Synthesis and spectral studies of the mesogenic Schiff-base, N,N′-di-(4′-pentyloxybenzoate) salicylidene-1,8-diamino-3,6-dioxaoctane and crystal structure of the Zn(II) complex

Article abstract of DOI:10.1016/j.poly.2007.09.002

A novel Schiff-base, N,N′-di-(4′-pentyloxybenzoate)salicylidene-1,8-diamino-3,6-dioxaoctane (H₂L₄) with the mesogenic phase, nematic droplets, was prepared and its structure studied by elemental analyses and mass, NMR and IR spectra. The di-negative tetra-dentate bonding of the Schiff-base in the non-mesogenic complex, [ZnL₄], as implied on the basis of IR and NMR spectral data, was confirmed in the crystal structure of the monomeric tetrahedral Zn(II) complex. As per the crystal structure of the complex, the dinegative species of the ligand, L₄^{2 -}, coordinates to the Zn(II) ion through two phenolate oxygens and two azomethine nitrogens, rendering the overall geometry around Zn(II) to distorted tetrahedron.

Full text of DOI:10.1016/j.poly.2007.09.002

Available online at www.sciencedirect.com
Polyhedron 27 (2008) 181–186
www.elsevier.com/locate/poly
Synthesis and spectral studies of the mesogenic Schiff-base,
N,N0-di-(40-pentyloxybenzoate) salicylidene-1,8-diamino-3,6-
dioxaoctane and crystal structure of the Zn(II) complex
a
a
b
b
a,
*
Angad Kumar Singh , Sanyucta Kumari , K. Ravi Kumar , B. Sridhar , T.R. Rao
a
Department of Chemistry, Banaras Hindu University, Varanasi 221 005, India
b
Indian Institute of Chemical Technology, Hyderabad, India
Received 27 June 2007; accepted 5 September 2007
Available online 18 October 2007
Abstract
A novel Schiff-base, N,N0-di-(40-pentyloxybenzoate)salicylidene-1,8-diamino-3,6-dioxaoctane (H2L4) with the mesogenic phase, nema-
tic droplets, was prepared and its structure studied by elemental analyses and mass, NMR and IR spectra. The di-negative tetra-dentate
bonding of the Schiff-base in the non-mesogenic complex, [ZnL4], as implied on the basis of IR and NMR spectral data, was confirmed in
the crystal structure of the monomeric tetrahedral Zn(II) complex. As per the crystal structure of the complex, the dinegative species of
the ligand, L24ꢀ, coordinates to the Zn(II) ion through two phenolate oxygens and two azomethine nitrogens, rendering the overall geom-
etry around Zn(II) to distorted tetrahedron.
ꢀ 2007 Elsevier Ltd. All rights reserved.
Keywords: Mesogenic Schiff-base; Tetrahedral zinc(II) complex; Crystal structure; NMR spectra
1. Introduction
corresponding Zn(II) complex. We have observed that
despite the mesogenic behaviour of the [CuL4] (unpub-
lished work), the zinc homologue, [ZnL4], does not exhibit
liquid crystalline properties presumably due to the tetrahe-
dral coordination of the latter [2].
Liquid crystals with transition metal core groups are
increasingly a topic of investigation since metals can impart
useful shapes and properties which are not easily produced
in totally organic liquid crystals [1]. A major distinction
between metallomesogens and most organic mesogens is
the greater tendency in the former type to exhibit intermo-
lecular dative coordination in the solid state [1]. However,
it should be noted that rigid tetrahedral molecules usually
seem to prevent mesophase formation [2]. As a part of
our investigation on systematic structural and spectro-
scopic studies of 3d metal complexes of a series of meso-
genic organic Schiff-bases, we report here, synthesis and
spectroscopic studies on the mesogenic Schiff-base, N,
N0-di-(40-pentyloxybenzoate)salicylidene-1,8-diamino-3,6-
dioxaoctane (H2L4) and crystal structure of the
2. Experimental
2.1. Materials
All reagents were purchased from the following and
used as received: 4-hydroxy benzoic acid, 1-bromopentane,
2,4-dihydroxy benzaldehyde, N,N0-dicyclohexylcarbodii-
mide (DCC), N,N-dimethylaminopyridine (DMAP) and
1,8-diamino-3,6-dioxaoctane from Sigma–Aldrich, USA,
and Zn(OAc)2 Æ 2H2O and KOH from Merck. The solvents
received from commercial sources were used as such with-
out further purification and were dried using standard
methods [3] when required.
