Bifunctional aluminum porphyrin complex: Soil tolerant catalyst for copolymerization of 2 and propylene oxide

Article abstract of DOI:10.1002/pola.27247

Due to the concern on residue toxic metal in biodegradable poly(propylene carbonate) (PPC), soil tolerant and heavy metal free aluminum complexes, that is, bifunctional aluminum porphyrin catalysts bearing quaternary ammonium salts on the ligand framework were prepared. Variation of the quaternary ammonium anion and the axial ligand had dramatic effects on the catalytic activity of resultant complex, among which complex 3b yielded perfectly alternative PPC with high molecular weight and relatively narrow polydispersity, and its TOF reached 3,407 h^-1 at [PO]/[cat.] ratio of 20,000 at 110 C, although the PPC selectivity was 71%. By introducing specific substituent on the ligand framework, the electronic environment at the active center can be changed, among which complex 5b bearing tertiary butyl-functionalized aryl substituents exhibited a TOF of 449 h^-1 at [PO]/[cat.] ratio of 5,000 at 70 C, with PPC selectivity of 92% and number average molecular weight of 36 kg mol^-1.

Full text of DOI:10.1002/pola.27247

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Bifunctional Aluminum Porphyrin Complex: Soil Tolerant Catalyst for
Copolymerization of CO2 and Propylene Oxide
Wei Wu,1,2 Xingfeng Sheng,1,2 Yusheng Qin,1 Lijun Qiao,1 Yuyang Miao,1 Xianhong Wang,1
Fosong Wang1
1Key Laboratory of Polymer Ecomaterials, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, Changchun
130022, People’s Republic of China
2University of Chinese Academy of Sciences, Beijing 100039, People’s Republic of China
Correspondence to: Y. S. Qin (E-mail: ysqin@ciac.jl.cn) or X. H. Wang (E-mail: xhwang@ciac.jl.cn)
Received 5 January 2014; accepted 10 May 2014; published online 12 June 2014
DOI: 10.1002/pola.27247
ABSTRACT: Due to the concern on residue toxic metal in biode-
gradable poly(propylene carbonate) (PPC), soil tolerant and
heavy metal free aluminum complexes, that is, bifunctional
aluminum porphyrin catalysts bearing quaternary ammonium
salts on the ligand framework were prepared. Variation of the
quaternary ammonium anion and the axial ligand had dramatic
effects on the catalytic activity of resultant complex, among
which complex 3b yielded perfectly alternative PPC with high
molecular weight and relatively narrow polydispersity, and its
TOF reached 3,407 h21 at [PO]/[cat.] ratio of 20,000 at 110 ꢀC,
although the PPC selectivity was 71%. By introducing specific
substituent on the ligand framework, the electronic environ-
ment at the active center can be changed, among which com-
plex 5b bearing tertiary butyl-functionalized aryl substituents
exhibited a TOF of 449 h21 at [PO]/[cat.] ratio of 5,000 at 70 ꢀC,
with PPC selectivity of 92% and number average molecular
weight of 36 kg mol21
.
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KEYWORDS: bifunctional aluminum porphyrin; carbon dioxide;
copolymerization; polycarbonates; catalysts
in Fig. 1), salenCoX (X 5OAc, O2CCCl3, O2CCCF3, Cl, OTs;
OTs 5 tosylate) complexes, as first developed by Coates in
2003,17 exhibited suitable activity and good PPC selectivity.
To improve the catalytic activity, Lu and Wang employed
ammonium salt cocatalystslike n-Bu4NY, Y 5Cl, Br, I, which
displayed significant increase in catalytic activity and carbon-
ate linkage of the copolymer.18 Coates and coworkers
employed bis-(triphenyiphosphine)iminium salt as cocata-
lysts,19 in addition to the improvement of catalytic activity,
stereoselective and regioselective PPC products were pre-
pared. However, higher activity is difficult to achieve for this
binary system, one reason lies in that the catalyst system
loses activity at a high [monomer]/[catalyst] ratio and
increased reaction temperature. To overcome the obstacle,
bifunctional catalyst was first proposed by Nozaki and
coworkers,20 and late developed by Lu via tethering the
cocatalyst to the ligand framework,21 moreover, Lee and
INTRODUCTION The alternating copolymerization between
epoxide and CO2, as first reported by Inoue and co-workers
in 1969,1,2 has been considered to be one of the most prom-
ising processes for CO2 fixation into polymers. Although a
variety of monomers can be employed in CO2/epoxides
copolymerization, alternating copolymer of propylene oxide
(PO) and CO2 (as shown in Scheme 1), poly(propylene car-
bonate) (PPC), is particularly attractive from the viewpoint
of industry, since PPC is melt processable with many other
biodegradable polymers displaying suitable mechanical per-
formance and biodegradability,3,4 which may create new
applications in use and throw away packages, biomedical
materials, and even in automotive parts.
Since the pioneer work of Inoue and coworkers, many heter-
ogeneous and homogeneous catalyst systems have been
developed for this transformation in the past decades.5–7
Among which, well-defined salen metal complex catalyst
receives the most intensive investigation due to its controlla-
ble tailoring of central metal, ligands, and axis group.8,9
Although initial investigations focused on Cr center,10–16 sub-
sequent research demonstrated that the cobalt complexes (1
coworkers
achieved
very
high
catalytic
activity
(TOF > 20,000 h21) by anchoring quaternary ammonium salt
on salenCoX framework (2 in Fig. 1),22–25 which helped salen
cobalt complex to be most effective catalyst for transforma-
tion of CO2 and PO into PPC. However, its potential
Additional Supporting Information may be found in the online version of this article.
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25 ꢀC in polystyrene standard on Waters 410 GPC instru-
ment with dichloromethane as the eluent, where the flow
rate was set at 1.0 mL min21
.
