Synthesis and spectral studies on 3d metal complexes of mesogenic schiff base ligands. Part 1. Complexes of N-(4-butylphenyl) salicylaldimine

Article abstract of DOI:10.1081/SIM-200055245

A novel mesogenic Schiff base, N-(4-butylphenyl) salicylaldimine (Hbpsal), was prepared and its ligational behavior towards some 3d metal ions studied. All the metal complexes synthesized were of the general formula, [M(bpsal) ₂(H₂O)₂] where M = Mn(II), Co(II), Ni(II), Cu(II) and Zn(II). The complexes were structurally characterized by elemental analyses, molar conductance, IR, NMR, and electronic spectral data. The metallo mesogenic phases of the Cu(II) complex were studied by optical microscope. The spectral data show that Hbpsal acts as a bidentate ligand towards the metal ions. Mesogenic studies indicate that the ligand shows a focalconic structure with smectic-A phase while the Cu(II) complex displays smectic-A as well as smectic-E phases. Copyright

Full text of DOI:10.1081/SIM-200055245

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Synthesis and Spectral Studies on 3d Metal Complexes
of Mesogenic Schiff Base Ligands. Part 1. Complexes of
N‐(4‐Butylphenyl) Salicylaldimine
T. R. Rao a & Archana Prasad a
a Department of Chemistry , Banaras Hindu University , Varanasi, India
Published online: 16 Aug 2006.
To cite this article: T. R. Rao & Archana Prasad (2005) Synthesis and Spectral Studies on 3d Metal Complexes of Mesogenic
Schiff Base Ligands. Part 1. Complexes of N‐(4‐Butylphenyl) Salicylaldimine, Synthesis and Reactivity in Inorganic, Metal-
Organic, and Nano-Metal Chemistry, 35:4, 299-304, DOI: 10.1081/SIM-200055245
To link to this article: http://dx.doi.org/10.1081/SIM-200055245
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Synthesis and Reactivity in Inorganic, Metal-Organic and Nano-Metal Chemistry, 35:299–304, 2005
Copyright # 2005 Taylor & Francis, Inc.
ISSN: 0094-5714 print/1532-2440 online
DOI: 10.1081/SIM-200055245
Synthesis and Spectral Studies on 3d Metal Complexes
of Mesogenic Schiff Base Ligands. Part 1. Complexes of
N-(4-Butylphenyl) Salicylaldimine
T. R. Rao and Archana Prasad
Department of Chemistry, Banaras Hindu University, Varanasi, India
coordination compounds of transition elements with various
A novel mesogenic Schiff base, N-(4-butylphenyl) salicylaldi- mesogens has been carried out (Duncan, 1993). It has been
mine (Hbpsal), was prepared and its ligational behavior towards
some 3d metal ions studied. All the metal complexes synthesized
were of the general formula, [M(bpsal)2(H2O)2] where
M 5 Mn(II), Co(II), Ni(II), Cu(II) and Zn(II). The complexes
were structurally characterized by elemental analyses, molar
conductance, IR, NMR, and electronic spectral data. The ation of our studies on coordination complexes of transition
observed that transition metal complexes of Schiff bases are
well suited for the purpose of testing their properties, due to
the synthetic ease in substituting the central metal atom as
well as peripheral groups (Chandrasekhar, 1992). In continu-
metallo mesogenic phases of the Cu(II) complex were studied by
optical microscope. The spectral data show that Hbpsal acts as a
bidentate ligand towards the metal ions. Mesogenic studies
indicate that the ligand shows a focalconic structure with
metals (Narang et al., 2000; Rao and Kumar 1994; Rao and
Sastry, 1994; Rao et al., 1994), we have undertaken a systema-
tic synthesis of mesogenic ligands and their metal complexes.
The present article deals with 3d metal complexes of the novel
mesogenic Schiff base, N-(4-butylphenyl) salicylaldimine,
Hbpsal (Figure 1).
smectic-A phase while the Cu(II) complex displays smectic-A as
well as smectic-E phases.
Keywords 3d metal complexes, complexes of salicylaldimine
derivative, liquid crystalline complexes, metallo
mesogens, synthesis and spectral studies of transition
metal complexes
EXPERIMENTAL
Materials
4-n-Butylaniline was purchased from Aldrich Chemical
Company, U.S.A. Salicylaldehyde and metal acetates were of
B.D.H. or E. Merck.
