Synthesis and structural characterization of trifluoromethyl-substituted β-ketoimino copper(II) complex and its catalytic behavior for methyl acrylate polymerization

Article abstract of DOI:10.1134/S1070328415080096

A new trifluoromethyl-substituted bis(β-ketoimino)copper complex (II) was synthesized and characterized by FTIR, elemental analysis, and X-ray diffraction (CIF file CCDC no. 1023178). XRD refinement revealed that the copper complex adopted a perfect centr

Full text of DOI:10.1134/S1070328415080096

ISSN 1070ꢀ3284, Russian Journal of Coordination Chemistry, 2015, Vol. 41, No. 8, pp. 543–548. © Pleiades Publishing, Ltd., 2015.
Synthesis and Structural Characterization of Trifluoromethylꢀ
Substituted
β
ꢀKetoimino Copper(II) Complex and Its Catalytic
1
Behavior for Methyl Acrylate Polymerization
a
a
a
b
a
a
a
a, c,
X. Xiao , Y. C. Wen , X. J. Wang , L. Lei , P. Xia , T. C. Li , A. Q. Zhang , and G. Y. Xie *
a
Key Laboratory of Catalysis and Materials Science of the State Ethnic Affairs Commission & Ministry of Education, Hubei
Province, SouthꢀCentral University for Nationalities, Wuhan, 430074 P.R. China
b
Baise University, Baise, 533000 P.R. China
c
State Key Laboratory of Fine Chemicals, Dalian University of Technology, Dalian, 116024 P.R. China
*
eꢀmail: xiegy@scuec.edu.cn
Received January 9, 2015
Abstract
characterized by FTIR, elemental analysis, and Xꢀray diffraction (CIF file CCDC no. 1023178). XRD reꢀ
finement revealed that the copper complex adopted a perfect central symmetric square planar structure with
the copper center coordinated by two trans ꢀketoimine ligands with delocalized double bonds. On activation
with modified methylaluminoxane (MMAO), the bis( ꢀketoimino)copper complex can polymerize methyl
—A new trifluoromethylꢀsubstituted bis(βꢀketoimino)copper complex (II) was synthesized and
β
β
acrylate effectively. Introduction of trifluoromethyl group into the Nꢀaryl ring of the ligands, which leads to
strong electronꢀwithdrawing effect, can improve significantly the catalytic activities. The activity of
II/MMAO can reach 89.4 kg/mol Cu h, which is among the highest values reported for copper complexes in
acrylic monomer polymerization.
DOI: 10.1134/S1070328415080096
1
INTRODUCTION
izations of ethylene and methyl acrylate [17–19]. The
activity of MAOꢀactivated [1,2ꢀbis(4,4ꢀdimethylꢀ2ꢀ
oxazolinꢀ2ꢀyl)ethane]copper(II) dichloride for the
polymerization of acrylates and their copolymerizaꢀ
tion with ethylene was reported in [20]. Bis(salicylalꢀ
diminate)copper(II) complexes can also be used to
catalyze the homo and copolymerizations of ethylene
and methyl methacrylate [21]. Wu’s group found that
Copper complexes have been widely used in many
fields [1–7]. For example, in life science, some copper
complexes have shown biological activities of anticancer,
antibacterial, antiviral, antitumor or carrying oxygen. In
the field of catalysis, copper complexes can catalyze
many organic reactions, such as the selective oxidation
of hydrocarbons, alcohols and phenols, the cycloproꢀ
panation of olefins, mimic enzyme catalysis and so on.
bis(
homoꢀ and copolymerization of methyl acrylate and
ꢀhexene [22]. The copper complexes ligated by
Nꢀtripodal [23], salicylaldiminate [24] or 2ꢀ(pyrazolꢀ
ꢀyl)ꢀ6ꢀ(pyrazolate)pyridine [25] ligands were also efꢀ
ficient catalyst precursors for methacrylate polymerꢀ
ization.
