Synthesis, characterisation and liquid crystalline behaviour of some lanthanides complexes...

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Synthesis, characterisation and liquid crystallinebehaviour of some lanthanides complexes containingtwo azobenzene Schiff baseKhalil K. Abida & Sadeem M. Al-barodya

a Department of Chemistry, College of Science, University of Al-Mustanseriyah, Baghdad,IraqPublished online: 19 May 2014.

To cite this article: Khalil K. Abid & Sadeem M. Al-barody (2014): Synthesis, characterisation and liquid crystalline behaviourof some lanthanides complexes containing two azobenzene Schiff base, Liquid Crystals, DOI: 10.1080/02678292.2014.919670

To link to this article: http://dx.doi.org/10.1080/02678292.2014.919670

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Synthesis, characterisation and liquid crystalline behaviour of some lanthanides complexescontaining two azobenzene Schiff base

Khalil K. Abid* and Sadeem M. Al-barody

Department of Chemistry, College of Science, University of Al-Mustanseriyah, Baghdad, Iraq

(Received 12 March 2014; accepted 27 April 2014)

Schiff base ligand N,N′-di-(4-butylsalicylidene)-1′,3′-diaminopropane [H2L] was synthesised by the reactionof substituted azobenzene and 1,3-diaminopropane in 2:1 molar ratio. Four mononuclear lanthanidecomplexes of the type [Ln(H2L)LCl] (Ln = LaIII, CeIII, SmIII and GdIII) were synthesised and charac-terised by 1H,13C NMR, fourier transform infrared (FT-IR) spectroscopy, elemental analysis (CHNO), gaschromotography-mass, magnetic susceptibility and molar conductivity. Thermal properties of the titlecompounds were studied using the thermogravimetric analysis/differential scanning calorimetry (TGA/DSC) and optical polarising microscopy (OPM). The ligand and coordination compounds exhibit liquid-crystalline properties (smectic A).

Keywords: liquid crystals; azobenzene Schiff base; lanthanides mesogens; smectic A phase

1. Introduction

The coordination chemistry of lanthanide ions hasbeen widely investigated in recent years because ofboth its useful magnetic and optical behaviours. Itcould exhibit coordination numbers 6–12, with 8 or 9being ubiquitous.[1] Wide variety of ligands such asSchiff bases ligands have been employed for the synth-esis of new classes of lanthanide mesogens.[2–4] Thelanthanide mesogens also have enjoyed lot of interestsowing to unusual geometries and novel properties suchas spin crossover, ferro electricity, photo refractivity,contrast agents for magnetic resonance imagining(MRI), luminescent stains for fluoro immune assays,catalysts for the selective cleavage of RNA and DNAand cancer radio therapeutic agents.[5–9]

Transition temperatures of the Schiff base com-plexes are greatly influenced by the choice of the coun-terion, and they were able to show that the lanthanidecontraction has a distinct influence on the transitiontemperatures of this type of compound. There is aninterest in developing lanthanide-containing liquidcrystals, mainly because of the unique magnetic prop-erties of some of the trivalent ions of the lanthanideseries.[10] Not only are these ions paramagnetic andhave a high magnetic moment but also more impor-tantly, the trivalent lanthanide ions can have a veryhigh magnetic anisotropy. A high value for the mag-netic anisotropy is desirable if one wants to switchliquid crystals by an external magnetic field.[11]Many researchers had studied in detail the complexesof Schiff base ligands with one aromatic ring.[12,13]Schiff base ligands complexes with two aromatic rings

have been studied much less intensively,[14] while anew Schiff base ligand contains three aromatic ringsand their lanthanide complexes were reported byBinnemans et al.[15] Schiff base ligands with N, Odonor sets have often been used since the Schiff baseligands may assemble coordination architectures direc-ted by the lanthanide (III) ions.[16–18] In the study oflanthanide complexes, many different types of ligandsare frequently used to link Ln(III). Moreover, thethermodynamic properties of lanthanide complexesare also important for the theoretical study and thepractical applications. The doubly deprotonatedligand contains strong donors, namely phenolato oxy-gen atoms as well as imine nitrogen atoms bearingexcellent coordination ability with transition/innertransition metal ions through its NxOy donor set.[19]

