3,6-didecyloxyphthalonitrile as a starting compound for the selective synthesis of phthalocyanines of the ABAB type

Article abstract of DOI:10.1023/A:1009576009714

Reactions of 3,6-dipentyloxy-and 3,6-didecyloxyphthalonitriles with lithium pentoxide in pentan-1-ol were studied. 3,6-Didecyloxyphthalonitrile can be used for the preparation of both the corresponding octasubstituted phthalocyanines and phthalocyanines of the ABAB type.

Full text of DOI:10.1023/A:1009576009714

Russian Chemical Bulletin, International Edition, Vol. 49, No. 12, December, 2000
2027
Organic Chemistry
3,6-Didecyloxyphthalonitrile as a starting compound
for the selective synthesis of phthalocyanines of the ABAB type
Å. V. Kudrik,« I. Yu. Nikolaev, and G. P. Shaposhnikov
Ivanovo State University of Chemistry and Technology,
7 prosp. F. Engel´sa, 153460 Ivanovo, Russian Federation.
Fax: +7 (093 2) 41 7995. E-mail: isl@icti.ivanovo.su
Reactions of 3,6-dipentyloxy- and 3,6-didecyloxyphthalonitriles with lithium pentoxide in
pentan-1-ol were studied. 3,6-Didecyloxyphthalonitrile can be used for the preparation of both
the corresponding octasubstituted phthalocyanines and phthalocyanines of the ABAB type.
Key words: phthalocyanine, asymmetrical substitution, selectivity, isoindole derivatives,
phthalonitrile.
Due to the presence of the unique two-circuit conju-
gation macrosystem and the high chemical and thermal
stability, phthalocyanines (Pc) find wide use as organic
dyes,1 materials for sensors2 and microelectronics,3 and
thermotropic and lyotropic liquid crystals.4
Recently, owing primarily to the discovery of the
nonlinear-optical effect,5 the synthesis and the nonlin-
ear-optical properties of unsymmetrically substituted
phthalocyanines containing simultaneously electron-do-
nating and electron-withdrawing substituents have been
extensively studied.6—8 It has also been found9 that some
compounds of this type are very convenient for the
formation of Langmuir—Blodgett films.
The synthesis in which one of the phthalogens intro-
duced into condensation is sterically hindered, i.e., can-
not undergo tetramerization to form the phthalocyanine
ring, seems to be a very promising procedure because it
substantially facilitates the separation of isomers. Previ-
ously, tetraphenylphthalonitrile10 and 2,3-dicyano-1,4-
diphenylnaphthalene11 have been used as sterically hin-
dered compounds.
With the aim of developing procedures for the selec-
tive synthesis of phthalocyanines of the ÀÂÀÂ type, we
studied the reactions of 3,6-dipentyloxy- and 3,6-di-
decyloxyphthalonitriles (1 and 2, respectively) with
lithium pentoxide in pentan-1-ol.
The major procedure for the preparation of unsym-
metrically substituted phthalocyanines involves statisti-
cal condensation of two phthalonitriles or 1,3-diimino-
isoindolines containing different substituents. However,
this procedure has a number of serious drawbacks the
major of which are the difficulties in the separation of
isomers and the difference in the reactivities of
phthalogens introduced into condensation.
3,6-Dialkoxyphthalonitriles have for a rather long
time been known as the starting compounds in the
synthesis of the corresponding octasubstituted phthalo-
cyanines soluble in organic solvents.12 This synthesis is
carried out by adding metallic lithium to boiling alco-
holic solutions of the dinitriles. Due to steric hindrances
caused by the presence of substituents at positions 3 and
6, the pronounced trans effect would be expected to
Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 12, pp. 2062—2065, December, 2000.
1066-5285/00/4912-2027 $25.00 © 2000 Plenum Publishing Corporation

