Article Cite This: Organometallics XXXX, XXX, XXX−XXX
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Heteroaryl Backbone Strategy in Bisphosphine Monoxide Palladium-Catalyzed Ethylene Polymerization and Copolymerization with Polar Monomers Junhao Ye,†,‡ Hongliang Mu,*,† Zhen Wang,*,†,‡,§ and Zhongbao Jian*,†,‡
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State Key Laboratory of Polymer Physics and Chemistry, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, Renmin Street 5625, Changchun 130022, China ‡ University of Science and Technology of China, Hefei 230026, China § Ningbo Institute of Materials Technology & Engineering, Chinese Academy of Sciences, Zhongguan West Road 1219, Ningbo 315201, China S Supporting Information *
ABSTRACT: The backbone structure of ligand is of critical importance to modulate the selectivity and the reactivity of catalyst. In this contribution, heteroaryl backbone (benzo)thiophene was for the first time installed on bisphosphinemonoxide (PO) ligands. Corresponding palladium complexes {κ2-2-P(2-OMe-Ph)2-3-P(O)(Ph)2-4−Br-C4HS}PdMeCl (2a) and {κ2-2-PR1R2-3-P(O)(Ph)2-C8H4S}PdMeCl {2b: R1 = R2 = 2-OMe-Ph; 2c: R1 = R2 = Cy; 2d: R1 = Ph, R2 = 2-(2′,6′(OMe)2C6H3)-C6H4} and the compared phenylene-linked palladium complex {κ2-1-P(2-OMe-Ph)2-2-P(O)(Ph)2-C6H4}PdMeCl (2e) were synthesized and fully characterized by 1H, 13 C, 31P, and 2D-NMR spectroscopy, elemental analysis, and single-crystal X-ray diffraction (2a, 2b, and 2d). In the presence of Na+B[3,5-(CF3)2C6H3]4− (NaBArF4), these complexes showed high activities (up to 10 000 kg mol−1 h−1) for ethylene polymerization that are comparable to the prototype phenylene-linked BPMO-Pd catalysts, producing moderate to high molecular weight (Mn up to 90 kDa) linear polyethylenes. These catalysts also copolymerized ethylene with various commercially available polar monomers, such as acrylates, acrylic acid, and vinyl butyl ether, to give linear functionalized polyethylenes with reasonable catalytic activities and moderate copolymer molecular weights and comonomer incorporations. This work provides new ligand backbones that span donor fragments, and points to new opportunities in catalyst designing for olefin (co)polymerization.
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INTRODUCTION The application of inherently nonpolar polyolefins in certain fields is restricted due to their limited printability, adhesion, and compatibility with other materials, which can be promoted significantly by incorporating small amounts (even 0.5 mol %) of polar functional groups.1 Thus, controlled synthesis of functional polyolefins has attracted much attention from both academic and industrial communities over the past two decades. Late transition metal catalyzed insertion copolymerization of commercially available polar monomers with olefins represents a rather promising method to this end.2 With significant effort, benchmark late transition metal catalysts ligated by α-diimine (Pd) and (P∧O) ligands (Ni or Pd) have been proven to be the most successful catalytic systems. αDiimine palladium catalysts reported by Brookhart and coworkers copolymerized olefins with acrylates,3 vinyl ketones,3 silyl vinyl ether,4 and vinylalkoxysilanes,5 giving copolymers with functional groups located at the ends of the branches. By taking advantage of the characteristic chain walking process, Ye © XXXX American Chemical Society