In our previous work,[20] the synthesis of lanthanidecomplexes with single arm N-aryl Schiff base ligand ofthe formula [LnH2L(HL)2Cl], L = N–(2–hydroxyphe-nyl)–4–n–butylsalicyl aldimine were reported, while inthis paper four lanthanide complexes of double armsazobenzene new Schiff base ligand N,N′-di-(4-butylsa-licylidene)-1′,3′-diaminopropane with the formula [Ln(H2L)LCl] were synthesised and investigated. Specialattention was made for studying thermal behaviour ofthe synthesised ligand and its complexes.

2. Materials and methods

All chemicals were analytical grade and used withoutany modification. Fourier transform infrared (FT-IR)spectra were recorded on a Perkin Elmer 100 Series FT-

*Corresponding author. Email: abidk56@yahoo.com

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IR Spectrometer (Waltham, MA, USA). 1H and13C NMR spectra were recorded onJEOLECA400 FT NMR spectrometer (Gangdong-gu,South Korea) using d6 – dimethyl sulfoxide (DMSO) asa solvent. Mass spectra were recorded by electron-impact mass spectrometry (EIMS) using a DirectInjection Probe on a Shimadzu GCMS – QP5050ASpectrometer (Shimadzu Europe, Duisburg,Germany), HRESIMS spectra were recorded onShimadzu UFLC-IT-TOF Mass Spectrometer.Magnetic susceptibility measurements were carried outby employing Magway MSB MK−1 susceptibility bal-ance at room temperature. Molar conductivity of thecomplexes were measured in DMSO as a solvent in0.001 M solutions using a CON 510 bench conductivitymeter, with 2-ring stainless steel conductivity electrode(cell constant, K = 1.0). Thermal stability (weightchanges) of the samples were recorded by MettlerToledo TGA851 and STA 6000 (Perkin Elmer,Schwerzenbach, Switzerland) in the temperature up to1000°C. The phase transition temperatures determinedby using a differential scanning calorimetry (DSC) 7(Perkin Elmer) equipped with a liquid nitrogen coolingsystem. Olympus BX50 Optical Polarising Microscope(OPM, Olympus America Inc., Corporate Parkway,Center Valley, PA, USA) equipped with aLinkamTHMSE-600 hot stage and aTetramethylsilane (TMS) 92 control unit was used toanalyse liquid crystal properties of the ligand and com-plexes. All measurements were carried out at Faculty ofScience, UPM University, Malaysia.

3. Experiment: preparation of AZO1, H2L andLn(III) complexes

3.1 Preparation of 5-((4-butyl phenyl)azo)-salicylaldehyde [AZO1]

A 33.5 mmol (0.149 g) of 4-butyl aniline was dissolvedin 6MHCl (15 mL) at 0–5°C, 24mmol (2.39 g) NaNO2

dissolved in cold water (15 mL) added dropwise to themixture for 30 min under constant stirring. Diazoniumsalt was obtained and used for coupling. 33.5 mmol(4.08 g) of salicylaldehyde was added to 10% NaOH(15 mL) in a 3-necked flask immersed in an ice-bath.Freshly prepared diazonium salt was added dropwisefor 1 h to the reactionmixture under constant stirring. Abrownish orange precipitate was formed, the reactionmixture temperature was kept at 0–5°C. Dilute aceticacid was then added to the reaction mixture and thebrownish orange precipitate was filtered off, washedwith ethanol and water. Recrystallised from (ethanol/benzene) (v/v, 1:1), yield: 64%; mp: 75–76°C, elementalanalysis. Found for C17H18N2O2 (282): C, 72.54%; H,6.68%; N, 10.02%; O, 11.4%. Calc.: C, 72.34%; H,6.38%; N, 9.92%; O, 11.34%.

