Aldimine-Thioether-Phenolate Based Mono- and Bimetallic Zinc Complexes as Catalysts for the Reaction of CO2 with Cyclohexene Oxide

Article abstract of DOI:10.1002/ejic.202000119

A synthetic strategy for the preparation of a new class of ligands is introduced. The bis(aldimine-thioether-phenolate) ligands bear two anionic oxygen donors from phenolate moieties, two neutral sulfur donors and two neutral nitrogen donors belonging, respectively, to thioether and aldimine functionalities. The so designed OSNNSO ligands show two coordinative pockets and thus should be able to host two metallic centers. Instead, the aldimine-thioether-phenolate OSN ligands are tridentate ligands which may form monometallic complexes. Two OSNNSO and one OSN ligands of this class, have been synthesized. Satisfyingly, by direct reaction of the ligands with one or two equivalents of the zinc precursor, the corresponding bimetallic Zn(II) complexes were prepared from the bis(aldimine-thioether-phenolate) ligands, while the monometallic zinc complex was prepared from the tridentate ligand. These zinc amido complexes act as single component catalysts in the reaction of CO₂ with cyclohexene oxide by furnishing polycyclohexene carbonate, while adding PPNCl as cocatalyst to the reaction medium, cyclohexene carbonates were obtained as the main products.

Full text of DOI:10.1002/ejic.202000119

DOI: 10.1002/ejic.202000119
Full Paper
Carbon Dioxide Fixation
Aldimine-Thioether-Phenolate Based Mono- and Bimetallic Zinc
Complexes as Catalysts for the Reaction of CO2 with
Cyclohexene Oxide
Mariachiara Cozzolino,[a] Flavia Melchionno,[a] Federica Santulli,[a] Mina Mazzeo,[a] and
Marina Lamberti*[a]
Abstract: A synthetic strategy for the preparation of a new have been synthesized. Satisfyingly, by direct reaction of the
class of ligands is introduced. The bis(aldimine-thioether- ligands with one or two equivalents of the zinc precursor, the
phenolate) ligands bear two anionic oxygen donors from corresponding bimetallic Zn(II) complexes were prepared from
phenolate moieties, two neutral sulfur donors and two neutral the bis(aldimine-thioether-phenolate) ligands, while the mono-
nitrogen donors belonging, respectively, to thioether and metallic zinc complex was prepared from the tridentate ligand.
aldimine functionalities. The so designed OSNNSO ligands show These zinc amido complexes act as single component catalysts
two coordinative pockets and thus should be able to host two in the reaction of CO2 with cyclohexene oxide by furnishing
metallic centers. Instead, the aldimine-thioether-phenolate OSN polycyclohexene carbonate, while adding PPNCl as cocatalyst
ligands are tridentate ligands which may form monometallic to the reaction medium, cyclohexene carbonates were obtained
complexes. Two OSNNSO and one OSN ligands of this class, as the main products.
Introduction
lysts to be used in sustainable processes. As for the ancillary
ligand, salen ligands show several advantages since they are
easy to synthesize, cheap and may be easily sterically and elec-
tronically modified.[3b,3c]
The presence of CO2 in the atmosphere makes possible life on
Earth. On the other hand, its uncontrolled growth, mainly due
to human activities, is causing several problems to the whole
ecosystem. For this reason, the fixation of massive quantities of
CO2 as a C1 building block into useful products is a challenging
goal.[1] The reaction with highly energetic molecules allows to
overcome the high thermodynamic stability of CO2. In particu-
lar, the relief of ring strain associated with epoxides is the driv-
ing force for chemical conversion of CO2 into cyclic carbonates
and/or polycarbonates.[2] To date, polycarbonates made from
carbon dioxide have been mainly used as binders, sacrificial
materials and as polyols in the manufacturing of polyurethane;
while cyclic carbonates find applications as non-protic polar sol-
vents, electrolyte in lithium ion batteries and as precursors for
the synthesis of drugs and polymers.
The formation of both products requires the presence of a
catalyst to reduce the energy demand of the whole process.[3]
The behaviour of a homogeneous catalyst basically depends on
the metal and on the ancillary ligand, which remains bound to
the metal through the whole catalytic cycle and allows to mod-
ify the reactivity of the catalyst. Inexpensive and biorelevant
metals, such as zinc,[4] constitute a valid choice to obtain cata-
Salen-type zinc complexes have been reported by some au-
thors as active catalysts for the production of cyclic carbon-
ates.[3c,5] On the other hand, polycarbonate synthesis has been
obtained by employing bimetallic zinc complexes bearing dif-
ferent classes of ligands. Starting from seminal works of Coates
which showed, for ꢀ-diiminate zinc complexes, the fundamental
role of a binuclear transition state to obtain the polycarbonate
products,[6] a number of different dizinc complexes active in the
fixation of CO2 in polycarbonates have been reported in the
literature.[7–9]
With the aim to combine the advantages of salen ligands
and zinc metal in a new catalytic system potentially active in
the production of polycarbonates, we tried to synthesize bime-
tallic salen-based zinc complexes by increasing the length of
the bridge between the two nitrogen atoms of the ligand skele-
ton, a strategy which worked well with aluminum complexes
bearing salen,[10a,10b] salan[10c] and also salalen ligands.[10d] Un-
fortunately, in the case of the zinc complexes, the monometallic
derivative was always obtained, reasonably because zinc cen-
ters in the dimetallic complexes would be tricoordinated while
the monometallic derivatives fix the zinc metal in a tetracoordi-
nate environment.[11] Following this reasoning we designed a
new class of ligands which resemble salen ligands, thus preserv-
ing some of their advantages, yet present two additional sulfur
atoms as neutral donors.
[a] Department of Chemistry and Biology “Adolfo Zambell”,
University of Salerno,
Via Giovanni Paolo II, 132, 84084 Fisciano, SA Italy
E-mail: mlamberti@unisa.it
http://docenti.unisa.it/marina.lamberti
Supporting information and ORCID(s) from the author(s) for this article are
available on the WWW under https://doi.org/10.1002/ejic.202000119.
In this paper we describe the synthetic strategy conceived
for the preparation of these new bis(aldimine-thioether-phenol-
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ate) OSNNSO ligands and the corresponding aldimine-thio- dry C6D6 and by MALDI-ToF spectrometry (Supporting Informa-
ether-phenolate OSN ligands, the synthesis and the characteri- tion Figures S3–S11).