General Copolymerization Procedure
The required catalyst and PO (5 mL) were added into a
bomb reactor (15 mL) in a glove box. The bomb reactor
was assembled and immersed in an oil bath whose temper-
ature was set at a given value. The CO2 gas was pressurized
and stirred for 15 min to allow the solution to reach the
bath temperature. The reaction was terminated by releasing
the CO2 gas. A small aliquot of the copolymerization mix-
ture was taken out for 1H NMR spectroscopy, and the
resulting mixture was poured into a flask and dried under
vacuum.
SCHEME 1 Reaction of PO and CO2 to yield PPC and PC.
application in industry was limited due to the use of highly
toxic cobalt, which was strictly limited or even should not be
detected in the biodegradable polymers, since the final com-
post product contains toxic Co element. Therefore, efforts
have been made to find a soil tolerant “green” metal cen-
tered complex to catalyze the copolymerization of PO/CO2 in
high activity and PPC selectivity, Rieger and coworkers,26
Sakai and coworkers,27 Williams and coworkers,28–31 and
Nozaki and coworkers32 employed the environmental
friendly metal centers like Fe, Zn, Mg, and Ti in combination
with ligand framework. All these systems either show poly-
mer formation only in the cyclohexene oxide/CO2 system, or
yield cyclic carbonate or show a low TOF value with PO/CO2
as monomers. More recently, Nozaki and coworkers reported
the Fe-Corrole complex catalyzed PO/CO2 copolymerization
with a high TOF value,33 the carbonate linkage of the
obtained copolymer was still low (Ca. 20%).
Synthesis of Complexes 3a, 4a, and 5a
Compound 6
Compound 6 was obtained as reported.34 1H NMR (400
MHz, CDCl3, d): 9.98 (s, 1H), 7.80 (d, J 5 8.0 Hz, 2H), 7.37 (d,
J 5 8.0 Hz, 2H), 3.17 (t, J 5 7.2 Hz, 2H), 2.82 (t, 2H), 2.15 (t,
2H). 13C NMR (100 MHz, CDCl3, d): 191.5, 147.7, 134.4,
129.7, 128.9, 36.2, 34.9, 6.3. The synthetic routes of the
bifunctional aluminum porphyrin catalysts and the 1H-NMR,
13C-NMR of Compound 6-15 and the aluminum porphyrin
complexes are shown in the Supporting Information.
As far as this manuscript is preparing, noteworthy successes
were achieved for the copolymerization of PO/CO2 using sale-
nCoX system, while catalysts based on non-toxic central metal
exhibiting high activity and excellent PPC selectivity, perfectly
alternating copolymer with high molecular weight, have not
yet been reported. Herein, we report a bifunctional porphyrin
complex based on aluminum metal center, which exhibits an
outstanding activity for the copolymerization of PO/CO2, and
yields alternating PPC with high molecular weight and narrow
polydispersity (PDI) under mild condition.
Compound 7
Compound 7 was obtained as reported.35 1H NMR (400
MHz, CDCl3, d): 7.93 (brs, 2H, NH), 7.18–7.35 (m, 5H), 6.72
(m, 2H), 6.18 (m, 2H), 5.92 (s, 2H), 5.49 (s, 1H). 13C NMR
(100 MHz, CDCl3, d): 142.09, 132.50, 128.68, 128.42, 127.02,
117.24, 108.47, 107.23, 44.01.
Compound 8
Compound 8 was synthesized in a similar procedure of com-
pound 7 by using 2,3,4,5,6-pentafluorobenzaldehyde in 82%
1
yield. H NMR (400 MHz, CDCl3, d): 8.10 (brs, 2H, NH), 6.74
(m, 2H), 6.15 (q, 2H), 6.00 (m, 2H), 5.88 (s, 1H). 13C NMR
(100 MHz CDCl3, d): 140.13, 133.5, 127.58, 127.33, 118.15,
117.74, 108.78, 108.23, 107.63.
EXPERIMENTAL
General Remarks
All reactions were carried out under argon atmosphere using
standard Schlenk techniques unless otherwise stated. Chemi-
cals were obtained from Aldrich and Acros, and used as
received without further purification unless otherwise stated.
Tetrahydrofuran, CH2Cl2, CH3CN, pyrrole, PO were purified
by distillation from CaH2 and stored under Ar atmosphere
prior to use. The CO2 gas (99.999%) was purchased and
used without further purification.
Compound 9
Compound 9 was synthesized in a similar procedure of com-
pound 7 by using 4-tert-butylbenzaldehyde in 85% yield. H
1
NMR (400 MHz, CDCl3, d): 7.89–7.95 (br, 2H), 7.32–7.35 (m,
2H), 7.13–7.16 (m, 2H), 6.68–6.70 (m, 2H), 6.15–6.17 (m,
2H), 5.94–5.96 (m, 2H), 5.45 (s, 1H), 1.31 (s, 9H), 13C NMR
(100 MHz, CDCl3, d): 149.9, 139.1, 132.9, 128.2, 125.7, 117.2,
108.5, 107.2, 43.6, 34.6, 31.5.
Solution NMR spectra were collected at ambient tempera-
tures using a Bruker AV400 or AV600 spectrometer. Solvent
proton shifts (ppm): CDCl3, 7.26 (s); DMSO-d6, 2.5 (s). Sol-
vent carbon shifts (ppm): CDCl3, 77.2 (t); DMSO-d6, 39.5
(sept). Matrix-assisted laser desorption/ionization time-of-
flight mass spectroscopy (MALDI-TOF/MS) was performed
on a Bruker atuoflex III mass spectrometer. The molecular
weight and molecular weight distribution of the PPC were
determined by gel permeation chromatography (GPC) at
FIGURE 1 SalenCoX complex (1) and its bifunctional analogue (2).
