Al(III) phthalocyanine catalysts for CO2 addition to epoxides: Fine-tunable selectivity for cyclic carbonates versus polycarbonates

Article abstract of DOI:10.1016/j.jorganchem.2021.121979

A sustainable synthetic methodology to prepare bifunctional cationic imidazolyl and neutral tert-butylphenoxy Al(III)-phthalocyanine complexes is described, using ultrasound irradiation to synthesize the phthalonitrile precursors, followed by microwave-as

Full text of DOI:10.1016/j.jorganchem.2021.121979

Journal of Organometallic Chemistry 950 (2021) 121979
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Journal of Organometallic Chemistry
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Al(III) phthalocyanine catalysts for CO addition to epoxides:
2
Fine-tunable selectivity for cyclic carbonates versus polycarbonates
∗
Andreia C.S. Gonzalez, Alexandre P. Felgueiras, Rafael T. Aroso, Rui M.B. Carrilho ,
∗
Mariette M. Pereira
Department of Chemistry, University of Coimbra, Coimbra Chemistry Centre, Rua Larga, 3004-535 Coimbra, Portugal
a r t i c l e i n f o
a b s t r a c t
Article history:
A sustainable synthetic methodology to prepare bifunctional cationic imidazolyl and neutral tert-
Received 23 March 2021
Revised 28 June 2021
Accepted 4 July 2021
Available online 8 July 2021
butylphenoxy Al(III)-phthalocyanine complexes is described, using ultrasound irradiation to synthesize
the phthalonitrile precursors, followed by microwave-assisted cyclotetramerization. The metallophthalo-
cyanine catalysts were applied in the coupling reaction of CO2 with epoxides, where the structure of
the phthalocyanine was shown to determine the reaction selectivity towards the formation of cyclic car-
bonates versus polycarbonates. The cationic imidazolyl phthalocyanines operate as efficient bifunctional
catalysts to promote the CO2 cycloaddition to terminal epoxides (styrene oxide, epichlorohydrin, propy-
lene oxide and allyl glycidyl ether), leading to TONs up to 1414 and 100% selectivity for cyclic carbonates,
without the need of using any co-catalyst. On the other hand, the neutral 4-tert-butylphenoxy Al(III) ph-
thalocyanine catalyzed the copolymerization reaction between CO2 and cyclohexene oxide, in the pres-
ence of PPNCl as co-catalyst, with TON = 814, providing the first example of a metallophthalocyanine
catalyst with full selectivity for formation of polycarbonates.
Keywords:
Carbon dioxide
Metallophthalocyanines
Bifunctional catalysts
Polycarbonates
Cyclic carbonates
©
2021 Elsevier B.V. All rights reserved.
1
. Introduction
cially useful, since these products play important roles as “green”
organic solvents or as polymers for the plastics industry, respec-
tively. These reactions have been mainly accomplished through
the use of metallic catalysts, such as metal complexes of salens,
[27–31] β-diiminates [32,33], aminotriphenolates [34–36] and por-
phyrins [37–43], as well as supported metal salts [44] and metal
organic frameworks [45,46]. Another group of attractive but less
explored metal complexes are metallophthalocyanines, which have
been efficiently used as homogeneous [47–51] and heterogeneous
catalysts [52–58] for CO2 addition reactions to propylene oxide,
styrene oxide, 1,2-epoxyhexane, epibromohydrin, epichlorohydrin,
(2,3-epoxypropyl)benzene and glycidyl ethers as substrates. In gen-
eral, cyclic carbonates are exclusively obtained as the main prod-
ucts for all types of substrates, while low conversions are observed
for cyclohexene oxide.
Carbon dioxide (CO ) is an abundant greenhouse gas consid-
2
ered as one of the main causes of climate changes, which bring
us serious social, health and economic burden in the present and
near future. In this context, there has been a growing interest in
the development of technologies that can reduce CO2 emissions
or, at least, mitigate its effects by means of its capture from at-
mosphere, storage and conversion into value-added products [1–9].
Indeed, the environmental urgency of reusing CO , combined with
2
its high availability and low toxicity make it an attractive build-
ing block for chemical synthesis. However, its high thermodynamic
and kinetic stability requires the use of efficient catalysts in or-
der to turn CO2 transformation into environmentally and econom-
ically viable processes [10–12]. To this effect, many strategies have
been reported on CO2 utilization for the synthesis of carboxylic
acids and their derivatives, carbamates and carbonates, or as a
precursor to carbon monoxide [13,14]. Among the most relevant
transformations, the catalytic addition of CO2 to epoxides, yield-
ing cyclic carbonates [15–20] or polycarbonates [21–26] is espe-
Most of metal catalysts are known to act as Lewis acids for ac-
tivating the epoxide [59]. In addition, a nucleophile is usually re-
quired as co-catalyst, to promote the epoxide ring opening upon
its coordination with the metal center [60]. The co-catalyst can be
an external agent added to the reaction (e.g. imidazole [47], DMAP
[
61,62], quaternary phosphonium [63,64] or ammonium salts [65–
8]), or it can be part of the catalyst molecule, forming a bifunc-
tional catalytic system (Fig. 1).
