Article Cite This: Organometallics XXXX, XXX, XXX−XXX
Concurrently Improving the Thermal Stability and Activity of Ferrous Precatalysts for the Production of Saturated/Unsaturated Polyethylene Qaiser Mahmood,†,‡ Jingjing Guo,§ Wenjuan Zhang,*,†,§ Yanping Ma,† Tongling Liang,† and Wen-Hua Sun*,†,‡ †
Key Laboratory of Engineering Plastics and Beijing National Laboratory for Molecular Science, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190, People’s Republic of China ‡ CAS Research/Education Center for Excellence in Molecular Sciences and International School, University of Chinese Academy of Sciences, Beijing 100049, People’s Republic of China § Beijing Key Laboratory of Clothing Materials R&D and Assessment, Beijing Engineering Research Center of Textile Nanofiber, School of Materials Science and Engineering, Beijing Institute of Fashion Technology, Beijing 100029, People’s Republic of China S Supporting Information *
ABSTRACT: The five unsymmetrical ligands 2-[1-(2,6-dibenzhydryl-4-tert-butylphenylimino)ethyl]-6-[1-(arylimino)ethyl]pyridine (aryl = 2,6-Me2C6H3 (L1), 2,6-Et2C6H3 (L2), 2,6iPr2C6H3 (L3), 2,4,6-Me3C6H2 (L4), 2,6-Et2-4-MeC6H2 (L5)) and the symmetrical ligand 2,6-bis(1-(2,6-dibenzhydryl-4-tertbutylphenylimino)ethyl)pyridine (L6) have been prepared and characterized by FT IR and 1H/13C NMR spectroscopy and elemental analysis. The treatment of L1−L6 with FeCl2·4H2O afforded the corresponding ferrous chloride complexes (Fe1− Fe6) in excellent yields. A distorted-square-pyramidal geometry with a τ5 value of 0.13 is a feature of the X-ray structure of Fe3; broad paramagnetically shifted peaks are revealed in the 1H NMR spectra for all the iron complexes in solution. On activation with either MAO or MMAO cocatalyst, all iron complexes displayed high activities with modest variations in activities at elevated temperature (up to 12.0 × 106, 12.4 × 106, 12.8 × 106, and 13.1 × 106 g of PE (mol of Fe)−1 h−1 at 50, 60, 70, and 80 °C, respectively). The activity at 80 °C is approximately 5 times higher than that of Brookhart/Gibosn’s classical iron precatalyst (Fe0) under identical conditions. Moreover, the nature of the methylaluminoxane cocatalyst employed had a marked effect in defining the end groups of the polymer chain. For example, using MAO cocatalyst, vinyl and n-propyl end groups are formed with a relative ratio of 0.66/0.44, whereas for the MMAO cocatalyst, polymer chains undergo termination with isobutyl, n-propyl, and vinyl end groups having a relative ratio of 0.49/0.33/0.18, indicating that both chain transfer to Al(R)3 and β-hydride elimination mechanisms are functional.
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INTRODUCTION The revolutionary discovery of α-diimino Ni(II) and Pd(II) catalysts for olefin polymerization/oligomerization by Brookhart and co-workers has evoked much interest in the development of late-transition-metal polymerization catalysts.1,2 Nearly two decades ago, Brookhart and Gibson independently reported bis(imino)pyridine-supported iron (A, Chart 1) and cobalt precatalysts that are highly effective for ethylene polymerization or oligomerization.3 Following that, many efforts have been made to extensively explore these catalysts.4−6 In general, two strategies are widely employed to exploit more promising precatalysts for ethylene polymerization and/or oligomerization: modifying the substituents of the parent bis(imino)pyridines5,6 or the ligand framework itself.7 To continue these efforts, many new NNN tridentate iron and cobalt catalysts have been investigated (B, Chart 1).8 In addition, an emerging class of highly active iron and cobalt © XXXX American Chemical Society
precatalysts bearing constrained cycloalkyl-fused pyridine ligand sets (C, Chart 1)9−11 such as 2-(1-(arylimino)ethyl)-7arylimino-6,6-dimethylcyclopentapyridines,9 2,3,7,8-tetrahydroacridine-4,5(1H,6H)-diimine,10 and α,α′-bis(arylimino)2,3:5,6-bis(pentamethylene)pyridines11 has also been exploited for iron based precatalysts for ethylene polymerization. These studies enabled the researchers to gain deeper insight into the mechanism of underpinning ethylene polymerization/oligomerization by monitoring active species and influences of ligand sterics, types of substituents, and choice of the late transition metal on the stability, reactivity, and selectivity of active species to improve the industrial process for more advanced polymeric materials.12 Received: December 26, 2017
A
DOI: 10.1021/acs.organomet.7b00909 Organometallics XXXX, XXX, XXX−XXX
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Organometallics
tempting to further explore the 4-R group to concurrently achieve high activities and high thermal stabilities of iron catalysts. With a view to further explore the type of 4-R substituents, herein we introduce the tert-butyl group in the N-2,6dibenzhydryl-4-R-phenyl group, considering increased positive inductive effects and additionally solubility effects. By modulation of the steric and electronic effects of the second N-aryl group, a series of unsymmetrical/symmetrical bis(imino)pyridine based iron complexes will be prepared. An in-depth ethylene polymerization will be performed to ascertain the effects of different reaction parameters such as reaction temperature, Al/Fe ratio, reaction time, and ethylene pressure on polymer properties (Mw, Mw/Mn, Tm) and catalytic efficiencies. The results will be compared with Brookhart/ Gibosn’s classical iron precatalyst (Fe0, Chart 2)3 under identical conditions. Moreover, a detailed microstructural analysis of the resultant polyethylene will be performed to determine the end groups of the polymer chain.
Chart 1. Structural Variations in Bis(imino)pyridyliron Chloride Precatalysts (A−E)
Despite a plethora of highly active precatalysts that have been developed, they often suffer from numerous limitations, especially poor thermal stability, yielding low-molecular-weight polyethylene at elevated temperatures between 80 and 100 °C. To tackle this impediment, Herrmann et al.13 and Wu et al.14 have increased steric crowding at axial positions of the metal center by incorporating anthracenyl and o-sec-phenethyl groups in the parent bis(imino)pyridines, respectively. The former gives high activities at lower temperature, and the latter iron catalysts displayed high activities (4.36 × 106 g of PE (mol of Fe)−1 h−1 as well as high-molecular-weight polyethylene at elevated temperature (70 °C). Similarly, significant catalytic systems such as α-diimino Ni(II)2i and Pd(II)2j−l and 2iminopyridine N-oxide Ni(II)2m,n catalysts have been developed by employing a benzhydryl group in their ligand frameworks. These catalysts are capable of producing highmolecular-weight polyethylenes (linear to highly branched) and tolerate and incorporate numerous polar comonomers. Meanwhile, we have modified the N-aryl group of a bis(imino)pyridine with a benzhydryl group (D and E, Chart 1)5e,g and achieved high activities with high-molecular-weight polyethylene at 80 °C. Furthermore, we observed that the nature of the 4-R substituents in the N-2,6-dibenzhydryl-4-R-phenyl group exerts a remarkable influence on the catalytic performance/polymer properties. For example, when R = Cl5g (E, Chart 1), iron catalysts display higher activity than when R = Me5e is used (D, Chart 1); even the former catalysts maintained an activity of 1.36 × 107 g of PE (mol of Fe)−1 h−1 with high molecular weight at 90 °C. It seems that the negative electronic effect of the 4-R group tends to improve the catalytic performance; however, it is counterintuitive to say that sole electron-withdrawing substituents play a role in improving the activities and thermal stabilities. Considering these results, it is
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RESULTS AND DISCUSSION Synthesis and Characterization of Fe0−Fe6. The ferrous complexes reported herein, 2-[1-(2,6-dibenzhydryl-4tert-butylphenylimino)ethyl]-6-[1-(arylimino)ethyl]pyridyliron(II) chloride: (aryl = 2,6-Me2C6H3 (Fe1), 2,6-Et2C6H3 (Fe2), 2,6-iPr2C6H3 (Fe3), 2,4,6-Me3C6H2 (Fe4), 2,6-Et2-4-MeC6H2 (Fe5), 2,6-{(C6H5)2CH}2-4-C(CH3)3-C6H2 (Fe6)), were prepared by the treatment of the corresponding ligands with FeCl2·4H2O in excellent yields (Scheme 1). The ligands are not commercially available and can be easily synthesized by classical Schiff base condensation reactions.5 Initially, diacetylpyridine was reacted with 1 equiv of 2,6-dibenzhydryl-4-tert-butylphenylaniline in the presence of p-toluenesulfonic acid for the monoketone compound 1. Subsequent reactions with the corresponding anilines led to a series of ligands (L1−L5, Scheme 1). The ligand L6 was isolated as a byproduct during the purification of compound 1. Furthermore, Brookhart/ Gibson’s classical iron complex Fe0 was synthesized by a procedure reported elsewhere.3 All of the organic compounds and Fe complexes thereof have been characterized by FTIR and NMR spectroscopy or elemental analysis. In addition, confirmation of the structure of the Fe3 complex was provided in the form of a single-crystal X-ray structure. A single crystal of complex Fe3 suitable for X-ray diffraction analysis was grown by the slow layering of heptane on its dichloromethane solution. The distortion of geometry was quantitatively calculated and compared with the typical fivecoordinate geometry index τ5.15 The parameter τ5 can be defined by the equation β − α/60, in which β is the largest Cl− Fe−Cl bond angle and α is the second largest angle. Typical
Chart 2. Comparison of a Reported Precatalyst (Fe0)3 with Unsymmetrical (Fe1−Fe5) and Symmetrical (Fe6) Iron Precatalysts
B
DOI: 10.1021/acs.organomet.7b00909 Organometallics XXXX, XXX, XXX−XXX
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Organometallics Scheme 1. Synthesis of Ligands (L1−L6) and Their Complexes (Fe1−Fe6)
trigonal-bipyramidal and square-pyramidal geometries have τ5 = 1, 0, respectively. The perspective view of Fe3 is shown in Figure 1; selected interatomic bond lengths and bond angles are collected in Table S1.
those expected for this functional group (N(1)−C(10) 1.438(2) Å vs N(3)−C(16) 1.440(2) Å). The average Cl(2)axial−Fe-Xbasal (X = N1, N2, N3, Cl1) basal angle is 100.3° with the Fe atom 0.52 Å pushed out from the basal plane. In addition, the imine vectors are essentially coplanar with the adjacent pyridine ring, whereas the 2,6-dibenzhydryl-4tert-butylphenylimino group is close to perpendicular with respect to the chelate plane (dihedral angle 83.8°) and the inclination of the second N-aryl group is found to be smaller (dihedral angle 73.5°). Related structures containing unsymmetrical bis(imino)pyridines have been previously reported to display similar features.5e−k By a comparison of the FT IR spectra of the complexes Fe1− Fe6 with those of the free ligands L1−L6, it is evident that ν(CN) vibrations for complexes appeared at lower wavenumbers (1638−1650 cm−1 vs 1602−1607 cm−1), highlighting an effective coordination of sp2 nitrogen atoms with the metal center. 1H NMR spectroscopy was employed to characterize these paramagnetic complexes in solution (recorded in deuterated chloroform (CDCl3) at ambient temperature), and the assignment of characteristic peaks has been made through a comparison with data recorded for related Fe(II) complexes and integration and proximity to the paramagnetic center (Figures S1−S6).3c,5e A single peak for the m-pyridyl proton appeared at 79.78 ppm with a relative peak area of two hydrogen atoms in the spectrum of the symmetrical complex Fe6, while two peaks at different chemical shifts of ca. 79.16 (av) and 77.9 (av) ppm was found for the unsymmetrical complexes Fe1−Fe3 and Fe5. However, a slightly broadened peak for the m-pyridyl protons is evident at 77.31 ppm with the relative peak area of two hydrogens in the spectrum of Fe4. Moreover, a prominent singlet at ca. 2.79 (av) ppm can be ascribed to the tert-butyl protons. The microanalytical results are consistent with the chemical compositions proposed for the complexes. Catalyst Evaluation for Ethylene Polymerization. In order to explore the catalytic potential of the prepared iron complexes Fe1−Fe6 for ethylene polymerization, the reaction parameters such as reaction temperature, Al/Fe ratio, reaction time, and ethylene feed pressure were systematically optimized using the Fe1 precatalyst. For this, two cocatalysts, namely methyl aluminoxane (MAO) and modified MAO (MMAO; AlMeO/Al-i-BuO = 3/1), were employed to rationalize the best combination of alkylating ability of the cocatalyst and their effect on polymer properties such as molecular weights, molecular weight distributions, and chain ends.5 The molecular
Figure 1. ORTEP drawing of complex Fe3 with thermal ellipsoids at the 50% probability level. All hydrogen atoms are omitted for clarity.
Complex Fe3 has a five-coordinate structure in which the Fe center is surrounded by three nitrogen atoms belonging to the 2,6-(bisarylimino)pyridine and two chloride atoms, and the τ5 value is equal to 0.13, highlighting a modest distorted-squarepyramidal geometry. Nitrogen atoms (N1, N2, and N3) and the chloride atom (Cl1) form the basal square plane, and Cl(2) is positioned at the axial position with an Cl(2)−Fe(1) distance of 2.3152(7) Å. The N(1)−Fe(1), N(3)−Fe(1), and Cl(1)− Fe(1) distances are almost similar to each other (2.2160(16), 2.2198(15), and 2.2570(6) Å, respectively) but are greater than the N(2)−Fe(1) distance (2.0941(16) Å), indicating stronger coordination bonding between Npyridine and the central Fe metal, an observation also made for structurally related iron complexes.5e−k,16 The average Fe−Xbasal distance with a value of ca. 2.1967 Å is slightly shorter than the Fe−Claxial distance. This might be due to more free space around the axial chloride atom. Despite the fact that both N-aryl groups are much different in size, the imine bond lengths are the same and are typical of C
DOI: 10.1021/acs.organomet.7b00909 Organometallics XXXX, XXX, XXX−XXX
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Organometallics Table 1. Ethylene Polymerization by Fe1/MAOa entry
temp (°C)
time (min)
Al/Fe
yield (g)
activityb
Mwc
Mw/Mnc
Tmd (°C)
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20e 21f
30 40 50 60 70 80 90 100 110 80 80 80 80 80 80 80 80 80 80 80 80
30 30 30 30 30 30 30 30 30 30 30 30 30 30 05 10 15 45 60 30 30
2000 2000 2000 2000 2000 2000 2000 2000 2000 1000 1500 1750 2250 2500 2000 2000 2000 2000 2000 2000 2000
2.12 5.07 5.70 7.39 8.01 12.88 12.39 7.79 2.82 2.11 7.50 8.27 11.12 10.85 4.40 8.69 10.18 13.62 15.26 0.6 8.2
2.12 5.07 5.70 7.39 8.01 12.88 12.39 7.79 2.82 2.11 7.50 8.27 11.12 10.85 26.51 26.10 20.36 9.08 7.63 0.60 8.02
318.2 179.4 114.1 111.6 63.9 23.2 15.8 14.9 8.5 135.5 52.0 29.0 13.3 11.2 5.0 9.8 12.9 38.8 46.6 1.7 12.0
53.3 30.1 21.8 11.6 12.4 3.0 2.3 2.6 2.6 4.0 3.7 2.9 1.5 2.5 1.7 2.2 2.2 5.9 5.6 1.4 3.0
134.2 132.3 132.0 133.1 131.3 131.3 131.2 130.5 128.7 133.5 132.2 131.4 129.6 130.3 127.8 129.3 130.1 130.5 131.2 118.6 129.6
General conditions: 2.0 μmol of Fe1, 100 mL of toluene, 10 atm of ethylene. bIn units of 106 g of PE (mol of Fe)−1 h−1. cMw in units of kg mol−1. Mw and Mw/Mn were determined by GPC. dDetermined by DSC. e1 atm of ethylene. f5 atm of ethylene.
a
Figure 2. GPC traces (a) and activity and Mw vs different reaction temperatures (b) of the resultant polyethylenes obtained using Fe1/MAO (entries 1−9, Table 1).
Table 1, Figure 2). Above 80 °C, the catalytic efficiency is slightly reduced to 12.39 × 106 g of PE (mol of Fe)−1 h−1 at 90 °C (entry 7, Table 1), an observation that can be attributed to the partial deactivation of active species5,17 or/and lower solubility of ethylene at the higher temperature.5,18 Nevertheless, the activities dropped at elevated temperature (above 90 °C); still, a high activity of 2.82 × 106 g of PE (mol of Fe)−1 h−1 was maintained at 110 °C, highlighting the high thermal stability. It was even higher in comparison with related precatalysts.5e,g In contrast to the correlation of activity with reaction temperature, molecular weights of the resultant PE steadily drop with an increase in reaction temperature, which has precedent in the literature.5e−k,19 This can be accredited to the more facile chain transfer with respect to the chain propagation yielding lower molecular weight PE (Mw = 8.5− 318.2 kg/mol). As can be seen in Figure 2a, the resultant PE
weights and molecular weight distributions of the resultant PE were determined by gel permeation chromatography (GPC), and melt temperatures were examined by differential scanning calorimetry (DSC); furthermore, the microstructure of a representative sample of PE was established by high-temperature 1H/13C NMR. Optimization Conditions for Fe1/MAO. With a view to establish optimal reaction conditions, namely reaction temperature, Al/Fe ratio, reaction time, and ethylene feed pressure, and to study the effect of these parameters on PE properties, Fe1 was used as a test precatalyst in conjunction with MAO cocatalyst and data are collected in Table 1. When the reaction temperature was increased from 30 to 110 °C with the Al/Fe ratio fixed at 2000 and the reaction run time at 30 min, the activity gradually increased to a maximum level of 12.88 × 106 g of PE (mol of Fe)−1 h−1 at 80 °C (entries 1−9, D
DOI: 10.1021/acs.organomet.7b00909 Organometallics XXXX, XXX, XXX−XXX
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Organometallics
Figure 3. GPC traces (a) and activity and Mw vs different Al/Fe ratios (b) of the resultant polyethylenes obtained using Fe1/MAO (entries 6 and 10−14, Table 1).
Figure 4. GPC traces (a) and activity and Mw vs different run times (b) of the resultant polyethylenes obtained using Fe1/MAO (entries 6 and 15− 19, Table 1).
samples at different molar equivalents of MAO displayed unimodal and narrow molecular weight distribution. To evaluate the lifetime of active species at 80 °C, the duration of the reaction was varied between 5 and 60 min (entries 6 and 15−19, Table 1). Examination of the data reveals a peak in activity (26.51 × 106 g of PE (mol of Fe)−1 h−1) occurred after 5 min. Beyond this reaction time, the activities steadily decreased on prolonging the reaction time. It is suggested that a short time frame was required to generate all active species and then the onset of partial deactivation of active species occurred over the course of the reaction.9,10,21 Nevertheless, despite the partial decay of active species over the course of the reaction, this catalyst still maintains a high activity of 7.63 × 106 g of PE (mol of Fe)−1 h−1, highlighting the long lifetime of this precatalyst. Regarding the molecular weights of the resultant PE, there were sufficient active species over longer run times leading to longer chains and in turn a gradual increase in the molecular weight of the resultant polymeric material; these grew linearly with reaction time and reached a maximum value of 46.6 kg/mol after 60 min (see Figure 4). Using the optimal reaction conditions of the temperature fixed at 80 °C with an Al/Fe ratio of 2000 for 30 min run time, the effect of the lower pressure of ethylene was studied (entries
displayed bimodal and broad molecular weight distribution at lower temperatures (30−70 °C), which shifted to a unimodal and narrow molecular weight distribution at elevated temperature (80−110 °C), suggesting single-site catalytically active species. Furthermore, high values of the melt temperature (Tm = 128.7−134.2 °C) indicate the linear characteristics of the resultant PE, which was further analyzed by high-temperature 1 H/13C NMR (vide infra). Likewise, the effect of reaction temperature on catalytic performance and changes in the Al/Fe ratio displayed a profound effect on the catalytic activities and characteristic properties of the resultant PE. When the Al/Fe ratio was increased from 1000 to 2500 and the temperature was kept constant at 80 °C (entries 6 and 10−14, Table 1) again an Al/ Fe ratio of 2000 was found to be the best; a lower or higher ratio of cocatalyst showed lower activities. Notably, as the molar ratio of Al/Fe of MAO is increased from 1000 to 2500, molecular weights of PE almost linearly drop; this correlation is exhibited in Figure 3. These findings are consistent with the literature and can be ascribed to the high molar ratio of Al/Fe increasing the rate of chain transfer in comparison to chain propagation, leading to a high rate of chain termination to form lower molecular weight PE.5,9−11,20 However, all of the polymer E
DOI: 10.1021/acs.organomet.7b00909 Organometallics XXXX, XXX, XXX−XXX
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Organometallics
dramatically lower activity of 2.55 × 106 g of PE (mol of Fe)−1 h−1. The overall activities follow the order Fe2 (2,6di(Et)) > Fe1 (2,6-di(Me)) > Fe4 (2,4,6-tri(Me)) > Fe5 (2,6di(Et)-4-Me) > Fe3 (2,6-di(iPr)). Fe6 was found to be essentially inactive, producing a trace amount of PE. This inactivity of Fe6 has precedent with a previously reported analogue.5e,g,22 The certain reason for this inactivity is unknown and may potentially be due to too great a steric hindrance that to a certain degree causes instabilities of the MAO-activated Fe species. In contrast, Fe2 displays the highest activity of 13.87 × 106 g of PE (mol of Fe)−1 h−1 with 2 μmol precatalyst loading (entry 3, Table 2) and 14.01 × 106 g of PE (mol of Fe)−1 h−1 with 0.5 μmol precatalyst loading (Table S2). Furthermore, unimodal with narrow molecular weight distributions (Mw/Mw = 2.6−4.5) and high values of melting temperature (>130 °C) are characteristic of the resultant polyethylene. The latter property indicates the highly linear nature of the polymer, which was further confirmed by employing 1H/13C NMR spectroscopy (vide infra). In comparison with Brookhart/Gibson’s classical catalyst Fe0 as well as other structurally related iron complexes (D5e and E,5g Chart 3), the current precatalysts Fe1−Fe5 display comparable catalytic performance. Under identical conditions, Fe0 precatalyst showed a good activity of 5.70 × 106 g of PE (mol of Fe)−1 h−1 but 2-fold lower activity in comparison with the precatalysts Fe1, Fe2, Fe4, and Fe5 and higher in comparison with Fe3 (entry 1 vs entries 2−6, Table 2). We attribute this lower activity to the smaller size of N-aryl group, which is not crowded enough to shield the axial sites of the Fe active species, thereby giving a comparatively lower stability of active species. On the other hand, precatalysts D and E exhibited slightly higher activities under similar conditions: namely, MAO as cocatalyst and 10 atm of ethylene pressure (Chart 3). Likewise, lower molecular weight polyethylene was produced by Fe1−Fe5 (Mw = 13.9−33.4) in comparison with that reported for precatalyst D. The origin of this variation is uncertain but may be due to the different reaction temperature used for the reported precatalysts: 60 °C for a series of D type precatalysts and 80 °C for the current precatalysts Fe1−Fe5. This phenomenon was further seen when molecular weights of the polymer were compared with those for E type precatalysts, which showed similar values of Mw (11−200 kg/mol) under similar conditions. Optimization Conditions for Fe1/MMAO. In a manner similar to that described for the screening of Fe1/MAO catalyst, optimum conditions, namely reaction temperature, Al/ Fe ratio, reaction run time, and ethylene pressure, were
6, 20, and 21, Table 1). The results reveal that a high pressure of ethylene is necessary to achieve promising activities, which can be attributed to mass transport limitations of the monomer at this low ethylene concentration. For example, at 1 atm pressure of ethylene, a dramatically lower activity of 0.60 × 106 g of PE (mol of Fe)−1 h−1 was observed, yielding lower molecular weight PE with narrow molecular weight distributions (entry 20, Table 1). In contrast, polymerizations conducted at 5 atm showed higher activity and produced PE having a higher molecular weight and comparatively broad molecular weight distributions (Mw/Mn = 3.0). The molecular weights grow linearly with an increase in ethylene feed pressure, in accordance with more facile insertion and greater solubility of the ethylene monomer in the reaction solvent at high ethylene pressure (Figure S7). Ethylene Polymerization using Fe0−Fe6/MAO. On the basis of the catalytic performance of the Fe1/MAO system, the optimal conditions were identified as a reaction temperature of 80 °C, an Al/Fe ratio of 2000, and 30 min run time. To establish the correlation of structural variations in the precatalysts with their catalytic performance in ethylene polymerization, all of the remaining iron complexes (Fe2− Fe6) were also studied under these conditions. For comparison, the model iron precatalyst (Fe0) reported by Gibson was synthesized and investigated for ethylene polymerization using identical conditions; the collective results are compiled in Table 2. Table 2. Ethylene Polymerization using Fe0−Fe6/MAOa entry
precat.
yield (g)
activityb
Mwc
Mw/Mnc
Tmd (°C)
1 2 3 4 5 6 7
Fe0 Fe1 Fe2 Fe3 Fe4 Fe5 Fe6
5.70 12.88 13.87 2.55 11.61 10.26 trace
5.70 12.88 13.87 2.55 11.61 10.26 trace
38.5 23.2 13.9 33.4 18.8 17.3
8.3 3.0 2.6 4.5 2.9 3.4
130.0 131.3 130.3 131.6 132.1 130.6
General conditions: 2.0 μmol of Fe, 100 mL of toluene, 10 atm of ethylene, 30 min, 80 °C, and 2000 Al/Fe ratio. bIn units of 106 g of PE (mol of Fe)−1 h−1. cMw in units of kg mol−1. Mw and Mw/Mn were determined by GPC. dDetermined by DSC. a
Complexes Fe1, Fe2, Fe4, and Fe5 are highly active in ethylene polymerization, displaying activities in the range of (10.26−13.87) × 106 g of PE (mol of Fe)−1 h−1 (entries 2, 3, 5, and 6, Table 2, Figure S8). However, Fe3 displayed a
Chart 3. Comparison of the Mw, PDI, and Activity of Previously Reported Iron Precatalysts (Fe0,3 D,5e E,5g and Fe2) with MAO as Activator under Related Conditions
F
DOI: 10.1021/acs.organomet.7b00909 Organometallics XXXX, XXX, XXX−XXX
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Organometallics Table 3. Ethylene Polymerization by Fe1/MMAOa entry
temp (°C)
time (min)
Al/Fe
yield (g)
activityb
Mwc
Mw/Mnc
Tmd (°C)
1 2 3 4 5 6 7 8 9 10 12 13 14 15 16 17 18 19 20 21e 22f
30 40 50 60 70 80 90 100 80 80 80 80 80 80 80 80 80 80 80 80 80
30 30 30 30 30 30 30 30 30 30 30 30 30 30 05 10 15 45 60 30 30
2000 2000 2000 2000 2000 2000 2000 2000 1000 1500 2500 2750 3000 3250 2750 2750 2750 2750 2750 2750 2750
1.50 8.90 12.01 12.39 12.84 13.08 5.28 2.97 2.11 10.36 13.68 14.08 11.21 9.47 5.96 6.97 7.66 14.80 15.08 0.7 8.10
1.50 8.90 12.01 12.39 12.84 13.08 5.28 2.97 2.11 10.36 13.68 14.08 11.21 9.47 35.06 21.12 15.32 9.86 7.54 0.7 8.10
12.6 9.0 7.7 7.7 7.3 7.2 3.4 2.1 41.6 22.3 7.1 6.7 5.2 4.3 3.2 3.6 4.2 6.8 9.3 0.6 4.2
13.3 5.3 1.9 1.7 1.7 1.7 1.8 1.5 7.6 8.2 1.9 2.0 1.7 2.3 1.5 1.5 1.6 1.8 2.3 1.1 1.8
126.7 123.7 128.9 128.4 129.1 130.4 124.8 119.8 129.4 131.2 129.2 129.0 127.8 126.7 125.8 124.9 126.5 128.1 129.0 125.1 125.2
General conditions: 2.0 μmol of Fe1, 100 mL of toluene, 10 atm of ethylene. bIn units of 106 g of PE (mol of Fe)−1 h−1. cMw in units of kg mol−1. Mw and Mw/Mn were determined by GPC. dDetermined by DSC. e1 atm of ethylene. f5 atm of ethylene.
a
Figure 5. GPC traces (a) and activity and Mw vs different reaction temperatures (b) of the resultant polyethylenes obtained using Fe1/MMAO (entries 1−8, Table 3).
temperature (30 and 40 °C). Indeed, the polydispersity index becomes narrow and unimodal as the high-molecular-weight fraction disappeared at an elevated temperature of 100 °C (Figure 5). Furthermore, the values of the melting temperature of the resultant polyethylene are slightly lower (Tm = ≤ 130 °C) than what was observed for the polymer obtained using the Fe1/MAO system. When the Al/Fe ratio was varied from 1000 to 3250 with the temperature fixed at 80 °C and a reaction duration of 30 min (entries 6 and 9−15, Table 3), activities initially increased to the highest value of 14.08 × 106 g of PE (mol of Fe)−1 h−1 with an Al/Fe ratio of 2750 and then consistently decreased to 9.47 × 106 g of PE (mol of Fe)−1 h−1 with an Al/Fe ratio of 3250. It was also found that the molecular weight of polyethylene drastically decreased from 41.6 to 4.3 kg/mol as the Al/Fe ratio was increased; this observation has precedent with a previously reported analogous system and can be ascribed to rapid
established for Fe1 using MMAO cocatalyst and data are collected in Table 3. Typically runs were carried out in toluene under 10 atm of ethylene with the Al/Fe ratio fixed at 2000 and the reaction temperature varied from 30 to 100 (entries 1−8, Table 3). The maximum level of activity was 13.08 × 106 g of PE (mol of Fe)−1 h−1 at 80 °C (entry 6, Table 3). Interestingly, changes in the reaction temperature from 50 to 80 °C showed a slight increase in productivity ((12.01−13.08) × 106 g of PE (mol of Fe)−1 h−1), indicating the high thermal stability of active species over a large range of reaction temperature (entries 3−6, Table 3). On the other hand, the molecular weight steadily decreased as a function of increasing reaction temperature and reached a minimum value of 2.1 kg/mol at 100 °C, an observation consistent with more rapid chain termination at elevated temperature.19 Likewise in the case of the Fe1/MAO system, the molecular weight distribution of the resultant polyethylene was broad and bimodal at lower G
DOI: 10.1021/acs.organomet.7b00909 Organometallics XXXX, XXX, XXX−XXX
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Organometallics
Figure 6. GPC traces (a) and activity and Mw vs different Al/Fe ratios (b) of the resultant polyethylenes obtained using Fe1/MMAO (entries 6 and 9−15, Table 3).
Figure 7. GPC traces (a) and activity and Mw vs different run times (b) of the resultant polyethylenes obtained using Fe1/MMAO (entries 6 and 16−20, Table 3).
relatively narrow (Mw/Mn = 1.5−2.3) over the different run times, consistent with an active species displaying single-site behavior. When the ethylene pressure was lowered from 10 to 5 atm, a lower activity of 8.7 × 106 g of PE (mol of Fe)−1 h−1 and molecular weight of 4.2 kg/mol were observed (entry 21, Table 3). At 1 atm, the activity and molecular weight further decreased to 0.8 × 106 g of PE (mol of Fe)−1 h−1 and 0.6 kg/ mol, respectively (entry 22, Table 3). The linear relationship of activity and molecular weight with the ethylene pressure can be seen in Figure S9. Ethylene Polymerization using Fe0−Fe6/MMAO. Using the optimum conditions established for the Fe1/ MMAO system (Al/Fe ratio 2750, 80 °C, run time 30 min, 10 atm ethylene), the remaining precatalysts Fe0 and Fe2−Fe6 were employed for ethylene polymerization (entries 2−6, Table 4, Figure S10). According to the data in Table 4, precatalyst Fe1 displayed marked activity of 14.08 × 106 g of PE (mol of Fe)−1 h−1, which is the highest value in the title complexes activated with either MAO or MMAO cocatalysts. In contrast, Fe0 was found to be almost inactive, yielding a trace amount of polymer; this observation is consistent with that seen in MAObased polymerization. The overall activity decreased in the order Fe1 (2,6-di(Me)) > Fe4 (2,4,6-tri(Me)) > Fe2 (2,6di(Et)) > Fe5 (2,6-di(Et)-4-Me) > Fe3 (2,6-di(iPr)). A clear
polymer chain transfer and termination to a lower molecular weight polymer.5e−k,20 Meanwhile, the molecular weight distribution also varied from broad to narrow and was unimodal on an increase in the Al/Fe ratio, as can be seen by the GPC traces in Figure 6. The lifetime of the active species of Fe1/MMAO was determined by carrying out polymerization over 5, 10, 15, 30, 45, and 60 min with the Al/Fe ratio fixed at 2750 and the temperature at 80 °C (entries 6 and 16−20, Table 3). After careful inspection of the data, it was revealed that the activity decreased and the molecular weight of the resultant polyethylene increased on prolonging the run time. A peak in activity of 35.06 × 106 g of PE (mol of Fe)−1 h−1 was found over 5 min, which is almost 3 times higher than that observed after 30 min (entry 16 vs entry 6, Table 3), which highlighted that a short period was required to generate all active species and onset of deactivation over the course of the reaction.21 Despite this apparent catalyst deactivation, still a high activity was maintained over 1 h run time, showing the long lifetime of active species at elevated temperature. Another striking advantage is that a high molecular weight of 9.3 kg/mol could be obtained over this prolonged reaction time (Figure 7). These findings are consistent with those in the literature.5e,g Furthermore, the molecular weight distribution remained H
DOI: 10.1021/acs.organomet.7b00909 Organometallics XXXX, XXX, XXX−XXX
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Organometallics Table 4. Ethylene Polymerization using Fe0−Fe6/MMAOa entry
precat.
yield (g)
activityb
Mwc
Mw/Mnc
Tmd (°C)
1 2 3 4 5 6 7
Fe0 Fe1 Fe2 Fe3 Fe4 Fe5 Fe6
3.18 14.08 10.74 6.12 11.50 9.52 trace
3.18 14.08 10.74 6.12 11.50 9.52 trace
4.4 6.7 6.6 6.0 5.6 5.1
3.1 2.0 2.0 2.1 1.8 1.8
124.3 129.0 127.6 127.5 127.8 127.8
protons, respectively, were unambiguously assigned to the vinyl end group (−CHCH2). Further confirmation was carried out via 13C NMR spectra (Figure 9), in which two distinct peaks at 114.4 and 139.6 ppm characteristically belong to the vinylenic carbons located at the end of the polymer chain.11b,23 In addition, three singlets at 14.2, 22.9, and 32.2 ppm in 13C NMR spectra can be assigned to the n-propyl end groups. It is found that the amounts of saturated chain ends are slightly higher than those of unsaturated chain ends (n-propyl/vinyl = 0.66/ 0.44) according to the areas of characteristic peaks for methyl (−CH3) and methylene carbons. Given the presence of npropyl and vinyl end groups, most likely polymer chain termination occurred via chain transfer to aluminum species and β-H elimination. However, the chain transfer mechanism seems to be dominating over β-H elimination during chain termination. To further explore the polymer chain termination, polyethylene samples obtained using Fe1/MMAO at 80 and 100 °C (entries 6 and 8, Table 3) were also inspected by hightemperature 1H and 13C NMR spectroscopy. The chemical shifts of all the peaks appearing in the 1H and 13C spectra of Fe1/MMAO-based polymer at 80 °C are very similar to those of the polyethylene obtained using the Fe1/MAO system (Figures S11 and S12); however, a comparatively small amount of vinyl end group was observed; the ratio of vinyl and n-propyl groups is found to be 0.23/0.77. The absence of peaks corresponding to isobutyl end groups excludes chain transfer and termination to Al(i-Bu)3 present in the MMAO cocatalyst. Most likely polymer chain termination occurred through β-H elimination as well as chain transfer to Al(Me)3. In addition to the peaks of the vinyl and n-propyl end groups, additional peaks at 22.9, 27.7, 28.3, and 39.5 ppm can be ascribed to isobutyl end groups appearing in the 13C NMR spectrum of Fe1/ MMAO-based polyethylene prepared at 100 °C. There are some variations in the relative end groups present: isobutyl/npropyl/vinyl end groups are found in the ratio 0.49/0.33/0.18 (determined by integration of relative peaks). Therefore, it would appear that the polymer contains high amounts of both saturated chain ends (isobutyl and n-propyl) formed through chain transfer to alkylaluminum species and their derivatives present in MMAO. 1H and 13C NMR spectra of polyethylene obtained using the Fe1/MMAO catalytic system can be seen in Figures 10 and 11, respectively.
General conditions: 2.0 μmol of Fe, 100 mL of toluene, 10 atm of ethylene, 30 min, 80 °C, and Al/Fe ratio 2750. bIn units of 106 g of PE (mol of Fe)−1 h−1. cMw in units of kg mol−1. Mw and Mw/Mn were determined by GPC. dDetermined by DSC. a
relation of activity with both steric and electronic effects imparted by the second N-aryl imine was observed. Typically, less steric hindrance is a dominant factor in improving the activities, as Fe3 is the most hindered precatalyst and shows the lowest activity of 6.12 × 106 g of PE (mol of Fe)−1 h−1 (entry 4, Table 4). Indeed, all complexes displayed higher activities; even Fe1 is approximately 5 times more productive than Fe0 precatalysts under identical conditions (entry 1, Table 4). Meanwhile, the molecular weight of the resultant polyethylene was found to fall in the range of 5.1−6.7 kg/mol. In comparison with previously reported complexes (D, Chart 3)5e in some cases the values of Mw are slightly higher under similar conditions. However, molecular weights were lower than those obtained with complex E (Chart 3)5g because a lower optimal temperature was used for polymerization (60 °C). Notably, all of the polyethylenes obtained display characteristically narrow molecular weight distribution (Mw/Mn = 1.8−3.1) and high melting temperatures in the range of 124.3−129.0 °C. The former characteristic highlights single-site catalytic behavior and the latter is the property of highly linear polymeric materials. Microstructural Properties of Polyethylenes. The microstructural properties of polyethylene obtained using Fe1/MAO at 110 °C (entry 9, Table 1) were studied by their high-temperature 1H and 13C NMR spectra (recorded in 1,1,2,2-[D2]tetrachloroethane at 135 °C) and interpreted according to the assignments listed in the literature. In the 1 H NMR spectra (Figure 8), two multiplets centered around 5.90 and 5.00 ppm with relative peak integrations of 1 and 2
Figure 8. 1H NMR spectrum of the polyethylene obtained using Fe1/MAO (entry 9, Table 1). I
DOI: 10.1021/acs.organomet.7b00909 Organometallics XXXX, XXX, XXX−XXX
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Organometallics
Figure 9. 13C NMR spectrum of the polyethylene obtained using Fe1/MAO (entry 9, Table 1).
Figure 10. 1H NMR spectrum of the polyethylene obtained using Fe1/MMAO at 100 °C (entry 8, Table 3).
Figure 11. 13C NMR spectrum of the polyethylene obtained using Fe1/MMAO at100 °C (entry 8, Table 3).
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CONCLUSIONS
(arylimino)ethyl)pyridine ligands have been successfully prepared and well characterized by a range of techniques including FT IR and 1H NMR spectroscopy, elemental analysis, and X-ray diffraction. On activation with either MAO or MMAO cocatalysts, all complexes displayed high activities in
A series of sterically and electronically enhanced Fe(II) chloride complexes ligated with symmetrical 2,6-bis(1-(2,6-dibenzhydryl-4-tert-butylphenylimino)ethyl)pyridine and unsymmetrical 2(1-(2,6-dibenzhydryl-4-tert-butylphenylimino)ethyl)-6-(1J
DOI: 10.1021/acs.organomet.7b00909 Organometallics XXXX, XXX, XXX−XXX
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Organometallics
(s), 1309 (w), 1237 (s), 1192 (w), 1116 (s), 1077 (s), 1029 (m), 997 (w), 949 (m), 892 (w), 867 (w), 814 (m), 765 (m), 739 (s). 1H NMR (400 MHz, CDCl3. TMS): δ 8.14 (d, J = 8.0 Hz, 1H, Py-Hm), 8.07 (d, J = 7.6 Hz, 1H, Py-Hm), 7.85 (t, J = 7.6 Hz, 1H, Py-Hp), 7.25−7.15 (m, 12H, aryl-H), 7.01 (t, J = 8.0 Hz, 8H, aryl-H), 6.89 (s, 2H, aryl-H), 5.26 (s, 2H, CHPh2), 2.67 (s, 3H, OCCH3), 1.11 (m, 9H, C(CH3)3), 1.10(m, 3H, NCCH3). 13C NMR (100 MHz, CDCl3. TMS): δ 200.3, 169.1, 155.5, 152.2, 145.7, 145.0, 143.7, 142.8, 137.0, 131.4, 129.8, 129.4, 128.2, 127.9, 126.1, 126.0, 125.1, 124.6, 122.2, 52.4, 34.3, 31.3, 25.6, 16.7. Anal. Calcd for C45H42N2O (626.84): C, 86.22; H, 6.75; N, 4.47. Found: C, 86.23; H, 6.98; N, 4.29. 2-(1-(2,6-Dibenzhydryl-4-tert-butylphenylimino)ethyl)-6-(1(2,6-dimethylphenylimino)ethyl)pyridine (L1). A solution of 2acetyl-6-[1-(2,6-dibenzhydryl-4-tert-butylphenylimino)ethyl]pyridine (2.00 g, 3.19 mmol) and 2,6-dimethylaniline (0.42 g, 3.50 mmol) in toluene (50 mL) was stirred and heated to reflux. After 1/2 h, a catalytic amount of p-toluenesulfonic acid was added and further refluxed for 6 h. On completion of the reaction (checked by silica TLC), the reaction mixture was cooled to room temperature and filtered and volatiles were removed using a rotary evaporator. The residual solid was purified by basic alumina column chromatography using petroleum ether/ethyl acetate (125/1 v/v) as an eluent, affording L1 as a light yellow powder (0.60 g, 45%). Mp: 142−144 °C. FTIR (KBr, cm−1): 3026 (w), 2957 (w), 1645 (ν(CN), s), 1570 (w), 1494 (m), 1451 (m), 1365 (s), 1326 (w), 1296 (w), 1245 (s), 1207 (m), 1124 (m), 1030 (m), 914 (w), 820 (m), 763 (s). 1H NMR (400 MHz, CDCl3, TMS): δ 8.39 (d, J = 7.6 Hz, 1H, Py-Hm), 8.04 (d, J = 7.6 Hz, 1H, Py-Hm), 7.82 (t, J = 8.0 Hz, 1H, Py-Hp), 7.25−7.07 (m, 15H, aryl-H), 7.03 (t, J = 7.2 Hz, 8H, aryl-H), 6.90 (s, 2H, aryl-H), 5.30 (s, 2H, CHPh2), 2.12 (s, 3H, NCCH3), 2.06 (s, 6H, 2 × CH3), 1.14 (s, 3H, NCCH3), 1.11(s, 9H, C(CH3)3). 13C NMR (100 MHz, CDCl3. TMS): δ 169.7, 167.4, 155.2, 154.9, 148.8, 145.8, 144.8, 143.8, 142.9, 136.6, 131.4, 129.8, 129.4, 128.2, 127.9, 127.9, 126.0, 125.9, 125.4, 125.0, 123.0, 122.3, 121.8, 52.3, 34.3, 31.3, 18.0, 17.0, 16.4. Anal. Calcd for C53H51N3 (730.01): C, 87.20; H, 7.04; N, 5.76. Found: C, 87.04; H, 7.15; N, 5.58. 2-(1-(2,6-Dibenzhydryl-4-tert-butylphenylimino)ethyl)-6-(1(2,6-diethylphenylimino)ethyl)pyridine (L2). On the basis of the procedure and molar ratio that were used for the synthesis of L1, L2 was prepared as a light yellow powder (1.02 g, 42%). Mp: 138−140 °C. FTIR (KBr, cm−1): 3027 (w), 2960 (w), 1638 (ν(CN), s), 1566 (w), 1494 (w), 1447 (s), 1365 (s), 1320 (w), 1294 (w), 1237 (s), 1191 (m), 1117 (s), 1029 (w), 865 (w), 823 (m), 761 (s). 1H NMR (400 MHz, CDCl3, TMS): δ 8.38 (d, J = 7.6 Hz, 1H, Py-Hm), 8.04 (d, J = 7.6 Hz, 1H, Py-Hm), 7.82 (t, J = 8.0 Hz, 1H, Py-Hp), 7.26−7.22 (m, 5H, aryl-H), 7.19−7.11 (m, 10H, aryl-H), 7.03 (t, J = 6.8 Hz, 8H, arylH), 6.90 (s, 2H, aryl-H), 5.30 (s, 2H, CHPh2), 2.48−2.31 (s, 4H, 2 × CH2), 2.13 (s, 3H, NCCH3), 1.18−1.14 (m, 9H, 2 × CH3, N CCH3), 1.11(s, 9H, C(CH3)3). 13C NMR (100 MHz, CDCl3, TMS): δ 169.58, 167.05, 155.13, 154.86, 147.77, 145.84, 144.73, 143.79, 142.89, 136.52, 131.36, 131.13, 129.79, 129.37, 128.11, 127.85, 125.94, 125.87, 125.85, 124.98, 123.20, 122.19, 121.74, 53.32, 52.26, 34.21, 31.28, 24.51, 16.85, 16.70, 13.66. Anal. Calcd for C55H55N3.EtOH (804.14): C, 85.14; H, 7.65; N, 5.23. Found: C, 85.46; H, 7.31; N, 40. 2-(1-(2,6-Dibenzhydryl-4-tert-butylphenylimino)ethyl)-6-(1(2,6-diisopropylphenylimino)ethyl)pyridine (L3). On the basis of the procedure and molar ratio that were used for the synthesis of L1, L3 was prepared as a light yellow powder (1.00 g, 40%). Mp: 204−206 °C. FTIR (KBr, cm−1): 3061 (w), 2960 (s), 1650 (ν(CN), s), 1570 (w), 1494 (m), 1450 (s), 1363 (s), 1326 (m), 1294 (m), 1241 (s), 1190 (s), 1123 (s), 1079 (m), 1033 (m), 817 (m), 764 (m). 1H NMR (400 MHz, CDCl3, TMS): δ 8.38 (d, J = 7.6 Hz, 1H, Py-Hm), 8.04 (d, J = 7.2 Hz, 1H, Py-Hm), 7.83 (t, J = 8.0 Hz, 1H, Py-Hp), 7.26−7.08 (m, 15H, aryl-H), 7.03 (t, J = 7.2 Hz, 8H, aryl-H), 6.90 (s, 2H, aryl-H), 5.31 (s, 2H, CHPh2), 2.84−2.73 (m, 2H, CHMe2), 2.15 (s, 3H, N CCH3), 1.19 (s, 6H, 2 × CH3), 1.17 (s, 9H, 2 × CH3, NCCH3), 1.11(s, 9H, C(CH3)3). 13C NMR (100 MHz, CDCl3, TMS): δ 169.7, 167.2, 155.2, 154.9, 146.5, 145.9, 144.8, 143.8, 142.9, 136.6, 135.8, 131.4, 129.8, 129.4, 128.2, 128.0, 126.0, 125.9, 125.0, 123.5, 123.0, 122.2, 121.8, 52.3, 34.3, 31.3, 28.3, 23.2, 23.0, 17.1, 16.9. Anal. Calcd
comparison to the benchmark precatalyst 2,6-bis(1-(2,6diisopropyl)ethyl)pyridyliron chloride (Fe0). What is more remarkable is the high thermal stabilities of title complexes at elevated temperature ranging between 80 and 110 °C. At 80 °C, a peak in activity of 14.01 × 106 g of PE (mol of Fe)−1 h−1 was obtained, which is almost 5 times higher than that of Brookhart/Gibosn’s classical iron precatalyst (Fe0) under identical conditions. Furthermore, Fe1/MAO-based polyethylene is highly linear with saturated and vinyl unsaturated end groups, and the ratio of vinyl and n-propyl end groups is 0.66/0.44. In contrast, polyethylene prepared with Fe1/ MMAO cocatalyst was terminated with saturated (n-propyl and isobutyl) and unsaturated vinyl terminal groups. The relative isobutyl/n-propyl/vinyl end group ratio is 0.49/0.33/ 0.18. Given the presence of saturated (either n-propyl or isobutyl) and unsaturated end groups, it would appear that these catalyst systems undergo termination by different mechanisms. The former end groups would be formed by chain transfer to aluminum species (AlR3, R = Me, i-Bu) and their derivatives (e.g., i-Bu2AlMe) present in MMAO. Vinyl end groups are generated by a β-H elimination mechanism of chain termination.
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EXPERIMENTAL SECTION
General Considerations. The synthesis and handling of air- and/ or moisture-sensitive compounds were carried out under an atmosphere of nitrogen using standard Schlenk techniques. For polymerization, freshly distilled toluene solution was used which was dried over sodium for approximately 10 h and distilled under a nitrogen atmosphere. Methylaluminoxane (MAO, 1.46 M in toluene) and modified methylaluminoxane (MMAO, 1.93 M in heptane) were purchased from Akzo Nobel Corp. High-purity ethylene was purchased from Beijing Yanshan Petrochemical Company and used as received. Other reagents were purchased from Aldrich, Acros, or local suppliers. 2,6-Dibenzhydryl-4-tert-butylaniline is not commercially available and was synthesized using a procedure reported elswhere.24 1H and 13C NMR spectroscopic measurements for the organic compounds as well as iron complexes (only 1H NMR) were performed on Bruker DMX 400 and 600 MHz instruments at room temperature using TMS as an internal standard. Chemical shifts and coupling constants are given in ppm and in Hz, respectively. C, H, and N analyses were performed on a Flash EA 1112 microanalyzer. FT-IR spectra were obtained using a PerkinElmer System 2000 FT-IR spectrometer. Molecular weights (Mw) and molecular weight distributions (MWD) of the polyethylenes were determined using a PL-GPC220 instrument at 150 °C with 1,2,4-trichlorobenzene as the solvent. The thermograms of the polyethylenes were obtained from the second scanning run on a PerkinElmer TA-Q2000 DSC analyzer under a nitrogen atmosphere. In the procedure, a sample of about 4.0−6.0 mg was heated to 160 °C at a heating rate of 20 °C min−1 and kept for 5 min at 150 °C to remove the thermal history and then cooled at a rate of 20 °C min−1 to −20 °C. The 13C NMR spectra of the polyethylenes were recorded on a Bruker DMX 300 MHz instrument at 135 °C in deuterated 1,2-dichlorobenzene with TMS as an internal standard. 2-Acetyl-6-[1-(2,6-dibenzhydryl-4-tert-butylphenylimino)ethyl]pyridine (1). To a boiling solution of 2,6-diacetylpyridine (3.26 g, 20 mmol) and 2,6-dibenzhydryl-4-(tert-butyl)aniline (9.63 g, 20 mmol) in toluene (150 mL) was added a catalytic amount of ptoluenesulfonic acid. The reaction mixture was stirred and refluxed for 12 h using a Dean−Stark trap. After it was cooled to room temperature, the reaction mixture was filtered and solvents were evaporated under reduced pressure. The residue was purified by basic alumina using petroleum ether/ethyl acetate (100/1) as the eluent, yielding 1 as a light yellow solid (5.01 g, 40%). Mp: 212−214 °C. FTIR (KBr, cm−1): 3024 (w), 2957 (m), 1700 (ν(CO), s), 1650 (ν(CN), s), 1573 (w), 1493 (s), 1448 (s), 1364 (s), 1413 (m), 1363 K
DOI: 10.1021/acs.organomet.7b00909 Organometallics XXXX, XXX, XXX−XXX
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Organometallics
N), w), 1582 (s), 1494 (m), 1470 (m), 1446 (m), 1370 (s), 1264 (s), 1209 (s), 1030 (m), 810 (m), 773 (s), 742 (m), 700 (s). Anal. Calcd for C53H51N3FeCl2·EtOH (902.83): C, 73.17; H, 6.36; N, 4.65. Found: C, 73.33; H, 6.05; N, 4.72. 2-(1-(2,6-Dibenzhydryl-4-tert-butylphenylimino)ethyl)-6-(1(2,6-diethylphenylimino)ethyl)pyridyliron Chloride (Fe2). The preparation of Fe2 was carried out using a procedure and molar ratio similar to those described for the synthesis of Fe1, but L2 (167 mg, 0.22 mmol) was used instead of L1. A blue powder was obtained (170 mg, 96%). 1H NMR (600 MHz, CDCl3, TMS): δ 78.13 (s, 1H, PyHm), 77.46 (s, 1H, Py-Hm), 72.03 (s, 1H, Py-Hp), 14.74 (s, 2H, arylHm), 14.07 (s, 2H, aryl-Hm), 7.15 (s, 4H, aryl-H), 6.75 (s, 2H, aryl-H), 5.95 (s, 4H, aryl-H), 4.90 (s, 2H, aryl-H), 4.78 (s, 4H, aryl-H), 3.85 (s, 4H, 2 × CH2), 2.81 (s, 9H, C(CH3)3), −4.02 (s, 4H, aryl-H), −4.49 (s, 6H, 2 × CH3), −12.55 (s, 1H, aryl-Hp), −14.60 (s, 2H, CHPh2), −29.35 (s, 3H, NCCH3), −41.59 (s, 3H, NCCH3). FTIR (KBr; cm−1): 3026 (w), 2966 (m), 1602 (ν(CN), w), 1577 (m), 1495 (w), 1447 (s), 1373 (s), 1314 (w), 1266 (s), 1205 (s), 1112 (m), 1079 (w), 1029 (s), 807 (s), 769 (m), 741 (s), 700 (s). Anal. Calcd for C55H55N3FeCl2·EtOH (930.88): C, 73.55; H, 6.61; N, 4.51. Found: C, 73.95; H, 6.30; N, 4.56. 2-(1-(2,6-Dibenzhydryl-4-tert-butylphenylimino)ethyl)-6-(1(2,6-diisopropylphenylimino)ethyl)pyridyliron Chloride (Fe3). The preparation of Fe3 was carried out using a procedure and molar ratio similar to those described for the synthesis of Fe1, but L3 (173 mg, 0.22 mmol) was used instead of L1. A blue powder was obtained (122 mg, 67%). 1H NMR (600 MHz, CDCl3, TMS): δ 81.83 (s, 1H, Py-Hm), 79.39 (s, 1H, Py-Hm), 76.53 (s, 1H, Py-Hp), 14.03 (s, 4H, aryl-Hm), 7.32 (s, 4H, aryl-H), 6.89 (s, 2H, aryl-H), 6.18 (s, 4H, arylH), 4.66 (s, 2H, aryl-H), 4.52 (s, 4H, aryl-H), 2.85 (s, 9H, C(CH3)3), −4.21 (s, 6H, 2 × CH3), −5.12 (s, 4H, aryl-H), −6.40 (s, 6H, 2 × CH3), −12.22 (s, 1H, aryl-Hp), −15.28 (s, 2H, CHPh2), −17.61 (s, 2H, 2 × CH), −35.04 (s, 3H, NCCH3), −42.42 (s, 3H, NCCH3). FTIR (KBr; cm−1): 3024 (w), 2960 (m), 1605 (ν(CN), w), 1576 (m), 1494 (m), 1447 (s), 1368 (s), 1321 (w), 1270 (s), 1201 (m), 1103 (m), 1030 (s), 937 (m), 807 (m), 767 (s), 743 (s), 700 (s). Anal. Calcd for C57H59N3FeCl2·EtOH (958.93): C, 73.90; H, 6.83; N, 4.38. Found: C, 74.06; H, 6.52; N, 4.37. 2-(1-(2,6-Dibenzhydryl-4-tert-butylphenylimino)ethyl)-6-(1(mesitylimino)ethyl)pyridyliron Chloride (Fe4). The preparation of Fe4 was carried out using a procedure and molar ratio similar to those described for the synthesis of Fe1, but L4 (164 mg, 0.22 mmol) was used instead of L1. A blue powder was obtained (155 mg, 89%). 1 H NMR (600 MHz, CDCl3, TMS): δ 77.31 (s, 2H, Py-Hm), 70.06 (s, 1H, Py-Hp), 23.08 (s, 3H, CH3), 14.06 (s, 2H, aryl-Hm), 13.91 (s, 2H, aryl-Hm), 9.51 (s, 6H, 2 × CH3), 7.12 (s, 4H, aryl-H), 6.83 (s, 2H, arylH), 5.73 (s, 4H, aryl-H), 4.87 (s, 2H, aryl-H), 4.81(s, 4H, aryl-H), 2.69 (s, 9H, C(CH3)3), −3.24 (s, 4H, aryl-H), −10.80 (s, 2H, CHPh2), −25.77 (s, 3H, NCCH3), −43.31 (s, 3H, NCCH3). FTIR (KBr; cm−1): 3024 (w), 2960 (m), 1607 (ν(CN), w), 1579 (m), 1475 (w), 1449 (s), 1370 (s), 1264 (s), 1218 (w), 1195 (s), 1078 (m), 1031 (s), 861 (s), 812 (m), 769 (m), 744 (s), 704 (s). Anal. Calcd for C54H53N3FeCl2·EtOH (916.85): C, 73.36; H, 6.49; N, 4.58. Found: C, 73.07; H, 6.09; N, 4.63. 2-(1-(2,6-Dibenzhydryl-4-tert-butylphenylimino)ethyl)-6-(1(2,6-diethyl-4-methylphenylimino)ethyl)pyridyliron Chloride (Fe5). The preparation of Fe5 was carried out using a procedure and molar ratio similar to those described for the synthesis of Fe1, but L5 (170 mg, 0.22 mmol) was used instead of L1. A blue powder was obtained (160 mg, 89%). 1H NMR (600 MHz, CDCl3, TMS): δ 78.26 (s, 1H, Py-Hm), 76.79 (s, 1H, Py-Hm), 76.32 (s, 1H, Py-Hp), 23.07 (s, 3H, CH3), 14.26 (s, 2H, aryl-Hm), 13.75 (s, 2H, aryl-Hm), 7.26 (s, 4H, aryl-H), 6.85 (s, 2H, aryl-H), 6.19 (s, 4H, aryl-H), 4.74 (s, 2H, aryl-H), 4.65 (s, 4H, aryl-H), 4.32 (s, 2H, CH2), 3.82 (s, 2H, CH2), 2.92 (s, 9H, C(CH3)3), −4.20 (s, 4H, aryl-H), −4.94 (s, 6H, 2 × CH3), −14.23 (s, 2H, CHPh2), −31.98 (s, 3H, NCCH3), −41.22 (s, 3H, NCCH3). FTIR (KBr; cm−1): 3029 (w), 2963 (s), 1605 (ν(CN), w), 1582 (m), 1495 (m), 1449 (s), 1425 (w), 1369 (s), 1264 (s), 1214 (m), 1075 (m), 1033 (s), 860 (s), 808 (m), 770 (m), 745 (s), 704 (s). Anal.
for C57H59N3 (786.12): C, 87.09; H, 7.57; N, 5.35. Found: C, 87.16; H, 7.67; N, 5.32. 2-(1-(2,6-Dibenzhydryl-4-tert-butylphenylimino)ethyl)-6-(1(mesitylphenylimino)ethyl)pyridine (L4). On the basis of the procedure and molar ratio that were used for the synthesis of L1, L4 was prepared as a light yellow powder (0.71 g, 30%). Mp: 146−148 °C. FTIR (KBr, cm−1): 3026 (w), 2960 (w), 1650 (ν(CN), s), 1573 (w), 1494 (w), 1450 (m), 1364 (s), 1325 (w), 1296 (w), 1245 (m), 1219 (s), 1150 (w), 1123 (s), 1079 (s), 1030 (m), 855 (m), 822 (m), 768 (m). 1H NMR (400 MHz, CDCl3, TMS): δ 8.39 (d, J = 7.6 Hz, 1H, Py-Hm), 8.04 (d, J = 7.2 Hz, 1H, Py-Hm), 7.81 (t, J = 7.6 Hz, 1H, Py-Hp), 7.26−7.13 (m, 12H, aryl-H), 7.03 (t, J = 6.8 Hz, 8H, aryl-H), 6.90 (s, 4H, aryl-H), 5.31 (s, 2H, CHPh2), 2.31 (s, 3H, CH3),, 2.12 (s, 3H, NCCH3), 2.03 (s, 6H, 2 × CH3), 1.16 (s, 3H, NCCH3), 1.11(s, 9H, C(CH3)3). 13C NMR (100 MHz, CDCl3, TMS): δ 169.6, 167.5, 155.1, 155.0, 146.2, 145.8, 144.7, 143.8, 142.9, 136.5, 132.1, 131.4, 131.3, 129.8, 129.4, 129.3, 128.5, 128.1, 127.9, 126.0, 126.0, 125.9, 125.2, 125.0, 122.2, 121.8, 52.3, 34.2, 31.3, 20.7, 17.8, 16.3. Anal. Calcd for C54H53N3 (744.04): C, 87.17; H, 7.18; N, 5.65. Found: C, 86.88; H, 7.53; N, 5.32. 2-(1-(2,6-Dibenzhydryl-4-tert-butylphenylimino)ethyl)-6-(1(2,6-diethyl-4-methylphenylimino)ethyl)pyridine (L5). On the basis of the procedure and molar ratio that were used for the synthesis of L1, L5 was prepared as a light yellow powder (0.86 g, 35%). Mp: 176−178 °C. FTIR (KBr, cm−1): 3028 (w), 2961 (m), 1640 (ν(C N), s), 1568 (m), 1494 (m), 1451 (s), 1362 (s), 1322 (w), 1294 (w), 1242 (m), 1209 (w), 1117 (s), 1077 (s), 1030 (m), 862 (m), 825 (m), 766 (m), 740 (s). 1H NMR (400 MHz, CDCl3, TMS): δ 8.39 (d, J = 8.0 Hz, 1H, Py-Hm), 8.04 (d, J = 7.6 Hz, 1H, Py-Hm), 7.81 (t, J = 8.0 Hz, 1H, Py-Hp), 7.26−7.13 (m, 12H, aryl-H), 7.03 (t, J = 7.2 Hz, 8H, aryl-H), 6.94 (s, 2H, aryl-H), 6.91 (s, 2H, aryl-H), 5.31 (s, 2H, CHPh2), 2.45−2.25 (m, 7H, 2 × CH2, CH3), 2.13 (s, 3H, NCCH3), 1.17−1.13 (m, 9H, 2 × CH3, NCCH3), 1.11(s, 9H, C(CH3)3). 13C NMR (100 MHz, CDCl3, TMS): δ 169.6, 167.2, 155.1, 155.0, 145.8, 145.2, 144.7, 143.8, 142.9, 136.5, 132.3, 131.4, 131.0, 129.8, 129.4, 128.1, 127.8, 126.6, 125.9, 125.9, 125.0, 122.1, 121.7, 52.2, 34.2, 31.2, 24.5, 20.9, 16.9, 16.6, 13.8. Anal. Calcd for C56H57N3 (772.09): C, 87.12; H, 7.44; N, 5.44. Found: C, 87.13; H, 7.49; N, 5.41. 2,6-Bis(1-(2,6-dibenzhydryl-4-tert-butylphenylimino)ethyl)pyridine (L6). During the purification process of the ligand L1 by basic alumina using petroleum ether/ethyl acetate (200/1) as the eluent, L6 was also collected as a light yellow solid (2.62 g, 12%). Mp: 208−210 °C. FTIR (KBr, cm−1): 3025 (w), 2955 (m), 1639 (ν(C N), s), 1599 (w), 1493 (s), 1448 (m), 1417 (w), 1363 (s), 1323 (w), 1242 (s), 1186 (m), 1117 (s), 1076 (m), 1029 (m), 915 (w), 866 (w), 825 (m), 767 (m), 741 (s). 1H NMR (400 MHz, CDCl3, TMS): δ 8.07 (d, J = 7.6 Hz, 2H, 2 × Py-Hm), 7.75 (t, J = 7.6 Hz, 1H, Py-Hp), 7.23 (t, J = 7.2 Hz, 8H, aryl-H), 7.18−7.08 (m, 16H, aryl-H), 7.04− 6.99 (m, 16H, aryl-H), 6.90 (s, 4H, aryl-H), 5.30 (s, 4H, 2 × CHPh2), 1.11 (s, 18H, 2 × C(CH3)3), 0.94 (s, 6H, 2 × NCCH3). 13C NMR (100 MHz, CDCl3, TMS): δ 169.9, 154.9, 144.7, 143.9, 142.8, 131.4, 129.8, 129.4, 128.1, 127.9, 125.9, 125.9, 125.0, 121.9, 52.2, 34.2, 31.3, 16.9. Anal. Calcd for C81H75N3 (1090.51): C, 89.21; H, 6.93; N, 3.85. Found: C, 88.85; H, 7.26; N, 3.80. 2-(1-(2,6-Dibenzhydryl-4-tert-butylphenylimino)ethyl)-6-(1(2,6-dimethylphenylimino)ethyl)pyridyliron Chloride (Fe1). To a solution of ligand L1 (161 mg, 0.22 mmol) in freshly distilled ethanol (10 mL) was added 1 equiv of FeCl2·4H2O (39.8 mg, 0.20 mmol) under a nitrogen atmosphere. Immediately, the color of the solution changed from yellow to blue. The reaction mixture was stirred for 8 h at room temperature to ensure the completion of the reaction. The precipitate was collected by filtration and washed with diethyl ether (3 × 5 mL), yielding Fe1 as a blue powder (158 mg, 92%). 1H NMR (600 MHz, CDCl3, TMS): δ 78.42 (s, 1H, Py-Hm), 78.03 (s, 1H, Py-Hm), 67.64 (s, 1H, Py-Hp), 14.93 (s, 2H, aryl-Hm), 13.71 (s, 2H, aryl-Hm), 9.19 (s. 6H, 2 × CH3), 7.05 (s, 4H, aryl-H), 6.77 (s, 2H, aryl-H), 5.47 (s, 4H, aryl-H), 4.98 (s, 2H, aryl-H), 4.92 (s, 4H, aryl-H), 2.66 (s, 9H, C(CH3)3), −3.16 (s, 4H, aryl-H), −11.44 (s, 2H, CHPh2), −13.46 (s, 1H, aryl-H), −23.49 (s, 3H, NCCH3), −44.35 (s, 3H, NCCH3). FTIR (KBr; cm−1): 3027 (w), 2961 (m), 1604 (ν(C L
DOI: 10.1021/acs.organomet.7b00909 Organometallics XXXX, XXX, XXX−XXX
Article
Organometallics Calcd for C56H57N3FeCl2·EtOH (944.91): C, 73.73; H, 6.72; N, 4.45. Found: C, 74.06; H, 6.36; N, 4.53. 2,6-Bis(1-(2,6-dibenzhydryl-4-tert-butylphenylimino)ethyl)pyridyliron Chloride (Fe6). The preparation of Fe6 was carried out using a procedure and molar ratio similar to those described for the synthesis of Fe1, but L6 (240 mg, 0.22 mmol) was used instead of L1. A blue powder was obtained (160 mg, 66%). 1H NMR (600 MHz, CDCl3, TMS): δ 79.78 (s, 2H, Py-Hm), 79.24 (s, 1H, Py-Hp), 13.04 (s, 4H, aryl-Hm), 6.49 (s, 8H, aryl-H), 6.42 (s, 6H, aryl-H), 4.80 (s, 4H, aryl-H), 4.52 (s, 8H, aryl-H), 3.28 (s, 6H, aryl-H), 2.36 (s, 18H, 2 × C(CH3)3), −4.84 (s, 8H, aryl-H), −19.44 (s, 4H, CHPh2), −33.06 (s, 3H, 2 × NCCH3). FTIR (KBr; cm−1): 3057 (w), 2962 (m), 1605 (ν(CN), w), 1581 (s), 1492 (m), 1446 (m), 1386 (m), 1264 (s), 1198 (s), 1144 (m), 1075 (m), 807 (m), 769 (m), 742 (s), 798 (s). Anal. Calcd for C81H75N3FeCl2·EtOH (1263.33): C, 78.91; H, 6.46; N, 3.33. Found: C, 79.16; H, 6.18; N, 3.56. X-ray Crystallographic Studies. An X-ray-quality crystal of complex Fe3 2as grown by the slow layering of a heptane solution on its dichloromethane solution at room temperature. X-ray determinations were carried out on a Rigaku Saturn 724+ CCD instrument with graphite-monochromated Mo Kα radiation (λ = 0.71073 Å) at 173(2) K; the cell parameters were obtained by global refinement of the positions of all collected reflections. Intensities were corrected for Lorentz and polarization effects and empirical absorption. The structure was solved by direct methods and refined by full-matrix least squares on F2. All hydrogen atoms were placed in calculated positions. Structure solution and refinement were performed by using the SHELXT (Sheldrick, 2015).25 The disorder displayed by the tertbutyl group and solvent molecule was also processed by the SHELXL97 software. Details of the crystal data and structure refinements for Fe3 are given in Table S3. Typical Procedure for Ethylene Polymerization. Ethylene Polymerization at 1 atm Ethylene Pressure. A calculated amount of Fe1 was placed in a Schlenk vessel, equipped with a magnetic stir bar, followed by freshly distilled toluene (30 mL). The required amount of cocatalyst, MAO or MMAO, was then added by using a syringe. The reaction mixture was stirred at 1 atm of ethylene pressure at the optimal temperature (80 °C). After the required time of reaction, the polymer was quenched with 10% hydrochloric acid in ethanol. The polymer was washed with ethanol and then dried under reduced pressure at 60 °C and weighed. Ethylene Polymerization at 5 or 10 atm Ethylene Pressure. The high-pressure polymerization (5 or 10 atm of ethylene) was carried out in a stainless steel autoclave having a capacity of 250 mL equipped with a mechanical stirrer, an ethylene pressure control system, and a temperature controller. At the required reaction time and temperature, a calculated amount of complex in 30 mL of toluene (30 mL) was injected into the autoclave, followed by the addition of 30 mL of neat toluene for washing purposes. In the next step, the required amount of cocatalyst (MAO or MMAO) was added and more toluene was then added successively to complete the volume to 100 mL. The autoclave was immediately pressurized with 10 atm feed pressure of ethylene, and the stirring commenced. After the specified reaction time, the ethylene pressure was vented and the reaction mixture was quenched with 10% hydrochloric acid in ethanol. The polymer was collected, washed with ethanol, dried under reduced pressure at 60 °C, and weighed.
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[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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AUTHOR INFORMATION
Corresponding Authors
*W.Z.: e-mail,
[email protected]. *W.-H.S.: e-mail,
[email protected]; tel, +86-10-62557955; fax, +86-10-62618239. ORCID
Wen-Hua Sun: 0000-0002-6614-9284 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 (Nos. 51473170, 21374123, and U1362204) and the National Key Research and Development Program of China (No. 2016YFB1100800). Q.M. is grateful to the CAS-TWAS president’s fellowship.
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REFERENCES
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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.7b00909. NMR spectra, GPC traces, crystal data, and additional polymerization data (PDF) Accession Codes
CCDC 1813395 contains the supplementary crystallographic data for this paper. These data can be obtained free of charge M
DOI: 10.1021/acs.organomet.7b00909 Organometallics XXXX, XXX, XXX−XXX
Article
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DOI: 10.1021/acs.organomet.7b00909 Organometallics XXXX, XXX, XXX−XXX
