New compounds with potential liquid crystal properties. Copper(II) and Nickel(II) complexes of N,N'-bis(4-decyloxysalicyliden-N-n-propyl)-Piperazine. Synthesis and characterization

Article abstract of DOI:10.1080/15421400701833999

The new Schiff base N,N'-bis(4-decyloxysalicyliden-N-n-propyl)-piperazine (ZOPP) was designed and synthesized. Structures were confirmed by elemental analysis and IR, 1H-NMR spectroscopy. ZOPP can act as ligand through the two set of donor atoms (one set:

Full text of DOI:10.1080/15421400701833999

Mol. Cryst. Liq. Cryst., Vol. 481, pp. 26–33, 2008
Copyright # Taylor & Francis Group, LLC
ISSN: 1542-1406 print=1563-5287 online
DOI: 10.1080/15421400701833999
New Compounds with Potential Liquid Crystal
Properties. Copper(II) and Nickel(II) Complexes
of N,N0-bis(4-decyloxysalicyliden-N-n-propyl)-
Piperazine. Synthesis and Characterization
Carmen Cretu1, Liliana Cseh1,2, Goerg H. Mehl2,
and Otilia Costisor1
1Institute of Chemistry of Timisoara, Romanian Academy,
Timisoara, Romania
2Department of Chemistry, University of Hull, Hull, UK
The new Schiff base N,N0-bis(4-decyloxysalicyliden-N-n-propyl)-piperazine (ZOPP)
was designed and synthesized. Structures were confirmed by elemental analysis
and IR, 1H-NMR spectroscopy. ZOPP can act as ligand through the two set of
donor atoms (one set: two nitrogen atoms and one oxygen) situated on each side
of the piperazine bridge. The nature of the complexes depends strongly on the metal
to ligand molar ratio. Thus, starting from metal to ligand of 1 to 1 ratio, where
=
M Cu(II), Ni(II), binuclear complexes with a polymeric structure have been
obtained, whereas at metal to ligand molar ratio of 2 to 1, discrete binuclear
complexes were formed.
Keywords: copper (II) and nickel (II) complexes; Schiff base; spectral properties
INTRODUCTION
In the area of metal containing liquid crystals, interesting develop-
ments have been reported recently [1–4]. These are mainly associated
with the particular properties associated with metal containing mate-
rials, such as magnetism, color, or electron transfer characteristicss
(e.g., FRET) usually not found for purely organic systems. Moreover
metal containing liquid crystals form a very interesting bridge
between the field of LC research and supra-molecular chemistry, an
area where spontaneous organization over extended domains can be
Address correspondence to Liliana Cseh, Institute of Chemistry of Timisoara,
Romanian Academy, Bv. M. Viteazu, No. 24, Timisoara, RO-300223, Romania. E-mail:
lili_cseh@yahoo.com
26

New Compounds with Potential Liquid Crystal Properties
27
achieved [4]. There ligands are characterized by two or more well
selected donor atoms able to close chelate cycles which are responsible
for shaping the metal containing core. For the design of mesomorphic
behavior this core is often designed to be planar. Mesomorphic phases
in such systems are induced by linking long hydrocarbon chains to the
aromatic cores placed at strategic positions. These are responsible for
lowering the melting points and obtaining the anisotrpic fluidic state.
For the investigation of novel metallomesogens the new Schiff base
ligand N,N0-bis(4-decyloxysalicyliden-N-n-propyl)piperazine (ZOPP) was
designed and synthesized. This moiety can act as a ligand through two
sets of donor atoms positioned symmetrically on each side of the
piperazine bridge. The conformation of the piperazine moiety estab-
lishes the nature of the complexes. Thus, when the piperazine adopts
a chair conformation, ZOPP acts as a binucleating ligand. Mononuclear
complexes can be obtained when piperazine adopts a bath conformation.
The nature of these complexes depends on the metal to ligand molar
ratio employed in the synthesis. Having in view the aim to obtain
metallomesogens, the synthesis and characterization of the copper (II)
and nickel (II) complexes containing ZOPP as ligand is reported.
EXPERIMENTAL
Chemical and Physical Measurements
Analytical data were obtained by a Perkin Elmer model 240 C elemen-
tal analyzer for C, N, and H. The thin layer chromatography (TLC):
Merck 60 F254 plates were used. Detection was by UV fluorescence
(254 nm). Column Chromatography: Flash or gravity column chroma-
tography was carried out using Merck Kieselgel C60 230–240 mesh
silica gel. IR spectra were recorded on a Jasco-400 Spectrum One
FT-IR spectrophotometer, as KBr pellet, in the 400–4000 cmꢀ1 range.
1H-NMR analysis was carried out using a Jeol -NMR (400 MHz)
spectrometer. UV=Vis spectra were recorded on a Perkin Elmer
16PC spectrophotometer. Thermomicroscopy was realised under
polarised light, using an Olympus BH-2 polarising microscope with
Mettler FP52 hot stage linked to an FP5 temperature control unit.
The accuracy of measurements is ꢁ1ꢂC.
All the reagents and solvents were purchased and used without
further purification.
Synthesis of 4(n-decyloxysalicylaldehyde) (1) (Fig. 1)
1 g (7.24 mmol) 4-hydroxysalicylaldehyde, 1.5 ml (7.24 mmol) 1-
bromodecane in DMF (10 ml) and 0.73 g (7.24 mmol) KHCO3 as base