During the last few years,
β
ꢀketoamino)copper/MAO could catalyze the
1
Copper complexes can also be used for the polyꢀ
merization or copolymerization of polar monomers,
which has been an interesting and challenging field [8, 9].
In the past decade, much attention has been focused
on the performance of lateꢀtransitionꢀmetal complexꢀ
es due to their weaker oxophilicity, greater tolerance to
3
βꢀketoimine ligands have
functional group and thus generally less sensitive to received some attention and been applied in organoꢀ
deactivation by polar species than earlyꢀtransitionꢀ metallic complexes due to their ease of preparation
metal complexes [10–16]. Among the lateꢀtransitionꢀ and modification of both the steric and electronic efꢀ
metal complexes, copper complexes are attractive candiꢀ fects [26–28]. However, copper complexes with ꢀkeꢀ
β
dates since they are cheaper, airꢀstable, and easy to preꢀ toimine ligand are rarely reported. Recently we have
pare. However, compared with the nickel and palladium been committed to finding some novel early and late
complexes, much less attention has been paid to copper transition nonꢀmetallocene catalysts with excellent
complexes for catalyzing the polymerization of polar catalytic performances by regulating the coordination
monomers. R.T. Stibrany et al. reported good activity of environments of the central metal with electronic efꢀ
bis(benzimidazole) copper(II)/MAO (MAO = methylꢀ fects or synergistic steric and electronic effect of subꢀ
aluminoxane) system for the homoꢀ and copolymerꢀ stituents on ligands [29–33]. The substituents in
ligands have great influence on the structure and cataꢀ
lytic activity of the complexes, which include the steric
1
The article is published in the original.
543

544
XIAO et al.
effects of alkyl substituents and electronic effects of (m., 4H, Ph), 5.47 (s., 1H, C
halogen substituents. However, relative to the steric efꢀ CHCOCH3), 2.21 (s., 3H, CHCNCH3).
fects of alkylꢀsubstituents, the electronic effects of haloꢀ
H), 2.30 (s., 3H,
–1
IR (KBr;
C=C).
ν
, cm ): 3432 (N–H), 1597 (C=O), 1560
gen substituents received much less attention in comꢀ
plexes design and catalytic behavior research [34–39].
Here we studied the electronic effects to the copper comꢀ
(
For C H NOF
3
12
12
plexes with
sis methylꢀ (
bis( ꢀketoimino)copper complexes, structure, and
βꢀketoimine ligands and reported the syntheꢀ
I
) and trifluoromethylꢀsubstituents (II
)
anal. calcd., %:
Found, %:
C, 59.26;
C, 59.37;
H, 4.97;
H, 5.02;
N, 5.76.
N, 5.83.
β
methyl acrylate (MA) polymerization activity of a new
complex II. We found that the trifluoromethylꢀsubstiꢀ
tution exerted significant influences on the structure
and MA polymerization activity of the copper comꢀ
plexes.
Synthesis of complex I. A mixture of ligand HL1
H O (0.38 g,
(
2
0.76 g, 4 mmol) and Cu(OAc)2
⋅
2
mmol) in methanol (50 mL) was refluxed for 5 h. Afꢀ
ter removal of methanol solvent, the crude product
was recrystallized in toluene to obtain 0.67 g black solꢀ
id with 76% yield.
EXPERIMENTAL
–
1
General procedures. All work involving airꢀ and/or
moistureꢀsensitive compounds was carried out with
standard Schlenk techniques. Modified methylalumiꢀ
noxane (MMAO), 7% aluminum in a heptane soluꢀ
tion, was purchased from Akzo Nobel Chemical, Inc.
IR (KBr; ν, cm ): 3422 w, 2921 w, 1577 s, 1530 s, 1414 s,
1274 w, 1187 w, 1019 w, 936 w, 781 m.
For C H N O Cu
28
24
2 2
anal. calcd., %:
C, 65.51;
C, 65.47;
H, 6.41;
H, 6.92;
N, 6.37.
N, 6.49.
All other commercial chemicals are used as received.
