Synthesis of (Imido) Vanadium (V) Dichloride Complexes Containing

May 9, 2016 - Introduction .... It is interesting to note that the borate moiety in 1 is pointing “upward” with a syn-orientation toward the imido...
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Synthesis of (Imido)Vanadium(V) Dichloride Complexes Containing Anionic N‑Heterocyclic Carbenes That Contain a Weakly Coordinating Borate Moiety: New MAO-Free Ethylene Polymerization Catalysts Atsushi Igarashi,† Eugene L. Kolychev,‡ Matthias Tamm,*,‡ and Kotohiro Nomura*,† †

Department of Chemistry, Tokyo Metropolitan University, 1-1 Minami Osawa, Hachioji, Tokyo 192-0397, Japan Institut für Anorganische und Analytische Chemie, Technische Universität Braunschweig, Hagenring 30, 38106 Braunschweig, Germany

‡

S Supporting Information *

ABSTRACT: Synthesis and structural analysis of (imido)vanadium(V) complexes containing anionic N-heterocyclic carbenes with a weakly coordinating borate [B(C6F5)3] moiety (WCA-NHC) of the type [V(NR)Cl2(WCA-NHC)] [R = 1-adamantyl, C6H5, 2,6-Me2C6H3; NHC = 1,3-bis(2,6dimethylphenyl)imidazolin-2-ylidene] have been explored; the WCA-NHC forms a σ-bond with vanadium as alkyl ligand (without strong π-donation) on the basis of crystallographic analysis [V−CNHC = 2.039(3)−2.049(2) Å]. [V(N2,6-Me2C6H3)Cl2(WCA-NHC)] exhibited remarkable catalytic activity (e.g., 11000 kg-PE/mol-V·h) for ethylene polymerization in the presence of AliBu3, and the activity was much higher than those in the presence of MAO and Et2AlCl.

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INTRODUCTION Metal-catalyzed olefin polymerization is the key reaction especially in polyolefin industry, and the design of efficient molecular catalysts that precisely control olefin insertion/ propagation has been one of the most attractive subjects.1−6 It has been recognized that cationic alkyl species, which are generally formed by alkyl abstraction from the dialkyl analogues by reacting with methylaluminoxane (MAO), borate, etc., play a key role as the catalytically active species in this catalysis. It has also been postulated that a stabilization of the cationic species would lead to the higher catalytic activity;1,2,4,5 these species are stabilized by anionic donor ligands through σ- and/ or π-donation and/or by delocalization of the ligand(s) and/or large counteranions. Frustrated carbene−borane Lewis pairs, NHC-B(C6F5)3, are known to exhibit a very strong propensity for the activation of small molecules,7−11 and the related anionic N-heterocyclic carbenes with a weakly coordinating borate moiety (WCANHC, Figure 1)10,11 have been considered as a new class of valuable anionic ligands in transition metal complexes, as demonstrated by the preparation of zwitterionic Au,10 Rh, and Ir complexes A and B (Figure 1).11 The latter complexes of the type [(WCA-NHC)M(1,5-cyclooctadiene)] exhibited unique intramolecular arene−metal interactions with the N-aryl groups of the carbene ligands. In all cases, coordination through the borate-flanking aryl groups was observed, which is substantially stabilized by short intramolecular C−F···Cipso contacts involving the metal-bound ipso-carbon atoms. Noteworthy, their overall neutral charge allowed the iridium complexes to be used as highly active olefin hydrogenation catalysts in nonpolar solvent © XXXX American Chemical Society

Figure 1. Anionic N-heterocyclic carbenes with a weakly coordinating borate moiety (WCA-NHC) and selected zwitterionic transition metal complexes.7,9−11

or in neat alkene substrate, which is not possible for analogous cationic Crabtree-type catalysts.11 In addition, zwitterionic palldaium(II)−allyl complexes of type C with similar Pd−arene interactions were recently prepared and used as precatalysts for the amination of aryl halides.12 (Imido)Vanadium(V) complexes containing anionic ancillary donor ligands of the type V(NR)Cl2(X) (X = aryloxo, ketimide, imidazolin-2-iminato, imidazolidin-2-iminato, etc., Chart 1) are known to be highly active for ethylene polymerization in the Received: March 9, 2016