6
∗
Corresponding authors.
E-mail addresses: rui.carrilho@uc.pt (R.M.B. Carrilho), mmpereira@qui.uc.pt
(
M.M. Pereira).
https://doi.org/10.1016/j.jorganchem.2021.121979
022-328X/© 2021 Elsevier B.V. All rights reserved.
0

A.C.S. Gonzalez, A.P. Felgueiras, R.T. Aroso et al.
Journal of Organometallic Chemistry 950 (2021) 121979
imidazolium-based catalysts [80]. There is also a report of reusable
bifunctional metallophthalocyanine-carbon nitride hybrid catalysts
(
metal = Cu or Co), which have been used in CO2 cycloaddition to
epoxides, in the absence of co-catalyst, and allowed to obtain the
corresponding cyclic carbonates in yields ranging from 12 to 98%
[
81]. However, to the best of our knowledge, there are no reports
of the application of bifunctional metallophthalocyanine homoge-
neous molecular catalysts in these reactions.
Fig. 1. Epoxide activation with a bifunctional catalyst.
Recently, we have described the synthesis of monoterpene-
based Zn (II) phthalocyanines [82] and their application as cata-
lysts in CO2 cycloaddition to styrene oxide [83], but the reactions
required the use of a co-catalyst and moderate TONs (up to 900)
were obtained.
As a part of our continuing research in this field, in this paper
we describe an efficient and sustainable synthetic methodology to
prepare bifunctional cationic imidazolyl-based Al(III) phthalocya-
nines and neutral tert-butylphenoxy Al(III) phthalocyanines. Fur-
thermore, their application as selective catalysts in CO2 addition
reactions to epoxides is reported and the effect of the phthalocya-
nine structure in the catalytic activity and selectivity is described.
2. Results and discussion
2
.1. Synthesis and characterization of Al(III) phthalocyanine catalysts
The Al(III)-phthalocyanine catalysts were prepared, through op-
timization of our recently developed sustainable synthetic method-
ology [82], which involves the synthesis of phthalonitrile precur-
sors using ultrasound irradiation, followed by microwave-assisted
cyclotetramerization reactions (Schemes 1–3). First, the 4-(1H-
imidazol-1-yl) phthalonitrile 1 was synthesized by reacting com-
mercially available 4-nitrophthalonitrile with imidazole in DMF,
Scheme 1. Synthesis of the phthalonitrile precursors.
Among the reported bifunctional catalysts, we highlight the
use of salens [69–71], metalloporphyrins [72–75], triphenolates
[
76], hetero-scorpionates [77], tris(2-aminoethyl)amine-based com-
using K CO3 as base, in an ultrasonic bath at 25 °C, over 9 h, under
2
plexes [78], as well as silanol [79] and heterogeneous Sn-SiO2
inert atmosphere. Similarly, phthalonitrile 2 was prepared by re-
Scheme 2. Synthetic pathway for cationic imidazole-based Al(III) phthalocyanine bifunctional catalysts.
2

A.C.S. Gonzalez, A.P. Felgueiras, R.T. Aroso et al.
Journal of Organometallic Chemistry 950 (2021) 121979
ing a hydroxyl group as axial ligand, which indicates that hydroly-
sis has occurred either during the synthesis or work-up procedure.
The subsequent cationization was carried out, through reaction of
2
with iodoethane in DMF at 80 °C, over 48 h (Scheme 2). After
direct precipitation from the reaction mixture by adding CH Cl ,
2
2
the tetra-cationic imidazolyl Al(III) phthalocyanine CAT 1 was iso-
lated in 82% yield. Likewise, the octa-cationic imidazolyl Al(III)
phthalocyanine CAT 2 was obtained by reaction of 2 with (3-
bromopropyl)trimethylammonium bromide in DMF, at 120 °C, over
72 h. The crude product was precipitated from the reaction mix-
ture with ethanol, followed by washing with ethanol, acetone and
dichloromethane and isolated in 53% yield.
