Article pubs.acs.org/Organometallics
Heterocycle-Substituted Phosphinesulfonato Palladium(II) Complexes for Insertion Copolymerization of Methyl Acrylate Zhongbao Jian, Philipp Wucher, and Stefan Mecking* †
Chair of Chemical Materials Science, Department of Chemistry, University of Konstanz, Universitätsstrasse 10, 78457 Konstanz, Germany S Supporting Information *
ABSTRACT: A family of heterocycle-substituted binuclear phosphinesulfonato Pd(II) complexes {[R2P(C6H4SO2O)]PdMeClLi(dmso)}2 (1a−d-LiCl-dmso: 1a-LiCl-dmso, R = 2-furyl; 1b-LiCl-dmso, R = 2-thienyl; 1c-LiCl-dmso, R = 2-(Nmethyl)pyrrolyl; 1d-LiCl-dmso, R = 2-benzofuryl) was synthesized, and the solid-state structures of 1a−c-LiCl-dmso were determined, which revealed various modes of bridging between the two metal fragments. 1a−d-LiCl-dmso further generated either the mononuclear Pd(II) complexes {[κ2P,O-R2P(C6H4SO2O)]PdMe(pyr)} (1a−d-pyr) by addition of pyridine or the more labile mononuclear Pd(II) complex {[κ2P,O-(2-thienyl)2P(C6H4SO2O)]PdMe(dmso)} (1b-dmso) by chloride abstraction with AgBF4. Stoichiometric methyl acrylate (MA) insertion experiments indicated that, in comparison with the other three substituents, the thienyl-substituted Pd(II) complexes undergo faster insertion of MA in a primary 2,1-fashion, and 1b-dmso possesses the fastest insertion rate due to the relative weakly coordinating dmso molecule. All palladium complexes were employed in ethylene polymerization, affording highly linear polyethylene with relatively low molecular weights (Mn = (0.5−7.4) × 103). In addition, under these pressure reactor conditions, the thienyl motif displays the highest activity (order: 1b-dmso > 1b-pyr > 1a-pyr > 1d-pyr > 1c-pyr ≫ 1a−d-LiCl-dmso). Copolymerization reactions of ethylene and MA further revealed that MA incorporation in the obtained linear copolymers depends moderately on the heterocyclic substituents.
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but also acrylonitrile, vinyl acetate, vinyl fluoride, vinyl chloride, allylic monomers, acrylic acid, and acrylamides.3a,b,d,5 Mechanistic insights into polar vinyl monomer insertion polymerization have revealed that the soft phosphine and the hard sulfonate oxygen σ-donor of the asymmetric anionic phosphinesulfonato ligand play a key role in the unique catalytic properties.5h Therefore, on the basis of the phosphinesulfonato framework, the effect of substituents at phosphorus on catalysis has been explored extensively and it has been found that not only catalytic activity but also molecular weight, molecular weight distribution, and polar vinyl monomer incorporation in polymerization reactions are significantly dependent on the substituents.5d,m,6 A number of symmetric or different substituents (R1 and R2) such as alkyls, cycloalkyl, various substituted aryls, and bulky aryls have been studied (Chart 1). Within this variety of substituents studied, aryl groups arguably result in the best catalytic performance overall. We have recently reported that electron-donating groups on the P-bound phenyl substituent enhance polymer molecular weights in ethylene polymerization and comonomer incorporation ratio in the copolymerization of ethylene and MA.6a This prompted us to study aromatic heterocycles as substituents.
INTRODUCTION
Catalytic insertion polymerization of ethylene and propylene by early-transition-metal catalysts is one of the most wellstudied chemical reactions. In terms of applications, it is employed for the production of a vast scale of polyolefins annually.1 An insertion (co)polymerization of electron-deficient polar-substituted vinyl monomers such as acrylates has remained elusive for a long time, however. In the mid-1990s, Brookhart et al. discovered that cationic Pd(II) diimine species can catalyze the insertion copolymerization of ethylene or 1-olefins and acrylates to generate highly branched copolymers containing acrylate units (≤25 mol %) at the end of branches preferentially.2 This capability for insertion of polar vinyl monomers is attributed to the low oxophilic nature of d8 metal (late-transition-metal) catalysts that renders them more tolerant toward polar moieties.3 In comparison with the aforementioned highly branched copolymers formed by extensive chain walking, linear ethylene−methyl acrylate (MA) copolymers were obtained with a neutral phosphinesulfonato palladium catalyst as reported by Drent et al.4 In the past decade, this phosphinesulfonato Pd(II) catalyst [(P∧O)PdMe(L)] (P∧O = κ2P,O-(2-MeOC6H4)2P(C6H4SO2O)) has been found to promote the formation of linear copolymers of ethylene with a broad scope of polar monomers, including not only acrylates © XXXX American Chemical Society
Received: April 15, 2014
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dx.doi.org/10.1021/om500400a | Organometallics XXXX, XXX, XXX−XXX
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Chart 1. Phosphinesulfonato Ligands Containing Various Substituents
Due to the presence of a heteroatom, the five-membered heterocycle has different electronic effects on the phosphorus atoms of the ligand in comparison to the common alkyl and aryl groups. Moreover, different heteroatoms (such as O, N, and S) in the heterocycle may undergo an additional weak interaction with the metal center and even bind to the metal center, thus influencing the polymerization reaction. For example, Rieger et al. have reported that the replacement of the o-MeO− in the complex 2-pyr (vide infra, Chart 2) with a MeS− group reduces the ethylene polymerization activity drastically but increases the polymer molecular weight significantly. An enhanced Pd−X (X = SMe) interaction was proposed due to the increased size and the softer donor nature of the sulfur atom in comparison to the corresponding harder oxygen donor (X = OMe).6e Intrigued by these factors, we now report on ethylene homopolymerization and copolymerization with MA by novel heterocycle-substituted phosphinesulfonato Pd(II) catalyst precursors.
