Copolymerization of cyclohexene oxide and carbon dioxide using (salen)Co(iii) complexes: Synthesis and characterization of syndiotactic poly(cyclohexene carbonate)

Article abstract of DOI:10.1039/b513107c

Synthetic routes to a series of new (salen)CoX (salen = N,N′-bis(salicylidene)-1,2-diaminoalkane; X = halide or carboxylate) species are described and the X-ray crystal structures of two (salen-1)CoX (salen-1 = N,N′-bis(3,5-di-tert-butylsalicylidene)-1,2-

Full text of DOI:10.1039/b513107c

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Copolymerization of cyclohexene oxide and carbon dioxide using
(salen)Co(III) complexes: synthesis and characterization of
syndiotactic poly(cyclohexene carbonate)†
Claire T. Cohen, Christophe M. Thomas, Kathryn L. Peretti, Emil B. Lobkovsky and Geoffrey W. Coates*
Received 15th September 2005, Accepted 2nd November 2005
First published as an Advance Article on the web 24th November 2005
DOI: 10.1039/b513107c
Synthetic routes to a series of new (salen)CoX (salen = N,Nꢀ-bis(salicylidene)-1,2-diaminoalkane;
X = halide or carboxylate) species are described and the X-ray crystal structures of two (salen-1)CoX
(salen-1 = N,Nꢀ-bis(3,5-di-tert-butylsalicylidene)-1,2-diaminocyclohexane; X = Cl, I) complexes are
presented. (R,R)-(salen-1)CoX (X = Cl, Br, I, OAc, pentafluorobenzoate (OBzF5)) catalysts are active
for the copolymerization of cyclohexene oxide (CHO) and CO2, yielding syndiotactic poly(cyclohexene
carbonate) (PCHC), a previously unreported PCHC microstructure. Variation of the salen ligand and
reaction conditions, as well as the inclusion of [PPN]Cl ([PPN]Cl = bis(triphenylphosphine)iminium
chloride) cocatalysts, has dramatic effects on the polymerization rate and the resultant PCHC tacticity.
Catalysts rac-(salen-6)CoX (salen-6 = N,Nꢀ-bis(3,5-di-tert-butylsalicylidene)-1,2-diaminopropane; X =
Br, OBzF5) have high activities for CHO/CO2 copolymerization, yielding syndiotactic PCHCs with up
to 81% r-centered tetrads. Using Bernoullian statistical methods, PCHC tetrad and triad sequences were
assigned in the 13C NMR spectra of these polymers in the carbonyl and methylene regions, respectively.
Introduction
The correlation between a polymer’s main-chain stereochemistry
and its physical properties has stimulated many research efforts
in the field of stereospecific polymerization. Ordered polymer
architectures, where sequential stereocenters are of the same
(isotactic) or alternating (syndiotactic) relative configuration
Scheme 1 Copolymerization of epoxides and CO2 to yield polycarbo-
nates.
are generally more crystalline than their atactic counterparts.
using hybrid imine-oxazoline zinc-based catalysts (Fig. 1).11–14,18 In
Although heterogeneous catalysts can be used to control polymer
microstructure in some cases, homogeneous systems are more
each system, isotactic poly(cyclohexene carbonate) (PCHC) with
an enantiomeric excess (ee) ∼70% is obtained. These isotactic
commonly employed for this purpose. Notably, these discrete
PCHCs have been structurally characterized by 13C NMR spec-
polymerization catalysts can be designed at the molecular level,
troscopy using both model compounds and statistical methods.14,19
providing for a tunable active site.
These systems show that chiral ligands allow the generation of
isotactic PCHC, consistent with a site-control copolymerization
The alternating copolymerization of epoxides and CO2 to
generate biodegradable polycarbonates has been a subject of much
mechanism.11–14
interest as CO2 is a readily available, inexpensive, non-toxic and
nonflammable reagent (Scheme 1).1 The first epoxide/CO2 copoly-
merization was achieved by Inoue using ZnEt2/H2O catalysts
in 1969.2 Following this discovery, several other heterogeneous
systems were developed for this transformation; however, catalyst
activities remained low with little control of polymer microstruc-
ture. To improve catalyst performance, recent research has focused
Fig. 1 Catalysts for the isospecific copolymerization of cyclohexene oxide
on single-site epoxide/CO2 copolymerization catalysts.1,3–18 To
and CO2; R1 = Me, CF3, or iPr; R2 = (R)-Ph, (S)-iPr, or (S)-tBu; R3 = Et
date, these homogeneous systems have achieved high activities for
epoxide/CO2 copolymerization and in some cases stereo- and/or
regiospecific monomer enchainment.5,6,11–18
The asymmetric alternating copolymerization of cyclohexene
oxide (CHO) and CO2 has been demonstrated first by Nozaki
using ZnEt2/chiral amino alcohol catalysts and later by our group
or iPr.
Recently we reported that (salen-1)CoX complexes (salen-1 =
N,Nꢀ-bis(3,5-di-tert-butylsalicylidene)-1,2-diaminocyclohexane;
X = Cl, Br, I, OAc, pentafluorobenzoate (OBzF5); Fig. 2) are
highly active catalysts for the copolymerization of propylene oxide
(PO) and CO2 to yield poly(propylene carbonate) (PPC) with
no detectable propylene carbonate byproduct.5,6,17 Using these
catalyst systems, we observed a variety of PPC microstructures, in
which the regio- and stereochemical composition of the resultant
Department of Chemistry and Chemical Biology, Baker Laboratory, Cornell
University, Ithaca, New York, 14853-1301, USA. E-mail: gc39@cornell.edu
† Electronic supplementary information (ESI) available: Crystallographic
details for (salen-1)CoX (X = Cl, I). See DOI: 10.1039/b513107c
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Fig. 2 (salen-1)CoX catalysts for the copolymerization of epoxides and
CO2 (X = Cl, Br, I, acetate, pentafluorobenzoate).
PPC was dependent on the relative stereochemistry of the catalyst
and monomer. Furthermore, through the addition of bulky,
organic ionic cocatalysts to these systems unprecedented activities
were observed, while maintaining excellent regioselectivity.6,17b
Based on our success with (salen-1)CoX catalysts for PO/CO2
copolymerization, we likewise applied these systems to CHO/CO2
copolymerization, with a focus on maximization of catalyst
activity and control of product PCHC microstructure.
Herein, we report the synthesis of a series of (salen)CoX (salen =
N,Nꢀ-bis(salicylidene)-1,2-diaminoalkane; X = halide or carboxy-
late) complexes that are active for CHO/CO2 copolymerization,
and we present the X-ray crystal structures of (R,R)-(salen-1)CoCl
and rac-(salen-1)CoI. In addition, we address the effects of the
salen ligand and catalyst axial ligand/initiator (X) used, the
reaction conditions, and the inclusion of [PPN]Cl ([PPN]Cl =
bis(triphenylphosphine)iminium chloride) cocatalysts on activity
and stereospecificity for the copolymerization. With these cata-
lysts, we obtain PCHC with various stereochemical architectures,
including syndiotactic PCHC, a previously unreported PCHC
microstructure. Using Bernoullian statistical methods, we assign
PCHC tetrad and triad sequences for the 13C NMR spectra of these
polymers in the carbonyl and methylene regions, respectively.
Fig. 3 (salen)CoX catalysts for cyclohexene oxide/CO2 copolymerization
(X = halide or carboxylate; cumyl = CMe2Ph).
Results and discussion
Fig. 4 ORTEP drawing of (R,R)-(salen-1)CoCl (non-hydrogen atoms)
Catalyst synthesis
with thermal ellipsoids drawn at the 40% probability level. Selected
◦
˚
bond lengths (A) and angles ( ): Co(1)–Cl(1) 2.330(2), Co(1)–O(1)
1.845(5), Co(1)–O(2) 1.844(5), Co(1)–N(1) 1.882(6), Co(1)–N(2) 1.887(6);
O(1)–Co(1)–Cl(1) 102.7(2), O(2)–Co(1)–Cl(1) 98.3(2), N(1)–Co(1)–Cl(1)
93.9(2), N(2)–Co(1)–Cl(1) 95.6(2).
Oxidation of commercially available (R,R)-(salen-1)Co readily
occurs in the presence of organic acids to yield the corre-
sponding (R,R)-(salen-1)CoOAc,20 (R,R)-(salen-1)CoOTs (OTs =
p-tolylsulfonate),21 and (R,R)-(salen-1)CoOBzF5.6 (R,R)-(salen-
1)CoOTs can be further modified through metathesis reactions
with the desired NaX salt, affording (R,R)-(salen-1)CoX (X = Cl,
Br, I).6,21 Rac-(salen-1)CoX catalysts were prepared by a 50 : 50
mixture of (R,R)-(salen-1)Co and (S,S)-(salen-1)Co. A variety of
other (salen)CoX complexes were synthesized, starting with an
aldehyde/diamine Schiff-base condensation to generate the de-
sired salen-ligand, followed by metallation with cobalt(II) acetate
tetrahydrate and subsequent oxidation reactions. The complete
series of (salen)CoX complexes used in this study is given in Fig. 3.
Although the majority of the (salen)CoX complexes were obtained
as powders, in limited cases X-ray quality crystals were obtained
from slow evaporation of benzene ((R,R)-(salen-1)CoCl) or by
slow evaporation of a methylene chloride/hexane (1:1) solution
(rac-(salen-1)CoI). The X-ray crystal structures of (R,R)-(salen-
1)CoCl and the (R,R)-(salen-1)CoI component of rac-(salen-
1)CoI with selected bond angles and bond lengths are shown in
Fig. 4 and 5, respectively. In both cases, the (salen-1)CoX complex
is pseudo square-pyramidal with the Co located slightly above the
plane of the salen ligand in the direction of the axial ligand.
Fig. 5 ORTEP drawing of the (R,R)-(salen-1)CoI component of rac-
(salen-1)CoI (non-hydrogen atoms) with thermal ellipsoids drawn at t◦he
˚
40% probability level. Selected bond lengths (A) and bond angles ( ):
Co(1)–I(1), 2.714(1), Co(1)–O(1) 1.828(3), Co(1)–O(2) 1.858(3), Co(1)–
N(1) 1.877(4), Co(1)–N(2) 1.866(4); O(1)–Co(1)–I(1) 100.6(1), O(2)–
Co(1)–I(1) 95.8(1), N(1)–Co(1)–I(1) 92.8(1), N(2)–Co(1)–I(1) 98.2(1).
