Anion variation on a cobalt(iii) complex of salen-type ligand tethered by four quaternary ammonium salts for CO2/epoxide copolymerization

Article abstract of DOI:10.1039/b920992a

Anion exchange of BF₄^- occurs by stirring a cobalt(iii) complex of salen-type ligand tethered by four quaternary ammonium BF₄^- salts over a slurry of NaX in CH₂Cl ₂, affording a complex containing four X's per cobalt (X = 2,4,5-trichlorophenolate, 6; X = 4-nitrophenolate, 10; X = 2,4- dichlorophenolate, 12). The ¹H and ¹³C NMR spectra are in agreement with an unusual imine uncoordinated structure. The two salen-phenoxys and the two X's persistently coordinate with cobalt(iii) to form a square planar cobaltate complex while the other two X's scramble through coordination and decoordination to the axial sites of the square plane. Another form of the complex (X = 2,4,5-trichlorophenolate, 14; X = 4-nitrophenolate, 15; X = 2,4-dichlorophenolate, 16) is also prepared, in which the scrambling two X's in 6, 10, or 12 are replaced with the corresponding [X...H...X] ^- homoconjugate. These complexes, which adopt an unusual imine uncoordinated structure, are excellent catalysts for CO₂/propylene oxide copolymerization (turnover frequency (TOF), 8300-16000 h^-1). In all cases, the complex containing the homoconjugate [X...H...X] ^- shows higher activity than the corresponding phenol-free complex. Among the prepared complexes, 4-nitrophenol-4-nitrophenolate homoconjugate complex 15 showed the best performance (TOF, 16000 h^-1; selectivity, 98%; M_n, 273000), allowing for replacement of the explosive 2,4-dinitrophenolate complex. The Royal Society of Chemistry 2010.

Full text of DOI:10.1039/b920992a

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Anion variation on a cobalt(III) complex of salen-type ligand tethered by four
quaternary ammonium salts for CO2/epoxide copolymerization†
Jina Yoo, Sung Jae Na, Hyeong Cheol Park, Anish Cyriac and Bun Yeoul Lee*
Received 7th October 2009, Accepted 17th December 2009
First published as an Advance Article on the web 28th January 2010
DOI: 10.1039/b920992a
-
Anion exchange of BF4 occurs by stirring a cobalt(III) complex of salen-type ligand tethered by four
-
quaternary ammonium BF4 salts over a slurry of NaX in CH2Cl2, affording a complex containing four
X’s per cobalt (X = 2,4,5-trichlorophenolate, 6; X = 4-nitrophenolate, 10; X = 2,4-dichlorophenolate,
12). The 1H and 13C NMR spectra are in agreement with an unusual imine uncoordinated structure.
The two salen-phenoxys and the two X’s persistently coordinate with cobalt(III) to form a square planar
cobaltate complex while the other two X’s scramble through coordination and decoordination to the
axial sites of the square plane. Another form of the complex (X = 2,4,5-trichlorophenolate, 14; X =
4-nitrophenolate, 15; X = 2,4-dichlorophenolate, 16) is also prepared, in which the scrambling two X’s
in 6, 10, or 12 are replaced with the corresponding [X ◊ ◊ ◊ H ◊ ◊ ◊ X]- homoconjugate. These complexes,
which adopt an unusual imine uncoordinated structure, are excellent catalysts for CO2/propylene oxide
copolymerization (turnover frequency (TOF), 8300–16 000 h-1). In all cases, the complex containing the
homoconjugate [X ◊ ◊ ◊ H ◊ ◊ ◊ X]- shows higher activity than the corresponding phenol-free complex.
Among the prepared complexes, 4-nitrophenol-4-nitrophenolate homoconjugate complex 15 showed
the best performance (TOF, 16 000 h-1; selectivity, 98%; Mn, 273 000), allowing for replacement of the
explosive 2,4-dinitrophenolate complex.
which corresponds to 28 000 g polymer per g of cobalt, which is
approximately 1000 times higher than that attained with the best
heterogeneous Zn-based catalyst.
The polymerization rate is also very high; the high TON
Introduction
Carbon dioxide/epoxide copolymerization was discovered four
decades ago by Inoue et al.1 The initial catalytic system, a Zn(II)-
based heterogeneous catalyst generated by mixing diethylzinc and
was achieved by running for 1.0 h (turnover frequency (TOF),
16 000 h-1), producing a strictly alternating copolymer with a
high molecular weight (Mn) of up to 300 000 and high selectivity
(>99%). Another advantage of 3 or 4 is that the catalyst can be
efficiently removed after polymerization from a polymer solution
through filtration over a short pad of silica gel. The collected
catalyst on the silica surface can then be recovered and reused.
Removing the catalyst residue is crucial not only because catalyst
residue colors the resin, but also because it causes toxicity as well
as severe degradation during thermal processing.10 All these merits
allow for the design of a continuous commercial process to produce
attractive CO2/epoxide copolymers.
The structure of 3 or 4 had been initially identified as that
of a conventional tetradentate salen-cobalt(III) complex, but we
subsequently determined that it adopts an unusual structure as
shown in Chart 1.11 The imine-nitrogen’s do not coordinate, but,
instead, counter anions of the quaternary ammonium cations, 2,4-
dinitrophenolates (DNPs), coordinate with cobalt. The extraordi-
narily high activity of 3 or 4 compared with 2 was attributed to
the unusual binding mode of 3 or 4. In this work, we report the
derivatives of 3 or 4 by replacing DNPs with other anions. Polymer
chains start to grow from DNPs in the catalysis of 3 or 4, and,
hence, all the polymer chains contain a DNP end group if the chain
transfer reactions are excluded. Dry 2,4-dinitrophenol is explosive
and so is sold as a hydrated product containing approximately
20% water. The dry sodium salt of 2,4-dinitrophenol, which is
necessarily for the preparation of 3 or 4, has also been reported to
be highly explosive.12 Complex 3 or 4 inherently has the potential
water in a 1 : 1 ratio, has been extensively improved through
various modifications. The best catalytic activities for the Zn-
based heterogeneous catalytic system are in the range of 350 g
polymer per g of zinc.2 Homogeneous catalysts of porphyrin-Al
and salen-Al, Co, and Cr complexes as well as b-diketiminatoZn
complexes have also been discovered.3 Mechanistic studies have
revealed or suggested that the two metal centers are involved in
the propagation reaction, with the carbonate anion appended
on a metal center attacking the epoxide that is activated by
the coordination on the other metal center.4 Based on these
studies, bimetallic catalysts have been constructed to improve
catalytic performance.5 In the case of homogeneous (salen)Co (1)
or (salen)Cr catalysts, the catalytic performance can be improved
by the addition of a cocatalyst such as quaternary ammonium
salt or amine base (Chart 1).6,7 We further improved catalysis by
binding the salen ligand and the quaternary ammonium unit in a
molecule (2 in Chart 1).8 Binding enables the two components
situated in proximity regardless of low catalyst concentration
or high polymerization temperature, consequently resulting in a
high turnover number (TON) and a high molecular weight (Mn).
