Complexes of tantalum with triaryloxides: Ligand and solvent effects on formation of hydride derivatives

Article abstract of DOI:10.1016/j.jorganchem.2005.05.002

A family of tantalum compounds supported by the triaryloxide [R-L] ^3- ligands are reported [H₃(R-L) = 2,6-bis(4-methyl-6-R- salicyl)-4-tert-butylphenol, where R = Me or ^tBu]. The reaction of H₃[Me-L] with TaCl₅ in toluene gave [(Me-L)TaCl ₂]₂ (1). The [^tBu-L] analogue [( ^tBu-L)TaCl₂]₂ (2) was synthesized via treatment of TaCl₅ with Li₃[^tBu-L]. A THF solution of LiBHEt₃ was added to 1 in toluene to provide [(Me-L)TaCl(THF)] ₂ (3), while treatment of 2 with 2 equiv of LiBHEt₃ or potassium in toluene followed by recrystallization from DME resulted in formation of [M(DME)₃][{(^tBu-L)TaCl}₂(μ-Cl)] [M = Li (4a), K (4b)]. When the amount of MBHEt₃ (M = Li, Na, K) was increased to 5 equiv, the analogous reactions in toluene afforded [{(bit- ^tBu-L)Ta}₂(μ-H)₃M] [M = Li(THF)₂ (5a), Na(DME)₂ (5b), K(DME)₂ (5c)]. During the course of the reaction, the methylene CH activation of the ligand took place. Dissolution of 5a in DME produced [{(bit-^tBu-L)Ta}₂(μ-H) ₃Li(DME)₂] (6), indicating that the coordinated THF molecules are labile. When the 2/LiBHEt₃ reaction was carried out in THF, the ring opening of THF occurred to yield [(^tBu-L)Ta(OBu ⁿ)₂]₂ (7) along with a trace amount of [Li(THF)₄][{(^tBu-L)TaCl}₂(μ-OBu ⁿ)] (8). Treatment of 2 with potassium hydride in DME yielded [{(^tBu-L)TaCl₂K(DME)₂}₂(μ- OCH₂CH₂O)] (9), in which the ethane-1,2-diolate ligand arose from partial C-O bond rupture of DME. The X-ray crystal structures of 2, 3, 4, 5a, 6, 7, and 9 are described.

Full text of DOI:10.1016/j.jorganchem.2005.05.002

Journal of Organometallic Chemistry 690 (2005) 5333–5345
www.elsevier.com/locate/jorganchem
Complexes of tantalum with triaryloxides: Ligand and solvent
effects on formation of hydride derivatives
*
Hiroyuki Kawaguchi , Tsukasa Matsuo
Coordination Chemistry Laboratories, Institute for Molecular Science, Myodaiji, Okazaki 444-8787, Japan
Received 31 January 2005; received in revised form 3 March 2005; accepted 2 May 2005
Available online 22 June 2005
Abstract
A family of tantalum compounds supported by the triaryloxide [R–L]3ꢀ ligands are reported [H3(R–L) = 2,6-bis(4-methyl-6-R-
t
salicyl)-4-tert-butylphenol, where R = Me or Bu]. The reaction of H3[Me–L] with TaCl5 in toluene gave [(Me–L)TaCl2]2 (1). The
[tBu–L] analogue [(tBu–L)TaCl2]2 (2) was synthesized via treatment of TaCl5 with Li3[tBu–L]. A THF solution of LiBHEt3 was
added to 1 in toluene to provide [(Me–L)TaCl(THF)]2 (3), while treatment of 2 with 2 equiv of LiBHEt3 or potassium in toluene
followed by recrystallization from DME resulted in formation of [M(DME)3][{(tBu–L)TaCl}2(l-Cl)] [M = Li (4a), K (4b)]. When
the amount of MBHEt3 (M = Li, Na, K) was increased to 5 equiv, the analogous reactions in toluene afforded [{(bit-tBu–
L)Ta}2(l-H)3M] [M = Li(THF)2 (5a), Na(DME)2 (5b), K(DME)2 (5c)]. During the course of the reaction, the methylene CH acti-
vation of the ligand took place. Dissolution of 5a in DME produced [{(bit-tBu–L)Ta}2(l-H)3Li(DME)2] (6), indicating that the
coordinated THF molecules are labile. When the 2/LiBHEt3 reaction was carried out in THF, the ring opening of THF occurred
to yield [(tBu–L)Ta(OBun)2]2 (7) along with a trace amount of [Li(THF)4][{(tBu–L)TaCl}2(l-OBun)] (8). Treatment of 2 with potas-
sium hydride in DME yielded [{(tBu–L)TaCl2K(DME)2}2(l-OCH2CH2O)] (9), in which the ethane-1,2-diolate ligand arose from
partial C–O bond rupture of DME. The X-ray crystal structures of 2, 3, 4, 5a, 6, 7, and 9 are described.
Ó 2005 Published by Elsevier B.V.
Keywords: Tantalum; Aryloxide; Hydride; Multidentate ligand; C–O bond cleavage
1. Introduction
linearly joined in the ortho position by the methylene
groups [H3(R–L) = 2,6-bis(4-methyl-6-R-salicyl)-4-tert-
t
butylphenol, where R = Me or Bu]. This ligand system
Multidentate ligands play an important role in orga-
nometallic and coordination chemistry, because of
enhancing the stability of complexes and enforcing con-
strained coordination environments in comparison to
their analogues containing monodentate ligands [1]. As
part of an ongoing research project in metal complexes
capable of activating small molecules, we have been
exploring the chemistry of complexes supported by
aryloxide-based multidentate ligands [2–5]. We are espe-
cially attracted by the tridentate, trianionic [R–L]3ꢀ
triaryloxide ligands in which three aryloxide rings are
is closely related to calixarene derivatives. Upon coordi-
nation to a metal center, the tridentate-ligand frame-
work assumes a U-shaped conformation, which is
reminiscent of the cone conformation of calix[4]arene.
A significant advantage of the [R–L]3ꢀ ligand system
is the ability to tailor steric properties to satisfy specific
requirements via derivatization of the ortho substituents,
enabling a degree of control to be gained over the chem-
istry that occurs at the metal center. Furthermore, there
is the opportunity for coordinative unsaturation as com-
pared to calix[4]arene. Recently, we [2–5] and other
groups [6–9] demonstrated the synthesis of coordination
compounds having these triaryloxide ligands.
*
Corresponding author. Tel.: +81 564 595 587; fax: +81 564 595 589.
E-mail address: hkawa@ims.ac.jp (H. Kawaguchi).
0022-328X/$ - see front matter Ó 2005 Published by Elsevier B.V.
doi:10.1016/j.jorganchem.2005.05.002

