Rare earth metal complexes of a mesogenic schiff-base, N,N′-di-(4- decyloxysalicylidene)-1′, 4′-diaminobutane: Synthesis and characterization

Article abstract of DOI:10.1080/15533174.2013.790056

A mesogenic Schiff-base, N,N′-di-(4-decyloxysalicylidene)-1′, 4′-diaminobutane, (H2L) that exhibits smectic-B and nematic mesophases, was synthesized and its structure studied by elemental analysis, mass spectrometry, NMR & IR techniques. H2L, upon conden

Full text of DOI:10.1080/15533174.2013.790056

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Rare Earth Metal Complexes of a Mesogenic Schiff-
Base, N,N′-di-(4-decyloxysalicylidene)-1′, 4′-
diaminobutane: Synthesis and Characterization
Pawan Raj Shakya a , Angad Kumar Singh a & T. R. Rao a
a Department of Chemistry , Banaras Hindu University , Varanasi , India
Accepted author version posted online: 12 Nov 2013.Published online: 16 Dec 2013.
To cite this article: Pawan Raj Shakya , Angad Kumar Singh & T. R. Rao (2014) Rare Earth Metal Complexes of a Mesogenic
Schiff-Base, N,N′-di-(4-decyloxysalicylidene)-1′, 4′-diaminobutane: Synthesis and Characterization, Synthesis and Reactivity
in Inorganic, Metal-Organic, and Nano-Metal Chemistry, 44:5, 739-747, DOI: 10.1080/15533174.2013.790056
To link to this article: http://dx.doi.org/10.1080/15533174.2013.790056
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Synthesis and Reactivity in Inorganic, Metal-Organic, and Nano-Metal Chemistry, 44:739–747, 2014
Copyright ꢀ Taylor & Francis Group, LLC
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ISSN: 1553-3174 print / 1553-3182 online
DOI: 10.1080/15533174.2013.790056
Rare Earth Metal Complexes of a Mesogenic Schiff-Base,
N,Nꢀ-di-(4-decyloxysalicylidene)-1ꢀ, 4ꢀ-diaminobutane:
Synthesis and Characterization
Pawan Raj Shakya, Angad Kumar Singh, and T. R. Rao
Department of Chemistry, Banaras Hindu University, Varanasi, India
One of the most unusual and intriguing properties of metal-
containing LCs, so-called metallomesogens,[9,10] is the presence
of magnetic moment which determines the behavior of the LCs
in a magnetic field. It is well known[11] that magnetic moments,
and especially magnetic anisotropy, are inherent properties of f-
block metal complexes. In continuation of the earlier work[12–16]
carried out in our laboratory on systematic structural and spec-
troscopic studies of 3d and 4f metal complexes of a series of
mesogenic organic Schiff-bases, we now report here the syn-
thesis and spectral studies of a mesogenic Schiff-base, N,Nꢁ-
di-(4-decyloxysalicylidene)-1ꢁ,4ꢁ-diaminobutane (H2L) and its
rare earth complexes. It has been observed that among all the
LnIII complexes of the mesogenic Schiff-base (H2L) synthe-
sized under the present study, only the PrIII complex exhibits
mesogenic property.
A mesogenic Schiff-base, N,Nꢀ-di-(4-decyloxysalicylidene)-1ꢀ,4ꢀ-
diaminobutane, (H2L) that exhibits smectic-B and nematic
mesophases, was synthesized and its structure studied by elemen-
tal analysis, mass spectrometry, NMR & IR techniques. H2L, upon
condensation with hydrated lanthanide(III) nitrates yields LnIII
complexes of the composition [Ln2(LH2)3(NO3)4](NO3)2, where
LnIII = La, Pr, Nd, Sm, Eu, Gd, Tb, Dy, and Ho. The PrIII complex
shows smectic-A and nematic phases. The IR and NMR spectral
data imply a bidentate bonding of H2L in zwitterionic form (as
LH2) to the LnIII ions through two phenolate oxygen atoms, ren-
dering the overall geometry of the complexes possibly to distorted
mono-capped octahedra.
