Synthesis and characterization of novel charge transfer complexes formed by N,N′-bis(ferrocenylmethylidene)-p-phenylenediamine and N-(ferrocenylmethylidene)aniline

Article abstract of DOI:10.1016/S0022-328X(98)00983-8

N,N′-Bis(ferrocenylmethylidene)-p-phenylenediamine 1 and N-(ferrocenylmethylidene) aniline 2 are readily synthesized by Schiff base condensation of appropriate units. Iodine (I₂), 2,3-dichloro-5,6-dicyano-1,4-benzoquinone (DDQ), tetrachloro-1,4-benzoquinone (CA), tetracyanoethylene (TCNE) and 7,7,8,8-tetracyanoquinodimethane (TCNQ) form charge transfer complexes with 1 and 2. IR spectroscopy suggests an increase in the amount of charge transferred from the ferrocenyl ring to the oxidant in the order, I₂a function of temperature. The larger Fe(II)/Fe(III) ratio at lower temperatures is best explained by a retro charge transfer from the iodide to the iron(III) metal center. There is negligible solvent effect on the formation of the iodine oxidized charge transfer complex of 1.

Full text of DOI:10.1016/S0022-328X(98)00983-8

Journal of Organometallic Chemistry 575 (1999) 108–118
Synthesis and characterization of novel charge transfer complexes
formed by N,N%-bis(ferrocenylmethylidene)-p-phenylenediamine and
N-(ferrocenylmethylidene)aniline
Sushanta K. Pal a, K. Alagesan a, Ashoka G. Samuelson a,*, J. Pebler b
a Department of Inorganic and Physical Chemistry, Indian Institute of Science, Bangalore 560 012, India
b Fachbereich Chemie der Philipps-Uni6ersita¨t Marburg, Hans-Meerwein-Straße, D-35032 Marburg, Germany
Received 7 September 1998
Abstract
N,N%-Bis(ferrocenylmethylidene)-p-phenylenediamine 1 and N-(ferrocenylmethylidene) aniline 2 are readily synthesized by
Schiff base condensation of appropriate units. Iodine (I2), 2,3-dichloro-5,6-dicyano-1,4-benzoquinone (DDQ), tetrachloro-1,4-ben-
zoquinone (CA), tetracyanoethylene (TCNE) and 7,7,8,8-tetracyanoquinodimethane (TCNQ) form charge transfer complexes with
1 and 2. IR spectroscopy suggests an increase in the amount of charge transferred from the ferrocenyl ring to the oxidant in the
order, I2BCABTCNQBTCNE:DDQ. EPR spectra of the oxidized binuclear complexes are indicative of localized species
containing iron- and carbon-centered radicals. The Mo¨ssbauer spectrum of the iodine oxidized complex of 1 reveals the presence
of both Fe(III) and Fe(II) centers. Variable temperature magnetic and Mo¨ssbauer studies show that the ratio of Fe(III)/Fe(II)
centers varies as a function of temperature. The larger Fe(II)/Fe(III) ratio at lower temperatures is best explained by a retro
charge transfer from the iodide to the iron(III) metal center. There is negligible solvent effect on the formation of the iodine
oxidized charge transfer complex of 1. © 1999 Elsevier Science S.A. All rights reserved.
Keywords: Ferrocene; CT complexes; Cyclic voltammetry; IR; UV–vis; EPR; Mo¨ssbauer spectroscopy
1. Introduction
shown that changing the counter anion in mixed-va-
lence biferrocenium salts leads to dramatic changes in
the electron transfer rates in the solid state [13,14].
Recently the effects of acetyl and napthylimine substi-
tutents on the cyclopentadienyl ring and variation of
counter anions I3−, CuI2− and FeCl4− on the valence
state of iron atoms in the biferrocenium derivatives
have been reported by Wei et al. [15]. The charge
transfer interactions between donor and acceptor moi-
eties have been studied in a series of organometallic
complexes derived from organometallic donors and var-
ious kinds of acceptors [16–18]. In all these studies, it
has been found that small changes in the donor or
acceptor, make a significant impact on the electronic
properties of the complex.
Charge-transfer (CT) complexes of ferrocene and
substituted ferrocenes have attracted considerable inter-
est in recent years [1–3]. Potential application of these
complexes in superconductivity [4], nonlinear optics [5]
and cooperative magnetics [6] have spurred research in
this area. In this context, CT complexes of bis(ferro-
cenyl) complexes should be even more interesting. A
number of symmetrical and unsymmetrical binuclear
ferrocenes have been investigated [7–12]. The electronic
properties of the oxidized binuclear ferrocenes depend
on a number of factors including the substitution of the
cyclopentadienyl ring and the counter ions. It has been
The present study was aimed at clarifying the effect
of varying the acceptors used to form CT complexes of
* Corresponding author. Tel.: +91-80-3092663; fax: +91-80-
3341683; e-mail: ashoka@ipc.iisc.ernet.in.
0022-328X/99/$ - see front matter © 1999 Elsevier Science S.A. All rights reserved.
PII: S0022-328X(98)00983-8

S.K. Pal et al. / Journal of Organometallic Chemistry 575 (1999) 108–118
109
N,N% - bis(ferrocenylmethylidene) - p - phenylenediamine
2.2. Synthesis of
and N-(ferrocenylmethylidene)aniline. Significant dif-
ferences were observed in the solid complexes formed
by I2, DDQ, TCNE, TCNQ and CA. An unique
polyiodide complex is formed by 1. The valence state of
iron in I2 oxidized compound was studied by Mo¨ss-
bauer spectroscopy, IR and magnetic susceptibility
measurements in the solid state. A retro charge transfer
in the solid state at low temperatures is proposed, to
explain the observed changes in the Mo¨ssbauer and
magnetic susceptibility data.