in the 31P NMR spectra of 1a−d-Li are in good agreement with the features of lithium phosphinesulfonate salts reported previously.5b,6a Synthesis of Palladium(II) Complexes. Lithium phosphinesulfonate salts 1a−d-Li can readily react with 1 equiv of [PdMeCl(COD)] at room temperature in dimethyl sulfoxide (dmso)/CH2Cl2 to straightforwardly generate the binuclear Pd(II) complexes 1a−d-LiCl-dmso in good isolated yields (70−85%) (Scheme 2). Doublets of the Pd−Me group arising from coupling with the phosphorus atom (3JPH) in the 1H NMR spectra of 1a−d-LiCl-dmso are observed at δ 0.83, 0.91, 0.51, and 0.94 ppm, respectively (Table 1). On the basis of Table 1. Selected Chemical Shifts (ppm) of Complexes 1 1
1a-LiCl-dmso 1b-LiCl-dmso 1c-LiCl-dmso 1d-LiCl-dmso 1a-pyr 1b-pyr 1c-pyr 1d-pyr 1b-dmso
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RESULTS AND DISCUSSION Synthesis of Lithium Phosphinesulfonate Salts. Heterocycle-substituted lithium phosphinesulfonate salts were prepared by slight modifications of known procedures.5b,6h Treatment of benzenesulfonic acid with 2 equiv of nBuLi at room temperature for 30 min followed by reaction with R2PCl or bis(N-methyl-2-pyrrolyl)methoxyphosphine7 at room temperature for another 4 h selectively afforded the targeted lithium phosphinesulfonate salts R(P∧O)Li (1a-Li, R = 2-furyl; 1b-Li, R = 2-thienyl; 1c-Li, R = 2-(N-methyl)pyrrolyl; 1d-Li, R = 2-benzofuryl) in high yields of 59−86% (Scheme 1). Note that,
H (Pd−Me) 0.83 0.91 0.51 0.94 0.69 0.73 0.46 0.79 0.67
(d) (d) (d) (d) (s) (s) (d) (s) (d)
13
C (Pd−Me) −0.47 1.04 −1.75 −0.61 −0.17 1.82
(s) (s) (s) (s) (s) (s)
0.26 (s) 1.84 (s)
31
P
−11.0 0.4 −18.0 −6.6 −8.8 2.3 −16.0 −4.7 3.3
NMR data, one cannot easily designate structures of 1a−dLiCl-dmso; thus, the solid-state structures were determined by X-ray diffraction. Suitable crystals of 1a−c-LiCl-dmso were grown by layering a CH2Cl2 solution of the complex with acetone and pentane.8 Interestingly, X-ray diffraction analysis reveals that the solid-state structures of 1a−c-LiCl-dmso (Figures 1−3 and Scheme 2) differ in the nature of the bridging ensemble, the atoms involved, and the structures involved, which are significantly different from known solid-state structures of acetone- or methanol-coordinated LiCl-bridged binuclear Pd(II) complexes with other phosphinesulfonato ligands.6b−d,9 Further, it is worth noting that neither furyl and thienyl groups nor the N-methylpyrrolyl group coordinates to the metal center. The corresponding Pd−O(furyl), Pd−S(thienyl), and Pd−N(pyrrolyl) bond distances are long and are in the ranges 3.536−4.618, 3.727−3.987, and 3.814−4.631 Å, respectively. By addition of pyridine to the binuclear Pd(II) complexes 1a−d-LiCl-dmso, mononuclear pyridine-coordinated phosphinesulfonato Pd(II) complexes 1a−d-pyr were formed directly
Scheme 1. Synthesis of Lithium Phosphinesulfonate Salts 1a−d-Li
for the anticipated further preparation of phosphinesulfonato Pd(II) complexes, it is not necessary to protonate the lithium phosphinesulfonate salts 1a−d-Li in this case. Singlets appearing at δ −54.9, −37.8, −59.4, and −48.8 ppm, respectively, B
dx.doi.org/10.1021/om500400a | Organometallics XXXX, XXX, XXX−XXX
Organometallics
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Scheme 2. Synthesis of P-Heterocycle-Substituted Phosphinesulfonato Palladium Complexes 1a−d-LiCl-dmso
Figure 2. ORTEP plot of 1b-LiCl-dmso drawn with 50% probability ellipsoids. Hydrogen atoms are omitted for clarity.
