Group IV compounds bearing a novel tridentate N-donor ligand: synthesis, structures and ethylene polymerization

Abstract

Treatment of the mixed amidinate-amido-lithium, [PhN(CH₂)₃NC(Ph)NSiMe₃]₂Li₄·2Et₂O, with group IV metal chlorides yields the corresponding compounds 3–5. Their structures were confirmed by X-ray diffraction analysis, showing a distorted octahedral geometry. The catalytic behaviors of compounds 3–5 were investigated in the presence of MAO as a co-catalyst. Compounds 3 and 4 showed high activities for ethylene polymerization.

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Introduction

Polyolefins, in particular polyethylene, are very important commercial synthetic polymers used in routine life. They possess diverse remarkable properties such as light weight, high mechanical strength, flexibility, processability, recyclability, and chemical inertness, leading to a wide range of applications. In addition, ultrahigh molecular-weight polyethylenes (UHMW-PEs: M_w = 10⁶ to 10⁷) are an important source of engineering plastics. Hence, the development of new olefin polymerization catalysts, is of immense interest in both academic and industrial research.¹ During the past decades, interest in the use of amidinate ligands in the preparation of main group element compounds stems from the recognition that these complexes can exhibit catalytic activity and that these species are useful for fundamental studies of the effects of ligand geometry on the coordination environments of transition elements. In this general scenario, group IV metallocene and half-metallocene olefin polymerization catalysts have been shown to exhibit very high activities and precise control over product macromolecular architectures, thereby stimulating intense worldwide basic and applied research efforts.²

Our efforts have focused on the (η³-azaallyl) zirconium complexes³ because of the great potential of zirconium complexes in producing poly-olefins with unique properties.⁴ The famous zirconium procatalysts⁵ are mainly divided as metallocene derivatives⁶ and half-metallocene derivatives.⁷ In fact, the bridged η⁵:η¹-coordinated half-metallocene, namely constrained geometry compounds (CGC), have been extensively investigated,⁷ and an analogous titanium catalyst has finally been commercialized.⁸ And the nonbridged half-metallocene compounds showed high efficiency both in catalytic activity and in precisely controlling copolymer properties.⁹ The amidinate [RC(NR′)₂] ligand, which is considered sterically equivalent to η⁵-cyclopentadienyl but, isoelectronic with π-allyl ligands, stabilize mononuclear main group, transition metal and lanthanide complexes, and can also serve as bridging ligands in di- or poly-nuclear metal complexes. And amidinate complexes have proven to be promising as potential catalysts for the oligomerization and polymerization of olefins, hydroamination, intramolecular hydroamination/cyclization and hydrosilylation.^10–15

With respect to the bridged biscyclopentadienyl η⁵:η⁵ ligands, the silyl-linked bis(amidinate) ligands can be considered as η³:η³ alternative systems. Since the latter are more electron-deficient than the former, it is supposedly helpful to enhance the electrophilic behavior of the metal center and then improve its activity for olefin polymerization. Moreover, the catalytic behavior of transition metal compounds bearing the silylamido motifs acting as versatile ligands in various reactive patterns has been widely explored.^16–18

In our recent work, we have reported the monoanionic bridged η³:η¹ ligand containing an amido with pendant amino group^19d,e and another monoanionic ligand of aminosilyl substituted aminopyridinato.^19a These Zr, Ti and Hf compounds show good activities toward ethylene (co)polymerization. On the other hand, in terms of the strained geometry compounds (CGC), the carbon chain-linked mixed amidinate-amido ligand can be considered as an alternative system. Herein, we report the group IV compounds from a novel tridentate N-donor, mixed amidinate-amido, and the synthesis and catalytic properties with regard to ethylene polymerization.

Experimental

General information

All manipulations and reactions were performed under an inert atmosphere of nitrogen using standard Schlenk techniques. Solvents were pre-dried over sodium, refluxed and distilled from sodium/potassium alloy (toluene), sodium/benzophenone (Et₂O, THF) and stored over a sodium mirror under nitrogen. Dichloromethane was distilled from CaH₂ and stored over molecular sieves (4 Å). The deuterated solvent CDCl₃ and C₆D₆ were dried over activated molecular sieves (4 Å). High-purity ethylene was purchased from Beijing Yansan Petrochemical Co. and used as received. All solvents were degassed prior to use. Chemicals were purified by distillation before use except for the commercial sample of n-butyllithium in hexane (2.2 mol dm⁻³, Alfa Aesar Corporation). ¹H, ¹³C-NMR spectra were recorded on a Bruker D8 Venture diffractometer. Elemental analyses were carried out using a Vario EL-III analyzer.

