Syntheses, characterization and mesomorphic properties of new bis(alkoxyphenylazo)-substituted N,N′ salicylidene diiminato Ni(II), Cu(II) and VO(IV) complexes

Article abstract of DOI:10.1039/b305278h

A series of tetradentate Schiff base ligands N,N′-bis[5-(4-n-alkoxy) phenylazosalicylidene]alkyldiamine-(n-alkoxy = butyloxy, octyloxy) homologues based on 1,2 diaminoethane and 1,3 diaminopropane have been synthesized and characterized by IR, NMR and mass spectroscopy. For all the ligands the nickel(II), copper(II) and oxovanadium(IV) complexes have been synthesized and characterized by elemental analyses and IR spectroscopy. The thermal behavior of these compounds was examined by differential scanning calorimetry (DSC) and polarizing microscopic observations. None of the Schiff-base ligands and butoxy containing complexes exhibit liquid crystalline properties but the octyloxy containing metal complexes show a smectic A mesophase.

Full text of DOI:10.1039/b305278h

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Syntheses, characterization and mesomorphic properties of new
bis(alkoxyphenylazo)-substituted N,N0 salicylidene diiminato
Ni(II), Cu(II) and VO(IV) complexes
Kamellia Nejati*a and Zolfaghar Rezvanib
a
Department of Chemistry, Payam Noor University-Tabriz center, Emamieh,
Hakim nezami street, Tabriz, Iran. E-mail: nejati_k@yahoo.com
Department of Chemistry, Faculty of Science, Azarbaijan University of Tarbiat Moallem,
Tabriz, Iran
b
Received (in London, UK) 12th May 2003, Accepted 11th August 2003
First published as an Advance Article on the web 3rd September 2003
A series of tetradentate Schiff base ligands N,N0-bis[5-(4-n-alkoxy)phenylazosalicylidene]alkyldiamine-
(n-alkoxy ¼ butyloxy, octyloxy) homologues based on 1,2 diaminoethane and 1,3 diaminopropane have been
synthesized and characterized by IR, NMR and mass spectroscopy. For all the ligands the nickel(II), copper(II)
and oxovanadium(IV) complexes have been synthesized and characterized by elemental analyses and IR
spectroscopy. The thermal behavior of these compounds was examined by differential scanning calorimetry
(DSC) and polarizing microscopic observations. None of the Schiff-base ligands and butoxy containing
complexes exhibit liquid crystalline properties but the octyloxy containing metal complexes show a smectic
A mesophase.
Sudhadevi and co-workers17 reported the azopyridine contain-
ing silver mesogens. We recently reported the synthesis and
liquid crystal character of Cu(II) bis(chelates) based on azo-
linked bidentate salicylaldimine.18 Only a few data are avail-
able from the literature about metallomesogens containing
the phenylazo fragment. In continuation of our interest we
reported herein the synthesis and investigation of liquid
crystalline behavior of symmetric tetradentate azo-linked
N,N-salicylidinediiminato Ni(II), Cu(II) and VO(IV) complexes
(Scheme 2).
1
Introduction
Investigation of metallomesogens is one of the most promising
areas in the field of liquid crystal research.1–6 This interest has
been stimulated by the potential applications of metal-contain-
ing liquid crystals.7–9 Much of this attention on the synthesis
and study of metallomesogens is due to the perceived advan-
tages of combining the properties of liquid crystal systems with
those of transition metals.10–12 Bidentate and tetradentate sal-
icylaldimine-based ligands (see Scheme 1) have been widely
used in the synthesis of metallomesogens. The majority of
these compounds described in the literature contain Cu(II),
Ni(II), Ag(I), Pt(II), Pd(II), VO(IV), lanthanide or actinide13 as
the central metal ion.