The H NMR spectrum of L1H2 (Supporting Information Fig-
ure S3) shows: a singlet for the protons of the hydroxyl groups
at 9.08 ppm, a singlet at 8.17 ppm due to the imine protons,
and six signals with predictable multiplicity between 7.45 and
6.43 ppm for the aromatic protons. The methylene protons
bound to the sulfur (SCH2Ar) and to the nitrogen (NCH2) were
observed as two singlets, each integrating for 4 protons, respec-
tively at 3.96 ppm and 3.40 ppm. Finally, two singlets, each
integrating for 18 protons, were observed at 1.72 and 1.30 ppm
for the protons of the tert-butyl groups. As expected, the 1H
NMR spectrum clearly indicated a high symmetry for the de-
scribed ligand. The number of the signals observed in the 13C
NMR spectrum was coherent with this observation (Supporting
Information Figure S4). As for the ligand precursor L2H2, both
the 1H and the 13C NMR (Supporting Information Figures S6
and S7) suggest a situation similar to that observed for L1H2,
with a high symmetry in solution. A detailed assignment is re-
ported in the Supporting Information part.
The MALDI-ToF spectrum of L1H2 showed three signals (Sup-
porting Information Figure S5): one at 737.417 m/z for the
molecular ion, another one at 517.234 m/z indicating the frag-
ment formed after the break of one of the S-CH2 bond and,
finally, a peak at 299.067 m/z indicating the fragment formed
after the breaking of both the S-CH2 bonds. A similar fragmen-
tation pattern was observed for L2H2 (Supporting Information
Figure S8).
1
zation of the corresponding zinc complexes and, finally, cata-
lytic experiments employing the zinc complexes in the reaction
of CO2 with cyclohexene oxide under different reaction condi-
tions.
Results and Discussion
A three step procedure (Scheme 1) was designed to prepare
the new class of OSNNSO ligands showing two anionic oxygen
donors from phenolate moieties, and four neutral donors: two
sulfur atoms and two nitrogen atoms belonging, respectively,
to thioether and aldimine functionalities. The proper choice of
the amine reagents in the third step allows the obtaining of
tridentate OSN ligands by the same synthetic strategy.
First, thiophenol, premixed with N,N,N′,N′-tetramethylethyl-
enediamine (TMEDA), reacted with n-butyllithium giving the
deprotonated product, which subsequently gave the 2-mercap-
tobenzaldehyde by reaction with dry dimethyl formamide in
hexane as solvent.[12] The second step was the reaction of 2-
mercaptobenzaldehyde with 2-(bromomethyl)-4,6-di-tert-butyl-
phenol, purposely synthesized,[13] using dry dimethyl forma-
mide as the solvent. Finally, the desired ligands were obtained
by condensation of two equivalents of the 2-(3,5 di-tert-butyl-2-
hydroxybenzyl) sulfanyl benzaldehyde (Supporting Information
Figures S1–S2) with the opportune diamine in dry acetonitrile.
The so designed bis(aldimine-thioether-phenolate) ligands in-
clude two coordinative pockets and thus should be able to host
The synthesis of the zinc complexes 1–3 was accomplished
two metallic centers. Obviously, to achieve the cooperative ac- by treating the ligands with 2 equivalents (L1H2 and L2H2) or
tion of the two metal centers, their distance is a critical parame- one equivalent (L3H) of zinc bis[bis(trimethylsilyl)amide] in
ter. For this reason, in the first instance, we chose two diamines benzene. After two hours at room temperature the benzene
with a different length of the alkylene chain between the nitro- was removed under vacuum and the product was washed with
gen atoms: The ligand precursor L1H2 is based on ethylenedi- cold hexane to remove any impurities including the bis(trimeth-
amine while the ligand precursor L2H2 is based on 1,3-propane- ylsilyl) amine formed as a co-product. The zinc complexes 1–3
diamine. In the case of the tridentate ligand L3H propanamine (Scheme 2) appeared as yellow powders (95 %, 91 % and 97 %
was used in the condensation reaction. The OSNNSO ligands yields, respectively). The three complexes were characterized by
1
and the OSN ligand were characterized by H and 13C NMR in NMR (Supporting Information Figures S12–S25). The 1H NMR
Scheme 1. Synthesis of the ligand precursors L1H2, L2H2 and L3H. (i) DMF; (ii) 0.5 equivalents of ethylenediamine or 1,3-propanediamine in CH3CN;
(iii) 1 equivalent of propanamine in CH3CN.
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spectrum of complex 1 (Supporting Information Figure S12)
showed the disappearance of the OH signal of the ligand and
the appearance of two new broad peaks, centered at 0.14 and
0.04 ppm and partially overlapping, integrating to a total of 36
protons, attributable to the protons of the silylamido groups.
Moreover, the narrow singlets observed for the methylene pro-
tons bound to the sulfur (SCH2Ar) and to the nitrogen
(NCH2CH2N) in the ligand spectrum, split in three broad signals
at 4.98, 4.51 and 3.57 ppm, integrating, respectively, for two,
four and two protons (see assignment in Figure S12 of the Sup-
porting information). All the other signals due to the protons
of the ligand skeleton were easily recognizable, most of them
shifting downfield with respect to the same signals in the spec-
trum of the free ligand.
Figure 1. Enlargement of 1H NMR spectrum (600 MHz, tol-d8, –10 °C) of com-
plex 1. * denotes signals due to toluene-d8 protons.
are present for the methylene protons, among them the four
doublets for the two AB patterns of the methylene protons
bound to the sulfur (SCH2Ar) are well recognizable (green circles
in Figure S19). Coherently, in the aromatic region, different sig-
nals are observable for each proton (see, for example, the two
Scheme 2. Zinc complexes 1–3 synthesized in this work.
A similar 1H NMR spectrum was obtained for complex 2 (Fig- peaks denoted with purple circles for the imine protons and
ure S17 Supporting Information), but in this case, the peaks the four signals denoted with light blue circles for the aromatic
of all the methylene protons, bound to the sulfur (SCH2Ar) or protons on the phenolate moieties). The whole picture suggests
corresponding to the bridge between the nitrogen atoms that, in the case of complex 2, the symmetry is lost at low
(NCH2CH2CH2N), appear as very broad signals in the range 2.5– temperature.
1
4.5 ppm. Moreover, only one broad signal, at 0.08 ppm, was
observed for the 36 protons of the silylamido groups.