*
Corresponding author. Tel.: +91 542 230726; fax: +91 542 2368127.
E-mail address: drtrrao@gmail.com (T.R. Rao).
0277-5387/$ - see front matter ꢀ 2007 Elsevier Ltd. All rights reserved.
doi:10.1016/j.poly.2007.09.002

182
A.K. Singh et al. / Polyhedron 27 (2008) 181–186
2.2. Synthesis
50 mL), DCC (11.35 g, 55 mmol in 100 mL) in dry
chloroform along with solid DMAP (0.3 g, 2.5 mmol as a
catalyst) were magnetically stirred at room temperature
for ꢁ12 h. The byproduct (dicyclohexyl urea) was filtered
off under suction and the solvent was removed on rotava-
por. The crude product was recrystallized from hot
solution of ethanol and purified by column chromatogra-
phy over SiO2 by eluting with a mixture of n-hexane and
chloroform (v/v, 1:1); evaporation of this eluent yielded
the ester, 2, in the form of a white solid. Yield: 10.17 g
(62%).
The synthesis of N,N0-di-(40-pentyloxybenzoate)-
salicylidene-1,8-diamino-3,6-dioxaoctane (H2L4) (3), was
achieved by proceeding through three major steps, viz.,
alkylation of 4-hydroxy benzoic acid with n-pentyl bro-
mide, followed by esterification and Schiff-base formation;
all the experimental details are given in Scheme 1.
2.2.1. Preparation of 4-pentyloxybenzoic acid (1)
Solutions of 4-hydroxybenzoic acid (11.05 g, 80 mmol)
in dry ethanol (150 mL) and of KOH (8.97 g, 160 mmol)
in dry ethanol (50 mL) were magnetically stirred with
simultaneous drop-wise addition of 1-bromopentane
(10.00 mL, 80 mmol). The reaction mixture was refluxed
for ꢁ14 h under dry atmosphere and allowed to come to
room temperature. The solid alkoxy potassium salt thus
obtained was separated out by filtration under suction
and treated with dilute HCl until the pH of the reaction
mixture reached to ꢁ2. The crude solid white product
was filtered off, washed thoroughly with water and recrys-
tallized successively from solutions of glacial acetic acid
and toluene. Yield: 11.65 g (70%).
2.2.3. Synthesis of N,N0-di-(40-pentyloxybenzoate)-
salicylidene-1,8-diamino-3,6-dioxaoctane (3)
Absolute ethanolic solutions of 4-pentyloxy-(40-formyl-
30-hydroxy)benzoate (6.56 g, 20 mmol, in 100 mL) and
1,8-diamino-3,6-dioxaoctane (1.50 mL, 10 mmol in
20 mL) were refluxed for ꢁ4 h in presence of a few drops
of acetic acid and the resultant solution was left over-night
in the reaction flask at room temperature. The micro-crys-
talline yellow coloured product, 3, was suction-filtered,
thoroughly washed with ethanol, recrystallized from a
solution of absolute ethanol/chloroform (v/v, 1/1) and
dried at room temperature. Yield: 4.61 g (60%); m.p.,
110 ꢁC. 1H NMR (300 MHz, CDCl3, 25 ꢁC, TMS):
2.2.2. Preparation of 4-pentyloxy (40-formyl-30-hydroxy)-
benzoate (2)
