Cerium-promoted formation of metal-free phthalocyanines

Article abstract of DOI:10.1016/S0040-4039(02)00756-6

Treatment of phthalonitriles with 6 mol% of CeCl₃ or Ce(acac)₃ (acac=acetylacetonate) in refluxing 1-pentanol affords the corresponding metal-free phthalocyanines in moderate yields. This non-alkaline pathway is complementary to the base-promoted cyclization methods which are commonly employed in the synthesis of phthalocyanines. Addition of 1,8-diazabicyclo[5.4.0]undec-7-ene (DBU) greatly shortens the reaction time and increases the yield of these reactions.

Full text of DOI:10.1016/S0040-4039(02)00756-6

TETRAHEDRON
LETTERS
Pergamon
Tetrahedron Letters 43 (2002) 4211–4214
Cerium-promoted formation of metal-free phthalocyanines
Chi-Hang Lee and Dennis K. P. Ng*
Department of Chemistry, The Chinese University of Hong Kong, Shatin, NT, Hong Kong, China
Received 11 December 2001; revised 28 March 2002; accepted 17 April 2002
Abstract—Treatment of phthalonitriles with 6 mol% of CeCl3 or Ce(acac)3 (acac=acetylacetonate) in refluxing 1-pentanol affords
the corresponding metal-free phthalocyanines in moderate yields. This non-alkaline pathway is complementary to the base-pro-
moted cyclization methods which are commonly employed in the synthesis of phthalocyanines. Addition of 1,8-diazabicy-
clo[5.4.0]undec-7-ene (DBU) greatly shortens the reaction time and increases the yield of these reactions. © 2002 Elsevier Science
Ltd. All rights reserved.
Phthalocyanines represent an important class of func-
tional dyes.1 Their characteristic blue–green color and
robustness allowed their traditional use as industrial
pigments. Their usage, however, has been much
extended recently to many high technological avenues.
With appropriate substitution, phthalocyanines can
function as optical recording materials,2 optical lim-
iters,3 field-effect transistors,4 catalysts for photo- or
oxidative-degradation of pollutants,5 and photosensitiz-
ers for photodynamic therapy,6 etc. The synthetic
methodologies for this class of compounds are rela-
tively well established. The metal-free analogues, which
can serve as the precursors for various metallophthalo-
cyanines, are usually prepared by base-promoted
cyclization of phthalonitriles. Some typical procedures
involve the treatment with DBU7 or lithium metal in
alcohol (followed by acidic work-up),8 and reaction in
N,N-dimethylethanolamine under an atmosphere of
ammonia.9 Preparation of metal-free phthalocyanines
under non-alkaline conditions is extremely rare.10 We
report herein a new and general approach to this class
of macrocyclic compounds using a cerium(III) salt as a
promoting agent.
The yields of these products were comparable, if not
better than those obtained by typical base-promoted
cyclization reactions.12 This method, however, could
not be applied to 3,6-dialkoxyphthalonitriles or tetra-
fluorophthalonitrile (1i); no significant amount of
phthalocyanines could be isolated for these dinitriles.
The phthalocyanines 2a–h were purified either by Soxh-
let extraction (for 2a and 2c) or by column chromato-
graphy (for the other phthalocyanines), and character-
ized using various spectroscopic methods.13
As DBU can also promote the tetramerization of
phthalonitriles,7 we examined the effects of this strong
base on these cerium-promoted reactions. It was found
that the addition of ca. 4 equiv. of DBU can greatly
shorten the reaction time (to 6–24 h) and increase the
reaction yield (Table 1). Under these conditions, tetra-
fluorophthalonitrile (1i) could also be cyclized to give 2i
in 73% yield. In comparison to the cyclization reactions
promoted solely by DBU,7,14 these reactions generally
proceed faster and give higher yields of product, show-
ing that the cerium(III) salt plays an essential role in
enhancing the formation of metal-free phthalocyanines.
Apart from phthalonitriles, 1,3-diiminoisoindoline
derivatives could also be used as the precursors. Thus,
treatment of the 1,3-diiminoisoindoline analogue of 1f
with CeCl3 in refluxing 1-pentanol gave 2f in 45% (24 h,
in the absence of DBU) or 53% yield (8 h, in the
presence of DBU).
The new methodology involves the treatment of
11
phthalonitriles with 6 mol% of anhydrous CeCl3 in
1-pentanol at ca. 160°C for 24–72 h. Under these
conditions, a range of phthalonitriles with different
functionalities including nitro, alkoxy, thioalkoxy, and
amino groups were converted into the corresponding
metal-free phthalocyanines in 20–64% yield (Table 1).
Due to the high solubility of 2b and the ease of
purification of this compound by column chromatogra-
phy, the conversion of 1b to 2b was examined in detail
under different reaction conditions (Table 2). It is
worth noting that treatment of 1b with lithium in
Keywords: phthalocyanines; cerium(III) compounds; cyclization.
* Corresponding author. Tel.: (852) 2609 6375; fax: (852) 2603 5057;
e-mail: dkpn@cuhk.edu.hk
0040-4039/02/$ - see front matter © 2002 Elsevier Science Ltd. All rights reserved.
PII: S0040-4039(02)00756-6