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Compound 10
146.55, 142.13, 140.91, 140.41, 138.66, 134.06, 132.08,
128.21, 127.10, 120.18, 35.66, 34.67, 8.32. (MALDI-TOF, m/
z): [M 2 Cl]1 calcd for C50H38AlI2N4, 975.1; found, 975.1.
Anal. Calcd for C50H38AlClI2N4: C 5 59.39, H 5 3.79, N 5 5.54;
Found: C 5 59.48, H 5 3.72, and N 5 5.63.
Compound 10 was obtained as reported.34 1H NMR (400
MHz, CDCl3, d): 8.85 (s, 8H; pyrr-H), 8.21–8.23 (d, J 5 6.4 Hz,
4H; 5,15-Ar-2,6-H), 8.13–8.15 (d, J 5 7.6 Hz, 4H; 10,20-Ar-
2,6-H), 7.77 (m, J 5 7.2 Hz, 6H; 10, 20-Ar-3,4,5-H), 7.57–7.59
(d, J 5 8.0 Hz, 4H; 5,15-Ar-3,5-H), 3.42 (t, J 5 6.8 Hz, 4H;
CH2-I), 3.08 (t, J 5 7.4 Hz, 4H; Ar-CH2), 2.44 (m, 4H; CH2),
22.76 (s, 2H; NH). 13C NMR (100 MHz, CDCl3, d): 142.39,
140.28, 134.76, 131.21, 127.87, 126.87, 120.30, 36.45, 35.20,
6.54. Matrix-assisted laser desorption/ionization time-of-
flight mass spectroscopy (MALDI-TOF), m/z: [M 1 H]1 calcd
Compound 14
Compound 14 was synthesized in a similar procedure of
1
compound 13 by using Compound 11 in 95% yield. H NMR
(400 MHz, CDCl3, d): 9.38 (d, 4H, pyrr-H), 9.11 (d, 4H, pyrr-
H), 8.17(m, 4H, ArAH), 7.71–7.74 (m, 4H, ArAH), 3.51 (t,
4H, CH2AI), 3.07 (t, 4H, ArACH2), 2.40 (m, 4H, CH2). 13C
NMR (100 MHz, DMSO-d6, d): 147.67, 146.13, 144.51,
142.45, 140.73, 138.07, 137.64, 135.86, 134.45, 133.59,
131.64, 127.00, 121.41, 102.14, 35.70, 34.31, 8.33. (MALDI-
TOF, m/z): [M 2 Cl]1 calcd for C50H28AlF10I2N4, 1155.0;
found, 1155.0. Anal. Calcd for C50H28AlClF10I2N4: C 5 50.42,
H 5 2.37, and N 5 4.70; Found: C 5 50.58, H 5 2.52, and
N 5 4.55.
for
C50H41I2N4 951.13, found 951.10. Anal. Calcd for
C50H40I2N4: C 5 63.17, H 5 4.24, N 5 5.89; Found: C 5 63.22,
H 5 4.30, and N 5 5.78.
Compound 11
Compound 11 was synthesized in a similar procedure of
compound 10 by using Compound 8 and Compound 6 in
1
24% yield. H NMR (400 MHz, CDCl3, d): 8.95 (d, J 5 4.0 Hz,
4H; pyrr-H), 8.79 (d, J 5 4.0 Hz, 4H; pyrr-H), 8.13 (d, J 5 8.0
Hz, 4H; 5,15-Ar-2,6-H), 7.62 (d, J 5 8.0 Hz, 4H; 5,15-Ar-3,5-
H), 3.44 (t, J 5 8.0 Hz, 4H; CH2-I), 3.10 (t, J 5 8.0 Hz, 4H, Ar-
CH2), 2.44 (m, 4H, CH2), 22.82 (s, 2H, NH). 13C NMR (100
MHz, CDCl3, d): 148.00, 145.51, 143.51, 142.28, 140.61,
139.30, 136.42, 134.97, 132.87, 128.67, 127.29, 124.89,
121.58, 121.58, 102.28, 36.46, 35.16, 6.41. (MALDI-TOF, m/
z): [M 1 H]1 C50H30F10I2N4 calcd for 1130.0; found, 1130.0.
Anal. Calcd for C50H30F10I2N4: C 5 53.12, H 5 2.67, N 5 4.96;
Found: C 5 53.18, H 5 2.60, N 5 4.90.
Compound 15
Compound 15 was synthesized in a similar procedure of
compound 13 by using Compound 12 in 97% yield. H NMR
1
(400 MHz, CDCl3, d): 9.00 (S, 8H, pyrr-H), 8.13 (d,
5,10,15,20-Ar-2,6-H), 7.88 (d, 4H, 10.20-Ar-3,5-H), 7.69 (t,
4H, 5,15-Ar-3,5-H), 3.50 (t, 4H, CH2-I), 3.02 (t, 4H, Ar-CH2),
2.39 (m, 4H, CH2), 1.59 (s, 18H, CH3). 13C NMR (100 MHz,
DMSO-d6, d): 150.42, 146.50, 142.10, 138.24, 138.00, 134.00,
132.06, 127.05, 123.95, 119.55, 34.67, 34.26, 31.39, 30.68.
(MALDI-TOF, m/z): [M 2 Cl]1 calcd for C58H54AlI2N4, 1087.2;
found, 1087.2. Anal. Calcd for C58H54AlClI2N4: C 5 62.01,
H 5 4.85, and N 5 4.99; Found: C 5 62.15, H 5 4.96, and
N 5 5.15.
