Self-assembled [2]catenane in trapezoidal metallacycles with [Cp?Ir]-corners

Article abstract of DOI:10.1039/c7cc08009c

A series of trapezoidal metallacycles were synthesized by the selective combination of a rigid with a flexible arm. [2]Catenane 3 was obtained by self-assembly when the cavity size of the trapezoidal rings was optimised.

Full text of DOI:10.1039/c7cc08009c

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Marilyn M. Olmstead, Alan L. Balch, Josep M. Poblet, Luis Echegoyenet al.
Reactivity differences of Sc3N@C2n (2n 68 and 80). Synthesis of the
first methanofullerene derivatives of Sc N@D5h-C80
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Self-assembled [2]Catenane in Trapezodial Metallacycles with
[Cp*Ir]-Corner
Naifang Liu,ab Sheng-Li Huang,*a Xiaogang Liu,ab He-Kuan Luo*a and T. S. Andy Hor*abc
Received 00th January 20xx,
Accepted 00th January 20xx
DOI: 10.1039/x0xx00000x
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A series of trapezoidal metallacycles were synthesized by the composed of a pair of interlocked rings, is among the best
selective combination of rigid with flexible arm. The candidates in exploring the spontaneously interlocking
[2]catenane 3 was obtained by self-assembly when the cavity size process.2 The C2 symmetry of [2]catenane provides a suitable
a
a
of the trapezodial rings is optimised.
template for C2 macrocycle with a single interlock. One typical
C2 symmetric ring is trapezoidal metallacycle (Fig. 1), which
was obtained by combining three components of cis-protected
Pd-corner and two types of bifunctional pyridyl ligands; it
however suffers from concurrent formation of other
thermodynamic products.10 Contrary to palladium, [Cp*Ir]-
based corners each holding three available coordination sites,
viz. a bis-chelating linker and a monodentate bifunctional
linker are needed to satisfy the requirements of coordination
saturation and cyclic structural formation.11 In contrast to
monodentate coordination interaction, the bonding energy of
chelating metal-ligand interaction has a geometric growth,
that decreases the possibility of the cleavage and re-bonding
of coordination bonds. Harness of the chelate effect can
simplify the formation and ease isolation in suppressing
minimal single linker assemblies. This shifts the attention to
judicious selection of bis-chelating linker as the
interpenetration outcome is sensitive to its length, spacer and
Interlocked superstructures are of topical interest, as
demonstrated by the molecular machines created by Sauvage,
Stoddart, Feringa and others.1 Catenanes as entangled
architectures are particularly well suited as designer molecular
devices that exploit the dynamics of the interlocked molecules.
As visually appealing they are, their syntheses still pose great
technical challenges due to the complex and constrained
topologies.2
The first synthesis of [2]catenane in 19603 was readily
followed by a variety of interlocked superstructures largely
attributed to the development of two effective synthetic
routes: template and self-assemble methods.2 The former can
provide obvious driving force responsible for catenation,4
whereas the latter, which is biomimetic in nature, is versatile
but more difficult to control the delicate intermolecular
interactions. Although several self-assembled molecular
topologies, such as, catenanes,2 Solomon links,5 Borromean
rings,6 interlocked cages7 and knots8 have been reported, most
of them are almost serendipitously obtained from the
metallasupramolecule synthesis.9 Inter-balance of many
influencing factors is spontaneous and complex thus difficult
to decipher or control.
aromaticity.6
Tetrapyrido[3,2-a:2',3'-c:3'',2''-h:2''',3'''-
LI1
j]phenazine
(tpphz)
and
9,11,20,22-tetraaza-
Owing to its comparatively simple topology, the [2]catenane,
the simplest type of catenane with Hopf link topology,
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tetrapyridopentacene (tatpp) LI2, which are long and rigid 842.7 and 512.5 corresponding to the cation [
1
-OTf]+, dication
aromatic bridging linker, may provide potential
-2OTf]2+ and trication [ -3OTf]3+ (Fig. S8, S9), respectively.
