Synthesis and Reaction of Neutral and Cationic Alkyltantalum

Oct 6, 2011 - Synthesis and Reaction of Neutral and Cationic Alkyltantalum. Complexes with a Linked Cyclopentadienyl-Carboranyl Ligand. Hayato Tsurugi...
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Synthesis and Reaction of Neutral and Cationic Alkyltantalum Complexes with a Linked Cyclopentadienyl-Carboranyl Ligand Hayato Tsurugi,† Zaozao Qiu,‡ Koji Yamamoto,† Rocio Arteaga-M€uller,† Kazushi Mashima,*,† and Zuowei Xie*,‡ †

Department of Chemistry, Graduate School of Engineering Science, Osaka University, Toyonaka, Osaka 560-8531, Japan, and CREST, JST, Japan ‡ Department of Chemistry and Center of Novel Functional Molecules, The Chinese University of Hong Kong, Shatin, New Territories, Hong Kong, People's Republic of China

bS Supporting Information ABSTRACT:

A trimethyltantalum complex with a Cp-carboranyl ligand was prepared via a salt metathesis reaction of TaCl2Me3 with [Li2{C5H4CMe2(C2B10H10)}(OEt2)2]. Two equivalents of carbodiimides reacted with Ta(Cp-carboranyl)Me3 to form fourmembered azametallacyclic compounds with an exocyclic imine group. A Lewis base stabilized cationic species was generated by the reaction of Ta(Cp-carboranyl)Me3 with B(C6F5)3(THF).

’ INTRODUCTION Early transition-metal constrained geometry catalysts (CGC), composed of cyclopentadienyl (Cp) and linked heteroatom ligands, have attracted much interest due to their ability to produce linear low-density poly(olefin)s. Various CGC-style complexes with different architectures of linked Cp-amide ligands have been studied.1 4 The carbanion attracts special interest as a linked donor atom because of its strong σ-donating and weak π-donating nature compared with nitrogen and oxygen atoms; however, the use of a carbanion as the linked donor atom is less developed due to the facile incorporation of the metal carbon bond into various catalytic transformations. We anticipated that the Cp-carboranyl ligand system might be a candidate of such a linked Cp-carbanion ligand for CGC because the metal carboranyl σ-bond might be stabilized due to carboranyl cage effects. We previously reported various Cp-carboranyl metal halide and amide complexes of group 4 metals.5 Because of the presence of electropositive boron atoms in the carborane cage, the metal center is highly Lewis acidic compared to N-, O-, P-, and S-based σ-donor ligated complexes. For the Cp-carboranyl metal alkyl complexes, three types of metal carbon bonds, the M C π-bond, the M C(cage) σ-bond, and the M alkyl σ-bond, are involved.6 8 A notable finding in this contribution was that the Ta alkyl σ-bond was selectively reacted with carbodiimines among three types of tantalum carbon bonds, in sharp contrast to the insertion of alkenes and alkynes into the early transitionmetal C(cage) σ-bond being often observed.9 Furthermore, we found that the ionic species, [Ta{η5-C5H4CMe2(C2B10H10)k1 C}Me 2 (THF)][B(C 6 F 5 )3 Me] was in equilibrium with r 2011 American Chemical Society

Ta{η5-C5H4CMe2(C2B10H10)-k1C}Me3 and B(C6F5)3(THF), indicating the highly electron-deficient nature of the tantalum atom supported by the Cp-carboranyl ligand.

