Aluminum porphyrin complexes via delicate ligand design: Emerging efficient catalysts for high molecular weight poly(propylene carbonate)

Article abstract of DOI:10.1039/c4ra10643a

Due to the deep concern over residual, toxic cobalt or chromium from catalysts in biodegradable poly(propylene carbonate) (PPC), bifunctional aluminum porphyrin complexes with quaternary ammonium salts anchored on the ligand framework were prepared, and a delicate design of the porphyrin ligand was obtained. An optimized catalyst was complex 6b, which had two para-bromine benzenes and two quaternary ammonium cations linked to benzene via a six-methylene spacer in the meso-position of the porphyrin framework and NO₃^- as axial ligand and quaternary ammonium anion, showed TOF of 560 h^-1 at 80 °C and 3 MPa to yield PPC with 94% carbonate linkage and number average molecular weight of 96 kg mol^-1. The PPC selectivity reached 93%, which was the highest record in this copolymerization for aluminum porphyrin complexes. The soil tolerant bifunctional aluminum porphyrin complexes are becoming increasingly competitive catalysts, since they can be left with the plastics without any extra separation. This journal is

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ARTICLE TYPE
Aluminum Porphyrin Complexes via Delicate Ligand Design: Emerging
Efficient Catalysts for High Molecular Weight Poly(propylene
carbonate)
a,b
a,b
a
a
a
Xingfeng Sheng, Yong Wang, Yusheng Qin,* Xianhong Wang,** Fosong Wang
5
Received (in XXX, XXX) Xth XXXXXXXXX 20XX, Accepted Xth XXXXXXXXX 20XX
DOI: 10.1039/b000000x
Due to the deep concern on residue toxic cobalt or chromium from catalyst in biodegradable
poly(propylene carbonate) (PPC), bifunctional aluminum porphyrin complexes with quaternary
ammonium salts anchored on the ligand framework were prepared, notably a delicate design of the
porphyrin ligand was carried out. An optimized catalyst was complex 6b, which had two paraꢀbromine
1
0
benzenes and two quaternary ammonium cations linked to benzene via sixꢀmethylene spacer in the mesoꢀ
ꢀ
position of porphyrin framework and NO as axial ligand and quaternary ammonium anion, showed TOF
3
ꢀ
1
o
of 560 h at 80 C and 3 MPa, yielding PPC with 94% carbonate linkage and number average molecular
ꢀ
1
weight of 96 kg mol , and the PPC selectivity reached 93%, which was the highest record in this
copolymerization for aluminum porphyrin complexes. The soil compostable bifunctional aluminum
porphyrin complexes are becoming increasingly competitive catalysts, since they can be left with the
1
5
plastics without any extra separation
.
Introduction
temperature. However, these highly active salen catalysts
contained toxic metal (Cr or Co), which are difficult to be
completely separated from final PPC products, significantly
limiting their potential application as biodegradable plastics,
since such plastics may fail to satisfy current strict heavy metal
limitation for soil compostable products.
1
Since the pioneering work by Inoue and coꢀworkers in 1969,
copolymerization of epoxides with carbon dioxide (CO2)
Scheme 1) has drawn much attention from both academic and
2
2
3
3
4
4
0
5
0
5
0
5
5
0
(
2
industrial viewpoints, mainly due to the environmental benefit
from utilization of renewable CO resource upon the growing
concern on the greenhouse effect. In the past decades, numerous
2
3
catalyst systems have been developed to promote this
4
transformation. Prominent among these are a variety of metal
salen complexes via delicate design of metal center, ligand as
well as axial group. In 2003, Coates and coworkers firstly
reported that (salen)CoX catalysts could effectively catalyze the
copolymerization of propylene oxide (PO) and CO at ambient
5
2
Scheme 1 Copolymerization reaction of propylene oxide and CO .
2
temperature and 55 atm, producing poly(propylene carbonate)
55
Considering environmentally benign metal complexes for
(PPC) with 90ꢀ99% carbonate linkage in 99% selectivity. A series
CO /epoxide coupling reaction, aluminum complex is a good
2
of (salen)CrX catalysts were also found to catalyze this
transformation effectively by Darensbourg and coworkers.
Bifunctional catalyst was designed with a piperidinium endꢀ
choice. In the literature, many (salen)AlX have been developed to
promote this transformation. In 2005, Darensbourg and
6
10
coworkers reported (salen)AlX complexes for copolymerization
7
8
capping arm in Nozaki’s group, and further developed by Lee
60 of cyclohexene oxide (CHO) with CO , which showed modest
2
9
and Lu, to improve the catalytic performance of metal salen
complex. A bifunctional (salen)CoX catalyst reported by Lee
activity but with high polymer selectivity, and the resultant
copolymer was highly alternated with 90ꢀ99% carbonate linkage.
