Two supramolecular isomers of dimeric cobalt(II) 5-aminoisophthalate: 2D (4,63) vs 3D Rutile nets

Article abstract of DOI:10.1016/j.molstruc.2007.06.032

The hydrothermal preparation via variations of the solvent conditions of Co(NO₃)₂·6H₂O with 5-aminoisophthalic dianion (5-NH₂-bdc) exhibits two new supramolecular isomerism of [Co(C₈NH₅O₄)(H₂O)]_n·2nH₂O (1) and [Co(C₈NH₅O₄)(H₂O)]_n (2). The reaction in EtOH/H₂O resulted in a layered structure of 1 featuring an unusual zigzag water chain between adjacent sheets, and that in H₂O led to an open 3D framework structure of 2. The structural data suggest that both the relative orientations in the ligand placement and the donor groups arrangement around the cobalt center are the key factor affecting the patterns of conformational supramolecular isomerism of 1 and 2. More interestingly, simplified by considering the cobalt dimer and NH₂-bdc ligand represented an octahedronal node and a triangular node, respectively, structural diversity of 1 and 2 exhibits a structure evidence of 3,6-connecting building blocks with different topologies from 2D (4,₆³) net to 3D Rutile net. Two Complexes exhibit the excellent thermal stability with the tolerate temperature ca. 440 °C for 1 and 410 °C for 2, respectively, which attribute to the presence of abundant coordination bonds of the μ₄ bridging mode dianion, and the chelate effect of carboxylate groups in the dimer SBUs.

Full text of DOI:10.1016/j.molstruc.2007.06.032

Available online at www.sciencedirect.com
Journal of Molecular Structure 876 (2008) 162–169
www.elsevier.com/locate/molstruc
Two supramolecular isomers of dimeric cobalt(II)
5-aminoisophthalate: 2D (4;36) vs 3D Rutile nets
a
a
a,
*
Qian-Jun Deng a,b, Mei-Chun Wu , Zhuang-Tian Liu , Ming-Hua Zeng
,
a
a,b,
*
Jian-Ye Huang , Hong Liang
a
Department of Chemistry and Chemical Engineering, GuangXi Normal University, Guilin 541004, PR China
College of Chemistry and Chemical Engineering, Central South University, Changsha, Hunan 410000, PR China
b
Received 1 May 2007; received in revised form 14 June 2007; accepted 16 June 2007
Available online 5 July 2007
Abstract
The hydrothermal preparation via variations of the solvent conditions of Co(NO3)2Æ6H2O with 5-aminoisophthalic dianion (5-NH2-
bdc) exhibits two new supramolecular isomerism of [Co(C8NH5O4)(H2O)]nÆ2nH2O (1) and [Co(C8NH5O4)(H2O)]n (2). The reaction in
EtOH/H2O resulted in a layered structure of 1 featuring an unusual zigzag water chain between adjacent sheets, and that in H2O led
to an open 3D framework structure of 2. The structural data suggest that both the relative orientations in the ligand placement and
the donor groups arrangement around the cobalt center are the key factor affecting the patterns of conformational supramolecular isom-
erism of 1 and 2. More interestingly, simplified by considering the cobalt dimer and NH2-bdc ligand represented an octahedronal node
and a triangular node, respectively, structural diversity of 1 and 2 exhibits a structure evidence of 3,6-connecting building blocks with
different topologies from 2D (4;36) net to 3D Rutile net. Two Complexes exhibit the excellent thermal stability with the tolerate temper-
ature ca. 440 ꢀC for 1 and 410 ꢀC for 2, respectively, which attribute to the presence of abundant coordination bonds of the l4 bridging
mode dianion, and the chelate effect of carboxylate groups in the dimer SBUs.
ꢁ 2007 Elsevier B.V. All rights reserved.
Keywords: Supramolecular isomerism; Cobalt(II); 5-Aminoisophthalic dianion; Water chain; Topology
1. Introduction
mine the outcome of the topology of the final structure [2].