N,N%-bis( ferrocenylmethylidene)-p-phenylenediamine
Fc–CHꢀN–C6H4–NꢀCH–Fc (1)
A mixture of ferrocenecarboxaldehyde (2.16 g, 10
mmol) and 1,4-phenylenediamine (0.54 g, 5 mmol), was
dissolved in 100 ml dry toluene and refluxed under
nitrogen atmosphere with azeotropic removal of water
using a Dean–Stark apparatus. After 18 h, the reaction
mixture was filtered hot. On cooling a red shiny flaky
precipitate was obtained. It was washed with ethanol
(3×10 ml) and 30 ml of acetonitrile. m.p. 238–240°C.
Yield: 93%. [Found: C, 66.96; H, 4.78; N, 5.33.
C28H24Fe2N2 1 requires C, (67.23); H, (4.83); N,
(5.60%)]. IR (KBr, cm−1): 1613 (CHꢀN str.), 811 (per-
2. Experimental details
1
pendicular C–H bend of Cp). H-NMR (CDCl3): 4.26
(s, 10H, C5H5), 4.52 (t, 4H, C5H4), 4.83 (t, 4H, C5H4),
7.30 (s, 4H, C6H4), 8.39 (s, 2H, CHꢀN). 13C-NMR
(CHCl3): 69.04 (Cp, s), 68.78 (Cp, h), 71.07 (Cp, i),
121.20 (phenyl, 4C), 150.00 (phenyl, 2C para), 160.83
(CHꢀN).
2.1. Physical measurements
C, H and N analysis of the complexes were obtained
from Heraeus Carlo Erba 1168, CDRI, Lucknow and
Carlo Erba Strumentazione elemental analyzer—model
1106. Iron percentages of the samples were obtained
from ARL, 3410 ICP with minitorch. Iron percentages
were also estimated by the standard gravimetric
method. The IR spectra of the complexes was recorded
either using a Bio Rad FT IR, FTS-7 or a Perkin Elmer
781 double beam spectrometer in the range 400–4000
cm−1 in KBr disc or nujol mull. UV–vis spectra of the
complexes were recorded on a Hitachi U-3400 spec-
trophotometer. The electron spin resonance (ESR)
spectra of the complexes were recorded in a X-band
Varian E10G line Century series spectrophotometer.
1H- and 13C-NMR spectra of the complexes were
recorded using a Bruker ACF 200 FT NMR spectrom-
eter and Jeol FX-90Q spectrometer, respectively. 57Fe
Mo¨ssbauer spectra were recorded with a constant accel-
eration spectrometer using 25 mCi 57Co source in Pd
(Amersham, UK) with respect to which isomer shifts
(IS) are reported. The spectra were computer-fitted
using a general fitting procedure to obtain the best fit to
the experimental data. Cyclic voltammograms of the
complexes 1 and 2 were recorded in EG&G PAR model
174A polarographic analyzer combined with a standard
three electrodes configuration. A platinum or a glassy
carbon electrode was used as the working electrode and
the counter electrode was a platinum wire. A saturated
Calomel electrode was used as a reference electrode. In
all electrochemical measurements 0.1 M Bu4NClO4
(TBAP) was used as a supporting electrolyte and the
redox potentials of the complexes were measured
against saturated calomel electrode. EDAX analysis of
the complexes were obtained using Cambridge
Stereoscan S-360. Magnetic susceptibility measurements
were obtained using George Associates Series 300,
Lewis coil Tm Force magnetometer.
2.3. Synthesis of N-( ferrocenylmethylidene)aniline
Fc–CHꢀN–C6H5 (2)
A mixture of ferrocenecarboxaldehyde, (1.07 g, 5
mmol), aniline (0.57 ml, 6 mmol) and five to ten drops
of dry pyridine in 100 ml of dry benzene was refluxed
under nitrogen atmosphere with azeotropic removal of
water using a Dean–Stark apparatus. After 18 h, the
reaction mixture was filtered and cooled. Following the
removal of benzene under reduced pressure, the excess
of aniline was removed under reduced pressure at a
temperature of 100–120°C. After drying the product in
vacuum for about 3 h, the crude product was extracted
using n-pentane in a Soxhlet apparatus. m.p. 70–71°C.
Yield: 77%. [Found: C, 70.61; H, 5.22; N, 4.84.
C17H15NFe 2 requires C, (70.55), H, (5.19); N, (4.43%)].
IR (KBr, cm−1): 1619 (CHꢀN str.), 818 (perpendicular
C–H bend of Cp). 1H-NMR (CDCl3): 4.15 (s, 5H,
C5H5), 4.35 (t, 2H, C5H4), 4.65 (t, 2H, C5H4), 6.85–7.40
(m, 5H, C6H5), 8.20 (s, 1H, CHꢀN). 13C-NMR
(CHCl3): 68.99 (Cp, s), 68.75 (2,2% Cp), 71.00 (3,3% Cp),
81.10 (1, Cp), 152.57 (h C6H5), 124.90 (i C6H5), 128.81
(k C6H5), 120.34 (l C6H5), 161.03 (CHꢀN).
2.4. Preparation of partially oxidized complexes
The Schiff base N,N%-bis(ferrocenylmethylidene)-p-
phenylenediamine 1 and the model compound N-(ferro-
cenylmethylidene)aniline 2 have been oxidized by iodine
in benzene, chloroform and ethanol to give 3a–c and
4a–c, respectively, to study the effect of solvent polar-
ity on the preparation of partially oxidized compounds.

110
S.K. Pal et al. / Journal of Organometallic Chemistry 575 (1999) 108–118
Table 1
2.4.1. Synthesis of [Fc–CHꢀN–C6H4–NꢀCH–Fc]·I4.5
(3a–c)
Electrochemical data for Fc–CHꢀN–C6H4–NꢀCH–Fc (1) and Fc–
CHꢀN–C6H5 (2) in propylene carbonate with glassy carbon and Pt as
working electrodea
A solution of iodine (1.52 g, 6 mmol) in 125 ml of
benzene was added to the vigorously stirred suspension
of compound 1 (2.00 g, 4 mmol) in 150 ml of dry
benzene at room temperature (r.t.). The reaction was
taken to completion by stirring the mixture for 8 h. A
small portion of precipitated iodide (0.44 g) was washed
with 2×30 ml portions of chloroform. The solvent was
removed under reduced pressure at a bath temperature
of 50°C. This yields the major portion of the black
polycrystalline iodide (2.00 g) which was washed with
chloroform and then dried in vacuum for about 3 h.
