Direct Evidence for a [4+2] Cycloaddition Mechanism of Alkynes to Tantallacyclopentadiene on Dinuclear Tantalum Complexes as a Model of Alkyne Cyclotrimerization

Article abstract of DOI:10.1002/chem.201501164

A dinuclear tantalum complex, [Ta₂Cl₆(μ-C₄Et₄)] (2), bearing a tantallacyclopentadiene moiety, was synthesized by treating [(η²-EtC≡CEt)TaCl₃(DME)] (1) with AlCl₃. Complex 2 and its Lewis base adducts, [Ta₂Cl₆(μ-C₄Et₄)L] (L=THF (3a), pyridine (3b), THT (3c)), served as more active catalysts for cyclotrimerization of internal alkynes than 1. During the reaction of 3a with 3-hexyne, we isolated [Ta₂Cl₄(μ-η⁴:η⁴-C₆Et₆)(μ-η²:η²-EtC≡CEt)] (4), sandwiched by a two-electron reduced μ-η⁴:η⁴-hexaethylbenzene and a μ-η²:η²-3-hexyne ligand, as a product of an intermolecular cyclization between the metallacyclopentadiene moiety and 3-hexyne. The formation of arene complexes [Ta₂Cl₄(μ-η⁴:η⁴-C₆Et₄Me₂)(μ-η²:η²-Me₃SiC≡CSiMe₃)] (7b) and [Ta₂Cl₄(μ-η⁴:η⁴-C₆Et₄RH)(μ-η²:η²-Me₃SiC≡CSiMe₃)] (R=nBu (8a), p-tolyl (8b)) by treating [Ta₂Cl₄(μ-C₄Et₄)(μ-η²:η²-Me₃SiC≡CSiMe₃)] (6) with 2-butyne, 1-hexyne, and p-tolylacetylene without any isomers, at room temperature or low temperature were key for clarifying the [4+2] cycloaddition mechanism because of the restricted rotation behavior of the two-electron reduced arene ligands without dissociation from the dinuclear tantalum center.

Full text of DOI:10.1002/chem.201501164

DOI: 10.1002/chem.201501164
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&
Reaction Mechanisms |Hot Paper|
Direct Evidence for a [4+2] Cycloaddition Mechanism of Alkynes
to Tantallacyclopentadiene on Dinuclear Tantalum Complexes as
a Model of Alkyne Cyclotrimerization
Keishi Yamamoto, Hayato Tsurugi,* and Kazushi Mashima*[a]
Abstract: A dinuclear tantalum complex, [Ta2Cl6(m-C4Et4)] (2),
bearing a tantallacyclopentadiene moiety, was synthesized
by treating [(h2-EtCꢀCEt)TaCl3(DME)] (1) with AlCl3. Complex
2 and its Lewis base adducts, [Ta2Cl6(m-C4Et4)L] (L=THF (3a),
pyridine (3b), THT (3c)), served as more active catalysts for
cyclotrimerization of internal alkynes than 1. During the re-
action of 3a with 3-hexyne, we isolated [Ta2Cl4(m-h4:h4-
C6Et6)(m-h2:h2-EtCꢀCEt)] (4), sandwiched by a two-electron re-
The formation of arene complexes [Ta2Cl4(m-h4:h4-
C6Et4Me2)(m-h2:h2-Me3SiCꢀCSiMe3)] (7b) and [Ta2Cl4(m-h4:h4-
C6Et4RH)(m-h2:h2-Me3SiCꢀCSiMe3)] (R=nBu (8a), p-tolyl (8b))
by treating [Ta2Cl4(m-C4Et4)(m-h2:h2-Me3SiCꢀCSiMe3)] (6) with
2-butyne, 1-hexyne, and p-tolylacetylene without any iso-
mers, at room temperature or low temperature were key for
clarifying the [4+2] cycloaddition mechanism because of the
restricted rotation behavior of the two-electron reduced
arene ligands without dissociation from the dinuclear tanta-
lum center.
duced m-h4:h4-hexaethylbenzene and
a
m-h2:h2-3-hexyne
ligand, as a product of an intermolecular cyclization be-
tween the metallacyclopentadiene moiety and 3-hexyne.
cyclotrimerization.[2] Although metallacyclopentadienes have
been established as key intermediates for the construction of
aromatic rings through [4+2] cycloaddition[3] or insertion/re-
ductive cyclization,[4] as representatively shown in Scheme 1,
the actual CÀC bond-forming process has rarely been manifest-
ed. One reliable mechanism is that a metallacyclopentadiene
undergoes a [4+2] cycloaddition with an incoming alkyne to
give a metallanorbonadiene (A) or a p-arene complex (B).
Aubert et al. investigated in detail the reaction pathway of
[2+2+2] alkyne trimerization catalyzed by [CpCoL2] (L=CO,
PR3, alkenes) based on DFT calculations, in which [4+2] cyclo-
addition of a cobaltacyclopentadiene and an alkyne was the
key reaction step for constructing the aromatic ring.[5] Direct
experimental evidence for the involvement of the [4+2] cyclo-
addition pathway in alkyne cyclotrimerization is, however, lim-
ited to the intramolecular case; Vollhardt et al. experimentally
demonstrated an intramolecular [4+2] cycloaddition of cobalt-
cyclopentadiene moiety in C with an internal coordinated CꢀC
bond, giving h4-arene complex D [Eq. (1)]:[6]
Introduction
Cyclotrimerization of alkynes assisted by various transition
metal catalysts is a straightforward synthetic method for con-
structing substituted aromatic compounds.[1] A well-established
reaction mechanism involves the oxidative coupling of two al-
kynes at a low-valent metal center, giving a metallacyclopenta-
diene, followed by further addition of an alkyne to produce
the corresponding benzene derivatives (Scheme 1). Many met-
allacyclopentadienes of a wide variety of transition metals
have been isolated, some of which act as catalysts for alkyne
Scheme 1. Proposed reaction mechanism of [2+2+2] alkyne cyclotrimeriza-
tion by transition metal complexes.
[a] K. Yamamoto, Dr. H. Tsurugi, Prof. Dr. K. Mashima
Department of Chemistry, Graduate School of Engineering Science
Osaka University and CREST, JST
Clear observation of the [4+2] cycloaddition pathway in in-
termolecular alkyne trimerization is rather complicated. Wigley
et al. reported that tantalum complex E reacted with alkynes
to give tantallanorbonadiene F [Eq. (2)], which acted as a cata-
lyst for tert-butylacetylene cyclotrimerization. Even after form-
Toyonaka, Osaka 560-8531 (Japan)
E-mail: mashima@chem.es.osaka-u.ac.jp
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/chem.201501164.
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ing an arene ring through insertion/reductive cyclization path-
way, recoordination of the arene ring to the low-valent metal
center leads to the formation of F. Bergman et al. first under-
took a kinetic study on the intermolecular [4+2] cycloaddition
mechanism for [CpCo(C4Me4)(PR3)] complexes;[3] however, de-
tection of a [4+2] cycloaddition product just after the [4+2]
cycloaddition has never been achieved.
