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Apr 19, 2016 - nPr) were synthesized by treating the mononuclear tantalum− alkyne complexes (η2-RC CR)TaCl3(dme) (1a, R = Et; 1b,. R = nPr) with 1 ...
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Alkyne-Induced Facile C−C Bond Formation of Two η2‑Alkynes on Dinuclear Tantalum Bis(alkyne) Complexes To Give Dinuclear Tantalacyclopentadienes Keishi Yamamoto, Hayato Tsurugi,* and Kazushi Mashima* Department of Chemistry, Graduate School of Engineering Science, Osaka University, Toyonaka, Osaka 560-8531, Japan S Supporting Information *

ABSTRACT: The dinuclear tantalum−alkyne complexes [(η2-RCCR)TaCl2]2(μ-OMe)2(μ-thf) (2a, R = Et; 2b, R = n Pr) were synthesized by treating the mononuclear tantalum− alkyne complexes (η2-RCCR)TaCl3(dme) (1a, R = Et; 1b, R = nPr) with 1 equiv of NaOMe in THF. We found that adding a catalytic amount (20 mol %) of 3-hexyne to 2a induced the spontaneous formation of Ta2Cl4(OMe)2(μC4Et4)(thf) (4a). Similarly, Ta2Cl4(OMe)2(μ-C4nPr4)(thf) (4b) was obtained by treatment of 2b with a catalytic amount (20 mol %) of 4-octyne. Reaction of 4a,b with 4dimethylaminopyridine gave 4-dimethylaminopyridine-coordinated complexes 6a,b, whose structures were elucidated by the X-ray structure of 6a. We conducted a control experiment in which 10 equiv of 4-octyne was added to 2a to give Ta2Cl4(OMe)2(μ-C4-2,3-nPr2-4,5-Et2)(thf) (7) in 90% yield, indicating that free 4-octyne reacted with the tantalacyclopropene moiety of 2a to form a dissymmetric tantalacyclopentadiene, followed by the release of 3-hexyne. The catalytic activity of 4a−6a for [2 + 2 + 2] cyclotrimerization of 3-hexyne was examined, and we found that their activities were in the order 5a > 4a ≫ 6a.

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INTRODUCTION Transition-metal-catalyzed [2 + 2 + 2] cyclotrimerization of alkynes is one of the most straightforward and atomeconomical synthetic methods to produce polysubstituted benzene derivatives; hence, intense efforts have been directed toward attaining high catalytic activity, regioselectivity, and chemoselectivity of alkyne cyclotrimerization.1 We have been especially interested in dinuclear tantalum complexes containing a metallacyclopentadiene fragment, as these dinuclear complexes exhibit higher catalytic activity in the alkyne cyclotrimerization reaction in comparison with the corresponding mononuclear tantalum complexes. We further elucidated a mechanism where the third alkynes were added in a facile [4 + 2] manner to the metallacyclopentadiene moiety on the dinuclear complexes A (Scheme 1).2 Even though such catalytic superiority of dinuclear tantalum complexes and the [4 + 2] cycloaddition mechanism were established, the details of the first step in constructing the dinuclear metallacyclopentadiene complex A in the catalytic reaction remained unclear. As outlined in Scheme 1, three possible pathways for the formation of A can be considered for the dinuclear catalyst system: (path a) one alkyne directly coordinates to a dinuclear skeleton to form the μ-η2:η2-(alkyne)-coordinated complex B, followed by insertion of the second alkyne into an M−C bond,3 (path b) alkyne coordinates to each metal center to form the bis(η2-alkyne) complex C, in which intramolecular coupling © XXXX American Chemical Society

Scheme 1. Proposed Pathways for the Formation of Dinuclear Metallacyclopentadiene Fragment A

between two alkyne ligands proceeds,4 and (path c) one of two metals first reacts with two alkynes to form metallacyclopentadiene, to which the other metal center coordinates to form D.5 To elucidate the first step for generating the dinuclear metallacyclopentadiene complex A, we prepared the doubly methoxy bridged alkyne−tantalum complexes [(η2-RCCR)Special Issue: Organometallics in Asia Received: March 3, 2016

A

DOI: 10.1021/acs.organomet.6b00182 Organometallics XXXX, XXX, XXX−XXX

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Organometallics TaCl2]2(μ-OMe)2(μ-thf) (2a, R = Et; 2b, R = nPr) and revealed that the addition of an alkyne induced the formation of dinuclear tantalacyclopentadienes, corresponding to path c for the formation of A.

Table 1. Selected Bond Lengths (Å) and Bond Angles (deg) of 2a Ta1···Ta2 Ta1−O2 Ta2−O2 Ta2−O3 C9−C10 Ta1−O2−Ta2 O1−Ta2−O2

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RESULTS AND DISCUSSION Synthesis and Characterization of [(η2-RCCR)TaCl2]2(μ-OMe)2(μ-thf) (2). Reaction of mononuclear alkyne complexes of tantalum, (η2-RCCR)TaCl3(dme)6 (1a, R = Et; 1b, R = nPr), with 1 equiv of NaOMe in THF afforded the corresponding dinuclear tantalum−alkyne complexes with the general formula [(η2-RCCR)TaCl2]2(μ-OMe)2(μ-thf) (2a, R = Et; 2b, R = nPr), in which two OMe ligands and one THF functioned as bridging ligands to hold two tantalum centers (eq 1). Their dinuclear structures were confirmed by their NMR

3.3194(8) 2.094(4) 2.118(4) 2.528(4) 1.306(7) 104.03(15) 73.98(14)

Ta1−O1 Ta2−O1 Ta1−O3 C3−C4

2.103(4) 2.080(4) 2.717(5) 1.336(8)

Ta1−O1−Ta2 O1−Ta1−O2 θa

105.04(15) 74.01(14) 80.09

θ = dihedral angle between the best planes of Ta1−C3−C4 and Ta2−C9−C10.

a

fragment of “Ta2(μ-OMe)2(μ-thf)” are similar to those of [TaCl3(OMe)]2(μ-OMe)2.9 The distance (3.3194(8) Å) between two tantalum centers is significantly longer than a typical Ta−Ta single-bond length (2.82−2.88 Å),10 suggesting no bonding interaction between the two tantalum centers. It is a notable structural feature that the two tantalacyclopropenes are twisted almost perpendicularly, the angle of the best planes of two tantalacyclopropenes being 80.09°. Although the two tantalum atoms in 2a,b were connected through two bridging methoxy ligands and one bridging THF ligand, we observed that these dinuclear complexes partially dissociated in solution. When the mixture of the two complexes 2a,b in C6D6 was monitored by 1H NMR spectroscopy, there were three sets of signals assignable to the original complexes 2a,b and a new complex 3 that was generated as a result of dissociation of dimers 2a,b to the corresponding monomers, followed by their spontaneous random dimerization. The 1H NMR spectrum of 3 displayed two dissymmetric signals due to the 3-hexyne and 4-octyne bound dissymmetrically to each tantalum center with chemical shift values different from those of the symmetric complexes 2a,b, with an estimated relative ratio of 2:1:1 for 3:2a:2b. The liberation and recoordination of alkyne ligands of 2a,b were ruled out because dinuclear tantalacyclopentadiene complexes are expected to form if there were liberated alkynes (vide infra).

