Structurally diverse copper complexes bearing NNO-tridentate schiff-base derivatives as efficient catalysts for copolymerization of carbon dioxide and cyclohexene oxide

Article abstract of DOI:10.1021/ic5002122

Structurally diverse copper acetate complexes based on NNO-tridentate Schiff-base ligands were synthesized and characterized as mono-, di-, and trinuclear complexes with respect to varied ancillary ligands. Treatment of the ligand precursors (L¹-H = 2-(1-((2-(dimethylamino)ethyl)imino)ethyl)- 4-methylphenol, L²-H = 4-chloro-2-(1-((2-(dimethylamino)ethyl)imino) ethyl)phenol, and L³-H = 2-(1-((2-(dimethylamino)ethyl)imino)ethyl)- 5-methylphenol) with Cu(OAc)₂·H₂O (1 equiv) in refluxing ethanol afforded five-coordinate mono- or bimetallic copper complexes ([(L¹)Cu(OAc)(H₂O)] (1); [(L²)Cu(OAc)(H ₂O)] (2); [(L³)₂Cu₂(OAc) ₂] (3)) in high yields. Dinuclear copper acetate analogue [(L ¹)₂Cu₂(OAc)₂] (4) resulted from treatment of L¹-H as the ligand precursor in refluxing anhydrous MeOH with equimolar proportions of metal acetate salt under a dry nitrogen atmosphere. However, a trinuclear complex, [(L⁴)₂Cu ₃(OAc)₄] (5), was obtained on utilizing 2-(1-((2-(dimethylamino)ethyl)imino)ethyl)-5-methoxyphenol (L⁴-H) as the proligand under the same synthetic route of 1-3; this complex was also synthesized in the reaction of L⁴-H and copper(II) acetate monohydrate in the ratio of 2:3, giving a quantitative yield. All complexes are active catalysts for copolymerization of cyclohexene oxide (CHO) and CO ₂ without cocatalysts. In particular, dinuclear Cu complex 3 performed satisfactorily to produce polycarbonates with controllable molecular weights and high carbonate linkages. These copper complexes are the first examples that are effective for both CO₂/CHO copolymerization and formation of polymers in a controlled fashion.

Full text of DOI:10.1021/ic5002122

Article
pubs.acs.org/IC
Structurally Diverse Copper Complexes Bearing NNO-Tridentate
Schiff-Base Derivatives as Efficient Catalysts for Copolymerization of
Carbon Dioxide and Cyclohexene Oxide
Chen-Yen Tsai,† Bor-Hunn Huang,† Mon-Wei Hsiao,† Chu-Chieh Lin,*,†,‡ and Bao-Tsan Ko*,†
†Department of Chemistry and ‡Center of Nanoscience & Nanotechnology, National Chung Hsing University, Taichung 402, Taiwan
S
* Supporting Information
ABSTRACT: Structurally diverse copper acetate complexes
based on NNO-tridentate Schiff-base ligands were synthesized
and characterized as mono-, di-, and trinuclear complexes with
respect to varied ancillary ligands. Treatment of the ligand
precursors (L1-H = 2-(1-((2-(dimethylamino)ethyl)imino)-
ethyl)-4-methylphenol, L2-H = 4-chloro-2-(1-((2-
(dimethylamino)ethyl)imino)ethyl)phenol, and L3-H = 2-(1-
((2-(dimethylamino)ethyl)imino)ethyl)-5-methylphenol) with
Cu(OAc)2·H2O (1 equiv) in refluxing ethanol afforded five-
coordinate mono- or bimetallic copper complexes ([(L1)Cu-
(OAc)(H2O)] (1); [(L2)Cu(OAc)(H2O)] (2); [(L3)2Cu2-
(OAc)2] (3)) in high yields. Dinuclear copper acetate
analogue [(L1)2Cu2(OAc)2] (4) resulted from treatment of
L1-H as the ligand precursor in refluxing anhydrous MeOH with equimolar proportions of metal acetate salt under a dry nitrogen
atmosphere. However, a trinuclear complex, [(L4)2Cu3(OAc)4] (5), was obtained on utilizing 2-(1-((2-(dimethylamino)ethyl)-
imino)ethyl)-5-methoxyphenol (L4-H) as the proligand under the same synthetic route of 1−3; this complex was also
synthesized in the reaction of L4-H and copper(II) acetate monohydrate in the ratio of 2:3, giving a quantitative yield. All
complexes are active catalysts for copolymerization of cyclohexene oxide (CHO) and CO2 without cocatalysts. In particular,
dinuclear Cu complex 3 performed satisfactorily to produce polycarbonates with controllable molecular weights and high
carbonate linkages. These copper complexes are the first examples that are effective for both CO2/CHO copolymerization and
formation of polymers in a controlled fashion.
satisfactorily with high turnover frequencies (TOF) and
selectivity of copolymerization.2 For instance, Williams et al.
reported bimetallic Co(II/III),3 Fe(III),4 Zn(II),5 and Mg(II)6
acetate catalysts in a series incorporating macrocyclic ligands
for copolymerization of cyclohexene oxide (CHO) and CO2.