A

DOI: 10.1021/acs.organomet.6b00200 Organometallics XXXX, XXX, XXX−XXX

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vanadium species upon activation by Al cocatalyst(s). In particular, we wish to emphasize that the complex [V(NXy)Cl2(WCA-NHC)] (3) exhibits remarkable catalytic activity for ethylene polymerization in the presence of AliBu3.17

Chart 1. Basic Structure of (Imido)Vanadium(V) Complexes Containing Anionic Donor Ligands as Catalyst Precursors for Ethylene (Co)polymerization2e,13

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RESULTS AND DISCUSSION 1. Synthesis and Structural Analysis of the (Imido)Vanadium Complexes [V(NR)Cl2(WCA-NHC)]. (Imido)Vanadium(V) dichloride complexes of the type [V(NR)Cl2(WCA-NHC)] [R = 1-adamantyl (Ad, 1), C6H5 (Ph, 2), 2,6Me2C6H3 (Xy, 3), Scheme 1] were prepared in toluene by treating V(NR)Cl3 with the corresponding lithium salt [Li(WCA-NHC)(toluene)], which was prepared according to the previous report.11 These complexes were isolated as microcrystals from a cooled toluene (or CH2Cl2) solution layered with n-hexane (−30 °C) and were fully characterized by 1 H, 13C, 19F, and 51V NMR spectroscopy, elemental analysis, and X-ray structure determination (Figure 2).18 The signals in the 51V NMR spectra of 1 (45.4 ppm), 2 (185.6 ppm), and 3 (302.7 ppm) were observed at significantly lower field compared to the other (imido)vanadium(V) dichloride complexes containing imidazolin-2-iminato and imidazolidin-2-iminato ligands, which are usually observed in the range between −268 and −77 ppm.13g This pronounced deshielding can be ascribed to the cationic nature of the carbene−vanadium complex fragment, which is compensated intramolecularly by the anionic charge of the borate moiety. ORTEP drawings of the molecular structures of complexes 1−3 determined by X-ray diffraction analysis are shown in Figure 2, and selected bond distances and angles are summarized in Table 1.18 All complexes display distorted tetrahedral geometries around the vanadium, as observed in other (imido)vanadium(V) dichloride complexes of type V(NR)Cl2(X) [X = NCtBu2,16a O-2,6-Me2C6H3,13d 1,3R′2(CHN)2CN,13g,16d and 1,3-R′2(CH2N)2CN;13g R′ = t Bu, 2,6-iPr2C6H3, and C6H5].18 The carbene ligands adopt vertical orientations with an almost perfectly coplanar arrangement of the imidazole and Ccarbene−V−Nimido planes in 1 as indicated by a dihedral angle of 1.90°, whereas larger deviations are established for 2 (18.91°) and 3 (12.35°). It is interesting to note that the borate moiety in 1 is pointing “upward” with a

presence of Al cocatalysts [e.g., MAO or halogenated Al alkyls such as Et2AlCl, Me2AlCl, etc.].2e,13−15 In these complexes, the anionic ancillary donor ligands act as both σ- and π-donors that would contribute to stabilization of the proposed cationic alkyl species. The related vanadium(V)−alkylidene complexes [V(CHSiMe3)(NR)(X)(L)n] (L = PMe3, etc.) are known be highly efficient catalysts for the ring-opening metathesis polymerization (ROMP) of cyclic olefins.16 In this report, we thus focus on the synthesis and structural characterization of (imido)vanadium(V) complexes of the type [V(NR)Cl2(WCA-NHC)] [R = 1-adamantyl (Ad, 1), C6H5 (2), 2,6-Me 2 C 6 H 3 (Xy, 3); NHC = 1,3-bis(2,6dimethylphenyl)imidazolin-2-ylidene, Scheme 1], since the Scheme 1. Synthesis of V(NR)Cl2(WCA-NHC) [R = Ad (1), C6H5 (2), 2,6-Me2C6H3 (3)]

presence of the WCA-NHC ligands might stabilize electrondeficient, yet neutrally charged (and/or cationic) alkyl

Figure 2. ORTEP drawings for [V(NR)Cl2(WCA-NHC)] [R = Ad (1), Ph (2), Xy (3)]. Thermal ellipsoids are drawn at the 30% probability level, and H atoms are omitted for clarity. Detailed data are shown in the Supporting Information.18 B

