1. Introduction

catalysts

Catalysts

2073-4344

MDPI

10.3390/catal3010261

catalysts-03-00261

Article

Phosphine-Thiophenolate Half-Titanocene Chlorides: Synthesis, Structure, and Their Application in Ethylene (Co-)Polymerization

Tang

Xiao-Yan

1 2 Liu

Jing-Yu

1 * Li

Yue-Sheng

1 State Key Laboratory of Polymer Physics and Chemistry, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, Changchun 130022, China; E-Mails: tangxiaoyan@ciac.jl.cn (X.-Y.T.); ysli@ciac.jl.cn (Y.-S.L.) 2 University of the Chinese Academy of Sciences, Changchun Branch, Changchun 130022, China

* Author to whom correspondence should be addressed; E-Mail: ljy@ciac.jl.cn; Tel.: +86-431-85262881; Fax: +86-431-85262039.

06 03 2013

03 2013

3 1 261 275 28 12 2012 08 02 2013 17 02 2013

2013

This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution license (http://creativecommons.org/licenses/by/3.0/).

A series of novel half-titanocene complexes CpTiCl₂[S-2-R-6-(PPh₂)C₆H₃] (Cp = C₅H₅, 2a, R = H; 2b, R = Ph; 2c, R = SiMe₃) have been synthesized by treating CpTiCl₃ with the sodium of the ligands, 2-R-6-(PPh₂)C₆H₃SNa, which were prepared by the corresponding ligands and NaH. These complexes have been characterized by ¹H, ¹³C and ³¹P NMR as well as elemental analyses. Structures for 2a–b were further confirmed by X-ray crystallography. Complexes 2a–b adopt five-coordinate, distorted square-pyramid geometry around the titanium center, in which the equatorial positions are occupied by sulfur and phosphorus atoms of the chelating phosphine-thiophenolate and two chlorine atoms, and the cyclopentadienyl ring is coordinated on the axial position. The complexes 2a–c were investigated as the catalysts for ethylene polymerization and copolymerization of ethylene with norbornene in the presence of MMAO or Ph₃CB(C₆F₅)₄/ⁱBu₃Al as the cocatalyst. All complexes exhibited low to moderate activities towards homopolymerization of ethylene. However, they displayed moderate to high activities towards copolymerization of ethylene with norbornene.

copolymerization catalysts ethylene

1. Introduction

The development of well-defined, single-site group 4 metal catalysts has attracted much attention for olefin polymerization [1,2,3]. Many studies have also been focused on the relationship between the structure and catalytic properties of a given catalyst with respect to polymer chain composition and architecture [4,5,6,7]. Recent studies of half-metallocenes, having one cyclopentadienyl moiety and one monoanionic ligand have led to new classes of catalysts. [8,9,10,11,12,13,14,15,16,17,18,19,20,21,22,23,24,25,26,27,28,29,30,31,32,33,34,35,36,37,38,39,40] The bridged half-sandwich group 4 metal complexes “constrained geometry” catalysts (CGCs) have proven to be a very successful class of catalysts which exhibit high activities and high comonomer incorporations [8]. Recently, non-bridged half-sandwich group 4 metal complexes containing anionic ancillary donor ligands have become a hot topic because of their easy modifications and remarkable catalytic activity for olefin (co)polymerization [9,10,11,12,13,14,15,16,17,18,19,20,21,22,23,24,25,26,27,28,29,30,31,32,33,34,35,36,37,38,39,40]. For instance, Nomura et al. found that the half-titanocenes containing aryloxo ligands exhibited remarkable catalytic activity in olefin polymerization and efficient comonomer incorporation in ethylene/α-olefin copolymerization [10,29]. Gibson and his coworkers reported a series of bis(phosphanylphenoxide) group 4 metal dichloride complexes, and further suggested that the use of ancillary ligands with softer L donors, such as phosphorus and sulfur, might offer beneficial stabilization of the highly reactive metal center [41,42]. Subsequently, our group found that high-temperature living ethylene/norbornene copolymerization could be achieved by using bis(phenoxy-phosphine)-titanium complexes as the catalysts in the presence of MMAO [43].

To combine the advantages of metallocene and phenoxy-phosphine ligands, we reported a series of o-di(phenyl)phosphanylphenolate-based half-titanocene complexes, CpTiCl₂[O-2-R¹-4-R²-6- (PPh₂)C₆H₂] (Cp = C₅H₅, A: R¹ = R² = H; B: R¹ = F, R² = H; C: R¹ = Ph, R² = H; D: R¹ = SiMe₃, R² = H; E: R¹ = ^tBu, R² = H; F: R¹= R² = ^tBu), supported by bidentate ligands with softer donor atoms (Chart 1) [28]. We found that all complexes exhibited low to moderate activities towards homopolymerization of ethylene, but displayed excellent ability to copolymerize ethylene with norbornene, and catalytic activity was more than 100 times greater than that of ethylene homopolymerization in the case of Ph₃CB(C₆F₅)₄/ⁱBu₃Al as cocatalyst, affording the copolymers with high comonomer incorporations. To further investigate the electronic effect of the ligand on polymerization behaviors, we synthesized some half-titanocene complexes containing phosphine-thiophenolate ligands. Herein, we thus described the synthesis and characterization of some novel phosphine-thiophenolate-based half-titanocene complexes, CpTiCl₂[S-2-R-6-(PPh₂) C₆H₃] (Cp = C₅H₅, 2a, R = H; 2b, R = Ph; 2c, R = SiMe₃), and explored their application in ethylene polymerization and ethylene/NBE copolymerization.

Chart 1

Structures of complexes A–F.

2. Results and Discussion

A series of novel half-titanocene complexes CpTiCl₂[S-2-R-6-(PPh₂)C₆H₃] (Cp = C₅H₅, 2a, R = H; 2b, R = Ph; 2c, R = SiMe₃) have been synthesized in high yields (76-85%) by treating CpTiCl₃ with the sodium of the ligands, 2-R-6-(PPh₂)C₆H₃SNa, which were prepared by the corresponding ligands and NaH, as shown in Scheme 1. The resultant complexes were identified by ¹H, ¹³C and ³¹P NMR spectra and elemental analysis. The ¹H NMR spectra of these complexes showed no complexity, and the integration of complexes confirms a 1:1 ratio of cyclopentadienyl to phosphine-thiophenolate ligand. A doublet for the Cp hydrogens was observed for complexes 2a–c (2a, δ 6.47 ppm, J_HP = 2.5 Hz; 2b, δ 6.45 ppm, J_HP= 2.3 Hz; 2c, δ 6.44 ppm, J_HP = 2.3 Hz), which suggests phosphorus is coordinated to titanium. The observed results were an interesting contrast to those found in the analogues containing o-di(phenyl)phosphanylphenolate ligands in which the phosphorus in A and C is not coordinated to Ti. In addition, all signals in the ³¹P NMR spectra of complexes 2a–c (2a, δ 44.33 ppm; 2b, δ 43.32 ppm; 2c, δ 42.53 ppm) were shifted substantially downfield from the values found for the corresponding ligands 1a–c (1a, δ -13.1 ppm; 1b, δ −12.20 ppm; 1c, δ –13.6 ppm).

2.1. Synthesis and Characterization of Half-Titanocene Complexes CpTiCl2[S-2-R-6-(PPh2)C6H3]

Crystals of 2a–b suitable for crystallographic analysis were grown from the chilled concentrated CH₂Cl₂-hexane mixture solution. The crystallographic data together with the collection and refinement parameters are summarized in Table 1. Selected bond distances and angles for 2a–b and C–D [28] were summarized in Table 2. If the centroid of the cyclopentadienyl ring is considered as a single coordination site, complexes 2a–b adopt five-coordinate, distorted square-pyramid geometry around the titanium center, where the equatorial positions are occupied by sulfur and phosphorus atoms of the ligands and two chlorine atoms. The cyclopentadienyl group is coordinated on the axial position, as shown in Figure 1, Figure 2. The configurations of 2a–b in the solid state were in line with the results observed in the ³¹P NMR spectra. These results are very different from those for complexes A–F, in which the steric bulk in R position is required in generation of five-coordinate, distorted square-pyramid geometry around the titanium center. However, for phosphine-thiophenolate-based half-titanocene complexes, the steric bulk in R position is not essential to generate distorted square-pyramid geometry around the titanium center.

Scheme 1

Synthesis of complexes 2a–c.

Ti(1)-S(1) bond distance in complex 2b (2.4359(9) Å) is slightly longer than that in 2a (2.4188(10) Å). Ti-Cp (centroid) distances in 2a–b (2d: 2.027, 2e: 2.029) are longer than that in C (2.016 Å), but somewhat shorter than that in D (2.034 Å). The five-membered C₂SPTi chelate ring in 2a–b has an envelope conformation with the metal lying ca. 1.132–1.136 Å (2a: 1.136, 2b: 1.132 Å) out of the C₂SP plane in the direction of Cp ring, whereas the metal lies ca. 0.549–0.603 Å (D: 0.549, E: 0.603, F: 0.593 Å) out of the C₂OP plane in D–F. The Ti(1)-P(1) bond length in 2a–b is slightly affected by the R group of the ligand (2a: 2.5769(9), 2b: 2.5630(10) Å), which is shorter than that in D–F (D: 2.6446(6), E: 2.6208(12), F: 2.6285(8) Å). The Ti(1)-P(1) bond distances in 2a–b appear in the range of 2.5630–2.5769 Å, indicative of significant coordination of phosphorus atom to the metal center in the solid state. These results indicate that Ti-P bond in 2a–b is stronger than that in D–F, which is also consistent with the results of ³¹P NMR. However, the S(1)-C(1) and P(1)-C(2) bond lengths in 2a–b change slightly with the variation in R group, as shown in Table 2. The two Ti(1)-Cl bond lengths in 2a–b are statistically identical (2a: 2.3121(10) and 2.3139(10) Å, 2b: 2.3106(10) and 2.3207(9) Å to Cl(1) and Cl(2), respectively). The bond angles for Cl(1)-Ti(1)-Cl(2) in 2a–b (2a: 91.73(4)°, 2b: 91.87(3)°, respectively) are much smaller than that for complex C (101.45(4)°), but larger than that for complex D (88.96(2)°). The bond angles for Ti(1)-S(1)-C(1) in 2a–b (2a: 110.27(11)°, 2b: 110.66(10)°, respectively) are much smaller than that for Ti(1)-O(1)-C(1) in D (134.99(11)º). However, the bond angles for Ti(1)-P(1)-C(2) in 2a–b (2a: 107.67(11)°, 2b: 109.24(11)°, respectively) are larger than that in D (97.11(6)º).

catalysts-03-00261-t001_Table 1

Table 1

Crystal data and structure refinements of complexes 2a–b.

	2a	2b
Empirical formula	C_23.50H₂₀Cl₃PSTi	C₂₉H₂₃Cl₂PSTi
Formula weight	519.68	553.30
Crystal system	Pna2(1)	P2(1)
Space group	orthorhombic	monoclinic
a (Å)	19.6852(10)	8.0269(7)
b (Å)	22.8729(12)	13.6834(11)
c (Å)	10.3207(6)	11.5902(9)
α (°)	90.00	90.00
β (°)	90.00	97.3240(10)
γ (°)	90.00	90.00
V (Å³), Z	4647.0(4), 8	1262.63(18), 2
Density_calcd(Mg/m³)	1.486	1.455
Absorption coefficient (mm^-1)	0.881	0.714
F (000)	2120	568
Crystal size (mm)	0.30 × 0.21× 0.15	0.30 × 0.24× 0.18
θ range for data collection (°)	1.78 to 26.01	1.77 to 26.03
Reflections collected	27237	8126
Independent reflections	9135	4698
Data/restraints/ parameters	9135/1/532	4698/1/307
Goodness-of-fit on F²	1.041	1.030
Final R indices [ I > 2σ (I)]: R1, wR2	0.0385, 0.0972	0.0350, 0.0847
Largest diff. Peak and hole (e Å^-3)	0.553 and −0.366	0.337 and −0.191

catalysts-03-00261-t002_Table 2

Table 2

Selected Bond Distances (Å) and Angles (deg) for complexes 2a-b and C-D [28].

	2a	2b	C	D
		Bond Distances in Å
Ti(1)- S(1)/O(1)	2.4188(10)	2.4359(9)	1.795(2)	1.8708(13)
Ti(1)-P(1)	2.5769(9)	2.5630(10)		2.6446(6)
Ti(1)-Cl(1)	2.3121(10)	2.3106(10)	2.2582(11)	2.3320(6)
Ti(1)-Cl(2)	2.3139(10)	2.3207(9)	2.2478(11)	2.3453(6)
Ti(1)-Cp(centroid)	2.027	2.029	2.016	2.034
S(1)/O(1)-C(1)	1.768(4)	1.778(3)	1.369(4)	1.360(2)
P(1)-C(2)	1.813(3)	1.808(3)	1.835(3)	1.7994(19)
		Bond Angles in °
Cl(1)-Ti(1)-Cl(2)	91.73(4)	91.87(3)	101.45(4)	88.96(2)
O(1)/S(1)-Ti(1)-P(1)	73.49(3)	72.69(3)		72.85(4)
Ti(1)-S(1)/O(1)-C(1)	110.27(11)	110.66(10)	161.39(19)	134.99(11)
Ti(1)-P(1)-C(2)	107.67(11)	109.24(11)		97.11(6)
Cl(1)-Ti(1)-S(1)/O(1)	129.59(4)	131.62(4)	104.48(7)	127.96(5)
Cl(2)-Ti(1)-S(1)/O(1)	81.09(4)	82.09(3)	103.92(7)	90.17(4)
Cl(1)-Ti(1)-P(1)	80.08(3)	79.39(3)		78.40(2)
Cl(2)-Ti(1)-P(1)	138.03(4)	135.69(4)		144.48(2)
S(1)/O(1)-C(1)-C(2)	121.0(2)	118.7(2)	117.8(3)	118.22(16)
P(1)-C(2)-C(1)	111.7(2)	112.8(2)	117.7(2)	111.70(14)

Figure 1

Molecular structure of complex 2a with thermal ellipsoids at 30% probability level. The hydrogen atoms and the solvent molecule are omitted for clarity.

Figure 2

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

2.2. Ethylene (Co)Polymerization Catalyzed by 2a–c

To explore the catalytic behaviors of complexes 2a–c, ethylene polymerizations were carried out in the presence of modified methylaluminoxane (MMAO). Complex 2a showed moderate catalytic activity (2a, 190 kg/mol_Ti·h) for ethylene polymerization (conditions: ethylene 4 atm, MMAO/Ti = 1000, V_total= 50 mL, 20 °C, 10 min). Introducing bulk group in R position decreased the activity (2c: 140 kg/mol_Ti·h). Complex 2b, bearing phenyl group in both R position, exhibited lowest activity among these catalysts (2b: 40 kg/mol_Ti·h). These data suggested that both the steric bulk effect and the electron-donating effect of R group play a key role to the enhanced catalytic activity. Using Ph₃CB(C₆F₅)₄/ⁱBu₃Al in place of MMAO as the cocatalyst, complexes 2a–c only produced trace polymers for ethylene polymerization under the similar conditions ([Al]/[B]/[Ti] = 100/2/1). Generally, the activities of ethylene homopolymerization by complexes 2a–c were lower than those by corresponding o-di(phenyl)phosphanylphenolate-based half-titanocene complexes [28]. Ethylene copolymerization with norbornene has also be carried out by complexes 2a–c in the presence of Ph₃CB(C₆F₅)₄/ⁱBu₃Al or MMAO under similar conditions with complexes A–F. The polymerization results are depicted in Table 3.

Copolymerizations of ethylene with NBE by complex 2a/Ph₃CB(C₆F₅)₄/ⁱBu₃Al took place at different NBE concentrations in feed. The activity increased upon the increasing the NBE concentration in feed from 0.3 to 0.5 mol/L (entries 1–2, Table 3), and decreased upon further increasing the NBE concentration (entries 3–4). This result was some different from the those by A/Ph₃CB(C₆F₅)₄/ⁱBu₃Al catalyst system, in which the highest activity was observed at NBE concentration of 1.0 mol/L in feed [28].The thiophenolate catalysts were less active than the phenolate catalysts. It was assumed that the electronegativity of sulfate is higher than that of oxygen, therefore, the cationic active species of thiophenolate catalysts is less electron deficiency. In addition, the NBE incorporation was lower than that obtained by complex A, especially at NBE concentration of 1.0 mol/L (2a: 33.8%, A: 53.2%, respectively). The resultant copolymers possessed relatively high molecular weights (MWs) with unimodal molecular weight distributions (MWDs), and the MW of the copolymers decreased upon the increasing NBE concentration (entries 1–4). The trace amount of polymer was obtained if changing the molar ratio of Ph₃CB(C₆F₅)₄/ⁱBu₃Al in system. Suitable amount of both Ph₃CB(C₆F₅)₄ and ⁱBu₃Al was thus required to optimize the polymerization conditions (entries 2 vs. 5). The copolymerization by 2a/MMAO was also carried out, and the observed activity calculated on the basis of the polymer yield was almost 20 times lower than that by 2a/Ph₃CB(C₆F₅)₄/ⁱBu₃Al system. However, either the MW or the NBE incorporation by 2a/MMAO system was much higher than that by 2a/Ph₃CB(C₆F₅)₄/ⁱBu₃Al.

catalysts-03-00261-t003_Table 3

Table 3

Copolymerization of ethylene with NBE using 2a–c/Ph₃CB(C₆F₅)₄/ⁱBu₃Al ^a.

Entry	Cat.	Al/Ti (molar ratio)	NBE (mol/L)	Yield (mg)	Activity (kg/mol_Ti·h)	M_w ^b (kg/mol)	M_w/M_n	NBE Incorp. (mol%) ^c
1	2a	100	0.3	387	1858	225	2.9	12.0
2	2a	100	0.5	605	2904	169	2.5	24.3
3	2a	100	0.7	337	1618	143	2.2	31.2
4	2a	100	1.0	135	648	112	1.8	33.8
5	2a	50	0.5	trace	-	-	-	-
6 ^d	2a	100	0.5	163	782	121	2.4	31.8
7 ^e	2a	1000	0.5	130	160	256	2.8	30.1
8	2b	100	0.5	90	432	109	2.0	26.4
9	2c	100	0.5	288	1382	299	1.8	28.5

^aConditions: Ph₃CB(C₆F₅)₄/ⁱBu₃Al as cocatalyst, catalyst 2.5 μmol, [B]/[Ti] = 2/1, ethylene pressure 4 atm., 20 °C, 5 min, V_total= 50 mL. ^bWeight-average molecular weights and polydispersity indexes determined by high temperature GPC at 150°C in 1,2,4-C₆Cl₃H₃vs. narrow polystyrene standards. ^cNBE content (mol%) estimated by ¹³C NMR spectra. ^d40ºC. ^e MMAO as cocatalyst, catalyst 5.0 μmol, 10 min.

Figure 3

¹³C NMR spectra of E/NBE copolymer with different NBE incorporations produced by 2a (A, 12.0%, entry 1; B, 24.3%, entry 2; C: 33.8%, entry 4 in Table 3, respectively).

Complex 2b with a phenyl group in R position showed much lower activity than complex 2a. The MWs of the resultant copolymers by 2b were also relatively lower than that those by 2a under similar condititions, whereas the NBE incorporation was higher than that by 2a (entries 2 vs. 9). Replacement of a phenyl group at the R position in complex 2b with a -SiMe₃ group (2c) led to higher activity. Moreover, 2c in combination with Ph₃CB(C₆F₅)₄/ⁱBu₃Al could provide a high molecular weight ethylene-norbornene copolymer, M_w 299 × 10³, with 1380 kg of polymer/mol of cat·h activity at a NBE content of 28.5 mol%.

The typical ¹³C NMR spectra of poly(ethylene-co-NBE)s with different NBE incorporations are illustrated in Figure 3. [44,45,46,47,48,49,50,51,52,53,54,55,56] Figure 3A showed that the microstructures of the COCs formed using 2a under low NBE concentration of 0.3 mol/L possessed alternating ethylene-NBE sequences and isolated NBE units. In contrast, resonances ascribed to NBE diads or triads were observed for the copolymers prepared at high NBE concentration of 0.5-1.0 mol/L (Figure 3B–C), and the microstructures thus possessed a mixture of NBE repeat units in addition to the alternating, isolated NBE sequences.

3. Experimental 3.1. General Procedures and Materials

All manipulation of air- and/or moisture-sensitive compounds was carried out under a dry argon atmosphere by using standard Schlenk techniques or under a dry argon atmosphere in an MBraun glovebox unless otherwise noted. All solvents were purified from an MBraun SPS system. The NMR data of the ligands and complexes used were obtained on a Bruker 300 MHz, 400 MHz or 600 MHz spectrometer at ambient temperature, with CDCl₃ as the solvent (dried by MS 4Å). The NMR data of the polymers were obtained on a Varian Unity-400 MHz spectrometer at 135 °C, with O-C₆D₄Cl₂ as a solvent. Elemental analyses were recorded on an elemental Vario EL spectrometer. Mass spectra were obtained using electron impact (EI-MS) and LDI-1700 (Linear Scientific Inc). The weight-average molecular weights (M_w) and the polydispersity indices (PDIs) of polymer samples were determined at 150 °C by a PL-GPC 220 type high-temperature chromatograph equipped with three Plgel 10-µm Mixed-B LS type columns. 1,2,4-Trichlorobenzene (TCB) was employed as the solvent at a flow rate of 1.0 mL/min. The calibration was made by polystyrene standard EasiCal PS-1 (PL Ltd., Kawasaki, Japan). CpTiCl₃were purchased from Aldrich. Modified methylaluminoxane (MMAO, 7% aluminum in heptane solution) was purchased from Akzo Nobel Chemical Inc. Commercial ethylene was directly used for polymerization without further purification. The other reagents and solvents were commercially available.

3.2. Synthesis of Half-Titanocene Complexes 3.2.1. Synthesis of Ligands <bold>1a–c</bold>

Various Phosphine-thiophenol ([S,P]) ligands bearing different substituents on R position, 2-R-6-(PPh₂)C₆H₃SH (1a, R = H; 1b, R = Ph; 1c, R = SiMe₃), prepared according to literature procedures [44,48]. Compounds 1b (R = Ph) was prepared via a procedure similar to that for 1a and the characterizations of the compound are followed:

Synthesis of ligands 1b (2-Ph-6-(PPh₂)C₆H₃SH). Yield: 68%. ¹H NMR (300 MHz, CDCl₃, 298K): δ 7.49–7.30 (m, 15H, Ar-H), 7.20 (dd, J = 7.6, 1.3 Hz, 1H, Ar-H), 7.08 (t, J = 7.6 Hz, 1H, Ar-H), 6.85–6.78 (m, 1H, Ar-H), 4.38 (s, 1H, SH).¹³C NMR (101 MHz, CDCl₃, 298K): δ 141.62 (d, J = 3.6 Hz), 141.19 (d, J = 2.2 Hz), 138.07, 137.78, 135.66 (d, J = 9.4 Hz), 134.10 (d, J = 19.6 Hz), 133.19, 130.88, 129.50, 129.18, 128.85 (d, J = 7.2 Hz), 128.57, 127.85, 125.06. ³¹P NMR (162 MHz, CDCl₃, 298K): δ −12.20. Anal. Calcd. For C₂₄H₁₉PS: C, 77.81; H, 5.17. Found: C, 77.85; H, 5.15.