Herein, we report the synthesis of dinuclear tantalum com-
plexes bearing arene ligands by the reaction of a dinuclear tan-
talum metallacyclopentadiene complex, [Ta2Cl4(m-C4Et4)(m-h2:h2-
Me3SiCꢀCSiMe3)] (6), with several alkynes. We have obtained
evidence for an intermolecular [4+2] cycloaddition pathway:
the restricted rotational manner of strongly coordinating
arenes without dissociation from the metal center is an im-
portant factor that determines the [4+2] cycloaddition
mechanism.
Figure 1. Molecular structures of dinuclear tantalum complexes 2 (top) and
3a (bottom) with thermal ellipsoids set at 50% probability. Hydrogen atoms
are omitted for clarity.
Results and Discussion
Table 1. Selected bond lengths (Š) and angles (8) of complexes 2 and
3a.
Reaction of [(h2-EtCꢀCEt)TaCl3(DME)] (1)[7] with anhydrous AlCl3
(1 equiv) in toluene afforded dinuclear complex 2 as a dark-
purple solid in 75% yield, along with the formation of
AlCl3(DME), in which anhydrous AlCl3 functioned to capture
DME [Eq. (3)]. The molecular structure of 2 was characterized
spectroscopically, and further established by an X-ray diffrac-
tion study (Figure 1 and Table 1), which clearly indicated the
formation of a dimetallacyclopentadiene structure bridging
two tantalum atoms. The distances of Ta1ÀC1 (2.031(14) Š) and
Ta1ÀC4 (2.011(14) Š) in the tantallacyclopentadiene unit are
shorter than those of [Ta(CEt=CEtCEt=CEt)(DIPP)3] (DIPP=2,6-
diisopropylphenoxy).[8] Coordination of the tantallacyclopenta-
diene moiety to the second tantalum atom results in deforma-
tion of the planar metallacyclopentadienyl moiety with a fold
angle of 44.48 between the C1-Ta1-C4 and C1-C2-C3-C4 planes.
Reports of halide cluster complexes of Nb and W bearing
a metallacyclopentadiene unit are rare,[9] although there are
many homo- or hetero-multinuclear transition metal carbonyl
complexes of Cr, Mo, W, Os, Fe, Ru, and Co containing a metal-
lacyclopentadiene moiety.[10]
2
3a
Ta1ÀTa2
3.011(2)
2.9877(4)
2.173(6)
2.157(6)
2.426(6)
2.424(5)
2.409(6)
2.421(6)
1.418(9)
1.448(9)
1.439(8)
7.2
Ta1ÀC1
2.031(14)
2.011(14)
2.319(13)
2.446(13)
2.426(13)
2.341(18)
1.442(19)
1.43(2)
Ta1ÀC4
Ta2ÀC1
Ta2ÀC2
Ta2ÀC3
Ta2ÀC4
C1ÀC2
C2-ÀC3
C3ÀC4
1.44(2)
44.4
dihedral angle between
C1-Ta1-C4 and C1-C2-C3-C4 planes
ular formulas of which were determined to contain one Lewis
base, based on their spectroscopic data as well as on the mo-
lecular structure of 3a by an X-ray diffraction study (Figure 1
and Table 1). In contrast to the deformed planarity of 2, the
tantallacyclopentadiene moiety of 3a is almost planar, as evi-
dent from the angle (7.28) between the C1-Ta1-C4 and C1-C2-
C3-C4 planes.
Lewis bases such as tetrahydrofuran (THF), pyridine, and tet-
rahydrothiophene (THT), coordinate to the tantalum center of
2 to give the corresponding adducts 3a–c [Eq. (4)], the molec-
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The mononuclear and dinuclear tantalum complexes 1, 2,
and 3a–c function as catalysts for the cyclotrimerization reac-
tion of internal alkynes (Table 2). Under ambient conditions in
[D6]benzene, complex 2 (0.5 mol%) converted 3-hexyne quan-
titatively into hexaethylbenzene within 15 min (Table 2,
entry 2), whereas complex 1 exhibited very low catalytic activi-
ty (entry 1). Complexes 3a–c showed less catalytic activity than
complex 2 due to coordination of the donor ligands to the di-
nuclear tantalum core (Table 2, entries 3–5). Internal alkynes
such as 2-butyne and 4-octyne were also trimerized by com-
plex 2 to give the corresponding hexaalkylbenzene in quantita-
tive yield (Table 2, entries 6 and 7).
appeared at dC =104.5 and 108.7 ppm, respectively, suggesting
that rotation of the C6Et6 ligand on the dinuclear tantalum
center was prevented. In contrast, fast rotation of the arene li-
gands on tantalum and dinuclear vanadium centers were ob-
served for previously reported [(h6-arene)Ta(OAr)3] and [(m-
h4:h4-C6H6)(CpV)2(m-H)2] complexes.[11] The 1H NMR spectrum
displayed resonances for two magnetically inequivalent ethyl
groups of the C6Et6 ligand: one due to the ethyl group bound
to the Ca was a quartet at dH =0.72 ppm [3J(H,H)=7.5 Hz,
CH2CH3] and
a
triplet at dH =0.39 ppm [3J(H,H)=7.5 Hz,
CH2CH3], and the second due to the ethyl group bound to Cb
appeared at dH =2.19 and 2.77 ppm [dq, 2J(H,H)=14.8 Hz,
3
3J(H,H)=7.4 Hz, CHHCH3] and dH =1.07 ppm [t, J(H,H)=7.4 Hz,
CH2CH3]. The upfield shift of the methylene protons bound to
Ca compared with that of Cb indicated the sp3 carbon charac-
ter of Ca. The m-h2:h2-3-hexyne moiety was characterized as
a dianionic alkyne ligation bridging two tantalum centers be-
Table 2. Catalytic trimerization of internal alkynes by tantalum complexes
1, 2, and 3a–c.
1
cause H NMR signals due to ethyl groups resonated at lower
magnetic field at dH =3.98 ppm [q, 3J(H,H)=7.4 Hz, CH2CH3]
and dH =1.53 ppm [t, 3J(H,H)=7.4 Hz, CH2CH3].[7,12] Although
the mechanism of the reaction shown in Eq. (5) is unclear, we
presume disproportionation of chloride ligands take place
during the formation of complex 4.[13]
Figure 2 and Table 4 show the molecular structure and se-
lected geometrical parameters of 4. The Ta1ÀTa2 distance of
Entry
Cat.
R
Yield [%][a]
1
2
3
4
5
6
7
1[b]
2
3a
3b
3c
2
Et
Et
Et
Et
Et
Me
nPr
7
>99
47
22
94
>99
>99
2
[a] Determined by 1H NMR spectroscopic analysis. [b] 1mol% of 1 was
used.
Terminal alkynes such as 1-hexyne and p-tolylacetylene were
trimerized by 2 to produce a mixture of trisubstituted benzene
derivatives in quantitative yield (Table 3).
Table 3. Catalytic trimerization of terminal alkynes by complex 2.