The latter dinuclear magnesium complexes displayed out-
standing activity even under CO2 at pressure of 1 atm and
showed excellent selectivity with high percentage of carbonate
linkages.6 For practical applications, copper(II)-based catalysts
seem to be a promising alternative by virtue of their inexpensive
nature and insensitivity to air, but most copper complexes give
only cyclic carbonates in the CO2/epoxide coupling.7
Among ancillary ligand systems employed in catalytic
applications, Schiff-base ligands are widely utilized due to
easily modifiable steric and electronic effects; magnesium and
zinc alkoxides supported by NNO-tridentate Schiff-base
derivatives were shown to be excellent initiators for the ring-
opening polymerization of lactide.8 Although mono- and
tetranuclear copper acetate complexes based on such Schiff-
base ligands have been reported,9 no bi- or trimetallic copper
INTRODUCTION
■
The catalytic transformation of carbon dioxide (CO2) to fine
chemicals has long attracted considerable attention because of
its abundant, inexpensive, almost nontoxic, and biorenewable
character. Metal-catalyzed epoxide/CO2 alternating copolymer-
ization provides a promising synthetic path to transform CO2
into aliphatic polycarbonates, as shown in Scheme 1a.1 Various
impressive catalytic systems including cobalt, chromium, iron,
magnesium, titanium, and zinc complexes bearing various
ancillary ligands were therefore developed to perform
Scheme 1. Copolymerization of CO2 and Epoxide
Received: January 28, 2014
Published: May 6, 2014
© 2014 American Chemical Society
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Scheme 2. Synthetic Routes of Copper Complexes 1−5
acetate complex of these NNO-tridentate Schiff-base ligands
has been isolated. We envisaged that modulating an electronic
or steric effect on the substituents might lead to the di- and
trinuclear conformations; we hence investigated the catalysis of
these complexes in CO2/CHO copolymerization (Scheme 1b).
Herein we report for the first time the use of well-defined
copper complexes as catalysts to form poly(cyclohexene
carbonate).
copper complexes [(L1)Cu(OAc)(H2O)] (1) and [(L2)Cu-
(OAc)(H2O)] (2), but the reaction of 2-(1-((2-
(dimethylamino)ethyl)imino)ethyl)-5-methylphenol (L3-H)
with Cu(OAc)2·H2O (1.0 mol equiv) under the same synthetic
conditions produced the bimetallic copper complex
[(L3)2Cu2(OAc)2] (3) in high yield. Interestingly, Cu(OAc)2
reacted with L1-H in equimolar proportions in refluxing
anhydrous MeOH to furnish the dinuclear copper acetate
analogue [(L1)2Cu2(OAc)2] (4) in good yield. However, a
trinuclear complex, [(L4)2Cu3(OAc)4] (5), resulted from
treatment of 2-(1-((2-(dimethylamino)ethyl)imino)ethyl)-5-
methoxyphenol (L4-H) as the proligand according to the
same synthetic route of 1−3 with a ligand/metal precursor ratio
of 1:1. Alternatively, complex 5 was prepared in quantitative
yield from the reaction of L4-H and copper(II) acetate
monohydrate with a ratio of 2:3. All complexes were obtained
as air-stable crystalline solids and were characterized on the
basis of mass, IR, and ultraviolet−visible (UV−vis) spectra as
well as elemental analysis. On the basis of electrospray
ionization (ESI) mass spectrum, a daughter peak related to
[(L1)Cu]+ (m/z = 282.1, 100%) was observed in 1. However, 4
displayed a main peak corresponding to [(L1)2Cu2(OAc)]+
(m/z = 623.2, 100%), indicating Cu complex 4 is dinuclear in
solution. The IR spectra displayed two sets of acetate groups in
RESULTS AND DISCUSSION
■
Synthesis and Characterization. NNO-tridentate Schiff-
base proligands were prepared in a series with good yields from
meta- or para-substituted 2-hydroxyacetophenone derivatives
with N,N′-dimethylethane-1,2-diamine in stoichiometric pro-
portions in refluxing ethanol solution.8b,c Copper complexes
coordinated with NNO-tridentate Schiff-base ligands and
acetate (−OAc) groups were readily synthesized via a one-
step procedure of copper acetate salt ([Cu(OAc)2·H2O]) with
the ligand precursors, as illustrated in Scheme 2. Treatment of
Cu(OAc)2·H2O with 2-(1-((2-(dimethylamino)ethyl)imino)-
ethyl)-4-methylphenol (L1-H) or 4-chloro-2-(1-((2-
(dimethylamino)ethyl)imino)ethyl)phenol (L2-H) (1.0 mol
equiv) in refluxing ethanol gave monomeric five-coordinate
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5: carbonyl-stretching bands at 1589 and 1426 cm−1 belong to
bridging bidentate acetate coordination modes, whereas strong
bands at 1526 and 1396 cm−1 are attributed to bridging
monodentate bound-acetate anion.10 On the basis of the
absorption spectra, the bands in the ranges of 280−330 and
350−400 nm might be assigned to the π−π* and n−π*
transitions. A broad band from 550 to 650 nm can be ascribed
to the d−d transition band, which is typical for a Schiff-base
Cu(II) complex (Supporting Information, Figures S1 and
S2).11 Molecular structures of all Cu complexes were confirmed
by single-crystal X-ray crystallography.
