Dinuclear Zinc Salen Catalysts for the Ring Opening Copolymerization of Epoxides and Carbon Dioxide or Anhydrides

Article abstract of DOI:10.1021/acs.inorgchem.5b02233

A series of four dizinc complexes coordinated by salen or salan ligands, derived from ortho-vanillin and bearing (±)-trans-1,2-diaminocyclohexane (L₁) or 2,2-dimethyl-1,3-propanediamine (L₂) backbones, is reported. The complexes are

Full text of DOI:10.1021/acs.inorgchem.5b02233

Article
pubs.acs.org/IC
Dinuclear Zinc Salen Catalysts for the Ring Opening
Copolymerization of Epoxides and Carbon Dioxide or Anhydrides
Arnaud Thevenon, Jennifer A. Garden, Andrew J. P. White, and Charlotte K. Williams*
Department of Chemistry, Imperial College London, London, SW7 2AZ, United Kingdom
S
* Supporting Information
ABSTRACT: A series of four dizinc complexes coordinated by
salen or salan ligands, derived from ortho-vanillin and bearing
( )-trans-1,2-diaminocyclohexane (L1) or 2,2-dimethyl-1,3-pro-
panediamine (L2) backbones, is reported. The complexes are
characterized using a combination of X-ray crystallography,
multinuclear NMR, DOSY, and MALDI-TOF spectroscopies,
and elemental analysis. The stability of the dinuclear complexes
depends on the ligand structure, with the most stable complexes
having imine substituents. The complexes are tested as catalysts
for the ring-opening copolymerization (ROCOP) of CO2/
cyclohexene oxide (CHO) and phthalic anhydride (PA)/CHO.
All complexes are active, and the structure/activity relationships
reveal that the complex having both L2 and imine substituents
displays the highest activity. In the ROCOP of CO2/CHO its activity is equivalent to other metal salen catalysts (TOF = 44 h−1
at a catalyst loading of 0.1 mol %, 30 bar of CO2, and 80 °C), while for the ROCOP of PA/CHO, its activity is slightly higher
than other metal salen catalysts (TOF = 198 h−1 at a catalyst loading of 1 mol % and 100 °C). Poly(ester-block-carbonate)
polymers are also afforded using the most active catalyst by the one-pot terpolymerization of PA/CHO/CO2.
catalysts.10d Since this pivotal finding, many highly active
INTRODUCTION
■
dinuclear catalysts have been developed including those based
Salen ligands, initially derived from the condensation of
salicylaldehyde and ethylenediamine, are of fundamental
importance in coordination chemistry and catalysis. They
on macrocyclic,11 BDI,10d,12 Trost type “pro-phenolate”,4d,13
porphyrin,14 and anilido−aldimine15 ligand scaffolds, typically
coordinated to zinc, although other transition metal systems
have been extensively studied since the pioneering work of
have also been investigated.16 Mechanistic studies and density
Jacobsen1 and Katsuki2 using a chiral [Mn(salen)] complex for
functional theory calculations have highlighted that the catalyst
the enantioselective epoxidation of alkenes. Since then, a range
activity seems to be highly dependent on the flexibility of the
of other transition metal catalysts have been reported for
ligand and the distance between the two metal centers.10a,c,11c,17
reactions as distinct as oxidations,3 polymerizations,4 and
epoxidations.5 More recently, a novel class of salen ligands
To date, one of the most active catalyst systems for the
synthesis of poly(cyclohexene carbonate) (PCHC), reported
have been developed incorporating a Lewis donor functional
group in the ortho position of the salicylaldehyde unit, enabling
earlier this year by Rieger and co-workers;18,19 is based on a
flexible dizinc complex, coordinated by two β-diiminate
the coordination of a second metal center within the ligand
scaffold.6 Dinuclear salen complexes, where two different
moieties that are linked through the phenyl rings (TOF =
155 000 h−1 at a catalyst loading of 0.0125 mol %, 100 °C, and
metals (usually a transition metal and a lanthanide) are
incorporated into the ligand, have been studied for their
30 bar of CO2).
magnetic properties7 and are even proposed as single-molecule
Mononuclear salen ligands coordinated to either Co(III) or
magnets.8 Such complexes are also attractive catalysts for
Cr(III) centers and combined with a Lewis base/ionic
cocatalyst have also been widely employed as catalysts for
asymmetric organic reactions, where it is proposed that they
CO2/epoxide ROCOP.4i,j,20 In this case, the separation
show better performances due to cooperative interactions
between the two metal centers.9 Exactly this type of metal
between the metal and the cocatalyst is of key of importance
for the high activity.21 Nozaki and co-workers pioneered a new
cooperation has also been proposed to be an important
criterion in the preparation of highly active catalysts for the
generation of “cocatalyst tethered” salen complexes, which
show a significant improvement in the catalyst activity.21 The
ring-opening copolymerization (ROCOP) of CO2 and
epoxides.10
combination of tethering together the salen catalyst and
One of the first well-characterized dinuclear catalysts for
CO2/epoxide ROCOP was a zinc β-diiminate (BDI) complex
Received: September 29, 2015
that was shown to adopt dimeric structures in the most active
© XXXX American Chemical Society
A
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cocatalyst was subsequently successfully applied by various
researchers,22 including Lu and Lee, to prepare some of the
most active catalysts reported for ROCOP (TOF > 20 000
h−1).23 A further benefit is that some of these tethered catalysts
can be reclaimed from the polymer and recycled without
significant loss in performance.23g
First, the amination of ortho-vanillin, using the relevant
diamine [( )-trans-1,2-diaminocyclohexane (L1) or 2,2-
dimethyl-1,3-propanediamine (L2)], gave the desired imine
product (L1aH, L2aH, Scheme 1). The ligands differ according
to the diamine “linker” group and were selected on the basis
that such C2 and C3 linkers are common components of
successful ROCOP catalysts.11a,f,21,23a Subsequent in situ
reduction of the imine moieties on the salen ligands was
performed, using NaBH4, to obtain the corresponding amines
and salan ligands (L1bH, L2bH, Scheme 1). All four ligands were
obtained in near quantitative yields and did not require
purification (Figure S1−8).
Another successful strategy, also initially developed by
Nozaki and co-workers, has been to join together two distinct
metal salen catalysts and to combine them with cocatalysts.24
Such dinuclear catalysts are coordinated by two salen ancillary
ligands that have been covalently linked to one another;25 the
catalysts show higher activity than mononuclear counterparts,
particularly at low catalyst loadings.24,25b Given the precedence
for salen ligands and the promise of dinuclear catalysts, it is
perhaps surprising that there is, so far, only a single report of
ROCOP catalysts where a single salen ancillary ligand
coordinates two metals.26 The catalyst comprises a multi-
dendate bis(benzotriazole iminophenol) ligand coordinated to
two Zn(II), Ni(II), or Co(II) metal centers. The best activities
were achieved with the di-Ni(II) catalyst, which for the
ROCOP of CHO/CO2 shows a TOF of 53 h−1, at 120 °C and
20 bar of CO2.26 These encouraging results using dinuclear
salen catalysts inspired the current development of new
dinuclear salen ligands and the investigation of their
coordination chemistry and catalytic applications for CO2/
CHO and phthalic anhydride (PA)/CHO ROCOP. When
developing new polymerization catalysts, it is attractive to target
low-cost ligands, ideally derived from renewables, which can be
coordinated to earth-abundant metal centers.27 ortho-Vanillin is
an interesting candidate, as it is relatively abundant and
straightforward to extract from a range of plants. In addition,
ortho-vanillin salen ligands have a good precedent for the
formation of dinuclear complexes, although these have not yet
been applied in this area of polymerization catalysis.28 Zinc was
also selected as the most desirable metal center as it is
abundant, nontoxic, and usually results in colorless complexes.
