Neutral Nickel(II) Complexes Bearing Aryloxide Imidazolin-2-imine

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Neutral Nickel(II) Complexes Bearing Aryloxide Imidazolin-2-imine Ligands for Efficient Copolymerization of Norbornene and Polar Monomers Mingyuan Li,† Zhengguo Cai,*,† and Moris S. Eisen*,‡ †

Organometallics Downloaded from pubs.acs.org by AUSTRALIAN NATL UNIV on 12/14/18. For personal use only.

State Key Laboratory for Modification of Chemical Fibers and Polymer Materials, College of Materials Science and Engineering, Donghua University, Shanghai 201620, People’s Republic of China ‡ Schulich Faculty of Chemistry, Technion-Israel Institute of Technology, Haifa 32000, Israel S Supporting Information *

ABSTRACT: A series of novel aryloxide imidazolin-2-imine bidentate neutral Ni(II) complexes with different substituents on the imidazolin-2-imine ligand were synthesized and characterized. The complex Ni2 bearing a 2,6-diisopropylphenyl substituent adopted an almost square planar geometry, while the bulkier 2,6-bis(diphenylmethyl)-4-methylphenyl substituent ligated complex Ni3 was obtained in an allyl adduct base-free η3 coordination mode. In the presence of B(C6F5)3, these Ni(II) complexes exhibited remarkably high activity (up to 2.7 × 107 g of PNB (mol of Ni)−1 h−1) and particularly good thermal stability toward the addition polymerization of norbornene. Most importantly, these catalysts were able to promote the direct copolymerization of norbornene with various polar monomers with high activity (∼105 g (mol of Ni)−1 h−1), reasonable comonomer incorporation (0.14−3.08%), and high copolymer molecular weight (Mn up to 2.0 × 105). The strategy of installing a strongly electron donating imidazolin-2-imine ligand on the nickel complex demonstrates a great advantage for the copolymerization of an olefin with polar monomers.

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INTRODUCTION Cyclic olefin copolymers (COCs) obtained via coordination polymerization are an attractive class of polyolefin with unique properties, such as high glass transition temperature, good chemical resistance, excellent transparency, and low water adsorption.1 Among them, the most common and versatile COC is the copolymer of norbornene and ethylene as a kind of commercial engineering thermoplastic.2 In addition, the properties of copolymers can be modified by changing the comonomer contents and sequence distributions as well as the comonomer structures employed.2,3 Numerous single-site catalysts have been developed in both academia and industry for the copolymerizations of norbornene with ethylene, propylene, and higher α-olefins to give various COCs.4 Despite these developments in the synthesis and the wide range of applications of COCs, their nonpolar nature remains a significant limitation. Incorporation of even a small amount of polar functional groups on the polyolefin backbone can significantly improve the performance of the polyolefins, such as adhesion, dye retention, printability, and compatibility.5 The direct copolymerization of an olefin with polar monomers is a desirable and economical synthetic strategy.5c−f However, early-transition-metal catalysts are easily poisoned by the polar functional groups.6 In contrast, the less electrophilic © XXXX American Chemical Society

late-transition-metal catalysts provide promising potentials to promote the concept for the synthesis of polar-functionalized polyethylene and polypropylene.7 A milestone discovery in this regard was made in the 1990s by Brookhart et al., who reported the copolymerization of olefins with acrylate monomers using α-diimine palladium catalysts.7a−c Later in the 2000s, neutral salicylaldimine nickel catalysts were reported by Grubbs et al., which are capable for the copolymerization of ethylene with polar-substituted norbornene and a few other special polar monomers.7d,e However, the activity and molecular weight are low and the scope of polar functional groups was limited in these catalyst systems. In addition, examples of catalysts for the copolymerization of norbornene with polar monomers are still limited, since late-transitionmetal complexes commonly show poor performance for the copolymerization of norbornene and higher α-olefins.8 Recent developments of late-transition-metal catalysts have achieved excellent performances in the copolymerization of ethylene or propylene with polar monomers,9 strategies for which can be classified into two categories. One is to block the metal center by the installation of sterically bulky substituents Received: October 15, 2018

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DOI: 10.1021/acs.organomet.8b00752 Organometallics XXXX, XXX, XXX−XXX

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on the ligand,9a−g,i−m and the other is to reduce the electrophilicity of the metal center by the incorporation of electron-donating ligands.9a,b,h−l Earlier observation of the former has been demonstrated extensively.9a−g,i−m The latter was successfully applied by Nozaki et al. using aryloxycarbenebased Ni/IzQO and Pd/IzQO catalysts supported by Nheterocyclic carbenes (NHCs) as strong σ donors (Chart 1, I).9i,j

