Lamellar columnar mesomorphism in a series of oxovanadium(IV) complexes derived from N, N/-di-(4-n-alkoxysalicylidene)diaminobenzene

Article abstract of DOI:10.1016/j.inoche.2011.01.041

A series of new nondiscoid shaped square pyramidal oxovanadium(IV)-Schiff base complexes containing 4-substituted alkoxy chains on the aromatic rings, [VO(4-C_nH_{2n + 1}O)₂salophen)] {n = 14, 16, 18; salophen = N, N/-phenylnebis(salicylideneiminato)} have been prepared and their mesogenic properties investigated. The ligands and the complexes were characterized by FT-IR, CHN, ¹H-NMR, UV-vis and FAB mass spectrometry. Presence of V=O stretching mode at ～980 cm^{- 1} indicated absence of any intermolecular?V=O?V= O?interactions with no dependence on the chain length. Variable temperature magnetic susceptibility measurements of the complexes also suggested lack of such exchange interaction. Cyclic voltammetry showed a quasireversible one-electron response at 0.75 V for the VO(IV)-VO(V) redox couple. DSC coupled with polarized optical microscopy showed the ligands to be nonmesogenic but the complexes exhibited an unprecedented thermally stable columnar mesophase in the 136-154°C. X-ray diffraction technique, confirmed that the structure of the mesophase is lamellar columnar in type (Col_l). DFT studies using DMol3 were carried out to obtain the optimized geometry of the complex.

Full text of DOI:10.1016/j.inoche.2011.01.041

Inorganic Chemistry Communications 14 (2011) 606–612
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Inorganic Chemistry Communications
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Lamellar columnar mesomorphism in a series of oxovanadium(IV) complexes
derived from N, N/-di-(4-n-alkoxysalicylidene)diaminobenzene
a
a
b
b
Chira R. Bhattacharjee a, , Gobinda Das , Paritosh Mondal , Subbarao Krishna Prasad , D. S. Shankar Rao
⁎
a
Department of chemistry, Assam University, Silchar 788011, Assam, India
Centre for Soft Matter Research, Jalahalli, Bangalore 560013, India
b
a r t i c l e i n f o
a b s t r a c t
Article history:
A series of new nondiscoid shaped square pyramidal oxovanadium(IV)-Schiff base complexes containing 4-
substituted alkoxy chains on the aromatic rings, [VO(4-CnH2n + 1O)2salophen)] {n=14, 16, 18; salophen=N,
N/-phenylnebis(salicylideneiminato)} have been prepared and their mesogenic properties investigated. The
ligands and the complexes were characterized by FT-IR, CHN, 1H-NMR, UV–vis and FAB mass spectrometry.
Presence of V=O stretching mode at ~980 cm−1 indicated absence of any intermolecular …V=O.....V=O…
interactions with no dependence on the chain length. Variable temperature magnetic susceptibility
measurements of the complexes also suggested lack of such exchange interaction. Cyclic voltammetry
showed a quasireversible one-electron response at 0.75 V for the VO(IV)–VO(V) redox couple. DSC coupled
with polarized optical microscopy showed the ligands to be nonmesogenic but the complexes exhibited an
unprecedented thermally stable columnar mesophase in the 136–154 °C. X-ray diffraction technique,
confirmed that the structure of the mesophase is lamellar columnar in type (Coll). DFT studies using DMol3
were carried out to obtain the optimized geometry of the complex.
Received 15 December 2010
Accepted 28 January 2011
Available online 20 February 2011
Keywords:
Vanadyl
Mononuclear complexes
Lamellar columnar
DFT
© 2011 Elsevier B.V. All rights reserved.
Incorporating metal centers to create unique geometric structures
has been widely exercised to generate metallomesogenic materials
with novel physical properties during the past decade [1–5].
