Copolymerization of Ethylene with Acrylate Monomers by Amide

Apr 20, 2017 - Department of Chemistry, The University of Chicago, 5735 South Ellis ... copolymerization behavior of a series of (N,N′-diaryl-α-dii...
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Copolymerization of Ethylene with Acrylate Monomers by AmideFunctionalized α‑Diimine Pd Catalysts Feng Zhai, Joseph B. Solomon, and Richard F. Jordan* Department of Chemistry, The University of Chicago, 5735 South Ellis Avenue, Chicago, Illinois 60637, United States S Supporting Information *

ABSTRACT: We report the ethylene homopolymerization and ethylene/methyl-acrylate (MA) and ethylene/acrylic-acid (AA) copolymerization behavior of a series of (N,N′-diaryl-αdiimine)Pd catalysts that contain secondary amide (−CONHMe) or tertiary amide (−CONMe2) substituents on the Naryl rings, including the “first-generation” catalysts {(2,6-iPr2Ph)NCMeCMeN(2-CONHMe-6- i Pr-Ph)}PdMeCl (1a,a′) and {(2,6-iPr2-Ph)NCMeCMeN(2-CONMe26-iPr-Ph)}PdMeCl (1b,b′) and the “second-generation” catalysts [{2,6-(CHPh 2 ) 2 -4-Me-Ph}NCMeCMeN(2CONHMe-6-iPr-Ph)]PdMeCl (1d,d′) and [{2,6-(CHPh2)2-4-Me-Ph}NCMeCMeN(2-CONMe2-6-iPr-Ph)]PdMeCl (1e,e′). Activation of 1d,d′ and 1e,e′ by NaB{3,5-(CF3)2C6H3}4 generates active ethylene polymerization catalysts that produce highly branched (77−81 br/1000 C) polyethylenes with number-average molecular weights (Mns) in the range 26−60 kDa. The replacement of two isopropyl units in 1a,a′ and 1b,b′ with benzhydryl groups in 1d,d′ and 1e,e′ leads to a significant improvement in ethylene homopolymerization performance. The secondary amide-functionalized catalyst 1d,d′ incorporates ca. twice as much MA and ca. three times as much AA as the iPr-substituted catalyst [{2,6-(CHPh2)2-4-Me-Ph}NCMeCMe N(2,6-iPr2-Ph)]PdMeCl (1f,f′) in copolymerization with ethylene. The reactions of 1a,a′ and 1b,b′ with metal salts that contain weakly coordinating anions lead to extrusion of CH4 and the formation of [{(μ-κ2-N,N′,κ-O-α-diimine)Pd}2(μ-CH2)]2+ complexes, in which the amide carbonyl O atoms coordinate to Pd centers.

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Chart 1a

INTRODUCTION The synthesis of polar polyethylenes by the direct copolymerization of ethylene and polar vinyl monomers is of significant fundamental and practical interest.1−6 One class of catalysts that are capable of incorporating polar monomers (PMs) by insertion copolymerization with ethylene are (αdiimine)Pd catalysts, 7,8 which can incorporate acrylate monomers,9−11 methyl vinyl ketone, 10 methyl acrolein dimethyl acetal,12 N-pentenylcarbazole,13 vinyl silyl ethers,14 vinyltrialkoxysilanes,15 and vinyl monomers with remote polar groups.16 However, the copolymerization of ethylene and PMs by (α-diimine)Pd catalysts has major limitations, including low levels of PM incorporation and retardation of copolymerization by chelate formation following PM insertion.9,17 Hydrogen bonding (H-bonding) interactions in the second coordination sphere can strongly influence the structures and ligand binding preferences of coordination and organometallic compounds, and their reactivity in transition-metal-catalyzed transformations.18−21 We are interested in how H-bonding interactions may influence the copolymerization of ethylene with PMs. We recently reported the synthesis, structures and coordination chemistry of the “first-generation” amidefunctionalized α-diimine Pd catalysts {(2,6-iPr2-Ph)N CMeCMeN(2-CONHMe-6-iPr-Ph)}PdMeCl (1a,a′) and {(2,6- i Pr 2 -Ph)NCMeCMeN(2-CONMe 2 -6- i Pr-Ph)}PdMeCl (1b,b′) (Chart 1). The secondary amide unit © XXXX American Chemical Society

a

The ′ denotes cis,trans isomers.