Also in the case of complex 3, the H NMR spectrum (Sup-
porting Information Figure S23) indicated the formation of the
These observations support, for both complexes, the coordi- desired complex by the disappearance of the hydroxyl proton
nation of two zinc centers to the anionic and neutral donors of and the presence of a signal at high field, integrating for the
the ligand, suggesting a tetravalent coordination sphere for 18 protons of the amido methyl groups and the presence of
both zinc centers, each of them resulting bound to: the oxygen two broad signals around 3.5 and 4.0 ppm, each integrating to
of the phenolate, the sulfur donor, the nitrogen of the aldimine 2 protons, attributable to the methylene protons respectively
and the nitrogen of the silylamido labile ligand. The spectra bound to the sulfur and the nitrogen donors. For complex 3, a
also suggest that the symmetry of the ligands was retained well resolved spectrum was recorded at –40 °C in toluene-d8
upon the formation of the complexes, at least in solution.
On the other hand, the broad resonances observed for all
the methylene protons in both spectra are consistent with the
(see Figure S24 and S25 in the Supporting Information for the
whole spectrum and for an enlargement of the range between
1.4 and 8.0 ppm).
1
formation of fluxional species. For this reason, H NMR spectra
With these three new complexes in hand, their behavior as
of both complexes were recorded at sub-ambient temperature catalysts for the fixation of CO2 in organic carbonates was stud-
1
in toluene-d8. In the H NMR spectrum of complex 1 at –10 °C ied.
(see Figure S13 in the Supporting Information for the whole
spectrum and Figure 1 for an enlargement of the range be-
tween 3 and 9 ppm) four sharp and well resolved signals, each
integrating for two of the protons of the methylene groups, can
be detected in the range 3.5–5.0 ppm. Accordingly, seven sig-
nals for the aromatic and imine protons, each integrating for
2H, are observable. This spectrum suggests that, at this temper-
ature, the fluxional process becomes slower than the NMR time-
scale and that the symmetry of complex 1 is preserved.[14]
Firstly, the dizinc complex 1 was tested in the reaction of
carbon dioxide and cyclohexene oxide under different reaction
conditions (Table 1). This reaction can give different possible
products (see Scheme 3): namely, cis-cyclohexene carbonate
(cis-CHC), trans-cyclohexene carbonate (trans-CHC) and poly-
(cyclohexene carbonate) (PCHC) eventually containing ether
linkages (PCHO). The conversion of the epoxide was measured
by 1H NMR spectroscopy by integrating the signal at δ =
4.6 ppm (methine protons of both PCHC and cis-CHC), the sig-
A well resolved 1H NMR spectrum of complex 2 was recorded nal at δ = 4.0 ppm (methine protons of trans-CHC) and the
at –50 °C in toluene-d8 (see Figure S18 and S19 in the Support- signal at δ = 3.5 ppm (methine protons of the ether linkages)
ing Information for the whole spectrum and for an enlargement with respect to the analogous protons of the CHO (δ =
of the range between 2.5 and 8.5 ppm). In this case, ten signals 3.1 ppm).
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Table 1. CO2/CHO reaction promoted by complexes 1–3.
Entry[a]
Complex
Temp [°C]
Pressure [bar] Time [h]
Conv[b]
PCHC
Trans-CHC
Ether
Mn
Ð[c]
GPC[c]
[mol-%]
linkages
[kDa]
1
2
3
4
5
6
7
8
1
1
1
1
1
1
2
3
3
1
1
80
30
30
30
30
20
10
30
30
30
30
30
16
16
16
64
16
16
16
16
16
16
16
10
20
29
52
22
13
24
13
26
13
32
97
90
82
93
90
85
91
86
86
85
91
3
3
10
3
2
7
2
5
5
<1
7
8
4
8
8
7
9
9
8.7
8.3
6.0
6.5
9.4
9.2
10.5
3.7
15.4
4.5
8.8
7.0
5.5
3.7
3.3
4.3
4.6
7.2
5.6
7.8
5.9
6.2
100
120
100
100
100
100
100
100
100
100
9[d]
10[e]
11[f]
<1
3
15
6
[a] General conditions: Complexes 1–3 = 19.8 μmol (0.05 mol-%), CHO = 40 mmol, (2000 equiv.). [b] The conversion and amount of CHC were determined by
integration of methine peaks in the 1H NMR spectra. [c] Determined by GPC, in THF, using polystyrene standards, for calibration. [d] Complex 3: 39.6 μmol.
[e] 0.05 mol-% of cyclohexane-1,2-diol (CHD) added in the reaction mixture. [f] Freshly bi-distilled CHO was used in this polymerization experiment.
sonable attributed to the decrease in the concentration of CO2
in solution.[2a]
The activity of complex 1 in the reaction of CO2 with CHO
can be expressed in terms of TOF (TurnOver Frequency) which
is calculated as the mole epoxide converted to product, per
mole metal per hour. From the results reported in Table 1, com-
Scheme 3. Possible products generated by the reaction of CO2 with cyclohex-
ene oxide (CHO).
plex 1 showed TOF up to 36 h–1 under the explored conditions,
indicating a moderate activity, according to the basis used by
Coates and Moore.[3a]
Subsequently, complexes 2 and 3 were tested in the reaction
of CO2 with CHO under the same conditions of entry 2
(0.05 mol-% of catalyst, 30 atm of CO2, 100 °C, 16 h). In both
cases, polycyclohexene carbonate was the most abundant
product (see entries 7 and 8), with complex 2 showing a double
activity with respect to complex 3. In order to have the same
concentration of metallic centers for the mono- and bi-metallic
complexes, another polymerization experiment (entry 9) was
carried out by doubling the mole amount of complex 3. Under
these conditions, the activity of complex 3 was comparable to
those obtained with both complexes 1 and 2 (cf entries 2, 7
and 9). This result suggests the absence of a cooperative effect
in the case of the bimetallic complexes 1 and 2, indicating that
each metallic center works independently from the other one.
The product ratio, corresponding to the reaction selectivity,
was determined through high-resolution 1H NMR spectra
(CDCl3, r.t., 600 MHz) after removing the residual CHO, by inte-
grating the signals due to the methylene protons of the differ-
ent products (see, for example, Figure S28 in the Supporting
Information).[10d,15] The first reaction trials were carried out in
neat CHO, by using complex 1 as a single component catalyst
(i.e. no cocatalyst was added to the reaction mixture), at a low
catalyst loading (0.05 mol-%) by varying reaction temperature
and CO2 pressure. In all cases poly(cyclohexene carbonate) was
obtained as the main product with a selectivity depending on
the experimental conditions. In addition, small percentages of
ether linkages and trans-cyclohexene carbonate were found in
the reaction mixture.
