Synthesis and reactivity of a strained silicon-bridged [1]ferrocenophanium ion

Full text of DOI:10.1002/anie.200901213

Angewandte
Chemie
DOI: 10.1002/anie.200901213
Cationic Metallocenophanes
Synthesis and Reactivity of a Strained Silicon-Bridged
[1]Ferrocenophanium Ion**
Georgeta Masson, David E. Herbert, George R. Whittell, Jason P. Holland, Alan J. Lough,
Jennifer C. Green,* and Ian Manners*
Ferrocene, [Fe(h-C5H5)2], first prepared in the 1950s as an
orange solid,[1] is the prototypical metallocene. Its remarkable
stability is attributed to the aromatic character of cyclo-
pentadienyl (Cp) ligands and an 18-electron count.[2] This
compound undergoes reversible one-electron oxidation to
generate the dark blue 17-electron FeIII ferrocenium ion,
[Fe(h-C5H5)2]+, which can be isolated as stable salts.[2a,3]
An interesting class of ferrocene derivatives is repre-
sented by [n]ferrocenophanes.[4] Among these, the most
explored are strained silicon-bridged [1]ferrocenophanes or
sila[1]ferrocenophanes, because of their diverse reactivity and
ability to be converted into polyferrocenylsilanes (PFSs) by a
variety of ring-opening polymerization (ROP) methods.[5]
Despite the large and rapidly increasing number of existing
strained [n]ferrocenophanes,[6] only species with two or more
atoms in the bridge, such as the 17-electron carbon-bridged
[2]ferrocenophanium ions in 1 and 2, have been successfully
synthesized by oxidation and isolated as stable salts.[7]
voltammetry, salts of 17-electron [1]ferrocenophanium cat-
ions have not been isolated and their stability has been
questioned.[9] On the other hand, recent studies by our
research group have shown that improvements to the stability
of oxidized PFS materials can be achieved by introducing
electron-donating Me or tBu groups on the Cp ligands.[10] This
prompted us to study the oxidation of a sila[1]ferrocenophane
precursor with such substituents.
Despite the reversible one-electron oxidation detected by
cyclic voltammetry (E1/2 = À0.24 V in 0.1m [Bu4N][PF6] in
CH2Cl2 vs ferrocene/ferrocenium), attempts to oxidize the
electron-rich tBu-substituted [1]ferrocenophane [Fe(h-
C5H3tBu)2SiPh2] (3)[10] to the corresponding 17-electron
cation using I2 or AgPF6 failed to afford the desired
[1]ferrocenophanium ion and only unidentified insoluble
products or ring-opened species were obtained.[11] We there-
fore explored the use of a one-electron oxidant with a
counteranion likely to be less reactive toward both FeIII Cp
À
À
and Si Cp bonds. Reaction of 3 with tris(4-bromophenyl)am-
moniumyl hexafluoroantimonate [N(C6H4-4-Br)3]C+[SbF6]À in
CH2Cl2 at À20 to À108C afforded the ferrocenophanium salt
4 in 74% yield (Scheme 1, top).[12] Surprisingly, the desired
To date, only electrochemical studies of the one-electron
oxidation of [1]ferrocenophanes have been reported.[8,9]
Although the process is reversible on the timescale of cyclic
[*] Dr. J. P. Holland, Prof. J. C. Green
Chemistry Research Laboratory, Department of Chemistry
University of Oxford, 12 Mansfield Road
Oxford, OX1 3TA (UK)
E-mail: jennifer.green@chem.ox.ac.uk
Scheme 1. Synthesis and ring-opening reactivity of ferrocenophanium
salt 4.
D. E. Herbert, Dr. G. R. Whittell, Prof. I. Manners
School of Chemistry, University of Bristol
Cantocks Close, Bristol BS8 1TS (UK)
Fax: (+44)117-9290509
product was isolated as red-brown crystals, very different in
color from the dark blue or green typical of ferrocenium salts.
The paramagnetic product was recrystallized and identified
by single-crystal X-ray diffraction and elemental analysis.
