The marriage of metallacycle transfer chemistry with suzuki-miyaura cross-coupling to give main group element-containing conjugated polymers

Article abstract of DOI:10.1021/ja402242z

A versatile and general synthetic route for the synthesis of conjugated main group element-based polymers, previously inaccessible by conventional means, is reported. These polymers contain five-membered chalcogenophene rings based on S, Se, and Te, and we demonstrate that optoelectronic properties can be readily tuned via controlled atom substitution chemistry. In addition, regioregular hybrid thiophene-selenophene-tellurophene and selenophene-fluorene copolymers were synthesized to provide a further illustration of the scope of the presented metallacycle transfer/cross-coupling polymerization method.

Full text of DOI:10.1021/ja402242z

Communication
pubs.acs.org/JACS
The Marriage of Metallacycle Transfer Chemistry with
Suzuki−Miyaura Cross-Coupling To Give Main Group
Element-Containing Conjugated Polymers
Gang He, Le Kang, William Torres Delgado, Olena Shynkaruk, Michael J. Ferguson,
Robert McDonald, and Eric Rivard*
Department of Chemistry, University of Alberta, 11227 Saskatchewan Drive, Edmonton, Alberta, Canada T6G 2G2
S
* Supporting Information
Scheme 1. Combined Metallacycle Transfer/Cross-Coupling
Route to Inorganic Heterocycle (Heterole)-Based Polymers
ABSTRACT: A versatile and general synthetic route for
the synthesis of conjugated main group element-based
polymers, previously inaccessible by conventional means,
is reported. These polymers contain five-membered
chalcogenophene rings based on S, Se, and Te, and we
demonstrate that optoelectronic properties can be readily
tuned via controlled atom substitution chemistry. In
addition, regioregular hybrid thiophene−selenophene−
tellurophene and selenophene−fluorene copolymers were
synthesized to provide a further illustration of the scope of
the presented metallacycle transfer/cross-coupling poly-
merization method.
onjugated polymers have attracted considerable attention
Cfrom the chemical community at large because of their
widespread use in photovoltaics, polymer light-emitting diodes
(PLEDs), and chemical sensing technologies.1−3 Moreover,
polymers are amenable to various processing/manufacturing
methods that are not readily translated to small-molecule-based
systems. The emergence of cross-coupling C−C bond-forming
methodologies has enabled the facile preparation of many
organic-based conjugated polymers, but the construction of
polymers featuring heavy inorganic elements as integral
components has been much less studied.4,5 Herein we report
a tandem synthetic strategy that combines the versatility of
Suzuki−Miyaura cross-coupling with Zr-mediated metallacycle
transfer chemistry (Scheme 1) to provide access to new poly-
meric frameworks containing inorganic rings such as seleno-
phenes and tellurophenes. This general strategy represents a
new addition to the ever-expanding synthetic toolkit available
for the synthesis of functional polymeric materials.
