Cobaltoporphyrin-catalyzed CO2/epoxide copolymerization: Selectivity control by molecular design

Article abstract of DOI:10.1021/ma301205g

A series of cobalt(III) chloride porphyrin complexes of the general formula 5,10,15,20-tetra(p-alkoxy)phenylporphyrin cobalt chloride (4b-e) and the related 5,10,15,20-tetra(p-nitro)phenylporphyrin cobalt chloride (4f) are presented and their reactivity toward propylene oxide (PO)/CO₂ coupling/copolymerization is explored. While the nitro-substituted complex (4f), in conjunction with an onium salt, shows moderate activity toward cyclization, the 4b-e/onium systems show superior copolymerization activity in comparison to tetraphenylporphyrin Co(III) chloride (4a) with high selectivity and conversion to poly(propylene carbonate) (PPC). A comprehensive copolymerization behavior study of the alkoxy-substituted porphyrin complexes 4b-e in terms of reaction temperature and CO₂ pressure is presented. Complexes bearing longer alkoxy-substituents demonstrate the highest polymerization activity and molecular weights, however all substituted catalyst systems display a reduced tolerance to increased temperature with respect to PPC formation. Studies of the resulting polymer microstructures show excellent head-to-tail epoxide incorporation and near perfectly alternating poly(carbonate) character at lower polymerization temperatures.

Full text of DOI:10.1021/ma301205g

Article
pubs.acs.org/Macromolecules
Cobaltoporphyrin-Catalyzed CO2/Epoxide Copolymerization:
Selectivity Control by Molecular Design
Carly E. Anderson, Sergei I. Vagin, Wei Xia, Hanpeng Jin, and Bernhard Rieger*
WACKER-Chair for Macromolecular Chemistry, Technical University of Munich, Lichtenbergstrasse 4, D-85748 Garching bei
Munich, Germany
S
* Supporting Information
ABSTRACT: A series of cobalt(III) chloride porphyrin
complexes of the general formula 5,10,15,20-tetra(p-alkoxy)-
phenylporphyrin cobalt chloride (4b−e) and the related
5,10,15,20-tetra(p-nitro)phenylporphyrin cobalt chloride (4f)
are presented and their reactivity toward propylene oxide
(PO)/CO2 coupling/copolymerization is explored. While the
nitro-substituted complex (4f), in conjunction with an onium
salt, shows moderate activity toward cyclization, the 4b−e/
onium systems show superior copolymerization activity in
comparison to tetraphenylporphyrin Co(III) chloride (4a)
with high selectivity and conversion to poly(propylene
carbonate) (PPC). A comprehensive copolymerization behavior study of the alkoxy-substituted porphyrin complexes 4b−e in
terms of reaction temperature and CO2 pressure is presented. Complexes bearing longer alkoxy-substituents demonstrate the
highest polymerization activity and molecular weights, however all substituted catalyst systems display a reduced tolerance to
increased temperature with respect to PPC formation. Studies of the resulting polymer microstructures show excellent head-to-
tail epoxide incorporation and near perfectly alternating poly(carbonate) character at lower polymerization temperatures.
ever increasing activities. To date, there remains a reliance on
heterogeneous Zn-based dicarboxylate systems for the
production of poly(carbonates) due to their ease of handling,
low toxicity and economic viability.12,13 However the active
sites of these compounds are often poorly defined and their
insoluble nature limits study of reaction mechanisms.14,15 It is
for these reasons that the development of coordination
complexes that comprise well-defined active sites and high
selectivity/activities for poly(carbonate) formation is an
ongoing area of research.
In terms of homogeneous systems suitable for poly-
(carbonate) synthesis there exist a variety of complex systems
capable of CO2/epoxide copolymerizations.3,6 Among these,
metalloporphyrins are currently subject to a resurgence of
interest which can be attributed to their facile synthesis and
ease of handling. The unusually high reactivity of the porphyrin
axial M−X bond16 and their well-defined coordination modes
with metal ions17 make them attractive targets for catalyst
development. Many previous studies have focused on
aluminum-based porphyrin systems, however these typically
suffer from lower activities (in comparison to other
homogeneous systems) and an increased tendency for the
resulting poly(carbonates) to contain significant amounts of
INTRODUCTION
■
Carbon dioxide is increasingly viewed as an attractive, low-cost
C1 carbon source for chemical processes due to its nontoxic,
nonflammable, and abundant nature. The widespread applica-
tion of carbon dioxide as a synthetic building block has
traditionally been limited by its stable, relatively unreactive
nature; however, major advances have been achieved in recent
years with the activation/usage of CO2 in transition metal-
mediated reactions.1,2 Foremost among these processes is the
use of carbon dioxide as a comonomer for the production of
aliphatic and aromatic poly(carbonates) in which CO2 has the
potential to replace the highly toxic phosgene in the synthesis
of these industrially important polymers.3−6
Although a variety of monomers can be employed in CO2/
epoxide copolymerizations, copolymers comprising aliphatic
epoxides are particularly attractive from an industrial viewpoint
due to their excellent physical properties7,8 and biodegrad-
ability.9 Specifically poly(propylene carbonate) (PPC) is of
interest due to its outstanding optical and mechanical
properties such as high UV stability, transparency and Young’s
modulus, which make it ideal for applications in the
automotive, food, and healthcare sectors. Consequently
catalysts that are capable of copolymerizing propylene oxide
(PO) and CO2 to produce high molecular weight PPC are
highly sought. Since the initial reports by Inoue and co-workers
on active metalloporphyrin systems for the synthesis of
poly(carbonates) with well-controlled molecular weight10,11
there has been a drive toward the development of catalysts with
Received: June 14, 2012
Revised: August 3, 2012
Published: August 21, 2012
© 2012 American Chemical Society
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poly(ether) linkages18 which negatively impact on the resulting
polymer properties.19
tion of propylene oxide and carbon dioxide and study of the
obtained PPC.
The realization that cobalt-based complexes with various
ligand types are able to catalyze epoxide/CO2 coupling and
copolymerization processes with high activities has led to a
large number of reports in this area.20 Furthermore, catalyst
systems that selectively form poly(carbonates) over the coupled
cyclic carbonate are favored as formation of the thermodynami-
cally stable cyclic carbonate (CC) by a “backbiting”
mechanism21,22 is a major limitation and its suppression is
desirable. Cobaltoporphyrins are advantageous in this respect as
the catalyst selectivity toward copolymerization/coupling
depends strongly on the nature of the employed cocatalyst23
which ranges from selective cyclization24,25 to production of
high molecular weight poly(carbonate)23,26 in accordance with
variation of reaction parameters. The employed cocatalyst
performs two principle roles in the polymerization reaction,
namely labilizing the axial metalloporphyrin M−X bond (Chart
1), thereby “tuning” the electronic properties of the central
metal,18 and assisting PPC chain growth by promoting CO2
insertion into the M−OR bond of a growing polymer chain.6
EXPERIMENTAL SECTION
■
Methods and Materials. All reactions were routinely carried out
under argon atmosphere using standard Schlenk techniques unless
otherwise stated. Chemicals were obtained from Aldrich, ABCR, or
Acros and used as received without further purification unless
otherwise stated. Pyrrole (Acros Organics, 99% extra pure) and
propylene oxide were purified by distillation from CaH2 and stored
under Ar atmosphere prior to use. Bis(triphenylphosphoranylidene)
ammonium chloride (PPNCl) was purchased from Aldrich and stored
under Ar atmosphere.
