Synthesis and physico-analytical studies of some novel ferrocenyl Schiff base derivatives

Article abstract of DOI:10.1016/j.jorganchem.2007.03.034

A series of ferrocenyl Schiff base derivatives was synthesized by condensation reactions of 1,1′-ferrocenedicarboxaldehyde and aromatic amines containing long chain alkyl groups as free ends which were characterized by their physical properties, elemental

Full text of DOI:10.1016/j.jorganchem.2007.03.034

Journal of Organometallic Chemistry 692 (2007) 3542–3546
www.elsevier.com/locate/jorganchem
Note
Synthesis and physico-analytical studies of some novel ferrocenyl
Schiff base derivatives
*
Zareen Akhter , Asifa Nigar, Muhammad Y. Razzaq, Humaira M. Siddiqi
Department of Chemistry, Quaid-i-Azam University, Islamabad 45320, Pakistan
Received 25 November 2006; received in revised form 2 March 2007
Available online 13 April 2007
Abstract
A series of ferrocenyl Schiff base derivatives was synthesized by condensation reactions of 1,10-ferrocenedicarboxaldehyde and aro-
matic amines containing long chain alkyl groups as free ends which were characterized by their physical properties, elemental, FTIR, 1H
NMR, 13C NMR spectral and thermal analysis. The thermal behaviour of the synthesized compounds was studied by differential scan-
ning calorimetry (DSC) which revealed that these compounds may exhibit mesomorphic properties. The DSC results of aromatic amines
and ferrocenyl Schiff bases were compared to study the effects of structure, i.e. rigid core and terminal chain length, on the phase tran-
sition behaviour.
Ó 2007 Elsevier B.V. All rights reserved.
Keywords: Ferrocene; Schiff base; Phase transitions
1. Introduction
sional conductors), strong optical [13] (birefringence,
dichroism, and nonlinear optical behaviour) and electro-
optical [14] (photoelectric behaviour, ferroelectric electro-
optical responses) properties. Many ferrocenyl Schiff base
derivatives have been studied for their biological activity
and found to be active against microbes and bacteria
[15,16]. In our research project, we have synthesized a ser-
ies of ferrocnyl Schiff bases as materials with great poten-
tials in the field of metallomesogens. In addition to a large
no of mesogenic ferrocenyl Schiff bases reported so far, it is
a novel series of compounds expected to exhibit the ther-
motropic liquid crystalline behaviour.
Ferrocene derivatives with good donor abilities (i.e.;
nitrogen, sulfur, oxygen, phosphorus, etc.) have attracted
much interest, as the coordination of a metal to these het-
eroatoms generates multicenter molecules which have
potential in many different areas. Amongst these N-substi-
tuted ligands ferrocenyl Schiff bases play pivotal role in the
field of catalysis [1–9]. Another important and widely stud-
ied property is the mesogenic behaviour of these com-
pounds. Ferrocene has a favourable balance of structure
and properties and is useful for incorporating a metal atom
into a molecule that requires certain degree of structural
rigidity such as liquid crystals. The ferrocenyl unit is not
only useful for modifying the shape of a molecule but also
the metal atom with its considerable electron density can
be used to modify physical properties such as colour,
polarisability and magnetism [10]. Liquid crystalline ferro-
cene based Schiff bases show interesting magnetic [11]
(paramagnetic liquid crystals, control of the molecular ori-
entation in a magnetic field), electrical [12] (one dimen-
2. Experimental
FTIR spectra (KBr pellets, 4000–400 cmꢀ1
) were
recorded on Bio-Rad Excalibur FTIR, Model FTS 3000
MX. Elemental analysis was conducted on Perkin Elmer
CHNS/02400, America. DSC curves were recorded on
1
DSC 404C (2 °C/min), Netzsch. H and 13C NMR spectra
were obtained in CDCl3 and DMSO-d6 solution on a Var-
ian Mercury 300 NMR spectrometer at 300 (1H) and 75.5
(13C) MHz.
*
Corresponding author. Tel.: +92 519219811; fax: +92 512873869.
E-mail address: zareenakhter@yahoo.com (Z. Akhter).
0022-328X/$ - see front matter Ó 2007 Elsevier B.V. All rights reserved.
doi:10.1016/j.jorganchem.2007.03.034

