Dehydrocoupling and Silazane Cleavage Routes to Organic–Inorganic Hybrid Polymers with NBN Units in the Main Chain

Article abstract of DOI:10.1002/anie.201602342

Despite the great potential of both π-conjugated organoboron polymers and BN-doped polycyclic aromatic hydrocarbons in organic optoelectronics, our knowledge of conjugated polymers with B?N bonds in their main chain is currently scarce. Herein, the first examples of a new class of organic–inorganic hybrid polymers are presented, which consist of alternating NBN and para-phenylene units. Polycondensation with B?N bond formation provides facile access to soluble materials under mild conditions. The photophysical data for the polymer and molecular model systems of different chain lengths reveal a low extent of π-conjugation across the NBN units, which is supported by DFT calculations. The applicability of the new polymers as macromolecular polyligands is demonstrated by a cross-linking reaction with Zr^IV.

Full text of DOI:10.1002/anie.201602342

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International Edition: DOI: 10.1002/anie.201602342
German Edition: DOI: 10.1002/ange.201602342
Inorganic Polymers
Dehydrocoupling and Silazane Cleavage Routes to Organic–Inorganic
Hybrid Polymers with NBN Units in the Main Chain
Thomas Lorenz, Artur Lik, Felix A. Plamper, and Holger Helten*
Abstract: Despite the great potential of both p-conjugated
organoboron polymers and BN-doped polycyclic aromatic
hydrocarbons in organic optoelectronics, our knowledge of
conjugated polymers with BÀN bonds in their main chain is
hydrocarbons (PAHs), has emerged as a viable strategy to
produce novel organic–inorganic hybrid compounds that
show structural similarities to their all-carbon congeners,
but in many cases fundamentally altered electronic proper-
[
3,4]
currently scarce. Herein, the first examples of a new class of
organic–inorganic hybrid polymers are presented, which
consist of alternating NBN and para-phenylene units. Poly-
condensation with BÀN bond formation provides facile access
ties.
This has led to molecular materials with intriguing
[
3,4]
properties, such as BN-doped nanographenes.
Linear
polymers with main-chain BÀN linkages involving tricoordi-
nate boron centers (B, C), however, are relatively undevel-
[5,6]
to soluble materials under mild conditions. The photophysical
data for the polymer and molecular model systems of different
chain lengths reveal a low extent of p-conjugation across the
NBN units, which is supported by DFT calculations. The
applicability of the new polymers as macromolecular poly-
oped.
Theoretical studies predict that the (partial) sub-
stitution of CC by BN units in semiconducting organic
polymers should result in an increase in the electronic band
[7–9]
gap of these materials.
It has been proposed that this
concept may be employed as an effective means of band-gap
IV
[7b]
ligands is demonstrated by a cross-linking reaction with Zr .
tuning.
Chujo and co-workers have devised a synthetic
method that uses a haloboration–phenylboration polymeri-
B
oron-containing p-conjugated polymers and oligomers are
zation sequence for the preparation of polymers of type B,
[10a,c]
currently attracting considerable attention owing to their
great potential for applications in organic electronics and
optoelectronics, for example, in (polymer-based) organic
light-emitting diodes (OLEDs/PLEDs), photovoltaics
which feature acyl groups at the nitrogen atoms.
How-
ever, for the polymers obtained, poorly extended conjugation
was deduced from UV/Vis absorption measurements. This
[
10c]
was attributed to the presence of cross-links
or meta-
[1b]
(
OPV), and organic field effect transistors (OFETs), or as
phenylene linkages in the polymer chains. Very recently,
Liu, Jäkle, and co-workers reported the synthesis of 1,2-
azaborine oligomers and a corresponding conjugated poly-
mer, which may be regarded as a derivative of a cyclolinear
[
1]
sensory or imaging materials. In such polymers, which
feature tricoordinate boron atoms linked to organic p-systems
in the main chain (A; Scheme 1), the conjugation path may be
extended via the vacant p orbital on boron.
