1D-3D mixed-ligand frameworks with an unusual dmp topology tuned by intersection angles of isomeric benzenedicarboxylates: Magnetic properties, gas-dependent calcination-thermolysis and energy storage performances

Article abstract of DOI:10.1039/c5dt00963d

In this work, three isomeric benzenedicarboxylates, 1,2-benzenedicarboxylic acid (o-H₂bdc), 1,3-benzenedicarboxylic acid (m-H₂bdc), and 1,4-benzenedicarboxylic acid (p-H₂bdc) have been utilized as the ancillary ligands to perform a systematic study on the structural diversity of mixed-ligand frameworks. The solvothermal reactions of Co(NO₃)₂ with these aromatic acids and the primary ligand 4,4′-bis(imidazolyl)biphenyl (bibp) afford three novel coordination polymers, {[Co₆(bibp)₃(o-bdc)₆(H₂O)](CH₃CN)_1.5}_∞ (1), [Co(bibp)(m-bdc)]_∞ (2), and [Co(bibp)(p-bdc)]_∞ (3). Owing to the different orientations of the carboxylate groups, the benzenedicarboxylates adopt various bridging modes to connect the Co^II ions into a series of 1D carboxylate∩cobalt architectures based on the 1D chain, binuclear and single-ion magnetic units, respectively. These 1D architectures are further decorated by the bibp ligand to afford a 1D belt for 1, 2D double-bridging (4,4) sheet for 2, and an unusual 3D dmp framework for 3. Significantly in 3, three equivalent frameworks are interlocked with each other to represent an unprecedented three-fold interpenetrating dmp network. The structural diversity indicates that the benzenedicarboxylate plays an essential role in the assembly of mixed-ligand frameworks, and the orientation of the carboxylate group exerts an important influence on the nucleation, dimensionality and also interpenetration. Furthermore, the magnetic properties of 1 and 2 have been studied by fitting the experimental data as possible, and the magneto-structural correlation of 2 has also been well discussed. Importantly, CoO and Co₃O₄ were obtained from the controllable thermolysis of crystals of 1 via simple calcination treatment under different gas environments. The as-synthesized cobalt oxides display good crystallinity and appear as micro- or nanoparticles, which can be applied as supercapacitor electrodes as demonstrated by their energy storage performance in 2 M KOH electrolyte.

Full text of DOI:10.1039/c5dt00963d

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Page 1 of 35
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DOI: 10.1039/C5DT00963D
1Dꢀ3D MixedꢀLigand Frameworks with an Unusual dmp Topology Tuned
by Intersection Angles of Isomeric Benzenedicarboxylates: Magnetic
Properties, GasꢀDependent Calcinationꢀthermolysis and Energy Storage
Performances
ZuoꢀXi Li,*[a, b] Gan Ye,[a] Juan Han,[a] Ying Yang,[a] KangꢀYu Zou,[a] Xin Wang,[a]
XiaoꢀLing Wang[b] and XiaoꢀFeng Gou[a]
aKey Laboratory of Synthetic and Natural Functional Molecule Chemistry (Ministry of
Education), Shaanxi Key Laboratory of PhysicoꢀInorganic Chemistry, College of Chemistry
and Material Sciences, Northwest University, Xi’an 710069, P.R.China;
bShannxi Key Laboratory of Phytochemistry, Baoji 721016, P.R.China.
*Corresponding author. Eꢀmail: Lizx@nwu.edu.cn
To Dalton Transactions (Article)
1

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DOI: 10.1039/C5DT00963D
ABSTRACT
In this work, three isomeric benzenedicarboxylates, 1,2ꢀbenzenedicarboxylic acid (oꢀH2bdc),
1,3ꢀbenzenedicarboxylic acid (mꢀH2bdc), and 1,4ꢀbenzenedicarboxylic acid (pꢀH2bdc), have been utilized
as the ancillary ligands to perform a systematic study on the structure diversity of mixedꢀligand frameworks.
