Synthesis of 2,5-diferrocenyl five-membered heterocyclic compounds and their electrochemistry

Article abstract of DOI:10.1080/00958972.2013.841902

A series of 2,5-diferrocenyl substituted five-membered heterocyclic compounds, 2,5-diferrocenyl-1-phenylpyrrole (1), 2,5-diferrocenyl-1-(4- fluorophenyl)-pyrrole (2), 2,5-diferrocenyl-1-(4-ethoxyphenyl)-pyrrole (3), 2,5-diferrocenyl-1-(4-ethylphenyl)-pyrrole (4), 2,5-diferrocenylthiophene (5), and 2,5-diferrocenylfuran (6), were synthesized using one-pot cycloaddition of ferrocenyl alkyne and characterized by elemental analysis, FT-IR, MS, and NMR. The molecular structures of 1, 2, 5, and 6 were determined using single-crystal X-ray diffraction. Electronic communication between two ferrocenyl units of 1-6 was investigated using cyclic voltammetry. These compounds have two well-resolved electrochemically reversible one-electron-transfer processes using [NBu4][PF6] as the supporting electrolyte. The electrochemical studies reveal that electronic communication between two ferrocenyl units depend on the heteroatoms.

Full text of DOI:10.1080/00958972.2013.841902

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Synthesis of 2,5-diferrocenyl five-
membered heterocyclic compounds
and their electrochemistry
Yu-Qiang Hua, Li-Min Hana, Ning Zhua, Hai-Long Honga & Rui-Jun
Xiea
a Chemical Engineering College, Inner Mongolia University of
Technology, Hohhot, P.R. China
Accepted author version posted online: 09 Sep 2013.Published
online: 18 Oct 2013.
To cite this article: Yu-Qiang Hu, Li-Min Han, Ning Zhu, Hai-Long Hong & Rui-Jun Xie (2013)
Synthesis of 2,5-diferrocenyl five-membered heterocyclic compounds and their electrochemistry,
Journal of Coordination Chemistry, 66:19, 3481-3497, DOI: 10.1080/00958972.2013.841902
To link to this article: http://dx.doi.org/10.1080/00958972.2013.841902
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Journal of Coordination Chemistry, 2013
Vol. 66, No. 19, 3481–3497, http://dx.doi.org/10.1080/00958972.2013.841902
Synthesis of 2,5-diferrocenyl five-membered heterocyclic
compounds and their electrochemistry
YU-QIANG HU, LI-MIN HAN*, NING ZHU, HAI-LONG HONG and RUI-JUN XIE
Chemical Engineering College, Inner Mongolia University of Technology, Hohhot, P.R. China
(Received 30 March 2013; accepted 21 August 2013)
A series of 2,5-diferrocenyl substituted five-membered heterocyclic compounds, 2,5-diferrocenyl-1-
phenylpyrrole (1), 2,5-diferrocenyl-1-(4-fluorophenyl)-pyrrole (2), 2,5-diferrocenyl-1-(4-ethoxy-
phenyl)-pyrrole (3), 2,5-diferrocenyl-1-(4-ethylphenyl)-pyrrole (4), 2,5-diferrocenylthiophene (5),
and 2,5-diferrocenylfuran (6), were synthesized using one-pot cycloaddition of ferrocenyl
alkyne and characterized by elemental analysis, FT-IR, MS, and NMR. The molecular structures of
1, 2, 5, and 6 were determined using single-crystal X-ray diffraction. Electronic communication
between two ferrocenyl units of 1–6 was investigated using cyclic voltammetry. These compounds
have two well-resolved electrochemically reversible one-electron-transfer processes using [NBu4]
[PF6] as the supporting electrolyte. The electrochemical studies reveal that electronic communication
between two ferrocenyl units depend on the heteroatoms.
Keywords: Diferrocenyl; Five-membered heterocyclic compounds; Electronic communication
1. Introduction
Multi-ferrocenyl (Fc) compounds have received attention because of their synthetic
accessibility and redox properties [1, 2]. In particular, π-covalently linked multi-ferrocenyl
derivatives have been extensively regarded as model molecules for studying intramolecular
electronic communication, because they have very good stability in the neutral state as well
as oxidized state during one-electron-transfer processes [3–16]. We have examined the
factors that affect electron transfer processes of aliphatic carbon-bridged diferrocenyl
derivatives and found that the charge density of the carbon bridge was the key factor for
intramolecular electron transfer [17, 18]. We also synthesized diferrocenylbenzenes and
studied the electronic communication of two ferrocenyl units bridged by phenyl, which
exhibited the strength of the charge transfer which depends on the electron-donating or
electron-withdrawing character of the substituents on phenyl and the planar character of the
central phenyl core [19]. Herein, we vary the heteroatom of the five-membered cyclic
core to modify the electronic properties without changing the carbon bridge setup and
investigate the electronic communication between the two ferrocenyls in heterocyclic
*Corresponding author. Email: hanlimin@imut.edu.cn
© 2013 Taylor & Francis

