Molecular networks based on dative boron-nitrogen bonds

Article abstract of DOI:10.1002/anie.201007225

The BN connection: Crystalline and soft molecular networks can be constructed using dative B-N bonds (see picture). The networks are obtained in a one-step, three-component reaction involving a triboronic acid, a catechol, and a bipyridyl linker. (Chemica

Full text of DOI:10.1002/anie.201007225

Communications
DOI: 10.1002/anie.201007225
Supramolecular Chemistry
Molecular Networks Based on Dative Boron–Nitrogen Bonds**
Erin Sheepwash, Vincent Krampl, Rosario Scopelliti, Olha Sereda, Antonia Neels, and
Kay Severin*
The controlled synthesis of crystalline polymers with two- or
three-dimensional network structures (crystal engineering)
can be achieved by connection of molecular building blocks
through noncovalent interactions.[1] Hydrogen bonds[2] and
coordination bonds[3] are most commonly used in this context,
but halogen bonds,[4] metal–metal,[5] CH–p,[6] and p–p inter-
actions[7] have been employed as well. Noncovalent inter-
actions are also crucial for the creation of gels from low-
molecular-weight gelators (LMGs).[8] In fact, the crystal
engineering approach of using supramolecular synthons[9]
has been an inspiration for the design of LMGs.[10] Crystalline
and soft molecular networks show a vast number of potential
applications. Novel strategies to generate such structures are
thus of high interest. Herein we describe crystalline organic
networks and an organogel that were obtained by connection
of triboronate esters with bipyridyl linkers. A unique feature
of these supramolecular polymers is the presence of dative
boron–nitrogen bonds as crucial structure-directing elements.
Boronate esters are Lewis acidic compounds, which can
form adducts with N-donor ligands.[11] This interaction results
in a distinct structural change from trigonal-planar to
tetrahedral geometry at the boron atom. The strength of the
triboronic acid 1 along with 4-tert-butylcatechol[17] and 4,4’-
bipyridine or 1,2-di(4-pyridyl)ethylene (Scheme 1). The tri-
boronic acid 1 was expected to undergo a triple condensation
reaction with the catechol to give a triboronate ester, which is
then linked by the bipyridyl linker.
À
B N interaction depends on the steric and electronic
characteristics of the reaction partners as well as on the
solvent.[11,12] Dative bonds between boronate esters and N-
donor groups have been employed in the context of materials
chemistry and structural supramolecular chemistry.[13] For
À
example, B N bonds were used to make molecularly defined
Scheme 1. Synthesis of the two-dimensional networks 2 and 3 by
polycondensation reactions.
macrocycles[14] and linear polymers.[15,16] The utilization of B
À
N bonds for the creation of molecular networks is, to best of
our knowledge, unprecedented.
Two-dimensional polymers can be accessed by connection
of tritopic building blocks with ditopic linkers. To implement
such a synthetic strategy with boronate esters, we used the
When a mixture of 1, 4-tert-butylcatechol, and 4,4’-
bipyridine (ratio 2:6:3; [1] = 3.3 mm) was heated in toluene/
THF (2:1) under reflux using a Dean–Stark trap, a homoge-
neous colorless solution was obtained. Upon cooling, polymer
2 precipitated in the form of an orange powder in 65%
yield.[18] Polymer 3 was obtained in a similar fashion using the
extended linker 1,2-di(4-pyridyl)ethylene (yield: 71%).
The polymers can be dissolved in organic solvents such as
[*] E. Sheepwash, V. Krampl, Dr. R. Scopelliti, Prof. K. Severin
Institut des Sciences et Ingꢀnierie Chimiques
Ecole Polytechnique Fꢀdꢀrale de Lausanne (EPFL)
1015 Lausanne (Switzerland)
Fax: (+41)21-693-9305
E-mail: kay.severin@epfl.ch
À
chloroform or toluene upon heating. In solution, the B N
bonds are broken. This was shown by NMR spectroscopy: the
signals observed in the 1H NMR spectra are identical to those
of the corresponding triboronate ester and the free bipyridyl
linker (see the Supporting Information). In the solid state,
however, the boron centers are tetrahedral, as evidenced by
signals in the 11B NMR spectrum (cross-polarization magic
angle spinning) at d = 14 ppm (3).[19]
O. Sereda, A. Neels
Centre Suisse d’Electronique et de Microtechnique (CSEM)
2002 Neuchꢁtel (Switzerland)
[**] This work was supported by funding from the Swiss National
Science Foundation and by the EPFL. We thank Dr. Diego Carnevale
(EPFL) for help with the solid-state 11B NMR measurements and Dr.
Marco Cantoni and Fabienne Bobard (EPFL) for help with the SEM
measurements.
The polymers can be crystallized by slow cooling of
toluene solutions or by vapor diffusion of pentane into
toluene solutions. Crystallographic analyses were performed
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201007225.
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Angew. Chem. Int. Ed. 2011, 50, 3034 –3037

