Copolymerization of CO2 and cyclohexene oxide: β-diketiminate-supported Zn(II)OMe and Zn(II)Et complexes as initiators

Article abstract of DOI:10.1021/om400977c

β-Diketiminate ligands with varying degrees of steric bulkiness and having -CN and -NO₂ groups on N-aryl substituents were synthesized. The catalytic activity of some of their [LZnEt] and [LZnOMe]₂ complexes (L = β-diketiminato) toward the copolymerization of carbon dioxide and cyclohexene oxide was explored. All the catalysts were found to be highly active and showed very good turnover frequency, and the polycarbonates produced have a very high percentage of carbonate linkages. The complex [L ^iPr,Me-CNZnEt] showed very high activity and selectivity on par with its zinc methoxide analogue. The polymers produced by [LZnEt] complexes displayed a bimodal molecular weight distribution.

Full text of DOI:10.1021/om400977c

Article
pubs.acs.org/Organometallics
Copolymerization of CO2 and Cyclohexene Oxide: β‑Diketiminate-
Supported Zn(II)OMe and Zn(II)Et Complexes as Initiators
N. M. Rajendran, A. Haleel, and N. Dastagiri Reddy*
Department of Chemistry, Pondicherry University, Pondicherry, India 605 014
S
* Supporting Information
ABSTRACT: β-Diketiminate ligands with varying degrees of steric bulkiness and
having −CN and −NO2 groups on N-aryl substituents were synthesized. The catalytic
activity of some of their [LZnEt] and [LZnOMe]2 complexes (L = β-diketiminato)
toward the copolymerization of carbon dioxide and cyclohexene oxide was explored. All
the catalysts were found to be highly active and showed very good turnover frequency,
and the polycarbonates produced have a very high percentage of carbonate linkages.
The complex [LiPr,Me‑CNZnEt] showed very high activity and selectivity on par with its
zinc methoxide analogue. The polymers produced by [LZnEt] complexes displayed a
bimodal molecular weight distribution.
INTRODUCTION
■
Production of polycarbonates via the alternating copolymeriza-
tion of CO2/epoxides is an attractive route, because carbon
dioxide is a cheap and nontoxic renewable resource.1 In addition,
CO2 contributes 31% w/w to poly(cyclohexene oxide) and 43%
w/w to poly(propylene oxide). The never-ending demand for
Figure 1.
organic polymers and ever-depleting petroleum resources, which
are conventional feedstock for the polymer industry, emphasize
the importance of CO2 fixation as a C1 source. Although the
catalytic copolymerization of CO2/epoxides has been known for
the past few decades,2 major advancements in catalytic
efficiencies have been achieved only since 1998 with the
discovery of discrete β-diketiminate zinc complexes by Coates.3
Recent developments in the copolymerization of CO2/epoxides
have been reviewed.4 Very recently, Rieger reported a flexibly
tethered β-diketiminate-based bimetallic zinc complex, which
shows a very high efficiency in catalyzing the copolymerization of
CO2 and cyclohexene oxide.5
β-Diketiminate ligands have emerged as one of the best
supporting ligands, and they have been utilized in stabilizing a
wide variety of transition metals,6 main group elements,7 and
lanthanide metals.8 Besides being innocent, these systems offer
easy fine-tuning of the steric and electronic environment in a
catalytic system. Findings by Coates and others emphasize the
importance of steric and electronic factors in β-diketiminate zinc
complexes employed for the copolymerization of CO2 and
epoxides. In these studies it has been found that cyano-
substituted β-diketiminate ligands with an appropriate steric
environment (Figure 1, L-CNR,R1) afford excellent catalysts.9 In
the course of time, a few more β-diketiminate zinc catalysts
having −OEt, −COOEt, and −CF3 groups on the ligand
backbone have also been explored for the catalysis of the
copolymerization of CO2 and epoxide.10 However, the activities
are found to be much lower than that of −CN-substituted β-
diketiminate zinc catalysts. It is noteworthy that all these groups
can directly influence the conjugate system of the ligand, because
they are connected to its skeleton, and hence result in a drastic
change in the activity. Nevertheless, introduction of electroni-
cally important groups on N-aryl substituents, which are
perpendicular to the plane of the ligand, could offer fine-tuning
of the donor ability of the ligand (Figure 1, LR‑CN/‑NO2,R1). So far
there is only one Zn complex with a β-diketiminate ligand
containing an electronically influential group (−Cl) on N-aryl
substituents reported in the literature.10b In this complex, the
−Cl group is present in the ortho position. Placing such groups in
ortho position affects the steric environment at the active center
and might even allow the group to have undesirable interaction
with the metal center and/or the monomers. Therefore, in this
work, β-diketiminate ligands with cyano and nitro groups in para
position of N-aryl substituents were synthesized, and zinc
complexes of some of these ligands were explored for their
catalytic activity toward the copolymerization of CO2 and
epoxide. Herein, the results of these reactions are discussed in
detail.
RESULTS AND DISCUSSION
■
4-Cyano- and 4-nitro-2,6-dialkylanilines, which were used in the
synthesis of β-diketimines, were prepared in moderate yields by
following standard procedures.11 Unsymmetrical β-diketimines
were synthesized in two steps. In the first step, monoketimines
were synthesized in good yields by refluxing appropriate 2,6-
Received: October 6, 2013
© XXXX American Chemical Society
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dialkylanilines with excess acetylacetone. The second step, which
involves condensation of monoketimines with 4-cyano- or 4-
nitro-2,6-dialkylanilines, was carried out under anhydrous
conditions using P2O5 as a water scavenger.12 Symmetrical β-
diketimines were synthesized directly in one step by con-
densation of acetylacetone with corresponding anilines in
toluene under reflux conditions in the presence of P2O5 (Scheme
1). All the ligands were characterized by using NMR (1H and
13C) and IR techniques.
