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Sterically-Controlled Self-Assembly of a Robust Multinuclear Palladium Catalyst for Ethylene Polymerization Qian Liu, and Richard F. Jordan J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.9b02465 • Publication Date (Web): 15 Apr 2019 Downloaded from http://pubs.acs.org on April 15, 2019
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Journal of the American Chemical Society
Sterically-Controlled Self-Assembly of a Robust Multinuclear Palladium Catalyst for Ethylene Polymerization Qian Liu and Richard F. Jordan* Department of Chemistry, The University of Chicago, 5735 South Ellis Avenue, Chicago, Illinois 60637, United States
Supporting Information Placeholder faces of the cage are blocked by the arene rings of the Pd-bound arenesulfonate groups, which are close-packed with the non-Pdbound arenesulfonate groups. 1 has S4 symmetry with SSRR configurations at the P atoms. 1 undergoes partial dissociation to Pd1 species in solution. Under conditions where 1 is partially dissociated (hexanes suspension at 80 °C or CH2Cl2 solution at 25 °C), 1 produces linear PE with a bimodal MW distribution (MWD) comprising a high-MW fraction formed by the Pd4 cage and a low-MW fraction formed by Pd1 species. 1 also copolymerizes ethylene with vinyl fluoride (VF) with a higher level of VF incorporation (up to 4 mol %) vs. monomeric (PO)PdRL catalysts (< 0.5 mol %).5 The self-assembly of (PO)PdR-type catalysts into multinuclear structures such as 1 may offer a general approach to achieving high MW PEs and high levels of polar monomer incorporation, provided that disassembly to monomeric species under catalytic conditions can be prevented. Here we show that increasing the steric profile of the OPO2- ligand enables the self-assembly of a tetranuclear {(OPO-Li)PdMe(py')}4Li2Cl2 complex (3) in which four (phosphine-sulfonate)PdMe(py') units are arranged around the periphery of an "expanded" Li4S4O12•Li2Cl2 cage that is more resistant to disassembly than 1. 3 functions as a single-site catalyst for the polymerization of ethylene to high-molecular-weight PE in hexanes suspension.
ABSTRACT: The self-assembly and reactivity of a robust multinuclear Pd catalyst based on the sterically-expanded phosphine-bisarenesulfonate ligand PPh(2-SO3--4,5-(OMe)2-Ph)2 (OPO2-, 2) are described. The reaction of Li2[2] with (COD)PdMeCl and 4-(5-nonyl)-pyridine (py') generates the tetranuclear complex {(OPO-Li)PdMe(py')}4Li2Cl2 (3) in which four (phosphine-sulfonate)PdMe(py') units are arranged around the periphery of a Li4S4O12•Li2Cl2 cage. The Pd atoms in 3 are arranged in pairs with a Pd–Pd distance of 6.6 Å within each pair. 3 is more resistant to disassembly to Pd1 species than previously studied {(OPO-Li)PdMe(py)}4 compounds based on Li4S4O12 cages. 3 is a single-site catalyst for the polymerization of ethylene to high-molecular weight polyethylene hexanes suspension at 80 oC.
(PO)PdRL complexes that contain phosphine-arenesulfonate ligands (PO-) polymerize ethylene to linear polyethylene (PE) and copolymerize ethylene with polar CH2=CHX monomers to functionalized PEs (Scheme 1).1,2 However, the activities of (PO)PdRL catalysts and the molecular weights (MWs) of the polymers they produce are lower than for other single-site catalysts, and the polar monomers often are inhibitors and chain transfer agents, further compromising catalyst performance.
Scheme 2. Reversible self-assembly of 1. py’ = 4-(5nonyl)pyridine. The lower (Li-OPO)PdMe(py’) units in the schematic structure of 1 are denoted by “Pd”.
Scheme 1. Ethylene/CH2=CHX copolymerization by (PO)PdRL catalysts. R = alkyl, aryl; L = pyridine or other labile ligand; X = C(O)OR, OAc, CN, OR, F, etc. R
py' Me
R P
Ph
Me Pd
X +
O
S O O
Pd
L
X ...
