Mild formation of cyclic carbonates using Zn(II) complexes based on N 2S2-chelating ligands

Article abstract of DOI:10.1016/j.poly.2011.05.025

We have prepared a series of Zn(II) complexes (1-3) based on a versatile N₂S₂-chelating ligand abbreviated as btsc [btsc = bis-(thiosemicarbazonato)] derived from simple and accessible building blocks. These complexes comprise a Lewis acidic Zn(II) center useful for substrate activation, and we have investigated the potential of these compounds in the cyclo-addition reaction of carbon dioxide to various epoxides yielding cyclic carbonate structures. Initial screening studies with complexes 1-3 showed that complex 3 is most suited for this CO₂ fixation reaction under particularly mild conditions (45 °C, pCO₂ = 10 bar) and low catalyst loadings (1 mol%). Furthermore, upon examination of the substrate scope, complex 3 shows appreciable catalytic turnover for a range of terminal epoxides, while for the sterically more challenging epoxides almost no conversion was achieved under comparable conditions. Additional experiments indicated that higher yields of cyclic carbonates may be realized by simply increasing the (co)catalyst loading up to 3%, while maintaining mild reaction conditions. The use of a relatively non-toxic and abundant metal and an environmentally benign solvent system (MEK, methyl ethyl ketone) mark this protocol as an attractive way for organic carbonate production.

Full text of DOI:10.1016/j.poly.2011.05.025

Polyhedron 32 (2012) 49–53
Contents lists available at ScienceDirect
Polyhedron
journal homepage: www.elsevier.com/locate/poly
Mild formation of cyclic carbonates using Zn(II) complexes
based on N2S2-chelating ligands
a
b
a
a
´
Daniele Anselmo , Vladica Bocokic , Antonello Decortes , Eduardo C. Escudero-Adán ,
Jordi Benet-Buchholz a, Joost N.H. Reek b, , Arjan W. Kleij a,c,
⇑
⇑
a Institute of Chemical Research of Catalonia (ICIQ), Av. Països Catalans 16, 43007 Tarragona, Spain
b University of Amsterdam, van’t Hoff Institute for Molecular Sciences, Science Park 904, 1098 XH, Amsterdam, The Netherlands
c Catalan Institute for Research and Advanced Studies (ICREA), Pg. Lluís Companys 23, 08010 Barcelona, Spain
a r t i c l e i n f o
a b s t r a c t
Article history:
Available online 2 June 2011
We have prepared a series of Zn(II) complexes (1–3) based on a versatile N2S2-chelating ligand abbrevi-
ated as btsc [btsc = bis-(thiosemicarbazonato)] derived from simple and accessible building blocks. These
complexes comprise a Lewis acidic Zn(II) center useful for substrate activation, and we have investigated
the potential of these compounds in the cyclo-addition reaction of carbon dioxide to various epoxides
yielding cyclic carbonate structures. Initial screening studies with complexes 1–3 showed that complex
3 is most suited for this CO2 fixation reaction under particularly mild conditions (45 °C, pCO2 = 10 bar)
and low catalyst loadings (1 mol%). Furthermore, upon examination of the substrate scope, complex 3
shows appreciable catalytic turnover for a range of terminal epoxides, while for the sterically more chal-
lenging epoxides almost no conversion was achieved under comparable conditions. Additional experi-
ments indicated that higher yields of cyclic carbonates may be realized by simply increasing the
(co)catalyst loading up to 3%, while maintaining mild reaction conditions. The use of a relatively
non-toxic and abundant metal and an environmentally benign solvent system (MEK, methyl ethyl
ketone) mark this protocol as an attractive way for organic carbonate production.
Keywords:
Carbon dioxide
Cyclic carbonates
Homogeneous catalysis
N,S-chelating ligands
Zinc
Ó 2011 Elsevier Ltd. All rights reserved.
1. Introduction
during the formation of the cyclic carbonate target is water.
