Supramolecular tetrahedra of phosphines and coinage metals

Article abstract of DOI:10.1002/anie.201200730

Treasure chests: The Group 11 metals Cu^I, Ag^I, and Au^I form tetrahedral supramolecular clusters with a rigid threefold-symmetric phosphine ligand. These supramolecular clusters, which assemble by metal-phosphine interactio

Full text of DOI:10.1002/anie.201200730

Angewandte
Chemie
DOI: 10.1002/anie.201200730
Coinage-Metal Supramolecules
Supramolecular Tetrahedra of Phosphines and Coinage Metals**
Sang Ho Lim, Yongxuan Su, and Seth M. Cohen*
Supramolecular metal–ligand clusters, sometimes referred to
as metal–organic polyhedra (MOPs), have been a major area
of chemical research for at least 20 years. The groups of
described with many other metal–ligand systems, have been
reported. The supramolecular tetrahedra prepared herein
(Scheme 1) with Cu , Ag , and Au are the first to realize this
challenging target in metal–ligand self-assembly.
[
1]
I
I
I
[
2]
[3]
[4]
Stang, Raymond, Fujita, and many others have discov-
[5]
ered an incredibly wide array of such molecules. Many of
the assemblies have demonstrated spectacular host–guest
chemistry, including the ability to catalyze chemical reac-
[
6]
tions. These clusters have also been linked together or
assembled in such a way as to generate solid-state materials,
such as metal–organic frameworks (MOFs, or “crystalline
[
7]
molecular flasks”). Despite the vast literature on these types
of self-assembled structures, such constructs have only been
rarely assembled from soft Lewis base ligands derived from
second-row heteroatoms (for example, sulfur and phospho-
rus). Such ligands would be expected to form a distinct class of
assemblies in conjunction with lower oxidation-state metal
ions or metalloids. Indeed, Johnson and co-workers have
successfully used thiol-based ligands to obtain self-assembled
III
III
III
III [8]
Scheme 1. Synthesis of the ligand tppeb and [M
TMS=trimethylsilyl.
(tppeb) ]X clusters.
4 4 4
clusters based on P , As , Sb , and Bi . Herein, we
present a unique series of tetrahedral clusters derived from
a rigid tris(diphenylphosphine) ligand and the soft Lewis acid
coinage-metal ions of Group 11. To best of our knowledge,
this is the first time isostructural supramolecular structures of
a platonic solid (tetrahedron) have been prepared with all
members of a transition metal group of the periodic table.
Some supramolecular assemblies have been described
with multidentate phosphine ligands. Of particular note is
By analogy to the many rigid, threefold-symmetric ligands
that have been successful used to prepare discreet supra-
[
13]
molecular complex ions,
phosphino)ethynyl)benzene (tppeb) was prepared as de-
the ligand 1,3,5-tris((diphenyl-
[14]
scribed previously (Scheme 1).
Combining a Group 11
[
9]
[10]
work from the groups of Yip and James, both of whom
metal iodide (CuI, AgI, AuI) with the tppeb ligand using
a 1:1 stoichiometry in CH Cl solvent results in the isolation of
I
have prepared self-assembled structures from M -phosphine
2
2
interactions (M = Ag, Au). From a variety of rigid, multi-
dentate phosphine ligands these groups have constructed
rings, helicates, and a small, adamantoid-shaped cluster. The
a pale yellow solid (after removal of solvent) in high yield (96,
95, and 86% for Cu, Ag, and Au, respectively). H NMR
analysis confirms that the isolated solids are a high-symmetry
species (see below).
