An efficient homogeneous chloro-aluminum-[N2O2] catalyst for the coupling of epoxides with carbon dioxide

Article abstract of DOI:10.1002/ejic.201300634

The coupling reaction of epoxides and CO₂ is in the focus of many research activities in academia as well as in the industry. In this contribution we report on a versatile aluminum-based homogeneous catalyst utilizing an easy-to-synthesize N

Full text of DOI:10.1002/ejic.201300634

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DOI:10.1002/ejic.201300634
An Efficient Homogeneous Chloro–Aluminum–[N2O2]
Catalyst for the Coupling of Epoxides with Carbon
Dioxide
Monica A. Fuchs,[a] Christiane Altesleben,[a] Thomas A. Zevaco,*[a]
and Eckhard Dinjus[a]
Keywords: Carbon dioxide fixation / Homogeneous catalysis / Aluminum / Copolymerization / N,O ligands
The coupling reaction of epoxides and CO2 is in the focus of
many research activities in academia as well as in the indus-
try. In this contribution we report on a versatile aluminum-
based homogeneous catalyst utilizing an easy-to-synthesize
N2O2 ligand system in combination with various cocatalysts,
all of them producing pure alternating poly(cyclohexene
carbonate)s with Mn values in the range 10000–15000 gmol–1.
This system also efficiently catalyzes the formation of propyl-
ene carbonate.
Introduction
Carbon dioxide is, from many points of view, an ideal
C1-building block and solvent because of its remarkable
characteristics, environmental innocuity, and sheer endless
availability.[1] Finding catalysts that are capable of activa-
ting the thermodynamically stable CO2 and making it a use-
ful starting material for valuable products is one of the cur-
rent challenges and the key to many “dream reactions”. For
a long time, the focus on CO2 chemistry was limited to a
few reactions that could be implemented in industrial pro-
cesses like, for example, the production of precipitated cal-
cium carbonate as an important functional additive (e.g.,
for the paper industry and polymers), the syntheses of urea,
salicylic acid, or, indirectly, that of methanol.[1] However,
since the 1990s a new trend dealing with the production of
organic carbonates has been rising: principally monomeric
cyclic carbonates (CC) and aliphatic polycarbonates (aPC)
obtained from the related reactive epoxides (Scheme 1).
Cyclic carbonates find numerous applications as, for ex-
ample, high-boiling solvents or electrolytes.[2] Aliphatic
poly(ether carbonates) are a useful complement to the al-
ready massively used aromatic, bisphenol-A-based polycar-
bonates because of their higher biodegradability (polypro-
pylene carbonate) and; they already find increasing use in
many technical applications as evaporative pattern castings,
ceramic binders, or mid-segments for polyurethanes.[3]
Scheme 1. Possible products of the reaction of carbon dioxide with
epoxides.
Among the reported catalysts, metal complexes based on
transition metals like chromium and cobalt seem to be the
most active; however, some catalysts using more “biocom-
patible”, abundant, and hence cheap, main group metals
are gaining an increasing interest.[4,5] For instance homo-
geneous aluminum complexes are known to catalyze pre-
dominantly the formation of cyclic carbonates.[5–8] Some of
them have even been immobilized on inorganic matrices
and successfully recycled.[9–12] Nevertheless, there is a sig-
nificantly smaller range of aluminum complexes known to
catalyze effectively the copolymerization of epoxides and
CO2. These complexes have to be divided into those effec-
tive for the “standard” test substrate propylene oxide
(PO)[13–15] and those active for cyclohexene oxide
(CHO).[15–19] Recently, a fine review giving a résumé on
these aluminum catalysts has been published by F. M. Ker-
ton et al.[20]
We already reported on the capability of an iron–N2O2
ligand system to produce quantitative yields of the corre-
sponding cyclic carbonates from different epoxides.[21] Now
we use the same ligand system with an organometallic alu-
minum precursor to get a highly active aluminum catalyst
giving excellent results in the copolymerization of cyclohex-
ene oxide.
[a] Institute of Catalysis Research and Technology,
Karlsruhe Institute of Technology (KIT)
Hermann-von-Helmholtz-Platz 1,
76344 Eggenstein-Leopoldshafen, Germany
E-mail: thomas.zevaco@kit.edu
eckhard.dinjus@kit.edu
Homepage: www.kit.edu
Supporting information for this article is available on the
WWW under http://dx.doi.org/10.1002/ejic.201300634.
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try but also to a trigonal bipyramidal one.[25] However, in
many studies the contribution of the background 27Al
NMR spectrum is underestimated, casting some doubt on
the assignment of the broad signals around 50–70 ppm.
