Synthesis and spectral studies of some rare earth metal complexes of a heterocyclic mesogenic Schiff-base

Article abstract of DOI:10.1080/24701556.2017.1284122

A heterocyclic liquid crystalline Schiff’s base, N-(4′-(4″-hexyl-1H-1″,2″,3″-triazol))-4-dodecyloxysalicylaldimine (HL), and a series of lanthanide(III) complexes of the type [Ln(LH)₃(NO₃)₃] (Ln^III = La, Pr, Nd, Sm, Eu, Tb, and Dy) have been synthesized and characterized by elemental analyses, mass spectrometry, FTIR, NMR, and UV-Vis spectral techniques. All the complexes involve binding of HL (through phenolate oxygen in zwitter-ionic form) and bidentate chelation of nitrato groups. The ligand exhibits smectic-A (SmA) mesophase over a wide temperature range as evidenced by DSC and polarizing optical microscopic studies. Fluorescence studies show emissions of HL and Tb^III complex.

Full text of DOI:10.1080/24701556.2017.1284122

Inorganic and Nano-Metal Chemistry
ISSN: 2470-1556 (Print) 2470-1564 (Online) Journal homepage: http://www.tandfonline.com/loi/lsrt21
Synthesis and spectral Studies of Some Rare Earth
Metal Complexes of a Heterocyclic Mesogenic
Schiff-base
M. Karunakar, Rajasekhar Yerrasani, Angad Kumar Singh, Ragini Dubey & T.
R. Rao
To cite this article: M. Karunakar, Rajasekhar Yerrasani, Angad Kumar Singh, Ragini Dubey
& T. R. Rao (2017): Synthesis and spectral Studies of Some Rare Earth Metal Complexes
of a Heterocyclic Mesogenic Schiff-base, Inorganic and Nano-Metal Chemistry, DOI:
10.1080/24701556.2017.1284122
To link to this article: http://dx.doi.org/10.1080/24701556.2017.1284122
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Synthesis and spectral Studies of Some Rare Earth Metal Complexes
of a Heterocyclic Mesogenic Schiff-base
M. Karunakar, Rajasekhar Yerrasani, Angad Kumar Singh, Ragini Dubey, and
T. R. Rao*
Department of Chemistry, Banaras Hindu University, Varanasi-221005, India
*Corresponding author. Email: drtrrao@gmail.com
Abstract:
A heterocyclic liquid crystalline Schiff’s base, N-(4'-(4''-hexyl-1H-1'',2'',3''-triazol))-4-
dodecyloxysalicylaldimine (HL) and a series of lanthanide(III) complexes of the type
[Ln(LH)3(NO3)3] (LnIII = La, Pr, Nd, Sm, Eu, Tb and Dy) have been synthesized and
characterized by elemental analyses, mass spectrometry, FT-IR, NMR, and UV/Vis spectral
techniques. All the complexes involve binding of HL (through phenolate oxygen in zwitter-ionic
form) and bidentate chelation of nitrato groups. The ligand exhibits smectic-A (SmA) mesophase
over a wide temperature range as evidenced by DSC and polarizing optical microscopic studies.
Fluorescence studies show emission of HL and of the TbIII complex.
Key words: Mesogenic Schiff’s base, zwitterionic coordination, LnIII complexes
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Introduction
Numerous liquid crystalline materials designed and synthesized with rings other than six-
membered rings have received relatively less attention owing to their non-linearity and less
mesogenic stability. It was found that some of the non-linear heterocyclic liquid crystals,
such as pyrazole, thiophene, furan and isoxazole derivatives with a terminally positioned
heterocyclic ring, form smectic/nematic phase over wide low-temperature range. In addition,
hetero atom plays a key role in increasing molecular dipole moment and dielectric
anisotropy, thereby producing molecular level polar organizations [1].