At the onset, our goal was to devise a rapid method for
generating selenophene- and tellurophene-containing polymers
for photovoltaic applications.1 Compared to their thiophene
congeners, polyselenophenes and thiophene−selenophene
copolymers are expected to have improved optoelectronic
properties due to the presence of lower band gaps and deeper
LUMO levels.5 Along these lines, Choi and co-workers have
constructed high-performance thin-film transistors based upon
selenophene copolymers,6 and the Seferos group has reported
a series of elegant studies involving the construction of novel
Se- and Te-based chalcogenophene block and alternate
copolymers.5e,7 In each case, the incorporation of selenophene
and tellurophene units led to a narrowing of the optical band
gap relative to their ubiquitous sulfur-based counterparts. It
should be noted that an efficient and high-yielding general
route to these materials is desired in order to advance the study
of these promising macromolecules.8
In 1988, Fagan and co-workers reported the synthesis of
a wide range of main group element (M) heterocycles via
metallacycle transfer from readily available zirconacyclopenta-
dienes in high yield.9 Later, Tilley reported the direct synthesis
of zirconacyclopentadiene-containing polymers from the reac-
tion of rigid aryl-spaced diynes [e.g., MeCCSiMe2(C6H4)-
SiMe2CCMe] with sources of “Cp2Zr” (Cp = η5-C5H5).10−12
Despite the promise of this method, there are challenges
associated with replacing all of the Cp2Zr groups along the
polymer backbone with main group elements to give air-stable
conjugated polymers, although successful examples are known.11
Furthermore, obtaining regioregular substitution about the
heterole rings can also be difficult.13
In line with the targets presented in Scheme 1, we sought to
develop a zirconacycle precursor containing pinacolborane
(BPin) cross-coupling groups at the 2 and 5 positions of the
ZrC4 ring. The desired zirconacycle 1 was prepared by reacting
the known pinacolborane-capped diyne (PinB)C
C(CH2)4CC(BPin)14 with in situ-generated “Cp2Zr” in
tetrahydrofuran (THF), as outlined in eq 1. Zirconacycle 1 was
isolated as an air- and moisture-sensitive red crystalline solid in
Received: March 3, 2013
© XXXX American Chemical Society
A
dx.doi.org/10.1021/ja402242z | J. Am. Chem. Soc. XXXX, XXX, XXX−XXX

Journal of the American Chemical Society
Communication
thermal stability in the solid state (mp = 180−181 °C) and
solution, thus expanding the temperature regime that can be
accessed during the subsequent polymerization chemistry.
Characterization of the pinacolborane-containing monomers
2−5 was accomplished by 1H, 11B, and 13C{1H} NMR spectro-
scopy; single-crystal X-ray crystallography (see Figures S2−S5 in
the Supporting Information),15 elemental analysis, UV−vis
spectroscopy, and thermogravimetric analysis (TGA).
65% yield, and the X-ray structure of 1 is shown in Figure 1.
Despite the presence of hindered BPin groups at flanking
Given the successful synthesis of 2−5, we were eager to
explore Suzuki−Miyaura cross-coupling methodologies to
incorporate conjugated diene and chalcogenophene units
into polymeric structures. To begin, we combined 2−5 with
2,5-dibromo-3-hexylthiophene to afford a series of hybrid
butadiene−thiophene (P1) or heterole−thiophene (P2−P4)
copolymers using Pd(PPh3)4 as a catalyst and nBu4NF as a phase-
transfer agent. Although productive cross-coupling was observed,
the molecular weights obtained were only 2−3 kg/mol. The
Pd2(dba)3/[HPtBu3]BF4 catalyst system (Scheme 3) used in
Scheme 3. Synthesis of the Hybrid Butadiene−Thiophene
a
Figure 1. Molecular structure of 1 with thermal ellipsoids presented at
the 30% probability level. H atoms have been omitted for clarity.
Selected bond lengths (Å) and angles (deg): Zr−C(1), 2.245(3); Zr−
C(8), 2.249(4); C(1)−B(1), 1.535(5); C(8)−B(2), 1.544(5); C(1)−
Zr−C(8), 79.62(13); Zr−C(1)−C(2), 109.8(2); Zr−C(8)−C(7),
109.5(2).
and Heterole−Thiophene Copolymers P1−P4
positions about the zirconacycle, the Zr center in 1 appeared
to have sufficient steric accessibility to allow for productive
metallacycle transfer chemistry with halides of main group
elements.