Recently, some metallomesogens containing isomerizable
azo moieties such the azo-substituted calixarenes containing
WO(IV),14 calamitic Cu(II) complexes of {1-[40-(400-hexyloxy-
phenylazo)phenyl-3-alkylamino-2-en-1-ones},15 and rodlike
bis(alkylphenylazo) substituted N,N-salicylidinediiminato
Ni(II), Cu(II) and VO(IV) complexes, have been reported.16
2
Experimental
2.1 Reagents
All reagents and solvents were used as supplied by Merck che-
mical company. In order to prepare the 4-alkoxyaniline homo-
logues, 4-alkoxynitrobenzene homologues were obtained by
standard etherification of 4-nitrophenol19 and reduced as
described in the literature.20
Scheme 1
DOI: 10.1039/b305278h
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Scheme 2
2.2 Physical measurements
2.3 Materials
Elemental (C, H and N) analyses were carried out on a Perkin-
Elmer automatic equipment model 240B. Electron impact
(70 eV) mass spectra were recorded on a Finnigen-mat GC-
MS-DS spectrometer model 8430. Infrared spectra were
recorded with a FT-IR Bruker, vector 22 spectrometer from
KBr pellets in the 400–4000 cmꢀ1 range. Transition tempera-
tures and enthalpy changes were determined using a Mettler
TA4000 system linked to a DSC-11 differential scanning
calorimeter, which was calibrated with indium. The optical
observations were made with a Leitz Orthoplan-Pol polariz-
ing microscope equipped with a Linkam THMSG 600 heat-
ing and cooling stage. A flow of liquid nitrogen was used
for the cooling. The heating or cooling rates were 10 K
minꢀ1. NMR spectra were measured in CDCl3 and (CD3)2SO
as solvent on a Bruker FT-NMR AC-400 (400 MHz) spectro-
meter. All chemical shifts are reported in d units downfield
from Me4Si.
All homologue materials were prepared in similar manners.
1a, 5-(4-Butoxyphenylazo)salicylaldehyde. 5-(4-butoxyphe-
nylazo)salicylaldehyde was obtained as described elsewhere.18
Yellow. Yield 75%. Mp 131 ꢁC. 1HNMR d 11.27(s,H8),
10.02(s,H9), 8.17(d,H3), 8.14(dd,H2), 7.91(dd,H4,7), 7.11(d,H1),
7.01(dd,H5,6), 4.05(t,H10), 1.81(p, O–C–CH2), 1.51(q, O–C–C–
CH2) 1(t, CH3).
1b, 5-(4-Octyloxyphenylazo)salicylaldehyde. Yellow. Yield
80%. Mp 126 ꢁC. 1HNMR
d
11.27(s,H8), 10.02(s,H9),
8.18(d,H3), 8.14(dd,H2), 7.91(dd,H4,7), 7.10(d,H1), 7.01
(dd,H5,6), 4.05(t,H10), 1.81–1.00 (15H, alkyl chain).
Syntheses of the ligands. 0.013 mole of related diamine and
0.026 mole of 5-(4-alkoxyphenylazo)salicylaldehyde were con-
densed by refluxing in 100 ml of absolute ethanol for 1 h. The
solution was left at room temperature. The ligands were
obtained as yellow micro crystals. The micro crystals were
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filtered off, washed with 15 ml of absolute ethanol and then
recrystallized from ethanol–chloroform(1 : 3, v/v).
6c. Brownish green. Yield 85%. Anal. Calcd for
C44H54N6O5V: C, 66.25; H, 6.77; N, 10.54. Found: C, 66.14;
H, 6.49; N, 10.38%.
7c. Brownish green. Yield 78%. Anal. Calcd for
C45H56N6O5V: C, 66.59; H, 6.90; N, 10.36. Found: C, 66.37;
H, 6.72; N, 10.10%.
2a. Yellow. Yield 90%. Mp 215 ꢁC. 1HNMR d 13.91
(s,H8), 8.48(s,H9), 7.94(dd,H2), 7.90–7.82(H3,4,7), 7.06(d,H1),
6.98(d,H5,6), 4.02(t,H10), 3.78(s, N–CH2), 1.84 (p, O–C–
=
CH2),1.55(q, O–C–C–CH2) 0.89(t, CH3).
2b. Yellow. Yield 85%. Mp 210 ꢁC. 1HNMR d 13.90
(s,H8), 8.49(s,H9), 7.93(dd,H2), 7.91–7.81(H3,4,7), 7.05(d,H1),
6.99(d,H5,6), 4.03(t,H10), 3.78(s, N–CH2), 1.83–0.90(30H,
=
3
Result and discussion
alkyl chain).
3a. Yellow. Yield 87%. Mp 158 ꢁC. 1HNMR d 13.92
3.1 Synthesis
(s,H8), 8.47(s,H9), 7.94(dd,H2), 7.91–7.82(H3,4,7), 7.05(d,H1),
6.99(d,H5,6), 4.02(t,H10), 3.79(s, N–CH2), 2.18(m, N–C–
CH2), 1.85 (p, O–C–CH2), 1.57(q, O–C–C–CH2) 0.91(t, CH3).
