Influencing the electronic interaction in diferrocenyl-1-phenyl-1H-pyrroles

Article abstract of DOI:10.1039/c1dt10997a

Functionalised diferrocenyl-1-phenyl-1H-pyrroles were synthesised using Negishi C,C cross-coupling reactions. The influence of different substituents at the phenyl moiety on the electronic interaction was studied using electrochemistry (cyclic and square-

Full text of DOI:10.1039/c1dt10997a

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PAPER
Influencing the Electronic Interaction in Diferrocenyl-1-Phenyl-1H-Pyrroles†
Alexander Hildebrandt and Heinrich Lang*
Received 27th May 2011, Accepted 16th August 2011
DOI: 10.1039/c1dt10997a
Functionalised diferrocenyl-1-phenyl-1H-pyrroles were synthesised using Negishi C,C cross-coupling
reactions. The influence of different substituents at the phenyl moiety on the electronic interaction was
studied using electrochemistry (cyclic and square-wave voltammetry) and spectro-electrochemistry (in
situ UV/Vis-NIR spectroscopy). The ferrocenyl moieties gave rise to two sequential, reversible redox
processes in each of the diferrocenyl-1-phenyl-1H-pyrroles. The observed DE1/2 values (DE1/2
=
difference between first and second oxidation) range between 420 and 480 mV. A linear relationship
between the Hammett constants s of the substituents and the separation of the redox potentials exists.
The NIR measurements confirm electronic communication between the iron centers as intervalence
charge transfer (IVCT) absorptions were observed in the corresponding mixed-valent monocationic
species. All compounds were classified as class II systems according to Robin and Day (M. B. Robin
and P. Day, Adv. Inorg. Chem., 1967, 10, 247–423). The oscillator strength of the charge transfer
transition highly depends on the electron donating or electron withdrawing character of the phenyl
substituents. This enables direct tuning of the intermetallic communication by simple modification of
the molecule’s functional group. Hence, this series of molecules may be regarded as model compounds
for single molecule transistors.
that modification of the p-system in the bridging units between
Introduction
the ferrocenyl moieties can have a significant impact on the
electrochemical behaviour of such diferrocenyl heterocycles. The
difference between the 1st and the 2nd ferrocenyl based oxidation
(DE1/2) increases from 150 mV in the 2,5-diferrocenyl thiophene to
280 mV in the appropriate 1,3-diferrocenyl benzo[c]thiophene.6,7
Despite using different electrolytes in those studies, which hinders
the comparability, these results clearly show that a modifica-
tion in the electronic structure of the bridging systems can
dramatically change the interaction between the redox-active
termini.
Molecules possessing at least two redox active metal centres
combined by p-conjugated connections offer the possibility to
generate mixed-valent compounds by oxidising one of the metals,
resulting in molecules in which the transition metals have different
oxidation states.1 Mixed valency has attracted interest during
recent decades as such species can be used as model systems
to study electron transfer properties of, for example, molecular
wires and conducting polymers.2 Moreover, these p-conjugated
redox active compounds may be suitable for the design of novel
electro-active materials.3 In this respect, the use of the ferrocene
Fe(II)/Fe(III) redox couple, as electron reservoir in conjugated
organometallics, is often beneficial due to its high degree of
reversibility and stability.4 Therefore, mixed-valent complexes
bearing two equivalent ferrocenyl substituents in different oxi-
dation states are excellent candidates for studying intramolecular
electron transfer.5
Recently, we reported on the synthesis and characterisation
of a series of ferrocenyl substituted five-membered heterocycles
c
revealing that the electronic properties of the C4E rings (E = S,
O, NR; R = CH3, C6H5) have strong influence on the intervalence
charge transfer in these systems.8–10 It was found that electron
rich pyrroles show much higher intermetallic interactions than,
for example, electron poor thiophenes.8–10 In continuation of this
work, we here report the synthesis and characterisation, as well as
the electro- and spectro-electrochemial behaviour, of substituted
2,5-diferrocenyl-1-phenyl-1H-pyrroles. Modifying the substituent
at the phenyl ring enables variation of the electronic properties
of the pyrrole core without changing the cC4N setup. Bobula and
Kotora have shown the direct relation between the redox properties
of substituted phenylferrocenes and the s Hammett constants
of the substituents.11 We herein show that the influence of such
substituents is not limited to the electrochemical characteristics
but also affects the electron transfer properties.
