Zirconium and hafnium complexes with new tetra-azane ligands: Synthesis, characterization and catalytic properties for ethylene polymerization

Polyhedron 102 (2015) 337–343

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Zirconium and hafnium complexes with new tetra-azane ligands: Synthesis, characterization and catalytic properties for ethylene polymerization Xiangdong Ji, Xuyang Luo, Wei Gao ⇑, Ying Mu ⇑ State Key Laboratory for Supramolecular Structure and Materials, School of Chemistry, Jilin University, 2699 Qianjin Street, Changchun 130012, People’s Republic of China

a r t i c l e

i n f o

Article history: Received 23 August 2015 Accepted 3 October 2015

Keywords: Ethylene polymerization Group 4 complexes Homogeneous catalysis Olefin polymerization Tetra-azane ligands

a b s t r a c t Two new anilido-imine tetra-azane ligands, 1,2-[(20 -(ArNH)C6H4HC@N)]2C6H4 (Ar = 2,6-Me2C6H3 (L1H2) and 2,6-iPr2C6H3 (L2H2)), were synthesized by the condensation reaction of o-phenylenediamine with the corresponding 2-(arylamino)benzaldehyde, and their zirconium and hafnium complexes, L1MCl2 (M = Zr (1b), Hf (1c)) and L2MCl2 (M = Zr (2b), Hf (2c)), were synthesized in high yields (61–66%) by the reactions of L1Li2 and L2Li2 with MCl4 in toluene. Direct HCl-elimination reactions of L1H2 with MCl4 (M = Ti, Zr, Hf) in toluene at 140 °C under vacuum afforded the products L1HMCl3 [M = Ti (1a0 ), Zr (1b0 ), Hf (1c0 )] with a partially deprotonated tridentate ligand in good to high yields (48–70%). All the new complexes were characterized by 1H and 13C NMR spectroscopy and the molecular structures of 1b and 2c were determined by single crystal X-ray diffraction analysis. The metal centers in both complexes are six-coordinated with a distorted octahedral geometry. Upon activation with MAO or AlR3/ Ph3CB(C6F5)4, complexes 1b–1c and 2b–2c all exhibit moderate catalytic activity for ethylene polymerization and produce linear polyethylene with ultra-high molar masses (100–184  104 g/mol). Ó 2015 Elsevier Ltd. All rights reserved.

1. Introduction Non-metallocene complexes of early transition metals as catalysts have attracted considerable attention in recent years as high performance catalysts for olefin polymerization reactions in both academic and industrial communities due to their easy preparation and structural modification [1–3]. A large number of complexes bearing didentate and tridentate chelating ligands with N, O, P and S donors have been developed and their catalytic properties been studied [4–7]. Early transition metal complexes carrying different tetradentate ligands have also been extensively investigated [8]. Especially, some group 4 metal complexes with tetradentate ligands have been found to show good catalytic properties for various olefin polymerization reactions [9]. For examples, group 4 metal complexes supported by a salen-type ligand were reported to be efficient catalysts for the polymerization reactions of ethylene, propylene and a-olefin [10]. Titanium and zirconium complexes bearing a C2-symmetric salan ligand were found to catalyze the polymerization of 1-hexene, 4-methyl-1-pentene and 1,5-hexadiene [11]. Group 4 metal complexes with a salalen ligand were also studied as catalysts for ethylene, propylene and ⇑ Corresponding authors. Tel./fax: +86 431 85168376. E-mail address: [email protected] (Y. Mu). http://dx.doi.org/10.1016/j.poly.2015.10.018 0277-5387/Ó 2015 Elsevier Ltd. All rights reserved.

1-hexene polymerization reactions [12]. Titanium and zirconium complexes with a chiral bipyrrolidine/bisphenolato-based salan ligand have been reported to show relatively high isospecific selectivity in 1-hexene polymerization [13]. Group 4 metal complexes carrying a [OSSO] ligand have also been systematically investigated for ethylene and 1-hexene polymerization [14]. In recent years, we have synthesized some main group and transition metal complexes supported by an anilido-imine type of ligand [15]. So far, bi-, tri- and tetra-dentate ligands containing the anilido-imine unit(s) have been developed. We have previously synthesized some chiral rare-earth metal complexes with tetraazane chelating ligands, (1R,2R)-N,N0 -bis(ortho-arylaminobenzylidene)-1,2-diaminocyclohexane, and investigated their catalytic properties for the intramolecular asymmetric hydroamination reaction of terminal aminoalkenes [16]. To extend the chemistry of this type of ligand, we have now synthesized two new achiral tetra-azane ligands, 1,2-[(20 -(ArNH)C6H4HC@N)]2C6H4 [Ar = 2,6Me2C6H3 (L1H2), 2,6-iPr2C6H3 (L2H2)], and their zirconium and hafnium complexes, L1MCl2 [M = Zr (1b), Hf (1c)] and L2MCl2 [M = Zr (2b), Hf (2c)], and have studied the catalytic performances of the new complexes for ethylene polymerization. It was found that these new Zr and Hf complexes, upon activation with MAO or AlR3/Ph3CB(C6F5)4, show moderate catalytic activity for ethylene polymerization and produce linear polyethylene with ultra-high

