Synthesis of novel tantalacalixarene complexes: First example of intramolecular CH activation of monodepleted aromatic ring

Article abstract of DOI:10.1021/om200140u

The alkylation of (dichlorotantala)-p-tert-butylcalix[4]arenes 1 and 3 by two equivalents of NpMgCl (Np = neopentyl) affords the corresponding unreported (bisneopentyltantala)-p-tert-butylcalix[4]arenes 4 and 5. These complexes can undergo dealkylation by

Full text of DOI:10.1021/om200140u

ARTICLE
pubs.acs.org/Organometallics
Synthesis of Novel Tantalacalixarene Complexes: First Example of
Intramolecular CH Activation of Monodepleted Aromatic Ring
Jeff Espinas,† Jꢀerꢀemie Pelletier,† Erwan Jeanneau,‡ Ulrich Darbost,‡ Kai C. Szeto,† Christine Lucas,†
Jean Thivolle-Cazat,† Christian Duchamp,§ Natali Henriques,§ Denis Bouchu,§ Jean-Marie Basset,†
Henry Chermette,*,|| Isabelle Bonnamour,*,‡ and Mostafa Taoufik*,†
†Laboratoire de Chimie Organomꢀetallique de Surface, CNRS, UMR 5265 C2P2, Universitꢀe Lyon 1, CPE Lyon, France
‡Chimie Supramolꢀeculaire Appliquꢀee (CSAp)/ICBMS, CNRS, UMR 5246, Universitꢀe Lyon 1, Villeurbanne, France
§Centre Commun de Spectromꢀetrie de Masse/ICBMS CNRS, UMR 5246, Universitꢀe Lyon 1, Villeurbanne, France
ꢀ
Chimie Physique Thꢀeorique, UMR 5180, Laboratoire de Sciences Analytiques, Equipe CHEMOD, Universitꢀe Lyon 1,
Villeurbanne, France
S Supporting Information
b
ABSTRACT:
The alkylation of (dichlorotantala)-p-tert-butylcalix[4]arenes 1 and 3 by two equivalents of NpMgCl (Np = neopentyl) affords the
corresponding unreported (bisneopentyltantala)-p-tert-butylcalix[4]arenes 4 and 5. These complexes can undergo dealkylation by modest
increase of the temperature to yield 6 and 7. The characterization and structural conformation of the complexes 6 and 7 were determined by
1H, 13C, NOESY, and COSY spectroscopic studies and single-crystal X-ray diffraction. 7 exhibits unprecedented use of an intramolecular
CꢀH activation of an aromatic ring and simultaneously releases of one equivalent of neopentane. DFT calculation supports a mechanism in
which the metal passes through a transition state involving a coplanar arrangement with its three ligands (CAr, HAr, CNp).
’ INTRODUCTION
ortho positions.5 The general interest in calix[4]arenes as ancillary
ligands can be explained by their unique combination of assets: great
stability, easily functionalized, relative rigidity, conformational con-
trol, and a cavity capable of hosting a wide range of molecules. All
these characteristics have led to abundant contributions in fields
as different as molecular recognition,6 metal sequestration,7 and
catalysis.8 In parallel with their increasing utilization as prime
components in catalytic systems, the calix[4]arene-based complexes
can also be considered as ideal substrates for the observation of
elementary organometallic transformations. This line of research can
be particularly interesting when models are required to enhance the
understanding of existent catalytic systems, notably that of a cataly-
tically active metal coordinated by several oxygens. Examples of in-
corporating early transition metals such as Zr,9 W,10 and Ta11 can be
found in the catalysts generated by surface organometallic chemistry.
As calix[4]arene substrates have been extensively characterized,
the analytical tools (e.g., NMR spectroscopy, X-ray diffraction) are
The effect produced by the derivatization of the ligand back-
bone of calix[4]arene-based complexes on the reactivity of the
coordinated metal is an issue gaining significance for catalysis and in
particular for modeling the structure and reactivity of complexes
grafted on the surface of oxides.1 One approach that has received less
attention is the study of supporting the metal on calixarene ligands
depleted on at least one of the hydroxyl groups.2,3 We have recently
demonstrated the potential of complexes in which such ligands are
coordinated to titanium and employed as precatalysts for the poly-
merization of ethylene.4 Considering oxophilic metals, one expected
benefit of such ligands consists in the enhancement of the metal
reactivity toward the activation of CH bonds. An unexpected, yet
fascinating reaction of these complexes can be observed when the
metal activates an aromatic proton from one depleted site. In the long
term, the activation of this position can allow the elaboration of novel
synthetic pathways directed toward an otherwise unreachable func-
tionalized calixarene backbone. Calix[4]arenes, by far the most pro-
minent members of the calixarene family, consist of four phenolic
subunits tethered to each other through methylene bridges at their
Received: February 14, 2011
Published: June 14, 2011
r
2011 American Chemical Society
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Scheme 1a
Chart 2. [[4þ]-1,2-(OH)2(O)2Mo] (I) and [[4þ]-1,2-(OH)2-
(O)2W] (II)
metals.12 In all cases, the metal interacts only with a CH bond from
the methylene bridge; nonetheless, the nature of this interaction is
strongly affected by the metal coordinated. Early transition metals,
such as group 6, appear to be prone to initiate agostic bonding
(i.e., three-centerꢀtwo-electron interaction), while those formed by
late transition counterparts (e.g., Pt,13,14 Rh15) such as a d8 metal
center are weaker and more reminiscent of hydrogen bonding.
Even early transition metals belonging to the same group can
exhibit distinct nuances toward CH bond activation. In the complex
[[4þ]-1,2-(OH)2(O)2Mo] (I), an agostic derivative that exhibits a
MoꢀHꢀC interaction with a d2 metal, has been identified. By
contrast, the tungsten analogue [[4þ]-1,2-(OH)2(O)2W] (II) is a
d0 trihydride formed by oxidative addition on the CH bond (see
Chart 2). In this context, high-electrophilic d0 tantalum can be
deemed as a metal of choice to observe intramolecular CH bond
activation of the calix[4]arene. We report herein the preparation of
new tantalum alkyl complexes with calix[4]arenes ligands of the type
of O2 and O3 (vide supra). Particular interest was paid to following
the CH bond activation by the insertion of CO.
a Conditions: (i) 1 equiv Me3SiCl, 3.5 equiv NEt3, toluene, RT, 16 h; (ii)
1.1 equiv nBuLi, 1.5equivClP(O)(OEt)2, toluene, RT, 4h;(iii) 50equivK,
NH3, Et2O, ꢀ78 °C, 1 h; (iv) 0.5 equiv K2CO3, 5 equiv MeI, CH3CN,
80 °C, 24 h; (v) 0.5 equiv K2CO3, 1 equiv BnBr, CH3CN, 80 °C, 2 h.
