 85-90.
(11) Lichtenberger, D. L.; Rai-Chaudhuri, A. J. Am. Chem. Soc. 1991, 113,
2923-2930.
1
) -36.6 ppm (s, JSiF ) 437 Hz). 29Si{1H} NMR (59.6 MHz,
1
C6D6): δ ) 109.4 ppm (t, JSiF ) 437 Hz). Anal. Calcd for
C14H10F2Fe2O4Si (420.01): C, 40.04; H, 2.40. Found: C, 39.54;
(12) (a) Malisch, W.; Vo¨gler, M.; Schumacher, D.; Nieger, M. Organo-
metallics 2002, 21, 2891-2897. (b) Malisch, W.; Vo¨gler, M.; Ka¨b,
H.; Wekel, H.-U. Organometallics 2002, 21, 2830-2832. (c) Malisch,
W.; Ries, W. Chem. Ber. 1979, 112, 1304-1315. (d) Malisch, W.;
Ries, W. Angew. Chem. 1978, 90, 140-141; Angew. Chem., Int. Ed.
Engl. 1978, 17, 120-121. (e) Ogino, H.; Tobita, H. AdV. Organomet.
Chem. 1998, 42, 223-290. (f) Luh, L. S.; Wen, Y. S.; Tobita, H.;
Ogino, H. Bull. Chem. Soc. Jpn. 1997, 70, 2193-2200. (g) Pannell,
K. H.; Sharma, H. Organometallics 1991, 10, 954-959. (h) Ueno,
K.; Hamashima, N.; Shimoi, M.; Ogino, H. Organometallics 1991,
10, 959-962. (i) Aylett, B. J.; Colquhoun, H. M. J. Chem. Res. Synop.
1977, 1, 148.
H, 2.63.
(13) Parr, R. G.; Yang, W. Density Functional Theory of Atoms and
Molecules; Oxford University Press: Oxford, U.K., 1989.
(14) Seminario, J. M.; Politzer, P. Modern Density Functional Theory, A
Tool for Chemistry; Elsevier Science B. V.: Amsterdam, The
Netherlands, 1995.
(15) (a) Berces, A.; Ziegler, T. J. Phys. Chem. 1994, 98, 13233-13242.
(b) Berces, A.; Ziegler, T. Top. Curr. Chem. 1996, 182, 41-85.
(16) (a) Jonas, V.; Thiel, W. J. Phys. Chem. 1995, 99, 8474-8484. (b)
Jonas, V.; Thiel, W. J. Phys. Chem. 1996, 100, 3636-3648.
Inorganic Chemistry, Vol. 42, No. 10, 2003 3275

Vo1gler et al.
Table 1. Crystallographic Data for Compounds 2-4
[Cp(OC)2Fe]2SiF2 (2)
C14H10F2Fe2O4Si
[Cp(OC)2Fe]2SiCl2 (3)
C14H10Cl2Fe2O4Si
[Cp(OC)2Fe]2SiBr2 (4)
C14H10Br2Fe2O4Si
empirical formula
fw
420.01
452.91
541.83
temp
123 ( 2 K
0.71073 Å
monoclinic
P2(1)/n (No. 14)
123 ( 2 K
0.71073 Å
monoclinic
C2/c (No. 15)
123 ( 2 K
0.71073 Å
monoclinic
C2/c (No. 15)
wavelength (Mo KR)
cryst syst
space group
unit cell dimensions
a
b
c
7.7190(1) Å
16.9176(2) Å
11.6193(2) Å
90°
19.0362(6) Å
7.4253(3) Å
13.5488(4) Å
90°
16.429(4) Å
7.624(2) Å
14.816(4) Å
90°
R
â
98.500(1)°
90°
â ) 120.102(2)°
116.91(2)°
90°
γ
90°
vol (Å3)
1500.66(4) Å3
4
1656.83(10) Å3
4
1654.8(7) Å3
4
Z
calculated density
abs coeff
F(000)
1.859 Mg/m3
2.050 mm-1
840
1.816 Mg/m3
2.159 mm-1
904
2.175 Mg/m3
6.668 mm-1
1048
cryst size
θ range
reflns (collected/unique)
R (int)
0.40 mm × 0.20 mm × 0.10 mm
2.93° < θ < 25.00°
28 456/2630
0.0488
0.25 mm × 0.20 mm × 0.15 mm
3.01° < θ < 28.29°
14 631/2047
0.0453
0.25 mm × 0.08 mm × 0.02 mm
2.78° < θ < 25.00°
6252/1455
0.1629
absorption correction
GOF on F2
R1 [I > 2σ(I)]
wR2 (all data)
largest diff. peak and hole
empirical
1.037
0.0164
empirical
1.078
0.0226
empirical
0.983
0.0597
0.0436
0.0578
0.1326
0.246, -0.249 e/Å3
0.356, -0.355 e/Å3
0.829, -1.054 e/Å3
[Cp(OC)2Fe]2SiBr2 (4). Tetrabromomethane (180 mg, 0.54
mmol) was added at room temperature to a solution of [Cp-
(OC)2Fe]2SiH2 (1) (188 mg, 0.49 mmol) in benzene (10 mL). After
the solution was stirred for 1 h, all volatiles were removed in vacuo
and the remaining yellow-orange residue of 4 was washed twice
with 5 mL of cold n-pentane and dried in vacuo. Yield: 240 mg
C6H6): δ ) 58.1 ppm (s). Anal. Calcd for C13H10F2Fe2O3Si
(392.00): C, 39.83; H, 2.57. Found: C, 39.41; H, 3.01.
[Cp(OC)Fe]2Si(µ-CO)(µ-SiBr2) (7). Using a procedure similar
to that used to synthesize 6, [Cp(OC)2Fe]2SiBr2 (4) (120 mg, 0.22
mmol) in 5 mL of benzene was irradiated with UV light (quartz
lamp model TQ 718, 700 W, Hanau) for 3 h. A dark-red solid of
7 precipitated when 3 mL of cold n-pentane was added; this
precipitate was filtered from the solution, washed twice with
n-pentane (each 1 mL), and dried in vacuo. Yield: 88 mg (76%).
