Lanthanide containing Schiff's base complexes with chloride counter-ions: mesomorphic properties

Materials Science and Engineering C 18 Ž2001. 211–215 www.elsevier.comrlocatermsec

Lanthanide containing Schiff’s base complexes with chloride counter-ions: mesomorphic properties Rik Van Deun ) , Koen Binnemans K.U. LeuÕen, Department of Chemistry, Coordination Chemistry DiÕision, Celestijnenlaan 200F, B-3001 LeuÕen, Belgium

Abstract Liquid crystalline lanthanide complexes Žmetallomesogens. with chloride counter-ions have been synthesized. The complexes have the stoichiometry wLnŽLH. 3 Cl 3 x, where Ln is a trivalent rare-earth ion ŽY, Pr, Sm, Eu–Lu., and LH is the ligand N-octadecyl-4-octyloxysalicylaldimine. Although the Schiff’s base ligands do not exhibit mesomorphism, the complexes do ŽSmA phase.. The mesomorphic behaviour of these compounds has been investigated by hot-stage polarized microscopy, differential scanning calorimetry ŽDSC. and high temperature X-ray diffraction measurements. The stoichiometry of the complexes remains constant throughout the lanthanide series. The complexes have rather high transition temperatures. q 2001 Elsevier Science B.V. All rights reserved. Keywords: Rare earths; Lanthanides; Liquid crystals; Metallomesogens

1. Introduction Liquid crystalline metal complexes Ž metallomesogens. have received a lot of attention in the past few decades. This is mainly due to the metal’s ability to influence the liquid crystalline properties of the materials in which it is embedded w1–9x. Metallomesogens can be obtained not only from liquid crystalline ligands because the metal can induce mesomorphism in non-mesomorphic ligands as well. The majority of the metallomesogens that have been studied up till now contain d-block metals like CuŽII., NiŽII., PtŽII. and PdŽII. because they coordinate in a square planar fashion with the ligands that were used. In this way, the molecular structure of the complexes can still be rod-like, and the structural anisotropy necessary for the formation of a mesophase is retained. However, only a limited number of metals exhibit square planar coordination, and as a result, metals other than those mentioned earlier were examined in order to obtain liquid crystalline metal complexes. Lanthanides have a very rich coordination chemistry, their ions obtaining coordination numbers between 6 and 12, with most complexes having coordination number 8 or 9. Such high coordination numbers seem to be incompati-

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Corresponding author. Tel.: q32-16-32-74-37; fax: q32-16-32-79-92. E-mail address: [email protected] ŽR. Van Deun..

ble with the structural anisotropy that is necessary to exhibit liquid crystalline behaviour. However, after the discovery of the first columnar liquid crystalline lanthanide complex by Piechocki et al. w10x in 1985, Galyametdinov et al. w11x synthesized the first calamitic lanthanide containing liquid crystal in 1991. These compounds consisted of a central trivalent lanthanide ion surrounded by three Schiff’s base ligands. The reported stoichiometry was wLnLX ŽLX H. 2 X 2 x, where Ln is the trivalent lanthanide ion, LX H is a Schiff’s base ligand and X is a counter-ion Žnitrate or chloride.. The ligand itself is mesomorphic, exhibiting a nematic phase: Cr P 43 P N P 71 P I. The complexes were shown to exhibit a highly viscous SmA mesophase, e.g. the Gd complex with nitrate counter-ions showed the following mesomorphic behaviour: Cr P 98 P SmA P 192 P I. Later on, mesomorphic lanthanide complexes with nonmesomorphic ligands LH were found w12x. Again, when Gd is taken as the central lanthanide and nitrate as the counter-ion, the complex wGdŽLH. 3 ŽNO 3 . 3 x with LH being the Schiff’s base depicted in Fig. 1, the transition temperatures are Cr P 121 P SmA P 150 P I. More recently, we performed a systematic study on lanthanide metallomesogens consisting of the ligand LH shown in Fig. 1 and nitrate as the counter-ion, and we found an overall stoichiometry wLnŽLH. 3 ŽNO 3 . 3 x for this type of compound w13,14x. Nitrate is often chosen to be the counter-ion because it can coordinate in a bidentate fashion, allowing the lanthanide ion to easily obtain a high coordination number.

