Elucidation of the structure of a highly active catalytic system for CO2/epoxide copolymerization: A salen-cobaltate complex of an unusual binding mode

Article abstract of DOI:10.1021/ic901584u

Salen-type ligands comprised of ethylenediamine or 1,2-cyclohexenediamine, along with an salicylaldehyde bearing a methyl substituent on its 3-position and a -[CR(CH₂CH₂CH₂N⁺Bu ₃)₂] (R = H

Full text of DOI:10.1021/ic901584u

Inorg. Chem. 2009, 48, 10455–10465 10455
DOI: 10.1021/ic901584u
Elucidation of the Structure of a Highly Active Catalytic System for CO2/Epoxide
Copolymerization: A salen-Cobaltate Complex of an Unusual Binding Mode
Sung Jae Na,† Sujith S,† Anish Cyriac,† Bo Eun Kim,† Jina Yoo,† Youn K. Kang,‡ Su Jung Han,§ Chongmok Lee,§
and Bun Yeoul Lee*,†
†Department of Molecular Science and Technology, Ajou University, Suwon 443-749 Korea, ‡Division of
Chemistry and Molecular Engineering, Department of Chemistry, College of Natural Sciences, Seoul National
§
University, Seoul 151-747 Korea, and Department of Chemistry and Nano Science, Ewha Womans University,
Seoul 120-750, Korea
Received August 8, 2009
Salen-type ligands comprised of ethylenediamine or 1,2-cyclohexenediamine, along with an salicylaldehyde bearing a
methyl substituent on its 3-position and a -[CR(CH2CH2CH2N+Bu3)2] (R = H or Me) on its 5-position, unexpec-
tedly afford cobalt(III) complexes with uncoordinated imines. In these complexes, two salen-phenoxys and two
2,4-dinitrophenolates (DNPs), which counter the quaternary ammonium cations, coordinate persistently with cobalt,
while two other DNPs are fluxional between a coordinated and an uncoordinated state in THF at room temperature.
The complexes of this binding mode show excellent activities in carbon dioxide/propylene oxide copolymerization
(TOF, 8 300-13 000 h-1) but with some fluctuation in induction times (1-10 h), depending on how dry the system is.
The induction time is shortened (<1.0 h) and activity is increased ∼1.5 times upon the replacement of the two fluxional
DNPs with 2,4-dinitrophenol-2,4-dinitrophenolate homoconjugation ([DNP
H
DNP]-). Imposing steric con-
3 3 3 3 3 3
gestion either by replacing the methyl substituent on the salicylaldehyde with tert-butyl or by employing H2NCMe2C-
Me2NH2 instead of ethylenediamine or 1,2-cyclohexenediamine results in conventional imine-coordinating complexes,
which show lower activities than uncoordinated imine complexes.
Introduction
of the resulting polymers.3 However, conventional binary
systems of [(salen)Co4 or (salen)Cr complex5]/(onium salt or
base), in which the two components are not bound, suffer low
catalytic performance at low catalyst concentrations and/or
high polymerization temperatures. Complex 3, which was
prepared through a routine method using a salen-type ligand
1 bearing small methyl substituents and four quaternary
ammonium salt units (eq 1), shows a turnover frequency
(TOF) exceeding 10 000 h-1 and produces a strictly alternat-
ing copolymer with a high molecular weight (Mn) of up to
The carbon dioxide/propylene oxide (CO2/PO) copolymer
has attracted much interest due to its favorable properties.1
The copolymer, which consists of alternating CO2 and PO
subunits, is composed of 44% CO2 by weight, making the
copolymer economical to prepare due to the abundance and
the cheapness of CO2 gas. The copolymer burns gently in air
without emitting toxic materials, decomposes at the relatively
low temperature of approximately 250 °C without producing
an ash residue, and adheres strongly to a cellulosic substrate.
We recently reported a highly active catalyst (3 in eq 1) with
the potential to be commercially applied.2 The success of this
would require the binding of two components or two metal
centers in proximity, regardless of low catalyst concentration
or high polymerization temperature, resulting in a high
turnover number (TON) and a high molecular weight (Mn)
(3) (a) Kember, M. R.; Knight, P. D.; Reung, P. T. R.; Williams, C. K.
Angew. Chem., Int. Ed. 2009, 48, 931. (b) Noh, E. K.; Na, S. J.; S, S.; Kim, S.-W.;
Lee, B. Y. J. Am. Chem. Soc. 2007, 129, 8082. (c) Lee, B. Y.; Kwon, H. Y.; Lee,
S. Y.; Na, S. J.; Han, S.-i.; Yun, H.; Lee, H.; Park, Y.-W. J. Am. Chem. Soc. 2005,
127, 3031. (d) Bok, T.; Yun, H.; Lee, B. Y. Inorg. Chem. 2006, 45, 4228. (e) Xiao,
Y.; Wang, Z.; Ding, K. Chem.;Eur. J. 2005, 11, 3668. (f) Piesik, D. F.-J.; Range,
S.; Harder, S. Organometallics 2008, 27, 6178. (g) Cui, D.; Nishiura, M.; Tardif,
O.; Hou, Z. Organometallics 2008, 27, 2428.
(4) (a) Cohen, C. T.; Chu, T.; Coates, G. W. J. Am. Chem. Soc. 2005, 127,
10869. (b) Qin, Z.; Thomas, C. M.; Lee, S.; Coates, G. W. Angew. Chem., Int. Ed.
2003, 42, 5484. (c) Paddock, R. L.; Nguyen, S. T. Macromolecules 2005, 38,
6251. (d) Sugimoto, H.; Kuroda, K. Macromolecules 2008, 41, 312.
(5) (a) Darensbourg, D. J.; Phelps, A. L.; Gall, N. L.; Jia, L. Acc. Chem.
Res. 2004, 37, 836. (b) Darensbourg, D. J.; Moncada, A. I. Inorg. Chem. 2008,
47, 10000. (c) Darensbourg, D. J.; Mackiewicz, R. M. J. Am. Chem. Soc. 2005,
127, 14026. (d) Darensbourg, D. J.; Bottarelli, P.; Andreatta, J. R. Macromole-
cules 2007, 40, 7727.
*To whom correspondence should be addressed. E-mail: bunyeoul@
ajou.ac.kr.
(1) (a) Luinstra, G. A. Polym. Rev. 2008, 48, 192. (b) Darensbourg, D. J.
Chem. Rev. 2007, 107, 2388. (c) Coates, G. W.; Moore, D. R. Angew. Chem., Int.
