Adapting N-heterocyclic carbene/azide coupling chemistry for polymer synthesis: Enabling access to aromatic polytriazenes

Article abstract of DOI:10.1002/anie.200901046

A click by any other name: Coupling bis(N-heterocyclic carbene)s with bis(azide)s afforded a novel class of conjugated polytriazenes. These polymers were rendered electrically conductive upon doping, and fluorene-containing variants exhibited luminescence

Full text of DOI:10.1002/anie.200901046

Angewandte
Chemie
DOI: 10.1002/anie.200901046
Polymers
Adapting N-Heterocyclic Carbene/Azide Coupling Chemistry for
Polymer Synthesis: Enabling Access to Aromatic Polytriazenes**
Daniel J. Coady, Dimitri M. Khramov, Brent C. Norris, Andrew G. Tennyson, and
Christopher W. Bielawski*
Aromatic polymers containing carbon–nitrogen double
bonds, such as poly(azine)s and poly(azomethine)s,[1] have
garnered tremendous interest across a range of scientific and
engineering disciplines.[2] These polymers commonly exhibit
high thermal stabilities[3] as well as good mechanical[4] and
electronic properties,[3,5] including nonlinear optical[6] and
semiconductive characteristics upon doping,[7] making them
good candidates for use in a variety of optoelectronic
applications. Since they can also coordinate to a variety of
transition metals and other electrophilic species, aromatic
across the triazeno linkage was efficient and could be tuned
by varying the nature of the coupling partners.[11,12] Further-
more, despite their relatively high nitrogen atom contents, the
triazene products are thermally stable. For example, deriva-
tives possessing bulky N-substituents (e.g., tert-butyl) were
found to be stable in the solid state at temperatures exceeding
1508C.[11]
Collectively, the unique features of the NHC/azide
coupling reaction is well suited for use in synthetic polymer
chemistry,[13] particularly for accessing aromatic polymers
=
=
polymers containing C N bonds are well-suited for applica-
containing C N bonds. We envisioned combining difunc-
tions ranging from catalysis to sensing.[8] In general, such
polymers are prepared by condensation polymerizations
involving difunctional monomers (typically, bisaldehydes
and bisamines), although the high reactivity between primary
amines and aldehydes often renders the polymerization
reaction uncontrollable, resulting in the formation of intract-
able products.[1,3,4] As a result, there is a need for the
development of new polymerization methods for forming the
aforementioned types of aromatic polymeric materials,[9]
particularly those that offer a broad range of options for
varying the structures as well as the functionalities of the
polymers produced.
tional NHCs (1) with complementary difunctional azides (2)
to create a new class of aromatic polymers (3) [Eq. (2); * =
arene linker].[14] As a corollary to this investigation, we sought
=
Recently, we reported a new method for forming C N
bonds which involves the combination of N-heterocyclic
carbenes (NHCs)[10] with organic azides to afford the respec-
tive acyclic triazenes [Eq. (1)].[11] The reaction tolerates a
wide variety of both NHC and azide coupling partners, and
proceeds in high yields. Through a comprehensive product
analysis, we determined that the electronic delocalization
to expand the utility of organic azides as building blocks in
synthetic polymer chemistry. Although metal-catalyzed
“click” cycloaddition reactions[15] have found tremendous
utility in such regards,[16] one important advantage of the
NHC/azide coupling chemistry is that it does not require a
catalyst, thereby facilitating the isolation of pure materials.
The structures of the various bis(NHC)s 1 and bis(azide)s
2 that were investigated as potential monomers are shown in
Figure 1. The bis(NHC)s were prepared from their respective
tetraamines and subsequent formylative cyclization and
deprotonation, in accord with published procedures.[17] The
N-substituents featured in these monomers were chosen to
enhance the solubilities of the respective polymers as well as
to probe their effects on the thermal and electronic properties
displayed by these materials (see below).[18] To maximize the
formation of polymers with electronically delocalized struc-
tures, aryl bis(azide)s were studied exclusively. In particular,
1,4-diazidobenzene (2a) as well as 4,4’-diazidobiphenyl (2b)
were synthesized. Considering that polymers containing
fluorene have been used in display devices,[19] the 2,7-diazido
derivative 2c, featuring two n-hexyl chains at the 9-position to
enhance photostability and solubility, was also studied.
