From diacetylferrocene to 1,1′-ferrocenyldiimines: Substituent effects on synthesis, molecular structure, electrochemical behavior and optical absorption property

Article abstract of DOI:10.1016/j.molstruc.2009.06.047

A series of 1,1′-ferrocenyldiimines [Fe{(η⁵-C₅H₄)-C(Me)N-R}₂ ], where R = n-hexyl 1a, cyclohexyl 1b, phenyl 1c, 4-methoxyphenyl 1d, 3-methoxyphenyl 1e, 4-nitrophenyl 1f, and 3-nitrophenyl 1g, have been synthesiz

Full text of DOI:10.1016/j.molstruc.2009.06.047

Journal of Molecular Structure 935 (2009) 102–109
Contents lists available at ScienceDirect
Journal of Molecular Structure
journal homepage: www.elsevier.com/locate/molstruc
From diacetylferrocene to 1,10-ferrocenyldiimines: Substituent effects
on synthesis, molecular structure, electrochemical behavior
and optical absorption property
Ting-Yu Lee a, , Pui-Ren Chiang a,b, Ming-Chen Tsai a,b, Che-Yu Lin c, Jui-Hsien Huang c
*
a Department of Applied Chemistry, National University of Kaohsiung, Kaohsiung 811, Taiwan
b Department of Chemistry, National Sun Yat-sen University, Kaohsiung 804, Taiwan
c Department of Chemistry, National Changhua University of Education, Changhua 500, Taiwan
a r t i c l e i n f o
a b s t r a c t
A series of 1,10-ferrocenyldiimines [Fe{( 5-C5H4)–C(Me)N–R}2], where R = n-hexyl 1a, cyclohexyl 1b, phe-
g
Article history:
Received 10 April 2009
Received in revised form 26 June 2009
Accepted 29 June 2009
nyl 1c, 4-methoxyphenyl 1d, 3-methoxyphenyl 1e, 4-nitrophenyl 1f, and 3-nitrophenyl 1g, have been
synthesized by reactions of ca. 1:2 M ratio of 1,10-diacetylferrocene and the corresponding amines. While
ca. 1:1 M ratio of the starting materials was employed, acetylferrocenylimines [Fe{(
C(CH3)@O}{(
5-C5H4)–C(CH3)@N–R}], where R = 4-nitrophenyl 2f, and 3-nitrophenyl 2g, were obtained.
g
5-C5H4)–
Available online 4 July 2009
g
Single crystal X-ray structural analysis revealed that the two cyclopentadienyl rings in 1d, 1e, 1g, 2f,
and 2g were antiperiplanar staggered, anticlinal eclipsed, anticlinal eclipsed/synclinal eclipsed, synclinal
eclipsed, and synclinal eclipsed to each other in solid state, respectively. All synthesized ferrocene deriv-
atives exhibited a reversible one-electron redox process in their cyclic voltammograms, and the values of
their redox potentials relied on the R groups. The correlation between the redox potential and the
Keywords:
Ferrocenyldiimine
Acetylferrocenylimine
Condensation reaction
Electrochemical property
Hammett constant
Hammett substituent constant,
tra showed that their optical property was also substituent dependent.
Ó 2009 Elsevier B.V. All rights reserved.
rp was quite well, with a correlation coefficient of 0.98. The UV–vis spec-
1. Introduction
to form monocyclopalladate compound containing a carbonyl group
on the other cyclopentadienyl ring [19,22]. Furthermore, expansion
of the number of coordination sites on the imino groups of the deriv-
atives might provide more diverse chelating modes as ligands with
other metal ions. For instance, a ferrocenyl-containing pyridyl-
imine-based bis-bidentate units and copper salt might undergo
self-assembly process to yield a helical coordination polymer [23].
Moreover, dinuclear double-helical and mononuclear copper com-
plexes with the ferrocenyl entity were prepared from the corre-
sponding ferrocene derivatives with azine backbone chains [24].
The 1,10-ferrocenyldiimines were also readily reduced to give ami-
no-ferrocenyl analogues, which was potentially as precursors for
molecular shuttle materials [25]. Although the method to prepare
1,10-ferrocenyldiimines was reported, the individual reaction condi-
tion needed to be modified upon the substituents on imines [19].
Owing to the importance of these substances, the development of
the strategy in preparation of 1,10-ferrocenyldiimines has been still
in progress [26]. Therefore, it is worthy to systematically study the
synthetic process from the common starting material, diacetylferro-
cene to 1,10-ferrocenyldiimines, especially from both the structural
and electronic point of view. In addition, this scrutiny might benefit
an opportunity to isolate possible intermediates, acetylferrocenyli-
mines, which were potentially able to serve as N,O-heteroatom
chelating ligands. On account of the potential application of
Since the discovery of ferrocene [1], (g
5-C5H5)2Fe, an increasing
interesthas beendevoted to this prominentcompoundand its deriv-
atives because of their wide application in catalysis, molecular elec-
tronics, polymer chemistry, nonlinear optical materials, and
bioorganometallic materials [2–14]. Most of the derivatives were
prepared by introducing functional groups into the cyclopentadi-
enyl rings of ferrocene in order to tune their properties. One of
intriguing research areas was therefore associated to 1,10-disubsti-
tuted ferrocenes bearing nitrogen atoms as ligands primarily due
to their distinctive geometry for application in organic synthesis
and self-assembly materials [15–18]. Among these, the imino deriv-
atives have demonstrated their fascinating coordination nature. For
example, the reaction of 1,10-ferrocenyldiimines with alkali-tetra-
chloropalladate salts produced cyclopalladated complexes contain-
ing r(PdACsp2,ferrocene) bonds [19,20], which showed high catalytic
activities for Heck reactions [21]. This metalation reaction followed
by treatment with Lewis base, e.g., PPh3, afforded two stereoisomers,
meso- and D,L-bis(cyclopalladated) derivatives [19,20]. Mono-meta-
lation might take place along with the hydrolysis of one imino group
* Corresponding author. Tel.: +886 7 5919465; fax: +886 7 5919348.