Results and Discussion
Synthesis
The ligand employed in our catalyst system is obtained
from the reaction of a diamine linker with diethyl ethoxy-
methylenemalonate. It was reported for the first time as li-
gand by the group of E. G. Jäger in the 1980s,[22a,22b] being
principally the product of the reaction of nucleophiles with
esters obtained by a Claisen condensation.[22c,22d]
The further reaction with the aluminum precursor was
realized in anhydrous tetrahydrofuran as
(Scheme 2). Once the ligand was dissolved in the solvent,
aluminum diethylchloride dissolved in THF was added
dropwise.
Screening the Catalytic Activity
Catalytic screening was done on a test bench with eight
75 mL autoclaves equipped with a PC-connected Agilent
multimeter for pressure and temperature data acquisition.
Cyclohexene oxide and propylene oxide are the standard
test substrates for the coupling reaction with CO2.
Table 1 shows firstly the blind tests with different catalyst
components alone and with the utilization of different cata-
lyst/cocatalyst combinations. In all cases, the catalyst and
the cocatalyst were used in an equimolar ratio. In order to
obtain reasonable polymer analytics and to rationally com-
pare the different combinations, it is principally important
to isolate definite, long-chained polycarbonate epoxide/CO2
copolymers. This is done according to the literature by
using the usual dichloromethane/methanol precipitation.[26]
However, the precipitation step is not an “exact science”
and depends on the performance of the catalyst and its abil-
ity to produce long-chain copolymers under the actual ex-
a solvent
Scheme 2. Synthesis of aluminum complex 2.
The successful reaction can be observed by the evolution perimental conditions. In our experiments we found some
of ethane and a color change of the reaction mixture from variation (10–15%) in the reproducibility of the isolated
colorless to bright yellow. In the NMR spectrum, the disap- yields; however, the general trends summarized in Table 1
pearing of the NH protons and a shift of all peaks relative remain valid. The values reported therein are the best values
to the spectrum of the ligand confirmed the formation of we gathered by using the experimental set-up at hand.
the aluminum species. The spectra also reveal that one mo-
Entry 1 shows that, under the chosen experimental con-
lecule of THF is bound to the aluminum–N2O2 complex. ditions, the epoxide and CO2 are not able to copolymerize
Comparison of the FTIR spectra of ligand 1 and complex without a catalyst. Cocatalysts TBAI, TBAB, and TBAC
2 (KBr pellets) shows the weakening of half of the ν(C=O) alone (Entries 2, 3, 4) lead to small amounts of monomeric
absorption bands for the ester moieties in compound 2, cyclohexene carbonate, whereas aluminum complex 2 pro-
which also evidences the coordination of the aluminum cen- duces a mixture containing cyclic carbonates, short-chained
ter to the ester groups.[23] At 618 cm–1 a band potentially polycarbonates, and polyethers in a very small overall yield
assigned to the aluminum–chloride vibration, is also ob- (Entry 5).
servable.[24] The resulting aluminum complexes are rela-
Using an equimolar mixture of catalyst 2 and different
tively stable solids, although storage for longer periods of cocatalysts yields poly(cyclohexene carbonate)s in very high
time should be under an inert atmosphere. 27Al NMR spec- yields with an efficient insertion of carbon dioxide into the
tra give the opportunity to evaluate the coordination geo- copolymer. The aluminum–N2O2/ammonium halide system
metry and the overall symmetry around the aluminum delivers actually pure alternating poly(cyclohexene carbon-
atom. An upfield shift of the peaks is often associated in ate), even if only atactic according to the 13C NMR spectro-
the literature to a growing coordination number, whereas scopic data (see Supporting Information). Comparing the
the narrowing of the signal is connected to a growing sym- effect of the cocatalysts, for example, the halide anions iod-
metry of the complex. The 27Al NMR spectrum of 2 ide, bromide, and chloride (TBAI, TBAB, and TBAC) (En-
showed, at different temperatures (ranging from –40 to tries 6–8), shows that the best results were obtained with
+40 °C), a broad signal around 48 ppm with a clear shoul- TBAB (Entry 7). The variation between the halides remains
der at about 7 ppm. With reference to the literature and by nevertheless in a narrow range. This gives a hint that the
taking into account the background 27Al NMR spectrum halide ion is not a preponderant factor in the mechanism.
of the probe head (broad signal around 60 ppm), the pres- The more sterically hindered bis(triphenylphosphoranylid-
ence of two aluminum species possibly involved in an equi- ene)ammonium cation with chloride as counterion (short:
librium, one pentacoordinate and one octahedral, hexaco- PPNCl) was reported by Darensbourg to be more active
ordinate species with coordinated THF can be inferred.
than the related ammonium salts.[27] In our case, its utiliza-
Aluminum salens are known, for instance, to display 27Al tion leads to a reactivity (yields and amount of CO3 pro-
NMR signals around 70 ppm, which principally can be at- duced) comparable to that obtained with TBAC (Entries 8
tributed to an expected tetragonal square pyramidal geome- and 9). To complement the ionic cocatalysts, we also tested
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Table 1. Catalytic screening of catalyst 2 in the copolymerization of cyclohexene oxide and CO2.
