Flexibly tethered dinuclear zinc complexes: A solution to the entropy problem in CO2/epoxide copolymerization catalysis?

Article abstract of DOI:10.1002/anie.201302157

Working in zinc: Kinetic investigations of a new zinc catalyst (see scheme) for the copolymerization of CO₂ and epoxides show a reversible shift in the rate-determining step from epoxide ring opening to CO₂ insertion, depending on the applied pressure. This shift is attributed to the similarity of the activation barriers and explains the exceptionally high activities for the copolymerization. Copyright

Full text of DOI:10.1002/anie.201302157

Angewandte
Chemie
DOI: 10.1002/anie.201302157
Copolymerization
Flexibly Tethered Dinuclear Zinc Complexes: A Solution to the
Entropy Problem in CO2/Epoxide Copolymerization Catalysis?**
Maximilian W. Lehenmeier, Stefan Kissling, Peter T. Altenbuchner, Christian Bruckmeier,
Peter Deglmann, Anna-Katharina Brym, and Bernhard Rieger*
World-wide research activities over the last decade have
demonstrated that carbon dioxide is a valuable carbon source
for a series of chemical products.[1–8] One very promising class
of products is aliphatic polycarbonates, synthesized by the
reaction of carbon dioxide and epoxides. These polymers
show interesting properties, as they are biodegradable, highly
transparent, UV stable, and have a high Youngꢀs modulus.[9–12]
Latest research even led to isotactic poly(cyclohexene
carbonates), opening a route to semi-crystalline thermoplastic
materials.[13]
Despite the fact that aliphatic polycarbonates were
already introduced 40 years ago by Inoue et al., the develop-
ment of an economic production process is still hampered by
the availability of efficient catalysts. The main focus of
numerous publications in this field was based on
a dinuclear catalysts, which accommodate the
epoxide ring opening and a CO2 insertion in
a single molecule.[8,10,12,14–21] Zinc as a catalytically
active species has tremendous advantages com-
pared to other transition metals. It is an eco-
nomic, ecofriendly element, and its ions are
colorless. For that reason since 2003 a variety of
dinuclear zinc catalysts has been developed.
However, most of the reported structures suffer
from poor solubility or low polymerization activ-
ity.[16,22–30]
step is the ring opening of the epoxide.[21] Williams et al. also
reported the incorporation of the epoxide as rate determining
for their rigid dinuclear zinc catalysts.[20] This implies that at
typical process conditions an increase in CO2 concentration
does not automatically enhance polymerization activity.
The concept presented herein also focuses on dinuclear
complexes. However, two BDI zinc units are connected by
a flexible tether. This helps to overcome the entropically
disfavored aggregation of two individual complex molecules
in dilute solutions in a way that the rate-determining step is
even shifted from ring opening to CO2 insertion.
The new macrocyclic ligand 1 (Scheme 1) was prepared in
a two-step synthesis in which each step involves the con-
densation of 4,4-diaminodiphenylmethane with acetylace-
One major breakthrough aside from zinc
phenoxides developed by Darensbourg is the
synthesis of b-diketiminato (BDI) zinc com-
plexes by Coates et al.[21,31–41] For these highly
active catalysts the mechanism involving a dime-
tallic intermediate was unambiguously demon- Scheme 1. Synthesis of the dinuclear catalyst 2. TMS=trimethylsilyl, TsOH=4-methyl
benzenesulfonic acid.
strated. Kinetic investigations using in situ IR
spectroscopy showed that the rate-determining
tone. A highly efficient synthesis of the desired complex 2 was
[*] M. W. Lehenmeier, S. Kissling, P. T. Altenbuchner, C. Bruckmeier,
carried out in toluene with bis[bis(trimethylsilyl)amido]zinc
at 908C.
Dr. A.-K. Brym, Prof. Dr. B. Rieger
Technische Universitꢀt Mꢁnchen
WACKER Lehrstuhl fꢁr Makromolekulare Chemie
Lichtenbergstrasse 4, 85748 Garching (Germany)
E-mail: rieger@tum.de
The dinuclear character of the complex was confirmed by
elemental analysis and NMR spectroscopy. The 1H NMR
spectrum shows broadened resonance signals for the trime-
thylsilyl groups at 258C. Upon heating the sample to 808C the
broad signals coalescence, which we attribute to a hindered
rotation of the N(TMS)2 groups caused by steric repulsion.
The other resonance signals of the complex are narrow and
confirm the symmetric configuration of the molecule (Fig-
ure S1 in the Supporting Information). The catalyst was tested
in the copolymerization of cyclohexene oxide and carbon
dioxide under various reaction conditions.