Ed. 2004, 43, 6618. (d) Inoue, S.; Koinuma, H.; Tsuruta, T. J. Polym. Sci. Part B:
Polym. Lett 1969, 7, 287. (e) Inoue, S.; Koinuma, H.; Tsuruta, T. Makromol.
Chem. 1969, 130, 210.
(2) S, S.; Min, J. K.; Seong, J. E.; Na, S. J.; Lee, B. Y. Angew. Chem., Int.
Ed. 2008, 47, 7306.
r
2009 American Chemical Society
Published on Web 09/25/2009
pubs.acs.org/IC

10456 Inorganic Chemistry, Vol. 48, No. 21, 2009
Na et al.
Figure 1. The1HNMRspectraof7 and 8inDMSO-d6. Thesignals markedwith“X” areassignedto2,4-dinitrophenolate. Thebox showsaselected region
of the 1H-1H COSY NMR spectrum of 7 at 20 °C.
300 000 and a high selectivity (>99%). Another advantage of
3 is that the catalyst can be efficiently removed after polym-
erization 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 for copolymerization not only
because catalyst residue colors the resin but also because it
causes toxicity as well as severe depolymerization during
thermal processing and storage.6
and broad NMR signals of 3 promoted us to investigate
thoroughly the structure of the highly active catalyst 3. We
reveal in this work that 3 adopts an unusual uncoordinated
imine binding mode.
The structure of 3 was identified as that of a conventional
tetradentate salen-cobalt(III) complex. The 1H and13CNMR
signals in DMSO-d6 at room temperature are very complex.
However, they are simplified to a set of broad signals at 40 °C
and become sharper after raising the temperature to 80 °C,
thereby allowing assignment to a conventional salen-cobalt-
(III) complex. Complex 4 is prepared from ligand 2, in which
the methyl substituent in 1 is replaced with tert-butyl. Un-
expectedly, 4 exhibits a low activity ∼1/20 of that of 3 (TOF,
1300 h-1) along with a lower selectivity (84%). This difference
in performance is contradictory to other reports. It was pre-
viously reported that a bulky tert-butyl group is required to
attain high activity in the case of the binary system [salen-
Co(III) complex]/[quaternary ammonium salt].7 salen-cobalt
complexes have been applied as versatile catalysts in various
asymmetric syntheses, such as the hydrolytic kinetic resolution
(HKR),8 the nitro-aldol reaction,9 the alkene hydrocyana-
tion,10 and the resolution of racemic N-benzyl R-amino acids.11
In all these reactions, the utilized complexes were construc-
ted from salicylaldehyde having a tert-butyl substituent on its
3-position. Complexes containing a methyl substituent instead
of a tert-butyl have rarely been reported.11 The large disparity
in activity difference between 3 and 4 as well as the complicated
Results
NMR Studies. We prepared ethylenediamine-derived
analogues using the same method and conditions as were
used for the preparation of 3 and 4, starting from the
corresponding salen ligands 5 and 6 (eq 2). Since 15N
nuclei-labeled ethylenediamine is commercially available,
15N NMR studies as well as H and 13C NMR studies
1
could be performed. The 1H, 13C, and 15N NMR spectra
of 7 and 8 in DMSO-d6 are significantly different from
each other (Figures 1-3). For 8, which bears a tert-butyl
1
substituent, the H NMR signals are normal; a set of
sharp aromatic signals that belong to a salen unit is
observed along with broad 2,4-dinitrophenolate (DNP)
signals. The integration ratio of the number of DNP per
salen unit is also as expected, ∼5.0. The 13C NMR
spectrum at 25 °C is also normal and belongs to the
structure of a conventional tetradentate salen-cobalt-
(III) complex. The 15N NMR signal at 25 °C is observed
at -163.43 ppm as a sharp singlet.
(6) Hongfa, C.; Tian, J.; Andreatta, J.; Darensbourg, D. J.; Bergbreiter,
D. E. Chem. Commun. 2008, 975.
(7) (a) Lu, X.-B.; Shi, L.; Wang, Y.-M.; Zhang, R.; Zhang, Y.-J.; Peng,
X.-J.; Zhang, Z.-C.; Li, B. J. Am. Chem. Soc. 2006, 128, 1664. (b) Cohen, C. T.;
Thomas, C. M.; Peretti, K. L.; Lobkovsky, E. B.; Coates, G. W. Dalton Trans.
2006, 237.
(8) (a) Zheng, X.; Jones, C. W.; Weck, M. J. Am. Chem. Soc. 2007, 129,
1105. (b) Kunaga, M. T.; Larrow, J. F.; Kakiuchi, F.; Jacobsen, E. N. Science 1997,
277, 936. (c) Schaus, S. E.; Brandes, B. D.; Larrow, J. F.; Tokunaga, M.; Hansen,
K. B.; Gould, A. E.; Furrow, M. E.; Jacobsen, E. N. J. Am. Chem. Soc. 2002, 124,
1307.
(9) Park, J.; Lang, K.; Abboud, K. A.; Hong, S. J. Am. Chem. Soc. 2008,
130, 16484.
(10) (a) Gaspar, B.; Carreira, E. M. Angew. Chem., Int. Ed. 2007, 46, 4519.
(b) Gaspar, B.; Carreira, E. M. Angew. Chem., Int. Ed. 2008, 47, 5758.
(11) Dzygiel, P.; Reeve, T. B.; Piarulli, U.; Krupicka, M.; Tvaroska, I.;
Gennari, C. Eur. J. Org. Chem. 2008, 1253.
On the contrary, the 1H NMRsignal pattern(DMSO-d6)
of 7 at room temperature is abnormal, broad, and very

Article
Inorganic Chemistry, Vol. 48, No. 21, 2009 10457
Figure 2. The 13C NMR spectra of 7 and 8 in DMSO-d6.
Figure 3. The 15N NMR spectra of 7 and 8 in DMSO-d6.
complicated, which differs significantly from that of 8.
Raising the temperature to 40 °C collapses some signals
into a fairly assignable spectrum, despite some remaining
quite broad. Raising the temperature to 80 °C even further
develops a set of sharp signals. The integration ratio of the
number of DNP per salen unit is not close to ∼5.0 but is to
∼4.0. The 13C NMR spectrum at 25 °C is also too com-
plicated to be simply assigned a conventional tetradentate
salen-cobalt(III) structure. At 40 °C, the pattern becomes
simplified, but still somevery broad signals are present. The
15N NMR spectrum is also strikingly different from that of
8. Two moderately sharp signals are observed at -156.32
and -159.21 ppm, which are shifted downfield from the
chemical shift of the tert-butyl complex (-163.43 ppm) at
room temperature. Raising the temperature to 40 °C
collapses the two signals into a broad single signal, which
then becomes sharp after the temperature is raised to 80°C.