The combined yield is 84% based on the amount of
iodine used. [Found: C, 32.83; H, 2.34; N, 3.03;
C28H24Fe2N2·I4.5 (3a) requires C, (31.39); H, (2.25); N,
(2.61%)]; similarly the complexes [Fc–CHꢀN–C6H4–
NꢀCH–Fc]·I4.5 (3b) and [Fc–CHꢀN–C6H4–NꢀCH–
Fc]. I4.5 (3c) were obtained by employing chloroform
and ethanol as reaction mediums, respectively. Their
elemental analytical data are as follows: [Found: C,
31.33; H, 2.21; N, 3.35; Fe, 10.33; I/Fe, 2.28 (EDAX).
C28H24Fe2N2·I4.5 (3b) requires C, (31.39); H, (2.25); N,
(2.61); Fe, (10.42); I/Fe, (2.25%)]. [Found: C, 32.08; H,
2.43; N, 2.78; Fe, 10.65; I/2Fe, 2.35 (EDAX).
C28H24Fe2N2·I4.5 (3c) requires C, (31.39); H, (2.25); N,
(2.61), Fe, (10.42); I/Fe, (2.25%)].
Complex
E1/2 (V)
Glassy carbon
Pt
1
2
0.55
0.56
0.54
0.54
0.82
0.92b
a All E1/2 values are relative to SCE.
b From differential pulse polarography.
period of 3 h. Then the reaction mixture was stirred for
8 h at r.t. and filtered. The filtrate was concentrated and
this gave 1.80 g of a green product, which was washed
with chloroform to remove unreacted starting material.
The yield obtained was 95% based on the amount of
TCNQ used. [Found: C, 67.41; H, 3.68; N, 15.76.
C52H32Fe2N10 (5) requires C, (68.26); H, (4.01); N,
(15.86%)].
2.4.4. Synthesis of [Fc–CHꢀN–C6H5]·(TCNQ) (6)
N-(Ferrocenylmethylidene)aniline (1.45 g, 5 mmol)
was dissolved in 50 ml of acetonitrile. To the above
solution, TCNQ (1.02 g, 5 mmol) in 50 ml of acetoni-
trile was added dropwise over a period of 0.5 h. Then
the resulting reaction mixture was stirred and heated to
80°C for 8 h. and then cooled and filtered. The precip-
itate was washed with chloroform, ether and dried. This
2.4.2. Synthesis of [Fc–CHꢀN–C6H5]·I3.5 (4a) and
[Fc–CHꢀN–C6H5]·I3 (4b,c)
A solution of iodine (1.90 g, 7.5 mmol) in 100 ml of
dry benzene was added slowly to the vigorously stirred
solution of 2 (1.45 g, 5 mmol) in 50 ml of dry benzene
over a period of 1.5 h and the reaction mixture was
stirred at r.t. for 9 h. The reaction mixture was filtered
and the solvent was removed under reduced pressure at
a bath temperature of 45°C, which afforded the black
precipitate. The crude product was washed with n-pen-
tane (4×25 ml) and dried in vacuum for 3 h. m.p.
112–115°C. Yield: 73% (decomposition). [Found: C,
27.38; H, 2.18; N, 2.03. C17H15NFe·I3.5 (4a) requires C,
(27.48); H, (2.06); N, (1.91%)]. Similarly the complexes
[Fc–CHꢀN–C6H5]·I3 (4b) and [Fc–CHꢀN–C6H5]·I3
(4c) were obtained by using chloroform and ethanol as
reaction mediums, respectively, and their elemental an-
alytical data are given below. [Found: C, 30.76; H, 2.39;
N, 2.14. C17H15NFe·I3 (4b) requires C, (30.48); H,
(2.25); N, (2.09). (Found: C, 31.45; H, 2.37; N, 1.98; Fe,
8.40; I/Fe,3.01% (EDAX). C17H15NFe·I3 (4c) requires
C, (30.48); H, (2.25); N, (2.09) Fe (8.33) I/Fe (3.0%)].
2.4.3. Synthesis of [Fc–CHꢀN–C6H4–NꢀCH–Fc]·
(TCNQ)2 (5)
Compound 1 (2.00 g, 4 mmol) was dissolved in 120
ml of chloroform. TCNQ (0.82 g, 4 mmol) in 100 ml of
THF was added to the above solution dropwise over a
Fig. 1. Cyclic voltammogram and differential pulse voltammogram
(inset) of Schiff base complex 1. Working electrode: Pt; solvent:
propylene carbonate.

S.K. Pal et al. / Journal of Organometallic Chemistry 575 (1999) 108–118
111
Table 3
UV–vis data for the Schiff base complexes (1–12)
Complex Solvent umax (log m)
1
CHCl3
272 (4.50), 350 (4.57), 460 (3.97)
2
CH3CN 281 (4.14), 309 (4.11), 448 (3.34)
3a–c
4a–c
5
CH3CN 248 (4.49), 291 (4.83), 363 (4.69), 510 (3.99)
CH3CN 243 (4.19), 292 (4.48), 355 (4.32), 525 (3.42)
CH3CN 259 (4.38), 393 (4.99), 476 (4.25)
6
CH3CN 374 (4.24), 393 (4.31), 470 (3.93)
7
8
CH3CN 254 (4.30), 347 (4.13), 417 (3.86), 582 (3.56)
CH3CN 253 (4.16), 349 (4.02), 565 (3.13)
9
CH3CN 302 (4.38), 378 (4.48), 479 (4.07)
10
11
12
CH3CN 261 (4.26), 367 (4.16), 434 (4.20), 565 (3.52)
CH3CN 312 (4.60), 358 (4.61), 393 (4.44), 570 (4.10)
CH3CN 273 (4.39), 351 (4.26), 435 (4.05), 560 (3.70)
continued at 60°C for about 8 h. The final reaction
mixture was filtered and a black precipitate was ob-
tained. It was washed with chloroform, ether and dried.