The structures of imidazole-based metallophthalocyanines CAT
1
and CAT 2 were elucidated by NMR spectroscopy and fur-
ther confirmed by HRMS (ESI). The spectroscopic data of CAT
was in agreement with that previously reported [84]. The
1H NMR spectrum of the new octa-cationic imidazolyl Al(III)-
1
Scheme 3. Microwave-assisted synthesis of tert-butylphenoxy Al(III) phthalocya-
nine.
phthalocyanine CAT 2, in DMSO-d , is presented in Fig. 2. This
6
spectrum shows broad signals, typical of cationic phthalocya-
nines, which may be attributed to aggregation. Additionally, since
the metallophthalocyanine is composed by four different regioi-
somers, this translates into a more complex signal pattern, both
in the aromatic and aliphatic regions. The aromatic signals range
from 8.4 to 10.9 ppm, which is a consequence of the exten-
action of 4-nitrophthalonitrile with 4-tert-butylphenol in an ultra-
sonic bath at 80 °C, for 2 h. Upon addition of water and ice, the de-
sired phthalonitriles 1 and 2 were precipitated and isolated in 62%
and 88% yields, respectively (Scheme 1). This synthetic strategy al-
lowed to significantly diminish reaction times when compared to
conventional synthesis and obtain the desired phthalonitrile pre-
cursors in higher yields than those previously reported in the lit-
erature [84,85].
sive π-conjugation of the macrocycle coupled with
a strong
electron-withdrawing effect caused by positively charged nitrogen
atoms.
The synthesis of CAT 3 was accomplished through microwave-
assisted cyclotetramerization of phthalonitrile 2 with Al(III) chlo-
ride, in 1-chloronaphthalene, at 260 °C for 1 h (Scheme 3). Af-
ter cooling to room temperature, the metallophthalocyanine was
precipitated by addition of a methanol/water (10:1) mixture, and
further purified by column chromatography on silica gel using
dichloromethane, followed by dichloromethane:methanol (95:5) as
eluent, to afford CAT 3 with 70% isolated yield. This synthetic strat-
egy allowed to obtain the desired Al(III) phthalocyanine in shorter
time and higher yield than those achieved by previously reported
conventional methods [88].
Then, to overcome the difficulties associated with Al(III) ph-
thalocyanine synthesis [86,87], the cyclotetramerization reaction
of phthalonitrile 1 with Al(III) chloride was performed in 1-
chloronaphthalene, a high boiling point solvent, at 260 ºC, under
microwave irradiation (200 W), over 1 h (Scheme 2). After pre-
cipitation of the product from the reaction medium by addition
of CH Cl2 and purification by consecutive washing with acetone,
2
water, acetone and dichloromethane (in this exact order), the cor-
responding tetra-imidazole β-substituted Al(III) phthalocyanine 3
was isolated in 60% yield. The HRMS (ESI) mass spectrum con-
firmed the formation of the Al(III)-phthalocyanine complex 3, bear-
Fig. 2. 1H NMR spectrum of CAT 2 in DMSO-d6 (expansion of the region 8.2–11.0 ppm).
3

A.C.S. Gonzalez, A.P. Felgueiras, R.T. Aroso et al.
Journal of Organometallic Chemistry 950 (2021) 121979
Table 1
Evaluation of Al(III)-phthalocyanine catalysts in CO2 addition to terminal epoxides.a
Yield (%)b
TONc
TOF (h 1)d
−
Entry
Substrate
Catalyst
Co-cat
Solvent
1
2
3
4
5
6
7
8
CAT 1
CAT 3
CAT 3
CAT 1
CAT 3
CAT 1
CAT 2
CAT 3
–
–
71
0
1014
–
42
–
–
–
PPNCl
–
81
69
38
72
19
80
1157
986
543
1029
271
1143
48
41
23
43
11
48
–
CH3OH
CH3OH
CH3CN
CH3CN
CH3CN
PPNCl
–
–
PPNCl
9e
CAT 1
–
–
94
1343
269
1
1
0e
1
CAT 1
CAT 1
–
–
–
–
99
90
1414
1286
283
53
a)Reaction conditions: styrene oxide (35 mmol, 4 ml), catalyst: 0.07 mol%, co-catalyst (when indicated):
PPNCl 0.07 mol%, solvent: 1 mL (when indicated), CO2 (10 bar), 80 °C, 24 h.
b)% Yield determined by 1H NMR.
c)Turnover number calculated as mol(catalysis products)/mol(catalyst).
d)Turnover frequency expressed as TON per hour.
e)5 h; Selectivity, determined by 1H NMR, was 100% for cyclic carbonates in all cases.