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the V−C double bonds in vanadium(V)−alkylidene species [1.829(3)−1.866(5) Å)].16a,c−g These observations indicate that the WCA-NHC ligands in 1−3 act as strong σ-donors toward the cationic [V(NR)Cl2] complex fragment. The electron-deficient nature of the latter is also revealed by the relatively short V−Cl bond distances in 1−3, i.e., 2.1544(14)− 2.1747(15) Å, indicating increased π-donation from the chloride ligands in comparison with related dichloride complexes of the type [V(NR)Cl2(X)], in which the V−Cl bond lengths range from 2.1901(8) to 2.2462(8) Å for X = NCtBu2,16a O-2,6-Me2C6H3,13d 1,3-R′2(CHN)2CN,13g,16d and 1,3-R′2(CH2N)2CN13g (R′ = tBu, 2,6-iPr2C6H3, C6H5). In comparison with the V−N bond lengths reported for these complexes, the V−N(3) distances of 1.621(2) Å (1), 1.634(3) Å (2), and 1.648(2) Å (3) fall in the expected ranges, with 1 exhibiting the shortest bond length despite significant bending of the imido moiety as indicated by the V−N(3)−C(38) angle of 154.53(19)°. Pronounced deviation from a linear orientation is also observed for 2 and 3, albeit to a smaller extent, with V− N(3)−C(38) angles of 170.8(3)° and 164.14(17)°, respectively (Table 1). On the basis of the above structural analysis, it seems likely that WCA-NHC ligands in 1−3 bind to the vanadium atoms as strong σ-donors, which creates an electron-deficient, cationic 12-electron complex fragment. Although we can probably consider additional π-donation from the imido ligand, the absence of an efficient π-donor ancillary ligand, as for instance present in related aryloxo,13d ketimide,16a and imidazolin-2iminato13g,16d complexes, would be expected to afford highly reactive complexes with an enhanced electrophilicity that might have a considerable impact on the performance of these complexes as precatalysts for olefin polymerization. 2. Ethylene Polymerization Promoted by the (Imido)Vanadium Complexes [V(NR)Cl2(WCA-NHC)] in the Presence of Al Cocatalyst. Table 2 summarizes the results of ethylene polymerizations promoted by V(NR)Cl2(WCANHC) [R = Ad (1), Ph (2), Xy (3)] conducted in toluene in the presence of MAO20 or AliBu3. It turned out that complex 3 showed higher catalytic activity than the others (1 and 2) in the presence of MAO cocatalyst at 25 °C (runs 1−5), and the

Table 1. Selected Bond Distances and Angles for [V(NR)Cl2(WCA-NHC)] [R = Ad (1), C6H5 (2), 2,6Me2C6H3 (3)]a complex (R) 1 (Ad) V−Ccarbene V−N(1) V−Cl(1) V−Cl(2) N(1)−C(1) Bond Angles (deg) Ccarbene−V−Cl(1) Ccarbene−V−Cl(2) Ccarbene−V−N(1) Cl(1)−V−Cl(2) V−N(1)−C(1) a

Bond Distances 2.049(2) 1.621(2) 2.1673(9) 2.1558(9) 1.441(3)

2 (Ph)

3 (Xy)

(Å) 2.039(3) 1.634(3) 2.1607(13) 2.1544(14) 1.377(5)

2.045(2) 1.648(2) 2.1747(15) 2.1553(13) 1.382(3)

114.45(10) 112.66(10) 98.12(14) 117.74(6) 170.8(3)

113.14(8) 111.55(8) 102.39(9) 116.26(5) 164.14(17)

114.64(7) 112.28(7) 105.75(10) 116.41(4) 154.53(19)