In this important family, a number of synthetic strategies
utilizes different bridging ligands and related species for
propagating new coordination polymers supramolecular
isomerisms [3–9]. Among them, systematic investigations
of open-framework structures constructed from different
kinds of dicarboxylates ligands are rather limited [9]. As
such, 1,3-benzenedicarboxylic acid (H2bdc) and 5-NO2-
bdc are expected to serve as a suitable building block in this
context, because of its multidentate nature, versatile coor-
dination modes and sensible correlation of coordination
characteristics with reaction parameters [9]. In this contri-
bution, we have chosen the structurally similar 5-NH2-
bdc analogue as a potential 3-connected node to serve as
a chelating/bridging ligand for this purpose. Two poly-
meric supramolecular isomers, triggered by the change in
the ligand placement orientation and the donor groups
Supramolecular isomerism refers to a crystalline mate-
rial that adopts two or more architectures but have identi-
cal chemical compositions in terms of building blocks and
molecular connectivity, i.e., the compound manifests more
than one topology [1]. As such, the different topologies for
the compounds provide an invaluable insight into the fac-
tors that govern the self-assembly and growth of crystals
[1,2]. In the architectural isomerism, the connectivities of
the organic building blocks as well as the geometries of
metal ions [or metal-cluster secondary building units
(SBUs)] represent two crucial factors that invariably deter-
*
Corresponding authors. Tel.: +86 773 5846 279; fax: +86 7735812 383.
E-mail address: zmh@mailbox.gxnu.edu.cn (M.-H. Zeng).
0022-2860/$ - see front matter ꢁ 2007 Elsevier B.V. All rights reserved.
doi:10.1016/j.molstruc.2007.06.032

Q.-J. Deng et al. / Journal of Molecular Structure 876 (2008) 162–169
163
Table 1
Crystal data and structure refinement of compounds 1 and 2
Empirical formula
Fw
C8H11CoNO7 (1)
292.11
Triclinic
C8H7CoNO5 (2)
256.08
Monoclinic
P21/c
8.807(1)
12.575(1)
7.971(1)
90
97.513(6)
90
875.2(1)
4
Crystal system
Space group
ꢀ
P1
˚
a (A)
7.723(1)
8.559(2)
8.568(2)
85.531(3)
75.893(3)
65.832(3)
501.0(2)
2
˚
b (A)
˚
c (A)
a (ꢀ)
b (ꢀ)
c (ꢀ)
Scheme 1. Structure–topology relations between supramolecular isomer-
ism of 1 and 2.
3
˚
V (A )
Z
T (K)
F (000)
273
298
1.936
1.078
2.5, 27.0
0.55, À0.44
1.739
4254
2144
1673
0.027
0.0352
0.0834
296
516
1.944
1.08
2.3, 28.4
0.42, À0.30
1.960
7036
2178
1902
0.022
0.0255
0.0650
arrangement around the metal center, are described in this
report (Scheme 1). Based on 6-connected dimeric cobalt(II)
center and 3-connected 5-aminoisophthalic dianion, their
topology from a 2D (4;36) net to a 3D (4162)2(4261083) net
exemplifies meaningful structure–topology correlations
between supramolecular isomerism and complex topologies.
Dc (g cmÀ3
)
Goodness-of-fit on F2
h Range (ꢀ)
Fmax/Fmin (e A
l (mmÀ1
Total reflections
Unique reflections
Observed reflections [I P 2r(I)]
Rint
À3
˚
)
)
2. Experimental
R1 [I P 2(I)]
wR2 (all data)
2.1. General methods
All chemicals were of reagent grade and were used as
received. Elemental analyses (C, H, N) were performed
on a Perkin-Elemer PE 2400 II CHN elemental analyzer.
IR spectra were recorded in the range 4000–400 cmÀ1 on
Perkin-Elmer PE Spectrum One FT/IR spectrometer using
a KBr disc. The TG experiments for the compounds were
performed using a Perkin-Elmer TGA7 under a dynamic
(%): C, 37.52; H, 2.76; N, 5.47. Found (%): C, 37.45; H,
2.88; N, 5.41.
2.4. Structure determination
Two diffraction data were collected on a Bruker Smart
Apex CCD diffractometer with graphite monochromated
nitrogen atmosphere with a heating rate of 10 ꢀC minÀ1
.
