Stepwise construction of a Re-Pd heterodinuclear core inside the cavity of p-tBu-calix[4]arene

Article abstract of DOI:10.1002/anie.200351530

A Re-Pd heterometallic core has been constructed inside the cavity of a calixarene ligand. An unprecedented calix[4]-arene-rhenium complex can incorporate a {Pd(η3-C₃H_s)⁺} fragment to afford the phenoxo-bridged heterodinuclear complex [ReO₂{p-t Bu-calix[4]arene-(O)₄} Pd (η³-C₃H ₅)] (see picture).

Full text of DOI:10.1002/anie.200351530

Communications
the calixarene scaffold, we have turned our attention to the
synthesis and reactivities of high-valent rhenium derivatives
of calixarenes. Herein we describe the synthesis and charac-
terization of mononuclear rhenium p-tBu-calix[4]arene com-
plexes and their use in the stepwise construction of a
phenoxo-bridged Re–Pd heterodinuclear core inside the
cavity of the calix[4]arene.
When a solution of p-tBu-calix[4]arene-(OH)4 (1) in THF
was treated with nBuLi (3 equiv) and then allowed to react
with [Ph4P][ReOCl4] (1.5 equiv),[8] an anionic complex tenta-
tively formulated as [Ph4P][ReCl(O){p-tBu-calix[4]arene-
(O)4}]·CH2Cl2·0.5Et2O (2·CH2Cl2·0.5Et2O) was obtained in
54% yield after repeated recrystallization (Scheme 1).[9]
Although the paramagnetism of the ReVI center (meff/mB =
1.3 in [D6]acetone solution) prevented spectroscopic full
characterization of 2, its analytical, EPR, and IR (n˜ =
ꢀ1
¼
929 cm , n(Re O)) data are in agreement with the chloro–
oxo structure. Further characterization of the rhenium com-
Re–Pd Calixarene Complexes
plex has been achieved by oxidation to a ReVII species.
Stepwise Construction of a Re–Pd
Heterodinuclear Core Inside the Cavity of
p-tBu-Calix[4]arene**
Kentarou Iwasa, Takuya Kochi, and Youichi Ishii*
Metallocalixarenes have recently received much interest as
molecular models of metal species bound on polyoxo
surfaces,[1] as well as metalloreceptors with controlled struc-
tures.[2,3] The majority of the metallocalixarenes so far
developed are oxophilic group 4–6 transition-metal deriva-
tives; few examples have been reported for late-transition-
metal complexes of unmodified calixarenes.[3–6] However,
recent studies have revealed that late transition metals can
also form stable aryloxo and alkoxo complexes that have
intriguing reactivities.[7] This background prompted us to
investigate the synthesis and properties of late-transition-
metal derivatives of calixarenes, and we have recently found
that calix[4]arenes undergo site-selective and stepwise com-
plexation with two {M(cod)+} fragments (M = Rh, Ir; cod =
1,5-cyclooctadiene), where the first metal center is coordi-
nated to the arene ring and the second is coordinated to the
phenolic oxygen atoms at the narrow rim.[4] With the intention
to construct heterodinuclear late-transition-metal cores on
Scheme 1. Synthesis of rhenium complexes 2 and 3a.
When complex 2 was treated with excess Ag2O in THF at
room temperature, the ReVII complex [Ph4P][ReO2{p-tBu-
calix[4]arene-(O)4}] (3a) was obtained in 89% yield
¼
(Scheme 1). The formation of Re O bonds is confirmed by
the characteristic strong IR bands at 902 and 910 cmꢀ1. The
1H NMR spectrum of 3a shows two aromatic and two tBu
signals at d = 7.06, 6.96 ppm and d = 1.29, 1.07 ppm, respec-
tively, as well as one set of CH2 signals at d = 4.51 and
3.29 ppm (d, J = 13.7 Hz). This spectral feature is in full
agreement with a formulation with an apparent C2v symmetry.
The structure of this anion was crystallographically deter-
mined via the PPN salt [PPN][ReO2{p-tBu-calix[4]arene-
(O)4}]·2Me2CO·0.5C6H6 (3b·2Me2CO·0.5C6H6) (Figure 1;
PPN = (Ph3P)2N).[10] The rhenium atom is coordinated by
the four phenolic oxygen atoms of the calix[4]arene ligand
and two cis oxo ligands, in a distorted octahedral geometry.
The Re(1)-O(5) and Re(1)-O(6) bond lengths at 1.717(5) and
[*] Prof. Dr. Y. Ishii
Department of Applied Chemistry
Faculty of Science and Engineering, Chuo University
Kasuga, Bunkyo-ku, Tokyo 112-8551 (Japan)
Fax: (+81)3-3817-1895
E-mail: ishii@chem.chuo-u.ac.jp
K. Iwasa, T. Kochi
Institute of Industrial Science
The University of Tokyo
Komaba, Meguro-ku, Tokyo 153-8505 (Japan)
[**] This work was supported by a Grant-in-aid for Scientific Research
from the Ministry of Education, Science, Sports, and Culture, Japan.
1.714(5) Š, respectively, are typical for ReVII O double
¼
bonds.[11] Two of the facing aromatic rings of the calix[4]arene
Supportinginformation for this article is available on the WWW
under http://www.angewandte.org or from the author.
ligand are very open, with a dihedral angle of 168.88, so that
3658
ꢀ 2003 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
DOI: 10.1002/anie.200351530
Angew. Chem. Int. Ed. 2003, 42, 3658 –3660