2
.2. CO2 addition to epoxides
The bifunctional cationic imidazolyl Al(III)-phthalocyanines
CAT 1-CAT 2, and the neutral 4–tert-butylphenoxy-based Al(III)-
phthalocyanine CAT 3 were evaluated as catalysts in CO2 addition
reaction to epoxides. The catalysts were initially tested in the CO2
cycloaddition reaction to styrene oxide. The reactions were per-
formed under previously optimized conditions (0.07 mol% catalyst,
1
0 bar CO , 80 °C, 24 h) [39,83].
2
The cationic tetra-imidazolyl Al(III) phthalocyanine catalyst CAT
1
was first tested in the absence of solvent, without addition of any
co-catalyst, leading to 71% yield of styrene carbonate (TON = 1014)
Table 1, entry 1). Under the same reaction conditions, the (4-tert-
(
butylphenoxy) Al(III) phthalocyanine catalyst CAT 3 was inactive in
the absence of a co-catalyst (Table 1, entry 2) while, in the pres-
ence PPNCl as co-catalyst, provided 81% yield of cyclic carbonates
Fig. 3. UV–Vis spectra of CAT 1, in DMSO, before and after CO2 addition to styrene
oxide (Table 1, entry 1).
(
Table 1, entry 3). When the reactions were carried out either in
the presence of protic (methanol) or aprotic (acetonitrile) polar
solvents, no significant differences were observed in the catalytic
activity or selectivity of the bifunctional tetra-imidazolyl Al(III) ph-
thalocyanine catalyst CAT 1 (Table 1, entries 4 and 6). On the other
hand, the catalytic system CAT 3/PPNCl produced only 38% yield
when the reaction was performed in methanol (Table 1, entry 5),
while the use of acetonitrile as solvent had a negligible effect on
the catalytic activity (Table 1, entry 8). The octa-cationic imidazolyl
Al(III) phthalocyanine bifunctional catalyst CAT 2 was also tested
in the reaction but this catalyst was only partially soluble in the
epoxide, so acetonitrile (1 mL) was used as solvent. However, only
ate products (Table 1, entry 11). Furthermore, the cationic tetra-
imidazolyl catalyst CAT 1 was easily recovered from the reaction
crude mixtures by precipitation with CH Cl , and reused in two
2
2
consecutive runs without loss of activity or selectivity, producing
similar yields of cyclic carbonates.
Catalyst stability studies were performed for CAT 1 by UV–
Vis (Fig. 3), through analysis of aliquots taken from the initial mix-
ture in styrene oxide and the final mixture after the catalytic CO2
addition reaction (10 μL sample/8 mL DMSO). From analysis of
Fig. 3, we concluded that ca. 80% of the catalyst remain intact,
which demonstrate its stability.
19% yield of styrene carbonate was obtained, which was attributed
In addition, the Al(III)-phthalocyanine catalysts were evaluated
in the catalytic CO2 addition to cyclohexene oxide, a more chal-
lenging disubstituted cyclic epoxide. Considering the lower reactiv-
ity of this substrate, the reactions were performed using a higher
CO2 pressure (40 bar), also at 80 °C and 0.07 mol% catalyst. The
results are presented in Table 2.
to a concentration decrease effect (Table 1, entry 7).
The promising results obtained with bifunctional catalyst CAT 1,
in absence of solvent and co-catalyst, led us to select this catalyst
for enlargement of the reaction’s scope to other terminal epoxides.
Remarkably, under the same reaction conditions, both CO2 cycload-
dition reactions to epichlorohydrin, and propylene oxide proceeded
with practically full conversions in 5 h, leading to the formation
of the corresponding cyclic carbonates in 94 and 99% yields, re-
spectively (Table 1, entries 9–10). Moreover, CO2 addition to allyl
glycidyl ether led to full selectivity and 90% yield of cyclic carbon-
The cationic tetra-imidazolyl Al(III)phthalocyanine complex CAT
1
was first evaluated in the absence of any co-catalyst and sol-
vent, leading to only 5% conversion in 24 h, with 100% selectiv-
ity for the formation of cyclohexane cyclic carbonate (Table 2, en-
try 1). When the reaction was performed under the same condi-
4

A.C.S. Gonzalez, A.P. Felgueiras, R.T. Aroso et al.
