Reaction of CO2 with propylene oxide and styrene oxide catalyzed by a chromium(iii) amine-bis(phenolate) complex

Article abstract of DOI:10.1039/c2dt31942j

A diamine-bis(phenolate) chromium(iii) complex, {CrCl[O ₂NN′]^BuBu}₂ catalyzes the copolymerization of propylene oxide with carbon dioxide. The synthesis of this metal complex is straightforward and it can be obtained in high yields. This catalyst incorporates a tripodal amine-bis(phenolate) ligand, which differs from the salen or salan ligands typically used with Cr and Co complexes that have been employed as catalysts for the synthesis of such polycarbonates. The catalyst reported herein yields low molecular weight polymers with narrow polydispersities when the reaction is performed at room temperature. Performing the reaction at elevated temperatures causes the selective synthesis of propylene carbonate. The copolymerization activity for propylene oxide and carbon dioxide, as well as the coupling of carbon dioxide and styrene oxide to give styrene carbonate are presented.

Full text of DOI:10.1039/c2dt31942j

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Reaction of CO2 with propylene oxide and styrene
oxide catalyzed by a chromium(III) amine-bis-
Cite this: Dalton Trans., 2013, 42, 9233
(phenolate) complex†
Rebecca K. Dean,a Katalin Devaine-Pressing,a Louise N. Daweb and
Christopher M. Kozak*a
A diamine-bis(phenolate) chromium(III) complex, {CrCl[O2NN’]BuBu
} catalyzes the copolymerization of
2
propylene oxide with carbon dioxide. The synthesis of this metal complex is straightforward and it can be
obtained in high yields. This catalyst incorporates a tripodal amine-bis(phenolate) ligand, which differs
from the salen or salan ligands typically used with Cr and Co complexes that have been employed as cata-
lysts for the synthesis of such polycarbonates. The catalyst reported herein yields low molecular weight
polymers with narrow polydispersities when the reaction is performed at room temperature. Performing
the reaction at elevated temperatures causes the selective synthesis of propylene carbonate. The copoly-
merization activity for propylene oxide and carbon dioxide, as well as the coupling of carbon dioxide and
styrene oxide to give styrene carbonate are presented.
Received 24th August 2012,
Accepted 6th November 2012
DOI: 10.1039/c2dt31942j
www.rsc.org/dalton
for epoxide/CO2 copolymerization includes the phenoxides,
where Zn is commonly the metal used.61–63 Coates and co-
Introduction
The copolymerization of epoxides and carbon dioxide to yield workers subsequently explored the use of β-diiminate (BDI)
polycarbonates has garnered growing interest as a process for ligand systems with Zn at lower pressures and temperatures
generating potentially valuable materials from renewable start- than had previously been reported.13 The most widely studied
ing materials.1–6 Due to its widespread availability, low cost, ligands for epoxide/CO2 copolymerization have been the
and nontoxicity carbon dioxide is appealing as a C1 feedstock7 salen3–5 and, more recently, the salan ligands, which have
and many homogeneous catalysts have been examined for use been primarily used with Cr22,24–29,32,35,37,64 and
in the copolymerization of CO2 and epoxides. These catalysts Co.38,42,43,50,51,65 Nucleophilic co-catalysts are typically required
can employ a variety of metal sites, such as Al,8–12 Zn,13–21 with the most commonly used being chlorides or azides paired
Cr,22–37 Co6,38–56 and most recently Fe.57 Most studies focus on with bulky cations such as PPN (PPN = bis(triphenylphosphor-
cyclohexene oxide (CHO) as the epoxide reagent with propy- anylidene)ammonium) or tetrabutylammonium, or neutral
lene oxide (PO) being much less studied. While PO has been a bases such as dimethylaminopyridine (DMAP) or N-methyl-
problematic reagent because of the propensity to form cyclic imidazole (N-MeIm). Due to their easily modifiable steric and
propylene carbonate (PC), its use is still warranted given poly- electronic properties, these catalyst systems exhibit some of
propylene carbonate possesses desirable physical properties.58 the highest activities reported to date, such as Darensbourg’s
Previous reports of epoxide and CO2 copolymerization report of a Cr(salen) catalyst, which exhibits a TOF of 1153 h−1
include such ligand classes as the porphyrins, where Cr23,59,60 for the copolymerization of CHO and CO2.29 Cr-salan com-
and Al12 complexes have been used. Another established plexes also show high TOFs and under certain conditions rival
ligand system introduced by Darensbourg and Holtcamp used the activities of the well-defined Cr-salen analogs.32 Darens-
bourg recently reported the use of a CrCl(salan) complex
which copolymerized propylene oxide and CO2 with a TOF of
405 h−1 using 1 equivalent of [PPN]N3 as the co-catalyst at
aDepartment of Chemistry, Memorial University of Newfoundland, St. John’s,
60 °C and 34 bar CO2 for 4 h.34
Newfoundland, A1B 3X7, Canada. E-mail: ckozak@mun.ca; Tel: +1-709-864-8082
bC-CART X-ray Diffraction Laboratory, Department of Chemistry, Memorial
Detailed mechanistic studies of the copolymerization of
University of Newfoundland, St. John’s, Newfoundland, A1B 3X7, Canada
CO2 and epoxides at salen and salan complexes have recently
†Electronic supplementary information (ESI) available: Crystallographic data for
been carried out18,21,38,64,66 and some possible initiation path-
2 (cif), selected MALDI-TOF MS, TGA and DSC plots, and 1H NMR spectrum of
ways for a model salen complex are described in Fig. 1. Mono-
metallic initiation pathways can occur via intermolecular
coupling of CO2 with styrene oxide (pdf). CCDC 897967. For ESI and crystallo-
graphic data in CIF or other electronic format see DOI: 10.1039/c2dt31942j
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Fig. 1 Initiation pathways for a model complex containing a planar salen
ligand, a nucleophilic co-catalyst, Nu, and an anionic leaving group, X, which is
typically a halide, azide or acetate.
