Synthesis of Alkyl and Alkylidene Complexes of Tungsten Bearing

Article pubs.acs.org/Organometallics

Synthesis of Alkyl and Alkylidene Complexes of Tungsten Bearing Imido and Redox-Active α‑Diimine or o‑Iminoquinone Ligands and Their Application as Catalysts for Ring-Opening Metathesis Polymerization of Norbornene Hiromasa Tanahashi, Hayato Tsurugi,* and Kazushi Mashima* Department of Chemistry, Graduate School of Engineering Science, Osaka University and CREST, JST, Toyonaka, Osaka 560-8531, Japan S Supporting Information *

ABSTRACT: We report a new strategy to synthesize tungsten imido complexes bearing bidentate redox-active ligands through reduction of high-valent tungsten imido complexes by 1-methyl-3,6-bis(trimethylsilyl)-1,4-cyclohexadiene (abbreviated MBTCD) without forming any metal salt waste. Reaction of W(NC6H3-2,6-iPr2)Cl4 and MBTCD in the presence of redox-active ligands, such as α-diimine and oiminoquinone, produced tungsten imido complexes with the corresponding redox-active ligands, (α-diimine)W(NC6H32,6-iPr2)Cl2 (1), [(o-iminoquinone)W(NC6H3-2,6-iPr2)Cl]2(μ-Cl)2 (3), and (o-iminoquinone)W(NC6H3-2,6-iPr2)Cl2(THF) (4), along with Me3SiCl and toluene as whole byproducts. Reaction of the brown complex 1 with [nBu4N][Cl] afforded intensely green single crystals of [nBu4N][(α-diimine)W(NC6H3-2,6-iPr2)Cl3] (2). The versatile coordination modes of the α-diimine and o-iminoquinone ligands were clarified by spectroscopic methods and X-ray diffraction studies. Treatment of complex 1 with 1 equiv of Mg(CH2Ph)2·Et2O resulted in the formation of (α-diimine)W(NC6H3-2,6-iPr2)(CH2Ph)2 (5), and thermolysis of 5 in the presence of PMe2Ph at 80 °C afforded the alkylidene complex (α-diimine)W(NC6H3-2,6-iPr2)( CHPh)(PMe2Ph) (6). On the other hand, thermolysis of 5 in the presence of CCl4 afforded the dissymmetric benzylidene complex (Cl3C-amido-imino)W(NC6H3-2,6-iPr2)(CHPh)Cl (7) via reductive cleavage of the C−Cl bond of CCl4. Isolated alkylidene complexes 6 and 7 served as catalysts for ring-opening metathesis polymerization of norbornene with 1 mol% of catalyst loading in toluene at 80 °C. Treatment of o-iminoquinone complex 4 with 2 equiv of LiCH2CMe2Ph afforded the dialkyl complex (o-iminoquinone)W(NC6H3-2,6-iPr2)(CH2CMe2Ph)2 (8). Dialkyl complexes 5 and 8 at 80 °C served as catalysts to give poly(norbornene) with rather broad polydispersity.

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breviated α-diimine) ligands to the imido alkylidene fragment of tungsten to introduce redox flexibility and versatility in coordination modes, i.e. neutral, monoanionic, σ2-enediamido, and σ2,π-enediamido modes, which affect the electron density and Lewis acidity of the metal center (Chart 1).11,12 In addition, structural changes of the ligand are possible via C−C coupling reactions between coordinated α-diimine ligands and organic fragments, leading to new metal complexes.13 Such reactivity of the α-diimine ligands prompted us to combine the α-diimine ligands with imido alkylidene fragments of tungsten as potent candidates for a new class of metathesis catalysts.

INTRODUCTION Imido alkylidene complexes of tungsten and molybdenum have attracted great interest, due to their versatility as catalysts for olefin metathesis reactions and their fundamental structural features.1 The steric and electronic properties of the supporting ligands on the catalysts have been finely tuned to control both the reactivity and stereochemistry toward olefin metathesis reactions. Alkoxy and diolate ligands are often used as supporting ligands for highly active homogeneous metathesis catalysts.1−4 Recently, imido complexes of tungsten and molybdenum bearing a σ-donor pyrrolyl ligand showed higher catalytic activity and stereoselectivity than traditional Schrocktype catalysts.5 Redox-active ligands were recently demonstrated to govern unique catalytic performance upon combination with transition metals,6−9 allowing the metal center to store excessive electrons and accept electrons on the ligand. However, there has been no report for the tungsten metathesis catalysts focusing on redox flexibility of the ligands, though structurally similar (o-phenylenediamido)metal complexes were reported.10 Therefore, we have been interested in the coordination of redox-active 1,4-diaza-1,3-butadiene (ab© XXXX American Chemical Society

Chart 1. Bidentate Coordination Modes of 1,4-Diaza-1,3butadiene Ligands to Transition Metals

Received: October 4, 2014

A

DOI: 10.1021/om501010n Organometallics XXXX, XXX, XXX−XXX

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Organometallics Well-established methods to introduce anionic α-diimine ligands into early-transition-metal complexes are classified into three categories: (i) reduction of high-valent early-transitionmetal complexes with metal reducing reagents in the presence of neutral α-diimine ligands,14 (ii) salt-metathesis reaction of high-valent early transition metals with alkaline- and alkalineearth-metal salts of α-diimine ligands,15 and (iii) reaction of isolated low-valent early-transition-metal complexes with neutral α-diimine ligands.16 Methods i and ii have been typically used in the last several decades, but they are accompanied by metal salt waste that may not only hamper the isolation of the desired complexes but also form ate complexes.15a,g,17 Method iii is considered ideal, using electron transfer from low-valent early-transition-metal complexes to neutral α-diimine ligands without any metal salt contamination, although only a few isolated low-valent early-transition-metal complexes are applicable to this method. Accordingly, the development of a versatile metal salt free synthetic method is highly desirable. We recently reported a new synthetic salt-free methodology to synthesize early-transition-metal complexes bearing redox-active α-diimine ligands through reduction of high-valent early-transition-metal chlorides by 1-methyl-3,6bis(trimethylsilyl)-1,4-cyclohexadiene (abbreviated MBTCD).13a,18,19 An advantage of MBTCD is that easily removable Me3SiCl and toluene are the sole byproducts in the reduction of early-transition-metal chlorides, as exemplified by the reduction of WCl6 with MBTCD in the presence of a neutral α-diimine ligand to give (α-diimine)WCl4.18 These achievements using a salt-free synthetic methodology prompted us to prepare tungsten imido complexes bearing redox-active ligands. Herein, we synthesized (α-diimine)W(NC6H32,6-iPr2)Cl2 (1), [(o-iminoquinone)W(NC6H3-2,6-iPr2)Cl]2(μ-Cl)2 (3), and (o-iminoquinone)W(NC6H3-2,6-iPr2)Cl2(THF) (4) by reducing W(NC6H3-2,6-iPr2)Cl4 with MBTCD in the presence of α-diimine and o-iminoquinone ligands (o-iminoquinone = N-(2,6-diisopropylphenyl)phenanthrene-o-iminoquinone). The unique coordination modes of the redox-active ligands were clarified by spectroscopic methods and X-ray diffraction studies. Dichloro complexes 1 and 4 were treated with Mg(CH2Ph)2·Et2O and LiCH2CMe2Ph, respectively, to give the corresponding dialkyl complexes (α-diimine)W(NC6H3-2,6-iPr2)(CH2Ph)2 (5) and (o-iminoquinone)W(NC6H3-2,6-iPr2)(CH2CMe2Ph)2 (8). Thermolysis of 5 in the presence of PMe2Ph at 80 °C afforded an alkylidene complex, (α-diimine)W(NC6H32,6-iPr2)(CHPh)(PMe2Ph) (6), along with 1 equiv of toluene. In addition, complex 5 reacted with CCl4 at 60 °C to give the dissymmetric alkylidene complex (Cl3C-amidoimino)W(NC6H3-2,6-iPr2)(CHPh)Cl (7). Isolated alkylidene complexes 6 and 7 showed catalytic activity for ringopening metathesis polymerization (ROMP) of norbornene.

