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. 2017 Dec 6;2(12):8660–8673. doi: 10.1021/acsomega.7b01225

Synthesis and Structural Analysis of (Imido)vanadium Dichloride Complexes Containing 2-(2′-Benz-imidazolyl)pyridine Ligands: Effect of Al Cocatalyst for Efficient Ethylene (Co)polymerization

Kotohiro Nomura †,*, Mari Oshima †, Takato Mitsudome §,*, Hitoshi Harakawa †, Peng Hao ‡, Ken Tsutsumi †, Go Nagai †, Toshiaki Ina ∥, Hikaru Takaya ⊥, Wen-Hua Sun ‡,*, Seiji Yamazoe #,*
PMCID: PMC6645497  PMID: 31457397

Abstract

graphic file with name ao-2017-01225b_0008.jpg

(Imido)vanadium(V) dichloride complexes containing 2-(2′-benzimidazolyl)-6-methylpyridine ligand (L) of type V(NR)Cl2(L) [R = 1-adamantyl (Ad, 1), C6H5 (2), and 2,6-Me2C6H3 (3)] have been prepared, and their structures were determined by X-ray crystallography as distorted trigonal bipyramidal structures around vanadium. Reactions with ethylene using 1–3 in the presence of methylaluminoxane (MAO) afforded a mixture of oligomer and polymers, and the compositions were affected by the imido ligand employed. By contrast, 1–3 exhibited remarkable catalytic activities for ethylene polymerization in the presence of Me2AlCl; the phenylimido complex (2) exhibited the highest activity [80 100 kg-PE/mol-V·h turn over frequency (TOF, 2 850 000 h–1, 792 s–1)]. The ethylene copolymerizations with norbornene afforded ultrahigh-molecular-weight copolymers with uniform molecular weight distributions and compositions [e.g., Mn = 1.71–2.66 × 106, Mw/Mn = 2.27–2.53]. On the basis of V nuclear magnetic resonance (51V NMR), electron spin resonance, and V K-edge X-ray absorption near-edge structure (XANES) spectra of the catalyst solution, the observed difference in the catalyst performance in the presence of (between) MAO and Me2AlCl cocatalyst should be due to the formation of different catalytically active species with different oxidation states. Apparent changes in the oxidation state were observed in the (especially in the NMR and XANES) spectra upon addition of Me2AlCl, whereas no significant changes in the spectra were observed in presence of MAO.

1. Introduction

Metal-catalyzed olefin polymerization/oligomerization is one of the key reactions in the chemical industry, and design of efficient molecular catalysts has been considered as an important subject.1−33 Because of the attractive characteristics (notable reactivity toward olefins) displayed by the classical Ziegler-type vanadium catalyst systems [V(acac)3, VOCl3 and Et2AlCl, EtAlCl2, nBuLi, etc.; employed as catalysts for production of EPDM (synthetic rubber) etc.],24,34−45 development of efficient vanadium complex catalysts is considered to be an attractive and important subject.24,42−45 It has been reported that (imido)vanadium(V) complexes containing the anionic ancillary donor ligand (aryloxo,46−49 imidazolidin-2-iminato,50 etc.)24,42−50 and the chelate anionic donor ligand,51−58 as shown in Chart 1, exhibit promising catalyst behaviors in the presence of the Al cocatalyst.59−64 In the reaction with ethylene using the chelate (2-anilidomethyl)pyridine analogues,52,54−58 V(NR)Cl2[2-ArNCH2(C5H4N)] [R = 1-adamantyl (Ad), C6H5, Ar(2,6-Me2C6H3), etc.], a steric bulk in the imido ligand affects the reactivity (dimerization vs polymerization) in the presence of methylaluminoxane (MAO) cocatalyst.52,54,56 For instance, adamantylimido/phenylimido complexes exhibited both high catalytic activity and selectivity in ethylene dimerization,54 whereas the 2,6-Me2C6H3 analogue afforded polyethylene under the same conditions.52 Moreover, on the basis of recent results in the reaction chemistry (of the dimethyl and the cationic methyl, etc.), nuclear magnetic resonance (NMR) and electron spin resonance (ESR) spectra, and X-ray absorption spectroscopy (XAS) analysis of the catalyst solution, it has been postulated that the catalytically active species in this catalysis should be the cationic vanadium(V) alkyl species preserving the basic ligand frameworks [imido, (2-anilidomethyl)pyridine].58

Chart 1. Selected Examples for Effective (Imido)vanadium(V) Dichloride Complexes for Ethylene Polymerization (Left) and Dimerization (Right)46−50,52,54−58.

Chart 1

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As described above, it has been suggested that a steric environment around vanadium plays a role for the reaction pathway (dimerization vs polymerization). Therefore, it could be assumed that use of the 2-(2′-benzimidazolyl)pyridine [or 2-(2′-pyridyl)benzimidazole] ligand65,66 would provide more open space around vanadium compared to the reported 2-(anilidomethyl)pyridine ligand (Chart 2, 1–3) because the ligand would construct a plane consisting of vanadium, three nitrogen (imido, pyridine, and benzimidazolyl ligands; marked with red), and the aromatic ring, which could be perpendicular to a plane consisting of two chloride and vanadium (and nitrogen) in the trigonal bipyramidal structure, as also demonstrated later in the text by their structural analyses. This should be a unique contrast to that in the 2-(anilidomethyl)pyridine ligand (4),52,54−58 in which the phenyl group connected to the nitrogen forms a plane (marked with blue, Chart 2) parallel to the plane consisting of two Cl atoms and nitrogen on the anilide ligand. It might be thus assumed that the structure would facilitate the β-hydrogen elimination after olefin insertion.52,54,56 In this paper, we thus prepared a series of (imido)vanadium(V) dichloride complexes containing 2-(2′-benzimidazolyl)-6-methylpyridine ligands and explored their catalyst performances in the reaction of ethylene in the presence of the Al cocatalyst. As we observed a unique effect of Al cocatalysts (MAO vs Me2AlCl) toward the activity and product distribution, we further explored more details in these catalyses by NMR and ESR spectra and XAS analysis.

Chart 2. List of (Imido)vanadium(V) Dichloride Complexes (1–3) Employed in This Study.

Chart 2

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2. Results and Discussion

2.1. Synthesis and Structural Analysis of (Imido)vanadium(V) Dichloride Complexes Containing 2-(2′-Benzimidazolyl)-6-methylpyridine Ligand

Three (imido)vanadium(V) dichloride complexes containing the 2-(2′-benzimidazolyl)-6-methylpyridine ligand (L) could be prepared from the corresponding trichloride complexes, V(NR)Cl3 [R = 1-adamantyl (Ad),67 C6H5,54 2,6-Me2C6H3 (Ar),68 in toluene by treating the ligand potassium salts (LKs), which were prepared in advance by treatment of 2-(2′-benzimidazole)-6-methylpyridine (LH)69 with KH in tetrahydrofuran (THF). This is a somewhat analogous procedure for the synthesis of titanium(IV) dichloride complexes containing the same ligands,65,66 although the syntheses were conducted in THF using the sodium salts. Resultant complexes were purified by recrystallization from the chilled dichloromethane solution layered by n-hexane. The isolated complexes were identified as V(NR)Cl2(L) [R = Ad (1), phenyl (2), and Ar (3); L = 2-(2′-benzimidazolyl)-6-methylpyridine] by NMR spectra and elemental analysis (Scheme 1).

Scheme 1. Synthesis of (Imido)vanadium(V) Dichloride Complexes Containing 2-(2′-Benzimidazolyl)-6-methylpyridine Ligand (1–3).

Scheme 1

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However, attempted reactions of V(NAr)Cl3 with 2-(2′-benzimidazolyl)pyridine70 conducted under the same conditions failed, affording a mixture of several complexes, which seemed difficult to separate on the basis of 51V NMR spectra of the reaction mixture [Figure S1, monitored the time course; additional experiments for the synthesis of 2-(2′-benzimidazolyl)-pyridine and attempted reactions with V(N-2,6-Me2C6H3)Cl3 are shown in the Supporting Information]. Therefore, we could not isolate the desired (methyl-free) complexes at this moment.

