Dynamic covalent assembly of tribenzotriquinacenes into molecular cubes

Article abstract of DOI:10.1039/c4cc01794c

Molecular cubes constructed from catechol-functionalized tribenzotriquinacenes and 1,4-phenylene diboronic acids were synthesized in a one-pot procedure by crosslinking 20 individual components through a dynamic covalent approach. Structural identity of t

Full text of DOI:10.1039/c4cc01794c

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Dynamic covalent assembly of
tribenzotriquinacenes into molecular cubes†
Cite this:DOI: 10.1039/c4cc01794c
Stefanie Klotzbach, Thorsten Scherpf and Florian Beuerle*
Received 10th March 2014,
Accepted 17th June 2014
DOI: 10.1039/c4cc01794c
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Molecular cubes constructed from catechol-functionalized tribenzo- dynamic molecular assemblies possess preferentially the highest
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triquinacenes and 1,4-phenylene diboronic acids were synthesized in a possible symmetry, formation of Archimedean or Platonic solids
one-pot procedure by crosslinking 20 individual components through should be preferred and as a matter of fact, solely organic repre-
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8,15
a dynamic covalent approach. Structural identity of the nanocubes was sentatives of tetrahedra, cubes, octahedra or cuboctahedra
confirmed by mass spectrometry and H-NMR spectroscopy.
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were synthesized in the last decade. The factual shape of the
cage is thereby encoded in the molecular symmetry of the
The formation of monodisperse molecular structures with implemented subunits. For instance, co-assembly of tritopic
ever increasing complexity is an ongoing desire in synthetic and ditopic building blocks in a 2 : 3 ratio is expected to yield
chemistry. Molecular cage compounds are intriguing examples either cuboctahedra (planar tritopic and ditopic molecules with
of such systems with dimensions in the nanometer regime. tetrahedral angles) or cubes for trigonal corner units and linear
Over the last few decades, hybrid systems assembled via metal ditopic struts. In contrast to the rich chemistry of octahedral
coordination of appropriate organic linker molecules have been metal complexes resulting in numerous examples of metal–
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presented in a large variety. However, reports on solely organic organic systems, the chiral cubes synthesized by Xu and
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cages are far more limited in numbers since tedious multiple Warmuth from aldehyde-functionalized cyclotriveratrylenes
step synthesis hampers the efficient formation of highly complex and linear diamines are the only reported cubic organic cage
systems. To circumvent these drawbacks, the concept of dynamic compounds so far. The actual absence of further examples is
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covalent chemistry offers the possibility of performing multiple most likely due to the lack of suitable organic building blocks
reaction steps under reversible conditions giving easy access to featuring the required rectangular arrangement of three func-
complex structures under thermodynamic control. The most tional units. One rare example of such a molecular framework
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prominent reactions in this regard are the formation of imines
is the tribenzotriquinacene (TBTQ) scaffold possessing three
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and boronate esters from aldehydes and amines or boronic acids condensed indane moieties. After being introduced by Kuck,
and 1,2-diols, respectively. Based on this approach, numerous functionalized TBTQ derivatives have been recently implemented
examples of organic cage compounds have been presented in the into fullerene receptors, microporous polymers or mimics of
literature in recent years. Potential applications can be found in odd-membered ring-defects in graphenes. However, despite the
the fields of host–guest chemistry or porous materials with favourable rigid arrangement of up to three functional sites, there
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À1
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reported BET surface areas of up to 3758 m g for organic is only one reported example of a TBTQ-based cage compound
cages in the solid state. However, it is still challenging to obtain and surprisingly, no attempts have yet been made to incorporate
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permanent porous materials and only a few examples of proces- such derivatives as corner units into covalent organic cages
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sing cage molecules into devices such as quartz microbalances
with cubic symmetry.
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0
or macroscopic inorganic porous beads have been reported.
Herein, we report on the synthesis of novel metal-free
Since it was shown theoretically that most equilibrium states of molecular cubes by the formation of 24 boronate esters by
crosslinking of catechol-functionalized tribenzotriquinacenes
Universit a¨ t W u¨ rzburg, Institut f u¨ r Organische Chemie & Center for Nanosystems
Chemistry (CNC), Am Hubland, 97074 W u¨ rzburg, Germany.