sharp contrast, when a salt-elimination reaction of W( NC6H3-2,6-iPr2)Cl4 with [(α-diimine)K2(μ-THF)(THF)4]12f,g in THF at −78 °C was conducted, we obtained a complicated reaction mixture from which no complexes, including 1, were isolated. The σ2,π-enediamido mode of the α-diimine ligand in complex 1 was elucidated by NMR spectral data: the 1H NMR spectrum displayed a singlet signal at δ 6.13 assignable to the olefinic proton of the dianionic α-diimine ligand that coordinated in a basal plane to the tungsten atom, and the 13 C{1H} NMR spectrum showed a carbon signal at δ 133.5 in the chemical shift of a typical olefinic carbon. Furthermore, since two conformations, i.e., supine and prone, were predicted for the σ2,π-enediamido ligand relative to the imido ligand, we conducted NOESY experiments to determine which conformation was adopted by complex 1. Protons due to iPr groups on the imido ligand were correlated to the other iPr group on the α-diimine ligand, suggesting that the α-diimine ligand on 1 had a supine conformation.12a,b,20 Because complex 1 could not be crystallized, we quantitatively prepared the anionic trichloride complex 2 by treating 1 with [nBu4N][Cl], from which intensely green single crystals of 2 were obtained and characterized crystallographically (eq 2). Transfer of an additional chloride anion to

the tungsten atom turned the coordination mode of the enediamido ligand from a symmetric coordination of 1 to a dissymmetric coordination of 2, on the basis of NMR spectroscopy. The 1H NMR spectrum of 2 displayed two doublet signals centered at δ 5.22 and 6.60 (3JH−H = 2.4 Hz) due to olefin protons of the enediamido moiety, while the 13 C{1H} NMR spectrum showed two signals at δ 133.4 and 143.4 attributed to the olefin carbons, clearly indicating the dissymmetric coordination of the enediamido ligand to the tungsten atom. In contrast to a band (λmax 310 nm) for 1, the UV−vis spectrum of 2 in toluene showed an intense LMCT band (λmax 621 nm), indicating that the filled π orbital at the nitrogen atoms donated to the empty dπ* orbital of the tungsten center.12b The structural features of 2 were confirmed by X-ray diffraction studies. Figure 1 shows an octahedral geometry around the tungsten atom supported by one imido, two nitrogen atoms of the enediamido ligand, and three chloride ligands. Selected bond distances and angles of 2 are summarized in Table 1. The W−N3 bond distance (1.760(10) Å) and the W−N3−C1 angle (172.9(10)°) are typical for the 6e-donating imido ligand to the metal center.21 The W−N1 (2.249(10) Å) and W−N2 distances (2.033(10) Å) lie in the range of W−N single-bond distances, though the

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RESULTS AND DISCUSSION Synthesis of Tungsten Imido Complexes Bearing αDiimine or o-Iminoquinone Ligands. We introduced αdiimine or o-iminoquinone ligands onto a fragment of Narylimido tungsten using a salt-free reduction procedure. Treatment of W(NC6H3-2,6-iPr2)Cl4 with 1 equiv of MBTCD in the presence of α-diimine in toluene at 80 °C for 12 h afforded dichloro complex 1 as a brown powder in 89% yield (eq 1), in which MBTCD worked as a reducing reagent for W(NC6H3-2,6-iPr2)Cl4 to give a nascent “W(NC6H32,6-iPr2)Cl2” species that was trapped by α-diimine.13a,18 In B


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Organometallics

included a UV−vis spectrum that displayed a typical LMCT band at 629 nm, indicating that 3 also adopted a metallacycle structure where there is no direct π interaction between the tungsten center and two carbons of the o-iminoquinone ligand. Complex 3 was silent to 1H NMR measurements even if cooled down to −50 °C. This was attributed to the partial paramagnetic nature of 3 in which the two electrons in each redox-active o-iminoquinone ligand were delocalized over the metal center in solution.18 Thus, its structure was determined by a single-crystal X-ray analysis. Figure 2 shows the chloridebridged dinuclear structure of 3, and the selected bond distances and angles are summarized in Table 2. The geometry

Figure 1. Molecular structure of 2 with 30% thermal ellipsoids. The anionic part and the hydrogen atoms are omitted for clarity.

Table 1. Selected Bond Distances (Å) and Angles (deg) for Complex 2 W−N1 W−N3 W−Cl2 N1−C13 C13−C14 dihedral anglea

2.249(10) 1.760(10) 2.427(4) 1.359(16) 1.384(17) 11.97

W−N2 W−Cl1 W−Cl3 N2−C14 W−N3−C1

2.033(10) 2.413(4) 2.427(4) 1.362(15) 172.9(10)

a

Dihedral angle: dihedral angle between the N1−W−N2 and N1− C13−C14−N2 planes.

Figure 2. Molecular structure of complex 3 with 30% thermal ellipsoids. The solvent and the hydrogen atoms are omitted for clarity.

W−N1 distance is longer than that of W−N2 due to the trans influence of the imido ligand. The elongated N−C distances (N1−C13 = 1.359(16) Å and N2−C14 = 1.362(15) Å) and the shortened C−C distance (C13−C14 = 1.384(17) Å) in the αdiimine ligand clearly indicate the dianionic ligation of the ligand in an enediamido mode in a typical long−short−long sequence to the metal center.13a,18 The dihedral angle between the best planes of N1−W−N2 and N1−C13−C14−N2 is 11.97°, consistent with the metalla-2,5-diazacyclopent-3-ene structure in which the metal center has no direct π interaction with the two carbons (C13 and C14) of the α-diimine ligand backbone.12b The coordinatively unsaturated metal center was ultimately stabilized by the donation of the filled N(pπ) orbital to the empty W(dπ*) orbital based on observation of the LMCT band at 621 nm (vide supra). We next focused on expanding the applicability of MBTCD to the preparation of a tungsten imido complex bearing a dianionic o-iminoquinone ligand using the same strategy as in the preparation of 1.22 Reaction of W(NC6H3-2,6-iPr2)Cl4 with 1 equiv of MBTCD in the presence of o-iminoquinone in toluene at 80 °C for 12 h resulted in the formation of a green powder of complex 3 in 91% yield (eq 3). Notable spectral data

Table 2. Selected Bond Distances (Å) and Angles (deg) for Complex 3 W−N1 W−O1 W−Cl2 N1−C2 C1−C2 W···W*

1.989(6) 2.068(5) 2.4672(17) 1.402(9) 1.407(9) 3.8023(4)

W−N2 W−Cl1 W−Cl2* O1−C1 W−N2−C27 dihedral anglea

1.775(6) 2.3432(18) 2.4960(16) 1.315(9) 174.5(5) 11.20

a Dihedral angle: dihedral angle between the O1−W−N1 and O1− C1−C2−N1 planes.

around the tungsten center is octahedral with an oxygen atom of o-iminoquinone trans to the imido ligand. The W−N1 (1.989(6) Å) and W−O1 distances (2.068(5) Å) reflect the σbond character of the W−N and W−O bonds. The N1−C2 (1.402(9) Å) and O1−C1 distances (1.315(9) Å) are longer than those of normal CN and normal CO bonds, and the C1−C2 distance (1.407(9) Å) lies in the range of aromatic C− C bonds, indicating the dianionic ligation of the oiminoquinone ligand in an amidophenolate mode in the solid-state structure. Two imido fragments in the dinuclear unit are directed trans to each other. The small dihedral angle (11.20°) between the best planes of O1−W−N1 and O1−C1− C2−N1 confirms the σ2-amidophenolate mode of the dianionic ligand without direct π interaction between the tungsten center and two carbons of the o-iminoquinone ligand. The long distance (3.8023(4) Å) between the doubly chloride bridged two tungsten centers suggests no bonding interaction between them.23 C


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Organometallics

tungsten atom shows the geometry of 4 as octahedral, and two chloride atoms are placed trans to each other. The coordination environment around the tungsten atom is essentially the same as that found for complex 3, where the tungsten atom keeps the dianionic σ2-amidophenolate ligand and the 6e-donating imido ligand intact. Preparation of Alkyl and Alkylidene Complexes. To synthesize alkylidene complexes from these imido tungsten chloride complexes, we began by preparing dibenzyl complex 5, which was obtained as a green powder in 75% yield by the reaction of complex 1 with 1 equiv of Mg(CH2Ph)2·Et2O in toluene (eq 5). The complex 5 showed fluxional behavior in

The mononuclear complex 4 was isolated in 77% yield as a green powder by the reaction of W(NC6H3-2,6-iPr2)Cl4 with 1 equiv of MBTCD in the presence of o-iminoquinone in THF at 55 °C for 20 h (eq 4). In contrast, simply dissolving the

dinuclear complex 3 in THF afforded the mononuclear complex 4 with the contamination of unreacted complex 3, which hampered the isolation of complex 4. The 1H NMR spectrum of 4 in C6D6 displayed broad signals at δ 1.28 and 4.35 assignable to the coordinated THF. In the 13C{1H} NMR spectrum of 4, we observed resonances due to the solution: in the 1H NMR in toluene-d8 at −50 °C, two AB quartet signals appeared at δ 2.60 and 2.78 (2JH−H = 13.6 Hz) and δ 3.43 and 3.73 (2JH−H = 8.8 Hz) due to two magnetically nonequivalent benzyl groups on complex 5, and each benzyl Hα

Figure 3. Molecular structure of complex 4 with 30% thermal ellipsoids. The solvent and the hydrogen atoms are omitted for clarity.

Table 3. Selected Bond Distances (Å) and Angles (deg) for Complex 4 W−N1 W−O1 W−Cl1 N1−C2 C1−C2 dihedral anglea

1.977(6) 2.057(5) 2.362(2) 1.426(8) 1.388(9) 14.26

W−N2 W−O2 W−Cl2 O1−C1 W−N2−C27

Figure 4. Molecular structure of complex 5 with 30% thermal ellipsoids. The hydrogen atoms are omitted for clarity.

1.767(6) 2.163(5) 2.368(3) 1.300(8) 178.6(5)

signal coalesced at −5 °C, from which ΔG⧧(268) = 11.7 kcal/ mol was obtained at the coalescence.24 The UV−vis spectrum of 5 in toluene displayed an absorption around 381 nm, though the structure of 5 in the solid state assumed a flat metallacycle form (vide infra).