Generally, (salen)AlX complexes typically provide alternating
8
group shows the highest activity for polycarbonate synthesis
even at high temperature and high [epoxide]/[catalyst] ratio,
copolymers from CO2 and CHO, but afford only propylene
ꢀ
1
11
giving turnover frequency (TOF) of 26,000 h , the resultant PPC 65 carbonate (PC) from PO and CO2. Thus it is still a great
shows remarkably high number average molecular weight (M ) of
challenge to design a suitable aluminum complex capable of
efficiently catalyzing the copolymerization of PO and CO2.
Since Inoue’s report in 1978 that Al prophyrin complex could
n
ꢀ
1
9
3
74 kg mol . Lu and coworkers reported that bifunctional
salen)CoX catalyst with 1,5,7ꢀtriabicyclo[4,4,0] decꢀ5ꢀene
(designated as TBD, a sterically hindered organic base) anchored
on the ligand framework showed high activity at high 70 catalysts have been studied extensively for epoxides/CO2
(
12
catalyze the copolymerization of PO and CO2, Al prophyrin
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13
copolymerization, and one common concept is that both the
using standard Schlenk technique or in a glove box.
catalytic activity and CO incorporation can increase dramatically 50 Dichloromethane (CH Cl ), chloroform (CHCl ), acetonitrile
2
2
2
3
14
with the use of quaternary onium salt. Ree and coworkers used
tetraphenylporphinato) aluminum chloride [(TPP)AlCl] or its PO
(CH CN), pyrrole, propylene oxide (PO) were distilled over CaH
3 2
under inert atmosphere. The CO gas (99.999%) was purchased
(
2
5
0
5
0
5
0
5
0
adduct with Et NBr as cocatalyst to catalyze the
copolymerization of PO and CO2, which only produced
and used without further purification. Other chemicals were
obtained from Aldrich and Acros, and used as received without
4
oligomeric products (M 1900ꢀ3300) with 70ꢀ75% carbonate 55 further purification unless otherwise stated.
n
1
5
linkage. Our early work indicated that the presence of a bulky
Lewis acid had positive effect for the copolymerization of CHO
Solution NMR spectra were collected at ambient temperatures
using a Bruker ARXꢀ300 spectrometer at room temperature in
1
1
2
2
3
3
4
and CO in (TPP)AlCl catalyst system. However, there is still a
deuterated chloroform (CDCl ) or dimethyl sulfoxide (DMSO)
2
3
long distance on catalytic performance of Al prophyrin
complexes in comparison with salen complexes.
with tetramethylsilane (TMS) as internal reference. Solvent
60 proton shifts (ppm): CDCl , 7.26 (s); DMSOꢀd , 2.50 (s). Solvent
3
6
Inspired by the success of bifunctional salenCo catalysts, we
prepared bifunctional cobalt porphyrin complexes for better
catalytic performance, which contained quaternary ammonium
carbon shifts (ppm): CDCl , 77.16 (t); DMSOꢀd , 39.52 (m).
3 6
Matrixꢀassisted laser desorption/ionization timeꢀofꢀflight mass
spectroscopy (MALDIꢀTOF/MS) was performed on a Bruker
atuoflex III mass spectrometer. The molecular weight and
1
6
salts tethered to the porphyrin ligand, and it provided a
ꢀ
1
moderate activity (TOF = 495 h ) for polycarbonate synthesis at 65 molecular weight distribution of the poly(propylene carbonate)
o
5
0
C and 40 atm. Most recently, we designed bifunctional
were determined by gel permeation chromatography (GPC) at 25
℃ in polystyrene standard on Waters 410 GPC instrument with
dichloromethane as the eluent, where the flow rate was set at 1.0
aluminum porphyrin complex, which exhibited good catalytic
o
17
performance at 70 C and 30 atm. However, the molecular
weight of PPC is still not high enough for practical application,
and the PPC selectivity of this catalyst decreased dramatically
ꢀ
1
mL min .
70
Synthesis of complex 6b.
o
with increasing temperature, i.e., it dropped to only 80% at 90 C.