The solvothermal reactions of Co(NO3)2 with these aromatic acids and the primary ligand
4,4'ꢀbis(imidazolyl)biphenyl
(bibp)
afford
three
novel
coordination
polymers,
{[Co6(bibp)3(oꢀbdc)6(H2O)](CH3CN)1.5}∞ (1), [Co(bibp)(mꢀbdc)]∞ (2), and [Co(bibp)(pꢀbdc)]∞ (3).
Owing to different orientations of carboxylate groups, the benzenedicarboxylates adopt various bridging
modes to connect CoII ions into a series of 1D carboxylate∩cobalt architectures based on the 1D chain,
binuclear and singleꢀion magnetic units, respectively. These 1D architectures are further decorated by the
bibp ligand to afford a 1D belt for 1, 2D doubleꢀbridging (4,4) sheet for 2, and 3D unusual dmp framework
for 3. Significantly in 3, three equivalent frameworks are interlocked with each other to represent an
unprecedented threeꢀfold interpenetrating dmp network. The structural diversity indicates that the
benzenedicarboxylate plays an essential role in the assembly of mixedꢀligand frameworks, and the
orientation of carboxylate group exerts an important influence on the nucleation, dimensionality and also
interpenetration. Furthermore, magnetic properties of 1 and 2 have been studied by fitting the experimental
data as possible, and the magnetoꢀstructural correlation of 2 has been also well discussed. Importantly, CoO
and Co3O4 were obtained from the controllable thermolysis of crystals of 1 via the simple calcination
treatment under different gas environments. The asꢀsynthesized cobalt oxides display good crystalline and
appear as microꢀ or nanoparticles, which can be applied as supercapacitor electrodes through the energy
storage performance in 2 M KOH electrolyte.
2

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Introduction
Coordination polymers (CPs) are assembled from the metal ion (or cluster) and organic ligand, where the
ligand acts as the “strut” bridging the metal unit (‘joint’) in turn to afford a multiꢀdimensional completely
ordered architecture.1 The intriguing topological feature as well as the potential candidate for the functional
material makes the design and synthesis of CPs attract a great deal of attention.2 Since the
physicoꢀchemical property is extremely dependant on the crystal structure, it is very important to control
over the topology framework and rationally construct CPs with predicted structures. According to the
definition of CPs, the topological structure seems most related to the ligand backbone, and the effect of
organic ligand is an indispensable and everlasting research topic.3
The rapid development of CPs has been experiencing great success in creating a number of molecular
magnets, which exhibit slow magnetic relaxation and quantum tunneling.4 Another major target in
molecular magnetism is the construction of novel magnetic unit, and then the exploration of
magnetoꢀstructural correlation by theoretical simulation of magnetic coupling.5 However, it is still a
challenging scientific endeavor to quantitatively analyze the strength of magnetic exchange interaction,
partly owing to the lack of reliable strict models for some complicated systems. On the other hand, the
large uniaxial magnetic anisotropy, for example, in the octahedral CoII ion, leads to significant spinꢀorbital
coupling effect and unquenched orbital angular momentum, undoubtedly making the magnetic fitting even
more difficult.6 Excitedly, the enhanced magnetic anisotropy could increase the blocking temperature Tb
2
2
according the equation U = ST |D| or (ST –1/4)|D|.7 Thus the CoII system with large anisotropic feature is
very interesting.
CPs have emerged as a class of functional materials with highly ordered architectures, large specific
surface and regular shapes. Solidꢀstate thermolysis of CPs as precursors may provide a very promising
opportunity to generate metal oxide nanomaterials with ultrahigh porosity.8 The calcination strategy is
simple, inexpensive, controllable and scalable. Despite the progress achieved to date, research on the facile
synthesis of nanostructure inorganic materials derived from CPs is still in its early stage.9 Thus it is highly
desirable to develop novel synthesis protocols and concepts utilizing CPs to prepare highꢀquality functional
nanomaterials with unique morphologies.