Solution NMR spectra were collected at ambient probe temper-
atures using a Bruker ARX300 or AV500 spectrometer. 1H and
13C{1H} NMR spectra were referenced to residual protio impurities in
the solvent (1H) or the 13C shift of the solvent (13C). Solvent proton
shifts (ppm): CDCl3, 7.26 (s); DMSO-d6, 2.50 (s). Solvent carbon
shifts (ppm): CDCl3, 77.2 (t); DMSO-d6, 39.5 (sept). 13C NMR
spectra were assigned with the aid of DEPT 90, DEPT 135 and
1H−13C correlation experiments. Chemical shifts are reported in ppm
and coupling constants in Hz. Mass spectrometry was performed on a
Varian LC-MS 500 (50−2000 Da) using isopropanol/ethyl acetate as
solvent. GPC was performed using a PolymerLaboratoriesGPC50 Plus
chromatograph at 35 °C with THF as elutant at a flow rate of 1.0 mL
min−1. Polystyrene standards were used for calibration. UV−vis
spectra were recorded on a Varian 50 Scan UV−visible spectropho-
tometer in dichloromethane or tetrahydrofuran solution. Differential
scanning calorimetry (DSC) measurements were performed on a TA
DSC Q2000 in a temperature range from −50 to 150 °C in three
cycles with a heating rate of 10 °C min−1. Elemental analyses were
performed by the microanalytic laboratory of the Department of
Inorganic Chemistry at the Technical University of Munich.
Chart 1. General M(III) Metalloporphyrin Structure,
Showing Substitution (Rn) at the meso-Aryl Rings, Central
Metal M and Axial Ligand X
General Procedure for Preparation of Aldehydes.30 p-Hydrox-
ybenzaldehyde (1 equiv) and K2CO3 (3.2 equiv) were added as solids
to 100 mL of Ar-purged DMF (ca. 100 mL) at room temperature. The
i
required bromoalkane (nPrBr or PrBr, 3.5 equiv) was added via
syringe, a reflux condenser was fitted to the reaction vessel, and the
resulting mixture was heated to 55 °C. When the starting material
could no longer be observed by TLC analysis, water was added to
dissolve the solid precipitate. The product was extracted with EtOAc
(4 × 50 mL) and the organic fractions combined, washed with water
(4 × 50 mL) and base washed with NaOH (aqueous 10%, 50 mL) to
remove traces of unreacted p-hydroxybenzaldehyde. The pale yellow
organic phase was separated, dried over MgSO4 and all volatiles
removed in vacuo. The resulting off-white liquid was purified by
column chromatography (SiO2, 2:1 light petroleum ether:diethyl
ether) to afford a colorless liquid.
Another commonly used method of varying the coordination
environment at the metal center is the concept molecular
design by introduction of specific ligand fragments that
influence the steric and electronic environment of the central
metal ion. The planar, symmetrical nature of the porphyrin ring
lends itself well to functionalization and this approach is
increasingly used to modulate the complex reactivity and the
resulting copolymerization behavior.27,28 Indeed, it has been
demonstrated that the judicious placement of electronically
differing aryl-functionalities in the meso-position of the
porphyrin framework raises the activity of the corresponding
complexes toward ring-opening polymerization.29
To this end, a series of porphyrins bearing various alkoxy-
functionalized aryl substituents in the meso-ring position have
been developed and the reactivity of their respective cobalt
complexes explored toward PO/CO2 copolymerization reac-
tions. As observed by Wang and co-workers, the placement of
functionalities in the ortho-position of the meso-ring may
interfere with monomer approach to the metal site,25 and thus
cobaltoporphyrins comprising substituents in the para-position
were employed in these studies. Here we report a full study of
effects of reaction conditions, specifically temperature and
pressure, on the activity and selectivity of these complexes, in
the presence of an onium cocatalyst, toward the copolymeriza-
p-n-Propoxybenzaldehyde (1d). p-hydroxybenzaldehyde, 10.0 g,
8.19 × 10−2 mol; n-propyl bromide, 26.1 mL, 0.29 mol; K2CO3, 36.2 g,
0.26 mol. Yield: 11.4 g, 85%. 1H NMR (CDCl3, 300.1 MHz) δ 0.98 (t,
3
3JHH = 8.0 Hz, 3H, OCH2CH2CH3), 1.76 (sext, JHH = 8.0 Hz, 2H,
OCH2CH2CH3), 3.92 (t, 3JHH = 8.0 Hz, 2H, OCH2CH2CH3), 6.92 (d,
3
3JHH = 9.0 Hz, 2H, ArH), 7.74 (d, JHH = 9.0 Hz, 2H, ArH), 9.80 (s,
1H, ArCHO). 13C{1H} NMR (CDCl3, 75.5 MHz) δ 10.4 (s,
OCH2CH2CH3), 22.4 (s, OCH2CH2CH3), 69.8 (s, OCH2CH2CH3),
144.7 (s, ArC), 129.7 (s, ArC), 131.9 (s, ArC), 164.2 (s, ArC), 190.6 (s,
ArCHO). MS (ESI+): m/z = 165.1 [M + H]+; 123.1 [M − Pr + H]+.
p-Isopropoxybenzaldehyde (1e). p-Hydroxybenzaldehyde, 10.04 g,
8.22 × 10−2 mol; isopropyl bromide, 27.0 mL, 0.29 mol; K2CO3, 36.5
g, 0.26 mol. Yield: 12.3 g, 91%. 1H NMR (CDCl3, 300.1 MHz) δ 1.35
3
3
(d, JHH = 6.0 Hz, 6H, OCH(CH3)2), 4.64 (sept, JHH = 6.0 Hz, 1H,
3
3
OCH(CH3)2), 6.93 (d, JHH = 9.0 Hz, 2H, ArH), 7.79 (d, JHH = 9.0
Hz, 2H, ArH), 9.84 (s, 1H, ArCHO). 13C{1H} (CDCl3, 75.5 MHz) δ
21.9 (s, OCH(CH3)2), 70.3 (s, OCH(CH3)2), 115.6 (s, ArC), 129.6 (s,
ArC), 132.1 (ArC), 163.2 (ArC), 190.8 (s, ArCHO). MS (ESI+): m/z
= 123.0 [M − Pr + H]+.
General Procedure for Porphyrin Preparation. Porphyrin synthesis
was performed according to an appropriate variation of a literature
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procedure.31 Under air, a three-neck 500 mL round-bottom flask fitted
with two pressure-equalizing dropping funnels was charged with
propionic acid (300 mL). Equimolar equivalents of freshly distilled
pyrrole and the appropriately substituted benzaldehyde were added via
syringe into the two dropping funnels, respectively, and the main
reaction vessel heated to 140 °C. The contents of the dropping funnels
were added to the refluxing acid dropwise over the course of 0.25 h
and the resulting mixture allowed to reflux for a further 2 h. Upon
cooling of the mixture to room temperature, the mixture was filtered
affording a purple crystalline solid. This precipitate was washed
sequentially with water (ca. 20 mL) and a minimum of cold methanol
(ca. 10 mL) and allowed to dry under air. The solid was recrystallized
twice from CHCl3/MeOH to afford a bright purple crystalline solid.
5,10,15,20-Tetraphenyl-21H,23H-porphyrin (2a). Benzaldehyde,
14.0 mL, 0.14 mol; pyrrole, 9.5 mL, 0.14 mol. Yield: 3.39 g, 16%.
1H NMR (CDCl3, 300.1 MHz) δ −2.73 (s, 2H, NH), 7.74−7.80 (m,
12H, PhH), 8.12−8.28 (m, 8H, PhH), 8.88 (s, 8H, β-porphH).
13C{1H} NMR (CDCl3, 75.5 MHz) δ 120.3 (s, ArC), 126.8 (s, ArC),
127.9 (s, ArC), 131.0 (s, β-porphC), 134.7 (s, ArC), 142.3 (meso-
porphC) [α-porphyrin carbon not observed]. MS (ESI+): m/z = 615.1
[M]+, 636.9 [M + Na]+. UV−vis (nm): 415.0, 515.0, 550.0, 590.0,
650.1, Anal. Calcd for C44H30N4: C, 85.96; H, 4.92; N, 9.12. Found: C,
85.29; H, 5.05; N, 8.99.