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Y.-Q. Hu et al.
Figure 1. The molecular structure of 1. Hydrogens have been omitted for clarity.
ring systematically. Hence, the electrochemical model molecules, diferrocenyl pyrrole,
thiophene, and furan were synthesized.
2,5-Diferrocenyl five-membered heterocyclic compounds, such as 2,5-diferrocenyl
pyrrole, thiophene or furan, have been prepared by Negishi cross-coupling reaction of
2,5-dibromo-substituted pyrrole, thiophene, or furan derivatives with FcZnCl in the
presence of Pd as a catalyst [20]. To avoid low-temperature and multi-step procedures
which are indispensable parts of the cross-coupling Negishi reaction, we explored a new
one-pot synthetic method to prepare the 2,5-diferrocenyl pyrrole, thiophene, and furan
derivatives. The electrochemical behaviors of all compounds were determined by cyclic
voltammetry and the electronic communication between two ferrocenyl units bridged by
pyrrole, thiophene, and furan cores are discussed.
2. Experimental
2.1. General procedures
All operations were carried out under an atmosphere of purified argon. All solvents were
dried and distilled according to standard procedures. The reactions were monitored by
thin-layer chromatography (TLC). 1,4-Diferrocenyl-1,3-diyne was prepared according to
literature methods [21]. Aniline, 4-ethylaniline, 4-ethoxyaniline, and 4-fluoroaniline were
commercially available. Infrared spectra were obtained with a Thermo Nicolet FT-IR
spectrometer using KBr pellets for sample preparation. Elemental analyzes were carried out
1
with an Elementar var III-type analyzer. H- and 13C-NMR spectra were recorded on a
Bruker Avance III 500 FT-MHz apparatus in deuterated CDCl3 solution. Mass spectra were
obtained on a Shimadzu LCMS-2020 mass spectrometer. Crystal (1) was determined on a
Bruker SMART APEX CCD diffractometer with graphite-monochromated Cu-K
(λ = 1.54184 Å) radiation, and other crystals (2, 5, and 6) were determined with graphite-
monochromated Mo-Kα (λ = 0.71073 Å) radiation. Data were collected using the φ and ω
scan techniques. The structures were solved using direct methods and expanded using
Fourier techniques. Absorption correction based on SADABS was applied. Structure
solution and refinement were performed using the SHELXSL 97 software.

2,5-diferrocenyl heterocyclic compounds
3483
Figure 2. The molecular structure of 2. Hydrogens have been omitted for clarity.
Cyclic voltammetry was performed on a platinum disk electrode in a dichloromethane
solution of 1–6 with tetra-n-butylammonium hexafluorophosphate [NBu4][PF6] (0.1 M)
as the supporting electrolyte, at a scan rate of 100 mV s−1. The reference electrode was an
Ag/Ag+ electrode and the auxiliary electrode was a coiled platinum wire. Oxygen was
purged from the one-compartment cell before each electrochemical run.
2.2. Synthesis of 1
1,4-Diferrocenyl-1,3-butadiyne (210.2 mg, 0.5 mM), aniline (2.5 mL, 27.4 mM), and CuCl
(2.4 mg, 0.024 mM) were added to a sealed tube under argon and stirred for 24 h at
100 °C. The reaction solution was cooled to room temperature, the solvent removed in
vacuo, and the residue was subjected to chromatographic separation on a silica gel column
(2.0 × 15 cm) using a mixture of dichloromethane/petroleum ether (1/4, v/v) to elute the
product at room temperature. The first orange band was 1,4-diferrocenyl-1,3-butadiyne. The
second yellow band was 1 (165 mg). The single crystal of 1 was obtained by recrystallizing
from hexane/dichloromethane (4/1, v/v) at room temperature. Yield: 64.7%, m.p.
192–195 °C. Anal. Calcd for C30H25NFe2: C, 70.48; H, 4.93; N, 2.74. Found: C, 70.49; H,
5.44; N, 2.66%. IR (KBr disk): 3089 cm−1 [Cp, νC–H]; 3050 cm−1 [Ph, νC–H]; 1602 cm−1
[Cp, νC=C]; 1497 cm−1 [C4H2N, νC=C]; 1419 cm−1 [Ph, νC=C]; 1104, 1000 cm−1 [Cp, δC–H];
1
820 cm−1 [Cp, γC–H]; 769, 695 cm−1 [Ph, γC–H]. H-NMR (CDCl3, δ): 7.31–7.50 (m, 5H,
C6H5), 6.38 (s, 2H, C4H2N), 3.84–4.01 (m, 18H, Cp). 13C-NMR (CDCl3, δ): 140.03,
129.91, 128.74, 128.64 (C6H5), 107.96 (C4H2N), 79.08 (Ci–C4H2N), 77.26, 77.01, 76.75,
69.39, 67.43, 66.64 (Cp). MS (ESI, relative abundance): 511.3 (M+, 100%).
2.3. Synthesis of 2
1,4-Diferrocenyl-1,3-butadiyne (210.9 mg, 0.5 mM), 4-fluoroaniline (2.5 mL, 26.1 mM),
and CuCl (2.4 mg, 0.024 mM) were added to a sealed tube under argon and stirred for 24 h

