Synthesis and characterization of new metallophthalocyanines bearing highly substituted imidazoles

Article abstract of DOI:10.1016/j.tetlet.2012.10.071

New mono-nuclear metallophthalocyanines (M = Co, Cu, Ni, and Zn) I-V bearing highly substituted imidazole moieties at peripheral positions have been synthesized from derivatives of 4-[4-(2,4,5-triphenyl-1H-imidazol-1-yl)phenoxy] phthalonitrile at reflux t

Full text of DOI:10.1016/j.tetlet.2012.10.071

Tetrahedron Letters 54 (2013) 45–48
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Tetrahedron Letters
journal homepage: www.elsevier.com/locate/tetlet
Synthesis and characterization of new metallophthalocyanines bearing
highly substituted imidazoles
⇑
Ali Reza Karimi , Fahimeh Bayat
Department of Chemistry, Faculty of Science, Arak University, Arak 38156-8-8349, Iran
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 5 June 2012
Revised 3 October 2012
Accepted 18 October 2012
Available online 24 October 2012
New mono-nuclear metallophthalocyanines (M = Co, Cu, Ni, and Zn) I–V bearing highly substituted
imidazole moieties at peripheral positions have been synthesized from derivatives of 4-[4-(2,4,5-tri-
phenyl-1H-imidazol-1-yl)phenoxy]phthalonitrile at reflux temperature. These complexes have been
characterized by UV/Vis, FT-IR, 1H NMR, MALDI-TOF mass spectrometry, and thermogravimetric analysis.
The aggregation behavior of these complexes was investigated under different conditions. Phthalocyanines
I, III, IV, and V show monomeric behavior in solution and phthalocyanine V has the highest molar extinction
coefficient in the visible region.
Keywords:
Soluble metallophthalocyanine
Highly substituted imidazole
Aggregation
Ó 2012 Elsevier Ltd. All rights reserved.
Microwave synthesis
Phthalocyanines (Pcs) have attracted significant interest be-
cause of their potential applications in fields such as optical disks,1
photodynamic therapy (PDT),2 gas sensing,3 liquid crystals,4 laser
dyes,5 electronic displays,6 and semiconducting devices.7
Initially, the preparation of highly substituted imidazoles with
one hydroxyl group was undertaken. 1,2,4,5-Substituted imidazoles
A1 and A2 were prepared via one-pot, four-component condensation
reactions of benzil (1), 4-aminophenol (2), an aldehyde, and ammo-
nium acetate using the heteropolyacid, H6P2W18O62Á24H2O (WD)
supported on silica (WD/SiO2) as the catalyst.20a The formation of
compounds A1 and A2 was confirmed by the presence of OH signals
and the disappearance of C@O signals in their analytical spectra (IR,
NMR).
The process of self-assembly which is driven by
p–p stacking
interactions causes a limitation in the use of phthalocyanines in
areas such as optics and PDT.8 Organo-soluble phthalocyanines
are therefore important and potentially useful materials. Insolubil-
ity can be overcome by introducing bulky substituents onto the
peripheral positions of the ring system.9
Imidazoles are an important class of heterocyclic system be-
cause of their interesting biological effects such as herbicidal,10
anti-inflammatory,11 antibacterial,12 antitumor,13 and analgesic.14
In particular, 4,5-diaryl substituted imidazoles are considered as
potent inhibitors of p38 MAP kinase.15 Highly substituted imida-
zoles can be used as light-sensitive materials in photography,16
and as fungicides,17 plant growth regulators,18 and other types of
therapeutic agents.
Introducing imidazole or benzimidazole moieties into phthalo-
cyanines may have a profound effect on their applications in
electron transfer processes.19
Encouraged by this information and due to our interest in the
synthesis of imidazoles20 and Pcs,21 herein, we report the prepara-
tion of new phthalocyanines (Pcs) containing highly substituted
imidazoles (Scheme 1). Moreover, the soluble Pcs synthesized in
this study may demonstrate new biological, optical, and electrical
properties.
The synthesis of dicyano compounds B1 and B2 was carried out
by base-catalyzed aromatic nitro displacement of 4-nitrophthalo-
nitrile with A1 and A2 in DMF; K2CO3 was used as the base for this
displacement. The IR spectra of B1 and B2 clearly indicated the
presence of CN vibrational peaks at 2233, 2227, and 2233 cmÀ1
,
respectively. The 1H NMR spectra were also in good agreement
with the structures of compounds B1 and B2. For instance the spec-
trum of B2 exhibited the aliphatic protons as two singlets at d 3.63
(6H) and 3.67 ppm (3H). The aromatic protons in the low field re-
gion appeared as a singlet at d 6.75 (2H), multiplets at d 7.18–7.21
(3H), d 7.25–7.29 (5H), d 7.34–7.35 (3H), d 7.42–7.45 (2H), d 7.50–
7.53 (2H), d 7.55–7.56 (1H), and a doublet at d 8.17 (1H).
The desired metallophthalocyanines I–V were obtained by
cyclotetramerization of the dinitriles B1 and B2 (3 mmol) in the pres-
ence of anhydrous metal salts [CoCl2, CuCl, NiCl2, and Zn(OAc)2]
(1 mmol) using DBU as the catalyst in 2-(dimethylamino)ethanol
(DMAE) (for B1) or DMF (for B2) at reflux temperature. Attempts to
obtain CoPc, NiPc, and CuPc from the dinitrile compound B2 were
unsuccessful (Scheme 1).
IR, 1H NMR, MALDI-TOF-MS, and UV–Vis analyses confirmed
the proposed structures of the compounds synthesized.
⇑
Corresponding author. Tel.: +98 861 4173400; fax: +98 861 4173406.
E-mail address: a-karimi@araku.ac.ir (A.R. Karimi).
0040-4039/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.tetlet.2012.10.071