Entry
R
Yield [%][a]
1,2,4/1,3,5[a]
1
2
nBu
p-tolyl
>99
>99
72:28
64:36
1
[a] Determined by H NMR spectroscopic analysis.
Figure 2. Molecular structure of 4 with thermal ellipsoids set at 50% proba-
To detect species present during the cyclotrimerization, we
conducted the reaction of 3a with 3-hexyne (10 equiv), from
which we isolated the new dinuclear tantalum complex 4 in
20% yield [Eq. (5)]. Notably, the main motif of complex 4 was
a unique sandwich structure, in which the dinuclear tantalum
center was sandwiched by a two-electron reduced m-h4:h4-hex-
aethylbenzene ligand and by a m-h2:h2-3-hexyne ligand, based
bility. Hydrogen atoms are omitted for clarity.
Table 4. Selected bond lengths (Š) and angles (8) of complex 4.
Ta1ÀTa2
C1ÀC2
C2ÀC3
C3ÀC4
2.8737(8)
1.475(18)
1.407(18)
1.460(16)
C4ÀC5
C5ÀC6
C6ÀC1
C21ÀC22
1.494(16)
1.434(18)
1.465(17)
1.391(18)
1
on the H and 13C NMR spectral data. In the 13C NMR spectrum,
dihedral angle between
37.5
two resonances assignable to ring carbon atoms of C6Et6, de-
C1-C2-C3-C4 and C1-C6-C5-C4 planes
noted as Ca and Cb, were observed: signals due to Ca and Cb
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2.8737(8) Š is typical for a TaIVÀTaIV single bond.[14] Four CÀC
bonds of the hexaethylbenzene ring, C1ÀC2, C3ÀC4, C4ÀC5,
and C1ÀC6, are significantly elongated compared with
a normal CÀC bond length of aromatic compounds, indicating
a 2,5-cyclohexadien-1,4-yl structure for the two-electron re-
duced hexaethylbenzene.[15] In addition, the bending structure
of the C6Et6 ligand is ascertained by the dihedral angle (37.58)
between the C1-C2-C3-C4 and C1-C6-C5-C4 planes, the value
of which is less acute than that of [(C6Et6)Ta(DIPP)2Cl] (34.48).[11a]
The bond length of C21ÀC22 (1.391(18) Š) in 4 is typical for
a dianionic h2-alkyne ligand found in mononuclear complexes,
[(h2-3-hexyne)TaCl3(L)2] (L=py; L2 =DME, TMEDA).[7,12b]
Complex 4 was alternatively prepared by reducing 2 by
using 2,3,5,6-tetramethyl-1,4-bis(trimethylsilyl)-1,4-diaza-2,5-cy-
clohexadiene (Me4-BTDP), which has a unique reducing ability
that does not lead to the formation of any reductant-derived
metal salts.[16] Treatment of complex 2 with Me4-BTDP, followed
by the addition of 3-hexyne (2 equiv) resulted in the formation
of 4 in 70% yield, along with 2,3,5,6-tetramethylpyrazine
(1 equiv re. 2) and ClSiMe3 (2 equiv to 2), both of which were
readily removed (Scheme 2). When Me4-BTDP was used in the
absence of alkynes, a yellow powder [Ta2Cl4(m-C4Et4)]n (5) pre-
cipitated quantitatively. This protocol for preparing 5 allowed
us to introduce bis(trimethylsilyl)acetylene to give m-h2:h2-{bis-
(trimethylsilyl)acetylene} derivative 6 in 64% yield, but no cycli-
zation with the tantallacyclopentadiene moiety proceeded due
to the steric bulk of bis(trimethylsilyl)acetylene (Figure 3 and
Table 5).
Figure 3. Molecular structure of complex 6 with thermal ellipsoids set at
50% probability. Hydrogen atoms are omitted for clarity.
Table 5. Selected bond lengths (Š) and angles (8) of complex 6.
Ta1ÀTa2
Ta1ÀC1
Ta1ÀC4
Ta2ÀC1
Ta2ÀC2
Ta2ÀC3
2.8403(4)
2.172(5)
2.183(6)
2.389(6)
2.387(6)
2.359(5)
Ta2ÀC4
C1ÀC2
C2ÀC3
C3ÀC4
C13ÀC14
2.427(6)
1.409(8)
1.466(9)
1.422(8)
1.393(8)
dihedral angle between
0.95
C1-Ta1-C4 and C1-C2-C3-C4 planes
Scheme 3. Proposed mechanism of a cyclization reaction between the met-
allacyclopentadiene moiety of 6 and an alkyne.
Scheme 2. Reaction of complex 2 with Me4-BTDP followed by addition of 3-
hexyne or bis(trimethylsilyl)acetylene.
is an insertion/cyclization pathway via metallacycloheptatriene
intermediate II, giving a mixture of two isomers III and IV
(Path B). When the formed arene ligand freely rotates on the
dinuclear tantalum center, the final product contains both iso-
mers, III and IV, and then the thermodynamically favored com-
plex is formed as a single species.
With complex 6 in hand, we conducted control experiments
to investigate the reaction mechanism for the formation of an
arene ligand on the dinuclear tantalum center. As outlined in
Scheme 3, there are two reasonable pathways for the intermo-
lecular cyclization: one is a [4+2] cycloaddition pathway via
transition state I, resulting in the exclusive formation of III just
after the formation of the arene ligand (Path A), and the other
When internal alkynes, such as 2-butyne and 4-octyne, were
added to 6, Cs-symmetric complexes 7a and 7b, correspond-
ing to III in Scheme 3, were isolated as a single product in
good yield in each case (Scheme 4 and Figure 4, top, for mo-
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lecular structure of 7b; Table 6 for bond lengths and angles).
When terminal alkynes, such as 1-hexyne and p-tolylacetylene,
were adapted to this arene formation reaction, complexes 8a’
and 8b’, corresponding to IV, were obtained as a single prod-
uct in good yield in each case.
Scheme 4. Reactions of complex 6 with several alkynes.
Isomeric rotations of the formed arene ligands were clearly
observed by variable-temperature 1H NMR measurements.
Heating 7a at 808C in C6D6 solution induced rotation of the
C6Et4Me2 ligand by 608 to afford C1-symmetric complex 7a’ as
another single product [Eq. (6), Figure 4, bottom, and Table 6],
which indicated that 7a’ is thermodynamically favored over 7a
due to the location of a less bulky Me group at the a-position.
Rotation of the arene ligands led to the less bulky H being lo-
cated at the a-position. Complexes 8a and 8b were detected
1
by H NMR measurements at À108C (for 8a) or À608C (for 8b)
as a single product, in which singlet olefin protons due to
C6Et4RH (dH =5.53 ppm for 8a and 6.04 ppm for 8b) were ob-
served in each case [Eq. (7), and the Supporting Information,
Figures S1 and S2]. The isomeric rotation of the C6Et4RH li-
gands proceeded at room temperature to produce complexes
8a’ and 8b’, which was clearly confirmed by the disappear-
ance of the olefin protons and by the appearance of the ali-
phatic protons (dH =1.15 ppm for 8a’ and 1.56 ppm for 8b’).
These results definitively showed that the reaction of the met-
allacyclopentadiene moiety of 6 with 2-butyne, 1-hexyne, and
p-tolylacetylene proceeded through intermolecular [4+2] cy-
Figure 4. Molecular structures of dinuclear tantalum complexes 7b (top) and
7a’ (bottom) with thermal ellipsoids set at 50% probability. Hydrogen atoms
are omitted for clarity.
Table 6. Selected bond lengths (Š) and angles (8) of complexes 7b and
7a’.
7b
2.8451(6)
1.418(13)
1.398(13)
1.493(12)
1.457(13)
1.418(13)
1.539(14)
7a’
2.8557(3)
1.472(7)
1.399(8)
1.481(7)
1.489(7)
1.391(7)
1.482(7)
Ta1ÀTa2
C1ÀC2
C2ÀC3
C3ÀC4
C4ÀC5
C5ÀC6
C6ÀC1
C21ÀC22 (for 7b)
C17ÀC18 (for 7a’)
dihedral angle between
C1-C2-C3-C4 and C1-C6-C5-C4 planes
1.443(14)
42.44
1.446(7)
43.24
cloaddition (Path A). In contrast, a similar rotational behavior
of the arene ligand for 7b was not observed at high tempera-
ture, probably due to the location of the smaller Et group at
the a-position. We could not rule out the involvement of
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Path B and isomeric rotation of the C6Et4nPr2 ligand in the reac-
tion of 6 with 4-octyne to form 7b.
Conclusion
Rate constants for the isomeric rotation of the C6Et4Me2
ligand at several temperatures were measured by 1H NMR
spectroscopy (Figure 5, top), and the activation parameters
We have synthesized several dinuclear tantalum complexes
bearing m-h4:h4-arene and m-h2:h2-alkyne ligands from tantalla-
cyclopentadiene complexes. The formation of complexes 7a,
8a, and 8b by the reaction of 6 with the corresponding al-
kynes confirmed the intermolecular [4+2] cycloaddition of the
tantallacyclopentadiene moiety with an external alkyne. The re-
stricted rotational behavior of the arene ligands on the dinu-
clear tantalum center enabled us to isolate 7a at room tem-
perature and to detect 8a and 8b at low temperature just
after the intermolecular [4+2] cycloaddition reaction. In this
context, our system represents a model for the catalytic inter-
molecular alkyne trimerization. Further investigation of dinu-
clear complexes of group 5 metals as catalysts is ongoing in
our laboratory.
(DG303K° =103.8(Æ4.4) kJmolÀ1
,
DH° =102.6(Æ2.3) kJmolÀ1,
DS° =À3.9(Æ6.9) JKÀ1 molÀ1) were estimated by using Eyring
plots (Figure 5, bottom). Such a negative DS° value indicated
that the dissociation mechanism during the rotation could be
ruled out.
These isolated arene dinuclear complexes were inactive for
catalytic cyclotrimerization of alkynes, presumably due to the
nonlability of the arene ligand and the m-h2:h2-alkyne ligand.
We tested the oxidation of the dinuclear tantalum center in an
attempt to release the arene ligands. The C6Et6 ligand of 4 was
quantitatively released upon the addition of oxidants such as
2,6-lutidine N-oxide, WCl6, and AgBArF (BArF=B{3,5-
(CF3)2C6H3}4). It is assumed that the catalytically active species
in the cyclotrimerization reaction contains a {Ta2Cl6} fragment
for releasing arenes as reaction products.
Experimental Section
General
All manipulations involving air- and moisture-sensitive tantalum
complexes were performed either under argon by using standard
Schlenk techniques or in an argon-filled glovebox. [(h2-EtCꢀCEt)-
TaCl3(DME)] (1)[7] and 2,3,5,6-tetramethyl-1,4-bis(trimethylsilyl)-1,4-
diaza-2,5-cyclohexadiene (Me4-BTDP)[16] were prepared according
to reported procedures. Alkynes were purchased from TCI or Al-
drich and distilled over CaH2 before use. Anhydrous hexane, tolu-
ene, 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.[17] Benzene,
[D6]benzene, [D8]toluene, and [D8]THF were distilled over CaH2 and
degassed before use. 1H NMR (400 MHz) and 13C NMR (100 MHz)
spectra were measured with BRUKER AVANCEIII-400 and JEOL JNM-
ESC400 spectrometers. Assignment of 1H and 13C NMR peaks for
1
1
some of the complexes were facilitated by 2D H-1H COSY, 2D 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)
with a Shimadzu GCMS-QP2010Plus. All melting points were mea-
sured in sealed tubes under an argon atmosphere. UV/Vis spectra
were recorded with an Agilent 8453. Elemental analyses were re-
corded with a PerkinElmer 2400 at the Faculty of Engineering Sci-
ence, Osaka University.
Preparation of dinuclear tantalum complex 2
Complex 1 (3.01 g, 6.57 mmol) and AlCl3 (876 mg, 6.57 mmol) were
placed in an argon-filled Schlenk tube, and toluene (20 mL) was
added by using a syringe. The reaction mixture was stirred at 808C
for 30 min. The color of the solution changed from deep-orange to
deep-purple. [AlCl3(DME)] precipitated after hexane (40 mL) was
added to the reaction mixture by using a syringe. The supernatant
was transferred to another Schlenk tube, then all volatiles were re-
moved under reduced pressure. The residue was recrystallized
from a mixture of toluene and hexane (1:2 v/v) to give 2 (1.82 g,
1
2.46 mmol, 75%) as purple crystals; m.p. 101–1038C (dec). H NMR
3
(400 MHz, C6D6, 303 K): d=0.91 (t, J=7.7 Hz, 6H; CbCH2CH3), 1.07
(t, 3J=7.3 Hz, 6H; TaCaCH2CH3), 2.81 (q, 3J=7.7 Hz, 4H; Cb
CH2CH3), 3.45 ppm (q, 3J=7.3 Hz, 4H; TaCaCH2CH3); 13C NMR
(100 MHz, C6D6, 303 K): d=13.6 (CbCH2CH3), 17.3 (TaCaCH2CH3),
24.4 (CbCH2CH3), 36.5 (TaCaCH2CH3), 127.5 (CbCH2CH3), 224.0 (Ta-
CaCH2CH3); UV/Vis (toluene): lmax (e, MÀ1 cmÀ1)=523 nm (4.0”102);
Figure 5. Representative plot of In([7a]) versus time (top) and Eyring plots
(bottom) for conversion of 7a into 7a’. First-order rate constants were deter-
mined by monitoring the decay of 7a by H NMR spectroscopy. k
(10À4 molsÀ1)=0.4440 (315.64 K), 0.7866 (320.23 K), 1.288 (324.72 K), 2.162
1
(329.31 K), 3.991 (333.59 K), 6.264 (338.08 K).
