Phase behavior of some mono-substituted ferrocene- and [3]ferrocenophane- containing nematics with the cyclohexane ring in the rigid core

Article abstract of DOI:10.1016/j.jorganchem.2011.03.013

Generally, incorporation of the cyclohexane rings into the rigid core of rod-like mesogens leads to improved technological parameters, i.e. low viscosity, ambient transition temperatures and stability of the nematic state. Taking this into consideration,

Full text of DOI:10.1016/j.jorganchem.2011.03.013

Journal of Organometallic Chemistry 696 (2011) 2429e2437
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Journal of Organometallic Chemistry
journal homepage: www.elsevier.com/locate/jorganchem
Phase behavior of some mono-substituted ferrocene- and [3]
ferrocenophane-containing nematics with the cyclohexane ring in the rigid core
,
b
,
a
a,
So Yeon Kim a, Oleg N. Kadkin a , Eun Ho Kim , Moon-Gun Choi
*
**
a Department of Chemistry, Yonsei University, 262 Seongsanno, Seodaemun-Gu, Seoul 120-749, Republic of Korea
b Department of Physical and Colloid Chemistry, Kazan State Technological University, 68, K. Marks Str., 420015 Kazan, Russia
a r t i c l e i n f o
a b s t r a c t
Article history:
Generally, incorporation of the cyclohexane rings into the rigid core of rod-like mesogens leads to
improved technological parameters, i.e. low viscosity, ambient transition temperatures and stability of
the nematic state. Taking this into consideration, a series of novel cyclohexane-containing derivatives
of ferrocene has been synthesized. The effect of various structural factors on liquid crystalline behavior of
the synthesized ferrocene-containing nematics has been examined. Ferrocenophane compounds
exhibited enhanced liquid crystalline properties in comparison with the derivatives of unbridged
ferrocene. Depending on thermal prehistory of the samples, some of the synthesized ferrocenomesogens
showed remarkable migration of the phase transition temperatures. In one case such behavior led to
stabilization of the initially monotropic nematic mesophase in subsequent heating cycles. In another
case, the phase transition shifts caused the lowering of the crystal-to-nematic transition temperature and
the broadening of the mesophase range. There was also a case of alteration from the initially enantio-
tropic to monotropic behavior. The obtained novel metallomesogens are important footsteps toward the
development of low-viscous and low-temperature materials for liquid crystal applications possessing
a chromophoric, redox-switchable, polarizable and chemically stable superaromatic ferrocene unit.
Ó 2011 Elsevier B.V. All rights reserved.
Received 28 January 2011
Received in revised form
27 February 2011
Accepted 7 March 2011
Keywords:
Liquid crystals
Metallomesogens
Synthesis design
Phase transitions
Mesophases
Ferrocene
1. Introduction
into account the above characteristics. Despite of bulky end-groups
mono-substituted ferrocenomesogens are able to show liquid crys-
The presence of transition metals brings unusual properties to
liquid crystals and opens wide possibilities for new applications. This
perception stimulated A.M. Giroud-Godquin and others to start
systematic studies of metal-containing liquid crystals in 1977 [1,2].
Ferrocene-containing liquid crystals among various types of metal-
lomesogenic systems attracted reasonable interest of researchers
during recent decades [3,4]. Certainly, a chemical and thermal
stability of superaromatic ferrocene moiety radically exceeds
a stability of other metal-containing coordination sites that are
traditionally used in the molecular design of metallomesogens.
Besides, ferrocene is a good chromophore, redox-switchable and
polarizable unit due to the increased electronic density around the
metal center. Hence, mesogenic ferrocene derivatives are valuable
candidates for potential applications in liquid crystal devices taking
talline properties. As a rule, they exhibit nematic mesomorphism
sometimes preceded by smectogenic behavior [4e14]. Introducing
asymmetric fragments into the mesogenic core of mono-substituted
ferrocenomesogens leads to chiral organization of the mesophases
[15e17]. Besides, chiral mesophases can be achieved in multi-
substituted ferrocene derivatives with planar chirality. The latter
compoundswerecharacterizedasweaklyferroelectricliquidcrystals
[18,19]. Bridged ferrocenes or ferrocenophanes with a minimal
structural variation from mono-substituted ferrocenomesogens
show enhanced mesomorphism [20,21]. Hexagonal columnar and
bicontinuous cubic mesophases were observed in supramolecularly
organized ferrocene-containing liquid crystals [22,23] and hex-
acatenar 1,10-bis substituted ferrocene derivatives [24]. Recently,
unusual mesophases showing ambidextrous chirality were found in
unsymmetrically 1,10-bis substituted ferrocenomesogens, which are
preliminarily assigned to mesophases with tetrahedral symmetry
[25]. Finally, ferrocene units can be successfully tailored into various
liquid crystalline polymer structures [26e31].
* Corresponding author. Department of Physical and Colloid Chemistry, Kazan
State Technological University, 68, K. Marks Str., 420015 Kazan, Russia.
Tel.: þ7 843 2314177; fax: þ7 843 2366523.
The phase transition temperatures in mono-substituted ferro-
cenes often far exceed 100 ꢀC. Regarding disubstituted liquid
crystalline ferrocene derivatives, they often show even higher
** Corresponding author. Tel.: þ82 2 2123 2645; fax: þ82 2 364 7050.
E-mail addresses: oleg.kadkin@bk.ru (O.N. Kadkin), choim@yonsei.ac.kr
(M.-G. Choi).
