1,3,5-Tris(functionalised-phenylethynyl)benzene-metal complexes: Synthetic survey of mesoporous coordination polymers and investigation of their carbonisation

Article abstract of DOI:10.1039/b715550f

A series of multicoordinate 1,3,5-tris(functionalised-phenylethynyl) benzenes (1-9) was synthesised, and coordination polymers were constructed from these organic linkers and copper ions in high yields. The carbonisation of the linkers 1-9 afforded a micr

Full text of DOI:10.1039/b715550f

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PAPER
www.rsc.org/materials | Journal of Materials Chemistry
1,3,5-Tris(functionalised-phenylethynyl)benzene–metal complexes:
synthetic survey of mesoporous coordination polymers and investigation
of their carbonisation†
*
Norifumi Kobayashi and Masashi Kijima
Received 9th October 2007, Accepted 3rd January 2008
First published as an Advance Article on the web 25th January 2008
DOI: 10.1039/b715550f
A series of multicoordinate 1,3,5-tris(functionalised-phenylethynyl)benzenes (1–9) was synthesised,
and coordination polymers were constructed from these organic linkers and copper ions in high
yields. The carbonisation of the linkers 1–9 afforded a microporous carbon that shows type I
adsorption–desorption isotherm. Although most of the coordination polymers prepared in this study
turned out to be low porous materials, the coordination polymer 7e prepared from the reaction of
1,3,5-tris(4-carboxyphenylethynyl)benzene tripotassium salt (K37) and copper(II) nitrate was
a microporous material in addition to the mesoporous materials (7c and 7d) prepared from the reaction
of K37 with copper(II) acetate and copper(II) chloride, respectively. The carbonisation of the
coordination polymers unexceptionally brought about an increase of micropore volume. A stepwise
analysis of 7c pyrolysed at 350, 600, and 900 ꢀC revealed that the mesoporosity hardly changed upon
heat treatment, which in other words, demonstrates that microporosity could be successfully added to
the mesoporous coordination polymer through the carbonisation process.
On the other hand, the porous metal–organic frameworks
Introduction
(MOFs) also attract a profound research interest because of their
shape-selective, predictable, topological architectures together
with their homogeneous metal dispersion.13 It is attractive to
introduce these features to the porous carbon materials.
To the best of our knowledge, despite many reports on the
synthesis of microporous MOFs (microMOFs),13e,14 mesoMOF
with permanent porosity exhibiting type IV adsorption–desorp-
tion isotherm has been reported only once.15 Hence, the synthesis
of novel mesoMOFs and transformation from the MOFs to
porous carbon are a great challenge in the field of chemistry of
coordination polymers and carbon materials.
Porous materials1 are generally classified into three groups, that
is, carbon materials,2 inorganic materials,3 and coordination
polymers.4 In particular, porous carbon materials have uniquely
electrical conductivity in addition to thermal, hydrothermal, and
mechanical stabilities compared to those that are uncarbonised,
and they are used for a wide variety of application purposes not
only in hydrogen fuel storage,5 gas separation,6 water purifica-
tion,7 catalysis or catalyst supports,8 but also in energy-storage
devices such as lithium ion secondary batteries9 and electric
double layer capacitors.10
A surface-activation procedure of the carbon or carbon
precursor has conventionally been used to produce activated
carbons,11 but it is hard to eliminate their inherancy of inhomo-
geneous pore distribution. On the contrary to the activation
method, the template synthesis is a promising method for con-
structing ordered nano-porous carbon materials.12 In this case,
the specific space of porous inorganic materials such as zeolite
has been utilised in porous carbon synthesis, but inevitably the
process is a reverse transcription of the template structure, and
the template must be removed by post-cleaning using hydro-
fluoric acid or alkaline. After all, the pore structure of carbon
is dependent on the template, and the mass production is difficult
due to the strict synthetic and removal processes.
We have recently reported that the pyrolytic carbonisation of
conjugated polymers having an alternative structure of the
phenylene and carbon–carbon triple bond portions could afford
microporous carbons in high yields above 80% under an argon
flow.16 More recent reports have successfully demonstrated
that rigid networks of poly(aryleneethynylene) can construct
microporous materials.17 It is also reported that large molecules
having six-fold phenyleneethynylene bonds such as hexakis
(4-functionalised-phenylethynyl)benzenes18 or hexakis[4-(40-
functionalised-phenylethynyl)phenyl]benzenes19 could serve as
guest-inclusion organic crystals directed to organic zeolites
because their rigid and starburst-like structures tend to prevent
network interpenetration.
For the reasons stated above, it is thought that multicoordi-
nate p-conjugated molecules are appropriate as a scaffold to
construct novel mesoMOFs which can be carbonised with reten-
tion of the framework in consequence of their rigid structures
and the higher carbon content than the general MOFs. In this
study, we initially investigate what kind of p-conjugated organic
linker would be appropriate as a pre-carbon material to construct
the carbonised MOFs. Next, we synthesised multicoordinate
Institute of Materials Science, Graduate School of Pure and Applied
Sciences, University of Tsukuba, Tsukuba, Ibaraki, 305-8573, Japan.