Alternating copolymerization of cyclohexene oxide and carbon
dioxide with (salen)CoX catalysts. The high PO/CO2 copolymer-
ization activities and selectivities observed with catalysts (R,R)-
(salen-1)CoX (X = Cl, Br, I, OAc, OBzF5) led us to investigate
2 3 8 | Dalton Trans., 2006, 237–249
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these systems for CHO/CO2 copolymerization.5,6 Initially, we
employed high CO2 pressures (54.4 atm) with a [CHO] : [Co]
loading of 500 : 1 using a series of catalyst axial ligands/initiators
(X). To our surprise, the 13C NMR spectra of the resultant
PCHCs were unlike those previously reported. Based on prior
PCHC 13C NMR assignments of the carbonyl region,13,14,19,22 our
PCHCs revealed a strong contribution correlating to r-centered
tetrads, indicative of syndiotactic PCHC (Fig. 6, see following
discussion and Fig. 8). Furthermore, we detected up to 81% r-
centered tetrads in our PCHCs depending on the catalyst’s ligand
structure.22 To the best of our knowledge, these are the first
CHO/CO2 copolymerization catalysts to produce syndiotactic
PCHC.
chain transfer agents. Notably, this was not observed with these
catalyst systems for PO/CO2 copolymerization under comparable
reaction conditions.6 As such, we are currently looking into
the possibilities of chain transfer to monomer or trace water
and/or the formation of cyclic polymer. The syndiotactic PCHCs
we obtain are amorphous, with no observable melting point
transition. In an effort to obtain more crystalline PCHC we are
presently synthesizing syndiotactic PCHCs with higher Mn values.
It has been shown previously that minor alterations of the
salen ligands in their chromium-, cobalt- or aluminium-based
complexes have dramatic effects on epoxide/CO2 copolymeriza-
tion activity.6,8–10,16,17 To explore the relationship of the structural
features of the salen ligand to catalyst performance in our systems,
we synthesized a diverse series of (salen)CoX complexes (Fig. 3) for
use in CHO/CO2 copolymerization. Table 2 lists the CHO/CO2
copolymerization data for a series of (salen)CoBr catalysts, in
which systematic changes in the diimine backbone have been
employed in order to study their impact on polymerization
behavior. In all cases a [CHO] : [Co] loading of 500 : 1 was
applied while using high CO2 pressures (54.4 atm). Catalyst (R,R)-
(salen-1)CoBr produces PCHC with 66% r-centered tetrads and
a TOF of 16 h−1 (entry 1). Alternatively, its racemic analogue
(rac-(salen-1)CoBr) is more syndiospecific, generating PCHC with
78% r-centered tetrads (entry 2). Increasing the steric bulk on
the catalyst backbone ((R,R)-(salen-2)CoBr) reduces the TOF to
2 h−1 and yields PCHC with only 61% r-centered tetrads (entry
3). Complex (salen-3)CoBr, with a phenylene diimine backbone,
has very low activity for the CHO/CO2 copolymerization with
only trace PCHC observed after 20 h (entry 4). Although the
similar complex (salen-3)CoOAc is highly active for isospecific
PO homopolymerization,23 (salen-3)CoBr does not catalyze this
transformation. The CHO/CO2 copolymerization rate is also
reduced compared to that observed with (R,R)-(salen-1)CoBr
when a 1,1-dimethyl substituted backbone is used ((salen-4)CoBr)
(entry 5). However, decreasing the steric bulk with (salen-5)CoBr
or rac-(salen-6)CoBr results in a substantial increase in catalyst
activity while maintaining the high syndiospecificity initially
observed (entries 6 and 7). In fact, complex rac-(salen-6)CoBr
produces PCHC with 81% r-centered tetrads, which is the greatest
syndiotacticity that we have observed in PCHC formation to date.
Interestingly, use of the most active catalysts ((salen-5)CoBr or rac-
(salen-6)CoBr) results in very high conversions even though the
polymerization is run in neat CHO (entries 6 and 7). We suspect
that at 54.4 atm, CO2 plays a role as a solvent and/or lowers
the viscosity of the reaction mixture such that the polymerization
can proceed to high PCHC yields. Finally, based on a shorter
Fig. 6 Carbonyl region of the 13C NMR spectrum (CDCl3, 125 MHz) of
poly(cyclcohexene carbonate) generated using catalyst (R,R)-(salen-1)CoI.
The CHO/CO2 copolymerization data for (R,R)-(salen-1)CoX
(X = Cl, Br, I, OAc, OBzF5) catalysts are listed in Table 1.
While most of the catalyst activities were quite similar (entries
1–3, 5), when acetate was used as an initiating group, catalyst
activity decreases substantially (entry 4). Overall, (R,R)-(salen-
1)CoX catalysts had turnover frequencies (TOF)s ≤16 h−1 for
CHO/CO2 copolymerization. As much higher activities were ob-
served with these catalyst systems for PO/CO2 copolymerization
under comparable reaction conditions,6 we suspect that reaction
rates are hindered by the more sterically bulky, less reactive CHO
monomer. Generally, variation of the axial ligand/initiator had
little influence on the syndiotacticity of the resultant PCHC, how-
ever, when iodide was used, the resultant PCHC was exceptionally
syndiotactic. Furthermore, the role of the iodide axial ligand in
the CHO/CO2 copolymerization is under current investigation.
In all cases, the PCHCs we obtained exhibit Mn values lower
than the corresponding theoretical values despite the absence of
Table 1 Effect of initiator: (R,R)-(salen-1)CoX catalyzed CHO/CO2 copolymerization (X = Cl, Br, I, OAc, OBzF5 (OBzF5 = pentafluorobenzoate))a
Yieldb (%)
TOFc/h−1
Mnd/kg mol−1
Mw/Mn
r-Centered tetradse (%)
d
Entry
Catalyst
1
2
3
4
5
(R,R)-(salen-1)CoCl
(R,R)-(salen-1)CoBr
(R,R)-(salen-1)CoI
(R,R)-(salen-1)CoOAc
(R,R)-(salen-1)CoOBzF5
62
66
51
27
53
15
16
13
7
19.7
20.3
17.3
9.8
1.27
1.23
1.30
1.28
1.30
68
66
75
64
66
13
16.4
a Copolymerizations run in neat cyclohexene oxide (CHO) at 22 ◦C for 20 h at 54.4 atm of CO2 with [CHO] : [Co] = 500 : 1. In all cases, cyclohexene
1
carbonate byproduct is not observed and product poly(cyclohexene carbonate)s (PCHC)s have ≥96% carbonate linkages as determined by H NMR
spectroscopy (CDCl3, 300 MHz). b Based on isolated PCHC yield. c Turnover f◦requency = mol CHO (mol Co)−1 h−1
.
d Determined by gel permeation
chromatography calibrated with polystyrene standards in tetrahydrofuran at 40 C. e Determined by 13C NMR spectroscopy (CDCl3, 125 MHz).
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Table 2 Effect of backbone structure: (salen)CoBr catalyzed CHO/CO2 copolymerizationa
d
Entry
Catalyst
Time/h
Yieldb(%)
TOFc/h−1
Mnd/kg mol−1
Mw/Mn
r-Centered tetradse (%)
1
2
3
4
5
6
7
8
(R,R)-(salen-1)CoBr
rac-(salen-1)CoBr
(R,R)-(salen-2)CoBr
(salen-3)CoBr
20
20
20
20
20
20
20
3
66
64
8
<1
35
90
87
59
16
16
2
NA
9
22
22
98
20.3
23.7
7.7
NA
14.2
26.3
30.3
21.0
1.23
1.22
1.46
NA
1.35
1.24
1.28
1.32
66
78
61
NA
75
75
80
81
(salen-4)CoBr
(salen-5)CoBr
rac-(salen-6)CoBr
rac-(salen-6)CoBr
a Copolymerizations run in neat cyclohexene oxide (CHO) at 22 ◦C with [CHO] : [Co] = 500 : 1 at 54.4 atm of CO2. In all cases, cyclohexene carbonate
byproduct is not observed and product poly(cyclohexene carbonate)s (PCHC)s have ≥96% carbonate linkages as determined by 1H NMR spectroscopy
(CDCl3, 300 MHz). b Based on isolated PCHC yield. c Turnover frequency = mol CHO (mol Co)−1 h−1
.
d Determined by gel permeation chromatography
calibrated with polystyrene standards in tetrahydrofuran at 40 ◦C. e Determined by 13C NMR spectroscopy (CDCl3, 125 MHz).
reaction time, catalyst rac-(salen-6)CoBr exhibited high activity
(TOF = 98 h−1) (entry 8). In general, we found that for (salen)CoBr
catalysts, the highest activity and syndiospecificity were both
achieved by using a racemic ligand backbone with minimal steric
hindrance.
As enantiomerically pure, racemic and achiral (salen)CoX
catalysts are all syndiospecific for CHO/CO2 copolymerization,
we suspect that monomer enchainment proceeds by a chain-end
control mechanism where the stereogenic center from the last
enchained monomer unit influences the stereochemical outcome
of subsequent monomer addition. Notably, this propagation
scheme becomes competitive with an enantiomorphic-site control
pathway in the case of some (salen)CoX catalysts, resulting in a
loss of syndiospecificity. Based on the results presented above,
we believe that an enantiomorphic-site control mechanism is
competitive for (salen)CoX catalysts with a sterically bulky diimine
backbone or cumyl ortho and para phenolate substituents. Also,
in the case of catalyst rac-(salen-8)CoOBzF5, where the steric bulk
of the salen ligand is minimal, all stereochemical direction is lost,
showing that moderate ligand steric bulk is necessary for chain-
end control. Overall, the optimized ligand structure for maximum
catalyst activity and syndiospecificity is achieved with minimal
steric bulk on the salen diimine backbone and tert-butyl moieties
present in the ortho and para positions of the phenolate.
