XAS Analysis of Reactions of (Arylimido)vanadium(V) Dichloride Complexes Containing Anionic NHC That Contains a Weakly Coordinating B(C6F5)3 Moiety (WCA-NHC) or Phenoxide Ligands with Al Alkyls: A Potential Ethylene Polymerization Catalyst with WCA-NHC Ligands.

(Arylimido)vanadium(V) dichloride complexes containing anionic N-heterocyclic carbene (NHC) ligands that contain weakly coordinating B(C6F5)3 moieties (WCA-NHC) of the type [V(NAr)Cl2(WCA-NHC-Ar')] (5, Ar = 2,6-Me2C6H3, Ar' = 2,6- i Pr2C6H3) showed significant catalytic activity for ethylene polymerization in the presence of Al cocatalysts (MAO and Al i Bu3); the activity by the 5-MAO catalyst (19 500 kg-PE/mol-V·h; TOF 11 600 min-1) is the highest among those reported using the other (imido)vanadium(V) complexes in the presence of MAO, and the 5-Al i Bu3 catalyst showed higher activity (66 000 kg-PE/mol-V·h; TOF 39 200 min-1). The V K-edge X-ray absorption near-edge structure (XANES) analyses (in toluene) strongly suggest the formation of certain vanadium(III) species by reduction with Al i Bu3 accompanying structural changes; the EXAFS analysis suggests the presence of the arylimido ligand and one V-Cl bond (2.34 ± 0.04 Å), which is longer than those [2.1901(8)-2.2462(8) Å] in the reported (imido)vanadium(V) complexes. The XANES analysis of [V(NAr)Cl2(OAr)] strongly suggests the formation of the other vanadium(III) species by reduction with Me2AlCl or Et2AlCl, and the EXAFS analysis suggests the presence of the arylimido ligand and two V-Cl bonds (2.45 ± 0.03 Å). The XANES spectra showed no significant changes in both the pre-edge peak(s) and the edge peak when these complexes were treated with MAO, suggesting that the basic geometry and the high oxidation state were preserved under these conditions.

Introduction

Transition-metal-catalyzed olefin polymerization is the core method in commercial production of polyolefin [exemplified as polyethylene (PE) and polypropylene (PP)], and the design of the molecular catalysts has been considered as an attractive subject especially in terms of synthesis of new polymers.[1−17] Classical Ziegler-type vanadium catalyst systems (e.g., VOCl3, VCl4, VCl3–AlBr3, AlCl3–AlPh3, AlBu3, and SnPh4) are known to exhibit high reactivity toward olefins (notable propagation for synthesis of ultrahigh-molecular-weight polymers with narrow distributions)[18−23] and are applied for commercial production of EPDM (ethylene propylene diene monomer) rubber.[8,17,24−26,28]

Although potentially high initial catalytic activities were exhibited using these catalyst systems, their rapid catalyst deactivation [probably associated with conversion into the inactive species by reduction, from vanadium(III) to vanadium(II)] causes very poor overall productivities. The concern, short catalyst life, could be overcome partly by addition of ethyl trichloroacetate (ETA),[27] and this would be assumed as due to the catalyst re-activation by re-oxidation with ETA (Scheme );[27] ethylene polymerizations with most of these catalysts were thus performed in the presence of large excess of ETA.[8,17,24−26] Therefore, vanadium(III) species have been assumed to play a role as the active species on the basis of electron spin resonance (ESR) spectra and titration study,[29−32] as shown in Scheme .[21,22,25] However, as described below, ESR spectroscopy, widely employed for analysis of paramagnetic compounds,[32−38] faces concerns for the observation of “ESR-silent” vanadium(III) species and the quantitative analysis and for obtainment of structural information.

Scheme 1

Assumed Ethylene Polymerization Mechanism in the Presence of Ethyl Trichloroacetate (ETA)[25]

Studies on synthesis of the efficient vanadium complex catalysts and the related organometallic chemistry have been considered as important subjects in the fields of catalysis, organometallic chemistry, and polymer chemistry.[8,16,17,28] (Adamantylimido)vanadium(V) complexes containing 2-anilidemethyl-pyridine, [V(NAd)X2(2-ArNCH2C5H4N)], [A; Ad = 1-adamantyl; Ar = 2,6-Me2C6H3; X = Cl, Me; Scheme ],[39,40] or 8-anilide-5,6,7-trihydroquinoline[41] ligands have been known to exhibit significant catalytic activities for selective ethylene dimerization in the presence of MAO cocatalysts, whereas the related complexes containing 2-(2′-benzimidazolyl)pyridine ligands (B) showed the notable activities for ethylene polymerization in the presence of Me2AlCl cocatalysts.[42] The (arylimido)vanadium(V) complexes that contain a monodentate anionic ancillary donor ligand (Y), [V(NAr)Cl2(Y)] [Y = phenoxide (C),[43−45] iminoimidazolide (D),[46] and iminoimidazolidide (E),[46]Scheme ], showed the high activities for ethylene polymerization and the copolymerization with norbornene (NBE) in the presence of MAO or Me2AlCl and Et2AlCl. Moreover, the complexes containing anionic N-heterocyclic carbenes (NHCs) that contain a weakly coordinating borate moiety (WCA-NHC, F) exhibited high catalytic activities for ethylene polymerization in the presence of AlBu3,[47] which has been considered as an ineffective Al cocatalyst in the metal-catalyzed olefin polymerization.[1−17,24−26,28,39−46]

Scheme 2

Selected (Imido)vanadium(V) Dichloride Complex Catalysts for Ethylene Polymerization and Dimerization[39−47]

It was demonstrated that cationic alkyl species play a role in the ethylene dimerization using the (A)–MAO catalyst, on the basis of (i) 51V NMR and electron spin resonance (ESR) spectra,[40] (ii) the effect of the Al cocatalyst and the ethylene pressure dependence,[40] (iii) syntheses of the cationic (and the dimethyl) complexes and their reaction chemistry,[48] and (iv) analysis through solution V K-edge XANES (XANES = X-ray absorption near-edge structure) and the FT-EXAFS (EXAFS = extended X-ray absorption fine structure) spectra.[48] Analysis by synchrotron X-ray absorption spectroscopy (XAS) has been known to provide important information of the oxidation states (through XANES analysis; pre-edge and edge peaks) and their coordination atoms around the centered metal (through FT-EXAFS analysis).[49,50] XAS analysis has been widely used in the field of heterogeneous catalysis,[49−53] and the reports for studies in the field of molecular catalysis are still limited.[42,48,54−60]