Using this concept, we recently discovered a highly active catalytic
system (3 and 4 in Chart 1).9 It shows a TON of up to 16 000,
Department of Molecular Science and Technology, Ajou University, Suwon,
443-749, Korea. E-mail: bunyeoul@ajou.ac.kr; Fax: +82-31-219-2394;
Tel: +82-31-219-1844
† Electronic supplementary information (ESI) available: NMR spectra of
6, 10, 12, 14, 15, and 16. See DOI: 10.1039/b920992a
2622 | Dalton Trans., 2010, 39, 2622–2630
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Chart 1 Salen-Co(III) catalysts for CO2/epoxide copolymerizations.
to be explosive because it contains many anhydrous DNP anions.13
Preparation of trichlorophenolate complexes
So, replacement of DNP in 3 or 4 with another and safe anion is
required when considering a scalable synthesis of the catalyst for
use in a pilot or commercial process. A scalable synthetic route for
the ligand system of 3 or 4 has been already developed.14
The next targeted anion for replacement of explosive DNP
anion in 3 was chlorinated phenolates, which are not explosive.
Oxidation with O2/4-chlorophenol or O2/2,4-dichlorophenol was
not successful, but cobalt(II) complex 5 was transformed to 2,4,5-
trichlorophenolato cobalt(III) complex under an O2 atmosphere
in the presence of one equivalent of 2,4,5-trichlorophenol (eqn
(2)). The reaction rate was so slow that 4 days were required
for complete oxidation. The anion exchange reaction with five
equivalents of 2,4,5-trichlorophenolate in CH2Cl2 produced the
desired salen-Co(III) complex 6.
Results
Trial for preparation of carboxylate complexes
The most attractive derivatization is a replacement of DNPs in 3
with carboxylates such as acetate. A similar type of active catalyst
in the Nozaki system is an acetate complex.15 A binary system
of salen-Co(III) complex/quaternary ammonium salt has been
reported to work well with an acetato complex as well as with
quaternary ammonium acetate.6 Presently, a cobalt(II) complex of
(2)
-
salen-type ligand tethered by four quaternary ammonium BF4
salts (5) was oxidized under an oxygen (O2) atmosphere in the
presence of various carboxylic acids such as acetic acid, hexanoic
acid, benzoic acid, trifluoroacetic acid, or trichloroacetic acid,
generating the corresponding salen-Co(III) complexes (eqn (1)). In
the case of acetic acid, trifluoroacetic acid, or trichloroacetic acid,
the reaction rate was so fast that the oxidation could be completed
in 3 h. For oxidation in the presence of hexanoic acid or benzoic
acid, the rate was so slow that 4 days were required for complete
oxidation. The 1H NMR spectra clearly showed that all the
paramagnetic cobalt(II) species were transformed to diamagnetic
In the 1H NMR spectrum (dmso-d6) of the crude prod-
uct obtained after the anion exchange reaction, three sets of
sharp 2,4,5-trichlorophenolate signals were clearly evident, along
with a set of salen-signals (Fig. 1 and ESI†). A set of 2,4,5-
trichlorophenolate signals comprised the coordinated one at
8.34 and 6.69 ppm with the integration value of two 2,4,5-
trichlorophenolates per cobalt. Another set comprised the un-
coordinated one at 7.10 and 6.36 ppm with the integration value
of two 2,4,5-trichlorophenolates per cobalt. The chemical shifts
of the uncoordinated one were observed to be close to that of
sodium 2,4,5-trichlorophenolate (7.08, 6.35 ppm in dmso-d6).
The third set was observed at 7.79 and 7.76 ppm along with
a signal at 6.15 ppm with an integration value of one 2,4,5-
trichlorophenolate per cobalt (ESI†). The third set of signals
could be removed by washing with diethyl ether, indicating that
these signals are attributed to a neutral species, not an anion. We
assigned the third set of signals to compound 7, generated by
the attack of 2,4,5-trichlorophenolate anion onto CH2Cl2 (eqn
(2)). In the 19F NMR spectrum, a negligible amount of BF4
-
cobalt(III) complexes. Unsatisfactorily, the next BF4 replacement
with acetate anion was not successful. In the preparation of 3,
-
anion exchange of BF4 was carried out by stirring a solution
-
of the oxidized cobalt(III) complex containing four BF4 over a
slurry of five equivalents of NaDNP in CH2Cl2. Under the same
conditions using sodium carboxylate instead of NaDNP, BF4- was
not replaced with carboxylate anion (eqn (1)).
-
signal was evident, indicating that all the BF4 was replaced
with 2,4,5-trichlorophenolate anion. Running the anion exchange
reaction at low temperature (<15 ◦C) significantly suppressed the
(1)
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sets of broad 2,4,5-trichlorophenolate signals were observed. The
coordinated 2,4,5-trichlorophenolate signals are sharply evident
at 6.66 ppm and very broadly evident at 8.15 ppm. The other set
of 2,4,5-trichlorophenolate signals were very broad at 7.21 and
6.80 ppm. This signal pattern was consistent with structure 6-B
in Chart 2, where two 2,4,5-trichlorophenolate and two salen-
phenoxy’s persistently coordinate with cobalt to provide a square
planar cobaltate complex. The other two 2,4,5-trichlorophenolates
scramble through coordination and decoordination to the axial
sites of the square plane. Due to the scrambling, the magnetic
environment on the ortho-proton of the persistently coordinated
2,4,5-trichlorophenolate is severely perturbed and the signal at
8.15 ppm becomes very broad, while the perturbation on the
meta-proton is negligible, producing the corresponding sharp
signal. Signals of the scrambling 2,4,5-trichlorophenolates were
very broad. Some tetradentate cobaltate(III) complexes bearing a
-1 charge on cobalt have been previously reported.17 They are also
prone to interconversion between four-, five-, and six-coordinate
states in the presence of additional neutral or anionic ligands.18
Almost the same signal pattern was observed in THF-d8 (Fig. 1).