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Russ.Chem.Bull., Int.Ed., Vol. 49, No. 12, December, 2000
Kudrik et al.
Scheme 1
occur in the synthesis of mixed phthalocyanines result-
ing in the predominant formation of compounds of the
ÀÂÀÂ type. Actually, it was demonstrated that the
selective synthesis of porphyrazines of the ÀÂÀÂ type
with the use of 4,7-diisopropoxy-1,3-diiminoisoindoline
as the starting compound sometimes afforded the
trans isomer in a yield several times higher than that of
the cis isomer (porphyrazine of the ÀÀÂÂ type) depend-
ing on the nature of the second component.13
CN
CN
AlkOH
LiOAlk
AlkO
OAlk
OAlk
N
N
+
–n AlkOH
(n = 4—8)
NH
NH2
Results and Discussion
Refluxing of phthalonitrile 1 with a solution of
n-C5H11OLi in pentan-1-ol afforded octasubstituted
phthalocyanine (3). Under analogous conditions, phtha-
lonitrile 2 did not produce even traces of phthalocya-
nine. At the same time, the IR spectrum of the reaction
mixture did not have bands corresponding to Ñ≡N
stretching vibrations of the nitrile groups, which is in-
dicative of their conversion with the possible formation
of the isoindole ring. The addition of a new portion of
metallic Li to a boiling solution of compound 2 in
pentan-1-ol containing C5H11OLi rapidly yielded
1,4,1´,4´,1″,4″,1´´´,4´´´-octakis(decyloxy)phthalocyanine
4, which is identical with that described previously.12
N
N
N
N
AlkCH2OH
–AlkCHO
Pc
N
N
N
N
Phthalonitrile 1 in the C5H11OLi—C5H11OH system
underwent cyclization to form dehydrophthalocyanine,
which was reduced yielding octapentyloxyphthalocyanine
3. Under analogous conditions, compound 2 cannot be
converted into the corresponding dehydrophthalocyanine
due to steric hindrances caused by "twisting" of the decyl
chains; instead, it formed only mono- and dialkoxy
derivatives. Apparently, the repeated addition of Li to
the reaction mixture caused reduction of the above-
mentioned alkoxy derivatives, giving rise to octa-
kis(decyloxy)phthalocyanine 4.
RO
OR
RO
RO
RO
RO
OR
OR
N
N
N
NH HN
N
CN
CN
N
N
The addition of a 10-fold excess of phthalonitrile to a
boiling solution of compound 2 in pentan-1-ol contain-
ing C5H11OLi afforded a mixture of unsubstituted phtha-
locyanine and an isomer of the ÀÂÀÂ type (5). The
mass spectrum (FAB-MS) of phthalocyanine 5 has two
major groups of peaks. The first group containing the
major peak at m/z 1139.7 corresponds to the molecular
ion (Ì = 1138.77). The second group is characterized
by an intense peak at m/z 577.2 [M + H+ – 4 Ñ10Í21].
1, 2
RO
OR
3, 4
R = C5H11 (1, 3); C10H21 (2, 4)
We believe that the above-mentioned differences in
the chemical behavior of compounds 1 and 2 result from
the following fact. It is known14 that the first stage of the
reaction of phthalonitrile with alcohols catalyzed by
alkali metal alkoxides affords monoalkoxy and dialkoxy
derivatives, which then undergo tetramerization to form
dehydrophthalocyanine (Scheme 1).
The transformation into phthalocyanine requires the
participation of a reducing agent. Either alcohol, which
is converted into the corresponding aldehyde, or alkali
metal can act as such a reducing agent. Thus it was
demonstrated14 that dehydrophthalocyanine could be
isolated from the reaction mixture in the absence of
protic solvents (lithium in anhydrous benzene). Subse-
quent treatment with protic solvents (alcohols or H2O)
afforded phthalocyanine.
H21C10O
OC10H21
N
N
N
NH HN
N
N
N
H21C10O
OC10H21
5