Yield: 47% (0.89 g). The stochiometry of the compound
obtained was independent of the ratio of 1 and DDQ.
[Found for 7: C, 54.34; H, 2.34; N, 8.51; Fe, 10.64.
C44H24Cl4Fe2N6O4 requires C, (55.38); H, (2.53); N,
(8.81); Fe, (11.71%)].
Fig. 2. Cyclic voltammogram and differential pulse voltammogram
(inset) of Schiff base complex 2. Working electrode: Pt; solvent:
propylene carbonate.
2.4.6. Synthesis of [Fc–CHꢀN–C6H5]·(DDQ)2 (8)
N-(Ferrocenylmethylidene)aniline (1.45 g, 5 mmol) in
75 ml of chloroform was added dropwise to a stirred
suspension of DDQ (1.34 g, 5 mmol) in 100 ml of
chloroform. Stirring was continued for 8 h at r.t. and
filtered. The black precipitate so obtained was washed
with chloroform, ether and dried. The yield obtained
was 79% (1.63 g) based on the amount of the DDQ
used. [Found for 8: C, 54.72; H, 2.13; N, 8.4; Fe, 6.52.
C33H15Cl4FeN5O4 requires C, (53.33); H, (2.03); N,
(9.42); Fe, (7.51%)].
yields a minor portion of the product (0.30 g). The
filtrate was concentrated under reduced pressure and
the solid product was washed with chloroform, ether
and dried. This yields the major portion of the product
(1.20 g). The total yield obtained was 76%. [Found: C,
69.29; H, 4.16; N, 13.72. C29H19FeN5 (6) requires C,
(70.60); H, (3.88); N, (14.19%)].
2.4.5. Synthesis of [Fc–CHꢀN–C6H4–NꢀCH–Fc]·
(DDQ)2 (7)
DDQ (0.91 g, 4 mmol) was dissolved in 100 ml of
chloroform at 50°C. To the hot solution, compound 1
(1.00 g, 2 mmol) in 100 ml of chloroform was added
dropwise over a period of 1 h and the stirring was
2.4.7. Synthesis of [Fc–CHꢀN–C6H4–NꢀCH–Fc]·
(TCNE)2 (9)
Tetracyanoethylene (0.0317 g, 0.25 mmol) dissolved
in 30 ml benzene was added dropwise over a 0.5 h
period to a stirred solution of compound 1 (0.060 g,
0.119 mmol) in benzene (15 ml). Stirring was continued
at r.t. for 8 h. The black precipitate obtained during the
reaction was filtered, washed with benzene and finally
dried. The yield obtained was 26.3% (0.025 g) [Found
for 9: C, 63.73; H, 3.45; N, 17.95. C40H24N10Fe2 re-
quires C, (63.51); H, (3.19); N, (18.52%)]
Table 2
Electrochemical data for the complex 1, 2 and different oxidants in
different solvents
Complex
E1/2 (V)
CH3CN
CH2Cl2
DMF
a
1
2
0.61
0.61
0.62
0.58
0.54
0.47
0.33
0.79
0.76
0.65
0.59
0.16
0.31
0.04
2.4.8. Synthesis of [Fc–CHꢀN–C6H5]·(TCNE) (10)
Compound 1 (50 mg, 0.1730 mmol) was dissolved in
20 ml of benzene. Then TCNE (0.022 g, 0.1733 mmol)
in 30 ml benzene was added dropwise to the stirred
solution of compound 1. After complete addition stir-
ring was continued at 55–60°C for 10 h. The dark red
solid formed was filtered, washed with benzene and
0.54
0.61
0.51
0.24
0.18
0.03
I2/I−3
DDQ/DDQ−
’
TCNE/TCNE
TCNQ/TCNQ−
CA/CA−
’
a Insoluble in CH3CN.

112
S.K. Pal et al. / Journal of Organometallic Chemistry 575 (1999) 108–118
ether and finally air dried. Yield: 22.2% (0.016 g).
[Found for 10: C, 66.42; H, 3.69; N, 16.09. C23H15N5Fe
requires C, (66.20); H, (3.62); N, (16.78%)].
plexes with CA, [Fc–CHꢀN–C6H4–NꢀCH–Fc]·(CA)2
11 and [Fc–CHꢀN–C6H5]·(CA)2 12 were synthesized
by mixing appropriate solutions of 1 and 2 with the
acceptor, CA.
2.4.9. Synthesis of [Fc–CHꢀN–C6H4–NꢀCH–Fc]·
(CA)2 (11)
3.1. Cyclic 6oltammetry
Compound 1 (60 mg, 0.119 mmol) was dissolved in
20 ml chloroform. CA (0.0589 g, 0.239 mmol) in 20 ml
chloroform was added dropwise to the stirred solution
of compound 1. Stirring was continued at r.t. for 8 h.
The color of the solution changes to dark red. The
reaction mixture was concentrated under reduced pres-
sure, and the dark red solid which precipitated was
filtered, washed with benzene and finally dried. Yield:
26.3% (0.031 g). [Found for 11: C, 47.77; H, 2.92; N,
2.89. C40H24N2Fe2O4Cl8 requires C, (48.43); H, (2.43);
N, (2.82%)].