Compound 12
Compound 12 was synthesized in a similar procedure of
compound 10 by using Compound 9 and Compound 6 in
18% yield. 1H NMR (400 MHz, CDCl3, d): 8.86 (d, J 5 12.0
Hz, 8H, pyrr-H), 8.16 (d, J 5 8.0 Hz, 8H, 5,10,15,20-Ar-2,6-H),
7.78 (d, J 5 8.0 Hz, 4H, 10,20-Ar-3,5-H), 7.58 (d, J 5 0.0 Hz,
4H, 5,15-Ar-3,5-H), 3.42 (t, J 5 8.0 Hz, 4H, CH2-I), 3.08 (t,
J 5 8.0 Hz, 4H, Ar-CH2), 2.44 (m, 4H, CH2), 1.62 (s, 18H,
CH3), 22.73 (s, 2H, NH). 13C NMR (100 MHz, CDCl3, d):
150.98, 140.38, 139.72, 139.40, 135.01, 131.59, 127.33,
124.10, 120.30, 36.72, 35.48, 32.20, 6.90. (MALDI-TOF, m/z):
[M 1 H]1 calcd for C58H56I2N4, 1062.3; found, 1062.3. Anal.
Calcd for C58H56I2N4: C 5 65.54, H 5 5.31, N 5 5.27; Found:
C 5 65.62, H 5 5.42, and N 5 5.33.
Complex 3a
A solution of Compound 13 (1.01 g, 1.0 mmol) and tributyl-
amine (7.40 g, 40 mmol) in anhydrous CHCl3 (5.0 mL) and
CH3CN (5.0 mL) was refluxed for 96 h under Argon atmos-
phere. After cooled to room temperature, the solvent was
removed by a rotary evaporator and the layer of Bu3N was
removed with a pipette. The residue was washed three times
by ether and the yield was 95%.
1H NMR (400 MHz, CDCl3, d): 9.00 (m, 8H, pyrr-H), 8.48 (m,
8H, ArAH), 8.21 (m, 4H, ArAH), 7.87 (m, 6H, ArAH), 3.20–
3.46 (m, 16H), 3.02 (m, 4H), 2.26 (m, 4H), 1.21–1.48 (m,
24H), 0.85–1.06 (m, 18H). 13C NMR (100 MHz, DMSO-d6, d):
146.90, 140.86, 140.28, 138.84, 133.98, 132.01, 128.55,
127.42, 120.44, 53.54, 34.25, 31.75, 29.32, 19.98, 19.22,
13.90. (MALDI-TOF, m/z): [M 2 I]1 calcd for C74H92AlClIN6,
Compound 13
A solution of Compound 10 (0.951 g, 1.0 mmol) in 20 mL
dry dichloromethane was degassed with a stream of argon
for 5 min in an ice-bath. After 1.3 mmol Et2AlCl was added
slowly, the reaction solution was heated to room tempera-
ture 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, dichlorome-
thane/methanol vol/vol 5 10:1) and obtained as a purple
solid in 98% yield.
1253.6; found, 1253.6. Anal. Calcd for
C 5 64.32, H 5 6.71, and N 5 6.08; Found: C 5 64.68,
H 5 6.99, and N 5 6.41.
C74H92AlClI2N6:
Complex 4a
Complex 4a was synthesized in a similar procedure of com-
plex 3a by using Compound 14 in 96% yield. 1H NMR (400
MHz, CDCl3, d): 9.42 (s, 4H, pyrr-H), 9.11 (m, 4H, pyrr-H),
8.20 (m, 4H, Ar-H), 7.81 (m, 4H, Ar-H), 3.23-3.47 (m, 16H),
1H NMR (400 MHz, CDCl3, d): 8.85–8.89 (m, 8H, pyrr-H),
7.53–8.2 (m, 18H, ArAH), 3.45 (t, 4H, CH2AI), 3.10 (t, 4H,
ArACH2), 2.46 (m, 4H, CH2). 13C NMR (100 MHz, CDCl3, d):
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3.02 (m, 4H), 2.29 (m, 4H), 1.21–1.69 (m, 24H), 0.80–1.01
(m, 18H). 13C NMR (100 MHz, DMSO-d6, d): 147.63, 146.15,
144.45, 142.43, 140.65, 139.15, 138.27, 135.87, 134.40,
133.50, 131.75, 127.45, 121.32, 102.19, 53.14, 34.32, 31.42,
28.91, 23.26, 19.32, 13.62. (MALDI-TOF, m/z): [M 2 I]1 calcd
for C74H82AlClF10IN6, 1433.5; found, 1433.5. Anal. Calcd for
1265.7. Anal. Calcd for C74H92AlB3F12N6: C 5 65.69, H 5 6.85,
and N 5 6.21; Found: C 5 65.14, H 5 6.68, and N 5 6.43.
Complex 3d
Complex 3d was synthesized in the general procedure by
using complex 3a and AgClO4 in 98% yield. 1H NMR (400
MHz, CDCl3, d): 9.00 (m, 8H, pyrr-H), 8.48 (m, 8H, ArAH),
8.20 (m, 4H, ArAH), 7.87 (m, 6H, ArAH), 3.22–3.47 (m,
16H), 3.02 (m, 4H), 2.27 (m, 4H), 1.20–1.68 (m, 24H), 0.83–
1.05 (m, 18H). 13C NMR (100 MHz, DMSO-d6, d): 146.53,
140.89, 140.30, 138.89, 134.00, 132.06, 128.22, 127.08,
120.14, 51.77, 34.28, 31.39, 25.03, 23.06, 19.30, 13.53.
(MALDI-TOF, m/z) [M 2 ClO4]1 calcd for C74H92AlCl2N6O8,
C74H82AlClF10I2N6: C 5 56.91, H 5 5.29, and N 5 5.38; Found:
C 5 57.28, H 5 5.56, and N 5 5.69.