˚
Mo-Ka radiation (k = 0.71073 A) at 293 K. Absorption
corrections were applied by SADABS [10]. All the struc-
tures were solved by direct methods and refined with full-
matrix least-squares technique using SHELXTL [11]. All
non-hydrogen atoms were refined with anisotropic dis-
placement parameters. For 1, the lattice water hydrogen
atoms were located from the difference Fourier map at
the final states of refinements. Each water molecule has a
full-occupancy H-atom. The other H-atom is positionally
disordered and treated as half-occupancy atoms during
refinement. The crystallographic details are provided in
Table 1 (Table 2).
2.2. Synthesis of 1
All chemicals and solvents used in the synthesis were of
reagent grade and used as purchased without further puri-
fication. Complex 1 was prepared from a mixture of Co(N-
O3)2Æ6H2O (1 mmol), 5-NH2-bdc (1 mmol), H2O (8 mL)
and ethanol (8 mL), sealed in a 23 mL Teflon-lined stain-
less steel bomb, and kept at 140 ꢀC for 5 days under autog-
enous pressure. After the reaction was slowly cooled to
room temperature, red block crystals were produced (yield:
32%, based on Co). Anal. Calcd for C8H11CoNO7 (%): C,
32.89; H, 3.80; N, 4.80. Found (%): C, 32.77; H, 4.02; N,
4.65.
3. Results and discussion
3.1. Crystal structures
2.3. Synthesis of 2
X-ray single-crystal diffraction reveals that 1 (red block,1
Fig. 1) crystallizes in the triclinic space group (P1). As
shown in Fig. 2a, the cobalt(II) atom is chelated by a
1-carboxylate group of 5-NH2-bdc; it is also linked to
Complex 2 was prepared from a mixture of Co(N-
O3)2Æ6H2O (1 mmol), 5-NH2-bdc (1 mmol), H2O (16 mL),
sealed in a 23 mL Teflon-lined stainless steel bomb, and
was heated at 140 ꢀC for 5 days under autogenous pressure.
Red column crystals were isolated after the reaction solu-
tion was cooled gradually and then washed with water
(yield: 20%, based on Co). Anal. Calcd for C8H7CoNO5
ꢀ
1
For interpretation of the references to colour in this text, the reader is
referred to the web version of this article.

164
Q.-J. Deng et al. / Journal of Molecular Structure 876 (2008) 162–169
Table 2
boxylate groups of four parallel adjacent dianions to form
an infinite double chain, which are further alternately inter-
connected through the amino group of the dianion from
adjacent chain into a pulldown 2D network. The dianion
is functioning in a l4 bridging mode (one of its carboxylate
groups bridges to two different CoII cations, and another
coordinates to the third CoII center as a bidentate ligand;
the amino group bonds to the fourth CoII cation)
(Scheme 2, Fig. 3). The host framework of layer motif is
similar to the reported compound with the 2D network
envisioned as being built up from the interlocking motifs
[A + B + C + D] through sharing cobalt atoms by 5-
NH2-bdc bridges [13]. The aromatic rings of neighboring
layers are too distant to take part in p–p interactions, with
˚
Selected bond lengths (A) and bond angles (ꢀ) for the complexes 1 and 2
C8H11CoNO7 (1)
Co1–O1
Co1–O1W
Co1–O2
Co1–N1d
Co1–O3c
Co1–O4b
O1–Co1–O1W
O1-Co1–O2
O1–Co1–O4b
O1–Co1–N1d
2.162(2)
2.048(3)
2.217(2)
2.236(3)
2.095(2)
2.015(2)
152.3(1)
59.7(1)
O1W–Co1–O4b
O1W–Co1–O3c
O1W–Co1–N1d
O2–Co1–O4b
O2–Co1–O3c
O2–Co1–N1d
O3c–Co1–O4b
O4b–Co1–N1d
O3c–Co1–N1d
O1W–Co1–O2
115.3(1)
90.0(1)
85.7(1)
150.4(1)
91.6(1)
87.5(1)
97.2(1)
85.7(1)
175.6(1)
92.8(1)
92.4(1)
96.1(1)
C8H7CoNO5 (2)
Co1–O1
Co1–O1W
Co1–O2
Co1–O3b
Co1–N1c
2.118(1)
2.100(2)
2.261(1)
2.015(1)
2.223(2)
1.988(1)
90.6(1)
59.8(1)
95.6(1)
93.0(1)
O1W–Co1–O2
O1W–Co1–O3b
O1W–Co1–N1c
O1W–Co1–O4e
O2–Co1–O3b
O2–Co1–N1c
O2–Co1–O4e
O3b–Co1–N1c
O3b–Co1–O4e
O4e–Co1–N1c
91.9(1)
90.5(1)
173.8(1)
88.5(1)
155.3(1)
85.6(1)
95.2(1)
94.1(1)
109.4(1)
86.1(1)
˚
the shortest distance between centroids being 4.197 A. The
stacking of the corrugated layers provides space large
enough to contain the associated water molecules (poten-
tial solvent-accessible volume 15.1%).
