Polymerization Catalyzed by (Adamantylimido

Article pubs.acs.org/Organometallics

Ethylene Dimerization/Polymerization Catalyzed by (Adamantylimido)vanadium(V) Complexes Containing (2Anilidomethyl)pyridine Ligands: Factors Affecting the Ethylene Reactivity Atsushi Igarashi, Shu Zhang, and Kotohiro Nomura* Department of Chemistry, Graduate School of Science and Engineering, Tokyo Metropolitan University, 1-1 minami Osawa, Hachioji, Tokyo 192-0397, Japan S Supporting Information *

ABSTRACT: V(N-1-adamantyl)Cl2[2-(2,6-Me2C6H3)NCH2(C5H4N)] showed both remarkable activity and selectivity (from 92 through >99.5% selectivity) in ethylene dimerization in the presence of MAO or MMAO, and the activity increased linearly upon increasing the ethylene pressure. The same reaction in the presence of Et2AlCl or Me2AlCl afforded ultrahigh molecular weight polyethylene. Both the ESR spectra and the 51V NMR spectra suggest that the chelate anionic donor ligand plays an important role in stabilization of the oxidation state in the catalyst solution even containing Al alkyls.

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INTRODUCTION Linear α-olefins are important intermediates for a variety of products (such as detergents, polymers, lubricants, surfactants, etc.)1 and are largely produced by ethylene oligomerization,2−10 exemplified by using a nickel complex catalyst containing a chelate P−O ligand (Shell higher olefin process, SHOP).2 In particular, recent examples for ethylene oligomerization, using nickel4 and iron complexes,5 or ethylene trimerization, using chromium complex catalysts,6,7 have been well-known. Designing vanadium complex catalysts should also be promising, because the classical Ziegler-type catalyst systems (V(acac)3, VOCl3, etc. and Et2AlCl, EtAlCl2, nBuLi, etc.) display unique high reactivity toward olefins in olefin coordination/insertion polymerization.11 We previously reported that (arylimido)vanadium(V) complexes containing anionic donor ligands (X), V(N-2,6Me2C6H3)Cl2(X) (shown in Scheme 1), exhibited remarkable catalytic activities for ethylene polymerization in the presence of Al cocatalysts.11c,e,12 The activity by the aryloxo analogue was highly dependent upon the Al cocatalyst; the activities in the presence of halogenated Al alkyls (iBu2AlCl, EtAlCl2, Me2AlCl, Et 2 AlCl) were higher than that in the presence of methylaluminoxane (MAO).12a−c Both the activity and the norbornene incorporation in the ethylene/norbornene copolymerization were also affected by the Al cocatalyst employed.12b,c We thus speculated that a reason for the observed difference could be formation of different catalytically active species, a catalyst/cocatalyst nuclearity effect13 in the two catalyst systems (assumed in Scheme 1).11e,12b The activity decreased upon addition of CCl3CO2Et, which can be © 2012 American Chemical Society

Scheme 1

commonly used as an effective additive, clearly suggesting that the active species were thus different from those prepared from vanadium(III) and -(IV) complexes.11e,12c Received: January 23, 2012 Published: May 1, 2012 3575

dx.doi.org/10.1021/om3000532 | Organometallics 2012, 31, 3575−3581

Organometallics

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Reactions with ethylene in the presence of V(NAd)Cl2[2-(2,6Me2C6H3)NCH2(C5H4N)] (1) and MMAO (modified methylaluminoxane, methylisobutylaluminoxane) were conducted in toluene, and the results are summarized in Table 1. The results in the presence of MAO are also shown for comparison. Complex 1 has been chosen for this study, because 1 showed the highest catalytic activity, affording 1-butene exclusively in the presence of MAO:14 the activity was affected by the Al/V molar ratios and ethylene pressure without significant changes in the selectivity of 1-butene.14 Both the activity and the selectivity increased when the reaction was conducted under low catalyst loading (run 4).14 The activity by 1−MMAO catalyst was lower than that by 1−MAO catalyst (e.g. run 1 vs run 8, run 2 vs run 6) without any significant changes in the selectivity of 1-butene.17 The activity increased upon increasing the ethylene pressure (runs 5−8), whereas no significant differences in their selectivities were observed under these conditions (C4′ selectivity 97.3− 98.7%). As shown in Figure 1, first-order relationships among