E-mail: florian.beuerle@uni-wuerzburg.de; Fax: +49 931 3187348;
Tel: +49 931 3183603
and 1,4-phenylene diboronic acids in a one-pot reaction.
Hexahydroxy TBTQ 1 has been recently used as a precursor for
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microporous polymers and based on the PM6-minimized
model in Fig. 1, an angle of 89.31 could be calculated between
two catechol units each indicating an almost rectangular orienta-
tion of the three indane moieties with respect to each other.
†
Electronic supplementary information (ESI) available: Experimental procedures
and spectral data for all new compounds, TGA of cube 4a, DOSY NMR spectra of
cube 4b. See DOI: 10.1039/c4cc01794c
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(see Fig. S13, ESI†) showed one peak at m/z = 4584 as the
dominant signal which correlates exactly to the molecular mass
of the desired organic cube 4a (see Fig. 1c). After filtration, the
measured elemental analysis of the precipitate is in good agreement
with the calculated one for 4a assuming the isolation of cube 4a in
53% yield. Unfortunately, the obtained crystals were either too small
or tended to grow into bigger agglomerates so we were not able to
determine the structure by X-ray diffraction. Since the obtained solid
material was virtually insoluble in most organic solvents, with the
exception of methanol where cleavage of the boronic esters occurred
and starting materials 2 and 3a were recovered in a 2 : 3 ratio (see
Fig. S15, ESI†), all attempts to recrystallize the reaction product also
failed. However, MALDI TOF-MS under solvent-free conditions of
the precipitate showed the molecular ion peak of 4a at m/z = 4584.49
as the only signal (see Fig. 3b).
In order to enhance the solubility of the molecular cages and
2
2
Fig. 1 (a) PM6-minimized model of 1 as a tritopic building block with an to definitely prove the structural assignment as cubes with O
h
orthogonal arrangement of the peripheral catechol units; (b) formation of
symmetry, we synthesized phenylene diboronic acid 3b with
two butyl chains as additional solubilizing groups (see ESI†
for synthetic procedures and characterization details). Again,
molecular cubes after reaction of eight tritopic corner units and twelve
ditopic linear spacer units under dynamic covalent conditions; (c) synthesis
of molecular cubes 4 from TBTQ derivative 2 and diboronic acids 3 in THF
under water removing conditions.
mixing 2 and 3b in a 2 : 3 ratio in THF-d resulted in an equilibrium
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of several reaction intermediates, whereas subsequent addition
of 4 Å molecular sieves led to quantitative conversion to a
As a matter of fact, this spatial arrangement of its functional highly symmetrical product after five days as indicated by NMR
groups identifies 1 as a well-suited precursor for the formation analysis (see Fig. 2 and Fig. S18, ESI†). The reaction product
of molecular cubes via boronate ester formation with linear could be precipitated with hexane and redissolved in CHCl
boronic acids. We therefore investigated the reaction of 1 with solvent in which both precursors are completely insoluble.
,4-phenylene diboronic acid 3a in a 2 : 3 molecular ratio in various Subsequent filtration, removal of the solvent and drying under
solvents and under different reaction conditions. However, in high vacuum gave cube 4b in 94% yield. Fig. 3d shows the
all cases either no reaction occurred or rapid precipitation of MALDI TOF mass spectrum of the CHCl solution revealing the
3
, a
1
3
presumably oligomeric side products was observed probably formation of cube 4b as the only detectable species. As depicted
1
due to low solubility of either the starting materials or the in Fig. 4, H NMR spectra show only one set of signals for the
formed reaction intermediates.
individual protons indicating the high symmetry of the structure.
symmetrical molecular
Most promising results were obtained in THF solution at Therefore, the successful formation of O
h
room temperature, indicating an equilibrium of unreacted 1 and cubes 4b is the only plausible conclusion in accordance with
1
one to threefold boronate esters as detected by monitoring the both MS and H-NMR results. DOSY NMR spectra of 4b showed
1
reaction progress by H-NMR spectroscopy. Addition of 4 Å mole- all relevant signals with the same diffusion coefficient (see
cular sieves shifted the equilibrium significantly towards higher Fig. S20, ESI†) and from that we could calculate the diameter of
condensation products, yet, primarily resulted in precipitation the cage as 3.3 nm via the Stokes–Einstein equation. This finding is
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of an insoluble material, which could not be further character- in good agreement with a PM6-optimized model of 4b exhibiting
ized. Since it was assumed that successful cage formation was a diagonal corner-to-corner distance of 3.2 nm (see Fig. 3c).
