Crystal structures and mesomorphic properties of Schiff base homologs and derivatives, and magnetic properties of their dimeric and dinuclear copper(II) complexes

Article abstract of DOI:10.1080/15421406.2015.1044695

Three N₂O₄ Schiff. base homologs, H₂L1 (n = 8), H₂L3 ((n = 9), and H₂L4 (n = 10) were obtained from the reactions of 2-HOC₆H₄CHO with H₂N(CH₂)_8-10

Full text of DOI:10.1080/15421406.2015.1044695

Molecular Crystals and Liquid Crystals
ISSN: 1542-1406 (Print) 1563-5287 (Online) Journal homepage: http://www.tandfonline.com/loi/gmcl20
Crystal structures and mesomorphic properties
of Schiff base homologs and derivatives, and
magnetic properties of their dimeric and dinuclear
copper(II) complexes
Norbani Abdullah, Yanti Yana Halid, Tee Jia Ti & Afiq Azil
To cite this article: Norbani Abdullah, Yanti Yana Halid, Tee Jia Ti & Afiq Azil (2016) Crystal
structures and mesomorphic properties of Schiff base homologs and derivatives, and
magnetic properties of their dimeric and dinuclear copper(II) complexes, Molecular Crystals
and Liquid Crystals, 624:1, 132-143
To link to this article: http://dx.doi.org/10.1080/15421406.2015.1044695
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Date: 19 February 2016, At: 03:36

MOL. CRYST. LIQ. CRYST.
, VOL. , NO. , –
http://dx.doi.org/./..
Crystal structures and mesomorphic properties of Schiff base
homologs and derivatives, and magnetic properties of their
dimeric and dinuclear copper(II) complexes
Norbani Abdullah, Yanti Yana Halid, Tee Jia Ti, and Afiq Azil
Department of Chemistry, Faculty of Science, University of Malaya, Kuala Lumpur, Malaysia
ABSTRACT
KEYWORDS
Copper(II) complexes;
crystals; magnetism;
mesomorphism; Schiff bases
Three N2O Schiff. base homologs, H2L1 (n = 8), H2L3 ((n = 9), and
H2L4 (n =4 10) were obtained from the reactions of 2-HOC6H4CHO
with H2N(CH2)8-10NH2, while a derivative of H2L1, namely, H2L2, was
obtained from the reaction of 3,5-X2-2-HOC6H2CHO (X = tert-butyl) with
H2N(CH2)8NH2. The Schiff bases H2L2 (triclinic; P-1), H2L3 (monoclinic;
C2/c), and H2L4 (monoclinic; P21/c) were single crystals with low melt-
ing temperatures (less than 100°C). The homologs exhibited mesomor-
phisms, while the derivative was not mesomorphic and did not reform
crystals from its melt. Copper(II) complexes of H2L1, H2L2 and H2L4 were
dimeric, while that of H2L3 was dinuclear with chelating CH3COO lig-
and. Thesecomplexeswereparamagneticwithinsignificantinteractions
between the copper(II) atoms and have high decomposition temper-
atures (Tdec = 268–304°C). Their melting temperatures (Tmelt = 138.9–
190.2°C) were higher than the corresponding Schiff bases, but they were
not mesomorphic.
Introduction
Schiff bases formed from polymethylenediamines with suitably selected carbonyls were
reported to form metal complexes exhibiting tunable nuclearity, geometry, and magnetic
properties [1–11]. As examples, Nathan et al. in 2003 [1] obtained crystals of [Cu(Sal-n)]
(H2Sal-n were Schiff bases formed from H2N(CH2)2-6NH2 and 2-HOC6H4CHO). The struc-
tures of these complexes were either dimeric cis planar (n = 2), monomeric tetrahedral (n =
3, 4), or dimeric trans planar (n = 5, 6). Later in 2009, Mitra et al.[8] reported an almost lin-
ear trinuclear Cu(II) complex, [Cu3(μ-L)2(ClO4)2][Cu3(μ-L)2(H2O)(ClO4)2] (H2L was Schiff
base formed from 1,3-diaminopentane and 2-HOC6H4CHO), which showed strong intra-
trimer (J ∼ −202 cm−1) but very weak intertrimer (zJ ∼ −0.1 cm−1) antiferromagnetic inter-
actions. The accumulated knowledge from such works has made it easier to design functional
materials with tunable properties.
Our research is focused on thermally stable molecular magnetic materials. The
impetus for the work presents in this paper arose from molecular structure of 2,2ꢀ-
[nonane-1,9-diylbis(nitrilomethylidyne)]diphenol (H2L3), formed from H2N(CH2)9NH2
and 2-HOC6H4CHO [12]. This paper reports the syntheses, characterizations, magnetic,
CONTACT Norbani Abdullah
norbania@um.edu.my
Universiti of Malaya,  Kuala Lumpur, Malaysia.
Norbani Abdullah, Department of Chemistry, Faculty of Science,
Color versions of one or more of the figures in the article can be found online at www.tandfonline.com/gmcl.
©  Taylor & Francis Group, LLC