Keywords LnIII complexes, mesogenic schiff-base, mono-capped oc-
tahedron, NMR and IR spectra, zwitterionic coordination
INTRODUCTION
EXPERIMENTAL
Schiff-base metal complexes, which are well known to have
played a key role in the development of coordination chemistry
since nineteenth century,[1–4] have been reported[5] in large num-
ber because they exhibit the phenomenon of mesomorphism.
The interest in the synthesis and structural investigations of
mesogenic Schiff-base liquid crystals (LCs) originates in their
potential applications as “non-linear optical and/or optical stor-
age materials.”[6,7] Of the metallomesogens, thermotropic LCs
have assumed great technological importance owing to their
specific applications as advanced functional materials.[8] The
design of novel thermotropic LCs involves suitable selection
of a core fragment, linking group, and terminal functionality.
Starting Materials
All the required reagents of analytical grade were ob-
tained from commercial sources and used without further
purification: 1-bromodecane, 2,4-dihydroxybenzaldehyde, and
1,4-diaminobutane are from Sigma–Aldrich, USA; all the
Ln(NO3)3.xH2O salts are from Indian Rare Earths Ltd. and
KI and KHCO3 are from Merck. The organic solvents obtained
from commercial vendors were dried using standard methods[17]
when required.
Physical Measurements
The metal contents of the complexes were determined by
complexometric titrations against EDTA using xylenol orange
as an indicator. Carbon, hydrogen and nitrogen were analyzed
Received 20 February 2013; accepted 23 March 2013.
The authors wish to acknowledge the recording of FAB Mass spec-
tra at the Central Drug Research Institute, Lucknow, India. The fi-
1
on an Exeter Analyzer, Model CE-440 CHN. The H and 13C
NMR spectra were recorded on a JEOL AL-300 MHz FT-NMR
nancial assistance received from the CSIR, New Delhi (vide Sanction multinuclear spectrometer. Infrared spectra were recorded on
letter No. 01(2321)/09/EMR-II) and the fellowship awarded to Mr. P.
a JASCO FT/IR (model-5300) spectrophotometer within the
R. Shakya by the University Grants Commission, Kathmandu, Nepal,
4000–400 cm−1 region using KBr pellets. Mass spectra were
are gratefully acknowledged.
recorded on JEOL SX-102 FAB mass spectrometer. UV–Vis
spectra were recorded on Shimadzu spectrophotometer, model
Pharmaspec-UV 1700. Magnetic susceptibility measurements
Address correspondene to T. R. Rao, Department of Chemistry,
Banaras Hindu University, Varanasi-221005, India. E-mail: drtrrao@
gmail.com
739

740
P. R. SHAKYA ET AL.
were made at room temperature on a Cahn–Faraday balance us- to the filtrate until the point of neutralization and the product
ing Hg[Co(NCS)4] as the calibrant. Mesophases were identified was then extracted twice with CHCl3 (100 mL portions). The
by the textures observed by using polarized hot-stage binoc- chloroform extracts were concentrated to obtain a straw-yellow
ular microscope (LOMO, USA) equipped with digital camera solid which was purified by column chromatography over SiO2
(Nikon Coolpix 4500). Differential Scanning Calorimetry stud- eluting first with n-hexane and then with a mixture of n-hexane
ies were made on METTLER DSC-25, Mettler STARe SW 9.00 and chloroform (v/v, 1/1); evaporation of this purified extract fi-
unit.
nally yielded 4-decyloxysalicylaldehyde in the form of a white
solid; yield: 8.63 g, 62%.
Synthesis and Analysis
The synthesis of N,Nꢁ-di-(4-decyloxysalicylidene)-1ꢁ,4ꢁ- Synthesis of N,Nꢀ-di-(4-decyloxysalicylidene)-
diaminobutane (H2L), 2, was achieved by proceeding through 1ꢀ,4ꢀ-diaminobutane, 2
The ligand, N,Nꢁ-di-(4-decyloxysalicylidene)-1ꢁ,4ꢁ-diamino-
butane, was prepared by refluxing absolute ethanolic solutions
of 4-decyloxysalicylaldehyde (8.34 g, 30 mmol in 50 mL) and 1,
4-diaminobutane (1.5 mL, 15 mmol) for ∼1.5 h in presence of
few drops of glacial acetic acid. The resulting mixture was left
over night; minute yellow-colored crystalline product, 2, thus
obtained was filtered off under suction, thoroughly washed with
cold ethanol and dried at room temperature. Yield: 4.39 g, 60%,
as shining yellow-colored microcrystalline solid; m.p. 112◦C.
