Anionic porous polymers with tunable structures and catalytic properties

Article abstract of DOI:10.1039/c6ta04917f

A series of boron-containing conjugated microporous polymers with hierarchical porous structures have been readily prepared via typical transition metal-catalyzed coupling reactions. The distribution of micro- and mesopores in the networks as well as the specific surface areas are tunable via tailoring the structures of the building blocks. The distinct capability of the resulting Lewis acid-based neutral porous polymers to selectively capture fluoride ions provides a high-efficiency conversion into stable anionic porous polymers. For the first time, fluoride anion binding to boron atoms in a solid sample was essentially characterized by solid-state ¹¹B MAS NMR spectroscopy, clearly revealing such an efficient conversion from a neutral network to a negatively charged one only through Lewis acid-base adduct formation. Upon a simple ion-exchange process, various heavy metal cations were facile to be loaded into the networks of the anionic porous polymers. Furthermore, the cobalt(ii)-loaded porous polymers were shown to promote the stoichiometric homocoupling reactions of the different aryl Grignard regents, and exert distinct size selectivities for the homocoupling products, highly dependent on their porous structures. Such a successful loading strategy might be used for design and synthesis of new types of zeolite-like porous polymers with desirable catalytic properties for a certain organic transformation, as well as other functional materials.

Full text of DOI:10.1039/c6ta04917f

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Anionic Porous Polymers with Tunable Structures and Catalytic
Properties
Received 00th January 20xx,
Accepted 00th January 20xx
Wuxue Zhao,a Fan Zhang,*a Lingyun Yang,b Shuai Bi,a Dongqing Wu, a Yefeng Yao,b Manfred
Wagner,c Robert Graf,c Michael Ryan Hansen,cd Xiaodong Zhuang*ae and Xinliang Fengae
DOI: 10.1039/x0xx00000x
www.rsc.org/
A series of boron-containing conjugated microporous polymers with hierarchical porous structures have
been readily prepared via typical transition metal-catalyzed coupling reactions. The distribution of micro-
and mesopores in the networks as well as the specific surface areas are tunable via tailoring the structures of
the building blocks. The distinct capability of the resulting Lewis acid-based neutral porous polymers to
selectively capture fluoride ions renders a high-efficiency conversion into stable anionic porous polymers. For
the first time, fluoride anions binding to boron atoms in a solid sample, was essentially characterized by
solid-state 11B MAS NMR spectroscopy, clearly revealing such an efficient conversion from a neutral network
to a negatively charged one only through Lewis acid-base adduct formation. Upon simple ion-exchange
performance, various heavy metal cations were facile to be loaded into the networks of the anionic porous
polymers. Furthermore, the cobalt(II)-loaded porous polymers were exemplified to promote the
stoichiometric homocoupling reactions of the different aryl Grignard regents, and exerting the distinct size
selectivities for the homocoupling products, highly dependent on their porous structures. Such a successively
loading strategy might be used for design and synthesis new types of zeolite-like porous polymers with
desirable catalytic properties for a certain organic transformation, as well as the other functional materials.
contrast, zeolites and MOFs6 possess anionic networks balanced
Introduction
with
extra-network
counter-cations
to
maintain
the
Porous materials1 have attracted tremendous academic and
commercial attention in numerous fields, such as membrane
separation,2 industrial catalysis,3 and energy conversion and
storage.4 Among them, are metal organic frameworks (MOFs),5
consisting of organic linkers and coordinated metal centers, that
have cationic networks with extra-framework counter anions. In
electroneutrality of the materials. The unique structural and porous
features of zeolites facilitate their use as heterogeneous catalysts in
many organic transformations, such as Friedel-Crafts alkylation,7
alkylaromatic isomerization8 and disproportionation,9 realized on an
industrial scale. However, due to the lack of controlled and versatile
functionalization at the molecular level, the structures of zeolites
cannot be finely tailored. A lot of work remains to be done to
understand the relationship between the catalytic mechanism and a
material’s structure.10
aSchool of Chemistry and Chemical Engineering, State Key Laboratoryof
Metal Matrix Composites, Shanghai Key Laboratory of Electrical Insulation
and Thermal Ageing, Shanghai Jiao Tong University, 800 Dongchuan Road,
Porous polymer networks are generally built up from light
elements, such as carbon, nitrogen, boron and oxygen, and are
formed by cross-linking organic-directing and building units through
covalent bonds.11 Their well-defined structure can be tailored at the
molecular level to optimize their textural and chemical properties.