Signal broadness of the scrambling anion was less in THF-d8
than in CD2Cl2, indicative of less staying in a coordination state
in THF. In dmso-d6, the signal of the scrambling anion became
very narrow, indicative of staying mostly in a decoordination state
in the highly coordinating dmso solvent (Chart 2).◦In variable
temperature 1H NMR studies over the range of -50 C to 50 ◦C
in THF-d6, the salen-aromatic signals and the coordinated 2,4,5-
trichlorophenolate signals were persistently sharp relative to the
signals of scrambling 2,4,5-trichlorophenolate which became very
broad by lowering the temperature (ESI†).
Fig. 1 1H NMR spectra of 2,4,5-trichlorophenolate (X) complex 6 and
its homoconjugate (X ◊ ◊ ◊ H ◊ ◊ ◊ X) complex 14 (“s” signals for salen-unit;
“∧” signals for coordinated X; “*” signals for scrambling X; “#” signals
for [X ◊ ◊ ◊ H ◊ ◊ ◊ X]).
As a comparison, we prepared a tert-butyl analogue 8 (eqn (3)),
which adopted the conventional imine coordinated structure.
The tert-butyl substituents blocked formation of the unusual
imine uncoordinated structure. Salen-cobalt complexes have been
applied as versatile catalysts in various asymmetric syntheses such
as hydrolytic kinetic resolution (HKR),19 nitro-aldol reaction,20
alkene hydrocyanation21 and resolution of racemic N-benzyl a-
amino acids.22 In all these reactions, the utilized complexes are
constructed from salicylaldehyde having a tert-butyl substituent
on its 3-position. After metallation of the ligand bearing tert-
butyl substituent with Co(OAc)2, the cobalt(II) complex was
completely oxidized to cobalt(III) species by the action of O2
and 2,4,5-trichlorophenol, which was proved by the absence of
any signal at the abnormal region below 0 ppm. After anion
exchange with sodium 2,4,5-trichlorophenolate, the signal pattern
formation of 7; a trace amount of the third set of signals could
be observed along with an increase of integration value of the
uncoordinated 2,4,5-trichlorophenolate signals from two to three
2,4,5-trichlorophenolates/Co (ESI†).
Formation of 7 supported the suggestion that the product
adopts an unusual imine uncoordinated octahedral structure
previously proposed for DNP complex 3.11 Because 2,4,5-
-
trichlorophenolate is more basic than DNP,16 all four BF4
would be replaced with 2,4,5-trichlorophenolate in CH2Cl2. Only
-
three among the four BF4 are replaced in the anion exchange
1
reaction with NaDNP. After all four BF4- are replaced with 2,4,5-
trichlorophenolate, we presume that an octahedral complex would
be formed in CH2Cl2, where the four 2,4,5-trichlorophenolates
and the two salen-phenoxys coordinate with cobalt (6-A in Chart
2). The remaining 2,4,5-trichlorophenolate exists as a free anion,
which is sufficiently nucleophilic to attack CH2Cl2 above approxi-
mately 15 ◦C. The coordinated four 2,4,5-trichlorophenolates are
not so nucleophilic that they are prevented from attacking CH2Cl2
to generate 6-B. Even when three equivalents of sodium 2,4,5-
trichlorophenolate was added in the final anion exchange reaction
observed in the H NMR spectrum of 8 in dmso-d6 was totally
different from that observed for 6 (Fig. 2). A set of salen signals
was evident at 7.59, 7.11, and 6.99 ppm, but a set of very
broad 2,4,5-trichlorophenolate signals was observed at 7.18 and
6.31 ppm, in contrast with the observation of two sets of sharp
1
2,4,5-trichlorophenolate signals in the H NMR spectrum of 6.
Paramagnetic signals were also evident at -2 to 0 ppm. This signal
behavior can be explained by the conventional imine-coordinated
structure as shown in eqn (3). In this structure, the coordinated
2,4,5-trichlorophenolates on the axial sites are exchangeable with
the uncoordinated 2,4,5-trichlorophenolates in the NMR time
scale. Consequently, the coordinated and the uncoordinated 2,4,5-
trichlorophenolate signals collapsed to a set of broad signals. In
this exchange reaction, the five coordinated, square pyramidal
◦
at 20 C, the formation of a small amount of 7 (~10 mol%) was
inevitable.
The 1H NMR spectrum of 6 in CD2Cl2 (Fig. 1) revealed sharp
salen-aromatic signals at 7.61 (1H) and 6.95 (2H) ppm while two
2624 | Dalton Trans., 2010, 39, 2622–2630
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Chart 2 Structures of 6 in various solvents.
complex is a paramagnetic mediator, which produces signals at
an abnormal region, -2 to 0 ppm. The pattern of the H NMR
spectrum in CD2Cl2 was similar to that observed in dmso-d6
(Fig. 2).
Cobalt(II) complex 5 could be transformed to a diamagnetic
cobalt(III) complex by the action of O2/2,4,6-trichlorophenol, even
though the reaction rate was slow, requiring 4 days for complete
oxidation (eqn (4)). The anion exchange reaction with five
equivalents of sodium 2,4,6-trichlorophenolate occurred, yielding
1
1
product 9, whose behavior in the H NMR spectra was totally
different from that observed for 2,4,5-trichlorophenolate complex
6 (Fig. 2). In dmso-d6, a set of broad 2,4,6-trichlorophenolate
signals and salen-signals were observed with an integration value
of [2,4,6-trichlorophenolate]/[Co] = 5. The chemical shift of the
2,4,6-trichlorophenolate signal (6.91 ppm) was almost identical
with that of sodium 2,4,6-trichlorophenolate (6.93 ppm), indicat-
ing uncoordinated behavior of 2,4,6-trichlorophenolate in dmso-
d6. In CD2Cl2 and THF-d8, paramagnetic signals at -2 to 0 ppm
were observed, while the aromatic salen-signals disappeared.