28
C. Cretu et al.
FIGURE 1 Structure of compound (1).
were refluxed under vigorous stirring in a nitrogen atmosphere for
4 hours. After cooling to room temperature, the solid was removed
by filtration. The filtrate was diluted with distilled water (20 ml) and
acidulated with aqueous HCl solution until pH ¼ 2. The organic layer
was separated and the aqueous layer was extracted with chloroform
(3 ꢃ 10 ml). The combined organic layers were dried over anhydrous
MgSO4 and the solvent was removed by concentration under reduced
pressure. The crude aldehyde was purified by column chromatography
(SiO2) using toluene as eluent (Rf ¼ 0.58).
IR (film) nmax[cmꢀ1]: 2916, 2880, 2852, 1691, 1644, 1636, 1630,
1580, 1510, 1466, 1376, 1354, 1334, 1292, 1218, 1204, 1192, 1180,
1168, 1136, 1116, 1018, 804, 760, 718, 636, 572, 540.
Synthesis of N,N 0-bis(4-decyloxysalicyliden-N-n-propyl)
piperazine (ZOPP) (2) (Fig. 2)
0.7 g (2.51 mmol) 4-hydroxysalicylaldehyde (1) dissolved in 10 ml
dry ethanol was treated with 0.2 ml (1.25 mmol) 1,4-bis(3-amino-propyl)-
piperazine. The mixture was refluxed for 2 h and then cooled slowly to
room temperature. The formed yellow solid was filtered and washed
with methanol. Yield: 0.58 g (64%); m.p. ¼ 65–66ꢂC;
Anal: calcd for C44H72N4O4 (721.08): C 73.29%, H 10.06%, N
7.77%; found C 72.98%, H 10.34%, N 7.88%;
1H-NMR (400 MHz),CDCl3=d [ppm]: 13.92 (s, 1H, H6); 8.13 (s, 1H,
H5); 7.06 (d, 1H, H7);0 6.35 (m, 2H, H8,9); 3.95 (t, 2H, H10); 3.58 (t, 2H,
H4); 2.57 (m, 6H, H1,1 ,2); 1.89 (m, 2H, H11); 1.77 (m, 2H, H3); 1.26–1.43
(m, 14H, H12–18); 0.88 (t, 3H, H19);
IR (KBr pellet) nmax [cmꢀ1]: 2941, 2918, 2850, 2812, 2794, 2769,
2360, 1625, 1575, 1521, 1473, 1437, 1415, 1356, 1302, 1274, 1228,
1173, 1147, 1055, 1017, 970, 856, 796, 756, 720, 660, 631, 582, 466.
FIGURE 2 Structure of compound (2).