Complex 5a
Complex 5a was synthesized in a similar procedure of com-
plex 3a by using Compound 15 in 97% yield. 1H NMR (400
MHz, CDCl3, d): 9.02 (m, 8H, pyrr-H), 8.16 (m, 8H, Ar-H),
7.60–7.91 (m, 8H, Ar-H), 3.25–3.48 (m, 16H), 3.05 (m, 4H),
2.32 (m, 4H), 1.20–1.71 (m, 42H), 0.79–1.02 (m, 18H). 13C
NMR (100 MHz, DMSO-d6, d): 150.40, 146.48, 140.24,
138.85, 137.96, 133.95, 132.00, 127.25, 123.89, 119.98,
57.69, 53.17, 34.63, 31.36, 28.98, 23.13, 19.22, 13.52.
(MALDI-TOF, m/z): [M 2 I]1 calcd for C82H107AlClIN6,
1364.7; found, 1364.7. Anal. Calcd for C82H107AlClI2N6:
C 5 65.97, H 5 7.22, and N 5 5.63; Found: C 5 65.65,
H 5 7.61, and N 5 5.78.
1289.6; found, 1289.7. Anal. Calcd for C74H92AlCl3N6O12
:
C 5 63.90, H 5 6.67, and N 5 6.04; Found: C 5 63.59,
H 5 6.33, and N 5 6.42.
Complex 4b
Complex 4b was synthesized in the general procedure by
using complex 4a and AgNO3 in 99% yield. 1H NMR (400
MHz, CDCl3, d): 9.44 (s, 4H, pyrr-H), 9.15 (m, 4H, pyrr-H),
8.25 (m, 4H, ArAH), 7.84 (m, 4H, ArAH), 3.20–3.49 (m,
16H), 3.06 (m, 4H), 2.31 (m, 4H), 1.19–1.71 (m, 24H), 0.89
(m, 18H). 13C NMR (100 MHz, DMSO-d6, d): 147.70, 146.23,
144.57, 142.49, 140.70, 139.23, 138.36, 135.90, 134.47,
133.60, 131.68, 127.46, 121.46, 102.24, 51.86, 34.37, 31.48,
25.08, 23.17, 19.35, 13.64. (MALDI-TOF, m/z): [M 2 NO3]1
calcd for C74H82AlF10N8O6, 1395.6; found, 1395.6. Anal.
General Procedure for Preparation of Complexes 3b–e,
4b, and 5b
To a stirred solution of required silver salt (4.5 mmol) in
CH2Cl2 (20.0 mL) and CH3CN (20.0 mL) was added 3a/4a/
5a (1.0 mmol) quickly. The reaction mixture was stirred for
12 h in dark at room temperature. After the solvent was
removed by a rotary evaporator, the residue was dissolved
by CH2Cl2 (10.0 mL), then filtered, the filtrate was concen-
trated under vacuum to give a purple product and the yield
was 98%.
Calcd for
C74H82AlF10N9O9: C 5 60.94, H 5 5.67, and
N 5 8.64; Found: C 5 60.43, H 5 5.82, and N 5 8.29.
Complex 5b
Complex 5b was synthesized in the general procedure by
using complex 5a and AgNO3 in 99% yield. 1H NMR (400
MHz, CDCl3, d): 9.03 (m, 8H, pyrr-H), 8.16 (m, 8H, ArAH),
7.68–7.90 (m, 10H, ArAH), 3.20–3.46 (m, 16H), 3.04 (m,
4H), 2.29 (m, 4H), 1.20–1.69 (m, 42H),0.76–1.00 (m, 18H).
13C NMR (100 MHz, DMSO-d6, d): 150.91, 146.55, 140.37,
138.51, 138.06, 134.07, 132.59, 127.34, 124.47, 120.51,
58.09, 52.25, 35.18, 31.90, 27.58, 23.57, 19.70, 14.08.
(MALDI-TOF, m/z): [M 2 NO3]1 C82H107AlN8O6, 1326.8;
found, 1326.8. Anal. Calcd for C82H107AlN9O9: C 5 70.87,
H 5 7.76, and N 5 9.07; Found: C 5 70.68, H 5 7.87, and
N 5 9.44.
Complex 3b
Complex 3b was synthesized in the general procedure by
using complex 3a and AgNO3 in 99% yield. 1H NMR (400
MHz, CDCl3, d): 9.00 (m, 8H, pyrr-H), 8.17–8.21 (m, 8H,
ArAH), 7.75–7.84 (m, 10H, ArAH), 3.40–3.47 (m, 16H), 3.04
(m, 4H), 2.27 (m,4H), 1.21–1.68 (m, 24H), 0.88–1.02 (m,
18H). 13C NMR (100 MHz, DMSO-d6, d): 146.60, 140.96,
134.09, 138.93, 134.09, 132.12, 128.26, 127.14, 120.15,
64.95, 51.83, 34.36, 31.45, 25.05, 23.12, 19.39, 15.18, 13.61.
(MALDI-TOF, m/z): [M 2 NO3]1 calcd for C74H92AlN8O6,
1215.7; found, 1215.7. Anal. Calcd for
C 5 69.52, H 5 7.25, and N 5 9.86; Found: C 5 69.86,
H 5 7.52, and N 5 9.51.
C74H92AlN9O9:
RESULTS AND DISCUSSION
Molecular Design of Bifunctional Aluminum Porphyrin
Complexes
Complex 3c
To replace the highly toxic cobalt center in the complex,
environmental friendly Aluminum centered complexes were
chosen, since the possibility for aluminum centered complex
for the transformation of PO/CO2 into PPC have been
explored before.36–41 As far as ligands are concerned, metal-
loporphyrins are currently subjected to a resurgence of
interest owing to the high reactivity of the porphyrin axial
M-X bond and their well-defined coordination modes with
metal ions.42 Therefore, complexes comprising aluminum
metal center and porphyrin ligand framwork were designed.