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Y.-Q. Hu et al.
Figure 3. The molecular structure of 5. Hydrogens have been omitted for clarity.
at 100 °C. The reaction solution was cooled to room temperature, the solvent removed in
vacuo, and the residue was subjected to chromatographic separation on a silica gel column
(2.0 × 15 cm) using a mixture of dichloromethane/petroleum ether (1/4, v/v) to elute the
product at room temperature. The first orange band was 1,4-diferrocenyl-1,3-butadiyne. The
second yellow band was 2 (180 mg). The single crystal of 2 was obtained by recrystallizing
from hexane/dichloromethane (4/1, v/v) at room temperature. Yield: 68.0%, m.p.
240–242 °C. Anal. Calcd for C30H24FNFe2: C, 68.09; H, 4.57; N, 2.65. Found: C, 67.78;
H, 4.94; N, 2.53%. IR (KBr disk): 3093 cm−1 [Cp, νC–H]; 1602 cm−1 [Cp, νC=C];
1513 cm−1 [C4H2N, νC=C]; 1415 cm−1 [Ph, νC=C]; 1104, 1003 cm−1 [Cp, δC–H]; 835 cm−1
1
[Cp, γC–H]; 816 cm−1 [Ph, γC–H]. H-NMR (CDCl3, δ): 7.14–7.24 (m, 4H, C6H4), 6.39 (s,
2H, C4H2N), 3.87–4.03 (m, 18H, Cp). 13C-NMR (CDCl3, δ): 163.35, 161.37, 135.97,
132.26, 131.37 (C6H4), 108.21 (C4H2N), 78.91 (Ci–C4H2N), 77.27, 77.01, 76.76, 70.26,
70.14, 70.11, 69.85, 69.72, 69.42, 67.55, 66.91 (Cp). MS (ESI, relative abundance): 529.1
(M+, 100%).
2.4. Synthesis of 3
1,4-Diferrocenyl-1,3-butadiyne (210.5 mg, 0.5 mM), 4-ethoxyaniline (2.5 mL, 19.4 mM),
and CuCl (2.4 mg, 0.024 mM) were added to a sealed tube under argon and stirred for 24 h
at 100 °C. The reaction solution was cooled to room temperature, the solvent removed in
vacuo, and the residue was subjected to chromatographic separation on a silica gel column
(2.0 × 15 cm) using a mixture of dichloromethane/petroleum ether (1/4, v/v) to elute the
product at room temperature. The first orange band was 1,4-diferrocenyl-1,3-butadiyne. The
second yellow band was 3 (181 mg). Yield: 56.1%, m.p. 217–219 °C. Anal. Calcd for
C32H29ONFe2: C, 69.22; H, 5.26; N, 2.52. Found: C, 67.78; H, 4.94; N, 2.53%. IR (KBr
disk): 3085 cm−1 [Cp, νC–H]; 2984, 2918 cm−1 [OC2H5, νC–H]; 1610 cm−1 [Cp, νC=C];
1509 cm−1 [C4H2N, νC=C]; 1104, 1049 cm−1 [Cp, δC–H]; 812 cm−1 [Cp, γC–H]; 754 cm−1
1
[Ph, γC–H]. H-NMR (CDCl3, δ): 6.99–7.25 (m, 4H, C6H4), 6.34 (s, 2H, C4H2N), 3.89–4.03
(m, 18H, Cp), 1.49–1.54 (m, 5H, OC2H5). 13C-NMR (CDCl3, δ): 159.09, 132.65, 130.82,
114.36 (C6H4), 107.63 (C4H2N), 79.19 (Ci–C4H2N), 77.27, 77.01, 76.76, 69.37, 67.48,
66.40 (Cp), 63.80, 14.86 (OC2H5). MS (ESI, relative abundance): 555.2 (M+, 100%).

2,5-diferrocenyl heterocyclic compounds
3485
Figure 4. The molecular structure of 6. Hydrogens have been omitted for clarity.
2.5. Synthesis of 4
1,4-Diferrocenyl-1,3-butadiyne (210.4 mg, 0.5 mM), 4-ethylaniline (2.5 mL, 20.1 mM), and
CuCl (2.4 mg, 0.024 mM) were added to a sealed tube under argon and stirred for 24 h at
100 °C. The reaction solution was cooled to room temperature, the solvent removed in
vacuo, and the residue was subjected to chromatographic separation on a silica gel column
Scheme 1. The syntheses of 1–4.