Figure 1 shows the Oak Ridge thermal ellipsoid plot (ORTEP) drawings for complexes 1–3, and the selected bond distances and angles are summarized in Table 1 (structure reports and xyz files for complexes 1–3 are shown in the Supporting Information). Both the adamantylimido complex (1) and the phenylimido complex (2) fold a distorted trigonal bipyramidal geometry around vanadium, with the nitrogen on pyridyl of the bidentate ligand (L) and the nitrogen atom in the imido ligand lying on the axis, and an equatorial plane consisted of two chloride ligands and the N atom in L. The axial N(1)–V–N(2) bond angles in 1 and 2 [178.37(8) and 179.1(2)°, respectively] are larger than that in the reported 2-(anilidomethyl)pyridine complex, V(NAd)Cl2[2-ArNCH2(C5H4N)] (4) [174.90(4)°],54 and the total bond angles of the equatorial Cl(1)–V–N(3) and Cl(2)–V–N(3) and Cl(1)–V–Cl(2) are 355.55 and 356.21° for 1 and 2, respectively. These results clearly suggest that the nitrogen atoms in the pyridine ligand locate at the trans position of the imido ligand. The vanadium–imido bond distances in 1 and 2 [V–N(1): 1.647(2) and 1.651(4) Å, respectively] are apparently shorter than the V–N bond distances in L [V–N(3): 1.918(2) and 1.896(5) Å, respectively] and those in the pyridine ligand [V–N(2): 2.279(2) and 2.264(5) Å, respectively]. These results also indicate that three nitrogen atoms coordinate with vanadium in different fashions. Moreover, the V–N bond distances in the pyridine ligand [1 and 2: V–N(2): 2.279(2) and 2.264(5) Å, respectively] are slightly longer than that in 4 [2.2241(11) Å]54 but apparently shorter that those in the (1-adamantylimido)vanadium(V) dichloride complexes with 2- or 8-(anilidomethyl)quinoline ligands [2.2911(14) and 2.3338(18) Å, respectively].56

Figure 1.

Figure 1

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ORTEP drawings for (top) V(NAd)Cl2(L) (1), (middle) V(NC6H5)Cl2(L) (2), and (bottom) V(NAr)Cl2(L) (3) [Ad = 1-adamantyl, Ar = 2,6-Me2C6H3; L = 2-(2′-benzimidazolyl)-6-methylpyridine]. Thermal ellipsoids are drawn at the 50% probability level, and H atoms are omitted for clarity.

Table 1. Selected Bond Distances and Angles for V(NR)Cl2(L) [R = Ad (1), C6H5 (2), and 2,6-Me2C6H3 (3); L = 2-(2′-Benzimidazolyl)-6-methylpyridine]a.

complex (R) 1 (Ad) 2 (C6H5) 3 (2,6-Me2C6H3)
Bond Distances (Å)
V–N(1) 1.647(2) 1.651(4) 1.679(6)
V–N(2) 2.279(2) 2.264(5) 2.263(6)
V–N(3) 1.918(2) 1.896(5) 1.909(5)
V–Cl(1) 2.2376(7) 2.248(2) 2.240(2)
V–Cl(2) 2.2532(7) 2.2418(18) 2.266(2)
Bond Angles (deg)
V–N(1)–C(10) 162.03(18) 168.3(4), V–N(1)–C(4) 164.7(5), V–N(1)–C(1)
Cl(1)–V–Cl(2) 124.04(3) 128.38(7) 125.01(8)
N(1)–V–N(2) 178.37(8) 179.1(2) 171.1(3)
N(1)–V–N(3) 101.27(10) 99.7(2) 98.2(3)
N(2)–V–N(3) 78.99(8) 79.33(19) 78.3(2)
Cl(1)–V–N(1) 96.12(7) 94.9(2) 101.6(2)
Cl(1)–V–N(2) 85.20(5) 85.38(15) 87.35(16)
Cl(1)–V–N(3) 114.99(6) 113.88(17) 112.02(19)
Cl(2)–V–N(1) 94.06(7) 95.16(17) 91.1(2)
Cl(2)–V–N(2) 84.40(5) 85.31(13) 83.59(16)
Cl(2)–V–N(3) 116.52(6) 113.95(17) 118.78(19)
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a

Structure reports and xyz files for complexes 1–3 are shown in the Supporting Information.

The V–N(1)–C (in the imido ligand) bond angles in 1 and 2 [162.03(18) and 168.3(4)°] are apparently smaller than that in 4 [170.94(10)°],54 and the Cl(1)–V–N(1) and Cl(2)–V–N(1) bond angles [1: 96.12(7)° and 94.06(7)°; 2: 94.9(2)° and 95.16(17)°] are also slightly smaller than those in 4 [98.90(3)° and 96.02(4)°].54 The imido substituents are slightly bent toward the chloride ligands, probably due to a steric influence of the phenyl group in the benzimidazolyl ligand. These Cl(1)–V–N(1) and Cl(2)–V–N(1) bond angles are larger than the Cl(1)–V–N(2) and Cl(2)–V–N(2) angles [1: 85.20(5)° and 84.40(5)°; 2: 85.38(15)° and 85.31(13)°], which are similar to those in 4 [85.49(3)° and 83.93(3)°].54

It turned out that the mean deviations in the plane consisting of vanadium, imido nitrogen, and three nitrogens in the 2-(2′-benzimidazolyl)-6-methylpyridine ligand including aromatic rings in 1–3 are 0.0315, 0.0234, and 0.00965 Å, respectively. The results thus clearly indicate that these ligand frames possessed a plane perpendicular to a plane consisting of two chlorine and vanadium atoms [dihedral angles in 1: 90.053° and 2: 89.298°], as observed previously in the related titanium complexes,65,66 although the dihedral angle in 3 is somewhat low [86.682°] due to a steric bulk, as described below. The results also indicate that these complexes provide more open space for the catalytic reaction compared to the reported 2-(anilidomethyl)pyridine complex (4).54

The 2,6-dimethylphenylimido analogue (3) also folds a distorted trigonal bipyramidal geometry around vanadium, as observed in 1 and 2. Probably because of a steric bulk in the two methyl groups in the imido ligand (Figure 1), the N(1)–V–N(2) bond angle [171.1(3)°] is apparently smaller than those in the others [178.37(8) and 179.1(2)°, for 1 and 2, respectively], and the V–N(1) bond distance [1.679(6) Å] is longer than those in others [1.647(2) and 1.651(4) Å, for 1 and 2, respectively]. Moreover, the Cl(1)–V–N(1) and Cl(1)–V–N(2) bond angles [101.6(2) and 87.35(16)°, respectively] are apparently larger than the Cl(2)–V–N(1) and Cl(2)–V–N(2) bond angles [91.1(2)° and 83.59(16), respectively]. Similarly, the Cl(1)–V–N(3) bond angle [112.02(19)°] is smaller than the Cl(2)–V–N(3) bond angle [118.78(19)°], and the V–Cl(2) distance [2.266(2) Å] is apparently longer than that in V–Cl(1) [2.240(2) Å]. These would be due to a steric bulk (the phenyl substituent in the imido ligand is bent).

2.2. Reaction with Ethylene in the Presence of Al Cocatalysts

Reactions with ethylene using V(NR)Cl2(L) [R = Ad (1), C6H5 (2), and 2,6-Me2C6H3 (3)] were conducted in toluene at 25 °C in the presence of the Al cocatalyst [Me2AlCl or the methylaluminoxane white solid prepared by removing toluene and AlMe3 from the commercially available sample, TMAO, 9.5 wt % (Al) toluene solution, Tosoh Finechem Co.].57 The results are summarized in Tables 2 and 3.

Table 2. Reaction with Ethylene Using V(NR)Cl2(L) [R = Ad (1), C6H5 (2), and Ar (3, Ar = 2,6-Me2C6H3); L = 2-(2′-Benzimidazolyl)-6-methylpyridine] in the Presence of Al Cocatalystsa.