supramolecular recognition sites for π-π stacking The absence of peaks assignable to odd-cation of dimeric
interactions.12
structure further confirmed no interpenetration. In Pd-based
The combination of [Cp*Ir]-corner and bis-chelating linker [2]catenanes, the distance between two methyl units of LII is
[
1
1
1
LIn easily offered the linear arm with two available 5.8 Å, and the length of Pd···Pd arm is about 11.0 Å. In ring
coordination sites, in order to form the trapezoidal the length of Ir-LI1-Ir is raised to 12.8 Å (The length was
metallacycle, one flexible monodentate bifunctional linker is obtained from the crystal structure of complex ). This creates
1,
2
the best choice. The methylene unit is widely utilized in the a greater length disparity between the two parallel sides, thus
design of flexible ligands,13 and this prompted us to design a decreasing the ladder height, and defying enough space to
series of bifunctional ligands LII that include the terminal accommodate the copy of itself. Compared with LII1, the
n
pyridyl groups with free-to-rotate methylene. Both the length distance between two methyl units in LII2 is increased to 10.5 Å,
of the central backbone and terminal pyridyl groups could then which is slightly shorter than the Ir-LI1-Ir arm. LII could hence
2
be tuned to influence the interpenetrating process (Scheme 1). be supportive of the catenane structure, but, somewhat
The flexible ligand LII1 has been successfully employed in the surprisingly, only the monomeric ring
2
was obtained. Similar
, no high upfield shift and odd-cation peaks of dimer were
, that observed (Fig. S11), proving no interlocking assembly formed.
was confirmed by NMR spectroscopy and Electrospray Slow evaporation of a MeOH solution of afforded single
Ionisation Mass Spectrometry (ESI-MS). NMR is used to crystal ·2MeOH·H2O suitable for data collection, and the
Pd-based [2]catenane synthesis.10b In contrast, treatment of to
Cp*Ir-LI1 with LII produced non-interlocked structure
1
1
1
2
2
evaluate the interpenetration outcome, as a very high upfield single-crystal X-ray diffraction was carried out in liquid
shift of the central aromatic proton signals caused by mutual nitrogen due to its instability. The macrocycle crystallized in
shielding between the two component rings would be the trigonal space group R-3 with structural details shown in
diagnostic for the catenane formation. The methylene protons Fig. S12. The structure revealed its expected trapezoidal
of
1 appear at 5.17 ppm and all the aromatic protons are pattern, with the trapezoidal bottoms about 8.6 Å apart,
above 7.10 ppm, indicating no upfield shift. ESI-MS analysis whereas two independent rings are separated by 4.3 Å which
could also be used to monitor the interlocking process. is too wide for interlocking (Fig. 2).
Although the even-cation of dimeric structure has same MS
The locking process from monomeric rings to catenated
peaks with one corresponding odd-cation of monomeric dimers must be accompanied by the distribution decrease and
structure, the odd-cation peak of dimeric structure is unique. symmetry increase of the whole system. In [2]catenane
The isotopic distributions are an unambiguous indication of formation, the binding energy between two independent rings
the exact mass of the MS fragment, that also allow must be sufficient to cover the entropy cost. Enhancing ring-
distinguishing between the dimeric and monomeric structure. ring interactions could increase the possibility of interlocking.
The mass spectrum of
1
showed relevant peaks at m/z= 1834.3, For LII2, its failure to form interlocked structure may be
explained by poor ring-ring interactions due to lengthy
terminal pyridyl groups (Fig. 2). This postulate was tested and
2 | J. Name., 2012, 00, 1-3
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confirmed by the use of LII3, which resulted in the absence of
interlocked structure or monomeric ring under a range of
terminal pyridyl groups. Appling the same synthetic method to
1
and 2, we were unable to find any evidence of mono-ring
formation through either NMR or MS analysis, suggesting that
the trapezoidal ring with enlarged cavity is unstable for
formation.
