Syntheses, topological analyses and magnetic properties of two 3D supramolecular nickel-organic frameworks constructed from 1,3,5-benzenetricarboxylate and flexible imidazole-based ligands

Article abstract of DOI:10.1016/j.inoche.2009.04.020

Two supramolecular nickel-organic frameworks, [Ni(HBTC)(bix)]_n (1) and [Ni₃(BTC)₂(mbix)₃(H₂O)₄]_n · 6nH₂O (2) (bix = 1,4-bis(imidazole-l-yl-methyl)benzene, mbix = 1,3-bis(imidazole-l-yl-methyl)benzene, and H₃BTC = 1,3,5-benzenetricarboxylate), have been hydrothermally prepared by the assembly of H₃BTC, Ni²⁺ with bix or mbix. Compound 1 shows a 2D layer structure, whose 3D supramolecular structure exhibits a fsc topology when hydrogen-bonging interactions between the adjacent layers are taken into account. Compound 2 manifests an unprecedented 2D (3, 4)-connected topological net. The 3D supramolecular framework of 2 is formed through π?π stacking interactions between the adjacent layers. The magnetic studies show that 1 features overall ferromagnetic property whilst 2 presents strong zero-field splitting (ZFS) when treated as a mononuclear model. Furthermore, the IR and TGA properties of 1 and 2 were also studied.

Full text of DOI:10.1016/j.inoche.2009.04.020

Inorganic Chemistry Communications 12 (2009) 558–562
Contents lists available at ScienceDirect
Inorganic Chemistry Communications
journal homepage: www.elsevier.com/locate/inoche
Syntheses, topological analyses and magnetic properties of two 3D
supramolecular nickel-organic frameworks constructed from
1,3,5-benzenetricarboxylate and flexible imidazole-based ligands
Jian-Di Lin a,b, Cui-Cui Jia a,b, Zhi-Hua Li a, Shao-Wu Du a,
*
a State Key Laboratory of Structural Chemistry, Fujian Institute of Research on the Structure of Matter, Chinese Academy of Sciences, Fuzhou, Fujian 350002, PR China
b Graduate University of Chinese Academy of Sciences, Beijing 100039, PR China
a r t i c l e i n f o
a b s t r a c t
Article history:
Two supramolecular nickel-organic frameworks, [Ni(HBTC)(bix)]n (1) and [Ni3(BTC)2(mbix)3(H2O)4]n
ꢀ
Received 17 February 2009
Accepted 16 April 2009
Available online 24 April 2009
6nH2O (2) (bix = 1,4-bis(imidazole-l-yl-methyl)benzene, mbix = 1,3-bis(imidazole-l-yl-methyl)benzene,
and H3BTC = 1,3,5-benzenetricarboxylate), have been hydrothermally prepared by the assembly of
H3BTC, Ni2+ with bix or mbix. Compound 1 shows a 2D layer structure, whose 3D supramolecular struc-
ture exhibits a fsc topology when hydrogen-bonging interactions between the adjacent layers are taken
into account. Compound 2 manifests an unprecedented 2D (3, 4)-connected topological net. The 3D
Keywords:
Metal-organic frameworks
Crystal structures
Topological analyses
Magnetic properties
supramolecular framework of 2 is formed through
p p stacking interactions between the adjacent lay-
ꢀ ꢀ ꢀ
ers. The magnetic studies show that 1 features overall ferromagnetic property whilst 2 presents strong
zero-field splitting (ZFS) when treated as a mononuclear model. Furthermore, the IR and TGA properties
of 1 and 2 were also studied.
Ó 2009 Elsevier B.V. All rights reserved.
