Urea-bridged diferrocene: Structural, electrochemical, and spectroelectrochemical studies

Article abstract of DOI:10.1007/s11426-015-5423-9

Urea-bridged diferrocene derivatives N,N′-diferrocenylurea (1) and N,N′-dimethyl-N,N′-diferrocenylurea (2) were prepared and characterized. Single-crystal X-ray analysis shows that Compound 1 has a trans-trans linear conformation whereas Compound 2 has a trans-cis conformation. Both compounds display two consecutive redox couples with, respectively, E_1/2 of +0.29 and +0.42 V vs. Ag/AgCl for 1 and +0.31 and +0.50 V for 2. Spectroelectrochemical studies show the presence of distinct intervalence charge transfer (IVCT) transitions for the one-electron-oxidized mixed-valent Compound 1 ⁺, with an estimated electronic coupling parameter of 190 cm^-1. By contrast, the one-electron-oxidized Compound 2 ⁺ shows much weaker IVCT transitions.

Full text of DOI:10.1007/s11426-015-5423-9

SCIENCE CHINA
Chemistry
September 2015 Vol.58 No.9: 1444–1450
doi: 10.1007/s11426-015-5423-9
• ARTICLES •
Urea-bridged diferrocene: structural, electrochemical,
and spectroelectrochemical studies
Zhong-Liang Gong & Yu-Wu Zhong*
Beijing National Laboratory for Molecular Sciences; CAS Key Laboratory of Photochemistry;
Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190, China
Received January 13, 2015; accepted March 16, 2015; published online August 11, 2015
Urea-bridged diferrocene derivatives N,N-diferrocenylurea (1) and N,N-dimethyl-N,N-diferrocenylurea (2) were prepared
and characterized. Single-crystal X-ray analysis shows that Compound 1 has a trans-trans linear conformation whereas Com-
pound 2 has a trans-cis conformation. Both compounds display two consecutive redox couples with, respectively, E1/2 of +0.29
and +0.42 V vs. Ag/AgCl for 1 and +0.31 and +0.50 V for 2. Spectroelectrochemical studies show the presence of distinct in-
tervalence charge transfer (IVCT) transitions for the one-electron-oxidized mixed-valent Compound 1+, with an estimated
electronic coupling parameter of 190 cm1. By contrast, the one-electron-oxidized Compound 2+ shows much weaker IVCT
transitions.
mixed-valent compounds, ferrocene, electrochemistry, urea, spectroelectrochemistry
1 Introduction
Since the pioneering experimental [1] and theoretical work
[2] by Creutz, Taube, Hush, and others, mixed-valence
chemistry has attracted much attention as a classic means to
examine the bridge-mediated electron-transfer processes
[3–5]. To date, a large number of mixed-valent compounds
with various bridges and redox sites have been reported
[3–10]. These studies have demonstrated that the degree of
charge delocalization is largely dependent on the electronic
and geometrical properties of the bridge. In addition,
mixed-valent compounds have been very useful in molecu-
lar electronics [11], electrochromism [12], and information
storage [13], among others.
Figure 1 Different conformers of diarylureas.
ganic synthesis and supramolecular chemistry [17–19]. Di-
arylurea is a cross-conjugated molecule in nature [20]. The
two aryl groups are conjugated with the central carbonyl
group but not to each other. Diarylurea has been used to
mediate charge separation between photoactive donors and
acceptors [21]. However, the use of diarylurea as a bridge in
mixed-valence chemistry has remained unexplored.
We recently reported that linear diarylurea is an efficient
bridge to mediate the electronic communication between
two triarylamine or cyclometalated ruthenium termini [22].
When N,N-dimethylurea was used as the bridge, through-
Urea is an important organic functional group. Depend-
ing on the substituents, diarylureas can have a linear
trans-trans, folded cis-cis, or trans-cis conformation (Figure
1) [14–16]. Urea are very important building blocks in or-
*Corresponding author (email: zhongyuwu@iccas.ac.cn)
© Science China Press and Springer-Verlag Berlin Heidelberg 2015
chem.scichina.com link.springer.com