GPC analysis of the polycarbonates showed, in all cases, low
molecular weights (Mn = 3.7-15.4 kDa) and broad molecular
weight distributions (Ð = 3.3-7.8, see Figure S29 in the Support-
ing Information) which could indicate either the formation of
multiple active centers and/or the occurrence of post-copoly-
merization reactions (such as hydrolysis, decarboxylation or
other degradation).[16]
At 80 °C and under 30 atmosphere of CO2 pressure, a conver-
sion of 10 % was obtained (Table 1, entry 1). By increasing the
reaction temperature, the conversion increased (cf entries 2 and
3 with entry 1) but at 120 °C an increase in the trans-CHC prod-
uct was also observed. Trans-CHC are usually generated by
back-biting reaction which can occur at the end of the free or
metal-bound copolymer, a process that is usually accelerated at
higher temperature.[2a]
Accordingly, the MALDI-ToF spectra showed the presence of
several molecular weight distributions. The MALDI-ToF spec-
trum of the PCHC obtained in entry 2 shows three main distri-
butions centered around 1050, 3970 and 6100 Da (Supporting
Prolonging the reaction time, the conversion increased
showing that catalyst 1 is still active after 64 h of reaction (entry
4).
Two catalytic reactions were carried out under 20 and 10 bar Information Figure S30). However each distribution is the result
of CO2 pressure at 100 °C. In these conditions, the conversions of the overlapping of several series. A careful analysis of the
decreased with respect to the reaction carried out at 30 bar observed peaks was conducted allowing the identification of
under the same reaction temperature (cfr entries 5 and 6 vs. three different end groups, namely trimethylsiloxide, cyclohex-
entry 2) while the percentage of polycarbonate product de- anol and cyclohexenolate together with the presence of cyclic
creases, mainly in favor of trans-CHC. Both effects can be rea- chains (Supporting Information Figure S31). In addition, three
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other minor peaks were observed corresponding to the tri- the spectrum of the polymer obtained after 64 hours (entry 4),
methylsiloxide, cyclohexanol and cyclohexenolate terminated suggesting that during longer polymerization times, the adven-
chains with one additional cyclohexenoxide unit (that is, a miss- titious presence of water can hydrolyze an increasing amount
ing carbon dioxide unit) in agreement with the presence of of CHO, generating CHD and thus promoting the formation of
ether linkages observed in the NMR spectra.
PCHC end-capped with two hydroxyl groups. This conclusion
was in agreement with the H NMR spectrum of the same co-
1
As previously proposed in the literature, the reaction of the
amido ligand with CO2, followed by migration of a trimethylsilyl
group and extrusion of trimethylsilylisocyanate, leads
to the formation of trimethylsiloxide end groups (pathway A
in Figure S32).[6b] The formation of cyclohexanol
H[OC6H10]m[CO2]nOC6H10OH end groups has been previously
ascribed to two different pathways. Either they can be due to
the presence of cyclohexane-1,2-diol (CHD, produced by hydrol-
ysis of CHO) which, acting as a chain transfer agent, leads to
the formation of a diol initiating species (pathway B in Figure
S32),[8d] or by the release of a CO2 molecule from the poly-
carbonate through a mechanism generating cyclohexenolate
H[OC6H10]m[CO2]nOC6H9 and cyclohexanol end groups (path-
way C in Figure S32).[16b] The observation of the cyclohexenol-
ate end group in the MALDI-ToF spectrum supported the occur-
rence of the latter mechanism, yet the first mechanism cannot
be ruled out. To get insight on the formation of cyclohexanol
end group by reaction of the zinc center with CHD, a new
polymerization experiment was carried out by adding one
equivalent of CHD (vs. complex 1) to the reaction mixture be-
fore the addition of CO2 (entry 10 in Table 1). The MALDI-ToF
spectrum of the polymer sample obtained in this experiment,
compared with the spectrum of the reaction carried out under
the same conditions but in the absence of the CHD (entry 10
vs. entry 2 in Table 1, Supporting Information Figure S33),
showed an increase of the peak due to the cyclohexanol termi-
nated chain, supporting the occurrence of pathway B detailed
in Figure S32. Finally, intramolecular transesterification at carb-
onate functionalities along the polymer chains (i.e. different
from the last inserted one) may explain the formation of cyclic
structures [OC6H10]n[CO2]m (pathway D in Figure S32). In this
respect, the presence of trans-CHC in the product mixtures, in-
dicates that the back-biting mechanism of the polymer chain is
operative, thus corroborating pathway D.
polymer: in addition to the main signals for the protons of the
copolymer chains, two peaks at 3.60 and 4.45 ppm were ob-
served, which have been assigned to the methine protons adja-
cent to the hydroxyl end groups[7f] (Supporting Information Fig-
ure S36). Signals of other end groups were not detectable by
NMR.
In order to minimize the chain transfer reaction due to the
presence of traces of water, a copolymerization experiment was
carried out in the same conditions of run 2 but using freshly
bi-distilled CHO (run 11, Table 1). However, in spite of an in-
crease in the reaction conversion (cf entries 2 and 11 in Table 1)
the results of these two copolymerization reactions (in terms of
product selectivity, molecular weights and dispersity) did not
show any significant difference.
The microstructure of the PCHC obtained in entry 2, was
studied by 13C NMR spectroscopy (Supporting Information Fig-
ure S37). In the carbonyl region, a peak at 153.94 ppm for the
m-centered tetrads and two peaks at 153.43 and 153.27 ppm
for the r-centered tetrads were observed.[17] The comparable
intensities of these two sets of peaks denotes that the obtained
polycarbonate is atactic.
Subsequently, we explored the effect of the addition of a
cocatalyst to complex 1 (Table 2). In particular, by adding one
equivalent of PPNCl (bis(triphenylphosphine)iminium chloride)
(i.e. 0.5 equivalents per Zn center) to the reaction carried out
at 100 °C and under 30 bar of CO2 (entry 12) the conversion
decreased while the selectivity of the catalyst changed and a
mixture of PCHC, cis-CHC, trans-CHC and ether linkages in the
ratio 25:23:48:4 was obtained (to facilitate the comparison, en-
try 2, carried out under the same conditions of entry 12 but in
the absence of PPNCl, was reported again in Table 2). A plausi-
ble explanation is related to the ability of the nucleophile to
favor the displacement of the growing chain and so favoring
the backbiting of the free copolymer, i.e. not-bound to the
metal (for the metal-bound carbonate and alkoxide, the back-
biting reaction was found to have higher barriers).[18]
Further information from the MALDI-ToF analysis, stems from
comparing the spectra of samples obtained under different
conditions. For example, the comparison of spectra of entries
1–3 (Supporting Information Figure S34), shows the predomi-
nance of the trimethylsiloxide end group (pathway A) at 80 °C,
while, by increasing the temperature, the relative intensity of
the other end groups is increased, indicating that the collateral
reactions (i.e. hydrolysis of CHO, decarboxylation and transester-
ification, described in pathways B-D) are favored by increasing
the temperature.