The design and synthesis of functional metal-organic frame-
works (MOFs) have been received extensive attention in recent
years owing to their potential as functional materials [1–4] and
intriguing topologies [5–7]. Consequently, a variety of coordination
polymers with complicated architectures and topologies have been
synthesized successfully [8–10]. It is widely acknowledged that the
coordinative bonding is normally considered to be the primary
interaction to sustain the network, while those supramolecular
our knowledge, MOFs constructed from aromatic carboxylates
and bix (bix = 1,4-bis(imidazole-l-yl-methyl)benzene) or mbix
(mbix = 1,3-bis(imidazole-l-yl-methyl)benzene) have not been
well explored [19–26]. Recently, we have successfully obtained
five novel d10 metal-organic coordination polymers containing
mixed ligands of polycarboxylates and bix (or mbix). These poly-
mers exhibit different topologies from one to another, in which
the polycarboxylate acids display their characteristic distinct coor-
dination modes [27]. As part of our ongoing effort in the design of
topological architecture, we have investigated the assemblies of Ni
ion and the mixed-ligands of H3BTC (H3BTC = 1,3,5-benzenetri-
carboxylate) and bix (or mbix), which led to the isolation of two
3D supramolecular MOFs, [Ni(HBTC)(bix)]n (1) and [Ni3(BTC)2-
(mbix)3(H2O)4]n ꢀ 6nH2O (2). The crystal structures, topological
analyses, thermal stability of these compounds, along with their
magnetic properties will be represented and discussed in this
paper.
Bix and mbix were synthesized by the literature method [28].
All the other chemicals were commercially available and used as
purchased. NiCl2 ꢀ 6H2O (0.125 mmol, 29.7 mg), H3BTC (0.125
mmol, 26.3 mg), bix (0.25 mmol, 59.5 mg), and H2O (10 mL) were
placed in a Teflon-lined stainless steel vessel and the mixture
was sealed and heated to 130 °C for 72 h. The reaction system
was cooled to room temperature during 36 h. Green prism crystals
of 1 were obtained, yield: 39 mg, 62% based on NiCl2 ꢀ 6H2O.
Blue prism crystals of 2 were obtained in a similar way as for 1
except that mbix was employed instead of bix and the reaction
interactions, such as hydrogen-bonding,
p p stacking interac-
ꢀ ꢀ ꢀ
tions and metal–metal interactions are also expected to control
the assembly and dimensionality of the resulting structures and di-
rect the overall supramolecular topology [11–17]. The topological
analysis could be a better insight into the supramolecular architec-
tures. Upon topological analysis, complicated metal-organic frame-
works can be reduced to simple equivalent networks [5].
According to our investigation, MOFs with 3-, 4-, or 6-connected
topologies in which d-block metal ions are used as nodes are com-
monly observed [18]. However, the 3D frameworks with mixed
connectivity, such as (3, 6)-, (4, 6)-, and (4, 8)-connected frame-
works, are considered to be difficult to achieve because of their
greater geometric limitations [18]. Our interests are focused on
designing metal-organic frameworks using transition metal ions
to assembly with mixed-ligands of polycarboxylates and flexible
N-donor ligands and carrying out their topological analyses. In
* Corresponding author. Tel./fax: +86 591 83709470.
E-mail address: swdu@ms.fjirsm.ac.cn (S.-W. Du).
1387-7003/$ - see front matter Ó 2009 Elsevier B.V. All rights reserved.
doi:10.1016/j.inoche.2009.04.020

J.-D. Lin et al. / Inorganic Chemistry Communications 12 (2009) 558–562
559
temperature was 160 °C, yield: 29.8 mg, 48% based on NiCl2 ꢀ 6H2O.