Dark-red solid; IR (cyclohexane): ν(CO) ) 1996 (vs), 1951 (s);
1787 (s) cm-1. Cis-isomer (7a): 1H NMR (300.4 MHz, C6D6): δ
) 4.06 ppm (s, H5C5). 13C{1H} NMR (75.5 MHz, C6D6): δ )
86.8 (s, C5H5), 213.3 ppm (s, CO). 29Si{1H} NMR (59.6 MHz,
C6D6): δ ) 181.1 ppm (s). Trans-isomer (7b): 1H NMR (300.4
MHz, C6D6): δ ) 4.25 ppm (s, H5C5). 13C{1H} NMR (75.5 MHz,
C6D6): δ ) 87.7 (s, C5H5), 210.3 ppm (s, CO). Anal. Calcd for
C13H10Br2Fe2O3Si (513.90): C, 30.39; H, 1.96. Found: C, 30.75;
H, 2.48.
1
(93%). Yellow-orange solid; mp: 45 °C (dec.). H NMR (300.4
MHz, C6D6): δ ) 4.28 ppm (s, H5C5). 13C{1H} NMR (75.5 MHz,
C6D6): δ ) 86.8 (s, C5H5), 214.7 ppm (s, CO). 29Si{1H} NMR
(59.6 MHz, C6D6): δ ) 141.4 ppm (s). Anal. Calcd for C14H10-
Br2Fe2O4Si (541.83): C, 31.03; H, 1.86. Found: C, 30.76; H, 2.04.
[Cp(OC)2Fe]2SiI2 (5). A solution of [Cp(OC)2Fe]2SiH2 (1) (410
mg, 1.07 mmol) in 10 mL of benzene was treated with an excess
of CH2I2 (2.70 g, 10.1 mmol). After the solution was stirred for 8
h at 50 °C, all volatiles were removed in vacuo and the remaining
residue of 5 was washed twice with cold n-pentane (each 5 mL)
and then dried in vacuo. Yield: 598 mg (88%). Yellow solid; mp:
1
92 °C (dec.). H NMR (300.4 MHz, C6D6): δ ) 4.27 (s, H5C5).
13C{1H} NMR (75.5 MHz, C6D6): δ ) 88.2 (s, C5H5), 215.7 ppm
(s, CO). 29Si{1H} NMR (59.6 MHz, C6D6): δ ) 105.0 ppm (s).
Anal. Calcd for C14H10Fe2I2O4Si (635.81): C, 26.45; H, 1.59.
Found: C, 26.71; H, 1.84.
X-ray Crystal Structure Determination. Crystallographic data
are summarized in Table 1. Suitable single crystals of 2-4 were
obtained by the slow evaporation of a saturated solution of the
respective compound in diethyl ether. The intensities were measured
with a Nonius-Kappa CCD diffractometer (Mo KR radiation, λ
) 0.71073 Å; graphite monochromator) at T ) 123 ( 2 K. The
structure was solved by the Patterson method (SHELXS-97)17a and
refined by least-squares methods based on F2 with all measured
reflections (SHELXL-97) (full-matrix least-squares on F2).17b Non-
hydrogen atoms were refined anisotropically; H atoms were
calculated according to the ideal geometry. An empirical absorption
correction was applied. Additional crystallographic data are given
in the Supporting Information.
[Cp(OC)Fe]2Si(µ-CO)(µ-SiF2) (6). A solution of [Cp(OC)2Fe]2-
SiF2 (2) (69 mg, 0.16 mmol) in 3 mL of benzene was irradiated
with UV light (quartz lamp model TQ 150, 150 W, Hanau) and
1
the reaction mixture was monitored by H NMR. After 0.5 h, all
insoluble material was removed by filtration through a pad of Celite
and the filtrate was condensed in vacuo to 1 mL. A dark-red solid
of 6 precipitated when 2 mL of cold n-pentane was added; this
precipitate was filtered from the solution, washed twice with
n-pentane (each 0.5 mL), and dried in vacuo. Yield: 35 mg (54%).
Dark-red solid; mp: 148 °C. IR (cyclohexane): ν(CO) ) 2031(m),
1999 (w), 1988 (s), 1961 (vs); 1794 (vs) cm-1. Cis-isomer (6a):
1H NMR (100 MHz, C6H6): δ ) 4.13 ppm (s, H5C5). 19F{1H}
Computational Details. The DFT calculations were performed
using the Gaussian 98 program package.18 Becke’s 1988 exchange
2
NMR (94.1 MHz, C6H6): δ ) 57.0 (d, JFSiF ) 85.1 Hz), 59.6
(17) (a) Sheldrick, G. M. Acta Crystallogr., Sect. A: Found. Crystallogr.
1990, A46, 467-473. (b) Sheldrick, G. M. SHELXL-97; University
of Go¨ttingen: Go¨ttingen, Germany, 1997.
ppm (d, 2JFSiF ) 85.1 Hz). Trans-isomer (6b): 1H NMR (100 MHz,
C6H6): δ ) 4.33 ppm (s, H5C5). 19F{1H} NMR (94.1 MHz,
3276 Inorganic Chemistry, Vol. 42, No. 10, 2003

Silylene-Bridged Dinuclear Iron Complexes
functional,19 in combination with the Perdew-Wang 91 correlation
functional (BPW91),20 was employed in the calculation. The
6-311G(d) Pople split valence basis set implemented in the Gaussian
98 program18 was chosen in the geometry optimization and normal
mode calculations of 1-4, taking into account that results obtained
with a split valence set are a significant improvement on those
obtained with a minimal basis set.21 Because of the presence of I
atoms in complex 5, the calculations were performed in this case
using the 3-21G(d) basis set18 for the I atoms and the 6-311G(d)
basis set for the other atoms. DFT calculations on harmonic
vibrational modes and Raman scattering activity of each band were
performed using the fully optimized molecular geometry without
symmetry restrictions, and the analytical harmonic vibrational
wavenumbers for all structures confirmed that local minima on the
potential energy surface had been found. The partial charges (given
in units of e) situated on selected atoms of 1-5 were then
determined by the natural population analysis (NPA)22 at the same
level of theory. To obtain further information regarding the
electronic structures, NBO (natural bond orbital) analyses were also
performed.23-26 The essential feature of NBO analysis is that the
electron density is represented, as far as possible, by localized core
orbitals, bonds, and lone pairs. These orbitals comprise Lewis
structures corresponding precisely to the chemist’s view of mol-
ecules built from atoms connected by localized two-electron bonds.