The cyclic voltammetry of 1, and 2 were studied
using glassy carbon or Pt working electrodes in various
solvents. The cyclic voltammogram recorded in propy-
lene carbonate using a glassy carbon working electrode
was quasi-reversible. The electrochemical data of com-
plexes 1 and 2 are given in Table 1. Surprisingly, when
Pt disk is used as the working electrode, there are two
distinct electrochemical events for complex 1. Figs. 1
and 2 show the cyclic voltammogram and differential
pulse voltammogram of the Schiff base complexes 1
and 2, respectively using a Pt working electrode and
propylene carbonate as the solvent. The half-wave po-
tentials of the first process is 0.54 V and the second
anodic peak appears at 0.82 V at a scan rate of 100 mV
2.4.10. Synthesis of [Fc–CHꢀN–C6H5]·(CA)2 (12)
Tetrachloro-1,4-benzoquinone (CA) (0.085 g, 0.346
mmol) in 30 ml chloroform was added dropwise to the
stirred solution of compound 2 (0.050 g, 0.173 mmol)
dissolved in 30 ml chloroform. After complete addition
the reaction mixture was kept stirred in a water bath
maintained at 55–60°C. After 9 h, the solution was
concentrated, the black colored solid formed was
filtered and washed with chloroform, ether and finally
dried under vacuum. Yield: 21.5% (0.029 g). [Found for
12: C, 45.36; H, 2.11; N, 1.98. C29H15NFeO4Cl8 re-
quires C, (44.60); H, (1.93); N, (1.79%)].
s
−1. While the first process is quasi reversible, the
second step is irreversible. The separation in the anodic
peaks of these two steps is 225 mV. It is tempting to
assign these two electrochemical events to the oxidation
of the Fe(II)–Fe(II) complex to the mixed-valence
Fe(II)–Fe(III) complex, and, at a more positive poten-
tial, oxidation to the Fe(III)–Fe(III) complex. Consid-
ering the fact that complex 1, has two well separated Fe
atoms, it would be very surprising if there is electronic
communication between the two iron centers separated
˚
by nearly 12 A. A better explanation emerges on exam-
ination of the CV of the monomer 2 in the same solvent
using a Pt working electrode. Complex 2 shows a
ferrocenium–ferrocene redox wave with a half potential
of 0.54 V. A small separate oxidation of the organic
moiety was observed at a more anodic potential 0.92 V
versus SCE [21] in the differential pulse polarography.
The presence of the second anodic wave in both cases
rules out the formation of a mixed valence species in
3. Results
Substituted ferrocenes 1 and 2 were obtained by the
condensation of ferrocenecarboxaldehyde with 1,4-
phenylenediamine and aniline, respectively using a
slightly different procedure from that of Sonogashira et
al. [19]. The complexes 3a–c and 4a–c were obtained
following the literature procedure for the preparation
of ferrocenium triodide [20]. Complexes 1 and 2 are
quite stable and highly soluble in common solvents
such as toluene, chloroform, benzene and THF, while
the iodine oxidized complexes 3a–c and 4a–c are solu-
ble only in CH3CN, THF or DMSO and were freshly
prepared as and when necessary. The charge transfer
Table 4
Selected IR stretching frequencies for the CT complexes of iodine,
DDQ, TCNE, CA and TCNQ with 1 and 2
Complex wCꢁN
(cm−1
wCꢀO
(cm−1
wCꢀN
(cm−1
ÞC–H bend
)
)
)
(cm−1
)
complexes [Fc–CHꢀN–C6H4–NꢀCH–Fc]·(TCNQ)2
5
3a–c
4a–c
5
6
7
8
9
10
11
12
–
–
–
–
–
–
1588
1588
–
1630
1633
1641
1646
1642
1638
1642
1645
1636
1648
823
823
834
833
833
828
833
830
831
828
and [Fc–CHꢀN–C6H5]·(TCNQ) 6 were obtained by
TCNQ oxidation of the complexes [Fc–CHꢀN–C6H4–
NꢀCH–Fc] 1 and [Fc–CHꢀN–C6H5] 2. Similarly, the
2174
2187
2224
2229
2198
2205
–
complexes [Fc–CHꢀN–C6H4–NꢀCH–Fc]·(DDQ)2
7
and [Fc–CHꢀN–C6H5]·(DDQ)2 8 were obtained by
oxidation of the complexes 1 and 2, respectively, with
DDQ. Reaction of 1 and 2 with TCNE gives complex
[Fc–CHꢀN–C6H4–NꢀCH–Fc]·(TCNE)2 9 and [Fc–
CHꢀN–C6H5]·(TCNE) 10, respectively. The com-
–
1606
1600
–

S.K. Pal et al. / Journal of Organometallic Chemistry 575 (1999) 108–118
113
Fig. 3. Variable temperature X-band EPR spectra of [Fc–CHꢀN–C6H4–NꢀCH–Fc]·I4.5 (3c).
lization of the cation formed by the Schiff base in the
solid state would depend on the size of the anion due to
solid state packing effects as well as the redox poten-
tials. Based on the elemental analysis of the various
complexes characterized, it is clear that the CT com-
plexes formed do not have the simple reduced form of
the acceptor, A−, but form larger anions through self
complexation such as An−. Although, there is a consid-
erable solvent effect on the oxidation potential of the
complexes 1 and 2 on going from CH3CN to DMF, the
stochiometry and the characteristics of the isolated
complexes formed from 1 and 2 were remarkably the
same. Variation of solvent polarity did not seem to
affect the formation of I2 oxidized complexes signifi-
cantly. One exception is the complex formed by 2 with
I2 in benzene. The stochiometry of this complex 4a is
2·I3.5 whereas in other solvents the complex formed
was only 2·I3 (4b,c). Surprisingly, in spite of this
difference in the stochiometry the spectroscopic proper-
ties of the complexes were essentially identical.
the oxidation of 1 and suggests that the second wave is
due to the Schiff base present in 1 and 2.
The oxidizing capacity of the acceptors increase in the
following order: CABTCNQBTCNEBDDQBI2
[22]. No solid complexes were formed when the acceptor
oxidation potential was less than 0.01 V versus SCE in
acetonitrile. Thus acceptors such as hexafluorobenzene,
napthaquinone and trinitrobenzene do not form solid
complexes with 1 and 2. The more positive the reaction
potential, the more favorable would be the formation of
an ionic complex. Increase in the polarity of the solvent
can also be expected to stabilize an ionic complex. Table
2 gives the redox potentials of the oxidants in acetonitrile,
dichloromethane and dimethylformamide solvent
mediums.
