Heterodinuclear Zn(II), Mg(II) or Co(III) with Na(I) Catalysts for Carbon Dioxide and Cyclohexene Oxide Ring Opening Copolymerizations

Article abstract of DOI:10.1002/chem.202101140

A series heterodinuclear catalysts, operating without co-catalyst, show good performances for the ring opening copolymerization (ROCOP) of cyclohexene oxide and carbon dioxide. The complexes feature a macrocyclic ligand designed to coordinate metals such as Zn(II), Mg(II) or Co(III), in a Schiff base ‘pocket’, and Na(I) in a modified crown-ether binding ‘pocket’. The 11 new catalysts are used to explore the influences of the metal combinations and ligand backbones over catalytic activity and selectivity. The highest performance catalyst features the Co(III)Na(I) combination, [N,N′-bis(3,3’-triethylene glycol salicylidene)-1,2-ethylenediamino cobalt(III) di(acetate)]sodium (7), and it shows both excellent activity and selectivity at 1 bar carbon dioxide pressure (TOF=1590 h^?1, >99 % polymer selectivity, 1 : 10: 4000, 100 °C), as well as high activity at higher carbon dioxide pressure (TOF=4343 h^?1, 20 bar, 1 : 10 : 25000). Its rate law shows a first order dependence on both catalyst and cyclohexene oxide concentrations and a zeroth order for carbon dioxide pressure, over the range 10–40 bar. These new catalysts eliminate any need for ionic or Lewis base co-catalyst and instead exploit the coordination of earth-abundant and inexpensive Na(I) adjacent to a second metal to deliver efficient catalysis. They highlight the potential for well-designed ancillary ligands and inexpensive Group 1 metals to deliver high performance heterodinuclear catalysts for carbon dioxide copolymerizations and, in future, these catalysts may also show promise in other alternating copolymerization and carbon dioxide utilizations.

Full text of DOI:10.1002/chem.202101140

Accepted Article
Accepted Article
Title: Heterodinuclear Zn(II), Mg(II) or Co(III) with Na(I) Catalysts
for Carbon Dioxide and Cyclohexene Oxide Ring Opening
Copolymerizations
Authors: Charlotte Katherine Williams, Wouter Lindeboom, Duncan
Fraser, and Christopher Durr
This manuscript has been accepted after peer review and appears as an
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of the final Version of Record (VoR). This work is currently citable by
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To be cited as: Chem. Eur. J. 10.1002/chem.202101140
Link to VoR: https://doi.org/10.1002/chem.202101140
01/2020

10.1002/chem.202101140
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PAPER
Heterodinuclear Zn(II), Mg(II) or Co(III) with Na(I) Catalysts
for Carbon Dioxide and Cyclohexene Oxide Ring Opening
Copolymerizations
Wouter Lindeboom[a], Duncan A. X. Fraser[a], Christopher B. Durr[a] and Charlotte K.
Williams*[a]
[a]
W. Lindeboom, Dr. D. A. X. Fraser, Dr. C. B. Durr, Prof. Dr. C. K. Williams
Department of Chemistry
University of Oxford
Chemistry Research Laboratory, Oxford, UK.
E-mail: charlotte.williams@chem.ox.ac.uk
Twitter Handle: @williamsgroupox
Supporting information for this article is given via a link at the end of the document.
Abstract: A series heterodinuclear catalysts, operating without co-
catalyst, show good performances for the ring opening
copolymerization (ROCOP) of cyclohexene oxide and carbon dioxide.
The complexes feature a macrocyclic ligand designed to coordinate
metals such as Zn(II), Mg(II) or Co(III), in a Schiff base ‘pocket’, and
Na(I) in a modified crown-ether binding ‘pocket’. The 11 new catalysts
are used to explore the influences of the metal combinations and
ligand backbones over catalytic activity and selectivity. The highest
performance catalyst features the Co(III)Na(I) combination, [N, N'-
bis(3,3’-triethylene glycol salicylidene)-1,2-ethylenediamino cobalt(III)
di(acetate)]sodium (7), and it shows both excellent activity and
selectivity at 1 bar carbon dioxide pressure (TOF = 1590 h-1, >99%
polymer selectivity, 1:10: 4000, 100 ºC), as well as high activity at
higher carbon dioxide pressure (TOF = 4343 h-1, 20 bar, 1:10:25000).