        PEc
 oligomer (C4–C22)f
  
run V(NR)Cl2(L), R (μmol) Al cocat. Al/Vb wt % yield/mg activityd TOFe/h–1 activityd TOFe/h–1 C4–C22g total C4g/wt % C6g/wt % C8g/wt % otherh/wt %
1 Ad (0.5) MAO 500 16.7 9.6 115 4100 574 20 500 68.1 30.5 15.8 11.8 15.1
2 Ad (0.5) MAO 1000 23.7 13.8 165 5890 532 19 000 66.4 26.6 15.2 9.8 10.0
3 Ad (0.5) MAO 2000 22.0 11.2 134 4780 476 17 000 64.5 26.0 14.8 9.5 13.3
4 Ad (0.2) MAO 500 29.5 7.7 231 8220 434 15 500 55.6 20.1 15.5 9.2 14.9
5 Ad (0.2) MAO 1000 28.4 8.7 261 9290 534 19 000 58.3 20.5 15.9 10.6 13.2
6 Ad (0.2) Me2AlCl 1250 >99 311 9320 332 000              
7 Ad (0.2) Me2AlCl 2500 >99 327 9790 349 000              
8 C6H5 (0.2) MAO 1000 49.5 7.6 228 8110 232 8290 30.7 12.5 7.9 6.7 19.8
9 Ar (0.2) MAO 500 96.1 102 3050 109 000 124 4420 1.9 1.0 0.2 0.6 2.0
10 Ar (0.2) MAO 1000 93.1 109 3270 117 000 244 8700 3.9 2.1 1.2 0.5 3.1
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a

Conditions: ethylene 8 atm, toluene 30 mL, 25 °C, 10 min.

b

Al/V molar ratio.

c

Isolated as MeOH/HCl insoluble portion; details are shown in the Experimental Section.

d

Activity in kg-ethylene reacted/mol-V·h [also kg-PE/mol-V·h].

e

TOF = (molar amount of ethylene reacted)/(mol-V·h).

f

Oligomer (analyzed by GC and CHCl3 extracted portion); details are shown in the Experimental Section.

g

Oligomer (C4–C22 fraction) analyzed by GC vs internal standards.

h

Wax fraction (isolated as CHCl3 soluble fraction after pouring the mixture into MeOH/HCl).

Table 3. Ethylene Polymerization Using V(NR)Cl2(L) [R = Ad (1), C6H5 (2), and Ar (3, Ar = 2,6-Me2C6H3); L = 2-(2′-Benzimidazolyl)-6-methylpyridine] in the Presence of Al Alkyl Cocatalystsa,f.

run V(NR)Cl2(L), R (μmol) Al cocat. Al/Vb ETA/Vc time/min yield/mg activityd TOFe/h–1 (s–1)
6 Ad (0.2) Me2AlCl 1250 0 10 311 9320 332 000 (92)
7 Ad (0.2) Me2AlCl 2500 0 10 327 9790 349 000 (97)
11 Ad (0.1) Me2AlCl 2500 0 10 221 13 300 472 000 (131)
12 Ad (0.1) Me2AlCl 2500 0 6 143 14 300 508 000 (141)
13 Ad (0.1) Me2AlCl 5000 0 6 265 26 500 946 000 (263)
14 Ad (0.05) Me2AlCl 5000 0 6 108 21 700 773 000 (215)
15 Ad (0.05) Me2AlCl 10 000 0 6 278e 55 700 1 990 000 (553)
16 Ad (0.05) Me2AlCl 10 000 10 6 155 31 000 1 110 000 (308)
17 Ad (0.05) Me2AlCl 10 000 50 6 234 46 700 1 670 000 (464)
18 Ad (0.05) Me2AlCl 10 000 100 6 410 82 000 2 930 000 (814)
19 Ad (0.05) Me2AlCl 10 000 1000 6 269 53 700 1 920 000 (533)
20 C6H5 (0.05) Me2AlCl 5000 0 6 400 80 100 2 850 000 (792)
21 C6H5 (0.05) Me2AlCl 10 000 0 6 332 66 400 2 370 000 (658)
22 C6H5 (0.05) Et2AlCl 10 000 0 6 7.3 1460 52 000 (14.5)
23 C6H5 (0.5) Et2AlCl 100 0 6 127 2540 90 700 (25.2)
24 C6H5 (0.5) Et2AlCl 1000 0 6 73.7 1470 52 400 (14.6)
25 C6H5 (0.05) AliBu3 5000 0 6 trace    
26 C6H5 (0.5) AliBu3 100 0 6 trace    
27 C6H5 (0.05) AlEt3 5000 0 6 trace    
28 C6H5 (0.5) AlEt3 100 0 6 trace    
29 Ar (0.1) Me2AlCl 2500 0 6 299 29 900 1 070 000 (297)
30 Ar (0.1) Me2AlCl 5000 0 6 186 18 600 662 000 (184)
31 Ar (0.05) Me2AlCl 5000 0 6 106 21 100 753 000 (209)
32 Ar (0.05) Me2AlCl 2000 0 6 178 35 700 151 000 (42)
33 Ar (0.05) Me2AlCl 1000 0 6 117 23 400 834 000 (232)
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a

Conditions: ethylene 8 atm, toluene 30 mL, 25 °C, 10 min.

b

Al/V molar ratio.

c

ETA = Cl3CCO2Et (molar ratio).

d

Activity in kg-PE/mol-V·h.

e

TOF = (molar amount of ethylene reacted)/(mol-V·h).

f

Melting temperature at 136 °C by DSC thermogram (shown in the Supporting Information).

It turned out that the reactions by the 1–MAO catalyst system afforded a mixture of oligomer [C4–C22 fractions, mostly 1-butene and 1-hexene analyzed by gas chromatography (GC) of the reaction mixture] in addition to higher oligomers [CHCl3 extracted portion (that could not be measured by GC) after pouring the reaction solution into a mixture consisting of MeOH and HCl aqueous solutions and removal of volatiles] and polyethylene. Although the activities [and turn over frequency (TOF) values] by the 1–MAO catalyst system were affected by the Al/V molar ratio employed (runs 1–5), significant differences in the compositions by varying the ratio were not observed. The resultant polymers isolated (MeOH insoluble fraction) were insoluble in ordinary high-temperature gel permeation chromatography (GPC) analysis (conducted in o-dichlorobenzene at 140 °C), which suggest a possibility of obtainment of polymers with an ultrahigh molecular weight.47−50,55,56 These results thus assume that at least two catalytically active species play roles (yielding oligomers and ultrahigh-molecular-weight polymer) in the reaction mixture.

It also turned out that the reactions by the V(NAr)Cl2(L) (3)–MAO catalyst system afforded polymers that were insoluble for ordinary GPC analysis in addition to small amount of oligomers (runs 9 and 10); this is the same observation in the reaction using the 2-(anilidomethyl)pyridine analogue, V(NAr)Cl2[2-ArNCH2(C5H4N)].52 The phenylimido analogue (2) exhibited the lowest activity affording a mixture of the polymer and oligomer (run 8), whereas another phenylimido analogue, V(NC6H5)Cl2[2-ArNCH2(C5H4N)], showed higher activity than the 2,6-arylimido analogue in the reaction with ethylene under similar conditions.52,54 The ratio of oligomer/polymer (composition) was thus affected by the imido ligand employed in these catalyses.

It should be noted that the reaction by the adamantylimido analogue (1) exhibited a remarkably high catalytic activity for ethylene polymerization in the presence of the Me2AlCl cocatalyst (runs 6 and 7, Table 2), and the amount of oligomers (should be detected by GC or extracted with CHCl3 from the reaction solution) formed were negligible. Noteworthy, the activities by 1 in the presence of Me2AlCl were much higher than those in the presence of MAO. Moreover, the activities by 1 (9320 and 9790 kg-PE/mol-V·h, runs 6 and 7) were higher than that by V(NAd)Cl2[2-ArNCH2(C5H4N)] (4, e.g., 704 kg-PE/mol-V·h)55 conducted under similar conditions. Although it has often been observed in ethylene polymerization and ethylene/propylene copolymerization using (especially, classical Ziegler-type) vanadium catalysts that the use of halogenated Al alkyls is more effective than MAO,24,42−50 the present catalyst systems exhibited remarkably high catalytic activities compared to the reported systems (shown in Table 3).