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Y.-Q. Hu et al.
(2.0 × 15 cm) using a mixture of dichloromethane/petroleum ether (1/4, v/v) to elute the
product at room temperature. The first orange band was 1,4-diferrocenyl-1,3-butadiyne. The
second yellow band was 4 (165 mg). Yield: 61.2%, m.p. 236–240 °C. Anal. Calcd for
C32H29NFe2: C, 71.27; H, 5.42; N, 2.60. Found: C, 67.78; H, 4.94; N, 2.53%. IR (KBr
disk): 3089 cm−1 [Cp, νC–H]; 2964, 2921 cm−1 [C2H5, νC–H]; 1513 cm−1 [C4H2N, νC=C];
1419 cm−1 [Ph, νC=C]; 1104, 1003 cm−1 [Cp, δC–H]; 843 cm−1 [Cp, γC–H]; 761 cm−1
1
[Ph, γC–H]. H-NMR (CDCl3, δ): 7.22–7.32 (m, 4H, C6H4), 6.36 (s, 2H, C4H2N), 3.87–4.01
(m, 18H, Cp), 2.77–2.82 (m, 2H, CH2), 1.33–1.36 (m, 3H, CH3). 13C-NMR (CDCl3, δ):
144.91, 137.52, 132.28, 129.63, 128.09 (C6H4), 107.80 (C4H2N), 79.19 (Ci–C4H2N),
77.27, 76.76, 69.39, 67.40, 66.52 (Cp), 28.59, 15.44 (C2H5). MS (ESI, relative abundance):
539.2 (M+, 100%).
2.6. Synthesis of 5
Method 1: 1,4-diferrocenyl-1,3-butadiyne (210.4 mg, 0.5 mM), sodium sulfide nonahydrate
(600.3 mg, 2.74 mM), CuCl (5.1 mg, 0.05 mM), and dry DMF (5 mL) were added to a
sealed tube under argon and stirred for 24 h at 150 °C, then cooled to room temperature.
The residue was washed with water (3 × 5 mL), extracted with CH2Cl2 (3 × 15 mL), and
dried over MgSO4. The solvents were removed in vacuo, and the residue was subjected to
chromatographic separation on a silica gel column (2.0 × 20 cm) using a mixture of
dichloromethane/petroleum ether (1/5, v/v) to elute the product at room temperature. The
first orange band was 1,4-diferrocenyl-1,3-butadiyne. The second orange band was 5
(27 mg). Single crystals of 5 were obtained by recrystallizing from hexane/dichloromethane
(1/1, v/v) at room temperature. Yield: 20.2%.
Method 2: Ferroceneacetylene (210.6 mg, 1.0 mM), sulfur powder (80.1 mg, 1.0 mM), Mo
(CO)6 (53.2 mg, 0.2 mM), and dry benzene (30 mL) were added to a sealed tube under
argon and stirred for 72 h at 80 °C, then cooled to room temperature. The solvents were
removed in vacuo, and the residue was subjected to chromatographic separation on silica
gel column (2.0 × 20 cm) using a mixture of dichloromethane/petroleum ether (1/5, v/v) to
elute the product at room temperature. The first orange band was ferroceneacetylene. The
second orange band was 5 (136 mg). Single crystals of 5 were obtained by recrystallizing
from hexane/dichloromethane (1/1, v/v) at room temperature. Yield: 60.1%.
Compound 5 obtained by methods 1 and 2 show same characterization data, m.p.
215 °C. Anal. Calcd for C24H20SFe2: C, 63.75; H, 4.46. Found: C, 63.95; H, 4.28.
Method 1
Method 2
Scheme 2. The synthesis of 5.