More recently, we demonstrated that (imido)vanadium(V) complexes containing the (2-anilidomethyl)pyridine ligand, V(NR)Cl2[2-ArNCH2(C5H4N)] (R = 1-adamantyl (Ad), cyclohexyl, phenyl; Ar = 2,6-Me2C6H3, 2,6-iPr2C6H3), efficiently dimerize ethylene with both notable catalytic activities and high selectivity in the presence of MAO cocatalyst (Scheme 2),14 whereas the 2,6-dimethylphenylimido analogue V(N-2,6Scheme 2

Me2C6H3)Cl2[2-ArNCH2(C5H4N)] showed moderate catalytic activity for ethylene polymerization.12e Through these facts, we thus assumed that (i) the steric bulk in the imido ligand directly affects the reactivity (between dimerization and polymerization) and that (ii) electronic factors also play a role in the activity. In general, two reaction mechanisms via a metal− hydride (or metal−alkyl) or metallacycle intermediate in ethylene oligomerization have been postulated, but we do not have any clear evidence/information for the mechanism. In this paper, we thus present our results concerning factors affecting the ethylene reactivity (polymerization vs dimerization) in this catalysis and some supporting data concerning the catalytically active species in this catalysis.15,16

Figure 1. Effect of ethylene pressure in ethylene dimerization by V(NAd)Cl2[2-(2,6-Me2C6H3)NCH2(C5H4N)] (1)−MMAO catalyst. Detailed data are shown in Table 1 (conditions: complex 1 0.5 μmol, MMAO 0.25 mmol, toluene 30 mL, 25 °C, 10 min), and additional data (plotted in this figure) showing the reproducibility are given in the Supporting Information.17

RESULTS AND DISCUSSION 1. Ethylene Dimerization Using V(NAd)Cl2[2-(2,6Me2C6H3)NCH2(C5H4N)] (1)−MAO, MMAO Catalysts: Factors Affecting both the Activity and the Selectivity.

the activity, TOF value, and the ethylene pressures were observed. It has been known that the ethylene oligomerization proceeds via a metal−hydride (metal−alkyl) or metallacycle intermediate (Scheme 3), and the ethylene pressure depend-

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Table 1. Ethylene Dimerization by V(N-1-adamantyl)Cl2[2-(2,6-Me2C6H3)NCH2(C5H4N)] (1)−MAO, MMAO Catalyst Systems (in Toluene at 25 °C)a run

complex 1 (μmol)

Al cocat.

Al/Vb

ethylene/atm

time/min

activityc

TOFd/(mol of V) h−1

C4′/%e

C6′/%e

f

0.5 0.5 0.5 0.1 0.5 0.5 0.5 0.5 0.5 0.5 0.5 0.5

MAO MAO MAO MAO MMAO MMAO MMAO MMAO MMAO MMAO MMAO MMAO

500 500 500 1500 500 500 500 500 500 500 500 1000

8 4 8 8 2 4 6 8 8 8 8 8

10 10 20 10 10 10 10 10 30 5 5 10

51 100 25 500 50 300 76 500 1 760 4 830 7 360 9 580 9 510 12 200 14 600 12 900

1 830 000 911 000 1 800 000 2 730 000 62 600 172 000 262 000 341 000 339 000 436 000 519 000 458 000