Table 3 summarizes the results of ethylene polymerization by 1–3 in the presence of the Me2AlCl cocatalyst. These reactions afforded polymers exclusively, which were insoluble for the ordinary GPC analysis (in o-dichlorobenzene at 140 °C), suggesting a possibility of formation of ultrahigh-molecular-weight polymers (as also suggested from the copolymerization results with norbornene (NBE), shown below in Table 4).47−50,55,56 Differential scanning calorimetry (DSC) thermogram in the resultant polymer possesses a melting temperature at 136 °C (sample run 15, shown in the Supporting Information), strongly suggesting that the resultant polymers are linear polyethylene. Attempts for reactions with ethylene by 1 in the presence of AliBu3 (1, 0.05 μmol, Al/V = 1000, molar ratio, in toluene, ethylene 8 atm for 10 min) afforded negligible amounts of polymers/oligomers. Moreover, attempts for reactions with ethylene by 2 in the presence of AlEt3 or AliBu3 afforded a negligible amount of polymers/oligomers (runs 25–28), and the activities by 2 in the presence of Et2AlCl (in place of Me2AlCl) were rather low (runs 22–24).

Table 4. Copolymerization of Ethylene with Cyclic Olefin Using the V(NC6H5)Cl2(L) [2; L = 2-(2′-Benzimidazolyl)-6-methylpyridine]–Me2AlCl Catalyst Systema.

run 2/μmol comonomer (M)b Al/Vc ETA/Vd time/min yield/mg activitye Mnf × 10–6 Mw/Mnf Tm (Tg)g/°C
34 0.05 NBE (0.25) 20 000 0 10 58.2 6970 1.74 2.27 71
35 0.05 NBE (0.25) 10 000 0 10 67.6 8100 1.78 2.41 71
36 0.05 NBE (0.25) 10 000 50 10 118 14 200 2.49 2.53 66
37 0.05 NBE (0.25) 10 000 100 10 272 32 600 2.66 2.36 67
38 0.05 NBE (0.25) 10 000 0 15 297 23 700 1.71 2.30  
39 0.05 NBE (0.25) 5000 0 15 78.8 5660      
40 0.05 NBE (0.5) 20 000 0 10 47.1 5640      
41 0.05 NBE (0.5) 10 000 0 10 78.8 9440      
42 0.05 NBE (0.5) 10 000 0 15 93.6 7490      
43 0.05 NBE (0.5) 5000 0 10 141 16 900 3.37h 2.73h 42 (−10)
44 0.05 NBE (0.5) 5000 0 15 198 15 900 3.44h 2.75h 45 (−8.5)
45 0.50 TCD (0.5) 1000 0 10 trace        
46 0.50 TCD (0.5) 5000 0 10 trace        
47I 0.50 CPE (5.0) 2000 0 10 trace        
48I 0.05 CHE (5.0) 20 000 0 10 trace        
49I 0.50 CHE (5.0) 2000   10 11.3 136 4.00 2.15 129
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a

Conditions: ethylene 8 atm, comonomer + toluene total 30 mL, 25 °C.

b

Initial cyclic olefin concentration in mmol/mL.

c

Al/V molar ratio.

d

ETA = Cl3CCO2Et (molar ratio).

e

Activity in kg-polymer/mol-V·h.

f

GPC data in o-dichlorobenzene at 140 °C vs polystyrene standard.

g

By DSC thermograms (shown in the Supporting Information).

h

Measured under low polymer concentration (data with hot o-dichlorobenzene soluble portion).

I

Cyclic olefin (CPE, CHE) + toluene total 10.0 mL, ethylene 2 atm. .

It turned out that the activities by V(NR)Cl2(L)–Me2AlCl catalysts were affected by the Al/V molar ratio employed, and the activity under the optimized conditions increased in the order 3 (R = 2,6-Me2C6H3, 35 700 kg-PE/mol-V·h, run 32) < 1 (R = Ad, 55 700, run 15) < 2 (R = C6H5, 80 100, run 20). The phenylimido analogue (2) exhibited the highest activities, and the activity [80 100 kg-PE/mol-V·h (TOF 2 850 000 h–1, 792 s–1)] is higher than those by V(N-2,6-Cl2C6H3)Cl2(OAr)–Et2AlCl (55 800 kg-PE/mol-V·h)49 and V(NAr)Cl2(OAr)–iBu2AlCl catalyst systems (64 800 kg-PE/mol-V·h),48 which are known to exhibit the highest activities in toluene among the reported aryloxo-modified (imido)vanadium dichloride complex catalysts. It would be interesting to note that the activity by the adamantylimido analogue (1) increased upon the addition of Cl3CCO2Et (ETA) that has been known as an effective promoter (reoxidants, for the reactivation of the assumed catalytically active species),24,42−44,71 whereas a similar effect was not observed in ethylene polymerization using the V(NAr)Cl2(OAr)–Et2AlCl catalyst system.48

Copolymerizations of ethylene with various cyclic olefins [NBE, tetracyclododecene (TCD), cyclopentene (CPE), and cyclohexene (CHE)] were conducted in the presence of the V(NC6H5)Cl2(L) (2)–Me2AlCl catalyst system in toluene at 25 °C. This is because the aryloxo-modified (imido)vanadium complexes, exemplified as V(NAr)Cl2(OAr), exhibited both high catalytic activities and efficient NBE incorporation in the ethylene/NBE copolymerization,47−49 and the resultant cyclic olefin copolymers are promising materials with a high thermal resistance [glass transition temperature (Tg)], transparency with low water absorption, and so forth.72−76 The results are summarized in Table 4. Selected 13C NMR spectrum (Figure S3) and DSC thermograms (Figure S4) in the resultant polymers are shown in the Supporting Information.

It was revealed that the copolymerizations with NBE afforded ultrahigh-molecular-weight polymers with unimodal molecular weight distributions (e.g., Mn = 1.71–2.66 × 106, Mw/Mn = 2.27–2.53, runs 34–38). The activity was affected by the Al/V molar ratio and the NBE concentration employed, and the activity seemed to decrease upon increasing the NBE concentration. This is consistent with the fact that the activity in the copolymerization became low compared with that in the ethylene homopolymerization [e.g., activity: 23 700 kg-polymer/mol-V·h (run 38) vs 80 100 kg-PE/mol-V·h (run 20)]. The Mn values in the copolymer increased upon increasing the NBE contents (runs 34–38 vs 43 and 44), and the Mn values were not affected by the Al/V molar ratio. As observed in the ethylene polymerization,24,42−44,71 the activities increased upon addition of ETA, affording ultrahigh-molecular-weight copolymers (runs 35–37). The resultant polymers possessed uniform molecular weight distributions (as described above, measured by GPC in o-dichlorobenzene at 140 °C) and compositions confirmed by DSC thermograms (single melting temperature, glass transition temperature, Figure S4). These results clearly suggest that the polymerization proceeded with uniform catalytically active species.

Figure S3 shows the selected 13C NMR spectrum in the resultant poly(ethylene-co-NBE) (in 1,1,2,2-tetrachloroethane-d2 at 110 °C). All resonances could be assigned according to the previous reports,73,74,77,78 and the resultant polymer possessed microstructures corresponding to the isolated NBE incorporations in addition to the alternating NBE incorporations to a small extent. The attempted measurement of samples prepared under a high NBE concentration failed (because of poor solubility for measurement of the NMR spectrum because of ultrahigh-molecular-weight). The NBE content (8.5 mol %) estimated by the NMR spectrum is close to that conducted by the aryloxo analogue under similar conditions,47−49 demonstrating that the present catalyst (2) exhibited a similar copolymerization ability. Moreover, both the melting temperature and glass transition temperature observed by DSC thermograms (especially, samples in runs 43 and 44, shown in the Supporting Information, Figure S4) well-corresponded to those in the samples with the same NBE content prepared by the reported catalysts.73,74,77,78 The results thus strongly suggest that the resultant polymers are copolymers with random NBE incorporations.