MOL. CRYST. LIQ. CRYST.
133
thermal, and mesomorphic properties of: (a) two homologs of H2L3, namely H2L1 (n =
8) and H2L4 (n = 10). The molecular structure of H2L4 was deduced from single-crystal
X-ray crystallography; (b) a derivative of H2L1, labeled as H2L2, formed as single crystals
from H2N(CH2)8NH2 and 3,5-X2-2-HOC6H2CHO (X = tert-butyl); and (c) complexes 1–
4, formed from the reactions of H2L1-H2L3 with [Cu2(CH3COO)4(H2O)2], and H2L4 with
[Cu2(CH3(CH2)14COO)4], respectively. These Schiff bases were of interest since their two
N,O-donors were separated by a long and linear alkyl chain (Scheme 1), and hence may form
multinuclear and/or ferromagnetic complexes. Accordingly, the main objectives of this work
were to study the effect of increasing the carbon chain length and substituent (tert-butyl)
on the structure, magnetic interaction, thermal stability and mesomorphisms of these com-
pounds. Copper(II) ion was specially selected in this study as it can forms geometrically labile
magnetic complexes in order to accommodate the steric dictate of these ligands.
Scheme . (a) Structural formulas of: (a) Schiff bases HL (n = ) and HL (n = ); and (b) HL.
Experimental
Reagents
All chemicals were analar grade and used without further purification.
Physical measurements
1H-NMR spectra were recorded in CDCl3 on a JEOL FT-NMR Lambda 400 MHz spectrom-
eter. Elemental analyses (CHN) were performed on a Perkin-Elmer 2400 Series II elemental
analyzer. IR spectra were recorded as KBr discs from 4000–400 cm−1 on a Perkin-Elmer RX 1
spectrophotometer. Electronic spectra were recorded on a Shimadzu UV-vis-NIR 3600 spec-
trometer. Single-crystal X-ray diffraction data were collected at 293(2) K on a Bruker SMART
APEX II CCD fitted with MoKα radiation so that θmax was 27.5°. The data set was corrected
for absorption based on multiple scans [13] and reduced using standard methods [14]. The
structures were solved by direct methods with SHELXS97 [15] and refined by a full-matrix
least-squares procedure on F2 using SHELXL97 with anisotropic displacement parameters for
2
non-hydrogen atoms and a weighting scheme of the form w = 1/[σ2(Fo ) + aP2 + bP], where
2
2
P = (Fo + 2Fc )/3). All hydrogen atoms were included in the final refinement in their calcu-
lated positions. Magnetic measurements at room temperature were performed on a Sherwood
scientific magnetic susceptibility balance, calibrated with Hg[Co(SCN)4]. The diamagnetic
corrections were made using Pascal’s constants. Temperature dependence of molar suscepti-
bility for 3 was carried out on a Quantum Design MPMS-7 SQUID magnetometer at a con-
stant field of 2000 Oe. Thermogravimetry (TG) was performed in the temperature range of
325–1173 K and scan rate of 20 K min−1 on a Perkin-Elmer Pyris Diamond TG/DTA thermal

134
NORBANI ABDULLAH ET AL.
Table . Crystal data and structure refinement for HL and HL.
Molecule
HL
HL
Chemical formula
Formula weight (g mol−
T (K)
CHNO
.
()
CHNO
.
()
)
˚
λ (A)
.
.
Crystal system
Space group
Triclinic
P-
Monoclinic
P/c
.()
.()
.()

.()

.()

.
.