3.2.2. Synthesis of Half-Titanocene Complexes <bold>2a–c</bold>

Synthesis of half-titanocene complex 2a (CpTiCl₂[S-6-(PPh₂)C₆H₄]). To a stirred solution of 6-(PPh₂)C₆H₄SH (0.294 g, 1.00 mmol) in dried tetrahydrofuran (THF) (15 mL) was added NaH (0.048 g, 2.00 mmol) at 0 °C, and the mixture was allowed to warm up to room temperature and stirred for an additional 6 hours. And then the mixture was filtered to remove residual NaH, and the filtrate cooled to 0°C, and 15 mL THF solution of CpTiCl₃ (0.219 g, 1.00 mmol) was added slowly. The cooler bath was removed, and the resultant mixture was warmed up to room temperature and stirred overnight. The solvents were removed under vacuum, and the residue was extracted with CH₂Cl₂ (3 × 10 mL). The combined filtrates were concentrated under vacuum to about 5 mL, and 20 mL hexane was layered with this solution for several days at −30 °C to precipitate dark red crystals of 2a (Yield: 85%). ¹H NMR (300 MHz, CDCl₃, 298K): δ 7.91–7.79 (m, 4H, Ar-H), 7.61–7.53 (m, 1H, Ar-H), 7.52—7.42 (m, 6H, Ar-H), 7.40-7.36 (m, 1H, Ar-H), 7.35–7.27 (m, 1H, Ar-H), 7.21–7.13 (m, 1H, Ar-H), 6.47 (d, J = 2.5 Hz, 5H, C₅H₅). ¹³C NMR (151 MHz, CDCl₃, 298K): δ 156.71 (d, J = 34.2 Hz), 135.64, 135.28, 133.90 (d, J = 7.4 Hz), 131.74, 131.28, 131.06, 129.25 (d, J = 7.7 Hz), 128.79 (d, J = 9.0 Hz), 125.16, 120.69. ³¹P NMR (162 MHz, CDCl₃, 298K): δ 44.33. Anal. Calcd. For C₂₃H₁₉Cl₂PSTi: C, 57.89; H, 4.01. Found: C, 57.98; H, 4.05.

Synthesis of half-titanocene complex 2b (CpTiCl₂[S-2-Ph-6-(PPh₂)C₆H₃]) was carried out according to the same procedure as that of 2a, except that 2-Ph-6-(PPh₂)C₆H₃SH (0.370 g, 1.00 mmol) was used in place of 6-(PPh₂)C₆H₄OH. Complex 2b was obtained as black red crystals in 80% yield.¹H NMR (600 MHz, CDCl₃, 298K): δ 7.93–7.82 (m, 4H, Ar-H), 7.55 (t, J = 6.7 Hz, 1H, Ar-H), 7.49 (m, 6H, Ar-H), 7.41–7.36 (m, 4H, Ar-H), 7.35-7.30 (m, 2H, Ar-H), 7.23 (td, J = 7.5, 1.6 Hz, 1H, Ar-H), 6.45 (d, J = 2.3 Hz, 5H, C₅H₅). ¹³C NMR (151 MHz, CDCl₃, 298K): δ 154.70 (d, J = 33.6 Hz), 141.88 (d, J = 8.5 Hz), 140.38, 137.56, 137.19, 134.15, 133.53, 131.26, 130.08, 129.52, 128.77 (d, J = 9.1 Hz), 128.19, 127.72, 125.61 (d, J = 6.0 Hz), 120.47. ³¹P NMR (162 MHz, CDCl₃, 298K): δ 43.32. Anal. Calcd. For C₂₉H₂₃Cl₂PSTi: C, 62.95; H, 4.19. Found: C, 62.90; H, 4.23.

Synthesis of half-titanocene complex 2c (CpTiCl₂[S-2-SiMe₃-6-(PPh₂)C₆H₃]) was carried out according to the same procedure as that of 2a, except that 2-SiMe₃-6-(PPh₂)C₆H₃SH (0.367, 1.00 mmol) was used in place of 6-(PPh₂)C₆H₄SH. Complex 2c was obtained as black red crystals in 76% yield.¹H NMR (300 MHz, CDCl₃, 298K): δ 7.86–7.81 (m, 4H, Ar-H), 7.61–7.32 (m, 8H, Ar-H), 7.13 (t, J = 7.4 Hz, 1H, Ar-H), 6.44 (d, J = 2.3 Hz, 5H, C₅H₅), 0.37 (s, 9H, Si(CH₃)₃). ¹³C NMR (151 MHz, CDCl₃, 298K): δ 163.77 (d, J = 33.6 Hz), 140.98 (d, J = 4.5 Hz), 137.83, 136.26, 135.90, 134.05, 131.92, 131.14, 128.70 (d, J = 9.0 Hz), 124.45 (d, J = 4.1 Hz), 120.45, −0.17. ³¹P NMR (162 MHz, CDCl₃, 298K): δ 42.53. Anal. Calcd. For C₂₆H₂₇Cl₂PSSiTi: C, 56.84; H, 4.95. Found: C, 56.91; H, 4.91.

3.2.3. Ethylene (Co-)Polymerization

A 200 mL stainless steel autoclave was heated under vacuum up to 150 °C for 6 h and then was cooled to and maintained at the desired reaction temperature. The reactor was charged with toluene and comonomer if desired under vacuum, and then the stirring motor was engaged to facilitate heat transfer. For single-component trials, a toluene solution of the catalyst was injected to the reactor, for co-catalyst-activated trials, a toluene solution of MMAO or Ph₃CB(C₆F₅)₄/ⁱBu₃Al was also added, after which ethylene was fed continuously, which was manually adjusted to maintain a desired constant pressure. Temperature control was conducted by internal cooling water coils. After the prescribed reaction time, the stirring motor was stopped and the reactor was vented. The solid polyethylene was obtained by filtration after precipitation from ethanol, washed with ethanol and acetone, dried at 60 °C for 10 h under vacuum.

3.2.4. Crystallographic Studies

Single crystals of complexes 2b, 2e and 3g suitable for X-ray structure determination were grown from CH₂Cl₂ and hexane solution in a glove box, thus maintaining a dry, O₂-free environment. The crystallographic data, collection parameters, and refinement parameters are listed in Table 1. The intensity data were collected with the ω scan mode (186 K) on a Bruker Smart APEX diffractometer with CCD detector using Mo Kα radiation (λ = 0.71073Å). Lorentz, polarization factors were made for the intensity data, and absorption corrections were performed using the SADABS program. The crystal structures were solved using the SHELXTL program and refined using full matrix least-squares. The positions of hydrogen atoms were calculated theoretically and included in the final cycles of refinement in a riding model along with attached carbons.

4. Conclusions

A series of novel half-titanocene complexes CpTiCl₂[S-2-R-6-(PPh₂)C₆H₃] (Cp = C₅H₅, 2a, R = H; 2b, R = SiMe₃; 2c, R = Ph) have been synthesized in high yields. The ¹H and ³¹P NMR spectra indicated that the phosphorus is coordinated to titanium in complexes 2a-c. Additionally molecular structures show that complexes 2a–b adopted five-coordinate distorted square-pyramid geometry around the titanium center. The complexes 2a–c were investigated as the catalysts for ethylene polymerization and copolymerization of ethylene with norbornene in the presence of MMAO or Ph₃CB(C₆F₅)₄/ⁱBu₃Al as the cocatalyst. All complexes exhibited low to moderate activities towards ethylene polymerization. Note that these complexes displayed moderate to high activities towards copolymerization of ethylene with norbornene, affording relatively high-molecular-weight copolymers with unimodal molecular weight distributions.

Acknowledgements

The authors are grateful for subsidy provided by the National Natural Science Foundation of China (Nos. 21074128 and 20923003). We also thank to Prof. Kotohiro Nomura (Tokyo Metropolitan University) for his kind discussion in preparing the manuscript.

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