Sustainable approaches to the synthesis of metallophthalocyanines in solution

Article abstract of DOI:10.3390/molecules26061760

This work aims to investigate more sustainable reaction conditions for the synthesis of metallophthalocyanines. Anisole, glycerol and their mixtures have been investigated as reaction media for the tetramerization of phthalonitriles. Acetates of three divalent first-transition metal cations, Co(II), Cu(II) and Zn(II), were used and several bases were tested, depending on the chosen substrates and reaction conditions, with a view to making the whole process more sustainable while ensuring its scalability. Unsubstituted phthalocyanines were synthesized to analyze the behavior of the different metal ions in terms of reactivity in the new reaction media, resulting in a general Cu > Co > Zn trend, while the nonpolar tetra-tert-butyl substitution was investigated to evaluate the synthesis of soluble derivatives in the new conditions. Furthermore, the potassium hydroxide (KOH)-aided statistical synthesis of the unsymmetrical 9(10), 16(17), 23(24)-tri-tert-butyl-2-iodophthalocyaninato zinc(II), starting from 4-tert-butylphthalonitrile and 4-iodophthalonitrile in a glycerol/anisole mixture, proceeded with a satisfactory 26% yield. Our results provide insights into the investigation of new reaction environments and the understanding of their strengths and weaknesses, with a view to further increasing the sustainability of the synthesis of metallomacrocycles with high added value while lowering their production cost.

Full text of DOI:10.3390/molecules26061760

molecules
Article
Sustainable Approaches to the Synthesis of
Metallophthalocyanines in Solution
Gloria Zanotti * , Patrizia Imperatori , Anna Maria Paoletti and Giovanna Pennesi
Istituto di Struttura della Materia (ISM) Consiglio Nazionale delle Ricerche (CNR), Via Salaria km 29.300,
0015 Monterotondo, Italy; patrizia.imperatori@ism.cnr.it (P.I.); annamaria.paoletti@ism.cnr.it (A.M.P.);
gianna.pennesi@ism.cnr.it (G.P.)
Correspondence: gloria.zanotti@ism.cnr.it; Tel.: +39-06-90672318
0
*
Abstract: This work aims to investigate more sustainable reaction conditions for the synthesis of
metallophthalocyanines. Anisole, glycerol and their mixtures have been investigated as reaction
media for the tetramerization of phthalonitriles. Acetates of three divalent first-transition metal
cations, Co(II), Cu(II) and Zn(II), were used and several bases were tested, depending on the chosen
substrates and reaction conditions, with a view to making the whole process more sustainable while
ensuring its scalability. Unsubstituted phthalocyanines were synthesized to analyze the behavior of
the different metal ions in terms of reactivity in the new reaction media, resulting in a general Cu > Co
>
Zn trend, while the nonpolar tetra-tert-butyl substitution was investigated to evaluate the synthesis
of soluble derivatives in the new conditions. Furthermore, the potassium hydroxide (KOH)-aided
statistical synthesis of the unsymmetrical 9(10), 16(17), 23(24)-tri-tert-butyl-2-iodophthalocyaninato
zinc(II), starting from 4-tert-butylphthalonitrile and 4-iodophthalonitrile in a glycerol/anisole mixture,
proceeded with a satisfactory 26% yield. Our results provide insights into the investigation of new
reaction environments and the understanding of their strengths and weaknesses, with a view to
further increasing the sustainability of the synthesis of metallomacrocycles with high added value
while lowering their production cost.
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ꢁꢂꢀꢃꢄꢅꢆꢇ
ꢀ
ꢁꢂꢃꢄꢅꢆ
Citation: Zanotti, G.; Imperatori, P.;
Paoletti, A.M.; Pennesi, G.
Sustainable Approaches to the
Synthesis of Metallophthalocyanines
in Solution. Molecules 2021, 26, 1760.
https://doi.org/10.3390/
Keywords: sustainability; alternative solvents; metallomacrocycles
molecules26061760
Academic Editors:
M. Salomé Rodríguez-Morgade
and Soji Shimizu
1
. Introduction
Sustainability keeps gaining increasing importance in many technological fields,
such as energy production, conversion, and storage on large- and small-scales, and the
synthesis of high-added value chemicals for a variety of technologies. Applying the green
chemistry principles to synthetic processes makes them more affordable both from a sus-
tainable and, as a consequence, an economical point of view, thanks to a rational design of
the synthesis aimed at reducing wastes, choosing reagents and reactants in order to retain
the most of their constituting atoms in the final products, and selecting appropriate and
low-impacting reaction media.