Comparison of the electrochemical behaviour of diferrocenyl
thiophene, which has been studied by Iyoda in 19976 and the
corresponding benzo[c]thiophenes of Ogawa and Sato,7 showed
Technische Universita¨t Chemnitz, Fakulta¨t fu¨r Naturwissenschaften, Institut
fu¨r Chemie, Lehrstuhl fu¨r Anorganische Chemie, Straße der Nationen 62,
09111, Chemnitz, Germany. E-mail: heinrich.lang@chemie.tu-chemnitz.de;
Fax: (+49)371-531-21219; Tel: (+49)371-531-21210
† Electronic supplementary information (ESI) available: NMR-simulation
and 2D-Spectra of 3e, NIR-Spectra of 3a, 3b, 3e and 3f. See DOI:
10.1039/c1dt10997a
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Results and discussion
Synthesis and Characterisation
1-Phenyl-1H-pyrroles 1a–f were synthesised using Ullmann-
type cross-coupling reactions of the appropriate bromobenzenes
and pyrrole according to procedures known in the literature
(Scheme 1).12 Reacting molecules 1a–f with two equivalents of
N-bromosuccinimide leads to the formation of the 2,5-dibromo-
1-phenyl-1H-pyrroles 2a–f,13 which were used in the next reaction
step without further purification. Treatment of 2a–f with FcZnCl
5
5
(Fc = Fe(h -C5H4)(h -C5H5)) in the presence of catalytic amounts
of [Pd(PPh3)4] gave diferrocenes 3a–f in 60 to 80% yield (Experi-
mental). A threefold excess of ferrocenyl zinc chloride is required
to reach complete conversion of the reactants.
1
Fig. 1 Correlation of the 13C{ H} NMR shifts of the CH carbon atoms
of the pyrrole core and the s Hammett parameter of the substituents at
the phenyl ring in 3a–f.
nature of the substituents R at the phenyl moiety, described by
the s Hammett parameter,15 strongly influence the electronic
properties of the pyrrole system. Similar relations are described
in the literature, showing linear slopes.16
Electrochemistry and Spectro-electrochemistry
The redox characteristics of 3a–f were studied by cyclic voltamme-
try (CV), square-wave voltammetry (SWV) (Fig. 2) and UV/Vis-
NIR spectroscopy (Fig. 4 and Fig. 5). As supporting electrolyte
0.1 mol L-1 dichloromethane solutions of [NnBu4][B(C6F5)4] were
used.17 The cyclovoltammetric studies were carried out at a scan
rate of 100 mV s-1 and are summarized in Table 1. All potentials
were referenced to the FcH/FcH+ redox couple (E0 = 0.0 mV).18
Compounds 3a–f show reversible redox events and the ferro-
cenyl moieties could be oxidised step-wise. Depending on the
compound the 1st oxidation occurs between -305 and -190 mV,
while the 2nd oxidation can be observed in the range of 170
and 230 mV. A special feature of 3a is an additional reversible
redox event at 890 mV (DEp = 75 mV) which is due to the
oxidation of the NMe2 group.19 The potential of the 1st oxidation
(E01) increases when the substituent functionality changes from
electron-donating (4-NMe2: 3a, 4-OMe: 3b, 4-Me: 3c) to electron-
withdrawing (3-F: 3e, 4-COOEt: 3f). Therefore, we can conclude
that the nature of the substituent strongly affects the electron
density at the iron centers. Plotting the E01 values versus the
Hammett parameter of the substituents reveals a linear slope
Scheme 1 Syntheses of 3a–f by consecutive Ullmann-type coupling,
dibromination and Negishi-ferrocenylation reactions. (i): In dimethyl-
sulfoxide, 18 h, 110 ◦C; (ii) in tetrahydrofuran, 2 h, 0 ◦C, NBS
(= N-bromo-succinimide); (iii) in tetrahydrofuran, [Pd] = [Pd(PPh3)4], 48
h, 60 ◦C.
Diferrocenyl pyrroles 3a–f are stable to air and moisture in
the solid state as well as in solution. They have been identified
1
by elemental analysis, IR, UV/Vis and NMR (1H, 13C{ H})
spectroscopy as well as high resolution ESI TOF mass spec-
trometry (Experimental). The electrochemical behaviour of all
compounds was determined by cyclic voltammetry (CV), square-
wave voltammetry (SWV) and in situ NIR experiments.