Crystal Structures of Complexes 1−5. Single crystals of
complexes 1−5 suitable for X-ray structural determination were
grown from their saturated solutions in ethanol or methanol. As
shown in Figures 1 and 2, the geometries around the copper
Figure 2. ORTEP drawing of complex 2 with probability ellipsoids
drawn at 50% level. Hydrogen atoms except for those in O(4) are
omitted for clarity. Selected bond lengths (Å) and angles (deg):
Cu(1)−O(1) 1.9236(16), Cu(1)−O(2) 1.9516(16), Cu(1)−N(1)
1.982(2), Cu(1)−N(2) 2.0441(19), Cu(1)−O(4) 2.2683(17), O(1)−
Cu(1)−O(2) 87.30(7), O(1)−Cu(1)−N(1) 90.38(7), O(2)−Cu(1)−
N(1) 171.63(8), O(1)−Cu(1)−N(2) 155.82(8), O(2)−Cu(1)−N(2)
93.33(7), N(1)−Cu(1)−N(2) 85.50(8), O(1)−Cu(1)−O(4)
103.16(7), O(2)−Cu(1)−O(4) 95.11(7), N(1)−Cu(1)−O(4)
93.25(8), N(2)−Cu(1)−O(4) 100.86(7).
Figure 1. ORTEP drawing of complex 1 with probability ellipsoids
drawn at 50% level. Hydrogen atoms except for those in O(4) are
omitted for clarity. Selected bond lengths (Å) and angles (deg):
Cu(1)−O(1) 1.9218(17), Cu(1)−O(2) 1.9660(19), Cu(1)−N(1)
1.979(2), Cu(1)−N(2) 2.048(2), Cu(1)−O(4) 2.2591(17), O(1)−
Cu(1)−O(2) 87.46(8), O(1)−Cu(1)−N(1) 90.76(8), O(2)−Cu(1)−
N(1) 172.30(8), O(1)−Cu(1)−N(2) 155.10(8), O(2)−Cu(1)−N(2)
3.42(9), N(1)−Cu(1)−N(2) 85.05(9), O(1)−Cu(1)−O(4)
105.37(7), O(2)−Cu(1)−O(4) 94.51(7), N(1)−Cu(1)−O(4)
93.19(7), N(2)−Cu(1)−O(4) 99.37(7).
Figure 3. ORTEP drawing of complex 3 with probability ellipsoids
drawn at 50% level. Hydrogen atoms are omitted for clarity. Selected
bond lengths (Å) and angles (deg): Cu(1)−O(1) 1.894(2), Cu(1)−
O(2) 1.9776(18), Cu(1)−N(1) 1.965(2), Cu(1)−N(2) 2.063(2),
Cu(1)−O(2A) 2.505(2), O(1)−Cu(1)−O(2) 90.11(8), O(1)−
Cu(1)−N(1) 91.69(9), O(2)−Cu(1)−N(1) 171.83(11), O(1)−
Cu(1)−N(2) 174.59(8), O(2)−Cu(1)−N(2) 93.15(8), N(1)−
Cu(1)−N(2) 84.48(9).
centers of five-coordinate complexes 1 and 2 are both distorted
square pyramids (τ = 0.29 for 1 and 0.26 for 2)12 constructed
by one O and two N atoms of the NNO-tridentate ligand, one
O atom from an acetate group, and a coordinated water. The
lengths of bonds between the Cu and O(1), O(2), N(1), N(2),
and O(4) atoms are 1.9218(17), 1.9660(19), 1.979(2),
2.048(2), and 2.2591(17) Å for 1 and 1.9236(16),
1.9516(16), 1.982(2), 2.0441(19), and 2.2683(17) Å for 2;
all these distances are within the normal ranges reported in the
literature.9 As a result, the electronic effect of substituents at the
para position of the phenoxy group seems to be not obvious.
The molecular structure of 3, presented in Figure 3, is a dimeric
and pentacoordinate copper complex containing a Cu(1)/
O(2)/Cu(1A)/O(2A) planar core with inversion symmetry in
which the Cu atom is located ca. 0.1029 Å above the O(1)/
O(2)/N(1)/N(2) plane. The geometry around each Cu atom
displays a slightly distorted square pyramid (τ = 0.05)12 formed
by O(1), N(1), and N(2) of an ancillary ligand and two μ2-
acetate oxygen atoms. The bond distances around the Cu
atoms for 3 are comparable to those observed in complex 1, as
listed in Table 1. The crystal structure of copper complex 4
(Figure 4) is isostructural with Cu analogue 3, except that a
methyl (−CH3) substituent is at the para position of the
phenoxy group. As expected, the average bond lengths of Cu−
O(phenoxy) = 1.894(2) Å, Cu−N(imine) = 1.970(3) Å, and
Cu−O(OAc) = 2.242(2) Å in 4 are similar to those found for
complex 3, as shown in Table 1. It is noteworthy that the solid-
state structures of the mono- and dinuclear complexes 1 and 4
are consistent with the results of their ESI mass spectra.
