J. MATER. CHEM., 1994, 4(5), 679-682 1,3=Disubstituted Ferrocene-containing Thermotropic Liquid Robert Deschenaux,*a Julio Santiago: Daniel Guillonb and Benoit Heinrichb a Universite de Neuchitel, lnstitut de Chimie, Av. de Bellevaux 51, 2000 Neuchiitel, Switzerland lnstitut de Physique et Chimie des Materiaux de Strasbourg, Groupe des Materiaux Organiques, 67 rue du Loess, 67037 Strasbourg Cedex, France The title compounds have been synthesized and their liquid-crystal properties investigated. The reported ferrocene derivatives exhibited enantiotropic nematic and/or smectic A phases associated with broad anisotropic domains. The molecular arrangement within the smectic A phases was studied by X-ray diffraction. The experimental data, compared to the values obtained from CPK models, suggested a monolayer molecular organization with a pronounced chain disorganization for the medium- and long-chain derivatives. The mesomorphic properties of ferrocene-containing thermo- tropic liquid crystals have recently been reviewed.' Among the structures reported so far,' 1,3-disubstituted ferrocene- containing liquid crystals' are of particular interest.Indeed, 1,3-substitution led to thermotropic materials which exhibited remarkable liquid-crystalline properties compared with those obtained for the 1,l'-and 1,2-isomeric analogues: the 1,3-disubstituted ferrocene derivatives showed enantiotropic nematic and/or smectic C phases: while their 1,l'-isomeric analogues (series I in ref. 3) gave only monotropic nematic phases (for the short-chain derivatives) and their 1,2-isomeric analogues (structure 18 in ref.1) were found to be non-mesomorphic. The origin of the strong liquid-crystalline character resulting from 1,3-disubstitution can be explained on the basis of structural considerations. First, the two substituents, located on each side of the cyclopentadienyl ring, are disposed in a coplanar arrangement. Such a situation, provided that the rigid rod is sufficiently long, can thwart the unfavourable steric effect of the bulky ferrocene core, which strongly decreases the mesomorphic behaviour (with respect to the ferrocene-free material) when incorporated into a mesogenic structure. The tendency of the ferrocene unit in decreasing the liquid-crystalline properties, was also clearly established in the case of 1,l'-disubstituted ferrocene derivative^.^ Secondly, the X-ray crystal structure determined for one derivative2" revealed a highly anisotropic structure for the 1,3-disubstituted ferrocene derivatives, so allowing favourable intermolecular attractions.More information is required to understand fully the structure-mesomorphic properties relationship for the 1,3-disubstituted ferrocene derivatives, particularly concerning their supramolecular organization in liquid-crystalline states. Therefore, to explore further the capability of such structures for forming liquid-crystalline materials, we describe the prep- aration, mesomorphic properties and X-ray diffraction studies of the new ferrocene derivatives I.These compounds differ from those previously reported2 by the orientation of the external ester functions. Results and Discussion Syntheses Ferrocene derivatives I were prepared by reaction of the ferrocene 1,3-diacid chloride' with the 4-alkoxyphenyl 4-hydro~ybenzoates~(n=5-8,10,12,14,16,18) in dry CH2C12, at reflux, in the presence of Et,N. Purification by column chromatography (Silica gel; CH2C12-AcOEt, 50:l v/v) and crystallization from CH,Cl,-EtOH gave the desired com- pounds in 70% yield. The structures were confirmed by 'H NMR spectroscopy and elemental analysis. Mesomorphic Properties The transition temperatures and enthalpy changes were deter- mined by differential scanning calorimetry (DSC) and are reported in Table 1.The mesophases were identified by a combination of thermal polarized optical microscopy and X-ray diffraction. The phase diagram of compounds I is illustrated in Fig. 1. All ferrocene derivatives I exhibited a strong liquid- crystalline character. The first member of the series, 1 (n=5), showed an enantiotropic nematic phase associated with a broad liquid-crystal range (62 "C). On cooling from the isotropic state, a monotropic smectic A phase formed after the nematic