3.2 Synthesis of N,N′-di-(4-butylsalicylidene)-1′,3′-diaminopropane [H2L]

2 mmol (0.564 g) of [AZO1] dissolved in ethanol(30 mL), 1 mmol (0.074 g) of 1,3-diaminopropanedissolved in ethanol (30 mL) and few drops ofglacial acetic acid as catalyst were mixed andrefluxed for 3 h. The precipitate was filtered offand recrystallised from (ethanol/ether) (v/v, 1:1),yield: 70%; mp: 102–104°C, elemental analysis.Found for C37H42N6O2 (602): C, 73.85%; H,7.07%; N, 14.35%; O, 5.5%. Calc.: C, 73.75%; H,6.97%; N, 13.95%, O, 5.31% (Scheme 1).

3.3 Synthesis of lanthanide complexes

To a stirred solution of 2 mmol (1.2 g) of theligand (H2L) in ethanol (40 mL) and 1 mmol ofthe LnCl3·XH2O (Ln = LaCl3·7H2O, CeCl3·7H2O,SmCl3·6H2O and GdCl3·6H2O] in ethanol (50 mL)were mixed and refluxed for 3 h then kept stirringfor 72 h at temperature 40–50°C. The yellow

OH

CHO

Reflux 3 hrethanol , acetic acid

[AZO 1]

H2N NH2

N N

HC

HOOH [H2L2]

LnCl3·XH2O(Ln = La, Ce, Sm, Gd)

Ln(H2L2)L2Cl

2

N N

N

N

C4H9

N

N

C4H9

1,3-Diaminopropane

Scheme 1. Summarised synthesis of [H2L].

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precipitate was filtered off, washed with cold etha-nol and dried in vacuum oven for 24 h at 70°C.The precipitate was further purified by columnchromatography over SiO2 by eluting with a mix-ture of (n-hexan: DMF: chloroform) (v/v, 1:1:1)evaporation of this eluent afforded the crystals ofthe complexes (see Table 1).

4. Results and discussion

4.1 FT-IR spectra

The (FT-IR) spectra were recorded in the region4000–400 cm−1 by universal attenuated total reflec-tion (UATR) technique using KBr disc.

The ligand (H2L) shows absorption band at3190 cm−1 to ν(O–H). The low frequency for thisband is probably due to considerable amount ofH-bonding to the ortho (>C=N) group.[21] Twostrong bands located at 1635 and 1618 cm−1

attributed to ν(C=N) and ν(N=N), ν(Ar–O) at1283 cm−1. Three Bands appeared at 3028 and1481 cm−1 for aromatic ν(C–H, C=C), ν(C–H)and for aliphatic at 2957–2857 cm−1 respectively.[22]In Ln(III) complexes, the disappearance of phenolicν(O–H) was observed with the appearance of newbroad band around 3376 cm−1 probably attributedto the formation of N–H bond by the immigrationof phenolic proton to the azomethines nitrogenforming a zwitter ion (Figure 1). Similar zwitterionic behaviour has been recorded elsewhere forlanthanide complexes.[23–25] The ν(C=N) in the

ligand was split to two bands around 1663 and1618 cm−1 for lanthanide ions complexes probablydue to the presence of two types of ν(C=N) stretch-ing and compression vibrations. The vibration athigher frequency is due to the coordination throughoxygen atom, while the lower value clearly indicatedcoordination of the complexes take place throughthe deprotonated phenolic oxygen (OH2) and azo-methine nitrogen atoms,[14,26] resulting two typesof coordination. A molecular structure suggested forthe lanthanide complexes is shown in Figure 6.