mainly prevented due to solubility issues of the small-size
reaction intermediates, we modified the apical position of the
TBTQ moiety by introducing a butyl chain in 2 (see ESI† for
synthetic procedures and characterization details) in order to
enhance the solubility of both the starting material and oligo-
meric condensation products. Analogous to 1, reaction of 2 and
3a in a 2 : 3 molecular ratio in THF at room temperature resulted
in an equilibrium of unreacted 2 and mono- to trisboronate esters
1
as monitored by H-NMR spectroscopy of the TBTQ methine
protons (see Fig. S11, ESI†).
The equilibrium state was reached after several minutes and
no further changes were observed even over the course of
several days. However, addition of 4 Å molecular sieves led to
precipitation of a microcrystalline material (see Fig. S12, ESI†)
1
Fig. 2 H-NMR monitoring (THF-d
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, 400 MHz, rt) of methine protons for
reaction of 2 and 3b in a 2 : 3 molecular ratio (top: start of reaction, middle:
after several hours and MALDI TOF-MS of the reaction mixture after one day, bottom: after five days).
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after washing the precipitate with dry THF and drying under
high vacuum at 100 1C for 20 hours showed no significant
porosity of the material. These findings indicate either kinetic
hindrance of gas absorption or a structural collapse of the cubes
after desolvation, a fact which is often observed for organic cage
5c
molecules. In ongoing work, we will address these issues and
look for alternative activation protocols in order to preserve
porosity of such materials and gain more insight into the inner
surfaces of the cubes.
In conclusion, we were able to synthesize molecular cubes 4a
and 4b through the concerted formation of 24 boronate esters in
a [8+12] cocondensation of twenty small molecule precursors in a
dynamic covalent approach. These cage compounds are the first
examples of purely organic cubes with the highest possible cubic
h
symmetry O . Due to the easy and modifiable synthesis of TBTQ
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precursors 2 and linear diboronic acids 3, this strategy will give
Fig. 3 (a) PM6-minimized
model of molecular cube 4a with non-
functionalized spacer units 3a; MALDI-TOF MS (TCNQ, solvent free, mole- access to a whole new family of structurally related cubic organic
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cular ratio 4a :matrix 1:500) of 4a in the solid state; (c) PM6-minimized
model of molecular cube 4b with spacer units 3b containing butyl chains as
solubilizing groups; (d) MALDI-TOF MS (DCTB, CHCl ) of 4b.
cage molecules and starting from lower symmetry analogues of
the corner units cuboid cages can also be envisaged. Currently,
we are working on the synthesis and characterization of modified
cubes and cuboids in order to test the scope and potential of this
synthetic approach for dynamic covalent cube formation.
The presented work was financially supported by the Fonds der
Chemischen Industrie (Liebig fellowship for FB and doctoral
fellowship for SK). The authors would like to acknowledge
assistance from Stefan Wachtler and Prof. Dr Anke Kr u¨ ger
for measuring TGA as well as Fabian Sch o¨ nfeld and Prof.
Dr Klaus M u¨ ller-Buschbaum for gas sorption measurements.
The authors would also like to thank Prof. Dr Frank W u¨ rthner for
general support and assistance within the Center of Nanosystems
chemistry in the framework of the Collaborative Research Net-
work ‘‘Solar Technologies Go Hybrid’’ of the Bavarian Ministry
of Science, Research and the Arts.
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Notes and references
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Fig. 4 H-NMR spectra (400 MHz, rt) of tritopic building block 2 (D
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COD,
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Altogether, these analytical findings provide evidence for the
efficient formation of molecular cubes 4b.
Due to the microcrystallinity of 4a, we investigated the
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precipitate in MeOH, 0.9 equivalents of THF remained in
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