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C.-H. Lee, D. K. P. Ng / Tetrahedron Letters 43 (2002) 4211–4214
Table 1. Cerium-promoted cyclization of phthalonitrilesa
1-pentanol did not lead to the formation of 2b. It can
be seen in entries 1–4 (Table 2) that the reaction yield is
greatly affected by the amount of CeCl3 used. About 6
mol% of CeCl3 with respect to the dinitrile 1b seems to
be the optimum conditions. If the reaction proceeded
by cerium-induced tetramerization followed by
removal of the metal center, a higher yield would be
expected when a larger amount of CeCl3 (ca. 0.25
a

C.-H. Lee, D. K. P. Ng / Tetrahedron Letters 43 (2002) 4211–4214
Table 2. Conversion of 1b to 2b in 1-pentanol
4213
Entry
Cerium salt (mol%)a
DBU (equiv.)a
Temp. (°C)
Time (h)
Yield (%)b
1
2
3
4
5
6
7
8
CeCl3 (12)
CeCl3 (6)
CeCl3 (3)
CeCl3 (1.5)
CeCl3 (6)
CeCl3 (6)
CeCl3 (6)
CeCl3 (6)
Ce(acac)3 (12)
Ce(acac)3 (6)
Ce(acac)3 (3)
4
4
4
4
8
8
8
8
4
4
8
160
160
160
160
150
120
150
120
160
160
160
24
24
24
24–48
24
24
48
48
24
18
71
47
0
56
37
69
52
9
9
10
11
24
48
56
15
a With respect to phthalonitrile 1b.
b For a mixture of 2b and its constitutional isomers.
equiv.) was used, however, the reverse was observed
(entries 1–2). The addition of a larger amount of CeCl3
led to the isolation of a substantial amount of the
double-decker complex Ce[Pc(OC7H15)4]2, which was
characterized by UV–vis (umax=315, 624, and 682 nm
in CHCl3) and mass spectrometry. The L-SIMS spec-
trum showed the M+1 peak at m/z 2078.9 together with
successive losses of C7H15 fragments (m/z 1979.8,
1880.6, 1781.5). These results suggest that the cerium-
(III) salt functions as a catalyst rather than simply a
template.
Structure and Function; Cambridge University Press:
Cambridge, 1998.
2. (a) Gu, D.; Chen, Q.; Shu, J.; Tang, X.; Gan, F.; Shen,
S.; Liu, K.; Xu, H. Thin Solid Films 1995, 88, 93; (b)
Birkett, D. Chem. Ind. 2000, March 6, 178.
3. (a) Perry, J. W.; Mansour, K.; Lee, I.-Y. S.; Wu, X.-L.;
Bedworth, P. V.; Chen, C.-T.; Ng, D.; Marder, S. R.;
Miles, P.; Wada, T.; Tian, M.; Sasabe, H. Science 1996,
273, 1533; (b) Dini, D.; Barthel, M.; Hanack, M. Eur. J.
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4. (a) Bao, Z.; Lovinger, A. J.; Dodabalapur, A. Adv.
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M.; Sugimori, T. Inorg. Chem. Commun. 2000, 3, 634; (c)
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Allen, C. M.; Sharman, W. M.; van Lier, J. E. J. Por-
phyrins Phthalocyanines 2001, 5, 161.
As expected, the yield of 2b increases with temperature
(entries 2, 5 and 6) and the reaction time (entries 5–8).
11
Apart from anhydrous CeCl3, Ce(acac)3 can also be
used as the catalyst (entries 9–11), but the efficiency in
promoting this cyclization is generally lower than that
of CeCl3 (compared with entries 1–3). We also
attempted the conversion of 1b to 2b in 1-chloronaph-
thalene instead of 1-pentanol. The reactions, in the
presence or absence of DBU, did not lead to the
expected product, showing that 1-pentanol plays an
important function in the catalytic cycle.
7. Wo¨hrle, D.; Schnurpfeil, G.; Knothe, G. Dyes Pigments
1992, 18, 91.
8. Cook, M. J.; Dunn, A. J.; Howe, S. D.; Thomson, A. J.;
Harrison, K. J. J. Chem. Soc., Perkin Trans. 1 1988,
2453.
In summary, we have developed a convenient method