Journal of Organometallic Chemistry 950 (2021) 121979
Table 2
Evaluation of Al(III)-phthalocyanine catalysts in CO2 addition to cyclohexene oxide.a
Product distributionc
Cyclic Carbonates (%)
Entry
Catalyst
Co-cat
Solvent
Conversion (%)b
TONd
TOFe
Polymers (%)
1
2
3
4
5
CAT 1
CAT 1
CAT 1
CAT 3
CAT 3
–
–
5
100
100
100
0
0
71
3
PPNCl
PPNCl
–
–
20
13
4
0
286
186
57
12
8
CH3CN
0
–
–
100
100
2
PPNCl
57
0
814
34
a)Reaction conditions: cyclohexene oxide (40 mmol, 4 ml), catalyst: 0.07 mol%, co-catalyst (when indicated): PPNCl 0.07 mol%, solvent: 1 mL acetonitrile
(
when indicated), CO2 (40 bar), 80 °C, 24 h.
b)% Conversion determined by 1H NMR.
c)% Selectivity determined by 1H NMR, through integral ratio of polycarbonate/cyclic carbonate peaks
d)
Turnover number calculated as
mol(catalysis products)/mol(catalyst).
e)Turnover frequency expressed as TON per hour.
tions, but with addition of PPNCl as co-catalyst, 20% conversion
was obtained in 24 h, also with full selectivity for cyclic carbon-
ates (Table 2, entry 2). When acetonitrile was used as solvent, a
lower conversion (13%) was observed (Table 2, entry 3), which was
attributed to the lower substrate concentration in the initial reac-
tion mixture. To appraise the effect of the phthalocyanine struc-
ture on the catalytic activity and selectivity, the studies proceeded
with the evaluation of 4–tert-butylphenoxy Al(III) phthalocyanine
cyclic carbonates, independently of the Al(III)-phthalocyanine cat-
alyst used. However, when cyclohexene oxide is used as substrate,
the cationic imidazolyl Al(III)-phthalocyanine bifunctional catalyst
CAT 1 is highly selective for the formation of cyclic carbonates,
while the tert–butylphenoxy Al(III)-phthalocyanine catalyst CAT 3,
in the presence of PPNCl as co-catalyst, provides the polycarbon-
ate product, which demonstrates that the catalyst structure has a
crucial effect on the reaction selectivity.
(
CAT 3) as catalyst in the CO2 addition to cyclohexene oxide, un-
As depicted in Fig. 4, both mechanisms involve the initial coor-
dination of the epoxide to the metal center, followed by ring open-
ing in the presence of a nucleophile, which may be included in
the catalyst (i.e., the iodide anion in the case of cationic imidazolyl
bifunctional catalyst CAT 1) or it can come from the co-catalyst
(the chloride anion, in the case of CAT 3/PPNCl catalytic system).
The subsequent CO2 insertion into the alkoxo species (B) results
in the formation of metal-carbonate (C), which may undergo back-
biting reaction, leading to intramolecular cyclization or it may un-
dergo chain-growing, upon consecutive epoxide and CO2 inser-
tions, leading to alternating copolymerization. In this case, when
the cationic imidazolyl Al(III)-phthalocyanine bifunctional catalyst
der the above mentioned conditions. In the absence of co-catalyst,
only 4% conversion was observed (Table 2, entry 4) but using PP-
NCl as co-catalyst, CAT 3 provided 57% conversion in 24 h, with a
remarkable 100% selectivity for the formation of polymers (Table 2,
entry 5). The polymer structure was analyzed by 1H and 13C NMR,
which confirmed a high carbonate incorporation (96%). In addition,
high performance size exclusion chromatography (HPSEC) (Fig. S7,
SI) revealed a number-average molecular weight (Mn) of 1626 and
a polydispersity (Mw/Mn) of 1.55.
These results clearly show that the developed Al(III) phthalo-
cyanines are talented catalysts for CO2 coupling reaction with
epoxides. On one hand, the TONs and TOFs obtained with bi-
functional catalyst CAT 1, without any co-catalyst, in CO2 cy-
cloaddition reactions to terminal epoxides (Table 1) were signif-
icantly higher than those obtained with Zn(II) phthalocyanines
−
CAT 1 is used, the nucleophile (I ) is a good leaving group, so it
promotes ring closure and the subsequent formation of cyclic car-
bonates [90,91]. On the other hand, when CAT 3 is used, in the
presence of PPNCl as co-catalyst, the poor leaving group ability of
TON = 900, TOF = 38 h−1) [83], and have the same magnitude
Cl possibly favors the growing of polymer chain and promotes the
−
(
of those achieved with other metallophthalocyanine catalysts in
formation of the polycarbonate [90,91].