. –.
− ࣘ hࣘ 
− ࣘ k ࣘ 
− ࣘ l ࣘ 

˚
a (A)
.()
.()
.()
.()
.()
.()
.()

˚
b (A)
˚
c (A)
α (o)
β (o)
γ (o)

˚
V (A )
Z
Dcalc (g cm−
)
.
.

Absorption coefficient (mm−
F()
)
θ range (o)
.–.
− ࣘ h ࣘ 
− ࣘ k ࣘ 
− ࣘ l ࣘ 

Index ranges
Reflections collected
Independent reflections
Data/restraints/parameters
Goodness-of-fit on F
 (.)
//
.
R = .
wR = .
R = .,
wR = .
., −.
 (.)
//
.
R = .
wR = .
R = .,
wR = .
., −.
Final R indices [I ࣙ .σ(I)]
R indices (all data)
ꢀρmax, ꢀρmin (e A
−
˚
)
instrument under N2 at a flow rate of 20 cm3 min−1. Differential scanning calorimetry (DSC)
was carried out on a Mettler Toledo DSC 822 instrument in the range 25–150°C under N2 at
a flow rate of 20 cm3 min−1 and scan rate of 10°C min−1. The onset temperatures were quoted
for all peaks observed. Polarizing optical microscopy (POM) was carried out on an Olympus
polarizing microscope equipped with a Mettler Toledo FP90 central processor and a Linkam
THMS 600 hot stage. The heating and cooling rates were 10°C and 1°C min−1, respectively,
and the magnification was 50×.
Materials
The synthesis and molecular structure of 2,2’-[nonane-1,9-diylbis(nitrilomethylidyne)]
diphenol (H2L3) were as previously reported [12].
,ꢀ-[Octane-,-diylbis(nitrilomethylidyne)]diphenol (HL)
–
A mixture of H2(NH2)8NH2 (1.44 g, 10.00 mmol), 2 HOC6H4CHO (2.44 g, 20.00 mmol) and
a few drops of glacial acetic acid in ethanol (100 cm3) was refluxed for 1 hr. The yellow powder
obtained from the cooled reaction mixture was filtered, washed with ethanol and left to dry
at room temperature. Yield 88.5%. Anal. Calc. for C22H28N2O2: C, 74.97; H, 8.01; N, 7.95;
Found: C, 74.95; H, 8.13; N, 8.26. 1H-NMR (400 MHz, CDCl3) δ 13.6 (s, 2H), 8.2 (s, 2H), 7.2
–
(m, 4H), 6.8 (m, 4H), 3.5 (t, 4H), 1.6 (m, 4H), 1.2 (m, 8H). IR (KBr): ν = 2919, 2849 (C H
−1
=
=
–
aliphatic), 1630 (C N, imine), 1608 (C C, aromatic), 1050 (C O, phenolic) cm .