The deprotonated Schiff’s base ligand L behavesas bidentate (through the two phenol oxygens only)and tetradentate (two phenol oxygen and two iminenitrogen) for the other [24] made the coordinationnumber of the lanthanide ion seven. It was alsopossible to find evidence for the presence of azwitterionic form from the 1H NMR spectral dataof the complexes. The formation of zwitter ion canincrease the tendency of Ln(III) ion to coordinateto the negatively charged oxygen atom.[27] Newbands were recorded at 513–585 and 460–496 cm −1 attributed to Ln–N and Ln–O bonds,respectively (see Table 2).

4.2 1H, 13C NMR spectra

NMR spectra were recorded using DMSO as sol-vent and TMS was used as an internal standard.Chemical shifts were reported in parts per million

Table 1. Some physical properties of lanthanide complexes of H2L.

Compound mp (°C) Mwt.

Elemental analysis % Found (%Calc.)

C H N O

LaC74H82N12O4Cl 106–107 1368 65.31 (64.91) 6.03 (5.99) 12.57 (12.23) 4.77 (4.67)CeC74H82N12O4Cl 98–99 1377 65.07 (64.48) 6.05 (5.95) 12.52 (12.20) 5.04 (4.64)SmC74H82N12O4Cl 107–109 1387 64.65 (64.02) 6.01 (5.91) 12.47 (12.12) 4.51 (4.61)GdC74H82N12O4Cl 103–105 1394 64.10 (63.70) 5.98 (5.88) 12.35 (12.05) 4.79 (4.59)

N

NN N

N

N

O OH H

N

NNN

N

N

O O

H H

Figure 1. Formation of zwitter ions in H2L.

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(ppm) and coupling constants were given inHertz (Hz).

The spectrum of the ligand showed two singletpeaks at 13.97 and 8.49 δ ppm assigned to (Ar–OH)and (–CH=N–), respectively. The quintet and thetriplet peaks at 3.71 and 2.50 δ ppm were attributedto (–CH2–C–CH2–) and (–C–CH2–C–), respectively.Multiple peaks were observed in the range of 7.05–7.89 δ ppm attributed to the aromatic protons.

Only 1H NMR spectrum for La(III) complexwas recorded, due to the paramagnetic properties

of the other lanthanide ions. The phenolic (–OH)signal disappeared while the signal correspondingto the imine hydrogen (–CH–N) was shifted upwardin the complex to 8.62 δ. New signal, probablycharacteristic of (–N+H) resonance, appeared at10.14 δ while the parent ligand does not show anysuch signal [28] (Figures 2 and 3). This couldassigned to the movement of phenolic protons incoordinated ligand to the two uncoordinatedimino nitrogen, to give rise to the zwitter ionicstructure (–N+–H…..O−).[29] This observations

Figure 2. 1H NMR spectrum of H2L.

Table 2. Major infrared absorption bands of H2L and complexes (cm−1).

Compound ν(O–H)ph. ν(N+H) ν(C–N+), (C=N) ν(C–O) ν(M–N) ν(M–O)

H2L 3190 – 1635 1283 – –

LaC74H82N12O4Cl – 3390 1663, 1618 1276 549, 522 480, 456CeC74H82N12O4Cl 3365 1661, 1616 1256 586, 530 496, 463SmC74H82N12O4Cl – 3376 1661, 1615 1277 585, 534 495, 460GdC74H82N12O4Cl – 3322 1669, 1619 1224 576, 513 496, 466

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have very similar structural properties in compari-son with the corresponding complexes of anotherSchiff’s bases reported recently [20,30,31] indicatingclearly that this signal corresponding to the protonof the (–N+H) group. Little shift for other signalswere recorded due to the coordination effect. The13C NMR spectra show a shift of the (–NCH) signalfrom δ, 165.48 (in the case of H2L) to δ, 166.07 inthe case of the La(III) complex. Similar shifts wereobserved in the case of the carbon atoms directlyattached to the bonding atoms (phenolate carbons)while those for the other carbons were of lessermagnitude.[25]