The reason may lie in that the Lewis acidity of Al center is not
compatible with porphyrin ligand, therefore, the ligand should be
designed delicately to adjust the Lewis acidity of Al to get high
molecular weight PPC. In another consideration, coulombic
interaction between the quaternary ammonium cation and the
8
chainꢀgrowing anion should be balanced for better catalytic
performance at high temperature, which may be adjusted through
altering the length of the spacer linked ligand framework and
quaternary ammonium units. Of course, the influence of the axial
ligands or the substituents on the porphyrin is also important to
adjust the electronic property of Al, which will also be the
concern in this work. Upon delicate design of the ligands,
aluminum porphyrin complex 6b (Fig. 1) with two paraꢀbromine
benzenes and two quaternary ammonium cations linked to
benzene via sixꢀmethylene spacer in the mesoꢀposition of
ꢀ
porphyrin framework, and NO3 as axial ligand and quaternary
ammonium anion, showed outstanding catalytic performance at
o
8
0
C and 3 MPa, yielding highly alternated PPC with high
molecular weight and narrow polydispersity (PDI).
Scheme 2 Synthesis of the bifunctional aluminum porphyrin complex 6b.
Compound 1
1
8
Compound 1 was obtained as reported in the literature.
A
Fig.1 Structure of the bifunctional aluminum porphyrin complex 6b.
75
solution of pyrrole (2 mol) and pꢀbromobenzaldehyde (20 mmol)
was degassed with a stream of argon for 10 min, then InCl (0.4
3
45
Experimental
g, 2.0 mmol) was added, and the mixture was stirred at room
temperature for 2 h. Then NaOH (0.2 mol) was added and the
mixture was stirred for another 45 min. After filtration, the filtrate
was concentrated under vacuum. The crude product was purified
Reagents and methods
All reactions of airꢀ and/or moistureꢀsensitive complexes and
product manipulations were performed under inert atmosphere
80
by
column
chromatography
(silica,
petroleum
2
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ether/dichloromethane v/v = 1:1). The product was obtained as a
dichloromethane was degassed with a stream of argon for 5 min
light yellow solid in 87% yield.
60 in an iceꢀbath. After 1.3 mmol Et AlCl was added slowly, the
2
1
H NMR (300 MHz, CDCl , δ): 7.91 (brs, 2H, NH), 7.43 (d,
reaction solution was heated to room temperature and stirred for 1
h. The mixture was concentrated using a rotary evaporator to
produce a residue which was purified by column chromatography
(neutral alumina, dichloromethane/methanol v/v = 10:1) and
3
J=9 Hz, 2H), 7.07 (d, J=9 Hz, 2H), 6.71 (m,2H), 6.16 (m,2H),
5.89 (s,2H), 5.43 (s,1H)
Compound 2
5
19
Compound 2 was synthesized as reported in the literature. To 65 obtained as a purple solid in 97% yield.
1
a solution of bromobenzene (10 mmol) in anhydrous diethyl ether
20 mL) under an argon atmosphere, nꢀbutyllithium (1.5 eq., 2.5
H NMR(300 MHz, DMSOꢀd , δ): 9.04 (m, 8H; pyrrꢀH), 8.05ꢀ
6
(
8.18 (m, 16H), 3.47ꢀ3.73 (m, 4H; CH ꢀBr), 2.97 (t, J=6 Hz, 4H;
2
o
13
1
1
2
2
3
3
4
4
5
5
0
5
0
5
0
5
0
5
0
5
M in hexane) was slowly added at 0 C under stirring followed by
addition of 1,6ꢀdibromohexane (4.0 eq.). After the mixture was
ArꢀCH ), 1.92ꢀ1.99 (m, 8H), 1.52ꢀ1.58 (m, 8H). C NMR (75
2
MHz, DMSOꢀd , δ): 146.46, 140.07, 135.83, 134.07, 132.27,
6
refluxed for 2 h, it was cooled to room temperature, subsequently 70 130.05, 127.01, 122.19, 118.84, 35.24, 35.10, 32.57, 30.81,
partitioned between diethyl ether (40 mL) and water (30 mL).
The aqueous phase was extracted with diethyl ether (2 × 20 mL),
28.87, 27.45.
+
MS (MALDIꢀTOF): m/z = 1119.1 [MꢀCl] (calcd. 1119.06).
Complex 6a
the combined organic layers were dried over MgSO , evaporated
4
in vacuum and purified by flash chromatography on silica gel
A solution of compound 4 (1.0 mmol) and tributylamine (40
using hexane as a mobile phase. Then 80 mmol 1ꢀbromoꢀ6ꢀ 75 mmol) in anhydrous CHCl (5.0 mL) and CH CN (5.0 mL) was
3
3
phenylhexane was dissolved in 120 ml dry dichloromethane and
the mixture was cooled to 0ꢀ5 C on an ice bath. A gasꢀtrap was
refluxed for 96 h under argon atmosphere. After cooled to room
temperature, the solvent was removed by a rotary evaporator,
o
connected to the setup. 24 g (128 mmol) TiCl4 was added
carefully but quickly, then dichloromethyl methyl ether (8 g, 67
mmol) was added dropwise in approximately 20 min to the cold
and the layer of Bu N was removed with a pipette. The residue
3
was washed 3 times by ether, and the yield was 96%.