Although, the redox potentials serve as a guide for
predicting the formation of solid complexes between the
Schiff bases and the acceptors, other factors affect the
extent of charge transfer in these systems and the
stochiometry of the solid complexes formed. The stabi-

114
S.K. Pal et al. / Journal of Organometallic Chemistry 575 (1999) 108–118
Table 5
EPR data of the oxidized Schiff base complexes (3–12)
Complex
gÞ (iron centerd)
g(iso) (carbon centerd)
[Fc–CHꢀN–C6H4–NꢀCH–Fc]·I4.5 (3c)
[Fc–CHꢀN–C6H5]·I3.5 (4c)
[Fc–CHꢀN–C6H4–NꢀCH–Fc]·(TCNQ)2 (5)
[Fc–CHꢀN–C6H5]·(TCNQ) (6)
[Fc–CHꢀN–C6H4–NꢀCH–Fc]·(DDQ)2 (7)
[Fc–CHꢀN–C6H5]·(DDQ)2 (8)
[Fc–CHꢀN–C6H4–CHꢀN–Fc]·(TCNE)2 (9)
[Fc–CHꢀN–C6H5]·(TCNE) (10)
4.278
4.280
4.230
4.311
4.112
4.311
4.169
4.135
4.007
4.203
2.006
2.070
2.008
1.989
1.996
2.093
1.984
1.981
1.973
1.988
[Fc–CHꢀN–C6H4–NꢀCH–Fc]·(CA)2 (11)
[Fc–CHꢀN–C6H5]·(CA)2 (12)
3.2. Electronic absorption spectra
of the CT complexes are shown in Table 4. Our assign-
ments of the various bands observed are based on
earlier assignments for ferrocene and related com-
pounds [27]. The perpendicular C–H bending fre-
quency has been considered to be most indicative of the
oxidation state of the iron in biferrocene type of com-
pounds [28–30]. The unoxidized Schiff base complexes
1 and 2 show a perpendicular C–H bend at 811 and
818 cm−1, respectively. The strong band observed be-
tween 1613 and 1619 cm−1 in these complexes is due to
the stretching frequency wCꢀN typical of the Schiff bases.
The electronic spectral data of complexes 1–12 are
summarized in Table 3. Neutral Schiff base complexes 1
and 2, have essentially identical spectra in comparison
with related Schiff base complexes [23], and not very
different from biferrocene, biferrocenylene and (1,1)-
ferrocenophanes [24,25]. In the case of ferrocene and
biferrocene compounds there are two bands in the
visible region at r.t., which can be assigned to d–d
transitions based on their position and intensity. Cal-
abrese et al. [26], reported the visible absorption spec-
The wCꢀN of 1 and 2 appear at 1613 and 1619 cm−1
,
trum
of
the
metallocenes
of
the
form
respectively.
[M(C5X5){C5H4–(CHꢀCH)n–C6H4–Y-p}] (M=Fe or
Ru; X=Me or H; n=1 or 2, Y=CN or NO2). These
compounds also show two bands in the visible region.
The lower energy transition was tentatively assigned to
a MLCT and the higher energy transition was assigned
to an essentially ligand centered y“y* transition with
some metal character. Based on the over all similarities
of these compounds and their absorption spectra to
those reported earlier it is reasonable to suppose that
molecular orbitals with similar compositions are in-
volved in the electronic transitions of the compounds 1
and 2.
All partially oxidized Schiff base complexes exhibit
multiple bands around 248, 291, 363, 510–532 nm as
shown in the Table 3. The band at 535–545 nm is
attributed to the ferricenium half of the molecule. The
UV spectra of the iodide complexes, 3a–c and 4a–c are
essentially identical and surprisingly there is no absorp-
tion around 700 nm which appears in biferrocenium
triiodide. In the case of the binuclear DDQ complex 7,
an additional band probably due to charge transfer is
observed at 582 nm.
The partially oxidized complexes 3a–c exhibit a sin-
gle absorption at 82393 cm−1. In the case of mononu-
clear oxidized Schiff base complexes 4a–c also, a single
absorption is seen at 82393 cm−1. In the case of the
ferrocenium triiodide the perpendicular C–H bend ap-
pears at 851 cm−1. The rather small shift in the perpen-
dicular C–H bending of the oxidized complexes 3 and
4 reflects the reduced electron density depletion from
the Cp ring of the ferrocene unit. The perpendicular
C–H bending vibration appears as a strong band
around 833 cm−1 for complexes 5–12. Compared to
the iodine oxidized Schiff base complexes, the C–H
bend occurs at a higher frequency (by about +10
cm−1) indicating greater amount of charge transfer
from the Cp ring to the acceptor in the complexes
5–12. The CꢀN stretching frequency also increases on
oxidation and follows the same trend as the perpendic-
ular C–H bending vibration. So based on the IR
spectral data, the degree of charge transfer from the
iron center to the acceptor falls in the following order
I2BCABTCNQBTCNE:DDQ.
In the case of Schiff base complexes oxidized by CA,
TCNE, DDQ and TCNQ, the presence of additional
bands diagnostic of the acceptor CꢀO and CꢁN groups
are helpful in understanding the nature of the com-
plexes formed. The ionic nature of the DDQ oxidized
complexes 7 and 8 are reflected both in wCꢁN and wCꢀO
3.3. Infrared spectra
All the bands characteristic of ferrocene and the
Schiff base linkage are observable in the IR spectra of
Schiff base complexes 1 and 2. The characteristic bands
stretching frequencies. DDQ and DDQ− have wCꢁN
’

S.K. Pal et al. / Journal of Organometallic Chemistry 575 (1999) 108–118
115
Table 6
Mo¨ssbauer data for the neutral Schiff base complexes 1 and 2
Neutral complex
T (K)
Isomer shift (mm s−1
)
Quadruple splitting (mm s−1
)
Fc–CHꢀN–C6H4–NꢀCH–Fc (1)
Fc–CHꢀN–C6H5 (2)
298
298
0.459
0.456
2.35
2.30
and wCꢀO stretching frequencies at 2233, 1680 cm−1 and
2230, 1600–1580 cm−1, respectively. The wCꢁN and wCꢀO
stretching vibrations of the complexes fall in the range
expected for the DDQ radical anion [30].