2,5-diferrocenyl heterocyclic compounds
3487
Scheme 3. The synthesis of 6.
IR (KBr disk): 3077 cm−1 [Cp, νC–H]; 1653 cm−1 [Cp, νC=C]; 1404 cm−1 [C4H2S, νC=C];
1
1100, 1026 cm−1 [Cp, δC–H]; 808 cm−1 [Cp, γC–H]. H-NMR (CDCl3, δ): 6.80 (s, 2H,
C4H2S), 4.07–4.60 (m, 18H, Cp). 13C-NMR (CDCl3, δ): 142.98, 140.64 (Ci–C4H2S),
122.33, 121.72 (C4H2S), 80.47, 77.26, 76.75, 69.93, 69.43, 68.58, 66.56 (Cp). MS (ESI,
relative abundance): 452.0 (M+, 100%).
2.7. Synthesis of 6
1,4-Diferrocenyl-1,3-butadiyne (210.3 mg, 0.5 mM), 2,5-norbornadiene (7.0 μL 0.75 mM),
Pd(PPh3)3Cl2 (26.7 mg, 0.025 mM), KOH (42.1 mg, 0.75 mM), and dry dioxane (3 mL)
were added to a sealed tube under argon and stirred for 24 h at 120 °C, then cooled to room
temperature. The residue was washed with water (3 × 5 mL), extracted with CH2Cl2
(3 × 15 mL), and dried over MgSO4. The solvents were removed in vacuo, and the residue
was subjected to chromatographic separation on a silica gel column (2.0 × 15 cm) using a
mixture of dichloromethane/petroleum ether (1/3, v/v) to elute the product at room tempera-
ture. The first orange band was 1,4-diferrocenyl-1,3-butadiyne. The second orange band
was 6 (12.4 mg). The single crystal of 6 was obtained by recrystallization from hexane/
dichloromethane (1/1, v/v) at room temperature. Yield: 5.7%, m.p. 224–226 °C. Anal. Calcd
for C24H20OFe2: C, 66.10; H, 4.62. Found: C, 66.22; H, 4.59. IR (KBr disk): 3081 cm−1
[Cp, νC–H]; 1581 cm−1 [Cp, νC=C]; 1432 cm−1 [C4H2O, νC=C]; 1105, 1013 cm−1 [Cp, δC–H];
1
807 cm−1 [Cp, γC–H]. H-NMR (CDCl3, δ): 6.15 (s, 2H, C4H2O), 4.15–4.67 (m, 18H, Cp).
13C-NMR (CDCl3, δ): 152.07 (Ci–C4H2O), 105.26 (C4H2O), 77.26, 77.00, 76.75, 69.38,
68.43, 65.17 (Cp). MS (ESI, relative abundance): 436.1 (M+, 100%).
3. Results and discussion
A series of 2,5-diferrocenyl heterocycles including pyrrole, thiophene, and furan (1–6) were
synthesized using one-pot cycloaddition of 1,4-diferrocenyl-1,3-butadiyne with aromatic
amine, Na2S·9H2O, or KOH, rather than by the traditional Nigishi cross-coupling reaction
[21, 22]. The synthetic processes and characterizations are discussed in detail.
3.1. Synthesis of 1–4
Compounds 1–4 must be synthesized in an inert environment, otherwise the main product
would be a self-coupled azo-benzene derivative if there is even a trace of oxygen in the
reaction system (scheme 1). Compounds 3 and 4 were synthesized in yields of 23.4% (3)
and 27.1% (4), lower than the that by Negishi reactions. The low yield could be ascribed to
trace oxygen which was not removed completely from the reaction solution. We performed
the experiments to synthesize 3 and 4 again, when oxygen was removed thoroughly from

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Y.-Q. Hu et al.
the reaction system by argon bubbling, then the isolated yields of 3 and 4 increased to 56.1
and 61.2%, close to that of the Negishi reactions.
To further prove that trace oxygen is the key factor for reducing the yields of the desired
product, we used 1,4-diferrocenyl-1,3-butadiyne reacting with 4-methoxyaniline in air, the
main product was 4-methoxyazobenzene (the molecular structure and synthetic procedure
of 4-methoxyazobenzene are described in the Supplementary material) and only a trace of
2,5-diferrocenyl-1-(4-methoxy-phenyl)-pyrrole was obtained.
These experimental results clearly show that oxygen promotes the self-coupled reaction
to form the azo-benzene derivative and hinders the cycloaddition reaction to form a pyrrole
ring. In summary, anaerobic condition is the key factor for one-pot cycloaddition.
3.2. Synthesis of 5
The synthesis of 5 can be obtained through cycloaddition of 1,4-diferrocenyl-1,3-butadiyne
with Na2S·9H2O catalyzed by CuCl (scheme 2 method 1) in a very low yield of 20.2%, even
if the mole ratio of Na2S·9H2O was five times more than that of 1,4-diferrocenyl-1,3-
butadiyne. Fortunately, 5 could be prepared in a yield of 63.5% when ferrocenyl acetylene
reacted with sulfur catalyzed by Mo (CO)6 (scheme 2 method 2) [23], which was higher than
the Nigeshi cross-coupling reactions of ferrocenylzinc chloride and dihalothiophene [24].
3.3. Synthesis of 6
Compound 6 was prepared by the reaction of 1,4-diferrocenyl-1,3-butadiyne with KOH in
the presence of Pd(PPh3)3Cl2 as catalyst (scheme 3) [25]. This one-pot cycloaddition
synthesis could avoid low-temperature and multi-step procedures required by cross-coupling
Negishi reactions. However, the yield of 6 is 5.7%, lower than Negishi reactions (62.1%)
[26]. The possible reason of low yield could be ascribed to inefficient Pd(PPh3)3Cl2
catalyst. Higher yield (65–92%) could be obtained in similar reactions adopting gold or
copper as catalysts [27, 28]. Hence, the high yield synthetic method using 1,4-diferrocenyl-
1,3-butadiyne as precursor to prepare furan derivatives would be studied through efficient
catalysts in our future works.
3.4. Characterization of 1–6
The compounds were confirmed by FT-IR, 1H-NMR, 13C-NMR, elemental analysis, and MS
(corresponding data listed in the experimental section). The molecular structures of 1, 2, 5,
and 6 were determined by single-crystal X-ray diffraction and are shown in figures 1–4.
Crystal data and relevant structural parameters are listed in table 1 and selected bond lengths,
angles, and torsion angles are listed in tables 2–4.
3.5. Intermolecular interactions of 6
The crystal structure of 6 includes two independent molecules. Hydrogen carbon bond
lengths of H(56A)–C(19), H(27A)–C(5), H(13A)–C(4), and H(47A)–C(14) are 2.827,
2.872, 2.884, and 2.816 Å (figure 5(A)), respectively, showing intermolecular interactions
exist in the twin molecules. The T-shaped conformation is edge-to-face C–H⋯π interactions
formed by C–H of one molecule and π-bond of another molecule. Similar C–H⋯π interac-
tions also appears in other molecules [29].

