A fully dynamic five-component triangle via self-sorting

Article abstract of DOI:10.1039/c0cc00191k

Clean fabrication of a five-component supramolecular triangle was elaborated via self-sorting. The robustness of the dynamic triangle against external stimuli was challenged by varying both the metal ions and their ratios. Self-sorting of five components proved to be superior to that of four components.

Full text of DOI:10.1039/c0cc00191k

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A fully dynamic five-component triangle via self-sortingw
Michael Schmittel* and Kingsuk Mahata
Received 2nd March 2010, Accepted 13th April 2010
First published as an Advance Article on the web 5th May 2010
DOI: 10.1039/c0cc00191k
Clean fabrication of a five-component supramolecular triangle
was elaborated via self-sorting. The robustness of the dynamic
triangle against external stimuli was challenged by varying both
the metal ions and their ratios. Self-sorting of five components
proved to be superior to that of four components.
It is an important mission of supramolecular science to mimic
the complexity and richness of biological self-assembly and
self-organisation. Hence, the fabrication of multicomponent
nano-assemblies has received considerable attention over the
past years.1,2 However, an increasing number of components
not only opens a venue for growing emergent properties, but
equally augments the intricacy of all competing interactions,3
in such a way that supramolecular self-assembly of multiple
building blocks into a single species becomes increasingly
tricky. It thus comes as no surprise that the majority of all
known metallo-supramolecular aggregates consists of just one
type of ligand and one sort of metal ion, whereas architectures
with multiple ligands (n Z 2) and multiple metal ions (n Z 2)
are found to be scarce.
Scheme 1 Self-sorting in a six-component library.
not yet realised well. To the best of our knowledge there is only
a single report on self-assembled isosceles triangles,9 realised
through a combination of the HETTAP10 and PHENLOCK11
methodologies. While two different metal–ligand combinations
were used, only one of them was truly dynamic, the other one
was kinetically locked. Using exclusively dynamically exchanging
components we present herein the fabrication of a fully
supramolecular isosceles triangle, an architecture, which needs
precise constitutional and positional control.
Nature artfully combines multiple components by means of
self-sorting.4 Thus, the transfer of connatural algorithms to
artificial supramolecular species has received considerable
attention over the past two decades.5–7 Self-sorting in metallo-
supramolecular structures is managed by various factors, such
as steric and electronic effects, p–p interactions, and metal–ion
coordination specifics. After extensive optimisation, we have
recently described a dynamic library comprising six different
components (metal ions and ligands) and allowing for twenty
different possibilities, from which only two metal complexes
emerged (Scheme 1). As a proof of concept, the formation of a
five-component (three ligands, two metal ions) supramolecular
trapezoid was exemplified.6 Herein, we report on the fabrication
of the first five-component triangle by exploiting the above
basis set of components in a different combination. The
resultant isosceles triangular species proved to be robust while
changing factors, such as excess, variation and ratio of
metal ions.
In order to establish the required positional control, we
implemented the coordination motifs of 1–4 (Scheme 1) into
the multitopic ligands 5–7 (Fig. 1), using a different combination
as in ref. 6. The information contained in ligand 3 was
implemented into the ditopic component 5, the latter being
readily accessible by means of a Sonogashira cross-coupling
reaction.12 The coordination properties of ligands 1 and 4 were
instated in the unsymmetrical bisphenanthroline 6, conveniently
prepared via stepwise Sonogashira cross-coupling reactions
(ESIw). The coordination features of ligands 2 and 4 were
implemented in the terpyridine–phenanthroline hybrid 7,
easily synthesised using a known procedure.6 With the ditopic
Among all conceivable cyclic 2D nano-architectures, the
triangle is the conceptually simplest supramolecular assembly
and therefore its fabrication has initiated ample efforts.8 Even
though the common equilateral triangle has been studied
in extenso, all other architectures (isosceles and scalene) are
Universitat Siegen - FB 8 (Chemie - Biologie), Adolf-Reichwein Str.,
Siegen D-57068, Germany. E-mail: schmittel@chemie.uni-siegen.de
w Electronic supplementary information (ESI) available: Synthetic
procedure and spectra for 6. DPV, 1H-, 13C- and DOSY-NMR spectra
of T and ESI-MS of all relevant compounds. See DOI: 10.1039/
c0cc00191k
Fig. 1 Ligands used in the current study.