As anticipated, zirconacycle 1 is a progenitor to various main
group element-containing heterocycles (Scheme 2). For future
a
Conditions: (i) Pd2(dba)3, [HPtBu3]BF4, K2CO3(aq), THF, 70 °C,
Scheme 2. Synthesis of Heterocyclic Polymer Precursors
2−5 via Metallacycle Transfer Chemistry
24 h; (ii) toluene, 70 °C, 24 h.
conjunction with 2,5-diiodo-3-hexylthiophene20,21 gave the target
regiorandom polymers with higher number-average molecular
weights (Mn) of 3.57−6.51 kg/mol (relative to polystyrene in
THF). The synthesis of polymers containing tellurophene repeat
units (P4) is noteworthy5e,7 and could be due to the higher
thermal stability of the bis(BPin)-substituted reagent 5, which
enabled efficient cross-coupling to transpire at 70 °C in THF/
toluene for 3 days. It should be mentioned that hybrid S/Se
and S/Te oligochalcogenophenes related to P3 and P4 have been
prepared in small quantities via electropolymerization and/or
oxidative polymerization routes, and optical properties similar
to those of our systems were noted (vide infra).5d,22 Our route
remains attractive because it offers a high degree of control over
the polymer composition. As a result of the versatile nature of
Suzuki−Miyaura cross-coupling, the chemistry described above
could be readily applied to the synthesis of various heterole−aryl
copolymers simply by altering the nature of the aryl dihalide and
BPin-grafted heterole being copolymerized.8,23
comparative studies, the butadiene-linked monomer 2 was first
synthesized as an air-stable colorless solid in 77% yield by
adding an aqueous HCl solution to 1.16 The thiophene hetero-
cycle 3 was then prepared via pre-established metallacycle
transfer chemistry using S2Cl2 as the chalcogen source. We
found that the heavier Se congener 4 (yellow solid) was best
generated from the reaction of 1 with metastable SeCl2.17
Initially we attempted to generate the tellurophene monomer 5
using “TeCl2” (prepared from TeCl4 and Me3SiSiMe3)18 as a
tellurium delivery agent, but no reaction between this reagent
and 1 was observed. We speculated that the lack of reactivity
was due to the poor solubility of “TeCl2” in THF, and there-
fore, we treated 1 with the soluble 2,2′-bipyridyl-sequestered
tellurium dihalide adduct bipy·TeCl2.19 Gratifyingly, this
reaction afforded 5 in a moderate 55% yield as green-yellow
crystals. In contrast to previously reported polytellurophene
precursors (e.g., diiodobitellurophene),7a 5 exhibits considerable
Figures 2 and S1115 show the UV−vis data for polymers P1−
P4. A general red shift of the optical absorption profiles upon
moving from the polythiophene (P2) to the selenophene (P3)
and tellurophene (P4) hybrids was observed, consistent with
a progressive decrease in optical band gap (Eg). Interestingly,
the butadiene-linked polymer P1 showed a longer absorption
wavelength (Eg = 2.36 eV in solution) relative to those in P2−
P4;24 therefore, monomer 2 could be a useful building block
for the construction of new conjugated polymer systems.25
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Figure 2. Absorption spectra of (left) P2−P4 and (right) P1, P5, and
P6 in THF; extrapolated optical band gaps (Eg) are shown as insets.24
Figure 3. (a) MALDI−TOF mass spectrum of P5. (b) UV−vis
absorption spectra of P5 in THF solution and as a film.
The polymers also showed high thermal stabilities (Tdec = 300−
350 °C by TGA; Figure S12),15 while clearly resolved melting
temperatures (Tm) were observed at 212−227 °C (Figure S14).
Weak luminescence (orange-red) by polymers P2 and P3 was
noted, but the tellurophene analogue P4 was nonemissive.26
As noted, polymers P1−P4 have regio-irregular structures,
which may limit their use in optoelectronic devices.1,2 To install
solubilizing hexyl groups at regiospecific sites along a polymer
backbone, the procedure outlined in Scheme 4 was developed.
presence of a regioregular material, P5 displays a single
thiophene-bound hydrogen environment in the 1H NMR
spectrum. P5 exhibited an absorption maximum at 429 nm in
THF (Figure 2b), which was red-shifted by ca. 10 nm relative
to the S/Te hybrid copolymer P4. When films of P5 were cast,
the absorption maximum became broader and was red-shifted
by ca. 80 nm relative to that in solution, suggesting an increase
in conjugation in the solid state (Figure 3b). Moreover con-
siderable tailing of the absorption to 750 nm in films of P5 was
noted.7c Like the polytellurophene P4, the S−Se−Te terpolymer
P5 was not luminescent. Further manipulation of the structure of
P5 via side-group placement/annealing should lead to improved
optoelectronic characteristics; we are currently in the process of
evaluating the charge carrier mobilities of P5 and its related
regioregular polychalcogenophenes.