46
A. R. Karimi, F. Bayat / Tetrahedron Letters 54 (2013) 45–48
CN
OCH3
OCH3
H3CO
CHO
NC
NC
HO
O2N
CN
CN
CN
MX
d
H3CO
Cl
CHO
O2N
CN
CN
O
O
N
Ph
OCH3
Ph
7
4
H3CO
5
7
MX
N
N
b
Ph
a
H3CO
H3CO
H3CO
b
a
Ph
c
N
Ph
Ph
N
N
N
Ph
Ph
OH
A2
A1
Cl
Cl
NH2
N
B1
O
O
B2
+
NH4OAc
+
Ph
Ph
1
2
3
OH
Cl
OCH3
OCH3
OCH3
N
N
N
N
N
O
N
N
N
O
O
N
O
N
N
H3CO
Cl
N
N
OCH3
N
H3CO
M
Cl
N
N
N
N
N
Zn
N
N
N
N
O
H3CO
N
O
N
OCH3
N
O
H3CO
N
N
I-III
O
H3CO
H3CO
H3CO
Cl
N
N
N
N
I: M=Co; II: M=Cu; III: M=Ni; IV: M= Zn
V
Scheme 1. Synthesis of metallophthalocyanines I–V. Reagents and conditions: (a) WD/SiO2, 140 °C (A1): 72%, (A2): 65%; (b) K2CO3, DMF, 24 h, rt, (B1): 83%, (B2): 89%; (c) metal
salt (MX), DBU, DMAE, N2, reflux, (I): 34%, (II): 36%, (III): 35%, (IV): 41%; (d) Zn(OAc)2, DBU, DMF, N2, reflux (V): 33%.
After conversion of the dinitriles into the metallophthalocya-
nines, the sharp peak for the CN vibration completely disappeared
in the IR spectra. The 1H NMR spectrum of IV exhibited aromatic
protons as multiplets at d 7.24–7.32, 7.40–7.45, and 7.51–7.56. In
the 1H NMR spectrum of V, the aliphatic CH3 group protons ap-
peared at d 3.6–3.70 and the aromatic protons were observed as
a multiplet at d 6.74–7.55.
The MALDI-TOF mass spectrum provided evidence for the struc-
ture of phthalocyanine IV with the presence of the molecular ion
peak at 2261.8.
The thermal properties of phthalocyanines I–V were analyzed
by thermal gravimetric analysis (TGA) in the temperature range
of 30–1000 °C under a nitrogen atmosphere with a heating rate
of 10 °C/min. The primary weight loss was related to the residual
solvent which is typical of a TGA heating run.
The initial decomposition temperatures and the amounts of to-
tal mass loss percentages determined from the TG curves are listed
in Table 1. The initial decomposition temperatures of the com-
pounds were in the order: II > I > V > III > IV. No obvious correla-
tion between the transition metal ion in the phthalocyanine and
the initial decomposition temperatures was evident.
The UV/Vis spectra of the metal Pc complexes consisted of two
strong absorption regions at 600–700 nm (Q band) and around
300–400 nm (B band). The UV/Vis spectra of phthalocyanines I–V
in THF are shown in Figure 1 and demonstrated single intense
bands at 658, 671, 668, 677, and 669 nm, respectively. There was
also a shoulder at a slightly higher energy for all the phthalocya-
nines. Weaker absorptions appeared at 597, 604, 602, 606, and
606 nm for phthalocyanines I–V, respectively. This is typical of me-
tal complexes of substituted and unsubstituted metallophthalocy-
anines with D4h symmetry.22 The B bands for I–V were observed at
298; 296, 253; 298, 253; 346, 296; 349, 296 nm, respectively (Fig. 1
and Table 2).
Aggregation is usually a coplanar association and is dependent
on the nature of the solvent, the concentration, the metal ion, the
nature of substituents, and the temperature.23 In this study the
aggregation behavior of these complexes was investigated at dif-
ferent concentrations in various solvents. The aggregation behavior
of phthalocyanines I, IV, and V at five concentrations (5 Â 10À5
,
4 Â 10À5, 3 Â 10À5, 2 Â 10À5, and 1 Â 10À5 M) in DMSO, DMF,
Phthalocyanines I, IV, and V displayed good solubility in DMSO,
DMF, chloroform, and THF. Phthalocyanine II was soluble in THF
but only slightly in DMF. Phthalocyanine III was soluble in THF
and DMF.
Table 1
Thermal analyses data for I–V
Pc
Initial dec temp (°C)
Mass loss up to 1000 °C (%)
I
320
340
280
246
300
65.80
69.33
93.97
58.92
80.95
II
III
IV
V
Figure 1. UV/Vis absorption spectra of I–V in THF (c = 3 Â 10À5 M).