Metal Complexations. L1aH, L1bH, and L2aH were reacted
with 2 equiv of [Zn(OAc)2·2(H2O)] to afford the correspond-
ing bis(zinc acetate) complexes (Scheme 2) in good yields
(62−68%).
Scheme 2. Complexation Reactions of the Ligands with Zinc
(i) 2 equiv of [Zn(OAc)2·2(H2O)], MeOH, 22 °C, 16 h.
Crystals suitable for X-ray diffraction studies were obtained
for L1aZn2(OAc)2 by slow diffusion of pentane into a
concentrated dichloromethane solution. The molecular struc-
ture of L1aZn2(OAc)2 shows that both the zinc centers are
pentacoordinated by the ligand framework (Figure 1 and Figure
RESULTS AND DISCUSSION
■
Using ortho-vanillin as a starting point, four new ligands were
synthesized, isolated, and characterized. As mentioned, ortho-
vanillin is an attractive starting material, as it is inexpensive and
commercially available; furthermore, the targeted ligands were
obtained on a large scale in only 1 d, through a simple, one-pot
synthetic method (Scheme 1).
Scheme 1. Ligand Syntheses
Figure 1. The crystal structure of (L1a)Zn2(OAc)2. Hydrogen atoms
and a dichloromethane molecule are omitted for clarity.
S9, Table 1); one within the “enclosed” phenolic pocket, and
the other in the “open” cavity. A κ2-coordinated acetate ligand
provides stabilization for the dinuclear complex by bridging
between the two Zn centers. The zinc center enclosed in the
small pocket is coordinated via two nitrogen centers, two
phenolic oxygen centers, and one acetate oxygen in a distorted
square pyramidal geometry (τ = 0.16).29 In contrast, the zinc
occupying the open pocket has a distorted trigonal bipyramidal
(i) 2 equiv of o-vanillin, 1 equiv of corresponding diamine, MeOH, 22
°C, 4 h, (100% yield for L1aH and L2aH); (ii) 3.5 equiv of NaBH4,
MeOH, 22 °C, 2 h; (iii) H2O, 22 °C, 16 h (97% yield for L1bH and
L2bH).
B
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weight close to the theoretical one (456 g/mol for 614 g/mol
and 729 g/mol for 633 g/mol, respectively), providing further
confirmation of the dinuclear complex structures. To confirm
the stability of L2aZn2(OAc)2 under typical polymerization
conditions, L2aZn2(OAc)2 was heated to reflux for 16 h in d8-
Table 1. Selected Bond Lengths (Å) and Angles (deg) for
L1aZn2(OAc)2
Zn(1)−O(1)
Zn(1)−N(16)
Zn(1)−O(30)
Zn(2)−O(1)
Zn(2)−O(32)
Zn(1)···Zn(2)
Zn(2)···O(25)
Zn(2)···O(27)
2.003(2)
2.036(2)
1.994(2)
2.055(2)
2.023(2)
2.9539(4)
2.764(2)
2.592(2)
Zn(1)−N(9)
Zn(1)−O(24)
Zn(2)−O(24)
Zn(2)−O(40)
2.027(3)
2.023(2)
2.021(2)
2.096(2)
2.192(2)
90.03(9)
115.57(10)
161.27(9)
1
THF. The H NMR spectra remained constant over time,
indicating that this species is thermally stable under the
polymerization conditions (Figure S19). Unfortunately, this
test could not be performed on L1aZn2(OAc)2 and
L1bZn2(OAc)2 as they were not soluble even in hot THF.
In contrast to the other ligands, the direct reaction of L2bH
with either Zn(OAc)2 or Zn(OAc)2·2(H2O) gave a mixture of
Zn(2)−O(42)
O(1)−Zn(1)−N(9)
O(30)−Zn(1)−N(16)
O(32)−Zn(2)−O(42)
1
products (Scheme 3). Although the H NMR spectrum in any
geometry (τ = 0.34) and is coordinated by two phenolic oxygen
atoms, one bridging acetate oxygen, and a terminal κ2 acetate.
In the solid state, the two methoxy groups on the salen ligand
do not coordinate to the zinc in the open pocket [Zn2−O25,
2.764(2) Å; Zn2−O27, 2.592(2) Å]. The distance between the
two zinc centers is 2.9539(4) Å, which is small compared to
other dinuclear catalysts (∼3.1 Å).10a,b,19 A similar molecular
structure has been reported for the closely related hydrate
complex, L1aZn2(OAc)2·H2O (Figure S10), where the two zinc
centers adopt distorted square pyramidal geometries.30 The
zinc center occupying the enclosed pocket of L1aZn2(OAc)2·
H2O has a similar coordination mode to that of L1aZn2(OAc)2.
However, the zinc center in the open pocket is coordinated to
just one phenolic oxygen, with one methoxy group, one water,
and two acetate ligands (one bridging and one terminal)
completing the coordination sphere of the second zinc. As a
result, the Zn···Zn distance is longer [3.358(1) Å] but still in
the range of common dinuclear catalysts for epoxide/CO2
ROCOP.10a,b,19 It is relevant to consider that the coordination
of water by L1aZn2(OAc)2·H2O indicates that the zinc
coordination geometry within L1aZn2(OAc)2 is likely to be
altered in the presence of a Lewis donor, suggesting that under
the polymerization conditions, the epoxide (CHO) could
coordinate to the zinc center in the open pocket. Furthermore,
the different coordination geometries of the zinc in the open
pocket suggest that the methoxy groups may play a role in
stabilizing the active sites during the polymerization.
Scheme 3. Reactivity of L2bH with Zn(OAc)2·2H2O Giving
Rise to a Mixture of Products Containing L2bZn2(OAc)2,
L2bZn3(OAc)4, and (L2b)2Zn3(OAc)2
(i) 2 equiv of Zn(OAc)2·2(H2O), MeOH, 22 °C, 16 h.
solvent was convoluted (Figures S20 and S21), no signals
corresponding to the free ligand were observed; instead, three
new sets of resonances were present. MALDI-TOF analysis of
the mixture revealed the presence of L2bZn2(OAc)2 (M+1-OAc
= 563 g/mol) along with two higher molecular weight species,
assigned as L2bZn3(OAc)4 [(M1+1-OAc = 745 g/mol) and
(L2b)2Zn3(OAc)2 (M2+1-OAc = 999 g/mol)] (Figure S22).
DOSY analysis of the product mixture in C6D6 showed a range
of diffusion coefficients [−9.1 × 10−10 m2/s to −9.3 × 10−10
m2/s)] (Figure S23). A comparison of the diffusion coefficients
obtained from a calibration plot of known standards gave a
predicted molecular weight range of 496 g/mol to 977 g/mol,
which is in agreement with the presence of products
L2bZn2(OAc)2 (621 g/mol), L2bZn3(OAc)4 (802 g/mol),
and (L2b)2Zn3(OAc)2 (1061 g/mol) in the solution state.