5334
H. Kawaguchi, T. Matsuo / Journal of Organometallic Chemistry 690 (2005) 5333–5345
We previously reported that the [tBu–L] ligand al-
lowed for isolation of hydride-bridged dinuclear com-
plexes of titanium(III) and zirconium(IV) [3], and
these complexes are rare examples of non-metallocene
group 4 metal hydrides [10]. Attempts to prepare the
niobium analogue by the reaction of [(tBu–L)NbCl2]2
with LiBHEt3 under dinitrogen resulted in formation
of the nitride-bridged Nb(V) dinuclear complex [4]. This
reaction presumably proceeds via a niobium hydride
intermediate, which could cleave the N„N bond along
with the reductive elimination of H2. Therefore we have
extended the use of the [R–L] ligand to tantalum com-
plexes and set out to investigate whether hydride com-
plexes supported by the [R–L] auxiliary could be
isolated. In this paper we describe the synthesis and
structural properties of tantalum complexes based on
the triaryloxide ligand. Portions of this work were com-
municated in the preliminary report [5].
Scheme 1. Synthesis of 1 and 2.
The desired complex [(tBu–L)TaCl2]2 (2) was isolated
as a yellow solid, which was crystallized from a warm
toluene solution. Upon isolation, these chloride deriva-
tives are moderately soluble in CH2Cl2 and THF, spar-
ingly soluble in toluene, and insoluble in hydrocarbons.
Although 2 could be obtained in moderate yield, the low
solubility in non-polar solvents renders isolation of
LiCl-free 2 difficult. Combustion analyses and 1H
NMR data of 1 and 2 are in good agreement with their
formulations. The 1H NMR spectrum of each com-
pound is indicative of a highly symmetric species in solu-
tion. The bridging methylene protons of the triaryloxide
ligands appear as a pair of doublets.
2. Results and discussion
2.1. Synthesis of [(R–L)TaCl2]2 [R = Me (1), tBu (2)]
Our objective was to obtain well-characterized tanta-
lum hydride complexes supported by aryloxide auxilia-
ries. The strategy was to use chloride derivatives
t
bearing the [R–L] ligands (R = Me, Bu) as a substrate
and triethylborohydrides MBHEt3 (M = Li, Na, K) as
a hydride source. We thought it would be advantageous
to use a linked-aryloxide [R–L] ligand, which would
leave little possibility of reorganizing the molecule and
leading to insoluble aryloxide-hydride polymeric prod-
ucts. As a hydride reagent, a stock THF solution of
MBHEt3 (Aldrich) was used without further
purification.
To prepare the substrates [(R–L)TaCl2]2 for the
present investigation, the reactions of 1 equiv of
H3[Me–L] with TaCl5 were examined in refluxing tolu-
ene. When the ligand precursor H3[Me–L] was used,
[(Me–L)TaCl2]2 (1) was obtained as an orange crystal-
line precipitate with evolution of HCl (Scheme 1). For
the tert-butyl-substituted ligand precursor H3[tBu–L],
the analogous reaction led to ligand degradation associ-
ated with loss of one of tert-butyl groups of [tBu–L]3ꢀ
.
The de-tert-butylation of the [tBu–L] ligand during the
reaction is not entirely unexpected given the fact that li-
gand degradation has already been observed in the reac-
tion of NbCl5 with H3[tBu–L] [2c]. In this case, the
tantalum complex is a strong enough Lewis acid to effect
a similar type of reaction chemistry. The synthesis of the
niobium chloride derivative having the [tBu–L] ligand
was previously achieved from the reaction of NbCl5
and the solvent-free trilithium salt Li3[tBu–L]. The sim-
ilar route has been employed in the present work start-
ing from TaCl5 and Li3[tBu–L] in refluxing toluene.
Fig. 1. Structure of [(tBu–L)TaCl2]2 (2). Methyl and tert-butyl groups
˚
have been omitted for clarity. Selected bond distances (A) and angles
(°): Ta(1)–O(1) 1.905(4), Ta(1)–O(2) 2.066(3), Ta(1)–O(3) 1.905(4),
Ta(1)–O(5) 2.197(4), Ta(2)–O(2) 2.190(4), Ta(2)–O(4) 1.904(4), Ta(2)–
O(5) 2.062(4), Ta(2)–O(6) 1.891(4), Ta(1)–Cl(1) 2.319(1), Ta(1)–Cl(2)
2.350(2), Ta(2)–Cl(3) 2.320(1), Ta(2)–Cl(4) 2.352(2), Ta(1)–Ta(2)
3.6098(4), O(1)–Ta(1)–O(3) 168.1(2), O(4)–Ta(2)–O(6) 166.9(2),
O(2)–Ta(1)–O(5) 63.9(1), O(2)–Ta(2)–O(5) 64.1(1), Ta(1)–O(2)–Ta(2)
116.0(1), Ta(1)–O(5)–Ta(2) 115.9(1), Cl(1)–Ta(1)–Cl(2) 104.79(5),
Cl(3)–Ta(2)–Cl(4) 105.57(6).

H. Kawaguchi, T. Matsuo / Journal of Organometallic Chemistry 690 (2005) 5333–5345
5335
R = Me
The solid-state structure of 2 was confirmed by a sin-
LiHBEt3, toluene
gle-crystal X-ray diffraction study (Fig. 1). The molecule
of 2 is a dimer, which derives from the sharing of a cen-
tral aryloxide group of the [tBu–L] ligand to yield a pla-
nar Ta2(l-O)2 ring. The bridging Ta–O distances reflect
a dissymmetry in the Ta2O2 ring with two longer
O
O
THF
O
O
O
R
But
Ta
Ta
Cl
R
O
THF
But
O
O
Cl
Cl
O
O
But
Ta
Ta
Cl
˚
[2.197(4), 2.190(4) A] and two shorter distances
˚
[2.066(3), 2.062(4) A]. This distortion is probably attrib-
Cl
O
Cl
But
uted to steric crowding about the metal centers and geo-
metrical constraint imposed by the tridentate ligand
framework. The terminal Ta–O distances (average
O
CDCl3
R
3
R
˚
1.901 A) are longer than the bridging ones (average
R = Me (1)
R = But (2)
R = But
t
2.129 A) as expected. Each Bu–L ligand meridionally
˚
2 equiv LiHBEt3, toluene
or
chelates a tantalum metal in a U-shaped conformation.
The coordination sphere of each octahedral tantalum is
completed by two mutually cis chloride ligands, with the
Cl–Ta–Cl angles of 104.79(5)° and 105.57(6)°. The
K, toluene
But
Cl
Cl
O
But
Cl
R = But
˚
O
Ta–Cl distances (average 2.335 A) are typical for
5 equiv MHBEt3,
toluene
Ta
O
Ta
O
Ta(V) complexes [11].
O
(M = Li, Na, K)
O
But
But
But
2.2. Reaction of [(Me–L)TaCl2]2 (1) with LiBHEt3
But
[M(dme)3]+
Addition of 2 equiv of LiBHEt3 in THF to a toluene
suspension of 1 gave a brown precipitate of a mixture of
LiCl and [(Me–L)TaCl(THF)]2 (3) with the concomitant
reduction of the metal center from Ta(V) to Ta(IV), in
which LiBHEt3 functions as a reductant (Scheme 2).
Analytically pure 3 was isolated in 67% yield after
recrystallization from a warm toluene. The analogous
reaction with excess LiBHEt3 did not proceed further
than the Ta(IV) complex 3. This is presumably due to
the very low solubility of 3 in the reaction media (tolu-
ene/THF). Complex 3 is diamagnetic, and the 1H
NMR spectrum revealed the presence of coordinated
THF as two broad signals. In addition, the resonance
pattern due to the Me–L ligand are entirely analogous
to that of 1. The complex 3 is readily oxidized. When
3 was dissolved in CDCl3, formation of the Ta(V) chlo-
ride dimer 1 was noticed according to its 1H NMR
spectrum.
The X-ray crystal structure of 3 confirms the dinu-
clear character of the compound in the solid state
(Fig. 2). A crystallographically imposed center of inver-
sion is located at the center of the Ta2O2 ring. The
geometry at tantalum is distorted octahedral, with a
chloride, a THF molecule, a tridentate [Me–L] ligand,
and a central aryloxide of another [Me–L] ligand. Simi-
lar to 2, the [Me–L] ligand is coordinated in a meridio-
nal manner and assumes a U-conformation. The
coordinated THF molecule is inside the cavity of the
U-shaped triaryloxide ligand and is bound trans to
the bridging aryloxide [O(2*)] of another triaryloxide li-
M = Li (4a), K (4b)
3 equiv LiBHEt3
toluene
L
L
But
But
M
O
O
H
H
H
But
O
Ta
Ta
O
But
O
O
[(But–L)TaX2]2
X = Cl (2), Br
But
But
CDCl3
or
PhCH2Br
M = Li (5a), Na (5b), K (5c)
M = Li (5a)
THF
DME
O
O
But
But
O
O
Li
O
O
H
But
O
Ta
H
H
Ta
O
But
O
O
But
But
6
Scheme 2. Synthesis of 3, 4, 5, and 6.
and chloride groups by the Ta(IV) center in 3 relative
to related Ta(V) species. In the planar Ta2O2 dimer ring
the angles at O [85.35(7)°] are smaller than those at Ta
[94.65(7)°], and the bridging Ta–O(2*) distance of
˚
gand. The terminal Ta–O(aryloxide) (average 1.923 A)
˚
and the Ta–Cl distances (2.4368(9) A) are increased
˚
2.027(2) A is significantly shortened as compared to
˚
the corresponding distances of 2 (average 2.194 A). This
observation is in keeping with the presence of a Ta–Ta
from those found in the Ta(V) complex 2, inferring a
diminished donation of electron density from aryloxide