Elemental analysis (%) Anal. Calc. For compound 1 (C23H18NiN4O6):
Calcd. C 54.69, H 3.59, N 11.09; Found: C 54.67, H 3.18, N 11.15(%)
and for compound 2 (C60H68Ni3N12O22): Calcd. C 48.52, H 4.61, N
11.32; Found: C 48.91, H 4.58, N 11.36(%). Empirical absorption
corrections were applied to the data using the CrystalClear pro-
gram [29]. The structures of 1 and 2 were solved by the direct
method and refined by the full-matrix least-squares on F2 using
the SHELXTL-97 program [30]. All of the non-hydrogen atoms were
refined anisotropically. The hydrogen atoms of organic ligands in 1
and 2 were treated by geometrical positions or located from differ-
ence Fourier maps. The hydrogen atoms of lattice and coordinated
water molecules in 2 were not added. Crystallographic data and
structural refinements for compounds 1 and 2 are summarized in
Table S1. The selected bond distances and bond angles for com-
pound 1 and 2 are summarized in Table S2 and Table S3, respec-
tively. The IR spectra of 1 (Figure S1) and 2 (Figure S2) show the
characteristic bands of the carboxylic groups in the usual region
at 1457–1343 cmꢁ1 for the symmetric vibrations and at 1630–
1535 cmꢁ1 for the asymmetric vibrations. The presence of the
characteristic band attributed to the protonated carboxylic group
is observed at 1720 cmꢁ1 for 1, indicating the incomplete deproto-
nation of the H3BTC ligand.
Compound 1 crystallizes in the triclinic space group P–1 and the
asymmetric unit contains one Ni ion, one bix and one HBTC2ꢁ an-
ion. There exists one carboxyl group in the HBTC2ꢁ anion. As
shown in Fig. 1a, the Ni1 centre is in a distorted octahedral coordi-
nation geometry, and is coordinated by two nitrogen atoms from
two distinct bix ligands (N1, N4B), two carboxylate oxygen atoms
in a bidentate mode from two symmetry-related HBTC2ꢁ ligands
(O3A, O4C) and one chelating carboxylate oxygen atoms (O5, O6)
from another HBTC2ꢁ ligand. The Ni–O/N bond lengths are all in
the normal range [31]. The bond angles around Ni1 centre range
from 60.71(4) to 169.65(6)°. The two nitrogen atoms occupy the
axial positions of the octahedron with the N1–Ni1–N4B bond angle
of 169.65(6)°, whereas the equatorial plane of the distorted octahe-
dral sphere is defined by the four carboxylate oxygen atoms. The
Ni1 center is approximately coplanar with the mean plane of the
four equatorial oxygen atoms with a deviation of 0.0191 Å Chart 1.
The HBTC2ꢁ ligand employs one chelating carboxylate group
and one bidentate carboxylate group, adopting a (k2)-(k1-k1)-l3
coordination mode (Chart 2a), to connect the Ni ions into a 1D
double chain motif with alternating 8-membered (A) and 16-mem-
bered (B) rings (Figure S3), which are common arrangements found
in other transition metal carboxylate complexes [23,32–35]. The
N
N
N
N
N
N
N
N
(a)
(b)
Chart 1.
Fig. 1. (a) View of the coordination environment of Ni ion in 1. Symmetry codes: (A)
x, y + 1, z; (B) x ꢁ 1, y, z ꢁ 1; (C) ꢁx, ꢁy, ꢁz + 2. Hydrogen atoms belonged to C atoms
are omitted for clarity. (b) View of the 2D topological network in 1. (c) View of the
3D topological network in 1.
Chart 2.

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J.-D. Lin et al. / Inorganic Chemistry Communications 12 (2009) 558–562
distance of the two Ni atoms in A ring is 4.4950(5) Å. The flexible
bix ligands bridge the metal centres of the 1D double chains in
an anti-conformation to build up a 2D layer (Figure S4). The dihe-
dral angle between two terminal imidazole rings is 43.79°. Finally,
these 2D layers are further extended into a 3D supramolecular
architecture with channels through strong hydrogen bonds involv-
ing the uncoordinated carboxylate group and the chelating carbox-
ylate group [O1–H0ꢀ ꢀ ꢀO5#1 2.706 Å; symmetry code: (#1) –x, –y,
1 – z], as illustrated in Figure S5.