and co-workers synthesized functionalized polyethylenes with various complex chain architectures.6 Guan7 and Chen8 further developed this system by introducing bulky substituents or side arm heteroatoms. Recently, phosphino-phenolate nickel complexes were reported by Shimizu9 and Li10 to be efficient catalysts for the copolymerization of ethylene with acrylates and acrylamides, affording high molecular weight linear functionalized polyethylenes. Among these (P∧O) bidentate catalysts, phosphine-sulfonate palladium catalysts represented the most thoroughly studied system due to their excellent catalytic properties for a much broader scope of polar comonomers.2a,11 Mechanism studies indicate that the combination of strong/weak σ-donors is the crucial structural feature for phosphine-sulfonate palladium catalysts.12 Following this principle, Nozaki and co-workers designed bisphosphine monoxide (BPMO) palladium catalysts (A) that Received: May 21, 2019
A
DOI: 10.1021/acs.organomet.9b00340 Organometallics XXXX, XXX, XXX−XXX
Article
Organometallics could copolymerize ethylene with a wide range of polar monomers (Chart 1).13 After this initial advance, much efforts
Scheme 1. Synthesis of Heteroaryl- and Aryl-Bridged BPMO Ligands and Palladium Complexes
Chart 1. Bisphosphine Monoxide Palladium Catalysts Bearing Various Backbones
by Jordan (B),14 Chen (C)15 and Carrow (D)16 has been devoted to the optimization of −P or −PO substituents, leading to several efficient new catalysts. Steric and electronic effects of one catalyst are decisive for reaction including olefin polymerization. While these effects could be readily tuned by changing the type of coordination sites and their corresponding substituents, the backbone structure of the bidentate ligand is also of critical importance. It is believed that backbone structures are responsible for modulating the bite angle, ligand flexibility, distance between coordination sites, and electron-donating ability of the ligand, thus influencing selectivity and reactivity of the catalysts.17 Several reports on late transition metal catalysts bearing αdiimine18,19 and phosphine-sulfonate20 chelate ligands also highlighted the pivotal role of backbone structures. As a result, Nozaki et al. designed new BPMO-Pd catalysts bearing a methylene (E) linker for efficient copolymerization of ethylene with various polar monomers, even challenging 1,1-disubstituted methyl methacrylate.21 Chen et al. also disclosed the diphosphazane monoxide palladium catalysts (F) with a Nlinker for copolymerization of ethylene and polar monomers and computationally compared this system with phosphinesulfonate catalysts.22 These ligand frameworks also provided efficient nickel copolymerization catalyst.23 In addition, other BPMO-Pd catalysts containing longer flexible linkers have also been tried, but only oligomerization occurred.24 In recent years, we have been concentrating on the design of late transition metal catalysts and polar monomers for the improvement on the copolymerization of olefins and polar monomers.25 In this contribution, we now report the synthesis of a new class of BPMO-Pd catalysts supported by heteroaryl backbones that have found applications in bisphosphine ligands for metal-catalyzed coupling reactions.26 The heteroaryl-based BPMO palladium catalysts in this work were all highly active for ethylene polymerization and produced linear copolymer of ethylene with various commercially available polar monomers.
