Amine-bis(phenolato)cobalt(II) Catalysts for the Formation of Organic Carbonates from Carbon Dioxide and Epoxides

Article abstract of DOI:10.1002/ejic.201403087

New monometallic amine-bis(phenolato)cobalt(II) [(ONNO)^RCo^II] (R = CMe₂Ph; Cl; Br) complexes have been synthesized and fully characterized including X-ray crystallographic analysis. These Co^II complexes show goo

Full text of DOI:10.1002/ejic.201403087

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DOI:10.1002/ejic.201403087
Amine-bis(phenolato)cobalt(II) Catalysts for the
Formation of Organic Carbonates from Carbon Dioxide
and Epoxides
Marina Reiter,[a][‡] Peter T. Altenbuchner,[a][‡] Stefan Kissling,[a]
Eberhardt Herdtweck,[b] and Bernhard Rieger*[a]
Keywords: Carbon dioxide fixation / Copolymerization / Cyclic carbonates / Epoxides / Cobalt
New monometallic amine-bis(phenolato)cobalt(II) [(ONNO)R-
CoII] (R = CMe2Ph; Cl; Br) complexes have been synthesized
and fully characterized including X-ray crystallographic
CO2 and propylene oxide/CO2. [(ONNO)ClCoII]*(MeOH)
was found to effectively copolymerize cyclohexene oxide
(CHO) and CO2. This is the first reported amine-bis(phen-
analysis. These CoII complexes show good activity for the for- olato) cobalt(II) complex to be active in the copolymerization
mation of cyclic propylene carbonate in combination with
tetrabutylammonium bromide (TBAB) as a co-catalyst. The
reaction parameters such as carbon dioxide pressure, co-cat-
alyst loadings and temperature were varied to determine the
of CO2 and CHO. In-depth stability studies were conducted
(Evan’s method) to validate CoII as the active species re-
quired for copolymerization. End-group analysis via NMR,
ESI-MS and MALDI-TOF revealed the presence of 4-(di-
ideal reaction conditions. These catalysts were also em- methylamino)pyridine (DMAP) and methoxy terminated
ployed in copolymerization reactions of cyclohexene oxide/
chains.
cyclic carbonate synthesis often rely on porphyrin, β-di-
iminate- and salen-ligands in combination with various
metals like zinc, iron, magnesium, aluminum, chromium,
and cobalt.[15,27–38] The adequate choice of catalyst is essen-
tial to achieving reaction selectivity. Owing to the desire for
high selectivities and activities, in recent years, catalyst de-
sign has shifted towards bifunctional and bimetallic cata-
lysts.[1,5,10,12] Bifunctional complexes combine the active
metal site, coordinated by an organic ligand framework,
with a covalently bound co-catalyst whereas for bimetallic
catalysts the co-catalyst is replaced by a second metal cen-
ter. The highest activities reported in the literature paired
with exceptional selectivities for the copolymerization of
propylene oxide/cyclohexene oxide and CO2 have been
achieved using ionic salen-cobalt(III) catalysts[32] and di-
nuclear zinc catalysts.[34]
Published reports of CoII catalysts for the coupling of
CO2 and epoxides are rare and limited to Co(OAc)2 with
acetic acid as co-catalyst for a substantial period of time
(TOF = 0.06 h–1).[39] In recent years Lu et al. were able to
utilize immobilized CoII catalysts for the coupling of ethyl-
ene oxide and carbon dioxide under harsh conditions for
24 h without loss of activity.[40] In 2010, Williams et al.
successfully synthesized bimetallic CoII (TOF = 172 h–1)
and mixed metallic CoII/CoIII (TOF = 159 h–1) catalysts
which were active for the copolymerization of CHO/CO2 at
ambient pressure.[41,42] Recently, amine-bis(phenolato) cata-
lysts were introduced for the coupling of CO2 and epoxides.
Promising results were obtained with chromium and
Introduction
Carbon dioxide is non-toxic, abundant, renewable, and a
low cost C1-building block. By virtue of these attributes,
CO2 is an increasingly attractive material for economic and
academic use.[1–4] The industrial application of CO2 is
currently limited to the production of urea, methanol, cyclic
carbonate, and salicylic acid.[2,3,5–10] Since Inoue et al. dis-
covered the coupling of thermodynamically stable CO2 with
epoxides in 1969, a plethora of homogeneous catalysts have
been developed.[5,10–13] Cyclic carbonates are in high indus-
trial demand,[14,15] given their use as precursors to poly-
carbonates,[14,16] electrolytes in lithium ion batteries,[17–19]
additives to gasolines,[20] thickeners of cosmetics,[18] and as
so called “green solvents”.[21–23] Moreover, polycarbonates
derived from CO2 have promising physical properties like
durability, biodegradability, high transparency, heat resis-
tance, and gas permeability which make them possible can-
didates for applications in the automotive, medical and elec-
tronics industries.[10,24–26] Catalysts for polycarbonate or
[a] WACKER-Lehrstuhl für Makromolekulare Chemie,
Technische Universität München,
Lichtenbergstraße 4, 85747 Garching, Germany
E-mail: rieger@tum.de
http://www.makro.ch.tum.de
[b] Anorganisch-Chemisches Institut, Lehrstuhl für Anorganische
Chemie, Technische Universität München,
Lichtenbergstraße 4, 85747 Garching, Germany
[‡] The authors contributed equally.
