Complexes of some 4f metal ions of the mesogenic Schiff-base, N,N -di-(4-decyloxysalicylidene)-1,3-diaminobenzene: Synthesis and spectral studies

Article abstract of DOI:10.1080/00958972.2012.719225

A mesogenic Schiff-base, N,N-di-(4-decyloxysalicylidene)-1,3- diaminobenzene, H2ddsdbz (abbreviated as H2L), that exhibits a nematic mesophase was synthesized and its structure was studied by elemental analysis, mass spectrometry, NMR, and IR spectral tec

Full text of DOI:10.1080/00958972.2012.719225

This article was downloaded by: [University of Tasmania]
On: 13 November 2014, At: 10:41
Publisher: Taylor & Francis
Informa Ltd Registered in England and Wales Registered Number: 1072954 Registered
office: Mortimer House, 37-41 Mortimer Street, London W1T 3JH, UK
Journal of Coordination Chemistry
Publication details, including instructions for authors and
subscription information:
http://www.tandfonline.com/loi/gcoo20
Complexes of some 4f metal ions
of the mesogenic Schiff-base, N,N′-
di-(4-decyloxysalicylidene)-1′,3′-
diaminobenzene: synthesis and
spectral studies
Pawan Raj Shakya a , Angad Kumar Singh a & T.R. Rao a
a Department of Chemistry , Banaras Hindu University , Varanasi –
221005 , India
Accepted author version posted online: 07 Aug 2012.Published
online: 24 Aug 2012.
To cite this article: Pawan Raj Shakya , Angad Kumar Singh & T.R. Rao (2012) Complexes of some
4f metal ions of the mesogenic Schiff-base, N,N′-di-(4-decyloxysalicylidene)-1′,3′-diaminobenzene:
synthesis and spectral studies, Journal of Coordination Chemistry, 65:20, 3519-3529, DOI:
10.1080/00958972.2012.719225
To link to this article: http://dx.doi.org/10.1080/00958972.2012.719225
PLEASE SCROLL DOWN FOR ARTICLE
Taylor & Francis makes every effort to ensure the accuracy of all the information (the
“Content”) contained in the publications on our platform. However, Taylor & Francis,
our agents, and our licensors make no representations or warranties whatsoever as to
the accuracy, completeness, or suitability for any purpose of the Content. Any opinions
and views expressed in this publication are the opinions and views of the authors,
and are not the views of or endorsed by Taylor & Francis. The accuracy of the Content
should not be relied upon and should be independently verified with primary sources
of information. Taylor and Francis shall not be liable for any losses, actions, claims,
proceedings, demands, costs, expenses, damages, and other liabilities whatsoever or
howsoever caused arising directly or indirectly in connection with, in relation to or arising
out of the use of the Content.
This article may be used for research, teaching, and private study purposes. Any
substantial or systematic reproduction, redistribution, reselling, loan, sub-licensing,
systematic supply, or distribution in any form to anyone is expressly forbidden. Terms &

Conditions of access and use can be found at http://www.tandfonline.com/page/terms-
and-conditions

Journal of Coordination Chemistry
Vol. 65, No. 20, 20 October 2012, 3519–3529
Complexes of some 4f metal ions of the mesogenic Schiff-base,
N,N9-di-(4-decyloxysalicylidene)-19,39-diaminobenzene:
synthesis and spectral studies
PAWAN RAJ SHAKYA, ANGAD KUMAR SINGH and T.R. RAO*
Department of Chemistry, Banaras Hindu University, Varanasi – 221005, India
(Received 22 December 2011; in final form 12 June 2012)
A
mesogenic Schiff-base, N,N0-di-(4-decyloxysalicylidene)-10,30-diaminobenzene, H2ddsdbz
(abbreviated as H2L), that exhibits a nematic mesophase was synthesized and its structure
was studied by elemental analysis, mass spectrometry, NMR, and IR spectral techniques. The
Schiff-base, H2L, upon condensation with hydrated lanthanide(III) nitrates yields LnIII
complexes, [Ln2(LH2)3(NO3)4](NO3)2, where Ln ¼ La, Pr, Nd, Sm, Eu, Gd, Tb, Dy, and Ho.
Analyses of the IR and NMR spectral data imply bidentate Schiff-base through two phenolate
oxygen atoms in its zwitterionic form to LnIII, rendering the overall geometry of the complexes
as a seven-coordinate polyhedron – possibly distorted mono-capped octahedron. Polarizing
optical microscope and differential scanning calorimetry studies reveal that despite H2L being
mesogenic, none of the LnIII complexes synthesized under this study exhibits mesomorphism.
Keywords: Mesogenic Schiff-base; LnIII Complexes; Zwitterionic-coordination; Mono-capped
octahedron; NMR and IR Spectra
1. Introduction
Liquid-crystalline metal complexes, called metallomesogens, are the subject of intense
research [1–4]. Liquid crystals with 3d and/or 4f metals have received increasing
attention because of the possibility of combining the physico-chemical properties of the
metal (color, magnetism, polarizability, redox behavior, etc.) with those of the organic
framework [5–9]. Bertolo et al. and Chandra et al. published [10, 11] complexes of
macrocyclic Schiff-base ligands with different size, number, and donors involving
coordination with a variety of metal centers. The research groups of Galyametdinov,
Bruce, and Binnemans [12–15] reported complexes of Schiff-base ligands with one
aromatic ring, but Schiff-base ligands with two and more than two aromatic rings were
much less studied [16, 17]. Hence, in continuation of earlier work [18–21] carried out in
our laboratory on systematic structural and spectroscopic studies of 3d and 4f metal
complexes of mesogenic Schiff-bases, we present here synthesis and spectroscopic
*Corresponding author. Email: drtrrao@gmail.com
Journal of Coordination Chemistry
ISSN 0095-8972 print/ISSN 1029-0389 online ß 2012 Taylor & Francis
http://dx.doi.org/10.1080/00958972.2012.719225

