Synthesis and structural, electrochemical, and stacking properties of conical molecules possessing buckyferrocene on the apex

Article abstract of DOI:10.1021/ja062757h

A series of conical molecules featuring a [60]fullerene/ferrocene hybrid and five aralkyl side chains (Fe[C₆₀{C₆H₄-(OCO-C₆H₃-(OR)₂-3,4)-4}₅]Cp) have been synthesized and examined for their structural and electrochemical properties as well as their ability to form supramolecular structures in crystals and liquid crystals. When the R group on the side is a methyl group, the compound forms crystals in which the dipolar conical molecules are stacked head-to-tail to form a columnar structure. When the R group is as long as a C₁₈H₃₈ group, the compound forms liquid crystals. Oxidation of the liquid crystalline compound by an aminium salt [(4-BrC₆H₄)₃N][SbCl₆] produces the corresponding paramagnetic Fe(III) compound that also exhibits liquid crystalline properties. Copyright

Full text of DOI:10.1021/ja062757h

Published on Web 07/04/2006
Synthesis and Structural, Electrochemical, and Stacking Properties of Conical
Molecules Possessing Buckyferrocene on the Apex
Yutaka Matsuo,† Ayako Muramatsu,‡ Yuko Kamikawa,# Takashi Kato,# and Eiichi Nakamura*,†,‡
Nakamura Functional Carbon Cluster Project, ERATO, Japan Science and Technology Agency, Department of
Chemistry, The UniVersity of Tokyo, Hongo, Bunkyo-ku, Tokyo 113-0033, Japan, and Department of Chemistry and
Biotechnology, School of Engineering, The UniVersity of Tokyo, Hongo, Bunkyo-ku, Tokyo 113-8656, Japan
Received April 20, 2006; E-mail: nakamura@chem.s.u-tokyo.ac.jp
There has been a growing interest in functional liquid crystal
materials.1 For instance, incorporation of metal atoms into liquid
crystalline materials (metallomesogens) provides new opportunities
in materials science through tuning of anisotropic optical, electronic,
and magnetic properties.2,3 Mixed fullerene/ferrocene mesogens4
have therefore attracted considerable attention for their donor/accep-
tor characteristics and potential redox switching activity.5 Though
the idea is simple, incorporation of a bulky fullerene and ferrocene
units as integrated parts of the bulk liquid crystal supramolecular
structure has been rather difficult in practice. We considered that
the compact, C5-symmetric buckyferrocene (fullerene and ferrocene
sharing one of their pentagons in common, cf. 2)6 would serve as
the core structure of such metallomesogens and that installation of
extended side chains (cf. 5) will allow the resulting conical, shuttle-
cock-like molecule to stack into a columnar supramolecular assem-
bly (Figure 1).7 Herein we report on the successful implementation
Figure 1. Columnar stacking of 4a and 4e. (a) Crystal structure of 4a‚
(toluene)4 from the a-axis. Toluene molecules in crystal packing are omitted
for clarity. Views from the b- and c-axes are shown in Supporting
Information. (b) A model of the stack of 4e in liquid crystals (based on
SAXD data described in the text).
of the design and describe the synthesis and electrochemical proper-
ties of buckyferrocene shuttlecocks and ordered supramolecular
structures of the Fe(II) (4) and Fe(III) (5) compounds (Scheme 1).
Synthesis of the conical molecules started by converting Ih-
symmetric C60 into Cs-symmetric C60(C6H4-OMe-4)5H (1) by treat-
ment of C60 with a (4-methoxyphenyl)copper reagent.8 Complex-
ation of 1 with [FeCp(CO)2]2 (Cp ) C5H5) in benzonitrile at 175
°C afforded a buckyferrocene, Fe[C60(C6H4-OMe-4)5]Cp (2),9 which
was deprotected with BBr3‚SMe2 in 1,2-dichlorobenzene at 165 °C
to obtain the phenol derivative, Fe[C60(C6H4-OH-4)5]Cp (3). Acyla-
tion of 3 with appropriate benzoic chloride derivatives possessing
alkyl side chains afforded the desired conical molecules, Fe[C60-
{C6H4-(OCO-C6H3-(OR)2-3,4)-4}5]Cp (4a, R ) methyl; 4b, R )
dodecyl; 4c, R ) tetradecyl; 4d, R ) hexadecyl; 4e, R ) octadecyl)
Scheme 1
1
(Scheme 1). Their H and 13C NMR spectra were consisted with
the expected η5-coordinated, C5-symmetric structure of 4a-e. The
elemental composition and the purity of the samples used for the
subsequent studies were established by elemental analysis.
The compound lacking long side chains (4a, R ) Me) gave
crystals. Red prism crystals of 4a suitable for X-ray crystallographic
study were obtained by slow diffusion of methanol into a chloroform
solution. The columnar structure of 4a (view from the a-axis) is
shown in Figure 1a. The bottom of the concave cavity formed by
the five aryl groups in 4a is filled by a ferrocene group (ca. 3.4 Å
in height) and therefore is shallower than that of the corresponding
compound lacking the ferrocene group (e.g., the protio compound
1 vs 2).7 The shallowness of the cavity made the intermolecular
stacking structure much less ordered than in the protio compound.
As a result, the molecular axis is tilted by 18.9° against the columnar
axis (as opposed to 0° in the protio compound; see the crystal pack-
ing view along the c-axis in the Supporting Information). The inter-
molecular stacking distance within a column (17.8 Å) is much long-
er (by 6.7 Å) than in the corresponding protio case (11.1 Å),7 hence
leaving a space between the molecules within a column. Note that,
in the protio case, the highly acidic cyclopentadienyl hydrogen10
(cf. 1) is in direct contact with the π-surface of the bottom of the
next fullerene molecule in the stack, as if there is hydrogen bonding.
The molecules bearing longer side chains (4d,e) showed liquid
crystalline behavior. The mesophases were identified by differential
scanning calorimetry (DSC), polarizing optical microscopy, and
small-angle X-ray diffraction analysis (SAXD). Compound 4e
showed clear mesomorphism over a wide temperature range. The
† ERATO, Japan Science and Technology Agency.
‡ Department of Chemistry, The University of Tokyo.
# School of Engineering, The University of Tokyo.
9
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10.1021/ja062757h CCC: $33.50 © 2006 American Chemical Society

