Designed synthesis of metal cluster-centered pseudo-rotaxane supramolecular architectures

Article abstract of DOI:10.1021/ja202294v

The designed synthesis and structural characterization of two metal cluster-centered metallosupramolecular architectures are reported. In complex [(CF₃SO₃)Ag₄(^tBuC=C)(Py8)](CF ₃SO₃)₂

Full text of DOI:10.1021/ja202294v

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pubs.acs.org/JACS
Designed Synthesis of Metal Cluster-Centered Pseudo-Rotaxane
Supramolecular Architectures
Cai-Yan Gao, Liang Zhao,* and Mei-Xiang Wang*
The Key Laboratory of Bioorganic Phosphorus Chemistry & Chemical Biology (Ministry of Education), Department of Chemistry,
Tsinghua University, Beijing 100084, China
S Supporting Information
b
[(CF3SO3)Ag4{CtCꢀ(m-C6H4)ꢀCtCꢀ(m-C6H4)ꢀCtCꢀ
ABSTRACT: The designed synthesis and structural char-
(m-C6H4)ꢀCtC}Ag4(CF3SO3)(Py8)2](CF3SO3)4 (2) (Py8 =
acterization of two metal cluster-centered metallosupramo-
lecular architectures are reported. In complex [(CF3SO3)-
Ag4(tBuCtC)(Py8)](CF3SO3)2 (1) and [(CF3SO3)Ag4-
{CtCꢀ(m-C6H4)ꢀCtCꢀ(m-C6H4)ꢀCtCꢀ(m-C6-
H4)ꢀCtC}Ag4(CF3SO3)(Py8)2](CF3SO3)4 (2), organic
acetylide ligands are utilized to induce the formation of
polynuclear silver aggregates, which are encapsulated into
the central cavity of the neutral macrocyclic compound
azacalix[8]pyridine (Py8). The tetrasilver cluster centered
[2]- and [3]-pseudo-rotaxane structures are obtained and
fully characterized by X-ray crystallography, ESI mass
spectrometry, and 1H NMR spectroscopy.
azacalix[8]pyridine), both featuring a pseudorotaxane structure
with the encapsulation of an acetylide-tetrasilver aggregate into a
neutral polypyridine macrocycle.
Considering the rigid linear geometry of acetylene units and
their ready chemical availability via the Pd-catalyzed coupling
reactions,12 we purposefully investigated the assembly of silver
acetylide complexes [AgCtCR]n with the neutral polydentate
macrocyclic ligand Py8. The polymeric silver acetylide complex
[AgCtCtBu]n usually has very poor solubility in common
solvents as reported previously.13 However, treatment of a
suspension of [AgCtCtBu]n and silver triflate with Py8 in a
mixed solvent of methanol and dichloromethane yielded a clear
pale yellow solution. Silver triflate is herein used to increase the
silver ion concentration, which is required for the formation of
multinuclear silver aggregate.13c Light yellow crystals were sub-
sequently acquired by the diffusion of diethyl ether into the
solution. X-ray crystallographic analysis provided the formula of
this crystalline complex 1 as [(CF3SO3)Ag4(tBuCtC)(Py8)]-
(CF3SO3)2.14 The asymmetrical unit of 1 consists of two
independent silver atoms, half Py8 ligand, half tert-butylacetylide
anion, and one and a half triflate groups. As shown in Figure 1a,
the two silver atoms (Ag1 and Ag2) and their inversion-related
ones (Ag1A and Ag2A) constitute a square planar tetranuclear
aggregate, which is held together by both σ- and π-bonding of the
tert-butylacetylide anion in the μ4-η1,η1,η1,η2 mode. The π-type
AgꢀC bonding distance at Ag1ꢀC2 = 2.704(16) Å is longer
than the σ-type ones in the range 2.130(14)ꢀ2.430(19) Å. The
triple bond length in tBuCtCꢀ is 1.195(9) Å, in good agreement
with the values observed in other silver tert-butylacetylide
complexes.15 Relative to the tert-butylacetylide anion, the triflate
group S1 is located at the opposite side of the Ag4 plane and binds
the Ag1ꢀAg2 edge through a simple μ2-O,O0 coordination
he study of metallosupramolecular complexes has emerged
T
as an active research field and attracted intense interest in the
past two decades. Such multicomponent complexes have shown
a variety of potential applications, acting for example as pre-
cursors of electronic,1 photophysical,2 and magnetic materials,3
or used in catalysis,4 molecular recognition,5 and transport.6
Supramolecular coordination assembly based on directional
bonding,7 symmetry interaction,8 and weak linkage9 has proven
to be an effective approach to achieve well-defined metallosu-
pramolecular architectures, wherein most of them are generally
constructed from single metal centers by virtue of their typical
octahedral, square planar, or tetrahedral coordination geome-
tries. In contrast, metallosupramolecular structures involving
polynuclear metal clusters as centers are rarely reported. Although
metal cluster entities have specific physical properties10 and the
incorporation of cluster aggregates into coordination assemblies
may endow abundant functions to the resulting metallosupra-
molecules, however, multivariate coordination geometries of
cluster aggregates make the designed synthesis of cluster-cen-
tered supramolecular structures a formidable task.
