Synthesis of metal-free phthalocyanines in functionalized ammonium ionic liquids

Article abstract of DOI:10.1080/00397910701470768

A convenient, fast, efficient, and ecofriendly synthesis of metal-free phthalocyanines from various substituted phthalonitriles in different hydroxyalkylammonium ionic liquids in the presence of 1,8-diazabicyclo-[5,4,0]- undec-7-ene (DBU) is reported in moderate yields. The effect of concentration of DBU and temperature on the synthesis of phthalocyanine in N-(2-hydroxyethyl)-N, N-dimethylbutylammonium bromide ionic liquid has been examined, and the ionic liquid has been recovered and reused conveniently. Copyright Taylor & Francis Group, LLC.

Full text of DOI:10.1080/00397910701470768

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Synthesis of Metal‐Free Phthalocyanines in
Functionalized Ammonium Ionic Liquids
S. M. S. Chauhan a , Shweta Agarwal a & Pratibha Kumari a
a Department of Chemistry, University of Delhi, Delhi, India
Version of record first published: 30 Aug 2007
To cite this article: S. M. S. Chauhan, Shweta Agarwal & Pratibha Kumari (2007): Synthesis of Metal‐Free
Phthalocyanines in Functionalized Ammonium Ionic Liquids, Synthetic Communications: An International
Journal for Rapid Communication of Synthetic Organic Chemistry, 37:17, 2917-2925
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Synthetic Communicationsw, 37: 2917–2925, 2007
Copyright # Taylor & Francis Group, LLC
ISSN 0039-7911 print/1532-2432 online
DOI: 10.1080/00397910701470768
Synthesis of Metal-Free Phthalocyanines in
Functionalized Ammonium Ionic Liquids
S. M. S. Chauhan, Shweta Agarwal, and Pratibha Kumari
Department of Chemistry, University of Delhi, Delhi, India
Abstract: A convenient, fast, efficient, and ecofriendly synthesis of metal-free phtha-
locyanines from various substituted phthalonitriles in different hydroxyalkylammo-
nium ionic liquids in the presence of 1,8-diazabicyclo-[5,4,0]-undec-7-ene (DBU) is
reported in moderate yields. The effect of concentration of DBU and temperature on
the synthesis of phthalocyanine in N-(2-hydroxyethyl)-N,N-dimethylbutylammonium
bromide ionic liquid has been examined, and the ionic liquid has been recovered and
reused conveniently.
Keywords: 1,8-diazabicyclo-[5,4,0]-undec-7-ene (DBU), ionic liquids, liquid crystals,
metal-free phthalocyanines, thermal heating
INTRODUCTION
Phthalocyanines are 18p-electron-conjugated macrocycles with unique physio-
chemical properties, and they have been used as dyes,[1] chemical sensors,[2]
photodynamic cancer drugs,[3] nonlinear optical materials,[4] catalysts,[5]
liquid crystals,[6] and other newer materials.[7] These versatile features have
stimulated attempts on the synthesis of different functionalized phthalocya-
nines. Metal-free phthalocyanines have been obtained from cyclocondensation
of diverse precursors such as phthalonitriles,[8,9] 1,3-diiminoisoindolines,[10]
phthalic anhydrides and phthalimides[11] under basic conditions in high-
boiling nonaqueous solvents such as pentanol, N,N-dimethylformamide, and
N,N-dimethylaminoethanol. Metal-free phthalocyanines have also been
prepared by an electrochemical method,[12] UV irradiation,[13] and microwave
Received in India January 29, 2007
Address correspondence to S. M. S. Chauhan, Department of Chemistry, University
of Delhi, Delhi, India. E-mail: smschauhan@chemistry.du.ac.in
2917