Solutions of 4-pentyloxy benzoic acid (10.40 g, 50 mmol
in 50 mL), 2,4-dihydroxybenzaldehyde (6.90 g, 50 mmol in
3
d = 0.945 (t, JH–H = 6.9 Hz, 3H, CH3), 1.847–1.390
(m, 6H, Hmethylene), 3.614 (s, 2H, H6 ), 3.721 (s, 4H,
00
00 00
000
H3 ,4 ), 4.035 (t, JH–H = 6.6 Hz, 2H, H1 ), 6.720 (dd,
3
Ethanol, KOH, Reflux, 14h
dry atmosphere; 6 N HCl / H3O+
H2O
C5H11Br
KBr
C5H11O
COOH
HO
COOH
1
DCC, DMAP
C5H11O
COOH
HO
CHO
Dry CHCl3, Stirring ~12 h at RT
OH
O
O
C5H11O
CHO
H2O
2
OH
O
O
C5H11O
EtOH, AcOH
CHO
O
H2N
O
O
NH2
Reflux ~4 h, dry atmosphere
OH
2'
5'
O
OH
1'
3'
4'
2'
HO
O
O
3'
4'
1'
2''
N
8''
O
4''
6''
10''
1
1''
6'
11''
1
6
5
12''
N
2
3
6'
O
3''
7''
9''
6
5
5''
2
3
5'
H
H
4
4
4"'
2"'
O
4"'
2"'
3
O
5"'
3"'
1"'
5"'
1"'
3"'
Scheme 1. Major reaction steps involved in the synthesis of N,N0-di-(40-pentyloxy-benzoate)salicylidene-1,8- diamino-3,6-dioxaoctane (3) (H2L4).

A.K. Singh et al. / Polyhedron 27 (2008) 181–186
183
0
4
4
3JH–H = 6.3 Hz, JH–H = 2.1 Hz, 1H, H6 ), (d, JH–H
=
minary lattice parameters and orientation matrices were
obtained from four sets of frames. Unit cell dimensions
were determined from the setting angles of 7224 reflections
in the range of 2.31ꢁ < h < 24.66ꢁ. Integration and scaling
of intensity data was accomplished using SAINT program
[4]. The structure was solved by direct methods using
SHELXS97 [5] and refinement was carried out by full-matrix
least-squares technique using SHELXL97 [5]. Anisotropic
displacement parameters were included for all non-hydro-
gen atoms. All other hydrogen atoms were positioned
geometrically and treated as riding atoms, with C–H
0
1.8 Hz, 1H, H2 ), 6.970 (d, JH–H = 8.7 Hz, 1H, H3),
3
0
7.277 (d, JH–H = 8.4 Hz, 1H, H5 ), 8.126 (d, JH–H
=
3
3
9.0 Hz, 1H, H2), 8.324 (s, 1H, N@CH), 13.813 (s, 1H,
/-OH) ppm. FAB Mass: the molecular ion (m/e,
769; 75% intensity) generates simultaneously four
fragments, M1–M4, (m/e, fragment, % intensity): M1:
398,
C5H11OC6H4COOC6H3ðOHÞCH@NðCH2Þ2OCH2-
CHþ, 15; M2: 370, C5H11OC6H4COOC6H3(OH)CH@
2
N(CH2)2O+, 11%; M3: 340, C5H11OC6H4COOC6H3ðOHÞ-
CH@NCHþ, 12; M4: 191, C5H11OC6H4CO+, 100; M4
0
2
(generated from M4): 121, HOC6H4CO+, 97. IR (CCl4,
distances in the range of 0.93–0.96 A and with Uiso(H)
˚
cmꢀ1): m(O–H)phenol 3450b, m(>C@O) 1742, m(C@N)
1633, m(C–O)phenol 1248, m(C(O)O) 1149; (KBr, cmꢀ1):
m(O–H)phenol 3439b, m(>C@O) 1722, m(C@N) 1628,
m(C–O)phenol 1246, m(C(O)O) 1145. Anal. Calc. for
C44H52N2O10(768.0): C, 68.75; H, 6.77; N, 3.65. Found:
C, 68.62; H, 6.50; N, 3.62%.
values of 1.5Ueq(C) for methyl hydrogens and 1.2Ueq(C)
for other hydrogen atoms. The geometries of the atoms
C17, C18 and C19 were restrained, where distances
˚
C17–C18@C18–C19 were set to the target value, 1.55 A.