Its rate law shows a first order dependence on both catalyst and
cyclohexene oxide concentrations and a zeroth order for carbon
dioxide pressure, over the range 10-40 bar. These new catalysts
eliminate any need for ionic or Lewis base co-catalyst and instead
exploit the coordination of earth-abundant and inexpensive Na(I)
adjacent to a second metal to deliver efficient catalysis. They highlight
the potential for well-designed ancillary ligands and inexpensive
Group 1 metals to deliver high performance heterodinuclear catalysts
for carbon dioxide copolymerizations and, in future, these catalysts
may also show promise in other alternating copolymerization and
carbon dioxide utilizations.
manufacture of household goods, home insulation and footwear,
or as high molar mass polymers and networks which are
elastomers or ductile plastics.[4] Life cycle assessments indicate
such utilizations save carbon dioxide emissions both directly and
by avoiding the use of the petrochemical raw materials.[4b] The
materials evolution and expansion to new applications requires
highly selective, active and controllable polymerization
catalysts.[3a, 3b, 5] Heterogeneous catalysts can show high activities
but with the trade-off of much lower carbon dioxide uptake, the
requirement for high carbon dioxide pressures and limited
polymerization control.[3a, 6] Homogeneous catalysts combine high
carbon dioxide uptake, outstanding activity and impressive
polymerization control facilitating production of sophisticated and
precise polymers and copolymers.[3a] Leading homogeneous
catalysts
include
β-diiminate
di-Zn(II)
complexes,
[Co(III)/Cr(III)(salen)X/PPNX, where X = halide / arylalkoxide /
carboxylate] catalyst systems which are either bicomponent or
feature ‘tethered’ ionic groups and organoboron/PPNX, where X
is defined as above, catalyst systems again either bicomponent
or tethered.[3a, 3b, 7] We have investigated dinuclear complexes of
Zn(II), Mg(II), Co(II/III) and Fe(III), all coordinated by a diphenolate
tetra(amine) macrocyclic ligand, the best of which combine high
activity at low carbon dioxide pressure (including at 1 bar CO2
pressure) and high polymerization control.[8] Recently, we have
discovered that some heterodinuclear catalysts, notably those of
Zn(II)Mg(II) or Co(II)Mg(II), show intermetallic synergy resulting in
9]
higher activity than the homodinuclear catalyst analogues.[8,
Nonetheless, this catalytic synergy remains restricted to specific
metal combinations, since catalysts featuring Zn(II) combined
with Li(I), Na(I), K(I), Ca(II), Al(III), Ga(III) or In(III) show lower
activity than the analogous di-Zn(II) catalyst.[10] The most active
heterodinuclear catalyst, Co(II)Mg(II), showed polymerization
kinetic data that indicates synergy arises from differentiated ‘roles’
for each metal in the catalysis; Mg(II) minimizes the transition
state entropy and Co(II) lowers the transition state enthalpy.[11]
This catalyst showed unprecedented low pressure activity of 1205
h-1 (120 ºC, 1 bar, 0.05 mol%) rising to a field-leading value of
12,500 h-1 at higher pressure (140 °C, 20 bar, 0.05 mol%).
Introduction
The efficient conversion of carbon dioxide to useful products is a
lynchpin of sustainable chemistry.[1] It is driven by the large
quantities of carbon dioxide emitted by a range of industries and
the need to both valorize and (re)cycle it into new products.[1, 2]
One promising carbon dioxide utilization is its copolymerization
with epoxides to form polycarbonates or polyether carbonates.[3]
These materials are useful either as low molecular weight polyols
for polyurethane synthesis, replacing petrochemical polyols in the
1
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Scheme 1. Synthesis of heterodinuclear complexes 1-11 (i) Na(OAc), diamine, 60 ºC, MeOH, 3 h. (ii) M(OAc)n, MeOH, 25 ºC, 1 h, 15-79%. (iii) 20 equiv. NaBH4,
MeOH, 25 ºC, 2 h. (iv) Na(OAc), Zn(OAc)2, 25 ºC, 3 h, 65%.
Okuda and Mashima also investigated hetero-multimetallic
catalysts, in particular a trizinc-cerium catalyst showed high
activity for CHO/CO2 ROCOP.[12] In 2020, Mashima and Nozaki
reported a Co(II)3Nd(III) catalyst that showed an activity of 1625
h-1 (130 ºC, 20 bar, 0.005 mol%).[13] Although these heteronuclear
polymerization catalysts show impressive performances, better
understanding of how to design heterometallic catalysts is needed
and, in particular, the influences of ancillary ligands and metal
combinations must be investigated.