1
I
I
use of Ag and Au has also engendered some of these
[
11]
supramolecular species with luminescent properties. Fur-
thermore, Hahn and co-workers have also prepared cylinder-
typed supramolecular structures combining polydentate N-
heterocyclic carbene (NHC) ligands with coinage-metal ions
The isolated powders could be recrystallized by various
methods. Slow diffusion of Et O into a saturated solution of
2
the material in CHCl , CH Cl , or acetone gave colorless
3
2
2
crystals in high yield (ca. 90% from the powder) within one
day. However, these crystals were not of suitable quality for
single-crystal X-ray diffraction. Alternatively, layering of
Et O or n-pentane over these same solutions in CHCl ,
[
12]
by metal-controlled self-assembly. Despite the impressive
progress made by these groups and others, no large, three-
dimensional structures, reminiscent of the platonic solids
2
3
CH Cl , or acetone gave crystals of similar quality. Crystals
2
2
suitable for single-crystal X-ray diffraction were produced
from a saturated (prepared hot) DMF solution of the clusters,
although the yields obtained were lower and the crystalliza-
tion was slower. The single-crystal X-ray structure of the three
clusters was determined, all of which crystallized in the chiral
space group P2 2 2 (Supporting Information, Table S1),
[
*] Dr. S. H. Lim, Y. Su, Prof. Dr. S. M. Cohen
Department of Chemistry and Biochemistry
University of California, San Diego
9500 Gilman Drive, La Jolla, CA 92093 (USA)
E-mail: scohen@ucsd.edu
1
1 1
[
**] We thank P. Dau (UCSD) for assistance with crystallography and J.
Cirera and Prof. F. Paesani (UCSD) for helpful discussions. This
work was supported by a grant from the Department of Energy (BES
grant no. DE-FG02-08ER46519).
revealing that this series of supramolecules were essentially
isostructural (Figure 1). In all of the clusters, the phosphine
ligands occupy a pyramidalized, threefold-symmetric coordi-
I
nation geometry around the M coinage metal. The overall
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201200730.
I
geometry of the M ions is tetrahedral, with the apical site
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.
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Figure 1. Crystal structure of the [M (tppeb) ]I clusters. Each cluster is
4
4 4
shown in a distinct depiction: the complete [Cu (tppeb) ]I cluster is
4
4 4
shown as sticks (left); to emphasize the cluster geometry, the [Ag4-
tppeb) ]I cluster is shown with phenyl groups removed and each
(
4
4
ligand in a different color (middle); and the [Au (tppeb) ]I cluster is
4
4 4
shown as a space-filling model (right).
occupied by a tightly bound iodide ligand (Figure 1). A minor
difference between the clusters is the range of the metal–
phosphine distances, which are 2.28–2.31 ꢀ, 2.49–2.52 ꢀ, and
2
.37–2.41 ꢀ in the Cu, Ag, and Au clusters, respectively.
Similarly, the metal–iodide distances are about 2.60, 2.76 ꢀ,
and 2.82 ꢀ in the Cu, Ag, and Au clusters, respectively. The
metal–phosphine and metal–iodide bond distances are in
good agreement with those of metal–phosphine and metal–
iodide bonds in other four-coordinate phosphine-based
Figure 2. High-resolution ESI-MS of the [Ag (tppeb) ]I cluster: The
4
4 4
positive-ion mode measured spectrum (top), magnification of the
+
+
[
M+Na] base peak (bottom left), and the simulated [M+Na]
[
15]
metal(I) iodide complexes. There do not appear to be any
strong interactions, such as hydrogen bonding, between the
clusters and the solvate molecules (DMF) present in the
crystals. No solvent guests are found inside of the clusters,
(bottom right) spectrum.
not the base peak, as this complex showed significantly more
fragmentation in the spectrum.
3
which are too small to accommodate such guests (< 60 ꢀ ).
1
A critically important feature of supramolecular coordi-
nation clusters is their persistence in solution, and not just in
the solid state. It is in solution that many such clusters
demonstrate their most interesting reactivity. Therefore, ESI-
Along with the aforementioned ESI-MS data, H NMR
3
1
and P NMR spectroscopy confirmed the robust nature of
these [M (tppeb) ]I clusters in solution. Figure 3 shows the
4
4
4
1
31
H and P NMR spectra of free tppeb and of [Cu (tppeb) ]I .