1
The H NMR spectra of ligands were recorded on a Found, %:
Bruker Avance III 400 MHz spectrometer with tetꢀ
ramethylsilane as an internal standard. IR spectra of
the ligands and copper complexes were collected on a a procedure similar to that for complex I in 70% yield.
Nicolet Nexus 470 FT–IR spectrometer. Elemental
analyses were carried out using Vario EL 111.
Synthesis of 4ꢀ(
ꢀtolylamino)pentꢀ3ꢀenꢀ2ꢀone (HL1). 1056 w, 946 w, 760 m.
A mixture of 2ꢀmethylaniline (2.14 g, 0.02 mol),
acetyl acetone (2.10 g, 0.021 mol) and ꢀtolueneꢀ
sulfonic acid (0.02 g) in toluene (100 mL) was refluxed
for 12 h, with azeotropic removal of water using a
Synthesis of complex II. Complex was prepared via
–1
IR (KBr;
ν
, cm ): 3416 w, 2965 w, 1573 s, 1522 s,
1453 s, 1411 s, 1316 s, 1264 w, 1196 w, 1158 s, 1113 s,
o
p
For C H N O CuF
2
4
22
2
2
6
anal. calcd., %:
C, 52.60;
C, 52.50;
H, 4.05;
H, 4.16;
N, 5.11.
N, 5.02.
Deanꢀstark trap. After removing the solvent, the crude Found, %:
product was washed by 30 mL water and extracted
three times each by 40 mL diethyl ether. The ether soꢀ
lution was washed one or two times by a little dilute hyꢀ
drochloric acid to eliminate raw materials and the byꢀ
product diketiminate. The organic solution was then
washed by water, dried by Na SO and removed of
Polymerization of MA. A flameꢀdried Schlenk flask
was thrice purged with N and a desired amount of
freshly distilled toluene was transferred into the flask
2
(
placed in an oil bath at a designated temperature).
2
4
MA monomer and MMAO were injected into the flask
using a syringe and the mixture was stirred for 5 min.
The polymerization was started by adding the copper
complex solution in toluene with a syringe. After a deꢀ
sired time, the polymerization was quenched with
acidified ethanol (100 mL, 10 vol % HCl in ethanol).
Et O. 2.90 g product was obtained with 76.7% yield.
2
1
H NMR (400 MHz; CDCl3;
δ
ppm): 12.35 (s., 1H,
), 7.19 (m., 4H, Ph), 5.21 (s., 1H, C ), 2.29 (s.,
H, PhCH3), 2.12 (s., 3H, CHCOCH3), 1.88 (s., 3H,
CHCNCH3).
IR (KBr;
C=C).
N
3
H
H
–1
ν
, cm ): 3432 (N–H), 1597 (C=O), 1560 The precipitated polymer was filtered off, washed with
(
ethanol, and then dried under vacuum overnight at
5
0
°
C to constant weight.
X-ray structure determinations. Crystal data
obtained with the –2 scan mode were collected on
a Bruker Smart Apex CCD diffractometer with graphꢀ
iteꢀmonochromated Mo radiation ( = 0.71073 ).
For C H NO
15
12
anal. calcd., %:
Found, %:
C, 76.16;
C, 76.07;
H, 7.99;
H, 7.78;
N, 7.40.
N, 7.44.
ω
θ
K
λ
Å
α
Synthesis of 4ꢀ((2ꢀ(trifluoromethyl)phenyl)amiꢀ Structure II was solved using direct methods, whereas
2
no)pentꢀ3ꢀenꢀ2ꢀone (HL ). Following a similar proꢀ further refinements with fullꢀmatrix least squares on
2
1
2
cedure, HL was obtained in 76.5% yield. H NMR
400 MHz; CDCl3; , ppm): 12.66 (s., 1H, N ), 7.41 package. All nonhydrogen atoms were refined anisoꢀ
F
were obtained with the SHELXLꢀ97 program
(
δ
H
RUSSIAN JOURNAL OF COORDINATION CHEMISTRY Vol. 41
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2015

SYNTHESIS AND STRUCTURAL CHARACTERIZATION
545
C(12)
C(11)
C(9)
C(10)
C(8)
F(1)
O(1)
N(1)
C(7)
C(5A)
C(6)
F(3A)
F(2A)
C(3)
C(6
A)
C(4A)
C(7
A)
C(4)
C(1)
C(2)
Cu(1)
C(3A)
F(2)
C(2A)
F(3)
C(1
A)
C(5)
N(1A)
O(1
A)
F(1
A)
C(8A)
C(10A)
C(9A)
C(12A)
C(11A)
Molecular structure of complex II
.