Most noticeable is the existence of a rare zigzag water
chain between adjacent sheets [14–16], formed by solvent
water as inclusion in compound 1. The water molecules
connect to form a chain of the following sequence:
O2Wf–O2W–O3W–O3Ws linked with each other via alter-
Co1–O4e
O1–Co1–O1W
O1–Co1–O2
O1–Co1–O3b
O1–Co1–N1c
(1) Symmetry codes: (b) 1 + x, y, À1 + z; (c) 1 À x, 1 À y, Àz; (d) 2 À x,
Ày, Àz.
(2) Symmetry codes: (b) 1 À x, 1/2 + y, 1/2 À z; (c) 1 À x, 1 À y, Àz; (e)
1 + x, 1/2 À y, 1/2 + z.
natively strong and weak hydrogen bonding [O2W–
˚
H2WBÁ Á ÁO2Wf
2.853(5) A,
O3W–H3WBÁ Á ÁO2W
˚
˚
˚
2.817(5) A, O3W–H3WCÁ Á ÁO3Ws2.922(4) A, Table 3].
The average OÁ Á ÁO distance is 2.86 A and the average
OÁ Á ÁOÁ Á ÁO angle is 109.7ꢀ. O3W is involved in four hydro-
gen bonds with two waterÁ Á Áwater interactions, the other
two interactions are water–carboxylate oxygen and
water–amido nitrogen atom of the 5-NH2-bdc ligand, in
contrast three waterÁ Á Áwater interactions, one water–car-
boxylate oxygen for O2W. Therefore, each water molecule
acts as both donor and a hydrogen acceptor along with
amido nitrogen to form a water tape by edges sharing cyc-
lic double hetero pentamer units (Fig. 4a) [15,16]. The
hydrogen bonding scheme is different from the example
of a metal-coordinated hexameric cluster in which the
coordinated water molecules behave only as single-hydro-
gen donors whereas the oxygen atom in the other water
molecules show four-coordination [15,16]. It is worthy of
note that there also exists extended host–host hydrogen
bonds between the adjacent layers among aqua ligands,
the oxygen atoms of carboxylate groups, and amino groups
(Fig. 4b). Finally, the adjacent layers are assembled into a
3D supramolecular structure by the above-mentioned
hydrogen-bonding interactions, which are rather extensive
and indicates that the water chain plays a crucial role in
the formation of the 3D structure. The shortest CoÁ Á ÁCo
Fig. 1. Photos of the single-crystals of 1 and 2. (For interpretation of the
references to colour in this figure legend, the reader is referred to the web
version of this article.)
one oxygen atom of the 3-carboxylate group of another
dianion as well as to the other oxygen atom of the 3-car-
boxylate group of yet another dianion. And the six-coordi-
nate geometry are completed by a coordinated water
molecule and the amino group of a third adjacent dianion
˚
interlayer distance is 5.150(1) A. Although various low-
dimensional hydrogen-bonded water networks have been
structurally characterized, the structures of mixed water
and other small hydrogen bonding molecules have been
still rarely reported. Nevertheless, structural elucidation
of the supramolecular networks of these hybrids is signifi-
cant in understanding the unusual properties of these water
solutions [17].
˚
˚
(Co–N 2.236(3) A; Co–O 2.015(2)–2.217(2) A). Each pair
of Co(II) ions is bridged by a pair of skew–skew syn–syn
carboxylate groups of 5-NH2-bdc into a dimeric subunit
˚
with an intradimer CoÁ Á ÁCo distance of 4.229(1) A. The
different adjacent dimers are doubly interlinked by the car-

Q.-J. Deng et al. / Journal of Molecular Structure 876 (2008) 162–169
165
Fig. 2. The Co(II) coordination environment in the dimeric subunit in 1 (left) and 2 (right). Lattice water molecules and hydrogen atoms have been
omitted for clarity. Color code: deep blue, CoII; red, O; blue, N; green, C. (For interpretation of the references to colour in this figure legend, the reader is
referred to the web version of this article.)