...
P 4
Pd S O O O
One potential strategy for enhancing the performance of olefin polymerization catalysts is to incorporate the active metal-alkyl unit into a multinuclear assembly.3 Previously we reported that (OPO-Li)PdR(py) catalysts that contain phosphine-bisarenesulfonate (OPO2-) ligands self-assemble into Pd4 species that are held together by a Li4S4O12 double-four-ring (D4R) cage, as exemplified by 1 (Scheme 2).4 The Pd units in 1 are arranged in pairs at the “top” and “bottom” faces of the cage. The four “side”
SO3Li Me
O O S P O
O
Li
S
O
S O O
N O
py' Me
O
Li O Pd
O Pd
Pd
O Ph P S O O
Li O S O O
S O
Li
1
The self-assembly of a multinuclear Pd catalyst based on the methoxy-substituted OPO2- ligand PPh(2-SO3--4,5-(OMe)2-Ph)2
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(2) is shown in Scheme 3. The reaction of Li2[2] with (COD)PdMeCl and 4-(5-nonyl)-pyridine (py') in CH2Cl2 generates a clear or slightly cloudy pale-yellow solution, from which tetranuclear complex 3 is isolated in crystalline form in 67% yield by layering pentane onto the solution and cooling to – 40 °C.
Scheme 3. Self-assembly and structure of 3. py’ = 4-(5nonyl)pyridine. The lower (Li-OPO)PdMe(py’) units in the schematic structure of 3 are denoted by “Pd”. py' Me
MeO MeO
SO3Li
4
P Ph SO3Li
MeO MeO
Li2[2]
+
CH2Cl2 -2LiCl
Pd
4 N
py' Me Pd X
Ph O Ph X O X P S O S P X O O S O Li O O O O S Li O MeO OMe O MeO OMe Cl Li Cl
O Li
Li
O
4 (COD)PdMeCl
+
is close to the ortho and meta-Hs (H9, 2.52 Å; H10, 2.94 Å) of the non-Pd-bound arenesulfonate ring on the top of the cage. Therefore, replacement of H17, H10 and the C20 and C13 methyl groups by methoxy groups in 3 would not be possible without significant structural distortion. Inclusion of the inner Li2Cl2 layer in the core of 3 elongates the cage enough to satisfy the steric requirements of the methoxy groups while maintaining close packing of the arene units (Figure 2b).
S
O Pd O
Pd
O
Li O
S 3, X = OMe
The solid-state structure of 3 is shown in Figure 1. As for 1, the four Pd centers in 3 are arranged in pairs with a Pd-Pd distance of 6.6 Å within each pair. The Pd–Me groups point toward the py' ring of the neighboring Pd unit. However, 3 differs from 1 in two ways. First, 3 contains a rhomboidal Li2Cl2 unit inserted between the top and bottom Li2S2O6 layers of the cage. The Li+ and Cl– ions in the Li2Cl2 unit adopt tetrahedral and seesaw geometries, respectively.6 Second, 3 is chiral, with C2 symmetry and SSSS (ent RRRR) configurations at the P atoms. 3 crystallizes as a racemic conglomerate, i.e. a mixture of individual crystals that are enantiomerically pure but together comprise a racemate.7
Figure 1. Solid-state structure of 3. Hydrogen atoms and the para-5-nonyl groups of the py' ligands are omitted. Atom color scheme: C: grey; H: white; O: red; P: orange; S: yellow; Li: purple; Cl: green; N: blue; Pd: teal. The structural differences between 3 and 1 provide insight to why 3 forms with the observed composition and structure. As noted above, the arene rings on the periphery of 1 are closepacked. In particular, as shown in Figure 2a, the meta-H (H17) on the Pd-bound arenesulfonate ring that blocks the side of the cage
Figure 2. (a) Space-fill and corresponding capped-stick views of 1 highlighting the close contacts between H17 of the Pd-bound arenesulfonate ring and H9 and H10 of the non-Pd-bound arenesulfonate ring. (b) Space-fill and corresponding capped-stick views of 3 highlighting the close contacts between the methoxy groups of the arenesulfonate rings. Inclusion of the inner Li2Cl2 layer in 3 has important stereochemical consequences. As shown in Figure 3a, assembly of 3 with SSRR configurations at the P atoms (as in 1) is not possible because the ArSO3– and Li+ units in the lower Li2S2O6 layer would be misaligned with the Li+ and Cl– ions in the inner Li2Cl2 layer. However, inversion of the configurations of the bottom two P atoms from RR to SS provides the proper alignment of the lower ArSO3– and Li+ atoms with the inner Li2Cl2 unit and enables cage formation.