Anal. Calcd for C38H60N2O4 (608.89) (%): C, 74.96; H;
two major steps, viz., alkylation of 2,4-dihydroxybenzaldehyde
with 1-bromodecane, followed by Schiff-base formation. All
the experimental details are given in Scheme 1. The LnIII com-
plexes, [Ln2(LH2)3(NO3)4](NO3)2 (Ln = La, Pr, Nd, Sm, Eu,
Gd, Tb, Dy, and Ho), 3, were prepared by adding together THF
solutions of the appropriate metal nitrate (2.0 mmol, in 20 mL)
and the ligand, H2L (1.83 g, 3.0 mmol in 30 mL) under magnetic
stirring. The resulting mixture was then stirred continuously for
∼3 h at room temperature. The solid product thus separated out
in each case was filtered, washed repeatedly with cold methanol
and dried over fused CaCl2.
1
9.93; N, 4.60. Found: C, 74.92; H, 9.97; N, 4.63. H NMR
(300 MHz; CDCl3; δ) 0.88 (t, J = 6.9, 3H, –CH3), 1.77–1.27
Preparation of 4-decyloxysalicylaldehyde, 1
(m, 16H, –(CH2)8-), 3.56 (s, 2H, –NCH2), 3.95 (t, J = 6.6,
To a solution of 100 mL of dry acetone, equimolar amounts of 2H, –OCH2), 6.37 (d, J = 8.1, 1H, Ar–H), 6.38 (s, 1H, Ar–H),
2,4-dihydroxy benzaldehyde (6.91 g, 50 mmol), 1-bromodecane 7.06 (d, J = 8.4, 1H, Ar–H), 8.14 (s, 1H, –N CH), 13.96
1
(10.4 mL, 50 mmol) and potassium bicarbonate (5.51 g, (s, br, Ph–OH); 13C{ H}NMR (300 MHz; δ, CDCl3): 166.0,
∼55 mmol) were added. The mixture was refluxed for ∼30 h 163.7, 163.3, 132.9, 112.0, 106.8, 101.8, 68.1, and 57.6; FAB
in presence of KI (0.1–0.2 g) as a catalyst and filtered while hot Mass (m/e, fragment, % intensity): the molecular ion as base
to remove insoluble solids. Hydrochloric acid (6N) was added peak (608, M+,100) generates simultaneously two fragments,
OH
OH
Dry acetone, KI
Reflux ~ 30 h
H2O
C10H21Br
+ CO2
+
+ KHCO3
KBr
+
+
HO
CHO
C10H21
O
CHO
1
OH
Ethanol / Acetic acid
Reflux , ~ 1.5 h
2
H2N
O
CHO
NH2
3
C10H21
4
O
OH
2
C10H21
H
2'
4'
6
3
N
1
5
5
1
N
1'
3'
2H2O
6
H
C10H21
HO 2
4 O
2 (H2L)
THF
2 Ln(NO3)3. xH2O
[Ln2(LH2)3(NO3)4](NO3)2
3 H2L
Stir, ~3 h.
3
SCH. 1. Reaction steps involved in the synthesis of 4-decyloxysalicylaldehyde, 1; N,Nꢁ-di-(4-decyloxysalicylidene)-1ꢁ,4ꢁ-diaminobutane, 2, (H2L), and LnIII
complexes, 3.