However, for conjugated systems, ideal Lewis structures are
obviously not adequate. Deviations from idealized Lewis struc-
tures due to conjugation are shown in NBO analysis as orbital
interactions between localized bonds and antibonds and between
lone pairs and antibonds. The energetic contributions from these
interactions can be quantified with the help of second-order
perturbation theory.
Scheme 1
for the generation of the fluoro-substituted derivatives, as
well as for the generation of the yet unknown bromo- and
iodo-substituted derivatives. This is due to the extreme
activation of the Si-H functions by the two transition-metal
fragments, with respect to Si-H/Si-Hal exchange, which
can be deduced from the very low ν˜SiH absorption (2037
cm-1).12b This value is even ca. 55 cm-1 lower than that in
Cp(OC)2Fe-SiH3 (2093 cm-1)27 and indicates a very electron-
rich SiH2 unit for 1.
In this context, 2 is obtained by the fluorination of 1 at
room temperature, using HBF4 in diethyl ether. The reaction
proceeds with the elimination of H2 and BF3 and produces
[Cp(OC)2Fe]2SiF2 (2) after 1 h in 50% yield (Scheme 1).
The dibromosilylene complex [Cp(OC)2Fe]2SiBr2 (4) is
generated from 1 after 1 h at room temperature, using a slight
excess of tetrabromomethane in benzene. The formation of
4, which can be isolated in excellent yield (93%), could be
easily followed using IR and NMR spectroscopy by the
disappearance of the SiH signals.
Results and Discussion
Attempts to generate [Cp(OC)2Fe]2SiI2 (5) in an analogous
manner, by treatment of 1 with CI4, leads predominantly to
the formation of Cp(OC)2Fe-I by cleavage of the Fe-Si
bond. However, when the milder iodination reagent CH2I2
was employed (in 10-fold excess at 50 °C), a controlled H/I
exchange could be achieved, leading to 5 after 8 h in 88%
yield.
Compounds 2, 4, and 5 are isolated as light-yellow (2, 5)
and yellow-orange (4) solids, which show good solubility
in benzene, diethyl ether, and tetrahydrofuran. The 29Si NMR
spectra of 2, 4, and 5 clearly reveal the influence of two
silicon-bonded Cp(OC)2Fe fragments. The paramagnetic term
of the metal fragments leads to 29Si NMR resonances
downfield of 100 ppm in the following order of decreasing
ppm values: 3 (146.2, X ) Cl) > 4 (141.2, X ) Br) > 2
(109.4, X ) F) > 5 (105.0, X ) I).
A characteristic feature of the dihalosilylene complexes
2-5 is their extreme sensitivity toward light. When 2 and 3
are exposed to light, slow decomposition is observed, leading
predominantly to [Cp(OC)2Fe]2 with extrusion of the silylene
“SiX2” (X ) F, Cl), giving rise to the formation of an
insoluble product.28 For 4 and 5, the major decomposition
product is Cp(OC)2Fe-X (X ) Br, I), as a result of the
relatively weak Si-Br and Si-I bonds (343 and 339 kJ/
mol, respectively).29
I. Preparative Results. In the series of dihalosilylene-
bridged dinuclear iron complexes [Cp(OC)2Fe]2SiX2, only
the fluoro- and chloro-substituted representatives 2 (X ) F)
and 3 (X ) Cl) have been prepared until now via chlorination
of [Cp(OC)2Fe]2Si(Cl)H with CCl4 to give 3, which, on
treatment with AgBF4 at 70 °C, is transformed to 2.12c
We now have found that the SiH2-substituted bis(ferrio)-
silane [Cp(OC)2Fe]2SiH2 (1)12b,i is a valuable starting material
(18) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb,
M. A.; Cheeseman, J. R.; Zakrzewski, V. G.; Montgomery, J. A., Jr.;
Stratmann, R. E.; Burant, J. C.; Dapprich, S.; Millam, J. M.; Daniels,
A. D.; Kudin, K. N.; Strain, M. C.; Farkas, O.; Tomasi, J.; Barone,
V.; Cossi, M.; Cammi, R.; Mennucci, B.; Pomelli, C.; Adamo, C.;
Clifford, S.; Ochterski, J.; Petersson, G. A.; Ayala, P. Y.; Cui, Q.;
Morokuma, K.; Malick, D. K.; Rabuck, A. D.; Raghavachari, K.;
Foresman, J. B.; Cioslowski, J.; Ortiz, J. V.; Stefanov, B. B.; Liu, G.;
Liashenko, A.; Piskorz, P.; Komaromi, I.; Gomperts, R.; Martin, R.
L.; Fox, D. J.; Keith, T.; Al-Laham, M. A.; Peng, C. Y.; Nanayakkara,
A.; Gonzalez, C.; Challacombe, M.; Gill, P. M. W.; Johnson, B. G.;
Chen, W.; Wong, M. W.; Andres, J. L.; Head-Gordon, M.; Replogle,
E. S.; Pople, J. A. Gaussian 98, revision A.7; Gaussian, Inc.:
Pittsburgh, PA, 1998.
(19) Becke, A. D. Phys. ReV. A 1988, 38, 3098-3100.
(20) Perdew, J. P.; Wang, Y. Phys. ReV. B 1992, B45, 13244-13249.
(21) Yamguchi, Y.; Frisch, M. J.; Gaw, J.; Schaefer, H. F.; Binkley, S. J.