Received: March 17, 2017

A

DOI: 10.1021/acs.organomet.7b00209 Organometallics XXXX, XXX, XXX−XXX

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Organometallics Scheme 1

(−CONHMe) in 1a,a′ is a H-bond donor and acceptor, while the tertiary amide unit (−CONMe2) in 1b,b′ is only a H-bond acceptor. 1a, in which the Cl ligand is cis to the amidefunctionalized arylimine unit, exhibits a H-bonding interaction between the amide NH group and the Pd−Cl ligand, while the trans Cl/amide-functionalized imine isomer 1a′ does not. However, the ethylene polymerization activity of 1a,a′ and 1b,b′ is poor compared to that of the reference catalyst 1c. In order to improve the polymerization performance to the point where ethylene/PM copolymerization studies would be feasible, we designed the second-generation catalysts [{2,6(CHPh2 ) 2-4-Me-Ph}NCMeCMeN(2-CONHMe-6- iPrPh)]PdMeCl (1d,d′) and [{2,6-(CHPh2)2-4-Me-Ph}N CMeCMeN(2-CONMe2-6-iPr-Ph)]PdMeCl (1e,e′) (Chart 1). Replacement of two isopropyl groups of 1a,a′ and 1b,b′ with the more sterically demanding benzhydryl (−CHPh2) groups in 1d,d′ and 1e,e′ was expected to significantly improve ethylene polymerization performance, based on recent studies of benzhydryl (α-diimine)Ni catalysts by Long and co-workers and (α-diimine)Pd catalysts by Chen and co-workers.22−25 Here we report the synthesis of 1d,d′ and 1e,e′ and the ethylene homopolymerization behavior and ethylene/acrylate copolymerization behavior of the first- and second-generation amide-functionalized (α-diimine)Pd catalysts. The secondary amide-functionalized catalyst 1d,d′ incorporates higher levels of methyl acrylate (MA) and acrylic acid (AA) in the copolymerization with ethylene than the analogous iPrsubstituted catalyst [{2,6-(CHPh 2 ) 2 -4-Me-Ph}N CMeCMeN(2,6-iPr2-Ph)]PdMeCl (1f,f′). We also report that (α-diimine)PdMe+ cations derived from 1a,a′ and 1b,b′ undergo facile elimination of methane with concomitant formation of dinuclear methylene-bridged [(μ-κ2-N,N′,κ-O-αdiimine)2(μ-CH2)Pd2]2+ complexes.

Figure 1. Molecular structure of 5 in 5·2(CHCl2CHCl2). Solvent molecules and hydrogen atoms except for H3 are omitted. Selected distances (Å): Pd1−Cl1, 2.277(2); Pd1−Cl2, 2.292(2); Pd1−N1, 2.040(6); Pd1−N2, 2.034(6); N3···Cl2, 3.541(9). Selected bond angles (deg): Cl1−Pd1−Cl2, 91.43(8); N1−Pd1−Cl1, 94.19(17); N2−Pd1−Cl2, 94.60(19); N2−Pd1−N1, 79.8(2).

structure of 5 was confirmed by X-ray diffraction (Figure 1) and features an intramolecular N−H···Cl H-bond between the amide NH unit and a Cl ligand. The H···Cl distance of 2.81 Å is shorter than the sum of van der Waals radii of H and Cl (2.95 Å), consistent with an N−H···Cl−M H-bond.26,27 The 1H NMR spectrum of 5 exhibits a doublet at δ 3.01 for the amide N−Me group, which is coupled to the NH signal (br q, J = 4.9) at δ 6.86. The 1H NMR spectrum also exhibits an upfield resonance at δ −0.34 for the diimine Me group that is adjacent to the benzhydryl units and shielded by the two proximal phenyl rings. Similar results were observed for other (αdiimine)PdMeCl complexes bearing o,o′-dibenzhydryl-p-tolyl unit(s).24,25 The reaction of 5 with SnMe4 yields a 7.7/1 mixture of isomeric (α-diimine)PdMeCl complexes 1d/1d′, which evolves to an equilibrium 5.2/1 mixture of 1d/1d′ in a few hours in CD2Cl2 at room temperature (Scheme 1). The structures of 1d and 1d′ were assigned based on 1H NMR data. The 1H NMR NH signals for 1d and 1d′ in CD2Cl2 appear at δ 7.50 and δ 6.08, respectively. Analogous to the case of 1a,a′, the more downfield NH resonance indicates the presence of intramolecular H-bonding and is thus assigned to 1d. Additionally, the 1H NMR Pd−Me signal of 1d appears at δ 0.88, 0.43 ppm downfield from that of 1d′ (δ 0.45). This difference in Pd−Me chemical shifts is unlikely to be solely caused by electronic