92.5 92.2 90.4 97.0 98.7 97.3 97.8 98.5 95.4 >99.5 >99.5 99.2

7.5 7.8 9.6 3.0 1.3 2.7 2.2 1.5 4.6 trace trace 0.8

1 2f 3f 4f 5 6 7 8 9 10 11 12 a

Conditions: toluene 30 mL, d-MAO white solid (methylaluminoxane prepared by removing AlMe3, toluene from PMAO-S) or d-MMAO white solid (modified methylaluminoxane (methylisobutyl aluminoxane), prepared by removing AlMe3, AliBu3, and n-hexane from MMAO-3AH (commercial sample, Me/iBu = 2.67)), 25 °C. bAl/V molar ratio. cActivity in (kg of ethylene reacted)/((mol of V) h). dTOF (turnover frequency) = (mol of ethylene reacted)/((mol of V) h). eBy GC analysis vs internal standard. fCited from our previous report.14 3576


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affects the ethylene reactivity (dimerization vs polymerization) in this catalysis. As described above, the products by 1 in the presence of MAO and MMAO cocatalysts were 1-butene (and 1-hexene) exclusively, and we assumed that the relatively small imido ligand facilitates β-hydrogen elimination. The above results showing the notable effect of Al cocatalyst toward ethylene reactivity could be explained by our assumption (shown in Scheme 1 as expressed by a catalyst/cocatalyst nuclearity effect)13 that the steric environment around V may also be affected by the Al cocatalyst employed. 3. ESR and NMR Spectra of a Toluene Solution Containing V(NAd)Cl2[2-(2,6-Me2C6H3)NCH2(C5H4N)] (1) in the Presence of Al Cocatalyst. Figure 2 shows selected ESR spectra of toluene solution containing complex 1 in the presence of 100 equiv of Et2AlCl or MMAO (0 °C).20 The spectra for the trichloride analogue V(NAd)Cl321 were also measured for comparison. Resonances ascribed to a paramagnetic species (vanadium(IV) species)22 were observed, when V(NAd)Cl3 was treated with Et2AlCl (100 equiv) (Figure 2a,b), and the intensities by V(NAd)Cl3 were (slightly) affected by the temperature as well as Al/V molar ratios.20 In contrast, note that a trace amount of or negligible resonances ascribed to paramagnetic (vanadium(IV)) species were observed when 1 was treated even with 100 equiv of Et2AlCl (Figure 2c,d). Similar spectra were observed when 1 or V(NAd)Cl3 was treated with 100 equiv of MMAO (Figure 2f,g). Moreover, also note that negligible resonances ascribed to paramagnetic (vanadium(IV)) species were observed when 1 was treated with 100 equiv of Et2AlCl (or Me2AlCl)20 under ethylene pressure (similar conditions for ethylene polymerization: ethylene 8 atm at 0 °C for 2 min, Figure 2e).20 These results strongly suggest that no paramagnetic species (ascribed to vanadium(IV) species) were generated in the catalyst solution and that the anionic ancillary donor ligand (2-anilidomethyl)pyridine plays an important role in the stabilization of the oxidation state in this catalysis. It has been reported that reactions of (oxo)vanadium(V) complexes containing a chelate tris(phenolate) ligand with Al alkyls (Et2AlCl etc.) afforded vanadium(IV) complexes,16 and the catalyst systems exhibited high catalytic activities for ethylene polymerization in the presence of Cl3CCO2Et, which is commonly used as a reoxidant, so-called “rejuvenators” or “promoters,”18 for ethylene polymerization using vanadium(IV), vanadium(III), the above (oxo)vanadium(V), or (ptolylimido)vanadium(V) complex Al alkyl catalyst systems.11,16,18,19 It has been postulated on the basis of ESR studies16 that the growth of concentration of vanadium(IV) species correlates with the increase of the ethylene polymerization activity of the catalyst systems. These facts are thus an interesting contrast with those observed in our catalyst systems