5336
H. Kawaguchi, T. Matsuo / Journal of Organometallic Chemistry 690 (2005) 5333–5345
Fig. 3. Structure of the anion in [K(DME)3][{(tBu-L)TaCl}2(l-
Cl)](4b). Methyl and tert-butyl groups have been omitted for clarity.
˚
Selected bond distances (A) and angles (°): Ta–O(1) 1.910(6), Ta–O(2)
2.070(6), Ta–O(3) 1.953(7), Ta–O(2*) 2.109(6), Ta–Cl(1) 2.410(2),
Ta–Cl(2) 2.552(3), Ta–Ta* 2.7105(4), O(1)–Ta–O(2) 89.8(3), O(1)–Ta–
O(3) 103.5(3), O(2)–Ta–O(3) 86.6(3), Cl(1)–Ta–Cl(2) 88.49(6), Cl(1)–
Ta–O(2) 169.9(2), Cl(2)–Ta–O(1) 166.1(2), Ta–O(2)–Ta* 80.9(2),
Ta–Cl(2)–Ta* 64.17(8), Cl(1)–Ta–Ta*–Cl(1*) 63.43(7), O(2)–Ta–
Ta*–O(2*) 146.0(4).
Fig. 2. Structure of [(Me–L)TaCl(THF)]2 (3). One set of disordered
tert-butyl groups have been omitted for clarity. Selected bond
˚
distances (A) and angles (°): Ta–O(1) 1.924(2), Ta–O(2) 2.105(2),
Ta–O(3) 1.922(2), Ta–O(4) 2.293(2), Ta–O(2*) 2.027(2), Ta–Cl
2.4368(9), Ta–Ta* 2.8015(2), O(1)–Ta–O(3) 158.40(8), O(2)–Ta–Cl
177.05(5), O(4)–Ta–Cl 89.35(6), O(2)–Ta–O(2*) 94.65(7), Ta–O(2)–
Ta* 85.35(7).
counterion, which is solvated by three DME molecules
and well separated from its ionic counterpart. The anion
has a crystallographically imposed C2 symmetry, the 2-
fold axis running through the Cl(2) atom and bisecting
the Ta–Ta bond. In contrast to the meridional structures
found in 2 and 3, the flexible [tBu–L] ligand facially caps
the tantalum atom in a U-shaped arrangement. The ste-
ric influence of the [tBu–L] ligand is demonstrated by a
comparison between the Ta(IV) complexes, 3 and 4b.
The [Me–L] ligand in 3 assumes a conformation allow-
ing the inclusion of a THF molecule that enters its cavity
and is bound to the metal, whereas the bulky tert-butyl
groups of the [tBu–L] ligand may prevent formation of
the [tBu–L] complex analogous to 3 due to the increased
congestions of the metal site. Instead, one chloride anion
is incorporated into the dimer frame to generate an an-
ionic [{(tBu–L)TaCl}2(l-Cl)] unit. Thus the dimer struc-
ture of the anion in 4 is formed by two octahedral
tantalum atoms bridged by one chloride atom and two
central aryloxide groups of the [tBu–L] ligands. The
˚
single bond of 2.8015(2) A, which rationalizes its dia-
magnetism [12].
2.3. Reactions of [(tBu–L)TaCl2]2 (2) with MBHEt3
(M = Li, Na, K) and KH
In order to investigate the effect of an increase in ste-
ric bulk upon the reaction course, a slurry of the [tBu–L]
complex 2 in toluene was treated with 2 equiv of LiB-
HEt3 in THF to give a green–brown solution. Upon re-
moval of the solvent a brown solid was obtained that
was recrystallized from DME/hexane to produce
diamagnetic blue crystals of [Li(DME)3][{(tBu–
L)TaCl}2(l-Cl)] (4a) in 28%. The metal center is reduced
from Ta(V) to Ta(IV). Alternatively, reduction of 2 with
a potassium film in DME resulted in formation of the
potassium analogue [K(DME)3][{(tBu–L)TaCl}2(l-Cl)]
(4b) in 70%, which was isolated from DME/hexane as
blue crystals suitable for X-ray crystallography (vide in-
fra). The use of excess potassium failed to further reduce
the complex. The 1H NMR spectra of 4a and 4b in
THF-d8 are very alike, and the methylene protons of
the [tBu–L] ligands appear as two pairs of mutually cou-
pled doublets. Additionally, there are three tert-butyl
and two methyl singlets. These features indicate that
the dimeric structure observed in the solid state is
retained in solution.
˚
˚
Ta–O distances [average 1.932 A (terminal), 2.090 A
(bridging)] are similar to those of the Ta(IV) complex
3, while the shortening of the triply bridged Ta–Ta dis-
˚
tance [2.7105(4) A] reflects the acute Ta–Cl(2)–Ta
[64.17(8)°] and Ta–O(2)–Ta angles [80.9(2)°]. The termi-
nal Ta–Cl(1) distance of 2.410(2) A is longer than the
˚
˚
bridging Ta–Cl(2) distance of 2.552(3) A, and it is com-
parable to that of the related Ta(IV) complex 3
˚
[2.4368(9) A].
When the amount of LiBHEt3 was increased, [{(bit-
tBu–L)Ta}2(l-H)3Li(THF)2] (5a) was obtained as yel-
low crystals along with 4a. Intramolecular metalation
The structure of the anion part of 4b is shown in Fig.
3. There is no direct connection with the potassium