MOL. CRYST. LIQ. CRYST.
135
o
˚
Table . Selected bond lengths (A) and angles ( ) for HL and HL.
Molecule
HL
HL
Bond length
O()-C()
O()-H(A)
N()-C()
N()-C()
.()
.
.()
.()
.()
.
.()
.()
Angle
C()-O()-H(A)
C()-N()-C()
O()-C()-C()
O()-C()-C()
N()-C()-C()
N()-C()-H(A)
N()-C()-C()
N()-C()-H(A)
N()-C()-H(B)
.
.()
.()
.()
.()
.
.()
.
.
.
.()
.()
.()
.()
.
.()
.
.
,ꢀ-[Octane-,-diylbis(nitrilomethylidyne)]-,ꢀ,,ꢀ-tetra(tert-butyl)diphenol (HL)
The procedure was the same as H2L1, replacing 2-HOC6H4CHO with 3,5-X2-2-
HOC6H2CHO (X = tert-butyl). The product was yellow crystals. Yield 90.1%. Anal.
Calc. for C38H60N2O2: C, 79.11; H, 10.48; N, 4.86; Found: C, 79.6; H, 11.1; N, 4.9. 1H-NMR
(400 MHz, CDCl3) δ 8.3 (s, 2H), 7.4 (d, 2H), 7.1 (d, 2H), 3.5 (t, 4H), 2.2 (s, 2H), 1.7 (m, 4H),
–
=
1.3 (m, 44H).ν = 2959, 2932, 2896, 2860, 2849 (C H aliphatic), 1634 (C N, imine), 1594
−1
=
–
(C C, aromatic), 1054 (C O, phenolic) cm .
,’-[Decane-,-diylbis(nitrilomethylidyne)]diphenol (HL)
The procedure was the same as H2L1, replacing H2(NH2)8NH2 with H2(NH2)10NH2. The
product was small yellow crystals. Yield 74.2%. Anal. Calc. for C24H32N2O2: C, 75.75; H, 8.48;
1
N, 7.36; Found: C, 75.5; H, 8.8; N, 7.3. H-NMR (400 MHz, CDCl3) δ 13.6 (s, 2H), 8.3 (s,
2H), 7.3 (d, 2H), 7.2 (t, 2H), 6.9 (d, 2H), 6.8 (t, 2H), 3.5 (t, 4H), 1.7 (m, 4H), 1.2 (m, 12H).
–
=
=
IR (KBr): ν = 2929, 2851 (C H aliphatic), 1632 (C N imine), 1610 (C C aromatic), 1048
−1
–
(C O, phenolic) cm .
[Cu(L)].HO (1)
An ethanolic solution of H2L1 (0.71 g, 2.00 mmol,) was added portionwise to a hot ethanolic
solution of [Cu2(CH3COO)4(H2O)2] (0.81 g, 1.00 mmol). The reaction mixture was refluxed
Figure . (a) ORTEP view of HL; and (b) its packing diagram viewed along the c-axis (H atoms are omitted
for clarity).

136
NORBANI ABDULLAH ET AL.
Figure . (a) ORTEP view of HL; and (b) its packing diagram viewed along the c-axis [].
for 3 hr, and the khaki-green powder obtained was filtered from the hot reaction mixture and
dried in a warm oven (60°C). Yield 82.7%. Anal. Calc. for C44H54Cu2N4O5: C, 62.47; H, 6.38;
–
N, 6.62; Found: C, 62.73; H, 6.37; N, 6.66. IR (KBr): ν = 3740 (O-H water), 2915, 2850 (C H
−1
=
=
–
aliphatic), 1619 (C N imine), 1598 (C C aromatic), 1145 (C O, phenolic) cm .
[Cu(L)].HO (2)
The procedure was the same as [Cu(L1)]2.H2O, replacing H2L1 with H2L2. The product was a
khaki-green powder. Yield 38.8%. Anal. Calc. for C76H118Cu2N4O5: C, 70.49; H, 9.20; N, 4.33;
–
Found: C, 70.54; H, 9.69; N, 4.4. IR (KBr): ν = 2950, 2928, 2906, 2858 (C H aliphatic), 1618
−1
=
=
–
(C N imine, C C aromatic), 1169 (C O, phenolic) cm .
[Cu(L)(CHCOO)].HO (3)
The procedure was the same as [Cu(L1)]2.H2O, replacing H2L1 with H2L3. The product was
a dark brown powder. Yield 32.4%. Anal. Calc. for C27H38Cu2N2O8: C, 50.22; H, 5.93; N, 4.34;
–
Found: C, 50.1; H, 6.0; N, 5.0. IR (KBr): ν = 3424 (O-H water), 2922, 2852 (C H aliphatic),
−1
=
=
1624, 1614 (C N imine, C C aromatic), 1535 (COO, asym), 1448 (COO, sym) cm .
[Cu(L)] ()
The procedure was the same as [Cu(L1)]2.H2O, replacing H2L1 with H2L4, and
[Cu2(CH3COO)4(H2O)2] with [Cu2(CH3(CH2)14COO)4]. The product was a khaki-green
powder. Yield 67.9%. Anal. Calc. for C48H60Cu2N4O4: C, 65.21; H, 6.84; N, 6.34; Found: C,
–
=
=
65.9; H, 7.0; N, 6.2. IR (KBr): ν = 2919, 2849 (C H aliphatic), 1616 (C N imine), 1598 (C C
−1
–
aromatic), 1147 (C O, phenolic) cm .
Figure . (a) ORTEP view of HL; and (b) its packing diagram, viewed along the c-axis (H atoms are omitted
for clarity).