phase. Owing to the high clearing temperature, slight decomposition was detected in the isotropic liquid. The ferrocene derivatives I (n=6) and I (n=7) exhibited two Table 1 Phase-transition temperatures/"C" and enthalpy changes/kJ mol-' of ferrocene derivatives I n C-SA SC-SA C-N SA-N SA-I N-I 186 (178)b,c -248 32.6 2.6 6 191 --219' -239 47.1 3.2 7 190 --228 -233 41.3 1.o 3.4 8 190 ---228 -45.0 7.4 -10 185 ---227 44.9 10.5 12 178 ---223 -43.3 11.6 14 174 ---219 -43.8 12.4 16 171 ---213 -40.4 13.7 18 168 ( 163)b3' --208 -43.2 13.0 C =crystal; N =nematic phase; SA=smectic A phase; Sc =smectic C phase; I =isotropic liquid.Monotropic transition. 'Observed by polarized optical microscopy only. J. MATER. CHEM., 1994, VOL. 4 To strengthen the interpretation given from the observations 270 obtained by polarized optical microscopy, and to gain infor- mation about the molecular organization within the liquid- .-F \ crystalline states, the smectic phases of the ferrocene deriva- 1501 I I I I I , (SC) I 6 8 10 12 74 16 18 20 alkyl chain length Fig.1 Phase diagram of ferrocenes I. 0,Melting point; +, clearing point; A, smectic A-nematic transition; 0,smectic A-smectic C transition; A, nematic-smectic A transition. enantiotropic mesophases, a smectic A phase and a nematic phase. Compounds I (n=8, 10, 12, 14, 16) gave rise to enantiotropic smectic A phases with large anisotropic domains. Finally, the last member of the family, I (n =18), showed two mesophases, a broad enantiotropic smectic A phase and a monotropic smectic C phase. Typical textures were observed by means of polarized optical microscopy. When I (n=5-7) were cooled from the isotropic liquid, the nematic phases appeared either in the schlieren texture or in the homeotropic one.In the latter case, bright flashes were observed when the preparation was touched with a spatula. The nematic to smectic A transition was clearly detected by polarized optical microscopy and in one case, i.e. for I (n=7), also by DSC. When ferrocene derivatives I (n=8, 10, 12, 14, 16, 18) were cooled from the isotropic liquid, focal-conic fan textures and, in several cases, homeotropic zones, both characteristic of the smectic A phases, were observed. For I (n= 18), the smectic A to smectic C transition was identified by the formation of a schlieren texture from the previous homeotropic zones, and by the transformation of the focal-conic fan texture (Plate 1) into the broken focal-conic fan texture (Plate 2).tives I (n=6, 8, 10, 18) were analysed by X-ray diffraction. The nature of the srnectic A phases was thus clearly confirmed. Each of these compounds gave similar data, i.e. X-ray patterns presenting a sharp ring in the low-angle region and a diffuse one in the wide-angle region. The monotropic smectic C phase of I (n =18), which crystallized during experiment owing to its short existence range (SA-Sc, 163 "C; Sc-C, 158 "C),could not be characterized by X-ray diffraction. The layer spacing, d, obtained by X-ray diffraction, the molecular length, L, measured from CPK models, and the corresponding d/Lratio are reported in Table 2. For the fer- rocene derivatives I (n=6) and I (n=8), the d/L ratio ranged between 0.9 and 1.0 and indicated a monolayer arrangement of the molecular units in the smectic A phases.On increasing the alkyl chain length, i.e. for compounds I (n= 10) and I (n=18), the d/L ratio decreased to 0.78 [I (n=18)]. The discrepancy between the layer spacing and the molecular length can be attributed to the pronounced disorganized state of the long alkyl chains which can fold easily owing to the lateral bulkiness of the ferrocene moiety. However, we cannot exclude that the low d/L values may also originate from intensive orientational fluctuations or from pre-existing smectic C correlations in the smectic A phase. These results are in agreement with literature data reported for other metallomesogens.6 Table 2 Layer spacing of ferrocene derivatives I 6 42.1 200 46 0.92 8 50.1 200 51 0.98 10 49.6 200 57 0.87 18 59.9 190 77 0.78 ~ ~~ a From X-ray diffraction.conformation. From CPK models in the fully extended Plate 1 Thermal optical micrograph of the focal-conic fan texture displayed by I(n= 18) in the smectic A phase upon cooling from the isotropic liquid to 