Scheme 3

ence toward the activity is different in these catalytic reactions.1,7m Therefore, the results (first-order dependence) would suggest that the metal−hydride (or metal−alkyl) species play a role as the active species in this catalysis. Although the activity did not decrease after 30 min under the optimized conditions (runs 8 and 9), the selectivity of 1-butene decreased over longer reaction times, and the selectivity of 1butene was exclusive after 5 min (>99.5%, runs 10 and 11). These results thus suggest that 1-hexene could be produced from 1-butene accumulated in the mixture by codimerization with ethylene. 2. Notable Effect of Al Cocatalyst toward the Ethylene Reactivity by V(NAd)Cl2[2-(2,6-Me2C6H3)NCH2(C5H4N)] (1). Reactions with ethylene in the presence of V(NAd)Cl2[2-(2,6-Me2C6H3)NCH2(C5H4N)] (1) and Et2AlCl or Me2AlCl in place of MAO or MMAO were conducted in toluene, and the results are summarized in Table 2. Note that the resultant products were not 1-butene (the reaction product by 1 in the presence of MAO or MMAO) but polyethylene in all cases. The activity was affected by Al cocatalyst, Al/V molar ratio, and the polymerization temperature: the activity in the presence of Me2AlCl was higher than that in the presence of Et2AlCl. The resultant polymer samples were insoluble in hot odichlorobenzene (140 °C) for conventional measurements by gel permeation chromatography (GPC), and the molecular weights were thus estimated by viscosity. The resultant polymers possessed ultrahigh molecular weights (Mη = (5.92−8.96) × 106), as previously observed in ethylene polymerization by V(N-2,6-Me2C6H3)Cl2(O-2,6-Me2C6H3)12c and V(N-2,6-Me2C6H3)Cl2[2-(2,6-Me2C6H3)NCH2(C5H4N)]12f in the presence of halogenated Al alkyls. This fact thus clearly indicates that the Al cocatalyst directly

Table 2. Ethylene Polymerization by V(NAd)Cl2[2-(2,6-Me2C6H3)NCH2(C5H4N)] (1)−Me2AlCl, Et2AlCl Catalyst Systemsa

a

run

complex 1/ μmol

cocat.

Al/Vb

temp/°C

time/min

yield/mg

activityc

Mηd × 10−6

13 14 15 16 17 18

5.0 5.0 5.0 0.5 1.0 1.0

Et2AlCl Et2AlCl Et2AlCl Me2AlCl Me2AlCl Me2AlCl

100 100 200 200 500 200

0 25 0 0 0 0

10 10 10 10 10 20

113.9 95.7 86.9 58.7 116.0 179.3

137 115 104 704 696 538

5.92

6.76 8.96

Conditions: toluene 30 mL, ethylene 8 atm, 10 min. bAl/V molar ratio. cActivity in (kg of PE)/((mol of V) h). dMolecular weight by viscosity. 3577


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Figure 2. ESR spectra (complex 2.5 μmol/mL in toluene) for (a) V(NAd)Cl3 + 100 equiv of Et2AlCl (at 0 °C), (b) V(NAd)Cl3 + 100 equiv of Et2AlCl (at 25 °C), (c) V(NAd)Cl2[2-(2,6-Me2C6H3)NCH2(C5H4N)] (1) + 100 equiv of Et2AlCl (at 0 °C), (d) 1 + 100 equiv of Et2AlCl (at 25 °C), (e) 1 + 100 equiv of Et2AlCl (at 0 °C) after treating with ethylene (8 atm for 2 min at 0 °C), (f) V(NAd)Cl3 + 100 equiv of MMAO (at 0 °C), and (g) 1 + 100 equiv of MMAO (at 0 °C). Experimental parameters: 9.16 GHz frequency, 0.79 mT modulation amplitude, power 1.0 mT, sweep time 240 s.

