Metal-free phthalocyanines are an important class of phthalocyanines,
which are usually prepared from phthalonitriles in
the presence of basic catalyst [14], CeCl3 [15], Na2S [16] and
HMDS [11a] at high temperature and very long reaction times.
As indicated in Scheme 2, application of microwave technology
to this reaction decreased the time required to obtain good
yields by a factor of about 240. For example, the time required
for the conversion of phthalonitrile to metal-free phthalocyanine
(72%), at 140e160 C after 1440 min is reduced to
5e6 min when microwave irradiation is used.
Optimization of reaction conditions, the yields and necessary
times for the syntheses of H2Pc were indicated in Table 1.
It was found that the molar ratio of the reactants influenced
the conversion as well as yields, and the best results were
obtained when 1:2:0.1:1 mole ratio of phthalonitrile, HMDS,
(NH4)2SO4 and DMF, respectively, were used.
It is important to note that in the absence of each of the
reagents, the reaction is completely stopped (Table 1, experiments
2, 4 and 7).
We also repeated the experiments in the presence of
Na2S$xH2O under MWirradiation and classical heating conditions.
As can be seen from Scheme 3, phthalonitrile is efficiently
converted into the H2Pc in relatively high yields
using MW irradiation, however, attempt for the synthesis of
H2Pc under classical heating conditions at 160e170 C after
180 min is not so successful.
As indicated in Table 2, the optimized amount of reagents
is 4:0.3:6.0 mole ratio of phthalonitrile, Na2S$xH2O and 1,2-
propylene glycol (PG), respectively. Thus, the reaction is not
carried out in the absence of Na2S$xH2O or PG. (Entries
1 and 2 in Table 2.)
2.2. Metallophthalocyanines
The reaction conditions were optimized with CuCl or/and
with various raw materials such as PN, PA and PI. We found
that the molar ratio of 1:4:4:1:0.41 of CuCl:phthalonitrile:
HMDS:DMF:p-TsOH$H2O obtained the best results.
In a typical experiment, the reaction was carried out by the
mixing of PN, PA or PI with HMDS, MXn, p-TsOH and DMF
in a glass tube and the reaction mixture was stirred at room
temperature for 20 min under a nitrogen atmosphere. Then,
the tube was sealed and the mixture was irradiated for
10e15 min in the microwave oven.
To explore the scope and limitations of this reaction further,
we extended the procedure to various raw materials and to different
metal salts such as Cu(OAc)2$H2O, CuCl2$2H2O,
CoCl2, NiCl2, FeCl2$4H2O, ZnCl2, PdCl2, PtCl4 and RuCl3.
As can be seen from Table 3, the different starting materials
such as PN, PA and PI are converted to MPcs with good yields
after several minutes. In general these results showed the PN
to produce the best yield.
It is important to note that the yield of the reaction in the
absence of HMDS and p-TsOH$H2O as a catalyst intensively
decreased and in the case of the PN the reaction is completely
stopped. In addition, the reaction in the absence of DMF with
each of the raw materials has not occurred.
It is important to note that the MPcs of two expensive
metals such as Pt and Ru are prepared efficiently from PN.
However, synthesis of these compounds from PA or PI was
not successful and yields of the reaction were very low or
reaction was completely stopped (entries 9 and 10).
In conclusion, we have developed a convenient and rapid
procedure for the synthesis of H2Pc and MPcs using microwave
irradiation within a few minutes. Investigation toward
extension of this procedure to other derivatives is in
progress.