Compounds containing five-membered [1,2,3]-triazole ring have attracted attention in the
fields of materials chemistry [2], organic chemistry [3] and drug discovery [4] based upon
their advantageous properties [5, 6]. Schiff-base ligands with N, O donor sets have often
been used since such ligands may assemble coordination architectures directed by the
lanthanide(III) ions [7-12] while the lanthanide ions can promote Schiff-base condensation
while providing access to complexes of otherwise inaccessible ligands [13]. The
technological applications of lanthanide complexes (including lanthanide metallomesogens)
are largely governed by their interaction with light. All these aspects were dealt with in detail
in a critical review [14] tailored for a broad audience of chemists, biochemists and materials
scientists. However, the design of lanthanide metallomesogens [15] is rather difficult because
their high coordination numbers seem to be incompatible with the structural anisotropy that
is necessary to exhibit liquid crystalline behavior.
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In continuation of our earlier work on metallomesogens [16-22] we now report here
synthesis and spectral studies of a [1,2,3]-triazole-containing mesogenic Schiff-base (having
terminal alkyl chain next to triazole ring) and of some Ln(III) complexes.
EXPERIMENTAL
Starting materials
All the required reagents of analytical grade, obtained from commercial sources, were
used without
further purification; 1-bromododecane, PMDETA and 2,4-
dihydroxybenzaldehyde are from Sigma–Aldrich, USA; all the Ln(NO3)3.xH2O salts are
from Alfa Aesar and 4-nitro aniline, 1-octyne, NaNO2, NaN3, CuBr, KI and KHCO3 are from
Merck. Organic solvents obtained from commercial vendors were dried when required using
standard methods [23].
Synthesis: All the experimental details are given in Scheme 1.
Synthesis of 1-azido-4-nitrobenzene, 1:
4-Nitro aniline (6.90g, 50 mmol) was suspended in hydrochloric acid (17%, 100 mL)
at room temperature followed by stirring with ethanol until a clear solution was obtained.
NaNO2 (5.18g, 75 mmol) was added to the solution at ~ 0 °C, stirred for 15-30 min, followed
by slow addition of NaN3 (4.88g, 75 mmol); the resultant solution was magnetically stirred
for ~2h at room temperature and the product was extracted thrice into ethereal portions, each
portion washed with saturated NaHCO3 solution and then with brine; the ether was removed
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1
after drying over MgSO4 under reduced pressure and the desired nitro-azide was isolated. H
NMR (300.40 MHz; CDCl3; Me4Si at 25 C, J (Hz), ppm) δ = 8.25 (d, J = 8.7, 2H, Ar-H),
13
7.15 (d, J = 8.7, 2H, Ar-H); C{1H} NMR (75.45 MHz; CDCl3; Me4Si at 25 C, ppm) δ =
119.4, 125.7, 144.7, 146.9; IR (KBr): cm-1 = 2126 (νazide), 1505(ν-NO).
Synthesis of 4-hexyl-1-(4ʹ-nitrophenyl)-1H-1,2,3-Triazole), 2, by ‘click’ chemistry:
Compound 2 was synthesized by adapting the reported method [24]; a light pink
colored solid was obtained. Yield - 70%, mp = 156 C. 1H NMR (300.40 MHz; CDCl3;
Me4Si at 25 C, J (Hz), ppm) δ = 0.89 (t, J = 5.1, 3H,-CH3), 1.34–1.75 (m, 8H,(-CH2)4), 2.84
(t, J = 6.1, 2H, triazole-CH2), 7.82 (s, 1H, triazole ring-H), 7.97 (d, J = 8.7, 2H, Ar-H), 8.42
13
(d, J = 9.0, 2H, Ar-H); C{1H} NMR (75.45 MHz; CDCl3; Me4Si at 25 C, ppm) δ =14.0,
22.5, 25.5, 28.8, 29.1, 31.5, 118.4, 120.1, 125.4, 141.3, 146.9, 150.2; IR (KBr, cm-1): 1598:
νtriazole ring,1522: νas (-NO2).