Gong et al. Sci China Chem September (2015) Vol.58 No.9
1445
space coupling was realized in the resultant folded con-
formers. Herein, we present electronic-coupling studies
between two ferrocene termini through a urea bridge. Fer-
rocene, a popular redox site for mixed-valency chemistry
[23–26], is characterized by a well-defined ferrocene/
ferrocenium (Fc0/+) redox pair. The structure modifications
of ferrocene and ferrocene derivatives are well established.
Urea-functionalized ferrocene derivatives and related
compounds are also well known [27,28]. However, direct
ferrocene-ferrocene coupling through the urea bridge has
not been examined in detail.
After 15 min, CH3I (85 mg, 0.60 mmol) was added drop-
wise. The resulting mixture was stirred for an additional 3 h.
When the reaction was complete as monitored by TLC, 50
mL dichloromethane was added to the mixture. The result-
ing organic phase was washed with water (3×30 mL) and
dried over anhydrous K2CO3. The solvent was removed
under reduced pressure. The resulting residue was purified
by column chromatography on silica gel (eluent: CH2Cl2/
ethyl acetate, 75:1) to give 75 mg compound 2 in 82% yield
as a yellow solid. 1H NMR (400 MHz, acetone-d6):  3.06 (s,
6H), 3.92 (s, 4H), 4.21 (s, 14H). 13C NMR (100 MHz,
CDCl3):  38.51, 61.57, 64.26, 68.71, 104.16, 160.75.
MALDI-TOF-HRMS calcd. for C23H24Fe2N2O (M+):
456.0587. Found: 456.0582. Anal. calcd. for C23H24Fe2N2O:
C, 60.56; H, 5.30; N, 6.14. Found: C, 60.47; H, 5.33; N,
6.03.
2 Experimental
2.1 Synthesis and general experimental section
NMR spectra were recorded in the designated solvent on
either a Bruker Avance 300 or a 400 MHz spectrometer
(Switzerland). Spectra are reported in ppm values from re-
sidual protons of a deuterated solvent. Mass data were ob-
tained with a Bruker Daltonics Inc. Apex II FT-ICR or an
Autoflex III MALDI-TOF (Germany) mass spectrometer.
The matrix for the MALDI-TOF measurement was -cyano-
4-hydro-xycinnamic acid. Microanalysis was done using a
Thermo Scientific Flash EA 1112 (USA) or a Carlo Erba
1106 analyzer (Italy) at the Institute of Chemistry of the
Chinese Academy of Sciences. All measurements were done
at the same institute. Iodoferrocene [29] and aminoferrocene
[30] were prepared according to known procedures.
2.2 Spectroscopic measurements
Spectroelectrochemical measurements were performed in a
thin-layer cell (optical length: 0.2 cm) in which a transpar-
ent ITO glass electrode (<10 /square) was set in the indi-
cated solvent that contained the compound to be studied
(about 5105 mol/L) and 0.1 mol/L Bu4NClO4 as the sup-
porting electrolyte. A platinum wire and Ag/AgCl in satu-
rated aqueous NaCl were respectively used as the counter
electrode and reference electrode. The cell was put into the
spectrometer to monitor the spectral change during elec-
trolysis. Absorption spectra were recorded on a PE Lambda
750 UV/Vis/NIR spectrophotometer (PerkinElmer, USA) at
room temperature.
2.1.1 N,N-diferrocenylurea (1)
To the mixture of triphosgene (89 mg, 0.30 mmol) and tri-
ethylamine (0.30 mL) in 10 mL dry dichloromethane in an
ice bath (0–5 °C) was added 1-aminoferrocene (60 mg, 0.30
mmol). After 1 h, another portion of 1-aminoferrocene (60
mg, 0.30 mmol) in 5 mL dry pyridine was added. The mix-
ture was heated to 95 °C and stirred at this temperature for
2 h before being cooled to room temperature. The solvent
was removed under vacuum. The crude product was puri-
fied by column chromatography on silica gel (eluent:
CH2Cl2/ethyl acetate, 15:1) to give 106 mg compound 1 in
83% yield as a yellow solid. 1H NMR (300 MHz, CDCl3): 
4.16 (s, 4H), 4.27 (s, 10H), 4.48 (s, 4H), 6.05 (s, 2H). 13C
NMR (100 MHz, CDCl3+DMSO-d6):  59.62, 62.88, 67.80,
95.96, 152.77. MALDI-TOF-HRMS Calcd. for C21H20-
Fe2N2O (M+): 428.0274. Found: 428.0690. Anal. calcd. for
C21H20Fe2N2O: C, 58.92; H, 4.71; N, 6.54. Found: C, 58.75;
H, 4.62; N, 6.62.
2.3 Electrochemical measurements
All electrochemical measurements were taken with a CHI
660D potentiostat with a one-compartment electrochemical
cell under a nitrogen atmosphere. All measurements were
taken in denoted solvents that contained 0.1 mol/L
nBu4NClO4 as the supporting electrolyte at a scan rate of
100 mV/s. The working electrode was a glassy carbon with
a diameter of 3 mm. The electrode was polished prior to use
with 0.05 m alumina and rinsed thoroughly with water and
acetone. A large-area platinum-wire coil was used as the
counter electrode. All potentials were referenced to an
Ag/AgCl electrode in saturated aqueous NaCl without re-
gard for the liquid junction potential. Potentials vs. ferro-
cene+/0 can be deduced by subtracting 0.45 V.
2.4 X-ray crystallography
2.1.2 N,N-dimethyl-N,N-diferrocenylurea (2)
The X-ray diffraction data were collected using a Rigaku
Saturn 724 diffractometer (Japan) on a rotating anode
(Mo-K radiation, 0.71073 Å) at 173 K. The structure was
solved by the direct method using SHELXS-97 [31] and
refined with Olex2 [32].
To a suspension of NaH (32 mg, 60%, 1.0 mmol, washed
with 3×3 mL hexane before use) in 3 mL dry DMF, a solu-
tion of the above-prepared compound 1 (85 mg, 0.20 mmol)
in 5 mL DMF was added dropwise at room temperature.