was treated with 10.0 equiv of Et2AlCl in C6D6 at 25 °C (Figure 3b). A significant decrease in the intensities in the 51V NMR spectra was not observed when these spectra were measured under the same vanadium concentrations (Figures 3). In contrast, it is interesting to note that no resonances were observed when V(NAd)Cl3 was treated with 10.0 equiv of Et2AlCl in C6D6 at 25 °C at the same concentration (shown in the Supporting Information).20 These facts (51V NMR spectra for the C6D6 solution of 1 with MMAO, Et2AlCl in addition to V(NAd)Cl3 with Et2AlCl)

and may support our assumption about the role of anionic donor ligands. Figure 3 shows 51V NMR spectra of C6D6 solution containing complex 1 in the presence of 10.0 equiv of Et2AlCl or MMAO (at 25 °C).20 A resonance at 32 ppm in addition to the resonance ascribed to 1 was observed if 1 (−114 ppm) was treated with MMAO (10.0 equiv) without a notable decrease in the peak intensity (Figure 3a): these may suggest that different vanadium(V) species were generated from 1 in the presence of MMAO. The resonance was slightly shifted to −76 ppm if 1 3578


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use. V(NAd)Cl321 and V(NAd)Cl2[2-(2,6-Me2C6H3)NCH2(C5H4N)] (1)14 were prepared according to our previous reports. Polymerization grade ethylene (purity >99.9%, Sumitomo Seika Co. Ltd.) was used as received. Toluene and AlMe3 in the commercially available methylaluminoxane (PMAO-S, 9.5 wt % (Al) toluene solution, Tosoh Finechem Co.) were removed under reduced pressure (at ca. 50 °C for removing toluene and AlMe3 and at >100 °C for 1 h for completion) in the drybox to give white solids. MMAO-3AH (Me/iBu = 2.67) was also supplied from Tosoh Finechem Co. and was used as a white solid after removing n-hexane, AlMe3, and AliBu3 in vacuo according to a procedure analogous to that for PMAO-S, except that the resultant solid was redissolved in n-hexane and then removed in vacuo to remove AliBu3 completely. GC analysis was performed with a Shimadzu GC-17A gas chromatograph (Shimadzu Co. Ltd.) equipped with a flame ionization detector. Molecular weights for samples which were poorly soluble in o-dichlorobenzene for the GPC measurements (in Table 2) were estimated by viscosity (Asahi Kasei Chemicals Co.) according to a procedure established for ultrahigh molecular weight polyethylene. Oligomerization/Polymerization of Ethylene. Ethylene oligomerizations were conducted in a 100 mL scale stainless steel autoclave. The typical reaction procedure is as follows. Toluene (29 mL) and the prescribed amount of MMAO solid (prepared from ordinary MMAO3AH by removing n-hexane, AliBu3. and AlMe3) were added into the autoclave in the drybox. The reaction apparatus was then filled with ethylene (1 atm), and V(NAd)Cl2[2-(2,6-Me2C6H3)NCH2(C5H4N)] (1) (0.5 μmol) in toluene (1.0 mL) was then added into the autoclave; the reaction apparatus was then immediately pressurized to 7 atm (total 8 atm), and the mixture was magnetically stirred for the prescribed time. After the above procedure, the remaining ethylene was purged at −78 °C, and 0.5 g of heptane was added as an internal standard. The solution was then analyzed by GC to determinate the activity and the product distribution. Ethylene polymerizations were also conducted similarly, except a prescribed amount of Et2AlCl or Me2AlCl (1 M in n-hexane) was used in place of MMAO. After the above procedure, the remaining ethylene was purged upon cooling, and the mixture was then poured into MeOH containing HCl. The resultant polymer (white precipitate) was collected on a filter paper by filtration and was adequately washed with MeOH. The resultant polymer was then dried in vacuo at 60 °C for 2 h. ESR Measurements. A typical procedure is as follows. A toluene solution containing V(NAd)Cl2[2-(2,6-Me2C6H3)NCH2(C5H4N)] (1) and a toluene solution containing Et2AlCl were mixed with their concentrations being kept at 2.5 μmol/mL for 1 and 12.5 or 250 μmol/mL for Et2AlCl. The reaction mixture was then quickly transferred into an ESR tube precooled to −30 °C and placed into a cold ice bath (at 0 °C, outside the drybox). The tube was placed into the instrument preset at 0 °C (ESR measurements were started were started in less than 10 min after the preparation). Experimental parameters: 9.16 GHz frequency, 0.79 mT modulation amplitude, power 1.0 mT, sweep time 240 s. ESR measurements after treatment of ethylene (Figure 2e) were conducted as follows. Into a toluene solution containing Et2AlCl in a 100 mL scale stainless steel autoclave was added a toluene solution containing V(NAd)Cl2[2-(2,6-Me2C6H3)NCH2(C5H4N)] (1) under atmospheric ethylene pressure. Their concentrations were kept at 2.5 μmol/mL for 1 and 12.5 or 250 μmol/mL for Et2AlCl. The reaction apparatus was then immediately pressurized to 7 atm (total 8 atm), and the mixture was magnetically stirred for 2 min at 0 °C. After the above procedure, the remaining ethylene was purged (to atmospheric pressure) at 0 °C, and the reaction solution was then quickly transferred into an ESR tube precooled to 0 °C and placed into a cold ice bath (at 0 °C, outside the drybox). The tube was placed into the instrument preset at 0 °C (ESR measurements were started were started in less than 10 min after the preparation). 51 V NMR Experiments in the Reaction of V(NAd)Cl2[2-(2,6Me2C6H3)NCH2(C5H4N)] (1) with MMAO, Et2AlCl in C6D6. A typical procedure is as follows. Into a frozen C6D6 solution (ca. 0.4 mL) containing 1 (25 μmol) placed in the freezer (0 °C) was added a