H and 4-Br sites lithiated by n-BuLi, the yield of pure ligand 1a formed by lithiation of the active 2-H site was low. This prompted us to choose an alternative synthetic pathway. Introduction of an additional phenyl ring into 4,5-positions of thiophene would eliminate the aforementioned side reactions and potentially improve the catalyst stability as well. Therefore, ligands 1b−d were further designed (Scheme 1). Additionally, phenylene linked BPMO ligand 1e was also designed and synthesized for comparison. All ligands were fully characterized by 1H, 13C, and 31P NMR spectroscopy. The reaction of the resultant BPMO ligands with PdMeCl(COD) in dichloromethane at room temperature gave desired palladium complexes 2a−e as white powders. The structures and purities of all these palladium complexes were characterized unambiguously by comprehensive 1H, 13C, 31P, and 2DNMR spectroscopy and elemental analysis. Upon coordination to palladium center, the 31P NMR chemical shifts of phosphine units moved downfield relative to the free ligands (for more details, see the Supporting Information). In the 1H NMR spectra, the doublets of newly formed Pd-Me protons arising from the coupling with P atom at 0.62, 0.66, 0.70, 0.33, and 0.41 ppm for 2a−e respectively clearly indicated the successful coordination of PdMeCl moieties to the (P∧O) bidentate ligands. The 3JPH values (2.7−3.5 Hz) indicate a cis arrangement between methyl group and phosphine moiety. An obvious difference between complex 2b and its phenylene analogue 2e was observed in 31P NMR spectra. 31P NMR spectrum of 2b contains two doublets at 30.58 and 9.4 ppm, while these signals for complex 2e are at 38.3 and 25.4 ppm, respectively, indicating possible influence of electron-donating nature of bezothiophene backbone on both −P and −PO donors. The solid-state structures of 2a, 2b, and 2d were further identified by X-ray single crystal diffraction analysis (Figure 1). In each case, a near-square-planar Pd(II) center is chelated by phosphorus and oxygen donor atoms. The bulky 2-[2′,6′(OMe)2C6H3)]C6H4 biaryl group in complex 2d effectively
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RESULTS AND DISCUSSION Thienylene or benzothienylene groups were designed as the BPMO ligand backbone because of their rigidity and tunable electronics at 2,3-positions. Thienylene- and benzothienylenebridged BPMO ligands 1a−d were synthesized according to Scheme 1. Due to the competitive side reaction of the active 5B
DOI: 10.1021/acs.organomet.9b00340 Organometallics XXXX, XXX, XXX−XXX
Article
Organometallics
Figure 1. Molecular structures of Pd(II) complexes 2a, 2b, and 2d drawn with 30% probability ellipsoids. Selected bond lengths (Å): 2a: Pd1− C31: 2.052(4), Pd1−O1: 2.194(3), Pd1−P2: 2.2071(10), Pd1−Cl1: 2.3644(10). 2b: Pd1−C35: 2.047(3), Pd1−O1: 2.193(2), Pd1−P1: 2.2108(8), Pd1−Cl1: 2.3670(8). 2d: Pd1−C41: 2.048(3), Pd1−O1: 2.186(2), Pd1−P1: 2.2068(8), Pd1−Cl1: 2.3695(8). Selected bond angles (deg): 2a: C31−Pd1−O1: 175.17(14), C31−Pd1−P2: 92.75(12), O1−Pd1−P2: 84.31(7), C31−Pd1−Cl1: 90.58(12), O1−Pd1−Cl1: 92.49(7), P2−Pd1−Cl1: 176.22(4). 2b: C35−Pd1−O1: 174.93(11), C35−Pd1−P1: 92.19(9), O1−Pd1−P1: 85.23(6), C35−Pd1−Cl1: 90.83(9), O1− Pd1−Cl1: 91.90(6), P1−Pd1−Cl1: 176.44(3). 2d: C41−Pd1−O1: 179.08(11), C41−Pd1−P1: 92.67(9), O1−Pd1−P1: 87.06(6), C41−Pd1− Cl1: 90.34(9), O1−Pd1−Cl1: 89.92(6), P1−Pd1−Cl1: 176.90 (3).
shields the metal center from the axial position. The bite angles of complexes 2a (84.31 (7)°), 2b (85.23 (6)°), and 2d (87.06 (6)°) are all smaller than those in phenylene-linked complex D (R1 = iPr, R2 = 2-OMe-C6H4, 89.02 (3)°)16 but slightly larger than those of A (R1 = 2-OMe-C6H4, R2 = tBu, 83.18 (4)°).13,16 The Pd−P bond lengths for 2a (2.2071 (10) Å), 2b (2.2108 (8) Å), and 2d (2.2068 (8) Å) are slightly shorter than those in A (R1 = 2-OMe-C6H4) (2.2239 (15) Å) and D (R2 = 2OMe-C6H4) (2.2181 (4) Å). In addition, the Pd−O1 bond lengths for 2a (2.194 (3) Å), 2b (2.193 (2) Å), and 2d (2.186 (2) Å) are longer than that in A (2.1784 (12) Å). It is also interesting that the Pd−CH3 distances for 2a (2.052 (4) Å), 2b (2.047 (3) Å), and 2d (2.048 (3) Å) are obviously longer those reported for arylene-linked complexes A (2.019(2)Å), B (2.0147(7)Å), and even D (2.0345(17)Å) bearing the same phosphine moieties, presumably reflecting enhanced trans effect of −PO donor originated from heteroaryl skeleton. These solid-state structures were also analyzed by SambVca 2.0 program to visualize and quantify the steric hindrance around the palladium center (Figure 2). As expected, complexes 2a and 2b, differing in the ligand backbone that remotes to the coordination center, showed almost identical filled space, mostly in the eastern hemisphere, which was also