0022-328X/$ e see front matter Ó 2011 Elsevier B.V. All rights reserved.
doi:10.1016/j.jorganchem.2011.03.013

2430
S.Y. Kim et al. / Journal of Organometallic Chemistry 696 (2011) 2429e2437
temperatures of mesophase transitions. It seems there are two
major ways for decreasing the temperatures of phase transitions in
ferrocene-containing liquid crystals: 1) incorporation of cyclo-
hexane fragments into the rigid core; 2) lowering symmetry of the
substituents in the ferrocene unit. The first approach is applicable
to both mono- and bis-substituted ferrocenomesogens. T. Hanasaki
et al. successfully applied the second approach in some unsym-
metrically bis-substituted ferrocenomesogens with the crystal-to-
nematic transition below 100 ꢀC [32]. The purpose of the present
studies was preparation of various mono-substituted ferrocene-
containing liquid crystals with a cyclohexane ring in the rigid core,
and investigation of the effect of some selected structural factors on
their liquid crystalline properties. In all cases, pure trans- isomers of
the disubstituted cyclohexanes were used for replacement of
benzene rings in the rigid core. In general, a cyclohexane structural
unit reduces viscosity and stabilizes the technologically important
nematic state [33,34]. Structural variations in the synthesized fer-
rocenomesogens comprised a different number of carbon atoms in
the terminal alkyl chains, the azomethine linking group, the lateral
fluoro-substituents, and the 1,3-propylidene bridge in the ferro-
cene fragment (Scheme 1).
SMP3. 1H NMR spectra were measured on a Bruker AM 400 with
internal TMS standard. Mass-spectra were obtained on a Bruker
microflex MALDI-TOF mass spectrometer. Elemental analyses were
performed on a Fisons instrument 2A1108 at Korea Institute of
Science and Technology. DSC thermographs were obtained on
Perkin Elmer Diamond DSC with various scanning rates. Thermo-
optical observations were carried out on a Nicon Eclipse E600 Pol
optical polarized microscope equipped with a Mettler Toledo FP82
HT hot stage system and Mettler FP90 central processor. Micro-
photographs were obtained with a Moticam 2300 digital camera.
The syntheses of 4-bromophenylferrocene and 4-(4-n-decylcyclo-
hexyl)-2,3-difluorobenzene as a mixture of cis- and trans- isomers
are described in our earlier publication [25].
2.1.1. 4-Bromophenylferrocenophane (8)
A mixture of 4-bromoaniline (2.02 g, 11.7 mmol), concentrated
H2SO4 (7.3 mL, 133 mmol) and water (15 mL) was cooled down
to ꢁ5 ꢀC and diazotized with a solution of NaNO2 (0.81 g,11.7 mmol)
in water (5 mL). Addition of NaNO2 solution was carried out in such
a way so temperature inside of the reaction mixture was not allowed
to rise higher than 0 ꢀC. The cold diazonium salt solution was added
portion wise to a cold solution (0 ꢀC) of [3]ferrocenophane (1.50 g,
6.6 mmol) in a mixture of diethyl ether (30 mL) and hexadecyl-
trimethylammonium bromide (0.15 g, 0.4 mmol). The reaction
mixture was stirred at 0 ꢀC for 3 h and then at room temperature for
overnight. After that the solution was neutralized by NaOH (aq.10%)
and the ether layer was separated with a separation funnel. The
aqueous phase was extracted with diethyl ether (ꢂ3) then the
combined organic extracts were dried over MgSO4, filtered and
evaporated under reduced pressure. The residue after evaporation
of diethyl ether was placed on an Al2O3 column and eluted with
hexane. The first fraction containing a mixture of aromatics together
with [3]ferrocenophane traces was rejected. The second fraction
was collected and evaporated to dryness. The obtained residue was
recrystallized from hexane. Yield 1.39 g (58%). Orange powder, m.p.
2. Experimental section
2.1. Materials and instruments
Reagent grade chemicals and solvents were purchased from
Aldrich (Yongin, Kyounggi-Do, Korea, Korea Branch). Solvents were
dried and freshly distilled just before use. Melting points were
determined by capillary method in a Stuart Scientific apparatus
CnH2n+1
Fe
F
1
F
F
98.2e99.0 ꢀC; 1H NMR (250 MHz, CDCl3):
d
¼ 7.42e7.22 (m, 5H), 4.47
(d, J ¼ 1.8 Hz, 2H), 4.21 (t, J ¼ 2.1 Hz, 2H), 4.13 (t, J ¼ 1.9 Hz, 1H),
3.96e3.86 (m, 1H), 3.44e3.36 (m, 1H), 2.16e1.83 (m, 6H); MS (EI):
380 (379.99) and Anal. Calcd for C19H17BrFe (%): C 59.88, H 4.50;
found: C 59.85, H 4.47.
CnH2n+1
Fe
Fe
F
2
2.1.2. 4-Aminophenyl ferrocene (9)
4-Nitrophenylferrocene (0.40 g, 1.3 mmol) was dissolved in
EtOH (5 mL), and 10% Pd/C (0.14 g) was added to the obtained
solution. The mixture was reduced in a hydrogenation apparatus
under pressure of H2 (3 atm) for 6 h. The resulting mixture was
filtered off through a Celite layer, evaporated to dryness and puri-
fied by column chromatography. Yield 0.28 g (78%). Yellowish
orange powder, m.p. 152e154 ꢀC (lit. [35] 157e159 ꢀC). 1H NMR
N
CnH2n+1
CH
F
F
F
F
3
(250 MHz, CDCl3):
2H), 4.53 (t, J ¼ 1.8 Hz, 2H), 4.23 (t, J ¼ 1.8 Hz, 2H), 4.03(s, 5H).
d
¼ 7.29 (d, J ¼ 8.9 Hz, 2H), 6.64 (d, J ¼ 8.5 Hz,
N
CnH2n+1
CH
Fe
Fe
2.1.3. 4-(4-n-Pentylcyclohexyl)-2,3-difluorobenzaldehyde (10a)
n-Butyllithium (11.8 mL of 2.5 M in cyclohexane, 30.0 mmol) was
added dropwise for 30 min to a cooled (ꢁ78 ꢀC) solution of 1,2-
difluorobenzene (2.74 g, 24.0 mmol) in dry THF (80 mL) under
vigorous stirring in dry argon atmosphere. The mixture was stirred
at ꢁ78 ꢀC for 2.5 h, and then a solution of 4-pentylcyclohexanone
(3.66 g, 21.8 mmol) in dry THF (10 mL) was added dropwise at the
same temperature for 30 min. The mixture was allowed to warm up
to room temperature overnight and the excess of organometallics
was quenched with aqueous saturated ammonium chloride (40 mL).