nucleophilic attack (for example, by an added neutral or
anionic nucleophilic co-catalyst, Nu) on a pre-coordinated
epoxide. This results in the co-catalyst nucleophile, Nu (such
as chloride, azide or DMAP) being incorporated into the chain
end-group of the growing polymer. Intramolecular ring
opening of the coordinated epoxide monomer by a nucleophile
may alternatively occur. The incoming nucleophile (Nu) binds
to the metal site increasing the leaving group ability of ligand
X, which would then become incorporated in the polymer
chain end group. As both Nu and X can serve as initiator
groups, either one may be observed in the chain end. However,
if the loss of the X group occurs rapidly (either during a cata-
lyst pre-activation step or as an equilibrium with epoxide
coordination), it would be possible for the X group to perform
an intermolecular ring opening of the coordinated epoxide.
Lastly, two metal complexes may follow an intermolecular
interaction, where an activated nucleophile (either X or Nu) on
one metal site may ring open an epoxide that is coordinated to
the metal site of a different complex.
Whereas compounds of the salen (and the related salan)
ligands represent the most investigated homogeneous systems
for CO2/epoxide copolymerization, the related tetradentate, tri-
podal amine-bis(phenolato) ligand class has yet to be investi-
gated for this reaction. These ligands can be easily synthesized
via a modified Mannich condensation reaction, and their
desired steric and electronic properties may be obtained
through changing the substituents on the phenolate groups or
pendant arms.67,68 We have studied these ligands in combi-
Fig. 2 Diamino-bis(phenolato) chromium chloride complexes used in this
study. R = t-Bu (1a), R = Me (1b) (left). Position of vacant site available for coordi-
nation of incoming nucleophilic co-catalyst, Nu, or epoxide relative to nucleophi-
lic ligand X. Phenolate donors may exhibit either cis or trans orientations (right).
Scheme 1 Copolymerization of PO and CO2.
metal complexes of tetradentate, tripodal amine-bis(pheno-
late) ligands, however, direct the incoming nucleophile or
monomer to bind cis to the X group and trans to either an
anionic phenolate donor or a neutral pendant donor (Fig. 2).
In this study, we report the use of Cr(III) amine-bis(phenolate)
complexes in the presence of ionic and neutral initiators as
catalysts for the coupling or copolymerization of propylene
oxide or styrene oxide with CO2.
Results and discussion
nation with various mid-to-late transition metals, including The synthesis and structure of 1a has been previously
Cr,69 Co70–72 and Fe,73–76 and they have shown a diverse range reported77 and 1b can be obtained a similar route, which is
of structural, spectroscopic and electronic properties.
reported here. Although the solid-state structure of 1b has not
We have recently reported the use of Cr(III) catalyst 1a been obtained at this time, its spectroscopic properties are
(Fig. 2) for the copolymerization of CO2 with cyclohexene oxide similar to those of 1a and it is therefore highly likely that it
to give polycyclohexene carbonate.77 We then sought to investi- also exists as a dichloro-bridged species. Furthermore, a
gate whether other epoxides could be used. For this study, dihydroxide-bridged product derived from 1b has been
bulky t-butyl substituents ortho to the C–O groups on the obtained during the course of this study and structurally auth-
phenolates were employed, and a pyridyl pendant arm com- enticated. Further details of this product are given below.
prised the second neutral donor site. This metal–ligand com-
Initial studies were conducted on propylene oxide (PO) with
bination offers new avenues for development of potentially CO2 catalyzed by complex 1a. As outlined in Scheme 1, either
highly active catalysts because of the modifiable nature of the the cyclic product, propylene carbonate (PC), and/or the
donor sites, their steric and electronic properties, and their polymer, poly(propylene carbonate) (PPC), were obtained for
geometry, which differs from that exhibited by the salen and this process depending on the conditions used. The copoly-
salan-based systems. For example, the planar salen-based merization reactions investigated utilized common co-catalysts
systems cause the nucleophilic co-catalyst, Nu, or epoxide previously reported in the literature to give high yields of co-
monomer to preferentially bind trans to the X ligand. Trivalent polymer.28,78 Neutral or ionic co-catalysts such as
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(4-dimethylamino)pyridine (DMAP), bis(triphenylphosphora- catalyst loading to 0.2% (entry 8) gave a much higher yield of
nylidene)iminium chloride or azide (PPNCl or PPNN3), and tet- polycarbonate and maintained selectivity. Generally, using
rabutylammonium bromide (TBABr) have been shown to play PPNCl as the co-catalyst (entries 8–12) gave good-to-excellent
an important role in the outcome of the desired product.
selectivity for polycarbonate, but required long reaction times.