From the topological point of view, the 2D layer in 1 is a (3, 5)-
connected net. The long connection is corresponding to a bix
bridge. Each Ni1 atom acts as a five-connected node and every
HBTC2ꢁ ligand functions as a 3-connected node in the ratio 1:1
(Fig. 1b). The 2D net is analyzed by OLEX program [36] and the re-
sult shows that the Schläfli symbol can be described as
(42 ꢀ 6)(42 ꢀ 67 ꢀ 8). The vertex symbol of the 3-connected node
and the five-connected node in this 2D net is 4 ꢀ 4 ꢀ 62 and 4 ꢀ 4 ꢀ 64 ꢀ
*
*
64 ꢀ 63 ꢀ 63 ꢀ 63 ꢀ 63 ꢀ ꢀ , respectively. To the best of our knowledge,
there is only one example of 2D topological net with Schläfli
symbol of (42 ꢀ 6)(42 ꢀ 67 ꢀ 8) reported to date [37]. If the hydro-
gen-bonding interactions are taken into account [38,39], the
HBTC2ꢁ ligand can be regarded as a 4-connected node. Therefore,
the connectivity of 1 can be simplified as a (4, 6)-connected 3D net-
work and the overall Schläfli symbol becomes (44 ꢀ 62)(44 ꢀ 610 ꢀ 8)
(Fig. 1c). The 4-connected node and 6-connected node of this 3D
net can be further specified by the vertex symbol of
Fig. 2. (a) View of the coordination environment of Ni ions in 2. (b) Polyhedral view of the 2D layer structure in 2. The zigzag chain and 24-membered rings are highlighted in
black. Lattice water molecules are omitted for clarity. (c) View of the 2D topological network in 2. Hydrogen atoms are omitted for clarity. Symmetry codes: (A) x, y, z ꢁ 1; (B)
ꢁx ꢁ 1, ꢁy, ꢁz + 1; (C) ꢁx ꢁ 2, ꢁy + 1, ꢁz + 1; (D) ꢁx ꢁ 1, ꢁy, ꢁz + 2; (E) ꢁx ꢁ 2, ꢁy + 1, ꢁz + 2; (F) x, y, z + 1.

J.-D. Lin et al. / Inorganic Chemistry Communications 12 (2009) 558–562
561
4 ꢀ 4 ꢀ 4 ꢀ 4 ꢀ 62 ꢀ 62
and
4 ꢀ 4 ꢀ 4 ꢀ 4 ꢀ 65 ꢀ 65 ꢀ 65 ꢀ 65 ꢀ 65 ꢀ 65 ꢀ 65 ꢀ
there is no significant loss up to about 370 °C. The TGA curve of 2
(Figure S10) reveals a weight loss of 7.31% from room temperature
to 134 °C, which can be assigned to the weight loss of the lattice
water molecules (calcd = 7.27%). The subsequent weight loss of
4.76% from 134 °C to 343 °C can be attributed to the weight loss
of the coordinated water molecules (calcd = 4.85%). And from
343 °C the decomposition of the framework of 2 starts.
The temperature-dependent magnetic susceptibility data of 1
and 2 have been measured for polycrystalline samples at an ap-
plied magnetic field of 10 kOe in the temperature range of 2–
300 K, as shown in Fig. 3. For 1, vm is the molar magnetic suscep-
*
* *
65 ꢀ
ꢀ ꢀ , respectively. According to O’Keeffe, this net with Schläfli
symbol (44 ꢀ 62)(44 ꢀ 610 ꢀ 8) is named as fsc net [36]. The coordina-
tion sequences of this fsc net are 4, 16, 38, 66, 102, 146, 198, 258,
326, 402 for the 4-connected node and 4, 18, 38, 66, 102, 146, 198,
258, 326, 402 for the 6-connected node [40]. As far as we know,
metal-organic frameworks with the fsc topology have been rarely
encountered [41].