Scheme 2. The bridging modes of 5-NH2-bdc and the coordination environments of cobalt dimers in 1 and 2.
Another 3D supramolecular isomer (2) was formed
when the reactants are crystallized from an aqueous solu-
tion (Fig. 1); the compound 2 crystallizes as red columns
in the monoclinic space group (P21/c, No. 14). X-ray crys-
tallography shows that 2 also has a centrosymmetrical
dimeric Co2(5-NH2-bdc)2(H2O)2 subunit. The coordina-
tion geometry around each CoII center is also a distorted
six-coordinated octahedron being coordinated by four 5-
NH2-bdc ligands and a coordinated water molecule (Co–
˚
˚
N: 2.223(2) A; Co–O: 1.988(1)–2.261(1) A) (Fig. 2b). Coor-
dination mode of 5-NH2-bdc ligand in 2 is similar to that of
1. Each dimeric subunit bridged by a pair of skew–skew
syn–syn 5-NH2-bdc carboxylate groups with an intradimer
˚
CoÁ Á ÁCo distance of 4.325(1) A, which are interlinked by
the carboxylate groups of 5-NH2-bdc ligands with peculiar
clockwise disposition of the ligand placement orientation
to form a wavy (4,4) net (Fig. 5), and further intercon-
nected through the anilinyl rings of 5-NH2-bdc (CoÁ Á ÁCo
Fig. 3. A view of 1 showing how the organization of the four different
subrings (marked with A, B, C, and D) contributes to the construction of
the sheet.

166
Q.-J. Deng et al. / Journal of Molecular Structure 876 (2008) 162–169
Table 3
˚
Selected hydrogen bond lengths (A) for 1 and 2
D–H–A
d(D–H)
d(H–A)
d(D–HÁ Á ÁA)
DHA
C8H11CoNO7 (1)
O1W–H1WAÁ Á ÁO2h
0.82(4)
0.86(3)
0.83(3)
0.83(4)
0.97(10)
0.75(8)
0.83(5)
0.84(4)
0.84(4)
0.87(5)
0.91(4)
1.92(4)
2.66(3)
2.20(4)
1.92(4)
2.09(9)
2.13(8)
2.03(5)
2.57(4)
2.07(5)
2.05(7)
2.06(5)
2.706(3)
3.291(3)
3.006(4)
2.742(4)
2.854(7)
2.817(5)
2.790(4)
3.188(4)
2.802(3)
2.817(5)
2.923(6)
162(4)
131(2)
164(3)
171(4)
135(7)
151(7)
152(5)
132(4)
146(4)
146(5)
159(6)
N1–H1AÁ Á ÁO2p
N1–H1BÁ Á ÁO3Wo
O1W–H1WBÁ Á ÁO2We
O2W–H2WBÁ Á ÁO2Wf
O2W–H2WCÁ Á ÁO3W
O2W–H2WAÁ Á ÁO1f
O3W–H3WAÁ Á ÁO1
O3W–H3WAÁ Á ÁO3c
O3W–H3WBÁ Á ÁO2W
O3W–H3WCÁ Á ÁO3Ws
C8H7CoNO5 (2)
O1W–H1WAÁ Á ÁO2x
O1W–1WBÁ Á ÁO1y
N1–1BÁ Á ÁO3z
0.74(3)
0.83(2)
0.82(2)
2.11(3)
1.95(2)
2.61(2)
2.845(2)
2.776(2)
3.258(2)
170(3)
177(2)
136(2)
Fig. 5. The (4,4) net constructed by the metal ions and 5-NH2-bdc groups
in 2 (omitting Co–N bonds).
(1) Symmetry codes: (c) À1 À x, 1 À y, Àz; (e) Àx, 1 À y, À1 À z; (f) Àx,
Ày, À1 À z; (h) Àx, 1 À y, Àz; (o) À1 À x, Ày, Àz; (p) x, À1 + y, z; (s)
À1 À x, 1 À y, À1 À z.
