Microporous materials of metal carboxylates

Article abstract of DOI:10.1006/jssc.2000.8675

Copper(II) terephthalate absorbs a large amount of gases such as N₂, Ar, O₂, and Xe. The maximum amounts of absorption of gases were 1.8, 1.9, 2.2, and 0.9 mole per mole of the copper(II) salt for N₂, Ar, O₂, and Xe, respectively, indicating that the gases were not adsorbed on the surface but occluded within the solid. Other microporous copper(II) dicarboxylates are also reviewed. The porous structure of copper(II) terephthalate, in which the gas is occluded, is deduced from the temperature dependence of magnetic susceptibilities and the linear structure of terephthalate. Microporous molybdenum(II) and ruthenium(II, III) dicarboxylates are discussed. We describe that rhodium(II) monocarboxylate bridged by pyrazine form stable micropores by self-assembly of infinite linear chain complexes. (C) 2000 Academic Press.

Full text of DOI:10.1006/jssc.2000.8675

Journal of Solid State Chemistry 152, 120}129 (2000)
doi:10.1006/jssc.2000.8675, available online at http://www.idealibrary.com on
Microporous Materials of Metal Carboxylates
Wasuke Mori*ꢀꢁ and Satoshi Takamizawa*ꢀ-
*Department of Chemistry, Faculty of Science, Kanagawa University, Hiratsuka, Kanagawa 259-1293, Japan; and
-Department of Chemistry, Graduate School of Science, Osaka University, Toyonaka, Osaka 560-0043, Japan
of weight at low temperature (5). Figure 1 shows the Fara-
Copper(II) terephthalate absorbs a large amount of gases day magnetometer.
such as N2, Ar, O2, and Xe. The maximum amounts of absorp-
tion of gases were 1.8, 1.9, 2.2, and 0.9 mole per mole of the
copper(II) salt for N2, Ar, O2, and Xe, respectively, indicating
that the gases were not adsorbed on the surface but occluded
within the solid. Other microporous copper(II) dicarboxylates
are also reviewed. The porous structure of copper(II) tereph-
thalate, in which the gas is occluded, is deduced from the temper-
ature dependence of magnetic susceptibilities and the linear
structure of terephthalate. Microporous molybdenum(II) and
ruthenium(II, III) dicarboxylates are discussed. We describe
that rhodium(II) monocarboxylate bridged by pyrazine form
The temperature dependence of the magnetic susceptibili-
ties of copper(II) terephthalate, as shown in Fig. 2, obeyed
the Bleaney}Bowers equation (6) for S"1/2 Heisenberg
model of dimer structure observed in many copper(II) com-
plexes (7) forming the same dinuclear structure as copper(II)
acetate (8) . It is reasonable to assume that the copper(II)
terephthalate comprises the same con"guration as the cop-
per(II) acetate monohydrate, so that considering the linear
structure of terephthalate, two-dimensional lattice structure
is proposed for copper(II) terephthalate (9) (see Fig. 3). The
stable micropores by self-assembly of in5nite linear chain com- temperature dependence of the amount of absorbed gases
plexes. ( 2000 Academic Press
were gravimetrically measured by a Cahn 1000 electric
balance. The amounts of absorbed N , Ar, and O were
almost completely saturated at the temperature of liquid
nitrogen as shown in Fig. 4. The amount of adsorbed gas is
ꢂ
ꢂ
1. INTRODUCTION
enormous. For example, over 30 g (30 L) of O was ab-
sorbed in the 100 g of copper(II) terephthalate. The pore size
ꢂ
The mineral zeolite was reported by Cronstedt in 1756
and Damour in 1840 (1). In the 20th century, numerous
porous compounds (either natural or arti"cial) became rec-
ognized. Zeolite, porous silica, Cray, activated carbon, fab-
ric carbon, etc., are well known classical porous materials
(2). They are classi"ed by their components (organic and
inorganic), the degree of crystallization, pore-size, and func-
tions.
was calculated by the Horvath}Kawazoe equation (10) from
the absorption isotherm of copper(II) terephthalate at the
temperature of liquid argon. The shape of isotherm was
Type I according the IUPAC classi"cation (11). A very
narrow peak at ca. 6 A suggests that regular and stable
ultramicropores are constructed by copper(II) terephthalate
in a solid state (see Fig. 5).
In the middle of the 20th century, study of the magnetic
behavior of copper(II) acetate derivatives (3) which have the
well-known dimer structure (4), became quite popular. Mori,
one of the authors of this paper, prepared the copper(II)
terephthalate as microcrystals for investigating magnetic
interaction of interdimers. When we were measuring the
temperature dependencies of the magnetic susceptibility of
copper(II) terephthalate by a Faraday balance with a Cahn
electric balance under a nitrogen atmosphere, we found
In this paper we report on the syntheses and gas occlusion
properties of the microporous complexes of metal car-
boxylates.
On the isomorphous zinc(II) terephthalate, the research
has been made by Yaghi (12).
2. DESIGN STRATEGY TO SYNTHESIZE
MICROPOROUS COMPLEXES
incidentally that a large amount of N gas was occluded in
the copper(II) salt. It was con"rmed by anomalous increase
ꢂ
Because we found that copper(II) terephthalate occluded
large amounts of nitrogen gas, several transition-metal
dicarboxylates were synthesized as the "rst step in making
new porous materials. Polydentate ligands are suitable for
constructing stable covalent networks: the softness of the
ꢁTo whom correspondence should be addressed. E-mail: wmori@
info.kanagawa-u.ac.jp.
120
0022-4596/00 $35.00
Copyright ( 2000 by Academic Press
All rights of reproduction in any form reserved.