Chem. Eur. J. 2015, 21, 11369 – 11377
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elemental analysis calcd (%) for C12H20Cl6Ta2: C 19.51, H 2.73;
found: C 19.65, H 2.69.
hexane (10 mL) was added to the mixture to form insoluble tanta-
lum species and a small amount of poly(3-hexyne) as precipitates.
The supernatant was transferred to another vial, then all volatiles
were removed under reduced pressure. The residue was washed
with hexane (3”5 mL), then the resulting precipitates were dried
under reduced pressure to give 4 (20.0 mg, 0.0240 mmol, 20%) as
a red powder; m.p. 217–2188C (dec); 1H NMR (400 MHz, C6D6,
Preparation of THF-coordinated dinuclear tantalum complex
3a
THF (1.0 mL, 12 mmol) was added by using a syringe to a solution
of complex 2 (100 mg, 0.135 mmol) in toluene (2 mL) at RT. The re-
action mixture was stirred for 5 min, then all volatiles were re-
moved under reduced pressure to give 3a (109 mg, 0.135 mmol,
3
3
303 K): d=0.39 (t, J=7.5 Hz, 6H; TaCaCH2CH3), 0.72 (q, J=7.5 Hz,
3
3
4H; TaCaCH2CH3), 1.07 (t, J=7.4 Hz, 12H; CbCH2CH3), 1.53 (t, J=
7.4 Hz, 6H; ꢀCCH2CH3), 2.19 (dq, 2J=14.8 Hz, 3J=7.4 Hz, 4H;
CbCHHCH3), 2.77 (dq, 2J=14.8 Hz, 3J=7.4 Hz, 4H; CbCHHCH3),
1
quantitative) as an orange powder; m.p. 115–1168C (dec); H NMR
3
3
3.98 ppm (q, J=7.4 Hz, 4H; ꢀCCH2CH3); 13C NMR (100 MHz, C6D6,
(400 MHz, CD2Cl2, 303 K): d=1.16 (t, J=7.4 Hz, 6H; TaCaCH2CH3),
3
1.29 (t, J=7.6 Hz, 6H; CbCCH2CH3), 2.14–2.32 (m, 4H; b-THF), 2.7–
303 K): d=15.5 (TaCaCH2CH3), 16.6 (ꢀCCH2CH3), 19.8 (CbCH2CH3),
23.8 (TaCaCH2CH3), 26.6 (CbCH2CH3), 38.7 (ꢀCCH2CH3), 104.5 (Ta-
CaCH2CH3), 108.7 (CbCH2CH3), 209.4 ppm (ꢀCCH2CH3); UV/Vis (tolu-
ene): lmax (e, MÀ1 cmÀ1)=318 (8.3”103), 381 (3.0”103), 446 nm
(6.3”103); elemental analysis calcd (%) for C24H40Cl4Ta2: C 34.63, H
4.84; found: C 34.79, H 4.81.
3.9 (br, 4H; CbCH2CH3), 3.5–3.8 (br, 4H; TaCaCH2CH3), 4.62–
4.88 ppm (m, 4H; b-THF); 13C NMR (100 MHz, CD2Cl2, 303 K): d=
17.4 (CbCH2CH3), 20.6 (TaCaCH2CH3), 24.4 (b-C of THF), 26.5
(CbCH2CH3) 40.7 (TaCaCH2CH3), 83.2 (a-C of THF), 159.0 (CbCH2CH3),
230.3 ppm (TaCaCH2CH3); UV/Vis (toluene): lmax (e, MÀ1 cmÀ1)=
451 nm (9.5”102); elemental analysis calcd (%) for C16H28Cl6OTa2: C
23.70, H 3.48; found: C 24.02, H 3.60.
Preparation of low-valent tantalum complex 5 by Me4-BTDP
Complex 2 (200 mg, 0.270 mmol) was placed in an argon-filled
Schlenk tube, then toluene (3 mL) was added by using a syringe. A
solution of Me4-BTDP (76.5 mg, 0.270 mmol) in toluene (2 mL) was
added to the Schlenk tube to form yellow precipitates. The reac-
tion mixture was stirred at RT for 2 h. After all volatiles were re-
moved under reduced pressure, the residue was washed with
hexane (2”2 mL). The yellow solids were dried under reduced
pressure to give 5 (180 mg, 0.270 mmol, quantitative) as a yellow
Preparation of pyridine-coordinated dinuclear tantalum
complex 3b
Pyridine (21.4 mg, 0.270 mmol) was added by using a syringe to
a solution of complex 2 (200 mg, 0.270 mmol) in toluene (3 mL) at
RT. The reaction mixture was stirred for 5 min at RT, then cooled to
À408C to form 3b (172 mg, 0.210 mmol, 78%) as orange crystals;
1
m.p. 120–1218C (dec); H NMR (400 MHz, C6D6, 303 K): d=0.68 (t,
3J=6.8 Hz, 6H; TaCaCH2CH3), 0.93 (t, 3J=7.5 Hz, 6H; CbCH2CH3),
2.42–2.64 (m, 2H; CbCHHCH3), 3.21–3.45 (m, 2H; CbCHHCH3), 3.57–
3.85 (m, 4H; TaCaCHHCH3, TaCaCHHCH3), 6.29–6.40 (m, 2H; m-py),
6.65–6.73 (m, 1H; p-py), 9.0–9.2 (br, 2H; o-py); 13C NMR (100 MHz,
C6D6, 303 K): d=16.9 (CbCH2CH3), 20.2 (TaCaCH2CH3), 24.1
(CbCH2CH3), 40.7 (TaCaCH2CH3), 124.6 (p-py), 140.5 (m-py), 152.1 (o-
py), 158.8 (CbCH2CH3), 233.0 ppm (TaCaCH2CH3); UV/Vis (toluene):
lmax (e, MÀ1 cmÀ1)=319 (8.0”103), 462 nm (1.6”103); elemental
analysis calcd (%) for C17H25Cl6NTa2: C 24.96, H 3.08, N 1.71; found:
C 25.03, H 3.16, N 1.81.
1
powder; m.p. 303–3048C (dec); H NMR (400 MHz, [D8]THF, 303 K):
d=0.85 (t, 3J=7.5 Hz, 6H; TaCaCH2CH3), 1.61 (t, 3J=7.7 Hz, 6H;
CbCH2CH3), 3.10 (q, 3J=7.7 Hz, 4H; CbCH2CH3), 3.16 ppm (q, 3J=
7.5 Hz, 4H; TaCaCH2CH3); 13C NMR (100 MHz, [D8]THF, 303 K): d=
12.8 (CbCH2CH3), 16.0 (TaCaCH2CH3), 27.8 (CbCH2CH3), 50.4 (Ta-
CaCH2CH3), 106.0 (CbCH2CH3), 235.1 (TaCaCH2CH3); UV/Vis (tolu-
ene): lmax (e, MÀ1 cmÀ1)=363 nm (1.6”103); elemental analysis
calcd (%) for C12H20Cl4Ta2: C 21.58, H 3.02; found: C 21.77, H 3.40.
Preparation of complex 4 from 5
A solution of Me4-BTDP (573 mg, 2.03 mmol) in toluene (15 mL)
was added by using a syringe to a Schlenk tube containing a solu-
tion of 2 (1.50 g, 2.03 mmol) in toluene (40 mL) at RT. The reaction
mixture was stirred at RT for 2 h to form yellow precipitates of 5.
After addition of 3-hexyne (463 mL, 4.06 mmol) to the yellow sus-
pension by using a syringe, the reaction mixture immediately
changed to a red suspension. All volatiles were removed under re-
duced pressure, then the residue was extracted with toluene (3”
30 mL). The solvent was evaporated to give a red powder, which
was washed with hexane (20 mL) and dried under reduced pres-
sure to give 4 (1.18 g, 1.42 mmol, 70%) as a red powder. The NMR
spectral data in C6D6 was identical to that of complex 4 prepared
from 3a with an excess amount of 3-hexyne (as mentioned above).
Preparation of THT-coordinated dinuclear tantalum complex
3c
Complex 3c was synthesized as described for 3a. The reaction of 2
(300 mg, 0.406 mmol) with THT (0.10 mL, 1.1 mmol) in toluene
(3 mL) gave 3c (335 mg, 0.406 mmol, quantitative) as a red-orange
1
powder; m.p. 106–1088C (dec); H NMR (400 MHz, C6D6, 303 K): d=
0.92 (t, 3J=7.6 Hz, 6H; CbCH2CH3), 1.05 (t, 3J=7.4 Hz, 6H; Ta-
CaCH2CH3), 1.30–1.43 (m, 4H; b-THT), 2.74–2.94 (m, 8H; CbCH2CH3,
a-THT), 3.58 ppm (q, 3J=7.4 Hz, 4H; TaCaCH2CH3); 13C NMR
(100 MHz, C6D6, 303 K): d=16.3 (CbCH2CH3), 18.8 (TaCaCH2CH3),
23.8 (CbCH2CH3), 30.3 (b-C of THT), 37.2 (a-C of THT), 39.6 (Ta-
CaCH2CH3), 149.5 (CbCH2CH3), 226.8 ppm (TaCaCH2CH3); elemental
analysis calcd (%) for C16H28Cl6STa2: C 23.24, H 3.41; found: C 23.28,
H 3.32.
Preparation of (Me3SiCꢀCSiMe3)-coordinated dinuclear tan-
talum complex 6
Preparation of a two-electron reduced m-h4:h4-hexaethyl-
benzene and a m-h2:h2-3-hexyne ligand-coordinated dinu-
clear tantalum complex 4 from complex 3a
A solution of Me4-BTDP (382 mg, 1.35 mmol) in toluene (5 mL) was
added to a solution of complex 2 (1.00 g, 1.35 mmol) and bis(tri-
methylsilyl)acetylene (460 mg, 2.70 mmol) in toluene (35 mL) at RT.
The reaction mixture was stirred at 808C for 16 h. The purple su-
pernatant was transferred to another Schlenk tube, then all vola-
tiles were removed under reduced pressure. The purple residue
was dried at 808C to give 6 (720 mg, 0.859 mmol, 64%) as
A solution of 3-hexyne (141 mL, 1.23 mmol) in toluene (5 mL) was
added by using a syringe to a solution of complex 3a (100 mg,
0.123 mmol) in toluene (15 mL) at RT. The reaction mixture imme-
diately became red. After stirring the reaction mixture for 1 h at RT,
Chem. Eur. J. 2015, 21, 11369 – 11377
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a purple powder; m.p. 105–1078C (dec); 1H NMR (400 MHz, C6D6,
CaCH2CH3), 26.0 (CbCH2CH3), 27.4 (CbCH2CH3), 27.8 (CbCH2CH3),
100.3 (TaCaMe), 105.8 (TaCaCH2CH3), 112.0 (CbMe), 115.4
(CbCH2CH3), 115.6 (CbCH2CH3), 116.7 (CbCH2CH3), 218.4 (ꢀCSiMe3),
223.4 ppm (ꢀCSiMe3); UV/Vis (toluene): lmax (e, MÀ1 cmÀ1)=306
(4.0”103), 418 (2.7”103), 477 ppm (2.5”103); elemental analysis
calcd (%) for C24H44Cl4Si2Ta2: C 32.30, H 4.97; found: C 31.93; H
4.64.
3
303 K): d=0.49 (s, 18H; SiMe3), 1.03 (t, J=7.6 Hz, 6H; CbCH2CH3),
3
1.17 (t, J=7.3 Hz, 6H; TaCaCH2CH3), 2.3–2.6 (br, 4H; TaCaCH2CH3),
2.6–2.8 ppm (br, 4H; CbCH2CH3); 13C NMR (100 MHz, C6D6, 303 K):
d=2.7 (ꢀCSiMe3), 17.4 (TaCaCH2CH3), 17.5 (CbCH2CH3), 24.6
(CbCH2CH3), 36.1 (TaCaCH2CH3), 149.1 (CbCH2CH3), 249.2 (Ta-
CaCH2CH3), 265.5 ppm (ꢀCSiMe3); UV/Vis (toluene): lmax (e,
M
À1 cmÀ1)=319 (6.5”103), 492 nm (1.7”103); elemental analysis
calcd (%) for C20H38Cl4Si2Ta2: C 28.65, H 4.57; found: C 28.45, H 4.26.
Preparation of dinuclear tantalum complex 8a’ from com-
plex 6
Preparation of dinuclear tantalum complex 7a from com-
plex 6
Complex 8a’ was synthesized as described for 7a. The reaction of
complex 6 (100 mg, 0.119 mmol) with 1-hexyne (14 mL, 0.12 mmol)
gave 8a’ (97.0 mg, 0.105 mmol, 88%) as an orange powder; m.p.
216–2178C (dec); 1H NMR (400 MHz, C6D6, 303 K): d=0.49 (t, 3J=
7.5 Hz, 3H; TaCaCH2CH3), 0.60 (s, 9H; SiMe3), 0.61 (s, 9H; SiMe3),
0.76 (t, 3J=7.1 Hz, 3H; CbCH2CH3), 0.80 (t, 3J=7.5 Hz, 3H;
CbCH2CH3), 0.91 (t, 3J=7.5 Hz, 3H; CbCH2CH3), 0.94 (t, 3J=7.5 Hz,
3H; CbCH2CH3), 1.08–1.29 (m, 6H; CbCH2CH2CH2CH3 and Ta-
CaCH2CH3), 1.21 (s, 1H; TaCaH), 2.09–2.64 ppm (m, 8H;
CbCHHCH2CH2CH3 and CbCHHCH3); 13C NMR (100 MHz, C6D6,
303 K): d=3.8 (2C, SiMe3), 13.9 (CbCH2CH2CH2CH3), 15.4 (Ta-
CaCH2CH3), 16.3 (CbCH2CH3), 17.0 (CbCH2CH3), 17.1 (CbCH2CH3),
23.4 (TaCaCH2CH3), 25.5 (CbCH2CH2CH2CH3), 26.5 (CbCH2CH3), 26.6
(CbCH2CH3), 29.5 (CbCH2CH3), 35.3 (CbCH2CH2CH2CH3), 36.6
(CbCH2CH2CH2CH3), 89.0 (1J(C,H)=160 Hz; TaCaH), 106.6 (Ta-
CaCH2CH3), 114.9 (CbCH2CH3), 115.2 (CbCH2CH3), 115.4 (CbCH2CH3),
115.6 (CbCH2CH2CH2CH3), 217.9 (ꢀCSiMe3), 219.1 ppm (ꢀCSiMe3);
UV/Vis (toluene): lmax (e, MÀ1 cmÀ1)=303 (3.9”103), 408 (2.7”103),
486 nm (2.8”103); elemental analysis calcd (%) for C24H44Cl4Si2Ta2:
C 33.92, H 5.26; found: C 33.71, H 5.47.
