Phosphine-Iminoquinoline Iron Complexes for Ethylene

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Phosphine-Iminoquinoline Iron Complexes for Ethylene Polymerization and Copolymerization Dan Zhang,† Yanlu Zhang,† Wenjun Hou,† Zhibin Guan,*,‡ and Zheng Huang*,† †

State Key Laboratory of Organometallic Chemistry, Shanghai Institute of Organic Chemistry, University of Chinese Academy of Sciences, Chinese Academy of Sciences, 345 Lingling Road, Shanghai 200032, People’s Republic of China ‡ Department of Chemistry, University of California, 1102 Natural Sciences 2, Irvine, California 92697-2025, United States S Supporting Information *

ABSTRACT: The synthesis and olefin polymerization behavior of a series of new phosphine-iminoquinoline iron complexes is described. Upon activation, the iron complexes are highly active for ethylene polymerization, producing low molecular weight (MW) linear polyethylene (PE) with activities up to 108 g of PE/((mol of Fe) h). This activity is comparable to that of the most active bis(imino)pyridine iron catalysts and heterogeneous Ziegler−Natta catalysts. The MWs and molecular weight distributions (MWDs) of the resulting polymers can be controlled by the modification of the catalyst structures. In addition, upon activation the iron complexes are capable of copolymerizing ethylene with 1-octene, giving copolymers with α-olefin incorporation up to 8.8%.

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INTRODUCTION The seminal discovery of α-diimine nickel and palladium complexes1 for ethylene polymerization and copolymerization by Brookhart and co-workers has inspired enormous interest in the development of late-transition-metal polymerization catalysts.2 In the late 1990s, Brookhart,3 Gibson,4 and DuPont5 have independently reported that bis(imino)pyridine iron and cobalt catalysts are highly active for ethylene oligomerization or polymerization. Following that, numerous iron catalysts containing bis(imino)pyridine ligands and their derivatives have been developed.6 Modifying the ligand backbone and directly changing the coordination atom are two widely used strategies for new catalyst design.7,8 However, previous studies showed that the substitution of one imino group by an amino,9 carbonyl,10 furanyl,11 thiophenyl,11 or alcohol group12 led to catalyst systems with much lower activity in comparison to the parent bis(imino)pyridine complexes. We envisioned that replacing one of the imino motifs with a phosphino moiety would create a PNN coordination environment with steric and electronic properties distinct from the bis(imino)pyridine type. One of our laboratories has previously synthesized PNN-type pincer iron or cobalt complexes with phosphinite-iminopyridine (PONN, 1, Chart 1),13 phosphineiminopyridine (PCNN, 2),14 and bipyridyl-phosphine15 ligands for highly efficient alkene hydrofunctionalizations. Unfortunately, attempts to apply the phosphinite-iminopyridine Fe complexes to ethylene polymerization resulted in very poor activity, partially due to facile ligand decomposition via O−P bond cleavage under the reaction conditions. Our early work showed that the substitution of the O linker between the P atom and the pyridine backbone in 1 with the methene (CH2) linker significantly enhances the stability of the resulting PCNN complexes 2.14 In this context, we sought to develop PNN © XXXX American Chemical Society

Chart 1. PNN Iron Complexes Studied for Ethylene Polymerization

chelating ligands with a rigid quinoline backbone, which are likely to be even more robust than the PCNN type. Here we report the synthesis and polymerization studies of PNN-type iron complexes 3 with new phosphine-iminoquinoline ligands. Upon activation, these complexes are highly active for ethylene polymerization, yielding low-MW linear polyethylene with activities comparable to those of the parent bis(imino)pyridine iron catalysts. Notably, the iron complexes are also active for the copolymerization of ethylene with α-olefin.

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RESULTS AND DISCUSSION The synthetic route to the PNN-Fe complexes is outlined in Scheme 1. The condensation of 2-acetyl-8-bromoquinoline (4)16 with arylamines bearing iPr or Me substituents at the 2,6aryl positions provided iminoquinolines 5. Cross-coupling reactions between 5 and various secondary diaryl- or dialkylphosphines R′2PH catalyzed by Pd(OAc)2/DiPPF generated the phosphine-iminoquinoline PNN ligands 6.17 Received: July 17, 2017