Although the 2–Me2AlCl catalyst system gave poly(ethylene-co-NBE)s, attempted ethylene copolymerizations with TCD afforded a negligible amount of polymers (runs 45 and 46). Attempted copolymerizations with CPE and CHE conducted under a low ethylene pressure (2 atm) with a high comonomer concentration (5.0 M) afforded trace amount of polymers (runs 47 and 48) or a small amount of polyethylene (confirmed by the DSC thermogram, run 49, Figure S4).

2.3. Analysis of the Catalyst Solution Consisting of V(NAd)Cl2(L) (1) and Al Cocatalysts in Toluene by NMR, ESR, and Solution-Phase XAS

51V NMR spectra of toluene-d8 solution containing V(NAd)Cl2(L) [1, L = 2-(2′-benzimidazolyl)-6-methylpyridine] and MAO or Me2AlCl (10 equiv) were measured at 25 °C, and the results are shown in Figure S2 (Supporting Information). It was revealed that a resonance ascribed to 1 at −87.0 ppm disappeared upon addition of Me2AlCl (Figure S2c), whereas no significant changes in the spectrum was observed upon addition of MAO (Figure S2b). As the disappearance of resonance upon addition of Me2AlCl suggests a possibility of formation of certain paramagnetic species, ESR spectra of similar solutions consisting of 1 and MAO or Me2AlCl were measured (Figure 2).55,79−83

Figure 2.

Figure 2

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ESR spectra (in toluene at 25 °C, vanadium 2.5 μmol/mL) for (a–c) V(NAd)Cl2(L) [1, L = 2-(2′-benzimidazolyl)-6-methylpyridine] upon addition of Al cocatalyst (10 equiv) and (d) V(NAd)Cl2[N(H)Me2]2.

It was revealed that no significant differences (no resonances ascribed to the formation of paramagnetic species) in the spectrum were observed when a toluene solution of 1 was added to MAO (10.0 equiv, Figure 2b). By contrast, resonances ascribed to the formation of a paramagnetic species were observed upon addition of Me2AlCl (10.0 equiv, Figure 2c). However, the intensity was low compared to that of the (imido)vanadium(IV) analogue, V(NAd)Cl2[N(H)Me2]280 (Figure 2d), under the same conditions. This would suggest that the percentage of ESR-observed species, probably vanadium(IV), might be low. Because disappearance of the signal in the 51V NMR spectrum was observed when 1 was added to Me2AlCl, a detailed analysis is required to explore the oxidation state of the probable catalytically active species.

We thus focus on the synchrotron XAS because the method (V–K edge analysis, 5.46 keV, through synchrotron radiation at SPring-8, BL01B1 beamline) provides information concerning the oxidation state [by V–K pre-edge and edge peaks in the X-ray absorption near-edge structure (XANES) analysis] and coordination atoms around the vanadium [by FT extended X-ray absorption fine structure (FT-EXAFS) analysis]. Figure 3 shows the XANES spectra of toluene solution containing V(NAd)Cl2(L) [1, L = 2-(2′-benzimidazolyl)-6-methylpyridine] and the solution in the presence of MAO or Me2AlCl (10.0 equiv). The spectra of V(NAd)Cl2[2-ArCH2(C5H4)] (4),58 V(NAd)Cl3,67 and the (imido)vanadium(IV) complex, V(NAd)Cl2[N(H)Me2]2,80 measured under the same conditions are also shown for comparison (Figure 3a).

Figure 3.

Figure 3

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Solution-phase V K-edge XANES spectra (in toluene at 25 °C) for (a) V(NR)Cl2(L) [R = Ad (1), C6H5 (2), L = 2-(2′-benzimidazolyl)-6-methylpyridine]. Spectra for V(NAd)Cl2[2-ArCH2(C5H4)],58 V(NAd)Cl3,58 and V(NAd)Cl2[N(H)Me2] measured under the same conditions are also placed for comparison. (b) V(NAd)Cl2(L) [1, L = 2-(2′-benzimidazolyl)-6-methylpyridine] in the presence of Al cocatalyst [MAO, Me2AlCl, Me2AlCl, and ETA10.0 equiv].

As shown in Figure 3a, V(NAd)Cl2(L) [1, L = 2-(2′-benzimidazolyl)-6-methylpyridine] shows two pre-edge peaks at 5465.5 and 5467.7 eV in addition to a small shoulder-edge peak at 5477.0 eV. Similarly, V(NPh)Cl2(L) (2) shows pre-edge peak(s) at 5465.7 and 5467.5 eV in addition to a small shoulder-edge peak at 5478.1 eV. These are similar to the observed facts that two pre-edge peaks were observed in V(NAd)Cl2[2-ArCH2(C5H4)] (4, 5465.2 and 5467.3 eV) and V(NAd)Cl3 (5465.6 and 5467.1 eV), and these complexes also showed a shoulder-edge peak (5477.8 and 5477.6 eV, respectively) that would be ascribed to the V–Cl bond.58 Two pre-edge peaks are probably due to a transition from 1s to 3d + 4p,84,85 although examples of solution V K-edge XANES spectra by synchrotron radiation still have been limited.86,87 By contrast, the related (imido)vanadium(IV) dichloride, V(NAd)Cl2[N(H)Me2]2, showed a broad pre-edge peak at 5466.4 eV and a shoulder-edge peak at 5477.5 eV. It is clear from the spectra that the edge peak of the vanadium(IV) dichloride complex low-shifted compared to the vanadium(V) dichloride complexes. These results also suggest that the observed shoulder-edge peak would be ascribed to the V–Cl bond.58

It was revealed that no significant differences in the XANES spectrum (pre-edge peaks and edge) from that in 1 was observed upon addition of MAO [5465.5 and 5467.7 eV (pre-edge), 5476.8 eV (shoulder-edge), Figure 3b]. These results thus suggest that the oxidation state of 1 was preserved upon addition of MAO, and the results would be in good agreement with those in the NMR and ESR spectra. By contrast, it should be noted that remarkable changes in the XANES (pre-edge and edge regions) spectrum were observed when 1 [5465.5 and 5467.7 eV (pre-edge), 5477.0 eV (shoulder-edge)] was treated with Me2AlCl [5466.0 eV (pre-edge), 5475.8 eV (shoulder-edge), Figure 4]. Apparently, the low-energy shift in the edge peak (in addition to change from two to one pre-edge peak) strongly suggests that complex 1 was reduced by reaction with Me2AlCl, and the result is consistent with that observed especially in the 51V NMR spectrum (disappearance of signal because of the generation of paramagnetic species). The remarkable changes in the XANES (pre-edge and edge regions) spectrum suggest the structural changes upon addition of Me2AlCl, especially by reduction. Moreover, the intensity in the shoulder-edge (5475.7 eV) increased upon addition of ETA on decreasing the intensity of the pre-edge peak (5465.5 eV), and this corresponds to the fact that the activity increased upon addition of ETA. The results thus suggest that the addition of ETA would be effective for the generation (increased percentage) of catalytically active species by certain structural changes with the reduction of 1.

Figure 4.

Figure 4

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(a) Solution-phase (in toluene at 25 °C) V K-edge EXAFS oscillations and the simulated spectrum for V(NAd)Cl2(L) (1). In this simulation (on the basis of X-ray crystallographic data), the contributions from the neighbor atoms within 3.34 Å distance from the V atom were considered, and 0.0036 Å2 of the Debye–Waller factor was applied. (b) Solution-phase V K-edge FT-EXAFS spectra (in toluene at 25 °C) for V(NAd)Cl2(L) (1) upon addition of Me2AlCl (10 equiv). Additional analysis data are shown in the Supporting Information.