Z. Akhter et al. / Journal of Organometallic Chemistry 692 (2007) 3542–3546
3543
2.1. Procedure for the synthesis of (4-aminophenyl)-
N-phenethylamide (1b)
(ph–) = 6.55d (2–20, 1H), 6.70d (3–30, 1H), 6.80d (6–60, 1H),
7.50d (7–70, 1H); d(–NHCO–) = 9.80s; d(–CH2–) = 1.24s,
d(–CH2CO–) = 2.40t; d (–NH2–) = 4.98s, d (–CH3) = 0.90t.
4-Nitrobenzoic acid (5 g, 30 mmol) was refluxed for 5 h in
the presence of distilled thionyl chloride (2.18 mL, 30 mmol)
and reacted with 2-phenethylamine (2.42 mL, 21 mmol) in
anhydrous toluene. An equimolar amount of Et3N (3 mL,
21 mmol) was also added to catalyze the reaction and
refluxed with stirring for 3 h. After cooling to room temper-
ature, the reaction mixture was allowed to stand for 16 h.
The resulting crude solid was filtered off, washed with water
and recrystallized from ethanol. The product 1a (3 g,
0.012 mmol) with (30 mL, 0.618 mmol) of hydrazine mono-
hydrate, 80 mL of ethanol and 0.1 g of 5% Pd–C were
charged in a flask for reduction. This mixture was refluxed
with stirring for 16 h and then filtered in hot state to remove
Pd–C. White semi-crystalline solid 1b was obtained on cool-
ing the filtrate to room temperature which was filtered and
dried (Scheme 1).
2.2.3. 4-[4-Aminophenyloxy)octadecanamide (5b)
White solid, yield (85%), m.p. 95–115 °C. DSC melt-
ing range 101–120 °C. Anal. Calc. for C30H46N2O2: C,
77.25; H, 9.44; N, 6.01. Found: C, 78.37; H, 8.96; N,
1
6.43%. IR (cmꢀ1) 1655, 3287, 3314. H NMR (DMSO-
d6, d ppm): d (ph–) = 6.48d (2–20, 1H), 6.65d (3–30, 1H),
6.80d (6–60, 1H), 7.45d (7–70, 1H); d(–NHCO–) = 9.80s;
d
(–CH2–) = 1.26s, d(–CH2CO–) = 2.40t;
d
(–NH2–)
= 5.0s, d(–CH3) = 0.90t.
2.3. General procedure for the synthesis of Schiff base
derivatives (6, 7, 8, 9)
A pre-baked two-neck round bottom flask equipped
with condenser and magnetic stirrer was charged with
the corresponding amine (1.6 mmol) in absolute ethanol
(20 mL) and treated with 1,10-ferrocene dicarboxaldehyde
(0.8 mmol). Concentrated hydrochloric acid (2–3 drops)
was added to the reaction mixture as a catalyst and
refluxed for 6 h. After cooling to room temperature, the
precipitates were collected by filtration and purified by
flash chromatography (eluent; hexane:acetone; 5:1) and
recrystallized from petroleum ether (Scheme 3). All the
reactions involved in the synthesis of Schiff bases were
carried out in inert atmosphere, using absolute ethanol
as solvent. Water is liberated as a byproduct during the
synthesis of all Schiff bases, which is removed continu-
ously by using Dean and Stark apparatus. Otherwise
the reaction is reversed to backward direction resulting
in the hydrolysis of C@N bond. To avoid the further
hydrolysis of the Schiff bases, the solvent is evaporated
on vacuum as soon as the product is formed and dried
in vacuum dessicator on CaCl2. The dried product can
be stored in dessicator for longer times. All the synthe-
sized ferrocene Schiff base derivatives were obtained in
excellent yields (ꢁ85%).
White semi-crystalline solid, yield (82%), m.p.
135–145 °C. DSC melting range 137–148 °C. Anal. Calc.
for C15H16ON2: C, 74.97; H, 6.71; N, 11.66. Found: C,
1
73.86; H, 5.89; N, 10.87%. IR (cmꢀ1) 1604, 3221, 3326. H
NMR (DMSO-d6, d ppm): d (ph–) = 6.60d (2–20, 1H),
7.63d (3–30, 1H); 7.23m. d (–NHCO–) = 8.18s; (–CH2–)
= 3.45m, 2.85m; d (–NH2–) = 5.65s.
2.2. General procedure for the synthesis of aromatic amines
(3b, 4b, 5b)
4-[4-Nitrophenyloxy]aniline
2
was synthesized by
reported method [17] and condensed with different acid chlo-
rides to get the desired products. A mixture of 4-[4-nitrophe-
nyloxy]aniline (5 g, 210 mmol), acid chloride (210 mmol)
and triethylamine (210 mmol) was stirred in dried toluene
and refluxed for 3 h. The reaction mixture was allowed to
stand at room temperature for 16 h. The resulting solid
(3a, 4a, and 5a) was filtered off, washed several times with
water to remove any salt impurities and recrystallized from
dichloromethane:toluene (1:1) mixture. The products
obtained were then reduced to get 3b, 4b and 5b following
the standard procedure as already described (Scheme 2).
2.3.1. 1,10-Bis[N-phenethylbenzamidoimine]ferrocene (6)
Brown solid, semi-crystalline; yield (82.5%), m.p.
160–186 °C. DSC melting range 161–202 °C. Anal. Calc.
for C42H36FeN4O2: C, 73.68; H, 5.26; N, 8.18. Found: C,
2.2.1. 4-[4-Aminophenyloxy)dodecanamide (3b)
White solid, yield (85%), m.p. 115–130 °C. DSC melting
range 115–134 °C. Anal. Calc. for C24H34N2O2: C, 75.39;
H, 8.91; 7.32. Found: C, 74.68; H, 7.96; N, 6.86%. IR
72.66; H, 5.34; N, 8.14%. IR (cmꢀ1) 1557, 1639, 500. H
1
NMR (DMSO-d6, d ppm): d (ph–) = 6.70d (2–20, 1H),
7.65d (3–30, 1H), 7.20m; d(–NHCO–) = 8.50s; d (–CH2–)
= 3.42m, 2.80m; d(–CH@N–) = 7.62s; d (Fe–cp) = 4.95s
(a-a0, 1H), 4.60s (b, b0, 1H).
1
(cmꢀ1) 1655, 3281, 3387. H NMR (DMSO-d6, dppm):d
(ph-) = 6.58d (2–20,1H), 6.75d (3–30,1H), 6.85d (6–60, 1H),
7.52d (7–70, 1H); d(–NHCO–) = 9.80s; d (–CH2–) = 1.3s,
d(–CH2CO–) = 2.35t; d (–NH2–) = 5.65s, d (–CH3) = 0.90t.
2.3.2. 1,10-Bis[4,40-dodecanamido-N-phenyloxybenzimine]-
ferrocene (7)
2.2.2. 4-[4-Aminophenyloxy)hexadecanamide (4b)
White solid, yield (84%), m.p. 111–120 °C. DSC melting
range 110–122 °C. Anal. Calc. for C28H42N2O2: C, 76.71;
H, 9.58; N, 6.39. Found: C, 75.48; H, 8.43; N, 6.54%. IR
Brown solid, semi-crystalline; yield (85%), m.p.
210–220 °C. DSC melting range 185–224 °C. Anal. Calc.
for C60H72FeN4O4: C, 74.38; H, 7.43; N, 5.78. Found: C,
75.07; H, 6.95; N, 6.12%. IR (cmꢀ1) 1655, 1619, 489.
1
(cmꢀ1) 1655, 3285, 3386. H NMR (DMSO-d6, d ppm): d

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Z. Akhter et al. / Journal of Organometallic Chemistry 692 (2007) 3542–3546
1H NMR (DMSO-d6, d ppm): d (ph–) = 6.72d (2–20, 1H),
2.3.3. 1,10-Bis[4,40-hexadecanamido-N- phenyloxy-
benzimine]ferrocene (8)
Dark brown solid, semi-crystalline; yield (86%), m.p.
205–213 °C. DSC melting range 185–210 °C. Anal. Calc.
for C68H88FeN4O4: C, 75.55, H, 8.14; N, 5.18. Found: C,
6.58d (3–30, 1H), 6.85d (6–60, 1H), 7.50d (7–70, 1H);
d(–NHCO–) = 9.85s;
d
(–CH2–) = 1.28s, d(–CH2CO–)
= 2.25t; d (–CH3) = 0.85t; (–CH@N–) = 7.60s; d (Fe-
cp) = 4.90s (a–a0, 1H), 4.75s (b,b0,1H).
O
O
SOCl2
O2N
C
OH
O2N
C Cl + H2N
Reflux
Reflux
Toluene/Et3N
Ethanol
5% Pd-C
hydrazine
2
2
3
3
O
7
6
O
C
1
9
4
H2N
C
N
H
5
8
11
O2N
N
10
Reflux
H
7
6
(1b)
(1a)
Scheme 1.
O
C
H2N
OH + F
NO2
NH2
HO
C H2
n n+1
Reflux
DMAc/K2CO3
SOCl2
Reflux
O
O2N
O
+ Cl
C C H2
n n+1
(2)
Reflux
Toluene/Et3N
O
3a: n = 11
4a: n = 15
5a: n = 17
O2N
O
N
H
C C H2
n n+1
Reflux
Ethanol/5% Pd-C/hydrazine
2
3
6
7
O
1
5
4
8
H2N
O
N
H
C
C H2
n n+1
9
2
3
7
6
3b: n = 11
4b: n = 15
20
10
24
10
5b: n = 17
26
10
Scheme 2.

Z. Akhter et al. / Journal of Organometallic Chemistry 692 (2007) 3542–3546
3545
1
74.67; H, 8.74; N, 4.89%. IR (cmꢀ1) 1655, 1618, 489. H
NMR (DMSO-d6, d ppm): d (ph-) = 6.75d (2–20, 1H),
6.55d (3–30, 1H), 6.82d (6–60, 1H), 7.52d (7–70, 1H);
exceptions. The IR spectra of the compounds 1b, 2, 3b,
4b, and 5b confirm the presence of different functional
groups along with the appearance of two absorption bands
at 3221 cmꢀ1 and 3366 cmꢀ1 characteristics of aromatic
amines. These bands are absent in the respective Schiff base
derivatives 6, 7, 8, 9, and 10 and a strong peak appeared at
1620 cmꢀ1 due to C@N stretch. These compounds also
show characteristic absorption peaks of the Fe-Cp stretch-
d(–NHCO–) = 9.80s;
d
(–CH2–) = 1.25s, d(–CH2CO–)
= 2.24t; d (–CH3) = 0.85t; (–CH@N–) = 7.65s; d (Fe-
cp) = 4.85s (a–a0, 1H), 4.72s (b, b0, 1H).
2.3.4. 1,10-bis[4,40-Octadecanamido-N-
phenyloxybenzimine]ferrocene (9)
1
ing vibrations at 483 cmꢀ1 or above. In H NMR, all the
Dark brown solid, semi-crystalline; yield (84.5%), m.p.
195–205 °C. DSC melting range 183–207 °C. Anal. Calc.
for C72H96FeN4O4: C, 76.05; H, 8.45; N, 4.92. Found: C,
aromatic amines give broad signals in the range of
5–5.6 ppm, which are shifted upfield at about 7.6 ppm in
case of their Schiff base derivatives, characteristics of the
proton attached with imine group. They have also shown
two different values for the chemical shift of ring protons
that confirm the chemical equivalence of the proton atomic
pairs present at a- and b-positions and hence its 1,10-disub-
stituted structures. The results of DSC provide some useful
information about the mesomorphic nature of these com-
pounds due to wide range of melting and clearing temper-
atures of the compounds 3b, 4b and 5b. There is reduction
in melting and clearing temperatures as the terminal chain
length increases which is most probably due to the high
entropic contribution of the long terminal chains on the
1
77.13; H, 7.64; N, 4.54%. IR (cmꢀ1) 1655, 1619, 490. H
NMR (DMSO-d6, d ppm): d (ph–) = 6.70d (2–20, 1H),
6.55d (3–30, 1H), 6.85d (6–60, 1H), 7.52d (7–70, 1H);
d(–NHCO–) = 9.82s;
d
(–CH2–) = 1.35s, d(–CH2CO–)
= 2.25; d (–CH3) = 0.85t; (–CH@N–) = 7.62s; d (Fe-
cp) = 4.90s (a–a0, 1H), 4.80s (b, b0, 1H).
3. Results and discussion
1
The FTIR, H NMR, and 13C NMR spectral data for
the synthesized compounds is self explanatory with few
O
C
H
H
2
3
3
α
Ethanol
(-H2O)
1
4
β
C
N
Ar
+
H2N
Ar
Fe
Reflux
2
Fe
C
O
Ar
N C
O
C
6
6
7
7
6: Ar =
7: Ar =
5
N
H
8
O
C
O
N
H
C11H23
O
8: Ar =
9: Ar =
O
N
H
C C15H31
O
O
N
H
C C17H35
Scheme 3.

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Z. Akhter et al. / Journal of Organometallic Chemistry 692 (2007) 3542–3546
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ously [18]. Furthermore, this trend coincides with the prin-
cipal of liquid crystal, i.e. mesomorphism depends on the
length ratio of rigid core and terminal chain. It is also
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We are thankful to HEC Pakistan for providing the
financial assistance to accomplish the reported work.
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