The 1H NMR spectra of 7 and 8 in THF-d8 or CD2Cl2
In the 15N NMR spectrum of 8 in THF-d8, a sharp singlet
signal is observed at -166.80 ppm. A sharp singlet signal is
also observed but significantly shifted downfield at -154.32
ppm in the 15N NMR spectrum of 7. This difference in
chemicalshift (Δppm = 12.5) is too big to be explainedwith
only a substituent effect. It was reported that the 15N NMR
chemical shifts of imine (-NdC-C4H4-X) and hydra-
zone (N-NdC-C4H4-X) fit well to the Hammett-type
equation with a slope of F = ∼10.12 Considering the σ
values of methyl and tert-butyl substituents (σm = -0.06,
σp = -0.14, and σ+ = -0.31 for methyl; σm = -0.09,
σp = -0.15, and σ+ = -0.26 for tert-butyl), the estimated
chemical shift difference (Δppm) should not be so large. In
fact, the chemical shift difference observed in the ligand
stage is only 2.86 ppm (-63.68 ppm for 5; -66.54 ppm for
6). The reported substituent effect on the 15N NMR
chemical shifts is also not as large for dipyrrolmethenes
and their Zn(II) complexes, as evidenced by only 2.0 ppm
difference when hydrogen was replaced with ethyl in the
pyrrol ring of the Zn(II) complex.13 Therefore, the large
chemical shift difference observed in the 15N NMR studies
(Δppm = 12.5), along with the significant differences
between the 1H and 13C NMR spectra, leads us to propose
that 7 adopts a binding mode that differs from the typical
tetradentate salen-Co(III) structure of 8. The 15N NMR
signal of 7 in THF-d8 becomes significantly broadened after
the temperature is lowered to -75 °C (full width at half-
maximum (fwhm), ∼10 ppm), while that of 8 is persistently
1
also have very different patterns (Figure 4). For the H
NMR spectrum of 8 in THF-d8, a set of salen signals is
observed along with very broad DNP signals. Some
very broad signals are observed at an abnormal region
(-2-0 ppm) duetothe presence ofa paramagneticspecies.
For the 1H NMR spectrum of 7 in THF-d8, a set of salen
signals is observed, but the chemical shifts of the aromatic
salen protons (6.90, 7.05, and 7.93 ppm) and that of the
NCH2CH2N unit (4.51ppm) are fairlydifferentfrom those
observed for the tert-butyl complex 8 (7.12, 7.31, 7.74, and
4.18 ppm, respectively). Interestingly, the DNP signals are
broad at 7.88, 8.01, and 8.59 ppm, and the integration ratio
:: ::
1
(12) Neuvonen, K.; Fulop, F.; Neuvonen, H.; Koch, A.; Kleinpeter, E.;
of number of DNP per salen unit is ∼2.0. The H NMR
Pihlaja, K. J. Org. Chem. 2003, 68, 2151.
(13) Wood, T. E.; Berno, B.; Beshara, C. S.; Thompson, A. J. Org. Chem.
2006, 71, 2964.
spectra of 7 and 8 in CD2Cl2 have features that are almost
identical with those observed in THF-d8 (Figure 4).

10458 Inorganic Chemistry, Vol. 48, No. 21, 2009
Na et al.
Figure 4. The 1H NMR spectra of 7 and 8 in THF-d8 and CD2Cl2.
sharp (fwhm, ∼1.5 ppm at -75 °C). These variable tem-
perature 15N NMR studies suggest that 7 adopts some
flexible structure.
The IR spectra of 7 and 8 are also significantly different
from each other, especially in the -NO2 symmetric
vibration region that is located between 1 200 and
1 400 cm-1 (Figure 5). In the spectrum of the methyl
complex 7, a very broad band at the region seems to be an
overlap of several sets of signals. In the spectrum of the
tert-butyl complex 8, there are signals that are similar to
those of sodium 2,4-dinitrophenolate, but are slightly
shifted to a lower frequency (main signal from 1 350 to
1 315 cm-1).
1
Proposal for Uncoordinated Imine Structure. The H,
13C, and 15N NMR spectra of the tert-butyl complex 8 can
be assigned to a structure of the conventional tetradentate
salen-cobalt(III) complex (Chart 1). One equivalent of
NaBF4 is incorporated during the salt exchange reaction,
which is supported by the observation of typical BF4
signals (two broad signals at -50.65 and -50.70 ppm) in
the 19F NMR spectrum and by the detection of an
equivalent amount of Na+ ions in the ICP-AES. DNP
proton signals in DMSO-d6 are broad and even broader
in THF-d6 and CD2Cl2, indicating DNP ligands are
fluxional between a coordinated and an uncoordinated
state. A paramagnetic, five-coordinating, square-pyrami-
dal structure is a mediator in this fluxional motion,
thereby causing the incidence of abnormal signals at
-2-0 ppm.14
Figure 5. The IR spectra of 7, 8, sodium 2,4-dinitrophenolate, and
tetrabutylammonium 2,4-dinitrophenolate.
ammonium cations, coordinate to cobalt instead of imine
donors. We performed the anion exchange reaction of
BF4- with DNPs by stirring the BF4- complex overnight
in slurry of 5 equivalents (per cobalt) of NaDNP in
CH2Cl2. The integration ratio of the number of DNP per
salen unit is ∼4.0, which is persistent even under conditions
of more excess NaDNP (10 equivalents) and longer reac-
tion times (2 days). That is, one of the four BF4- remains
present and unexchanged with DNP. We also observe a
typical BF4 signal in the 19F NMR spectrum of 7. The ICP-
AES study of 7 indicates a negligible amount of Na+ ion
contained in 7.
We propose that 7 adopts an unusual uncoordinated
imine structure (7-A in Chart 1) as opposed to a conven-
tionalone, 7-B. DNP anions, whichcounterthe quaternary
Complex 9 is obtained from a similar salen-type ligand
containing six quaternary ammonium salt units (Chart 1).
In complex 9, only three of the six BF4 anions are
(14) (a) Konig, E.; Kremer, S.; Schnakig, R.; Kanellakopulos, B. Chem.
::
ꢀ
Phys. 1978, 34, 379. (b) Kemper, S.; Hrobarik, P.; Kaupp, M.; Schlorer, N. E.