A very broad 2,4,6-trichlorophenolate signal was observed at
6.8–7.3 ppm. Cobalt(III) complexes of five coordinated, square-
pyramidal structure were reported to be high spin paramagnetic,
causing abnormal signals in NMR spectra.23 We presume that the
conventional imine-coordinated square-pyramidal salen-Co(III)
complex is formed in CH2Cl2 (eqn (4)). Since the coordinat-
ing power of 2,4,6-trichlorophenolate is weaker than that of
2,4,5-trichlorophenolate or DNP, 2,4,6-trichlorophenolate anions
around the cobalt center cannot substitute the imine-coordination.
(3)
(4)
Fig. 2 The 1H NMR spectra of 8 and 9 of the conventional imine-coor-
dinated structure (“s” signals for salen-unit; “a” signals for CH2Cl2-attack
product, 7).
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Preparation of 4-nitrophenolate complex
(dd, J = 10, 3.6 Hz), and 6.28 (d, J = 10 Hz), of which the
chemical shift indicated that DNPs exist as an uncoordinated free
anion. Because 2,4-dichlorophenolate is more basic than 2,4,5-
trichlorophenolate, scrambling of the two 2,4-dichlorophenolate is
more severe, consistent with the very broad corresponding signals
even in dmso-d6 (ESI†).
Cobalt(II) complex 5 was also oxidized with O2/4-nitrophenol with
a slow rate (4 days for complete oxidation). Anion exchange re-
action with sodium 4-nitrophenolate occurred in CH2Cl2 yielding
the desired complex 10 (eqn (5)). Integration value in the 1H NMR
-
spectrum indicated that three among the four BF4 were replaced
with 4-nitrophenolate and that the side reaction of the attack
onto CH2Cl2 was absence. The 1H NMR patterns in dmso-d6 and
THF-d8 were similar to that observed for 2,4,5-trichlorophenolate
1
complex 6 (ESI†). In the H NMR spectrum in dmso-d6, two
sets of broad 4-nitrophenolate signals – a coordinated one (7.31
and 6.91 ppm) and a scrambling one (7.75 and 6.21 ppm) –
were observed along with broad salen-signals at 7.66, 7.97, and
6.90 ppm. In THF-d8, the signal pattern was the same as that
obtained in dmso-d6 except for sharpened signals. In CD2Cl2, the
salen-signals were sharp while both sets of 4-nitrophenolate signals
were very broad.
(6)
Preparation of homoconjugate complexes
(5)
In previous studies with DNP complexes, we fortuitously found
that the activity, selectivity, and induction time were improved by
employing five equivalents of 60 mol% NaDNP (that is, three
equivalents of NaDNP + two equivalents of DNP-H) in the
anion exchange reaction. We presumed that a complex involv-
ing two 2,4-dinitrophenolate-2,4-dinitrophenol homoconjugates
([DNP ◊ ◊ ◊ H ◊ ◊ ◊ DNP]-) loosely bound on cobalt was created.
It is well-established that a hydrogen bond between a proton
donor and an acceptor whose DpKa is 0 is extraordinarily
strong, especially in an aprotic solvent.24 The formation constant
of homoconjugate for [DNP ◊ ◊ ◊ H ◊ ◊ ◊ DNP]- was previously de-
termined electrochemically in acetonitrile to be approximately
100.16 Presently, the same type of complexes (14–16), 2,4,5-
trichlorophenolate, 4-nitrophenolate, and 2,4-dichlorophenolate
were prepared by carrying out the anion exchange reaction using a
mixture of three equivalents of sodium salt and two equivalents of
the corresponding phenol (eqn (7)). The side reaction of CH2Cl2-
attack of the anion did not take place in these cases.
Preparation of 2,4-dichlorophenolate complex
Cobalt(II) complex 5 cannot be oxidized under O2 atmosphere
in the presence of phenol, 4-chlorophenol, or 2,4-dichlorophenol.
-
Accordingly, anion exchange of BF4 was tried with cobalt(III)
complex 11 that was obtained through oxidation of 5 with
O2/2,4-dinitrophenol (eqn (6)). Anion exchange reaction of BF4
-
occurs with sodium phenolate or sodium 4-chlorophenolate, but
concomitant reduction of Co(III) complex to Co(II) species is
accompanied, which is inferred from the observation of param-
1
agnetic signals at -2 to 0 ppm in the H NMR spectra in dmso-
d6. Furthermore, diamagnetic 1H NMR signals indicate that two
anions among the exchanged four attack CH2Cl2 to generate PhO-
CH2-OPh or ClC6H4O-CH2-OC6H4Cl.
