Article Cite This: Organometallics XXXX, XXX, XXX−XXX
A Bulky Pd(II) α‑Diimine Catalyst Supported on Sulfated Zirconia for the Polymerization of Ethylene and Copolymerization of Ethylene and Methyl Acrylate Damien B. Culver,‡ Hosein Tafazolian,‡ and Matthew P. Conley* Department of Chemistry, University of California, Riverside, California 92521, United States S Supporting Information *
ABSTRACT: The reaction of (N∧N)PdMe2 (N∧N is Ar−NCMeMeCN−Ar; Ar = 2,6-bis(diphenylmethyl)-4-methylbenzene) and sulfated zirconia (SZO) in diethyl ether forms organometallic Pd-sites that polymerize ethylene and copolymerize ethylene and methyl acrylate. The Pd-sites bind CO and were studied by infrared and solid-state NMR spectroscopies. Analysis of the reaction mixture shows that more methane than expected evolves during the grafting reaction, suggesting that some Pdsites do not contain a Pd-Me group. Consistent with this observation, deuterium labeling experiments show that ∼9% of palladium sites are active in polymerization reactions. (N∧N)PdMe2/SZO polymerizes ethylene with activity as high as 1342 kgPE/(molactive Pd*h) and incorporated up to 0.46% methyl acrylate in copolymerization reactions.
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INTRODUCTION Most polyolefins are synthesized in the presence of a heterogeneous catalyst. The heterogeneous catalyst controls the morphology of the polymer product and prevents reactor fouling.1 Classic examples of heterogeneous catalysts for polyolefin synthesis are the Ziegler−Natta (TiCl4/AlR3/ MgCl2)2 and Phillips (CrOx/SiO2)3 catalysts that produce high-density polyethylene from ethylene. These catalysts produce broad molecular weight distributions, reflecting the multisite behavior of these catalysts. More modern heterogeneous catalysts for gas-slurry-phase polymerizations are generated by the reaction of early transition metal precatalysts with an oxide containing alkyl aluminum fragments, which generates electrophilic metal sites that are likely bound to the oxide by electrostatic interactions (Figure 1).1 Organometallic complexes of Pd(II) are active olefin polymerization catalysts in solution, and these catalysts can incorporate vinyl polar monomers into the polymer chain.4 This result is significant because incorporation of only ∼2% of a polar functionality into a polyethylene chain can affect polymer properties.5 Though catalyst activities are often reduced in the presence of polar monomers, several polar monomers can be incorporated into polyethylene chains using late-transition metal catalysts,4c,6 and in some cases even early-transition-metal catalysts can incorporate certain polar monomers.7 Supporting late-transition-metal complexes containing bulky α-diimine ligands on silica functionalized with methaluminoxane (MAO/SiO2) results in active polymerization catalysts, albeit with broader molecular weight polymer products and lower activities than those obtained with solution catalysts. A very active example of late-metal complexes supported on MAO/ © XXXX American Chemical Society
Figure 1. Generation of polymerization sites on MAO/SiO2.
SiO2 is shown in Figure 1. Functionalized (α-diimine)NiBr2 containing hydroxyl groups on the ligand react with MAO/ SiO2 and polymerize ethylene when activated with exogenous Et3Al2Cl3 to yield broad molecular weight distributions of polyethylene.8 In some cases, this strategy can also apply to copolymerizations of ethylene and monomers containing polar functionalities.9 However, supporting palladium complexes using these strategies is underexplored, possibly due to the instability of organopalladium complexes in the presence of methylaluminoxane activators.10 Received: January 9, 2018
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DOI: 10.1021/acs.organomet.8b00016 Organometallics XXXX, XXX, XXX−XXX
Article
Organometallics
NCMeMeCN−Ar (Ar = 2,6-bis(diphenylmethyl)-4-methylbenzene, N∧N), a robust diimine ligand that stabilizes Niand Pd-catalysts at high polymerization temperatures.21 Palladium sites supported on SZO catalyze the polymerization of ethylene and the copolymerization of ethylene and methyl acrylate, incorporating up to 0.46% of the polar monomer.