Figure 4a shows the EXAFS oscillations and the simulated spectrum of 1 in toluene.82 The observed spectrum shows a good fitting with that estimated from the X-ray crystallographic data (Figure 4a). Three nitrogen atoms are coordinated to vanadium [coordination number (CN) 1.7(2), V–N = 1.683 Å; CN 1.2(8), V–N = 2.290(42) Å], which probably corresponds to three vanadium–nitrogen bonds [V–N(1): 1.647(2), V–N(2): 2.279(2), and V–N(3): 1.918(2) Å, Table 1]. The spectrum also suggests the presence of two V–Cl bonds [CN 1.6(2), V–Cl = 2.293(3) Å], which also correspond well to the two vanadium–chloride bonds [V–Cl(1): 2.2376(7) and V–Cl(2): 2.2532(7) Å]. The results clearly indicate that complex 1 preserves the basic trigonal bipyramidal structure (determined by X-ray crystallography) in solution. It was revealed that the CN of the vanadium–nitrogen bond decreased upon addition of Me2AlCl [CN 0.9(3), V–N: 1.64(2) Å, additional analysis data are shown in the Supporting Information], suggesting that two nitrogen bonds (maybe in L) would probably be dissociated upon addition of Me2AlCl (Figure 4b). We are unsure about the reason for the presence of two/three V–Cl bonds [CN 2.6(1), V–Cl = 2.455(7) Å], which became apparently weak compared to those in the original (complex 1, V–Cl = 2.293(3) Å). Taking into account the result in Figure 4, we can at least say that reduction occurred from 1 (probably by dissociation of two vanadium–nitrogen bonds) upon addition of Me2AlCl, although the detailed structure in the real catalytically active species is still unclear at this moment.

Through 51V NMR, ESR, and XAS analyses, it was revealed that (i) no resonances in the 51V NMR spectrum were observed when V(NAd)Cl2(L) [1, L = 2-(2′-benzimidazolyl)-6-methylpyridine] was treated with 10.0 equiv of Me2AlCl in toluene-d8 at 25 °C, (ii) formation of paramagnetic species (but weak intensity) was observed in the toluene solution of 1 upon addition of Me2AlCl (10 equiv) in the ESR spectrum, whereas no resonances were observed upon addition of MAO in place of Me2AlCl, and (iii) remarkable changes in the XANES (pre-edge and edge regions) spectrum was observed when 1 was added with Me2AlCl in toluene, whereas no significant differences were observed upon addition of MAO. These results strongly suggest the observed difference as the effect of the Al cocatalyst (MAO vs Me2AlCl) in the reaction with ethylene using 1 because of the formation of different catalytically active species with different oxidation states. Because the observed XANES spectra upon addition of Me2AlCl are different (especially, the edge region) from those in (adamantylimido)vanadium(IV) dichloride, V(NAd)Cl2[N(H)Me2]2, and the intensity of resonances observed in the ESR spectrum (Figure 3c) was weak, it thus seems likely and may be assumed that certain vanadium(III) species by reduction was generated from 1 upon addition of Me2AlCl, although we could not come to any detailed conclusion (structure, oxidation state) at this stage from the EXAFS spectrum.

3. Concluding Remarks

Three (imido)vanadium(V) dichloride complexes containing 2-(2′-benzimidazolyl)-6-methylpyridine ligand (L) of type, V(NR)Cl2(L) [R = 1-adamantyl (Ad, 1), C6H5 (2), 2,6-Me2C6H3 (3)], were prepared, and their structures could be determined by X-ray crystallographic analysis. These complexes fold distorted trigonal bipyramidal structures around vanadium and a plane consisting of three nitrogen atoms in the 2-(2′-benzimidazolyl)pyridine ligand and the imido ligand positioned perpendicular to a plane consisting of two chloride and vanadium. These complexes (1–3) exhibited catalytic activities for the reaction with ethylene in the presence of MAO, affording a mixture of oligomer and polymers, and the compositions were affected by the imido ligand employed. By contrast, the complexes (1–3) exhibited remarkable catalytic activities for ethylene polymerization in the presence of Me2AlCl. The phenylimido complex (2) exhibited an exceptionally high activity [80 100 kg-PE/mol-V·h (TOF 2 850 000 h–1, 792 s–1)], which was higher than those reported previously by the aryloxo-modified (imido)vanadium dichloride complexes, demonstrating its promising capability as the efficient ethylene polymerization catalyst. Complex 2 was also effective for ethylene copolymerization with NBE to afford ultrahigh-molecular-weight copolymers with uniform molecular weight distributions and compositions, suggesting that the polymerization proceeds with uniform catalytically active species.

On the basis of the studies by NMR and ESR spectra and solution V K-edge XAS (XANES and EXAFS) analysis, the observed difference in the catalyst performances in the presence of (between) MAO and Me2AlCl cocatalyst should be due to the formation of different catalytically active species with different oxidation states. In particular, significant changes in the oxidation state were observed upon addition of Me2AlCl in the XANES spectrum (in addition to NMR and ESR spectra), and the intensity in the shoulder-edge increased upon addition of ETA with decreasing intensity of the preedge peak, whereas no significant changes in the spectra were observed upon the presence of MAO. Although the detailed structural analysis of the catalyst solution consisting of 1 and Me2AlCl could not be done at this moment, the observed fact should provide important information for a better understanding of the catalysis mechanism and catalyst design. Moreover, we believe that a combination of all these analyses (NMR and ESR spectra and XAS analysis) should be helpful in providing more clear information for better understanding and explanation. We are now exploring more details, including further analyses of the catalyst solutions (and complexes) through solution XAS (XANES, EXAFS) analysis, including assignments of resonances; these would be introduced in the future.

4. Experimental Section

4.1. General Procedure

All experiments were conducted in a Vacuum Atmospheres drybox under a nitrogen atmosphere. Toluene, n-hexane, and dichloromethane of anhydrous grade (Kanto Kagaku Co., Ltd.) were stored in a bottle containing molecular sieves (a mixture of 3A 1/16, 4A 1/8, and 13X 1/16) in the drybox; solvents were further purified by passing through an alumina short column under a nitrogen stream prior to use. V(NAd)Cl3 (Ad = 1-adamantyl),67 V(NC6H5)Cl3,54 V(N-2,6-Me2C6H3)Cl3,68 and V(NAd)Cl2[N(H)Me2]280 were prepared according to a published method. Me2AlCl (1.0 M in n-hexane, Kanto Kagaku Co., Ltd.) and ethylene for polymerization (purity >99.9%, Sumitomo Seika Co. Ltd.) were used as received. Toluene and AlMe3 in methylaluminoxane [TMAO, 9.5 wt % (Al) toluene solution, Tosoh Finechem] were removed in vaccuo (at ca. 50 °C for removal of toluene and AlMe3, and then heated at >100 °C for 1 h for completion) in the drybox to afford white solids.57

Elemental analyses were performed by using a EAI CE-440 CHN/O/S elemental analyzer (Exeter Analytical, Inc.). All 1H, 13C, and 51V NMR spectra were recorded on a Bruker AV500 spectrometer (500.13 MHz for 1H, 125.77 MHz for 13C, and 131.55 MHz for 51V). All spectra were obtained in the solvent indicated at 25 °C, unless otherwise noted. Chemical shifts are given in ppm and are referenced to SiMe4 (δ 0.00 ppm, 1H, 13C) and VOCl3 (δ 0.00 ppm, 51V). Coupling constants and half-width values, Δν1/2, are given in Hz. 13C NMR spectra for the resultant polymers were recorded with proton decoupling, the pulse interval was 5.2 s, the acquisition time was 0.8 s, the pulse angle was 90°, and the number of transients accumulated was about 6000. The copolymer samples for analysis were prepared by dissolving the polymers in 1,1,2,2-tetrachloroethane-d2 solution, and the spectra was measured at 110 °C. GC analysis was performed with a SHIMADZU GC-2025AF gas chromatograph (Shimadzu Co. Ltd.) equipped with a flame ionization detector.