E-mail: kijima@ims.tsukuba.ac.jp; Fax: +81 29 853 4490; Tel: +81 29
853 6905
† Electronic supplementary information (ESI) available: Syntheses,
preparations, NMR spectra, thermogravimetric analysis plots, and N2
adsorption isotherms. See DOI: 10.1039/b715550f
This journal is ª The Royal Society of Chemistry 2008
J. Mater. Chem., 2008, 18, 1037–1045 | 1037

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linkers, 1,3,5-tris(functionalised-phenylethynyl)benzene deriva-
tives, and their coordination polymers with copper salts.
Last, they were pyrolysed to investigate behaviors of the trans-
formation from coordination polymers to porous carbonised
materials.
Scanning electron microscopy (SEM) images were acquired
with a JSM-5610 (JEOL).
BET surface area was estimated from the results of N2 adsorp-
tion isotherm at 77 K measured with a SA 3100 (Coulter). The
pore characterisations were also carried out by the as-plots for
micropores and the DH method for mesopores, respectively.20
The subtracting pore effect (SPE) analysis of the as-plots
provides the total surface area (Atotal) from the slope of the
narrow linear region around as ¼ 0.5, the external surface area
(Aext) from the slope of the linear region between as ¼ 0.7 and
1.1, using eqn (1). The total micropore volume (Vmicro) is also
obtained from the intercept of the latter straight line. Further,
the average micropore width (wa) can be evaluated from the
Atotal, Aext, and Vmicro according to eqn (2), assuming that the
pore shape is slit-like.
Experimental
General method and instrumentations
Synthetic manipulations shown in Scheme 1 were performed
under an argon atmosphere using Schlenk glassware. The disk-
pellet sample (f ¼ 13 mm, ca. 50 mg) was prepared under a pres-
sure of 3 MPa using a press unit (Hitachi). The pellet sample on
a quartz boat was carbonised in a quartz tube by heating from
room temperature to 900 ꢀC at a rate of 5 ꢀC minꢁ1 under flowing
argon with a furnace EKRO-12 K (Isuzu), then allowed to cool
to room temperature and stored in a desiccator.
Stotal ¼ 2.14 ꢂ (slope) [m2 gꢁ1
]
(1)
(2)
IR and NMR spectra were recorded on a JASCO FT/IR 550
spectrophotometer and a JEOL FT-NMR (270 MHz) spectrom-
eter, respectively. Elemental analyses were carried out with a
Perkin–Elmer type 2400. Inductively coupled plasma (ICP)
atomic emission spectroscopy were done on an ICAP 757
(Jarrell–Ash) plasma spectrometer. Thermal analyses were car-
ried out by an Extra 6000 TG/DTA (Seiko) thermogravimetric
analyser at a rate of 10 ꢀC minꢁ1 except for hexakis(phenylethy-
wa ¼ 2Vmicro/(Atotal ꢁ Aext) [nm]
Materials
Triethylamine and THF were distilled after drying with CaH2 or
sodium, respectively, under an argon atmosphere. The other
solvents and reagents commercially available were used without
further purification. Hexakis(phenylethynyl)benzene,21 hexakis
(4-cyanophenyl)benzene,19 1,3,5-triethynylbenzene,22 and 4-(tert-
butyldimethylsilyloxy)iodobenzene23 were prepared according to
the established procedure. 1,3,5-Tris(4-cyanophenylethynyl)
nyl)benzene (from 210 to 370 ꢀC, 2 C minꢁ1) under an argon
ꢀ
atmosphere. X-Ray diffraction (XRD) patterns were performed
˚
using a Rint 2100 (Rigaku) with Cu Ka (1.6418 A) radiation.
24
25
benzene (1),
1,3,5-tris(4-pyridylethynyl)benzene (2),
1,3,5-
tris[4-(ethoxycarbonyl)phenylethynyl]benzene (6), 1,3,5-tris(4-
carboxyphenylethynyl)benzene (H37)26 are known literature
compounds.
Synthesis
1,3,5-Tris(5-pyrimidinylethynyl)benzene (3). To a mixture of
CuI (49.5 mg, 0.260 mmol), Pd(PPh3)4 (300 mg, 0.260 mmol),
1,3,5-triethynylbenzene (1.30 g, 8.66 mmol), 5-bromopyrimidine
(6.19 g, 39.0 mmol) were added Et3N (15 mL) and THF (70 mL),
ꢀ
ꢀ
and the mixture was stirred at 55 C for 12 h and then at 70 C
for 14 h. The reaction mixture was concentrated and extracted
with CHCl3. The organic layer was washed with H2O and brine,
and dried over Na2SO4. Reprecipitation from CHCl3–EtOAc–
1
hexane gave 3 (3.13 g, 94.0% yield) as a white solid. H-NMR
(CDCl3) d 9.17 (s, 3 H), 8.86 (s, 6 H), 7.75 ppm (s, 3 H);
13C-NMR (CDCl3) d 158.6, 157.1, 135.0, 123.1, 119.1, 93.7,
84.2 ppm; IR (KBr) n 3033 (m), 2220 (w), 1542 (s), 1426 (s),
1406 (s), 877 (m), 631 (m) cmꢁ1
;
Anal. Calcd for
C24H12N6$0.2EtOAc: C, 74.09; H, 3.41; N, 20.90. Found: C,
74.01; H, 3.59; N, 20.74.