(2) Symmetry codes: (x) x, Ày + 5/2, z + 1/2; (y) Àx À 1, Ày + 3, Àz + 1;
(z) Àx À 2, y + 1/2, Àz À 1/2.
˚
at 8.050 A) to lead to a 3D extended network (Fig. 6) with
the aqua ligands pointing into the troughs of the channels
˚
(ca. 10.3 · 4.5 A, CoÁ Á ÁCo and CoÁ Á Áaromatic ring dis-
tance) [18]. There also exist hydrogen bonds among aqua
ligands and the oxygen atoms of carboxylate groups/the
nitrogen atom of amino groups [O1W–H1WAÁ Á ÁO2g
Fig. 6. The 3D framework of 2.
˚
˚
2.845(2) A, O1W–H1WBÁ Á ÁO1f 2.776(2) A, N1–H1BÁ Á Á
˚
O3Wj 3.258(2) A]. Thus the water content tunes the coor-
metal center and organic ligand under such rigorous
conditions.
dination environment of the metal centers and the confor-
mation of organic ligands, leading to the formation of 2D
and 3D architectures in the products (Scheme 1). Interest-
ingly, the influence of the water poor condition, the forma-
tion of 1 exhibiting water chains is unexpected. Finally, it is
note worthy that under hydrothermal conditions, high-
dimensional (2D and 3D) networks are preferred for 5-
NH2-bdc based coordination polymers. This might be
attributed to the enhancement of interactions between the
The network topology of 1 and 2 can be simplified by
considering just the cobalt atoms as a single node located
between the two metal ions (represented by an octahedro-
nal node) and the tri-connected 5-NH2-bdc ligands (repre-
sented by a triangular node). A topological analysis of this
net was performed with OLEX. The resulting nets, showing
in Fig. 7, is a rare 2D (3,6)-connected (4;36) net with trian-
Fig. 4. (a) Representation of the 1D water chain linked via edges sharing cyclic double hetero pentamer units. (b) The hydrogen-bonding network of a lay
for 1 (the dashed bond showing hydrogen-bonding interaction). Only (N) and (H2O), and (CO2) group are shown.

Q.-J. Deng et al. / Journal of Molecular Structure 876 (2008) 162–169
167
wise disposition of the ligand placement orientation among
1, 2, 3, 4, the consecutive units formed a wavy sheet and
again interconnected up and down through coordination
bonds between the nitrogen atom of the 5-NH2-bdc and
the cobalt center leading to a 3D coordination network.
Obviously, the relative orientations of the 5-NH2-bdc
groups affect the presence or absence of interlayer connec-
tion may play a key role leading to the 2D structure of 1
and the 3D structure of 2 [25]. On the other hand, although
the Co(II) ion was similarly bound by a NO5 donor set, the
Co–O and Co–N distance in 1 and 2 were identical, some
positions of coordination atoms arrangement in the two
complexes is different. It is noted that the Co–N bond
and Co–O1W bond are at perpendicular sites of the octa-
hydron (N1d–Co1–O1W, 85.7(1)ꢀ) in 1, whereas in 2,
amido group and the coordination water occupy the oppo-
site sites of cobalt center with N1b–Co1–O1W ca. 173.8(1)ꢀ
(Scheme 2). By being able to adopt different binding modes
and relative orientations, as described above, the ligands
and the metal center in allowed for supramolecular isomer-
ism, and interesting structural diversity were obtained.
Fig. 7. Schematic illustrating the connectivity of the topology of the
network observed for 1 ((a) (4;3) net), 2 ((b) (4162)2(4261083) net). Six-
6
connecting nodes (Co2) and 3-connecting nodes (5-NH2-bdc) are repre-
sented by yellow balls and green balls, respectively. (For interpretation of
the references to colour in this figure legend, the reader is referred to the
web version of this article.)
gular and octahedronal nodes in the ratio 2:1 [19]. The net
is different to commonly observed (4,4), (6,3), (8,4) net-
works [20]. For 2, this network has a 3D (4162)2(4261083)
net similar to tetragonal rutile (TiO2) (Fig. 7) [19,21]. As
only a limited number of simple 3,6-connected 2D or 3D
nets have been reported so far, the two isomeric complexes
with different dimensional architectures and topologic nets
represent an useful contribution to this field as they give
some insight into the reason that the building block in 1
and in 2 can extend into either an unique 2D (4,63) net
or a 3D Rutile net. Indeed, this supramolecular isomerism
represents quite an interesting example, which cannot be
observed in the symmetrical bridging ligands, involves
structural diversity and topology from 2D to 3D in MOFs
[3–8,21–25].