1
80
H NMR(300 MHz, DMSOꢀd , δ): 9.04 (m, 8H; pyrrꢀH), 8.08ꢀ
6
o
mixture, while the temperature was kept between 0 and 2 C. The
8.16 (m, 12H), 7.69 (s, 4H), 2.97ꢀ3.34 (m, 20H), 1.15ꢀ1.92 (m,
13
mixture was stirred for 5 min, slowly heated to room temperature
40H), 0.87ꢀ0.98 (m, 18H). C NMR (75 MHz, DMSOꢀd , δ):
6
o
and subsequently stirred at 35 C for 15 min. Late the reaction
146.47, 140.09, 135.88, 134.13, 132.29, 130.08, 127.05, 122.22,
57.62, 53.21, 35.07, 32.09, 30.89, 29.03, 25.79, 23.19, 19.29,
mixture was slowly poured in a beaker filled with ice and
subsequently transferred to a separation funnel and extracted with 85 13.56.
+
dichloromethane. The organic layer was collected and the
aqueous phase was extracted two more times with
dichloromethane. The combined organic layers were washed with
MS (MALDIꢀTOF): m/z = 1445.6 [MꢀBr] (calcd. 1445.52).
Complex 6b
To a stirred solution of AgNO (4.5 mmol) in ethanol (20.0
3
a
saturated NaHCO3 solution, and dichloromethane was
mL) and acetone (20.0 mL), complex 6a was added (1.0 mmol)
evaporated in vacuum. The crude product was purified by column 90 quickly. The reaction mixture was stirred for 12 h in dark at room
chromatography (silica, petroleum ether/EtOAc gradient) to
temperature. After the solvent was removed by a rotary
evaporator, the residue was dissolved by CH Cl (10.0 mL), then
filtered, the filtrate was concentrated under vacuum to give a
obtain the pure product.
2
2
1
H NMR (300 MHz, CDCl , δ): 9.97 (s, 1H), 7.81 (d, J=9 Hz,
3
2
2
H), 7.34 (d, J=6 Hz, 2H), 3.39 (t, J=6 Hz, 2H), 2.69 (t, J=9 Hz,
H), 1.85 (m, 2H), 1.66 (m, 2H), 1.47 (m, 2H), 1.36 (m, 2H).
purple product and the yield was 98%.
1
95
H NMR(300 MHz, DMSOꢀd , δ): 9.03 (m, 8H; pyrrꢀH), 8.05ꢀ
6
Compound 3
8.19 (m, 12H),7.69 (m, 4H), 2.98ꢀ3.40 (m, 20H), 1.28ꢀ1.92 (m,
1
6
13
Compound 3 was obtained as reported. A solution of
compound 1 (1.9 mmol) and compound 2 (1.9 mmol) in 380 mL
dry dichloromethane was degassed with a stream of argon for 10
40H), 0.88ꢀ1.00 (m, 18H). C NMR (75 MHz, DMSOꢀd , δ):
6
146.42, 140.09, 135.85, 134.09, 132.18, 130.05, 127.02, 122.18,
57.61, 51.84, 35.03, 30.80, 27.53, 25.04, 23.14, 19.26, 13.52.
+
min, the solution was stirred for 1 h after trifluoroacetic acid 100 MS (MALDIꢀTOF): m/z = 1455.6 [MꢀNO ] (calcd. 1455.61).
3
(
0.37 mL) was added, then 2,3ꢀdichloroꢀ5,6ꢀdicyanoꢀpꢀ
General Copolymerization Procedure
benzoquinone (DDQ) (0.9 g) was added and the solution was
stirred for another 1 h. After filtration, the filtrate was
concentrated using a rotary evaporator to produce a residue which
was purified by column chromatography (silica, petroleum
The required catalyst and PO were added to a 15 mL stainlessꢀ
steel autoclave with a magnetic stirrer in a glove box. CO
pressurized to this mixture and the reaction was operated under
was
2
ether/dichloromethane v/v = 1:1) to obtain a purple solid in 22% 105 determined condition. After polymerization, the autoclave was
yield.