Our objective is to generate electrophilic palladium sites supported on a high surface area oxide to form an active heterogeneous polymerization catalyst. The chemisorption of organometallic complexes onto partially dehydroxylated oxide forms well-defined active sites for catalytic reactions.11 This reaction is general and usually involves the protonolysis of a M−R group to anchor the complex to the support. Figure 2
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RESULTS AND DISCUSSION To form active Pd-sites on SZO, we synthesized (N∧N)PdMe2 (2) shown in Scheme 1. Alkylation of (N∧N)PdCl2 (1) with Scheme 1. Synthesis of (N∧N)PdMe2 and Generation of [(N∧N)PdMe(OEt2)][B(C6F5)4]
Figure 2. Products generated from the reaction of LnMR2 with partially dehydroxylated metal oxides.
shows this reaction for LnMR2, which can form three distinct surface species. The distribution of products depends on the oxide used in this reaction. Surface species generated from the reaction of LnMR2 with the −OH groups on the surface of partially dehydroxylated silica forms SiO−MR(Ln) shown in Figure 2, path 1.11a,e,12 Though SiO−MR(Ln) is more electrophilic than LnMR2,13 these surface sites often require further activation with Lewis acids to form polymerization sites.14 These highly electrophilic surface sites are active polymerization catalysts, but decompose by alkyl transfer to the surface,15 or by pathways commonly encountered with homogeneous catalysts.16 The reaction of highly dehydroxylated alumina, which contains Lewis acid sites on the surface, and LnMR2 results in abstraction of an alkyl group to form [LnMR][RAlOx] ion pairs,11b,e,12 shown in Figure 2, path 2. In contrast to the silica supported examples mentioned above, [LnMR][RAlOx] initiates polymerization in the absence of exogenous Lewis acids. However, only a small quantity of [LnMR][RAlOx] are active in catalytic reations.11b Treating an oxide with H2SO4 results in a sulfated oxide. High-surface-area sulfated aluminum, zirconium, hafnium, iron, and tin oxides are known, and all contain Brønsted-acid sites.17 The reaction of LnMR2 with a sulfated oxide forms [LnMR][oxide] ion pairs by protonolysis of a M−R group, and small quantities of [LnMR][R−oxide] formed by alkyl abstraction, shown in Figure 2, path 3.11c,d The well-defined sites generated on sulfated oxides are exceptionally active in the polymerization of ethylene and the hydrogenation of arenes,11d,18 and in some cases up to 100% of these sites are catalytically active.19 We recently reported that (α-diimine)NiMe2 (α-diimine = (2,6-iPr2−C6H3)-NCMeMeCN-(2,6-iPr2−C6H3)) reacts with partially dehydroxylated sulfated zirconium oxide (SZO) to form Ni-sites with high polymerization activity.20 Unlike the Ni-catalysts supported on MAO/SiO2 mentioned above, the well-defined Ni-sites supported on SZO produce polymers with narrow molecular weight distributions (2.0 < Đ < 2.4). These well-defined catalysts also incorporate small quantities of methyl 10-undecenoate, an ester containing monomer. Here we show that SZO supports cationic Pd-sites containing Ar−
MeMgBr in Et2O/dioxane at 0 °C results in the formation of (N∧N)PdMe2 (2) as a brown powder in 44% yield. Et2O solutions of 2 react with [H(OEt2)][B(C6F5)4] to form methane and [(N∧N)PdMe(OEt2)][BC(6F5)4] (3). The 1H NMR spectrum of 3 in CD2Cl2 at 25 °C contains two signals at 0.32 and 0.86 ppm for the methyls from the diimine backbone and a signal for the Pd-Me group at 0.39 ppm. The 13C NMR spectrum of 3 contains signals assigned to the Pd-Me at 8.2 ppm, methyls from the ligand backbone at 19.3 and 20.6 ppm, methine carbons at 51.68 and 51.74 ppm, and coordinated Et2O at 15.2 and 65.8 ppm. For further details, refer to the Supporting Information (Figures S4−S7). 3 has nearly identical ethylene polymerization activity as (N∧N)PdMeCl activated with Na[B(C6F5)] at 40 °C.21b The reaction of 2 with SZO partially dehydroxylated at 300 °C results in the formation of 0.059 mmol CH4/g SZO. Elemental analysis of 2/SZO contains 0.4% Pd (0.039 mmol Pd/g) and 0.11% N, corresponding to 2.0 ± 0.05 N/Pd. The amount of CH4 evolved per Pd is ∼1.5 times higher than the expected CH4:Pd ratio of 1 for the exclusive formation of [(N∧N)PdMe(OEt2)][SZO] (4), Scheme 2. This result suggests that 2 reacts with SZO to form species lacking Pd− Me groups on some surface sites and indicates that less than 100% of surface palladium sites will be active in polymerization reactions (vide infra). The 13C cross-polarization magic angle Scheme 2. Preparation of 4 and 5