Homepage: http://www.makro.ch.tum.de
Dr. P. Deglmann
BASF SE, GM
B001, 67056 Ludwigshafen (Germany)
[**] The financial support of the BMBF (grant 01RC0902A) is gratefully
acknowledged. We thank Thomas Neubauer and Sergej Vagin for
discussion and Carly Anderson for proofreading the manuscript.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201302157.
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For a detailed understanding of this catalyst system kinetic
studies were performed with online ATR (attenuated total
reflection) IR measurements. For these measurements it is
important to confirm that the reaction mixture is homoge-
neous and the IR signal increases linearly with increasing
amount of polycarbonate.[7,8] The homogeneity of the reaction
mixture in the IR autoclave was demonstrated by an experi-
ment without stirring. Therefore, covering of the probe by
precipitation of copolymer during the reaction can be
excluded (Figure S5).[42] The IR signal grows linearly with
increasing amount of polycarbonate. The kinetic order with
respect to catalyst was obtained by performing the copoly-
merization at different catalyst concentrations. By plotting
the initial rate against the catalyst concentration in a double
logarithmic scale the first-order dependence in catalyst is
obtained (Figure 1).
Figure 2. Rate dependence on carbon dioxide pressure.
Figure 3. Order with respect to cyclohexene oxide.
Figure 1. Determination of the order with respect to catalyst.
catalyst. The groundbreaking work of Coates et al. showed
that two mononuclear complexes and consequently two metal
centers are necessary for chain propagation.[21] Therefore, the
first order dependence on catalyst is in accordance to the
dinuclearity of the described catalyst. Kinetic measurements
on other zinc catalysts show that the rate-determining step for
the copolymerization is the ring opening of the epoxide.[20,21]
Thus, herein the first zinc-based epoxide/CO2 copolymeriza-
tion catalyst is reported which shifts the rate-determining step
from ring opening of the epoxide to carbon dioxide insertion
at technically relevant CO2 pressures (Scheme 2 and Fig-
ure S9). Considering the first order dependence with respect
to epoxide of all the other catalysts, this suggests that the
flexible tether between the two zinc centers accelerates the
cooperative ring-opening step in an unprecedented manner
and to an extent that a pressure depenency of the rate
equations occurs. This assumption is strongly supported by
the synthesis of a similar complex 3 without such a flexible
tether which exhibits very low activities for this copolymer-
ization under identical conditions (Table S5).
This shift in the rate-determining step and the involve-
ment of two cooperating zinc centers is further supported by
quantum chemical computations, which were performed to
rationalize the above findings. Calculations were performed
at the B3-LYP/def2-TZVP jj BP86/SV(P) level of theory,
involving enthalpic and entropic corrections and a treatment
of solvation by COSMO-RS as described elsewhere.[43] As
solvent, toluene was used and a temperature of 1008C as well
as CO2 pressures of 5 and 50 bar. The motivation for the
method applied and details on enthalpic and entropic
With this confirmation of the expected catalyst concen-
tration dependence, kinetic investigations concerning the CO2
pressure and the cyclohexene oxide concentration were
undertaken. Measurements for different pressures were
carried out in the same manner as the assignment of the
order with respect to catalyst. At first, the CO2 uptake in the
cyclohexene oxide/toluene mixtures was studied, and a clearly
linear increase of the IR signal with increasing pressure was
found (Figure S8). With constant catalyst and cyclohexene
oxide concentration the order with respect to carbon dioxide
was determined to be one in the range of 5–25 bar CO2
pressure. For 25–45 bar the order with respect to CO2
concentration changes surprisingly from one to zero
(Figure 2).
The assignment of the order with respect to cyclohexene
oxide was performed at two different pressures 10 and 30 bar.
The determination of the order with respect to epoxide at
10 bar reveals a value of zero and a reaction order of one at
30 bar (Figure 3).
These results lead to the rate laws given in Equations (1)
and (2) where CHO = cyclohexene oxide:
0
1
1
ð1Þ
ð2Þ
r ¼ k ½CHOꢀ ½CO2ꢀ ½Catalystꢀ
5ꢁ25 bar CO2
25ꢁ45 bar CO2
1
0
1
r ¼ k ½CHOꢀ ½CO2ꢀ ½Catalystꢀ
The different aspects of the rate laws are discussed in the
following parts, beginning with the first order with respect to
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correction terms can be found in the Supporting
Information. As starting point of the catalytic
cycle, a k2,k2-dicarbonato complex was chosen,
assuming that catalyst activation can and will
readily take place at both sides of the plane made
up by the ligand and the two-coordinated zinc
atoms. From this species, the reaction is expected
to proceed by coordination of epoxide, epoxide
ring opening to form an alkoxide, and subsequent
addition of this coordinated alkoxide to CO2 (i.e.