cooled to room temperature, and the CO pressure was released
by opening the outlet valve. small aliquot of the
copolymerization mixture was taken out for
spectroscopy, and the resulting mixture was poured into a flask
2
1
H NMR(300 MHz, CDCl , δ): 8.84 (m, 8H; pyrrꢀH), 8.09 (m,
A
3
1
8
2
H), 7.91 (m, 4H), 7.57 (m, 4H), 3.51 (t, J=6 Hz, 4H; CH ꢀBr),
H NMR
2
.97 (t, J=6 Hz, 4H; ArꢀCH ), 1.98 (m, 8H), 1.62 (m, 8H), ꢀ2.82
2
13
(
s, 2H; NH). C NMR (75 MHz, CDCl , δ): 142.37, 141.10, 110 and dried in vacuum.
3
1
39.45, 135.98, 134.73, 130.11, 126.94, 122.67, 118.88, 35.94,
34.21, 32.94, 31.51, 28.77, 28.29.
MS (MALDIꢀTOF): m/z = 1095.1 [M+H] (calcd. 1095.09).
Compound 4
Results and Discussion
+
Molecular Design of Bifunctional Aluminum Porphyrin
Complexes
A solution of compound 3 (1.0 mmol) in 20 mL dry
Considering environmentally benign metal complexes for
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CO /epoxide coupling reaction, aluminum complex is a good
noteworthy that cyclic carbonate byproduct with high boiling
2
choice. Though (salen)MX (M = Co, Cr) complexes showed high 60 point is hard to be separated from PPC product. Therefore,
2
catalytic activity for CO /PO copolymerization, (salen)AlX
though high temperature copolymerization generally increases
catalytic activity which may be the reason to carry out
copolymerization at high temperature, it is frustrated by the low
PPC selectivity. To improve highꢀtemperature catalytic
2
worked only for CO /CHO copolymerization and not for a more
2
1
0ꢀ11
5
0
5
0
5
0
5
0
5
0
industrially viable CO /PO copolymerization.
As is known,
2
coordinating ligands greatly influence the reactivity of metal
complexes. Porphyrin ligand which has wellꢀdefined coordination 65 performance of aluminum porphyrin complexes, we prepared
modes with metal ions is a good choice for aluminum complexes
complexes 1aꢀ5a with different length of the spacer linked ligand
framework and quaternary ammonium units (Fig. 2), trying to
change coulombic interaction between the quaternary ammonium
to achieve the ability to catalyze copolymerization of CO and
2
2
0
1
1
2
2
3
3
4
4
5
PO. Therefore, complexes comprising aluminum metal center
and porphyrin ligand framework were designed.
cation anchored on the ligand framework and the chainꢀgrowing
8
Binary catalytic system of (aluminum porphyrin 70 anion.
complex)/(onium salt or base) show good catalytic performance,
but has a great drawback where it cannot be highly active at a
high [monomer]/[catalyst] ratio. Considering that tethering the
cocatalyst to the ligand framework may improve the activity and
1
6ꢀ17
selectivity at a high [monomer]/[catalyst] ratio,
bifunctional
aluminum porphyrin complexes with quaternary ammonium salts
anchored on the ligand framework were prepared, where the
chainꢀgrowing carbonate unit always hangs around the metal
center through coulombic interaction between the quaternary
ammonium cation and the chainꢀgrowing anion. Moreover,
coulombic interaction may be adjusted through altering the
length of the spacer linked ligand framework and quaternary
ammonium units to further change the catalytic performance.
Therefore, bifunctional aluminum porphyrin complexes with
different length of the spacer linked ligand framework and
quaternary ammonium units were prepared (Fig. 2).
Fig.2 Structures of the bifunctional aluminum porphyrin catalysts 1aꢀ5a.
a
Table 1 PO/CO
2
copolymerization catalyzed by complexes 1a-5a.
Additionally, the Lewis acidity of the central metal ion
TOF Selectivity Carbonate Linkage
M
n
e
Entry Catalyst
ꢀ1
b
c
d
ꢀ1
e
PDI
1
3c, 17, 21
(h ) (%PPC)
(%)
(kg mol )
influenced the catalytic performance significantly,
which
could be adjusted by introduction of specific substituents on
ligand framework. In this regard, porphyrin ligand has another
merit where introduction of specific substituents in the mesoꢀ
position of porphyrin framework only affect electronic property
of the metal center without any significant steric influence. On
the basis of this concept, complexes with electronically different
arylꢀfunctionalities in the mesoꢀposition of porphyrin framework
1
2
3
4
5
1a
2a
250
330
360
380
400
72
75
82
82
83
93
97
97
98
98
23.0
43.0
48.6
49.1
49.6
1.16
1.13
1.16
1.16
1.17
3
a
4a
5a
13c
a
Reaction conditions: PO (5 mL), [PO]/[catalyst] = 5000, 3 MPa CO
2
o
b
c
75
pressure, 70 C, 5 h. Turnover frequency of PO to products. Selectivity
d 1 e
(Fig. 3) were prepared.