β-Diketimine ligands [LH] were treated with an excess of
ZnEt2 (1.5−2 times) in toluene at 80 °C. All −CN-substituted
ligands yielded [LZnEt] complexes as pale yellow, oily liquids/
solids, which were pure enough for carrying out further reactions
(Scheme 2). The absence of an NH resonance and the
appearance of Zn−Et resonances in 1H NMR spectra confirmed
the formation of zinc complexes. Under similar conditions,
−NO2-substituted ligands gave some red-colored products,
which showed complex NMR spectra. No pure substances could
be isolated from these mixtures. The −CN-substituted [LZnEt]
complexes were then treated with one equivalent of methanol in
toluene. The complexes [LiPr,Et‑CNZnEt], [LiPr,Me‑CNZnEt], and
[LEt‑CN,Et‑CNZnEt] afforded [LZnOMe]2 (1−3, respectively)
complexes in good yields (Scheme 2). While the presence of
Scheme 1. Syntheses of Ligands
1
the methoxy group in the H NMR spectra was evident by the
appearance of a singlet resonance around δ 3.2 ppm, the absence
of ZnEt resonances confirmed the conversion. Under similar
conditions, ligand regeneration was observed in the case of
[LiPr,iPr‑CNZnEt], while [LEt,Et‑CNZnEt] gave a bis(chelate)
complex [(LEt,Et‑CN)2Zn] (4), which is similar to [(LiPr,H)2Zn]
reported in the literature,13 and [LiPr‑CN,iPr‑CNZnEt] did not react
at all. However, when excess MeOH (5 equiv) was used,
[LiPr‑CN,iPr‑CNZnEt] gave the demetalated product [CH2(C-
(Me)N(2,6-diisopropyl-4-cyanophenyl))2] (5), which is a
tautomer of the ligand LiPr‑CN,iPr‑CNH. A similar observation
(ligand regeneration) is reported in case of bimetallic anilido-
aldimine zinc methyl complexes and some β-diketiminate-ZnEt
complexes.13,14 An analogous ligand (both sides ortho isopropyl
groups) with −CN in γ position is found in the literature to form
the complex [LZnOMe]2 under similar conditions.15
The molecular structures of [LiPr,Et‑CNZnOMe]2,
[LiPr,Me‑CNZnOMe]2, and [LEt‑CN,Et‑CNZnOMe]2 were confirmed
by single-crystal X-ray data analysis. ORTEP diagrams are shown
in Figures 2−4. Important bond lengths and bond angles are
listed in Table 1. While the 1H NMR spectra of
[LiPr,Me‑CNZnOMe]2 and [LiPr,Et‑CNZnOMe]2 show a mixture of
dimer and monomer in solution state, the solid-state structures of
all three complexes are dimeric. The Zn−Zn distance, which is
similar in all three structures (av 3.0 Å), is in the range (3.0−3.1
Scheme 2. Syntheses of [LZn(OMe)]2 Complexes
B
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compound also confirms that it exists as diimine tautomer rather
than an imine−enamine tautomer even in solution state.
Polymerization Studies. All the three zinc methoxide
complexes have been explored for catalytic activity toward the
copolymerization of cyclohexene oxide and CO2. Polymerization
reactions were carried out in cyclohexene oxide with catalyst
loadings of 0.1% at 50 °C and a 100 psi of CO2 for 30 min. The
results are listed in Table 2. All the complexes were found to be
highly active. The reactions had progressed with very good
turnover frequency (TOF), and the polycarbonates produced
had a very high percentage of carbonate linkages.
The activity of the complexes prepared in this study is
compared with that of similar complexes reported in the
literature (Table 2). It is found that the activity of complexes 1−3
is significantly higher than that of the complexes having no −CN
group in the ligand ([LZnOMe]2) (entries 2, 5, and 8).13
However, the catalysts with a −CN group on the ligand
backbone (([L-CN)ZnOMe]2) are much superior to these
catalysts (entries 3, 6, and 9).9 This observation clearly
demonstrates that the electronic influence of the −CN group
on the activity is moderate when it is placed on N-aryl
substituents. It is also noteworthy that the complex
[LiPr,Me‑CNZnOMe]2 having unsymmetrical ligand geometry is
more active and selective. This observation is similar to the
literature reports. These results demonstrate that electronically
influential groups in the para position of N-aryl substituents can
be used for fine-tuning of the catalyst without disturbing the
steric environment at the active center. A moderate increase in
TOF and carbonate linkages was observed when the
copolymerization reactions were carried out at a higher pressure
of 150 psi of CO2 (entries 10−12). A literature report has also
shown a similar observation.10b No further improvement was
observed when the pressure was raised to 200 psi with the
catalyst [LiPr,Me‑CNZnOMe]2.
Figure 2. Molecular structure of [LiPr,Et‑CNZn(OMe)]2. Hydrogen atoms
and the solvent molecule CH2Cl2 are omitted for clarity. Thermal
ellipsoids are drawn at the 30% probability level.
Å) of the reported alkoxy-bridged β-diketiminate Zn complex-
es.3a,6d,9,13,16 Zn atoms adopt a distorted tetrahedral geometry
and deviate from the mean plane of the rest of the atoms of the
chelate ring by ∼0.4 Å. The Zn2O2 ring is planar, and all the bond
parameters are the same as those for similar structures reported
in the literature.3a,6d,9,13,16 The molecular structure of [CH2(C-
(Me)N(2,6-diisopropyl-4-cyanophenyl))2] (5) was also con-
firmed by single-crystal X-ray studies. The crystal data and
ORTEP diagram along with selected bond parameters are given
in Table S1 and Figure S1 (see Supporting Information). The
absence of an NH resonance and presence of a CH2 resonance at
δ 3.02 ppm (integration for 2 H) in the 1H NMR spectrum of the
The zinc complexes that have been used as catalysts for the
copolymerization of CO2 and CHO are typically acetates,
alkoxides,4a or alkylsulfinates.3g,14,17 Zinc alkyl complexes,
especially mononuclear complexes, are not reported to initiate
this reaction.13 However, there are at least two articles that report