The replacement of CC units by isoelectronic and isosteric
BN units, primarily applied to mono- and polycyclic aromatic
[
2]
[11]
version of polymer type B. To date, poly(boronic carbama-
p
[10b]
[10a,c]
te)s,
poly(boronic carbamoyl chloride)s,
and related
copolymers, which were reported by Chujo and co-workers,
are the only known examples of organoboron polymers that
clearly fall into category C. The derivatives that have been
[12]
prepared contain NBN units bridged by meta-phenylene
groups. A few examples of conjugated polymers comprising
1
,3,2-benzodiazaboroline units have also been prepared. This
building block was incorporated into polymer chains either
[13]
via the benzo core or via the benzo core and the boron
center; chain linking through the N atoms has not been
investigated thus far.
Scheme 1. Generalized structural units of boron-containing p-conju-
gated polymers or oligomers (p=organic p system; R,R’=organic
substituents).
[
14]
We are interested in the development of mild and
environmentally benign methods for the construction of
extended molecular architectures. Polymerization by BÀN
bond formation may provide elegant access to BÀN contain-
[
*] T. Lorenz, A. Lik, Dr. H. Helten
Institute of Inorganic Chemistry
RWTH Aachen University
ing macromolecules. The incorporation of diaminoborane
Landoltweg 1, 52056 Aachen (Germany)
E-mail: holger.helten@ac.rwth-aachen.de
Homepage: http://www.ac.rwth-aachen.de/extern/helten/
index.html
functional groups appeared as an attractive target because
2À
their dianionic form, [RB(NR’) ] , is a versatile bidentate
2
[15]
ligand system. Diaminoboryl units may also coordinate to
[16]
a metal center via the boron atom. Herein, we report on the
exploration of two conceivable routes to poly[N-(para-
phenylene)diimidoborane(3)]s (PPP-DIBs), namely dehydro-
Priv.-Doz. Dr. F. A. Plamper
Institute of Physical Chemistry
RWTH Aachen University
[5,17,18]
Landoltweg 2, 52056 Aachen (Germany)
coupling
and silazane cleavage with Si/B exchange; the
latter led to the first soluble derivatives of this class of
Supporting information for this article can be found under:
http://dx.doi.org/10.1002/anie.201602342.
7
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polymers, which can function as macromolecular polyligands,
as demonstrated by cross-linking with Zr .
ing 6 with THF, it dissolved with vigorous bubbling, pointing
IV
to H2 production. However, in the further course of the
reaction, the formation of a colorless precipitate was
observed, and the product obtained after work-up was
Manners and co-workers recently showed that aniline–
borane (1) undergoes spontaneous dehydrocoupling with
aniline (2) in solution at ambient temperature to selectively
[21]
found to be insoluble in common organic solvents.
[19]
afford dianilinoborane (3; Scheme 2). The phenyl rings of 3
With a view to enhance the solubility of the target
polymers, we decided to incorporate organic side groups
through the introduction of substituents at the boron atoms.
For this purpose, an alternative strategy for BÀN bond
formation proved to be more convenient, namely silazane
SiÀN bond cleavage with chloroboranes. We chose 2,4,6-
trimethylphenyl (mesityl, Mes) as the substituent at boron,
and we first explored the feasibility of our approach in the
synthesis of two molecular model compounds, 7 and 8
(
Scheme 3). Compound 7 was prepared by condensation of
N-trimethylsilylaniline (9; 2 equiv) and dichloro-
mesityl)borane (10). In the same manner, the reaction of
(
N,N’-bis(trimethylsilyl)-para-phenylenediamine (11) with 10
(2 equiv) and the subsequent reaction of the intermediate, 12,
with 9 (2 equiv) afforded the extended system 8, which has
two diaminoborane building blocks. Both model compounds
were isolated in excellent yields of 95 (7) and 92% (8). Their
identity was unambiguously ascertained by NMR spectros-
copy and mass spectrometry; the molecular structure of 7 was
determined by single-crystal X-ray diffraction (Figure 1). In
the solid state, compound 7 is E,Z-configured, as observed in
other structurally characterized B,N,N’-trisubstituted diami-
[
19]
Scheme 2. Synthesis of 3 and 5 and attempted dehydropolymeriza-
tion of 6.
Table 1: Photophysical data for 3, 5, 7, 8, and polymer 13’ and calculated
vertical singlet excitations for 3, 5, 7, and 8 (TD-DFT, B3LYP/6-31G(d,p)).
[
a]
[a]
[b]
[b]
[b]
3
5
7
8
13’
labs,max [nm]
272
272
325
290
308
365
267
275
420
280
293
465
295
–
455
[
22]
labs,TD-DFT [nm]
noboranes.
