Active O,Npy,N-Titanium(IV) Fluoride Precatalysts for Ethylene

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Active O,Npy,N-Titanium(IV) Fluoride Precatalysts for Ethylene Polymerization: Exploring “Fluoride Effects” on Polymer Properties and Catalytic Performance Luka A. Wright,† Eric G. Hope,† Gregory A. Solan,*,† Warren B. Cross,†,‡ and Kuldip Singh† †

Department of Chemistry, University of Leicester, University Road, Leicester LE1 7RH, United Kingdom School of Science and Technology, Nottingham Trent University, Clifton Lane, Nottingham NG11 8NS, United Kingdom

‡

S Supporting Information *

ABSTRACT: Three examples of discrete six-coordinate O,Npy,N-bearing titanium(IV) trifluoride complexes, [{2(C5H4N)-6-(CMe2O)C5H3N}TiF3] (1), [{2-(C6H4-2′-O),6(CMeN(2,6-i-Pr2C6H3))C5H3N}TiF3] (2a), and [{2-(3C12H8-2-O)-6-(CMeN(2,6-i-Pr2C6H3))C5H3N}TiF3] (3a), were prepared using a one-step HF elimination protocol from cis-[(THF)2TiF4] and the corresponding pincer ligand precursor (HL1, HL2H, and HL2Ph). A mer configuration of the three fluoride ligands is adopted in their solid-state structures, which is mirrored in solution, as shown by mutual two-bond F−F coupling in their 19F{1H} NMR spectra. For purposes of comparison, the chloride counterparts of 2a and 3a, [{2-(C6H4-2′-O),6-(CMeN(2,6-i-Pr2C6H3)C5H3N}TiCl3] (2b) and [{2-(3-C12H8-2-O)-6-(CMeN(2,6-i-Pr2C6H3))C5H3N}TiCl3] (3b), are also reported. On treatment with excess MAO, 1 is scarcely active in ethylene polymerization, 2a is more active, and the most sterically protected system, 3a, represents the most active nonmetallocene metal-fluoride precatalyst reported to date (340 g mmol−1 h−1 bar−1) and produces ultra-high-molecular-weight polyethylene (UHMWPE). Similar structure−activity correlations are displayed for chloride-containing 2b and 3b, but, in general, they are significantly more active than their fluoride counterparts, with 3b being the most productive of the series (990 g mmol−1 h−1 bar−1). Using fluoride rather than chloride in the precatalyst has a marked effect on not only the molecular weight but also the molecular weight distribution, with broad dispersities (Đ) being a feature of the polymers obtained from 2a and 3a, whereas those for chloride-containing 2b and 3b are appreciably narrower. Single-crystal X-ray structures are reported for 1, 2a, 2b, and 3a.

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comparative difficulty in synthesizing fluoro-containing precatalysts of the type (L1)MF3, (L2)MF2, or (L1)2MF2 (M = group IV metal; L1 and L2 = mono- and dianionic chelating ligands, respectively) in sufficient purity may, in part, explain this dearth of reported catalytic investigations. Nevertheless, thermodynamic considerations regarding their catalytic activation raise some intriguing possibilities when put in comparison with their chloride counterparts.14 For instance, the bond enthalpies for Ti−F and Ti−Cl bonds are 569 and 494 kJ mol−1, respectively, thus implying that an alkyl-aluminumpromoted abstraction of a titanium fluoride should be less facile than abstraction of a chloride ligand. On the other hand, the bond enthalpies of a Al−X bond differ more significantly (X = Cl 511 kJ mol−1 vs F 663 kJ mol−1), suggesting that the resultant Al−F bond-forming step should present a considerable driving force. Indeed, investigations involving metallocene and half-metallocene fluorides in polymerization applications15−17 have revealed some unusual findings, and

INTRODUCTION There have been some significant advances in the development of nonmetallocene group IV transition metal catalysts for olefin polymerization, with some examples not only rivalling the performance characteristics disclosed for metallocenes themselves 1−9 but also allowing some notable operational advantages (e.g., applications in high-temperature stereospecific polymerizations).10,11 These developments have, in large measure, been driven by a growing appreciation of the role played by the multidentate ancillary ligand in influencing catalytic performance. To allow rapid screening, metal halide (e.g., Cl, Br) complexes bearing one or more chelating ligands are often used in conjunction with a suitable co-catalyst such an alkyl aluminum (e.g., methylaluminoxane (MAO)) to facilitate in situ formation of the active catalyst through a combined alkylation and abstraction process.2,12 By contrast, metalfluoride nonmetallocene precatalysts have received scant attention for this application.13 For example, Roesky et al. reported NacNac-bearing Ti(IV) fluoride species that exhibit only poor activity in ethylene polymerization.13a The © XXXX American Chemical Society

Received: October 27, 2015

A

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notably the term “fluoride effect” has been coined.15a,b The disclosure of rac-(ebthi)ZrF2/i-Bu3Al as an active catalyst system for ethylene polymerization while its chloride analogue is inactive under comparable conditions represents an example of this fluoride effect. In an attempt to prepare active metal-fluoride precatalysts for alkene polymerization, we report herein several examples of well-defined titanium(IV) trifluorides bearing two new classes of pyridine-based unsymmetrical O,Npy,N-pincer ligands,18 namely, the bipy-alkoxide, L1, and the phenolate-pyridylimines, L2H and L2Ph (Figure 1). These pincer ligands have been

Article

RESULTS AND DISCUSSION

Synthetic and Characterization Aspects. The proligands, 2-(C6H4-2′−OH),6-{CMeN(2,6-i-Pr2C6H3)}C5H3N (HL2H) and 2-(3-C12H8-2-OH)-6-{CMeN(2,6-i-Pr2C6H3)}C5H3N (HL2Ph), were prepared in good yields using methods described elsewhere,19 whereas 2-(C5H4N)-6-(CMe2OH)C5H3N (HL1) was prepared by treating 6-acetyl-2,2′bipyridine20 sequentially with trimethylaluminum and water (see Supporting Information). Recently, Levason et al. have shown that cis-[(L)2TiF4] (L = MeCN, THF) can be prepared by the dissolution of polymeric TiF4 in the corresponding donor solvent.21 The resulting solvent adducts have proved to be useful precursors to other complexes such as [(L′)2TiF4] (L′ = Ph3PO, Ph3AsO) and [(L-L)TiF4] [L-L = 2,2′-bipy, N,N,N′,N′-tetramethylethylenediamine] on reaction with L′ or L−L in dichloromethane. We have also found that cis[(THF)2TiF4] is an attractive starting material that reacts via an HF elimination protocol with suitably acidic pro-ligands. Hence, reaction of HL1, HL2H, and HL2Ph with cis[(THF)2TiF4] in dichloromethane gave, on workup, [{2(C5H4N)-6-(CMe2O)C5H3N}TiF3] (1), [{2-(C6H4-2′-O),6(CMeN(2,6-i-Pr2C6H3)C5H3N}TiF3] (2a), and [{2-(3C12H8-2-O)-6-(CMeN(2,6-i-Pr2C6H3))C5H3N}TiF3] (3a) in moderate to good yields with concomitant elimination of THF and HF (Scheme 1). The reactions can also be performed in THF but with some decrease in the yield. Complexes 1, 2a, and 3a have been characterized by NMR (1H, 13C, and 19F) and IR spectroscopies and by mass spectrometry and give satisfactory microanalytical data (see Experimental Section). Single crystals of 1, 2a, and 3a suitable for X-ray determination were grown from chloroform solutions on prolonged standing or by layering with petroleum ether. There are three independent molecules (A, B, and C) in the unit cell of 3a, with the main differences among molecules being the relative inclinations of the neighboring pyridine and phenolate units within the tridentate ligand (vide infra).

