Letter Cite This: ACS Macro Lett. 2018, 7, 305−311
pubs.acs.org/macroletters
Methylene-Bridged Bisphosphine Monoxide Ligands for PalladiumCatalyzed Copolymerization of Ethylene and Polar Monomers Yusuke Mitsushige,† Hina Yasuda,† Brad P. Carrow,‡ Shingo Ito,† Minoru Kobayashi,§ Takao Tayano,§ Yumiko Watanabe,∥ Yoshishige Okuno,∥ Shinya Hayashi,⊥ Junichi Kuroda,⊥ Yoshikuni Okumura,⊥ and Kyoko Nozaki*,† †
Department of Chemistry and Biotechnology, Graduate School of Engineering, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-8656, Japan ‡ Department of Chemistry, Princeton University, Princeton, New Jersey, United States § Japan Polychem Corporation, 1 Toho-cho, Yokkaichi, Mie 510-0848, Japan ∥ Computational Science and Technology Information Center, Showa Denko K.K., 1-1-1 Ohnodai, Midori-ku, Chiba, Chiba 267-0056, Japan ⊥ Institute for Advanced and Core Technology, Showa Denko K.K., 2 Nakanosu, Oita, Oita 870-1809, Japan S Supporting Information *
ABSTRACT: A series of palladium complexes bearing a bisphosphine monoxide with a methylene linker, that is, [κ2P,O-(R12P)CH2P(O)R22]PdMe(2,6-lutidine)][BArF4] (Pd/ BPMO), were synthesized and evaluated as catalysts for the homopolymerization of ethylene and the copolymerization of ethylene and polar monomers. X-ray crystallographic analyses revealed that these Pd/BPMO complexes exhibit significantly narrower bite angles and longer Pd−O bonds than Pd/BPMO complexes bearing a phenylene linker, while maintaining almost constant Pd−P bond lengths. Among the complexes synthesized, menthyl-substituted complex 3f (R 1 = (1R,2S,5R)-2-isopropyl-5-methylcyclohexan-1-yl; R2 = Me) showed the best catalytic performance in the homo- and copolymerization in terms of molecular weight and polymerization activity. Meanwhile, complex 3e (R1 = t-Bu; R2 = Me) exhibited a markedly higher incorporation of comonomers in the copolymerization of ethylene and allyl acetate (≤12.0 mol %) or methyl methacrylate (≤0.6 mol %). The catalytic system represents one of the first examples of late-transition-metal complexes bearing an alkylene-bridged bidentate ligand that afford high-molecular-weight copolymers from the copolymerization of ethylene and polar monomers.
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and polar monomers for synthesizing functional polyolefin materials; seminal discoveries in this area include palladium/αdiimine,3,4 palladium/phosphine−sulfonate,5−7 and palladium/ IzQO catalysts.8 Given that the latter two systems are able to copolymerize a wider range of comonomers, various bidentate ligands have been designed and synthesized in an attempt to mimic their unique structural features,9 that is, electronically unsymmetric coordination sites consisting of a strong σdonating group with bulky substituents and a weak σ-donating group. This unsymmetric structure is responsible for suppressing β-hydride elimination and subsequent chaintransfer, which leads to an increase in molecular weight of the resulting polymers.10 In comparison, less attention has been focused on the backbone structure of the bidentate ligands. While almost all of the group-10-metal-based copolymerization
iven that the reactivity of homogeneous transition-metal catalysts depends not only on the metal center, but also largely on the nature of the ligands, the development of bespoke ligands is a central pillar of research in this area. In order to develop new ligands, the stereoelectronic effects of the ligands can be optimized by changing the type of coordination site, as well as substituents on or around the coordination site. In the case of bidentate ligands, the backbone structure of the ligand is also an important factor to modulate the distance between two coordination sites and the bite angle, which may influence the reactivity and selectivity of the catalysts.1 The backbone structure of the ligand may also affect the electrondonating ability of bidentate ligands.2 Thus, acquiring in-depth knowledge on the effect of the backbone structure on the catalytic performance is essential to the design of novel bidentate ligands for active and selective homogeneous transition-metal-based catalysts. Intensive efforts have been devoted to the development of late-transition-metal-catalyzed copolymerizations of ethylene © XXXX American Chemical Society