RARE EARTH METAL COMPLEXES
741
M1-M2; M1: 332, C10H21OC6H3(OH)CH–N(CH2)3CH2+, 45%;
shown in Table 1) have been found to be higher than the reported
van Vleck values presumably on account of metal-metal inter-
actions that may be present in all such complexes showing the
abnormal μeff values. Jean-Pierre Costes et al.[18] in their first
example of a polynuclear gadolinium complex observed that
χMT increases steadily from 40 to 10 K and then more sharply
down to 2 K and attributed that such a behavior was typical
of a ferromagnetic interaction between the gadolinium ions;
further, in the introduction part of the same publication, they
claimed to have reported lately two dinuclear (Er, Er) and (Gd,
Gd) complexes which showed a ferromagnetic behavior unam-
biguously attributable to lanthanide–lanthanide interaction. On
these lines, the present complexes may be considered to involve
ferromagnetic interaction at the room temperature itself.
M2: 192, HOC6H3(OH)CH–N(CH2)3CH2+, 25%; IR (cm−1
,
KBr disk): 3448 (ν–OH), 1619 (ν–C N), 1286 (ν–Cph–O).
Synthesis of LaIII Complex, [La2(LH2)3(NO3)4](NO3)2
The LaIII complex was prepared by adding drop-wise THF
solution of La(NO3)3.6H2O (0.87 g, 2.0 mmol in 20 mL) to a
THF solution of the ligand, H2L (1.83 g, 3.0 mmol in 30 mL)
under magnetic stirring. The resultant solution turned cloudy
after ∼15 min; stirring was continued for ∼3 h at room tem-
perature when a solid product separated out which was filtered,
washed repeatedly with cold methanol and dried over fused
CaCl2. Yield: 74%, 1.84 g, as a light yellow-colored solid; m.p.
225◦C (decompose).
Anal. Calcd for La2C114H180N12O30 (2476.52) (%): C, 55.29;
H; 7.33; N, 6.79; La, 11.22. Found: C, 55.32; H, 7.28; N, 6.80;
La, 11.20. 1H NMR (300 MHz; CDCl3; δ): 0.88 (t, J = 6.0, 3H,
-CH3), 1.57–1.25 (m, 16H, –(CH2)8-), 3.41 (s, 2H, –NCH2),
3.44 (t, J = 6.6, 2H, –OCH2), 6.06 (d, J = 6.9, 1H, Ar–H),
6.48 (s, 1H, Ar–H), 6.92 (d, J = 6.0, 1H, Ar–H), 8.04 (s, 1H,
Electronic Spectra
The electronic spectra (qualitative solution-state spectra in a
mixed solvent (CHCl3 and DMSO; 3:1 v/v) in the 200–1100 nm
region) recorded for only the PrIII, NdIII, SmIII, and DyIII com-
plexes show a considerable red shift in the λmax values in com-
parison with those of their corresponding aquo ions.[19] These
red shifts are presumably due to the Nephelauxetic effect[20] and
are regarded as a measure of covalency of the bonding between
the metal ions and the ligands. Various bonding parameters (Ta-
ble 2), viz., Nephe1auxetic ratio (β), bonding parameter (b1/2),
Sinha’s parameter (%δ), and covalence angular overlap parame-
ter (η), calculated by the procedures as reported in literature,[21]
suggest a weak covalent nature of the metal-ligand bonds.
The structures and purity of the Schiff-base ligand and of the
metal complexes were studied by IR and NMR spectroscopy
and elemental analyses. The structure of the ligand was further
confirmed by FAB Mass spectrum. The mass spectral features
of H2L were characterized by the base peak as well as molecular
ion peak corresponding to the m/e value of 608, which matches
with the molecular weight of 608.89 of H2L of the molecular
formula, C38H60N2O4. The 100% intensity of the molecular ion
peak is as expected for the molecule on the basis of its predom-
inant aromatic character; the major fragment peaks at m/e =
332, 192 are due to C10H21OC6H3(OH)CH–N(CH2)3CH2+ and
HOC6H3(OH)CH–N(CH2)3CH2+, respectively.
1
–N CH), 12.20 (br-s, 1H, –N+H); 13C{ H} NMR (300 MHz;
CDCl3; δ) 177.9, 164.4, 167.9, 134.4, 114.6, 108.9, 104.5, 67.9
and 57.2; IR (cm−1, KBr disk): 3199 (ν–N+H), 1652 (ν–C N),
1229 (ν–Cph–O).