Compared with MOFs, porous organic polymers exhibit much
higher chemical and thermal stabilities.12 In general, porous
polymers are featured with neutral character, but in the past few
years, some cationic porous polymers have been reported,13 while
the anionic porous polymers remain rarely explored. Very recently,
the anionic microporous polymers were prepared by using tetraaryl
boron anion units, suitable for being used as catalyst, solid
Shanghai
zhuang@sjtu.edu.cn;
200240,
China.
*E-mail:
fan-zhang@sjtu.edu.cn;
bPhysics Department & Shanghai Key Laboratory of Magnetic Resonance,
East China Normal University, North Zhongshan Road 3663, 200062
Shanghai, China.
cMax Planck Institute for Polymer Research, Ackermannweg 10, D-55128
Mainz, Germany.
dInterdisciplinary Nanoscience Center (iNANO) and Department of
Chemistry, Aarhus University, Gustav WiedsVej 14, DK-8000 Aarhus C,
Denmark.
eCenter for Advancing Electronics Dresden (cfaed)& Department of
Chemistry and Food Chemistry, Technische Universitaet Dresden,01062
Dresden, Germany.
†Electronic Supplementary Informaꢀon (ESI) available: [details of any
supplementary information available should be included here]. See
DOI: 10.1039/b000000x/
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electrolytes or absorbent, and thus arousing much more attention Synthesis of B-Ph-ae-3: Starting from 1, 3, 5-Triethynylbenzene (174
to the charged porous polymers.14
mg, 1.16 mmol), tris(bromoduryl)borane(500 mg, 0.77 mmol),
(triphenylphosphine)palladium (200 mg, 0.174 mmol), and copper
In this paper, we present a concise synthetic strategy toward a (I) iodide (34.0 mg, 0.174 mmol) were added in a mixture of THF (15
series of neutral Lewis acid boron-containing porous polymers, mL) and Et3N (10 mL). Porous polymer B-Ph-ae-3 was obtained as
which can be readily converted into anionic porous polymers via yellow powder (522 mg, 91% yield) via the same procedure as
Lewis acid–base interaction with fluoride ions. Further, various described above.
alkali or heavy metals can be selectively loaded on such anionic
porous polymers by simple ion-exchange performance, General protocol for the synthesis of anionic porous polymers
representing the first example of the successively loading approach MF@B-Ph-ae-n:
to functional porous materials. The as-prepared neutral and anionic
porous polymers exhibit high permanent surface areas and Synthesis of anionic porous polymer F@B-Ph-ae-n: The neutral
hierarchical porous structures. Remarkably, as an example, the polymer B-Ph-ae-n was added in the solution of thiocarbamide,
cobalt (II)-loaded porous polymers can effectively promote stirred 2h at 80 °C in order to remove the palladium catalyst. Then,
stoichiometric homocoupling reactions of aryl Grignard regents B-Ph-ae-n and excess amount of tetrabutylammonium fluoride
with molecular size selectivity depending on the porous features of (Bu4NF), were mixed in anhydrous THF and stirred for 12 h. After
the networks.
centrifuged, the precipitate was collected, and stirred in a fresh
anhydrous THF solution of Bu4NFagain for 12 h. Finally, the resulting
precipitate was collected, washed with THF, and dried for 1 h under
high vacuum, affording the F@B-Ph-ae-n.