[M]+, 868.9 [M + Na]+. UV−vis (nm): 419.9, 520.0, 554.9, 594.9,
650.1, Anal. Calcd for C56H54N4O4: C, 79.39; H, 6.44; N, 6.62. Found:
C, 79.25; H, 6.65; N, 6.53.
Synthesis of 5,10,15,20-Tetra(nitro)phenyl-21H,23H-porphyrin
(2f).32 Under air, a solution of 4-nitrobenzaldehyde (4.0 g, 2.6 ×
10−2 mol) and acetic anhydride (2 mL) in propionic acid (100 mL)
was heated to 100 °C. Pyrrole (1.85 mL, 2.6 × 10−2 mol) was added
dropwise to the hot solution and the mixture was maintained at 100
°C for a further hour. The resulting solution was allowed to cool and
stand in air for 18 h. Filtration of the tar-like mixture afforded a black
solid which was washed with water (5 × 50 mL) and dried in vacuo.
The residue was taken into pyridine (50 mL) and heated to 120 °C for
1 h. The resulting mixture was hot filtered to remove any residual solid
and the pyridine removed under vacuum. Soxhlet extraction of the
black solid with acetone afforded a purple solid (1.0 g, 19%).
1H NMR (CDCl3/DMSO-d6) δ 0.02 (s, 2H, NH), 8.59 (s, 8H, β-
3
3
porphH), 8.84 (d, JHH = 9 Hz, 8H, ArH), 8.87 (d, JHH = 9 Hz, 8H,
ArH). MS (ESI+): m/z = 794.8 [M]+. UV−vis (nm): 427.9, 516.0,
552.0, 592.0, 646.0. Anal. Calcd for C44H26N8O8: C, 66.50; H, 3.50; N,
14.10. Found: C, 65.81; H, 3.30; N, 13.95.
[The limited solubility of 2f in CDCl3/DMSO precluded further
purification and study by 13C{1H} NMR spectroscopy.]
General Procedure for Porphyrin Complexation/Oxidation.
Porphyrin Complexation with Co(OAc)2. A 100 mL three neck
round-bottom flask was charged with DMF (ca. 50 mL) and the
solvent degassed with Ar for 5 min. The porphyrin (1.0 equiv) was
added to the main reaction vessel as a solid, a reflux condenser fitted
and the mixture heated to 110 °C. Anhydrous Co(OAc)2 (1.5 equiv)
was then added to the mixture as a solid under a flow of Ar and the
mixture allowed to stir at 110 °C for a further 1 h until TLC analysis
(SiO2, CHCl3 elutant) showed no trace of free porphyrin. The
resulting mixture was cooled to 0 °C and quenched by the addition of
ice−water. The purple precipitate was collected by filtration, washed
with water and allowed to dry in air. The obtained material was used in
the subsequent oxidation without further purification.
5,10,15,20-Tetra(p-methoxy)phenyl-21H,23H-porphyrin (2b). p-
Anisaldehyde, 9.5 mL, 0.14 mol; pyrrole, 16.6 mL, 0.14 mol. Yield:
1
3.42 g, 14%. H NMR (CDCl3, 300.1 MHz) δ −2.75 (s, 2H, NH),
4.10 (s, 3H, OCH3), 7.29 (d, 3JHH = 9.0 Hz, 8H, ArH), 8.13 (d, 3JHH
=
9.0 Hz, 8H, ArH), 8.86 (s, 8H, β-porphH). 13C{1H} NMR (75.5
MHz): δ 55.8 (s, OCH3), 112.3 (s, ArC), 119.9 (s, ArC), 131.3 (s, β-
porphC), 134.8 (s, meso-porphC), 135.7 (s, ArC), 159.5 (s, ArC) [α-
porphyrin carbon not observed]. MS (ESI+): m/z = 734.9 [M]+, 756.9
[M + Na]+. UV−vis (nm): 419.9, 520.0, 554.9, 594.9, 650.1, Anal.
Calcd for C48H38N4O4: C, 78.45; H, 5.22; N, 7.63. Found: C, 78.27;
H, 5.17; N, 7.25.
5,10,15,20-Tetra(p-ethoxy)phenyl-21H,23H-porphyrin (2c). p-
Oxidation of [Porphyrin−CoII] to [Porphyrin−CoIIICl]. Under air, a
methanolic solution (ca. 40 mL of MeOH) of the [porphyrin−CoII]
complex was treated with HCl (37%, aq.) and the resulting mixture
allowed to stir at 50 °C until the formation of a precipitate was noted
and the [porphyrin−CoII] complex could not be detected by UV−
visible spectroscopy or TLC analysis (SiO2, CHCl3 elutant). The
precipitates were collected by filtration, washed with a minimum of
cold methanol and thoroughly dried in vacuo. The obtained solids were
recrystallized twice from CH2Cl2/pentane affording bright purple
crystalline solids.
[5,10,15,20-Tetraphenylporphyrin]cobalt(II) (3a). 5,10,15,20-Tet-
raphenyl-21H,23H-porphyrin, 3.38 g, 5.50 × 10−3 mol; Co(OAc)2,
1.46 g, 8.25 × 10−3 mol. Yield: 3.40 g, 92%. MS: ESI+ m/z = 671.0
[M]+, 687.9 [M−OH]+, 710.9 [M−OH+Na]+. UV−vis (nm) (THF
solution): 415.0, 515.0/550.0, 589.0, 650.9.
[5,10,15,20-Tetraphenylporphyrin]cobalt(III) Chloride (4a).
[5,10,15,20-Tetraphenylporphyrin]cobalt(II), 2.15 g, 3.20 × 10−3
mol; HCl (2 mL). Yield: 2.04 g, 90%. 1H NMR (CDCl3, 300.1
MHz); δ 7.93−8.08 (m, 12H, ArH), 8.56−8.67 (m, 16H, β-porphH +
ArH). 13C{1H} NMR (CDCl3, 75.5 MHz) δ 122.7 (s, ArC), 128.1 (s,
ArC), 128.5 (s, ArC), 130.1 (s, α-porphC), 139.1 (s, ArC), 140.0 (s, β-
porphC), 146.1 (s, meso-porphC). MS: ESI+ m/z = 670.9 [M − Cl]+.
UV−vis (nm) (CH2Cl2 solution): 445.0, 664.9. FT-IR (ATR): 3412 w,
2958 w, 2871 w, 1485 m, 1439 m, 1230 m, 1176 m, 1071 m, 1001 m,
979 m, 841 m, 802 s, 762 m, 704 s. Anal. Calcd for C44H28N4CoCl: C,
74.73; H, 4.00, N; 7.92. Calcd for C44H28N4CoCl + H2O: C, 72.87; H,
4.18; N, 7.72. Found: C, 73.25; H, 4.38; N, 7.58.
Ethoxybenzaldehyde, 15 mL, 0.11 mol; pyrrole, 7.5 mL, 0.11 mol.
1
Yield: 2.75 g, 13%. H NMR (CDCl3, 300.1 MHz) δ −2.76 (s, 2H,
NH), 1.62 (t, 3JHH = 6.0 Hz, 12H, OCH2CH3), 4.35 (q, 3JHH = 6.0 Hz,
8H, OCH2CH3), 7.29 (d, 3JHH = 9.0 Hz, 8H, ArH), 8.11 (d, 3JHH = 9.0
Hz, ArH), 8.86 (s, 8H, β-porphH). 13C{1H} NMR (75.5 MHz): δ 15.2
(s, OCH2CH3), 63.9 (s, OCH2CH3), 112.8 (s, ArC), 114.6 (s, ArC),
127.7 (s, β-porphC), 134.7 (s, meso-porphC), 135.8 (s, ArC), 146.5 (s,
α-porphC), 158.9 (s, ArC). MS (ESI+): m/z = 791.0 [M]+, 812.8 [M +
Na]+. UV−vis (nm): 419.9, 520.0, 554.9, 594.9, 650.1, Anal. Calcd for
C52H46N4O4: C, 78.96; H, 5.87; N, 7.08. Found: C, 78.28; H, 6.05; N,
6.82.