The Oak Ridge thermal-ellipsoid plot (ORTEP) in Figure 5
shows that two distinct environments of Cu centers constitute
the linear trinuclear complex 5 with respect to one
hexacoordinate Cu(2) and two pentacoordinate Cu(1) and
Cu(1A). Few trinuclear copper acetate complexes are
reported.13 The Cu(2) center exhibits a distorted octahedral
geometry formed by two O atoms from two different NNO−
backbones, two O atoms from two bridged OAc groups, and
two different μ2-acetate oxygen atoms; a C2 symmetry axis
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Table 1. Selected Bond Lengths (Å) for Complexes 1−5
1
2
3
4
5
Cu(1)−Ophenoxy
Cu(1)−N(1)
Cu(1)−N(2)
Cu(1)−OH2O
Cu(1)−Omonodentate OAc
Cu(1)−Obridging monodentate OAc
Cu(1A)−Obridging monodentate OAc
Cu(1)−Obridging bidentate OAc
Cu(2)−Obridging monodentate OAc
Cu(2)−Obridging bidentate OAc
1.9218(17)
1.979(2)
1.9236(16)
1.982(2)
1.894(2)
1.965(2)
2.063(2)
1.894(2)
1.970(3)
2.073(3)
1.956(2)
1.960(2)
2.055(3)
2.048(2)
2.0441(19)
2.2683(17)
1.9516(16)
2.2591(17)
1.9660(19)
1.9776(18)
2.505(2)
1.982(2)
2.502(2)
1.956(2)
2.255(2)
2.490(2)
1.936(2)
Figure 5. ORTEP drawing of complex 5 with probability ellipsoids
drawn at 50% level. Hydrogen atoms are omitted for clarity. Selected
bond lengths (Å) and angles (deg): Cu(1)−O(1) 1.956(2), Cu(1)−
O(3) 1.956(2), Cu(1)−N(1) 1.960(3), Cu(1)−N(2) 2.055(3),
Cu(1)-O(6A) 2.255(3), Cu(2)−O(3) 2.490(2), Cu(2)−O(5)
1.936(2), Cu(2)−O(1) 2.019(2), O(3)−Cu(1)−O(1) 87.10(9),
O(3)−Cu(1)−N(1) 171.78(10), O(1)−Cu(1)−N(1) 90.03(10),
O(3)−Cu(1)−N(2) 98.78(10), O(1)−Cu(1)−N(2) 174.04(10),
N(1)−Cu(1)−N(2) 84.01(11), O(3)−Cu(1)−O(6A) 89.09(9),
O(1)−Cu(1)−N(6A) 90.23(9), N(1)−Cu(1)−O(6A) 98.64(10),
N(2)−Cu(1)−O(6A) 90.78(10), O(5A)−Cu(2)−O(1) 89.07(9),
O(5)−Cu(2)−O(1) 90.93(9).
Figure 4. ORTEP drawing of complex 4 with probability ellipsoids
drawn at 50% level. Hydrogen atoms are omitted for clarity. Selected
bond lengths (Å) and angles (deg): Cu(1)−O(1) 1.894(2), Cu(1)−
O(2) 1.982(2), Cu(1)−N(1) 1.970(3), Cu(1)−N(2) 2.073(3),
Cu(1)−O(2A) 2.502(2), O(1)−Cu(1)−O(2) 91.74(10), O(1)−
Cu(1)−N(1) 90.96(11), O(1)−Cu(1)−N(2) 170.18(11), O(2)−
Cu(1)−N(1) 177.05(9), O(2)−Cu(1)−N(2) 92.93(10), N(1)−
Cu(1)−N(2) 84.22(11).
passes through that center. The geometry around pentacoordi-
nated Cu(1) and Cu(1A) is a slightly distorted square pyramid
(τ = 0.04)12 in which both copper atoms are ca. 0.071 Å above
the mean base plane of the pertinent square pyramid. The bond
distances between Cu(2) and its neighboring oxygen atoms are
all within the normal range reported for six-coordinate copper
acetate complexes bearing the NNO-tridentate Schiff-base
ligand.13 In comparison, the five-coordinate Cu-involving
bond with Cu(1)−O(1) = 1.956(2) Å is longer than those in
complexes 1 and 2, which is attributed to the phenoxy oxygen
atoms bridged to two copper atoms. Four ancillary acetate
groups in complex 5 adopt both bridging monodentate and
bridging bidentate bonding modes to coordinate the metal
center(s) in the solid state, which is consistent with the
observations of the IR spectra.
Copolymerization of Cyclohexene Oxide (CHO) with
Carbon Dioxide (CO2). Copper complexes 1−5 were
performed to catalyze the coupling of CO2/CHO; representa-
tive results appear in Table 2. When the reaction temperature
was set at 100 °C (entry 1), the conversion of copolymerization
catalyzed with 1 was low (<20%). On raising the temperature
to 120 °C with pCO20 = 300 psi (entry 2), the TOF increased
from 3.75 to 9.38 h−1, and a narrowly dispersed copolymer
(polydispersity index (PDI) = 1.19) with low molecular weight
was obtained. Increasing the concentration of catalyst 1
induced a larger CHO conversion (74%), but the PDI
broadened to 1.40 (entry 3). Adjusting the initial pressure of
CO2 also affected the CO2/CHO copolymerization (entries 4−
5): a higher (500 psi) or lower (100 psi) pressure decreased the
TOF and selectivity of copolymerization. However, increasing
the duration of reaction also slightly decreased the TOF and
broadened the molecular weight distribution (Table 2, entry 6
vs 2). Under the optimal conditions ([CHO]0/[catalyst]0 =
0
500/1, pCO2 = 300 psi, 120 °C) evaluated from complex 1,
bimetallic Cu complexes 3 and 4 displayed high productivity
(CHO conversion = 88%, TON = 440) (TON = number of
moles of CHO consumed per mole of catalyst) for 48 h among
complexes 1−5, compared with entries 6−10 in Table 2. The
selectivities (Supporting Information, Figure S3) and control of
molecular weights of the copolymers with Cu catalyst 3 are
superior to those of other Cu catalysts in this work, and a
copolymer resulted with the higher molecular-weight copoly-
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Table 2. Copolymerization of CHO and CO2 Catalyzed by Using Cu Complexes 1−5
a
b
b
b
c
d
e
f
f
entry catalyst (mol %) time (h) % CHO conv.