In the H NMR spectrum, the proton signals of free tppeb
appear at 7.37–7.41 (for H and H ) and 7.64–7.68 (for H and
4
4
4
1
31
1
MS, H NMR, and P NMR analysis of the [M (tppeb) ]I
4
4
4
clusters was performed to determine their stability in solution.
Samples of all three clusters dissolved in CH Cl and diluted
a
b
c
1
H ). In contrast, the H NMR spectrum of [Cu (tppeb) ]I
4
2
2
d
4
4
with acetone yielded high-resolution ESI-MS spectra that
unambiguously showed the persistence and elemental com-
position of these clusters in solution. As shown in Figure 2,
the positive-ion mode ESI-MS spectrum of [Ag (tppeb) ]I
4
shows a significant shift of the resonances and thus was easily
distinguishable from the free ligand. The signals of Hortho (Hc’
and H ’’) were observed at d = 7.83 and 7.97 ppm as broad
c
1
multiplets. Indeed, all of the H NMR resonances of the
4
4
shows several peaks indicative of the fully intact supra-
cluster indicate an asymmetry between the two phenyl rings
on each phosphorous atom (two distinct environments,
Hmeta = H ’ and H ’’ at d = 6.82, 7.35 ppm; H = H ’ and
molecule. The base peak at m/z 3773.9502 corresponds to the
+
[
M+Na] exact mass, which confirms the molecular formula
b
b
para
a
of the cluster and is further supported by the simulated
isotopic spectrum of this peak (Figure 2). The [M+Na] base
H ’’ at d = 7.09, 7.39 ppm), which is consistent with X-ray
a
+
structure of the clusters (peak assignments were facilitated by
a COSY NMR experiment; Supporting Information, Fig-
ure S4). The singlet at 7.57 ppm is attributed to the protons of
peak indicates a strong association of the iodide ligands with
the cluster, suggesting that in solution the complex is best
considered as a neutral molecule (that is, Ag (tppeb) I as
3
1
the central benzene ring in tppeb (H ). In the P NMR
4
4
4
d
opposed to the complex ion [Ag (tppeb) ]I ). Additional
spectrum of [Cu (tppeb) ]I , the peak of free tppeb
4
4
4
4
4
4
+
peaks are seen in the ESI-MS corresponding to [M+K] (m/z
(À34.02 ppm) is no longer present and a new resonance at
+
+
3
(
787.9175), [MÀI+acetone] (m/z 3680.0117), and [MÀI]
8.06 ppm was observed (Figure 3). For Ag (tppeb) ]I , similar
4
4
4
1
31
m/z 3623.0429), all providing conclusive evidence for the
patterns were observed in H and P NMR spectra (Support-
ing Information, Figure S5) with all of the chemical shifts
being quite similar, except the protons of central benzene
stability of [Ag (tppeb) ]I in solution (Supporting Informa-
4
4
4
tion, Figure S1). Similar positive-ion mode ESI-MS were
obtained for [Cu (tppeb) ]I and [Au (tppeb) ]I . For [Cu -
moiety (H ), which exhibit more upfield shifts (d = 7.19 ppm)
4
4
4
4
4
4
4
d
+
(
tppeb) ]I , a [M+Na] (m/z 3595.0447) base peak was also
than those of [Cu (tppeb) ]I . Unfortunately, efforts to obtain
4
4
4
4
4
1
observed (Supporting Information, Figure S2), while for
H NMR of [Au (tppeb) ]I were not successful owing to
4 4 4
poorer solubility of this cluster. However, P NMR in hot
+
31
[
Au (tppeb) ]I a [M+Na] (m/z 4129.1965) peak was
4
4
4
observed (Supporting Information, Figure S3), but it was
[D ]DMF solution gave a resonance at 7.19 ppm consistent
7
2
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from the [M (tppeb) ]I clusters. [Ag (tppeb) ](OTf)
4
4
4
4
4
4
also crystallizes in a noncentrosymmetric space group,
P6 , with the cluster sitting on a threefold symmetry axis
3
such that that one third of the cluster resides in
I
asymmetric unit. There are two distinct Ag centers in
I
asymmetric unit. One Ag ion is tightly bound to one of
oxygen atoms of a triflate counter ion (2.43 ꢀ) and three