MICROPOROUS MATERIALS OF METAL CARBOXYLATES
121
FIG. 3. Molecular structure of copper(II) acetate monohydrate (a) and
the deduced two-dimensional structure of copper(II) terephthalate (2) and
its micropore (b).
FIG. 1. Magentic balance.
cally, and the nature of their multiple bonds has been
structure will be required. The use of molecular assembly is theoretically and spectroscopically investigated. They are
also favored to produce porous complexes in a solid state. reviewed by Cotton and Walton (18).
The general method to synthesizing a 3D porous complex is
Generally, a two- or three-dimensional network is built
drawn in Fig. 6. The intermolecular interactions in con- up of bridges of dicarboxylate or polycarboxylate ligands
structing the porous structure composed of lower dimen- (19). In the case of the linear dicarboxylate bridge, it is
sional building blocks. In this study, we selected dinuclear obvious that two-dimensional lattices are constructed, and
carboxylates as the building blocks.
in"nite linear micropores are created by stacking the two-
Dinuclear metal carboxylates, M (O CR) , represent an dimentional lattices as shown in Fig. 7b and 7c.
ꢂ
ꢂ
ꢄ
important class of transition metal complexes with respect
to the study of metal}metal interaction. The most represen-
tative series is the carboxylates for M"Crꢁꢅ, Moꢁꢄ, Wꢁꢆ,
Reꢁꢃ, Ruꢁꢇ. They are represented by the general formula
2.1. Metal Dicarboxylate
Copper(II) dicarboxylates. Copper(II) terephthalate (2)
M (O CR) L and have a lantern-like structure, as shown is the "rst transition-metal complex capable of adsorbing
ꢂ
ꢂ
ꢄ ꢂ
in Fig. 7a. Dinuclear complexes bearing metal}metal bonds gases reversibly. This complex has opened a new area of
have been synthesized and characterized crystallographi-
FIG. 2. Temperature dependence of magnetic susceptibilities of cop-
per(II) terephthalate (2). The solid line is the best-"t curve of the
Bleaney}Bowers equation with 2J"! 321 cm\ꢁ, g"2.20, and Na"
60;10\ꢃ emu mol\ꢁ.
FIG. 4. Isobar of copper(II) terephthalate (2) for N ()), Ar (᭹),
ꢂ
O
(᭝), and Xe (*) at 20 Torr.
ꢂ