All the other rare-earth complexes (Ln = Pr, Nd, Sm, Eu,
Gd, Tb, Dy, and Ho) were synthesized in an analogous way by
using the appropriate hydrated salt of LnIII nitrate; the physi-
cal properties and the analytical data of all the complexes are
given in Table 1. The infrared spectral data of two representative
complexes are given below:
[Gd2(LH2)3(NO3)4](NO3)2: IR (cm−1
,
KBr disk):
3175 (ν–N+H), 1653 (ν–C N), 1229 (ν–Cph–O);
[Ho2(LH2)3(NO3)4](NO3)2: IR (cm−1, KBr disk): 3193
(ν–N+H), 1652 (ν–C N), 1229 (ν–Cph–O).
RESULTS AND DISCUSSION
The shining yellow-colored microcrystalline Schiff-base lig-
and (H2L) was found to react with Ln(NO3)3.xH2O to yield LnIII
complexes; the data obtained (elemental analyses, magnetic mo-
ments, important physical properties, and general behavior) on
the ligand and the complexes are given in Table 1. The analytical
data of the complexes indicate 2:3 metal to ligand stoichiome-
try with the general formula [Ln2(LH2)3(NO3)4](NO3)2 where
Ln = La, Pr, Nd, Sm, Eu, Gd, Tb, Dy, and Ho. The ligand
(H2L) in all the complexes under discussion coordinates to the
metal ion in the form of non-deprotonated zwitter-ionic species
as evidenced by NMR and IR spectra discussed later while the
nitrate groups (both within and outside the coordination sphere)
counter balance the positive charge of the LnIII ion(s).
NMR Spectral Study
1
A comparison of the NMR spectral [1H and 13C{ H}] data
(Table 3, Figure 1) of the ligand, with that of the LaIII complex
shows that the bonding of the phenolic oxygen of the ligand
to the metal ion is substantiated by the fact that the phenolic-
OH signal, appearing at δ, 13.96 ppm in the ligand, disappears
upon complexation. It may be noted that the composition of
the LaIII complex, [La2(LH2)3(NO3)4](NO3)2, implies coordi-
nation of H2L as a neutral species; however, it may be inferred
1
Magnetic Study
from the H NMR spectrum of the compound that the pheno-
The μeff values (at R.T.) of all the present LnIII complexes lic protons are shifted to the two uncoordinated imino nitro-
(3.72, 3.83, 1.92, 3.80, 11.69, 14.49, 15.22, and 15.30 B.M. gen atoms, which then get intramolecularly hydrogen-bonded
for Ln = Pr, Nd, Sm, Eu, Gd, Tb, Dy, and Ho, respectively, as to the metal-bound phenolate oxygen atoms to give rise to the

742

RARE EARTH METAL COMPLEXES
743
TABLE 2
Electronic spectral data of various metal complexes of H2L
PrIII
NdIII
Transitions/bonding
parameters
λmax (cm−1
)
λmax(cm−1
complex
)
Transitions/bonding
parameters
λmax (cm−1
)
λmax (cm−1
)
complex
aq. ion
aq. ion
3
4
1G4 ← H4
9,900
16,850
20,800
9,681
16,863
20,661
4F3/2 ← I9/2
11,450
12,500
13,500
14,800
17,400∗
19,500
11,468
12,484
13,423
14,706
17,123
19,531
0.996
0.045
0.402
0.002
1D2
←
4F5/2, 2H9/2
4S3/2, 4F7/2
←
←
∗
3P0 ←
4F9/2
2G7/2
4G9/2
←
∗
←
←
β
0.991
0.067
0.908
0.004
b1/2
%δ
η
SmIII
DyIII
6
6
6F9/2← H5/2
9,200
10,500
17,900
26750∗
9,372
9,843
17,391
25,840
0.974
0.114
2.669
0.013
6H5/2 ← H15/2
10,200
11,000
12,400
13,200
10,249
10,977
12,346
13,123
0.998
0.032
0.200
0.001
6F11/2
4G5/2
←
←
←
6F7/2
6F5/2
6F3/2
←
←
←
6P7/2
∗
β
b1/2
%δ
η
∗Hypersensitive band.