We recently introduced that the method should be useful for analysis of the vanadium catalyst solution, especially for ESR-silent paramagnetic species, which cannot be observed by both NMR and ESR spectra.[42,55,56,58,59] For example, 51V NMR (in toluene-d8) and ESR (in toluene) spectra showed negligible resonances when complex B was treated with Me2AlCl (10.0 equiv) or when the toluene solution of complex F was treated with AlBu3 (10.0 equiv). In contrast, as shown in Figure (XANES spectra), apparent changes in the oxidation state were observed when the toluene solution containing B was treated with Me2AlCl (10.0 equiv, Figure a)[42] or F (complex 3 in Scheme ) was treated with AlBu3 (10.0 equiv, Figure b);[59] the spectra did not change when these complexes were treated with MAO. The low-energy shifts in the edge absorption accompanied by changes in the pre-edge peak strongly suggest that these complexes (B and F) were reduced by Al alkyls to form certain vanadium(III) species. These results are thus consistent with those observed in the 51V NMR and ESR spectra.[42,58]

Figure 1

V K-edge XANES spectra (in toluene at 25 °C; 5.46 keV) for (a) [V(NAd)Cl2(L)] [B; Ad = 1-adamantyl; L = 2-(2′-benzimidazolyl)-6-methylpyridine][42] and (b) [V(N-2,6-Me2C6H3)Cl2(WCA-NHC)] (F)[58,59] in the presence of cocatalysts (MAO, Me2AlCl, AlBu3, 10.0 equiv).

Scheme 3

(Arylimido)vanadium(V) Complexes Coordinated with Anionic N-Heterocyclic Carbenes (NHCs) Containing a Weakly Coordinating tris(Pentafluorophenyl) Borane Moiety [WCA-NHC-Ar (1–3), Ar = 2,6-Me2C6H3; WCA-NHC-Ar′ (4, 5), Ar′ = 2,6-Pr2C6H3]

The above results clearly suggest the formation of certain vanadium(III) species by treatment of Me2AlCl or AlBu3,[42,58,59] whereas the high oxidation state was preserved by addition of MAO.[42,58] Moreover, it is clear that the XANES spectrum of the toluene solution consisting of B-Me2AlCl should be different from that consisting of F-AlBu3, especially in the edge region (Figure ). In contrast, the oxidation state was preserved when the dichloro complex containing the 2-anilidemethylpyridine ligand (A) was treated with Me2AlCl (10.0 equiv), which afforded ultrahigh-molecular-weight polymers in the reaction with ethylene; the formation of another vanadium(V) species was thus suggested from 51V NMR spectra and V K-edge XANES spectra.[48] Since, as described below, a new (arylimido)vanadium(V) dichloride complex containing the WCA-NHC-Ar′ ligand (5, Scheme , Ar′ = 2,6-Pr2C6H3) showed promising characteristics as the ethylene polymerization catalyst, we thus explored the effect of Al cocatalysts on the oxidation state of the formed vanadium species by the solution V K-edge XANES and EXAFS analyses (especially with complexes 3 and 5). Moreover, the studies with the phenoxide analogues [exemplified as [V(NAr)Cl2(OAr)] (C), Scheme ] were also explored because the effect of the Al cocatalyst (MAO vs Me2AlCl) on both the activity and comonomer incorporation by C was significant in the ethylene/NBE copolymerization.[44,45] Through this study, we explored for the obtainment of the information of the active species, especially vanadium(III), formed by reaction with Al alkyls, which should be important for understanding the catalysis mechanism in the ethylene polymerization.

Results and Discussion

Synthesis of (Arylimido)vanadium(V) Complexes Containing Anionic N-Heterocyclic Carbenes That Contain a Weakly Coordinating B(C6F5)3 (WCA-NHC), [V(NR′)Cl2(WCA-NHC)], and Their Use as the Catalyst Precursors for Ethylene Polymerization

Synthesis and Structural Analysis of [V(NR′)Cl2(WCA-NHC-Ar′)] (Ar′ = 2,6-Pr2C6H3)

Anionic N-heterocyclic carbene–borate (WCA-NHC) ligands have been promising anionic ligands[47,61−66] because it has been postulated that electron-deficient neutral or cationic early transition-metal-alkyl species with high oxidation states can be stabilized with the WCA-NHC ligands as anionic donor ligands through σ- and/or π-donation and/or by delocalization.[47,66]

(Arylimido)vanadium(V) dichloride complexes containing 2,6-diisopropylphenyl-modified WCA-NHC ligands of type [V(NR′)Cl2(WCA-NHC-Ar′)] [R′ = C6H5 (4), Ar (5); Ar = 2,6-Me2C6H3; Ar′ = 2,6-Pr2C6H3] were prepared according to the reported procedure for syntheses of [V(NR′)Cl2(WCA-NHC-Ar)] [R′ = Ad (1), C6H5 (2), Ar (3); Scheme ][47] by treating V(NR′)Cl3 with lithium salts of the 2,6-diisopropylphenyl analogue[64,65] in toluene. The resultant complexes were isolated as microcrystals grown from cold CH2Cl2–n-hexane (−30 °C); the complexes were identified by NMR spectra and elemental analysis, and their structures were determined by X-ray diffraction analysis (Figure ).a It was revealed that the resonance ascribed to 5 (410.5 ppm) in the 51V NMR spectrum was shifted low field compared to those in 1 (45.4 ppm), 2 (185.6 ppm), 3 (302.7 ppm), and 4 (256.6 ppm).

Figure 2

ORTEP drawings of [V(NC6H5)Cl2(WCA-NHC-Ar′)] (4, left, Ar′ = 2,6-Pr2C6H3) and [V(NAr)Cl2(WCA-NHC-Ar′)] (5, right, Ar = 2,6-Me2C6H3). Thermal ellipsoids were drawn at 30% probability level, and H atoms were omitted for clarity. Selected bond lengths (Å) for 4: V–Ccarbene 2.060(2), V–N(1) 1.646(3), V–Cl(1) 2.1641(7), V–Cl(2) 2.1655(7), and N(1)–C(1) 1.380(4). Selected bond angles (°) for 4: Ccarbene–V–Cl(1) 113.65(6), Ccarbene–V–Cl(2) 113.55(7), Ccarbene–V–N(1) 103.36(10), Cl(1)–V–Cl(2) 114.92(3), and V–N(1)–C(1) 165.31(18). Selected bond lengths (Å) for 5: V–Ccarbene 2.076(3), V–N(1) 1.654(3), V–Cl(1) 2.1620(10), V–Cl(2) 2.1559(9), and N(1)–C(1) 1.386(5). Selected bond angles (deg) for 5: Ccarbene–V–Cl(1) 106.53(8), Ccarbene–V–Cl(2) 117.61(8), Ccarbene–V–N(1) 103.66(12), Cl(1)–V–Cl(2) 114.68(4), and V–N(1)–C(1) 169.1(2). Detailed data are shown in the Supporting Information (see footnote a).