Supporting information for this article is available on the
WWW under http://dx.doi.org/10.1002/ejic.201403087.
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CoII/III systems although only the chromium-based amine-
bis(phenolato) systems generated polycarbonates.[43–45] The
amine-bis(phenolate) ligand structure is versatile and offers
numerous possibilities for electronic and steric alter-
ation.[46–53] Contradicting reports exist in the literature re-
garding the role and respective activity of CoII and CoIII
catalysts for the copolymerization of epoxides and carbon
dioxide.[54–57] In some cases, complete loss of activity upon
reduction to CoII is reported whereas in other cases, activi-
ties remain in the same order of magnitude for the CoII and
CoIII species.[42–44,55,56] The increased Lewis acidity of CoIII
is generally advantageous for copolymerization; it facilitates
epoxide activation and ring opening and concomitantly
suppresses the undesired dissociation of the growing poly-
mer chain from the metal and subsequent back-biting. The
tendency of CoIII to be reduced to CoII is detrimental, espe-
cially during the copolymerization, since it changes the elec-
tronics and therefore the carefully designed characteristics
of the catalyst. Accordingly, a loss or decisive drop in ac-
tivity is observed.
Figure 1. Structure of amine-bis(phenolato)cobalt(II) complexes 1–
4.
Crystallography
Recrystallization from hexane (catalyst 2) or methanol
(catalysts 3, 4) yields purple crystals, suitable for single-crys-
tal X-ray diffraction studies. The molecular structures and
selected bond lengths of catalysts 2·H2O, 3·MeOH and
4·MeOH are shown in Figures 2, 3 and 4 and are given
in Table 1. (For more detailed information see Supporting
Information.) An equatorial plane is formed by the CoII
ion, two phenolate oxygens O(2) and O(3) and a dimeth-
ylamino donor N(1) in all presented crystal structures. The
We were interested in designing a CoII catalyst system
for the copolymerization of carbon dioxide and epoxides
based on the work of Kozak et al. that circumvents in situ
degradation of the active species. We were curious to see if
amine-bis(phenolato)cobalt(II) catalysts, in particular, can
be modified and subsequently used for copolymerization of
CO2 and epoxides.
Results and Discussion
Substituted amine-bis(phenolate) (LigH1–LigH4) com-
pounds were prepared by modified literature procedures
employing Mannich condensation of the corresponding
amine, phenol and formaldehyde in a one pot reaction with
water or methanol as solvent (Scheme 1).[58,59]
Scheme 1. Synthesis of amine-bis(phenolate) ligands LigH1–LigH4.
CoII complexes were prepared by reacting 2.2 equiv. of
potassium hydroxide with the respective ligand in a meth-
anol/toluene solution (1:1) followed by addition of cobalt
acetate. After refluxing for 18 h under an inert atmosphere
purple solids were formed and catalysts 2–4 were isolated
following recrystallization. Catalyst 1 was synthesized ac-
cording to a previously published procedure (Figure 1).[47]
Figure 2. ORTEP view of the molecular structure of catalyst 2 with
50% thermal ellipsoid probability. Selected bond lengths [Å] and
angles [°]: Co1–O1 2.1538(11), Co1–O2 1.9607(10), Co1–O3
1.9563(9), Co1–N1 2.1199(12), Co1–N2 2.1262(11), O1–Co1–O2
89.26(5), O1–Co1–O3 86.33(4), O2–Co1–O3 130.66(4), O1–Co1–
N1 92.78(5), O1–Co1–N2 175.61(5), O2–Co1–N1 118.23(4), O3–
Co1–N1 111.06(4), O2–Co1–N2 94.10(4), O3–Co1–N2 93.61(4),
N1–Co1–N2 83.13(4).
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nitrogen donor N(2) of the ligand backbone and the oxygen bromo- or chloro-substituted complexes compared to a
of the water molecule O(1) (or methanol molecule, respec- Co(1)–O(2) distance of 1.9607 Å of complex 2 with a cumyl
tively) are located in apical positions. Therefore, these com- substituent in the ligand backbone (Table 1).
plexes exhibit a distorted trigonal bipyramidal coordination
sphere. Regarding the Co(1)–O(2) or Co(1)–O(3) distances,
there is an observable difference between complex 2 and
complexes 3 and 4. Shorter Co(1)–O(2) bonds of 1.9345 Å
(complex 3) and 1.9364 Å (complex 4) are observed for the
Table 1. Selected bond lengths [Å] for 2, 3 and 4.
