Copper-Free Sonogashira Coupling for High-Surface-Area Conjugated Microporous Poly(aryleneethynylene) Networks

Article abstract of DOI:10.1002/chem.201600783

A modified one-pot Sonogashira cross-coupling reaction based on a copper-free methodology has been applied for the synthesis of conjugated microporous poly(aryleneethynylene) networks (CMPs) from readily available iodoarylenes and 1,3,5-triethynylbenzene. The polymerization reactions were carried out by using equimolar amounts of halogen and terminal alkyne moieties with extremely small loadings of palladium catalyst as low as 0.65 mol %. For the first time, CMPs with rigorously controlled structures were obtained without any indications of side reactions, as proven by FTIR and solid-state NMR spectroscopy, while showing Brunauer-Emmett-Teller (BET) surface areas higher than any poly(aryleneethynylene) network reported before, reaching up to 2552 m² g^-1. A modified one-pot Sonogashira cross-coupling reaction based on a copper-free methodology has been applied for the synthesis of conjugated microporous polymers (CMPs). Not only are the resulting structures much more defined than previously published materials, but they also show far superior Brunauer-Emmett-Teller (BET) surface areas of up to 2552 m² g^-1, setting a new record for this type of materials (see scheme).

Full text of DOI:10.1002/chem.201600783

DOI: 10.1002/chem.201600783
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&
Microporous Materials |Hot Paper|
Copper-Free Sonogashira Coupling for High-Surface-Area
Conjugated Microporous Poly(aryleneethynylene) Networks
Matthias Trunk,* Anna Herrmann, Hakan Bildirir, Ali Yassin, Johannes Schmidt, and
Arne Thomas*[a]
Abstract: A modified one-pot Sonogashira cross-coupling
reaction based on a copper-free methodology has been ap-
plied for the synthesis of conjugated microporous poly(aryl-
eneethynylene) networks (CMPs) from readily available io-
doarylenes and 1,3,5-triethynylbenzene. The polymerization
reactions were carried out by using equimolar amounts of
halogen and terminal alkyne moieties with extremely small
loadings of palladium catalyst as low as 0.65 mol%. For the
first time, CMPs with rigorously controlled structures were
obtained without any indications of side reactions, as
proven by FTIR and solid-state NMR spectroscopy, while
showing Brunauer–Emmett–Teller (BET) surface areas higher
than any poly(aryleneethynylene) network reported before,
reaching up to 2552 m2 gÀ1.
ly.[19–21] The importance of these materials is emphasized by the
recent commercialization of CMP-1 by Sigma–Aldrich.[22]
Introduction
Over the last two decades, microporous organic materials have
seen remarkable development due to their potential applica-
tions in gas storage, separation, sensing, and catalysis.[1,2] This
development has been accelerated by the general trend to-
wards sustainability and a call for less energy-intensive alterna-
tives for industrial procedures, such as cryogenic distillation.
Aside from ordered materials, such as metal–organic frame-
works (MOFs)[3] and covalent organic frameworks (COFs),[4,5] mi-
croporous polymer networks (MPNs) have emerged as a family
of amorphous yet highly promising materials for a range of ap-
plications.[6–10] Apart from polymeric materials, porous organic
molecules have gained an increasing amount of attention re-
cently.[11–16] Extensive studies have produced unique combina-
tions of high accessible surface areas and chemical robustness,
as well as thermal stability, often surpassing MOFs and COFs in
those respects, although in exchange for ordered struc-
tures.[1,2,17] A plethora of porous polymers has been obtained
through a variety of polymerization methods. Especially the
palladium-catalyzed procedure following the Sonogashira-type
protocol published by Cooper et al.[6] to produce conjugated
microporous poly(aryleneethynylene) networks (CMPs) is a com-
monly employed method to incorporate functional groups
into porous polymer backbones.[18] Further functional groups
can be grafted onto the polymer backbones postsynthetical-
Since 2007, different protocols for CMP syntheses with vary-
ing solvents, temperature, starting materials, and stoichiometry
have been reported.[23–26] Initially, ethynyl-functionalized aro-
matic compounds were reacted with aryl iodides, which were
later often succeeded by more readily available bromides,
whereby lower degrees of condensation accompanied by
slightly lower surface areas were observed.[18,27] Surprisingly
and solely based on empirical findings, the highest surface