Numerous metallomesogenic structures exhibiting novel mesomor-
phic properties have continuously been reported and their physical
properties studied [6–11]. Coordination compounds of redox active
metals exhibit paramagnetism, and often undergo electronic excita-
tions with visible light. Such properties can be exploited to produce
novel conductors, magnetic materials, and display devices [12]. Unlike
other first row transition metals, oxovanadium(IV) complexes owing
to the weakened intermolecular …V=O........V=O… nonpolar inter-
action often frustrates exhibition of mesogenic properties [13–17].
Five coordinate vanadyl complexes are susceptible to intermolecular
dative interactions through the VO2+ moiety which in square
pyramidal vanadyl complexes form a linear chain-like (―V=
O―V=O―)n) structures generating a polar order along the column
axis [12]. Such polar order is quite significant in the context of novel
ferroelectric/piezoelectric and NLO materials [12]. Despite the
inherent difficulties in synthesizing mesogenic vanadyl(IV) com-
plexes, a number of vanadyl(IV) liquid crystalline complexes have
been documented [11–30]. Rich but complex mesomorphism exhib-
ited by the salicylaldimines and their metal complexes including
vanadium has attracted considerable interests. The Schiff base
complexes of vanadium(IV) that possess a calamitic (rod-like)
mesogenic group to which terminal flexible alkyl or alkoxy chains
are attached, results in mesogens which either exhibit nematic or
smectic phases, have been rather extensively studied [20–30].
However, by coordinating non-discoid ligands, a molecular shape
with a reduced length-to-breadth ratio is formed, mimicking disk-like
metallomesogens [31]. Such structures will have a reduced tendency
to form calamitic mesophases and favor formation of columnar
mesophases [31].Complexes containing Schiff base ligands with long
alkoxy chain at 5-position on aromatic rings have been reported to
exhibit smectic A (SmA )or C (SmC) phase [17,32]. Four coordinated
Ni(II), Cu(II)-salen complexes with 5-substituted long alkoxy chains
forming a linear rod like (1-D) structure also show SmA mesophases
[33–35]. In contrast, metallomesogens of Schiff bases with long alkoxy
chain at 4-position have been sparsely investigated [14,15,19]. Abe et
al. reported a series of vanadyl complexes, possessing an aliphatic
(―CH2)n rigid core showing an unusual rectangular (Colr) columnar
mesophase [15]. Complexes with shorter alkoxy substituent, [VO(4-
CnH2n+1O)2salen)]ClO4 (n=3, 8, 10) with (―CH2)2-rigid core lacked
liquid crystalline behavior [14,15,19]. Metallomesogens exhibiting
Coll mesophase are not ubiquitous [19,36]. Quite surprisingly,
metallomesogen with 4-substituted Schiff base ligands with an
aromatic rigid core are not known till date. Very recently we have
reported synthesis of a series of novel fluorescent liquid crystalline Zn
(II)-salophen complexes with methyl substituted phenylenediamine
spacer [37]. Choice of metal ion, nature and position of the
substituents on aromatic ring are known to greatly influence the
⁎
Corresponding author. Tel.: +91 03842 270848; fax: +91 03842 270342.
E-mail address: crbhattacharjee@rediffmail.com (C.R. Bhattacharjee).
1387-7003/$ – see front matter © 2011 Elsevier B.V. All rights reserved.
doi:10.1016/j.inoche.2011.01.041

C.R. Bhattacharjee et al. / Inorganic Chemistry Communications 14 (2011) 606–612
607
H2n+1CnO
CHO
i
HO
CHO
n=14,16,18
OH
OH
ii
H2N
NH2
7
8
9
6
N
8
7
4
H
H
9
6
O
V
4
N
O
H
H
1
N
N
iii
1
2
O
5
OH
2
HO
3
OCnH2n+1
H2n+1CnO
3
H2n+1CnO
OCnH2n+1
Scheme 1. (i) CnH2n + 1Br, KHCO3, KI, dry acetone, Δ, 40 h, and (ii) glacial AcOH, absolute EtOH Δ, 4 h (iii) VOSO4.5H2O, MeOH, TEA. Δ, 1 h.