+
−
oxygen atoms of the ligand in the zwitter-ionic form to the LaIII
metal ion.
. . . . .
zwitterionic structure ( N –H
O ) and the macrocycle un-
der this condition is designated as LH2.[22] We found that the
signal corresponding to the imine hydrogen, –CH–N, was broad-
ened in the LaIII complex (δ, 8.04 ppm) when compared with that
of the ligand (δ, 8.14 ppm). Additionally, a new signal, character-
istic of –N+H resonance, appears in the spectrum of the complex
at δ, 12.20 ppm which is not shown by the parent ligand. These
observations are in accordance with those made by Binnemans
et al.,[23] who, while reporting their work on rare earth containing
magnetic LCs of the formula, [Ln(LH)3(NO3)3], where LH =
4-alkoxy-N-alkyl-2-hydroxy benzaldimine, found that selective
irradiation of the signal at δ 12.29 ppm 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 –N+H group. Thus, it may be confirmed that the present
Schiff-base (H2L) exists in the metal complex in a zwitter-ionic
form, with the phenolic oxygen deprotonated and the imine
nitrogen protonated (Scheme 2).
IR Spectral Study
Assignments regarding bonding through specific functional
groups have been made by a careful comparison of the vibra-
tional bands in the spectra of the ligand and the metal complexes
(Table 4). The broad absorption centered on 3448 cm−1 in the IR
O
C10H21
H
H
OH
N
N
C10H21
HO
O
O
C10H21
The 13C{ H} NMR spectra show a significant shift of the
1
H
–NCH signal from δ, 166.01 ppm (in the case of H2L) to δ,
177.85 ppm (in the case of the LaIII complex). Similar shifts
were observed in the case of the carbon atoms directly attached
to the bonding atoms (phenolate carbons). Other carbons of the
metal complex showed only smaller changes in their chemical
shift differences on comparing with the neutral ligand. Thus, the
NMR spectral data imply bonding through the two phenolate
H
O
HN
N
H
C10H21
O
O
SCH. 2. Depiction of migration of phenolic protons to imine nitrogen atoms
of the ligand, H2L, during the formation of zwitter ion.

744
P. R. SHAKYA ET AL.
TABLE 3
NMR Spectral data∗ of H2L and the LaIII complex
Ligand
LaIII complex
Coupling constant J
Coupling constant J
1
1H/13C{ H} (multiplicity) Peak position (δ) ppm
(Hz)
Peak position (δ) ppm
(Hz)
ꢀ–OH (s)
–N+H (br s)
–N=CH (s)
–C6H (d)
–C3H (s)
13.962
–
8.141
–
–
–
8.4
–
–
12.199
8.044
–
–
–
6.0
–
7.073, 7.045
6.378
6.926, 6.906
6.477
–C5H (d)
6.368, 6.341
3.970, 3.948, 3.927
3.564
8.1
6.6
–
6.075, 6.052
3.466, 3.444, 3.424
3.409
6.9
6.6
–
ꢁꢁ
–C1 H (t)
–NCH2 (s)
ꢁ
–(CH2)8(m) &–C2 H (d)
1.769–1.269
0.901, 0.880, 0.858
166.01
–
6.3
–
1.570–1.254
0.898, 0.878, 0.856
177.85
–
6.0
–
–CH3 (t)
–NCH
–C4
163.54
–
164.44
–
–C2
163.27
–
167.93
–
–C6
132.45
–
134.38
–
–C1
112.04
–
114.62
–
–C5
106.83
–
108.88
–
–C3
101.80
68.09
–
–
104.52
67.91
–
–
ꢁꢁ
–C1
ꢁ
ꢁ
ꢁ
–C1 /–C4
57.56
28.48
–
–
57.23
28.98
–
–
ꢁ
–C2 /–C3
ꢁꢁ
ꢁꢁ
–C2 –C9
31.89–22.67
14.10
–
–
31.90–22.67
14.10
–
–
ꢁꢁ
–C10
1
Spectra (300 MHz) recorded in solutions of CDCl3; ∗spectrum recorded at 22.7◦C; 1H NMR spectral data are given in δ (w.r.t. TMS); 13C{ H}
NMR data measured in ppm w.r.t. CDCl3 signal at 77.00 ppm.