% CHC
% copolymer (% carbonate)
TON TOF (h−1
)
Mn (calcd.) Mn (obsd.) (PDI)
g
1
2
3
4
5
6
7
8
9
1(0.2)
1(0.2)
1(0.4)
1(0.2)
1(0.2)
1(0.2)
2(0.2)
3(0.2)
4(0.2)
5(0.2)
3(0.2)
3(0.2)
3(0.2)
24
24
24
24
24
48
48
48
48
48
12
24
36
18
45
74
16
33
55
25
88
88
24
45
63
83
29(trans)
34(trans)
20(trans)
59(trans)
35(trans)
20(trans)
48(trans)
16(trans)
17(trans)
19(trans)
29(trans)
22(trans)
16(trans)
71(97)
66(97)
80(94)
41(81)
65(95)
80(96)
52(89)
84(96)
83(97)
81(38)
71(95)
78(96)
90
225
185
80
3.75
9.38
7.71
3.33
6.88
5.73
2.60
9.17
9.17
2.50
9100
21 100
21 000
4700
4800(1.19)
4700(1.40)
h
I
j
165
275
125
440
440
120
225
315
415
15 200
31 300
9200
3200(1.29)
4900(1.40)
1900(1.54)
9100(1.42)
6800(1.56)
26 300
26 300
3500
10
11
12
13
18.8
11 300
17 500
24 800
3400(1.31)
5600(1.46)
7000(1.42)
13.1
11.5
84(96)
a
b
0
Copolymerization conditions: CHO 5.0 mL, 0.1 mmol of catalyst, pCO2 = 300 psi, T = 120 °C. Determined by comparison of the integrals of
signals arising from the methylene protons in the 1H NMR spectra, including PCHC carbonate (δ = 4.65 ppm), PCHC ether (δ = 3.3−3.5 ppm),
and CHC (δ = 3.9 (trans) or 4.63 ppm (cis)).4 TON = number of moles of CHO consumed per mole of catalyst. TOF = TON per hour.
c
d
e
[Mw(CHO + CO2)] (142.06 g/mol) times [CHO]0/n[catalyst]0 times conversion yield times % copolymer (n = 1 for complexes 1 and 2; n = 2 for
f
g
h
I
0
complexes 3 and 4; n = 4 for complex 5). Determined in THF by GPC. T = 100 °C. 2.5 mL of CHO, 0.1 mmol catalyst. pCO2 = 100 psi.
j
0
pCO2 = 500 psi.
1
Figure 6. H NMR spectrum of the purified copolymer produced by using copper acetate complex 3 (Table 2, entry 8) in CDCl3.
Figure 7. A typical 13C NMR spectrum of the resultant copolymer (Table 2, entry 8) in CDCl3. (inset) Expanded carbonyl region.
mer (Mw > 10 000 g/mol). Despite the moderate selectivity of
the copolymers (copolymer/CHC = 84/16) of CHO/CO2
coupling with 3, the constituents of carbonate linkages in the
polymeric product have a high content (96%). As shown in
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●
▼
Figure 8. Relationship of Mn ( ) and PDI ( ) (determined from GPC analysis) vs time for the copolymerization of cyclohexene oxide and CO2 by
using dicopper acetate complex 3.
1
Figure 6, the H NMR signal at 4.65 ppm is assigned to the
methine protons in the carbonate linkages; the broad signals at
3.3−3.4 ppm correspond to the 4% of ether linkages in the
copolymer. Nevertheless, the copolymer produced was almost
atactic, based on the 13C NMR spectrum illustrated in Figure 7.
It is noteworthy that the observed molecular weight
(Mn(GPC)) is significantly lower than the theoretical value
based on the acetate anion(s). This can be attributed to the
presence of water or alcohol of coordination or crystallization
in the catalyst and a small amount of water from the CHO
monomer. Trace quantities of protic species that are present in
the catalytic system will behave as the chain-transfer agent,
resulting in lower molecular weight in comparison with the
calculated molecular weight.14 The superior catalytic perform-
ance of dicopper complex 3 might result from a cooperative
ring-opening copolymerization via a bimetallic mechanism.2j,15
The controllable character of 3 was further demonstrated with
kinetic experiments that involved variation of Mn(GPC) and
the molecular weight distribution (PDI) with various durations,
as depicted in entries 11−13 of Table 2. A proportional
relationship (Figure 8) between Mn(GPC) and time for four
parallel reactions, along with the remaining narrow PDI
(<1.50), implied a controllable copolymerization by dicopper
acetate catalyst 3. To test catalytic activity of these bimetallic
copper complexes in copolymerization of propylene oxide
(PO) and CO2, coupling of CO2/PO catalyzed by complex 3
was performed under the conditions of 50 °C and 150 psi initial
CO2 pressure for 24 h. However, almost negligible PO
CONCLUSIONS
■
Five diverse copper complexes containing mono-, di-, and
trinuclear configurations that depend on the nature of the
NNO-tridentate Schiff-base ligands or the conditions of
moisture in the environment have been synthesized and fully
characterized by X-ray single-crystal structural analyses. The
molecular structures differ in the solid state under varied
synthetic conditions with treatment of L1-H with copper
acetate salt in equimolar proportions. Monomeric copper
complex 1 containing one coordinated water was formed in a
highly moist environment, whereas a dry nitrogen atmosphere
afforded dinuclear complex 4 that is a dimeric species
containing Cu2O2 core bridging through monodentate acetate
groups. Tri-Cu complex 5 is a rare solid-state example in which
four ancillary acetate groups assume both bridging mono-
dentate and bridging bidentate bonding modes to coordinate
metal center(s). Bimetallic Cu complex 3 effectively copoly-
merized CO2 with CHO in a controlled manner; the
copolymers were obtained with controllable molecular weights
and a high alternating microstructure up to 96% carbonate
linkages. Improvement of the catalytic activity of copolymeriza-
tion utilizing dicopper catalyst 3 in the presence of cocatalysts is
under investigation.