Since solvents usually represent a significant portion of the whole material involved
in a chemical process, their replacement for “greener” alternatives is an effective way to
improve the sustainability of the entire synthesis. In principle, solvent-free conditions
would be the optimum choice; however, most of the synthetic applications consistently
benefit from the use of a solvent or a solvent mixture for several reasons. Dissolving
reagents in a liquid medium (i) minimizes inhomogeneities both in the temperature of
the reaction and in the reagents ratios, (ii) promotes the selectivity of the reaction due to
interactions between the solvent molecules and the reagents and inducing the activation of
specific molecular positions among others, and (iii) enables an easier handling, processing
and purification of the resulting crude mixture [1,2].
Received: 15 February 2021
Accepted: 17 March 2021
Published: 21 March 2021
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Phthalocyanines have been among the most important family of dyes and pigments
since their discovery, around 100 years ago. The central cavity of their aromatic structure
4
.0/).
Molecules 2021, 26, 1760. https://doi.org/10.3390/molecules26061760
https://www.mdpi.com/journal/molecules

Molecules 2021, 26, 1760
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can host a wide variety of metal ions, and the macrocycle periphery can be rationally
tailored to tune their chemical, optical, and electronic properties according to their final
application, resulting in an ever-increasing range of structures that are widely implemented
in a variety of sectors, such as catalysis [
3,4
], gas sensing [
5], photodynamic therapy [6],
organic electronics [ ] and hybrid photovoltaics [9–
7
,
8
12]. The synthesis of phthalocyanines
can be performed in several ways: a cheap and scalable protocol, currently used to synthe-
size industrially relevant pigments such as the phthalocyanine-blue, involves the reaction
of phthalic anhydride with urea as the nitrogen source and a catalytic amount of ammo-
nium heptamolybdate. On the lab scale, substituted and unsubstituted phthalocyanines are
usually synthesized starting from phthalonitriles, either with or without a templating salt
depending on the desired coordination of their central cavity. From the perspective of green
chemistry, the atom economy, namely, the amount of atoms of the reagents that are retained
in the final products, of the “phthalonitrile route” is consistently higher than that of the
“
phthalic anhydride route”. Nevertheless, it still often requires an organic base, commonly
1
,8-diazabicyclo(5.4.0)undec-7-ene (DBU), to catalyze the reagent tetramerization and re-
action media such as alkyl alcohols (pentanol, hexanol), dimethylaminoethanol (DMAE)
nitrobenzene, chlorobenzene, quinoline and α-chloronaphthalene [13–21], most of which
are non-environmentally friendly. Given their extreme technological importance, many
unconventional synthetic approaches have been studied and developed in parallel with
their applications to increase their yields and sustainability while reducing their produc-
tion cost, such as (i) microwave-assisted [22] and UV-assisted [23,24] processes, which are
usually much faster and cleaner than their “standard” counterparts but requiring specific
instrumentation and lacking scalability, and (ii) room-temperature reactions promoted by
strong organic bases such as lithium diisopropylamide (LDA) [25] and pre-formed lithium
alcoholate solutions [26]. With a view to increase the sustainability of the phthalocya-
nine synthesis on the lab scale, we systematically studied the effect of solvent and base
replacement on the yield and cost of the macrocyclization reaction of phthalonitrile and
4
-tert-butylphthalonitile templated by three first-transition metal ions: Co(II), Cu(II) and
Zn(II). We focused our attention on two main solvents, anisole, glycerol and their mixtures,
for several reasons. The former is a low-cost, non-toxic and biodegradable compound
that ranks very well in recent solvent selection guides [27,28] and can be obtained from
renewable sources such as lignin and guaiacol [29 31]. Its aromatic nature can in principle
–
guarantee the good solubility of a variety of phthalonitrile precursors, and its boiling point
is suitable for macrocyclizations in solution. The latter is available on a large scale from the
vegetable oil industry, is non-toxic, and can dissolve inorganic salts as well as a variety of
organic molecules that would be immiscible with water [32].
Furthermore, we tested potassium hydroxide (KOH) as a possible and cheaper alter-
native to the use of DBU given its lower cost and wide availability, bearing in mind the
-
possibility of side-reactions on the nitrile groups due to the strong nucleofilicity of the OH
ion. When testing our alternative protocols, hereinafter named A-DBU (anisole + DBU),
A-KOH (anisole + KOH) and GA-KOH (glycerol/anisole + KOH), both unsubstituted
and 2,9(10),16(17),23(24)-tetra-tert-butyl phthalocyanines formation showed an evident
dependence on the nature of the central metal: copper gave the best results in terms of yield,
with values up to 76% for the KOH-aided tetramerization of phthalonitrile in anisole, and
both its KOH-based synthetic protocols were up to 73.43 EUR/kg cheaper than standard
methods, as highlighted in the cost analysis reported in Section 2.3. On the contrary, the
reactivity of zinc was very poor whenever the inorganic base replaced DBU. Interestingly,
when extending the GA-KOH approach to the synthesis of a non-symmetrically substituted
zinc phthalocyanine, specifically 9(10), 16(17), 23(24)-tri-tert-butyl-2-iodophthalocyaninato
zinc(II), the reaction provided the expected mixture of mono- and poly-iodinated phthalo-
cyanines along with 2,9(10),16(17),23(24)-tetra-tert-butylphthalocyaninato zinc(II), obtained
in higher yields than when reacting 4-tert-butylphthalointrile in the same conditions.