mode. Every edge of the Ag4 square (Ag1 Ag2 = 2.941(1)
3 3 3
Å; Ag1 Ag2A = 2.997(1) Å) is much shorter than twice the
3 3 3
van der Waals radius of the silver atom (3.4 Å), suggesting the
existence of argentophilic interaction.16 This Ag4 square aggre-
gate is encapsulated by the central cavity of the Py8 ligand and is
stabilized by the coordination of its four alternate pyridyl
nitrogen atoms, which are coplanar but make a dihedral angle
of 16.3° with the Ag4 square (Figure 1b). Furthermore, a
[(tBuCtC)Ag4(CF3SO3)] moiety with the tert-butylacetylide
anion and the triflate group attached to the Ag4 plane on either
On the other hand, the template-directed approach is often
employed to access multinuclear clusters through the induction of
anionic inner bonding sites of macrocyclic ligands.11 Inspired by such
a cluster-macrocycle model, we envisioned that adscititious anionic
ligands can be judiciously used to dictate the assembly of metal
cluster aggregates upon a neutral macrocyclic ligand as a template
(Scheme 1). Moreover, the peripheral coordination of a macrocyclic
ligand is likely to simplify the coordination modes of metal clusters.
We herein describe the designed synthesis and structural character-
ization of two cluster-centered metallosupramolecular complexes,
namely [(CF3SO3)Ag4(tBuCtC)(Py8)](CF3SO3)2 (1) and
Received: March 13, 2011
Published: May 11, 2011
r
2011 American Chemical Society
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dx.doi.org/10.1021/ja202294v J. Am. Chem. Soc. 2011, 133, 8448–8451
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Scheme 1. Structure of Azacalix[8]pyridine (Py8) and Its
Role in Protecting the Coordination Sites of a Metal Cluster
Figure 3. Partial 1H NMR spectra (600 MHz, methanol-d4) of the pyridyl
and bridging N-Me protons of the Py8 ligand in 1 at different temperatures.
Formation of complex 1 was also confirmed by the electro-
spray ionization (ESI) mass spectrometry, which displays two
isotopically resolved peaks at m/z = 1659.55 and 755.34 corre-
sponding to [1ꢀOTf]þ and [1ꢀ2OTf]2þ, respectively (Figure S1
in the Supporting Information). The 1H NMR spectroscopy of 1
at 298 K shows two broad singlets at 6.87 and 7.81 ppm due to
the pyridyl β- and γ-proton signals of the Py8 ligand, respec-
tively. Both signals experience significant downfield shifts (Δδ =
0.16 and 0.47 ppm, respectively) compared with the chemical
shifts of the free Py8 in solution.19 However, only one set of
proton signals for the Py8 ligand in the solution of 1 conflicts
with its coordination behavior in the crystalline structure, where-
in two kinds of pyridine rings, coordinated and uncoordinated,
can be clearly discriminated. We subsequently collected the
proton NMR of 1 at lower temperatures. As the temperature
decreases from 298 to 213 K (Figure 3), the broad pyridyl
γ-proton signal at 7.81 ppm is gradually split into seven sharp
triplets ranging from 7.6 to 8.3 ppm, and meanwhile the
resonance of the pyridyl β-protons exhibits over ten doublet
and multiplet peaks in the range 6.5ꢀ7.5 ppm. The ratio of the
proton signals in the above two regions is close to 1:2, equal to
the proportion of γ- to β-protons in Py8. Seven singlets for the
methyl groups in the bridging N-Me moieties are observed as
well at 213 K. Moreover, the interconversion of several triplet
and doublet peaks takes place upon the variation of temperature.