Figure 3. (a) Model structure generated by insertion of a Li2Cl2 unit between the Li2S2O6 layers of 1 and inclusion of meta and
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Journal of the American Chemical Society para OMe groups on the Pd-bound arenesulfonate rings. Cage formation is precluded by misalignment of the inner Li+ and Cl– ions with the Li+ and sulfonate ions on the lower layer. (b) Inversion of the P configurations in the lower Pd units enables cage formation. NMR data provide strong evidence that the cage structure of 3 is retained in chlorocarbon solution at room temperature. The 31P{1H} spectrum of 3 in CD Cl at room temperature contains 2 2 only one resonance (δ 34.1) indicative of a highly symmetric structure. The 7Li{1H} spectrum contains two resonances (δ –0.9 and –1.1) in a 2:1 intensity ratio (Figure 4a), consistent with the presence of two types of Li+ ions. The 1H spectrum contains four resonances for the methoxy groups, which indicates the two arenesulfonate rings are inequivalent. The Pd-Me 1H NMR resonance appears at high field (δ –0.17), consistent with the anisotropic shielding by the adjacent py' ring expected for the solid-state structure.4 The 13C{1H} spectrum contains two sets of resonances for the butyl chains of the 5-nonyl group, which is ascribed to steric inhibition of rotation of the nonyl group by the cage. Moreover, the 1H-1H NOESY spectrum of 3 in CD2Cl2 at 263 K contains a cross-peak at δ 3.21/4.19, corresponding to the correlation between the hydrogens in the O19 and O7 methoxy groups (Figure 2b; see SI for further discussion). This result is consistent with the arrangement of OMe groups observed in the solid-state structure. In contrast, in CD3OD solution, the 7Li{1H} NMR spectrum comprises a singlet at δ 0.2, the Pd-Me 1H NMR resonance of 3 shifts downfield to δ 0.58, and the 13C{1H} NMR spectrum contains only one set of butyl resonances, as expected for disassembly of 3 to a monomeric (OPO-Li)PdMe(py') species in this coordinating solvent. Similarly, the reaction of 3 with Cryptand211 to sequester the Li+ ions generates a Pd1 species with a Pd-Me 1H NMR resonance at δ 0.62 in CDCl2CDCl2. The hydrodynamic volume of 3 determined by PGSE-NMR in CD2Cl2 at room temperature is 8.0×103 Å3, which is four time larger than the value determined in CD3OD (2.0×103 Å3) and somewhat larger than that of 1 (6.2×103 Å3) in CD2Cl2. The thermal stability of 3 was assessed by variable temperature 1H NMR in CDCl CDCl solution (Figure 4b). Upon heating 2 2 above 25 °C, a minor Pd-Me resonance at ca. δ 0.6 appears, which disappears when the solution is cooled back to 25 °C. This resonance is assigned to a monomeric Pd1 species, based on the Pd-Me chemical shifts observed for 3 in CD3OD and the product of the reaction of 3 with Cryptand211. Because the speciation of the LiCl that presumably is released upon disassembly of 3 is unknown, it is not possible to specify an equilibrium constant expression and determine a Keq,dissoc value for comparison to values for 1.4 However, an informative assessment of the relative stability of 3 and 1 toward cage dissociation is provided by comparison of the percentage of cage dissociation at a given initial concentration of the cage ([Pd4]0). At [3]0 = 4.7 mM, only 6.5% of 3 is dissociated at 80 °C in CDCl2CDCl2. In contrast, under these conditions, 38% of 1 is dissociated. Thus, inclusion of the Li2Cl2 layer significantly