3520
P.R. Shakya et al.
characterization of some LnIII complexes with a Schiff-base containing three aromatic
rings, namely N,N0-di-(4-decyloxysalicylidene)-10,30-diaminobenzene (H2L) (2).
2. Experimental
2.1. Materials
We obtained reagents of analytical grade (AR) from commercial vendors and used them
without purification: 1-bromodecane, 2,4-dihydroxy-benzaldehyde, and 1,3-diamino-
benzene are from Sigma-Aldrich, USA; Ln(NO3)3 ꢀ xH2O salts are from Indian Rare
Earths Ltd., while KI and KHCO3 are from Merck. The solvents obtained from
commercial sources were dried using standard methods [22] when required.
2.2. Synthesis and analysis
Synthesis of N,N0-di-(4-decyloxysalicylidene)-10,30-diaminobenzene (H2L) (2), was
achieved by a two-step synthesis, alkylation of 2,4-dihydroxybenzaldehyde with
1-bromodecane followed by condensation with aromatic diamine as per the experimental
details given in scheme 1. We prepared the LnIII complexes, [Ln2(LH2)3(NO3)4](NO3)2
(Ln ¼ La, Pr, Nd, Sm, Eu, Gd, Tb, Dy, and Ho) (3) by reacting solutions of the
appropriate metal nitrate and H2L at room temperature.
2.2.1. Preparation of 4-decyloxysalicylaldehyde (1). To a solution of 100 mL of dry
acetone we added equimolar amounts of 2,4-dihydroxy benzaldehyde (50 mmol, 6.91 g),
1-bromodecane (50 mmol, 10.4 mL), and potassium bicarbonate (ꢁ55 mmol, 5.51 g) and
Scheme 1. Reaction steps involved in the synthesis of 4-decyloxysalicylaldehyde (1), N,N0-di-
(4-decyloxysalicylidene)-10,30-diaminobenzene (H2L) (2), and LnIII complexes (3).

