Received 27 August 2008
Synthesis and physical properties of ferrocene derivatives. XXI. Crystal structure of a liquid crystalline ferrocene derivative, 1,1'-bis[3-[4-(4-methoxyphenoxycarbonyl)phenoxy]propyloxycarbonyl]ferrocene
Takashi Okabe,a+ Keisuke Nakazaki,a Tsuyoshi Igaue,a Naotake Nakamura,a* Bertrand Donnio,b Daniel Guillonb and Jean-Louis Gallanib
aDepartment of Applied Chemistry, College of Science and Engineering, Ritsumeikan University, 1-1-1 Nojihigashi, Kusatsu, Shiga 525-8577, Japan, and bInstitut de Physique et Chimie des Matériaux de Strasbourg, UMR 7504, Groupe des Matériaux Organiques, 23 rue du Loess, BP43, F-67034 Strasbourg Cedex 2, France
The crystal structure of the title 1,1'-disubstituted ferrocene derivative was determined by X-ray diffraction using a single crystal. This compound exhibits a nematic phase only. The X-ray structure analysis revealed that the two substituents lie in the same direction (`U' shape) and the flexible spacer adopts a gauche conformation. These conformations are strongly related to the formation of a rod-like shape, which favors liquid crystalline behavior. In the crystal structure, C-H, - and T-stacking interactions were observed. It is considered that these interactions play a major role in stabilizing the molecular packing arrangement and the mesomorphism.
Ferrocene is a metallocene that shows a remarkable aromaticity combined with thermal stability. It is well known that ferrocene derivatives have the potential to be liquid crystalline materials with unique properties, such as electrochemical (Carano et al., 2002) and photoinduced electron transfer (Even et al., 2001) properties. As substitution reactions occur easily at the cyclopentadienyl rings in the ferrocene molecule, just like in benzene, many different types of liquid crystalline ferrocene derivative have been synthesized and investigated. A number of papers concerned with ferrocene-containing liquid crystals have been published already. These papers deal with the thermal and mesomorphic properties of monosubstituted ferrocenes (Malthête & Billard, 1976; Imrie et al., 2001, 2003; Nakamura, Hanasaki & Onoi, 1993; Nakamura, Hanasaki, Onoi & Oida, 1993; Hanasaki et al., 1993; Nakamura et al., 1994, 1995; Nakamura & Takayama, 1997; Nakamura & Setodoi, 1998a,b, 1999a,b; Nakamura & Oida, 1999; Nakamura, Maekawahara et al., 2000; Nakamura, Setodoi & Hanasaki, 2000; Nakamura, Setodoi & Takayama, 2000; Nakamura et al., 2002; Nakamura et al., 2006a,b; Zhao et al., 2001; Deschenaux, Marendaz et al., 1995; Loubser et al., 1993; Loubser & Imrie, 1997), 1,1'-, 1,3- and 1,2-disubstituted ferrocenes (see next paragraph), 1,1',3-trisubstituted ferrocene derivatives (Deschenaux, Kosztics & Nocolet, 1995), polycatenar ferrocenes (Deschenaux, Monnet et al., 1998), and ferrocene-containing liquid crystalline dendromers (Carano et al., 2002; Even et al., 2001; Chuard & Deschenaux, 2003; Deschenaux et al., 1997; Dardel et al., 1999; Deschenaux, Even & Guillon, 1998; Campidelli et al., 2004) [see also the reviews by Donnio et al. (2003) and Bruce et al. (2007a)].
From the structural viewpoint, disubstituted ferrocene derivatives can be classified into three families according to the positions of the substituent: 1,1'-, 1,2- and 1,3-disubstituted ferrocene derivatives. It is well known that the 1,1'-disubstituted ferrocene derivatives can adopt an `S' (Nakamura et al., 2005), a `U' (Nakamura & Okabe, 2004; Nakamura et al., 2007) or a `Z' (Nakamura & Nishikawa, 2005) shaped conformation. On the other hand, the 1,3-disubstituted ferrocene derivatives give a `T' (Deschenaux et al., 1993) shaped conformation. In the `S', the `T' and the `Z' shaped conformations, the two substituents lie in opposite directions with respect to the ferrocenyl group, but the `U' shape means that the two substituents lie in the same direction. In general, it is advantageous for the appearance of liquid crystallinity that the aspect ratio of molecular length to breadth should be relatively large. Therefore, it is considered that the `S' and the `T' shaped conformations are typical for liquid crystals of 1,1'-disubstituted ferrocene derivatives. In addition, it has been reported that 1,2-disubstituted ferrocene derivatives do not show liquid crystalline properties because of their unique hairpin structures (Bruce et al., 2007b).