=
=
The 5-(4-alkoxyphenylazo)salicylaldehyde homologues (1a,b),
Schiff-base ligand homologues (2a,b, 3a,b) and related metal
complexes were obtained in good yields and purity. Aldehyde
and ligand homologues were characterized by IR and 1H
NMR spectroscopy and complexes were characterized by C,
H, N elemental analysis and IR spectroscopy. Some physical
and characterization data for ligands and complexes are given
in the experimental section and some selected IR data are
reported in Table 1. For the IR spectra of metal complexes,
3b. Yellow. Yield 91%. Mp 152 ꢁC. HNMR d 13.90(s,H8),
1
8.48(s,H9),
7.94(dd,H2),
7.91–7.81(H3,4,7),
=
7.06(d,H1),
=
6.98(d,H5,6), 4.02(t,H10), 3.78(s, N–CH2), 2.17(m, N–C–
CH2), 1.85–0.90 (30H, alkyl chain).
Syntheses of the copper and nickel complexes. Copper and
nickel complexes were prepared in a similar manner. A solu-
tion of 0.004 mole of Ni(OAc)2ꢂ4H2O or Cu(OAc)2ꢂH2O in
10 ml of ethanol was added to an ethanol–chloroform(1 : 1
v/v) solution containing 0.004 mole of ligand and refluxed
for 2 h. The obtained solution was left at room temperature.
Copper complexes were obtained as brown micro crystals,
and nickel complexes were obtained as green micro crystals.
The micro crystals were filtered off, washed with absolute
ethanol and then recrystallized from ethanol–chloroform
(1 : 3 v/v).
4a. Brown. Yield 80%. Anal. Calcd for C36H38N6O4Cu:
C, 63.39; H, 5.57; N, 12.32. Found: C, 63.16; H, 5.33; N,
12.18%.
4b. Green. Yield 85%. Anal. Calcd for C36H38N6O4Ni:
C, 63.84; H, 5.61; N, 12.41. Found: C, 63.66; H, 5.30; N,
12.25%.
uC N shifted to lower wavenumbers by 10–30 cmꢀ1 upon coor-
=
dination. On the other hand, the disappearance of the OH
band of free ligands in metal complexes indicates that the
OH group has been deprotonated and connected to the metal
ion as –Oꢀ. On the basis of these observations it can be con-
cluded that the Schiff-base ligands are coordinated to metal
atoms as tetradentate ONNO ligands.
It is well known that the Ni(II) coordination geometry in
salen type ligands is usually square planar.21,22
We recently reported the crystal structure of [bis(5-pheny-
lazo salicylaldehyde)-trimethylendiiminato] copper(II)23 which
has a similar ONNO coordination environment to that of
the title complex of this work. However it is suggested that
the Cu(II) coordination is square planar or nearly square pla-
nar from its similar coordination environment to a recently
reported copper(II) complex.21,22,24
5a. Brown. Yield 75%. Anal. Calcd for C37H40N6O4Cu:
C, 63.84; H, 5.75; N, 12.07. Found: C, 63.61; H, 5.49; N,
11.86%.
5b. Green. Yield 86%. Anal. Calcd for C37H40N6O4Ni:
C, 64.28; H, 5.79; N, 12.16. Found: C, 64.03; H, 5.54; N,
11.94%.
6a. Brown. Yield 74%. Anal. Calcd for C44H54N6O4Cu:
C, 66.54; H, 6.80; N, 10.59. Found: C, 66.39; H, 6.50; N,
10.47%.
6b. Green. Yield 75%. Anal. Calcd for C44H54N6O4Ni:
C, 66.94; H, 6.85; N, 10.65. Found: C, 66.73; H, 6.55; N,
10.40%.
VO complexes with salen type ligands can give rise either to
monomeric structure25–27 with square pyramidal coordination
geometry or to polymeric structures26–29 with [V Oꢂ ꢂ ꢂV O]
=
=
interactions which afford a distorted octahedral geometry.
=
However on the basis of the position of the V O band in IR
spectra25–31 we can distinguish between the monomeric or
polymeric nature of these complexes. In this work, the VO
complexes with –CH2CH2– bridge show uV
at 995.93
=
O
cmꢀ1, according to which result, a square pyramid coordina-
tion is assigned to the salen derivative. In the case of complexes
7a. Brown. Yield 80%. Anal. Calcd for C45H56N6O4Cu:
C, 66.87; H, 6.93; N, 10.40. Found: C, 66.68; H, 6.65; N,
10.22%.
Table 1 Selected IR data for Schiff base ligands and metal complexes
n/cmꢀ1
7b. Green. Yield 78%. Anal. Calcd for C45H56N6O4Ni:
C, 67.27; H, 6.97; N, 10.46. Found: C, 66.95; H, 6.69; N,
10.24%.