An alternative synthetic route was investigated using a
stepwise procedure via the synthesis of a monometallic
intermediate, L2bZn (Scheme 4), followed by its reaction
with Zn(OAc)2, and was expected to afford the dinuclear
complex L2bZn2(OAc)2 (Scheme 5). Accordingly, when L2bH
was deprotonated, using either Zn(OAc)2 or Et2Zn in dry
MeOH or THF, respectively, the formation of a white
precipitate was observed. The solid-state structure of L2bZn
was determined as both the THF and methanol solvates
(Figures S24−26). Although the two solvates are very distinct
crystallographically (with the structure of the THF solvate
containing one independent Ci-symmetric complex, whereas
that of the methanol solvate contains two independent Ci-
symmetric complexes), the conformations of all three
In the solution state, the 1H NMR spectra of imine
complexes L1aZn2(OAc)2 and L2aZn2(OAc)2 confirm the two
metals are coordinated, with all the resonances experiencing
upfield shifts in comparison to the free ligands (Figure S11−
14). A particularly significant shift was observed for the imine
signal in CDCl3 [from 13.84 ppm in L1aH to 8.23 ppm for
L1aZn2(OAc)2 (Figure S11) and from 14.14 ppm in L2aH to
8.08 ppm for L2aZn2(OAc)2 (Figure S13)]. The broadness of
the methylene resonances and the loss of the phenolic proton
resonance also confirm the deprotonation of the ligand and the
coordination of a zinc center. The appearance of two new
singlet resonances at 2.00 and 1.97 ppm are assigned to the two
1
distinct acetate coligands. Similarly, the H NMR spectrum of
the amine complex L1bZn2(OAc)2 exhibits upfield resonances
in comparison to those of the free ligand (Figures S15 and
S16). In d5-pyridine, it can be seen that the N−H resonances
are broad (4.28 ppm) and that a significant shift of the benzylic
resonance occurs (from 4.01 to 4.19 ppm), indicating that the
zinc center is located within the small pocket. The acetate
resonance at 2.11 ppm is assigned to both the zinc-coordinated
acetate ligands. DOSY analysis of L2aZn2(OAc)2 in CDCl3
(Figure S17) and of L1bZn2(OAc)2 in d5-pyridine (Figure S18)
1
shows that, in each case, all the H NMR resonances possess
the same diffusion coefficient and give a predicted molecular
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centers adopt a trigonal bipyramidal geometry (τ = 0.70) and
are bound to three phenolic oxygens in a fac fashion. The
complexation of the zinc leads to the formation of six-
membered rings, providing stabilization for the dimeric
complex. The methoxy groups do not coordinate to zinc, and
so the open pocket remains available for the coordination of
another metal center. Multinuclear (1H, 13C) NMR spectros-
copy along with two-dimensional (2D) NMR and DOSY
experiments suggest that the dimeric structure is retained in
solution (Figures S27−29)
Scheme 4. Reaction of L2bH with 1 equiv of Et2Zn and the
Formation of the Dimeric Structure (L2b)2Zn2
To investigate whether the dimer (L2b)2Zn2 can be cleaved
by the coordination of a second metal, the reaction of
(L2b)2Zn2 with 1 equiv of Et2Zn was performed (Scheme 5).
This reaction was selected because reactions involving Et2Zn
are generally fast and because the formation of dinuclear
(i) 1 equiv of Et2Zn in THF, 22 °C, 16 h or 1 equiv of Zn(OAc)2 in
MeOH, 22 °C, 16 h.
Scheme 5. Reaction of (L2b)Zn2 with Et2Zn and Zn(OAc)2
1
complexes can be monitored by following the distinctive H
NMR signals of the Zn-ethyl groups. The 1H NMR spectrum of
the product, in C6D6, showed no trace of the unreacted dimer,
and all resonances experienced an upfield shift, consistent with
the incorporation of a second zinc center within the ligand
scaffold (Figures S30 and S31). Two different sets of
characteristic Zn-ethyl resonances are observed, with two
triplets at 2.05 and 1.79 ppm and the corresponding quartets at
0.94 and 0.61 ppm. These Zn-ethyl chemical shifts are
significantly different than those of diethylzinc [1.11 ppm (t)
and 0.12 ppm (q) in C6D6],31 confirming the coordination of
the second zinc center within the ligand scaffold.
(i) 1 equiv of Et2Zn in THF, 22 °C, 16 h (ii) 1 equiv of L2bH in C6D6.
(iii) 1 equiv of Zn(OAc)2 in THF, 22 °C, 16 h.
1H DOSY NMR spectroscopy shows that all the resonances
possess the same diffusion coefficient (Figure S32). The
estimation of the molecular weight based on the diffusion
coefficient, using a calibration plot, gave a calculated molecular
weight of 503 g/mol, which is consistent with the formation of
a monomeric L2bZn2Et2 product (561 g/mol). Nevertheless,
block colorless crystals, obtained from a saturated THF
solution of L2bZn2Et2 at 22 °C, corresponded to the starting
dimer (L2b)2Zn2; this finding suggests that the second metal is
weakly coordinated within the ligand scaffold and that an
equilibrium exists between L2bZn2Et2, Et2Zn, and (L2b)2Zn2.
To emphasize the lability of the metal centers within this ligand
system, 1 equiv of free ligand was added to a solution of
L2bZn2Et2, in C6D6 (Scheme 5). The instantaneous formation
of a precipitate was observed, with concomitant release of
ethane. The white precipitate was isolated, dissolved in d8-THF,
and the resultant spectrum correspond to the dimer (L2b)2Zn2.
Finally, when the dimer (L2b)2Zn2 was reacted with 1 equiv of
Zn(OAc)2 at room temperature, the same product mixture was
obtained as when the parent ligand was reacted with 2 equiv of
Zn(OAc)2 (Scheme 5).
complexes are very similar. For simplicity, only the THF solvate
will be discussed here, with full crystallographic details being
provided in the Supporting Information. The molecular
structure of L2bZn reveals that it is dimeric and possesses Ci
symmetry, with a central {Zn2O2} ring, where one phenolic
oxygen from each ligand bridges between the two zinc metal
centers (Figure 2 and Table 2). The symmetry-related zinc
Further investigations were performed to quantify the ratio
of the reaction products L2bZn2(OAc)2, L2bZn3(OAc)4, and
(L2b)2Zn3(OAc)2. Complexes L2bZn3(OAc)4 and
(L2b)2Zn3(OAc)2 were independently synthesized by the
reaction of L2bH with 3 equiv or 1.5 equiv of Zn(OAc)2 in
THF, respectively (Scheme 6). Colorless block crystals of
L2bZn3(OAc)4 suitable for X-ray diffraction were obtained by
slow diffusion of pentane into a saturated dichloromethane
solution of L2bZn3(OAc)4 (Figure 3, Table 3, and Figure S33).
The molecular structure reveals that Zn1 and Zn2 each possess
an octahedral coordination sphere, while Zn3 is tetrahedral.
Zn1 occupies the enclosed phenolic pocket and is coordinated
by both phenolic oxygen centers and both amine nitrogens,
with two bridging acetate ligands completing its coordination
sphere. Zn2 occupies the open phenolic pocket and is
Figure 2. The crystal structure of (L2b)2Zn2-THF. Hydrogen atoms
and three THF molecules are omitted for clarity.
Table 2. Selected Bond Lengths (Å) and Angles (deg) for
(L2b)2Zn2
Zn(1)−O(1)
Zn(1)−O(21′)
Zn(1)−O(21)
Zn(1)−N(9)
Zn(1)−N(13)
C(20)−O(21)
Zn(1)···Zn(1)
1.925(2)
2.004(2)
2.088(2)
2.160(2)
2.109(2)
1.336(3)
3.1557(5)
O(1)−Zn(1)−N(13)
O(1)−Zn(1)−O(21′)
O(21′)−Zn(1)−N(13)
O(21′)−Zn(1)−N(9)
O(21)−Zn(1)−N(13)
O(21′)−Zn(1)−O(21)
O(1)−Zn(1)−O(21)
131.60(9)
116.99(8)
110.84(8)
173.79(7)
90.70(8)
79.11(7)
91.19(7)
D
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Scheme 6. Reaction of L2bH with (i) 1.5 equiv and (ii) 3 equiv of Zn(OAc)2
(i) 1.5 equiv of Zn(OAc)2 in THF, 22 °C, 16 h; (ii) 3 equiv of Zn(OAc)2 in THF, 22 °C, 16 h.
acetate ligands, with coordination by an additional, terminal
acetate ligand. The ligand adopts a bowl shape and has a very
similar structure to the related trizinc complex coordinated by a
bis(phenolate) tetra(amine) macrocycle.11f The 1H NMR
spectrum of L2bZn3(OAc)4 is consistent with the formation
of a trizinc complex (Figure S34). Although the acetate
resonance overlaps with the NH and methylene ligand
resonances, the integration gives good agreement with the
presence of four acetate coligands coordinated to the metal
centers. In addition, the DOSY spectrum shows that all the
resonances possess the same diffusion coefficient, suggesting
that the trizinc structure is retained in C6D6 solution (Figure
S36). The estimation of the complex molecular weight, based
on the diffusion coefficient, gave a calculated molecular weight
of 725 g/mol, which is in close agreement with the calculated
value (802 g/mol). The product is also stable, under reflux
conditions in THF solution, over a period of 16 h (Figure S37).