2918
S. M. S. Chauhan, S. Agarwal, and P. Kumari
irradiation.[14] These routes are not very common for peripherally functiona-
lized phthalocyanines. Moreover, these methods suffer from certain disadvan-
tages such as slow reaction rate, low yields, use of high-boiling solvents, and
tedious isolation of products, and generally employed drastic reaction con-
ditions often cause contamination with undesired side products.[15]
Ionic liquids offer an environmentally benign reaction media in place of
conventional organic solvents because of their low vapor pressure, thermal
stability, high ionic conductivity, and ease of recyclability. Different functio-
nalized ionic liquids have been used in various chemical transformations
including Knoevenagel condensation,[16] aldol condensation,[17] and Heck
reaction.[18] Tetrabutylammonium bromide[19,20] and 1,1,3,3-N,N,N0,N0-teter-
amethylguanidinium trifluoroacetate[21] have been used as ionic liquids for
phthalocyanines synthesis. Recently, hydroxylated quaternary ammonium
salts[22] have attracted increasing interest because they are less expensive,
moisture stable, and base stable and have been used effectively in the electro-
deposition of thick, crack-free films[23] and Diels-Alder reactions.[24] Herein,
we report a simple, efficient, and ecofriendly synthesis of free-base phthalo-
cyanines from corresponding phthalonitriles in the presence of 1,8-diazabicy-
clo-[5,4,0]-undec-7-ene (DBU) in N-(2-hydroxyethyl)-N,N-dimethylbutyl
amonium bromide (1a) and N-(2-hydroxyethyl)-N,N-dimethylbutylamonium
hexafluorophosphate (1b) ionic liquids (Fig. 1) at 1408C.
RESULTS AND DISCUSSION
Different metal-free phthalocyanines have been prepared by cyclotetramer-
ization of various phthalonitriles in the presence of DBU in hydroxyalky-
lammonium ionic liquids (1a and 1b) at 1408C (Scheme 1). The presence
of DBU is important for the synthesis of free-base phthalocyanines under
the condition of the experiment, as no phthalocyanine formation was
observed in the reaction of phthalonitrile (2a) in ionic liquid 1a in the
absence of DBU at 1408C (Table 1). Further, the concentration of DBU in
the reaction of 2a affected not only the rate of cyclotetramerization of 2a
but also the yield of phthalocyanine (3a) (Table 1). The reaction of 2a in
presence of 2 equivalents of DBU in ionic liquid 1a at 1408C gave 3a in
47% yield in 10 min, whereas only 27% yield of 3a was obtained with 1
equivalent of DBU in 30 min. The rate of phthalocyanine formation as
Figure 1. Structure of ionic liquids.

Metal-Free Phthalocyanines
2919
Scheme 1. Synthesis of phthalocyanines in ionic liquids.
well as the yield of 3a enhanced with increasing DBU concentration in the
reaction of 2a in ionic liquid 1a at 1408C. A decrease in reaction temperature
affected more the rate of phthalocyanine formation than the yield of 3a as
the reaction of 2a in the presence of DBU (2 equiv) in 1a gave 3a in 45%
yield at 1208C after 2 h. Similarly, the reaction of 2a in the presence of
DBU (2 equiv) in ionic liquid 1b gave 3a in 24% yields at 1408C. The
lower yield of 3a in 1b than in ionic liquid 1a could be attributed to
decomposition of PF6 anion to hydrofluoric acid or fluoride in the
presence of a base.[25]
The formation of phthalocyanine (3a) in the reaction of 2a in the presence
of DBU in different ionic liquids (1a and 1b) was confirmed by different spec-
troscopic analysis.[1] The UV-visible spectrum of 3a showed characteristic
absorption pattern consisting of two Q bands at 656 nm and 693 nm in N,N-
dimethylformamide. The disappearance of the band due to -CN stretching at
ꢀ2200 cm21 and the appearance of N-H stretching band at 3278 cm21 in IR
spectrum of 3a suggested the formation of 3a by cyclotetramerization of 2a
in the presence of DBU in ionic liquids. The appearance of a peak at m/z
514.1470 in the ESI-MS spectrum of 3a corresponding to its [Mþ] molecular
Table 1. Synthesis of phthalocyanine (3a) in different ionic liquids (1a and 1b) from
phthalonitrile (2a) under different reaction conditions
Yield of
3aa(%)
Entry
Ionic liquid
Conc. of DBU
Temp (8C)
Time
24 h
1
2
3
4
5
6
7
1a
1a
1a
1a
1a
1a
1b
0 equ.
140
140
140
140
140
120
140
0
11
17
27
47
45
24
Catalytic amount
0.5 equ.
1 equ.
2 h
1 h
30 min
10 min
2 h
2 equ.
2 equ.
2 equ.
1 h
aIsolated yield.