In an effort to investigate the relationship between PCHC
tacticity and the applied CO2 pressure, we proceeded to explore a
series of (salen)CoX catalysts for the CHO/CO2 copolymerization
at low CO2 pressures (6.8 atm) (Table 4). In all cases, catalyst
activity decreased notably and reaction times were increased to
48 h or longer in order to achieve appreciable conversion. In
addition, the CHO/CO2 copolymerizations at low CO2 pressures
generally resulted in a loss of syndiospecificity and decreased
carbonate incorporation in the resultant PCHC. Catalyst (R,R)-
(salen-1)CoBr showed no syndiospecificity yielding atactic PCHC
with only 90% carbonate incorporation (entry 1). Using our
Using the highly active rac-propyldiimine ligand backbone, we
investigated the influence of the phenolate ortho and para sub-
stituents on (salen)CoX catalyzed CHO/CO2 copolymerization
(Table 3). It is notable that the catalyst axial ligand X = OBzF5
was used in this study (instead of X = Br as above) because
the complexes rac-(salen-8)CoBr and rac-(salen-9)CoBr proved
too soluble to purify by washing with pentane. The CHO/CO2
copolymerization using catalyst rac-(salen-6)CoOBzF5 is highly
active with a TOF of 93 h−1 yielding PCHC with 76% r-centered
tetrads (entry 1). These values are similar to those observed with
the analogous X = Br complex (Table 2, entry 8). Increasing the
steric bulk of the phenolate ortho and para substituents using
rac-(salen-7)CoOBzF5 is detrimental, decreasing catalyst activity
to a TOF of 5 h−1 and reducing syndiotacticity to 65% (entry
2). Alternatively, using catalyst rac-(salen-8)CoOBzF5 where the
steric bulk of the ortho and para positions is virtually nonexistent
results in a complete loss in stereospecificity yielding atactic PCHC
(entry 3). In the intermediate cases which incorporating tert-butyl
groups in the ortho positions while varying the para positions of the
phenolate to either H or Br (rac-(salen-9)CoOBzF5 or rac-(salen-
10)CoOBzF5, respectively) catalyst activity and syndiospecificity
are decreased (entries 4 and 5). Largely, the salen ligand with tert-
butyl groups in both the ortho and para positions of the phenolate
moiety provides the ideal steric bulk for use in the CHO/CO2
copolymerizations, giving high activity and syndiospecificity.
Table 3 Effect of phenolate substituents: (salen)CoOBzF5 ([OBzF5] = pentafluorobenzoate) catalyzed CHO/CO2 copolymerizationa
Yieldb (%)
TOFc/h−1
Mnd/kg mol−1
Mw/Mn
r-Centered tetradse(%)
d
Entry
Catalyst
Time/h
1
2
3
4
5f
rac-(salen-6)CoOBzF5
rac-(salen-7)CoOBzF5
rac-(salen-8)CoOBzF5
rac-(salen-9)CoOBzF5
rac-(salen-10)CoOBzF5
3
20
3
3
3
56
22
19
39
30
93
5
32
65
50
22.3
11.7
10.9
14.5
14.6
1.35
1.40
1.22
1.42
1.38
76
65
50
66
75
a Copolymerizations run in neat cyclohexene oxide (CHO) at 22 ◦C with [CHO] : [Co] = 500 : 1 at 54.4 atm of CO2. In all cases, cyclohexene carbonate
byproduct is not observed and product poly(cyclohexene carbonate)s (PCHC)s have ≥ 96% carbonate linkages as determined by 1H NMR spectroscopy
(CDCl3, 300 MHz). b Based on isolated PCHC yield. c Turnove◦r frequency = mol CHO (mol Co)−1 h−1
.
d Determined by gel permeation chromatography
calibrated with polystyrene standards in tetrahydrofuran at 40 C. e Determined by 13C NMR spectroscopy (CDCl3, 125 MHz). f 93% carbonate linkages.
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Table 4 Effect of catalyst structure: (salen)CoX catalyzed CHO/CO2 copolymerization at low CO2 pressures (X = Br, OBzF5 (OBzF5
=
pentafluorobenzoate))a
Entry Catalyst
e
Time/h Yieldb (%) TOFc/h−1 Carbonate linkagesd (%) Mne/kg mol−1 Mw/Mn r-Centered tetradsf (%)
1
2
3
4
5
(R,R)-(salen-1)CoBr
(R,R)-(salen-11)CoBr
(R,R)-(salen-2)CoBr
rac-(salen-6)CoOBzF5 48
rac-(salen-6)CoBr 72
48
72
72
35
32
10
35
17
4
2
<1
4
90
99
20
88
86
12.6
8.9
10.8
9.3
1.23
1.19
1.83
1.27
1.33
50
35
NA
60
51
1
6.5
a Copolymerizations run in neat cyclohexene oxide (CHO) at 22 ◦C with [CHO] : [Co] = 500 : 1 at 6.8 atm of CO2. In all cases, cyclohexene carbonate
byproduct is not observed as determined by 1H NMR spectroscopy (CDCl3, 300 MHz). b Based on isolated poly(cyclohexene carbonate) yield. c Turnover
1
frequency = mol CHO (mol Co)−1 h−1
.
d Determined by H NMR spectroscopy (CDCl3, 300 MHz). e Determined by gel permeation chromatography
calibrated with polystyrene standards in tetrahydrofuran at 40 ◦C. f Determined by 13C NMR spectroscopy (CDCl3, 125 MHz).
most sterically bulky complex (R,R)-(salen-11)CoBr, however,
surprisingly produced isoenriched PCHC with 65% m-centered
tetrads and 99% carbonate incorporation (entry 2). Interestingly,
the sterically bulky catalyst (R,R)-(salen-2)CoBr incorporated
very little CO2, producing predominantly poly(cyclohexene oxide)
(entry 3). One of our best catalysts at high pressure (rac-(salen-
6)CoOBzF5) showed a decrease in syndiospecificity with CO2 at 6.8
atm generating PCHC with only 60% r-centered tetrads and 88%
carbonate linkages (entry 4), whereas changing its axial ligand to
Br completely eliminated its syndiospecificity (entry 5). To further
correlate CO2 pressure to catalyst activity and syndiospecificity,
we continued to investigate the CHO/CO2 copolymerization
using rac-(salen-6)CoOBzF5 at a series of different CO2 pressures.
Results from these experiments are presented graphically in Fig. 7.
It is notable◦that 54.4 atm is highest pressure we are able to
obtain at 22 C using our standard copolymerization setup and
was therefore the highest pressure applied. Decreasing the CO2
pressure from 54.4 atm to 47.6 atm caused a dramatic loss
in catalyst activity, consistent with the results we observe with
(salen)CoX catalysts for PO/CO2 copolymerization.5 Further
reductions in CO2 pressure resulted in a slow but steady decrease
in catalyst activity. The syndiospecificity of catalyst rac-(salen-
6)CoOBzF5 is optimized at 54.4 atm of CO2 and parallels the
catalyst activity at all pressures employed.
Alternating copolymerization of cyclohexene oxide and car-
bon dioxide with (salen)CoX catalysts and [PPN]Cl cocatalysts.
Previous investigations have shown the addition of organic
ionic cocatalysts to porphyrin- or salen-supported catalysts for
epoxide/CO2 copolymerization often results in increased reac-
tion rates.3,4,6–8,17b In our earlier work, addition of [PPN]Cl to
(R,R)-(salen-1)CoOBzF5 for PO/CO2 copolymerization led to
a dramatic increase in reaction rate as compared with the use
of (R,R)-(salen-1)CoOBzF5 alone.6 In addition, (R,R)-(salen-
1)CoOBzF5/[PPN]Cl was most active for PO/CO2 copolymeriza-
tion at low CO2 pressures, which has allowed the development of
optimized systems functioning under only 6.8 atm of CO2. Based
on these findings, we decided to test the effect of [PPN]Cl in the
CHO/CO2 copolymerization using a series of (salen)CoX (X =
Br, OBzF5) under various reaction conditions (Table 5).
We initially applied [PPN]Cl cocatalysts to the (salen)CoX
catalyzed CHO/CO2 copolymerization at high CO2 pressures in
an effort to maximize the catalyst syndiospecificity. Interestingly,
catalyst system (R,R)-(salen-1)CoBr/[PPN]Cl showed a loss in
syndiospecificity from use of (R,R)-(salen-1)CoBr alone. Addi-
tionally, only a small increase in activity was observed upon
the addition of the [PPN]Cl cocatalyst. By varying the catalyst
axial ligand in (R,R)-(salen-1)CoX/[PPN]Cl from Br to OBzF5
we observe little change in activity and syndiospecificity (entries
1 and 2). Surprisingly, catalyst rac-(salen-6)CoOBzF5, one of the
fastest catalysts without an additive, was less active for CHO/CO2
copolymerization upon the inclusion of [PPN]Cl (entry 3). Similar
to the (R,R)-(salen)CoX/[PPN]Cl (X = Br, OBzF5) catalyst
systems, little syndiospecificity was realized in this case.
The loss of syndiospecificity upon the addition of [PPN]Cl to
the (salen)CoX catalyzed CHO/CO2 copolymerization indicates
that a chain-end control mechanism exerts less control on the
stereospecificity of the polymerization. We therefore explored
(salen)CoOBzF5 catalysts with sterically bulky salen ligands in an
effort to invoke a site-control mechanism for isotactic PCHC. Ap-
plication of (R,R)-(salen-2)CoOBzF5/[PPN]Cl, with a more ster-
ically bulky diimine backbone, had comparable activity to (R,R)-
(salen-1)CoOBzF5/[PPN]Cl however the resultant PCHC was
atactic (entry 4). Additionally, employing a more sterically bulky
phenolate moiety ((R,R)-(salen-11)CoOBzF5/[PPN]Cl) also gen-
erated atactic PCHC with slightly lower activity (entry 5). In
general, addition of [PPN]Cl cocatalysts to (salen)CoX cat-
alyzed CHO/CO2 copolymerization at high CO2 pressures re-
sults in a loss in stereospecificity with little or no increase in
activity.
Fig. 7 Effect of CO2 pressure on catalyst activity and syndiospecificity of
CHO/CO2 copolymerization with rac-(salen-6)CoOBzF5.