3.4. Electron paramagnetic resonance spectroscopy
In mixed-valence biferrocenium complexes, it has
been found that there is an empirical qualitative corre-
lation of Dg and delocalization [33,34]. A mixed-valence
biferrocenium cation which is valence detrapped has a
g-tensor anisotropy, DgB0.7, the mixed-valence cation
has essentially no potential energy barrier for electron
transfer between the two iron centers and is electroni-
cally delocalized. Complexes with Dg\1.5 have been
found to remain valence trapped at all temperatures.
The large anisotropy observed in valence localized sys-
tems are similar to what is observed for ferrocenium
triiodide which exhibits a relatively large anisotropic
g-tensor, gII=4.35, gÞ=1.26 and anisotropy, Dg=
gII−gÞ=3.09 at 20 K [35].
Fig. 3 shows the variable temperature X-band EPR
spectrum of the powdered complex [Fc–CHꢀN–C6H4–
NꢀCH–Fc]·I4.5 3c recorded in the temperature ranges
from 4.3 to 290 K. For the complexes 3c and 4c, two
features are observed. In addition to the iron-centered
radical, there was an isotropic signal corresponding to a
carbon-centered radical which seem to indicate two
different centers are possible for the unpaired electron
in these complexes. The intensity of the high field signal
for which only the gÞ line is visible was significantly
weaker than the isotropic signal at g=2.000. The high
field signal has significant intensity only at liquid he-
lium temperature. As the temperature increases, the
intensities of the both signals in the complexes 3c and
4c decrease drastically without changing their position.
For complex 3c the high field signal completely disap-
peared at 250 K while for complex 4c, it disappeared at
290 K. The change in the intensities are presumably due
to the change in the spin-lattice relaxation time (T1)
since variable temperature Mo¨ssbauer spectra indicate
an increase in the concentration of Fe(III) centers at r.t.
However, the g-tensor anisotropy of the complexes 3c
and 4c, could not be computed since the gII line is too
weak to be observed.
The apparent difference in the order of the oxidizing
capacity, as measured by the electrochemical potentials
and the degree of charge transferred from the Cp ring
using the IR technique needs an explanation. In most
ferrocenium complexes, it is assumed that the electron
is transferred from the iron to the acceptor. However,
in complexes such as 1 and 2 where an additional aryl
ring is present, electron density could be transferred to
the acceptor through the aromatic ring which functions
as a donor antenna. Acceptors which overlap with the
antenna efficiently allow for greater degree of charge
transfer. The changes in the IR stretching frequencies in
these complexes appear to be sensitive to the region
from which the electron density is removed and not
necessarily to the total charge transferred to the accep-
tor. In the iodine oxidized complexes, the C–H bending
frequency changes by small amounts. The shifts in the
wCꢀN are larger, due to their proximity to the site of
charge transfer but follow the same trend.
An interesting fact comes to light on examination of
the electronic structure of these complexes through
extended Hueckel theory (EHT) calculations [31]. Using
the program CACAO [32] EHT calculations were car-
ried out on complexes 1 and 2 using the geometrical
parameters available for well characterized Schiff bases
[23]. The nature of the highest occupied molecular
orbital (HOMO) in ferrocene is the d orbital localized
2
z
on iron. The HOMO in 1 is as expected having consid-
erable contribution from the d orbital of the two iron
2
z
atoms. But apart from the two iron atoms, the HOMO
has significant contributions from the aromatic bridge.
Hence, removal of electron density from this molecule
is efficiently accomplished by organic acceptors which
have LUMOs of the right symmetry and energy to
overlap with the aromatic bridge. Even oxidation with
iodine in this case occurs by removal of electron density
from the aromatic ring and not from the iron as in the
case of ferrocenium triiodide. This factor explains the
small changes in the perpendicular C–H bending fre-
quency in 3a–c and 4a–c.
Powder X-band EPR spectra of the complexes [Fc–
CHꢀN–C6H4–NꢀCH–Fc]·(TCNQ)2 (5), [Fc–CHꢀN–
C6H5]·(TCNQ) (6), [Fc–CHꢀN–C6H4–NꢀCH–Fc]·
(DDQ)2 (7) and [Fc–CHꢀN–C6H5]·(DDQ) (8) were

116
S.K. Pal et al. / Journal of Organometallic Chemistry 575 (1999) 108–118
Fig. 4. Variable temperature Mo¨ssbauer spectra of [Fc–CHꢀN–C6H4–NꢀCH–Fc]·I4.5 (3c).
recorded at liquid nitrogen temperature (77 K), and are
similar. The g values are summarized in Table 5.
Similar results are obtained for the CT complexes
formed by TCNE and CA.
shows two quadruple-split doublets, one for Fe(II) ion
with a quadruple splitting DEq=2.1 mm s−1 and the
other for Fe(III) ion with DEq in the range 0–0.5 mm
s
−1. If the mixed-valence biferrocenium cation is de-
trapped that is, it is interconverting at a rate in excess
of 107 to 108 s−1, then a single doublet with average
properties (DEq=1.1 mm s−1) is seen. The Mo¨ssbauer
spectra of the unoxidized complexes were as expected
for subsituted ferrocene complexes. The data for com-
plexes 1 and 2 are given in Table 6.