H. Kawaguchi, T. Matsuo / Journal of Organometallic Chemistry 690 (2005) 5333–5345
5337
of the methylene linkers in the [(tBu–L)Ta] complex
took place to provide a new [bit-tBu–L]4ꢀ ligand. After
experimenting with various amounts of LiHBEt3, we
found that optimum yields (62% isolated) could be ob-
tained by reaction of 1 with 5 equiv of LiBHEt3 followed
by filtration and concentration of the solution. The anal-
ogous reactions of 2 with NaBHEt3 and KBHEt3 gave
[(bit-tBu–L)Ta]2(l-H)3M(THF)2 [M = Na (5b), K (5c)]
in moderate yields, after recrystallization from DME/
hexane. This suggests that formation of the [{(bit-
tBu–L)Ta}2(l-H)3]ꢀ dimeric core (vide infra) is indepen-
dent of the nature of the counterion.
The molecular structures of 5a, 5b, and 5c were estab-
lished by X-ray analysis. Since their structures and geo-
metrical parameters are very similar, those of the lithium
adduct 5a are described here (Fig. 4). The molecular
structure of 5a shows the dimer to be bridged by three
hydrides, which were located and refined isotropically
L)Ta] ends of the molecule to be eclipsed. The short
˚
Ta–Ta distance of 2.9313(4) A relative to that of 2
˚
[3.6098(4) A] is imposed by the three hydride bridges
and not indicative of any metal–metal interaction be-
cause two metal centers are Ta(V) in this formalism.
However, the metal–metal distance bridged by three hy-
drides in 5a is substantially longer than the Ta(IV)–
Ta(IV) single bonds found in 3 and 4.
The hydride complexes 5a–c show similar patterns
in the 1H NMR spectra, which are consistent with
their solid-state structures. The metal-bound methine
group appears as a singlet, and the remaining methy-
lene protons of the ligand are observed a pair of dou-
blets. In addition, the bridging hydrides appear
equivalent at ca. 11 ppm, the downfield shift being
characteristic of hydride ligands on a highly electro-
philic metal center surrounded by hard donor groups
[14] (cf. ½ðMe3SiCH2Þ ðArN ¼ÞTaðl ꢀ HÞðl ꢀ g1 : g3ꢀ
2
(Ta–H, 1.80–1.90 A). The metalacyclic Ta–C(7) and –
Pri2 ꢀ tacnÞLiꢁ, 11.03 ppm [15]). These features are
essentially invariant over the temperature range from
ꢀ80 to 20 °C. On warming up to 60 °C, these signals
of methine, methylene, and hydride protons are ob-
served to coalesce along with the partial decomposi-
tion of the hydride complexes due to their thermal
instability [5,16]. This indicates that the methylene
CH activation process is reversible. Thus the cyclo-
˚
˚
C(21) distances of 2.267(4) and 2.264(3) A are normal
for Ta(V)–alkyl compounds [11a,13]. Each Cispo carbon
of the central aryloxide of the [bit-tBu–L] ligand forms
close contact with the metal center [2.420(4) and
˚
2.426(3) A]. This is a consequence of the conformational
constraint imposed by the [bit-tBu–L] framework in 5a.
One lithium cation, solvated by two THF molecules, is
connected to two central aryloxides [O(2), O(5)] of the
bit-tBu–L ligands. This requires the two [(bit-tBu–
metalated-hydride complex
masked-form of low-valent tantalum dimer.
5
could provide
a
The role of MBHEt3 (M = Li, Na, K) in the reaction
with 2 is not only as a reducing agent but also as a hy-
dride source. During the course of the reaction, 4 equiv
of MBHEt3 is consumed with the reductive loss of H2.
Subsequently, intramolecular addition of a methylene
CH bond to a Ta(III)–Ta(III) intermediate may take
place. The remaining triethylborohydride reactant ap-
pears to be incorporated as a hydride ligand in the final
product. The proposal that the Ta(IV) dimer 4a is an
intermediate in the reaction can be confirmed by the
preparation of 5a from 4a by addition of LiBHEt3 in
toluene. It is noted that 4a is not found to undergo intra-
molecular CH activation across a Ta(IV)–Ta(IV) bond,
while there are precedents for oxidative addition of a
CH bond to Ta(III) [17]. It was also reported that at-
tempt to prepare the d2–d2 ditantalum(III) allene com-
plex by reduction with sodium amalgam resulted in
formation of the propynylidene-dihydride-bridging
complex along with a double CH activation of the allene
ligand [17d]. Therefore we prefer the mechanism via a
Ta(III)–Ta(III) intermediate which undergoes CH acti-
vation of the ligand (Scheme 3). This mechanism is also
in accordance with the observation that no elimination
of HD and H2 occurred in the reaction mixture of 2
and LiBDEt3. However, in the 2/LiBDEt3 reaction, it
is not clear into which position of the final product the
deuteride reagent is incorporated due to the reversible
CH metalation process.
Fig. 4. Structure of [{(tBu-L)Ta}2(H)3Li(THF)2] (5a). Methyl and
tert-butyl groups have been omitted for clarity. Selected bond
˚
distances (A) and angles (°): Ta(1)–O(1) 1.958(2), Ta(1)–O(2)
2.038(3), Ta(1)–O(3) 1.946(2), Ta(2)–O(4) 1.974(3), Ta(2)–O(5)
2.028(3), Ta(2)–O(6) 1.947(2), Ta(1)–C(7) 2.267(4), Ta(1)–C(8)
2.420(4), Ta(2)–C(41) 2.264(3), Ta(2)–C(42) 2.426(3), Ta(1)–Ta(2)
2.9313(4), Li–O(2) 1.946(8), Li–O(5) 1.959(7), Ta(1)–H 1.80–1.85,
Ta(2)–H 1.81–1.90, O(1)–Ta(1)–O(2) 142.8(1), O(1)–Ta(1)–O(3)
96.25(10), O(2)–Ta(1)–O(3) 89.61(10), O(4)–Ta(2)–O(5) 141.97(10),
O(4)–Ta(2)–O(6) 93.8(1), O(5)–Ta(2)–O(6) 90.48(10), O(2)–Li–O(5)
119.3(4), Ta(1)–H–Ta(2) 102.9–108.5, O(2)–Ta–Ta–O(5) 6.47(10).

5338
H. Kawaguchi, T. Matsuo / Journal of Organometallic Chemistry 690 (2005) 5333–5345
LiBDEt3 did not show any sign of the H/D exchange.
Attempts to prepare the deuterated analogues such as
[(bit-tBu–L)Ta]2(l-D)3Li(THF)2 have been unsuccessful.
The THF molecules coordinated at lithium in 5a can
be readily replaced by DME. Treatment of 5a in DME
and subsequent addition of hexane produced yellow
crystals formulated as [(bit-tBu–L)Ta]2(l-H)3Li(DME)2
(6), which was identified by X-ray crystallography
(Fig. 5). It is obvious that the molecule contains a
[{(bit-tBu–L)Ta}2(l-H)3] dimeric unit, like the structure
of 5a. The local geometry of the monomeric [(bit-tBu–
L)Ta] units is similar to that found in 5a, and the
˚
Ta–O distances of 6 [Ta–O(central), average 2.024 A;
˚
Ta–O(outer), 1.963 A] resemble those of 5a [2.033 and
˚
1.956 A, respectively]. However, the dimeric unit has
to rearrange its conformation to accommodate a large
[Li(DME)2]+ unit. The unique feature of 6 is that as
the lithium atom is coordinated by chelating DME in-
stead of THF, one of the Li–O(aryloxide) bridging
˚
bonds is missing [Li–O(5) 5.052(7) A] due to the increas-
ing coordination number and steric crowding about
small Li. One notable difference can be seen in the
O(2)–Ta(1)–Ta–(2)–O(5) torsion angle. While the
opposing [bit-tBu–L] ligands on Ta(1) and Ta(2) in 5a
Scheme 3. Proposed mechanism for the formation of 5.
A comparison of niobium and tantalum complexes in
the reactions with LiBHEt3 merits comment. The analo-
gous reaction of [(tBu–L)NbCl2]2 with LiBHEt3 may
proceed via the Nb(II) species of the type [(tBu–
L)NbLi]n along with reductive elimination of hydrides,
which subsequently binds, reduces, and cleaves dinitro-
gen to produce the Nb(V) nitride complex [4]. Since tan-
talum is known to stabilize higher oxidation states better
than niobium, it is expected that the metal center is
hardly reduced to Ta(II) in the 2/triethylborohydride
reaction system. When the metal center is reduced to
Ta(III), the tantalum complex is prone to undergo intra-
molecular CH activation, resulting in formation of the
Ta(V) hydride complex. On the other hand, the preli-
minary result shows that the tantalum complexes 5a–c
are inert toward dinitrogen (1 atm).
When 5a was dissolved in CDCl3, 1H NMR assays re-
vealed near quantitative conversion to 2 and the signal
assigned to the hydride protons immediately disap-
peared. During the course of the reaction, the hydride li-
gand has migrated back to the methine carbon of the
Fig. 5. Structure of [{(tBu-L)Ta}2(H)3Li(DME)2] (6). Methyl and tert-
˚
butyl groups have been omitted for clarity. Selected bond distances (A)
and angles (°): Ta(1)–O(1) 1.973(3), Ta(1)–O(2) 2.049(2), Ta(1)–O(3)
1.954(2), Ta(2)–O(4) 1.968(3), Ta(2)–O(5) 2.002(3), Ta(2)–O(6)
1.956(3), Ta(1)–C(7) 2.254(4), Ta(1)–C(8) 2.411(4), Ta(2)–C(41)
2.276(4), Ta(2)–C(42) 2.416(4), Ta(1)–Ta(2) 2.9458(3), Li–O(2)
2.004(7), Ta(1)–H 1.80–1.85, Ta(2)–H 1.81–1.90, O(1)–Ta(1)–O(2)
139.2(1), O(1)–Ta(1)–O(3) 95.7(1), O(2)–Ta(1)–O(3) 89.1(1), O(4)–
Ta(2)–O(5) 139.0(1), O(4)–Ta(2)–O(6) 94.9(1), O(5)–Ta(2)–O(6)
90.0(1), Ta(1)–H–Ta(2) 96.3–101.7, O(2)–Ta–Ta–O(5) 65.89(8).
t
bit-tBu–L ligand, restoring the tridentate Bu–L ligand.
This reaction could be used in preparing a Ta(V) halide
starting material. For example, treatment of 5a with
PhCH2Br in toluene gave the bromide analogue [(tBu–
L)TaBr2]2. On the other hand, these hydride complexes
did not react with D2 gas. The mixture of 5a and