E-mail address: tingyulee@nuk.edu.tw (T.-Y. Lee).
0022-2860/$ - see front matter Ó 2009 Elsevier B.V. All rights reserved.
doi:10.1016/j.molstruc.2009.06.047

T.-Y. Lee et al. / Journal of Molecular Structure 935 (2009) 102–109
103
1,10-ferrocenyldiimines in biosensor and nonlinear optical material
[27,28], it is also useful to investigate their electrochemical and
optical properties. In this article, the synthesis, structures, electro-
chemical behavior, and optical absorption property of a series of
1,10-ferrocenyldiimines and two novel acetylferrocenylimines are
reported.
2.2.4. [Fe{(g
5-C5H4)AC(CH3)@NAC6H4-4-OCH3}2] (1d)
Without MgSO4. 48 h. Red solid (yield = 61%). 1H NMR
(300 MHz, CDCl3, rt): d (ppm) 6.83 (4H, dd, J = 8.7 Hz), 6.68 (4H,
dd, J = 8.7 Hz), 4.76 (4H, t, J = 1.8 Hz), 4.44 (4H, t, J = 1.8 Hz),
3.78 (6H, s, OMe), 2.13 (6H, s, CH3). 13C{1H} NMR (75 MHz, CDCl3,
rt): d (ppm) 167.10 (CN), 155.75, 144.75, 120.93, 114.13 (aryl),
85.32, 71.73, 69.42 (cyclopentadienyl), 55.46 (OMe), 18.09 (CH3).
EI mass spectrum (m/z): 480 (M+). m.p. = 177–179 °C.
2. Experimental
2.2.5. [Fe{(g
5-C5H4)AC(CH3)@NAC6H4-3-OCH3}2] (1e)
Without MgSO4. 48 h. Red solid (yield = 62%). 1H NMR (300 MHz,
CDCl3, rt): d (ppm) 7.18 (2H, t, J = 8.1 Hz), 6.59 (2H, dd, J = 4.8 Hz),
6.33 (2H, d, J = 0.9 Hz), 6.30 (2H, d, J = 1.8 Hz), 4.77 (4H, t,
J = 2.1 Hz), 4.45 (4H, J = 2.1 Hz), 3.73 (6H, s, OMe), 2.13 (6H, s,
CH3). 13C{1H} NMR (75 MHz, CDCl3, rt): d (ppm) 167.01 (CN),
160.21, 153.01, 129.67, 112.03, 108.72, 105.30 (aryl), 84.94, 71.88,
69.59 (cyclopentadienyl), 55.22 (OMe), 18.17 (CH3). EI mass spec-
trum (m/z): 480 (M+). Anal. calcd. for C28H28N2O2Fe: C, 70.01; H,
5.83; N, 5.87; found: C, 70.10; H, 5.82; N, 5.76. m.p. = 123–125 °C.
2.1. Materials
Aniline (Aldrich), hexylamine (Aldrich), m-anisidine (Alfa
Aesar), p-anisidine (Alfa Aesar), 3-nitroaniline (Alfa Aesar), 4-nitro-
aniline (Alfa Aesar), cyclohexylamine (Sigma–Aldrich), ferrocene
(Alfa Aesar), toluene (Mallinckrodt Chemicals) and anhydrous
magnesium sulfate (Shimakyu’s pure chemicals) were used as
received. Activated Al2O3 (Macherey–Nagel) was heated at 110 °C
in an oven overnight before used. 1,10-Diacetylferrocene was syn-
thesized according to literature [29].
2.2.6. [Fe{(g
5-C5H4)AC(CH3)@NAC6H4-4-NO2}2] (1f)
With MgSO4. 48 h. Yellow–orange solid (yield = 10%). 1H NMR
(300 MHz, CDCl3, rt): d (ppm) 8.16 (4H, dd, J = 8.7 Hz), 6.81 (4H, dd,
J = 8.7 Hz), 4.78 (4H, t, J = 3 Hz), 4.53 (4H, t, J = 3 Hz), 2.11 (6H, s,
CH3). 13C{1H} NMR (75 MHz, CDCl3, rt): d (ppm) 167.89 (CN), 157.66,
143.66, 125.10, 119.75 (aryl), 83.84, 72.46, 70.03 (cyclopentadienyl),
18.57 (CH3). EI mass spectrum (m/z): 510 (M+). m.p. = 212–214 °C.
2.2. General procedure for synthesis of 1,10-ferrocenyldiimines (1a–g)
and acetylferrocenylimines (2f and 2g)
Preparation of 1,10-ferrocenyldiimines were modified by litera-
ture methods [19,30]. A Schlenk flask equipped with a Dean–Stark
trap was charged with 1equiv of 1,10-diacetylferrocene, 2.2 equiv of
the selected amine, and toluene in the presence of freshly activated
Al2O3 with or without MgSO4. The mixture was refluxed from 24 to
72 h under nitrogen atmosphere. After filtration through Celite, the
solvent and excess amines were removed to give compound 1. The
solid products were recrystallized from CH2Cl2/hexane. Under the
similar reaction conditions, 1:1.1 M ratio of 1,10-diacetylferrocene
and the corresponding amines was employed to give compound
2 after recrystallization.
2.2.7. [Fe{(g
5-C5H4)AC(CH3)@NAC6H4-3-NO2}2] (1g)
Without MgSO4. 48 h. Yellow–orange solid (yield = 50%).