When only 1a with no co-catalyst was used, 100% conver- After 12 or 6 h (entries 10 and 11, respectively) modest yields
sion of PO to PC was observed regardless of reaction tempera- of polymer were obtained. Since the PPNX salts were found to
ture as shown in Table 1, entries 1 and 2. However, complex 1a be sparingly soluble in epoxides at room temperature, a reac-
combined with PPNN3 co-catalyst gives high selectivity for tion was carried out where the autoclave was brought into the
polycarbonate at room temperature and moderate pressure of glove box in order to add complex 1a and the desired equiva-
CO2. The polymer obtained is of low molecular weight but also lents of co-catalyst and epoxide accurately. This procedure
narrow polydispersity (entry 3). Without 1a, no PPC was pro- showed an improvement in the yield, while still maintaining
duced, observing an 85% conversion to PC after 24 hours good Mn and PDI (entry 12).
(entry 4). It has previously been shown that PPN salts with
Mass transfer limitations inhibited the ability to increase
strong nucleophilic anions, e.g. PPNCl, are good catalysts in yields due to the solidification of the polymer. Therefore, the
their own right for the coupling of CO2 with neat epoxides.79 use of a non-coordinating solvent, toluene, was explored to
Changing the amount of co-catalyst from 1 equivalent (entry 3) prevent precipitation of the polymer in the autoclave during
to 0.5 equivalents (entry 5), did not prove to have a significant the reaction. Unfortunately, no copolymer or cyclic carbonate
effect on the activity, however, a lower number average molecu- was produced in the presence of a co-solvent (entry 13).
lar weight (Mn) was observed when using a higher loading of Heating the reaction to 80 °C for 4 h (entry 14) did not
PPNN3. Generally, comparable results were obtained using improve the yield of polymer. Under these conditions, quanti-
PPNN3 (entries 5 and 6) or PPNCl (entries 8 and 12) as the co- tative conversion of propylene oxide to cyclic propylene car-
catalyst under otherwise similar conditions. The catalyst bonate was observed instead. This result was not surprising as
loading also proved to be an important parameter. Using a Darensbourg and co-workers have shown that cyclic propylene
0.1% catalyst loading vs. epoxide monomer (entry 7) gave good carbonate
selectivity for polycarbonate, but low yield. The polymer temperatures.25
obtained was found to have a similar molecular weight to that DMAP (entries 15 and 16) and TBABr (entries 17 and 18)
preferentially
forms
at
higher
reaction
obtained when employing a 0.2% catalyst loading with PPNN3 were screened as Lewis basic co-catalysts. However, they were
(entry 5), but was more polydisperse. These molecular weight found to be inferior to PPNCl under the conditions investi-
values were reproducible over three runs. Increasing the gated in entry 8. Using DMAP as co-catalyst at room
a
Table 1 Copolymerization or coupling of PO and CO2
[Cr] : [PO] : [Co-Cat.]
(per Cr centre)
Time
(h)
Temp.
(°C)
Pressure
(bar)
Yieldb
PPC (%)
Selectivityc
(%)
Conversionb
(%)
TOFd
Mn
e
e
Entry
TONd
(h−1
)
(kg mol−1
)
Mw/Mn
1f
1 : 270 : 0
1 : 270 : 0
24
48
24
24
24
6
24
24
16
12
6
6
4
4
24
24
24
24
24
24
60
25
23
23
25
25
25
23
22
23
25
25
25
80
24
60
25
60
22
22
47
40
38
45
45
40
42
37
39
40
40
40
37
47
39
47
40
48
37
39
0
0
83
0
83
59
33
85
69
42
41
59
0
0
73
0
42
0
0
0
0
88
0
93
96
82
93
95
51
93
92
0
0
73
0
46
0
0
100
100
94
85
89
61
40
100
91
81
44
64
0
100
100
99
0
0
414
0
422
280
315
418
334
223
200
287
0
0
386
0
212
0
0
0
0
17
0
18
47
13
17
21
19
33
48
0
0
16
0
9
0
0
14
NAg
NAg
5.5
NA
NA
1.14
NA
1.15
1.19
1.40
1.40
1.27
1.21
1.19
1.17
NA
NA
1.32
NA
ND
NA
NA
1.19
2f
3
1 : 500 : 1 (PPNN3)
0 : 500 : 1 (PPNN3)
1 : 500 : 0.5 (PPNN3)
1 : 500 : 0.5 (PPNN3)
1 : 1000 : 0.5 (PPNCl)
1 : 500 : 0.5 (PPNCl)
1 : 500 : 0.5 (PPNCl)
1 : 500 : 0.5 (PPNCl)
1 : 500 : 0.5 (PPNCl)
1 : 500 : 0.5 (PPNCl)
1 : 500 : 0.5 (PPNCl)
1 : 500 : 0.5 (PPNCl)
1 : 500 : 0.5 (DMAP)
1 : 500 : 0.5 (DMAP)
1 : 500 : 0.5 (TBABr)
1 : 500 : 0.5 (TBABr)
1 : 500 : 1 (TBABr)
1 : 500 : 1 (PPNCl)
4f
NA
5
21.1
12.4
18.7
13.3
10.9
8.7
12.4
11.5
NA
6h
7
8
9
10
11
12h
13i
14f
15
16f
17
18f
19f,j
20j
NA
12.7
NA
65
NDg
NA
100
100
80
NA
6.4
66
83
330
a Polymerization reactions were carried out in neat PO (2.8 mL) using 1a, except where noted. b Yield calculated for PPC formation, conversion is
% PO consumed as determined by 1H NMR. c Selectivity of polycarbonate over cyclic carbonate in units of % as determined by 1H NMR of the
crude product. d Turnover number (TON) is moles of repeating units of PPC produced per mol of Cr, turnover frequency (TOF) is moles of PPC
produced per mol of Cr per hour. e Determined by gel permeation chromatography in CHCl3, calibrated with polystyrene standards. f No PPC
formed only PC. g NA = not applicable (no polymer obtained), ND = not determined due to lack of polymer and/or work-up related issues.