The reaction of 1.5 equiv of Zn(OAc)2 with L2bH gave a
Figure 3. The crystal structure of (L2b)Zn3(OAc)4. Hydrogen atoms
omitted for clarity.
Table 3. Selected Bond Lengths (Å) of L2bZn3(OAc)4
1
convoluted H NMR spectrum in C6D6 (Figure S38). No
Zn(1)−O(1)
Zn(1)−O(19)
Zn(1)−O(30)
Zn(1)−N(8)
Zn(1)−N(12)
Zn(2)−O(19)
Zn(1)···Zn(2)
2.241(3)
2.043(4)
2.191(4)
2.087(5)
2.095(4)
1.994(4)
3.0546(9)
Zn(2)−O(1)
Zn(2)−O(20)
Zn(2)−O(24)
Zn(2)−O(32)
Zn(3)−O(1)
Zn(3)···Zn(2)
Zn(3)···Zn(1)
2.139(4)
2.576(4)
2.330(4)
1.950(4)
2.005(3)
3.2936(9)
3.6005(8)
signals from the unreacted ligand, dimer (L2b)2Zn2, or the
trimetallic species L2bZn3(OAc)4 were observed, suggesting
that a new product is formed. DOSY spectroscopy reveals that
the new resonances all possess the same diffusion coefficient
(Figure S39), corresponding to a molecular weight of 978 g/
mol, which is in close agreement with the value for
(L2b)2Zn3(OAc)2 (1063 g/mol). Although crystals of
(L2b)2Zn3(OAc)2 suitable for X-ray diffraction experiments
could not be obtained, it was possible to grow crystals for the
closely related trizinc bis(chloride) complex, (L2b)2Zn3Cl2
(Figure S40), providing further support for the formation of
a trizinc complex coordinated by the ancillary ligands.32
coordinated by both methoxy groups, although the bond
distance is quite long [Zn2−O24, 2.330(4) Å and Zn2−O20,
2.576(4) Å], with two bridging acetate ligands providing further
stabilization. Zn3 is only coordinated to one phenolic oxygen
and is coordinated to the other zinc centers via two κ2-bridging
a
Table 4. Copolymerization of CO2 and CHO at 1 bar of CO2
b
c
d
d
e
e
entry
catalyst
L1aZn2(OAc)2
TON
TOF (h−1
)
% carbonate
% selectivity
Mn , (g/mol−1
)
Đ
time
1
2
15
14
45
9
0.2
0.1
2
71
84
83
87
80
82
92
93
77
>99
97
400
400
1500
-
1.09
1.10
1.3
-
4 d
4 d
1 d
4.5 h
4 d
6 h
24 h
1 d
6 h
1 d
L1bZn2(OAc)2
77
3
L2aZn2(OAc)2
>99
>99
88
4
L2bZn2(OAc)2 mix
L2bZn2(OAc)2 mix
2
5
30
19
24
23
99
96
0.3
3
1000
-
1.17
-
f
6
L2bZn3(OAc)4
45
f
7
L2bZn3(OAc)4
1
52
-
-
8
(L2b)2Zn3(OAc)2
1
85
400
1300
3400
1.17
1.23
1.21
g
dizinc macrocycle catalyst11f
17
4
>99
>99
,
9
g
trizinc macrocycle catalyst11f
,
10
a
b
Copolymerization conditions: catalyst/CHO = 1 mol %, 80 °C, 1 bar of CO2. TON = number of moles of epoxide consumed per mole of catalyst.
c
d
1
TOF = TON/h. Determined by comparison of the integrals of signals arising from the methylene protons in the H NMR spectra due to
copolymer carbonate linkages (δ = 4.65 pmm), copolymer ether linkages (δ = 3.45 ppm), and the signals due to the cyclic carbonate byproduct (δ =
e
f
g
4.00 ppm). Determined by SEC in THF calibrated using polystyrene standards. Catalyst/CHO = 0.5 mol %. Catalyst/CHO = 0.1 mol %.
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Comparison of 1H NMR spectra of L2bZn3(OAc)4 and
(L2b)2Zn3(OAc)2 with the spectrum of products obtained from
the reaction of L2bH with 2 equiv of Zn(OAc)2·2(H2O)
suggests that L2bZn3(OAc)4 and (L2b)2Zn3(OAc)2 are the two
major products (Figure S41). DOSY spectroscopy of the
mixture provides further support for this observation, as two
major species are observed with diffusion coefficients that
correspond exactly to the values obtained for the independently
prepared complexes L2bZn3(OAc)4 and (L2b)2Zn3(OAc)2
(Figures S23, S36, and S39). A third product is also present,
albeit in very low concentration, which is likely to be the
dinuclear complex L2bZn2(OAc)2. These findings suggest that
using this ligand, the formation of the dinuclear complex is not
favorable and that metal rearrangement occurs to form
L2bZn3(OAc)4 and (L2b)2Zn3(OAc)2. This was further
supported as L2bZn3(OAc)2 was isolated as a white precipitate
from the reaction of L2bH and 2 equiv of Zn(OAc)2 in dry
THF at −78 °C. As the dinuclear complex could not be
isolated, the L2bZn2(OAc)2 mixture, L2bZn3(OAc)4, and
(L2b)2Zn3(OAc)2 were each tested within polymerization
studies.