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molar masses (viscosity-averaged molecular weight Mg up to 184  104 g/mol). In addition, in an attempt to develop a new synthetic method for the new complexes, complexes L1HMCl3 [M = Ti (1a0 ), Zr (1b0 ), Hf (1c0 )], with a partially deprotonated tridentate ligand, were obtained in good to high yields (48–70%) from the one-pot HCl elimination reactions of MCl4 with L1H2. In the present paper, we report these results in detail. 2. Experimental 2.1. General considerations All manipulations involving air- and/or moisture-sensitive compounds were carried out under a nitrogen atmosphere using either standard Schlenk or glove box techniques. Toluene and n-hexane were dried over sodium/benzophenone and distilled under nitrogen prior to use. CH2Cl2 was dried and distilled over calcium hydride before use. Polymerization grade ethylene was further purified by passage through columns of 4 Å molecular sieves and MnO. TiCl4, ZrCl4, HfCl4, n-BuLi, o-phenylenediamine, Pd(OAc)2, NaOtBu, DPEphos, 2,6-dimethylaniline and 2,6-diisopropylaniline were purchased from Aldrich or Acros. 1H and 13C NMR spectra were recorded on a Bruker Avance III-400 NMR spectrometer at room temperature in CDCl3. 13C NMR spectra of the polyethylenes were recorded at 135 °C with o-C6D4Cl2 as the solvent. The elemental analysis was performed on a Vario EL cube analyzer. The intrinsic viscosity (g) values of the polyethylenes were measured in decahydronaphthalene at 135 °C using an Ubbelohde viscometer. Viscosity average molecular weight [17] (Mg) values of the polyethylenes were calculated according to the following equation: [g] = (6.77  104) Mg0.67. The melting points of the polyethylenes were measured by differential scanning calorimetry (DSC) on a NETZSCH DSC 204 at a heating/cooling rate of 10 °C/min from 35 to 160 °C and the data from the second heating scan were used. 2-(20 ,60 -Dimethylphenylamino)-benzaldehyde, 2-(20 ,60 diisopropylphenylamino)benzaldehyde [18] and Ph3CB(C6F5)4 [19] were prepared according to the literature procedures. 2.2. Synthesis of the compounds 2.2.1. Synthesis of 1,2-[(20 -(200 ,600 -Me2C6H3NH)C6H4H C@N)]2C6H4 (L1H2) To a solution of 2-(20 ,60 -dimethylphenylamino)benzaldehyde (2.25 g, 10.0 mmol) and o-phenylenediamine (0.54 g, 5.0 mmol) in toluene was added p-TsOH (0.10 g) at room temperature. The mixture was refluxed with a Dean–stark trap for 10 h, cooled to room temperature and the solvent was then removed by rotary evaporation to give a deep yellow solid. The crude product was purified by column chromatography on silica gel eluting with ethyl acetate/petroleum ether (v/v = 1:15, 1% Et3N) to give the pure product (2.11 g, 4.04 mmol, 81%). Anal. Calc. for C36H34N4 (522.28): C, 82.72; H, 6.56; N, 10.72. Found: C, 82.70; H, 6.52; N, 10.89%. 1H NMR (400 MHz, CDCl3, 298 K, d, ppm): 10.34 (s, 2H, NH), 8.62 (s, 2H, CH@NAr), 7.04–7.28 (m, 14H, ArH), 6.58 (t, J = 12 Hz, 2H, ArH), 6.19 (d, J = 8.4 Hz, 2H, ArH), 1.95 (s, 12 H, ArCH3). 13C NMR (100 MHz, CDCl3, 298 K, d, ppm): 165.31, 148.21, 144.01, 137.47, 136.53, 134.69, 132.10, 128.17, 126.08, 125.78, 121.63, 117.18, 115.47, 111.72, 18.12. 2.2.2. Synthesis of 1,2-[(20 -(200 ,600 -iPr2C6H3NH)C6H4HC@N)]2C6H4 (L2H2) L2H2 was synthesized in the same way as described above for the synthesis of L1H2 with 2-(20 ,60 -diisopropylphenylamino)benzaldehyde (2.81 g, 10.0 mmol) and o-phenylenediamine (0.54 g, 5.0 mmol) as the starting materials. After the crude product was