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Table 5 Effect of [PPN]Cl cocatalyst: (salen)CoX/[PPN]Cl catalyzed copolymerization of CHO and CO2 (X
=
Br, OBzF5 (OBzF5
=
pentafluorobenzoate))a
d
Entry Catalyst
CO2 Pressure/atm Time/h Yieldb (%) TOFc/h−1 Mnd/kg mol−1 Mw/Mn r-Centered tetradse (%)
1
2
3
4
5
6
7
8
9
(R,R)-(salen-1)CoBr
54.4
54.4
54.4
54.4
6
6
6
6
6
2
2
2
2
34
40
18
36
27
22
32
18
13
44
44
57
67
30
60
45
110
160
90
65
31
14.8
15.0
10.1
11.8
10.6
8.2
14.9
7.4
6.6
1.38
1.36
1.44
1.38
1.41
1.24
1.14
1.22
1.18
1.17
1.23
62
(R,R)-(salen-1)CoOBzF5
rac-(salen-6)CoOBzF5
(R,R)-(salen-2)CoOBzF5
59
62
50
51
47
49
56
42
44
47
(R,R)-(salen-11)CoOBzF5 54.4
(R,R)-(salen-1)CoBr
(R,R)-(salen-1)CoOBzF5
rac-(salen-6)CoOBzF5
(R,R)-(salen-2)CoOBzF5
(R,R)-(salen-11)CoOBzF5
(R,R)-(salen-1)CoOBzF5
6.8
6.8
6.8
6.8
6.8
6.8
10
14
1
15.3
11.9
11f
440
a Copolymerizations run in neat cyclohexene oxide (CHO) at 22 ◦C with [CHO] : [Co] : [[PPN]Cl] = 1000 : 1 : 1. In all cases, cyclohexene carbonate
byproduct is not observed and product poly(cyclohexene carbonate)s have ≥97% carbonate linkages as determined by 1H NMR spectroscopy (CDCl3,
300 MHz). b Based on isolated PCHC yield. c Turnover frequency = mol CHO (mol Co)−1 h−1
.
d Determined by gel permeation chromatography calibrated
with polystyrene standards in tetrahydrofuran at 40 ◦C. e Determined by 13C NMR spectroscopy (CDCl3, 125 MHz). f 70 ◦C.
At low CO2 pressures (6.8 atm), catalyst activities increase
dramatically upon the addition of [PPN]Cl to the copolymeriza-
tion, although product PCHCs become virtually atactic. Catalyst
systems (R,R)-(salen-1)CoX/[PPN]Cl (X = Br, OBzF5) are highly
active with TOFs of 110 and 160 h−1, respectively, producing
atactic PCHC (entries 6 and 7). rac-(salen-6)CoOBzF5/[PPN]Cl
was less active for CHO/CO2 copolymerization with a TOF of
90 h−1 (entry 8), while use of the more sterically bulky (R,R)-(salen-
2)CoOBzF5/[PPN]Cl or (R,R)-(salen-11)CoOBzF5/[PPN]Cl re-
sulted in even lower TOFs of 65 and 31 h−1 (entries 9 and 10). Over-
all, when using the cocatalyst [PPN]Cl, (R,R)-(salen-1)CoOBzF5
is the most active catalyst for CHO/CO2 copolymerization at low
CO2 pressures.
Although use of elevated temperatures for (R,R)-(salen-
1)CoOBzF5/[PPN]Cl catalyzed PO/CO2 copolymerization pre-
dominantly forms propylene carbonate, we did not observe the
analogous cyclohexene carbonate upon heating the CHO/CO2
copolymerization. Furthermore, at the optimized temperature of
70 ◦C, catalyst system (R,R)-(salen-1)CoOBzF5/[PPN]Cl gener-
Fig. 8 Classification of poly(cyclohexene carbonate) according to the
ates PCHC with a TOF of 440 h−1 (entry 11). Overall, this is the
fastest catalyst activity we observe with the (salen)CoX/[PPN]Cl
systems, although it still is inferior to zinc- and chromium-based
alternatives.1
relative stereochemistry of the carbon at which the main chain enters the
cyclohexane unit (a), the carbon at which the main chain enters and leaves
the cyclohexane unit (b) and adjacent carbons (c).
of the carbonate unit (Fig. 8(c)). Therefore a racemic diad ([r])
represents two entire monomer units that have been incorporated
in the opposite stereochemical orientation.
Stereochemical characterization and analysis of poly(cyclohexene
carbonate). In previous accounts, stereoregular PCHC has been
classified according to the relative stereochemistry of the car-
bons on the cyclohexane units at which the main chain enters
(Fig. 8(a)).14,19 In all reported cases to date, the main chain
bonds entering and leaving the enchained cyclohexane units of
PCHC are oriented trans to each other. Furthermore, although
stereoregular PCHC is conventionally a ditactic polymer,24 as-
signing the relative stereochemistry for the carbons at which
the main chain leaves provides no additional stereochemical
information and has therefore been generally omitted (Fig. 8(b)).
In keeping with the previously established conventions in this
field, it is important to note that [m] and [r] assignments used
herein represent the relative stereochemistry of the carbons of
the cyclohexane units at which the main chain enters (Fig. 8(a)),
not the relative stereochemistry of the two carbons on either side
In earlier reports, we and Nozaki assigned the carbonyl region of
the 13C NMR spectrum for isotactic PCHC,13,14 which was further
corroborated using statistical methods.14 Additionally, Nozaki
has assigned the resonances corresponding to [mmm] and [rrr]
tetrad sequences by synthesizing model polycyclohexene carbon-
ate oligomers.19 In each case, all [mmm], [mmr] and [rmr] tetrads
were correlated to one central resonance at 153.7 ppm and the
remaining r-centered tetrads resided in the 153.3–153.1 ppm range.
Isotactic PCHC was therefore characterized by a strong resonance
at 153.7 ppm with a smaller series of resonances from 153.3–
153.1 ppm due to stereoerrors. Statistically (enantiomorphic-site
control), the most prominent stereoerrors in isotactic PCHC are
[mrr] and [mmr] tetrads, with much smaller contributions from the
remaining [rrr], [rmr] and [mrm] tetrads. In contrast, the PCHC
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that we obtain with catalyst rac-(salen-6)CoOBzF5 revealed a
13C NMR spectrum with four distinct resonances at 152.94,
153.06, 153.12 and 153.70 ppm integrating to 9, 56, 24 and 11%,
respectively (Fig. 9(a)).22 The peak at 152.94 ppm is unique to this
polymer and was unobserved in isotactic PCHC.14,19 Based on the
previous assignments, we correlated the most intense resonance
at 153.06 ppm to the [rrr] tetrad, with a relative integration of
56%. Using Bernoullian (chain-end control) statistical methods,
such that the calculated probability of a racemo diad (Pr) was
determined by the maximized weighted fit between calculated and
observed (obs) tetrad concentrations we calculate 56% [rrr] tetrads,
24% [rrm] tetrads and 12% [rmr] tetrads, with the remaining
tetrads: [mmr] = 5%, [mrm] = 2% and [mmm] = 1% (Table 6).25 The
previous reports assigning all m-centered tetrads to a central reso-
nance at 153.7 ppm do not agree with these statistical calculations.
We therefore propose that the [rmr] tetrad corresponds to the most
upfield resonance at 152.94 ppm in our spectrum, an unobservable
stereoerror in isotactic PCHC samples, due to its low probability,
leading to a previously incorrect assignment. It is also important
to note that the [rmr] tetrad was not investigated in previous
syntheses of model oligomers,19 precluding its direct assignment
through chemical means. Finally, we attribute the resonance at
153.11 ppm to the [rrm] sequence, leaving the remaining [mmr]
and [mmm] tetrads collectively at 153.70 ppm and the negligible
[mrm] tetrad absent. It is notable that the [rrm] resonance in the 13
C
NMR spectrum of isotactic PCHC (Fig. 9(d)) is shifted 0.1 ppm
upfield from that of syndiotactic PCHC (Fig. 9(a) and (b)), which
we attribute to the minor influence of the long-range polymer
tacticity on this shift.
To further support the PCHC 13C NMR assignments that
we propose, we applied Bernoullian statistical methods and
compared the observed concentration of each tetrad with its
corresponding calculated amount. Table 6 compares observed
tetrad concentrations to their calculated statistical occurrences
for a series of syndiotactic PCHCs. In most cases, the observed
and calculated concentrations are in close agreement, and are
within the expected error of the deconvolution methods used.
Notably, the PCHCs with the highest observed % r-centered
tetrads overall have relative tetrad concentrations that best agree
with the corresponding calculated values.
The 13C NMR spectrum of less syndiotactic PCHC, as generated
from catalyst (R,R)-(salen-1)CoOBzF5 shows the [rrr] and [rrm]
tetrad resonances approach each other in magnitude with a
more significant contribution from the [mmm] and [mmr] tetrads
(Fig. 9(b)). Using Bernoullian statistical methods, we calculate
relative tetrad occurrences of [mmm] + [mmr] = 15%, [rmr] =
15%, [rrr] = 35%, [rrm] = 29% and [mrm] = 6% (Table 6). This is
indeed consistent with what we observe, where a previously minor
resonance at 153.18 ppm is now more prominent, which we assign
to the [mrm] tetrad. In this spectrum we detect fine splitting of
various resonances in the 153.2–152.9 ppm region and hypothesize
that due to the increase of PCHC stereoerrors we are able to
Fig. 9 Carbonyl region of the 13C NMR spectra (125 MHz, CDCl3)
of poly(cyclohexene carbonate)s generated using CHO/CO2 (54.4 atm)
with catalyst rac-(salen-6)CoOBzF5 (a), CHO/CO2 (54.4 atm) with
catalyst (R,R)-(salen-1)CoOBzF5 (b), CHO/CO2 (54.4 atm) with cat-
alyst rac-(salen-8)CoOBzF5 (c) and CHO/CO2 (6.8 atm) with catalyst
(R,R)-(salen-11)CoBr (d).
Table 6 Comparison of observed tetrad concentrations to corresponding calculated statistical occurrences in syndiotactic poly(cyclohexene carbonate)a
Pr
[mmm] + [mmr]
[rmr]
[rrr]
[rrm]
Table/entry Calc. (%) Obs. (%)
Calc. (%) Obs. (%)
Calc. (%) Obs. (%)
Calc. (%) Obs. (%)
Calc. (%) Obs. (%)
2/7
3/1
3/5
2/2
2/6
1/5
1/2
3/2
82
82
81
79
77
71
68
66
80
76
75
78
75
66
66
65
6
6
7
8
9
15
17
19
10
9
12
12
13
13
14
15
15
15
12
11
10
11
14
13
16
15
55
56
53
49
46
35
32
29
55
56
53
49
46
35
32
29
24
24
25
26
27
29
30
30
24
24
26
29
26
26
28
31
11
11
15
25
24
26
a Calculated (calc.) statistical occurrences for each tetrad were determined through application of Bernoullian (chain-end control) statistical methods
where the calculated probability of a racemo diad (Pr) was determined by the maximized weighted fit between calculated and observed (obs.) tetrad
concentrations. All observed values determined by quantitative 13C NMR spectroscopy (CDCl3, 125 MHz) with deconvolution of 13C NMR resonances.