3. Results and discussion
2.2.4. Synthesis of the zinc complex, [ZnL4]
3.1. Spectral investigation
Anhydrous solutions of N,N0-di-(40-pentyloxybenzo-
ate)salicylidene-1,8-diamino-3,6-dioxaoctane (3), (0.77 g,
1.0 mmol) in dichloromethane (30 mL) and of zinc acetate
(0.22 g, 1.0 mmol) in ethanol (20 mL) were refluxed for
ꢁ6 h and the reaction mixture left over-night in the reac-
tion flask after reducing the volume to ꢁ10 mL. The crude
solid complex, [ZnL4], was filtered off under suction,
washed repeatedly with cold ethanol, recrystallised from
the solution of chloroform/ethanol and dried over fused
CaCl2 in a desiccator. Yield: 0.68 g (82%); m.p. 210 ꢁC.
1H NMR (300 MHz, CDCl3, 25 ꢁC, TMS): d = 0.947 (t,
The Schiff-base ligand (H2L4), 3, is yellow-coloured
while the Zn(II) complex, [ZnL4] is colourless. Both
Table 1
Crystal data and structure refinement for aq74m, [ZnL4]
Empirical formula
Formula weight
Cell axes
C44H50N2O10Zn
832.23
˚
a (A)
8.7660 (6)
41.322(3)
11.5379(8)
˚
b (A)
3JH–H = 6.9 Hz, 3H, CH3), 1.850–1.392 (m, 6H, Hmethylene),
˚
c (A)
Cell angles
000
3
3
4.040 (t, JH–H = 6.9 Hz, 2H, H1 ), 6.973 (d, JH–H
=
0
8.1 Hz, 1H, H3), 7.185 (d, JH–H = 8.4 Hz, 1H, H5 ),
3
a (ꢁ)
b (ꢁ)
90.000 (0)
99.358 (1)
8.133 (d, JH–H = 7.8 Hz, 1H, H2), 8.197 (s, 1H, N@CH)
3
ppm. IR (KBr, cmꢀ1): m(>C@O) 1730, m(C@N) 1535,
m(C–O)phenol 1253, m(C(O)O) 1151. Anal. Calc. for
C44H50N2O10Zn (832.23): C, 63.50; H, 6.06; N, 3.37; Zn,
7.86. Found: C, 63.27; H, 6.01; N, 3.22; Zn, 7.79%.
c (ꢁ)
90.000 (0)
4123.8 (5)
3
˚
Cell volume V (A)
Crystal system
Crystal size
monoclinic
0.22 · 0.17 · 0.09 mm
Temperature (K)
Wavelength (A)
294(2)
0.71073
˚
Space group
F(000)
P21=n
1752
1.340
4
2.3. Physical measurements
Density (Mg/m3)
No. form Unit Z
Infrared spectra were recorded on a JASCO-5300 FT
IR spectrophotometer and the H and 13C NMR spectra
1
Absorption coefficient (mmꢀ1
Reflections collected/unique
Completeness to h = 25.00
Absorption correction
Refinement method
Data/restraints/parameters
Hmin,max
)
0.656
39614/7261 [R(int) = 0.0251]
100.0%
were recorded on a JEOL AL300 FT NMR spectrometer.