spectroscopic data together with an X-ray analysis of 2a (vide infra). The 1H NMR spectra of 2a,b displayed singlet resonances at δH 4.07 (2a) and 4.11 (2b), respectively, due to two magnetically equivalent μ-OMe ligands as well as two multiplet resonances assignable to a bridging THF ligand. Four alkyl groups of two η2-alkyne ligands were observed as magnetically equivalent, indicating the fast rotation of the alkyne ligands on each tantalum atom. In the 13C{1H} NMR spectra of 2a,b, signals due to the quaternary carbons of the alkyne moiety appeared around δC 250 (δC 252.7 for 2a; δC 251.4 for 2b), the chemical shift values of which indicated the coordination of the alkyne ligand to the metal center as a fourelectron-donating ligand, with a metallacyclopropene of Ta(V) being the best described canonical form.7 The structure of complex 2a was determined by an X-ray diffraction study (Figure 1 and Table 1). Complex 2a has a bifacial octahedral dinuclear structure. The CC bond lengths (1.336(8) and 1.306(7) Å) of each alkyne coordinating to the tantalum atom are typical for the dianionic η2-alkyne ligands found for other tantalum alkyne complexes,6,8 adopting a Ta(V) metallacyclopropene canonical form. The bond lengths (2.080(4)−2.118(4) Å) of the Ta−O(methoxy) bonds in a

Synthesis and Characterization of Ta2Cl4(OMe)2(μC 4 R 4 )(thf) (4), Ta 2 Cl 5 (OMe)(μ-C 4 R 4 )(thf) (5), and Ta2Cl4(OMe)2(μ-C4R4)(dmap) (6). When complex 2a was heated in C6D6 at 60 °C for 16 h, an alkyne coupling reaction proceeded to give the dinuclear tantalacyclopentadiene complex 4a in 84% yield with a small contamination of monomethoxy derivative 5a in 8% yield (eq 2). Under the same conditions, complex 2b was converted into complexes 4b and 5b in 81% and 9% yields, respectively. Complexes 5a,b were thermally decomposed products of 4a,b, respectively (see the Supporting Information). The dinuclear tantalacyclopentadiene structures of 4a,b were characterized by NMR spectral data. We observed an unexpected effect when free alkyne was introduced: when a catalytic amount of 3-hexyne (20 mol %) was added to complex 2a at −40 °C in toluene and the solution was stirred at room temperature for 30 min, the dinuclear tantalacyclopentadiene complex 4a was selectively obtained in

Figure 1. Molecular structure of dinuclear tantalum complex 2a with 50% thermal ellipsoids. All hydrogen atoms are omitted for clarity. B

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Organometallics

Figure 2. Molecular structures of (a) 5a and (b) 6a with 50% thermal ellipsoids. All hydrogen atoms and solvent molecules are omitted for clarity. In case of 6a, the corresponding enantiomer is observed in the unit cell.

high yield (78%) without any 5a contamination (eq 3). Under the same reaction conditions, addition of 4-octyne (20 mol %) to 2b selectively afforded 4b in 77% yield. Additionally, monomethoxy complexes 5a,b were prepared by treating 4a,b with 1 equiv of SiCl4 in toluene at 60 °C for 12 h (eq 4), suggesting that the liberated chloride under thermal decomposition conditions reacted with 4a,b. The molecular structure of 5a was determined by spectral data together with an X-ray diffraction study (vide inf ra). All attempts to crystallize complexes 4a,b failed. We then prepared a 4-dimethylaminopyridine (DMAP)-coordinated adduct, Ta2Cl4(OMe)2(μ-C4Et4)(dmap) (6a), by treating 4a with 1 equiv of DMAP in toluene (eq 5). In the 1H NMR

complexes 5a and 6a, and the selected bond lengths and angles are summarized in Table 2. Complex 5a is a CsTable 2. Selected Bond Lengths (Å) and Bond Angles (deg) of 5a and 6a Ta1−Ta2 Ta1−O1 Ta2−O1 Ta1−C1 Ta1−C4 Ta2−C1 Ta2−C2 Ta2−C3 Ta2−C4 C1−C2 C2−C3 C3−C4 C1−Ta1−C4 Ta1−C1−C2 C1−C2−C3 C2−C3−C4 C3−C4−Ta1 θa

spectrum of 6a at −60 °C, the resonance pattern due to the tantalacyclopentadiene moiety and two methoxy ligands was similar to that of complex 4a, indicating that the core moiety of Ta2Cl4(OMe)2(μ-C4Et4) remained intact upon the replacement of THF by DMAP. Comparably, reaction of 4b with 1 equiv of DMAP afforded 6b in good yield. We then obtained crystals of 5a and 6a suitable for X-ray diffraction studies. Figure 2 shows ORTEP drawings of

5a

6a

2.8561(9) 1.968(5) 2.110(6) 2.156(8) 2.152(7) 2.379(8) 2.428(7) 2.423(7) 2.420(7) 1.424(11) 1.442(10) 1.421(10) 73.6(3) 119.1(5) 113.9(6) 113.2(6) 119.9(5) 4.26

2.8537(8) 1.992(8) 2.083(8) 2.143(11) 2.167(10) 2.439(11) 2.425(11) 2.410(10) 2.387(11) 1.428(15) 1.440(16) 1.396(15) 73.4(4) 118.8(8) 114.7(10) 112.7(9) 120.4(8) 2.62

θ = the dihedral angle between the best planes of C1−Ta1−C4 and C1−C2−C3−C4. a

C

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Organometallics symmetric dinuclear tantalacyclopentadiene complex bearing a μ-C4Et4 ligand and one μ-OMe ligand. In contrast, complex 6a has a C1-symmetric structure bearing a μ-C4Et4 ligand, one μOMe ligand, and one κ1-OMe ligand. The distance between the two tantalum atoms are 2.8561(9) Å for 5a and 2.8537(8) Å for 6a, both of which are typical for a Ta−Ta single-bond length.10 The bond lengths (1.968(5) Å for 5a; 1.992(8) Å for 6a) of Ta1−O1 are shorter than those (2.110(6) Å for 5a; 2.083(8) Å for 6a) of Ta2−O1. Although there are no reports of dinuclear tantalum complexes containing a dative metal−metal interaction, we assumed that 5a and 6a have a dative metal−metal interaction from low-valent Ta2 to high-valent Ta1 because related dinuclear tungsten complexes involving metallacyclopentadiene fragments are considered to have a dative metal− metal interaction.11 Due to the coordination of the second tantalum atom to the tantalacyclopentadiene fragment, the bond lengths of three C−C bonds in the ring of the tantalacyclopentadienes were similar to each other; on the other hand, the corresponding bond lengths found for (C4Et4)Ta(OAr)3 (OAr = 2,6-OiPr2C6H3) were 1.32(1), 1.49(1), and 1.35(1) Å, indicating a typical metallacyclopentadiene fragment (Figure 3).12

