Formation of boronate ester polymers with efficient intrastrand charge-transfer transitions by three-component reactions

Article abstract of DOI:10.1002/ejic.200700723

The three-component reaction of aryl boronic acids with 1,2,4,5-tetrahydroxybenzene and 1,2-bis(4-pyridyl)ethylene or 4,4′-bipyridine leads to the formation of dark-purple boronate ester polymers. Crystallographic analyses show that the polymer strands have a zig-zag geometry, and the bis(dioxaborole) units are connected by dipyridyl linkers through dative B-N interactions. Upon dissolution of the polymers in hot chloroform, most of the B-N connections are broken, which indicates that polymer formation is a reversible process. A computational study provides evidence that the strong color of the polymers is due to efficient intrastrand charge-transfer excitations from the tetraoxobenzene to the dipyridyl linker. Wiley-VCH Verlag GmbH & Co. KGaA, 2007.

Full text of DOI:10.1002/ejic.200700723

FULL PAPER
DOI: 10.1002/ejic.200700723
Formation of Boronate Ester Polymers with Efficient Intrastrand Charge-
Transfer Transitions by Three-Component Reactions
Nicolas Christinat,[a] Emmanuel Croisier,[a] Rosario Scopelliti,[a][‡] Michele Cascella,[a]
Ursula Röthlisberger,[a] and Kay Severin*[a]
Keywords: Boron / Charge transfer / Polymers / Self-assembly
The three-component reaction of aryl boronic acids with
1,2,4,5-tetrahydroxybenzene and 1,2-bis(4-pyridyl)ethylene
or 4,4Ј-bipyridine leads to the formation of dark-purple bo-
ronate ester polymers. Crystallographic analyses show that
the polymer strands have a zig–zag geometry, and the
bis(dioxaborole) units are connected by dipyridyl linkers
through dative B–N interactions. Upon dissolution of the
polymers in hot chloroform, most of the B–N connections are
broken, which indicates that polymer formation is a revers-
ible process. A computational study provides evidence that
the strong color of the polymers is due to efficient intrastrand
charge-transfer excitations from the tetraoxobenzene to the
dipyridyl linker.
(© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim,
Germany, 2007)
coworkers who made polymers by the reaction of the bo-
rylated ferrocene [(C5H4BMe2)2Fe] with 4,4Ј-bipyridine or
pyrazine[13] and by the group of Jäkle who assembled poly-
mers with borane end groups by the reaction with 4,4Ј-bi-
pyridine.[14,15] In the following we show that boronic acid–
diol condensations can be used in parallel with dative B–N
interactions to generate novel polymer architectures by
three-component reactions. The resulting crystalline poly-
mers are deeply colored due to efficient intrastrand charge-
transfer transitions.
Introduction
Boronic acids are known to easily and reversibly con-
dense with diols to form boronate esters. This type of reac-
tion has been exploited extensively in carbohydrate chemis-
try where boronic acids are used as an integral part of re-
ceptors and sensors or as protecting groups.[1] The revers-
ible formation of boronic esters has also been employed to
build complex structures such as macrocycles,[2] dendri-
mers,[3] helicates,[4] nanotubes,[5] or porous covalent organic
frameworks.[6] The labile boron–oxygen bonds are used here
in a similar fashion to metal–ligand interactions in supra-
molecular coordination chemistry. Constitutional dynamic
polymers[7] with boronate ester backbones represent an-
other interesting class of compounds. Lavigne and cowork-
ers showed that poly(dioxaborolane)s[8] and poly(dioxabor-
ole)s[9] can be obtained by the condensation of diboronic
acids with pentaerythritol or 1,2,4,5-tetrahydroxyben-
zene.[10] An interesting feature of these polymers is that hy-
drolytic degradation can be reversed by annealing under
vacuum (“self-repairing polymers”).[8] The polycondensa-
tion of diboronic acids with sugar derivatives as linkers has
been investigated by the groups of Shinkai[11] and Shim-
izu.[12] Again it was found that the polymerization is a re-
versible process, which depends on the conditions.