Our study, far from being exhaustive, aims to be a first step towards the investigation of
alternative reaction protocols for the synthesis of state-of-the-art and novel phthalocyanines,

Molecules 2021, 26, 1760
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with the aim of providing alternatives with increased sustainability and lower costs while
preserving their effectiveness.
2
2
2
. Results and Discussion
.1. Synthesis
.1.1. Unsubstituted and 2,9(10),16(17),23(24)-tetrasubstituted Phthalocyanines
The general reaction for the synthesis of unsubstituted and tetrasubstituted derivatives
is sketched in Scheme 1.
Scheme 1. General synthetic procedure for unsubstituted and tetra-tert-butyl phthalocyanines. n = 1
◦
(
1
Cu), 2 (Zn), 4 (Co). General conditions: 1 eq phthalonitrile, 0.30 eq hydrated salt.
54 C (anisole), 175 C (anisole/glyerol).
∆
= 135 C (DMAE),
◦ ◦
Anisole has been used with DBU to decouple any solvent effect with changing the
base, and with KOH in order to move towards greener and cheaper conditions. When
◦
using pure glycerol at temperatures above 130 C, a consistent amount of phthalonitrile
crystallized on the neck of the reaction flask. The addition of anisole as a cosolvent, whose
solubility in glycerol is tabulated as a function of quantity and temperature [33], favored
the dissolving of the organic reagent, and a 10% w/w solution proved to be suitable to our
scopes. In parallel with the alternative protocols reactions in standard conditions, i.e., in
dimethylaminoethanol with DBU as the base, have been carried out for comparison for all
the derivatives. In all cases, the yields have been calculated after washing the crudes with
diluted HCl, water and methanol, and purifying the resulting solids in a Soxhlet extractor
with acetic acid. To reduce the number of variables we always worked with acetate salts.
We firstly focused on the synthesis of unsubstituted cobalt(II), copper(II) and zinc(II)
phthalocyanines, to evaluate the influence of the central metal on our new reaction condi-
tions. The best yielding ones are summarized in Table 1.
Table 1. Best-yielding reactions for the considered unsubstituted phthalocyanines.
Product
CoPc
DMAE-DBU
87%
A-DBU
72%
A-KOH
24%
GA-KOH
37%
CuPc
83%
49%
76%
73% *
ZnPc
75%
56%
n/a **
n/a **
*
*
With respect to the experimental procedure described in 3.1, the best result was achieved with 0.27 eq KOH.
* No formation of the expected product was observed.
The nature of our products has been confirmed by standard spectroscopic techniques
and powder X-ray diffraction. The latter will be discussed in Section 2.2; the UV-Vis spectra
in tetrahydrofuran (THF) showed phthalocyanine’s typical absorption Q and Soret bands,
and are reported in the Supplementary Materials as Figure S10a,b. The narrow shape
of the Q bands indicates the absence of aggregation in solution, in accordance with the
1
coordinating nature of THF. Furthermore, the structure of ZnPc was confirmed by H NMR
(
Figure S1), in which the two multiplets centered at 9.44 and 8.28 ppm can be assigned to

Molecules 2021, 26, 1760
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the protons of the aromatic ring. As a first comment, all the tested alternative conditions
depend on the templating metal much more than the standard protocols. As expected
given its 3d10 shell, Zn(II) is in general the less reactive ion, and did not react at all in
KOH-based procedures, while in the same conditions copper gave the best results. The
generally lower yield of the DBU-assisted tetramerization in anisole with respect to that
in DMAE, even if performed at slightly higher temperatures, might be due to the weak
polar aprotic nature of the solvent that could not favor an efficient solubilization of acetate
salts nor a suitable environment for the coordination of the +2 metal ion to the reaction
intermediates. Contrary to what was expected, the yield for the formation of CuPc in the
A-DBU conditions was the lowest, and the color of the resulting mixture was dark-brown
rather than purple as in other cases. It is probable that the chemical properties of the
solvent allow the Cu(II) ions to catalyze some oxidation processes rather than templating
the formation of the macrocycle. The better results obtained with cobalt and zinc in the
same conditions may support this hypothesis. In the A-KOH conditions, CoPc was formed
only in a 24% yield, less than when we worked with the glycerol/anisole mixture, due to
the very scarce solubility of the inorganic base in anisole, along with the probable reduced
complexation capability of the cobalt cation. The formation of CuPc in the same conditions
provided more satisfactory results, consistently with the templating properties of Cu(II),
which is known to provide the highest yields for the synthesis of phthalocyanines. The
slightly lower yield obtained with the GA-KOH approach with respect to the A-KOH
approach is mainly due to the side-reactions of phthalonitrile that will be discussed later in
this paragraph.