These complexes showed a distorted tetrahedral geometry around vanadium (Figure ); the V–Ccarbene bond distances [2.060(2) and 2.076(3) Å for 4 and 5, respectively] are longer than those in complexes 1–3 [2.039(3)–2.049(2) Å][47] but are shorter than those in the other complexes such as [VOCl3(NHC)][67] and [V(CHSiMe3)(NAd)(CH2SiMe3)(NHC)][68] [V–Ccarbene = 2.137 and 2.172(2) Å, respectively]. The results thus indicate that the WCA-NHC-Ar′ ligands in 4 and 5 also act as a σ-donor toward vanadium. It seems that the degree of the σ-donation especially in 5 would be rather weak compared to that in 1–3, as also seen from the chemical shift in the 51V NMR spectra (described above). The V–Cl bond distances in 4 and 5 [2.1559(9)–2.1655(7) Å] are within the range of those in 1–3 [2.1544(14)–2.1747(15) Å],[47] but, as discussed previously for 1–3,[47] the distances are shorter than those in the related dichloride complexes [2.1901(8)–2.2462(8) Å] containing monodentate anionic donor ligands (exemplified as complexes C–E in Scheme ).[46,69−71] The Cl(1)–V–Cl(2) bond angles in 4 and 5 [114.92(3) and 114.68(4)° for 4 and 5, respectively] are smaller than those in 1–3 [116.26(5)–117.74(6)°],[47] probably due to a steric bulk on the diisopropylphenyl substituent on the NHC-B(C6F5)3 ligand. The V–N(1) distances in 4 [1.646(3) Å] and 5 [1.654(3) Å] are relatively close to those in 2 and 3 [1.634(3) and 1.648(2) Å for 2 and 3, respectively],[47] whereas the V–N(1)–C(1) bond angle in 5 [169.1(2)°] is larger than that in 3 [164.14(17)°] and the angle in 4 [165.31(18)°] is smaller than that in 2 [170.8(3)°].[47] Taking into account these analysis data, as discussed for 1–3,[47] the WCA-NHC-Ar′ ligands play a role as σ donors in electron-deficient, cationic 12-electron complexes in 4 and 5.

Ethylene Polymerization by [V(NR′)Cl2(L)] in the Presence of Al Cocatalysts

Table summarizes the results of ethylene polymerization conducted in toluene at 25 °C using [V(NR′)Cl2(WCA-NHC-Ar′)] [R′ = C6H5 (4), 2,6-Me2C6H3 (5)] in the presence of MAO (d-MAO white solid, prepared by removal of toluene and AlMe3 from TMAO, commercially available from Tosoh Finechem Co.)[72−74] or AlBu3 cocatalysts. The results obtained using [V(NR′)Cl2(WCA-NHC-Ar)] [R′ = Ad (1), C6H5 (2), and Ar (3)][47] are also shown for comparison.

Table 1

Ethylene Polymerization Catalyzed by Vanadium(V) Dichloro Complexes Containing WCA-NHC Ligands (1–5)–Al Cocatalysts (Ethylene 8 atm, 25 °C, 10 Min)a

run	V cat. (μmol)	Al cocat. (μmol)	Al/Vb	yield/mg	activityc	TOFd/min	Mne × 10–4	Mw/Mne
1f	1 (1.0)	d-MAO (500)	500	6	35	21
2e	2 (1.0)	d-MAO (100)	100	8	48	29
3f	3 (1.0)	d-MAO (200)	200	264	1580	939	1.53	1.93
4f	3 (1.0)	d-MAO (500)	500	137	824	490
5	4 (1.0)	d-MAO (100)	500	17	102	61
6	5 (1.0)	d-MAO (200)	200	895	4210	2500
7	5 (0.2)	d-MAO (100)	500	14	420	250
8	5 (0.2)	d-MAO (200)	1000	136	4080	2420
9	5 (0.2)	d-MAO (400)	2000	664	19 900	11 800
10	5 (0.2)	d-MAO (600)	3000	645	19 500	11 600	9.46	1.56
11g	C (1.0)	d-MAO (2500)	2500	488	2930	1740	175	1.64
12f	3 (0.2)	AliBu3 (10.0)	50	365	11 000	6540	1.80	1.76
13f	3 (0.2)	AliBu3 (20.0)	100	161	4830	2870
14f,h	3 (0.2)	Et2AlCl (500)	2500	65.8	1970	1170
15	4 (1.0)	AliBu3 (100)	100	95	570	339
16	4 (1.0)	AliBu3 (200)	200	99	594	353
17	5 (0.2)	AliBu3 (10.0)	50	13	390	232	5.11	1.42
18	5 (0.2)	AliBu3 (20.0)	100	67	2010	1190
19	5 (0.2)	AliBu3 (40.0)	200	187	5610	3330
20	5 (0.2)	AliBu3 (200)	100	223	6690	3970
21	5 (0.2)	AliBu3 (400)	200	390	11 700	6950
22	5 (0.1)	AliBu3 (400)	400	263	15 780	9380
23	5 (0.1)	AliBu3 (600)	600	373	22 380	13 300
24	5 (0.02)	AliBu3 (400)	2000	157	47 100	28 000
25	5 (0.02)	AliBu3 (500)	2500	220	66 000	39 200	13.7	2.35
26	5 (0.02)	AliBu3 (550)	2750	187	56 100	33 300
27	5 (0.02)	AliBu3 (600)	3000	59	17 700	10 500
28g,h	C (0.05)	Me2AlCl (250)	5000	229	27 500	16 300	(898)i
29g,h	C (0.01)	iBu2AlCl (250)	25 000	108	64 800	38 500	(1250)i

Conditions: toluene 30 mL, d-MAO [used as a white solid prepared from TMAO (Tosoh Finechem Co., Ltd.) by removing toluene and AlMe3] or AlBu3.

Al/V molar ratio.

Activity in kg-PE/mol-V·h.

TOF (min–1) = (molar amount of ethylene reacted)/(mol-V)·(min).

GPC data in o-dichlorobenzene vs polystyrene standards.

Cited from ref (47).

Result by V(NAr)Cl2(OAr) cited from refs (43) (run 11) or[45](runs 28, 29).

Conducted at 0 °C.