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Scheme
2 Synthesis of the triangular assembly T (only syn
Fig. 3 Partial 1H NMR (400 MHz, 298 K, CD3CN) spectra of an
equimolar mixture of 5, 6, 7 in the presence of (a) 3 equivalents of
Cu+, (b) 3 equivalents of Zn2+ and (c) 3 equivalents of a Cu+ and
Zn2+ (2 : 1) metal salt mixture.
diastereomer is shown).
ligands at hand, self-sorting was expected to lead to
preferential formation of the [Zn(6phenAr2)(7terpy)]2+ subunit
and consequential closure of the triangle T via [Cu(5)(7phen)]+
and [Cu(5)(6phen)]+ (Scheme 2).z
constitutional difference in the chiral heteroleptic copper(I)
complex [Cu(6)(7)]+ and due to the occurrence of two
diastereomers.13 The 1H NMR of the assembly indeed showed
eight singlets between 2.73–3.06 ppm (Fig. 3c). Integration of
the two sets of singlets allowed us to determine the ratio of the
In a first set of experiments, we combined the ligands 5, 6, 7 in
an equimolar ratio in the presence of two equivalents of Cu+ and
one equivalent of Zn2+ and refluxed the mixture in acetonitrile/
dichloromethane (3 : 1) for 3 h. After obtaining a clear red
solution we characterised the reaction mixture using mass
spectroscopy, 1H and DOSY (diffusion-ordered spectroscopy)
NMR, differential pulse voltammetry (DPV) and elemental
analysis. The electrospray ionisation mass spectrum (ESI-MS)
of the reaction mixture evidenced the clean formation of the
triangular species T = [Cu2Zn(5)(6)(7)](OTf)2(PF6)2 (Scheme 2).
In the accessible spectral region of m/z = 150–2000 only three
intense peaks were observed, all of them corresponding to
triangle T (Fig. 2). The most abundant peak at m/z = 685.8
(four-charged) was assigned to [Cu2Zn(5)(6)(7)]4+, whereas the
diastereomers as approximately 3
: 2. As ligand 7 can
connect via the terpyridine or phenanthroline unit to 6, thus
involving either [M(6phenAr2)(7terpy)]n+ or [M(6phenAr2)(7phen)]n+
linkages,z verifying the correct connectivity of ligands 5–7 in
the triangular assembly requires special attention. Comparison
of the proton resonances of T with those of [M(phenAr2)(phen)]n+
or [M(phenAr2)(terpy)]n+, units from similar compounds,
provides convincing evidence that 7 is connected to 6 via the
[M(6phenAr2)(7terpy)]n+ unit. For example, in a rectangle
serving as a reference,6 the methoxy protons appear at
3.19–3.39 ppm for [M(phenAr2)(phen)]n+ while they arise at
2.68–2.82 ppm for [M(phenAr2)(terpy)]n+ moiety. Similarly,
the proton resonances for methoxy groups arise at 2.71–2.85 ppm
for the [M(phenAr2)(terpy)]n+ unit in a supramolecular
isosceles trapezoid.6 As the MeO proton resonances in T show
up at 2.73–3.06 ppm, they point to the occurrence of a
[M(phenAr2)(terpy)]n+ unit while ruling out the existence of
a [M(phenAr2)(phen)]n+ complex. Thus, self-assembly occurs
in full agreement with the strategy conceived on the basis of
our model compounds (Scheme 1).
triply charged one at m/z
[Cu2Zn(5)(6)(7)](OTf)3+
and the doubly charged one at
m/z = 1519.5 to [Cu2Zn(5)(6)(7)](OTf)2
= 963.7 was attributed to
,
2+
.
To corroborate the clean self-assembly process we carefully
interrogated the DOSY and 1H NMR of T. As for the
ESI-MS, both sets of data unambiguously supported the
presence of only one species, e.g. the DOSY spectra showed
only a single diffusion coefficient (ESIw). Additional information
1
could be derived from the H NMR signals of the methoxy
protons, as they appear in a diagnostic region. In T, eight
DPV is a good tool to characterise supramolecular species
with redox active subunits. The knowledge that copper(I) ions
show distinct oxidation potentials in combination with ligands
1–4 allows to analyse for the presence of different copper(I)
centres in T. In the mononuclear complex [Cu(1)(4)](PF6) the
copper(I) oxidation wave is observed at +0.29 VSCE, whereas
for [Cu(3)(4)](PF6) and [Cu(1)(2)](PF6) the oxidation is seen at
+0.44 VSCE and ꢁ0.21 VSCE, respectively.6 For T, only one
type of copper(I) complex is expected, but the presence of two
diastereomers may lead to two closely spaced oxidation waves.