X-ray crystallography confirmed the bent-sandwich struc-
ture of the [1]ferrocenophanium ion in 4 (Figure 1). The two
Cp ligands remain essentially planar, and the tilt angle (a =
28.9(13)8) between their planes is considerably greater than
for the 18-electron precursor 3 (a = 18.69(9)8).[10] Other
E-mail: ian.manners@bristol.ac.uk
G. Masson, Dr. A. J. Lough
Department of Chemistry, University of Toronto
80 St. George Street, Toronto, Ontario M5S 3H6 (Canada)
[**] This work was supported by the NSERC of Canada. I.M. thanks the
European Union for a Marie Curie Chair and the Royal Society for a
Wolfson Research Merit Award.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.200901213.
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To allow more in-depth compara-
tive studies, the 1,1’-di-tert-butylferro-
cenium salt [Fe(h-C5H4tBu)2]+[SbF6]À
(9) was synthesized and characterized
by X-ray diffraction. UV/Vis spectra
of 3–5 (R = H) and 9 were acquired at
room temperature in CH2Cl2 solution
(5 ꢀ 10À3 m). The spectra are shown in
Figure 2a (key data are summarized in
the Supporting Information, Table 2).
While the UV/Vis spectra of 5 (R = H)
and 9 are typical of ferrocenium ions,[3]
that of the sila[1]ferrocenophanium
ion in 4 is very unusual, as implied by
the red-brown color of the species.
As shown previously, the ligand-
to-metal charge-transfer (LMCT)
band characteristic of ferrocenium
Figure 1. Side (a) and top (b) ORTEP views of 4 with thermal ellipsoids drawn at the 30%
probability level. Hydrogen atoms have been omitted for clarity.
significant differences in angles and iron displacement values
are noted in the Supporting Information.[13]
Based on the substantially increased tilt-angle in the
cation in 4 relative to that in 3, ring-opening reactions would
be expected to be even more favorable. Indeed, in contrast to
3 which, as previously reported,[10] does not react with either
water or methanol at 258C, the [1]ferrocenophanium salt 4
was found to undergo both hydrolysis and methanolysis under
similar conditions (Scheme 1). The product resulting from the
hydrolysis of 4 was identified as 5 (R = H) by single-crystal X-
ray diffraction. Following reduction with decamethylferro-
cene, the neutral compound, 1-(diphe-
nylhydroxysilyl)-3,1’-di-tert-butylferro-
cene (6; R = H), was identified by NMR
spectroscopy and mass spectrometry. The
reaction of 4 with methanol followed by
reduction with decamethylferrocene
yielded an orange product identified by
1H NMR spectroscopy and mass spec-
trometry as 1-(diphenylmethoxysilyl)-
3,1’-di-tert-butylferrocene (6; R = Me).
Further support was provided by 13C
and 29Si NMR spectroscopy and the sili-
con connectivity was confirmed by 2D
gHMBC 29Si NMR spectroscopy.[12]
Ring-opening polymerization of 4
would be expected to lead to polymers
constituted entirely of ferrocenium
repeat units with silicon spacers. Notably,
while ferrocenophane 3 undergoes ROP
Figure 2. a) UV/Vis spectra of 3, 4, 5 (R=H), and 9. b) Photographs of these species in
CH2Cl2 solutions.
above 1908C, the ring-opening reaction of [1]ferrocenopha-
nium 4 occurs at a much lower temperature (658C). However,
the product was not macromolecular, and following reduc-
tion, mass spectrometry indicated the presence of a molecular
ion at m/z 498, corresponding to the molecular ferrocene
derivative, 1-(diphenylfluorosilyl)-3,1’-di-tert-butylferrocene
(7). The corresponding cation 8 was also obtained in the
ions undergoes a red shift from 620 to 650 nm upon
substitution of Cp ligands with electron-donating groups
such as nBu.[3] In agreement with this, the LMCT band of the
di-tert-butylferrocenium ion in 9 appeared at 645 nm and that
of 5 (R = H) was located at 660 nm. Remarkably, the
maximum absorption of ferrocenophanium salt
4 was
observed at 435 nm as a more intense band than any observed
for either the FeII precursor 3 or the ferrocenium derivatives 5
(R = H) and 9. Also, the LMCT band for the ferrocenopha-
nium ion in 4 appeared as a broad shoulder on a shorter-
[12]
attempted synthesis of 4 by the reaction of 3 with AgPF6
and 7 may form through an initial thermally induced fluoride
transfer reaction between electrophilic 4 and [SbF6]À.[14]
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wavelength d–d band,[3] blue-shifted relative to the character-
istic position of LMCT bands in unstrained ferrocenium ions.
