Synthesis, characterisation and mesomorphic

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Synthesis, characterisation and mesomorphic properties of ester containing aroylhydrazones and their nickel(II) complexes a

b

Sachin Kumar Singh , K. Vikram & Bachcha Singh a

a

Department of Chemistry, Faculty of Science, Banaras Hindu University, Varanasi, India

b

Department of Physics, Faculty of Science, Banaras Hindu University, Varanasi, India Version of record first published: 09 Sep 2011.

To cite this article: Sachin Kumar Singh , K. Vikram & Bachcha Singh (2011): Synthesis, characterisation and mesomorphic properties of ester containing aroylhydrazones and their nickel(II) complexes, Liquid Crystals, 38:9, 1117-1129 To link to this article: http://dx.doi.org/10.1080/02678292.2011.597880

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Liquid Crystals, Vol. 38, No. 9, September 2011, 1117–1129

Synthesis, characterisation and mesomorphic properties of ester containing aroylhydrazones and their nickel(II) complexes Sachin Kumar Singha , K. Vikramb and Bachcha Singha * a

Department of Chemistry, Faculty of Science, Banaras Hindu University, Varanasi, India; b Department of Physics, Faculty of Science, Banaras Hindu University, Varanasi, India

Downloaded by [Banaras Hindu University BHU] at 07:09 03 April 2013

(Received 25 May 2011; final version received 14 June 2011) A new series of mesogenic aroylhydrazone-based ligands, N-[4-(4 -alkoxy)benzoyloxybenzylidene]-N -[4 -alkoxybenzoyl]hydrazine with either the same or different peripheral alkyl chains, and nickel(II) complexes of some of them have been synthesised. They were characterised by elemental analyses, Fourier transform infrared, proton and carbon nuclear magnetic resonance and ultraviolet-visible spectroscopy. The mesomorphic properties of these compounds were investigated by differential scanning calorimetry and polarising optical microscopy. All the aroylhydrazones, except those with no lateral chains on either end of the molecule and where m = n = 14, 16, exhibit a monotropic or enantiotropic smectic C mesophase, which are almost insensitive to the peripheral alkoxy chain length. The square planar nickel(II) complexes of the ligands show only an isotropic phase at higher temperature (>175◦ C) and no mesogenic nature is observed. Density functional theory calculations have been performed using the GAUSSIAN-03 program at the Becke, three-parameter, Lee–Yang–Parr level to obtain the stable electronic structure of the ligand. Keywords: aroylhydrazone; enantiotropic mesophase; differential scanning calorimetry; polarising optical microscopy; density functional theory

1. Introduction Aroylhydrazones, hydrazine derivatives and their metal complexes are known to exhibit a broad spectrum of biological, bactericidal and fungicidal activities and are of great interest due to their analytical, industrial and pharmacological importance [1–8]. Aroylhydrazones and their parent hydrazine compounds form stable chelates with transition metals. The tuberculostatic activity of these compounds has been attributed to the formation of stable chelates with transition metals present in the cell [9–11]. Therefore, aroylhydrazone-based mesogenic ligands have been used to form a novel family of metal containing liquid crystals (LCs) with high thermal stability and high variety of LC phases [12–14] in contrast to many of the currently known metallomesogens [15]. N-Alkylidenearoylhydrazones (RCONHN= CHR ) can coordinate to a divalent metal ion either via the enolic form (1) or the ketonic form (2 and 3) [16–25] and the preferred form of the chelate is strongly dependent on the coordination ability of the counter anion and the metal salt used [26]. For instance, aroylhydrazones react with nickel(II) acetate to yield the corresponding square-planar bis(aroylhydrazinato)nickel(II) complex 1 (M = Ni), with the deprotonation of the secondary imino ∗ Corresponding author. Email: [email protected] ISSN 0267-8292 print/ISSN 1366-5855 online c 2011 Taylor & Francis DOI: 10.1080/02678292.2011.597880 http://www.informaworld.com

hydrogen; whereas with nickel(II) chloride it gives the octahedral dichlorobis(aroylhydrazone)nickel(II) compound. The square-planar complexes 1 and 2 would show the greatest promise as LC as the planar portion of such molecules would more easily align side by side than the non-planar octahedral complex 3 (Figure 1). McCabe et al. provided a systematic investigation into the LC properties of aroylhydrazone-based metallomesogens [12]. They reported, for the first time, a series of aroylhydrazinato-nickel(II) and copper(II) complexes; a novel class of metallomesogens which exhibit smectic C (SmC) and nematic (N) phases. The nickel(II) complexes were found to be highly stable even in the isotropic (I) phase, which is in contrast with the rapid decomposition of the copper(II) complexes soon after entering into the LC phase. The mesomorphic nature of these complexes was found to be strongly dependent on the nature of the substituents at the azomethine moiety. A substituent on the azomethine moiety increases the molecular width substantially and thus prevents the molecule coming into close contact to form an ordered fluid, however the incorporation of a polar substituent would result in an increased intermolecular attraction and therefore forcing the molecule to come into closer contact. Therefore, by

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S.K. Singh et al. 2+ N

O

N

HN

M N

O

N

1

O

N M

N

O

2

HN NH

N

Cl

O

M N

O

NH

Cl

3


Figure 1.

substitution with an alkoxy-substituted aryl moiety such as –N=CHC6 H4 (OR), –N=CHC6 H3 (OR)2 and –N=CHC6 H2 (OR)3 the LC nature increases [13, 27]. Lai et al. synthesised three series of aroylhydrazone-based ligands having an alkoxysubstituted benzoyl moiety such as –CO–C6 H4 (OR), –COC6 H3 (OR)2 , –COC6 H2 (OR)3 and a dialkoxysubstituted benzylidene moiety –N=CHC6 H4 (OR)2 and their nickel(II) complexes. They found that the mesomorphic properties of the nickel(II) complexes were strongly dependent on the number of side chains around the core group. The complexes with a total of ten and eight side chains exhibited columnar phases; however, complexes with six and four side chains exhibited crystalline phases [14]. Thus, only a few aroylhydrazone-based mesogenic ligands and their metal complexes have been studied to date. The mesogenic properties of these ligands and their metal complexes are strongly dependent on the nature of substituent at the benzoyl and benzylidene (azomethine) moieties. To the best of our knowledge, the effect of an ester (linking group) containing substituent at the benzylidene ring on the mesomorphic properties of aroylhydrazones and their metal complexes has not been reported to date. Here, we report the synthesis, characterisation and mesomorphic properties of a series of ester containing aroylhydrazone-based ligands N-[4-(4 -alkoxy)benzoyloxybenzylidene] H2m+1 Cm OC6 H4 N -(4 -alkoxybenzoyl)hydrazine CONHN=CHC6 H4 O(OC)C6 H4 OCn H2n+1 (m = 0, 5–8, 10, 12, 14, 16; n = 0, 5–8, 10, 12, 14, 16) and their nickel(II) complexes (m = 10, 12, 14; n = 10, 12).