In 1H NMR spectroscopy the expected signals for all com-
pounds were observed. However, 3e shows a more complex signal
pattern, as the protons of the phenyl ring are coupling to each other
and to the fluoro atom resulting in four doublets-of-doublets-of-
doublets-of-doublets (dddd). The protons for 3e were assigned by
simulation of the spectrum and 2D experiments (Experimental,
Supporting Information†). For the cyclopentadienyl groups two
pseudo-triplets with JHH = 1.9 Hz (C5H4) centred at 3.90 and 4.02
ppm are found as expected for AA¢XX¢ spin systems.14 In most
cases the pseudo-triplet at 4.02 ppm is overlapping with the singlet
for the C5H5 protons. Within this series of compounds the pyrrole
protons are observed at increasing chemical shifts: 6.34 (3a), 6.36
(3b), 6.36 (3c), 6.39 (3d), 6.40 (3e), 6.43 (3f). More accurately, this
Table 1 Cyclic voltammetry data (potentials vs. FcH/FcH+), scan rate
100 mV s-1 at a glassy-carbon electrode of 0.5 mmol L-1 solutions of 3a–f
in dry dichloromethane containing 0.1 mol L-1 of [N(nBu)4][B(C6F5)4] as
supporting electrolyte at 25 ◦
C
E01 in mV
E02 in mV
DE1/2
in mV
b
Compd.
(DEp in mV)a
(DEp in mV)a
s
3a
3b
3c
3d
3e
3f
-305 (61)
-255 (60)
-250 (71)
-240 (68)c
-210 (61)
-190 (62)
175 (68)
205 (69)
205 (75)
210 (75)c
215 (65)
230 (69)
480
460
455
450c
425
420
-0.83
-0.27
-0.17
0
0.34
0.45
1
trend could be shown by the 13C{ H} NMR shifts of the respective
a E01 = Potential of the 1st oxidation; E02 = Potential of the 2nd oxidation.
C–H carbon atoms at the pyrrole ring. As it can be seen from Fig.
1, the chemical shifts of this atoms are increasing from 3a to 3f
indicating that the electron-withdrawing and electron-donating
s = Hammett parameter of the substituent on the phenyl ring.15 c Values
b
have previously been published, see ref. 11
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Fig. 4 NIR spectra of 3c at increasing potentials (bottom: -200 to
300 mV; top: 250 to 900 mV vs. Ag/AgCl) at 25 ◦C, in dichloromethane,
supporting electrolyte 0.1 mol L-1 [NnBu4][B(C6F5)4]. Arrows indicate
increasing or decreasing of the absorption.
Fig.
2
Left: Cyclic voltammograms of 3a–f; scan rate: 100 mV.
Right: Square-wave voltammograms of 3a–f. Measurement conditions:
dichloromethane, analyte concentration 1.0 mmol L-1, 25 ◦C, supporting
electrolyte [NnBu4][B(C6F5)4] (0.1 mol L-1). For DE1/2 values see Table 1.
Fig. 5 Deconvolution of the NIR absorption of 3c+ using three Gaus-
sian-shaped bands determined by spectro-electrochemistry in an Optically
Transparent Thin-Layer Electrode (OTTLE) cell.
The spectro-electrochemical studies were conducted by stepwise
increase of the potential from -500 to 1500 mV vs. Ag/AgCl
in an Optically Transparent Thin-Layer Electrode (OTTLE) cell
containing dichloromethane solutions of 3a–f (1.0 mmol L-1) and
[NnBu4][B(C6F5)4] (0.1 mol L-1) as supporting electrolyte. This
procedure allows in situ generation of 3a–f+ and 3a–f2+, respec-
tively, and in the case of diferrocenyl-(4-dimethyl-aminophenyl)-
pyrrole even the generation of the three-fold charged cation 3a3+.