A

DOI: 10.1021/acs.organomet.7b00537 Organometallics XXXX, XXX, XXX−XXX

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Organometallics

Information for details) and high melting temperature (Tm) determined by DSC. The complexes showed very high polymerization activities up to 108 g of PE/((mol of Fe) h), which is comparable to the most active bis(imino)pyridine iron catalysts and heterogeneous Ziegler−Natta catalysts.3a,4b The number-averaged molecular weights (Mn) for the polymers range from 600 to over 4000, which are significantly lower than those obtained using the o-iPr-substituted bis(imino)pyridine iron complex.3a,4b The MW reduction is likely due to steric effects. The Fe−P bond (2.4941(12) Å) in the solid-state structure of 3a is much longer than the Fe− N(imino) bond (2.225(5) Å) observed for the bis(imino)pyridine iron complex, and the Fe−N(quinoline) bond for 3a (2.206(3) Å) is also considerably longer than the Fe− N(pyridine) bond (2.091(4) Å) for the bis(imino)pyridine complex.3a In addition, a comparison between the phosphino and the N-aryl group in the structure of 3a shows that the former is seemingly less effective than the latter regarding the steric protection along the axial direction. Taken together, the axial access of ethylene toward the metal center in the PNN-Fe system is generally less blocked in comparison to that in the oiPr-substituted bis(imino)pyridine system, which presumably leads to greater chain transfer rates in the former and thus the formation of low-MW polymers. A unique feature of the PNN ligands is that the steric and electronic properties of the complexes can be tuned by the modification of the substituents at the N-aryl group and the phosphorus atom. Comparison of the results shown in entries 1−10 of Table 1 reveals that both the phosphino substituents and the substituents at the 2,6-positions of the N-aryl ring have important effects on the activities and MWs of the resultant polyethylene. A reduction of steric bulk at the N-aryl group results in decreases in activities and MWs. For complexes containing the same phosphino substituent, those with isopropyl groups at the ortho positions of the N-aryl rings in general offer higher MWs and higher activities than those with methyl groups (Table 1, entry 1 vs entry 2, entry 4 vs entry 5, and entry 6 vs entry 7). For example, the Mn drops from 840 to 600 and the activity from 4.1 × 106 to 2.8 × 106 g of PE/((mol of Fe) h) on changing the ortho substituents from iPr (3a) to Me (3b) in the complexes with the iPr2P moiety (Table 1, entry 1 vs entry 2). An exception is the results obtained in the runs using 3h,i. The more sterically hindered complex 3h is less active than the latter, while the former gives a polymer with higher MW (Table 1, entry 8 vs entry 9). The effects of the phosphino substituents on the polymerization behavior were also evaluated. The iPr to tBu substitution on the phosphorus atom yields a polymer with higher MW (Mn = 3530), albeit with reduced activity (Table 1, entry 1 vs entry 3). Note that the complexes containing dialkylphosphino groups iPr2P produce polymers with much narrower unimodal MWD (Đ = 1.6−2.2, Table 1, entries 1 and 2) in comparison with those obtained using the complexes containing diarylphosphino groups (Ar2P) (Table 1, entries 4, 5, 8, and 10). In particular, complex 3d bearing the Ph2P group affords a polymer with a very broad MWD (Đ = 18.8, entry 4). Substituents in the para positions of the aryl groups in Ar2P were modified to tune the electronic properties of the phosphine. For complexes with the o-iPr substituents in the N-aryl rings, replacing the para aryl proton in Ar2P with an MeO (3f) or CF3 (3h) group both results in higher activity and furnished polymers with higher MWs (Table 1, entries 4, 6, and 8). However, for complexes with the o-Me substituents in the

Scheme 1. Synthesis of the (PNN)Fe Complexes

Treatment of the ligands with anhydrous FeCl2 in THF formed the neutral Fe(II) dichloride complexes (RPNNR′)FeCl2 in high yields (R = iPr, R′ = iPr, 3a; R = Me, R′ = iPr, 3b; R = iPr, R′ = tBu, 3c; R = iPr, R′ = Ph, 3d; R = Me, R′ = Ph, 3e; R = iPr, R′ = p-MeOC6H4, 3f; R = Me, R′ = p-MeOC6H4, 3g; R = iPr, R′ = p-CF3C6H4, 3h; R = Me, R′ = p-CF3C6H4, 3i; R = iPr, R′ = oMeOC6H4, 3j). Magnetic susceptibility measurements using the Evans NMR method18 indicated that these blue iron complexes are high-spin, paramagnetic Fe(II) species. Because of this, NMR spectroscopy was not effective in establishing the structures of the complexes. Elemental analysis confirmed the composition of the complexes. For complex 3a, single crystals were prepared from CH2Cl2/hexane and the structure of 3a in the solid state was characterized by X-ray diffraction analysis, which reveals a distorted-square-pyramidal geometry (Figure 1).

Figure 1. ORTEP plots of complex 3a. Hydrogen atoms are omitted for clarity. Selected bond distances (Å) and angles (deg) for 3a: Fe(1)−P(1), 2.4941(12); Fe(1)−N(1), 2.206(3); Fe(1)−N(2), 2.187(3); Fe(1)−Cl(1), 2.2823(11); Fe(1)−Cl(2), 2.3409(11). N(1)−Fe(1)−P(1), 76.76(7); N(2)−Fe(1)−N(1), 73.04(10); N(2)−Fe(1)−P(1), 144.93(8); N(1)−Fe(1)−Cl(1), 144.51(8); Cl(1)−Fe(1)−Cl(2), 114.84(4); N(1)−Fe(1)−Cl(2), 100.61(7).

The catalytic activities of the series of iron complexes 3a−j were assessed by performing polymerizations at 1 bar of ethylene at 20 °C for 0.5 h upon in situ activation with modified methylalumoxane (MMAO). The results are summarized in Table 1. The PNN iron catalysts reported here produce linear polyethylene, as indicated by the low branching density determined by NMR analysis (3−11 branches per 1000 carbons; see Table S3 in the Supporting B

DOI: 10.1021/acs.organomet.7b00537 Organometallics XXXX, XXX, XXX−XXX

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Organometallics Table 1. Results of Ethylene Polymerization Using Iron Complexes 3a−j and 2a,ba entry 1 2 3 4 5 6 7 8 9 10 11h 12h 13i 14i 15 16 17 18 19 20j 21k

complex iPr

iPr

( PNN )FeCl2 (3a) (iPrPNNMe)FeCl2 (3b) (tBuPNNiPr)FeCl2 (3c) (PhPNNiPr)FeCl2 (3d) (PhPNNMe)FeCl2 (3e) (p‑MeOC6H4PNNiPr)FeCl2 (3f) (p‑MeOC6H4PNNMe)FeCl2 (3g) (p‑CF3C6H4PNNiPr)FeCl2 (3h) (p‑CF3C6H4PNNMe)FeCl2 (3i) (o‑MeOC6H4PNNiPr)FeCl2 (3j) (iPrPCNNiPr)FeCl2 (2a) (iPrPCNNMe)FeCl2 (2b) 3a 3a 3a 3a 3a 3a 3a 3a 3a

MMAO (equiv) 2000 2000 2000 2000 2000 2000 2000 2000 2000 2000 2000 2000 2000 2000 200 1000 4000 2000 2000 2000 2000

Pb 1 atm 1 atm 1 atm 1 atm 1 atm 1 atm 1 atm 1 atm 1 atm 1 atm 200 200 200 500 1 atm 1 atm 1 atm 1 atm 1 atm 1 atm 1 atm

t (min) 30 30 30 30 30 30 30 30 30 30 30 30 30 30 30 30 30 10 240 30 30

yield (g)

activityc

4.06 2.75 2.15 1.80 1.64 3.21 1.68 2.05 3.40 2.71 26.70 21.23 5.58 10.15 2.13 3.50 6.12 2.63 8.90 1.15 0.17

× × × × × × × × × × × × × × × × × × × × ×

4.1 2.8 2.2 1.8 1.6 3.2 1.7 2.1 3.4 2.7 1.1 8.5 5.6 1.0 2.1 3.5 6.1 7.9 1.1 1.2 0.2

6

10 106 106 106 106 106 106 106 106 106 107 106 107 108 106 106 106 106 106 106 106

Mw (Mn)d

Đd

1820 (840) 970 (600) − (3530)f 20600 (1090) 3300 (810) − (2690)f 6070 (860) − (2920)f 5450 (830) 5500 (940) 2500 (1000) 900 (600) 8000 (2770) 8600 (2800) 12700 (4400) 6800 (1380) 1905 (780) 1300 (770) 12500 (1400) 2200 (660) - (910)f

2.2 1.6

700 560

18.8 4.1

1100 570

7.1

570

6.6 5.9g 2.5g 1.5 2.9 3.1 2.9 4.9g 2.5 1.7 9.0g 3.4

Mpd

580 5000 770 680 6600 7200 12800 1300 640 710 720 610

Tm (°C) 122e 119e 127 126 121e 129 123e 127 121e 128 124e 118e 129 129 132 126 121e 120e 127 122

Conditions unless specified otherwise: 2 μmol of a complex in 100 mL of toluene, 20 °C. Yields are average of two runs. bPressure in units of psig unless otherwise noted. cIn units of g of PE/((mol of Fe) h). dMolecular weight distribution (MWD, or Đ) as determined by GPC analysis unless otherwise noted. eVery broad melting transition. fMn was determined by end group analysis using 13C NMR spectroscopy because the sample was unsuitable for GPC analysis due to its limited solubility. gBimodal MW distribution. hUsed 5 μmol of phosphine-iminopyridine iron precatalyst 2a or 2b. iUsed 0.2 μmol of 3a. jConducted at 60 °C. kConducted at 90 °C. a

N-aryl rings, complex 3g with p-OMe substituents in Ar2P provides an activity similar to that obtained with the parent complex 3e, while the incorporation of p-CF3 substituents leads to a 2-fold increase in the activity (Table 1, entries 5, 7, and 9). Complex 3j with o-OMe substituents in Ar2P and o-iPr substituents in the N-aryl rings produces a polymer with a bimodal MWD (Table 1, entry 10). Thus, the modifications of the phosphino substituents and the ortho substituents in the Naryl ring lead to the production of low-MW linear PEs with varying MWD. Iron complexes ligated by phosphine-iminopyridine ligands, (iPrPCNNiPr)FeCl2 (2a) and (iPrPCNNMe)FeCl2 (2b), proved to be also effective for ethylene polymerization (Table 1, entries 11 and 12). The polymerization using 2a,b as precatalysts at 200 psi of ethylene produces polymers with Mn values of 1000 and 600, respectively. The MWDs are rather narrow, similar to those obtained using 3a,b with the same phosphino substituents. In contrast, the related phosphinite-iminopyridine iron complexes 1 were demonstrated to be inactive for ethylene polymerization. These data indicate that the replacement of the linker from an O atom in 1 to a C atom in 2 and 3 clearly has a beneficial effect on generating an active polymerization catalyst. A comparison of 3a and 2a, which have the same substituents, shows that the former with a rigid quinoline backbone is 4 times more active than the latter with a pyridine backbone for polymerization under the same pressure (Table 1, entry 13 vs 11). Increasing the ethylene pressure leads to an enhanced activity as demonstrated in entries 1, 13, and 14 in Table 1 using 3a as the precatalyst. At 500 psi of ethylene, the activity is 1.0 × 108 g of PE/((mol of Fe) h) with a turnover frequency of 3.6 × 106 h−1. This activity can be compared to that of the most active bis(imino)pyridine iron catalysts.3a,4b Moreover, the MWs of

PE increase as the pressure increases: the peak MW is shifted from 700 at 15 psi to 6600 at 200 psi and to 7200 at 500 psi. The MWs and MWDs are also influenced by the loadings of the MMAO. At 200 equiv of MMAO, a relatively high MW was observed (Table 1, entry 15 vs entry 1), centered at 12800. As the [MMAO]/[Fe] ratio is increased to 1000/1, the MWD becomes clearly bimodal, with a lower MW fraction centered at 1300 and a higher molecular weight fraction at around 8000 (Table 1, entry 16). At 4000 equiv (Table 1, entry 17) of MMAO, a unimodal MWD appears again, with Mp at 640. The effects of the ethylene pressure and the amounts of MMAO on the MWs and MW distributions are consistent with a Cossee-type propagation mechanism,19 in which two chaintransfer processes coexist: the β-H transfer to the metal or the monomer gives a high-MW polymer with unsaturated chain ends (vinyl end groups) and the chain transfer to aluminum gives low-MW PE with saturated polymer chains (methyl end groups).4b The fraction of high-MW PE increases as the amount of MMAO decreases or the reaction times are prolonged. Indeed, when the reaction times were increased from 10 min to 4 h, a major change in MWD was observed. The former gave a unimodal distribution with a peak centered at 710 (Table 1, entry 18), while the latter gave a bimodal distribution with the high-MW fraction centered at 10000 (Table 1, entry 19). Similar observations have been made in other homogeneous single-site polymerization catalytic systems.3a,4b When the reaction temperature is increased to 60 °C, the iron complex 3a still shows activity above 106 g of PE/((mol of Fe) h) (Table 1, entry 20). However, at 90 °C, the activity drops to 0.2 × 106 g of PE/((mol of Fe) h) (Table 1, entry 21). These data indicate moderate thermal stability of the PNN-Fe catalyst system. C

DOI: 10.1021/acs.organomet.7b00537 Organometallics XXXX, XXX, XXX−XXX

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Organometallics A time dependence study has been also conducted at 20 °C. As shown in Figure 2, the yield of the polymer continues to

Me-substituted bis(imino)pyridine Fe complex yielded the copolymer with a relatively low 1-hexene incorporation (∼3.5 mol %).22 13C NMR analysis of the waxy products reveals a linear backbone structure. The copolymers exhibit a narrow MWD (Đ = 1.8−2.1) with MWs of around 1000. The incorporation of 1-octene clearly leads to low melting points (Tm = 37−58 °C) (Table 2, entries 4−6), and the copolymer with the highest content of 1-octene incorporation has the lowest melting points (Table 2, entry 6).

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SUMMARY In conclusion, we have described the synthesis of a series of phosphine-iminoquinoline PNN iron complexes that are highly active for ethylene polymerization, producing linear, low molecular weight polyethylene with varying molecular weight distributions. Among them, the iron complexes with the iPr2P groups (3a) enable the formation of polyethylene with narrow molecular weight distribution. In addition, we demonstrated that using a sterically less bulky and electronically deficient iron complex allows for efficient copolymerization of ethylene with 1-octene.

Figure 2. Plot of polymer yield as a function of reaction time for ethylene polymerization.

increase after 12 h, albeit at a slower rate in comparison to the initial rates (see Table S4 in the Supporting Information for details). The reduced activity at a prolonged reaction time in part can be attributed to the limited diffusion of ethylene. The results show that the PNN-Fe catalyst has a reasonable lifetime (>12 h at 20 °C), which can be compared to that of the classic o-iPr-substituted bis(imino)pyridine iron catalyst that becomes completely inactive in 30 min at 35 °C.20 Moreover, the PNN iron complexes enable highly efficient copolymerization of ethylene with α-olefins. Catalyst screening revealed that the steric and electronic properties of the PNN-Fe complexes have a profound effect on the incorporation of αolefin. Complexes 3a,b with iPr2P groups yield polymers with no incorporation of 1-octene in the copolymerization reaction of ethylene (1 atm) in neat 1-octene with 200 equiv of MMAO, indicating the exclusive selectivity of 3a,b for ethylene insertion relative to 1-octene insertion (Table 2, entries 1 and 2). Replacement of the iPr2P group with the Ph2P group leads to the incorporation of 1-octene (Table 1, entry 3). In particular, the less sterically hindered, o-Me-substituted complex 3e is effective for copolymerization of 1-octene with ethylene, yielding a polymer with a 5.1% incorporation of α-olefin (Table 2, entry 4).21 The introduction of the p-MeO group in the Ar2P moiety leads to a decrease in 1-octene incorporation (3.9%) (Table 2, entry 5), whereas the complex containing a pCF3 group yielded a polymer with a decent content of 1-octene incorporation (8.8%) (Table 2, entry 6). Taken together, the data indicate that the use of an electron-deficient and sterically less demanding complex has an advantageous effect on the synthesis of copolymers with a high degree of 1-octene incorporation. For comparison, earlier studies showed that the copolymerization of ethylene with 1-hexene catalyzed by an o-

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EXPERIMENTAL SECTION

General Considerations. All air- or moisture-sensitive manipulations were carried out under an argon atmosphere either by using Schlenk techniques or in a glovebox. 1H NMR, 13C NMR, 31P NMR, and 19F NMR spectra were recorded on a Agilent or Varian 400 MHz spectrometer with chemical shifts reported in ppm relative to the residual deuterated solvent, the internal standard tetramethylsilane, or external 85% H3PO4 for 31P. Elemental analysis was performed by the Analytical Laboratory of Shanghai Institute of Organic Chemistry (CAS). X-ray crystallographic data were collected using a Bruker AXSD8 X-ray diffractometer. Mn, Mw, and Mw/Mn values of polymers were determined using a Waters Alliance GPC 2000 series at 150 °C (using polystyrene calibration, 1,2,4-trichlorobenzene as the solvent at a flow rate of 1.0 mL/min). 1H NMR and 13C NMR data for polymers were obtained using 1,1,2,2-tetrachloroethane-d2 as the solvent at 110 °C. Modified methylaluminoxane (MMAO) was purchased from Akzo Chemical as a 1.88 M heptane solution. Polymerization-grade ethylene was purified by passing through Et3Al before use. Toluene and THF were distilled over sodium/benzophenone ketyl prior to use. Compounds 3a−j were prepared according to the known procedure.12 Preparation of the Ligands and Iron Complexes. General Procedure for the Synthesis of the Ligands. According to the literature,12 an oven-dried resealable Schlenk tube was charged with Pd(OAc)2 (16.8 mg, 0.075 mmol), DiPPF (37.6 mg, 0.09 mmol), NaOtBu (173.0 mg, 1.8 mmol), and the 8-bromo-2-iminoquinolines 5a,b (1.5 mmol). The solution was stirred for 1 h at room temperature. The secondary phosphine (1.0 mmol) was added by syringe. The Schlenk tube was sealed with a Teflon valve, heated to 80 °C, and stirred for 24 h. The reaction mixture was concentrated under reduced pressure, and the residue was purified by flash column

Table 2. Copolymerization of Ethylene with 1-Octene by Iron Complexesa entry

precat.

yield (g)

1 2 3 4 5 6

(iPrPNNiPr)FeCl2 (3a) (iPrPNNMe)FeCl2 (3b) (PhPNNiPr)FeCl2 (3d) (PhPNNMe)FeCl2 (3e) (p‑MeOC6H4PNNMe)FeCl2 (3g) (p‑CF3C6H4PNNMe)FeCl2 (3i)

1.51 2.40 1.67 3.21 4.01 6.52

activityb

Mwc

× × × × × ×

2110e 980e 1670 720 1250 700

1.5 2.4 1.7 3.2 4.0 6.5

106 106 106 106 106 106

Đc

2.3 1.8 2.1 1.8

Mpc

Tm (°C)

incorporation (%)d

830 450 580 460

131 122 125 48f 58f 37f

0 0 0.1 5.1 3.9 8.8

Conditions: 2 μmol of precat., 200 equiv of MMAO, 20 °C, 0.5 h, 1 atm of ethylene, 60 mL of 1-octene. bIn units of g of polymer/((mol of Fe) h). Determined by GPC analysis unless otherwise noted. dPercentage of 1-octene incorporation. Determined by 13C NMR analysis. eMn determined by 13 C NMR analysis. fVery broad melting transition. a c

D

DOI: 10.1021/acs.organomet.7b00537 Organometallics XXXX, XXX, XXX−XXX

Article

Organometallics

Ligand p‑CF3C6H4PNNMe (6i). Yellow solid (690.0 mg, 77%). 1H NMR (400 MHz, CDCl3): δ 8.51 (d, 3JH,H = 8.8 Hz, 1H), 8.27 (d, 3JH,H = 8.4 Hz, 1H), 7.92 (d, 3JH,H = 8.0 Hz, 1H), 7.60 (d, 3JH,H = 8.0 Hz, 4H), 7.55−7.46 (m, 5H), 7.12 (m, 1H), 7.05 (d, 3JH,H = 7.2 Hz, 2H), 6.93 (t, 3JH,H = 7.2 Hz, 1H), 1.94 (s, 6H), 1.83 (s, 3H). 31P{1H} NMR (CDCl3, 162 MHz): δ −12.51. 19F NMR (CDCl3, 376 MHz): δ −62.82. HRMS-ESI (m/z): calcd for [(C33H25F6N2P + H)+], 595.1732; found, 595.1733. Ligand o‑MeOC6H4PNNiPr (6j). Yellow solid (210.0 mg, 60%). 1H NMR (400 MHz, CDCl3): δ 8.45 (d, 3JH,H = 8.8 Hz, 1H), 8.22 (d, 3 JH,H = 8.8 Hz, 1H), 7.83 (d, 3JH,H = 8.0 Hz, 1H), 7.47 (t, 3JH,H = 7.6 Hz, 1H), 7.32 (m, 2H), 7.25 (m, 1H), 7.15 (m, 2H), 7.11 (m, 1H), 6.93 (m, 2H), 6.81 (m, 4H), 3.77 (s, PhOMe, 6H), 2.68 (m, CHMe2, 2H), 1.90 (s, NCMe, 3H), 1.15 (d, 3JH,H = 6.8 Hz, CHMe2, 6H), 1.09 (d, 3JH,H = 6.8 Hz, CHMe2, 6H). 31P{1H} NMR (CDCl3, 162 MHz): δ −34.86. Anal. Calcd for C37H39N2O2P: C, 77.33; H, 6.84; N, 4.87. Found: C, 77.30; H, 6.94; N, 4.80. General Procedure for Preparation of Iron Complexes. In an argon-filled glovebox, FeCl2 (0.254 g, 2 mmol, 1 equiv) and THF (50 mL) were placed in a 100 mL Schlenk tube. After the mixture was stirred for 30 min, a solution of PNN ligands (2.2 mmol, 1.1 equiv) in THF (10 mL) was added. The orange solution turned to dark blue immediately. After the mixture was stirred for 24 h, the resulting suspension was filtered. The solid was washed with 10 mL of ether and dried under vacuum to afford the products as blue solids. Complex (iPrPNNiPr)FeCl2 (3a). Blue solid (120.0 mg, 88%). Anal. Calcd for C29H39Cl2FeN2P: C, 60.75; H, 6.86; N, 4.89. Found: C, 61.20; H, 7.04; N, 4.61. μeff (Evans NMR method, CDCl3, 25 °C) = 5.19 μB. Complex (iPrPNNMe)FeCl2 (3b). Blue solid (107.1 mg, 83%). Anal. Calcd for C25H31Cl2FeN2P: C, 58.05; H, 6.04; N, 5.42. Found: C, 57.71; H, 6.03; N, 5.42. μeff (Evans NMR method, CDCl3, 25 °C) = 5.21 μB. Complex (tBuPNNiPr)FeCl2 (3c). Blue solid (157.0 mg, 87%). Anal. Calcd for C31H43Cl2FeN2P: C, 61.91; H, 7.21; N, 4.66. Found: C, 62.31; H, 7.36; N, 4.78. μeff (Evans NMR method, CDCl3, 25 °C) = 5.11 μB. Complex (PhPNNiPr)FeCl2 (3d). Blue solid (240.0 mg, 92%). Anal. Calcd for C35H35Cl2FeN2P: C, 65.54; H, 5.50; N, 4.37. Found: C, 65.74; H, 5.71; N,4.02. μeff (Evans NMR method, CD2Cl2, 25 °C) = 5.17 μB. Complex (PhPNNMe)FeCl2 (3e). Blue solid (156.2 mg, 83%). Anal. Calcd for C31H27Cl2FeN2P: C, 63.62; H, 4.65; N, 4.79. Found: C, 63.14; H, 5.03; N, 4.60. μeff (Evans NMR method, CD2Cl2, 25 °C) = 5.21 μB. Complex (p‑MeOC6H4PNNiPr)FeCl2 (3f). Blue solid (160.0 mg, 93%). Anal. Calcd for C37H39Cl2FeN2O2P: C, 63.35; H, 5.60; N, 3.99. Found: C, 63.17; H, 5.66; N, 3.89. μeff (Evans NMR method, CDCl3, 25 °C) = 5.23 μB. Complex (p‑MeOC6H4PNNMe)FeCl2 (3g). Blue solid (109.7 mg, 85%). Anal. Calcd for C33H31Cl2FeN2O2P: C, 61.42; H, 4.84; N, 4.34. Found: C, 60.70; H, 5.27; N, 4.58. μeff (Evans NMR method, CDCl3, 25 °C) = 5.40 μB. Complex (p‑CF3C6H4PNNiPr)FeCl2 (3h). Blue solid (170.0 mg, 90%). Anal. Calcd for C37H33Cl2F6FeN2P: C, 57.17; H, 4.28; N, 3.60. Found: C, 57.49; H, 4.64; N, 3.20. μeff (Evans NMR method, CD2Cl2, 25 °C) = 5.36 μB. Complex (p‑CF3C6H4PNNMe)FeCl2 (3i). Blue solid (124.1 mg, 86%). Anal. Calcd for C33H25Cl2F6FeN2P: C, 54.95; H, 3.49; N, 3.88. Found: C, 54.63; H, 3.86; N, 4.06. μeff (Evans NMR method, CD2Cl2, 25 °C) = 5.27 μB. Complex (o‑MeOC6H4PNNiPr)FeCl2 Complex (3j). Blue solid (94.3 mg, 96%). Anal. Calcd for C37H39Cl2FeN2O2P: C, 63.35; H, 5.60; N, 3.99. Found: C, 63.63; H, 5.38; N, 3.82. μeff (Evans NMR method, CD2Cl2, 25 °C) = 5.06 μB. Ethylene Polymerization. General Procedure for Ethylene Polymerization at 1 atm Ethylene. A 100 mL portion of freshly distilled toluene was transferred into a 200 mL Schlenk flask and saturated with ethylene at polymerization temperature. Modified methylaluminoxane (MMAO) was syringed into the flask in sequence,

chromatography on silica gel to afford the desired PNN ligands as yellow solids. Ligand iPrPNNiPr (6a). Yellow solid (225.0 mg, 85%). 1H NMR (400 MHz, CDCl3): δ 8.55 (d, 3JH,H = 8.4 Hz, 1H), 8.25 (d, 3JH,H = 8.8 Hz, 1H), 7.87 (m, 2H), 7.59 (t, 3JH,H = 8.0 Hz, 1H), 7.22−7.12 (m, 3H), 2.81 (m, CHMe2, 2H), 2.54 (m, PCHMe2, 2H), 2.43 (s, NCMe, 3H), 1.22 (dd, 3JP,H = 13.2 Hz, 3JH,H = 6.8 Hz, PCHMe2, 6H), 1.19 (d, 3 JH,H = 6.8 Hz, CHMe2, 12H), 1.08 (dd, 3JP,H = 13.2 Hz, 3JH,H = 6.8 Hz, PCHMe2, 6H). 31P{1H} NMR (CDCl3, 162 MHz): δ 4.94. Anal. Calcd for C29H39N2P: C, 77.99; H, 8.80; N, 6.27. Found: C, 77.43; H,8.79; N, 5.73. Ligand iPrPNNMe (6b). Yellow solid (450.0 mg, 74%). 1H NMR (400 MHz, CDCl3): δ 8.57 (d, 3JH,H = 8.4 Hz, 1H), 8.24 (d, 3JH,H = 8.8 Hz, 1H), 7.89−7.83 (m, 2H), 7.59 (t, 3JH,H = 7.6 Hz, 1H), 7.12 (d, 3 JH,H = 7.6 Hz, 2H), 6.97 (t, 3JH,H = 7.6 Hz, 1H), 2.54 (m, PCHMe2, 2H), 2.40 (s, NCMe, 3H), 2.08 (s, PhMe, 6H), 1.22 (dd, 3JP,H = 13.2 Hz, 3JH,H = 6.8 Hz, PCHMe2, 6H), 1.07 (dd, 3JP,H = 13.2 Hz, 3JH,H = 6.8 Hz, PCHMe2, 6H). 31P{1H} NMR (CDCl3, 162 MHz): δ 5.13. Anal. Calcd for C25H31N2P: C, 76.89; H, 8.00; N, 7.17. Found: C, 76.99; H, 8.11; N, 7.19. Ligand tBuPNNiPr (6c). Yellow solid (240.0 mg, 84%). 1H NMR (400 MHz, CDCl3): δ 8.53 (d, 3JH,H = 8.4 Hz, 1H), 8.22 (m, 2H), 7.88 (d, 3 JH,H = 8.0 Hz, 1H), 7.60 (t, 3JH,H = 7.6 Hz, 1H), 7.22−7.12 (m, 3H), 2.81 (m, CHMe2, 2H), 2.45 (s, NCMe, 3H), 1.31 (d, 3JH,H = 11.2 Hz, PCMe3, 18H), 1.19 (d, 3JH,H = 6.8 Hz, CHMe2, 12H). 31P{1H} NMR (CDCl3, 162 MHz): δ 16.90. Anal. Calcd for C31H43N2P: C, 78.44; H, 9.13; N, 5.90. Found: C, 78.71; H, 9.36; N, 5.96. Ligand PhPNNiPr (6d). Yellow solid (378.8 mg, 92%). 1H NMR (400 MHz, CDCl3): δ 8.47 (d, 3JH,H = 8.8 Hz, 1H), 8.24 (d, 3JH,H = 8.4 Hz, 1H), 7.86 (d, 3JH,H = 8.0 Hz, 1H), 7.47 (t, 3JH,H = 7.6 Hz, 1H), 7.40 (m, 4H), 7.32 (m, 6H), 7.13 (m, 3H), 7.10 (m, 1H), 2.64 (m, CHMe2, 2H), 1.86 (s, NCMe, 3H), 1.12 (d, 3JH,H = 6.8 Hz, CHMe2, 6H), 1.06 (d, 3JH,H = 7.2 Hz, CHMe2, 6H). 31P{1H} NMR (CDCl3, 162 MHz): δ −11.79. Anal. Calcd for C35H35N2P: C, 81.68; H, 6.85; N, 5.44. Found: C, 81.19; H, 6.70; N, 5.65. Ligand PhPNNMe (6e). Yellow solid (510.0 mg, 84%). 1H NMR (400 MHz, CDCl3): δ 8.39 (d, 3JH,H = 8.4 Hz, 1H), 8.14 (d, 3JH,H = 8.4 Hz, 1H), 7.76 (d, 3JH,H = 8.4 Hz, 1H), 7.38 (t, 3JH,H = 7.6 Hz, 1H), 7.33− 7.28 (m, 4H), 7.24 (m, 6H), 7.04 (m, 1H), 6.95 (d, 3JH,H = 7.6 Hz, 2H), 6.83 (t, 3JH,H = 7.6 Hz, 1H), 1.86 (s, 6H), 1.76 (s, 3H). 31 1 P{ H}NMR (CDCl3, 162 MHz): δ −12.02. HRMS-ESI (m/z): calcd for [(C31H27N2P + H)+], 459.1985; found, 459.1985. Ligand p‑MeOC6H4PNNiPr (6f). Yellow solid (480.0 mg, 74%). 1H NMR (400 MHz, CDCl3): δ 8.49 (d, 3JH,H = 8.0 Hz, 1H), 8.23 (d, 3 JH,H = 8.8 Hz, 1H), 7.83 (d, 3JH,H = 8.0 Hz, 1H), 7.46 (t, 3JH,H = 8.0 Hz, 1H), 7.31 (m, 4H), 7.15−7.06 (m, 4H), 6.86 (d, 4H), 3.78 (s, PhOMe, 6H), 2.65 (m, CHMe2, 2H), 1.91 (s, NCMe, 3H), 1.12 (d, 3 JH,H = 6.8 Hz, CHMe2, 6H), 1.06 (d, 3JH,H = 6.8 Hz, CHMe2, 6H). 31 1 P{ H} NMR (CDCl3, 162 MHz): δ −14.16. Anal. Calcd for C37H39N2O2P: C, 77.33; H, 6.84; N, 4.87. Found: C, 77.05; H, 6.88; N, 4.84. Ligand p‑MeOC6H4PNNMe (6g). Yellow solid (560.0 mg, 72%) 1H NMR (400 MHz, CDCl3): δ 8.46 (d, 3JH,H = 8.8 Hz, 1H), 8.21 (d, 3 JH,H = 8.8 Hz, 1H), 7.82 (d, 3JH,H = 8.0 Hz, 1H), 7.46 (t, 3JH,H = 7.6 Hz, 1H), 7.33 (t, 3JH,H = 8.0 Hz, 4H), 7.14 (m, 1H), 7.04 (d, 3JH,H = 7.6 Hz, 2H), 6.92 (t, 3JH,H = 7.6 Hz, 1H), 6.86 (d, 3JH,H = 8.4 Hz, 4H), 3.79 (s, 6H), 1.96 (s, 6H), 1.90 (s, 3H). 31P{1H} NMR (CDCl3, 162 MHz): δ −15.42. HRMS-ESI (m/z): calcd for [(C33H31N2O2P + H)+], 519.2196; found, 519.2196. Ligand p‑CF3C6H4PNNiPr (6h). Yellow solid (530.0 mg, 77%). 1H NMR (400 MHz, CDCl3): δ 8.57 (d, 3JH,H = 8.8 Hz, 1H), 8.30 (d, 3 JH,H = 8.8 Hz, 1H), 7.95 (d, 3JH,H = 8.0 Hz, 1H), 7.62 (d, 3JH,H = 8.0 Hz, 4H), 7.56−7.49 (m, 5H), 7.17−7.08 (m, 4H), 2.64 (m, CHMe2, 2H), 1.87 (s, NCMe, 3H), 1.14 (d, 3JH,H = 6.8 Hz, CHMe2, 6H), 1.06 (d, 3JH,H = 6.8 Hz, CHMe2, 6H). 31P{1H} NMR (CDCl3, 162 MHz): δ −11.39. 19F NMR (CDCl3, 376 MHz): δ 77.02. Anal. Calcd for C37H33F6N2P: C, 68.30; H, 5.11; N, 4.31. Found: C, 68.09; H, 5.12; 4.55. E

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Organometallics

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and the mixture was stirred for 10 min. Then the precatalyst solution in toluene was injected via a syringe. The polymerization was carried out for the desired time and then quenched with acidified ethanol and poured into a large amount of acidified ethanol (300 mL, 10% HCl (v/ v) in ethanol). The precipitated polymer was collected, washed with ethanol, and then dried at 60 °C under reduced pressure until a constant weight. General Procedure for High-Pressure Ethylene Polymerizations. A 300 mL autoclave with a magnetic stirrer was heated under vacuum to 110 °C and then was cooled to the desired reaction temperature and back-filled with ethylene. The autoclave was charged with toluene (100 mL) and modified methylaluminoxane (MMAO) and was stirred for 10 min. In an argon glovebox, the precatalyst was weighed and dissolved in 10 mL of toluene. Then the precatalyst solution in toluene was injected via a syringe. The autoclave was sealed and pressurized to the desired level. After the prescribed reaction time, the reactor was vented and the mixture was poured into a large amount of acidified ethanol (300 mL, 10% HCl (v/v) in ethanol). The precipitated polymer was collected, washed with ethanol, and then dried at 60 °C under reduced pressure until a constant weight.

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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.7b00537. Ethylene polymerization procedure, ligands and complex syntheses, characterization data, crystallographic data for complex 3a, and polymer characterization (PDF) Accession Codes

CCDC 1562753 contains 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 data_ [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

*E-mail for Z.G.: [email protected]. *E-mail for Z.H.: [email protected]. ORCID

Zheng Huang: 0000-0001-7524-098X Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We gratefully acknowledge financial support from the National Key R&D Program of the MOST of China (2016YFA0202900, 2015CB856600), the National Natural Science Foundation of China (21272255, 21422209, 21432011, 21421091, 21602236), and the Strategic Priority Research Program of the Chinese Academy of Sciences (XDB20000000).

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REFERENCES

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DOI: 10.1021/acs.organomet.7b00537 Organometallics XXXX, XXX, XXX−XXX