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RESULTS AND DISCUSSION Synthesis of 1d−f,d′−f′. The reaction of α-ketaimine 2 with o-aminobenzamide 3 yields the 2,3-dihydroquinazolin4(1H)-one compound 4 (Scheme 1) via initial imine condensation and subsequent ring closure between the −CONHMe group and the adjacent imine CN unit. The 13 C NMR spectrum of 4 in CD2Cl2 exhibits four sets of signals for the phenyl groups and two signals for the isopropyl Me groups, consistent with the C1-symmetry that results from the presence of the quaternary carbon atom (δ 76.8 in CD2Cl2). The reaction of 4 with (MeCN)2PdCl2 at 40 °C results in opening of the dihydroquinazolinone ring and formation of the secondary amide-substituted (α-diimine)PdCl2 complex 5. The B

DOI: 10.1021/acs.organomet.7b00209 Organometallics XXXX, XXX, XXX−XXX

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Organometallics Table 2. Ethylene Polymerizationa

Scheme 2

entry

cat

C2H4 (psi)

1 2 3 4 5c 6 7 8 9 10 11 12

1a,a′ 1b,b′ 1b,b′ 1b,b′ 1c 1d,d′ 1d,d′ 1d,d′ 1e,e′ 1e,e′ 1f,f′ 1f,f′

200 200 200 350 200 200 200 350 200 200 200 350

time (h)

yield (g)

activity (kg PE mol−1 h−1)

20 2 20 20 16 2 20 2 2 20 2 2

0.20 0.39 0.76 0.68 8.77 0.68 1.54 0.59 0.78 2.75 2.67 3.23

1.0 20 3.8 3.4 55 34 7.7 30 39 14 133 162

Mn (103 Da)

Mw/ Mn

Bb

2.4 3.5 3.3 2.9 138 26.3 44.0 24.2 45.1 59.7 292 327

1.85 1.52 1.61 1.56 2.16 1.81 1.82 1.75 1.53 1.75 1.44 1.39

93 92 91 92 98 79 79 81 77 79 73 74

Conditions: 10 μmol Pd, 20 μmol NaBArF4, CH2Cl2 (40 mL), 25 °C. Branches per 1000 C. Determined by 1H NMR. Corrected for terminal groups of low-MW polymer (Mn < 5000 Da). cRef 31. a b

Table 3. Ethylene/MA Copolymerizationa entry

cat

yield (g)

1 2c 3 4c 5

1d,d′ 1d,d′ 1e,e′ 1e,e′ 1f,f′

0.31 0.040 0.22 0.020 0.13

Scheme 3

MA TON

C2H4 TON

MA incorp. mol %

Mn (103 Da)

Mw/ Mn

Bb

14 3.3 7.3 1.0 2.4

1060 130 760 68 460

1.3 2.4 0.95 1.4 0.61

8.1 0.98 4.4 0.79 6.9

2.13 1.70 1.76 1.56 2.06

74 82 73 78 77

Conditions: 10 μmol Pd, 20 μmol NaBArF4, C2H4 (14 psi), MA (1.0 M), galvinoxyl (5 μmol), CH2Cl2 (total volume 25 mL), 20 h, room temperature (23 °C). bBranches per 1000 C. Determined by 1H NMR. Corrected for terminal groups for low-MW polymers (Mn < 5000 Da). c4/1 toluene/CH2Cl2 mixture (total volume 25 mL) as solvent. a