2.

Chemicals, USA and were used as received. All other solvents and reagents were purchased from Merck. The solvents were dried using standard methods when required [28]. 2.2 Techniques Elemental analyses were performed using a CE440 Exeter Analytical CHN analyser. Fourier transform infra-red (FT-IR) spectra (4000–100 cm−1 ) were recorded on a Varian 3100 FT-IR Excalibur series spectrophotometer. Proton (1 H) and carbon (13 C) nuclear magnetic resonance (NMR) spectra were obtained on a JEOL FT-NMR AL 300 MHz spectrometer using tetramethylsilane as the internal standard. Electronic spectra were recorded on a (UV)-1700 Pharma Spec. Shimadzu ultravioletvisible (UV-vis) spectrophotometer. Room temperature magnetic susceptibility measurements were performed on a Cahn Faraday balance using Co[Hg(SCN)4 ] as the standard. The magnetic susceptibility was corrected for diamagnetism using Pascal’s constants. Differential scanning calorimetry (DSC) thermograms were recorded with a Mettler Toledo TC 15 TA DSC at the rate of 5.0 K min−1 under a nitrogen atmosphere using spec pure grade indium as the standard by taking samples in closedlid aluminium pans. The transition temperatures from the DSC thermograms have been determined with an accuracy of ±0.1 K. The mesophase type was identified by visual comparison with known phase standards using an HT 30.01 NTT 268 Lomo polarising optical microscope (POM) fitted with a hot stage with a temperature controlling accuracy of 0.1 K. Quantum chemical calculations were carried out using density functional theory (DFT) as implemented in the GAUSSIAN-03 package.

Experimental

2.1 Materials Ethylbenzoate, ethyl-4-hydroxybenzoate, benzoic acid, 4-hydroxybenzoic acid, 4-hydroxy benzaldehyde, hydrazine monohydrate, bromoalkanes and nickel(II) acetate were purchased from Aldrich

2.3 Synthesis 2.3.1 Synthesis of ethyl-4-decyloxybenzoate n-Bromodecane (10.0 mmol, 2.07 mL), ethyl-4hydroxybenzoate (10.0 mmol, 1.66 g), and potassium


Liquid Crystals

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carbonate (40.0 mmol, 5.52 g) were taken in 50 mL of 2-butanone. A small amount of potassium iodide was added as a catalyst. This mixture was refluxed for 48 h, and then cooled to room temperature and 100 mL of water was added. The compound was extracted with diethyl ether (3 × 30 mL) and the organic layer was washed with water (twice) and brine. The organic solutions were dried over MgSO4 and solvent was removed under reduced pressure. Yield: 77%. IR (KBr, cm−1 ): 2933, 2864 (aliphatic C–H), 1713 (ester, C=O), 1607, 1514 (Ph), 1306, 1255 (OPh). 1 H NMR (300 MHz, CDCl3 , 25◦ C): δ 7.98 (d, J 1 (H,H) = 8.7 Hz, 2H, –C6 H4 ), 6.89 (d, J 1 (H,H) = 8.7 Hz, 2H, –C6 H4 ), 4.33 (q, 2H, –COOCH2 ), 3.99 (t, 3H, –OCH2 ), 1.77 (m, 2H, –OCH2 CH2 ), 1.43 (t, 3H, –COOCH2 CH3 ), 1.37–1.18 (m, 14H, – [CH2 ]7 ), 0.88 (t, 3H, –CH3 ). 13 C NMR (75 MHz, CDCl3 , 25◦ C): δ c 166.22, 162.74, 131.35, 122.49, 113.82, 68.00, 60.39, 31.78, 29.45, 29.26, 29.21, 29.00, 25.87, 22.57, 14.24, 13.98. All other members of the homologous series 4pentyloxy-, hexyloxy-, heptyloxy-, octyloxy-, decyloxy-, dodecyloxy-, tetradecyloxyand hexadecyloxy-benzoates were prepared using the above procedures.

2.3.3 Synthesis of 4-decyloxybenzoic acid

2.3.2 Synthesis of 4-decyloxybenzoylhydrazine To a solution of ethyl-4-decyloxybenzoate (10.0 mmol, 3.06 g) in absolute ethanol (50 mL) was added hydrazine monohydrate (25 mL) drop wise with continuous stirring. The reaction mixture was refluxed for 6 h and the solvent was removed under reduced pressure. The white solid obtained was washed with water several times and dried under vacuum. The crude product was recrystallised from absolute ethanol. Yield: 85%. IR (KBr, cm−1 ): 3424 ν a (N–H), 3321 ν s (N–H), 2919, 2854 (aliphatic C–H), 1641 (amide-I, C=O), 1570 (amide-II, N–H), 1617, 1507 (Ph), 1305, 1253 (OPh), 1027 (N–N). 1 H NMR (300 MHz, CDCl3 , 25◦ C): δ 7.69 (d, J 1 (H,H) = 8.7 Hz, 2H, –C6 H4 ), 7.40 (b, 1H, NHNH2 ), 6.91 (d, 2H, J 1 (H,H) = 8.4, –C6 H4 ), 3.98 (t, 2H, –OCH2 ), 3.47 (b, 2H, NHNH2 ), 1.79 (m, 2H, –OCH2 CH2 ), 1.45–1.27 (m, 14H, – [CH2 ]7 ), 0.87 (t, 3H, –CH3 ). 13 C NMR (75 MHz, CDCl3 , 25◦ C): δ c 168.31, 162.06, 131.46, 128.61, 124.53, 114.35, 68.18, 31.85, 29.51, 29.33, 29.27, 29.08, 25.95, 22.63, 14.07. All other members of the homologous series 4-pentyloxy-, hexyloxy-, heptyloxy-, octyloxy-, decyloxy-, dodecyloxy-, tetradecyloxyand hexadecyloxy-benzoyl hydrazines were prepared using the above procedures.