Mesogenic Schiff-base
3521
refluxed the mixture for ꢁ30 h in the presence of KI (0.1–0.2 g) as a catalyst and filtered
while hot to remove insoluble solids; subsequently, we added 6N hydrochloric acid to
the filtrate until neutralization and extracted the product twice with 100 mL portions of
CHCl3. By concentrating the chloroform extracts we obtained a straw-yellow solid
which was purified by column chromatography over SiO2 by eluting first with n-hexane
and then with a mixture of n-hexane and chloroform (v/v, 1/1); evaporation of this
purified extract finally yielded 4-decyloxysalicylaldehyde (1) in the form of a white
solid; yield: 62% (8.63 g).
2.2.2. Synthesis of N,N0-di-(4-decyloxysalicylidene)-10,30-diaminobenzene (H2L) (2). We
prepared N,N0-di-(4-decyloxysalicylidene)-10,30-diaminobenzene (H2L) (2) by refluxing
together absolute ethanolic solutions of 4-decyloxysalicylaldehyde (1) (8.34 g, 30 mmol
in 50 mL) and 1,3-diaminobenzene (1.62 g, 15 mmol in 15 mL) for ꢁ1.5 h in the presence
of a few drops of glacial acetic acid. Leaving the resulting mixture overnight yielded a
yellow solid, 2, which was filtered off under suction, thoroughly washed with cold
ethanol and dried at room temperature. Yield: 75%, m.p. 105ꢂC. Anal. Calcd for
C40H56N2O4 (%): C, 76.39; H; 8.98; N, 4.45. Found (%): C, 76.33; H, 9.05; N, 4.42.
1H NMR (300 MHz; DMSO-d6): ꢀ ¼ 0.86 (t, 3H, –CH3, J ¼ 6.3 Hz), 1.72–1.27 (m, 16H,
Hmethylene), 4.04 (t, 2H, –OCH2, J ¼ 6.6 Hz), 6.46 (s, 1H, H3), 6.54 (d, 1H, H5,
0
0
0
J ¼ 7.8 Hz), 7.24 (d, 1H, H4 , J ¼ 7.5 Hz,), 7.36 (s, 1H, H2 ), 7.46 (d, 1H, H5 , J ¼ 6.6 Hz),
7.51 (d, 1H, H6, J ¼ 9.0 Hz), 8.89 (s, 1H, –N ¼ CH), 13.38 (s, br, 1H, ph-OH);
13C{1H}NMR: (75.45 MHz; DMSO-d6) 163.02 (–NCH), 162.85 (–C4), 162.46 (–C2),
0
0
0
0
148.02 (–C3 ), 133.73 (–C5 ), 130.02 (–C6), 129.22 (–C4 ), 118.80 (–C2 ), 112.69 (–C1),
106.89 (–C5), 101.20 (–C3), and 67.62 (ꢃOCH2). LC/MS mass: the molecular ion (m/e,
631, 12% intensity) generated simultaneously three fragments, M1–M3 (m/e, fragment,
%
intensity): M1: 613, C10H21OC6H3(OH)CH¼N(C6H4)N¼CHC6H3(OH)O
(CH2)8CHþ2 , 15%; M2 (generated from M1): 571, C10H21O-C6H3(OH)CH¼
N(C6H4)N¼CH-C6H3(OH)O(CH2)5CHþ2 , 17%; M3: 276, C10H21OC6H3-(OH)CH¼
Nþ, 100%; IR (KBr) (ꢁmax/cmꢃ1): ꢁ(O–H)phenol 3450(br), ꢁ(C¼N) 1623(s), ꢁ(C–O)phenol
1291(s).
2.2.3. Synthesis of [La2(LH2)3(NO3)4](NO3)2 (3). Dropwise addition of THF solution
of La(NO3)3 ꢀ 6H2O (0.87 g, 2.0 mmol in 20 mL) to a THF solution of H2L (1.89 g,
3.0 mmol in 30 mL) under magnetic stirring resulted in formation of the LaIII complex.
The resultant solution turned cloudy after ꢁ15 min; a solid product separated upon
continuous stirring for ꢁ3 h at room temperature, which was filtered, washed
repeatedly with cold methanol, and dried over fused CaCl2. Yield: 66% as yellow
solid; m.p. 256ꢂC (decompose); Anal. Calcd for La2C120H168N12O30 (%): C, 56.82; H;
6.68; N, 6.63; La, 10.95; Found (%): C, 56.89; H, 6.70; N, 6.59 and La, 11.01, 1H NMR
(300 MHz; DMSO-d6): 0.86 (t, 3H, CH3, J ¼ 5.7 Hz), 1.72–1.27 (m, 16H, Hmethylene),
4.04 (t, 2H, –OCH2, J ¼ 6.3 Hz), 6.46 (s, 1H, H3), 6.53 (d, 1H, H5, J ¼ 8.4 Hz), 7.23
0
0
0
(d, 1H, H4 , J ¼ 8.1 Hz), 7.35 (s, 1H, H2 ), 7.45 (d, 1H, H5 , J ¼ 7.8 Hz), 7.51 (d, 1H, H6,
J ¼ 8.7 Hz), 8.88 (s, 1H, –N ¼ CH), 13.34 (br-s, 1H, –NþH); 13C{1H} NMR
0
(75.45 MHz; DMSO-d6): 175.28 (–NCH), 162.82 (–C4), 168.08 (–C2), 148.91 (–C3 ),
0
0
0
133.69 (–C5 ), 129.78 (–C6), 129.20 (–C4 ), 118.76 (–C2 ), 112.70 (–C1), 106.85 (–C5),
00
101.20 (–C3), and 67.61 (–C1 ); IR (KBr) (ꢁmax/cmꢃ1): ꢁ(–NþH, str.) 3195(w), ꢁ(C¼N)
1636(s), ꢁ(C–O)phenol 1243(m).