In all cases, the [mrm] tetrad concentrations (calc values between 2–7%) were too small for analysis by the deconvolution methods used and were therefore
omitted.
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observe signals at greater than tetrad resolution. Furthermore,
the 13C NMR spectrum of the same PCHC sample using a lower
resolution spectrometer showed only the five resonances at 152.95,
153.06, 153.12, 153.17 and 153.70 ppm.
relative to the [mr] triad resonances and additional downfield reso-
nances are detected which we assign to the [mm] triads (Fig. 10(b)).
Alternatively, the 13C NMR spectrum of atactic PCHC reveals
more intense [mm] triad resonances compared to the syndiotactic
PCHC samples, with the [mr] and [rr] triad resonances also present
(Fig. 10(c)). In this case, we observe some tetrad resolution where
the [rr] triad resonances are split into their [rrm] and [rrr] compo-
The 13C NMR spectrum of PCHC, as generated from catalyst
rac-(salen-8)CoOBzF5, reveals that the [mmm] + [mmr] tetrads
integrate equally to those of the remaining tetrads, indicative
of the formation of atactic polymer (Fig. 9(c)). Finally, isoen-
riched PCHC, as generated from catalyst (R,R)-(salen-11)CoBr
while carrying out the CHO/CO2 copolymerization at low CO2
pressures (6.8 atm), has a 13C NMR spectrum similar to that of
previously reported isotactic PCHC.13,14 In this case we observe
a large resonance correlating to the combined [mmm] + [mmr]
tetrads at 153.70 ppm and additional resonances representing
the [rrm] and [mrm] stereoerrors with no detectable [rrr] or [rmr]
tetrads (Fig. 9(d)).
The methylene regions in the PCHC 13C NMR spectra also vary
with PCHC stereochemistry (Fig. 10). In the case of syndiotactic
PCHC, two series of two adjacent resonances are observed which
we assign to the [rr] and [mr] triads corresponding to the non-
equivalent methylene carbons in the PCHC backbone (Fig. 10(a)).
In each case, the [mr] and [rm] triad resonances cannot be distin-
guished. Furthermore, we suspect that one of these two triads has
the same 13C NMR shift as the [rr] triad. The 13C NMR spectrum
of less syndiotactic PCHC shows [rr] triads of decreased intensity
nents and the [mr] triad resonances are broadened. Finally, the 13
C
NMR spectrum of isoenriched PCHC shows that the downfield
[mm] triad resonances in the methylene region have the greatest
intensities relative to the [mr] and [rr] triad signals (Fig. 10(d)).
Conclusions
We have explored a series of (salen)CoX catalysts for the alternat-
ing copolymerization of CHO and CO2. These catalysts provide a
viable route to syndiotactic PCHC, a previously unreported PCHC
microstructure. The structural properties of the salen ligand in
these catalysts dictate the syndiotacticity of the resultant PCHC, a
correlation which we attribute to a competition of chain-end and
enantiomorphic-site control mechanisms. Additionally, catalyst
syndiospecificity is pressure dependent, with the best selectivity
realized employing high CO2 pressures. PCHC with as high as
81% r-centered tetrads was obtained using this class of catalyst.
Inclusion of [PPN]Cl cocatalysts to the (salen)CoX catalyzed
CHO/CO2 copolymerization results in an increase in catalyst
activity in most cases with an overall loss in syndiospecificity.
Under optimized conditions, these systems achieve a TOF of
440 h−1 for the generation of PCHC.
Experimental
Materials and physical measurements
All air- or water-sensitive reactions were carried out under dry ni-
trogen using an MBraun Labmaster drybox or standard Schlenk-
line techniques. Methylene chloride and diethyl ether were dried
and degassed by passing through a column of activated alumina
and by sparging with dry nitrogen. (S,S)- and (R,R)-N,Nꢀ-
bis(3,5-di-tert-butylsalicylidene)-1,2-diaminocyclohexane cobalt
((S,S)-(salen-1)Co and (R,R)-(salen-1)Co) were purchased from
Aldrich and recrystallized from methylene chloride and methanol.
Bis(triphenylphosphine)iminium chloride ([PPN]Cl) was pur-
chased from Strem and recrystallized from dry methylene chloride
and diethyl ether under nitrogen before use. CHO was dried over
calcium hydride and vacuum transferred before use. CO2 (99.998%
purity) was purchased from Airgas and passed over a column of
˚
4 A molecular sieves. All other reagents were purchased from com-
mercial sources and used as received. Varian Mercury (300 MHz)
and Inova (500 MHz) spectrometers were used to record 1H NMR
spectra, which were referenced vs. residual nondeuterated solvent
shifts. An Inova (500 MHz) spectrometer was used to record 13
C
NMR spectra (at 125 MHz), which were referenced vs. residual
deuterated solvent shifts. C6F6 (−162.90 ppm) was used as an
internal reference for all 19F NMR spectra. GPC analyses were
carried out using a Waters instrument (M515 pump, U6 K injector)
equipped with Waters UV486 and Waters 2410 detectors, and four
Fig. 10 Methylene region of the 13C NMR spectra (125 MHz, CDCl3)
of poly(cyclohexene carbonate)s generated using CHO/CO2 (54.4 atm)
with catalyst rac-(salen-6)CoOBzF5 (a), CHO/CO2 (54.4 atm) with
catalyst (R,R)-(salen-1)CoOBzF5 (b), CHO/CO2 (54.4 atm) with cat-
alyst rac-(salen-8)CoOBzF5 (c) and CHO/CO2 (6.8 atm) with catalyst
(R,R)-(salen-11)CoBr (d).
˚
5-lm PL Gel columns (Polymer Laboratories; 100, 500, 1000 A
and Mixed C porosities) in series. The GPC columns were eluted
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◦
with THF at 40 C at 1 mL min−1 and were calibrated using
alcohol (10 mL) were used, and the reaction was refluxed for
12 h. Following the removal of solvent in vacuo, the crude yellow
oil was used for the synthesis of rac-(salen-8)Co without further
23 monodisperse polystyrene standards. IR spectra were measured
on a Mattson Research Series FTIR. High resolution mass spectra
were obtained from the Mass Spectrometry Laboratory, School of
Chemical Sciences, University of Illinois. All (salen)CoX (X = Br
or OBzF5) complexes reveal axial ligand (X) loss in the (EI) mass
spectra attributable to the poor stability of this complex under the
applied conditions. In the case of all (salen)CoOBzF5 complexes,
carbons on the phenyl group of pentafluorobenzoate were not
assigned in the 13C NMR spectra due to complex carbon fluorine
splitting patterns.
1
3
purification. H NMR (CDCl3, 500 MHz): 1.38 (d, J = 5 Hz,
3H), 2.23 (s, 3H), 2.24 (s, 3H), 3.69 (m, 2H), 3.83 (m, 1H), 6.74 (t,
3J = 7.5 Hz, 1H), 6.75 (t, 3J = 7.5 Hz, 1H), 7.05 (d, 3J = 7.5 Hz,
3
2H), 7.14 (d, J = 7.5 Hz, 2H), 8.29 (s, 1H), 8.33 (s, 1H), 13.52
(br s, 2H).
rac-(Salen-9)H2. 3-tert-Butyl-2-hydroxybenzaldehyde (1.0 mL,
5.4 mmol), 1,2-diaminopropane (0.23 mL, 2.7 mmol) and ethyl
alcohol (10 mL) were used, and the mixture was refluxed for
12 h. Following the removal of solvent in vacuo, the crude yellow
oil was used for the synthesis of rac-(salen-9)Co without further
Ligand preparation
Schiff-base condensation of salicylaldehydes with 1,2-diamines to
generate salen-type ligands is readily achieved by refluxing 2 eq of
the parent aldehyde and 1 equiv of the parent diamine in absolute
ethyl alcohol.26 In most cases, 3 h reaction time is sufficient for
obtaining high product yields. Following the removal of solvent
in vacuo, the crude yellow solid is recrystalized from ethyl alcohol
yielding the product salen ligand in most cases. Alternatively, the
tartrate salt of the parent diamine can be employed in the synthesis,
where the diamine tartrate is stirred with K2CO3 in water until dis-
solution prior to reaction with the aldehyde.27 The analytical data
and synthetic modifications for new salen ligands are listed below:
1
purification. H NMR (CDCl3, 500 MHz): 1.42 (s, 9H), 1.43 (s,
9H), 1.44 (m, 3H), 3.70–3.75 (m, 2H), 3.82–3.87 (m, 1H), 6.78 (t,
3J = 7.5 Hz, 2H), 7.08 (d, 3J = 7.5 Hz, 2H), 7.30 (d, 3J = 7.5 Hz,
2H), 8.34 (s, 1H), 8.38 (s, 1H), 13.79 (br s, 2H).
rac-(Salen-10)H2. 5-Bromo-3-tert-butyl-2-hydroxybenzalde-
hyde29 (0.20 g, 0.78 mmol), 1,2-diaminopropane (33 lL,
0.39 mmol) and ethyl alcohol (10 mL) were used, and the reaction
was refluxed for 12 h. Following the removal of solvent, the crude
1
yellow solid was dried in vacuo (0.20 g, 93%). H NMR (CDCl3,
500 MHz): d 1.40 (s, 9H), 1.41 (s, 9H), 1.44 (m, 3H), 3.68–3.74
(m, 2H), 3.85–3.86 (m, 1H), 7.19 (s, 2H), 7.37 (s, 2H), 8.24 (s, 1H),
8.27 (s, 1H), 13.84 (s, 2H). 13C NMR (CDCl3, 125 MHz): d 20.46,
29.28, 29.33, 35.26, 35.28, 64.92, 65.54, 109.98, 110.02, 119.91,
131.81, 131.88, 132.62, 132.69, 140.21, 140.33, 159.56, 159.69,
164.39, 166.17. HRMS (ESI) m/z calc. (C25H32 Br2N2O2 + H+)
551.0909, found 551.0910.