MOL. CRYST. LIQ. CRYST.
137
Figure . Proposed structural formulas of: (a)  (n = ),  (n = ); and (b) .
Results and discussion
Syntheses and structures of Schiff bases
The Schiff bases H2L1-H2L4 were obtained from the condensation reaction of
H2N(CH2)8-10NH2 with 2-HOC6H4CHO or 3,5-X2-2-HOC6H2CHO (X = tert-butyl).
H2L2 (n = 8; two tert-butyl groups) is a derivative of H2L1, while H2L3 and H2L4 are its
homologs. These Schiff bases were crystalline solids. However, H2L1 (n = 8) lost its crys-
tallinity almost immediately once removed from its crystallizing solvents (EtOH-CHCl3). Its
structure was deduced from combined instrumental data (Experimental) and by comparison
with its derivative, H2L2 (n = 8) and homolog H2L4 (n = 10), which formed stable single
crystals. The crystallographic and structure refinement data for both crystals are given in
Table 1 and selected bond lengths and angles are given in Table 2. The molecular structures
for H2L2-H2L4 are shown in Figs. 1–3 (the molecular structure and packing diagram of H2L3
[12] are reproduced for comparison).
H2L2 was triclinic, and its space group was P-1. The two aromatic rings of each molecule
were almost coplanar to each other, while the two OH groups pointed in the opposite
direction. There were strong intramolecular N-H-O hydrogen bonding interactions, and the
molecules packed so as to optimize the van der Waals interaction between the tert-butyl
groups at the aromatic rings of neighboring molecules.
The molecular structures and intramolecular N-H-O hydrogen bonding interactions of
H2L3 (n = 9) [12] and H2L4 (n = 10) were quite similar to that of H2L2. However, both
molecules were monoclinic, and the space group for H2L3 was C2/c, while that for H2L4 was
P21/c. Finally, the packing of their molecules were quite different. Currently, we are unaware
of single crystal structures of homologs and derivatives of these Schiff bases.
˚
Table . Hydrogen bonds for HL and HL (A and °).
Molecule
HL
D-H …A
D-H
H …A
D …A
(DHA)
O()-H(A) …N()
C()-H(B) …O()#
O()-H(A) …N()
.
.
.
.
.
.
.()
.()
.()
.
.
.
HL

138
NORBANI ABDULLAH ET AL.
Table . Analytical data for –.
λmax/nm
(εmax/M− cm−
Complex
)
μeff/BM
Tdec/oC
[Cu(L)].HO (1)
[Cu(L)].HO (2)
[Cu(L)(CHCOO)].HO (3)
[Cu(L)] (4)
 ()∗
 ()#
 (-)
.
.
.
.
.