Chem. Phys. 1986, 84, 2262-2278.
(22) Reed, A. E.; Weinstock, R. B.; Weinhold, F. J. Chem. Phys. 1985,
83, 735-746.
(23) Reed, A. E.; Curtiss, L. A.; Weinhold, F. Chem. ReV. 1988, 88, 899-
926.
(24) Foster, J. P.; Weinhold, F. J. Am. Chem. Soc. 1980, 102, 7211-7218.
(25) Reed, A. E.; Weinhold, F. J. Chem. Phys. 1985, 83, 1736-1740.
(26) Salzner, U.; Kiziltepe, T. J. Org. Chem. 1999, 64, 764-769.
(27) Malisch, W.; Mo¨ller, S.; Fey, O.; Wekel, H.-U.; Pikl, R.; Posset, U.;
Kiefer, W. J. Organomet. Chem. 1996, 507, 117-124.
Inorganic Chemistry, Vol. 42, No. 10, 2003 3277

Vo1gler et al.
Figure 1. (a) ORTEP view of [Cp(OC)2Fe]2SiF2 (2). Ellipsoids are at the 50% probability level. (b) View along the Fe1-Si1 axis of [Cp(OC)2Fe]2SiBr2
(4).
the intensity of the two Cp signals in the 1H NMR spectrum.
Scheme 2
Moreover, in the case of 6, a 19F{1H} NMR spectrum clearly
proves the stereochemistry, with the cis-isomer showing an
AB spectrum with resonances at 57.0 and 59.6 ppm (2JFSiF
) 85.1 Hz), and the trans-isomer showing a singlet at 58.1
ppm.
UV irradiation of the iodo derivative 5 does not lead to
the formation of a µ-CO-bridged diiodo iron complex that
is analogous to 6 and 7. In this case, only the decomposition
product Cp(OC)2Fe-I is formed almost quantitatively.
II. X-ray Analyses of [Cp(OC)2Fe]2SiX2 (X ) F (2), Cl
(3), Br (4)). The most characteristic feature of all three solid-
state structures of 2-4 is the coordination geometry of the
We also performed controlled UV irradiation experiments
of 2, 4, and 5, to determine if a cyclization process under
CO elimination is possible, analogous to the formation of
[Cp(OC)Fe]2(µ2-SiH2)(µ2-CO) from 112b or [Cp(OC)Fe]2[µ2-
Si(Me)(SiMe3)](µ2-CO) from [Cp(OC)2Fe-SiMe2]2.12g,h In-
deed, in the case of the difluoro derivative 2, a mixture of
the CO-bridged complex [Cp(OC)Fe]2(µ2-SiF2)(µ2-CO) (6),
[Cp(OC)2Fe]2, and other byproducts is formed after UV
irradiation for 0.5 h in benzene; from this mixture, 6 can be
isolated, in 54% yield, as a dark-red solid (Scheme 2).
Irradiation of the dibromosilylene complex 4 in benzene
proceeds more selectively and, after 3 h, leads to the
formation of 7, which is isolated, after removal of the
byproduct Cp(OC)2Fe-Br, in a yield of 76%. Compounds 6
and 7 form mixtures of cis-(6a/7a) and trans-(6b/7b) isomers,
with the cis-isomer presumably dominating in both cases,
as previously observed for 1.12b Isomer ratios (cis:trans) of
57:43 (6) and 89:11 (7), respectively, can be determined from
central Si atom, which deviates significantly from the ideal
tetrahedral form, as shown for the example of the fluoro
derivative 2 (Figure 1a). For the Fe-Si-Fe angle, values
of 125.3° (2), 125.4° (3), and 122.2° (4) are found, in
accordance with Bent’s rule, which predicts an enlargement
of the angles including the two most electropositive silicon
substituents. Consequently, the X-Si-X angles between the
electronegative halogen substituents are considerably smaller
(98.7°-99.3°).
In the solid state, 2-4 show C2 symmetry. The view along
one of the Fe-Si bonds, as shown in Figure 1b for the ex-
ample of the bromo derivative 4, reveals a distorted staggered
conformation of the substituents in the bis(metallo)silanes
2-4, with the following pairs adopting a mutually trans
position: Cp/X, CO/X, and CO/Fe.
The Fe-Si bond distances in the bismetalated silanes 2-4
(2.2924-2.342 Å) are longer than those in comparable
mono(ferrio)silanes Cp(OC)2Fe-SiR3 (2.20-2.29 Å).11,27 This
is probably the result of a much lower Fe-Si π-type back-
bonding in the bis(ferrio)silanes, which is due to the
competition of two electron-releasing Cp(OC)2Fe fragments
on the same Si atom. Similar observations have been made
for two other structurally characterized bis(ferrio)silanes:
[Cp(OC)2Fe]2SiH2 (2.34 Å)12b and [Cp(OC)2Fe]2Si(H)OH
(28) (a) Koe, J. R.; Powell, D. R.; Hayase, S.; Buffy, J. J.; West, R. Angew.
Chem. 1998, 110, 1514-1515; Angew. Chem., Int. Ed. Engl. 1998,
37, 1441-1442. (b) Hengge, E.; Kovar, D. Z. Anorg. Allg. Chem.
1979, 458, 163-167.
(29) (a) Gaydon, A. G. Dissociation Energies and Spectra of Diatomic
Molecules, 3rd ed.; Chapman and Hall: London, 1968. (b) Kondratiev,
V. N. Bond Dissociation Energies, Ionization Potentials and Electron
Affinities; Mauka Publishing House: Moscow, 1974.