Table 1. Selected Bond Lengths and Bond Angles for [LZn(OMe)]2 Complexes
[LiPr,Et‑CNZn(OMe)]2
[LiPr,Me‑CNZn(OMe)]2
[LEt‑CN,Et‑CNZn(OMe)]2
Bond Distances/Å
Zn1−O1
Zn1−O1′
Zn1−N1
Zn1−N2
C1−N1
C1−C2
C2−C3
C3−N2
1.944(2)
1.978(2)
1.993(2)
1.993(2)
1.326(3)
1.400(4)
1.383(4)
1.333(3)
3.001(1)
0.450
1.952(2)
1.978(2)
1.991(2)
1.988(2)
1.321(3)
1.400(4)
1.381(4)
1.332(3)
3.011(1)
0.362
1.950(1)
1.983(1)
1.985(1)
1.991(1)
1.330(2)
1.398(2)
1.400(2)
1.327(2)
3.002(0)
0.406
Zn1···Zn1′
Zn1···mean plane (N1,C1,C2,C3,N2)
Bond Angles/deg
Zn1−O1−Zn1′
O1−Zn1−O1′
N1−Zn1−N2
N1−Zn1−O1
N2−Zn1−O1
N1−Zn1−O1′
N2−Zn1−O1′
99.83(7)
80.17(7)
100.00(8)
80.00(8)
99.49(5)
80.51(5)
96.25(8)
96.47(9)
95.82(5)
126.69(8)
119.67(8)
119.40(8)
116.53(8)
124.99(8)
120.72(8)
118.17(8)
118.51(8)
124.73(5)
121.21(5)
119.96(5)
116.76(5)
C
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a
Table 2. Copolymerization of CHO/CO2 Using Zn Complexes
b
c
d
e
entry
complex
time
TON
TOF (h−1
)
carbonate (%)
Mn (kg/mol)
Mw/Mn
ref
1
[LiPr,Et‑CNZnOMe]2
[LiPr,EtZnOMe]2
[L-CNiPr,EtZnOMe]2
[LiPr,Me‑CNZnOMe]2
[LiPr,MeZnOMe]2
[L-CNiPr,MeZnOMe]2
[LEt‑CN,Et‑CNZnOMe]2
[LEt,EtZnOMe]2
[L-CNEt,EtZnOMe]2
[LiPr,Et‑CNZnOMe]2
[LiPr,Me‑CNZnOMe]2
[LEt‑CN,Et‑CNZnOMe]2
0.50
2.00
0.17
0.50
2.00
0.17
0.50
2.00
0.17
0.50
0.50
0.50
368
515
362
379
418
382
345
477
282
389
407
374
736
257
91
99
89
93
94
90
79
96
93
95
97
83
22.6
32.1
22.8
19.7
17.0
22.9
19.4
23.7
17.8
27.2
32.6
21.0
1.28
1.14
1.11
1.27
1.21
1.09
1.12
1.14
1.08
1.23
1.21
1.37
this work
2
13
3
2170
758
9
4
this work
13
5
209
6
2290
690
9
7
this work
13
8
239
9
10f
1690
778
9
this work
this work
this work
f
11
814
f
12
748
a
b
All reactions were performed in neat cyclohexene oxide (CHO) with [monomer]:[Zn] = 1000:1 at 50 °C and 100 psi of CO2 . TON = moles of
c
polymer formed (PCHC (carbonate) + ether linkage) per mole of Zn, and in refs 9 and 13, TON = moles of CHO consumed per mole of Zn. TOF
= moles of polymer formed (PCHC + ether linkage) per mole of Zn per hour, and in refs 9 and 13, TOF = moles of CHO consumed per mole of Zn
per hour. Calculated by integration of methine resonances of the carbonate linkage (δ = 4.6 ppm) and ether linkage (δ = 3.4 ppm) in HNMR
d
1
e
f
(CDCl3 400 MHz). Determined by gel permeation chromatography calibrated with polystyrene standard in THF. 150 psi of CO2.
a
Table 3. Copolymerization of CHO/CO2 Using ZnEt Complexes
b
c
d
e
entry
13
complex
[LiPr,iPr‑CNZnEt]
pressure (psi)
100
TON
142
TOF(h−1
284
)
carbonate (%)
56
area (%)
Mn (kg/mol)
Mw/Mn
30
70
40
60
23
77
60
40
96.5
23.5
70.2
23.2
105.7
32.1
83.0
1.40
1.35
1.05
1.35
1.04
1.39
1.11
1.23
14
15
16
[LiPr,Me‑CNZnEt]
[LiPr,Me‑CNZnEt]
[LiPr,Me‑CNZnEt]
100
150
200
286
316
374
572
632
748
94
96
98
28.3
a
b
All reactions were performed in neat cyclohexene oxide (CHO) with [monomer]:[Zn] = 1000:1 at 50 °C for 0.5 h. TON = moles of polymer
c
d
formed (PCHC + ether linkage) per mole of Zn. TOF = moles of polymer formed (PCHC + ether linkage) per mole of Zn per hour. Calculated
from integration of methine resonances of the carbonate linkage (δ = 4.6 ppm) and ether linkage (δ = 3.4 ppm) in the 1HNMR (CDCl3 400 MHz).
e
Determined by gel permeation chromatography calibrated with polystyrene standard in THF.
that bimetallic β-diketiminate ZnEt2 complexes show moderate
to good activity toward the copolymerization of CO2/CHO,
present in the literature.18 Since we could not convert some of
the zinc alkyl complexes into the corresponding zinc alkoxide
complexes, we were interested in examining them for the
catalytic activity. Polymerization reactions were carried out at 50
°C and 100 psi of CO2 for 30 min, and the monomer/catalyst
ratio was 1000:1. The complex containing two −CN groups
[LiPr‑CN,iPr‑CNZnEt] produced merely a small amount of polymer
that is found to contain only 5% polycarbonate linkages. On the
other hand, the [LiPr,iPr‑CNZnEt] complex, which contains one
−CN group, showed very high activity and gave a good yield of
the polymer, although the polycarbonate linkages were found to
be only 56%. To compare the activity of zinc alkyl complexes
with that of corresponding zinc methoxide complexes, we have
employed the complex [LiPr,Me‑CNZnEt], which is a precursor to
our most efficient zinc methoxide catalyst [LiPr,Me‑CNZnOMe]2.
Under similar conditions, [LiPr,Me‑CNZnEt] afforded poly-
(cyclohexene carbonate) with very good TOF (572 h−1) and a
high percentage of carbonate linkages (94%). Further, upon
increasing the CO2 pressure from 100 to 200 psi, the catalyst
activity and the selectivity increased (TOF = 748 h−1; carbonate
linkages = 98%). The results are listed in Table 3. Contrary to the
literature reports, the zinc alkyl complexes show very good
activity toward the copolymerization of CO2/CHO.
[LiPr,Me‑CNZnEt] is found to be almost as active as the analogous
zinc methoxide complex [LiPr,Me‑CNZnOMe]2. Unlike the
Figure 3. Molecular structure of [LiPr,Me‑CNZn(OMe)]2. Hydrogen
atoms are omitted for clarity. Thermal ellipsoids are drawn at the 30%
probability level.
polymers obtained in the case of Zn methoxide catalysts, in all
these experiments (entries 13 to 16) the polycarbonates
produced have a bimodal molecular weight distribution, which
D
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[(2,6-dialkylphenyl)amino]pent-3-en-2-one12 were prepared by follow-
ing reported procedures. NMR spectra were recorded on a Bruker 400
MHz spectrometer using CDCl3 or benzene-d6 as solvent. IR spectra
were recorded on a Perkin-Elmer 882 FT-IR spectrophotometer in KBr.
Elemental analyses were performed using a Flash 2000 organic
elemental analyzer. High-resolution mass spectral (HRMS) data were
obtained on a Micromass QTOF mass spectrometer.