J. Am. Chem. Soc. 2009, 131, 4172.

Article
Inorganic Chemistry, Vol. 48, No. 21, 2009 10459
Chart 1. Structures of salen-Co(III) complexes by variation of ligand structures (X = 2,4-dinitrophenolate)
exchanged with DNPs, even though salt exchange is
performed using 10 equivalents of NaDNP. Formation
of an octahedral complex with a formal -3 charge on
cobalt, coordinated by four DNPs and two salen-phe-
noxys, may drive the anion exchange reaction. The
cobalt(III) ion is classified as a hard acid, preferring to
be coordinating with hard oxygen donors such as DNPs
as opposed to soft imine bases. For 8, the bulky tert-butyl
group blocks formation of such an octahedral structure,
yielding a conventional imine-coordinating complex.
Octahedral cobaltate(III) complexes with a formal -3
charge on cobalt are not rare.15
NMR signal behaviors of the highly active catalyst 3,
which was previously assigned a conventional imine-
coordinating structure, are almost identical to those of 7
(Supporting Information). Therefore, we here reassign 3
as having an uncoordinated imine structure. Recently, we
developed an efficient synthetic route for 10 in which the
-CH(CH2CH2CH2N+Bu3)2 units in 3 are replaced with
-CMe(CH2CH2CH2N+Bu3)2.16 Complex 10 is easily
prepared in 10s-gram scale compared to that of the
relatively difficult preparation of 3. The NMR signal
behaviors of 10 and hexaammonium complex 9 are
similar to those of 7.
structure. Complex 11 is prepared from a salen-type ligand
constructed from H2N-CMe2CMe2-NH2 and salicylal-
dehyde containing a methyl substituent on its 3-position.17
Interestingly, 11 shows the same features in its H NMR
1
spectrum as does 8: a sharp aromatic signal as well as a
paramagnetic signal at -2-0 ppm in DMSO-d6. There-
fore, complex 11 should be assigned a conventional imine-
coordinating structure.
DFT Calculations. We performed DFT calculations to
compare the energy level of the proposed uncoordinated
imine structure 7-A to that of its isomeric form 7-B, a
conventional imine-coordinating structure. In structure
7-A, two salen-phenoxys are fixed to be coordinating in
cis configuration with cobalt along with four DNPs at other
sites of the octahedron. In structure 7-B, the conventional
salen-Co(III) architecture forms an octahedron with two
DNPs on the axial sites. Two additional DNP anions as
-
well as a BF4 are around in the outer sphere as counter
anions. The tributylammonium units are abbreviated as
trimethylammonium units in order to reduce the calcula-
tion time. According to computational calculations, the
structure 7-A is more stable than the structure 7-B by ca.
34 kcal/mol. This amount of energy difference is substan-
tial; the structure of the complex prepared from the methyl
ligand 5 by eq 2 can, thus, be reasonably assigned to 7-A.
Figure 6 shows the energy-minimized conformations of the
structure 7-A and -B obtained by the DFT calculations.
Status of DNP Ligands. In methylene chloride, the
medium of the final anion exchange reaction, broad
The NMR signal behaviors of 4, which exhibits a sig-
nificantly lower activity than 3, are almost identical to those
of 8, which exhibits a conventional imine-coordinating
(15) (a) Yagi, T.; Hanai, H.; Komorita, T.; Suzuki, T.; Kaizaki, S.
J. Chem. Soc., Dalton Trans. 2002, 1126. (b) Fujita, M.; Gillards, R. D.
Polyhedron 1988, 7, 2731.
(16) Min, J.; Seong, J. E.; Na, S. J.; Cyriac, A.; Lee, B. Y. Bull. Korean
Chem. Soc. 2009, 30, 745.
(17) Boettcher, A.; Elias, H.; Jaeger, E. G.; Langfelderova, H.; Mazur,
M.; Mueller, L.; Paulus, H.; Pelikan, P.; Rudolph, M.; Valko, M. Inorg.
Chem. 1993, 32, 4131.

10460 Inorganic Chemistry, Vol. 48, No. 21, 2009
Na et al.
Figure 6. The most stable conformation of the structures of 7-A and -B calculated by DFT calculations (-CH(CH2CH2CH2N+Bu3)2 units are deleted for
simplification).
Figure 7. Cartoons showing the status of DNP ligands at room temperature when solvent and ligand structure are varied (X = 2,4-dinitrophenolate).
DNP signals are observed at 8.4, 8.1, and 7.9 ppm in the
1H NMR spectra of 7, 9, and 10, which adopt an
uncoordinated imine structure. The integration ratio of
number of DNP per salen unit is ∼2.0, which is half of
that observed in DMSO-d6 (Figure 4). This means that
only two of the four DNPs are detected in the 1H NMR
spectra, whereas the other two are missing possibly due to
severe broadening. For complex 3, all four DNP signals
are observed at the same region. Since the chemical shifts
of the observed DNP signals are significantly different
from those of [Bu4N]+[DNP]- (8.77, 7.85, and 6.48 ppm),
they can be assigned to coordinating DNPs. These NMR
data indicate that all four DNPs of 3 stay in a persistently
coordinated state in CH2Cl2. However, for 7, 9, and 10,
two of the four DNPs persistently stay in a coordinated
state, while the other two are fluxional between a coordi-
nated and an uncoordinated state. Fluxional movement
occurs in NMR time scales and with almost unbiased
probability, making average signals unobservably broad.
Figure 7 shows cartoons describing the status of DNP
ligands when the solvent and the ligand structures are
varied.
DNP signal, and at 6.5 ppm isolated from other signals.
Since these chemical shifts are close to those of
[Bu4N]+DNP- (8.68, 7.78, and 6.38 ppm in THF-d8),
we assign them to a mostly uncoordinated, fluxional
DNP signal. When the temperature is raised to 70 °C,
all four DNP signals become severely broadened as a
single set of signals at 9.3, 9.0, and 7.8 ppm. Figure 8
1
shows the variable temperature H NMR spectra with
cartoons that describe the status of DNPs at each tem-
perature. As the temperature increases, DNPs more
tightly interact with the cobalt center. Since uncoordi-
nated DNPs are prone to being solvated, the decoordi-
nating process is, therefore, entropy sacrificing and more
favorable at low temperatures. Lowering the temperature
causes a similar equilibrium shift from contact ion pairs
toward solvent separated ion pairs in other systems.18
Variation of the DNP signal pattern in the variable
1
temperature H NMR studies of 8 in thf-d8 is different
from that observed for 7 (Supporting Information). Low-
ering the temperature to 0 °C severely broadens the DNP
signals such that almost all become missing. At -25 °C, a
set of relatively sharp DNP signals is observed at 8.1, 7.6,
and 6.8 ppm with an integration ratio of the number of
DNP per salen unit being ∼2.0. Another set of very broad
DNP signals are present at 8.9, 8.0, and 6.8 ppm whose
chemical shifts are close to those of uncoordinated DNP in
7 (8.7, 8.0, and 6.8 ppm). At -50 °C, the very broad signals
are sharpened and revealed clearly two sets of DNP
signals. We assign one set of these (8.1, 7.6, and 6.8 ppm)
to DNPs coordinating at the axial sites of a conventional
tetradentate salen-structure, whereas the other set (8.9, 8.0,
and 6.8 ppm) is assigned to an uncoordinated one.