2,5-diferrocenyl heterocyclic compounds
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Y.-Q. Hu et al.

2,5-diferrocenyl heterocyclic compounds
3491

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Y.-Q. Hu et al.
Table 4. Selected bond lengths and angles of 6.
Selected bond lengths (Å) of 6
Selected bond angles (°) of 6
Molecule 1 Molecule 2
C3–O1–C11 C4–O2–C6
Molecule 1
Molecule 2
O1–C3
1.388(10)
1.333(11)
1.418(11)
1.337(11)
1.390(8)
O2–C4 1.388(9)
105.7(7)
109.8(7)
107.1(7)
107.7(8)
109.7(7)
116.4(8)
115.4(7)
107.1(6)
109.2(7)
107.2(7)
107.0(7)
109.5(6)
116.4(6)
117.4(7)
C3–C31
C16–C31
C11–C16
O1–C11
C3–C18
C11–C23
C4–C20
C20–C30
C6–C30
O2–C6
1.359(10)
1.438(11)
1.359(11)
1.395(8)
C16–C11–O1
C11–C16–C31
C3–C31–C16
C31–C3–O1
O1–C3–C18
O1–C11–C23
C30–C6–O2
C6–C30–C20
C4–C20–C30
C20–C4–O2
O2–C4–C8
1.457(10)
1.457(11)
C4–C8
C6–C24
1.442(10)
1.415(11)
O2–C6–C24
Intermolecular interactions not only lead to repeating unit in a cell, but also influence the
crystal data. The bond lengths of C3–C31, C16–C31, C11–C16, and O1–C11 are 1.333
(11), 1.418(11), 1.337(11), and 1.390(8) Å, respectively, shorter than C4–C20, C20–C30,
C6–C30, and O2–C6. The C3–C18 and C11–C23 bond lengths are 1.457(10) and 1.457
(11) Å longer than C4–C8 and C6–C24 (table 4).
There is experimental evidence that torsion angles have been affected by C–H⋯π interac-
tions. Stacking of this type of intermolecular interaction does not have to be a perfect
face-to-face alignment of the atoms, but can also be an offset or slipped packing [30]; hence,
the packing gives torsion angles having different values in twin molecules. For example, the
torsion angles O1–C3–C18–C13 (−9.32°), O2–C4–C8–C7 (9.88°), O1–C11–C23–C17
(7.96°), and O2–C6–C24–C26 (−8.57°) (table 3) exhibit different values which could be
attributed to intermolecular interactions of C–H⋯π system as reported [31]. The dihedral
angle between the plane C4–O2–C6–C30–C20 and the plane O1–C3–C11–C16–C31 is
77.12°, which also can be attributed to C–H⋯π interactions (figure 5(B)).
3.6. Electrochemistry
The redox potential of 1–6 was determined using cyclic voltammetry. All compounds
display two well-resolved redox waves from 0 to 0.8 V (figures 6 and 7), which are
assigned to the two FeII/FeIII redox couples. The electrochemical data are listed in table 5.
From the Ipa/Ipc (≈1) values of each couple, the redox processes were electrochemically
reversible one-electron-transfer processes [32].
Figure 5. The figures of intermolecular interaction of 6.

