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1

Copolymerization of CO2 and Cyclohexene Oxide
Mediated by Yb(salen) Based Complexes
Antonello Decortes,† Robert M. Haak,† Carmen Martín,† Marta Martínez Belmonte,† Eddy
Martin,† Jordi Benet-Buchholz,† Arjan W. Kleij*,†,‡
†

Institute of Chemical Research of Catalonia (ICIQ), Av. Països Catalans 16, 43007 – Tarragona,
Spain.

‡

Catalan Institute of Research and Advanced Studies (ICREA), Pg. Lluís Companys 23, 08010 –
Barcelona, Spain.

Table of contents entry:

2

ABSTRACT: New catalysts based on Yb(salen) complexes active for the copolymerization of
cyclohexene oxide (CHO) and CO2 to give poly(cyclohexene)carbonate (PCHC) are reported. In
combination with co-catalytic, nucleophilic chloride additives these new (binary) catalysts
provided good conversion and selectivity for PCHC formation with average turnover frequencies
of up to 35 h-1 and narrow molecular weight distributions. The best results were obtained with the
binary catalyst system 1 (0.1 mol%)/NBu4Cl (0.05 mol%), and at 90 °C a conversion of 57% was
reached after 18 h with a TOF of 31 h1, and the polycarbonate had an Mn of 10.2 Kg/mol and a
PDI of 1.54. Comparative catalysis studies have also been performed with a series of literature
systems based on transition metal/lanthanide salen complexes, and the newly presented catalysts
show comparatively good activity as well as copolymerization selectivity. MALDI-ToF mass
spectrometric analysis revealed that trace water contamination and/or traces of 1,2-cyclohexanediol were responsible for chain transfer effects limiting to some extent the maximum molecular
weights that can be achieved in the current reactor set up.

3

INTRODUCTION
In recent years, reactions using carbon dioxide as starting material have been the focus of
increasing research efforts.1 Amongst the numerous reported transformations, reactions of CO2
with epoxides to produce either cyclic2 or poly-carbonates have received special attention due to
the industrial and economic importance of these products.3 Among the catalysts used for such and
related processes, salen-type trivalent cobalt and chromium complexes are especially prominent.4
Other metal complexes that have been reported to successfully mediate the copolymerization of
epoxides and CO2 include, for instance, aluminum,5 zinc,6 magnesium,7 iron8 and manganese.9 A
limited number of homogeneous catalysts based on rare earth metals have been described for the
copolymerization of epoxides and carbon dioxide. Hultzsch and coworkers, for example, have
developed catalysts based on lanthanum and yttrium ions supported by -diketiminate ligands,10
whereas Hou and coworkers reported on cyclopentadienyl-ligated yttrium, dysprosium, lutetium
and scandium catalysts.11 The group of Cui reported catalysts based on yttrium, lutetium and
scandium catalysts with -diimine ligands.12 It should also be noted that lanthanide based catalysts
have previously also been applied as catalysts in the formation of other types of related copolymers
such as those incorporating epoxides and/or cyclic anhydrides or lactides.13
To the best of our knowledge, the combination of rare-earth metals and salen-type ligands has
not been explored in great detail for the coupling reaction between epoxides and CO2 to afford
polycarbonates. Therefore, our primary aim was to develop catalysts active towards this
copolymerization reaction and to study their catalytic efficiencies and polymer-derived products.

4

A key aspect to control with these lanthanide based salen-type complexes is their strong
aggregation behavior mediated by bridging ligands. The formation of mononuclear complexes is
hence clearly preferred if catalytic applications are pursued. Here we describe these efforts and a
simple, one-pot synthetic procedure has been developed that allows for the isolation a small series
of discrete mononuclear Yb(salen) complexes with a pendant, hemi-labile phenoxide ligand. These
complexes have been tested in the copolymerization of cyclohexene oxide (CHO) and CO2 giving
rise to polycarbonates with high selectivity control. A detailed comparison of the activity and
selectivity of these systems with known literature catalysts under similar reaction conditions
allowed for a realistic estimate of the activity and selectivity profiles of the newly presented Yb
complexes.

EXPERIMENTAL SECTION
General comments: Unless otherwise stated, all manipulations were carried out using standard
Schlenk vacuum line techniques under a N2 atmosphere or using a nitrogen-filled glove box.
Starting materials were obtained from Aldrich and used as received unless stated otherwise. PPNCl
(PPN = bis(triphenylphosphine)iminium), NBu4Cl and DMAP (4-dimethylamino-pyridine) were
obtained from Aldrich and dried under vacuum at 80 °C or at room temperature (DMAP)
overnight.

PPNBr, PPNI, PPNN3 and PPN(OAc) were prepared according to literature

procedures.14,15 Complexes 5, 6 and 9-12 were prepared according to published procedures.16-18
CHO and C6D6 were dried over CaH2, distilled under reduced pressure and stored under a nitrogen
atmosphere before use. Cyclohexanol was distilled over CaO prior to use. Commercial CO 2 was
obtained from Praxair and used without further purification. Solvents were dried by passing them
through dried alumina towers and stored under nitrogen atmosphere.

5

Elemental analyses was performed by the Unidád de Análisis Elemental at the Universidad de
Santiago de Compostela (Spain). Mass spectrometric analyses (MALDI-ToF mass for polymer
and MALDI(+) and MALDI()-MS for new complexes) studies were carried out by the Research
Support Group at the ICIQ using a BRUKER Autoflex matrix assisted laser desorption ionization
(MALDI) time-of-flight mass spectrometer. Typical solutions of 1 mg/mL were prepared in
dichloromethane using dithranol (10 mg/mL THF) as matrix and NaAcO (1 mg/mL MeOH) as
additive. Samples were prepared by taking 5 L of analyte and combining it with 25 L of matrix
and 1 L of additive.
FT-IR measurements were performed on a Bruker Optics FTIR Alpha spectrometer equipped
with a DTGS detector, KBr beam-splitter at 4 cm-1 resolution. UV-Vis measurements were carried
out on a Shimadzu UV-2401PC spectrophotometer equipped with a photomultiplier detector,
double beam optics, and D2 and W light sources. NMR spectra were recorded on a Bruker AV400 spectrometer and referenced to the residual NMR solvent signals.
Size exclusion chromatography (SEC/GPC) was measured on a Waters Alliance system
equipped with a Waters 2695 separation module, a Waters 2414 refractive index detector (35 °C),
a Waters 2487 dual absorbance detector, and a PSS SDV 5 m guard column followed by two PSS
SDV linear XL columns in series of 5 m (8 × 300) at 40 °C. THF stabilized with BHT (Biosolve)
with 1% v/v acetic acid was used as eluent at a flow rate of 1.0 mL·min−1. The molecular weights
were calculated with respect to polystyrene standards (Polymer Laboratories, Mp = 580 Da up to
Mp = 7.1 × 106 Da).
X-ray crystallographic studies: X-ray diffraction studies were carried out by the Research
Support Group at the ICIQ. Crystals for the diffraction studies were prepared under inert conditions
and immersed in perfluoro-polyether as protecting oil for further manipulation. Data Collection:

6

measurements were made on a Bruker-Nonius diffractometer equipped with an APPEX 2 4 K
CCD area detector, a FR591 rotating anode with Mo-K radiation, Montel mirrors as monochromator and a Kryoflex low temperature device (T = 173 °C). Full-sphere data collection was
used with  and  scans. Programs used: data collection Apex2 V2009.1-0 (Bruker-Nonius 2009),
data reduction Saint+ V7.60A (Bruker-Nonius 2009) and absorption correction SADABS Version
2008/1. In the case of twinned crystals, TWINABS Version 2008/4 Bruker AXS was applied.19
Structure Solution and Refinement: this was performed with SHELXTL Version 6.14 (Sheldrick,
2008).20
Synthesis of complexes 1–4: In a typical experiment, the appropriate diamine (0.91 mmol) and
Yb(NO3)3∙5H2O (1.00 mmol) were mixed together in MeOH (15 mL). Upon complete dissolution,
the respective salicylaldehyde (2.93 mmol) was added together with NEt3 (4.55 mmol). A yellow
precipitate readily formed and the resulting suspension was left to stir at reflux temperature for 3
hours. Afterwards, the reaction was allowed to cool down to r.t. and stirred for another 3 hours.
Finally, the suspension was filtered and the resulting residue was washed with MeOH (three times)
before it was dried under vacuum. In the case of complexes 1 and 4, crystals were obtained by
slow evaporation of the mother liquor left after isolation of the first fraction of product, and
analyzed by single crystal X-ray diffraction. In general, the isolated Yb complexes were
manipulated and characterized without any special precautions.
Complex 1: Yellow solid. Yield: 60%. 1H NMR (CDCl3, 400 MHz, RT, paramagnetic
spectrum):  = 32.17, 24.54, 18.29, 15.29, 15.05, 13.13, 12.52, 10.32, 8.59, 7.58, 3.73, 3.31, 1.94,
1.41, 0.76, -1.58, -2.64, -3.20, -3.72, -9.90; MS (MALDI+, pyrene): m/z = 777.4 [M – MeOH]+
(calcd. 777.3), 825.4 [M + H2O  H]+ (calcd. 825.4). HRMS (MALDI+, dctb): found; m/z =
777.2590 [M – MeOH]+ (calcd. 777.2611); Anal. calcd. for C39H43N2O4Yb·MeOH: C 59.40, H

7

5.86, N 3.46; found: C 59.18, H 6.27, N 3.41. IR (cm-1):  = 3531, 2944, 2902, 2854, 2774, 1629,
1610, 1581, 1535, 1428, 1409, 1385, 1329, 1319, 1183, 1141, 1084, 1006, 869, 753, 640, 531, 486
; UV/Vis (CH2Cl2, 0.04 mM, 25 ºC, ε = L·mol-1·cm-1):  = 281 nm (29850), 296 nm (31025), 382
(25300).
Complex 2: Yellow solid. Yield: 67%. %. 1H NMR (CDCl3, 400 MHz, RT, paramagnetic
spectrum):  = 13.77, 11.72, 9.80, 8.18, 7.14, 6.88, 6.61, 3.98, 3.23, 1.78, 1.30, -15.17; MALDI(+)
(dctb): m/z = 783.4 [M – MeOH]+ (calcd. 783.3). HRMS (MALDI+, pyrene): m/z = 783.2893 [M
– MeOH]+ (calcd. 783.3081); Anal. calcd. for C39H49N2O4Yb·4H2O: C 54.79, H 6.72, N 3.28;
found: C 54.66, H 6.67, N 3.20. IR (cm-1):  = 3529, 2945, 2905, 2868, 1629, 1610, 1535, 1409,
1385, 1329, 1184, 1141, 1084, 870, 753, 640 ; UV/Vis (CH2Cl2, 0.04 mM, 25 ºC, ε = L·mol-1·cm1

):  = 268 nm (29480), 354 nm (12860).
Complex 3: Yellow solid. Yield: 68%. 1H NMR (CDCl3, 400 MHz, RT, paramagnetic

spectrum):  = 37.97, 32.59, 22.20, 17.99, 15.91, 14.17, 13.73, 11.69, 10.93, 9.78, 8.33, 4.33, 3.75,
1.55, 1.35, -2.57, -3.66, -4.13, -5.49, -15.23, -19.90; MALDI(+) (dctb): m/z = 722.4 [M – MeOH
– salicylaldehyde]+ (calcd. 722.2), 916.5 [M – MeOH – H + 2H2O]+ (calcd. 916.3). HRMS
(MALDI+, pyrene): m/z = 722.2346 [M – MeOH – salicylaldehyde + H2O]+ (calcd. 722.2429).
Anal. calcd. for C36H38N2O2Yb·2MeOH·2H2O: C 56.78, H 6.27, N 3.48; found: C 56.35, H 6.00,
N 3.70. IR (cm1):  = 3331, 3280, 2948, 2904, 2867, 1627, 1598, 1538, 1431, 1406, 1337, 1186,
1141, 1059, 980, 871, 753, 696, 543, 438; UV/Vis (CH2Cl2, 0.04 mM, 25 ºC, ε = L·mol-1·cm-1): 
= 273 nm (19550), 375 nm (11675).
Complex 4: Yellow solid. Yield: 43%. %. 1H NMR (CDCl3, 400 MHz, RT, paramagnetic
spectrum):  = 34.66, 30.85, 21.60, 4.36, 1.12, 0.98, -2.36, -2.89; MALDI(+) (pyrene): m/z =
1117.8 (M  NEt3H)+ (calcd. 1117.8). HRMS (MALDI, pyrene): m/z = 1117.7965 [M –

8

NEt3H]+] (calcd. 1117.7879). Anal. calcd. for C46H40Br4N5O4Yb·5H2O: C 42.19, H 3.85, N 5.35;
found: C 42.55, H 3.30, N 4.97. IR (cm-1):  = 3049, 2988, 2890, 1605, 1577, 1516, 1442, 1387,
1202, 1170, 1124, 1070, 839, 750, 737, 655, 453; UV/Vis (CH2Cl2, 0.04 mM, 25 ºC, ε = L·mol1

·m-1):  = 292 nm (9575), 324 nm (13000), 375 (16125).
Synthesis of complexes 7 and 8: Complexes 7 and 8 were prepared by stirring a mixture of 5

or 6 (0.1 mmol) and dry cyclohexanol (0.1 mmol) in DCM (5 mL) for 24 hours, followed by
removal of the solvent in vacuo and drying of the resulting residue for a few hours under reduced
pressure to give a yellow powder. The disappearance of the septet at 4.82 ppm (for 5) and 4.96
ppm (for 6) and the simultaneous appearance of a broad singlet at 3.71 and 4.07 ppm, respectively,
demonstrated the formation of 7 and 8. Complex 7: 1H NMR (400 MHz, C6D6): δ = 7.70 (s, 1H,
CH=N), 7.65 (s, 1H, CH=N), 7.56 (d, 2H, aryl-H), 6.99 (d, 2H, aryl-H), 3.71 (m, 1H, C5H10-CHO), 3.47 (dd, 2H, N(CH2)2N), 2.71 (dd, 2H, N(CH2)2N), 1.68 (s, tBu), 1.38 (s, tBu) 1.59-1.41 (m,
cyclohexyl-H). Complex 8: 1H NMR (400 MHz, C6D6): δ = 7.79 (s, 2H, CH=N), 7.68 (d, 2H, arylH), 7.06 (d, 2H, aryl-H), 4.07 (br s, 1H, C5H10-CH-O), 3.86 (br s, 4H, thf), 3.56 (br s, 4H,
N(CH2)2N), 2.74 (br s, 4H, thf), 1.67 (s, 18H, CH3), 1.40 (s, 18H, CH3), 1,50-1.25 (m, cyclohexylH).
General catalytic procedure for the copolymerization of CO2 and CHO: In a typical
experiment, the catalyst (0.02 mmol), co-catalyst (0.02 mol) and CHO (2.0 mL, 19.77 mmol) were
added inside a glove box into the teflon reaction vessel (a 25 mL stainless-steel Berghof autoclave)
which had been previously dried in vacuo for 18 h at 140 °C. Three cycles of pressurization and
depressurization of the reactor (with CO2 at 10 bar) were carried out before finally stabilizing the
pressure at 20 bar and the solution left stirring at the appropriate temperature for 18h. After the
reaction was stopped, the autoclave was allowed to cool down to room temperature before it was

9

vented. The crude reaction mixture was analyzed by 1H NMR and FT-IR spectroscopy to
determine the conversion to polymer, ether linkages and cyclic carbonate.
Isolation of the polymers: The crude reaction mixture was extracted with dichloromethane and
the solution allowed evaporating. This unpurified mixture was then re-dissolved in a minimal
amount of dichloromethane and the polymer precipitated with a 1 M solution of hydrochloric acid
in methanol. Upon settling, the supernatant solution containing catalyst, co-catalyst and cyclic
carbonate was decanted and discarded. The obtained purified polymer was dried in vacuo at 60 °C
overnight and analyzed by 1H and 13C{1H} NMR, IR, MS (MALDI+) and GPC.

Scheme 1. Synthetic route for the synthesis of the Yb(salen) complexes 1–4.

10

RESULTS AND DISCUSSION
Preparation and analysis of complexes: A small number of complexes based on ytterbium (1–
4, see Scheme 1) have been prepared. The synthetic approach is based on the combination of a
mixture of an appropriate diamine, 3-R-butylsalicylaldehyde [R = tBu (for 1-3) or Br (for 4)] and
ytterbium nitrate in an approximate 1:3:1 ratio in methanol in the presence of triethylamine. It is
noteworthy that this one-pot synthesis reproducibly leads to discrete monometallic ytterbium
complexes. If the reaction is performed with pre-formed salen or salphen ligands and combined
with Yb(NO3)3 under comparable reaction conditions, only catalytically inactive polymeric
material is produced. Apparently in the latter cases the isolated products are based on multinuclear
Yb complexes incorporating bridging ligands, which is a common feature encountered in
lanthanide chemistry. Complexes 1-4 were isolated in reasonably good yields of up to 68%.
Complexes 1 and 4 were first analyzed by single crystal X-ray crystallography (see Figures 1
and 2). Pertinent bond lengths and angles are collected in the Figure´s legends, whereas crystal
data/parameters are summarized in Table 1. The substitution on the aldehyde reagent in the 3position determines the nature of the complex that is formed. In the case of complex 1, having a
bulky tBu group, a mononuclear Yb(III)-salen complex is formed in which a deprotonated
salicylaldehyde fragment (cf, a formyl-phenolate ligand) is coordinating to the metal center in a
bidentate fashion. The seven-membered coordination sphere around Yb(1) is completed by a
neutral MeOH molecule giving rise to a distorted pentagonal bipyramidal geometry. The bidentate
coordination of the formyl-phenolate may be regarded as hemi-labile which may be of use in
catalytic applications. Also, the neutral MeOH ligand could potentially be displaced by other,
appropriate initiating species (cf., copolymerization reactions, vide infra).

11

Figure 1. Molecular structure for Yb-complex 1. Hydrogen atoms are omitted for clarity.
Selected bond lengths (Å) and angles (º) with esd´s in parentheses: Yb(1)-O(1) = 2.1521(11),
Yb(1)-O(2) = 2.1394(13), Yb(1)-O(4) = 2.1804(12), Yb(1)-O(5) = 2.3314(12), Yb(1)-O(6) =
2.3871(15), Yb(1)-N(1) = 2.3999(13), Yb(1)-N(2) = 2.4218(14); O(2)-Yb(1)-O(1) = 96.83(5),
O(2)-Yb(1)-O(4) = 95.43(5), O(1)-Yb(1)-O(4) = 158.27(5), O(2)-Yb(1)-O(5) = 83.78(5), O(1)Yb(1)-O(5) = 87.10(5), O(5)-Yb(1)-O(6) = 76.54(5), N(2)-Yb(1)-O(5) = 147.97(5), N(2)-Yb(1)O(4) = 82.36(5), N(1)-Yb(1)-O(5) = 145.63(5), N(1)-Yb(1)-O(4) = 112.56(5), N(1)-Yb(1)-N(2)
= 65.30(5).

12

Figure 2. Molecular structure for Yb-complex 4. Hydrogen atoms and co-crystallized
HNEt3NO3 molecules are omitted for clarity. Selected bond lengths (Å) and angles (º) with esd´s
in parentheses: Yb(1)-O(1) = 2.202(2), Yb(1)-O(2) = 2.221(2), Yb(1)-O(3) = 2.2161(19), Yb(1)O(4) = 2.2211(19), Yb(1)-N(1) = 2.488(2), Yb(1)-N(2) = 2.502(2), Yb(1)-N(3) = 2.526(2),
Yb(1)-N(4) = 2.478(2); O(1)-Yb(1)-O(2) = 150.49(8), O(3)-Yb(1)-O(4) = 150.61(7), N(1)Yb(1)-N(3) = 136.58(7), N(2)-Yb(1)-N(4) = 135.68(8), O(1)-Yb(1)-O(3) = 91.79(7), O(2)Yb(1)-O(4) = 92.67(7), O(1)-Yb(1)-N(1) = 72.99(7), O(2)-Yb(1)-N(2) = 71.09(8), N(1)-Yb(1)N(2) = 65.32(8), N(3)-Yb(1)-N(4) = 65.10(8).

13

Table 1. Crystal data for complexes 1 and 4.
Complex 1:

Complex 4:

Empirical formula

C40H47N2O5Yb

C58H72Br4N9O10Yb

Formula weight

808.84

1547.93

Temperature

100(2) K

100(2) K

Wavelength

0.71073 Å

0.71073 Å

Crystal system

Orthorhombic

Monoclinic

Space group

P2(1)2(1)2(1)

P2(1)/n

Unit cell dimensions

a = 13.3478(4) Å

a = 16.2698(11) Å

b = 15.5479(4) Å

b = 15.1873(9) Å

c = 17.2278(5) Å

c = 25.1775(16) Å

=  =  = 90°

=  = 90°



 = 96.682(2)°

Volume

3575.29(18) Å3

6179.0(7) Å3

Z

4

4

Density (calculated)

1.503 Mg/m3

1.664 Mg/m3

Absorption coefficient

2.662 mm1

4.163 mm1

F(000)

1644

3092

Crystal size

0.15 × 0.15 × 0.15 mm3

0.40 × 0.05 × 0.05 mm3

 range for data collection

2.3337.04°

1.5730.14°

Reflections collected

56735

39318

Independent reflections

18177 [R(int) = 0.0509]

15248 [R(int) = 0.0363]

Completeness to  = 30.14°

99.8%

83.5%

Unit cell angles

14

Absorption correction

Empirical

Empirical

Refinement method

Full-matrix least-squares on F2 Full-matrix least-squares on F2

Data/restraints/parameters

18177/0/444

15248/0/748

Goodness-of-fit on F2

1.02

1.019

Final R indices [I>2(I)]

R1 = 0.0248, wR2 = 0.0606

R1 = 0.0325, wR2 = 0.0649

R indices (all data)

R1 = 0.0265, wR2 = 0.0616

R1 = 0.0511, wR2 = 0.0706

Flack parameter

x = 0.026(4)



Largest diff. peak and hole

1.816 and 0.625 e∙Å3

0.915 and 0.645 e∙Å3

In complex 4, the presence of a less bulky substituent in the aldehyde reagent (R = Br, Scheme
1) also leads to discrete formation of a mononuclear Yb complex, however in this case two
dianionic salphen units are found in the coordination sphere of the metal ion with both acting as
tetradentate ligands wrapped around the Yb center in a rather symmetrical fashion giving a square
antiprismatic geometry. Due to the anionic nature of the complex a charge-compensating HNEt3
cation co-crystallizes originating from the synthesis upon deprotonation of the salicylaldehyde
reagent. It should also be noted that two molecules of co-crystallized HNEt3NO3 are found in the
unit cell (not shown in Figure 2) with the nitrate anions coming from the Yb reagent. The structural
dependence of these Yb complexes on the substitution pattern (size of the 3-positioned group) of
the aldehyde reagent and the incorporation of one or two salen ligands could lead to different
reactivity and polymerization initiation properties. The influence of the aldehyde reagent on the
structural properties and/or aggregation of oxophilic Zn(II)21 and Mg(II)22 centered salen
complexes has also been described previously.

15

Beside the use of X-ray crystallography, mass analysis proved to be rather useful for identifying
the molecular compositions of complexes 1-4 (see Supporting Information). In all cases either
single isotope patterns for the complete structures (after loss of the neutral MeOH ligand) could
be identified and/or major peaks related to the molecular ions having the MeOH ligand
exchanged/replaced by H2O molecules. For complex 4 the intact anionic part of the molecular
structure was easily identified and only the loss of the associated cation (HNEt3+) could be
observed. Complexes 1-4 were also further analyzed by UV-Vis and IR spectroscopy, 1H
paramagnetic NMR, and exact mass/elemental analyses (see also Supporting Information). The
hemi-labile, bidentate ortho-formyl phenolate ligands in complexes 1-3 derived from the
salicylaldehyde reagent likely prevents these complexes of forming multinuclear assemblies
through bridging ligands. The phenolate moiety could potentially act as an initiator group during
the catalytic copolymerization of epoxides and carbon dioxide, with the growing polymer chain
formally displacing this anionic ligand after initiation.
Complexes 5 and 6, based on scandium and yttrium, respectively, were synthesized following
the extended silylamide route developed by Anwander and coworkers,16 while complexes 7 and 8
were prepared conveniently from precursors 5 and 6, respectively, by treatment with dry
cyclohexanol (Scheme 2). The formation of complexes 7 and 8 was inferred from their 1H NMR
spectra; 5 and 6 both show a septuplet at 4.82 and 4.96 ppm, respectively, which disappeared upon
reaction with cyclohexanol. Furthermore, the appearance of a broad singlet (CHOLn unit) at
3.71 ppm and 4.17, respectively, is indicative of coordination of the cyclohexanolate ligand to the
metal center.

16

Scheme 2. Synthesis of complexes 7 and 8 from known precursors 5 and 6 by ligand exchange.

Figure 3. Benchmark complexes 9-12 for comparison with Yb complexes 1-4 in the catalytic
copolymerization of CHO and CO2.

Finally, the dinuclear yttrium complex 9 (Figure 3) was prepared according to a procedure
reported by Evans and coworkers,17 whereas the metallosalen complexes 10–12 (M = Co or Cr;
used as benchmark catalysts in the copolymerization studies, vide infra) were prepared according
to procedures published by Jacobsen and coworkers.18

17

Copolymerization studies with Yb-complex (1) : For our initial experiments, cyclohexene
oxide (CHO) was chosen as a benchmark substrate, and Yb-complex 1 was first tested as a
potential catalyst for poly(cyclohexene)carbonate (PCHC) formation using an initial CO2 pressure
of 20 bar, a variety of co-catalysts and DCM as solvent (Table 2, entries 14). However, in these
experiments only the formation of the corresponding cyclic carbonate product (CHC) was
observed with generally low to moderate CHO conversion levels. The cis-configuration of the
cyclic carbonate product CHC indicates the occurrence of a double inversion pathway (two
separate SN2 type substitution reactions at the same carbon center),23 and thus the CHC product
does not likely originate from a typical backbiting process in a pre-formed polycarbonate leading
to trans-CHC (cf., entry 9). When performing the copolymerization reaction under solvent-free
conditions using Yb complex 1 as catalyst and lowering both the catalyst and nucleophile
concentration ten-fold (entries 610), the carbonate selectivity switched dramatically and virtually
only PCHC formation was observed with generally low polydispersity indices. It should be noted
that in the absence of a co-catalytic additive (entry 5) no conversion could be observed suggesting
that the complex 1 itself cannot initiate the copolymerization reaction under these conditions.24
The use of five different co-catalysts was probed (entries 610) at 70ºC, and the PCHC properties
were compared. Whereas the use of DMAP, PPNBr and PPNI give copolymers of good quality
(98% carbonate linkages), the molecular weight (Mn) values only reach about 7.2 Kg/mol. As
may be expected, the poorer leaving group ability of chloride allows for higher molecular weight
polymers to be formed (entries 9 and 10), with an appreciable 11.4 Kg/mol when NBu4Cl is used.
Although under these conditions the use of NBu4Cl gives higher molecular weight material, the
presence of PPNCl allows for significantly higher conversion levels (22 versus 50%).

18

Table 2. Screening phase of the copolymerization between CHO and CO2 using Yb-complex 1 as
catalyst and various nucleophilic additives (co-cat). N.a. refers to not applicable and Nu is the
nucleophilic additive used.a

Entry

T

Conv. TONd

TOFe

%CHCc %PCHCc,f

Mn g,h

1

Nu

[mol%]b

[mol%]

1

1.0

DMAP (1.0)

80

2i

2

<1

>99 (cis)

0

n.a.

2

1.0

PPNBr (1.0)

80

14i

14

<1

>99 (cis)

0

n.a.

3

1.0

PPNI (1.0)

100

48i

47

3

>99 (cis)

0

n.a.

4

1.0

PPNI (1.0)

70

46i

45

3

>99 (cis)

0

n.a.

5

0.1



70

0

0

0

trace

0

n.a.

6

0.1

DMAP (0.1)

70

36

356

20

1 (trans)

98 (98)

7

0.1

PPNBr (0.1)

70

45

445

25

2 (trans)

98 (99)

8

0.1

PPNI (0.1)

70

47

465

26

1 (trans)

99 (99)

9

0.1

PPNCl (0.1)

70

50

494

27

2 (trans)

98 (99)

10

0.1

NBu4Cl (0.1)

70

22

217

12

0

99 (99)

[°C] [%]c

(Mw/Mn)g

3.70
(1.25)
4.80
(1.42)
7.20
(1.53)
9.10
(1.66)
11.4
(1.35)

a

Reactions were carried out in a 25 mL autoclave in neat cyclohexene oxide (CHO; 20 mmol), t =
18 h, p(CO2)º = 20 bar, DCM as solvent (5 mL; only entries 1-4). b Mol% of metal. c From the
normalized integrals in the 1H NMR spectra (CDCl3) of the methine resonances, including PCHC

19

(: 4.65 ppm), CHC [: 3.904.00 ppm (trans) and 4.63 ppm (cis)], and the ether signal in impure
PCHC (: 3.45 ppm). d TON = (mol epoxide converted)/(mol catalyst). e TOF = TON/h. f In
brackets the % of carbonate linkages; remaining % are ether linkages. g Determined by GPC
analysis. h In Kg·mol-1. i Numbers refer to isolated yield.

Comparative catalysis studies with Yb-complex (1) and Ln-salen complexes 59: Next, we
investigated the catalytic properties of known lanthanide salen complexes 59 and compared the
activity and selectivity features of these reactions with those obtained with Yb-complex 1 in the
copolymerization of CHO and CO2 (Table 3). As observed for Yb complex 1, for lanthanide
complexes 59 the use of a chloride based initiator (nucleophile) also proved to be vital to access
PCHC specifically at temperatures lower than 80ºC. When scandium complex 5 was probed, both
the use of PPNCl and PPNOAc initiators gave PCHC polymers of up to 4.5 Kg/mol with narrow
polydispersities (1.41.6; entries 5 and 6). Upon exchange of the silylamide ligand in 5 for a
cyclohexanolate (i.e., Sc complex 6) and combining with PPNX (X = Cl, N3 or OAc; entries 710), the copolymerization reaction gave access to PCHC of higher molecular weight but at the
expense of the chemoselectivity which was lower than observed for Yb complex 1 (cf., entries 1
and 2). For the yttrium complex 7 the use of NBu4Cl fully reversed the chemoselectivity in favor
of CHC (cf., entry 12). The presence of PPNCl (entry 11), compared with the results obtained with
Yb complex 1 (entry 1), gave a polymer of an appreciable molecular weight (10.5 Kg/mol) but
with poorer control (PDI = 6.21). The increase in initial CO2 pressure to 35 bar (entry 13) did not
improve on this result showing similar polymer properties. When the silylamide ligand in 6 was
replaced by a cyclohexanolate one (complex 8), even poorer control over the PHCH quality was
noted. Finally, bis-yttrium complex 9 was also subjected under the conditions reported for Yb
complex 1 (entries 1 and 2) and again a PCHC polymer with a reasonable molecular weight could

20

be produced when PPNCl was present as initiator though the isolated polymer was poorly defined
(PDI = 5.77). As had been

21

Table 3. Comparative studies for the copolymerization of CHO and CO2 using Yb-complex 1 and
lanthanide based catalysts 59. N.a. refers to not applicable and Nu is the nucleophilic additive.a

Entry

T

Conv. TONd TOFe

%CHCc %PCHCc,f

Cat.

Nu

[mol%]b

[mol%]

1

1 (0.1)

PPNCl (0.1)

70

50

494

27

2 (trans)

98 (99)

2

1 (0.1)

NBu4Cl (0.1)

70

22

217

12

0

99 (99)

[°C] [%]c

Mn g,h
(Mw/Mn)g
9.10
(1.66)
11.4
(1.35)

99
3

5 (0.1)

PPNBr (0.1)

100

75

741

41

(87%
cis)

0

n.a.

n.a.

99
4

5 (0.1)

PPNCl (0.1)

100

70

692

38

(88%
cis)

0

5

5 (0.1)

PPNCl (0.1)

70

64

640

36

5 (trans)

95 (99)

6

5 (0.1)

PPNOAc (0.1)

70

31

306

17

2 (trans)

98 (99)

7

6 (0.1)

PPNCl (0.1)

70

72

711

39

12
(trans)

85 (99)

8

6 (0.1)

PPNCl (0.05)

70

53

523

29

11
(trans)

89 (99)

9

6 (0.1)

PPNN3 (0.1)

70

65

642

36

8 (trans)

92 (99)

4.00
(1.62)
4.50
(1.46)
5.40
(1.91)
7.00
(1.41)
4.40

22

Entry

Cat.

Nu

[mol%]b

[mol%]

T

Conv. TONd TOFe

%CHCc %PCHCc,f

[°C] [%]c

Mn g,h
(Mw/Mn)g
(1.58)

10

6 (0.1)

PPNOAc (0.1)

70

61

602

34

8 (trans)

92 (99)

11

7 (0.1)

PPNCl (0.1)

70

27

266

15

1 (trans)

99 (99)

7.80
(1.57)
10.5
(6.21)

>99
12

7 (0.1)

NBu4Cl (0.1)

70

23

227

13

(98%
cis)

0

13

7 (0.1)i

PPNCl (0.1)

70

25

247

14

3 (trans)

97 (99)

14

8 (0.1)

PPNCl (0.05)

70

30

297

16

trace

99 (99)

15

9 (0.1)

PPNCl (0.1)

70

9

89

5

4 (trans)

96 (85)

n.a.
10.0
(7.83)
10.4
(15.8)
9.00
(5.77)

>99
16

9 (0.1)

NBu4Cl (0.1)

70

12

119

7

(97%
cis)

0

n.a.

a

Reactions were carried out in a 25 mL autoclave in neat cyclohexene oxide (CHO; 20 mmol), t =
18 h, p(CO2)º = 20 bar, neat. b Mol% of metal. c From the normalized integrals in the 1H NMR
spectra (CDCl3) of the methine resonances, including PCHC (: 4.65 ppm), CHC [: 3.904.00
ppm (trans) and 4.63 ppm (cis)], and the ether signal in impure PCHC (: 3.45 ppm). d TON =
(mol epoxide converted)/(mol catalyst). e TOF = TON/h. f In brackets the % of carbonate linkages;
remaining % are ether linkages. g Determined by GPC analysis. . h In Kg·mol-1. i Initial CO2
pressure was 35 bar.

noted for yttrium complex 7, the combination of NBu4Cl with complex 9 led only to the formation
of the cyclic carbonate product (entry 16). Comparatively, Yb complex 1 shows somewhat lower
reactivity in relation to 59 but with much better copolymerization control combining high

23

molecular weight, narrow polydispersities and very little formation of the CHC by-product.
Comparison with other rare-earth metal based catalysts in the literature shows that Yb-complex 1
shows a combination of appreciable activity and good polymer selectivity in the coupling of CO2
and CHO. For instance, Hou et. al reported on lanthanide complexes containing cyclopentadienyl
ligands having high activity towards the synthesis of high molecular weight polycarbonates (13.7–
25.6 Kg/mol) without using a co-catalytic additive. However, these catalysts typically exhibited
lower (average) TOF values (between 3.7–9.4 h-1) than the ones reported here for complex 1, and
also gave rise to polycarbonates with higher polydispersities between 1.65–6.15.11b

24

Count

m/z

Figure 4. MALDI-ToF spectrum of the PCHC product from Table 3 (entry 5) using scandium
complex 5 as catalyst and PPNCl as initiator. Sampling conditions: dithranol matrix, NaOAc as
additive in CH2Cl2/THF/MeOH.

In order to assess the properties of the produced PCHC in more detail, MALDI-TOF mass
spectrometric analysis was performed on some representative samples (Figures 4 and 5, see also
the Supporting Information for other examples)25 which allowed for end-group analysis. In several
cases where lower molecular weight polymers were formed we found evidence for chain-transfer

25

effects. Typically, the MALDI spectra recorded for these samples included isotopic distributions
of polymers with different end-groups such as Cl, OH and cyclohexene (see inserts Figures 4 and
5). The latter two are indicative for 1,2-cyclohexanediol (CHD) acting as a chain transfer agent
(CTA) of the growing polycarbonate as previously reported by Williams et al.25a

n = 16

n = 15

Count

n = 17

m/z

Figure 5. Expansion of the MALDI-ToF spectrum reported in Figure 4 (region m/z 2200-3300).
Some of the repeating units and the corresponding assigned peaks have been added.

26

As a result, two propagating species are present with one growing from Cl-induced polymer
initiation whereas the other relates to CHD initiation (after chain transfer) that can propagate from
both ends. This typically leads to the observation of bimodal distributions in the GPC analyses and
this was indeed confirmed in most of the cases studied (Supporting Information for more details).
Thus, it seems that traces of water or the presence of (traces of) CHD may be held responsible for
the observation of some limitations of the polymer properties reported in Tables 2 and 3, and
particularly the PCHC molecular weight. Most PCHC polymers produced were in fact found to
have at least 98 % carbonate linkage content. The tacticity of the polymers was also determined
using 13C NMR and showed the presence of syndiotactic (153.1 – 153.3 ppm) as well as isotactic
(153.7 ppm) diads,26 with the polymers thus being atactic (Supporting Information).
In order to assess whether further improvement of the PCHC properties could be realized, the
copolymerization of CHO and CO2 was probed with the other Yb(salen) complexes 24 and the
results compared with those obtained with Yb complex 1 (Table 4, entries 1 and 2). Structurally
related Yb(salen) complexes 2 and 3 incorporating a cyclohexyl and a 1,2-diphenyl-ethylene
bridging unit, respectively, showed inferior activities and polymer properties (lower Mn´s, and for
3 a much higher PDI; entries 3-5). This seems to indicate that the more rigid nature of the phenyl
bridging group in Yb complex 1 positively influences the control over the copolymerization
process. For Yb-bis-salen complex 4 we found that relative high molecular weight material may
be accessed (about 15 Kg/mol) using either PPNBr or PPNI as initiator, however under these
conditions the copolymerization control was extremely poor (entries 8 and 9). Slightly improved
control over the polydispersity of the produced PCHC was noted when PPNCl was used as initiator
(entry 6) but in this case the molecular weight was significantly reduced (3919). The much higher

27

PDI values reported for Yb-bis-salen complex 4 may be a result of various propagating species
present after initiation by the nucleophilic additive, and complexes based on mono- or bis-Ln salen
derivatives with more than one salen unit present do not seem to provide a good starting point for
the development of highly active and selective copolymerization catalysts (cf., complex 9 Table
3). In complexes 1-3 the presence of a bidentate phenolate ligand derived from the salicylaldehyde
reagent leads to more defined catalyst species after initiation. In the case of 4, lacking such a ligand,
multiple active catalytic species may be responsible for the lower quality of polycarbonate formed
under comparatively similar reaction conditions reported for catalysts based on 1-3.
Finally, to benchmark the catalysis features of our best-performing catalyst 1, we tested known
catalysts 10–12 reported by the groups of Darensbourg and Coates (Table 5).27,28 Both the activity
and selectivity profiles for Yb salen complex 1 were evaluated against those obtained for Cr(III)
and Co(III) salen complexes 1012 under similar experimental conditions and using the same
reactor set up. Complexes 1012 are well-known copolymerization catalysts for the CHO/CO2
coupling reaction typically giving rise to PCHC having molecular weights of up to 30 Kg/mol and
initial high reactivity with TOFs in the range 250450 h-1 using polymerization initiators of type
PPNX (X = Cl, N3, OAc).27,28
The activities and selectivity profiles of the Cr and Co salen catalysts 1012 (Table 5, entries 811) were determined under comparable reaction conditions (cf., entries 1 and 2) and our reactor
system generally gave PCHC products of somewhat higher molecular weight (Mn´s between
13.213.7 Kg/mol, PDI´s 1.31.7) than the polymers obtained by using Yb salen complex 1
without too much difference between the three systems. These results show that under the applied
copolymerization conditions (i.e., the best ones for Yb complex 1) there is no significant influence
of the metal ion (Cr versus Co) and axial ligand (Cl versus OAc versus N 3) bound to the metal

28

center. The average TOF values reported for the complexes 1012 were also higher (3644 h-1)
than noted for 1 (entry 1; 27 h-1). Better reactivity for 1 could be realized by

29

Table 4. Comparative studies for the copolymerization of CHO and CO2 using Yb-complexes 14. N.a. refers to not applicable and Nu stands for nucleophilic additive.a

Entry

T

Conv. TONd

TOFe

%CHCc %PCHCc,f

Cat.

Nu

[mol%]b

[mol%]

1

1 (0.1)

PPNCl (0.1)

70

50

494

27

2 (trans)

98 (99)

2

1 (0.1)

NBu4Cl (0.1)

70

22

217

12

0

99 (99)

3

2 (0.1)

PPNCl (0.1)

70

34

340

14

99 (cis)

traces

4

2 (0.1)

PPNCl (0.05)

70

22

217

12

1 (trans)

99 (99)

5

3 (0.1)

PPNCl (0.1)

70

34

336

19

3 (trans)

97 (99)

6

4 (0.1)

PPNCl (0.1)

70

39

385

21

4 (trans)

96 (99)

7

4 (0.1)

NBu4Cl (0.1)

70

32

316

18

5 (trans)

95 (99)

8

4 (0.1)

PPNBr (0.1)

70

35

346

19

2 (trans)

98 (98)

9

4 (0.1)

PPNI (0.1)

70

55

544

30

5 (trans)

95 (99)

[°C] [%]c

Mn g,h
(Mw/Mn)g
9.10
(1.66)
11.4
(1.35)
n.a.
4.60
(1.26)
6.10
(5.12)
4.00
(10.1)
9.00
(19.5)
15.1
(12.1)
15.3
(14.0)

a

Reactions were carried out in a 25 mL autoclave in neat cyclohexene oxide (CHO; 20 mmol), t =
18 h, p(CO2)º = 20 bar, neat. b Mol% of metal. c From the normalized integrals in the 1H NMR

30

spectra (CDCl3) of the methine resonances, including PCHC (: 4.65 ppm), CHC [: 3.904.00
ppm (trans) and 4.63 ppm (cis)], and the ether signal in impure PCHC (: 3.45 ppm). d TON =
(mol epoxide converted)/(mol catalyst). e TOF = TON/h. f In brackets the % of carbonate linkages;
remaining % ether linkages. g Determined by GPC analysis. . h In Kg·mol-1.

31

Table 5. Comparative studies for the copolymerization of CHO and CO2 using Yb-complex 1 and
Co/Cr-salen complexes 10-12. N.a. refers to not applicable and Nu stands for nucleophilic
additive.a

Entr
y

TONd TOFe %CHCc %PCHCc,

Cat.

Nu

T

Conv.

[mol%]b

[mol%]

[°C]

[%]c

1

1 (0.1)

PPNCl (0.1)

70

50

494

27

2 (trans)

98 (99)

2

1 (0.1)

NBu4Cl (0.1)

70

22

217

12

0

99 (99)

3

1 (0.01)

PPNCl (0.01)

70

10

1000

55

99 (cis)

traces

4

1 (0.05)

NBu4Cl
(0.05)

70

40

800

44

2 (trans)

98 (99)

5

1 (0.1)

NBu4Cl
(0.05)

70

13

128

7

traces

99 (99)

6

1 (0.1)

NBu4Cl
(0.05)

90

57

563

31

traces

99 (99)

7

1 (0.01)

PPNCl (0.01)

90

15

1500

83

4 (trans)

96 (99)

8

10 (0.1)

PPNCl (0.1)

70

75

741

41

7 (trans)

93 (99)

9

11 (0.1)

PPNCl (0.1)

70

81

801

44

1 (trans)

98 (99)

f

Mn g,h
(Mw/Mn)
g

9.10
(1.66)
11.4
(1.35)
n.a.
3.00
(1.78)
3.40
(1.26)
10.2
(1.54)
4.60
(5.37)
13.2
(1.69)
13.6

32

Entr
y

Cat.

Nu

T

Conv.

[mol%]b

[mol%]

[°C]

TONd TOFe %CHCc %PCHCc,

[%]c

f

Mn g,h
(Mw/Mn)
g

(1.33)

10

12 (0.1)

PPNCl (0.1)

70

74

731

41

traces

99 (99)

11

12 (0.1)

PPNN3 (0.1)

70

66

652

36

1 (trans)

99 (99)

12

1 (0.1)

PPNN3 (0.1)

70

55

544

30

1 (trans)

99 (99)

13

1 (0.1)

PPNOAc
(0.1)

70

68

672

37

3 (trans)

97 (99)

13.7
(1.87)
13.1
(1.59)
6.50
(1.35)
7.50
(1.57)

a

Reactions were carried out in a 25 mL autoclave in neat cyclohexene oxide (CHO; 20 mmol), t =
18 h, p(CO2)º = 20 bar, neat. b Mol% of metal. c From the normalized integrals in the 1H NMR
spectra (CDCl3) of the methine resonances, including PCHC (: 4.65 ppm), CHC [: 3.904.00
ppm (trans) and 4.63 ppm (cis)], and the ether signal in impure PCHC (: 3.45 ppm). d TON =
(mol epoxide converted)/(mol catalyst). e TOF = TON/h. f In brackets the % of carbonate linkages;
remaining % are ether linkages. g Determined by GPC analysis. h In Kg·mol-1.

variation of the co-catalytic additive and the type of anion (entries 4, 6, 7, 12 and 13) but in each
case at the expense of the molecular weight of the isolated PCHC. The best compromise between
reactivity and polymer weight is the use of Yb complex 1 with NBu4Cl as initiator at 90ºC giving
a TOF value of 31 h-1 and an Mn of 10.2 Kg/mol (PDI = 1.54). The benchmark studies carried out
for 1 and comparing its reactivity and selectivity features with those of 1012 in the
copolymerization of CHO and CO2 demonstrate clearly that the Yb salen complex 1 has good
reactivity and allows for the highly selective formation of PCHC of excellent quality unlike the
other lanthanide based systems reported in this work.

33

34

CONCLUSION
We here describe the synthesis of discrete mononuclear Yb complexes based on salen ligands and
their full characterization. These complexes have been used as (pre)catalysts for the
copolymerization reaction of CHO and CO2 to afford polycarbonates,29 and their reactivity and
selectivity profiles, in particular for complex 1, have been benchmarked using the measured
profiles of a series of known lanthanide salen complexes, and the well-known Cr(III)- and
Co(III)salen catalysts. Whereas the reactivity and selectivity features of 1 are clearly much more
attractive than observed for the other lanthanide complexes 5-9, lower reactivity (TOFs) and
somewhat lower Mn values were achieved when evaluating against the Co and Cr(salen) based
catalyst systems using similar reaction conditions in the same reactor set up. Nonetheless, Yb
complex 1 reports high quality PCHC product of appreciable molecular weight (1011 Kg/mol)
under mild conditions (70ºC, 0.1 mol% cat/PPNCl, p(CO2) = 20 bar) and further optimization of
the reactivity of such Yb based catalysts derived from other types of aminophenolate ligands
should be feasible, including the design of binary and bifunctional type catalysts.

ASSOCIATED CONTENT
Supporting Information. Copies of analytical data for complexes 1–4, spectra of isolated
PCHC polymer samples, representative MALDI-ToF and corresponding GPC traces, and cif files
for the X-ray molecular structures reported for complexes 1 and 4. This material is available free
of charge via the Internet at http://pubs.acs.org.

35

AUTHOR INFORMATION
Corresponding Author
akleij@iciq.es
Notes
The authors declare no competing financial interest.

ACKNOWLEDGMENTS
We thank the Spanish MINECO (projects CTQ2011-27385 and CTQ-2014–60419-R) and
support through Severo Ochoa Excellence Accreditation 2014-2018 (SEV-2013-0319), ICIQ,
ICREA, and Sabic Ltd. for generous support to the results described herein. CM thanks the Marie
Curie COFUND action from the European Commission for co-financing a postdoctoral fellowship.
Dr. Noemí Cabello, Sofía Arnal, and Vanessa Martínez are acknowledged for the MALDI-ToF
analyses, Dr. L. Jasinska for GPC measurements and Prof. Sandro Gambarotta, Dr. Rob
Duchateau, Dr. Jason Morton, Dirk Beetstra, Dr. Atieh Abu Raqabah for valuable discussions
concerning the copolymerization results.

36

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Supporting Information for details.

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