The variation in color of CH2Cl2 solutions from orange for 3,
to unexpected red-brown for [1]ferrocenophanium ion in 4,
and to green or blue for the acyclic ferrocenium derivatives 5
(R = H) and 9, respectively, can be seen in Figure 2b.
The LMCT band at 620 nm in the spectrum of the
ferrocenium ion is assigned to a transition from the e1u
p
orbitals of the Cp ligands to the hole in the metal-based e2g
level, and its intensity is attributed to the charge-transfer
nature of the transition. Subsequent bands are vibronic d–d
transitions; hence, they are of lower intensity. In the lower-
symmetry metallocenophanium structure of 4, d–d transitions
become dipole-allowed, and as observed, are of greater
intensity. The other effect on the electronic absorption
spectrum on bending the Cp rings from a parallel config-
uration is the perturbation of the d levels.[15]
The shift in the first band is reflected in the differences
between the photoelectron (PE) spectra of ferrocene and
10.[16] A PE spectrum maps selected ion states of the
molecular cation: the onset of the first band corresponds to
the ground state of the cation and subsequent bands
correspond to excited states where the hole has been filled
by promotion of an electron from one of the occupied
molecular orbitals. Thus, the separation of the first adiabatic
ionization energy and the e1u band of the PE spectrum of
ferrocene should correspond in energy to the e1u to e2g
transition discussed above. For ferrocene, the band separation
is 2.07 eV, which is roughly equivalent to 600 nm.[17] For 10,
the separation of the bands increases to 2.39 eV (correspond-
ing to 520 nm) in the region of the broad shoulder exhibited
by 4.[16] The PE spectrum of 3 has not been measured, but that
of the methyl-substituted analogue (11) has a band separation
of 2.1 eV (corresponding to 590 nm), showing a similar blue
shift from the band position in the ferrocenium ion. The PE
spectra indicate that the differences between the electronic
absorption spectra of 4 and unbridged ferrocenium ions (e.g.,
9) are primarily due to the increase in energy of the metal d
orbitals on bending, rather than a perturbation of the Cp
p levels.
Time-dependent density functional theory (TD-DFT)
calculations were used to investigate the nature of the
unexpected electronic absorption spectrum obtained for 4.
The less computationally demanding dimethylsila[1]ferroce-
nophane (10) and the corresponding dimethylsila[1]ferroce-
nophanium (12), which do not contain ring substituents
besides the ansa bridge, were chosen for the calculations. The
DFT-optimized structure for 10 (C2v) is in excellent agree-
ment with the X-ray crystal structure with a weighted root-
mean-square deviation (RMSD) calculated for all heavy
atoms of only 0.0517 ꢁ.[15] The structure of 12 was optimized
without symmetry constraints as the C1 structure was found to
be 10 kJmolÀ1 more stable than the C2v structure. The
calculated structures show the same trends upon oxidation
as the X-ray structures of 3 and 4 (see the Supporting
Information, Table 1).
Figure 3. a) Overlay of the best fit of the simulated spectrum (b)
with the experimental electronic absorption spectrum (c; [10]=
7.8 mm in hexanes). The fit was optimized using a scaling factor of
0.996 for the calculated transition energies. b) Plot of the calculated
electronic absorption intensity vs. wavelength for the C1 structure of 12
(b: simulated, c: experimental 4). Several minor transitions have
been combined. The calculated wavelengths have been corrected by
+32 nm to provide the best fit with experiment (in CHCl3). MO
contributions to bands A–E are presented in the Supporting
Information.
tronic absorption spectra for the ferrocenophanium ion 12
with the experimental electronic absorption spectrum of 4.