the presence of a co-catalyst, such as copper-based (TOF = 502)
In sum, the cationic imidazolyl-based Al(III) phthalocyanines
were shown to be active bifunctional catalysts for the CO2 ad-
dition to terminal epoxides, without the need of using any co-
catalyst. Furthermore, the tert–butylphenoxy Al(III)-phthalocyanine
catalytic system CAT 3/PPNCl was active in CO2 addition reactions
to terminal epoxides, showing high selectivity for cyclic carbonates,
−1
−1
[
(
[
50], aluminum (TOF = 586 h ) [51], TOF = 188 h ) [55], iron
−
1
−1
)
TOF = 900 h ) [53] and cobalt-based catalysts (TOF = 47 h
−1
57], TOF = 12 h ) [81], under similar reaction conditions. More-
over, the activity of the catalytic system CAT 3/PPNCl in CO addi-
2
tion to cyclohexene oxide (TON = 814, TOF = 34 h−1) is superior
to that achieved with other metallophthalocyanine-based catalysts
and also in CO /cyclohexene oxide coupling, leading to an unprece-
2
[
49,53,81], showing a markedly different selectivity for cyclohex-
dented selectivity for the formation of cyclohexane polycarbonate.
ane polycarbonate formation, from that reported with metalloph-
thalocyanine catalysts.
3. Conclusions
We have previously observed that, with Mn(III) [38] and Cr(III)
[
39] porphyrin catalysts, the reaction selectivity is substrate-
We have disclosed an efficient synthetic methodology to mod-
dependent, with cyclic carbonates being mainly formed when ter-
minal epoxides are used as substrates, while polycarbonates are
predominantly obtained with cyclohexene oxide, since the ring
closure is less favored due to high geometric strain [89]. In the
present study, the CO2 addition reactions to terminal epoxides
were also found to be substrate-dependent, giving exclusively
ulate the structure of Al(III) phthalocyanine complexes bearing
cationic imidazolyl or tert–butylphenoxy moieties. It was demon-
strated that the use of alternative energy sources, such as ul-
trasounds for the synthesis of the phthalonitrile precursors and
microwave irradiation for the subsequent phthalonitrile cyclote-
tramerization with AlCl3 allowed to obtain the desired Al(III) ph-
5

A.C.S. Gonzalez, A.P. Felgueiras, R.T. Aroso et al.
Journal of Organometallic Chemistry 950 (2021) 121979
Fig. 4. Mechanistic pathways for the formation of cyclohexane cyclic carbonates, using CAT 1 versus polycarbonates using CAT 3/PPNCl.
spectrometer, operating at 400.13 MHz for 1H and 100.61 MHz for
13C. Chemical shifts for 1H and 13C are expressed in ppm, rela-
tively to a TMS as internal standard, or relatively to residual peaks,
thalocyanines in shorter times and higher yields, when compared
with previous conventional methods. The Al(III) phthalocyanine
complexes have efficiently catalyzed the coupling reaction between
CO2 and terminal epoxides (styrene oxide, epichlorohydrin, propy-
lene oxide and allyl glycidyl ether), where the bifunctional cationic
imidazolyl metallophthalocyanines were shown to be active (using
only 0.07 mol%) and selective catalysts, to promote the cycloaddi-
tion for the formation of cyclic carbonates, even in the absence of
any co-catalyst and recyclable, being recovered by simple precipi-
tation from the reaction crude mixture and reused in two consecu-
tive runs without loss of catalytic activity or selectivity. In addition,
the use of tert–butylphenoxy Al(III)-phthalocyanine/PPNCl catalytic
system in the CO2 addition to cyclohexene oxide constitutes the
first example ever reported of a metallophthalocyanine-based cat-
present in the deuterated solvents used, CDCl3 or DMSO–d . The
6
MALDI-TOF and ESI mass spectra were acquired using an Autospec
Micromass and Bruker Microtof, respectively. Microwave-assisted
experiments were performed in an appropriate thick-walled glass
vial (10 mL) under closed-vessel conditions, using a CEM Discov-
er® SP Microwave System. The reactions carried out in a bath
with ultrasound irradiation were conducted in a Bandelin Sonorex
RK 100H device. High performance size exclusion chromatography
(HPSEC) was performed for polycarbonate samples, using a Vis-
cotek (ViscotekTDAmax) with a differential viscometer (DV), right-
angle laser-light scattering (RALLS, Viscotek), and refractive index
(RI) detectors. The column set was composed by a PL 10 mm guard
column followed by one MIXED-E PLgel column and one MIXED-C
PLgel column. Previously filtered THF was used as an eluent at a
flow rate of 1.0 mL/min at 30 °C. The samples were filtered through
a polytetrafluoroethylene (PTFE) membrane with 0.2 mm pore be-
fore injection and the system was calibrated with narrow PS stan-
dards. The MnSEC and Ð of the synthesized polymers were deter-
mined by using a conventional calibration (OmniSEC software ver-
sion: 5.0).
alyst capable to promote CO /epoxide copolymerization.