Figure 1. Monoanionic bipy-alkoxide, L1, and phenolate-pyridylimines, L2H and L2Ph; dipp = 2,6-i-Pr2C6H3.

targeted as they differ not only in their steric and electronic properties but also in their distinct chelating properties. All of the new titanium-fluoride complexes have been evaluated, in the presence of MAO, as catalysts for ethylene polymerization, and the properties of the resultant polymers have been investigated using gel permeation chromatography (GPC) and differential scanning calorimetry (DSC). To complement this study and to probe potential fluoride effects, selected titanium(IV) trichlorides have also been prepared and screened, with their catalytic performances compared against their TiF3ligand analogues. Scheme 1a

a

Reagents and conditions: (i) THF, rt, 3 h; (ii) HL1, CH2Cl2, 3 h; (iii) HL2H, CH2Cl2, 1.5−3 h; (iv) HL2Ph, CH2Cl2, 1.5−3 h. B

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bonding. In 1, the exterior pyridine donor is the longest of the three metal−L1 distances [2.216(4) Å], whereas in 2a and 3a, that involving the central pyridine is the longest [2.228(3) Å (2a); 2.232(4)A, 2.233(4)B, 2.217(4)C (3a) Å], again highlighting the differences in binding properties exerted by L1 and L2. Of the three fluoride ligands, the axial Ti−F distances [Ti− Faxial 1.827(3)−1.852(3) Å vs Ti−Fequatorial 1.793(3)−1.809(3) Å] are the longest and show some modest deviations from linearity [Faxial−Ti−Faxial 162.51(13)° (1); 167.34(15)° (2a); 167.34(15)A°, 165.32(16)B°, 165.06(14)C° (3a)]; notably, this bending is, in each case, away from the anionic oxygen donor of the pincer ligand. In 1 and 2a, the three distinct donor sections of the pincer ligand (viz. pyridine−pyridine−alkoxide (1) and phenolate−pyridine−imine (2a)) are close to coplanarity, whereas in 3a, there are some variations between the independent molecules. This is most evident on inspection of the relative inclination of the adjacent phenolate and pyridine planes within L2Ph, as shown by the C(1)−C(2)−C(13)−N(1) torsion angles that vary between 2.4° (molecule A) and 18.6° (molecule B). The 3-phenyl group in 3a is tilted [tors. C(12)− C(7)−C(6)−C(1) 56.27A, 46.59B, 50.36C] with respect to the adjacent phenolate group, whereas the N-2,6-diisopropylphenyl rings in 2a and 3a are oriented almost orthogonally to the neighboring imine group. There are no intermolecular contacts of note. The spectroscopic data for 1, 2a, and 3a are agreement with their solid-state properties being maintained in solution. Coordination of the imine arm of the ligands in 2a and 3a is confirmed by a shift of the imine stretching frequency by ca. 20 cm−1 to lower wavenumber on coordination. Likewise, the 1H NMR spectra of 2a and 3a display characteristic splitting of the isopropyl methyl signals into two distinct 6H doublets that occurs on coordination of the N-diisopropylphenyl group, a feature that can be attributed to either two distinct Ar-CHMe2 groups or to inequivalent CHMeAMeB methyl substituents (Figures S4 and S9). The ToF mass spectra for each complex further support the X-ray structures with fragmentation peaks corresponding to the loss of one or two fluoride ligands. The 19F{1H} NMR spectra are particularly useful in confirming the mer configrations adopted by the fluoride ligands in 1, 2a, and 3a with two mutually coupled triplet and doublet resonances for the Fequatorial and Faxial ligands, respectively, in a 1:2 ratio (Table 4 and Figures S3, S6, and S11). To the knowledge of the authors, well-defined merconfigured mononuclear titanium(IV) trifluoride complexes have not been reported, with the closest nonmetallocene comparators being fluoride-bridged [(NacNac)TiF2(μ-F)]213a and two examples of fac-coordinated titanium trifluoride complexes.22 The variations in the values of the fluoride chemical shifts further emphasize the differences in the binding properties of L1 and L2, with the corresponding fluoride resonances in L2-containing 2a and 3a being notably more downfield. In order to confirm the fate of the eliminated HF during the reaction, we also conducted the reaction of cis[(THF)2TiF4] with HL2H in CDCl3 in a Young’s tap NMR tube. After 3 h, the 19F{1H} NMR spectrum revealed peaks due to 2a along with a more upfield signal at −19.0 ppm that can be attributed to a chlorofluorocarbon, which is the product of the in situ fluorination of the solvent by the eliminated HF.23 For purposes of comparison, the chloride analogues of 2a and 3a, [{2-(C6H4-2′-O),6-(CMeN(2,6-i-Pr2C6H3)C5H3N}TiCl 3 ] (2b) and [{2-(3-C 12 H 8 -2-O)-6-(CMeN(2,6-iPr2C6H3))C5H3N}TiCl3] (3b), were prepared in good yield

Perspective views are shown in Figures 2, 3, and 4; selected bond distances and angles for all three structures are collected

Figure 2. Molecular structure of 1 with atom labeling scheme and ellipsoids at 30% probability. All hydrogen atoms have been omitted for clarity.

Figure 3. Molecular structure of 2a with atom labeling and ellipsoids at 30% probability. All hydrogen atoms have been omitted for clarity.