Molecular weight by viscosity.[45]

It should be noted that the 5–MAO catalyst exhibited 10 times higher catalytic activity than the 3–MAO catalyst [e.g., activity: 1580 kg-PE/mol-V·h (by 3, run 3)[47] vs 19 500 kg-PE/mol-V·h (by 5, run 10)]. The catalyst afforded high-molecular-weight polymers with unimodal molecular weight distributions, suggesting the formation of uniform catalytically active species in the solution. The activity by 5 was affected by the amount of the Al cocatalyst employed (runs 7–10), and the observed trend was somewhat different from that observed in 3 (runs 3, 4). Importantly, the activity by 5 (19 500 kg-PE/mol-V·h, run 10) is higher than those reported by [V(NAr)Cl2(OAr)] (C, 2930 kg-PE/mol-V·h, run 11),[44] [V(NAr)Cl2{1,3-Ar2(CHN)2C=N}] (507 kg-PE/mol-V·h),[46] [V(NAr)Cl2{1,3-Ar2(CH2N)2C=N}] (627 kg-PE/mol-V·h),[46] and [V(NAd)Cl2{N=CBu2}] (516 kg-PE/mol-V·h).[68] The results thus demonstrate that 5 should be the most active (imido)vanadium(V) complex catalyst in the presence of the MAO cocatalyst, although the Mn values in the resultant polymers were lower than those by C (Table ). It also turned out that the activity by the phenylimido analogue (4)–MAO catalyst was low (run 5), as observed in 2.

It was revealed that the 5–AlBu3 catalyst exhibited higher activities than the 5–MAO catalyst under the similar conditions [e.g., activity: 19 500 kg-PE/mol-V·h (run 10) vs 66 000 kg-PE/mol-V·h (run 25)]. Note that the 5–AlBu3 catalyst showed higher activity than the 3–AlBu3 catalyst [e.g., activity: 11 000 kg-PE/mol-V·h (by 3, run 12)[47] vs 66 000 kg-PE/mol-V·h (by 5, run 25)], affording high-molecular-weight polymers with unimodal molecular weight distributions. The activity by 5 was affected by Al/V molar ratio (runs 17–27), and the observed trend was somewhat different from that observed in 3 (runs 12, 13).[47] It also turned out that the activities by the 4–AlBu3 catalyst were low (runs 15, 16), as seen in the presence of MAO.

The activity by the 5–AlBu3 catalyst (66 000 kg-PE/mol-V·h, run 25) was higher than that by the [V(NAr)Cl2(OAr)]–Me2AlCl catalyst (C, 27 500 kg-PE/mol-V·h, run 28)[45] but was comparable to that by the C–Bu2AlCl catalyst (64800 kg-PE/mol-V·h, run 29), which afforded ultrahigh-molecular-weight polymers under these conditions.[45] However, these polymerizations by C were conducted at 0 °C, and the activity decreased significantly at 25 °C.[43] A similar effect of retardations (by increasing the temperature from 0 to 25 °C) was also observed in the (arylimido)vanadium(V) complexes containing iminoimidazolide or iminoimidazolidide ligands.[46] As described in the Introduction section, AlBu3 has been considered as an ineffective Al cocatalyst in the metal-catalyzed olefin polymerization,[1−17,24−26,28,39−46] but the use of AlBu3 should be favored in terms of practical viewpoints, especially for the establishment of a halogen-free process (without using halogenated Al) and for using an inexpensive Al cocatalyst compared to MAO.

Solution V K-Edge XANES and EXAFS Analyses of Reactions of (Arylimido)vanadium(V) Dichloride Complexes Containing Phenoxide and WCA-NHC Ligands with Al Alkyls. Effects of Anionic Ancillary Donor Ligands and Al Cocatalysts

As described in the Introduction section, metal-alkyl species play an important role in olefin polymerization/dimerization. It was demonstrated that cationic vanadium(V) alkyl species play a role in the ethylene dimerization using [V(NAd)X2(2-ArNCH2C5H4N)] (A, X = Cl, Me; Ar = 2,6-Me2C6H3)–MAO catalysts, whereas another cationic vanadium(V) alkyl species plays a role in the ethylene polymerization using A–Me2AlCl or Et2AlCl catalysts.[40,48] The different catalyst behaviors in the reactions with ethylene (in the presence of MAO or Me2AlCl cocatalysts) could be thus explained as due to the catalyst/cocatalyst nuclearity effect (the formation of isolated or associated cationic alkyl species due to different cation/anion interactions).[10,12,40,56,75,76] In contrast, as also described in the Introduction section (Figure ), certain vanadium(III) species were formed when [V(NAd)Cl2(L)] [B, L = 2-(2′-benzimidazolyl)-6-methylpyridine] was treated with Me2AlCl (Figure a)[42] or [V(NAr)Cl2(WCA-NHC-Ar)] (3) was treated with AlBu3 (Figure b).[58,59] The high oxidation states were preserved when complex B or 3 was treated with MAO, suggesting that vanadium(V) species play a role in the ethylene polymerization.[42,48] Since the XANES spectra, especially in the edge region (shoulder-edge), are apparently different between B–Me2AlCl and 3–AlBu3 catalysts (Figure ), we thus herein explore solution XANES and EXAFS analyses of complexes 3 and 5 to obtain more information concerning the active species.

Figure shows V K-edge XANES spectra of [V(NAr)Cl2(WCA-NHC-Ar)] (3) and [V(NAr)Cl2(WCA-NHC-Ar′)] (5, Ar′ = 2,6-Pr2C6H3) in the presence of MAO or AlBu3 (in toluene at 25 °C, 50 μmol-V/mL). As reported previously,[58] the XANES spectrum of 3 shows sharp pre-edge peak(s), which are typically observed in the complex with the tetrahedral geometry, at 5466.6 eV (and 5465.1 eV) ascribed to a transition from 1s to 3d + 4p.[55,56,77−79] A shoulder-edge peak, ascribed to an absorption of the V–Cl bond,[55,56] was also observed at 5478.1 eV. The diisopropylphenyl analogue (5) also shows the pre-edge peak(s) at 5466.9 eV (and 5465.1 eV) and the shoulder-edge peak at 5478.0 eV. It turned out that the pre-edge peaks in 5 were shifted slightly at 5464.5 eV with a decrease in the intensity of the shoulder-edge peak by addition of MAO (10 equiv, Figure ), suggesting that the oxidation state and the basic framework (geometry) were preserved. In contrast, as observed in 3 (Figures a, 3), significant changes in the XANES spectrum (pre-edge and edge peaks) were observed when 5 was treated with AlBu3 (100 equiv, Figure ). An apparent decrease in the pre-edge intensity (observed as a small shoulder at 5465.1 eV) upon addition of AlBu3 clearly suggests a structural change, and the large low-energy shift in the edge absorption, which is very close to that observed in the 3–AlBu3 system, strongly suggests the formation of vanadium(III) species by reduction.b

Figure 3

V K-edge XANES spectra of [V(NAr)Cl2(WCA-NHC-Ar)] (3, Ar = 2,6-Me2C6H3) and [V(NAr)Cl2(WCA-NHC-Ar′)] (5, Ar′ = 2,6-Pr2C6H3) in the presence of MAO or AlBu3 (in toluene at 25 °C).