[a] Catalyst/cocatalyst = 1:1; TBAI = tetrabutylammonium iodide; TBAB = tetrabutylammonium bromide; TBAC = tetrabutylammonium
chloride; PPNCl = bis(triphenylphosphane)iminium chloride; DMAP = 4-dimethylaminopyridine. [b] Determined by 1H NMR spec-
troscopy as follows: 100*[Int@4.8 ppm/(Int@4.8 ppm + Int@3.5 ppm)]. [c] Determined on precipitated polymer: 100% yield being equiva-
lent to 14.05 g copolymer, which means a complete conversion of 0.099 mol of CHO (10 mL) into pure alternating PCHC. [d] Determined
by gel permeation chromatography (GPC) calibrated with polystyrene standard in THF at 40 °C. [e] Measured with differential scanning
calorimetry (DSC). [f] Screening was done with dichloromethane as cosolvent (CHO/DCM = 1:1).
the effect of neutral bases like, for example, 4-dimethyl- except for the catalyst concentration had a negative in-
aminopyridine (DMAP, Entry 10). This cocatalyst leads to fluence on the Mn values, which sank under 10000 gmol–1.
lower polycarbonate yields, probably because of the forma- In order to evaluate the practicability of the catalytic sys-
tion of a stable aluminum–N2O2–DMAP complex, as found tem, some experiments were performed under very low CO2
earlier in the case of iron–N2O2–pyridine and iron-N2O2– pressure, at about 2 bar (Entry 15). Concurrently raising the
MeIm (1-methyl imidazole) derivatives.[21,28] The formation catalyst concentration and the reaction time made it pos-
of an Al–N2O2–DMAP complex would therefore, on the sible to reach a 29% yield of a fully alternating polycarbon-
one hand, compete unfavorably with the coordination of ate. The Mn of the resulting, precipitated polycarbonate was
the epoxide at the aluminum center and, on the other hand, around 3000 gmol–1 with a good polydispersity index (PDI)
“trap” DMAP, which is also able to activate carbon dioxide of 1.4 and a somewhat lower Tg of 94 °C. The fact that,
as transient zwitterionic carbamic acid derivative.[27]
despite a very low CO2 pressure, we still have pure alternat-
As reported above, the copolymers show a pure alternat- ing polycarbonate with no polyether regions indicates that
ing polycarbonate backbone, displaying glass transition the N2O2 ligand succeeds in tuning the Lewis acidity of the
temperatures (Tg) over 100 °C (as observed in DSC curves) metal center. To the best of our knowledge, complex 2 is
and an average molecular weight (Mn) above 10000 gmol–1. the only aluminum–N2O2 system that was tested at low
The isolated copolymers displayed average molecular pressures of 2 bar and showed a conversion to fully alter-
weights and polydispersity indexes rivaling the best systems nating polycarbonates, although not as successful as the
reported for CHO like, for example, the aluminum salens FeIII–, CoII– and ZnII–N4O2 macrocyclic catalysts reported
and β-ketoiminates reported by Sugimoto and co- by C. K. Williams and co-workers.[30]
workers.[16,29] The average molecular weights are in our case
The GPC traces recorded for copolymers, obtained un-
higher than those reported in the newest system of Sugi- der higher CO2 pressure, displayed a common bimodal dis-
moto (around 15000 vs. 7000 gmol) with also a tad higher tribution probably due to the presence of traces of water
polydispersity indexes (PDI ≈ 1.5 vs. 1.2). Further studies and consequently of cyclohexane-1,2 diol acting as chain-
were performed, investigating the influence of reaction tem- transfer reagent.[30d] Interestingly, the trace recorded for the
perature, catalyst concentration, solvent, and pressure. A experiment run at 2 bar CO2 displays a unimodal distribu-
higher reaction temperature (100 °C) had a negative impact, tion (see Supporting Information).
