Synthesis and reaction of neutral and cationic alkyltantalum complexes with a linked cyclopentadienyl-carboranyl ligand

Article abstract of DOI:10.1021/om200773m

A trimethyltantalum complex with a Cp-carboranyl ligand was prepared via a salt metathesis reaction of TaCl₂Me₃ with [Li ₂{C₅H₄CMe₂(C₂B ₁₀H₁₀)}(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).

Full text of DOI:10.1021/om200773m

ARTICLE
pubs.acs.org/Organometallics
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
S Supporting Information
b
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 four-
membered 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
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.
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.1À4 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 metalÀcarbon 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 metalÀcarboranyl
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 metalÀcarbon bonds, the MÀC
π-bond, the MÀC(cage) σ-bond, and the MÀalkyl σ-bond,
are involved.6À8 A notable finding in this contribution was that
the TaÀalkyl σ-bond was selectively reacted with carbodiimines
among three types of tantalumÀcarbon bonds, in sharp contrast
to the insertion of alkenes and alkynes into the early transition-
metalÀC(cage) σ-bond being often observed.9 Furthermore, we
found that the ionic species, [Ta{η5-C5H4CMe2(C2B10H10)-
k1C}Me2(THF)][B(C6F5)3Me] was in equilibrium with
’ 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
1H 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 TaÀC(1) bond (2.265(3) Å) is in
the range typically observed for a σ-bound metalÀcarboranyl
moiety.5c The elongation of the TaÀC(13) bond (2.212(3) Å)
compared to others (2.182(3) Å for TaÀC(11), 2.181(4) Å for
Received: August 18, 2011
Published: October 06, 2011
r
2011 American Chemical Society
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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)ÀC(19)ÀN(4) ligand coordinates to the tantalum
atom as an acetoamidinate-k2N,N ligand, which is confirmed by
almost the same distances of N(3)ÀC(19) and N(4)ÀC(19)
bonds (1.327 (7) and 1.319(7) Å). The distance of the TaÀC(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)ÀTaÀN(1) 99.79(17)°; C(1)ÀTaÀ
N(3) 93.79(16)°). The bond length of TaÀC(11) is a typical
σ-bonded tantalumÀcarbon bond.
Figure 1. ORTEP drawing of the molecular structure of 1. All hydrogen
atoms are omitted for clarity. Selected distances (Å) and angles (deg.):
TaÀC1 = 2.265(3), TaÀC11 = 2.182(3), TaÀC12 = 2.181(4),
TaÀC13 = 2.212(3); C1ÀTaÀC11 = 114.94(11), C1ÀTaÀC12 =
115.14(12), C11ÀTaÀC12 = 119.44(13).
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 TaÀCH3 affords an k2-acetami-
dinate 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-acetami-
dinate-azatantalacyclobutane 2. The other path (b) proceeds
through the formation of methylÀmethylidene, 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 diisopropyl-
carbodiimide in toluene at 50 °C to give the corresponding com-
plexes 3a and 3b in good yields, where 1 equiv of the carbodiimide
inserted into one of three TaÀCH3 bonds (eq 3). In each 1H NMR
spectrum, aresonancecorrespondingtomethylacetoamidinate-k2N,
Figure 2. ORTEP drawing of the molecular structure of 2b. All
hydrogen atoms are omitted for clarity. Selected distances (Å) and
angles (deg.): TaÀC1 = 2.354(5), TaÀC11 = 2.228(5), TaÀN1 =
2.032(4), TaÀN3 = 2.250(4), TaÀN4 = 2.153(5), N3ÀC19 =
1.327(7), N4ÀC19
=
1.319(7); C1ÀTaÀN1
= 99.79(17),
C1ÀTaÀN3 = 93.79(16), C1ÀTaÀN4 = 148.99(16), C1ÀTaÀC11 =
81.31(17).
TaÀC(12)) ascribes to the strong trans influence of the cyclo-
pentadienyl ring.
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 simi-
lar 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
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resonances newly appeared at δ À131.6, À163.2, and À165.9
assignable to the four-coordinated borate anion of the cationic
tantalum complex,17 but the four-coordinated Lewis acidÀbase
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]À 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 =
À2.0(1) kcal/mol, ΔSr = À6.4(5) e.u.) (see Figure S7, Support-
ing 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 electro-
positive carboranyl ligand, and thus, the cationic tantalum center
competes with B(C6F5)3 for possessing the methyl group.
Figure 3. ORTEP drawing of the molecular structure of 3b. All
hydrogen atoms are omitted for clarity. Selected distances (Å) and
angles (deg.): TaÀC1 = 2.426(5), TaÀC11 = 2.251(4), TaÀC12 =
2.255(5), TaÀN1 = 2.081(3), TaÀN2 = 2.228(4), N1ÀC16 =
1.370(6), N2ÀC16
=
1.312(5); C1ÀTaÀC11
= 88.43(15),
C1ÀTaÀC12 = 158.32(18), C1ÀTaÀN1 = 87.17(13), C1ÀTaÀN2 =