Compared to LII1, LII and LII3, the ligand LII has medium-
2
4
length backbone and shorter terminal pyridyl groups, which
should generate enough ring-ring interactions to balance the
entropy expense in the catenane formation. Mix of an
equimolar of Cp*Ir-LI1 and LII in MeOH at r.t. resulted in pure
4
light-yellow solids of [2]catenane complex
contrast to the 1H NMR spectra of
3
upon isolation. In
1 and 2, the protons of
phenylene and methylene of LII moieties are significantly
4
shifted upfield, which is diagnostic of interlock of the two rings.
Further, evidence for [2]catenane
through ESI-MS analysis (Fig. S16). The mass spectrum of a
dilute solution of revealed peaks at m/z=1165.6, which
corresponds to the ion [
-3OTf]3+. The isotopic distributions for
the peaks of the corresponding ions were consistent with
simulated distributions (Fig. 3). LII has similar structure as LII
3 formation was obtained
3
3
,
4
5
but atomic link between methylene and central backbone is
nitrogen replacement of carbon. Although the positive sites
are located at the terminal of central backbones, the positive
charge could also transfer to the central site thus creating
positive charge repulsion when two central backbones meet.
The replacement of LII with LII led to the non-interlocked
[2]catenane
3
, but the combined use of Cp*Ir-LI2 and LII
4
produced a mono-ring
5
(Fig. S23-S24). The reason of un-
the
locking may be similar to the formation of mono-ring
1
―
extension of one parallel side of trapezoid leads to a decreased
ring size which does not favour interpenetration.
5
4
By the Le Chatelier's principle, when a system at equilibrium
is subjected to change in concentration, the system readjusts
itself to counteract the effect of the applied change. The
equilibrium mixture of non-interlocked and interlocked
assemblies is disrupted following concentration change due to
the dynamic behaviour of metal-ligand interaction and weak
binding between independent individuals in interlocked
metallasupramolecules.10b,14 Accordingly, higher concentration
could favour the interlocking path, whereas lower
concentration could weaken such potential. This phenomenon
structure
4 as expected (Fig. S19-S21). Besides the influence of
flexible linker, the influence of bis-chelating linker was also
investigated. The treatment of Cp*Ir-LI1 and LII gave
4
was observed in the combination of Cp*Ir-LI1 and LII with
4
variable concentrations. At high concentrations (0.1 mM, with
respect to Cp*Ir), [2]catenane
3 is the exclusive product. At
medium concentration (0.04 mM), the equilibrium mixture of
interlocked- and non-interlocked was observed. Only the
mono-ring 3' was found at low concentrations (0.005 mM).
This concentration-dependent dynamic system is illustrated
through NMR spectra changes (Fig. 4).
In summary, we have designed and synthesized a series of
trapezoidal metallacycles of Cp*Ir bearing one rigid bis-
chelating linker and one flexible bifunctional pyridyl linker.
Optimal shape and size of the trapezoidal metallacycles are
critical for interlocking. The results showed that overly small
ring cannot provide sufficient space for interlocking, and
enlarged ring cannot create efficient ring-ring interaction with
poor structural stability. Only with optimal ring sizes can
favour interlocking through sufficient compensation of entropy
cost by binding energy of the two independent rings. This
points to the central importance of intermolecular interaction
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H. Seki, O. Kamo, M. Imanari, K. Ogura, J. Am. Chem. Soc.,
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14 N. Singh, D. Kim, D. H. Kim, H. Kim, E. H. Kim, M. S. Lah, K. W.
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in governing the interlocking assembly in the formation of
[2]catenane structure. We are currently investigating other
factors especially supplementary hydrogen bonding and metal-
metal interactions.
We acknowledge IMRE-A*STAR for infrastructure support and
financial assistance (IMRE/14-1C0120). Technical support from
staff at CMMAC of NUS Department of Chemistry is also
appreciated.
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