Table 4. Selected Bond Distances (Å) and Angles (deg) for Complex 5

phenanthrene backbone at the aromatic region but no signals due to carbonyl carbon, clearly indicating that the oiminoquinone ligand coordinated to the tungsten center in a σ2-amidophenolate mode. The UV−vis spectrum of 4 exhibited a typical LMCT band at 620 nm, indicating the interaction of pπ orbitals of nitrogen and oxygen atoms on the oiminoquinone ligand with the dπ* orbital of the tungsten center. The X-ray diffraction study of 4 revealed its octahedral geometry as depicted in Figure 3, and selected bond distances and angles are given in Table 3. Coordination of THF to the

W−C39 W−N1 W−N3 N2−C2 W···C40 W−C39−C40 W−N3−C3

2.172(11) 2.175(8) 1.779(9) 1.411(13) 2.70(1) 93.1(6) 173.6(8)

W−C46 W−N2 N1−C1 C1−C2 W···C47 W−C46−C47 dihedral anglea

2.196(10) 1.977(8) 1.413(13) 1.327(15) 3.22(1) 121.4(8) 9.93

a

Dihedral angle: dihedral angle between the N1−W−N2 and N1− C1−C2−N2 planes. D


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Organometallics

The ORTEP drawing of the molecular structure of the alkylidene complex 6 is shown in Figure 5, and selected bond

The molecular structure of 5 was determined by X-ray diffraction study, as shown in Figure 4, and the geometrical parameters are summarized in Table 4. The tungsten atom possesses a five-coordinated trigonal-bipyramidal geometry: the nitrogen atom of the imido ligand and one of two nitrogen atoms of the enediamido ligand are located at the axial positions, and two carbon atoms and the other nitrogen atom of the enediamido ligand are located in the trigonal plane. The enediamido ligand coordinates to the tungsten center in a flat metallacycle form without any direct π interaction between the tungsten center and two carbons of the α-diimine ligand backbone: the dihedral angle between the N1−W−N2 and N1−C1−C2−N2 planes is 9.93°. The shorter C1−C2 bond distance (1.327(15) Å) and the longer N1−C1 (1.413(13) Å) and N2−C2 distances (1.411(13) Å) in comparison with the corresponding distances of 2 might be due to the electrondonating benzyl ligands. Notably, one of the two benzyl ligands coordinates to the tungsten atom in an η2 fashion to compensate for the coordinative unsaturation, as is evident from the acute W−C39−C40 angle (93.1(7)°) and the short W−C40 distance (2.70(1) Å).25 Thermolysis of 5 in the presence of PMe2Ph at 80 °C for 1 h in toluene afforded the alkylidene complex 6 in 85% yield along with the release of 1 equiv of toluene (eq 6). In the 1H NMR


Table 5. Selected Bond Distances (Å) and Angles (deg) for Complex 6 W−C39 W−N2 W−P1 N2−C2 W−C39−C40 dihedral anglea

spectrum of 6, Hα of the benzylidene moiety was observed at δ 12.3 as a doublet (3JH−P = 9.6 Hz) due to the adjacent phosphorus atom, the coupling constant of which showed the coordination of PMe2Ph cis to the benzylidene moiety.26 The 13 C NMR spectrum displayed a resonance of the benzylidene carbon as a multiplet signal at δ 257.8 (1JC−H = 133 Hz, 2JC−P = 16 Hz, 1JC−W = 157 Hz). In general, tungsten and molybdenum imido alkylidene complexes have syn and anti conformations: in the syn isomer, the alkylidene substituent lies in the N(imido)−M−C(alkylidene) plane and points toward the imido nitrogen atom, while in the anti isomer, the alkylidene substituent lies in the N(imido)−M−C(alkylidene) plane and points away from the imido nitrogen atom.27 Furthermore, DFT calculations on imido alkylidene complexes of tungsten and molybdenum confirmed that the presence of a C−H agostic interaction crucially increased the stability of the syn isomer more than that of the corresponding anti isomer with no such C−H agostic interaction.28 The relatively large JC−H (133 Hz) and small JC−W values (157 Hz) of the benzylidene carbon resonance of complex 6 suggested no agostic interaction between the tungsten center and the alkylidene C−H bond, judged as an anti conformation. We assumed that the anti conformation of complex 6 was due to the steric hindrance of PMe2Ph. The 31P{1H} NMR spectrum of 6 showed a resonance of PMe2Ph coordinated to the tungsten center at δ 4.38 (1JP−W = 128 Hz).26b When thermolysis of complex 5 was conducted in the presence of PPh3 or PCy3 with cone angles (145 and 170°, respectively) larger than that of PMe2Ph (122°), or without any supporting ligand, no alkylidene complexes were isolated.

1.982(9) 2.101(7) 2.531(2) 1.381(10) 131.6(6) 3.44

W−N1 W−N3 N1−C1 C1−C2 W−N3−C27

2.081(7) 1.784(8) 1.394(11) 1.332(13) 176.8(7)

a Dihedral angle: dihedral angle between the N1−W−N2 and N1− C1−C2−N2 planes.

distances and angles are summarized in Table 5. The geometry around the tungsten center is trigonal bipyramidal with one of two nitrogen atoms of the enediamido ligand and PMe2Ph at the axial positions. The W−C39 distance (1.982(9) Å) is within the range typically observed for WC bonds.29 The anti conformation around the alkylidene moiety toward the imido ligand is consistent with the NMR observations (vide supra). The long−short−long sequence of the enediamido backbone for N1−C1, C1−C2, and N2−C2 indicates a two-electron reduction of the α-diimine ligand to form the W(VI) alkylidene complex 6. The dihedral angle between the best planes of N1− W−N2 and N1−C1−C2−N2 is 3.44°, indicating that the tungsten center does not interact with C1 and C2 of the αdiimine ligand backbone. Similar to the reactivity of (α-diimine)TaCl3 toward alkyl halides,13a dibenzyl complex 5 reacted with CCl4 to give new organometallic species. Upon stirring the mixture of dialkyl complex 5 and 1.2 equiv of CCl4 in toluene at 60 °C, we obtained a CCl3-added alkylidene complex 7 together with 1 equiv of toluene (eq 7). The 1H NMR spectrum of 7 displayed singlet resonances at δ 5.63 and 9.15, respectively, due to an amine proton and an imine proton of the Cl3C-amido-imino ligand. The Hα resonance of the alkylidene moiety was observed at δ 9.94, shifted upfield in comparison with that of complex 6 (δ 12.3). The 13C NMR spectrum displayed a resonance of the benzylidene carbon as a singlet signal at δ E


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Organometallics

Table 6. Selected Bond Distances (Å) and Angles (deg) for Complex 7 W−C40 W−N2 W−Cl1 N2−C2 C1−C3

281.0 with coupling constants of 1JC−H = 116 Hz and 1JC−W = 172 Hz, consistent with the syn conformation of 7. The stereochemistry at the methine next to CCl3 could not be determined in the NMR data but was revealed by an X-ray diffraction study (vide infra). Scheme 1 shows a possible

1.937(12) 2.320(9) 2.361(3) 1.279(15) 1.564(15)

W−N1 W−N3 N1−C1 C1−C2 W−N3−C28

2.031(10) 1.734(9) 1.480(15) 1.487(17) 173.8(8)

which the alkylidene moiety locates in a position trans to the imino nitrogen, N2. The W−C40 distance (1.937(12) Å) is a typical WC distance. The long N1−C1 (1.480(15) Å) and C1−C2 distances (1.487(17) Å) and short N2−C2 distance (1.279(15) Å) indicate the amido-imino structure of the monoanionic bidentate ligand. The W−N1 distance (2.031(10) Å) reflects the σ-bond character of the W−N bond. Next, we conducted alkylation of o-iminoquinone complex 4 using LiCH2CMe2Ph as an alkylation reagent. Treatment of complex 4 with 2 equiv of LiCH2CMe2Ph in toluene afforded dialkyl complex 8 in 52% yield as a green powder (eq 8). Two

Scheme 1. Plausible Mechanism for Generation of Complex 7

reaction pathway beginning with α-hydrogen elimination, followed by the release of toluene to give a coordinatively unsaturated four-coordinated alkylidene species that activated a C−Cl bond of CCl4 to generate a tungsten chloride alkylidene complex of a π-radical monoanionic α-diimine ligand and a CCl3 radical and then attack of the CCl3 radical at the monoanionic α-diimine backbone to afford complex 7. The syn conformation of the alkylidene complex 7 was clarified by X-ray diffraction studies, as shown in Figure 6, where the phenyl group and CCl3 point up toward the imido moiety. Selected bond distances and angles are summarized in Table 6. The tungsten atom adopts a square-pyramidal geometry with an imido ligand in the apical position, in

doublet signals of Hα of the neophyl groups were observed at δ 1.41 and 3.70 (2JH−H = 11.6 Hz) in the 1H NMR spectrum of complex 8. Two singlet signals of the methyl moieties were observed at δ 1.26 and 1.61. In the 13C{1H} NMR spectrum, a signal of Cα of the neophyl groups was observed at δ 97.3 and two singlet signals of the methyl carbons were observed at δ 32.7 and 33.3. These NMR observations clearly indicated that the neophyl groups of complex 8 were magnetically equivalent. The UV−vis spectrum of 8 in toluene displayed absorption around 338 and 400 nm, though the structure of 8 in the solid state had a flat metallacyclic form (vide infra). Thermal treatment of complex 8 in the presence of PMe2Ph produced tert-butylbenzene, but no alkylidene complex was isolated. The X-ray diffraction study of complex 8 revealed the trigonal-bipyramidal molecular geometry with an imido ligand and oxygen atom of o-iminoquinone in axial positions, as shown in Figure 7. A longer−shorter−longer sequence of the ligand backbone (O1−C1 = 1.338(4) Å, C1−C2 = 1.364(6) Å, and N1−C2 = 1.430(5) Å) is observed in comparison with those of complex 4 (O1−C1 = 1.300(8) Å, C1−C2 = 1.388(9) Å, and N1−C2 = 1.426(8) Å), indicating that the oiminoquinone in complex 8 is more reduced in comparison to that in complex 4 because of the electron-donating neophyl ligands (Table 7). The small dihedral angle (5.36°) between the best planes of O1−W−N1 and O1−C1−C2−N1 confirms the σ2-amidophenolate mode of the dianionic ligand without direct π interaction between the tungsten center and two carbons of the o-iminoquinone ligand. Ring-Opening Metathesis Polymerization of Norbornene. We examined the catalytic application of newly prepared alkylidene and dialkyl complexes to a ring-opening metathesis polymerization of norbornene with 1 mol% of catalyst loading in toluene at 80 °C and room temperature. Obviously, coordination of phosphine to the tungsten center retarded