This work targets new ancillary ligands which are dinucleating
macrocycles featuring two different coordination ‘pockets’ each
targeting different metals. The ligands combine a Schiff base site,
featuring diphenolate-di(imine/amine) donors, for the coordination
of first row transition metals, M(II) or M(III), or Mg(II) and a second
binding pocket featuring a tetra-ether moiety to coordinate Group
1 metals. These ligands, and derivatives, have been widely
explored for coordination of UO2, lanthanides(III) or M(II) ions (M
= Ni, Cu, Zn, Co) with Group 1 (Li, Na, K) or 2 (Ba) metals; the
transition metal redox potentials depended upon the second
metal selection.[14] Various transition metal (II) or Group 1 or 2
metals with lanthanides (III) were used to investigate intermetallic
magnetic/electronic interactions.[14e, 15] Recently, Yang and co-
shows excellent tolerance to chain transfer agents (diols) allowing
for the preparation of either high molecular weight PPC or low
molecular weight polyols.
Here, heterodinuclear catalysts featuring either M(II) or M(III)
centres with Na(I) are investigated capitalizing on the low cost,
light-weight and earth-abundance of sodium. So far, in carbon
dioxide/epoxide ring-opening polymerization catalysis there is
scant research into the use of Group 1 metals either on their own
or in heterodinuclear combinations.[17] Yet, the commonly
accepted dinuclear polymerization mechanisms propose one
metal should be sufficiently Lewis acidity to coordinate and active
the epoxide and, in this regard, sodium(I) complexes should be
explored since the metal has a good precedent for epoxide
coordination within crown-ether moieties.[18] This work therefore
targets these new complexes as a means to prepare high activity
and selectivity cyclohexene oxide/carbon dioxide ROCOP
catalysts.
Results and Discussion
The macrocyclic pro-ligand, LH2, was synthesized, according to
14g]
literature procedures, and isolated in 43% yield.[14a,
The
workers
investigated
heterodinuclear
complexes
of
Co(II)/M(IV)/Ni(II)/Fe(II) with alkali/ne earth metals [Na(I), K(I),
Ca(II), Sr(II) and Ba(II)] with particular focus on moderation of
redox potential values.[16] In 2020, our team reported new
Co(III)/M(I) complexes (M = Na, K, Rb, Cs), coordinated by one
of these macrocyclic ligands, as high activity PO/CO2 ROCOP
catalysts (800 h-1, 70 °C, 30 bar, 0.025 mol%).[17] The best catalyst
complexes were synthesized using a new approach where LH2
was first reacted with sodium acetate, the relevant diamine and,
subsequently, the second metal acetate to form the new
heterodinuclear complexes in good/excellent yields (Scheme 1
and SI for further information on the syntheses). Two systematic
series of complexes were targeted, either featuring the same
2
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backbone diamine linker and coordinated to Zn(II), Mg(II) or
Co(III) (1-3; 5-7; 8-10) or featuring a particular metal combination
with different backbone linkers (e.g. Zn(II)Na(I) with L1 = 1, L2 =
5, L3 = 8 or L´1 = 11). When preparing the Co(III)/Na(I) complexes
(3, 7, 10), the ligand was reacted with Co(II)(OAc)2 and after
oxidation, in air, the desired Co(III)/Na(I) complexes were isolated
(occasionally residual Co(II) complexes were removed during
purification). Complex 11 features a macrocyclic ligand with a
diamine linker, and was prepared by reduction of the free Schiff
base macrocycle (NaBH4) with the new ligand being reacted with
sodium and zinc acetate, at room temperature in methanol, to
yield 11 in 65% yield (see SI for experimental details).
Figure 1. Molecular structures, determined by X-ray diffraction experiments, presented as thermal displacement ellipsoid plots (50% probability)
for 1, 1(EtOH), 3 and 7. H-atoms are removed for clarity and color coding used is M (pink), Na (orange), O (red), N (blue), C (grey).
These macrocyclic ligands are particularly useful for isolating pure
heterodinuclear complexes since the different donors (Schiff
base/ether) allow for selective and specific metal coordination in
the two ‘pockets’. It is also worth emphasis that all the
heterodinuclear complexes were isolated pure, i.e. without any
detectable contamination by homodinuclear counterparts.