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R. Van Deun, K. Binnemansr Materials Science and Engineering C 18 (2001) 211–215

212

Fig. 1. Schiff’s base ligand discussed in this paper.

In this paper, we report on a study concerning liquid crystalline lanthanide complexes with chloride counterions. Complexes have been made with most of the lanthanides, including the rare-earth ion yttrium ŽY, Pr, Sm–Lu.. For this particular type of chloride-containing lanthanide complex, a comparable stoichiometry is found: wLnŽLH. 3 Cl 3 x.

2. Experimental All 1 H NMR spectra were recorded on either a Bruker Avance 300 spectrometer Ž300 MHz. or a Bruker AMX-400 spectrometer Ž400 MHz. using CDCl 3 as solvent and tetramethylsilane ŽTMS. as internal standard. Elemental analyses ŽCHN. were performed on a CE-Instrument EA1110 elemental analyzer. Differential scanning calorimetry ŽDSC. measurements were done on a Mettler-Toledo DSC821e module Žscan rate of 10 8C miny1 under a nitrogen flow.. Optical textures of the mesophases were observed with an Olympus BX60 polarized microscope equipped with a Linkam THMS600 hot stage and a Linkam TMS93 programmable temperature controller. High temperature X-ray diffraction was measured on a STOE Transmission Powder Diffractometer System STADI P, with a high temperature attachment version 0.65.1 Žtemperature range from room temperature to 1000 8C.. Monochromatic ˚ . was obtained with the Cu K a 1 radiation Ž l s 1.5406 A aid of a curved germanium primary monochromator. Diffracted X-rays were measured by a linear position sensitive detector ŽPSD.. The sample was placed in a quartz glass capillary Žouter diameter of 0.3 mm and wall thickness of 0.01 mm. and spun during the measurement. In general, data were collected in the range 1 - 2 u - 308. Organic reagents were purchased from ACROS; lanthanide salts were obtained from Aldrich. All solvents and chemicals were used as received. All complexes were synthesized by direct addition of the hydrated lanthanide chloride LnCl 3 P 6H 2 O in absolute ethanol to a stirred absolute ethanolic solution of the ligand LH ŽFig. 1. at a temperature of about 50 8C. Ligand LH was prepared by condensation of stearylamine with 2-hydroxy-4-octyloxybenzaldehyde in absolute ethanol. 2.1. Synthesis of ligand LH N-octadecyl-4-octyloxysalicylaldimine A solution of 2,4-dihydroxybenzaldehyde Ž50 mmol, 6.91 g., 1-bromooctane Ž50 mmol, 9.66 g. and KHCO 3 Ž50