2
Therefore, we can conclude that these Al(III) phthalocyanines
are promising catalysts for CO2 addition reactions to epoxides, and
their structural modulation allows to selectively obtain cyclic car-
bonates versus polycarbonates. This work paves the way for the
future development of highly active and selective catalysts through
fine-tuning of the phthalocyanine structure.
4. Experimental
4
.1. General
4.2. Synthesis and characterization of catalysts
ꢀ
ꢀ
All reagents were purchased from Sigma-Aldrich, dried over
4-(1 H-imidazol-1 -yl) phthalonitrile (1)
alumina and stored under inert atmosphere. The solvents used
in the synthesis reactions were, whenever necessary, purified or
dried according to the methods described in the literature. Air and
moisture sensitive reagents were manipulated using Schlenk tech-
niques, under a nitrogen or argon atmosphere, in a vacuum sys-
tem. All glass material was dried in an oven at 100 °C. 1H and
Imidazole (1.26 g, 18.5 mmol) was added to a solution of
4-nitrophthalonitrile (2.56 g, 14.8 mmol) and K2CO3 (10.2 g,
74 mmol) in DMF (38 ml) and the reaction mixture was stirred in
an ultrasound bath, at 25 °C, for 9 h. At the end, the mixture was
filtered to remove the K CO . Then, a mixture of water and ice (ca.
2
3
200 ml) was added to the reaction mixture until a precipitate was
formed. The yellowish precipitate was then filtered under vacuum
13
C NMR spectra were recorded in CDCl3 on a Bruker DRX 400
6

A.C.S. Gonzalez, A.P. Felgueiras, R.T. Aroso et al.
Journal of Organometallic Chemistry 950 (2021) 121979
using a sintered plate glass funnel, and dried at 80 °C for 12 h.
Then, the solid was recrystallized from methanol (50 ml), and the
compound was filtered again under vacuum, using a sintered plate
glass funnel and, finally, dried in vacuum at 80 °C for 4 h. Yield:
heated to 260 °C, under microwave irradiation (P = 200 W), for
1 h. After cooling to room temperature, the product was precipi-
tated by addition of a solution of methanol/water (10:1) and it was
left at 4 °C overnight. The blue precipitate was purified by column
chromatography on silica gel using dichloromethane, followed by
dichloromethane:methanol (95:5) as eluent. Yield: 555 mg (70%)
of a blue solid. Spectroscopic data was in accordance with the
literature [88].
1.78 g (62%), obtained as a white solid. Spectroscopic data was in
accordance with that previously reported [84].
4
-(4–tert-butylphenoxy) phthalonitrile (2)
–tert-Butylphenol (2.25 g, 15 mmol) was added to a solution of
-nitrophthalonitrile (2.08 g, 12 mmol) and K CO3 (9.8 g, 71 mmol)
4
4
2
4.3. General procedure of catalytic CO2 addition reactions to epoxides
in DMF (40 ml). The reaction mixture stirred in an ultrasound bath,
at 80 °C, for 2 h, under inert atmosphere. After cooling, ice water
was added, and the precipitate formed was filtered. Yield 2.96 g
The catalytic reactions of CO2 addition to epoxides were car-
ried out in a stainless steel 120 mL autoclave. The catalyst (0.07
mol%) and co-catalyst (0.07 mol%) (when indicated) were placed in
a glass beaker, inside the autoclave and it was left in vacuum for
approximately 3 h, at 80 °C. Then, the epoxide substrate (4 ml),
previously dried over alumina, was injected into the autoclave via
cannula. The autoclave was then pressurized with CO2 (10–40 bar)
and the reaction proceeded at the desired temperature (80 °C). Af-
ter 24 h, the autoclave was cooled and slowly depressurized. The
(
88%), obtained as a white solid. Spectroscopic data was in accor-
dance with that previously reported [85].
ꢀ
ꢀ
2
(3),9(10),16(17),23(24)-tetrakis(1 H-imidazol-1 -yl)phthalocyaninato
hydroxyaluminium (III) (3)
ꢀ ꢀ
4
-(1 H-imidazol-1 -yl)phthalonitrile (1) (500 mg, 2.57 mmol)
and AlCl3 (171 mg, 1.28 mmol) were dissolved in 1-
chloronaphthalene (5 ml), under inert atmosphere. Then, the
mixture was heated, under microwave irradiation (P = 200 W),
to 260 °C for 1 h. After cooling to room temperature, the product
was precipitated by addition of dichloromethane and filtered-off.