h Loaded reagents into autoclave in glove box. i Reaction performed in toluene (16 mL). j 1b used as initiator.
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temperature and a CO2 pressure of 39 bar gave 73% yield of
polycarbonate in good selectivity after 24
h (entry 15).
However, raising the temperature of the reaction to 60 °C led
to nearly quantitative yield of propylene carbonate instead
(entry 16). A similar trend was observed when TBABr was
employed as the co-catalyst. Modest yield of polymer was
obtained at room temperature (entry 17) but raising the temp-
erature, again, gives quantitative yield of propylene carbonate
and no polymer (entry 18). This temperature dependence is an
intriguing feature of the reaction mechanism. The selectivity
of a combination of catalyst and co-catalyst can be switched
very easily by varying the temperature of the reaction. In short,
performing the reaction at room temperature produces poly-
carbonate whereas raising the temperature produces propylene
carbonate in high yield. When complex 1b was used as
initiator using TBABr as co-catalyst at room temperature, excel-
lent yields of PC were obtained but no PPC was observed
(entry 19). Using PPNCl was more successful in producing
polycarbonate (entry 20).
Fig. 3 1H NMR of the copolymerization of PO and CO2 (Table 1, entry 3,
300 MHz, CDCl3).
Comparing to recent results in the literature, Darensbourg
and co-workers34 reported an effective catalyst system using 1
equivalent of PPNN3 in rac-PO with a CrCl(salan) derivative.
Poly(propylene carbonate) was obtained at room temperature
and under relatively low pressure yielding a polymer with a
molecular weight of 8.0 kg mol−1 and a turnover frequency of
21 h−1. By comparison, entries 5 and 11 in Table 1 (using 0.5
equivalents of PPNN3 and PPNCl, respectively) achieved TOFs
of 47 and 48 h−1. Lu, Zhang and co-workers have observed the
use of a Cr(salan) complex in the presence of 1 equivalent of
DMAP gave TOFs of 86 h−1 at room temperature and low-
pressure.64
Although this new catalyst system shows similar catalytic
activity to Cr salen and salan systems for PO/CO2 copolymeri-
zation, Co(salen) catalysts are observed to have the highest
activities, best stereochemical control and highest molecular
weights reported in the literature.45,51,56
Fig. 4 1H NMR of the depolymerization of PPC to PC with time (300 MHz,
CDCl3).
allowed for the determination of regiochemistry in PPC
(Fig. 5). Similar to related catalysts34,80 complex 1a is highly
selective for head-to-tail (HT) linkages (Fig. 6) showing
80–100% regioselectivity, whereas the polymer obtained using
1b (entry 20) showed nearly exclusively HT regioregularity
(Fig. 5 inset).
NMR spectroscopy
1H NMR was used in the initial analysis of the product
formed, whereby an aliquot of the crude reaction mixture was
analyzed immediately after the reaction was terminated, in
order to determine the conversion/yield and selectivity of the
products produced. Integration of the methine protons in the
1H NMR spectra allowed quantitative determination of the
Mass spectrometry
amount of cyclic carbonate produced relative to polycarbonate The MALDI-TOF spectrum of poly(propylene carbonate) pro-
formation (Fig. 3). Interestingly it was also noticed that if the duced in Table 1, entry 8 (using 1a and PPNCl as the catalytic
catalyst was not totally removed from the polymer in CDCl3 system), along with end group assignments is shown in Fig. 7.
(aliquot samples for NMR analysis), depolymerization occurred The MALDI-TOF spectrum of polymer produced using 1a and
causing propylene carbonate formation to increase over the PPNN3 (Table 1, entry 5) is given in ESI.† The observed mass
period of 1 week at room temperature (Fig. 4).
spectra show the presence of three polymer chains. The lowest
In addition, 1H NMR allowed for the identification of ether- molecular weight corresponds to series (a), [Cl (35) + 102n
linkages, which were just discernable in the spectra. The copo- (repeating unit) + C3H7O (59)] where PPC is found to have end
lymerization of PO and CO2 by complex 1a, therefore, afforded groups with a chloride as the initiator and a hydroxyl at the ter-
approximately 99% carbonate linkages. MALDI-TOF MS minating end. Series (b) is found to have both ends terminated
studies have also provided evidence towards a highly alternat- by Cl, [Cl (35) + 102n (repeating unit) + C3H6Cl (77)]. The
ing copolymer. Quantitative 13C NMR of the carbonyl region highest molecular weight series (c), [Cl (35) + 102n (repeating
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Fig. 5 Carbonyl region of the 13C NMR spectrum in CDCl3 of PO/CO2 copoly-
mer prepared with 1a and PPNCl (Table 1, entry 7). Inset: Carbonyl region of 13
NMR spectrum of copolymer prepared with 1b and PPNCl (Table 1, entry 20).