A series of disubstituted ferrocene derivatives, 1,1'-bis[-[4-(4-methoxyphenoxycarbonyl)phenoxy]alkyloxycarbonyl]ferrocene (abbreviated hereafter as bMAF-n, n = 2-12, where n is the number of C atoms in the methylene chain), have been prepared in our laboratory. The general structure of bMAF-n is shown in Fig. 1. In the homologous series, bMAF-3 and bMAF-5-bMAF-12 show liquid crystallinity, with nematic and smectic (except n = 3) phases. In contrast, bMAF-2 and bMAF-4 do not exhibit any liquid crystallinity (Hanasaki et al., 1994; Nakamura et al., 1998). The crystal structures of bMAF-2 (Nakamura & Nishikawa, 2005), bMAF-5 (Nakamura et al., 2005), bMAF-9 (Nakamura et al., 2007) and bMAF-10 (Nakamura & Okabe, 2004) have already been determined. In the crystal of bMAF-2 (Nakamura & Nishikawa, 2005), the molecule adopts the `Z' shaped conformation, and bMAF-5 (Nakamura et al., 2005) adopts the `S' shaped one, whereas in the crystals of bMAF-9 (Nakamura et al., 2007) and bMAF-10 (Nakamura & Okabe, 2004), the molecules exhibit the `U' shaped conformation.
| || Figure 1 |
General structure of bMAF-n (n = 2-12).
Correlations between the crystal structure and some physical properties can be deduced from the structure analyses.
In this study, the crystal and molecular structures of bMAF-3 were determined by X-ray diffraction methods using single crystals. Some interesting interactions will be discussed.
Single crystals of the title compound were obtained from a mixed solvent solution of ethylacetate and heptane (1:1) by slow evaporation. The single crystals obtained are orange in color and plate-like. The sample, which had approximate dimensions of 0.75 × 0.10 × 0.05 mm, was mounted on a goniometer.
Data collection was carried out at 294 K, using a Rigaku AFC5R four-circle diffractometer with graphite-monochoromated Cu K ( = 1.54178 Å) radiation, operated at 50 kV and 200 mA.
The unit-cell parameters were obtained from a least-squares refinement using the setting angle of 22 reflections in the range 18.27 < 2 < 26.62°. The data were collected using the -2 scan technique to a maximum 2 value of 140.3°.
6153 reflections were measured, of which 4059 were unique. The intensities of three standard reflections were measured after every 150 reflections. Over the course of the data collection, the intensity of the standards decreased by 0.88%. The data were also corrected for Lorentz and polarization effects and for absorption [numerical (Coppens et al., 1965); the minimum and maximum transmission factors were 0.60091 and 0.83876, respectively].
All calculations were performed using the WinGX crystallographic software package (Farrugia, 1999). The atomic scattering factors were taken from Cromer & Waber (1974). The structure was solved by direct methods (SIR92; Altomare et al., 1994) and expanded using the Fourier technique. All non-H atoms were refined anisotropically, and all H atoms were refined isotropically. The H atoms were introduced at their theoretical positions and allowed to ride with the C atoms to which they are attached. The final refinement was made by full-matrix least-squares analysis based on 4059 observed reflections. The refinement cycles converged to R = 0.038, wR(F2) = 0.098, w = 1/[2(Fo2) + (0.0211P)2 + 1.2943P], where P = [Max(Fo2, 0) + 2Fc2]/3, and S = 1.052. The maximum shift s.u. ratio for all atoms in the final cycle is less than 0.001. The final cycles of the refinement resulted in a residual electron density in the range -0.369-0.346 eÅ-3.
Experimental details are summarized in Table 1.1
| || Figure 2 |
ORTEPIII (Johnson & Burnett, 1996) views of the molecular structure excluding H atoms in bMAF-3. Displacement ellipsoids are drawn at the 50% probability level. Showing (a) the crystallographic numbering scheme, (b) a side view onto the cyclopentadienyl rings, (c) a view from above, onto the cyclopentadienyl rings, and (d) a projection along the long axis of the molecule. C atoms: black; O atoms: red; Fe atoms: brown.
In the ferrocenyl group, the two cyclopentadienyl rings are almost parallel; the dihedral angle is 1.0 (2)° (Table 2). The average values of the Fe-C and C-C bond distances in the ferrocenyl unit of bMAF-3 are 2.050 (5) and 1.421 (7) Å, respectively. The C-C-C bond angles in the cyclopentadienyl rings are 108.0 (4)°. These values agree with those of ferrocene reported elsewhere (Nakamura & Setodoi, 1998a,b, 1999a,b; Nakamura et al., 2002, 2005, 2006a,b, 2007; Nakamura, Setodoi & Takayama, 2000; Nakamura & Okabe, 2004; Nakamura & Nishikawa, 2005; Dunitz et al., 1956) within experimental error.