However, since CO2 has a tremendously high kinetic and thermody-
The synthesis of organic carbonates is an area of research that has
received a great deal of attention in the last decade [1–7]. These
carbonate structures find many useful applications including new
non-protic solvents, as substitutes for toxic reagents in organic syn-
thesis and as valuable precursors for their polymeric counterparts
[1,2,4]. Cyclic carbonates form a sub-family of carbonates which
can be derived from epoxides; in the past these were primarily pre-
pared by treatmentof the oxiranes by phosgene followedby ringclo-
sure [8]. This protocol, however, is characterized by the use of a
highly toxic and hazardous reagent and concomitantly produces
large amounts of corrosive by-products, which are difficult to han-
dle. For this reason, alternative synthetic pathways have been inves-
tigated and the cycloaddition of carbon dioxide (CO2) to epoxides
represents an attractive and green process. Such a process has a
number of advantages; CO2 is an economical, abundant, non-toxic
and renewable C1 reagent and the only by-product that is formed
namic stability, conversion into a cyclic carbonate requires catalytic
activation [9]. Various research groups explored homogeneous and
heterogenous catalytic solutions that are known to effectively cata-
lyze this transformation [6]. A major drawback of most of these
proposed catalysts is the need for harsh reaction conditions which
either translates into elevated CO2 pressures, high reaction temper-
atures or a combination of both. Obviously, such demanding process
conditions compromise the degree of sustainability, the net con-
sumption of CO2 and the overall energy balance.
Recent breakthroughs have demonstrated the potential that
metal-catalysis can have for the design of more sustainable options
focusing on (nearly) ambient conversion of CO2 [10–12]. In this re-
spect, metallosalen structures have emerged as powerful catalysts
for cyclic carbonate synthesis [6] with the Al(III) [11], Co(III) [13]
and Cr(III) complexes [14] among the most active systems. We
have recently designed Zn-based salen complexes that have shown
great promise in cyclic carbonate synthesis under extremely mild
(25 °C, p(CO2) = 2 bar) and green reaction conditions [15,16]. While
the salen ligands comprise typical N2O2 coordination environ-
ments around the complexed metal ion, other combinations of
hetero-atoms within the ligand structure could give rise to new
yet unexplored reactivity patterns. In our search for such ligands,
⇑
Corresponding authors. Address: Institute of Chemical Research of Catalonia
(ICIQ), Av. Països Catalans 16, 43007 Tarragona, Spain. Tel.: +34 977920247; fax:
+34 977920224 (A.W. Kleij), tel.: +31 (0) 20 5256437; fax: +31 (0) 20 5255604
(J.N.H. Reek).
E-mail addresses: J.N.H.Reek@uva.nl (J.N.H. Reek), akleij@iciq.es (A.W. Kleij).
0277-5387/$ - see front matter Ó 2011 Elsevier Ltd. All rights reserved.
doi:10.1016/j.poly.2011.05.025

50
D. Anselmo et al. / Polyhedron 32 (2012) 49–53
and a Kryoflex low temperature device (T = ꢁ173 °C). Full-sphere
data collection was used with and / scans.
R
R
x
1. R= Me, R1 = Ph
N
S
N
S
Programs used: data collection Apex2 V2011.3 (Bruker-Nonius
2008), data reduction Saint + Version 7.60A (Bruker AXS 2008)
and absorption correction SADABS V. 2008-1 (2008).
Structure solution: SHELXTL Version 6.10 (Sheldrick, 2000) was
used [23].
N
N
2.
3.
R= R1 = Me
R= R1 = Ph
Zn
Me
R1
N
N
H
Me
Scheme 1. Schematic structures for Zn(btsc) complexes 1–3.
Structure refinement: SHELXTL-97-UNIX Version.