The asymmetric unit of 2 contains four crystallographically
independent Ni atoms, two kinds of unique BTC3ꢁ ligands, three
kinds of unique mbix ligands, and four coordinated water mole-
cules and six lattice water molecules. All the Ni atoms are six-coor-
dinated. The Ni1 (or Ni3) is coordinated by three carboxylate
oxygen atoms from two BTC3ꢁ, two nitrogen atoms from two mbix
ligands and a water molecule. In the distorted octahedral sphere of
Ni1 and Ni3 centres, the four oxygen atoms make up of the equa-
torial plane, whereas the two nitrogen atoms locate at the axial
positions of the octahedron. The bond angles of N1–Ni1–N4B and
N9–Ni3–N12D are 178.22(9) and 176.84(9)°, respectively
(Fig. 2a). The Ni1 and Ni3 centres are approximately coplanar with
the mean plane of their four equatorial oxygen atoms with a devi-
ation of 0.0572 and 0.0359 Å, respectively. As for Ni2 and Ni4 cen-
tres, they are both surrounded by one monodentate carboxylate
oxygen atom from BTC3ꢁ, one nitrogen atom from mbix and one
water molecule along with their corresponding symmetry-related
atoms (Fig. 2a). The arrangement sequences of the coordination
atoms around Ni2 and Ni4 are similar to that of Ni1 and Ni3. All
the coordination bonds are in normal distances [31].
tibility for per Ni2 unit (A ring in Figure S3). The experimental vmT
value at room temperature is 2.45 cm3 molꢁ1 K, which is as ex-
pected for two magnetically quasi-isolated S = 1 ions (g > 2.00).
Upon cooling, the
vmT value increases to a maximum of
3.12 cm3 molꢁ1 K at 5.55 K, following with a sharp decrease to
2.56 cm3 molꢁ1 K at 2 K, suggesting an overall ferromagnetic inter-
action. Fitting all data to the Curie–Weiss law gave Weiss constant
of h = 3.42 K (Figure S11), also indicative of ferromagnetic ex-
change interactions. With the consideration of the intermolecular
exchange interaction (ZJ0) in the molecular-field approximation,
the magnetic analysis was carried out by using the spin Hamilto-
nian H ¼ ꢁ2JS1S2 ꢁ DðS2 þ S2Þ, where J is the intramolecular ex-
1
2
change integral, D the zero-field splitting parameter. And the
expression of magnetic susceptibility reported in the literature
can be used to fit the data [42]. A good fit was achieved with the
fitting parameters as follows: g = 2.204, J = +2.70 cmꢁ1
, D =
ꢁ0.035 cmꢁ1, ZJ0 = ꢁ0.133 cmꢁ1 and the agreement factor R =
Each BTC3ꢁ ligand utilizes one chelating carboxylate group and
two monodentate carboxylate groups, adopting a (k2)-(k1)-(k1)-l3
coordination mode (Chart 2b), to join the Ni ions into a 1D ladder
structure (Figure S6). Three crystallographically distinct mbix li-
gands in 2 all adopt a syn-conformation with the dihedral angles be-
tween two terminal imidazole rings varying from 19.465° to
81.161°. The Ni2 and Ni4 along with their corresponding symme-
try-related atoms are linked by a mbix ligand to form an infinite zig-
zag chain with the distance of Ni2ꢀ ꢀ ꢀNi4 being 10.1750(2) Å. The
other two mbix ligands each form a 24-membered ring via coordina-
tion to Ni1 or Ni3 with the distances of Ni1ꢀ ꢀ ꢀNi1B and Ni3ꢀ ꢀ ꢀNi3D
being 8.2014(6) and 7.9828(5) Å, respectively (Figure S7). The zig-
zag chains pass through the centres of the 1D ladders, which are fur-
ther extended by the 24-membered rings to produce a 2D layer
R
[(v
mT)obsd – (
v
mT)calcd]2/
R
(v
mT)o2bsd is 1.28 ꢂ 10ꢁ4
.