Entry Complex
Co1–O1
Co1–O2
Co1–O3
1
2
3
2·H2O
3·MeOH
4·MeOH
2.1538(11)
2.0947(11)
2.0980(15)
1.9607(10)
1.9345(10)
1.9364(14)
1.9563(9)
1.9449(10)
1.9485(14)
Coupling Reactions of CO2 and Epoxides
Catalysts 1–4 were employed in coupling reactions of
carbon dioxide and propylene oxide under various reaction
conditions (Table 2). Catalyst 1 was developed in the group
of Kozak et al. and preliminary tests of the coupling of
CO2 and propylene oxide were conducted.[44] Due to the
purification via crystallization, complex 2 was only used as
2·H2O, and complexes 3 and 4 were employed as 3·MeOH/
3·Acetone and 4·MeOH species. Since tetrabutylammonium
halides are claimed to be catalysts for the formation of
cyclic carbonates from CO2 and epoxides, an experiment
was conducted using only TBAB.[24] Neither TBAB, nor
catalyst 1, by themselves, enabled formation of cyclic carb-
onates, polyethers or polycarbonates at 60 °C and 30 bar.
Catalyst 1 and TBAB were found to be active in coupling of
PO and CO2 and were used to screen for optimal pressure,
temperature and co-catalyst. The CO2 pressure was varied
from 10 to 40 bar at 60 °C ([Cat]:[CoCat] = 1:1). The results
of these trials revealed an independence of the reaction
Figure 3. ORTEP view of the molecular structure of catalyst 3 with
50% thermal ellipsoid probability. Solvent molecule (MeOH) is re-
fined for clarity. Selected bond lengths [Å] and angles [°]: Co1–
O1 2.0946(11), Co1–O2 1.9345(10), Co1–O3 1.9446(10), Co1–N1
2.1129(13), Co1–N2 2.1965(12), O1–Co1–O2 90.34(4), O1–Co1–
O3 91.44(4), O2–Co1–O3 121.77(4), O1–Co1–N1 93.80(5), O1– velocity from CO2 pressure within the tested pressure range
Co1–N2 175.92(5), O2–Co1–N1 129.82(5), O3–Co1–N1 108.11(4),
(Table 2). This suggest that the rate-determining step during
O3–Co1–N2 90.80(4), O2–Co1–N2 91.37(4), N1–Co1–N2 82.26(5).
the reaction is not the insertion of CO2, but rather, the co-
ordination and ring opening of the epoxide, as is consistent
with literature.[34,55,60] Reaction temperatures evaluated
ranged from room temperature up to 80 °C (30 bar,
[Cat]:[CoCat] = 1:1). With elevated temperatures the pro-
duction of cyclic propylene carbonate (cPC) increased in
Table 2. Optimization of reaction temperature, pressure and co-
catalyst to catalyst ratio for formation of cyclic propylene carb-
onate.[a]
Entry Catalyst
T [°C]
p(CO2) [bar] TBAB/cat TON[b] Yield [%][c]
1
2
3
4
5
6
7
8
1
1
1
1
1
1
1
1
60
60
60
60
25
40
80
60
60
60
80
80
10
20
30
40
30
30
30
30
30
30
30
30
1
1
1
1
1
1
1
0.5
2
1
485
520
500
510
130
340
620
415
630
1400
2060
2050
20
25
20
20
5
15
30
20
30
60
85
85
9
1
Figure 4. ORTEP view of the molecular structure of catalyst 4 with
50% thermal ellipsoid probability. Selected bond lengths [Å] and
angles [°]: Co1–O1 2.0980(15), Co1–O2 1.9364(14), Co1–O3
1.9485(14), Co1–N1 2.1134(17), Co1–N2 2.2002(16), O1–Co1–O2
10
11
12
3·MeOH
3·MeOH
4·MeOH
1
1
90.26(6), O1–Co1–O3 91.96(6), O2–Co1–O3 121.40(6), O1–Co1– [a] Reaction conditions: cat/PO
=
1:2150, cobalt catalyst
N1 93.87(6), O1–Co1–N2 175.67(6), O2–Co1–N1 129.31(6), O3–
Co1–N1 108.93(6), O3–Co1–N2 90.71(6), O2–Co1–N2 91.25(6),
N1–Co1–N2 82.04(6).
(0.03 mmol), TBAB (0.03 mmol), 18 h, 5 mL of PO (neat). [b] Mol
of propylene carbonate produced per mol cobalt complex.
[c] Yields determined on basis of mass.