2-Butyne (28.0 mL, 0.357 mmol) was added by using a microsyringe
to a solution of complex 6 (100 mg, 0.119 mmol) in toluene (2 mL)
at À408C, then the reaction mixture was stirred at RT for 16 h.
After evaporation of all volatiles, the residue was washed with
hexane (5 mL). Drying the precipitates gave 7a (80.0 mg,
0.0896 mmol, 75%) as an orange powder; m.p. 264–2658C (dec);
1H NMR (400 MHz, C6D6, 303 K): d=0.44 (t, 3J=7.4 Hz, 6H; Ta-
3
CaCH2CH3), 0.60 (s, 18H; SiMe3), 0.91 (t, J=7.4 Hz, 6H; CbCH2CH3),
1.05 (q, 3J=7.4 Hz, 4H; TaCaCH2CH3), 2.03 ppm (s, 6H; CbMe);
13C NMR (100 MHz, C6D6, 303 K): d=4.2 (SiMe3), 15.7 (TaCaCH2CH3),
18.7 (CbCH2CH3), 20.2 (CbMe), 25.3 (TaCaCH2CH3), 27.7 (CbCH2CH3),
107.9 (TaCaCH2CH3), 111.0 (CbMe), 116.1 (CbCH2CH3), 224.3 ppm (ꢀ
CSiMe3); UV/Vis (toluene): lmax (e, MÀ1 cmÀ1)=306 (3.8”103), 426
(2.5”103), 470 nm (2.1”103); elemental analysis calcd (%) for
C24H44Cl4Si2Ta2: C 32.30, H 4.97; found: C 32.60, H 4.95.
Preparation of dinuclear tantalum complex 7b from com-
plex 6
Complex 7b was synthesized as described for 7a. The reaction of
Preparation of dinuclear tantalum complex 8b’ from com-
plex 6
complex
6
(100 mg, 0.119 mmol) with 4-octyne (52.4 mL,
0.357 mmol) gave 7b (90.0 mg, 0.098 mmol, 80%) as an orange
1
Complex 8b’ was synthesized as described for 7a. The reaction of
complex 6 (100 mg, 0.119 mmol) with p-tolylacetylene (13.2 mL,
0.120 mmol) gave 8b’ (102 mg, 0.107 mmol, 90%) as an orange
powder; m.p. 263–2648C (dec); H NMR (400 MHz, C6D6, 303 K): d=
0.45 (t, 3J=7.5 Hz, 6H; TaCaCH2CH3), 0.62 (s, 18H; SiMe3) 0.75 (t,
3
3J=7.3 Hz, 6H; CbCH2CH2CH3), 0.98 (t, J=7.4 Hz, 6H; CbCH2CH3),
1
1.05 (q, 3J=7.5 Hz, 4H; TaCaCH2CH3), 1.23–1.42 (m, 2H;
CbCH2CHHCH3), 1.49–1.69 (m, 2H; CbCH2CHHCH3), 2.09–2.30 (m,
4H; CbCHHCH2CH3, CbCHHCH3), 2.38–2.60 ppm (m, 4H; CbCHHCH3,
CbCHHCH2CH3); 13C NMR (100 MHz, C6D6, 303 K): d=4.3 (SiMe3),
14.3 (CbCH2CH2CH3), 15.3 (TaCaCH2CH3), 18.9 (CbCH2CH3), 25.7 (Ta-
CaCH2CH3), 27.8 (CbCH2CH3), 28.1 (CbCH2CH2CH3), 37.2
(CbCH2CH2CH3), 107.5 (TaCaCH2CH3), 116.7 (CbCH2CH2CH3), 117.8
(CbCH2CH3), 224.5 ppm (ꢀCSiMe3); UV/Vis (toluene): lmax (e,
powder; m.p. 227–2288C (dec); H NMR (400 MHz, C6D6, 303 K): d=
0.54 (s, 9H; SiMe3), 0.55 (t, 3J=7.4 Hz, 3H; TaCaCH2CH3), 0.64 (s,
3
3
9H; SiMe3), 0.72 (t, J=7.4 Hz, 3H; CbCH2CH3), 0.93 (t, J=7.5 Hz,
3H; CbCH2CH3), 0.96 (t, 3J=7.5 Hz, 3H; CbCH2CH3), 1.28 (q, 3J=
7.4 Hz, 2H; TaCaCH2CH3), 1.63 (s, 1H; CH), 2.05 (s, 3H; MeC6H4),
2.16–2.25 (m, 2H; CbCHHCH3, CbCHHCH3), 2.34–2.48 (m, 2H;
CbCHHCH3, CbCHHCH3), 2.72 (dq, 2J=15.4 Hz, 3J=7.6 Hz, 1H;
2
3
CbCHHCH3), 3.20 (dq, J=14.9 Hz, J=7.5 Hz, 1H; CbCHHCH3), 6.91
(d, 3J=8.1 Hz, 2H; m-MeC6H4), 7.50 ppm (d, 3J=8.1 Hz, 2H; o-
MeC6H4); 13C NMR (100 MHz, C6D6, 303 K): d=3.8 (2C, SiMe3), 15.2
(CbCH2CH3), 15.4 (TaCaCH2CH3), 15.5 (CbCH2CH3), 16.8 (CbCH2CH3),
21.0 (MeC6H4), 25.7 (TaCaCH2CH3), 26.0 (CbCH2CH3), 26.8
(CbCH2CH3), 29.1 (CbCH2CH3), 87.8 (1J(C,H)=158 Hz, TaCaH), 105.1
(TaCaCH2CH3), 111.4 (Cbtolyl), 114.8 (CbCH2CH3), 116.6 (CbCH2CH3),
116.9 (CbCH2CH3), 129.3 (m-C of tolyl), 130.9 (o-C of tolyl), 134.7
(ipso-C of tolyl), 139.5 (MeC), 220.1 (ꢀCSiMe3), 220.1 ppm (ꢀCSiMe3);
UV/Vis (toluene): lmax (e, MÀ1 cmÀ1): 306 (1.0”104), 414 (6.5”103),
483 nm (4.9”103); elemental analysis calcd (%) for C24H44Cl4Si2Ta2:
C 36.49, H 4.86; found: C 36.16, H 4.81.