Crystal data for 3 at 100 K: C26H25N7S2Zn1, Fw = 565.02 g molꢁ1
,
monoclinic, P21/c, a = 7.7782(6) Å, b = 32.165(2) Å, c = 10.1938(8)
Å, V = 2549.8(3) Å3, Z = 4, calcd = 1.472 Mg/m3, R1 = 0.0437
(0.0705), wR2 = 0.1007 (0.1200), for 3193 reflections with I > 2 (I)
we considered that bis-(thiosemicarbazonato) ligands (abbreviated
as btsc) could be useful to construct N2S2-chelated complexes as
these can be readily assembled from available precursors [17–
20]. It should be mentioned here that the choice for btsc ligands
opposing analogous salen systems having N2S2 donor sets can be
justified from the viewpoint of their synthetic accessibility; as a
testament of this postulation, only a few of these N2S2-salen li-
gands have been reported to date and their synthesis is less devel-
oped [21].
q
r
(for 4240 reflections [Rint: 0.0671] with a total measured of
14 756 reflections), 332 parameters, goodness-of-fit on F2 = 1.088,
largest difference in peak (hole) = 0.455 (ꢁ0.748) e Åꢁ3
.
3. Results and discussion
Herein we report on the use of Zn(II)–btsc complexes (Scheme 1)
as effective catalysts in cyclic carbonate synthesis under mild and
environmentally friendly conditions using comparably low loadings
of catalyst. This protocol features an easily synthesized catalyst
comprising a cheap and abundant metal. The scope of cyclic carbon-
ate synthesis has been investigated in detail and shows that the
Zn(II) based complexes are active mediators for this synthetic con-
version under mild conditions.
In order to establish the ability of complexes 1–3 to act as Lewis
acid activators we first examined their structural properties. As a
representative example, we combined complex 3 with various
epoxides to obtain crystals suitable for X-ray diffraction. Crystals
were obtained from CH3CN upon heating 3 in the presence of an
epoxide and allowing the mixture to cool to ambient temperature.
Small, needle-shaped crystals were obtained which were, unfortu-
nately, not suitable for X-ray diffraction. Recrystallization at 55 °C
afforded, however, better quality crystals in the case where epoxy-
hexane was utilized. These crystals were subjected to X-ray diffrac-
tion studies. An isolated sample of these crystals was dried in vacuo
and then examined by 1H NMR (DMSO-d6); surprisingly, the spec-
trum did not show the expected signals for the epoxide substrate
as only 3 was present.1 The crystal structure was then determined
and the result is presented in Fig. 1. In line with the 1H NMR obser-
vation, the structure does not contain an axially ligated epoxide,
but shows an unexpected self-assembled dimer mediated through
2. Experimental
2.1. Complex synthesis
The preparation of Zn(II)–btsc complexes 1–3 is straightforward
and consists of two steps. The first step involves preparation of the
(pro)ligand by condensation of a dicarbonyl compound with two
thiosemicarbazide molecules, followed by metallation with an
appropriate metal salt. Full details on the synthesis of 1–3 are pro-
vided elsewhere, see also Supporting Information [22].
l2-S bridging ligands [24,25–27].
Complex 3 crystallizes in the centro-symmetric space group
P2(1)/c. The Zn ions in each of the two units of 3 are pentacoordi-
nate with the chelating N2S2 fragment occupying the basal plane of
the approximate square pyramidal geometry. Both S2 and S2a sul-
fur donor atoms act as anionic bridging ligands between the Zn1a
and Zn1 metal centers completing the pentacoordination. The for-
mation of the central S2Zn2 unit causes the Zn center to displace
from the N2S2 coordination plane as illustrated by the difference
in Zn–S bond distances (Zn1–S1 = 2.35 Å and Zn1–S2a = 2.42 Å)
within each monomeric unit.