Molecular weights and molecular weight distributions (MWDs) of polyethylene were determined by a PLGPC220 instrument at 150 °C, with 1,2,4-trichlorobenzene as the solvent. Melting points of polyethylenes were measured from the second scanning run on a PerkinElmer DSC-7 differential scanning calorimetry (DSC) analyzer under a nitrogen atmosphere; in the procedure, a sample of about 2.6–5.7 mg was heated to 180 °C at a rate of 20 °C min⁻¹, kept for 5 min at 180 °C to remove the thermal history, and then cooled at a rate of 20 °C min⁻¹ to −40 °C.

Procedure for the synthesis of [PhNH(CH₂)₃NHSiMe₃]. A flame dried Schlenk flask, equipped with a magnetic stirring bar, was charged with PhNH(CH₂)₃NH₂ (13.78 mL, 100 mmol), 100 mL of Et₂O were added under a constant stream of nitrogen and the colorless solution was cooled to 0 °C (ice water bath). LiBuⁿ (45.45 mL, 100 mmol, 2.2 M in hexane) was added dropwise via syringe to the PhNH(CH₂)₃NH₂ solution under a constant stream of nitrogen. The white reaction mixture was warmed to room temperature for 4 hours and then Me₃SiCl (12.66 mL, 100 mmol) was slowly added at −78 °C (acetone/dry ice bath). After stirring for a further 8 hours at room temperature, the mixture was filtered, and the filtrate was distilled by distillation at 140 °C under 75 mmHg to give colorless viscous oil [PhN¹HCH₂CH₂CH₂N²H₂] 1. ¹H-NMR (C₆D₆, 300.00 MHz, ppm): δ 0.11 (s, 9H, Si(CH₃)₃), 0.47 (s, N²H₂), 1.75 (m, 2H, CH₂CH₂CH₂), 3.00–2.89 (t, 2H, N²CH₂), 3.21–3.12 (t, 2H, N¹CH₂), 3.917 (s, N¹H), 7.22–6.65 (m, 5H, Ph). ¹³C-NMR (C₆D₆, 75.00 MHz): δ −0.01 (Si(CH₃)₃), 33.94 (CH₂CH₂CH₂), 39.97 (N²CH₂), 42.12 (N¹CH₂), 148.61–112.74 (Ph). Elemental analysis calculated: C: 64.81, H: 9.97, N: 12.60%. Found C: 64.80, H: 9.96, N: 12.48%.

Procedure for the synthesis of [PhN¹(CH₂)₃N²C(Ph)N³(SiMe₃)]₂Li₄·2Et₂O. To a stirred solution of 1 (1.112 g, 5 mmol) in Et₂O (30 mL) was treated with LiBuⁿ (4.55 mL, 10 mmol, 2.2 M in hexane) at 0 °C. The yellow reaction mixture was stirred at room temperature for 4 hours and then PhCN (0.51 mL, 5 mmol) was slowly added at −78 °C (acetone/dry ice bath). After stirring for a further 8 hours at room temperature, the orange solution was concentrated and crystallized as colorless crystals 2 (1.644 g, 80%). ¹H-NMR (C₆D₆, 300.00 MHz, ppm): δ 0.20 (s, 9H, Si(CH₃)₃), 1.09–1.04 (t, 6H, CH₃, Et₂O), 1.76–1.72 (t, 2H, N³CH₂), 2.85–2.81 (m, 2H, CH₂CH₂CH₂), 3.18–3.15 (t, 2H, N¹CH₂), 3.28–3.21 (m, 4H, CH₂, Et₂O), 7.16–6.58 (m, 5H, Ph). ¹³C-NMR (C₆D₆, 75.00 MHz): δ 15.96 (CH₃, Et₂O), 5.27 (Si(CH₃)₃), 40.41 (CH₂CH₂CH₂), 50.27 (N³CH₂), 65.72 (N¹CH₂), 86.63 (N²C(Ph)N³), 137.25–125.44 (N²C(Ph)N³), 143.51–117.3 (PhN¹). Elemental analysis calculated: C: 67.18, H: 8.32, N: 11.19%. Found C: 67.08, H: 8.32, N: 11.15%.

Procedure for the synthesis of Ti[PhN¹(CH₂)₃N²C(Ph)N³(SiMe₃)]₂. To a solution of 2 (2.466 g, 3 mmol) in Et₂O (30 mL) at −78 °C was added TiCl₄(THF)₂ (1.002 g, 3 mmol) and the reaction mixture was warmed to room temperature and stirred for a further 12 hours, then the mixture was filtered, the filtrate concentrated and crystallized as red crystals 3 (0.967 g, 1.39 mmol) in 46.4% yield. ¹H-NMR (C₆D₆, 300.00 MHz, ppm): δ −0.01 (s, 9H, Si(CH₃)₃), 1.64 (m, 2H, CH₂CH₂CH₂), 2.69–2.51 (t, 2H, N¹CH₂), 3.25–3.02 (m, 4H, CH_2, Et₂O), 7.18–6.56 (m, 5H, Ph). ¹³C-NMR (C₆D₆, 75.00 MHz): δ 2.64 (Si(CH₃)₃), 25.82 (CH₂CH₂CH₂), 30.56 (N³CH₂), 37.70 (N¹CH₂), 57.70 (N²C(Ph)N³), 137.09–128.09 (N²C(Ph)N³), 156.68–113.18 (PhN¹). Elemental analysis calculated: C: 65.49, H: 7.52, N: 12.06%. Found C: 65.38, H: 7.49, N: 12.05%.