phosphorous atoms of the tppeb ligands (2.46, 2.48, and
I
2
.50 ꢀ, respectively). The other Ag ion is bound to three
identical phosphorous atoms (AgÀP distance 2.51 ꢀ)
generated by threefold symmetry; however, no close
I
contact between this Ag ion and a triflate anion is
present, suggesting that indeed the effective charge of
the cluster can be modulated by changing the counter-
I
ion. Interestingly, this Ag center still shows a tetrahedral
geometry despite a vacant apical coordination site, with
P-Ag-P bond angles of about 109.88, which is close to
ideal tetrahedral geometry. The formation of the cluster
1
in organic solution was confirmed by H NMR spectros-
copy (Supporting Information, Figure S8); however, the
spectrum is not as well resolved as for [Ag (tppeb) ]I ,
4
4
4
perhaps suggesting lower stability in solution. This
observation is supported by ESI-MS experiments on
[
Ag (tppeb) ](OTf) , which did not yield the expected
4 4 4
ions. This is also consistent with a hypothesis that with
weakly bound anions a more labile cluster species is
produced that cannot survive under the ESI ionization
conditions.
In conclusion, a new class of supramolecular metal–
ligand clusters has been prepared. These clusters have
several unusual features, including being comprised of
metal–phosphine bonds, utilizing Group 11 coinage
metals, modulation of composition/charge by employing
of different counteranions, and formation an isostruc-
tural series over an entire group of the periodic table.
These findings suggest that these and related clusters can
be prepared that have several modular features that can
be controlled by choice of ligand, metal, and counterion.
Furthermore, the growing use of coinage metals as
catalysts suggest such clusters might serve as unusual
1
31
Figure 3. H and P NMR (inset) spectra of free tppeb (top) and of
Cu (tppeb) ]I (bottom). The PPh reference is labeled with an asterisk.
[
4
4
4
3
[
16]
with the spectra of the other clusters (Supporting Informa-
tion, Figure S6).
scaffolds for new catalyst design. Studies along these lines
are presently underway.
As noted above, the [M (tppeb) ]I series of compounds
4
4
4
appear to behave as neutral clusters, based on the strong
association of the halide ligand with the Group 11 metal ions.
This is supported by both the X-ray structures and positive-
ion mode, high-resolution ESI-MS of all three clusters. Based
on this observation, it was hypothesized that synthesizing
these supramolecules from a metal salt with a weakly
coordinating anion could modulate the composition and
charge of the cluster. This would provide an unusual but
simple way to control the physiochemical properties of these
self-assembled systems. Indeed, combining AgOTf with tppeb
under similar reaction conditions as described above gave the
Received: January 26, 2012
Published online: && &&, &&&&
Keywords: clusters · Group 11 metals · phosphines ·
.
self-assembly · tetrahedra
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Coinage-Metal Supramolecules
S. H. Lim, Y. Su,
S. M. Cohen*
&&&& — &&&&
Supramolecular Tetrahedra of
Phosphines and Coinage Metals
Treasure chests: The Group 11 metals
by metal–phosphine interactions, are the
first to form across an entire group of the
I
I
I
Cu , Ag , and Au form tetrahedral supra-
molecular clusters with a rigid threefold-
symmetric phosphine ligand. These
+
+
periodic table. Cu green, Ag pale blue,
+
À
Au green, I red, P purple.
supramolecular clusters, which assemble
Angew. Chem. Int. Ed. 2012, 51, 1 – 5
ꢀ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.angewandte.org
5
These are not the final page numbers!