CO2 insertion). Computed results for this reactive
pathway are summarized in Figure 4. The catalytic
cycle given does not contain all the intermediates
involved in the reaction mechanism; however,
further intermediates, for example, from dissocia-
tive ligand exchange were found to be not relevant
for overall reaction kinetics (Figure S15). From
the dicarbonato starting point I, creation of a free
coordination site and epoxide coordination, lead-
ing to II, is exothermic, but not favorable with
respect to Gibbs free energy for entropic reasons.
The subsequent epoxide ring-opening transition
state TS(a) is around 50 kJmolꢁ1 higher in
enthalpy H than I, which should correspond
Scheme 2. Proposed mechanism for the copolymerization with the pressure depend-
ence of the rate determining step.
Figure 4. Enthalpy and Gibbs free energy profile for copolymerization catalyzed by 2 at 1008C and a CO2 pressures of 5 and 50 bar.
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more or less to Ea in case of an Arrhenius evaluation of
temperature dependent reaction rates, whereas the overall
Gibbs free energy G to overcome for epoxide ring opening is
around 100 kJmolꢁ1. The immediately resulting bridged
alkoxide species III is computed to be about as stable as II.
Another rather stable alkoxide species (see Supporting
Information) which has a m2-oxo bridge could be identified,
but needs no further consideration owing to a higher relative
G if compared to I and III. Addition of CO2 to III leads to IV,
which by a nucleophilic attack of the alkoxide at the carbonyl
carbon atom, TS(b), regenerates the propagated carbonato
species I + 1. Altogether, all the alkoxide species are signifi-
cantly higher in G than I, therefore it is concluded that,
independent of the rate-limiting step, the resting state is this
dicarbonato species. Thus, for the incorporation of CO2, the
effective activation barriers have to be calculated with respect
to I, this gives a computed H which suggests an Ea of around
20 kJmolꢁ1, whereas the relative barrier with respect to G is
again around 100 kJmolꢁ1 and depends on the applied CO2
pressure. This striking similarity in the computed G value of
both potentially highest lying transition states means that
both transition states are similarly difficult to overcome
(considering potential sources of error from the quantum
chemical energy method as well as the solvation treatment),
which is in line with the observed change in rate-limiting step
upon variation of the CO2 pressure: Whereas in the first part
of the catalytic cycle CO2 is not involved (TS(a)), increasing
or decreasing the CO2 pressure will shift the second (TS(b))
part downwards or upward, respectively. For high CO2
pressures as high as the 50 bar considered in Figure 4, the
epoxide ring opening TS(a) with its Ga of 101.0 kJmolꢁ1 is the
highest point of the G profile compared to the Ga of
95.3 kJmolꢁ1 for the CO2 insertion TS(b); at lower CO2
pressure (e.g. 5 bar), alkoxides are formed in a pre-equilib-
rium and undergo the rate-limiting CO2 insertion, with for
example, a Ga of 102.4 kJmolꢁ1 for TS(b) (Figure 4). The
point where TS(b) is identical to the Ga of 101.0 kJmolꢁ1 of
TS(a) is computed to be at a CO2 pressure of around 8 bar,
which is somewhat below the experimental 25 bar where both
elementary reactions are “equally rate limiting”; at 25 bar,
a Ga of 97.4 kJmolꢁ1 is obtained for TS(b). This means
however still a remarkably good agreement between experi-
ment and theory considering typical errors of quantum
chemical calculations for larger molecules in solution.
species into the catalyst-bound polymer chain. On the other
hand, CO2 addition is effectively a trimolecular reaction,
which consumes both one liquid epoxide and one gaseous
CO2. Consequently, the Gibbs free energy G for epoxide ring
opening is higher than the corresponding enthalpy H (around
50 kJmolꢁ1), but the difference between G and H for CO2
insertion is even larger (around 80 kJmolꢁ1). This, of course
means that Arrhenius prefactors are significantly smaller for
CO2 insertion than for epoxide ring opening (for the
measured Ea, a ratio of around 5000 in prefactors would be
required at 1008C to obtain similar rate coefficients at
standard concentrations of all reactants).
The flexibility of the tether between the two b-diketimi-
nato zinc centers is also supported by the calculations:
Whereas for the bis[bis(trimethylsilyl)amido] precursor com-
plex a Zn–Zn distance of 7.77 ꢁ is computed, all the Zn–Zn
distances within the actual catalytic cycle are significantly
smaller but also exhibit a variation of more than 1 ꢁ (the
following distances are computed for the presented inter-
mediates and transition states: I 4.50 ꢁ, II 5.66 ꢁ, TS(a)
5.31 ꢁ, III 5.29 ꢁ, IV 5.40 ꢁ and TS(b) 4.92 ꢁ).
It can be assumed that the accessibility of such a large
range of Zn–Zn distances allows every transition state to
adopt optimal structures and thus avoids additional barriers
from catalyst rigidity, which means that the entropic benefit
from not having to group two b-diketiminato zinc units
together is fully exploited. The acceleration of the epoxide
ring-opening step leads to the high copolymerization activ-
ities of the catalyst (Table 1).