,
After evaluating the formation of unsubstituted phthalocyanines, we extended our
approach to the synthesis of tetra-tert-butyl-substituted derivatives, following the same
procedures described above. A summary of the best-yielding reactions is reported in
Table 2.
Table 2. Best-yielding reactions for the considered tetra-tert-butylphthalocyanines.
Product
t-Bu CoPc
DMAE-DBU
62%
A-DBU
50%
A-KOH
traces
GA-KOH
traces
4
t-Bu CuPc
58%
38%
43% *
26%
4
t-Bu ZnPc
49%
10%
n/a **
16%
4
Yields have been calculated after chromatographic purification for t-Bu
4
CoPc and t-Bu
4
ZnPc, and after washings
4
with hot methanol for t-Bu CuPc. * Addition of catalytic amount of 95% ethanol. ** No formation of the expected
product was observed.
Generally speaking, the yields and yield ratios were lower than those of the unsubsti-
tuted derivatives, in spite of the better solubility of 4-tert-butylphthalonitrile in the reaction
media. Contrary to the CoPc general trend, t-Bu CoPc was synthesized in appreciable
4
yields only with the A-DBU protocol. Instead, t-Bu CuPc was successfully synthesized
4
using the A-KOH and GA-KOH approaches, even though in lower yields with respect to
the standard procedure, as a further confirmation that Cu(II) is the most reactive among the
chosen metal ions. The A-KOH protocol was found to benefit from adding a small amount
of ethanol (1 drop per 1.63 mmol of phthalonitrile), which promoted a better availability of
KOH in the reaction medium along with the partial formation of the ethanoate anion, and
raised the yield from 10 to 43%. On the contrary, adding an excess of ethanol lowered the
yield to 32%, as it favored the undesired formation of hydration byproducts. t-Bu ZnPc
4
synthesized with the GA-KOH procedure was obtained as statistical mixtures of regioiso-
1
mers, clearly visible by the multiplicity of the methyl signal centered at 1.76 ppm in the H
NMR spectrum of a diluted sample in DMSO-d (Figure S2). By extension, the formation
6
of regioisomeric mixtures for the Co(II) and Cu(II) tetrasubstituted derivatives is very
probable, although not experimentally proven. As for the unsubstituted phthalocyanines,
the UV-Vis spectra of THF solutions were compatible with the structure of our target
products, and are reported in Figure S11.

Molecules 2021, 26, 1760
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For both unsubstituted and tetrasubstituted phthalocyanines synthesized in the
GA-KOH conditions, the concomitant formation of (4-tert-butyl)phthalimide could not be
1
avoided and was confirmed by H-NMR (see Figure S4, S5). The hydration of nitrile groups
-
in polar environments in the presence of OH ions is reported in the literature [34
,
35], and
proceeds through a nucleophilic attack of the hydroxide anion on the nitrile carbon group.
As sketched in Scheme 2, in the case of o-dicyanobenzenes, a five-membered ring closure
can be promoted by an intramolecular nucleophilic attack followed by acidic hydrolisis, as
reported in the presence of several nucleophiles such as ethanoates [36,37].
Scheme 2. Proposed hydration reaction of phthalonitriles to phthalimides.
2.1.2. Statistical Synthesis of Unsymmetrical Phthlaocyanines
Lastly, we extended our approach to the synthesis of unsymmetrical phthalocyanines,
in which more than one type of phthalonitrile is involved and a variety of products with
different degrees of substitutions is expected, to evaluate any variations in terms of yield
and imbalance towards one of the possible products. Specifically, we investigated the sta-
tistical synthesis of tri-tert-butyl-iodophthalocyaninatozinc(II) (t-Bu IZnPc), starting from
3
4
-tert-butylphthalonitrile and 4-iodophthalonitrile, given its versatility as an intermediate
for the synthesis of a variety of compounds for several technological applications [38 40].
–
The related reaction is reported in Scheme 3; the glycerol/anisole mixture was chosen
as solvent, with KOH as a base and a 5:1 ratio between 4-tert-butylphthalonitrile and
4-iodophthalonitrile, respectively.
Scheme 3. Statistical reaction between 4-tert-butylphthalonitrile and 4-iodophthalonitrile.