C
Fig. 7 (A) MALDI-TOF MS of PPC produced by 1a (Table 1, entry 8). (B)
Magnified region of the spectrum (n = 44–48). Modeled isotopic masses (when
n = 47) and images for polymer chains with end groups (a–c).
Fig. 6 Possible regiochemistry of PPC.
results, bimodal distributions were also observed in some GPC
unit) + C6H12O (100) + Cl (35)] is modeled to have an ether-con-
taining group within the polymer, likely resulting from double
insertion of an epoxide unit during the reaction. Isotopic
masses modeled for series a–c are found to agree well with the
experimental peaks. In addition, all co-catalysts were observed
to give the same three polymer chains in their MALDI-TOF
spectra. This suggests ring opening of the epoxide is induced
by the Cl group of the metal complex rather than by the co-
catalyst because no DMAP, bromide or azide-containing end-
groups were identified in the MALDI-TOF spectra when those
nucleophiles were used as co-catalysts.
traces making accurate analysis of Mn values difficult.
Thermal analysis (DSC and TGA)
Thermal analysis was also performed on the polymers
obtained. Differential scanning calorimetry (DSC) measure-
ments of the PPC afforded glass transitions (Tg) ranging from
35 °C to 40 °C. The DSC trace and observable Tg for PPC pro-
duced by complex 1a, Table 1, entry 8 is given in ESI.† The Tg
values obtained are found to be within the ranges reported in
the literature.53,81 Thermal gravimetric analysis (TGA) indi-
cated 50% decomposition (T50) at 146–187 °C. A representative
TGA trace and derivative plot is shown in ESI.† Darensbourg
and coworkers have shown a similar TGA value where the
onset of weight loss at 175 °C was determined for PPC.34
Gel permeation chromatography
Gel permeation chromatography (GPC) was utilized in the
determination of polymer molecular weights and polydisper-
Coupling of styrene oxide and CO2
sity indices (PDIs). Resulting molecular weights listed in Preliminary studies were also conducted on the reaction of
Table 1 were found to be dependent on reaction time. Higher styrene oxide (SO) and carbon dioxide (Scheme 2). All reactions
molecular weights were generally obtained over longer reaction were performed in neat SO and initiated by complex 1a. In all
times. PDIs were usually observed to be narrow, with most cases, only the cyclic product, styrene carbonate (SC) was
1
ranging between 1.1 and 1.4. Similar to the MALDI-TOF MS obtained. The H NMR spectra of the products were found to
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The proposed mechanism for this system is shown in
Scheme 3. Based on the results obtained, preliminary mechan-
istic insights can be proposed. For instance, experimental find-
ings from MALDI-TOF MS have suggested that a chloride
(originating from the Cr complex) can initiate the ring-opening
of the epoxide whether a neutral or ionic co-catalyst is utilized.
In the copolymerization of PO/CO2 only chloride end groups
were observed as possible initiators in the MALDI-TOF MS.
However, in our previous study involving CHO, hydroxyl and
chloride initiators were observed.77 The presence of the OH
end groups along with lower than expected Mn values suggests
chain transfer reactions are occurring, likely due to trace
water.33,85,86 Therefore, the initiation step in the mechanism
likely involves the co-catalyst (Nu) binding to Cr, displacing
the Cl− from the metal. This Cl− can now ring-open the bound
epoxide at the methine carbon (R′) and start the polymeriz-
ation process. Through this newly formed chromium alkoxide
(Cr–OR) bond, CO2 can insert at R producing a polycarbonate
chain with HT regiochemistry. Sequential additions of epoxide
and CO2 allow propagation of the growing polymer chain. The
copolymerization can then be terminated by addition of
alcohol or water (pathway A, Scheme 3). From the MALDI-TOF
MS analysis, the expected OH end groups were observed in the
polypropylene carbonate, however, chloride-terminated groups
were also found on these polymers. These may be a result of
chain transfer occurring between two growing polymer frag-
ments, producing a polymer with two Cl-groups as the chain
ends as shown in pathway B. This pathway would also result in
a HH or TT linkage where the chain combination occurs.
Although an intermolecular process is shown for pathway B in
Scheme 3, an intramolecular chain transfer could also occur
for two polymer chains growing on a single metal site as has
been proposed for salen complexes that catalyze the copoly-
merization of PO with CO2.38 The formation of ether linkages
is produced through double epoxide insertion (pathway C).
Pathway D illustrates a possible route to the formation of
polymer series (c), which contains chloro-terminated groups as
well as an ether linkage.
Scheme 2 Coupling of SO and CO2 to give styrene carbonate.
be consistent with previously characterized SC (see ESI†).82 An
aliquot was taken from the reaction mixture in order to calcu-
late the % yield.