3278 Inorganic Chemistry, Vol. 42, No. 10, 2003

Silylene-Bridged Dinuclear Iron Complexes
Table 2. Selected Experimental Crystal Structure Data of [Cp(OC)2Fe]2SiX2 with X ) H (1), F (2), Cl (3), and Br (4), in Comparison with the
Calculated Structural Parameters of 1-5
1 (X ) H)
expa calcb
2 (X ) F)
3 (X ) Cl)
exp
4 (X ) Br)
exp
exp
calcb
calcb
calcb
5 (X ) I), calcc
Bond Lengths (pm)
Si(1)-Fe(1)
Si(1)-Fe(2)
Si(1)-X(1)
234.2
234.0
234.0
237.2
151.7
151.3
174.0
174.1
116.4
116.4
229.24(4)
229.55(4)
164.12(8)
163.23(9)
175.65(16)
175.34(15)
115.21(19)
115.28(19)
172.9(1)
229.3
232.2
166.9
166.9
174.5
174.4
116.1
116.2
230.48(3)
230.48(3)
213.66(4)
213.66(4)
175.78(15)
175.12(15)
114.23(18)
114.64(18)
172.85(7)
231.7
234.6
217.2
216.5
175.1
174.8
115.9
116.1
230.21(19)
230.21(19)
231.8(2)
231.8(2)
175.8(10)
173.9(9)
115.6(10)
116.0(9)
173.3
233.9
233.9
234.0
234.0
175.3
174.0
115.9
116.5
233.4
233.4
260.5
260.5
175.4
174.1
115.8
116.5
Si(1)-X(2)
Fe(1)-C(1A)
Fe(1)-C(1B)
C(1A)-O(1A)
C(1B)-O(1B)
Fe(1)-Z(Cp)
174.2(5)
175.5(4)
113.6(5)
114.5(7)
172.3(6)
Bond Angles (degrees)
X(2)-Si(1)-X(1)
104.00
123.48(6)
103.78
94.0(2)
178.1(4)
178.1(5)
102.6
124.8
107.6
92.9
177.6
178.3
99.22(5)
125.269(18)
108.45(3)
95.99(7)
177.16(14)
178.55(14)
101.9
126.8
107.9
93.0
176.3
177.3
99.31(3)
101.2
125.8
107.4
93.4
175.3
176.1
98.74(12)
122.16(14)
108.25(5)
95.0(4)
178.3(7)
178.5(7)
100.3
122.6
105.8
93.9
174.5
175.9
101.0
122.8
105.2
94.1
174.1
175.5
Fe(1)-Si(1)-Fe(2)
Fe(1)-Si(1)-X(1)
C(1B)-Fe(1)-C(1A)
O(1A)-C(1A)-Fe(1)
O(1B)-C(1B)-Fe(1)
125.44(2)
108.606(11)
93.80(7)
177.61(13)
178.10(14)
a Data taken from a previous work.12b b Calculated with BPW91/6-311G(d); for 1 and 4, symmetry transformations were used to generate equivalent
atoms. #1 -x+1,y,-z+1/2 and #1 -x+1,y,-z+1/2. c Calculated using two basis sets: BPW91/3-21G(d) for the I atoms, and 6-311G(d) for the other atoms.
O1B with the H2 atom (Figure 3a). In addition, hydrogen
bridges between the cyclopentadienyl hydrogen atom (H3)
and the carbonyl groups (O1A) cross-link the chains of 3 in
a plane (Figure 3b).
The solid-state structure of [Cp(OC)2Fe]2SiF2 (2) is more
complex. Two molecules are linked, via hydrogen bridges
between a cyclopentadienyl hydrogen atom (H2) and a
carbonyl oxygen atom (O2B), to a “dimer”. Each dimer is
connected to other dimers via strong hydrogen bridges
between the fluoro substituents and the cyclopentadienyl
hydrogen atoms (H3, H8), with each fluoro atom forming
two hydrogen bonds to different molecules, resulting in a
three-dimensional network (Figure 4).
The variety in the solid-state structures of 2-4 can be
explained by the different strength of the H‚‚‚X hydrogen
bonds, which is greatest for 2, because of the small and hard
F atoms, and decreases in the following order: 2 . 3 > 4.
This can also be deduced from the fact that, in the case of
2, every F atom forms two hydrogen bonds to cyclopenta-
dienyl hydrogen atoms. Recently, weak hydrogen bonding
has also been studied by theoretical calculations.30
III. Vibrational Spectroscopy and DFT Calculations.
The most-significant calculated and experimental fundamen-
tal vibrational modes, together with their Raman scattering
activities, are summarized in Table 3. As one can notice,
the calculated values agree well with the experimental data;
however, some deviations were observed for the vibrations
involving lone-pair atoms and the SiH2 vibrations in 1, for
which anharmonicity is large. The results were then scaled
to better approach the experimental spectra in these regions
(Table 3). All other calculated wavenumbers were in very
good agreement with the experimental values and should not
be scaled. The best conformity between calculation and
experimental data was observed for the cyclopentadienyl ring
breathing and ν(CC) modes, for which the maximum
deviations were only 4 and 6 cm-1, respectively (Table 3).
Figure 2. Hydrogen bridges Br1-H4 (3.20 Å) and O1B-H5 (2.56 Å)
forming the chain structure of 4.
(2.34 Å).12a It is also remarkable that the Fe-Si bond length
in 2-4 is almost invariant, with respect to the type of halogen
substituent on the silylene bridge, which means that no
significant shortening of the Fe-Si bond is observed as the
electronegativity of the substituents increases (Table 2).
As expected, all three halogen-substituted bis(ferrio)silanes
2-4 form intermolecular hydrogen bonds. In the case of
[Cp(OC)2Fe]2SiBr2 (4), a simple chain structure can be
observed. The two strongest hydrogen bridges are found
between the cyclopentadienyl hydrogens (H4, H5) and the
Br or O1B atoms, respectively (Figure 2).
A similar arrangement is found for [Cp(OC)2Fe]2SiCl2 (3),
which involves the interaction of the Cl substituents with
the H1 atom and the interaction of the carbonyl oxygen atom
(30) Calhorda, M. J. Chem. Commun. 2000, 801-809.
Inorganic Chemistry, Vol. 42, No. 10, 2003 3279

Vo1gler et al.