122
MORI AND TAKAMIZAWA
FIG. 5. Pore diameter of copper(II) terephthalate (2).
complex chemistry and has become recognized as a princi-
pal complex in the construction of microporous complexes.
This copper(II) salt forms during the reaction of copper(II)
formate with terephthalic acid in methanol in the presence
of formic acid. After standing several weeks in a moisture-
free environment, the blue plates were precipitated (Scheme
1). Heating the crystals for 2 h at 1103C in a vacuum,
produced greenish blue microcrystals, which are capable of
absorbing gases. The greenish blue microcrystals show very
sharp powder X-ray di!raction, indicating that the adsor-
bent consists of a rigid crystalline material. The other cop-
FIG. 7. Lantern-like structure (a), two-dimensional lattice structure (b),
and porous structure (c).
Several copper(II) dicarboxylates (20) (fumarate (1), 2,6-
per(II) dicarboxylates were synthesized in a similar manner. naphthalenedicarboxylate (3), and trans-1,4-cyclohexane-
dicarboxylate (4)) capable of adsorbing gases such as N ,
ꢂ
O , Ar, and CH are obtained. Those complexes reversibly
ꢂ
ꢄ
adsorb gases without any structural damages. The amounts
of gas adsorbed and the pore sizes of the copper complexes
are summarized in Table 1. The pore sizes of copper(II)
dicarboxylates are controllable by the size and structure of
dicarboxylate ligands.
Molybdenum(II) dicarboxylates (21). Molybdenum(II)
dicarboxylates are successfully obtained by a ligand ex-
change reaction of acetate (22) in dry methanol media under
an argon atmosphere (Scheme 2). Molybdenum(II) dicar-
boxylates (5}9) are obtained as an insoluble powder. The
bands in the Raman and infrared spectra of 5}9 are sum-
FIG. 6. Three-dimensional lattice organized by the assembly of lower
dimensional building blocks.
SCHEME 1

MICROPOROUS MATERIALS OF METAL CARBOXYLATES
123
TABLE 1
Adsorbent Properties of Copper(II) Dicarboxylates 1+4
Amount of occluded N
(mol/mol of copper atoms)
Averaged pore diameter?
ꢂ
Copper(II) dicarboxylate
(A)
1
2
0.8
1.8
5.4
6.0
3
4
1.1
1.1
4.9
Not measured
?Horvath}Kawazoe method.
marized in Table 2. The strongest bands in the Raman are also very similar to that of copper(II) dicarboxylates,
spectra of 5}9 occur at 386}398 cm\ꢁ in the solid. These described in the previous section. This similarity of gas-
bands are assigned to l(Mo}Mo) by comparison with the adsorption behavior suggests that molybdenum(II) dicar-
acetate spectrum in the assignment of the strong band boxylates 5}9 have the isomorphous structure of copper(II)
406 cm\ꢁ to the A mode consisting predominantly of dicarboxylates.
ꢁ'
Mo}Mo stretching (23). The decrease of about several cm\ꢁ
Solid-state magic angle spinning (MAS) NMR probes the
in going from the acetate to 5}9 may indicate a slight local structure of the host lattice complexes and the environ-
weakening of the metal}metal bond. This phenomenon may ment of the guest molecules. The ꢁꢅC CP/MAS NMR in-
be due to interdinuclear overlap of MOs in the axial direc- vestigation of the complexes was conducted at a spinning
tion, but is not due to the e!ect of n-conjugation in the speed of 5 kHz at room temperature. The shifts in chemical
ligands, because the same decrease is observed in 9, which values observed for 5 through 9 are listed in Table 4. The
has no n-conjugation. Especially in the case of 8, axial ꢁꢅC CP/MAS NMR spectra of complex 6 with adsorbed
coordination of the pyridine nitrogen in the ligand to mol- n-butane (a) and the dried complex (b) are shown in Fig. 8.
ybdenum in the neighboring dinuclear moiety may occur. In Two additional peaks of spectrum (a) corresponding to the
the infrared spectra the strongest two bands were observed methyl and methylene groups of n-butane appeared at 24.8
in the C}O stretching region; they were assigned to the and 13.4 ppm, respectively. The line width of the n-butane is
asymmetric and symmetric stretching modes. The infrared small, suggesting rapid motion of n-butane molecules adsor-
spectra are very similar to those of molybdenum(II) acetate bed in complex 6. Three spectra of 6 for the shift region of
(24).
the benzene ring (120}140 ppm) are shown in Fig. 9. In the
The maximum amounts adsorbed, which were evaluated spectrum (a) of the dried 2, two signals located at 130.1 and
from the saturated amount with isobars (p"20 Torr), are 133.0 ppm were clearly assigned to the C and C carbons,
ꢂ
ꢁ
summarized in Table 3. The maximum amount of adsorbed respectively, by the use of a dipolar-dephasing spectrum (b)
nitrogen for 2 is 2.0 mol, which is ever so slightly more than in which the signal of the C carbon bonded to proton
ꢂ
that of copper(II) terephthalate. The temperature depend- nuclei disappeared and the signal of the carbon bonded to
encies of gas adsorption for molybdenum(II) complexes 5}9 the carboxyl group remained. Spectrum (c) shows a signi"-
cant change in the chemical environment of the C carbon
ꢂ
of the phenyl ring of the terephthalate ligand by occluding
the n-butane molecules in complex 6. The signal of the
C carbon shifts to a lower "eld by 0.8 ppm, while other
ꢂ
signals, including that of carboxyl carbon, remain almost
constant. This result shows that the adsorbed molecules
apparently interact with C carbons. The line width showed
ꢂ
no signi"cant broadening, suggesting a homogeneous
SCHEME 2
change in the environment of the benzene rings of all the