Figure 4. Molecular structure of 3a (molecule A) with atom labeling scheme and ellipsoids at 30% probability. All hydrogen atoms have been omitted for clarity.

in Tables 1, 2, and 3. In each structure, a single titanium(IV) center is bound by a monoanionic O,Npy,N-chelate and three fluoride ligands to complete a distorted octahedral geometry. The fluoride ligands adopt a mer configuration that is enforced by the constraints of the respective tridentate ligands. The main difference in binding properties is that bipy alkoxide L1 forms two five-membered chelate rings whereas phenolate-pyridylimine L2 forms five- and six-membered chelate rings. In all three structures, the Ti−O distances [1.809(2) Å (1), 1.798(3) Å (2a), 1.808(4)av Å (3a)] are the shortest of the three metal− O,Npy,N contacts, consistent with the ionic contribution to the C

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Organometallics Table 1. Selected Bond Distances (Å) and Angles (deg) for 1a bond distances (Å) Ti(1)−O(1) Ti(1)−N(1) Ti(1)−N(2) Ti(1)−F(1) Ti(1)−F(2)

1.809(3) 2.216(4) 2.140(4) 1.809(3) 1.849(2)

Ti(1)−F(2A) C(11)−C(10) C(11)−C(12) C(11)−C(12A) C(11)−O(1)

1.849(2) 1.522(6) 1.524(4) 1.524(4) 1.432(5)

bond angles (deg) N(1)−Ti(1)−O(1) N(1)−Ti(1)−N(2) N(1)−Ti(1)−F(1) N(1)−Ti(1)−F(2) N(1)−Ti(1)−F(2A) N(2)−Ti(1)−O(1) a

147.19(15) 71.69(14) 106.08(13) 81.39(6) 81.39(6) 75.50(14)

N(2)−Ti(1)−F(1) N(2)−Ti(1)−F(2) F(1)−Ti(1)−F(2) F(1)−Ti(1)−F(2A) F(2)−Ti(1)−F(2A) C(10)−C(11)−O(1)

177.77(13) 88.75(7) 90.91(7) 90.91(7) 162.51(13) 105.3(4)

The ‘A’ atoms have been generated by symmetry; symmetry operation: x, −y + 1/2, z.

Table 2. Selected Bond Distances (Å) and Angles (deg) for 2a and 2b 2a

Table 3. Selected Bond Distances (Å) and Angles (deg) for 3a molecule A

2b

bond distances (Å) Ti(1)−O(1) Ti(1)−N(1) Ti(1)−N(2) Ti(1)−X(1) Ti(1)−X(2) Ti(1)−X(3) C(12)−N(2) O(1)−Ti(1)−N(1) O(1)−Ti(1)−N(2) O(1)−Ti(1)−X(1) O(1)−Ti(1)−X(2) O(1)−Ti(1)−X(3) N(1)−Ti(1)−N(2) N(1)−Ti(1)−X(1) N(2)−Ti(1)−X(2) N(2)−Ti(1)−X(3) X(1)−Ti(1)−X(2) X(1)−Ti(1)−X(3) X(2)−Ti(1)−X(3) C(13)−C(12)−N(2)

1.798(3) 2.228(3) 2.188(3) 1.852(3)X = F 1.800(3)X = F 1.845(3)X = F 1.277(5) bond angles (deg) 83.88(13) 157.98(13) 94.90(13)X = F 105.96(12)X = F 93.85(13)X = F 74.15(12) 85.65(12)X = F 95.99(12)X = F 82.77(12)X = F 93.77(12)X = F 166.32(12)X = F 93.91(12)X = F 124.7(4)

molecule B

molecule C

bond distances (Å) 1.809(9) 2.149(11) 2.219(10) 2.317(5)X = Cl 2.260(4)X = Cl 2.303(5)X = Cl 1.270(15)

Ti(1)−O(1) Ti(1)−N(1) Ti(1)−N(2) Ti(1)−F(1) Ti(1)−F(2) Ti(1)−F(3) C(18)−N(2) C(6)−C(7)

85.8(4) 159.8(4) 90.4(3)X = Cl 105.6(3)X = Cl 90.1(3)X = Cl 74.1(4) 83.0(3)X = Cl 94.5(3)X = Cl 86.7(3)X = Cl 94.23(17)X = Cl 169.00(18)X = Cl 96.22(18)X = Cl 125.9(13)

O(1)−Ti(1)−N(1) O(1)−Ti(1)−N(2) O(1)−Ti(1)−F(1) O(1)−Ti(1)−F(2) O(1)−Ti(1)−F(3) N(1)−Ti(1)−N(2) N(1)−Ti(1)−F(1) N(2)−Ti(1)−F(2) N(2)−Ti(1)−F(3) F(1)−Ti(1)−F(2) F(1)−Ti(1)−F(3) F(2)−Ti(1)−F(3) C(25)−C(18)−N(2)

by treating HL2H and HL2Ph with TiCl4 in dichloromethane (Scheme 1). Both complexes were fully characterized (see Experimental Section), and in addition, 2b was the subject of a single-crystal X-ray diffraction study (Figure 5 and Table 2). The structure shows similar features to those for 2a, with a sixcoordinate titanium center bound by a tridentate L2H ligand and three mer-configured chloride ligands. Exchanging a fluoride for a chloride has the effect of shortening the Ti− Npy distance [Ti(1)−N(1) 2.140(11) Å (2b) vs 2.228(3) Å (2a)], presumably as a consequence of the poorer trans influence of a chloride. This change also appears to affect the Ti−Nim distance, which becomes elongated [Ti(1)−N(2) 2.219(10) Å (2b) vs 2.188(3) Å (2a)], whereas the Ti−O distance is little changed. The cumulative effect of these variations is to cause some twisting in L2H that can be best seen in the C(1)−C(6)−C(13)−N(1) torsion angle at 21.7° (cf. 0.54° in 2a). As expected, the 1H NMR spectra for 2b and 3b resemble those of their fluoride counterparts, whereas their mass spectra

1.808(4) 1.803(4) 2.232(4) 2.233(4) 2.197(4) 2.193(4) 1.835(3) 1.827(3) 1.793(3) 1.801(3) 1.832(3) 1.829(4) 1.279(7) 1.290(7) 1.482(8) 1.487(7) bond angles (deg) 83.23(16) 156.90(17) 94.04(16) 106.07(16) 93.29(16) 73.69(17) 86.49(15) 96.98(16) 83.63(15) 94.30(16) 167.34(15) 93.56(16) 124.4(5)

1.815(4) 2.217(4) 2.188(4) 1.846(3) 1.796(3) 1.824(3) 1.270(6) 1.472(7)

83.70(16) 157.88(17) 94.92(17) 106.32(16) 93.92(17) 74.19(16) 84.26(16) 95.79(16) 83.67(16) 94.22(16) 165.32(16) 94.52(17) 124.4(5)

84.39(16) 158.28(16) 94.28(15) 106.26(16) 94.94(16) 73.90(16) 83.53(15) 95.45(15) 82.93(15) 94.79(15) 165.06(14) 93.88(15) 125.6(5)

Table 4. Comparison of 19F NMR Chemical Shifts and Coupling Constantsa

a

complex

Fequatorial (δ)

Faxial (δ)

1 2a 3a [(NacNac)TiF2(μ-F)]2b

+141 +202 +201 +4

+100 +131 +132 +242

2

JFF (Hz)

ref

36 28 32 not resolved

this work this work this work 13a

Spectra recorded in CDCl3 at ambient temperature. recorded in toluene-d8 at 183 K

b

Spectrum

show fragmentation peaks corresponding to the loss of one or two chloride ligands. A key difference between the chloride and fluoride complexes is their moisture sensitivity. Whereas the 1H NMR spectra for 2b and 3b could be recorded only in rigorously dried NMR solvent, those for fluorides 2a and 3a D