Scheme 2. Proposed Mechanism of a Mutual Exchange in 4a and 6a

Information). The negative ΔS⧧ value ruled out any dissociation processes in the mutual exchange. Because no fluxional behavior was observed for signals due to a κ1-OMe ligand and a μ-OMe ligand, it was assumed that the mutual exchange likely proceeded through a trigonal-bipyramidal state (TS1). Similar fluxional behavior was reported for some halfmetallocene complexes of group 5 metals.13 Similarly, VTNMR measurements and simulations were conducted for complex 6a in the temperature range of −58 to +2 °C, giving the thermodynamic parameters (ΔG⧧303 K = 58.0(±6.9) kJ/ mol, ΔH⧧ = 24.5(±3.1) kJ/mol, and ΔS⧧ = −110.6(±12.7) J/ (mol K)) (Scheme 2 and Figures SI4 and SI5 in the Supporting Information). Mechanism of Formation of Dinuclear Tantalacyclopentadiene Complexes from Dinuclear Tantalum Bis(alkyne) Complexes. The unexpected additive effects of a catalytic amount of 3-hexyne induced the smooth conversion of 2a into 4a (vide supra). Aiming to elucidate such effects, we conducted some control experiments. We first added 1 equiv of 4-octyne to a solution of 2a in C6D6 and then monitored its 1H NMR spectrum after 15 min, displaying signals due to 4a and 7 at an intensity ratio of 4a (39%) and 7 (48%) along with those due to 2a (eq 7). Complex 7 was a product of the coupling

Figure 3. Schematic drawing and the selected bond lengths of (C4Et4)Ta(OAr)3.

We observed fluxional behavior for complex 4a on the basis of VT-NMR measurements in toluene-d8 in the temperature range of −43 to +29 °C, suggesting a mutual exchange between two enantiomers, as shown in eq 6. The 1H NMR spectrum of

4a at −43 °C in the range of δH 2.1−3.6 displayed eight resonances in an ABX3 pattern (2JAB = ca. 14 Hz, 3JAX = ca. 7 Hz, 3JBX = ca. 7 Hz) assignable to eight methylene protons attached to the tantalacyclopentadiene fragment, three signals of which overlapped around δH 3.2 (Figure SI3 in the Supporting Information). As the temperature increased, the signals gradually broadened and eight peaks coalesced around 0 °C to become four peaks at 29 °C; two broad signals centered at δH 3.06 and 3.29 were assignable to methylene protons bound to α-carbons of the tantalacyclopentadiene skeleton, and two other signals centered at δH 2.40 and 3.22 appeared as an ABX3 pattern (2JAB = ca. 14 Hz, 3JAX = ca. 7 Hz, 3JBX = ca. 7 Hz) due to methylene protons bound to β-carbons of the tantalacyclopentadiene moiety. Because the last two signals between δH 2.0 and 2.5 at 30 °C were well separated into two signals at −43 °C, simulations of the 1H NMR spectra provided the thermodynamic parameters (ΔG⧧303 K = 54.3(±8.0) kJ/ mol, ΔH⧧ = 42.2(±3.7) kJ/mol, and ΔS⧧ = −40.1(±14.2) J/ (mol K)) (Scheme 2 and Figure SI5 in the Supporting

reaction between 3-hexyne and 4-octyne. When the amount of 4-octyne was increased up to 10 equiv, the yield of 7 was considerably increased up to 90%. On the basis of the additive effects of alkynes for the dinuclear tantalacyclopentadiene formation, we proposed the reaction mechanism shown in Scheme 3. The first step is the coordination of 4-octyne to one of the two tantalum atoms of 2a to give the transient species I, followed by the insertion of 4octyne to the tantalacyclopropene to form the dissymmetric tantalacyclopentadiene species II. The other tantalum center D

DOI: 10.1021/acs.organomet.6b00182 Organometallics XXXX, XXX, XXX−XXX

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Organometallics Scheme 3. Proposed Mechanism for the Formation of 7 by the Reaction of 2a with 4-Octyne

Scheme 4. Intramolecular Coupling of Two 2-Butyne Ligands at a Dinuclear Tungsten Center, Reported by Chisholm et al.4

can be migrated to coordinate to the tantalacyclopentadiene moiety to generate another transient species III, from which 3hexyne was released to generate 7. The released 3-hexyne can react with 2a to produce 4a in the same manner. It was another conceivable mechanism that the complex 2a partially dissociated into the mononuclear tantalum complexes (η2EtCCEt)TaCl2(OMe) and (η2-EtCCEt)TaCl2(OMe)(thf), and then 4-octyne reacted with (η2-EtCCEt)TaCl2(OMe) to form (C4-2,3-nPr2-4,5-Et2)TaCl2(OMe), followed by coordination of the C4-2,3-nPr2-4,5-Et2 moiety to the tantalum center of (η2-EtCCEt)TaCl2(OMe)(thf) to afford 7 with release of 3-hexyne. However, the mechanism was less plausible because the unsaturated mononuclear complexes would show good catalytic activity for alkyne cyclotrimerization and the added 4-octyne would be converted immediately into hexapropylbenzene. Interestingly, Chisholm et al. reported that thermal reaction of a bis(η2-alkyne) dinuclear tungsten complex, [(η2-MeCCMe)W(OiPr)(CH2SiMe3)]2(μ-OiPr)2 (E), in C6D6 resulted in the formation of two dinuclear tungstacyclopentadiene complexes, R 1 W 2 (μ-CSiMe 3 )(μC4Me4)(OiPr)4 (F, R1 = H; G, R1 = CH2SiMe3), even in the presence of other alkynes (Scheme 4).4 In this case, intramolecular rearrangement of two alkyl groups in complex E induced C−C bond formation. Some Reactions of Dinuclear Tantalacyclopentadiene Complexes. We previously reported that a dinuclear tantalacyclopentadiene complex, Ta2Cl6(μ-C4Et4), was a catalyst for alkyne cyclotrimerization.2 Accordingly, we tested the catalytic performance of 4a−6a toward the cyclotrimerization of 3-hexyne. When a solution of complex 4a and 20 equiv of 3-hexyne in C6D6 was maintained at room temperature for 30 h, its 1H NMR spectrum revealed the formation of hexaethylbenzene in 93% yield (eq 8). Complex 5a produced hexaethylbenzene quantitatively; on the other hand, complex 6a showed little catalytic activity. The cyclotrimerization activity was thus ranked in the order of Ta2Cl6(μ-C4Et4) > 5a > 4a ≫ 6a. It was considered that coordination of Lewis bases to the tantalum center decreased the catalytic activity, and strong

coordination of DMAP definitely prohibited the cyclotrimerization reaction.14 A dinuclear tantalacyclopentadiene fragment easily reacted with MeOH due to the high oxophilicity of the tantalum atoms.15 Complex 4a reacted with 1 equiv of CH3OH to give the dinuclear butadienyl tantalum complex 8 in 85% yield (eq 9): the protonation of the μ-C4Et4 ligand afforded a bridging

butadienyl μ-HC4Et4 ligand and a new terminal OMe ligand. In the 1H NMR spectrum of 8, a broad signal assignable to the terminal butadienyl proton was observed at δH 2.13, and three singlet signals due to two κ1-OMe ligands and one μ-OMe ligand were observed at δH 4.33, 4.59, and 4.73, respectively. When methanol-d4 was used for the protonation of 4a, complex 8-d4 with a μ-DC4Et4 ligand was selectively obtained, as evidenced by the disappearance of the signal at δH 2.13. Although three singlet signals assignable to three methoxy ligands were observed at the same chemical shift as those of 8, the integral values of two κ1-OMe were half those of 8, indicating that scrambling of the κ1-OCH3 and κ1-OCD3 ligands occurred without involvement of the μ-OCH3 ligand, although the mechanism of the scrambling was as yet unclear. In the 13C{1H} NMR spectrum of 8, the signal of the δ-carbon of the butadienyl tantalum moiety was observed at higher magnetic field (δC 106.8) in comparison with that of the αcarbon (δC 219.8). E