4-Decyloxybenzoic acid (5.0 mmol, 1.39 g) was dissolved in dry chloroform (30 mL) and heated under reflux with thionyl chloride (15.0 mmol, 1.10 mL) for 8 h. The solvent was removed under reduced pressure to give a yellow oil. It was further treated with a solution of 4-hydroxy benzaldehyde (5.0 mmol, 0.60 g) in dry chloroform (35 mL) in the presence of 2–3 drops of pyridine. The mixture was then refluxed for 5 h. A light yellow solution was obtained. It was washed with water (2 × 25 mL), dried over anhydrous MgSO4 , and then concentrated to form cream coloured crystals of 4-(4 decyloxybenzoyloxy)benzaldehyde. Yield: 66%. IR (KBr, cm−1 ): 2923, 2853 (aliphatic C–H), 1733 (ester, C=O), 1698 (aldehyde, C=O), 1605, 1511 (Ph), 1308, 1260 (OPh). 1 H NMR (300 MHz, CDCl3 , 25◦ C): δ 10.02 (s, 1H, CHO), 8.06 (dd, J 1 (H,H) = 8.7 Hz, J 2 (H,H) = 8.4 Hz, 4H, –C6 H4 ), 6.89 (dd, J 1 (H,H) = 8.7 Hz, J 2 (H,H) = 8.4 Hz, 4H, –C6 H4 ), 4.04 (t, 2H, –OCH2 ), 1.80 (m, 2H, –OCH2 CH2 ), 1.46–1.27 (m, 14H, –[CH2 ]7 ), 0.88 (t, 3H, –CH3 ). 13 C NMR (75 MHz, CDCl3 , 25◦ C): δ c 195.46, 163.91, 163.19, 134.45, 118.58, 114.44, 114.16, 110.88, 68.41, 31.90, 29.60, 29.57, 29.34, 29.33, 29.06, 25.96, 22.67, 14.10. All the other members of the series were prepared following the above procedure.

Ethyl-4-decyloxybenzoate (10.0 mmol, 3.06 g) was dissolved in 50 mL of ethanol and KOH (15.0 mmol, 0.84 g) was added. This was refluxed for 4 h. The mixture was poured into an acidic water solution (pH = 1, HCl). The precipitate was filtered and washed thoroughly with water. The crude product was recrystallised from absolute ethanol. Yield: 74%. IR (KBr, cm−1 ): 3481 (OH), 2921, 2851 (aliphatic C–H), 1680 (acid, C=O), 1610, 1502 (Ph), 1321, 1259 (OPh). 1 H NMR (300 MHz, CDCl3 , 25◦ C): δ 12.12 (b, 1H, –COOH), 8.02 (d, J 1 (H,H) = 9 Hz, 2H, –C6 H4 ), 6.91 (d, 2H, J 1 (H,H) = 8.4 Hz, –C6 H4 ), 4.01 (t, 3H, –OCH2 ), 1.78 (m, 2H, –OCH2 CH2 ), 1.43–1.21 (m, 14H, – [CH2 ]7 ), 0.89 (t, 3H, –CH3 ). All other members of the homologous series 4-hexyloxy-, heptyloxy-, octyloxy-, decyloxy-, dodecyloxy-, tetradecyloxy- and hexadecyloxybenzoic acids were prepared using the above procedures.

2.3.4 Synthesis of 4-(4 -decyloxybenzoyloxy) benzaldehyde


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2.3.5 Synthesis of N-[4-(4 -decyloxy)benzoyloxybenzylidene]-N -(4 -decyloxybenzoyl)hydrazine The ligand, (C10,10 LH) was prepared by refluxing together an absolute ethanol solution of 4-(4 decyloxybenzoyloxy)benzaldehyde (10 mmol, 3.82 g) and 4-decyloxybenzoylhydrazine (10 mmol, 2.92 g) for ∼8 h in the presence of few drops of acetic acid and leaving the solution overnight in a closed flask with guard tube. The microcrystalline cream coloured solid was filtered off by suction, thoroughly washed with cold ethanol, and recrystallised from a mixture of absolute ethanol and chloroform (1 : 2 v/v) and dried at room temperature. All the other members of the series were prepared in a similar manner. The yield, IR, NMR and elemental data for the compounds are summarised as follows.

2.3.6 N-[4-(4 -tetradecyloxy)benzoyloxybenzylidene]-N -benzoylhydrazine, C0,14 LH (1a) Yield: 86 %. IR (KBr, cm−1 ): 3214 ν s (N–H), 2931, 2860 (aliphatic C–H), 1727 (ester, C=O), 1636 (amide-I, C=O), 1557 (amide-II, N–H), 1605, 1504 (Ph), 1284, 1253 (OPh). 1 H NMR (300 MHz, DMSO-d6 , 25◦ C): δ 11.73 (s, 1H, –NH), 8.49 (s, 1H, –CH=N), 8.09 (d, J 1 (H,H) = 9.0 Hz, 2H, –C6 H4 ), 7.81 (dd, J 1 (H,H) = 8.7 Hz, J 2 (H,H) = 8.7 Hz, 4H, –C6 H4 ), 7.34 (d, J 1 (H,H) = 8.7 Hz, 2H, –C6 H4 ), 7.08 (dd, J 1 (H,H) = 9.0 Hz, J 2 (H,H) = 8.7 Hz, 4H, –C6 H4 ), 4.08 (m, 4H, –OCH2 ), 1.75 (m, 4H, – OCH2 CH2 ), 1.44–1.27 (m, 22H, – [CH2 ]11 ), 0.93 (m, 6H, –CH3 ). 13 C NMR (75 MHz, DMSO-d6 , 25◦ C): δ c 163.34, 163.03, 161.63, 160.44, 151.21, 132.08, 131.68, 128.95, 127.86, 124.65, 121.63, 120.54, 114.48, 113.87, 67.84, 67.79, 30.58, 30.49, 28.48, 28.27, 28.18, 25.01, 24.62, 21.74, 21.49, 13.72, 13.46. Elemental analyses: calculated for C35 H44 N2 O4 (%), C 75.56, H 7.96, N 5.03; Found: C 75.60, H 7.89, N 5.01.