Figure 3. 51V NMR spectra (complex 25 μmol/0.6 mL in C6D6 at 25 °C) for (a) V(NAd)Cl2[2-(2,6-Me2C6H3)NCH2(C5H4N)] (1) + 10.0 equiv of MMAO, (b) 1 + 10.0 equiv of Et2AlCl, (c) complex 1.

are closely related to the observations of the corresponding ESR spectra in toluene solution. Taking into account these facts, it is thus suggested that the anionic ancillary donor ligand (2-anilidomethyl)pyridine plays an essential role in stabilization of the oxidation state in this catalysis, even in the presence of Al alkyls.

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CONCLUDING REMARKS The experimental results observed through this study can be summarized as follows. (1) The catalytic activity in ethylene dimerization using V(NAd)Cl 2 [2-(2,6-Me 2C 6 H3 )NCH 2 (C5 H 4N)] (1)− MMAO catalyst increased upon increasing the ethylene pressure, and a first-order relationship between the activity and the ethylene pressure was observed. (2) The ethylene reactivity was strongly affected by the Al cocatalyst employed. The reaction by 1 in the presence of MAO or MMAO cocatalyst afforded 1-butene exclusively, whereas the resultant products by 1−Et2AlCl and −Me2AlCl catalysts were polyethylene with ultrahigh molecular weights. We assumed that the observed difference could be explained by using the assumption shown in Scheme 1 (catalyst/cocatalyst nuclearity effect).13 (3) On the basis of ESR and 51V NMR spectra under various conditions, the chelate anionic donor ligand (2anilidomethyl)pyridine in 1 plays an important role in the stabilization of the oxidation state, even in the presence of an excess amount (100 equiv) of MMAO or Et2AlCl. Taking into account the facts summarized above, it seems strongly likely (but has not been clearly elucidated yet) that the cationic vanadium(V) alkyl species could play an important role in this catalysis, and the nature of the catalytically active species (steric bulk of the imido ligand, counteranion) affects the ethylene reactivity. We believe this information is potentially important for designing efficient molecular catalysts with vanadium for precise olefin polymerization as well as oligomerization.

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EXPERIMENTAL SECTION

General Procedure. All experiments were carried out under a nitrogen atmosphere in a Vacuum Atmospheres drybox. Anhydrous grade toluene and n-hexane (Kanto Kagaku Co., Ltd.) were transferred into a bottle containing molecular sieves (a mixture of 3A 1/16, 4A 1/ 8, and 13X 1/16) in the drybox under a nitrogen stream and were passed through a short alumina column under an N2 stream prior to 3579


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C6D6 solution (ca. 0.2 mL) containing Et2AlCl (250 μmol, 10.0 equiv). The mixture was then measured by a 51V NMR spectrum at 25 °C within 10 min after the preparation.