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obtained in runs 2−4 using DSC. The melting temperatures (Tm) fall in the range 136.0−137.1 °C, with the UHMWPE obtained using 3a falling at the top end of the range and the HDPE using 2a at the bottom. By contrast, the degree of crystallinity (Xc) using 3a is the lowest of the three runs, which is also reflected in the magnitude of the enthalpy of fusion (ΔHf). These trends are in line with previously reported Tm and ΔHf data for commercial samples of UHMWPE and HDPE.27 Chloride-containing 2b and 3b were also evaluated using the same protocol (runs 5 and 6, Table 5). Several points emerge from inspection of these results. First, 3b is more active than its less sterically protected analogue, 2b, and moreover is the most active of all of the precatalysts screened in this study (990 g mmol−1 h−1 bar−1). Second, the presence of the 3-phenyl group in 3b again has the effect of increasing the molecular weight but not into the UHMWPE regime as seen for fluoride-containing 3a (cf. runs 6 and 3). Third, the dispersities of the polymers obtained using chloride-based precatalysts are significantly more narrow (Đ = 7−9) than those of their fluoride counterparts. The melting temperatures (Tc) of these polymers are also lower in agreement with the lower molecular weight and narrower dispersities, whereas the degree of crystallinity is increased. In an attempt to probe the mode of activation of the titanium fluoride precatalyst, we used 19F NMR spectroscopy (recorded in toluene using a C6D6 insert) to investigate the effect of MAO addition to a sample of 3a. As commercial MAO solutions can contain high percentages (30−35%) of trimethylaluminum (TMA),29 we also conducted a parallel study to examine the amenability of 3a to methylation with TMA. Unsurprisingly, the results of the MAO addition at molar equivalents approaching those used for polymerization (300 equiv) were unclear, with broad and weak peaks just discernible at ca. δ −139.0 that could be tentatively assigned to a fluoroaluminate anion or suchlike.30 With 30 equiv of MAO, the same weak peak at δ −139.0 could be seen along with a quadrupolar broadened signal at δ −145.0 that can be ascribed to tetrameric FAlMe2.31 Resonances due to a Ti−F unit could not be detected in the downfield region at either concentration. By contrast, treatment of 3a at room temperature with various molar ratios (Ti/Al ratio between 1:2 and 1:50) of TMA gave, in each case, two main broad peaks: one corresponding to [FAlMe2]4 (at δ −145.0) and a more upfield peak at δ −149.9. Recording the spectrum at lower temperature (280 K) resulted in some sharpening of these two peaks and the appearance of further minor Al−F peaks in the same region. We tentatively assign the resonance at δ −149.9 to the adduct [{(L2Ph)TiMeF2}(AlMe3)] in which the fluoride ligands bridge electropositive metal centers in a manner similar to that

Figure 5. Molecular structure of 2b with atom labeling and ellipsoids at 30% probability. All hydrogen atoms have been omitted for clarity.

could be run in bench-quality solvents without any appreciable degradation. Catalytic Evaluation for the Polymerization of Ethylene. The three titanium trifluoride complexes, 1, 2a, and 3a, were initially screened as precatalysts for the polymerization of ethylene (runs 1−4, Table 5). Typically, the complex was dissolved in toluene and treated with 300 equiv of MAO, and after 5 min, the reaction mixture purged with ethylene gas (1 bar) over 30 min. Complex 1 gave trace quantities of polyethylene (run 1), whereas the phenolate−pyridylimine systems, 2a and 3a (runs 2 and 3), were considerably more productive, with 3a being classified as highly active (340 g mmol−1 h−1 bar−1)2a and indeed being the most active nonmetallocene metal fluoride precatalyst reported to date; no evidence for low-molecular-weight oligomeric materials could be obtained for any run. Complex 3a was also screened at 50 °C, which had the effect of decreasing the activity (run 4: 236 g mmol−1 h−1 bar−1). The enhanced catalytic performance of 3a over 2a can be attributed to the presence of the 3-phenyl group in 3a that sterically protects the anionic phenoxy-O donor from coordination with the MAO.24 Similar structure−activity correlations have been observed elsewhere involving related phenolate-containing nonmetallocene catalysts.25 Furthermore, the molecular weight (Mw) of the polymers was found to significantly increase from 149 400 for 2a (run 2, typical of HDPE) to 1 089 800 for 3a (run 3), characteristic of ultra-highmolecular-weight polyethylene (UHMWPE).26 Indeed, the molecular weights for run 3 were on the edge of the reliable detection window of the GPC column. As with all of the runs performed using 2a and 3a, the molecular weight distributions are broad (Đ = Mw/Mn = 25−48), indicative of multisite behavior. To further probe the polymer characteristics, we investigated the thermophysical properties of the polyethylenes Table 5. Catalytic Evaluation of 1−3a run

precat.

T/oC

yield/g

activityb

Mwc

Đd

T me

ΔHfe

Xcf

1 2 3 4 5 6

1 2a 3a 3a 2b 3b

20 20 20 50 20 20

0.014 0.209 1.669 1.182 1.766 4.950

3 42 340 236 353 990

149 400 1 089 800 349 100 22 800 209 700

41 48 26 9 7

136.0 137.1 136.8 130.3 133.9

193.3 158.1 180.9 223.7 208.5

66 54 62 76 71

Conditions: 0.01 mmol of [Ti] per run, 40 mL of toluene, 300 equiv of MAO per run, 1 bar of ethylene, 30 min. bIn g mmol−1 h−1 bar−1. cIn g mol−1; GPC data using DRI analysis. dĐ = Mw/Mn. eTm in °C; ΔHf in J g −1, determined by DSC. fIn %, XC = 100 × ΔHf(sample)/ΔHf(crystalline PE).28 a