3522
P.R. Shakya et al.
We synthesized all the other rare-earth complexes (Ln ¼ Pr, Nd, Sm, Eu, Gd, Tb, Dy,
and Ho) in an analogous way by using the appropriate hydrated salt of LnIII nitrate; the
physical properties and the analytical data of all the complexes are given in table 1.
Infrared spectral data of the ligand and complexes are given in table 2; the data of two
representative complexes are given below:
[Gd2(LH2)3(NO3)4](NO3)2: IR (KBr) (ꢁmax/cmꢃ1): ꢁ(–NþH, str.) 3205(w), ꢁ(C¼N)
1637(s), ꢁ(C–O)phenol 1228(m); [Ho2(LH2)3(NO3)4](NO3)2: IR (KBr) (ꢁmax/cmꢃ1):
ꢁ(–NþH, str.) 3195(w), ꢁ(C¼N) 1640(s), ꢁ(C–O)phenol 1238(m).
2.3. Physical measurements
To determine the metal content of the complexes, we carried out complexometric
titrations against EDTA using xylenol orange as indicator. Carbon, hydrogen, and
1
nitrogen were analyzed on an Exeter Analyzer, Model CE-440 CHN. The H and
13C{1H} NMR spectra were recorded on a JEOL AL-300 MHz FT-NMR multinuclear
spectrometer; infrared spectra (as KBr pellets from 4000 to 400 cmꢃ1) on a JASCO
FT-IR (model-5300) spectrophotometer; mass spectrum on a LC/MS (ESI Mode-3000)
mass spectrometer, and UV-Vis spectra on a Shimadzu spectrophotometer (model
Pharmaspec-UV 1700). We measured molar conductances of the complexes in
0.001 mol Lꢃ1 solutions, magnetic susceptibility data at room temperature on a
Cahn–Faraday balance with Hg[Co(NCS)4] as the calibrant, and identified the
mesophases under polarized binocular microscope (LOMO, USA) equipped with hot-
stage and digital camera (Nikon Coolpix 4500). Differential scanning calorimetry
(DSC) studies were made on a METTLER DSC-25, Mettler STARe SW 9.00 unit.
3. Results and discussion
3.1. Magnetic and spectral studies
The yellow Schiff-base (H2L) reacts with Ln(NO3)3 ꢀ xH2O to yield LnIII complexes;
table 1 includes the data obtained (elemental analyses, magnetic moments, important
physical properties, and general behavior) on the ligand and the complexes. The
analytical data of the complexes imply 2 : 3 metal to ligand stoichiometry with the
general formula [Ln2(LH2)3(NO3)4](NO3)2, where Ln ¼ La, Pr, Nd, Sm, Eu, Gd, Tb,
Dy, and Ho. In all the complexes H2L coordinates as a non-deprotonated zwitterionic
species as evidenced by NMR and IR spectra while the counter ions (nitrate groups
both within and outside the coordination sphere) balance the positive charge of the
LnIII ion(s); the molar conductance data (107–121 ꢀꢃ1 cm2 per mole) imply 2 : 1
electrolytic behavior [23].
The ꢂeff values (at room temperature) of the LnIII complexes (3.93, 3.97, 1.90, 4.87,
11.18, 12.61, 14.56, and 15.31 B.M. for Ln ¼ Pr, Nd, Sm, Eu, Gd, Tb, Dy, and Ho,
respectively) are higher than the reported van Vleck values; we attribute these higher
values to metal–metal interactions in analogy to similar complexes reported [24–26]
with abnormal ꢂeff values.
We have recorded the electronic spectra (qualitative solution-state spectra from 200
to 1100 nm) of only the PrIII, NdIII, SmIII, and DyIII complexes (table 3) in view of their