mesogenic behavior [38]. Usually macrocyclic compounds possessing
discoid molecular shape are known to stack and self assemble into
columnar mesophase [39]. In the present study a series of new Schiff
base [N2O2] donor with an aromatic rigid core and 4-substituted long
alkoxy group at the side aromatic rings have been synthesized and its
vanadyl(IV) complexes were accessed. Though the ligands are
nonmesogenic, the complexes showed lamellar columnar meso-
phases (Coll) hitherto unreported for such compounds with 4-
substituted alkoxy group.
Accordingly, we describe herein the synthesis, structures and
mesogenic properties of some nondiscoid Schiff base-oxovanadium
(IV) complexes with different 4-substituted long alkoxy group.
Synthesis of the compounds (n-opd and VO-nopd) could be
achieved through a facile and straightforward procedure [40]. The
synthetic strategy for the ligands [(L=N,N/-bis(4-(4/-n-alkoxy)-
salicylidene)1,2phenylenediamine), hereafter abbreviated as n-opd,
where n indicates the number of carbon atoms in alkyl chains, n=14,
16, 18 and opd=1,2phenylenediamine] and the mononuclear
oxovanadium(IV) complexes (VO-nopd) are presented in Scheme 1.
The mononuclear oxovanadium(IV) complexes, VO-nopd (n=14,16,18),
were prepared by the reaction of appropriate ligand with vanadyl
sulfate in hot ethanol (1:1 molar ratio) and recrystallized from
methanol/CH2Cl2; the complexes were isolated as greenish solids in
good yields.
The characterizations of the compounds were made by elemental
analyses, FT-IR, UV–vis, 1H-NMR, and mass spectrometry [40]. The
analytical data of the compounds are in good agreement with the
proposed formulae of the compounds. The shift of νCN vibrational
stretching frequency at ca.1625 cm− 1 to lower wave number
(Δν~30 cm−1) and absence of νOH mode and phenolate oxygen
upon chelation, clearly suggested the coordination of azomethine N
and phenolate oxygen to the metal. The νC=N stretching frequency is
rather independent of the length of alkoxy side chain in both ligands
and their complexes. The vanadyl (V=O) stretching at ca.980 cm−1 is
suggestive of the absence of any intermolecular (…V=O…V=O…)
interaction indicating the monomeric nature of the complexes. The
presence of linear chain interactions usually cause νV=O to shift to
lower wave number (ca.870 cm−1) [14,15]. The IR data also revealed
V=O stretching frequency to be insensitive to the length of the alkoxy
side chain. 1H NMR spectra of ligands showed a signal at δ=13.4–
13.8 ppm, corresponding to the OH-proton. The imine proton
appeared at 8.5 ppm. The FAB-mass spectra of the vanadyl(IV)
complexes are concordant with their formula weights. The electronic
absorption spectrum (Fig. 1) of ligand 18-opd showed a strong band
at ~337 nm, (ε=5400 l mol−1 cm−1) attributed to π–π* transition
localized on the aromatic rings. Upon complexation with VO(IV), this
0.6
1
. 18-opd
1
2. VO-18opd.
0.5
0.4
0.3
0.2
0.1
0.0
Table 1
2
The UV–visible absorption data of n-opd and VO-nopd.
Compound
λmax(nm)
π→π*
MLCT
18-opd
VO-18opd
16-opd
VO-16opd
14-opd
VO-14opd
337
329
331
328
331
328
400
398
398
250
300
350
400
450
500
550
Wave length
Fig. 1. UV–visible spectra of 18-opd and VO-18opd.

608
C.R. Bhattacharjee et al. / Inorganic Chemistry Communications 14 (2011) 606–612
Fig. 4. X-ray difractogram of VO-18opd.