Procedure for the synthesis of Zr[PhN¹(CH₂)₃N²C(Ph)N³(SiMe₃)]₂. Using the same procedure as compound 3, the compound 4 was isolated as colorless crystals 4 (0.974 g, 1.32 mmol) in 44% yield. ¹H-NMR (C₆D₆, 300.00 MHz, ppm): δ −0.09 (s, 9H, Si(CH₃)₃), 1.95–1.63 (m, 2H, CH₂CH₂CH₂), 2.93–2.83 (t, 2H, N¹CH₂), 3.19–3.10 (m, 4H, CH₂, Et₂O), 7.18–6.56 (m, 5H, Ph). ¹³C-NMR (C₆D₆, 75.00 MHz): δ 2.64 (Si(CH₃)₃), 31.82 (CH₂CH₂CH₂), 46.56 (N³CH₂), 57.70 (N¹CH₂), 77.70 (N²C(Ph)N³), 137.09–128.09 (N²C(Ph)N³), 156.68–113.18 (PhN¹). Elemental analysis calculated: C: 61.66, H: 7.08, N: 11.35%. Found C: 61.56, H: 7.02, N: 11.25%.

Procedure for the synthesis of Hf[PhN¹(CH₂)₃N²C(Ph)N³(SiMe₃)]₂. Using the same procedure as compound 3, the compound 5 was isolated as colorless crystals 5 (1.134 g, 1.37 mmol) in 54.8% yield. ¹H-NMR (C₆D₆, 300.00 MHz, ppm): δ −0.05 (s, 9H, Si(CH₃)₃), 1.79 (m, 2H, CH₂CH₂CH₂), 3.18–2.99 (t, 2H, N¹CH₂), 3.51–3.30 (m, 4H, CH₂, Et₂O), 7.23–6.47 (m, 5H, Ph). ¹³C-NMR (C₆D₆, 75.00 MHz): δ 0.01 (Si(CH₃)₃), 30.26 (CH₂CH₂CH₂), 31.37 (N³CH₂), 42.90 (N¹CH₂), 48.85 (N²C(Ph)N³), 126.67–112.68 (N²C(Ph)N³), 129.19–112.84 (PhN¹). Elemental analysis calculated: C: 55.15, H: 6.33, N: 10.16%. Found C: 55.05, H: 6.13, N: 10.15%.

X-ray crystallographic studies

X-Ray diffraction data of 2–5 were collected with graphite monochromated Mo-Kα radiation (λ = 0.71073 Å) on a Bruker Smart Apex CCD diffractometer, equipped with an Oxford Cryosystems CRYOSTREAM device. The collected frames were processed with the proprietary software SAINT and absorption corrections were applied (SADABS) to the collected reflections.^20,21 The structures of the molecules were solved by direct methods and expanded by standard difference Fourier syntheses using the software SHELXTL.²² Structure refinements were made on F² using the full-matrix least-squares technique. All non-hydrogen atoms were refined with anisotropic displacement parameters. Hydrogen atoms were placed in idealized positions and allowed to ride on the respective parent atoms. Crystal data and refinement parameters for all crystalline complexes are given in Table 1.

Table 1 Crystal data and structure refinement for complexes 2–5

Compound reference	2	3	4	5
Chemical formula	C46H69Li4N6O2Si2	C38H50N6Si2Ti	C38H50N6Si2Zr	C38H50N6Si2Hf
Formula weight	822.01	694.92	738.24	827.52
Crystal system	Triclinic	Triclinic	Monoclinic	Monoclinic
a/Å	12.2130(5)	9.8027(4)	10.5821(4)	10.5582(5)
b/Å	13.3811(5)	12.2995(4)	17.9611(8)	17.9443(8)
c/Å	16.3014(7)	17.4058(6)	20.3816(9)	20.4584(9)
α/°	84.4800(10)	99.9200(10)	90.00	90.00
β/°	72.5300(10)	94.7200(10)	95.5500(10)	95.1510(10)
γ/°	83.8350(10)	109.6330(10)	90.00	90.00
Unit cell volume/Å^3	2520.70(18)	1924.68(12)	3855.7(3)	3860.4(3)
Temperature/K	200(2)	194(2)	194(2)	200(2)
Space group	P1̄	P1̄	P21/c	P21/c
No. of formula units per unit cell, Z	2	2	4	4
ρcalc (g cm^−3)	1.083	1.199	1.272	1.424
No. of reflections collected	25 182	14 852	28 637	28 442
No. of independent reflections	12 118	6666	6806	6813
No. of parameters	587	430	418	430
Rint	0.0312	0.0262	0.0288	0.0327
Final R1 values (I > 2σ(I))	0.0739	0.0379	0.0618	0.0282
Final wR(F^2) values (I > 2σ(I))	0.2073	0.0980	0.1384	0.1097
Final R1 values (all data)	0.0998	0.0475	0.0679	0.0324
Final wR(F^2) values (all data)	0.2326	0.1054	0.1442	0.1155
Goodness of fit on F^2	1.105	1.043	1.012	0.978

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General procedure for ethylene polymerization

A 500 mL autoclave stainless steel reactor equipped with a mechanical stirrer and a temperature controller was heated under vacuum for at least 3 h at 80 °C. It was allowed to cool to the required reaction temperature and then charged with an ethylene atmosphere. 50 mL of toluene was first injected into the autoclave, another 30 mL toluene containing the dissolved complex and the required amount of co-catalyst (MAO), plus the residual toluene were successively added using a syringe. The total volume was 100 mL. The reaction mixture was stirred for the desired time under 10 atm pressure of ethylene throughout the entire experiment. The polymerization reaction was quenched by the addition of a solution of ethanol containing HCl. The precipitated polymer was collected by filtration, washed with ethanol several times and dried in a vacuum.

Results and discussion

Synthesis and characterization

The precursor N-phenyl-N′-(trimethylsilyl)propane-1,3-diamine [PhNH(CH₂)₃NHSiMe₃] 1 was prepared by reaction of PhNH(CH₂)₃NH₂ with butyllithium followed by one equivalent of Me₃SiCl in Et₂O in high yield. Subsequently the diamine 1 reacted with double equivalent of LiBuⁿ in Et₂O to afford a solution of lithium intermediate, and then reacted with PhCN to get the adduct lithium salt 2, which was used in situ in the further reactions. Complexes 3–5 were prepared from the reaction of the lithium salt (2) with an equivalent of TiCl₄(THF)₂, ZrCl₄ and HfCl₄, respectively.

The complexes 3–5 have similar structures and six nitrogen atoms of both mixed amidinate-amido ligands are bound to metal centres (Ti⁴⁺, Zr⁴⁺ or Hf⁴⁺). Compared with mixed amidinate-amido ligand, only a few examples of corresponding titanium,^23a ytterbium,^23b germanium and stannum^23c,d complexes have been reported so far. The metallic compounds 3–5 are dianionic, in which the mixed amidinate-amido group acts in a η³-coordination fashion.

The crystalline metal complexes 3–5 were isolated in good to medium yields and were highly soluble in CH₂Cl₂ and THF, respectively. Complexes 2–5 were fully characterized by ¹H-NMR, ¹³C-NMR spectroscopy, elemental analysis and X-ray diffraction. The synthetic procedures are summarized in Scheme 1. As shown in the scheme, the new ligand presents tridentate modes toward different metal ions.

Scheme 1 Synthesis of the complexes 1–5.

Crystal structures

In the molecular structure of compounds 2–5 are shown in Fig. 1–4 and details of the structural parameters are given in Tables 2–6. All the hydrogen atoms were located in the Fourier difference map and were subsequently refined. The crystal structure of lithium compound 2 is illustrated in Fig. 1 with selected bond lengths and angles in Table 2.

Fig. 1 Molecular structure of complex 2 with thermal ellipsoids drawn at 30% probability level. Hydrogen atoms are omitted for clarity.

Fig. 2 Molecular structure of complex 3 with thermal ellipsoids drawn at 30% probability level. Hydrogen atoms are omitted for clarity.

Fig. 3 Molecular structure of complex 4 with thermal ellipsoids drawn at 30% probability level. Hydrogen atoms are omitted for clarity.

Fig. 4 Molecular structure of complex 5 with thermal ellipsoids drawn at 30% probability level. Hydrogen atoms are omitted for clarity.

Table 2 Selected bond lengths and angles for complex 2

Li(1)–O(1)	1.965(5)	Li(3)–O(2)	1.960(6)
Li(1)–N(4)	2.044(5)	Li(3)–N(5)	2.224(5)
Li(1)–N(1)	2.081(5)	Li(3)–N(2)	2.289(6)
Li(2)–N(1)	1.954(5)	Li(3)–N(6)	2.290(5)
Li(2)–N(6)	1.997(5)	Li(4)–N(4)	1.954(5)
Li(2)–N(2)	2.052(5)	Li(4)–N(3)	2.014(5)
Li(2)–N(5)	2.580(5)	Li(4)–N(5)	2.048(5)
Li(3)–N(3)	2.267(5)	Li(4)–N(2)	2.483(5)
N(4)–Li(1)–N(1)	129.0(3)	N(3)–Li(3)–N(2)	59.79(15)
N(1)–Li(2)–N(6)	143.4(3)	N(5)–Li(3)–N(6)	60.55(14)
N(1)–Li(2)–N(2)	103.7(2)	N(3)–Li(3)–N(6)	142.4(3)
N(6)–Li(2)–N(2)	112.4(2)	N(2)–Li(3)–N(6)	94.6(2)
N(1)–Li(2)–N(5)	124.8(2)	N(4)–Li(4)–N(3)	142.3(3)
N(6)–Li(2)–N(5)	57.99(14)	N(4)–Li(4)–N(5)	103.9(2)
N(2)–Li(2)–N(5)	96.35(19)	N(3)–Li(4)–N(5)	109.5(2)
N(5)–Li(3)–N(3)	95.2(2)	N(4)–Li(4)–N(2)	131.1(3)
N(5)–Li(4)–N(2)	99.53(19)	N(3)–Li(4)–N(2)	59.59(14)
N(5)–Li(3)–N(2)	100.5(2)

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Table 3 Selected bond lengths and angles for complex 3

Ti(1)–N(1)	1.9726(17)	Ti(1)–N(5)	2.0778(16)
Ti(1)–N(4)	1.9735(16)	Ti(1)–N(6)	2.1947(16)
Ti(1)–N(2)	2.0616(16)	Ti(1)–N(3)	2.2168(17)
N(1)–Ti(1)–N(4)	96.81(7)	N(2)–Ti(1)–N(6)	98.59(7)
N(1)–Ti(1)–N(2)	84.94(7)	N(5)–Ti(1)–N(6)	62.16(6)
N(4)–Ti(1)–N(2)	111.63(7)	N(1)–Ti(1)–N(3)	146.67(7)
N(1)–Ti(1)–N(5)	116.80(7)	N(4)–Ti(1)–N(3)	93.03(7)
N(4)–Ti(1)–N(5)	85.39(6)	N(2)–Ti(1)–N(3)	61.87(6)
N(2)–Ti(1)–N(5)	151.40(7)	N(5)–Ti(1)–N(3)	95.64(6)
N(1)–Ti(1)–N(6)	97.39(7)	N(6)–Ti(1)–N(3)	90.92(6)
N(4)–Ti(1)–N(6)	147.54(6)

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Table 4 Selected bond lengths and angles for complex 4

Zr(1)–N(4)	2.127(3)	Zr(1)–N(5)	2.243(3)
Zr(1)–N(1)	2.131(3)	Zr(1)–N(3)	2.272(3)
Zr(1)–N(2)	2.242(3)	Zr(1)–N(6)	2.276(3)
N(4)–Zr(1)–N(1)	97.00(13)	N(2)–Zr(1)–N(3)	58.83(12)
N(4)–Zr(1)–N(2)	123.19(13)	N(5)–Zr(1)–N(3)	99.82(13)
N(1)–Zr(1)–N(2)	79.28(12)	N(4)–Zr(1)–N(6)	137.32(12)
N(4)–Zr(1)–N(5)	78.78(13)	N(1)–Zr(1)–N(6)	100.18(13)
N(1)–Zr(1)–N(5)	120.82(14)	N(2)–Zr(1)–N(6)	98.36(12)
N(2)–Zr(1)–N(5)	150.27(14)	N(5)–Zr(1)–N(6)	58.86(12)
N(4)–Zr(1)–N(3)	101.79(13)	N(3)–Zr(1)–N(6)	91.00(12)
N(1)–Zr(1)–N(3)	137.83(12)

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Table 5 Selected bond lengths and angles for complex 5

Hf(1)–N(4)	2.115(4)	Hf(1)–N(2)	2.219(4)
Hf(1)–N(1)	2.119(4)	Hf(1)–N(3)	2.253(4)
Hf(1)–N(5)	2.214(4)	Hf(1)–N(6)	2.255(4)
N(4)–Hf(1)–N(1)	95.38(15)	N(5)–Hf(1)–N(3)	99.04(13)
N(4)–Hf(1)–N(5)	80.21(14)	N(2)–Hf(1)–N(3)	58.99(14)
N(1)–Hf(1)–N(5)	121.6(14)	N(4)–Hf(1)–N(6)	138.76(13)
N(4)–Hf(1)–N(2)	119.78(15)	N(1)–Hf(1)–N(6)	101.45(15)
N(1)–Hf(1)–N(2)	79.85(15)	N(5)–Hf(1)–N(6)	58.87(13)
N(5)–Hf(1)–N(2)	150.96(18)	N(2)–Hf(1)–N(6)	100.24(15)
N(4)–Hf(1)–N(3)	100.02(17)	N(3)–Hf(1)–N(6)	91.85(13)
N(1)–Hf(1)–N(3)	138.45(14)

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Table 6 Ethylene polymerization results by complexes 3–5/MAO systems^a

Entry	Cat.	Al/M	t (min)	T (°C)	Polym. (g)	Act^b	Mw^c^,^d	Mw/Mn^d	Tm^e (°C)
a Conditions: 5 μmol catalyst, toluene (total volume 100 mL), 10 atm, 30 min.b Activity: 10^4 g PE per mol (Cat.) per h.c 10^5 g mol^−1.d Determined by GPC.e Determined by DSC.
1	3	1000	30	30	0.196	7.84	9.71	9.40	133.28
2	3	1500	30	30	0.210	8.40	6.99	6.21	133.28
3	3	2500	30	30	0.294	11.76	10.66	8.11	132.71
4	3	3000	30	30	0.278	11.12	8.79	8.51	133.06
5	3	2500	30	50	0.305	12.20	6.62	11.32	134.33
6	3	2500	30	70	0.382	15.28	6.59	12.06	134.91
7	3	2500	30	80	0.446	17.84	2.79	9.11	132.36
8	3	2500	30	90	0.149	5.96	5.19	12.72	132.47
9	3	2500	15	80	0.192	15.36	5.09	15.39	132.10
10	3	2500	45	80	0.513	13.68	5.36	16.29	133.93
11	4	500	30	30	0.008	0.32	15.79	2.48	133.35
12	4	1000	30	30	0.014	0.56	4.74	18.23	134.07
13	4	1500	30	30	0.008	0.32	14.28	2.28	134.00
14	4	2000	30	30	0.007	0.28	—	—	—
15	4	1000	30	50	0.032	1.28	11.17	21.51	134.49
16	4	1000	30	70	0.087	3.48	11.14	19.74	133.59
17	4	1000	30	80	0.065	2.60	5.62	16.23	133.58
18	5	1000	30	50	Trace	—	—	—	—
19	5	1000	30	70	Trace	—	—	—	—

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In the solid state, lithium 2 crystallizes in the triclinic space group P1̄ with two independent molecules in the unit cell. And the lithium 2 fixes the four Li ions in the close coplanarity (mean deviation 0.06 Å). The three nitrogen atoms of each ligand are coplanar, and the dihedral angle of 63.99°. And the dihedral angle between N(1)–N(2)–N(3) and Li(1)–Li(2)–Li(3)–Li(4) is 36.95° and between N(4)–N(5)–N(6) and Li(1)–Li(2)–Li(3)–Li(4) is 38.92°, respectively. Li1 atom is coordinated with N1, N4 and ether; Li2 and Li3 are coordinated with N1, N2, N5, N6 and N2, N3, N4, N5, respectively; and Li5 is coordinated with N2, N3, N5, N6 and ether. The Li–N distances (1.954(5)–2.580(5) Å) were longer than Li–N distances of normal amidinate 1.961–1.962 Å^19c,f and 2.01–2.18 Å.²⁴

In the solid state, compound 3 crystallizes in the triclinic space group P1̄ with two independent molecules in the unit cell, however, compounds 4 and 5 crystallize in the monoclinic space group P21/c with four independent molecules in the unit cell. The three nitrogen atoms of each ligand are coplanar, and the dihedral angle of 87.76° for compound 3 (82.23° for compound 4, and 83.07° for compound 5).

In the solid state of 3, the titanium atom is bound to two tridentate [NNN] ligands in a distorted octahedral geometry. The titanium atom is located in the plane defined by three N atoms from both mixed amidinate-amido ligands, namely [N1N2N3] and [N4N5N6]. The Ti1 atom slightly deviates from the planes constructed by the coordinated atoms (N1, N2, N3 and N4, N5, N6) by about 0.077 and 0.016 Å, respectively. The sum of N(3)–Ti(1)–N(5) = 95.64(6)°, and N(1)–Ti(1)–N(5) = 116.80(7)° bond angles is nearly 360° (mean deviation 0.13 Å). Moreover, the mean planes of the two tridentate mixed amidinate-amido ligands coordinated to the titanium center are nearly orthogonal with a dihedral angle of 87.8°. The dihedral angle between N(1)–Ti(1)–N(2) and N(4)–Ti(1)–N(5), N(2)–Ti(1)–N(3) and N(5)–Ti(1)–N(6) are 86.41° and 89.96°, respectively. The average Ti–ligand bond distances (Ti1–N1 = 1.9726(17) Å, Ti1–N2 = 2.0616(16) Å, Ti1–N3 = 2.2168(17) Å, Ti1–N4 = 1.9735(16) Å, Ti1–N5 = 2.0778(16) Å, Ti1–N6 = 2.1947(16) Å) are typical of titanium amido bonds and amidinate bonds.^19a–c,25 In addition, in complex 3, the amidinate C–N bond existed a distinctive bond character between single and double-bond (N2–C10 = 1.306(5) Å, N3–C10 = 1.346(5) Å, N5–C29 = 1.314(5) Å, N6–C29 = 1.343(5) Å).