groups (Figures S2 and S4). These complexes were
characterized at higher temperatures (328 – 398 K) where
averaged resonances were observed. In contrast, 3, produces
well-defined NMR spectra, at room temperature, due to the two
acetate ligands resulting in a coordinatively saturated ‘locked’
structure, consistent with that observed in the solid state (Figures
1 and S3). 13C{1H} NMR spectra for all complexes were fully
assigned with the use of HSQC and HMBC. Each shows a distinct
acetate resonance, at around 180 ppm, and imine peaks, from
160 - 170 ppm (Figures S12-S21). The complexes featuring L3 (8-
10) displayed very low solubility, between 298 K and 398 K, and
so were characterization by CP-MAS 13C NMR spectroscopy. For
10, this solid state 13C NMR spectrum shows two different acetate
peaks in excellent agreement with the two distinct acetate
environments observed in the solid-state structure characterized
by X-ray diffraction of 7 (Figure 1 and S51). The complexes show
molecular ions in the ESI mass spectra consistent with ionization
resulting in loss of an acetate ligand (Figure S52 and S53). The
cobalt catalysts also undergo reduction under the mass
spectrometry conditions to form cobalt(II) complexes; this
phenomenon was observed previously for the Co(III)/K(I)
1
Complex formation was confirmed using H NMR spectroscopy,
which were characterized by the disappearance of the pro-
ligand’s aldehyde resonance and the formation of the desired
imine resonance (8.67-8.29 ppm) and by the formation of new
acetate and linker resonances (Figures S1-S10). Complexes
1
were fully characterized by H, 13C, COSY, HSQC and HMBC
NMR spectroscopy (Figures S1-50). All complexes show 1H NMR
spectra with only a single imine resonance and most show three
crown-ether resonances indicating they have symmetrical, time-
averaged structures. Complexes 4, 6, 9 and 11 all display
additional ether methylene resonances, presenting as a large
multiplet in the corresponding region, consistent with lower
symmetry complexes likely due to particular conformations of the
ether moieties being favored (Figures S4, S6, S9 and S11).
Complexes 2 and 4 show fluxional NMR spectra at room
temperature (CDCl3 or C2D2Cl4), due to the flexible propene linker
3
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catalyst.[17] All complexes also have IR spectra showing
symmetric and asymmetric acetate stretches (Figure S54-S56).
Single crystals, suitable for X-ray diffraction experiments, were
obtained for 1, 3, and 11 by the slow evaporation of chloroform
solutions; 1(EtOH) by the slow evaporation of an ethanol solution
and 7 by the slow diffusion of pentane into a methylene chloride
solution of catalyst. All structures confirm the formation of
metallate complexes at Zn(II) or Co(III), respectively (Figure 1).
Both phenolate groups are anionically coordinated to the
transition metal (resulting in shorter M-O1 and M-O2 bond lengths
than the Na1-O1 and Na1-O2 bond lengths). Also, the acetate
ligands are also anionically coordinated by the transition metal, as
evidenced by the unsymmetrical acetate ligand C-O bond
distances (C30-O7 vs. C30-O8 or C32-O9 vs. C32-O10) and
shorter M-O7 (M-O9) bond distances compared with Na-O8 (or
Na-O10) bond lengths (Figure S57-S60, Table S2-S8). This effect
is also observed in the structure of 1(EtOH), where the acetate is
anionically coordinated at the zinc centre and the ethanol
molecule is coordinated at the sodium. This complex shows a
greater distance between the Zn(II) and Na(I) atoms compared to
1 which features a bridging acetate ligand. The formation of
metallate complexes is significant for the catalysis because the
sodium ion is expected to show higher Lewis acidity and may,
therefore, be pre-disposed towards epoxide coordination. Support
for this notion comes from the coordination of the ethanol
molecule at the sodium, rather than zinc site in complex 1(EtOH).
It is also relevant to note that cobaltate catalysts were long-
proposed in carbon dioxide, as well as anhydride, epoxide ring
onset of trans-cyclic carbonate formation only becomes
significant above 100 °C, although the barrier was somewhat
lower for selected catalysts (Table S8). The series of
heterodinuclear zinc catalysts were all moderately active, with
TOF = 23 – 42 h-1, and all showed high selectivity. For these zinc
complexes, the flexible C3 backbone linker, e.g. in 1, results in a
higher polymerization rate (Table 1, entries 1-3), while use of the
diamine variant 11 led to a slight decrease in activity alongside a
concomitant decrease in selectivity (Table 1, entries 1 and 4). All
the magnesium catalysts show very low activity, with TOF = 6 – 7
h-1, and low selectivity for PCHC formation, 43 – 73%, the latter
driven by competitive trans-cyclic carbonate formation (Table 1,
entries 5-7).
Table 1. Polymerization data for CHO/CO2 ROCOP[a]
PCHC
Selec.