With less basic sodium 2,4-dichlorophenolate, the exchange re-
action occurred without the reductive side reaction (eqn (6)). In the
1H NMR spectrum in dmso-d6, three sets of 2,4-dichlorophenolate
signals and a set of DNP signals were observed along with a set of
salen-signals at the aromatic region (ESI†). A set of sharp signals
at 7.59 (1H), 7.42 (2H), and 6.02 (1H), which could be removed
by washing with diethyl ether and assigned to Cl2C6H3O-CH2-
OC6H3Cl2 (13), is indicative of the generation of the compound
by the attack of 2,4-dichlorophenolate anion onto CH2Cl2. The
ether-extracted compound was isolated and purified by silica gel
(7)
column chromatography. The H and 13C NMR data and high
In the 1H and 13C NMR spectra (THF-d8) of 2,4,5-tri-
chlorophenol-2,4,5-trichlorophenolate homoconjugate complex
(14), we observed the signals of the salen-unit and the two
coordinated 2,4,5-trichlorophenolates at the same chemical shifts
1
resolution mass data agreed with the proposed structure. The
signals of the second set were sharp at 7.88 (d, J = 9.2 ppm),
6.57 (dd, J = 9.2, 2.0 Hz), and 6.50 (d, J = 2.0 Hz) with an
integration value of two 2,4-dichlorophenolates per cobalt, which
were assignable to the coordinated ones. The signals of the third
set were observed to be very broad at 7.10, 6.79, and 6.27 ppm,
with an integration value of two 2,4-dichlorophenolates per cobalt,
characteristic of scrambling 2,4-dichlorophenolates. DNP signals
were observed as sharp signals at 8.58 (d, J = 3.6 Hz), 7.76
observed in the H and 13C NMR spectra of phenol-free 2,4,5-
1
trichlorophenolate complex 6. The only observable difference
between the H and 13C NMR spectra of 14 and those of 6 was
1
the replacement of the broad scrambling 2,4,5-trichlorophenolate
signals in the spectra of 6 with a set of sharp 2,4,5-trichlorophenol-
2,4,5-trichlorophenolate homoconjugate signals in the spectra of
2626 | Dalton Trans., 2010, 39, 2622–2630
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Table 1 CO2/(propylene oxide) copolymerization resultsa
d
Entry
Cat
TOFb
Selectivityc
Mn (¥ 10-3)
Mw/Mn
1e
2e
3f
4
5
6
7
8
9
10
3 (DNP)
11 000
15 000
10 000
0
96
>99
94
140
270
176
1.17
1.26
1.21
4 (DNP ◊ ◊ ◊ H ◊ ◊ ◊ DNP)
6 (2,4,5-Cl3C6H2O)
8 (tBu, 2,4,5-Cl3C6H2O)
9 (2,4,6-Cl3C6H2O)
10 (4-NO2C6H4O)
12 (2,4-Cl2C6H3O)
14 (2,4,5-Cl3C6H2O ◊ ◊ ◊ H ◊ ◊ ◊ X)
15 (4-NO2C6H4O ◊ ◊ ◊ H ◊ ◊ ◊ X)
16 (2,4-Cl2C6H3O ◊ ◊ ◊ H ◊ ◊ ◊ X)
0
8800
8300
11 000
16 000
13 000
96
94
96
98
97
374
253
310
273
182
1.27
1.17
1.16
1.26
1.15
a Polymerization condition: PO (10 g, 170 mmol), [PO]/[Cat] = 100 000, CO2 (2.0–1.7 MPa), 70–75 ◦C, 60 min. b Calculated based on the weight of the
isolated polymer including the cyclic carbonate. c Selectivity of polycarbonate over cyclic carbonate in units of % as determined by 1H NMR spectroscopy
of the crude product. d Determined on GPC using a polystyrene standard. e Data from reference 11. f Induction time of 140 min was observed.
1
14. The same trend was evident in the H and 13C NMR spectra
of 15 and 16.
but the anion exchange reaction with an excess (five equiva-
lents) of sodium 2,4,5-trichlorophenolate, 4-nitrophenolate, or
2,4-dichlorophenolate occurred in CH2Cl2 to give the desired
complexes (6, 10, and 12, respectively). In each complex, the
number of 2,4,5-trichlorophenolate, 2,4-dichlorophenolate, or 4-
nitrophenolate per cobalt is four. In case of 4-nitrophenolate, sub-
stitution reaction stops at the stage of four 4-nitrophenolate/Co.
In case of 2,4,5-trichlorophenolate or 2,4-dichlorophenolate, the
excessively substituted anions over four anions/Co react with
CH2Cl2. This observation is consistent with the formation of an
imine uncoordinated octahedral complex, which is coordinated
by four anions and two salen-phenoxys, in the anion exchange
reactions in CH2Cl2 (Chart 1).
CO2/propylene oxide (PO) copolymerization studies
Complexes 8 and 9, which adopt the conventional imine co-
ordinated structure, did not show any activity for CO2/PO
copolymerization at conditions of [PO]/[Cat] = 100 000, 70–75 ◦C,
and PCO = 17–20 bar (entries 4 and 5 in Table 1), whereas the other
2
complexes adopted an unusual imine uncoordinated structure
exhibit excellent activities (TOF, 8300–16 000 h-1). Complexes
bearing homoconjugate anions 14, 15, or 16 were found to be more
active than the corresponding phenol-free complexes 6, 10, or
12. In the case of the 4-nitrophenolate and 2,4-dichlorophenolate
complexes, the activity difference between the homoconjugate
complex (15 or 16) and its corresponding phenol-free complex (10,
or 12) was pronounced with activities 1.6–1.8 times higher being
observed for the homoconjugate complexes. In case of phenol-free
complex 6, an induction time was observed (entry 3). Among the
complexes, the 4-nitrophenol-4-nitrophenolate homoconjugate
complex (15) displayed the highest activity (TOF, 16 000 h-1,
entry 9), which was slightly higher than that attained with
2,4-dinitrophenol-2,4-dinitrophenolate homoconjugate complex 4
(entry 2). A viscous polymerization solution that was incapable of
being stirred was obtained by running the polymerization for 1.0 h
with 15 (ESI†). All the complexes screened in this study displayed
selectivity for formation of polycarbonate over cyclic carbonate
above 90%. Complex 15, which displayed the highest activity, also
showed the highest selectivity (98%). High Mn’s (>180 000) were
attained due to the high activities. The molecular weight of the
polymer obtained with the most highly active catalyst 15 was
satisfactorily high (Mn, 273 000, entry 9).
The 1H and 13C NMR spectra of the obtained cobalt(III)
complexes of 2,4,5-trichlorophenolate, 2,4-dichlorophenolate, and
4-nitrophenolate are in agreement with an unusual imine un-
-
coordinated structure. The two salen-phenoxys and two BF4 -
replacing anions persistently coordinate to cobalt(III) to form a
-
square planar cobaltate structure. The other two BF4 -replacing
anions scramble through coordination and decoordination to the
axial sites of the square plane. The signals of the scrambling
1
anion are broad in the H and 13C NMR spectra while those of
persistently coordinated ones are sharp. Another form of 2,4,5-
trichlorophenolate, 4-nitrophenolate, and 2,4-dichlorophenolate
complexes (14, 15, and 16, respectively) could also be pre-
pared, in which the scrambling anions in 6, 10, and 12 are re-
placed with the corresponding phenol-phenolate homoconjugates,
respectively.
Substitution of methyls with bulky tert-butyl on the ortho-
position of the salen-phenoxys blocks formation of the unusual
imine uncoordinated structure, affording a conventional imine
coordinated complex (8 in eqn (3)). The anion exchange reaction
occurs with sodium 2,4,6-trichlorophenolate, but its coordinated
power is not enough to replace the imine-coordination, conse-
quently affording the conventional imine-coordinated structure 9.