Single-Crystal X-ray Experiments. X-ray diffraction data for
[LZn(OMe)]2 complexes were collected on a Bruker AXS Kappa APEX
II diffractometer, and the data for compound [CH2(C(Me)N(2,6-
diisopropyl-4-cyanophenyl))2] were obtained on an Oxford XCALI-
BUR diffractometer. Both the diffractometers were equipped with a
CCD area detector, and the data were collected using graphite-
monochromated Mo Kα (λ = 0.71073 Å) radiation. In order to avoid
quality degradation, all the single crystals were coated with
perfluoropolyalkyl ether oil/paraffin oil and mounted on a glass fiber
using cyanoacrylate glue. All the structure solutions were performed
using SHELXS-97 and SHELXL-97.19 The structures were solved by
direct methods and successive interpretation of the difference Fourier
maps, followed by full matrix least-squares refinement (against F2). All
non-hydrogen atoms were refined anisotropically. The contribution of
the hydrogen atoms, in their calculated positions, was included in the
refinement using a riding model. Upon convergence, the final Fourier
difference map of the X-ray structures showed no significant peaks. All
the data sets were collected to 2θ values of >50°. Relevant data
concerning crystallographic data, data collection, and refinement details
for compounds 1−3 and 5 are summarized in Table S1 (see Supporting
Information). CCDC 817173−817175 and 962796 contain supple-
mentary crystallographic data for compounds 1−3 and 5, respectively.
These data can be obtained free of charge from the Cambridge
Crystallographic Data Centre via www.ccdc.cam.ac.uk/data_request/
cif.
Figure 4. Single-crystal X-ray structure for [LEt‑CN,Et‑CNZn(OMe)]2.
Hydrogen atoms are omitted for clarity. Thermal ellipsoids are drawn at
the 30% probability level.
indicates a simultaneous operation of two mechanisms.
Presumably, one mechanism involves a dimeric zinc carboxylate
complex, which is formed in situ by the insertion of CO2 into the
Zn−Et bond. Another mechanism could probably utilize a single
active metal center. It is noteworthy that the polymers show a
narrow molecular weight distribution (low PDI), which points to
controlled polymerization in both the pathways.
CONCLUSION
General Procedure for the Synthesis of Unsymmetrical
Ligands (ref 12). To a slurry of P2O5 and toluene (20 mL) in a
round-bottom flask was added an equimolar mixture of 4-[(2,6-
dialkylphenyl)amino]pent-3-en-2-one and 4-cyano/nitro-2,6-dialkyla-
niline. The reaction mixture was refluxed under a nitrogen atmosphere
for 18 h and then allowed to cool to room temperature. To this was
slowly added 50 mL of a saturated sodium bicarbonate solution, and the
resulting mixture was extracted with 50 mL of dichloromethane. The
organic layer was washed with 3 × 30 mL of brine solution, dried over
anhydrous sodium sulfate, and filtered. The solvent was removed using a
rotary evaporator to obtain a solid residue, which was chromatographed
over a neutral alumina column using a mixture of ethyl acetate−hexane
(∼10:90) as eluent to obtain a white solid. The final product was
recrystallized from a dichloromethane−hexane mixture to obtain
colorless crystals.
■
In summary, several β-diketiminate ligands with varying degrees
of steric bulkiness and having −CN and −NO2 groups on N-aryl
substituents were synthesized. Our attempts to synthesize Zn
complexes of −NO2-substituted ligands were unsuccessful.
Though all the six −CN-substituted ligands afforded [LZnEt]
complexes with ZnEt2, only three of them could be converted to
zinc methoxide complexes [LZnOMe]2. Catalytic activity of
these complexes toward the copolymerization of carbon dioxide
and cyclohexene oxide was explored. All three catalysts are highly
active and showed a very good turnover frequency, and the
polycarbonates produced bore a very high percentage of
carbonate linkages. The activity of these complexes was
compared with that of similar complexes having no −CN
group and complexes having a −CN group on the ligand
backbone, and it was found that these complexes stood in
between the two categories reported in the literature. Finally,
zinc alkyl complexes of a few β-diketiminate ligands reported in
this work were also examined for their activity. Interestingly, the
complex [LiPr,Me‑CNZnEt] showed very high activity and
selectivity on par with its zinc methoxide analogue. The
polymers produced by [LZnEt] complexes displayed a bimodal
molecular weight distribution, which indicates a simultaneous
operation of two mechanisms.
[LiPr,iPr‑CNH]. 4-[(2,6-Diisopropylphenyl)amino]pent-3-en-2-one
(0.71 g, 2.74 mmol), 4-cyano-2,6-diisopropylaniline (0.56 g, 2.77
mmol), and P2O5 (3.50 g, 24.65 mmol) were used. Yield: 0.82 g, 68%.
1
Mp: 209−210 °C. H NMR (C6D6, 400 MHz, ppm): δ 12.02 (1H, s,
NH), 7.46 (2H, s, ArH), 7.27 (1H, m, ArH), 7.16 (2H, d, ArH), 4.90
(1H, s, β-CH), 3.34 (2H, m, CHMe2), 3.13 (2H, m, CHMe2), 1.65 (6H,
s, α-Me), 1.24 (6H, d, CHMeMe), 1.16 (6H, d, CHMeMe), 1.08 (12H,
m, CHMeMe). 13C NMR (C6D6, 100 MHz, ppm): δ 166.59, 157.16,
150.81, 146.40, 140.45, 135.32, 128.16, 127.57, 123.86, 120.33, 107.59,
94.05, 28.72, 28.33, 24.69, 23.45, 23.15, 22.82, 21.94, 19.73. HRMS
(ESI-TOF) m/z: [M + H]+ calcd for C30H42N3 444.3379, found
444.3380. IR (KBr): ν 2221 (CN) cm−1.
[LiPr,iEt‑CNH]. 4-[(2,6-Diisopropylphenyl)amino]pent-3-en-2-one
(0.70 g, 2.70 mmol), 4-cyano-2,6-diethylaniline (0.47 g, 2.72 mmol),
and P2O5 (3.50 g, 24.65 mmol) were used. Yield: 0.82 g, 73%. Mp: 156−
157 °C. 1H NMR (CDCl3, 400 MHz, ppm): δ 11.70 (1H, s, NH), 7.34
(2H, s, ArH), 7.26 (1H, m, ArH), 7.14 (2H, d, ArH), 4.89 (1H, s, β-CH),
3.16 (2H, m, CHMe2), 2.53 (2H, m, CHHCH3), 2.40 (2H, m,
CHHCH3), 1.71 (3H, s, α-Me), 1.70 (3H, s, α-Me), 1.25 (6H, d,
(CHMeMe), 1.17 (6H, t, CH2CH3), 1.10 (6H, d, CHMeMe). 13C NMR
(CDCl3, 100 MHz, ppm): δ 165.95, 157.10, 152.58, 146.24, 135.15,
134.78, 129.50, 127.66, 123.42, 120.37, 105.42, 93.29, 28.42, 24.65,
24.19, 22.94, 21.87, 19.81, 13.51. HRMS (ESI-TOF) m/z: [M + H]+
EXPERIMENTAL SECTION
■
General Procedures. Manipulations involving air/moisture-
sensitive compounds were carried out under a N2 atmosphere using a
Schlenk line and a glovebox. Diethylzinc (1.0 M in heptane) solution,
anhydrous methanol, and copper(I) cyanide were procured from
Aldrich and used as received. 2,6-Dialkylanilines (from CaH2), toluene,
and hexane (from Na/benzophenone ketyl) were distilled fresh and
when required. Benzene-d6 was condensed from Na/benzophenone
ketyl, and CDCl3 was condensed from CaH2 and stored in a glovebox. 4-
Cyano-2,6-dialkylanilines,11a 4-nitro-2,6-diisopropylaniline,11b and 4-
E
dx.doi.org/10.1021/om400977c | Organometallics XXXX, XXX, XXX−XXX

Organometallics
Article
calcd for C28H38N3 416.3066, found 416.3063. IR (KBr): ν 2217 (C
N) cm−1.