5,10,15,20-Tetra(p-n-propoxy)phenyl-21H,23H-porphyrin (2d). p-
n-Propoxybenzaldehyde, 12.5 g, 7.61 × 10−2 mol; pyrrole, 5.3 mL, 7.61
1
× 10−2 mol. Yield: 2.86 g, 12%. H NMR (CDCl3, 300.1 MHz) δ
−2.74 (s, 2H, NH), 1.21 (t, 3JHH = 7.5 Hz, 12H, OCH2CH2CH3), 2.02
(sext, 3JHH = 7.5 Hz, 8H, OCH2CH2CH3), 4.22 (t, 3JHH = 7.5 Hz, 8H,
OCH2CH2CH3), 7.28 (d, 3JHH = 9.0 Hz, 8H, ArH), 8.12 (d, 3JHH = 9.0
Hz, 8H, ArH), 8.87 (s, 8H, β-porphH). 13C{1H} NMR (CDCl3, 75.5
MHz) δ 11.0 (s, OCH2CH2CH3), 23.0 (s, OCH2CH2CH3), 70.0 (s,
OCH2CH2CH3), 112.8 (s, ArC), 120.0 (s, ArC), 131.0 (s, β-porphC),
134.6 (s, meso-porphC), 135.7 (s, ArC), 159.0 (s, ArC), [α-porphyrin
carbon not observed]. MS (ESI+): m/z = 847.0 [M]+, 868.8 [M +
Na]+. UV−vis (nm): 419.9, 520.0, 554.9, 594.9, 650.1, Anal. Calcd for
C56H54N4O4: C, 79.39; H, 6.44; N, 6.62. Found: C, 78.89; H, 6.48; N,
6.46.
5,10,15,20-Tetra(p-isopropoxy)phenyl-21H,23H-porphyrin (2e).
p-Isopropoxybenzaldehyde, 13.5 g, 8.22 × 10−2 mol; pyrrole, 5.7
1
mL, 8.22 × 10−2 mol. Yield: 4.22 g, 24%. H NMR (CDCl3, 300.1
[5,10,15,20-Tetra(p-methoxy)phenylporphyrin]cobalt(II) (3b).
5,10,15,20-Tetra(p-methoxy)phenyl-21H,23H-porphyrin, 1.47 g, 2.00
× 10−3 mol; Co(OAc)2, 0.532 g, 3.01 × 10−3 mol. Yield: 1.36 g, 86%.
MS: ESI+ m/z = 791.0 [M]+, 807.8 [M − OH]+, 830.8 [M − OH +
Na]+. UV−vis (nm) (THF solution): 419.9, 518.0/550.0, 596.0, 652.0.
[5,10,15,20-Tetra(p-methoxy)phenylporphyrin]cobalt(III) Chlor-
ide (4b). 5,10,15,20-Tetra(p-methoxy)phenylporphyrin cobalt(II),
3
MHz) δ −2.72 (s, 2H, NH), 1.57 (d, JHH = 6.0 Hz, 24H,
3
OCH(CH3)2), 4.86 (sept, JHH = 6.0 Hz, 4H, OCH(CH3)2), 7.27
(d, 3JHH = 9.0 Hz, 8H, ArH), 8.12 (d, 3JHH = 9.0 Hz, 8H, ArH), 8.90 (s,
8H, β-porphH). 13C{1H} NMR (CDCl3, 75.5 MHz) δ 22.5 (s,
OCH(CH3)2), 70.3 (s, OCH(CH3)2), 114.0 (s, ArC), 120.0 (s, ArC),
131.0 (s, β-porphC), 134.5 (s, meso-porphC), 135.9 (s, ArC), 157.8 (s,
ArC), [α-porphyrin carbon not observed]. MS (ESI+): m/z = 846.9
1
1.20 g, 1.52 × 10−3 mol; HCl (2 mL). Yield: 1.17 g, 93%. H NMR
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Scheme 1. Synthesis of Porphyrin−Cobalt(III) Chloride Complexes 4a−f
(CDCl3, 300.1 MHz) δ 4.16 (s, 12H, OCH3), 7.53 (d, 8H, 3JHH = 9.0
Hz, ArH), 8.46−8.64 (m, 16H, ArH + β-porphC). 13C{1H} NMR
(CDCl3, 75.5 MHz) δ 56.0 (s, OCH3), 114.2 (s, ArC), 127.9 (s, ArC),
133.5 (s, ArC), 140.5 (s, β-porphC), 146.4 (s, meso-porphC), 161.6 (s,
ArC) [α-porphyrin carbon not observed]. MS: ESI+ m/z = 791.0 [M-
Cl]+. UV−vis (nm) (CH2Cl2 solution): 453.0, 691.1. FT-IR (ATR):
3429 w, 3357 w, 2959 w, 2833 w, 1597 m, 1506 m, 1483 m, 1349 m,
1292 m, 1235 s, 1174 s, 1010 m, 984 m, 803 s, 724 m, Anal. Calcd for
C48H36N4O4CoCl: C, 69.69; H, 4.40, N, 6.77; Calcd for
C48H36N4O4CoCl + H2O: C, 68.20; H, 4.54; N, 6.63. Found: C,
67.67; H, 4.49; N, 6.48.
[5,10,15,20-Tetra(p-ethoxy)phenylporphyrin]cobalt(II) (3c).
5,10,15,20-Tetra(p-ethoxy)-phenyl-21H,23H-porphyrin, 1.50 g, 1.90
× 10−3 mol; Co(OAc)2, 0.50 g, 2.85 g × 10−3 mol. Yield: 1.47 g, 91%.
MS: ESI+ m/z = 847.0 [M]+, 863.8 [M−OH]+. UV−vis (nm) (THF
solution): 417.9, 540.0/550.0, 598.9, 653.0.
1173 s, 1108 m, 964 m, 800 s, 737 m, Anal. Calcd for
C56H52N4O4CoCl: C, 70.24; H, 5.70; N, 5.85. Calcd for
C56H52N4O4CoCl + H2O: C, 71.59; H, 5.59; N, 5.97. Found: C,
69.77; H, 5.90; N, 5.46.
[5,10,15,20-tetra(p-isopropoxy)phenylporphyrin]cobalt(II) (3e).
[5,10,15,20-Tetra(p-isopropoxy)phenyl-21H,23H-porphyrin, 2.28 g,
2.69 × 10−3 mol; Co(OAc)2, 0.71 g, 4.03 × 10−3 mol. Yield: 2.01 g,
83%. MS: ESI+ m/z = 903.0 [M]+, 919.9 [M−OH]+. UV−vis (nm)
(THF solution): 419.0, 528.0, 602.1, 653.0.
[5,10,15,20-tetra(p-isopropoxy)phenylporphyrin]cobalt(III) Chlor-
ide (4e). [5,10,15,20-Tetra(p-isopropoxy)phenylporphyrin]cobalt(II),
1
1.67 g, 1.85 × 10−3 mol; HCl, 2 mL. Yield: 1.63 g, 94%. H NMR
3
(300.1 MHz, CDCl3) δ 1.57 (d, JHH = 6.0 Hz, 24H, OCH(CH3)2),
3
4.86 (sept. JHH = 6.0 Hz, 4H, OCH(CH3)2), 7.73 (m [solvent
3
overlap], ArH), 8.08 (d, JHH = 9.0 Hz, 8H, ArH), 9.06 (s, 8H, β-
porphH). 13C{1H} NMR (75.5 MHz, CDCl3/DMSO-d6) δ 22.4 (s,
OCH(CH3)2), 70.2 (s, OCH(CH3)2), 114.1 (s, ArC), 120.4 (s, α-
porphC), 134.2 (s, ArC), 135.3 (s, β-porphC), 144.0 (s, ArC), 144.9 (s,
meso-porphC), 157.8 (s, ArC). MS: ESI+ m/z = 903.2 [M − Cl]+. UV−
vis (nm) (CH2Cl2 solution): 429.0, 546.1. FT-IR (ATR): 3480 w,
2973 w, 2905 w, 1605 m, 1501 s, 1349 m, 1281 m, 1243 s, 1182 m,
1103 m, 1101 m, 950 m, 798 s, 713 m, Anal. Calcd for
C56H52N4O4CoCl: C, 70.24; H, 5.70; N, 5.85. Calcd for
C56H52N4O4CoCl + H2O: C, 71.59; H, 5.59; N, 5.97. Found: C,
70.78; H, 5.98; N, 5.61.