C–H C–H
(aromatic) (aliphatic)
C–O
=
(etheric) C O V O
=
=
Compound O–H
C
–
N
1(a,b)
2(a,b)
3(a,b)
3185
(br,m)
3420
(br,m)
3441
(br,m)
3055(m)
3040(m)
3060(m)
2952,
2866(s)
2955,
2872(s)
2965,
2871(s)
1239(s) 1660 —
1637(s) 1248(s)
1634(s) 1247(s)
—
—
—
Syntheses of the oxovanadium(IV) complexes. 0.004 mol of
VO(acac)2 was added to 70 ml of dichloromethane hot solu-
tion containing 0.004 mol of ligand and a few drops of triethy-
lamine and refluxed for 1 h. The resulting yellow precipitate
was collected by filtration and washed with dichloromethane
and ether and recrystallized from ethanol–chloroform (1 : 3
v/v).
4c. Brownish green. Yield 85%. Anal. Calcd for
C36H38N6O5V: C, 63.07; H, 5.55; N, 12.26. Found: C, 62.88;
H, 5.34; N, 12.04%.
—
(4,6)a
(4,6)b
—
—
3042(m)
3055(m)
2925(s)
2955,
1626(s) 1250(s)
1618(s) 1249(s)
—
—
—
—
2870(s)
2936,
2869(s)
(4,6)c
—
3047(m)
1620(s) 1255(s)
—
995(s)
(5,7)a
(5,7)b
—
—
3037(m)
3050(m)
2924(s)
2923,
1616(s) 1252(s)
1630(s) 1250(s)
—
—
—
—
2834(s)
2 2935,2864(s) 1620(s) 1248(s)
5c. Brownish green. Yield 80%. Anal. Calcd for
C37H40N6O5V: C, 63.52; H, 5.72; N, 12.02. Found: C, 66.43;
H, 5.51; N, 11.88%.
(5,7)c
—
3066(m)
—
842(s)
s: strong, m: medium
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with the trimethylene bridge (see Scheme 2), uV O appeared at
=
842.91 cmꢀ1 which indicated a polymeric structure with an
octahedral coordination for the vanadyl complex. These
results are similar to the bis(alkylphenylazo)-N,N salicyliden-
diiminato VO(IV) complexes, as reported by Iolinda Aiello
and co-workers.16
3.2 Mesomorphism
The mesomorphic properties of the Schiff ligands and related
Cu(II), Ni(II) and VO(IV) complexes have been investigated
by polarizing microscopic observations using a heating–cool-
ing stage. The phase transition temperature and enthalpy data
were obtained by Differential Scanning Calorimetry (DSC).
None of the Schiff-base ligands, despite the two mesogenic
alkoxyphenylazo fragments they contain, exhibit liquid crys-
talline properties. All ligands clearly melted and transformed
into isotropic liquids as seen optically. The melting points of
the ligands decrease with either increasing bridge size or alkyl
chain length. On the basis of the literature data we propose
that this behaviour results from the transoid N,N conformation
of Schiff base ligands (see Scheme 2) which stabilizes a stepped
molecular geometry that prevents mesomorphism.16 This
behavior is similar to that of the tetradentate Schiff base
ligands N,N0-bis[5-(40-n-alkyl)phenylazosalicylidene]alkyldia-
mine and N,N0-bis[5-(40-n-alkoxy-50-fluoro)salicylidene]ethyle-
nediimine as reported by Iolinda Asello and co-workers16
and A. B. Blake and co-workers22 respectively.
Fig. 1 Batonnet texture of 6a at 315 ꢁC, which was obtained with a
polarizing microscope on cooling from the isotropic liquid.
Acknowledgements
We thank Professor M. Marcos, Zaragoza University, Spain
for useful discussions on mesophase assignment.
Copper(II), nickel(II) and oxovanadium(IV) complexes of
Schiff Base ligands (except 7c, see Scheme 2), containing octy-
loxy substitution, exhibit liquid crystalline character as seen
optically. Metal complexation of Schiff base ligands induces
the mesomorphism because the transoid N,N conformation
of Schiff base ligands is converted to the rod like molecular
shape (see Scheme 2) upon complexation.16
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Table 2 Transition temperatures and enthalpy changes of copper(II),
nickel(II) and oxovanadium(IV) complexes
Compound
Transisiona
Tb /ꢁC
dHb /kJ molꢀ1
16.21
6a
6b
K–SA
SA–Ic (dec.)
K–SA
295
317
292
312
303
321
230
255
239
261
380
17.25
SA–Ic (dec.)
K–SA
6c
7a
7b
18.23
14.36
15.29
SA–Ic (dec.)
K–SA
SA–Ic (dec.)
K–SA
SA–Ic (dec.)
K–Ic (dec.)
7c
a
K: Crystal, SA : Smectic A, I: Isotropic liquid. b Data obtained from
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