stabilizes the cage in 3. It is reasonable to expect the trend for resistance to cage disassembly for the chain-growing species derived from 3 or 1 under polymerization conditions (toluene solution, CH2Cl2 solution or hexanes slurry) will parallel that observed for 3 and 1 in CDCl2CDCl2 solution. Consistent with its enhanced resistance to cage disassembly, 3 produces linear PE with a higher MW and a higher proportion of high-MW fraction (vs. low-MW fraction) compared to 1 in CH2Cl2 solution at 25 oC, toluene solution at 80 oC and hexanes suspension at 80 oC (Figure 5 and Table 1). In particular, under the latter condition, 3 exhibits nearly ideal single-site behavior,
producing high-MW PE with a monomodal narrow MWD (PDI = 2.3). In contrast, as noted above, under this condition 1 produces PE with a broad bimodal MWD due to competing polymerization by the intact cage catalyst and mononuclear species generated by cage disassembly.4a
Figure 4. (a) 7Li{1H} NMR spectrum of 3 (CD2Cl2). (b) Pd-Me region of the variable temperature 1H NMR spectra of 3 (CDCl2CDCl2). The signal labeled Pd4 is due to intact 3 and the signal labeled Pd1 is assigned to the monomeric Pd1 species.
Figure 5. Molecular weight distributions of PEs generated by 1 and 3 determined by high temperature GPC. Polymerization conditions: 410 psi ethylene, hexanes or toluene solvent, 80 °C, 2 h; 410 psi ethylene, CH2Cl2 solvent, 25 °C, 24 h. This work shows that the use of a sterically-expanded OPO2ligand enables the self-assembly of Pd4 catalyst 3, which is held together by a Li4S4O12•Li2Cl2 cage that is more robust than the Li4S4O12 cage in 1. Close packing of arene units around the cage periphery is a key control element in the self-assembly process. This work also shows that the formation of high-MW PE is a general property of Pd4 cage catalysts. Chain transfer may be retarded for these catalysts by steric blockage of one axial face of each Pd center by the cage, which should inhibit associative chain transfer,8 binding of the Li+ to the sulfonate groups, which may enhance the electrophilic character of the Pd centers as observed for binding of B(C6F5)3 to the sulfonate groups of mononuclear (PO)PdRL catalysts,9 and possible cooperative effects between neighboring Pd centers, from which the polymer chains would grow in close proximity if both Pd centers are activated. The availability of robust cage catalysts like 3 will enable mechanistic studies to probe these issues.
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Table 1. Comparison of ethylene polymerization performance for 1 and 3.a
Entry
Catalyst
T (°C)
Time (h)
Solvent
Yield (g)
Activity (kgmol-1h-1)
Mwc (103 Da)
PDIc
Tmd (°C)
3
80 80 80 25 80 80 25
2 2 1 24 2 2 24
hexanes toluene toluene CH2Cl2 hexanes toluene CH2Cl2
6.35
318
1473
2.3
136.5
1.25
62
412
14.3
137.3
0.716
72
136
6.6
135.2
0.424
1.8
1015
12.5
138.7
5.18
259
1000
60
138.7
6.25 1.91
312 8.0
7.87 915
2.6 29
131.7 136.5
1 2 3 4 5b 6b 7b aConditions:
3 3 3 1 1 1
410 psi ethylene, 10 μmol Pd. bData from ref 4a. cDetermined by GPC. dDetermined by DSC.
ASSOCIATED CONTENT Supporting Information Experimental procedures and characterization data for compounds (PDF).Crystallographic data for 3 (cif). The Supporting Information is available free of charge on the ACS Publications website.
AUTHOR INFORMATION Corresponding Author
[email protected] Notes The authors declare no competing financial interests.