Detailed data are shown in the Supporting Information.18

syn-orientation toward the imido group, whereas an antiorientation is found for 2 and 3 in the solid state. In all cases, πstacking between one C6F5 moiety and the neighboring N-2,6dimethylphenyl (Xy) substituent is observed, affording particularly short distances between the respective ipso-carbon atoms of ca. 3.07 Å. In contrast to the Rh(I), Ir(I), and Pd(II) systems B and C (Figure 1),11,12 intramolecular arene−metal interaction is not observed in 1−3, despite a somewhat asymmetric binding of the WCA-NHC ligands, which bend away from the imido unit due to a steric repulsion. Accordingly, the adamantylimido derivative 1 features the strongest asymmetry and therefore the shortest V−Cipso distance (3.098 Å); however, this distance is too long to be considered as an indication of vanadium−arene interaction. The V−Ccarbene bonds of 2.039(3)−2.049(2) Å are markedly shorter than those found in other NHC−vanadium complexes, cf. [V(CHSiMe3)(NAd)(CH2SiMe3)(NHC)] [V−Ccarbene = 2.172(2) Å]16c and [VOCl3(NHC)] (V−Ccarbene = 2.137 Å);19 they are similar to vanadium(V)−alkyl bond distances [2.0267(18)−2.069(3) Å],13g,16c,d but appreciably longer than

Table 2. Ethylene Polymerization by [V(NR)Cl2(WCA-NHC)] [R = Ad (1), Ph (2), Xy (3)]−Al Cocatalyst Systemsa run

cat. (μmol)

cocat.

Al/Vb

temp/ °C

yield/ mg

activityc

1 2 3 4 5 6 7 8 9 10

1 2 3 3 3 1 1 1 2 2

(1.0) (1.0) (1.0) (1.0) (1.0) (1.0) (1.0) (1.0) (1.0) (1.0)

d-MAO d-MAO d-MAO d-MAO d-MAO AliBu3 AliBu3 AliBu3 AliBu3 AliBu3

500 100 100 200 500 50 100 200 50 100

25 25 25 25 25 0 0 0 0 0

6 8 27 264 137 97 212 83 19 20

35 48 160 1580 824 582 1270 558 114 120

11 12 13 14 15

2 3 3 3 3

(1.0) (0.2) (0.2) (0.2) (0.2)

AliBu3 AliBu3 AliBu3 AliBu3 AliBu3

200 25 50 50 100

0 0 0 0 0

13 248 197 212 166

75 7430 5910 6360 4970

Mnd

Mw/Mnd

15 300

1.93

4530

2.67

411 000 2830

4.14 5.34

14 300

1.72

23 200

1.42

a Conditions: toluene 30 mL, ethylene 8 atm, 10 min. bMolar ratio of Al/V. cActivity in kg-PE/mol-V·h. dGPC data in o-dichlorobenzene vs polystyrene standards.

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Organometallics Table 3. Ethylene Polymerization by [V(NXy)Cl2(WCA-NHC)] (3)−Al Cocatalyst Systemsa run 12 16e 14 17e 18 19 20 21 22 23 24 25 26 27 a

cocat. i

Al Bu3 AliBu3 AliBu3 AliBu3 AliBu3 AliBu3 AliBu3 AliBu3 AliBu3 AlMe3 AlEt3 Et2AlCl Et2AlCl Et2AlCl

Al/Vb

temp/ °C

yield/ mg

activityc

Mnd

25 25 50 50 25 50 100 25 50 25 25 500 1000 2500

0 0 0 0 25 25 25 50 50 0 0 0 0 0

248 101 212 144 271 365 161 114 74.5 trace 31.8 36.4 46.8 65.8

7430 6030 6360 8610 8120 11 000 4830 3410 2240

14 300 22 100 23 200 21 800

1.72 1.38 1.42 1.44

18 000

1.76

16 500

1.79

13 100

1.89

954 1090 1400 1970

Mw/Mnd

insoluble

Conditions: complex 3 0.2 μmol, toluene 30 mL, ethylene 8 atm, 10 min (runs 16, 17 5 min). Molar ratio of Al/V. cActivity in kg-PE/mol-V·h. GPC data in o-dichlorobenzene vs polystyrene standards. ePolymerization time 5 min. b