Figure a shows V K-edge XANES spectra (in toluene at 25 °C) of [V(N-2,6-R21C6H3)Cl2(O-2,6-R22C6H3)] [R1, R2 = Me, Me (6a);[43] Me, Ph (6b);[44] and Pr, Pr (6c)43] and [V(NAr)(OAr)3] (7).[80] The dichloride complexes containing phenoxide ligands (6a–c) have been chosen in this study because 6a (corresponds to C in Scheme ) showed the highest activities for ethylene polymerization[43−45] and ethylene copolymerization with norbornene (NBE)[44,45] in the presence of Al cocatalysts. It was demonstrated that the activities and the NBE incorporation in the (co)polymerization were highly affected by the Al cocatalyst employed (MAO vs Me2AlCl or Et2AlCl);[44,45] the 6a,b–Et2AlCl catalyst showed the higher catalytic activities but less NBE incorporation than the 6a,b–MAO catalyst in the ethylene/NBE copolymerization.[44]

Figure 4

V K-edge XANES spectra (in toluene at 25 °C) of (a) [V(N-2,6-R21C6H3)Cl2(O-2,6-R22C6H3)] [R1, R2 = Me, Me (6a); Me, Ph (6b); and Pr, Pr (6c)], [V(NAr)(OAr)3] (7, Ar = 2,6-Me2C6H3), and 6a,b in the presence of MAO (10 equiv), (b) 6a in the presence of Me2AlCl or Et2AlCl (50 equiv) and norbornene (NBE, 50 equiv), (c) 6a,c in the presence of Me2AlCl (50 equiv), and (d) [V(NAr)Cl2(WCA-NHC-Ar)] (3) or [V(NAr)Cl2(WCA-NHC-Ar′)] (5, Ar′ = 2,6-Pr2C6H3) in the presence of AlBu3 (100 equiv).

As reported previously (for 6a,b),[59] the XANES spectra of 6a–c showed pre-edge peaks (and shoulder peaks) [5466.8 (and 5465.3) eV (6a), 5467.4 (and 5465.2) eV (6b), and 5466.9 (and 5465.7) eV (6c), respectively], which are similar to those in 3, 5, and [V(NAr)Cl3] [5466.8 (and 5465.0) eV][48] and shoulder-edge peaks (marked in the dashed circle in Figure a; 5478.2, 5478.2, and 5478.9 eV, respectively). The tris(phenoxide) analogue (7) showed only a sharp pre-edge peak at 5467.1 eV, clearly suggesting that the shoulder-edge peaks in 6a–c are ascribed to an absorption of the V–Cl bond.[55,56] The pre-edge peak positions in the spectra of 6a,b did not change (5466.8 and 5467.0 eV, respectively) upon addition of MAO (10 equiv) with an increase in the intensity, and the edge absorptions shifted slightly (Figures a and S2–1).c The results thus suggest that the oxidation state and the basic framework were preserved even upon addition of MAO; the facts also suggest that cationic alkyl species with vanadium(V) play a role in this catalysis, as demonstrated previously.[42,48,55,56,58]

Note that notable changes in the edge peaks (shoulder-edge at 5475.7 eV) were observed when 6a was treated with Me2AlCl or Et2AlCl (50 equiv, Figure b), and the spectra did not change upon the further addition of NBE (50 equiv). It seems that the pre-edge peak in 6a [5466.8 (and 5465.3) eV] shifted to low energy or became one absorption band upon addition of Me2AlCl (5465.7 eV) or Et2AlCl (5465.9 eV). A similar XANES spectrum was observed when 6c was treated with Me2AlCl (50 equiv, Figure c), and these edge peaks (absorptions) were observed in a low-photon-energy region compared to that in the (imido)vanadium(IV) complex. The results thus suggest that 6a,c were reduced by Me2AlCl to afford certain vanadium(III) species, as suggested by the reaction of 3 and 5 with AlBu3 (as shown in Figures b and 3).

It should also be noted that the observed XANES spectra (in both intensities in the pre-edge peaks and edge peaks) of 6a with addition of Me2AlCl or Et2AlCl are apparently different from those of 3 or 5 with AlBu3. The spectra (6a–Me2AlCl), especially in the pre-edge intensities, are also different from those of [V(NAd)Cl2(L)] (B) containing 2-(2′-benzimidazolyl)-6-methylpyridine ligands with addition of Me2AlCl (Figure a);[42] the pre-edge intensity by B–Me2AlCl further decreased upon addition of ETA (Cl3CCO2Et), whereas the intensity decreased slightly by the reaction of 6a with Me2AlCl.[42] It has been known that pre-edge intensities are affected by the basic geometry around vanadium; a compound in Td symmetry generally shows much higher pre-edge peak intensity than that in Oh symmetry due to a difference in the possibility of a p–d orbital hybridization.[77] These results thus suggest that different vanadium(III) species (geometry and ligands) play roles in the ethylene (co)polymerization (see footnote b).

Moreover, as shown in Figure S2-2 (see footnote c), the shoulder-edge intensity (at 5475.7 eV) decreased when ETA (50 equiv) was added into a toluene solution of 6a, Me2AlCl (50 equiv), and NBE (50 equiv), whereas the intensity (at 5475.7 eV) increased with a decrease in the pre-edge intensity (at 5465.5 eV) when a toluene solution of B and Me2AlCl was added with ETA.[42] The observed facts also correspond to the facts that the activity in ethylene polymerization by 6a decreased upon addition of ETA,[45] whereas the activity by B increased upon addition of ETA,[42] as seen in most vanadium complex and classical Ziegler-type vanadium catalyst systems (as described in the Introduction section).[8,16,17,23−28] These also suggest that different catalytically active vanadium(III) species would play roles in these catalyses.

Figure shows V K-edge EXAFS oscillations and FT-EXAFS spectra (in toluene at 25 °C) of 3 with addition of AlBu3 (100 equiv) and 6a with addition of MAO (10 equiv) or Me2AlCl (50 equiv) (see footnote c). Tables and 3 summarize the analysis results for observed neighboring atoms and bond distances around vanadium (see footnote c). The result for B with addition of Me2AlCl is also shown for comparison.[42]

Figure 5

V K-edge (a, c) EXAFS oscillations and (b, d) FT-EXAFS spectra (in toluene at 25 °C) for reactions of (a, b) [V(NAr)Cl2(WCA-NHC-Ar)] (3, Ar = 2,6-Me2C6H3) in the presence of AlBu3 (100 equiv) and (c, d) [V(NAr)Cl2(OAr)] (6a) in the presence of MAO (10 equiv) and Me2AlCl (50 equiv). Additional spectra including fitting curves are shown in the Supporting Information (see footnote c).