area for a given CMP was obtained by use of a 50% excess of
alkyne functions, which should actually give unreacted alkyne
end groups within the resulting network structures,[23] and be
detrimental for reaching high surface areas in a MPN. A recent
article addresses the issue of this counterintuitive stoichiome-
try in depth, focusing on the network formation with special
attention paid to open end groups within the network.[28]
Herein, the empirically found necessity for an excess of ethynyl
groups for the formation of high-surface-area materials is ra-
tionalized by an ongoing reaction of ethynyl end groups,
which are trapped within the precipitated material. These eth-
ynyl groups cross-link during elongated reaction times, where-
as halide end groups would remain unreactive after gelation,
giving dead ends within the polymer. The steady decrease in
terminal ethynyl functions was shown to be accompanied by
a rise in nitrogen-accessible surface area. Nevertheless, the use
of equimolar monomer ratios can also be found in the litera-
ture.[10,24,29,30] Recently, Son and co-workers used the correct
stoichiometry to create impressive surface areas of almost
1800 m2 gÀ1 by systematic variation of the phosphine ligand
accompanying the palladium catalyst.[26]
[a] M. Trunk, A. Herrmann, Dr. H. Bildirir, Dr. A. Yassin, Dr. J. Schmidt,
Prof. Dr. A. Thomas
Department of Chemistry, Functional Materials
Technische Universität Berlin
Hardenbergstrasse 40, 10623 Berlin (Germany)
E-mail: arne.thomas@tu-berlin.de
To elucidate the exact formation mechanism and the nature
of the homocoupling product, Bunz and co-workers created
a homocoupled CMP from a tetrahedral tin-based monomer,
which was digested after polymerization. The fragments were
matthias.trunk@tu-berlin.de
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/chem.201600783.
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analyzed to shed light on the structure of the struts linking the
former nodes, and thereby on the mechanism of internal cross-
linking. The struts were found to consist of isomeric enynes
originating from dimers, trimers, and tetramers of alkyne
groups.[31]
To make the conventional homogeneous Sonogashira cou-
pling less susceptible to oxygen and to prevent the formation
of side-products, copper-free variants have been developed, al-
though the copper-free mechanism is yet unknown.[32,33] Moti-
vated by these routes and to avoid the aforementioned side-
reactions during CMP formation, we developed a copper-free
polymerization method employing the exact stoichiometry of
alkyne and halide functions.
Figure 1. Varying states of 1,3,5-triethynylbenzene: a) as-purchased; b) after
column chromatography from dichloromethane; and c) after sublimation.
that for aryl iodides and polyalkynes, high surface areas could
be obtained when the amount of palladium catalyst
[Pd(PPh3)4] was reduced to 0.65 mol% per iodide/alkyne
moiety, whereas the co-catalyst, copper(I) iodide, was omitted
completely.[34] Reaction of TEB with 1,3,5-triiodobenzene, 1,4-
diiodobenzene, 4,4’-diiodobiphenyl, and tetrakis(4-iodophenyl)-
methane in a mixture of DMF/NEt3 (2:1) gave highly electro-
static, spongy materials P1–P4 (Scheme 1). The resultant pow-
ders range from off-white to yellow, according to the length
and geometry of the conjugated system. Materials P1 and P4,
consisting only of weakly electronically conjugated 1,3-con-
nected aromatics, were obtained as beige and off-white, re-
spectively. In contrast, polymers emerging from 1,4-functional-
ised arylenes form longer conjugated aromatic systems, giving
rise to different hues of yellow (P2 and P3). It should be noted
that materials with structures similar to these materials are de-
scribed in literature—P1 corresponds to CMP-X,[24] P2 and P3
correspond to CMP-1 and -2,[6] respectively, and a structure
similar to P4 was also reported.[35]
From a practical point of view, the addition of the catalyst
into the hot reaction mixture in the form of a slurry, as it was
reported before, seemed to us unfeasible.[6] Residues of insolu-
ble catalyst remaining inside the syringe, cannula, or the vessel
used for the preparation of the slurry can prove detrimental to
reproducibility and rinsing aforementioned vessel with solvent
makes the reaction prone to oxygen contamination. Thus, the
herein presented route was originally developed to provide
a facile and robust one-pot procedure, using equimolar
amounts of ethynyl and halide groups, thus avoiding side-reac-
tions while keeping the conversion efficiency of functional
groups as high as possible.