1,3,5-Tris[4-(tert-butyldimethylsilyloxy)phenylethynyl]benzene
(4). A mixture of CuI (23.8 mg, 0.125 mmol), Pd(PPh3)4 (144 mg,
0.125 mmol), 1,3,5-triethynylbenzene (625 mg, 4.16 mmol),
4-(tert-butyldimethylsilyloxy)iodobenzene (5.97 g, 18.ꢀ8 mmol),
and THF (40 mL) in Et3N (10 mL) was stirred at 45 C for 18
h. Hexane was added to the reaction mixture, and the separated
solid was filtered out. The filtrate was subjected to column
Scheme 1 Syntheses of functionalised 1,3,5-tris(substituted-phenylethy-
nyl)benzenes.
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chromatography on silica gel eluted with hexane–CH2Cl2 (4 : 1)
to give 4 (2.75 g, 85.9% yield) as a light yellow solid. 1.89 g of the
overloaded iodide was recovered. 1H-NMR (CDCl3) d 7.58 (s, 3
H), 7.42 (d, J ¼ 8.7 Hz, 6 H), 6.83 (d, J ¼ 8.7 Hz, 6 H), 1.00 (s, 27
H), 0.23 ppm (s, 18 H); 13C-NMR (CDCl3) d 156.1, 133.4, 133.0,
124.1, 120.2, 115.5, 90.4, 86.9, 25.7, 18.3, ꢁ4.2 ppm; IR (KBr): n
2929 (m), 2211 (w), 1604 (m), 1579 (m), 1507 (s), 1256 (s), 912 (s),
837 (s), 781 (m), 680 (w) cmꢁ1; Anal. Calcd for C48H60O3Si3: C,
74.95; H, 7.86; N, 0.00. Found: C, 74.94; H, 7.55; N, 0.00.
H), 7.50 (d, J ¼ 8.4 Hz, 6 H), 7.37 ppm (d, J ¼ 8.4 Hz, 6 H);
13C-NMR (CDCl3) d 134.0, 133.0, 131.6, 123.7, 122.9, 121.5,
89.6, 88.7 ppm; IR (KBr) n 2955 (w), 2922 (w), 2857 (w), 2210
(w), 1589 (s), 1486 (s), 1070 (s), 1011 (s), 875 (m), 820 (s), 752
(m) cmꢁ1; Anal. Calcd for C30H15Br3: C, 58.57; H, 2.46; N,
0.00; Br, 38.97. Found: C, 58.27; H, 2.78; N, 0.00; Br, 38.24.
1,3,5-Tris[4-(dihydroxyboryl)phenylethynyl]benzene (9). To
a solution of 8 (200 mg, 0.325 mmol) in THF (12 mL) was added
dropwise a hexane solution of n-BuLi (2.60 M, 750 mL, 1.95
ꢀ
ꢀ
1,3,5-Tris(4-hydroxyphenylethynyl)benzene (H35). A mixture
of 4 (4.08 g, 5.30 mmol), THF (25 mL), MeOH (100 mL), and
KOH (1.40 g, 21.2 mmol) in H2O (5 mL) was stirred at room
temperature for 12 h. The reaction mixture was diluted with
H2O, and the organic solvents were evaporated. The aqueous
residue was acidified with 0.5 M HCl and extracted with EtOAc.
The organic layer was washed with H2O and brine and dried over
Na2SO4 to give 5 (2.26 g, 99.9% yield) as a light yellow solid.
1H-NMR (DMSO-d6) d 10.05 (s, 3 H), 7.61 (s, 3 H), 7.46 (d, J ¼
8.6 Hz, 6 H), 6.87 ppm (d, J ¼ 8.6 Hz, 6 H); 13C-NMR (DMSO-
d6) d 158.2, 133.1, 132.4, 124.0, 115.7, 111.8, 91.4, 85.7 ppm; IR
(KBr) n 3336 (m), 2209 (w), 1606 (m), 1578 (s), 1509 (s), 1262 (m),
1220 (m), 1170 (m), 830 (s), 680 (w) cmꢁ1; Anal. Calcd for
C30H18O3$0.2 H2O: C, 83.78; H, 4.31; N, 0.00. Found: C,
83.59; H, 4.46; N, 0.14.
mmol) at ꢁ78 C. After stirring at ꢁ78 C for 1 h and then at
ꢀ
ꢁ20 C for 1.5 h, to the resulting solution was added B(OMe)3
(435 mL, 3.90 mmol) at ꢁ78 ꢀC. The resulting mixture was
warmed to room temperature for overnight with stirring, and
then hydrolysed with 2 M HCl at 0 ꢀC. After evaporation of
THF, the residue was extracted with EtOAc. The organic layer
was washed with 2 M HCl, H2O, and brine, and dried over
Na2SO4. After evaporation of EtOAc, the residue was triturated
with CH2Cl2 and filtered. The solid was recrystallised from ben-
zene–CHCl3–MeOH to give 9 (125 mg, 75.5% yield) as a light
1
yellow solid. H-NMR (DMSO-d6) d 8.20 (s, 6 H), 7.84 (d, J ¼
8.1 Hz, 6 H), 7.76 (s, 3 H), 7.55 ppm (d, J ¼ 8.1 Hz, 6 H);
13C-NMR (DMSO-d6) d 135.2, 134.2, 133.7, 130.3, 123.6,
123.0, 91.1, 88.1 ppm; IR (KBr) n 3400 (s), 3075 (w), 2208 (w),
1604 (s), 1341 (s), 1016 (w), 834 (w), 749 (w) cmꢁ1; Anal. Calcd
for C30H21B3O6: C, 70.66; H, 4.15; N, 0.00. Found: C, 70.62;
H, 4.45; N, 0.00.
1,3,5-Tris(4-hydroxyphenylethynyl)benzene tripotassium salt
(K35). A mixture of H35 (206 mg, 0.483 mmol) and CH3OK
(102 mg, 1.45 mmol) in MeOH 8 mL was concentrated to give
Preparation of 7c (7$2Cu$K$2CH3CO2$MeOH). To a solution
of K37 (120 mg, 0.192 mmol) in MeOH (40 mL) was added drop-
wise a solution of Cu(CH3COO)2 (105 mg, 0.576 mmol) in
MeOH (25 mL) at refluxing temperature for 2 h and then slowly
cooled to room temperature. A blue precipitate was collected by
filtration and evacuated at 80 ꢀC for 12 h (135 mg, 85.5% yield).
This compound was insoluble in common organic solvents such
as DMAc and DMSO. IR (KBr) n 3389 (m), 1695 (w), 1604 (s),
1578 (s), 1533 (s), 1411 (s), 1177 (w), 1098 (w), 1016 (w), 860 (m),
779 (m) cmꢁ1; Anal. Calcd for (C38H25Cu2O11K)n: C, 55.40; H,
3.06; N, 0.00; Cu, 15.4. Found: C, 55.29; H, 3.25; N, 0.20;
Cu, 16.3. Metal concentration analyses were performed by ICP
analysis for all coordination polymers.
1
K35 (256 mg, 98.1% yield) as a yellow–brown solid. H-NMR
(DMSO-d6) d 7.11 (s, 3 H), 6.98 (d, J ¼ 8.6 Hz, 6 H), 6.06
ppm (d, J ¼ 8.6 Hz, 6 H); 13C-NMR (DMSO-d6) d 173.4,
133.0, 128.3, 125.5, 119.3, 97.3, 96.0, 84.8 ppm; IR (KBr): n
3434 (s), 2200 (m), 1596 (m), 1575 (s), 1498 (s), 1296 (s), 1167
(m), 841 (m) cmꢁ1; Anal. Calcd for C30H15O3K3$6.5 H2O: C,
54.77; H, 4.29; N, 0.00. Found: C, 55.04; H, 4.33; N, 0.02.
1,3,5-Tris(4-carboxyphenylethynyl)benzene tripotassium salt
(K37). A mixture of H37 (800 mg, 1.57 mmol) and CH3OK
(330 mg, 4.70 mmol) in MeOH 120 mL was concentrated to
give K37 (980 mg, 100% yield) as a white solid. 1H-NMR
(CD3OD) d 7.96 (d, J ¼ 8.1 Hz, 6 H), 7.68 (s, 3 H), 7.55 ppm
(d, J ¼ 8.1 Hz, 6 H); 13C-NMR (CD3OD) d 174.1, 139.3,
134.8, 132.0, 130.2, 125.48, 125.46, 91.5, 89.3 ppm; IR (KBr): n
3372 (s), 2214 (w), 1603 (s), 1584 (s), 1540 (s), 1385 (s), 1279
(w), 842 (w), 786 (m) cmꢁ1; Anal. Calcd for C33H15O6K3$3.83
H2O: C, 57.13; H, 3.29; N, 0.00. Found: C, 57.19; H, 3.46; N,
0.31.
Preparation of 7d (7$1.5Cu$0.25 KCl$1.5 MeOH). To a solu-
tion of K37 (120 mg, 0.192 mmol) in MeOH (40 mL) was added
dropwise a solution of CuCl2$2 H2O (98.2 mg, 0.576 mmol) in
MeOH (5 mL) at refluxing temperature for 2 h and then slowly
cooled to room temperature. A blue precipitate was collected by
filtration and evacuated at 80 ꢀC for 12 h (114 mg, 88.6% yield).
This compound is insoluble in most of solvents. IR (KBr) n 3045
(m), 1695 (m), 1604 (s), 1580 (s), 1535 (m), 1411 (s), 1281 (w),
1176 (w), 1099 (w), 1017 (w), 859 (w), 779 (m) cmꢁ1; Anal. Calcd
for (C138H84ClCu6O30K)n: C, 61.89; H, 3.16; N, 0.00; Cl, 1.32;
Cu, 14.24. Found: C, 61.88; H, 3.42; N, 0.50; Cl, 2.82; Cu, 14.3.
1,3,5-Tris(4-bromophenylethynyl)benzene (8). A mixture of
CuI (56.2 mg, 0.295 mmol), Pd(PPh3)4 (340 mg, 0.295 mmol),
1,3,5-triethynylbenzene (1.47 g, 9.82 mmol), 1-bromo-4-iodo-
benzene (25.0 g, 88.4 mmol), and THF (75 mL) in Et3N (22
mL) was stirred at room temperature for 40 h. The reaction mix-
ture was concentrated and extracted with CH2Cl2. The organic
layer was washed with H2O and brine, and subjected to column
chromatography on silica gel eluted with hexane–CH2Cl2 (7 : 1)
to give 8 (5.92 g, 98.1% yield) as a white solid. 16.3 g of the over-
Preparation of 7e (7$1.5Cu$6 H2O). A solution of K37 (70 mg,
0.112 mmol) in DMF–MeOH (7 : 1 v/v, 80 mL) was placed in
a long Pyrex tube with a 15 mm internal diameter. A solution
of Cu(NO3)2$3 H2O (54.1 mg, 0.224 mmol) in DMF–MeOH
(1 : 1 v/v, 4 mL) was carefully layered on the top of the K37
1
loaded iodide was recovered. H-NMR (CDCl3) d ¼ 7.63 (s, 3
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solution, followed by layering 1 mL of the mixed solvent DMF–
MeOH (2 : 1 v/v) of intermediate density. The mixture was
allowed to stand for 10 days at room temperature. The mixture
was heated to a refluxing temperature for 5 h and then slowly
cooled to room temperature. A blue precipitate was collected
by filtration and evacuated at 80 ꢀC for 12 h (76.0 mg, 95.4%
yield). This compound was insoluble in water and common
organic solvents. IR (KBr) n 3421 (s), 1604 (s), 1581 (s), 1537
(m), 1409 (s), 1177 (w), 1099 (w), 1017 (w), 861 (w), 780 (m)
cmꢁ1; Anal. Calcd for (C66H54Cu3O24)n: C, 55.76; H, 3.83; N,
0.00; Cu, 13.41. Found: C, 56.02; H, 3.84; N, 0.32; Cu, 14.9.