Figure 3. (a) View along the hydrogen-bridged chains of 3 with Cl1-H1 (2.88 Å) and O1B-H2 (2.58 Å). (b) View onto the cross-linked chains of 3 with
O1A-H3 (2.56 Å).
could be measured for [Cp(OC)2Fe]2SiI2 (5), because of its
decomposition under the laser light.
A closer examination of the 2100-1850 cm-1 spectral
region in the FT-Raman spectra of 1-4 indicates the
presence of two or more symmetrical and asymmetrical CO
vibrational modes (Table 3, Figure 5). More exactly, 3 and
4 show only two ν(CO) vibrations, whereas more than two
bands can be observed for 1 and 2 (Table 3, Figure 5). In
the Raman spectrum of 1, the bands at 2074 and 2043 cm-1
were assigned to symmetrical SiH stretching modes. For the
other peaks at 2028, 1987, 1978, and 1928 cm-1, the
theoretical calculations and literature indicate CO stretching
modes strongly mixed with symmetrical and asymmetrical
SiH2 stretching vibrations.32,33 For 2-4, these vibrations are
“pure” CO stretchings and no other internal coordinates are
involved.32-37 As expected, the DFT calculations, which were
performed without any symmetry restriction, led to more than
two ν(CO) modes in each case (1-5) (Table 3). Taking into
account that the X-ray structure determination indicated a
(31) Shillady, D. D.; Craig, J.; Rutan, S.; Rao, B. Int. J. Quantum Chem.
2002, 90, 1414-1420.
(32) Malisch, W.; Jehle, H.; Mo¨ller, S.; Thum, G.; Reising, J.; Gbureck,
A.; Nagel, V.; Fickert, C.; Kiefer, W.; Nieger, M. Eur. J. Inorg. Chem.
1999, 1597-1605.
(33) Nagel, V.; Fickert, C.; Hofmann, M.; Vo¨gler, M.; Malisch, W.; Kiefer,
W. J. Mol. Struct. 1999, 480-481, 511-514.
(34) (a) Jetz, W.; Graham, W. A. G. J. Am. Chem. Soc. 1967, 89, 2773-
Figure 4. Segment of the three-dimensional network of 2. Only the three
2775. (b) Malisch, W.; Kuhn, M. Chem. Ber. 1974, 107, 2835-2851.
strongest hydrogen bonds are shown: O2B-H2 (2.50 Å), F2-H3 (2.42
Å), and F2-H8 (2.42 Å).
(c) Malisch, W.; Panster, P. Chem. Ber. 1975, 108, 2554-2573.
(35) Dalton, J.; Paul, I.; Stone, F. G. A. J. Chem. Soc. A 1969, 2744-
2749.
(36) Ho¨fler, M.; Scheuren, J.; Spilker, D. J. Org. Chem. 1975, 40, 205-
The FT-Raman spectra of 1-4 in the CO spectral region
211.
are presented in Figure 5. Unfortunately, no Raman spectrum
(37) Thum, G.; Malisch, W. J. Organomet. Chem. 1984, 264, C5-C9.
3280 Inorganic Chemistry, Vol. 42, No. 10, 2003

Silylene-Bridged Dinuclear Iron Complexes
Inorganic Chemistry, Vol. 42, No. 10, 2003 3281

Vo1gler et al.
Table 4. Partial Chargesa Situated on Selected Atoms of
[Cp(OC)2Fe]2SiX2 with X ) H, F, Cl, Br, and I (1-5), Determined by
the Natural Population Analysis
atom
1b
2b
3b
4b
5c
X
Si
Fe
C
-0.063
-0.076
+0.740
-0.026
-0.196
-0.402
+0.676
+0.698
-0.036
-0.186
-0.336
+0.167
+0.767
-0.020
-0.175
-0.265
+0.100
+0.766
+0.011
-0.162
-0.128
-0.330
+0.805
+0.034
-0.159
O
a Given in units of e. b Calculated with BPW91/6-311G(d). c Calculated
using two basis sets: BPW91/3-21G(d) for the I atoms, and 6-311G(d) for
the other atoms.
atom in 2 still bears the highest electron density. The reasons
for these observations can be rationalized assuming a π-type
bonding from the nonbonding halogen valence electrons to
the unoccupied d(Si) or σ*(Fe-Si) orbitals. Such a π-type
overlap is more efficient for smaller orbitals (e.g., in F) than
for the more diffuse orbitals of the larger halogen substituents
and might therefore overcompensate the opposite ionic
contributions to the Fe-Si bond.
For a better understanding of the π-type bonding from the
nonbonding halogen valence electrons to the unoccupied
d(Si) or σ*(Fe-Si) orbitals, we performed an NBO analysis
for quantitative information regarding these second-order
interactions. Table 5 shows the occupancy of the lone-pair
orbitals nx at the halogen atoms and that of the σ*(Fe-Si)
orbitals. As one can notice, the ionic contribution decreases
in the order F > Cl > Br > I, as indicated by the presence
of four lone-pair orbitals of the fluoro and chloro derivatives
(2 and 3); the most electronegative compound 2 presents a
higher occupancy. As we have already mentioned, the NPA
analysis indicates that the Fe atom in 2 has the highest
electron density, in comparison with 3-5 (Table 4), which
is further substantiated by the NBO results (Table 5) that
show a decrease in the population of the σ1*(Fe-Si) orbitals
when going from 2 to 5. The NBO analysis reveals that
σ1*(Fe-Si) orbitals are a mixture of Fe- and Si-centered
hybrids, which have sd form at the Fe atom and sp form at
the Si atom. In contrast to this, σ2*(Fe-Si) orbitals have
almost pure Si character (Table 5). Moreover, a lone-pair
orbitalscomposed of a hybrid of a high p-character and an
occupancy of 0.63 and 0.59 e, respectively (Table 5)sat the
Si atom was detected only for 2 and 3. We would like to
mention that the possible discrepancies in the calculated
values of complex 5 (in comparison with the other com-
plexes, 2-4) may be caused by the different basis set
employed in this case for the halogen atom (3-21G(d) for
the I atoms and 6-311G(d) for the other atoms).