C O M M U N I C A T I O N S
Ph5)Cp lacking the side chains (Figure 2d).6 A reversible one-
electron oxidation of the metal center was observed at 0.57 V (vs
Fc/Fc+). This potential is higher than that of Fe(C60Ph5)Cp, which
lacks the ester side chains (0.50 V). Upon reduction, 4e showed
three-step, one-electron reduction at -1.30, -1.88, and -2.35 V
(vs Fc/Fc+), which indicates that the compound is slightly easier
to reduce than Fe(C60Ph5)Cp, as one would expect.
We synthesized stable ferrocenium cations 5c and 5e. Com-
pounds 4c and 4e were treated with an aminium salt [(4-BrC6H4)3N]-
[SbCl6] in CH2Cl2 to obtain the corresponding ferrocenium
derivatives 5c,e (Scheme 1). The ferrocenium 5e was paramagnetic
character, showing g-values of 3.75 (g|) and 1.70 (g ) at 4.4 K,
and showed liquid crystalline behavior at the temperature range
from 25 to 161 °C. Further characterization is underway.
In conclusion, we have synthesized a new class of metallome-
sogens that show reversible multielectron redox behavior, accepting
and giving up total of at least four electrons. Given a vast variety
of η5-fullerene metal complexes available at this time,11 we expect
that a whole new generation of fullerene/metal complexes would
be built into mesophases and utilized for a variety of applications.
Supporting Information Available: Procedures for synthesis of
new compounds, crystallographic data of 2 and 4a, optical textures and
DSC data of 4b-e and 5e, and XRD data of 4d,e (PDF, CIF). This
material is available free of charge via the Internet at http://pubs.acs.org.
Figure 2. Characterization and electrochemical properties of 4e. (a) DSC
trace of 4e recorded during the first cooling. (b) DSC trace of 4e recorded
during the second heating. (c) SAXD data for 4e at 100 °C. (d) Cyclic
voltammogram of 4e at 25 °C in a THF solution containing n-Bu4NClO4
as a supporting electrolyte (scan rate 100 mV/s).
first cooling after heating to 170 °C (50 °C above the clearing point)
showed two exothermic peaks at 97 and 10 °C (Figure 2a), and
the second heating process showed two endothermic peaks at 21
and 120 °C (Figure 2b).
Noteworthy was a large transition enthalpy at the clearing
temperature (77 and 88 kJ/mol), which was much larger than those
of the corresponding pentaaryl-C60 protio compounds (5-21 kJ/
mol),7a related silicon-tethered conical protio analogues (27-35 kJ/
mol),7b and other nonconical fullerene/ferrocene hybrid mesogens
(11.7 kJ/mol).4a We ascribe the observed supercoolability and the
high transition enthalpies partially to the polarity of the buckyfer-
rocene moiety.6a
Compound 4d (see Supporting Information for details) also
exhibited supercoolability and showed, during the second heating,
a crystal-to-liquid crystal transition at -3 °C and a liquid crystal-
to-isotropic liquid transition at 123 °C. The transition enthalpy at
the clearing temperature was also very large (93 kJ/mol). As found
in the protio series,7 the isotropization temperature that is related
to the fullerene/fullerene stacking did not change much as the side
chains got longer, while the melting temperature that is related
largely to the side chain/side chain interactions became lower (4d,
-3 °C; 4e, 20 °C). Compounds 4b,c, having shorter side chains,
did not show typical liquid crystalline behavior. Crystallization (at
85 °C for 4b, 101 °C for 4c) and melting (at 124 °C for 4b, 120
°C for 4c) peaks were observed during the heating.
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(200), and 18.3 Å (001) and a broad halo typical of molten aliphatic
moieties at 4.5 Å. Diffraction indexed to (110) was not observed.
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distance and is larger than the d-spacing values of 38.3 Å for 4d.
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