While neutral 3a–f do not exhibit any absorption in the range
between 800 and 3000 nm (NIR region), upon stepwise increase of
the potential broad bands are observed indicating the formation
of mixed-valent 3a–f+ species. Further increase of the potential
(above 500 mV vs. Ag/AgCl) leads to the disappearance of these
absorptions and new LMCT bands (LMCT = Ligand-to-Metal
Charge Transfer) at ca. 1000 and 1150 nm can be observed. The
absorptions between 1500 and 2500 nm are characteristic for
intervalence transfer exitations.20 Determination of the physical
parameters (wavenumber (nmax), extinction (emax), Full-Width-at-
Half-Maximum (FWHM) (Dn1/2)) was achieved by deconvolution
of the NIR data using three Gaussian-shaped overlapping absorp-
tions (Fig. 5, Supporting Information†). The sum of these spectral
components closely matches with the measured spectra. In all
compounds two transitions were observed contributing to the NIR
absorption. Between 4000 and 4150 cm-1 a sharp LMCT band was
detected. Furthermore, in the range of 4500 to 5100 cm-1 a strong
Fig. 3 Left ordinate (black): correlation of the DE1/2 values and the s
Hammett constant of 3a–f, linear fit (R2 = 0.978, dashed line). Right
ordinate (grey): correlation of E01 and the s Hammett constant of 3a–f,
linear fit (R2 = 0.995, dotted line).
(Fig. 3). This corresponds well with the substituents having
a similar influence on the ferrocenyl oxidations in modified
ferrocenyl benzenes.11
Besides having an influence on the ferrocenyl redox potentials
the separation of the redox events (DE1/2) also varies depending
on the substituent effects. The DE1/2 values increase, compared
to non-substituted 3d, when an electron-donating functionality is
introduced at the phenyl moiety. Hence, 3a (480 mV), 3b (460 mV)
and 3c (455 mV) show a higher separation of the ferrocenyl-based
redox processes, while the introduction of electron-withdrawing
groups in 3e (425 mV) and 3f (420 mV), respectively, leads
to a decrease in DE1/2. The influence of the substituents can
nicely be demonstrated by plotting DE1/2 vs. s, revealing a linear
relationship (R2 = 0.978). These results are in good agreement with
the observation that electron-rich diferrocenyl heterocycles show
a higher degree of intermetallic electron transfer.10
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Table 2 NIR data of 3a–f+ in dry dichloromethane containing 0.1 mol L-1 of [N(nBu)4][B(C6F5)4] as supporting electrolyte at 25 ◦C
Compd.
3a+
Transition
nmax (cm-1) (emax (L mol-1 cm-1))
Dn1/2 (cm-1)
(Dn1/2
3417
3354
3339
3340
3254
3243
)
theo.(cm-1)
C
f ¥103 (cm-1)
50.72
IVCT
LMCT
IVCT
LMCT
IVCT
LMCT
IVCT
LMCT
IVCT
LMCT
IVCT
5055 (4678)
4134 (2789)
4870 (4296)
4050 (2205)
4825 (4279)
4130 (2112)
4820 (4200)a
4256 (1805)b
4586 (3584)
4056 (1527)
4554 (3445)
4027 (1023)
2358
1099
2360
1100
2337
1037
2369a
690b
2413
1045
2438
962
0.31
0.30
0.30
0.30
0.26
0.25
3b+
46.64
3c+
46.00
3d+
45.76
3e+
39.76
3f+
38.63
LMCT
a Values have previously been published, see ref. 9. b See ref. 10.
absorption with Dn1/2 values typical for IVCT absorption was
found (Table 2). For all compounds 3a–f the FWHM is in the same
range (2337–2438 cm-1), the extinction increases with the electron
donating capabilities of the phenyl substituent. This tendency
supports the results which were observed in the voltammetric
studies (vide supra).
coupling increases with the electron-donating capability of the
substituents.
1/2
(Dn1/2
)
= (2310nmax
)
(2)
(3)
theo.
C = 1 - ((Dn1/2)/(Dn1/2
)
)
theo.
The strength of the intervalence charge transfer (IVCT) ab-
sorption is expressed by the oscillator strength f , which can be
calculated from emax and Dn1/2 (FWHM) assuming Gaussian-
shaped transitions (eqn (1)).21
Conclusions
A series of phenyl-substituted 2,5-diferrocenyl-1H-pyrroles was
synthesised using palladium-catalysed Negishi C,C cross-coupling
reactions. Within this series a systematic change from electron-
donating (R = NMe2, OMe, Me) to electron neutral (R = H) to
electron-withdrawing (R = F, CO2Et) functionalities was realized.