’ RESULTS AND DISCUSSION Treatment of TaCl2Me3 with 1 equiv of [Li2{C5H4CMe2(C2B10H10)}(OEt2)2] in toluene at room temperature afforded Ta{η5-C5H4CMe2(C2B10H10)-k1C}Me3 (1) in 58% yield as gray powders (eq 1). In contrast, the alkane elimination reaction of Ta(CH2Ph)5 with the Cp-carborane ligand, C5H5CMe2(C2B10H11), in toluene did not proceed even if heated. In the 1 H NMR spectrum, three methyl groups bound to tantalum were observed as a singlet at δ 1.48, and the cyclopentadienyl ring protons were observed as a broad signal centered at δ 5.10 in four hydrogen intensity. The 13C NMR spectrum of 1 displayed one resonance at δ 82.5 assignable to CH3 bound to the tantalum atom. The molecular structure of 1 was determined by X-ray analysis.10 Figure 1 reveals that the tantalum atom adopts a distorted trigonal-bipyramidal geometry, where the centroid of the Cp ring and one of three methyl groups bound to tantalum occupies the apical position, and two methyl groups and one carbon of the carboranyl moiety attach in trigonal corners. The distance of the Ta C(1) bond (2.265(3) Å) is in the range typically observed for a σ-bound metal carboranyl moiety.5c The elongation of the Ta C(13) bond (2.212(3) Å) compared to others (2.182(3) Å for Ta C(11), 2.181(4) Å for Received: August 18, 2011 Published: October 06, 2011 5960

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azatantalacyclobutane with an exocyclic imine group were formed (molecular structure of 2b in Figure 2; see Figure S1 (Supporting Information) for 2a).11 In the solid-state structures of 2, the tantalum atom has a pseudooctahedral geometry. The N(3) C(19) N(4) ligand coordinates to the tantalum atom as an acetoamidinate-k2N,N ligand, which is confirmed by almost the same distances of N(3) C(19) and N(4) C(19) bonds (1.327 (7) and 1.319(7) Å). The distance of the Ta C(1) bond (2.354(5) Å) is longer than that of 1 due to the increase of steric hindrance between the carboranyl moiety and other equatorial ligands (C(1) Ta N(1) 99.79(17)°; C(1) Ta N(3) 93.79(16)°). The bond length of Ta C(11) is a typical σ-bonded tantalum carbon bond. Figure 1. ORTEP drawing of the molecular structure of 1. All hydrogen atoms are omitted for clarity. Selected distances (Å) and angles (deg.): Ta C1 = 2.265(3), Ta C11 = 2.182(3), Ta C12 = 2.181(4), Ta C13 = 2.212(3); C1 Ta C11 = 114.94(11), C1 Ta C12 = 115.14(12), C11 Ta C12 = 119.44(13).

Figure 2. ORTEP drawing of the molecular structure of 2b. All hydrogen atoms are omitted for clarity. Selected distances (Å) and angles (deg.): Ta C1 = 2.354(5), Ta C11 = 2.228(5), Ta N1 = 2.032(4), Ta N3 = 2.250(4), Ta N4 = 2.153(5), N3 C19 = 1.327(7), N4 C19 = 1.319(7); C1 Ta N1 = 99.79(17), C1 Ta N3 = 93.79(16), C1 Ta N4 = 148.99(16), C1 Ta C11 = 81.31(17).

Ta C(12)) ascribes to the strong trans influence of the cyclopentadienyl ring.

The formation of 2 is likely assumed to proceed through two different pathways outlined in Scheme 1. In path (a), an insertion of 1 equiv of carbodiimide into Ta CH3 affords an k2-acetamidinate compound, and then subsequent methylidene formation via α-proton abstraction with the liberation of CH4, followed by the cycloaddition of another carbodiimide, gives an k2-acetamidinate-azatantalacyclobutane 2. The other path (b) proceeds through the formation of methyl methylidene, as has been reported for “Cp2Ta” derivatives,13 followed by the subsequent cycloaddition and insertion processes. Recently, we reported that a four-membered azametallacycle similar to 2 was generated through the cycloaddition reaction of carbodiimides to a Ta-alkylidene intermediate.12 To clarify the mechanism, we conducted the controlled reaction of 1 with 1 equiv of dicyclohexylcarbodiimide and diisopropylcarbodiimide in toluene at 50 °C to give the corresponding complexes 3a and 3b in good yields, where 1 equiv of the carbodiimide inserted into one of three Ta CH3 bonds (eq 3). In each 1H NMR spectrum, a resonance corresponding to methyl acetoamidinate-k2N, Scheme 1. Plausible Reaction Mechanisms for the Formation of 2