tropically. Hydrogen atoms were introduced in calcuꢀ
lated positions with the displacement factors of the
host carbon atoms.
RESULTS AND DISCUSSION
By referring previous works [30], we synthesized
the ꢀketoimino ligands bearing methylꢀ and trifluoꢀ
β
romethylꢀsubstituents by reacting acetylacetone with
substituted aniline in toluene. The corresponding copꢀ
per complexes were obtained in good yields by reacting
the ligands with cupric acetate in methanol according
to the following Scheme:
Crystallographic data (excluding structure factors) for
structure II have been deposited with the Cambridge
Crystallographic Data Centre (CCDC no. 1023178; deꢀ
posit@ccdc.cam.ac.uk; http://www.ccdc.cam.ac.uk).
NH2
R
R
N
O
N
Cu(OAc)2
⋅
H O
2
Cu
NH O
O
O
O
R
R
HL
1
I, II
HL ,
I
: R = Me;
2
HL , II: R = CF3
Scheme.
These black copper complexes are stable in air and ture was shown in figure. The crystallographic data toꢀ
soluble in toluene, CH Cl and THF. The ligands were gether with the collection and refinement parameters
2
2
1
are summarized in Table 1.
characterized by H NMR, FTIR and elemental
analysis, and the copper complexes were measured by
FTIR and elemental analysis. The crystal of II suitable
The crystal structure of complex II showed that the
complex had a 2 : 1 ligand to metal ratio. Furthermore,
for Xꢀray crystallography were obtained by slow evaꢀ the two
βꢀketoimino ligands acted as bidentate
poration from toluene solution. The molecular strucꢀ N,Oꢀchelators and lie in the trans conformation to
RUSSIAN JOURNAL OF COORDINATION CHEMISTRY Vol. 41
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2015

546
XIAO et al.
Table 1. The crystal data and structure refinement parameꢀ fect square planar geometry and central symmetric strucꢀ
ters for complexes II
ture. The dihedral angle between Cu(1)O(1)N(1) and
C(1)C(2)C(3)C(4)C(5)C(6) planes is 86.79 , indicating
that the two imineꢀaryl rings are almost perpendicular
to the N–Cu–O plane. The Cu–O and Cu–N bond
lengths are 1.893(2) and 1.986(2) , respectively, and
the angle of NCuO is 91.36(9) , siminar to that of anaꢀ
°
Parameter
Value
Formula weight
Temperature
Crystal system
Space group
547.98
296(2)
Å
°
Orthorhombic
Pbca
logous [N,O] copper(II) complexes.
Copper complexes can be used as catalyst precurꢀ
sors for polar monomer polymerization. Recently Wu
et al. reported that bis(ketoamino)copper complexes
with alkyl substituents could catalyze the polymerizaꢀ
tion of MA with moderate activity when activated with
MAO [22]. We found that the introduction of elecꢀ
tronꢀdrawing trifluoromethyl group into Nꢀaryl rings
of ligand can improve greatly the catalytic activities,
and the results are summarized in Table 2.
a
, Å
, Å
7.902(2)
14.504(3)
2432.6(10)
4
b
Volume, Å3
Z
ρ
, mg/m3
, mm–1
1.496
μ
0.967
F
(000)
Crystal size
Range, deg
1116
The Al/Cu molar ratio and reaction temperature
apparently influence the catalytic activity of II. It canꢀ
not catalyze the polymerization of MA without
MMAO, showing that the cocatalyst MMAO has a
pivotal role. With the usage of MMAO increased, the
activities of complex II increased to a maximum
0.45
×
0.40 × 0.40
θ
1.92–25.02
Index ranges
–9
17
24
≤
h
k
≤
≤
≤
8,
–
≤
12,
25
–
≤
l
(
52.6 kg/mol Cu h) at an Al/Cu molar ratio of 100 and
Reflections collected
12570
then decreased (entry 2–5 of Table 2). The molecular
weights of obtained PMA have similar change trends
with the increase of Al/Cu molar ratio, but the molecꢀ
ular weight distributions decreased gradually.