Surprisingly, differently from the tetramerization of 4-tert-butylphthalonitrile in the
same conditions, the reaction started quickly and the presence of the expected product
mixture, including t-Bu ZnPc, was confirmed by thin-layer chromatography (TLC). After
4
purification by column chromatography on silica gel, we isolated t-Bu IZnPc in a satisfac-
3
tory 26% yield, along with a 41% yield for the tetra-tert-butyl derivative, similarly to what
we obtained in our laboratory using standard procedures with the same phthalonitriles mo-
1
lar ratio. H NMR and electrospray ionization mass spectrometry (ESI-MS) confirmed the
structure of both compounds. Although we cannot provide a rigorous explanation for this
evidence, regarding the unsymmetrical derivatives we may infer that 4-iodophthalonitrile,
which is slightly more reactive than an alkyl-phthalonitrile, may efficiently undergo nucle-
ophilic attack on one nitrile group, and form a sufficiently reactive intermediate to attack
4-tert-butylphthalonitrile and involve it in the tetramerization process.

Molecules 2021, 26, 1760
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2
.2. XRD Characterization
X-ray diffraction (XRD) spectra are a useful further confirmation of the structure of
our products. The powder XRD patterns of Cu-, Zn- and Co-phthalocyanine are reported in
Figure 1. The three compounds were identified as phase, according to the JCPDS cards n.
9-1881, 39-1882 and 14-0948 for the Cu-, Zn- and Co-phthalocyanine, respectively. The
Rietveld fit of the data, performed using the monoclinic structure of the phase, is shown
β
3
β
β
β
β
in Figure 1 as well (red line). The lattice parameters obtained from the fit are reported in
Table 3.
Figure 1. X-ray diffraction (XRD) patterns of Cu-, Zn- and Co-phthalocyanine. The red line is the
Rietveld fit of the experimental data.
Table 3. Lattice parameters of Cu-, Zn- and Co-phthalocyanine.
◦
a (Å)
c (Å)
c (Å)
β ( )
CuPc
ZnPc
CoPc
14.698 (1)
14.569(1)
14.642(2)
4.8137(8)
4.8676(3)
4.804(2)
19.595(2)
19.279(2)
19.496(3)
121.06(1)
120.62(1)
120.60(1)
The XRD patterns of tetra-tert-butyl-substituted Cu-, Zn- and Co-phthalocyanine are
shown in Figure 2. The substituted phthalocyanines present a lack of reflections when
compared to the corresponding non substituted ones, indicating a lower crystallinity, even
though the logarithmic scale of intensity reveals the presence of weak reflections. Moreover,
data are probably an average of the four t-Bu MPc isomers.
4
◦
◦
An intense low-angle reflection at 2
θ
of 5.50 (d = 17.04 Å) and 5.18 (d = 16.07 Å) for
t-Bu ZnPc and t-Bu CuPc, respectively, is present. The t-Bu CoPc sample only shows a
4
4
4
◦
weak broad peak at 2θ = 5.80 (d ~ 15.24 Å), indicating an amorphous structure.
The crystal structures of t-Bu MPc are not reported in the literature and the few data
4
on powder samples are often discordant. Lebedeva et al. [41] presented powder XRD
data on either
α- and β- t-Bu ZnPc phases, indicating for both phases a d spacing of
4 100
16.97Å, in quite good agreement with our value. Dong et al. [42] showed an XRD pattern

Molecules 2021, 26, 1760
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◦
for powder t-Bu ZnPc with only a low angle peak at 2
θ
= 6.0 , which was a slightly higher
4
value than ours.
Figure 2. XRD patterns of tetra-tert-butyl-substituted Cu-, Zn- and Co-phthalocyanine.
G. Sfyri et al. [43] reported the grazing incidence XRD pattern of t-Bu CuPc film,
4
◦
◦
showing two low-angle reflections at 5.18 and 5.92 , well in agreement with our data (see
Figure 2).
2.3. Cost Analysis
Since we have stressed the importance of lowering the fabrication costs of our target
materials while increasing their sustainability, we propose a cost-per-kilogram analysis for
our new experimental conditions and compare the obtained results with the “standard”
protocol that we used and with the current price of CoPc, CuPc, ZnPc. The comparison
among the tetra-tert-butylated phthalocyanines is not reported because the high price/kg
of 4-tert-butylphthalonitrile, along with generally lower yields with respect to the standard
protocol, far outweighs the economic advantage of using less expensive chemicals. The
total material cost (in EUR) for our phthalocyanines has been estimated according to a
paper published in 2013 by Osedach et al. [44]. For the sake of simplicity, Merck has
been chosen as the only supplier for reagents, solvents and purification materials. In all
cases, the bulkiest batches of suitable purity available on the online catalogs have been
considered, the list of which is reported in Table S1 in the supporting information, along
with further technical details. Moving from Lab to Fab requires a complete cost analysis
that must include parameters like energy, facilities maintenance and personnel costs, taxes,
and other charges that we have not taken into account. Furthermore, we are aware that
large-scale synthesis may significantly differ from small-scale preparations in many ways
(homogeneity of the reaction mixture, temperature uniformity, type of stirring, purification
methods), but as a first approximation, the costs of our syntheses have been estimated
by assuming linearity when scaling the procedure from grams to kilograms. Prices were
estimated by calculating the required amounts of chemicals to produce and purify 1 kg of
the desired product in the obtained yield, and multiplying them by the respective prices.