Table 2

Summary of Data for [V(NAr)Cl2(WCA-NHC-Ar)] (3) in the Presence of AlBu3 (100 Equiv)a

	complex 3		3 + AliBu3 (100 equiv)
atom	C.N.	r (Å)	C.N.	r (Å)
N(C)	2.1 ± 0.2	1.6 2± 0.03	0.8 ± 0.3	1.66 ± 0.17
Cl	1.0 ± 0.2	2.16 ± 0.04	1.0 ± 0.2	2.34 ± 0.04
Cl	1.0 ± 0.2	2.34 ± 0.05

Atom: neighbor atom, C.N.: coordination number, r: bond length.

Table 3

Summary of Data for the Complex [V(NAr)Cl2(OAr)] (6a) or [V(NAd)Cl2(L)] [B, L = 2-(2′-Benzimidazolyl)-6-methylpyridine] in the Presence of Me2AlCl or MAOa

	complex 6a		6a + MAO (10 equiv)		6a + Me2AlCl (50 equiv)		complex Bb		B + Me2AlCl (10 equiv)b
atom	C.N.	r (Å)	C.N.	r (Å)	C.N.	r (Å)	C.N.	r (Å)	C.N.	r (Å)
N(O)	2.4 ± 0.3	1.80 ± 0.05	1.8 ± 0.2	1.73 ± 0.04	1.3 ± 0.2	1.64 ± 0.04	1.7(2)	1.683(5)	0.9(3)	1.64(2)
N							1.2(8)	2.290(42)
Cl	1.9 ± 0.2	2.1 8± 0.03	1.4 ± 0.2	2.17 ± 0.03	2.0 ± 0.2	2.45 ± 0.03	1.6(2)	2.293(3)	2.6(1)	2.455(7)

Atom: neighbor atom, C.N.: coordination number, r: bond length.

Data cited from ref (42).

As summarized in Table , the observed V–N distance by the EXAFS analysis (1.62 ± 0.03 Å) in 3 is close to that determined by X-ray crystallography [1.654(3) Å], and one of the observed V–Cl bond distances by the EXAFS analysis (2.16 ± 0.04 Å) is close to those determined by X-ray crystallography [2.1559(9) and 2.1620(10) Å]. Another V–Cl bond distance (2.34± 0.05 Å) was much longer than those in the related dichloride complexes [2.1901(8)–2.2462(8) Å, by X-ray crystallography][46,69−71] containing monodentate anionic donor ligands (such as phenoxides, ketimides, iminoimidazolides, etc.). However, the V–Ccarbene bond [2.076(3) Å] was not observed or may be overlapped with the V–N bond (coordination number, 2.1 ± 0.2, Table ). This could be assumed as due to the flexibility of the anionic WCA-NHC ligand in solution or explained by a simple speculation that the anionic NHC ligand binds to vanadium strongly in solution (the short V–Ccarbene bond distance compared to that in the solid state).

It should be noted that both EXAFS oscillation and the FT-EXAFS spectrum of 3 changed drastically by treatment with AlBu3 (100 equiv, Figure a,b), as observed in the XANES spectra (Figures b and 3). It turned out that the imido ligand (V–N bond, 1.66 ± 0.17 Å) was preserved, and one V–Cl bond (2.34 ± 0.04 Å), which is longer than those in the reported (imido)vanadium complexes with anionic donor ligands (shown above),[46,69−71,81] was also observed. However, the V–Calkyl bond, which should be present in catalysis, could not be defined in this analysis. Since the formed species are considered as vanadium(III) species on the basis of 51V NMR and ESR spectra, and the XANES spectra in solution,[58] it is thus suggested that the Cl atom would be coordinated to vanadium (containing imido and alkyl ligands) as a neutral donor ligand probably bridged through Al.

It should also be noted that the imido ligand (V–N bond, 1.64 ± 0.04 Å, Table ) was preserved after treatment of 6a with Me2AlCl. It also turned out that two V–Cl bonds were observed as the coordinating atoms to vanadium, and the long V–Cl bond distances (2.45 ± 0.03 Å) compared to those in 6a (2.18 ± 0.03 Å) would suggest that these Cl ligands coordinated to vanadium (containing imido and alkyl ligands) as neutral (or weak anionic) donor ligands (bridged through Al).[81] In contrast, no significant changes in both the EXAFS oscillation and the FT-EXAFS spectrum were observed when 6a was treated with MAO, as observed in the XANES spectra; no significant differences in the both V–N and V–Cl bond distances were observed in the EXAFS analysis, except a decrease in the coordination number of Cl by treatment with MAO. These results thus suggest that the basic framework including the oxidation state was preserved by treatment of 6a with MAO.

Taking into account these analysis results, one V–Cl bond (2.34 ± 0.04 Å) was observed upon treatment of 3 with AlBu3 and two V–Cl bonds (2.45 ± 0.03 Å) were observed in the reaction of 6a with Me2AlCl; three V–Cl bonds [2.455(7) Å] were observed in the reaction of [V(NAd)Cl2(L)] [B, L = 2-(2′-benzimidazolyl)-6-methylpyridine] with Me2AlCl.[42] In all cases, vanadium(III) species containing the imido V–N bond were formed on the basis of XANES and EXAFS analyses. Although we could not define the V–Calkyl bond, which should play a role in the ethylene (co)polymerization, the observed differences strongly suggest that three different active vanadium(III) species were formed depending upon the anionic donor and Al cocatalyst employed.

Concluding Remarks

(Arylimido)vanadium(V) dichloride complexes that contain anionic N-heterocyclic carbenes (NHCs) coordinated with a weakly coordinating B(C6F5)3 moiety, [V(NAr)Cl2(WCA-NHC-Ar′)] (5, Ar = 2,6-Me2C6H3, Ar′ = 2,6-Pr2C6H3), exhibited remarkable catalytic activity for ethylene polymerization in the presence of Al cocatalysts (MAO and AlBu3). The activity by the 5–MAO catalyst (19 500 kg-PE/mol-V·h) was much higher than that by the dimethylphenyl analogue, [V(NAr)Cl2(WCA-NHC-Ar)] (3) and those reported using the reported (imido)vanadium(V) complexes containing monodentate anionic ligands.[44,46,67] The 5–AlBu3 catalyst showed higher activity (66 000 kg-PE/mol-V·h) than 5–MAO and 3–AlBu3 catalysts, affording rather high-molecular-weight polymers with unimodal molecular weight distributions (Mn = 1.37 × 105, Mw/Mn = 2.35).

The solution V K-edge XANES analyses of 3 and 5 strongly suggest the formation of vanadium(III) species by reduction with AlBu3, accompanying structural changes (geometry) around vanadium (supported by changes in the pre-edge peak and decrease in the intensity) (see footnote b). The solution EXAFS analysis suggests the formation of vanadium species containing arylimido ligands and one V–Cl bond (2.34 ± 0.04 Å), which is longer than those in the reported (imido)vanadium(V) dichloride complexes that contain monodentate anionic ligands [2.1901(8)–2.2462(8) Å, by X-ray crystallography];[46,69−71]the V–Calkyl bond, which should be present in catalysis, could not be defined in this analysis.