EXPERIMENTAL SECTION
■
General Considerations. Solvents and reagents were dried by
refluxing for at least 24 h over sodium/benzophenone (hexane,
toluene, tetrahydrofuran (THF)), over phosphorus pentoxide
(CH2Cl2), or over calcium hydride (methanol (MeOH)). Deuterated
solvents were dried over 4 Å molecular sieves. Cu(OAc)2·H2O,
ethanol (EtOH, 95%), 1-(2-hydroxy-5-methylphenyl)ethanone, 1-(5-
chloro-2-hydroxyphenyl)ethanone, 1-(2-hydroxy-4-methylphenyl)-
ethanone, 1-(2-hydroxy-4-methoxyphenyl)ethanone, and N,N′-dime-
thylethane-1,2-diamine were purchased and used without further
1
conversion (<5%) was observed by H NMR spectrum, and
this result indicates that complex 3 is an inactive catalyst for
copolymerization of PO and CO2. Although the best catalytic
activity of CHO/CO2 copolymerization is only moderate (TOF
= 18.8 h−1), the results presented here are the first examples to
produce copolymers with controllable molecular weights and
high carbonate linkages through the use of a bimetallic
copper(II) acetate complex containing NNO-tridentate Schiff-
base ligands. The theoretical investigation of the mechanism of
copper-catalyzed CHO/CO2 copolymerization is in progress.
1
purification. Cyclohexene oxide (CHO) was purified before use. H
NMR and 13C NMR spectra were recorded on Varian Mercury 400
1
(400 MHz for H and 100 MHz for 13C) spectrometer with chemical
shifts given in parts per million from the peak of internal
tetramethylsilane. Gel permeation chromatography (GPC) measure-
ments were performed on a Jasco PU-2080 plus system equipped with
a RI-2031 detector using THF (high-performance liquid chromatog-
raphy grade) as an eluent (flow rate 1.0 mL/min, at 40 °C). The
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Inorganic Chemistry
Article
chromatographic column was Phenomenex Phenogel 5 μm 103 Å, and
the calibration curve used to calculate Mn(GPC) was produced from
polystyrene standards. The GPC results were calculated using the
Scientific Information Service Corporation (SISC) chromatography
data solution 3.1 edition. The Fourier transform IR (FT-IR) spectra
were recorded on a PerkinElmer Spectrum 100 FT-IR spectrometer in
the range of 4000−600 cm−1. UV−vis spectra were determined by
JASCO U-530 UV−vis spectrometer at 25 °C using CH2Cl2 as
solvent. Mass analyses were performed using positive electron spray
ionization (ESI+ or APCI+) technique on a Thermo Finnigan TSQ
Quantum mass spectrometer for all these complexes upon dissolving
in dimethyl sulfoxide (DMSO) solvent.
Synthesis of Complex [(L1)2Cu2(OAc)2] (4). All manipulations for
synthesis of compound 4 were carried out under a dry nitrogen
atmosphere. A mixture of 2-{1-[2-(dimethylamino)ethyl)imino]-
ethyl}-4-methylphenol (1.12 g, 5.1 mmol) and Cu(OAc)2 (0.900 g,
5.0 mmol) was stirred in refluxing anhydrous methanol (30 mL) for 24
h. The solution was then cooled to room temperature. The solution
was concentrated to ca. 5 mL, followed by filtration to yield blue
solids. Yield: 1.28 g (75%). Anal. Calcd for C30H44Cu2N4O6·2MeOH:
N, 7.49; C, 51.39; H, 7.01%. Found: N, 7.55; C, 51.34; H, 6.09%. m/z
(ESI-MS, DMSO): 623.2 ([M−CH3COO]+, 100%, calcd. 623.2).
Synthesis of Complex [(L4)2Cu3(OAc)4] (5). A mixture of 4-
methoxy-2-{1-[2-(dimethylamino)ethylimino]ethyl}phenol (1.54 g,
4.1 mmol) and Cu(OAc)2·H2O (1.19 g, 6.0 mmol) was dissolved in
ethanol (30 mL), and the solution was heated under reflux for 24 h.
The solution was then cooled to room temperature. Volatile materials
were removed to leave about 5 mL under vacuum to yield blue solids.
The blue powder was obtained after filtration. Yield: 1.52 g (85%).
Anal. Calcd for C34H50Cu3N4O12: N, 6.24; C, 45.50; H, 5.62%. Found:
N, 6.20; C, 45.70; H, 5.74%. m/z (ESI-MS, DMSO): 655.3 ([M−
Cu(OAc)3]+, 100%, calcd. 655.13). Characteristic IR absorptions
(cm−1, neat): 1610 (νCN), 1589 (νasymmetric bridging bidentate acetate), 1526
(νasymmetric monodentate acetate), 1426 (νsymmetric bridging bidentate acetate), 1396
(νsymmetric monodentate acetate).