The Soxhlet purification has not been included in the discussion since the removal of
impurities can be carried out with the same solvent batch multiple times. The resulting

Molecules 2021, 26, 1760
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costs include reagents, solvents and purification materials, and are reported in Table 4,
given as EUR per kilogram of synthesized product.
Table 4. Cost analysis for unsubstituted phthalocyanines.
Reagents
Solvents
(EUR/kg)
Workup/Purification
(EUR/kg)
Total
Total (no workup/purification)
(EUR/kg)
Product
Protocol
(
EUR/kg)
238.57
340.78
1016.34
671.47
119.62
199.41
131.02
134.11
135.03
179.19
(EUR/kg)
CoPc
standard
A-DBU
221.08
165.43
492.00
240.17
228.45
241.56
157.82
100.53
252.52
212.43
356.03
419.42
1247.47
835.75
368.81
612.47
400.15
408.81
408.32
538.64
815.68
925.63
2755.81
1747.39
716.88
1053.44
688.99
643.45
785.97
930.26
459.65
506.21
1508.34
911.74
348.07
440.97
288.84
234.64
387.55
391.62
A-KOH
GA-KOH
standard
A-DBU
CuPc
ZnPc
A-KOH
GA-KOH
standard
A-DBU
As a general remark, the reagents and purification costs increase whenever the yield
lowers with respect to the standard conditions, given the increased amount of chemicals
needed to obtain 1 kg of product. Reagents for the synthesis of CoPc in standard conditions
have a higher cost than those of CuPc and ZnPc due to the higher price of cobalt acetate.
Conversely, in some cases the alternative solvents positively affect the overall price since
their cost per kg is considerably lower than that of DMAE. It is noteworthy to point out
the huge impact that the work-up and purification steps have on the price of the final
product, as highlighted by the differences in the related net and gross total costs. The
optimization of any synthetic process is mandatory in order to scale it up, and materials
recycling is an essential part of it, so we are confident that most of the purification solvents,
e.g., methanol, could be recovered and used more than once, significantly lowering the
overall cost and waste production. Among the others, both syntheses of CuPc with KOH
are less costly than their standard counterpart, by up to 73.43 EUR/kg, and are much
cheaper than the alternatives on the market even when the purification costs are included
in the price. Furthermore, the similarity between the net prices of ZnPc is encouraging
and suggests that a careful optimization of the reaction parameters, along with a rational
strategy to minimize the impact of the workup in terms of costs, could further lower the
economic impact of the A-DBU protocol.
3. Materials and Methods
All reagents and solvents were purchased from Merck Life Science S.r.l (Milano,
Italy), TCI Chemicals (Zwijndrecht, Belgium) and Carlo Erba Reagents (Cornaredo, Italy)
and used without further purification. Reactions were purged and refilled three times,
performed under nitrogen and monitored by thin-layer chromatography (TLC) employing
a polyester layer coated with 250 mm F254 silica gel. Chromatographic purifications,
when needed, were performed using silica gel 60A 35–70
depending on the nature of the target molecule. H spectra were recorded on a Bruker
AVANCE 600 NMR spectrometer (Billerica, MA, USA) operating at a proton frequency
µ and several mobile phases,
1
of 600.13 MHz in DMSO-d ; chemical shifts (
δ
) are given in ppm relative to TMS. UV-Vis
spectra were recorded on a Perkin-Elmer Lambda 950 UV-vis/NIR spectrophotometer
Perkin Elmer Italia, Milano, Italy) using THF as solvent. Infrared spectra were recorded
6
(
on a Shimadzu FT-IR prestige-21 spectrometer (Kyoto, Japan) using an attenuated total
reflectance (ATR) unit. ESI-MS -spectra were recorded at the Toscana Life Science facility
with a Thermo Fisher Scientific Q-Exactive Plus (Geel, Belgium). The samples have been
analyzed by direct infusion in positive mode with a spray voltage of 3.5 kV, sheat gas
10 a.u., and a flow rate of 5 ul/min. XRD measurements were carried out on a Seifert XRD
3003 TT diffractometer (Ahrensburg, Germany) in the Bragg–Brentano geometry, using Cu

Molecules 2021, 26, 1760
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◦
K
0
α
radiation (
λ
= 1.5418 Å).