Synthesis of functionalised 1,3,5-tris(substituted-
phenylethynyl)benzenes
The Sonogashira coupling reaction is a powerful method for the
synthesis of differentially functionalised 1,3,5-tris(phenylethy-
nyl)benzenes because the reaction is tolerant of functional
groups such as cyano and pyridyl groups.29 1,3,5-Triethynylben-
zene was synthesised in 96% total yield by the reactions of 1,3,5-
tribromobenzene with trimethylsilylacetylene and subsequent
desilylation. The Sonogashira coupling reaction of 1,3,5-triethy-
nylbenzene with a functionalised phenylhalide in Et3N–THF
in the presence of Pd(PPh3)4 and CuI was carried out to synthe-
sise 1,3,5-tris(substituted-phenylethynyl)benzenes (Scheme 1).
4-Cyanophenyl (1), 4-pyridyl (2), 5-pyrimidinyl (3), 4-(tert-butyl-
dimethylsilyloxy)phenyl (4), 4-(ethoxycarbonyl)phenyl (6), and
4-bromophenyl (8) groups were successfully introduced to
1,3,5-triethynylbenzene in high yields. The deprotection of the
silyl ether of 4 and the ethyl ester of 6 with KOH gave 1,3,5-
tris(4-hydroxyphenylethynyl)benzene (H35) and 1,3,5-tris(4-
carboxyphenylethynyl)benzene (H37), respectively. Successively,
K35 and K37 were produced quantitatively by treatment with
potassium methoxide in methanol. The lithiation of 8 with
n-BuLi followed by the reaction with B(OMe)3 gave 1,3,5-
tris[4-(dihydroxyboryl)phenylethynyl] benzene (9). The linkers,
1, 2, 3, K35, H37, K37, and 9, synthesised in this section are
subjected to reaction with metal ions to form 2- or 3-dimensional
coordination polymers.
Results and discussion
Thermal analysis of organic linkers
To select a thermally stable organic linker, thermogravimetric
analysis (TGA) and differential thermal analysis (DTA) of
multicoordinate frameworks of hexakis(phenylethynyl)benzene,
hexaphenylbenzene, and 1,3,5-tris(phenylethynyl)benzene were
performed and the results are shown in Fig. 1. These compounds
are highly symmetrical (D6h or D3h), which can support radial
coordination-bonding sites to form a predictable network with
cavity based on the multipoint cooperative interactions, and
can make an orthogonal arrangement of the interactive
benzene-centered moieties in the network.27 Judging from their
TGA and DTA traces, hexakis(phenylethynyl)benzene is not
appropriate for an organic linker because the Bergman-type
cyclisation28 occurs with an intensive exothermic peak at about
295 ꢀCꢀ, although it showed the high carbonised residual ratio
at 900 C (78%). Therefore, the multiple bonds of the molecule
must have distance from each other to prevent the Bergman-
type cyclisations in the pyrolytic process. In the case of the
hexakis(4-cyanophenyl)benzene, the exothermic peak was
scarcely observed, but it is also inadequate for an organic linker
because thermal cracking of the phenyl–phenyl bonds occurred
at around 550 ꢀC, which was reflected in its relatively low
Carbonisation of organic linkers
To investigate the carbonisation behavior of the organic linkers
themselves, the compounds were carbonised by heating from
room temperature to 900 ꢀC at a rate of 5 ꢀC minꢁ1 under flowing
argon in a furnace, and then the surface analysis of the carbon-
ised samples was carried out by N2 adsorption method. The N2
adsorption isotherms and the surface-analysis data of the
carbonised 1,3,5-tris(substituted-phenylethynyl)benzenes (C-1–
C-9) are summarised in Fig. 2 and Table 1. All isotherms of
C-1–C-9 show characteristic of type I according to IUPAC clas-
sification.30 The surface-analysis data suggests that C-1–C-9 are
microporous materials with average pore widths (wa) of ca. 0.7
nm estimated from the as plot by SPE method31 and have little
ꢀ
carbonisation yield (49% at 900 C). From this standpoint, the
star-shaped 1,3,5-tris(4-cyanophenylethynyl)benzene could have
an optimal structure. As a consequence, it showed the mild
pyrolysis process similar to hexakis(4-cyanophenyl)benzene in
addition to the high carbonisation yield equivalent to hexakis
(phenylethynyl)benzene.