were used. Yield: 0.656 g, 31%. Mp: 228−229 °C. 1H NMR (CDCl3, 400
MHz, ppm): δ 11.75 (1H, s, NH), 8.02 (4H, s, ArH), 5.01 (1H, s, β-
CH), 3.13 (4H, m, CHMe2), 1.74 (6H, s, α-Me), 1.28 (12H, d,
CHMeMe), 1.16 (12H, d, CHMeMe). 13C NMR (CDCl3, 100 MHz): δ
161.33, 146.90, 145.88, 144.37, 119.24, 95.18, 28.78, 24.07, 22.91, 21.19.
HRMS (ESI-TOF) m/z: [M + H]+ calcd for C29H41N4O4 509.3128,
found 509.3121. IR (KBr): ν1540, 1321 (NO2) cm−1.
[LiPr,Me‑CNH]. 4-[(2,6-Diisopropylphenyl)amino]pent-3-en-2-one
(1.23 g, 4.75 mmol), 4-cyano-2,6-dimethylaniline (0.70 g, 4.79
mmol), and P2O5 (6.00 g, 42.25 mmol) were used. Yield: 1.12 g, 61%.
Mp: 124−125 °C. 1H NMR (CDCl3, 400 MHz, ppm): δ 11.81 (1H, s,
NH), 7.33 (2H, s, ArH), 7.28 (1H, m, ArH), 7.18 (2H, d, ArH), 4.92
(1H, s, β-CH), 3.16 (2H, m,CHMe2), 2.12 (6H, s, o-CH3), 1.73 (3H, s,
α-Me), 1.72 (3H, s, α-Me), 1.28 (6H, d, CHMeMe), 1.15 (6H, d,
CHMeMe). 13C NMR (CDCl3, 100 MHz, ppm): δ 165.97, 157.26,
153.87, 146.32, 135.05, 131.57, 129.63, 127.77, 123.50, 120.21, 105.09,
93.24, 28.67, 24.86, 22.75, 21.76, 19.82, 18.07. HRMS (ESI-TOF) m/z:
[M + H]+ calcd for C26H34N3 388.2753, found 388.2749. IR (KBr): ν
2220 (CN) cm−1.
General Procedure for the Synthesis of [LZnEt] (ref 3a). To a
stirred solution of LH in toluene (20 mL) at 0 °C was added diethylzinc
(1.0 M in heptane) dropwise. After being stirred overnight at 80 °C, the
clear solution was dried under vacuum for several hours to obtain a
yellow semisolid/solid.
[LiPr,iPr‑CNZnEt]. Diethylzinc (1.7 mL, 1.70 mmol) and [LiPr,iPr‑CNH]
(0.50 g, 1.13 mmol) were used. It was isolated as a yellow semisolid.
1
[LEt,Et‑CNH]. 4-[(2,6-Diethylphenyl)amino]pent-3-en-one (1.54 g,
6.67 mmol), 4-cyano-2,6-diethylaniline (1.16 g, 6.67 mmol), and P2O5
(8.50 g, 59.86 mmol) were used. Yield: 1.65 g, 64%. Mp: 128−129 °C.
1H NMR (CDCl3, 400 MHz, ppm): δ 11.64 (1H, s, NH), 7.25 (2H, s,
ArH), 7.11 (1H, m, ArH), 7.02 (2H, m, ArH), 4.80 (1H, s, β-CH), 2.57
(2H, m, CHHCH3), 2.50 (4H, m, CHHCH3), 2.36 (2H, m, CHHCH3)
1.61 (6H, s, α-Me), 1.09 (12H, m, CH2CH3). 13C NMR (CDCl3, 100
MHz, ppm): δ 165.83, 156.96, 152.45, 141.72, 136.62, 135.22, 129.56,
127.20, 126.28, 120.37, 105.40, 93.45, 24.72, 24.24, 21.84, 19.70, 14.71,
13.57. HRMS (ESI-TOF) m/z: [M + H]+ calcd for C26H34N3 388.2753,
found 388.2744. IR (KBr): ν 2214 (CN) cm−1.
Yield: 0.49 g, 81%. H NMR (C6D6, 400 MHz, ppm): δ 7.28 (2H, s,
ArH), 7.17 (1H, m, ArH), 7.09 (2H, m, ArH), 4.93 (1H, s, β-CH), 3.03
(4H, m, CHMe2), 1.65 (3H, s, α-Me), 1.51 (3H, s, α-Me), 1.21 (6H, d,
CHMeMe), 1.13 (6H, d, CHMeMe), 1.03 (6H, d, CHMeMe), 0.90 (6H,
d, CHMeMe), 0.82 (3H, t, CH2CH3), 0.16 (2H, m, J = 8.0 Hz,
CH2CH3). 13C NMR (C6D6, 100 MHz, ppm): δ 168.45, 166.07, 149.45,
144.50, 143.55, 141.28, 126.31, 123.96, 119.72, 110.17, 95.71, 28.54,
28.43, 24.15, 23.61, 23.45, 23.34, 23.24, 22.79, 12.02, −1.24. Anal. Calcd
for C32H45N3Zn: C, 71.56; H, 8.45; N, 7.82. Found: C, 71.24; H, 8.70;
N, 7.96.
[LiPr,Et‑CNZnEt]. Diethylzinc (2.0 mL, 2.00 mmol) and [LiPr,Et‑CNH]
(0.53 g, 1.27 mmol) were used. It was isolated as a yellow semisolid.
Yield: 0.52 g, 81%. 1H NMR (C6D6, 400 MHz, ppm): δ 7.12 (3H, m,
ArH), 7.07 (2H, s, ArH), 4.91 (1H, s, β-CH), 3.06 (2H, m, CHMe2),
2.37 (2H, m, CH2CH3), 2.18 (2H, m, CH2CH3), 1.65 (3H, s, α-Me),
1.42 (3H, s, α-Me), 1.19 (6H, d, CHMeMe), 1.14 (6H, d, CHMeMe),
0.93 (6H, t, CH2CH3), 0.83 (3H, t, CH2CH3), 0.15 (2H, m, CH2CH3).