1
At room temperature, the H NMR spectrum of 7 in
THF-d8 contains only two coordinating DNPs, which are
detected as broad signals at 8.6, 8.1, and 7.9 ppm
(Figure 8). The other two DNPs are missing due to signal
broadening. When the temperature is lowered to 0 °C, the
DNP signals become sharpened and display a coupling
pattern. By correlating the coupling pattern and the
analysis of the 1H-1H COSY spectrum (Supporting
Information), the coordinating DNP signals are able to
be unambiguously assigned (Figure 8). When the tem-
perature is lowered further to -25 °C, a new set of broad
DNP signals, which are marked as “*” in Figure 8, is
observed at 8.8 ppm as a shoulder of the coordinating
DNP signal, at 7.9 ppm collapsed with the coordinating
(18) (a) Streitwieser, A., Jr.; Chang, C. J.; Hollyhead, W. B.; Murdoch, J.
R. J. Am. Chem. Soc. 1972, 94, 5288. (b) Hogen-Esch, T. E.; Smid, J. J. Am.
::
Chem. Soc. 1966, 88, 307. (c) Lu, J.-M.; Rosokha, S. V.; Lindeman, S. V.;
Neretin, I. S.; Kochi, J. K. J. Am. Chem. Soc. 2005, 127, 1797.

Article
Inorganic Chemistry, Vol. 48, No. 21, 2009 10461
Figure 8. Variable temperature 1H NMR spectra of 7 in THF-d8.
1
along with the splitting of aromatic signals in the H
The status of the DNPs in THF varies depending upon
ligand structure. In the 1H NMR spectrum of 7, a set of
coordinating DNP signals is observed with an integration
value of ∼2 for the number of DNP per salen unit. The
other two DNPs are missing, indicating they are fluxional
between a coordinated and an uncoordinated state with
almost unbiased probabilities. On the contrary, all the
four DNPs are observed in 3, 9 and 10. Specifically, two
sets of DNP signals are clearly present at an almost 1:1
ratio in the 1H NMR spectrum of 3 at room temperature.
One set (8.7, 8.2, and 7.8 ppm) is assigned to the coordi-
nating DNPs, while the other set (8.8, 7.8, and 6.4 ppm) is
assigned to fluxional, mostly uncoordinated DNPs. In the
1H NMR spectra of 9 and 10, two sets of DNP signals are
also observed. However, the fluxional DNP signals are
much broader, indicating those in 9 and 10 stay a little
more in a coordinated state than those in 5. Therefore,
the order of binding affinity of the two fluxional DNPs is
7 > 9 and 10 > 3.
and 13C NMR spectra (Figures 1 and 2). The signals for
NCH2CH2N in the 1H NMR spectrum are observed split
1
at 4.3, 4.15, and 4.1 ppm in a 1:1:2 ratio. The H-1H
COSY NMR spectrum proves that the three signals arise
from a NCH2CH2N unit, not from a mixture of three
different conformations or configurations (Figure 1). In
the energy-minimized conformation of 7-A, flip of the
cobalt octahedron from a side of the macrocycle to the
other side is not allowed (Figure 6) under which condi-
tion the cobalt square plane formed by a DMSO, a DNP,
and two salen-phenoxys has a planar chirality. Similar
planar chirality in cyclophane-type imidazole was re-
cently reported.19 Upon generation of a chiral center,
two protons attached to a N-CH2 unit become diaster-
eotopic to each other, making it possible for them to be
observed separately. For complexes such as 3 and 10,
which contain chiral centers in their salen-type ligand,
planar chirality in the cobalt octahedron results in
1
At 40 °C in DMSO-d6, the chemical shifts of DNP (8.6,
7.8, and 6.4 ppm) in the 1H NMR spectra of 3, 7, 9, and 10
are similar to those of [Bu4N]+DNP- (8.58, 7.80, and
6.35 ppm in DMSO-d6). However, the signals of DNP are
broad, while those of [Bu4N]+DNP- are very sharp. At
room temperature, another set of DNP signals, which can
be assignable to a coordinating DNP, is observed at 8.5,
8.1, and 7.8 ppm with roughly one-third the intensity of the
main uncoordinated DNP signals (Figure 1 and Support-
ing Information). Therefore, as shown in Figure 7, one
DNP stays in a coordinated state, while the other three
DNPs stay mostly in uncoordinated state when in DMSO
at room temperature.
generation of two diastereomers, which make the H
and 13C NMR signals more complicated (see Supporting
Information).
Raising the temperature to 40 °C collapses the coordi-
nating DNP signals into a set of very broad signals. In this
situation, the asymmetrical coordination environment is
broken, and hence, a set of simplified salen-ligand signals
is observed. In THF and CH2Cl2, the coordination
environment surrounding cobalt is symmetrical even at
room temperature (Figure 7), and hence, a set of sharp
1
salen signals is observed in the H, 13C, and 15N NMR
spectra.
Cyclic Voltammetric Studies. Cyclic voltammetric (CV)
studies also suggest that 3 and 4 adopt different binding
modes. If 3 and 4 had the same binding mode, then a more
facile reduction of methyl complex 3 would be expected
because methyl is less electron-donating than tert-butyl.
The opposite result is observed in the CV studies: reduc-
tion of tert-butyl complex 4 is easier than that of methyl
complex 3. The E1/2 values for Co(III/II) are -0.076 and
1
Analysis of the NMR Spectra in DMSO-d6. The H,
13C, and 15N NMR spectra of 7 in DMSO-d6 at room
temperature are very complicated but explainable by the
proposed structure of 7-A as well as the status of DNPs.
In Figure 7, the octahedral cobalt is coordinated by three
DMSOs, one DNP, and two salen-phenoxys. In this
situation, the two phenoxys in a salen unit are inequiva-
lent; one is situated in trans with DMSO, while the other is
in trans with DNP. Hence, we observe two signals in the
15N NMR spectrum at room temperature (Figure 3)
(19) Ishida, Y.; Iwasa, E.; Matsuoka, Y.; Miyauchi, H.; Saigo, K. Chem.
Commun. 2009, 3401.

10462 Inorganic Chemistry, Vol. 48, No. 21, 2009
Na et al.