Previously, it has been reported that some dizinc complexes
show activities that are dependent on the CO2 pressure; indeed,
such effects were usually observed for complexes coordinated
by flexible ligands.17 Therefore, the catalysts were also tested at
30 bar of CO2 pressure. In all cases, an improvement in the
catalytic activity was observed, as all the complexes produced
PCHC at much lower catalyst loading (0.1 mol % catalyst vs
CHO). On the one hand, the activity of L1aZn2(OAc)2,
L
1 b Zn2 (OAc)2 , the L2 b Zn2 (OAc)2 mixture, and
(L2b)2Zn3(OAc)2 were still very low (Table 5, Entries 1, 2, 4,
a
Table 5. Copolymerization of CO2 and CHO at 30 bar of
CO2
c
TOF
b
d
d
entry
catalyst
TON
(h−1
)
% carbonate % selectivity
1
2
3
4
L1aZn2 (OAc)2
L1bZn2 (OAc)2
L2aZn2 (OAc)2
34
16
2
1
>99
>99
>99
>99
98
89
94
94
706
29
44
2
L2bZn2 (OAc)2
mix
5
6
L2bZn3 (OAc)4
147
28
9
2
75
87
89
(L2b)2Zn3
(OAc)2
>99
Polymerization Catalysis. The series of dizinc complexes
were tested as catalysts for the ROCOP of CO2 and CHO, at 1
bar of pressure of CO2 and 80 °C (Table 4). As other dizinc
catalysts are active under these conditions, the reaction
provides a useful benchmark for comparison.11f,13a,18 All
catalysts displayed a low activity; therefore, high catalyst
loadings were required to convert CHO and CO2 to PCHC. In
particular, complexes containing the L1 ligand backbone
[L1aZn2(OAc)2, and L1bZn2(OAc)2] showed negligable TOF
values (∼0.1 h−1; Entries 1 and 2) at a catalyst loading of 1 mol
% versus CHO. These low activities are attributed to the poor
solubility of the catalyst in CHO. With the L2 backbone, the
L2bZn2(OAc)2 mixture displayed improved solubility in hot
CHO and showed slightly higher TOF values of 2 h−1 (Entry
4). However, after the first 4.5 h of reaction, a white precipitate
formed, and the TOF decreased to 0.3 h−1. Similarly,
L2bZn3(OAc)4 was active during the first 6 h of polymerization
(TOF = 3 h−1, Entry 6) with similar activity to that of the
trizinc macrocycle catalyst reported previously (Entry 10);11f
however, the polymers have only low carbonate linkage
content, with 45% of ether linkages. Again, formation of a
white precipitate was observed after 6 h, and the activity of the
catalyst decreased (TOF 1 h−1 after 24 h, Entry 7). The white
a
Copolymerization conditions: catalyst/CHO = 0.1 mol %, 80 °C, 30
b
bar of CO2, 16 h. TON = number of mole of epoxide consumed per
c
d
mole of catalyst. TOF = TON/h. Determined by comparison of the
integrals of signals arising from the methylene protons in the 1H NMR
spectra due to copolymer carbonate linkages (δ = 4.65 pmm),
copolymer ether linkages of moles of epoxide (δ = 3.45 ppm), and the
signals due to the cyclic carbonate byproduct (δ = 4.00 ppm).11g
and 6, respectively). On the other hand, L2aZn2(OAc)2 (Entry
3) and L2bZn3(OAc)4 (Entry 5) showed a significant
enhancement in their catalytic activities (TOF = 44 and 9
h−1, respectively) and polymer selectivities. The best catalyst,
L2aZn2(OAc)2, has comparable activity to other metal salen
catalysts4g,20e and to the dinickel salen complex that was
previously reported by Ko and co-workers (TOF = 56 h−1).26
At 80 °C, L2aZn2(OAc)2 formed PCHC with a very high
carbonate linkage content and with much better polymer
selectivities (Mn = 18 800 g/mol, Đ = 1.6). These results show
that the more flexible L2 backbone gave catalysts with higher
activity than those with the L1 backbone.
The series of catalysts was also tested for the ROCOP of PA
and CHO. In 2014, Lu et al. reported that L1aZn2(OAc)2·H2O
could polymerize maleic anhydride and CHO when a cocatalyst
such as N,N-dimethylaminopyridine was present.30 In this case,
all the catalysts could fully polymerize PA, in 16 h, without the
need for a cocatalyst (Table 6). While the imine complexes
afforded the corresponding polyesters (PE) with high selectivity
and no ether linkages, the amine complexes provided the
polymers with a greater proportion of ether linkages. A possible
explanation for this trend is that the more electron-donating
imine decreases the Lewis acidity of the metal center, thereby
decreasing the likelihood of successive ring opening two CHO
molecules. Comparing the catalyst activities, L1aZn2(OAc)2
(Entry 1) gave activities (TOF = 70 h−1) similar to some of
the best catalysts based on chromium, cobalt, or aluminum
salen/salophen complexes,16a all of which require the use of a
cocatalyst. However, L2aZn2(OAc)2 (Entry 3) was significantly
more active (TOF = 198 h−1) than the majority of other known
catalysts for this reaction, under similar conditions.33 Both
1
solid was isolated, and its H NMR spectrum showed catalyst
decomposition (Figure S42). This observation suggests that the
trizinc complex L2bZn3(OAc)4 decomposes during the
polymerization reaction, most likely to form the dimer
(L2b)2Zn2 and Zn(OAc)2, which could explain the low activity
observed. Under these conditions, the catalysts could only
produce oligomers with low molecular weights. However,
slightly higher activities (TOF of 2 and 1 h−1) were obtained
with the more stable L2aZn2(OAc)2 (Entry 3) and
(L2b)2Zn3(OAc)2 (Entry 8). The imine complex L2aZn2(OAc)2
also showed an improved CO2 uptake compared to the rest of
the series, producing almost perfectly alternating copolymers of
low molecular weight (1500 g/mol). Despite the low activities
recorded for all the catalysts, the observations from this set of
experiments suggest that complexes containing the L1 back-
bone tend to have lower activity compared to those with the L2
backbone, probably due to their lack of solubility in neat CHO
at high loadings.
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Inorganic Chemistry
Article
a
Table 6. Copolymerization of PA and CHO
b
c
d
e
f
f
entry
catalyst
conv [%]
TON
TOF (h−1
)
% ester
time
1 h
Mn , (g mol−1
)
Đ
1
2
3
4
5
6
L1aZn2(OAc)2
L1bZn2(OAc)2
L2aZn2(OAc)2
L2bZn2(OAc)2 mix
L2bZn3(OAc)4
(L2b)2Zn3(OAc)2
70
64
99
59
28
61
70
64
99
59
28
61
70
11
>99
27
4400
6400
5300
8200
7400
7700
1.15
2.18
1.23
2.17
2.75
2.24
6 h
198
10
>99
21
30 min
6 h
28
13
1 h
10
26
6 h
a
b
1
Copolymerization conditions: catalyst/PA/CHO = 1/100/800, 100 °C. Determined by H NMR spectroscopy (deuterated dimethyl sulfoxide
c
(DMSO-d6)) by integrating the normalized resonances for PA (7.97 ppm) and the phenylene signals in polyester (7.30−7.83 ppm). TON =
number of moles of epoxide consumed per mole of catalyst. TOF = TON/h. Determined by H NMR spectroscopy (DMSO-d6) by integrating
the normalized resonances for ester linkages (4.80−5.26 ppm) and ether linkages (3.22−3.64 ppm). Determined by SEC in THF calibrated using
polystyrene standards.
d
e
1
f
L1aZn2(OAc)2 and L2aZn2(OAc)2 afford polymers with high
ester linkages and with narrow dispersities.
L2aZn2(OAc)2 was also used to successfully afford poly(ester-
block-carbonate) from the one-pot terpolymerization of PA/
CO2/CHO, providing further evidence that this simple system
is selective and is able to synthesize polymers with more
complicated structures.
Terpolymerization of PA/CHO/CO2. The promising
results of L2aZn2(OAc)2 toward CHO/CO2 and PA/CHO
ROCOP prompted the investigation of the terpolymerization
of PA/CHO/CO2. The reaction was performed at 100 °C and
30 bar of CO2 pressure with a 100/1000 mixture of PA/CHO.
The reaction was monitored by taking aliquots after 2 and 18 h.
After 2 h, 1H NMR analysis showed selective polyester
formation, with the appearance of a distinctive resonance at
5.06 ppm (74% conversion) and with no traces of PCHC
observed (Figure S43). 1H NMR analysis after 18 h showed full
conversion of PA (with loss of the resonances at 7.84 and 7.95
ppm) and the formation of PCHC, shown by the increase of
intensity of the resonance at 4.64 ppm. Between 2 and 18 h, a
clear increase in the molecular weight was observed, from 3598
to 7085 g/mol, along with conservation of the polydispersity
index (1.20). 13C NMR and DOSY analysis of the crude
polymer suggests that the PE and PCHC blocks are connected,
as only two carbon signal can be found in the carbonate region
(162.7 and 166.6 ppm); both sets of resonances have a
diffusion coefficient of −9.81 × 10−10 m2/s (Figures S44 and
S45). This evidence suggests that L2aZn2(OAc)2 can be used to
synthesize poly(ester-block-carbonates) from a one-pot reaction
through a PA/CHO/CO2 terpolymerization.