mmol, 5.01 g. in DMF was left to reflux for 3 h. The crude 2-hydroxy-4-octyloxybenzaldehyde was purified by column chromatography, using a 90:10 mixture of n-heptanerethylacetate as the eluens w15x. Yield 63% Ž7.88 g.. 1 H NMR d ŽCDCl 3 .: 0.9 Žt, 3H, CH 3 .; 1.0–1.6 Žm, 10H, CH 2 .; 1.8 Žm, 2H, CH 2 –CH 2 –O.; 4.0 Žt, 2H, CH 2 –O.; 6.4 Žd, 1H, H arom .; 6.5 Ždd, 1H, H arom .; 7.4 Žd, 1H, H arom .; 9.7 Žs, 1H, CHO.; 11.5 Žs, 1H, OH.. Elemental analysis calculated for C 15 H 22 O 3 Ž M w s 250.33.: C, 72.0% and H, 8.9%; found: C, 72.2% and H, 8.9%. The purified aldehyde Ž10 mmol, 2.50 g. was converted into the Schiff’s base by reaction with octadecyl amine Žstearylamine, 10 mmol, 2.70 g. in absolute ethanol, with a few drops of acetic acid as the catalyst. After refluxing for 3 h and cooling, the yellow precipitate was filtered, crystallized from absolute ethanol and dried in a vacuum desiccator. Yield 80% Ž4.01 g.. 1 H NMR d ŽCDCl 3 .: 0.9 Žm, 6H, CH 3 .; 1.2–1.5 Žm, 40H, CH 2 .; 1.7 Žm, 2H, N–CH 2 –CH 2 .; 1.8 Žm, 2H, CH 2 –CH 2 –O.; 3.5 Žt, 2H, N–CH 2 .; 3.9 Žt, 2H, CH 2 –O.; 6.3 Ždd, 1H, H arom .; 6.4 Žd, 1H, H arom .; 7.0 Žd, 1H, H arom .; 8.1 Žs, 1H, CH-N.; 14.1 Žs, 1H, OH. Žmp. 50.3 8C.. Elemental analysis calculated for C 33 H 59 NO 2 Ž M w s 501.83.: C, 79.0%; H, 11.8%; and N, 2.8%; found: C, 79.1%; H, 11.8%; and N, 2.7%.. 2.2. Synthesis of complex [Tb(LH)3 Cl 3 ] A solution of TbCl 3 P 6H 2 O Ž1.03 mmol, 0.39 g. in absolute ethanol was added dropwise to a stirred absolute ethanolic solution of the ligand LH ŽFig. 1. Ž1.03 mmol, 0.52 g. at 50 8C. After addition, the solution was left to stir overnight. A pale yellow precipitate formed, which was filtered on a crucible, washed with absolute ethanol and dried in vacuo. Yield 88% Ž0.53 g.. Elemental analysis calculated for C 99 H 177 N3 O6 Cl 3Tb Ž M w s 1770.76.: C, 67.2%; H, 10.1%; and N, 2.4%; found: C, 67.2%; H, 10.1%; and N, 2.3%. All the other lanthanide complexes wLnŽLH. 3 Cl 3 x were obtained via the same synthetic method as wTbŽLH. 3 Cl 3 x. 3. Results and discussion The starting material for our investigation was the Schiff’s base ligand LH ŽFig. 1., which we prepared by condensing 2-hydroxy-4-octyloxybenzaldehyde with octadecylamine. Reaction of excess LnCl 3 P 6H 2 O with LH in absolute ethanol then led to the formation of wLnŽLH. 3 Cl 3 x. Looking at the elemental analysis results in Table 1, it is clear that three Cl anions are present in each complex. The question arises whether the ligands LH coordinate to the lanthanide ion in a monodentate Žthrough the phenol oxygen only. or a bidentate way Žphenol oxygen and imine nitrogen.. It has been shown earlier by means of single crystal X-ray diffraction as well as by NMR studies that in the case of the analogous nitrate complexes, coordination occurs through the phenol oxygen only, the ligand being

R. Van Deun, K. Binnemansr Materials Science and Engineering C 18 (2001) 211–215 Table 1 Yields and elemental analysis data for wLnŽLH. 3 Cl 3 x Compound wYŽLH. 3 Cl 3 x wPrŽLH. 3 Cl 3 x wSmŽLH. 3 Cl 3 x wEuŽLH. 3 Cl 3 x wGdŽLH. 3 Cl 3 x wTbŽLH. 3 Cl 3 x wDyŽLH. 3 Cl 3 x wHoŽLH. 3 Cl 3 x wErŽLH. 3 Cl 3 x wTmŽLH. 3 Cl 3 x wYbŽLH. 3 Cl 3 x wLuŽLH. 3 Cl 3 x

Yield Ž%.

Elemental analysis: calculated Žfound. Ž%. C H N

80 73 77 84 81 88 89 89 88 95 92 88

69.9 Ž69.9. 67.8 Ž67.8. 67.5 Ž67.2. 67.4 Ž67.4. 67.2 Ž67.2. 67.2 Ž67.2. 67.0 Ž66.8. 66.9 Ž66.8. 66.8 Ž66.7. 66.8 Ž66.7. 66.6 Ž66.5. 66.6 Ž66.6.

10.5 Ž10.5. 10.2 Ž10.1. 10.1 Ž10.1. 10.1 Ž10.1. 10.1 Ž10.1. 10.1 Ž10.1. 10.0 Ž10.0. 10.0 Ž10.0. 10.0 Ž10.0. 10.0 Ž10.0. 10.0 Ž10.0. 10.0 Ž10.0.