Experimental section
All starting materials were purchased from Aladdin and Aldrich. All
air-sensitive reactions were carried out under nitrogen atmosphere Synthesis of heavy metal-loaded anionic porous polymer MF@B-
and performed using Schlenk techniques. All solvents were dried Ph-ae-n: The ion exchange was carried out by treatment of F@B-
before use. Tetrahydrofuran (THF) was refluxed with sodium. Ph-ae-n with excess amount of one or several M(Ac)2 salts (M: Co,
Triethylamine (TEA) and dichloromethane were refluxed with Fe, Cu, Ni,)in a solvent mixture of ethanol and THF, for 2 days, and
calcium hydroxide. Tris(bromoduryl)borane,15 4,4-diethynyl-1,1- then centrifuged. The resulting solid powder was collected, washed
biphenyl,16 1,4-diethynylbenzene,16 1,3,5-triethynylbenzene,17 were with ethanol for several times and dried under high vacuum,
synthesized according to the reported procedures.
affording the MF@B-Ph-ae-n.
General protocol for the synthesis of neutral B-Ph-ae-n:
All polymerization reactions were carried out at 75 °C for 72 h. The
molar ratio of ethynyl to bromo functionalities in the monomer
feed was set at 1.5 : 1. The typical experimental procedure is given
as follows:
Synthesis of B-Ph-ae-1: 1, 4-Diethynylbenzene (219 mg, 1.74
mmol),
tris(bromoduryl)borane(500
mg,
0.77
mmol),
(triphenylphosphine) palladium (200 mg, 0.174 mmol), and copper
(I) iodide (34.0 mg, 0.174 mmol) were added in a mixture of
anhydrous THF (15 mL) and Et3N (10 mL). The reaction mixture was
heated to 75°Cand stirred for 72 h under a nitrogen atmosphere.
Afterwards, the mixture was cooled to room temperature, and the
resulting precipitate was collected by filtration and washed
sequentially with chloroform, water, ethanol, and acetone to
remove any un-reacted monomers or catalyst residues, and further
purified by Soxhlet extraction with methanol for 24 h. Finally, the
product was dried under vacuum for 24 h at 60 °C to afford the
porous polymerB-Ph-ae-1as a yellow powder (512 mg, 87% yield).
Scheme 1. Synthesis of Lewis acid-based neutral porous polymers and
corresponding ionic porous polymers.i) Et3N, Pd(PPh3)4, CuI, 75°C, 72h,
tetrahydrofuran. ii) Tetrabutylammonium fluoride (Bu4NF), THF, room
temperature (RT). iii) Ethanol and THF, cobalt acetate, nickel acetate, iron
nitrate, at RT.
Synthesis of B-Ph-ae-2: Started from 4, 4-Diethynyl-1, 1-biphenyl
(352 mg, 1.74 mmol), tris(bromoduryl)borane(500 mg, 0.77 mmol),
(triphenylphosphine)palladium (200 mg, 0.174 mmol), and copper
(I) iodide (34.0 mg, 0.174 mmol) were added in a mixture of THF (15
mL) and Et3N (10 mL). Porous polymerB-Ph-ae-2 was obtained as a
yellow powder (634 mg, 88% yield) via the same procedure as
described above.
Results and discussion
The synthetic route toward Lewis acid boron-based porous
polymers B-Ph-ae-n (n = 1, 2, 3) is presented in Scheme 1. The key
monomers of tris(bromoduryl)borane (1), 1, 4-diethynylbenzene(2),
4, 4-diethynyl-1,1-biphenyl (3), and 1, 3, 5-triethynylbenzene (4)
2 | J. Name., 2012, 00, 1-3
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were synthesized according to previous reports (see the Supporting
X-ray diffraction (XRD) (Fig. S2b) demonstrates the amorphous
Information). Under a typical Sonogashira cross-coupling condition, features that are typical for porous polymers built up from metal-
monomer 1 was reacted with monomers 2-4, to afford the porous catalyzed coupling reactions. Thermogravimetric analysis (TGA) (Fig.
polymers B-Ph-ae-1, B-Ph-ae-2and B-Ph-ae-3, all as yellow powders S2c) under a nitrogen atmosphere manifests a weight loss of only
in high yields (from 87% to 91%). The as-synthesized porous 5% at 315, 312 and 267°C for B-Ph-ae-1, B-Ph-ae-2, and B-Ph-ae-3,
polymers were revealed to exhibit excellent stabilities after respectively, suggesting good thermal stability for these polymers.
treatment with water (Fig. S1). The duryl moiety containing four The morphology and microstructure of the polymer networks were
sterically bulky methyl groups at the 2, 3, 5, and 6-positions of the investigated by transmission electron microscopy (TEM) (Fig. S3).
phenyl ring in monomer 1 is clearly beneficial for achieving high The TEM images of polymers show the presence of a vermicular
polymerization efficiency as well as to improve the inertness of the texture. The alternating areas of light and dark contrast in the TEM
resulting porous polymers to moisture and oxygen.
images disclose a disordered porous structure for the porous
polymers.