Copolymerization of CO2 and CHO Catalyzed by Complexes 1−
5. A representative procedure for the copolymerization of cyclohexene
oxide with CO2 (Table 2, entry 8) was exemplified. Cu catalyst 3
(0.0682 g, 0.1 mmol) was dissolved in 5.0 mL of neat cyclohexene
oxide under a dry nitrogen atmosphere. Under CO2 atmosphere, the
mixed solution was added to the 100 mL autoclave with magnetic
stirrer. CO2 was then charged into the reactor until the pressure of 300
psi was reached, and the stirrer was started. The reaction was
performed at 120 °C for 48 h. Then the reactor was placed into ice
water, and excess CO2 was released. The CHO conversion (88%) was
analyzed by 1H NMR spectroscopic studies. Spectral characteristics of
cyclohexene carbonate: poly(cyclohexene carbonate) (PCHC) carbo-
nate (δ: 4.65 ppm), PCHC ether (δ: 3.3−3.5 ppm), and CHC (δ: 3.9
(trans) or 4.63 ppm (cis)). The mixture was diluted with CH2Cl2 (50
mL), followed by the addition of 1 N HCl solution (1.0 mL) to
quench this reaction, and the final mixture was passed through a short
column of neutral alumina to remove the metal salt. After precipitation
by adding polymer solution in CH2Cl2 into methanol (50 mL) three
times, the off-white polymer was collected by filtration and dried under
vacuum overnight.
Synthesis of NNO-Schiff-Base Proligands. General procedures
for ligand precursor preparation were conducted by the reaction of
corresponding 2-hydroxyacetophenone (5.0 mmol) with N,N-
dimethylethane-1,2-diamine (5.1 mmol) in refluxing ethanol (30.0
mL) for 24 h. The mixture was cooled to room temperature, and the
volatile components were removed in vacuo to give the products.16
2-(1-((2-(Dimethylamino)ethyl)imino)ethyl)-4-methylphenol (L1-
1
H). Yield: 0.91 g (85%). H NMR (CDCl3, 400 MHz, ppm): δ 16.02
(1H, s, −OH), 7.29 (1H, d, Ar-H), 7.08 (1H, dd, Ar-H), 6.82 (1H, d,
Ar-H), 3.66 (2H, t, −CN−CH2−), 2.71(2H, t, −CH2−N(CH3)2),
2.34(6H, s, −N(CH3)2), 2.33 (3H, s, Ph−CH3), 2.27 (3H, s, −N
C−CH3). 13C NMR (CDCl3, 100 MHz, ppm): δ 171.6, 161.5, 133.3,
127.9, 125.8, 118.9, 118.4, 59.7, 48.0, 45.8, 20.7, 14.5.
4-Chloro-2-(1-((2-(dimethylamino)ethyl)imino)ethyl)phenol (L2-
H). Yield: 0.99 g (83%). 1H NMR (CDCl3, 400 MHz, ppm): δ
16.41(1H, s,−OH), 7.38(1H, d, Ar-H), 7.16 (1H, dd, Ar-H), 6.80(1H,
d, Ar-H), 3.60 (2H, t, −CN−CH2−), 2.64 (2H, t, −CH2−
N(CH3)2), 2.27 (9H, s, −CH3). 13C NMR (CDCl3, 100 MHz, ppm):
δ 171.0, 163.3, 132.3, 127.2, 120.9, 120.5, 119.4, 59.2, 47.4, 45.6, 14.4.
2-(1-((2-(Dimethylamino)ethyl)imino)ethyl)-5-methylphenol (L3-
1
H). Yield: 0.93 g (85%). H NMR (CD3Cl, 400 MHz, ppm): δ 16.34
(1H, s, −OH), 7.38 (1H, d, Ar-H), 6.72 (1H, s, Ar-H), 6.55 (1H, d,
Ar-H), 3.30 (2H, t, −CN−CH2-), 2.71 (2H, t, −CH2−N(CH3)2),
2.34 (6H, s, −N(CH3)2), 2.24 (3H, s, −CN−CH3), 2.17 (3H, s,
Ar−CH3). 13C NMR (CDCl3, 100 MHz, ppm): δ 171.40, 161.5, 133.0,
127.6, 125.2, 118.4, 118.1, 59.2, 47.3, 47.4, 20.3, 14.0.
2-(1-((2-(Dimethylamino)ethyl)imino)ethyl)-5-methoxyphenol
1
(L4-H). Yield: 0.92 g (78%). H NMR (CDCl3, 400 MHz, ppm): δ
16.88 (1H, s, −OH), 7.33(1H, d, Ar-H), 6.31(1H, d, Ar-H), 6.19(1H,
dd, Ar-H), 3.76(3H, s, −O(CH3)), 3.60(2H, t, −CN−CH2−),
2.65(2H, t, −CH2−N(CH3)2), 2.33(3H, s, −CN−CH3), 2.29(6H,
s, −N(CH3)2). 13C NMR (CDCl3, 100 MHz, ppm): δ 171.0, 157.6,
150.6, 124.0, 118.9, 113.2, 112.3, 59.5, 55.8, 48.0, 45.6, 14.4.