leading to lower polycarbonate yields, shorter polymer
Aluminum complex 2 was also tested as a catalyst for
chains with Mn values around 7000 gmol–1, a higher poly- propylene oxide. These tests revealed that it is a good, selec-
dispersity, and a lower Tg (Entry 11). A lower catalyst con- tive catalyst for the production of propylene carbonate, as
centration also lowers the polycarbonate yield to only 66% was the case for the related iron–N2O2/pyridine deriva-
(Entry 12). A shorter reaction time of 10 h reduced the yield tive.[21] The addition of a cocatalyst is, however, mandatory
to 46%, showing that a longer reaction time is definitely to obtain satisfying results.
needed to get high polycarbonate yields (Entry 13). Dilut-
The results are summarized in Table 2. We compared the
ing the reaction mixture with dichloromethane as a cosol- three different tetrabutylammonium cocatalysts TBAI,
vent also resulted in a reduced yield of only 32% (Entry TBAB, and TBAC and found the cocatalysts to have some
14). Altogether all the changes in the reaction conditions catalytic activity on their own: the iodide shows the highest
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Mettler Toledo DSC822e instrument. For inductively coupled
plasma atomic emission spectroscopy (ICP-OES), a 720/725-ES
emission spectrometer with a CCD detector from Agilent Technol-
ogies was used. The plasma was generated with a 40 MHz quartz-
controlled generator with argon as the carrier gas.
activity with 11% cyclic carbonate in the blind tests (Entries
1–3). Catalyst 2 in combination with the bromide cocata-
lyst, TBAB, delivered the best yields with 94% propylene
carbonate (Entry 5). The other two cocatalysts used in com-
bination with 2 show a little lower efficiency, lying however
in a narrow range (Entry 4, Entry 6).
The starting materials [o-phenylene diamine, diethyl (ethoxymeth-
ylene)malonate (99%), methanol, dichloromethane, dry dichloro-
methane, dry tetrahydrofuran, epoxides, aluminum precursor
Al(Et)2Cl, and the cocatalysts] were purchased from Sigma Ald-
rich. Except for the epoxides, all of them were used without further
purification. The epoxides were distilled from CaH2 and stored un-
der an argon atmosphere before being used in the catalytic tests.
Ligand 1 was synthesized according to a procedure established by
E. G. Jäger et al.[22] Standard Schlenk techniques were applied in
the synthesis and further utilization of aluminum complex 2. Com-
plex 2 should be stored under an inert atmosphere. For the general
procedure used in the catalytic screening tests, see the Supporting
Information.
Table 2. Conversion of propylene oxide and CO2 to propylene
carbonate.[a]
Entry
Catalyst
Cocatalyst
Yield
TON
1
2
3
4
5
6
–
–
–
2
2
2
TBAI
TBAB
TBAC
TBAI
TBAB
TBAC
11
5
6
91
94
85
55
25
30
455
468
425
[a] t = 20 h, T = 80 °C, p = 50 bar; 0.2 mol-% cat., cat./cocat. =
1:1; TBAI = tetrabutylammonium iodide, TBAB = tetrabutyl-
ammonium bromide TBAC = tetrabutylammonium chloride.
Compound 2: Ligand 1 (3.00 g, 6.7 mmol) was dissolved in dry
THF (40 mL) under an argon atmosphere. Then, aluminum dieth-
ylchloride (0.81 g, 0.84 mL, 6.7 mmol) was dissolved in THF
(20 mL) and added dropwise to the ligand solution. Little gas evol-
ution was observed. The colorless reaction mixture started to
change color into a light yellow. Then the mixture was heated at
reflux overnight to yield a deep yellow solution. After removing
the solvents, a deep yellow solid was obtained (3.91 g, 6.7 mmol,
100%). IR (KBr): 1713, 1675 [νas(C=O)Al-bound], 1607, 1585
[νas(C=O)free and ν(C=N)], 1437 [ν(C=C)], 1280, 1261 [νs (C=O)],
Conclusions
We investigated the catalytic possibilities of a new alumi-
num complex involving a ligand with a N2O2 chelating
pattern (amine and ester functions), less common than the
widespread salen ligands (with phenol and aldimine func-
tionalities). This compound catalyzes very efficiently the re- 1094 [ν(C–O)], 797, 748 [δ (C–H)], 618 [ν(Al–Cl)] cm–1. MS (ESI+):
m/z (%) = 491 (100) [M – THF – Cl + H2O]+, 1005 (57) [2(M –
action of epoxides with carbon dioxide. The use of an ionic
cocatalyst is mandatory to get the most out of the system,
even though the nature of the halide anion seems to have
no direct effect on the efficiency of the catalytic system.