Results and Discussion
To evaluate the possibility to generate boronate polymers
with ditopic N-donor ligands, we heated a mixture of 1,2-
bis(4-pyridyl)ethylene, two equivalents of 4-ethylphenylbo-
ronic acid, and one equivalent of 1,2,4,5-tetrahydroxyben-
zene in benzene with the use of a Dean–Stark trap. A
slightly yellow solution was obtained, from which, upon
cooling, a dark-purple solid (1) precipitated (Scheme 1). A
similar behavior was observed when 4-tert-butylphenylbo-
ronic acid was employed: when the hot benzene solution
was cooled, a dark precipitate formed (2).
At room temperature, products 1 and 2 were found to
display low solubility in common organic solvents such as
benzene, chloroform, acetonitrile, and tetrahydrofuran.
Upon heating, however, it was possible to dissolve them in
chloroform. This process was accompanied with a strong
change in color: the dark-purple solids gave rise to weakly
yellow solutions. When the hot chloroform solutions were
cooled to room temperature, the purple polymers started to
precipitate after a few minutes, which indicated that the
color change was due to a reversible process.
An alternative way to generate dynamic polymers with
boron centers in the backbone is the polymerization by
dative B–N bonds. This was demonstrated by Wagner and
[a] Institut des Sciences et Ingénierie Chimiques, École Polytech-
nique Fédérale de Lausanne
1015 Lausanne, Switzerland
Fax: +41-21-693-9305
E-mail: kay.severin@epfl.ch
[‡] X-ray crystal structural analysis.
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FULL PAPER
Scheme 1. Synthesis of polymers 1 and 2.
Scheme 2. When dissolved in CHCl3, polymer 1 splits into 1,2-
bis(4-pyridyl)ethylene and bis(dioxaborole) 3.
A CDCl3 solution of polymer 1 was analyzed by 1H
NMR spectroscopy, and its spectrum was nearly a superpo-
sition to those of 1,2-bis(4-pyridyl)ethylene and bis(di-
oxaborole) 3 (∆δ Ͻ 0.15 ppm). The latter compound was
prepared for comparison by condensing 4-ethylphenyl-
boronic acid with 1,2,4,5-tetrahydroxybenzene.[16] The 11B
NMR spectrum of 1 showed a peak at δ = 28.8 ppm, which
was similar to that of 3 (δ = 34.6 ppm) and very distinct
from the expected signal of a tetracoordinated boron cen-
ter.[17] This data suggested that the chloroform solution of
polymer 1 contained mainly dissociated 1,2-bis(4-pyridyl)-
ethylene and bis(dioxaborole) 3 (Scheme 2). The small dif-
ferences in chemical shifts can be explained by the forma-
tion of minor amounts of B–N adducts, which are in fast
equilibrium with 1,2-bis(4-pyridyl)ethylene and 3. A related
situation was observed for the ferrocene polymers prepared
by Wagner et al.: upon dissolution at high temperature, the
polymer was split into its monomers, [(C5H4BMe2)2Fe] and
4,4Ј-bipyridine.[13]
The reversible formation of 1 from 1,2-bis(4-pyridyl)-
ethylene and 3 allowed single crystals to be obtained from
CHCl3/pentane solutions, and crystallographic analysis
confirmed the polymeric structure (Figure 1). The ditopic
1,2-bis(4-pyridyl)ethylene acts as a bridge between the bo-
ronate esters. The average boron–nitrogen bond length of
1.677 Å is long relative to what was observed for other crys-
tallographically characterized adducts between sp2-nitrogen
donors and tetracoordinated boron centers.[18] Accordingly,
the tetrahedral character (THC)[18] of the boron center is
Figure 1. Molecular structure of polymer 1 in the crystal. Top: view
of the repeating unit – only one of the two crystallographically
independent subunits is shown; bottom: view of the polymer chain.
The solvent molecule (CHCl3) is not shown for clarity.
The modular nature of the three-component polymeriza-
relatively low (THCav = 72.5%). This data is in line with tion reactions offers structural flexibility as demonstrated
the results of the NMR spectroscopic experiments, which by the formation of polymer 4, which contains 4,4Ј-bipyr-
suggest that the dative B–N bond in 1 is weak. The B–O idine instead of 1,2-bis(4-pyridyl)ethylene as the bridging
bonds (B–Oav 1.479 Å) are, as expected, longer than N-donor ligand (Scheme 3). The physical properties of 4 are
what was observed for 1,2,4,5-tetrahydroxybenzene-based similar to those of 1 and 2: the polymer has a dark-purple
bis(dioxaborole)s with trigonal planar boron (B–O 1.388– color and it splits into 4,4Ј-bipyridine and the respective
1.395 Å).[14]
boronate ester upon dissolution in hot chloroform.