θ
-2
θ
scans were performed in the 2
θ
range 5–50 with step size
◦
.04 and counting 8 s/step. Rietveld refinements were done using GSAS II software [45].
3.1. General Synthesis of Unsubstituted Phthalocyanines
In a two-necked 50 mL round bottom flask equipped with a reflux condenser and a
magnetic stirrer, 1.0 g (1 eq) of phthalonitrile, 0.30 eq of hydrated salt and two Pasteur-
drops (approx. 32 mg, 0.21 mmol, 0.027 eq) of DBU (or 0.21 eq of KOH) were added and
stirred in 5 mL of solvent (or in a 5.5 g of anisole/glycerol mixture) under reflux for 3 h.
The mixture was then cooled, treated with 15 mL of HCl 1N, filtered and washed with
water (2
×
20 mL) and methanol (2
×
15 mL). It was then purified in a Soxhlet extractor
with glacial acetic acid until the extracts were colorless.
CoPc: purple microcrystalline powder; UV (THF) λabs 324, 593, 656 nm.
CuPc: purple microcrystalline powder; UV (THF)
λ
342, 601, 666 nm. *scarcely soluble
abs
λ
1
ZnPc: purple microcrystalline powder; UV (THF)
DMSO-d , 600 MHz) δ 9.44 (8H, m), 8.28 (8H, m).
342, 601, 666 nm [46]; H NMR
abs
(
6
3.2. General Synthesis of 2,9(10),16(17),23(24)-Tetra-tert-butyl-Substituted Phthalocyanines
In a two-necked 50 mL round bottom flask equipped with a reflux condenser and a
magnetic stirrer, 1.0 g (5.43 mmol, 1 eq) of 4-tert-butylphthalonitrile, 0.30 mmol of hydrated
salt and two Pasteur-drops (approx 32 mg, 0.21 mmol, 0.0387 eq) of DBU (or 0.21 to 0.3 eq
of KOH) were added and stirred in 5 mL of solvent (or in a 5.5 g of anisole/glycerol
mixture) under reflux for 3 h. The mixture was then cooled, treated with 15 mL of HCl
1
N, filtered, and washed with water (2
×
20 mL) and methanol (2
×
15 mL). The resulting
Zn and Co crudes were then purified by filtration on a silica pad (10g) using DCM as a
solvent, while the Cu derivative was washed with hot methanol until washings were pale
yellow/colorless.
t-Bu CoPc: shiny purple crystals; UV (THF)
λ
abs
328, 598, 661 nm; ESI m/z 795.84 [M]+
4
+
t-Bu CuPc: shiny purple crystals; UV (THF)
λ
abs
344, 605, 671 nm; ESI m/z 800.33 [M]
4
1
t-Bu ZnPc: shiny purple crystals; UV (THF)
λ
abs
348, 606, 672 nm [47]; H NMR
4
(
DMSO-d , 600 MHz)
δ
9.38–9.16, (8H, m), 8.32–8.24 (4H, m), 1.76 (36H, m, C-(CH ) ),
6
3 3
+
regioisomers); ESI m/z 802.83 [M]
3.3. Synthesis of Tri-tert-butyl-iodophthalocyaninatozinc(II)
In a two-necked 50 mL round bottom flask equipped with a reflux condenser and
a magnetic stirrer, 520 mg (5 eq, 2.822 mmol) of 4-tert-butylphthalonitrile, 140 mg of
4
-iodophthtalonitrile (0.55 mmol, 1 eq), 120 mg (0.688 mmol, 1.25 eq) of Zn(OAc) and
2
5
0 mg KOH (0.89 mmol, 2.62 eq) were mixed in a 10% w/w anisole-glycerol mixture (5.5 g
)
◦
at 175 C for 18 h. After removing anisole under reduced pressure, the mixture was
treated with HCl 1N and the resulting solid was washed with water and methanol to
provide 380 mg of crude. The desired product and 2,9(10),16(17),23(24)-tetra-tert-butyl
phthalocyanine were purified by column chromatography on silica gel using several
petroleum ether–THF mixtures (98:2 to 4:1), and they were obtained in 26% (125 mg) and
4
1% (180 mg) yields, respectively.
t-Bu IZnPc: shiny purple crystals; UV (THF)
λ
350, 606, 672 nm; ESI m/z 872.67 [M]+.