Independent reflections (Rint
)
2151 (0.0343)
1476
Reflections with
I
> 2
σ
(I)
Completeness to
θ
, %
99.9
Goodnessꢀofꢀfit on F 2
Number of refinement parameters
Final indices ( > 2 ))
indices (all data)
Largest diff. peak and hole,
1.016
A clear change in activity of II/MMAO at different
temperatures was also observed, as shown in entry 4
and 6–8 of Table 2. When the reaction temperatures
162
R
I
σ(
I
0.0358, 0.0878
0.0593, 0.1017
0.341, –0.292
increased from 25 to 75
ties increased gradually to a maximum at 65
then decreased, while the polymers have the highest
molecular weight of 119 kg/mol at 45 C, and higher
°C, the polymerization activiꢀ
R
°
C and
e
Å–3
°
and lower temperatures would lower the molecular
weight. The catalyst need to be activated at a certain
temperature, so increase of temperatures can increase
the activity. But too high a temperature may destroy
the structure of copper complex, resulting in the reꢀ
duction of catalytic activity. Similar phenomenon had
create two sixꢀmembered chelate rings (Cu–O–C–
C–C–N). The bond distances of C(8)–C(9) and
C(9)–C(10) are 1.406(4) and 1.361(4) , respectively,
which are between that of single and double bonds,
suggesting that the double bond of C=C is delocalized.
The bond length of C(10)–O(1) is 1.276(4) , which
is longer than ordinary C=O bond (1.23 ), showed
that the C=O double bond was also delocalized. Thus
an N–C–C–C–O conjugated big ꢀbond has been the
formed.
plexes (mono
and bis( ꢀketoimino) titanium complexes [30]) and
Å
been found in alkylꢀsubstituted bis(βꢀketoamino)copꢀ
Å
per complex by Wu et al. [22].
Å
We have previously introduced fluorine atoms into
ꢀaryl moiety of some early transition metal comꢀ
ꢀdiiminato titanium complexes [29]
π
N
β
β
Fourꢀcoordinated Cu(II) complexes are usually
characterized by a square planar coordination that
may distort to a pseudoꢀtetrahedral geometry [40].
Furthermore, the steric hindrance and the electronic
characteristics of the substituents on the imine moiꢀ
eties also influence the planarity. For example, 2,6ꢀdiꢀ
observed significant electronic effect of the fluoroꢀ
substituents upon the structure and catalytic activity of
the titanium complexes as well as the property of the
obtained polymer. The trifluoromethyl substitution of
the
bis(
ization. Complex
showed an activity of 20.7 kg/mol Cu h, which was
planar coordination geometry. However, unlike the most similar to the results reported by Wu et al. [22]. Introꢀ
N,O] chelated copper complexes with distorted square duction of trifluoromethyl group into the
ꢀaryl ring
planar structures, the ꢀCF derivative II exhibited perꢀ of ligand, however, appreciably improved the activiꢀ
N
β
ꢀaryl moiety also exerted great influence to
ꢀketoimino)copper complexes for MA polymerꢀ
with the methyl substitution
isopropylꢀsubstituted
βꢀketoimino copper complex
I
reported by Wu’s group adopted a distorted square
[
N
o
3
RUSSIAN JOURNAL OF COORDINATION CHEMISTRY Vol. 41
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2015

SYNTHESIS AND STRUCTURAL CHARACTERIZATION
547
Table 2. The results of MA polymerization at different polymerization conditions*
Temperature,
Time,
h
Product,
g
Activity,
kg/mol Cu h
Entry Catalyst
Al/Cu
Mw
×
104
Mw/Mn
°
C
1
2
3
4
5
6
7
8
9
0
1
I
65
65
65
65
65
25
45
75
65
65
65
100
0
3
3
3
3
3
3
3
3
1
2
4
1.2454
0
20.7
0
8.2
2.27
II
II
II
II
II
II
II
II
II
II
50
0.2676
3.1574
2.1985
2.4811
2.9728
2.9081
1.7886
2.5664
3.3207
4.5
3.4
6.0
4.1
5.9
11.9
4.9
6.1
6.9
6.8
2.26
1.83
1.46
2.39
2.20
2.02
1.77
2.22
1.48
100
200
100
100
100
100
100
100
52.6
36.6
41.4
49.6
48.5
89.4
64.2
41.5
1
1
*
Reaction conditions: toluene solvent, catalyst 20 µmol, MA 5 mL, solution total volume 25 mL.