New Compounds with Potential Liquid Crystal Properties
29
Synthesis of Cu2(ZOPP-2H)(CH3COOꢀ)22C2H5OH (3)
0.3 g (0.4 mmol) (2) dissolved in 20 ml ethanol was treated with a
solution of 0.16 g (0.8 mmol) Cu(CH3COO)2ꢄH2O dissolved in 10 ml
ethanol. The mixture was stirred at 55–60ꢂC for 3 h and then cooled
slowly to room temperature. The solid, which had precipitated was
filtered, washed several times with H2O and purified by recrystalliza-
tion from dichloromethane and diethyl ether. A brown-green solid was
obtained. Yield: 0.27 g (64.28%) m.p. ¼ 156–160ꢂC.
Anal: calcd for C52H88N4O10Cu2 (1054.51): C 59.17; H 8.41; N 5.31;
found: C 59.21; H 9.18; N 5.26.
IR (KBr pellet) nmax[cmꢀ1]: 3460, 2923, 2851, 1608, 1589, 1530, 1470,
1431, 1400, 1365, 1346, 1306, 1218, 1121, 1034, 835, 795, 686, 619, 417.
UV=Vis (CH2Cl2) k[nm]: 727.50, 471.14(s).
Synthesis of Cu(ZOPP-2H) (4)
0.3 g (0.4 mmol) (2) dissolved in 20 ml ethanol was treated with a
solution of 0.08 g (0.4 mmol) Cu(CH3COO)2ꢄH2O in 10 ml ethanol.
The mixture was stirred at 55–60ꢂC for 4 h and then cooled to room
temperature. The solid was filtered and green solid was obtained.
Yield: 0.2 g (66.6%) m.p. ¼ 155–164ꢂC.
Anal: calcd. for C44H70N4O4Cu (781.46): C 67.57; H 9.03; N 7.17;
found: C 67.57; H 9.21; N 7.06.
IR (KBr pellet) nmax[cmꢀ1]: 2925, 2853, 2808, 2767, 2360, 1620,
1605, 1529, 1466, 1435, 1400, 1322, 1241, 1175, 1148, 1015, 840,
773, 610, 453, 417, 406.
UV=Vis (CH2Cl2) k[nm]: 605.87.
Synthesis of Ni2(ZOPP-2H)(CH3COOꢀ)2 4H2O (5)
0.3 g (0.4 mmol) (2) dissolved in 20 ml ethanol was treated with a
solution of 0.2 g (0.8 mmol) Ni(CH3COO)2ꢄ4H2O in 10 ml ethanol. The
resulting mixture was stirred at 50ꢂC for 3 h and then the solvent
removed under reduced pressure. The solid was purified by recrystal-
lization from chloroform= diethyl ether. A green solid was obtained.
Yield: 0.22 g (51.4%) m.p. ¼ 144–149ꢂC.
Anal: calcd for C48H84N4O12Ni2 (1024.48): C 56.22; H 8.26; N 5.47;
found: C 56.16; H 8.89; N 5.86.
IR (KBr pellet) nmax[cmꢀ1]: 2924, 2853, 1605, 1538, 1487, 1467,
1433, 1401, 1352, 1305, 1209, 1119, 1000, 847, 789, 658, 615, 462, 407.
UV=Vis (CHCl3) k[nm]: 670.18, 507.65(s).
Synthesis of Ni(ZOPP-2H) (6)
0.3 g (0.4 mmol) (2) dissolved in 20 ml ethanol was treated with a
solution of 0.103 g (0.4 mmol) Ni(CH3COO)2ꢄ4H2O in 7 ml ethanol.

30
C. Cretu et al.
The resulted mixture was refluxed for 3 h. The solid was filtered and
washed with hot methanol. A green solid was obtained. Yield: 0.2 g
(62.5%) m.p. ¼ 87–98ꢂC.
Anal: calcd for C44H70N4O4Ni (776.47): C 68.00; H 9.09; N 7.21;
found: C 68.29; H 9.24; N 6.90.
IR (KBr pellet) nmax[cmꢀ1]: 2923, 2852, 2806, 2767, 1605, 1531,
1467, 1437, 1398, 1373, 1315, 1231, 1200, 1142, 1120, 1001, 827,
787, 620, 459, 435.
UV=Vis (CHCl3) k[nm]: 614.60.
RESULTS AND DISCUSSIONS
The Schiff base N,N0-bis(4-decyloxysalicyliden-N-n-propyl)-piperazine
(ZOPP) (2) was designed with the aim to obtain binuclear metal
complexes with potential mesogenic properties. It contains two
coordination sites linked by the piperazine bridge, forming a symmetri-
cal molecule. The nature of the complexes depends strongly on the
initial metal to ligand molar ratio.
The ligand ZOPP was synthesized following the routed shown in the
Scheme 1.
N,N0-bis(4-decyloxysalicyliden-N-n-propyl)piperazine
(2)
was
synthesized in two steps. The first step consists in the synthesis
of 4(n-decyloxysalicylaldehyde) from 4-hydroxysalicylaldehyde and
1-bromodecane and in the second step 4(n-decyloxysalicyl-aldehyde)
forms in a direct condensation reaction with 1,4-bis(3-aminopropyl)-
piperazine. The formation of 4(n-decyloxysalicylaldehyde) was con-
trolled by TLC. Confirmation of the structure of the compounds was
SCHEME 1 Synthetic route to ZOPP.