2.5 Ž2.4. 2.4 Ž2.2. 2.4 Ž2.3. 2.4 Ž2.3. 2.4 Ž2.2. 2.4 Ž2.3. 2.4 Ž2.2. 2.4 Ž2.2. 2.4 Ž2.2. 2.4 Ž2.2. 2.4 Ž2.2. 2.4 Ž2.2.

Table 2 Transition temperatures and thermal data for wLnŽLH. 3 Cl 3 x complexes Žsecond heating run. Ln

Transitiona

T Ž8C.

DH ŽkJ moly1 .

DS ŽJ moly1 Ky1 .

Y

Cr ™SmA SmA ™ I Cr ™SmA SmA ™ I Cr ™SmA SmA ™ I Cr ™SmA SmA ™ I Cr ™SmA SmA ™ I Cr ™SmA SmA ™ I Cr ™SmA SmA ™ I Cr ™SmA SmA ™ I Cr ™SmA SmA ™ I Cr ™SmA SmA ™ I Cr ™SmA SmA ™ I Cr ™SmA SmA ™ I

175 192 158 187 168 191 170 193 171 194 169 191 174 192 174 191 174 188 174 190 176 193 175 190

41.1 11.4 35.0 12.9 42.0 13.2 41.4 12.8 43.4 13.5 41.8 13.0 42.3 12.5 42.2 13.2 41.7 12.0 44.9 12.6 45.4 12.7 46.6 11.8

91.7 24.6 81.1 28.0 95.2 28.5 93.3 27.6 97.9 28.9 94.6 28.2 94.6 27.0 94.4 28.4 93.2 25.9 100.3 27.2 101.2 27.3 104.0 25.4

Pr Sm Eu Gd Tb Dy

present in a zwitter-ionic form w15x. The three nitrate counter-ions coordinate in a bidentate fashion, bringing the coordination number of the lanthanide ion to nine. Obviously, chloride coordinates in a monodentate way, so this is one significant difference between the chloride complexes and the nitrate compounds. We have already made numerous efforts to crystallize a short chain analogue derived from the ligand N-butyl-4-methoxysalicylaldimine for X-ray diffraction studies. However, up till now, no single crystals have been obtained. In this way, it would be possible to determine the coordination of the ligands. It seems to be a lot easier to crystallize the more covalent nitrate complexes, than it is to dissolve the more ionic chloride complexes in a suitable solvent out of which single crystals could be obtained by slow evaporation. Very recently, we studied similar complexes with dodecyl sulphate ŽDOS. counter-ions w16x. They all have the stoichiometry wLnŽLH. 3 ŽDOS. 3 x and turned out to be very similar to the nitrate compounds. 1 H NMR studies of the diamagnetic complex wLaŽLH. 3 ŽDOS. 3 x also showed that the coordination of the ligands to the lanthanum ion is identical to that of the analogous nitrate complex: the ligands are present in the zwitter-ionic form. Since dodecyl sulphate can coordinate in a bidentate way, we assumed these compounds to be similar to the nitrate complexes. Until good single crystals of the chloride complexes can be obtained, several assumptions regarding their coordination properties can be made. Firstly, CHN analysis does not show evidence of crystal water molecules, so the coordination number of the lanthanide ion must be 6, which is very low, when three monodentate chloride counter-ions and three monodentate ligands LH are present, coordinating through the phenol oxygen only. The second possibility is that the ligands LH have to be present in a bidentate form. Thus, a coordination number of 9 can be obtained. A third possibility is the presence of bridging chlorides, where one chloride ion is shared by more than one lanthanide. Investigation of the IR data of the yttrium complex indicate a shift of the C-N peak from 1623 cmy1 in the

213

Ho Er Tm Yb Lu a

Cr s crystalline solid, SmA ssmectic A mesophase, I s isotropic liquid.

Schiff’s base ligand to 1658 cmy1 in the complex. This behaviour is opposite to that observed in copperŽII. complexes with Schiff’s base ligands w15x. This, together with the appearance of an additional band at about 3300 cmy1 ŽC-Nq–H. reflects the zwitter-ionic presence of the ligand. As for the thermal properties, all complexes show an enantiotropic SmA phase ŽTable 2.. The DSC trace of a typical compound wPrŽLH. 3 Cl 3 x ŽFig. 2. shows two transitions: the melting point at 158 8C Ž D H s 35.0 kJ moly1 .