Results and Discussion
To understand and perhaps eliminate the empirically found ne-
cessity for a large excess of alkyne functionalities, the reaction
parameters were optimized for commercial starting materials,
1,4-diiodobenzene and 1,3,5-triethynylbenzene (TEB). However,
the first problem for a controlled polymer synthesis was posed
by the varying states in which TEB arrives after pur-
chase (Figure 1a). Purification of the as-purchased
monomer by column chromatography gave a yellow,
crystalline powder (Figure 1b). Further investigation
showed that the monomer can be purified by
a simple sublimation procedure (408C10À3 mbar) to
give pristine, colorless crystals, which correspond to
the expected appearance (Figure 1c).
However, not only are the materials reported herein much
closer to their ideal structures, but they also exhibit significant-
ly enhanced BET surface areas. Furthermore, P1–P4 are fluores-
cent under UV light (l=254 nm, light fluorescence; 366 nm,
Storage of the alkyne at 88C showed no evidence
of decomposition over several months, whereas elon-
gated exposure to ambient temperature caused the
material to turn brown, even when stored under
inert conditions. We attribute this behavior to a high
inherent reactivity of multi-alkyne compounds, which
could in part account for the need of an excess of
these compounds when non-purified monomers are
applied, because parts of the ethynyl groups have al-
ready reacted and thus are not available for further
polymerization.
It is a known fact that the use of aryl iodides over
bromides facilitates
a smoother reaction, which
lowers the probability of unreacted end groups.
Therefore, iodoarylenes were exclusively used in this
Scheme 1. Copper-free synthetic procedure towards CMPs based on 1,3,5-triethynylben-
zene. All reactions were carried out in a mixture of dimethylformamide/triethylamine
study. Upon testing different conditions, it was found (2:1) at 1008C for 20 h.
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strong fluorescence), whereas reproduced CMP-1 was found to
be non-fluorescent.
peak intensities for internal alkynes (2200 cmÀ1) and terminal
alkynes (2100 and 3300 cmÀ1, dashed lines) showed low con-
centrations of terminal alkyne moieties for P1 and P4, and
traces for P2 and P3, which is in good agreement with the
NMR data. Furthermore, structural differences between P2 and
CMP-1, prepared by the conventional method by hot injection
under excess of TEB, can be observed (Figure 3, shaded insets).
The occurrence of a shoulder at 1600 cmÀ1 and the additional
band at 1440 cmÀ1 in the CMP-1 spectrum resulted from the
formation of enyne moieties, which is in agreement with IR
data reported before[36] and the NMR data acquired by Bunz
and co-workers.[31]
Compound P1 was synthesized by reaction of TEB with
1,3,5-triiodobenzene. According to its highly symmetrical
(ideal) structure, P1 exhibits three major peaks at d=131, 121,
and 86 ppm in the 13C CP/MAS NMR spectrum (Figure 2).
These enyne groups are the product of coupling of the re-
maining terminal alkyne groups, which can therefore no
longer be observed for CMP-1, despite the huge excess of TEB
employed in the synthesis. Further differences can be observed
in the fingerprint region around 700 cmÀ1.
As was pointed out previously, the same network structure
as P1 has been reported before.[24] However, the previous
solid-state 13C NMR spectrum exhibited an additional sharp
peak at d=131 ppm, which we attribute to only partially con-
sumed starting material due to non-optimal reaction condi-
tions and less reactive 1,3,5-tribromobenzene. Consequently,
the resulting network would exhibit a lower degree of conver-
sion. Testament to this assumption are the comparatively low
reported surface areas, which lie in the range of 370 to
400 m2 gÀ1, whereas for P1 a surface area of 914 m2 gÀ1 was de-
tected. For P2 and P3, 1720 and 873 m2 gÀ1 were obtained
(Figure 4). These values surpass the surface areas of structurally
Figure 2. 13C NMR spectra of P1–P4; asterisks (*) denote spinning sidebands.