2.3.7 N-[4-(4 -dodecyloxy)benzoyloxybenzylidene]-N -benzoylhydrazine, C0,12 LH (1b) Yield: 84 %. IR (KBr, cm−1 ): 3209 ν s (N–H), 2926, 2857 (aliphatic C–H), 1727 (ester, C=O), 1637 (amide-I, C=O), 1559 (amide-II, N–H), 1606, 1506 (Ph), 1287, 1255 (OPh). 1 H NMR (300 MHz, DMSO-d6 , 25◦ C): δ 11.70 (s, 1H, –NH), 8.47 (s, 1H, –CH=N), 8.07 (d, J 1 (H,H) = 9.0 Hz, 2H, –C6 H4 ), 7.81 (dd, J 1 (H,H) = 8.7 Hz, J 2 (H,H) = 8.7 Hz, 4H, –C6 H4 ), 7.34 (d, J 1 (H,H) = 8.7 Hz, 2H, –C6 H4 ), 7.07 (dd, J 1 (H,H) = 9.0 Hz, J 2 (H,H) = 8.7 Hz, 4H, –C6 H4 ), 4.08 (m, 4H, –OCH2 ), 1.75 (m, 4H,

–OCH2 CH2 ), 1.43–1.26 (m, 18H, –[CH2 ]9 ), 0.93 (m, 6H, –CH3 ). 13 C NMR (75 MHz, DMSO-d6 , 25◦ C): δ c 163.30, 163.09, 161.64, 160.47, 151.20, 132.18, 131.98, 129.07, 127.66, 124.75, 121.83, 120.58, 114.47, 113.88, 67.80, 67.76, 30.59, 30.49, 28.47, 28.37, 28.28, 25.12, 24.67, 21.74, 21.49, 13.73, 13.44. Elemental analyses: calculated for C33 H40 N2 O4 (%), C 75.02, H 7.62, N 5.30; Found: C 75.10, H 7.67, N 5.29.

2.3.8 N-(4-benzoyloxybenzylidene)-N -(4 tetradecyloxybenzoyl) hydrazine, C14,0 LH (1c) Yield: 82 %. IR (KBr, cm−1 ): 3212 ν s (N–H), 2922, 2853 (aliphatic C–H), 1727 (ester, C=O), 1641 (amide-I, C=O), 1547 (amide-II, N–H), 1606, 1505 (Ph), 1287, 1254 (OPh). 1 H NMR (300 MHz, DMSO-d6 , 25◦ C): δ 11.43 (s, 1H, –NH), 8.41 (s, 1H, –CH=N), 8.03 (d, J 1 (H,H) = 9.0 Hz, 2H, – C6 H4 ), 7.81 (dd, J 1 (H,H) = 8.7 Hz, J 2 (H,H) = 8.7 Hz, 4H, –C6 H4 ), 7.34 (d, J 1 (H,H) = 8.7 Hz, 2H, –C6 H4 ), 7.01 (dd, J 1 (H,H) = 9.0 Hz, J 2 (H,H) = 8.7 Hz, 4H, –C6 H4 ), 4.06 (m, 4H, –OCH2 ), 1.72 (m, 4H, –OCH2 CH2 ), 1.47–1.23 (m, 22H, –[CH2 ]11 ), 0.89 (m, 6H, –CH3 ). 13 C NMR (75 MHz, DMSOd6 , 25◦ C): δ c 163.67, 163.56, 161.17, 160.19, 151.81, 131.80, 131.60, 129.75, 127.76, 125.27, 121.87, 120.64, 114.44, 113.78, 67.89, 67.79, 30.79, 30.67, 28.58, 28.43, 28.25, 28.27, 28.17, 28.08, 25.08, 24.68, 21.59, 21.49, 13.38, 13.29. Elemental analyses: calculated for C35 H44 N2 O4 (%), C 75.56, H 7.96, N 5.03; Found: C 75.57, H 8.01, N 5.10.

2.3.9 N-[4-(4 -pentyloxy)benzoyloxybenzylidene]N -(4 -pentyloxybenzoyl)hydrazine, C5,5 LH (1d) Yield: 87 %. IR (KBr, cm−1 ): 3212 ν s (N–H), 2924, 2854 (aliphatic C–H), 1728 (ester, C=O), 1643 (amide-I, C=O), 1554 (amide-II, N–H), 1607, 1507 (Ph), 1286, 1256 (OPh). 1 H NMR (300 MHz, DMSO-d6 , 25◦ C): δ 11.48 (s, 1H, –NH), 8.46 (s, 1H, –CH=N), 8.07 (d, J 1 (H,H) = 9.0 Hz, 2H, –C6 H4 ), 7.83 (dd, J 1 (H,H) = 8.7 Hz, J 2 (H,H) = 8.7 Hz, 4H, –C6 H4 ), 7.33 (d, J 1 (H,H) = 8.7 Hz, 2H, –C6 H4 ), 7.06 (dd, J 1 (H,H) = 9.0 Hz, J 2 (H,H) = 8.7 Hz, 4H, –C6 H4 ), 4.03 (m, 4H, –OCH2 ), 1.73 (m, 4H, – OCH2 CH2 ), 1.46–1.24 (m, 8H, –[CH2 ]2 ), 0.87 (m, 6H, –CH3 ). 13 C NMR (75 MHz, DMSO-d6 , 25◦ C): δ c 163.44, 163.13, 161.33, 160.04, 151.41, 131.68, 131.49, 129.35, 127.96, 125.15, 121.53, 120.84, 114.44, 113.33, 67.94, 67.79, 30.86, 28.47, 28.75, 28.25, 28.18, 24.56, 21.41, 13.61, 13.86. Elemental analyses: calculated for C31 H36 N2 O5 (%), C 72.12, H 7.02, N 5.42; Found: C 72.08, H 7.07, N 5.34.