B
DOI: 10.1021/acs.organomet.8b00016 Organometallics XXXX, XXX, XXX−XXX
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Organometallics spinning (CPMAS) spectrum of 4 recorded at 10 kHz and −20 °C contains signals at 140, 136, and 129 from aromatic residues; 52 ppm from the methine carbons; 19 ppm from the backbone methyl and p-tolyl carbons; and a signal at 11.5 ppm assigned to the Pd-Me. The signals corresponding to coordinated Et2O appear at 68 and 13 ppm. These chemical shift values are close to those observed for 3 in CD2Cl2 solution. The reaction of 4 with 13CO forms [(N∧N)PdMe(13CO)][SZO] (5). The FTIR of 5 contains an intense νCO band at 2079 cm−1. The νCO for [(N∧N)PdMe(CO)][B(C6F5)4] is 2129 cm−1, which is close to that expected for 5 using the harmonic oscillator approximation (2125 cm−1 at natural abundance) .21b The 13C CPMAS spectrum of 5 shows that signals for coordinated Et2O are no longer present, consistent with the formation of a Pd−13CO adduct. The Pd−13CO appears at 173 ppm, and the Pd-Me signal appears at 11.3 ppm. Recording 13C CPMAS spectra of 5 heated to 50 °C under excess 13CO for 1 h does not result in formation of a Pd(COMe)(CO) surface species, indicating that migratory insertion of CO does not occur under these conditions. The mass balance and elemental analysis data show that 2/ SZO cannot contain 100% active Pd-sites because more CH4 than expected is evolved in this reaction. Landis and co-workers showed that deuterium labels, either installed by quenching polymerizations with CH3OD or by initiating polymerizations with M−CD3 groups, report the number of active polymerization sites in catalytic mixtures.22 We prepared 4-CD3 from the reaction of 2-d6 and SZO to determine the quantity of PdCD3 groups that can initiate polymerization. Polymer chains initiated by a Pd-CD3 group will incorporate −CD3 end groups, or −CD2H end groups and −D in to the PE chain through chainwalking (Figure 3). Under these conditions, the (α-
Table 1. Ethylene Polymerization Activity of 4a entry T (°C) 1 2 3 4
20 40 60 80
activity (kgPE/(molactivePd*h))b
Mn (kg/mol)c
Đ
Bd
879 1342 487 434
158 210 93 20
1.4 1.6 2.1 3.9
44 41 41 42
Reaction conditions: catalyst (0.07 μmol active Pd in 4), 5 mL of toluene, and 150 psi ethylene. bDetermined after 1 h. cDetermined by GPC at 140 °C in trichlorobenzene. dNumber of branches per 1000 carbons determined by 1H NMR in tetrachloroethane-d2 at 120 °C.24 a
catalyzed at 25 °C. Higher temperatures result in decreases in activities and lower-molecular-weight polymer products (Table 1, entries 3 and 4). Under these conditions, 4 produces polymers with narrow Đ values ranging from 1.4−2.1 up to 60 °C. 4 shows steady gas uptake over 15 h at 40 °C (Figure S13). We also conducted control experiments with SZO dehydroxylated at 300 °C. The OH sites on SZO are often claimed to be superacidic, suggesting they could initiate polymerization of ethylene. Under these polymerization conditions, SZO does not initiate the polymerization of ethylene, showing that the acidic sites on this support are not responsible for polymerization catalysis. The results of copolymerizations of ethylene and methyl acrylate (eq 1) catalyzed by 4 are shown in Table 2. The
Table 2. Copolymerization of Ethylene and Methyl Acrylatea entry
T (°C)
activity (kgPE/(molactivePd*h))b
XMAc
Mn (kg/mol)d
Đ
Be
1 2 3
25 40 60
5.0 14.9 14.9
0.25 0.33 0.46
33 44 16
1.9 1.9 2.3
33 31 39
Reaction conditions: catalyst (0.35 μmol active Pd in 4), 5 mL of toluene, 2 M methyl acrylate under 80 psi ethylene pressure. b Determined after 15 h. cAmount methyl acrylate incorporated in to the polymer (mol %). dDetermined by GPC at 140 °C in trichlorobenzene. eNumber of branches per 1000 carbons determined by 1H NMR in tetrachloroethane-d2 at 120 °C.24 a
Figure 3. Quantification of catalytically active polymerization sites with Pd-CD3; branching is omitted for clarity.