spectrum of the ligand, characteristic of phenolic O–H stretch- of the complexes due to the shifting of the phenolic proton to the
ing,[24] may be understood to involve H-bonding (to the ortho azomethine nitrogen atom resulting in the formation of zwitter
>C N group presumably of intramolecular type) under the ex- ion. The weak/medium intensity bands centered on 1286 cm−1
perimental conditions; this band totally disappears in the spectra are assignable to phenolic C–O stretching vibration. The strong
TABLE 4
IR Spectral data∗ (cm−1) of H2L and of LnIII metal complexes
ν (NO3)
ν (O–H)
νasCH
νsCH
ν (C–O)
H2L1/complex
phenolic ν (N+H) CH3/CH2 CH3/CH2 ν(C N) phenolic
ν5
Ionic
–
ν1
ν2 ν5–ν1
H2L
3448 b
–
2,922
2,926
2,925
2,925
2,926
2,926
2,926
2,926
2,926
2,926
2,854
2,855
2,855
2,855
2,855
2,855
2,855
2,855
2,855
2,855
1,619
1,652
1,649
1,649
1,649
1,653
1,653
1,653
1,652
1,652
1,286
1,229
1,225
1,227
1,229
1,229
1,229
1,229
1,229
1,229
–
–
–
–
[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
–
–
–
–
–
–
–
–
–
3199w
3182w
3190w
3178w
3170w
3175w
3173w
3191w
3193w
1,472 1,383 1,290 846 182
1,472 1,381 1,289 846 183
1,473 1,381 1,289 845 184
1,472 1,381 1,290 846 182
1,472 1,382 1,291 846 181
1,468 1,378 1,288 845 180
1,472 1,378 1,288 845 184
1,472 1,381 1,291 846 181
1,470 1,379 1,289 846 181
∗Spectra recorded as KBr pellets; b: broad; as: asymmetric; s: symmetric; w: weak.

RARE EARTH METAL COMPLEXES
745
1
FIG. 1. (a) and (b): 1H NMR spectra of ligand (H2L) and its LaIII complex; (c) and (d): 13C{ H} NMR spectra of ligand (H2L) and its LaIII complex.
intensity band occurring at 1619 cm−1, assignable[25] to νC
itating coordination to the lanthanide ion. The initial evidence
for zwitter-ionic formation was obtained from 1H NMR spectra
while further evidence was given by infrared spectroscopy and,
more particularly, by the band frequencies of the C N stretch-
ing vibration. The shift to higher wave numbers upon com-
plexation implies the presence of C–N+ group[27] and the non-
involvement of nitrogen atom in complex formation. Thus, all
the present complexes are characterized by a strong band due to
C N stretching vibration at 1649–1653 cm−1 and a weak broad
band at about 3199–3170 cm−1 due to the hydrogen-bonded
N
absorption of the azomethine moiety, undergoes a hypsochromic
shift in all the complexes on account of zwitter-ion formation.
Thus, the complexation to the LnIII ion results in migration of
the phenolic protons onto the two uncoordinated imino nitrogen
atoms, which then get intramolecularly hydrogen-bonded to the
metal-bound phenol+ate oxygen atoms to give rise to the zwit-
−
O . Similar zwitter-ionic behavior
. . . .
terionic structure, N –H
has been reported by others for acyclic Schiff-base lanthanide
complexes.[26] The formation of a zwitterionic form can be ra-
tionalized by the tendency of the lanthanide ions to coordinate
to negatively charged ligands with a preference for O-donor lig-
ands. By transfer of the phenolic proton to the imine nitrogen,
the phenolic oxygen becomes negatively charged thereby facil-
+
−
O vibration of the protonated imine.[27] Thus, it may
. . . .
N –H
be understood that the ligand coordinates to the metal ion via
the negatively charged phenolic oxygen with no bond formation
between the lanthanide ion and the imine nitrogen.