With decreasing catalyst loading the activity slightly
increases, which can be attributed to better diffusion
(Table 1, entries 1–3). The dependence of the activity on
CO2 pressure is also documented (entries 3–6). The results for
the selectivity for polycarbonate over cyclic carbonate are in
accordance with those in the literature.[12,21,39] The selectivity
increases with higher CO2 pressure. The catalyst shows an
unusual temperature dependence. The temperature of 1008C
seems to be the optimum with regard to polymerization
activity and selectivity. The decreased activity at 1208C could
be explained by catalyst decomposition and a higher proba-
bility for side reactions (Table 1, entry 7). This catalyst
exhibits a turnover frequency (TOF) of 9130 hꢁ1 at 1008C
and 40 bar CO2 pressure, a result of a good balance in
activation barriers for the two potentially rate-determining
steps.
Kinetic studies at various temperatures were performed to
further probe this mechanistic picture (Figure S16–S19).
Experimental activation energies at 10 and 30 bar are found
to be 8.8 and 35.9 kJmolꢁ1, respectively. The computationally
estimated values demonstrate the same trend. For low
pressure (5 bar) the CO2 insertion step is rate limiting with
an activation energy of 19.2 kJmolꢁ1, at higher pressure
(50 bar) the epoxide ring opening becomes rate limiting with
52.3 kJmolꢁ1. At a first glance, it appears surprising that
elementary steps with such dissimilar activation energies can
both be rate limiting. However, this is a consequence of Gibbs
free activation energies depending on both activation enthal-
pies and activation entropies. As the resting state is in all cases
the dicarbonato complex I, epoxide ring opening is a bimo-
lecular reaction, that is, the incorporation of one solution
In summary we report the first dinuclear zinc catalyst
which shows a shift in the rate-determining step from ring
opening of the epoxide to carbon dioxide insertion for the
copolymerization of cyclohexene oxide and carbon dioxide.
The reason for this behavior is the similarity of the activation
barriers of the ring opening and insertion reactions, which is
achieved by linking the two active centers with a flexible
tether. These attributes lead to very high activities for the
copolymerization reaction. To achieve even higher activities,
according to our study, an additional decrease of the
activation energy of the ring-opening step is necessary. The
match between experimental and theoretical results delivers
a tool for further catalyst development. Currently, modifica-
tion of these catalysts is under investigation.
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[9] M. W. Lehenmeier, C. Bruckmeier,
Table 1: Copolymerization of carbon dioxide and cyclohexene oxide with catalyst 2.
S. Klaus, J. E. Dengler, P. Degl-
mann, A.-K. Ott, B. Rieger, Chem.
Eur. J. 2011, 17, 8858 – 8869.
T
[8C]
p(CO2)
[bar]
TON[b]
TOF[c]
% poly-
Conversion
[%]
carbonate[d]
1
2
3
4
5
6
7
8
9
100
100
100
100
100
100
120
80
10
10
10
5
20
40
10
14
14
10
628[e]
1649
1750[f]
1194
2529
4015
1190
1251[k]
9440[g]
127[h]
1255[e]
4946
99
98
98
97
>99
>99
92
62.8
41.2
21.9
29.9
63.2
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5249[f]
3581
7587
9130
1785
29.8
31
18.9
3.2
5003[k]
2860[g]
127[h]
99
80
100
79[g]
0[h]
10
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[a] Copolymerizations were conducted in a preheated Parr reactor for 20 min at a catalyst loading of
catalyst/epoxide of 1:4000 in a toluene/cyclohexene oxide mixture (1:1 v/v) (unless otherwise stated).
[b] The turnover number (TON) is calculated by the number of mole of consumed epoxide divided by the
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Experimental Section
The synthesis of the ligand and the complexes are described in the
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Catalyst 2 (24.0 mg, 0.025 mmol, 0.1 mol%) was dissolved in
cyclohexene oxide (2.5 mL, 24.8 mmol, 1.0 equiv). The autoclave was
pressurized with 10 bar carbon dioxide and heated to 1008C. The
reaction was terminated after 20 min by the addition of methanol
(1.0 mL) and teh reaction mixture dissolved in methylene chloride.
The organic phase was washed with HCl (1m) and dried with Na2SO4.
After evaporation of the solvent the poly(carbonate) was dried in an
oven to constant weight.
1H NMR (300 MHz, CDCl3, 292 K): d = 4.58 (s, 2H), 1.30–
2.04 ppm (m, 8H).
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Received: March 14, 2013
Revised: May 7, 2013
Published online: July 19, 2013
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Keywords: carbon dioxide · copolymerization ·
dinuclear complexes · epoxides · zinc
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