The signal pattern in the 1H NMR spectrum of complexes 8 and
9 adopting the conventional imine coordinated structure is totally
different from that observed for the complexes of an unusual imine
uncoordinated structure (Fig. 1 versus 2).
Summary and discussion
-
Attempts for BF4 -exchange reaction of a cobalt(III) complex
of salen-type ligand tethered by four quaternary ammonium
BF4 salts with caboxylate anion such as acetate, hexanoate,
benzoate, or trifluoroacetate anion were not presently successful,
-
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The Royal Society of Chemistry 2010
Dalton Trans., 2010, 39, 2622–2630 | 2627
©

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=
Complexes 8 and 9 of the conventional imine-coordinated
(s, 2H, N CH), 7.16 (s, 2H, salen), 7.14 (s, 2H, scrambling X),
6.96 (s, 2H, salen), 6.79 (br, 2H, scrambling X), 6.56 (s, 2H,
coordinated X), 4.27 (s, 2H, cyclohexyl-CH), 3.31–2.83 (br, 32H,
NCH2), 2.72 (s, 6H, CH3), 2.14 (s, 2H, cyclohexyl), 1.97 (s, 2H,
cyclohexyl), 1.87–1.14 (m, 74H, CH2), 0.89 (t, J = 7.2 Hz, 36H,
structure do not show any activity in CO2/PO copolymerization
◦
at the condition of [PO]/[Cat] = 100 000, 70–75 C, and PCO
=
2
17–20 bar. However, 2,4,5-trichlorophenolate, 4-nitrophenolate,
and 2,4-dichlorophenolate complexes (6, 10, and 12, respectively),
which adopt an unusual imine uncoordinated structure, and their
homoconjugate analogues (14, 15, and 16, respectively), show
excellent activities for CO2/PO copolymerization (TOF, 8300–
16 000 h-1). In all cases, the complex containing the homo-
conjugate [X ◊ ◊ ◊ H ◊ ◊ ◊ X]- shows higher activities than the corre-
sponding phenol-free complex. Among the prepared complexes,
4-nitrophenol-4-nitrophenolate homoconjugate complex 15
shows the best performance (TOF, 16 000 h-1; selectivity, 98%;
Mn, 273 000).
CH3) ppm. 13C{ H} NMR (THF-d8): d 164.93, 164.18 (scrambling
1
X), 163.39, 160.54, 132.28, 131.45, 130.67, 130.49, 129.80, 129.52,
128.34 (scrambling X), 127.53 (scrambling X), 125.81, 124.61,
123.27 (scrambling X), 120.61 (scrambling X), 118.43, 112.76,
111.98 (scrambling X), 70.19, 59.89, 59.06, 40.42, 39.65, 30.94,
30.81, 26.50, 24.63, 20.70, 19.05, 18.12, 14.31 ppm. Anal. Calcd.
(C110H168Cl13CoN6O6): C, 60.32; H, 7.73; N, 3.84%. Found: C,
60.71; H, 8.22; N, 4.12%.
2,4,5-Trichlorophenol is expensive. 2,4-Dichlorophenol is inex-
pensive but is classified as a highly toxic and possible carcinogenic
chemical. Furthermore, chlorinated aromatic compounds are
typically resistant in environmental degradation. 4-Nitrophenol
or its anion is neither explosive nor expensive. Furthermore,
bioaccumulation of this compound rarely occurs. So, the at-
tachment of 4-nitrophenol unit in each polymer chain end by
employing 4-nitrophenolate complex as a catalyst is not a prob-
lem. Consequently, 4-nitrophenol-4-nitrophenolate homoconju-
gate complex 15 is the best choice for replacement of explosive
2,4-dinitrophenolate complex 4 in the view of activity, cost, and
environmental impact.
Complex 8
Cobalt(II) acetate (39.4 mg, 0.223 mmol) and tert-butyl salen
ligand (401 mg, 0.223 mmol) were dissolved in ethanol inside
a glove box. The resulting red solution was stirred for 3.5 h at
room temperature. The solvent was removed under vacuum to
give a red solid that was subsequently triturated twice in diethyl
ether to remove the acetic acid that had been generated. The
solid (50 mg, 0.028 mmol) was dissolved in CH2Cl2 containing
2,4,5-trichlorophenol (5.6 mg, 0.028 mmol), and the solution
was stirred under an O2 atmosphere for 4 days. Sodium 2,4,5-
trichlorophenolate (42.6 mg, 0.141 mmol) was added. After the
solution was stirred overnight at room temperature, it was filtered
over Celite. The solvent was removed under vacuum to give a
brown powder.
Experimental
General remarks
Complex 9
All manipulations were performed under an inert atmosphere
using standard glove box and Schlenk techniques. THF and diethyl
ether were distilled from benzophenone ketyl. Ethanol was dried
as previously described using sodium and diethyl phthalate.25
CH2Cl2, and CDCl3 were dried by stirring over CaH2, and were
subsequently vacuum-transferred to reservoirs. CO2 gas (99.999%
This compound was synthesized using the same conditions and
procedure as those for 6 with 2,4,6-trichlorophenol and its sodium
salt instead of 2,4,5-trichlorophenol and its sodium salt. M.p.
◦
1
=
82 C. H NMR (dmso-d6): d 7.99 (s, 2H, N CH), 7.47 (s, 2H,
salen), 7.32 (s, 2H, salen), 6.92 (s, 10H, uncoordinated X), 3.56
(s, 2H, cyclohexyl-CH), 3.26–2.91(br, 32H, NCH2), 2.63 (s, 6H,
CH3), 2.01 (s, 2H, cyclohexyl), 1.81 (s, 2H, cyclohexyl), 1.72–1.16
˚
purity) was dried by storing in a column of molecular sieves 3 A
at a pressure of 30 bar. Propylene oxide was dried by stirring
over CaH2 for several days and was vacuum-transferred to a
1
(m, 74H, CH2), 0.88 (t, J = 7.2 Hz, 36H, CH3) ppm. 13C{ H}
NMR (dmso-d6): d 163.92, 160.85, 159.18, 132.37, 131.76, 130.11,
129.18, 126.14, 123.38, 117.22, 108.01, 69.41, 58.02, 57.43, 55.89,
55.79, 29.42, 24.37, 23.95, 23.02, 21.54, 21.38, 19.16, 17.37, 16.77,
13.44, 11.02, 10.68 ppm. Anal. Calcd. (C116H170Cl15CoN6O7): C,
59.25; H, 7.29; N, 3.57%. Found: C, 59.59; H, 7.68; N, 3.95%.