[%][b]
Mn
t
[h]
TOF
Entry
1
Cat.
TON[c]
[g.mol-1]
[Ð][e]
[h-1][d]
335
(±17)
42
(±2)
2500
[1.16]
1
8
96
235
(±12)
29
(±1)
1700
[1.13]
2
3
4
5
6
5
8
8
94
97
93
58
48
265
(±13)
33
(±2)
2000
[1.17]
8
318
(±16)
23
(±1)
2500
[1.13]
11
2
14
24
24
168
(±8)
7
(±1)
900
[1.14]
177
(±9)
7
(±1)
400
[1.43]
6
opening
copolymerizations
using
catalysts/co-catalyst
140
(±7)
6
(±1)
400
[1.33]
combinations but so-far these species eluded structural
characterization.[7a, 7h, 7q, 19] Hence, the structures isolated in this
work are likely to be relevant to the active site species present
using M(III)(salen)X/PPNX co-catalyst systems.
7
8
9
9
4
3
24
24
14
43
-
0
0
n.d.
581
(±29)
42
(±2)
3000
[1.17]
95
Almost all the complexes show dinuclear, but monomeric,
structures in the solid state, except 11 which forms acetate
bridged polymeric structure (Figures S60). All the zinc complexes
(1, 1(EtOH), and 11) show square pyramidal geometries at the
Zn(II) site and heptacoordinate Na(I). The Co(III), which has two
acetate ligands, show different binding modes depending on the
imine linker. Catalyst 3 features two bridging κ2 acetates and a
short Co(III)-Na(I) separation of 3.194 Å. In contrast, 7 shows both
a κ2 and κ1 acetate ligand and has a greater Co(III)-Na(I)
separation of 3.388 Å. Both complexes feature octahedral
cobaltate centres, but show distinctly different coordination
geometries at sodium due to the different acetate binding modes.
11 forms a coordination polymer with bridging acetate ligands,
either causing, or resulting from, significant distortion of the solid-
state structure, with highly unsymmetrical binding of sodium to the
crown-ether moiety (Figure S60).
The new complexes were tested as catalysts for the
copolymerization of cyclohexene oxide (CHO) and CO2, with
experiments conducted at 1 bar pressure and 100 ºC using 10
equivalents of trans-1,2-cyclohexanediol as chain transfer agent
and neat epoxide (Table 1). For most complexes, the catalyst
loading was 0.1 mol% (i.e. 1:1000, catalyst:CHO), but for the
more active Co(III)Na(I) catalysts lower loadings were applied to
avoid entering viscosity limited kinetic regimes over fixed time
period experiments. For the series of complexes, a range of
activity values were observed with turnover frequencies (TOFs)
varying from 0 – 1590 h-1 and selectivity for poly(cyclohexene
carbonate) (PCHC) formation from 43 - >99%. In general, the
5300
[1.07]
795
(±40)
1590
(±80)
10[f]
7
0.5
>99
2200
[1.05]
15700
[1.03]
4343
(±43
4)
4343
(±434)
11[g]
12[h]
13[i]
7
10
1
2
6
>99
94
6700
[1.17]
176
(±9)
88
(±4)
900
[1.19]
12700
[1.04]
LaMgZn
>99
438
98
5100
[1.16]
0.6
7
1600
[1.15]
14[j]
15[k]
16[l]
LaMgCoII
LbZn3Ce
>99
>99
>99
465
900
522
699
300
87
15000
[1.20]
3
6
LcCoIII(X)/
nBu4NX
19100
[1.17]
[a] Catalysis conditions: catalyst : CHD : CHO 1 : 10 : 1000, 100 °C, 1 bar
pressure CO2 and in neat epoxide; [b] Selectivity for PCHC against trans-
cyclohexene carbonate (no ether observed). Measured by integration of 1H
NMR resonances for cyclic carbonate (δ 4.00 ppm) and ether linkages (δ 3.45
ppm) against PCHC (δ 4.65 ppm); [c] Turnover number (TON) = moles of CHO
consumed/moles catalyst, moles of CHO consumed determined by, determined
by the addition of integrals of 1H NMR resonances of cyclic carbonate (δ 4.00
ppm) and PCHC (δ 4.65 ppm) over addition of CHO (δ 3.05 ppm), cyclic
carbonate (δ 4.00 ppm) and PCHC (δ 4.65 ppm), multiplied by initial moles of
CHO; [d] Turnover frequency (TOF) = TON/time; [e] Determined by SEC, in
THF, calibrated against narrow Mn polystyrene standards; dispersity given in
º
square brackets; [f] 1:10:4000; [g] 1:10:25000, 120 C, 20 bar CO2; [h] 0.033
4
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ZnMg complexes.[8d, 10a, 11] The lack of reactivity could be due to
coordinative saturation of the magnesium(II) center, which has a
strong preference for octahedral coordination, preventing epoxide
coordination.[20] Furthermore, sodium(I) has a much weaker
binding association than Mg(II), which has among the strongest
binding associations to oxygens, which may be relevant to the
metal’s abilities to accelerate epoxide coordination and insertion
mol% catalyst loading; [i] This literature catalyst was tested at 0.1 mol% catalyst,
80 ºC and 1 bar CO2[10a]; [j] 0.05 mol% catalyst loading, 20 equiv. CHD, 100 ºC
and 1 bar CO2[11]; [k] This literature catalyst was tested at 0.05 mol% catalyst,
º
100 C and 3 bar CO2[12]; [l] This literature catalyst was tested at 0.02 mol%
catalyst, 50 ºC and 1 bar CO2[7e]. For the chemical structures of all the literature
catalysts, see Figure S61. GPC traces can be found Figures S62-S64.