Liquid Crystals 2.3.10 N-[4-(4 -hexyloxy)benzoyloxybenzylidene]N -(4 -hexyloxybenzoyl)hydrazine, C6,6 LH (1e) Yield: 87 %. IR (KBr, cm−1 ): 3219 ν s (N–H), 2924, 2851 (aliphatic C–H), 1729 (ester, C=O), 1644 (amide-I, C=O), 1555 (amide-II, N–H), 1608, 1509 (Ph), 1284, 1261 (OPh). 1 H NMR (300 MHz, DMSO-d6 , 25◦ C): δ 11.46 (s, 1H, –NH), 8.46 (s, 1H, –CH=N), 8.07 (d, J 1 (H,H) = 9.0 Hz, 2H, –C6 H4 ), 7.82 (dd, J 1 (H,H) = 8.7 Hz, J 2 (H,H) = 8.7 Hz, 4H, –C6 H4 ), 7.31 (d, J 1 (H,H) = 8.7 Hz, 2H, –C6 H4 ), 7.04 (dd, J 1 (H,H) = 9.0 Hz, J 2 (H,H) = 8.7 Hz, 4H, –C6 H4 ), 4.03 (m, 4H, –OCH2 ), 1.73 (m, 4H, –OCH2 CH2 ), 1.45–1.24 (m, 12H, –[CH2 ]3 ), 0.87 (m, 6H, –CH3 ). 13 C NMR (75 MHz, DMSOd6 , 25◦ C): δ c 163.47, 163.43, 161.23, 160.15, 151.91, 131.80, 131.58, 129.45, 127.76, 125.28, 121.84, 120.59, 114.54, 113.87, 67.85, 68.09, 30.76, 30.47, 28.28, 28.07, 24.81, 24.62, 21.64, 21.59, 13.34, 13.29. Elemental analyses: calculated for C33 H40 N2 O5 (%), C 72.82, H 7.40, N 5.14; Found: C 72.74, H 7.28, N 5.08.

2.3.11 N-[4-(4 -heptyloxy)benzoyloxybenzylidene]-N -(4 -heptyloxybenzoyl)hydrazine, C7,7 LH (1f) Yield: 85 %. IR (KBr, cm−1 ): 3217 ν s (N–H), 2920, 2852 (aliphatic C–H), 1728 (ester, C=O), 1647 (amide-I, C=O), 1559 (amide-II, N–H), 1607, 1508 (Ph), 1283, 1257 (OPh). 1 H NMR (300 MHz, DMSO-d6 , 25◦ C): δ 11.45 (s, 1H, –NH), 8.46 (s, 1H, –CH=N), 8.07 (d, J 1 (H,H) = 9.0 Hz, 2H, –C6 H4 ), 7.83 (dd, J 1 (H,H) = 8.7 Hz, J 2 (H,H) = 8.7 Hz, 4H, –C6 H4 ), 7.33 (d, J 1 (H,H) = 8.7 Hz, 2H, –C6 H4 ), 7.02 (dd, J 1 (H,H) = 9.0 Hz, J 2 (H,H) = 8.7 Hz, 4H, –C6 H4 ), 4.03 (m, 4H, –OCH2 ), 1.73 (m, 4H, – OCH2 CH2 ), 1.45–1.25 (m, 16H, –[CH2 ]4 ), 0.87 (m, 6H, –CH3 ). 13 C NMR (75 MHz, DMSO-d6 , 25◦ C): δ c 163.74, 163.43, 161.19, 160.74, 151.68, 131.89, 131.59, 129.45, 127.65, 125.23, 121.88, 120.64, 113.44, 113.37, 67.94, 67.69, 30.99, 30.87, 28.68, 28.28, 28.20, 28.18, 25.01, 24.82, 21.79, 13.39, 13.36. Elemental analyses: calculated for C35 H44 N2 O5 (%), C 73.45, H 7.73, N 4.89; Found: C 73.48, H 7.68, N 5.01.

2.3.12 N-[4-(4 -octyloxy)benzoyloxybenzylidene]N -(4 -octyloxybenzoyl) hydrazine, C8,8 LH (1g) Yield: 86 %.IR (KBr, cm−1 ): 3214 ν s (N–H), 2931, 2860 (aliphatic C–H), 1728 (ester, C=O), 1647 (amide-I, C=O), 1557 (amide-II, N–H), 1605, 1504 (Ph), 1287, 1253 (OPh). 1 H NMR (300 MHz, DMSO-d6 , 25◦ C): δ 11.45 (s, 1H, –NH), 8.46 (s, 1H,

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–CH=N), 8.05 (d, J 1 (H,H) = 9.0 Hz, 2H, –C6 H4 ), 7.81 (dd, J 1 (H,H) = 8.7 Hz, J 2 (H,H) = 8.7 Hz, 4H, –C6 H4 ), 7.34 (d, J 1 (H,H) = 8.7 Hz, 2H, –C6 H4 ), 7.02 (dd, J 1 (H,H) = 9.0 Hz, J 2 (H,H) = 8.7 Hz, 4H, –C6 H4 ), 4.03 (m, 4H, –OCH2 ), 1.73 (m, 4H, – OCH2 CH2 ), 1.46–1.25 (m, 20H, –[CH2 ]5 ), 0.86 (m, 6H, –CH3 ). 13 C NMR (75 MHz, DMSO-d6 , 25◦ C): δ c 163.68, 163.43, 161.17, 160.11, 150.61, 132.08, 131.79, 129.34, 127.65, 125.35, 122.03, 1201.04, 114.48, 113.84, 67.88, 67.69, 30.78, 30.49, 28.68, 28.46, 28.25, 28.22, 28.17, 28.08, 25.01, 24.72, 21.74, 21.48, 13.42, 13.39. Elemental analyses: calculated for C37 H48 N2 O5 (%), C 74.01, H 8.04, N 4.66; Found: C 73.94, H 8.01, N 4.58.