Synthesis of Copper Complexes 1−5. Synthesis of Complex
[[(L1)Cu(OAc)(H2O)] (1). A mixture of 2-{1-[2-(dimethylamino)ethyl)-
imino]ethyl}-4-methylphenol (1.12 g, 5.1 mmol) and Cu(OAc)2·H2O
(0.995 g, 5.0 mmol) was dissolved in ethanol (30 mL), and the
solution was heated under reflux for 24 h. The solution was then
cooled to room temperature. Volatile materials were removed to leave
about 10 mL under vacuum to yield blue solids. The blue powder was
obtained after filtration. Yield: 1.49 g (83%). Anal. Calcd for
C15H24CuN2O4: N, 7.78; C, 50.06; H, 6.72%. Found: N, 7.68; C,
50.34; H, 6.97%. m/z (ESI-MS, DMSO): 282.1 ([M−CH3COO−
H2O]+, 100%, calcd. 282.08). Characteristic IR absorptions (cm−1,
neat): 1615 (νCN), 1533 (νasymmetric acetate), 1393 (νsymmetric acetate).
Synthesis of Complex [(L2)Cu(OAc)(H2O)] (2). The synthetic route
for complex 2 was the same as that of 1. Yield: 1.61 g (85%). Anal.
Calcd for C14H23ClCuN2O5·H2O: N, 7.03; C, 42.21; H, 5.82%. Found:
N, 6.94; C, 41.66; H, 5.99%. Characteristic IR absorptions (cm−1,
neat): 1602 (νCN), 1522 (νasymmetric acetate), 1403 (νsymmetric acetate).
Synthesis of Complex [(L3)2Cu2(OAc)2] (3). The synthetic route for
complex 3 was the same as that of 1. Yield: 1.36 g (80%). Anal. Calcd
for C30H44Cu2N4O6·2H2O: N, 7.78; C, 50.06; H, 6.72%. Found: N,
7.80; C, 50.28; H, 6.27%. m/z (ESI-MS, DMSO): 623.3 ([M−
CH3COO]+, 100%, calcd. 623.17). Characteristic IR absorptions
(cm−1, neat): 1606 (νCN), 1566 (νasymmetric acetate), 1407
(νsymmetric acetate).
X-ray Crystallographic Studies. Single crystals of complexes 1−
5 were obtained from their saturated ethanol or methanol solutions.
Suitable crystals were immersed in FOMBLINY under nitrogen
atmosphere and mounted on an Oxford Xcalibur Sapphire-3 CCD
Gemini diffractometer employing graphite-monochromated Mo Kα
radiation (λ = 0.710 73 Å), and intensity data were collected with ω
scans. The data collection and reduction were performed with the
CrysAlisPro software,17 and the absorptions were corrected by the
SCALE3 ABSPACK multiscan method.18 The space-group determi-
nation was based on a check of the Laue symmetry and systematic
absences, and it was confirmed using the structure solution. The
structure was solved and refined with the SHELXTL package.19 All
non-H atoms were located from successive Fourier maps, and
hydrogen atoms were refined using a riding model. Anisotropic
thermal parameters were used for all non-H atoms, and fixed isotropic
parameters were used for H atoms. Crystallographic data of complexes
1−5 are summarized in Supporting Information, Table S1.
ASSOCIATED CONTENT
■
S
* Supporting Information
1
Absorption spectra of complexes 1−5, H NMR spectrum of
copolymer selectivity by 3, and crystallographic details. This
material is available free of charge via the Internet at http://
pubs.acs.org. CCDC 980348−980351 and −992731 (for 1−5)
contain the supplementary crystallographic data in this paper.
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Inorganic Chemistry
Article
Am. Chem. Soc. 2005, 127, 10869−10878. (c) Ren, W.-M.; Liu, Z. W.;
Wen, Y. Q.; Zhang, R.; Lu, X.-B. J. Am. Chem. Soc. 2009, 131, 11509−
11518. (d) Darensbourg, D. J.; Wu, G.-P. Angew. Chem., Int. Ed. 2013,
52, 10602−10606.
AUTHOR INFORMATION
Corresponding Authors
*E-mail: btko@dragon.nchu.edu.tw. Phone: 886-4-22840411-
715. Fax: 886-4-22862547 (B.T.K.).
*E-mail: cchlin@mail.nchu.edu.tw. Phone: 886-4-22840411-
718. Fax: 886-4-22862547 (C.C.L.).
Notes
■
(15) Moore, D. R.; Cheng, M.; Lobkovsky, E. B.; Coates, G. W. J.
Am. Chem. Soc. 2003, 125, 11911−11924.
(16) Villa, A. C.; Guastini, C.; Blech, P.; Floriani, C. Dalton Trans.
1990, 3557−3561.
(17) CrysAlisPro, Version 1.171; Agilent Technologies: Santa Clara,
CA, 2009.
The authors declare no competing financial interest.
(18) CrysAlisPro SCALE3 ABSPACK, a scaling algorithm for
empirical absorption correction using spherical harmonics; Oxford
Diffraction (Agilent Technologies): Santa Clara, CA, 2005.
(19) Sheldrick, G. M. Acta Crystallogr. 2008, A64, 112−122.
ACKNOWLEDGMENTS
■
We gratefully acknowledge the financial support from the
National Science Council, Taiwan (NSC101-2113-M-005-010-
MY3 to C.C.L. and NSC101-2113-M-005-020-MY3 to B.T.K.).
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