H. Kawaguchi, T. Matsuo / Journal of Organometallic Chemistry 690 (2005) 5333–5345
5339
are eclipsed with a negligible O(2)–Ta(1)–Ta(2)–O(5)
But
torsion angle of 6.47(10)°, the structure of 6 is assumed
to have the two [(bit-tBu–L)Ta] ends staggered with re-
spect to each other [65.89(8)°]. This structural feature
contrasts with the eclipsed geometries found in 5b and
5c, where larger sodium and potassium solvated by
two DME molecules can bridge the central aryloxides
But
O
BuO
O
O
But
Ta
Ta
OBu
O
But
BuO
O
OBu
But
O
But
˚
of the ligands. The Ta–Ta separation of 2.9458(3) A is
slightly elongated relative to that of 5a.
7
LiHBEt3, THF
The 1H NMR spectrum of 6 in THF-d8 indicates that
5a is regenerated along with free DME, because the Li-
bound ligands are labile. However, we have not been
successful in preparing their ion-separated salts and
replacing the alkali cations by organic cations. The
counter cations appear to be bound strongly to arylox-
ide oxygen atoms. Additionally, the counter ions might
play an important role in stabilizing this dimeric struc-
ture. For example, addition of 18-crown-6 to a THF
solution of 5c was found to lead to facile C–O bond
cleavage of the triaryloxide ligand [5].
[(But–L)TaCl2]2
+
2
But
But
O
Cl
Cl
O
O
Ta
O
Ta
O
O
KH
O
But
But
But
But
DME
[Li(dme)3]+
8
2.4. C–O bond cleavage reactions of THF and DME
But
The choice of solvent and hydride reagent is crucial to
the successful synthesis of the Ta(V) hydride complexes
5a–c. For instance, the reaction of 2 with 4 equiv of LiB-
HEt3 was carried out in THF to produce a mixture of
colorless 7 (major) and blue 8 (Scheme 4). Compounds
7 and 8 could not be isolated by fractional crystalliza-
tion due to their similar solubility in toluene and
THF, but large colorless crystals of 7 were manually
separated from blue needle crystals of 8 with the aid
of a microscope. Coupled with crystallographic data
DME
Cl
Ta
Cl
O
DME
But
K
But
DME
DME
But
O
O
Cl
Ta
Cl
O
K
O
O
O
But
O
But
9
Scheme 4. Synthesis of 7, 8, and 9.
1
(vide infra), the H NMR spectrum confirmed the for-
mulation of 7 as [(tBu-L)Ta(OBun)2]2. The 1H NMR
spectrum of 7 features two sets of resonances assigned
to inequivalent n-butoxide ligands which arise from ring
opening of THF, and this NMR data is consistent with
the solid-state structure. Analytically pure 7 was inde-
pendently prepared in 77% yield by treatment of 2 with
4 equiv of sodium butoxide in toluene.
The solid-state structure of 7 was determined by a
single-crystal X-ray diffraction study (Fig. 6). The
molecular structure reveals the dimeric nature of 7 with
the halves related by a crystallographic center of symme-
try. This dimeric species 7 has an overall geometry sim-
ilar to the chloride derivatives 2, with n-butoxide groups
in place of chloride ligands. The bond distances and
angles for 7 all fall within the typical ranges for triaryl-
oxide ligands found in Ta(V) complexes. The Ta–O(n-
acquisition of a high quality X-ray structure of this spe-
cies. A low-resolution was obtained which at least con-
firmed connectivity (Scheme 4) [18]. The blue complex
8 is formulated as [Li(THF)4][{(tBu–L)TaCl}2(l-OBun)].
The structure of the anion part shows a dimer, which is
closely related to that found in the chloride-bridging
Ta(IV) dimer 4. In 8, the tantalum centers are bridged
by one n-butoxide ligand and two central aryloxides of
the tBu–L ligands. During the formation of 8, reduction
from Ta(V) to Ta(IV) took place along with the ring-
opening reaction of THF. The reaction of 7 with LiBHEt3
in THF provided a mixture of unidentified products, and
the conversion of 7–8 has been unsuccessful. This implies
that formation of the n-butoxide Ta(V) complex 7 dose
not proceed via the Ta(IV) complex 8.
˚
butoxide) distances of average 1.860 A are shorter than
the Ta–O(aryloxide) distances.
The reaction of 2 with KH in DME was of interest in
that 5c could be regarded as the adduct of [{(bit-tBu–
L)Ta}2(l-H)2] with a neutral KH reagent. Although 5c
did not form, a remarkable reaction occurred nonethe-
less. The ethane-1,2-diolate bridging complex [{(tBu–
L)TaCl2K(DME)2}2(l-OCH2CH2O)] (9), is obtained
The minor component 8 was invariably contaminated
by 7, whose presence precluded NMR and elemental
analyses of 8. The crystals of 8 that were obtained after
manual separation did not diffract well, preventing the

5340
H. Kawaguchi, T. Matsuo / Journal of Organometallic Chemistry 690 (2005) 5333–5345
Fig. 7. Structure of the anion in [{(tBu–L)TaCl2K(DME)2}2(l-
OCH2CH2O)] (9). Methyl and tert-butyl groups have been omitted
˚
for clarity. Selected bond distances (A) and angles (°): Ta–O(1)
Fig. 6. Structure of [(tBu-L)Ta(OBun)2]2 (7). Methyl and tert-butyl
groups, and one set of disordered n-butyl groups have been omitted for
1.870(6), Ta–O(2) 1.904(5), Ta–O(3) 1.984(6), Ta–O(4) 1.912(5), Ta–
Cl(1) 2.484(2), Ta–Cl(2) 2.484(2), K–Cl(1) 3.166(3), K–Cl(2) 3.196(3),
K–O(3) 2.678(5), K–O(4) 2.711(7), K–O(5) 2.770(8), K–O(6) 2.78(1),
K–O(7) 2.675(9), C(1)–C(1*) 1.48(1), O(1)–Ta–O(2) 95.2(2), O(2)–Ta–
O(3) 92.7(2), O(2)–Ta–O(4) 98.3(2) O(3)–Ta–O(4) 91.0(2), Cl(1)–Ta–
Cl(2) 85.00(6), Cl(1)–K–Cl(2) 63.69(6), Ta–O(3)–K 110.4(2), Ta–Cl(1)–
K 84.95(7), Ta–Cl(2)–K 84.31(7).
˚
clarity. Selected bond distances (A) and angles (°): Ta–O(1) 1.968(4),
Ta–O(2) 2.104(5), Ta–O(3) 1.964(4), Ta–O(4) 1.857(4), Ta–O(5)
1.862(4), Ta–O(2*) 2.167(4), Ta–Ta* 3.5905(4), O(1)–Ta–O(3)
169.7(1), O(4)–Ta–O(5) 109.0(2), O(2*)–Ta–O(4) 165.2(2), O(2)–Ta–
O(5) 151.3(2), O(2)–Ta–O(2*) 65.6(1), Ta–O(2)–Ta* 114.4(2).
and 2-methoxy-ethoxide ligands has also precedent
[22,23], whereas the generation of the ethane-1,2-diolate
ligand via fragmentation of DME is less common. It is
likely that the formation of n-butoxide and ethane-1,2-
diolate complexes includes formation of an oxonium
ion by coordination of THF and DME to the Lewis acidic
Ta(V) center in 2, which is opened and cleaved by nucle-
ophilic attack by LiBHEt3 and KH, respectively. This
first step is supported by the observation that the chloride
complex 2 forms the labile solvent adducts with THF and
as light blue crystals in 54% yield. The presence of the
1
ethane-1,2-diolate ligand is confirmed by the H NMR
spectrum, which exhibits a sharp singlet at 3.46 ppm
with the appropriate intensity ratio. The ethane-1,2-dio-
late ligand in 9 is likely formed by C–O bond cleavage of
a DME molecule of the solvent.
The molecular structure of 9 is shown in Fig. 7. The
C(1)–C(1*) bond of the ethane-1,2-diolate linker lies on
a crystallographic inversion center so that the halves of
the molecule are identical. It is obvious that the com-
pound contains a pair of the [(tBu–L)Ta]2+ units, in
which the U-shaped [tBu–L] ligand bind to the metal
center facially. The octahedral coordination sphere of
tantalum is completed by two chlorides and one oxygen
atom of the ethane-1,2-diolate linker. The potassium
cation, solvated by two DME molecules, caps the trian-
gle face defined by two chloride [Cl(1), Cl(2)] and one
central aryloxide [O(2)] of the [tBu–L] ligand. Thus the
formulation assigns a formal 5+ oxidation state to the
tantalum atom. The Ta–O and Ta–Cl bond distances
are similar to the corresponding distances found in the
Ta(V) complexes [11].
The nature of the byproducts of these reactions
shown in Scheme 4 is not yet definitely established and
requires further study. However, degradation of ethereal
solvents such as THF and DME is often found in the
synthesis of early transition metal and f-element com-
plexes [19–24]. Ring-opening reaction of THF is well
known to form n-butoxide compounds [19]. The C–O
bond cleavage of DME to produce oxo, methoxide,
1
DME according to H NMR analysis. Although metal
hydrides and low-valent metal complexes occasionally
undergo C–O bond cleavage of ethereal molecules in mild
conditions [19d,20,22], the Ta(IV) complexes (3 and 4)
and the hydride complexes (5a–c) proved inert to C–O
bond cleavage of THF and DME.
3. Conclusions
Tantalum complexes supported by triaryloxide li-
gands have been synthesized and structurally character-
ized. The [R–L] ligands allow for isolation of Ta(V) and
Ta(IV) chloride complexes, Ta(V) alkoxide complexes,
and Ta(V) hydride complexes. The C–O bond rupture
of THF and DME shows that the Ta(V) center having
the [tBu–L] ligand is highly Lewis acidic. Although this
fragmentation of ethereal solvent proved troublesome,
the hydride complexes 5a–c could be prepared in moder-
ate yields by the reactions of 2 with triethylborohydrides