Figure 1. Parts of the two-dimensional network structures of 2 (a, b)
and 3 (c, d) as determined by single-crystal X-ray diffraction. Space-
filling (a, c) and wireframe views (b, d) along the crystallographic
z axis. Catechol groups in green, bipyridyl linker in blue, and triphenyl-
benzene in red.
Figure 2. The (A-B-C)n layer structure of 3 in the solid state. a) Wire-
frame view along the crystallographic x axis. b,c) Space-filling represen-
tation of two adjacent layers viewed along the crystallographic z and
y axes. The layers are tightly interwoven, but there is no catenation. For
clarity, the catechol groups have been removed in (b) and (c).
for both compounds,[20] and graphic representations of the
solid-state structures are shown in Figure 1. The polymers
form two-dimensional networks in which the triboronate
esters act as nodes that are connected by the bipyridyl linkers.
The compounds are related to some covalent organic frame-
works (COFs), as they feature boronate esters as integral
components of their framework.[21] In contrast to COFs,
however, network formation is achieved through dative
vent-free polymer 3 did not show significant permanent
porosity (ca. 13 m2 gÀ1). The structural collapse seems to occur
in two steps. Upon careful removal of solvent from crystalline
3 at ambient conditions (room temperature, no vacuum), the
diffraction peaks at 108 ꢀ 2Vꢀ 208 disappear, but a dominant
low-angle peak at 2Vꢁ 58 remains. The latter is attributed to
the d spacing of 15.75 ꢀ of the (101) plane, which is slightly
smaller (d = 16.68 ꢀ) than the interchain spacing in the
original 3. This first step can be reversed by addition of
toluene, which results in the re-formation of crystalline 3 (see
the Supporting Information). However, more forcing con-
ditions (prolonged standing after removal of solvent, or
application of vacuum) irreversibly produce an amorphous
material (step 2). A similar behavior was observed for 2.
We have also investigated condensation reactions with the
extended triboronic acid 4 (Scheme 2). This compound was
obtained from 1,3,5-tris(4’-bromobiphenyl)benzene using a
standard procedure (see the Supporting Information). Con-
trary to what was observed for the smaller building block 1,
À
boron–nitrogen bonds. The B N bond lengths are
1.676(5) ꢀ for 2 and 1.678(7) ꢀ for 3. These values are
similar to what has been observed for the 4-picoline adduct of
phenylcatecholborane (1.651(3) ꢀ).[22] The trigonal triphe-
nylbenzene core of the boronate ester shows a propeller-like
conformation, with a torsion angle between the peripheral
and the central benzene rings of 458 (2) and 328 (3).
The individual layers of the networks can be described as
connections of large macrocyclic structures with ring sizes of
126 (2) or 138 (3) atoms. The macrocycles of 3 appear more
compact when viewed along the crystallographic z axis (Fig-
ure 1d vs. Figure 1b). However, the diameter of the macro-
cycles, as defined by the longest B···B distance, is rather
similar (36.6 ꢀ for 2 and 37.2 ꢀ for 3).
The polymer layers of 2 and 3 show an (A-B-C)n repeat
pattern (Figure 2a). The individual layers are tightly inter-
woven, but there is no catenation of adjacent layers (Fig-
ure 2b,c).[23]
Crystalline 2 and 3 contain voids, which are filled with
disordered toluene molecules. Thermogravimetric analysis
(TGA) indicates that the solvent can be removed under