Complex 3c was synthesized in the general procedure by
using complex 3a and AgBF4 in 99% yield. 1H NMR (400
MHz, CDCl3, d): 8.84 (m, 8H, pyrr-H), 8.12–8.23 (m, 8H,
ArAH), 7.75–7.86 (m, 10H, ArAH), 3.28–3.41 (m, 16H), 3.00
(m, 4H), 2.24 (m, 4H), 1.26–1.65 (m, 24H), 0.88–1.03 (m,
18H). 13C NMR (100 MHz, DMSO-d6, d): 146.61, 141.91,
140.99, 139.93, 134.13, 131.27, 127.89, 126.95, 119.80,
51.55, 34.33, 31.44, 24.88, 23.12, 18.93, 13.54. (MALDI-TOF,
m/z): [M-BF4]1 calcd for C74H92AlB2F8N6, 1265.7; found,
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SCHEME 2 Synthetic routes to the bifunctional aluminum porphyrin catalysts.
Moreover, considering that tethering the cocatalyst to the
ligand framework may result in enhanced activity and selec-
tivity in high monomer/catalyst ratio,34 bifunctional porphy-
rin complexes were prepared by anchoring the quaternary
ammonium salt on the porphyrin ligand via a one-step reac-
tion with tributylamine.
in Fig. 2) fragments and/or electron-donating fragments
(complexes 5a,b in Fig. 2) in the meso-ring position were
designed, and their corresponding aluminum complexes
were also used for PO/CO2 copolymerization.
Copolymerization Behavior of Complex 3b: Effect of
Copolymerization Temperature and CO2 Pressure
As far as the axial ligand is concerned, Coates and coworkers
clearly pointed out the influence of axial ligand and cocata-
lyst on catalytic activity in salenCoX/cocatalyst system, and
observed a significant copolymerization rate dependence on
the composition of the axial ligand and cocatalyst.19 Our ear-
lier work on tppCoX/cocatalyst system also indicated that
the feature of axial group X as well as cocatalyst had strong
influence on the catalytic activity.43 Therefore, as shown in
scheme 2 for the synthetic route, we prepared a series of
complexes with various quaternary ammonium anions and
axial ligand (see complexes 3a–d in Fig. 2), to investigate the
effect of the quaternary ammonium anions and axial ligand
on catalytic activity, PPC selectivity and PPC structure.
Using complex 3b as catalyst, the influence of reaction condi-
tion such as CO2 pressure, reaction temperature and [PO]/
[cat.] ratio etc, on copolymerization behavior was summar-
ized in Table 1. Entries 2–4 showed the effect of CO2 pres-
sure on catalyst activity at [PO]/[cat.] ratio of 5000, where
ꢀ
the reaction temperature was kept at 70 C. When the reac-
tion pressure increased from 2.0 to 4.0 MPa, only slight
change of turnover frequency (TOF; <5%) was observed,
TOF increased when CO2 pressure increased from 2.0 to 3.0
MPa, it reached 305 h21 after 5 h, but further increase in
pressure to 4.0 MPa led to negative impact on the activity
(TOF of 292 h21). This phenomenon can be attributed to the
dilution effect of the reaction mixture by CO2 at higher pres-
sure. Meanwhile, number average molecular weight and PDI,
polymer selectivity and carbonate linkage did not show any
change with the pressure change, indicating that copolymer-
ization window on CO2 pressure was quite broad.
In another consideration, the Lewis acidity of the central
metal ion has significant influence on catalytic activity and
PPC selectivity,41,44–47 which can be adjusted by introduction
of specific ligand to change the electronic environment of
the central metal ion. On the basis of this concept, a series
of porphyrins bearing electron withdrawing (complexes 4a,b
The effect of reaction temperature on copolymerization
behavior was studied at 3.0 MPa under [PO]/[cat.] of 5000,
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FIGURE 2 Bifunctional aluminum porphyrin complexes for the copolymerization of PO/CO2.
as shown in Entries 1, 3, 5 and 6, when the copolymerization
was carried out at 25 ꢀC, TOF was only 20 h21, and 5%
cyclic by-product propylene carbonate (PC) was produced.
Though the resultant polycarbonates had carbonate linkage
over 99%, it showed number average molecular weight (Mn)
of 26 kg mol21 with narrow PDI (5 1.12). TOF increased to
305 h21 at 70 ꢀC, whereas the polymer selectivity reduced
to 88%, it was encouraging that the molecular weight of the
obtained copolymer increased to 48 kg mol21, while the PDI
was still very narrow. TOF increased to 985 h21 at 90 ꢀC,
whereas the polymer selectivity reduced to 80%, while the
carbonate linkage and PDI were similar to those at 70 ꢀC.
Increase in reaction temperature gave higher TOF, but
resulted in more PC production, which was attributed to the
acceleration of back-bitting reaction, leading to degradation
of PPC at higher temperature. Therefore, the subsequent
ꢀ
copolymerization was conducted at 70 C due to the accepta-
ble activity and product selectivity. To evaluate the effect of
tethering the cocatalyst to the ligand framework on the com-
plex activity, tetraphenylporphyrin aluminum nitrate complex
was prepared. As shown in Entry 9, Table 1, TPPAlNO3 with
2 equivalent of n-Bu4NNO3 (tetrabutyl-ammonium nitrate)
TABLE 1 Copolymerizationa of PO/CO2 Catalyzed by Complex 3b
Entry PO/cat.b T (ꢀC) P (MPa) Time (h) TOF (h21 c
)
Selectivity (%PPC)d Carbonate Linkage (%)e Mn (kg mol21 f
)
PDIf
1
2
3
4
5
6
7
8
9g
5000
25
70
70
70
90
90
110
3
2
3
4
3
3
3
3
3
5
20
95
88
88
89
80
82
71
70
19
99
99
99
99
99
99
98
99
96
26
46
48
48
25
42
36
42
10
1.12
1.09
1.10
1.10
1.13
1.15
1.25
1.19
1.18
5000
5
286
305
292
985
972
3407
3128
152
5000
5
5000
5
5000
0.5
2
10,000
20,000
2
100,000 110
5000 70
5
5
a
e
f
Copolymerization was carried out in neat PO (5 mL), unless otherwise
was specifically noted.