Figure 3. ORTEP plot of 1c-LiCl-dmso drawn with 50% probability ellipsoids. Hydrogen atoms are omitted for clarity.
Scheme 3. Synthesis of Phosphinesulfonato Palladium Complexes 1a−d-pyr
Figure 1. ORTEP plot of 1a-LiCl-dmso drawn with 50% probability ellipsoids. Hydrogen atoms are omitted for clarity. Coordinating acetone and H2O molecules originate from the solvent.
in 79−87% yield (Scheme 3). Complexes 1a−d-pyr were fully identified by one- (1H, 13C, and 31P) and two-dimensional NMR spectroscopy and elemental analysis. In addition, X-ray diffraction analysis of 1a-pyr revealed that the palladium methyl group and the phosphorus atom are located cis to each other in the square-planar coordination sphere around the palladium center, as expected (Figure 4). The Pd1−C15(Me) bond distance (2.026(3) Å) is close to that of Pd−C (2.027(5) Å) in the analogous anisyl-phosphinesulfonato complex {[κ2P,O-(2MeOC6H4)2P(C6H4SO2O)]PdMe(pyr)}, as anticipated, given that the sulfonate donor in a trans position is identical in both cases. The corresponding Pd1−P1 bond distance is slightly shorter (2.216(7) vs 2.232(1) Å), in line with a slightly stronger interaction with the more electron rich phosphine donor in
Figure 4. ORTEP plot of complex 1a-pyr drawn with 50% probability ellipsoids. Hydrogen atoms are omitted for clarity. Selected bond lengths (Å): Pd1−O3 2.183(2), Pd1−P1 2.216(7), Pd1−C15 2.026(3), Pd1−N1 2.111(2). C
dx.doi.org/10.1021/om500400a | Organometallics XXXX, XXX, XXX−XXX
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1a-pyr.5n Moreover, both furyl groups do not bind to the center metal with long distances between Pd1 and O4 or O5 (3.721 and 4.631 Å). These bond distances are comparable with the analogous distances between the Pd atom and the anisyl methoxy groups (3.604 and 5.125 Å) in {[κ2P,O-(2MeOC6H4)2P(C6H4SO2O)]PdMe(pyr)}.5n As a more labile ligand in comparison to pyridine, dmso was also studied. According to the procedure reported previously,6c addition of a silver(I) salt (AgBF4) to a methylene chloride solution of the binuclear Pd(II) complex 1b-LiCl-dmso resulted in direct formation of the corresponding mononuclear dmsocoordinated phosphinesulfonato Pd(II) complex 1b-dmso in 87% yield (Scheme 4). In comparison with the complex
Table 2. Rate Constants of MA Insertion into Palladium Complexesa
Scheme 4. Synthesis of Phosphinesulfonato Palladium Complex 1b-dmso
complex
kfirst (10−5 s−1)
ksecond (10−5 s−1)
1a-LiCl-dmso 1b-LiCl-dmso 1c-LiCl-dmso 1d-LiCl-dmso 1a-pyrc 1b-pyrc 1c-pyrc 1d-pyrc 2-pyrc 1b-dmso 2-dmso 3d
∼0.7 1.6 ∼0.2b ∼0.4b ∼0.2b ∼0.4b ∼0.2b 0.2 1.2 11 70 10
2.0 9.2 0.3
b
Conditions: Pd (0.025 mol L−1), MA (20 equiv), CD2Cl2, T = 25 °C, unless otherwise noted. bExact determination not possible due to overlapping or broad resonances. cC2D2Cl4, T = 50 °C. dReference 6a. a
at 25 °C (cf. the Supporting Information, Figure S26). This is comparable to the rate constant for 3 (kfirst = 10 × 10−5 s−1) and is lower than the rate constant of the benchmark complex 2-dmso (kfirst = 70 × 10−5 s−1) (cf. the Supporting Information, Figure S30).6a In line with the results of MA insertion into 2-dmso or 3, the 2,1-mode dominates in the first MA insertion into 1b-dmso; meanwhile, both β-H elimination and the second insertion of MA are observed as well (Scheme 5). In addition, the
