Transformative binding and release of gold guests from a self-assembled Cu8L4 tube

Article abstract of DOI:10.1002/anie.201108450

Totally tubular: A linear M₈L₄⁸⁺ receptor, which binds tightly and selectively to the dicyanoaurate anion, was assembled from simple organic subcomponents and copper(I) ions. The guest complex is not bound unchanged, but instead is transformed into a longer linear complex where two dicyanogold units are bridged by a central cation of copper/silver (see scheme). This complex optimally fills the cavity of the receptor but is not observed in the absence of the host.

Full text of DOI:10.1002/anie.201108450

Angewandte
Chemie
DOI: 10.1002/anie.201108450
Host–Guest Systems
Transformative Binding and Release of Gold Guests from a Self-
Assembled Cu8L4 Tube**
Wenjing Meng, Jack K. Clegg, and Jonathan R. Nitschke*
The highly specific binding, transformation and protection of
chemical compounds are functions associated with biomolec-
ular systemsꢀ inner phases,[1] pockets of space that are well-
isolated from the external environment. A growing number of
abiological host molecules have been developed to emulate
these functions.[2] Container molecules have been developed
that can encapsulate xenon[3] and sulfur hexafluoride[4] with
the specificity that hemoglobin and myoglobin exhibit when
binding and transporting dioxygen within the body.[5] The
ability of enzymes to transform substrates by binding to the
transition state of a reaction has inspired the use of container
molecules to catalyze reactions[6] and the protection of the
highly reactive active sites of nitrogenases[7] from atmospheric
oxidation,[8] has been mimicked, allowing sensitive com-
pounds to be stabilized within synthetic hosts.[9]
crystallized separately, allowing the structures of both to be
determined (Figure 2). In both isomers, four self-assembled
ligands, each formed from one residue of A and four 2-formyl-
6-methylpyridine residues, are observed to wrap around eight
CuI template ions to create tube-like complexes with approx-
imate D2d and D4 point symmetries, in which the copper(I)
ions form an elongated cuboidal structures. The ligands adopt
different conformations in these two diastereomers of 1, as
shown in Figure 1. In 1-D2d, the long faces of the cuboid form
isosceles trapezoids, with the shorter faces forming rectangles.
The parallel ligands of 1-D2d thus come together in such a way
as to eliminate internal void volume, as shown in Figure 2c
and d. In 1-D4, the cuboid approximates a right square prism
in which one of the square faces is twisted by 408 with respect
to the other. This ligand arrangement results in a narrow
tubular channel having a radius of ca. 2.1 ꢁ and a volume of
193 ꢁ3.[15] In the crystal structure two acetonitrile molecules
were found encapsulated in this channel (Figure 2a and b).
The 1-D2d and 1-D4 diastereomers in the solid state were
Whereas nature makes use of narrow tubular channels for
purposes ranging from carbon monoxide reduction[10] to ion
transport,[11] most synthetic capsules have compact binding
cavities.[2a,12] In order to investigate the specific binding and
transformation of linear substrates within rigid tubular hosts,
we designed and synthesized tetramine subcomponent A
(Figure 1). Based on modeling studies[13] and our prior
experience with copper(I)-templated subcomponent self-
assembly,[14] we predicted A to have the correct geometry to
1
also observed in solution by H and 13C NMR spectroscopy.
The different symmetries of these isomers led to different
NMR peak multiplicities. Kinetic studies (described in the
Supporting Information) revealed activation enthalpies and
entropies of 148 Æ 5 kJmolÀ1 and 134 Æ 15 JKÀ1 molÀ1 respec-
tively for the isomerization from 1-D4 to 1-D2d, and 85 Æ
7 kJmolÀ1 and À62 Æ 21 JKÀ1 molÀ1 for the reverse trans-
formation (from 1-D2d to 1-D4). The rate constants for both
transformations were identical at 323 K, marking 1-D4 as the
dominant species in solution below this temperature, and 1-
D2d above.
As the interior of 1-D4 was observed to accommodate two
acetonitrile molecules in the crystal, we reasoned that other
linear guests[16] might also bind within this host. No new peaks
were observed in the 1H NMR spectrum, however, following
the addition to an acetonitrile solution of 1 (1.8 mm) of either:
1) the potassium salts of Ag(CN)2À, Cu(CN)2À, CNÀ, OCNÀ,
SCNÀ, SeCNÀ, N3À, H2FÀ, or FÀ (1 equiv in each case),
2) CuCN, Ni(CN)2, Hg(CN)2, CS2, 1,4-dichlorobut-2-yne,
succinonitrile, butyronitrile, C6F6, or but-2-yne (5 equiv), or
3) N2O, C2H4, or C2H2, (by bubbling the gas through the
acetonitrile solution for 5 min at 258C), suggesting that no
guest binding occurred.