Complex 1a does exhibit some activity for the cyclic carbon-
ate product, styrene carbonate at high temperatures and press-
ures (Table 2). Reactions performed at room temperature using
DMAP as co-catalyst and approximately 40 bar CO2 pressure
did not yield any carbonate-containing product (entry 1).
Increasing reaction temperature to 80 °C yielded 16% SC after
3 hours under 50 bar CO2 (entry 2). Increasing the temperature
further to 100 °C (entry 3), gave an improved yield and TOF by
almost double that of entry 2. Since the reactions were
observed to proceed slowly even at high temperatures, longer
reaction times were investigated. Using DMAP as co-catalyst at
80 °C and 50 bar over 18 h gave 38% yield of styrene carbonate
(entry 4). Changing the co-catalyst to PPNCl yielded 65% SC
over 18 h (entry 5). Unfortunately, no copolymer was produced
with this Cr initiator and these co-catalysts. This result was not
surprising as the copolymerization of SO/CO2 to form poly
(styrene carbonate) is known to be very difficult to achieve due
to the occurrence of the ring-opening at the methine Cα–O
bond. The electron-withdrawing nature of styrene oxide causes
backbiting to occur through intramolecular cyclic elimin-
ation.82 Recent advances with Co(salen) catalysts82–84 in con-
junction with PPNX [X = acetate or 2,4-dinitrophenoxy] co-
catalysts, has produced alternating poly(styrene carbonate)
with an increase in thermal stability.82
Mechanistic considerations
Epoxide/CO2 copolymerization has been proposed to proceed
by a coordination-insertion mechanism via a monometallic
Isolation of Cr hydroxide complex
single site or through a bimetallic dual site fashion. Darens- Single crystals from the reaction of CO2 and PO in the presence
bourg and co-workers for example, have performed detailed of 1b were obtained upon venting the system and allowing
mechanistic studies for Cr(salen)X and Cr(salan)X complexes some of the unreacted PO to evaporate (Table 1, entry 20).
towards epoxide/CO2 copolymerization.4
Analysis of these crystals by X-ray diffraction showed them to
Table 2 Coupling reaction of SO and CO2 catalyzed by 1aa
Entry
[Cr] : [SO] : [Co-Cat.]
Time (h)
Temp. (°C)
Pressure (bar)
Yield (%)
TONb
TOFc
1
2
3
4
5
1 : 500 : 0.5 (DMAP)
1 : 500 : 0.5 (DMAP)
1 : 500 : 0.5 (DMAP)
1 : 500 : 0.5 (DMAP)
1 : 500 : 0.5 (PPNCl)
24
3
3
18
18
21
80
100
80
40
50
50
50
45
0
NAd
79
140
190
313
NAd
26
47
11
17
16
28
38
65
80
a All reactions were carried out in neat SO (4 mL). b Calculated by 1H NMR. c Turnover frequency is moles of SC produced per mol of Cr per hour.
d NA = not applicable (no carbonate-containing product obtained).
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Scheme 3 Proposed mechanism for the copolymerization of CO2 with epoxides by Cr amine-bis(phenolate) complexes.
be the hydroxide-bridged dimer 2 (Fig. 8). This compound
The complex lies on an inversion centre located within the
likely results from the hydrolysis of complex 1b by adventitious Cr2O2 rhomboid and the Cr(III) centres exhibit distorted octa-
moisture.
hedral geometries. Related dimeric Cr(III) complexes
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Experimental
General
Unless otherwise stated, all manipulations were performed
under an atmosphere of dry oxygen-free nitrogen by means of
standard Schlenk techniques or using an MBraun Labmaster
DP glove box. PO and SO were purchased from Aldrich and
freshly distilled from CaH2 and vacuum transferred into a
sealed ampule prior to use. All solvents were purified by an
MBraun Manual Solvent Purification System. 1a and
H2[O2NN′]BuMe were prepared according to the previously
reported procedures.74,77 PPNN3 was prepared according to the
literature procedure.88 Supercritical Fluid Chromatography (4.8
Grade) CO2 was supplied from Praxair in a high-pressure cylin-
1
der equipped with a liquid dip tube. All H and 13C NMR were
performed in CDCl3 purchased from Cambridge Isotope Labo-
ratories, Inc. 1H NMR and 13C NMR spectra were recorded on a
Bruker AVANCE III 300 MHz spectrometer at 300 and
Fig. 8 Molecular structure (ORTEP) and partial numbering scheme of {Cr(μ-OH)-
[O2NN’]BuMe
}
2
2. Ellipsoids are drawn at 50% probability. Hydrogen atoms
omitted for clarity. Selected bond distances (Å) and angles (°): Cr1–O1 = 1.912(2),
Cr1–O2 = 1.939(2), Cr1–O3 = 1.989(2), Cr1–O3* = 2.014(2), Cr1–N1 = 2.081(3), 75.5 MHz, respectively. Chemical shifts are given in ppm rela-
Cr1–N2 = 2.081(3), Cr1⋯Cr1* = 3.0673(13); O1–Cr1–O2 = 92.01(10), O1–Cr1–
O3 = 91.16(10), O2–Cr1–N1 = 92.02(10), O2–Cr1–N2 = 86.80(10), N1–Cr1–N2 =
tive to TMS. All copolymerization reactions were carried out in
a 300 mL stainless steel Parr® 5500 autoclave reactor with a
Parr® 4836 controller. N.B. Caution should be taken when
80.83(12), N1–Cr1–O3 = 173.64(10), N2–Cr1–O3 = 96.71(11), O2–Cr1–O3 =
93.70(9), O3–Cr1–O3* = 79.97(3), Cr1–O3–Cr1* = 100.03(10). Symmetry oper-
operating such high-pressure equipment.
ation used to generate equivalent atoms: −x + 1, −y, −z + 2.