EXPERIMENTAL SECTION
■
All solvents and reagents were obtained from commercial sources
(Aldrich and Merck) and used as received unless stated otherwise. The
solvents THF, toluene, and hexane were predried over activated
molecular sieves (THF) or potassium hydroxide (toluene and hexane)
then refluxed over sodium/benzophenone. All dry solvents and
reagents were stored under nitrogen and degassed by several freeze−
pump−thaw cycles. When stated, manipulations were performed using
a double-manifold Schlenk vacuum line under nitrogen atmosphere or
a nitrogen-filled glovebox. Cyclohexene oxide was dried over calcium
hydride, filtered, purified by fractional distillation prior to use, and
stored under an inert nitrogen atmosphere. Phthalic anhydride was
purified by dissolving in benzene, filtering off impurities, recrystallizing
from chloroform, and then subliming. Research-grade carbon dioxide
was used for copolymerization studies.
NMR. 1H, 13C, and 2D NMR (COSY, HMQC) spectra were
recorded using a Bruker AV 400 MHz spectrometer at 298 K (unless
otherwise stated). DOSY analysis was performed using a Bruker AV
500 MHz spectrometer at 298 K (see the DOSY NMR section in the
Supporting Information for more details).
MALDI-TOF MS. MALDI TOF analysis was performed on
Micromass MALDI micro MX spectrometer. The matrix used was
trans-2-[3-(4-tert-butylphenyl)-2-methyl-2-propenylidene]-
malononitrile, and sodium acetate was used as an additive, when
necessary. The samples were prepared as follows: 10 mg/mL THF
solutions of the complex, matrix, and additive were separately
prepared. Then, 20 μL of the complex and 20 μL of the matrix
solution were mixed, along with 10 μL of the additive solution, if
required. This mixture (2 μL) was then spotted on the MALDI plate
and allowed to dry.
X-ray Diffraction. Data were collected using an Agilent Xcalibur
3E diffractometer, and the structures were refined using the SHELXTL
and SHELX-2013 program systems. The crystallographic data table of
all the compounds can be found in the Supporting Information (Table
S4).
CONCLUSIONS
■
A series of dinuclear zinc salen complexes that are active
catalysts for carbon dioxide/cyclohexene oxide and phthalic
anhydride/cyclohexene oxide ROCOP were investigated.
These studies highlight the influence of the ligand structures
on the catalytic activity and the stability of the dinuclear
complexes. Ligands containing imine moieties resulted in
complexes that were more stable than the amine analogues.
Consequently, a mixture of products, deriving from metal
disproportionation, was obtained with the more flexible amine
L2bH ligand, highlighting its complex solution coordination
chemistry. The catalytic studies indicate that complexes with
the more flexible 2,2-dimethyl propylene backbones are more
active than those with the trans-1,2-cyclohexylene backbones.
Further, the complexes with imine substituents showed much
better selectivities, affording perfectly alternating copolymers,
whereas complexes with amine substitutents gave higher ether-
linkage content. The best catalyst, L2aZn2(OAc)2, combines
imine donors with a flexible 2,2-dimethyl propylene backbone.
It shows higher activity than other dizinc salen catalysts for
carbon dioxide/CHO copolymerizations and higher activities
than other reported catalysts for PA/CHO copolymerizations.
Elemental Analysis. Elemental analysis was determined by
Stephen Boyer at London Metropolitan University.
Size-Exclusion Chromatography. Two Mixed Bed PSS SDV
linear S columns were used in series, with THF as the eluent, at a flow
rate of 1 mL/min, on a Shimadzu LC-20AD instrument at 40 °C.
Polymer molecular weight (Mn) was determined by comparison
against polystyrene standards. The polymer samples were dissolved in
SEC-grade THF and filtered prior to analysis.
Typical Procedure for the Amination of o-Vanillin (L1aH and
L2aH). The appropriate diamine [( )-trans-1,2-diaminocyclohexane
(1.87 g, 0.016 mol) or 2,2-dimethyl-1,3-propanediamine (1.67 g, 0.016
mol)] in MeOH (5 mL) was added dropwise to a pale yellow solution
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of o-vanillin (5 g, 0.033 mol) in MeOH (50 mL). Upon addition of the
diamine, a deep yellow solution was obtained. The mixture was stirred
for 4 h at 22 °C, and the solvent was subsequently removed under
vacuum to give a yellow powder. (Yields: L1aH: 6.1 g, 100% and L2aH:
5.9 g, 100%). 1H and 13C NMR spectra are in accordance with
reported literature values.30,34
L1aH: Elemental analysis for C22H26N2O4 (382.86 g/mol):
Calculated: C, 69.09; H, 6.85; N 7.32%. Found: C, 69.21; H, 6.93;
N, 7.38%.
(OAc). Elemental analysis for C26H34N2O8Zn2 (633 g/mol):
Calculated: C, 49.31; H, 5.41; N, 4.42%. Found: C, 49.18; H, 5.34;
N, 4.39%.
Synthesis of L2aZn2(OAc)2. A solution of Zn(OAc)2·2(H2O) (440
mg, 2 mmol) in MeOH (15 mL) was added to a solution of L2aH (370
mg, 1 mmol) in MeOH (3 mL), and the reaction mixture was stirred
at 22 °C for 16 h. After removal of all volatiles in vacuo, L2aZn2(OAc)2
was isolated as a yellow powder. The pure product was obtained after
washing with pentane followed by crystallization from THF/pentane
at −40 °C (yield: 68%, 420 mg).
L2aH: Elemental analysis for C21H26N2O4 (370.49 g/mol):
Calculated: C, 68.09; H, 7.07; N, 7.56%. Found: C, 68.21; H, 6.99;
N, 7.45%.
1H NMR (400 MHz, CDCl3, 298 K) δ 8.08 (s, 2H, NCH), 6.84
(d, J = 7.9 Hz, 2H, m-Ph), 6.74 (dd, J = 8.0 Hz, 1.7 Hz, 2H, m-Ph),
6.60 (t, J = 7.8 Hz, 2H, p-Ph), 3.87 (s, 6H, O−CH3), 3.72 (s, 4H, N−
CH2-C), 1.97 (s, 6H, OAc), 1.05 (s, 6H, C-(CH3)2). 13C NMR (101
MHz, CDCl3, 298 K) δ 171.1 (CHN), 150.4 (i-Ph), 126.4 (m-Ph),
118.1 (o-Ph), 115.1 (p-Ph), 113.8 (m-Ph), 74.7 (CHN), 55.9 (O−
CH3), 35.3 ((CH2)2−C-(CH3)2), 24.8 (C-(CH3)2), 23.0 (N-CH2−C).
Elemental analysis for C25H30N2O8Zn2 (617 g/mol): Calculated: C,
48.64; H, 4.90; N 4.54%. Found: C, 48.66; H, 5.03; N, 4.43%.
Synthesis of (L2b)2Zn2. Under a nitrogen atmosphere, L2bH (1 g,
2.64 mmol) was dissolved in dry THF (35 mL). Subsequently, ZnEt2
(326 mg, 2.64 mmol) was dissolved in dry THF (5 mL) and added
dropwise into the ligand solution. The reaction mixture was then
stirred at 22 °C for 16 h. (L2b)2Zn2 was isolated via canular transfer of
the solvent as a white powder (yield: 62%, 726 mg).