Mesogenic Schiff-base
3523

3524
P.R. Shakya et al.
Table 2. IR spectral data (cmꢃ1) of H2L and of Ln(III) metal complexes.a
ꢁ(NO3)
ꢁ(O–H)
ꢁ
asCH
ꢁsCH
ꢁ(C–O)
H2L/Complex
phenolic ꢁ(NþH) CH3/CH2 CH3/CH2 ꢁ(C¼N) phenolic ꢁ5 Ionic ꢁ1
ꢁ2 ꢁ5–ꢁ1
H2L
3450b
–
2925
2925
2927
2926
2925
2925
2926
2926
2926
2925
2855
2854
2856
2855
2855
2855
2855
2855
2855
2855
1623
1636
1640
1642
1637
1643
1637
1640
1638
1640
1291
1243
1242
1244
1237
1237
1238
1238
1237
1238
–
–
–
–
–
[La2(LH2)3(NO3)4](NO3)2
[Pr2(LH2)3(NO3)4](NO3)2
[Nd2(LH2)3(NO3)4](NO3)2
[Sm2(LH2)3(NO3)4](NO3)2
[Eu2(LH2)3(NO3)4](NO3)2
[Gd2(LH2)3(NO3)4](NO3)2
[Tb2(LH2)3(NO3)4](NO3)2
[Dy2(LH2)3(NO3)4](NO3)2
[Ho2(LH2)3(NO3)4](NO3)2
–
–
–
–
–
–
–
–
–
3195w
3200w
3200w
3205w
3210w
3205w
3210w
3210w
3195w
1462 1385 1287 850 175
1465 1384 1292 852 173
1463 1385 1289 852 174
1473 1385 1292 837 181
1472 1385 1295 849 177
1475 1384 1293 841 182
1472 1384 1293 842 179
1472 1384 1293 842 179
1472 1384 1292 846 180
aSpectra recorded as KBr pellets; b: broad; as: asymmetric; s: symmetric; w: weak.
Table 3. Electronic spectral data of selected metal complexes of H2L.
Transitions/Bonding
parameters
ꢃmax (cmꢃ1
aq. ion
)
ꢃmax (cmꢃ1
complex
)
Transitions/Bonding
parameters
ꢃmax (cmꢃ1
aq. ion
)
ꢃmax (cmꢃ1
)
complex
PrIII
NdIII
1G4 3H4
9900
16,850
20,800
9794
16,835
–
4F3/2 4I9/2
11,450
12,500
13,500
17,400
–
1D2
3P0
4F5/2
,
2H9/2
12346
13,459
17,182
0.991
0.067
0.908
0.004
a
4S3/2
,
4F7/2
2G7/2
a
ꢄ
0.994
0.055
0.604
0.003
B1/2
%ꢀ
ꢅ
SmIII
DyIII
6F9/2 6H5/2
9200
10,500
17,900
26,750
9174
10,482
–
6F9/2
6H5/2
6F7/2
6F5/2
,
6H5/2 6H15/2
9100
10,200
11,000
12,400
9099
10,122
10,989
12,346
0.997
0.039
0.301
0.001
6F11/2
4G5/2
6P7/2
ꢄ
–
a
0.998
0.032
0.200
0.001
B1/2
%ꢀ
ꢅ
aHypersensitive band.
ability to show hypersensitive bands; the ꢃmax values of the complexes show
considerable red shift in comparison with those of their corresponding aqua ions [27]
due to the nephelauxetic effect [28] which is regarded as a measure of covalency of the
bonding between the metal ions and the ligands. Various bonding parameters (table 3),
namely nephelauxetic ratio (ꢄ), bonding parameter (b1/2), Sinha’s parameter (%ꢀ), and
covalency angular overlap parameter (ꢅ), calculated by procedures reported [29],
suggest a weak covalent nature of the metal–ligand bonds.
On the basis of IR, NMR, and electronic spectral data, we propose that the mesogenic
N,N0-di-(4-decyloxysalicylidene)-10,30-diaminobenzene
Schiff-base,
(H2L)
(2),

Mesogenic Schiff-base
3525
Table 4. NMR spectral data of H2L and the La(III) complex.a
Ligand
La(III) complex
Peak
position
Peak
position
(ꢀ) ppm
Coupling
constant
J (Hz)
Coupling
constant
J (Hz)
1H/13C{1H}
(multiplicity)
(ꢀ) ppm
È–OH (s)
–NþH (br s)
–N¼CH (s)
–C6H (d)
0
–C5 H (d)
0
–C2 H (s)
13.380
–
–
–
9.0
6.6
–
–
–
–
–
8.7
7.8
–
–
13.343
8.883
8.888
7.529, 7.499
7.467, 7.445
7.358
0 7.527, 7.498
7.466, 7.440
7.352
0
–C4 H (d)
–C5H (d)
–C3H (s)
7.249, 7.224
6.550, 6.524
6.458
7.5
7.8
–
7.247, 7.220
6.546, 6.518
6.456
8.1
8.4
–
00
–C1 H (t)
4.057, 4.035, 4.015
1.720–1.268
0.880, 0.859, 0.836
163.02
162.85
162.46
148.02
133.53
130.02
129.22
6.6
–
6.3
–
–
–
–
–
–
–
4.058, 4.037, 4.015
1.721–1.267
0.879, 0.860, 0.838
175.28
162.82
168.08
148.91
133.69
129.78
129.20
6.3
–
5.7
–
–
–
–
–
–
–
–(CH2)8 (m)
–CH3 (t)
–NCH
–C4
–C2
–C1 /–C3
0
0
0
–C5
–C6
0
–C4 /–C6
0
0
–C2
118.80
112.69
106.89
101.20
–
–
–
–
118.76
112.70
106.85
101.20
–
–
–
–
–C1
–C5
–C3
00
–C1
–C2 –C9
67.62
30.81–21.56
13.35
–
–
–
67.61
30.78–21.53
13.30
–
–
–
00
00
00
–C10
Spectra (300 MHz) recorded in solutions of DMSO-d6.
aSpectrum recorded at 75.5ꢂC; 1H NMR spectral data are given in ꢀ (w.r.t. TMS); 13C{1H} NMR data measured in ppm
w.r.t. DMSO-d6 signal at 39.50 ppm.
coordinates neutral bidentate to LnIII to yield seven-coordinate complexes,
[Ln2(LH2)3(NO3)4](NO3)2, where Ln ¼ La, Pr, Nd, Sm, Eu, Gd, Tb, Dy, and Ho,
possibly in a distorted mono-capped octahedron.
We studied the Schiff-base and metal complexes by IR and NMR spectroscopy and
elemental analyses and confirmed the molecular weight of the ligand by LC/MS mass
spectrum. The mass spectrum of H2L shows the base peak (m/e ¼ 276, corresponding to
the fragment, C10H21OC6H3(OH)CH¼Nþ), the molecular ion peak with ꢁ12%
intensity (m/e ¼ 631), and the major fragment peaks (m/e ¼ 613 and 571) due to
C10H21OC6H3(OH)CH¼N-(C6H4)N¼CHC6H3(OH)O(CH2)8CH2þ
and
C10H21OC6H3(OH)-CH¼N(C6H4)N¼CH-C6H3(OH)O(CH2)5CHþ2 , respectively.
A comparison of NMR spectral [1H and 13C{1H}] data (table 4, figure S1 in
Supplementary material section) of the ligand, with those of the LaIII complex, shows
that the bonding of the phenolic oxygen of the ligand to metal is substantiated by the
phenolic–OH signal, appearing at ꢀ, 13.38 in the ligand, disappears upon complexation.
The composition of the LaIII complex, [La2(LH2)3(NO3)4](NO3)2, implies coordination
of H2L as a neutral species; further, the 1H NMR spectral data imply that the phenolic
protons are shifted to the two uncoordinated imino nitrogen atoms, which then are