for PPC over PC. Determined by H NMR spectroscopy. Determined
The affinity of the five coordinate Al(III) center toward PO is
by gel permeation chromatography in CH
2 2
Cl at 25℃, calibrated with
greatly influenced by both the nature of the porphyrin and the
trans ligand, and axial ligand strongly influences the catalytic
performance in not only (salen)MX system but also
polystyrene standards.
As expected, the bifunctional aluminum porphyrin complexes
(1a-5a) with quaternary ammonium units anchored on the ligand
framework could effectively catalyze the copolymerization to
8
0
1
3a, 13b, 22
(
porphyrin)MX system.
Our earlier work indicated that
ꢀ
bifunctional aluminum porphyrin complexes with NO3 as axial
ligand and quaternary ammonium anion had the best PPC
selectivity and moderate activity. Therefore, as shown in Fig. 3,
we prepared bifunctional aluminum porphyrin complexes with
selectively afford PPC with high carbonate linkages (93ꢀ98%) at
o
7
0 C and 3 MPa even at a high [PO]/[catalyst] ratio of 5000
1
7
(Table 1). Considering that the chainꢀgrowing carbonate unit was
85
attracted to the metal center through coulombic interaction
between the quaternary ammonium cation and chainꢀgrowing
ꢀ
NO3 as axial ligand and quaternary ammonium anion, to
investigate the effect of axial ligand and quaternary ammonium
anion on catalytic activity and PPC selectivity.
8
anion, the length of the spacer linking ligand framework and
quaternary ammonium units may play an important role in
improving high temperature catalytic activity. When the number
90 of the spacer (n) (Fig. 2) increased from 2 to 6, the catalytic
Copolymerization Behavior of Bifunctional Aluminum
Porphyrin Complexes
ꢀ
1
activity increased from 250 to 400 h , and the PPC selectivity
increased from 72 to 83%, while the carbonate linkage content
increased from 93 to 98%. A flexible and sufficiently long spacer
is usually a prerequisite for obtaining excellent catalytic
performance. However, when n is over 4, improvements of
catalytic activity, PPC selectivity and carbonate linkage content
Bifunctional aluminum porphyrin complexes for PO/CO
2
7
1
copolymerization has been reported by our group very recently ,
which provided PPC selectivity around 90% and TOF around 400
55
ꢀ
1
o
h
at 70 C. However, the PPC selectivity of bifunctional
95
aluminum porphyrin complex dramatically decreased with
o
increasing temperature, and it was only 80% at 90 C. It is
4
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slowed down significantly. The aluminum porphyrin complex 20 b, 6aꢀb, 7aꢀb with electronically different arylꢀfunctionalities in
ꢀ
1
(
5a) showed relatively high catalytic activity (TOF = 400 h ) and
the mesoꢀposition of porphyrin framework (Fig. 3) were prepared,
PPC selectivity (83%), while producing PPC with 98% carbonate
whose catalytic performances were listed in Table 2, where
copolymerization was conducted at 70 C under 3 MPa for 5 h
ꢀ
1
o
linkage and M of 49.6 kg mol in molecularꢀweight distribution
n
5
(PDI) below 1.2. The M value is lower than the expected values
with a [PO]/[cat.] ratio of 5000.
n
due to the existence of chain transfer aroused by adventitious
water during the reaction.
25
Complex 6a in which the porphyrin ligand bearing two
brominated aryl substituents displayed relatively lower
ꢀ
1
copolymerization rate (TOF = 330 h ) in comparison with the
ꢀ
1
unsubstituted complex 5a (TOF = 400 h ). However, complex 6a
produced PPC with 92% carbonate linkage in higher PPC
selectivity (88%) than that achieved by complex 5a. In complex
3
3
4
4
5
5
0
5
0
5
0
5
6
a, since the porphyrin ligand bearing brominated aryl
substituents, the central metal aluminum showed enhanced Lewis
acidity, which enhanced the interaction between the anionic
propagating species and the metal center resulting in improved
PPC selectivity. Furthermore, lower carbonate content indicates
that complex 6a promoted consecutive insertion of PO into AlꢀO
bond, which reduced the catalytic activity because the relative
rates of ringꢀopening of PO by Al porphyrin is favored by the
Fig.3 Structures of the bifunctional aluminum porphyrin catalyst 5aꢀb,
6aꢀb, 7aꢀb.