 ()#
 nm (-)
[Cu(sal-)] []
∗In THF. #In CHCl. -Solid sample.
Copper(II) complexes of H2L1-H2L4
The Schiff bases H2L1-H2L3 reacted with [Cu2(CH3COO)4(H2O)2] to form [Cu(L1)]2.H2O
(1), [Cu(L2)]2.H2O (2), and [Cu2(L3)(CH3COO)2].2H2O (3), respectively, while H2L4
reacted with [Cu2(CH3(CH2)14COO)4] to form [Cu(L4)]2 (4). In these reactions, the dimeric
paddle-wheel structure of [Cu2(CH3COO)4(H2O)2] and [Cu2(CH3(CH2)14COO)4] com-
pletely dissociated, while the role of carboxylate ion was to deprotonate the Schiff bases, and
to act as a chelating ligand for 3 only.
The proposed structural formulas for these complexes (Fig. 4) are based on a combination
of several analytical data (Table 4). Hence, 1, 2, and 4 were dimeric, similar to [Cu(Sal-8)]2
(H2Sal-8 was the Schiff base formed from H2N(CH2)8NH2 and 2-HOC6H4CHO [1]), while
3 was dinuclear.
−1
– =
The IR spectral data (Experimental) showed the C N peaks at 1630–1635 cm for
H2L1-H2L4, and at 1616-1620 cm−1 for their complexes [16]. The shifts to lower energies indi-
cate coordination of the azomethine nitrogen to copper(II) atom in these complexes. Addi-
tionally, the spectrum for 3 showed peaks for v¯ asCOO at 1535 cm−1 and v¯ sCOO at 1448 cm−1.
The difference (ꢀ) in the two values was 87 cm−1, indicating a chelating CH3COO− ligand
[17].
The electronic absorption spectra of 1, 2, and 4 showed d-d bands at 674, 677, and 600 nm,
respectively. These indicate a square pyramidal geometry at Cu(II)[18] for 1 and 2, and square
planar geometry [19,20] for 4. The spectrum for 3 was recorded for solid sample as it was par-
tially soluble in most solvents. It shows a broad d-d band at 850 nm, suggesting a tetrahedral
geometry at Cu(II) [21]. The geometrical change for Cu(II) atom in these complexes seems to
accord with the molecular structures of their Schiff bases, and reflects an increased flexibility
of the alkyl chain, as similarly suggested by Nathan et. al.[1].
Figure . Plot of χM vs. T for .

MOL. CRYST. LIQ. CRYST.
139
Figure . Plot of χMT vs. T for .
TG was used to ascertain the purity of complexes 1–4, and to determine their thermal
stability. For 1, there was an initial weight loss of 2.7% at 132°C due to loss of lattice H2O (calc.
2.1%). The higher temperature at which this occurred suggests that H2O was H-bonded to the
complex. The next weight loss of 76.5% in the temperature range of 268°C–900°C is assigned
to loss of L12− ligand (calc. 82.9%). However, the trace did not plateau out at temperatures
above 900°C, suggesting incomplete decomposition of the organic ligand.
For 2, there was an initial weight loss of 1.1% from 50°C to 280°C due to loss of lattice H2O
(calc. 1.4%). This was followed by a two-step weight loss of 85.9% in the temperature range
of 280°C–680°C due to loss of L22− ligand (calc. 88.8%). The amount of residue above this
temperature was 13.0% (calc. 12.4%, assuming pure CuO [22–25]).
For 3, there was an initial weight loss of 5.0% in the temperature range of 30°C–300°C due
to loss of lattice H2O (calc. 5.6%). The next weight loss occurred in steps of 18.6%, 23.5%,
and 28.1% in the temperature range of 300°C–900°C due to loss of CH3COO− ion (calc.
18.3%), H2N(CH2)9NH2 molecule (calc. 23.8%), and 2-HOC6H4CHO molecule (calc. 27.6%),
respectively. The amount of residue above this temperature was 24.8% (calc. 24.6%, assuming
pure CuO).
Finally for 4, there was an initial weight loss of 85.4% in the range of 304°C–850°C due to
loss of L42− ion (calc. 85.6%). The amount of residue above this temperature was 14.4% (calc.
18.0%, assuming pure CuO).
These thermal data support indicate high purities of these complexes, and actually sup-
port their proposed structural formulas. The thermal decomposition temperatures were in
the range of 268–304°C, and increasing in the following order: 1 < 2 < 3 < 4. From this, we
infer that the thermal stability of these complexes increased with the number of CH2 group
Table . DSC data for HL-HL and complexes –.
T
(°C)
ꢀH (kJ
mol−
)
Compound
HL
HL
HL
HL
[Cu(L)].HO (1)
[Cu(L)].HO (2)
.
.
.
+.
+.
+.
+.
+.
+.
+.
.
.
.
.
.
[Cu(L)(CHCOO)].HO (3)
+.
+.
.
[Cu(L)] (4)
.∗
+.
∗Two overlapping peaks.