E

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Organometallics

cis-[(THF)2TiF4] was performed according to a literature procedure.21 The pro-ligands, HL2H and HLPh,19 were prepared as described elsewhere, and the synthesis of HL1 is given in the Supporting Information. All other reagents were obtained commercially and used without further purification. GPC analyses were performed on a Waters Alliance 2000 system equipped with a dual flow refractive index (DRI) detector and a UV detector. The samples were analyzed in 1,2,4-trichlorobenzene at 145 °C using a flow rate of 1.0 mL/min. All polymers were injected at a concentration of 1 mg mL−1 in 1,2,4-trichlorobenzene, after filtration through a 2 μm stainless steel filter. Separation was performed with a guard column and three PL gel 5 μm MIXED-C (7 μm, 300 × 7.5 mm). The average molar masses (number-average molar mass Mn and weight-average molar mass Mw) and the dispersity (Đ = Mw/Mn) were derived from the RI signal by a calibration curve based on polystyrene standards. DSC was performed using a TA DSC model Q200. The samples were analyzed under a nitrogen atmosphere according to the following cycles: in the first cycle, the sample was heated from 25 to 180 °C, at a heating rate of 20 °C min−1, leaving the material at 180 °C for 5 min; the second cycle was done using a cooling rate of 10 °C min−1, until 20 °C; in the third cycle, the sample was heated from 20 to 180 °C, at a heating rate of 10 °C min−1. The crystalline melting temperature (Tm) and the degree of crystallinity (Xc) of the polyethylenes were obtained considering the second heating curves. The Xc was determined based on the ratio between the melting enthalpy (ΔHf) of the polyethylene and the 100% crystalline HDPE (293 J g −1).28 Synthesis of 1. TiF4 (0.060 g, 0.49 mmol) was added to a Schlenk flask in the glovebox. On removal from the glovebox, THF (10 mL) was added and the slurry was stirred until dissolution. The solvent was removed under reduced pressure, yielding cis-[(THF)2TiF4] as a white solid. Dichloromethane (10 mL) was added to a second Schlenk flask that had been preloaded with 2-(C5H4N)-6-(CMe2OH)C5H3N (HL1) (0.105 g, 0.49 mmol). The resulting solution of HL1 was transferred by cannula to cis-[(THF)2TiF4], forming a milky suspension that was stirred at room temperature for 3 h. Following this period, the solvent was removed under reduced pressure and bench chloroform (20 mL) added. Any insoluble material was then removed by filtration, and hexane (30 mL) was added to precipitate 1 as a white solid (0.040 g, 26%). Single crystals suitable for X-ray determination were grown by slow evaporation of a solution of 1 in chloroform. mp > 260 °C. 1H NMR (CDCl3, 500 MHz): δ 1.73 (s, 6H, CMe2), 7.43 (d, 3JHH 8.0, 1H, PyH), 7.64 (td, 3JHH 5.6, 4JHH 2.8, 1H, PyH), 7.96 (d, 3JHH 7.8, 1H, PyH), 8.08−8.09 (m, 2H, PyH), 8.11 (t, 3JHH 7.8, 1H, PyH), 8.81 (d, 3 JHH 5.2, 1H, PyH). 13C{1H} NMR (CDCl3, 125 MHz): δ 28.7 (CMe2), 94.1 (CMe2), 119.0 (CH), 121.4 (CH), 121.5 (CH), 126.9 (CH), 140.5 (CH), 142.1 (CH), 147.9 (CH), 150.5 (C), 151.7 (C), 176.0 (C). 19F NMR (CDCl3, 375 MHz): δ 100 (d, 2JFF 36.5, 2F), 141 (t, 2JFF 35.5, 1F). TOFMS (ASAP): m/z 299 [M − F]+. HR-ToFMS (ASAP): m/z calcd for (C13H13F2N2OTi), 299.0477; found, 299.0699 [M − F]+. Synthesis of 2a. cis-[(THF)2TiF4] was prepared from TiF4 (0.129 g, 1.04 mmol) as described for 1. A solution of 2-(C6H4-2′−OH),6(CMeN(2,6-i-Pr2C6H3))C5H3N (HL2H) (0.388 g, 1.04 mmol) in dichloromethane (10 mL) was transferred to cis-[(THF)2TiF4] by cannula, forming a yellow solution that was stirred at room temperature for 3 h. Following this period, the solvent was removed under reduced pressure, chloroform (20 mL) was added, and the slurry was stirred for 10 min. Any insoluble material was removed by filtration, and hexane (30 mL) added to the filtrate to precipitate 2a as a yellow solid (0.167 g, 34%). Single crystals suitable for X-ray determination were grown by slow diffusion of hexane into a chloroform solution of 2a. 1H NMR (CDCl3, 300 MHz): δ 1.04 (d, 3 JHH 7.0, 6H, CHMe2), 1.26 (d, 3JHH 6.7, 6H, CHMe2), 2.28 (s, 3H, CH3CN), 3.17 (sept, 3JHH 6.9, 2H, CHMe2), 6.79 (dd, 3JHH 8.3, 4 JHH 1.2, 1H, ArH), 6.98 (ddd, 3JHH 7.2, 3JHH 8.2, 4JHH 1.2, 1H, ArH), 7.21−7.23 (m, 2H, ArH), 7.26−7.29 (m, 1H, ArH), 7.35 (ddd, 3JHH 8.4, 3JHH 7.3, 4JHH 1.6, 1H, ArH), 7.72 (dd, 3JHH 8.2, 4JHH 1.5, 1H, ArH), 7.81 (dd, 3JHH 7.6, 4JHH 0.9, 1H, PyH), 8.10 (dd, 3JHH 7.7, 3JHH 7.7, 1H, PyH), 8.22 (d, 3JHH 7.9, 1H, PyH). 13C{1H} NMR (CDCl3,

reported elsewhere.30 Furthermore, related adduct formation has been observed by Roesky et al. from the reaction of Cp*TiF3 with TMA, initially affording Cp*TiF2Me and [AlMe2F]4 before going on to form (Cp*TiF2Me)(AlMe2F).32 Notably, on standing of the reaction mixture of 3a with TMA for 19 h at room temperature, the signal at δ −149.9 disappears, leaving behind the peak due to [FAlMe2]4 as the main resonance. It is worthy of note that hydrolytic workup of samples of 3a/MAO (and 3a/TMA) gave HL2Ph along with some precursor ketone, highlighting the robustness of the ligand framework to further reaction such as nucleophilic attack by TMA present in the MAO.19b

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CONCLUDING REMARKS A general methodology was developed for the synthesis of a range of well-defined titanium(IV) trifluoride pincer complexes, 1, 2a, and 3a, that have proved to be air- and moisture-stable. 19 F NMR spectroscopy was used to confirm the mer configuration of the fluoride ligands and to highlight the difference in donor properties of the two classes of pincer ligand. Whereas the bipy−alkoxide system (1) proved to be barely active for ethylene polymerization, phenolate−pyridylimine precatalysts 2a and 3a were significantly more active, with sterically protected 3a being the most productive while also affording UHMWPE. A similar structure−activity trend was observed for chloride-containing 2b and 3b, but these are in general more productive than their fluoride counterparts (2a and 3a) and produce polyethylenes of appreciably narrower molecular weight distributions and lower molecular weight. The differences in catalytic performance (and polymer properties) between these (L2)TiX3/MAO systems offers a further manifestation of the fluoride effect as has been identified in other polymerization studies. Attempts to establish the mode of precatalyst activation points to fluoride-bridged species playing a key role such that the differences in catalytic performance probably arise from the well-known propensity for fluoride to form strong, short bridges between electropositive metal centers.