Synthesis
1b: A 1.6 M hexanes solution of n-butyllithium (11.7 mL,
18.7 mmol) was added via syringe to a stirring solution of
H2[O2NN′]BuMe (3.92 g, 8.50 mmol) in THF (50 mL) at −78 °C.
The reaction mixture was warmed to room temperature and
stirred for 2 h to give a clear orange solution. This mixture was
transferred via cannula to a solution of CrCl3(THF)3 (3.29 g,
8.86 mmol) in THF (50 mL) at −78 °C to give a purple suspen-
sion. Upon warming to room temperature under nitrogen a
dark purple mixture remained. The solvent was removed in
vacuo and the residue was extracted into toluene. The mixture
was filtered through Celite and the solvent was removed under
vacuum. The product was washed with pentane and dried to
yield 2.37 g of green powder (49% yield). Anal. Calcd for
C30H38ClCrN2O2: C, 65.98; H, 7.01; N, 5.13. Found: C, 65.83; H,
7.10; N, 5.24. MS (MALDI-TOF) m/z (%, ion): 1090.4 (12, [(CrCl-
[L])2]+), 545.2 (96, [CrCl[L]]+), 510.2 (25, [Cr[L]]+). Single crystals
possessing salen and acacen (N,N′-bis(t-butylacetylacetone)-
1,2-ethylenediamine) ligands and dibridging-hydroxo groups
have been used for copolymerization of cyclohexene oxide and
CO2.87 The Cr(1)–O(3)–Cr(1)* angle in 2 is 100.04(10)° and the
hydroxides are orientated cis to the nitrogen on the pendant
pyridyl group. This angle is typical of what is reported for
other HO-bridged Cr(III) dimers. The Cr–OH bond distances
are asymmetric at 1.989(2) and 2.014(2) Å and are within the
range observed for five or six-coordinate chromium(III) hydrox-
ide-bridged species.
Conclusions
The
chromium
chlorido
diamine-bis(phenolate) of 2 were obtained from a solution of 1b in propylene oxide
complex, {CrCl[O2NN′]BuBu
}
2
(1a), was shown to effectively after performing the polymerization procedure described
couple PO with CO2 in the presence of co-catalysts below.
(DMAP, PPNCl, PPNN3). Good activities were observed for
X-ray crystallography
the formation of PPC at low temperatures but at elevated temp-
eratures the reaction was selective for PC formation. Diffraction data for compound 2 were collected on a Rigaku
The
number
average
molecular
weights
of
the Saturn CCD area detector with a SHINE optic and Mo-Kα radi-
polycarbonate were found to be lower than expected, but ation and solved on an AFC8-Saturn 70 single crystal X-ray
the polydispersities observed were generally narrow. Prelimi- diffractometer from Rigaku, equipped with an X-stream 2000
nary results suggest the chloride on the Cr complex plays an low temperature system. Crystallographic and structure refine-
important role in the initiation of the epoxides in order to gen- ment data are given in Table 3. The data were processed using
erate polycarbonate. Further investigation into the influence of CrystalClear89 software and corrected for Lorentz and polariz-
the pendant donor and substituents on the phenolate frag- ation effects and absorption.90 Neutral atom scattering factors
ments of the amine-bis(phenolate) ligand are currently for all non-hydrogen atoms were taken from the International
underway.
Tables for X-ray Crystallography.91 The structure was solved by
9240 | Dalton Trans., 2013, 42, 9233–9244
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then charged with the appropriate pressure of CO2 and left to
stir at the desired temperature and time period. After the
desired time the autoclave was cooled in an ice bath and
vented in the fume hood. An aliquot for NMR was taken
immediately after opening for the determination of yield and
conversion. The copolymer was extracted with CH2Cl2 and re-
precipitated in cold acidic methanol. For reactions done in the
presence of onium salts, complex 1a and the co-catalyst were
first activated in 4 mL of CH2Cl2 and allowed to stir for
30 minutes and then dried in vacuo. The remaining procedure
was followed in the same manner as the neat reactions.