Fig. 2. Optical micrographs of the VO-18op.
Noteworthy here is to mention that most vanadyl-mesogens show
either rectangular or hexagonal mesomorphism [12–14,20,23,27].
DSC thermograms (Fig. 3) of the complexes exhibited three transi-
tions (Table 2) in heating and one in cooling cycle. The phase
transition for the complex VO-18opd at 154 °C (ΔH=3.2 kJ mol−1) is
due to I-Col.
45
40
35
30
25
To confirm the structure of the mesophase, powder XRD measure-
ments were carried out using an image plate detector (Mac Science
DIP1030, Japan) equipped with double mirror focusing optics, and the
sample contained in a Lindmann capillary tube. The XRD pattern (Fig. 4)
for VO-18opd carried out at 130 °C showed overall four sharp
reflections, and a diffuse peak in the wide angle region. The d-spacings
corresponding to these reflections are given in Table 3. The broad and
diffuse scattering peaking at 4.98 Å (hch) corresponds to the short range
order of the molten chains. The four sharp reflections can be indexed in
two ways. Considering the first reflection to have a hkl value of (100),
the other three reflections, can be indexed as (010), (500) and (700),
with lattice dimensions of a=41.05 Å and b=20.23 Å of a rectangular
lattice. The l value being zero for all the reflections suggests the fluid
nature of the structure in one direction. The need for the second
possibility arises owing to the fact that the spacing of the second
reflection is nearly half of that of the first one, a feature characteristic of
lamellar structures. In fact, the spacings of the four reflections turn out
to be in the ratio of 1:2.03:4.8:7, supporting such a conjecture with a
layer thickness of ~41 Å. It may be noted that the molecular length for
the extended molecule as computed from DFT studies is 42.1 Å. This
value is comparable to the longer dimension for the rectangular lattice
as well as the interlayer spacing for the columnar structure. Owing to
the fact that only a few sharp reflections are seen, we cannot assert
either of the structures. However, noting that the intensity of even the
second reflection is very weak, we tend to believe the layered order and
keeping in view of the shape of the molecule propose a lamellar
columnar structure [19,41,42]. In such a scenario, the half-disk shaped
molecules pack themselves into a layer, but order in a face-to-face
80
100
120
140
160
Temperature(°C)
Fig. 3. DSC thermogram of the VO-18opd.
band is blue shifted to a lower wave length at ~328 nm. In addition,
the vanadyl complex also showed a strong band at ~400 nm,
(ε=2400 l mol−1 cm−1) due to MLCT transition. A similar observa-
tion (Table 1) was noted for 16-opd and 14-opd and their complexes.
The ligands lack mesogenicity, while their vanadyl complexes
exhibited monotropic mesmorphism in the temperature range 136–
150 °C. Mesomorphism induced by the vanadyl ion is quite unique as
in many cases the mesophase of organic ligands are destroyed or
destabilized upon complexation. A classical fan like texture of a typical
columnar mesophase (Fig. 2) appeared at 150 °C upon cooling from
the isotropic melt that remained stable down to room temperature.
Table 2
DSC data of the VO-nopd complexes.
Compounds
VO-18-opd
T (°C)
Transition
ΔH (kJ mol−1
)
Table 3
XRD data of VO-18opd (s: sharp peak, d: diffuse peak).