2920
S. M. S. Chauhan, S. Agarwal, and P. Kumari
Table 2. Synthesis of different phthalocyanines in the presence of DBU in ionic
liquid 1a
Entry
Phthalonitriles (2)
Time (h)
Product
Yield of 3a (%)
1
2
3
4
5
6
2b
2c
2d
2e
2f
5
3
3bb
3cb
3db
3eb
3f
18
25
24
20
17
12
3
6
48
48
2g
3g
aIsolated yield.
bMixture of constitutional isomers.
mass further confirmed the structure of 3a. The reaction of 4-methoxyphthalo-
nitrile (2b) at 1408C in the presence of DBU in ionic liquid 1a gave 3b in 18%
yield (Table 2). Similarly other phthalocyanines (3c–3g) were prepared in
moderate yields (Table 2) by the reaction of corresponding phthalonitriles in
the presence of DBU in ionic liquid 1a at 1408C. It has been observed
that on increasing the chain length of the alkoxy group of phthalonitrile,
yields of phthalocyanines decreased, but in this series, yield of 2(3),9(10),
16(17),23(24)-tetramethoxyphthalocyaine (3b) is less than that of 2(3),9(10),
16(17),23(24)-tetrapentoxyphthalocyaine (3c) (18% and 25 % yields, respect-
ively). The structures of various phthalocyanines (3b–3g) were characterized
by different spectroscopic techniques.
For phthalocyanine 3a, recyclability of ionic liquid 1a was examined. The
ionic liquid 1a was recovered from the reaction mixture by washing the crude
product with distilled water. Water was evaporated at 708C under reduced
pressure, and the resulting ionic liquid was reused three times for phthalocya-
nine synthesis. In the second run, phthalocyanine 3a was obtained in 42%
yield by cyclotetramerization of 2a in the presence of DBU in recovered
ionic liquid 1a. The reactivity of ionic liquid reduced appreciably in the
third run as 3a was isolated in 20% yield.
The results suggested that ionic liquid is playing an important role in
cyclotetramerization of phthalonitriles. It was believed that DBU, like other
bases, abstracts protons from the hydroxyalkylammonium ionic liquid, and
the resulting alkoxide ion[25] attacks one of the cyano groups of phthalonitrile,
leading to formation of phthalocyanine under the conditions of the reaction.[8]
In conclusion, the synthesis of metal-free phthalocyanines by
cyclotetramerization of different phthalonitriles in the presence of DBU in
hydroxyalkylammonium ionic liquids is simple and efficient. The method
offers various advantages such as fewer side products, easy isolation of
products, and recyclability of ionic liquid. The moderate yields of various
functionalized phthalocyanines have been obtained under the condition of
the experiment.