Scheme 4. Synthesis of Regioregular Thiophene−
a
Selenophene−Tellurophene Copolymer P5
The general synthetic route shown in Scheme 1 could be
further applied to construct fluorene−chalcogenophene co-
polymers, such as the Se analogue P6 (eq 2). The absorption
maximum of this polymer (389 nm) is significantly hypso-
chromically shifted relative to those P1−P5, consistent with the
behavior of other blue-emitting polyfluorenes found in the
literature.28 Accordingly, P6 is luminescent in the blue spectral
region with λem = 458 nm, a property that is highly desirable for
PLEDs. We are actively exploring P6 and related systems to
this end in our laboratory.
a
Conditions: (i) Pd2(dba)3, [HPtBu3]BF4, K2CO3(aq), THF, 70 °C,
24 h; (ii) toluene, 70 °C, 48 h.
In conclusion, we have reported a highly efficient and modular
synthetic route to conjugated polymers featuring heterole units
with tunable atomic composition. To illustrate this concept, we
synthesized a number of S-, Se- and Te-containing polymers
with varying compositions and optoelectronic properties. This
combination of Zr-mediated ring-forming chemistry with
Suzuki−Miyaura cross-coupling should provide access to an
immense range of new polymer classes via judicious selection
of the heterole and aryl cross-coupling partners. Future work will
involve extending this synthetic concept to include conjugated
materials containing new inorganic heterocycles.23
Specifically, the thiophene-capped diyne 6 was first reacted with
Cp2Zr(py)(Me3SiCCSiMe3)27 to give zirconacycle 7 as a hexanes-
soluble orange-red solid (see Figure S6 for its crystallographically
determined structure).15 Replacement of the Cp2Zr unit in 7 by Se
followed by selective bromination of the thiophene residues using
N-bromosuccinimide (NBS) afforded the regioregular thiophene−
selenophene monomer 9 as a yellow oil. The direct Suzuki−
Miyaura copolymerization of 9 with tellurophene 5 led to the
formation of the novel regioregular thiophene−selenophene−
tellurophene copolymer P5 (Mn = 4.30 kg/mol; PDI = 1.62) in
69% yield (Scheme 4).
Further evidence for the structural composition of P5 was
obtained via MALDI−TOF mass spectrometry (Figure 3a),
which gave the expected spectral pattern in line with the
assigned structure (repeat unit of m/z 750.11). To the best of
our knowledge, P5 is the first example of a conjugated polymer
containing three different chalcogenophenes (S, Se, and Te)
as part of a single polymer repeat unit. Consistent with the
ASSOCIATED CONTENT
* Supporting Information
Experimental details; crystallographic data for 1−5 and 7
(CIF); and UV−vis, fluorescence, DSC, TGA, and MALDI
data for the new polymers. This material is available free of
charge via the Internet at http://pubs.acs.org.
■
S
C
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Journal of the American Chemical Society
Communication
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(e) Zhou, W.-M.; Tomita, I. J. Inorg. Organomet. Polym. 2009, 19, 113.
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AUTHOR INFORMATION
Corresponding Author
erivard@ualberta.ca
Notes
■
n
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The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
This work was supported by the Natural Sciences and
Engineering Research Council (NSERC) of Canada, the
Canada Foundation for Innovation, and Alberta Innovates-
Technology Futures (New Faculty Award to E.R.). We thank
William Ramsay and Jason Brandt for their initial investigations
of metallacycle transfer chemistry.
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