Fig. 2. DSC trace Žsecond heating and cooling run. of wPrŽLH. 3 Cl 3 x. Endothermic peaks are pointing upwards.

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R. Van Deun, K. Binnemansr Materials Science and Engineering C 18 (2001) 211–215

confirmed by the fact that the layer spacing decreases with increasing temperature in the mesophase, together with the observation of the typical focal-conic texture with homeotropic regions in the microscope.

4. Conclusions

Fig. 3. Evolution of the melting and clearing temperatures of complexes wLnŽLH. 3 Cl 3 x as a function of the lanthanide ion Žtemperatures are taken from the second DSC heating run..

and the clearing point at 187 8C Ž D H s 12.9 kJ moly1 .. Although these transition temperatures are rather high, the compounds do not decompose at temperatures several tens of degrees above the clearing point, not even if these temperatures are maintained for several hours. The transitions are very sharp and well defined, indicating that all parts of the compound Žalkyl chains and core. melt at the same temperature. The effect of changing the counter-ion is clearly illustrated: the similar nitrate complex wPrŽLH. 3 ŽNO 3 . 3 x shows significantly lower transition temperatures: Cr P 90 P SmA P 163 P I, while the decrease of the temperature is even more spectacular in the case of the dodecyl sulphate compound wPrŽLH. 3 ŽDOS. 3 x: Cr P 66 P SmA P 88 P I. Changing the counter-ion has been shown to be a very efficient way of influencing the transition temperatures w17–19x. The alkoxy chain length of the ligand LH used to synthesize this DOS compound is slightly longer ŽC 14 H 29 O. than that of the chloride and nitrate compounds ŽC 8 H 17 O., but we have shown earlier that for these kinds of compound, the influence of the chain length is negligible in comparison to the effect of the counter-ion w15x. A plot of the transition temperatures as a function of the lanthanide ion in compounds wLnŽLH. 3 Cl 3 x is shown in Fig. 3. The compound wSmŽLH. 3 Cl 3 x has also been investigated by high temperature X-ray diffraction. At room temperature, the compound exists in a lamellar state with ˚ crystallized chains. The d-spacing at 25 8C is 33.5 A, Ž which is less than the all-trans length of the ligand calcu˚ .. This means that the complex molecules lated to be 40 A are interpenetrated, as is often seen for this kind of com˚ when pound. The value of d varies by no more than 1.0 A heating the lamellar solid phase towards the melting point. At the melting point however, there is a sharp decrease of ˚ At 190 8C, a the lamellar layer spacing by more than 4 A. temperature close to the clearing point, the value of d is ˚ The identity of the mesophase being SmA is 29.0 A.

Lanthanide containing complexes of Schiff’s base ligands with chloride counter-ions have been synthesized by direct addition of hydrated lanthanide chloride salts to the Schiff’s base N-octadecyl-4-octyloxysalicylaldimine in absolute ethanol. All complexes have the stoichiometry wLnŽLH. 3 Cl 3 x, where Ln is a trivalent rare-earth ion ŽY, Pr, Sm–Lu., LH is the Schiff’s base ligand and Cl is the chloride counter-ion. These compounds all exhibit an enantiotropic SmA phase, as shown by polarized optical microscopy, differential scanning calorimetry ŽDSC. and high temperature X-ray diffraction measurements. The complexes are examples of metallomesogens with rather high melting and clearing temperatures. However, they remain stable at these high temperatures.

Acknowledgements RVD is indebted to the Flemish Institute for the Encouragement of Scientific and Technological Research in the Industry ŽIWT. for financial support. KB is a Postdoctoral Fellow of the FWO-Flanders ŽBelgium.. Financial support by the FWO-Flanders ŽG.0243.99. is gratefully acknowledged. The authors wish to thank Prof. C. Gorller¨ Walrand ŽK.U. Leuven. for providing laboratory facilities and Prof. G. Meyer ŽUniversity of Koln, ¨ Germany. for access to high temperature XRD equipment. X-ray diffractograms were recorded by Dr. D. Hinz ŽUniversity of Koln, ¨ Germany.. CHN microanalyses were done by Ms. P. Bloemen ŽK.U. Leuven..

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