Synthesis of phthalocyanines of the ABAB type
Russ.Chem.Bull., Int.Ed., Vol. 49, No. 12, December, 2000 2029
D
The formation of octakis(decyloxy)phthalocyanine 4
and isomers of the ÀÂ3 and ÀÀÂÂ types was not ob-
served and the À3Â isomer was obtained only as an
admixture. An unexpectedly high selectivity of the pro-
cess is attributed to the fact that compound 2 cannot
undergo tetramerization under conditions used for the
synthesis.
1.0
0.8
0.6
0.4
0.2
Under conditions of statistical condensation of a
mixture of two dinitriles, mixtures of all six possible
isomers are always formed except for rare cases in which
one of dinitriles is sterically hindered.15 Steric hin-
drances in one of the starting dinitriles result in the
formation of mixtures of only three isomers, viz., À4,
À3Â, and ÀÂÀÂ,10 where À is the non-hindered unit.
Therefore, the fact that the mass spectra do not have
signals corresponding to molecular ions of the ÀÂ3 and
Â4 isomers allows one to conclude with a fair degree of
assurance that the mixture does not contain an isomer of
the ÀÀÂÂ type whose spectrum is analogous to the mass
spectrum of the target product 5 and also supports the
previous suggestion that 3,6-didecyloxyphthalonitrile can
be used as a sterically hindered dinitrile for the synthesis
of phthalocyanines of the ABAB type. The structure of
compound 5 was also confirmed by the 1Í NMR spec-
300
400
500
600
700
λ/nm
Fig. 1. UV spectrum of compound 5 (solution in ÑHCl3).
of the molecule resulting in a substantial bathochromic
shift of the Q band. From comparison of the electronic
absorption spectra of phthalocyanines 5 and 3, it may be
concluded that the electron density redistribution takes
place in molecule 5. In this case the fragments contain-
ing the decyloxy groups act as electron donors, whereas
the fragments without these groups act as electron ac-
ceptors. The UV spectral data also confirm the proposed
structure of phthalocyanine 5, which follows from com-
parison of its spectra with the spectra of the structurally
very similar phthalocyanine of the ÀÂÀÂ type prepared
based on compound 1 and tetraphenylphthalonitrile.15
Therefore, we demonstrated that the use of 3,6-di-
decyloxyphthalonitrile as a sterically hindered dinitrile
allows one to perform the selective synthesis of phthalo-
cyanines of the ÀÂÀÂ type and of the corresponding
octasubstituted phthalocyanine.
1
tral data. The Í NMR spectrum of phthalocyanine 5
has only one triplet corresponding to eight protons of
the α-ÑÍ2 groups, which is indicative of the equivalence
of all four α-methylene groups.
The presence of the low-intensity signals with
m/z [M – 70] and [Ì – 140] in the mass spectrum is
attributable to the fact that the synthesis of phthalocya-
nine 5 can be accompanied by transetherification and
one or two decyloxy fragments are replaced by the
pentyloxy groups.
Experimental
The electronic absorption spectrum of phthalocya-
nine 5 (Fig. 1) has two groups of signals. The Soret band
(a2u—b*1g, π—π*-transition) for this compound is shifted
bathochromically by 6 nm compared to its position in
the spectrum of phthalocyanine 3 (337.7 and 331.5 nm,
respectively). The replacement affects only slightly the
position of this band because the à2u orbital is localized
primarily on the nitrogen atoms of the phthalocyanine
ring and the introduction of substituents at the periphery
of the molecule exerts only an insignificant perturb-
ing effect. The position of the Q band (a1u—b*1g,
π—π*-transition) depends on the nature of the substitu-
ents to a substantially greater extent due to localization
of the à1u orbital on the carbon atoms of the isoindole
fragments. This band is split into two components with
approximately equal intensities, which is also character-
istic of unsubstituted phthalocyanine. It should be noted
that the long-wavelength component of the Q band for
compound 5 is hypsochromically shifted by 33 nm com-
pared to its position in the spectrum of compound 3
(737.1 and 770.0 nm, respectively). It is known that the
introduction of the electron-donating substituents into
the phthalocyanine molecule leads to an increase in the
electron density in the single conjugation macrosystem
The electronic absorption spectra were measured on a
Hitachi UV-2000 instrument in CHCl3. The 1Í NMR spectra
were recorded on a Bruker AM-200 spectrometer.
The IR spectra of compound 2, which has been refluxed in
the pentan-1-ol—lithium 1-pentoxide mixture for 2 h, were
measured on a Specord M-60 instrument in the frequency
–1
region of 2400—2100 cm at ∼20 °C.
Compounds 1 and 2 were synthesized according to a proce-
dure reported previously,13 which was insignificantly modified,
viz., the temperature of the process was increased to 100 °Ñ.
The resulting compounds were purified by double recrystalliza-
tion successively from light petroleum and DMF. The com-
pounds were identified based on the data of elemental analysis
and comparison of the melting points of the resulting com-
pounds with the published data.12
1,4,1´,4´,1″,4″,1´´´,4´´´-Octapentyloxyphthalocyanine (3).
3,6-Dipentyloxyphthalonitrile (1) (2 g, 0.67 mmol) was added
to
a solution of C5H11OLi (prepared from Li (0.15 g,
2.14 mg-at.)) in pentan-1-ol (50 mL). The resulting solution
was refluxed with stirring for 30 min and cooled. Then ÀñÎÍ
(5 mL) and acetone (100 mL) were added. The precipitate that
formed was filtered off, dissolved in ÑÍÑl3, and chromato-
graphed on a column with Al2O3 (the 20 : 1 ÑHCl3—acetone
mixture as the eluent). The eluate was concentrated. The target
product was recrystallized from DMF, washed with acetone,
and dried at 60 °Ñ. Phthalocyanine 3 was obtained as a crystal-
line bright-green powder in a yield of 1.13 g (56%). Found (%):

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Kudrik et al.
Ñ, 71.73; Í, 8.28; N, 9.37. Ñ72H98N8O8. Calculated (%):
Ñ, 71.85; H, 8.21; N, 9.31. UV, λmax/nm (log ε): 770.0 (5.16),
694.0 (4.58), 458.0 (4.19), 419.0 (4.24), 331.5 (4.78). FAB-MS
(m-nitrobenzyl alcohol), m/z: 1203.9 [Ì + H]+.
1.82 g (62%). The UV spectra of the resulting compound are
identical with those reported previously.16
We thank Professor T. Torres (Autonomous Univer-
sity of Madrid) for help in measuring the FAB mass
spectra.
1,4,1´,4´,1″,4″,1´´´,4´´´-Octakis(decyloxy)phthalocyanine (4).
3,6-Didecyloxyphthalonitrile (2) (1 g, 0.23 mmol) was added to
a solution of C5H11OLi (prepared from Li (0.075 g, 1.07 mg-at.))
in pentan-1-ol (20 mL) and the reaction mixture was refluxed
with stirring for 2 h. The solution remained colorless. Then an
additional amount of lithium (0.075 g) was added and the
mixture was refluxed for 20 min. After cooling, ÀñÎÍ (5 mL)
and acetone (100 mL) were added to the bright-green solution.
The precipitate that formed was filtered off, dissolved in ben-
zene, and chromatographed on a column with Al2O3 (Ñ6Í6 as
the eluent). The eluate was concentrated. The residue was
washed with acetone and dried at 60 °Ñ. Phthalocyanine 4 was
obtained as a waxy bright-green substance in a yield of 0.43 g
(43%), m.p. 47—49 °Ñ. Found (%): Ñ, 75.94; Í, 10.48; N, 6.83.
Ñ112H178N8O8. Calculated (%): Ñ, 76.23; H, 10.92; N, 6.35.
UV, λmax/nm (D): 775.0 (1.579), 331.0 (1.397).
1,4,1″,4″-Tetrakis(decyloxy)phthalocyanine (5). 3,6-Dide-
cyloxyphthalonitrile (2) (1 g, 0.23 mmol) was added to a
solution of C5H11OLi (prepared from Li (0.15 g, 2.14 mg-at.))
in pentan-1-ol (50 mL). The reaction mixture was refluxed for
2 h. Then phthalonitrile (2.6 g, 2.03 mmol) was added and the
mixture was refluxed for 4 h. The resulting suspension was
cooled, AcOH (5 mL) was added, and the mixture was diluted
with MeCN (200 mL). The precipitate that formed was filtered
off, the products were extracted with ÑÑl4 using a Soxhlet
apparatus, and the extract was chromatographed on a column
with Al2O3 (benzene as the eluent). The eluate was concen-
trated and the residue was washed with acetone and dried at
40 °Ñ. Tetrasubstituted phthalocyanine 5 was obtained as a blue
powder in a yield of 0.091 g (7%). Found (%): Ñ, 74.92;
Í, 8.48; N, 10.02. Ñ72H98N8O4. Calculated (%): Ñ, 75.88;
H, 8.67; N, 9.83. UV, λmax/nm (log ε): 337.7 (4.82), 434.4
(4.08), 644.5 (4.48), 712.3 (4.87), 737.1 (4.84). 1Í NMR
(ÑDCl3), δ: 0.72—0.85 (m, 12 Í, CH3); 1.10—1.23 (m, 56 Í,
ÑÍ2); 1.55—1.65 (m, 8 Í, β-ÑÍ2); 4.02 (t, 8 Í, α-ÑÍ2).
FAB-MS (m-nitrobenzyl alcohol), m/z: 1139.7 [Ì + H]+,
577.2 [Ì + H – 4 Ñ10Í21]+.
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After completion of extraction, the residue was withdrawn
from the Soxhlet apparatus and reprecipitated from concen-
trated H2SO4 (50 mL) by pouring into ice water. Then the
precipitate was filtered off, washed with water, and dried at
120 °Ñ. Unsubstituted phthalocyanine was obtained in a yield of
Received January 21, 1999;
in revised form July 6, 2000