Received: January 18, 2018 Accepted: February 6, 2018
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DOI: 10.1021/acsmacrolett.8b00034 ACS Macro Lett. 2018, 7, 305−311
Letter
ACS Macro Letters catalysts bear an unsymmetric bidentate ligand with an arylene linker between the two coordination sites,9,11 fewer examples of group-10-metal-based catalysts with an alkylene-bridged bidentate ligand that catalyzed (co)oligo- and polymerizations of ethylene are known:9a,12 Notable examples include nickel/2(di-tert-butylphosphino)-1-phenylethan-1-one catalysts for ethylene/methyl 10-undecenoate copolymerization,13 and palladium/cyclopentane-1,2-diyl-bridged phosphine−sulfonate complexes for ethylene/methyl acrylate and ethylene/vinyl fluoride copolymerization.14 Our group has previously developed phenylene-bridged bisphosphine monoxide (BPMO) ligands of the type R12PC6H4PR22O (R1 = i-Pr, Ph, 2-MeOC6H4, 2-CF3C6H4; R2 = t-Bu, i-Pr, Me), which can promote the palladiumcatalyzed coordination−insertion copolymerization of ethylene with various polar monomers.15 Further investigations on this ligand platform revealed that changing the ligand backbone significantly influences the catalytic performance. Herein we describe the synthesis of novel palladium/BPMO complexes that bear a methylene linker (Figure 1)16 and their catalytic
Scheme 1. Synthesis of Methylene-Bridged BPMO Ligands and Their Palladium Complexes
structures of 2c and 2f were moreover characterized by singlecrystal X-ray diffraction analysis (Figure 2). For comparison, the corresponding palladium complex 4,15a which bears a phenylene-bridged BPMO ligand (R1 = i-Pr, R2 = t-Bu), is also shown in Figure 2c. The bite angles of complexes 2c (85.05(8)°) and 2f (88.41 (10)°) are narrower than that of complex 4 (91.19(6)°), which is consistent with the bite angles of well-known diphosphine ligands that form five-membered rings or six-membered rings (1,2-bis(diphenylphosphino)ethane: 85°; and 1,2-bis(diphenylphosphino)propane: 91°) upon chelation.1b The narrower bite angles of complexes 2c and 2f relative to complex 4 induces an elongation of the Pd1− O1 bond by about 0.11 Å. As a result, electron donation from the phosphine oxide moiety seems to be slightly weakened, which results in a contraction of the Pd1−C1 bond in 2c and 2f compared to that in 4. It is notable that the Pd1−P1 bond length and P1−Pd1−C1 angle are comparable in 2c, 2f, and 4. These results suggest that changing the backbone from phenylene to methylene modulates the environment around the phosphine oxide moiety electronically and sterically without changing the environment around the phosphine moiety. Palladium complexes 3a−f were examined in the homopolymerization of ethylene (Table 1). Complexes 3a and 3b, which bear aryl groups on the phosphine moiety, exclusively afforded oligoethylenes, indicating inferior performance relative to the corresponding palladium complexes bearing a phenylenebridged BPMO ligand (5a and 5b) (compare entries 1 and 2 with entries 7 and 8). Alkyl-substituted complex 3c also exhibited a poorer performance than the corresponding phenylene-bridged complex (5c; compare entries 3 and 9). Considering the X-ray structures of 2c and 4, this difference could arise from the change in the steric environment around the palladium center caused by the smaller bite angle of the methylene-bridged ligands. The decrease of “steric protection” is known to cause relatively faster β-hydride elimination followed by chain transfer leading ultimately in lower catalytic activity.6b,c It is noteworthy that 3a−3c afforded highly linear polyethylenes with less than 1−4 methyl branches per 103
Figure 1. Palladium complexes bearing phenylene- and methylenebridged BPMO ligands.