[*] Dr. D. J. Coady, Dr. D. M. Khramov, B. C. Norris, Dr. A. G. Tennyson,
Prof. C. W. Bielawski
Department of Chemistry & Biochemistry
The University of Texas at Austin, Austin, TX 78712 (USA)
E-mail: bielawski@cm.utexas.edu
[**] We are grateful to the NSF (CHE-0645563), the ARO (W911NF-05-1-
0430), the ONR (N00014-08-1-0729), the Robert A. Welch Founda-
tion (F-1621), the Arnold and Mabel Beckman Foundation, and the
Research Corporation for their generous financial support.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.200901046.
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Figure 2. A) Gel permeation chromatogram of 3a. Conditions: 408C,
THF/pyridine (3:1) as eluent, 1 mLminÀ1. B) Thermogravigrams of 3a
(dotted line) and 3g (solid line). Conditions: 108C minÀ1, nitrogen
atmosphere. C) Normalized absorption (solid line) and emission
(dashed line) spectra for 3c in THF at 25oC.
Table 1: Key properties of the polytriazenes prepared from the mono-
mers indicated.[a]
ꢀ
ꢀ
Polymer
Yield
[%][b]
Mn;NMR
Mn;GPC
PDI
Td
lmax
[nm][f]
[kDa][c]
[kDa][d]
[8C][e]
Figure 1. Structures of the bis(NHC)s 1 and bis(azide)s 2 explored in
this study.
3a (1a + 2a)
3b (1a + 2b)
3c (1a + 2c)
3d (1b + 2c)
3e (1c + 2a)
3 f (1c + 2b)
3g (1c + 2c)
98
95
76
81
96
95
72
29.4
24.6
21.5
8.3
22.1
20.3
17.7
34.8
27.5
23.9
17.6
27.6
22.7
19.9
1.8
1.6
1.8
1.6
2.3
2.3
1.9
154
157
160
262
282
282
278
485
434
450
476
437
379
436
In an initial experiment, the bis(NHC) 1a was added to an
equimolar quantity of bis(azide) 2a in CD2Cl2 ([total
monomer]0 = 0.1m) and then stirred at 25oC. Gratifyingly,
the expected polymer 3a was observed by 1H NMR spectros-
copy (through development of broad signals that were shifted
downfield) shortly after the monomers were combined.
Polymer formation was monitored over time by integrating
signals diagnostic of the monomer 2a (d = 7.00 ppm) to its
analogous signals found in the repeat units of 3a (d =
7.58 ppm). Furthermore, by comparing signals attributed to
[a] All polymerization reactions were performed in either tetrahydrofuran
or dichloromethane using equimolar quantities of monomers 1 and 2
(total initial concentration of monomer=0.1m). [b] Yields of isolated
polymeric products obtained after precipitation in n-pentane. Polymers
containing 2c were partially soluble in n-pentane, which resulted in lower
yields of the isolated polymer. Monomer conversion into polymer, as
1
determined by H NMR spectroscopy, was quantitative. [c] Determined
by end-group analysis (see text). [d] Performed using THF/pyridine (9:1)
as the eluent at 408C using light scattering, refractive index, and
viscometry detectors. [e] Performed under an atmosphere of nitrogen at
a scan rate of 108CminÀ1. [f] UV/Vis spectra were recorded in THF at
258C.
À
the end groups of 3a (terminal triazene Ar H, d = 7.05 ppm)
to the signals associated with its main chain, the number-
ꢀ
average molecular weight (Mn)of the polymer was calculated.
After 24 hours, the reaction appeared to be complete,
producing a polymer having a Mn = 29.4 kDa.[20] The reaction
ꢀ
mixture was then poured into an excess of n-pentane, a poor
solvent for the polymer but a good solvent for its composite
monomers. The precipitated solids were collected and then
dried under high vacuum to afford 3a in 98% yield. Analysis
of the polymer by gel permeation chromatography (GPC)
to 29.4 kDa and compared well to those obtained by GPC
methods. In general, polymerizations involving 1a produced
higher molecular weight (MW) polymers than analogous
reactions involving 1c.[22] The difference may be explained by
the reduced conformational freedom associated with the
former monomers, thus reducing chances for molecular
weight limiting cyclization or related side reactions. The
polymers 3 were of high purity, as determined by elemental
analysis, displayed good solubilities in common organic
solvents, and were stable toward oxygen and moisture.