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Boronate Ester Polymers with Intrastrand Charge-Transfer Transitions
with one trigonal and one tetragonal boron center. Appar-
ently, the methyl boronate ester is not sufficiently Lewis
acidic to promote extended polymerization.
A striking feature of polymers 1, 2, and 4 is their very
dark-purple color. Because the color disappears upon dis-
solution in chloroform, it must be the fully assembled poly-
mer, which gives rise to the strong absorption. To under-
stand this phenomenon, we performed single-point second-
order approximate coupled-cluster (CC2)[19] calculations of
the electronic excitations on model systems of the acid and
base pairs 1,2-bis(4-pyridyl)ethylene and 4,5-dihydroxy-
phenyl-4-ethylphenylborole and of their respective conden-
sate in vacuo. Both the uncomplexed acid and base showed
the first absorption peak in the near-UV region (4.2 and
3.8 eV, respectively), which is consistent with the finding
that the solution containing nonassembled monomers is
colorless. In contrast, the assembled acid–base complex
showed one transition in the yellow-green region (2.2 eV),
which is in agreement with the observed purple coloration
of its crystals.
Scheme 3. Synthesis of polymer 4.
Decomposition of the optical excitation in the visible re-
gion onto a molecular orbital basis shows that it has an
almost pure HOMO–LUMO π–π* character (88%). This
transition corresponds to an intrastrand charge-transfer ex-
citation from the hydroxy–phenyl ring of the dioxaborol
moiety to the 1,2-bis(4-pyridyl)ethylene ring (Figure 3).[20]
Charge-transfer transitions internal to the dioxaborol
strand, specifically between the hydroxy–phenyl and the
ethyl–phenyl rings, are not strongly affected by the presence
of the pyridyl base; therefore, they do not contribute to the
absorption in the visible region, and they remain confined
in the UV region of the spectrum. Furthermore, it should
be noted that there is no orbital contribution of boron and
thus no extended conjugation as it was observed for poly-
mers containing tricoordinate boron.[9b]
The molecular structure of polymer 4 in the crystal is
depicted in Figure 2. As it was observed for 1, a zig–zag
chain is formed. With B1–N1 1.702(5) Å, B1–O1
1.470(4) Å, and B1–O2 1.479(4) Å, the bond lengths of 4
are very similar to those of 1. Again, the long B–N bond
correlates with a low tetrahedral character of 69.5%.
Figure 2. Molecular structure of polymer 4 in the crystal. Top: view
of the repeating unit; bottom: view of the polymer chain.
Figure 3. Left: Theoretical cluster-model of the polymeric acid–
base pair. Center-right: HOMO–LUMO orbitals responsible for
the charge-transfer optical transition in the yellow-green region.
The solid-state 11B NMR spectra of polymers 1, 2, and
4 are in agreement with the depicted structures and show
peaks around 10 ppm. An interesting difference was ob-
served when we attempted to make a polymer from methyl
boronic acid, 4,4Ј-bipyridine, and 1,2,4,5-tetrahydroxyben-
zene. Again, a dark-purple precipitate was formed (5), but
its solid-state 11B NMR spectrum showed two peaks at ca.
Conclusions
We showed that condensation reactions of aryl boronic
32 and ca. 9 ppm. The former peak is characteristic for a acids with 1,2,4,5-tetrahydroxybenzene can be used in paral-
trigonal planar boron center.[16] We therefore propose that lel with dative B–pyridyl interactions to generate novel
compound 5 is the monoadduct [MeB(C6H2O4)BMe(bipy)] polymer architectures by three-component reactions. The
Eur. J. Inorg. Chem. 2007, 5177–5181
© 2007 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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R. Scopelliti, K. Severin et al.
