Chromium(iii) amine-bis(phenolate) complexes as catalysts for copolymerization of cyclohexene oxide and CO2

Article abstract of DOI:10.1039/c3cy01002c

Amine-bis(phenolate) chromium(iii) complexes catalyze the copolymerization of cyclohexene oxide with carbon dioxide. These catalysts incorporate tetra- or tridentate amine-bis(phenolate) ligands and when combined with dimethylaminopyridine (DMAP) or bis(triphenylphosphoranylidene)ammonium chloride or azide, (PPNCl or PPNN₃), yield low molecular weight polymers with moderately narrow polydispersities. The tetradentate ligand containing catalysts show higher conversions and yields of polycyclohexene carbonate, but with a wide variety of polymer end groups as shown by MALDI-TOF mass spectrometry. This journal is the Partner Organisations 2014.

Full text of DOI:10.1039/c3cy01002c

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Chromium(III) amine-bis(phenolate) complexes as
catalysts for copolymerization of cyclohexene
oxide and CO2†
Cite this: Catal. Sci. Technol., 2014,
4, 1547
a
Hua Chen,a Louise N. Dawe‡ab and Christopher M. Kozak
*
Amine-bis(phenolate) chromium(III) complexes catalyze the copolymerization of cyclohexene oxide with
carbon dioxide. These catalysts incorporate tetra- or tridentate amine-bis(phenolate) ligands and when
combined with dimethylaminopyridine (DMAP) or bis(triphenylphosphoranylidene)ammonium chloride or
azide, (PPNCl or PPNN3), yield low molecular weight polymers with moderately narrow polydispersities.
The tetradentate ligand containing catalysts show higher conversions and yields of polycyclohexene
carbonate, but with a wide variety of polymer end groups as shown by MALDI-TOF mass spectrometry.
Received 3rd December 2013,
Accepted 24th December 2013
DOI: 10.1039/c3cy01002c
www.rsc.org/catalysis
the salen3–5 and, more recently, the salan ligands, which have
been primarily used with Cr23,25–30,33,36,38,66 and Co.39,44,45,52,53,67
Introduction
The copolymerization of epoxides and carbon dioxide to yield
polycarbonates has become an important process that gener-
ates potentially valuable materials from renewable starting
materials.1–6 Carbon dioxide is an appealing C1 feedstock
because it is widely available, inexpensive, and nontoxic.7
Many homogeneous catalysts containing metals such as Mg,8
Al,9–13 Zn,14–22 Cr,23–38 Co6,39–58 and Fe59 have been examined
for use in the copolymerization of carbon dioxide (CO2) and
epoxides, particularly cyclohexene oxide (CHO).
Homogeneous catalysts applied to CHO and CO2 copoly-
merization have involved a variety of ligand classes including
Cr24,60,61 and Al13 porphyrins. Darensbourg and Holtcamp
introduced the use of Zn phenoxides for epoxide/CO2 copoly-
merization,62–64 whereas Coates and co-workers explored the
use of β-diketiminate (BDI) ligand systems with Zn at lower
pressures and temperatures than had previously been
reported.14 Nozaki and co-workers reported tetravalent group
4 (Ti and Zr) and 14 (Ge and Sn) metals supported by planar
trianionic bis(phenolato) ligands have shown activity towards
epoxide and CO2 copolymerization.65 By far the most widely
studied ligands for epoxide/CO2 copolymerization have been
These catalysts typically require nucleophilic co-catalysts
such as chlorides, bromides or azides paired with bulky cat-
ions such as PPN (PPN = bis(triphenylphosphoranylidene)-
ammonium) or tetrabutylammonium, or neutral bases such
as dimethylaminopyridine (DMAP) or N-methylimidazole
(N-MeIm).
Whereas compounds of the salen (and the related salan)
ligands represent the most investigated homogeneous sys-
tems for CO2/epoxide copolymerization, we recently reported
the use of the related tetradentate amine-bis(phenolato)
ligand class.68,69 Synthesis of these ligands occurs by a modified
Mannich condensation reaction, and changing the substitu-
ents on the phenolate groups or pendant arms alters the
steric and electronic properties.70,71 Work in our group has
involved investigating the catalytic properties of various
mid-to-late transition metal complexes of these ligands.
Metals such as Cr,72 Co73,74 and Fe,75–78 have shown diverse
structural, spectroscopic and electronic properties, whereas
Co(II) and Co(III) complexes of these ligands were shown to
couple CO2 with propylene oxide under neat conditions
to give propylene carbonate.79
Because Cr(III) catalysts have shown excellent activity
towards CO2/epoxide copolymerization, we began a program
to study the potential of our Cr(III) amine-bis(phenolato)
complexes to catalyze this reaction. In previous studies, bulky
t-butyl substituents on the phenolate groups were employed,
and a pyridyl pendant arm comprised the second neutral
donor site. This metal–ligand combination offers new avenues
for development of potentially highly active catalysts because
of the modifiable nature of the donor sites, their steric and
electronic properties, and their geometry, which differs from
that exhibited by the salen and salan-based systems. In this
a Department of Chemistry, Memorial University of Newfoundland, St. John's,
Newfoundland, A1B 3X7, Canada. E-mail: ckozak@mun.ca; Tel: +1 709 864 8082
b C-CART X-ray Diffraction Laboratory, Department of Chemistry, Memorial
University of Newfoundland, St. John's, Newfoundland, A1B 3X7, Canada
† Electronic supplementary information (ESI) available: MALDI-TOF MS and
UV-vis spectra of 1 and 2, representative 1H and 13C NMR and MALDI-TOF
mass spectra of PCHC, and crystallographic information file (CIF) for
(1)2·LiCl·THF. CCDC 974612. For ESI and crystallographic data in CIF or other
electronic format see DOI: 10.1039/c3cy01002c
‡ Current address: Department of Chemistry, Wilfrid Laurier University,
75 University Ave. W. Waterloo, Ontario, N2L 3C5, Canada.
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study, the influence of the pendant donor is evaluated, first by
replacing the pyridyl-containing arm with a non-coordinating
benzyl group. Two of these tridentate ligands were assessed
with one possessing t-butyl groups in the 2 and 4 position
relative to the phenolic OH group. The other tridentate ligand
employs the mildly larger t-amyl (t-pentyl) groups in these
positions. Also, a tetradentate ligand having a tetrahydro-
furanyl group in place of the pyridyl donor was examined
for its copolymerization activity. Herein, we report the use
of these three Cr(III) amine-bis(phenolate) complexes in the
presence of ionic and neutral initiators as catalysts for the
co-polymerization of cyclohexene oxide with CO2.
is of limited use.82,83 The MALDI-TOF mass spectrum of
complex 1 shows peaks at m/z 628.28 corresponding to the
[CrCl[L1]]+˙ ion (see Fig. S1–S3†). Ligand fragment ion peaks
are shown at m/z 538.15 and the fragment ion corresponding
to the loss of the chloride, [Cr[L1]]+ occurs at m/z 593.33. The
isotopic distribution pattern of the experimental [CrCl[L1]]+˙
is in agreement with the theory for 1. In addition to peaks
corresponding to monomeric complexes, higher masses were
observed at m/z 1221.6 suggesting the presence of bimetallic
fragments corresponding to [{[Cr[L1]}2Cl]+˙ and others. Crys-
tallographic evidence supporting the existence of bimetallic
species will be discussed below. Similarly, compound 2 shows
the loss of the chloride complex from the parent ion at
m/z 649.31 in the mass spectrum and an intense peak at
m/z 684.27 corresponding to [CrCl[L2]]+˙ (Fig. S4–S6†).
Electronic absorption spectra of complexes 1 and 2 in
dichloromethane show multiple bands in the UV and visible
regions. The absorption maxima observed in the UV region
(below 300 nm) are caused by π → π* transitions involving
the phenolate units. Bands are also observed in the region
between 300 and 400 nm, which are assigned to charge trans-
fer transitions from the pπ orbital (HOMO) of the phenolate
oxygen to the empty dx2−y2/dz2 and the half-filled dπ, orbitals
of Cr(III). Weaker bands arising from d–d transitions were
observed in the visible region at 560 and 720 nm.
Results and discussion
Synthesis and characterization of Cr complexes
The amine-bis(phenol) compounds benzylamino-N,N-bis(2-
methylene-4,6-di-tert-butylphenol) (H2[L1]), benzylamino-N,
N-bis(2-methylene-4,6-di-tert-amylphenol)
(H2[L2])
and
2-tetrahydrofurfurylamino-N,N-bis(2-methylene-4,6-tert-butylphenol)
(H2[L3]), were prepared according to previously reported pro-
cedures.71,80,81 Cr(III) complexes 1 and 2 were synthesized
via reaction of the protonated ligands H2[L1] and H2[L2],
respectively, at −78 °C in THF with n-butyllithium followed
by reaction of the resulting dilithium salt with CrCl3(THF)3
in THF at −78 °C to produce green solids in 80% yield
(Scheme 1). The complexes were characterized using MALDI-TOF
mass spectrometry and UV-vis spectroscopy (see ESI†).
Elemental analysis was unable to unequivocally identify
whether 1 and 2 contained 1 or fewer THF molecules per
CrCl[L] formula unit. Given the preference of Cr(III) com-
plexes of these ligands to adopt six-coordinate geometries,
some degree of THF coordination is expected. The synthesis
and characterization (including single crystal diffraction) of
complex 3 was previously reported.72
Dark green single crystals of 1 suitable for X-ray diffrac-
tion analysis were isolated by slow evaporation of a toluene
solution under a N2 atmosphere in a glove box. The resulting
structure is shown in Fig. 1 and selected bond distances
and angles are listed in Table 1. In the solid state, 1 is an
unsymmetric, dichromium complex with both transition
metal atoms exhibiting distorted face-sharing octahedral
MALDI-TOF mass spectrometry is useful for the characteri-
zation of coordination and organometallic complexes, partic-
ularly for paramagnetic compounds where NMR spectroscopy
Fig. 1 Molecular structure (ORTEP) and partially labelled numbering
scheme of (1)2·LiCl·THF. Ellipsoids are shown at 50% probability.
Hydrogen atoms and methyl groups on phenolate t-butyl groups are
omitted for clarity. Symmetry operations used to generate equivalent
atoms (*): −x + 2, −x + y + 1, −z + 1/3.
Scheme 1 Synthesis of 1–3.
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Table 1 Interatomic distances (Å) and angles (°) for (1)2·LiCl·THF
contain a six-coordinate CrCl(THF)[L3] formulation, bearing
the tetradentate ligand [L3].72
Bond lengths (Å)
Cr(1)–O(2)
Cr(1)–O(1)
Cr(1)–N(1)
Li(1)–Cl(1)
Li(1)–O(3)
1.870(5)
1.921(6)
2.108(7)
2.06(7)
Cr(1)–Cl(1)
Cr(1)–Cl(2)
Cr(1)⋯Cr(1)*
Li(1)–O(1)
2.425(3)
2.386(2)
3.1615(13)
1.70(5)
Copolymerization of CO2 and cyclohexene oxide
The copolymerization of CHO and CO2 was investigated using
complexes 1–3. Neutral or ionic co-catalysts such as (4-dimethyl-
amino)pyridine(DMAP), bis(triphenylphosphoranylidene)ammonium
1.44(7)
Cr(1)⋯Li(1)
2.40(8)
−
chloride or azide, (PPNX where X = Cl− or N3 ) were employed
Bond angles (°)
in this study and were shown to be important for the forma-
tion of the desired polymer. For each of 1, 2 or 3, strictly
poly(cyclohexene carbonate) (PCHC) was produced with no evi-
dence of cyclohexene carbonate (CHC) formation (Scheme 2).
Initial reactions were performed by combining the
tridentate ligand-containing compounds 1 and 2 with DMAP,
PPNCl or PPNN3 co-catalysts in neat epoxide and placing the
mixture under elevated pressures of CO2 and heating to
60 °C (Table 2). The combination of 1 and DMAP at 45 bar
CO2 and 60 °C showed only 33.8% conversion to PCHC by
NMR (entry 1). The yield of polymer obtained by precipitation
from acidified methanol was only 12.7%, which suggests that
the oligomers obtained are of low molecular weight and
highly soluble in methanol. Only the precipitated products
obtained were characterized by MALDI-TOF MS and gel per-
meation chromatography (GPC). Conducting the reaction at
room temperature shows no conversion of CHO (entry 2).
The alternating copolymerization of CHO with CO2 to form
PCHC typically requires higher temperatures,1,3,5,68 and such
is the case for catalyst 1 (and 3, see below). Using PPNCl or
PPNN3 as cocatalyst shows improved conversion and yield of
PCHC (entries 3 and 4). Complex 2 combined with DMAP
proved inferior to complex 1 (entry 5), likely because of the
increased steric hindrance caused by the t-amyl (t-pentyl)
groups. For both catalysts 1 and 2, only low molecular weight
polymers were obtained ranging between 1.1 to 3.8 kg mol−1
O(2)–Cr(1)–N(1)
O(1)–Cr(1)–N(1)
O(1)–Cr(1)–Cl(2)
N(1)–Cr(1)–Cl(2)
O(2)–Cr(1)–Cl(1)
O(2)–Cr(1)–O(1)
O(1)–Cr(1)–Cl(1)
N(1)–Cr(1)–Cl(1)
91.3(3)
92.6(3)
164.4(2)
100.5(2)
96.59(19)
95.7(3)
83.6(2)
Cl(2)–Cr(1)–Cl(1)
O(2)–Cr(1)–Cl(1)
O(1)–Cr(1)–Cl(1)
N(1)–Cr(1)–Cl(1)
Cl(2)–Cr(1)–Cl(1)
Cl(1)–Cr(1)–Cl(1)
Cr(1)–Cl(1)–Cr(1)
Cr(1)–Cl(2)–Cr(1)*
O(1)–Cr(1)–Cl(1)
82.25(7)
174.19(19)
89.5(2)
91.2(2)
81.98(7)
81.26(11)
81.08(8)
82.98(10)
89.5(2)
171.5(2)
coordination. Each chromium atom is coordinated by an
amine-bis(phenolate) ligand and three bridging chlorides,
thus leading to seven anionic ligands surrounding two Cr(III)
ions. The surplus negative charge is offset by a THF-bound
lithium ion. This Li(THF) fragment is disordered in the
asymmetric unit such that it exists in two sites of 50% occu-
pancy bound to two of the bridging chlorides and one
phenolate oxygen of each amine-bis(phenolate) ligand. The
Cr–Cl distances in (1)2·LiCl·THF are 2.425(3) and 2.386(2) Å
and fall within the range of other complexes of chromium(III)
bearing three bridging chlorides.84–87 Of the three Cl atoms,
two are related by symmetry. The longer of the two Cr–Cl
bonds, Cr(1)–Cl(1), is likely due to the coordination of the Li
ion rather than due to any trans influence. Metal–metal bonds
between Cr(III) centres are not expected and the observed
Cr⋯Cr interatomic distance of 3.1615(13) Å supports the
absence of any such interaction. Therefore, the formation
of what can be described as a LiCl adduct of a dimer of
compound 1 must be the result of the thermodynamic sta-
bility of an octahedral ligand field for Cr(III). As mentioned
above, the elemental analysis of the bulk material for 1 and
2
are consistent with formulations possessing between
0 and 1 molecules of THF per CrCl[L] formula unit, but does
not support the presence of significant amounts of LiCl.
The structurally authenticated complex 3 was also shown to
Scheme 2 The copolymerization of CHO and CO2 producing PCHC.
Table 2 Results of the copolymerization of CHO and CO2 by catalysts 1 and 2 with DMAP or PPNX saltsa
f
Entry Catalyst Co-catalyst Temp (°C) Pressure (bar) % conversionb
% yieldc
TONd
TOFe (/h) Mn (kg mol−1
)
PDIf (Mw/Mn)
1
1
1
1
1
2
2
DMAP
DMAP
PPNCl
PPNN3
DMAP
DMAP
60
25
60
60
60
60
45
45
45
42
40
42
33.8
0
51.5
44.8
18.7
13.7
12.7
0
44.3
27.0
6.63
5.41
169
0
258
224
93.5
68.5
7.1
0
11
9.3
3.9
2.9
2.3
—
2.8
3.8
1.1
ND
1.58
—
1.56
1.48
2.10
ND
2
3g
4g
5
6h
a
b
Polymerization reactions were carried out in neat CHO (5 mL) for 24 h using catalyst and co-catalyst of loading of 0.2%. Calculated by 1H
c
d
e
f
NMR. Isolated yield. Calculated by conversion. Turnover frequency is moles of PCHC produced per mol of Cr per hour. Determined by
g
gel permeation chromatography in CHCl3, calibrated with polystyrene standards. Dissolved in 4 mL dichloromethane prior to adding CHO.
h
Duplicate run of previous entry. ND = not determined.
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and possessing moderate molecular weight distributions
(PDIs of 1.5 to 2.1).
13C NMR. Examining the carbonyl region shows the polymer
is atactic, consisting of isotactic and syndiotactic PCHC
(Fig. S9†).88,89 The relative stereochemistry of the polymer
obtained using achiral 1 is similar to that obtained using
racemic tetradentate salen cobalt complexes with chiral diamine
backbones.57 Similarly, although 3 possesses chirality in the
2-position of the tetrahydrofuranyl group, no stereo control
is observed and again atactic polymer is obtained.
MALDI-TOF MS studies were performed in order to gain
insight into experimental values of the number average
molecular weights of the copolymers and analysis of possible
end-groups providing mechanistic details for this system. In
the MALDI-TOF mass spectra of these copolymers, multi-
modal molecular weight distributions were observed showing
multiple end-group series, all separated with a repeating unit
of m/z 142. The different end-groups can be attributed to the
mechanism, in particular the initiation step involved in the
copolymerization process, and the catalyst systems used. In
addition to the use of an appropriate acidic matrix, sodium
trifluoroacetate was added as cationizing agent, however,
models of the spectra obtained suggest polymer cations could
also be obtained by protonation and NaTFA was not always
required to obtain a useful MALDI-TOF spectrum.
For catalyst 1, several polymer chains possessing different
end-groups were observed when PPNCl or PPNN3 was used as
co-catalyst according to the conditions shown in Table 2,
entries 3 and 4, respectively. The MALDI-TOF spectrum
obtained using PPNCl is shown in Fig. 2 and shows a multi-
modal molecular weight distribution. In the 2000 to 4000 m/z
region of the mass spectrum, four series of polymer chains
are detected. Series (a) consists of an ether linkage located
within the polymer (not necessarily at a chain end as illus-
trated) and two chloride end-groups [35 (Cl) + 142n (repeating
unit) + 180 (C12H20O) + 35 (Cl)], which may be a result of chain
transfer.90 Series (b) consists of two hydroxyl end-groups
[17 (OH) + 142n (repeating unit) + 82 (C6H10) + 17 (OH)] sug-
gesting evidence of chain-transfer as a result of hydrolysis.36,91
Series (c) consists of one chloride and hydroxyl-end-group
[35 (Cl) + 142n (repeating unit) + 82 (C6H10) + 17 (OH)]. This
series occurs over a wide mass range from 2000 to 7000 m/z
in the spectrum and is the expected polymer if epoxide
Catalyst 3 (possessing a tetradentate ligand) proved supe-
rior to both 1 and 2. When combined with DMAP at 60 °C
and 40 bar CO2, good conversions and yields of polycarbonate
were obtained (Table 3, entries 1 and 2). The activity was very
sensitive to catalyst loading as lowering it to 0.1% Cr vs.
CHO showed a decrease in CHO conversion (entry 3). The
polymer obtained was also shown by GPC to have lower
molecular weight than that obtained under the higher cata-
lyst loading conditions. Using PPNCl or PPNN3 co-catalysts
gave the best results (76.3% conversion of PCHC) (entries 4
and 5, respectively). The poor solubility of these co-catalysts
in CHO required dissolving the catalyst and co-catalyst in
dichloromethane, whereupon removal of solvent gives a mate-
rial now soluble in CHO. This procedure has also been used
when these ionic co-catalysts are used with salen- or salan-based
Cr(III) catalysts35,37 and with related diamine-bis(phenolate)
Cr(III) catalysts.68 Similar levels of conversion and polymer
yield were reported by our group using an analogue of 3 with
PPNCl. In that study, the neutral pendent donor was a pyridyl
group rather than a tetrahydrofuranyl donor.68 Although fairly
high yielding in polymer of moderate weights (approximately
6 kg mol−1), the catalysts exhibit slow TOFs. Shortening reac-
tion times to 6 and 3 h (entries 6 and 7, respectively) showed a
decrease in conversions and commensurate decreases in iso-
lated polymer yield. An inspection of the molecular weights
shows a trend towards increasing polymer chain lengths over
time, which is consistent with the lower isolated yield of poly-
mer due to increased solubility of the lower molecular weight
material. Lastly, the use of a cocatalyst and elevated tempera-
tures appear crucial for copolymerization of CO2 with CHO by
3, since no polymer was obtained in the absence of co-catalyst
(entry 8) or at room temperature (entry 9).
Structural analysis of the copolymers
As shown by 1H NMR of the resulting polymer, carbonate
linkages dominate and the existence of ether linkages is in
very low abundance (see Fig. S8†). The stereochemistry of
the isolated poly(cyclohexene carbonate) was determined by
Table 3 Results of the copolymerization of CHO and CO2 for catalyst 3 with DMAP or PPNX saltsa
Entry Co-catalyst Time (h) Temp (°C) Pressure (bar) % conversionb % yieldc TONd TOFe (h−1
)
Mn (kg mol−1
)
PDIf (Mw/Mn)
f
1
DMAP
DMAP
DMAP
PPNCl
PPNN3
PPNCl
PPNCl
None
24
24
24
24
24
6
3
6
24
60
60
60
60
60
60
60
60
25
40
43
43
42
41
41
43
42
40
68.0
59.9
28.6
76.3
76.3
49.5
33.0
0
58.9
60.8
16.4
73.7
64.1
37.3
22.8
0
340
300
286
382
382
248
165
0
14
12
12
16
16
41
55
0
5.7
ND
2.8
5.7
6.4
4.5
3.0
—
1.48
ND
2g
3h
4i
5i
6i
7i
8
1.67
1.46
1.42
1.39
1.63
—
9
DMAP
0
0
0
0
—
—
a
b
Polymerization reactions were carried out in neat CHO (5 mL), using catalyst and co-catalyst of loading of 0.2%. Calculated by 1H NMR.
c
d
e
f
Isolated yield. Calculated from % conversion. Turnover frequency is moles of PCHC produced per mol of Cr per hour. Determined by
g
h
gel permeation chromatography in CHCl3, calibrated with polystyrene standards. Duplicate run of previous entry. Catalyst and co-catalyst
of loading of 0.1%. Dissolved in 4 mL dichloromethane prior to adding CHO. ND = not determined.
i
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The polymer obtained using complex 2 with DMAP as
described in Table 2, entry 5 showed three end-group types
in its MALDI-TOF mass spectrum (Fig. S11†). Series (a) con-
sists of a chloride-initiated and hydroxyl-terminated polymer.
Series (b) and (c) suggest DMAP-initiated, hydroxy-terminated
polymers with series (b) containing an ether linkage. Therefore
series (b) consists of [122 (C7H10N2) + 142n (repeating unit) +
180 (C12H20O) + 17 (OH)] chains and series (c) of [122 (C7H10N2) +
142n (repeating unit) + 82 (C6H10) + 17 (OH)] chains.
The mass spectra of polymers formed by 3 were more
complicated than those obtained using catalyst 1 or 2 and
some peaks could not be satisfactorily identified. The mass
spectrum of the polymer obtained using catalyst 3 and DMAP
as described in Table 3, entry 1 is shown in Fig. 3 and six
different end-groups could be identified (Fig. S12†). The lower
molecular weight series (a) contains an ether linkage and
two hydroxyl end groups [17 (OH) + 142n (repeating unit) +
180 (C12H20O) + 17 (OH)], likely resulting from intermolecular
chain transfer. Series (b) contains DMAP-initiated, hydroxyl-
terminated polymer [122 (C7H10N2) + 142n (repeating unit) +
180 (C12H20O) + 17 (OH)]. Series (c) consists of the commonly
observed chloride-initated, hydroxyl-terminated chain [35 (Cl) +
142n (repeating unit) + 82 (C6H10) + 17 (OH)]. Series (d) con-
tains Cl groups at both termini of the polymer, again likely
resulting from intermolecular chain transfer [35 (Cl) + 142n
(repeating unit) + 82 (C6H10) + 35 (Cl)] and is observed at
higher mass as well. In the higher molecular weight region,
series (e) consists of chloride and hydroxyl chain-ends and an
ether linkage [35 (Cl) + 142n (repeating unit) + 180 (C12H20O) +
17 (OH)]. Series (f) ends possesses two hydroxyl groups,
[17 (OH) + 142n (repeating unit) + 82 (C6H10) + 17 (OH) + 7 (Li)],
suggesting evidence of chain-transfer as a result of protonolysis.
The presence of Li+ likely arises from adventitious LiCl
remaining from the synthesis of 3. The highest molecular
weight polymer (between 7500 and 12000 m/z) consists of
chloride and hydroxyl end groups and an ether linkage
[35 (Cl) + 142n (repeating unit) + 180 (C12H20O) + 17 (OH)].
Using PPNN3 as co-catalyst also leads to polymers con-
taining various end-groups (Fig. 4). The differing end groups
Fig. 2 (A) MALDI-TOF MS of PCHC produced according to Table 2,
entry 3. (B) Lower mass region (2750–3350 m/z, n = 18–21) of the
spectrum with modeled polymer chains, a–d.
ring-opening is initiated by nucleophilic attack by a chloride
and the copolymerization is terminated by protonolysis of
the metal-alkoxide. Series (d) contains Cl groups at both
termini of the polymer [35 (Cl) + 142n (repeating unit) +
82 (C6H10) + 35 (Cl)], which could arise from intermolecular
chain transfer of two chloride-initiated polymer chains.
For the polymer obtained in the presence of PPNN3, the
spectrum obtained (Fig. S10†) shows two end-group types in
the lower molecular weight series. Series (a) consists of an
ether linkage and two chloride end-groups as shown above,
which may be a result of chain transfer. Series (b) consists of
two chloride chain ends but lacking an ether linkage as in
series (d) above. The higher molecular weight series (c) is the
expected chloride-initiated and hydroxyl-terminated polymer
as described previously. Perhaps surprisingly, no azide-containing
end-groups were observed in the mass spectrum despite the
use of PPNN3 as co-catalyst.
Fig. 3 MALDI-TOF spectrum of PCHC produced by 3 and DMAP
(Table 3, entry 1).
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the polymers obtained generally possessed lower molecular
weights and broader polydispersities. It is believed that the
coordination sphere around the metal must be stabilized by
the presence of strongly electron donating ligands. Tridentate
variants of amine-bis(phenolates) are less electron donating
than tetradentate ligands, and the tetrahydrofuranyl donor
is more weakly coordinating than our previously reported
pyridyl-containing analogue. Further investigations into the
influence of the pendant donor and substituents are being
conducted.
Experimental section
General experimental conditions
Fig. 4 MALDI-TOF spectrum of PCHC produced by 3 and PPNN3
Unless otherwise stated, all manipulations were performed
under an atmosphere of dry oxygen-free nitrogen by means of
standard Schlenk techniques or using an MBraun Labmaster
glove box. CrCl3(THF)3,92 the proligands H2[L1], H2[L2],
H2[L3]71,80,81 and complex 372 were prepared according to pre-
viously reported procedures. CHO was purchased from Aldrich
and distilled from CaH2. THF was purified by distillation from
sodium/benzophenone ketyl under nitrogen. All other solvents
were purified by an MBraun Manual Solvent Purification
System. PPNN3 was prepared according to the literature pro-
cedure.93 99.998% (4.8 Supercritical fluid chromatography
grade) CO2 was supplied from Praxair in a high-pressure
(Table 3, entry 5).
result from seven polymer chains attributed to various
molecular weights (Fig. S13†). In the lower molecular weight
region, series (a) is modeled to have an ether linkage and
two hydroxyl end groups [17 (OH) + 142n (repeating unit) +
180 (C12H20O) + 17 (OH)]. Series (b) suggests initiation by
the azide nucleophile and termination by protonolysis
[42 (N3) + 142n (repeating unit) + 180 (C12H20O) + 17 (OH)].
Series (c) contains two hydroxyl groups and is cationized by
adventitious Li+ [17 (OH) + 142n (repeating unit) + 82 (C6H10) +
17 (OH) + 7 (Li)]. Series (d) also appears to be initiated by
an azide anion and terminated by the expected hydroxyl
group [42 (N3) + 142n (repeating unit) + 82 (C6H10) + 17 (OH)].
In the higher molecular weight region, assignment of the
end groups becomes less straightforward and the presence
of azide (from the PPNN3), chloride (from 3) and hydroxyl
end groups can be found. Chain transfer leads to compli-
cated scrambling of chain ends as well as the incorporation
of ether linkages as described above. As was observed in the
polymer obtained using DMAP (Fig. 3, Table 3, entry 1), the
highest molecular weight polymer (between 6000 and 9000 m/z)
consists of chloride and hydroxyl end groups and an ether
linkage [35 (Cl) + 142n (repeating unit) + 180 (C12H20O) + 17 (OH)].
1
cylinder equipped with a liquid dip tube. All H and 13C NMR
spectra were obtained in CDCl3 purchased from Cambridge
Isotope Laboratories, Inc.
Instrumentation
1H and 13C NMR spectra were recorded on a Bruker AVANCE
III 300 MHz spectrometer. All copolymerization reactions
were carried out in a 100 mL stainless steel Parr® 5500 auto-
clave reactor with a Parr® 4836 controller. N.B. Caution
should be taken when operating high-pressure equipment.
UV-vis spectroscopy was conducted on an Ocean Optics
USB4000+ fiber optic spectrophotometer. Elemental analysis
was performed at Guelph Chemical Laboratories, Guelph,
ON, Canada. Gel permeation chromatography (GPC) analysis
was performed on a Viscotek VE 2001 GPCMax at 35 °C equipped
with a Viscotek VE 3580 RI Detector, Phenogel narrow-bore
5 μm 100 Å, 5 μm Linear(2) columns (300 × 4.60 mm) and
a 5 μm guard column. Samples were prepared at a concen-
tration of 2 mg mL−1 and left to equilibrate for ~2 h. The
samples were filtered through 0.2 μm syringe filters before
analysis. The GPC columns were eluted with chloroform
(HPLC grade) at a flow rate of 0.30 mL min−1 with a 100 μL
injection volume. Eight polystyrene standards were used in
making the calibration curve, bracketing molecular ranges
from 1050 to 3 800 000 Da. No further corrections were
performed on the molecular weights obtained. MALDI-TOF
mass spectrometry was performed by using an Applied
Biosystems 4800 MALDI TOF/TOF Analyzer equipped with a
reflectron, delayed ion extraction and high performance
nitrogen laser. Anthracene was used as the matrix for analysis
Conclusions
Chromium amine-bis(phenolate) complexes 1–3 showed
promising activity for the copolymerization of CHO and CO2
in the presence of three co-catalysts (DMAP, PPNCl, and
PPNN3) to give PCHC with no evidence of cyclic carbonate
being formed and with a high degree of carbonate linkages.
The steric effect of substituents on the phenolate fragments
of the tridentate amine-bis(phenolate) ligand appeared to
influence the catalyst activity. Complex 2, containing t-amyl
substituents on the phenolate, showed lower activity than 1,
possessing t-butyl groups. However, both these tridentate
ligand-containing complexes showed lower activity than 3,
which contained a tetradentate ligand. The activity of 3 was
found to be similar to that of a related chromium(III)
diamine-bis(phenolate) complex previously reported by us, but
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of 1 and 2. The matrix and chromium complex were dissolved
in toluene at a concentration of 20 mg mL−1 separately. The
matrix and chromium complex solutions were mixed together
at a ratio of 1 : 1, and the mixture was spotted on the MALDI
plate and left to dry. Samples were prepared in the glove box
and sealed under nitrogen in a Ziploc™ bag for transport to
the spectrometer. For polymer analysis, either 2,5-dihydroxy-
benzoic acid (DHBA) or 1,8,9-trihydroxyanthrancene94 was
used as the matrix with sodium trifluoroacetate (NaTFA) as
cationizing agent. The matrix and NaTFA were dissolved in
THF at a concentration of 2 mg mL−1. The polymer was
dissolved at approximately 2 mg mL−1. The matrix and poly-
mer solutions were mixed together in a ratio of 1 to 1.1 μL
and this mixture was spotted on the MALDI plate and
left to dry.
X-ray crystallography
Diffraction data for compound 1 were collected on a Rigaku
Saturn CCD area detector with a SHINE optic and Mo-Kα
radiation and solved on an AFC8-Saturn 70 single crystal
X-ray diffractometer from Rigaku, equipped with an X-stream
2000 low temperature system. Crystallographic and structure
refinement data are given in ESI.† The data were processed
using CrystalClear95 software and corrected for Lorentz and
polarization effects and absorption.96 Neutral atom scat-
tering factors for all non-hydrogen atoms were taken from
the International Tables for X-ray Crystallography.97 The struc-
ture was solved by direct methods using SIR9298 and expanded
using Fourier techniques (DIRDIF99).99 All non-hydrogen
atoms were refined anisotropically. Hydrogen atoms were
refined using the riding model. Anomalous dispersion
effects were included in Fcalc
100
;
the values for Δf′ and Δf′′
were those of Creagh and McAuley.101 The values for the mass
attenuation coefficients are those of Creagh and Hubbell.102
All calculations were performed using the CrystalStructure103
crystallographic software package except for refinement, which
was performed using SHELXL-97.104 Disordered lattice solvent
for 1 was removed through applying Platon's Squeeze105 pro-
cedure. Structural illustrations were created using ORTEP-III
(v. 2.02) for Windows.106
Synthesis of Cr(III) complexes
1: H2[L1] (4.89 g, 9.00 mmol) was dissolved in THF (50 mL)
and cooled to −78 °C. n-Butyllithium (1.6 M in hexanes, 12.37 mL,
19.79 mmol) was slowly added to give a clear red solution,
which was warmed to room temperature and further stirred
for 2 h. This mixture was transferred via cannula to a sus-
pension of CrCl3(THF)3 (3.37 g, 9.00 mmol) in THF (50 mL)
cooled to −78 °C to give a dark green mixture. Upon warming
to room temperature and stirring for 18 h the solids dissolved
giving a dark green solution. The solvent was removed in vacuo
and the solid residue was extracted into toluene. The mixture
was filtered through Celite and the solvent was removed under
vacuum. The product was washed with pentane and dried to
yield 6.01 g (95% for CrCl(THF)[L1]) of dark green powder.
Elemental analysis is unable to unequivocally differentiate
between the THF-free or the solvent-containing complex. Anal.
calcd for C37H51ClCrNO2: C, 70.62; H, 8.17; N, 2.23. Anal. calcd
for C37H51ClCrNO2·(C4H8O): C, 70.21; H, 8.48; N, 2.00. Found:
C, 70.57; H, 8.33; N, 1.96. MS (MALDI-TOF) m/z (%, ion):
628.4 (100, [CrCl[L]+]), 593.4 (95, [Cr[L]+]).
2: H2[L2] (4.06 g, 6.77 mmol) was dissolved in THF (50 mL)
and cooled to −78 °C. n-Butyllithium (1.6 M in hexanes, 9.31 mL,
14.90 mmol) was slowly added to give a bright red solution,
which was warmed to room temperature and further stirred
for 2 h. This mixture was transferred via cannula to a sus-
pension of CrCl3(THF)3 (2.54 g, 6.77 mmol) in THF (50 mL)
cooled to −78 °C to give a dark green mixture. Upon warming
to room temperature and stirring for 18 h the solids dissolved
giving a dark green solution. The solvent was removed in vacuo
and the residue was extracted into toluene. The mixture was
filtered through Celite and the solvent was removed under
vacuum. The product was washed with pentane and dried to
yield 4.57 g (98.6% for CrCl(THF)[L2]) of dark green powder.
Elemental analysis is unable to unequivocally differentiate
between the THF-free or the solvent-containing complex. Anal.
calcd for C41H59ClCrNO2: C, 71.85; H, 8.68; N, 2.04. Anal. calcd
for C41H59ClCrNO2·(C4H8O): C, 71.35; H, 8.92; N, 1.85. Found:
C, 71.61; H, 8.45; N, 1.90. MS (MALDI-TOF) m/z (%, ion): 628.4
(83, [CrCl[L]+]), 593.4 (100, [Cr[L]+]).
Copolymerization procedure
The appropriate amount of monomer was added to the cata-
lyst and co-catalyst in a glove box. The reactant solution was
then added via a long-needled syringe to a Parr® autoclave,
which was pre-dried under vacuum overnight at 80 °C. The
autoclave was then charged with the appropriate pressure of
CO2 and left to stir at the desired temperature and time
period. After the desired time, the autoclave was cooled in an
ice bath until the temperature reached below 4 °C and then
vented in the fume hood. An aliquot was taken immediately
after opening the reactor for the determination of conversion
by NMR. The contents of the reactor were extracted with
CH2Cl2 and the polymer precipitated using cold acidic methanol.
For reactions done in the presence of PPNCl or PPNN3, the cata-
lyst and co-catalyst were first combined in 4 mL of CH2Cl2 in the
glove box, stirred for 30 minutes then dried in vacuo. This
generated a material that was soluble in cyclohexene oxide.
The remaining procedure was followed as described above.
Acknowledgements
Financial support was provided by the Canada Foundation for
Innovation (CFI), Newfoundland and Labrador Research Devel-
opment Corporation, the Natural Sciences and Engineering
Research Council (NSERC) of Canada and Memorial University
of Newfoundland. C. M. K. thanks Suncor Energy for a Terra
Nova Young Innovator Award.
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