d

chain transfer to Al. The assumption would be suggested by the fact that the Mn values were not significantly affected by varying the Al/V molar ratio. The activity of 3 was affected by the Al cocatalyst employed. The activity by 3 in the presence of AliBu3 (run 12) was higher than that in the presence of AlEt3 (run 24); the activity in the presence of AlMe3 was negligible (run 23). The activities by 3 in the presence of Et2AlCl (runs 25−27) were similar to that in the presence of MAO (run 4). The resultant polymer formed by 3−Et2AlCl was insoluble in ordinary GPC analysis in odichlorobenzene at 140 °C, suggesting formation of ultra-high molecular weight polymers.13c,g,h,14b The effect of Al cocatalyst demonstrated here is different from those observed by (imido)vanadium(V) dichloride complexes reported previously, which can be regarded as a unique characteristic of the WCANHC ancillary ligand introduced in this contribution. We have prepared (imido)vanadium(V) dichloride complexes containing an anionic N-heterocyclic carbene ligand with a weakly coordinating borate moiety (WCA-NHC). The X-ray crystallographic data reveal that the WCA-NHC ligand in 1−3 binds to vanadium as a strong σ-bond with the absence of πdonation as observed in the other anionic ancillary ligands (aryloxo, ketimide, imidazolin-2-iminato, etc.). This creates rather electron-deficient (12-electron) species, in which short V−Cl bond distances indicate a strong degree of π-donation from the chloride ligands. It is noteworthy that these complexes are capable of polymerizing ethylene even in the presence of AliBu3, which is usually an inefficient Al cocatalyst in olefin polymerization promoted by ordinary metallocenes, half-titanocenes, and previously reported (imido)vanadium(V) complexes. In particular, the activity shown by the 3−AliBu3 catalyst system is remarkable, affording linear polyethylene with unimodal molecular weight distribution (activity 11 000 kg-PE/mol-V·h, Mn = 18 000, Mw/Mn = 1.76, run 19). At present, we speculate that the reason for exhibiting such high activity could be ascribed to the stabilization of (formally) cationic alkyl species by the WCA-NHC ligand.21 We are now exploring the role of such alkyl complexes in more detail by attempting their isolation and characterization and by exploiting their potential to serve as single-source catalysts without the requirement of any additional (Al) cocatalysts.

activity was affected by the Al/V molar ratio. The polymer resulting from 3 possesses a rather high molecular weight with a unimodal molecular weight distribution (run 4), suggesting that the polymerization proceeded with uniform catalytically active species. It is particularly noteworthy that these complexes exhibited catalytic activities for ethylene polymerization in the presence of AliBu3 (in toluene at 0 °C, runs 6−15), although other (imido)vanadium(V) dichloride complexes exemplified by [V(NR)Cl2(O-2,6-Me2C6H3)]12 showed negligible catalytic activities under the same conditions. The activities were affected by the Al/V molar ratios, and the activity shown by 1−3 under the optimized Al/V molar ratio increased in the order 120 kg-PE/mol-V·h (2, R = Ph, run 10) < 1270 (1, R = Ad, run 7) < 7430 (3, R = Xy, run 12). As observed for the polymerization in the presence of MAO cocatalyst (run 4), the Xy analogue 3 showed the highest catalytic activity under these conditions. The resultant polymers prepared by 3 possess rather high molecular weights with unimodal molecular weight distributions (Mn = 14 300−23 200, Mw/Mn = 1.42−1.72). Although these Mn values were much lower than those prepared by the aryloxo analogues, e.g., [V(N-2,6-Me2C6H3)Cl2(O-2,6-Me2C6H3)],13a−c these results clearly suggest that the complexes showed unique catalyst behaviors, especially by exhibiting high catalytic activities even in the presence of AliBu3 (with small amount, 25−50 equiv, etc.), which was commonly considered as an ineffective cocatalyst in the polymerization not only using ordinary metallocenes1,2 and half-titanocenes,4 but also using the other (imido)vanadium(V) dichloride complexes with anionic ancillary donor ligands.2e,13 As summarized in Table 3, the activity by the 3−AliBu3 catalyst system was affected by the polymerization temperature (runs 12, 14, 18−22): the highest catalytic activity was observed at 25 °C under the optimized conditions (11 000 kg-PE/mol-V· h, run 19), but the activity decreased at 50 °C (runs 21, 22). Significant differences in the Mn values were not observed by varying the Al/V molar ratios as well as polymerization temperature (Mn = 14 300−23 200), and the distributions were unimodal (Mw/Mn = 1.38−1.79), which clearly suggest that polymerizations took place with uniform catalytically active species. The reason for the rather low Mn values may be explained by the assumption that the major chain transfer with these catalysts proceeds by β-hydrogen elimination and not by D