NMR studies confirmed that the substituents R have a significant
effect on the electron density in the pyrrole core system which could
f = 4.6 ¥ 10-9 ¥ emax ¥ Dn1/2
(1)
Due to their similar structures, the electrostatic interactions
of the ferrocenyls in this series of molecules are very similar.
We previously have shown that these conditions enable a direct
correlation of the electrochemical and the spectro-electrochemical
results.10 A plot of f vs. the DE1/2 values shows a linear relation
(R2 = 0.9899, Fig. 6) proving that 3a–f+ can be classified as class
II mixed-valent species. This relationship confirms similar obser-
vations in a series of diferrocenyl heterocycles.10 Furthermore, the
classification criterion of Brunschwig, Creutz and Sutin22 derived
from the theoretical and the experimental FWHM (eqn (2) and
eqn 3) confirm that these are moderately to strongly coupled
class II systems. It is noteworthy that C also increases when
the s Hammett parameter decreases, showing that the electronic
1
be monitored nicely by the 13C{ H} shift of the pyrrolic carbon
atoms. Electrochemical studies (cyclic voltammetry and square-
wave voltammetry) were performed to investigate the redox be-
haviour of the ferrocenyl-substituted heterocyclic compounds. The
electron-donating or electron-withdrawing character of the sub-
stituents affects the redox potentials of the ferrocenyl oxidations,
as a linear relationship between the s Hammett constants and the
appropriate E01 values was observed. In addition, the electronic
influence is not limited to the redox potentials but the separation
of the redox events DE1/2 also correlates linearly with the Hammett
constants. In situ UV/Vis-NIR measurements revealed significant
electron transfer in the mixed valence oxidation state for all
compounds in this series, which allows their classification as
class II systems according to Robin and Day.23 Furthermore, the
electron transfer properties are tuneable as the oscillator strength
f of the IVCT absorption varies with the substituent on the phenyl
group. It is obvious that the pyrrole core has to be electron-
rich to achieve a high degree of intermetallic communication as
the highest f values were observed in compounds with electron-
donating functionality (3a–c). Due to the possibility to adjust the
electron transfer, and hence the “conductivity” of the heterocyclic
core, this series of compounds can be regarded as a model system
for single molecule transistors. For substituted diferrocenyl-1-
phenyl-1H-pyrroles a linear relationship between DE1/2 and f
(IVCT) was found which is in agreement with the theoretical
Fig. 6 Correlation of the oscillator strength f of the IVCT absorption
and the DE1/2 values of 3a–f+ determined by NIR and cyclic voltammetry,
respectively.
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hypothesis for a series of molecules with similar geometries and
hence, similar electrostatic properties.
then manipulated in a Microsoft Excel worksheet to set the formal
reduction potentials of the FcH/FcH+ couple to 0.0 V. Under our
conditions the FcH/FcH+ couple was at 220 mV vs. Ag/Ag+,
DEp = 61 mV.25
Experimental
General conditions
Reagents
1a–f were prepared according to published procedures.12 All other
chemicals were purchased from commercial suppliers and were
used as received.
All reactions were carried out under an atmosphere of nitrogen
using standard Schlenk techniques. Tetrahydrofuran, toluene,
n-hexane, and n-pentane were purified by distillation from
sodium/benzophenone ketyl; dichloromethane was purified by
distillation from calcium hydride.
Synthesis
Gerneral Procedure – Synthesis of 2,5-diferrocenyl-1-phenyl-1H-
pyrroles (3a–f). To 1.5 mmol of the substituted phenylpyrrole 1a–
f dissolved in 50 mL of tetrahydrofuran, and 534 mg (3.0 mmol)
of N-bromosuccinimide were added in a single portion at -20 ◦C.
After 1 h of stirring at this temperature the solvent was evaporated
and the resulting solid was suspended in tetrachloromethane.
Filtration through a pad of Celite and removal of all volatiles in
vacuum gave 2a–f as brown-yellow oils. Without further purifica-
tion the appropriate oil was dissolved in 30 mL of tetrahydofuran
and added in a single portion to a solution of 4.5 mmol ferrocenyl
zinc chloride prepared from monolithiated ferrocene and [ZnCl2·2
thf] in tetrahydrofuran.13 To this mixture 17 mg (0.015 mmol)
of [Pd(PPh3)4] was added in a single portion. After stirring at
60 ◦C for 48 h all volatiles were evaporated and the precipitate was
dissolved in 100 mL of toluene and washed three times with 50 mL
portions of water. The organic phase was dried over MgSO4 and
the solvent was evaporated to dryness in oil-pump vacuum. The
remaining solid was purified by column chromatography (column
size 20 cm ¥ 5 cm) on alumina using a n-hexane-toluene mixture of
ratio 1 : 1 (v/v) as eluent. All volatiles were removed under reduced
pressure. The title compounds were obtained as orange solids.