structure (Fig. 2b). Close
p p contacts occur between aromatic
ꢀ ꢀ ꢀ
rings of BTC3ꢁ and the phenyl rings of mbix ligands of the adjacent
layers with face-to-face distance of 3.6519 Å (with centroid-to-cen-
troid distance of 3.935 Å). Thus, the structure of 2 can also be de-
scribed as a 3D supramolecular network (Figure S8). If the mbix
ligands are considered as connectors, and both crystallographically
independent BTC3ꢁ ligands are simplified as 3-connected nodes,
there exist two kinds of 3-connected nodes (Ni1/Ni3 and BTC3ꢁ
)
and one kind of 4-connected node (Ni2/Ni4). The topology of this
2D network can be described as a (3, 4)-connected net (Fig. 2c).
Therefore, the Schläfli symbol of this net is (5 ꢀ 62)(52 ꢀ 6)(54 ꢀ 82).
To the best of our knowledge, the 2D metal-organic frameworks
with this topology net are previously unknown.
In comparison, when different symmetries of flexible imidaz-
ole-based ligands (bix and mbix) were employed as coligands to
construct 1 and 2, absolutely distinct frameworks and topologies
are emerged in these metal-organic frameworks. In 1, twofold
deprotonated HBTC2ꢁ ligand employs two carboxylate groups,
adopting a (k2)-(k1-k1)-l3 coordination mode (Chart 2a). Whereas
in 2, the coordination mode of triply deprotonated BTC3ꢁ ligand
is (k2)-(k1)-(k1)-l3 (Chart 2b), utilizing all the three carboxylate
groups.
Fig. 3. Temperature dependence of
The solid lines correspond to the best-fit curves using the parameters described in
the text.
v
mT ( )and vm (s) values for 1 (a) and 2 (b).
r
The thermal stability of 1 and 2 was examined by TGA in a
nitrogen atmosphere. The TGA curve of 1 (Figure S9) indicates that

562
J.-D. Lin et al. / Inorganic Chemistry Communications 12 (2009) 558–562
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(2000) 1095.
perature is 1.11 cm3 molꢁ1 K and increase slowly to a maximum
of 1.16 cm3 molꢁ1 K at 22.5 K. Below 22.5 K, there is a clear de-
crease to approximately 0.53 cm3 molꢁ1 K at 2 K. Although the
structure of 2 is two-dimensional, from the magnetic point of view,
it can be analyzed with a mononuclear model owing to the long
Niꢀ ꢀ ꢀNi distances between the organic linkers. The resulting mag-
netic susceptibility is given by Eq. (1), where h represents all the
weak interactions and the other symbols have their usual mean-
ings. The best fitting gave the parameters g = 2.12, D = 12.67 K,
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R vmT)obsd –
vmT value be-
[(
(v
mT)calcd]2/
R
(v
mT)2obsd is 1.3 ꢂ 10ꢁ4. The decrease of
low 22.5 K might be mainly attributed to the presence of zero-field
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ꢀ
ꢁ
2Ng2b2 2kT=D ꢁ 2kT expðꢁD=kTÞ=D þ expðꢁD=kTÞ
vm
¼
ð1Þ
3kðT ꢁ hÞ
1 þ 2 expðꢁD=kTÞ
In summary, we have successfully obtained two nickel-organic
frameworks using mixed-ligand of H3BTC and bix (or mbix) under
hydrothermal conditions. Compounds 1 and 2 both exhibit a 3D
supramolecular network with a novel topology formed by hydro-
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p p stacking interactions. The differ-
ꢀ ꢀ ꢀ
ent symmetries of bix and mbix play an important role in tuning
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between the Ni2+ ions whereas 2 presents strong zero-field split-
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This work was supported by the Grants from the State Key Lab-
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Structure of Matter, Chinese Academy of Sciences (CAS, SZD08002-
2), National Basic Research Program of China (973 Program,
2007CB815306), the National Natural Science Foundation of China
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Appendix A. Supplementary material
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