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Organometallics Table 4. Crystal Data and Collection Parameters of [V(NR)Cl2(WCA-NHC)] [R = Ad (1), Ph (2), Xy (3)]a formula fw cryst color, habit cryst size (mm) cryst syst space group a (Å) b (Å) c (Å) α (deg) β (deg) γ (deg) V (Å3) Z value Dcalcd (g/cm3) F000 temp (K) μ(Mo Kα) (cm−1) no. of reflns measd (Rint) 2θmax (deg) no. of observations [I > 2.00σ(I)] no. of variables R1 [I > 2.00σ(I)] wR2 [I > 2.00σ(I)] goodness of fit

1b

2b

3c

C50.50H38BCl2F15N3V 1104.49 red, block 0.20 × 0.19 × 0.18 monoclinic P21/n (#14) 12.5185(9) 18.3345(13) 20.9291(15) 90 104.275(3) 90 4655.3(6) 4 1.576 2236 93(2) 4.29 total: 48 872 unique: 10 648 (0.0327) 55.0 10 648 678 0.0559 0.1533 1.051

C53.50H36BCl2F15N3V 1138.50 red, block 0.300 × 0.060 × 0.040 monoclinic P21/n (#14) 14.696(3) 19.462(4) 18.195(4) 90 109.032(4) 90 4919.5(18) 4 1.537 2300 93(2) 4.09 total: 41 436 unique: 10 998 (0.0667) 55.0 10 998 682 0.0696 0.1582 1.167

C46H30BCl4F15N3V 1113.28 black, block 0.28 × 0.20 × 0.08 monoclinic P21/c (#14) 10.806(8) 20.402(13) 20.961(15) 90 101.884 90 4522(5) 4 1.635 2232 93(2) 5.57 total: 46 992 unique: 10 291 (0.0433) 55.0 10 291 650 0.0458 0.1201 1.081

a

Detailed structural data are shown in the Supporting Information.18 bStructures for 1 and 2 contain toluene. cStructure for 3 contains dichloromethane.

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was filtered and washed with toluene. The lithium salt containing toluene (521 mg) was obtained. Synthesis of [V(NAd)Cl2(WCA-NHC)] (1). Into a toluene solution (20 mL) containing V(NAd)Cl3 (151 mg, 0.493 mmol), lithium carbene complex (433 mg, prepared above) was added at −30 °C. The reaction mixture was warmed slowly to room temperature, and the mixture was then stirred overnight. The solution was passed through a Celite pad, and the filter cake was washed with toluene. The combined filtrate and the wash were placed in a rotary evaporator to remove volatiles. The resultant solid was dissolved in a minimum amount of toluene. The chilled solution layered with n-hexane placed in the freezer (−30 °C) afforded red microcrystals (380 mg, yield: 72.8%). 1 H NMR (C6D6): δ 6.90 (t, 1H, J = 7.85 Hz, Ar-H), 6.82 (t, 1H, J = 7.55 Hz, Ar-H), 6.82 (s, 1H, CHC-B), 6.68 (d, 2H, J = 7.55 Hz, ArH), 6.65 (d, 2H, J = 7.85 Hz, Ar-H), 1.96 (s, 6H, Ar-CH3), 1.93 (s, 6H, Ar-CH3), 1.53 (brs, 3H, Ad-H), 1.28 (brs, 6H, Ad-H), 1.11 (brd, 3H, J = 12.45 Hz, Ad-H), 1.01 (brd, 3H, J = 12.45 Hz, Ad-H). 13C NMR (C6D6): δ 149.1 (dm, J = 240 Hz), 139.73 (dm, J = 249 Hz), 137.9, 137.4 (dm, J = 236 Hz), 136.4, 134.8, 131.4, 130.8, 129.6, 129.6, 41.6, 34.8, 32.0, 29.6, 23.1, 18.8, 17.6, 14.3. 19F NMR (C6D6): δ −127.78 (d, J = 14.1 Hz), −159.06 (t, J = 18.8 Hz), −164.71. 51V NMR (C6D6): δ 45.39 (Δν1/2 = 1078 Hz). Anal. Calcd for C47H34BCl2F15N3V·0.6 (hexane): C, 54.74; H, 3.85; N, 3.79. Found: C, 55.03; H, 3.89; N, 3.76. Microcrystals for the X-ray crystallographic analysis were grown from the chilled toluene solution. Synthesis of V(NC6H5)Cl2(WCA-NHC) (2). The synthetic procedure for [V(NC6H5)Cl2(WCA-NHC)] (2) is similar to that in 1, except that V(NC6H5)Cl3 (202 mg, 0.813 mmol) in place of V(NAd)Cl3 and lithium carbene complex (721 mg) were used. The reaction mixture was passed through a Celite pad, and the filter cake was washed with toluene. The combined filtrate and the wash were placed in a rotary evaporator to remove volatiles. The resultant solid was dissolved in a minimum amount of toluene, and the chilled solution (−30 °C) afforded black microcrystals (120 mg, yield: 15%).