This ‘‘self-dimerization’’ resembles the well-known self-assem-
bly behavior of Zn(salen) complexes (having N2O2 donor sets) for
which exists ample literature precedent [28–30]. We recently re-
ported extensive studies on the features that control the strength
of this self-assembly process [31,32]; one key feature is the steric
control when, ortho to the O-donor atoms, large fragments are
introduced. In these cases dimer formation may be minimized
and monomeric complexes prevail. Connected with the substitu-
tion pattern of these salen ligands is their ability to act as Lewis
acid activators. We have reported on a series of Zn(II)salphen com-
plexes [salphen = N,N-bis(salicylidene)imine-1,2-diaminobenzene]
that were applied as efficient catalysts for the cycloaddition of car-
bon dioxide (CO2) to terminal epoxides under ambient conditions
[15,16]. Within these studies, those Zn-complexes comprising
sterically demanding groups near the O-donor atoms showed the
2.2. Catalytic protocol
2.2.1. Typical procedure
Catalyst 3 (2.0 ꢀ 10ꢁ5 mol, 10.4 mg) and (co-catalyst) NBu4I
(2 ꢀ 10ꢁ5 mol, 7.4 mg) were added in a two-necked round-bot-
tomed flask under a gentle flux of argon. Then, methyl ethyl ketone
(5.0 mL) and mesitylene (280
the solution stirred until complete dissolution occurred. To this
mixture, propylene oxide (140
the solution transferred to a 25 mL stainless-steel reactor previ-
ously purged with argon. Three cycles of pressurization and
depressurization of the reactor (with CO2 at 5 bar) were carried
out before finally stabilizing the pressure at 10 bar and the solution
left stirring at 45 °C for 18 h. A sample of the solution was then
analyzed by means of 1H NMR spectroscopy (DMSO-d6) and the
yield determined using mesitylene as an internal standard.
l
L, 2.0 ꢀ 10ꢁ3 mol) were added and
l
L, 2.0 ꢀ 10ꢁ3 mol) was added and
2.3. X-ray structure determination
Crystals of compound 3 were obtained by slow evaporation in
CH3CN at 55 °C. The measured crystals were stable under atmo-
spheric conditions; nevertheless they were prepared under inert
conditions immersed in perfluoropoly-ether as protecting oil for
manipulation.
Data collection: measurements were made on a Bruker-Nonius
diffractometer equipped with an APPEX 2 4 K CCD area detector,
a FR591 rotating anode with Mo K
1
This results contrasts our previous findings for related Zn(salphen) complexes
where the coordination of various epoxides was crystallographically characterized
[see Ref. [15]]. Apparently, under these conditions the Zn(btsc) complex 3 preferen-
tially crystallizes as a dimer.
a radiation, Montel mirrors

D. Anselmo et al. / Polyhedron 32 (2012) 49–53
51
Fig. 1. X-ray molecular structure for (3)2 with the adopted numbering scheme. Co-crystallized solvent molecules and H-atoms are omitted for clarity. Selected bond lengths/
angles: Zn(1)–S(1) = 2.3546(9) Å, Zn(1)–S(2a) = 2.4236(11) Å, Zn(1)–N(3) = 2.113(3) Å, Zn(1)–N(1) = 2.107(3) Å, Zn(1)–S(2) = 2.4592(13) Å, S(2a)–Zn(1)–S(1) = 108.82(4)°,
S(2a)–Zn(1)–S(2) = 96.77(4)°, S(2a)–Zn(1)–N(3) = 80.91(9)°, S(2a)–Zn(1)–N(1) = 147.16(11)°, S(1)–Zn(1)–N(3) = 142.61(9)°, S(1)–Zn(1)–S(2) = 108.57(4)°, S(1)–Zn(1)–
N(1) = 79.54(8)°.
highest activities, whereas Zn(salphen) building blocks with smal-
ler groups proved to be less efficient ascribed to their competing
self-dimerization. This self-assembly behavior thus limits the
effectiveness in Lewis acid mediated catalysis. The occurrence of
Zn(salphen) self-assembly is easily recognized by UV–Vis spectros-
copy and titration of these complexes with pyridine leads to signif-
icantly different absorption spectra [31,32]. Therefore, the possible
dimer formation of Zn complex 3 was additionally studied by UV–
Vis titration with a suitable titrant (epoxyhexane) in different sol-
vents (methyl ethyl ketone) to evaluate the formation of mono-
meric 3 upon addition of the epoxide. The addition of epoxide
did not lead to any observable changes in the UV–Vis spectrum
which may be interpretated as 3 already existing as a monomer
under these conditions. Alternatively, when we titrated complex
3 with pyridine (Supporting Information), clear 1:1 complexation
was observed and the stability constant Ks is in the same order
as for pyridine binding at Zn(salphen) structures. This further sup-
ports the preferential presence of monomeric 3 in solution.