When the magnesium is localized in the tetra-ether coordination
‘pocket’ in complex 4, there was no activity at all (Table 1, entry
8) which is unexpected given the success of other macrocyclic
Figure 2. Plots used to analyse the polymerization kinetics and determine the reaction orders in various monomers. (A) Semilogarithmic plot of cyclohexene oxide
concentration vs. time with a linear fit to data indicative of a first order dependence on cyclohexene oxide concentration. (B) Plot of activity (TOF) vs. pressure of
carbon dioxide, over the range 10-40 bar with a constant value consistent with zero order in CO2 pressure. (C) Logarithmic plot of pseudo first order rate coefficient,
kobs vs. concentration of 7 and the linear fit to the data, used to determine a first order dependence on catalyst concentration.
steps in the catalytic cycle.[20, 21] Thus, the ZnMg complex is inert
using this ligand but the same ligand yields an active catalyst for
the ZnNa combination.
system, such as LcCoIII(X)/nBu4NX, 7 remains stable at higher
temperatures resulting in higher activities and these are achieved
without any co-catalyst.[7b] Catalyst 7 also shows five times
greater activity than the tetranuclear Zn(II)Ce(III) catalyst,
LbZn3Ce.[12]
The variation in activities for the cobalt series of complexes shows
a greater than 30-fold rate acceleration for the catalysts with
ethene linkers, and to a lesser extent higher rates for phenylene
linkers compared to propene. The most active catalyst, 7, shows
a TOF of 1523 h-1 and >99% selectivity for carbon dioxide uptake
(Table 1, entry 10). To optimize its performance, polymerizations
were conducted in a stainless-steel reactor, with improved stirring
efficiency and 20 bar carbon dioxide, and this allowed the catalyst
to reach a TOF of 4343 h-1 at 120 °C (Table 1, entry 11). Indeed,
it’s amongst the most highly active catalysts yet reported in this
field using 1 bar pressure of carbon dioxide.[11-13] Comparisons
with literature catalysts are more complex since studies are not
run under identical conditions, but selected data taken using
conditions closest to those used in this study reveals the excellent
performance of 7 (Table 1). For example, compared with the
previously reported heterodinuclear LaMgZn catalyst, 7 shows
>10 times greater activity and it shows approximately twice the
activity of the recently reported heterodinuclear LaMgCoII
catalyst.[11] Although at higher temperatures (120 °C) LaMgCoII
shows a higher activity of 1205 h-1 and outperforms catalyst 7
(Table S8). Compared with literature Co(III)(salen)(X)/co-catalyst
In comparison to other catalysts operating at CO2 pressures of
>10 bar, 7 shows more average performances. For example,
LaMgCoII achieved a TOF of 12,462 h-1, at 140 °C and 20 bar CO2
pressure, and it maintained >99% carbonate selectivity and 0.05
mol% catalyst loading.[11] This performance equates with a three-
fold higher rate than 7 at >10 times lower loading. Under
equivalent pressure, catalyst loading and at 120 °C, a Co(III)Co(II)
catalyst showed a similar activity to 7 of 4200 h-1.[9b, 11] Rieger and
co-workers reported a di-zinc catalyst showing a record-breaking
activity of 155,000 h-1, at 30 bar CO2 pressure and 100 °C,
although this value was achieved in the initial phases of
polymerization (low conversions) and for conditions where
carbonate selectivity was slightly lower (88%).[22] Notably,
subsequent reports from the same group using similar di-zinc
catalysts showed more consistent TOFs of around 6000 h-1.[23]
To better understand the enhanced activity of 7, its polymerization
kinetics were studied using in situ ATR-IR spectroscopy allowing
the collection of polymerization versus time data (Figure S65).