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EXPERIMENTAL SECTION

General Procedures. All operations, unless otherwise stated, were carried out under an inert atmosphere of dry, oxygen-free nitrogen using standard Schlenk and cannular techniques or in a nitrogenpurged glovebox. Solvents were distilled under nitrogen from appropriate drying agents33 or were employed directly from a solvent purification system (Innovative Technology, Inc.) unless otherwise stated. Mass spectra (including high resolution) were recorded on a Waters Xevo QToF mass spectrometer equipped with an atmospheric solids analysis probe (ASAP). The infrared spectra were recorded in the solid state with universal ATR sampling accessories on a PerkinElmer Spectrum One FTIR instrument. NMR spectra were recorded on either a Bruker DPX 300 spectrometer operating at 300.03 (1H) and 75.4 MHz (13C), a Bruker DRX400 spectrometer at 400.13 (1H), 376.46 (19F), and 100.61 MHz (13C), or a Bruker Avance III 500 spectrometer at 125 MHz (13C), at ambient temperature unless otherwise stated. Chemical shifts (ppm) for the 1H and 13C NMR spectra are referenced using the residual protic solvent peaks and are reported relative to tetramethylsilane; 19F NMR spectra were referenced to external CFCl3. Coupling constants are expressed in hertz (Hz). Elemental analyses were performed at the Science Technical Support Unit, London Metropolitan University. The reagents TiF4, TiCl4 (1 M solution in dichloromethane), MAO (10 wt % solution in toluene), and TMA (2 M solution in toluene) were purchased from Sigma-Aldrich and used without further purification. Ethylene was supplied by BOC and used as received. The synthesis of F

DOI: 10.1021/acs.organomet.5b00900 Organometallics XXXX, XXX, XXX−XXX

Article

Organometallics 100 MHz): δ 18.1 (CHMe2), 24.6 (CHMe2), 24.6 (CHMe2), 28.1 (CH3CN), 117.5 (CH), 122.2 (CH), 123.4 (CH),123.5 (C),124.8 (CH), 126.6 (CH), 128.2 (CH), 129.3 (CH), 133.5 (CH), 140.6 (C), 140.6 (CH), 141.0 (C), 151.6 (C), 156.0 (C), 160.5 (C), 172.4 (C Nimine). 19F NMR (CDCl3 375 MHz): δ 131 (d, 2JFF 27.0, 2F), 202 (t, 2 JFF 29.3, 1F). IR (cm−1): ν(CN)imine 1638. ToFMS (ASAP): m/z 457 [M − F]+, 437 [M − 2F − H]+. HR-ToFMS (ASAP): m/z calcd for (C25H27N2OF2Ti), 457.1573; found, 457.1455. Anal. Calcd for (C25H27N2OTiF3·CHCl3): C, 52.42; H, 4.74; N, 4.70. Found: C, 52.20; H, 4.67; N, 4.99%. Synthesis of 2b. HL2H (0.372 g, 1.0 mmol) was added to a Schlenk flask that had previously been evacuated and backfilled with nitrogen. The solid was dissolved in dry dichloromethane (10 mL) before TiCl4 (1 mL, 1.0 mmol; 1.0 M solution in dichloromethane) was introduced by syringe. The resultant mixture was stirred at room temperature for 1.5 h, at which point the volatiles were removed under reduced pressure to yield the title compound as a red solid (0.51 g, 99%). Single crystals suitable for X-ray determination were grown from a saturated solution of 2b in chloroform. 1H NMR (CDCl3, 400 MHz): δ 1.02 (d, 3JHH 6.8, 6H, CHMe2), 1.33 (d, 3JHH 6.5, 6H, CHMe2), 2.37 (s, 3H CH3CN), 3.63 (sept, 3JHH 6.7, 2H, CHMe2), 6.70 (d, 3JHH 8.4, 1H, ArH), 7.10 (td, 3JHH 8.2, 4JHH 1.1, 1H, ArH), 7.25−7.32 (m, 3H, ArH), 7.41 (td, 3JHH 8.7, 4JHH 1.5, 1H, ArH), 7.75 (dd, 3JHH 8.2, 4JHH 1.3, 1H, ArH), 7.83 (d, 3JHH 7.6, 1H, PyH), 8.10 (dd, 3JHH 8.1, 3JHH 8.1, 1H, PyH), 8.19 (d, 3JHH 8.4, 1H, PyH). 13 C{1H} NMR (CDCl3, 125 MHz): δ 18.9 9 (CHMe2), 23.9 (CH3), 24.3 (CH3), 27.6 (CH3), 115.6 (CH), 124.4 (CH), 124.8 (CH), 124.9 (C), 125.2 (CH), 126.9 (CH), 128.4 (CH), 129.3 (CH), 133.6 (CH), 139.9 (C), 140.6 (CH), 143.4 (C), 150.9 (C), 153.9 (C), 161.4 (C), 173.2 (CNimine). ToFMS (ASAP): m/z 489 [M − Cl]+, 454 [M − 2Cl − H]+. HR-ToFMS (ASAP): m/z calcd for (C25H27N2OCl2Ti), 489.0980; found, 489.1004 [M − Cl] + . Anal. Calcd for (C25H27N2OTiCl3): C, 57.12; H, 5.18; N, 5.33. Found: C, 57.18; H, 5.40; N, 5.59%. Synthesis of 3a. cis-[(THF)2TiF4] was prepared from TiF4 (0.129 g, 1.04 mmol) as described for 1. A solution of 2-(3-C12H8-2-OH)-6(CMeN(2,6-i-Pr2C6H3))C5H3N (HL2Ph) (0.300 g, 0.67 mmol) in dry dichloromethane (10 mL) was transferred to cis-[(THF)2TiF4] by cannula, instantly forming a yellow solution that was stirred at room temperature for 3 h. Following this period, the solvent was removed under reduced pressure, chloroform (20 mL) was added, and the resultant slurry was stirred for 10 min. Insoluble material was removed by filtration, and hexane (30 mL) added to the filtrate to precipitate 3a as a yellow solid (0.254 g, 67%). Single crystals suitable for X-ray determination were grown by slow diffusion of petroleum ether (40− 60) into a chloroform solution of 3a. 1H NMR (CDCl3, 400 MHz): δ 1.10 (d, 3JHH 7.2, 6H, CHMe2), 1.33 (d, 3JHH 6.6, 6H, CHMe2), 2.36 (s, 3H, CH3CN), 2.34 (sept, 3JHH 6.9, 2H, CHMe2), 7.13 (dd, 3JHH 8.5, 3 JHH 8.5, 1H, ArH), 7.28 (dd, 3JHH 8.4, 4JHH 1.0, 2H, ArH), 7.30 (t, 3 JHH 6.6, 1H, ArH), 7.33 (dd, 3JHH 8.5, 3JHH 7.8, 1H, ArH), 7.39 (dd, 3 JHH 7.9, 3JHH 7.5, 2H, ArH), 7.52 (dd, 3JHH 7.5, 4JHH 1.6, 1H, ArH), 7.64 (ddd, 3JHH 6.7, 4JHH 2.6, 4JHH 1.1, 2H, ArH), 7.75 (dd, 3JHH 8.1, 4 JHH 1.4, 1H, ArH), 7.89 (dd, 3JHH 7.7, 4JHH 0.7, 1H, PyH), 8.19 (dd, 3 JHH 8.1, 3JHH 8.1, 1H, PyH), 8.30 (d, 3JHH 8.3, 1H, PyH). 13C{1H} NMR (CDCl3, 125 MHz): δ 18.1 (CH3CN), 24.5 (CHMe2), 24.7 (CHMe2), 28.1 (CHMe2), 122.3 (CH), 123.2 (CH), 124.3 (C), 124.8 (CH), 127.1 (CH), 127.3 (CH), 128.1 (CH), 128.1 (CH), 128.7 (CH), 129.6 (CH), 130.1 (C), 134.5 (CH), 137.4 (C), 140.5 (C), 140.6 (CH), 140.9 (C), 151.6 (C), 156.6 (C), 158.0 (C), 172.4 (C Nimine). 19F NMR (CDCl3, 375 MHz): δ 132 (d, 2JFF 31.6, 2F), 201 (t, 2 JFF 32.9, 1F). IR (cm−1): ν(CN)imine 1630. ToFMS (ASAP): m/z 533 [M − F] + , 413 [M − 2F − H] + . Anal. Calcd for (C31H31N2OTiF3): C, 67.40; H, 5.66; N, 5.07. Found: C, 67.15; H, 5.42; N, 5.00%. Synthesis of 3b. HL2Ph (0.250 g, 1.0 mmol) was added to a Schlenk flask that had previously been evacuated and backfilled with nitrogen. The solid was dissolved in dry dichloromethane (10 mL) before TiCl4 (1 mL, 1 mmol 1.0 M solution in dichloromethane) was introduced by syringe. The resultant mixture was stirred at room