Fig. 1 a) Solid-state 13CCP/MAS NMR spectra of B-Ph-ae-1 (black), B-Ph-ae-2
(red), and B-Ph-ae-3 (blue); b) Solid-sate 11B MAS NMR spectra of B-Ph-ae-n;
c) Solid-state 11B MAS NMR spectra of F@B-Ph-ae-n.
Fig. 2 Nitrogen adsorption (open symbols) and desorption (solid symbols)
isotherms of a) B-Ph-ae-n, b) F@B-Ph-ae-n, and c) CoF@B-Ph-ae-n measured
at 77 K. Pore size distribution curves for d) B-Ph-ae-1 and CoF@B-Ph-ae-1, e)
B-Ph-ae-2 and CoF@B-Ph-ae-2, and f) B-Ph-ae-3 and CoF@B-Ph-ae-3 as
calculated by NL-DFT.
The structures of the polymeric networks B-Ph-ae-1, B-Ph-ae-2,
and B-Ph-ae-3 were characterized by Fourier transform infrared
(FTIR) spectroscopy (Fig. S2a) and 13C solid-state nuclear magnetic
resonance (NMR) spectroscopy (Fig. 1a). In the FTIR spectra, a
typical C−B stretching signal at around 1105 cm−1 was observed for
all three porous polymers. A peak with a very low intensity at 2110
cm–1 corresponds to –C≡C– stretching. The broad peaks at 3300
cm−1 can be attributed to the –C≡C– linkages. The signals at 2870-
3050 cm−1 originate from aromatic C−H stretching.18 In the solid-
state 13C {1H} cross-polarization/magic angle spinning (CP/MAS)
NMR spectra, the well-resolved resonances at around 20.0 ppm are
ascribed to the methyl carbons (Car–CH3). The peaks at~136.3 and
131.0 ppm can be attributed to the substituted aromatic carbons
(Car–CH3) and the hydrogen-bearing aromatic carbons (Car–H),
respectively. The signals located at ~123.3 and 91.6 ppm can be
assigned to (Car–C≡C–Car’) (Car and Car’ are the carbon atoms of dury
and phenyl rings, respectively; they cannot be distinguished under
the current measurement conditions) and the carbon atoms of the
quaternary alkynes (Car–C≡C–Car), respectively. There are no
detectable resonances at about 83.0 and 77.0 ppm from the
The porous nature of the polymer networks was further
confirmed by nitrogen physisorption measurements. It was found
that these porous polymers exhibit type IV isotherms.20 The distinct
hysteresis phenomena between the adsorption and desorption
cycles at relatively low pressure for B-Ph-ae-1, and, in particular, for
B-Ph-ae-2, stems from the deformation and swelling of the polymer
networks.21. The Brunauer–Emmet–Teller (BET) surface areas of
these polymer networks increased in the sequence of B-Ph-ae-2
(226 m2 g–1)＜B-Ph-ae-1 (432 m2 g–1)＜B-Ph-ae-3 (935 m2 g–1). The
pore size distribution curves for these polymers from the
adsorption branches of the isotherms were calculated using non-
local density functional theory (NL-DFT) (Fig. 2). A set of peaks from
0.88 to 1.94 nm are found for B-Ph-ae-2, demonstrating its distinct
microporous features. However, two main peaks at 1.62 and 3.85
nm for B-Ph-ae-1 and three narrowly distributed peaks with
maxima at 1.70, 2.70, and 3.90 nm for B-Ph-ae-3 were observed,
signifying the presence of both micro- and meso-pores in the
terminal acetylenic groups (Car–C≡C–H), suggesting
polymerization efficiency for the porous polymers.19
a high
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networks. This result is in good agreement with the profiles of N2 polymers and model compound were also verified by 19F NMR
absorption isotherm. Compared with B-Ph-ae-3, B-Ph-ae-2 spectroscopy (Fig. S7). The signal at -148.8 ppm can be clearly
constructed from the conjugated monomer with a longer chain attributed to F atoms in F-B bonds, highly in line with the 19F NMR
length, B-Ph-ae-1 exhibits a lower surface area and smaller pore chemical shift of the model compound (Fig. S8) and the reported
sizes, which can be ascribed to the possible formation of an literatures.24 All these characterizations strongly indicate that the
interpenetrating network and a coiled structure as the backbone ionic porous polymers F@B-Ph-ae-n have been successfully
rigidity declined.