evidenced by the similar percent buried volume value. In contrast, complex 2d provided a significantly more crowded environment around the palladium center, consistent with the percent buried volume of 52.3%. This steric protection is important for late transition metal catalysts in suppressing chain transfer reactions. After the abstraction of chloride unit with NaBArF4, all palladium precursors 2a-d were used for ethylene polymerization. All these complexes displayed very high activities at a level of 106−107 g mol−1 h−1. Among these palladium catalysts, 2c with flexible cyclohexyl substituents showed the lowest catalytic activities (1.9−3.3 × 106 g mol−1 h−1), which were still among the most active BPMO palladium catalysts.13−16,21,23,27 The molecular weights of the polymer products given by 2c were very low (Mn = ca. 1000, Table 1, entries 7−9). Catalyst 2d bearing the more bulky biaryl group showed the highest activity of up to 1.1 × 107 g mol−1 h−1 (Table 1, entry 12) under optimized conditions (20 bar, 80 °C), and produced polymers with the highest molecular weights (Mn = 25 100) in these heteroaryl-based catalysts, suggesting a crucial role of the bulky biaryl group in suppressing chain transfer. Moreover, complexes 2a and 2b, bearing the same BPMO substituents but different backbones, showed similar catalytic activities under identical conditions. The activities and the molecular weights of the polymers produced by 2b bearing an additional phenyl group in the backbone were slightly higher (Table 1, entries 1−3 vs 4−6). As a comparison with 2a, 2b, and 2d, N-linked catalysts F bearing the same BPMO substituents were much less active under similar conditions (80 °C, 8 bar, R = R1 = Ph, R2 = R3 = 2-OMe-C6H4: 2.5 × 106 g mol−1 h−1; R = R1 = R2 = Ph, R3= 2(2′,6′-OMe2-C6H3)-C6H4: 4.0 × 105 g mol−1 h−1),23 but the Mn of the obtained polymers was similar (ca. 10 000). As a direct comparison, phenylene-based complex 2e (A, R1 = oOMePh, R2 = Ph) with the same BPMO substituents as 2a and 2b was also very active for ethylene polymerization and produced polymers with even higher molecular weights (Table 1, entries 15 and 16). This seems to be completely different from the results reported by Nozaki using the original BPMO system where the catalyst with Ph2PO donor (A, R1 = Ph, R2 = Ph) showed low activity and produced very low molecular
Figure 2. Topographic steric maps of palladium catalysts (2a, 2b, and 2d) based on heteroaryl BPMO ligands. C
DOI: 10.1021/acs.organomet.9b00340 Organometallics XXXX, XXX, XXX−XXX
Article
Organometallics Table 1. Ethylene Polymerizationa entry
cat.
p (bar)
yield (g)
act. (106)b
Mn (104)c
Mw/Mnc
Tm (°C)d
Brse
1 2 3 4 5 6 7 8 9 10 11 12 13f 14f 15 16
2a 2a 2a 2b 2b 2b 2c 2c 2c 2d 2d 2d 2d 2d 2e 2e
5 10 20 5 10 20 5 10 20 5 10 20 10 20 10 20
3.6 10.7 12.8 5.5 11.3 13.1 2.8 4.7 4.9 9.5 15.0 16.7 1.9 2.1 11.3 15.3
2.4 7.1 8.5 3.7 7.5 8.7 1.9 3.2 3.3 6.3 10.0 11.2 1.3 1.4 7.5 10.2
0.77 0.82 0.95 1.10 1.26 1.18 0.09 0.10 0.11 2.03 2.51 2.14 9.00 8.26 3.20 2.60
2.2 2.3 2.6 2.2 2.5 2.3 2.2 2.2 1.6 2.3 2.1 2.2 1.8 1.9 2.1 2.1
130 130 130 130 131 132 117 118 120 130 135 131 135 136 129 129
1 1 1 1 2 2 2 2 2 1