Complexes 1–3 were screened as catalysts for the formation of
cyclic carbonates using CO2 and propylene oxide as reagents, under
relatively mild reaction conditions (T = 45 °C, p(CO2) = 10 bar) as a
starting point using tetrabutylammonium iodide as co-catalyst and
methyl ethyl ketone (MEK) as medium (Table 1).
Table 1
Screening of reaction conditions and catalysts (1–3) using propylene oxide as
standard substrate, a p(CO2) of 10 bar, NBu4I as co-catalyst and a reaction time of
18 h, [Zn(btsc)] = 3.7–11.1 mM.
CO2
Catalysts 1-3
O
O
O
Me
NBu4I, solvent
25-65º, pCO2 = 10 bar
18 h
O
Me
Entry
Complex (mol%)
Co-cat (mol%)
T (°C)
Solvent
Yielda (%)
1
2
3
4
5
6
7
8
1 (1)
1 (1)
1 (1)
2 (1)
2 (1)
2 (1)
3 (1)
1
2.5
1
1
2.5
1
45
45
65
45
45
65
45
45
25
45
65
45
45
45
45
45
45
MEK
MEK
MEK
MEK
MEK
MEK
MEK
MEK
MEK
MEK
MEK
MEK
MEK
THF
15b
72b
70b
18b
54b
52b
0
1
1
1
1
1
1
1
1
1
1
0
9
3 (1)
3 (1)
3 (1)
3 (2)
3 (3)
3 (1)
3 (1)
3 (1)
3 (1)
18
66
89
76
90
16
5c
10
11
12
13
14
15
16
17
All three complexes proved to be active, although in those reac-
tions employing either complex 1 or 2 (entries 1–6) heterogeneous
mixtures were noted that may have affected catalytic turnover.2 In
all cases conversion to propylene carbonate was achieved, with the
best performance noted for complex 3 (see entries 9–11). It should
be noted that in the absence of either the catalyst (3) or co-catalyst
DCM
CH3CN
acetone
19
54
a
Yield determined by 1H NMR (DMSO-d6) using mesitylene as an internal
standard. No other products were observed and therefore the selectivity toward the
cyclic carbonate is at least 99%.
b
Note that in these cases only partial dissolution of the complexes is achieved
before as well as after the reaction.
c
After the reaction a heterogeneous mixture was observed.
2
Attempts to solubilize complexes 1-2 at higher temperatures failed which
indicates that during the reactions with these catalysts only partial dissolution is
achieved. Furthermore, we analyzed these precipitates in order to identify any
possible decomposition products: in these cases only the starting complexes 1-2 were
identified by 1H NMR spectroscopy.
(entries 7–8) no conversion was observed, which emphasized the
need for both catalyst components for effective turnover. As may

52
D. Anselmo et al. / Polyhedron 32 (2012) 49–53
Table 2
Screening of the substrate scope at 45 °C, pCO2 = 10 bar, NBu4I as co-catalyst (1 mol%), MEK as solvent and a reaction time of 18 h using catalyst 3 (1 mol%).
CO2
Catalyst 3
O
O
R´
O
R
R
O
NBu4I (1%), MEK
45º, pCO2 = 10 bar
18 h
R´
Entry
1
Substrate (R)
Me
Substrate (R0)
H
Catalyst (mol%)
1
Product
Yielda (%)
66
O
O
Me
Me
Me
Me
O
2
Me
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
2
76b
90c
90d
62
O
O
O
3
Me
3
O
O
O
4
Me
2.5
1
O
O
O
5
CH2CH2CH@CH2
Bn
O
O
CH2CH2CH=CH2
Ph
O
6
1
64
O
O
O
7
Bn
2
94b,e
54
O
Ph
O
O
8
CH2OH
CH2Cl
1
O
OH
Cl
O
O
9
1
60
O
O
O
10
11
12
13
14
15
16
17
CH2OMe
Ph
1
37
O
OMe
O
O
1
29
O
O
Ph
O
n-Bu
1
27f
80d
57
O
O
nBu
O
n-Bu
2.5
1
O
O
nBu
O
CH2OCH2Ph
CH2OCH2Ph
CH2OCH2Ph
1,1-di-Me
O
O
CH2OCH2Ph
CH2OCH2Ph
O
2
98b,g
83d
1
O
O
O
2.5
1
O
O
CH2OCH2Ph
Me
O
O
O
O
Me
Me
18
19
Me
Me
H
1
1
1
O
O
O
Me
CH@CH2
30
O
O
O
a
b
c
d
e
f
Yield determined by 1H NMR (DMSO-d6) using mesitylene as an internal standard.