For example, the ratio of three triplets at 7.80, 7.75, and 7.65
ppm is approximately 1:1.5:1.5 at 233 K, whereas it is changed to
1:2:2 at 213 K. The NMR studies indicate that the conformation
of the Py8 ligand in 1 is fluxional in solution at room temperature
Figure 1. (a) Tetranuclear silver aggregate coordinated by tert-butyla-
cetylide and triflate in complex [(CF3SO3)Ag4(tBuCtC)(Py8)]-
(CF3SO3)2 (1) with atom labeling (the thermal ellipsoids set at 50%
probability level). The CF3SO3ꢀ and tBuCtCꢀ groups are disordered
and can be located at either side of the Ag4 plane. Only one pair of
CF3SO3ꢀ and tBuCtCꢀ is shown here. (b) Encapsulation of the Ag4
aggregate into the Py8 macrocyclic ligand in 1. Hydrogen atoms and two
1
free triflate groups are omitted for clarity. Symmetry code: A /2ꢀx,
11/2ꢀy, ꢀz. Selected bond lengths and distances (Å): C1ꢀC2
1.195(9); C1ꢀAg1 2.130(14), C1ꢀAg1A 2.407(18); C1ꢀAg2
2.241(17); C1ꢀAg2A 2.430(19); C2ꢀAg1 2.704(16); Ag1ꢀN1
2.245(7); Ag2ꢀN5 2.217(7); Ag1 Ag2 2.941(1); Ag1 Ag2A
3 3 3
3 3 3
2.997(1).
1
relative to the H NMR time scale. The eight pyridyl nitrogen
t
atoms undergo a rapid dissociationꢀrecombination equilibrium
to bond to the central Ag4 aggregate, analogous with the contact
between thread and ring in the reported organometallic rotaxane
structures.17,18
Figure 2. Pseudorotaxane structure in 1 with the BuCtC, CF3SO3,
and Ag4 aggregate shown as a space-filling model. Color scheme for
atoms: Ag, purple; C, black; H, gray; O, red; N, blue; F, cyan.
We next embarked on the employment of a specific angled
organic ligand to direct the construction of cluster-centered
metallosupramolecular architectures. The extending conjugated
ligand 1,3-bis((3-ethynylphenyl)ethynyl)benzene was synthe-
sized, and it reacted with silver nitrate in the presence of trieth-
ylamine to generate a new silver acetylide complex [AgCtCꢀ
(m-C6H4)ꢀCtCꢀ(m-C6H4)ꢀCtCꢀ(m-C6H4)ꢀCtCAg]n
as a white powder (see Supporting Information). A synthetic
procedure similar to that of 1 with [AgCtCꢀ(m-C6H4)ꢀ
CtCꢀ(m-C6H4)ꢀCtCꢀ(m-C6H4)ꢀCꢂCAg]n in place
side is threaded through the Py8 macrocyclic ring, thus giving rise
to a pseudorotaxane structure (Figure 2). To the best of our
knowledge, this structure represents the first example of a cluster-
centered organometallic rotaxane among the reported organo-
metallic17 and hybrid organicꢀinorganic18 rotaxanes. Moreover,
in contrast to the parallelogram 1,3,4,6-alternate conformation of
Py8 in its free crystalline state,19 the Py8 ligand in 1 is folded into
a 1,5-planar-2,4,7-alternate configuration, thus affording a cylin-
der belt (8.86 Å ꢁ 8.97 Å) surrounding the [(tBuCtC)Ag4-
(CF3SO3)] central chain (Figure 2).
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COMMUNICATION
Scheme 2. Synthetic Procedure of Complex 2
Figure 5. Hexagonal nanosized catenane-like structure in 2 built
through the linkage of two semicircle cluster-centered units by F
interactions. Color scheme for atoms: Ag, purple; C, black; H, gray; O,
F
3 3 3
red; N, blue; F, cyan.
2.148(9)ꢀ2.198(8) Å for the π and σ interactions, respectively.
The lengths of the carbonꢀcarbon triple bonds are in the range
1.176(12)ꢀ1.218(13) Å, being consistent with the value ob-
served in complex 1. Each Ag4 aggregate in 2 is encircled by a Py8
ligand through the coordination of six pyridyl nitrogen atoms
with the AgꢀN distances in the range 2.196(8)ꢀ2.690(7) Å,
being different from the alternate 4-fold coordination fashion in
1. The two dissimilar coordination modes of Py8 in 1 and 2
support our assumption that multiform conformations of Py8
coexist in the [CtCAg4Py8] coordination moiety as observed in
1
the cryogenic H NMR of 1. At each terminal of 1,3-bis((3-
ethynylphenyl)ethynyl)benzene in 2, a [(CtC)Ag4(CF3SO3)]
chain is also threaded through a Py8 macrocycle to construct a
pseudorotaxane structure. Two such cluster-centered pseudor-
otaxane units are further bridged by the central phenylene-
acetylene moiety to afford a semicircle structure. This cluster-
centered metallosupramolecular structure can be described as a
[3]pseudorotaxane with reference to the reported organometallic