4.3 Mass spectra

Mass spectra were recorded using a Direct InjectionProbe. The mass spectral features of the ligand H2Las illustrated in Figure 4 was described by molecularion peak at m/z = 602 which confirm the molecularweight m/z value of 602 that agree with the empiricalformula of the ligand (C37H42N6O2). The intensitywas 100% for the base peak 106 which is assign tothe cleavage of [CH2CH2C6H4]

+ main aromaticcharacter in the molecule. The peak at 55 (parentpeak) is assign to the cleavage of [C4H9]

+, and theparent peak at 133 is assign to the cleavage of[C4H9C6H5]

+. Several peaks were observed in the mass

Figure 3. 1H NMR spectrum of LaIII complex.

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spectrum at m/z [fragment, intensity %] 603 [M + 1, 5],602 [M, 40], 308 [C4H9C6H4N2C6H3OHCH=NCH2CH2CH3, 50]. The mass spectrum of Sm(III)complex confirmed the metal/ligand molar ratio 1:2.As shown in Figure 5, the mass spectrum wasdescribed by molecular ion peak at m/z = 1388 whichconfirm the molecular weight m/z value of 1387 thatagree with the empirical formula of the complex(SmC74H82N12O4Cl). The intensity was 100% for thebase peak 322 which assign to the cleavage of[C4H9C6H4N2]

+. The fragmentation started with theloose of first molecule of the ligand before the dissocia-tion of the ligand itself.

4.4 Magnetic measurements

Magnetic susceptibility for the complexes wasrecorded in the solid state at 298 K using Gouey

method in emu/mole units. All the trivalent lantha-nide ions, except lutetium, have unpaired f electrons.However the magnetic moments deviate considerablyfrom the spin-only values because of strong spin–orbitcoupling.[32] The magnetic properties of rare earthions are strongly influenced by this coupling; in parti-cular the magneto-crystalline anisotropy is generallylarge. As far as the magnetic properties are con-cerned, the large anisotropic moments arise from thelarge spin–orbital coupling, making lanthanide ionsattractive building blocks in the synthesis of magneticmolecular materials.[33] The magnetic moment dataof Ln(III) complex show that lanthanum(III) chloridecomplexes is diamagnetic in nature while all othercomplexes are paramagnetic, as expected. Magneticmoment of 2.52 B.M. was recorded for Ce(III) com-plex due to the presence of one electron. However inthe case of Sm(III) complex, a slight variation from

Figure 4. (colour online) Mass spectrum of H2L.

Figure 5. Mass spectrum of SmIII complex.

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VanVleck values is observed due to low J–J separa-tion which leads to thermal population of higherenergy levels. In Gd(III) complex, a magneticmoment of 7.87 B.M. was recorded which suggest amaximum number of unpaired electrons is 7, with allthe electrons have parallel spin. This property isimportant for the use of gadolinium complexes ascontrast reagent in MRI scans.[17,27]

4.5 Molar conductivity measurements

The molar conductivity measurements for the com-plexes were carried out using a concentration of10−3 M and DMSO as a solvent and CON 510bench conductivity meter (cell constant, K = 1.0)built-in temperature sensor (Table 3). All complexesshowed a behaviour of non-electrolyte type indicatingthe presence of chloride ion inside the coordinationsphere, and these results agree with the suggestedconfigurations of the lanthanide complexes.[31]

4.6 Thermo gravimetric analyses

Thermo gravimetric analyses (weight changes) wereperformed in the temperature range from room tem-perature up to 1000°C under air atmosphere at theheating rate of 20°C/min. Thermal properties of thecomplexes are largely defined by the Schiff’s baseligands, rather than by the lanthanide ions. This isdifferent from what is observed for the Schiff’s baseligands with one aromatic ring. The lanthanidomeso-gens derived of the two-ring or three-ring Schiff’s baseligands are not very suitable for exploration of theirspectroscopic, electric or magnetic properties, due totheir high transition temperatures, highly viscousmesophases and low thermal stability.[34] Thermaldecomposition of lanthanide complexes is very attrac-tive, since it consists of a moderate temperature calci-nation step and yields high purity oxides without anyunwanted by-products. Recently, synthetic methodsfor producing oxides with specific morphologieshave been reported.[35]