M
À1 cmÀ1): 307 (3.7”103), 426 (2.2”103), 472 nm (1.9”103); elemen-
tal analysis calcd (%) for C28H52Cl4Si2Ta2: C 35.45, H 5.53; found: C
35.10, H 5.00.
Isomerization of 7a to produce 7a’
Complex 7a (65.0 mg, 0.0728 mmol) was placed in a Schlenk tube,
then toluene (3 mL) was added. The solution was stirred at 808C
for 1 h, then all volatiles were removed under reduced pressure to
give 7a’ (65.0 mg, 0.0728 mmol, quantitative) as a red powder; mp
1
3
260–2618C (dec.); H NMR (400 MHz, C6D6, 303 K): d=0.47 (t, J=
7.5 Hz, 3H; TaCaCH2CH3), 0.59 (s, 9H; SiMe3), 0.60 (s, 9H; SiMe3),
0.66 (t, 3J=7.4 Hz, 3H; CbCH2CH3), 0.84 (t, 3J=7.4 Hz, 3H;
CbCH2CH3), 0.88 (t, 3J=7.4 Hz, 3H; CbCH2CH3), 0.94 (s, 3H;
TaCaMe), 1.04–1.14 (m, 2H; TaCaCH2CH3), 1.92 (s, 3H; CbMe), 2.06–
2.44 ppm (m, 6H; CbCH2CH3); 13C NMR (100 MHz, C6D6, 303 K): d=
4.2 (2C, SiMe), 15.5 (TaCaCH2CH3), 15.6 (CbCH2CH3), 15.6
(CbCH2CH3), 17.8 (TaCaMe), 17.8 (CbMe), 17.9 (CbCH2CH3), 25.7 (Ta-
Detection of 8a and 8b by low-temperature NMR measure-
ments
In J-young NMR tubes, 1-hexyne (1.4 mL, 1 equiv., 0.012 mmol) or
p-tolylacetylene (1.4 mL, 1 equiv., 0.012 mmol) was added by using
a microsyringe at À788C to a solution of 6 in [D8]toluene. These
Chem. Eur. J. 2015, 21, 11369 – 11377
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11376
ꢀ 2015 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

Full Paper
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NMR tubes were kept at À788C until NMR measurements. 8a:
1H NMR (400 MHz, [D8]toluene, 263 K): d=0.45 (t, 3J=7.5 Hz, 3H;
TaCaCH2CH3), 0.48 (s, 18H; SiMe3), 0.51 (t, 3J=7.5 Hz, 3H; Ta-
CaCH2CH3), 0.71–1.70 (m, 17H; CbCH2CH2CH2CH3 and TaCaCH2CH3,
CbCH2CH3), 2.08–2.80 (m, 6H; CbCHHCH2CH2CH3 and CbCHHCH3),
5.53 ppm (s, 1H; CbH); 13C NMR (100 MHz, [D8]toluene, 263 K): d=
3.7 (SiMe3), 4.0 (SiMe3), 14.0, 14.1, 15.2, 17.7, 18.4, 23.2, 24.4, 25.3,
26.3, 28.1, 34.5, 35.3 (for 12 alkyl carbons), 102.2, 107.1, 107.3,
114.8, 115.1, 118.6 (for 6 arene ring carbons), 224.2 (ꢀCSiMe3),
224.9 ppm (ꢀCSiMe3). 8b: 1H NMR (400 MHz, [D8]toluene, 213 K):
3
3
d=0.09 (t, J=7.3 Hz, 3H; TaCaCH2CH3), 0.41 (t, J=7.3 Hz, 3H; Ta-
CaCH2CH3), 0.66 (s, 9H; SiMe3), 0.70 (s, 9H; SiMe3), 0.82 (t, 3J=
7.3 Hz, 6H; CbCH2CH3), 0.91–1.38 (m, 4H; TaCaCH2CH3), 2.02 (s, 3H;
3
Me), 2.13–2.30 (m, 4H; CbCH2CH3), 6.04 (s, 1H; CbH), 6.78 (d, J=
3
8.0 Hz, 2H; m-MeC6H4), 7.65 ppm (d, J=8.0 Hz, 2H; o-MeC6H4).
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Catalytic alkyne trimerization catalyzed by tantalum com-
plexes 1, 2 and 3a–c (Tables 2 and 3)
In J-young NMR tubes, tantalum complexes 1, 2, or 3a–
c (0.877 mmol, 0.5 mol% to alkynes), alkynes (0.175 mmol) and hex-
amethylbenzene or anthracene as an internal standard were dis-
solved in C6D6 (0.6 mL). The NMR tubes were allowed to stand at
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1
RT for 15 min, then H NMR spectra of the samples were recorded
to determine the product yield and the ratio of regioisomers.
Kinetic studies for isomeric reaction from 7a to 7a’
The rate constants for the isomerization of 7a to 7a’ were deter-
mined by recording 1H NMR spectra at several temperatures
(Figure 5, top), and the activation parameters were estimated
based on Eyring plots (Figure 5, bottom). Sample solutions for
1
their H NMR measurements ([7a]0 =2.80 mmolLÀ1, [anthracene]=
33.6 mmolLÀ1 as an internal standard) were prepared by dissolving
complex 7a (16.6 mg, 0.0186 mmol) and anthracene (10.2 mg,
0.0572 mmol) in C6D6 in a measuring flask (5 mL), and then the so-
lution was transferred into four J-young NMR tubes by using a sy-
ringe. The temperatures of the samples in a NMR probe were cali-
brated with ethylene glycol.[18] 1H NMR spectra were then recorded
every 31 seconds. The plots from 500 seconds to the half-life of 7a
were used to calculate the rate constants.
[13] Although alkynes are known reductants for transition metal halides by
the generation of the corresponding halogenated alkenes, no chlorinat-
ed products of 3-hexyne were detected in the reaction mixture.
[14] a) P. D. W. Boyd, A. J. Nielson, C. E. F. Rickard, J. Chem. Soc. Dalton Trans.
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J. Am. Chem. Soc. 2014, 136, 5161–5170.
Acknowledgements
K.Y. thanks the JSPS Research Fellowships for Young Scientists
for financial support. H. T. acknowledges financial support
through a Grant-in-Aid for Scientific Research on Innovative
Areas “New Polymeric Materials Based on Element-Blocks
(No.2401)”. This work was supported by the Core Research for
Evolutional Science and Technology (CREST) program of the
Japan Science and Technology Agency (JST), Japan.
[17] A. B. Pangborn, M. A. Giardello, R. H. Grubbs, R. K. Rosen, F. J. Timmers,
Organometallics 1996, 15, 1518–1520.
[18] C. Ammann, P. Meier, A. Merbach, J. Magn. Reson. 1982, 46, 319.
Keywords: alkynes · bridging ligands · cycloaddition · reaction
mechanisms · tantalum
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Schore, Chem. Rev. 1988, 88, 1081–1119; c) Transition-Metal-Mediated
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Received: March 25, 2015
Published online on June 26, 2015
Chem. Eur. J. 2015, 21, 11369 – 11377
www.chemeurj.org
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