2,5-diferrocenyl heterocyclic compounds
3493
The difference in the first oxidation potentials of ferrocenyl unit in 1–4 can be ascribed
to the electronic effect of nitrogen. The charge density of nitrogen in pyrrole is affected by
the substituents (scheme 1). The first oxidation potential increases when the substituent
functionality changes from electron-donating 3 (4-OC2H5) 17 mV and 4 (4-C2H5) 55 mV to
1 (4-H) 58 mV to electron-withdrawing 2 (4-F) 89 mV. Therefore, the nature of the
substituent affects the electron density of ferrocenyls via interaction of nitrogen.
The first oxidation potentials of 5 (217 mV) and 6 (184 mV) are higher than those of
1–4, illustrating that thiophene and furan rings have stronger electron-withdrawing effects
than the pyrrole ring. However, according to the electronegativity principle of Pauling (elec-
tronegativity: O 3.5 > N 3.0 > S 2.5) [33], the first oxidation potential of 5 should be the
smallest of the compounds. In order to explain the unexpected first oxidation potential of 5
and 6, we investigated the π-conjugation of 5 and 6. Both dihedral angles of the two
cyclopentadiene planes to the thiophene plane of 5 are 8.93°. The dihedral angles of the
four cyclopentadiene planes within twin molecules (6) to the furan plane are different; the
dihedral angle formed by plane O2–C4–C6–C20–C30 to C12–C14–C24–C26–C41 is
11.20°, the plane O2–C4–C6–C20–30 to C5–C7–C8–C19–C43 is 10.40°, the plane
O1–C3–C11–C16–C31 to C9–C10–C17–C23–C35 is 11.47°, and the plane O1–C3–C11–
C16–C31 to C13–C15–C18–C25–C2 is 10.88°. Hence, the average dihedral angles are
10.64° and 11.33°. The two dihedral angles of 1 have a small difference with each other,
the plane N1–C11–C12–C13–C14 to C15–C16–C17–C18–C19 is 40.11° and N1–C11–
C12–C13–C14 to C1–C2–C3–C4–C5 is 47.35° (Supplementary material). In theory, the
smaller value of dihedral angle indicates more coplanarity between the cyclopentadienyl
planes and the heterocyclic plane, increasing π-conjugation effects. Therefore, the order of
π-conjugation effect between two ferrocenyl units bridged by heterocycle is
5
(Ea1 = 217 mV) > 6 (Ea1 = 184 mV) > 1 (Ea1 = 58 mV), which indicates the electron-with-
drawing inductive effect of sulfur can be transmitted to ferrocenyls more effectively than to
oxygen.
The electronic communication effect of 1–6 was discussed through comparing the
oxidation potential differences (ΔE) between two ferrocenyl units. For 1–4, the ΔE values
are larger than 290 mV, showing strong electronic communication exist in 1–4 (table 5) [1].
The ΔE values increase, compared to 1 (314 mV), when an electron-donating group is
introduced into the phenyl, hence, 3 (4-OC2H5) (328 mV) and 4 (4-C2H5) (324 mV) show
4
2
0
-2
0
0.5
E / V vs Ag| Ag +
Figure 6. The cyclic voltammograms of 1 (green line), 2 (black line), 3 (blue line) and 4 (red line) (see http://
dx.doi.org/10.1080/00958972.2013.841902 for color version).

3494
Y.-Q. Hu et al.
4
2
0
-2
0
0.5
E / V vs Ag| +Ag
Figure 7. The cyclic voltammograms of 1 (black line), 5 (red line) and 6 (blue line) (see http://dx.doi.org/
10.1080/00958972.2013.841902 for color version).
larger separation of ferrocenyl-based redox processes, while the introduction of
electron-withdrawing group in 2 (4-F) (292 mV) leads to a decrease in ΔE value. These
experimental results are in agreement with the substituent effects and deeply influence
ferrocenyl electronic communication via interaction of nitrogens.
The ΔE values are also affected by π-conjugation, as investigated by the torsion angles of
1 and 2 (table 3). The torsion angles Fe1–C1–C11–N1 (−138.34°) and Fe2–C15–C14–N1
(−133.01°) of 1 are smaller than Fe1–C15–C10–N1 (−142.24°) and Fe2–C22–C7–N1
(−138.94°) of 2, showing ferrocenyl units had stronger conjugation with the pyrrole core in
1 than 2. At the same time, the torsion angle C24–N1–C25–C26 (−66.75°) of 1 is also
smaller than C3–C4–N1–C10 (−74.02°) of 2, showing phenyl had stronger conjugation
with pyrrole in 1 than 2. Hence, the total conjugation of 1 is stronger than 2, suggesting a
stronger degree of electronic interaction between two ferrocenyl units in 1; therefore,
stronger electronic communication exists in 1 (ΔE = 314 mV) than 2 (ΔE = 292 mV).
The ΔE of 6 (ΔE = 161 mV) is larger than 5 (ΔE = 139 mV), in agreement with electroneg-
ativity principle of Pauling (electronegativity: O 3.5 > S 2.5). The torsion angles (table 3)
of two cyclopentadiene planes to heterocycle plane of 1, 5, and 6 have been investigated to
measure the conjugation effect. The torsion angles N1–C14–C15–C19 (−39.73°) and
N1–C11–C1–C2 (−47.25°) of 1 are larger than the torsion angles O1–C3–C18–C13
(−9.32°), O1–C11–C23–C17 (7.96°), O2–C4–C8–C7 (9.88°), and O2–C6–C24–C26
(−8.57°) of 6, and the torsion angles S1–C11–C9–C10 (−5.31°) and S1–C11–C9–C10
Table 5. The electrochemical data of 1–6.
Compounds
Ea1 (mV)
Ea2 (mV)
ΔE (mV)
ΔE (mV) Ref.
1
2
3
4
5
6
58
89
17
55
217
184
372
381
345
379
356
345
314
292
328
324
139
161
315a, 450b [20, 33]
–
–
–
260a [20, 33]
290b [20, 33]
Notes: aSupporting electrolyte: [NBu4] [PF6].
bSupporting electrolyte: [NBu4] [B(C6F5)4].