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Gong et al. Sci China Chem September (2015) Vol.58 No.9
Table 1 Data collection and refinement statistics of the 2′-Se-A DNA
3 Results and discussion
1
C21H20Fe2N2O
C1 2/c1
Monoclinic
18.621(5)
17.523(4)
23.757(6)
90
2
C23H24Fe2N2O
P1 21/c1
Monoclinic
7.4647(15)
12.015(2)
22.065(4)
90
Empirical formula
Space group
3.1 Synthesis and structural studies
Crystal system
We selected the urea-bridged diferrocene 1 and the
N,N-dimethylurea-bridged diferrocene 2 as the target mol-
ecules (Scheme 1), because they may have different con-
formations and thus different degrees of electronic coupling.
According to the reported procedures, ferrocene was first
successively treated with t-butyl lithium and iodine to give
iodoferrocene [29], which was then transformed to amino-
ferrocene with aqueous NH3 as the amination reagent in the
presence of CuI and Fe2O3 [30]. The reaction of amino-
ferrocene with triphosgene in the presence of NEt3 gave an
isocyanate intermediate, which was treated with another
equivalent of aminoferrocene to afford the desired com-
pound 1 in 83% yield. The straightforward N-methylation of
a (Å)
b (Å)
c (Å)
(°)
(°)
109.364(3)
90
91.62(3)
90
(°)
vol (Å3)
7313(3)
16
1978.1(7)
4
Z
Reflections collected
Density (mg/m3)
Temperature (K)
Wavelength (Å)
24525
14349
1.555
1.532
173.1500
173.1500
0.71073
R1=0.0897,
wR2=0.1314
R1=0.1087,
wR2=0.1384
1042199
0.71073
R1=0.0422,
wR2=0.0913
R1=0.0458,
wR2=0.0934
1042200
Final R indices
1 gave 2 in good yield. Only one set of H and 13C NMR
signals were recorded for both ferrocene units of 1 and 2.
This means that the two ferrocene units in both compounds
are chemically equivalent on the NMR time scale.
1
R indices (all data)
CCDC No.
The structures of 1 and 2 were confirmed by single-
crystal X-ray analysis (Figure 2 and Table 1). Compound 1
has an expected trans-trans conformation. The two cyclo-
pentadiene (Cp) rings directly connected to the urea unit
have respective torsion angles of 13.9° and 56.6° with re-
spect to the urea plane. A typical urea tape structure is ob-
served in the crystal packing, which is assembled via the
intermolecular NHO hydrogen bonds (Figure 2(b)). In
contrast, Compound 2 displays a trans-cis conformation.
The steric bulkiness of the ferrocene units may suppress the
formation of a folded cis-cis conformation in the solid state.
Figure S1 (see Supporting Information online) shows the
1H NMR spectra of 2 at various temperatures from 223 to
323 K. It seems that the molecule has different confor-
mations at low and high temperatures. The Cp and methyl
protons at 323 K all slightly shifted to the low field region
with respect to those at 223 K; the signals at 273 K are
broadened due to conformation exchange. We deduced that
Compound 2 has a trans-cis conformation at high tempera-
ture and cis-cis conformation at low temperature, because
the aromatic protons of trans-cis N,N-dimethyl diarylurea
compounds are known to locate at the lower field with
respect to cis-cis conformers [15]. The possibility of form-
ing the trans-trans conformer is believed low due to the
potential methyl-methyl steric hinderance.
3.2 Electrochemical studies
Figure 3 shows the cyclic voltammograms (CVs) and dif-
ferential pulse voltammograms (DPVs) of 1 and 2. Both
compounds show two consecutive redox couples with E1/2
of +0.29 and +0.42 V vs. Ag/AgCl for 1 and +0.31 and
+0.50 V for 2, respectively. The peak separation of each
redox couple of 1 is around 60 mV, which is well in agree-
ment with the one-electron Nernstain processes. However,
the peak separation of 2 is relatively large (around 90 mV),
which may be caused by the slower electron transfer kinet-
ics of 2.
3.3 Spectroelectrochemical studies
Compounds 1 and 2 were subjected to stepwise oxidative
electrolysis at a transparent indium-tin-oxide (ITO) glass
electrode in 0.1 mol/L Bu4NClO4/CH2Cl2. The absorption
spectral changes are shown in Figure 4. When the potential
was gradually increased from +0.20 to +0.40 V vs. Ag/AgCl
to transform 1 into 1+, new absorptions between 600 and
2200 nm appeared with a main band at 820 nm and a broad
shoulder band in the near-infrared (NIR) region (Figure
4(a)). When the potential was further increased from
+0.40 to +0.55 V to transform 1+ into 12+, the absorption at
Scheme 1 Synthesis of Compounds 1 and 2.