3526
P.R. Shakya et al.
Scheme 2. Depiction of migration of phenolic protons to imine nitrogen atoms of H2L during the formation
of zwitterion.
intramolecularly hydrogen-bonded to the metal-bound phenolate oxygen atoms to give
zwitterionic structure (¼Nþ–Hꢀ ꢀ ꢀOꢃ) and the macrocycle under this condition is
designated as LH2 [30]. The signal corresponding to the imine hydrogen, –CH¼N,
broadened in the LaIII complex (ꢀ, 8.88) when compared with that of the ligand
(ꢀ, 8.89); further, a new signal, characteristic of –NþH resonance, appears at 13.34 ꢀ
which is absent in H2L. These observations are in accord with those made by
Binnemans et al. [31], who, while reporting their work on rare earth containing
magnetic liquid crystals, [Ln(LH)3(NO3)3], where LH ¼ 4-alkoxy-N-alkyl-2-hydroxy
benzaldimine, found that selective irradiation of the signal at 12.29 ꢀ removed the
broadening of the imine signal, thereby inferring that the signal does not correspond to
the proton of the –OH group, but to the proton of the –NþH group. Thus, H2L exists in
the metal complex in a zwitterionic form, with the phenolic oxygen deprotonated and
the imine nitrogen protonated (scheme 2).
13C{1H} NMR spectra show a significant shift of the –NCH signal from ꢀ, 163.02
(H2L) to ꢀ, 175.28 (in the LaIII complex); the carbons directly attached to the phenolate
showed similar shifts while the shifts of the other carbon signals were of lower
magnitude. Thus, NMR spectral data imply bonding through two phenolate oxygen
atoms of the ligand in the zwitterionic form to LaIII.
The broad absorption at 3450 cmꢃ1 in the IR spectrum of the ligand, characteristic of
ꢁ(O–H)phenolic [32], involves considerable H-bonding (to the ortho 4C¼N group
presumably of intramolecular type) under the experimental conditions; this band
disappears in spectra of the complexes due to shifting of the phenolic proton to the
azomethine nitrogen, resulting in formation of the zwitterion. Weak/medium intensity
bands centered at 1291 cmꢃ1 are assigned to ꢁ(C–O)phenolic. The strong band at
1623 cmꢃ1, assignable [33] to ꢁ(C¼N) of azomethine, undergoes a hypsochromic
shift (13–17 cmꢃ1) in all the complexes on account of zwitterion formation.
Thus, complexation to LnIII results in migration of phenolic protons onto the
two uncoordinated imino nitrogen atoms, which then are intramolecularly hydrogen-
bonded to metal-bound phenolate oxygen atoms to give the zwitterionic structure,
Nþ–Hꢀ ꢀ ꢀOꢃ. Binnemans et al. [34] reported similar zwitterionic behavior for acyclic