H. Kawaguchi, T. Matsuo / Journal of Organometallic Chemistry 690 (2005) 5333–5345
5341
in toluene. Once isolated, these hydride complexes are
found to be inert towards THF and DME but thermally
unstable. Complexes 5a–c undergo reversible intramo-
lecular CH activation of the methylene groups of the
[tBu–L] ligand, which suggests that they functions as a
masked low-valent dinuclear complex of tantalum. In
this context, we expect these complexes to demonstrate
a rich and varied chemistry.
L]. Solid TaCl5 (0.78 g, 2.18 mmol) was added to the
resulting mixture, and the solution immediately turned
brown. The mixture was stirred for 4 h at reflux, dur-
ing which time a yellow crystalline powder was
formed. After standing at room temperature over-
night, the yellow precipitate was isolated by filtration.
Recrystallization from hot toluene gave 2 Æ (toluene)2
as yellow blocks in 47% yield (0.86 g). The product
is slightly soluble in THF, DME, toluene, and chlori-
nated solvents.
4. Experimental
Elemental analysis. Found: C, 57.90; H, 5.87%. Calc.
for [C82H102O6Cl4Ta2]: C, 58.37; H, 6.09. 1H NMR
(CDCl3): d 1.21 (s, 18H, Bu), 1.42 (s, 36H, Bu), 2.35
(s, 12H, Me), 2.39 (s, 6H, toluene), 3.36 (d,
J = 13.6 Hz, 4H, CH2), 5.03 (d, J = 13.6 Hz, 4H, CH2),
7.0–7.3 (m, 22H, Ar–H + toluene).
t
t
4.1. General methods
All manipulations of air- and/or moisture-sensitive
compounds were carried out under an atmosphere of ar-
gon or of dinitrogen using conventional Schlenk tech-
niques and an MBraun dry-box (<1 ppm H2O/O2).
4.2.3. [(Me–L)TaCl(THF)]2 (3)
t
The ligand precursors H3(R–L) (R = Me, Bu) was pre-
A slurry of 1 (0.25 g, 0.19 mmol) in toluene (10 ml)
was cooled to ꢀ78 °C, and 2 equiv of LiBHEt3 (1.0 M
solution in THF; 0.38 mL) was added. The mixture
was allowed to warm to room temperature and stirred
for 10 h, during which time formation of a brown solid
was observed. Filtration and drying under vacuum
afforded 0.20 g of 3 Æ (toluene)2 as a brown microcrystal-
line solid (67%). Analytically pure samples were ob-
tained from slow cooling of a warm (60 °C) saturated
toluene/THF solution to room temperature.
pared according to the reported method [2a,6,25]. All
dried solvents and chemicals commercially available
were used as received without further purification. Deu-
terated solvents were dried over appropriate drying re-
agents and then distilled trap-to-trap in vacuo. NMR
spectra were recorded on JEOL Lambda-500 and JEOL
JNM-GX500 spectrometers, and referenced internally
to residual solvent resonances. Elemental analyses were
measured using LECO–CHNS and Yanaco MT-6 and
MSU-32 microanalyzers.
Elemental analysis. Found: C, 58.41; H, 5.72%. Calc.
for [C78H94O8Cl2Ta2]: C, 58.83; H, 5.95. 1H NMR
t
4.2. Syntheses
(THF-d8): d 0.64 (br, 8H, THF), 1.35 (s, 18H, Bu),
2.05 (s, 12H. Me), 2.15 (s, 6H, toluene), 2.63 (s, 12H,
Me), 3.29 (br, 8H, THF), 3.66 (d, J = 13.3 Hz, 4H,
CH2), 5.29 (d, J = 13.3 Hz, 4H, CH2), 7.0–7.3 (m,
22H, Ar–H + toluene).
4.2.1. [(Me–L)TaCl2]2 (1)
Toluene (15 ml) was added to a mixture of H3[Me–
L] (0.28 g, 0.67 mmol) and TaCl5 (0.24 g, 0.67 mmol).
The resulting orange solution was refluxed for 3 h to
drive off HCl gas and then allowed to cool, affording
a orange crystalline solid. The solvent was decanted
away from this solid, which was subsequently washed
with hexane two times and dried under vacuum
affording an orange solid of 1 (0.43 g, 96%). Complex
1 is slightly soluble in THF, DME, toluene, and chlo-
rinated solvents.
4.2.4. [Li(DME)3][{(tBu–L)TaCl}2(l-Cl)] (4a)
A slurry of 2 (0.54 g, 0.32 mmol) in toluene (10 ml)
was cooled to ꢀ78 °C, and 2 equiv of LiBHEt3 (1.0 M
solution in THF; 0.64 mL) was added. The mixture
was allowed to warm to room temperature and stirred
for 20 h, during which time a color changed from yellow
to brown. Removal of the volatiles and extraction with
DME afforded a brown solution, from which 0.16 g of
3 precipitated as blue needles (28% yield).
Elemental analysis. Found: C, 50.22; H, 4.63%. Calc.
for [C56H62O6Cl4Ta2]: C, 50.39; H, 4.68. 1H NMR
t
(CDCl3): d 1.19 (s, 18H, Bu), 2.37 (s, 12H, Me), 2.41
Elemental analysis. Found: C, 54.41; H, 6.52%.
Calc. for [C80H116O12Cl3LiTa2]: C, 55.06; H, 6.70.
1H NMR (THF-d8): d 1.15 (s, 18H, tBu), 1.17 (s,
(s, 12H, Me), 3.45 (d, J = 13.4 Hz, 4H, CH2), 4.97 (d,
J = 13.4 Hz, 4H, CH2), 6.89 (s, 4H, Ar), 7.02 (s, 4H,
Ar), 7.18 (s, 4H, Ar).
t
t
18H, Bu), 1.18 (s, 18H, Bu), 2.11 (s, 6H, Me), 2.17
(s, 6H, Me), 2.92 (d, J = 13.5 Hz, 2H, CH2), 3.23 (s,
18H, DME), 3.31 (d, J = 13.8 Hz, 2H, CH2), 3.38 (s,
12H, DME), 5.13 (d, J = 13.5 Hz, 2H, CH2), 5.64
(d, J = 13.8 Hz, 2H, CH2), 6.75 (d, J = 1.9 Hz, 2H,
Ar), 6.80 (s, 2H, Ar), 6.81 (s, 2H, Ar), 6.93 (d,
J = 1.9 Hz, 2H, Ar), 7.03 (d, J = 2.1 Hz, 2H, Ar),
7.06 (d, J = 2.1 Hz, 2H, Ar).
4.2.2. [(tBu–L)TaCl2]2 (2)
A solution of butyllithium (1.60 M in hexane;
4.10 ml, 6.56 mmol) was added dropwise to H3[tBu–
L] (1.10 g, 2.19 mmol) in toluene (20 ml) at 0 °C.
The solution was warmed to room temperature and
stirred for 1 h giving a white suspension of Li3[tBu–