Gong et al. Sci China Chem September (2015) Vol.58 No.9
1447
Figure 2 Thermal ellipsoid plots of the single-crystal X-ray structures of 1 (a, b) and 2 (c) at 30% probability (color online).
The potential splitting (E) between two redox couples
are 130 and 190 mV for 1 and 2, respectively. The compro-
portionation constant Kc for the equation of [Fc0-Fc0]+
[Fc+-Fc+]  2[Fc0-Fc+] is 160 and 1660 for 1 and 2, respec-
tively, as determined by Kc=10E/59) for a room-temperature
case.
During the similar stepwise oxidations of 2, the appear-
ance of LMCT bands around 800 nm was observed. How-
ever, the NIR absorptions of 2+ were very weak. Some weak
and broad IVCT transitions seemed to be present in the
one-electron-oxidized state 2+, which increased in single
oxidation and decreased in double oxidation. These results
can be seen in the enlarged NIR plots in Figures 4(c, d).
The NIR transitions of 1+ were fitted to Gaussian func-
tions. The blue curve in Figure 5(a) is the deconvoluted
IVCT transition, which has a max at 8900 cm1 with max of
200 L/(mol cm). The observed full width at half-height
(1/2,obs) is 3000 cm1, which is narrower yet in the same
Figure 3 CVs and DPVs of 1 (a) and 2 (b) at a Pt disk electrode in 0.1
mol/L Bu4NClO4/CH3CN (color online). E1/2=+0.29 and +0.42 V for 1 and
+0.31 and +0.50 V for 2, respectively.
820 nm continued to increase and slightly shifted to 780 nm.
In contrast, the NIR band decreased significantly (Figure
4(b)). The absorption band at 820 nm of 1+ is assigned to
the ligand-to-metal charge transfer (LMCT) transition [33]
and the NIR band is ascribed to the intervalence charge
transfer (IVCT) transition. When the potential was de-
creased from +0.55 V to +0.20 V, reverse-absorption spec-
tral changes could be observed, which suggests the reversi-
bility of the redox reactions.
order of magnitude with respect to the theoretical value
1/2
(4530 cm1) determined by 1/2,theo=(2310max
)
.
The electronic coupling parameter Hab is calculated to be
190 cm1, according to the Hush formula Hab=0.0206
(maxmax1/2)1/2/(rab) [2], where the electron transfer rab is
taken to be the crystallographic Fe–Fe distance (8.1 Å). The
true rab value is very likely shorter than the geometrical
Fe–Fe distance, due to charge delocalization. The Hab value