INTRODUCTION
Synthesis of the Ligand
Molecular design and synthesis of new metallo mesogenic
complexes with novel mesophases represent an active
research area in metallomesogenic materials (Ghedini, et al.,
1987; Hoshino, et al., 1991; Marcos, et al., 1989; Morrone
and Ghehini, 1991). An extensive amount of work on
Hbpsal was prepared by refluxing absolute ethanolic
solution (25 mL) of salicylaldehyde (15 mmol, 1.83 mL) and
an ethanol solution (25 mL) of 4-butyl aniline (15 mmol,
2.23 mL) for ꢀ2 h and leaving the solution at room temperature
overnight. The resulting crude solid product was filtered,
washed repeatedly with ethanol and recrystallized from
absolute alcohol; yield, 2.65 g, 70%; m.p, 558C. Anal. found:
C, 80.05; H, 7.45; N, 5.43%; calc. for C17H19NO (FW,
253.17): C, 80.60; H, 7.56; and N, 5.53%.
Received January 21, 2005; accepted March 18, 2005.
The authors thank the Head of the Department of Chemistry,
Faculty of Science, Banaras Hindu University, for providing labora-
tory facilities, the Director of the Central Drug Research Institute,
Lucknow, for elemental analyses, and Dr. N.V.S. Rao, Head of the
Chemistry Department, Assam University, Silcher, India, for provid-
ing the optical microscope facility in identifying the mesogenic
phases. One of the authors (TRR) thanks the Council of Scientific
and Industrial Research, New Delhi, for financial assistance in the
form of a research project (No.01(1834)/03/EMR-II).
Synthesis of the Complexes
To a methanolic solution (50 mL) of Hbpsal (4 mmol, 1.0 g)
was slowly added a methanolic solution (20 mL) of the respect-
ive metal acetate (2 mmol, ꢀ0.49 g) with stirring over a period
of 10 min. The reaction mixture was magnetically stirred for
2 h at room temp. During this period, the solid complex
started separating. The solution was then reduced to half of
Address correspondence to T. R. Rao, Department of Chemistry,
Banaras Hindu University, Varanasi 221005, India. E-mail:
trraobhu@hotmail.com
299

300
T. R. RAO AND A. PRASAD
measurements were made at room temperature on a Cahn-
Faraday electrobalance using Hg[Co(NCS)4] as the calibrant.
Infrared spectra (4000–400 cm21) were recorded on an FT
IR (Jasco 5300) spectrophotometer in KBr pellets while the
electronic spectra were recorded on a Cary 2390 spectropho-
1
tometer in Nujol mulls. The H and 13C NMR spectra of all
the organic ligands and the Zn(II) complex were recorded in
DMSO-d6 solutions on a Jeol FX-90Q multinuclear spec-
trometer. The water content of the complexes was determined
by thermal analyses over the temperature range 80–1608C.
Nitrogen was determined by micro-analysis. The mesogenic
phases were detected with the help of a polarizing optical
microscope (Leitz) equipped with a Mettler FP 82 hot stage.
FIG. 1. Structure of Hbpsal.
its volume under reduced pressure and left overnight at room
temp. The solid complex that separated out was filtered by
suction and recrystallized in ethanol and dried over fused
CaCl2 in a desiccator.
RESULTS AND DISCUSSION
The formation of the complexes may be represented by the
following equation:
Reflux2 h
M(OAc)2 ꢀ nH2O þ 2Hbpsal ꢁꢁꢁꢁ!
MeOH
Analysis of the Complexes and Physical Measurements
In order to determine the metal content, a weighed quantity
(ꢀ80 mg) of the complex was heated successively with aqua
regia (0.5 mL) and conc. H2SO4 (2 drops to destroy the
organic matter. The resultant aqueous extract (weakly acidic
with respect to HCl) was titrated against a standard solution
M(bpsal)2 ꢀ 2H2O þ 2AcOH þ ðn-2ÞH2O
ðM ¼ Mn; Co; Ni; Cu and ZnÞ
ð1Þ
All of the complexes are microcrystalline in their
(0.025 M) of the disodium salt of EDTA using a literature appearance and are yellow/brown/green/light cream in color
procedure (Jeffery et al., 1989). (Table 1); they are fairly stable at room temperature and
The molar conductances of the complexes in 0.001 M are soluble in common organic solvents such as methanol,
DMSO solutions were measured at room temperature on a ethanol, acetone, benzene, acetonitrile, carbon tetrachloride
WTW conductivity meter. Magnetic susceptibility DMF and DMSO, but are insoluble in water. The
TABLE 1
Analytical and physico-chemical data of M(II) complexes of Hbpsal
Ligand/complex (emp.
formula, formula wt.)
(m.p., 8C)
Wt. loss found
(calcd.) at
1508C
Found (calcd.) %
Color
(% yield)
Molar
conductancea
C
H
N
M
[Mn(bpsal)2(H2O)2]
(C34H40N2O4Mn, 595.62)
(230b)
Brown
(80)
66.94
(68.56)
6.60
(6.71)
4.58
(4.70)
9.11
(9.22)
6.05
(6.04)
11.2
12.0
15.6
10.8
08.4
[Co(bpsal)2(H2O)2]
(C34H40N2O4Co, 599.61)
(170)
Yellow
(77)
67.23
(68.10)
6.67
(6.69)
4.50
(4.67)
9.67
(9.83)
5.91
(6.00)
[Ni(bpsal)2(H2O)2]
(C34H40N2O4Ni, 599.37)
(210b)
Green
(83)
67.08
(68.12)
6.54
(6.67)
4.59
(4.67)
9.59
(9.79)
5.94
(6.00)
[Cu(bpsal)2(H2O)2]
(C34H40N2O4Cu, 604.22)
(110)
Brown
(84)
66.39
(67.58)
6.50
(6.62)
4.53
(4.63)
10.42
(10.52)
5.79
(5.96)
[Zn(bpsal)2(H2O)2]
(C34H40N2O4Zn, 606.07)
(180)
Cream
(70)
66.14
(67.38)
6.52
(6.60)
4.50
(4.63)
10.69
(10.79)
5.83
(5.94)
aMolar conductance (ohm21 cm2/mole) in 1023 M methanol solution.
bDecomposition temperature.

SYNTHESIS AND SPECTRAL STUDIES ON 3D METAL COMPLEXES
301
melting/decomposition temperature of the complexes were
TABLE 2
Mesophase recognition by optical microscope
determined on a manually controlled melting point apparatus
with the rate of heating fixed at 28C/min in the transition
region. Thermal analyses of the complexes correspond to the
loss of lattice water in the temperature range, 80–1008C.
Compound
Hbpsal
Transition temp (8C) and mesophase (s)
42W C
49-55W C
K ꢁꢁꢁꢁꢁꢁꢁ! SA ꢁꢁꢁꢁꢁꢁꢁ! 1
The electrical conductivity values measured in 1023
M
98W C
105W C
110W C
[Cu(bpsal)2(H2O)2] K ꢁꢁꢁꢁꢁꢁꢁ! SA ꢁꢁꢁꢁꢁꢁꢁ! SE ꢁꢁꢁꢁꢁꢁꢁ! 1
K ¼ Crystalline, S ¼ smectic, I ¼ isotropic liquid.
methanol solutions of all the complexes lie in the range
8.40–15.60 ohm21 cm2/mol, indicating their non-ionic nature
(Geary, 1971). The solutions kept at this concentration for
several days did not show any change in their appearance
and in the conductance values. This indicates that no ionic
species are formed even after keeping the solutions over
several days. Further, the absence of acetate groups in the com-
plexes implies that the di-positive charge of the metal ion is
balanced by the deprotonated form of the ligand.
association and thus promote a smectic phase (Hoshino,
et al., 1990).
Magnetic Moments and Electronic Spectra
The room temperature magnetic moments and electronic
spectral data of the complexes are included in Table 3.
The meff values of the Co(II) and Ni(II) complexes imply an
octahedral geometry around the metal ion, while that of the
Mn(II) and Cu(II) complexes fall within the normal ranges
and provide no information about the stereochemistry. All of
the three expected transitions for the octahedral geometry of
the Co(II) and Ni(II) complexes are observed in their electronic
spectra; further, the n2/n1 ratio in their spectra supports the
assigned stereochemistry (Figgis, 1976). The transition assign-
ments and the ligand-field parameters, viz., 10 Dq, B0, b and
LFSE values calculated from the absorptions of the Co(II)
and Ni(II) complexes are also included in Table 3; the b
values imply that the nature of M-L bond in the present com-
plexes is partially covalent (Lever, 1968).
Mesomorphism
The ligand upon heating shows a hazy phase over the 38–
498C range with isotropic melting point at 558C, and the
fluid displays a focalconic texture resembling discotic liquid
crystals over the range 42–498C. The Cu(II) complex, upon
heating/cooling from the isotropic point (1108C), displays a
smectic-A phase at 988C (Figure 2) and smectic-E phase at
1058C. The details are shown in Table 2.
The data in Table 2 is consistent with the fact that incorpor-
ation of a metal center into the mesogenic ligand leads to a
more ordered mesophase of the metal complex (Ghedini,
et al., 1987; Hoshino, et al., 1991; Lai, and Lin, 1997;
Marcos, et al., 1989; Morrone and Ghehini, 1991). The
change in molecular geometry apparently is due to the fact
that the Cu(II) complex being more symmetrical than the
ligand due to the presence of alkyl chains on both ends.
Further, the polarity of the coordination bond in the Cu(II)
complex is believed to enhance transverse inter-molecular
Infrared Spectra
The IR spectrum of N-(butyl)phenylsalicylaldimine,
Hbpsal, is dominated by (1) C-H stretching and bending
vibrations of the methyl/methylene groups of the butyl
chain, (2) O-H stretching and bending and C-O stretching
vibrations of the phenolic group, (3) .C¼N stretching of the
azomethine moiety, and (4) C-H stretching, out-of-plane C-H
bending and overtone and combination bands of disubstituted
aromatic rings. Of all the above bands, only those diagnostic
of coordination to the metal ion, are listed in Table 4 along
with the nas(CH3) and ns(CH2) bands. The prominent IR
spectral data of the metal complexes of Hbpsal may be dis-
cussed as follows: (1) The n(OH)phenolic band [arising at
3490 cm21 in the spectrum of the ligand] disappears in the
spectra of the complexes and the n(C-O)phenolic undergoes a
bathochromic shift upon metal complexation by ꢀ20 cm21
.
These features imply deprotonation of the phenolic group
and coordination through the phenolate anion; (Silverstein,
and Webster, 2002) (2) The n(C¼N) band is shifted from
1620 to ꢀ1610 cm21 in the spectra of the complexes; coordi-
nation through this group to a metal ion is usually implied by
a bathochromic shift of the band; but the exact magnitude of
the bathochromic shift may be visualized only when the
position is compared to that of the free ligand devoid of any
FIG. 2. Black/white photomicrograph of [Cu(bpsal)2(H2O)2] in smetic-A
phase at 988C obtained on cooling the isotropic liquid.

302
T. R. RAO AND A. PRASAD
TABLE 3
Electronic spectral data (cm21) and ligand field parameters of M(II) complexes of Hbpsal
Spectroscopic terms
of the complex
lmax (cm21
(n1, n2, n3)
)
Dq
(cm21
B0
Complex [meff (B.M)]
SLJ
6A1g
S0L0J0
)
(cm21
)
b
b0 (%)
LFSEa
—
[Mn(bpsal)2(H2O)]
[5.89]
[Co(bpsal)2(H2O)]b
[4.95]
15,625
4T1g(G)
—
—
—
9,090
18,348
20,920
10,526
17,094
26,315
14,084
4T1g(F)
4T1g(F)
4T1g(F)
3A2g(F)
3A2g(F)
3A2g(F)
2Eg
4T2g(F)
4A2g(F)
4T1g(P)
3T2g(F)
3T1g(F)
3T1g(P)
2T2g
1030
869
0.89
0.75
11
25
98.1
150.0
100.6
[Ni(bpsal)2(H2O)]c
[3.05]
1052
1408
788
—
[Cu(bpsal)2(H2O)]
[1.90]
—
aLFSE values are given in kJmole21
bn2/n1 ¼ 2.01.
.
cn2/n1 ¼ 1.62.
TABLE 4
IR spectral dataa (cm21) of Hbpsal and the metal complexes
n(C-O)
(phenolic)
n(O-H)
(water)
Ligand/complexes
n(C¼N)
nas(CH3)
ns(CH2)
Hbpsalb
1620 s
1608 s
1610 s
1610 s
1610 s
1608 s
2955 s
2955 s
2950 s
2952 s
2955 s
2952 s
2852 s
2856 s
2858 s
2852 s
2856 s
2854 s
1363
1348
1340
1348
1348
1340
—
[Mn(bpsal)2(H2O)2]
[Co(bpsal)2(H2O)2]
[Ni(bpsal)2(H2O)2]
[Cu(bpsal)2(H2O)2]
[Zn(bpsal)2(H2O)2]
3434 b
3454 b
3445 b
3445 b
3454 b
ab ¼ broad, s ¼ strong, m ¼ medium and w ¼ weak.
bn(OH)phenolic occurs at 3490 b.
hydrogen bonding; and (3) An additional prominent and broad
band, in contrast to the shallow nature of the n(OH)phenolic band
in the free ligand, arises in the spectra of all the complexes due
to the n(OH) of coordinated water molecules.
NMR Spectra
1
The 1H NMR and the 13Cf Hg NMR spectral data of Hbpsal
and the Zn(II) complex along with the assignments are given
in Tables 5 and 6, respectively. The numbering scheme of
the carbon atoms of Hbpsal is as shown in Figure 3. The proton
NMR spectrum of the ligand (Hbpsal) is expected to show
a broad peak (due to phenolic-OH), a singlet (due to
–N¼CH), four doublets (due to C02H, C300 H, C3H, and C6H),
three triplets (due to C040H C4H and C5H) and a multiplet due
FIG. 3. Numbering scheme of Hbpsal.

SYNTHESIS AND SPECTRAL STUDIES ON 3D METAL COMPLEXES
303
TABLE 5
1H NMR spectral dataa of Hbpsal and the Zn(II) complex
Proton
Hbpsal
[Zn(bpsal)2(H2O)2]
Ar-OH
–N¼CH
–CH3
14.40
8.70 s
absent
9.00 s
0.88 t
0.87 t
1.30 m
6.2–8.0 m
–CH2
Ar-H
1.35 m
6.5–8.3 m
a1H NMR Spectral data are given in d with respect to tetramethyl-
silane; s ¼ singlet; d ¼ doublet; t ¼ triplet; m ¼ multiplet.
to the methylene groups of the alkyl chain. The prominent
features in the spectrum of the corresponding Zn(II)
complex, as underlined in Table 6, are: (1) the disappearance
of the phenolic-OH signal, and (2) a slight downfield shift of
the –N¼CH signal while retaining its singlet nature. These
features imply coordination through the phenolate oxygen
and the azomethine nitrogen. Further, a new and broad
signal, observed in the spectrum of the complex at 3.4 ppm,
could be attributed to the coordinated water molecule of the
complex.
FIG. 4. Proposed structure of [M(bpsal)2(H2O)2].
The –N¼CH signal appearing at 162.58 in the spectrum of
the ligand undergoes downfield shift by 0.98 ppm in the
spectrum of the complex, which can be inferred on account
of coordination to the metal ion through the azomethine
nitrogen (Domiano et al., 1984). The slight downfield shift of
the C2 resonance signal is indicative of bonding through the
phenolate oxygen. However, the methyl and methylene
carbon signals remain almost at the same positions as in
Hbpsal. The slight upfield shift in some of the resonance
signals may be attributed to electron delocalization (Paolucci
et al., 1980).
1
The 13Cf Hg NMR spectra of the ligand (Hbpsal) as well as
the Zn(II) complex show all the signals expected for the 15
non-equivalent carbon atoms of the ligand. The signal assign-
ments shown in Table 6 were based on the expected positions,
which were calculated by applying the principle of substituent
additivity.
TABLE 6
1
13Cf Hg NMR spectral dataa of Hbpsal and the Zn(II) complex
Carbon
Hbpsal
[Zn(bpsal)2(H2O)2]
CONCLUSION
In the complexes of Mn(II), Co(II), Ni(II), Cu(II) and Zn(II)
the ligand, Hbpsal, acts as a uninegative bidentate ligand coor-
dinating through the azomethine nitrogen and phenolate
oxygen where the M:L ratio was found to be 1:2. The metal
ion is believed to have an octahedral environment and the
additional two sites in the polyhedron are provided by water
molecules. The proposed structures are shown in Figure 4.
The Cu(II) complex shows mesogenic activity displaying a
smectic-A phase at 988C (Figure 2) and smectic-E phase at
1058C.
–N¼CH
C1
C2
162.58
119.03
159.00
116.54
133.80
121.80
129.20
119.23
145.58
141.34
132.21
32.70
163.56
119.00
160.02
116.19
133.78
121.61
129.11
119.22
145.51
141.32
132.20
32.54
C3
C4
C5
C6
0
C1
C2
C3
C4
C1
C2
C3
C4
0
0
0
00
00
00
00
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