ACKNOWLEDGMENT This work was supported by the National Science Foundation (CHE-1709159). We thank Drs. Antoni Jurkiewicz, Alexander Filatov and C. Jin Qin for assistance with NMR, X-ray crystallography and mass spectrometry.
REFERENCES (1) Nakamura, A.; Anselment, T. M. J.; Claverie, J.; Goodall, B.; Jordan, R. F.; Mecking, S.; Rieger, B.; Sen, A.; van Leeuwen, P. W. N. M.; Nozaki, K., Ortho-Phosphinobenzenesulfonate: A Superb Ligand for Palladium-Catalyzed Coordination–Insertion Copolymerization of Polar Vinyl Monomers. Acc. Chem. Res. 2013, 46, 1438-1449. (2) (a) Nakano, R.; Chung, L. W.; Watanabe, Y.; Okuno, Y.; Okumura, Y.; Ito, S.; Morokuma, K.; Nozaki, K., Elucidating the Key Role of Phosphine−Sulfonate Ligands in Palladium-Catalyzed Ethylene Polymerization: Effect of Ligand Structure on the Molecular Weight and Linearity of Polyethylene. ACS Catalysis 2016, 6, 6101-6113. (b) Liang, T.; Chen, C., Side-Arm Control in Phosphine-Sulfonate Palladium- and Nickel-Catalyzed Ethylene Polymerization and Copolymerization. Organometallics 2017, 36, 2338-2344. (c) Jian, Z. B.; Wucher, P.; Mecking, S., Heterocycle-Substituted Phosphinesulfonato Palladium(II) Complexes for Insertion Copolymerization of Methyl Acrylate. Organometallics 2014, 33, 2879-2888. (d) Wucher, P.; Goldbach, V.; Mecking, S., Electronic Influences in Phosphinesulfonato Palladium(II) Polymerization Catalysts. Organometallics 2013, 32, 4516-4522. (e) Piche, L.; Daigle, J. C. Rehse, G.; Claverie, J. P., Structure–Activity Relationship of Palladium Phosphanesulfonates: Toward Highly Active Palladium‐Based Polymerization Catalysts. Chem.—Eur. J. 2012, 18, 3277-3285. (f) Skupov, K. M.; Marella, P. R.; Simard, M.; Yap, G. P. A.; Allen, N.; Conner, D.; Goodall, B. L.; Claverie, J. P., Palladium Aryl Sulfonate Phosphine Catalysts for the Copolymerization of Acrylates with Ethene. Macromol. Rapid Commun. 2007, 28, 2033-2038. (g) Vela, J.; Lief, G. R.; Shen, Z.; Jordan, R. F., Ethylene Polymerization by Palladium
Alkyl Complexes Containing Bis(aryl)phosphino-toluenesulfonate Ligands. Organometallics 2007, 26, 6624-6635. (3) (a) McInnis, J. P.; Delferro, M.; Marks, T. J., Multinuclear Group 4 Catalysis: Olefin Polymerization Pathways Modified by Strong Metal– Metal Cooperative Effects. Acc. Chem. Res. 2014, 47, 2545-2557. (b) Delferro, M.; Marks, T. J., Multinuclear Olefin Polymerization Catalysts. Chem. Rev. 2011, 111, 2450-2485. (c) Takeuchi, D.; Chiba, Y.; Takano, S.; Kurihara, H.; Kobayashi, M.; Osakada, K., Ethylene polymerization catalyzed by dinickel complexes with a double-decker structure. Polymer Chemistry 2017, 8, 5112-5119. (d) Sampson, J.; Choi, G.; Akhtar, M. N.; Jaseer, E. A.; Theravalappil, R.; Al-Muallem, H. A.; Agapie, T., Olefin Polymerization by Dinuclear Zirconium Catalysts Based on Rigid Teraryl Frameworks: Effects on Tacticity and Copolymerization Behavior. Organometallics 2017, 36, 1915-1928. (e) Chiu, H.-C.; Koley, A.; Dunn, P. L.; Hue, R. J.; Tonks, I. A., Ethylene polymerization catalyzed by bridging Ni/Zn heterobimetallics. Dalton Trans. 