1448
Gong et al. Sci China Chem September (2015) Vol.58 No.9
Figure 4 Absorption spectral changes of 1 (a, b) and 2 (c, d) upon stepwise oxidative electrolysis at an ITO glass electrode in 0.1 mol/L Bu4NClO4/CH2Cl2.
(a, c): one-electron oxidation; (b, d) the second one-electron oxidation. The applied potentials were referenced vs. Ag/AgCl. The insets in (c, d) show the
enlarged plots between 900 and 2500 nm (color online).
Figure 5 NIR absorption spectra of 1+ (a) and 2+ (b) vs. wavenumbers. The blue curve in (a) is the Gaussian-fitted IVCT transition; the red curve is the
sum of green and blue curves; the black curve is the experimental data (color online).
is thus underestimated.
resonance (due to electronic coupling), electrostatic repul-
sion, and inductive effect [34]. Although 2+ shows slightly
larger electrochemical splitting than 1+ (190 vs. 130 mV),
the IVCT transition of 2+ is much weaker than 1+. Single-
crystal X-ray analysis indicated that the two ferrocene units
of 2 are much closer with respect to those of 1 (Fe–Fe dis-
tance: 6.4 vs. 8.1 Å). The through-space electrostatic repul-
Figure 5(b) shows the NIR transitions of 2+ vs. wave-
numbers. No satisfactory deconvolution could be performed
because the possible IVCT transition is very weak com-
pared to the significant noise in the NIR region. It is known
that the electrochemical splitting of mixed-valent com-
pounds is determined by many factors, including charge

Gong et al. Sci China Chem September (2015) Vol.58 No.9
1449
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9
4 Conclusions
Two urea-bridged diferrocene compounds were prepared
and studied. Compound 1 with the pristine urea bridge has
the linear trans-trans conformation and Compound 2 with
the N,N-dimethyl urea bridge displays a trans-cis confor-
mation in the solid state. Most known N,N-dimethyl dia-
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conformation. The urea bridge is found to be an efficient
cross-conjugated bridge to mediate the electronic coupling
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the nature of electronic coupling is dependent on the con-
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Supporting information
The supporting information is available online at chem.scichina.com and
link.springer.com/journal/11426. The supporting materials are published as
submitted, without typesetting or editing. The responsibility for scientific
accuracy and content remains entirely with the authors.
This work was supported by the National Natural Science Foundation of
China (21271176, 91227104, 21472196, 21221002), the National Basic
Research Program of China (2011CB932301), and the Strategic Priority
Research Program of the Chinese Academy of Sciences (XDB 12010400).
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