The crude product was extracted into ether (ꢂ2), and the combined
organic layers were dried over anhydrous MgSO4. The obtained
solutionwas filtered, and solvents were evaporated. The residuewas
4
N
C10H21
CH
5
a: n=5
b: n=10
Scheme 1. Chemical structures of the mesogenic ferrocenes 1e5 with a cyclohexane
fragment in the rigid core.

S.Y. Kim et al. / Journal of Organometallic Chemistry 696 (2011) 2429e2437
2431
purified on a silica column and the mixture of cis- and trans- isomers
of 4-n-pentyl-1-(2,3-difluorophenyl)cyclohexen-2-ol-1 was
obtained as a colorless oil (5.78 g, 20.6 mmol, 94%). 1H NMR
(250 MHz, CDCl3):
¼ 7.42e7.23 (m, 1H), 7.21e6.80 (m, 2H),
colorless oil. 1H NMR (250 MHz, CDCl3):
d
¼ 6.86 (td, J1 ¼ 1.3 Hz,
J2 ¼ 2.5 Hz, J3 ¼ 10.2 Hz, 1H), 5.98 (dd, J2 ¼ 2.5 Hz, J3 ¼ 10.2 Hz, 1H),
2.28e2.56 (m, 4H), 2.11 (m, 1H), 1.70 (m, 2H), 1.55e1.14 (m, 16H),
0.89 (t, J ¼ 6.4 Hz, 3H). A solution of n-phenyllithium (1.8 M in di-n-
butylether, 37 mL, 20.5 mmol) was added dropwise during 20 min
to a cooled solution (ꢁ78 ꢀC) of previously obtained 4-n-decylcy-
clohexen-2-one-1 in dry THF (50 mL) under vigorous stirring in dry
argon atmosphere. The mixture was allowed to warm up to room
temperature overnight and excess of organometallics were
quenched with aqueous saturated ammonium chloride (100 mL).
The crude product was extracted into ether (ꢂ2), and the combined
organic layers were dried over anhydrous MgSO4. The obtained
solution was filtered, and solvents were evaporated. The residue
was purified on a silica gel column, and 11.15 g (45%) of 4-n-decyl-
1-phenylcyclohexen-2-ol-1 (16) was obtained as a mixture of cis-
d
2.48e2.28 (m, 2H), 2.12 (s,1H), 2.00e1.80 (m, 2H),1.81e1.53 (m, 3H),
1.52e1.11 (m, 11H), 0.89 (t, J ¼ 6.4 Hz, 3H). The obtained mixture of
isomers without separation dehydrated by dissolving inTHF (60 mL)
and slow addition of 95% sulfuric acid (10 mL), and then stirring for
1 h to eliminate OH group. The mixture was quenched in water and
extracted into ether (ꢂ2). The combined ethereal extracts were
washed with saturated NaHCO3 and dried over MgSO4. The obtained
solution was filtered and evaporated to dryness. The obtained
residue was dissolved in EtOH and hydrogenated to completion in
a hydrogenation apparatus in the presence of 10 wt% Pd/C (1.96 g) for
6 h at 3 atm. The crude product was filtered through a Celite layer
and ethanol was evaporated. The mixture of cis- and trans- isomers
of 1-(4-n-pentylcyclohexyl)-2,3-difluorobenzene was obtained
after the purification on a SiO2 column with hexane as an eluent.
Yield 3.69 g (60%), colorless oil. 1H NMR (250 MHz, CDCl3):
and trans- isomers. 1H NMR (250 MHz, CDCl3):
d
¼ 7.85e7.43 (m,
2H), 7.42e7.12 (m, 2H), 6.07e5.88 (m, 1H), 5.85e5.63 (m, 1H), 2.16
(s,1H), 2.11e1.71 (m, 4H),1.58e1.09 (m,18H), 0.88 (t, J ¼ 6.3 Hz, 3H).
The obtained material was dissolved in ethanol under dry argon
and reduced in a hydrogenation apparatus in the presence of 10%
Pd/C (1.60 g) for 6 h at 3 atm. After completion of the reaction the
mixture was filtered through a Celite layer and ethanol was evap-
orated. The residue was purified by SiO2 column chromatography
with hexane as an eluent. Yield 7.66 g (33%), colorless oil. 1H NMR
d
¼ 7.10e6.88 (m, 3H), 3.03e2.74 (m, 1H), 1.87 (d, J ¼ 9.6 Hz, 2H),
1.80e1.58 (m, 4H), 1.51e1.02 (m, 12H), 0.90 (t, J ¼ 5.6 Hz, 3H). n-
Butyllithium (6.0 mL of 2.5 M in hexane, 15 mmol) was added
dropwise to a stirred and cooled (ꢁ78 ꢀC) solution of unseparated
isomers of 1-(4-n-pentylcyclohexyl)-2,3-difluorobenzene in dry
THF under a dry argonpad. The mixture was stirred at ꢁ78 ꢀC for 2 h,
and then a solution of freshly distilled DMF (0.96 mL, 0.93 g,
12.55 mmol) in absolute THF (10 mL) was added dropwise at ꢁ78 ꢀC.