[5,10,15,20-Tetra(p-ethoxy)phenylporphyrin]cobalt(III) Chloride
(4c). 5,10,15,20-Tetra(p-ethoxy)phenylporphyrin cobalt(II), 1.20 g,
1
1.42 × 10−3 mol; HCl (2 mL). Yield: 1.11 g, 89%. H NMR (CDCl3,
300.1 MHz) δ 1.66 (t, 3JHH = 6.0 Hz, 12H, OCH2CH3), 4.40 (q, 3JHH
3
= 6.0 Hz, 8H, OCH2CH3), 7.52 (d, JHH = 9.0 Hz, 8H, ArH), 8.43−
8.58 (m, 16H, β-porphH + ArH). 13C{1H} NMR (CDCl3, 75.5 MHz)
δ 15.1 (s, OCH2CH3), 64.2 (s, OCH2CH3), 114.6 (s, ArC), 127.9 (s,
ArC), 133.3 (s, ArC), 140.4 (s, β-porphC), 146.4 (s, meso-porphC),
161.1 (s, ArC) [α-porphyrin carbon not observed]. MS: ESI+ m/z =
846.9 [M-Cl]+. UV−vis (nm) (CH2Cl2 solution): 455.0, 692.9. FT-IR
(ATR): 3426 w, 2972 w, 2869 w, 1594 m, 1482 m, 1292 m, 1231 s,
1174 s, 1035 m, 819 s, 795 s, 693 m, Anal. Calcd for
C52H44N4O4CoCl: C, 70.70; H, 5.03; N. 6.34. Calcd for
C52H44N4O4CoCl + H2O: C, 69.29; H, 5.15; N, 6.22. Found: C,
67.87; H, 5.15; N, 5.93.
[5,10,15,20-Tetra(p-nitro)phenylporphyrin]cobalt(II) (3f).
5,10,15,20-Tetra(p-nitro)phenylporphyrin (2f), 0.21 g, 2.64 × 10−4
mol; Co(OAc)2, 0.07 g, 3.96 × 10−4 mol. Yield: 0.18 g, 81%. MS: ESI+
m/z = 850.8 [MII]+. UV−vis (nm) (THF solution): 422.0, 516.1,
591.9, 646.9.
[5,10,15,20-Tetra(p-nitro)phenylporphyrin]cobalt(III) (4f).
[5,10,15,20-Tetra(p-nitro)phenylporphyrin]cobalt(II) (3f), 0.11 g,
1.29 × 10−4 mol; HCl, 0.5 mL. Yield: 0.096 g, 84%. 1H NMR
[5,10,15,20-Tetra(p-n-propoxy)phenylporphyrin]cobalt(II) (3d).
5,10,15,20-Tetra(p-n-propoxy)-phenyl-21H,23H-porphyrin, 2.02 g,
2.39 × 10−3 mol; Co(OAc)2, 0.63 g, 3.58 × 10−3 mol. Yield: 1.59 g,
74%. MS: ESI+ m/z = 903.0 [M]+, 919.9 [M − OH]+. UV−vis (nm)
(THF solution): 417.9, 530.0, 606.1, 669.0.
3
3
(CDCl3/DMSO-d6): δ 8.35 (d, JHH = 6 Hz, ArH), 8.60 (d, JHH = 6
Hz, ArH), 8.98 (s, 8H, β-porphH). 13C{1H} (CDCl3/DMSO-d6): δ
118.1 (s, α-porphC), 122.1 (s, ArC), 134.5 (s, ArC), 134.9 (s, β-
porphC), 143.5 (s, meso-porphC), 147.9 (s, ArC), 148.1 (s, ArC). MS
(ESI+): m/z = 850.4 [M−Cl]+. UV−vis (nm) (CH2Cl2 solution):
454.9, 664.0; CHN: FT-IR (ATR): 3492 w, 2941 w, 2862 w, 1596 m,
1489 s, 1338 s, 1234 m, 1107 m, 1002 m, 979 m, 864 m, 817 s, 771 m,
749 m, 709 s. Calcd for C44H24N8O8CoCl: C, 59.57; H, 2.73; N, 12.63.
Calcd for C44H24N8O8CoCl + H2O: C, 58.39; H, 2.90; N, 12.38.
Found: C, 58.13; H, 3.37; N, 12.23.
[5,10,15,20-Tetra(p-n-propoxy)phenylporphyrin]cobalt(III) Chlor-
ide (4d). [5,10,15,20-Tetra(p-n-propoxy)phenylporphyrin]cobalt(II),
1
1.35 g, 1.49 × 10−3 mol; HCl (2 mL). Yield: 1.21 g, 86%. H NMR
(300.1 MHz, CDCl3) δ 1.19 (t, 3JHH = 6.0 Hz, 12H, OCH2CH2CH3),
1.99 (sext., 3JHH = 6.0 Hz, 8H, OCH2CH2CH3), 4.20 (t, 3JHH = 6.0 Hz,
8H OCH2CH2CH3), 7.23 (m, [solvent overlap], ArH), 8.06 (d, 3JHH
=
6.0 Hz, 8H, ArH), 9.05 (s, 8H, β-porphH). 13C{1H} NMR (75.5 MHz,
CDCl3/DMSO-d6) 10.8 (s, OCH2CH2CH3), 22.9 (s,
δ
Polymerization Procedure. General Procedure for Propylene
Oxide Homopolymerizations. Under Ar, a 50 mL Schlenk flask was
charged with a porphyrin catalyst (0.06 mol %) and the PPNCl
cocatalyst (0.06 mol %) if required. Two mL PO (2.86 × 10−2 mol)
was added to the reaction vessel via syringe and the resulting mixture
allowed to stir for 18 h at 25 °C. A small aliquot was then removed for
OCH2CH2CH3), 69.8 (s, OCH2CH2CH3), 112.9 (s, ArC), 134.2 (s,
ArC), 135.4 (s, β-porphC), 142.0 (s, ArC), 144.8 (s, meso-porphC),
159.0 (s, ArC) [α-porphyrin carbon not observed]. MS: ESI+ m/z =
903.0 [M-Cl]+. UV−vis (nm) (CH2Cl2 solution): 428.0, 545.0. FT-IR
(ATR): 3414 w, 2960 w, 2873 w, 1603 m, 1501 m, 1466 m, 1241 m,
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analysis by 1H NMR spectroscopy. Because of the low conversions, the
formed polymer was not isolated.
a
Table 1. Homopolymerization of PO with Catalysts 4a−f
b
c
d
e
entry
catalyst system
PPO (%)
TON
TOF /h−1
General Procedure for Propylene Oxide/Carbon Dioxide Copoly-
merizations. The required catalyst (0.05 mol %) together with an
equimolar amount of PPNCl cocatalyst was added to a dry 250 mL
stainless steel autoclave and propylene oxide (5 mL, 7.15 × 10−2
mol)/CH2Cl2 (5 mL) were added via syringe under a stream of argon.