EXPERIMENTAL SECTION

General Procedure. All experiments were carried out under a nitrogen atmosphere in a Vacuum Atmospheres drybox. Anhydrousgrade toluene and n-hexane (Kanto Kagaku Co., Ltd.) were transferred into a bottle containing molecular sieves (a mixture of 3A 1/16, 4A 1/ 8, and 13X 1/16) in the drybox under a nitrogen stream and were passed through an alumina short column under a N2 stream prior to use. V(NAd)Cl3 (Ad = 1-adamantyl),13d V(NC6H5)Cl3,14a and V(N2,6-Me2C6H3)Cl322 were prepared according to a published method. NHC [NHC = 1,3-bis(2,6-dimethylphenyl)imidazolin-2-ylidene] was prepared according to the reported procedure23 and replaced dimethylaniline. AlMe3, AlEt3, AliBu3, and Et2AlCl were purchased from Kanto Kagaku Co., Ltd. Polymerization-grade ethylene (purity >99.9%, Sumitomo Seika Co. Ltd.) was used as received. Toluene and AlMe3 in the commercially available methylaluminoxane [TMAO, 9.5 wt % (Al) toluene solution, Tosoh Finechem Co.] were removed under reduced pressure (at ca. 50 °C for removing toluene and AlMe3 and then heated at >100 °C for 1 h for completion) in the drybox to give white solids.13,14 Elemental analyses were performed by using an 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, 470.59 MHz for 19F, 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), CFCl3 (δ 0.00 ppm, 19F), and VOCl3 (δ 0.00 ppm, 51V). Coupling constants and half-width values, Δν1/2, are given in Hz. Preparation of Lithium Carbene Complex.11 Into a toluene solution (20 mL) containing NHC [1,3-bis(2,6-dimethylphenyl)imidazolin-2-ylidene, 276 mg, 1.00 mmol] was added nBuLi (0.60 mL, 1.00 mmol, 1.67 M in n-hexane) in the drybox, and the mixture was stirred overnight. To the solution was then added B(C6F5)3 (512 mg, 1.00 mmol), and the mixture was stirred overnight. The precipitation E