FULL PAPER
3
3
(q, J = 8 Hz, 4 H, CH2), 7.31 (s, 2 H, phenyl), 7.34 (d, J = 8 Hz,
4 H, phenyl), 8.00 (d, 3J = 8 Hz, 4 H, phenyl) ppm. 13C NMR
(101 MHz, CDCl3): δ = 15.45, 29.36, 98.36, 128.08, 135.15, 143.95,
149.18 ppm. 11B NMR (128 MHz, CDCl3): δ = 34.6 (h1/2 = 870 Hz)
ppm. C22H20B2O4 (370.01): calcd. C 71.41, H 5.45; found C 71.25,
H 5.57.
resulting crystalline polymers are deeply colored due to ef-
ficient intrastrand charge-transfer transitions. An intrinsic
advantage of the multicomponent assembly strategy is its
modular nature, which allows structural modifications to be
easily incorporated. This was evidenced by utilizing dif-
ferent boronic acids and dipyridyl linkers. Because of the
weak B–N interaction, the polymers are not stable in solu-
tion. However, the reversible formation of the polymer
backbone could be interesting from a processing point of
view. Furthermore, single crystals can be grown and thus
structural information can be obtained. Overall, the work
further highlights the potential of boronic acids as building
blocks for the generation of polymers with interesting new
structures and properties.
Polymer 4:
(135 mg,
A
suspension of 3,5-dimethylphenylboronic acid
0.90 mmol), 1,2,4,5-tetrahydroxybenzene (64 mg,
0.45 mmol), and 4,4Ј-bipyridyl (70 mg, 0.45 mmol) in distilled ben-
zene (90 mL) was heated under reflux with the use of a Dean Stark
trap. After 1 h, the suspension was filtered hot, and the filtrate was
cooled to room temperature. Upon cooling, a purple solid precipi-
tated. The precipitate was filtered, washed with pentane, and dried
under vacuum. Yield: 176 mg, 74%. (C32H28B2N2O4)n (526.20)n:
calcd. C 73.04, H 5.36, N 5.32; found C 72.62, H 5.43, N 5.45.
Crystals were obtained by slow diffusion of pentane into a solution
of 4 in chloroform.
Adduct 5: A suspension of methylboronic acid (54 mg, 0.90 mmol),
1,2,4,5-tetrahydroxybenzene (64 mg, 0.45 mmol), and 4,4Ј-bipyr-
idyl (70 mg, 0.45 mmol) in distilled benzene (90 mL) was heated
under reflux with the use of a Dean Stark trap. After 1 h, the sus-
pension was filtered hot, and the filtrate was cooled to room tem-
perature. Upon cooling, a purple solid precipitated. The precipitate
was filtered, washed with pentane, and dried under vacuum. Yield:
110 mg, 71%. C18H16B2N2O4 (345.95)n: calcd. C 62.49, H 4.66, N
8.10; found C 62.03, H 4.46, N 7.82.
Experimental Section
General: 1,2-Bis(4-pyridyl)ethylene, 4,4Ј-bipyridyl, and the boronic
acids were obtained from commercial sources. 1,2,4,5-Tetrahy-
droxybenzene was prepared as described in the literature.[21] All
reactions were carried out under an atmosphere of dry nitrogen by
using standard Schlenk techniques. The 1H, 13C and 11B spectra
were recorded with a 400 MHz spectrometer by using the residual
protonated solvents (1H, 13C) as internal standards or BF3·OEt2
(11B) as an external standard. 1H NMR spectra of the polymers
were obtained by dissolving the samples in hot chloroform and
then rapidly measuring the solutions before the polymers started
to precipitate. The solid-state NMR spectra were recorded with a
Bruker DRX 400 spectrometer with a 7.0-widebore magnet by uti-
lizing a 3-mm CPMAS probehead. A solution of boric acid in H2O
was used as the external standard (δ = 19.3 ppm).
X-ray Crystallography: Details about the crystals and their struc-
ture refinement are listed in Table 1. Data collection was performed
at 100(2) K by means of a Bruker APEX II CCD detector. Data
reduction was carried out with EvalCCD.[22] Absorption correction
Table 1. Crystallographic data for polymers 1 and 4.
Polymer 1: A suspension of 4-ethylphenylboronic acid (135 mg,
0.90 mmol), 1,2,4,5-tetrahydroxybenzene (64 mg, 0.45 mmol), and
1,2-bis(4-pyridyl)ethylene (82 mg, 0.45 mmol) in distilled benzene
(90 mL) was heated under reflux with the use of a Dean Stark trap.