room temperature. Similarly, ligand 8 reacts with (cod)PdMeCl to afford the corresponding (α-diimine)PdMeCl complexes 1f,f′ (Scheme 3) as reported by Chen and co-workers.25 1f/1f′ is formed as a 1/1.2 mixture of isomers that evolves to a 1/3.4 equilibrium 1f/1f′ mixture over ca. 2 months at room temperature in CD2Cl2. This isomerization is therefore much slower than those of 1d,d′ and 1e,e′. Influence of Hydrogen Bonding on Isomer Ratios. Equilibrium isomer ratios 1a−f/1a′−f′ are listed in Table 1. The isomer ratios are influenced by the difference in the donor abilities of the imine ligands. In square planar (α-diimine)PdMeCl complexes that contain two different imine donors, the Me ligand is favored to be trans to the more weakly electrondonating of the two imine ligands because Me exerts a stronger trans influence than Cl. In the case of 1f,f′, the preference for 1f′ is consistent with the expected weaker donor ability of the 2,6-(CHPh2)2-4-Me-Ph unit versus 2,6-iPr2-Ph. The same effect also contributes to higher isomer ratios observed for 1a/1a′ and 1b/1b′ versus 1d/1d′ and 1e/1e′, respectively. The higher values for the 1a/1a′ and 1d/1d′ ratios compared to the 1b/1b′ and 1e/1e′ ratios, respectively, are ascribed to the stabilization of 1a and 1d by intramolecular H-bonding.29 The difference between ratios 1d/1d′ and 1e/1e′ corresponds to an additional stabilization in 1d by ΔΔG = 0.9 kcal mol−1, which is a measure of the strength of intramolecular N−H···Cl

Table 1. Equilibrium Isomer Ratios of {ArNCMeCMe N−(2-R-6-iPr-Ph)}PdMeCl Complexes R= CONHMe i

Ar = 2,6- Pr2-Ph Ar = 2,6-(CHPh2)2-4-MePh a

16/1 (1a/ 1a′)a 5.2/1 (1d/ 1d′)

R = CONMe2 4.5/1 (1b/ 1b′)a 1.2/1 (1e/1e′)

R = iPr 1/1 (1c) 1/3.4 (1f/ 1f′)

Ref 29.

effects (cf. Δδ = 0.11 ppm for 1a vs 1a′). The Pd−Me ligand in 1d is expected to be close to the edges of the two adjacent −CHPh2 phenyl rings based on the solid-state structure of 5. Therefore, the Pd−Me chemical shift difference is ascribed to anisotropic deshielding from the two adjacent −CHPh2 phenyl rings in 1d.28 The condensation reaction between 2 and 6 yields α-diimine ligand 7 (Scheme 2). 7 reacts with (cod)PdMeCl to form a 1/ 4.2 mixture of isomeric complexes 1e/1e′, which evolves to an equilibrium 1.2/1 mixture within a few hours in CD2Cl2 at C

DOI: 10.1021/acs.organomet.7b00209 Organometallics XXXX, XXX, XXX−XXX

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Organometallics Table 4. Ethylene/AA Copolymerizationa entry

cat

yield (g)

AA TON

C2H4 TON

AA incorp. mol %

Mn(103 Da)

Mw/Mn

Bb

1 2c 3 4c 5

1d,d′ 1d,d′ 1e,e′ 1e,e′ 1f,f′

0.060 trace 0.054 trace 1.29

3.1 − 1.7 − 21

210 − 190 − 4600

1.5 − 0.88 − 0.45

3.0 − 3.2 − 65.3

1.66 − 1.56 − 1.63

73 − 74 − 77

Conditions: 10 μmol Pd, 20 μmol NaBArF4, C2H4 (14 psi), AA (1.0 M), galvinoxyl (5 μmol), CH2Cl2 (total volume 25 mL), 20 h, room temperature (23 °C). bBranches per 1000 C. Determined by 1H NMR. Corrected for terminal groups of low-MW polymer (Mn < 5000 Da). c4/1 toluene/CH2Cl2 mixture (total volume 25 mL) as solvent. a