While the N-phenyl substituent at the E-
[
c]
lem,max [nm]
configured B1ÀN2 bond of 7 adopts a largely coplanar
[
a] In THF. [b] In CH Cl . [c] Irradiated at the absorption maximum.
2 2
arrangement with the N-B-N plane (twist angle: 3.8(3)8), the
[23]
phenyl group at N1 is twisted out of that plane by 46.5(3)8.
adopt a largely coplanar arrangement with the N-B-N
moiety in the solid state, thus potentially pointing to
extended
p-conjugation across the NBN unit. We repeated the
reported synthesis to obtain photophysical data for 3
(
Table 1). The lowest-energy absorption band in the UV/
Vis spectrum of 3 in CH Cl2 showed a maximum at
2
labs,max = 272 nm. Fluorescence was detected at lem,max
25 nm. We then targeted the synthesis of an extended
molecular system with two diaminoborane units connected
=
3
[20]
through a para-phenylene linkage. The bisborane adduct
of para-phenylenediamine (4) was reacted with excess
aniline, which enabled the isolation of compound 5 in 82%
yield. The UV/Vis spectrum of 5 displayed an absorption
band at labs,max = 290 nm. This bathochromic shift with
respect to 3, albeit moderate, confirms that the p-
conjugation has been extended to a certain degree by the
elongation of the chain length.
We anticipated that the monoborane adduct of para-
phenylenediamine (6), which contains both a BH coordi-
3
nated and a free amino group within the same molecule,
should undergo spontaneous dehydrocoupling with itself
in the same manner. This process should provide access to
the parent derivative of a new class of organic–inorganic
hybrid polymers featuring alternating para-phenylene and
diaminoborane groups in the main chain, poly[N-(para-
phenylene)diimidoborane(3)]s (PPP-DIBs). Upon treat- Scheme 3. Synthesis of compounds 7 and 8 and polymers 13 and 13’.
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and gel permeation chromatography (GPC) and dynamic
light scattering (DLS) to determine its molecular weight. The
11
1
B{ H} NMR spectrum of 13’ in CDCl displayed a broad
3
resonance at d = 31 ppm. As for the model systems 7 and 8,
1
the H NMR spectrum of 13’ at 258C showed well-resolved
mesityl resonances and broad signals for the phenylene and
NH protons as well as for the SiMe end groups. At À408C,
3
the signals decoalesced to give sets of multiple closely spaced
peaks (Figure S19), which is likely due to the large number of
possible relative configurations at the BÀN bonds along the
1
polymer chain. The integral ratio of the H resonances of the
SiMe (d = 0.00–0.30 ppm) and the tert-butyl end groups (d =
3
1
.31 ppm) was 1:1, which indicates that quantitative end
capping has occurred. Even after prolonged evacuation (ca.
À2
1
0
mbar), residual n-pentane was still detectable, which
suggests that it is confined by the polymer chains.
Figure 1. Molecular structure of 7 in the solid state with thermal
ellipsoids set at 50% probability (only one of two independent
molecules with similar structural data shown; C-bonded hydrogen
atoms omitted for clarity).
The macromolecular nature of 13’ was confirmed by GPC
(Figure 2a). This revealed an average molecular weight of
NMR spectroscopy provided evidence that the E,Z
1
configuration persists in solution. In the H NMR spectra of
7, the phenyl and the NH proton resonances showed dynamic
line broadening. Upon lowering the temperature, the signals
split into two well-resolved sets of resonances of equal
1
intensity. An analysis of the 2D H COSY and NOESY
spectra recorded at À408C allowed to assign the resonances
to the substituents at the E- and the Z-configured BÀN bond.
1
Analysis of variable-temperature H NMR spectra revealed
that the dynamic process can be described as the simultaneous
rotation about both BÀN bonds, thus equilibrating two
degenerate states with interchanged configuration
(
E,ZÐZ,E). The activation parameters for this process were
° À1 °
estimated from an Eyring plot, DH = 60 kJmol and DS =
À1 À1
À47 Jmol
K
(see the Supporting Information, Figure S5).