The reaction of 1 with excess amounts of RNdCdNR (R = Cy, iPr) in toluene at 90 °C afforded 2 in good yield, in which 2 equiv of carbodiimides was incorporated (eq 2), though a similar reaction of 1 with excess amounts of alkynes and nitriles resulted in the decomposition of 1. In the 1H NMR spectrum of 2, the resonance corresponding to one methyl group inserted into carbodiimides increased as the signal assignable to TaCH3 disappeared; however, the fate of other two methyl groups could not be spectroscopically determined. The molecular structures of 2 were clarified by single-crystal X-ray analyses of 2, in which a four-membered diazatantalacyclobutane and a four-membered 5961

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Figure 3. ORTEP drawing of the molecular structure of 3b. All hydrogen atoms are omitted for clarity. Selected distances (Å) and angles (deg.): Ta C1 = 2.426(5), Ta C11 = 2.251(4), Ta C12 = 2.255(5), Ta N1 = 2.081(3), Ta N2 = 2.228(4), N1 C16 = 1.370(6), N2 C16 = 1.312(5); C1 Ta C11 = 88.43(15), C1 Ta C12 = 158.32(18), C1 Ta N1 = 87.17(13), C1 Ta N2 = 78.97(15), C1 Ta C11 = 88.43(15), C1 Ta C12 = 158.32(18).

N complexes 3a and 3b appeared at δ 1.78 for 3a and 1.60 for 3b, while signals due to the TaCH3 moieties were observed at δ 0.49 and 0.19 for 3a and δ 0.42 and 0.11 for 3b, suggesting that one of the three Ta CH3 groups of 1 selectively inserted into the CdN bond. Figure 3 shows the molecular structure of 3b.14 The tantalum atom has a distorted octahedral geometry coordinated by η5-cyclopentadienyl, σ-carboranyl, acetamidinate-k2N,N, and two methyl groups. The bond lengths of tantalum and σ-carbons [Ta C(1), Ta C(11), and Ta C(12)] are longer than those of 1, and among the tantalum carbon bonds, the distance of the tantalum σ-carboranyl bond is significantly elongated (ca. 0.14 Å) compared with 1 and 2b due to the steric congestion between the carboranyl moiety, acetamidinate-k2N,N, and one of two Ta-Me ligands (C1 Ta N1 87.17(13)°; C1 Ta C11 88.43(15)°). The distance of the Ta N(1) bond (2.089(3) Å) is in the range typically observed for σ-bonded tantalum nitrogen bonds, while the length of the Ta N(2) bond (2.228(4) Å) is a typical coordinating tantalum nitrogen bond.15 On the basis of the bond distances of N(1) C(16) (1.376(6) Å) and N(2) C(16) (1.312(5) Å), the N(1) C(16) N(2) ligand is bound to the metal center as an amido-imino ligand.

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resonances newly appeared at δ 131.6, 163.2, and 165.9 assignable to the four-coordinated borate anion of the cationic tantalum complex,17 but the four-coordinated Lewis acid base adduct, B(C6F5)3(THF), still remained, as observed in an approximately 1:2 ratio at 303 K, indicating that the Lewis base stabilized cationic species was in equilibrium with 1 and B(C6F5)3(THF) (eq 4).18,19 Equilibrium constants for the reaction were determined by integration of the 19F NMR intensities for [B(C6F5)3Me] and B(C6F5)3(THF) at four different temperatures (248, 273, 303, and 323 K) and were used in determining thermodynamic values for the reaction (ΔHr = 2.0(1) kcal/mol, ΔSr = 6.4(5) e.u.) (see Figure S7, Supporting Information, for the Van’t Hoff plot). Similar solution dynamics for the methyl abstraction reaction was reported for Cp*TiMe(OC6F5)2 and B(C6F5)3.20 The unusual behavior of the methyl abstraction reaction is probably due to the strong Lewis acidic nature of the tantalum center having the electropositive carboranyl ligand, and thus, the cationic tantalum center competes with B(C6F5)3 for possessing the methyl group.

’ CONCLUSION We prepared and characterized a neutral trimethyltantalum complex 1 supported by a unique Cp-carboranyl ligand, where the three types of metal carbon bonds, that is, the M C π-bond, the M cage C σ-bond, and the M alkyl σ-bond, were involved. Among the three types of metal carbon bonds, the metal alkyl σ-bond is most reactive toward the insertion of carbodiimides to give acetoamidinato-k2N,N and four-membered azatantalacyclobutane compounds with an exocyclic imine group. By the addition of B(C6F5)3(THF) to the complex 1, the corresponding ionic species, [Ta{η5-C5H4CMe2(C2B10H10)k1C}Me2(THF)][B(C6F5)3Me], was generated, and it was in equilibrium with the complex 1 and B(C6F5)3(THF), indicating the highly electron-deficient nature of the tantalum atom supported by the Cp-carboranyl ligand. ’ EXPERIMENTAL SECTION

It has been reported that the group 4 metal Cp-carboranyl complexes were active catalysts for ethylene polymerization upon activation with MAO, and accordingly, we were interested in the generation of cationic alkyl species from 1. Treatment of 1 with B(C6F5)3 resulted in facile decomposition, presumably due to instability of the coordinatively unsaturated tantalum species.16 In contrast, the reaction of 1 with B(C6F5)3(THF) in C6D5Br generated the corresponding Lewis base stabilized cationic complex 4. The 1H NMR spectrum displayed broad signals for the Cp ring and methyl groups bound to the bridging carbon and the tantalum atom. In the 19F NMR spectrum, one set of

General Procedures. All manipulations involving air- and moisture-sensitive organometallic compounds were operated using the standard Schlenk or glovebox techniques under argon. Complexes TaCl2Me321 and [Li2{C5H4CMe2(C2B10H10)}(OEt2)2]22 were prepared according to the literature. Toluene, hexane, tetrahydrofuran, and diethyl ether were dried and deoxygenated by distillation over sodium benzophenone ketyl under argon or by using Grubbs columns (Glass Counter Solvent Dispensing System, Nikko Hansen & Co., Ltd., Konohana, Osaka, Japan). Diisopropylcarbodiimide and dicyclohexylcarbodiimide were purchased and used as received. The 1H (300 MHz, 400 MHz), 19F (376 MHz), 13C (75 MHz, 100 MHz), and 11B (128 MHz) NMR spectra were measured on VARIAN UNITY INOVA-300 and BRUKER AVANCEIII-400 spectrometers. When C6D6 was used as the solvent, the spectra were referenced to the residual solvent protons at δ 7.15 in the 1H NMR spectra and residual solvent carbons at δ 128.0 in the 13C NMR spectra. When tetrahydrofuran-d8 was used as the solvent, 5962

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Organometallics the spectra were referenced to the residual solvent protons at δ 3.58 in the 1H NMR spectra and residual solvent carbons at δ 67.4 in the 13C NMR spectra.11B NMR spectra were referenced to external BF3 3 OEt2 at δ 0.0. 19F NMR spectra were referenced to external C6H5CF3 at δ 63.9. All melting points were measured in sealed tubes under an argon atmosphere. The elemental analyses were recorded by using a PerkinElmer 2400 at the Faculty of Engineering Science, Osaka University. Preparation of Ta{η5-C5H4CMe2(C2B10H10)-j1C}Me3 (1). In a glovebox, [Li2{C5H4CMe2(C2B10H10)}(OEt2)2] (410 mg, 1.0 mmol) and TaCl2Me3 (297 mg, 1.0 mmol) were placed in a Schlenk tube, and toluene (15 mL) was added. The reaction mixture was stirred at room temperature for 2 h, and the salt was removed by filtration. All the solvent was evaporated in vacuo to give 1 as a gray solid (273 mg, 58%). mp: 135 140 °C (dec.). 1H NMR (300 MHz, C6D6, 35 °C) δ 5.15 (s, 4H, Cp), 1.48 (s, 9H, TaCH3), 1.09 (s, 6H, C(CH3)2). 13C{1H} NMR (75 MHz, C6D6, 35 °C) δ 147.8 (Cp), 114.4 (Cp), 103.9 (Cp), 82.5 (br, TaCH3), 73.7 (cage carbon), 38.7 (C(CH3)2), 31.9 (C(CH3)2), another cage carbon was not observed. 11B{1H} NMR (128 MHz, C6D6, 30 °C) δ 1.0 (1B), 2.2 (1B), 6.5 (2B), 7.7 (2B), 9.8 (2B), 13.7 (2B). Anal. Calcd for C13H29B10Ta: C, 32.91; H, 6.16. Found: C, 32.88; H, 5.88.

Preparation of Ta{η5-C5H4CMe2(C2B10H10)-j1C}(RNCMeNRk N,N){CH2C(NR)NR-k2C,N} (2a, R = Cy; 2b, R = iPr). In a glovebox, 2

1 (95 mg, 0.20 mmol) and dicyclohexylcarbodiimide (83 mg, 0.40 mmol) were placed in a Schlenk tube, and toluene (5 mL) was added. The reaction mixture was stirred at 50 °C overnight and then at 90 °C for 9 h. After the reaction mixture was cooled to room temperature, the orange powder was collected and washed with hexane to give 2a as a yellow solid (76 mg, 43%). mp: 260 265 °C (dec.). 1H NMR (300 MHz, tetrahydrofuran-d8, 35 °C) δ 6.45 (m, 1H, Cp), 6.40 (m, 1H, Cp), 6.10 (m, 1H, Cp), 6.05 (m, 1H, Cp), 4.4 4.6 (m, 1H, CH of Cy), 4.2 4.3 (m, 1H, CH of Cy), 3.6 3.7 (m, 1H, CH of Cy), 3.49 (br, 1H, CH of Cy), 2.9 3.3 (m, 2H, CH2 of Cy), 2.11 (s, 3H, CCH3), 1.1 2.2 (m, 18H, CH2 of Cy). 13C{1H} NMR (75 MHz, C6D6, 35 °C) δ 168.0, 166.8, 147.7 (Cp), 117.9 (Cp), 110.4 (Cp), 109.1 (Cp), 106.4 (cage carbon), 104.0 (Cp), 71.2 (CH of Cy), 60.8 (CH of Cy), 57.1 (TaCH2), 55.9 (CH of Cy), 55.6 (CH of Cy), 41.1 (C(CH3)2), 35.9 (CH2 of Cy), 35.8 (CH2 of Cy), 35.5 (CH2 of Cy), 35.1 (CH2 of Cy), 34.6 (CH2 of Cy), 34.49 (C(CH3)2), 34.46 (CH2 of Cy), 34.2 (CH2 of Cy), 31.8 (C(CH3)2), 30.6 (CH2 of Cy), 28.4 (CH2 of Cy), 28.22 (CH2 of Cy), 28.21 (CH2 of Cy), 28.0 (CH2 of Cy), 27.7 (CH2 of Cy), 27.5 (CH2 of Cy), 27.31 (CH2 of Cy), 27.27 (CH2 of Cy), 27.22 (CH2 of Cy), 26.6 (CH2 of Cy), 25.8 (CH2 of Cy), 25.7 (CH2 of Cy), 19.2 (CCH3), another cage carbon was not observed. 11B{1H} NMR (128 MHz, C6D6, 30 °C) δ 1.3 (1B), 2.8 (1B), 6.3 (2B), 9.1 (2B), 11.6 (2B), 14.1 (2B). Anal. Calcd for C38H69N4B10Ta: C, 52.40; H, 7.98; N, 6.43. Found: C, 52.17; H, 7.84; N, 6.31. The complex 2b was prepared in a similar procedure to 2a. 83% yield. mp: 143 148 °C (dec.). 1H NMR (300 MHz, C6D6, 35 °C) δ 5.91 (m, 1H, Cp), 5.83 (m, 1H, Cp), 5.36 (m, 1H, Cp), 5.23 (m, 1H, Cp), 4.5 4.6 (m, 1H, CH(CH3)2), 4.4 4.5 (m, 1H, CH(CH3)2), 3.8 3.9 (m, 1H, CH(CH3)2), 3.4 3.5 (m, 1H, CH(CH3)2), 2.08 (d, J = 15.6 Hz, TaCH2), 1.83 (d, J = 6.8 Hz, 3H, CH(CH3)2), 1.71 (d, J = 6.8 Hz, 3H, CH(CH3)2), 1.39 (s, 3H, CH3), 1.33 (d, J = 15.6 Hz, TaCH2), 1.33 (s, 3H, CH3), 1.30 (s, 3H, CH3), 1.28 (d, J = 6.3 Hz, 3H, CH(CH3)2), 1.24 (d, J = 6.3 Hz, 3H, CH(CH3)2), 1.14 (d, J = 7.2 Hz, 3H, CH(CH3)2), 1.04 (d, J = 7.0 Hz, 3H, CH(CH3)2), 0.91 (d, J = 6.8 Hz, 3H, CH(CH3)2), 0.83 (d, J = 6.8 Hz, 3H, CH(CH3)2). 13C{1H} NMR (75 MHz, C6D6, 35 °C) δ 166.7, 164.8, 146.9 (Cp), 117.0 (cage carbon), 116.1 (Cp), 108.5 (Cp), 107.4 (Cp), 105.3 (cage carbon), 102.3 (Cp), 60.7 (NCH(CH3)2), 56.4 (TaCH2), 50.3 (NCH(CH3)2), 47.6 (NCH(CH3)2), 45.6 (NCH(CH3)2), 40.0 (C(CH3)2), 33.8, 31.1, 25.1 (2C), 24.3, 23.7, 23.5 (2C), 23.0, 21.3, 18.3. 11B{1H} NMR (128 MHz, C6D6, 30 °C) δ 1.4 (1B), 3.1 (1B), 6.4 (2B), 9.4 (4B), 11.6 (2B). Anal. Calcd for C26H53N4B10Ta: C, 43.93; H, 7.52; N, 7.88. Found: C, 43.46; H, 7.55; N, 7.57.

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Preparation of Ta{η5-C5H4CMe2(C2B10H10)-j1C}(RNCMeNRk N,N)Me2 (3a, R = Cy; 3b, R = iPr). In a glovebox, 1 (352 mg, 0.74 2

mmol) and dicyclohexylcarbodiimide (184 mg, 0.89 mmol) were placed in a Schlenk tube, and toluene (10 mL) was added. The reaction mixture was stirred at 50 °C overnight, and the insoluble material was removed by filteration. All the solvent was evaporated in vacuo, and the residue was washed with hexane (10 mL  2) to give 3a as a brown solid (337 mg, 63%). mp: 112 117 °C (dec.). 1H NMR (300 MHz, C6D6, 35 °C) δ 6.01 (br s, 1H, Cp), 5.52 (br s, 2H, Cp), 5.24 (br, 1H, Cp), 4.5 4.6 (m, 1H, CH of Cy), 3.25 3.35 (m, 1H, CH of Cy), 2.35 2.60 (m, 2H, CH2 of Cy), 1.73 (s, 3H, CCH3), 1.30 (s, 3H, C(CH3)2), 1.29 (s, 3H, C(CH3)2), 0.44 (s, 3H, TaCH3), 0.14 (s, 3H, TaCH3), 0.7 1.8 (m, 18H, CH2 of Cy). Anal. Calcd for C26H51N2B10Ta: C, 45.87; H, 7.55; N, 4.12. Found: C, 44.70; H, 7.52; N, 3.65. The complex 3b was prepared in a similar procedure to 3a. 65% yield. mp: 150 159 °C (dec.). 1H NMR (300 MHz, C6D6, 35 °C) δ 5.98 (m, 1H, Cp), 5.46 (m, 1H, Cp), 5.21 (m, 1H, Cp), 5.15 (m, 1H, Cp), 4.80 4.90 (m, 1H, CH(CH3)2), 3.45 3.55 (m, 1H, CH(CH3)2), 1.55 (s, 3H, CCH3), 1.41 (d, J = 7.2 Hz, 3H, CH(CH3)2), 1.24 (s, 6H, C(CH3)2), 1.29 (d, J = 7.2 Hz, 3H, CH(CH3)2), 1.05 (d, J = 7.2 Hz, 3H, CH(CH3)2), 0.74 (d, J = 7.2 Hz, 3H, CH(CH3)2), 0.37 (s, 3H, TaCH3), 0.06 (s, 3H, TaCH3). Anal. Calcd for C20H43N2B10Ta: C, 39.99; H, 7.22; N, 4.66. Found: C, 39.78; H, 7.20; N, 4.33. Due to the poor solubility of 3a and 3b in organic solvents, we could not get their 13C NMR and 11B NMR spectra. The 1H NMR spectra are included in Figures S2 and S3 (Supporting Information).

