УДК 541.49
SYNTHESIS AND STRUCTURAL CHARACTERIZATION OF TRIFLUOROMETHYL-SUBSTITUTED p-KETOIMINO COPPER(II) COMPLEX AND ITS CATALYTIC BEHAVIOR FOR METHYL ACRYLATE POLYMERIZATION
© 2015 X. Xiao1, Y. C. Wen1, X. J. Wang1, L. Lei2, P. Xia1, T. C. Li1, A. Q. Zhang1, and G. Y. Xie1, 3, *
1Key Laboratory of Catalysis and Materials Science of the State Ethnic Affairs Commission & Ministry of Education, Hubei Province, South-Central University for Nationalities, Wuhan, 430074 P.R. China 2Baise University, Baise, 533000 P.R. China 3State Key Laboratory of Fine Chemicals, Dalian University of Technology, Dalian, 116024 P.R. China
*E-mail: xiegy@scuec.edu.cn Received January 9, 2015
A new trifluoromethyl-substituted bis(P-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 central symmetric square planar structure with the copper center coordinated by two trans P-ketoimine ligands with delocalized double bonds. On activation with modified methylaluminoxane (MMAO), the bis(P-ketoimino)copper complex can polymerize methyl acrylate effectively. Introduction of trifluoromethyl group into the N-aryl ring of the ligands, which leads to strong electron-withdrawing 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: 10.7868/S0132344X15080095
INTRODUCTION
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 cyclopro-panation of olefins, mimic enzyme catalysis and so on.
Copper complexes can also be used for the polymerization 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 late-transition-metal complexes due to their weaker oxophilicity, greater tolerance to functional group and thus generally less sensitive to deactivation by polar species than early-transition-metal complexes [10—16]. Among the late-transition-metal complexes, copper complexes are attractive candidates since they are cheaper, air-stable, and easy to prepare. However, compared with the nickel and palladium complexes, much less attention has been paid to copper complexes for catalyzing the polymerization of polar monomers. R.T Stibrany et al. reported good activity of ¿«■(benzimidazole) copper(II)/MAO (MAO = methylaluminoxane) system for the homo- and copolymer-izations of ethylene and methyl acrylate [17—19]. The
activity of MAO-activated [1,2-bis(4,4-dimethyl-2-oxazolin-2-yl)ethane]copper(II) dichloride for the polymerization of acrylates and their copolymeriza-tion with ethylene was reported in [20]. $/s(salicylal-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 b/s(P-ketoamino)copper/MAO could catalyze the homo- and copolymerization of methyl acrylate and 1-hexene [22]. The copper complexes ligated by N-tripodal [23], salicylaldiminate [24] or 2-(pyrazol-3-yl)-6-(pyrazolate)pyridine [25] ligands were also efficient catalyst precursors for methacrylate polymerization.
During the last few years, P-ketoimine ligands have received some attention and been applied in organo-metallic complexes due to their ease of preparation and modification of both the steric and electronic effects [26—28]. However, copper complexes with P-ke-toimine ligand are rarely reported. Recently we have been committed to finding some novel early and late transition non-metallocene catalysts with excellent catalytic performances by regulating the coordination environments of the central metal with electronic effects or synergistic steric and electronic effect of sub-stituents on ligands [29—33]. The substituents in ligands have great influence on the structure and cata-
lytic activity of the complexes, which include the steric effects of alkyl substituents and electronic effects of halogen substituents. However, relative to the steric effects of alkyl-substituents, the electronic effects of halogen substituents received much less attention in complexes design and catalytic behavior research [34—39]. Here we studied the electronic effects to the copper complexes with P-ketoimine ligands and reported the synthesis methyl- (I) and trifluoromethyl-substituents (II) ¿/«(P-ketoimino)copper complexes, structure, and methyl acrylate (MA) polymerization activity of a new complex II. We found that the trifluoromethyl-substi-tution exerted significant influences on the structure and MA polymerization activity of the copper complexes.
EXPERIMENTAL
General procedures. All work involving air- and/or moisture-sensitive compounds was carried out with standard Schlenk techniques. Modified methylalumi-noxane (MMAO), 7% aluminum in a heptane solution, was purchased from Akzo Nobel Chemical, Inc. All other commercial chemicals are used as received. The XH NMR spectra of ligands were recorded on a Bruker Avance III 400 MHz spectrometer with tet-ramethylsilane as an internal standard. IR spectra of the ligands and copper complexes were collected on a Nicolet Nexus 470 FT—IR spectrometer. Elemental analyses were carried out using Vario EL 111.
Synthesis of 4-(o-tolylamrno)pent-3-en-2-one (HL1).