204.7 "C J. MATER. CHEM., 1994, VOL. 4 681 Plate 2 Thermal optical micrograph of the broken focal-conic fan texture displayed by I(n= 18) in the smectic C phase upon cooling from the smectic A phase (see Plate 1) to 154.8 "C Fe I (n=5-8,10,12,14,16,18) The strong liquid-crystal character shown by ferrocene derivatives I can be interpreted on the basis of structural features.In fact, in a previous report,% by comparing the thermal properties of two families of ferrocene derivatives substituted in the 1,3-positions, we demonstrated that meso- morphism develops from an l/l' ratio>5-7 (l=length of the rigid rod; I' =distance between the two cyclopentadienyl rings of the ferrocene unit). In the present study, the length I of the rigid segment in I was found to be ca. 27.5 A (in the most extended conformation) from CPK molecvlar models. The depth E' of the ferrocene core being ca. 3.3 A,7 an l/E' ratio of 8.3-8.4 is obtained. Therefore, this value confirms that ferro- cene derivatives I possess the required structural character- istics for exhibiting pronounced mesomorphism. The ferrocene derivatives I and those reported in ref.2 differ in the orientation of the external ester functions. It is interes- ting to point out that, whereas compounds I exhibit smectic A and nematic phases, their isomeric structures gave smectic C and nematic phases.2 These observations, which show the strong influence of the organic substituents on the nature and stability of the mesophases, can be explained in terms of electronic effects, Owing to the C, symmetry of compounds I (and of their isomeric analogues2), local effects can be considered. The organic fragments 1and 2 were used for constructing ferrocene derivatives I and their isomeric analogues: respect-ively. In structure 1,electron delocalization takes place in the interior of the organic fragment. In structure 2, electron delocalization occurs in the opposite direction, from the 0 atom of the alkoxy chain to the ester function.Consequently, (1) electron delocalization is more extended in 2 than in 1, and (2) the 0 atom of the ether group is more polar in 2 than in 1. The electron delocalization presented above which leads to different intermolecular interactions for each organic fragment is, most likely, at the origin of the different mesomorphic behaviour observed between the two isomeric series. Such results, which were also observed for isomeric 1,l'-disubstituted ferrocene derivatives: are in agreement with literature data reported for wholly organic liquid crystak8 Finally, the liquid-crystal ranges reported herein, and those exhibited by the first family of 1,3-disubstituted ferrocene derivatives,2 represent the largest anisotropic domains observed to date in case of ferrocene-containing thermotropic liquid crystals.1 2 Conclusions The synthesis and characterization of the second family of homologous 1,3-disubstituted ferrocene-containing thermo- tropic liquid crystals are described. Ferrocene derivatives I exhibited broad enantiotropic smectic A and/or nematic phases. X-Ray investigations suggested a molecular organk- ation into monolayers with an important chain disorganiz- ation for the medium and long-chain derivatives (n= 10, 18) within the smectic A phases. The present results, and those recently described,' clearly show that 1,3-disubstituted ferro- cene-containing thermotropic liquid crystals, owing to their high thermal stability and pronounced mesomorphic charac- ter, are valuable metallome~ogens.~ Furthermore, the electro- chemical characteristics of the ferrocene unit,7 which can be reversibly oxidized into the ferrocenium species, combined with the mesomorphic properties of the 1,3-disubstituted ferrocene derivatives, make such compounds interesting candi- dates for elaborating liquid-crystalline materials from elec- troactive molecular units.Experimental General Ferrocene-l,3-diacid chloride,' and the 4-alkoxyphenyl 4-hydroxyben~oates~were prepared as in the literature. Column chromatography (CC) used Silicagel 60 (0.063-0.200 mm, Merck) and TLC used Silicagel plates (Merck).Transition temperatures and enthalpies were deter- mined with a differential scanning calorimeter (Mettler DSC 30) connected to a Mettler-TA 3000 system, rate 