Indeed, deconvolution of the broad peak at 0.62 VSCE revealed
oxidation waves (Fig. 4) at 0.59 and 0.64 VSCE in a ratio of to
ca. 3 : 2, which agrees well with the diastereomer ratio
detected in the 1H NMR. The oxidation values agreed well
singlets are expected for the four methoxy groups due to their
Fig. 2 ESI-MS of the triangle T in acetonitrile along with its
isotopic distribution (black: experimental; red: calculated for
[Cu2Zn(5)(6)(7)]4+).
with those found in similar copper(I) complexes, as for example
6
in a supramolecular trapezoid with 0.61 and 0.67 VSCE
.
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four-component assembly (see the homometallic triangles).
Moreover, the homometallic triangle [Cu3(5)(6)(7)](PF6)3 was
easily converted into the heterobimetallic T simply by adding
one equivalent of Zn2+, lending extra proof for the dynamic
nature of all triangles [M3(5)(6)(7)]n+
.
In conclusion, we have described the clean formation of the
five-component triangular species T. The structure was
established by ESI-MS, 1H NMR, DOSY, DPV and elemental
analysis. Self-sorting was interrogated in the presence of
different metal ion scenarios. Importantly, self-sorting does
not only lead to T when all components are applied in a
stoichiometric manner, but equally when excess amounts of
the required metal ions are available. The latter aspect may
become important for repairing defect metal ion corners in T,
and thus is reminiscent of biological multi-component assemblies
being in exchange with their surrounding. Moreover, self-
sorting also led to ligand reorientation inside the supra-
molecular assembly, a switching process possibly of use for
the fabrication of complex molecular machines.14
Fig. 4 Differential pulse voltammogram of T in acetonitrile with
0.1 M nBu4NPF6 as electrolyte against a Ag wire used as a quasi-
reference electrode and 1,10-dimethylferrocene (left wave) as internal
standard (scan rate of 20 mV sꢁ1 and a pulse height of 2 mV).
The latter observation also unambiguously validated the
location of the metal ions inside the triangle T, as absence of
any peak at negative potential excluded the possibility of a
[Cu(phenAr2)(terpy)]+ unit.z Thus 7 is coordinated to 6 via a
[Zn(phenAr2)(terpy)]2+ complex, while the remaining corners
are occupied by Cu+ metal ions (Scheme 2).
Notes and references
z phen = [1,10]phenanthroline; phenAr2 = 2,9-bis(2,6-dimethoxy-
phenyl)-3-ethynyl-[1,10]-phenanthroline; terpy = [2,20;60,200]terpyridine.
1 M. Schmittel and K. Mahata, Angew. Chem., Int. Ed., 2008, 47,
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From the above experiments and analytical data the
exclusive formation of the triangular assembly T is proven
to arise from a combination of three different ligands in the
presence of three equivalents of metal ions (one equiv. of Zn2+
ions and two equiv. of Cu+ ions). One may, however, argue
that it is the potentially cooperative binding in the cyclic ligand
array of T that drives self-assembly in the presence of any
metal ions. We thus prepared the triangular assemblies using
either exclusively Zn2+ or Cu+ ions. ESI-MS data confirmed
the formation of the triangular [M3(5)(6)(7)]n+ assemblies
(ESIw; Fig. S8 and S9). However, the 1H spectra of the
homometallic triangles [M3(5)(6)(7)]n+ (Fig. 3a and b) appear
much more complicated than that of the heterometallic case
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1
(Fig. 3c). A comparison of all H NMR spectra clearly shows
that the number of signals for methoxy groups and thus of
constitutional isomers increases in case of homometallic
scenarios. To demonstrate this case, we reacted the homometallic
triangle [Cu3(5)(6)(7)](PF6)3 with one equivalent of Zn2+ and
kept it at reflux for 3 h. The colour faded from dark red to
1
light red and the H NMR became akin to that observed for
T = [Cu2Zn(5)(6)(7)](OTf)2(PF6)2 (ESIw; Fig. S6).
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The observations can be rationalised in the following way. For
the homometallic copper(I) or zinc(II) self-assembly, the ligands
organise into differently connected triangular arrays. As the
connection of ligands 5 and 6 is firmly instated, the terpyridine
part of 7 may connect with either 5 or 6 leading to two
dissimilar triangular species. On the contrary, in the mixed
metal scenario the terpyridine of 7 has to connect with 6, as
demonstrated in Scheme 1. Thus, the self-assembly process clears
up through self-sorting. Interestingly, due to the beauty of
self-sorting the five-component assembly (five different starting
materials, mixed metal scenario) was flawless as compared to the
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Chem. Commun., 2010, 46, 4163–4165 | 4165