-0.013 V versus SCE for 3 and 4, respectively. The
observed potential difference (ΔE1/2 for Co(III/II)) of
63 mV is not trivial considering that a potential difference
of 59 mV means the ratio of [Co(II)] to [Co(III)] differs by
10-fold at the same electrochemical potential, which is
inferred from the Nernst equation (E = Eo + (0.0592)
log{[ox]/[red]}).
negligible, and the integration ratio of [spiro-Meisenhei-
mer anion]/[DNP] is 1.1 at 2 h.
Complexes 12 and 13 (chart 1) contain five BF4 anions
but no DNP anions. We can expect the same binding
mode of a conventional imine-coordinating structure,
especially in a noncoordinating solvent such as CH2Cl2.
CV studies in CH2Cl2 find the same E1/2 value for
Co(III/II) for both complexes at 0.63 V versus SCE.
This result implies that changing the methyl substituent
with tert-butyl has a negligible influence on the E1/2
value for Co(III/II) once the complexes adopt the same
binding mode. By changing the solvent from CH2Cl2 to
DMSO, the E1/2 values for Co(III/II) of 12 and 13
become differentiated at -0.074 and -0.011 V versus
SCE, respectively. These values are almost identical with
those observed for 3 and 4 in DMSO (-0.076 and -0.013
V versus SCE, respectively). DMSO is a good ligand
especially for a hard Co(III) center. Therefore, a con-
ventional imine-coordinating binding mode switches to
an uncoordinated imine binding mode in DMSO, where
cobalt is coordinated by four DMSOs and two sale-
n-phenoxys. A single sharp signal is observed at
-159.11 ppm (DMSO-d6) in the 15N NMR spectrum
of the complex containing five BF4 anions, which is
prepared from a 15N-labeled ligand 5. This observed
chemical shift is close to that of 7 (-158.35 ppm at 40 °C)
but not to that of 8 (-163.43 ppm), which is evidence of
four coordinated DMSOs and an uncoordinated imine
binding mode in DMSO.
While the conversion rate is not altered by the addition
of 5 equivalents of water per cobalt, it is significantly
reduced by the addition of more water. In the presence
of 25 and 50 equivalents of water, the integration ratio
of [spiro-Meisenheimer anion]/[DNP] is reduced from
1.0 to 0.68 and 0.47, respectively, at 1 h. In the presence
of 50 equivalent of water, the [spiro-Meisenheimer anion]/
[DNP] ratios are only 0.53 and 0.63 at reaction times
of 2 and 4 h, respectively, not reaching a 1.0 value
(Figure 9).
The reaction pattern between PO and 8 is different from
that between PO and uncoordinated imine complex 10.
The same spiro-Meisenheimer anion is generated, but its
amount gradually increases and exceeds the stage of
[spiro-Meisenheimer anion]/[DNP] = 1.0. The [spiro-
Meisenheimer anion]/[DNP] ratio is 0.96 (1 h), 1.4 (2 h),
1.8 (7 h), and 2.0 (20 h). Another striking difference is that
a significant amount of paramagnetic species (1H NMR
signals at -1 - 0.5 ppm) is formed, which amount
gradually increases by the time.
Initiation Reaction. Nothing is changed in the 1H
NMR spectrum of 10 when CO2 gas is added to the
THF-d6 solution. This implies that DNPs in 10 do not
attack CO2. However, 10 reacts with PO, increasing new
signals that are marked with “*” in Figure 9. These new
signals are assigned to an anion illustrated in structure
14 (eq 2). Attack of PO by the DNP anion generates
an alkoxide anion, which then intramolecularly attacks
the ipso-carbon. A salt containing this type of anion
has been previously reported and termed the spiro-
Meisenheimer complex.20 Due to the stereochemistry
of the methyl group, a diastereomerism is created
that produces two sets of signals. The salen-aromatic
signals observed at 7.0-7.4 ppm are very complicated
but not due to destruction of the salen unit. Upon
addition of acetic acid, the salen aromatic signals be-
come a simple set of broad signals, whereas and the
spiro-Meisenheimer anion becomes protonated to form
a mixture of 2,4-(NO2)2C6H3-OCH2CH(Me)OH and
2,4-(NO2)2C6H3-OCH(Me)CH2OH (eq 3). Conversion
of DNPs into the spiro-Meisenheimer anion is stopped
when the integration values of DNP (6.3 ppm) and the
spiro-Meisenheimer anion (6.8 ppm) signals are almost
equal (∼1 h). Following this, the conversion rate is
CO2/Propylene Oxide Copolymerization Studies. Com-
plexes 6, 8, and 11 show poor catalytic performance
(entries 2, 4, and 7 in Table 1), whereas complexes 3, 7,
and 10 are excellent at polymerization conditions of [PO]/
[Cat] = 1 Â 105, temperature = 70-75 °C, and P
=
CO2
17-20 bar (entries 1, 3, and 6). Hexaammonium salt
complex 9 does not produce any polymer at these condi-
tions even though its coordination mode is identical to 3,
7, and 10 (entry 5). The order of activities (TOF) is 3 >
10 > 7, which, interestingly, is opposite to the order
observed for the binding affinity of the two loosely
bound, fluxional DNPs with cobalt in THF.
Complex 10 is easily prepared in 10s-gram scale allow-
ing feasibility in a commercial application.16 In extensive
polymerization studies with 10, severe fluctuation of
induction time (1-10 h) has been observed especially in
hot and humid weather. After initiation, rather consistent
TOFs are attained in the range of 9 000-11 000 h-1
.
Typically, bimodal molecular weight distributions are
observed for all copolymers obtained in this study, even
though the Mw/Mn values are only in the range of 1.2-1.3
(Figure 10). The higher molecular weight portion is
attributed to biaxial chain growth from water molecules,
whereas the other lower portion is due to uniaxial chain
(20) (a) Fendler, E. J.; Fendler, J. H.; Byrne, W. E.; Griff, C. E. J. Org.
Chem. 1968, 33, 4141. (b) Bernasconi, C. F.; Cross, H. S. J. Org. Chem. 1974, 39,
1054.

Article
Inorganic Chemistry, Vol. 48, No. 21, 2009 10463
Figure 9. The 1H NMR spectra (DMSO-d6) showing the reaction between the complexes and the propylene oxide (* signals from spiro-Meisenheimer
anion; ∧ signals from DNP anion; # signals from protonated compound of the spiro-Meisenheimer anion).