Table 2. CO2/CHO reaction promoted by complex 1/PPNCl.
PPNCl
[equiv.]
Conv[b]
[mol %]
PCHC
Cis-CHC Trans-
Ether
linkages
Entry[a]
CHC
2
0
1
2
4
20
6
10
40
90
25
20
6
<1
23
59
81
3
7
4
1
<1
12
13
14
48
20
13
[a] General conditions: Complex 1: 19.8 μmol (0.05 mol-%), CHO = 40 mmol
(2000 equiv.), temperature = 100 °C, PCO2 = 30 bar, reaction time = 16 hours.
[b] The conversion and the product ratio were determined through high-
resolution 1H NMR spectra (CDCl3, 600 MHz).
The molecular weight of the polymer obtained after 64 h
(entry 4 in Table 1) is lower than that obtained after 16 h (entry
2 in Table 1), suggesting that some kinds of chain transfer reac-
tions occur during the long polymerization times. Accordingly,
a decrease of the dispersity value is observed (Ð = 3.3 and 5.2
Accordingly, by increasing the amount of PPNCl to two
respectively for entries 4 and 2). The comparison of the MALDI- equivalents (i.e. one equivalent per Zn center) (entry 13 in
ToF spectra of these polymers (Supporting Information Figure Table 2) the total percentage of cyclic carbonates increased
S35) shows the prevalence of the cyclohexanol end groups in with respect to the polymer. Furthermore, in the presence of 4
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equivalents of PPNCl, the main product was cis-CHC (entry 14 complex 2, having a propylene bridge between the nitrogen
in Table 2). As known, cis-CHC may also be formed independ- atoms, and with complex 3, a monometallic analogue, only
ently from chain growth (or even in alternative), that is, follow- small differences were observed both in the activity and in the
ing initial epoxide opening by an anionic initiator and the sub- selectivity of the catalysts in the CO2/CHO reaction. This could
sequent CO2 insertion.[18] The same change in selectivity by be reasonably due to the observed flexibility of the bimetallic
adding a cocatalyst to the reaction mixture was already ob- complexes in which, as a consequence, the two metallic centers
served by other authors both with iron based[19a,19b] and zinc may act independently.
based[19c] catalysts. As already observed in some cases, the ac-
tivity of the catalytic system does not depend linearly on the
amount of the cocatalyst.[19a]
Experimental Section
Finally, we explored the scope of complex 1 in the reaction
Materials and General Methods: All manipulations of air- and/or
of CO2 with other epoxides, namely propylene oxide (PO) and
water-sensitive compounds were carried out under a dry nitrogen
styrene oxide (SO) representative of terminal and aromatic ep-
atmosphere using a Braun Labmaster glovebox or standard Schlenk
oxides, respectively. Carrying out the reaction in the same con-
line techniques. The glassware and autoclave used in the polymeri-
zation were dried in an oven at 120 °C overnight. All solvents and
reagents were obtained from commercial sources (Aldrich and
ditions of entry 2 and in the presence of 2900 equivalents of
PO and 2000 equivalents of SO (see Table S1 in the Supporting
Information for more details) complex 1 showed no activity. Merck). The zinc precursor, zinc(trimethyl silyl) amido, were pur-
chased from Aldrich and used as received. Benzene and hexane,
were distilled from sodium benzophenone. Cyclohexene oxide
(98 %; Sigma-Aldrich), propylene oxide (≥99.5 %; Sigma-Aldrich)
and styrene oxide (97 %; Sigma-Aldrich) were dried with CaH2 for
24 h at room temperature, then distilled under reduced pressure
and stored in a sealed flask in a glove-box. Deuterated solvents
were purchased from Cambridge Isotope Laboratories, Inc., de-
gassed, and dried with activated 3 Å molecular sieves prior to use.
This is not surprisingly since, except for BDI-Zn complexes, no
homogeneous Zn-based systems have been reported to be ac-
tive for the copolymerization of CO2/PO.[2e] Rieger ascribed the
inactivity of a binuclear BDI tethered-zinc complex to the for-
mation of an energetically highly stable intermediate generated
in the presence of CO2 and PO.[9c]
Conclusion
Instruments and Measurements: NMR spectra were recorded on
Bruker Advance 300, 400 and 600 MHz spectrometers at 298 K,
unless otherwise stated. The chemical shifts (δ) are expressed as
parts per million and coupling constants (J) are in Hertz. 1H NMR
spectra are referenced using the residual solvent peak at δ =
7.16 ppm for C6D6 and δ = 7.27 ppm for CDCl3. 13C NMR spectra
are referenced using the residual solvent peak at δ = 128.06 ppm
for C6D6 and δ = 77.23 ppm for CDCl3. The molecular weights (Mn
and Mw) and the dispersity (Đ = Mw/Mn) of polymer samples were
measured by gel permeation chromatography (GPC) at 30 °C, using
In this paper we describe the synthetic strategy to prepare new
bis(aldimine-thioether-phenolate) OSNNSO ligands, which
present two anionic oxygen donors and four neutral donors,
such as two nitrogen atoms and two sulfur atoms, the same
strategy works also to prepare the related aldimine-thioether-
phenolate ONS ligands.
The first ONS ligand and two OSNNSO ligands, with a differ-
ent length of the alkyl bridge between the nitrogen atoms,
have been synthesized and characterized. A monometallic com- THF as the solvent, an eluent flow rate of 1 mL min–1, and narrow
polystyrene standards as the reference. The measurements were
performed on a Waters 1525 binary system equipped with a Waters
2414 RI detector using four Styragel columns (range: 1000–1000
000 Å). Mass spectra were acquired using a Bruker solariX XR Fourier
transform ion cyclotron resonance mass spectrometer (Bruker Dal-
tonik GmbH, Bremen, Germany) equipped with a 7 T refrigerated
actively-shielded superconducting magnet (Bruker Biospin, Wissem-
bourg, France). The samples were ionized in positive ion mode us-
ing the MALDI ion source. The samples of ligands were prepared at
a concentration of 1.0 mg mL–1 in toluene, while the samples of
polymers were prepared at a concentration of 1.0 mg mL–1 in THF.