The autoclave was then pressurized with CO2 and heated to the
required temperature in an oil bath, if required. The reaction was
terminated after 18 h by cooling the reactor to 0 °C and slow release
of pressure. A small aliquot of the copolymerization mixture was
removed for analysis by 1H NMR spectroscopy and the resulting
mixture dissolved in CH2Cl2. The crude reaction mixture was poured
into methanolic HCl solution (0.5 mol dm−3) and the polymer
precipitated by addition of excess methanol. Residual co-containing
impurities were removed from the precipitated polymer by heating a
CHCl3 solution of the polymer to reflux with activated charcoal
followed by hot filtration of the colorless solution. After evaporation of
the solvent, the poly(propylene carbonate) was dried in a vacuum
oven to constant weight.
1
2
3
4
5
6
4a
4b
4c
4d
4e
4f
−
2
−
30
−
2
8
6
4
9
135
105
75
7
5
−
−
−
a
b
All reactions carried out in neat PO (2 mL), at 25 °C. [catalyst]:
[PPNCl]:[PO] = 1:1:1500, 18 h reaction time. Determined by H
NMR spectroscopy. Moles of PO consumed per mole of Co. Moles
of PO consumed per mole of Co per hour.
c
1
d
e
in the gas phase (in comparison to related (TPP)Cr+ and
(TPP)Al+ cations),35 however, the introduction of four electron
donating alkoxy- fragments clearly enhances the reactivity of
the overall metal center toward epoxide binding, despite the
relatively low Lewis acidity of the Co(III) ion. Indeed,
porphyrin cobalt(III) complexes bearing meso-methoxy-sub-
stituents display catalytic activity toward α-epoxide ring-
opening with a high degree of selectivity.36 Although the
alkoxy-substituted compounds 4b−e display significantly
reduced activities toward epoxide homopolymerization than
related Al37 and Cr35 metalloporphyrins, it is noteworthy that
even relatively minor modifications of the ligating porphyrin
can alter the reactivity of the resulting Co complexes.
Copolymerization Behavior of Catalysts 4a-f: Effect of
CO2 Pressure. As a next step the reactivity of complexes 4a−f
toward PO/CO2 copolymerization was explored. Experiments
were initially investigated with variation of reaction pressure
with a [catalyst]:[propylene oxide] ratio of 1:2000, Table 2. All
copolymerization experiments were performed in the presence
of a noncoordinating solvent (CH2Cl2) in order to reduce
monomer diffusion limitations. To probe the effect of the
cocatalyst on the polymerization system all metalloporphyrins
were first tested for catalytic activity in the absence of the
PPNCl cocatalyst. In contrast to the more Lewis acidic Al-
containing porphyrins, which demonstrate moderate activity
toward epoxide/CO2 copolymerization in the absence of
additives,18,35 the Co complexes 4a−f demonstrated no
catalytic activity in this reaction.
RESULTS AND DISCUSSION
■
Synthesis of Complexes. The free porphyrins 2a−f were
synthesized by variations of acid-catalyzed Rothemund
procedures with pyrrole and an appropriately substituted
benzaldehyde, Scheme 1.30−32 To establish the effect of ligand
substitution on the complex activity, the unsubstituted
tetraphenylporphyrin (2a) was also prepared. Formation of
the CoIIICl porphyrin complexes 4a−f was easily achieved in a
two-step procedure via complexation with Co(OAc)2 and
subsequent oxidation of the metal center in methanolic HCl
solution at 50 °C.
The NMR spectra of the air stable diamagnetic porphyrin
Co(III) complexes were largely similar to those of the parent
free-base porphyrins 2a−e with only moderate shifts of the
resonances attributed to the β-porphyrin protons observed.33
CHN microanalyses of the oxidized Co(III) complexes 4a−f
corresponded to the incorporation of one molecule of water per
metal center which could not be removed by drying of the
complexes under high vacuum. The presence of this
coordinated water was confirmed by both the appearance of
absorption bands in the region 3300−3500 in their IR spectra34
1
and resonances corresponding to H2O by H NMR spectros-
copy (Supporting Information, S47).
Propylene Oxide Homopolymerization of Catalysts
4a−e. To establish the reactivity of the substituted complexes
4b−f as catalysts in epoxide ring-opening homopolymerization
experiments with propylene oxide (PO) were performed.
Consistent with previous observations,35 the unsubstituted
cobaltoporphyrin 4a showed no activity toward PO polymer-
ization, even in the presence of PPNCl cocatalyst. By contrast,
complexes 4b−e/PPNCl systems show low reactivity toward
the formation of poly(propylene oxide) (PPO) with small
TON/TOF numbers, Table 1. Experiments with the nitro-
substituted complex 4f showed no activity toward PO ring-
opening, even in the presence of PPNCl additive. These results
lead to the conclusion that the activity of the central cobalt ion
toward epoxide ring-opening is significantly influenced by the
electronic environment of the porphyrin as the incorporation of
electronically differing substituents into the periphery of the
ligand framework clearly modulates the overall binding
environment at the metal center. Previous studies of
unsubstituted 5,10,15,20-tetraphenylporphyrin (TPP) M(III)+
complexes by Chisholm and co-workers have indicated that the
(TPP)Co+ cation has a surprisingly low affinity for PO binding
Addition of one molar equivalent of PPNCl cocatalyst to the
copolymerization reaction afforded significant reactivity of the
complexes with PO/CO2. The binary 4a−e/PPNCl catalysts
systems demonstrate high activity toward copolymerization, as
reflected in their TON/TOF values, however it is striking that
the 4f/PPNCl only afforded cyclic carbonate in low yield
(Table 2, entry 17). This is noteworthy in light of previous
studies with Al-porphyrin binary catalyst systems where the
incorporation of meso-fluoro substituents in the porphyrin
framework afforded more active and polymer selective systems
than analogous porphyrins bearing unsubstituted or β-ethyl
substituents.27 In the case of 4f the exclusive formation of cyclic
carbonate can be attributed to the electron withdrawing nature
of the nitro-substituents in the catalyst framework.
The formation of cyclic carbonate is dominant in systems
where the backbiting activation barriers are low enough to
compete with chain propagation38 and is known to occur by
depolymerization of growing polymer chains from either the
alkoxy- and carbonato- chain ends,22 Scheme 2. In the electron-
rich BDI-Zn complexes, the use of electron withdrawing groups
promotes PO/CO2 polymerization activity over cyclization by
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Table 2. Selected PO/CO2 Copolymerization Data for 4a−f at 30 and 50 bar CO2 Pressure
b
conversion
a
c
d
e
f
g
g
entry
catalyst system
P/bar
PPC (%)
PPO (%)
CC (%)
TON
TOF /h−1
H−T (%)
Tg /°C
Mn /kg mol−1
PD
1
4a
30
30
50
30
30
50
30
30
50
30
30
50
30
30
50
30
30
−
−
<1
<1
−
<1
<1
-
−
5
−
−
−
−
−
−
2
4a/PPNCl
4a/PPNCl
4b
50
41
−
68
74
-
1112
930
−
1420
1540
-
62
52
−
79
86
-
93
93
−
93
93
-
41.5
40.7
−
40.7
39.4
-
36
33
−
39
45
-
1.14
1.38
−
1.17
1.23
-
3
5
4
−
3
5
4b/PPNCl
4b/PPNCl
4c
6
3
7
-
8
4c/PPNCl
4c/PPNCl
4d
77
74
-
<1
1
4
1620
1540
-
90
86
-
93
94
-
41.4
40.8
-
38
52.5
-
1.16
1.26
-
9
3
10
11
12
13
14
15
16
17
-
-
4d/PPNCl
4d/PPNCl
4e
78
73
−
85
81
−
<1
2
2
1620
1520
−
1760
1680
−
90
85
−
98
93
−
93
94
−
93
93
−
39.6
40.6
−
40.6
38.4
−
27.5
43
−
38
34.5
−
1.13
1.23
−
1.17
1.23
−
3
−
<1
1
−
3
4e/PPNCl
4e/PPNCl
4f
3
−
−
−
13
4f/PPNCl
−
263
11
−
−
1
−
−
a
b
c
[catalyst]:[PO] = 1:2000/[catalyst]:[cocatalyst]:[PO] = 1:1:2000, 18 h reaction time, 25 °C. Determined by H NMR spectroscopy. Moles of
d
e
f
PO consumed per moles of Co. Moles of PO consumed per mole of Co per hour. Determined by 13C{1H} NMR spectral integration. Tg values
g
determined from DSC second heating cycle. Determined by size exclusion chromatography using polystryrene standard.