The green precipitate was washed with 20 ml of acetone, water,
acetone and dichloromethane, in that order. Yield: 316 mg (60%)
of a green solid. Spectroscopic data was in accordance with that
previously reported [84].
of conversion was determined by 1H NMR of the crude mixture,
%
using mesitylene as standard. Selectivity was calculated by inte-
gral ratio between polycarbonate and cyclic carbonate peaks. Prod-
uct isolation and purification was carried out only when polymeric
products were obtained (see below).
4
.4. Isolation and characterization of poly(cyclohexanecarbonate)
The reaction crude was evaporated and the residue was dried
ꢀ
ꢀ
ꢀ
2
(3),9(10),16(17),23(24)-tetrakis(3 -ethyl-1 H-imidazol-1 -
in vacuum at 100 °C for 5 h. Then, the solid was washed with n-
hexane, filtered and dried under vacuum at 100 °C for 12 h. The
polymer was obtained in 50% yield (3.44 g), calculated from the
mass of the isolated product relative to the weighted mass of epox-
yl)phthalocyaninato hydroxy aluminium (III) iodide (CAT
1
)
Phthalocyanine 3 (120 mg, 0.14 mmol) and iodoethane (1 ml,
2 mmol) were dissolved in DMF (2 ml). The reaction proceeded
at 80 °C for 48 h under inert atmosphere and an additional 3 ml
36 mmol) of iodoethane were added during this period. The crude
1
ide and the CO wt of the catalyst and co-catalyst. The CO content
2
2
(
%) was calculated from 1H NMR data by the integral ratio between
(
copolymer carbonate linkages (δ = 4.64 ppm) with respect to the
was precipitated with the addition of 20 ml dichloromethane and
filtered. Yield: 165 mg (82%) of a green solid. Spectroscopic data
was in accordance with that previously reported [84].
1
ether linkage signals (δ = 3.57 ppm). H NMR (400 MHz, CDCl ):
3
δ (ppm): 4.65 (br s, 2H), 2.12 (br s, 2H), 1.71 (br s, 2H), 1.57–1.22
br m, 4H). 13C NMR (100 MHz, CDCl ): δ (ppm): 153.8, 77.2, 29.7,
(
3
2
9.4, 23.1, 22.8.
ꢀ
ꢀꢀ
2
(3),9(10),16(17),23(24)-tetrakis((3 -(3 -
ꢀ
ꢀ
trimethylammonium)propyl)−1 H-imidazol-1 -yl)phthalocyaninato
Declaration of Competing Interest
hydroxyaluminium (III) bromide (CAT 2)
The phthalocyanine
3
(85 mg, 0.11 mmol) and (3-
mmol)
The authors declare that they have no competing interests.
bromopropyl)trimethylammonium bromide (261 mg,
1
were dissolved in DMF (5 ml). The reaction proceeded under inert
atmosphere, at 120 °C, for 72 h. After cooling to room temper-
ature, the product was precipitated with the addition of 40 ml
ethanol and filtered-off. The green precipitate was washed with
Acknowledgments
This work was supported by national funds from FCT
–
2
0 ml of ethanol, acetone and dichloromethane, in that order.
Fundação para a Ciência e a Tecnologia, I.P., through projects
UID/QUI/00313/2019 to the Coimbra Chemistry Center (CQC) and
PTDC/QUI-OUT/27996/2017 (DUALPI). ACSG and RTA also thank FCT
for PhD grants UI/BD/150804/2020 and PD/BD/143123/2019, re-
spectively.
Yield: 95 mg (53%) of a green solid. UV–Vis (DMSO): εabs, nm
−
1cm ): 346 (4.61 × 10 ), 611 (2.28 × 10 ), 678 (1.38 × 10 ).
−1
4
4
5
(
M
1
H NMR (400 MHz, DMSO-d ): δ (ppm): 10.98–10.66 (m, 4H),
6
1
0.65–10.09 (m, 4H), 10.04–9.83 (m, 4H), 9.32–8.90 (m, 8H),
.47–8.36 (m, 4H), 4.67–4.55 (m, 8H), 3.70–3.59 (m, 8H), 3.27–3.20
m, 36H), 2.72–2.58 (m, 8H). 13C NMR (101 MHz, DMSO-d ): δ
8
(
(
(
(
[
References
6
ppm): 62.1 (–CH –N(CH ) ), 52.8 (-CH –N
), 52.6 (-CH ), 23.0
3
2
3
3
2
Imid
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