Fig. 1 TGA–DTA curves of hexakis(4-cyanophenyl)benzene, hexaki-
Fig. 2 Adsorption isotherms of nitrogen at 77 K for the carbonised
C-1,3,5-tris(substituted-phenylethynyl)benzenes (1–9).
s(phenylethynyl)benzene, and 1,3,5-tris(4-cyanophenylethynyl)benzene.
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Table 1 Surface parameters of C-1,3,5-tris(substituted-phenylethynyl)benzenesa
b
c
d
e
Vmeso/Vtotal (%)
f
wa /nm
Sample
Yield (%)
ABET /m2 gꢁ1
Vtotal /mL gꢁ1
Vmicro /mL gꢁ1
1
2
3
H35
K35
H37
K37
9
82
84
70
495
113
459
840
1040
67
0.23
0.068
0.20
0.43
0.59
0.054
0.30
0.13
0.22
1.2
0.9
0
7.0
6.5
38
0.72
0.85
0.74
0.64
0.78
0.68
0.72
0.71
0.040
0.19
0.37
59g
56g
53g
53g
77
0.45
0.025
0.23
0.048
534
150
8.3
37
a
b
All the samples were carbonised by heating from rt to 900 ꢀC at a heating rate of 5 ꢀC minꢁ1
.
The surface area was estimated by the Brunauer–
d
Emmett–Teller (BET) method. c The total pore volume was estimated from the maximum adsorbed amount of N2 in the adsorption isotherm. The
micropore volume was estimated by the as method. e The mesopore ratio was estimated by the Dollimore–Heal (DH) method. f The average micropore
g
width was determined by the as plot and the SPE analysis. Annealed at 900 ꢀC for 3 h.
meso- and macropores. The carbonisation yields of 1–9 were
moderately high in the range of 53–83%. Brunauer–Emmett–
Teller (BET) surface areas (ABET) of C-1–C-9 were scattering
between 67 and 1040 m2 gꢁ1. Since polymers have generally
been used as starting substances of carbon materials, it is inter-
esting that such small molecules show the high carbonisation
yields and lead to the carbonaceous materials with high surface
areas. Annealing at 900 ꢀC is effective to develop microporosity
in particular in the cases of C-5 and C-7 but is accompanied with
a considerable mass loss, which could be related to the degree
of conversion from the amorphous carbon network material to
the carbonised material that consists of the basic structure unit
of carbonaceous layers.16a C-H37, C-2 and C-9 showed low
surface areas, which is probably due to an intermolecular filling
of the carbon precursors with softening properties during the
pyrolysis. On the contrary, the potassium salts, C-K35 and
C-K37, showed relatively high surface areas, because K35 and
K37 were rigid solids with ionic interaction and, successively, K
sublimed at around 760 ꢀC after hardening of the carbonaceous
precursors.
complexes are satisfactory for mass production of the coordina-
tion polymers.
In order to trace variation of the porosity through the pyroly-
sis, surface analysis of the coordination polymers 2a–7e,
as-heat-treated carbonised samples, and acid-washed carbonised
C-2a–C-7e was carried out by N2 adsorption method. The yields
in each process and the extracted data on the surface analysis are
summarised in the right three quarters of Table 2. Since the
carbonised materials listed in Table 2 are basically microporous
solids with mesoporosity in some degree, each adsorption volume
such as Vmicro and Vmeso is roughly forecasted from the ABET and
the mesopore ratio (Vmeso/Vtotal). Most of the coordination poly-
mers were low porous materials showing small ABET below 50 m2
gꢁ1, which suggests that it is hard to keep the network structure
on the occasion of solvate removal.14e,34 Generally, MOFs con-
structed from a larger ligand tend to be less stable.13c Hence,
utilisation of a strong metal–linker bond is desirable to form a
variety of MOFs with a stable porous structure.35 Actually, 7c–7e
prepared from the conjugated carboxylate linker K37 with cop-
per ions had considerable permanent total pore volumes (Vtotal),
0.261, 0.469, and 0.266 mL gꢁ1, respectively, which suggests that
a large linker 7, 1,3,5-tris[4-carboxyphenylethynyl]benzene, is
notably adapted to construct stable MOFs.
Synthesis and carbonisation of coordination polymers
In order to narrow down the specific conditions for obtaining
permanent porous coordination polymers by solution reaction,
in the beginning a variety of coordination polymers were
prepared by a combinatorial reaction of the multicoordinated
1,3,5-tris(phenylethynyl)benzene derivatives with common
transition metal salts such as Cu(CH3CO2)2, CuCl2$2 H2O,
Cu(NO3)2$3 H2O, Ni(CH3CO2)2$4 H2O, Ni(NO3)2$6 H2O,
and AgOTf. All metal–organic coordination polymers were
prepared from the slow intermixing method32 or rapid mixing
method described in the experimental section. Chemically inert
solvents such as 1,4-dioxane and THF were preferentially used
in the preparations. All the coordination polymers were dried
After the carbonisation of 2a–7e, the as-heat-treated materials
were washed with 8 M HNO3 to remove the isolated copper that
was sintered on the surface during pyrolysis, affording the acid-
washed carbons, C-2a–C-7e. Removal of the sintering metal on
the surface by the post-treatment was confirmed by the disap-
pearance of the XRD diffraction peaks of the metal. The differ-
ence between the ABET of the acid-washed carbonised materials
and those simply estimated from the ABET of the as-heated sam-
ples and the yields of acid-washing (shown in Table 2) suggests
the presence of a considerable amount of metal included in the
materials.