Figure 5. FT-Raman spectra of 1-4 in the CO spectral region. Excitation
line is located at 1064 nm.
C2 symmetry for all complexes, we believe that the appear-
ance of more than two bands in the CO spectral region of 2
(1999, 1983, 1956, 1944, and 1931 cm-1) is mostly due to
the existence of different conformers, as previously reported
for similar compounds.34
For complexes 2-4, the ν(CO) vibration is coupled only
with other ν(CO) vibrations; therefore, the wavenumber shift
can be taken as a direct measure of bond strength. As one
can notice (Figure 5, Table 3), the sequential replacement
of the H atoms with either F, Cl, or Br atoms on the Si atom
causes the ν(CO) values to increase. Furthermore, the
magnitude of this effect depends on the substituents in the
following order: F < Cl < Br (i.e., 1999, 2003, 2006 cm-1
and 1931, 1941, 1948 cm-1). The same trend was observed
in the IR spectra and confirmed by the DFT calculations (F
< Cl < Br e I), although the experimental values differ in
this case by almost 30 cm-1 from the experimental data
(Table 3). However, when scaled by 0.9827 in this spectral
region,31 the calculated CO stretching modes come very close
to the experimental values (Table 3). This is a very surprising
result, because it is contrary to the tendency observed for
other homologous silyl complexes (OC)LnM-SiR3,11 where
the ν(CO) values decreased as the electronegativity of the
silyl substituent decreased.
This unexpected shift of the carbonyl stretching mode to
higher wavenumbers could be explained by an increase of
the electron density on the metal available for d-π-type back-
bonding to the π*(CO) orbitals. The NPA (Table 4) actually
reveals that the Fe atoms in 1-5 are positively charged, with
the lowest formal charge being found for the fluorine
derivative 2 (+0.698) and significantly higher charges being
observed for the other halogen derivatives 3-5. Despite the
undoubtedly higher electronegativity of the SiF2 unit, the Fe
Table 6 presents the strengths of the perturbative donor-
acceptor interactions, which involve occupied and formally
unoccupied NBOs. A closer examination of the stabilization
energies in Table 6 reveals stronger π-type back-bonding
for 2 and 3, showing slightly higher energy values for the
chlorine compound 3 (417.35 kJ/mol) and very small values
for the other derivatives (4 and 5). However, for the σ1*(Fe-
Si) orbital, the highest stabilization energy (120.88 kJ/mol)
was found in the fluorine compound 2. In addition, in the
case of the fluoro derivative 2, we also observed a strong
nSi f σ*(Fe-Si) interaction (929.27 kJ/mol) that should lead
3282 Inorganic Chemistry, Vol. 42, No. 10, 2003

Silylene-Bridged Dinuclear Iron Complexes
Table 5. Orbital Occupancy (e) and Contributions of Atomic Orbitals (%) for [Cp(OC)2Fe]2SiX2 with X ) F, Cl, Br, and I (2-5)
2a
3a
4a
5b
σ1*(Fe-Si)
Fe
Si
0.46858 e (sd1.87, sp0.23
52.30%
47.70%
)
0.40984/0.38207 e (sd1.93, sp1.30
56.56%
43.44%
)
0.38570/0.36686 e (sd1.81, sp2.25
55.78%
44.22%
)
0.34184 e (sd1.92, sp1.21
47.25%
52.75%
)
σ2*(Fe-Si)
Fe
0.54904/0.38657 e (d, p)
4.84%
0.46016 e (d, p)
4.83%
0.48267/0.90064 e (d, p)
4.24%
Si
nSi
nx (1)
nx (2)
nx (3)
nx (4)
95.16%
95.17%
95.76%
0.62590 e (sp5.62
1.97890 e (sp0.67
1.93219 e (p)
)
)
0.58662 e (sp7.43
1.97684 e (sp0.35
1.94419 e (p)
)
)
1.97774 e (sp0.27
1.93461 e (p)
1.92902 e (p)
)
1.98099 e (sp0.17
1.93833 e (p)
1.93250 e (p)
)
1.92907 e (p)
1.93430 e (p)
1.79020 e (sp1.48
)
1.57587 e (sp2.86
)
a Calculated with BPW91/6-311G(d). b Calculated using two basis sets: BPW91/3-21G(d) for the I atoms, and 6-311G(d) for the other atoms.
Table 6. Results of the NBO Analysis for [Cp(OC)2Fe]2SiX2 with X )
H (1), F (2), Cl (3), Br (4), and I (5)
2a
3a
4a
5b
nx f σ2*(Fe-Si)
E(2) (kJ/mol)c
ꢀ(j) - ꢀ(i) (a.u.)
F(i, j) (a.u.)
271.29
417.35
0.38
72.59
1.11
0.141
0.74
0.205
0.174
nx f σ2*(Fe-Si)
E(2) (kJ/mol)c
ꢀ(j) - ꢀ(i) (a.u.)
F(i, j) (a.u.)
92.88
106.60
0.75
73.97
0.05
0.031
0.81
0.133
0.140
nx f σ1*(Fe-Si)
E(2) (kJ/mol)c
ꢀ(j) - ꢀ(i) (a.u.)
F(i, j) (a.u.)
120.88
21.51
0.56
25.94
0.24
0.036
2.26
0.36
0.013
0.80
0.146
0.049
n
Si f σ2*(Fe-Si)
E(2) (kJ/mol)
ꢀ(j) - ꢀ(i) (a.u.)
F(i, j) (a.u.)
929.27
0.01
0.059
Figure 6. FT-Raman spectra of 1-4 between wavenumbers of 1600 and
200 cm-1. Excitation line is located at 1064 nm.
nSi f σ1*(Fe-Si)
E(2) (kJ/mol)
ꢀ(j) - ꢀ(i) (a.u.)
F(i, j) (a.u.)