The matrix (anthracene for the ligands and trans-2-[3-(4-tert-butyl-
phenyl)-2-methyl-2-propenylidene]malononitrile, DCTB, for poly-
mers) was mixed at a concentration of 10.0 mg mL–1 to promote
desorption and ionization.
plex was obtained by reaction of the tridentate ligand with one
equivalent of the zinc precursor while two bimetallic zinc com-
plexes were successfully prepared by direct reaction of the
bis(aldimine-thioether-phenolate) ligands with two equivalents
of the zinc precursor. The NMR characterization indicated that
the complexes are flexible in solution at room temperature.
Complex 1 was able to convert CO2 and CHO in the corre-
sponding polycarbonate product with a good selectivity (up to
97 %) and a moderate activity (TOF up to 36 h–1). The catalyst
did not require the use of an external nucleophile and/or of a
solvent. The obtained polycarbonates showed low molecular
masses (Mn: 3.7–15.4 kDa) with broad dispersities (Ð = 3.3–7.8).
The MALDI-ToF analysis of the obtained polymers allowed the
observation of some terminal groups, thereby indicating both
the formation of multiple active centers for the explored cata-
lytic systems and the occurrence of post-copolymerization reac-
tions.
Synthesis and Characterization of the Ligands: A representative
procedure for the preparation of ligands is given. Synthetic details
and full characterization are reported in the Supporting Informa-
tion. The synthetic strategy, which allowed the preparation of
OSNNSO ligands, involves three steps.
By adding PPNCl as a cocatalyst, the selectivity diverted to-
wards the formation of cyclic carbonates (up to 94 % of cis- and
trans-CHC in the presence of two equivalents of PPNCl per zinc
center) with a slightly higher activity (TOF = 50 h–1).
Synthesis of 2-Mercaptobenzaldehyde: The 2-mercaptobenzalde-
hyde was prepared according to literature procedure.[12]
By comparing the behaviour of the bimetallic complex 1,
Synthesis of 2-(3,5 Di-tert-butyl-2-hydroxybenzyl)sulfanylbenz-
having an ethylene bridge between the nitrogen atoms, with aldehyde: To
a
stirred solution of 2-mercaptobenzaldehyde
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(1.041 g, 7.535 × 10–6) and K2CO3 (4.165 g, 30.1 mmol) in DMF 1.59 (s, 9H, tBu), 1.70 (m, 2H, CH2), 3.47 (t, 2H, N-CH2), 3.81 (s, 2H,
(83 mL) at room temperature was added dropwise a solution of 2-
S-CH2), 6.77 (s, 1H, OH), 6.81 (s, 1H, Ar-H), 6.85 (t, 1H, Ar-H), 6.91 (t,
(bromomethyl)-4,6-di-tert-butyl-phenol (2.255 g, 7.535 × 10–6 1H, Ar-H), 7.25 (d, J = 7.62 Hz, 1H, Ar-H), 7.39, (s, 1H, Ar-H), 7.82 (d,
mmol) in dry DMF (18 mL). The flask was left to stir at room tem-
perature for 3 h. 50 mL of water and 50 mL of diethyl ether were
added. The organic layer was washed with water (3 × 30 mL) and
brine (3 × 30 mL). The solution was dried with sodium sulfate and
then filtered. The solvent was removed under vacuum yielding a
white yellow solid that was recrystallized from pentane as a white
solid, and collected by vacuum filtration. Yield 63 %. 1H NMR
(300 MHz, CD2Cl2, 298 K): δ = 1.18 (s, 9H, tBu), 1.40 (s, 9H, tBu), 4.15
(s, 2H, S-CH2), 5.76 (s, 1H, OH), 6.79 (s, 1H, Ar-H), 7.20 (s, 1H, Ar-H),
7.36 (s, 1H, Ar-H), 7.40 (t, 1H, Ar-H), 7.52 (m, 2H, Ar-H), 7.79 (d, J =
8.05 Hz, 1H, Ar-H), 10.22 (s, 1H, CH=O). 13C NMR (400 MHz, CD2Cl2,
298 K): δ = 30.01 (C(CH3)3), 31.61 (C(CH3)3), 34.47 (C(CH3)3), 35.19
(C(CH3)3), 37.05 (S-CH2), 121.70 (Cq),124.40 (CH), 125.96 (CH), 127.67
(CH), 131.86 (CH), 132.68 (CH), 134.35 (CH), 136.21 (Cq), 137.16 (Cq),
139.06 (Cq), 143.07 (Cq), 151.47 (Cq), 191.60 (CH=O).
J = 7.55 Hz, 1H, Ar-H), 8.52 (s, 1H, CH=N). 13C NMR (400 MHz, C6D6,
298 K): δ = 12.08 (CH3), 24.48 (CH2), 30.12(C(CH3)3), 31.74 (C(CH3)3),
34.32 (C(CH3)3), 35.33 (C(CH3)3), 38.53 (N-CH2), 64.16 (S-CH2), 122.09
(Cq), 123.90 (CH), 125.79 (CH), 130.10 (CH), 130.37(CH), 134.68 (Cq),
135.06 (CH), 136.93 (Cq), 138.19 (Cq), 142.22 (Cq), 152.37 (Cq),
159.44 (CH=N). MS (ESI) m/z (ion): [M + H+] calcd. for C25H36NOS
398.251, found 398.252.
Synthesis and Characterization of Complexes 1–3: The represent-
ative procedure for the preparation of complex 1 is given, see Sup-
porting Information for further details for all other complexes.
Synthesis of Complex 1: To a benzene solution (6 mL) of L1H2
(0.150 g, 2.03 × 10–4 mol) was added a benzene solution (3.0 mL)
of Zn[N(SiMe3)2]2 (0.1572 g, 4.07 × 10–4 mol). The reaction mixture
was stirred at room temperature for 2 hours. Afterwards the solvent
was removed in vacuo and the solid residue was washed with pent-
ane. Complex 1 was obtained as a yellow powder in 95 % yield.