performance in the homopolymerization of ethylene and the copolymerization of ethylene and polar monomers. The introduction of bulky R1 groups, such as tert-butyl and menthyl, on the phosphine moiety is thereby essential to obtain highmolecular-weight (co)polymers. Methylene-bridged BPMO ligands 1a−f were synthesized according to Scheme 1. Ligands 1a−c and 1e−f were obtained from reactions of the corresponding chlorophosphines (ClPR12) with the respective (phosphorylmethyl)lithium (LiCH2P(O)R22) compounds that were prepared via the deprotonation of the corresponding methylphosphine oxides with alkyllithium (Scheme 1a). Ligand 1d was prepared by the monoxidation of bis(di-tert-butylphosphanyl)methane (Scheme 1b). A subsequent reaction between ligands 1a−f and PdMeCl(cod) afforded complexes 2a−f, which were purified by recrystallization (Scheme 1c). Finally, complexes 2a−f were converted to complexes 3a−f with sodium tetrakis[3,5bis(trifluoromethyl)phenyl]borate (NaBArF4) in the presence of 2,6-lutidine. The structures of all these palladium complexes were characterized by multinuclear NMR spectroscopy, as well as mass spectrometry and/or elemental analysis. The molecular 306
DOI: 10.1021/acsmacrolett.8b00034 ACS Macro Lett. 2018, 7, 305−311
Letter
ACS Macro Letters
Figure 2. X-ray structures of palladium/BPMO complexes (a) 2c, (b) 2f, and (c) 4 with 50% thermal ellipsoids. Hydrogen atoms are omitted for clarity. Selected bond lengths (Å) and angles (°): (a) 2c: Pd1−P1 2.2054(13), Pd1−O1 2.236(3), Pd1−C1 2.033(5), P1−Pd1−O1 85.05(8), P1− Pd1−C1 92.33(14). (b) 2f: Pd1−P1 2.2188(16), Pd1−O1 2.224(5), Pd1−C1 2.027(7), P1−Pd1−O1 88.45(12), P1−Pd1−C1 91.55(18). (c) 4: Pd1−P1 2.2285(10), Pd1−O1 2.120(2), Pd1−C1 2.068(3), P1−Pd1−O1 91.19(6), P1−Pd1−C1 92.51(11).
behavior stands in stark contrast to that discussed in our previous report, in which a decrease of molecular weight was observed upon changing the substituents on the phosphine oxide moiety from tert-butyl to methyl.15b Finally, complex 3f, which has menthyl groups on the phosphine moiety and methyl groups on the phosphine oxide moiety, exhibited the highest catalytic activity and linearity among complexes synthesized in this study (2,000 kg mol−1 h−1, < 1 methyl branch per 103 carbon atoms; entry 6). With the best catalyst 3f in hand, the copolymerization of ethylene with various polar monomers was investigated (entries 1−10; Table 2). Copolymerization of ethylene (3.0 MPa) and allyl acetate (AAc; 20 vol % in toluene) at 80 °C afforded an ethylene/AAc copolymer with an AAc incorporation of 0.4 mol % (entry 1). When the concentration of AAc was increased to 80 vol % and the temperature was raised to 100 °C, the incorporation ratio of AAc increased almost 5-fold (2.1 mol %) as compared to that in entry 1 (entry 2). We subsequently explored the copolymerization of ethylene and polar monomers of the type CH2CHOR, such as vinyl acetate (VAc) and butyl vinyl ether (BVE) (entries 3−8). Copolymers of ethylene and VAc or BVE were successfully obtained with incorporation ratios of 0.7−1.6 mol % of the polar monomer (entries 3−6). It should be noted that the incorporation ratio of BVE could be doubled under high-concentration conditions (compare entries 5 and 6). The catalyst system was also applied to acrylic monomers such as acrylonitrile (AN) and methyl acrylate (MA). The use of 20 vol % of AN at 80 °C resulted in the formation of polyethylene without any functional groups (entry 7); however, under high-concentration conditions, AN was successfully incorporated (1.0 mol %; entry 8). MA was also incorporated into the linear polyethylene, and the incorporation ratio could be increased 3-fold under high-concentration conditions (0.5 to 1.6 mol %; entries 9 and 10). During the investigation of the copolymerization, we serendipitously discovered a significant increase of comonomer incorporation efficiency when complex 3e was used as a catalyst (entries 11−22 in Table 2). First, we examined the copolymerization of ethylene and AAc using 3e (entries 11− 16). As the concentration of AAc was increased from 6.7 to 80 vol %, the incorporation ratios also increased from 1.4 to 12.0 mol %. In order to compare the efficiency of the incorporation of AAc, we plotted the AAc incorporation ratios (mol %) as a function of the concentration of AAc (mol·L−1) divided by the
Table 1. Homopolymerization of Ethylene by Palladium/ BPMO Complexesa
entry catalyst 1 2 3 4 5 6 7d 8d 9d
3a 3b 3c 3d 3e 3f 5a 5b 5c
yield (g)
activity (kg mol−1 h−1)
Mnb (103)
Mw/Mnb
Me br.c (103 C)
0.70 0.59 0.10 0.12 0.67 1.50 1.74 0.79 2.00
930 790 130 160 890 2000 2300 1100 2700
2.9 2.3 1.5 24 24 29 12 21 31
2.1 2.1 1.8 2.2 2.0 2.1 4.2 2.8 3.1
2 1 4 9 2