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H. Kawaguchi, T. Matsuo / Journal of Organometallic Chemistry 690 (2005) 5333–5345
4.2.5. [K(DME)3][{(tBu–L)TaCl}2(l-Cl)] (4b)
A slurry of 2 (0.44 g, 0.26 mmol) in toluene (40 ml)
was added to a vessel containing an excess of potassium
mirror. The mixture was stirred vigorously for 24 h.
After centrifugation, the brown solution was evaporated
to dryness. The residue was dissolved in DME and
cooled to ꢀ30 °C to give 4b as blue crystals (0.32 g,
70%).
2H, Ar), 6.85 (s, 2H, Ar), 6.87 (s, 2H, Ar), 7.07 (s, 2H,
Ar), 10.9 (s, 3H, Ta–H).
4.2.8. [{(bit-tBu–L)Ta}2(l-H)3K(DME)2] (5c)
A
100 mL flask was charged with 2 (0.41 g,
0.24 mmol), KHBEt3 (1 M in THF; 1.2 ml, 1.2 mmol),
and toluene (60 ml) at ꢀ78 °C. Upon stirring the mix-
ture at room temperature for 12 h, a dark yellow solu-
tion was obtained. After centrifugation to remove an
insoluble solid, the supernatant was evaporated to dry-
ness. The residue was extracted with DME. Addition
of hexane resulted in the precipitation of 5c as yellow
crystals (0.21 g, 54%).
Elemental analysis. Found: C, 53.77; H, 6.39%. Calc.
1
for [C80H116O12Cl3KTa2]: C, 54.07; H, 6.58. H NMR
t
t
(THF-d8): d 1.15 (s, 18H, Bu), 1.17 (s, 18H, Bu), 1.18
(s, 18H, Bu), 2.12 (s, 6H, Me), 2.18 (s, 6H, Me), 2.93
t
(d, J = 13.3 Hz, 2H, CH2), 3.23 (s, 18H, DME), 3.31
(d, J = 13.9 Hz, 2H, CH2), 3.40 (s, 12H, DME), 5.13
(d, J = 13.3 Hz, 2H, CH2), 5.66 (d, J = 13.9 Hz, 2H,
CH2), 6.76 (d, J = 1.9 Hz, 2H, Ar), 6.80 (s, 2H, Ar),
6.81 (s, 2H, Ar), 6.93 (d, J = 1.9 Hz, 2H, Ar), 7.04 (d,
J = 2.2 Hz, 2H, Ar), 7.07 (d, J = 2.2 Hz, 2H, Ar).
Elemental analysis. Found: C, 57.46; H, 6.62%. Calc.
for [C76H107O10KTa2]: C, 57.71; H, 6.82. 1H NMR
t
t
(THF-d8): d 1.06 (s, 18H; Bu), 1.11 (s, 18H, Bu), 1.20
(s, 18H, Bu), 2.28 (s, 6H; Me), 2.32 (s, 6H; Me), 3.40
t
(s, 12H, DME), 3.50 (d, J = 13.5 Hz, 2H, CH2), 3.56
(s, 8H, DME), 4.09 (s, 2H; CH), 4.73 (d, J = 13.5 Hz,
2H, CH2), 6.46 (s, 2H, Ar), 6.74 (s, 2H, Ar), 6.76 (s,
2H, Ar), 6.86 (s, 2H, Ar), 6.87 (s, 2H, Ar), 7.07 (s, 2H,
Ar), 10.8 (s, 3H, Ta–H).
4.2.6. [{(bit-tBu–L)Ta}2(l-H)3Li(THF)2] (5a)
A THF solution of LiHBEt3 (1 M; 2.7 ml, 2.7 mmol)
was added dropwise to a suspension of 2 (0.89 g,
0.52 mmol) in toluene (40 ml) that had been precooled
to ꢀ78 °C. The mixture was allowed to warm up to
room temperature and stirred for 8 h. The resulting dark
yellow solution was concentrated to ca. 5 mL and centri-
fuged to remove an insoluble material. The supernatant
was layered with hexane to afford 5a as yellow crystals
(0.52 g, 62%).
4.2.9. [{(bit-tBu–L)Ta}2(l-H)3Li(DME)2] (6)
After 5a (0.21 g, 0.13 mmol) was dissolved in DME
(20 ml), the solution was concentrated to 3 ml and lay-
ered with hexane to afford yellow crystals of
6 Æ (DME)0.5 (0.17 g, 82%).
Elemental analysis. Found: C, 58.19; H, 7.15%. Calc.
1
for [C78H112O11LiTa2]: C, 58.75; H, 7.08. The H NMR
spectrum of 6 in THF-d8 exhibited a pattern identical to
that of 5a along with resonances ascribed to DME.
Elemental analysis. Found: C, 61.89; H, 7.11%. Calc.
for [C83H111O8LiTa2]: C, 62.09; H, 6.97. 1H NMR
t
t
(THF-d8): d 0.84 (s, 18H, Bu), 0.98 (s, 18H, Bu), 1.31
(s, 18H, Bu), 1.77 (m, THF), 2.18 (s, 6H, Me), 2.19 (s,
t
6H, Me), 3.34 (d, J = 13.0 Hz, 2H, CH2), 3.62 (m,
THF), 3.76 (s, 2H, CH), 4.64 (d, J = 13.0 Hz, 2H,
CH2), 6.30 (s, 2H, Ar), 6.51 (s, 2H, Ar), 6.65 (s, 2H,
Ar), 6.77 (s, 2H, Ar), 6.80 (s, 2H, Ar), 6.91 (s, 2H,
Ar), 10.7 (s, 3H, Ta–H).
4.2.10. Reaction of 2 with LiBHEt3 in THF
To a slurry of 2 (1.81 g, 1.07 mmol) in THF (40 ml) at
ꢀ78 °C was added 6 equiv of LiBHEt3 (1.0 M solution
in THF; 6.4 ml). The mixture was allowed to warm up
to room temperature, and the yellow color discharged
to give a colorless homogeneous solution. After stirring
for 4 h, the volatile components were removed in vacuo.
The gray residue was extracted with toluene and centri-
fuged to remove an insoluble material. The toluene
supernatant was concentrated and cooled to ꢀ30 °C,
yielding colorless blocks of [(tBu–L)Ta(OBun)2]2 (7)
along with a trace amount of pale blue needles of
[Li(THF)4][{(tBu–L)TaCl}2(l-OBun)] (8).
4.2.7. [{(bit-tBu–L)Ta}2(l-H)3Na(DME)2] (5b)
A THF solution of NaHBEt3 (1 M; 1.6 ml, 1.6 mmol)
was added dropwise to a suspension of 2 (0.55 g,
0.33 mmol) in toluene (40 ml) that had been precooled
to ꢀ78 °C. The mixture was allowed to warm up to
room temperature and stirred for 8 h. The resulting dark
yellow solution was centrifuged to remove an insoluble
material. The supernatant was evaporated to dryness.
The brown residue was recrystallized from DME/hexane
to yield 5b as yellow crystals (0.24 g, 47%).
4.2.11. [(tBu–L)Ta(OBun)2]2 (7)
A mixture of 1-butanol (0.27 mL, 2.95 mmol) and
NaH (70 mg, 2.9 mmol) in THF (30 ml) was stirred for
1 h and then the solvent was removed in vacuo. A sus-
pension of 2 (1.25 g, 0.74 mmol) in toluene (60 ml) was
added to the residue. The mixture was stirred at 80 °C
for 3 h, during which time the yellow color discharged
to give a colorless solution and NaCl precipitated. The
solution was centrifuged to remove an insoluble
Elemental analysis. Found: C, 57.96; H, 6.67%. Calc.
for [C76H107O10NaTa2]: C, 58.31; H, 6.89. 1H NMR
t
t
(THF-d8): d 1.06 (s, 18H, Bu), 1.10 (s, 18H, Bu), 1.22
(s, 18H, Bu), 2.29 (s, 6H, Me), 2.32 (s, 6H, Me), 3.38
t
(s, 12H, DME), 3.52 (d, J = 13.6 Hz, 2H; CH2), 3.55
(s, 8H, DME), 4.11 (s, 2H, CH), 4.71 (d, J = 13.6 Hz,
2H, CH2), 6.44 (s, 2H, Ar), 6.73 (s, 2H, Ar), 6.75 (s,