Chart 1. Complexes with Derived Anions, O4, O3, and O2 of
Calix[4]arenes (X = H, OR)
’ RESULTS AND DISCUSSION
readily available to identify the behavior of the metal toward any given
position of its ligand. Calix[4]arene-based complexes usually adopt a
cone conformation in which the oxygen set is quasiplanar. The parent
calix[4]arene can act as a tetraanionic ligand, whereas the tri- or
dianionic counterparts (O4, O3, and O2, respectively) are readily
available by either alkylation or depletion of the hydroxyl sites
(Scheme 1). By coordination with suitable calix[4]arenes, the metal,
under the electronic and steric constraints of the ligand, can see its
reactivity directed toward reactions otherwise difficult to pursue.
Intramolecular activation of a CH bond, on the calix[4]arene frame-
work, by a coordinated transition metal is a particularly interesting
area. The activated complex can then serve as a starting point of new
synthetic routes toward calix[4]arenes with novel functionalizations
(e.g., introduction of a carbon-bearing function).
To that end, selecting both the metal employed and calix[4]arene
ligand is pivotal. The coordination mode of the metal as enforced by
its ligand set (i.e., O4, O3, and O2) can affect the reactivity by modify-
ing their electronic properties. In addition, the interplay between the
metal and its calix[4]arene ligand can allow enough proximity for the
targeted position to be activated. Complexes of the O3 type appear
particularly relevant in the approach for the trianionic anchoring site,
which can maintain the metal close enough to react with a modified
site (i.e., the X group in Chart 1, X = H or OCH3).
Synthesis of the p-tert-Butylcalix[4]arene Ligands L1ꢀL4.
A series of p-tert-butylcalix[4]arenes derivatized on the lower rim
(L1ꢀL4) have been prepared using different synthetic strategies
with a view to offer different coordination modes to TaCl5. The
1,3-methoxy-functionalized p-tert-butylcalix[4]arene compound
[[4þ]-1,3-(OMe)2(OH)2] (L1)16 was synthesized by treatment
of the p-tert-butylcalix[4]arenewithK2CO3 as base in dry acetonitrile
before addition of iodomethane. The mono-OH-depleted [[4þ]-
(H)(OH)3] (L2) was first described by Biali et al. as a byproduct
from the reductive cleavage of the diphosphorylated derivative
[[4þ]-1,3-(OP(O)(OEt)2)2(OH)2] with K/NH3 in refluxing am-
monia at ꢀ33 °C, in low yield.17 Thereafter, L2 was also obtained in
poor yield (<10%) via either Bohmer’s route, consisting in a [1þ4]
convergent synthesis,18 or the monospirodienone route.19 A more
efficient synthetic pathway, though surprisingly undocumented, can
be proposed to provide L2 by using the monophosphorylated
calix[4]arene derivative [[4þ]-(OP(O)(OEt)2)(OH)3] for the
cleavage step by K/NH3. The direct synthesis from p-tert-butylcalix-
[4]arene with phosphorylating agent ClP(O)(OEt)2 leads to several
multisubstituted derivatives mixed with unconverted starting material.
In order to obtain solely L2, we report a novel and selective three-
step route (Scheme 1). The first step passes by the selective protec-
tion of three of the four hydroxyl groups of the p-tert-butylcalix-
[4]arene with MeSiCl3 in the presence of NEt3, following Lattman’s
method.20 The unprotected OH group is then easily deprotonated
with 1.1 equivalent of nBuLi before reaction with ClP(O)(OEt)2, to
give the protected monophosphorylated derivative [[4þ]-(OP(O)-
(OEt)2)(O3SiMe)] in 65% yield. The latter was then treated with an
The intramolecular activation of CH bonds can be performed by
various metals whose electronic properties are determinant. For
example, Parkin compared the structures of complexes, in both the
liquid and the solid state, formed by the same p-tert-butylcalix-
[4]arene ligand (noted [4þ]) coordinated to various transition
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Table 1. Crystallographic Data for Compounds L4, 3, and 6
generate [[4þ]-(OMe)(O)3TaCl2], 1. This reactivity is char-
acteristic of Ta complexes with ether-bearing calix[4]arene: the
metal center can easily cleave the OꢀCH3 bond to accommodate
its coordination sphere.22 However, employing L2, L3, and L4 in
similar conditions revealed a more complicated reactivity. For
instance, L2 does not lead to the expected 3, but to a mixture of
products difficult to either identify or isolate. Mediating the
complexation reaction with alternative solvents such as toluene
or ether did not permit any progress toward the targeted
compounds. In addition, another synthetic route in which L2
is deprotonated with 3 equivalents of nBuLi prior to the reaction
with TaCl5 also proved unsuccessful. The reaction of L3 with
TaCl5 at reflux in toluene for 2 h gives [[4þ]-(OMe)(H)-
1,3-(O)2TaCl3] (2), which shows thermal (110 °C overnight)
and UV-irradiation stability, without further methoxy group
cleavage. The presence of the methoxy group appears necessary
to facilitate the complexation of the tantalum on the calix-
[4]arene. Arguably the precoordination of the oxygen to the
metal by the methoxy group can position it favorably to form
bonds in an O2 environment. L4 undergoes complexation and
debenzylation in the reaction with TaCl5 at 25 °C in toluene to
afford 3 in 82% yield. The 1H NMR and the 13C NMR spectra
lack the signals (i.e., 5.22 and 81.0 ppm, respectively) corre-
sponding to the methylene of the benzyl group pendant on the
calixarene. The preferential cleavage of the benzyloxy group
rather than the methoxy group by TaCl5 may be explained by the
bond strength (the PhCH2ꢀO bond being weaker than the
MeꢀO bond) or possible coordination of the aromatic ring to
the tantalum atom, allowing greater proximity of the metal and
the ether group.23
A crystal of 3 suitable for X-ray diffraction studies was obtained
by slow crystallization of a saturated solution in pentane. Its solid-
state structure can be compared with that of complex 1 described
by Floriani.22 Crystallographic data and details associated with
data collection are given in Table 1, while selected bonds angles
and lengths for complexes 1 and 3 are listed in Table 2. The solid-
state structure of 3 consists of a dimeric compound incorporating
two tantalum atoms, bridged by two chlorines and each chelated
by one calixarene ligand (Figure 1). This dimer appears to be
centrosymmetric with a centroid approximately localized on that
of the square formed by the tantalum Ta(1) and Ta(2) and the
bridging chlorine atoms Cl(2) and Cl(3). Ta(1) and Ta(2) are
quasi equivalent and follow a distorted octahedral geometry, in
which the equatorial ligands can be identified as the bridging
chlorines Cl(2) and Cl(3) and the oxygens of the phenols facing
each other, O(1) and O(2), and O(4) and O(5), respectively.
The remaining oxygen, O(3) and O(6), and the terminal
chlorines, Cl(1) and Cl(4), occupy the axial positions of Ta(1)
and Ta(2), respectively (Table 2). A quasi plane passing through
Ta(1) and Ta(2) and containing all the equatorial ligands [viz.,
Cl(2), Cl(3), O(2), O(3), O(4), and O(5)] can also be drawn.