1
0
13c
Table 2 PO/CO
aꢀb.
2
copolymerization catalyzed by complexes 5aꢀb, 6aꢀb,
alkylcarbonate over the alkoxide.
a
7
For complex 7a in which the porphyrin ligand bearing
methoxyꢀfunctionalized aryl substituents and the center
aluminum have reduced Lewis acidity than complex 5a,
significant improvement in catalytic activity (TOF = 440 h ) was
observed (Entry 5), which is consistent with the result of our
recent work. In this case, the PPC selectivity can reach 80% and
the resultant copolymer was highly alternated with 98%
carbonate linkage.
These results lead to the conclusion that the catalytic
performance of the aluminum porphyrin complex is significantly
influenced by the electronic environment of the porphyrin ligand.
It is noteworthy that even relatively minor modifications of
porphyrin can obviously alter the catalytic performance of the
resulting Al complexes. In general, porphyrin ligand with
electronꢀwithdrawing substituents for Al complexes was
beneficial for PPC selectivity, while both the catalytic activity
and carbonate linkage selectivity dropped.
TOF Selectivity Carbonate Linkage
M
n
(kg mol )
Entry Catalyst
ꢀ1
ꢀ1 PDI
(h ) (%PPC)
(%)
ꢀ
1
b
1
5a
5b
6a
6b
7a
7b
400
83
87
88
93
80
88
98
98
92
93
98
98
49.6
35.9
53.6
40.9
50.5
45.5
1.17
1.15
1.14
1.11
1.12
1.09
2
290
330
270
440
340
17
3
4
5
6
a
Reaction conditions: PO (5 mL), [PO]/[catalyst] = 5000, 3 MPa CO
2
o
b
pressure, 70 C, 5 h. Entry 5 from Table 1 has been reproduced here for
comparative purposes.
1
5
Specific substituents were introduced on ligand framework to
change the steric and electronic environment of the metal center
and adjust the catalytic performance, and similar strategy has
1
3c, 21b, 21c, 23
been used in the literature.
Therefore, complexes 5aꢀ
a
Table 3 Copolymerization of PO/CO
2
catalyzed by complex 6b.
PO/Cat.
mol)
T
( C)
P
Time
(h)
b
TOF
(h )
Selectivity
(%PPC)
M
n
(kg mol )
Entry
o
TON
ꢀ1
Carbonate Linkage (%)
ꢀ1
PDI
(
(MPa)
c
1
5000
5000
5000
5000
5000
5000
5000
2500
10000
10000
70
70
70
60
80
90
100
80
80
80
3
2
4
3
3
3
3
3
3
3
5
5
1400
1500
1450
800
280
300
290
160
490
950
1230
620
660
560
93
85
95
95
93
89
77
90
92
91
93
90
93
91
95
95
93
91
95
94
40.9
33.9
34.7
24.3
65.5
55.1
62.2
30.0
59.0
96.0
c
1.11
1.16
1.16
1.12
1.11
1.11
1.16
1.11
1.14
1.22
2
3
4
5
6
7
8
9
5
5
5
2450
1900
2460
930
2
2
1.5
5
3300
5600
1
0
10
a
b
Copolymerization was carried out in neat PO (5 mL), unless otherwise was specificly noted. Turnover number of PO to products. Entry 4 from Table
has been reproduced here for comparative purposes.
2
6
0
The affinity of the five coordinate Al(III) center toward PO is
greatly influenced by both the nature of the porphyrin and the
trans ligand. Minor alteration of axial ligand and cocatalyst 65 showed that simple changes in the axial group and the anion of
structure can significantly affect catalytic performance of
17, 22
porphyrin or salen metal complexes.