8+
assemble into a Cu8L4 host with a narrow central channel.
Indeed, A, 6-methyl-2-formylpyridine and tetrakis(aceto-
nitrile)copper(I) tetrafluoroborate reacted in the ratios
shown in Figure 1 to form the deep red-purple product 1 in
acetonitrile. Electrospray ionization mass spectra (ESI-MS)
and elemental analysis of 1 were consistent with the formula
[Cu8L4](BF4)8, but 1H and 13C NMR spectra indicated the
presence of two distinct product structures.
Vapor diffusion of diethyl ether into an acetonitrile
solution of 1 led to the isolation of opaque crystals having
two different crystalline aspects. Single-crystal X-ray diffrac-
tion experiments revealed that two isomeric structures had
[*] W. Meng, Dr. J. K. Clegg, Dr. J. R. Nitschke
University of Cambridge, Department of Chemistry
Lensfield Road, Cambridge, CB2 1EW (UK)
E-mail: jrn34@cam.ac.uk
Homepage: http://www-jrn.ch.cam.ac.uk/
[**] This work was supported by the European Research Council, the US
Army Research Office, and the Marie Curie IIF Scheme of the 7th EU
Framework Program. We thank the EPSRC Mass Spectrometry
Service at Swansea for FT-ICR MS experiments, Dr. Peter Grice for
help with NMR experiments, and the EPSRC National Crystallog-
raphy Service and Dr. John E. Davis for crystallographic data
collection.
Despite these other guestsꢀ failure to bind, the addition of
KAu(CN)2 to an acetonitrile solution of 1 produced a new
host–guest complex 2, as identified by NMR spectroscopy
(Figure S40, Supporting Information) and ESI-MS. Mass
spectra indicated that the dicyanoaurate adduct of 1 was not
a simple 1:1 complex, however, but rather that the guest
species was the complex anion Cu(Au(CN)2)2À, leading to the
formulation of 2 as [Cu(Au(CN)2)2ꢀ1-D4]7+ (Figure 3). The
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201108450.
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Figure 1. Formation of the two diastereoisomers of Cu8L4 complex 1, 1-D4 and 1-D2d, by subcomponent self-assembly.[17]
When a 1:1 stoichiometry of KAu(CN)2:1 was employed,
the CuI required to generate the Cu(Au(CN)2)2 guest was
À
obtained through decomposition of a portion of the host
molecules present. This partial host consumption was indi-
cated by the appearance of free 6-methyl-2-formylpyridine,
which was observed by 1H NMR in proportion to the amount
of 2 produced, together with 2, 1-D4, and 1-D2d in solution
(Figure S22). When host 1, KAu(CN)2, and Cu(CH3CN)4BF4
were mixed in a 1:2:1 ratio, host decomposition was sup-
pressed; analysis of the product of this reaction enabled the
calculation of the stability constant of 2 to be 1.21 ꢂ 109 mÀ3
.
Further evidence for the structure of 2 was provided
through its preparation from 13C-labeled KAu(CN)2. In the
13C NMR spectrum (Figure S26), the 13C-labeled guest gave
rise to a pair of coupled doublets (2JC-C = 47.6 Hz), consistent
with the two inequivalent carbon environments of the NC-
Au-CN-Cu-NC-Au-CN anion observed in solution, whereas
free KAu(13CN)2 showed one singlet. The addition of excess
KAu(13CN)2 allowed for the observation of both [Cu(Au-
(13CN)2)2ꢀ1-D4]7+ and free KAu(13CN)2 simultaneously.
When KAu(13CN)2 and Cu(CH3CN)4BF4 were mixed in a
molar ratio of 2:1 in CD3CN or dimethylsulfoxide (DMSO) in
the absence of 1 an insoluble precipitate formed and only free
dicyanoaurate was observed in solution; no evidence of
Cu(Au(CN)2)2À was observed. This observation suggests that
the linear binding pocket of 1-D4 served as a protective cover
Figure 2. Crystal structures of the two diastereoisomers of 1. All views
omit solvent molecules and anions, and non-space-filling views omit
hydrogen atoms, for clarity. a,b) 1-D4, with one ligand highlighted and
the two encapsulated acetonitrile molecules emphasized as space-
filling representations: a) side view down the pseudo-C2 axis; b) space-
filling representation of top view down the pseudo-C4 axis. c,d) 1-D2d
with two ligands of different orientations highlighted in lighter shades:
c) view down the pseudo-mirror plane; d) space-filling representation
of view down the pseudo-C2 axis.