Moreover, the solution XANES analysis of [V(NAr)Cl2(OAr)] (6a) also suggests the formation of vanadium(III) species by reduction with Me2AlCl or Et2AlCl (supported by changes in the edge peaks). The observed XANES spectra especially in the edge region (shoulder-edge at 5475.7 eV) are different from those observed in the toluene solution containing 3 or 5 and AlBu3. The solution EXAFS analysis suggests the formation of vanadium species containing arylimido ligands and two V–Cl bonds (2.45 ± 0.03 Å), although the presence of the V–Calkyl bond could not be defined.

In contrast, no significant changes in both XANES and EXAFS spectra were observed by treatment of these complexes (3, 6a) with MAO, suggesting the preservation of the basic framework (a distorted 4-coordinate tetrahedral geometry) and the oxidation state; these results thus suggest that cationic (arylimido)vanadium(V) species play a role in the ethylene polymerization in the presence of the MAO cocatalyst. Results concerning the effect of the Al cocatalyst in ethylene polymerization and XANES analysis of 3, 5, and 6a are summarized in Scheme .

Scheme 4

Summary of the Effect of the Al Cocatalyst and the Anionic Donor Ligand in Ethylene Polymerization Using [V(NAr)Cl2(WCA-NHC-Ar)] (3, Ar = 2,6-Me2C6H3), [V(NAr)Cl2(WCA-NHC-Ar′)] (5, Ar′ = 2,6-Pr2C6H3), and [V(NAr)Cl2(OAr)] (6a), and Reaction Chemistry through Solution XANES Analysis

Results for [V(NAd)Cl2(L)] [B, L = 2-(2′-benzimidazolyl)-6-methylpyridine][42] are also shown for comparison.

The facts observed for 3, 5, and 6a by treatment with AlBu or Me2AlCl demonstrate a unique contrast to the fact in the EXAFS analysis that more than two (three) V–Cl bonds [2.455(7) Å] were observed when [V(NAd)Cl2(L)] [B, L = 2-(2′-benzimidazolyl)-6-methylpyridine] was treated with Me2AlCl, which showed remarkable activity for ethylene polymerization.[42] In addition to the observed difference in the XANES spectra as well as the catalyst behavior (the effect of Cl3CCO2Et in 6a and B in ethylene polymerization, Scheme ), it should be considered that three different vanadium(III) species play roles in the ethylene polymerization depending upon the anionic donor and the Al cocatalyst employed. Moreover, we can at least say that these vanadium(III) species could be stabilized by the coordination of neutral donor ligands exemplified as Cl.

As described in the Introduction section,[42,55,56,58,59] reported examples of analysis of (NMR and ESR-silent) vanadium(III) species as well as the study of homogeneous catalysis using solution XANES and EXAFS spectra are limited. As far as we know, this should be the first demonstration of the formation of different vanadium(III) species generated by reactions of vanadium complexes with Al alkyls through XAS analysis. We highly believe that the information should be potentially important for better understanding the study of the catalysis mechanism and organo-vanadium chemistry. Moreover, these data also demonstrate that the solution XAS analysis should be a powerful method for the obtainment of information concerning the catalytically active species in solution. More detailed analysis including the observation of V–Calkyl and V–CNHC bonds will be expected by the development of the analysis method (or measurements at low temperature), and the simulation of the active species on the basis of EXAFS spectra might also be possible; we thus expect further progress in this project.

Experimental Section

General Procedure

All experiments were conducted under a nitrogen atmosphere in a vacuum atmospheres dry box. Toluene, dichloromethane, and n-hexane of anhydrous grades (Kanto Kagaku Co., Ltd.) were transferred into a bottle containing molecular sieves (a mixture of 3A 1/16, 4A 1/8, and 13 × 1/16) in the dry box; solvents were passed through an alumina short column prior to use. V(NC6H5)Cl3[39] and V(N-2,6-Me2C6H3)Cl3[82] were prepared according to a published method. WCA-NHC-Ar′ (NHC-Ar′ = 1,3-bis(2,6-diisopropyl)-imidazolin-2-ylidene] and Li(WCA-NHC-Ar′)(toluene) were prepared according to the reported procedure.[64,83] AlBu3, Me2AlCl, and Et2AlCl (Kanto Kagaku Co., Ltd) and ethylene (polymerization grade, purity >99.9%, Sumitomo Seika Co. Ltd.) were used as received. Solid MAO (d-MAO) samples were prepared by the removal of toluene and AlMe3 from the commercially available TMAO [9.5 wt % (Al) toluene solution, Tosoh Finechem Co.] in the dry box under reduced pressure (at ca. 50 °C for removing toluene, AlMe3, and then heated at >100 °C for 1 h for the completion).[72−74]

All 1H, 13C, 19F, and 51V NMR spectra were recorded on a Bruker AV500 spectrometer (500.13 MHz for 1H, 125.77 MHz for 13C, 470.59 MHz for 19F, and 131.55 MHz for 51V). All spectra were recorded 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), CFCl3 (δ 0.00 ppm, 19F), and VOCl3 (δ 0.00 ppm, 51V). Coupling constants and half-width values, Δν1/2, are given in hertz. Elemental analyses were performed by an EAI CE-440 CHN/O/S elemental analyzer (Exeter Analytical, Inc.).

Synthesis of [V(NC6H5)Cl2(WCA-NHC-Ar′)] (4, Ar′ = 2,6-Pr2C6H3)