withdrawing amide groups is counteracted by enhanced steric crowding resulting from the presence of the benzhydryl groups. In 2 h experiments at 200 psi (entries 6 and 9), both 1d,d′ and 1e,e′ exhibit higher activity and higher MW than 1b,b′. The resulting polymers exhibit less branching (77−81 br/1000 C) than those produced by the first-generation catalysts. The apparent activities of 1d,d′ and 1e,e′ decrease significantly when the polymerization time is extended from 2 to 20 h (entries 7 and 10), but 1e,e′ is more thermally stable than 1b,b′. The performance of 1d,d′ is essentially identical at 200 and 350 psi ethylene pressure (entries 6 and 8). The reference catalyst 1f,f′ is ca. 4 times more active than 1d,d′ and 1e,e′ and produces high-MW polyethylene (Mn = ca. 300 kDa) with a narrow MW distribution. Copolymerization of Ethylene with MA. Ethylene/MA copolymerization results are summarized in Table 3. The secondary amide catalyst 1d,d′ incorporates 1.3 mol % MA (entry 1), which is ca. twice the level of MA incorporation observed for 1f,f′ (entry 5). The tertiary amide catalyst 1e,e′ incorporates 0.95 mol % MA (entry 3), which is intermediate between those of 1d,d′ and 1f,f′. The 1H NMR spectra of the copolymers establish that the MA units are incorporated predominantly in branch ends (Supporting Information), which results from chain walking following MA insertion.10 All of the catalysts studied exhibit significant inhibition in the presence of MA, presumably due to formation of chelate species following MA insertion. However, the activity trend in ethylene/MA copolymerization (1d,d′ > 1e,e′ > 1f,f′) is opposite to that for ethylene homopolymerization (1f,f′ > 1e,e′ > 1d,d′). It is notable that 1d,d′ exhibits the highest MA incorporation level yet the lowest extent of inhibition among all three catalysts. One possible explanation for these observations is that the H-bond donor properties of 1d,d′ assist the insertion of MA into Pd−R bond and the opening of chelate formed after MA insertion. Steric and electronic effects may also contribute to this reactivity trend. However, previous studies of 2,6-iPr2-Ph substituted and cyclophane-type (α-diimine)Pd catalysts by Guan and co-workers have shown that para-electron-withdrawing groups on the N-aryl units lead to decreased MA incorporation in copolymerization with ethylene.32,33 The catalysts studied here display the opposite trend, i.e., the electron-withdrawing ortho-amide groups lead to increased acrylate incorporation compared to iPr. Therefore, it is unlikely that the higher acrylate incorporation associated with 1d,d′ and 1e,e′ vs iPr-substituted 1f,f′ results from the weaker electrondonating ability of the amide groups vs iPr. Further studies of amide-functionalized catalysts that display larger reactivity differences will be required to understand these trends. For 1d,d′ and 1e,e′, changing the solvent from CH2Cl2 to a 4/1 toluene/CH2Cl2 mixture leads to an increase in MA

Scheme 4

hydrogen bond in 1d. 1a is stabilized over 1b by a similar amount (ΔΔG = 0.7 kcal mol−1).29 Ethylene Homopolymerization. Ethylene polymerization results for the first- and second-generation amide-functionalized (α-diimine)Pd catalysts are provided in Table 2. 1a,a′ and 1b,b′ are activated by NaBArF4 (ArF = 3,5-(CF3)2-Ph) to generate single-site catalysts for ethylene polymerization that produce highly branched (91−93 br/1000 C), low MW (Mn = 2.4−3.5 kDa) polyethylenes (entries 1−4) at room temperature in CH2Cl2 solvent. Increasing the ethylene pressure from 200 to 350 psi does not change the activity or the microstructure of resulting polymer significantly (entry 4), showing that chain growth and chain transfer are both zero-order in [ethylene], in agreement with the established mechanistic picture for (αdiimine)Pd-catalyzed ethylene polymerization.30 The activity of 1b,b′ decreases by 81% when the polymerization time is extended from 2 to 20 h (entry 3), indicating that decomposition of the catalytically active species is significant. 1a,a′ and 1b,b′ exhibit lower activity by an order of magnitude and produce polyethylene with lower MW by 2 orders of magnitude compared to the benchmark catalyst 1c. Guan and co-workers studied a series of (α-diimine)Pd catalysts bearing four ortho-isopropyl groups and different para-substituents on the N-aryl units and found that electron-withdrawing substituents reduced catalyst activity and polymer MW in ethylene polymerization. The electron-withdrawing effect of the amide groups likely accounts for the poor performance of 1a,a′ and 1b,b′. The poor performance of the first-generation catalysts prompted us to study the second-generation catalysts 1d,d′ and 1e,e′, in which the detrimental effect of the electronD