3.2. TGA analyses
Thermogravimetric analysis (TGA) was carried out to
examine the thermal stability of the MOFs of 1 and 2
Although the two networks containing an identical
chemical composition and the common building block
[Co(5-NH2-bdc)(H2O)] with almost the same structural
parameters of the metal ions, the networks in 1 and 2 differ
in the dimensions and topology. One of obviously differ-
ence is the conformation of the 5-NH2-bdc ligand.
Although the 1,3-l-bidentate and the 1,3-chelating carbox-
ylate groups were twisted by 15.1ꢀ, 29.2ꢀ, relative to the
central phenyl ring, the six 5-NH2-bdc ligands around the
cobalt dimer in 1 give rise to the parallel mode between
the central phenyl rings (Scheme 2, Fig. 2a). The 5-NH2-
bdc units located on the two opposite orientation and lin-
early arranged into a double chain, which further intercon-
nected through coordination bonds between the nitrogen
atom of the 5-NH2-bdc and the cobalt centers of adjacent
dimers leading to a 2D coordination networks. In marked
contrast, in the present case of 2, the 1,3-l-bidentate and
the 1,3-chelating carboxylate groups twisted by 38.0ꢀ,
29.3ꢀ, relative to the central phenyl ring. Whereas four of
the six phenyl rings from 5-NH2-bdc ligands (1,3,5,6) con-
nected to the cobalt(II) dimer are parallel with the dihedral
of 0ꢀ, and the phenyl ring in ligand 2 and 4 parallel each
other, on the other hand, the phenyl ring from the above
two groups and strongly tilted each other with the dihedral
of 23.2ꢀ (Fig. 2b). As the consequence of a peculiar clock-
Fig. 8. The TG–DTG curves of 1 and 2.

168
Q.-J. Deng et al. / Journal of Molecular Structure 876 (2008) 162–169
(Fig. 8). The crushed single-crystal sample was heated up
to 900 ꢀC in N2 at a heating rate of 10 ꢀC minÀ1. The
DTG and TG curves for 1 show that the first weight loss
of 11.9% between 40 and 110 ꢀC corresponds to the loss
of the water molecules (ca. 12.0% calc. 12.3%) between
the layers, followed by a loss of 6.0% during 160–
240 ꢀC, corresponding to the loss of one coordinated
water molecule per formula (ca. 6.3% calc. 6.2%). The
anhydrous layered MOF remained stable up to
ꢀ440 ꢀC without any weight loss, showing that the lam-
inated framework could retain structural integrity to ca.
440 ꢀC. From 440 to 750 ꢀC, there was the total loss of
54.8% consistent with the pyrolysis of coordinated
NH2-BDC ligands. The final residual weight is 24.6%
(calc. 25.7%) corresponding to CoO. For 2, a total
weight loss of ca. 7.1% (calc. 7.03%) occurred over the
temperature range 210–270 ꢀC, corresponding to the loss
of one coordinated water molecule per formula, showing
the coordinated water molecules escape more slowly
from the limited porosity in 2 than the water molecules
located between the layer of 1. In the temperature range
410–600 ꢀC, the pyrolysis of 5-NH2-bdc ligands occurred
by a loss of 61.8%, and the final residue was probably
CoO (remaining weight: found: 28.4%, calc. 29.2%).
The above different thermal behaviors may attribute to
their structural features. It can be concluded that 1
and 2 have excellent thermal stability in the class of
MOFs [12,20]. The high stability of the MOFs may be
attributed to the presence of abundant coordination
bonds of the l4 bridging mode dianion, the intermolecu-
lar hydrogen bond between the oxygen atoms of carbox-
ylate groups, and amino groups, and the chelate effect of
carboxylate groups in the dimer SBUs.
Appendix A. Supplementary data
Supplementary data associated with this article can be
found, in the online version, at doi:10.1016/j.molstruc.
2007.06.032.
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