Table 1
Experimental data for the X-ray diffraction of 2, 3, 4, 5a, 6, 7, and 9
2
3
4
5a
6
7
9
Formula
M
Space group
C
82H102O6Cl4Ta2
C78H94O8Cl2Ta2
1592.40
C80H116O12Cl3KTa2
1777.14
C83H111O8LiTa2
1605.62
P1(bar) (no. 2)
12.481(3)
12.884(3)
24.519(6)
81.175(5)
82.109(6)
86.360(5)
3855(1)
2
C80H117O12LiTa2
1639.63
C84H122O10Ta2
1653.78
C90H140O18Cl4K2Ta2
2091.99
P1(bar) (no. 2)
12.103(5)
12.866(5)
16.699(6)
79.544(13)
78.68(2)
82.13(2)
2493.6(17)
1
1687.41
P1(bar) (no. 2)
13.529(5)
14.237(4)
21.365(8)
73.911(9)
90.036(10)
72.469(9)
3754.4(22)
2
P21/c (no. 14)
11.336(3)
18.802(5)
C2/c (no. 15)
29.924(10)
16.009(5)
P21/n (no. 14)
17.687(3)
18.452(3)
P21/n (no. 14)
13.309(5)
18.628(7)
˚
a (A)
˚
b (A)
˚
c (A)
16.537(5)
17.555(6)
24.435(4)
17.471(7)
a (°)
b (°)
c (°)
97.061(3)
106.025(4)
93.231(3)
113.600(4)
3
˚
V (A )
3497.8(17)
2
1.512
8083.1(46)
4
1.460
7961.7(25)
4
1.368
3969.0(27)
2
1.384
Z
Dc (g cmꢀ3
)
1.493
31.02
1.383
28.84
1.393
24.40
l (Mo Ka) (cmꢀ1
)
32.53
29.09
27.99
28.06
Collected reflections
Unique reflections
Parameters
29,255
16,408
27,109
7985
471
65,070
9578
501
30,172
16,787
60,921
17,884
982
31,926
8854
460
38,508
10,637
968
0.045
917
0.032
593
0.071
R1[I > 2r(I)]
wR2 (all)
0.025
0.058
0.077
0.209
0.031
0.072
0.043
0.116
0.088
1.066
0.075
1.058
0.201
1.002
Goodness-of-fit
Largest residual
peak and hole (e A
1.053
1.30 and ꢀ1.10
1.000
7.05 and ꢀ3.63
1.055
1.93 and ꢀ1.31
1.025
3.02 and ꢀ3.74
3.98 and ꢀ3.63
1.78 and ꢀ0.97
4.54 and ꢀ2.45
ꢀ3
˚
)

5344
H. Kawaguchi, T. Matsuo / Journal of Organometallic Chemistry 690 (2005) 5333–5345
material. Concentration of the supernatant and cooling
to ꢀ30 °C provided 7 as colorless crystals (0.94 g, 77%
yield).
and refined isotropically. Anisotropic refinement was
applied to all non-hydrogen atoms except for disordered
atoms, and hydrogen atoms were put at calculated posi-
˚
Data for 7: Elemental analysis. Found: C, 59.52; H,
7.20%. Calc. for [C84H122O10Ta2]: C, 61.01; H, 7.44.
1H NMR (CDCl3): d ꢀ0.35 (t, J = 7.8 Hz, 6H,
O(CH2)3CH3), ꢀ0.11 (m, 4H, CH2), 0.04 (t, J =
7.3 Hz, 6H, O(CH2)CH3), 0.61 (m, 4H, CH2), 0.75 (m,
4H, CH2), 1.14 (s + m, 22H, overlapping CH2 and
tions with C–H distances of 0.97 A.
5. Supplementary material
Crystallographic data for the structural analyses have
been deposited with the Cambridge Crystallographic
Data Centre, CCDC nos. 261910–261916 for com-
pounds 2, 3, 4b, 5b, 6, 7, and 9, and CCDC nos.
225528 and 225530 for 5a and 5c, respectively. Copies
of this information may be obtained free of charged from
The Director, CCDC, 12 Union Road, Cambridge CN2
1EZ, UK (Fax: +44 1223 336033; e-mail: deposit@ccdc.
cam.ac.uk or www: http://www.ccdc.cam.ac.uk).
t
tBu), 1.25 (s, 36H, Bu), 2.13 (s, 12H, Me), 2.43 (t,
J = 6.8 Hz, 4H, OCH2), 3.15 (d, J = 12.9 Hz, 4H,
CH2), 3.75 (m, 4H, OCH2), 5.46 (d, J = 12.9 Hz, 4H,
CH2), 6.76 (s, 4H, Ar), 6.91 (s, 4H, Ar), 7.22 (s, 4H, Ar).
4.2.12. [{(tBu–L)TaCl2K(DME)2}2(OCH2CH2O)]
(9)
A slurry of 2 (0.39 g, 0.23 mmol) in DME (30 ml) was
added to a vessel containing potassium hydride (58 mg,
1.5 mmol) at ꢀ78 °C. The mixture was allowed to warm
up to room temperature and was stirred vigorously for 2
days. After centrifugation, the greenish brown solution
was concentrated to 5 ml and layered with hexane to
give 9 Æ (DME) as light blue crystals (0.26 g, 54%).
Elemental analysis. Found: C, 51.48; H, 6.74%. Calc.
Acknowledgments
Institute for Molecular Science is thanked for finan-
cial support. This work was also supported by a
Grant-in-Aid for Scientific Research [Nos. 14703008
and 14078101 (Priority Areas ‘‘Reaction Control of
Dynamic Complexes’’)] from Ministry of Education,
Culture, Sports, Science, and Technology, Japan.
1
for [C90H140O18Cl4K2Ta2]: C, 51.67; H, 6.75. H NMR
t
t
(THF-d8): d 1.16 (s, 18H, Bu), 1.35 (s, 36H, Bu), 2.33
(s, 12H, Me), 3.22 (s, 30H, DME), 3.32 (d,
J = 13.5 Hz, 4H, CH2), 3.42 (s, 20H, DME), 3.46 (s,
4H, OCH2CH2O), 5.05 (d, J = 13.5 Hz, 4H, CH2), 6.78
(s, 4H, Ar), 7.03 (s, 4H, Ar), 7.18 (s, 4H, Ar).
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