to prepare metal-free phthalocyanines under neutral
conditions. This procedure is complementary to the
commonly used base-promoted cyclization reactions in
the preparation of this important class of functional
dyes.
9. Brach, P. J.; Grammatica, S. J.; Ossanna, O. A.; Wein-
berger, L. J. Heterocycl. Chem. 1970, 7, 1403.
10. The use of hydroquinone to enhance the formation of
metal-free phthalocyanines has been reported. See for
example: (a) Snow, A. W.; Jarvis, N. L. J. Am. Chem.
Soc. 1984, 106, 4706; (b) George, R. D.; Snow, A. W.
Chem. Mater. 1994, 6, 1587; (c) Walsh, C. J.; Mandal, B.
K. Chem. Mater. 2000, 12, 287.
Acknowledgements
11. Anhydrous CeCl3 was prepared by drying white crystals
of CeCl3·7H2O in vacuo to give white lumps which were
grounded into a fine powder and dried again in vacuo for
2 days. Anhydrous Ce(acac)3 was prepared by drying the
brown powder of Ce(acac)3·nH2O in vacuo for 2 days.
The presence of water in the salts greatly reduced the
yield of the reactions.
We thank The Chinese University of Hong Kong and
the Hong Kong Research Grants Council for support.
References
12. All the phthalocyanines shown in Table 1 (except 2b, 2e,
and 2h) are known compounds. See (a) Christie, R. M.;
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4214
C.-H. Lee, D. K. P. Ng / Tetrahedron Letters 43 (2002) 4211–4214
09,152,729, 1997 (Chem. Abstr. 1997, 127, 155984p) (for
C, 74.20; H, 7.68; N, 11.54. Found: C, 73.97; H, 7.70; N,
11.14%. 2e: H NMR (CDCl3, 300 MHz) l 9.16–9.24 (m,
1
2d); (c) Hanack, M.; Gu¨l, A.; Hirsch, A.; Mandal, B. K.;
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Kokai Tokkyo Koho JP 08,64,492, 1996 (Chem. Abstr.
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Lin, L.; Sugimori, T.; Handa, M.; Abe, K.; Kikkawa, T.;
4H, Ar-H), 8.78–8.79 (m, 4H, Ar-H), 7.69–7.73 (m, 4H,
Ar-H), 4.81–4.85 (m, 4H, OCH), 2.04–2.09 (m, 16H,
CH2), 1.23–1.29 (m, 24H, Me), −0.89 (br s, 2H, NH);
L-SIMS m/z 858.5 (M+); UV–vis (CHCl3) [umax, nm
(log m)] 344 (4.87), 393 (4.58), 611 (4.45), 646 (4.65), 672
(5.06), 709 (5.13). 2h: 1H NMR (CDCl3, 300 MHz) l 8.88
(s, 8H, Ar-H), 4.09 (t, J=6.6 Hz, 16H, OCH2), 3.79–3.82
(m, 16H, OCH2), 3.65–3.70 (m, 32H, OCH2), 3.44 (t,
J=6.7 Hz, 16H, SCH2), 1.51 (quintet, J=7.1 Hz, 16H,
CH2), 1.21–1.33 (m, 16H, CH2), 0.80 (t, J=7.3 Hz, 24H,
Me), −2.60 (br s, 2H, NH); 13C{1H} NMR (CDCl3, 75.4
MHz) l 148.3, 140.2, 133.5, 122.1, 71.3, 70.6, 70.1, 69.8,
33.9, 32.7, 19.2, 13.8; L-SIMS m/z 1922.6 (M+); UV–vis
(CHCl3) [umax, nm (log m)] 326 (4.57), 361 (4.61), 440
(4.31), 631 (4.29), 666 (4.41), 698 (4.94), 730 (5.01).
14. (a) Wo¨hrle, D.; Eskes, M.; Shigehara, K.; Yamada, A.
Fujiwara, T. Bull. Chem. Soc. Jpn. 1997, 70, 1859.
1
13. Selected data for 2b: H NMR (CDCl3, 300 MHz) l 9.10
(d, J=7.5 Hz, 4H, Ar-H), 8.08 (t, J=7.5 Hz, 4H, Ar-H),
7.72 (d, J=7.5 Hz, 4H, Ar-H), 4.70 (t, J=5.4 Hz, 4H,
OCH), 2.57–2.68 (m, 8H, CH), 1.52 (d, J=6.6 Hz, 24H,
Me), 1.22 (d, J=6.6 Hz, 24H, Me), −0.06 (br s, 2H, NH);
13C{1H} NMR (CDCl3, 75.4 MHz) l 158.9, 150.3 (two
overlapping signals), 140.2, 130.9, 123.9, 115.1, 114.0,
89.6, 29.7, 20.4, 18.5; L-SIMS m/z 970.6 (M+); UV–vis
(CHCl3) [umax, nm (log m)] 319 (4.69), 633 (4.38), 667
(4.51), 700 (5.02), 731 (5.05). Anal. calcd for C60H74N8O4:
8
Synthesis 1993, 194; (b) Gu¨rek, A. G.; Bekaroglu, O. J.
Chem. Soc., Dalton Trans. 1994, 1419.