The order in epoxide was determined, using a 3.33 M CHO
5
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solution, 1.66 mM of catalyst, in diethyl carbonate (total volume
10 mL) at 100 ºC, by monitoring the increase in concentration of
an absorption due to polycarbonate formed (1330, 1160 and 988
cm-1). The concentration data was plotted using a semi-
logarithmic integrated rate law treatment (5-70 % conversion) and
the linear fit indicates a first order dependence of the rate on the
concentration of cyclohexene oxide. The dependence of the rate
on carbon dioxide pressure was determined, using neat epoxide
and 0.395 mM catalyst concentration at 100 ºC, over the pressure
range 10 – 40 bar CO2. This range was chosen as it is the
standard used for many other catalysts for CO2/epoxide
ROCOP.[11, 23, 24] All reactions were performed in steel reactors
which utilize mechanical stirring, unfortunately it is not feasible to
apply this set-up with a constant pressure below 10 bar. Over the
pressure range studied, there was no statistically significant
correlation between CO2 pressure and rate, indicating a zero-
order dependence; it should be noted that because the reactor
set-up is static after charging with gas there is some error in data
points particularly at lower pressures where gas consumption
carbonate.[27] In future, other di- or polynucleating ligands should
be explored making use of the Group 1 metal concept to replace
co-catalysts and enhance activity and selectivity values. A further
benefit of catalyst 7 is that it maintains high activity and Co(III)
speciation even at elevated temperatures. In contrast,
bicomponent Co(III)(salen)(X)/PPNX catalyst systems were
shown to undergo irreversible thermally activated Co(III)
reduction at temperatures above ~60 °C.[7e, 28]
This investigation of new heterodinuclear catalysts resulted in the
isolation and structural characterization of new complexes and in
all cases the transition metal is speciated as a metallate species.
This finding is significant since metallates were previously
proposed as the active sites for M(III)(salen)X/PPNX catalyst
29]
systems.[7b,
For example, a
[Cr(III)(salen)(azide)]-/PPN+
complex was isolated, and structurally characterized, by reaction
of the Cr(III) complex with the PPN(azide) salt.[30] Lee and co-
workers reported a Co(III)(salen)(X) catalyst system featuring four
specially tethered quaternary ammonium salts and proposed that
the complex operated via a cobaltate intermediate, although such
a species was not isolated or structurally characterized.[7a, 7q] Both
in the previous literature work and in the case of these
heterodinuclear catalysts, it is proposed that the formation of a
cobaltate(III) complex enhances the nucleophilicity of the Co(III)-
carbonate intermediate and accelerates epoxide ring-opening. It
is proposed that the high activity resulting from catalyst 7 arises
from the different roles for the two metals and is enhanced by a
short Co(III)-Na(I) (3.388 Å). In future, this new class of catalyst
should be explored using other combinations of metals, for
example replacing Co(III) with Fe(III), Al(III), Ti(III) or Cr(III)
centres, which all have some precedent in this carbon dioxide
copolymerization catalysis, as well as investigating the optimum
Group 1 metal combination.[8a, 30, 31] These new heterodinuclear
catalysts are also expected to show high activity for related
heterocycle/heterocumulene ring-opening copolymerizations,
including epoxide/anhydride, epoxide/COS and/or heavier
congener combinations to access other oxygenated and
heteroatom containing polymers. It has also been observed that
many of the successful ROCOP catalysts may also be used to
copolymerize mixtures of epoxide, anhydride, carbon dioxide and
lactone, using switch catalysis, to access multi-block polymers.[32]
Thus, these new catalysts should be explored using a range of
other monomers to diversify the product scope and range for
carbon dioxide containing polymers.
occurs rapidly (Figure 2B).