96.9
115.6
167.9
154.0
84.3
118.4
170.9
146.3
88.4
Cr–Cr1
Cr1–Cr2
Cr2–I
61.7
11.2
16.0
3.3
17.2
10.7
12.2
2.5
13.7
6.0
14.6
2.0
Observed
spacing (Å)
Rectangular
lattice calculated
spacing (Å)
Rectangular
lattice Miller
indices (h k l)
Lamellar
structure Miller
indices (h k l)
I–Coll
VO-16-opd
VO-14-opd
Cr–Cr1
Cr1–Cr2
Cr2–I
41.02s
20.23s
8.66s
41.05
20.23
8.21
(100)
(010)
(500)
(700)
Lattice constant (Å)
a=41.05
b=20.23
(100)
(200)
(500)
(700)
Layer spacing (Å)
a=41.02
I–Coll
5.83s
5.86
Cr–Cr1
Cr1–Cr2
Cr2–I
4.98d
123.1
176.7
136.5
I–Coll

C.R. Bhattacharjee et al. / Inorganic Chemistry Communications 14 (2011) 606–612
609
Fig. 5. Antiparallel arrangement of molecules in the lamellar columnar mesophase.
antiparallel dimeric fashion (Fig. 5) within the layer resembling a
columnar structure.
As efforts to obtain a single crystal of the complexes failed, the
density functional theory (DFT) calculation is performed to
investigate the electronic structure of the V(IV) complex ,VO-18-
opd (Fig. 8). Full geometry optimization of the complex without
symmetry constraint has been carried out with DMol3 program
package [43] using the Kohn–Sham Theory [43,44]. We used the
generalized gradient approximation (GGA) in our calculations. At
the GGA level, we have chosen the BLYP functional which
incorporates Becke's exchange and the Lee–Yang–Parr correlation
[43–45]. The length and bent angle of the molecule are found to be
~42 Å and 92.8°, respectively, from DFT calculation. Some of the
selective geometric parameters of the optimized VO(IV) complex,
evaluated by DFT calculation at BLYP/DNP level are shown in
(Table 4). From DFT study, it has been found that the complex has
average V―O and V―N bond lengths of 2.0 and 2.1 Å, respectively,
which matched well with the other related vanadium salen
complexes [14,15]. The bond angles 92.8° and 76.7° for O―V―O
and N―V―N, respectively, suggest a distorted square pyramidal
geometry around the vanadyl centre. The 3D isosurface plots of the
lowest unoccupied molecular orbital (LUMO) and the highest
occupied molecular orbital (HOMO) of the complex are shown in
Figs. 9 and 10 respectively. The HOMO and LUMO energies of the
The variable temperature magnetic susceptibility measurements
were carried out for a representative complex, VO-18-opd (Fig. 6). The
compound follows the Curie–Weiss law. Absence of any maximum in
χM vs T indicated nonexistence of strong exchange interactions
between the spin centers. The strength of the efficient superexchange
path is presumably hindered by the coordinating ability of metal ions.
The vanadium complexes can thus be considered as magnetically
isolated spin centers.
The redox natures of the complexes were probed by cyclic
voltammetry in acetonitrile solution in the potential range −1.2 to
1.2 V versus SCE electrode. The voltammogram (Fig. 7) displayed a
quasireversible (peak-to-peak separation N100 mV) one-electron
response at 0.75 V (Epc=0.64 V, Epa=0.86 V, ΔEp=0.22 V) assigned
to the VO(V)/VO(IV) couple. On scanning to negative potential (0.0 to
−1.2 V), no response is observed implying redox innocent character-
istics of the ligand, supported also by the free ligand cyclic
voltammetry.
Fig. 6. Variation of magnetic susceptibility of ( VO-18opd) with temperature.
Fig. 7. Cyclic voltamogram of VO-18opd.

610
C.R. Bhattacharjee et al. / Inorganic Chemistry Communications 14 (2011) 606–612
Fig. 8. DFT structure of VO-18opd.
decreasing carbon chain length of alkoxy substituent. The cyclic
voltammetry of a representative complex VO-18-opd displayed a
quasireversible (peak-to-peak separation N100 mV) one-electron
response at 0.75 V for the VO(V)/VO(IV) redox couple. Based on
DFT, spectral and magnetic studies, a square pyramidal five-
coordinate structure has been proposed. The synthetic strategy
adopted herein can be effectively employed to access newer
mesogenic vanadyl complexes.