After 8 h, the suspension was filtered hot, and the filtrate was
cooled to room temperature. Upon cooling, a purple solid precipi-
tated. The precipitate was filtered, washed with pentane, and dried
under vacuum. Yield: 196 mg, 79%. (C34H30B2N2O4)n (552.24)n:
calcd. C 73.95, H 5.48, N 5.07; found C 74.25, H 5.47, N 5.16.
Crystals were obtained by slow diffusion of pentane into a solution
of 1 in chloroform.
1·CHCl3
4
Empirical formula
Mol. weight / gmol–1
Crystal size / mm
Crystal system
Space group
a / Å
b / Å
c / Å
α / °
β / °
C35H31B2Cl3N2O4
671.59
C16H14BNO2
263.09
0.52ϫ0.15ϫ0.10
monoclinic
P21/n
8.104(5)
11.507(7)
14.356(8)
90
95.82(6)
90
1331.8(14)
4
1.312
100(2)
0.085
0.59ϫ0.29ϫ0.16
triclinic
¯
P1
9.989(2)
12.797(4)
13.452(4)
100.01(3)
91.234(17)
102.769(19)
1648.3(8)
2
1.353
100(2)
0.320
3.32 to 25.02
–11 Ǟ 11
–15 Ǟ 15
–16 Ǟ 16
28027
γ / °
Volume / Å3
Z
Polymer 2:
A
suspension of 4-tert-butylphenylboronic acid
(160 mg,
0.90 mmol), 1,2,4,5-tetrahydroxybenzene (64 mg,
Density / gcm–3
Temperature / K
Absorption coeff. / mm–1
Θ range / °
Index ranges
0.45 mmol), and 1,2-bis(4-pyridyl)ethylene (82 mg, 0.45 mmol) in
distilled benzene (90 mL) was heated under reflux with the use of
a Dean Stark trap. After 8 h, the suspension was filtered hot, and
the filtrate was cooled to room temperature. Upon cooling, a pur-
ple solid precipitated. The precipitate was filtered, washed with
pentane, and dried under vacuum. Yield: 222 mg, 81%.
(C38H38B2N2O4)n (608.34)n: calcd. C 75.02, H 6.30, N 4.60; found
C 75.18, H 6.39, N 4.39.
3.36 to 25.00
–9 Ǟ 9
–13 Ǟ 13
–17 Ǟ 17
16398
Reflections collected
Independent reflections
Absorption correction
Max. / min. transmission
Data / restraints / param.
Goodness-of-fit on F2
5765 (Rint = 0.0729) 2338 (Rint = 0.1178)
semiempirical
1.0000 / 0.8119
5765 / 0 / 452
1.127
semiempirical
1.0000 / 0.4708
2338 / 0 / 181
1.118
R1 = 0.0678
wR2 = 0.1207
R1 = 0.1249
wR2 = 0.1408
0.264 / –0.233
Boronate Ester 3: A suspension of 4-ethylphenylboronic acid
(135 mg, 0.90 mmol) and 1,2,4,5-tetrahydroxybenzene (64 mg,
0.45 mmol) in distilled benzene (90 mL) was heated under reflux
with the use of a Dean Stark trap. After 8 h, the suspension was
filtered hot, and the filtrate was cooled to room temperature. Upon
cooling, white crystals formed. The crystals were filtered, washed
with pentane, and dried under vacuum. Yield: 93 mg, 58%. 1H
Final R indices [I Ͼ 2σ(I)] R1 = 0.0564
wR2 = 0.0860
R indices (all data)
R1 = 0.1081
wR2 = 0.1025
Larg. diff. peak/hole / eÅ–3 0.307 / –0.279
3
NMR (400 MHz, CDCl3): δ = 1.29 (t, J = 7 Hz, 6 H, CH3), 2.73
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Boronate Ester Polymers with Intrastrand Charge-Transfer Transitions
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H atoms were placed in calculated positions by using the “riding
model”. CCDC-653486 and -653487 contains the supplementary
crystallographic data for this paper. These data can be obtained
free of charge from The Cambridge Crystallographic Data Centre
via www.ccdc.cam.ac.uk/data_request/cif.
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Acknowledgments
We gratefully acknowledge the support of the Swiss National Sci-
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Dr. Anuji Abraham for measuring the solid-state 11B NMR spec-
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Received: July 10, 2007
Published Online: September 12, 2007
Eur. J. Inorg. Chem. 2007, 5177–5181
© 2007 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.eurjic.org
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