Similar dynamic line-broadening effects were also observed
for 8. In the UV/Vis spectra, the lowest-energy absorption
bands were detected at labs,max = 267 (7) and 280 nm (8), which
points to some degree of extension of the conjugation length
with chain elongation. Both compounds fluoresced with
Figure 2. a) Molar mass distribution of 13’ as determined by GPC (in
THF, versus polystyrene standards). b) Intensity-weighted size distribu-
tion of the particles in a 13’ sample in THF as determined by DLS
(q=908). c) SIMS spectrum of 13’.
[
24]
a large Stokes shift (Table 1).
With the aim to synthesize polymer 13, compound 11 was
reacted with 10 in a 1:1 ratio in CH Cl at ambient temper-
ature (Scheme 3). An B{ H} NMR spectrum of the reaction
mixture obtained after 15 min revealed that 10 had already
been consumed, and a resonance at d = 31 ppm indicated the
formation of diaminoborane groups. Concomitant formation
M = 20400 and M = 7900, respectively, according to
2
2
w
n
11
1
a number average degree of polymerization (DP ) of 33,
n
and a fairly large polydispersity index (PDI) of 2.6, typical of
a polymer formed in a step-growth process. The GPC trace
showed a tail with a shoulder, pointing to the presence of
a certain number of lower-molecular-mass species, which
could not be separated by repeated precipitation. Evidence
for the presence of macrocycles of 6–12 repeat units in the
sample of 13’ was obtained by secondary ion mass spectrom-
etry (SIMS; Figure 2c). This may also explain why the
of the volatile condensation byproduct, Me SiCl, was con-
3
1
firmed by the detection of its H NMR resonance at d =
0
.45 ppm. After the mixture had been stirred for 18 h at
room temperature, (p-tBu)C H NHSiMe (10 mol%) was
6
4
3
added to destroy the reactive BÀCl end groups of 13. The
1
product was then purified by precipitation with n-pentane
concentration of end groups estimated by H NMR spectros-
(
repeated twice). This afforded the end-capped PPP-DIB 13’
copy was actually lower than expected from the GPC-derived
[25]
as a white solid in 83% yield, which proved to be soluble in
solvents of moderate polarity, such as CH Cl , CHCl , and
M value.
n
DLS measurements revealed a hydrodynamic
radius (R ) of 5.1 nm for particles of 13’ in THF solution
2
2
3
h
THF. The polymer was characterized by multinuclear NMR,
UV/Vis, and fluorescence spectroscopy, mass spectrometry,
(Figure 2b). The lowest-energy absorption maximum in the
UV/Vis spectrum of 13 (labs,max = 295 nm) is slightly red-
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shifted with respect to those of the molecular model
characterized as p–p* excitations, despite the noted deviation
compounds, which corroborates the hypothesis that the
conjugation length is somewhat extended upon polymer
formation. The fluorescence spectrum of 13 showed a max-
imum at lem,max = 455 nm.
from coplanarity.
We then explored the potential of PPP-DIBs to function
as macromolecular polyligand frameworks; we chose polymer
IV
[27]
13’ and Zr for our proof-of-concept studies. Heating 13’
To gain more insight into the geometric and electronic
and substoichiometric amounts of Zr(NMe ) in the presence
2
4
[26]
structures of the PPP-DIBs, DFT calculations were carried
out at the B3LYP/6-31 + G(d,p) level of theory for the
molecular model compounds 3, 5, 7, and 8 (see the Supporting
Information for the complete results). These calculations
revealed that the preferred diastereomers of 3 and 5 are
E-configured at each BÀN unit. In both (E,E)-3 and
of THF in 1,2-dichlorobenzene at 1208C yielded a yellow
solid, which swells in organic solvents (Figure S43), a charac-
teristic feature of cross-linked polymers (Scheme 4). The
(
E,E,E,E)-5, the aromatic rings and the N-B-N planes adopt
an almost perfectly coplanar arrangement (maximum twist
angle: 0.198). For compound 7, our calculations confirmed the
preference of the E,Z configuration, as observed in the
crystal. For 8, we found three diastereomers of comparable
free energy, with differing configurations at adjacent BÀN
bonds.
The wavelengths of the lowest-energy vertical singlet
excitations of 3, 5, 7, and 8 obtained by TD-DFT are in fairly
good agreement with their experimental UV/Vis absorption
maxima (Table 1). The transitions correspond to HOMO!