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an approximately linear fashion as was expected and has
previously been observed for salen systems.[37,61] Addition-
ally, the co-catalyst ratio was varied from 0.5 to 2 equiv.
at 60 °C and 30 bar (Table 2, Entries 3, 8, 9). Again, an
approximately linear dependence between turn over number
(TON) and co-catalyst ratio was observed. The highest
TON of 2060 for the formation of cPC was obtained with
catalyst 3·MeOH and TBAB at 80 °C and 30 bar (Table 2,
Entry 11). The high activity of complex 3 relative to the
other catalysts can be explained by the electron-with-
drawing property of the chloro substituents. The Lewis
acidity of the cobalt center is increased and, therefore, the
propylene oxide is coordinated more strongly to the metal
center. Similar activities could be observed utilizing com-
plex 4. Kleij et al. observed the same trend for chloro sub-
stituents on amino-tris(phenolato)aluminum complexes
with respect to cyclic carbonate formation.[30]
Figure 5. Intensity progress of the carbonyl vibration band of cyclic
propylene carbonate at 1797 cm–1 against time in the ATR-IR. Re-
action conditions: p(CO2) = 30 bar, 60 °C, cat/PO = 1:2150, cobalt
catalyst (0.03 mmol), TBAB (0.03 mmol), 5 mL of PO (neat). De-
termination of the initial slopes for catalyst 1–4: νobsЈ: 0.50 h–1 (cat-
alyst 1), 0.15 h–1 (catalyst 2·H2O), 0.83 h–1 (catalyst 3·MeOH),
0.81 h–1 (catalyst 4·MeOH).
In an in situ ATR IR, which can be applied for con-
tinuous monitoring experiments using IR spectroscopy and
an attenuated total reflectance probe, the decrease of the
stretching carbonyl vibration of carbon dioxide at
2350 cm–1 and the increase of the carbonyl vibration of
cyclic carbonate at 1797 cm–1 was monitored during the re- ated. Some experiments were performed with DMAP as the
action of CO2 and propylene oxide for all four catalysts 1– co-catalyst instead of TBAB (Table 3, Entries 6, 7). The use
4. The initial linear slope of each curve obtained with cata- of DMAP and catalyst 1 or 3·MeOH failed to afford poly-
lyst 1–4 at 60 °C, 30 bar and using a 1:1 co-catalyst to cata- (propylene carbonate) (PPC), polyether or the cyclic prod-
lyst ratio was measured (Figure 5). Notably, catalyst 3 ex- uct. However, reactions at 50 bar CO2, 80 °C, cat/PO =
hibited the highest activity (νobsЈ = 0.83 h–1), whereas cata- 1:500 with TBAB as co-catalyst afforded cPC in 90–95%
lyst 2 displayed the lowest activity (νobsЈ = 0.15 h–1).
yield (Table 3, Entries 8, 9), but no polycarbonate. Under
This increase can be attributed to the positive influence these conditions neither catalyst 1 nor catalyst 3·MeOH
of electron-withdrawing groups in ortho- and para-positions with TBAB or DMAP as co-catalyst were able to produce
of the ligands. The comparison of chloro- and bromo-sub- PPC.
stituted complexes 3 and 4 reveals no significant difference
The inability of catalysts 1–4·MeOH to polymerize prop-
in activity as was expected (R = Cl, νobsЈ = 0.83 h–1; R = ylene oxide and carbon dioxide led us to cyclohexene oxide
Br, νobsЈ = 0.81 h–1). This is likely attributable to the similar- as a viable alternative for use in further investigating the
ity of the chloride and bromide moieties.
capabilities of the CoII-catalyst system. The reason behind
The main focus of this work was to develop enhanced the switch of monomer, from propylene oxide to cyclohex-
synthesis of polycarbonates with amine-bis(phenolato)co- ene oxide, is the reluctance of CHO and carbon dioxide to
balt(II) catalysts. Therefore, reaction variables such as tem- form cyclic carbonates. The depolymerization (back-biting)
perature, pressure and nature of the co-catalyst were evalu- is several orders of magnitude slower for poly(cyclohexene
Table 3. CO2 coupling reactions with cyclohexene oxide and propylene oxide.[a]
.
Entry Catalyst
Co-catalyst [cat]/[cocat]/
[epoxide]
Epoxide
Product Polycarbonate/cyclic carb- TON[b]
onate/ether linkages [%]
Yield
[%][c]
Mn
PDI[d]
[g/mol][d]
1
2
3
4
5
6
7
8
3·MeOH
3·MeOH
1
3·MeOH
3·Acetone
3·MeOH
1
3·MeOH
1
3·MeOH
1
DMAP
DMAP
DMAP
DMAP
DMAP
DMAP
DMAP
TBAB
1:1:500
1:3:500
1:1:500
1:1:100
1:1:500
1:1:500
1:1:500
1:1:500
1:1:500
1:1:500
1:1:500
CHO
CHO
CHO
CHO
CHO
PO
PO
PO
PO
CHO
CHO
PCHC
PCHC
–
PCHC
PCHC
–
99:1:0
96:4:0
–
99:1:0
97:3:0
–
120
95
0
20
60
0
25
20
0
20
13
0
4000
1.48
1600[e]
1.49[e]
–
n.d.
6700
–
–
–
–
–
–
–
n.d.