2.3.13 N-[4-(4 -decyloxy)benzoyloxybenzylidene]N -(4 -decyloxybenzoyl)hydrazine, C10,10 LH (1h) Yield: 87 %. IR (KBr, cm−1 ): 3209 ν s (N–H), 2926, 2857 (aliphatic C–H), 1725 (ester, C=O), 1641 (amide-I, C=O), 1553 (amide-II, N–H), 1606, 1506 (Ph), 1277, 1255 (OPh). 1 H NMR (300 MHz, DMSO-d6 , 25◦ C): δ 11.46 (s, 1H, –NH), 8.45 (s, 1H, –CH=N), 8.07 (d, J 1 (H,H) = 9.0 Hz, 2H, –C6 H4 ), 7.81 (dd, J 1 (H,H) = 8.7 Hz, J 2 (H,H) = 8.7 Hz, 4H, –C6 H4 ), 7.34 (d, J 1 (H,H) = 8.7 Hz, 2H, –C6 H4 ), 7.02 (dd, J 1 (H,H) = 9.0 Hz, J 2 (H,H) = 8.7 Hz, 4H, –C6 H4 ), 4.03 (m, 4H, –OCH2 ), 1.72 (m, 4H, –OCH2 CH2 ), 1.43–1.26 (m, 28H, –[CH2 ]7 ), 0.86 (m, 6H, –CH3 ). 13 C NMR (75 MHz, DMSO-d6 , 25◦ C): δ c 163.60, 163.23, 161.13, 160.15, 151.51, 131.78, 131.57, 129.33, 127.68, 125.21, 121.88, 121.54, 114.44, 113.88, 67.84, 67.79, 30.77, 30.47, 28.68, 28.40, 28.23, 28.20, 28.18, 28.07, 25.21, 24.52, 21.64, 21.48, 13.38, 13.29. Elemental analyses: calculated for C41 H56 N2 O5 (%), C 74.96, H 8.58, N 4.26; Found: C 75.05, H 8.51, N 4.28.

2.3.14 N-[4-(4 -decyloxy)benzoyloxybenzylidene]N -(4 -dodecyloxybenzoyl)hydrazine, C12,10 LH (1i) Yield: 83 %.IR (KBr, cm−1 ): 3214 ν s (N–H), 2922, 2854 (aliphatic C–H), 1729 (ester, C=O), 1641 (amide-I, C=O), 1557 (amide-II, N–H), 1605, 1506 (Ph), 1278, 1254 (OPh). 1 H NMR (300 MHz, DMSO-d6 , 25◦ C): δ 11.46 (s, 1H, –NH), 8.45 (s, 1H, –CH=N), 8.07 (d, J 1 (H,H) = 9.0 Hz, 2H, –C6 H4 ), 7.83 (dd, J 1 (H,H) = 8.7 Hz, J 2 (H,H) = 8.7 Hz, 4H, –C6 H4 ), 7.34 (d, J 1 (H,H) = 8.7 Hz, 2H, –C6 H4 ), 7.00 (dd, J 1 (H,H) = 9.0 Hz, J 2 (H,H) = 8.7 Hz, 4H, –C6 H4 ), 4.02 (m, 4H, –OCH2 ), 1.72 (m, 4H, –OCH2 CH2 ), 1.44–1.26 (m, 32H, –[CH2 ]8 ), 0.83 (m, 6H, –CH3 ). 13 C NMR (75 MHz, DMSO-d6 , 25◦ C): δ c 163.64, 163.13, 161.33, 160.14, 151.63, 131.83,

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S.K. Singh et al.


131.59, 129.35, 127.67, 125.35, 121.83, 120.57, 114.44, 113.83, 67.84, 67.59, 30.79, 30.47, 28.48, 28.44, 28.25, 28.20, 28.15, 28.08, 25.06, 24.62, 21.58, 21.79, 13.39, 13.36. Elemental analyses: calculated for C43 H60 N2 O5 (%), C 73.45, H 8.82, N 4.09; Found: C 75.39, H 8.77, N 4.16.

2.3.15 N-[4-(4 -dodecyloxy)benzoyloxybenzylidene]-N -(4 -dodecyloxybenzoyl)hydrazine, C12,12 LH (1j) Yield: 85 %. IR (KBr, cm−1 ): 3212 ν s (N–H), 2924, 2854 (aliphatic C–H), 1727 (ester, C=O), 1643 (amide-I, C=O), 1557 (amide-II, N–H), 1609, 1507 (Ph), 1286, 1259 (OPh). 1 H NMR (300 MHz, DMSO-d6 , 25◦ C): δ 11.46 (s, 1H, –NH), 8.44 (s, 1H, –CH=N), 8.01 (d, J 1 (H,H) = 9.0 Hz, 2H, –C6 H4 ), 7.81 (dd, J 1 (H,H) = 8.7 Hz, J 2 (H,H) = 8.7 Hz, 4H, –C6 H4 ), 7.34 (d, J 1 (H,H) = 8.7 Hz, 2H, –C6 H4 ), 7.01 (dd, J 1 (H,H) = 9.0 Hz, J 2 (H,H) = 8.7 Hz, 4H, –C6 H4 ), 4.01 (m, 4H, –OCH2 ), 1.72 (m, 4H, –OCH2 CH2 ), 1.42–1.26 (m, 36H, –[CH2 ]9 ), 0.83 (m, 6H, –CH3 ). 13 C NMR (75 MHz, DMSO-d6 , 25◦ C): δ c 163.68, 163.08, 161.23, 160.74, 151.81, 131.89, 131.69, 129.39, 127.74, 125.05, 121.87, 120.56, 114.54, 113.87, 67.88, 67.59, 30.70, 30.46, 28.47, 28.43, 28.21, 28.20, 28.17, 28.11, 25.01, 24.62, 21.58, 21.49, 13.33, 13.28. Elemental analyses: calculated for C45 H64 N2 O5 (%), C 75.75, H 9.03, N 3.92; Found: C 75.81, H 8.91, N 4.03.