Typical Reduction Procedure for the Synthesis of L1bH and
L2bH. The reduction was performed in situ following the synthesis of
L1aH or L2aH in methanol solution, by slow addition of 3.5 equiv of
NaBH4 (Caution! Exothermic). The reaction mixture was stirred for 2
h at 22 °C. Water was then added until a white precipitate was
obtained. The suspension was left to stir for 16 h before the product
was isolated via filtration as a white powder. (Yields: L1bH, 6.0 g, 97%
and L2bH, 6.0 g, 97%)
L1bH: 1H NMR (400 MHz, CDCl3, 298 K) δ 6.78 (dd, 2H, J = 8.1,
1.6 Hz, m-Ph), 6.72 (t, J = 7.5 Hz, 2H, p-Ph), 6.63 (dd, J = 7.5, 1.6 Hz,
2H, m-Ph), 4.01 (d, J = 13.6 Hz, 2H, NH−CH2-Ph), 3.88 (d, J = 13.6
Hz, 2H, NH−CH2-Ph), 3.85 (s, 6H, O−CH3), 2.40 (m, CH-
cyclohexyl), 2.11 (m, 2H, CH2−cyclohexyl), 1.67 (m, 2H, CH2−
cyclohexyl), 1.20 (m, 4H, CH2−cyclohexyl). 13C NMR (101 MHz,
CDCl3, 298 K) δ 147.9 (i-Ph), 146.8 (o-Ph), 124.1 (o-Ph), 120.7 (m-
Ph), 118.9 (p-Ph), 110.8 (m-Ph), 60.5 (CH chiral-cyclohexyl), 56.0
(O−CH3), 49.1 (C−CH2-NH), 30.6 (CH2−cyclohexyl), 24.3 (CH2−
cyclohexyl). m/z (ES): 387 ([L1bH + H]+, 100%). Elemental analysis
for C22H30N2O4 (386.49 g/mol): Calculated: C, 68.37; H, 7.82; N,
7.18%. Found: C, 68.23; H, 7.91; N, 7.16%.
1H NMR (400 MHz, d8-THF, 298 K) δ 6.98 (dd, J = 7.9, 1.7 Hz,
1H, m-Ph), 6.83 (dd, J = 7.5, 1.8 Hz, 1H, m-Ph), 6.50 (t, J = 7.7 Hz,
1H, p-Ph), 6.46 (dd, J = 7.8, 1.7 Hz, 1H, m-Ph), 6.21 (dd, J = 7.5, 1.7
Hz, 1H, m-Ph), 6.03 (t, J = 7.6 Hz, 1H, p-Ph), 4.90 (t, J = 11.6 Hz, 1H,
Ph−CHH-NH), 4.37 (s, 3H, O−CH3), 3.82−3.65 (m, 2H, CHH-NH
+ NH), 3.50 (s, 3H, O−CH3), 3.38 (t, J = 14.5, 11.9, 1.9 Hz, 2H, Ph−
CHH-NH + NH−CHH-C), 3.08−2.84 (m, 2H, NH−CHH-C + NH),
2.56−2.43 (m, 1H, Ph−CHH-NH), 2.33 (t, J = 12.5 Hz, 1H, NH−
CHH−C), 1.97 (d, J = 11.9 Hz, 1H, NH−CHH-C), 1.14 (s, 3H,
C(CH3)2), 0.96 (s, 3H, C(CH3)2). 13C NMR (101 MHz, d8-THF, 298
k) δ 159.4 (i-Ph), 158.0 (i-Ph), 152.5 (o-Ph), 152.0 (o-Ph), 130.9 (o-
Ph), 126.7 (m-Ph), 123.9 (m-Ph), 122.7 (m-Ph), 122.4 (o-Ph), 116.9
(p-Ph), 112.9 (m-Ph), 111.8 (p-Ph), 62.9 (OCH3), 60.2 (Ph−CH2−
NH), 56.3 (Ph-CH2−NH), 56.0 (Ph-CH2−NH), 55.9 (O−CH3), 54.5
(NH-CH2−C), 33.6 (C(CH3)2), 26.9 (C(CH3)2), 26.7 (C(CH3)2).
Elemental Analysis for C21H28N2O4Zn (875 g/mol): Calculated: C,
57.61; H, 6.45; N, 6.40%. Found: C, 57.38; H, 6.31; N, 6.34%.
Synthesis of L2bZn2(OAc)2 Mixture (from L2bH). To a solution
of L2bH (200 mg, 0.53 mmol) in MeOH (10 mL), a methanol solution
of Zn(OAc)2·2(H2O) (235 mg, 1.07 mmol in 10 mL) was added. The
reaction mixture was stirred at 22 °C for 16 h, prior to removal of all
solvent, under reduced pressure. The resultant white powder was
washed with pentane and dried under reduced pressure (yield: 71%,
235 mg). From dimer (L2b)2Zn2: Under a nitrogen atmosphere, the
dimer (L2b)2Zn2 (100 mg, 0.23 mmol) was suspended in dry THF (3
mL). Subsequently, a suspension of Zn(OAc)2 (41.9 mg, 0.23 mmol)
in dry THF (2 mL) was added in one portion to the ligand solution.
After the mixture stirred for 16 h at ambient temperature, all volatiles
were removed under vacuum, and the product mixture was isolated as
a white powder (yield: 64%, 92 mg). Elemental Analysis for
C25H34N2O8Zn2 (621 g/mol): Calculated: C, 48.33; H, 5.52; N,
4.51%. Found: C, 48.18; H, 5.63; N, 4.34%.
L2bH: 1H NMR (400 MHz, CDCl3, 298 K) δ 6.84 (dd, J = 7.5, 1.6
Hz, 2H, m-Ph), 6.75 (t, J = 7.8 Hz, 2H, p-Ph), 6.65 (dd, J = 7.5 Hz, 1.6
Hz, 2H, m-Ph), 4.79 (s, 4H, NH, and Ph−OH), 3.98 (s, 4H, NH−
CH2-Ph), 3.87 (s, 6H, O−CH3), 2.56 (s, 4H, NH−CH2−C), 1.00 (s,
6H, C-(CH3)). 13C NMR (101 MHz, CDCl3, 298 K) δ 148.1 (i-Ph),
147.2 (o-Ph), 123.1 (o-Ph), 120.6 (m-Ph), 118.8 (p-Ph), 111.0 (m-Ph),
57.7 (C-CH2−NH), 55.9 (O−CH3), 53.2 (NH-CH2−Ph), 34.7
((CH3)2-C-(CH2)2), 24.4 (C-(CH3)2). m/z (ES): 375 ([L2bH +
H]+, 100%). Elemental analysis for C21H30N2O4 (374.48 g/mol):
Calculated: C, 67.35; H, 8.078; N 7.48%. Found: C, 67.19; H, 7.90; N
7.49%.
Synthesis of L1aZn2(OAc)2 and L1bZn2(OAc)2. A solution of
Zn(OAc)2·2(H2O) (2 equiv) in MeOH (15 mL) was added in one
portion into a solution of L1aH or L1bH (500 mg) in MeOH (3 mL).
The reaction mixture was stirred at 22 °C for 16 h. The dizinc
complexes L1aZn2(OAc)2 and L1bZn2(OAc)2 were isolated as a white
powder via filtration (yields: L1aZn2(OAc)2: 65%, 398 mg and
L1bZn2(OAc)2: 62%, 506 mg).