In this paper, 4,4′ꢀbis(imidazolꢀ1ꢀyl)ꢀbiphenyl (bibp) is chosen as the primary ligand, where a series of
benzenedicarboxylates, 1,2ꢀbenzenedicarboxylic acid (oꢀH2bdc), 1,3ꢀbenzenedicarboxylic acid (mꢀH2bdc),
and 1,4ꢀbenzenedicarboxylic acid (pꢀH2bdc), are utilized as ancillary ligands to react with Co(NO3)2 under
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the solvothermal environment. Finally, three novel CPs with mixedꢀligand frameworks,
{[Co6(bibp)3(oꢀbdc)6(H2O)](CH3CN)1.5}∞ (1), [Co(bibp)(mꢀbdc)]∞ (2) and [Co(bibp)(pꢀbdc)]∞ (3), have
been obtained and characterized. The relationship between the topological structure and benzoate backbone
was discussed, and magnetic properties of 1 and 2 have been well investigated. Significantly, CoO and
Co3O4 particles were obtained from the gasꢀcontrollable calcinationꢀthermolysis of crystals of 1, which
have been employed as electrode materials for supercapacitors.
Experimental Section
Materials and General Methods. All the solvents and reagents were commercially available and used
as received. The ligand bibp was synthesized according to the reported procedure.10 Elemental analyses of
C, H, and N were performed on a PerkinꢀElmer 240C analyzer. IR spectra were measured on a TENSOR 27
(Bruker) FTꢀIR spectrometer with KBr pellets. The thermogravimetry (TG) experiment was carried out on
a Rigaku standard TG analyzer under N2 atmosphere with a heating rate of 10 °C·minꢀ1 in the temperature
range of 15ꢀ600 °C. The powder Xꢀray diffraction (XRD) was recorded on a Rigaku D/Maxꢀ2500
diffractometer at 40 kV, 100 mA for a Cuꢀtarget tube and a graphite monochromater. Magnetic data were
collected on a Quantum Design MPMSꢀXL SQUID magnetometer equipped with a 5T magnet using the
crushed crystal sample. The diamagnetic corrections were calculated using Pascal's constants. The
morphologies and structures of CoO and Co3O4 samples were observed by scanning electron microscopy
(SEM, SHIMADZU, SSXꢀ550) and field emission scanning electron microscopy (FEꢀSEM, JEOL,
JSMꢀ6700F), respectively.
Syntheses of CPs 1ꢀ3.
Single crystals of 1ꢀ3 suitable for Xꢀray analysis were all obtained by the solvothermal method.
{[Co6(bibp)3(oꢀbdc)6(H2O)](CH3CN)1.5}∞ (1). The suspension of Co(NO3)2·6H2O (0.2 mmol, 58 mg),
bibp (0.1 mmol, 29 mg) and oꢀH2bdc (0.2 mmol, 33 mg) in 14 mL component solvent (CH3CN:H2O = 4:3)
was sealed in a Teflonꢀlined autoclave and heated to 125 °C for 3 days. After the autoclave was cooled to
room temperature at 5 °C·hꢀ1, purple needle crystals with suitable diameters for single crystal Xꢀray
crystallographic analysis were obtained. The mother liquor was decanted, and crystals were rinsed three
times with acetonitrile (8 mL × 3) and dried in air for 4 h. yield: ~70% (handꢀpicking crystals, based on
bibp). Anal. Calcd. for C105H72.5Co6N13.5O25: C, 55.39; H, 3.21; N, 8.30%. Found: C, 55.15; H, 3.28; N.
8.41%. IR (KBr, cmꢀ1): 3553w, 3413s, 3123s, 2361m, 1556s, 1520s, 1393s, 1310s, 1262m, 1126m, 1066s,
963m, 818s, 754s, 652m, 462w (Figure S1ꢀa).
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[Co(bibp)(mꢀbdc)]∞ (2). A mixture of Co(NO3)2·6H2O (0.2 mmol, 58 mg), bibp (0.1 mmol, 29 mg) and
mꢀH2bdc (0.2 mmol, 33 mg) was suspended in 14 mL component solvent (C2H5OH:H2O = 4:3). After
addition of 0.1 mL ammonia water, the suspension was heated in a Teflonꢀlined autoclave (25 mL) at
185 °C for 3 days. After the autoclave was cooled to room temperature at 5 °C·hꢀ1, pink block crystals with
a small amount of powder were obtained. The mother liquor was decanted, and crystals were rinsed three
times with ethanol (8 mL × 3) and dried in air for 4 h. yield: ~20% (handꢀpicking crystals, based on bibp).