The ligand and complexes exhibit a smectic A(SmA) phase. A thermogravimetric analysis (TGA)

curve of Sm(III) complex revealed that the complexis stable up to 123–185°C and do not show anyweight loss below this temperature. It is strong evi-dence, which represent that the complex is devoid oflattice water as well as coordinated water in thecoordination sphere.[26] The first weight loss of thecomplexes was observed 43.39% (1.28 mg), whichoccurs between the temperatures 123–185°C, corre-sponding to one molecule of ligand. After the elim-ination of the first molecule, the decomposition ofthe second molecule of the complex started simulta-neously between the temperatures 185–391°C, andthe observed weight loss 45.78% (1.3505 mg) isequivalent to (L) + 0.5Cl2.

A further mass loss recorded up to 300°C indi-cates the formation of a thermally stable metal oxide(Figure 6). In last step comprising deanionation withstimulantaneous oxidation of Sm to Sm2O3 arefound to be very broad in nature, however in major-ity of the cases all these steps are not generallyisolated but overlapped to varying extent and fre-quently result in fractional losses of the constituentmoieties.[36]

4.7 Liquid crystal properties of H2L and Ln(III)complexes

Texture observations for all prepared compoundswere done under a polarised optical microscopeattached to a hot stage. All compounds were exam-ined for texture changes before they were sent forDSC to determine their phase transition temperatureand transition enthalpies (ΔH).

The ligand exhibits SmA phases during heatingto isotropic liquid (I) and the reverse cooling processto crystalline solid (Cr), see Figure 7. Cr-SmA andSmA-I phase transitions were observed at 77°C (Cr-SmA) and 102° (SmA-I). The enthalpy changes forthose transitions were 39.7 and 11.9 kJ/mol, on thereverse process, I-SmA and SmA-Cr phase transi-tions happened at 92°C (I-SmA) and 46°C (SmA-Cr). The enthalpies for these transitions were 34.1and 28.3 kJ/mol. Lanthanide(III) complexes exhibitSmA broken focal-conic fan texture. Therefore, DSC

Table 3. Magnetic moment and molar conductivity data of H2L2 complexes.

Complexes μeff μeff Van VleckMolar conductance

(Ω−1 cm2/mol) Geometry

LaC74H82N12O4Cl 0.0 X = −0.18 × 10–6 cm g sec 12.4 Mcoh*CeC74H82N12O4Cl 2.50 2.54 10.36 McohSmC74H82N12O4Cl 1.56 0.85 11.40 McohGdC74H82N12O4Cl 7.98 7.94 12.84 Mcoh

Note: *Monocapped octahedron.

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analysis was done for the complexes to determinetheir phase transition temperature and transitionenthalpies value (ΔH). La(III) complex is an enan-tiotropic liquid crystal exhibiting SmA phases duringheating to isotropic liquid (I) and the reverse coolingprocess to crystalline solid (Cr). Cr-SmA and SmA-Iphase transitions were observed at 81.34°C and106.35°C, respectively. The enthalpy changes forthose transitions were 56.2 and 2.7 kJ/mol. On the

reverse process, I-SmA and SmA-Cr phase transi-tions happened at 95.36°C and 48.96°C, respectively.The enthalpies for these transitions were 24.3 and29.6 kJ/mol. Upon cooling the complex from isotro-pic liquid, typical batonnets are formed which coa-lesce to give rise to a highly birefringent fan-liketexture, characteristic of the SmA phase at 48°C,which quickly reverts to crystalline phase at 20°C(see Table 4 and Figures 8–11).