The molecular structures of complexes 4 and 5, as shown in Fig. 3 and 4, are similar to that of complex 3. In the solid state, complexes 4 and 5 also exhibit a distorted octahedral geometry. Three nitrogen atoms of each mixed amidinate-amido ligand are coplanar too. The Zr1 atom slightly deviates from the planes constructed by the coordinated atoms (N1, N2, N3 and N 4, N5, N6) by about 0.135 and 0.148 Å, respectively (for Hf1: 0.159 and 0.145 Å). The average metal–ligand bond distances (Zr1–N1 = 2.131(3) Å, Zr1–N2 = 2.242(3) Å, Zr1–N3 = 2.272(3) Å, Zr1–N4 = 2.127(3) Å, Zr1–N5 = 2.243(3) Å, Zr1–N6 = 2.276(3) Å; Hf1–N1 = 2.119(4) Å, Hf1–N2 = 2.219(4) Å, Hf1–N3 = 2.253(4) Å, Hf1–N4 = 2.115(4) Å, Hf1–N5 = 2.214(4) Å, Hf1–N6 = 2.255(4) Å) of complexes 4 and 5 are slightly longer than those observed for complex 3. In addition, the amidinate C–N bond distances are comparable to complex 3, which have a distinctive bond character between single and double-bond. The dihedral angle between N(1)–Zr(1)–N(2) and N(4)–Zr(1)–N(5), N(2)–Zr(1)–N(3) and N(5)–Zr(1)–N(6) are 79.56° and 89.25°, respectively (for 5: N(1)–Hf(1)–N(2) and N(4)–Hf(1)–N(5), N(2)–Hf(1)–N(3) and N(5)–Hf(1)–N(6) are 79.39° and 90.00°).

Catalytic behavior for ethylene polymerization

The influences of various catalytic systems formed from 3–5 in the presence of cocatalysts such as methylaluminoxane (MAO) on the ethylene activation were evaluated. To demonstrate what factors and how they may affect the ethylene polymerization, the activity of catalyst precursors 3 and 4 were particularly investigated at varied Al/Ti (or Zr) molar ratio and temperature, respectively. The detailed results are summarized in Table 6.

For the 3/MAO system, increasing the Al/Ti molar ratio from 1000 to 3000 at 30 °C gave the highest activity of 1.176 × 10⁵ g per mol (Ti) per h at an Al/Ti molar ratio of 2500 (entry 3). However, a further increase to 3000 led to a decrease of catalytic activity (entries 3 and 4). Probably, the active species was suppressed by an excess of MAO, as suggested in the literature,²⁵ moreover, the amount of AlMe₃ contained in MAO led to deactivation. At low reaction temperature (30 °C), the polymers show high molecular weights but broad molecular weight distributions, which is coherent with the literature.²⁶ And complex 3 was selected for a series of polymerizations at varied reaction temperature from 30 to 90 °C while other reaction parameters were not changed. Apparently, the activity of the catalyst increased in this whole process with increased reaction temperature. With the temperature from 30 to 80 °C, the activity of 3 increased from 1.176 × 10⁵ to 1.784 × 10⁵ g per mol (Ti) per h. Regarding the obtained polyethylenes, the higher molecular weights were achieved with lower reaction temperatures (entries 3 and 5–8 in Table 6). It is assumed that the higher chain transfer and termination easily takes place at elevated temperatures and slightly lower solubility of ethylene in toluene solution at elevated temperatures. Overall, the catalytic system displays good activity over a wide range of reaction temperature. Moreover, the resultant PEs has broader molecular weight distributions at higher reaction temperature, indicating that uniform formation of active sites requires higher reaction temperatures. Since the polymerization is a highly exothermic reaction, the reaction temperature will rise and accelerate the formation of the active species. Polyethylenes with broader molecular weight distributions would be expected compared with the results for mononuclear catalyst precursors like zirconium complexes.^19d In the activation process of complex 3 with MAO, different metal–nitrogen bonds can be cleaved. Furthermore, the spacer moiety can be transferred to the co-catalyst MAO. At the beginning of the polymerization experiment, the different species may all be present in the reaction mixture.^27,28 Molecular weight, on the other hand, increased as temperature was raised. This suggests that single molecule might show higher catalytic activity at higher temperature, although deactivation or decomposition proceeds faster at higher temperature.

Ethylene polymerization using procatalyst 4 was also investigated. For the 4/MAO system, increasing the molar ratio of Al/Zr from 500 to 1500 at 30 °C led to an increase in polymerization activity from 0.32 × 10⁴ g per mol (Zr) per h to 0.56 × 10⁴ g per mol (Zr) per h (entries 11–13), further increasing the Al/Zr ratio to 2000 led to the decrease of catalytic activities (entries 14). The reaction temperature was varied from 30 to 80 °C and the highest activity 3.48 × 10⁴ g per mol (Zr) per h was observed at 70 °C (entries 12, 15–17).

In our case, the change of central metal affected the catalytic activity of the compounds significantly. Ti compound 3 showed a catalytic activity of 1.784 × 10⁵ g per mol (Ti) per h, while Zr compound 4 showed a relatively lower value of 3.48 × 10⁴ g per mol (Zr) per h and Hf compound 5 showed tiny polymers. Without consideration of the polymerization conditions and types of co-catalysts, the ethylene polymerization activities of pro-catalysts 3 and 4 are comparable or higher than that of the phenylamido aniline zirconium or titanium complexes,²⁹ but lower than the electron-rich bisamidinate titanium complexes^19b and polydentate nitrogen ligand zirconium complex.^19c–f

The GPC data indicated that some obtained polyethylene had a molecular weight of higher than a million for pro-catalysts 3 and 4 (Table 6), respectively. In comparison to most of the polydentate nitrogen ligand zirconium^19c–g or titanium^19c complexes, the new catalysts produce PEs with much higher molecular weights, except for the electron-rich bisamidinate titanium complexes.^19b It is likely that the planar structure of the mixed amidinate-amido ligand of pro-catalysts 3 and 4 provide more space for monomer coordination and result in PEs showing high molecular weights. All the melting points of the resultant polyethylene are within relatively narrow ranges and higher than 132 °C, which indicates the formation of high-density polyethylene (HDPE) obtained by 3 and 4, respectively.³⁰ The ¹H-NMR and ¹³C-NMR spectrums for the PE sample listed in entry 7 of Table 6 analysis of the obtained PE confirms no presence of end vinyl groups (see ESI, Fig. 1 and 2†), which implies that chain termination occurs with aluminum alkyls transfer for polyethylene.³¹ The ¹³C-NMR spectrum of the methylene and methyl region of polyethylene obtained by the 3/MAO system shows only one strong signal at 30 ppm for methylene units, indicating that the polyethylene is highly linear. DSC and ¹³C-NMR studies indicated that the polyethylene produced is highly crystalline and highly linear.³²

Regarding the lifetime of the 3/MAO system, the ethylene polymerization was conducted over different periods, namely 15, 30 and 45 min (entries 7 and 9–10); the catalytic activities decreased significantly with longer reaction times. The obtained polyethylene showed higher molecular weights and wider polydispersities along with increased reaction times, illustrating partial deactivation of active sites over extended reaction times; meanwhile, the increased viscosity of the reaction medium due to the initial formation of PE hinders the diffusion of ethylene into the reaction medium.

Conclusion

The new group IV (Ti, Zr, Hf) compounds from tridentate N-donor, mixed amidinate-amido containing carbon chain are synthesized and characterized by single-crystal X-ray diffraction. In the solid state, the compounds 3–5 are distorted octahedral. The pro-catalysts 3–5 were screened for ethylene polymerization and compound 3 exhibited good activity toward (up to 1.784 × 10⁵ g per mol (Ti) per h), producing polymers with high molecular weights, broad molecular distributions and relatively narrow ranges and higher than 132 °C, which indicates the formation of high-density polyethylene (HDPE). More importantly, the current titanium procatalyst showed excellent thermal stability at 80 °C regarding the aspect of industrial requirement.

Acknowledgements

The authors acknowledge the financial support from the Natural Science Foundation of China (No. 21272142) and the Natural Science Foundation of Shanxi Province (No. 2015011015).

Notes and references

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Footnote

† Electronic supplementary information (ESI) available: The ¹H-NMR and ¹³C-NMR spectra for the PE sample listed in entry 7 of Table 6. Crystallographic data for the structural analysis of complexes 2–5. And CIF files giving crystallographic data for complexes 2 (CCDC ID 1444243), 3 (CCDC ID 1444236), 4 (CCDC ID 1444240) and 5 (CCDC ID 1444241). For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/c6ra01365a

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