Table 1. CO2/PO Copolymerization Resultsa
entry
cat
induction time (min)
TOFb
selectivityc
head-to-tail linkage (%)
Mnd ( Â 10-3
)
Mw/Mn
1
2
3
4
5
6
7
8
3
4f
7
8
9
10
11
14
15
15
60e
0
13 000
1 300
8 300
5 000
0
11 000
0
13 000
15 000
16 000
92
84
97
85
80
85
74
82
210
38
113
120
1.26
2.34
1.23
1.41
120e
0
260e
96
87
140
1.17
30g
0g
0
>99
>99
>99
87
84
82
170
270
300
1.21
1.26
1.31
9
10h
a Polymerization condition: PO (10 g, 170 mmol), [PO]/[Cat] = 1 Â 105, CO2 (2.0-1.7 MPa), temperature = 70-75 °C, and reaction time = 60 min.
b Calculated based on the weight of the isolated polymer including the cyclic carbonate. c Selectivity of the polycarbonate over the cyclic carbonate in unit
of % as determined by 1H NMR spectroscopy of the crude product. d Determined on GPC using the a polystyrene standard. e Induction times severely
fluctuate batch to batch in the range of 1-10 h. f Polymerization data in [PO]/[Cat] = 2.5 Â 103 from ref 2. g Induction times of <1 h are observed in many
other batches. h Large scale polymerization using 230 g of PO.
growth from DNPs.21 An interesting trend is observed in
the GPC traces: the longer the induction time, the lower
the molecular weight (Mn) and the larger the portion
of polymer chains that are grown from water (Figure 10
and Table 1). This feature indicates that the long induc-
tion time is related to water. Diffusion of water into
a glovebox through gloves and a polycarbonate panel
are substantial in a hot and humid climate.22 Failure in
maintaining water levels in the glovebox, in which the
reactor is assembled, may be a cause of a fluctuating
induction time.
A slow rate of initiation (DNP attack onto PO) in the
presence of water is also found in the NMR studies
(Figure 9). Complex 14 (Chart 2) is prepared by reacting
(21) Nakano, K.; Kamada, T.; Nozaki, K. Angew. Chem., Int. Ed. 2006,
45, 7274.
(22) Shriver, D. F.; , Drezdzon, M. A. The Manipulation of Air-Sensitive
Compounds 2nd ed.; John Wiley & Sons: Singapore, 1986; pp 52-54.

10464 Inorganic Chemistry, Vol. 48, No. 21, 2009
Na et al.
Chart 2. Salen-Co(III) Complex Showing an Excellent Catalytic Per-
formance (X = 2,4-Dinitrophenolate)
∼1.5 times higher TOFs (15 000-16 000 h-1) in small
scale (several g-polymer scale) as well as in large scale
polymerization (70 g-polymer scale) (entries 9 and 10).
In the 1H NMR studies of the reaction between 15 and
PO, a set of signals indicating a protonated version of the
spiro-Meisenheimer anion 2,4-(NO2)2C6H3-OCH2CH-
(Me)OH and 2,4-(NO2)2C6H3-OCH(Me)CH2OH is
observed at the initial stage (10 min). After 30 min, signals
representing both a spiro-Meisenheimer anion and its
protonated version are observed. After 1 h, the integra-
tion ratio of [spiro-Meisenheimer anion + its protonated
compound]/[DNP] is 1.6. At 2 h, the ratio reaches ∼2.0.
At a reaction time of 4 h, the ratio remains at a value of
∼2.0. Since 2,4-dinitrophenol does not react with PO
at room temperature, we presume that 2,4-(NO2)2C6H3-
OCH2CH(Me)OH and 2,4-(NO2)2C6H3-OCH(Me)-
CH2OH are generated through the protonation of the
spiro-Meisenheimer anion or the alkoxide. The rate of PO
attack is not altered in the presence of 50 equivalents of
water. This contrasts with the rate of 10, which is sig-
nificantly reduced in the presence of 50 equivalents of
water. In fact, 10 does not generate 14 even after a 4 h
reaction (Figure 9).
Figure 10. The GPC traces of polymers.
a high concentration of 10 with/in PO for 1 h such that the
ratio of [residual water in PO]/[10] is negligible. Complex
14 is not as sensitive as 10 and is more consistent in its
polymerization for induction times below 1 h (entry 8).
Dinitrophenol-Dinitrophenolate Homoconjuation as
the Initiating Anion. Better and consistent catalytic per-
formances are obtained when 60 mol % NaDNP contain-
ing 40 mol % 2,4-dinitrophenol (DNP-H) is employed at
the final anion exchange reaction in Equation 1. A student
mistakenly added commercial 60% NaH instead of oil-
washed 100% NaH during the preparation of five equiva-
lents of pure NaDNP from DNP-H.23 Therefore, approxi-
mately three equivalents of NaDNP plus two equivalents
of DNP-H were added to the anion exchange reaction. In
the 1H NMR spectrum of the complex prepared using the
60 mol % NaDNP, a set of broad DNP-related signals is
observed in DMSO-d6 with a [DNP + DNP-H]/[salen]
signal ratio of ∼6. We presume that a complex, where two
2,4-dinitrophenolate-2,4-dinitrophenol homoconjuga-
tions ([DNP
H
DNP]-) are loosely bound on cobalt,
Summary and Discussion
3 3 3 3 3 3
was created by the mistake (15 in chart 2). It is well-
established that a hydrogen bond between a proton donor
and an acceptor whose ΔpKa = 0 is extraordinarily strong,
especially in an aprotic solvent.24 The distance between the
oxygens in the solid structure of (18-crown-6-κ6O) potas-
sium2,4-dinitrophenolate-2,4-dinitrophenol homoconju-
Cobalt(III) complexes, prepared from salen-type ligands
bearing four quaternary ammonium salt units, can adopt an
unusual binding mode that differs from that of a conven-
tional salen-Co(III) complex. Imine donors in salen-type
ligand do not coordinate with cobalt. Instead, DNPs, which
counter the quaternary ammonium cation, coordinate with
cobalt creating a cobaltate complex with a formal negative
˚
gation is very short (2.453(4) A), supporting a very strong
hydrogen bond.25 The formation constant of homoconju-
1
charge on cobalt. This is supported by H, 13C, and 15N
gation for [DNP
H
DNP]- was determined electro-
3 3 3 3 3 3
NMR spectroscopy, IR spectroscopy, DFT calculations, and
cyclic voltammetric (CV) studies. Adoption of such an
unusual binding mode is allowed when the substituent on
the 3-position of salicylaldehyde in the salen-type ligand is
not bulky (3, 7, and 10 in Chart 1). Salen-type ligands,
prepared from either salicylaldehyde containing a bulky
tert-butyl substituent on the 3-position or H2NCMe2CMe2-
NH2, afford complexes of a conventional imine-coordinated
binding mode (4, 8, and 11 in Chart 1). Unusual uncoordi-
nated imine complexes show extraordinarily high activities,
while those of a conventional imine-coordinated structure
show relatively low or negligible activities in CO2/PO copoly-
merization.
chemically in acetonitrile to be ∼100.26 In the H NMR
spectrum of 15 in THF-d6, two sets of DNP signals are
observed; one set is assigned to a coordinating DNP, while
the other set, whose chemical shifts resemble a mixture of
[Bu4N]+[DNP]- and 2,4-dinitrophenol (1:1) in THF-d8, is
assigned to the homoconjugation (Supporting Infor-
mation).