The mixture was allowed to warm up to 25 ꢀC over 2 h, quenched
with aqueous saturated ammonium chloride (40 mL) and extracted
with diethyl ether (ꢂ3), and the combined organic layers were dried
over anhydrous MgSO4. The obtained solution was filtered, and
solvents were evaporated. The residue was purified on a silica
column. Yield 2.47 g (39%) of colorless oil as a mixture of cis- and
trans- isomers. Pure trans-isomer of 4-(4-n-pentylcyclohexyl)
benzaldehyde was obtained by multiple recrystallization from
pentane. Yield 0.74 g (12%). White crystals, m.p. 66.4e68.8 ꢀC; 1H
(250 MHz, CDCl3):
2.09e1.81 (m, 2H), 1.80e1.57 (m, 4H), 1.51e0.99 (m, 18H), 0.89 (t,
J ¼ 6.7 Hz, 3H).
d
¼ 7.50e7.09 (m, 5H), 2.68e2.38 (m, 1H),
2.1.5. 4-(4-n-Decylcyclohexyl)phenyl bromide (18) (as a mixture of
cis and trans- isomers)
Bromine (1.20 mL, 23.4 mmol) was added dropwise to a stirred
and ice-cooled solution of 4-n-decyclohexylbenzene (3.10 g,
10.4 mmol) and I2 (0.03 g, 0.11 mmol) in CH2Cl2 (5 mL) in the
presence of iron powder (5 g) with the rigorous exclusion of light.
After stirring for 1 day at room temperature aq. KOH (10 mL, 20%)
was added. The mixture was shaken and extracted with CH2Cl2
(ꢂ3). Combined extracts were dried over anhydrous Na2SO4,
filtered and the solvent was evaporated under vacuum. The crude
product was purified by silica gel column. Yield 2.09 g (51%). White
NMR (400 MHz, CDCl3):
d
¼ 10.29 (s, 1H), 7.60e7.53 (m, 1H),
7.15e7.09 (m,1H), 2.90 (tt, J1 ¼12.0 Hz, J2 ¼ 3.4 Hz,1H),1.94e1.83 (m,
4H),1.52e1.43 (m, 2H),1.37e1.16 (m,12H), 0.90 (t, J ¼ 7.0 Hz, 3H); MS
(EI): 294 (294.18) and Anal. Calcd for C18H24F2O (%): C 73.44, H 8.22;
found: C 73.30, H 8.22.
solid, m.p. 38e41 ꢀC 1H NMR (250 MHz, CDCl3):
d
¼ 7.39 (trans) (d,
J ¼ 8.4 Hz, 2H), 7.40 (cis) (d, J ¼ 8.5 Hz, 2H), 7.08 (trans) (d, J ¼ 8.4 Hz,
2H), 7.10 (cis) (d, J ¼ 8.8 Hz, 2H), 3.23 (trans) (tt, J ¼ 12.2 Hz,
J ¼ 3.1 Hz, 1H), 2.52 (m, 1H), 1.86 (d, J ¼ 10.5 Hz, 3H), 1.78e0.98 (m,
20 Hz), 0.88 (t, J ¼ 6.7 Hz, 3H).
2.1.4. 4-n-Decylcyclohexyl benzene (17) (as a mixture of cis- and
trans- isomers)
A mixture of n-lauryl aldehyde (14.31 g, 77.7 mmol), methyl
vinyl ketone (9.60 mL, 8.06 g, 115.0 mmol) and freshly distilled
diethylamine (1.60 mL, 1.13 g, 15.5 mmol) were dissolved in dry THF
(100 mL) and then stirred in a high pressure apparatus (autoclave)
at 80 ꢀC in argon atmosphere for 50 h. After completion of the
reaction an excess of methyl vinyl ketone and THF were evaporated.
The crude product was obtained as an oily brown colored solid. A
small amount of material (0.10 g) was purified for 1H NMR char-
acterization by SiO2 column chromatography with toluene/ether
2.1.6. 4-(4-n-Decylcyclohexyl)benzaldehyde (12)
A solution of n-butyllithium (2.5 M in hexane, 2.50 mL,
6.2 mmol) was added dropwise to a stirred and cooled (ꢁ78 ꢀC)
solution of 4-(4-n-decylcyclohexyl)phenylbromide (18) (2.09 g,
5.4 mmol) in dry THF under a dry argon pad. The mixture was
stirred at ꢁ78 ꢀC for 30 min, and then a solution of freshly distilled
DMF (0.38 mL, 0.36 g, 4.9 mmol) in absolute THF (10 mL) was added
dropwise at ꢁ78 ꢀC. The mixture was allowed to warm up to 25 ꢀC
over 2 h, quenched with aqueous saturated ammonium chloride
(40 mL) and extracted with diethyl ether (ꢂ3). The combined
organic layers were dried over anhydrous MgSO4. The obtained
solution was filtered, and solvents were evaporated. The residue
was purified on a silica column. Yield 0.71 g (40%) of colorless oil as
a mixture of cis- and trans- isomers. The pure trans isomer was
isolated by multiple recrystallization from pentane. Yield 0.17 g
(10:1) as an eluent. 1H NMR (250 MHz, CDCl3):
d
¼ 9.55 (d,
J ¼ 2.8 Hz, 1H), 2.45 (m, 2H), 2.25 (m, 1H), 2.13 (s, 3H), 1.91e1.72 (m,
2H), 1.71e1.40 (m, 2H), 1.36e1.19 (m, 16H), 0.88 (t, J ¼ 13.7 Hz, 3H).