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RESULTS AND DISCUSSION Synthesis and Characterization of the Ligands and Complexes. The aryloxide imidazolin-2-imine ligands L1−L3 were prepared by the reaction of o-aminophenol (4) with substituted dichloroimidazolium salts (3) in the presence of trimethylamine.13 These dichloroimidazolium salts (3) can be easily obtained by the deprotonation of NHC precursors (1) with 1.2 equiv of sodium hydride using potassium tert-butoxide as a catalyst in THF,14 followed by the addition of 1.1 equiv of hexachloroethane at −35 °C.15 In order to prepare nickel complexes, the ligands were deprotonated with 1.1 equiv of potassium hydride in THF, respectively. The obtained potassium salt of each ligand was then treated with an appropriate nickel precursor (trans[NiMeCl(PMe3)2]), which led to the formation of the expected complexes Ni1 and Ni2 in good yields (80.9− 85.7%). To our surprise, the bulkier 2,6-bis(diphenylmethyl)4-methylphenyl-substituted ligand L3 did not react by the same procedure, which may be due to a larger steric hindrance effect. Therefore, the potassium salt of L3 was treated with 0.5 equiv of [Ni(Br)allyl]2 dimer, and the allyl adduct base-free complex Ni3 was successfully prepared in a good yield of 81.6%. All of these novel ligands and complexes were characterized by NMR spectroscopy, HRMS, and elemental analysis. In addition, the molecular structures of the ligands L1−L3 (Figure 1) and the complexes Ni2 and Ni3 (Figures 2 and 3) were determined by single-crystal X-ray diffraction. In Figure 1, all of the ligands showed a similar [N,O] bidentate chelating structure, in which the N1−C1 distance (∼1.29 Å) and C2−C3 distance (∼1.33 Å) are similar to those of typical NC double bonds (∼1.28 Å) and CC double bonds (∼1.34 Å), respectively.10 The structure in Figure 2

Chart 1. Selected Examples of Previously Reported Catalysts for Olefin Polymerization

Stimulated by these strategies,9 we speculate that an imidazolin-2-iminato(imine) ligand would serve as a promising ligand, owing to its strong 2σ,4π-electron N donor with mesomeric structures (Chart 2).10 In addition, the steric Chart 2. Mesomeric Forms of Imidazolin-2-iminato(imine) Ligands

hindrance at the axial position on the metal can also be tailored by the substituent of the corresponding R groups.10 Excellent performances of their early-transition-metal complexes with the monoanionic forms were observed in the field of olefin polymerization (Chart 1, II−IV).11 However, the study of latetransition-metal complexes combined with these ligands for olefin polymerization is still very limited, although they have already been used in organometallic and coordination chemistry.10−12 Our recent work gave rare examples of neutral Ni(II) and Pd(II) complexes bearing aryloxide imidazolidin-2-imine ligands toward the vinyl-type polymerization of norbornene (Chart 1, V).13 However, the addition of polar monomers quenched the polymerization. Here, we report the synthesis of neutral Ni(II) complexes bearing strongly electron donating imidazolin-2-imine fragments and their excellent performances in the copolymerization of norbornene with polar monomers. To the best of our knowledge, this is the first example of neutral Ni(II) complexes bearing aryloxide imidazolin-2-imine ligands with different substituents for olefin copolymerization, affording functionalized COCs with various polar functional groups and suitable comonomer contents.

Figure 1. ORTEP representation of the ligands L1−L3 (50% thermal ellipsoids). Hydrogen atoms have been omitted for clarity. Selected bond lengths (Å) and angles (deg): L1, C1−N1 1.288(3), C1−N2 1.382(3), C1−N3 1.384(3), C2−C3 1.326(4), N2−C1−N3 105.1(2); L2, C1−N1 1.303(3), C1−N2 1.384(3), C1−N3 1.376(3), C2−C3 1.333(4), N2−C1−N3 105.1(2); L3, C1−N1 1.286(4), C1−N2 1.393(4), C1−N3 1.385(4), C2−C3 1.329(4), N2−C1−N3 103.9(3). B

DOI: 10.1021/acs.organomet.8b00752 Organometallics XXXX, XXX, XXX−XXX

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Organometallics Table 1. Polymerization of Norbornene with Ni(II) Complexesa

Figure 2. ORTEP representation of complex Ni2 (50% thermal ellipsoids). Hydrogen atoms are removed for clarity. Selected bond lengths (Å) and angles (deg): Ni1−P1 2.1037(8), Ni1−O1 1.8864(16), Ni1−N1 1.9590(18), Ni1−C34 1.921(2), N1−C1 1.328(3), C2−C3 1.328(3); O1−Ni1−P1 90.14(6), O1−Ni1−N1 85.19(7), O1−Ni1−C34 168.75(10), N1−Ni1−P1 171.11(6).

entry

cat.

T (°C)

yield (g)

Ab

105Mnc

PDIc

1d 2e 3 4f 5 6 7 8 9 10 11 12 13 14 15 16 17 18

Ni2 Ni2 Ni2 Ni2 Ni2 Ni2 Ni2 Ni2 Ni3 Ni3 Ni3 Ni3 Ni3 Ni1 Ni1 Ni1 Ni1 Ni1

25 25 25 25 40 60 80 100 25 40 60 80 100 25 40 60 80 100

0.147 0.157 0.226 0.195 0.352 0.343 0.246 0.093 0.245 0.267 0.302 0.186 0.085 0.409 0.448 0.362 0.239 0.077

8.82 9.42 13.56 11.70 21.12 20.58 14.76 5.58 14.70 16.02 18.12 11.16 5.10 24.54 26.88 21.72 14.34 4.62

5.37 5.45 5.02 4.76 4.85 4.78 4.22 3.06 3.53 4.10 4.65 4.73 3.35 6.06 6.19 5.24 4.03 3.17

1.71 1.70 1.74 1.79 1.89 1.79 1.82 1.99 1.79 1.88 1.82 1.85 2.03 1.64 1.69 1.74 1.83 2.11

a

Polymerization conditions unless specified otherwise: complex, 0.5 μmol; cocat., B(C6F5)3, 10 μmol, 20 equiv; norbornene, 10 mmol; Vtotal(toluene) = 10 mL; t = 2 min. bAcitivity in units of 106 g of PNB (mol of Ni)−1 h−1. cGPC data in 1,2,4-trichlorobenzene at 165 °C with polystyrene standards. dB(C6F5)3, 5.0 equiv. eB(C6F5)3, 10 equiv. fB(C6F5)3, 50 equiv.

106g of PNB (mol of Ni)−1 h−1 with an increase in the B/Ni ratio from 5 to 20 (entries 1−3, Table 1). Further increase in the B/Ni ratio to 50 slightly decreased the activity (entry 4, Table 1). Therefore, the B/Ni ratio was fixed at 20. The polymerization temperature significantly affected the activity, where the highest activities of Ni1 (26.88 × 106 g of PNB (mol of Ni)−1 h−1), Ni2 (21.12 × 106 g of PNB (mol of Ni)−1 h−1), and Ni3 (18.12 × 106 g of PNB (mol of Ni)−1 h−1) were obtained at 40 and 60 °C, respectively, indicating the better thermal stability of Ni3 by the bulkier substituent. However, all of the nickel complexes still maintained high activity (107 g of PNB (mol of Ni)−1 h−1) even at 80 °C, suggesting the great thermal stability of the aryloxide imidazolin-2-imine nickel catalysts (entries 7, 12, and 17, Table 1). Ni1 bearing a 2,4,6trimethylphenyl substituent showed higher activity in comparison to complexes Ni2 and Ni3 at the same polymerization temperature range from 25 to 80 °C (entries 3, 5, 6, 9−12, and 14−17, Table 1), indicating that the bulky substituent seems to hinder the coordination site to prohibit the insertion of the norbornene monomer. A similar steric effect of ligands, in the norbornene polymerization, has also been observed in many previous reports.8a,13 Interestingly, a different phenomenon was observed that the activity and molecular weight of Ni2 and Ni3 are slightly higher than those of Ni1 at 100 °C (entries 8, 13, and 18, Table 1). This observation indicate higher thermal stability of Ni2 and Ni3, which is possibly ascribable to the greater steric hindrance effects of Ni2 and Ni3 at high temperature. The similar microstructures of PNBs obtained were characterized by 1H NMR and 13C NMR spectra in CDCl3, at 25 °C (see the Supporting Information). The 1H NMR spectra of PNBs indicates that the PNBs were vinyl addition type products. On the other hand, these complexes showed poor behaviors toward ethylene polymerization. The reason is not yet clear;

Figure 3. ORTEP representation of complex Ni3 (50% thermal ellipsoids). Hydrogen atoms are removed for clarity. Selected bond lengths (Å) and angles (deg): Ni1−O1 1.827(3), Ni1−N1 1.972(4), Ni1−C76 2.000(6), Ni1−C77 1.940(6), Ni1−C78 1.969(5), N1−C1 1.339(5), C76−C77 1.399(9), C77−C78 1.384(9), C2−C3 1.332(5); O1−Ni1−N1 87.28(15), O1−Ni1−C76 92.2(2), N1− Ni1−C78 106.61(19), C76−Ni1−C78 73.1(2).

revealed that complex Ni2 adopted an almost square planar geometry, in which the distance from the metal center to the coordination plane is 0.029 Å. The phosphine in complex Ni2 is trans to the nitrogen atom, which is consistent with previously reported aryloxide imidazolidin-2-imine Ni(II) complexes.13 The Ni−C(allyl) distances in complex Ni3 (2.000(6) Å for Ni1−C76; 1.940(6) Å for Ni1−C77; 1.969(5) Å for Ni1−C78) are similar to those in previously reported phosphine sulfonate allyl Ni(II) complexes, suggesting a typical η3 coordination mode in Ni3.8b,16 Although different nickel precursors were installed in complexes Ni2 and Ni3, not much difference was observed in the corresponding bond lengths and angles. Norbornene Polymerization. The catalytic behaviors of Ni1−Ni3 toward norbornene homopolymerization were studied in the presence of B(C6F5)3 as a phosphine scavenger (Table 1). We selected Ni2 to investigate the effects of the polymerization conditions in detail, at room temperature (25 °C). The activity of Ni2 increased from 8.82 × 106 to 13.56 × C

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Organometallics Table 2. Copolymerization of Norbornene with Polar Monomersa

Polymerization conditions unless specified otherwise: complex, 5 μmol; cocat., B(C6F5)3, 10.0 equiv, 50 μmol; norbornene, 10 mmol, 0.9415 g; Vtotal(toluene) = 10 mL; time = 30 min. bAcitivity in units of g (mol of Ni)−1 h−1. cDetermined by 1H NMR spectra in CDCl3 at room temperature. d GPC data in 1,2,4-trichlorobenzene at 165 °C with polystyrene standards. etime = 60 min. a

one plausible reason can be attributed to the addition of the electron density to the Ni center by the installation of the strongly electron donating imidazolin-2-imine ligand, which reduces the electrophilicity of the metal center to prohibit ethylene coordination. Norbornene Copolymerization. On the basis of the norbornene homopolymerization results, the Ni(II) complexes were employed in the copolymerization of norbornene with various polar monomers (Table 2). In this system, the addition of some commercial polar monomers, such as methyl acrylate, ethyl acrylate, n-butyl acrylate, and allyl chloride, completely

shuts down polymerization. Although the copolymerization of norbornene and vinyl acetate was performed with moderate polymerization activity, no comonomer incorporation was observed. When a spacer was put between the double bond and the polar functional groups, high activity, high copolymer molecular weight ,and reasonable comonomer incorporation were achieved during the copolymerization of norbornene with allyl acetate, allyl ethyl ether, allyl benzene, and some OH/ COOMe/Br/Cl-containing long-chain monomers. The complexes Ni1 and Ni2 bearing Mes and Dipp substituents conducted efficient copolymerization of norborD

DOI: 10.1021/acs.organomet.8b00752 Organometallics XXXX, XXX, XXX−XXX

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Figure 4. Plots of copolymerization activity and comonomer incorporation against polar monomer concentration in the feed: (a) copolymerization of norbornene with 10-undecen-ol; (b) copolymerization of norbornene with methyl 10-undecenoate (Tables S1 and S2).

Figure 5. GPC curves of copolymers obtained with complexes Ni2 and Ni3: (a) norbornene and 10-undecen-1-ol copolymers; (b) norbornene and methyl 10-undecenoate copolymers (Tables S1 and S2).

nene and polar monomers with considerable activities and copolymer molecular weights. In contrast to the homopolymerization of norbornene, complex Ni3 bearing the bulkiest 2,6bis(diphenylmethyl)-4-methylphenyl substituent showed the highest activity toward the copolymerization of norbornene with polar monomers, indicating that the effective blockage of the axial position on the metal is efficient for enhancing the tolerance of the nickel complex toward polar functional groups. This could also be proved by our previous study,13 in which the polymerization of norbornene was quenched when polar monomers were added to the system by using similar aryloxide imidazolidin-2-imine Ni(II) complexes bearing a smaller methyl substituent. As shown in Table 2, the polar comonomer incorporation in the copolymers can be improved by increasing the concentration of the feeding polar monomers (from 1.67% to 2.40% for allyl acetate (entries 1 and 2, Table 2); or from 0.70% to 1.06% for 11-chloro-1-undecene (entries 13 and 14, Table 2). Therefore, the effects of the concentration of the feeding polar monomers on the copolymerization activity, molecular weight, and polar comonomer incorporation in the copolymers were studied in detail with complexes Ni2 and Ni3 using 10-undecen-1-ol and methyl 10-undecenoate as the polar monomers. The results are shown in Figures 4 and 5 (see also Tables S1 and S2).

When the polar monomer concentration was increased from 0.1 to 0.4 mol/L, the polar monomer incorporation in the copolymer was effectively increased regardless of the complex and monomer used, accompanied by a decrease in the activities (Figure 4). However, it should be noted that both catalytic systems still maintained considerable activities (∼104 g (mol of Ni)−1 h−1) to give high-molecular-weight copolymers (4 × 104 g mol−1), indicating the good tolerance of these Ni(II) complexes toward polar functional groups. The higher activity, polar monomer incorporation, and molecular weight of Ni3 (Figures 4 and 5) even at high polar monomer concentration also indicated the improved tolerance of Ni3 toward polar functional groups by the large steric hindrance effect around the nickel center. Furthermore, GPC curves of all the copolymers were unimodal with a narrow molecular weight distribution (∼2.0), indicating the single-site catalytic behavior of these catalysts (Figure 5). The glass transition temperatures (Tg) of PNB and copolymers were determined by DSC (Figure 6). The Tg value of PNB was not observed below 350 °C, while each copolymer with the highest comonomer incorporation exhibited a corresponding Tg value (328.2 °C for norbornene−10-undecen-1-ol copolymer (entry 8, Table S1); 327.4 °C for norbornene−methyl 10-undecenoate copolymer (entry 8, Table S2); 324.8 °C for norbornene−6-bromo-1-hexene copolymer (entry 17, Table 2); 326.3 °C for norbornene−allyl E

DOI: 10.1021/acs.organomet.8b00752 Organometallics XXXX, XXX, XXX−XXX

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imidazolidin-2-imine ligand and their behaviors toward olefin (co)polymerization are in progress.

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EXPERIMENTAL SECTION

All manipulations of air-sensitive materials were performed with the rigorous exclusion of oxygen and moisture in flamed Schlenk-type glassware or J. Young Teflon valve sealed NMR tubes on a dualmanifold Schlenk line interfaced to a high-vacuum (10−5 Torr) line or in a nitrogen-filled Innovative Technologies glovebox with a mediumcapacity recirculator (1−2 ppm of O2). Materials. Argon and nitrogen were purified by passage through an MnO oxygen-removal column and a Davison 4 Å molecular sieve column. Dichloromethane, tetrahydrofuran, diethyl ether, toluene, and n-hexane were purified by a PS-MD-5 (Innovative Technology) solvent purification system. Benzene-d6 (Cambridge Isotopes) was distilled under vacuum from Na/K alloy. Norbornene was dissolved in dry toluene and stirred with CaH2 for 24 h, and then the solution was distilled under reduced pressure. Comonomers (polar α-olefins) were dried with CaH2 overnight and then freshly distilled before use. 2Aminophenol, hexachloroethane, trimethylphosphine (1.0 M in toluene), anhydrous nickel(II) chloride, Ni(COD)2, potassium hydride, and tris(pentafluorophenyl)boron were purchased from Sigma-Aldrich and used without any purification. Norbornene (Co)polymerization. Norbornene (co)polymerization was performed in a 50 mL flask equipped with a magnetic stirrer and carried out by the following methods. At first, the flask was charged with a certain amount of norbornene, tris(pentafluorophenyl)boron, polar α-olefins (for copolymerization), and toluene under nitrogen. After the mixture was kept at the desired temperature for 10 min, (co)polymerization was initiated by introduction of the Ni(II) complexes in toluene into the flask via syringe, and the reaction was started. The desired time later, the (co)polymerization was terminated by addition of 10% HCl in ethanol. The precipitated polymer was washed with ethanol and water and dried at 60 °C under vacuum to a constant weight. For all of the (co)polymerization procedures, the reaction volume in total was constant, which can be achieved by variation of the added toluene when necessary. Measurements. The molecular weights and distributions of polymers were determined by a Polymer Laboratory PL GPC-220 instrument equipped with a triple-detection array consisting of a differential refractive index (DRI) detector, a two-angle (45, 90°) light scattering (LS) detector at a laser wavelength of 658 nm, and a four-bridge capillary viscosity detector. This system included one guard column (PL# 1110-1120) and three 30 cm columns (PLgel 10 μm MIXED-B 7.5 × 300 mm). Polymer characterization was carried out at 165 °C using 1,2,4-trichlorobenzene as eluent and calibrated by polystyrene standards. The NMR spectra were measured on Bruker Avance 200, Bruker Avance 300, Bruker Avance III 400, and Bruker Avance III 500 spectrometers. Elemental analysis was performed using an Element Vario EL III elemental analyzer. HRMS experiments were performed at 200 °C (source temperature) on a Maxis Impact (Bruker) mass spectrometer with an APCI solid probe method. The glass-transition temperatures (Tg) of the copolymers were measured with a TA Q2000 Differential Scanning Calorimeter at a heating rate of 10 °C/min from 40 to 350 °C. The TGA measurements were performed on a NETZSCH TG 209F3 instrument from 40 to 550 °C at a rate of 10 °C/min under a nitrogen atmosphere. Crystal Structure Determinations. A single crystal was immersed in perfluoropolyalkylether oil and quickly mounted on a Kappa CCD diffractometer underflow of liquid nitrogen. Data collection was performed using monochromated Mo Kα radiation using φ and ω scans to cover the Ewald sphere. Accurate cell parameters were obtained with the amount of indicated reflections. The structure was solved by SHELXS-97 direct methods and refined by the SHELXL-97 program package. The atoms were refined anisotropically. Hydrogen atoms were included using the riding model.

Figure 6. DSC curves of selected copolymers: (a) norbornene and 10undecen-1-ol copolymer (entry 8, Table S1); (b) norbornene and methyl 10-undecenoate copolymer (entry 8, Table S2); (c) norbornene and 6-bromo-1-hexene copolymer (entry 17, Table 2); (d) norbornene and allyl acetate copolymer (entry 3, Table 2).

acetate copolymer (entry 3, Table 2)). The lower Tg value of these copolymers in comparison to PNB also indicated that various functionalized COCs were obtained by direct copolymerization of norbornene with polar monomers. The thermal stabilities of selected copolymers were studied by TGA analysis. According to the TGA data (see the Supporting Information), all of the copolymers exhibited similar thermal stabilities and decomposition temperatures (Td) up to 421.5 °C, suggesting the good thermal stability of the copolymers prepared in this catalytic system. In general, the copolymerization of ethylene with polar monomers catalyzed by Ni(II) catalysts exhibit significantly decreased activity (∼104 g (mol of Ni)−1 h−1) and copolymer molecular weight (∼104 g mol−1) because of the poisoning effect of polar groups on the metal center.8a The polymerization of norbornene was commonly quenched when polar monomers were involved.7,8 In this paper we present a promising catalytic system in which high activity (∼105 g (mol of Ni)−1 h−1) and high copolymer molecular weight (Mn up to 197600) were achieved during the copolymerization of norbornene with allyl acetate, allyl ethyl ether, allyl benzene, and some OH/COOMe/Br/Cl-containing long-chain monomers. The high performances of these nickel catalysts may originate from the lower electrophilicity of the nickel center due to the installation of the electron-donating ligand.

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CONCLUSION In summary, the synthesis of novel neutral Ni(II) complexes bearing strongly electron donating imidazolin-2-imine bidentate ligands and their catalytic behavior toward norbornene copolymerization with various polar monomers is reported. Upon activation with B(C6F5)3, these Ni(II) complexes showed good thermal stabilities toward norbornene polymerization. Most importantly, excellent performance of the nickel catalysts including high activity, reasonable comonomer incorporation, high copolymer molecular weight, and good solubility of the copolymers were observed in the copolymerization of norbornene with various polar monomers without the use of large amounts of a cocatalyst or any protecting reagents, affording functionalized COCs. Further studies on other late-transition-metal complexes bearing this F

DOI: 10.1021/acs.organomet.8b00752 Organometallics XXXX, XXX, XXX−XXX

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Organometallics Synthesis of Aryloxide Imidazolin-2-imine ligands. The Nheterocyclic carbene precursors (compound 1, Scheme 1) with

bis(2,6-diisopropylphenyl)-2-dichloroimidazolium chloride (20 mmol) in MeCN (60 mL) at 0 °C with strong agitation. After the whole mixture was refluxed overnight, volatiles were removed by evaporation under vacuum. The mixture was then added to Et2O (200 mL), stirred for 0.5 h, and filtered. Then the light yellow Et2O solution was collected and evaporated under vacuum. The crude product was dissolved in the minimum amount of MeCN (∼50 mL) and stored at −20 °C overnight to recrystallize. The ligands were used as white crystals or a microcrystalline powder. Yield: 45.8%. 1H NMR (400 MHz, CDCl3): δ 7.35−7.28 (m, 2H, Ar-CH), 7.20−7.10 (m, 4H, Ar-CH), 6.56−6.48 (m, 1H, Ar-CH), 6.42 (s, 2H, NCHCHN), 6.40−6.32 (m, 1H, Ar-CH), 6.04−5.97 (m, 1H, Ar-CH), 5.95−5.88 (m, 1H, Ar-CH), 3.11−2.98 (m, 4H, CH(CH3)2), 1.24 (dd, J = 12.9, 6.9 Hz, 24H, CH(CH3)2). 13C NMR (100 MHz, CDCl3): δ 149.1, 146.6, 145.1, 133.9, 129.5, 124.0, 120.0, 119.8, 117.8, 115.9, 111.6, 28.9, 24.9, 22.9. Anal. Calcd for C33H41N3O: C, 79.96; H, 8.34; N, 8.48. Found: C, 79.84; H, 8.42; N, 8.37. HRMS (APCI, m/z): calcd for C33H42N3O [M + H]+ 496.3322, found 496.3376. Ligand L3. The 2-aminophenol (5 mmol) and triethylamine (10 mmol) in MeCN (40 mL) were added to a solution of N,N’-1,3bis(2,6-bis(diphenylmethyl)-4-methylphenyl)-2-dichloroimidazolium chloride (5 mmol) in MeCN (80 mL) at 0 °C with strong agitation. After the whole mixture was refluxed overnight, a large amount of white precipitate appeared in the hot MeCN solution. The pure ligand was obtained by filtration and washed with a small amount of MeCN. Yield: 82.5%. 1H NMR (400 MHz, CDCl3): δ 7.19−7.09 (m, 24H, Ar-CH), 7.01−6.93 (m, 8H, Ar-CH), 6.91−6.85 (m, 8H, ArCH), 6.75 (s, 4H, Ar-CH), 6.74−6.72 (m, 1H, Ar-CH), 6.50−6.39 (m, 1H, Ar-CH), 6.05−5.93 (m, 1H, Ar-CH), 5.90−5.76 (m, 1H, ArCH), 5.60 (s, 4H, CH(Ph)2), 4.93 (s, 2H, NCHCHN), 2.16 (s, 6H, CH3). 13C NMR (100 MHz, CDCl3): δ 149.6, 144.2, 143.8, 143.5, 142.0, 138.6, 133.4, 133.2, 130.1, 129.9, 129.5, 128.4, 128.3, 126.5, 126.4, 119.7, 119.3, 117.9, 115.2, 111.9, 51.9, 21.8. Anal. Calcd for C75H61N3O: C, 88.29; H, 6.03; N, 4.12. Found: C, 88.17; H, 5.98; N, 4.23. HRMS (APCI, m/z): calcd for C75H62N3O [M + H]+ 1020.4815, found 1020.4887. Synthesis of Complexes. The nickel(II) precursors used in this work are trans-NiMeCl(PMe3)2 and [Ni(Br)allyl]2 dimer, which were prepared according to the work reported previously.17,18 Complex Ni1. To a solution of L1 (0.5 mmol) in dry THF (20 mL) was added KH (0.55 mmol, 1.1 equiv). After it was stirred at 25 °C for 6 h, the resulting mixture was filtered, and the filtrate was evaporated. The light yellow-green residue was washed with n-hexane (5 mL × 3) and dried in vacuo. This salt was used for the next synthesis immediately without further purification. A solution of this salt in dry THF (10 mL) was added dropwise at 25 °C into a flask containing trans-NiMeCl(PMe3)2 (0.5 mmol) in dry THF (10 mL). A rapid color change from yellow to red-orange was observed. After it was stirred overnight at 25 °C, the reaction mixture was filtered. The volatiles were then partially removed from the filtrate in vacuo, and to the residue was added excess n-hexane. A yellow-orange powder of complex Ni1 was obtained by filtration. Yield: 80.9%. 1H NMR (300 MHz, C6D6): δ 6.96−6.86 (m, 1H, Ar-CH), 6.86−6.72 (m, 1H, ArCH), 6.65 (s, 4H, Ar-CH), 6.62−6.54 (m, 1H, Ar-CH), 6.37−6.11 (m, 1H, Ar-CH), 5.96 (s, 2H, NCHCHN), 2.48 (s, 12H, CH3), 1.99 (s, 6H, CH3), 0.91 (d, J = 10.0 Hz, 9H, P(CH3)3), −1.25 (d, J = 6.7 Hz, 3H, Ni-CH3). 13C NMR (75 MHz, C6D6): δ 165.1, 165.0, 141.9, 137.4, 134.4, 133.4, 129.7, 120.5, 116.9, 115.6, 114.8, 109.5, 20.4, 20.1, 12.7 (d, J = 27.2 Hz, P(CH3)3), −18.0 (d, J = 43.7 Hz, Ni-CH3). 31 P NMR (240 MHz, C 6 D 6 ): δ −6.02. Anal. Calcd for C31H40N3NiOP: C, 66.45; H, 7.20; N, 7.50. Found: C, 66.24; H, 7.37; N, 7.38. HRMS (ESI, m/z): calcd for C31H41N3NiOP [M + H]+ 560.2341, found 560.2340. Complex Ni2. To a solution of L2 (0.5 mmol) in dry THF (20 mL) was added KH (0.55 mmol, 1.1 equiv). After it was stirred at 25 °C for 6 h, the resulting mixture was filtered, and the filtrate was evaporated. The yellow residue was washed with n-hexane (5 mL × 3) and dried in vacuo. This salt was used for the next synthesis immediately without further purification. Then a solution of this salt in dry THF (10 mL) was added dropwise at 25 °C into a flask

Scheme 1. Synthesis of Aryloxide Imidazolin-2-imine Ligands and Complexesa

a

Abbreviations: Mes, 2,4,6-trimethylphenyl; Dipp, 2,6-diisopropylphenyl; IPr*, 2,6-bis(diphenylmethyl)-4-methylphenyl.

different substituents (2,4,6-trimethylphenyl (Mes), 2,6-diisopropylphenyl (Dipp), and 2,6-bis(diphenylmethyl)-4-methylphenyl (Ipr*)) were prepared according to literature procedures.14a The corresponding N-heterocyclic carbenes (compound 2, Scheme 1) were obtained by the deprotonation of these imidazolium salts, which has been reported before.14a N,N′-1,3-Bis(2,4,6-trimethylphenyl)-2-dichloroimidazolium chloride (compound 3, Mes, Scheme 1)15a and N,N′-1,3bis(2,6-diisopropylphenyl)-2-dichloroimidazolium chloride (compound 3, Dipp, Scheme 1)15b were prepared according to published procedures. N,N′-1,3-Bis(2,6-bis(diphenylmethyl)-4-methylphenyl)2-dichloroimidazolium chloride (compound 3, Ipr*, Scheme 1) was prepared by the same method as for N,N′-1,3-Bis(2,6-diisopropylphenyl)-2-dichloroimidazolium chloride (compound 3, Dipp, Scheme 1).15b Ligand L1. The 2-aminophenol (20 mmol) and triethylamine (40 mmol) in MeCN (40 mL) were added to a solution of N,N’-1,3bis(2,4,6-trimethylphenyl)-2-dichloroimidazolium chloride (20 mmol) in MeCN (60 mL) at 0 °C with strong agitation. After the whole mixture was refluxed overnight, sodium hydroxide (40 mmol) in 10 mL of H2O was added. Volatiles were removed by evaporation under vacuum. Then, 50 wt % potassium hydroxide in H2O (10 mL) was added. The products were extracted with MeCN, and the organic phase was collected and dried with anhydrous MgSO4. Then the dry MeCN solution was evaporated under vacuum. The crude product was dissolved in the minimum amount of MeCN (∼35 mL) and stored at −20 °C overnight to recrystallize. The ligands were used as colorless crystals or a white microcrystalline powder. Yield: 33.2%. 1H NMR (400 MHz, CDCl3): δ 6.83 (s, 4H, Ar-CH), 6.56−6.49 (m, 1H, Ar−CH), 6.40−6.34 (m, 1H, Ar-CH), 6.33 (s, 2H, NCHCHN), 6.11−6.02 (m, 2H, Ar-CH), 2.24 (s, 6H, CH3), 2.20 (s, 12H, CH3). 13 C NMR (100 MHz, CDCl3): δ 148.8, 144.4, 138.3, 135.7, 134.7, 133.3, 129.1, 120.0, 119.6, 117.9, 114.53, 111.6, 21.0, 18.0. Anal. Calcd for C27H29N3O: C, 78.80; H, 7.10; N, 10.21. Found: C, 78.67; H, 7.02; N, 10.33. HRMS (APCI, m/z): calcd for C27H30N3O [M + H]+ 412.2383, found 412.2390. Ligand L2. The 2-aminophenol (20 mmol) and triethylamine (40 mmol) in MeCN (40 mL) were added to a solution of N,N’-1,3G