As representative examples, N2 adsorption isotherms of the
low-porous coordination polymers 2c, 3a, microporous MOF
7e, and their acid-washed carbonised samples C-2c, C-3a, C-7e
are depicted in Fig. 3. The ABET of C-2c, C-3a, and C-7e (780,
910, and 747 m2 gꢁ1) were considerably larger than those of 2c,
3a, and 7e (12, 2, and 333 m2 gꢁ1). The increase of ABET is con-
sidered to be reasonable, because 2, 3, and K37 (the linkers of 2c,
3a, and 7e, respectively) were pyrolysed to a microporous carbon
as shown in Fig. 2 and Table 1. Nevertheless, it should be noted
ꢀ
in vacuum at 80 C for 12 hours, and their empirical formulas
of the complexes were estimated from the results of the elemen-
tal, ICP, and thermogravimetric analyses. The preparation
conditions and the synthetic yields of 2a–7e are listed in the
left quarter of Table 2.33 The majority of the coordination
polymers were obtained in good yields over 80%. Considering
the total yields (ca. 60%) from the starting 1,3,5-tribromoben-
zene, it could be said that the synthetic procedure of these
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Fig. 3 N2 adsorption isotherms of the coordination polymers (low
porous 2c, 3a, microporous 7e), and their carbonised (C-) samples.
that the ABET of all the acid-washed carbonised samples C-2a–C-
7e (Table 2) were larger than those of corresponding carbonised
linkers (Table 1). Hence, constructing a coordination network in
advance must be available to prepare a porous carbon with
a high porosity, even if the coordination polymer is low-porous
or non-porous. On the other hand, the mesopore volumes
(Vmeso) of 2c, 3a, and 7e (0.036, 0.012, and 0.078 mL gꢁ1) were
almost equivalent to the Vmeso of their carbonised samples,
C-2c, C-3a, and C-7e (0.14, 0.055, and 0.15 mL gꢁ1). In the P/P0
range of 0.4–1, the shape of isotherms of all samples before and
after carbonisation is similar to each other, which indicates that
the coordination polymers retain the meso- and macroporosity
through the pyrolysis. Origin of the mesoporosity must be inter-
spaces of aggregated nanoparticles as well as mesoporous poly-
mer gels and carbonised ones.36
In this work, three mesoporous coordination polymers, 7b, 7c,
and 7d, were obtained by the solution reaction of 7 with copper
salts. The N2 adsorption isotherms of 7b, 7c, and 7d are shown in
Fig. 4 together with the acid-washed carbonised samples C-7b,
C-7c and C-7d. In analogy with the low-porous coordination
polymers, increases of Vmicro and retention of the mesoporous
features were observed through the carbonisation with no excep-
tions. Exhibiting the steep N2 uptakes at a high relative pressure
(P/P0) and the hysteresis loop extending from 0.8 to 1 in P/P0
provides an evidence of existence of large mesopores. As
Fig. 4 N2 adsorption–desorption isotherms of the mesoporous coordi-
nation polymers and their carbonised samples.
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a consequence, atypical porous carbons that have both of the
mesoporosity of the coordination polymers and the microporos-
ity of carbonised organic linkers were created. The Vtotal of C-7b,
C-7c and C-7d came to 0.47, 0.98, 0.74 mL gꢁ1, respectively.
These values are comparable to the porosity of carbon gels,36
which are representative advanced mosoporous carbon species.
begins to increase when the temperature is above 500 ꢀC,37 which
is different from the tendency observed in 7c. TG/DTA of 7c
(ESI†, Fig. S4) showed an exothermal peak accompanied by
a mass loss at around 300 ꢀC, which is due to decomposition
or elimination of the counter acetate anion. Therefore, the unex-
ꢀ
pected increase of the ABET of 7c when heat-treated at 350 C
(7c(350)) could be due to desorption of the acetate anion from
the original building construction. On the contrary, the Vmeso
of 7c (ca. 0.2 mL gꢁ1) remained unchanged during the heat treat-
ment from the starting temperature to 900 ꢀC. These results
mean that the mesoporosity of the mesoporous coordination
polymer was kept during the process of transforming to a micro-
porous carbon by heat-treatment.
Pyrolytic carbonisation behavior of a mesoporous coordination
polymer
In order to confirm that the mesoporosity of the carbonised
coordination polymers C-7b–d were derived from those of pre-
carbonised ones, a step-wise pyrolysis of 7c that showed type
IV adsorption–desorption isotherm was performed at 350 ꢀC
The generation process of the microporosity and the thermal
behavior of the inclusion metal are traced by XRD analysis.
The XRD patterns of the coordination polymer 7c, the heat-
treated samples at different temperatures, and the acid-washed
carbonised sample C-7c are shown in Fig. 6. The coordination
polymer 7c has diffraction peaks at 2q ¼ 4.5 and 7.9ꢀ, which
suggests that 7c has a periodic structure with 2.0 and 1.0 nm
distances as well as the microporous carbon synthesised by a
template method.38 The original diffraction peaks were com-
pletely lost under the heat treatment by 350 ꢀC. At the same
time, peaks of isolated copper(I) indexing Cu2O newly appeared
in 7c(350). The periodic microporous structure of 7c could be
disarranged by the increase of microporosity with the elimina-
tion of acetate during the pyrolysis. In the case of 7c(600),
diffraction peaks of copper(0) appeared instead of the peaks of
Cu2O, which was attributed to reduction by carbon. The Cu
ꢀ
ꢀ
(7c(350)), 600 C (7c(600)), and 900 C (7c(900)). Their surface
analyses by N2 adsorption method, XRD, and SEM were carried
out in this section.