A. R. Karimi, F. Bayat / Tetrahedron Letters 54 (2013) 45–48
47
Table 2
Absorption data for phthalocyanines I–V in THF (c = 3 Â 10À5 M)
Pc
kmax (nm) (loge)
I
658 (4.59), 597 (4.10), 298 (4.97)
II
III
IV
V
671 (4.53), 604 (3.90), 296 (4.89), 253 (4.76)
668 (4.34), 602 (3.95), 298 (4.95), 253 (4.79)
677 (4.22), 606 (3.75), 346 (4.24), 296 (4.73)
669 (4.98), 606 (4.46), 349 (4.8), 296 (4.93)
Figure 4. UV/Vis absorption spectra of II in THF and DMF (c = 3 Â 10À5 M).
and there were no new bands due to aggregated species. Hence,
phthalocyanines I, IV, and V did not show aggregation in these sol-
vents (Figs. 2 and 3).
In addition, the aggregation behavior of phthalocyanines II and
III was investigated in DMF and THF at different concentrations, as
above. While phthalocyanine II did not show aggregation in THF, it
showed aggregation in DMF as was apparent by the decrease in the
absorbance intensity24 (Fig. 4).
Phthalocyanine III did not show aggregation in THF or DMF. It is
noteworthy that among the phthalocyanines synthesized in this
study, phthalocyanine V has the highest molar extinction coeffi-
cient in the visible region in all the considered solvents.
In conclusion, we have synthesized and characterized five new
mononuclear phthalocyanines I–V. Phthalocyanines I, IV, and V
had the best solubility and did not show aggregation in DMSO,
DMF, THF, or CHCl3. Phthalocyanine II did not show aggregation
in THF, but did aggregate in DMF. Phthalocyanine III displayed
monomeric behavior in THF and DMF. Phthalocyanine V had the
highest molar extinction coefficient in the visible region and thus
may show better results in PDT.
Acknowledgments
We gratefully acknowledge the financial support from the
Research Council of Arak University.
Figure 2. UV/Vis absorption spectra of IV and V in THF at different concentrations.
Supplementary data
Supplementary data associated with this article can be found, in
the online version, at http://dx.doi.org/10.1016/j.tetlet.2012.10.
071.
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