Instrumentation
Infrared spectra were recorded using FT-Nicolet IR 200 equip-
ment. The 1H NMR spectra were recorded with a Bruker Avance
III 500 spectrometer operating at 500.303 MHz in the Fourier
transform mode; the 13C{ H} NMR spectra were recorded at
1
125.800 MHz. Chemical shifts are reported in d (parts per
million) downfield from tetramethylsilane with the solvent as
reference signal (1H NMR: CHCl3, d 7.26; 13C{ H} NMR:
1
CDCl3, d 77.00). The melting points of analytical purity samples
(sealed off in nitrogen-purged capillaries) were determined using
Gallenkamp MFB 595 010 M melting point apparatus. Micro-
analyses were performed using a Thermo FLASHEA 1112 Series
instrument. Spectro-electrochemical measurements were carried
out in an OTTLE cell similar to that described previously,24
from dichloromethane solutions containing 0.1 mol L-1 of
[NnBu4][B(C6F5)4] as supporting electrolyte using a Varian Cary
5000 spectrometer. High resolution mass spectra were recorded
using a micrOTOF QII Bruker Daltonite workstation.
Electrochemistry
Measurements on 1.0 mmol L-1 solutions of 3a–f in dry, air-
free dichloromethane containing 0.1 mol L-1 of [NnBu4][B(C6F5)4]
as supporting electrolyte were conducted under a blanket of
purified argon at 25 ◦C utilizing a Radiometer Voltalap PGZ 100
electrochemical workstation interfaced with a personal computer.
A three electrode cell, which utilized a Pt auxiliary electrode, a
glassy carbon working electrode (surface area 0.031 cm2) and
an Ag/Ag+ (0.01 mol L-1 [AgNO3]) reference electrode mounted
on a Luggin capillary was used. The working electrode was pre-
treated by polishing using a Buehler microcloth first with 1 micron
Data
for
2,5-diferrocenyl-1-(4-dimethylaminophenyl)-1H-
pyrrole 3a. Yield: 582 mg (1.05 mmol, 70% based on 1a) Anal.
Calc. for C32H30Fe2N2 (554.28): C, 69.34; H, 5.46; N, 5.05 Found:
C, 69.33; H, 5.56.; N, 4.96 Mp.: 224 ◦C (decomposition). IR
data (KBr): 3084 m, 2922 s, 2847 m, 2810 w, 1614 w, 1524 s,
1358 m, 1328 w, 1101 m, 1022 w, 999 m, 815 s, 805 s, 755 m.
1H NMR (CDCl3, d); 7.18 (m, 2H, C6H4), 6.80 (m, 2H, C6H4),
6.34 (s, 2H, H2C4N), 4.02 (m, 14 H, C5H5/C5H4), 3.93 (pt, JHH
=
1.9 Hz, 4H, C5H4), 3.09 (s, 6H, CH3); 13C { H} NMR (CDCl3, d):
150.4 ((CH3)2N-iC-C6H4), 132.4 (iC-C4H2N), 130.3 (C6H4), 128.7
(N-iC-C6H4), 111.8 (C6H4), 107.3 (C4H2N), 79.4 (iC-C5H4) 69.3
(C5H5), 67.4 (C5H4), 66.2 (C5H4), 40.6 (CH3). HR-ESI-MS [m/z]:
calc’d for C32H30Fe2N2: 554.1103, found: 554.1082 [M]+.
1
1
and then with micron diamond paste. The reference electrode
4
was constructed from a silver wire inserted into a solution of
0.01 mol L-1 [AgNO3] and 0.1 mol L-1 [N(nBu)4][B(C6F5)4] in ace-
tonitrile, in a luggin capillary with a vycor tip. This luggin capillary
was inserted into a second luggin capillary with vycor tip filled with
a 0.1 mol L-1 [N(nBu)4][B(C6F5)4] solution in dichloromethane.