Synthesis of 4-(4ʹ-hexyl-1H-1ʹ, 2ʹ, 3ʹ-triazol-1-yl)aniline 3:
Compound 3 was obtained using the method described earlier [25]; the solid product
1
was re-crystallized from ethanol; Yield 65%; mp 96 ºC. H NMR (300.40 MHz; CDCl3;
Me4Si at 25 C, J (Hz), ppm) δ = 0.89 (t, J = 5.4, 3H,-CH3), 1.34–1.71 (m, 8H,(-CH2)4), 2.77
(t, J = 7.5, 2H, triazole-CH2), 3.81 (s, 2H, -NH2), 7.47 (d, J = 8.7, 2H, Ar-H), 6.76 (d, J = 8.4,
2H, Ar-H), 7.57 (s, 1H, triazole ring-H); 13C{1H} NMR (75.45 MHz; CDCl3; Me4Si at 25
C, ppm) δ = 14.0, 22.5, 25.6, 28.9, 29.4, 31.5, 115.2, 118.9, 122.1, 128.9, 146.7, 148.74.
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Synthesis of Schiff base (HL), 4:
A mixture of compound 3 (2.40g, 10 mmol) and 4-dodecyloxysalicylaldehyde
(3.06g, 10 mmol) was refluxed in absolute ethanol (15 mL) in presence of a catalytic amount
of glacial acetic acid for ~ 4h. The mesogen, 4, obtained as a precipitate from the hot reaction
mixture, was recrystallized from a mixture of ethanol and chloroform and dried under
1
vacuum; yield 70%. mp 210 ºC, H NMR (300.40 MHz; CDCl3; Me4Si at 25 C, J (Hz),
ppm) (Fig.S1) δ = 0.88 (t, J = 4.8, 6H,(-CH3)2), 1.27-1.77 (m, 28H,-(CH2)14-), 2.80 (t, J =
7.5, 2H, triazole -CH2), 4.01 (t, J = 6.3, 2H, -OCH2), 6.41 (s, 1H, Ar-H), 6.51 (d, J = 7.2, 2H,
Ar-H), 7.33 (d, J = 7.8, 1H, Ar-H), 7.40 (d, J = 8.1, 1H, Ar-H), 7.71 (1H, triazole ring-H),
7.78 (d, J = 8.1, 2H, Ar-H), 8.56 (s, 1H, -N=CH), 13.40 (s, 1H, phenolic-OH). 13C{1H} NMR
(75.45 MHz; CDCl3; Me4Si at 25°C, ppm) (Fig.S2) δ = 68.38 (-OCH2), 104.21 (-C3), 108.11
(-C5), 113.04 (-C1), 118.69 (triazole ring-C4”), 122.20 (-C2’), 128.03 (-C4’), 132.10 (triazole
ring-C5”), 133.20 (-C6), 136.46 (-C3’), 149.33 (-C1’), 162.14 (-C4), 165.83 (C-OH); 168.37 (-
N=CH); IR (KBr, cm-1): 1230, νC–O; 1622, νC=N, 3423, ν(OH)phenolic; formation of HL was
further confirmed by ESI mass spectrum (Fig. S3) on the basis of [M+H] base peak (m/e
value of 534).
Synthesis of LaIII complex, [La(LH)3(NO3)3], 5:
Anhydrous solutions of HL (0.47g, 0.9 mmol in 30 mL dichloromethane) and of
La(NO3)3.6H2O (0.12 g, 0.3 mmol in 20 mL methanol) were refluxed together for ~ 10h and
the reaction mixture left overnight. The micro-crystalline solid complex obtained was filtered
off under suction, washed repeatedly with methanol and dried over fused CaCl2 in a
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1
desiccator to get as a light yellow colored solid; Yield: 60%; mp 225 °C (decompose). H
NMR (300.40 MHz; CDCl3; Me4Si at 25 C, J (Hz), ppm) (Fig.S4) δ = 0.89 (t, J = 5.1, 3H, -
CH3), 1.77–1.27 (m, 28H, -(CH2)14-), 2.82 (t, J = 9.0, 2H, Triazole-CH2), 4.03 (t, J = 5.4, 2H,
-OCH2), 6.48 (s, 1H, Ar-H), 6.5 (d, J = 8.4, 2H, Ar-H), 7.34 (d, J = 8.4, 1H, Ar-H), 7.44 (d,
J = 6.3, 1H, Ar-H), 7.83 (d, J = 7.8, 2H, Ar-H), 7.77 (s, 1H, triazole ring-H), 8.61 (s, 1H,
HC=N-), 13.45 (br, s, 1H, –N+H); 13C{1H} NMR (75.45 MHz; CDCl3; Me4Si at 25 C, ppm)
(Fig.S5) δ = 67.99 (-OCH2), 101.26 (-C3), 107.43 (-C5), 112.49 (-C1), 118.53 (triazole ring-
C4”), 120.97 (-C2’), 121.86 (-C4’), 132.01(triazole ring-C5”), 133.61 (-C6), 136.66 (-C3’),
148.14 (-C1’), 162.01 (-C4), 163.67 (-C-OH); 181.60 (-CH=N+H); IR (KBr, cm-1): 1214, νC–O
;
+
1635, νC=N; 3153, νN H. The other LnIII complexes (LnIII = Pr, Nd, Sm, Eu, Tb and Dy) were
synthesized in an analogous way by using the appropriate hydrated LnIII nitrate.
Physical measurements
The metal contents of the complexes were determined by complexometric
titrations against EDTA-disodium salt solution using xylenol orange as indicator. C, H and N
1
were micro-analyzed on an Exeter (Model CE-440) CHN Analyzer. The H and 13C{1H}
NMR spectra were recorded on a JEOL AL-300 MHz multinuclear FTNMR spectrometer.
Infrared spectra were recorded on a Perkin-Elmer spectrophotometer over 4000-400 cm−1
range. Mass spectrum was recorded on a WATERS Q-TOF Premier mass spectrometer. UV-
Vis spectra were recorded on Shimadzu Pharmaspec-UV 1700 spectrometer. Molar
conductance of the complexes was determined in 0.001M DMF solutions at room
temperature on a CON-510 bench conductivity meter, using a 2-ring stainless steel ultem
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body conductivity electrode (cell constant, K = 1.0). Room temperature magnetic
susceptibility measurements were carried out on a Cahn-Faraday balance. Mesophases were
identified by the textures observed on a polarized hot-stage (heating and cooling rates, 2
°C/min) binocular microscope (LOMO, USA) equipped with digital camera (Nikon Coolpix
4500). Differential scanning calorimetry studies were made on METTLER DSC-25 unit
(with heating and cooling cycle rate of 10 °C/min).
Results and discussion
Synthesis and structural studies
The data on elemental analyses, important physical properties and general behavior of
the LnIII complexes are given in table 1. The molar conductivity measurements in 10-3M
DMF solutions (14.80 − 16.50 Ω-1 cm2 mol-1) are consistent with non-ionic (coordinated)
nature of the nitrate groups [26] and non-electrolytic behaviour of the complexes. The μeff
values of all the present LnIII complexes have been found to be in good agreement with the
reported van Vleck values [27].
The important infrared spectral data of HL and its LnIII complexes are presented in
table 2. The broad peak, centered at 3423 cm-1, characteristic of ν(O–H)phenolic [28],
disappears in the spectra of the complexes due to shifting of the phenolic proton to the
azomethine nitrogen atom resulting in the formation of zwitter ion. The weak/medium
intensity band (of HL) at 1230 cm-1 is assignable to ν(C–O)phenolic. The strong intensity band
occurring at 1622 cm-1, assignable [29] to azomethine moiety, undergoes a hypsochromic
shift in all the complexes on account of zwitter-ion formation. Thus, the complexation of HL
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to the LnIII ion results in migration of the phenolic protons onto the three uncoordinated
imino nitrogens, which then get intramolecularly hydrogen-bonded to the metal-bound
phenolate oxygens to give rise to zwitter-ionic structure, N+–H··O− [30]. The observed shift
to higher wave numbers upon complexation implies presence of −C=N+ group [31]. Further,
all the present complexes are characterized by a strong band at 1634-1636 cm-1 due to νC=N
and a weak broad band at about 3153–3157 cm-1 due to H-bonded N+–H···O− vibration of
the protonated imine [32]. Thus, the ligand may be considered to coordinate to the metal ion
via the negatively charged phenolate oxygen. The LnIII complexes also exhibit three
additional bands around 1478–1482, 1287–1290 and 835-836 cm-1 which can be assigned to
the vibrational modes of the coordinated nitrate groups (C2v symmetry) [33, 34].