DOI: 10.1021/acs.organomet.7b00209 Organometallics XXXX, XXX, XXX−XXX

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Organometallics

Figure 2. (a) Molecular structure of 9a-OTf in 9a-OTf·4(CH2Cl2). Solvent molecules and anions are omitted. The minor position of the disordered amide group is not shown. Left: ORTEP view. Hydrogen atoms except for the H atoms on C1 and N3 are omitted. Middle: Space-filling view showing the Pd2(μ-CH2) unit and the two aryl rings in close proximity. Right: Space-filling view showing the environment of an amide group that is engaged in H-bonding with a OTf− anion. The O-cis amide N−Me group (C28) is positioned close to the adjacent aryl ring. Selected distances (Å) and angles (deg): Pd1−N1, 2.131(6); Pd1−N2, 2.012(5); Pd1−C1, 1.989(7); Pd1−O1, 2.11(3); Pd1···Pd1, 2.9999(10); N3···O4, 2.980(16); H3··· O4, 2.11; Pd1−C1−Pd1, 97.9(4); N3−H3···O4, 170. (b) Molecular structure of 9b-OTf in 9b-OTf·2(CH2Cl2). Solvent molecules and anions are omitted. Left: ORTEP view. Hydrogen atoms except for the H atoms on C1 are omitted. Middle: Space-filling view showing the Pd2(μ-CH2) unit and the two aryl rings in close proximity. Right: Space-filling view showing the environment of the two amide groups. The O-cis amide N−Me groups (C16 and C57) are positioned close to the adjacent aryl rings. Selected distances (Å) and angles (deg): Pd1−C1, 1.979(4); Pd2−C1, 1.990(4); Pd1−O2, 2.068(3); Pd2−O1, 2.073(3); Pd1−N1, 2.161(3); Pd2−N4, 2.154(3); Pd1−N2, 2.001(3); Pd2−N5, 2.006(3); Pd1···Pd2, 3.0142(4); Pd1−C1−Pd2, 98.85(18).

Chart 2. Crystallographically Characterized (μ-CH2)Pd2 Complexesa

Scheme 5. Formation of Meyer’s μ-CH2 Complex 1242

a

incorporation but a decrease in reaction rate and MW (entries 2 and 4).34 Copolymerization of Ethylene with AA. Catalysts 1d,d′, 1e,e′, and 1f,f′ produce ethylene/AA copolymer, with 1.5 mol

dipp =2,6-iPr2-Ph.

E

DOI: 10.1021/acs.organomet.7b00209 Organometallics XXXX, XXX, XXX−XXX

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Organometallics

reported that the reaction of complex 14 with excess SnMe4 forms a μ-CH2 complex 12 and methane, likely through the intermolecular C−H activation of a dinuclear Pd2Me2(μ-Cl) intermediate (Scheme 5).42 1a,a′ and 1b,b′ also react with 1 equiv NaBArF4 to form 9a,bBArF4 and methane, albeit with lower selectivity than observed for the AgOTf reactions (NMR yields 63% for 9a-BArF4; 57% for 9b-BArF4).45 In contrast, 1a,a′ reacts cleanly with AgOAc to form the corresponding (α-diimine)PdMe(η1-OAc) complex.29 These results, and the clean formation of (α-diimine)PdMeL+ species in the presence of Lewis bases as noted above, suggest that weakly coordinating anions and open coordination sites are crucial for the conversion of (α-diimine)PdMe+ species to (αdiimine)Pd(μ-CH2)Pd(α-diimine)2+ products. The formation 9a,b-OTf may proceed through intermolecular Pd-mediated C−H activation of a Pd−Me ligand, as observed for 11-OTf and 12. The facile formation of these Pd2(μ-CH2) species from in situ generated (α-diimine)PdMe+ species suggests that they are possible catalyst deactivation products.

% AA incorporation observed for secondary amide catalyst 1d,d′ and lower levels of incorporation observed for 1e,e′ (0.88 mol %) and 1f,f′ (0.45 mol %) at 14 psi ethylene pressure and 1 M AA in CH2Cl2 solvent at room temperature (Table 4, entries 1, 3, and 5).35−37 AA is a stronger inhibitor than MA for 1d,d′ and 1e,e′. Interestingly, 1f,f′ is quite active and yields high-MW copolymer in ethylene/AA copolymerization, although the AA incorporation level is lower than observed for 1d,d′ and 1e,e′. The 1H NMR spectra of the copolymers produced by all three catalysts show that the AA units are incorporated predominantly in branch ends (Supporting Information), which is similar to the case of MA copolymerization. For 1d,d′ and 1e,e′, changing the solvent from CH2Cl2 to a 4/1 toluene/ CH2Cl2 mixture leads to only trace polymer (entries 2 and 4). Reaction of (α-Diimine)PdMeCl Complexes with Metal Salts. The reactions of 1a,a′ and 1b,b′ with metal salts with weakly coordinating anions were investigated to generate discrete (α-diimine)PdMe+ species, which are the presumed active species in the polymerization reactions discussed above.30,38 As reported previously, 1a,a′ and 1b,b′ react with metal salts in the presence of Lewis bases to form (αdiimine)PdMeL+ complexes (L = pyrazole, CD3CN).29 In contrast, in the absence of Lewis bases, 1a,a′ and 1b,b′ react quantitatively with 1 equiv of AgOTf at room temperature in CH2Cl2 to form 0.5 equiv methane and the blue-green μ-CH2 complexes 9a-OTf and 9b-OTf (Scheme 4), in which the amide-substituted α-diimine ligands function as bridging ligands to support the Pd2(μ-CH2) unit. The solid-state structures of 9a,b-OTf are shown in Figure 2. The dications in 9a,b-OTf are C2-symmetric; in particular, the solid-state structure of 9a-OTf contains a crystallographic C2-axis running through the μ-CH2 C atom. 9a-OTf exhibits a Pd−C−Pd angle of 97.9(4)° and a Pd···Pd distance of 2.9999(10) Å. The corresponding data for 9b-OTf are similar (98.85(18)°, 3.0142(4) Å). Both Pd···Pd distances are ca. 0.25 Å shorter than the sum of the van der Waals radii of two Pd atoms (3.26 Å). The μ-CH2 units in 9a,b-OTf are jacketed by the two adjacent aryl rings that are attached to the bridging amide groups (Figure 2a, middle; Figure 2b, middle). The secondary amide groups of 9a-OTf are engaged in H-bonding interactions with the triflate anions (Figure 2a, right). The 1H and 13C NMR signals of the μ-CH2 unit of 9a-OTf in CD2Cl2 appear at δ 2.11 (s) and δ 67.3, respectively, and are correlated in the HMQC spectrum. The 13C{1H−gated} NMR spectrum exhibits a triplet (J = 142 Hz) for the μ-CH2 C atom. Similarly, the 1H and 13C NMR signals of the μ-CH2 unit of 9bOTf in CD2Cl2 appear at δ 1.76 (s) and δ 66.6 (t, J = 142 Hz as determined by 13C{1H−gated} NMR), respectively. The O-cis amide N−Me groups in both 9a,b-OTf are shielded by the 2,6-iPr2-Ph rings (Figure 2a, right; Figure 2b, right), and accordingly, their 1H NMR resonances appear at unusually high field (δ 2.05 for 9a-OTf; δ 2.23 for 9b-OTf; cf. δ 2.94 for 1d). Several crystallographically characterized Pd2(μ-CH2) complexes have been reported previously and are shown in Chart 2.39−44 The Pd···Pd distances in these compounds range from 2.70 Å for 11-OTf, in which the presence of a Pd−Pd bond was suggested, to 3.3 Å for 12, which a Pd−Pd bond is clearly ruled out. The Pd···Pd distances in 9a,b-OTf are intermediate within this range, and therefore any Pd···Pd electronic interactions, if present, must be weak. Baird and co-workers reported the formation of μ-CH2 complex 11-OTf and extrusion of CH4 in the reaction of {(2,6-iPr2-Ph)NCMeCMeN(2,6-iPr2-Ph)}PdMe2 with 0.5 equiv of [Ph3C]OTf.41 Meyer and co-workers