Elemental analysis calcd. for C58H94N4O2S2Si4Zn2: C 58.71, H 7.98, N
4.72, S 5.40; found C 59.02, H 8.02, N, 4.74, S 5.42. 1H NMR (600 MHz,
tol-d8, 263 K): δ = –0.01 (s, 18H, Si(CH3)3), 0.13 (s, 18H, Si(CH3)3),
1.49 (s, 18H, tBu), 1.86 (s, 18H, tBu), 3.53 (d, J = 11.97 Hz, 2H, N-
CH2), 4.40 (d, J = 7.03 Hz, 2H, S-CH2), 4.45 (d, J = 11.73 Hz, 2H, N-
CH2), 4.89 (d, J = 7.19 Hz, 2H, S-CH2), 6.48 (t, 2H, Ar-H), 6.54 (t, 2H,
Ar-H), 7.11 (s, 2H, Ar-H), 7.28 (d, J = 7.44 Hz, 2H, Ar-H). 7.49 (d, J =
7.76 Hz, 2H, Ar-H), 7.60 (s, 2H, Ar-H), 8.54 (s, 2H, CH=N). 13C NMR
(600 MHz, tol-d8, 263 K): δ = 1.38 (Si(CH)3)3), 3.09 (Si(CH)3)3), 30.45
(2C, C(CH3)3), 32.44 (2C, C(CH3)3), 34.44 (2C, C(CH3)3), 35.80 (2C,
C(CH3)3), 53.31 (2C, CH2), 61.58 (2C, CH2), 122.37 (2C, CH), 124.57
(2C, Cq) 125.11 (2C, CH), 127.21 (2C, CH) 131.40 (2C, CH), 135.66
(2C, Cq), 136.50 (2C, CH), 139.01 (2C, CH), 150.10 (2C, Cq), 164.71
(2C, Cq), 176.87 (2C, CH=N).
Synthesis of Ligand Precursor L1H2: Compound L1H2 was ob-
tained by condensation of two equivalents of the 2-(3,5-di-tert-
butyl-2-hydroxybenzyl) sulfanyl benzaldehyde (1.628 g, 4.565 × 10–
mmol) with ethylenediamine (153 μL, 2.289 × 10–6 mmol), in dry
6
acetonitrile (57 mL). The characterization of L1H2 was done by
means of 1H and 13C NMR and MALDI-ToF mass spectrometry. Yield:
51 %. 1H NMR (400 MHz, C6D6, 298 K): δ = 1.29 (s, 18H, tBu), 1.71
(s, 18H, tBu), 3.42 (s, 4H, N-CH2), 3.97 (s, 4H, S-CH2), 6.44 (t, 2H, Ar-
H), 6.67 (t, 2H, Ar-H), 6.82 (d, J = 2.44 Hz, 2H, Ar-H), 7.27 (d, J =
7.84 Hz; 2H, Ar-H), 7.37 (d, J = 7.79 Hz, 1H, Ar-H), 7.44 (d, J = 2.41 Hz,
1H, Ar-H), 8.10 (s, 1H, OH) 9.07 (s, 1H, CH=N). 13C NMR (400 MHz,
C6D6, 298 K): δ = 30.48 (C(CH3)3), 31.82 (C(CH3)3), 34.40 (C(CH3)3),
35.65 (C(CH3)3), 38.65 (N-CH2), 61.61 (S-CH2), 123.62 (CH),125.38
(Cq), 125.93 (CH), 129.05 (CH), 130.60 (CH), 135.27 (CH), 135.69 (Cq),
138.09 (Cq), 139.20 (Cq), 143.03 (Cq), 152.36 (CH), 165.19 (CH=N).
MS (MALDI-ToF) m/z (ion): [M + H+] calcd. for C46H61N2O2S2 737.42,
found 737.417; calcd. for C31H37N2OS2 517.23, found 517.234; calcd.
for C16H15N2S2 299.07, found 299.067.
Synthesis of Complex 2: The same procedure used for complex 1
was followed using the ligand precursor L2H2 (0.200 g, 2.67 × 10–4
mol) and Zn[N(SiMe3)2]2 (0.212 g, 5.33 × 10–4 mol). Complex 2 was
obtained as a yellow powder in 91 % yield. Elemental analysis calcd.
for C59H96N4O2S2Si4Zn2: C 59.02, H 8.06, N 4.67, S 5.34; found C
Synthesis of Ligand Precursor L2H2: Compound L2H2 was
obtained by condensation of two equivalents of the 2-(3,5-
di-tert-butyl-2-hydroxybenzyl) sulfanyl benzaldehyde (0.7050 g,
1
59.29, H 8.07, N, 4.65, S 5.32. H NMR (600 MHz, tol-d8, 223 K): δ =
1.9774 × 10–3
mol) with 1,3-propanediamine (83.3 μL,
–0.01 (s, 18H, Si(CH3)3), 0.17 (s, 9H, Si(CH3)3), 0.26 (s, 9H, Si(CH3)3),
1.59 (s, 9H, tBu), 1.62 (s, 9H, tBu), 1.82 (s, 9H, tBu), 1.92 (s, 9H, tBu),
2.68 (br, 1H, CH2), 3.37 (m, 2H, CH2), 3.48 (m, 2H, CH2), 3.88 (m, 1H,
CH2), 4.29 (d, J = 11.96 Hz, 1H, S-CH2), 4.31 (br, 2H, CH2), 4.51 (d,
J = 11.76 Hz, 1H, S-CH2), 6.70 (m, 3H, Ar-H), 6.76 (m, 2H, Ar-H), 6.79
(d, 1H, Ar-H), 7.02 (s, 1H, Ar-H), 7.10 (s, 1H, Ar-H), 7.65 (s, 1H, Ar-H),
7.69 (s, 1H, Ar-H), 7.82 (t, 2H, Ar-H), 7.91 (s, 1H, CH=N), 8.40 (s, 1H,
CH=N). 13C NMR (300 MHz, C6D6, 298 K): δ = 2.44 (2C, Si(CH)3)3),
30.52 (2C, C(CH3)3), 32.24 (CH2), 32.37 (2C, C(CH3)3), 34.31 (2C,
C(CH3)3), 35.81 (2C, C(CH3)3), 52.96 (2C, N-CH2), 58.21 (2C, S-CH2),
122.18 (2C, CH), 123.07 (2C, Cq) 124.38 (2C, CH), 127.08 (2C, CH),
129.52 (2C, Cq), 131.27 (2C, CH), 135.54 (2C, Cq), 136.99 (2C, CH),
137.97 (2C, Cq), 138.14 (2C, CH), 150.82 (2C, Cq), 164.82 (2C, Cq),
174.69 (2C, CH=N).