13C NMR (C6D6, 100 MHz, ppm): δ 168.41, 165.73, 151.20, 144.49,
141.35, 138.60, 130.44, 129.34, 126.30, 125.70, 123.96, 119.53, 109.46,
95.77, 28.48, 24.75, 24.22, 23.40, 23.29, 22.99, 13.78, 12.05, −1.71. Anal.
Calcd for C30H41N3Zn: C, 70.78; H, 8.12; N, 8.25. Found: C, 70.49; H,
7.94; N, 8.28.
[LEt,iPr‑NO2H]. 4-[(2,6-Diethylphenyl)amino]pent-3-en-one (0.71 g,
3.05 mmol), 4-nitro-2,6-diisopropyl aniline (0.68 g, 3.05 mmol), and
P2O5 (4.00 g, 28.17 mmol) were used. Yield: 0.71 g, 54%. Mp: 158−159
°C. 1H NMR (CDCl3, 400 MHz, ppm): δ 11.62 (1H, s, NH), 8.00 (2H,
s, ArH), 7.20 (1H, m, ArH), 7.12 (2H, d, J = 7.6 Hz, ArH), 4.92 (1H, s, β-
CH), 3.04 (2H, m, CHMe2), 2.61 (4H, m, CH2CH3), 1.74 (3H, s, α-
Me), 1.71 (3H, s, α-Me), 1.20 (18H, m, CH2CH3 and CHMe2). 13C
NMR (CDCl3, 100 MHz, ppm): δ 166.12, 157.03, 153.19, 143.76,
141.84, 139.92, 136.37, 127.39, 126.28, 119.10, 93.51, 28.31, 24.64,
23.46, 22.82, 22.19, 19.71, 14.60. HRMS (ESI-TOF) m/z: [M + H]+
calcd for C27H38N3O2 436.2964, found 436.2976. IR (KBr): ν 1540,
[LiPr,Me‑CNZnEt]. Diethylzinc (4.0 mL, 4.00 mmol) and [LiPr,Me‑CNH]
(1.05 g, 2.71 mmol) were used. It was isolated as a pale yellow solid.
Yield: 1.16 g, 89%. 1H NMR (C6D6, 400 MHz, ppm): δ 7.09 (3H, m,
ArH), 6.91 (2H, s, ArH), 4.91 (1H, s, β-CH), 3.05 (2H, m, CHMe2),
1.83 (6H, s, o-CH3), 1.64 (3H, s, α-Me), 1.37 (3H, s, α-Me), 1.18 (6H, d,
CHMeMe), 1.14 (6H, d, CHMeMe), 0.84 (3H, t, CH2CH3), 0.14 (2H,
m, CH2CH3). 13C NMR (C6D6, 100 MHz, ppm): δ 168.41, 165.37,
152.36, 144.52, 141.30, 132.92, 132.16, 126.30, 123.95, 119.27, 108.79,
95.81, 28.47, 24.23, 23.28, 22.48, 18.14, 11.90, −2.26. Anal. Calcd for
C28H37N3Zn: C, 69.92; H, 7.75; N, 8.74. Found: C, 69.73; H, 7.47; N,
8.97.
1328 (NO2) cm−1
.
Genernal Procedure for the Synthesis of Symmetrical
Ligands. A mixture of acetyl acetone and 4-cyano/nitro-2,6-dialkylani-
line was added to a slurry of P2O5 and toluene (20 mL) in a round-
bottom flask. The reaction mixture was refluxed under a nitrogen
atmosphere for 72 h and allowed to cool to room temperature. To this
was added slowly 50 mL of a saturated sodium bicarbonate solution, and
the resulting mixture was extracted with 50 mL of dichloromethane. The
organic layer was washed with 3 × 30 mL of brine solution, dried over
anhydrous sodium sulfate, and filtered. The solvent was removed using a
rotary evaporator to obtain a solid residue, which was chromatographed
over a neutral alumina column using a mixture of ethyl acetate−hexane
as eluent to obtain a solid. The final product was recrystallized from a
dichloromethane−hexane mixture to obtain crystals.
[LiPr‑CN,iPr‑CNZnEt]. Diethylzinc (2.2 mL, 2.20 mmol) and
[LiPr‑CN,iPr‑CNH] (0.53 g, 1.13 mmol) were used. It was isolated as a
1
yellow solid. Yield: 0.52 g, 83%. H NMR (C6D6, 400 MHz, ppm): δ
[LiPr‑CN,iPr‑CNH]. Acetyl acetone (0.20 g, 2.00 mmol), 4-cyano-2,6-
diisopropylaniline (0.89 g, 4.40 mmol), and P2O5 (3.00 g, 21.13 mmol)
were used. Yield: 0.44 g, 47%. Mp: 265−268 °C. 1H NMR (CDCl3, 400
MHz, ppm): δ 11.79 (1H, s, NH), 7.40 (4H, s, ArH), 4.96 (1H, s, β-
CH), 3.08 (4H, m, CHMe2), 1.71 (6H, s, α-Me), 1.22 (12H, d,
CHMeMe), 1.11 (12H, d, CHMeMe). 13C NMR (CDCl3, 100 MHz,
ppm): δ 161.35, 145.11, 144.15, 127.65, 119.85, 109.10, 94.88, 28.46,
24.06, 22.93, 21.11. HRMS (ESI-TOF) m/z: [M + H]+ calcd for
C31H41N4 469.3331, found 469.3329. IR (KBr): ν 2220 (CN) cm−1.
[LEt‑CN,Et‑CNH]. Acetyl acetone (0.20 g, 2.00 mmol), 4-cyano-2,6-
diethylaniline (0.77 g, 4.43 mmol), and P2O5 (3.00 g, 21.13 mmol) were
used. Yield: 0.43 g, 52%. Mp: 225−226 °C. 1H NMR (CDCl3, 400 MHz,
ppm): δ 11.88 (1H, s, NH), 7.38 (4H, s, ArH), 4.97 (1H, s, β-CH), 2.53
(8H, m, CH2CH3), 1.70 (6H, s, α-Me), 1.17 (12H, t, CH2CH3). 13C
NMR (CDCl3, 100 MHz, ppm): δ 161.06, 146.76, 139.32, 129.96,
119.63, 108.46, 94.97, 24.51, 20.87, 14.02. HRMS (ESI-TOF) m/z: [M
+ H]+ calcd for C27H33N4 413.2705, found 413.2700. IR (KBr): ν 2221
(CN) cm−1.