124
MORI AND TAKAMIZAWA
TABLE 2
Results from Raman and IR Spectra of Molybdenum(II) Dicarboxylates 5+9
IR
Raman
Complex
lMo}Mo/cm\ꢁ
l
OCO/cm\ꢁ
1500
l
OCO/cm\ꢁ
1435
!ꢀꢁꢂ
ꢀꢁꢂ
406
398
5
1498
1397
6
7
399
389
1506
1487
1387
1372
8
9
386
399
1523
1505
1397
1420
terephthalate ligands, which make up the micropores. The solid-state physics and chemistry (26) because of their halo-
change in the local structure of complex 6 due to the gen bridged }Ru}Ru}X} mixed-valence structure and the
adsorption of n-butane is reversible. These results strongly #2.5 oxidation state of the ruthenium atom with a
indicate that gas molecules are adsorbed homogeneously in (p)ꢂ(n)ꢄ(d)ꢂ(n*)ꢂ(d*)ꢁ con"guration per dinuclear (27) (see
micropores surrounded by a quartet of terephthalate Fig. 10a). We have focused on ruthenium (II,III) dicar-
ligands.
boxylate derivatives, which will make up a more rigid net-
work structure, assisted by the axial-halogen bridge, than
Ruthenium(II, III) dicarboxylates (25). Since the dis- those of copper(II) dicarboxylates and molybdenum(II)
covery of binuclear tetra-k-carboxylates of ruthenium(II,III) dicarboxylates.
Ru (O CR) Cl, various studies have prompted much inter-
est in ruthenium(II,III) acetate derivatives in the "eld of Ru (trans-trans-O C}CH"CH}CH"CH}CO ) Cl (11),
Complexes of Ru (trans-O C}CH"CH}CO ) Cl (10),
ꢂ
ꢂ
ꢄ
ꢂ
ꢂ
ꢂ ꢂ
ꢂ
ꢂ
ꢂ ꢂ
TABLE 3
Amounts of Adsorbed Gases in Molybdeum(II) Dicarboxylates 5+9
Amount of occlusion (mol/mol of molydenum(II))
Complex
N
Ar
O
CH
ꢂ
ꢂ
ꢄ
5
6
1.1
2.0
1.2
1.9
1.4
0.9
2.0
2.1
1.7
1.2
7
8
1.0
0.8
1.2
1.0
1.2
0.6
9
07
0.8
Not measured
Not measured