Molecular weights and molecular weight distributions of the prepared polymers were measured by GPC (Tosoh HLC-8121GPC/HT) using an RI-8022 detector (for high temperature; Tosoh Co.) with a polystyrene gel column (TSK gel GMHHR-H HT32, 30 cm, 37.8 mm i.d.), ranging from <102 to <2.8 × 108 MW) at 140 °C using o-dichlorobenzene containing 0.05 w/v % 2,6-di-tert-butyl-p-cresol as the solvent. DSC data for the polymer were recorded by means of a Hitachi DSC-7020 instrument under a nitrogen atmosphere [preheating from 30 to 300 °C (20 °C/min), cooling to −100 °C under N2, and measurement from −100 to 300 °C (20 °C/min) under N2]. This heating and cooling was repeated two times. Tg values were determined from the middle point of the phase transition of the second heating scan.

4.1.1. Synthesis of V(NAd)Cl2(L) [1, L = 2-(2′-Benzimidazolyl)-6-methylpyridine]

(i) Synthesis of LK. Into the THF solution (15 mL) containing 2-(2′-6′-methylpyridiyl)-benzimidazole (836 mg, 4.0 mmol),69 KH powder (160 mg, 4.0 mmol) was added at −30 °C in the drybox. The reaction mixture was warmed slowly to room temperature and stirred for 4 h. The resultant white solid (insoluble in THF) was then collected and dried in vacuo (935 mg, 94%). (ii) Synthesis of complex 1. To a toluene solution (20 mL) containing V(NAd)Cl3 (568 mg, 1.82 mmol), LK (450 mg, 1.82 mmol) was added at −30 °C. The reaction mixture was warmed slowly to room temperature and stirred overnight (ca. 16 h). The resultant red brown solution was filtered and extracted with toluene. The extracts were filtered through a Celite pad, and the filtercake was washed with toluene. The toluene extract (combined filtrate and the wash) was then placed in vacuo to remove volatiles, giving deep red solids. The resultant solids were then dissolved in a minimum amount of dichloromethane layered by n-hexane. The chilled solution placed in the freezer (−30 °C) yielded deep red microcrystals, which were dried in vacuo. Yield 403 mg (46%). 1H NMR (CDCl3): δ 8.32 (d, J = 7.55 Hz, 1H), 8.16 (s, 1H), 7.85 (s, 1H), 7.66 (s, 1H), 7.35–7.41 (m, 3H), 2.94 (s, 3H), 2.61 (br s, 6H), 2.28 (br s, 1H), 1.82 (br s, 2H), 1.79 (br s, 2H), 1.74 (br s, 2H), 1.71 (br s, 1H). 13C NMR (CDCl3): δ 159.5, 147.4, 142.9, 139.5, 128.4, 126.2, 125.4, 118.9, 116.0, 42.8, 35.8, 29.4, 23.9. 51V NMR (CDCl3): δ −78.33 (Δν1/2 = 1950 Hz). Anal. Calcd for C23H25Cl2N4V: C, 57.63; H, 5.26; N, 11.69%. Found: C, 57.81; H, 5.46; N, 11.80%.

4.1.2. Synthesis of V(NC6H5)Cl2(L) (2)

The procedure for the synthesis of 2 was similar to that for 1, except that V(NC6H5)Cl3 (485 mg, 1.95 mmol) in place of V(NAd)Cl3 and LK (483 mg, 1.95 mmol) were used. After the reactions and extraction with toluene, the resultant solids after the removal of volatiles were dissolved with a minimum amount of dichloromethane layered by n-hexane. The chilled solution placed in the freezer (−30 °C) yielded deep red microcrystals, which were dried in vacuo. Yield 621 mg (75%). 1H NMR (CDCl3): δ 8.10 (d, J = 8.05 Hz, 1H), 7.93 (d, J = 7.70 Hz, 1H), 7.75 (d, J = 7.75 Hz, 2H), 7.71 (d, J = 7.85 Hz, 1H), 6.99 (t, J = 7.58 Hz, 1H), 6.84 (td, J = 7.71 Hz, 0.98, 1H), 6.73–6.78 (m, 3H), 6.65 (t, J = 7.53 Hz, 1H), 6.38 (d, J = 7.65 Hz, 1H), 2.87 (s, 3H, CH3). 13C NMR (CDCl3): δ 24.0, 117.5, 118.9, 125.9, 126.2, 128.7, 129.2, 129.7, 131.4, 139.9, 142.8, 145.34, 147.4, 158.2, 159.7, 162.0. 51V NMR (CDCl3): δ 41.70 (Δν1/2 = 1470 Hz). Anal. Calcd for C19H15Cl2N4V: C, 54.18; H, 3.59; N, 13.30%. Found: C, 53.89; H, 3.57; N, 13.22%.

4.1.3. Synthesis of V(N-2,6-Me2C6H3)Cl2(L) (3)

The procedure for synthesis of 3 was similar to that for 1, except that V(N-2,6-Me2C6H3)Cl3 (206 mg, 0.744 mmol) in place of V(NAd)Cl3 and LK (184 mg, 0.744 mmol) were used. After the reactions and extraction with toluene, the resultant solids after the removal of volatiles were dissolved with a minimum amount of dichloromethane layered by n-hexane. The chilled solution placed in the freezer (−30 °C) yielded deep red microcrystals, which were dried in vacuo. Yield 184 mg (55%). 1H NMR (CDCl3): δ 8.18 (br s, 1H), 7.88 (br s, 1H), 7.74 (d, 1H, J = 7.8 Hz), 7.64 (br s, 1H), 7.42 (br s, 1H), 7.07 (br s, 4H), 3.05 (s, 3H), 2.92 (s, 6H). 13C NMR (CDCl3): δ 20.1, 24.2, 117.1, 118.8, 125.9, 126.0, 128.5, 128.7, 130.8, 139.7, 143.0, 144.9, 146.4, 147.3, 158.6, 159.8, 162.8. 51V NMR (CDCl3): δ 131.19 (Δν1/2 = 1950 Hz). Anal. Calcd for C21H19Cl2N4V·0.5 toluene: C, 59.41; H, 4.68; N, 11.31%. Found: C, 58.91; H, 4.61; N, 11.55%.

4.2. Reaction with Ethylene

Reactions with ethylene were conducted in a 100 mL scale stainless steel autoclave, and the typical reaction procedure is as follows. Toluene (29.0 mL) and the prescribed amount of MAO or Me2AlCl (1.0 M n-hexane) were added into the autoclave in the drybox. The reaction apparatus was then filled with ethylene (1 atm), and the prescribed amount of complex, V(NR)Cl2(L) [L = 2-(2′-benzimidazolyl)-6-methylpyridine], in toluene (1.0 mL) was added into the autoclave. The reaction apparatus was then immediately pressurized to 7 atm (total 8 atm, kept constant during the reaction), and the mixture was magnetically stirred for 10 or 15 min. After the reaction, the remaining ethylene was purged at −30 °C, and 0.5 g of n-heptane (internal standard) was added. Both the activity and product distribution were then analyzed by GC. After the above oligomerization procedure, the mixture remaining in the reactor was poured into MeOH containing HCl; the resultant white precipitate was collected on a filter paper by filtration, and the powder was adequately washed with MeOH. The resultant polymer was then dried in vacuo at 60 °C for 2 h. The MeOH soluble portion was extracted with CHCl3, washed with water, and dried through Na2SO4. Removal of volatiles afforded a high-molecular-weight oligomer (or a low-molecular-weight PE).

4.3. Copolymerization of Ethylene with Cyclic Olefins by V(NC6H5)Cl2(L) (2)–Me2AlCl Catalyst System

The polymerization procedures conducted were similar to those conducted in ethylene polymerization/oligomerization. Toluene (29.0 or 9.0 mL) and the prescribed amount of cyclic olefin (NBE etc.) and Me2AlCl (1.0 M n-hexane) were added into the autoclave in the drybox. The reaction apparatus was then filled with ethylene (1 atm), and the prescribed amount of the complex, V(NC6H5)Cl2(L) (2), in toluene (1.0 mL) was added into the autoclave. The reaction apparatus was then immediately pressurized to 1 or 7 atm (total 2 or 8 atm), and the mixture was stirred for 6 or 10 min. After the reaction, the mixture in the reactor was poured into MeOH containing HCl, and the resultant polymer was collected on a filter paper by filtration, which was adequately washed with MeOH. The resultant polymer was then dried in vacuo at 60 °C for 2 h. Selected 13C NMR spectra (Figure S3) and DSC thermograms (Figure S4) in the resultant polymers are shown in the Supporting Information.