1
Complex 15 is not so sensitive toward water as 10,
providing consistent polymerization performance with
low induction times (<1 h). Furthermore, it provides
(23) In previous report (ref 2), we mistakenly used 60 mol % NaDNP in
preparation of 5.
The status of DNPs in uncoordinated imine complexes
depends on the solvent, ligand structure, and temperature. In
THF, whose medium effect resembles that of the polymeriza-
tion medium, the two DNPs are situated in a coordinated state,
while the other two are scrambling between a coordinated and
(24) Shan, S.-o.; Loh, S.; Herschlag, D. Science 1996, 272, 97.
(25) (a) Barnes, J. C.; Weakley, T. J. R. Acta Crystallogr. 2003, E59, m160.
(b) Paola, G. P.; Bertolasi, V.; Ferretti, V.; Gilli, V. J. Am. Chem. Sec. 1994, 116,
909.
(26) Magonski, J.; Pawlak, Z.; Jasinski, T. J. Chem. Soc., Faraday Trans.
1993, 89, 119.

Article
Inorganic Chemistry, Vol. 48, No. 21, 2009 10465
a uncoordinated state. It was documented that neutral dia-
magnetic octahedral cobalt(III) complexes remain very inert
in regard to ligand substitution reactions.27 However, for
uncoordinated imine cobaltate complexes, cobalt possesses a
negative charge in order to expel the negatively charged ligand,
which cannot move far from the cobalt center due to ion-ion
interactions with a quaternary ammonium cation. The un-
coordinated negatively charged anion is thermodynamically
unstable and prone to recoordinating to the cobalt center.
Balancing between these two statuses results in scrambling.
Some tetradentate cobaltate(III) complexes bearing a -1
charge on cobalt have been previously reported.28 They are
also prone to interconversion between four-, five-, and six-
coordinate states in the presence of additional neutral or
anionic ligands.29,15a The extraordinarily high activities may
be attributed to the scrambling of the negatively charged
ligand. Equation 4 shows a plausible chain-growing mechan-
ism for CO2/PO copolymerization. In this mechanism, a key
step is the nucleophilic attack of the terminal carbonate anion
on a coordinated epoxide. A facile back-side attack is easily
allowed, if the carbonate anion is able to scramble between a
coordinated and a uncoordinated state.30
times higher activity than 10, which has four DNP anions.
Another advantage of 15 is that it is not so sensitive to water
impurity and, therefore, consistently provides an excellent
polymerization activity (TOF 1.5 Â 103 h-1) at both small
(10 g PO scale) and large scale (230 g PO scale) polymeriza-
tions. Induction times are below 1 h, even when PO is dried
with molecular sieves and a sacrificial drying agent such as
CaH2 is not used.
Complexes with four DNPs such as 3 and 10 show fluctuat-
ing induction times (1-10 h), depending on the dryness of the
polymerization system. It is observed in the 1H NMR studies
with 10 that the rate of DNP attack on PO is diminished by a
certain amount of water. We guess that stabilization of
uncoordinated DNPs by hydrogen bonding with water is
responsible for the decrease in reaction rate. It may be
suggested that the slower rate is due to competitive coordina-
tion of water with the cobalt center, which consequently
hampers the PO coordination. However, water and epoxide
reportedly bind with similar affinity to the cobalt center of a
salen-Co(III) complex.31 The [DNP
H
DNP]- homo-
3 3 3 3 3 3
conjugation is not as hygroscopic as pure DNP-, and hence,
the catalytic performance of 15 is not as highly affected by
water impurity as is 10. Variations in the induction time of 10
are minimized by use of 14, which is prepared by reacting a
high concentration of 10 with/in PO for 1 h, such that the ratio
of [residual water in PO]/[10] is negligible.
Acknowledgment. This work was supported by the
Korea Research Foundation Grant funded by the Korean
Government (MOEHRD, Basic Research Promotion
Fund) (KRF-2008-314- C00186).
The binding affinity of the two fluxional DNPs is sensitive
to the ligand structure (7 > 10 > 3). In addition, it may
influence catalytic activity in that the more loosely DNP binds
cobalt, the higher the activity. A more loosely bound status
Supporting Information Available: Synthetic details and
characterization data for all new compounds, polymerization
procedures, and NMR spectra. This material is available free of
charge via the Internet at http://pubs.acs.org.
can be attained by employing the [DNP
H
DNP]-
3 3 3 3 3 3
homoconjugation instead of the fluxional DNPs, as in 15
(Chart 2). Complex 15 has two coordinated DNPs and two
loosely bound [DNP
H
DNP] anions, providing ∼1.5
(30) For (salen)Cr-based binary system, it was proposed that the chain
propagation step proceeds via a concerted insertion of epoxide into a metal-
bound carbonate, but in another DFT study, a bimolecular process in which
a metal-bound carbonate attacks a metal-bound epoxide was suggested. See:
(a) Darensbourg, D. J.; Yarbrough, J. C. J. Am. Chem. Soc. 2002, 124, 6335.
(b) Luinstra, G. A.; Haas, G. R.; Molnar, F.; Bernhart, V.; Eberhardt, R.; Rieger, B.
Chem.;Eur. J. 2005, 11, 6298.
3 3 3 3 3 3
(27) Becker, C. A. L.; Motladiile, S. Synth. React. Inorg. Met.-Org. Chem.
2001, 31, 1545.
(28) (a) Collins, T. J.; Richmond, T. G.; Santarsiero, B. D.; Treco, B. G.
R. T. J. Am. Chem. Soc. 1986, 108, 2088. (b) Gray, H. B.; Billig, E. J. Am.
Chem. Soc. 1963, 85, 2019.
(29) Langford, C. H.; Billig, E.; Shupack, S. I.; Gray, H. B. J. Am. Chem.
Soc. 1964, 86, 2958.
(31) Nielsen, L. P. C.; Stevenson, C. P.; Blackmond, D. G.; Jacobsen, E.
N. J. Am. Chem. Soc. 2004, 126, 1360.