Mesogenic Schiff-base
3527
Figure 1. Proposed polyhedron (mono-capped octahedron) for [Ln2(LH2)3(NO3)4](NO3)2: Ln ¼ La, Pr, Nd,
Sm, Eu, Gd, Tb, Dy, and Ho.
Schiff-base lanthanide complexes. The formation of a zwitterion can be rationalized by
the tendency of lanthanides to coordinate to negatively charged ligands with a
preference for O-donors by transfer of the phenolic proton to imine nitrogen, the
phenolic oxygen becomes negatively charged facilitating coordination to lanthanide.
Further evidence was given by the band frequencies of ꢁ(C¼N) shifting to higher
wavenumbers upon complexation, implying the presence of the C–Nþ [35] and the non-
involvement of nitrogen in complex formation. The present complexes are characterized
by a strong band (1643–1636 cmꢃ1) due to ꢁ(C¼N) and a weak broad band at 3210–
3195 cmꢃ1 due to hydrogen-bonded Nþ–Hꢀ ꢀ ꢀOꢃ vibration of the protonated imine [35].
Thus, the ligand coordinates to metal via the negatively charged phenolic oxygen with
no binding between lanthanide and the imine nitrogen.
IR spectra of the complexes also show three additional characteristic frequencies of
coordinating nitrate (C2v) at 1475–1462, 1295–1287, and 852–837 cmꢃ1 [36]. The profile
and magnitude of separation of the modes associated with asymmetric nitrate
vibrations have been used as criteria to distinguish between mono- and bidentate
chelating nitrates; accordingly, the magnitude of splitting (182–173 cmꢃ1) at higher
energies indicates a bidentate coordinated nitrate [36, 37]. Additional bands at 1385–
1384 cmꢃ1 may be attributed to non-coordinated nitrate present outside the coordina-
tion sphere. A distorted mono-capped octahedron with coordination number ¼ 7 may
be tentatively proposed for the complexes (figure 1).

3528
P.R. Shakya et al.
Table 5. Thermodynamic data (transition temperatures and enthalpy and entropy changes).
Compound
H2L
Transitiona
Tb (ꢂC)
ꢀHb (kJ molꢃ1
)
DS (J molꢃ1 Kꢃ1
)
Cr–Cr
Cr–N
N–I
I–N
N–Cr
Cr–Cr
47.52
94.85
104.74
99.00c
75.41
65.67
10.67
18.20
1.77
–
17.22
3.13
33.29
49.48
4.69
–
49.42
9.24
ꢀH (kJ molꢃ1) ¼ DH ꢄ mw/1000; DS (J molꢃ1 Kꢃ1) ¼ DH (kJ molꢃ1) ꢄ 1000/T K.
aCr: Crystal, N: Nematic; I: Isotropic liquid.
bData as obtained from the second DSC cycle.
cData as inferred from polarizing optical microscope.
The polarizing optical microscope (POM) studies imply the nematic phase of H2L
and the corresponding transition temperatures, enthalpy, and entropy changes are given
in table 5 while the texture is shown in ‘‘Supplementary material’’ section. All LnIII
complexes reported here were non-mesogenic. The plausible explanation may be that
the thermal energy required to melt the alkoxy chains is so high that the layered
structure breaks down before the alkoxy chains are completely molten; in such a
situation, the liquid-crystalline properties of the materials are lost.
4. Conclusion
The mesogenic Schiff-base, N,N0-di-(4-decyloxysalicylidene)-l0,30-diaminobenzene
(H2L), coordinates to LnIII as a neutral bidentate species to yield seven-coordinate
complexes, [Ln2(LH2)3(NO3)4](NO3)2, where Ln ¼ La, Pr, Nd, Sm, Eu, Gd, Tb, Dy,
and Ho. Unlike the case with 3d metal ions, the neutral bidentate H2L coordinates to
LnIII in a zwitterionic form through two phenolate oxygen atoms. POM and DSC
studies reveal that only H2L shows mesogenic activity (nematic phase), not the LnIII
complexes presumably due to breakdown of the structure of the complexes before the
isotropic (clear melting) point.
Acknowledgments
We wish to acknowledge the recording of LC/MS mass spectrum at the Central Drug
Research Institute, Lucknow. The financial assistance received from the CSIR, New
Delhi (vide Sanction letter No. 01(2321)/09/EMR-II) and the fellowship awarded to Mr
P.R. Shakya by the University Grants Commission, Kathmandu, Nepal, are gratefully
acknowledged.
References
[1] A.M. Giroud-Godquin, P.M. Maitlis. Chem. Int. Ed. Engl., 30, 375 (1991).
[2] S.A. Hudson, P.M. Maitlis. Chem. Rev., 93, 861 (1993).