The metals lie out of this plane by 0.046 Å for Ta(1) and 0.040 Å
for Ta(2). In contrast, 1 is a monomeric species in which the
tantalum metal also adopts an octahedral coordination with
two chlorines and the oxygen atoms from the three phenol
units and that of the methoxy group. Presumably, additional
stabilization can be furnished to the metal by the methoxy
group completing its coordination sphere, hence preventing
the formation of a dimeric form of 1. The position of the
tantalum center in 1 is significantly out of the equatorial plane
with values of 0.146 Å. This suggests that the octahedral
geometry is more distorted in 1 than in 3.
3
6
L4
Color
Yellow
1.5418
C88H106
Colorless
1.5418
Colorless
1.5418
Wavelength (Å)
formula
-
C49H63O4Ta,
2.5(C7H8)
1126.83
0.33 ꢁ 0.25
ꢁ 0.18
monoclinic
P21/c
C51H62O3
Cl4O6Ta2
1763.51
fw
723.01
0.35 ꢁ 0.08
ꢁ 0.03
monoclinic
Cc
cryst size (mm3)
0.19 ꢁ 0.04
ꢁ 0.03
triclinic
P1
cryst syst
space group
ꢀ
a (Å)
16.3590(10)
17.9810(10)
19.7810(10)
116.445(6)
98.994(5)
97.308(5)
5016.6(6)
2
14.653(5)
19.076(5)
21.946(5)
90
13.245(2)
22.608(3)
15.123(3)
90
ꢀ
b (Å)
ꢀ
c (Å)
R (deg)
β (deg)
γ (deg)
V (Å3)
Z
105.974(5)
90
102.71(2)
90
5897(3)
4
4417.5(13)
4
T (K)
150
150
293
F (g cmꢀ3
)
1.167
1.269
1.087
F(000)
1792
2346
1568
μ(Mo KR)
5.253
1.909
0.501
(mmꢀ1
reflns collected
indep reflns
Rint
)
14 984
13 990
0.066
47 482
13 666
0.046
6825
4222
0.054
80/542
restraints/
params
172/982
24/676
final R indices
[I > 2σ(I)]a
all data
0.0763
0.0332
0.0807
0.0912
1.0755
0.0890
1.1336
0.0987
0.9765
goodness-of-fit
on F2
a R = ||Fo| ꢀ |Fc||/|Fo|; wR2 = [w(Fo ꢀ Fc2)2/w(Fo ) ]
.
2
2 2 1/2
excess of K/NH3 at ꢀ78 °C; deprotection of the siloxane function
occurs directly in the reductive media to finally afford pure L2 in
44% yield.
The ligand [[4þ]-1,3-(OH)2(OMe)(H)] (L3) was synthe-
sized according to the method previously described by Fukazawa
et al.21 Its benzyloxy analogue [[4þ]-1,3-(OH)2(OBn)(H)]
(L4) was obtained by treatment of L2 with 0.5 equivalent of
K2CO3 and 1 equivalent of BnBr. Single crystals suitable for
X-ray studies have been obtained and revealed the undescribed
Cs symmetric L4 (Table 1 and see Supporting Information). Of
note is the proximity of the pendant benzyl group to the phenolic
hydroxyls (ex infra). The four ligands synthesized were selected
for their potential to provide two different types of ligand sets as
defined in Scheme 1: O2 for L1, L3, and L4 and O3 for L2.
Synthesis of the p-tert-Butylcalix[4]arene-Based Com-
plexes 1ꢀ3. All ligands (L1-L4) were reacted with TaCl5 to
obtain a series of complexes (1ꢀ3) in good yield, with 2 and 3
being unreported to our knowledge (Scheme 2). 1ꢀ3 were
characterized by NMR studies and elemental analyses; a crystal
of 3 was submitted to X-ray diffraction studies. NMR data
support that the tantalum center had performed a monode-
methylation of L1, which possesses two methoxy groups, to
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Scheme 2a
a Conditions: (i) 1 equiv TaCl5, toluene, 110 °C, 16 h; (ii and iii) 1 equiv TaCl5, toluene, 110 °C, 2 h.
Table 2. Comparison of Selected Bond Lengths [Å] for 1 and 3
1
3
Ta(1)ꢀCl(1)
Ta(1)ꢀCl(2)
Ta(1)ꢀCl(3)
Ta(1)ꢀO(1)
Ta(1)ꢀO(2)
Ta(1)ꢀO(3)
Ta(1)ꢀO(4) (OMe)
Ta(2)ꢀCl(2)
Ta(2)ꢀCl(3)
Ta(2)ꢀCl(4)
Ta(2)ꢀO(4)
Ta(2)ꢀO(5)
Ta(2)ꢀO(6)
2.354(6)
2.384(6)
2.363(4)
2.567(4)
2.538(4)
1.818(13)
1.829(14)
1.899(13)
1.80(2)
1.826(9)
1.866(11)
2.27(2)
2.579(4)
2.570(4)
2.359(5)
1.814(13)
1.835(14)
1.891(14)
Several attempts to characterize 3 by NMR spectroscopic
techniques were unsuccessful due to its low solubility in most
solvents employed; only resorting to acetonitrile permitted the
solubilization of 3. This increase in solubility can be interpreted
as the presence of a more soluble monomeric species, as for other
complexes described by Redshaw et al.24 At room temperature,
the 1H NMR spectrum of 3 is typical of calixarene compounds
with several series of peaks that can be easily assigned as follow:
three large signals from the tBu groups (1.0ꢀ2.0 ppm), eight
main signals from the methylene protons (3.4ꢀ5.0 ppm), and
large multiplets from the protons at the meta position of the
aromatic rings (7.0ꢀ7.5 ppm).5 It is noteworthy that three
signals are centered at 8.8 ppm, with only one remaining when
increasing the temperature to 343 K (Figure 2).
Figure 1. Molecular structure of 3. Hydrogen and solvent molecules
have been omitted for clarity.
proximity with this CH bond or the presence of the acetonitrile
ligand in the coordination sphere.24
The corresponding proton is assumed to be located on the
depleted site of the calixarene ligands (namely, the ipso position).
HSQC experiments at that temperature reveal a correlation
between the signals at 8.8 ppm and that of an aromatic signal
at 132 ppm (Figure 3), thus confirming without doubt the ipso
proton assignment. However, the value of this chemical shift is
surprisingly high for such a proton, probably due to the metal
At ambient temperature, eight stronger signals, each identified
as one of the protons borne by the methylene bridges (i.e., 4.85,
4.63, 4.38, 4.21, 3.60, 3.55, 3.45, and 3.45 ppm), indicate a highly
unsymmetrical molecule (Figure 4). Increasing the temperature
simplifies the spectrum that only contains four averaged signals,
thus indicating the occurrence of a fluxional process with the
coalescence temperature around 323 K. The HSQC experiments
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Figure 4. Temperature dependence of the 1H NMR spectra for 3 (300
MHz, CD3CN).
Figure 2. HSQC experiments at 333 K for 3 (300 MHz, CD3CN).
Figure 3. HSQC experiments at 333 K for 3 (300 MHz, CD3CN).