temperature for 1.5 h, at which point the solvent was removed under reduced pressure to yield 3b as a red solid (0.33 g, 98%). mp > 260 °C. 1 H NMR (CDCl3, 400 MHz): δ 1.02 (d, 3JHH 6.8, 6H, CHMe2), 1.33 (d, 3JHH 6.6, 6H, CHMe2), 2.37 (s, 3H, MeCN), 3.64 (sept, 3JHH 6.6, 2H, CHMe2), 7.16−7.20 (m, 3H, ArH), 7.23−7.31 (m, 3H, ArH), 7.36 (app. t, 3JHH 7.8, 2H, ArH), 7.53 (dd, 3JHH 7.7, 4JHH 1.5, 1H, ArH), 7.66 (m, 2H, ArH), 7.71 (dd, 3JHH 8.2, 4JHH 1.5, 1H, ArH), 7.83 (dd, 3 JHH 7.8, 4JHH 1.0, 1H, PyH), 8.11 (dd, 3JHH 7.8, 1H, PyH), 8.22 (d, 3 JHH 8.0, 1H, PyH). 13C{1H} NMR (CDCl3, 125 MHz): δ 19.9 (CHMe2), 24.8 (CH3), 25.4 (CH3), 28.7 (CH3), 124.3 (CH), 124.9 (CH), 125.2 (CH), 126.8 (C), 127.4 (CH), 127.9 (CH), 128.3 (CH), 128.4 (CH), 128.6 (CH), 129.1 (C), 129.7 (CH), 134.5 (CH), 136.6 (C), 140.8 (CH), 140.9 (C), 144.2 (C), 152.1 (C), 155.4 (C), 160.0 (C), 174.4 (CN). ToFMS (ASAP): m/z 565 [M − Cl]+, 529 [M− 2Cl − H]+. HR-ToFMS (ASAP): m/z calcd for (C31H31N2OCl2Ti), 565.1293; found, 565.1306. General Screening for Ethylene Polymerization. An ovendried 200 mL Schlenk vessel equipped with magnetic stir bar was evacuated and backfilled with nitrogen. The vessel was charged with the precatalyst (0.01 mmol) and dissolved in toluene (40 mL). MAO (3.0 mmol, 300 equiv) was introduced, and the reaction mixture was left to stir for 5 min. The vessel was purged with ethylene, and the contents were magnetically stirred under 1 bar ethylene pressure at a preset temperature (controlled using an external oil bath) for 30 min. The run was terminated by the addition of dilute aqueous hydrogen chloride (5 mL). The polymer was isolated by filtration, washed with methanol, and dried in a vacuum oven set at 40 °C until it reached a constant weight. In Situ NMR Study of the Reaction of 3a with MAO or AlMe3. In a typical procedure, an oven-dried 20 mL Schlenk vessel equipped with magnetic stir bar was evacuated and transferred to a glovebox. The vessel was loaded with 3a (5.53 mg, 0.010 mmol), the required aluminum alkyl (in toluene) was introduced at the ratio required, and the vessel was topped-up (if necessary) with dry toluene. The mixture (0.5 mL) was added to a NMR Teflon valve-sealed NMR tube containing a C6D6 glass insert and transferred directly to the NMR spectrometer. Crystallographic Studies. Data for 1, 2a, 2b, and 3a were collected on a Bruker APEX 2000 CCD diffractometer. Details of data collection, refinement, and crystal data are listed in Table S1. The data were corrected for Lorentz and polarization effects, and empirical absorption corrections were applied. Structure solution by direct methods and structure refinement based on full-matrix least-squares on F2 employed SHELXTL, version 6.10.34 Hydrogen atoms were included in calculated positions (C−H = 0.95−1.00 Å) riding on the bonded atom with isotropic displacement parameters set to 1.5 Ueq(C) for methyl H atoms and 1.2 Ueq(C) for all other H atoms. All non-H atoms were refined with anisotropic displacement parameters. Data are deposited under CCDC reference numbers 1432734−1432737.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.organomet.5b00900. Synthetic details for HL1; 1H, 13C{1H}, and 19F{1H} NMR spectra along with GPC and DSC traces; and crystallographic and data processing parameters for 1, 2a, 2b, and 3a (PDF) Crystallographic data for 1 (CIF) Crystallographic data for 2a (CIF) Crystallographic data for 2b (CIF) Crystallographic data for 3a (CIF) checkCIF/PLATON report for 1 (PDF) checkCIF/PLATON report for 2a (PDF) checkCIF/PLATON report for 2b (PDF) checkCIF/PLATON report for 3a (PDF) G

DOI: 10.1021/acs.organomet.5b00900 Organometallics XXXX, XXX, XXX−XXX

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Organometallics

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R.; Talarico, G.; Froese, R. D. J.; Vosejpka, P. C.; Hustad, P. D.; Macchioni, A. Organometallics 2009, 28, 5445. (c) Zuccaccia, C.; Macchioni, A.; Busico, V.; Cipullo, R.; Talarico, G.; Alfano, F.; Boone, H. W.; Frazier, K. A.; Hustad, P. D.; Stevens, J. C.; Vosejpka, P. C.; Abboud, K. A. J. Am. Chem. Soc. 2008, 130, 10354. (12) Bochmann, M. Organometallics 2010, 29, 4711. (b) Chen, E.Y.X.; Marks, T. J. Chem. Rev. 2000, 100, 1391. (13) (a) Nikiforov, G. B.; Roesky, H. W.; Jones, P. G. J. Fluorine Chem. 2008, 129, 376. (b) Jäger, F.; Roesky, H. W.; Dora, H.; Shak, S.; Noltemeyer, M.; Schmidt, H.-G. Chem. Ber. 1997, 130, 399. (c) Nikiforov, G. B.; Roesky, H. W.; Koley, D. Coord. Chem. Rev. 2014, 258−259, 16. (14) Kerr, J. A. In CRC Handbook of Chemistry and Physics 1999− 2000: A Ready-Reference Book of Chemical and Physical Data, 81st ed.; Lide, D.R., Ed.; CRC Press: Boca Raton, FL, 2000. (15) (a) Arndt, P.; Jager-Fiedler, U.; Klahn, M.; Baumann, W.; Spannenberg, A.; Burlakov, V. V.; Rosenthal, U. Angew. Chem., Int. Ed. 2006, 45, 4195. (b) Arndt, P.; Spannenberg, A.; Baumann, W.; Burlakov, V. V.; Rosenthal, U.; Becke, S.; Weiss, T. Organometallics 2004, 23, 4792. (c) Becke, S.; Rosenthal, U. Patent DE 199 32 409 A1, Jan 1, 2001. (d) Becke, S.; Rosenthal, U. Patent US 6,303,718 B1, Oct 16 2001. (e) Becke, S.; Rosenthal, U.; Arndt, P.; Baumann, W.; Spannenberg, A. Patent DE 101 10 227 A1, Sept 5, 2002. (16) (a) Pedeutour, J. N.; Cramail, H.; Deffieux, A. J. Mol. Catal. A: Chem. 2001, 174, 81. (b) Schwecke, C.; Kaminsky, W. Macromol. Rapid Commun. 2001, 22, 508. (c) Qian, Y.; Zhang, H.; Qian, X.; Huang, J.; Shen, C. J. Mol. Catal. A: Chem. 2003, 192, 25. (17) (a) Kaminsky, W.; Lenk, S.; Scholz, V.; Roesky, H. W.; Herzog, A. Macromolecules 1997, 30, 7647. (b) Kaminsky, W. J. Chem. Soc., Dalton Trans. 1998, 1413. (c) Murphy, E. F.; Murugavel, R.; Roesky, H. W. Chem. Rev. 1997, 97, 3425. (d) Shah, S. A. A.; Dorn, H.; Voigt, A.; Roesky, H. W.; Parisini, E.; Schmidt, H.-G.; Noltemeyer, N. Organometallics 1996, 15, 3176. (18) For related O,N,N ligands on titanium(IV), see (a) Paolucci, G.; Zanella, A.; Sperni, L.; Bertolasi, V.; Mazzeo, M.; Pellecchia, C. J. Mol. Catal. A: Chem. 2006, 258, 275. (b) Huang, W.; Zhang, W.; Liu, S.; Liang, T.; Sun, W.-H. J. Polym. Sci., Part A: Polym. Chem. 2011, 49, 1887. (c) Wang, Y.; Zhang, W.; Huang, W.; Wang, L.; Redshaw, C.; Sun, W.-H. Polymer 2011, 52, 3732. (d) Cariou, R.; Gibson, V. C.; Tomov, A. K.; White, A. J. P. J. Organomet. Chem. 2009, 694, 703. (19) (a) Wright, L. A.; Hope, E. G.; Solan, G. A.; Cross, W. B.; Singh, K. Dalton Trans. 2015, 44, 6040. (b) Alkarekshi, W.; Armitage, A. P.; Boyron, O.; Davies, C. J.; Govere, M.; Gregory, A.; Singh, K.; Solan, G. A. Organometallics 2013, 32, 249. (c) Giesbrecht, G. R.; Solan, G. A.; Davies, C. J. PCT Int. Appl. WO 2009082556 A1, July 2, 2009. (d) Davies, C. J.; Gregory, A.; Griffith, P.; Perkins, T.; Singh, K.; Solan, G. A. Tetrahedron 2008, 64, 9857. (e) Solan, G. A.; Davies, C. J. PCT Int. Appl. WO 2005095469 A1, Oct 13, 2005. (20) van der Vlugt, J. I.; Demeshko, S.; Dechert, S.; Meyer, F. Inorg. Chem. 2008, 47, 1576. (21) Jura, M.; Levason, W.; Petts, E.; Reid, G.; Webster, M.; Zhang, W. Dalton Trans. 2010, 39, 10264. (22) (a) Decken, A.; Ilyin, E. G.; Jenkins, H. D. B.; Nikiforov, G. B.; Passmore, J. Dalton Trans. 2005, 3039. (b) Il'in, E. G.; NiKiforov, G. B.; Aleksandrov, G. G.; Roesky, G. V.; Buslaev, Y. A. Dokl. Chem. 2001, 376, 58. (23) Dungan, C. H.; van Wazer, J. R. Compilation of Reported 19F NMR Chemical Shifts; Wiley-Interscience: New York, 1970. (24) Yoshida, Y.; Matsui, S.; Fujita, T. J. Organomet. Chem. 2005, 690, 4382. (25) (a) Mitani, M.; Nakano, T.; Fujita, T. Chem. - Eur. J. 2003, 9, 2396. (b) Furuyama, R.; Saito, J.; Ishii, S. I.; Mitani, M.; Matsui, S.; Tohi, Y.; Makio, H.; Matsukawa, N.; Tanaka, H.; Fujita, T. J. Mol. Catal. A: Chem. 2003, 200, 31. (26) (a) Sobieraj, M. C.; Rimnac, C. M. J. Mech Beh. Biomed. Mater. 2009, 2, 433. (b) Chanda, M.; Roy, S. K. Plastics Technology Handbook, 4th ed.; CRC Press: Boca Raton, FL, 2007. (27) (a) Zhu, Y.; Chang, L.; Yu, S. J. Therm. Anal. 1995, 45, 329. (b) Ahmad, M.; Wahit, M. U.; Kadir, M. R. A.; Dahlan, K. Z. M.;

AUTHOR INFORMATION

Corresponding Author

*Tel.: +44 (0) 116 2522096. Fax: +44 (0) 116 2523789. Email: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We are grateful to the University of Leicester for financial assistance. Dr. T. Boller (ExxonMobil) is thanked for obtaining the DSC and high-temperature GPC data for the polyethylene samples.

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DOI: 10.1021/acs.organomet.5b00900 Organometallics XXXX, XXX, XXX−XXX

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DOI: 10.1021/acs.organomet.5b00900 Organometallics XXXX, XXX, XXX−XXX