Synthesis of Titanium Complexes Containing an Amine Triphenolate

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Synthesis of Titanium Complexes Containing an Amine Triphenolate Ligand of the Type [TiX{(O-2,4‑R2C6H2)‑6-CH2}3N] and the Ti−Al Heterobimetallic Complexes with AlMe3: Effect of a Terminal Donor Ligand in Ethylene Polymerization Kotohiro Nomura,* Udomchai Tewasekson, and Yuki Takii Department of Chemistry, Faculty of Science and Engineering, Tokyo Metropolitan University, 1-1 Minami Osawa, Hachioji, Tokyo 192-0397, Japan S Supporting Information *

ABSTRACT: The synthesis of various titanatranes containing tris(aryloxo)amine ligands, TiX[{(O-2,4-R2C6H2)-6-CH2}3N] [R = Me (1), tBu (2); X = OiPr (a), OtBu (b), OCH(CF3)2 (c), OC(CF3)3 (d), Cl (e)], and some reactions with Al alkyls have been explored. The Ti− Al heterobimetallic complexes [TiMe{(O-2,4-Me2C6H2-6-CH2)2(μ2-O2,4-Me2-C6H2-6-CH2)}N]-[Me2Al(μ2-OR′)] [R′ = tBu (3b), CH(CF3)2 (3c), C(CF3)3 (3d)] were isolated by reaction of 1b−d with 1 equiv of AlMe3, whereas reaction of 2b with AlMe3 in toluene did not take place under the same conditions, and the reaction of 2c afforded the methyl complex TiMe[{(O-2,4-tBu2C6H2)-6-CH2}3N]. Structures of 3b,c were determined by X-ray crystallography. These complexes exhibited high catalytic activities for ethylene polymerization in n-octane in the presence of MAO, and the activity by 1 and 2 was affected by the terminal donor ligand (X) as well as the substituent on the atrane ligand (R). The activities by 1c,d increased upon addition of AlMe3, and the corresponding heterobimetallic complexes (3c,d) showed high activities, whereas an increase in the activity upon addition of AlMe3 was not observed by 1b,e, and 2c,e.

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temperature (100−120 °C).11a The isolated Ti−Al heterobimetallic complexes, [TiMe{(μ2-O-2,4-R2C6H2-6-CH2)(O2,4-R2C6H2-6-CH2)2N}][Me2Al(μ2-OiPr)] [R = Me (3a), tBu (4a)], exhibited from moderate to high activities in toluene in the presence of MAO at 80−120 °C.11b In particular, 4a itself polymerizes ethylene without any cocatalysts to afford high molecular weight polymer with unimodal molecular weight distribution.11b Moreover, another Ti−Al heterobimetallic complex, [TiMe{(O-2,4-Me2C6H2-6-CH2)2(μ2-OCH2CH2)N}][Me2Al(μ2-OtBu)], also showed activity without cocatalyst at 120 °C.13a These results clearly suggest a hypothesis that the cationic species generated by cleavage of the Ti−O bonds play an important role as the active species in this catalysis (Scheme 1). Effects of an aryloxo terminal ligand and AlMe3 in ethylene polymerization using a series of Ti(OAr)[{(O-2,4-Me2C6H2)6-CH2}3N] [Ar = 2,6-Me2C6H3, 2,6-iPr2C6H3, 2,6-Ph2C6H3, 2,6-F2C6H3, C6F5] were explored in the presence of MAO.11c It turned out that reaction of the complexes, which exhibited increases in the activity upon addition of AlMe3 [Ar = 2,6F2C6H3, C6F5, 2,6-iPr2C6H3], with AlMe3 afforded the corresponding Ti−Al heterobimetallic complexes in moderate