140
NORBANI ABDULLAH ET AL.
in the chain, and for the same number of CH2, was higher for the ligands with alkyl groups at
the aromatic ring.
The room-temperature effective magnetic moments (μeff) for 1–4 were calculated from the
relationship: μeff = [8(χMcorr - Nα)T] , where χM = corrected molar magnetic suscepti-
bility, T = 298 K, and Nα = temperature-independent paramagnetism (60 × 10−6 c.g.s per
Cu(II) ion). The μeff values were 2.1 B.M. for 1, 1.9 B.M. for 2, 2.61 B.M. for 3, and 2.0 B.M. for
4 (the values for 1, 2, and 4 were calculated for the monomers). The experimental values for 1,
2, and 4 were similar to crystals of [Cu(Sal-8] [1], which was dimeric with trans-N2O2 donors
and distorted planar geometry at Cu(II) in the solid state. Hence, we infer similar structure
for 1, 2, and 4. The results also suggest insignificant interaction between the two Cu(II) atoms
in the dimers, a consequent of the large separation between them by the alkyl chain.
In contrast, the μeff value for 3 was higher than the spin-only value expected for a dinuclear
complex (2.45 B.M.) [26]. To probe further the magnetic interaction of 3, its low-temperature
magnetic susceptibility was recorded using SQUID magnetometer in the temperature range
of 5–300 K. The results, shown as plots of χM vs. T and 1/χM vs. T in Fig. 5, and as χMT vs.
T in Fig. 6, indicate that it deviated from purely paramagnetic behavior as the temperature
was lowered, first exhibiting increasing susceptibility with decreasing temperature, and then
a sharp decrease below 10 K due to the zero-field effect. Hence, 3 obeyed the Curie-Weiss law
(Curie constant, C = 16.03 emu K mol−1; Weiss constant, ꢁ = −7.24 K). The small negative
value for the Weiss constant suggests a weak antiferromagnetic interaction between the two
Cu(II) centers, postulated to occur through the lattice H2O [27,28].
½
corr
Mesomorphic properties
The phase transitions for H2L1-H2L4 and their corresponding complexes were studied by
DSC (Table 5), while their optical textures were captured by POM on cooling from the respec-
tive isotropic liquid phase (I).
The DSC of H2L1 (Fig. 7(a)) showed an endothermic peak at 75.6°C (ꢀH = +48.1 kJ
mol−1) for the crystal-to-mesophase transition, and an exothermic peak at 43.9°C (ꢀH =
−40.4 kJ mol−1) for the mesophase-to-crystal transition. Hence, the liquid strongly super-
cooled. Its POM showed a smectogenic optical texture at 78.0°C (Fig. 8(a)).
In contrast, its derivative H2L2 showed an endothermic peak at a higher temperature of
89.7°C (ꢀH = +61.3 kJ mol−1) for crystal-to-isotropic liquid transition, but its POM did not
show any optical textures.
The DSC of its homolog H2L3 showed an endothermic peak at a lower temperature of
61.2°C (ꢀH = +101.7 kJ mol−1) for assigned to crystal-to-mesophase transition, while its
POM showed an optical texture similar to H2L1 at 52.5°C (Fig. 8(b)).
Finally, the DSC of H2L4 (Fig. 7(b)) showed an endothermic peak at 57.6°C (ꢀH = +46.8
kJ mol−1) for the crystal-to-mesophase transition, and an exothermic peak at 43.9°C (ꢀH
= −40.4 kJ mol−1) for mesophase-to-crystal transition. Its POM showed an optical texture
similar to H2L1 at 57.0°C (Fig. 8(c)). Hence, these Schiff bases have melting temperatures
lower than 100°C; their melting temperatures decreased with increasing alkyl chain length.
The homologs H2L1, H2L3, and H2L4 exhibited mesomorphic properties in a narrow temper-
ature range (2–3°C), while the derivative H2L2 was not mesomorphic. This is a consequent
of the unfavorable packing pattern dictated by the two tert-butyl groups at the aromatic ring,
as indicated from its molecular structure.
For comparison, the DSC scan for complex 1 showed an endothermic peak at 190.2°C
(ꢀH = +41.6 kJ mol−1, assigned as its melting temperature. For 2, the scan showed two