Mesogenic Schiff-base
3529
[3] J.L. Serrano (Ed.). Metallomesogens, Synthesis, Properties and Applications, VCH, Weinheim (1996).
[4] B. Donnio, D.W. Bruce. Struct. Bonding, 95, 193 (1999).
[5] B. Donnio, D. Guillon, R. Deschenaux, D.W. Bruce. In Metallomesogens in Comprehensive Coordination
Chemistry, J.A. McCleverty, T.J. Meyer (Eds), p. 6, Elsevier, Oxford (2003).
[6] M.H. Chisholm. Acc. Chem. Res., 33, 53 (2000).
[7] P. Kirsch, M. Bremer. Angew. Chem. Int. Ed. Engl., 39, 4216 (2000).
[8] I. Aiello, M. Ghedini, M. La Deda, D. Pucci, O. Francescangeli. Eur. J. Inorg. Chem., 1367 (1999).
[9] B. Donnio, D. Guillon, R. Deschenaux, D.W. Bruce. In Comprehensive Coordination Chemistry II,
J.A. McCleverty, J.J. Meyer, M. Fujita, A. Powell (Eds), Vol. 7, p. 353, Elsevier, Oxford (2003).
[10] E. Bertolo, R. Bastida, A.D. Blas, D.E. Fenton, C. Loderio, A. Macias, A. Rodriguez, T. Rodriguez-
Blas. J. Inclusion Phenom. Macrocyclic Chem., 35, 191 (1999).
[11] S. Chandra, K. Gupta. Transition Met. Chem., 27, 329 (2002).
[12] K. Binnemans, Yu.G. Galyametdinov, S.R. Collinson, D.W. Bruce. J. Mater. Chem., 8, 1551 (1998).
[13] K. Binnemans, R. van Deun, Yu.G. Galyametdinov, D.W. Bruce. Chem. Phys. Lett., 300, 509 (1999).
[14] K. Binnemans, D.W. Bruce, S.R. Collinson, R. van Deun, Y.G. Galyametdinov. Philos. Trans. R. Soc.
Lond., Ser. A, 357, 3063 (1999).
[15] Yu.G. Galyametdinov, G.I. Ivanova, I.V. Ovchinnikov, K. Binnemans, D.W. Bruce. Russ. Chem. Bull.,
48, 385 (1999).
[16] F. Martin, S.R. Collinson, D.W. Bruce. Liq. Cryst., 27, 859 (2000).
[17] K. Binnemans, K. Lodewyckx, R. van Deun, Yu.G. Galyametdinov, D. Hinz, G. Meyer. Liq. Cryst., 28,
279 (2001).
[18] A.K. Singh, S. Kumari, K. Ravi Kumar, B. Sridhar, T.R. Rao. Polyhedron, 27, 181 (2008).
[19] A.K. Singh, S. Kumari, K. Ravi Kumar, B. Sridhar, T.R. Rao. Polyhedron, 27, 1937 (2008).
[20] S. Kumari, A.K. Singh, T.R. Rao. Mater. Sci. Eng. C, 29, 2454 (2009).
[21] S. Kumari, A.K. Singh, K. Ravi Kumar, B. Sridhar, T.R. Rao. Inorg. Chim. Acta, 362, 4205 (2009).
[22] A. Weisberger, F.S. Praskver. Organic Solvents, International Publishers Inc., New York (1956).
[23] W.J. Geary. Coord. Chem. Rev., 7, 110 (1971).
[24] W. Plass, G. Fries. Z. Anorg. Allg. Chem., 623, 1205 (1997).
[25] J.P. Costes, F. Dahan, A. Dupuis, S. Lagrave, J.P. Laurent. Inorg. Chem., 37, 153 (1998).
[26] J.P. Costes, A. Dupuis, J.P. Laurent. Inorg. Chim. Acta, 268, 125 (1998).
[27] R. Reisfeld, C.K. Jorgensen. Lasers and Excited States of Rare Earths, Springer-Verlag, Berlin (1977).
[28] W.T. Carnall, P.R. Fields, K. Rajnak. J. Chem. Phys., 49, 4424 (1968).
[29] T.R. Rao, P.A. Kumar. Synth. Rect. Inorg. Met. Org. Chem., 25, 1011 (1995).
[30] P. Bag, U. Florke, K. Nag. Dalton Trans., 3236 (2006).
¨
[31] K. Binnemans, Y.G. Galyametdinov, R. van Deun, D.W. Bruce, S.R. Collison, A.P. Polishchuk,
I. Bikechantaev, W. Hasse, A.V. Prosvirin, L. Tinchurina, I. Litvinov, A. Gubajdullin,
A. Rakhamatullin, K. Uytterhoeven, L. VanMeervelt. J. Am. Chem. Soc., 122, 4335 (2000).
[32] R.M. Silverstein, F.X. Webster. Spectrometric Identification of Organic Compounds, 6th Edn, Wiley &
Sons, New York, NY (2002).
[33] N.B. Colthup, L.H. Daly, S.E. Wiberley. Introduction to Infrared and Raman Spectroscopy, 3rd Edn,
Academic Press, New York, NY (1990).
[34] K. Binnemans, D.W. Bruce, S.R. Collison, R. van Deun, R. Galyametdinov, Y.G. Martin. Philos. Trans.
R. Soc. Lond., Ser. A, 357, 3063 (1999).
[35] K. Binnemans, R. van Deun, Christiane G. Walrand, W. Haase, D.W. Bruce, L. Malykhina,
Y.G. Galyametdinov. Mater. Sci. Eng. C, 18, 247 (2001).
[36] B. Keshavan, P.G. Chandrashekara, N.M. Made Gowda. J. Mol. Struct., 553, 193 (2000).
[37] J. Nawrocka, V. Patroniak. J. Alloy Compounds, 380, 159 (2004).