78.97(15), C1ÀTaÀC11 = 88.43(15), C1ÀTaÀC12 = 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 TaÀCH3 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
[TaÀC(1), TaÀC(11), and TaÀC(12)] are longer than those of
1, and among the tantalumÀcarbon bonds, the distance of the
tantalumÀσ-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 (C1ÀTaÀN1 87.17(13)°; C1ÀTaÀC11 88.43(15)°).
The distance of the TaÀN(1) bond (2.089(3) Å) is in the range
typically observed for σ-bonded tantalumÀnitrogen bonds,
while the length of the TaÀN(2) bond (2.228(4) Å) is a typical
coordinating tantalumÀnitrogen bond.15 On the basis of the
bond distances of N(1)ÀC(16) (1.376(6) Å) and N(2)ÀC(16)
(1.312(5) Å), the N(1)ÀC(16)ÀN(2) ligand is bound to the
metal center as an amido-imino ligand.
’ CONCLUSION
We prepared and characterized a neutral trimethyltantalum
complex 1 supported by a unique Cp-carboranyl ligand, where
the three types of metalÀcarbon bonds, that is, the MÀC
π-bond, the MÀcage C σ-bond, and the MÀalkyl σ-bond, were
involved. Among the three types of metalÀcarbon bonds, the
metalÀalkyl 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 sup-
ported by the Cp-carboranyl ligand.
’ EXPERIMENTAL SECTION
General Procedures. All manipulations involving air- and moist-
ure-sensitive organometallic compounds were operated using the stan-
dard Schlenk or glovebox techniques under argon. Complexes
21
TaCl2Me3 and [Li2{C5H4CMe2(C2B10H10)}(OEt2)2]22 were pre-
pared 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 dicyclohexyl-
carbodiimide 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,
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
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the spectra were referenced to the residual solvent protons at δ 3.58 in
Preparation of Ta{η5-C5H4CMe2(C2B10H10)-j1C}(RNCMeNR-
k2N,N)Me2 (3a, R = Cy; 3b, R = iPr). In a glovebox, 1 (352 mg, 0.74
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 Â 2) to give 3a as a brown solid (337
mg, 63%). mp: 112À117 °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), 4.5À4.6 (m,
1H, CH of Cy), 3.25À3.35 (m, 1H, CH of Cy), 2.35À2.60 (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), 0.7À1.8 (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: 150À159 °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),
4.80À4.90 (m, 1H, CH(CH3)2), 3.45À3.55 (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.
the 1H NMR spectra and residual solvent carbons at δ 67.4 in the 13
C
NMR spectra.11B NMR spectra were referenced to external BF3 OEt2 at
3
δ 0.0. 19F NMR spectra were referenced to external C6H5CF3 at δ
À63.9. All melting points were measured in sealed tubes under an argon
atmosphere. The elemental analyses were recorded by using a Perki-
nElmer 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: 135À140 °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 À1.0 (1B), À2.2 (1B), À6.5 (2B), À7.7 (2B), À9.8 (2B), À13.7 (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}(RNCMeNR-
k2N,N){CH2C(NR)NR-k2C,N} (2a, R = Cy; 2b, R = iPr). In a glovebox,
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: 260À265 °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),
4.4À4.6 (m, 1H, CH of Cy), 4.2À4.3 (m, 1H, CH of Cy), 3.6À3.7 (m, 1H,
CH of Cy), 3.49 (br, 1H, CH of Cy), 2.9À3.3 (m, 2H, CH2 of Cy), 2.11 (s,
3H, CCH3), 1.1À2.2 (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 À1.3 (1B),
À2.8 (1B), À6.3 (2B), À9.1 (2B), À11.6 (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: 143À148 °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),
4.5À4.6 (m, 1H, CH(CH3)2), 4.4À4.5 (m, 1H, CH(CH3)2), 3.8À3.9
(m, 1H, CH(CH3)2), 3.4À3.5 (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 À1.4 (1B), À3.1 (1B),
À6.4 (2B), À9.4 (4B), À11.6 (2B). Anal. Calcd for C26H53N4B10Ta: C,
43.93; H, 7.52; N, 7.88. Found: C, 43.46; H, 7.55; N, 7.57.
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 À35 °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 À131.6 (d, 3J = 22 Hz,
3
6F, B(o-C6F5)3Me), À132.3 (d, J = 24 Hz, 6F, B(o-C6F5)3(THF)),
À154.2 (t, 3J = 22 Hz, 3F, B(p-C6F5)3(THF)), À161.8 (m, 6F, B(m-
C6F5)3(THF)), À163.2 (t, 3J = 21 Hz, 3F, B(p-C6F5)3Me), À165.9 (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 S4ÀS6, Supporting
Information).
’ ASSOCIATED CONTENT
S
Supporting Information. Molecular structure of 2a; the
b
1H 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: zxie@cuhk.edu.hk (Z.X.); mashima@chem.es.osaka-u.
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 Fellow-
ships for Young Scientists. This work was supported by the
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dx.doi.org/10.1021/om200773m |Organometallics 2011, 30, 5960–5964

Organometallics
ARTICLE
Core Research for Evolutional Science and Technology
(CREST) program of the Japan Science and Technology
Agency (JST), Japan.
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(10) Crystal data for 1: C13H29B10Ta(C7H8), fw = 566.56, mono-
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12.1677(5) Å, β = 92.3848(16)°, V = 2407.28(18) Å3, T = 113 K, Z = 4,
Dcalcd = 1.563 g cmÀ3, 2θmax = 61°, μ = 4.566 mmÀ1, R1 and wR2 =
0.0337 and 0.0739 (all), GOF = 0.841.
(11) Crystal data for 2b: C26H53B10N4Ta(C4H8O), fw = 782.89,
orthorhombic, space group P212121, a = 10.72858(19) Å, b =
18.3278(4) Å, c = 18.7448(4) Å, V = 3685.82(12) Å3, T = 113 K, Z =
4, Dcalcd = 1.411 g cmÀ3, 2θmax = 60.9°, μ = 3.008 mmÀ1, R1 and wR2 =
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space group P1, a = 10.9495(3) Å, b = 16.0243(5) Å, c = 16.2839(5) Å, α =
102.0810(10)°, β = 105.6842(9)°, γ = 99.3235(10)°, V = 2616.33(14) Å3,
T = 113 K, Z = 2, Dcalcd = 1.525 g cmÀ3, 2θmax = 55°, μ = 4.208 mmÀ1, R1
and wR2 = 0.0328 and 0.0754 (all), GOF = 1.107.
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dx.doi.org/10.1021/om200773m |Organometallics 2011, 30, 5960–5964