(Salen-4)H2. To a solution of 3,5-di-tert-butyl-2-hydroxy-
benzaldehyde (3.0 g, 13 mmol) in ethyl alcohol (60 mL) was
added 2-methyl-1,2-propanediamine (0.67 mL, 6.4 mmol) and◦the
mixture was refluxed for 3 h. The reaction was cooled to 22 C,
and the solvent was removed in vacuo. The◦ crude yellow solid
was recrystalized from ethyl alcohol at −20 C affording yellow
needles (3.1 g, 93%). 1H NMR (CDCl3, 500 MHz): d 1.28 (s, 9H),
1.29 (s, 9H), 1.43 (s, 24H), 3.71 (s, 2H), 7.07 (d, 4J = 4.5 Hz, 1H),
(R,R)-(Salen-11)H2. (R,R)-1,2-Diaminocyclohexane-L-tartrate
(0.74 g, 2.8 mmol) and K2CO3 (0.77 g, 5.6 mmol) were stirred in
H2O (8 mL) until all was dissolved. To it was added a solution
4
4
7.09 (d, J = 4.5 Hz, 1H), 7.35 (d, J = 4.5 Hz, 1H), 7.36 (d,
4J = 4.5 Hz, 1H), 8.35 (s, 1H), 8.39 (s, 1H) 13.67 (s, 1H), 14.21 (s,
1H). 13C NMR (CDCl3, 125 MHz): d 25.73, 29.60, 29.63, 31.63,
31.66, 34.26, 35.17, 35.19, 60.14, 70.71, 117.99, 118.08, 126.22,
126.35, 126.90, 127.18, 136.78, 136.80, 139.98, 140.12, 158.32,
158.52, 162.88, 167.78. HRMS (ESI) m/z calc. (C34H52N2O2 +
H+) 521.4107, found 521.4110.
ꢀ
of 3,5-bis(a,a -dimethylbenzyl)-2-hydroxybenzaldehyde28 (2.0 g,
5.6 mmol) in ethyl alcohol (35 mL) and the mixture was refluxed
◦
for 3 h. The reaction mixture was then cooled to 22 C, filtered,
and washed thoroughly with H2O and then with cold ethyl
alcohol. The crude yellow solid was dried and collected (1.8 g,
81%). 1H NMR (CDCl3, 500 MHz): d 1.29 (m, 2H), 1.52 (m, 2H),
1.59 (s, 6H), 1.67 (s, 12H), 1.68 (s, 6H), 1.73 (m, 4H), 3.11 (m,
2H), 6.94 (d, 4J = 2.5 Hz, 2H), 7.16 (tt, 3J = 7.0 Hz, 4J = 1.5 Hz,
2H), 7.16–7.29 (m, 20H), 8.08 (s, 2H), 13.21 (br s, 2H). 13C NMR
(CDCl3, 125 MHz): d 24.31, 28.71, 30.38, 30.95, 31.04, 33.22,
42.25, 42.44, 72.25, 118.01, 125.11, 125.68, 125.70, 126.78, 127.74,
127.92, 128.11, 129.27, 135.82, 139.46, 150.64, 150.76, 157.77,
165.38. HRMS (ESI) m/z calc. (C56H62N2O2 + H+) 795.4890,
found 795.4900.
ꢀ
rac-(Salen-7)H2. 3,5-Bis(a,a -dimethylbenzyl)-2-hydroxy-
benzaldehyde28 (0.59 g, 1.6 mmol), 1,2-diaminopropane (68 lL,
0.80 mmol) and ethyl alcohol (10 mL) were mixed and the react◦ion
was heated at reflux for 12 h. The reaction was cooled to 22 C,
and the solvent was removed in vacuo. Following recrystallization
from ethyl alcohol, the product was collected as yellow needles
(0.41 g, 68%). 1H NMR (CDCl3, 500 MHz): d 1.25 (d, 3J = 4 Hz,
3H), 1.595 (s, 3H), 1.599 (s, 3H), 1.61 (s, 3H), 1.62 (s, 3H), 1.698
(s, 3H), 1.702 (s, 3H), 3.58–3.75 (m, 3H), 7.13–7.32 (m, 24H),
8.23 (s, 1H), 8.27 (s, 1H), 13.34 (br s, 2H). 13C NMR (CDCl3, 125
MHz): d 20.56, 29.29, 29.73, 29.92, 31.05, 31.06, 31.08, 42.27,
42.31, 42.54, 65.08, 65.80, 118.01, 125.17, 125.70, 125.73, 125.76,
126.84, 127.91, 127.98, 127.15, 129.34, 136.00, 136.17, 139.62,
139.69, 150.71, 150.84, 150.85, 157.69, 157.88, 165.17, 166.97.
HRMS (ESI) m/z calc. (C53H58N2O2 + H+) 755.4577 found
755.4571.
Preparation of (salen)Co complexes
Complexes (R,R)-(salen-1)Co and (S,S)-(salen-1)Co are commer-
cially available. All other (salen)Co complexes were prepared
according to the general procedure for the synthesis of (salen-
4)Co. Complexes (R,R)-(salen-2)Co,30 (salen-3)Co,31 and (salen-
5)Co32 prepared by this method are in agreement with literature
characterization. The analytical data and synthetic modifications
for new (salen)Co complexes are listed below:
rac-(Salen-8)H2. 2-Hydroxy-3-methylbenzaldehyde (0.50 g,
3.7 mmol), 1,2-diaminopropane (0.16 mL, 1.9 mmol) and ethyl
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(Salen-4)Co. (Salen-4)H2 (2.3 g, 4.4 mmol) and cobalt acetate
tetrahydrate (1.3 g, 5.2 mmol) were added to a Schlenk flask
charged with a Teflon stir bar under N2. A 1 : 1 mixture of toluene
and methanol (100 mL); (degassed for 20 min by sparging with
dry N2) was added and stirred at 22 ◦C for 2 h. The resultant
red precipitate was filtered in air and washed with distilled water
(50 mL) and methanol (50 mL) and collected as a crude solid (2.3 g,
90% yield). IR (KBr, cm−1): 786, 842, 871, 1178, 1255, 1318, 1363,
1390, 1464, 1528, 1595, 2870, 2909, 2959. HRMS (ESI) m/z calc.
(C34H50CoN2O2) 577.3204, found 577.3226.
Preparation of (salen)CoX (X = Cl, Br, I, OAc, OBzF5) complexes
The synthesis of (salen-1)CoX (X = Cl, Br, I, OAc, OBzF5) have
been described previously.6,20,21 A modified procedure as reported
by Jacobsen and coworkers21 for the synthesis of (R,R)-(salen-
1)CoCl was applied to the synthesis of all (salen)CoBr complexes
with the substitution of NaBr for NaCl.
(R,R)-(Salen-2)CoBr. (R,R)-(salen-2)Co (0.25 g, 0.36 mmol)
and p-toluenesulfonic acid monohydrate (68 mg, 0.36 mmol) were
added to a 50 mL round bottomed flask charged with a Teflon
stir bar. Methylene chloride (10 mL) was added to the reaction
mixture and stirred for 2 h open to air at 22 ◦C. The solvent was
removed by rotary evaporation at 22 ◦C, and the crude solid was
washed with pentane (100 mL) and filtered. The crude material was
dissolved in methylene chloride (25 mL) and added to a 125 mL
separatory funnel. The organic layer was rinsed with saturated
aqueous NaBr (3 × 25 mL). The organic layer was dried over
Na2SO4 and evaporated under reduced pressure. The solid was
washed with pentane (100 mL) and filtered to afford (R,R)-(salen-
2)CoBr (0.12 g, 43%). 1H NMR (DMSO-d6, 500 MHz): d 1.22 (s,
18H), 1.76 (s, 18H), 5.62 (s, 2H), 6.97 (s, 2H), 7.23 (s, 2H), 7.41–
7.48 (m, 12H). 13C NMR (DMSO-d6, 125 MHz): d 30.37, 31.35,
33.31, 35.73, 76.61, 117.64, 128.48, 129.18, 129.90, 134.93, 136.07,
142.02, 162.31, 166.50. HRMS (EI) m/z calc. (C44H54BrCoN2O2 −
Br) 701.3517 found 701.3502.
rac-(Salen-6)Co. Employing the same reaction conditions as
for (salen-4)Co, rac-(salen-6)10 (2.8 g, 5.5 mmol) and cobalt acetate
tetrahydrate (1.7 g, 6.8 mmol) in a 1 : 1 mixture of degassed toluene
and methanol (150 mL) were stirred for 6 h to afford a crude red
solid (2.9 g, 95%). IR (KBr, cm−1): 787, 837, 874, 1179, 1204, 1255,
1320, 1361, 1385, 1466, 1528, 1596, 2871, 2909, 2956. HRMS (ESI)
m/z calc. (C33H48CoN2O2) 563.3048, found 563.3046.
rac-(Salen-7)Co. Employing the same reaction conditions as
for (salen-4)Co, rac-(salen-7) (0.30 g, 0.40 mmol) and cobalt
acetate tetrahydrate (0.12 g, 0.48 mmol) in a 1 : 1 mixture of
degassed toluene and methanol (10 mL) were stirred for 3 h to
afford a crude red solid (0.25 g, 77%). IR (KBr, cm−1) 765, 777,
787, 818, 873, 957, 1031, 1106, 1166, 1226, 1323, 1370, 1427, 1441,
1497, 1526, 1583, 1607, 2904, 2926, 2966, 3060. HRMS (ESI) m/z
calc. (C53H56CoN2O2) 811.3674, found 811.3698.
(R,R)-(Salen-2)CoOBzF5. To a 50 mL round-bottomed flask
charged with a Teflon stirbar was added (R,R)-(salen-2)Co (0.20 g,
0.29 mmol), pentafluorobenzoic acid (62 mg, 0.29 mmol) and
toluene (10 mL). The mixture was stirred open to air for 12 h
at 22 ◦C. The toluene was removed by rotary evaporation and
the crude solid was washed with pentane (50 mL), filtered, and
rac-(Salen-8)Co. Employing the same reaction conditions as
for (salen-4)Co, the crude yellow oil rac-(salen-8) (2.1 g, 6.8 mmol)
was dissolved in a 1 : 1 mixture of degassed toluene and methanol
(40 mL) and cobalt acetate tetrahydrate (2.0 g, 8.0 mmol) was
added under N2 and stirred for 1 h to afford a crude red–brown
solid (1.9 g, 76%). IR (KBr, cm−1) 739, 888, 947, 1106, 1120, 1234,
1318, 1376, 1419, 1451, 1545, 1604, 2920, 2966, 3020, 3439. HRMS
(ESI) m/z calc. (C19H20CoN2O2) 367.0857, found 367.0843.