abs
3
4. Conclusions
To summarize, we have explored the potentials of greener synthetic approaches for the
Co(II), Cu(II) and Zn(II) templated synthesis of unsubstituted and 2,9(10),16(17),23(24)-tetra-
tert-butylphthalocyanines in solution, either using DBU or KOH as bases. Our experimental
protocols showed a remarkable dependence on the nature of the metal ions, with the best
performances obtained by Cu(II) consistently with its excellent templating properties. The
yields obtained for the synthesis of CuPc with both KOH-assisted approaches, even though
slightly lower than that in standard conditions, are economically advantageous, while
the synthesis of ZnPc with the A-DBU protocol, a few EUR/kg more expensive than that

Molecules 2021, 26, 1760
10 of 12
in the DMAE net of the purification costs, could become so with a further optimization
of the procedure. 2,9(10),16(17),23(24)-tetra-tert-butylphthalocyanines were obtained in
appreciable yields only with the A-DBU protocol. Further optimizations of the KOH-aided
approaches, as in the case of the A-KOH synthesis of t-Bu CuPc, may provide better results.
4
When extending the GA-KOH protocol to the synthesis of unsymmetrical t-Bu IZnPc,
3
the result exceeded our expectations and confirmed the potential of our approach, which
will be further investigated to better understand the involved phenomena and allow its
extension to the synthesis of other macrocyclic targets. To our knowledge, this is the first
work in which the results of a systematic synthesis of metallophthalocyanines in solution
with more environmentally sustainable reaction conditions are analyzed, and, although
not exhaustive, in our opinion it shows considerable potential for application, in principle
even on a large scale.
Supplementary Materials: The following are available online. Figure S1: 1H NMR of ZnPc,
Figure S2: 1H NMR of t-Bu4ZnPc, Figure S3: 1H NMR of t-Bu3IZnPc, Figure S4: 1H NMR of
phthalimide, Figure S5: 1H NMR of 4-tert-butylphthalimide, Figure S6: ESI spectrum of t-Bu4CoPc,
Figure S7: ESI spectrum of t-Bu4CuPc, Figure S8: ESI spectrum of t-Bu4ZnPc, Figure S9: ESI spectrum
of t-Bu3IZnPc, Figure S10: UV-Vis spectra of unsubstituted phthalocyanines, Figure S11: UV-Vis spec-
trum of (t-bu)4substituted phthalocyanines, Figure S12: UV-Vis spectrum of t-bu3IZnPc, Figure S13:
IR spectrum of CoPc, Figure S14: IR spectrum of CuPc, Figure S15: IR spectrum of ZnPc, Figure S16:
IR spectrum of t-Bu4CoPc, Figure S17: IR spectrum of t-Bu4CuPc, Figure S18: IR spectrum of
t-Bu4ZnPc, Figure S19: IR spectrum of t-bu3IZnPc, Figure S20: flowchart for the synthesis of 1 kg
of CoPc in standard conditions, Figure S21: flowchart for the synthesis of 1 kg of CuPc in standard
conditions, Figure S22: flowchart for the synthesis of 1 kg of ZnPc in standard conditions, Figure S23:
flowchart for the synthesis of 1 kg of CoPc in A-DBU conditions, Figure S24: flowchart for the
synthesis of 1 kg of CuPc in A-DBU conditions, Figure S25: flowchart for the synthesis of 1 kg of ZnPc
in A-DBU conditions, Figure S26: flowchart for the synthesis of 1 kg of CoPc in A-KOH conditions,
Figure S27: flowchart for the synthesis of 1 kg of CuPc in A-KOH conditions, Figure S28: flowchart
for the synthesis of 1 kg of CoPc in GA-KOH conditions, Figure S29: flowchart for the synthesis of
1
kg of CuPc in GA-KOH conditions, Table S1: Overview of the quoted materials.
Author Contributions: Conceptualization, G.Z., A.M.P., G.P.; experimental work G.Z.; XRD char-
acterization, P.I.; writing—original draft preparation, G.Z.; writing—review and editing, G.Z., P.I.,
A.M.P., G.P; All authors have read and agreed to the published version of the manuscript.
Funding: This research was funded by MINISTERO DELL’ISTRUZIONE, DELL’UNIVERSITA’ E
DELLA RICERCA (MIUR) through grant PRIN2017 BOOSTER (protocol number 2017YXX8AZ).
Institutional Review Board Statement: Not applicable.
Informed Consent Statement: Not applicable.
Data Availability Statement: The data presented in this study are available in this article and in the
Supplementary Materials file.
Acknowledgments: G.Z. thanks Anatoly Sobolev for assistance and clarifications on NMR measure-
ments, and Dario Pasini and Andrea Nitti for fruitful scientific discussions. All the authors thank
Laura Salvini for the ESI-MS analysis.
Conflicts of Interest: The authors declare no conflict of interest.
Sample Availability: Samples of all the synthesized compounds are available from the authors.
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