purified by column chromatography on silica gel eluting with ethyl acetate/petroleum ether (v/v = 1:15, 1% Et3N), 2.36 g of pure product (3.72 mmol, 74%) was obtained. Anal. Calc. for C44H50N4 (634.40): C, 83.24; H, 7.94; N, 8.82. Found: C, 83.20; H, 7.86; N, 8.78%. 1H NMR (400 MHz, CDCl3, 298 K, d, ppm): 10.32 (s, 2H, NH), 8.65 (s, 2H, CH@NAr), 7.04–7.28 (m, 14H, ArH), 6.58 (t, J = 16 Hz, 2H, ArH), 6.14 (d, J = 8.4 Hz, 2H, ArH), 2.98 (m, 4H, CHMe2), 1.01 (d, J = 6.8 Hz, 12H, CH(CH3)2), 0.87 (d, J = 6.8 Hz, 12H, CH(CH3)2). 13C NMR (100 MHz, CDCl3, 298 K, d, ppm): 165.64, 149.78, 147.45, 143.99, 134.82, 134.62, 132.06, 127.29, 125.72, 123.63, 122.42, 116.91, 115.31, 112.11, 28.40, 24.42, 22.81. 2.2.3. Synthesis of complex 1a0 To a stirred solution of L1H2 (0.52 g, 1.0 mmol) in 30 mL of toluene was added TiCl4 (0.11 mL, 1.0 mmol) in toluene (10 mL) at 0 °C, during which period a red suspension was formed immediately. The mixture was allowed to warm to room temperature, the solvent was removed under reduced pressure and the residue was heated to 140 °C under vacuum for 3 h. The crude product was extracted with CH2Cl2 (2  10 mL) and recrystallized from CH2Cl2/n-hexane to give the pure product 1a0 (0.47 g, 0.69 mmol, 70%) as deep red crystals. Anal. Calc. for C36H33Cl3N4Ti (674.13): C, 63.97; H, 4.92; N, 8.29. Found: C, 64.12; H, 5.05; N, 8.35%. 1H NMR (400 MHz, CDCl3, 298 K, d, ppm): 9.86 (s, 1H, CH@NAr), 8.83 (s, 1H, CH@NAr), 5.93–7.82 (m, 18H, ArH), 5.93 (s, 1H, NH), 2.25 (s, 6H, ArCH3), 1.87 (s, 6H, ArCH3). 13C NMR (100 MHz, CDCl3, 298 K, d, ppm): 170.27, 161.88, 150.96, 146.08, 143.48, 141.98, 141.78, 137.66, 137.25, 136.86, 136.49, 133.19, 129.84, 129.50, 129.25, 129.18, 128.52, 126.66, 125.71, 123.75, 121.60, 118.95, 118.35, 117.92, 114.52, 113.13, 99.99, 20.00, 18.22. 2.2.4. Synthesis of complex 1b0 To a stirred solution of L1H2 (0.52 g, 1.0 mmol) in 30 mL of toluene was added ZrCl4 (0.23 g, 1.0 mmol) in toluene (10 mL) at 0 °C. The mixture was stirred at 50 °C for 2 h, during which period a red suspension was formed. The solvent was removed under reduced pressure and the residue was heated to 140 °C under vacuum for 3 h. The crude product was extracted with CH2Cl2 (2  10 mL) and recrystallized from CH2Cl2/n-hexane to give the pure product 1b0 (0.38 g, 0.53 mmol, 54%) as reddish brown crystals. Anal. Calc. for C36H33Cl3N4Zr (716.08): C, 60.12; H, 4.62; N, 7.79. Found: C, 60.23; H, 4.61; N, 7.85%. 1H NMR (400 MHz, CDCl3, 298 K, d, ppm): 9.72 (s, 1H, CH@NAr), 8.79 (s, 1H, CH@NAr), 6.09–7.92 (m, 18H, ArH), 5.79 (s, 1H, NH), 2.34 (s, 6H, ArCH3), 1.93 (s, 6H, ArCH3). 13C NMR (100 MHz, CDCl3, 298 K, d, ppm): 170.05, 163.72, 149.79, 145.84, 144.44, 142.71, 137.64, 137.29, 136.98, 136.83, 133.97, 130.79, 130.23, 130.21, 129.97, 129.31, 129.05, 128.60, 126.88, 122.95, 121.61, 120.03, 119.11, 118.41, 117.95, 116.08, 113.55, 19.67, 18.22. 2.2.5. Synthesis of complex 1c0 Complex 1c0 was synthesized in the same way as described above for the synthesis of complex 1b0 with the free ligand L1H2 (0.52 g, 1.0 mmol) and HfCl4 (0.32 g, 1.0 mmol) as starting materials. Recrystallization of the crude product from CH2Cl2/n-hexane gave the pure product 1c0 (0.38 g, 0.47 mmol, 48%) as reddish brown crystals. Anal. Calc. for C36H33Cl3N4Hf (806.12): C, 53.61; H, 4.12; N, 6.95. Found: C, 53.29; H, 4.21; N, 6.86%. 1H NMR (400 MHz, CDCl3, 298 K, d, ppm): 9.66 (s, 1H, CH@NAr), 8.12 (s, 1H, CH@NAr), 6.15–7.67 (m, 18H, ArH), 5.73 (s, 1H, NH), 2.31 (s, 6H, ArCH3), 1.95 (s, 6H, ArCH3). 13C NMR (100 MHz, CDCl3, 298 K, d, ppm): 170.26, 166.59, 148.95, 146.89, 145.52, 142.34, 137.75, 137.54, 136.79, 136.68, 133.28, 131.64, 130.72, 130.44, 129.66, 129.26, 128.90, 128.64, 126.96, 122.93, 121.76, 119.47, 119.18, 118.69, 118.02, 117.17, 113.70, 19.54, 18.21.

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2.2.6. Synthesis of complex 1b n BuLi (1 mL, 2.0 mmol) in n-hexane was added dropwise to a solution of L1H2 (0.52 g, 1.0 mmol) in 20 mL of toluene at 78 °C. The reaction mixture was allowed to warm to room temperature and stirred for 1 h, then canula transferred into a solution of ZrCl4 (0.23 g, 1.0 mmol) in 10 mL of toluene at 40 °C. The reaction mixture was allowed to warm to room temperature, stirred for 3 h and then filtered to remove the insoluble impurities. After the solvent was removed under reduced pressure, the crude product was recrystallized from CH2Cl2/n-hexane to give the pure product 1b (0.42 g, 0.62 mmol, 62%) as deep red crystals. Anal. Calc. for C36H32Cl2N4Zr (680.11): C, 63.33; H, 4.72; N, 8.21. Found: C, 63.45; H, 4.81; N, 8.29%. 1H NMR (400 MHz, CDCl3, 298 K, d, ppm): 8.73 (s, 2H, CH@NAr), 6.90–7.51 (m, 14H, ArH), 6.68 (t, J = 14.8 Hz, 2H, ArH), 5.45 (d, J = 8.8 Hz, 2H, ArH), 2.01 (s, 12H, ArCH3). 13C NMR (100 MHz, CDCl3, 298 K, d, ppm): 164.02, 153.16, 144.02, 140.46, 138.56, 137.75, 136.88, 130.19, 129.00, 128.18, 128.10, 121.92, 117.89, 117.82, 116.56, 19.53. 2.2.7. Synthesis of complex 1c Complex 1c was synthesized in the same way as described above for the synthesis of 1b with the free ligand L1H2 (0.52 g, 1.0 mmol) and HfCl4 (0.32 g, 1.0 mmol) as starting materials. Recrystallization of the crude product from CH2Cl2/n-hexane gave the pure product 1c (0.49 g, 0.64 mmol, 64%) as red brown crystals. Anal. Calc. for C36H32Cl2N4Hf (770.15): C, 56.15; H, 4.19; N, 7.28. Found: C, 56.08; H, 4.13; N, 7.43%. 1H NMR (400 MHz, CDCl3, 298 K, d, ppm): 8.78 (s, 2H, CH@NAr), 6.63–7.49 (m, 16H, ArH), 5.54 (d, J = 8 Hz, 2H, ArH), 1.97 (s, 12H, ArCH3). 13C NMR (100 MHz, CDCl3, 298 K, d, ppm): 164.68, 155.09, 144.00, 140.08, 138.75, 138.39, 137.04, 130.03, 129.05, 128.97, 128.23, 127.52, 121.72, 117.76, 117.53, 19.41. 2.2.8. Synthesis of complex 2b n BuLi (1 mL, 2.0 mmol) in n-hexane was added dropwise to a solution of L2H2 (0.63 g, 1.0 mmol) in 20 mL of toluene at 78 °C. The reaction mixture was allowed to warm to room temperature and stirred for 1 h, then canula transferred into a solution of ZrCl4 (0.23 g, 1.0 mmol) in 10 mL of toluene at 40 °C. The reaction mixture was allowed to warm to room temperature, stirred for 3 h and then filtered to remove the insoluble impurities. After the solvent was removed under reduced pressure, the crude product was recrystallized from CH2Cl2/n-hexane to give the pure product 2b (0.48 g, 0.61 mmol, 61%) as deep red crystals. Anal. Calc. for C44H48Cl2N4Zr (792.23): C, 66.47; H, 6.09; N, 7.05. Found: C, 66.42; H, 6.15; N, 7.19%. 1H NMR (400 MHz, CDCl3, 298 K, d, ppm): 8.99 (s, 2H, CH@NAr), 7.59 (d, J = 8 Hz, 2H, ArH), 7.08–7.37 (m, 12H, ArH), 6.75 (d, J = 8 Hz, 2H, ArH), 6.11 (d, J = 8 Hz, 2H, ArH), 2.71 (m, 4H, CH(CH3)2), 0.90 (d, J = 8 Hz, 12H, CH(CH3)2), 0.85 (d, J = 8 Hz, 12H, CH(CH3)2). 13C NMR (100 MHz, CDCl3, 298 K, d, ppm): 165.67, 149.81, 147.48, 144.62, 134.85, 134.62, 132.09, 129.08, 128.27, 128.16, 127.33, 125.26, 123.66, 122.45, 115.35, 112.14, 28.44, 24.45, 22.84. 2.2.9. Synthesis of complex 2c Complex 2c was synthesized in the same way as described above for the synthesis of complex 2b with the free ligand L2H2 (0.63 g, 1.0 mmol) and HfCl4 (0.32 g, 1.0 mmol) as starting materials. Recrystallization of the crude product from CH2Cl2/n-hexane gave the pure product 2c (0.58 g, 0.66 mmol, 66%) as red brown crystals. Anal. Calc. for C44H48Cl2N4Hf (882.27): C, 59.90; H, 5.48; N, 6.35. Found: C, 59.84; H, 5.39; N, 6.42%. 1H NMR (400 MHz, CDCl3, 298 K, d, ppm): 9.03 (s, 2H, CH@NAr), 7.55 (d, J = 12 Hz, 2H, ArH), 7.08–7.37 (m, 12H, ArH), 6.71 (t, 2H, ArH), 6.17 (d, J = 8 Hz, 2H, ArH), 2.90 (m, 4H, CH(CH3)2), 0.90 (d, J = 8 Hz, 12H, CH(CH3)2), 0.88 (d, J = 8 Hz, 12H, CH(CH3)2). 13C NMR

(100 MHz, CDCl3, 298 K, d, ppm): 165.65, 149.79, 147.46, 143.99, 134.83, 134.63, 132.21, 128.22, 127.35, 125.73, 123.64, 122.43, 115.32, 112.12, 28.41, 24.42, 22.81.

2.3. Ethylene polymerization The ethylene polymerization experiment was carried out as follows. A dry 250 mL steel autoclave with a magnetic stirrer was charged with 60 mL of toluene, thermostated at the desired temperature and saturated with ethylene (1.0 atm). The polymerization reaction was started by the addition of a solution of AlR3 in toluene (10 mL) and a mixture of a catalyst and Ph3CB(C6F5)4 in toluene (10 mL) or a solution of MAO in toluene (10 mL) and a solution of a catalyst in toluene (10 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 a certain period of time. The polymerization was then quenched by injecting acidified methanol [HCl (3 M)/methanol = 1:1], and the polymer was collected by filtration, washed with water and methanol, and dried at 60 °C in vacuo to a constant weight under vacuum. 2.4. Data collection and structural refinement of 1b and 2c Single crystals of complexes 1b and 2c suitable for X-ray crystal structural analysis were obtained from a CH2Cl2/n-hexane (v/v = 1–2:10) mixed solvent system. The diffraction data were collected with the x–2h scan mode on a Bruker SMART 1000 CCD diffractometer with graphite-monochromated Mo Ka radiation (k = 0.71073 Å). The structures were solved by direct methods, and refined with full-matrix least squares on F2. All non-hydrogen atoms were refined anisotropically and hydrogen atoms were introduced in calculated positions with the displacement factors of the host carbon atoms. All calculations were performed using the SHELXTL crystallographic software packages [20]. Details of the crystal data, data collections, and structure refinements are summarized in Table 1.

Table 1 Crystallographic data for complexes 1b and 2c.

Formula Fw Crystal system Space group a (Å) b (Å) c (Å) a (°) b (°) c (°) v (Å3) Z Dcalc (g cm3) l (mm1) F(0 0 0) hmax (°) Reflections collected Unique reflections Rint Goodness-of-fit (GOF) R1 wR2 Largest diff peak, hole (e A3)

1b

2c

C36H32Cl2N4Zr 682.78 monoclinic P2(1)/n 12.1236(9) 13.8121(10) 19.2819(14) 90 106.3730(10) 90 3097.9(4) 4 1.464 0.560 1400 1.84–26.01 13 935 5996 0.0341 1.128 0.0441 0.1005 0.472 and 0.432

C46H52Cl6HfN4 1052.11 triclinic  P1 11.5775(5) 11.5882(5) 19.0303(8) 104.4390(10) 99.4300(10) 105.8560(10) 2303.51(17) 2 1.517 2.649 1060 1.89–26.03 12 421 8843 0.0162 1.130 0.0320 0.0858 1.010 and 0.920

R1 = R||F0|  |Fc||/R|F0|, wR2 = [R[ w (F20  F2c )2]/R[ w (F20)2]]1/2.

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3. Results and discussion 3.1. Synthesis and characterization of the tetra-azane ligands According to the literature methods for the synthesis of the anilido-imine type of ligands [15,16,21], the free ligands L1H2 and L2H2 were conveniently synthesized in high yields (74–81%) by refluxing a 2-(arylamino)benzaldehyde compound with o-phenylenediamine in toluene in the presence of a catalytic amount of p-toluene sulfonic acid, as shown in Scheme 1. The free ligands were characterized by 1H and 13C NMR spectroscopy and elemental analyses. The 1H NMR spectra of L1H2 and L2H2 show similar characteristic resonances for the HC@N protons (d 8.62–8.65 ppm) and the Ar–NH protons (d 10.33– 10.34 ppm). The methyl protons of CH(CH3)2 in L2H2 exhibit two sets of doublets centered at d 0.87 and 1.01 ppm, indicating that the rotation of the 2,6-iPr2C6H3 group around the C–N bond is restricted on the NMR timescale at room temperature [4b,21].

3.2. Synthesis and characterization of the complexes Group 4 metal complexes can be synthesized by different methods, such as salt metathesis [22], amine elimination [23], Me3SiCl elimination [24] and alkane elimination reactions [25]. Some titanium complexes bearing polydentate ligands with O or S donors can also be synthesized by direct HCl elimination reactions of TiCl4 with the corresponding free ligands [4a]. Recently, we and other groups have found that titanium complexes carrying N-containing polydentate ligands can also be synthesized in high yields by the direct reactions of TiCl4 with weak acidic amine-containing free ligands [21]. In the present work, attempts to synthesize the complexes 1a–1c by the one-pot HCl elimination reactions of L1H2 with MCl4 (M = Ti, Zr, Hf) were not successful. 1H NMR spectra and elemental analyses indicate that the products obtained from the HCl elimination reactions after the reaction mixtures were heated at 140 °C for 3 h under vacuum are complexes bearing a partial deprotonated tridentate ligand, L1HMCl3 [M = Ti (1a0 ), Zr (1b0 ) and Hf (1c0 )], as shown in Scheme 2. As seen from Fig. S1, the 1H NMR spectrum of L1H2 shows characteristic resonances for the HC@N protons (d 8.62 ppm) and the ArNH protons

Ar

2

NH

O

toluene

+ H 2N

NH2

N

ref lux

NH Ar

N HN Ar

Ar = 2,6-Me2 C 6H 3 (L1 H 2), 2,6- iPr 2C 6 H3 (L2H 2 ) Scheme 1. Synthesis of the free ligands L1H2 and L2H2.

R

N

N

NH

HN

RR

1. MCl 4 in toluene R

2. 140oC in vacuo

(d 10.34 ppm) [21a]. After the reaction mixture of L1H2 and TiCl4 was heated at 140 °C, the 1H NMR spectrum of the formed 1a0 shows two singlets for two different HC = N protons at d 8.83 and 9.86 ppm, and one singlet assigned to an uncoordinated ArNH proton at d 5.93 ppm [26]. The 1H NMR spectra of 1b0 and 1c0 also show two singlets (d 8.79 and 9.72 ppm for 1b0 , d 8.12 and 9.66 ppm for 1c0 ) for the two different HC@N protons and a broad singlet (d 5.79 ppm for 1b0 and d 5.73 ppm for 1c0 ) for the residual NH protons. The presence of the NH resonance indicates that one of the two amine groups is not deprotonated and bonded to the metal centre. In addition, two singlets (d 1.93 and 2.34 ppm for 1b0 , d 2.31 and 1.95 ppm for 1c0 ) for the methyl protons of the two 2,6-Me2C6H3 groups in these complexes were observed, which further confirm that the two 2,6-Me2C6H3N groups in these complexes are inequivalent and one of them should be uncoordinated. Attempts to force the HCl elimination reaction to go to completion by elevating the reaction temperature to 160 °C failed with the formation of unidentifiable black solids. The failure of a further HCl elimination reaction for complexes 1a0 –1c0 may result from the saturated six-coordinated structures of these complexes which do not allow the remaining free amine group to coordinate to the metal centre and approach to one of the three chloride atoms. Finally, the zirconium and hafnium complexes 1b, 1c, 2b and 2c were synthesized in good yields (61–66%) by the reactions of L1Li2 and L2Li2 with ZrCl4 and HfCl4 in toluene, respectively, as shown in Scheme 3. Attempts to synthesize the corresponding titanium complexes L1TiCl2 and L2TiCl2 by the reactions of TiCl4 with L1Li2 and L2Li2 were unsuccessful and unidentifiable mixtures were obtained. The complexes 1b–1c and 2b–2c were characterized by 1 H and 13C NMR spectroscopy and elemental analyses. As seen from their 1H NMR spectra, the disappearance of the NH signals and the shift of the HC@N resonances (d 8.73–9.03 ppm) toward low field in comparison to those in their free ligands (d 8.62– 8.65 ppm) demonstrate the coordination of the amido and imine groups to the metal centres. For the complexes 2b–2c, only two sets of doublets for the methyl protons of CH(CH3)2 were observed, indicating that complexes 2b–2c have a Cs symmetric configuration in solution. 3.3. Molecular structures of complexes 1b and 2c The molecular structures of 1b and 2c were determined by X-ray crystallography. Their ORTEP views are shown in Figs. 1 and 2, respectively, and their selected bond lengths and angles are listed in Table 2. The central metal atoms in both complexes are six-coordinated and have a distorted octahedral coordination environment. In the complex 1b, the four N atoms of the dianionic ligand occupy the four equatorial positions and the two chloride atoms lie in the two axial positions, with the whole molecule having a Cs symmetric configuration. However, in the complex 2c, the ligand is twisted with the two chloride atoms being located in two cis-positions and the whole molecule possesses a C1 symmetric

N H N Cl M Cl N Cl N

> 140oC unidentifiable black solids in vacuo

M = Ti (1a'), Zr ( 1b'), Hf (1c') Scheme 2. Attempted synthesis of the Ti, Zr and Hf complexes 1a–1c, with complexes 1a0 –1c0 bearing a partial deprotonated tridentate ligand being obtained.

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R

N

N

NH

HN

RR

1. 2 eq nBuLi 2. 1 eq MCl 4 R

in toluene

N N R

Cl M

N

Cl RR

N R

R = Me, M = Zr (1b), Hf (1c) R = i Pr, M = Zr (2b), Hf (2c) Scheme 3. Synthesis of the Zr and Hf complexes 1b–1c and 2b–2c.

their existence. In the structure of 1b, the Zr–Namido bond distances of 2.156(3) and 2.154(3) Å are obviously shorter than the Zr–Nimine bond distances [2.287(3) and 2.325(3) Å]. The Zr–Nimine bond distances are slightly shorter than the corresponding Zr–Nimine bond lengths [2.313(5) and 2.336(5) Å] observed in the salanand salen–zirconium complexes. The Zr–Cl bond distances of 2.4607(8) and 2.4387(8) Å are slightly shorter than those of 2.4994(16) and 2.4884(16) Å observed in salen–zirconium complexes [27]. Noteably, the two 2,6-disubstituted ary rings attached to the two amido nitrogens are nearly parallel to each other, with a small dihedral angle of 11.21(15)°. In similar way, the Hf–Namido bond distances of 2.119(3) and 2.132(4) Å in 2c are also shorter than the Hf–Nimine bond distances of 2.230(3) and 2.235(4) Å. The Hf–Nimine bond distances are slightly shorter than the corresponding Hf–Nimine bond length (2.281(5) Å) observed in a salalen– hafnium complex, while the Hf–Cl bond distances of 2.4274(11) and 2.4694(10) Å in 2c are in line with the corresponding Hf–Cl bond distances of 2.394(2) and 2.4799(16) Å in the reported salalen–hafnium complex [12a]. 3.4. Ethylene polymerization