Further investigation
1
3c
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Page 6 of 8
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DOI: 10.1039/C4RA10643A
quaternary ammonium salt drastically affect the PPC/PC 45 basically depended on the TON value. As noted in Table 3, the
selectivity. In our recent work, bifunctional aluminum porphyrin
complexes with NO3 as axial ligand and quaternary ammonium
anion showed the best PPC selectivity. Therefore, complexes 5b,
number average molecular weight increased as the increasing
ꢀ
TON, while complex 6b was capable of providing high
ꢀ
1
molecularꢀweight polymers (M = 96.0 kg mol ) in addition to
n
5
0
5
0
5
0
5
6b, 7b were prepared by the reaction of complexes 5a, 6a, 7a and
high TONs. Importantly, the TOFs have only slight change upon
ꢀ
3
equivalent of AgNO , respectively. The complexes with Cl as 50 reducing the catalyst loading, which is an encouraging feature of
3
ꢀ
the axial group and Br as quaternary ammonium anion (5a, 6a
and 7a) showed higher catalytic activity (400 h , 330 h and 380
h ) and lower PPC selectivity (83%, 88% and 80%), while the
complexes with NO3 as the axial group and quaternary
ammonium anion (5b, 6b and 7b) showed lower activity (290 h , 55 relationship was observed between M (GPC) and reaction time
2
8
this one component bifunctional catalyst system compared to
traditional binary system. Furthermore, the superior catalytic
performance of complex 6b further reflected the controllable
ꢀ
1
ꢀ1
ꢀ
1
ꢀ
1
1
2
2
3
3
character for copolymerization of PO and CO . A proportional
2
ꢀ
1
n
ꢀ
1
ꢀ1
70 h and 340 h ) and higher PPC selectivity (87%, 93% and
9%), which was consistent with the result reported in the
for six parallel reactions (Fig. 4), along with the narrow PDI (<
1.25), implying a controllable copolymerization by complex 6b.
literatures, that the axial group and quaternary ammonium anion
1
7, 24
having poorer leaving ability gave higher selectivity for PPC.
Conclusion
To optimize the reactivity of these aluminum porphyrin
complexes with quaternary ammonium units anchored on the
A series of aluminum porphyrin complexes with quaternary
ammonium salts anchored on the ligand framework have been
synthesized, which showed good catalytic performance for
copolymerization of propylene oxide and carbon dioxide. The
PPC selectivity was up to 95% and the resultant copolymer was
highly alternated with 90ꢀ98% carbonate linkage. The catalytic
performance could be adjusted by the length of the spacer linked
ligand framework and quaternary ammonium units, the axial
ligand and the substituents on the porphyrin. Complex 6b
6
6
7
0
5
0
ligand framework for the copolymerization of CO and PO, the
2
influence of catalyst loading, reaction temperature, CO pressure
2
and reaction time was investigated utilizing complex 6b as an
example which exhibited the highest PPC selectivity. The results
are summarized in Table 3. Only slight change of catalytic
activity (TOF change less than 10%) was observed when the CO2
o
pressure increased from 2 MPa to 4 MPa (Entries 1ꢀ3) at 70 C.
However, the PPC selectivity and carbonate linkage content of
the resultant copolymer showed a modest increase. When the
copolymerization was carried out at 4 MPa, the PPC selectivity
could reach 95%, and the resultant copolymer was highly
alternated with 93% carbonate linkage. By contrast, the reaction
temperature showed a strong influence on the catalytic activity
o
exhibited the best catalytic performance at 80 C and 3 MPa in
ꢀ
1
ꢀ1
TOF of 560 h with 95% PPC selectivity and M of 96 kg mol ,
n
which was the highest record for porphyrin metal complexes.
Acknowledgements
(
Entry 1 and Entries 4ꢀ7), and the activity increased dramatically
The authors thank financial support from the National Natural
Science Foundation of China (Grant No. 51321062, 51273197
and 21134002).
o
o
as the temperature increased from 60 C to 100 C, resulting in a
dramatic increase of TOF from 160 h to 1230 h . However, the
PPC selectivity decreased as with increasing temperature, though
it still maintained 89% at 90 C. When the temperature further
increased to 100 C, the PPC selectivity sharply dropped to 77%
ꢀ
1
ꢀ1
o
7
5
Notes and references
o
a
Key Laboratory of Polymer Ecomaterials, Changchun Institute of
because of the acceleration of backingꢀbitting reaction, leading to
degradation of PPC at higher temperature.
Applied Chemistry, Chinese Academy of Sciences, Changchun 130022,
People’s Republic of China. Fax: +86 431 85262252; Tel: +86 431
8
5262252; E-mail: ysqin@ciac.jl.cn. Fax: +86 431 85689095; Tel: +86
80
431 85262250; E-mail: xhwang@ciac.jl.cn
University of Chinese Academy of Sciences, Beijing 100039, People’s
b
Republic of China
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Fig.4 Relationship of M
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DOI: 10.1039/C4RA10643A
Due to the deep concern on residue toxic cobalt or chromium from catalyst in
biodegradable poly(propylene carbonate) (PPC), bifunctional aluminum porphyrin
complexes with quaternary ammonium salts anchored on the ligand framework were
prepared, which showed good catalytic performance for copolymerization of
propylene oxide and carbon dioxide.