746
P. R. SHAKYA ET AL.
TABLE 5
Transition temperatures and enthalpy changes
Compound
Transitiona Tb/◦C δHb/kJ mol−1
H2L
Cr Cr
Cr SmB 82.04
SmB N 105.97
N I
N SmB 108.89
SmB-Cr 72.99
76.97
33.93
6.45
69.54
–
68.35
38.48
23.94
50.93
-
111.00c
[Pr2(LH2)3(NO3)4](NO3)2 Cr SmA 134.90
SmA N 167.86
N I
242.00c
aCr: Crystal, N: Nematic; SmA: Smectic A; SmB: Smectic B; I:
Isotropic liquid.
bData as obtained from the first DSC cycle.
cData.as inferred from Polarizing Optical Microscope.
higher energies, 184–180 cm−1, may be indicative of a bidentate
interaction of the coordinated nitrate anions with the lanthanide
ions.[28,29] The additional bands observed at 1383–1378 cm−1
may be attributed to the non-coordinated nitrate groups present
outside the coordination sphere of the complexes. At this junc-
ture, a distorted mono-capped octahedron with CN = 7 may be
tentatively proposed for the complexes (Figure 2).
FIG.
2. Proposed
polyhedron
(Monocapped
Octahedron)
for
[Ln2(LH2)3(NO3)4](NO3)2: Ln = La, Pr, Nd, Sm, Eu, Gd, Tb, Dy, and
Ho.
Optical and Thermal Study
The mesophases were identified by polarized hot-stage mi-
croscopy (POM) and DSC studies. The POM studies imply
smectic-B and nematic phases of H2L and smectic-A and ne-
matic phases of the PrIII complex, respectively; the correspond-
ing transition temperatures and enthalpy changes are given in Ta-
ble 5 while the textures are shown in Figure 3. All the other LnIII
complexes under present study are found to be non-mesogenic.
The IR spectra of the complexes also show three addi-
tional characteristic frequencies of the coordinating nitrate
groups (C2v) in the range of 1473–1468, 1291–1288, and
846–845 cm−1 [28]
The profile and magnitude of separation of
.
the modes associated with asymmetric nitrate vibrations have
been used as a criterion to distinguish between mono- and biden-
tate chelating nitrates. Accordingly, the magnitude of splitting at
FIG. 3. Optical textures of: (a) H2L (SmB) at 92◦C and (b) PrIII complex (SmA) at 150◦C.

RARE EARTH METAL COMPLEXES
747
CONCLUSION
11. Galyametdinov, Y.G.; Haase, W.; Malykhina, L. Chem. Eur. J. 2001, 7, 99.
12. Singh, A.K.; Kumari, S.; Kumar, K.R.; Sridhar, B.; Rao, T.R. Polyhedron
2008, 27, 181.
The mesogenic Schiff-base, N,Nꢁ-di-(4-decyloxysali-
cylidene)-lꢁ,4ꢁ-diaminobutane, (H2L) exhibits smectic-B and
nematic phases and coordinates to LnIII as a neutral bidentate
species to yield seven-coordinate complexes (the polyhedron
being possibly a distorted monocapped octahedron) of the
general formula, [Ln2(LH2)3(NO3)4](NO3)2, where Ln = La,
Pr, Nd, Sm, Eu, Gd, Tb, Dy, and Ho. In all cases, the zwitterionic
species of the neutral bidentate ligand, H2L, coordinates to the
LnIII metal ion through two phenolate oxygen atoms. Among
the complexes, only the PrIII complex exhibits liquid crystalline
property (smectic-A and nematic phases).
13. Singh, A.K.; Kumari, S.; Kumar, K.R.; Sridhar, B.; Rao, T.R. Polyhedron
2008, 27, 1937.
14. Singh, A.K.; Kumari, S.; Guru Row, T.N.; Jai, P.; Kumar, K.R.; Sridhar,
B.; Rao, T.R. Polyhedron 2008, 27, 3710.
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Acta 2009, 362, 4205.
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18. Costes, J.-P.; Juan, J.-M.C.; Dahan, F.; Nicode`me, F. Dalton Trans. 2003,
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