2.3.16 N-[4-(4 -dodecyloxy)benzoyloxybenzylidene]-N -(4 -tetradecyloxybenzoyl) hydrazine, C14,12 LH (1k) Yield: 82 %. IR (KBr, cm−1 ): 3214 ν s (N–H), 2923, 2854 (aliphatic C–H), 1729 (ester, C=O), 1645 (amide-I, C=O), 1553 (amide-II, N–H), 1606, 1507 (Ph), 1286, 1258 (OPh). 1 H NMR (300 MHz, DMSO-d6 , 25◦ C): δ 11.45 (s, 1H, –NH), 8.44 (s, 1H, –CH=N), 8.03 (d, J 1 (H,H) = 9.0 Hz, 2H, –C6 H4 ), 7.81 (dd, J 1 (H,H) = 8.7 Hz, J 2 (H,H) = 8.7 Hz, 4H, –C6 H4 ), 7.34 (d, J 1 (H,H) = 8.7 Hz, 2H, –C6 H4 ), 7.02 (dd, J 1 (H,H) = 9.0 Hz, J 2 (H,H) = 8.7 Hz, 4H, –C6 H4 ), 4.00 (m, 4H, –OCH2 ), 1.71 (m, 4H, –OCH2 CH2 ), 1.41–1.25 (m, 40H, –[CH2 ]10 ), 0.83 (m, 6H, –CH3 ). 13 C NMR (75 MHz, DMSO-d6 , 25◦ C): δ c 163.67, 163.08, 161.18, 160.19, 151.68, 131.89, 132.69, 129.85, 128.66, 125.85, 122.83, 121.54, 114.54, 113.88, 67.88, 67.69, 30.78, 30.67, 28.48, 28.45, 28.29, 28.24, 28.18, 27.88, 25.11, 24.98, 21.64, 21.49, 13.38, 13.36. Elemental analyses: calculated for C47 H68 N2 O5 (%), C 76.12, H 9.24, N 3.78; Found: C 76.20, H 9.21, N 3.73.

2.3.17 N-[4-(4 -tetradecyloxy)benzoyloxybenzylidene]-N -(4 -tetradecyloxybenzoyl) hydrazine, C14,14 LH (1l) Yield: 81 %. IR (KBr, cm−1 ): 3219 ν s (N–H), 2924, 2851 (aliphatic C–H), 1727 (ester, C=O), 1647 (amide-I, C=O), 1559 (amide-II, N–H), 1608, 1509 (Ph), 1284, 1261 (OPh). 1 H NMR (300 MHz, DMSO-d6 , 25◦ C): δ 11.41 (s, 1H, –NH), 8.43 (s, 1H, –CH=N), 8.01 (d, J 1 (H,H) = 9.0 Hz, 2H, –C6 H4 ), 7.81 (dd, J 1 (H,H) = 8.7 Hz, J 2 (H,H) = 8.7 Hz, 4H, –C6 H4 ), 7.34 (d, J 1 (H,H) = 8.7 Hz, 2H, –C6 H4 ), 7.01 (dd, J 1 (H,H) = 9.0 Hz, J 2 (H,H) = 8.7 Hz, 4H, –C6 H4 ), 4.01 (m, 4H, –OCH2 ), 1.71 (m, 4H, –OCH2 CH2 ), 1.42–1.26 (m, 44H, –[CH2 ]11 ), 0.81 (m, 6H, –CH3 ). 13 C NMR (75 MHz, DMSO-d6 , 25◦ C): δ c 163.67, 163.08, 161.18, 160.17, 151.71, 131.87, 131.58, 129.45, 128.06, 125.27, 121.88, 121.54, 114.74, 113.88, 67.88, 67.69, 30.78, 30.48, 28.49, 28.33, 28.28, 28.20, 28.17, 28.03, 25.47, 25.43, 24.86, 24.68, 21.84, 21.69, 13.39, 13.36. Elemental analyses: calculated for C49 H72 N2 O5 (%), C 76.47, H 9.42, N 3.64; Found: C 76.53, H 9.48, N 3.65.

2.3.18 N-[4-(4 -hexadecyloxy)benzoyloxybenzylidene]-N -(4 -hexadecyloxybenzoyl) hydrazine, C16,16 LH (1m) Yield: 79 %. IR (KBr, cm−1 ): 3224 ν s (N–H), 2923, 2851 (aliphatic C–H), 1731 (ester, C=O), 1647 (amide-I, C=O), 1556 (amide-II, N–H), 1608, 1509 (Ph), 1283, 1260 (OPh). 1 H NMR (300 MHz, DMSO-d6 , 25◦ C): δ 11.39 (s, 1H, –NH), 8.42 (s, 1H, –CH=N), 8.00 (d, J 1 (H,H) = 9.0 Hz, 2H, –C6 H4 ), 7.81 (dd, J 1 (H,H) = 8.7 Hz, J 2 (H,H) = 8.7 Hz, 4H, –C6 H4 ), 7.34 (d, J 1 (H,H) = 8.7 Hz, 2H, –C6 H4 ), 7.03 (dd, J 1 (H,H) = 9.0 Hz, J 2 (H,H) = 8.7 Hz, 4H, –C6 H4 ), 4.02 (m, 4H, –OCH2 ), 1.71 (m, 4H, –OCH2 CH2 ), 1.43–1.25 (m, 52H, –[CH2 ]13 ), 0.81 (m, 6H, –CH3 ). 13 C NMR (75 MHz, DMSO-d6 , 25◦ C): δ c 163.64, 163.07, 161.15, 160.19, 151.76, 131.98, 131.60, 129.39, 127.56, 125.23, 121.88, 120.74, 114.49, 113.88, 67.88, 67.79, 30.99, 30.67, 28.50, 28.48, 28.41, 28.23, 28.17, 28.13, 25.71, 25.66, 25.07, 24.67, 21.44, 21.34, 13.37, 13.33. Elemental analyses: calculated for C53 H80 N2 O5 (%), C 77.09, H 9.76, N 3.39; Found: C 77.15, H 9.73, N 3.42.

2.3.19 General procedures for preparation of nickel complexes bis[N-{4-(4 -decyloxy)benzoyloxybenzylidene}-N -{4 - decyloxybenzoyl}hydrazinato]nickel(II), (C10,10 L)2 Ni (2a) A suspension of N-[4-(4 -decyloxy)benzoyloxybenzylidene]-N -(4 -decyloxybenzoyl) hydrazine (1.0