1
dried in vacuo to afford a green solid (0.19 g, 72%). H NMR
(DMSO-d6, 500 MHz): d 1.22 (s, 18H), 1.76 (s, 18H), 5.62 (s, 2H),
6.97 (s, 2H), 7.23 (s, 2H), 7.41–7.48 (m, 12H). 13C NMR (DMSO-
d6, 125 MHz): d 30.38, 31.35, 33.40, 35.80, 76.64, 117.59, 128.69,
129.24, 129.96, 134.87, 136.16, 141.97, 162.34, 166.67. 19F NMR
(DMSO-d6, 470 MHz): d −163.66, −162.81, −144.85. HRMS (EI)
m/z calc. (C51H54CoF5N2O4 − C7F5O2) 701.3517, found 701.3533.
rac-(Salen-9)Co. Employing the same reaction conditions as
for (salen-4)Co, the crude yellow oil rac-(salen-9) (2.2 g, 5.6 mmol)
was dissolved in a 1 : 1 mixture of degassed toluene and methanol
(40 mL) and cobalt acetate tetrahydrate (1.6 g, 6.4 mmol) was
added under N2 and stirred for 3 h to afford a crude red solid
(1.4 g, 55%). IR (KBr, cm−1) 753, 869, 886, 952, 1057, 1086, 1147,
1200, 1235, 1266, 1316, 1386, 1408, 1466, 1535, 1597, 2873, 2921,
2953. HRMS (ESI) m/z calc. (C25H32CoN2O2) 451.1796, found
451.1776.
(Salen-3)CoBr. Employing the same reaction conditions as
for (R,R)-(salen-2)CoBr, (salen-3)Co (1.0 g, 1.7 mmol) and p-
toluenesulfonic acid monohydrate (0.32 g, 1.7 mmol) were used
1
to produce (salen-3)CoBr (0.35 g, 30%). H NMR (DMSO-d6,
4
500 MHz): d 1.35 (s, 18H), 1.78 (s, 18H), 7.55 (d, J = 2.5 Hz,
rac-(Salen-10)Co. Employing the same reaction conditions as
for (salen-4)Co, rac-(salen-10) (0.22 g, 0.40 mmol) and cobalt
acetate tetrahydrate (0.12 g, 0.48 mmol) in a 1 : 1 mixture of
degassed toluene and methanol (7 mL) were used to afford a crude
red solid (0.10 g, 41%). IR (KBr, cm−1) 734, 783, 865, 1179, 1310,
1385, 1402, 1525, 1592, 2873, 2922, 2957. HRMS (ESI) m/z calc.
(C25H30Br2CoN2O2) 607.0006, found 607.0010.
2H), 7.56 (td, 3J = 6.5 Hz, 4J = 3.5 Hz, 2H), 7.66 (d, 4J = 2.5 Hz,
3
4
2H), 8.63 (dd, J = 6.5 Hz, J = 3.5 Hz, 2H), 8.95 (s, 2H). 13C
NMR (DMSO-d6, 125 MHz): d 30.34, 31.34, 33.79, 36.01, 117.40,
117.45, 128.14, 129.97, 131.00, 136.59, 142.07, 144.72, 161.60,
165.59. HRMS (EI) m/z calc. (C36H46BrCoN2O2 − Br) 597.2891,
found 597.2878.
(Salen-4)CoBr. (salen-4)Co (0.30 g, 0.52 mmol) and p-
toluenesulfonic acid monohydrate (99 mg, 0.52 mmol) were added
to a 50 mL round-bottomed flask charged with a Teflon stir bar.
Methylene chloride (30 mL) was added to the reaction mixture
and stirred for 2 h open to air at 22 ◦C. The solvent was removed
by rotary evaporation at 22 ◦C, and the crude dark green solid
was dissolved in pentane (50 mL) and filtered. The solvent was
removed by rotary evaporation, and the material was dissolved in
(R,R)-(Salen-11)Co. Employing the same reaction conditions
as for (salen-4)Co, (R,R)-(salen-7) (0.69 g, 0.87 mmol) and cobalt
acetate tetrahydrate(0.26g, 1.0mmol)in a1:1mixture of degassed
toluene and methanol (30 mL) were used to afford a crude red
solid (0.59 g, 80%). IR (KBr, cm−1): 766, 809, 1034, 1105, 1246,
1325, 1340, 1362, 1459, 1528, 1605, 2872, 2937, 2968, 3026, 3061,
3453. HRMS (ESI) m/z calc. (C56H60CoN2O2) 851.3987, found
851.3972.
2 4 6 | Dalton Trans., 2006, 237–249
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methylene chloride (50 mL) and added to a 250 mL separatory
funnel. The organic layer was shaken vigorously with saturated
aqueous NaBr (3 × 50 mL). The organic layer was dried over
Na2SO4 and evaporated under reduced pressure to afford a crude
black solid (92 mg, 27%). 1H NMR (DMSO-d6, 500 MHz): d 1.30
(s, 9H), 1.32 (s, 9H), 1.63 (s, 6H), 1.73 (s, 9H), 1.74 (s, 9H), 4.02 (s,
2H), 7.36 (d, 4J = 2.5 Hz, 1H), 7.45 (d, 4J = 2.5 Hz, 1H), 7.475 (s,
1H), 7.482 (s, 1H), 7.88 (s, 1H), 8.03 (s, 1H). 13C NMR (DMSO-
d6, 125 MHz): d 27.10, 30.35, 31.28, 31.32, 31.55, 31.61, 33.43,
33.50, 35.72, 35.77, 66.98, 70.93, 118.36, 119.57, 128.05, 128.75,
128.98, 129.35, 135.85, 136.41, 141.42, 142.12, 161.11, 161.96,
166.31, 168.37. HRMS (EI) m/z calc. (C34H50BrCoN2O2 − Br)
577.3204, found 577.3199.
0.25 mmol) and pentafluorobenzoic acid (53 mg, 0.25 mmol) were
used to afford the crude green product (0.13 g, 51%). H NMR
1
(CDCl3, 500 MHz): d 1.48 (d, 3J = 6.0 Hz, 3H), 1.57 (s, 6H), 1.58
(s, 6H), 1.93 (s, 3H), 2.02 (s, 3H), 2.05 (s, 3H), 2.07 (s, 3H), 3.68
(dd, 2J = 13.0 Hz, 3J = 6.0 Hz, 1H), 4.01 (dd, 2J = 13.0 Hz, 3J =
6.0 Hz, 1H), 4.15 (m, 1H), 6.98 (s, 1H), 7.04 (s, 1H), 7.14–7.07 (m,
2H), 7.16–7.30 (m, 18H), 7.34 (s, 2H), 7.91 (s, 1H), 8.04 (s, 1H).
13C NMR (CDCl3, 125 MHz): d 18.48, 29.87, 30.12, 30.33, 30.80,
41.37, 41.39, 43.29, 43.40, 62.91, 64.40, 118.58, 119.13, 125.11,
125.38, 125.43, 126.14, 126.19, 126.22, 127.78, 127.90, 129.97,
130.59, 132.89, 133.29, 135.11, 135.36, 140.95, 141.24, 150.57,
150.61, 151.12, 151.21, 161.69, 161.96, 167.52, 168.82. 19F NMR
(DMSO-d6, 470 MHz): d −163.73, −162.97, −144.96. HRMS (EI)
m/z calc. (C60H56CoF5N2O4 − C7F5O2) 811.3674, found 811.3683.
(Salen-5)CoBr. Employing the same reaction conditions as
for (salen-4)CoBr, (salen-5)Co (0.30 g, 0.55 mmol) and p-
toluenesulfonic acid monohydrate (0.10 g, 0.55 mmol) were used.
Following the salt metathesis with NaBr, the crude product
rac-(Salen-8)CoOBzF5. Employing the same reaction condi-
tions as for (R,R)-(salen-2)CoOBzF5, rac-(salen-8)Co (0.70 g,
1.9 mmol) and pentafluorobenzoic acid (0.40 g, 1.9 mmol) were
1
1
(salen-5)CoBr was obtained (86 mg, 25%). H NMR (DMSO-
used to afford the crude brown product (1.0 g, 91%). H NMR
(CDCl3, 500 MHz): d 1.55 (d, 3J = 6.0 Hz, 3H), 2.63 (s, 3H), 2.65
(s, 3H), 3.93 (dd, 2J = 14.0 Hz, 3J = 6.0 Hz, 1H), 4.24 (dd, 2J =
d6, 500 MHz): d 1.30 (s, 18H), 1.73 (s, 18H), 4.14 (s, 4H), 7.31 (d,
4J = 3.0 Hz, 2H), 7.45 (d, 4J = 3.0 Hz, 2H), 8.12 (s, 2H). 13C NMR
(DMSO-d6, 125 MHz): d 30.36, 31.52, 33.43, 35.77, 58.24, 118.51,
128.27, 128.74, 135.93, 142.05, 162.13, 168.65. HRMS (EI) m/z
calc. (C32H46BrCoN2O2 − Br) 549.2891, found 549.2885.
3
3
14.0 Hz, J = 6.0 Hz, 1H), 4.42 (m, 1H), 6.57 (t, J = 7.5 Hz,
2H), 7.32 (d, 3J = 7.5 Hz, 1H), 7.33 (d, 3J = 7.5 Hz, 1H), 7.36 (d,
3J = 7.5 Hz, 1H), 7.41 (d, 3J = 7.5 Hz, 1H), 8.17 (s, 1H), 8.27 (s,
1H). 13C NMR (CDCl3, 125 MHz): d 16.98, 18.81, 63.11, 64.62,
114.48, 114.62, 117.62, 118.07, 130.53, 130.81, 132.02, 132.56,
134.25, 134.34, 162.96, 163.34, 166.90, 168.20. 19F NMR (DMSO-
d6, 470 MHz): d −163.25, −162.42, −144.35. HRMS (EI) m/z calc.