N

N

N N

N

N

N

N

NN

N

N

Ln

HH

O O

O O

R*

N N

R*

Ln

O O

R*

N N

R*

HH

O O

R*

N N

R*O O

+

0.5 Cl2

Cl

Sm2O3

Mwt. 60243.40%

Mwt. 63545.77%

Mwt. 63545.77%

Residue

Step 1

Cl

NN

Step 2

*R = C4H9

Figure 6. Fragmentation pattern of SmIII complex.

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Figure 7. (colour online) Optical photomicrograph of H2L smectic texture on heating to 80°C (cross polarisermagnification × 100).

Figure 8. (colour online) Optical photomicrograph of La(III) complex: smectic texture on cooling to 90°C.

Table 4. Phase transition and enthalpies obtained of H2L and its Ln(III) complexes.

Symbols Transition temperature (T/°C) and associated transitionEnthalpy values(ΔH/kJ/mol)

H2L (C23H23N3O2) 101°C (Cr-SmA1), 147°C (SmA1–SmA2), 168°C (SmA2-I) (heating). 14.1, 28.4, 2.0165°C (I-SmA2), 129°C (SmA2–SmA1), 80 °C (SmA-Cr) (cooling). 2.0, 28.4, 13.4

La(C69H67N9O6Cl) 127°C (Cr-SmA), 143°C (SmA-I) (heating). 51.6, 1.46141°C (I-SmA1), 120°C (SmA1–SmA2), 112 °C (SmA2-Cr) (cooling). 1.6, 25.8, 25.0

Ce(C69H67N9O6Cl) 145°C (Cr-SmA1), 160°C (SmA1–SmA2), 165°C (SmA2-I) (heating). 9.61, 81.8120.10

166°C (I-SmA2), 160°C (SmA2–SmA1), 146°C (SmA1-Cr) (cooling). 81.77, 9.6018.95

Sm(C69H67N9O6Cl) 50°C (Cr-SmA1), 80°C (SmA1–SmA2), 125°C (SmA2–SmA3),130 (SmA3–SmA4), 135(SmA4-I) (heating).

38.21, 14.48, 5.07,97.00, 56.2

137°C (I-SmA4), 130°C (SmA4–SmA3), 122°C (SmA3–SmA2),80 °C (SmA2-Cr) (cooling)

97.00, 5.10, 14.46,38.22

Gd(C69H67N9O6Cl) 125°C (Cr-SmA1), 183°C (SmA1–SmA2), 215°C (SmA2–SmA3),218 (SmA3-I) (heating).

7.12, 601, 14.0010.50

220°C (I-SmA3), 214°C (SmA3–SmA2), 223°C (SmA2–SmA1),124°C (SmA1-Cr) (cooling)

7.12, 601, 14.0013.12

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5. Conclusion

Double arms lanthanide Schiff base complexes bear-ing azobenzene nuclei were synthesised and

characterised. Investigations of the collected resultsare consistent with the stoichiometry [Ln(H2L)LCl]as two forms of ligands exist in the complexes:

Figure 9. (colour online) Optical photomicrograph of Ce(III) complex: smectic texture on cooling to 90°C.

Figure 10. (colour online) Optical photomicrograph of Sm(III) complex: smectic texture on heating to 91°C.

Figure 11. (colour online) Optical photomicrograph of Gd(III) complex: smectic texture on heating to 75°C.

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bidentate through two phenolic oxygen and tetraden-tate through the deprotonated phenolic oxygen andazomethine nitrogen to complete seven coordinationgeometry. In our previous report [20] using samelanthanide ions with single arm Schiff base ligand,the stoichiometry was found to be [Ln(H2L)2HLCl]and nine coordination. It is clearly indicated that thestoichiometry depends on number and differentlengths of the terminal chains on the ligands. All thecomplexes form a smectic A mesophase anddetermined by polarising microscopy.

Funding

The authors would like to thank University of PutraMalaysia for physical measurements and Al-MustansiriyahUniversity for the financial support.

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