7.27 (4H, s, ArH), 4.88 (1H, s, β-CH), 2.89 (4H, m, J = 6.8 Hz, CHMe2),
1.46 (6H, s, α-Me), 0.98 (12H, d, CHMeMe), 0.88 (12H, d, CHMeMe),
0.74 (3H, t, CH2CH3), 0.07 (2H, m, CH2CH3). 13C NMR (C6D6, 100
MHz, ppm): δ 167.22, 148.83, 143.32, 119.54, 110.55, 95.98, 28.45,
23.52, 23.29, 22.74, 11.94, −1.32. Anal. Calcd for C33H44N4Zn: C, 70.51;
H, 7.89; N, 9.97. Found: C, 70.69; H, 7.83; N, 10.00.
[LEt‑CN,Et‑CNZnEt]. Diethylzinc (2.0 mL, 2.00 mmol) and
[LEt‑CN,Et‑CNH] (0.53 g, 1.29 mmol) were used. It was isolated as a
1
yellow solid. Yield: 0.56 g, 86%. H NMR (C6D6, 400 MHz, ppm): δ
7.05 (4H, s, ArH), 4.83 (1H, s, β-CH), 2.27 (4H, m, CH2CH3), 2.13
(4H, m, CH2CH3), 1.38 (6H, s, α-Me), 0.88 (12H, d, CH2CH3), 0.78
(3H, t, CH2CH3), 0.07 (2H, m, CH2CH3). 13C NMR (C6D6, 100 MHz,
ppm): δ 166.83, 150.56, 138.39, 130.55, 119.36, 109.81, 96.04, 24.68,
22.98, 13.78, 11.98, −2.16. Anal. Calcd for C29H36N4Zn: C, 68.84; H,
7.17; N, 11.07. Found: C, 68.68; H, 7.26; N, 11.35.
[LEt,Et‑CNZnEt]. Diethylzinc (1.0 mL, 1.00 mmol) and [LEt,Et‑CNH]
(0.23 g, 0.59 mmol) were used. It was isolated as a yellow solid. Yield:
0.22 g, 77%. 1H NMR (C6D6, 400 MHz, ppm): δ 7.15 (2H, s, ArH), 7.06
(3H, m, ArH), 4.87 (1H, s, β-CH), 2.54 (2H, m, CH2CH3), 2.38 (4H, m,
CH2CH3), 2.18 (2H, m, CH2CH3), 1.58 (3H, s, α-Me), 1.42 (3H, s, α-
[LiPr‑NO2,iPr‑NO2H]. Acetyl acetone (0.41 g, 4.10 mmol), 4-nitro-2,6-
diisopropylaniline (2.10 g, 9.46 mmol), and P2O5 (6.00 g, 42.25 mmol)
F
dx.doi.org/10.1021/om400977c | Organometallics XXXX, XXX, XXX−XXX

Organometallics
Article
1
Me), 1.12 (6H, t, CH2CH3), 0.89 (9H, m, CH2CH3 and Zn-CH2CH3),
0.15 (2H, m, CH2CH3). 13C NMR (C6D6, 100 MHz, ppm): δ 168.08,
165.68, 151.16, 146.33, 138.63, 136.67, 130.46, 126.91, 125.76, 119.54,
109.43, 95.78, 25.16, 24.74, 23.98, 23.52, 14.55, 13.84, 12.08, −2.05.
Anal. Calcd for C28H37N3Zn: C, 69.92; H, 7.75; N, 8.74. Found: C,
69.89; H, 7.73; N, 8.72.
to obtain colorless crystals. Yield: 0.34 g, 77%. H NMR (CDCl3, 400
MHz, ppm): δ 7.34 (4H, s, ArH), 4.72 (1H, s, β-CH), 3.23 (3H, s,
OCH3), 2.38 (4H, m, CH2CH3), 2.05 (4H, m, CH2CH3), 1.37 (6H, s, α-
Me), 1.08 (12H, t, CH2CH3). 13C NMR (CDCl3, 100 MHz, ppm): δ
168.34, 151.48, 138.73, 129.60, 119.48, 108.73, 95.06, 55.44, 23.73,
23.25, 13.01. Anal. Calcd for C56H68N8O2Zn2: C, 66.20; H, 6.75; N,
11.03. Found: C, 66.23; H, 6.70; N, 10.94.
Attempted Synthesis of [LiPr,iPr‑CNZn(OMe)]2. To a solution of
[LiPr,iPr‑CNZnEt] (0.30 g, 0.56 mmol) in toluene (10 mL) was added
methanol (0.022 mL, 0.56 mmol) at −78 °C, and the mixture was stirred
for 1 h at room temperature. The resultant turbid solution was filtered
over a frit using Celite. All the volatiles were removed from the filtrate,
and the residue was dried under vacuum. The 1H NMR spectrum of the
residue showed only the ligand resonances.
Attempted Synthesis of [LEt,Et‑CNZn(OMe)]2. To a solution of
[LEt,Et‑CNZnEt] (0.21 g, 0.44 mmol) in toluene (10 mL) was added
methanol (0.018 mL, 0.44 mmol) at −78 °C, and the mixture was stirred
for 2 h at room temperature. The resultant mixture was filtered over a
frit, and the solvent was removed from the filtrate to obtain
[(LEt,Et‑CN)2Zn] (4) in quantitative yield. 1H NMR (CDCl3, 400
MHz, ppm): δ 7.21 (6H, m, ArH), 7.00 (4H, d, ArH), 4.63 (2H, s, β-
CH), 2.34 (8H, m, J = 8.0 Hz, CH2CH3), 2.12 (8H, m, CH2CH3), 1.33
(12H, d, α-Me), 1.04 (24H, m, CH2CH3). 13C NMR (CDCl3, 100 MHz,
ppm): δ 169.35, 166.50, 151.67, 146.33, 138.55, 136.49, 129.78, 126.00,
125.07, 120.35, 107.94, 94.02, 23.66, 23.63, 23.25, 13.72, 13.23. Anal.
Calcd for C52H64N6Zn: C, 74.49; H, 7.69; N, 10.02. Found: C, 74.63; H,
7.81; N, 9.94.
[LiPr,Et‑CNZn(OMe)]2 (1). To a solution of [LiPr,Et‑CNZnEt] (0.32 g, 0.63
mmol) in toluene (15 mL) was added methanol (0.028 mL, 0.69 mmol)
at −78 °C, and the mixture was stirred for 2 h at room temperature to
obtain a white precipitate, which was filtered using a frit and dried under
vacuum. Single crystals for X-ray diffraction analysis were grown from a
hexane−dichloromethane solution by the slow evaporation method
under a nitrogen atmosphere. Yield: 0.24 g, 63%. The 1H NMR
spectrum at room temperature shows a dimer−monomer equilibrium in
a 6:1 ratio. Only the resonances related to the dimer are reported here.