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(e) McGuinness, D. S.; Suttil, J. A.; Gardiner, M. G.; Davies, N. W. Organometallics 2008, 27, 4238. (f) Zhang, J.; Braunstein, P.; Hor, T. S. A. Organometallics 2008, 27, 4277. (g) Hanton, M. J.; Tenza, K. Organometallics 2008, 27, 5712. (h) Kirillov, E.; Roisnel, T.; Razavi, A.; Carpentier, J.-F. Organometallics 2009, 28, 2401. (i) Zhang, J.; Li, A.; Hor, T. S. A. Organometallics 2009, 28, 2935. (j) Dulai, A.; de Bod, H.; Hanton, M. J.; Smith, D. M.; Downing, S.; Mansell, S. M.; Wass, D. F. Organometallics 2009, 28, 4613. (k) Tenza, K.; Hanton, M. J.; Slawin, A. M. Z. Organometallics 2009, 28, 4852. (l) Vidyaratne, I.; Nikiforov, G. B.; Gorelsky, S. I.; Gambarotta, S.; Duchateau, R.; Korobkov, I. Angew. Chem., Int. Ed. 2009, 48, 6552. (m) McGuinness, D. S. Chem. Rev. 2011, 111, 2321. (8) For Ti and Zr complexes, see: (a) Wielstra, Y.; Gambarotta, S.; Chiang, Y. Organometallics 1988, 7, 1866. (b) Deckers, P. J. W.; Hessen, B.; Teuben, J. H. Angew. Chem., Int. Ed. 2001, 40, 2516. (c) Deckers, P. J. W.; Hessen, B.; Teuben, J. H. Organometallics 2002, 21, 5122. (d) You, Y.; Girolami, G. S. Organometallics 2008, 27, 3172. (e) Otten, E.; Batinas, A. A.; Meetsma, A.; Hessen, B. J. Am. Chem. Soc. 2009, 131, 5298. (9) For Ta catalysts, see: (a) Fellmann, J. D.; Rupprecht, G. A.; Schrock, R. R. J. Am. Chem. Soc. 1979, 101, 5099. (b) Andes, C.; Harkins, S. B.; Murtuza, S.; Oyler, K.; Sen, A. J. Am. Chem. Soc. 2001, 123, 7423. (c) Arteaga-Müller, R.; Tsurugi, H.; Saito, T.; Yanagawa, M.; Oda, S.; Mashima, K. J. Am. Chem. Soc. 2009, 131, 5370 Ligandfree TaCl5−cocatalyst systems were used in refs 9b,c.. (10) For V complex catalysts, see:7g (a) Brussee, E. A. C.; Meetsma, A.; Hessen, B.; Teuben, J. H. Chem. Commun. 2000, 497. (b) Schmidt, R.; Welch, M. B.; Knudsen, R. D.; Gottfried, S.; Alt, H. G. J. Mol. Catal. A 2004, 222, 17. Vanadium complexes containing chelate bis(imino) pyridine ligands yielded polymers/oligomers with Schultz−Flory distribution.7g,10b (11) For recent reviews (vanadium catalysts), see: (a) Hagen, H.; Boersma, J.; van Koten, G. Chem. Soc. Rev. 2002, 31, 357. (b) Gambarotta, S. Coord. Chem. Rev. 2003, 237, 229. (c) Nomura, K.; Zhang, W. Chem. Sci. 2010, 1, 161. (d) Redshaw, C. Dalton Trans. 2010, 39, 5595. (e) Nomura, K.; Zhang, S. Chem. Rev. 2011, 111, 2342. (12) (a) Nomura, K.; Sagara, A.; Imanishi, Y. Macromolecules 2002, 35, 1583. (b) Wang, W.; Nomura, K. Macromolecules 2005, 38, 5905. (c) Wang, W.; Nomura, K. Adv. Synth. Catal. 2006, 348, 743. (d) Nomura, K.; Wang, W.; Yamada, J. Stud. Surf. Sci. Catal. 2006, 161, 123. (e) Onishi, Y.; Katao, S.; Fujiki, M.; Nomura, K. Organometallics 2008, 27, 2590. (f) Zhang, S; Katao, S.; Sun, W,-H.; Nomura, K. Organometallics 2009, 28, 5925. (13) (a) Macchioni, A. Chem. Rev. 2005, 105, 2039. (b) Nuclearity and cooperativity effects in binuclear catalysts and cocatalysts: Li, H.; Marks, T. J. Proc. Natl. Acad. Sci. U.S.A. 2006, 103, 15295. (c) Bochmann, M. Organometallics 2010, 29, 4711. (14) Zhang, S.; Nomura, K. J. Am. Chem. Soc. 2010, 132, 4960. (15) Pioneering ESR study for monitoring the catalytically active species (VCl4−Et2AlCl catalyst system): Lehr, M. H.; Carman, C. J. Macromolecules 1969, 2, 217. (16) For recent examples of the ESR study of vanadium species formed by treatment with Al alkyls,11d see: (a) Soshnikov, I. E.; Semikolenova, N. V.; Bryliakov, K. P.; Zakharov, V. A.; Redshaw, C.; Talsi, E. P. J. Mol. Catal. A 2009, 303, 23. (b) Soshnikov, I. E.; Semikolenova, N. V.; Shubin, A. A.; Bryliakov, K. P.; Zakharov, V. A.; Redshaw, C.; Talsi, E. P. Organometallics 2009, 28, 6714. (17) Additional data in ethylene dimerization by the V(NAd)Cl2[2(2,6-Me2C6H3)NCH2(C5H4N)] (1)−MMAO catalyst system (for reproducibility in the experiments) are given in the Supporting Information. The catalyst system in the presence of MMAO has been chosen due to the better reaction control (in terms of exotherm, reproducibility), as shown in the Supporting Information. (18) For example: Christman, D. L. J. Polym. Sci., Part A-1: Polym. Chem. 1972, 10, 471. (19) For example:16 (a) Redshaw, C.; Rowan, M. A.; Homden, D. M.; Dale, S. H.; Elsegood, M. R. J.; Matsui, S.; Matsuura, S. Chem. Commun. 2006, 3329. (b) Wu, J.-Q.; Mu, J.-S.; Zhang, S.-W.; Li, Y.-S.