1
reservoir. H NMR (400 MHz), 13C NMR (100 MHz), and 19F
(376 MHz) NMR spectra were recorded on a Varian Mercury Plus
400 apparatus. The 19F NMR spectra were calibrated and reported
downfield from external a,a,a-trifluorotoluene. Gel permeation
chromatograms (GPC) were obtained at room temperature in
CHCl3 using a Waters Millennium apparatus with polystyrene
standards.
Complex 10
This compound was synthesized using the same conditions and
procedure as those for 6 with 4-nitrophenol and its sodium salt
instead of 2,4,5-trichlorophenol and its sodium salt. M.p. 75 ◦C.
1H NMR (dmso-d6): d 7.86 (s, 4H, scrambling X), 7.77 (s, 2H,
Complex 6
Cobalt(II) complex
5 (200 mg, 0.120 mmol) and 2,4,5-
=
trichlorophenol (24 mg, 0.12 mmol) were dissolved in CH2Cl2
inside a glove box. The resulting solution was stirred for 4 days
under an O2 atmosphere. After sodium 2,4,5-trichlorophenolate
(182 mg, 0.600 mmol) was added, the reaction mixture was stirred
overnight at room temperature. The solution was filtered over
Celite, and the solvent was removed under vacuum to give a brown
powder, which was washed with diethyl ether several times. M.p.
N CH), 7.48 (s, 4H, coordinated X), 7.10 (s, 2H, salen), 7.02
(s, 6H, coordinated X, salen), 6.32 (s, 4H, scrambling X), 3.97
(s, 2H, cyclohexyl-CH), 3.15–2.85 (br, 32H, NCH2), 2.61 (s, 2H,
cyclohexyl), 2.09 (s, 2H, cyclohexyl), 1.89 (s, 6H, CH3), 1.60–0.92
1
(br, 74H, CH2), 0.83 (t, J = 7.2 Hz, 36H, CH3) ppm. 13C{ H}
NMR (dmso-d6): d 177.08, 173.56 (scrambling X), 162.61, 160.35,
133.19 (scrambling X), 132.17, 130.90, 129.96, 129.62, 127.95,
126.59 (scrambling X), 123.68, 119.88, 117.47 (scrambling X),
◦
1
85 C. H NMR (THF-d8): d 8.41 (s, 2H, coordinated X), 7.75
2628 | Dalton Trans., 2010, 39, 2622–2630
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©

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116.34, 68.82, 57.99, 57.39, 29.46, 24.62, 23.77, 22.93, 19.11, 17.72,
16.71, 13.35 ppm. 19F NMR (dmso-d6): d -50.56, -50.62 ppm.
Anal. Calcd. (C110H176BCoF4N10O14): C, 65.78; H, 8.83; N, 6.97%.
Found: C, 65.29; H, 8.54; N, 6.41%.
d -50.64, -50.70 ppm. Anal. Calcd. (C122H174BCl18CoF4N6O8): C,
55.58; H, 6.65; N, 3.19%. Found: C, 55.12; H, 6.33; N, 3.03%.
Complex 15
This compound was synthesized using the same conditions and
procedure as those for 10 with a mixture of 3 equivalents of
sodium 4-nitrophenolate and 2 equivalents of 4-nitrorophenol
instead of 5 equivalents of pure sodium 4-nitrorophenolate. M.p.
Complex 12
Cobalt(III) complex 11 and sodium 2,4-dichlorophenolate were
dissolved in CH2Cl2 inside a glove box. After the resulting solution
was stirred overnight at room temperature, it was filtered over
Celite. The solvent was removed under vacuum to give a brown
powder which was washed several times with diethyl ether. M.p.
78 ◦C. 1H NMR (dmso-d6): d 8.60 (d, J = 3.6 Hz, 1H, DNP), 7.89
(d, J = 8.8 Hz, 2H, coordinated X), 7.77 (dd, J = 9.6, 3.2 Hz,
◦
1
75 C. H NMR (dmso-d6): d 7.96 (s, 8H, homoconjugate), 7.77
=
(s, 2H, N CH), 7.49 (s, 4H, coordinated X), 7.28–6.84 (br, 8H,
coordinated X, salen), 6.56 (s, 8H, homocomjugate), 3.97 (s, 2H,
cyclohexyl-CH), 3.25–2.74 (br, 32H, NCH2), 2.61 (s, 6H, CH3),
2.09 (s, 2H, cyclohexyl), 1.89 (s, 2H, cyclohexyl), 1.68–1.02 (m,
1
74H, CH2), 0.83 (t, J = 7.2 Hz, 36H, CH3) ppm. 13C{ H} NMR
=
1H, DNP), 7.64 (s, 2H, N CH), 7.10 (s, 2H, scrambling X), 7.05
(dmso-d6): d 177.06, 176.08, 170.63 (homoconjugate), 162.63,
161.84, 160.30, 135.33 (homoconjugate), 132.19, 130.87, 130.02,
129.62, 127.88, 126.38 (homoconjugate), 123.70, 119.87, 116.74
(homoconjugate), 116.31, 68.79, 57.95, 57.37, 29.48, 24.59, 23.56,
22.95, 19.12, 17.77, 16.70, 13.38 ppm. 19F NMR (dmso-d6): d
-50.63, -50.69 ppm. Anal. Calcd. (C122H186BCoF4N12O20): C,
64.08; H, 8.20; N, 7.35%. Found: C, 64.20; H, 8.22; N, 7.72%.