Fig. 1. Perspective view of complex 1b with thermal ellipsoids drawn at the 30% probability level. Hydrogen atoms are omitted for clarity.

Ethylene polymerization reactions with complexes 1b–1c and 2b–2c as pro-catalysts under different conditions were studied and the results are summarized in Table 3. Upon activation with MAO or AlR3/Ph3CB(C6F5)4, these complexes show moderate catalytic activities for ethylene polymerization and produce high or ultra-high molecular weight polyethylene. For all complexes, the highest catalytic activity was observed at Al/M molar ratios of about 3000 with MAO activated systems and 250 with Al iBu3/Ph3CB(C6F5)4 activated systems. For the same complex, the catalytic activity of the MAO activated system is higher than that of the AlR3/Ph3CB(C6F5)4 activated system under similar conditions. These catalytic systems show relatively long lifetimes and good thermostability, with the highest catalytic activity being observed at 80 °C. Under similar conditions, the catalytic activity of these complexes decreases in the order of 2b > 2c and 1b > 1c. The fact that complex 1b, with two chloride ligands in trans-positions in its solid state structure, shows even higher catalytic activity than complex 2c, with the two chloride ligands in cis-positions, for ethylene polymerization implies an easy configuration transition of these complexes in solution. The results that the complexes with the ligand L2 exhibit higher catalytic activity than the complexes

Table 2 Selected bond lengths (Å) and bond angles (°) in 1b and 2c.

Fig. 2. Perspective view of complex 2c with thermal ellipsoids drawn at 30% probability level. Hydrogen atoms and the uncoordinated solvent are omitted for clarity.

configuration, probably due to the repulsion between the two large 2,6-iPr2C6H3 groups on the two amido N atoms. The 1H NMR observation of the Cs configuration for 2c in solution reveals that the molecule of 2c in solution is in a fast dynamic process on the NMR timescale. The configurational difference observed for 2c in solution and the solid state demonstrates that the configuration of these complexes can change depending on the conditions of

N1–M N2–M N3–M N4–M Cl1–M Cl2–M N1–M–N2 N1–M–N3 N1–M–N4 N2–M–N3 N2–M–N4 N3–M–N4 N1–M–Cl1 N2–M–Cl1 N3–M–Cl1 N4–M–Cl1 N1–M–Cl2 N2–M–Cl2 N3–M–Cl2 N4–M–Cl2 Cl1–M–Cl2

1b (M = Zr)

2c (M = Hf)

2.325(3) 2.287(3) 2.156(3) 2.154(3) 2.4607(8) 2.4387(8) 71.19(9) 151.13(10) 81.89(9) 80.21(9) 152.70(9) 126.92(10) 80.77(6) 89.17(7) 95.30(7) 90.95(7) 88.52(6) 81.47(7) 91.09(7) 93.67(7) 167.62(3)

2.235(4) 2.230(3) 2.132(4) 2.119(3) 2.4274(11) 2.4694(10) 88.18(9) 91.91(13) 79.54(13) 76.82(13) 148.88(13) 109.33(13) 171.86(9) 102.78(9) 89.16(10) 107.70(10) 88.18(9) 80.24(9) 155.46(10) 94.82(10) 87.50(4)

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Table 3 Results of ethylene polymerization with Zr and Hf complexes.a

a b c d

Enter

Catalyst

Co-catalyst

t/min

Al/M ratio

T (°C)

Yield (g)

Activityb

Mgc  104

Tmd (°C)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20

1b 1c 2b 2c 2b 2b 2b 2b 2b 1b 1c 2b 2c 2b 2b 2b 2b 2b 2b 2b

MAO MAO MAO MAO MAO MAO MAO MAO MAO AlMe3/B AlMe3/B AlMe3/B AlMe3/B AlEt3/B Al(iBu)3/B Al(iBu)3/B Al(iBu)3/B Al(iBu)3/B Al(iBu)3/B Al(iBu)3/B

15 15 15 15 15 15 15 15 120 15 15 15 15 15 15 15 15 15 15 120

3000 3000 3000 3000 3000 3000 1500 4500 3000 250 250 250 250 250 250 400 100 250 250 250

80 80 80 80 60 100 80 80 80 80 80 80 80 80 80 80 80 60 100 80

0.23 0.12 0.28 0.14 0.15 0.08 0.22 0.23 0.86 0.15 0.12 0.17 0.09 0.19 0.25 0.12 0.16 trace 0.17 0.74

920 480 1120 560 600 320 880 920 430 600 480 680 360 760 1000 480 640 – 680 370

125.31 104.96 153.58 138.21 145.98 100.45 127.09 102.87 114.87 130.28 158.89 175.89 126.78 133.45 183.87 119.03 176.45 – 121.92 170.93

136.4 137.8 135.5 137.4 136.8 137.2 138.6 135.4 136.2 136.9 137.5 137.2 138.4 137.6 135.9 137.1 138.6 138.2 137.3 137.8

Polymerization conditions: 80 mL toluene; catalyst 1 lmol; ethylene pressure 5 atm; B/M molar ratio = 1.2, B = Ph3CB(C6F5)4. Units of kg PE (mol Cat)1 h1. Measured in decahydronaphthalene at 135 °C. Determined by DSC.

with the ligand L1 may be caused by both of steric and electronic effects. It is well known that electron-donating substituents on the ligands of a catalyst would stabilize the catalytically active cationic species and improve the catalytic activity of the catalyst [28]. On the other hand, moderately bulky ligands would weaken the interaction between the catalyst and the co-catalyst species, which leaves more space for ethylene to coordinate to the central metal and thus enhance the catalytic activity of the catalyst too [29]. The polyethylene samples produced by these catalyst systems possess high or ultra-high molecular weight (100–184  104 g/mol), depending on the structure of the catalyst. As can be seen from the results in Table 3, a catalyst with a bulkier ligand produces polyethylene with higher molecular weight, which can be attributed to the fact that a catalyst with bulkier coordination environment would slow down the rate of both the b-hydride elimination [4b] and the chain transfer reactions [30]. As expected, the molecular weight of the obtained polyethylene decreases with the increase in the Al/M molar ratio and the elevation in the polymerization temperature due to the acceleration of both the chain transfer reaction to alkylaluminum and the b-hydride elimination reaction. In addition, the polyethylenes produced by the Al iBu3/Ph3CB(C6F5)4 activated systems possess higher molecular weight values than those produced by the corresponding MAO activated systems. 13C NMR analysis indicates that the resultant polymers are linear polyethylene with a characteristic resonance at d 30.1 ppm. The melting points of the obtained polyethylene samples range from 135.4–138.6 °C, being typical for linear polyethylene. In comparison to known zirconium and hafnium complexes with salen-, salalen- and salphen-type ligands, these new complexes show comparable catalytic activities for ethylene polymerization under similar conditions. However, the polyethylene produced by these new complexes possesses ultra-high molecular weight, while the complexes with salen-, salalen- and salphen-type ligands usually produce oligoethylene [31] or polyethylene [10a] with relatively low molecular weight. 4. Conclusions Two new anilido-imine tetra-azane ligands L1H2 and L2H2 were synthesized and characterized, and their zirconium and hafnium

complexes L1MCl2 (M = Zr (1b), Hf (1c)) and L2MCl2 (M = Zr (2b), Hf (2c)) were synthesized in high yields by the reactions of L1Li2 and L2Li2 with MCl4 in toluene. Direct HCl-elimination reactions of L1H2 with MCl4 (M = Ti, Zr, Hf) at 140 °C afforded the products L1HMCl3 [M = Ti (1a0 ), Zr (1b0 ), Hf (1c0 )] with a partial deprotonated tridentate ligand in good yields. Single crystal X-ray diffraction analysis on complexes 1b and 2c reveals that the central metal atoms in these complexes have a distorted octahedral coordination environment. Upon activation with MAO or AliR3/Ph3CB(C6F5)4, these complexes exhibit moderate catalytic activity for ethylene polymerization and produce polyethylene with ultra-high molecular weight. The catalytic activity of complexes 1b–1c, 2b–2c for ethylene polymerization vary in the order 2b > 2c and 1b > 1c. Acknowledgment This work was supported by the National Natural Science Foundation of China (Nos. 51173061, 21274050 and U1462111). Appendix A. Supplementary data CCDC 1418717 and 1418718 contain the supplementary crystallographic data for complexes 1b and 2c. These data can be obtained free of charge via http://www.ccdc.cam.ac.uk/conts/ retrieving.html, or from the Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: (+44) 1223336-033; or e-mail: [email protected]. Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.poly.2015.10.018. References [1] (a) G.J.P. Britovsek, V.C. Gibson, D.F. Wass, Angew. Chem. Int. Ed. 38 (1999) 428; (b) C. Redshaw, Y. Tang, Chem. Soc. Rev. 41 (2012) 4484; (c) T. Matsugi, T. Fujita, Chem. Soc. Rev. 37 (2008) 1264. [2] (a) H. Makio, N. Kashiwa, T. Fujita, Adv. Synth. Catal. 344 (2002) 447; (b) Y. Ma, E.B. Lobkovsky, G.W. Coates, Dalton Trans. 44 (2015) 12265. [3] (a) V.C. Gibson, S.K. Spitzmesser, Chem. Rev. 103 (2003) 283; (b) T. Elkin, M.S. Eisen, Catal, Sci. Technol. 5 (2015) 82. [4] (a) S. Yuan, X. Wei, H. Tong, L. Zhang, D. Liu, W.-H. Sun, Organometallics 29 (2010) 2085; (b) S. Matsui, M. Mitani, J. Saito, Y. Tohi, H. Makio, N. Matsukawa, Y. Takagi, K.

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