DOI: 10.1021/acs.organomet.8b00752 Organometallics XXXX, XXX, XXX−XXX

Article

Organometallics ORCID

containing trans-NiMeCl(PMe3)2 (0.5 mmol) in dry THF (10 mL). A rapid color change from yellow to dark red was observed. After it was stirred overnight at 25 °C, the reaction mixture was filtered. The volatiles were then partially removed from the filtrate in vacuo, and to the residue was added excess n-hexane. A red-orange powder of complex Ni2 was obtained by filtration. Yield: 85.7%. The single crystal for XRD was obtained in n-hexane/THF (3/1 v/v). 1H NMR (600 MHz, C6D6): δ 7.13−7.06 (m, 6H, Ar-CH), 6.88−6.83 (m, 1H, Ar-CH), 6.77−6.70 (m, 1H, Ar-CH), 6.48 (s, 2H, NCHCHN), 6.47− 6.46 (m, 1H, Ar-CH), 6.26−6.19 (m, 1H, Ar-CH), 3.57−3.49 (m, 4H, CH(CH3)2), 1.52 (d, J = 6.6 Hz, 12H, CH(CH3)2), 1.08 (d, J = 6.6 Hz, 12H, CH(CH3)2), 0.88 (d, J = 10.0 Hz, 9H, P(CH3)3), −1.39 (d, J = 6.7 Hz, 3H, Ni−CH3). 13C NMR (75 MHz, C6D6): δ 165.2, 165.1, 153.6, 145.1, 141.4, 133.8, 129.2, 124.4, 120.7, 117.8, 115.5, 114.1, 110.0, 28.8, 25.9, 22.7, 12.6 (d, J = 27.0 Hz, P(CH3)3), −19.1 (d, J = 42.6 Hz, Ni-CH3). 31P NMR (240 MHz, C6D6): δ −6.52. Anal. Calcd for C37H52N3NiOP: C, 68.95; H, 8.13; N, 6.52. Found: C, 68.81; H, 8.17; N, 6.38. HRMS (ESI, m/z): calcd for C37H53N3NiOP [M + H]+ 644.3280, found 644.3256. Complex Ni3. To a solution of L3 (0.2 mmol) in dry THF (20 mL) was added KH (0.2 mmol, 1.0 equiv). After the mixture was stirred at 25 °C overnight, a large amount of yellow precipitate appeared and the resulting mixture was filtered. The yellow powder was washed with n-hexane (5 mL × 3) and dried in vacuo. This salt was used for the next synthesis immediately without further purification. A suspension of this salt in dry THF (10 mL) was added dropwise at 25 °C into a flask containing [Ni(Br)allyl]2 (0.1 mmol) in dry THF (10 mL). A rapid color change from yellow to dark red was observed. After it was stirred overnight at 25 °C, the reaction mixture was filtered. The volatiles were then partialy removed from the filtrate in vacuo and to the residue was added excess nhexane. An orange powder of complex Ni3 was obtained by filtration. Yield: 81.6%. Single crystals for XRD were obtained in n-hexane/ THF/CHCl2 (5/1/1 v/v/v). 1H NMR (300 MHz, C6D6): δ 7.83− 7.63 (m, 7H, Ar-CH), 7.37−7.25 (m, 1H, Ar-CH), 7.20−6.86 (m, 34H, Ar-CH), 6.86−6.75 (m, 4H, Ar-CH), 6.72−6.62 (m, 1H, ArCH), 6.25 (bs, 4H, CH(Ph)2), 6.06−5.89 (m, 1H, Ar-CH), 4.86 (s, 2H, NCHCHN), 4.66−4.45 (m, 1H, allyl), 3.94 (br, 1H, allyl), 2.72 (br, 1H, allyl), 1.79 (br, 1H, allyl), 1.76 (s, 6H, CH3), 1.52 (br, 1H, allyl). 13C NMR (125 MHz, C6D6): δ 166.2, 147.0, 144.9, 144.4, 144.3, 144.0, 142.7, 141.3, 138.9, 133.8, 131.2, 130.9, 129.5, 128.9, 128.5, 128.4, 127.1, 126.6, 124.3, 121.1, 117.9, 112.5, 109.7, 21.3. Anal. Calcd for C78H65N3NiO: C, 83.72; H, 5.85; N, 3.75. Found: C, 83.49; H, 5.96; N, 3.68. HRMS (ESI, m/z): calcd for C78H66N3NiO [M + H]+ 1118.4559, found 1118.4550.

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Zhengguo Cai: 0000-0001-5784-3920 Moris S. Eisen: 0000-0001-8915-0256 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China (21774018), Program for New Century Excellent Talents in University, the Program for Professor of Special Appointment (Eastern Scholar) at Shanghai Institutions of Higher Learning, and “Shu Guan” project supported by Shanghai Municipal Education Commission and Shanghai Education Development Foundation and the Fundamental Research Funds for the Central Universities. We thank Dr. Heng Liu (Technion-Israel Institute of Technology) for helpful discussions. This research was supported by the Israel Science Foundation underContract No. 184/18.

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S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.organomet.8b00752. Polymerization tables, NMR spectra, and HRMS report of the ligands and complexes and NMR spectra, GPC data, and DSC data of the (co)polymers (PDF) Accession Codes

CCDC 1872890−1872894 contain the supplementary crystallographic data for this paper. These data can be obtained free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by emailing [email protected], or by contacting The Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033.

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*E-mail for Z.C.: [email protected]. *E-mail for M.S.E.: [email protected]. H

DOI: 10.1021/acs.organomet.8b00752 Organometallics XXXX, XXX, XXX−XXX

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DOI: 10.1021/acs.organomet.8b00752 Organometallics XXXX, XXX, XXX−XXX