Scheme 2. Backbiting Mechanism from Alkoxy- (A) or
Carbonato- (B) Poly(carbonate) Intermediates
Chart 2. PPC yield (%) with Variation of CO2 Pressure at 30
°C: 4a (Black Squares), 4b (Red Points), 4c (Green
Triangles), 4d (Dark Blue Triangles), and 4e (Light Blue
Rhombs)
promoting epoxide insertion into the polymer chain.39
However, it is clear in the case of 4f that the combination of
an electron poor Co3+ ion with a nitro-substituted ligand favors
cyclization of the substrates over polymer chain propagation as
a result of the electron deficient environment at the active metal
site. Indeed, theoretical calculations with Zn complexes have
indicated that the insertion of CO2 into a metal-alkoxide bond
has a considerably smaller energy barrier than epoxide
enchainment22 which provides further indication that the ease
of epoxide incorporation into the growing polymer chain is of
key importance.
For the catalyst systems capable for polymer synthesis (4a−
e), systematic polymerization test runs with increasing CO2
pressure allowed the determination of pressure-dependent
activity profiles for each catalyst, Chart 2. Further copoly-
merization experiments were not undertaken with 4f due to its
propensity for cyclization. All reactions were performed with
identical catalyst/cocatalyst loading to permit direct compar-
ison and establish structure−activity patterns to be drawn. The
binary 4a−e/PPNCl systems were demonstrated to be highly
active and selective copolymerization catalysts, furnishing
copolymer with approaching 50% CO2 content in high yields.
From the data in Table 2, it is readily apparent that 4b−e, in
which the porphyrin framework bears four alkoxy-fragments,
displayed an overall increased PO consumption in comparison
with the unsubstituted catalyst 4a. It is clear that the inclusion
of electron-donating fragments in the catalyst systems has a
beneficial effect on the catalytic activity of these complexes.
These substituents are presumed to have two advantageous
effects on the cobaltoporphyrin systems, namely modulating
both the electronic environment at the Co center and the
solubility of the overall complexes. The electron donating
nature of the alkoxy groups in complexes 4b−e increases the
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Chart 3. Schematic Description of α-/β-Modes of Epoxide Cleavage Leading to Insertion Regioselectivity and Representative
13C{1H} NMR Spectrum Showing Resulting Polymer Regiochemistry (Carbonate Region Shown Only) (Catalyst 4b, Table 2,
Entry 5, 92% H−T)
yield of copolymer in comparison to the unsubstituted 4a. This
increase in activity compares well with the observation of
Darensbourg and co-workers with Cr−salen complexes in
which a similar increase in catalyst activity was noted with the
introduction of OMe groups to the salen−phenolate rings.40
Furthermore, the solubility of porphyrin complexes is well-
known to be influenced by substituents present at the
macrocyclic framework41 and the incorporation of four
alkoxy-fragments results in an increased solubility of the
metalloporphyrins in the reaction medium. Indeed, these
higher activities for 4b−e mirror studies by Inoue where the
activities of Al-porphyrins substituted with methoxy groups
exceeded those of unsubstituted TPPAlCl in the polymerization
of PO.29 By examination of the pressure-dependence profiles of
the complexes 4b−e (Chart 2), it is immediately apparent that
the catalysts comprising the longer ethoxy (4c)/propoxy (4d−
e) substituents demonstrated the highest PO consumption, as
reflected in their greater TON/TOF numbers, thus indicating
complex solubility in the reaction medium is of crucial
importance to achieve high monomer conversions. Contrasting
the PPC yield of the propoxy-substituted catalysts 4d (78%)
and 4e (85%) at 30 bar CO2 pressure (Table 2, Entries 11 and
14) with that of the methoxy-substituted system 4b (Table 2,
Entry 5, 68%) provides an indication of the solubility influence
of the longer alkoxy-functionalities on the catalyst turnover.
By variation of reaction pressure, the optimum pressure for
the copolymerization reaction for the systems 4a−e could be
determined. Chart 2 clearly shows a maximum turnover around
30 bar with a further increase in pressure having little or
negative impact on the activity. The ready explanation for this
observation is a dilution of the reaction mixture by the CO2
monomer at higher pressure which causes a volumetric increase
of the copolymerization medium.42 In all copolymerization
experiments, total PO consumption was not achieved due to
reduced monomer diffusion in the reaction medium at higher
conversions although low CC conversion (<5%) was typically
observed with both the substituted and unsubstituted porphyrin
systems at 30 °C.
the copolymerization of PO and CO2 afforded almost perfectly
alternating poly(carbonate) structure with minimal polyether
content with moderate molecular weight (Mn). The sub-
stitution pattern of the porphyrin framework did not
significantly affect the obtained polymer Mn, however GPC
elugrams showed exclusively bimodal PPC character. Bimo-
dality of poly(carbonate)s produced by porphyrin-based
catalysts is regarded as a characteristic feature.18 This attribute
has been proposed to arise from the growth of poly(carbonate)
chain from both sides of the rigid metalloporphyrin
coordination plane,43 however it is highly likely that the
presence of water in the polymerization mixture gives rise to
chain transfer reactions, which prevents exclusively monomodal
polymer character. In studies with Al−porphyrin catalysts it has
been suggested that the presence of adventitious water leads to
complexes of reduced activity,27 however with the air/moisture
tolerant Co-porphyrins this is considered to be less likely.
Elemental analysis/IR spectroscopy of 4a−e was consistent
with the presence of coordinated water to the Co center which
presumably occupies an axial position of the metal’s
coordination sphere. Over the course of the copolymerization
reaction, this water causes chain transfer reactions, resulting in
these characteristic bimodal polymers. The polycarbonates
obtained from the cobaltoporphyrins all exhibited narrow PD
indices at 30 bar which were observed to broadened slightly
upon increase in pressure to 50 bar. This further evidence a
controlled, living-type polymerization mechanism in these Co-
systems, thus reflecting the studies of Al-based metalloporphyr-
ins.44
Independent of catalyst substitution pattern, head-to-tail
(H−T) connectivity45 in the obtained PPC, resulting from
repeated α- or β-cleavage of the epoxide ring, was observed to
reach 93% above 30 bar (Table 2). The comparably high
degree of H−T incorporation across this series of catalysts
demonstrate that the substitution of the catalyst does not
influence the nature of epoxide enchainment. Indeed, the
placement of the alkoxy- functionalities at the periphery of the
rigidly planar porphyrin ring evidently provides a significant
electronic influence to the catalytic metal center without
introducing steric interference. The consistent PPC stereo-
As indicated by the low TOFs in PO homopolymerization,
sequential epoxide enchainment is not favored with 4a−e and
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Table 3. Selected Polymerization data at 30 and 60 °C for 4a−e at 30 bar CO2 pressure
b
conversion
a
c
d
e
f
g
g
entry
catalyst system
T/°C
PPC (%)
PPO (%)
CC (%)
TON
TOF /h−1
H−T (%)
Tg /°C
Mn /kg mol−1
PD
1
2
3
4
5
6
7
8
9
10
4a/PPNCl
4a/PPNCl
4b/PPNCl
4b/PPNCl
4c/PPNCl
4c/PPNCl
4d/PPNCl
4d/PPNCl
4e/PPNCl
4e/PPNCl
30
60
30
60
30
60
30
60
30
60
54
39
68
12
85
25
88
20
90
10
<1
<1
<1
1
4
1172
1233
1440
640
65
91
85
91
85
92
82
93
81
93
41.7
38.6
40.8
37.5
41.1
36.0
40.7
35.1
38.8
34.4
41.5
21.5
38
8
1.19
1.25
1.18
1.23
1.20
1.25
1.18
1.22
1.18
1.14
22
4
69
80
20
3
36
<1
1
1760
860
98
46.5
14
47
9
18
2
48
<1
2
1800
740
100
41
17
3
<1
1
1860
560
103
31
39
6
18
80
a
b
c
1
[catalyst]:[cocatalyst]:[PO] = 1:1:2000, 18 h reaction time. Determined by H NMR spectroscopy. Moles of PO consumed per moles of Co.