2,5-diferrocenyl heterocyclic compounds
3495
Figure 8. The charge transfer model of the 2,5-diferrocenyl five-membered heterocyclic compounds.
(5.31°) of 5 are the smallest among these three compounds. If the ferrocenyl cyclopentadie-
nyls and the heterocyclic core have more coplanarity, there is more intermetallic electron
transfer interaction. Hence, the order of ΔE should be 5 > 6 > 1 deduced by torsion angles;
however, the order is 1 (314 mV) > 6 (161 mV) > 5 (139 mV), indicating the heteroatom is
the key factor for electronic communication rather than conjugation.
Similar electrochemical models of 2,5-diferrocenyl five-membered heterocyclic
derivatives have been studied [20, 34, 35]. Compound 1 displayed the same ΔE using the
same supporting electrolyte ([NBu4][PF6]) as in our work, and the ΔE increases to 450 mV
(table 5) with [NBu4][B(C6F5)4] as supporting electrolyte [34]. Similar derivatives have also
been determined using [NBu4][B(C6F5)4] as supporting electrolyte, exhibiting the same
trend of ΔE to substituents as with our experiment results. For example, ΔE decreased when
the substituent functionality changed from electron-donating 4-methoxyl (460 mV) and
4-methyl (455 mV) to 4-H (450 mV) to electron-withdrawing 3-fluorophenyl (425 mV)
[35]. The ΔE of 2,5-diferrocenylfuran (290 mV) is larger than 2,5-diferrocenylthiophene
(260 mV) [20], and the same results were observed in this article.
In general, the shorter distance of the bimetal center indicates stronger electrostatic
interaction and expressed larger ΔE [7, 36]. However, the order of Fe1–Fe2 distances is 1
(8.518 Å) > 5 (7.084 Å) > 6 (6.611, 6.614 Å), which disagrees with the ΔE variation trend 1
(314 mV) > 6 (161 mV) > 5 (139 mV). Hence, the electronic communication of two
ferrocenyl units is not affected by the Fe–Fe distance [37, 38].
NMR studies confirmed that electronic communication is not affected by aromaticity of
the central five-membered heterocyclic rings. The chemical shifts of β-H of 1, 5, and 6 are
6.383, 6.806, and 6.152 ppm (Supplementary material), respectively. Hence, the aromaticity
order of five-membered heterocycles is 5 > 1 > 6 [39]. The π-covalently linked bridge with
more aromatic properties should have stronger intermetallic electron transfer interaction [40].
Hence, the order of ΔE should be 5 > 1 > 6 according to aromaticity. However, the order is
1 > 6 > 5 in our experimental results. The NMR studies confirmed no electron transfer within
the C4 unsaturated spacers between terminal ferrocenyl moieties (figure 8(A)).
Electronic character of the heteroatom is for the electronic communication of the 2,5-dif-
errocenyl five-membered heterocycle compounds. There is a non-significant relationship
between electrochemistry and the distance of bimetal or chemical shifts of β-H, indicating
that electron transfer does not select the aromatic bridge spacer. Hence, electron transfer
between two ferrocenyl units of 1–6 selects a relatively shorter heteroatom-linked bridge
(figure 8(B)).
4. Conclusion
A
series of 2,5-diferrocenyl substituted five-membered heterocyclic compounds,
2,5-diferrocenyl-1-phenylpyrrole (1), 2,5-diferrocenyl-1-(4-fluorophenyl)-pyrrole (2),

3496
Y.-Q. Hu et al.
2,5-diferrocenyl-1-(4-ethoxyphenyl)-pyrrole (3), 2,5-diferrocenyl-1-(4-ethylphenyl)-pyrrole
(4), 2,5-diferrocenylthiophene (5), and 2,5-diferrocenylfuran (6), were synthesized by one-
pot cycloaddition with ferrocenyl alkynes as precursors. Electronic communication between
two ferrocenyl units was investigated by cyclic voltammetry. Charge transfers of 1–6 were
from the shorter heteroatom bridge, rather than the electrostatic or aromatic rings.
Supplementary material
Crystallographic data for the structural analysis have been deposited at the Cambridge
Crystallographic Data Center, CCDC No. 947276, 878311, 926209, and 930796 for 1, 2, 5,
and 6, respectively. Copies of this information may be obtained free of charge from the
Director, CCDC, 12 Union Road, Cambridge CB2 1EZ, UK (Fax: t44-1223-336-033;
E-mail: deposit@ccdc.cam.ac.uk). Supplemental data for this article can be accessed http://
dx.doi.10.1080/00958972.2013.841902.
Acknowledgements
We are grateful to the Natural Science Foundation of China (NSFC-21062011), the Natural
Science Foundation of Inner Mongolia (2012ZD01), and the Natural Science Foundation of
Inner Mongolia University of Technology (X201207).
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