Mass spectra were recorded on JEOL SX-102 (EI/CI/
FAB) mass spectrometers. C, H, and N were micro-ana-
lyzed on Elementar Vario EL III Carlo Erba 1108
analyzer.
none
full-matrix least-squares on F2
7261/2/515
ꢀ10, 10
Kmin,max
Lmin,max
ꢀ49, 49
ꢀ13, 13
2.4. X-ray crystallographic data collection and refinement of
the structure
Hmin,max
0.99, 25.0
Goodness-of-fit on F2
Final R indices
1.193
[I > 2r(I)] R1 = 0.0468,
wR2 = .1259
R1 = 0.0540, wR2 = 0.1338
0.682 and ꢀ0.553
X-ray data for the compound ZnL4, [aq74m], was col-
lected at room temperature using a Bruker Smart Apex
CCD diffractometer with graphite monochromated Mo
R indices (all data)
Largest diffraction peak and hole
ꢀ3
˚
(e A
)
˚
Ka radiation (k = 0.71073 A) with x-scan method. Preli-

184
A.K. Singh et al. / Polyhedron 27 (2008) 181–186
the parent ligand and the metal complex are soluble in
chloroform, dichloromethane, DMF and DMSO but are
insoluble in water, methanol, ethanol, and acetonitrile.
The structures and purities of the Schiff-base ligand, 3,
and of the metal complex, [ZnL4], were confirmed by IR
H2L4 the broad absorption centered on 3439 and
3450 cmꢀ1, in the solid and solution states, respectively,
may be assigned to the O–H stretching vibration of the
phenolic group [7]. The strong intensity band occurring
at 1633 cmꢀ1 may be assigned [8] to m(C@N) absorption
of the azomethine moiety. The strong bands occurring at
1722 cmꢀ1 and 1145 cmꢀ1 in the solution state of the par-
ent ligand may be assigned [7] to m(>C@O) and m(C–O)
stretching modes of aromatic ester. The characteristic
response of ether in the infrared is associated with the
stretching vibration of the C–O–C system. The bands
appearing at 1246 and 1064 cmꢀ1 may be assigned [7] to
mas(C–O–C) and ms(C–O–C) stretching of aryl alkyl ether.
Bonding of the parent ligand with the metal ions through
phenolate oxygen and azomethine nitrogen was inferred
[8] on the basis of (i) disappearance of the m(O–H)
(phenolic) band, and (ii) bathochromic shifts observed in
the m(˜C@N) frequencies in the spectra of the metal
complexes.
1
and H NMR spectroscopy and elemental analyses. The
structure of the ligand was further confirmed by FAB Mass
spectrum. The mass spectral features of H2L4 were charac-
terized by the base peak (m/e = 191 corresponding to the
fragment, C5H11OC6H4CO+), the molecular ion peak with
ꢁ75% intensity (m/e = 769) and the major fragment
peaks (m/e = 398, 370, 340, 121) due to C5H11OC6-
H4COOC6H3ðOHÞCH@NðCH2Þ OCH2CHþ, C5H11OC6-
2
2
H4COOC6H3(OH)CH@N(CH2)2O+, C5H11OC6H4COO-
C6H3ðOHÞCH@NCHþ and HOC6H4CO+. A comparison
2
1
of the H and 13C{1H} NMR spectral data of the ligand
with those of the Zn(II) complex shows the absence of
the phenolic–OH signal (13.813 ppm in H2L4) and a signif-
icant shift (8.324–8.197 ppm) in the peak position of the
–N@CH in the spectrum of the metal complex, which
implies bonding through the phenolate anion and the
azomethine nitrogen atom [6] of the ligand to the metal
ion. Similar shifts observed in the 13C{1H} NMR
spectra were of considerable magnitude in the case of the
carbon atoms directly attached to the bonding atoms
(phenolate–O and azomethine–N) while those observed
for the carbons close to the bonding atoms were of lesser
magnitude. Thus, the NMR spectral data imply tetra-
dentate bonding of H2L through the two ˜C@N groups
and two phenolate oxygen atoms. In the IR spectrum of
3.2. Crystal structure and molecular association
The crystal data and structure refinement details for
[ZnL4], [aq74m], are given in Table 1. The molecular struc-
ture and atomic labeling scheme of the complex, [ZnL4], is
as shown in Fig. 1.
As per the crystal structure of the complex, the
ligand, H2L4, coordinates to the Zn(II) ion through
two phenolate oxygens (O(1), O(5)) and two azome-
thine nitrogens (N(1), N(2)), rendering the overall
Fig. 1. Molecular structure and atomic labeling scheme of the Complex [ZnL4].