Table 3 Crystallographic and structure refinement data for 2
Chemical formula
Formula weight
T/K
[(C31H39N2O6)Cr]2·2CH2Cl2
1225.16
163
Color, habit
Crystal dimensions/mm
Crystal system
Space group
a/Å
b/Å
c/Å
α/°
Black-red, prism
0.40 × 0.32 × 0.12
Monoclinic
P21/c (#14)
14.239(7)
13.204(7)
18.420(9)
90
β/°
95.586(6)
90
3447(3)
2
1.180
5.17
γ/°
V/Å3
Mass spectrometry
Z
Dc/g cm−3
An Applied Biosystems 4800 MALDI TOF/TOF Analyzer
equipped with a reflectron, delayed ion extraction and high
performance nitrogen laser (200 Hz operating at 355 nm) was
used for polymer analysis. 2,5-Dihydroxybenzoic acid (DHBA)
was used as the matrix. The matrix was dissolved in THF at a
concentration of 10 mg mL−1. Polymer was dissolved in THF at
approximately 1 mg mL−1. The matrix and polymer solutions
were mixed together at a ratio of 5 to 1, respectively; 1 μL of
this was spotted on the MALDI plate and left to dry. Images of
mass spectra were prepared using mMass™ software (http://
www.mmass.org).
μ(MoKα)/cm−1
F(000)
1292
Reflections collected
Independent reflections
R(int)
32 578
7115
0.0707
0.0903, 0.2384
0.0791, 0.2262
1.064
R, wR2 (all)a
R, wR2 [I > 2σ(I)]a
GOF on F2
CCDC reference
897967
a R1 = Σ(|Fo| − |Fc|)/Σ|Fo|); wR2 = [Σ(w(Fo2 − Fc2)2)/Σw(Fo2)2]1/2
.
direct methods using SIR9292 and expanded using Fourier
techniques (DIRDIF99).93 All non-hydrogen atoms were refined
anisotropically. Hydrogen atoms were refined using the riding
model, except H3A. H3A was located in its difference map posi-
tion and was refined with a fixed displacement ellipsoid 1.5
times that of O3, and was refined positionally with a distance
restraint. All atoms were introduced prior to the application of
Gel permeation chromatography
Gel permeation chromatography (GPC) analysis was performed
on a Viscotek VE 2001 GPCMax at 35 °C equipped with a Visco-
tek VE 3580 RI Detector, and two phenogel 5μ Linear Mixed
Bed columns purchased from Phenomenex (300 × 4.60 mm
column in series with a 100 Å, 300 × 4.60 mm column).
Samples were prepared at a concentration of 2 mg mL−1 and
left to equilibrate for ∼2 h. The samples were filtered through
syringe filters before analysis. The GPC columns were eluted
with chloroform (HPLC grade) at a flow rate of 0.35 mL min−1
with a 100 μL injection volume. Six polystyrene standards were
used in making the calibration curve, bracketing molecular
ranges from 1050 to 3 800 000 Da. No further corrections were
performed on the molecular weights obtained.
Squeeze. Anomalous dispersion effects were included in
94
Fcalc
;
the values for Δf′ and Δf′′ were those of Creagh and
McAuley.95 The values for the mass attenuation coefficients are
those of Creagh and Hubbell.96 All calculations were per-
formed using the CrystalStructure97 crystallographic software
package except for refinement, which was performed using
SHELXL-97.98 The Platon Squeeze99 procedure was applied to
recover 173 electrons per unit cell in one void (total volume
1009 Å3); that is 86.5 electrons per formula unit. Disordered
solvent lattice dichloromethane molecules (42 electrons per
CH2Cl2; 2 molecules per formula unit) were present prior to
the application of Squeeze; however, a satisfactory point atom
model could not be achieved. The application of Squeeze gave
a good improvement in the data statistics and allowed for a
full anisotropic refinement of the structure. Structural illus-
trations were created using ORTEP-III (v. 2.02) for Windows.100
CCDC reference number 897967 (2) contains the supplemen-
tary crystallographic data for this paper.
Differential scanning calorimetry
Glass transition (Tg) temperatures were measured using a
Mettler Toledo DSC 1 STARe System equipped with a Julabo FT
100 immersion cooling system, using a R1150 refrigerant in an
EtOH bath with a working range of −100 to +20 °C. Samples
(∼4 mg) were weighed into 40 μL aluminum pans and sub-
jected to two heating cycles. The first heating cycle consisted
of heating from 0 to 100 °C at a rate of 10 °C min−1, held for
2 min at 100 °C and then cooled back to 0 °C at 10 °C min−1
.
The sample was held at this temperature for 2 min and sub-
jected to a second heating cycle from 0 to 180 °C at a rate of
Polymerization procedure
For all reactions carried out in neat epoxide, the appropriate 10 °C min−1
amount of monomer was added to the catalyst (0.05 g) and co-
.
Thermal gravimetric analysis
catalyst in a glove box. The reactant solution was added via a
long-needled syringe to a 300 mL Parr® autoclave, which was TGA was performed with a TA Instrument Q500. Samples
pre-dried under vacuum overnight at 80 °C. The autoclave was (8 mg) were loaded onto a platinum pan and subjected to a
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dynamic high-resolution scan, with an initial heating rate of
20 °C min−1. Each sample was heated from room temperature
to 400 °C.
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11, 3668.
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Acknowledgements
Financial support was provided by the Canada Foundation for
Innovation (CFI), Newfoundland and Labrador Research Devel-
opment Corporation, the Natural Sciences and Engineering
Research Council (NSERC) of Canada and Memorial University
of Newfoundland. R. K. D. thanks NSERC for a postgraduate
scholarship. C. M. K. thanks Suncor Energy for a Terra Nova
Young Innovator Award.
27 D. J. Darensbourg, R. M. Mackiewicz, J. L. Rodgers,
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