Metal-Free Phthalocyanines
2921
EXPERIMENTAL
1
The H NMR spectra were recorded on a Brucker 300-MHz spectrometer
using TMS as internal standard and CDCl3 as solvent, and the chemical
shifts were expressed in parts per million (ppm). IR spectra were recorded
on a Perkin-Elmer Spectrum 2000 infrared in nujol and CHCl3, and nmax
were expressed in cm21. UV-visible spectra were recorded on a Perkin-
Elmer Lambda 35 UV/VIS spectrophotometer, and absorption maxima
have been expressed in nm. ESI-MS spectra were recorded on a Micromass
LCT KC 455 using electrospray positive ion mass spectra. Melting points
were taken on a Thomas Hoover capillary melting-point apparatus, and all
the melting points are uncorrected. All reactions were carried out under an
N2 atmosphere except where mentioned. The phthalonitriles 2b [3], 2c
[26]
,
were prepared by
[27]
[24]
2d [23], 2e [26], 2f [27], and 2g
and ionic liquid 1a
minor modifications of literature procedures. Phthalonitrile and
4-methylphthalonirile were obtained from Spectrochem. Pvt. Ltd. and
Aldrich, respectively. All other solvents and reagents were used as received.
Preparation of Butyl-(2-hydroxyethyl)-dimethylammonium
Hexafluorophosphate (1b)
Butyl-(2-hydroxyethyl)-dimethylammonium bromide (1a) (2.26 g, 10 mmol)
was dissolved in dry acetone (30 mL) and stirred with KPF6 (2.42 g,
11 mmol) at room temperature for 24 h to exchange the anion. The reaction
mixture was filtered, evaporated, and dried under vacuum for 6 h. Yield:
94.2% (3.08 g). IR (neat) nmax (cm21): 3397, 2969, 1659, 1485, 1421, 1389,
1132, 1087, 977, 922, 838, 440. 1H NMR d (CDCl3): 3.0–3.2 (m, 2H,
CH2OH), 2.8 (m, 2H, NþCH2CH2OH), 2.3 (m, 6H, 2CH3), 1.9–2.1 (m, 2H,
C3H7CH2Nþ), 1.7–1.8 (m, 2H, NþCH2CH2C2H5), 1.5 (s, 5H, 3 CH3CH2).
ESI-MS (m/z): 146.2468 (Mþ-PF6).
Preparation of Phthalocyanines
Procedure A: For Phthalocyanine (3a)
A mixture of phthalonitrile (2a) (200 mg, 1.56 mmol) and ionic liquid (1a and
1b) (1.0 g) was heated at 1408C for 10 min. After addition of DBU (470 mL,
3.12 mmol), the reaction mixture was further heated for additional 10 min
with stirring under a nitrogen atmosphere to give a deep green color. The
reaction mixture was cooled to room temperature, and distilled water
(50 ml) was added to it. The product was separated by filtration as a green
solid, and the filtrate (A) was collected in a round-bottomed flask. The
green solid was washed with aqueous methanol (50:50) followed by