ASSOCIATED CONTENT

S Supporting Information *

Figures and a table giving additional data in ethylene dimerization by the V(N-1-adamantyl)Cl2[2-(2,6-Me2C6H3)NCH2(C5H4N)] (1)−MMAO catalyst system, additional ESR spectra measured under various conditions (in toluene with 5.0 or 100 equiv of Et2AlCl, Me2AlCl, at 0 and 25 °C), and 51V NMR spectra (in C6D6 at 25 °C) for V(NAd)Cl3 with 10.0 equiv of Et2AlCl. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*Corresponding Authors, tel.: +81-42-677-2547, fax: +81-42677-2547, E mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This project was partially supported by The Ube Foundation of Japan. We express our heartfelt thanks to Prof. Tohru Nishinaga, Mr. Chuanwen Lin, and Mr. Masaki Tateno (Tokyo Metropolitan University) for their assistance in ESR analysis, to Tosoh Finechem Co. for donating MAO and MMAO, and to Asahi Kasei Chemicals Co. for measurements of ultrahigh molecular weight polyethylene samples by viscosity. S.Z. expresses his thanks to the JSPS for a postdoctoral fellowship (P08361). K.N. also expresses his thanks to Prof. Moris S. Eisen (Technion, Israel Institute of Technology) for helpful discussions.

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REFERENCES

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