2 mol% of co-catalyst used.
3 mol% of co-catalyst used.
Taken from Ref. [15]; here a Zn(salphen) complex was used.
The calculated conversion using signal integration for both the remaining epoxide as well the carbonate product was 92%.
The duplicate experiment gave a 29% yield of the product.
g
The calculated conversion was 97%.

D. Anselmo et al. / Polyhedron 32 (2012) 49–53
53
be expected, increasing co-catalyst concentration (entries 2 and 5),
Appendix A. Supplementary data
increasing the reaction temperature (entries 3, 6 and 11) or
increasing the catalyst loading (entries 12 and 13) gives higher
yield of propylene carbonate. The influence of other solvents on
the product yield was also observed (entries 14–17); unlike in pre-
vious work [16], both DCM as well as CH3CN turned out to be poor
solvents, whereas acetone proved to be a rather good alternative.
The catalyst 3 was further employed to study the substrate
scope in the formation of cyclic carbonates using various terminal
epoxides (Table 2), MEK as solvent and using mild reaction condi-
tions (45 °C, p(CO2) = 10 bar) and a low catalyst loading. When
using 1 mol% (3.7 mM) of 3, yields of up to 66% are achieved (entry
1). These yields can simply be improved by increasing the catalyst
load up to 3 mol% (11.1 mmol/l) giving, for instance, propylene car-
bonate in high(er) yield (entry 3, 90%). The same effect is observed
for the conversion of other epoxides (entries 7 and 15). As also pre-
viously observed with Zn(salphen) based catalysts with sterically
more congested epoxides [15,16], conversion of 1,1-dimethyl-
and 1,2-dimethyl-oxirane (entries 17 and 18) proved to be difficult,
which relates to the steric impediment in the ring-opening step of
the coordinated epoxide upon using a bulky nucleophile (i.e., io-
dide). As may be expected for the less reactive epoxidic substrates
(entries 11 and 19) lower yields under comparable conditions are
achieved, while conversely, the benchmark substrate epoxyhexane
unexpectedly gives only 27% yield (entry 12, 27%). This is lower
than achieved with a similar substrate (entry 5) or using Zn(sal-
phen) under similar reaction conditions [15,16].
CCDC 817975 contains the supplementary crystallographic data
for 1–3. These data can be obtained free of charge via http://
www.ccdc.cam.ac.uk/conts/retrieving.html, or from the Cambridge
Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ,
UK; fax: (+44) 1223-336-033; or e-mail: deposit@ccdc.cam.ac.uk.
Supplementary data associated with this article can be found, in
the online version, at doi:10.1016/j.poly.2011.05.025.
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4. Conclusions
In this work we have presented application of a new type of Zn
complex based on btsc [btsc = bis-(thiosemicarbazonato)] ligands
as effective catalysts for the cyclo-addition of CO2 to various termi-
nal expoxides under relatively mild reaction conditions. Complex 3
proved to be the most useful among the series 1–3 under our reac-
tion conditions. Although X-ray diffraction revealed the formation
of an unexpected dimeric assembly, the monomeric form is domi-
nant in solution, as only the monomer is catalytically active. Also
the UV–Vis experiments suggest that in solution the monomeric
complex likely prevails. The use of a cheap and abundant metal in
combination with an environmentally tolerable solvent such as
MEK and mild reaction conditions further mark this catalytic pro-
cess as a potentially sustainable way for cyclic carbonate synthesis.
publication.
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This work was supported by ICIQ, ICREA, the Spanish Ministry
of Science and Innovation (CTQ 2008-02050), Consolider Ingenio
2010 (CSD 2006-0003), the Marie Curie Research and Training Net-
work ‘‘RevCat’’ and the Dutch National School Combination Catal-
ysis Controlled by Chemical Design (NRSC-Catalysis).