New Compounds with Potential Liquid Crystal Properties
31
obtained from elemental analyses, IR and 1H-NMR data. The new
band which appears at 1625 cmꢀ1 in the IR spectrum of (2) compared
=
to the spectrum of (1), is attributed to the new formed C N bond [5].
Also, the IR spectrum of compound (2) does not contain the character-
istic bands of the reactants, namely that at 3360 and 3280 cmꢀ1 in
the spectrum of 1,4-bis-(3-aminopropyl) piperazine and that of the
corresponding aldehyde at 1691 cmꢀ1(C O). The bands in the 2740–
=
2825 cmꢀ1 region are assigned to the n(C-H) of N-CH2 (alkylic) modes
of the piperazinic fragment [6,7]. The identity of the Schiff base was
confirmed by 1H-NMR spectroscopy. The signal at 8.13 ppm in the
1H-NMR spectrum of (4) is due to the formation of an azomethinic
bond. In addition, new signals at 3.58, 2.57 and 1.77 ppm appear,
which are attributed to protons belonging to the bis-(n-propyl) pipera-
zine fragment.
Copper (II) and nickel (II) complexes (3), (4), (5) and (6) were
obtained following a direct metal-ligand synthesis. It was estabilished
that the structure of the complexes depends strongly on the metal to
ligand molar ratio.
Thus, starting from a 2:1 metal to ligand ratio, the copper (3) and
nickel (5) complexes were obtained.
In the infrared spectrum of the complexes, the characteristic bands
of the ligand can be observed and some of them are shifted or are not
present as a result of coordination. The spectra of (3) and (5) show
the peaks assigned to the nC mode at 1608 and 1605 cmꢀ1, respect-
BN
ively. Also, the band assigned to the nC–O mode is shifted to 1218
and 1209 cmꢀ1 for the two complexes, respectively. The coordination
of the piperazine nitrogens is evidenced by the changes in the region
of 2750 cmꢀ1 in the spectra of the complexes. The bands attributable
to n(CꢀH) of NꢀCH2 modes of the piperazinic moiety disappear,
suggesting the involvement of piperazine nitrogen in coordination.
The copper and nickel-nitrogen vibrations appear in the region
between 450–600 cmꢀ1 and vibrational modes associated with
metal(II) – oxygen(acetate) have been identified at 686 cmꢀ1 and
658 at cmꢀ1 for (3) and (5) complexes, respectively.
The electronic spectra of the complexes were recorded with the aim
to obtain information regarding the geometry of the metal ions. The
UV=Vis spectra of freshly prepared dichloromethane and chloroform
solutions were recorded. The spectra of the complexes (3) and (5)
recorded at room temperature show a broad band centered at 728 nm
for (3) and 670 nm for (5) and also a shoulder at 471 nm and 507 nm for
(3) and (5) respectively. The geometry around copper(II) and nickel(II)
is five-coordinate: one piperazinic nitrogen atom and one azomethinic
nitrogen atom, one phenolic and two carbonylic oxygen atoms from

32
C. Cretu et al.
FIGURE 3 The proposed structure of the complexes (3) and (5). (See COLOR
PLATE II)
acetate group (Fig. 3) accommodate these coordination requirements.
The bands are assigned to the A1 E transition for the complex (3)
and 1E0 1A01, 1E00 1A01 for the complex (5), altogether this indicates a
trigonal bipyramide structure [10] for these complexes.
The copper (4) and nickel (6) complexes were obtained using a ratio
of 1:1 of metal to ligand. The elemental analyses indicate however that
for one ion metal corrsponding one ligand. The way of coordination of
metals was established on the IR, UV-Vis spectrum and melting point.
In the infrared spectrum of the complexes (4) and (6), fairly strong
nC N bands are found at 1605 cmꢀ1. A shift of this band towards lower
B
frequencies in the spectrum of the complex compared with that of the
free ligand indicates the involvement of the nitrogen lone electron pair
of the azomethine group of the Schiff base in the bonding to the central
metal atom and the formation of a coordinating linkage. The phenolic
m
CꢀO band found at 1228 cmꢀ1 in the spectrum of the Schiff base shows
a slight shift towards lower frequency and appears at 1241 cmꢀ1 in the
spectrum of the complex (4) and at 1231 cmꢀ1 the spectrum of the com-
plex (6). This may be due to the change in environment of the CꢀO
bond due to the formation of the CꢀOꢀCu linkage. An examination
of the region around 2750 cmꢀ1 shows that the bands due to n(CꢀH)
of NꢀCH2 (alkyls) appear at almost the same positions as in the free
base, which indicates that the nitrogen atoms of the piperazinic
fragment remains uncoordinated.
The geometry around the copper(II) and nickel(II) for the complexes
(4) and (6) is essentially four-coordinate. This is achieved by the donor
atoms, namely two azomethinic nitrogen atoms and two phenolic
oxygen atoms (Figure 4). The bands are tentatively assigned to the
2
transition A1g 2B1g, which indicates a square-planar structure [8]
for these complexes.

New Compounds with Potential Liquid Crystal Properties
33
FIGURE 4 The proposed structure of complexes (4) and (6).
The polymeric nature [9,10] of these two complexes is also sup-
ported by their thermal behavior observed on heating (they do not
show a sharp melting points).
CONCLUSIONS
A new Schiff base and four new complexes were obtained and charac-
terized.
The compounds do not show mesomorphism. However, we consider
that a slight modification of the ligand structure, such as extending
the aromatic group of the ligand by one further phenyl ring should
result in stable liquid crystalline phase behaviour in such complexes.
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