synthesized by the simply treatment of the neutral porous polymers
B-Ph-ae-n withnBu4NF in some common organic solvents.
Table 1. Physical properties for B-Ph-ae-n, F@B-Ph-ae-n, and CoF@B-Ph-ae-
n polymer networks.
The loading degree of F– was next examined by fluorescence
spectroscopy. A decreased intensity of the fluorescence emission
was found (Fig. S9), provided the residual empty orbitals of boron
atoms in the polymer networks were gradually occupied by the
fluoride anions.25 The very low dissociation constant of the F–
bonded triduryl boron moiety,26 essentially ensures that the
framework consisting of such a kind of moiety is able to be kept in a
relatively stable charged state. The content of F– remained
unchanged after washing many times with THF for thoroughly
removing off the free nBu4NF. Furthermore, considering that the
exact containing of fluoride in a sample, normally is difficult to be
determined directly by elemental analysis,27 alternatively, we can
measure the content of nitrogen (from the counter cation
tetrabutyl ammonium ions, equal molar amount relative to the
fluoride binding to boron) in the as-prepared F@B-Ph-ae-n samples.
As an example, F@B-Ph-ae-3 was examined to have a content of N
(1.52 wt%), thus the content of F (2.05 wt%,) can be evaluated.
Meanwhile, by compared with the content of B (1.48 wt%) from
elemental analysis, we can confirm that F and B exist nearly in a 1:1
molar ratio in F@B-Ph-ae-3. The detailed B, N and F contents of
F@B-Ph-ae-n were summarized in Table S1. After ionization, the
surface areas for F@B-Ph-ae-n decreased 10.4% (n=1), 22.6% (n=2)
and 54.7% (n=3) in comparison with B-Ph-ae-n. This result can be
easily attributed to the increased weight after ionization. The
relatively poor binding performance of B-Ph-ae-1 and B-Ph-ae-2 can
be attributed to their lower surface area and higher ratio
microporous structural characters. These results are well in
agreement with the 11B NMR spectra analyses above. The N2
adsorption isotherm manifested significantly the reduced surface
areas and pore volumes for the anionic F@B-Ph-ae-n compared
with the neutral B-Ph-ae-n (Table 1). This can be explained by the
increased molecular mass and space filling resulting from the
loaded potassium ions. Interestingly, the pore volume of 0.62 cm3 g–
[a]
[b]
Vmicro
[c]
SBET
Vtot
Sample
(m2 g–1
)
(cm3 g–1
)
(cm3 g–1
)
B-Ph-ae-1
B-Ph-ae-2
B-Ph-ae-3
F@B-Ph-ae-1
F@B-Ph-ae-2
F@B-Ph-ae-3
CoF@B-Ph-ae-1
CoF@B-Ph-ae-2
CoF@B-Ph-ae-3
432
226
935
387
175
424
424
186
490
0.14
0.12
0.20
0.10
0.08
0.13
0.11
0.10
0.15
0.26
0.42
0.71
0.15
0.22
0.47
0.17
0.26
0.53
[a] Surface areas calculated from the N2 adsorption isotherm using the BET
method. [b] The micropore volume derived using the non-local density
functional theory. [c] Total pore volume at p/p0 = 0.99.
Given that the Lewis-acid boron center in tridurylborane has a
highly selective and strong binding capacity for F–,22 we consider
that the as-prepared neutral porous polymers can be readily
converted into corresponding anionic porous polymers, as shown in
Scheme 1. Thus, the neutral polymers B-Ph-ae-n (n=1, 2, 3) were
suspended in the THF solution of tetrabutylammonium fluoride
(Bu4NF), and stirred at room temperature for 2 days to afford the
anionic porous polymers F@B-Ph-ae-n (n=1, 2, 3), respectively.
In order to reveal the difference between the samples before
and after nBu4NF treatment, B-Ph-ae-n and F@B-Ph-ae-n were
studied by X-ray photoelectron spectroscopy (XPS). For the neutral
B-Ph-ae-n, all XPS spectra of these samples show B1s region of
188.4 eV, which can be attributed to typical B-C bond (Fig. S4). After
ionization, two B1s peaks at 190.3 and 188.4 eV can be observed for
F@B-Ph-ae-n,23 which can be ascribed to B-F and B-C bonds,
respectively (Fig. S5). B-Ph-ae-n and F@B-Ph-ae-n were further
characterized by 11B solid-state nuclear magnetic resonance (SS-
NMR) spectroscopy (see Fig. 1b and 1c). From the 11B MAS NMR
spectra of B-Ph-ae-n, the broad signal centered at around 55 ppm
can be assigned to B incorporated into the conjugated polymer
networks at regular positions located in symmetric
environments.11d As a contrast, in the 11B MAS NMR spectra of
F@B-Ph-ae-n (Fig. 1c), a new relatively sharp signal at -15.8 ppm in
the high field regions, can be observed and the peak at around 55
ppm almost disappeared in the case of F@B-Ph-ae-3, essentially
suggestive of approximately all boron atoms coordinated with F– in
the network. This result coincides well with the 11B NMR chemical
shift of the model compound (Fig. S6). Notably, for F@B-Ph-ae-1 or
F@B-Ph-ae-2, the main signal at -15.8 ppm, with the tailed peaks in
the low field regions, might be attributed to the presence of no F--
binding boron atoms. Such a phenomenon is highly related to the
porous structure character in the different porous polymer samples.
Furthermore, the binding of F– to boron atoms in the porous
1
for F@B-Ph-ae-3 is much higher than that of 0.48 cm3 g–1 for
recently reported anionic porous polymer with the counter cation
Na+.14
Ion exchange is a favorable strategy to incorporate various ions
into the charged materials, e.g. zeolites, for achieving the desired
properties, such as catalysis28 and gas adsorption.13a, 29 As expected,
the tetrabutyl ammonium cations in the networks of the as-
prepared F@B-Ph-ae-n can be readily exchanged with the different
heavy metal ions by the simple treatment with the corresponding
heavy metal salt in a solvent mixture of ethanol and THF, resulting
in the heavy metal ion loaded porous polymers MF@B-Ph-ae-n (see
the Supporting Information). Here, CoF@B-Ph-ae-3 was selected as
a typical example for the detailed investigation. According to ICP-
OES analysis, the loading content of cobalt in Co@B-Ph-ae-3 is 2.43
4 | J. Name., 2012, 00, 1-3
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wt%, which is the nearly half molar amount of B (1.11 wt%) in the ion exchange constants.31 Therefore, the as-synthesized Lewis acid-
corresponding F@B-Ph-ae-3 sample, suggesting that tetrabutyl based neutral porous polymers might have a potential application
ammonium ions have been almost completely replaced by Co(II) to selectively collect F– and heavy metal ions in the polluted
ions to maintain the charge balance. In addition, acetate anion environment through a one-pot performance .
signal was not observed from the in the FTIR spectra (Fig. S10),
Table 2. Homocoupling reaction of aromatic Grignard reagents by Co(II)-
confirming that there is no the free Co(Ac)2 left in the networks.
loaded ionic porous polymers.[a]
Reactant
Product[b]
CoF@B-Ph-ae-1
CoF@B-Ph-ae-2
CoF@B-Ph-ae-3
CoCl2
85%
80%
91%
89%
43%
n. d.