Upon synthesis, the thermal stabilities of 3 were evaluated
using thermogravimetric analysis (TGA). Despite the rela-
tively large number of nitrogen atoms present in the main
chains of these materials, they were found to be thermally
stable and exhibited decomposition temperatures (Td) that
were dependent on their structures. For example, the Td of 3a,
which features tertiary N-alkyl groups, was found to occur at
1548C (Figure 2b); in contrast, 3g which features primary
N-alkyl groups did not decompose until 2788C. In this latter
material, the first decomposition proceeded cleanly with a
ꢀ
revealed an absolute Mn = 31.8 kDa and a broad polydispers-
ity (PDI = 1.8),[21] as expected for a step-growth polymeri-
zation (Figure 2a). As a testament to its high purity, the
elemental composition of 3a (C, 72.95; H, 10.04; N, 17.01) was
found to be in good agreement with its calculated values
(C, 72.67; H, 10.24; N 17.09).
As summarized in Table 1, a series of analogous polymer-
izations were performed using various combinations of bis-
(NHC)s 1 and bis(azide)s 2. Each of these reactions were
performed on a preparative scale in THF or CH2Cl2 for
24 hours, after which the reaction mixture was poured into
n-pentane, quenching the reaction and causing the respective
polymers to precipitate. The structures of the polytriazenes
obtained from these reactions were confirmed by NMR
ꢀ
spectroscopy. Their NMR-derived Mn values ranged from 8.3
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mass loss nearly equal to two units of molecular nitrogen (N2)
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3d–g presumably afforded the respective polyguanidines,
which appear to be stable to temperatures approaching
4008C; additional studies of these materials is underway.
Next, efforts shifted toward examining the electronic and
photophysical properties of 3. In THF, their absorption
maxima ranged between lmax = 379 and 485 nm, depending
on their structure. Polymers prepared from 2b absorbed at
shorter wavelengths compared to those prepared from 2a or
2c, presumably because of the conjugation disrupting effect of
the biphenyl linkage. Interestingly, the lmax of a relatively low
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The polymers 3c and 3g, comprising fluorenyl units in their
main chains, exhibited fluorescence at 546 nm in THF at 25oC
(F < 0.2%; Figure 2c).
Finally, to explore the suitability of these materials for
optoelectronic applications, we sought to determine their
electrochemical and conductive properties. The polymer 3a
was found to exhibit multiple, irreversible oxidations ranging
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voltammetry.[25] Prompted by this result, the polymer was spin
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using a multipoint probe station.[26] Whereas virgin films were
found to be insulating (s < 10À10 ScmÀ1), they were rendered
conductive (s = 4 ꢀ 10À3 ScmÀ1) after exposure to iodine
vapor for 24 hours. This value compares well with other
classes of doped conjugated aromatic polymers, containing
carbon–nitrogen double bonds, reported in the literature.[1a,7]
In conclusion, we have developed a novel, efficient, and
practical route to a new class of polymers by using NHC/azide
coupling chemistry. The combination of bis(NHC)s with
complementary bis(azide)s produced a variety of thermally
stable polytriazenes which exhibited good optical and elec-
tronic properties, as well as high solubilities in common
organic solvents. To the best of our knowledge, this is the first
example of using stable, free bis(NHC)s as polymer building
blocks and effectively opens a new design strategy in synthetic
polymer chemistry for accessing aromatic polymers contain-
ing carbon–nitrogen double bonds. Considering the large
number of methods known for modifying the structures and
electronic properties of NHCs[10] as well as organic azides,[27]
the polymers reported herein are well-adapted for use in a
variety of optoelectronic applications.
˙
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Received: February 23, 2009
Revised: May 5, 2009
Published online: June 3, 2009
Keywords: azides · carbenes · click chemistry ·
.
coupling reactions · polymers
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[21] Polytriazenes 3 were found to undergo protonation in the
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1b and 2b were found to be insoluble in common organic
solvents. The Td values of these materials were found to be
2738C.
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alkyl substituents are molecular nitrogen and its respective
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