13
A comparison of the NMR spectral data [1H and C{1H} NMR] of the ligand with
that of the LaIII complex shows absence of the phenolic–OH signal in the latter. We found
that the signal corresponding to the imine hydrogen, –CH=N-, got broadened in the LaIII
complex (δ, 8.61) when compared with that of the ligand (δ, 8.56); further, a new signal,
characteristic of –N+H resonance, appeared in the spectrum of the LaIII complex at 13.45 δ
while the parent ligand does not show any such signal. These observations are in accordance
with those made by Binnemans et al. [35] who reported similar results. Thus, the IR and
NMR spectral data provide sufficient evidence to show that the present Schiff-base (HL)
exists in the metal complex in a zwitter-ionic form, (as shown in Scheme 2) and provides
monodentate coordination through phenolate oxygen. The observed shift of the –N=CH-
signal (δ, 168.37 in the case of HL and 181.60 in the LaIII complex) provides experimental
evidence for coordination of the ligand with the metal ion. Similar shifts were observed in the
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case of the carbon atoms directly attached to the bonding atoms (phenolate carbons) while
those for the other carbons were of lesser magnitude.
Spectrophotometric and spectrofluorimetric studies
The qualitative electronic spectral bands (Fig.1a) of the ligand (at 246 & 294 nm) in
chloroform solution are assignable due to π-π* intra-ligand transitions of the aromatic rings
while the single peak of the selected LnIII complexes (in a mixed solvent of CHCl3 + DMSO;
3:1 v/v) around 350 nm is due to π-π* transition of the imine (-C=N) chromophore. The
considerable red shifts in the λmax values of the LnIII complexes (Ln = PrIII, NdIII, SmIII and
DyIII), compared to those of the corresponding aqua ions, implies Nephelauxetic effect [36],
which is regarded as a measure of covalency of the bonding between the metal ions and the
ligands [37]. Various bonding parameters (Table S1), viz., Nephelauxetic ratio (β), bonding
parameter (b1/2), Sinha's parameter (%δ) and covalency angular overlap parameter (η),
calculated by the procedures as reported in literature [38], suggest weak covalent nature of
the metal–ligand bonds.
Fluorescence emission spectra (Fig.1b, 1c) recorded in CHCl3/DMSO solutions (3:1
v/v ratio; 10-6 mol L-1; ex, 350 nm) show an emission band at 405 nm in the case of HL due
to intra ligand transition; the four typical metal-centered emission bands of the TbIII complex
5
at 490, 575, 619 and 682 nm imply emission due to D4→7F6, 5D4→7F5, 5D4→7F3 and
5D4→7F0 transitions respectively.
Optical and Thermal Studies
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The liquid crystalline studies were made on DSC (recorded in the second heating and
second cooling cycle with heating rate of 10 °C/min; Fig.2; thermal data in table 3) and
polarizing optical microscope (heating and cooling rates of 2 °C/min); the texture of HL
(Fig.3) at 175 °C implies SmA mesophase while the LnIII complexes were found to be non-
mesogenic.
Conclusion
The mesogenic (SmA) Schiff-base, N-(4'-(4''-hexyl-1H-1'',2'',3''-triazol))-4-
dodecyloxysalicylaldimine (HL), coordinates to LnIII ions (Ln = La, Pr, Nd, Sm, Eu, Tb and
Dy) forming non-mesogenic complexes of the formula, [Ln(LH)3(NO3)3]. HL bonds through
exclusively phenolate oxygen while the nitrato groups act as bidentate chelating ligands.
Luminescence of the ligand arises due to intra-ligand transition and that of the TbIII complex
originates in metal-centered emission.
Acknowledgments
One of the authors, T. R. Rao, gratefully acknowledges the financial grant received
from the Council of Scientific & Industrial Research, New Delhi [vide grant No.