(s, 2H, salen), 6.94 (s, 2H, salen), 6.78 (s, 2H, scrambling X), 6.58
(dd, J = 8.8, 2.8 Hz, 2H, coordinated X), 6.51 (d, J = 2.4 Hz,
2H, coordinated X), 6.31 (d, J = 10 Hz, 1H, DNP), 6.25 (s, 2H,
scrambling X), 4.16 (s, 2H, cyclohexyl-CH), 3.24–2.79 (br, 32H,
NCH2), 2.53 (s, 6H, CH3), 2.09 (s, 2H, cyclohexyl), 1.83 (s, 2H,
cyclohexyl), 1.68–1.06 (m, 74H, CH2), 0.86 (t, J = 7.2 Hz, 36H,
CH3) ppm. 13C{ H} NMR (dmso-d6): d 169.99 (DNP), 163.24,
1
162.23 (scrambling X), 160.91, 158.93, 135.76 (DNP), 129.91,
129.66, 129.11, 127.38 (DNP), 127.26 (DNP), 126.98, 126.79,
126.63, 126.25, 125.43 (DNP), 125.25, 124.78 (DNP), 124.67,
123.52, 119.39 (scrambling X), 116.84, 113.22, 111.49 (scrambling
X) ppm. Anal. Calcd. (C116H175Cl8CoN8O11): C, 63.32; H, 8.02; N,
5.09%. Found: C, 62.86; H, 7.72; N, 4.92%.
Complex 16
This compound was synthesized using the same conditions and
procedure as those for 12 with a mixture of 4 equivalents of sodium
2,4-dichlorophenolate and 2 equivalents of 2,4-dichlorophenol in-
stead of 5 equivalents of pure sodium 2,4-dichlorophenolate. M.p.
83 ◦C. 1H NMR (dmso-d6): d 8.59 (d, J = 3.2 Hz, 1H, DNP), 7.89
(d, J = 8.4 Hz, 2H, coordinated X), 7.77 (dd, J = 9.6, 3.2 Hz,
Compound 13
=
1H, DNP), 7.64 (s, 2H, N CH), 7.19 (s, 4H, homoconjugate),
7.70 (s, 2H, salen), 6.60–6.94 (m, 6H, homoconjugate, salen), 6.71
(d, J = 8.4 Hz, 4H, homoconjugate), 6.58 (d, J = 8.4 Hz, 2H,
coordinated X), 6.51 (d, J = 1.6 Hz, 2H, coordinated X), 6.31 (d,
J = 9.6 Hz, 1H, DNP), 4.16 (s, 2H, cyclohexyl-CH), 3.19-2.83 (br,
32H, NCH2), 2.53 (s, 6H, CH3), 2.09 (s, 2H, cyclohexyl), 1.84 (s,
2H, cyclohexyl), 1.68–1.06 (m, 74H, CH2), 0.86 (t, J = 7.2 Hz,
The extracted ether solution in the preparation of 12 was collected,
and the solvent was removed under vacuum. It was purified by
column chromatography on silica gel eluting with hexene and ethyl
acetate (v/v, 10 : 1). 1H NMR (CDCl3): d 7.40 (d, 4J = 2.4 Hz, 2H,
m-H), 7.26 (d, J = 8.8 Hz, 2H, o-H), 7.22 (dd, J = 8.8, 2.4 Hz,
1
2H, m-H), 5.78 (s, 2H, O-CH2-O) ppm. 13C{ H} NMR (CDCl3): d
1
36H, CH3) ppm. 13C{ H} NMR (dmso-d6): d 170.00, 163.25,
151.08, 130.33, 130.24, 128.00, 124.81, 117.79, 92.26 ppm. HRMS
(EI): m/z calcd for ([M]+ C13H8Cl4O2) 335.9278, found 335.9277.
160.91, 158.69, 158.54 (homoconjugate), 129.87, 129.69, 129.14,
127.90 (homoconjugate), 127.20, 126.97,126.88 (homoconjugate),
126.21, 125.44, 124.65, 122.58, 122.10 (homoconjugate), 118.56
(homoconjugate), 116.84, 116.46 (homoconjugate), 113.24, 68.49,
66.81, 57.98, 57.37, 54.67, 29.42, 25.02, 23.52, 22.90, 19.02, 17.65,
16.66, 13.26 ppm. Anal. Calcd. (C128H183Cl12CoN8O13): C, 60.86;
H, 7.30; N, 4.44%. Found: C, 61.20; H, 7.22; N, 4.82%.
Complex 14
This compound was synthesized using the same conditions
and procedure as those for 6 with a mixture of 3 equivalents
of sodium 2,4,5-trichlorophenolate and 2 equivalents of 2,4,5-
trichlorophenol instead of 5 equivalents of pure sodium 2,4,6-
trichlorophenolate. M.p. 85 ◦C. 1H NMR (THF-d8): d 8.38 (s, 2H,
Acknowledgements
=
coordinated X), 7.71 (s, 2H, N CH), 7.28 (s, 4H, homocoujugate),
This work was supported by the Korea Science and Engineering
Foundation (KOSEF) grant funded by the Korea government
(MEST) (No. 2009-0079207) and by Priority Research Centers
Program through the National Research Foundation of Korea
(NRF) funded by the Ministry of Education, Science and Tech-
nology (2009-0093826).
7.06 (s, 2H, salen), 7.03 (s, 2H, coordinated X), 6.94 (s, 2H, salen),
6.56 (s, 4H, homoconjugate), 4.24 (s, 2H, cyclohexyl-CH), 3.38–
2.83 (br, 32H, NCH2), 2.72 (s, 6H, CH3), 2.13 (s, 2H, cyclohexyl),
1.96 (s, 2H, cyclohexyl), 1.69–1.07 (m, 74H, CH2), 0.90 (t, J =
1
7.6 Hz, 18H, CH3), 0.89 (t, J = 7.2 Hz, 18H, CH3) ppm. 13C{ H}
NMR (THF-d8): d 164.99, 163.26, 160.36 (homoconjugate),
132.41, 131.28, 130.93 (homoconjugate), 130.46, 129.52, 127.98
(homoconjugate), 127.53, 125.83, 124.56, 122.37 (homoconju-
gate), 119.64 (homoconjugate), 118.37, 116.54 (homoconjugate),
112.87, 100.14, 59.71, 59.01, 40.41, 40.22, 39.95, 30.89, 26.44,
24.81, 24.59, 20.67, 19.06, 18.02, 14.28 ppm. 19F NMR (dmso-d6):
Notes and references
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