These peaks can be unambiguously assigned to CArÀH (130–
133 ppm, very broad), the quaternary carbon atom CArÀCꢀC
(121 ppm), and the acetylenic carbon atoms CꢀC (86 ppm).
Compounds P2, P3, and P4 were synthesized by reaction of
TEB with 1,4-diiodobenzene, 4,4’-diiodobiphenyl, and tetra-
kis(4-iodophenyl)methane, respectively. As can be seen, the
peak positions of the triethynylbenzene motif in P2, P3, and
P4 are largely unchanged from that in P1 (Figure 2, dashed
lines). All remaining peaks of P2, P3, and P4 can be assigned
accordingly (see the Supporting Information). Additionally, for
P1 and P4, small amounts of unreacted alkyne groups were
found at d=78 ppm.
The high conversion efficiency of this method was con-
firmed by FTIR measurements (Figure 3). Comparison of the
Figure 4. N2 uptake at 77 K for P1 (blue), P2 (green), P3 (red), and P4
(purple).
related materials obtained by the conventional method of
834 m2 gÀ1 for CMP-1 and 634 m2 gÀ1 for CMP-2 (Table 1). The
most significant surface-area enhancement was found for P4,
which exhibited an impressive surface area of 2552 m2 gÀ1. This
equals more than five times the accessible surface area of the
structurally related material published previously,[35] and is the
highest BET surface area of any poly(aryleneethynylene) net-
work reported to date, surpassing even those formed by dy-
namic alkyne metathesis.[10]
Figure 3. FTIR spectra of P1 (blue), P2 (green), P3 (red), and P4 (purple) ex-
hibiting very low concentrations of terminal alkyne moieties (2100 and
3300 cmÀ1, dashed lines), and conventional CMP-1 (pink). Structural differen-
ces between CMP-1 and P2 are highlighted (shaded insets).
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further mechanistic insights into the polymerization reaction
and widen the applicability of this facile and highly efficient
synthesis protocol.
Table 1. Surface-area comparison of P1–P4 and structurally related mate-
rials, as well as CO2 and H2 uptake capacities of the materials reported
herein.
SBET (reported) SBET (this work) CO2 uptake H2 uptake
[a]
[m2 gÀ1
]
[m2 gÀ1
]
[mmolgÀ1
]
[wt%][b]
Experimental Section
P1/CMP-X[24] 397
914
1720
873
3.01
2.53
2.12
3.36
1.32
1.36
1.00
1.59
P2/CMP-1[6]
P3/CMP-2[6]
P4/E2[35]
834
634
488
Materials: All chemicals were used as received unless otherwise
noted. Tetrakis(4-iodophenyl)methane and 1,3,5-triiodobenzene
were synthesized according to a literature procedure with slight
modifications (see the Supporting Information). 1,3,5-Triethynyl-
benzene was purchased from TCI and sublimated before use
(408C10À3 mbar). Tetrakis(triphenylphosphine)palladium(0) (99.9%),
1,4-diiodobenzene (99%), 1,3,5-tribromobenzene (98%), anhydrous
tetrachloromethane (ꢁ99.5%), anhydrous dimethylformamide
(99.8%), and triethylamine (ꢁ99.5%) were purchased from Sigma–
Aldrich. 4,4’-Diiodobiphenyl (99%) was purchased from Alfa Aesar.
Bis(trifluoroacetoxy)iodobenzene (98%) and iodine (99.5%) were
purchased from Acros Organics. Tetraphenylmethane (96%) was
purchased from Manchester Organics.
2552
[a] CO2 sorption experiments were performed at 273 K and 1 bar. [b] H2
sorption experiments were carried out at 77 K and 1 bar.
The high accessible surface area of P4 also gives rise to con-
siderable sorption capacity towards other gases; the total
uptake values for H2 at 77 K and 1 bar, as well as for CO2 at
273 K and 1 bar are among the highest reported for as-synthe-
sized MPNs.[2,37] Especially the almost linear shape of the CO2
adsorption curve suggests remarkable uptake under high pres-
sure (see the Supporting Information).