A range of different catalyst
concentrations, from 1.10 to 3.31 mM, using 3.33 M CHO in
diethyl carbonate (DEC), at 1 bar pressure and 100 ºC, were
tested. In each case, the pseudo first order rate coefficient, kobs
,
was determined from semi-logarithmic plots of conversion vs.
time. The double logarithmic plot of rate coefficient vs.
concentration shows a linear fit, consistent with a first order
dependence on catalyst concentration. Overall, the data is
consistent with a second order rate law (eq. 1) and is similar to
that observed for the Co(III)K(I) catalyst for PO/CO2 ROCOP.[17]
0
[
]1[
]1[
]
푅푎푡푒 = 푘 퐶푎푡 퐶퐻푂 퐶푂2
(1)
The polymerization kinetics and rate law suggests the rate
determining step likely involves epoxide ring-opening by a metal
carbonate intermediate.[3c, 8c] Such a rate law and mechanistic
interpretation was previously observed for other heterodinuclear
catalysts and is consistent with differentiated roles for the metals
in the catalysis.[11, 17] On the basis of the solid state structure and
the formation of cobaltate (or zincate) complexes in all cases, it is
proposed that the Lewis acidic sodium serves as the site for
epoxide coordination (Figure 1) and the carbonate nucleophile is
provided by the second metal with reactivity/activity following the
trend Mg(II) < Zn(II) < Co(III).
Comparing the most active Co(III)Na(I) catalyst with well-known
Co(III)(salen)(X)/PPNX catalyst systems, which were very
successfully optimized in the literature, reveals a new catalyst
design: replacing the ionic co-catalyst with a Group 1 metal centre
coordinated close to the cobaltate by exploiting a dinucleating
ancillary ligand.[7a, 7m, 25] Catalyst 7 also benefits from the use of
inexpensive, light-weight and abundant Na(I). Though the ‘18-
crown-6’-like tetra-ether cavity size is more commonly applied to
potassium, it is known that strong coordination to sodium is also
possible.[26] By correctly employing an ancillary ligand to
coordinate Na(I) adjacent to the Co(III) active site, addition or
tethering of ionic co-catalysts is obviated. The ability to replace
and remove co-catalysts is important since the best catalysts
feature rather expensive and esoteric salts, most notably PPNX
species. By incorporating the ’18-crown-6’-like moiety into the
macrocycle the catalyst shows increased rate and metal
cooperativity as compared to use of Co(III)salen complexes
applied with crown ether coordinated Group 1 metals, as shown
by Li and co-workers, where the major product was cyclic
Conclusions
A series of high activity heterodinuclear catalysts, combining Na(I)
with Zn(II), Mg(II) or Co(III), for the ring-opening copolymerization
of carbon dioxide and cyclohexene oxide were investigated. The
use of a macrocyclic ligand featuring two binding pockets allowed
for selective synthesis of only the heterodinuclear complexes: the
Na(I) is coordinated by a tetra-ether moiety, while the second
metal is coordinated by the schiff base portion of the macrocyle.
The most active catalyst, Co(III)/Na(I), showed field-leading rates
and selectivity values particularly when applied under low carbon
dioxide pressures. The rate law and solid-state structural data
support a rate determining step in which sodium coordinates the
cyclohexene oxide and is attacked by the Co(III)-carbonate
intermediate. The new catalyst types are beneficial compared to
6
This article is protected by copyright. All rights reserved.

10.1002/chem.202101140
Chemistry - A European Journal
FULL PAPER
[5]
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a) M. R. Kember, A. Buchard, C. K. Williams, Chem.
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PPNCl, and obviate complex salt ‘tethering’ processes which
complicate catalyst preparations. In contrast, these catalysts
feature the Co(III) and Na(I) centres ligated by a common
macrocyclic ligand and positioned close to one another so as to
modify and moderate reactivity appropriately. Future
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catalysts, combining transition metals/main group elements with
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heterocycles and cyclic anhydrides, as well as to lactone or lactide
ring-opening polymerizations allowing access to new materials
and structures derived from carbon dioxide.
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Acknowledgements
The EPSRC (EP/S018603/1, EP/R027129/1, EP/V003321/1),
the Oxford Martin School (Future of Plastics) and Econic
Technologies (CASE award to Wouter Lindeboom) are
acknowledged for research funding.
Keywords: Heterodinuclear complexes • ROCOP • Carbon
dioxide (CO2) • Sodium • Catalysis
Experimental Section
Experimental and characterisation details can be found in the
supporting information.
Deposition
numbers
^^<url
href="https://www.ccdc.cam.ac.uk/services/structures?id=doi:10.100
2/chem.202101140">2073147-2073150 (for 1, 1(EtOH), 3 and 11)
</url> contain(s) the supplementary crystallographic data for this
paper. These data are provided free of charge by the joint Cambridge
Crystallographic Data Centre and Fachinformationszentrum
Karlsruhe <url href=" http://www.ccdc.cam.ac.uk/structures ">Access
Structures service</url>.
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