The remaining crude 2-(3-oxobutyl)dodecanal was refluxed with
3 M HCl (200 mL) for 3 h. The product was extracted into ether
(ꢂ2), the combined ether layers were dried over anhydrous sodium
carbonate, filtered and evaporated to dryness. The obtained dark
brown liquid was eluted from SiO2 column with toluene, the eluate
was concentrated in a rotary evaporator and decolorized with
activated carbon. The remaining toluene was removed under
vacuum with heating and stirring (w50 ꢀC) during 3e4 h to give
(10%). 1H NMR (250 MHz, CDCl3):
d
¼ 9.97 (s, 1H), 7.80 (d, J ¼ 8.1 Hz,
2H), 7.36 (d, J ¼ 8.0 Hz, 2H) 2.55 (tt, J ¼ 12.2 Hz, J ¼ 2.9 Hz, 1H), 1.90
(d, J ¼ 10.4 Hz, 2H), 1.82e1.00 (m, 22H), 0.88 (t, J ¼ 6.2 Hz, 3H); MS
(EI): 328 (328.54) and Anal. Calcd for C23H36O (%): C 84.08, H 11.04;
found: C 83.96, H 11.23.
dry 4-n-decylcyclohexen-2-one-1 (15). Yield 13.62
g (69%),

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S.Y. Kim et al. / Journal of Organometallic Chemistry 696 (2011) 2429e2437
Pd[P(Ph)3]4; Na2CO3 (aq. 2M)
toluene; Δ
H2n+1Cn
Br
1
+
B(OH)2
Fe
F
F
6
7
Pd[P(Ph)3]4; Na2CO3 (aq. 2M)
toluene; Δ
Br
2
+
7
Fe
Fe
8
O
CnH2n+1
EtOH; Δ
NH2
NH2
+
C
3
[-H2O]
H
F
F
10
9
EtOH; Δ
+
10
4
[-H2O]
Fe
11
O
EtOH; Δ
C10H21
5
11
+
C
[-H2O]
H
12
Scheme 2. Synthetic paths to cyclohexane-containing ferrocenomesogens 1e5.
2.1.7. 4-[4-(trans-4-n-Penthylcyclohexyl)-2,3-difluorophenyl]
phenyl ferrocene (1a)
n-penthylcyclohexyl)-2,3-difluorobenzaldehyde
(0.046
g,
0.16 mmol) in hot ethanol (5 mL). The reaction mixture was
refluxed for 30 min until the formation of orange precipitate. The
product was filtered off. Yield 0.0215 g (25%). 1H NMR (250 MHz,
A solution of 4-bromophenylferrocene (0.106 g, 0.31 mmol) was
added to a stirred mixture of 4-(trans-4-n-penthylcyclohexyl)-2,3-
difluorophenylboronic acid (0.115 g, 0.37 mmol), tetrakis-(triphe-
nylphosphine) palladium(0) (0.015 g) in toluene (10 mL) and
aqueous sodium carbonate (2.0 M, 2 mL) in argon atmosphere. The
stirred reaction mixture was refluxed for 3 days. TLC controlled the
reaction completion. The product was extracted into diethyl ether
(ꢂ2), the ethereal extracts were washed with water and dried over
MgSO4. The residue after evaporation of solvents was placed on an
Al2O3 column, and eluted with hexane. Yield 0.026 g (16%) of
CDCl3):
d
¼ 8.76 (s, 1H), 7.94e7.79 (m, 1H), 7.44 (d, J ¼ 8.37 Hz, 2H),
7.14 (d, J ¼ 8.4 Hz, 2H), 7.07 (t, J ¼ 7.2 Hz,1H), 4.52 (d, J ¼ 2.2 Hz, 2H),
4.23 (t, J ¼ 2.0 Hz, 2H), 4.14 (t, J ¼ 1.8 Hz, 1H), 3.90 (t, J ¼ 1.7 Hz, 1H),
3.43 (t, J ¼ 1.2 Hz, 1H), 2.88 (tt, J ¼ 11.9 Hz, J ¼ 2.5 Hz, 1H), 2.26e1.77
(m, 11H), 1.75e1.00 (m, 15H), 0.90 (t, J ¼ 6.6 Hz, 3H); MS (MALDI-
TOF): 593.507 (593.26) and Anal. Calcd for C37H41F2FeN (%): C 74.87,
H 6.96, N 2.36; found: C 74.68, H 7.03, N 2.40.
orange powder. 1H NMR (250 MHz, CDCl3):
d
¼ 7.53 (d, J ¼ 8.3 Hz,
3. Results and discussion
2H), 7.46 (d, J ¼ 8.2 Hz, 2H), 7.16 (m, 1H), 7.04 (m, 1H), 4.69 (s, 2H),
4.36 (s, 2H), 4.09 (s, 5H), 2.87 (tt, J ¼ 12.2 Hz, J ¼ 2.9 Hz, 1H), 1.90 (d,
J ¼ 9.55 Hz, 2H), 1.82e1.00 (m, 14H), 0.90 (t, J ¼ 6.6 Hz, 3H); MS
(MALDI-TOF): 526.358 (526.21) and Anal. Calcd for C33H36F2Fe (%):
C 75.28, H 6.89; found: C 74.96, H 7.03.
3.1. Syntheses
Synthetic routes towards the ferrocene derivatives containing
a cyclohexane ring are sketched in Scheme 2. 4-Bromophenyl
ferrocene 6 was obtained by applying a GombergeBachman proce-
dure between ferrocene and 4-bromophenyl diazonium salt in the
presence of a phase transfer catalyst. Suzuki-Miyaura cross-coupling
reaction between 6 and boronic acid 7 yielded compounds 1(aeb)
(n ¼ 5 or 10). The same sequence of reactions starting from [3]
2.1.8. 3-{4-[4-(trans-4-n-Penthylcyclohexyl)-2,3-
difluorobenzaldimino]phenyl}[3]ferrocenophane (3a)
4-Aminophenyl[3]ferrocenophane (0.050 g, 0.16 mmol) was
dissolved in hot ethanol (5 mL) and added to a solution of 4-(4-
CnH2n+1
CnH2n+1
DMF
n-BuLi
Li
10
F
F
13
F
F
14
OH
H2; [Pd/C]
C10H21
PhLi
C10H21
C10H21
O
17
15
16
n-BuLi
Br2; [Fe]
benzene
C10H21
C
10H21
DMF
17
Br
12
Li
19
18
Scheme 3. Synthetic path to cyclohexane-containing aldehydes 10 and 12.

S.Y. Kim et al. / Journal of Organometallic Chemistry 696 (2011) 2429e2437
2433
ferrocenophane [36] led to compounds 2(aeb) (n ¼ 5 or 10). Ferro-
cene and [3]ferrocenophane derivatives 3e5 with the azomethine
linkage were obtained by condensation of ferrocene-containing
anilines 9 or 11 with cyclohexane-containing aldehydes 10 or 12.