9.887 × 10–4 mol), in dry acetonitrile (24 mL). The characterization
of L2H2 ligand precursor was done by means of 1H and 13C NMR
and MALDI-ToF mass spectrometry. Yield: 54 %. H NMR (400 MHz,
1
C6D6, 298 K): δ = 1.23 (s, 9H, tBu), 1.58 (s, 9H, tBu), 2.20 (m, 2H, N-
CH2), 3.72 (t, 2H, N-CH2), 3.81 (s, 2H, S-CH2), 6.76 (s, 1H, OH), 6.82,
(d, J = 2.36 Hz, 1H, Ar-H), 6.86 (t, 1H, Ar-H), 6.91 (t, 1H, Ar-H), 7.24
(d, J = 7.76 Hz, 1H, Ar-H), 7.39 (d, J = 2.43 Hz, 1H, Ar-H), 7.82 (d, J =
7.90 Hz, 1H, Ar-H), 8.60 (s, 1H, CH=N). 13C NMR (400 MHz, C6D6,
298 K): δ = 30.15(C(CH3)3), 31.98 (C(CH3)3), 32.39 (C(CH3)3), 34.34
(C(CH3)3), 35.33 (CH2), 38.47 (N-CH2), 59.97 (S-CH2), 122.09
(CH),123.91 (CH), 125.82 (CH), 130.21 (CH), 130.43 (CH), 134.84 (Cq),
136.95 (Cq), 138.05 (Cq), 142.25 (Cq), 152.35 (Cq), 160.09 (CH=N).
MS (MALDI-ToF) m/z (ion): [M + H+] calcd. for C47H63N2O2S2 751.43,
found 751.428; calcd. for C32H39N2OS2 531.25, found 531.247; calcd.
for C17H17N2S2 313.08, found 313.08.
Synthesis of Complex 3: The same procedure used for complex 1
was followed using the ligand precursor L3H (0.150 g, 3.77 × 10–4
mol) and Zn[N(SiMe3)2]2 (0.152 g, 3.77 × 10–4 mol). Complex 3 was
obtained as a yellow powder in 97 % yield. Elemental analysis calcd.
for C31H52N2OSSi2Zn: C 59.82, H 8.42, N 4.50, S 5.15; found C 60.18,
Synthesis of Ligand Precursor L3H: Compound L3H was obtained
by condensation of two equivalents of the 2-(3,5-di-tert-butyl-2-hy-
droxybenzyl) sulfanyl benzaldehyde (0.5028 g, 1.41 × 10–3 mol)
with propylamine (116 μL, 1.41 × 10–3 mol), in dry acetonitrile
(20 mL). The characterization of L3H ligand precursor was done by
1
H 8.47, N, 4.52, S 5.17. H NMR (400 MHz, tol-d8, 233 K): δ = –0.08
(s, 9H, Si(CH3)3), 0.17 (s, 9H, Si(CH3)3), 0.80 (t, 3H, CH3), 1.33 (br, 1H,
CH2), 1.40 (s, 9H, tBu), 1.89 (s, 9H, tBu), 2.33 (br, 1H, CH2), 2.80 (br,
1
1
means of H and 13C NMR and mass spectrometry. Yield: 62 %. H
NMR (400 MHz, C6D6, 298 K): δ = 0.91 (t, 3H, CH3) 1.23 (s, 9H, tBu), 1H, N-CH2), 3.37 (d, J = 11.86, 1H, S-CH2), 4.13 (br, 1H, N-CH2), 4.34
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(d, J = 11.88 Hz, 1H, CH2), 6.74 (m, 3H, Ar-H), 7.56 (s, 1H, Ar-H), 7.62
(s, 1H, CH=N) 7.82 (d, J = 7.27 Hz, 2H, Ar-H). 13C NMR (600 MHz,
C6D6, 298 K): δ = 2.34 (Si(CH)3)3), 11.44 (CH3), 25.73 (CH2), 30.20
(C(CH3)3), 32.37 (C(CH3)3), 34.29 (C(CH3)3), 35.69 (C(CH3)3), 52.93 (N-
CH2), 63.13 (S-CH2), 121.90 (CH), 124.29 (CH), 124.30 (CH), 127.90
(CH), 128.22 (Cq), 129.64 (Cq), 131.08 (CH), 135.36 (Cq), 137.30 (CH),
137.54 (CH), 138.31 (Cq), 172.82 (CH=N).
[6]
CO2/Epoxide Reaction Procedure: In a typical experiment, in a
glove-box, the catalyst (19.8 μmol, 0.05 mol-%), and when used, the
cocatalyst (PPNCl, from 1 to 4 equivalents) were dissolved in the
epoxide (4 mL) and then transferred into the autoclave. The auto-
clave was pressurized to the appropriate pressure of CO2 and was
heated to the appropriate temperature. The mixture was stirred for
the necessary reaction time. After the prescribed time, the reaction
mixture was quenched by dipping the autoclave in an ice bath and
adding CH2Cl2 (0.5 mL) under air and a 1H NMR spectrum of the
crude reaction mixture was used to calculate the conversion. The
residual epoxide was removed under vacuum. The polymer was
isolated by precipitation in methanol. The precipitated solid was
filtered and dried in a vacuum oven overnight at 40 °C.
[7]
Acknowledgments
The authors acknowledge the Cariplo Foundation (Apollo
project 2016-0643) for financial support, Dr Patrizia Oliva for
NMR technical assistance, Dr Mariagrazia Napoli for GPC meas-
urements, and Dr Patrizia Iannece for MALDI-ToF analysis.
[8]
Keywords: Carbon dioxide fixation · Cyclohexene oxide ·
Ligand design · Renewable resources · Zinc
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Carbon Dioxide Fixation
M. Cozzolino, F. Melchionno,
F. Santulli, M. Mazzeo,
M. Lamberti* .................................... 1–10
Aldimine-Thioether-Phenolate
Based Mono- and Bimetallic Zinc
Complexes as Catalysts for the Reac-
tion of CO2 with Cyclohexene Oxide
A new class of aldimine-thioether-phe- tion by reaction with cyclohexene ox-
nolate ligands and the corresponding ide, operating at a low catalyst loading
mono- and bi-metallic zinc complexes and in the absence of solvents. MALDI,
have been synthesized. The zinc com- GPC, and NMR analysis of the corre-
plexes are active catalysts in CO2 fixa- sponding polymers are discussed.
DOI: 10.1002/ejic.202000119
Eur. J. Inorg. Chem. 0000, 0–0
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© 2020 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim