Article pubs.acs.org/Organometallics
Chromium Complexes with N,N,N‑Tridentate Quinolinyl AnilidoImine Ligand: Synthesis, Characterization, and Catalysis in Ethylene Polymerization Zhiqiang Hao,† Bin Xu,† Wei Gao,*,† Yuxi Han,† Guang Zeng,‡ Jingshun Zhang,† Guanghua Li,‡ and Ying Mu*,† †
State Key Laboratory for Supramolecular Structure and Materials and ‡State Key Laboratory of Inorganic Synthesis and Preparative Chemistry, Jilin University, Changchun 130012, China S Supporting Information *
ABSTRACT: The treatment of 2-(ArNCH)C 6 H 4 -HNC 9 H 6 N ([NQNANIMe]H, Ar = 2,6-Me2C6H3; [NQNANIEt]H, Ar = 2,6-Et2C6H3; [NQNANIiPr]H, Ar = 2,6-iPr2C6H3) with nBuLi and CrCl3(THF)3 achieves the hetero-dinuclear complexes [NQNANIR]CrCl(μ-Cl)2Li(THF)2 (1a, R = Me; 1b, R = Et; 1c, R = iPr) or bisligated complex κ5-[NQNANIiPr]2CrCl (2c) depending on the reactant ratios used. Reactions of these ligands with nBuLi and CrCl2(THF)2 could achieve the square-pyramidal complexes [NQNANIR]CrCl(THF) (3b, R = Et; 3c, R = iPr). Complex 3c can be oxidized by alkyl chloride to dimeric complex {[NQNANIiPr]CrCl}2(μ-Cl)2 (4) through a single-electrontransfer mechanism. Similar reaction of 2-(C9H6N)NCHC6H4-HNAr ([NQNINAMe]H, Ar = 2,6-Me2C6H3; [NQNINAEt]H, Ar = 2,6-Et2C6H3; [NQNINAiPr]H, Ar = 2,6-iPr2C6H3) with nBuLi and CrCl3(THF)3 furnishes [NQNINAR]CrCl(μ-Cl)2Li(THF)2 (5a, R = Me; 5b, R = Et; 5c, R = iPr) in high yields. However, a chromium complex with a butyl-substituted ligand, [NQBuNINAiPr]CrCl2 (6), is obtained when 2 equiv of nBuLi is used. The molecular structures of 1c, 2c, 3c, 4, 5c, and 6 are confirmed by X-ray crystallography. Upon activation with MAO, the Cr(III) complexes (1a−1c, 4, and 5a−5c) show moderate catalytic activities (50 to 218 kg of PE·mol(Cr)−1·h−1) in ethylene polymerization, whereas the Cr(II) complexes 2c, 3b, and 3c are inert under the same conditions.
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efficient shield to stabilize the active centers and suppress βhydrogen elimination in the polymerization. Many nitrogenbased tridentate ligands have been used to support chromium complexes for olefin polymerization. For example, the bis(2pyridylmethyl)amino Cr(III) complexes (A in Chart 1), upon activation with methylaluminoxane (MAO), show moderate to high activity for ethylene polymerization, depending on the size of R1 and R2.9 The Cr(III) complexes bearing 2-imino-1,10phenanthrolines and similar ligands (B in Chart 1) can catalyze ethylene oligomerization with high activities (up to 1.15 × 107 g mol−1·(Cr)·h−1) in the presence of MAO. Their activities are very sensitive to R on the imino-C, as well as the groups on the imino-N aryl ring.10 N-(Pyridine-2-methyl)salicylaldimino chromium complexes (C in Chart 1)11 and quinolinyl salicylaldimino Cr(III) complexes (D in Chart 1)11,12 exhibit higher activities for ethylene oligomerization than bidentate analogues under the same conditions. We are interested in using N,N,N-tridentate quinolinyl anilido-imine ligands to prepare transition metal and rareearth metal complexes and in studying the impact of ligand geometry on their catalytic performance. Such ligands feature
INTRODUCTION Spurred by the extensive use of Phillips1 catalyst and Union Carbide2 catalyst in commercial ethylene polymerization, chromium complexes with well-defined molecular structure have received increasing attention as catalysts for olefin polymerization. Studies on these complexes that show catalytic activity would provide better insight into the mechanism and nature of the catalytic species.3 Meanwhile, the ligands in the complexes play a vital role in the catalytic properties. Careful tuning of the ligand environment can greatly affect the catalytic behaviors and properties of the products.4 Many ligands have been explored and used to support the chromium complexes, wherein non-Cp ligands have attracted special interest because of their diversity and easy tuning of the stereo- and electronic properties.3,5 In the literature, many chromium complexes supported by anionic or neutral bidentate ligands, such as N∧O (salicyaldiminato or amino-phenol),6 N∧N (imino-pyrrolide or β-diketiminate),7 and P∧P,8 have shown moderate to high activities in the polymerization or oligomerization of olefins. Apart from bidentate analogues, tridentate chromium complexes have also been explored. Tridentate ligands that possess an additional donor site allow more precise tuning of the environment at the metal center. Moreover, in comparison with bidentate analogues, tridentate ligands can provide a more © XXXX American Chemical Society
Received: January 20, 2015
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DOI: 10.1021/acs.organomet.5b00247 Organometallics XXXX, XXX, XXX−XXX
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of CrCl3(THF)3 resulted in an immediate change in solution color from orange to brown. After evaporation of the mixture to dryness, the residue was treated with toluene and crystallized with THF/hexane to afford the chromium complexes [NQNANIR]CrCl(μ-Cl)2Li(THF)2 (R = Me (1a), R = Et (1b), and R = iPr (1c)) as red-brown solids in good yields (Scheme 1). All complexes were characterized by IR spectroscopy and elemental analyses. No informative NMR spectra of these complexes were obtained because of their paramagnetic nature. In solution, these complexes exhibited magnetic moments (μeff = 3.6 to 3.9 μB, Evans’ method17) characteristic of the expected S = 3/2 ground spin state for chromium(III) centers. The crystals of 1c that are suitable for X-ray diffraction analysis were grown from a THF/hexane mixed solution. The molecular structure of 1c is shown in Figure 1, with selected
Chart 1. Typical Chromium Complexes Supported by Tridentate Ligands
easy preparation and fine-tuning of the steric and electronic properties at the metal centers. We have developed two kinds of quinolinyl N,N,N-tridentate anilido-imine ligands ([NQNANIR]H and [NQNINAR]H in Chart 1) by exchanging the quinolinyl and 2,6- disubstituted aryl groups on the amine and imine nitrogens in the parent anilido-imine backbone.13 The repulsion between the two adjacent H atoms on the quinolinyl ring and parent ring in [NQNANIR]H makes it more twisted than [NQNINAR]H when coordinated to a metal. Recently, we have reported on quinolinyl anilido-imino rareearth-metal complexes,14 aluminum complexes,15 and zinc complexes.15 These complexes show moderate to high activities in ring-opening polymerization of ε-caprolactone or lactide. The quinolinyl moiety in the ligand plays an important role in preventing the backbiting reaction in the polymerization.14 The nickel complexes with such ligands show high activities and good molecular weight controllability in norbornene polymerization.16 More recently, the iron(II) complexes supported by these ligands have been demonstrated to be active for the atom transfer radical polymerization (ATRP) of MMA.13 In extending this research, the current study reports on the synthesis and characterization of several chromium complexes supported by quinolinyl N,N,N-tridentate anilido-imine ligands. The catalytic behaviors of these chromium complexes toward ethylene polymerization are also presented.
Figure 1. Perspective view of 1c with thermal ellipsoids drawn at the 30% probability level. Hydrogens and uncoordinated solvent are omitted for clarity. Selected bond lengths (Å) and angles (deg): Cr(1)−Cl(1) 2.3174(17), Cr(1)−Cl(2) 2.3760(17), Cr(1)−Cl(3) 2.402(2), Cr(1)−N(1) 1.976(5), Cr(1)−N(2) 2.081(5), Cr(1)−N(3) 2.091(5); Cl(1)−Cr(1)−Cl(2) 176.31(7), Cl(1)−Cr(1)−Cl(3) 89.78(7), Cl(2)−Cr(1)−Cl(3) 87.73(6), N(1)−Cr(1)−N(2) 88.1(2), N(1)−Cr(1)−N(3) 80.3(2), N(2)−Cr(1)−N(3) 167.6(2).
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RESULTS AND DISCUSSION Synthesis and Characterization of Chromium Complexes. Reactions of the N,N,N-tridentate ligands 2-(ArN CH)C6H4-HNC9H6N ([NQNANIMe]H, Ar = 2,6-Me2C6H3; [NQNANIEt]H, Ar = 2,6-Et2C6H3; [NQNANIiPr]H, Ar = 2,6-iPr2C6H3) with nBuLi and subsequent addition of 1 equiv
bond lengths and angles in the caption. The crystallographic data in CIF format are shown in the Supporting Information. Complex 1c is heterodinuclear with the chromium atom in an octahedral geometry. As expected, the N,N,N-tridentate ligand
Scheme 1. Reaction of [NQNANIR]Li with CrCl3(THF)3
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DOI: 10.1021/acs.organomet.5b00247 Organometallics XXXX, XXX, XXX−XXX
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Organometallics binds to the metal center in a mer fashion. The whole ligand is greatly twisted with a dihedral angle between the quinolinyl ring and anilido-imino parent plane of 50.63(18)°. The chromium center is linked with the lithium atom by two bridging chlorides, one of which is located trans to the terminal one [bite angle 176.31(7)°]. The Cr−Clbridge bond lengths [2.3760(17) and 2.402(2) Å] are slightly longer than that of Cr−Clterm [2.3174(17) Å] and those of 2.2915(8)−2.3205(7) Å in the NCN-pincer Cr(III) complexes.18 The N−Cr bond lengths [1.976(5)−2.091(5) Å)] are comparable with those of 1.9873(3)−2.158(3) Å in the 2-imino-1,10-phenanthroline chromium complexes,10 as well as those of 1.981(3)− 2.141(3) Å in bis(imino)pyridyl chromium(III) complexes.19 When 0.5 equiv of CrCl3(THF)3 was used for preparing 1c, the bisligated complex κ5-[NQNANIiPr]2CrCl (2c) was obtained as a purple powder in 82.8% yield after workup. The crystals of 2c that are suitable for X-ray diffraction analysis were grown from the THF solution. The molecular structure of 2c is depicted in Figure 2, together with selected bond lengths and
two nitrogen atoms from the amine-quinolinyl moiety. The ligand in mer fashion is also twisted with a dihedral angle between the quinolinyl ring and amino-imino plane of 38.33(10)°. The N−Cr bond lengths span a relatively narrow range from 1.993(5) to 2.084(5) Å. The chlorine atom is located at a trans position to N(4) with a Cr−Cl bond distance of 2.3439(18) Å, which is also comparable with those in 1c. The Cr(II) complexes with [NQNANIR] ligands were also prepared in a similar procedure to that for 1a−1c as shown in Scheme 2. Treatment of [NQNANIR]H with nBuLi in THF and subsequent addition of CrCl2(THF)2 resulted in a brown solution. After workup, 3b and 3c were obtained as paramagnetic brick red solids in moderate yields. NMR measurements are precluded because of the paramagnetic nature of the Cr(II) core. The molecular structure of 3c was well established by X-ray crystallography (Figure 3). In 3c, the
Figure 3. Perspective view of 3c with thermal ellipsoids drawn at the 30% probability level. Hydrogens are omitted for clarity. Selected bond lengths (Å) and angles (deg): Cr(1)−Cl(1) 2.3420(19), Cr(1)−N(1) 2.028(5), Cr(1)−N(2) 2.065(6), Cr(1)−N(3) 2.088(7), Cr(1)−O(1) 2.4261(16); Cl(1)−Cr(1)−N(1) 176.31(7), N(1)−Cr(1)−N(2) 90.1(2), N(1)−Cr(1)−N(3) 79.4(2), N(2)−Cr(1)−N(3) 169.2(2). Figure 2. Perspective view of 2c with thermal ellipsoids drawn at the 30% probability level. Hydrogens are omitted for clarity. Selected bond lengths (Å) and angles (deg): Cr(1)−N(1) 1.993(5), Cr(1)−N(2) 2.081(5), Cr(1)−N(3) 2.091(5), Cr(1)−N(4) 2.026(5), Cr(1)−N(6) 2.084(5), Cr(1)−Cl(1) 2.3439(18); N(1)−Cr(1)−N(2) 89.2(2), N(1)−Cr(1)−N(3) 81.11(19), N(2)−Cr(1)−N(3) 165.6(2), N(4)−Cr(1)−N(6) 78.9(2), N(4)−Cr(1)−Cl(1) 169.88(15).
ligand is coordinated to chromium(II) in a mer fashion and the environment around the metal can be best described as a distorted square pyramid with the three N atoms and one Cl atom in the basal plane and the THF molecule in the apical position. The basal coordination environment around the Cr(II) is very close to the plane (the sum of the angles is 359.35°). The whole ligand is slightly twisted, with the dihedral angle between the anilido-imino parent plane and quinolinyl ring being 36.87(24)°, which is smaller than that in 1c. The N− Cr bond distances of 2.028(5)−2.088(7) Å are in a normal
angles in the caption. The Cr(III) in 2c is sitting in an octahedral geometry. One ligand coordinates to Cr(III) in a normal mer fashion, while the other bonds to the metal with Scheme 2. Reaction of [NQNANIR]Li with CrCl2(THF)2
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DOI: 10.1021/acs.organomet.5b00247 Organometallics XXXX, XXX, XXX−XXX
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46.09(8)°. The Cr−Cl bond distances of 2.3219(8)−2.4304(8) Å are comparable with those in 1c. Similar reactions of [N Q N I N A R ]H with n BuLi and CrCl3(THF)3 afford 5a−5c as pale green powders in moderate yields (Scheme 3). All complexes were well characterized by elemental analysis and IR spectroscopy. Complexes 5a−5c are paramagnetic and exhibit similar magnetic moments (μeff = 3.7 to 3.9 μB) that are typical for the d3 Cr(III) complexes. The solid structure of 5c was well established by X-ray diffraction analysis (Figure 5). Complex 5c is heterodinuclear, and its
range and comparable with those in other chromium complexes.5,18 Notably, the axial Cr−O bond length of 2.4261(16) Å is apparently longer than those in the squareplanar Cr(II) complexes,20 but very similar to that of 2.442(3) Å in the square-pyramidal [Cr(Nacnac)Cl(THF)]2.21 The long Cr−O bond in 3c may be attributed to the Jahn−Teller effect in the d4 Cr(II) complex. Interestingly, crystallization of 3c with hexane in the presence of CH2Cl2 yielded some red crystals, which were further identified as a Cr(III) complex (4). The Cr(II) in 3c is oxidized by CH2Cl2 to Cr(III). Complex 4 can also be achieved by treating 3c with excess benzyl chloride for 72 h. The bibenzyl (PhCH2CH2Ph) in the mixture was detected by GC-MS, suggesting that the reaction was performed by a radical oneelectron-transfer oxidation process.22 Complex 4 exists as a dimer in the solid state comprising a central Cr 2 Cl 2 metallacycle, in which the two halves are related by crystallographic symmetry (Figure 4). The environment around both
Figure 5. Perspective view of 5c with thermal ellipsoids drawn at the 30% probability level. Hydrogens are omitted for clarity. Selected bond lengths (Å) and angles (deg): Cr(1)−Cl(1) 2.3717(7), Cr(1)−Cl(2) 2.3406(7), Cr(1)−Cl(3) 2.3728(7), Cr(1)−N(1) 2.0408(19), Cr(1)− N(2) 2.0132(19), Cr(1)−N(3) 2.016(2); Cl(1)−Cr(1)−Cl(2) 91.89(2), Cl(1)−Cr(1)−Cl(3) 87.63(2), Cl(2)−Cr(1)−Cl(3) 171.33(3), N(1)−Cr(1)−N(2) 89.90(8), N(1)−Cr(1)−N(3) 169.46(8), N(2)−Cr(1)−N(3) 79.60(8).
Figure 4. Perspective view of 4 with thermal ellipsoids drawn at the 30% probability level. Hydrogens and uncoordinated solvent are omitted for clarity. Selected bond lengths (Å) and angles (deg): Cr(1)−Cl(1) 2.4304(8), Cr(1)−Cl(2) 2.3219(8), Cr(1)−Cl(1A) 2.4304(8), Cr(1)−N(1) 2.079(2), Cr(1)−N(2) 1.981(2), Cr(1)− N(3) 2.050(2); Cl(1)−Cr(1)−Cl(2) 92.12(3), Cl(1)−Cr(1)−Cl(1A) 83.25(3), N(1)−Cr(1)−N(2) 83.64(9), N(1)−Cr(1)−N(3) 99.23(9), N(2)−Cr(1)−N(3) 79.92(9).
molecular structure resembles that of 1c. Most of the metric parameters in 5c are very close to those in 1c, except that the ligand in 5c is more planar with a relatively small dihedral angle between the anilido-imino parent plane and the quinolinyl ring (23.47(6)°). Notably, when 2 equiv of nBuLi was used in the synthesis of 5c, a new chromium complex (6) was isolated in moderate yield. The X-ray crystallography of 6 (see Figure 6) reveals that a butyl group is incorporated into the ortho position of the quinolinyl moiety. Similar incorporation of alkyl into the
Cr(III) atoms in 4 can be best described as an octahedron. In contrast to those in 1c and 2c, the ligands in 4 are bound to the Cr(III) in a fac manner with a dihedral angle between the anilido-imino coordination plane and the quinolinyl ring of Scheme 3. Reaction of [NQNINAR]Li with CrCl3(THF)3
D
DOI: 10.1021/acs.organomet.5b00247 Organometallics XXXX, XXX, XXX−XXX
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X-ray diffraction analysis reveals that 6 adopts a distorted octahedral coordination geometry at the chromium center, with N2, O1, Cl1, and Cl2 atoms in the equatorial plane and N1 and N3 in axial positions [N1−Cr1−N3, 169.5(23)°]. The tridentate ligand coordinated to the metal center in a normal mer fashion, and the two chlorine atoms are located in cis position. Most of the bond angles in the equatorial plane are very close to right angle. The axial Cr1−N3 bond distance [2.174(8) Å] is longer by about 0.116 Å than the Cr1−N1 bond length [2.058(7) Å]. The butyl group at the 1-position of the quinolinyl moiety is stretched in the Cr−Cl2 direction, which makes the metal center more crowded. Ethylene Polymerization. The new chromium complexes were evaluated as precatalysts for ethylene polymerization. The polymerization reactions were carried out in toluene, and the preliminary results are summarized in Table 1. When activated with MAO, the Cr(III) complexes (1a−1c, 4, and 5a−5c) show moderate catalytic activities, whereas 2c, 3b, and 3c are totally inert in ethylene polymerization and almost no polymer was obtained. The activities of the Cr(III) complexes are slightly sensitive to the spatial sterics of the ancillary ligands in both the [NQNANIR] and [NQNINAR] series. Complexes 1b and 5b, bearing ligands with an ortho-ethyl-substituted N-aryl ring, are more active than those with ortho-methyl-substituted analogues (1a and 5a), respectively. When the ortho substituents are replaced by the more bulky isopropyl in the case of 1c and 5c, a slight decrease in catalytic activity is observed (Table 1, entries 2, 16). These results may be attributed to the balance between the benefits of bulky substituents in preventing the deactivation of the catalyst and their disadvantages in taking up the monomers. The 1c/MAO system was used to study the other factors influencing the polymerization reaction. The results show that catalytic activity decreased apparently with the increase in polymerization temperature from 25 to 70 °C probably because of the decomposition of these complexes at high temperature. The catalytic activity is also influenced by the Al/Cr ratio. The catalytic activity increases with the increase in Al/Cr ratio from 50 to 200 and then decreases slowly with further increase in Al/Cr ratio. As observed in other catalyst
Figure 6. Perspective view of 6 with thermal ellipsoids drawn at the 30% probability level. Hydrogens are omitted for clarity. Selected bond lengths (Å) and angles (deg): Cr(1)−Cl(1) 2.299(3), Cr(1)−Cl(2) 2.304(3), Cr(1)−N(1) 2.058(7), Cr(1)−N(2) 2.007(7), Cr(1)−N(3) 2.174(8), Cr(1)−O(1) 2.120(7); Cl(1)−Cr(1)−Cl(2) 97.63(12), Cl(1)−Cr(1)−O(1) 166.7(2), N(1)−Cr(1)−N(2) 90.0(23), N(1)− Cr(1)−N(3) 169.5(23), N(2)−Cr(1)−N(3) 79.6(3).
quinolinyl group was also observed by Enders and co-workers in their attempts to alkylate the quinolinyl-functionalized Cpchromium with BzMgCl at low temperature.23 Thus, butyl substitution in our case may be mediated by complex 5c. To investigate the reaction in detail, a control experiment was carried out. Approximately 2 equiv of nBuLi is added to the ligand in THF, followed by hydrolysis of the mixture with water to give a solution. No butyl-ortho-substituted compound was detected in HPLC-MS, suggesting that butyl-nucleophilic substitution did not occur in the absence of chromium. Meanwhile, addition of 1 equiv of nBuLi to 5c in THF at low temperature after workup afforded 6 in high yield. Notably, similar butyl substitution on the quinolinyl group was also found in the preparation of 1a−1c when excess nBuLi was used. Table 1. Polymerization of Ethylene under Various Conditionsa entry
cat
Tp (°C)
Al/Cr
time (min)
yield (g)
activityb
Mwc (×104)
PDI
DSCd
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17
1c 1c 1c 1c 1c 1c 1c 1a 1b 2c 3b 3c 4 5a 5b 5c 6
25 25 25 25 25 50 70 25 25 25 25 25 25 25 25 25 25
100 200 300 200 200 200 200 200 200 200 200 200 200 200 200 200 200
60 60 60 30 120 60 60 60 60 60 60 60 60 60 60 60 60
0.53 0.88 0.84 0.43 0.81 0.35 0.25 0.91 1.07
106 186 168 172 81 70 50 182 214
2.56 1.71 1.51 1.38 1.50 1.53 1.29 1.08 4.54
2.9 3.2 3.3 3.0 3.1 3.4 3.1 2.7 4.1
139.7 140.0 138.8 137.9 138.4 139.1 138.6 141.1 139.3
0.84 0.75 1.09 0.68 0.26
168 150 218 136 52
1.15 2.08 4.89 2.53 11.3
3.3 2.9 4.1 3.1 3.5
138.4 140.3 139.6 138.7 137.7
Polymerization conditions: toluene (60 mL), cat (5 μmol based on Cr); ethylene pressure, 5 bar. bActivity in units of kg of PE·mol(Cr)−1·h−1. Determined by GPC relative to polystyrene standards. dDetermined by DSC at a heating rate of 10 °C min−1.
a c
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DOI: 10.1021/acs.organomet.5b00247 Organometallics XXXX, XXX, XXX−XXX
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Organometallics
[NQNANIEt]CrCl(μ-Cl)2Li(THF)2 (1b). Complex 1b was synthesized in the same way as described above for the synthesis of 1a using [NQNANIEt]H (0.38 g, 1.00 mmol), nBuLi (2.00 M in hexane, 0.50 mL, 1.00 mmol), and CrCl3(THF)3 (0.38 g, 1.00 mmol). Complex 1b was obtained as red-brown powders (yield 0.59 g, 86.2%). Anal. Calcd for C34H40Cl3CrLiN3O2 (%): C, 59.36; H, 5.86; N, 6.11. Found: C, 59.29; H, 5.92; N, 6.15. UV/vis (toluene; λmax, 528.9 nm; ε, 2.94 × 10−3 M−1 cm−1). μeff = 3.8 μB. IR (KBr): ν (cm−1) 3061m, 2959m, 2866m, 1601s, 1542w, 1501m, 1460vs, 1382m, 1330s, 1208w, 1156s, 822w, 744 m, 670w. [NQNANIiPr]CrCl(μ-Cl)2Li(THF)2 (1c). Complex 1c was synthesized in the same way as described above for the synthesis of 1a using [NQNANIiPr]H (0.41 g, 1.00 mmol), nBuLi (2.00 M in hexane, 0.50 mL, 1.00 mmol), and CrCl3(THF)3 (0.38 g, 1.00 mmol). Complex 1c was obtained as red-brown powders (yield 0.63 g, 87.5%). The crystals of 1c that are suitable for X-ray structural determination were grown from a toluene/hexane mixed solution. Anal. Calcd for C36H44Cl3CrLiN3O2 (%): C, 60.38; H, 6.19; N, 5.87. Found: C, 60.32; H, 6.26; N, 5.94. UV/vis (toluene; λmax, 520.9 nm; ε, 3.22 × 10−3 M−1 cm−1). μeff = 3.9 μB. IR (KBr): ν (cm−1) 3061m, 2959s, 2866s, 1604s, 1539w, 1503m, 1459vs, 1434s, 1378m, 1325s, 1155m, 804w, 744m. κ5-[NQNANIiPr]2CrCl (2c). Complex 2c was synthesized in the same way as described above for the synthesis of 1a using [NQNANIiPr]H (0.41 g, 1.00 mmol), nBuLi (2.00 M in hexane, 0.50 mL, 1.00 mmol), and CrCl3(THF)3 (0.19 g, 0.5 mmol). Complex 2c was obtained as purple powders (yield 0.75 g, 82.8%). The crystals of 2c that are suitable for X-ray structural determination were grown from a hexane solution. Anal. Calcd for C56H56ClCrN6 (%): C, 74.69; H, 6.27; N, 9.33. Found: C, 74.60; H, 6.23; N, 9.42. UV/vis (toluene; λmax, 512.9 nm; ε, 4.46 × 10−3 M−1 cm−1). μeff = 3.8 μB. IR (KBr): ν (cm−1) 3055m, 2973m, 2361s, 1626m, 1564m, 1504s, 1456s, 1379s, 1327s, 1159w, 1107m, 811s, 773m, 734m. [NQNANIEt]CrCl(THF) (3b). A hexane solution of nBuLi (2.00 M in hexane, 0.50 mL, 1.00 mmol) was added dropwise to [NQNANIEt]H (0.38 g, 1.00 mmol) in 30 mL of THF at −78 °C. The mixture was allowed to warm slowly to −30 °C and stirred for an additional 30 min. After addition of CrCl2(THF)2 (0.27 g, 1.00 mmol) the mixture was allowed to warm to room temperature and stirred overnight. Volatile materials were removed under vacuum. The solid was extracted with toluene (20 mL), filtered through Celite, and evaporated to dryness to yield 3b as brick red powders (yield 0.45 g, 83.3%). Anal. Calcd for C26H24ClCrN3 (%): C, 67.02; H, 5.19; N, 9.02. Found: C, 67.10; H, 5.25; N, 9.10. UV/vis (toluene; λmax, 514.0 nm; ε, 2.00 × 10−3 M−1 cm−1). IR (KBr): ν (cm−1) 3061w, 2939m, 2855m, 1605vs, 1540w, 1505w, 1458s, 1428s, 1380m, 1316s, 1214w, 1155m, 783m, 746m. [NQNANIiPr]CrCl(THF) (3c). Complex 3c was synthesized in the same way as described above for the synthesis of 3b using nBuLi (2.00 M in hexane, 0.50 mL, 1.00 mmol), [NQNANIiPr]H (0.41, 1.00 mmol), and CrCl2(THF)2 (0.27 g, 1.00 mmol). Complex 3c was obtained as brick red powders (yield 0.49 g, 86.5%). The crystals of 3c that are suitable for X-ray structural determination were grown from a THF/hexane mixed solution. Anal. Calcd for C28H28ClCrN3 (%): C, 68.08; H, 5.71; N, 8.51. Found: C, 68.01; H, 5.78; N, 8.59. UV/vis (toluene; λmax, 515.9 nm; ε, 2.42 × 10−3 M−1 cm−1). IR (KBr): ν (cm−1) 3681m, 2950w, 2857m, 1600s, 1537w, 1498m, 1465s, 1433m, 1375m, 1320vs, 1147w, 934w, 782w, 742m. {[NQNANIiPr]CrCl}2(μ-Cl)2 (4). A mixture of 3c (0.30 g, 0.6 mmol) and PhCH2Cl (10 equiv) in 20 mL of toluene was stirred at 25 °C under a nitrogen atmosphere for 3 days. Evaporating the solvent to dryness and crystallizing the residue in hexane afford 4 as a red solid (yield 0.05 g, 16%). The crystals of 4 that are suitable for X-ray structural determination were grown from a CH2Cl2/hexane mixed solution. Anal. Calcd for C56H56Cl4Cr2N6 (%): C, 63.52; H, 5.33; N, 7.94. Found: C, 63.57; H, 5.36; N, 8.08. UV/vis (toluene; λmax, 520.0 nm; ε, 4.23 × 10−3 M−1 cm−1). μeff = 3.9 μB. IR (KBr): ν (cm−1) 3371m, 2956m, 2871m, 1606s, 1538w, 1503m, 1461s, 1440s, 1384m, 1324s, 1150m, 780w, 750m, 610w.
systems,24 the molecular weight of the resulting polymers in the present system decreases slightly with the increase in Al/Cr ratio from 100 to 300 because of increased probability of chain transfer from chromium to aluminum at high Al/Cr ratios. Notably complex 6 shows relatively lower activity, but the polymer with much higher molecular weight was obtained, suggesting that the outstretched butyl group may suppress βhydrogen elimination, which in turn affords a high molecular weight polymer. Gel permeation chromatography (GPC) analysis on the resultant polymers exhibits a relatively narrow polydispersity index (PDI) of 2.7−4.1, indicating the single-site nature of these catalyst systems. The melting points of the obtained polyethylenes range from 137.7 to 141.1 °C, which is typical for linear polyethylene.
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CONCLUSION Several chromium complexes based on quinolinyl tridentate anilido-imine ligands [NQNANIR] and [NQNINAR] were synthesized, some of which were structurally characterized by X-ray diffraction analysis. All Cr(III) complexes possess an octahedral geometry at the metal center; however, the Cr(II) center in 3c is in a square-pyramidal coordination environment. Complex 3c can be oxidized by benzyl chloride, affording the dimeric Cr(III) complex 4. Upon activation with MAO, the Cr(III) complexes 1a−1c, 4, and 5a−5c show moderate catalytic activity for ethylene polymerization, while the Cr(II) complexes 2c, 3b, and 3c are inert under the same conditions.
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EXPERIMENT SECTION
General Considerations. All manipulations involving air- and moisture-sensitive compounds were carried out under an atmosphere of dried and purified nitrogen using standard Schlenk and vacuum-line techniques. Toluene and hexane were dried over sodium metal and distilled under nitrogen. Elemental analyses were performed on a Varian EL microanalyzer. NMR spectra were carried out on a Varian 300 MHz instrument at room temperature in CDCl3 solution for ligands. The molecular weights and polydispersity indices of the polymer samples were determined at 150 °C by a PL-GPC 220 type high-temperature gel permeation chromatography. 1,2,4-Trichlorobenzene was employed as the solvent at a flow rate of 1.0 mL/min. UV−vis spectra were recorded on a Varian Cary 50 UV−vis analyzer. The differential scanning calorimetry (DSC) measurements were performed on a Netzsch DSC 204 instrument under a N2 atmosphere. The samples were heated at a rate of 10 °C/min and cooled at a rate of 10 °C/min. The ligands were synthesized according to the literature.13,14 The CrCl3 was prepared according to the literature procedure,25 and CrCl3(THF)3 prepared by Soxhlet extraction of the anhydrous CrCl3 with THF in the presence of a catalytic amount of zinc dust.26 The CrCl2 was prepared according to the literature,27 and CrCl2(THF)2 was obtained by complexing CrCl2 with THF. [NQNANIMe]CrCl(μ-Cl)2Li(THF)2 (1a). A hexane solution of nBuLi (2.00 M in hexane, 0.50 mL, 1.00 mmol) was added dropwise to [NQNANIMe]H (0.35 g, 1.00 mmol) in THF (30 mL) at −78 °C. The mixture was allowed to warm to −30 °C and stirred for an additional 30 min. After addition of CrCl3(THF)3 (0.38 g, 1.00 mmol), the mixture was allowed to warm to room temperature and stirred overnight. Volatile materials were removed under vacuum. The solid was extracted with toluene (20 mL), filtered through Celite, and evaporated to dryness to yield 1a as red-brown powders, which was further purified by recrystallization with THF/hexane (yield 0.56 g, 84.5%). Anal. Calcd for C32H36Cl3CrLiN3O2 (%): C, 58.24; H, 5.50; N, 6.37. Found: C, 58.17; H, 5.56; N, 6.43. UV/vis (toluene; λmax, 524.9 nm; ε, 2.37 × 10−3 M−1 cm−1). μeff = 3.6 μB. IR (KBr): ν (cm−1) 3066s, 2954m, 2919m, 1605s, 1504w, 1457vs, 1448s, 1388m, 1313m, 1221w, 1157s, 1097w, 825w. F
DOI: 10.1021/acs.organomet.5b00247 Organometallics XXXX, XXX, XXX−XXX
Article
Organometallics [NQNINAMe]CrCl(μ-Cl)2Li(THF)2 (5a). Complex 5a was synthesized in the same way as described above for the synthesis of complex 1a with the ligand [NQNINAMe]H (0.35 g, 1.00 mmol), nBuLi (2.00 M in hexane, 0.50 mL, 1.00 mmol), and CrCl3(THF)3 (0.38 g, 1.00 mmol). Complex 5a was obtained as a pale green powder (yield 0.57 g, 86.3%). Anal. Calcd for C32H36Cl3CrLiN3O2 (%): C, 58.24; H, 5.50; N, 6.37. Found: C, 58.18; H, 5.44; N, 6.42. UV/vis (toluene; λmax, 563.9 nm; ε, 3.18 × 10−3 M−1 cm−1). μeff = 3.7 μB. IR (KBr): ν (cm−1) 3383m, 3061w, 2959s, 2857m, 1607s, 1532w, 1502m, 1457s, 1439s, 1373m, 1320s, 1156s, 819w, 784m, 744w. [NQNINAEt]CrCl(μ-Cl)2Li(THF)2 (5b). Complex 5b was synthesized in the same way as described above for the synthesis of complex 5a with [NQNINAEt]H (0.38 g, 1.00 mmol), nBuLi (2.00 M in hexane, 0.50 mL, 1.00 mmol), and CrCl3(THF)3 (0.38 g, 1.00 mmol). Complex 5b was obtained as a pale green powder (yield 0.61 g, 87.8%). Anal. Calcd for C34H40Cl3CrLiN3O2 (%): C, 59.36; H, 5.86; N, 6.11. Found: C, 59.41; H, 5.95; N, 6.16. UV/vis (toluene; λmax, 551.0 nm; ε, 3.00 × 10−3 M−1 cm−1). μeff = 3.9 μB. IR (KBr): ν (cm−1) 3661w, 2959s, 2934m, 2871m, 1619w, 1575m, 1508vs, 1458s, 1421m, 1401w, 1254w, 1120w, 1026m, 882s, 805w, 755m. [NQNINAiPr]CrCl(μ-Cl)2Li(THF)2 (5c). Complex 5c was synthesized in the same way as described above for the synthesis of complex 5a with [NQNINAiPr]H (0.41 g, 1.00 mmol), nBuLi (2.00 M in hexane, 0.50 mL, 1.00 mmol), and CrCl3(THF)3 (0.38 g, 1.00 mmol). Complex 5c was obtained as a pale green powder (yield 0.64 g, 89.4%). The crystals of 5c that are suitable for X-ray structural determination were grown from a THF/hexane mixed solution. Anal. Calcd for C36H44Cl3CrLiN3O2 (%): C, 60.38; H, 6.19; N, 5.87. Found: C, 60.32; H, 6.22; N, 5.91. UV/vis (toluene; λmax, 527.0 nm; ε, 2.67 × 10−3 M−1 cm−1). μeff = 3.8 μB. IR (KBr): ν (cm−1) 3303w, 3072w, 2956s, 2971m, 2861m, 1603m, 1586w, 1511vs, 1448w, 1432s, 1378m, 1323s, 1153s, 1078w, 791m, 745m. [NQBuNINAiPr]CrCl2 (6). The synthesis of 6 was carried out according to complex 5c, using [NQNINAiPr]H (0.41 g, 1.00 mmol), nBuLi (2.00 M in hexane, 1.00 mL, 2.00 mmol), and CrCl3(THF)3 (0.38 g, 1.00 mmol). Yield: 0.43 g (72.6%) of a pale green solid. The crystals of 6 that are suitable for X-ray structural determination were grown from a THF/hexane mixed solution. Anal. Calcd for C32H36Cl2CrN3 (%): C, 65.64; H, 6.20; N, 7.18. Found: C, 65.57; H, 6.18; N, 7.23. UV/vis (toluene; λmax, 563.9 nm; ε, 2.73 × 10−3 M−1 cm−1). μeff = 3.6 μB. IR (KBr): ν (cm−1) 3324w, 2952m, 2356w, 1604m, 1574m, 1513s, 1433m, 1375m, 1327s, 1149s, 792m, 749m. Control Experiment of [NQNINAiPr]H with 2 equiv of nBuLi in the Absence of CrCl3(THF)3. A hexane solution of nBuLi (2.00 M in hexane, 1.00 mL, 2.00 mmol) was added dropwise to [NQNINAiPr]H (0.41 g, 1.00 mmol) in 30 mL of THF at −78 °C. The mixture was allowed to warm slowly to −30 °C and stirred for an additional 30 min; then 10 mL of water was added. The mixture was extracted with ether, and the combined organic solution was subjected to HPLC-MS analysis. Ethylene Polymerization Experiments. The ethylene polymerization experiments were carried out as follows: a dry 250 mL steel autoclave with a magnetic stirrer was charged with 50 mL of toluene, thermostated at the desired temperature, and saturated with ethylene (1.0 atm). The polymerization reaction was started by addition of a mixture of the catalyst and MAO in toluene (5 mL) at the same time. The vessel was pressurized to 5 atm with ethylene immediately, and the pressure was maintained by continuous feeding of ethylene. The reaction mixture was stirred at the desired temperature for the desired time period. The polymerization was then quenched by injecting an acidic ethanol solution containing HCl (3 M). The polymer was collected by filtration, washed with water and ethanol, and dried to a constant weight under vacuum. X-ray Crystal Structural Determination. The crystals were mounted on a glass fiber using the oil drop method. Data obtained with the ω−2θ scan mode were collected on a Bruker SMART 1000 CCD diffractometer with graphite-monochromated Mo Kα radiation (λ = 0.710 73 Å). The structures were solved using direct methods, while further refinement with full-matrix least-squares on F2 was obtained with the SHELXTL program package. All non-hydrogen
atoms were refined anisotropically. Hydrogen atoms were introduced in calculated positions with the displacement factors of the host carbon atoms. The large solvent accessible void(s) was found in 2c due to the unsolved disorder of the uncoordinated solvent molecules.
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ASSOCIATED CONTENT
S Supporting Information *
X-ray crystallographic data and refinements for complexes 1c, 2c, 3c, 4, 5c, and 6 in CIF format. The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.organomet.5b00247.
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AUTHOR INFORMATION
Corresponding Authors
*E-mail (W. Gao):
[email protected]. *E-mail (Y. Mu):
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS We thank the National Natural Science Foundation of China (21374035, 21074043, 21274050, and 51173061) and Jilin University (J1103302) for financial support.
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REFERENCES
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DOI: 10.1021/acs.organomet.5b00247 Organometallics XXXX, XXX, XXX−XXX