4.4. Crystallographic Analysis

All measurements were made on a Rigaku XtaLAB P200 diffractometer using multilayer mirror monochromated Mo Kα radiation. The crystal collection parameters are listed in Table 5. The data were collected and processed using CrystalClear (Rigaku)88 or CrysAlisPro (Rigaku Oxford Diffraction),89 and the structure was solved by direct methods90 and expanded using Fourier techniques. The nonhydrogen atoms were refined anisotropically. Hydrogen atoms were refined using the riding model. All calculations were performed using the CrystalStructure91 crystallographic software package, except for refinement, which was performed using SHELXL version 2014/7.92,93 Structure reports and xyz files for V(NR)Cl2(L) [R = 1-adamantyl (Ad, 1), C6H5 (2), 2,6-Me2C6H3 (3); L = 2-(2′-benzimidazolyl)-6-methylpyridine] are shown in the Supporting Information. CCDC reference numbers of 1–3 are CCDC 1544623–1544625, respectively. These data can be obtained free of charge from The Cambridge Crystallographic Data Centre.

Table 5. Crystal Data and Collection Parameters of [V(NR)Cl2(L)] [R = Ad (1), C6H5 (2), 2,6-Me2C6H3 (3); L = 2-(2′-Benzimidazolyl)-6-methylpyridyl]a.

  1b 2 3
formula C24H27Cl4N4V C19H15Cl2N4V C42H38Cl4N8V2
formula weight 564.26 421.20 898.51
crystal color, habit red, block red, plate red, block
crystal size (mm) 0.120 × 0.110 × 0.060 0.100 × 0.080 × 0.060 0.100 × 0.060 × 0.020
crystal system monoclinic triclinic orthorhombic
space group P21 (#4) P1̅ (#2) P212121 (#19)
a (Å) 8.3276(11) 7.4884(7) 7.2388(3)
b (Å) 11.0967(15) 9.3222(9) 15.3494(8)
c (Å) 13.592(2) 13.6821(15) 36.6025(16)
α (deg)   70.369(10)  
β (deg) 99.934(4) 87.341(8)  
γ (deg)   88.592(8)  
V (Å3) 1237.2(3) 898.60(17) 4066.9(3)
Z value 2 2 4
Dcalcd (g/cm3) 1.515 1.557 1.467
F000 580.00 428.00 1840.00
temp (K) 93(2) 93(2) 93(2)
μ (Mo Kα) (cm–1) 8.533 8.590 7.642
no. of reflections measured (Rint) total: 10 045, unique: 4251 (0.0327) total: 7640, unique: 3837 (0.0756) total: 31 734, unique: 8255 (0.1129)
2θmax (deg) 55.0 55.0 55.0
no. of observations [I > 2.00σ(I)] 4251 3837 8255
no. of variables 298 235 505
R1 [I > 2.00σ(I)] 0.0214 0.0730 0.0657
wR2 [I > 2.00σ(I)] 0.0538 0.1938 0.1595
goodness of fit 1.019 0.999 1.007
Open in a new tab
a

Structure reports and xyz files for complexes 1–3 are shown in the Supporting Information. CCDC reference numbers for 1–3 are 1544623–1544625, respectively.

b

Structure for 1 contains CH2Cl2.

4.5. 51V NMR Experiments in the Reaction of V(NAd)Cl2(L) [1, L = 2-(2′-Benzimidazolyl)-6-methylpyridine] with Al Cocatalysts in Toluene-d8

The typical procedure is as follows. Into a toluene-d8 solution (ca. 0.6 mL) containing 1 (25 μmol, ca. 42 μmol/mL) placed in the freezer (−30 °C), Al cocatalyst (Me2AlCl or MAO; 250 μmol, 10.0 equiv) was added. The mixture was then measured by 51V NMR spectrum at 25 °C within 10 min after the preparation.

4.6. ESR Measurements

ESR measurement was performed with a Bruker ER073 spectrometer. A toluene solution containing V(NAd)Cl2(L) (1) and a toluene solution containing MAO (10.0 equiv) or Me2AlCl (10.0 equiv) were mixed with their concentrations being kept at 2.50 μmol/mL for 1 and 25.0 μmol/mL for MAO or Me2AlCl. The tube was then placed into the instrument preset (ESR measurements were started in less than 10 min after the preparation). The experimental parameters were as follows: 9.4 GHz frequency, 0.10 mT modulation amplitude, and power 1.0 mW for 1 + Me2AlCl and 1 + MAO [9.85 GHz frequency for 1 and V(NAd)Cl2{N(H)Me2}2].

4.7. Analysis of the Catalyst Solution by Solution-Phase XAS

V K-edge XANES and XAFS measurements were carried out at the BL01B1 beam line at the SPring-8 facility of the Japan Synchrotron Radiation Research Institute (proposal nos. 2016A1455 and 2016B1509). The measurements were conducted at room temperature. A Si(111) two-crystal monochromator was used for the incident beam. V K-edge XAFS spectra of V complex samples (prepared as toluene solution, 50 μmol/mL) were recorded in the fluorescence mode using an ionization chamber as the I0 detector and 19 solid-state detectors as the I detector. The X-ray energy was calibrated using V2O5, and the data analysis was performed with REX2000 ver. 2.5.9 software package (Rigaku Co.). The XANES data were analyzed by removing the atomic absorption background using a cubic spline from the χ spectra and normalization of them to the edge height.

Acknowledgments

This project was partly supported by grant-in-aid for Scientific Research on Innovative Areas (“3D Active-Site Science”, no. 26105003) from The Ministry of Education, Culture, Sports, Science and Technology (MEXT), Japan and grant-in-aid for Scientific Research (B) from the Japan Society for the Promotion of Science (JSPS, no. 15H03812). The synchrotron XAFS analysis was performed at SPring-8 beam lines of BL01B1 with the approval of JASRI (2016A1455, 2016B1509, and 2017A1512). The authors also express their heartfelt thanks to Prof. Z. Maeno, Prof. K. Jitsukawa, and Prof. K. Kaneda (Osaka University) for their big support for collaboration of XAFS analysis at SPring-8. K.N. expresses his thanks to Takumi Yamada, Takuya Omiya, and Dr. Shunsuke Sueki (Tokyo Metropolitan Univ., TMU) for helping with the measurement of synchrotron XAS analysis at SPring 8, to Profs. S. Komiya and A. Inagaki (TMU) for discussions, and to Tosoh Finechem Co. for donating MAO. K.N. expresses his thanks to the Chinese Academy of Sciences, President’s International Fellowship Initiative (PIFI) for the support to conduct international collaboration research.

Supporting Information Available

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsomega.7b01225.

  • Additional experimental results in the attempted reaction of V(NAd)Cl3 with the potassium salt of 2-(2′-benzimidazolyl)pyridine (51V NMR data), selected 13C NMR spectrum and DSC thermograms in the resultant polymers in the (co)polymerization using the V(NR)Cl2(L) [R = Ad (1), C6H5 (2), L = 2-(2′-benzimidazolyl)-6-methylpyridine]–Me2AlCl catalyst system, additional EXAFS spectra and curve fittings for V(NAd)Cl2(L) (1) by calculations on the basis of X-ray crystallographic structure (PDF)

  • Structure reports for V(NR)Cl2(L) [R = 1-adamantyl (Ad, 1), C6H5 (2), 2,6-Me2C6H3 (3); L = 2-(2′-benzimidazolyl)-6-methylpyridine] (XYZ)

The authors declare no competing financial interest.

Supplementary Material

ao7b01225_si_001.pdf (1.9MB, pdf)
ao7b01225_si_002.xyz (8.1KB, xyz)

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Supplementary Materials

ao7b01225_si_001.pdf (1.9MB, pdf)
ao7b01225_si_002.xyz (8.1KB, xyz)

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