Angewandte
Chemie
The molecular structure of 4 was established by X-ray
crystallography.[10] An ORTEP drawing is shown in Figure 2,
which clearly confirms that the {Pd(h3-C3H5)+} fragment is
situated in the cavity and coordinated by the two phenoxy
Figure 1. ORTEP diagram for the anionic part in 3b·2Me2CO·0.5C6H6.
Thermal ellipsoids were set at 50%.
the calix[4]arene ligand adopts an elliptical cone conforma-
tion where the O(2) and O(4) atoms of the splayed phenoxo
residues are mutually cis and occupy the trans positions of the
oxo ligands. It should be noted that complexes 1 and 2 are the
first calixarene–rhenium complexes in which the metal atom
is directly bound to the body of calixarene, although several
calixarene derivatives having a rhenium complex moiety as a
pendant group have been synthesized.[12]
Figure 2. ORTEP diagram for complex 4. Thermal ellipsoids were set
at 50%.
Taking advantage of the anionic nature of the dioxorhe-
nium species, complexation of a second transition-metal
fragment with 3a was examined. When an EtOH solution of
3a was treated with the solvated allyl–palladium complex
¼
oxygen atoms situated trans to the Re O groups. The
heterobimetallic {RePdO2} core is planar, and the long
Re(1)···Pd(1) interatomic separation at 3.4383(8) Š excludes
any metal–metal bonding interaction. The dihedral angle of
the two bridging phenoxo groups at 170.78 is comparable to
that of 3b, and the conformation of the calix[4]arene moiety is
deformed only slightly by the coordination of the {Pd(h3-
C3H5)+} fragment. Although a few alkali-metal salts of anionic
calixarene complexes such as [Ta(OPh)2{p-tBu-calix[4]arene-
(O)4}Na(thf)2] have been reported to adopt a similar coordi-
nation structure,[14] complex 4 provides the first example
where a heterodinuclear core composed of two different
transition metals has been constructed inside the cavity of a
calixarene ligand. Interestingly, complex 4 was the only
bimetallic product detected in the reaction of 3a with
[Pd(h3-C3H5)(Me2CO)x]+, and coordination of the terminal
[Pd(h3-C3H5)(Me2CO)x](OTf)
Re–Pd complex
C3H5)] (4) was obtained in 75% yield (Scheme 2). The
(OTf = OSO2CF3),[13] the
[ReO2{p-tBu-calix[4]arene-(O)4}Pd(h3-
¼
Re O groups in 3a to the palladium center has not been
observed.
Scheme 2. Synthesis of heterobimetallic complex 4.
In conclusion, we have synthesized mononuclear ReVI and
ReVII complexes of 1 and found that the ReVII dioxo complex 3
can be used to construct a Re–Pd heterodinuclear core inside
the cavity of the calixarene. Although several homopolyme-
tallic complexes of calixarenes have been described in
literature,[15] a methodology to synthesize heteropolynuclear
transition-metal cores on the calixarene scaffold still remains
to be developed. The present study opens a potential
synthetic route for such a class of complexes.
1H NMR signals for the allyl protons of 4 appear at d = 4.01
(tt, J = 11.7, 6.6 Hz, 1H), 2.20 (d, J = 6.6 Hz, 2H), and
1.20 ppm (d, J = 11.7 Hz, 2H), and exhibit an extraordinary
high-field shift compared with those of [{Pd(h3-C3H5)Cl}2]
(Dd = 1.5–1.9 ppm). This observation strongly suggests that
the {Pd(h3-C3H5)+} fragment is encapsulated in the calixarene
pocket and surrounded by the aromatic rings. The signals for
the calix[4]arene CH2 protons are observed as a pair of
doublets at room temperature (d = 4.61, 3.53 ppm, J =
14.4 Hz), indicating the dynamic behavior of the allyl group.
These signals coalesce at ꢀ408C and split into four doublets at
ꢀ808C (d = 3.48, 3.58, 4.32, 4.44 ppm, J = 14.8 Hz), which is in
full agreement with the expected Cs symmetry of 4.
Received: March 31, 2003 [Z51530]
Keywords: bimetallic complexes · calixarenes · palladium ·
.
rhenium · transition metals
Angew. Chem. Int. Ed. 2003, 42, 3658 –3660
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ꢀ 2003 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
3659

Communications
24.763(3), b = 9.308(2), c = 19.239(2) Š, b = 108.154(10)8, V=
4213(1) Š3, Z = 4, 1calcd = 1.593 gcmꢀ3
2qmax = 55.08, T=
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294(2) K; 5233 reflections measured, 4856 unique (R(int) =
0.018), and 3885 I > 2s(I); Rigaku AFC7R four-circle automated
diffractometer, MoKa radiation (l = 0.71069 Š), graphite mono-
chromator, Lorentz-polarization and absorption corrections
(m = 33.46 cmꢀ1, range of transmission = 0.61–1.00). The struc-
ture was solved by heavy-atom Patterson methods and refined
with the full-matrix, least-squares method based on F2 (teXsan),
236 parameters, R2 = 0.102, wR = 0.159 for all data; residual
electron density 2.51/ꢀ1.80 eŠꢀ3. The allyl group was found to
be disordered around the C2 axis, and the corresponding C and H
atoms were included as fixed atoms with a 50% occupancy.
CCDC-205916 (3b·2Me2CO·0.5C6H6) and CCDC-205917 (4)
contain the supplementary crystallographic data for this paper.
These data can be obtained free of charge via www.ccdc.cam.a-
c.uk/conts/retrieving.html (or from the Cambridge Crystallo-
graphic Data Centre, 12, Union Road, Cambridge CB21EZ,
UK; fax: (+ 44)1223-336-033; or deposit@ccdc.cam.ac.uk).
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[9] The preparative procedures and data for complexes 2–4 are
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[10] a) 3b·2Me2CO·0.5C6H6: C89H97NO8P2Re: (Mr = 1556.90); crys-
3
¯
tal size 0.20 ” 0.35 ” 0.35 mm , triclinic, space group P1, a =
13.044(2), b = 14.168(3), c = 22.880(5) Š, a = 80.51(2), b =
88.96(2), g = 72.17(1)8, V= 3967(1) Š3, Z = 2, 1calcd
=
1.303 gcmꢀ3
, 2qmax = 45.08, T= 294(2) K; 10865 reflections
measured, 10361 unique (R(int) = 0.027) and 7693 I > 3s(I);
Rigaku AFC7R four-circle automated diffractometer, MoKa
radiation (l = 0.71069 Š), graphite monochromator, Lorentz-
polarization and absorption corrections (m = 16.28 cmꢀ1, range of
transmission = 0.80–1.00). The structure was solved by heavy-
atom Patterson methods and refined with the full-matrix, least-
squares method based on F (teXsan, Crystal Structure Analysis
Package, Molecular Structure Corporation, 1985 and 1999), 838
parameters, R = 0.042, Rw = 0.043 for I > 3s(I); residual elec-
tron density 0.76/ꢀ0.68 eŠꢀ3. The C and O atoms in the two
solvating acetone molecules were included as fixed atoms; one
of these molecules was found to be disordered and the
corresponding C and O atoms were treated with 60% and
40% occupancies; b) 4: C47H57O6PdRe (Mr = 1010.57); crystal
size 0.15 ” 0.40 ” 0.35 mm3, monoclinic, space group C2/c, a =
3660
ꢀ 2003 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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Angew. Chem. Int. Ed. 2003, 42, 3658 –3660