Reaction of Ta{η5-C5H4CMe2(C2B10H10)-j1C}Me3 (1) with B(C6F5)3(THF). In a glovebox, to a solution of 1 (20.0 mg, 0.042 mmol)

in C6D5Br was added a solution of B(C6F5)3(THF) (13.2 mg, 0.042 mmol) at 35 °C to generate cationic complex [Ta{η5-C5H4CMe2(C2B10H10)-k1C}Me2(THF)][B(C6F5)3Me] (4). 1H NMR (400 MHz, C6D5Br, 30 °C) δ 5.58 (br, 2H, Cp), 5.46 (br, 2H, Cp), 3.77 (br, 4H, THF), 1.54 (br, 4H, THF), 1.41 (br, 6H, CMe2), 1.21 (m, 9H, TaMe and BMe). 19F NMR (376 MHz, C6D5Br, 30 °C) δ 131.6 (d, 3J = 22 Hz, 6F, B(o-C6F5)3Me), 132.3 (d, 3J = 24 Hz, 6F, B(o-C6F5)3(THF)), 154.2 (t, 3J = 22 Hz, 3F, B(p-C6F5)3(THF)), 161.8 (m, 6F, B(mC6F5)3(THF)), 163.2 (t, 3J = 21 Hz, 3F, B(p-C6F5)3Me), 165.9 (m, 6F, B(m-C6F5)3Me). Resonances for B(C6F5)3Me and B(C6F5)3(THF) were detected in the ratio of 48:52 (248 K), 41:59 (273 K), 32:68 (303 K), and 30:70 (323 K) (see Figures S4 S6, Supporting Information).

’ ASSOCIATED CONTENT S b 1

Supporting Information. Molecular structure of 2a; the H and 19F NMR spectra of the reaction of Ta{η5-C5H4CMe2(C2B10H10)-k1C}Me3 (1) with B(C6F5)3(THF); the Van’t Hoff plot; crystal data and data collection parameters of 1, 2a, 2b, and 3b; and their CIF format are included in the Supporting Information. This material is available free of charge via the Internet at http://pubs.acs.org.

’ AUTHOR INFORMATION Corresponding Authors

*E-mail: [email protected] (Z.X.); [email protected]. ac.jp (K.M.).

’ ACKNOWLEDGMENT K.Y. thanks the Global COE (Center of Excellence) Program “Global Education and Research Center for Bio-Environmental Chemistry” of Osaka University and the JSPS Research Fellowships for Young Scientists. This work was supported by the 5963

dx.doi.org/10.1021/om200773m |Organometallics 2011, 30, 5960–5964

Organometallics Core Research for Evolutional Science and Technology (CREST) program of the Japan Science and Technology Agency (JST), Japan.

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ARTICLE

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dx.doi.org/10.1021/om200773m |Organometallics 2011, 30, 5960–5964