THF – Cl + H2O) + Na]+. 1H NMR (400 MHz, CD2Cl2): δ = 1.33
(t, J = 7.1 Hz, 6 H, CH3), 1.46 (t, J = 7 Hz, 6 H, CH3), 1.78 [m, 4
H, CH2(THF)], 3.63 [t, J = 6.1 Hz, 4 H, O–CH2(THF)], 4.24 (m,
4 H, CH2), 4.51 (m, 4 H, CH2), 7.23 (dd, J = 3.3, 6.0 Hz, 2 H,
CHaromat), 7.51 (m, 2 H, CHaromat), 9.15 (s, 2 H, N–CH=C) ppm.
Depending on the test substrates, polycarbonates or cyclic
carbonates can be selectively isolated in very high yields.
The poly(cyclohexene carbonate)s obtained from cyclohex-
ene oxide are fully alternating, displaying Mn values above
10000 gmol–1, with low PDI values and Tg values over
13C NMR (100 MHz, CD2Cl2): δ = 14.51 (CH3), 14.59 (CH3),
25.78 [CH2(THF)], 60.44 (CH2), 64.22(CH2), 68.48 [OCH2(THF)],
90.73 [=Cq(COOEt)2], 114.60 (CHarom), 126.13 (CHaromat), 137.06
(Cq,aromat), 159.86 (CH), 165.33 (C=O), 172.56(C=O) ppm. 27Al
100 °C. Most interestingly, even under a pressure of 2 bars (100 MHz, CD2Cl2): δ = 47.77, 7.07 ppm. ICP-OES: calcd. for
C22H26AlClN2O8: Al 5.30; found 5.29. We performed many elemen-
tal analyses unfortunately with diverging results. This might be ex-
plained by a high moisture sensitivity, although we observed the same
1H NMR spectra from complexes stored under argon or kept for a
while under a normal atmosphere. Another tentative explanation
might be the formation of transient, refractory aluminum species in-
fluencing the complete conversion of C,H,N to CO2, H2O, and NOx.
of CO2, the screening tests lead to fully alternating poly-
carbonates with 29% yield. In comparison, utilizing propyl-
ene oxide as substrate leads selectively to propylene carbon-
ate with yields up to 94%.
Experimental Section
Supporting Information (see footnote on the first page of this arti-
cle): Procedure of the catalytic screening tests and spectroscopic
data.
General
The 1H and 13C NMR spectra were recorded at 399.91 and
100.56 MHz, respectively, by means of a Varian Inova Unity 400
spectrometer (software VNMRJ 3.2) equipped with an Oxford
Magnet (9.4 T). IR spectra were obtained with a Varian 660-IR
FTIR spectrometer. Mass spectra were measured with a Bruker
ApexQe hybrid 9.4 T FT-ICR. Gel permeation chromatography
(GPC) was performed with Polymer Standard Service (PSS) SDV
(5 μ, 1000 Å) and SDV (5 μ, 100 Å) columns. The molecules were
detected with a RI Detector L-7490 (Merck). Tetrahydrofuran was
used as eluent, and toluene was used as an internal standard. Data
processing was performed with the software PSS WINGPC 6. For
the determination of the glass transition temperature of the poly-
carbonates, differential scanning calorimetry was performed with a
Acknowledgments
We authors would like to thank the Helmholtz Research School
“Energy-related catalysis” for financial support as well as the Bun-
desministeriums für Bildung und Forschung (BMBF)-Project
“Dream Reactions
– stoffliche CO2-Verwertung” (Förderk-
ennzeichen 01RC0901A).
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Received: May 16, 2013
Published Online: ᭿
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Job/Unit: I30634
/KAP1
Date: 07-08-13 16:23:48
Pages: 6
www.eurjic.org
SHORT COMMUNICATION
Aluminum Catalysts
M. A. Fuchs, C. Altesleben, T. A. Zevaco,*
E. Dinjus ........................................... 1–6
A new homogeneous aluminum catalyst
based on a N2O2 ligand obtained from di-
ethyl ethoxymethylenemalonate was used
in the conversion of epoxides with carbon
dioxide. Copolymerization with cyclohex-
ene oxide led to alternating polycarbonates
in the presence of a cocatalyst even at only
2 bar CO2 pressure. Propylene oxide af-
forded selectively the corresponding cyclic
carbonate in high yields.
An Efficient Homogeneous Chloro–Alumi-
num–[N2O2] Catalyst for the Coupling of
Epoxides with Carbon Dioxide
Keywords: Carbon dioxide fixation
/
Homogeneous catalysis / Aluminum / Co-
polymerization / N,O ligands
Eur. J. Inorg. Chem. 0000, 0–0
6
© 0000 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim