Copper‐Free Sonogashira Coupling for High‐Surface‐Area Conjugated Microporous Poly(aryleneethynylene) Networks [original]

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Trunk, M., Herrmann, A., Bildirir, H., Yassin, A., Schmidt, J., & Thomas, A. (2016). Copper-Free

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

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Trunk, M.; Herrmann, A.; Bildirir, H.; Yassin, A.; Schmidt, J.; Thomas,

Copper-Free Sonogashira Coupling for

High-Surface-Area Conjugated

Microporous Poly(aryleneethynylene)

Network

Accepted manuscript (Postprint)Journal article |

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 m2g1.

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-

ly.[19–21] The importance of these materials is emphasized by the

recent commercialization of CMP-1 by Sigma–Aldrich.[22]

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 m2g1by systematic variation of the phosphine ligand

accompanying the palladium catalyst.[26]

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

[a] M. Trunk, A. Herrmann, Dr. H. Bildirir, Dr. A. Yassin, Dr. J. Schmidt,

Prof. Dr. A. Thomas

Department of Chemistry, Functional Materials

Technische Universitt Berlin

Hardenbergstrasse 40, 10623 Berlin (Germany)

E-mail: arne.thomas@tu-berlin.de

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 conventionalh

omogeneous 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.

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 1 b). Further investigation

showed that the monomer can be purified by

a simple sublimation procedure (40 8C103mbar) to

give pristine, colorless crystals, which correspond to

the expected appearance (Figure 1c).

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

study. Upon testing different conditions, it was found

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]

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,

Figure 1. Varying states of 1,3,5-triethynylbenzene: a) as-purchased; b) after

column chromatography from dichloromethane; and c) after sublimation.

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

(2:1) at 1008C for 20 h.

strong fluorescence), whereas reproduced CMP-1 was found to

be non-fluorescent.

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 13CC

P/MASN

MR spectrum (Figure 2).

These peaks can be unambiguously assigned to CArH (130–

133 ppm, very broad), the quaternary carbon atom CArCC

(121 ppm), and the acetylenic carbon atoms CC(86 ppm).

Compounds P2,P3,andP4 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,andP4 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

peak intensities for internal alkynes (2200 cm1) and terminal

alkynes (2100 and 3300 cm1, 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 cm1and the additional

band at 1440 cm1in 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]

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 cm1.

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 m2g1, whereas for P1 a surface area of 914 m2g1was de-

tected. For P2 and P3, 1720 and 873 m2g1were obtained

(Figure 4). These values surpass the surface areas of structurally

related materials obtained by the conventional method of

834 m2g1for CMP-1 and 634 m2g1for CMP-2 (Table 1). The

most significant surface-area enhancement was found for P4,

which exhibited an impressive surface area of 2552 m2g1. 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 2. 13C NMR spectra of P1–P4; asterisks (*) denote spinning sidebands.

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 cm1, dashed lines), and conventional CMP-1 (pink). Structural differen-

ces between CMP-1 and P2 are highlighted (shaded insets).

Figure 4. N2uptake at 77 K for P1 (blue), P2 (green), P3 (red), and P4

(purple).

The high accessible surface area of P4 also gives rise to con-

siderable sorption capacity towards other gases; the total

uptake values for H2at 77 K and 1 bar, as well as for CO2at

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).

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.

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 m2g1were obtained. We will continue to gain

further mechanistic insights into the polymerization reaction

and widen the applicability of this facile and highly efficient

synthesis protocol.

Experimental Section

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

(408C103mbar). 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.

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.

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.

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-

Table 1. Surface-area comparison of P1–P4 and structurally related mate-

rials, as well as CO2and H2uptake capacities of the materials reported

herein.

SBET (reported)

[m2g1]

SBET (this work)

[m2g1]

CO2uptake

[mmolg1][a]

H2uptake

[wt%][b]

P1/CMP-X[24] 397 914 3.01 1.32

P2/CMP-1[6] 834 1720 2.53 1.36

P3/CMP-2[6] 634 873 2.12 1.00

P4/E2[35] 488 2552 3.36 1.59

[a] CO2sorption experiments were performed at 273 K and 1 bar. [b] H2

sorption experiments were carried out at 77 K and 1 bar.

Figure 5. Pore-size distributions of P1 (blue), P2 (green), P3 (red), and P4

(purple).

minous pale yellow precipitate formed after several minutes. The

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

the vacuum oven at 808C overnight.

Synthetic procedure for reproduced CMP-1: Inside the glovebox,

[Pd(PPh3)4] (50.1 mg, 43 mmol) and CuI (15.0 mg, 79 mmol) were

suspended in dimethylformamide (1.5 mL) and taken up into a sy-

ringe. A 5 mL glass vial was charged with 1,3,5-triethynylbenzene

(150.2 mg, 1.00 mmol), 1,4-diiodobenzene (330.0 mg, 1.00 mmol),

dimethylformamide (1.5 mL), and triethylamine (1.5 mL). The vessel

was equipped with a silicone septum and a balloon, the vial and

syringe were extracted from the glovebox, and the vial was im-

mersed in an oil bath preheated to 1008C. After 5 min, the catalyst

mixture was added into to the vial through the septum. The color-

less solution turned yellow instantly, and a brown precipitate

formed after a few seconds. The mixture was kept at 1008Cfor

20 h, quenched by addition of methanol, and filtered. The resulting

brown solid was purified by Soxhlet extraction from methanol

overnight and dried in the vacuum oven at 808C overnight.

Further experimental and analytical data are given in the Support-

ing Information.

Acknowledgements

This work was funded by the ERC Project ORGZEO (Grant-Nr.:

278593) and the DFG (Cluster of Excellence UniCat) . We thank

Christina Eichenauer, Maria Unterweger, and Caren Gçbel for

the sorption experiments, XRD measurements, and TEM and

EDX measurements, respectively.

Keywords: CO2sorption ·conjugated microporous polymers ·

copper-free Sonogashira ·covalent organic frameworks ·

hydrogen storage

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