LUMO processes. For (E,E)-3 and (E,E,E,E)-5, these MOs
can be clearly identified as p orbitals that are extended over
the whole molecule (Figures 3 and S48). Furthermore, it is
Scheme 4. Cross-linking of 13’. o-DCB=ortho-dichlorobenzene.
volatile byproduct, Me NH, was separated by distillation
2
1
during the reaction and subsequently identified by H NMR
spectroscopy. The solid product was characterized by FTIR
spectroscopy. NMR-spectroscopic analysis was precluded by
[28]
its insolubility in common organic solvents. Therefore, 14 is
given as a tentative structural proposal for the cross-linked
material.
In conclusion, the first derivatives of a new class of
organic–inorganic
hybrid
polymers,
poly[N-(para-
phenylene)diimidoborane(3)]s (PPP-DIBs), 13 and 13’, and
a series of related model compounds have been prepared.
Polycondensation with BÀN bond formation through silazane
cleavage with Si/B exchange proceeded smoothly under mild
conditions and provided facile access to soluble macromo-
lecular materials. Comparison of the photophysical data for
polymer 13’ and its molecular model systems revealed
moderate p-conjugation across the NBN units with some
extension of the conjugation path with increasing chain
length. DFT calculations provided deeper insight into this
effect. The possibility of metal coordination by the polymer is
expected to render the new materials broadly applicable.
Figure 3. Calculated frontier orbitals (isovalue: 0.022) of 5 and 7.
noteworthy that the boron p orbitals contribute significantly
p
to the LUMOs but not to the HOMOs of 3 and 5. Therefore, Acknowledgements
the HOMO–LUMO transitions are unambiguously charac-
terized as being of p–p* type, with some degree of charge
transfer to the boron atom. For compounds 7 and 8, the
definite classification of molecular orbitals as s- or p-type is
not possible owing to the deviation of the aromatic rings and
the N-B-N units from coplanarity (Figures 3 and S52–S54).
However, the frontier orbitals clearly show their largest
contributions above and below the planes of the phenyl (and,
in the case of 8, the phenylene) rings and the N-B-N units,
whereas the constituting nuclei are located in the nodal planes
of these orbitals. Therefore, we conclude that the classifica-
tion of these MOs as being predominantly of p-type is
reasonable, and that the corresponding transitions should be
We thank the German Research Foundation (DFG) for
funding through the Emmy Noether Programme. Support by
COST action (CM1302, SIPs) is gratefully acknowledged. We
thank Qianqian Guo for collecting the X-ray diffraction data,
Rebecca Liffmann for assistance with UV/Vis measurements,
Dr. Klaus Beckerle for assistance with GPC measurements,
and Prof. Dr. Jun Okuda for generous support and helpful
discussions.
Keywords: boron · p-conjugated polymers · hybrid materials ·
inorganic polymers · main-group polymers
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240 www.angewandte.org
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[25] Accurate H NMR end-group analysis was hampered by the
above-mentioned line broadening caused by dynamic processes
and, additionally, by an overlap of the proton resonances of the
tBu end groups with those of residual n-pentane in the sample. A
rough estimate yielded DP ꢀ 90, which was considered as an
n
overestimation in view of the presence of cyclic molecular
species.
[
[
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Supporting Information.
IV
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[
[
21] The insoluble product was not analyzed further. Rigid con-
jugated polymers and oligomers, such as the expected product,
often show poor solubility when no solubilizing side groups have
been attached.
2 4
[28] Even with very low amounts of Zr(NMe ) (4 mol%), the
obtained product was insoluble. Furthermore, we carried out
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11 1
2
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During B{ H} NMR reaction monitoring, a resonance at d =
37 ppm was detected (Figure S22), which is downfield-shifted by
Dd = 5–6 ppm compared to the resonance of 7. A downfield shift
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asymmetric unit are 9.8(3) and 52.0(3)8.
24] For polymers featuring 1,3,2-benzodiazaboroline units, unusu-
ally large Stokes shifts have also been reported, which were
[
[
[27b]
B(Ph)ÀNtBu) Zr(THF).
2
[14b]
attributed to intramolecular charge-transfer processes.
The
large Stokes shifts of aminoboranes have been attributed to
emission from twisted intramolecular charge transfer (TICT)
Received: March 7, 2016
Published online: May 6, 2016
Angew. Chem. Int. Ed. 2016, 55, 7236 –7241
ꢀ 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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7241