2922
S. M. S. Chauhan, S. Agarwal, and P. Kumari
methanol (30 ml) until the washings were colorless. The solid was air dried to
give 94 mg (47%) of phthalocyanine (3a). Water was evaporated from filtrate
(A) at 708C by rotatory evaporator to recover the ionic liquid, which was
reused for phthalocyanine synthesis.
Spectroscopic Data for 3a
Phthalocyanine (3a):[8] UV-VIS [DMF, lmax/nm, (Amax)]: 276.76 (2.45),
603.82 (0.06), 639.39 (0.088), 656.39 (0.18), 693.29 (0.19). IR (nujol) nmax
(cm21): 3278, 1608, 1499, 1320, 1276, 1157, 1118, 1093, 1004, 873, 735,
714. ESI-MS (m/z): 514.1470 (Mþ).
Spectroscopic Data for 3b–3g
The procedure is same as for phthalocyanine 3a. For the workup of phthalo-
cyanines (3b–3g), the reaction mixture was quenched with water and
extracted with CHCl3. The organic layer was washed with water, dried over
anhydrous Na2SO4, concentrated, and purified by column chromatography
over neutral alumina to give pure phthalocyanines (3b–3g).
Data
2(3),9(10),16(17),23(24)-Tetramethoxyphthalocyanine (3b):[8] UV-VIS
[CHCl3, lmax/nm, (Amax)]: 288.79 (0.18), 344.03 (0.32), 392.04 (0.16),
607.78 (0.12), 642.18 (0.22), 666.18 (0.48), 702.61 (0.55). IR (nujol) nmax
(cm21): 3286, 1613, 1345, 1242, 1098, 1012, 821, 743, 721. ESI-MS (m/z):
634.0889 (Mþ).
2(3),9(10),16(17),23(24)-Tetrapentyloxyphthalocyanine (3c):[28] UV-VIS
[CHCl3, lmax/nm, (Amax)]: 289.7 (0.21), 344.04 (0.36), 389.28 (0.18),
607.78 (0.14), 668.98 (0.55), 706.03 (0.65). IR (nujol) nmax (cm21): 3289,
1726, 1663, 1612, 1345, 1241, 1098, 1013, 835, 753. ESI-MS (m/z):
858.1573 (Mþ).
2(3),9(10),16(17),23(24)-Tetraoctyloxyphthalocyanine (3d):[23] UV-VIS
[CHCl3, lmax/nm, (Amax)]: 289.51 (0.25), 343.93 (0.44), 389.77 (0.22),
607.79 (0.17), 646.35 (0.29), 668.76 (0.69), 705.80 (0.81). IR (nujol) nmax
(cm21): 3294, 1726, 1663, 1611, 1349, 1242, 1100, 1023, 832, 750. ESI-
MS (m/z): 1026.2902 (Mþ).
2(3),9(10),16(17),23(24)-Tetrakis(dodecyloxy)phthalocyanine (3e): UV-VIS
[CHCl3, lmax/nm, (Amax)]: 289.53 (0.31), 343.72 (0.50), 389.76 (0.26),
608.10 (0.20), 668.85 (0.76), 705.78 (0.90). IR (nujol) nmax (cm21): 3293,
1
1612, 1345, 1244, 1116, 1097, 1010, 824, 748, 721. H NMR d (CDCl3):

Metal-Free Phthalocyanines
2923
7.79–6.76 (m, 12H, ArH), 3.98 (m, 8H, OCH2), 1.96 (m, 8H, OCH2CH2), 1.59–
1.27 (m, 80H, CH2), 0.97 (m, 12H, CH3). ESI-MS (m/z): 1251.2595 (Mþ).
2,3,9,10,16,17,23,24-Octakis(hexadecyloxy)phthalocyanine (3f):[27] UV-
VIS [CHCl3, lmax/nm, (Amax)]: 295.62 (0.21), 348.44 (0.22), 425.04 (0.08),
603.86 (0.05), 648.02 (0.10), 665.23 (0.26), 702.77 (0.32). IR (film) nmax
(cm21): 3204, 2919, 2851, 1594, 1466, 1383, 1283, 1094, 1026, 854, 743,
721. ESI-MS (m/z): 852.2440 (M3þ þ K).
2,3,9,10,16,17,23,24-Octakis(octadecyloxy)phthalocyanine (3g):[27] UV-
VIS [CHCl3, lmax/nm, (Amax)]: 290.26 (0.65), 347.96 (0.49), 421.17 (0.11),
603.18 (0.11), 648.36 (0.24), 664.72 (0.54), 702.41 (0.67). IR (film) nmax
(cm21): 3437, 2919, 2850, 1591, 1467, 1382, 1279, 1096, 1023, 854, 742,
721. ESI-MS (m/z): 668.340 (M4þ þ 2H).
ACKNOWLEDGMENTS
Pratibha Kumari and Shweta Agarwal acknowledge the Council of Scientific
and Industrial Research (CSIR), New Delhi, for awards of senior research
fellowship and junior research fellowship. S. M. S. Chauhan acknowledges
CSIR and the Department of Science and Technology (DST), New Delhi,
for financial support.
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