Liquid Crystals mmol, 0.65 g) and nickel acetate (1.2 mmol, 0.30 g) in 50 mL of absolute ethanol was heated under reflux overnight. The orange solid was filtered off, and then washed with hot ethanol (3 × 20 mL). The product was isolated as orange crystals after twice recrystallisation from ethanol. Yield: 68%. IR (KBr, cm−1 ): 2926, 2852, 1723, 1600, 1491, 1373, 1281, 1259, 1202, 1161, 1064. 1 H NMR (300 MHz, CDCl3 , 25◦ C): δ 8.43 (d, 2H, – CH=N), 8.15 (d, 4H, –C6 H4 ), 7.92 (dd, 8H, –C6 H4 ), 7.33 (d, 4H, –C6 H4 ), 6.93 (dd, 8H, –C6 H4 ), 4.01 (m, 8H, –OCH2 ), 1.82 (m, 8H, –OCH2 CH2 ), 1.57– 1.26 (m, 56H, –[CH2 ]14 ), 0.89 (m, 12H, –CH3 ). UVvisible (CHCl3 ): λmax = 412, 397, 359, 309, 280 nm. Elemental analyses: calculated for (C41 H55 N2 O5 )2 Ni (%), C 71.90, H 8.08, N 4.09, Ni 4.28; Found: C 71.83, H 7.94, N 4.01, Ni 4.13. 2.3.20 Bis [N-{4-(4 -decyloxy)benzoyloxybenzylidene}-N -{4 -dodecyloxybenzoyl}hydrazinato] nickel(II), (C12,10 L)2 Ni (2b) Yield: 70 %. IR (KBr, cm−1 ): 2924, 2854, 1726, 1606, 1495, 1370, 1278, 1256, 1209, 1165, 1067. 1 H NMR (300 MHz, CDCl3 , 25◦ C): δ 8.46 (d, 2H, – CH=N), 8.15 (d, 4H, –C6 H4 ), 7.93 (dd, 8H, –C6 H4 ), 7.36 (d, 4H, –C6 H4 ), 6.94 (dd, 8H, –C6 H4 ), 4.03 (m, 8H, –OCH2 ), 1.80 (m, 8H, –OCH2 CH2 ), 1.55– 1.27 (m, 64H, –[CH2 ]16 ), 0.89 (m, 12H, –CH3 ). UVvisible (CHCl3 ): λmax = 407, 392, 362, 311, 278 nm. Elemental analyses: calculated for (C43 H59 N2 O5 )2 Ni (%), C 72.44, H 8.33, N 3.93, Ni 4.11; Found: C 72.38, H 8.30, N 3.84, Ni 4.03. 2.3.21 [N-{4-(4 -dodecyloxy)benzoyloxybenzylidene}-N -{4 -tetradecyloxybenzoyl}hydrazinato] nickel(II), (C14,12 L)2 Ni (2c) Yield: 66%. IR (KBr, cm−1 ): 2923, 2852, 1724, 1609, 1495, 1371, 1279, 1263, 1207, 1163, 1056. 1 H NMR (300 MHz, CDCl3 , 25◦ C): δ 8.39 (d, 2H, –CH=N), 8.11 (d, 4H, –C6 H4 ), 7.90 (dd, 8H, –C6 H4 ), 7.35 (d, 4H, –C6 H4 ), 6.90 (dd, 8H, –C6 H4 ), 4.00 (m, 8H, –OCH2 ), 1.79 (m, 8H, –OCH2 CH2 ), 1.56–1.23 (m, 80H, –[CH2 ]20 ), 0.88 (m, 12H, –CH3 ). UVvisible (CHCl3 ): λmax = 417, 395, 364, 312, 286 nm. Elemental analyses: calculated for (C47 H67 N2 O5 )2 Ni (%), C 73.46, H 8.78, N 3.64, Ni 3.81; Found: C 73.34, H 8.69, N 3.67, Ni 3.76.

3.

Results and discussion

3.1 Synthesis and characterisation The synthetic route for preparation of aroylhydrazone based ligands [15] N-[4-(4 alkoxy)ben-

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zoyloxybenzylidene]-N -(4 -alkoxybenzoyl) hydrazines 1a–m and the nickel(II) complexes 2a–c is outlined in Scheme 1. Alkylation of ethyl 4-hydroxybenzoate ester with an appropriate alkyl bromide in the presence of mild base in 2-butanone allowed us to obtain a reasonable yield of ethyl4-alkoxybenzoate esters. Base hydrolysis of ethyl-4-alkoxybenzoate esters by refluxing in the presence of KOH in ethanol/H2 O solution leads to formation of 4-alkoxybenzoic acids, which on further esterification with 4-hydroxy benzaldehyde gave 4-alkoxybenzoyloxy benzaldehydes. Reactions of ethyl-4-alkoxybenzoate esters with hydrazine hydrate in refluxing absolute ethanol gave 4-alkoxybenzoylhydrazines as white solid. The Schiff bases 1a–m were obtained by the condensation reaction of these hydrazine derivatives with appropriate aldehydes in refluxing ethanol. Aroylhydrazone derivatives [17, 18] exist in two different keto–enol tautomeric forms (Scheme 2). The conjugating ability [21] of the substituents on the ketone moiety was found to influence the ratio of these two isomers. The increasing conjugating ability of aryl substituents preferred the enolic tautomer form. The square planar nickel(II) complexes 2 a–c studied in this work were prepared by the reaction of aroylhydrazone-based ligands with Ni(OAc)2 in refluxing ethanol. The nickel complexes were then isolated as orange crystals by recrystallisation from absolute ethanol in good yields (66–70%). All the complexes and free ligands were characterised by elemental analyses and standard spectroscopic techniques. The absorption bands appearing at 3214, (2922, 2854), 1729, (1641, 1557), (1605, 1506, 1471), (1278, 1254) cm−1 in the FT-IR spectrum of N-[4-(4 -decyloxy)benzoyloxybenzylidene]N -(4 -dodecyloxybenzoyl) hydrazine are attributed to ν(N–H), ν(aliphatic C–H), ν(C=O, ester), ν(C=O, amide-I), ν(N–H, amide-II), ν(Ph) and ν(OPh) modes, respectively. The occurrence of characteristic N–H stretching vibration (ν N–H ) band at 3214 cm−1 and the strong absorption of amide-I (ν C=O ) at 1641 cm−1 clearly revealed the interaction of the N–H groups with C=O groups through H-bonds (C=O···N–H) [29]. This assignment is supported by the fact that a non H-bonded (free) N–H stretching band appears at about 3400 cm−1 [30]. The nickel complexes of N-[4-(4 -decyloxy) benzoyloxybenzylidene]-N -(4 -dodecyloxybenzoyl) hydrazine show a strong band at 1606 cm−1 , representing the ν(>C=N–N=C