Both calculations give excellent qualitative and quantitative
fits with experiment. Minor transitions to four excited states
in the calculated structure of 12 were combined to reproduce
the UV/Vis spectrum of 4. If we compare the energy of the
b LUMO in [FeCp2]+ with that of 12, it increases by 0.87 eV
whereas the energy of the highest occupied Cp p level only
changes by 0.1 eV (see the Supporting Information). This
confirms that the main origin of the blue shift is the
perturbation on bending of the Fe d orbitals.[19]
In summary the first strained [1]ferrocenophanium ion
was successfully isolated as a [SbF6]À salt (4) by one-electron
oxidation of sila[1]ferrocenophane 3. Both the considerable
structural changes and the unexpected electronic absorption
spectrum of 4 were reproduced by TD-DFT calculations.
Consistent with the presence of additional ring strain, and in
contrast to the 18-electron precursor 3, ferrocenophanium 4
undergoes both hydrolysis and methanolysis at 258C. Under
thermal ROP conditions, a ring-opening reaction of 4 occurs,
Figure 3a shows an overlay of the experimental and
simulated absorption spectra of 10 (Gaussian03 B3LYP/6-
31 + G(d)), while Figure 3b compares the calculated elec-
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Communications
[10] G. Masson, P. Beyer, P. W. Cyr, A. J. Lough, I. Manners,
Macromolecules 2006, 39, 3720.
but this does not generate a polymer, presumably because of
competitive fluoride transfer involving the counteranion
[SbF6]À.[14] As a result, in our future attempts to induce the
ROP of sila[1]ferrocenophanium cations we will replace
[SbF6]À with an even more stable counterion such as [BPh4]À.
[11] The reaction of 3 with AgPF6 in CH2Cl2 led to the formation of a
green solid product which was identified as [Fe(h-C5H4tBu){h-
C5H3(tBu)SiFPh2}][PF6] (8). X-ray diffraction and mass spec-
trometry characterized the product as a ring-opened ferroce-
nium derivative with a fluorine atom at silicon (see the
Supporting Information). This unexpected result confirmed
that [PF6]À was a non-innocent anion acting as a source of HF,
presumably formed by hydrolysis (see R. Fernandez-Galan,
B. R. Manzano, A. Otero, M. Lanfranchi, M. A. Pellinghelli,
Received: March 3, 2009
Published online: June 2, 2009
Keywords: iron · metallocenophanes · oxidation ·
.
À
Inorg. Chem. 1994, 33, 2309), which cleaves the Si Cp bond and
ring-opening polymerization · ring strain
opens the ring with formation of the fluorosilane ferrocenium
derivative.
[12] Synthetic procedures and characterization data can be found in
the Supporting Information.
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[13] Crystal structure data for 4: C30H34F6FeSbSi, 714.26 gmolÀ1
,
orthorhombic, space group Pmn21; a = 11.172(2), b = 10.877(2),
c = 12.196(2) ꢁ, a = b = g = 908, V= 1482.0(5) ꢁ3; Z = 2, 1calcd
1.601 gcmÀ3; crystal dimensions: 0.30 ꢀ 0.22 ꢀ 0.06 mm3; diffrac-
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54.968; 6163 reflections, 2837 independent (Rint = 0.0776), direct
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correction: semi-empirical from equivalents; refinement
(against F2
) with SHELXTL V6.1, 253 parameters, 113
0
restraints, R1 = 0.0601 (I > 2s) and wR2 = 0.1648 (all data),
Goof = 1.020, residual electron density: 1.210/À0.795 eꢁÀ3
.
CCDC 690321 (4) contains the supplementary crystallographic
data for this paper. These data can be obtained free of charge
from The Cambridge Crystallographic Data Centre via www.
ccdc.cam.ac.uk/data_request/cif.
[14] This product might also arise from thermally generated HF by
hydrolysis of the [SbF6]À counteranion.
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