Figure 5. HSQC experiments at 333 K for 3 (300 MHz, CD3CN).
at 333 K show that two types of carbons, (i.e., 34 and 40 ppm) are
correlated with two protons signals (i.e., 3.4 and 4.8 ppm, and 3.6
and 4.4 ppm, respectively) (Figure 5). The chemical shift of the
methylene carbons indicates a syn and an anti relative disposition
of the attached phenyl groups; a partial cone conformation for 3
can then be proposed conclusively.
Our short study on the complexation reactions of TaCl5 on
L1ꢀL4 has illustrated its complicated reactivity. On one hand,
TaCl5 has demonstrated the capability to rearrange selectively its
coordination sphere. The assistance of an ether group (methoxy
or benzyloxy) seems to be necessary to drive the reaction. As a
result, all ligands have been transformed during the complexa-
tion; hence, and rather unexpectedly, the complexes 1 and 3
are O3, while 2 is O2. These complexes can serve as substrates
for alkylation studies probing the effect of ether groups or
depleted sites.
Synthesis of the Calix[4]arene-Based Complexes 4 and 5.
Complexes 1 and 3 were alkylated at ꢀ40 °C by 2 equivalents of
NpMgCl to give the corresponding bisneopentyl 4 (yield 46%)
and 5 (yield 53%), respectively (Scheme 3). 4 and 5, in contrast
with their precursors 1 and 3, are soluble in benzene, thus
1
allowing facile characterizations by H, 13C, DEPT, HMBC, and
HSQC NMR spectrometry.
The 1H NMR and 13C NMR spectra of 4 and 5 are consistent
with the presence of only one Cs symmetric conformer in each
case. For both 4 and 5, the two neopentyl ligands are equivalent;
their methylenic groups show respectively one singlet at 2.50 and
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Scheme 3a
a Conditions: (i) 2 equiv NpMgCl, toluene, RT, 1 h; (ii) toluene, 80 °C.
2.55 ppm in 1H NMR and one signal at 94.7 and 97.9 ppm in 13
C
Figure 6. Molecular structure of 6. Hydrogen atoms and solvent
NMR. In addition, the HMBC spectrum allows to undoubtedly
differentiate the tert-butyl of the neopentyl ligand from that displayed
by the calixarene scaffold. The conformation of 4and 5can be reliably
deduced from the chemical shift of the carbon atom of the methylene.
With two signals at 34.2 and 34.6 ppm, a typical range of a methylenic
bridge linking two syn-oriented phenolic units, 4 adopts a cone
conformation.5 The spectrum of 5 contains two peaks at 34.3 and
39.9 ppm, with the latter consistent with the anti-oriented OH-
depleted unit, thus indicating a partial-cone conformation.5 The co-
ordination of the OMe group to the tantalum center in 4, as indicated
by the chemical shifts of the methoxy protons and carbon,25 could
explain the slight upfield shift of the neopentyl signals compared to 5.
Synthesis of the Calix[4]arene-Based Complexes 6 and 7.
Both complexes 4 and 5 are stable at room temperature, but can be
easily converted at 80 °C to 6 and 7, respectively (Scheme 3). The
products were systematically characterized by NMR spectroscopy
and elemental analysis. In addition, single crystals of 6 suitable for
X-ray diffraction studies were obtained from a saturated toluene
solution at room temperature. Crystallographic data and details
associated with data collection are given in Table 1.
molecules have been omitted for clarity.
bond, while the signals corresponding to the methylene bridged
is shifted by 45 ppm. The gas evolved from the reaction was
clearly identified by GC and 1H NMR as neopentane at 0.9 ppm.
All these characterizations support that the Ta center undergoes
a CꢀH activation on the depleted site of the calix[4]arene ligand
via σ-bond metathesis of [TaꢀCH2tBu] with [CipsoꢀH]
(Scheme 3). This reactivity is reminiscent of cyclometalations
via σ-bond metathesis observed for tantalum systems with
phenolate ligands ortho-substituted with tBu, iPr, or phenyl
groups. For instance Bercaw et al.26 reported similar activation
but on the ipsoArꢀH of the middle aromatic part. However,
intramolecular CꢀH activation of a calix[4]arene unit was
documented only on the methylene bridges using W complexes.
To our knowledge, this is the first Csp2ꢀH induced by a
transition metal on the calix[4]arene framework.
The full optimization of the molecular geometries and transi-
tion states geometries (TS) and calculation of the energy profile
of the complexes 5 and 7 involved in the CꢀH activation
reactions were carried out using DFT calculations. Figure S1
(see Supporting Information) shows the relative energy profiles
for the transformation of 5 into 7. From the high-electrophilic
complex 5, the reaction proceeds by σ-bond metathesis via the
transition state with a barrier of 59 kcal molꢀ1. This value is larger
than the experimental conditions (the reaction occurs at 80 °C
overnight). However, one cannot rule out the mechanism for
that reason, because the model used for the calculation may lead
to significantly higher energies of stable states in the potential
energy surface along the reaction path. In other words, the
presence of the tert-butyl groups attached to the phenyl groups
may raise the energy of the reactants while keeping roughly
unchanged the energy of the transition state, therefore leading to
a significantly lowered energy barrier. The structure of the
transition state, on the contrary, is highly plausible, given the
high single imaginary frequency found for it (i.e., 945 cmꢀ1).
This transition state corresponds to a metal center surrounded by
three ligands (CAr, HAr, and CNp), which are in a coplanar
arrangement (Figure 7). This is reminiscent of the CH activation
of methane by zirconium hydride supported on silica.27 Similarly,
Comparing the 1H spectrum of 6 with its precursor 4 revealed
that the singlet at 3.65 ppm—assigned to the methoxy group—
had disappeared. The tantalum metal appears to rearrange the
calixarenic ligand by formation of new TaꢀO bonds. Given that
no carbene intermediate was observed, a mechanism in which the
metal undergoes σ-bond metathesis can be reasonably proposed.25
The presence of a neopentyl ligand on the tantalum, compared to the
analogue aryl complexes,22 may favor the elimination reaction so as to
form a monomeric O4 complex (probably due to reduced steric
hindrance). The solid-state structure of 6 consists of a monomer
displaying C4v symmetry order (Figure 6). The tantalum center
follows a pyramidal coordination in which the square base is formed
by the four oxygens of the O4 core and its top by the methylenic
carbon of the neopentyl group. The dihedral angles formed by
O(1)ꢀTa(1)ꢀO(2), O(2)ꢀTa(1)ꢀO(4), O(4)ꢀTa(1)ꢀO(3),
and O(3)ꢀTa(1)ꢀO(1) are of similar values, 86.31ꢀ86.55°, with
the corresponding bond lengths in a similar range of 1.904ꢀ1.918 Å.
Comparing the 1H spectrum of 7 with that of its precursor 5
revealed that the signal of the ipso ArꢀH at 7.47 ppm of 5 had
disappeared. Moreover, the 13C spectrum shows a new signal at
196 ppm that is consistent with the formation of a new CArꢀTa
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ARTICLE
Scheme 4a
a Conditions: (i) 500 Torr *CO, benzene, RT, 15 min.
backbone. Complex 7, containing two types of TaꢀC bonds (Taꢀ
Csp3 vs TaꢀCsp2), provides an interesting platform for reactivity
studies. For instance, the direct reaction with 13CO can result in the
carbonylation of 7 by migratory insertion on either the Csp2, the
Csp3, or both.
The reaction of 7 with 2 equivalents of 100% 13C-labeled carbon
monoxide reaches completion within 15 min at room temperature
to give 8 (Scheme 4). In the 13C NMR spectrum, the peak (at 196
ppm) corresponding to the carbon bound to the tantalum is absent,
whereas a new peak has appeared at 314 ppm, a chemical shift
consistent with a CO bridging an aromatic ring and a tantalum
metal.28 Moreover, the peak at 107.4 ppm in the 13C NMR of 7
assigned to the methylene group from the neopentyl ligand of the
tantalum has shifted downfield to 84.1 ppm in 8.
Figure 7. Optimized structure of TS.
’ CONCLUSION
Our work on the chemistry of tantalacalixarene-based com-
plexes has permitted us to collect some remarkable results. Central
to our study is the capability of the metal to rearrange its coordina-
tion sphere by modifying its ligand set. A series of calixarene ligands
(L1ꢀL4) were prepared by displacement of one or two hydroxyl
groups so as to accommodate an ether function or a hydrogen.
These tailored coordination sites (i.e., O3 vs O2) were found to
affect the reactivity of tantalum chloride toward the calixarenic
scaffold. The dichloro or trichloro tantalacalixarene complexes 1ꢀ3
obtained illustrate the extent by which the ether function (methoxy
or benzyloxy) can direct the coordination of the tantalum center. Of
particular interest is the ease demonstrated by the metal to eliminate
the ether group to afford 1 and 3. Some structural insights of 3 were
gained by X-ray diffraction studies and NMR spectroscopy. 1 and 3
were then alkylated to generate the dialkyl complexes 4and 5. These
complexes were then converted to the monoalkyl equivalent 6and 7
by moderate heating. In this process, the tantalum reacts also on the
calixarene by either removing the methoxy group of 6 or an
unprecedented activation of the aromatic CH bonds of 7. The
reactivity of the latter has been further examined by treatment of
13C-labeled CO and observing its insertion by the metal on the
calixarene backbone depleted position. These preliminary results are
supporting our interest in using the calixarene as a platform for the
observation of organometallic transformations. As examples of the
activation of aromatic CH bonds are generally scarce, this short
study is indeed rewarding. This methodology, when optimized, has
the potential to initiate derivatization strategies to introduce carbon-
bearing functionalities on the lower rim of the calixarene backbone.
Figure 8. Optimized structure of 7.
the HOMO shows two lobes of opposite sign, centered on the
carbon atoms involved in the mechanism, separated by a nodal
surface crossing the H atom migrating from the calixarene group
toward the neopentyl one (see Supporting Information Figure S2).
The optimized structure of 7 has been modeled and is presented in
Figure 8. It seems that the calixarene ligand accommodates the
coordination of the metal by allowing the phenyl ring on which a CH
bond is activated to lean toward the tantalum center. The estimated
bond length of TaꢀCAr is 2.29 Å, a value close to that reported by
Bercaw.26 Interestingly, the TaꢀCNp bond length is 2.15 Å in 7, a
value in the range of TaꢀC bonds in several complexes, whereas its
bond length is 2.49 Å in 5, a high value that may be attributed to
some steric repulsion between the two neopentyl groups. As
expected, the TaꢀCAr bond formation generates strong strains that
lead to significant stretching of the TaꢀO bonds (þ0.04 Å for the
oxygen trans to the CAr bond and þ0.07 Å for the two others), but
this stretching occurs after the transition-state step (see Supporting
Information Table S1).
Comparing the reactivity and the structure of 6 and 7 suggests
that the interplay between the tantalum and the calixarene scaffold
favors the formation of covalent bonding by the tantalum (i.e.,
TaꢀO in 6 and TaꢀC in 7). This behavior can be seen as promising
in regard to functionalization of the ispo position of the calixarene
’ EXPERIMENTAL SECTION
General Procedures. All manipulations were carried out under a
dry argon atmosphere using standard Schlenk techniques. All solvents
were dried by standard methods and were freshly distilled before use.
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Organometallics
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NMR spectra were recorded on DRX400 and DRX300 Bruker instru-
ments. 13C-labeled (99%) carbon monoxide was purchased from Aldrich
and used as received.
H þ OH). 13C, DEPT NMR (CD2Cl2, 125 MHz): δ 31.3 (CH3-iBu),
31.4 (CH3-iBu), 31.7 (CH3-iBu), 32.9 (CH2-calix), 34.1 (tBu-quat),
34.7 (tBu-quat), 34.7 (tBu-quat), 38.2 (CH2-calix), 81.0 (CH2-Bn),
123.3 (CAr), 125.2 (CAr), 126.0 (CAr), 126.2 (CAr), 127.0 (CAr), 128.3
(CAr), 129.3 (CAr), 129.7 (CAr), 129.9 (CAr), 130.1 (CAr), 133.4 (CAr),
134.9 (CAr), 141.7 (CAr), 143.6 (CAr), 148.3 (CAr), 149.8 (CAr), 150.1
(CAr), 151.0 (CAr). LRMS (ESþ): [M þ Na]þ = 745.3. HRMS (ESþ):
calcd for C51H62O3Na 745.4597, found 745.4600. Crystals suitable for
X-ray analysis were obtained at room temperature from a saturated
methanol/dichloromethane solution.
Synthesis of 5,11,17,23-Tetra-tert-butyl-25-monohy-
droxy-26,27,28-trioxo(methylsilane). 5,11,17,23-Tetra-tert-bu-
tyl-25,26,27,28-tetraoxocalix[4]arene (11.000 g, 16.952 mmol) was
suspended in toluene (1000 mL) before addition of NEt3 (8.27 mL,
59.331 mmol). Cl3SiMe (4.03 mL, 33.904 mmol) was then added dropwise.
The resulting mixture was stirred overnight at ambient temperature. After
filtration of the salts, the residue was washed with n-hexane (2 ꢁ 20 mL)
and dried to afford a white solid (8.900 g, 76%). 1H NMR (CDCl3, 300
MHz): δ ꢀ0.34 (s, 3H, OSiMe), 1.25 (s, 9H, tBu), 1.30 (s, 9H, tBu), 1.32
(s, 18H, tBu), 3.42 (d, 2H, calix-CH2, 2J(H,H) = 13.8 Hz), 3.77 (d, 2H,
Synthesis of 5,11,17,23-Tetra-tert-butyl-25,26,27-trioxo-
28-methyloxycalix[4]arenetantalum(V) Dichloride (1). TaCl5
(1.069 g, 2.984 mmol) and 5,11,17,23-tetra-tert-butyl-25,26,27-trihy-
droxy-28-methyloxycalix[4]arene (2.000 g, 2.954 mmol) were sus-
pended in toluene (40 mL). The reaction mixture was refluxed for 36
h, and the volatiles were removed in vacuo. The resultant residue was
washed with pentane to provide a bright yellow solid (2.500 g, 85%). 1H
NMR (C6D6, 125 MHz): δ 0.66 (s, 9H, tBu), 0.72 (s, 9H, tBu), 1.33 (s,
2
2
calix-CH2, J(H,H) = 16.5 Hz), 4.20 (d, 2H, calix-CH2, J(H,H) = 16.5
2
Hz), 4.41 (d, 4H, calix-CH2, J(H,H) = 13.8 Hz), 4.59 (s, 1H, OH),
7.07ꢀ7.13 (m, 8H, ArꢀH).
Synthesis of [p-tBu-calix[4]-(O3SiMe)(OP(O)(OEt)2)]. nBuLi
(7.8 mL, 12.453 mmol, 1.6 M in hexanes) was added dropwise to a
solution of 5,11,17,23-tetra-tert-butyl-25-monohydroxy-26,27,28-trioxo-
(methylsilane) (7.800 g, 11.321 mmol) in toluene (300 mL). The resulting
bright yellow mixture was stirred for 30 min. ClP(O)(OEt)2 (2.5 mL, 16.981
mmol) was then added, and the resulting mixture was stirred for 4 h at
ambient temperature. After filtration of the salts, the filtrate was evaporated
and the residue washed with n-hexane (20 mL) to give a white solid (6.040 g,
65%). 1H NMR (CDCl3, 300 MHz): δ ꢀ0.40 (s, 3H, OSiMe), 0.72 (t, 6H,
OCH2CH3, 3J(H,H) = 7.2 Hz), 1.21 (s, 9H, tBu), 1.28 (s, 9H, tBu), 1.34 (s,
18H, tBu), 3.48 (m, 2H þ 4H, calix-CH2 þ OCH2CH3), 3.67 (d, 2H, calix-
2
18H, tBu), 3.15 (d, 2H, calix-CH2, J(H,H) = 13.3 Hz), 3.29 (d, 2H,
calix-CH2, 2J(H,H) = 13.8 Hz), 3.94 (s, 3H, OMe), 4.36 (d, 2H, calix-
CH2, 2J(H,H) = 13.3 Hz), 4.96 (d, 2H, calix-CH2, 2J(H,H) = 13.8 Hz),
6.82 (s, 2H, Ar-H), 6.83 (s, 2H, Ar-H), 7.18 (s, 4H, Ar-H).
Synthesis of 5,11,17,23-Tetra-tert-butyl-25,26,27-trioxo-
tantalum(V) Dichloride (3). TaCl5 (0.500 g, 0.692 mmol) and
5,11,17,23-tetra-tert-butyl-25,26,27-trioxo-28-benzyloxycalix[4]arene
(0.248 g, 0.692 mmol) were dissolved in toluene (10 mL) to give
immediately an orange-yellow, clear solution. After 5 min, an orange
solid precipitated, and the resulting suspension was stirred for 5 h at
room temperature.
After evaporation of the volatiles, the residue was washed with
pentane (2 ꢁ 5 mL) and dried under vacuum to provide a yellow solid
(0.498 g, 82%). 1H NMR (CD3CN, 300 MHz, 343 K): δ 1.11 (s, 9H, tBu),
1.13 (s, 9H, tBu), 1.32 (s, 18H, tBu), 3.46 (d, 2H, calix-CH2, 2J(H,H) = 13.2
Hz), 3.59 (d, 2H, calix-CH2, 2J(H,H) = 12.6 Hz), 4.36 (d, 2H, calix-CH2,
2J(H,H) = 12.6 Hz), 4.80 (d, 2H, calix-CH2, 2J(H,H) = 13.2 Hz), 6.99 (s,
2H, Ar-H), 7.00 (s, 2H, Ar-H), 7.31 (s, 4H, Ar-H), 8.80 (s, 1H, ipso-Ar-H).
13C, DEPT NMR (CD3CN, 125 MHz, 343 K): δ 31.9 (CH3-iBu), 32.1
(CH3-iBu), 32.1 (CH3-iBu), 34.3 (CH2-calix), 35.3 (tBu-quat), 35.4 (tBu-
quat), 35.7 (tBu-quat), 40.0 (CH2-calix), 123.9 (CAr), 125.9 (CAr), 127.4-
(CAr), 127.6 (CAr), 130.4 (CAr), 132.2 (CAr), 134.9 (CAr), 135.9 (CAr),
141.5 (CAr), 147.86 (CAr), 149.4 (CAr), 153.1 (CAr), 157.3 (CAr), 157.6
(CAr). Anal. Calcd for C44H53Cl2O3Ta: C, 59.93; H, 6.06. Found: C, 60.02;
H, 6.20. Crystals suitable for X-ray analysis were obtained at room
temperature from a saturated pentane solution.
2
2
CH2, J(H,H) = 13.8 Hz), 4.42 (d, 4H, calix-CH2, J(H,H) = 14.7 Hz),
7.08ꢀ7.13 (m, 8H, ArꢀH). 13C, DEPT NMR (CDCl3, 125 MHz): δ ꢀ8.4
(OSiMe), 15.6 (OCH2CH3), 15.7 (OCH2CH3), 31.5 (CH3-tBu), 31.6
(CH3-tBu), 31.6 (CH3-tBu), 34.0 (tBu-quat), 34.3 (tBu-quat), 34.3 (tBu-
quat), 35.2 (calix-CH2), 37.2 (calix-CH2), 64.0 (OCH2CH3), 64.0 (OCH2-
CH3), 124.XXX (CAr), 125.4 (CAr), 126.5 (CAr), 127.4 (CAr),130.0 (CAr),
130.5 (CAr), 130.7 (CAr), 132.3 (CAr), 132.3 (CAr), 144.4 (CAr), 145.9
(CAr), 146.9 (CAr), 147.4 (CAr), 148.6 (CAr). LRMS (ESþ): [M þ H]þ
=
825.2. HRMS (ESþ): calcd for C49H66O7PSi 825.4315, found 825.4316.
Synthesis of 5,11,17,23-Tetra-tert-butyl-25,26,27-trihy-
droxycalix[4]arene (L2). NH3 (125 mL) was condensed over K
(14.000 g, 0.366 mol) at ꢀ78 °C to give a dark blue solution. A solution
of [p-tBu-calix[4]ꢀ(O3SiMe)(OP(O)(OEt)2)] (6.040 g, 7.320 mmol)
in 60 mL of dry diethyl ether was added dropwise to the blue solution.
The mixture was stirred for 1 h at ꢀ78 °C. After quenching of the
solution with NH4Cl until complete transformation of the blue solution
into a white suspension, NH3 was left to evaporate at ambient
temperature. The resulting residue was extracted with CH2Cl2 (3 ꢁ
50 mL), and the filtrate washed with water until neutrality and dried over
MgSO4. After evacuation of the volatiles, the residue was triturated into
Synthesis of 5,11,17,23-Tetra-tert-butyl-25,26,27-trioxo-
28-methyloxycalix[4]arenetantalum(V) Dineopentyl (4).
NpMgCl (3.41 mL, 1.43 M in Et2O, 4.881 mmol) was added dropwise to a
solution of 5,11,17,23-tetra-tert-butyl-25,26,27-trioxo-28-methyloxycalix-
[4]arenetantalum(V) dichloride in toluene (100 mL) (2.450 g, 2.441 mmol)
at room temperature. The reaction was stirred for 1 h, and 5 mL of 1,4-
dioxane was then added. After a further stirring of 1 h, the salts were filtered
1
MeOH to give a white solid (2.456 g, 53%). H NMR (CDCl3, 300
MHz): δ 1.28 (s, 18H, tBu), 1.29 (s, 9H, tBu), 1.34 (s, 9H, tBu), 3.92 (s,
4H, CH2), 3.97 (s, 4H, CH2), 7.08ꢀ7.28 (m, 9H, ArꢀH).
Synthesis of 5,11,17,23-Tetra-tert-butyl-25,27-dihydroxy-
26-benzyloxycalix[4]arene (L4). L2 (1.490 g, 2.194 mmol) and
K2CO3 (0.195 g, 1.413 mmol) were suspended in dry CH3CN (40 mL).
BnBr (0.31 mL, 2.590 mmol) was then added, and the mixture was
stirred under reflux for 2 h. After evaporation of the solvent, the residue
was dissolved in CH2Cl2 (30 mL), washed successively with HCl (1 M),
by water until neutrality, and with brine and dried (MgSO4). The solvent
was removed under vacuum, and the brown resin was tritured with hot
MeOH to precipitate a white powder, which was filtrated when cold and
washed with cold MeOH (0.930 g, 55%). 1H NMR (CD2Cl2, 300 MHz,
298 K): δ 1.18 (s, 9H, tBu), 1.19 (s, 9H, tBu), 1.20 (s, 18H, tBu), 3.29 (d,
2H, calix-CH2, 2J(H,H) = 12.3 Hz), 3.55 (d, 2H, calix-CH2, 2J(H,H) =
13.6 Hz), 4.05 (d, 2H, calix-CH2, 2J(H,H) = 13.6 Hz), 4.18 (d, 2H, calix-
CH2, 2J(H,H) = 12.3 Hz), 5.22 (s, 2H, CH2Ph), 6.99ꢀ7.72 (m, 16H, Ar-
1
off and the volatiles evaporated to give a yellow solid (1.215 g, 46%). H
NMR (C6D6, 300 MHz): δ 0.70 (s, 9H, calix-tBu), 0.79 (s, 9H, calix-tBu),
1.36 (s, 18H, calix-tBu), 1.55 (s, 18H, Np-tBu), 3.32 (d, 2H, calix-CH2, 2J(H,
H) = 12.4 Hz), 3.44 (d, 2H, calix-CH2, 2J(H,H) = 13.2 Hz), 3.65 (s, 3H,
OMe), 4.60 (d, 2H, calix-CH2, 2J(H,H) = 12.4 Hz), 5.06 (d, 2H, calix-CH2,
2J(H,H) = 13.2 Hz),), 6.83 (s, 2H, Ar-H), 6.84 (s, 2H, Ar-H), 7.26 (s, 4H,
Ar-H). 13C, DEPT NMR (C6D6, 75 MHz): δ 30.6 (CH3-tBu), 30.9 (CH3-
tBu), 31.9 (CH3-tBu), 33.5 (tBu-quat), 33.7 (tBu-quat), 34.2 (calix-CH2),
34.4 (tBu-quat), 34.6 (calix-CH2), 36.6 (CH3-Np), 37.6 (Np-quat), 65.2
(OMe), 94.7 (Np-CH2), 124.5 (CAr), 125.7 (CAr), 125.8 (CAr), 126.9
(CAr), 127.5 (CAr), 132.5 (CAr), 132.6 (CAr), 133.1 (CAr), 145.2 (CAr),
145.6 (CAr), 148.4 (CAr), 152.8 (CAr), 156.1 (CAr), 157.6 (CAr). Anal. Calcd
for C55H77O4Ta: C, 67.19; H, 7.89. Found: C, 66.89; H, 7.57.
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Synthesis of 5,11,17,23-Tetra-tert-butyl-25,26,27-trioxo-
tantalum(V) Dineopentyl (5). Route A: NpMgCl (0.54 mL, 0.644
mmol, 1.2 M in diethyl ether) was added dropwise at ꢀ40 °C to a solution
of 3 (0.300 g, 0.314 mmol) in toluene (20 mL). The resulting solution was
allowed to warm to ambient temperature. After stirring for 30 min, 0.6 mL
of 1,4-dioxane was added. After 30 min, the resulting mixture was filtered.
The filtrate was evaporated until dryness, washed with Et2O (5 mL), and
dried under vacuum at ambient temperature to provide a pale yellow solid
(0.096 g, 32%). Route B: TaCl5 (0.453 g, 1.264 mmol) and L4 (0.914 g,
1.264 mmol) were dissolved in toluene (15 mL) to give immediately an
orange-yellow, clear solution. After 5 min, an orange solid precipitated, and
the resulting suspension was stirred for 5 h at room temperature. After
evaporation of the volatiles, fresh toluene was added, and NpMgCl
(2.20 mL, 2.591 mmol, 1.2 M in diethyl ether) was added dropwise
at ꢀ40 °C. The resulting solution was allowed to reach RT and then stirred
30 min. A 2.4 mL portion of 1,4-dioxane was added, and after 30 min, the
resulting mixture was filtered. The filtrate was evaporated until dryness,
washed with Et2O (2 ꢁ 5 mL), and dried under vacuum at RT to provide a
pale yellow solid (0.640 g, 53%). 1H NMR (C6D6, 300 MHz): δ 0.95 (s,
9H, calix-tBu), 0.97 (s, 9H, calix-tBu), 1.18 (s, 18H, calix-tBu), 1.39 (s, 18H,
Np-tBu), 2.55 (s, 4H, Np-CH2), 3.46 (d, 2H, calix-CH2, 2J(H,H) = 13.3 Hz),
3.62 (d, 2H, calix-CH2,2J(H,H) = 13.8 Hz), 4.55 (d, 2H, calix-CH2,2J(H,H) =
13.8 Hz), 4.80 (d, 2H, calix-CH2, 2J(H,H) = 13.2 Hz), 6.90 (d, 2H, meta-
Ar-H), 6.97 (s, 2H, meta-Ar-H), 7.14 (s, 4H, meta-Ar-H), 7.48 (s, 1H, ipso-
Ar-H). 13C, DEPT, HMBC, HSQC NMR (C6D6, 125 MHz): δ 31.1 (CH3-
iBu), 31.2 (CH3-iBu), 31.5 (CH3-iBu), 31.9 (tBu-quat), 34.1 (tBu-quat),
34.2 (CH2-calix), 34.3 (tBu-quat), 35.7 (CH3-Np), 37.0 (Np-quat), 39.9
(CH2-calix), 97.9 (CH2-Np), 123.5 (CAr), 124.8 (CAr), 126.4 (CAr), 127.0
(CAr), 127.1 (CAr), 129.2 (CAr), 131.2 (CAr), 131.8 (CAr), 139.9 (CAr),
146.2 (CAr), 146.4 (CAr), 151.5 (CAr), 152.3 (CAr), 157.1(CAr). Anal. Calcd
for C54H75O3Ta: C, 68.05; H, 7.93. Found: C, 68.32; H, 8.03.
(s, 18H, tBu), 1.22 (s, 9H, tBu), 1.28 (s, 9H, tBu), 1.64 (s, 9H, CH3-Np),
2
3.15 (d, 2H, calix-CH2, J(H,H) = 13.6 Hz), 3.22 (s, 2H, CH2-Np),
2
3.28 (d, 2H, calix-CH2, J(H,H) = 12.3 Hz), 3.94 (d, 2H, calix-CH2,
2J(H,H) = 13.6 Hz), 5.03 (d, 2H, calix-CH2, 2J(H,H) = 12.3 Hz), 6.80
(d, 2H, Ar-H, 4J(H,H) = 2.4 Hz), 6.94 (d, 2H, Ar-H, 4J(H,H) = 2.4 Hz),
7.14 (s, 2H, Ar-H), 7.19 (s, 2H, Ar-H). 13C, DEPT NMR (C6D6, 125
MHz): δ 31.1 (CH3-iBu), 31.4 (CH3-iBu), 31.8 (CH3-iBu), 33.7 (tBu-
quat), 33.9 (CH2-calix), 34.3 (Np-CH3), 34.3 (Np-quat), 34.6 (tBu-
quat) 34.8 (tBu-quat) 35.5 (CH2-calix), 84.1 (Np-CH2), 123.6 (CAr),
124.5 (CAr), 126.2 (CAr), 126.5 (CAr), 130.5 (CAr), 131.1 (CAr), 132.7
(CAr), 133.1 (CAr), 136.8 (CAr), 145.2 (CAr), 145.6 (CAr), 152.2 (CAr),
152.3 (CAr), 153.8 (CAr), 314.4 (Ar-COꢀ).
DFT Calculations. DFT calculations were performed on complexes
5 and 7, as well as the TS linking them according to the reaction
described in Scheme 3 using the ADF09 packages.29ꢀ31 The PBE
gradient-corrected exchangeꢀcorrelation functional32 and the TZP
(triple zeta plus polarization) basis sets were retained for all the
calculations, with small frozen cores. The scalar relativist ZORA
approximation has been used in this work for all calculations.33,34 All
the structures were characterized by vibrational analysis in the harmonic
approximation, with no imaginary frequency for stable states and one
imaginary frequency for transition states.
’ ASSOCIATED CONTENT
S
Supporting Information. (a) CIF files giving crystal data
b
for compounds L4, 3, and 6. (b) Calculated bonding distances
and energies of the optimized starting compounds 5, product
complex 7, and the transition states (TS). (c) Orbital contribu-
tions to the HOMO of TS. These materials are available free of
charge via the Internet at http://pubs.acs.org.
Synthesis of 5,11,17,23-Tetra-tert-butyltetraoxocalix-
[4]arenetantalum(V) Mononeopentyl (6). A sample of 4 heated
in toluene at reflux for 16 h was fully converted into 5,11,17,23-tetra-tert-
’ AUTHOR INFORMATION
1
butyltetraoxocalix[4]arenetantalum(V) neopentyl (6). H NMR (C6D6,
Corresponding Authors
*E-mail: taoufik@cpe.fr, henry.chermette@univ-lyon1.fr, isabelle.
bonnamour@univ-lyon1.fr.
300 MHz): δ 1.12 (s, 36H, calix-tBu), 1.42 (s, 9H, Np-tBu), 2.75 (s, 2H,
CH2-Np), 3.34 (d, 2H, calix-CH2, 2J(H,H) = 12.3 Hz), 5.07 (d, 2H, calix-
CH2, 2J(H,H) = 12.3 Hz), 7.11 (s, 8H, Ar-H). 13C, DEPT NMR (C6D6,
300 MHz): δ 31.4 (CH3-tBu), 33.6 (CH3-Np), 34.1 (tBu-quat), 34.2
(calix-CH2), 34.8 (Np-quat), 90.4 (Np-CH2), 125.2 (Ar-meta), 132.1 (Ar-
’ ACKNOWLEDGMENT
ortho), 146.1 (Ar-para), 152.1 (Ar-O). Anal. Calcd for C49H63O4Ta
3
This work was supported by the French Government through
the “MATCALCAT” ANR Project (contract ANR 07-CP2D-21-
02) and the Competitiveness Cluster on Chemistry and the
Environment of Lyon 1 Rh^one-Alpes (Axelera). The authors
gratefully acknowledge the GENCI/CINES for HPC resources/
computer time (Project cpt2130).
(toluene)2: C, 69.98; H, 7.36. Found: C, 69.59; H, 7.59. Crystals suitable for
X-ray analysis were grown from a saturated solution in toluene at RT.
Synthesis of 5,11,17,23-Tetra-tert-butyl-25,26,27-trioxo-
tantalum(V) Neopentyl (7). In an NMR tube, a sample of 5 was
totally converted into 5,11,17,23-tetra-tert-butyl-25,26,27-trioxotanta-
lum(V) neopentyl (7) at 80 °C overnight, with NpH evolution (NMR
evidence, checked by GC injection of the gas phase). 1H NMR (C6D6,
300 MHz): δ 0.96 (s, 9H, calix-tBu), 0.98 (s, 9H, calix-tBu), 1.34 (s, 18H,
calix-tBu), 1.39 (s, 9H, Np-tBu), 2.37 (s, 2H, CH2-Np), 3.33 (d, 2H,
calix-CH2, 2J(H,H) = 12.3 Hz), 3.50 (d, 2H, calix-CH2, 2J(H,H) = 12.0
Hz), 4.86 (d, 2H, calix-CH2, 2J(H,H) = 12.0 Hz), 5.11 (d, 2H, calix-CH2,
2J(H,H) = 12.3 Hz), 7.01 (s, 2H, Ar-H); 7.09 (s, 2H, Ar-H); 7.25 (d, 2H,
Ar-H, 2J(H,H) = 2.3 Hz), 7.31 (d, 2H, Ar-H, 2J(H,H) = 2.3 Hz). 13C,
DEPT NMR (C6D6, 300 MHz): δ 30.7 (CH3-tBu), 31.2 (CH3-tBu),
31.9 (CH3-tBu), 33.8 (tBu-quat), 34.1 (CH3-Np), 34.2 (tBu-quat), 34.4
(calix-CH2), 34.5 (tBu-quat), 35.9 (Np-quat), 44.9 (calix-CH2), 107.8
(Np-CH2), 121.8 (CAr), 124.0 (CAr), 125.7 (CAr), 127.4 (CAr), 128.8
(CAr), 132.1 (CAr), 132.4 (CAr), 145.8 (CAr), 146.7 (CAr), 150.7 (CAr),
150.9 (CAr), 152.8 (CAr), 153.4 (CAr), 196.4 (Ta-CAr). Anal. Calcd for
C49H63O3Ta: C, 66.80; H, 7.21. Found: C, 66.91; H, 7.52.
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