10 "C min-' under N2. Optical studies were conducted using a Zeiss-Axioscop polarizing microscope equipped with a Linkam-THMS-600 variable temperature stage under N2, A Bruker AMX 400 spectrometer at 400.13 MHz was used for 'H NMR spectra. X-Ray diffraction patterns of powder samples in Lindemann capillaries were recorded photographically at several temperatures using a Guinier focusing camera equipped with a bent quartz monochromator (Cu-Ka, radi- ation from a Philips PW-1009 generator) and an electrical oven. Elemental analyses were conducted by Ciba SA, Marly, Switzerland. Syntheses The general synthetic procedure of bis [4-(4-alkoxyphenoxy-carbony1)phenyll ferrocene-1,3-dicarboxylatesI is exemplified by the preparation of bis [4-( 4-pentyloxyphenoxy- carbony1)phenyll ferrocene-1,3-dicarboxylateI (n=5).A mix- ture of ferrocene-1,3-diacid chloride (0.1g, 0.32 mmol), 4-pentyloxyphenyl 4-hydroxybenzoate (0.194 g, 0.65 mmol), Et,N (66 mg, 0.65 mmol), a catalytic amount of 4-pyrrolidinopyridine and CH,Cl, (10 cm3) was heated at reflux for 3 h. The solution was cooled to room temperature and evaporated. Purification of the resulting residue by CC (Silica gel, CH,Cl,-AcOEt 50:1, v/v) and crystallization from CH2C12-EtOH gave the desired compound in 70% yield. 'H NMR (CDCl,, TMS) dH: 0.94 (6 H, t, 2 x CH3), 1.42 [8 H, m, 2x(CH,),], 1.80 (4H, m, 2xCH2CH,0), 3.97 (4H, t, 2 x CH2CH20), 4.48 (5 H, S, Cp), 5.28 (2 H, d, Cp), 5.80 (1 H, J.MATER. CHEM., 1994, VOL. 4 Table 3 Elemental analytical data of ferrocene derivatives I (calculated values in parentheses) n C(%) H(%) 5 68.76( 68.74) 5.59( 5.53) 6 69.06 (69.29) 5.58( 5.81) 7 69.87( 69.80) 6.14( 6.08) 8 70.27( 70.28) 6.42( 6.34) 10 70.86( 71.16) 6.87( 6.80) 12 7 1.84( 7 1.94) 7.30( 7.21) 14 72.55 (72.65) 7.39( 7.57) 16 73.22( 73.28) 7.78( 7.91) 18 73.87( 73.86) 8.06(8.21) t, Cp), 6.94 (4 H, d, 2 x 2H-arom.), 7.13 (4 H, d, 2 x 2H-arom.), 7.35 (4 H, d, 2 x 2H-arom.), 8.29 (4 H, d, 2 x 2H-arom.). IR (KBr) v,Jcm-': 3132, 2933, 2869, 1733, 1604, 1467, 1415, 1120, 1103, 869, 834, 814.Found: C, 68.76, H, 5.59; calc. for C48H46010Fe (838.74): C, 68.74, H, 5.53%. Ferrocene derivatives I (n=6-8, 10, 12, 14, 16, 18) gave analytical data which were in agreement with their structure (Table 3). One of the authors (R.D.) acknowledges Ciba S.A, Marly, Switzerland, for the elemental analyses, Chemische Betriebe Plutopeba Oel AG, Germany, for a generous gift of acetyl ferrocene used to prepare ferrocene-1,3-dicarboxylicacid, and the Swiss National Science Foundation for financial support. References 1 R. Deschenaux and J. W. Goodby, in Ferrocenes. From Homogeneous Catalysis to Materials Science, ed. T. Hayashi and A. Togni, VCH Verlagsgesellschaft, Weinheim, in the press. 2 (a) R. Deschenaux, I.Kosztics, J-L. Marendaz and H. Stoeckli- Evans, Chimia, 1993, 47, 206; (b) R. Deschenaux and J-L. Marendaz, J. Chem. SOC., Chem. Commun. 1991,909. R. Deschenaux, J-L. Marendaz and J. Santiago, Helv. Chim. Acta, 1993,76, 865. N. J. Thompson, J. W. Goodby and K. J. Toyne, Liq. Cryst., 1993, 13,381. M. Hisatome, 0. Tachikawa, M. Sasho and K. Yamakawa, J. Organomet. Chem., 1981, 217, C17; A. Kashara, T. Izumi, Y.Yoshida and I. Shimizu, Bull. Chem. SOC. Jpn., 1982,55,1901. 6 M. Ghedini, D. Pucci, E. Cesarotti, P. Antogniazza, 0. Francescangeli and R. Bartolino, Chem. Muter., 1993, 5, 883; E. Campillos, M. Marcos, J. L. Serrano, J. Barbera, P. J. Alonso and J. I. Martinez, Chem. Mater., 1993,5, 1518. 7 C. Elschenbroich and A. Salzer, in Organometal~ics,Verlag Chemie, Weinheim, 1989. 8 Y. Sakurai, S. Takenaka, H. Miyake, H. Morita and T. Ikemoto, J. Chem. SOC., Perkin Trans. 2, 1989, 1199; H. Takeda, Y. Sakurai, S. Takenaka, H. Miyake, T. Doi, S. Kusabayashi and T. Takagi, J. Chem. SOC., Faraday Trans., 1990, 86, 3429; R. Centore, M. R. Ciajolo, A. Roviello, A. Sirigu and A. Tuzi, Liq. Cryst., 1991, 9, 873. 9 S. A. Hudson and P. M. Maitlis, Chem. Rev., 1993, 93, 861; D. W. Bruce, in Inorganic Materials, ed. D. W. Bruce and D. OHare, Wiley, Chichester, 1992; P. Espinet, M. A. Esteruelas, L. A. Oro, J. L. Serrano and E. Sola, Coord. Chem. Rev., 1992,117, 215; A-M. Giroud-Godquin and P. M. Maitlis, Angew. Chem. Int. Ed. Engl., 1991,30,375. Paper 3/07224J; Received 7th December, 1993