Synthetic procedure for P1: Inside the glovebox, a 50 mL glass
vial was charged with 1,3,5-triethynylbenzene (100.6 mg,
670 mmol), 1,3,5-triiodobenzene (305.4 mg, 670 mmol), [Pd(PPh3)4]
(15.1 mg, 13 mmol), dimethylformamide (12 mL), and triethylamine
(6 mL). The vessel was closed with a silicone septum, extracted
from the glovebox, and immersed in an oil bath preheated to
1008C. The colorless solution turned increasingly yellow, and a volu-
minous pale yellow precipitate formed after several minutes. The
mixture was kept at 1008C for 20 h, quenched by addition of
methanol, and filtered. The resulting beige solid was purified by
Soxhlet extraction from methanol overnight and dried in the
vacuum oven at 808C overnight.
In compounds P1–P3, two dominant pore sizes around 0.6
and 0.9 nm were found according to non-local (NL) DFT calcu-
lations derived from the nitrogen-sorption measurements
(Figure 5). A decrease in the relative intensity of the smaller
pores is accompanied by an increase in surface area. The same
tendency was reported previously[26] and culminates in P4 for
which pores at 1.10 nm were observed exclusively, and which
has the highest BET surface area.
Synthetic procedure for P2: Inside the glovebox, a 5 mL glass vial
was charged with 1,3,5-triethynylbenzene (100.1 mg, 667 mmol),
1,4-diiodobenzene (329.9 mg, 1.00 mmol), [Pd(PPh3)4] (7.5 mg,
6.5 mmol), dimethylformamide (3 mL), and triethylamine (1.5 mL).
The vessel was closed with a silicone septum, extracted from the
glovebox, and immersed in an oil bath preheated to 1008C. The
colorless solution turned increasingly yellow, and a voluminous
yellow precipitate formed after several minutes. The mixture was
kept at 1008C for 20 h, quenched by addition of methanol, and fil-
tered. The resulting yellow solid was purified by Soxhlet extraction
from methanol overnight and dried in the vacuum oven at 808C
overnight.
Synthetic procedure for P3: Inside the glovebox, a 50 mL glass
vial was charged with 1,3,5-triethynylbenzene (100.3 mg,
668 mmol), 4,4’-diiodobiphenyl (406.7 mg, 1.00 mmol), [Pd(PPh3)4]
(15.1 mg, 13 mmol), dimethylformamide (12 mL), and triethylamine
(6 mL). The vessel was closed with a silicone septum, extracted
from the glovebox, and immersed in an oil bath preheated to
1008C. The colorless solution turned increasingly yellow, and a volu-
minous yellow precipitate formed after several minutes. The mix-
ture was kept at 1008C for 20 h, quenched by addition of metha-
nol, and filtered. The resulting yellow solid was purified by Soxhlet
extraction from methanol overnight and dried in the vacuum oven
at 808C overnight.
Figure 5. Pore-size distributions of P1 (blue), P2 (green), P3 (red), and P4
(purple).
Conclusion
We herein present an improved protocol for the synthesis of
conjugated microporous polymers based on the Sonogashira
cross-coupling reaction. Experiments were carried out in
a facile one-pot procedure, and the amounts of halide and
alkyne groups were adjusted to 1:1. By reduction of the
amount of palladium catalyst to 0.65 mol% and removal of the
co-catalyst, copper(I) iodide, for the first time, CMPs with rigor-
ously controlled chemical structure and high BET surface areas
of up to 2552 m2 gÀ1 were obtained. We will continue to gain
Synthetic procedure for P4: Inside the glovebox, a 50 mL glass
vial was charged with 1,3,5-triethynylbenzene (20.1 mg, 134 mmol),
tetrakis(4-iodophenyl)methane (82.8 mg, 101 mmol), [Pd(PPh3)4]
(3.1 mg, 2.7 mmol), dimethylformamide (24 mL), and triethylamine
(12 mL). The vessel was closed with a silicone septum, extracted
from the glovebox, and immersed in an oil bath preheated to
1008C. The colorless solution turned increasingly yellow, and a volu-
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mixture was kept at 1008C for 20 h, quenched by addition of
methanol, and filtered off. The resulting off-white solid was puri-
fied by Soxhlet extraction from methanol overnight and dried in
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Keywords: CO2 sorption · conjugated microporous polymers ·
copper-free Sonogashira · covalent organic frameworks
·
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