84%
87%
71%
68%
81%
83%
n. d.
n. d.
14%
86%
Fig. 3 a) Typical scanning electron microscopy (SEM) image of CoF@B-Ph-ae-
3 and the corresponding elemental mapping images of b) carbon, c) boron,
and d) cobalt. The XPS spectra of CoF@B-Ph-ae-3: e) Co 2p core-level
spectrum and f) the survey spectrum.
[a] Reaction conditions: 1.6 mmol Grignard reagents, CoF@B-Ph-ae-n
containing 0.8 mmol cobalt,10 ml of THF at 55°C, 1 h. [b] Isolated yield after
chromatographic purification.32
It is well known that transition metal ion-loaded porous
materials are promising heterogeneous catalysts.33 The size and
shape selectivity3a, 34 for a porous catalyst plays a key role in the
The compositions of the resulting Co(II) loaded anionic polymers
were further analyzed via X-ray photoelectron spectroscopy (XPS)
(Fig. 3 and Fig. S11，S12). There are two characteristic peaks at
782.2 and 798.5 eV, corresponding to the 2p3/2 and 2p1/2 spin-
orbit peaks of Co(II). The peaks at around 787.2 and 803.3 eV are
associated with the shake-up type peaks of the 2p3/2 and
2p1/2 edges, respectively.30 XPS also indicates a cobalt content of
2.40 wt%, which is in good agreement with the value obtained from
ICP-OES analysis. Of note, elemental mapping images show the
homogeneous distribution of carbon, boron, and cobalt elements in
CoF@B-Ph-ae-3 (Fig. 3). The surface area and pore volume of
CoF@B-Ph-ae-3 were determined to be 468 m2 g-1 and 0.51 cm3 g-1,
respectively, which are slightly lower than those of F@B-Ph-ae-3
(Table 1) due to the a lower molecule weight of Co2+ (58.93) in
comparison with Bu4N+ (242.28). Therefore, one can recognize the
remarkable differences in N2 isotherm profiles, surface areas, and
pore volumes for B-Ph-ae-n, F@B-Ph-ae-n, and CoF@B-Ph-ae-n (Fig.
2 and Table 1), implying that the physisorption properties of porous
polymers can be tuned by introducing the fluoride anion and
different metal cations, without changing the backbone of the
polymer networks.
controlled synthesis of molecules with
a certain size or
configuration. Thus, the as-prepared MF@B-Ph-ae-n porous
polymers, loaded with different transition metal ions, presumably
possess heterogeneous catalytic properties highly related to their
unique porous structures. With these in our mind, homocoupling
reactions of aromatic Grignard reagents catalyzed by employing
CoF@B-Ph-ae-n loaded with a stoichiometric amount of cobalt
cations (calculated on the basis of the Grignard reagent) were
carried out. To elucidate the catalytic activity and size selectivity of
CoF@B-Ph-ae-n, a series of substrates of different sizes were
examined: phenyl Grignard reagent (0.43×0.84 nm2), 3-
methylphenyl Grignard reagent (0.47×0.84 nm2), 4-methylphenyl
Grignard reagent (0.43×0.93 nm2), and 4-ethylphenyl Grignard
reagent (0.43×1.10 nm2). The homocoupling products were
isolated, and the catalytic data were summarized in Table 2.
CoF@B-Ph-ae-3, with the largest mesoporous structure, provided
the homocoupling products of biphenyl, 4, 4′-dimethylbiphenyl, 3,
3′-dimethylbiphenyl in high yields of 91%, 84% and 81%,
respectively, but a poor yield of 14% for 4, 4′-diethylbiphenyl with a
larger molecular size. While, the microporous CoF@B-Ph-ae-2 only
resulted in a high conversion of 80% for biphenyl, but no larger size
molecule 4, 4′-dimethylbiphenyl or 4, 4′-diethylbiphenyl found and
isolated as the product. Interestingly, using 3-methylphenyl
Grignard reagent as the substrate, which has a smaller size than 4-
methylphenyl Grignard reagent, the microporous CoF@B-Ph-ae-2
enable delivering a remarkable catalytic activity up to yield of 68%.