3.5. Mo¨ssbauer and magnetic studies of the iodine
oxidized complexes
It has been shown that 57Fe Mo¨ssbauer spectroscopy
is particularly useful in monitoring the valence state of
iron in biferrocenium complexes [36]. The Mo¨ssbauer
spectrum for a valence trapped biferrocenium cation
Variable temperature 57Fe Mo¨ssbauer spectrum of
the iodine oxidized complex 3c [Fc–CHꢀN–C6H4–

S.K. Pal et al. / Journal of Organometallic Chemistry 575 (1999) 108–118
117
Table 7
Mo¨ssbauer data for [Fc–CHꢀN–C6H4–NꢀCH–Fc]·I4.5 complex 3c
T (K)
Fe(II)
Fe(III)
IS (mm s−1 a
)
EQ1 (mm s−1 b
)
B (mm s−1 c
)
I1 (%)d
IS (mm s−1
)
EQ1 (mm s−1
)
B (mm s−1
)
I2 (%)
4.2
20.0
79.0
145.0
190.0
250.0
295.0
0.255(8)
0.250(8)
0.251(8)
0.238(8)
0.230(8)
0.221(8)
0.313(8)
2.18(1)
2.18(1)
2.17(1)
2.17(1)
2.13(1)
2.16(1)
2.05(1)
0.31(1)
0.30(1)
0.28(1)
0.28(1)
0.32(1)
0.30(1)
0.33(1)
88.1(5)
86.0(5)
77.7(5)
60.7(7)
49.8(7)
38.0(7)
27.4(7)
−0.168(8)
−0.188(8)
−0.207(8)
−0.240(8)
−0.288(8)
−0.228(8)
−0.231(8)
0.359(8)
0.395(8)
0.195(8)
0.133(8)
0.160(8)
0.176(8)
0.170(8)
0.36(1)
0.38(1)
0.38(1)
0.38(1)
0.34(1)
0.37(1)
0.41(1)
7.9(7)
10.5(7)
16.4(7)
25.4(5)
41.0(5)
55.0(5)
63.0(3)
Fe(II) high-spin contamination
T (K)
IS (mm s−1
)
EQ1(mm s−1
)
B (mm s−1
)
I2 (%)
79.0
250.0
1.45(1)
1.40(1)
2.17(1)
1.91(1)
0.35(1)
0.35(1)
5.9(7)
7.0(7)
a IS, isomer shift“Fe(Rh).
b EQ, quadruple splitting.
c B, line-width.
d I, intensity ratio.
NꢀCH–Fc]·I4.5 is shown in Fig. 4. This spectra was
least square fit with Lorenzian line shapes, the spectral
fitting parameters for the complex is given in Table 7.
The 57Fe Mo¨ssbauer spectrum of this complex was
collected at six temperatures between 4.2 and 295 K. At
4.2 K, 57Fe Mo¨ssbauer spectra shows quadruple dou-
blets corresponding to Fe(II) center (88%) (IS=0.255
mm s−1, QS=2.18 mm s−1) with a small amount of
an Fe(III) ferromagnetic impurity. As the temperature
increases, the intensity of the inner quadruple doublets
corresponding to the Fe(III) center increases with de-
crease in intensity of the Fe(II) center. At 190 K, both
Fe(II) and Fe(III) quadruple doublets have almost
equal intensity. At r.t., the inner quadruple doublets
corresponding to Fe(III) has very high intensity (63%)
compared to the Fe(II) center (27.4%). This suggests
that at low temperature (4.2 K), the complex undergoes
a retro charge-transfer from the polyiodide anion In− to
the ferrocenium part of the molecule [37].
[Fc–CHꢀN–C6H4–NꢀCH–Fc]+·In−
l[Fc–CHꢀN–C6H4–NꢀCH–Fc]·In
Magnetic susceptibility studies confirm that such a
retro charge transfer is occurring at low temperatures.
The calculated magnetic moment for the iodine oxi-
dized complex is less than that expected for the fully
oxidized (3,3) complex but greater than that computed
for a (2,3) complex. The effective magnetic moment
decreases with decrease in temperature as expected for
a complex in which retro charge transfer is taking
place. The magnetic moment calculated from the sus-
ceptibility as a function of temperature for the iodine
oxidized complex is given in Fig. 5. In principle, the
reduced magnetic moment at low temperatures could
also be explained by invoking antiferromagnetic cou-
pling. If this was true, Curie-Weiss behavior would be
−1
expected and a plot of 
versus temperature would
be linear. Since this is not the case, retrocharge transfer
appears to be a more suitable explanation for the
change in the magnetic moment as a function of
temperature.
Fig. 5. Magnetic moment (BM) versus temperature (K) plot of
[Fc–CHꢀN–C6H4–NꢀCH–Fc]·I4.5 (3c).

118
S.K. Pal et al. / Journal of Organometallic Chemistry 575 (1999) 108–118
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The Schiff base complexes 1 and 2 form interesting
charge transfer complexes with a variety of acceptors.
The solution state redox potentials of the complexes
and the oxidants are not indicative of the type of
complex that could be formed in the solid state but
could be used to provide a guide for the synthesis of
solid CT complexes. The size, shape and charge distri-
butions of the acceptors used in this study are com-
pletely different, and all of these factors are ultimately
important in determining the stochiometry, charge
transfer and other properties of the isolated complexes.
IR spectra of the complexes reveal the extent of charge
transfer in these complexes and suggest the regions of
electron density depletion. Mo¨ssbauer spectroscopy of
the iodine oxidized complexes is interesting. Surpris-
ingly, it shows the presence of both oxidized and unox-
idized iron centers. The ratio of these two species is
temperature dependent and a retro charge transfer is
indicated at low temperatures. The extent of electron
density transferred from the biferrocene to the oxidant
is determined by the nature of complex formed in the
solid state, which in turn is critically dependent, among
other things, upon the nature of the HOMO and the
size of the anion formed. Since oxidation removes an
electron from a MO having significant contributions
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Acknowledgements
The authors would like to thank Prof. S.V. Bhat for
the EPR spectra and K. Kannan for the Mo¨ssbauer
spectra of the neutral complexes reported here. They
would also like to thank the Department of Science and
Technology for providing financial support. A.G. Sa-
muelson thanks the AVH foundation for a fellowship
and Prof. S. Vasudevan for useful discussions.
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