1HNMR (300 MHz, CDCl3, rt): d (ppm) 7.88 (2H, dd, J = 9.3 Hz),
7.57 (2H, t, J = 2.1 Hz), 7.41 (2H, t, J = 8.1 Hz), 7.04 (2H, dd, J =
8.1 Hz), 4.80 (4H, J = 1.8 Hz), 4.53 (4H, t, J = 1.8 Hz), 2.14 (6H, s,
CH3). 13C{1H} NMR (75 MHz, CDCl3, rt): d (ppm) 168.79 (CN),
152.49, 148.77, 129.68, 125.99, 117.97, 114.57 (aryl), 84.15,
72.307, 69.91 (cyclopentadienyl), 18.34 (CH3). EI mass spectrum
(m/z): 510 (M+). Anal. calcd. for C26H22N4O4Fe: C, 61.19; H, 4.34;
N, 10.98; found: C, 61.07; H, 4.81; N, 10.68. m.p. = 195–197 °C.
2.2.1. [Fe{(g
5-C5H4)AC(CH3)@NAC6H13}2] (1a)
Without MgSO4. 72 h. Red oil (yield = 95%). 1H NMR (300 MHz,
CDCl3, rt): d (ppm) 4.56 (4H, t, J = 1.8 Hz), 4.24 (4H, t, J = 2.1 Hz),
3.30 (4H, t, J = 7.5 Hz), 2.04 (6H, s, CH3), 1.71–1.53 (4H, m),
1.44–1.15 (12H, m), 0.89 (6H, t, J = 6.6 Hz). 13C{1H} NMR
(75 MHz, CDCl3, rt): d (ppm) 164.68 (CN), 87.04, 70.78, 68.44
(cyclopentadienyl), 51.98, 31.84, 30.86, 27.43, 22.72, 14.12 (n-hex-
yl), 15.84 (CH3). Anal. calcd. for C26H40FeN2: C, 71.55; H, 9.73; N,
6.42; found: C, 72.13; H, 9.73; N, 6.51%.
2.2.8. [Fe{(g g
5-C5H4)AC(CH3)@O}{( 5-C5H4)AC(CH3)@NAC6H4-4-
NO2}] (2f)
Without MgSO4. 48 h. Yellow–orange solid (yield = 50%).
1HNMR (300 MHz, CDCl3, rt): d (ppm) 8.23 (2H, dd, J = 9 Hz), 6.92
(2H, dd, J = 9 Hz), 4.80 (4H, d, J = 1.8 Hz), 4.55 (2H, t, J = 2.1 Hz),
4.50 (2H, t, J = 0.9 Hz), 2.42 (3H, s), 2.06 (3H, s). 13C{1H} NMR
(75 MHz, CDCl3, rt): d (ppm) 201.29 (CO), 167.55 (CN), 157.62,
143.59, 125.09, 120.14 (aryl), 84.12, 80.48, 73.36, 72.38, 70.99,
69.98 (cyclopentadienyl), 27.63 (CH3ACO), 18.45 (CH3). EI mass
spectrum (m/z): 390 (M+). m.p. = 169–171 °C.
2.2.2. [Fe{(g
5-C5H4)AC(CH3)@NAC6H11}2] (1b)
With MgSO4. 72 h. Red oil (yield = 66%). 1H NMR (300 MHz,
CDCl3, rt): d (ppm) 4.52 (4H, t, J = 1.8 Hz), 4.22 (4H, t, J = 2.1 Hz),
3.41–3.29 (2H, m), 2.09 (6H, s, CH3), 1.84–1.17 (20H, m). 13C{1H}
NMR (75 MHz, CDCl3, rt): d (ppm) 162.21 (CN), 87.49, 70.69,
68.70 (cyclopentadienyl), 59.60, 33.69, 25.80, 24.99 (cyclohexyl),
15.79 (CH3). Anal. calcd. for C26H36FeN2: C, 72.22; H, 8.39; N,
6.48; found: C, 71.98; H, 8.12; N, 6.32%.
2.2.9. [Fe{(g g
5-C5H4)AC(CH3)@O}{( 5-C5H4)AC(CH3)@NAC6H4-3-
NO2}] (2g)
Without MgSO4. 48 h. Yellow–orange solid (yield = 60%). 1HNMR
(300 MHz, CDCl3, rt): d (ppm) 7.94 (1H, d, J = 7.2 Hz), 7.67 (1H, s),
7.50 (1H, t, J = 7.5 Hz), 7.18 (H, d, J = 7.8 Hz), 4.82 (4H, s), 4.56 (2H,
s), 4.49(2H, s), 2.43(3H, s), 2.07(3H, s). 13C{1H}NMR(75 MHz, CDCl3,
rt): d (ppm) 201.60 (CO), 168.49 (CN), 152.50, 148.83, 129.76,
126.33, 118.06, 114.82 (aryl), 84.44, 80.44, 73.37, 72.34, 70.94,
69.90 (cyclopentadienyl), 27.66 (CH3–CO), 18.23 (CH3). EI mass
spectrum (m/z): 390 (M+). m.p. = 150–152 °C.
2.2.3. [Fe{(g
5-C5H4)AC(CH3)@NAC6H5}2] (1c) [19]
With MgSO4. 24 h. Orange solid (yield = 87%). 1H NMR
(300 MHz, CDCl3, rt): d (ppm) 7.28 (4H, t, J = 7.8 Hz), 7.03 (2H,
t, J = 7.8 Hz), 6.73 (4H, dd, J = 8.4 Hz), 4.78 (4H, t, J = 2.1 Hz),
4.46 (4H, t, J = 2.1 Hz), 2.12 (6H, s, CH3). 13C{1H} NMR
(75 MHz, CDCl3, rt):
d (ppm) 166.87 (CN), 151.57, 128.86,
123.06, 119.65 (aryl), 85.04, 71.86, 69.53 (cyclopentadienyl),
18.16 (CH3). EI mass spectrum (m/z): 420 (M+). Anal. calcd. for
C26H24FeN2: C, 74.30; H, 5.75; N, 6.66; found: C, 74.13; H,
6.02; N, 6.65. m.p. = 143–145 °C.