INTRODUCTION Polyolefins [high-density polyethylene (HDPE), linear lowdensity polyethylene (LLDPE), etc., polypropylene] produced by metal-catalyzed olefin coordination polymerization are important commercial synthetic polymers in our daily life, and the market capacity is still increasing every year. The design of efficient molecular catalysts for precise olefin polymerization has been one of the most attractive research subjects since the discovery of Ziegler−Natta catalysts.1−5 Transition-metal complexes are generally activated by different types of cocatalysts in the polymerization to afford cationic alkyl species, which are considered to be an active species in this catalysis.3f,6,7 Al reagents, such as Al alkyl and methylaluminoxane (MAO), play an important role as cocatalysts,6−8 and these Al complexes are also involved in the catalysis by forming heterobimetallic complexes or counteranions with titanium complexes.6,9,10 These bimetallic complexes exhibit high catalytic activities to produce polymers with different microstructures.3f,i,7b We reported that titanium complexes containing tris(aryloxo)amine ligands11,12 (so-called titanatranes)12−16 of the type Ti(OiPr)[{(O-2,4-R2C6H2)-6-CH2}3N] [R = Me (1a), tBu (2a)] exhibited catalytic activities for ethylene polymerization in the presence of MAO,11 and the activities increased upon addition of a small amount of AlMe3 as well as at higher © XXXX American Chemical Society

Received: April 11, 2015

A

DOI: 10.1021/acs.organomet.5b00303 Organometallics XXXX, XXX, XXX−XXX

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Organometallics Scheme 1. Plausible Activation Mechanism for Ethylene Polymerization by [TiMe{(O-2,4-Me2C6H2-6-CH2)2(μ2OCH2CH2)N}][Me2Al(μ2-OtBu)]11b

Scheme 2. Effect of Terminal Aryloxo Ligands in Ethylene Polymerization by [Ti(OAr){(O-2,4-Me2C6H2)-6-CH2}3N]11c

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yields.11c In contrast, monomeric methyl complex TiMe[{(O2,4-Me2C6H2)-6-CH2}3N] and/or [Me2Al(μ2-OAr)]2 were isolated from the reaction mixture when the complexes, which showed a decrease in activity upon addition of AlMe3 [Ar = 2,6-Me2C6H3, C6F5], were treated with AlMe3 under the same conditions (Scheme 2).11c The isolated heterobimetallic complexes exhibited high catalytic activities in the presence of MAO, suggesting that the bimetallic species play a role in this catalysis. It was thus assumed that steric bulk on the bridged ligands (substituent in the atrane ligand and terminal ligand) would play a role.11c In this paper, we focus on the synthesis of titanium complexes containing tris(aryloxo)amine ligands of the type TiX[{(O-2,4-R2C6H2)-6-CH2}3N] [R = Me, tBu; X = OiPr, OtBu, OCH(CF3)2, OC(CF3)3, etc.] and some reactions especially with Al alkyls, including synthesis and structural analysis of heterobimetallic complexes [TiMe{(O-2,4-R2C6H26-CH2)2(μ2-O-2,4-R2-C6H2-6-CH2)}N][Me2Al(μ2-X)]. Moreover, these complexes were employed as the catalyst precursors for ethylene polymerization in the presence of MAO (and AlMe3). On the basis of these results, we explore a role of the heterobimetallic complexes including ligand effect in this catalysis.17

RESULTS AND DISCUSSION

1. Synthesis of Titanatranes Containing Alkoxo Terminal Ligands, Ti(OR′)[{(O-2,4-R2C6H2)-6-CH2}3N], and Reactions with Al Alkyls. Reactions of reported Ti(OiPr)[{(O-2,4-R2C6H2)-6-CH2}3N] [R = Me (1a), tBu (2a)], prepared by reaction of Ti(OiPr)4 with [(HO-2,4R2C6H2-6-CH2)3N],11c,12a with (CF3)2CHOH (10 equiv) in toluene at 25 °C afforded corresponding Ti[OCH(CF3)2][{(O-2,4-R2C6H2)-6-CH2}3N] (1c, 2c, Scheme 3). A similar reaction of 1a with (CF3)3COH (5 equiv) afforded Ti[OC(CF3)3][{(O-2,4-Me2C6H2)-6-CH2}3N] (1d); however, the reaction with the tBu analogue (2a) did not take place. This may be explained by an assumption that coordination of (CF3)3COH onto Ti for the ligand exchange would be difficult due to steric bulk [tBu substituent of the atrane ligand and (CF3)3COH].18 Reaction of 2e (the chloro analogue described below) with LiOC(CF3)3 in toluene did not take place either. Reactions of Ti(OtBu)4 with [(HO-2,4-R2C6H2-6-CH2)3N] in toluene afforded Ti(OtBu)[{(O-2,4-R2C6H2)-6-CH2}3N] [R = Me (1b), tBu (2b)12e]. These newly prepared complexes (1b− d, 2c) were identified by NMR spectra and elemental analysis. It turned out that the chloro analogues, TiCl[{(O-2,4-R2C6H2)6-CH2}3N] [R = Me (1e), tBu (2e)], were prepared by reactions of TiCl4(THF)2 with [(HO-2,4-R2C6H2-6-CH2)3N] in toluene, and their formations were confirmed by comparison of their NMR spectra with refs 11c, 12f, and 12h. As reported B

DOI: 10.1021/acs.organomet.5b00303 Organometallics XXXX, XXX, XXX−XXX

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Scheme 3. Synthesis of a Series of Titanatranes Containing Tris(aryloxo)amine Ligands, TiX[{(O-2,4-R2C6H2)-6-CH2}3N]

Scheme 4. Some Reactions with AlMe3 and AlEt3

previously, 2e was prepared from 2a by treating with Me2AlCl.11c Synthetic protocols conducted for 1e and 2e [using TiCl4(THF)2 in place of TiCl4, Ti(OiPr)4, etc.] seem to be more simple and straightforward than the other reported procedures.11c,12f−h It was reported that reactions of 1a and 2a with AlMe3 (1 equiv) afforded [TiMe{(O-2,4-R2C6H2-6-CH2)2(μ2-O-2,4-R2C6H2-6-CH2)}N][Me2Al(μ2-OiPr)] [R = Me (3a), tBu (4a)].11b Although reaction of 1a with AlEt3 afforded the

heterobimetallic [TiEt{(O-2,4-Me2C6H2-6-CH2)2(μ2-O-2,4Me2-C6H2-6-CH2)}N][Et2Al(μ2-OiPr)],11b a similar reaction of 2a afforded TiEt[{(O-2,4-tBu2C6H2)-6-CH2}3N] (2f) as the sole isolated product (Scheme 4); 2f was identified by NMR spectra and elemental analysis, and the structure was confirmed by X-ray crystallography.19 Reactions of 1b−d with AlMe3 (1.0 equiv) in toluene (under similar conditions for the synthesis of 3a and 4a) afforded the corresponding heterobimetallic complexes [TiMe{(O-2,4C

DOI: 10.1021/acs.organomet.5b00303 Organometallics XXXX, XXX, XXX−XXX

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Figure 1. ORTEP drawings for [TiMe{(O-2,4-Me2C6H2-6-CH2)2(μ2-O-2,4-Me2-C6H2-6-CH2)}N][Me2Al(μ2-OR′)] [R′ = tBu (3b), CH(CF3)2 (3c)]. Thermal ellipsoids are drawn at the 50% probability level, and H atoms are omitted for clarity.21

one methyl and four oxygen atoms including three oxygen atoms from the atrane ligand, and the titanium atom in 3b,c is ligated via a transannular interaction stemming from the bridgehead amino nitrogen. The average O−Ti−N bond angles for 3b and 3c are 84.40° and 84.35°, respectively, suggesting a displacement of the titanium atom toward the axial oxygen, similar to the observation made in the case of other Ti−Al heterobimetallic complexes.11b,c Note that Al atoms in 3b,c are ligated through oxygen atoms in the terminal alkoxo ligand and the atrane ligand, and Ti− O(in μ2-O) bond distances in 3b,c [2.043(3)−2.219(2) Å, corresponding to Ti−O(1) and Ti−O(2) distances] are thus longer than the other Ti−O distances in the aryloxo ligands [1.8110(19)−1.8419(19) Å, corresponding to Ti−O(3) and Ti−O(4) distances]. These distances are also longer than those in Al−O bond distances [1.836(2)−1.858(3) Å, corresponding to Al−O(1) and Al−O(2) distances]. Moreover, the bond angle of O(1)−Ti(1)−O(2) [71.20(7)° for 3b, 68.99(7)° for 3c] is smaller than that for O(3)−Ti(1)−O(4) [103.24(8)° for 3b, 108.39(11)° for 3c], as observed in 3a [70.48(5)°, 104.40(6)°, respectively]. These are similar observations to those reported previously,1b,c and the results in 3b,c thus clearly indicate that the resultant complexes possess Ti−Me bonds by replacement of the OR′ group with AlMe3. These complexes (3a−c) also have a distorted tetrahedral geometry around Al, and the C(2)−Al(1)−C(3) bond angles in these complexes are 115.37(11)−116.67(16)°, although the O(1)−Al(1)−O(2) bond angles are small [81.24(6)−81.97(8)°]. As described above, Al−O bond distances [1.836(2)−1.858(3) Å] are shorter than the Ti−O(in μ2-O) bond distances [2.043(3)− 2.219(2) Å].

Me2C6H2-6-CH2)2(μ2-O-2,4-Me2-C6H2-6-CH2)}N][Me2Al(μ2OR′)] [R′ = OtBu (3b), OCH(CF3)2 (3c), OC(CF3)3 (3d)] in high yields (Scheme 4). These complexes were identified on the basis of NMR spectra and elemental analysis, and the structures of 3b,c were determined by X-ray crystallography (Figure 1, shown below).20 Since 3d is isolated as an oil, its structure is drawn on the basis of those in 3a−c. In contrast, reaction of the tBu analogue (2c) with AlMe3 afforded TiMe[{(O-2,4-tBu2C6H2)-6-CH2}3N] (2g) and [Me2Al{OCH(CF3)2}]n (Scheme 3);20 the methyl complex (2g) could also be prepared by treating 2a with MeMgBr in toluene. Attempted reaction of 2b with AlMe3 in toluene failed and recovered 2b even after 24 h (at 25 °C).20 We assume that these would be due to steric bulk of the tBu substituent in the atrane ligand. 2. Structural Analysis for [TiMe{(O-2,4-Me2C6H2-6CH2)2(μ2-O-2,4-Me2-C6H2-6-CH2)}N][Me2Al(μ2-OR′)] [R′ = t Bu (3b), CH(CF3)2 (3c)]. Figure 1 shows ORTEP drawings for the Ti−Al heterobimetallic complexes [TiMe{(O-2,4Me2C6H2-6-CH2)2(μ2-O-2,4-Me2-C6H2-6-CH2)}N][Me2Al(μ2OR′)] [R′ = tBu (3b), CH(CF3)2 (3c)], determined by X-ray crystallography, and the selected bond distances and angles are summarized in Table 1.21 Results for 3a (R′ = OiPr) and 4a are also shown for comparison. These complexes have a distorted octahedral geometry around titanium consisting of a C−Ti−N axis [bond angles for N(1)−Ti(1)−C(1): 170.09(9)° for 3b; 174.54(8)° for 3c] and a distorted plane of one alkoxo ligand and three aryloxo chelate ligands [total bond angles of the O(1)−Ti(1)−O(2), O(2)− Ti(1)−O(4), O(3)−Ti(1)−O(4), and O(1)−Ti(1)−O(3) for 3b,c were 358.90° and 357.75°, respectively], as observed in 3a (357.06°).11b The coordination sphere of titanium consists of D

DOI: 10.1021/acs.organomet.5b00303 Organometallics XXXX, XXX, XXX−XXX

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Table 1. Selected Bond Distances and Angles for [TiMe{(O-2,4-R2C6H2-6-CH2)2(μ2-O-2,4-R2-C6H2-6-CH2)}N][Me2Al(μ2OR′)] [R = Me, R′ = iPr (3a),11c tBu (3b), CH(CF3)2 (3c); R = tBu, R′ = iPr (4a)11b]a R′

a

3ab

3b

i

t

Pr

Ti(1)−O(1) Ti(1)−O(2) Ti(1)−O(3) Ti(1)−O(4) Ti(1)−N(1) Ti(1)−C(1) Al(1)−O(1) Al(1)−O(2)

2.0714(13) 2.0716(11) 1.8195(12) 1.8416(14) 2.4391(15) 2.1065(19) 1.8273(13) 1.8446(15)

O(1)−Ti(1)−O(2) O(1)−Al(1)−O(2) O(1)−Ti(1)−O(4) O(2)−Ti(1)−O(3) O(1)−Ti(1)−O(3) O(2)−Ti(1)−O(4) O(3)−Ti(1)−O(4) O(1)−Ti(1)−N(1) O(2)−Ti(1)−N(1) O(1)−Ti(1)−C(1) O(2)−Ti(1)−C(1) O(3)−Ti(1)−C(1) N(1)−Ti(1)−C(1) Ti(1)−O(1)−Al(1) Ti(1)−O(2)−Al(1) C(2)−Al(1)−C(3)

70.48(5) 81.24(6) 160.13(5) 156.71(5) 90.57(5) 91.61(5) 104.40(6) 89.11(5) 83.20(4) 96.01(7) 95.62(6) 99.86(6) 174.04(7) 102.93(6) 102.32(6) 115.50(9)

Bu

Selected Bond Distances (Å) 2.103(2) 2.0445(15) 1.8242(16) 1.8419(19) 2.477(2) 2.119(3) 1.8398(17) 1.842(2) Selected Bond Angles (deg) 71.20(7) 81.97(8) 162.72(6) 152.87(7) 92.93(7) 91.53(8) 103.24(8) 97.35(7) 80.61(6) 92.27(9) 100.40(8) 102.13 (8) 170.09(9) 102.08(9) 104.25(9) 115.37(11)

3c CH(CF3)2

4ab i

Pr

2.219(2) 2.043(3) 1.827(2) 1.8110(19) 2.419(2) 2.107(3) 1.858(3) 1.836(2)

2.0673(19) 2.095(2) 1.8283(19) 1.8237(19) 2.381 (2) 2.113(3) 1.825(2) 1.845(2)

68.99(7) 81.78(10) 162.48(9) 150.43(8) 86.23(9) 94.14(9) 108.39(11) 91.26(8) 82.11(8) 92.26(10) 95.29(12) 101.77(11) 174.54(8) 99.36(8) 106.82(8) 116.67(16)

70.99(7) 82.38(9) 163.60(9) 155.50(9) 89.71(8) 94.53(8) 101.95(8) 88.79(7) 83.80(7) 94.73(10) 95.88(10) 100.74(10) 176.15(10) 103.66(9) 101.90(9) 114.90(14)

The details are shown in the Supporting Information.21 bCited from ref 11b.

Ti−Al heterobimetallic complex (3a), cleanly isolated by reaction of 1a with AlMe3, showed moderate activity in toluene in the presence of MAO at 100 °C,22 and the observed activity is close to that by 1a upon addition of AlMe3 conducted under the same conditions.11a,b Ethylene polymerizations by TiX[{(O-2,4-R2 C6 H2 )-6CH2}3N] [R = Me (1), tBu (2); X = OiPr (a), OtBu (b), OCH(CF3)2 (c), OC(CF3)3 (d), Cl (e), Et (f), Me (g)], [TiMe{(O-2,4-Me 2 C 6 H 2 -6-CH 2 ) 2 (μ 2 -O-2,4-Me 2 -C 6 H 2 -6CH2)}N][Me2Al(μ2-OR′)] [R′ = tBu (3b), CH(CF3)2 (3c), C(CF3)3 (3d)] were thus conducted in n-octane in the presence of MAO. The results are summarized in Table 1. As described above, data for 1a11a conducted under the same conditions are shown for comparison. MAO white solid (dMAO) prepared by removing AlMe3 and toluene from the commercially available samples (TMAO, 9.2 wt % in toluene, Tosoh Finechem Co.) was chosen, because it was effective in the preparation of high molecular weight copolymers with unimodal molecular weight distributions, as reported previously.11,23 In contrast to the fact observed in 1a and 3a (X = OiPr), the activity by 1b (X = OtBu) decreased upon addition of AlMe3 (runs 9−11), although the activity was affected by the Al/Ti molar ratio; the heterobimetallic complex (3b) showed similar activity in the presence of MAO (run 12). The activity by the t Bu analogue (2b) was lower than that by 1b, and only a slight increase in the activity was observed upon addition of AlMe3 (runs 13, 14). A similar trend was observed between 1a and 2a (runs 1, 3−6). As assumed from the results by 1a (in the presence of AlMe3), the resultant polymers prepared by 1b and

Note that Ti(1)−O(1) distance for 3b,c [2.103(2), 2.219(2) Å, respectively] are apparently longer than those in 3a and 4a [2.0714(13), 2.0673(19) Å, respectively], and Ti(1)−O(2) bond distance in 3b,c [2.0445(15), 2.043(3) Å, respectively] is apparently shorter than those in 3a and 4a [2.0716(11), 2.095(2) Å, respectively]. Moreover, the Ti(1)−N(1) bond distance in 3a−c becomes longer in the following order: 2.419(2) Å [3c, R′ = CH(CF3)2] < 2.4391(15) Å (3a, R′ = iPr) < 2.477(2) Å (3b, R′ = tBu). The bond angle of O(1)−Ti(1)− O(2) in 3c [68.99(7)°] is smaller than the others [3a,b: 70.48(5)°, 71.20(7)°, respectively], whereas the bond angle of O(3)−Ti(1)−O(4) in 3c [108.39(11)°] is apparently larger than the others [3a,b: 104.40(6)°, 103.24(8)°, respectively]. These would be explained as the influence of the OCH(CF3)2 group (in 3c) in place of the OiPr (3a) or OtBu (3b) group. 3. Ethylene Polymerization by TiX[{(O-2,4-R2C6H2)-6CH2}3N] [R = Me (1), tBu (2); X = OiPr (a), OtBu (b), OCH(CF3)2 (c), OC(CF3)3 (d), Cl (e), Et (f), Me (g)], [TiMe{(O-2,4-Me 2 C 6 H 2 -6-CH 2 ) 2 (μ 2 -O-2,4-Me 2 -C 6 H 2 -6CH2)}N][Me2Al(μ2-OR′)] [R′ = tBu (3b), CH(CF3)2 (3c), C(CF3)3 (3d)] in the presence of Al cocatalysts. As reported previously, 11a Ti(O i Pr)[{(O-2,4-Me 2 C 6 H 2 )-6CH2}3N] (1a) exhibits high catalytic activity [expressed as kg-PE/mol-Ti·h, on the basis of polymer yields] for ethylene polymerization in n-octane in the presence of methylaluminoxane (run 1), and the activity increased upon addition of a small amount of AlMe3 (runs 2−4). As also shown in the Supporting Information, these polymerization results are reproducible. The resultant polymers were linear high molecular weight polyethylene with unimodal molecular weight distributions.11a The E

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suggest that, as assumed previously,11b,c the Ti−Al heterobimetallic complexes formed by reaction with AlMe3 play a role as catalyst precursors to generate assumed catalytically active species in this catalysis. As observed in both 2a and 2b (runs 5−8, 13, 14), the activity by 2c (runs 20, 21) was lower than that by 1c (runs 15−18), and no significant improvements in the activity were observed upon addition of AlMe 3 . Unfortunately, the resultant Ti−Al bimetallic complexes demonstrated here (3b−d) showed negligible catalytic activities for ethylene polymerization in the absence of MAO. It turned out that the activity by the chloro analogue (1e, run 27) was lower than the (fluorinated) isopropoxy analogues (runs 9, 15), and the activity decreased upon addition of AlMe3 (runs 28, 29), whereas the activity by another chloro analogue (2e) increased upon addition of AlMe3 (run 30 vs run 31). Interestingly, the activities by the methyl (2g) and ethyl (2f) analogues increased upon addition of a small amount of AlMe3 (runs 33, 35); thus, it may be assumed that formed bimetallic complexes play a role in this catalysis containing a tBu substituent on the atrane ligand.

Table 2. Ethylene Polymerization by TiX[{(O-2,4-R2C6H2)6-CH2}3N] [R = Me (1), tBu (2); X = OiPr (a), OtBu (b), OCH(CF3)2 (c), OC(CF3)3 (d), Cl (e), Et (f), Me (g)], [TiMe{(O-2,4-Me2C6H2-6-CH2)2(μ2-O-2,4-Me2-C6H2-6CH2)}N][Me2Al(μ2-OR′)] [R′ = tBu (3b), CH(CF3)2 (3c), C(CF3)3 (3d)]−MAO Catalyst Systemsa run

catalyst

1 2 3 4 5 6 7 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

1a 1a 1a 1a 2a 2a 2a 2a 1b 1b 1b 3b 2b 2b 1c 1c 1c 1c 3c 2c 2c 1d 1d 1d 1d 3d 1e 1e 1e 2e 2e 2f 2f 2g 2g

AlMe3/μmol (Al/Ti)b 1.0 (10) 0.5 (5.0) 0.3 (3.0) 1.0 (10) 0.5 (5.0) 0.3 (3) 1.0 (10) 0.35 (3.5)

1.0 (10) 1.0 (10) 0.5 (5.0) 0.3 (3.0)

1.0 (10.0) 1.0 (10) 0.5 (5.0) 0.3 (3.0)

1.0 (10) 0.35 (3.5) 1.0 (10) 1.0 (10) 1.0 (10)

yield/mg

activityc

149.7 163.3 180.9 181.5 129.2 142.1 132.5 132.7 236.6 212.7 200.6 213.6 191.1 208.8 243.3 243.2 252.9 270.7 251.3 178.5 177.2 201.2 177.6 206.9 229.9 285.8 200.2 178.5 167.7 168.5 226.7 180.2 208.8 158.8 193.9

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SUMMARY In this paper, we prepared and identified a series of titanatranes containing tris(aryloxo)amine ligands of the type TiX[{(O-2,4R2C6H2)-6-CH2}3N] [R = Me (1), tBu (2); X = OiPr (a), OtBu (b), OCH(CF3)2 (c), OC(CF3)3 (d), Cl (e)] and some reactions with Al alkyls. The Ti−Al heterobimetallic complexes [TiMe{(O-2,4-Me 2 C 6 H 2 -6-CH 2 ) 2 (μ 2 -O-2,4-Me 2 -C 6 H 2 -6CH2)}N][Me2Al(μ2-OR′)] [R′ = tBu (3b), CH(CF3)2 (3c), C(CF3)3 (3d)] were isolated by reaction of 1b−d with 1 equiv of AlMe3; structures by 3b,c were determined by X-ray crystallography. In contrast, attempted reaction of 2b with AlMe3 under similar conditions recovered the original complex (2b), and the reaction with 2c afforded a mixture of the methyl complex TiMe[{(O-2,4-tBu2C6H2)-6-CH2}3N] (2g) and the corresponding Al-alkoxide (Scheme 4). These complexes (1a−e, 2b,c,e−g) exhibited high catalytic activities for ethylene polymerization in n-octane in the presence of MAO, and the activity was affected by the terminal donor ligand (X) and the substituent on the atrane ligand (R). In particular, the activities by 1c,d [X = OCH(CF3)2 (1c), OC(CF3)3 (1d)] increased upon addition of AlMe3 in a small amount, and the isolated heterobimetallic complexes (3c,d) showed high activities, suggesting that formation of the bimetallic complexes plays a role in this catalysis. In contrast, an increase in the activity upon addition of AlMe3 was not observed by 1b,e, and by 2c,e, whereas an improvement in the activity was observed by the alkylated analogues (2f,g). These observations are somewhat related to the results in the reaction chemistry with AlMe3. On the basis of crystallographic analysis data, the Ti(1)− O(1) bond distance in 4a (which polymerizes ethylene without MAO) is shorter than those in 3b,c [2.0673(19) Å for 4a vs 2.103(2), 2.219(2) Å, respectively] and is close to that in 3a [2.0714(13) Å], whereas the Ti(1)−O(2) bond distance in 4a [2.095(2) Å] is longer than those in 3a−c [2.043(3)− 2.0716(11) Å], and no significant differences in the Al(1)− O(2) distances were seen [1.836(2)−1.845(2) Å]. Taking into account these facts and an assumed activation scheme, a preferred dissociation of the Ti(1)−O(2) bond (for generation of the assumed catalytically active cationic species) rather than the Ti(1)−O(1) bond (leading to formation of a neutral Ti− Me complex and Al-alkoxide) would probably be required in

