Organopalladium complexes of oxacalixarenes: Selecting the lid for the three-dimensional scaffold

Article abstract of DOI:10.1021/ic400925y

The first organometallic (palladium) complexes of oxacalixarene molecules were prepared via oxidative addition of the C-I bond at the lower rim. The unique geometry of the oxacalixarene scaffold allowed for the selective introduction of new ligands at the

Full text of DOI:10.1021/ic400925y

Communication
pubs.acs.org/IC
Organopalladium Complexes of Oxacalixarenes: Selecting the Lid for
the Three-Dimensional Scaffold
Yulia Visitaev, Israel Goldberg, and Arkadi Vigalok*
School of Chemistry, The Raymond and Beverly Sackler Faculty of Exact Sciences, Tel Aviv University, Tel Aviv 69978, Israel
S
* Supporting Information
Scheme 1. Stepwise Synthesis of Oxacalixarene Ligands
ABSTRACT: The first organometallic (palladium) com-
plexes of oxacalixarene molecules were prepared via
oxidative addition of the C−I bond at the lower rim.
The unique geometry of the oxacalixarene scaffold allowed
for the selective introduction of new ligands at the top of
the calixarene scaffold. Such coordination can be used to
coordinatively link the opposing aromatic rings.
The ethyl ester function in the para position was introduced to
igid three-dimensional transition-metal calixarene com-
plexes have received considerable attention as catalysts,1
increase the solubility of the metal complexes. The reaction
between 1 and the corresponding palladium(0) precursors
resulted in clean formation of the oxidative addition products 2
and 3 (Scheme 2), which could be purified by crystallization. The
R
supramolecular hosts,2 chemosensors3 and in biomimetics.4 In
most cases, the metal center is coordinated to the oxygen atoms
at the calixarene lower rim (Figure 1a).5 While suitable for the
Scheme 2. Synthesis of Palladium(II) Complexes of
Oxacalixarene 1
Figure 1. Metal coordination to calix[4]arenes.
early- and middle-row metals, such complexation generally limits
the use of less oxophilic late transition metals because they tend
to bind softer sites, such as the calixarene’s π system.6
Alternatively, the introduction of soft donors at the upper rim
can lead to stable late-transition-metal complexes (Figure 1b).7
To our knowledge, there are no examples of late-transition-metal
calixarene complexes with the metal directly attached to the
lower rim of a calixarene moiety. While such coordination is
essential for the successful cross-coupling of the triflate groups,8
the arylpalladium complexes were not isolated or characterized.9
Against this background, it is surprising that the direct metalation
of oxacalixarene molecules by late transition metals remained
completely unexplored.10 Only very recently have copper
complexes of somewhat related pyridine-based azacalixarenes
been reported;11 however, intramolecular pyridine coordination
was an integral part of the overall structure. In this work, we
present the first studies of the aryl-bound palladium oxacalixar-
ene complexes that show very interesting coordination proper-
ties relative to the calixarene cavity.
X-ray structure of complex 3 was determined and showed that
the palladium atom is located in the center of a distorted square
with a Pd−C distance of 1.998(4) Å. The presence of the bulky
neutral donors forces the iodo ligand to “cap” the oxacalixarene
cavity while the palladium center is surrounded by the nitro
groups of the neighboring aromatic rings. These surroundings
also prevent the associative substitution of the nitrogen-based
ligands in 2 and 3, which remain unchanged even upon stirring
overnight with a 5-fold excess of Ph3P. The cyclometalated
aromatic ring and ring opposite to it are nearly parallel, with the
distance between the opposing carbon atoms at the upper rim
(C24---C48) being ca. 4.976 Å.
Unlike parent calix[4]arenes, oxacalixarenes can readily be
assembled via a stepwise approach.12 This method allows for the
selective introduction of aromatic rings containing a readily
cleavable halide function.13 Thus, we prepared the iodocalixarene
1 in high yield using the synthetic approach shown in Scheme 1.
Received: April 15, 2013
© XXXX American Chemical Society
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dx.doi.org/10.1021/ic400925y | Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Communication
Figure 3. Molecular structures of 9a and 9b. The hydrogen atoms and
oxygen atoms of the nitro groups are omitted for clarity.
Scheme 3. Palladium Complexes of the Hydroxyoxacalixarene
6
Figure 2. Molecular structures of the cations of 4a and 5c−5e. The
hydrogen atoms and oxygen atoms of the nitro groups are omitted for
clarity.
predisposed to place the incoming ligand in the area between
the two opposing aromatic rings, we decided to explore an
oxacalixarene system where an additional binding group is
attached to the aromatic ring opposed to the carbopalladated
one. To this end, we prepared the oxacalixarene 6, which has the
phenolic moiety at the aromatic ring opposed to the iodine
anchor. Upon the addition of the palladium(0) precursor,
complexes 7 and 8 were obtained as dark-yellow crystalline
materials (Scheme 3). The 1H NMR spectra of the new
complexes were similar to those in 2 and 3 except for the lack of a
signal of the aromatic hydrogen atom, replaced by the OH group.
The X-ray structure of the dmap complex 8 was also similar to