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Organometallics

reduced butadiene fragment by Ta2 so that Ta1 and Ta2 adopt the oxidation states +5 and +5, respectively.16 The Ta1···H1 and Ta1···C4 lengths (2.17(11), 2.477(9) Å) imply the presence of an agostic interaction of C4−H1 with Ta1. The agostic interaction was also observed in the nondecoupling 13C NMR spectrum, showing a doublet resonance for TaCαH with a coupling constant of 125 Hz, a value smaller than those of typical C(sp2)−H bonds without an agostic interaction, even when the partial contribution of the canonical form 8-B containing C(sp3)−H is taken into account.17

X-ray diffraction analysis of 8 elucidated the overall molecular structure, in which the position of the terminal butadienyl proton was located on the Fourier density map and refined without constraint (Figure 4 and Table 3). Notably, the length

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CONCLUSION We found an unexpected effect of an alkyne on doubly methoxy bridged dinuclear tantalum bis(η2-alkyne) complexes 2a,b: coordination of an additional alkyne to one of two tantalum centers of 2a,b induced C−C bond formation of two alkynes on the dinuclear tantalum scaffolds to produce the dinuclear tantalacyclopentadiene complexes 4a,b. We found that the dinuclear tantalum complexes 4a and 5a showed equally high catalytic activities for the cyclotrimerization of 3-hexyne, while complex 6a showed poor activity in the order 5a > 4a ≫ 6a. Accordingly, the tightly bound DMAP retarded the catalytic activity, in contrast to the labile THF ligand for 4a and 5a.

Figure 4. Molecular structure of complex 8 with 50% thermal ellipsoids. All hydrogen atoms except for H1 and solvent molecules are omitted for clarity.

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Ta1−Ta2 Ta1−O2 Ta1−C1 Ta2−C1 Ta2−C3 C1−C2 C3−C4 C1−Ta−C4 C1−C2−C3 C3−C4−Ta1 C3−C4−H1

2.9662(11) 1.988(7) 2.147(8) 2.486(8) 2.347(9) 1.431(12) 1.457(12) 69.5(3) 115.6(7) 114.1(6) 103(5)

Ta1···H1 Ta2−O2 Ta1···C4 Ta2−C2 Ta2−C4 C2−C3 C4−H1 Ta1−C1−C2 C2−C3−C4 Ta1−C4−H1

EXPERIMENTAL SECTION

General Considerations. All manipulations involving air- and moisture-sensitive tantalum complexes were performed under argon using a standard Schlenk technique or an argon-filled glovebox. (η2EtCCEt)TaCl3 (dme) (1a) was prepared according to the literature.6 3-Hexyne and 4-octyne were purchased from TCI or Aldrich and distilled over CaH2 before use. 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.18 1,2-Dimethoxyethane, benzene-d6, toluene-d8, CD2Cl2, and THF-d8 were distilled over CaH2 or Na/benzophenone and then degassed before use. 1H NMR (400 MHz) and 13C{1H} NMR (100 MHz) spectra were measured on Bruker AVANCEIII-400 and JEOL JNM-ESC400 spectrometers. Assignment of 1H and 13C{1H} NMR peaks for some of the complexes were aided by 2D 1H−1H COSY, 1H−1H ROESY, 2D 1 H−13C HMQC, and 2D 1H−13C HMBC spectra. GC-MS measurements were performed using a DB-1 capillary column (0.25 mm × 30 m) on a Shimazu GCMS-QP2010Plus instrument. All melting points were measured in sealed tubes under an argon atmosphere. UV−vis spectra were recorded on an Agilent 8453 instrument. Elemental analyses were recorded by using s PerkinElmer 2400 instrument at the Faculty of Engineering Science, Osaka University. Synthesis of (η2-nPrCCnPr)TaCl3(dme) (1b). Toluene (40 mL) was added to TaCl5 (5.00 g, 14.0 mmol) in an 80 mL Schlenk tube, and then DME (20 mL) was slowly added to the reaction mixture. Zn powder (1.37 g, 21.0 mmol) was placed into the Schlenk tube in one portion at room temperature. After the mixture was stirred at room temperature for 1 h, 4-octyne (2.19 mL, 14.9 mmol) was added via a syringe. The suspension was stirred at 50 °C for 2 h. All volatiles were removed under reduced pressure, and then the residue was extracted with toluene (30 mL). By layering hexane (30 mL) onto the toluene solution and storing at −20 °C, orange crystals were formed. The supernatant was decanted, and then the crystals were washed with hexane (10 mL × 3). All volatiles were removed under reduced pressure to give complex 1b (4.54 g, 9.33 mmol) as an orange powder in 67% yield, mp 87−90 °C dec. 1H NMR (400 MHz, C6D6, 30 °C): δ 1.00 (t, 3J = 7.5 Hz, 6H, CH2CH2CH3), 1.91 (sextet, 3J = 7.5 Hz, 4H, CH2CH2CH3), 3.09−3.20 (m, 4H, CH3OCH2CH2OCH3), 3.20 (s, 3H, CH3OCH2CH2OCH3), 3.63 (s, 3H, CH3OCH2CH2OCH3), 3.67 (t, 3J = 7.5 Hz, 4H, CH2CH2CH3). 13C{1H} NMR (100 MHz, C6D6, 30 °C): δ 15.0 (CCH2CH2CH3), 22.7 (CCH2CH2CH3), 42.7 (CCH2CH2CH3), 62.4 (CH3OCH2CH2OCH3), 68.0

Table 3. Selected Bond Lengths (Å) and Bond Angles (deg) of 8 2.17(11) 2.070(7) 2.477(9) 2.491(9) 2.359(10) 1.427(12) 1.19(10) 126.0(6) 114.3(8) 61(5)

(2.477(9) Å) of Ta1···C4 was considerably longer than that (2.147(8) Å) of Ta1−C1, indicating no σ bond between Ta1 and C4. The Ta···Ta distance (2.9662(11) Å) is longer by 0.11 Å than those found for 5a (2.8561(9) Å) and 6a (2.8537(8) Å), which suggested a weak Ta···Ta interaction. The C1−C2, C2− C3, and C3−C4 bond lengths were almost the same as those found for 5a and 6a. In light of these geometric data as well as NMR spectroscopic data, two canonical forms could be hypothesized for complex 8, as shown in Scheme 5. The canonical form 8-A has two π bonds between Ta2 and the C1− C2−C3−C4 fragment so that Ta1 and Ta2 adopt the oxidation states +5 and +3, respectively, together with a dative metal− metal bond. The canonical form 8-B has a two-electronScheme 5. Two Canonical Structures for Complex 8

F

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Article

Organometallics

1H, CH2CH2CH3), 3.79 (s, 3H, κ1-OMe), 4.33 (m, 4H, α-thf), 5.03 (s, 3H, μ-OMe). 13C{1H} NMR (100 MHz, toluene-d8, −60 °C): δ 15.1− 15.3 (4C, CH2CH2CH3), 25.9 (Cβ of thf), 26.0 (CH2CH2CH3) 26.8 (CH2CH2CH3), 28.6 (2C, CH2CH2CH3), 32.5 (CH2CH2CH3), 32.9 (CH2CH2CH3), 45.1 (CH2CH2CH3), 47.7 (CH2CH2CH3), 63.8 (κ1OMe), 70.1 (μ-OMe), 82.2 (Cα of thf), 143.2 (CβCH2CH2CH3), 152.8 (C β CH 2 CH 2 CH 3 ), 213.9 (C α CH 2 CH 2 CH 3 ), 219.4 (CαCH2CH2CH3). UV−vis (toluene) λmax/nm (ε/M−1 cm−1): 375 (1.6 × 103). Anal. Calcd for C22H42Cl4O3Ta2: C, 30.79; H, 4.93. Found: C, 30.82; H, 5.11. Synthesis of Ta2Cl5(OMe)(μ-C4Et4)(thf) (5a). SiCl4 (64.0 mg, 0.377 mmol) was added to a solution of 4a (300 mg, 0.374 mmol) in toluene (5 mL) in a 20 mL Schlenk tube. After the reaction mixture was stirred at 60 °C for 12 h, the solvent was evaporated to give a green-black residue. The residue was washed with hexane (5 mL), and evaporation to dryness gave complex 5a (290 mg, 0.359 mmol) as a green powder in 96% yield, mp 248−250 °C dec. 1H NMR (400 MHz, C6D6, 30 °C): δ 0.88 (t, 3J = 7.5 Hz, 6H, CH2CH3), 0.97 (t, 3J = 7.5 Hz, 6H, CH2CH3), 1.21 (m, 4H, β-thf), 2.50 (dq, 2J = 14.9 Hz, 3J = 7.5 Hz, 2H, CH2CH3), 3.25 (dq, 2J = 14.9 Hz, 3J = 7.5 Hz, 2H, CH2CH3), 3.35 (dq, 2J = 14.9 Hz, 3J = 7.5 Hz, 2H, CH2CH3), 3.45 (dq, 2J = 14.9 Hz, 3J = 7.5 Hz, 2H, CH2CH3), 4.26 (m, 4H, α-thf), 5.09 (s, 3H, μOMe). 13C{1H} NMR (100 MHz, C6D6, 30 °C): δ 17.1 (CH2CH3), 20.9 (CH2CH3), 23.7 (CH2CH3), 25.7 (CH2CH3), 40.3 (Cβ of thf), 73.2 (μ-OMe), 80.7 (Cα of thf), 154.6 (CβCH2CH3), 219.5 (CαCH2CH3). UV−vis (toluene) λmax/nm (ε/M−1 cm−1): 406 (1.7 × 103). Anal. Calcd for C17H31Cl5O2Ta2: C, 25.31; H, 3.87. Found: C, 25.26; H, 3.87. Synthesis of Ta2Cl5(OMe)(μ-C4nPr4)(thf) (5b). Complex 5b was synthesized as described for 5a. The reaction of 4a (321 mg, 0.374 mmol) with SiCl4 (64.0 mg, 0.377 mmol) in toluene (5 mL) gave 5b (308 mg, 0.357 mmol) as a green powder in 95% yield, mp 187−189 °C dec. 1H NMR (400 MHz, C6D6, 30 °C): δ 0.86 (t, 3J = 7.2 Hz, 6H, CH2CH2CH3), 0.92 (t, 3J = 7.1 Hz, 6H, CH2CH2CH3), 0.96−1.12 (m, 2H, CH2CH2CH3), 1.27 (m, 4H, β-thf), 1.30−1.41 (m, 4H, CH2CH2CH3), 1.62−1.78 (m, 2H, CH2CH2CH3), 2.55−2.70 (m, 2H, CH2CH2CH3), 3.24−3.35 (m, 2H, CH2CH2CH3), 3.35−3.54 (m, 4H, CH2CH2CH3), 4.29 (m, 4H, α-thf), 5.12 (s, 3H, μ-OMe). 13 C{1H} NMR (100 MHz, C6D6, 30 °C): δ 14.9 (CH2CH2CH3), 15.1 (CH2CH2CH3), 25.8 (CH2CH2CH3), 26.5 (CH2CH2CH3), 29.7 (CH2CH2CH3), 33.7 (CH2CH2CH3), 50.0 (Cβ of thf), 73.2 (μOMe), 80.9 (C α of thf), 153.6 (C β CH 2 CH 2 CH 3 ), 218.2 (CαCH2CH2CH3). UV−vis (toluene) λmax/nm (ε/M−1 cm−1): 406 (1.5 × 103). Anal. Calcd for C21H39Cl5O2Ta2: C, 29.24; H, 4.56. Found: C, 29.48; H, 4.30. Synthesis of Ta2Cl4(OMe)2(μ-C4Et4)(dmap) (6a). In a 20 mL Schlenk tube, complex 4a (100 mg, 0.125 mmol) and DMAP (15.0 mg, 0.122 mmol) were dissolved in toluene (5 mL). After the reaction mixture was stirred at room temperature for 1 h, the solvent was removed under reduced pressure. The residue was washed with hexane (5 mL), and all volatiles were evaporated under reduced pressure. A red-brown powder of 6a (100 mg, 0.117 mmol) was obtained in 94% yield, mp 198−200 °C dec. 1H NMR (400 MHz, CD2Cl2, −60 °C): δ 0.63 (t, 3J = 7.2 Hz, 3H, CH2CH3), 0.72 (t, 3J = 7.2 Hz, 3H, CH2CH3), 1.12−1.26 (m, 6H, CH2CH3), 2.36 (m, 2H, CH2CH3), 2.77−2.95 (m, 4H, CH2CH3), 3.12 (s, 6H, NMe2), 3.17 (m, 2H, CH2CH3), 4.20 (s, 3H, κ1-OMe), 5.14 (s, 3H, μ-OMe), 6.53 (d, 3J = 6.7 Hz, 2H, Ar), 8.67 (d, 3J = 6.7 Hz, 2H, Ar). 13C{1H} NMR (100 MHz, CD2Cl2, −60 °C): δ 14.0 (CH2CH3), 16.0 (CH2CH3), 17.0 (CH2CH3), 18.9 (CH2CH3), 22.5 (CH2CH3), 22.7 (CH2CH3), 35.1 (CH2CH3), 37.4 (CH2CH3), 39.6 (NMe2), 64.1 (κ1-OMe), 69.2 (μ-OMe), 105.2 (Ar), 128.8 (Ar), 142.0 (Ar), 154.0 (C β CH2 CH3 ), 154.5 (C β CH2 CH 3), 221.2 (CαCH2CH3), 223.0 (CαCH2CH3). UV−vis (toluene) λmax/nm (ε/ M−1 cm−1): 390 (3.6 × 103). Anal. Calcd for C21H36Cl4N2O2Ta2: C, 29.60; H, 4.26; N, 3.29. Found: C, 29.35; H, 4.26; N, 3.27. Synthesis of Ta2Cl4(OMe)2(μ-C4nPr4)(dmap) (6b). Complex 6b was synthesized in the same manner as that of 6a. Reaction of 4b (100 mg, 0.117 mmol) with DMAP (14.2 mg, 0.117 mmol) gave complex 6b (99.5 mg, 0.110 mmol) in 94% yield, mp 163−165 °C dec. 1H NMR (400 MHz, toluene-d8, −60 °C): δ 0.82−1.13 (m, 12H,

(CH 3 OCH 2 CH 2 OCH 3 ), 70.4 (CH 3 OCH 2 CH 2 OCH 3 ), 75.9 (CH3OCH2CH2OCH3), 253.5 (CCH2CH2CH3). Anal. Calcd for C12H24Cl3O2Ta: C, 29.56; H, 4.96. Found: C, 29.04; H, 5.40. There were large differences between C, H found and C, H calculated values, presumably due to the contamination of Zn salts. The 1H and 13C{1H} NMR spectra are shown in the Supporting Information. Synthesis of [(η2-EtCCEt)TaCl2]2(μ-OMe)2(μ-thf) (2a). In an 80 mL Schlenk tube containing 1a (3.83 g, 8.33 mmol) in THF (40 mL) was added a solution of NaOMe (450 mg, 8.33 mmol) in THF (20 mL) at −78 °C, and then the reaction mixture was stirred at room temperature for 4 h. The solvent was removed under reduced pressure, and then the orange-oil residue was extracted with hexane (200 mL). Evaporation of the solvent gave a yellow powder. The yellow powder was washed with hexane (10 mL) and then dried under reduced pressure to give 2a as a yellow powder. The washing solution was kept at −40 °C for 1 day to give microcrystals of 2a as a second crop. Complex 2a was obtained in 64% yield (total yield, 2.15 g, 2.68 mmol), mp 98−100 °C dec. 1H NMR (400 MHz, C6D6, 30 °C): δ 1.18 (t, 3J = 7.5 Hz, 12H, CCH2CH3), 1.66 (br, 4H, β-thf), 3.36 (q, 3J = 7.5 Hz, 8H, CCH2CH3), 4.07 (s, 6H, OMe), 4.23 (br, 4H, α-thf). 13 C{1H} NMR (100 MHz, C6D6, 30 °C): δ 13.9 (CCH2CH3), 24.2 (Cβ of thf), 32.8 (CCH2CH3), 66.1 (OMe), 74.3 (Cα of thf), 252.7 (CCH2CH3). Anal. Calcd for C18H34Cl4O3Ta2: C, 26.95; H, 4.27. Found: C, 26.97; H, 4.31. Synthesis of [(η2-nPrCCnPr)TaCl2]2(μ-OMe)2(μ-thf) (2b). Complex 2b was synthesized in the same manner as that of 2a. The orange crystals of 2b (2.01 g, 2.34 mmol) were obtained in 54% yield, mp 75−78 °C dec. 1H NMR (400 MHz, C6D6, 30 °C): δ 0.93 (t, 3J = 7.5 Hz, 12H, CH2CH2CH3), 1.67 (br, 4H, β-thf), 1.72 (sextet, 3J = 7.5 Hz, 8H, CH2CH2CH3), 3.41 (t, 3J = 7.5 Hz, 8H, CH2CH2CH3), 4.11 (s, 6H, OMe), 4.30 (br, 4H, α-thf). 13C{1H} NMR (100 MHz, C6D6, 30 °C): δ 14.9 (CCH2CH2CH3), 22.7 (CCH2CH2CH3), 24.2 (Cβ of thf), 41.7 (CCH2CH2CH3), 66.0 (OMe), 74.4 (Cα of thf), 251.4 (CCH2CH2CH3). Anal. Calcd for C22H42Cl4O3Ta2: C, 30.79; H, 4.93. Found: C, 30.47; H, 5.18. Synthesis of Ta2Cl4(OMe)2(μ-C4Et4)(thf) (4a). To a solution of complex 2a (830 mg, 1.03 mmol) in toluene (15 mL) was added 3hexyne (23.6 μL, 0.207 mmol) via a microsyringe at −40 °C. The reaction mixture was stirred at room temperature for 30 min. During the reaction, the color of the solution changed from orange to purple. All volatiles were removed under reduced pressure, and then the purple residue was washed with hexane (5 mL × 3). The residue was dried under reduced pressure to give 4a (650 mg, 0.810 mmol) as a purple powder in 78% yield, mp 173−175 °C dec. 1H NMR (400 MHz, toluene-d8, −60 °C): δ 0.91 (t, 3J = 7.0 Hz, 3H, CH2CH3), 0.95 (t, 3J = 7.0 Hz, 3H, CH2CH3), 1.01−1.11 (m, 6H, CH2CH3), 1.14 (m, 4H, β-thf), 2.23 (dq, 2J = 14.0 Hz, 3J = 7.0 Hz, 1H, CH2CH3), 2.41 (dq, 2J = 14.0 Hz, 3J = 7.0 Hz, 1H, CH2CH3), 2.73 (dq, 2J = 14.0 Hz, 3 J = 7.0 Hz, 1H, CH2CH3), 3.07 (dq, 2J = 14.0 Hz, 3J = 7.0 Hz, 1H, CH2CH3), 3.11−3.31 (m, 3H, CH2CH3), 3.51 (dq, 2J = 14.0 Hz, 3J = 7.0 Hz, 1H, CH2CH3), 3.78 (s, 3H, κ1-OMe), 4.29 (m, 4H, α-thf), 5.02 (s, 3H, μ-OMe). 13C{1H} NMR (100 MHz, toluene-d8, −60 °C): δ 16.5 (CH2CH3), 17.4 (CH2CH3), 19.9 (CH2CH3), 20.0 (CH2CH3), 22.8 (CH2CH3), 23.1 (CH2CH3), 25.8 (Cβ of thf), 35.3 (CH2CH3), 38.3 (CH2CH3), 63.9 (κ1-OMe), 70.1 (μ-OMe), 80.5 (Cα of thf), 144.2 (CβCH2CH3), 153.7 (CβCH2CH3), 215.9 (CαCH2CH3), 220.7 (CαCH2CH3). UV−vis (toluene) λmax/nm (ε/M−1c m−1): 372 (5.1 × 102). Anal. Calcd for C18H34Cl4O3Ta2: C, 26.95; H, 4.27. Found: C, 26.99; H, 4.53. Synthesis of Ta2Cl4(OMe)2(μ-C4nPr4)(thf) (4b). Complex 4b was synthesized in the same manner as that of 4a. Reaction of 2b (300 mg, 0.350 mmol) with 4-octyne (10.3 μL, 0.0701 mmol) gave complex 4b (230 mg, 0.268 mmol) in 77% yield, mp 115−118 °C dec. 1H NMR (400 MHz, toluene-d8, −60 °C): δ 0.80−0.92 (br, 3H, CH2CH2CH3), 0.90−1.01 (br, 3H, CH2CH2CH3), 0.99−1.11 (br, 6H, CH2CH2CH3), 1.07−1.20 (br, 2H, CH2CH2CH3), 1.21 (m, 4H, β-thf), 1.30−1.57 (br, 4H, CH2CH2CH3), 1.75−1.95 (br, 2H, CH2CH2CH3), 2.37−2.50, (m, 1H, CH2CH2CH3), 2.50−2.65 (m, 1H, CH2CH2CH3), 2.78−2.95 (m, 1H, CH2CH2CH3), 3.05−3.18 (m, 1H, CH2CH2CH3), 3.17−3.33 (m, 2H, CH2CH2CH3), 3.33−3.45 (m, 1H, CH2CH2CH3), 3.45−3.58 (m, G

DOI: 10.1021/acs.organomet.6b00182 Organometallics XXXX, XXX, XXX−XXX

Organometallics

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CH2CH2CH3), 1.13−1.32 (m, 4H, CH2CH2CH3), 1.65 (s, 6H, NMe2), 1.65−1.94 (m, 4H, CH2CH2CH3), 2.55−2.80 (m, 2H, CH2CH2CH3), 2.93−3.08 (m, 1H, CH2CH2CH3), 3.26−3.41 (m, 2H, CH2CH2CH3), 3.40−3.53 (m, 1H, CH2CH2CH3), 3.53−3.74 (m, 2H, CH2CH2CH3), 3.92 (s, 3H, κ1-OMe), 5.25 (s, 3H, μ-OMe), 5.63 (brs, 2H, Ar), 9.01 (m, 2H, Ar). 13C{1H} NMR (100 MHz, toluene-d8, −60 °C): δ 15.10 (CH2CH2CH3), 15.14 (CH2CH2CH3), 15.2 (CH2CH2CH3), 15.3 (CH2CH2CH3), 23.3 (CH2CH2CH3), 26.0 (CH2CH2CH3), 27.0 (CH2CH2CH3), 28.9 (CH2CH2CH3), 32.8 (CH 2 CH 2 CH 3 ), 33.1 (CH 2 CH 2 CH 3 ), 37.8 (NMe 2 ), 45.4 (CH2CH2CH3), 47.9 (CH2CH2CH3), 63.8 (κ1-OMe), 69.2 (μOMe), 105.0 (Ar), 142.4 (CβCH2CH2CH3), 151.8 (CβCH2CH2CH3), 154.1 (Ar), 155.8 (Ar), 218.0 (C α CH 2 CH 2 CH 3 ), 223.3 (CαCH2CH2CH3). UV−vis (toluene) λmax/nm (ε/M−1 cm−1): 393 (2.6 × 103). Anal. Calcd for C25H44Cl4N2O2Ta2: C, 33.06; H, 4.88; N, 3.08. Found: C, 32.93; H, 4.99; N, 2.87. Reaction of Complex 2a with 4-Octyne. In a J. Young NMR tube, complex 2a (10 mg, 0.012 mmol) and anthracene as an internal standard were dissolved in C6D6. One equivalent of 4-octyne (1.8 μL, 0.012 mmol) was added at room temperature. After the NMR tube was kept at room temperature for 15 min, the 1H NMR spectrum was measured, and then the yields of 4a and 7 were determined by the integral ratio to the internal standard. Similarly, the reaction of 2a with 10 equiv of 4-octyne was carried out to calculate the ratio of 4a and 7. Data for 7 are as follows. 1H NMR (400 MHz, C6D6, 30 °C): δ 0.91 (t, 3 J = 6.6 Hz, 3H, CH2CH2CH3), 0.94−1.03 (m, 6H, CH2CH2CH3 and CH2CH3), 1.09 (t, 3J = 6.7 Hz, 3H, CH2CH3), 1.31 (m, 4H, β-thf), 1.28−1.51 (m, 3H, CH2CH2CH3), 1.72−1.88 (m, 1H, CH2CH2CH3), 2.34−2.55 (m, 1H, CH2CH3, 1H, CH2CH2CH3), 2.94−3.16 (m, 1H, CH2CH2CH3, 1H, CH2CH3), 3.14−3.43 (m, 2H, CH2CH2CH3, 2H, CH2CH3), 3.98 (s, 3H, κ1-OMe), 4.37 (m, 4H, α-thf), 5.07 (s, 3H, μOMe). Synthesis of Ta2Cl4(OMe)3(μ-HC4Et4) (8). To a solution of complex 4a (300 mg, 0.374 mmol) in toluene (5 mL) at −78 °C was added MeOH (15.0 μL, 0.370 mmol) via a microsyringe. After the reaction mixture was stirred at room temperature for 1 h, all volatiles were removed under reduced pressure. The red residue was washed with hexane (5 mL) to give 8a (240 mg, 0.316 mmol) as a red powder in 85% yield, mp 126−128 °C dec. 1H NMR (400 MHz, CD2Cl2, 30 °C): δ 1.16 (t, 3J = 7.1 Hz, 3H, CH2CH3), 1.23 (t, 3J = 7.7 Hz, 3H, CH2CH3), 1.31 (t, 3J = 7.9 Hz, 3H, CH2CH3), 1.33 (t, 3J = 7.9 Hz, 3H, CH2CH3), 2.09 (dq, 2J = 13.9, 3J = 7.1 Hz, 1H, CH2CH3), 2.13 (br, 1H, TaCH), 2.42 (dq, 2J = 14.5, 3J = 7.3 Hz, 1H, CH2CH3), 2.76 (dq, 2 J = 15.0, 3J = 7.5 Hz, 1H, CH2CH3), 2.91 (dq, 2J = 13.9, 3J = 7.1 Hz, 1H, CH2CH3), 2.92 (dq, 2J = 14.5, 3J = 7.1 Hz, 1H, CH2CH3), 3.06 (dq, 2J = 14.5, 3J = 7.3 Hz, 1H, CH2CH3), 3.15 (dq, 2J = 14.5, 3J = 7.1 Hz, 1H, CH2CH3), 3.69 (dq, 2J = 15.0, 3J = 7.5 Hz, 1H, CH2CH3), 4.33 (s, 3H, κ1-OMe), 4.59 (s, 3H, κ1-OMe), 4.73 (s, 3H, μ-OMe). 13 C{1H} NMR (100 MHz, CD2Cl2, 30 °C): δ 15.3 (CH2CH3), 18.6 (CH2CH3), 18.7 (CH2CH3), 18.9 (CH2CH3), 23.6 (CH2CH3), 25.4 (CH2CH3), 28.0 (CH2CH3), 35.0 (CH2CH3), 65.2 (κ1-OMe), 66.1 (κ1-OMe), 66.7 (μ-OMe), 106.8 (TaCH), 139.8 (CγCH2CH3), 156.3 (CβCH2CH3), 219.8 (CαCH2CH3). UV−vis (toluene) λmax/nm (ε/ M−1 cm−1): 482 (2.0 × 103). Anal. Calcd for C15H30Cl4O3Ta2: C, 23.64; H, 3.97. Found: C, 23.35; H, 4.04. Thermal Rearrangement of 2a or 2b in C6D6 at 60 °C. In J. Young NMR tubes, 2a or 2b and naphthalene as an internal standard were dissolved in C6D6. The ratio of the complex and naphthalene was determined by 1H NMR measurements. The NMR tube was heated at 60 °C for 16 h, and then 1H NMR spectra were measured. Catalytic Alkyne Cyclotrimerization Catalyzed by Tantalum Complexes 4a−6a. In J. Young NMR tubes, tantalum complexes 4a−6a (8.77 μmol, 5 mol % to alkynes), 3-hexyne (20.0 μL, 0.175 mmol), and anthracene as an internal standard were dissolved in C6D6 (0.6 mL). After the NMR tubes stood at room temperature for 30 h, 1 H NMR spectra of the samples were measured to determine the product yield.

Article

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.organomet.6b00182. Crystallographic data for complexes 2a, 5a, 6a, and 8 (CIF) Thermolysis results of complexes 4a,b, VT-NMR spectra of complexes 4a and 6a, and crystallographic data for complexes 2a, 5a, 6a, and 8 (PDF)

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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, +81-66850-6245. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS K.Y. acknowledges financial support by the JSPS Research Fellowship for Young Scientists and the JSPS Japanese-German Graduate Externship Program. This work was supported by a Grant-in-Aid for Scientific Research on Innovative Areas “Precise Formation of a Catalyst Having a Specified Field for Use in Extremely Difficult Substrate Conversion Reactions (No, 2702)” from the MEXT, Japan.

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REFERENCES

(1) For recent reviews for [2 + 2 + 2] cyclotrimerization, see: (a) Vollhardt, K. P. C. Angew. Chem., Int. Ed. Engl. 1984, 23, 539−556. (b) Boese, R.; Van Sickle, A. P.; Vollhardt, K. P. C. Synthesis 1994, 1994, 1374−1382. (c) Saito, S.; Yamamoto, Y. Chem. Rev. 2000, 100, 2901−2915. (d) Yamamoto, Y. Curr. Org. Chem. 2005, 9, 503−519. (e) Kotha, S.; Brahmachary, E.; Lahiri, K. Eur. J. Org. Chem. 2005, 2005, 4741−4767. (f) Chopade, P. R.; Louie, J. Adv. Synth. Catal. 2006, 348, 2307−2327. (g) Gandon, V.; Aubert, C.; Malacria, M. Chem. Commun. 2006, 2209−2217. (h) Varela, J. A.; Saa, C. Chem. Rev. 2003, 103, 3787−3802. (i) Omae, I. Appl. Organomet. Chem. 2007, 21, 318−344. (j) Tanaka, K. Chem. - Asian J. 2009, 4, 508−518. (2) Yamamoto, K.; Tsurugi, H.; Mashima, K. Chem. - Eur. J. 2015, 21, 11369−11377. (3) (a) Knox, S. A. R.; Stansfield, R. F. D.; Stone, F. G. A.; Winter, M. J.; Woodward, P. J. Chem. Soc., Dalton Trans. 1982, 173−185. (b) Knox, S. A. R.; Stansfield, R. F. D.; Stone, F. G. A.; Winter, M. J.; Woodward, P. J. Chem. Soc., Chem. Commun. 1978, 221−223. (c) Baxter, R. J.; Knox, G. R.; Pauson, P. L.; Spicer, M. D. Organometallics 1999, 18, 197−205. (d) Dickson, R. S.; McLure, F. I.; Nesbit, R. J. J. Organomet. Chem. 1988, 349, 413. (4) (a) Chisholm, M. H.; Eichhorn, B. W.; Folting, K.; Huffman, J. C. Organometallics 1989, 8, 49−66. (b) Chisholm, M. H.; Eichhorn, B. W.; Folting, K.; Huffman, J. C. Organometallics 1989, 8, 67−79. (c) Chisholm, M. H.; Eichhorn, B. W.; Huffman, J. C. Organometallics 1989, 8, 80−89. (5) (a) Tasi, M.; Powell, A. K.; Vahrenkamp, H. Angew. Chem., Int. Ed. Engl. 1989, 28, 318−319. (b) Tasi, M.; Powell, A. K.; Vahrenkamp, H. Chem. Ber. 1991, 124, 1549−1557. (6) Oshiki, T.; Tanaka, K.; Yamada, J.; Ishiyama, T.; Kataoka, Y.; Mashima, K.; Tani, K.; Takai, K. Organometallics 2003, 22, 464−472. (7) Templeton, J. L.; Ward, B. C. J. Am. Chem. Soc. 1980, 102, 3288− 3290. (8) (a) Oshiki, T.; Yamada, A.; Kawai, K.; Arimitsu, H.; Takai, K. Organometallics 2007, 26, 173−182. (b) Parker, K. D. J.; Fryzuk, M. D. Organometallics 2014, 33, 6122−6131. (c) Cotton, F. A.; Hall, W. T.

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DOI: 10.1021/acs.organomet.6b00182 Organometallics XXXX, XXX, XXX−XXX

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Organometallics Inorg. Chem. 1981, 20, 1285−1287. (d) Cotton, F. A.; Hall, W. T. Inorg. Chem. 1980, 19, 2352−2354. (e) Cotton, F. A.; Hall, W. T. Inorg. Chem. 1980, 19, 2354−2356. (f) Hill, A. F.; Smith, M. K. Organometallics 2007, 26, 4688−4691. (9) Marchetti, F.; Pampaloni, G.; Zacchini, S. Dalton Trans. 2009, 8096−8106. (10) (a) Boyd, P. D. W.; Nielson, A. J.; Rickard, C. E. F. J. Chem. Soc., Dalton Trans. 1987, 307−314. (b) Cotton, F. A.; Daniels, L. M.; Murillo, C. A.; Wang, X. J. Am. Chem. Soc. 1996, 118, 12449−12450. (11) (a) Chisholm, M. H.; Hoffman, D. M.; Huffman, J. C. J. Am. Chem. Soc. 1984, 106, 6806−6815. (b) Chisholm, M. H.; Folting, K.; Huffman, J. C.; Rothwell, I. P. J. Am. Chem. Soc. 1982, 104, 4389− 4399. (c) Chisholm, M. H.; Folting, K.; Huffman, J. C.; Klang, J. A.; Streib, W. E. Organometallics 1989, 8, 89−93. (12) Strickler, J. R.; Wexler, P. A.; Wigley, D. E. Organometallics 1988, 7, 2067−2069. (13) Herrmann, W. A.; Baratta, W. J. Organomet. Chem. 1996, 506, 357−361. (14) Oshiki, T.; Nomoto, H.; Tanaka, K.; Takai, K. Bull. Chem. Soc. Jpn. 2004, 77, 1009−1011. (15) Strickler, J. R.; Wexler, P. A.; Wigley, D. E. Organometallics 1991, 10, 118−127. (16) Yasuda, H.; Tatsumi, K.; Okamoto, T.; Mashima, K.; Lee, K.; Nakamura, A.; Kai, Y.; Kanehisa, N.; Kasai, N. J. Am. Chem. Soc. 1985, 107, 2410−2422. (17) (a) Brookhart, M.; Green, M. L. H.; Wong, L. Prog. Inorg. Chem. 1988, 36, 1−124. (b) Crabtree, R. H.; Hamilton, D. G. Adv. Organomet. Chem. 1988, 28, 299−338. (18) Pangborn, A. B.; Giardello, M. A.; Grubbs, R. H.; Rosen, R. K.; Timmers, F. J. Organometallics 1996, 15, 1518−1520.

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DOI: 10.1021/acs.organomet.6b00182 Organometallics XXXX, XXX, XXX−XXX