2.3. Physical measurements
1H and 13C NMR spectra were obtained on a Varian Mercury 300
Plus spectrometer. Chemical shifts for 1H and 13C spectra were

104
T.-Y. Lee et al. / Journal of Molecular Structure 935 (2009) 102–109
Table 1
Summary of crystallographic data for compounds 1d, 1e, 1g, 2f, and 2g.
1d
1e
1g
2f
2g
Formula
Mw
C28H28FeN2O2
480.37
C28H28FeN2O2
480.37
C52H44Fe2N8O8
1020.65
C20H18FeN2O3
390.21
C20H18FeN2O3
390.21
T (K)
150(2)
150(2)
150(2)
150(2)
150(2)
Crystal system
Space group
a (Å)
b (Å)
c (Å)
Monoclinic
P2(1)/n
6.9333(3)
11.6867(5)
14.1767(5)
90
Orthorhombic
P2(1)2(1)2(1)
8.5576(2)
11.0220(3)
24.0862(7)
90
Monoclinic
C2/c
15.1756(7)
21.2914(10)
14.5515(8)
90
Monoclinic
P2(1)/n
10.6022(2)
12.6707(3)
13.5818(3)
90
Monoclinic
P2(1)
9.209(2)
9.962(3)
9.314(2)
90
a
(°)
b (°)
100.656(2)
90
1128.89(8)
2
1.413
0.697
90
90
100.008(3)
90
4630.2(4)
4
1.464
0.693
111.3050(10)
90
1699.85(6)
4
1.525
0.910
91.670(12)
90
854.2(4)
2
1.517
0.906
c
(°)
V (Å3)
2271.86(10)
4
1.404
Z
dc (Mg/m3)
l
(mmꢀ1)
0.693
F(0 0 0)
504
1008
2112
808
404
Cryst. size
k (Å)
No. of rflns collected
Index rflns
0.28 ꢁ 0.25 ꢁ 0.21
0.38 ꢁ 0.28 ꢁ 0.22
0.36 ꢁ 0.29 ꢁ 0.22
0.38 ꢁ 0.33 ꢁ 0.28
0.33 ꢁ 0.25 ꢁ 0.18
0.71073
9030
4141 (Rint = 0.0399)
4141/1/237
1.053
0.0496, 0.1293
0.0614, 0.1340
0.956, ꢀ0.582
0.71073
0.71073
0.71073
0.71073
12,066
24,956
26,412
16,442
2718 (Rint = 0.0163)
2718/0/153
1.057
0.0267, 0.0780
0.0292, 0.0795
0.402, ꢀ0.290
5421 (Rint = 0.0405)
5421/0/302
1.031
0.0340, 0.0747
0.0467, 0.0782
0.272, ꢀ0.240
5990 (Rint = 0.0340)
5990/0/319
1.067
0.0411, 0.1000
0.0903, 0.1128
0.418, ꢀ0.389
3244 (Rint = 0.0196)
3244/0/237
1.051
0.0258, 0.0681
0.0293, 0.0701
0.308, ꢀ0.256
Data/restraints/params
Goodness-of-fit on F2
b
R1a, wR2 (I > 2
r(I))
b
R1a, wR2 (all data)
Largest diff peak, hole (e/Å3)
a
R1
wR2 = [
=
R
|F0| ꢀ |Fc|/
R|F0|.
b
2
R
[
x
(F0 ꢀ Fc2)2]/
R[x .
(F02)2]1/2
recorded in ppm relative to the residual protons and 13C of CDCl3 (d
7.24, 77.7). Mass spectra were recorded on Thermo Electron Corpo-
ration PolarisQ. Elemental analyses were performed on Heraeus
CHN-OS Rapid at the Instrument Center, NCHU. Melting points
(m.p.) were measured on a Fargo MP-2D melting point apparatus.
Cyclic voltammetry (CV) measurements were performed on a CHI
611A Electrochemical Analyzer with an Ag/AgCl reference elec-
trode, a Pt working electrode, and a Pt counterelectrode, the scan
rate at 50 mVsꢀ1, measured for 10ꢀ3 M solutions in 0.1 M tetrabu-
tylammonium hexafluorophosphate as electrolyte in 1:1 of CH2Cl2/
CH3CN. UV–vis spectra were recorded on a Beckman Coulter
DU730 spectrophotometer.
2.4. Single crystal X-ray structure determination
Good diffraction quality single crystals of 1d, 1e, 1g, 2f, and 2g,
obtained by recrystallization from layering hexane over CH2Cl2
solutions, were mounted on a Bruker SMART CCD diffractometer
equipped with graphite-monochromated Mo-K radiation (k =
a
0.71073 Å), respectively. Crystal data were collected at 150 K with
an Oxford Cryosystems Cryostream. No significant crystal decay
was found. Data were corrected for absorption empirically by
means of
w scans. All non-hydrogen atoms were refined with
anisotropic displacement parameters. For all the structures, the
hydrogen atom positions were calculated and they were con-
strained to idealized geometries and treated as riding where the
H atom displacement parameter was calculated from the equiva-
lent isotropic displacement parameter of the bound atom. Data
were processed using the CRYSALIS-CCD and -RED [31] programs.
The structures of the complexes were determined by direct
methods procedures in SHELXS [32a], and refined by full-matrix
least-squares methods, on F2’s, in SHELXL [32b]. All the relevant
crystallographic data and structure refinement parameters for 1d,
1e, 1g, 2f, and 2g are summarized in Table 1.
2.2 RNH2, Al2O3
N
N
R
R
O
Fe
Dean-Stark
apparatus
Fe
O
1
1.1 RNH2, Al2O3
Dean-Stark
apparatus
3. Results and discussion
3.1. Reaction scheme and NMR spectra of compounds 1a–g, 2f, and 2g
N
O
R
A
series of 1,10-ferrocenyldiimines [Fe{( 5-C5H4)AC(Me)@
g
Fe
NAR}2] with R = n-hexyl 1a, cyclohexyl 1b, phenyl 1c, 4-methoxy-
phenyl 1d, 3-methoxyphenyl 1e, 4-nitrophenyl 1f, and 3-nitro-
phenyl 1g, were prepared by reactions of about 1:2 M ratio of
1,10-diacetylferrocene and the corresponding amines, RNH2, in the
presence of freshly activated Al2O3 with or without MgSO4 in reflux-
ing toluene, equipped with a Dean–Stark apparatus (Scheme 1). This
method was successful in the syntheses of the 1,10-diimines, and no
large excesses of the amines were necessary [19]. The increasing
2f, 2g
Scheme 1. R = n-hexyl (a), cyclohexyl (b), phenyl (c), 4-methoxyphenyl (d), 3-
methoxyphenyl (e), 4-nitrophenyl (f), and 3-nitrophenyl (g).

T.-Y. Lee et al. / Journal of Molecular Structure 935 (2009) 102–109
105
order of the difficulties of preparing theses ferrocenyldiimines was
1a < 1c < 1d ꢂ 1e < 1b < 1g << 1f according to the reaction time
and yield. The electronic effect and the acid strength of the corre-
sponding amines might play important roles in the condensation
reaction, and therefore result in the very low yield (10%) of 1f. The
pKa value of 4-nitroaniline is 1.00 [33]. The high yield of 1a might
be attributed to the steric, the electronic effect, and the high pKa
value of n-hexylamine (10.56) [33]. The protons on the
g
5-C5H4
(Cp) rings of 1a–g appear as two sets of signals in the range of d
4.22–4.53 and d 4.52–4.80 ppm respectively in their NMR spectra.
Moreover, these signals were found to be downfield shifted as
R = aryl with respect to those of R = n-hexyl and cyclohexyl. While
substituents on the aryl moiety possess more electron-donating
character, the proton resonance on g
5-C5H4 rings is upfield shifted.
The 13C NMR resonance of the imino carbon also shows downfield
shifting as R = aryl, and further downfield shifting for compounds
with electron-withdrawing groups on the aryl rings. The shifts of
the NMR signals indicate that R groups possess noticeable electronic
influence on both ferrocenyl and imino moieties.
Under the similar condition, the reaction of about 1:1 M ratio of
1,10-diacetylferrocene with 4-nitroaniline and 3-nitroaniline
resulted in 2f and 2g, respectively. The good yield (50%) of 2f
implied that the first acetyl group in diacetylferrocene would more
readily experience the condensation reaction with 4-nitroaniline
than the second one in the sequential reaction, which produced
1f in low yield. On the other hand, the yields on preparing 1g
and 2g were similar. The protons on the Cp rings of 2f and 2g show
three signals in about 4.81, 4.55, and 4.50 ppm in their NMR spec-
tra. The 13C NMR resonance of the carbonyl (C@O) and the imino
(C@N) group in 2f and 2g exhibits at ca. 201 and 168 ppm, respec-
tively. Attempt to prepare other acetylferrocenylimines afforded a
mixture of 1,10-diacetylferrocene, the analogous 1,10-ferrocenyldii-
mines, as well as the desired products, and further purification was
not successful. The results are consistent with the facilitation of the
condensation reaction as the R groups with more electron-rich
character and higher pKa value.
Fig. 2. Molecular structure of 1e. The thermal ellipsoids are drawn at 50%
probability levels and the hydrogen atoms were omitted for clarity.
3.2. Crystal structures of compounds 1d, 1e, 1g, 2f, and 2g
Crystals of 1d, 1e, 1g, 2f, and 2g suitable for X-ray structure
analysis were obtained by recrystallization from layering hexane
over CH2Cl2 solutions of 1d, 1e, 1g, 2f, and 2g. Their molecular
structures are shown in Figs. 1–5. The selected bond lengths and
Fig. 3. Molecular structure of 1g. The thermal ellipsoids are drawn at 30%
probability levels and the hydrogen atoms were omitted for clarity.
angles are listed in Table 2. In view of the structures of the isomers
of 1d and 1e, the Cp rings are approximately parallel with the tilt
angles of 0° and 2.86°, respectively. Compound 1d exhibits stag-
gered Cp rings with an antiperiplanar arrangement of the two
arylimino substituents. On the other hand, the two substituted
Cp rings in 1e are anticlinal eclipsed [34]. Similarly, the Cp rings
of 1,10-diacetylferrocene also exhibit anticlinal eclipsed arrange-
ment [35]. The N-phenyl rings are almost perpendicular to their
attached Cp rings for 1e, the dihedral angles being 89.93° and
87.02°, however, those for 1d are less upright with the dihedral
angles of 79.58° and 79.58°. This arrangement renders little conju-
gation between the phenyl rings and the ferrocene motifs in the
Fig. 1. Molecular structure of 1d. The thermal ellipsoids are drawn at 50%
probability levels and the hydrogen atoms were omitted for clarity.

106
T.-Y. Lee et al. / Journal of Molecular Structure 935 (2009) 102–109
Table 2
Selected bond lengths (Å) and angles (°) for compounds 1d, 1e, 1g, 2f, and 2g.
1d
C(5)AC(6)
C(6)AN(1)
1.467(2)
C(8)AC(13)
C(8)AN(1)
C(14)AO(1)
1.3922(17)
1.4158(18)
1.4262(17)
1.2853(15)
1.3708(16)
1.5144(17)
116.66(11)
118.73(11)
116.77(11)
C(11)AO(1)
C(6)AC(7)
N(1)AC(6)AC(5)
C(5)AC(6)AC(7)
C(11)AO(1)AC(14)
N(1)AC(6)AC(7)
C(6)AN(1)AC(8)
124.60(13)
120.90(11)
1e
C(1)AC(6)
C(6)AC(7)
C(10)AO(1)
C(19)AC(20)
C(20)AC(21)
C(26)AO(2)
N(1)AC(6)AC(1)
C(6)AN(1)AC(8)
N(2)AC(20)AC(19)
C(20)AN(2)AC(22)
1.478(3)
1.492(3)
1.360(3)
1.475(3)
1.502(3)
1.357(3)
116.19(19)
121.71(18)
116.10(19)
121.92(19)
C(6)AN(1)
C(8)AN(1)
C(14)AO(1)
C(20)AN(2)
C(22)AN(2)
C(28)AO(2)
N(1)AC(6)AC(7)
C(6)AN(1)AC(8)
N(2)AC(20)AC(21)
C(26)AO(2)AC(28)
1.279(3)
1.422(3)
1.429(3)
1.282(3)
1.415(3)
1.431(3)
125.79(19)
121.71(18)
125.7(2)
117.07(18)
Fig. 4. Molecular structure of 2f. The thermal ellipsoids are drawn at 50%
probability levels and the hydrogen atoms were omitted for clarity.
1g
C(1)AC(3)
1.475(3)
C(3)AN(1)
1.277(2)
C(3)AC(4)
1.506(3)
C(3)AC(4)
1.506(3)
C(5)AN(1)
1.417(2)
C(14)AC(17)
1.477(3)
C(17)AN(3)
1.270(2)
C(19)AN(3)
1.425(2)
N(1)AC(3)AC(1)
C(1)AC(3)AC(4)
N(3)AC(17)AC(14)
C(14)AC(17)AC(18)
117.93(17)
116.56(18)
118.35(18)
116.42(18)
N(1)AC(3)AC(4)
C(3)AN(1)AC(5)
N(3)AC(17)AC(18)
C(17)AN(3)AC(19)
125.51(18)
119.08(16)
125.22(19)
119.79(17)
2f
C(1)AC(6)
C(8)AN(1)
C(14)AC(19)
C(1)AC(6)AN(1)
O(1)AN(2)AO(2)
C(14)AC(19)AO(3)
1.470(2)
1.407(19)
1.475(2)
117.48(13)
123.28(15)
120.03(16)
C(6)AN(1)
C(19)AO(3)
1.279(2)
1.220(2)
C(8)AN(1)AC(6)
C(11)AN(2)AO(1)
121.50(13)
118.40(16)
2g
C(1)AC(6)
C(8)AN(1)
C(14)AC(19)
C(1)AC(6)AN(1)
O(1)AN(2)AO(2)
C(14)AC(19)AO(3)
1.473(5)
1.417(4)
1.470(5)
116.8(4)
123.5(4)
120.3(4)
C(6)AN(1)
C(19)AO(3)
1.280(5)
1.226(5)
C(8)AN(1)AC(6)
C(12)AN(2)AO(1)
120.9(3)
118.6(3)
Fig. 5. Molecular structure of 2g. The thermal ellipsoids are drawn at 50%
probability levels and the hydrogen atoms were omitted for clarity.
solid state. Such conformation was also found in the structures of
the monoimino analogy, phenyliminoferrocene (Fc-Ph) and
[1-[(4-chlorophenyl)imino]ethyl]ferrocene (Fc-4-ClPh) [30,36]. In
comparison with the structures of 1,10-bis(N-aryl-formimidoyl)
ferrocenes (aryl = phenyl Fc-Ph2, and 2,6-diisopropylphenyl Fc-
(ipr-Ph)2), both N-aryl rings in Fc-(ipr-Ph)2) show the similar per-
pendicularity; only one N-aryl rings in Fc-Ph2 is perpendicular to
its Cp rings, but the other possesses a parallel conformation, which
relationship of the two Cp rings is fluxional and the energy barrier
for the interconversion of the different conformations between Cp
rings is low. It has also been reported that the energy difference
between the D5h and D5d conformations of ferrocene were
0.9 0.3 kcal/mol [38]. A theoretical calculation also demonstrated
that the energy difference between the eclipsed and staggered con-
figurations of the monosubstituted ferrocenes is in
a range
between ꢀ0.54 and 0.41 kcal/mol [39]. The angles between the
N-phenyl rings and their attached Cp rings for the two species in
1g are quite distinct, being 66.67° and 84.50°, respectively. The
conformation of Fe(1) species in 1g suggests that little conjugation
phenomenon between the phenyl rings and the ferrocene motifs
might occur. The Cp rings and the imino moieties are roughly
coplanar, as the dihedral angle C(2)AC(1)AC(3)AN(1) ca. 11.68°
and C(15)AC(14)AC(17)AN(3) ca. 10.95°.
The X-ray structure of 2f revealed that the two Cp rings are paral-
lel (the tilt angle ca. 1.61°) and synclinal staggered (the twist angle
ca. 30.5°) to each other (Fig. 4). The N-phenyl ring is nearly perpen-
dicular to its attached Cp ring at the angle ca. 84.33°. The carbonyl
group are almost coplanar with its attached Cp ring (the dihedral an-
gle C(15)AC(14)AC(19)AO(3) ca. 13.54°) and so is the imino group
(the dihedral angle C(5)AC(1)AC(6)AN(1) ca. 7.81°). The similar
geometry was observed in 2g, of which the tilt angle of the Cp rings
isca. 3.24°, thetwistangle(synclinalstaggered)is ca. 23–24°, andthe
anglebetweentheN-phenylringanditsattachedCpringis ca. 74.67°
(Fig. 5). The dihedral angles C(18)AC(14)AC(19)AO(3) being ca.
may result from a CAH. . .arene–p-interaction [37]. Moreover, the
configuration of the two Cp rings in Fc-(ipr-Ph)2 is anticlinal stag-
gered, and that in Fc-Ph2 is synperiplanar eclipsed. The C@N bond
length of the imino groups in 1d and 1e is ca. 1.28 Å, slightly longer
than that in Fc-Ph (1.269(5) Å), and Fc-4-ClPh (1.263(6) Å), and Fc-
(ipr-Ph)2 (1.273(2) Å), which can be attributed to the difference of
the substituents on the phenyl moiety. The imino functional
groups in 1d are oriented almost coplanar with their attached Cp
rings with a dihedral angle C(4A)AC(5)AC(6)AN(1) ca. 22.53°
(Fig. 1). The similar configuration was observed in 1e, of which
the dihedral angles C(2)AC(1)AC(6)AN(1) and C(18)AC(19)A
(20)AN(1) are ca. 22.37° and ca. 10.89°, respectively (Fig. 2).
In the case of 3-nitro derivative, 1g, two independent molecules
are present in the unit cell (Fig. 3). The Cp rings in both species are
planar and nearly parallel as the tilt angles in the range between
2.71° and 4.79°. Moreover, the first species with Fe(1) encloses
synclinal eclipsed iminic Cp rings, and the second species contains
anticlinal eclipsed substituted Cp rings. The results imply that the

T.-Y. Lee et al. / Journal of Molecular Structure 935 (2009) 102–109
107
ꢀ2.31° and C(2)AC(1)AC(6)AN(1) being ca. ꢀ18.34° illustrated the
coplanar relation between the carbonyl group and the imino group
and their attached Cp rings, respectively.
interaction between the two groups. Although the Cp rings in 2f
and 2g are staggered, the twist angles (23–30°) in both compounds
are relatively smaller than the typical one, such as ca. 36° in 1d.
With focus on the relative orientations of the bis(substituted)
motifs in 1d, 1e, 1g, 2f, and 2g, the conformational features are
various by projecting down the Cp(centroid)AFeACp(centroid)
axis, sketched in Fig. 6 [40]. For the 1,10-ferrocenyldiimines, the
imino groups are closer to each other in 1g, further in 1e, and anti
to each other in 1d. The 3-methoxy and 3-nitro analogues favor an
eclipsed configuration, but the 4-methoxy derivative retains a
staggered form. The antiperiplanar and anticlinal conformations
of the two imino moieties are observed in 1d, 1e, and 1f. Therefore,
the geometry of 1e and 1f is similar to that of 1,10-diacetylferro-
cene [35]. However, the carbonyl and imino groups in 2f and 2g
exhibit only the synclinal conformation, which might imply some
The CAC bond lengths within the g
5-Cp rings of the compound
1d, 1e, 1f, 2f, and 2g are in a classic range between 1.400(4) Å
and 1.442(3) Å as well as the FeAC(Cp) bond distances of the five
compounds are in
a typical range between 2.027(2) Å and
2.063(5) Å. The results indicate that all the compound 1d, 1e, 1f,
2f, and 2g possess the typical ferrocene structure [41].
3.3. Electrochemical and optical properties of compounds 1a–g, 2f,
and 2g
All compounds 1 and 2 exhibit a typical reversible one-electron
redox process attributed to a ferrocene/ferrocenium couple in their
Fig. 6. Conformation of the cyclopentadienyl rings of the ferrocene motifs in each of (a) 1d, (b) 1e, (c, d) 1g, (e) 2f, and (f) 2g. The diagrams illustrate the relative dispositions
of the five-membered rings as well as the orientations of the imino moieties in the five compounds.

108
T.-Y. Lee et al. / Journal of Molecular Structure 935 (2009) 102–109
Table 3
Table 4
Measured E1/2 values and the Hammett constants (
2g.
r
p) for compounds 1a–g, 2f, and
The electronic transition bands in the UV/vis spectra of compounds 1a–g, 2f, 2g, and
1aox–1gox in CH2Cl2.
a
Compound
E1/2 (V)
Substituent (R)
rp
Compound
kmax/nm (
e
max/molꢀ1 dm3 cmꢀ1
268 (21,100)
)
b
1a
1b
1c
1d
1e
1f
1g
2f
0.640
0.597
0.718
0.674
0.690
0.864
0.819
0.831
0.818
0.429
0.897
(CH2)5CH3
Cyclo-C6H11
C6H5
C6H4A4-OMe
C6H4A3-OMe
C6H4A4-NO2
C6H4A3-NO2
–
1a
1b
1c
1d
1e
1f
1g
232 (48,950)
232 (40,500)
232 (30,500)
235 (41,500)
232 (27,050)
232 (57,700)
235 (23,600)
235 (13,150)
234 (13,150)
238 (9700)
328 (4100)
338 (3050)
330 (5800)
328 (11,200)
322 (5150)
330 (45,400)
333 (4000)
364 (28,350)
364 (28,050)
361(19,300)
363 (25,500)
362 (20,450)
349 (27,200)
358 (13,250)
444 (1100)
459 (700)
446 (850)
452 (1200)
450 (650)
448 (2750)
446 (400)
505 (2000)
508 (1900)
503 (2150)
503 (3050)
500 (2550)
496 (2100)
499 (2000)
ꢀ0.15
ꢀ0.01
ꢀ0.08
268 (19,150)
270 (14,600)
269 (20,900)
274 (12,000)
280 (24,300)
273 (12,940)
294 (48,800)
290 (48,000)
294 (25,350)
293 (35,600)
293 (29,650)
299 (19,450)
286 (22,250)
b
ꢀ
0.26
0.23
1aox
1box
1cox
1dox
1eox
1fox
1gox
2g
Ferrocene
Diacetylferrocene
234 (16,400)
234 (16,150)
233 (22,200)
236 (29,650)
a
Ref. [45].
Not found.
b
cyclic voltammograms [42]. The data of their redox potentials are
listed in Table 3. The results show the order of their redox potentials
as the following: ferrocene < 1b < 1a < 1d < 1e < 1c < 2g < 1g < 2f <
1f < diacetylferrocene (Table 3). Generally, the aryl derivatives of
the ferrocenyldiimines possess higher E1/2 values than the alkyl
derivatives. For the aryl-substituted ferrocenyldiimines, the species
with the electron-withdrawing group (NO2) on the phenyl rings
have much larger redox potentials than those with the electron-
donating group (OMe), indicating the electron-rich group facilitates
the oxidation of the systems. The redox potential of compounds 1 is
higher than that of ferrocene, but lower than that of diacetylferro-
cene, which illustrate that the electron-attracting ability of the imi-
no group is stronger than hydrogen (H), but weaker than acetyl
(C(O)Me) group. The correlation of the ferrocene/ferrocenium redox
potential with Hammett constants has been used as a probe in study
of the electronic properties of the substituents in ferrocenyl units
[43]. Our results show that the redox potentials of the ferrocenyldii-
transfer transitions showing at 200, 240, along with 265 nm [45–
47]. The d–d transition bands were found to be substituent depen-
dent [48,49]. Thedata of theUV/visabsorptionspectra of compounds
1a–g, 2f, and 2g are listed in Table 4. The bands (ca. 450 nm) are red-
shifted in the order of 1a < 1c ꢂ 1g < 2g < 1f < 1e < 1d < 2f < 1b;
however, the difference of the shifts are small. Only little change is
observed at the bands (ca. 330 nm) as well. The results might reflect
the substitution effect on the HOMO and LUMO energy levels as well
as the consequential HOMO–LUMO gaps of ferrocenyldiimines and
acetylferrocenylimines[39,50]. Itisworthytomeasuretheelectronic
spectraof theferrocenium ions dueto their importance tothestudies
of electronic structure [51], biosensors [52], and antitumor activity
[53]. The ferrocene derivatives could be oxidized by I2 to form triio-
dide salts [51]. Our premier study showed that the optical absorption
bands of the ferrocenium species 1aox, which resulted from the reac-
tionof 1a andappropriateamountof I2 were red-shifted compared to
those of 1a (Table 4 and Fig. 8). Similar phenomena were observed in
2box–2gox, the corresponding ferrocenium derivatives of 2b–2g, and
their data of the UV/vis absorption spectra were listed in Table 4. The
mines linearly correlate with the Hammett
stituents R (Table 3 and Fig. 7) [44]: E1/2 = 0.575
Although the p value of n-hexyl group is not available, the pKa value
r
p constants of the sub-
rp + 0.706, r = 0.984.
position and theintensity of thebands at ca. 500 and 360 nm in 2box
–
r
2gox are almost the same, and therefore are assigned as essentially
metal d–d transition [45,51]. No absorption bands were observed
at lower-energy area (>600 nm) even at high concentration of the
compounds. According to the UV–vis spectroscopy, the electronic
structures of the ferrocenyldiimines are more complicated than
those of the typical ferrocene derivatives.
(10.66)of cyclohexylaminehigher thanthatof n-hexylamine(10.56)
[33] is consistent with the lower E1/2 value of 1b. The correlation
indicates that the influence of the R groups onto the ferrocene cen-
ters is efficient. The E1/2 value of 1f is higher than that of 2f and 2g,
which suggests that the electronic effect of the 4-nitro group plays
a significant role during the oxidation process and leads to the pre-
dicament of oxidation.
Optical spectroscopy is a power tool for studying the electronic
structure of ferrocene. Basically, two kinds of electronic transition
bands are observed: the Laporte-forbidden d–d character of ligand
field transitions appearing at 325 nm (arising from the unresolved
1A1g ? a1E1g and 1A1g ? 1E2g transitions) along with 440 nm (arising
from the 1A1g ? b1E1g transition), and the intramolecular charge
4. Conclusion
We demonstrated that Schiff-base condensation reaction of
1:2 M ratio of 1,10-diacetylferrocene and the analogous amines in
1
0.8
0.6
0.4
0.2
0
-0.2
-0.1
0
0.1
0.2
0.3
Fig. 8. UV–vis spectra of 1a (pink,
ferrocenium (blue,
), 1aox (red,
), ferrocene (black, —), and
) for 2.5 x 10ꢀ4 M solutions in CH2Cl2. (For interpretation of
σp
Fig. 7. Correlation between E1/2 and the Hammett constants (
the references to color in this figure legend, the reader is referred to the web version
of this paper.)
rp’s).

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Acknowledgements
We thank the National Science Council of Taiwan for financial
support and the National Center for High Performance Computing
for databank search. J.H.H. also thank the National Changhua Uni-
versity of Education for the purchase of the CAD-4 diffractometer
system.
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