The synthesis of alkylcyclohexyl-terminated boronic acid 7 and
its precursor 13 has been described earlier [25]. Being a valuable
intermediate for preparation of boronic acid compound 13 also can
be transformed into aldehyde 10 by quenching corresponding
organolithium reagent 14 with DMF (Scheme 3). Required trans-
isomer 10 was isolated from a mixture of isomers by crystallization
from hexane, while a cis-isomer remains in the mother liquor.
A synthetic path to lithium reagent 19 was less straightforward
in comparison with 14. Contrary to fluorinated aromatic rings, non-
fluorinated analogues cannot be lithiated directly. Therefore, it was
necessary to prepare a brominated precursor 18 for subsequent
lithiation. The cross-coupling reaction of cyclohexenone 15 with
phenyllithium allowed obtaining cyclohexenol 16. Hydrogenation
of 16 in the presence of 5% Pd on charcoal under pressure of
3e5 atm led to a mixture of stereoisomers 17. Ionic bromination of
17 with the powdered iron catalyst afforded p-brominated
compound 18. Lithiation and subsequent quenching of organo-
lithium reagent 19 with DMF finally gave aldehyde 12 as a mixture
of cis- and trans- isomers together with by-products. Required
trans-isomer 12 crystallizes in hexane.
can be considered as a part of the rigid core, so the structure 1 can
serve as a basic three-ring system for the comparative studies.
The following trends are observed depending on the structural
variations from the basic structure 1. The introduction of the 1,3-
propylidene bridge in the ferrocene fragment of compounds 2(aeb)
leads to appearance of the monotropic nematic mesophases (see
Table 1). Thus, the [3]ferrocenophane moiety brings larger
geometric anisotropy and closer resemblance to terminal alkyl
chains favoring liquid crystalline behavior. Further increase of the
geometric anisotropy by addition of the azomethine linking group
in compounds (3aeb) leads to additional enhancement of their
liquid crystalline properties in comparison with 2(aeb).
Thermal behavior of 3a is worth a separate consideration. This
compound showed an interesting phenomenon of migration of the
phase transition points in the subsequent heating cycles. The
melting point of the initial crystal form Cr1 in the first heating cycle
exceeds by 35 ꢀC the analogous value in the second heating cycle
and by 10 ꢀC the isotropization point (see Fig. 2a and b). It should be
noted the absence of a crystallization peak in the cooling course and,
accordingly, vitrification of the nematic state at room temperature.
By all accounts, crystallization in the subsequent heating path
occurs into a metastable crystal form Cr2 with the exothermic heat
effect. Apparently, the observed Cr2, Cr3 and enantiotropic nematic
mesophase in the subsequent heating cycles must be regarded as
a metastable with respect to Cr1 from the thermodynamic point of
view. In all probability exceptionally high energy barrier for initial
Elemental analyses, 1H NMR and mass-spectra of the synthe-
sized compounds are in a good agreement with the proposed
chemical compositions and structure.
3.2. Thermal behavior
Liquid crystal properties of the synthesized ferrocenomesogens
were investigated by means of polarized light microscopy (POM)
and differential scanning calorimetry (DSC).
According to POM observations almost all of the compounds
1e5 showed the nematic mesophases with characteristic schlieren
and marbled textures excluding non-mesomorphic compounds 1
(aeb). Typical examples of the optical textures are represented in
Fig. 1. Appropriate nematic-to-isotropic and isotropic-to-nematic
phase transitions were detected on the DSC thermograms as minor
peaks (see Supplementary information). In many cases, the
obtained compounds showed thermotropic crystal polymorphism.
Compound 1a is non-mesomorphic but shows a crystal-to-
crystal polymorphic transition just before a transition to the
isotropic liquid state (see DSC thermograms in Supplementary
information). Increasing the length of the terminal alkyl chain in
1b does not lead to inducing liquid crystallinity as well. Though 1
(aeb) do not exhibit liquid crystal properties the absence of
chemically and hydrolytically unstable groups is exceptionally
valuable feature of these compounds. Taking into account their
promesogenic architecture, the obtained compounds must be
highly adaptable for the mixed liquid crystal compositions.
Earlier investigations of mono-substituted ferrocenomesogens
have allowed outlining several structural criteria that are crucial for
generating liquid crystallinity and increasing stability of the
mesomorphic state [4e14]. Some of those factors are the presence
of at least three aromatic rings in the rigid core not counting
cyclopentadienyl rings, additional linking groups for widening
mesophases and halogen substituents in a lateral position for
decreasing transition temperatures. Additionally, all previously
studied mono-substituted ferrocenomesogens contained one or
several carboxyl, azomethine, azine or azo linking groups. In
contrast, compounds 1(aeb) include two adjacent benzene rings
and one cyclohexane ring in the ferrocene substituent without any
linking groups. Normally, a cyclohexane ring in rod-like mesogens
Fig. 1. Microphotographs of a schlieren texture of the nematic mesophase with the
growing crystal spherulites obtained at 84 ꢀC on cooling 2a from the isotropic liquid
state (on the top) and a marbled texture of 4b at 114 ꢀC (on the bottom).

2434
S.Y. Kim et al. / Journal of Organometallic Chemistry 696 (2011) 2429e2437
Table 1
Thermal properties of compounds 1e5 by DSC and POM.