Figure 6. Molecular structure of complex 7 with 30% thermal ellipsoids. The hydrogen atoms are omitted for clarity. F


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Organometallics

Table 8. ROMP of Norbornene Catalyzed by Benzylidene Complexes 6 and 7a

entry 1 2 3 4 5 6

cat. 6 6 6/CuCl2 (5 mol%) 6/B(C6F5)3 (5 mol%) 7 7

temp

yield

(°C)

(%)

rt. 80 rt. rt. rt. 80

trace >99 98 >99 trace 95

103Mnb

Mw/Mnb

cis (%)c

20 133 681

1.2 1.8 2.8

78 79 64

132

2.6

59

a

Reaction conditions: catalyst:monomer = 0.01:1.00 (all in mmol) in toluene, reaction time 1 h. The total volume was 3 mL. bDetermined by gel permeation chromatography. cDetermined by 1H NMR spectroscopy.


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Table 7. Selected Bond Distances (Å) and Angles (deg) for Complex 8 W−C39 W−N1 W−O1 O1−C1 W−C39−C40 W−N2−C27

2.146(3) 2.004(3) 2.008(3) 1.338(4) 118.0(3) 173.0(2)

W−C49 W−N2 N1−C2 C1−C2 W−C49−C50 dihedral anglea

CONCLUSION We have reported a salt-free synthetic methodology for preparing tungsten imido complexes bearing redox-active αdiimine and o-iminoquinone ligands. The coordination mode of α-diimine in complex 1 was elucidated to adopt a supine-σ2,πenediamido mode of α-diimine, while α-diimine in anionic complex 2 favors the σ2-enediamido mode. The coordinative unsaturation around the metal center in complex 2 was stabilized by donation of the filled N(pπ) orbital to the empty W(dπ*) orbital, on the basis of the LMCT band at 621 nm. Thermolysis of 5 in the presence of PMe2Ph afforded the alkylidene complex 6, in which PMe2Ph is located in a position cis to the benzylidene moiety. NMR and X-ray diffraction studies of 6 confirmed that the benzylidene moiety adopted an anti conformation to avoid steric repulsion with PMe2Ph. The reaction of complex 5 and CCl4 at 60 °C afforded the dissymmetric benzylidene complex 7 via reductive cleavage of the C−Cl bond of CCl4. The benzylidene moiety of 7 adopted a syn conformation to stabilize the tungsten center via a C−H agostic interaction. The benzylidene complex 6 served as a catalyst for ROMP of norbornene at 80 °C to give a cis-rich poly(norbornene) with narrow polydispersity. When CuCl2 (5 equiv to the catalyst) was added as a phosphine scavenger to generate coordinatively unsaturated alkylidene species, the combined catalyst smoothly polymerized at room temperature. The dinuclear o-iminoquinone complex 3 and the mononuclear o-iminoquinone complex 4 were obtained by using a salt-free synthetic methodology, depending on the reaction solvents. oIminoquinone in complexes 3 and 4 adopted an σ2amidophenolate mode without a direct π interaction between the tungsten center and two carbons of the o-iminoquinone ligand. The UV−vis spectra of 3 and 4 exhibited a typical LMCT band around 620−629 nm, indicating the interaction of pπ orbitals of nitrogen and oxygen atoms on the oiminoquinone ligand with the dπ* orbital of the tungsten center. Although we did not successfully isolate any alkylidene complexes bearing an o-iminoquinone ligand from thermolysis of the dialkyl complex 8, complex 8 at 80 °C served as a catalyst for the ROMP of norbornene. Further studies of earlytransition-metal imido complexes bearing redox-active ligands are ongoing in our laboratory.

2.110(4) 1.771(3) 1.430(5) 1.364(6) 129.6(3) 5.36


the catalytic activity: alkylidene complex 6 at room temperature resulted in almost no catalytic activity (Table 8, entry 1), and at 80 °C complex 6 served as a better catalyst to give a cis-rich poly(norbornene) with narrow polydispersity (Mw/Mn = 1.2) (entry 2).30,31 Accordingly, we assumed that the generation of phosphine-free alkylidene species was key to promoting the polymerization; therefore, we added CuCl2 (5 mol %, 5 equiv to the catalyst) as a phosphine scavenger to generate coordinatively unsaturated alkylidene species that smoothly catalyzed the polymerization at room temperature with narrow polydispersity (Mw/Mn = 1.8) (entry 3).32,33 Addition of B(C6F5)3 also gave a poly(norbornene) in quantitative yield, although the polydispersity (Mw/Mn = 2.8) was broader than that of the polymer obtained in entry 3 (entry 4). Complex 7 exhibited no polymerization activity at room temperature (entry 5), whereas at 80 °C the polymerization proceeded to give a polymer with rather broad polydispersity (Mw/Mn = 2.6) (entry 6), though the activation process for complex 7 was uncertain. The in situ generated alkylidene species by thermolysis of dialkyl complexes 5 and 8 also worked as catalysts for the polymerization of norbornene. Dialkyl complex 5 heated at 80 °C for 1 h in the presence of norbornene produced poly(norbornene) (cis content 61%) with rather broad polydispersity (Mn = 2.03 × 105; Mw/Mn = 2.4).34 Similarly, complex 8 at 80 °C for 24 h served as a catalyst to give poly(norbornene) (cis content 79%; Mn = 1.97 × 105; Mw/ Mn = 4.2), but with relatively low activity due to the thermal stability of 8. G


Article

Organometallics

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3

JH−H = 6.8 Hz, 6H, CH(CH3)2), 1.68 (d, 3JH−H = 6.8 Hz, 6H, CH(CH3)2), 2.59 (br, 8H, NCH2CH2CH2CH3), 4.47 (sept, 3JH−H = 6.8 Hz, 2H, CH(CH3)2), 4.58 (sept, 3JH−H = 6.8 Hz, 2H, CH(CH3)2), 4.80 (sept, 3JH−H = 6.8 Hz, 2H, CH(CH3)2), 5.22 (d, 3JH−H = 2.4 Hz, 1H, HCCH), 6.60 (d, 3JH−H = 2.4 Hz, 1H, HCCH), 6.80 (t, 3 JH−H = 7.6 Hz, 1H, p-H), 7.03 (t, 3JH−H = 7.6 Hz, 1H, p-H), 7.09 (d, 3 JH−H = 7.6 Hz, 2H, m-H), 7.11 (t, 3JH−H = 7.6 Hz, 1H, p-H), 7.21 (d, 3 JH−H = 7.6 Hz, 2H, m-H), 7.37 (d, 3JH−H = 7.6 Hz, 2H, m-H). 13C NMR (100 MHz, C6D6, 303 K): δ 14.0 (NCH2CH2CH2CH3), 19.9 (NCH2CH2CH2CH3), 23.7 (CH(CH3)2), 24.4 (NCH2CH2CH2CH3), 24.6 (CH(CH3)2), 27.5 (CH(CH3)2), 27.9 (CH(CH3)2), 28.1 (CH(CH 3 ) 2 ), 28.1 (CH(CH 3 ) 2 ), 28.3 (CH(CH 3 ) 2 ), 59.3 (NCH2CH2CH2CH3), 122.2 (m-Ar), 123.2 (m-Ar), 124.4 (m-Ar), 125.0 (p-Ar), 126.7 (p-Ar), 127.2 (p-Ar), 133.4 (NCCN), 143.4 (NCCN), 146.0 (o-Ar), 148.3 (ipso-Ar), 150.2 (ipso-Ar), 150.5 (oAr), 150.9 (o-Ar), 152.5 (ipso-Ar). Anal. Calcd for C54H89Cl3N4W: C, 59.80; H, 8.27; N, 5.17. Found: C, 59.66; H, 8.67; N, 5.10. UV−vis (toluene) λmax/nm (ε/M−1 cm−1): 308 (1.7 × 104), 422 (8.7 × 103), 621 (2.4 × 103). Synthesis of [(o-iminoquinone)W(NC6H3-2,6-iPr2)Cl]2(μCl)2 (3). A solution of o-iminoquinone (734 mg, 2.00 mmol) in toluene (5 mL) was added to a solution of W(NC6H3-2,6-iPr2)Cl4 (1.00 g, 2.00 mmol) in toluene (30 mL). 1-Methyl-3,6-bis(trimethylsilyl)-1,4-cyclohexadiene (0.57 mL, 2.0 mmol) was added to the mixture at room temperature. The bright green solution was stirred for 12 h at 80 °C, resulting in a dark green solution. After all of the volatiles were removed under reduced pressure, the product was washed with hexane (2 × 10 mL) and dried in vacuo to give complex 3 (1.45 g, 0.909 mmol) as a green powder in 91% yield: mp 196 °C dec. Anal. Calcd for C76H84Cl4N4O2W2: C, 57.23; H, 5.31; N, 3.51. Found: C, 57.61; H, 5.69; N, 3.33. UV−vis (toluene) λmax/nm (ε/M−1 cm−1): 629 (6.6 × 103). Synthesis of (o-iminoquinone)W(NC6H3-2,6-iPr2)Cl2(THF) (4). 1-Methyl-3,6-bis(trimethylsilyl)-1,4-cyclohexadiene (0.29 mL, 1.0 mmol) was added to a mixture of W(NC6H3-2,6-iPr2)Cl4 (500 mg, 1.00 mmol) and o-iminoquinone (367 mg, 1.00 mmol) in THF (10 mL). The dark green solution was stirred for 20 h at 55 °C. After all the volatiles were removed under reduced pressure, the product was washed with hexane (3 × 5 mL) and dried in vacuo to give complex 4 (668 mg, 0.768 mmol) as a green powder in 77% yield: mp 253 °C dec. 1H NMR (400 MHz, THF-d8, 303 K): δ 0.49 (d, 3JH−H = 6.4 Hz, 6H, CH(CH3)2), 0.92 (d, 3JH−H = 6.8 Hz, 6H, CH(CH3)2), 1.08 (br, 12H, CH(CH3)2), 3.67 (sept, 3JH−H = 6.4 Hz, 2H, CH(CH3)2), 3.94 (sept, 3JH−H = 6.4 Hz, 2H, CH(CH3)2), 5.84 (d, 3JH−H = 8.8 Hz, 1H, Ar), 6.76 (t, 3JH−H = 8.4 Hz, 1H, Ar), 6.91 (t, 3JH−H = 7.6 Hz, 1H, Ar), 7.01 (t, 3JH−H = 7.6 Hz, 1H, Ar), 7.14−7.24 (m, 5H, Ar), 7.42 (t, 3JH−H = 7.6 Hz, 1H, Ar), 7.59 (t, 3JH−H = 7.6 Hz, 1H, Ar), 8.09 (d, 3JH−H = 8.0 Hz, 1H, Ar), 8.56 (d, 3JH−H = 8.4 Hz, 1H, Ar), 8.63 (d, 3JH−H = 8.4 Hz, 1H, Ar). 13C NMR (100 MHz, THF-d8, 303 K): δ 24.5 (CH(CH3)2), 24.6 (CH(CH3)2), 26.4 (THF), 28.2 (CH(CH3)2), 28.8 (CH(CH3)2), 68.2 (THF), 122.4 (Ar), 122.9 (Ar), 123.2 (Ar), 124.5 (Ar), 125.5 (Ar), 125.9 (Ar), 126.1 (Ar), 126.2 (Ar), 126.3 (Ar), 127.1 (Ar), 127.3 (Ar), 128.5 (Ar), 129.5 (Ar), 129.7 (Ar), 130.0 (Ar), 133.8 (Ar), 136.8 (Ar), 144.2 (Ar), 147.5 (Ar), 150.3 (Ar), 153.5 (Ar), 161.2 (Ar). Anal. Calcd for C42H50Cl2N2O2W(C4H8O)2: C, 59.24; H, 6.56; N, 2.76. Found: C, 59.35; H, 6.64; N, 3.03. UV−vis (toluene) λmax/nm (ε/M−1 cm−1): 620 (6.0 × 103). Synthesis of (α-diimine)W(NC6H3-2,6-iPr2)(CH2Ph)2 (5). A suspension of Mg(CH2Ph)2·Et2O (104 mg, 0.371 mmol) in toluene (10 mL) was added dropwise to a solution of complex 1 (300 mg, 0.372 mmol) in toluene (10 mL) at −78 °C, resulting in a green suspension. The reaction mixture was stirred for 1 h at room temperature to give a dark yellow-green suspension. After filtration, the solvent was removed under reduced pressure. The oily product was washed with hexane (3 × 10 mL) and dried under reduced pressure to give complex 5 (261 mg, 0.280 mmol) as a pale yellowgreen powder in 75% yield: mp 135 °C dec. 1H NMR (400 MHz, CD2Cl2, 303 K): δ 0.69−1.33 (br), 2.88 (br), 3.44 (br), 6.53 (br), 6.95 (m), 7.02 (s), 7.03 (br d, J = 1.2 Hz, Ar), 7.12 (br), 7.36 (br). 1H NMR (400 MHz, CD2Cl2, 203 K): δ 0.28 (br d, 3JH−H = 5.6 Hz, 3H,

EXPERIMENTAL SECTION

General Considerations. All manipulations involving air- and moisture-sensitive compounds were carried out under argon using standard Schlenk techniques or an argon-filled glovebox. 1-Methyl-3,6bis(trimethylsilyl)-1,4-cyclohexadiene (MBTCD),35 α-diimine,36 oiminoquinone,22b and W(NC6H3-2,6-iPr2)Cl437 were prepared according to the literature procedures. Mg(CH2Ph)2·Et2O was prepared according to the modified procedure of the literature by replacing THF with Et2O.38 LiCH2CMe2Ph was prepared following the literature procedure to synthesize LiCH2CMe3.39 PMe2Ph, PPh3, PCy3, CuCl, CuCl2, CuBr2, Ni(cod)2, and B(C6F5)3 were purchased and used as received. Anhydrous hexane, toluene, and THF were purchased from Kanto Chemical and further purified by passage through activated alumina under positive argon pressure as described by Grubbs et al.40 Benzene-d6, CD2Cl2, toluene-d8, CCl4, and pyridine were distilled over CaH2 and degassed before use. [nBu4N][Cl] was purified by recrystallization. Norbornene (2-norbornene) was refluxed over sodium and distilled prior to use. 1H NMR (400 MHz), 13C NMR (100 MHz), and 31P NMR (161 MHz) spectra were measured on Bruker AVANCEIII-400 spectrometers. The 31P NMR spectrum was calculated with reference to 85% H3PO4. Assignment of 1H and 13 C NMR peaks for some of the complexes was facilitated by 2D 1 H−1H COSY, 2D 1H−1H NOESY, 2D 1H−13C HMQC, and 2D 1 H−13C HMBC spectra. Gel permeation chromatographic analysis was carried out at 40 °C by using a Shimadzu LC-20AD liquid chromatograph system and a RID 10A refractive index detector, equipped with a Shodex KF-806L column and KF-406L column, which were calibrated versus commercially available polystyrene standards (SHOWA DENKO). The elemental analyses were recorded by using a PerkinElmer 2400 instrument at the Faculty of Engineering Science, Osaka University. All melting points were measured in sealed tubes under an argon atmosphere. UV−vis spectra were recorded on an Agilent 8453 instrument. Synthesis of (α-diimine)W(NC6H3-2,6-iPr2)Cl2 (1). A solution of α-diimine (226 mg, 0.600 mmol) in toluene (5.0 mL) was added to a solution of W(NC6H3-2,6-iPr2)Cl4 (300 mg, 0.599 mmol) in toluene (5.0 mL). 1-Methyl-3,6-bis(trimethylsilyl)-1,4-cyclohexadiene (0.17 mL, 0.59 mmol) was added to the solution. The reaction mixture was stirred for 12 h at 80 °C. All of the volatiles were removed under reduced pressure to give a brown residue. The product was extracted with hexane (10 mL). Removing the hexane under reduced pressure gave 1 (432 mg, 0.536 mmol) as a dark brown powder in 89% yield: mp 131 °C dec. 1H NMR (400 MHz, CD2Cl2, 303 K): δ 0.94 (d, 3 JH−H = 6.8 Hz, 12H, imide CH(CH3)2), 1.16 (d, 3JH−H = 6.8 Hz, 12H, α-diimine CH(CH3)2), 1.19 (d, 3JH−H = 6.8 Hz, 12H, α-diimine CH(CH3)2), 2.90 (sept, 3JH−H = 6.8 Hz, 2H, imide CH(CH3)2), 3.38 (sept, 3JH−H = 6.8 Hz, 4H, α-diimine CH(CH3)2), 6.13 (s, 2H, HC CH), 6.97 (t, 3JH−H = 7.6 Hz, 1H, imide p-H), 7.12 (d, 3JH−H = 7.6 Hz, 2H, imide m-H), 7.18−7.21 (m, 2H, α-diimine p-H), 7.26−7.28 (m, 4H,δδ□α-diimine m-H). 13C NMR (100 MHz, CD2Cl2, 303 K): δ 23.4 (α-diimine CH(CH3)2), 25.2 (imide CH(CH3)2), 27.1 (α-diimine CH(CH3)2), 28.9 (imide CH(CH3)2), 29.1 (α-diimine CH(CH3)2), 122.3 (imide m-Ar), 124.2 (α-diimine m-Ar), 128.2 (α-diimine p-Ar), 129.5 (imide p-Ar), 133.5 (NCCN), 145.0 (α-diimine o-Ar), 147.6 (α-diimine ipso-Ar), 150.0 (imide o-Ar), 150.2 (imide ipso-Ar). Anal. Calcd for C38H53Cl2N3W: C, 56.58; H, 6.62; N, 5.21. Found: C, 56.34; H, 6.80; N, 4.98. UV−vis (toluene) λmax/nm (ε/M−1 cm−1): 310 (1.4 × 104). Synthesis of [nBu4N][(α-diimine)W(NC6H3-2,6-iPr2)Cl3] (2). A solution of 1 (300 mg, 0.372 mmol) in toluene (5.0 mL) was added to a suspension of [nBu4N][Cl] (104 mg, 0.374 mmol) in toluene (5.0 mL) at room temperature. The reaction mixture was stirred for 12 h at 80 °C. All of the volatiles were removed under reduced pressure to give a green residue. The residue was washed with hexane (3 × 5 mL). Removing the hexane under reduced pressure gave 2 (368 mg, 0.339 mmol) as an intensely green powder in 91% yield: mp 151 °C dec. 1H NM R ( 40 0 MH z , C 6 D 6 , 303 K): δ 0.74 (br, 12H , NCH2CH2CH2CH3), 0.99 (br, 16H, NCH2CH2CH2CH3), 1.32 (br, 18H, CH(CH3)2), 1.40 (d, 3JH−H = 6.8 Hz, 6H, CH(CH3)2), 1.43 (d, H


Article

Organometallics CH(CH3)2), 0.44 (br d, 3JH−H = 5.2 Hz, 3H, CH(CH3)2), 0.52 (br d, 3 JH−H = 5.6 Hz, 3H, CH(CH3)2), 0.73 (br d, 3JH−H = 6.4 Hz, 3H, CH(CH3)2), 0.86 (br d, 3JH−H = 6.4 Hz, 3H, CH(CH3)2), 0.89 (br d, 3 JH−H = 6.0 Hz, 3H, CH(CH3)2), 0.93 (br d, 3JH−H = 5.6 Hz, 3H, CH(CH3)2), 1.02 (br d, 3JH−H = 6.0 Hz, 3H, CH(CH3)2), 1.14 (br d, 3 JH−H = 5.6 Hz, 3H, CH(CH3)2), 1.19 (m, 6H, CH(CH3)2), 1.38 (br d, 3JH−H = 6.4 Hz, 3H, CH(CH3)2), 2.29 (m, 2H, WCHHPh), 2.51 (m, 1H, CH(CH3)2), 2.65 (m, 1H, CH(CH3)2), 2.98 and 3.31 (ABq, 2 JH−H = 8.8 Hz 2H, WCHHPh), 3.11 (m, 1H, CH(CH3)2), 3.37 (m, 1H, CH(CH3)2), 3.61 (m, 1H, CH(CH3)2), 6.46 (br, 1H, Ar), 6.55 (br, 1H, Ar), 6.62 (br, 2H, Ar), 6.76 (br, 2H, Ar), 6.85−6.95 (m, 4H, Ar), 7.00 (m, 1H, Ar), 7.03−7.13 (m, 3H, Ar), 7.25−7.40 (m, 5H, Ar). Anal. Calcd for C52H67N3W: C, 68.04; H, 7.36; N, 4.58. Found: C, 67.93; H, 7.63; N, 4.61. UV−vis (toluene) λmax/nm (ε/M−1 cm−1): 381 (2.7 × 104). Synthesis of (α-diimine)W(NC 6 H 3 -2,6- i Pr 2 )(CHPh)(PMe2Ph) (6). To a solution of complex 5 (400 mg, 0.429 mmol) in toluene (5.0 mL) was added a solution of PMe2Ph (59.0 mg, 0.427 mmol) in toluene (5.0 mL). The reaction mixture was stirred for 1 h at 80 °C. After filtration, all of the volatiles were removed under reduced pressure, resulting in a waxy deep green solid. Hexane (10 mL) was added and removed in vacuo to give complex 6 (353 mg, 0.366 mmol) as a green powder in 85% yield: mp 136 °C dec. 1H NMR (400 MHz, C6D6, 303 K): δ 0.58 (d, 3JH−H = 6.8 Hz, 3H, CH(CH3)2), 0.63 (d, 3 JH−H = 6.8 Hz, 3H, CH(CH3)2), 0.68 (d, 3JH−H = 6.8 Hz, 3H, CH(CH3)2), 0.74 (d, 3JH−H = 6.8 Hz, 3H, CH(CH3)2), 0.81 (d, 3JH−H = 6.8 Hz, 3H, CH(CH3)2), 1.07 (d, 3JH−H = 6.8 Hz, 3H, CH(CH3)2), 1.13 (d, 3JH−H = 8.0 Hz, 3H, CH(CH3)2), 1.22 (m, 12H, CH(CH3)2 and P(CH3)2Ph), 1.39 (m, 6H, CH(CH3)2), 1.44 (d, 3JH−H = 6.8 Hz, 3H, CH(CH3)2), 1.51 (d, 3JH−H = 6.8 Hz, 3H, CH(CH3)2), 3.63 (m, 2H, CH(CH3)2), 3.71 (sept, 3JH−H = 6.8 Hz, 1H, CH(CH3)2), 3.85 (sept, 3JH−H = 6.8 Hz, 1H, CH(CH3)2), 3.93 (sept, 3JH−H = 6.8 Hz, 1H, CH(CH3)2), 4.10 (sept, 3JH−H = 6.8 Hz, 1H, CH(CH3)2), 6.62 (s, 2H, HCCH), 6.77−7.39 (m, 19H, Ar), 12.3 (d, 3JH−P = 9.6 Hz, 1H, W=CH). 13C NMR (100 MHz, C6D6, 303 K): δ 14.7 (d, 1JC−P = 30 Hz, P(CH3)2Ph), 18.3 (d, 1JC−P = 22 Hz, P(CH3)2Ph), 21.8 (iPr), 22.2 (iPr), 22.9 (iPr), 23.5 (iPr), 23.6 (iPr), 25.8 (iPr), 26.1 (iPr), 26.1 (iPr), 27.1 (iPr), 27.3 (iPr), 27.7 (iPr), 27.7 (iPr), 27.8 (iPr), 27.9 (iPr), 28.0 (iPr), 28.1 (iPr), 28.4 (iPr), 30.4 (iPr), 122.8 (Ar), 122.8 (Ar), 123.0 (Ar), 123.2 (Ar), 124.3 (Ar), 125.8 (Ar), 126.0 (Ar), 126.2 (Ar), 127.0 (d, JC−P = 2.0 Hz, PMe2Ph), 127.1 (d, JC−P = 1.2 Hz, PMe2Ph), 127.6 (Ar), 127.7 (Ar), 127.9 (NCCN), 128.0 (NCCN), 128.6 (Ar), 128.7 (Ar), 129.4 (d, JC−P = 2.1 Hz, PMe2Ph), 130.2 (Ar), 130.3 (Ar), 139.6 (Ar), 140.1 (Ar), 144.5 (d, JC−P = 1.9 Hz, PMe2Ph), 144.8 (Ar), 145.1 (Ar), 145.3 (Ar), 145.5 (Ar), 148.0 (2C, Ar), 148.1 (Ar), 152.4 (d, JC−P = 3.1 Hz, PMe2Ph), 153.9 (Ar), 154.9 (d, JC−P = 3.5 Hz, PMe2Ph), 257.8 (1JC−H = 133 Hz, 2JC−P = 16 Hz, 1JC−W = 157 Hz, WCHPh). 31P NMR (161 MHz, C6D6, 303 K): δ 4.38 (1JP−W = 257 Hz). Anal. Calcd for C53H70N3PW: C, 66.04; H, 7.32; N, 4.36. Found: C, 65.98; H, 7.41; N, 4.35. UV−vis (toluene) λmax/nm (ε/M−1 cm−1): 358 (3.0 × 104). Synthesis of (Cl3 C-amido-imino)W(NC6H 3-2,6-iPr 2)( CHPh)Cl (7). A solution of CCl4 (59.0 mg, 0.384 mmol) in toluene (10 mL) was added to a dark green solution of complex 5 (300 mg, 0.322 mmol) in toluene (10 mL) at room temperature. The mixture was stirred for 18 h at 60 °C. After all of the volatiles were removed under reduced pressure, the product was washed with hexane (3 × 5 mL). Removing the hexane under reduced pressure gave complex 7 (114 mg, 0.116 mmol) as a yellow-green powder in 35% yield: mp 147 °C dec. 1H NMR (400 MHz, C6D6, 303 K): δ 0.52 (d, 3JH−H = 6.4 Hz, 3H, CH(CH3)2), 0.80 (d, 3JH−H = 6.8 Hz, 3H, CH(CH3)2), 0.88 (d, 3 JH−H = 6.4 Hz, 3H, CH(CH3)2), 0.93 (d, 3JH−H = 6.8 Hz, 3H, CH(CH3)2), 0.99 (d, 3JH−H = 6.8 Hz, 3H, CH(CH3)2), 1.06 (d, 3JH−H = 6.8 Hz, 3H, CH(CH3)2), 1.25 (d, 3JH−H = 6.8 Hz, 3H, CH(CH3)2), 1.42 (m, 9H, CH(CH3)2), 1.53 (d, 3JH−H = 6.8 Hz, 3H, CH(CH3)2), 1.67 (d, 3JH−H = 6.8 Hz, 3H, CH(CH3)2), 3.20 (m, 2H, CH(CH3)2), 3.38 (sept, 3JH−H = 6.8 Hz, 1H, CH(CH3)2), 3.79 (sept, 3JH−H = 6.8 Hz, 1H, CH(CH3)2), 3.89 (sept, 3JH−H = 6.8 Hz, 1H, CH(CH3)2), 4.43 (sept, 3JH−H = 6.8 Hz, 1H, CH(CH3)2), 5.63 (s, 1H, NCHCCl3),

6.39 (t, 3JH−H = 7.2 Hz, 1H, Ar), 6.77 (m, 2H, Ar), 6.89 (m, 2H, Ar), 6.96−7.14 (m, 9H, Ar), 9.15 (s, 1H, NCH), 9.94 (s, 1H, W=CH). 13 C NMR (100 MHz, C6D6, 303 K): δ 20.0 (CH(CH3)2), 23.5 (CH(CH3)2), 23.7 (CH(CH3)2), 24.2 (CH(CH3)2), 24.5 (CH(CH3)2), 26.0 (CH(CH3)2), 26.2 (CH(CH3)2), 26.3 (CH(CH3)2), 26.4 (CH(CH3)2), 26.6 (CH(CH3)2), 27.3 (CH(CH3)2), 27.7 (CH(CH3)2), 28.2 (CH(CH3)2), 28.4 (CH(CH3)2), 28.4 (CH(CH3)2), 28.5 (CH(CH3)2), 29.6 (CH(CH3)2), 29.7 (CH(CH3)2), 94.2 (CHCCl3), 101.6 (CCl3), 123.1 (Ar), 123.3 (Ar), 124.2 (Ar), 124.3 (Ar), 126.0 (Ar), 126.1 (Ar), 126.9 (Ar), 127.9 (Ar), 128.3 (Ar), 128.4 (Ar), 129.5 (Ar), 131.8 (Ar), 139.9 (Ar), 140.3 (Ar), 142.3 (Ar), 144.4 (Ar), 145.3 (Ar), 145.5 (Ar), 146.5 (Ar), 149.6 (Ar), 153.1 (Ar), 160.4 (Ar), 176.8 (NCH), 281.0 (1JC−H = 116 Hz, 1JC−W = 172 Hz, WCHPh). Anal. Calcd for C46H59Cl4N3W: C, 56.40; H, 6.07; N, 4.29. Found: C, 56.31; H, 6.07; N, 4.27. UV−vis (toluene) λmax/nm (ε/M−1 cm−1): 341 (2.3 × 104). Synthesis of (o-iminoquinone)W(NC 6 H 3 -2,6- i Pr 2 )(CH2CMe2Ph)2 (8). A solution of LiCH2CMe2Ph (483 mg, 3.44 mmol) in toluene (15 mL) was added to a solution of complex 4 (1.50 g, 1.72 mmol) in toluene (30 mL) at −78 °C. The reaction mixture was warmed to room temperature and stirred for 2 h, resulting in a greenish brown suspension. After filtration, the solvent was removed under reduced pressure to give a sticky brown product. Washing the product with hexane (5 × 10 mL) and drying in vacuo gave complex 8 (882 mg, 0.888 mmol) as a green powder in 52% yield: mp 137 °C dec. 1H NMR (400 MHz, C6D6, 303 K): δ 0.62 (d, 3JH−H = 6.4 Hz, 6H, o-iminoquinone CH(CH3)2), 0.88 (d, 3JH−H = 6.8 Hz, 6H, oiminoquinone CH(CH3)2), 0.98 (d, 3JH−H = 6.8 Hz, 12H, imide CH(CH3)2), 1.26 (s, 6H, CH2CMe2Ph), 1.41 and 3.70 (ABq, 2JH−H = 11.6 Hz, 4H, CH2CMe2Ph), 1.61 (s, 6H, CH2CMe2Ph), 3.21 (sept, 3 JH−H = 6.8 Hz, 2H, o-iminoquinone CH(CH3)2), 3.66 (sept, 3JH−H = 6.8 Hz, 2H, imide CH(CH3)2), 6.35 (d, 3JH−H = 8.8 Hz, 1H, Ar), 6.84 (t, 3JH−H = 7.6 Hz, 1H, Ar), 6.93−7.05 (m, 6H, Ar), 7.09−7.15 (m, 3H, Ar), 7.29 (t, 3JH−H = 7.6 Hz, 4H, Ar), 7.39 (m, 4H, Ar), 7.46 (t, 3 JH−H = 7.2 Hz, 1H, Ar), 7.74 (t, 3JH−H = 7.6 Hz, 1H, Ar), 8.56 (t, 3 JH−H = 7.6 Hz, 2H, Ar), 8.93 (d, 3JH−H = 8.4 Hz, 1H, Ar). 13C NMR (100 MHz, C6D6, 303 K): δ 23.7 (o-iminoquinone CH(CH3)2), 24.7 (o-iminoquinone CH(CH3)2), 25.4 (imide CH(CH3)2), 28.0 (CH(CH 3 ) 2 ), 28.6 (CH(CH 3 ) 2 ), 32.7 (WCH 2 CMe 2 Ph), 33.3 (WCH2CMe2Ph), 43.7 (WCH2CMe2Ph), 97.3 (WCH2CMe2Ph), 122,6 (Ar), 123.1 (Ar), 123.7 (Ar), 124.1 (Ar), 124.2 (Ar), 125.1 (Ar), 125.3 (Ar), 125.5 (Ar), 126.0 (Ar), 126.1 (Ar), 126.7 (Ar), 127.8 (Ar), 128.0 (Ar), 128.3 (Ar), 128.5 (Ar), 128.8 (Ar), 129.1 (Ar), 139.8 (Ar), 142.4 (Ar), 149.2 (Ar), 150.2 (Ar), 151.9 (Ar), 153.7 (Ar). Anal. Calcd for C58H68N2OW: C, 70.15; H, 6.90; N, 2.82. Found: C, 69.92; H, 7.31; N, 2.82. UV−vis (toluene) λmax/nm (ε/M−1 cm−1): 338 (2.0 × 104), 400 (1.8 × 104). Polymerization of Norbornene. In a typical reaction, a solution of tungsten complex (0.010 mmol) was added to a solution of norbornene (94.0 mg, 1.00 mmol) in toluene (2 mL) at room temperature. The reaction mixture was stirred at room temperature or at 80 °C for the prescribed period. Benzaldehyde was added to the reaction mixture, and the solution was added dropwise to 50 mL of stirred MeOH. The polymer was collected, washed with MeOH, and dried at 70 °C under reduced pressure. 1H NMR (400 MHz, C6D6, 303 K): δ 1.15 (br), 1.43 (br), 1.84 (br), 2.02 (br), 2.51 (br), 2.89 (br), 5.34 (br), 5.49 (br), 6.41 (br d, 3JH−H = 11.2 Hz, CHPh), 7.04−7.30 (m, CHPh). X-ray Diffraction Study. All crystals were handled similarly. The crystals were mounted on a CryoLoop (Hampton Research Corp.) with a layer of light mineral oil and placed in a nitrogen stream at 113(1) K. Measurements were made on a Rigaku R-AXIS RAPID imaging plate area detector Rigaku AFC7R/Mercury CCD detector with graphite-monochromated Mo Kα (1.71075 Å) radiation. Crystal data and structure refinement parameters are given in Table S3 in the Supporting Information. The structures were solved by direct methods (SHELXS-97).41 The structures were refined on F2 by full-matrix least-squares methods, using SHELXL-97.42 Non-hydrogen atoms were anisotropically refined. Hydrogen atoms were included in the refinement on I


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Organometallics calculated positions riding on their carrier atoms. The function minimized was [∑w(Fo2 − Fc2)2] (w = 1/[σ2(F02) + (aP)2 + bP]), where P = (Max(F02,0) + 2Fc2)/3 with σ2(Fo2) from counting statistics. The functions R1 and wR2 were (∑||Fo| − |Fc||)/∑|Fo| and [∑w(Fo2 − Fc2)2/∑(wFo4)]1/2, respectively. The ORTEP-3 program was used to draw the molecules.43 Complex 2 has two isotropic non-H atoms (C53 and C54) in the main residue (Alert B in CheckCIF) due to the disordered nBu group of the cationic part. For complex 6, large solvent-accessible voids in the lattice were involved in the crystal packing (Alert A in CheckCIF), but we could not find suitable solvent molecules. Complex 7 has the isotropic non-H atom C2 in the main residue (Alert B in Check CIF) due to the disordered imido carbon.

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(4) For representative studies regarding enantioselective metathesis catalyzed by imido alkylidene complexes bearing diolate ligands, see: (a) Hoveyda, A. H.; Schrock, R. R. Chem. Eur. J. 2001, 7, 945. (b) Fujimura, O.; Grubbs, R. H. J. Org. Chem. 1998, 63, 824. (c) Fujimura, O.; Grubbs, R. H. J. Am. Chem. Soc. 1996, 118, 2499. (d) Alexander, J. B.; La, D. S.; Cafalo, D. R.; Hoveyda, A. H.; Schrock, R. R. J. Am. Chem. Soc. 1998, 120, 4041. (e) La, D. S.; Alexander, J. B.; Cafalo, D. R.; Graf, D. D.; Hoveyda, A. H.; Schrock, R. R. J. Am. Chem. Soc. 1998, 120, 9720. (f) Zhu, S. S.; Cefalo, D. R.; La, D. S.; Jamieson, J. Y.; Davis, W. M.; Hoveyda, A. H.; Schrock, R. R. J. Am. Chem. Soc. 1999, 121, 8251. (5) For representative studies, see: (a) Malcolmson, S. J.; Meek, S. J.; Sattely, E. S.; Schrock, R. R.; Hoveyda, A. H. Nature 2008, 456, 933. (b) Sattely, E. S.; Meek, S. J.; Malcolmson, S. J.; Schrock, R. R.; Hoveyda, A. H. J. Am. Chem. Soc. 2009, 131, 943. (c) Singh, R.; Schrock, R. R.; Müller, P.; Hoveyda, A. H. J. Am. Chem. Soc. 2007, 129, 12654. (6) Representative reviews for redox-active ligands: (a) Lyaskovskyy, V.; de Bruin, B. ACS Catal. 2012, 2, 270. (b) Luca, O. R.; Crabtree, R. H. Chem. Soc. Rev. 2013, 42, 1440. (c) Kaim, W.; Schwederski, B. Coord. Chem. Rev. 2010, 254, 1580. (d) Kaim, W. Coord. Chem. Rev. 1987, 76, 187. (d) Dzik, W. I.; van der Vlugt, J. I.; Reek, J. N. H.; de Brin, B. Angew. Chem., Int. Ed. 2011, 50, 3356. (7) For representative studies regarding C−C bond formation reactions catalyzed by complexes bearing redox-active ligands, see: (a) Chirik, P. J.; Wieghardt, K. Science 2010, 327, 794. (b) Bouwkamp, M. W.; Bowman, A. C.; Lobkovsky, E.; Chirik, P. J. J. Am. Chem. Soc. 2006, 128, 13340. (c) Sylvester, K. T.; Chirik, P. J. J. Am. Chem. Soc. 2009, 131, 8772. (d) Russell, S. K.; Lovkovsky, E.; Chirik, P. J. J. Am. Chem. Soc. 2011, 133, 8858. (e) de Bruin, B.; Bill, E.; Bothe, E.; Weyhermüller, T.; Wieghardt, K. Inorg. Chem. 2000, 39, 2936. (8) For representative studies regarding oxidation reactions catalyzed by complexes bearing redox-active ligands, see: (a) Lippert, C. A.; Arnstein, S. A.; Sherrill, C. D.; Soper, J. D. J. Am. Chem. Soc. 2010, 132, 3879. (b) Lippert, C. A.; Riener, K.; Soper, J. D. Eur. J. Inorg. Chem. 2012, 554. (9) For representative studies regarding nitrene transfer reactions catalyzed by complexes bearing redox-active ligands, see: (a) Blackmore, K. J.; Lal, N.; Ziller, J. W.; Heyduk, A. F. J. Am. Chem. Soc. 2008, 130, 2728. (b) Nguyen, A. I.; Zarkesh, R. A.; Lacy, D. C.; Thorson, M. K.; Heyduk, A. F. Chem. Sci. 2011, 2, 166. (c) Heyduk, A. F.; Zarkesh, R. A.; Nguyen, A. I. Inorg. Chem. 2011, 50, 9849. (10) (a) VanderLende, D. D.; Abboud, K. A.; Boncella, J. M. Organometallics 1994, 13, 3378. (b) Vaughan, W. M.; Abboud, K. A.; Boncella, J. M. J. Am. Chem. Soc. 1995, 117, 11015. (c) Wang, S. -Y. S.; VanderLende, D. D.; Abboud, K. A.; Boncella, J. M. Organometallics 1998, 17, 2628. (11) (a) van Koten, G.; Vrieze, K. Adv. Organomet. Chem. 1982, 21, 151. (b) Vrieze, K. J. Organomet. Chem. 1986, 300, 307. (12) (a) Mashima, K.; Matsuo, Y.; Tani, K. Chem. Lett. 1997, 767. (b) Mashima, K.; Matsuo, Y.; Tani, K. Organometallics 1999, 18, 1471. (c) Matsuo, Y.; Mashima, K.; Tani, K. Angew. Chem., Int. Ed. 2001, 40, 960. (d) Mashima, K.; Nakamura, A. J. Organomet. Chem. 2001, 621, 224. (e) Tsurugi, H.; Ohno, T.; Kanayama, T.; Arteaga-Müller, R. A.; Mashima, K. Organometallics 2009, 28, 1950. (f) Panda, T. K.; Kaneko, H.; Pal, K.; Tsurugi, H.; Mashima, K. Organometallics 2010, 29, 2610. (g) Panda, T. K.; Kaneko, H.; Michel, O.; Pal, K.; Tsurugi, H.; Törnroos, K. W.; Anwander, R.; Mashima, K. Organometallics 2012, 31, 3178. (13) (a) Tsurugi, H.; Saito, T.; Tanahashi, H.; Arnold, J.; Mashima, K. J. Am. Chem. Soc. 2011, 133, 18673. (b) Kaupp, M.; Stoll, H.; Preuss, H.; Kaim, W.; Stahl, T.; van Koten, G.; Wissing, E.; Smeets, W. J. J.; Spek, A. L. J. Am. Chem. Soc. 1991, 113, 5606. (c) Wissing, E.; van der Linden, S.; Rijnberg, E.; Boersma, J.; Smeets, W. J. J.; Spek, A. L.; van Koten, G. Organometallics 1994, 13, 2602. (d) Wissing, E.; Rijnberg, E.; van der Schaaf, P. A.; van Gorp, K.; Boersma, J.; van Koten, G. Organometallics 1994, 13, 2609. (e) De Waele, P.; Jazdzewski, B. A.; Klosin, J.; Murray, R. E.; Theriault, C. N.; Vosejpka, P. C. Organometallics 2007, 26, 3896. (f) Froese, R. D. J.;

ASSOCIATED CONTENT

S Supporting Information *

Figures, tables, and CIF files giving the NOESY spectrum of complex 1, VT-NMR spectra of complex 5, additional experimental results for ring-opening metathesis polymerization of norbornene, and crystallographic data for complexes 2−8. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Authors

*H.T.: e-mail, [email protected]; tel, +81-6-68506247. *K.M.: e-mail, [email protected]; tel/fax, +81-66850-6245. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS H.T. expresses special thanks for the financial support provided by the JSPS Research Fellowships for Young Scientists. H.T. acknowledges financial support by a Grant-in-Aid for Young Scientists (A) of The Ministry of Education, Culture, Sports, Science, and Technology of Japan. This work was supported by the Core Research for Evolutional Science and Technology (CREST) program of the Japan Science and Technology Agency (JST) of Japan.

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REFERENCES

(1) Representative books and reviews for olefin metathesis reaction catalyzed by imido alkylidene complexes of tungsten and molybdenum: (a) Ivin, K. J.; Mol, J. C. Olefin Metathesis and Metathesis Polymerization; Academic Press: San Diego, CA, 1997. (b) Schrock, R. R.; Hoveyda, A. H. Angew. Chem., Int. Ed. 2003, 42, 4592. (c) Schrock, R. R. Chem. Rev. 2009, 109, 3211. (d) Buchmeiser, M. R. Chem. Rev. 2000, 100, 1565. (2) For representative studies regarding imido alkylidene complexes bearing alkoxy ligands, see: (a) Schrock, R. R.; Murdzek, J. S.; Bazan, G. C.; Robbins, B. J.; DiMare, M.; O’Regan, M. J. Am. Chem. Soc. 1990, 112, 3875. (b) Bazan, G. C.; Oskam, J. H.; Cho, H.-N.; Park, L. Y.; Schrock, R. R. J. Am. Chem. Soc. 1991, 113, 6899. (c) Murdzek, J. S.; Schrock, R. R. Organometallics 1987, 6, 1373. (d) Schrock, R. R.; DePue, R. T.; Feldman, J.; Schaverien, C. J.; Dewan, J. C.; Liu, A. H. J. Am. Chem. Soc. 1988, 110, 1423. (3) For representative studies regarding polymerization catalyzed by imido alkylidene complexes bearing diolate ligands, see: (a) O’Dell, R.; McConville, D. H.; Hofmeister, G. E.; Schrock, R. R. J. Am. Chem. Soc. 1994, 116, 3414. (b) McConville, D. H.; Wolf, J. R.; Schrock, R. R. J. Am. Chem. Soc. 1993, 115, 4413. (c) Totland, K. M.; Boyd, T. J.; Lavoie, G. G.; Davis, W. M.; Schrock, R. R. Macromolecules 1996, 29, 6114. (d) Schrock, R. R.; Lee, J.-K.; O’Dell, R.; Oskam, J. H. Macromolecules 1995, 28, 5933. J


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