that of 3. The addition of AgBF4 in acetone in the presence of a
donor molecule gave the corresponding cationic complexes 9
and 10 (Scheme 3). Although the hydroxy group can participate
in hydrogen bonding with the −NMe2 substituent of the dmap
ligand, no such interaction was observed in the X-ray structure of
the tmeda complex 9a (Figure 3), suggesting that the tilt at the
palladium center, which minimizes steric repulsions with the
calixarene scaffold, prevents formation of the hydrogen bond.
Similarly, using a smaller pyrazine ligand did not provide
evidence for such interaction in 9b (Figure 3). It is likely that a
more flexible hydrogen donor in place of the OH group is
required to provide a better fit to the incoming nitrogen donor.
Interestingly, when complex 8 bearing the monodentate dmap
ligands was treated with AgBF4 in acetone in the presence of
dmap, a light-gray precipitate of the presumed cationic 10a was
formed. The solid was insoluble in tetrahydrofuran or CH2Cl2;
however, it was possible to dissolve it in dimethyl sulfoxide
(DMSO), hot MeOH, or N,N-dimethylformamide (DMF). In
the last two cases, dissolution took place after several minutes.
The enforced geometry in complexes 2 and 3, where the iodo
ligand points toward the center of the calixarene rim, opens up
opportunities to introduce various ligands at this position. After
removal of the iodide with AgBF4, several donors can be
selectively placed at the top of the calixarene scaffold, giving new
cationic complexes 4 and 5 (Scheme 2). Interestingly, the
aromatic groups of the cavity-oriented ligands in 4a and 5c
(Figure 2) lie flat atop the calixarene unit, making a perfect lid at
the metal-bound side of its pinched cone. The X-ray structure of
4a shows that the presence of the large N,N-dimethylaminopyr-
idine (dmap) ligand has a relatively minor effect on the opening
of the oxacalixarene cavity compared with 3, with the metalated
ring still being roughly parallel to the opposing one. Additionally,
in contrast to the chelating tetramethylenediamine (tmeda, 4),
the presence of the monodentate dmap allows for the selective
placement of the incoming ligand in the position trans to the
aromatic carbon atom. For example, the reaction of 3 with AgBF4
followed by the addition of Ph3P gave the triphenylphosphine
complex 5d, where the bulky phosphine ligand occupies the
above position. While the stronger trans influence of Ph3P causes
elongation of the Pd−C bond [2.082(8) Å vs 1.998(4) and
2.016(6) Å in 3 and 4a, respectively), the calixarene cavity
remains largely intact (Figure 2). In all cases, the metalated ring is
nearly parallel to the opposing aromatic ring of the oxacalixarene
moiety, with the distance between C75 and C55 at the upper
opening being 4.980 Å. Significantly, when the small and weakly
bound acetone ligand is coordinated to the palladium center
(5e), the complex geometry dramatically changes in comparison
to complexes 3, 5a, and 5b. In particular, the internal dmap ligand
no longer points toward the center of the oxacalixarene rim.
Instead, it becomes involved in apparent π-stacking interactions
with one of the nitro-substituted aromatic rings of the
oxacalixarene scaffold (Figure 3). Distortion of the oxacalixarene
moiety is further manifested in the loss of the nearly parallel
alignment of the metalated ring and ring opposite to it, leading to
the significantly smaller opening of the upper rim of only 3.896 Å.
The selective introduction of various ligands at the top of the
calixarene cavity can lead to the development of new supra-
molecular structures, recognition of small donor molecules, and
catalysis. Because the oxacalixarene scaffold is generally
1
The H NMR spectrum of the DMSO-d6 solution of the
precipitate showed very broad lines for both the calixarene
scaffold and the palladium-coordinated dmap ligands. Because all
other palladium complexes reported in this work, including the
structurally similar 5c, showed very good solubility in acetone
and exhibited sharp signals in the 1H NMR spectra, we
tentatively propose that the observed precipitate of 10a has a
polymeric structure because of the hydrogen bonding between
the dmap ligands and the phenolic OH group. Because there are
three dmap molecules per palladium, we do not know which of
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dx.doi.org/10.1021/ic400925y | Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Communication
Scheme 4. Synthesis of the Iodo-Bridged Bimetallic Palladium
Complexes 12 and 13
AUTHOR INFORMATION
Corresponding Author
*E-mail: avigal@post.tau.ac.il..
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a
Author Contributions
All authors have given approval to the final version of the
manuscript.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
We thank the Israel Science Foundation for financial support and
Dr. Sofia Lipstman for help with X-ray analysis.
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a
All hydrogen atoms and oxygen atoms of the nitro groups are
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ASSOCIATED CONTENT
* Supporting Information
Experimental details for compounds 1−13 and X-ray analysis
data (CIF). This material is available free of charge via the
Internet at http://pubs.acs.org.
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dx.doi.org/10.1021/ic400925y | Inorg. Chem. XXXX, XXX, XXX−XXX