245.60
0.12
0.101
38.28
0.20
0.054
(symmetrical ν(Fe-Si)), and in that of 2, as a medium-strong
peak at 350 cm-1 (asymmetrical ν(Fe-Si)) (Table 3, Figure
6). The visualization of these vibrations shows a strong
coupling with the symmetrical and asymmetrical Fe-Cp
stretching mode, respectively. Hence, these motions should
be better ascribed to a symmetrical and asymmetrical
ν(SiFeCp) mode (Table 3). Nevertheless, the calculated
values are consistent with the assumed weakening of the Fe-
Si bond, because of the π-type bonding of the nonbonding
halogen valence electrons with the unoccupied d(Si) or σ*-
(Fe-Si) orbitals. More exactly, the asymmetrical ν(SiFeCp)
modes of 2-5 were found to decrease in the order F, Cl,
Br, and I (385, 379, 376, and 370 cm-1). A similar tendency
is present in the symmetrical ν(SiFeCp) modes, which were
theoretically determined at 361, 350, 344, and 342 cm-1 in
2-5.
Because of its relative intensity and characteristic wave-
number (1110 ( 10 cm-1), the ring breathing mode of the
cyclopentadienyl ring can be considered to be the most
evident vibration of the group (Figure 6). As also indicated
by the calculated harmonic vibrational wavenumbers, the
cyclopentadienyl ring breathing mode of 1-5 does not
change substantially upon the substitution of X (Table 3,
Figure 6). A similar behavior is present in the CC stretching
vibrations of the cyclopentadienyl ring, which strongly couple
with the CH in-plane deformations and give rise to the peaks
at 1435-1360 cm-1 in the Raman spectra of 1-5 (Figure
a Calculated with BPW91/6-311G(d). b Calculated with BPW91/3-21G(d)
for the I atoms and 6-311G(d) for the other atoms. c For each lone pair,
only the donor-acceptor interactions with the highest E(2) values have
been taken into consideration. E(2) ) qi[F(i,j)2/(ꢀj - ꢀi)], where E(2) is the
second-order perturbational energy stabilization. F(i,j) is the Fock matrix
element between an occupied orbital i and an unoccupied orbital j. F(i,j) is
proportional to the overlap between orbitals i and j. ꢀ(j) - ꢀ(i) is the energy
difference between orbitals i and j, and qi is the occupancy of donor orbital
i. Thus, NBO analysis can be used to examine conjugation effects in
π-conjugated systems quantitatively.23-26
to a significant π-stabilization of 2. This effect is negligible
for the other three compounds (3-5).
The introduction of halogen atoms at the Si site can be
monitored in a straightforward manner by the disappearance
of the SiH stretching modes of 1 at 2074 and 2043 cm-1
and the simultaneous formation of new bands in the IR and
Raman spectra of 2-5, which is characteristic to the
symmetrical and asymmetrical SiX2 stretching vibrations
(Table 3, Figures 5 and 6). One can notice that, upon
substitution of the H atom with either a F, Br, or Cl atom
on the Si atom, the ν(SiX2) modes are shifted significantly
to lower wavenumbers, because of mass effects. Moreover,
they strongly mix with other vibrations. Consequently, the
wavenumber shift cannot give us any further information.
Unfortunately, the very interesting symmetrical and asym-
metrical Fe-Si stretching modes can be observed only in
the Raman spectrum of 1, as a medium signal at 390 cm-1
Inorganic Chemistry, Vol. 42, No. 10, 2003 3283

Vo1gler et al.
6, Table 3). So, we can conclude that the changes in the
electronic density on the metal have no influence on these
bonds.
following order: F < Cl < Br e I. It must be assumed that
the π-type back-bonding of the nonbonding orbitals of the
halogen substituent X into unoccupied σ*(Fe-Si) orbitals
is responsible for this effect. All observed structural and
vibrational properties are clearly indicating a strong electron-
donating influence of the iron fragments on the silylene unit.
This transition-metal effect previously has been invoked to
rationalize some unusual characteristics and reactivities of
mono- and bis(ferrio)silanes, e.g., the stabilization of Si(OH)
units toward condensation or the activation of SiH moieties
for halogenation and oxygenation reactions.
Conclusion
This paper describes the synthesis and characterization of
bis(ferrio)silanes of the type [Cp(OC)2Fe]2SiX2 (X ) F, Cl,
Br, I). The availability of a full series of homologous
compounds gave us the opportunity to perform a systematic
study of the bonding situation in these complexes using X-ray
crystallography, vibrational spectroscopy, and density func-
tional theory calculations. In this context, the nature of the
Fe-Si bond and the influence of the various substituents H,
F, Cl, Br, and I on the silylene bridge seemed to be of
particular interest.
The Fe-Si distance can be taken as a measure of the bond
strength in these complexes. The experimental and calculated
Fe-Si bond distances in 1-5 are generally larger than those
in other mono(ferrio)silanes Cp(OC)2Fe-SiR3; however,
significant variation of the bond length, relative to the type
of halogen substituent, could not be observed. The calculated
Fe-Si stretching vibrations predict an increase in wave-
number and, hence, bond strength, in the following order: I
< Br < Cl < F. This is a typical consequence of the ionic
bonding contribution between the iron fragment and the
silylene unit SiX2, with increasing electronegativity of X.
A remarkable result was found for the wavenumbers of
the CO stretching modes, which are increasing in the
Further studies concerning the reactivity of this interesting
class of compounds are currently in progress and are focused
on the preparation of unusual metal-silicon species, such
as bismetalated silylenes [Cp(OC)2Fe]2Si and silanones
[Cp(OC)2Fe]2SidO.
Acknowledgment. We gratefully acknowledge financial
support from the Deutsche Forschungsgemeinschaft (Schwer-
punktprogramm “Spezifische Pha¨nomene in der Silicium-
chemie”; SFB 347 “Selektive Reaktionen metallaktivierter
Moleku¨le”), as well as support from the Fonds der Chemis-
chen Industrie.
Supporting Information Available: X-ray crystallographic files
for 2-4 in CIF format. This material is available free of charge
via the Internet at http://pubs.acs.org.
IC026138V
3284 Inorganic Chemistry, Vol. 42, No. 10, 2003