General Procedure for the Copolymerization of Cyclohexene
Oxide and Carbon Dioxide. In a glovebox under a N2 atmosphere, a
100 mL high-pressure reactor was charged with [LiPr,Et‑CNZn(OMe)]2
(0.02 g, 0.039 mmol), cyclohexene oxide (3.84 g, 39 mmol), and a
magnetic stir bar. The vessel was brought out of the glovebox and
pressurized with the preferred CO2 pressure. The mixture was stirred at
50 °C for about 0.5 h, followed by the addition of 20 mL of methanol,
which precipitated the polymer. The polymer was filtered off and dried
under vacuum to constant weight. Polymerization reactions involving
[LZnEt] complexes were also carried out in the same way.
1
The solid-state structure shows a methoxy-bridged dimer. H NMR
(CDCl3, 400 MHz, ppm): δ 7.32 (1H, m, ArH), 7.11 (4H, m, ArH), 4.67
(1H, s, β-CH), 3.31 (3H, s, OCH3), 2.96 (2H, m, CHMe2), 2.40 (2H, m,
CH2CH3), 1.88 (2H, m, CH2CH3), 1.53 (3H, s, α-Me), 1.29 (3H, s, α-
Me), 1.05 (12H, m, CHMeMe and CH2CH3), 0.77 (6H, d, CHMeMe).
13C NMR (CDCl3, 100 MHz, ppm): δ 170.48, 166.90, 152.43, 145.52,
141.70, 139.40, 130.01, 125.56, 123.95, 120.30, 108.20, 94.48, 55.55,
28.14, 24.33, 24.22, 24.13, 23.69, 22.90, 13.57. Anal. Calcd for
C58H78N6O2Zn2: C, 68.16; H, 7.69; N, 8.22. Found: C, 68.06; H,
7.61; N, 8.16.
ASSOCIATED CONTENT
* Supporting Information
■
[LiPr,Me‑CNZn(OMe)]2 (2). To a solution of [LiPr,Me‑CNZnEt] (0.18 g,
0.38 mmol) in toluene (10 mL) was added methanol (0.016 mL, 0.40
mmol) at −78 °C, and the mixture was stirred for 2 h at room
temperature to obtain a white precipitate, which was filtered through a
frit. The precipitate was washed with hexane and dried under vacuum.
The crude material was recrystallized from dichloromethane at −20 °C
to obtain colorless crystals. Yield: 0.14 g, 76%. The 1H NMR spectrum
shows a dimer−monomer equilibrium in a 4:1 ratio. Only the
resonances related to the dimer are reported here. The solid-state
structure shows a methoxy-bridged dimer. 1H NMR (CDCl3, 400 MHz,
ppm): δ 7.35 (1H, m, ArH), 7.14 (2H, d, ArH), 7.04 (2H, s, ArH), 4.68
(1H, s, β-CH), 3.32 (3H, s, OCH3), 2.96 (2H, m, CHMe2), 1.82 (6H, s,
o-CH3), 1.56 (3H, s, α-Me), 1.32 (3H, s, α-Me), 1.03 (6H, d,
CHMeMe), 0.82 (6H, d, CHMeMe). 13C NMR (C6D6, 100 MHz, ppm):
δ 170.54, 166.21, 153.28, 145.46, 141.88, 141.69, 134.38, 134.17, 132.46,
132.35, 125.63, 124.02, 123.77, 119.96, 107.51, 94.41, 55.46, 28.09,
24.35, 24.22, 23.86, 23.54, 17.58. Anal. Calcd for C54H70N6O2Zn2: C,
67.15; H, 7.30; N, 8.70. Found: C, 67.04; H, 7.41; N, 8.66.
S
Crystallographic information files (CIF) for compounds 1−3
and 5, ORTEP diagrams for compound 5, crystal data for
compounds 1−3 and 5, and 1H NMR spectra and GPC curves of
polycarbonates produced by catalysts [LiPr,Me‑CNZnEt] and
[LiPr,Me‑CNZn(OMe)]2 are available free of charge via the Internet
at http://pubs.acs.org.
AUTHOR INFORMATION
Corresponding Author
■
*Tel: 91 413 2654484. Fax: 91 413 2656740. E-mail: ndreddy.
che@pondiuni.edu.in.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
Attempted Synthesis of [LiPr‑CN,iPr‑CNZn(OMe)]2. To a solution of
[LiPr,iPr‑CNZnEt] (0.21 g, 0.37 mmol) in toluene (10 mL) was added
methanol (0.075 mL, 1.85 mmol) at −78 °C, and the mixture was stirred
for 2 h at room temperature. A cloudy solution was obtained, and this
was filtered over a frit using Celite. Volatiles were removed from the
filtrate, and the residue was dried in vacuo. The 1H NMR spectrum of the
residue showed a mixture of ligand tautomers (10% imine−enamine
tautomer and 90% diimine tautomer). The crude material was
recrystallized from toluene at −20 °C to obtain colorless crystals of
We gratefully acknowledge the Council of Scientific and
Industrial Research (CSIR) and Department of Science and
Technology (DST), New Delhi, for financial support and
Alexander von Humboldt Stiftung of Germany for donation of a
glovebox. We thank Dr. Babu Varghese and Dr. R. Jagan, SAIF,
IIT-Madras, for X-ray structure analysis, Department of
Chemistry, IIT-Madras, for HRMS, and Pondicherry University
for NMR and IR spectra and elemental analysis. N.M.R. thanks
UGC and CSIR for providing a fellowship.
1
the diimine tautomer of [LiPr‑CN,iPr‑CNH] (5). Yield: 0.13 g, 74%. H
NMR (C6D6, 400 MHz, ppm): δ 7.28 (4H, s, ArH), 3.02 (2H, s, β-CH2),
2.67 (4H, m, CHMe2), 1.25 (6H, s, α-Me), 0.94 (12H, d, CHMeMe),
0.87 (12H, d, CHMeMe). 13C NMR (CDCl3, 100 MHz, ppm): δ 168.20,
150.16, 137.77, 119.95, 108.39, 51.99, 28.20, 23.02, 22.39, 20.83.
[LEt‑CN,Et‑CNZnOMe]2 (3). To a solution of [LEt‑CN,Et‑CNZnEt] (0.44 g,
0.87 mmol) in toluene (20 mL) was added methanol (0.039 mL, 0.96
mmol) at −78 °C, and the mixture was stirred for 2 h at room
temperature to obtain a white precipitate, which was filtered through a
frit. The precipitate was washed with hexane and dried under vacuum.
The crude product was recrystallized from dichloromethane at −20 °C
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