1.35
–
–
–
–
–
–
–
–
0
0
cPC
cPC
–
0:100:0
0:100:0
–
480
450
0
95
90
0
9
10
11
TBAB
TBAB
TBAB
–
–
0
0
[a] Reaction conditions: cat/epoxide = 1:500, cat/DMAP = 1, 18 h, 80 °C, 50 bar, 4 mL of epoxide (neat). [b] mol of PCHC per mol cobalt
complex or mol of cyclic propylene carbonate produced per mol cobalt complex. [c] Yield determined by weight of the precipitated
polymer or by weight of the produced cyclic propylene carbonate. [d] Determined by GPC, calibrated with polystyrene standard in CHCl3.
[e] Unprecipitated polymer was used for GPC analysis.
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carbonate) (PCHC) than for PPC. Therefore, the deleteri- capped polymer chains are positively charged, the intensity
ous pathway to cyclic product can be suppressed by switch- of other polymer chains without a charge is probably very
ing from CHO to PO.[37,62,63]
low compared to the charged polymer. Therefore, 1H NMR
We tested our catalyst systems 1 and 3·MeOH for the spectroscopy and HSQC NMR spectroscopy were per-
copolymerization reaction of cyclohexene oxide and carbon formed. These analyses revealed the presence of methoxy
dioxide.[56] First the adequate co-catalyst had to be found end-groups for the produced PCHC (Figure 8, Supporting
1
and copolymerization experiments with catalysts 1 and Information FigureS7). The H NMR spectra of isolated
3·MeOH, in combination with TBAB were conducted. PCHC showed no signals corresponding to DMAP-
These experiments did not yield PCHC (Table 3, Entries 10, initiated polymer chains. Therefore, we conducted an ex-
11). However, using DMAP as co-catalyst with compound periment with catalyst 3·MeOH and a three-fold excess of
3·MeOH enabled copolymerization of carbon dioxide and DMAP to indirectly show the effect of DMAP on polymeri-
cyclohexene oxide at 80 °C and a pressure of 50 bar whereas zation. The molecular mass of PCHC dropped significantly
reference system 1 failed to generate any PCHC (Table 3, relative to samples prepared with only one equiv. DMAP
Entry 3). In this case, the Lewis acidity at the cobalt center (Table 3, Entries 1, 2), indirectly showing that DMAP can
might be too low for CHO/CO2 copolymerization. Catalyst act as the initiator for copolymerization.
3 is the first example of an amine-bis(phenolato)cobalt(II)
catalyst able to produce polycarbonate through copoly-
merization of CO2 and CHO. It exhibits very high selecti-
vity for carbonate linkages (99%), a TON of 120 and
provides a reaction yield of 24% (Table 3, Entry 1). GPC
measurements show a monomodal Mn distribution with a
narrow PDI of 1.48 (Supporting Information Figure S12).
The isolated poly(cyclohexene oxide) was subjected to 13C
NMR analysis which revealed that the isolated polymer is
atactic (Figure 6); it consists of mainly isotactic and a sig-
nificant percentage of syndiotactic PCHC (Table 3, Entry
1). ESI-MS-based end-group analyses of oligomeric cyclo-
hexene carbonate, obtained using catalyst 3·MeOH and
DMAP as the co-catalyst, (Table 3, Entry 4) revealed sig-
nals corresponding to [17 (OH) + 142 n (repeating unit) +
82 (C6H10) + 122 (C7H10N2)] (Supporting Information Fig-
ure S6).
Figure 7. Top = a) MALDI-TOF MS of PCHC produced accord-
ing to Table 3, Entry 1. Bottom = b) Labeled section of MALDI
spectrum a (600–1400 g/mol) with modeled polymer chain contain-
ing a DMAP initiating group.
Figure 6. Carbonyl region of the 13C spectrum of atactic poly-
(cyclohexene carbonate) prepared at 80 °C, 50 bar and DMAP as
co-catalyst (Table 3, Entry 1).
As already mentioned, complex 3·MeOH was always iso-
lated through crystallization meaning that two molecules of
MALDI-TOF analysis of the PCHC sample confirmed methanol were concomitantly present for every molecule of
the ESI-MS results and gave a series consisting of [17 (OH) catalyst. Alcohols are known to function as chain transfer
+ 142 n (repeating unit) + 82 (C6H10) + 122 (C7H10N2)], in- agents during catalytic copolymerizations and can influence
dicating that DMAP is the only initiating group and that activity, selectivity and polymer characteristics such as mo-
no chain transfer reaction is occurring (Figure 7). ESI-MS lecular weight and PDI.[25–28,64] Therefore, catalyst
and MALDI-TOF are very sensitive methods for detection 3·MeOH was isolated as 3·Acetone from a saturated acet-
of positive charged molecules and as the DMAP end- one solution and used for copolymerization.