Conditions: Ti complex 0.1 μmol, n-octane 30.0 mL, MAO 3.0 mmol, ethylene 8 atm, 100 °C, 60 min. bAl/Ti molar ratio. cActivity in kg-PE/mol-Ti·h. a

3b were insoluble in hot o-dichlorobenzene (140 °C) for GPC measurement (molecular weights and molecular weight distributions). These suggest that the resultant polymers possess ultrahigh molecular weights. It turned out that the catalytic activity for ethylene polymerization by 1c [X = OCH(CF3)2] increased upon addition of a small amount of AlMe3 (runs 17, 18), as observed in 1a;11a,b the activity was affected by the Al/Ti molar ratio, and further addition of Al led to a decrease in the activity (run 16). The activity by the isolated 3c showed similar activity by 1c upon addition of AlMe3 under the same conditions (runs 17− 19). Moreover, importantly, an increase in the activity upon addition of a small amount of AlMe3 was observed by 1d [run 25, X = OC(CF3)3], and the isolated heterobimetallic complex (3d) exhibited higher catalytic activity than 1d even upon addition of AlMe3 (run 26 vs runs 23−25). These results would F

DOI: 10.1021/acs.organomet.5b00303 Organometallics XXXX, XXX, XXX−XXX

Article

Organometallics the desired Ti−Al heterobimetallic catalyst precursor. MAO would also play a role for generation and/or stabilization of the catalytically active species. Therefore, the design of both the terminal donor ligand and substituent on the atrane ligand should play an important key role. The fact demonstrated here should be promising and important not only for a basic understanding of this catalysis but also for the design of a more active catalyst for the desired purpose.

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Table 3. Crystal Data and Collection Parameters of [TiMe{(O-2,4-Me2C6H2-6-CH2)2(μ2-O-2,4-Me2-C6H2-6CH2)}N][Me2Al(μ2-OR′)] [R′ = tBu (3b), CH(CF3)2 (3c)]a 3bb formula fw cryst color, habit cryst size (mm)

EXPERIMENTAL SECTION

cryst syst space group a (Å) b (Å) c (Å) α (deg) β (deg) γ (deg) V (Å3) Z value Dcalcd (g/cm3) F000 temp (K) μ(Mo Kα) (cm−1) no. of reflns measd 2θmax (deg) no. of observations [I > 2.00σ(I)] no. of variables R1 [I > 2.00σ(I)] wR2 [I > 2.00σ(I)] goodness of fit

General Procedures. All experimental procedures were carried out under an atmosphere of nitrogen using standard Schlenk techniques or using a Vacuum Atmospheres drybox unless otherwise specified. All chemicals used were of reagent grade and were purified by standard purification procedures. n-Octane (anhydrous grade, Sigma-Aldrich Co., LLC) for polymerization was stored in a bottle in the drybox in the presence of molecular sieves (a mixture of 3A 1/16, 4A 1/8, and 13X 1/16). Polymerization-grade ethylene (purity >99.9%, Sumitomo Seika Co., Ltd.) was used as received. Toluene and AlMe3 from commercially available methylaluminoxane [TMAO, 9.5 wt % (Al) toluene solution, Tosoh Finechem Co.] were removed under reduced pressure (at ca. 50 °C for removing toluene and AlMe3 and then heated at >100 °C for 1 h for completion) in the drybox to give white solids. Tris(2-hydroxy-3,5-dimethylbenzyl)amine and tris(2hydroxy-3,5-di-tert-butylbenzyl)amine were prepared according to a published procedure.12a Ti(OiPr)[{(O-2,4-Me2C6H2)-6-CH2}3N] (1a),12a Ti(OiPr)[{(O-2,4-tBu2C6H2)-6-CH2}3N] (2a),12a and Ti(OtBu)[{(O-2,4-tBu2C6H2)-6-CH2}3N] (2b)12h were prepared according to a published method. Molecular weights and molecular weight distributions for polyethylene were measured by gel permeation chromatography (Tosoh HLC-8121GPC/HT) with a polystyrene gel column (TSK gel GMHHR-H HT × 2, 30 cm × 7.8 mm ⦶, ranging from