L1aZn2(OAc)2: 1H NMR (400 MHz, CDCl3, 298 K) δ 8.23 (s, 2H,
CHN), 6.84 (d, J = 7.5 Hz, 2H, m-Ph), 6.79 (d, J = 8.2 Hz, 2H, m-
Ph), 6.60 (t, J = 7.8 Hz, 2H, p-Ph), 3.88 (s, 6H, O−CH3), 3.35 (br m,
2H, CH chiral cyclohexyl), 2.43 (br m, 2H, CH2−cyclohexyl), 2.00 (br
m, 6H OAc and 2H CH2−cyclohexyl), 1.46 (br m, 4H, CH2−
cyclohexyl). 13C NMR (101 MHz, CDCl3) δ 165.4 (HCN), 150.8
(i-Ph), 126.2 (m-Ph), 119.2 (o-Ph), 115.0 (p-Ph), 113.5 (m-Ph), 110.1
(o-Ph), 65.6 (CH-cyclohexyl), 55.7 (O−CH3), 27.9 (CH2−cyclo-
hexyl), 24.4 (CH2−cyclohexyl), 22.9 (OAc). Elemental analysis for
C26H30N2O8Zn2 (629 g/mol): Calculated: C, 49.62; H, 4.81; N,
4.45%. Found: C, 49.50; H, 4.89; N, 4.55%.
Synthesis of (L2b)2Zn3(OAc)4. A THF solution of Zn(OAc)2 (88
mg, 0.4 mmol in 3 mL) was injected into a solution of L2bH (100 mg,
0.27 mmol) in THF solvent (3 mL). The reaction mixture was stirred
for 16 h at 22 °C, and then all solvent was removed in vacuo. The
resultant white solid was washed with pentane and was dried under
reduced pressure (yield: 85%, 244 mg). Elemental Analysis for
C46H62N4O12Zn3 (621 g/mol): Calculated: C, 52.16; H, 5.90; N,
5.29%. Found: C, 52.08; H, 5.59; N, 5.19%.
Synthesis of L2bZn3(OAc)4. L2bH (400 mg, 1.06 mmol) was
dissolved in 20 mL of THF, and Zn(OAc)2 (391 mg, 2.14 mmol) was
subsequently added as a solid. After the mixture stirred for 16 h at
ambient temperature, all solvent was removed under reduced pressure
L1bZn2(OAc)2: 1H NMR (400 MHz, d5-pyr, 298 K) δ 6.75 (m, 4H,
m-Ph), 6.61 (t, J = 7.7 Hz, 2H, p-Ph), 4.28 (br s, 4H, NH−CH2-Ph,
and NH), 4.19 (br s, 2H, NH−CH2-Ph), 3.60 (s, 6H, O−CH3), 2.63
(s, 2H, chiral cyclohexyl), 2.32 (m, 2H, CH2−cyclohexyl), 2.09 (s,
11H, OAc + HOAc), 1.60 (s, 2H, CH2−cyclohexyl), 1.09 (m, 4H,
CH2−cyclohexyl). 13C NMR (101 MHz, d5-pyr, 298 K) δ 176.1 (i-
Ph), 153.4 (o-Ph), 150.1 (o-Ph), 122.8 (m-Ph), 115.1 (p-Ph), 110.4
(m-Ph), 59.7 (chiral CH-cyclohexyl), 54.9 (O−CH3), 50.3 (NH-
CH2−Ph), 29.9 (CH2−cyclohexyl), 24.9 (CH2−cyclohexyl), 22.3
H
DOI: 10.1021/acs.inorgchem.5b02233
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
to give L2bZn3(OAc)4 as a white powder. The pure product was
obtained by washing with pentane.
under reduced pressure. Polymers were purified by precipitation from
a THF solution in MeOH to yield a white powder.
1H NMR (400 MHz, C6D6, 298 K) δ 6.66 (m, 6H, Ph), 4.68 (br s,
2H, Ph−CHH-NH), 3.67 (s, 6H, O−CH3), 2.98 (d, J = 11.6 Hz, 2H,
Ph−CHH-NH), 2.82 (t, J = 12.7, 2H, NH−CHH-C), 1.93 (m, 6H,
NH−CHH-C, and OAc), 1.82 (br t, J = 14.0 Hz, 3H, NH, and OAc),
0.45 (d, J = 6.4 Hz, 6H, C(CH3)2). 13C NMR (101 MHz, C6D6, 298
K) δ 150.3 (i-Ph), 150.0 (o-Ph), 126.0 (o-Ph), 123.2 (m-Ph), 118.1 (p-
Ph), 111.8 (m-Ph), 61.4 (NH-CH2−C), 55.9 (O−CH3), 54.0 (Ph-
CH2−NH), 33.7 (C(CH3)2), 27.9 (C(CH3CH3)), 22.4 (OAc), 20.0
(C(CH3CH3)). Elemental Analysis for C29H40N2O12Zn3 (621 g/mol):
Calculated: C, 48.33; H, 5.52; N, 4.51%; Found: C, 48.17; H, 5.63; N,
4.37%.
ASSOCIATED CONTENT
■
S
* Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.inorg-
chem.5b02233.
The complete experimental procedures and character-
ization data for all new compounds and copolymers.
(PDF)
Synthesis of L2bZn2Et2. Under a nitrogen atmosphere, (L2b)2Zn2
(200 mg, 0.46 mmol) was suspended in dry THF (7 mL). A solution
of ZnEt2 (56 mg, 0.456 mmol) in dry THF (3 mL) was added
dropwise into the previous solution to obtain a colorless solution. The
reaction mixture was stirred at ambient temperature for 16 h, and then
all volatiles were removed under reduced pressure to give L2bZn2Et2 as
a white powder (yield: 75%, 190 mg).
Single-crystal X-ray structure information in CIF format.
(CIF)
AUTHOR INFORMATION
■
Corresponding Author
*E-mail: c.k.williams@imperial.ac.uk.
1H NMR (400 MHz, C6D6, 298 K) δ 6.74 (dd, J = 7.1, 2.1 Hz, 2H,
m-Ph), 6.65 (m, 4H, m-Ph + p-Ph), 4.29 (t, J = 11.7 Hz, 2H, Ph−
CHH-NH), 3.52 (s, 6H, O−CH3), 2.77 (d, J = 11.2 Hz, 2H, Ph−
CHH-NH), 2.60 (t, J = 13.8, 11.3 Hz, 2H, NH−CHH-C), 2.05 (t, J =
8.1 Hz, 3H, Zn-CH2CH3), 1.79 (t, J = 8.1 Hz, 3H, Zn-CH2CH3), 1.69
(d, J = 11.50 Hz, 2H, NH−CHH-C), 0.94 (q, J = 8.1 Hz, 2H, Zn−
CH2CH3), 0.86 (t, J = 13.3 Hz, 2H, NH), 0.61 (q, J = 8.1 Hz, 2H, Zn−
CH2CH3), 0.35 (s, 3H, C(CH3)2), −0.37 (s, 3H, C(CH3)2). 13C NMR
(101 MHz, C6D6) δ 154.10 (i-Ph), 149.3 (o-Ph), 124.9 (o-Ph), 123.1
(m-Ph), 113.9 (m-Ph), 111.1 (p-Ph), 61.7 (NH−CH2−C), 55.9 (O−
CH3), 54.8 (Ph−CH2−NH), 33.6 (C(CH3)2), 27.8 (C(CH3) (CH3)),
20.5 (C(CH3)(CH3)), 15.0 (Zn-CH2CH3), 13.6 (Zn-CH2CH3), −1.7
(Zn-CH2CH3), −3.8 (Zn-CH2CH3). Elemental Analysis for
C25H38N2O4Zn2 (561 g/mol): Calculated: C, 53.49; H, 6.82; N,
4.99%. Found: C, 53.35; H, 7.19; N, 4.84%.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
The EPSRC (EP/K035274/1; EP/K014070/1; EP/K014668;
EP/L017393/1), Chemistry Innovation (Project No. 101688),
and Climate KIC (studentship to A.T.) are acknowledged for
research funding.
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CO2/CHO Polymerization Reactions at 1 bar of CO2 Pressure.
The zinc catalyst (1 equiv) was suspended in CHO (1 mL, 100 equiv)
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