Compound
Phase transitionsa, oC (
DH, kJ/mol)
1st heating
1st cooling
2nd heating
2nd cooling
1a
1b
Crb 122.6 I (116.8)
Cr1 54.1 Cr2 (9.8)
Cr1 121.7 I (20.5)
Cr1 139.1 I (21.6)
I 65.8 Cr (ꢁ84.8)
I 112.3 Cr2 (ꢁ19.9)
Cr2 50.4 Cr1 (ꢁ8.9)
I [86.4]c N (ꢁ0.1)
N 83.0 Cr þ G
e
e
e
e
2a
2b
3a
Cr þ G 55.9 Cr2 (ꢁ0.9)
Cr2 133.0 Cr1 (5.9)
Cr1 139.8 I (14.8)
Cr3 124.2 (33.7) I
I [86.4] N (ꢁ0.1)
N 83.0 Cr þ G
Cr1 54.0 Cr2 (1.0)
Cr2 58.7 Cr3 (5.5)
Cr3 124.8 I (32.6)
Cr1 155.3 I (39.7)
I
d [81.8] N (ꢁ0.6)
I [82.3] N (ꢁ0.7)
N 61.0 Cr3 (ꢁ32.2)
N 61.3 Cr3 (ꢁ31.0)
I [143.5] N (ꢁ0.5)
G 82.0 Cr2 (ꢁ7.3)
Cr2 121.0 N (27.6)
N 144.9 I (0.6)
I [143.6] N (ꢁ0.6)
3b
4a
Cr 122.6 I (28.1)
I [116.5] N (ꢁ0.3)
N 60.8 Cr (ꢁ16.7)
I 133.8 N (ꢁ1.2)
Cr 120.3 I (27.5)
I [116.5] N (ꢁ0.5)
N 61.6 Cr (ꢁ18.5)
I 134.0 N (ꢁ1.2)
Cr1 119.2 N (26.2)
N 135.2 I (1.3)
G 99.4 Cr (ꢁ2.7)
N 63.6 Cr1þCr2 (ꢁ10.0)
Cr1þCr2119.0N þ Cr2 (19.1)
N þ Cr2 125.0 Cr2 (ꢁ24.7)
Cr2 142.3 I (26.9)
N 63.5 Cr1þCr2 (ꢁ10.2)
4b
Cr1 109.6 N (30.2)
N 117.1 I (1.2)
I 116.4 N (ꢁ1.4)
Cr3 99.4 (21.1) N
N 117.5 I (1.2)
I 116.3 N (ꢁ1.3)
N 81.2 Cr2 (ꢁ12.2)
Cr2 66.3 Cr3 (ꢁ2.3)
N 81.1 Cr2 (ꢁ11.0)
Cr2 65.0 Cr3 (ꢁ4.8)
5
Cr 143.4 I (11.2)
I
d 134.3 N (ꢁ0.2)
N 126.6 Cr (ꢁ14.3)
a
Cr e crystal, N e nematic, I e isotropic phases, G e vitrified nematic.
Additionally a crystal polymorphic transition was observed just before isotropisation.
Square brackets are standing for the monotropic phase transitions.
The monotropic phase is detectable with the cooling rate 20 ꢀC/min.
b
c
d
Fig. 2. DSC traces of 3a (enthalpy values in kJ/mol are given in parentheses): a) the first heatingecooling cycle at 5 ꢀC/min; b) the second heatingecooling cycle at 5 ꢀC/min; c) the
same sample after 3 days annealing in the third heatingecooling cycle at 1 ꢀC/min; d) the fourth heatingecooling cycle at 1 ꢀC/min.

S.Y. Kim et al. / Journal of Organometallic Chemistry 696 (2011) 2429e2437
2435
nucleation of Cr1 phase from the other crystal polymorphic modi-
fications and nematic liquid leads to slow kinetics of crystallization
to the Cr1 form. Even annealing the sample during 2e3 h at
130e135 ꢀC did not lead to appearance of the stable crystal modi-
fication Cr1. Nevertheless, the system returns to an initial state by
keeping the nematic glass at room temperature for several days
(Fig. 2c). Changing the scanning rate from 5 ꢀC/min to 1 ꢀC/min
revealed the existence of another metastable crystal form Cr3 with
the melting point exceeding by 10 ꢀC the previous metastable form
Cr2 (Fig. 2d). Moreover, even at this slow heating rate compound 3a
shows enantiotropic mesomorphism contrary to its initial mono-
tropic behavior. The sharpness of the appropriate DSC peaks
undoubtedly indicates the presence of the above crystal forms Cr2
and Cr3 as individual modifications but not the mixtures. Compli-
cated thermal behavior of ferrocenyl compounds with multiple
melting points is not uncommon [37]. It is noteworthy that the
analogous compound 3b with a longer terminal alkyl chain shows
the regular monotropic nematic mesophase in repeated DSC cycles
(see Supplementary information).
Compounds 4(aeb) without the 1,3-propylidene bridge exhibi-
ted enantiotropic nematic mesomorphism contrary to 3(aeb)
showing metastable mesophases. Normally ferrocenophane deriv-
atives displayed better mesophase characteristics with reference to
analogous derivatives of the unbridged ferrocene [20,21]. A positive
effect of the 1,3-proppylidene bridge in the reported cases was
connected with lowering the mesophase transitions and broadening
mesophases. In the present case [3]ferrocenophane derivatives 3
(aeb) have higher melting points in comparison with ferrocene
derivatives 4(aeb) and comparable clearing points. The latter
parameter characterizes thermal stability of the nematic phase. It
looks like the higher melting points are the main motive for the
comparatively poor mesomorphism of 3(aeb) in spite of their larger
geometric anisotropies and resemblance of the 1,3-propylidene
bridge to alkyl substituents. On the other hand, ferrocenophane
derivatives as alkylidene terminated mesogens follow in this case
a well-known rule for homologous series of liquid crystals with the
mesophase transition temperatures below 120 ꢀC, when the longer
alkyl chains increase the phase transition temperatures.
Both compounds 4(aeb) also show crystal polymorphism and
migration of the melting points in subsequent heating cycles.
Compound 4a exhibited the enantiotropic nematic mesophase as it
can be seen from its DSC thermogram at the scanning rate of 5 ꢀC/
min (Fig. 3a). Unexpectedly, on the second heatingecooling cycle
the transition to the nematic state was immediately followed by
exothermic crystallization of the sample (Fig. 3b). Apparently, the
latter crystal form Cr2 is thermodynamically more stable than the
initial Cr1 and the nematic states observed in the first heating cycle.
It is noteworthy that the nematic-to-isotropic transition is absent in
the second DSC cycle and replaced by a large crystal-to-isotropic
peak. The melting point of the Cr2 phase exceeds by 7 ꢀC the
nematic-to-isotropic liquid phase transition in the first heating
cycle. Obviously, the crystal form Cr2 was not observed in the first
heating cycle due to the slow kinetics of the crystallization process.
We suggest that the initial crystal form Cr1 is present in the virgin
sample. The formation of the second crystal form Cr2 from the
nematic state in this case must be extremely slow in the absence of
crystal seeds. However, both crystal forms may be present in the
sample after cooling from the isotropic liquid state. So, the second
heating process leads to the melting of the Cr1 phase to the nematic
state which immediately crystallizes to the stable Cr2 state in the
presence of unmelted Cr2 centers. In fact, DSC measurements at
1
ꢀC/min allowed detecting the Cr2 phase even in the first hea-
Fig. 3. DSC traces of 4a (enthalpy values in kJ/mol are given in parentheses): a) the first
heatingecooling cycle at 5 ꢀC/min; b) the second heatingecooling cycle at 5 ꢀC/min; c)
the first heatingecooling cycle at 1 ꢀC/min in the range of 110e150 ꢀC.
tingecooling cycle (Fig. 3c). Moreover, the transition Cr1-to-Cr2 in
the latter case is hidden and occurs with a negligible heat effect.
Thus, we observe in 4a a delicate thermodynamic balance between

2436
S.Y. Kim et al. / Journal of Organometallic Chemistry 696 (2011) 2429e2437
Fig. 4. DSC traces of 4b (enthalpy values in kJ/mol are given in parentheses): a) the first heatingecooling cycle at 5 ꢀC/min; b) the second heatingecooling cycle at 5 ꢀC/min; c) the
third heatingecooling cycle at 5 ꢀC/min after annealing the sample at room temperature for 3 days; d) the fourth heatingecooling cycle at 1 ꢀC/min in the range of 90e130 ꢀC.
stable and metastable crystal forms, which leads in due order to the
rate-depended alteration between enantiotropic and monotropic
nematic mesomorphism.
Thus, a remarkable drift of the melting points was observed in
compounds 3a and 4(aeb). According to DSC experiments the
presence of metastable crystal forms and extremely slow kinetics of
transition to the stable crystal states have been recognized as being
responsible for this unusual behavior.
Compound 4b with the longer terminal alkyl substituent also
shows complex behavior connected with the crystal poly-
morphism. The enantiotropic nematic mesophase was observed by
POM and the corresponding phase transitions are measured by DSC
at the scanning rate of 5 ꢀC/min (Fig. 4a). A significant drop
approximately by 10 ꢀC down was detected for the crystal-to-
nematic transition in the second heatingecooling cycle (Fig. 4b). It
appears that the crystal form Cr1 of 4b obtained on crystallization
from the solvent is not formed on crystallization from the isotropic
melt. The enthalpy of the crystal-to-nematic transition in the
second DSC cycle is significantly lower than in the first heating
course (see Table 1). Consequently, during crystallization from the
nematic melt the metastable crystal phases Cr2 and Cr3 with higher
energy were obtained. And a transition of the latter Cr3 phase to the
thermodynamically stable Cr1 state is again prevented by slow
kinetics of the crystallization processes. However, after annealing
the sample at room temperature for several days a small peak of the
Cr1-to-nematic transition appears in a DSC thermogram at the
scanning rate 5 oC/min (Fig. 4c). The slower scanning rate of 1 ꢀC/
min revealed exothermic crystallization processes after a Cr3-to-
nematic peak at 99 ꢀC and an increased peak of the Cr1-to-nematic
transition at 109 ꢀC (Fig. 4d).
4. Conclusions
A series of novel mono-substituted ferrocenomesogens with
a cyclohexane ring incorporated in the rigid core has been inves-
tigated by using DSC and thermal polarisational microscopy. A basic
structure with two aromatic and one carbocyclic ring in the ferro-
cene substituent does not exhibit liquid crystallinity. The addition
of the 1,3-propylidene bridge in the ferrocene moiety led to
appearance of the monotropic nematic mesophase. Inclusion of the
azomethine linkage group into the rigid core significantly enhances
liquid crystallinity and results in the enantiotropic nematic meso-
phase. Lateral fluoro-substituents in the benzene rings play a posi-
tive role in lowering transition temperatures and stabilizing the
nematic state. Migration of the phase transition points was
observed in some of the synthesized compounds. A delicate ther-
modynamic balance between the metastable and stable crystal
forms leads to alteration between enantiotropic and monotropic
mesomorphism depending on the thermal prehistory of the
sample. Chemically resistant, low-viscous and low-temperature
ferrocene-containing liquid crystals potentially can be used as
individual compounds or as mixtures in various technological
applications.
A comparison of 5 with 4b clearly demonstrates a positive role
of lateral fluorination in the pursuance of low-temperature ferro-
cene-containing nematics. The latter fact has been already
emphasized in systematic investigations of Loubser and Imrie [14].
Dissimilar to fluorinated 4b showing enantiotropic mesomorphism
its non-fluorinated counterpart 5 exhibits only the monotropic
nematic phase. The considerably higher melting point of 5 could be
a reasonable explanation of its poor mesomorphism.
Acknowledgements
This work has been supported by a grant from the Korea Science
and Engineering Foundation (KOSEF) through the Center for

S.Y. Kim et al. / Journal of Organometallic Chemistry 696 (2011) 2429e2437
2437
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Appendix A. Supplementary material
Supplementary data related to this article can be found online at
doi:10.1016/j.jorganchem.2011.03.013.
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