MICROPOROUS MATERIALS OF METAL CARBOXYLATES
125
TABLE 4
Isotropic Chemical Shifts of 13CP/MAS NMR Spectra of 5+9
Complex
d /ppm (assignment)
*ꢀꢃ
5
6
7
8
9
131.0 (ene), 177.0 (carbonyl)
130.1, 133.0 (benzene), 176.8 (carbonyl)
126.8, 141.8 (diene), 177.1 (carbonyl)
127.2, 138.6, 149.9 (pyridine ring), 174.0 (carbonyl)
30.2, 42.1, 45.5 (cyclohexane ring), 187.9 (carbonyl)
and Ru (p-O C}C H }CO ) Cl (12) were synthesized by
ꢃ ꢄ ꢂ ꢂ
a ligand exchange method between Ru (O CCH ) Cl and
ꢂ
ꢂ
ꢂ
ꢂ
ꢅ ꢄ
appropriate dicarboxylic acids in the presence of lithium
chloride. (Scheme 3). A high yield of insoluble brown pow-
der was obtained.
Raman, far-IR, and magnetic measurements revealed that
the electronic structures of 10}12 are very similar to that of
Ru (O CCH ) Cl (28), as shown in Table 5. In Raman
spectra, l(Ru}Ru) occurs at around 330 cm\ꢁ. In far-IR
spectra, bands are observed in two regions at around 400
and 340 cm\ꢁ, which are assigned to Ru}O and Ru}Cl
stretching modes, respectively (29). Raman and far-infrared
FIG. 9. ꢁꢅC CP/MAS NMR spectra of 6 magni"ed in the shift of the
benzene region; (a) 2, (b) dipolar dephased spectrum of 6, (c) 6 occluding
n-butane.
ꢂ
ꢂ
ꢅ ꢄ
spectra indicate that these compounds have a dinuclear ruthenium ions to construct linear chains. Thus the deduced
structure that combines with the halide ion at the axial structure is a three-dimensional network bridged by the
coordination site. The e!ective magnetic moments at room dicarboxylate ligands and halide ions, as shown in Fig. 10b.
temperature are in the range 3.75 and 4.04 BM per dinuc-
The ruthenium(II,III) dicarboxylates 10}12 have dis-
lear, consistent with the values obtained previously for played the ability to adsorb a large amount of gas. The
Ru(II)}Ru(III) compounds and species containing three un- maximum amounts of nitrogen, oxygen, and argon adsor-
paired electrons per dinuclear with the ground electronic bed are summarized in Table 6. The maximum amount
con"guration of (p)ꢂ(n)ꢄ(d)ꢂ(n*)ꢂ(d*)ꢁ. In view of their sim- increases in the order: 10(11(12. The gas adsorption
ilarity to mixed-valence Ru (O CR) Cl derivatives, it is properties depend upon the diameter of the capillary, which
ꢂ
ꢂ
ꢄ
most likely that the chloride anions in 10}12 link two is controlled by the size and structure of the dicarboxylate
ligands, as was also recognized in previous studies of
FIG. 10. Linear structure of mixed-valence ruthenium(II,III) acetate
FIG. 8. ꢁꢅC CP/MAS NMR spectra of complex (6); (a) 6 with n-butane bridged by chloride (a) and three-dimensional structure of ruthenium(II,III)
and (b) dried 6. *, Springing side bands; **, bands of acetate impurity.
dicarboxylate (b).

126
MORI AND TAKAMIZAWA
TABLE 6
Amounts of Adorbed Gases in Ruthenium(II) Dicarboxylates 10+12
Amount of occlusion
(mol/mol of Ru atoms)
Complex
N
O
Ar
0.8
1.2
ꢂ
ꢂ
10
11
0.7
1.1
1.0
1.4
SCHEME 3
copper(II) and molybdenum(II) dicarboxylates. This result
suggests that complexes 10}12 would form a capillary
structure similar to that of copper(II) dicarboxylates and
molybdenum(II) dicarboxylates.
12
1.3
1.7
1.4
2.2. Dinuclear Transition-Metal Carboxylates Bridged by yellowish-brown microcrystals, which are capable of oc-
Ligands in the Axial Direction
cluding gases. The maximum amounts of adsorbed N gas
for the rhodium(II) complexes are summarized in Table 7.
The maximum amount of adsorbed gas for rhodium(II)
benzoate-pyrazine is almost equal to that for copper(II)
terephthalate (2). The magnetic susceptibilities were mea-
sured by the Gouy method at room temperature. The e!ec-
ꢂ
The building blocks bearing noncovalent interaction sites
can form network structure. Such interactions build and
stabilize the micropore structure occluding gases.
All of the metal dicarboxylates described in the previous
sections have two- or three-dimensional network structures.
We found that the microporous structure constructed also
by self-assembly of in"nite linear chain complexes, transi-
tion-metal benzoates, and substituted benzoate bridged by
pyrazine (M (O CC H ) (pyz)). The van der Waals interac-
tive magnetic moments (k ) of the rhodium(II) complexes
%&&
are almost equal to those for dinuclear rhodium(II) car-
boxylates, such as rhodium(II) acetate, indicating the exist-
ence of the same dinuclear structure in the present
rhodium(II) carboxylates which have pyrazine bridges,
forming the one-dimensional structure shown in Fig. 11a.
The one-dimensional polymer complex is the elementary
component used to construct the micropores in which
ꢂ
ꢂ
ꢃ ꢆ ꢄ
tion between phenyl groups of ligands, so called n}n stack,
may act as driving force for the self-assembly of the one-
dimensional complexes.
Rhodium(II) complexes (30). Rhodium(II) benzoate and a large amount of gases can be occluded.
substituted benzoates are synthesized by the ligand ex-
The three-dimensional structure, which has a large micro-
change reaction between rhodium(II) acetate and corre- pores, as shown in Fig. 11b, is a reasonable proposal for our
sponding carboxylic acids in diglym at 1803C. (Scheme 4) rhodium(II) carboxylates}pyrazine by analogy with the
Rhodium(II) carboxylates bridged by pyrazine are quantit- structure of copper(II) terephthalate (2), which shows ad-
atively yielded as yellowish-brown microcrystals. Heating sorption behavior that is similar to the rhodium(II) salt. We
the microcrystals for 2 h at 1003C in vacuum produced determined the crystal structure of the microporous cop-
per(II) benzoate}pyradine (31) that constructs micropores
as shown in Fig. 11b.
The TG-DTA curves, shown in Fig. 12, clearly indicate
the thermal stability of rhodium(II). There is neither
any endo- nor any exothermic peak nor is there any remark-
TABLE 5
Raman and For-IR Spectra and E4ective Magnetic Moments (leff)
of Ruthenium(II,III) Dicarboxylates 10+12
Far IR
able weight loss at temperatures below 520 K (thermal
Raman
k
?/BM
%&&
Complex
l(Ru}Ru)/cm\ꢁ l(Ru}Cl)/cm\ꢁ l(Ru}O)/cm\ꢁ (at rt)
Ru (O CCH ) Cl
326@
334
336
336
340A
340
325
334
403A
410
364m, 485m
385s, 411w
4.16B
3.75
3.78
4.04
ꢂ
ꢂ
ꢅ ꢄ
10
11
12
?Gouy method
@Ref. 29a.
ARef. 29b.
BMeasured by carrying magnetometer for complexes.
SCHEME 4