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Organometallics

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H NMR (C6D6): δ 6.72 (s, 1H, CHC-B), 6.65 (t, 1H, J = 7.65 Hz, Ar-H), 6.50−6.37 (m, 10H, Ar-H), 1.94 (s, 6H, Ar-CH3), 1.89 (s, 6H, Ar-CH3). 13C NMR (C6D6): δ 149.2 (dm, J = 241 Hz), 139.8 (dm, J = 239 Hz), 137.4 (dm, J = 250 Hz), 137.9, 137.0, 136.6, 135.7, 131.8, 131.0, 129.3, 129.1, 129.1, 128.9, 128.6, 127.7, 126.3, 125.7, 53.3, 21.4, 18.6, 17.5, 17.0. 19F NMR (C6D6): δ −127.68, −158.92 (t, J = 21.2 Hz) −164.70. 51V NMR (C6D6): δ 185.56 (Δν1/2 = 960 Hz). Anal. Calcd for C43H24BCl2F15N3V·2(toluene): C, 57.79; H, 3.40; N, 3.55. Found: C, 58.04; H, 3.40; N, 3.56. Synthesis of V(N-2,6-Me2C6H3)Cl2(WCA-NHC) (3). The synthetic procedure for [V(N-2,6-Me2C6H3)Cl2(WCA-NHC)] (3) is similar to that in 1, except that V(N-2,6-Me2C6H3)Cl3 (225 mg, 0.813 mmol) in place of V(NAd)Cl3 and lithium carbene complex (721 mg) were used. The reaction mixture was passed through a Celite pad, and the filter cake was washed with toluene. The combined filtrate and the wash were placed in a rotary evaporator to remove volatiles. The resultant solid was dissolved in a minimum amount of CH2Cl2, and the solution was layered with n-hexane. The chilled solution (−30 °C) gave red microcrystals (216 mg, yield 26%). 1H NMR (C6D6): δ 6.73 (s, 1H, CHC-B), 6.70 (t, 1H, J = 7.65 Hz, Ar-H), 6.49 (d, 2H, J = 7.65 Hz, Ar-H), 6.41−6.34 (m, 3H, Ar-H), 6.29 (t, 1H, J = 7.65 Hz, Ar-H), 6.15 (d, 2H, J = 7.90 Hz, Ar-H), 2.10 (s, 6H, Ar-CH3), 1.94 (s, 6H, Ar-CH3), 1.90 (s, 6H, Ar-CH3) 13C NMR (C6D6): δ 149.2 (dm, J = 239 Hz), 142.4, 139.8 (dm, J = 250 Hz), 137.4 (dm, J = 253 Hz), 137.2, 136.7, 135.4, 135.3, 131.4, 131.0, 130.7, 129.0, 127.3, 126.9, 19.0, 18.9, 18.0. 19F NMR (C6D6): δ −127.57, −158.99 (t, J = 20.8 Hz), −164.75. 51V NMR (C6D6): δ 302.69 (Δν1/2 = 973 Hz). Anal. Calcd for C45H28BCl2F15N3V·0.65(CH2Cl2): C, 50.60; H, 2.73; N, 3.88. Found: C, 50.40; H, 2.69; N, 3.83. Ethylene Polymerization by [V(NR)Cl2(WCA-NHC)] [R = Ad (1), Ph (2), 2,6-Me2C6H3 (Xy, 3)]−Al Cocatalyst Systems. A typical procedure is as follows. Toluene and the aluminum cocatalyst were added into the autoclave in the drybox (total amount of solvent was 29 mL). The reaction apparatus was then filled with ethylene (1 atm), and a toluene solution (1.0 mL) containing the prescribed amount of catalyst (1.0 or 0.2 μmol/mL) was then added into the autoclave; the reaction apparatus was then immediately pressurized to 7 atm (total 8 atm), and the mixture was magnetically stirred for 10 (or 5) min. After polymerization, the chilled remaining 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. Crystallographic Analysis. The measurements for 1 and 3 were carried out on a Rigaku XtaLAB mini imaging plate diffractometer with graphite-monochromated Mo Kα radiation. The measurement for 2 was made on a Rigaku Micro Max-007HF imaging plate diffractometer with graphite-monochromated Mo Kα radiation. The crystal collection parameters are given in Table 4. All structures were solved by direct methods and expanded using Fourier techniques, and the nonhydrogen atoms were refined anisotropically.24,25 Hydrogen atoms were refined using the riding model. All calculations were performed using the Crystal Structure26 crystallographic software package, except for refinement, which was performed using SHELXL-97.27 1

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Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This project was partly supported by a 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 a Grant-in-Aid for Scientific Research (B) from the Japan Society for the Promotion of Science (JSPS, No. 15H03812). This project was also partly supported by TA 189/9-1 from the Deutsche Forschungsgemeinschaft (DFG). A.I. expresses his thanks to JSPS for a predoctoral fellowship (DC2 26·7313). We wish to thank Prof. Dr. Peter G. Jones and Dr. Matthias Freytag (Technische Universität Braunschweig) for crystallographic support. A.I. and K.N. express their thanks to Prof. K. Tsutsumi (Tokyo Metropolitan University) for technical support and discussion.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.organomet.6b00200. Structure reports for [V(NR)Cl2(WCA-NHC)] [R = 1adamantyl (1), C6H5 (2), 2,6-Me2C6H3 (3)] (PDF) Crystallographic data for 1−3 (CIF)

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REFERENCES

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DOI: 10.1021/acs.organomet.6b00200 Organometallics XXXX, XXX, XXX−XXX

Article

Organometallics

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DOI: 10.1021/acs.organomet.6b00200 Organometallics XXXX, XXX, XXX−XXX