In the flexible spacers (C7-C9 and C30-C32), the C-C-C bond angles are 111.3 (4) and 113.1 (5)°, the average values of the C-C bond length are 1.509 (7) and 1.496 (8) Å, and the lengths of the chains are 2.488 (7) and 2.497 (7) Å, respectively. These values are nearly equal to the values determined for n-paraffin [e.g. the average C-C bond distance is approximately 1.54 Å and the length of the -CH2-CH2-CH2- unit is 2.54 Å (Bunn, 1939)].
The molecular structure of bMAF-3 is somewhat different from that of the previously analyzed `U' shaped homologues bMAF-9 (Nakamura et al., 2007) and bMAF-10 (Nakamura & Okabe, 2004). The dihedral angles of the two phenyl rings (Plane 2-Plane 5 and Plane 3-Plane 6) in the mesogenic group are estimated to be 65.8 (2) and 58.6 (2)°, respectively, as shown in Table 2. In contrast, the dihedral angles between the two phenyl rings of bMAF-9 (Nakamura et al., 2007) and bMAF-10 (Nakamura & Okabe, 2004) are 3.5-11.2°, indicating that these phenyl rings are almost parallel.
The two substituents of bMAF-3 lie in the same direction (`U' shape), and the molecular length of bMAF-3 (C4-C23) is estimated to be 20.856 (16) Å. As the C6-O1-C7-C8, C7-C8-C9-O3, C29-O7-C30-C31 and C30-C31-C32-O9 fragments adopt a gauche conformation in the flexible spacer, the molecules can be considered as rod-like molecules, as shown in Table 3. A new shape of bMAF-n molecule was found by Nakamura & Nishikawa (2005), who reported a novel molecular structure for bMAF-2, the name of which was associated with a `Z' shape. The aspect ratio of bMAF-2 (Nakamura & Nishikawa, 2005) is rather low, which is not advantageous for the appearance of liquid crystallinity. Therefore, the rod-like shape obtained with the molecules analyzed in this paper is an important factor for obtaining a mesomorphic behavior.
Projections of the ab and bc planes of bMAF-3 are shown in Figs. 3(a) and 3(b), respectively. In both projections, herringbone layer structures were observed. Among the other homologues of bMAF-n for which the molecular and crystal structures have been analyzed, only bMAF-3 exhibits such a structure.
| || Figure 3 |
Crystal structure of bMAF-3. (a) Projection onto the ab plane and (b) projection onto the bc plane.
The following intermolecular interactions were identified in bMAF-3:
(i) Two C-H interactions. One is between the H atoms on C8 and C9 of the methylene chains and the C40-C45 benzene ring of the mesogenic group (Fig. 4a). The other is between the H atom on C32 and the C10-C15 benzene ring (Fig. 4b).
| || Figure 4 |
Illustration of intermolecular interactions of bMAF-3.
In addition, two intramolecular interactions are identified between the two substituents of the molecule. As shown in Fig. 5, they are two T-stacking interactions between H atoms (on C18, C19 and C37, C38) of the phenyl groups of the mesogenic units and the benzene rings C40-C45 and C10-C15, respectively. Therefore, bMAF-3 has four intermolecular interactions and two intramolecular interactions.
| || Figure 5 |
Illustration of intramolecular interactions of bMAF-3.
bMAF-3 exhibits only a monotropic nematic phase. On the other hand, bMAF-9 (Nakamura et al., 2007), for example, possesses a monotropic nematic phase and a smectic one. The difference between the occurrences of these liquid crystal phases is attributed to the different features of the inter- and intramolecular interactions in bMAF-3 and bMAF-9 (Nakamura et al., 2007). In the cooling process from the isotropic liquid phase, bMAF-3 gradually aggregates through the above-mentioned intramolecular interactions between the two substituents of bMAF-3 and the intermolecular interactions between neighboring molecules, while bMAF-9 (Nakamura et al., 2007) aggregates through the only intermolecular interactions between the two substituents of bMAF-9. Consequently, the presence of the different interactions may play an important role in giving rise to the mesomorphism.
A High-Tech Research Center Project for Private Universities matching subsidy from the Ministry of Education, Culture, Sports, Science and Technology, 2006-2010, is gratefully acknowledged for partial financial support of the present work. We are grateful to Associate Professor T. Hanasaki and Dr K. Uno, Ritsumeikan University, Japan, for useful discussions.
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