MICROPOROUS MATERIALS OF METAL CARBOXYLATES
127
TABLE 7
transition-metal complex 3D system building block, metal
tris(biimidazolato) complex, in which each of the three n-
conjugated biimidazolate ligands provide an intermolecular
NH2N-type hydrogen-bonding site. The complexes or-
ganized to form alternative 2D structures.
Compound
Amount of occlusion
(mol/mol of
rhodium atoms)
k
%&&
(BM/Rh at rt)
The 3D networks frequently interpenetrate them in the
solid state. Inorganic interpenetrating 3D system was re-
viewed by Batten (36). Fujita (37) and Kitagawa (38) each
developed the area of the design of metal bipyridine com-
plex with an open framework. Yaghi (19,39) synthesized
zinc(II) polycarboxylate system with an open framework. In
the chemistry of complexes, porous complexes are known as
a thermally unstable. Their porous structures are only stable
with crystal molecules and/or counter ions in their micro-
H
o-OH
m-OH
0.38
0.27
0.21
0.30
0.25
0.33
0.19
1.8
1.1
0.8
0.6
0.2
0.5
0.6
p-OH
R"
o-CH
ꢅ
m-CH
ꢅ
G
p-CH
ꢅ
decomposition temperature, 540}600 K). It is noteworthy pores. The generation of microporous transition-metal
that the micropores of the rhodium(II) carboxylates}pyra- complexes with thermal stability (stable micropore structure
zine are surprisingly thermally stable, although the micro- under even drying condition) is of great interest.
pores are formed from one-dimensional polymer complexes
Since 1995, after we had applied to patent adsorbent
with weak van der Waals force between phenyl groups (32). microporous complexes (40), we have continued to study
adsorbent microporous complexes. There are many excel-
3. OUTLOOK AND REMARKS
lent works of adsorbent porous complexes reported by
Yaghi (12, 41), Kitagawa (42), and Williams (43).
The noncovalent bond networks whose structures are
constructed through inter- and/or intramolecular interac-
tion are frequently seen in the biological systems (33).
Aoyama (34) reported that self-assembly of organic molecu-
4. FUTURE PROSPECTS
Complex chemistry consists of the characteristics of
les such as anthracene and porphyrin derivatives forms transition metals (inorganic species) and organic ligands
a hydrogen-bonded network together with a variety of guest (organic species). Considering the distinguishable features of
molecules in their cavities. Tadokoro (35) reported a unique inorganic compounds and organic compounds, the network
FIG. 11. Linear structure of rhodium(II) benzoate}pyrazine (a) and proposed porous structure in crystalline state (b).

128
MORI AND TAKAMIZAWA
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