A.K. Singh et al. / Polyhedron 27 (2008) 181–186
185
Table 2
4. Conclusion
˚
Bond lengths (A) and angles (ꢁ) involved in the tetrahedral unit of the
complex [ZnL4]
The Schiff-base, N,N0-di-(40-pentyloxybenzoate)salicy-
lidene-1,8-diamino-3,6-dioxaoctane (H2L4) with the meso-
genic phase, nematic droplets, coordinates to Zn(II) as a
di-negative tetra-dentate species, to yield the non-meso-
genic tetrahedral complex, [ZnL4]. As per the crystal struc-
ture of the complex, the dinegative species of the ligand,
L24ꢀ, coordinates to the Zn(II) ion through two phenolate
oxygens and two azomethine nitrogens, rendering the over-
all geometry around Zn(II) to distorted tetrahedron.
N(1)–Zn(1)
O(1)–Zn(1)
O(1)–Zn(1)–O(5)
O(5)–Zn(1)–N(2)
O(5)–Zn(1)–N(1)
2.003(2)
1.9146(19)
121.12(9)
96.78(8)
N(2)–Zn(1)
O(5)–Zn(1)
O(1)–Zn(1)–N(2)
O(1)–Zn(1)–N(1)
N(2)–Zn(1)–N(1)
1.994(2)
1.9213(18)
107.78(9)
95.86(9)
111.10(9)
126.37(9)
Table 3
˚
Hydrogen bonds geometry (A, ꢁ) for the complex [ZnL4]
D–H. . .A
d(D–H)
0.93
0.97
d(H. . .A)
2.53
2.47
d(D. . .A)
3.232(14)
3.350(4)
d(D. . .A)
132
151
C(30)–H(30). . .O2i
C(39)–H(39B). . .O10i
Acknowledgements
The authors wish to acknowledge the recording of FAB
Mass spectra and elemental analyses at the Central Drug
Research Institute, Lucknow. One of the authors, Prof.
T.R. Rao, gratefully acknowledges the financial grant re-
ceived from the Council of Scientific & Industrial Research,
New Delhi [vide Grant No. 01(1834)/03/EMR-II dated 12-
03-2003] and the University Grants Commission [vide
Grant No. F.12-26/2003 (SR) dated 31 Mar 2003].
Symmetry code: (i) 1/2 + x, 1/2 ꢀ y, 1/2 + z.
geometry around Zn(II) to distorted tetrahedron where
the bond lengths and the bond angles are as shown in
Table 2. The hydrogen bond geometry is tabulated
in Table 3. The crystal structure is stabilized by
intermolecular C–Hꢂ ꢂ ꢂO interactions which exist
between the atom C(30) of the benzoate ring and the
carbonyl oxygen atom O(3). Similarly, the aliphatic
atom C(39) forms an intermolecular C–Hꢂ ꢂ ꢂO interac-
tion with the ethereal oxygen atom O(10), thereby
forming an infinite glide-related chain running along
crystallographic c-axis. The packing diagram is depicted
in Fig. 2. Considerable torsion has been observed
between benzoate ring and salicylaldimine ring on
either side of the metal atom.
Appendix A. Supplementary material
CCDC 649423 contain the supplementary crystallo-
graphic data for this paper. These data can be obtained free
of charge via http://www.ccdc.cam.ac.uk/conts/retriev-
ing.html, or from the Cambridge Crystallographic Data
Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax:
(+44) 1223-336-033; or e-mail: deposit@ccdc.cam.ac.uk.
Supplementary data associated with this article can be found,
in the online version, at doi:10.1016/j.poly.2007.09.002.
Fig. 2. Part of the crystal structure for [ZnL4], viewed down a-axis. Dashed lines indicate C–Hꢂ ꢂ ꢂO interaction. H atoms not involved in hydrogen bonding
have been omitted for clarity.

186
A.K. Singh et al. / Polyhedron 27 (2008) 181–186
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