Molar ratio.
Determined by 1H NMR spectroscopy.
Determined by GPC in CH2Cl2 at 25 ꢀC, calibrated with polystyrene
b
standards.
c
g
TOF of PO to products.
TPPAlNO3 with
catalyst.
2 equivalent of tetrabutyl-ammonium nitrate as
d
Selectivity for PPC over PC.
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TABLE 2 Copolymerizationa of PO/CO2 Catalyzed by Complexes 3a–d
Selectivity
(%PPC)d
Carbonate
Linkage (%)e
Entry
Catalyst
PO/cat.b
T (ꢀC)
P (MPa)
Time (h)
TOF (h21 c
)
Mn (kg mol21 f
)
PDIf
1
2
3
4
3a
3b
3c
3d
5000
5000
5000
5000
70
70
70
70
3
3
3
3
2
2
2
2
422
306
298
330
82
87
88
88
99
99
99
75
26
25
26
15
1.11
1.09
1.10
1.21
a
d
e
f
Copolymerization was carried out in neat PO (5 mL), unless otherwise
was specificly noted.
Selectivity for PPC over PC.
Determined by 1H NMR spectroscopy.
b
Molar ratio.
Determined by GPC in CH2Cl2 at 25 ꢀC, calibrated with polystyrene
c
TOF of PO to products.
standards.
can efficiently catalyze the coupling of PO and CO2, although
a TOF of 152 h21 was obtained, the PPC selectivity and the
number average molecular weight was only 19% and 10 kg
mol21 respectively. This result directly demonstrated that
tethering the cocatalyst to the ligand can significantly
enhance not only the catalytic activity, but also the copoly-
mer selectivity.
our bifunctional aluminum porphyrin system. Aluminum por-
phyrin reacted with tributylamine initially gave the complex
3a, and further reacted with 3 equivalent of AgNO3, AgBF4,
AgClO4 gave complex 3b, 3c, 3d, respectively. The catalytic
performance of complexes 3a–d for PO/CO2 copolymeriza-
tion was listed in Table 2, copolymerization was conducted
at 70 ꢀC under 3.0 MPa for 2 h, with a [PO]/[cat.] ratio of
5,000. Catalyst 3a with I2 as quaternary ammonium anion
showed TOF value of 422 h21, while previous study for
binary porphyrin system using n-Bu4NI as cocatalyst in com-
bination with tppCoCl afforded no monomer conversion at
all.43 In comparsion to catalyst 3a, catalyst 3b showed a
decrease in copolymerization rate as reflected in its smaller
TOF value. However, in respect to product selectivity, catalyst
3b exhibited an increase versus catalyst 3a, which was con-
sistent with the conclusion summarized by Lu and
coworkers,18 where anion in quaternary ammonium salt hav-
ing poorer leaving ability gave higher product selectivity. In
comparison to catalyst 3b, catalyst 3c showed a slight
decrease in activity while an increase in selectivity.
A notable advantage of bifunctional salenCoX catalyst lies in
that it can catalyze the copolymerization reaction at very
high [monomer]/[cat.] ratio, owing to the anchoring cocata-
lyst species on the ligand framework.21,22 This was true
again in our bifunctional aluminum porphyrin catalyst sys-
tem. As listed in Table 1, when the copolymerization was
conducted at a [PO]/[cat.] ratio of 20,000 at 110 ꢀC under
3.0 MPa, TOF as high as 3407 h21 was obtained after 2 h,
though the selectivity of PPC dropped to 71%, while the
obtained PPC maintained high carbonate linkage of 98%.
Further raising the [PO]/[cat.] ratio to 100,000, the activity
of the catalyst was still high with a slight TOF decrease,
while the obtained copolymer was still perfectly alternative.
Copolymers generated by catalysts 3a–c had carbonate link-
age of over 99% with number average molecular weight of
about 26 kg mol21 L21 and relatively narrow PDIs
(PDI 5 1.10). In the case of catalyst 3d, although larger TOF
value was obtained, the generated PPC had only 75% car-
bonate linkage, suggesting that PO and CO2 insertion into
the propagating alkoxide may be competitive in this case.
Copolymerization Behavior of Complexes 3a–d: Effect of
Axial Ligand and Quaternary Ammonium Anion
It has been shown that minor alteration of axial ligand and
cocatalyst structure can result in large difference in catalytic
performance of porphyrin or salen-type metal complexes for
PO/CO2 copolymerization.19,48 Similar study was done for
TABLE 3 Copolymerizationa of PO/CO2 Catalyzed by Complexes 3a,b, 4a,b, and 5a,b
Selectivity
(%PPC)d
Carbonate
Linkage (%)e
Entry
Catalyst
PO/cat.b
T (ꢀC)
P (MPa)
Time (h)
TOF (h21 c
)
Mn (kg mol21 f
)
PDIf
1
2
3
4
5
6
3a
3b
4a
4b
5a
5b
5000
5000
5000
5000
5000
5000
70
70
70
70
70
70
3
3
3
3
3
3
2
2
2
2
2
2
422
306
466
392
485
449
82
99
99
99
99
99
99
26
25
29
29
35
36
1.11
1.09
1.12
1.11
1.09
1.08
87
76
88
87
92
a
d
e
f
Copolymerization was carried out in neat PO (5 mL), unless otherwise
was specifically noted.
Molar ratio.
Selectivity for PPC over PC.