vacuum at room temperature for 12 h (see the Supporting
Information). However, removal of the solvent leads to a loss
of crystallinity, as evidenced by powder X-ray diffraction
analysis. Furthermore, N2 sorption measurements with sol-
À
Scheme 2. Synthesis of an organogel with B N linkages.
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Communications
reaction of 4 with 4-tert-butylcatechol and 4,4’-bipyridine in
bipyridyl linkers. Polymer formation can be reversed by
increasing the temperature, a feature which could be of
interest from a processing point of view. It appears likely that
related two- and three-dimensional networks can be obtained
by variation of the building blocks. Furthermore, it should be
possible to tune the stability of the networks by modulating
toluene resulted in the formation of an orange gel. All three
components are necessary for gel formation, as evidenced by
control reactions in which one of the reactants was omitted.
The system thus represents an example of a multicomponent
gel.[24,25] Gel formation was expected to proceed by conden-
sation of the triboronic acid 4 with 4-tert-butylcatechol to give
the corresponding triester 5, which is then linked by 4,4’-
À
the strength of the dative B N interaction through steric and
electronic effects.[28] Investigations in this direction are
currently being performed in our laboratory.
À
bipyridine through dative B N bonds. This assumption was
verified by reaction of preformed 5 (1 equiv) with 4,4’-
bipyridine (1.5 equiv), which likewise resulted in gel forma-
tion.
Received: November 17, 2010
Revised: January 4, 2010
Published online: February 23, 2011
Gelation of toluene can be achieved with as low as
0.5 wt% of the two components. The system thus qualifies as
a supergelator.[26] Gel formation with 5 and 4,4’-bipyridine
was also observed in benzene and THF but not in mesitylene.
Additional evidence for the formation of a 3-dimensional
network was obtained by scanning electron microscopy
(SEM). SEM images of the xerogel obtained from toluene
show a network of entangled nanofibers with diameters less
than 40 nm. This observation is consistent with the trans-
parency of the gel.[27]
Keywords: boron · crystal engineering ·
dynamic covalent chemistry · networks · self-assembly
.
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[20] X-ray data for 2: C28.25H27BNO2, M = 423.32 gmolÀ1, space group
3
ꢀ
R3, a = b = 29.9203(12), c = 17.0275(15) ꢀ, V= 13201.2(14) ꢀ ,
Z = 18, 1 = 0.958 gcmÀ1, m = 0.059 mmÀ1, F(000) = 4041, crystal
size 0.23 ꢆ 0.20 ꢆ 0.15 mm3, 39206 reflections collected, 5972
independent reflections, Rint = 0.0872, R1 [I > 2s(I)] = 0.0950,
wR2 (all data) = 0.2740, largest difference peak 0.389 eꢀÀ3
,
largest difference minimum À0.323 eꢀÀ3. X-ray data for 3:
C38H38BNO2, M = 551.50 gmolÀ1
,
space group R3, a = b =
ꢀ
28.036(6), c = 22.600(4) ꢀ, V= 15384(6) ꢀ3, Z = 18, 1 =
1.072 gcmÀ1, m = 0.065 mmÀ1, F(000) = 5292, crystal size 0.30 ꢆ
0.25 ꢆ 0.19 mm3, 277448 reflections collected, 5989 independent
reflections, Rint = 0.1245, R1 [I > 2s(I)] = 0.1333, wR2 (all data) =
0.3899, largest difference peak 0.473 eꢀÀ3, largest difference
minimum À0.439 eꢀÀ3. For both structures, scattering contribu-
tions from diffuse solvent were removed using the SQUEEZE
routine in PLATON (see the Supporting Information).
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www.angewandte.org
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