MOL. CRYST. LIQ. CRYST.
141
Figure . DSC scans for (a) HL; and (b) HL for one heating-and-cooling cycle.
endothermic peaks at 138.9°C (ꢀH = + 41.6 kJ mol−1) and 183.6°C (ꢀH = +0.6 kJ mol−1),
assigned to its melting and clearing temperatures, respectively. Next, the scan for 3 showed
two endothermic peaks at 53°C (ꢀH = +77.7 kJ mol−1) and 102°C (ꢀH = +5.2 kJ mol−1),
assigned to the energy required to break the van der Waals forces between L3 ligands and
the Cu-OOCCH3 chelating bonds, and to evaporation of lattice H2O, respectively. Finally for
4, the scan showed two overlapping endothermic peaks at 142.0°C (ꢀHcombined = +71.0 kJ
Figure . Photomicrographs of: (a) HL at .°C; (b) HL at .°C; and (c) HL at .°C.

142
NORBANI ABDULLAH ET AL.
mol−1) assigned as its melting and dissociation temperature. However, POM did not show
any optical textures for these complexes, and it was noted that the samples remained fluid
on cooling from their isotropic temperatures to room temperature. Combining the DSC and
POM results, it is suggested that on heating at temperatures above 100°C, the imine bonds
of these complexes were hydrolyzed by the lattice H2O to the corresponding diaminoalkanes
and 2-HOC6H4CHO (a liquid at 30°C).
Conclusions
In this paper, we presented the syntheses, structures and mesomorphic studies of four N2O4-
Schiff bases and their corresponding Cu(II) complexes. The Schiff bases were obtained
from the reactions of 2-HOC6H4CHO with H2N(CH2)nNH2, where n = 8 (H2L1), 9
(H2L3), 10 (H2L4), and from the reaction of 3,5-X2-2-HOC6H2CHO (X = tert-butyl) with
H2N(CH2)8NH2 (H2L2). H2L2, H2L3, and H2L4 were single crystals. These Schiff bases have
melting temperatures lower than 100°C. The homologs (H2L1, H2L3, and H2L4) exhibited
mesomorphisms, while the derivative of H2L1, namely, H2L2, was not mesomorphic. The cop-
per(II) complexes of H2L1, H2L2, and H2L4 were dimeric, while the complex of H2L3 was din-
uclear with chelating CH3COO ligand. These complexes were paramagnetic with insignificant
interactions between the copper(II) atoms, have high decomposition temperatures, and their
melting temperatures were significantly higher than the corresponding Schiff bases. However,
they were not mesomorphic.
Supplementary Information
CCDC-1027255 (H2L2) and 1031111 (H2L4) contain the supplementary crystallographic data for this
paper. These data can be obtained free of charge from the Cambridge Crystallographic Data Centre via
www.ccdc.cam.ac.uk/data_request/cif.
Acknowledgments
This research was funded by the Malaysia Ministry of Education for High Impact Research Grant
(UM.C/625/1/HIR/MOHE/05) and Fundamental Research Grant Scheme (FP031-2013B), and Uni-
versiti Malaya for Postgraduate Research Grants (PV056-2012A and PG023-2013A). We would like
to record our thanks to Prof. M. H. Chisholm for the use of research facilities and instruments at the
Department of Chemistry, Ohio State University, Columbus, USA.
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