The N2 adsorption–desorption isotherms are shown in Fig. 5,
and the surface parameters are listed in Table 3. The specific sur-
face areas monotonically increased in sequence of 7c < 7c(350) <
7c(600) < 7c(900) < 7c(900, 3 h) as raising the heat-treatment
temperature. In the case of pyrolysis of conjugated polymers
that have carbon–carbon triple bond, a specific surface area
ꢀ
peaks became sharper after the heat treatment at 900 C, which
indicates that the particle size of the isolated metal grows larger.
In the case of C-7c, the peaks of isolated copper observed in
7c(900, 3 h) completely disappeared by leaching with nitric
acid, and the broad peaks of carbon became apparent. Dimen-
sions of the basic structure unit of carbon perpendicular and
parallel to the basal plane and graphitic interlayer spacing were
estimated from the (002) and (110) diffraction peaks by means of
the Bragg formula and Scherrer equation39 to be 1.6 nm (Lc), 2.5
nm (La), and 0.40 nm (di), respectively. The large di and small L
values for C-7c are comparable to the hard carbon derived from
the conjugated polymers16,34 and are different from graphitic
Fig. 5 N2 adsorption–desorption isotherms of the mesoporous coordi-
nation polymer 7e and the heat-treated samples at 350, 600, 900 ꢀC,
and 900 ꢀC for 3 h.
Table 3 Surface parameters of 7c, heat-treated samples, and acid-
washed C-7ca
Temperature
(ꢀC)
ABET
/
Vtotal
/
Vmicro
/
Vmeso
/
wa/
nm
m2 gꢁ1
mL gꢁ1
mL gꢁ1
mL gꢁ1
900b
900c
900
600
350
80d
1250
724
637
419
320
105
0.98
0.59
0.51
0.38
0.41
0.26
0.22
0.27
0.23
0.16
0.11
0.057
0.30
0.22
0.15
0.19
0.20
0.19
0.74
0.73
0.61
0.65
1.0
NAe
a
All the samples were treated at a heating rate of 5 ꢀC minꢁ1. The Vtotal
was estimated from the maximum adsorbed volume, and the Vmeso was
estimated by the DH method. The Vmicro and wa were determined by the
b
as plots. C-7c, the acid-washed carbon obtained from 7c(900, 3 h).
c
d
e
Annealed at 900 ꢀC for 3 h. 7c, as-prepared complex. NA ¼ not
Fig. 6 XRD patterns of 7c, heat-treated samples, and the acid-washed
C-7c.
applicable.
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Fig. 7 SEM images of 7c, the heat-treated samples, and the acid-washed C-7c.
materials such as carbon black.40 Additionally, the acid-washed
C-7c must include a highly dispersed copper, since the presence
of copper (0.63 wt%) in C-7c was confirmed by ICP analysis
despite XRD peaks of Cu were not observed.
of the coordination polymers synthesised by the solution reac-
tion method are low porous materials, those synthesised from
tricarboxy K37 as the linker exceptionally show either micropo-
rosity or mesoporosity. Nevertheless, the coordination polymers
can be transformed to microporous carbons by pyrolysis without
exception even if the pre-carbon materials are low-porous or
nonporous, because the organic linkers 1–9 themselves can be
converted to microporous carbons by pyrolysis. Consequently,
the carbonised coordination polymers have higher porosity
than the carbonised organic linkers. On the other hand, it was
found that mesoporosity of the coordination polymers are
hardly changed by the pyrolytic processes. Thus, it is realised
that mesoporous coordination polymers can be converted to
mesoporous carbons with microporosity.
Finally, the change of the surface morphology is checked by
SEM. Fig. 7 shows the SEM images of the coordination polymer
7c, the heat-treated samples at different temperatures, and the
acid-washed C-7c. The surface morphology of 7c suggests pres-
ence of aggregates consisted of particles having an average diam-
eter of ca. 100 nm. The mesopores that should be interspaces
between the aggregated particles are observed as countless black
dots with a hole diameter smaller than 50 nm. The nano-particles
seem to be gradually agglomerated as raising the heat-treated
temperatures, and the size of the agglomerations observed in
7c(600) came to the range of micron scales. The agglomeration
is due to leaching out of the metal salt and following metalation
on the surface, and the black dots can be observed unchanged.
After the agglomeration, a thin layer metal covered the surface
of 7c(900), and successively the sintering metal grew to about
250 nm-sized particles on the surface of 7c(900, 3 h). The surface
of C-7c renovated by acid-washing using nitric acid showed
a morphology similar to those observed in 7c and 7c(350). This
result supports that the mesoporosity of 7c is hardly changed
by the pyrolysis process, which has already been observed in
the N2 adsorption analysis (Fig. 5 and Table 3).
Acknowledgements
This work was supported by a JSPS Research Fellowships (No.
18.3824) and Ogasawara Foundation. We thank to the Research
Facility Center for Science and Technology, Chemical Analysis
Division, University of Tsukuba, for NMR, ICP, elemental,
thermal, and surface-analysis data.
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