Successive experiments under the same experimental conditions
showed that all formal reduction and oxidation potentials were
reproducible within 5 mV. Experimental potentials were referenced
against an Ag/Ag+ reference electrode but results are presented
referenced against ferrocene as an internal standard as required by
IUPAC.18 To achieve this, since the ferrocene couple FcH/FcH+
interferes with the ferrocenyl signals of 3a–f, each experiment was
first performed in the absence of any internal standard, and then
repeated in the presence of 1 mmol L-1 of ferrocene. Data were
Data for 2,5-diferrocenyl-1-(4-methoxyphenyl)-1H-pyrrole 3b.
Yield: 540 mg (1.00 mmol, 66% based on 1b) Anal. Calc. for
C31H27Fe2NO (541.24): C, 68.79; H, 5.03; N, 2.59 Found: C, 68.66;
H, 5.10.; N, 2.54 Mp.: 230 ◦C. IR data (KBr): 3089 m, 3006 w, 2953
m, 2924 w, 2901 w, 2892 w, 2828 m, 1608 m, 1594 s, 1439 m, 1419
m, 1293 m, 1247 s, 1104 s, 1040 s, 1025 s, 1001 s, 836 m, 819 s, 758,
m. 1H NMR (CDCl3, d); 7.25 (m, 2H, C6H4), 7.02 (m, 2H, C6H4),
6.36 (s, 2H, H2C4N), 4.02 (m, 14 H, C5H5/C5H4), 3.93 (s, 3H,
CH3), 3.90 (pt, JHH = 1.9 Hz, 4H, C5H4); 13C { H} NMR (CDCl3,
1
d): 159.6 (CH3O-iC-C6H4), 132.8 (N-iC-C6H4), 132.4 (iC-C4H2N),
130.8 (C6H4), 113.8 (C6H4), 107.7 (C4H2N), 79.2 (iC-C5H4) 69.4
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(C5H5), 67.5 (C5H4), 66.5 (C5H4), 55.5 (CH3). HR-ESI-MS [m/z]:
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calcd for C31H27Fe2NO: 541.0787, found: 541.0773 [M]+.
Data for 2,5-diferrocenyl-1-(4-tolyl)-1H-pyrrole 3c. Yield:
593 mg (1.13 mmol, 75% based on 1c) Anal. Calc. for C31H27Fe2N
(525.24): C, 70.89; H, 5.18; N, 2.67 Found: C, 70.67; H, 4.89;
N, 2.59 Mp.: 235 ◦C. IR data (KBr): 3098 m, 3090 m, 3030 w,
2915 w, 1643 w, 1515 s, 1420 s, 1329 m, 1106 s, 1037 w, 1023
1
m, 1001 s, 821 s, 806 m, 764 s. H NMR (CDCl3, d); 7.29 (m,
2H, C6H4), 7.21 (m, 2H, C6H4), 6.36 (s, 2H, H2C4N), 4.01 (m,
14 H, C5H5/C5H4), 3.88 (pt, JHH = 1.9 Hz, 4H, C5H4), 2.5 (s, 3H,
1
CH3); 13C { H} NMR (CDCl3, d): 138.6 (N-iC-C6H4), 137.9 (CH3-
iC-C6H4), 137.4 (iC-C4H2N), 129.6 (C6H4), 129.4 (C6H4), 107.8
(C4H2N), 79.2 (iC-C5H4) 69.4 (C5H5), 67.4 (C5H4), 66.5 (C5H4),
21.4 (CH3). HR-ESI-MS [m/z]: calc’d for C31H27Fe2N: 525.0838,
found: 525.0821 [M]+.
Data for 2,5-diferrocenyl-1-(3-fluorophenyl)-1H-pyrrole 3e.
Yield: 572 mg (1.08 mmol, 72% based on 1e) Anal. Calc. for
C30H24FFe2N (529.21): C, 68.09; H, 4.57; N, 2.65 Found: C, 68.17;
◦
H, 4.56.; N, 2.60 Mp.: 212 C. IR data (KBr): 3091 m, 2923 m,
1596 m, 1489 s, 1450 m, 1412 s, 1264 m, 1189 m, 1104 m, 879 m,
1
3
845 m, 815 s, 773 m. H NMR (CDCl3, d); 7.41 (dddd, JHH
=
4 P. Stepnicka, Ferrocenes: Ligands, Materials and Biomolecules; John
Wiley & Sons Ltd: Chichester, 2008.