diimine)Pd-catalyst will chain transfer by β-H elimination to generate a Pd−H, which initiates a new polymer chain. Polymers initiated by Pd−H after chain transfer will not contain deuterium. Therefore, quantification of deuterium in polyethylene generated by 4-CD3 will give the quantity of active sites in 4-CD3. Contacting 4-CD3 with 1 atm of ethylene at room temperature results in the formation of PE. Quantitative 2H NMR spectrum of this PE shows that 9.4% of Pd sites in 4-CD3 are active in ethylene insertion reactions. The activity of 4 in the polymerization of ethylene is summarized in Table 1.23 At 25 °C in toluene, 4 has an activity of 879 kgPE/(molactivePd*h) forming high-molecular-weight PE (Mn = 158 000 g/mol; Đ = 1.37) with 44 branches/1000C (Table 1, entry 1). Increasing the temperature to 40 °C results in higher polymerization activity (1342 kgPE/(molactivePd*h)) and higher molecular weight polymer (Mn = 210 000 g/mol; Đ = 1.37) with a similar number of branches as polymerizations
activity of 4 in copolymerization is roughly 2 orders of magnitude lower than activity in ethylene homopolymerization, a common feature of Pd-catalyzed copolymerization reactions. The Mn of the copolymers follows a similar trend as observed in ethylene homopolymerization. Under these conditions, 4 produces polymers with narrow dispersities and a similar number of branches/1000C as obtained in polymerizations of ethylene. The incorporation of methyl acrylate steadily increases from 0.25% at 25 °C to 0.46% at 60 °C. Copolymerization reactions at 80 °C did not yield measurable quantities of polymer, suggesting that the catalyst is unstable under these conditions. The polymerization reactions are performed in toluene, which is likely not polar enough to rupture the electrostatic interaction between Pd and the sulfated oxide support. To determine if leached Pd is responsible for catalysis, we subjected the supernatants from ethylene polymerizations to C
DOI: 10.1021/acs.organomet.8b00016 Organometallics XXXX, XXX, XXX−XXX
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Organometallics
yield). 1H NMR (C6D6, 300 MHz): 7.75 (d, 3JH−H = 7.5 Hz, 8H, Ar), 7.21−7.17 (m, 20H, Ar), 7.02 (t, 3JH−H = 7.5 Hz, 4H, Ar), 6.89−6.87 (m, 12H, Ar), 6.30 (s, 4H, −CHPh2), 1.87 (s, 6H, p-CH3), 0.87 (s, 6H, Pd-CH3), 0.29 (s, 6H, NC−CH3); 13C {1H} NMR (C6D6, 75.4 MHz): 172.4, 144.5, 143.6, 142.8, 135.2, 134.6, 130.5, 130.3, 130.0, 128.7, 127.5, 127.2, 127.0, 126.5, 52.0, 21.2, 19.7, −1.4. Anal. Calcd for C72H66N2Pd·0.2C4H8O2 C: 80.71%, H: 6.29%, N: 2.59%. Found C: 80.66%, H: 6.01%, N: 2.66. Synthesis of (N∧N)Pd(CD3)2 (2-d6). (N∧N)PdCl2 (400 mg, 0.36 mmol) in diethyl ether (50 mL) was added to a solution of CD3Li (27 mg, 1.08 mmol, 3 equiv) in Et2O (10 mL) at 0 °C. This was allowed to warm up to room temperature and stirred for 30 min. The supernatant was separated from the solid by cannula filtration, and the filtrate was concentrated to dryness under vacuum. The solid was dissolved in toluene (40 mL) was added, and the mixture was filtered. After removal of volatiles, the dark red residue was dissolved in minimum amount of dichloromethane and the product precipitated by slow addition of pentane. The brown precipitate was isolated by filtration and dried under vacuum. (135 mg, 35% yield). 