Determined by 1H NMR spectroscopy.
b
Determined by GPC in CH2Cl2 at 25 ꢀC, calibrated with polystyrene
c
TOF of PO to products.
standards.
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lysts with BF 24 and ClO 42 as quaternary ammonium anion
were not prepared.
Copolymerization results from complexes 3a,b, 4a,b, and
5a,b were listed in Table 3 (Entries 1–6), complexes 4a,b in
which the porphyrin ligand bearing fluorine-functionalized
aryl substituents displayed increased copolymerization rate
in comparison with the unsubstituted catalyst 3a,b, this was
consistent with the result of Chatterjee and Chisholm,41
where TFPPAlCl [TFPP 5 5,10,15,20-tetrakis (pentafluoro-
phenyl) porphyrin] complex showed enhanced activity ver-
sus TPPAlCl (TPP 5 5,10,15,20-tetraphenylporphyrin) toward
copolymerization of PO/CO2. Therefore, introduction of elec-
tron withdrawing fragment in the catalyst system was bene-
ficial to improve the catalytic activity. For complexes 5a,b
bearing tertiary butyl-functionalized aryl substituents on the
framework, significant improvement in activity with high
PPC selectivity was observed, especially for catalyst 5b
where aluminum ion was incorporated with porphyrin ligand
having reduced Lewis acidity, TOF reached 449 h21, and PPC
selectivity was 92%, while the number average molecular
weight was 36 kg mol21, indicating that the catalytic per-
formance can be significantly influenced by changing elec-
tronic environment of porphyrin ligand, and porphyrin
ligand with electron donating substituent was better for
higher activity and PPC selectivity than that with electron
withdrawing substituent.
FIGURE 3 GPC profile for PPC prepared from complex 3b.
These results directly led to the conclusion that the copoly-
merization behavior of the bifuncational catalyst was signifi-
cantly influenced by the quaternary ammonium anion and
axial ligand.
Copolymerization Behavior of Complexes 3a,b, 4a,b, and
5a,b: Effect of Electronic Environment at the Metal
Center
As an active center, cobalt(III) has shown superior perform-
ance in the coupling of PO and CO2, but the toxic nature
causes a common concern on its possible residue in biode-
gradable polymer, especially in application as compostable
product. Currently, although much effort has been made,
replacement of cobalt ion with other environmental friendly
metal is still a harsh task, especially for the copolymerization
of PO/CO2. As far as aluminum is concerned, since the Lewis
acidity of Al31 is greater than Co31, introduction of electron
donating groups may reduce the Lewis acidity of the alumi-
num ion, resulting in increased copolymerization rate. This
was confirmed by the catalytic performance of complexes
5a,b, for complex 5b, its TOF reached 449 h21, and PPC
selectivity was 92%, while the number average molecular
weight of PPC was 36 kg mol21. Although a big margin in
Introducing specific substituents on the ligand framework
may vary the coordination environment at the active metal
center, and this approach was extensively used in metal por-
phyrin,41,44,46 metal salen,45,48 and metal b-diiminate sys-
tems47 to adjust the catalytic performance. With regard to
metal porphyrin system, owing to the significant distance
from the periphery of the framework to the active center,
the specific ligand at meso-ring of the rigid porphyrin ligand
can have electronic influence on the central metal without
introducing steric interference. Therefore, complexes 4a,b
bearing fluorine-functionalized aryl substituents and com-
plexes 5a,b bearing tertiary butyl-functionalized aryl sub-
stituents on the meso-ring of the ligand were prepared. Due
to the negative effect on the activity of catalyst 3c,d, cata-
TABLE 4 Copolymerizationa of PO/CO2 Catalyzed by Complex 3b
Selectivity
(%PPC)d
Carbonate
Linkage (%)e Conversion (%) Mn (kg mol21 f
Entry Catalyst PO/cat.b T (ꢀC) Time (h) TOF (h21 c
)
)
PDIf
1
2
3
4
5
3b
3b
3b
3b
3b
5000
5000
5000
5000
5000
70
70
70
70
70
2
306
308
272
253
236
87
99
99
99
99
99
10.5
21.3
28.1
34.5
38.5
25
43
53
64
57
1.11
1.09
1.10
1.11
1.28
4
88
87
86
83
6
8
10
a
d
e
f
Copolymerization was carried out in neat PO (5 mL), unless otherwise
Selectivity for PPC over PC.
was specifically noted.
b
Determined by 1H NMR spectroscopy.
Molar ratio.
Determined by GPC in CH2Cl2 at 25 ꢀC, calibrated with polystyrene
c
TOF of PO to products.
standards.
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were highly active for the living-type copolymerization of
PO/CO2, yielding perfectly alternated PPC with high molecu-
lar weight. The copolymerization rate can be adjusted by
variation of axial ligand and quaternary ammonium anion.
Additionally, the influence of the electronic environment at
the active center on the copolymerization behavior was stud-
ied by introducing specific substituent on the ligand frame-
work, catalysts bearing electron donating groups may reduce
the Lewis acidity of the aluminum ion, resulting in increased
copolymerization rate. To the best of our knowledge, this
catalyst system exhibits the highest TOF value among cata-
lysts, which incorporated with nontoxic metal center for the
copolymerization of PO/CO2.
FIGURE 4 Dependence of Mn of PPC on PO conversion.
ACKNOWLEDGMENTS
catalytic activity and molecular weight existed in comparison
with cobalt centered system, bifunctional aluminum complex
may be very promising system in replace of cobalt centered
analogue for the copolymerization of PO/CO2. Further effort
is still necessary, by means of varying the coordination envi-
ronment at the aluminum center, to improve its catalytic
activity and product selectivity; moreover, raising molecular
weight of PPC is another target.
The financial support from the National Natural Science Foun-
dation of China (Grant No: 51321062, 21134002, and
51273197) is greatly appreciated.
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