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susceptibility of a substance in solution.[65,66] Experiments
employing the Evan’s NMR method to study the magnetic
susceptibility of catalyst 3·MeOH during reaction were per-
formed to observe the catalyst oxidation behavior (detailed
information are given in the Supporting Information). The
oxidation of catalysts 1 and 3·MeOH with 3 equiv. AgNO3
in CDCl3 at 40 °C was monitored by periodic measure-
ments of the magnetic moment. Catalyst 1 displayed a
faster rate of oxidation than did compound 3·MeOH. The
magnetic susceptibility of 1 dropped from 3.7 μB to 3.4 μB
within 5 h whereas catalyst 3·MeOH displayed a smaller de-
crease (4.3 μB to 4.2 μB) over the same period of time. After
24 h at 40 °C the magnetic moment of 3·MeOH dropped
significantly and reached a value of 3.5 μB; this was ac-
companied by a color change to dark brown. In contrast,
complete oxidation could be observed with catalyst 1 at
40 °C after the same period of time. Hence, catalyst
3·MeOH exhibits remarkable stability failing to undergo
oxidization to CoIII even under forcing conditions com-
pared to catalyst 1 (Supporting Information Table S6, S7).
Further stability measurements were conducted by system-
atically varying conditions of the cobalt(II) catalyst
3·MeOH; conditions such as temperature, pressure and
composition of the solution were varied. Catalyst 3·MeOH
in both CDCl3 and [D2]tetrachloroethane was found to pos-
sess a solution magnetic moment of 4.3 μB. In the presence
of one equiv. DMAP, this value was unchanged. Experi-
ments at elevated temperatures were conducted in tetra-
chloroethane to allow the monitoring of the magnetic
susceptibility up to those of established copolymerization
conditions. Catalyst 3·MeOH, in combination with one
equiv. DMAP and an excess of CHO under argon atmo-
sphere, showed no immediate change in its magnetic mo-
ment. Prolonged heating of the sample to 80 °C and mea-
surements at regular intervals revealed that μB stayed con-
stant at 4.3 even after 15 h. To exclude CO2 as a possible
oxidizing agent, a Young tube was charged with catalyst
3·MeOH, co-catalyst (DMAP), cyclohexene oxide under an
argon atmosphere, subsequently pressurized with 1 bar of
CO2 and heated to 80 °C. The solution magnetic moment
stayed constant at 4.3 μB over a period of 21 h. The same
experiment was conducted under an air atmosphere; under
these aerobic conditions a slight change of 0.1 μB after 21 h
Figure 8. 1H NMR of the polycarbonate catalyzed by 3·MeOH
(500 MHz, CDCl3, 298 K).
The role and capabilities of methanol as a chain transfer
agent in the copolymerization with 3·MeOH should be
evaluated. Catalyst 3·Acetone proved to be active for the
copolymerization of CHO/CO2 (Table 3, Entry 5) and, as
expected, produced PCHC with increased molecular
1
weight.[25,64] Comparison of the polymer H NMR spectra
between catalyst 3·MeOH-and 3·Acetone-derived material
showed that no methoxy end-groups are present in the case
of poly(cyclohexene carbonate) produced with 3·Acetone
(Supporting Information Figure S9). MALDI-TOF analy-
sis showed signals corresponding to a DMAP end-group
[17 (OH)
+ 142 n (repeating unit) + 82 (C6H10) +
122 (C7H10N2)] (Supporting Information Figure S8). Thus,
the direct comparison of 3·MeOH and 3·Acetone in the co-
polymerization of CHO and carbon dioxide is in good
agreement with previously described effects of chain trans-
fer agents in copolymerization reactions.[25–27,64] Increased
activity for the copolymerization with 3·MeOH is observed
compared to 3·Acetone. Altogether the polymerization
results of both catalysts are similar and show the minor
influence of methanol on the copolymerization in case of
3·MeOH.
Identifying the Active Species for Copolymerization of
Carbon Dioxide and Cyclohexene Oxide
The stability of our amine-bis(phenolato)cobalt(II) cata- was noted. Since copolymerizations are conducted at 50 bar
lyst system 3·MeOH against in situ oxidation during co- CO2 pressure, the stability of complex 3·MeOH was further
polymerizations was evaluated using NMR spectroscopy, evaluated at 50 bar CO2 pressure in the presence of one
ESI-MS and susceptibility measurements (Evan’s equiv. DMAP and an excess of CHO in tetrachloroethane.
method).[65,66] Following copolymerization of CHO and All components were charged in a suitable glass vessel, put
CO2 with catalyst 3·MeOH, the catalyst could be separated into a standard autoclave and pressured with 50 bar CO2.
from the oligomer via filtration, as the solubility of the After 15 h at 80 °C the autoclave was cooled to –78 °C to
oligomer in methanol is better than that of the catalyst avoid loss of volume, solvent or CHO, and the CO2 was
1
(Table 3, Entry 4). H NMR spectra taken before and after slowly released. The magnetic susceptibility of the catalyst
reaction were paramagnetic in nature and the constant was measured and found to have undergone no change in
purple color of the reaction mixture indicated that the cata- solution magnetic moment (4.3 μB). Moreoever, this post-
lyst did not change its oxidation state during the polymeri- reaction sample retained its original purple color. These ex-
zation; even small quantities of CoIII are strongly coloring periments indicate the profound stability of catalyst
(Supporting Information Figure S15). The Evan’s method 3·MeOH against oxidation under all tested conditions and
is an NMR technique for determining paramagnetic especially under copolymerization reaction conditions.