As a comparison, CoCl2 (its amount was calculated relative to the
content of cobalt in CoF@B-Ph-ae-n) as catalyst to undergo the
homocoupling of the aromatic Grignard reagents under the same
reaction conditions, which offered the coupling products biphenyl,
4, 4′-dimethylbiphenyl, 3, 3′-dimethylbiphenyl or 4, 4′-
Encouraged by the success in the loading of cobalt cation, we
further investigated the inclusion of other metal cations in these
anionic porous polymers. For instance, nickel (II) acetate or iron (II)
nitrate was added to the dispersion of F@B-Ph-ae-3 in THF, and
followed by stirring for 2 days, respectively. The resulting Ni or Fe
containing porous polymers (NiF@B-Ph-ae-3 and FeF@B-Ph-ae-3)
exhibit the loading amount of 1.05 wt% and 3.11 wt% respectively.
Moreover, the capability of neutral porous polymers B-Ph-ae-n to
selectively capture metal ions from a mixture system was further
examined (Fig. S13). Reasonably, the ratios of the transition metals
loaded in an anionic sample, were governed by the corresponding
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diethylbiphenylin good yields without efficient size selectivity. the Natural Science Foundation of China (21574080, 51403126 and
Therefore, the porous structures had the major impact on the 21102091), the Shanghai Committee of Science and Technology
catalytic behavior through spatially controlling the formation of the (15JC1490500 ) and an ERC grant on 2DMATER. We also thank the
species in the process of homocoupling reaction (e.g. the Instrumental Analysis Center of Shanghai Jiao Tong University for
intermediates formed in the transmetallation step).
provide some measurements.
To further probe the catalytic activities and selectivities with
respect to the porous structures, a mixture containing phenyl, 4-
methylphenyl, and 4-ethylphenyl Grignard reagents in 1:1:1 eq.
were treated with CoF@B-Ph-ae-n containing a stoichiometric
amount of cobalt cations in THF under mild conditions (55°C). Gas
chromatography mass–spectrometry (GC-MS) analyses revealed
that the distribution of coupling products is highly dependent on
the pore size of the as-made anionic porous polymers, and the
microporous CoF@B-Ph-ae-2 showed the best size selectivity for
the formation of biphenyl homocoupling product (Table S2, Fig.
S14-S18). Undoubtedly, such results will be helpful for developing
new catalytic systems with high size and shape selectivity.
Notes and references
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a high-efficiency
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ions, can be rationally incorporated into the as-prepared anionic
porous polymers, resulting into new heterogeneous catalytic
systems. One can expect that the catalytic behavior are tunable not
only by the pore structures of the porous polymers, but also
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polymers may provide synergetic effects for certain catalytic
organic transformations with the size and shape control. On the
other hand, the pronounced capability to selectively recycle and
reuse environmentally harmful fluoride anions and heavy metals
makes the as-prepared boron-based porous polymers promising
candidates for potential applications in the comprehensive
management of the environment.
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Acknowledgements
This work was financially supported by the National Basic Research
Program of China (973 Program: 2013CBA01602, 2012CB933404),
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Graphical Table of Contents
Anionic porous polymers: A series of boron-containing conjugated polymers with hierarchical porous structures
have been prepared via Sonogashira-Hagihara coupling. The distinct capability of the resulting neutral porous
polymers to selectively capture fluoride ions renders a high-efficiency conversion into stable anionic porous
polymers balanced with counter-cationic metals. Such as-prepared Lewis acid-based neutral porous polymers
might have the potential to selectively uptake F– and heavy metal ions, as well as can effectively catalyze
stoichiometric homocoupling reactions of aryl Grignard regents with size selectivity depending on the porous
features of the networks.
Anionic Porous Polymers with Tunable Structures and Catalytic Properties
Wuxue Zhao, Fan Zhang, Lingyun Yang, Shuai Bi, Dongqing Wu, Yefeng Yao, Manfred Wagner, Robert Graf,
Michael R. Hansen, Xiaodong Zhuang and Xinliang Feng