The basic synthetic procedure of 4 was analogous to that of [V(NC6H5)Cl2(WCA-NHC-Ar)] (2, Ar = 2,6-Me2C6H3),[47] except that Li(WCA-NHC-Ar′) was used in place of Li(WCA-NHC-Ar). Into a toluene solution (40 mL) containing V(NC6H5)Cl3 (177 mg, 0.713 mmol), Li(WAC-NHC-Ar′)(toluene) (715 mg, 0.716 mmol) was added at −30 °C. The reaction mixture was warmed slowly to room temperature, and the mixture was continuously stirred at room temperature overnight. The resultant mixture was passed through a Celite pad; the filter cake was washed with toluene. After the removal of volatiles from the combined filtrate and the wash, the resultant solid was dissolved in a minimum amount of dichloromethane. The chilled solution layered with n-hexane afforded red microcrystals in the freezer (−30 °C, 83 mg, 74.6 μmol, yield: 10%). 1H NMR (500 MHz, C6D6 at 25 °C): δ 7.19 (t, J = 7.9 Hz, 1H), 7.03 (d, J = 7.8 Hz, 2H), 6.80 (s, 1H), 6.61 (d, J = 7.6 Hz, 2H), 6.51–6.44 (m, 6H), 2.95 (sep, J = 6.6 Hz, 2H), 2.52 (sep, J = 6.5 Hz, 2H), 1.25 (d, J = 6.6 Hz, 6H), 1.17 (d, J = 6.4 Hz, 6H), 0.91 (d, J = 5.7 Hz, 12H). 13C NMR (125 MHz, C6D6 at 25 °C): δ 149.6 (dm, 1JC–F = 233 Hz, C6F5), 147.7 (s, N–C–N), 146.4 (br, CH=–B), 140.0 (dm, 1JC–F = 255 Hz, C6F5), 137.6 (dm, 1JC–F = 249 Hz, C6F5), 137.3 (s, Ar), 133.0 (s, Ar), 132.1 (s, Ar), 131.3 (s, H=C–B), 130.9 (s, Ar), 129.4 (br, ipso-C6F5), 127.5 (s, Ar), 127.1 (s, Ar), 125.1 (s, Ar), 124.4 (br, Ar), 29.6 (s), 28.4 (s), 27.8 (s), 26.9 (s), 22.8 (br), 21.2 (s). 19F NMR (470 MHz, C6D6 at 25 °C): δ −158.7, −164.7. 51V NMR (131 MHz, C6D6 at 25 °C): δ 256.6 (Δν1/2 = 1121.0 Hz). Anal. calcd for C51H40BCl2F15N3V: C, 55.06; H, 3.62; N, 3.78; found: C, 53.18; H, 3.81; N, 3.51. The structure of 4 could be determined by X-ray crystallographic analysis (CCDC 1949138).

Synthesis of [V(NAr)Cl2(WCA-NHC-Ar′)] (5, Ar = 2,6-Me2C6H3)

The basic synthetic procedure of 5 was analogous to that of [V(NAr)Cl2(WCA-NHC-Ar)] (3),[47] except that Li(WCA-NHC-Ar′) was used in place of Li(WCA-NHC-Ar). Into a toluene solution (40 mL) containing V(NAr)Cl3 (200 mg, 0.723 mmol), Li(WAC-NHC-Ar′)(toluene) (722 mg, 0.723 mmol) was added at −30 °C. The reaction mixture was continuously stirred at room temperature overnight. The resultant mixture was passed through a Celite pad; the filter cake was washed with toluene. After the removal of volatiles from the combined filtrate and the wash, the resultant solid was dissolved in a minimum amount of dichloromethane. The chilled solution layered with n-hexane afforded red microcrystals in the freezer (−30 °C, 116 mg, 102 μmol, yield: 14%). Yield 116 mg (0.102 mmol, 14%). 1H NMR (500 MHz, C6D6 at 25 °C): δ 7.22 (t, J =7.8 Hz,1H), 6.96 (t, J = 7.5 Hz, 3H), 6.74 (s, 1H), 6.71 (d, J = 7.7 Hz, 2H), 6.32 (t, J = 7.6 Hz, 1H), 6.22 (d, J = 7.6 Hz, 2H), 3.27 (sep, J = 6.2 Hz, 2H), 2.66 (sep, J = 6.5 Hz, 2H), 1.92 (m, 6H), 1.30 (s, 6H), 1.02 (s, 6H), 0.95 (d, J = 6.5, 6H), 0.76 (s, 6H). 13C NMR (125 MHz, C6D6 at 25 °C): δ 149.6 (dm, 1JC–F = 239 Hz, C6F5), 146.9 (s, N–C–N), 146.5 (br, CH=–B), 142.5 (s, Ar), 139.8 (dm, 1JC–F = 245 Hz, C6F5), 137.6 (dm, 1JC–F = 253 Hz, C6F5), 135.0 (s, Ar), 134.8 (s, Ar), 132.4 (s, Ar), 132.4 (br, H=C–B), 132.2 (s, Ar), 131.8 (s, Ar), 127.6 (s, Ar), 125.6 (s, ipso-C6F5), 124.8 (s, Ar), 28.9 (s), 28.6 (s), 26.8 (s), 23.5 (br), 21.4 (s), 18.8 (s). 51V NMR (131 MHz, C6D6 at 25 °C): δ 410.5 (Δν1/2 = 1450.0 Hz). Anal. calcd for C53H44BCl2F15N3V (+0.6 × n-hexane): C, 56.97; H, 4.40; N, 3.52; found: C, 57.14; H, 4.54; N, 3.46. The structure of 5 could be determined by X-ray crystallographic analysis (CCDC 1949093).

Typical Ethylene Polymerization Procedure

Into a stainless-steel autoclave (100 mL scale), toluene and the Al cocatalyst (total 29 mL) were added in the dry box. A toluene solution (1.0 mL) containing a prescribed amount of the catalyst (1.0 or 0.2 μmol/mL) was then added into the autoclave filled with ethylene (1 atm); the reactor was then pressurized immediately to 7 atm (total 8 atm), and the mixture was stirred magnetically for 10 min. After the reaction, the chilled mixture in the autoclave 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.

Analysis of Catalyst Solution by Solution-Phase X-ray Absorption Spectroscopy (XAS)

V K-edge X-ray absorption near-edge structure (XANES) and X-ray absorption fine structure (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, 2016B1509, 2017A1512, 2018A1245, and 2018B1335). V K-edge XAFS spectra of V complex samples [toluene solution, 50 μmol/mL, at 25 °C; a Si (111) two-crystal monochromator was used for the incident beam] 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 the REX2000 Ver. 2.5.9 software package (Rigaku Co.). The XANES data was analyzed using a cubic spline from the χ spectra with the removal of the atomic absorption background and their normalization to the edge height.

Crystallographic Analysis

The measurements for [V(NC6H5)Cl2(WCA-NHC-Ar′)] (4, CCDC 1949138) and [V(N-2,6-Me2C6H3)Cl2(WCA-NHC-Ar′)] (5, CCDC 1949093) were performed using a Rigaku XtaLAB P200 diffractometer with multilayer mirror monochromated Mo Kα radiation. The crystal collection parameters are given in the Supporting Information (see footnote a). The data were collected at 180 °C and processed using CrystalClear (Rigaku)[84] or CrysAlisPro (Rigaku Oxford Diffraction),[85] and the structure was solved by direct methods[86] and expanded using Fourier techniques. The nonhydrogen atoms were refined anisotropically, and some hydrogen atoms were refined using the riding model. All calculations were performed using the Crystal Structure[87] crystallographic software package, except for refinement, which was performed using SHELXL Version 2014/7.[88,89] Cif and xyz files are shown in the Supporting Information, and the crystallographic data was deposited at the Cambridge Crystallographic Data Centre (CCDC 1949093 and 1949138).