A mixture of 2-methylaniline (2.14 g, 0.02 mol), acetyl acetone (2.10 g, 0.021 mol) and ^-toluene-sulfonic acid (0.02 g) in toluene (100 mL) was refluxed for 12 h, with azeotropic removal of water using a Dean-stark trap. After removing the solvent, the crude product was washed by 30 mL water and extracted three times each by 40 mL diethyl ether. The ether solution was washed one or two times by a little dilute hydrochloric acid to eliminate raw materials and the byproduct diketiminate. The organic solution was then washed by water, dried by Na2SO4 and removed of Et2O. 2.90 g product was obtained with 76.7% yield. 1H NMR (400 MHz; CDCl3; 8 ppm): 12.35 (s., 1H, NH), 7.19 (m., 4H, Ph), 5.21 (s., 1H, CH), 2.29 (s., 3H, PhCH3), 2.12 (s., 3H, CHCOCH3), 1.88 (s., 3H, CHCNCH3).
IR (KBr; v, cm-1): 3432 (N-H), 1597 (C=O), 1560 (C=C).
For C12H15NO anal. calcd., %: Found, %:
C, 76.16; C, 76.07;
H, 7.99; H, 7.78;
N, 7.40. N, 7.44.
Synthesis of 4-((2-(trifluoromethyl)phenyl)ami-no)pent-3-en-2-one (HL2). Following a similar procedure, HL2 was obtained in 76.5% yield. 1H NMR
(400 MHz; CDCl3; 8, ppm): 12.66 (s., 1H, NH), 7.41 (m., 4H, Ph), 5.47 (s., 1H, CH), 2.30 (s., 3H, CHCOCH3), 2.21 (s., 3H, CHCNCH3).
IR (KBr; v, cm-1): 3432 (N-H), 1597 (C=O), 1560 (C=C).
For C12H12NOF3 anal. calcd., %: Found, %:
C, 59.26; C, 59.37;
H, 4.97; H, 5.02;
N, 5.76. N, 5.83.
Synthesis of complex I. A mixture of ligand HL1 (0.76 g, 4 mmol) and Cu(OAc)2 • H2O (0.38 g, 2 mmol) in methanol (50 mL) was refluxed for 5 h. After removal of methanol solvent, the crude product was recrystallized in toluene to obtain 0.67 g black solid with 76% yield.
IR (KBr; v, cm-1): 3422 w, 2921 w, 1577 s, 1530 s, 1414 s, 1274 w, 1187 w, 1019 w, 936 w, 781 m.
For C24H28N2O2Cu
anal. calcd., %: C, 65.51; H, 6.41; N, 6.37. Found, %: C, 65.47; H, 6.92; N, 6.49.
Synthesis of complex II. Complex was prepared via a procedure similar to that for complex I in 70% yield.
IR (KBr; v, cm-1): 3416 w, 2965 w, 1573 s, 1522 s, 1453 s, 1411 s, 1316 s, 1264 w, 1196 w, 1158 s, 1113 s, 1056 w, 946 w, 760 m.
For C24H22N2O2CuF6
anal. calcd., %: C, 52.60; H, 4.05; N, 5.11. Found, %: C, 52.50; H, 4.16; N, 5.02.
Polymerization of MA. A flame-dried Schlenk flask was thrice purged with N2 and a desired amount of freshly distilled toluene was transferred into the flask (placed in an oil bath at a designated temperature). 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 desired time, the polymerization was quenched with acidified ethanol (100 mL, 10 vol % HCl in ethanol). The precipitated polymer was filtered off, washed with ethanol, and then dried under vacuum overnight at 50°C to constant weight.
X-ray structure determinations. Crystal data obtained with the ®-29 scan mode were collected on a Bruker Smart Apex CCD diffractometer with graph-ite-monochromated Mo^a radiation (X = 0.71073 A). Structure II was solved using direct methods, whereas further refinements with full-matrix least squares on F2 were obtained with the SHELXL-97 program
SYNTHESIS AND STRUCTURAL CHARACTERIZATION
509
Molecular structure of complex II.
package. All nonhydrogen atoms were refined aniso-tropically. Hydrogen atoms were introduced in calculated positions with the displacement factors of the host carbon atoms.
Crystallographic data (excluding structure factors) for structure II have been deposited with the Cambridge Crystallographic Data Centre (CCDC no. 1023178; de-posit@ccdc.cam.ac.uk; http://www.ccdc.cam.ac.uk).
RESULTS AND DISCUSSION
By referring previous works [30], we synthesized the P-ketoimino ligands bearing methyl- and trifluo-romethyl-substituents by reacting acetylacetone with substituted aniline in toluene. The corresponding copper complexes were obtained in good yields by reacting the ligands with cupric acetate in methanol according to the following Scheme:
nh2
YY
O O
R
NH O
R
Cu(OAc)2 ■ H2O
HL HL1, I: R = Me;
HL2, II: R = CF3
N O
C/
/ \
ON
I, II
R
Scheme.
These black copper complexes are stable in air and soluble in toluene, CH2Cl2 and THF. Th
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