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CONCLUSIONS Activation of the amide-functionalized (α-diimine)Pd complexes 1a,a′ and 1b,b′ by NaBArF4 generates (α-diimine)PdMe+ species that produce low-MW polyethylenes with low activity. The poor polymerization performance of 1a,a′ and 1b,b′ is ascribed to the electron-withdrawing effect of the amide units. Replacement of two iPr units in 1a,a′ and 1b,b′ with −CHPh2 groups in 1d,d′ and 1e,e′ leads to a significant improvement in ethylene homopolymerization performance. This trend is ascribed to the enhanced steric crowding resulting from the presence of −CHPh2 groups, which counteracts the detrimental effect of electron-withdrawing amide units. The secondary amide catalyst 1d,d′ incorporates higher levels of MA and AA in the copolymerization with ethylene than 1e,e′ and 1f,f′. (αdiimine)PdMe+ species generated in situ from 1a,a′ and 1b,b′ in the absence of Lewis bases or strongly coordinating anions undergo facile elimination of methane and formation of methylene-bridged [(μ-κ2-N,N′,κ-O-α-diimine)2(μ-CH2)Pd2]2+ complexes.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.organomet.7b00209. Experimental procedures, NMR spectra of polymers, formation of 9a,b-BArF4, X-ray crystallography, NMR spectra of compounds (PDF) Detailed crystallographic data (PDF) Crystallographic data for 5·2(CHCl2CHCl2) (CIF) Crystallographic data for 9a-OTf·4(CH2Cl2) (CIF) Crystallographic data for 9b-OTf·2(CH2Cl2) (CIF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Feng Zhai: 0000-0001-7892-3188 Richard F. Jordan: 0000-0002-3158-4745 F

DOI: 10.1021/acs.organomet.7b00209 Organometallics XXXX, XXX, XXX−XXX

Article

Organometallics Notes

(34) Chen and co-workers reported that 1f,f′ copolymerized ethylene with MA to form copolymer with higher MW (Mn = 21 kDa), higher MA incorporation (2.6 mol%), and higher activity in 4/1 toluene/ CH2Cl2 mixture compared to the results in CH2Cl2 reported here. However, using this solvent mixture for ethylene/MA copolymerization by 1d,d′ and 1e,e′ led to a decrease in MW and activity, although enhancements in MA incorporation were observed. See ref 25. (35) (o-R2P-C6H4-SO3)PdMe and {o-R2P-C6H4-PO(OEt)2}PdMe+ catalysts copolymerize ethylene with AA to yield linear copolymers with in-chain or chain-end incorporation. See refs 36 and 37. (36) Rünzi, T.; Fröhlich, D.; Mecking, S. J. Am. Chem. Soc. 2010, 132, 17690−17691. (37) Contrella, N. D.; Sampson, J. R.; Jordan, R. F. Organometallics 2014, 33, 3546−3555. (38) Johnson, L. K.; Killian, C. M.; Brookhart, M. J. Am. Chem. Soc. 1995, 117, 6414−6415. (39) Balch, A. L.; Hunt, C. T.; Lee, C.-L.; Olmstead, M. M.; Farr, J. P. J. Am. Chem. Soc. 1981, 103, 3764−3772. (40) Klopfenstein, S. R.; Kluwe, C.; Kirschbaum, K.; Davies, J. A. Can. J. Chem. 1996, 74, 2331−2339. (41) Brownie, J. H.; Baird, M. C.; Zakharov, L. N.; Rheingold, A. L. Organometallics 2003, 22, 33−41. (42) Sachse, A.; John, M.; Meyer, F. Angew. Chem., Int. Ed. 2010, 49, 1986−1989. (43) Schnetz, T.; Rominger, F.; Hofmann, P. Acta Crystallogr., Sect. E: Struct. Rep. Online 2010, 66, m453−m454. (44) Puddephatt, R. J. Polyhedron 1988, 7, 767−773. (45) The identities of 9a,b-BArF4 were confirmed by salt metathesis reactions between 9a,b-OTf and NaBArF4, although incomplete conversions were observed. See the Supporting Information for details. 9a,b-OTf are inactive in ethylene polymerization (80 psi) in CH2Cl2 at room temperature, likely due in part to the detrimental effect of triflate anions.

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank Alexander Filatov for assistance with X-ray crystallography and Kelsey Brown for assistance with the synthesis of 4. This work was supported by the National Science Foundation (CHE-0911180 and CHE-1048528).

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REFERENCES

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DOI: 10.1021/acs.organomet.7b00209 Organometallics XXXX, XXX, XXX−XXX