Eur. J. Inorg. Chem. 0000, 0–0
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FULL PAPER
prior to polymerization. NMR spectra were recorded with a Bruker
AVIII-300, AVIII-500 spectrometer. 1H NMR spectroscopic chemi-
cal shifts δ are reported in ppm relative to tetramethylsilane and
calibrated to the residual proton signal of the deuterated solvent.
Deuterated solvents were obtained from Sigma Aldrich and used
as received. ESI-MS analytical measurements were performed in
acetonitrile solutions with a Varian 500 MS spectrometer. Magnetic
measurements in solution were performed using Evan’s method
with a Bruker AVIII-300 spectrometer with a coaxial insert (Wil-
mad).[65,66] CDCl3 and C2D2Cl4 were used as solvent and TMS as
reference. In-situ IR measurements were carried out under an ar-
gon atmosphere using an ATR-IR Mettler Toledo system. IR spec-
tra were performed with a Bruker Vertex 70 spectrometer with a
Bruker Platinum ATR setup and the integrated MCT detector.
MALDI-TOF mass spectrometry was carried out using a Bruker
Ultraflex III, with a dithranol matrix and sodium trifluroacetate as
cationizing agent. Polymer, Matrix and sodium trifluoroacetate
were dissolved at a concentration of 5 mgmL–1 in THF and pre-
mixed at a ratio of 1:1:1. Gel permeation chromatography (GPC)
analysis was performed with a Varian PL-GPC 50 Plus at 30 °C
equipped with a PLgel Olexis Column Set (300ϫ7.5 mm). CDCl3
(HPLC grade) was used as solvent together with tetrabutylammo-
nium tetrafluoroborate at a flow rate of 1 mL/min and a polystryr-
ene standard. Elemental analysis was performed at the microana-
lytic laboratory of the Department of Inorganic Chemistry at the
Technical University of Munich.
Conclusions
We investigated the formation of cyclic propylene carb-
onate from carbon dioxide and propylene oxide, as well as
the copolymerization of carbon dioxide and cyclohexene
oxide with new monometallic amine-bis(phenolato)cobalt-
(II) (2–4) complexes. Catalysts 2–4 were used with their co-
ordinated solvent molecules in the copolymerization experi-
ments. Nevertheless, all catalysts enabled the coupling reac-
tion of PO and carbon dioxide to afford cPC. The dichloro-
substituted catalyst 3·MeOH achieved turnover numbers up
to 2060 for the formation of cPC. Reaction parameters for
the coupling of propylene oxide and CO2, such as tempera-
ture, pressure range and catalyst to co-catalyst ratio were
varied to optimize the reaction conditions (80 °C, 30 bar
CO2, [Cat]:[TBAB] = 1:1). Complexes 1 and 3·MeOH, in
combination with TBAB, did not yield any polycarbonate
in copolymerization experiments with PO/CO2 and CHO/
CO2. However, DMAP and catalyst 3·MeOH readily
copolymerized CHO and CO2 to generate atactic, low mo-
lecular weight polycarbonates with narrow polydispersities
(80 °C, 50 bar CO2, [Cat]:[TBAB] = 1:1, PDI = 1.48). End-
group analyses using ESI-MS, MALDI-TOF and NMR
measurements of the produced poly(cyclohexene) carbonate
unveiled DMAP and methoxy terminated polymer chains.
This finding indicates that DMAP initiates the copoly-
Coupling Reaction of CO2 and PO: All standard coupling reactions
were performed in 100 mL steel autoclaves equipped with glass in-
merization and that, due to the presence of an excess lay, magnetic stirring, and oil bath heating. The autoclaves were
heated to 130 °C and dried in an oven under vacuum prior to use
to remove trace water. The catalyst and co-catalyst were transferred
into the autoclave in the desired ratio, followed by addition of PO
(5 mL). No additional solvent was used. The reactor was closed,
pressurized with CO2 and heated to the desired temperature for
18 h. After cooling down the reaction vessel to 0 °C, CO2 was
slowly released. The remaining PO was removed under vacuum and
afterwards yield was determined by mass weight.
amount of methanol, chain-transfer reactions occur. Cata-
lyst 3·MeOH and 3·Acetone were used in copolymerization
reactions to investigate the effect of methanol as a chain
transfer agent. Increased activity and reduced molecular
weight products were obtained with catalyst 3·MeOH which
can be attributed to the presence of methanol in the copoly-
merization. Catalyst 3 is the first example of an amine-
bis(phenolato)cobalt(II) catalyst active in copolymerization
reactions of CHO/CO2. Therefore, particular attention was
given to the oxidation state of cobalt in this catalyst. The
nature of the active species during copolymerization was
scrutinized using Evan’s NMR method under various con-
ditions; no observable oxidation of catalyst 3·MeOH was
noted under polymerization conditions. Hence, all experi-
ments conducted with catalyst [(ONNO)ClCoII]*(MeOH)
indicate that the catalyst retains the CoII state under poly-
merization conditions and that this represents the active
species. Therefore, we conclude that the copolymerization
of CHO and CO2 is, in fact, catalyzed by the newly devel-
oped catalyst [(ONNO)ClCoII]*(MeOH).