01/2321/09/EMR/II] and M. Karunakar is grateful to the University Grants Commission
(UGC), New Delhi, India for providing financial assistance under research fellowship
scheme for Meritorious Students.
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Table 1. General and analytical data of HL and of LnIII metal complexes
HL / Complex
Formula weight. (Emp.
Formula)
Colour,
Yield
m.p.
(C)
μeff
Found (Calc) %
(van Vleck)
value, B.M.
C
H
N
M
HL
Yellow
70%
210
74.42
9.09
10.54
-
532.76,C33H48N4O2
(74.40) (9.08) (10.52)
61.88 7.56 10.95
[La(LH)3(NO3)3],1923.2, Light yellow 225d
Diamag.
7.25
LaC99H144N15O15
[Pr(LH)3(NO3)3],1925.20, Light yellow 235 d
PrC99H144N15O15 65%
[Nd(LH)3(NO3)3],1928.5, Light yellow 220 d
NdC99H144N15O15 72%
[Sm(LH)3(NO3)3],1934.6, Light yellow 260 d
SmC99H144N15O15 69%
[Eu(LH)3(NO3)3],1936.2, Light yellow 240 d
EuC99H144N15O15 75%
[Tb(LH)3(NO3)3, 1943.22, Light yellow 262 d
TbC99H144N15O15 65%
[Dy(LH)3(NO3)3],1936.2, Light yellow 245 d
DyC99H144N15O15 65 %
d = decomposition
60%
(61.83) (7.55) (10.92) (7.22)
61.85 7.58 10.96 7.45
(61.76) (7.54) (10.91) (7.32)
6169 7.59 10.94. 7.51
(61.66) (7.53) (10.89) (7.48)
61.48 7.56 10.81 7.80
(61.46) (7.50) (10.86) (7.77)
61.54 7.55 10.89 7.89
(61.41) (7.50) (10.85) (7.85)
61.25 7.51 10.85 8.25
(61.19) (7.47) (10.81) (8.18)
61.15 7.49 10.85 8.41
3.45
(3.40-3.60)
3.44
(3.50-3.60)
1.26
(1.50-1.60)
3.58
(3.40-3.60)
9.60
(9.10-9.60)
10.45
(10.40-10.50) (61.08) (7.46) (10.79) (8.35)
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Table 2. IR spectral data (cm-1) of HL and of LnIII metal complexesa
ν(O-H)
ν
νas
ν
νC-O
HL / Complex
νs
phenolic (N+H)
(CH3)
2924
2926
2927
2925
2926
2925
2925
2924
(CH2)
2854
2856
2856
2854
2856
2855
2855
2855
(C=N) (phenolic)
HL
3423
-
1622
1635
1634
1636
1635
1634
1635
1635
1230
1214
1215
1215
1215
1216
1216
1215
[La(LH)3(NO3)3]
[Pr(LH)3(NO3)3]
[Nd(LH)3(NO3)3]
[Sm(LH)3(NO3)3]
[Eu(LH)3(NO3)3]
[Tb(LH)3(NO3)3]
[Dy(LH)3(NO3)3]
-
-
-
-
-
-
-
3153
3155
3155
3153
3155
3155
3157
aSpectra recorded as KBr pellets; as: asymmetric; s: symmetric
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Table 3. Thermodynamic data (transition temp. and enthalpy changes)
Compound Transitiona Tb/ °C δHb/ kJ mol−1
Cr— Cr1
Cr1__SmA
SmA—I
I—SmA
SmA—Cr
42.8
100.9
205.5
203.1
82.6
3.8
29.1
2.9
4.8
19.9
HL
a Cr: crystal, SmA; smectic-A, I: isotropic liquid.
b Data as obtained from the DSC cycle
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Fig. 1a & 1b. UV-Vis and Fluorescence spectra of HL in CHCl3
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Fig. 1c. Photoluminescence spectrum of TbIII complex
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Fig. 2. DSC Thermogram of HL
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Fig. 3. Optical Texture of ligand (HL) at 175 ºC
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