3
4
5
8.1 Hz, JHH = 8.1 Hz, JFH = 6.3 Hz, JHH = 0.5 Hz, 1H, 5-H-
3
3
4
5 for example: (a) R. G. Hadt and V. N. Nemykin, Inorg. Chem., 2009, 48,
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C6H4), 7.19 (dddd, JHH = 8.1 Hz, JFH = 8.4 Hz, JHH = 2.5 Hz,
4JHH = 0.9 Hz, 1H, 4-H-C6H4), 7.06 (ddd, JHH = 8.1 Hz, JHH
=
3
4
0.9 Hz, 4JHH = 2.1 Hz, 1H, 6-H-C6H4), 7.03 (dddd, 3JFH = 9.2 Hz,
4JHH = 2.5 Hz, JHH = 2.1 Hz, JHH = 0.5 Hz, 1H, 2-H-C6H4),
4
5
6.40 (s, 2H, H2C4N), 4.05 (m, 14 H, C5H5/C5H4), 3.90 (pt, JHH
=
=
1
1
1.9 Hz, 4H, C5H4); 13C { H} NMR (CDCl3, d): 162.4 (d, JCF
249 Hz, 3-C-C6H4F), 141.4 (d, 3JCF = 9.5 Hz, 1-C-C6H4F), 132.4
(iC-C4H2N), 129.7 (d, 3JCF = 8.9 Hz, 5-C-C6H4F), 125.8 (d, 4JCF
=
3.0 Hz, 6-C-C6H4F), 117.4 (d, 2JCF = 21.9 Hz, 2-C-C6H4F), 115.6
2
(d, JCF = 20.8 Hz, 4-C-C6H4F), 108.5 (C4H2N), 78.8 (iC-C5H4)
69.5 (C5H5), 67.5 (C5H4), 67.0 (C5H4). HR-ESI-MS [m/z]: calcd
for C30H24FFe2N: 529.0587, found: 529.0560 [M]+.
Data for ethyl 4-(2,5-diferrocenyl-1H-pyrrol-1-yl)benzoate 3f.
Yield: 700 mg (1.20 mmol, 80% based on 1f) Anal. Calc. for
C33H29Fe2NO2 (583.28): C, 67.95; H, 5.01; N, 2.40 Found: C, 67.53;
H, 5.06; N, 2.37. Mp.: 267 ◦C. IR data (KBr): 3093 m, 2923 w,
1718 s, 1653 m, 1606 m, 1419 m, 1411 m, 1272 s, 1114 m, 1105 m,
1097 m, 819 m, 766 m. 1H NMR (CDCl3, d); 8.11 (m, 2H, C6H4),
7.30 (m, 2H, C6H4), 6.43 (s, 2H, H2C4N), 4.43 (q, 3JHH = 7.1 Hz,
2H, CH2CH3), 4.04 (s, 10 H, C5H5), 4.02 (pt, JHH = 1.9 Hz, 4H,
10 A. Hildebrandt, D. Schaarschmidt, R. Claus and H. Lang, Inorg.
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ˇ
11 T. Bobula, J. Hudlicky´, P. Nova´k, R. Gyepes, I. C´ısarova´, P. Steˇpnicˇka
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3
C5H4), 3.88 (pt, JHH = 1.9 Hz, 4H, C5H4), 1.44 (t, JHH = 7.1 Hz,
1
3H, CH2CH3); 13C { H} NMR (CDCl3, d): 166.0 (CO), 144.0 (N-
iC-C6H4), 132.0 (iC-C4H2N), 130.3 (CO-iC-C6H4), 129.9 (C6H4),
129.7 (C6H4), 108.8 (C4H2N), 78.9 (iC-C5H4) 69.5 (C5H5), 67.6
(C5H4), 67.3 (C5H4), 61.3 (CH2CH3), 14.3 (CH2CH3). HR-ESI-
MS [m/z]: calc’d for C33H29Fe2NO2: 583.0893, found: 583.0846
[M]+.
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Acknowledgements
We are grateful to the Deutsche Forschungsgemeinschaft and the
Fonds der Chemischen Industrie for generous financial support.
17 For information concerning the use of [NnBu4][B(C6F5)4] as supporting
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