ties. Under the same reaction conditions, the catalytic
activity of II can reach 52.6 kg/mol Cu h, more than
3. Li, S.W., Wang, Y.F., and Zhao, J.S., Russ. J. Coord.
Chem., 2014, vol. 40, p. 653.
4. Levchenkov, S.I., Shcherbakov, I.N., Popov, L.D., et al.,
twice that of
I. The calculated activity can even reach
89.4 kg/mol Cu h at one hour reaction time. To the
Russ. J. Coord. Chem., 2014, vol. 40, p. 523.
best of our knowledge, this is the highest value reportꢀ
ed so far for copper complexes in acrylic monomer poꢀ
lymerization.
5. Saljooghi, A.S., Rudbari, H.A., Nicol
J. Coord. Chem., 2014, vol. 40, p. 424.
. Levchenkov, S.I., Shcherbakov, I.N., Popov, L.D., et al.,
Russ. J. Coord. Chem., 2014, vol. 40, p. 69.
ò, F., et al., Russ.
6
In summary, a new
βꢀketoimine ligand with trifluꢀ
oromethylꢀsubstitution on the
N
ꢀaryl moiety and its
7. Xia, H., Jiang, X., Jiang, C., et al., Russ. J. Coord.
Chem., 2014, vol. 40, p. 93.
8. Boffa, L.S. and Novak, B.M., Chem. Rev., 2000,
corresponding copper complex were synthesized in
good yields and characterized. XRD characterization
revealed that the copper(II) were coordinated by two
vol. 100, p. 1479.
trans
βꢀketoimino ligands with delocalized double
9. Nakamura, A., Ito, S., and Nozaki, K., Chem. Rev.
,
bonds and the copper complex II adopted a perfect
central symmetric square planar structure with copper
as the center. The MMAOꢀactivated copper complexꢀ
es can polymerize MA effectively. The electronꢀwithꢀ
drawing effect of trifluoromethyl group improved sigꢀ
nificantly the catalytic activities. The activity of
II/MMAO can reach 89.4 kg/mol Cu h, which is the
highest value reported so far for copper complexes in
acrylic monomer polymerization.
2009, vol. 109, p. 5215.
0. Wang, S., Sun, W.ꢀH., and Redshaw, C., J. Organomet.
1
Chem., 2014, vol. 751, p. 717.
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1
12. Bianchini, C., Giambastiani, G., Luconi, L., et al.,
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ACKNOWLEDGMENTS
15. Johnson, L.K., Mecking, S., and Brookhart, M., J. Am.
Chem. Soc., 1996, vol. 118, p. 267.
The authors are grateful for the financial support
from the Natural Sciences Foundation of China
21172269, 51373201), the Applied Fundamental Reꢀ
search Project of Wuhan City (2014010101010017)
and the Open Fund of State Key Laboratory of Fine
Chemicals (KF1207).
16. Wang, J., Ye, Z., and Joly, H., Macromolecules, 2007,
vol. 40, p. 6150.
(
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2015