d
e
f
Moles of PO consumed per mole of Co per hour. Determined by 13C{1H} NMR spectral integration. Tg values determined from DSC second
g
heating cycle. Determined by size exclusion chromatography using polystryrene standard.
chemistry indicates the inherently regioselective nature of the
cobaltoporphyrin systems.
therefore the selectivity of the catalyst systems with increased
temperature. Chart 5 demonstrates a clear drop in PPC yield
Effect of Copolymerization Temperature. A further set
of experiments were performed to probe the effect of
temperature on the porphyrin catalyst systems at 30 bar CO2
pressure, Table 3. Overall an increase of reaction temperature
from 30 to 60 °C resulted in a reduced PO consumption. This
loss of activity was accompanied, as expected, by increased
formation of cyclic carbonate as a result of the higher activation
barriers for epoxide/CO2 cyclization.46,47 Chart 4 depicts a plot
Chart 5. PPC Yield with Variation of Temperature for 4a−e
at 30 bar CO2 Pressure; 4a (Black Squares), 4b (Red
Points), 4c (Green Triangles), 4d (Dark Blue Triangles), 4e
(Light Blue Rhombs)
Chart 4. Overall PO Consumption as a Function of
Temperature (30 bar CO2 Pressure): 4a (Black Squares), 4b
(Red Points), 4c (Green Triangles), 4d (Dark Blue
Triangles), and 4e (Light Blue Rhombs)
with higher reaction temperature with this being more
pronounced with 4b−e. As expected by the living catalytic
nature of this process, the fall in PPC yield was accompanied by
a significant reduction in polymer molecular weight, Table 3.
Interestingly, the PPC obtained at higher temperature with 4a−
e remained highly alternating with only small degrees of
poly(ether) linkages although a rise in polymerization temper-
ature afforded a minor decrease in the H-T connectivity of the
poly(carbonate)s. Epoxide incorporation proceeds by com-
petitive attack at the methylene and methine ring carbons by
metalloporphyrin catalysts48 although ring-opening is favored at
the least sterically hindered carbon of the ring (β-cleavage).6,47
An increase in reaction temperature clearly leads to a greater
competition between these ring-opening modes. This temper-
ature-induced decrease of PPC regioregularity is consistent with
the findings of Wang’s studies with the unsubstituted 4a/
PPNCl system.26 Associated with the reduction in the Mn of the
of total PO conversion as a function of increasing temperature
which reveals that the unsubstituted system 4a was least
affected by a rise in reaction temperature (Chart 4, black
squares) whereas the alkoxy-substituted systems exhibited
dramatically reduced activity. Specifically, the activities of the
higher substituted systems 4c−e were most affected by an
increase in temperature with their general PO consumption
dropping from >85% at 30 °C to ca. 20% at 60 °C.
The overall PO consumption vs pressure can be further
analyzed to demonstrate the relative product formation and
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PPCs was a decrease in the glass transition temperatures (Tg)
of the polymers. The Tg values of poly(carbonate)s are known
to be affected by the polymer chain length as well as the
polymer microstructure49 as PPC with more uniform head-to-
tail character demonstrates higher Tg values than those with
more regiorandom microstructures50 which is consistent with
the here presented findings.
CONCLUSION
■
Here we have presented a series of cobaltoporphyrin systems
with systematically varied substitution pattern and their
activities in both PO homopolymerization and PO/CO2
copolymerization. The ease of synthesis of these rigid,
symmetrical porphyrin catalysts combined with their oxygen
and moisture stability presents numerous advantages over more
sensitive catalyst systems.
The decrease in polymer yield was accompanied by a clear
increase in cyclic carbonate formation, Chart 6. Interestingly
The activity and selectivity of these cobaltoporphyrins is
strongly dependent on the substitution pattern of the ligand
framework and enables tailoring of the product selectivity;
electron withdrawing substituents afford exclusive cyclization
(forming cyclic carbonate) while electron donating fragments
afford highly active copolymerization catalysts, producing
poly(carbonate) with excellent properties. By placement of
these electronically differing substituents at the periphery of the
rigidly planar porphyrin, the electron influence of these
fragments is maintained while minimizing steric clashes at the
active metal site. Furthermore, our studies indicate that an
increase of catalyst solubility directly impacts on the polymer-
ization activity, as indicated by the higher turnovers for the
ethoxy-/propoxy-substituted catalysts 4c−e, which emphasizes
the importance of the homogeneity of the polymerization
mixture with these systems. Overall, we have demonstrated that
molecular catalyst design by simple ligand modification
represents a useful and underused strategy in porphyrin-based
catalyst systems and has great potential to generate catalysts
with high activities and tailored selectivities.
Chart 6. CC Yield with Variation of Temperature for 4a−e at
30 bar CO2 Pressure: 4a (Black Squares), 4b (Red Points),
4c (Green Triangles), 4d (Dark Blue Triangles), and 4e
(Light Blue Rhombs)
ASSOCIATED CONTENT
* Supporting Information
■
S
Characterization of catalysts and precursors, representative
polymer analyses and full tables of polymerization data to
accompany Charts 2 and 4−6. This material is available free of
charge via the Internet at http://pubs.acs.org/.
the produced amount of CC was independent of catalyst
substitution pattern with a linear increase from ca. 4% (30 °C)
to ca. 20% (60 °C) for all systems 4a−e. This rise in cyclization
can be broadly attributed to either an accelerated rate of
depolymerization or deactivation of the cobaltoporphyrin
catalysts with increased temperature. Indeed, higher reaction
temperatures have been demonstrated to promote the
formation of reduced Co(II) salen compounds in the
copolymerization mixture,51 a situation which may occur with
the present Co(III) porphyrins as indicated by the overall fall in
monomer conversion at increased temperature (Chart 4).
Polymers comprising 1-alkyl-substituted epoxides are more
susceptible to cyclic carbonate formation than their aromatic
counterparts through chain backbiting22,52,53 however it has
previously been demonstrated that CC is capable of ligating the
metal center in Zn-based systems and hindering the catalyst
turnover.38 This “self-inhibition” by side product formation
would further explain the loss of activity observed with higher
reaction temperatures. In addition, increase of reaction
temperature may disfavor the coordination equilibrium of the
epoxide to the active catalytic center and the presence of the
relatively electropositive Co(III) ion is assumed to move the
position of this equilibrium further to the left. The combination
of these latter two effects clearly results in a reduced overall
catalytic activity of the polymerization systems at elevated
temperature.
AUTHOR INFORMATION
Corresponding Author
■
*Telephone +49 89 289 13571. Fax +49 89 289 13562. E-mail:
rieger@tum.de.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
We gratefully acknowledge the King Abdullah University of
Science and Technology (KAUST), Award No. UK-C0020,
́
KSA-C0069, for financial support. Ms. A. Jonovic is thanked for
technical support with TGA and GPC measurements. Mr. V.
Bretzler is acknowledged for helpful comments with the
preparation of this manuscript.
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