(C26H20CoF5N2O4 − C7F5O2) 367.0857, found 367.0845.
rac-(Salen-6)CoBr. Employing the same reaction conditions
as for (salen-4)CoBr, rac-(salen-6)Co (1.0 g, 1.8 mmol) and p-
toluenesulfonic acid monohydrate (0.34 g, 1.8 mmol) were used
1
to afford the crude product rac-(salen-6)CoBr (0.50 g, 43%). H
NMR (DMSO-d6, 500 MHz): d 1.30 (s, 18H), 1.61 (d, 3J = 6.5 Hz,
3H), 1.73 (s, 18H), 3.86 (m, 1H), 4.21 (m, 1H), 4.32 (m, 1H), 7.33
rac-(Salen-9)CoOBzF5. Employing the same reaction condi-
tions as for (R,R)-(salen-2)CoOBzF5, rac-(salen-9)Co (0.42 g,
0.92 mmol) and pentafluorobenzoic acid (0.20 g, 0.92 mmol) were
stirred for 3 h to afford the crude brown product (0.49 g, 80%). 1H
NMR (CDCl3, 500 MHz): d 1.61 (d, 3J = 6.0 Hz, 3H), 1.73 (s, 18H),
3.89 (dd, 2J = 13.5 Hz, 3J = 6.0 Hz, 1H), 4.23 (dd, 2J = 13.5 Hz,
3J = 6.0 Hz, 1H), 4.36 (m, 1H), 6.59 (t, 3J = 7.5 Hz, 2H), 7.37–7.44
(m, 4H), 8.00 (s, 1H), 8.15 (s, 1H). 13C NMR (CDCl3, 125 MHz): d
18.44, 30.11, 35.47, 35.49, 63.16, 64.47, 114.53, 118.98, 119.29,
131.11, 132.94, 133.51, 142.29, 142.52, 163.83, 164.08, 167.00,
168.45. 19F NMR (DMSO-d6, 470 MHz): d −163.27, −162.47,
−144.43. HRMS (EI) m/z calc. (C32H32CoF5N2O4 − C7F5O2)
451.1796, found 451.1776.
4
4
(d, J = 2.0 Hz, 1H), 7.40 (d, J = 2.0 Hz, 1H), 7.44 (s, 1H),
7.45 (s, 1H), 7.93 (s, 1H), 8.09 (s, 1H). 13C NMR (DMSO-d6, 125
MHz): d 18.45, 30.34, 30.38, 31.51, 31.54, 33.39, 33.43, 35.71,
35.73, 62.99, 64.57, 118.57, 118.88, 128.15, 128.67, 128.74, 128.82,
135.84, 136.01, 141.73, 142.01, 161.67, 161.94, 167.03, 168.55.
HRMS (EI) m/z calc. (C33H48BrCoN2O2 − Br) 563.3048, found
563.3037.
rac-(Salen-6)CoOBzF5. To a 50 mL round-bottomed flask
charged with a Teflon stir bar was added rac-(salen-6)Co (0.50 g,
0.89 mmol), pentafluorobenzoic acid (0.19 g, 0.89 mmol) and
methylene chloride (10 mL). The mixture was stirred open to
air for 12 h at 22 ◦C. The methylene chloride was removed by
rotary evaporation, and the crude solid was dissolved in toluene
and filtered. The toluene was then removed, and the green solid
was washed with pentane (50 mL) and filtered. Drying in vacuo
rac-(Salen-10)CoOBzF5. Employing the same reaction condi-
tions as for (R,R)-(salen-2)CoOBzF5, rac-(salen-10)Co (0.10 g,
0.17 mmol) and pentafluorobenzoic acid (36 mg, 0.17 mmol) were
1
afforded a green solid (0.32 g, 46%). H NMR (DMSO-d6, 500
1
stirred for 3 h to afford the crude brown solid (94 mg, 67%). H
MHz): d 1.297 (s, 9H), 1.301 (s, 9H), 1.61 (d, 3J = 6.0 Hz,
NMR (DMSO-d6, 500 MHz): d 1.62 (d, 3J = 7.0 Hz, 3H), 1.69 (s,
2
3
3H), 1.727 (s, 9H), 1.735 (s, 9H), 3.86 (dd, J = 12.5 Hz, J =
2
3
9H), 1.70 (s, 9H), 3.88 (d of d, J = 13.5 Hz, J = 7.0 Hz, 1H),
2
3
6.0 Hz, 1H), 4.21 (dd, J = 12.5 Hz, J = 6.0 Hz, 1H), 4.32
(m, 1H), 7.33 (s, 1H), 7.40 (s, 1H), 7.45 (s, 2H), 7.93 (s, 1H),
8.09 (s, 1H). 13C NMR (DMSO-d6, 125 MHz): d 18.47, 30.36,
31.52, 33.43, 35.76, 63.05, 64.66, 118.58, 118.90, 128.20, 128.86,
136.17, 141.79, 142.04, 162.15, 167.13, 168.60. 19F NMR (DMSO-
d6, 470 MHz): d −163.53, −162.90, −144.86. HRMS (EI) m/z calc.
(C40H48CoF5N2O4 − C7F5O2) 563.3048, found 563.3042.
2
3
4.21 (dd, J = 13.5 Hz, J = 7.0 Hz, 1H), 4.34 (m, 1H), 7.37 (d,
4J = 2.5 Hz, 1H), 7.38 (d, 4J = 2.5 Hz, 1H), 7.61 (d, 4J = 2.5 Hz,
4
1H), 7.69 (d, J = 2.5 Hz, 1H), 8.06 (s, 1H), 8.21 (s, 1H). 13C
NMR (CDCl3, 125 MHz): 17.89, 29.54, 29.74, 29.77, 35.67, 35.73,
35.75, 63.40, 64.69, 104.93, 105.00, 120.50, 120.70, 133.51, 133.53,
134.26, 134.82, 145.00, 145.21, 162.87, 163.16, 168.08, 166.61. d
19F NMR (DMSO-d6, 470 MHz): d −163.66, −162.75, −144.89.
HRMS (EI) m/z calc. (C32H30CoF5N2O4 − C7F5O2) 608.9986,
found 608.9990.
rac-(Salen-7)CoOBzF5. Employing the same reaction condi-
tions as for (R,R)-(salen-2)CoOBzF5, rac-(salen-7)Co (0.20 g,
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◦
(R,R)-(Salen-11)CoBr. Employing the same reaction condi-
tions as for (R,R)-(salen-2)CoBr, (R,R)-(salen-11)Co (0.50 g,
0.59 mmol) and p-toluenesulfonic acid monohydrate (0.11 g,
0.59 mmol) were stirred for 5 h. Following the salt metathesis a
crude black solid was obtained (0.16 g, 29%). 1H NMR (DMSO-
d6, 500 MHz): d 1.50 (m, 2H), 1.57 (s, 6H), 1.58 (s, 6H), 1.76
(s, 6H), 1.84 (m, 2H), 1.93 (m, 2H), 2.29 (s, 6H), 2.92 (m, 2H),
˚
−1
14.5088(11), b = 10.1068(6), c = 29.765(2) A, b = 105.834(9) ,
3
˚
V = 4355.9(5) A , Z = 2, l = 0.494 mm ; 23614 reflections
collected and 8926 independent reflections (Rint = 0.0679). Final
R indices [I > 2r(I)] R1 = 0.0589, wR2 = 0.1325; R indices (all
data) R1 = 0.0808, wR2 = 0.1408.
Crystallographic details for (salen-1)CoX (X = Cl, I) are
summamized in the ESI.†
CCDC reference numbers 289855 and 289856.
For crystallographic data in CIF or other electronic format see
DOI: 10.1039/b513107c
4
3
3.39 (m, 2H), 6.98 (d, J = 3.0 Hz, 2H), 7.06 (tt, J = 9.0 Hz,
4
4J = 2.0 Hz, 2H), 7.12–7.28 (m, 18H), 7.45 (d, J = 2.5 Hz,
2H), 7.75 (s, 2H). 13C NMR (DMSO-d6, 125 MHz): d 24.19,
27.92, 27.98, 29.56, 30.31, 30.42, 30.51, 30.54, 32.55, 41.43, 41.57,
41.90, 43.36, 69.07, 118.92, 125.09, 125.35, 125.39, 125.52, 126.17,
126.20, 126.37, 127.66, 127.74, 127.88, 127.94, 130.80, 133.02,
135.11, 135.14, 141.02, 150.72, 151.20,161.90, 164.70. HRMS (EI)
m/z calc. (C56H60BrCoN2O2 − Br) 851.3987, found 851.3984.
Acknowledgements
G. W. C. gratefully acknowledges a Packard Foundation Fellow-
ship in Science and Engineering, and funding from the NSF (CHE-
0243605), the Swiss National Science Foundation Fellowship to
C. M. T., the Cornell Center for Materials Research (supported
through the NSF MRSEC program, DMR-0079992), Sumitomo
Chemicals, the Cornell University Nanobiotechnology Center,
and the Cornell University Center for Biotechnology, a New York
State Center for Advanced Technology.
(R,R)-(Salen-11)CoOBzF5. Employing the same reaction
conditions as for (R,R)-(salen-2)CoOBzF5, (R,R)-(salen-11)Co
(0.25 g, 0.29 mmol) and pentafluorobenzoic acid (62 mg,
0.29 mmol) were used to afford the crude green product (0.21 g,
1
68%). H NMR (DMSO-d6, 500 MHz): d 1.48 (m, 2H), 1.57 (s,
6H), 1.58 (s, 6H), 1.75 (s, 6H), 1.85 (m, 2H), 1.94 (m, 2H), 2.29 (s,
6H), 2.92 (m, 2H), 3.38 (m, 2H), 6.98 (s, 2H), 7.07 (t, 3J = 7.0 Hz,
2H), 7.12–7.28 (m, 18H), 7.45 (d, 4J = 2.5 Hz, 2H), 7.76 (s, 2H). 13C
NMR (DMSO-d6, 125 MHz): d 24.09, 28.80, 29.45, 30.19, 30.30,
32.47, 41.33, 43.25, 68.96, 118.82, 124.97, 125.23, 126.06, 127.63,
127.76, 130.71, 132.90, 135.03, 140.90, 150.61, 151.10, 161.80,
164.61. 19F NMR (DMSO-d6, 470 MHz): d −163.66, −162.78,
−144.92. HRMS (EI) m/z calc. (C63H60CoF5N2O4 − C7F5O2)
851.3987, found 851.3984.
References
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