1b-LiCl-dmso, the doublet signal of the Pd−Me group in the 1 H NMR spectrum of 1b-dmso is shifted upfield from 0.91 to 0.67 ppm (Table 1). Stoichiometric Insertion of Methyl Acrylate. The insertion reaction with MA has been well studied for phosphinesulfonato Pd(II) methyl complexes, and rate constants are available for various substitution patterns at phosphorus. A stoichiometric study on MA insertion has revealed that the first insertion of MA into the anisyl-substituted complex 2-dmso or the phenyl-substituted complex 3 respectively (Chart 2) takes
Scheme 5. Reaction of 1b-dmso with Excess Methyl Acrylate (MA)
Chart 2. Reported Phosphinesulfonato Pd(II) Methyl Complexes 2 and 3
observed rate constants (kfirst) of MA 2,1-first insertion for 1a−d-pyr are within (0.2−0.4) × 10−5 s−1 at 50 °C (Table 2), which are again 3−6 times smaller than the observed rate constant (kfirst = 1.2 × 10−5 s−1) for 2-pyr (cf. the Supporting Information, Figure S31) and are again considerably smaller than those for the corresponding dmso complexes, reflecting the strong binding of pyridine. Polymerization of Ethylene. Beyond the stoichiometric studies of MA insertion, the catalytic properties of all complexes were investigated. Complexes 1a−d-LiCl-dmso show low to no activity toward ethylene polymerization even at high temperature (92 °C) and high pressure (20 bar) (cf. the Supporting Information, Table S1). In contrast, pyridine complexes 1a−d-pyr exhibit higher activities (TOF = (2.3−69.4) × 103 molE molPd−1 h−1). Either with increasing ethylene pressure from 5 to 20 bar or with increasing temperature from 80 to 92 °C, the catalytic activity increases correspondingly. Under the same conditions, the heterocyclic substituents at phosphorus have a significant influence on the catalytic activity. The thienyl-functionalized catalyst 1b-pyr displays a higher activity than the furyl-substituted catalyst 1a-pyr and a much higher
place primarily in a 2,1-fashion, and then this insertion product can either undergo β-H elimination to methyl crotonate or insert another molecule of MA to form a double-insertion product. 5b,j,6a,b,12 The insertion reactions of an excess of MA (ca. 20 equiv) with complexes 1a−d-LiCl-dmso were monitored through 1H NMR at 25 °C over a period of 15 h (cf. the Supporting Information, Figures S22−S25). A first insertion of MA into the Pd−Me bond of 1a−d-LiCl-dmso occurs without prior halide abstraction, but the net rate is very slow. Under the pseudofirst-order conditions, the observed rate constants (kfirst) of the first insertion are in the range (0.2−1.6) × 10−5 s−1 for the four complexes, with 1b-LiCl-dmso being subject to the fastest insertion of MA (Table 2). For 1b-dmso, generated from AgBF4 and 1b-LiCl-dmso, the rate constant kfirst = 11 × 10−5 s−1 is observed D
dx.doi.org/10.1021/om500400a | Organometallics XXXX, XXX, XXX−XXX
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Table 3. Ethylene Polymerization by Phosphinesulfonato Pd(II) Complexesa Mn entry
cat.
p (bar)
T (°C)
yield (mg)
TOF (103 molE molPd−1 h−1)
NMRb
GPCc
Mw/Mnc
branching (/1000C)d
3-1 3-2 3-3 3-4 3-5 3-6 3-7 3-8e 3-9e 3-10e 3-11 3-12 3-13 3-14 3-15 3-16 3-17f 3-18f 3-19g
1a-pyr 1a-pyr 1a-pyr 1a-pyr 1b-pyr 1b-pyr 1b-pyr 1c-pyr 1c-pyr 1c-pyr 1d-pyr 1d-pyr 1d-pyr 1b-dmso 1b-dmso 1b-dmso 2-pyr 2-dmso 3
10 5 10 20 5 10 20 5 10 20 5 10 20 5 10 20 5 5 5
80 92 92 92 92 92 92 92 92 92 92 92 92 92 92 92 80 80 80
851 834 1300 2543 930 1666 3472 159 282 477 360 571 1305 1324 3214 6750 1490 4920 4200
17.0 16.7 26.0 50.9 18.6 33.3 69.4 2.3 4.0 6.8 7.2 11.4 26.1 26.5 64.3 135.0 30.0 100.0 65.0
1650 1200 1000 1150 3460 3900 3800 14900 17300 18100 1100 900 900 3250 3600 3400
700 600 650 650 2100 1950 2100 6850 6850 7360 570 600 580 1700 1730 1900 16600 12600 1500
1.8 1.7 1.7 1.7 2.4 2.5 2.4 2.8 2.7 2.4 1.6 1.6 1.7 2.2 2.3 2.3 2.2 2.2 2.2