Table 4
Selected bond length and bond angles of VO-18opd, from DFT study.
Structure parametera
Distance/angles
V―N
V―O
2.1
2.0
V=O
1.8
N―V―O
O―V―O
N―V―N
84.0
92.8
76.7
a
Bond lengths are reported in Å and bond angles in degrees.
Acknowledgments
The authors thank SAIF, NEHU and CDRI, Lucknow for analytical
and spectral data. GD acknowledges financial support from DST and
UGC, Government of India. Dr. R. C. Deka, Tezpur University, India, is
acknowledged for computational facility.
complex are calculated to be −5.4 eV and −3.5 eV, respectively,
ΔE=1.9 eV. The LUMO is a n* MO localized on the salophen ligand
and the HOMO is mainly a 3d orbital of the V metal center.
In summary, a series of new nondiscoid mononuclear oxovana-
dium(IV) complexes of tetradentate [N2O2] donor Schiff base with 4-
alkoxy substituent on the aromatic ring have been successfully
synthesized. The complexes exhibited unprecedented lamellar co-
lumnar mesophase in the temperature range 136–154 °C. Quite
notable is the trend of progressive hike in isotropisation temperature
and a steady decrease in the mesophase appearance as a function of
Appendix A. Supplementary data
Supplementary data to this article can be found online at
doi:10.1016/j.inoche.2011.01.041.
Fig. 9. LUMO energy diagram of VO-18opd.

C.R. Bhattacharjee et al. / Inorganic Chemistry Communications 14 (2011) 606–612
611
Fig. 10. HOMO energy diagram of VO-18opd.
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chloroform-ethanol(1:1).75%. FAB Mass (m/e, fragment): m/z: calc. 917; found:
918[M+H+]; Anal. Calc. for C56H86N2O5V(917): C, 73.2; H, 9.3; N 3.0. Found: C,
73.1; H, 9.2; N, 3.1%. IR (νmax, cm−1, KBr): 2920(νas(C―H), CH3), 2850(νas
(C―H), CH2), 1612(νC=N), 981(νV=O).
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1 mmol) was added to an ethanolic solution of 1,2 phenylenediamine (0.06 g,
0.5 mmol). The solution mixture was refluxed with a few drops of acetic acid as
catalyst for 3 h to yield the yellow Schiff base N,N/-bis (4-(4/-n-alkoxy)-
salicylidene) 1,2-phenylenediamine. The compound was collected by filtration
and recrystallised from absolute ethanol to obtain a pure compound.Yield:0.28 g,
75%. FAB Mass (m/e, fragment): m/z: calc. 852.7; found: 858[M+H+]; Anal. Calc.
for C56H88N2O4 (852.7): C, 78.8; H, 10.3; N 3.2. Found: C, 78.1; H, 10.2; N, 3.1%.1H
NMR (400 MHz, CDCl3): δ=13.01 (s, 1 H, H5), 8.77 (s, 1 H, H4), 7.74 (d,
J=8.5 Hz, H9), 7.20 (d, 2 H, H6), 7.12 (t, J=8.4 Hz, 2 H, H1), 7.13 (dd, J=2.3 Hz,
J=9.1, 2 H, H8), 6.61 (d, J=2.4 Hz, 2 H, H3), 6.49 (dd, J=2.4 Hz, J=8.29 Hz, 2 H,
H2), 3.97 (t, J=6.8 Hz, 2 H, ―OCH2), 0.91 (t, J=6.8 Hz, 6 H, CH3), 0.87 (m, ―CH2
of methylene proton in side chain); IR (νmax, cm−1, KBr): 3510 (νOH), 2922 (νas
(C―H),CH3), 2920 (νas(C―H), CH2), 2871 (νs(C―H),CH3), 2849 (νas(C―H), CH2),
1627 (νC=N), 1295 (νC―O). Synthesis of mononuclear oxovanadium(IV)
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