À
for the complex Cu(Au(CN)2)2 anion, suppressing the
presence of bridging cyanides was confirmed by the observa-
formation of cyanide-bridged insoluble polymeric and oligo-
meric species.[18,19] In seeking thermodynamic equilibrium,
this host–guest system may thus be observed not only to
stabilize an otherwise inaccessible guest,[9] but to generate an
optimal guest from amongst the different possibilities latent
ꢁ
tion of two C N resonances in the infrared spectrum (2144
and 2190 cmÀ1) at higher wavenumbers than free dicyanoau-
rate (2142 cmÀ1).[18]
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Angewandte
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guests of 2 and 3 were not stable in DMSO solution;
dissolution of 2 or 3 prepared with 13C-labeled
Au(CN)2 in DMSO gave only a simple 13C NMR
À
signal corresponding to free Au(CN)2À. Evaporation of
the DMSO and addition of acetonitrile resulted in the
partial regeneration of 1-D4 and 2 or 3 along with
insoluble material which we infer to be oligomer or
[18,19c] We attribute
À
polymer generated from Au(CN)2
.
the instability of the host–guest complexes 2 and 3 in
DMSO to the greater ability of this more polar solvent
À
to solvate the separated 18+ and Au(CN)2 ions; less-
polar acetonitrile would be expected to favour the ion
pairing required for host–guest complex formation.
DMSO also appears too large to fit into the cavity of 1-
D4, thus depriving this diastereomer of the inner-cavity
solvation that likely stabilizes it in acetonitrile.
Host 1 thus displays an ability to bind gold with
high affinity and remarkable selectively, not through
simple encapsulation of Au(CN)2À, but by transform-
ing this anion into an optimal guest that is not observed
in the hostꢀs absence. This behavior emerges from the
complex interactions of the parts of a system, as it
reconstitutes itself for binding during the process of
thermodynamic equilibration. This process comprises
several distinct chemical events, including 1) the elimination
of the D2d isomer of 1, the cavity of which is too small, 2) the
consumption of part of the host, if required, to provide a
necessary copper(I) building block for the optimal guest, and
3) the construction of this optimal guest by arranging two
dicyanoaurate units around a single linearly coordinated
copper(I) or silver(I) ion. This adaptive response to pertur-
bation is much less complex than what is observed in
biological systems, but the response nonetheless occurs at a
system-wide level and achieves the potentially useful function
of tight and selective binding of dicyanoaurate, a substrate of
considerable economic value. Given current record gold
prices, the ability to specifically bind and release dicyanoau-
rate, the form in which gold is extracted from ores in modern
mines,[22] might allow for economic and environmental
benefits to be realized during the extraction and refining of
this increasingly useful metal.[23]
Figure 3. Transformations between host and host–guest complexes.
in the system, although this guest has no observed independ-
ent existence.
À
The systemꢀs ability to discriminate between Au(CN)2À
Ag(CN)2À is also remarkable. We hypothesize that Ag(CN)2
is not a competent guest for 1 because of the ability of silver(I)
to adopt a wide range of coordination geometries, whereas
gold(I) complexes tend to adopt a linear 2-coordinate
geometry.[20]
Although no evidence of interaction was observed
between KAg(CN)2 and 1, AgI was able to take the place of
the central CuI ion within 2 to generate complex 3,
[Ag(Au(CN)2)2ꢀ1-D4]. This complex could be prepared
under identical conditions to those used to make 2 when
AgBF4 was substituted for Cu(CH3CN)4BF4. The titration of
AgBF4 into an acetonitrile solution of 2 resulted in the
displacement of CuI by AgI within the guest, allowing the
stability constant of 4.2 ꢂ 1010 mÀ3 for 3 to be obtained, 34
times greater than that of 2; the larger AgI center appears
better stabilized by van der Waals contacts and favorable
interactions with the p-electron density[21] of the host frame-
work than the smaller CuI. The 13C NMR spectrum of 3
prepared using 13C-labeled dicyanoaurate was consistent with
a NC-Au-CN-Ag-NC-Au-CN guest structure, wherein the
signals of the carbon atoms closest to silver were further split
due to coupling to the NMR-active silver centers (Figures S34
and S35).
CCDC 846044 and 846045 contain the supplementary
crystallographic data for this paper. These data can be
obtained free of charge from The Cambridge Crystallo-
graphic Data Centre via www.ccdc.cam.ac.uk/data_request/
cif.
Received: November 30, 2011
Published online: January 17, 2012
Keywords: coordination chemistry · host–guest systems ·
The addition of excess KAu(CN)2 to 1 did not result in
.
À
metal–organic capsules · self-assembly ·
supramolecular chemistry
generation and encapsulation of the Au(Au(CN)2)2 guest;
only [Cu(Au(CN)2)2ꢀ1-D4]7+ was observed, followed by
precipitation of the complex once five equivalents had been
added. The use of other Au+ precursors with less strongly
bound ligands is under investigation.
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