2017, 5513-5517. (f) Ji, P.; Solomon, J. B.; Lin, Z.; Johnson, A.; Jordan, R. F.; Lin, W., Transformation of Metal–Organic Framework Secondary Building Units into Hexanuclear Zr-Alkyl Catalysts for Ethylene Polymerization. J. Am. Chem. Soc. 2017, 139, 11325-11328. (4) (a) Shen, Z.; Jordan, R. F., Self-Assembled Tetranuclear Palladium Catalysts That Produce High Molecular Weight Linear Polyethylene. J. Am. Chem. Soc. 2010, 132, 52-53. (b) Analogues of 1 with different substituents para to the sulfonate group exhibit similar properties: Wei, J.; Shen, Z.; Filatov, A. S.; Liu, Q.; Jordan, R. F., Self-Assembled Cage Structures and Ethylene Polymerization Behavior of Palladium Alkyl Complexes That Contain Phosphine-Bis(arenesulfonate) Ligands. Organometallics 2016, 35, 3557-3568. (5) (a) Shen, Z.; Jordan, R. F., Copolymerization of Ethylene and Vinyl Fluoride by (Phosphine-bis(arenesulfonate))PdMe(pyridine) Catalysts: Insights into Inhibition Mechanisms. Macromolecules 2010, 43, 87068708. (b) Weng, W.; Shen, Z.; Jordan, R. F., Copolymerization of Ethylene and Vinyl Fluoride by (Phosphine-Sulfonate)Pd(Me)(py) Catalysts. J. Am. Chem. Soc. 2007, 129, 15450-15451. (c) Wada, S.; Jordan, R. F., Olefin Insertion into a Pd-F Bond: Catalyst Reactivation Following B-F Elimination in Ethylene/Vinyl Fluoride Copolymerization." Angew. Chem. 2017, 129, 1846-1850. (6) The Li2Cl2 unit in 3 is structurally similar to those in [LiCl(THF)2]2, [LiCl(Et2O)]4 and related compounds. (a) Hahn, F. E.; Rupprecht, S., Synthese und Kristallstruktur von [LiCl • 2THF]2. Z. Naturforsch., Teil B, 1991, 46, 143-146. (b) Mitzel, N. W.; Lustig, C., Crystal Structure of a Lithium Chloride Cubane Cluster Solvated by Diethyl Ether. Z. Naturforsch., Teil B, 2001, 56, 443-445. (7) Flack, H. D., Chiral and Achiral Crystal Structures. Helv. Chim. Acta 2003, 86, 905-921. (8) Little, S. D.; Johnson, L. K.; Brookhart, M., Late-Metal Catalysts for Ethylene Homo- and Copolymerization. Chem. Rev. 2000, 100, 11691204. (9) (a) Johnson, A. M.; Contrella, N. D.; Sampson, J. R.; Zheng, M.; Jordan, R. F., Allosteric Effects in Ethylene Polymerization Catalysis. Enhancement of Performance of Phosphine-Phosphinate and PhosphinePhosphonate Palladium Alkyl Catalysts by Remote Binding of B(C6F5)3. Organometallics 2017, 36, 4990-5002. (b) Cai, Z.; Shen, Z.; Zhou, X.; Jordan, R. F., Enhancement of Chain Growth and Chain Transfer Rates in Ethylene Polymerization by (Phosphine-sulfonate)PdMe Catalysts by
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Journal of the American Chemical Society Binding of B(C6F5)3 to the Sulfonate Group. ACS Catalysis 2012, 2, 11871195.
Insert Table of Contents artwork here
py' Me
MeO MeO 4
P Ph SO3Li
MeO MeO 4
self-assembly -2LiCl, COD
+
py' Me Pd O Ph X X O X P S O S P X O O S O Li O O O O S Li O MeO OMe O MeO OMe Cl Li Ph
SO3Li
Pd
Cl
(COD)PdMeCl
+
O
4 4-(5-nonyl)-py (py')
Li
Li
O S
O Pd O
Pd
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O Li O S X = OMe