1H NMR (C6D6, 600 MHz): 7.75 (d, 3JH−H = 7.5 Hz, 8H, Ar), 7.21−7.17 (m, 20H, Ar), 7.02 (t, 3JH−H = 7.5 Hz, 4H, Ar), 6.89−6.87 (m, 12H, Ar), 6.30 (s, 4H, −CHPh2), 1.87 (s, 6H, p-CH3), 0.29 (s, 6H, NC−CH3); 2H NMR (C6D6, 92.1 MHz): 0.79; 13C {1H} NMR (C6D6, 150.9 MHz): 172.4, 144.5, 143.6, 142.8, 135.2, 134.6, 130.5, 130.3, 130.0, 128.7, 127.5, 127.2, 127.0, 126.5, 52.0, 21.2, 19.7. Anal. Calcd for C72H60D6N2Pd· 1.6CH2Cl2 C: 73.57%, H: 5.80%, N: 2.33%. Found C: 73.21%, H: 5.69%, N: 2.50%. Generation of [(N∧N)PdMe(OEt2)][B(C6F5)4] (3). 2 (15 mg, 0.014 mmol) and [(Et2O)2H][B(C6F5)4] (10 mg, 0.013 mmol) were placed in a J-Young NMR tube. The NMR tube was evacuated and diethyl ether (about 0.5 mL) was vac-transferred at −40 °C. The NMR tube was shaken a few times, but kept cold with an ice bath. After 5 min, a deep red-colored solution was obtained. The volatiles were removed under vacuum and CD2Cl2 was transferred under vacuum for NMR experiments, which established the composition of 3. 1H NMR (CD2Cl2, 600 MHz): 7.38−7.04 (m, 40H, Ar), 7.00 (s, 2H, Ar), 6.90 (s, 2H, Ar), 5.55 (s, 2H, −CHPh2), 5.54 (s, 2H, −CHPh2), 3.33 (q, 3 JH−H = 9.2 Hz, 4H, Et2O), 2.25 (s, 3H, p-CH3), 2.24 (s, 3H, p-CH3), 1.05 (t, 3JH−H = 9.2 Hz, 6H, Et2O), 0.86 (s, 3H, NC−CH3), 0.39 (s, 3H, Pd-CH3), 0.32 (s, 3H, NC−CH3); 13C {1H} NMR (CD2Cl2, 150.9 MHz) non aromatic peaks: 183.0 (NC−), 175.5 (NC−), 66.3 (Et2O), 52.24 (−CHPh2), 52.18 (−CHPh2), 21.7 (p-CH3), 21.6 (p-CH3), 21.0 (NC−CH3), 19.8 (NC−CH3), 15.7 (Et2O), 8.7 (Pd-CH3). Grafting 2 onto SZO300 To Form 4. SZO300 (100−500 mg) and 2 (0.5 equiv based on the 0.132 mmol OH/g SZO) were transferred to one arm of a double-Schlenk flask inside an argon-filled glovebox. Addition of more 2 before, or during, the grafting reaction does not result in higher palladium loadings. Diethyl ether (ca. 3−5 mL) was transferred under vacuum to the flask at 77 K. The mixture was warmed to 0 °C and gently stirred for 15 min. The brown solution was filtered to the other side of the double Schlenk. The Pd-containing SZO300 was washed by condensing solvent from the other arm of the double Schlenk at 77 K, warming to room temperature and stirring for 2 min, and filtering the solvent back to the other side of the flask. This was repeated two times; the final washing was clear and colorless, indicating that all palladium on the SZO surface is chemibsorbed onto the oxide surface. The volatiles were transferred to a 2 L flask at 77 K and allowed to equilibrate into the gas phase. Analysis of the gas phase by gas chromatography showed that methane (0.059 ± 0.006 mmol/ g) evolved during the grafting reaction. Further details on gas quantification in grafting reactions are given in ref 11e. The orange solid was dried under vacuum (10−6 Torr) at room temperature and stored in an argon-filled glovebox at −20 °C. 13C CP-MAS NMR (150.9 MHz): 184.7 (NC−), 174.9 (NC−), 140.3 (Ar.), 136.0 (Ar.), 128.7 (Ar.), 68.8 (Et2O), 51.7 (2X −CHPh2), 18.9 (2X p-CH3), 18.9 (2X NC−CH3), 12.8 (Et2O), 11.5 (Pd-CH3). Elemental Analysis: Pd: 0.4%, C: 3.87%, H: 0.34%, N: 0.12%. Reaction of 4 with 13CO To Form 5. A 4 mm solid-state NMR rotor containing 4 was contacted with excess 13CO (∼500 equiv/Pd)
AAS after 15 h of polymerization. These results indicate that