Copolymerization Procedure: All standard copolymerization experi-
ments were performed in 100 mL steel autoclaves equipped with
glass inlay, magnetic stirring, and oil bath heating. The autoclaves
were heated to 130 °C and dried in an oven under vacuum prior to
use to remove trace water. The catalyst and co-catalyst were trans-
ferred into the autoclave in the desired ratio, followed by addition
of CHO (4.0 mL) and no additional solvent. The reactor was
closed, pressurized with CO2 and heated to 80 °C for 18 h. After
cooling down the reaction vessel to 0 °C, CO2 was slowly released.
After a small sample of the crude material was removed for charac-
terization, the product was dissolved in CH2Cl2, and the polymer
was precipitated from methanol. The product was then dried in
vacuo to constant weight.
Ligand Synthesis:
A solution of the disubstituted phenol
(2.00 equiv.), N,N dimethylethylenamine (1.00 equiv.) and 37%
aqueous formaldehyde (2.00 equiv.) in dist. water (LigH2) or meth-
anol (LigH4) was stirred and refluxed for 18 h. The mixture was
cooled to room temperature and the supernatant liquid decanted.
The remaining oil was recrystallized from ethanol, affording color-
less crystals.
Experimental Section
Methods and Materials: Unless otherwise stated, all manipulations
were performed under an argon atmosphere using standard
Schlenk techniques or an MBraun glovebox. Chemicals were pur-
chased from Sigma–Aldrich or Acros Organics and used without
further treatment if not otherwise stated. All glassware was heat-
dried under vacuum prior to use. Toluene, CH2Cl2, Et2O, pentane
and THF were dried applying an MBraun SPS-800 and used as
received. Monomers were dried with calcium hydride and distilled
H2[ONNO]CMe2Ph (LigH2): Yield 40%. C54H64N2O2 (773.11):
1
calcd. C 83.89, H 8.34, N 3.62; found C 84.13, H 8.49, N 3.83. H
NMR (300 MHz, CDCl3, 298 K): δ = 9.49 (br. s, 2 H, OH), 7.24–
7.04 (m, 22 H, HAr), 6.69 (d, 3J = 3 Hz, 2 H, HAr), 3.36 [s, 4 H,
3
3
N(CH2)2], 2.27 (t, J = 6 Hz, 2 H, NCH2), 2.08 (t, J = 6 Hz, 2 H,
Eur. J. Inorg. Chem. 0000, 0–0
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FULL PAPER
NCH2), 1.65 (s, 12 H, ArCH3), 1.62–1.61 [m, 18 H, ArCH3,
N(CH3)2] ppm. 13C NMR (75 MHz, CDCl3, 298 K): δ = 153.42,
152.16, 151.89, 139.93, 136.13, 128.30, 127.82, 127.54, 127.21,
126.44, 125.78, 125.43, 125.05, 122.88, 56.86, 56.25, 49.13, 44.48,
42.81, 42.40, 31.28, 29.52 ppm. MS (ESI) m/z (%, ion): 773.7 g/mol
[[M – H]+].
Acknowledgments
The authors thank Richard Reithmeier for support with the prepa-
ration of this paper, Dr. Malte Winnacker for measuring MALDI-
TOF experiments and Dr. Alexander Pöthig for additional crystal-
lographic measurements and editorial corrections.
H2[ONNO]Br (LigH4): Yield 55%. C18H20Br4N2O2 (615.98): calcd.
1
C 35.10, H 3.27, N 4.55; found C 35.45, H 3.25, N 4.52. H NMR
(300 MHz, CD2Cl2, 298 K): δ = 7.56 (d, 3J = 3.0 Hz, 2 H, HAr),
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Received: November 13, 2014
Published Online: ᭿
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Job/Unit: I43087
/KAP1
Date: 26-02-15 14:11:22
Pages: 10
www.eurjic.org
FULL PAPER
Cobalt Copolymerization Catalysts
M. Reiter, P. T. Altenbuchner, S. Kissling,
E. Herdtweck, B. Rieger* ............... 1–10
New amine-bis(phenolato)cobalt(II) com-
plexes were synthesized and employed in
the coupling and copolymerization of
carbon dioxide and epoxides. In combi-
nation with DMAP, complex 3 enabled
poly(cyclohexene) carbonate generation.
End-group analysis found DMAP and
methoxy terminated polymer chains due to
chain-transfer and the active species for
CHO/CO2 copolymerization was found to
be CoII.
Amine-bis(phenolato)cobalt(II) Catalysts
for the Formation of Organic Carbonates
from Carbon Dioxide and Epoxides
Keywords: Carbon dioxide fixation / Co-
polymerization / Cyclic carbonates / Epox-
ides / Cobalt
Eur. J. Inorg. Chem. 0000, 0–0
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