rotaxanes.17,18 As shown in Figure 5, two cluster-centered semi-
circles are arranged in the face-to-face fashion and are mutually
Figure 4. (a) Formation of tetranuclear silver aggregates at two
terminals in complex [(CF3SO3)Ag4{CtCꢀ(m-C6H4)ꢀCtCꢀ(m-
C6H4)ꢀCtCꢀ(m-C6H4)ꢀCtC}Ag4(CF3SO3)(Py8)2](CF3SO3)4
(2). (b) Coordination mode of the Py8 ligand and top view of the
semicircle cluster-centered structure in 2. The free triflate groups are
omitted for clarity. Selected bond lengths and distances (Å): C97ꢀC98
1.176(12); C121ꢀC122 1.218(13); C105ꢀC106 1.179(13); C113ꢀ
C114 1.208(15); C97ꢀAg1 2.198(8); C97ꢀAg2 2.172(9); C97ꢀAg3
2.399(8); C97ꢀAg4 2.484(10); C98ꢀAg3 2.506(9); C122ꢀAg5
2.148(9); C122ꢀAg6 2.532(9); C122ꢀAg7 2.370(8); C122ꢀAg8
2.157(9); C121ꢀAg7 2.491(9); Ag1ꢀN27 2.229(7); Ag1ꢀN25
2.581(8); Ag2ꢀN31 2.209(7); Ag3ꢀN17 2.690(7); Ag3ꢀN19
2.250(7); Ag4ꢀN23 2.304(9); Ag5ꢀN13 2.205(7); Ag5ꢀN15
2.608(8); Ag6ꢀN1 2.276(7); Ag7ꢀN5 2.196(8); Ag7ꢀN7 2.637(8);
connected by the F F interactions (2.815 Å) between the
3 3 3
coordinated triflate groups, thus generating a hexagonal nanosized
catenane-like structure (24.912 Å ꢁ 33.352 Å). It is remarkable that
the F F contact herein is quite shorter than the sum of the van der
3 3 3
Waals radius of the fluorine atom (2.94 Å) and is among the shortest
fluorineꢀfluorine interactions reported in literatures.20 This cate-
nane-like structure in 2 constructed by silver acetylide clusters is
reminiscent of the reported gold thiolate21 or acetylide catenanes,22
although the latter ones just have a single gold atom as centers.
In summary, we have demonstrated a viable synthetic strategy
to designedly construct metal cluster-centered metallosupramo-
lecular architectures by combining anionic directing ligands with
macrocyclic protecting compounds. The pseudorotaxane unit
composed of polynuclear metal clusters and macrocycles can be
feasibly arranged onto various organic skeletons to form desired
structures. Efforts to construct high-order and mixed cluster-
involved metallosupramolecular architectures and to acquire new
cluster-based functional materials are continuing.
Ag8ꢀN9 2.220(7); Ag Ag 2.866(1)ꢀ3.058(1).
3 3 3
of [AgCtCtBu]n was then applied to yield complex 2
(Scheme 2), which was formulated as [(CF3SO3)Ag4{CtCꢀ
(m-C6H4)ꢀCtCꢀ(m-C6H4)ꢀCtCꢀ(m-C6H4)ꢀCtC}-
Ag4(CF3SO3)(Py8)2](CF3SO3)4 based on ESI-MS and single
crystal X-ray analysis vide infra. The proton NMR spectrum of
2 at 298 K reveals only one set of signals for the Py8 ligand as
well, wherein two broad singlet peaks at δ = 6.73 and 7.73
ppm can be ascribed to the pyridyl β- and γ-protons, respectively.
The ESI-MS analysis of 2 shows two peaks at m/z = 1740.55 and
1111.09 corresponding to the [2ꢀ2OTf]2þ and [2ꢀ3OTf]3þ
species, respectively, and their isotopic resolution is in excellent
agreement with the theoretical distribution (Figure S1).
In the crystal structure of 2 (Figure 4a), the two terminal
acetylide anions each bonds to an approximately planar Ag4
aggregate (deviation 0.054ꢀ0.060 Å) in the μ4-η1,η1,η1,η2
mode. The AgꢀC bond distances between the Ag4 caps and
the acetylide groups lie in the range 2.370(8)ꢀ2.532(9) Å and
’ ASSOCIATED CONTENT
S
Supporting Information. Experimental procedures and
b
crystal structure determination details. ESI mass spectra of 1 and 2.
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Journal of the American Chemical Society
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X-ray crystallographic data for 1 and 2 in CIF format. This
material is available free of charge via the Internet at http://pubs.
acs.org.
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’ AUTHOR INFORMATION
Corresponding Author
zhaolchem@mail.tsinghua.edu.cn (L.Z.); wangmx@mail.tsinghua.
edu.cn (M.-X.W.)
’ ACKNOWLEDGMENT
This work is dedicated to the centennial celebration of
Tsinghua University. Financial support by the National Natural
Science Foundation of China (21002057) and the National Basic
Research Program of China (973 program, 2011CB932501) is
gratefully acknowledged. We are grateful to Prof. Thomas C. W.
Mak at the Chinese University of Hong Kong for helpful discus-
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