Anal. Calcd. for C26H18CoN4O4: C, 61.31; H, 3.56; N, 11.00%. Found: C, 61.04; H, 3.62; N. 11.14%. IR
(KBr, cmꢀ1): 3706m, 3412w, 3107s, 3067s, 2361s,1624s, 1602s, 1580s, 1515s, 1453s, 1391s, 1242s, 1125m,
1062m, 979w, 961w, 915m, 755s, 720s, 523w, 414w (Figure S1ꢀb).
[Co(bibp)(pꢀbdc)]∞ (3). The suspension of Co(NO3)2·6H2O (0.2 mmol, 58 mg), bibp (0.1 mmol, 29 mg)
and pꢀH2bdc (0.2 mmol, 33 mg) in 14 mL component solvent (C2H5OH:H2O = 4:3) was sealed in a
Teflonꢀlined autoclave and heated to 165 °C for 3 days. After the autoclave was cooled to room
temperature at 5 °C·hꢀ1, purple spindle crystals with quite an amount of powder were obtained. The mother
liquor was decanted, and crystals were rinsed three times with ethanol (8 mL × 3) and dried in air for 4 h.
yield: ~15% (handꢀpicking crystals, based on bibp). Anal. Calcd. for C26H18CoN4O4: C, 61.31; H, 3.56; N,
11.00%. Found: C, 61.05; H, 3.61; N. 11.13%. IR (KBr, cmꢀ1): 3691m, 3412w, 3129s, 2361s, 2340s,
1630m, 1593s, 1515s, 1390s, 1309s, 1239s, 1125s, 1007m, 961m, 829s, 755s, 654w, 559m, 524w. (Figure
S1ꢀc).
Syntheses of CoO and Co3O4 Particles.
During the synthesis of 1, the product appears as highꢀyield needle crystals with lengths of about 0.5 cm
in the maximum. Thus 1 is easy to be collected in abundance, and suitable as a precursor to prepare cobalt
oxides through the calcination treatment.
For the preparation of CoO particles, the asꢀsynthesized crystals were placed in a tube furnace and heated
under N2 gas flow at 400 °C for 3 h with a heating rate of 10 °C·hꢀ1. Then the tube furnace was naturally
cooled to room temperature. Black powder was obtained, which was placed in a furnace at 900 °C for 1 h
in the air with a heating rate of 5 °C·minꢀ1 to remove the carbon residue. After cooled under N2 gas, gray
CoO particles were gained. XRD peaks (2θ): 36.662, 42.571, 61.643, 73.798, and 77.652°.
For the Co3O4 particles, the asꢀsynthesized crystals were placed in a furnace and heated at 400 °C for 3 h
with a heating rate of 10 °C·hꢀ1 when both ends opening to the air. Then the furnace was naturally cooled to
room temperature, and black Co3O4 powder was obtained. XRD peaks (2θ): 19.091, 31.365, 36.938, 38.697,
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44.883, 55.497, 59.469, and 65.280°.
Xꢀray Data Collection and Structure Determinations. Xꢀray singleꢀcrystal diffraction data of 1ꢀ3 were
collected on a Rigaku MMꢀ007/Saturn 70 with graphite monochromatic MoꢀKα radiation ( = 0.71073 Å).
λ
The program SAINT11 was used for integration of the diffraction profiles. All the structures were solved by
direct methods using the SHELXS program of the SHELXTL package and refined by fullꢀmatrix
leastꢀsquares methods with SHELXL.12 Metal atoms in each complex were located from the Eꢀmaps and
other nonꢀhydrogen atoms excluded in counterions were located in successive difference Fourier syntheses
and refined with anisotropic thermal parameters on F2. The hydrogen atoms of organic ligands were
generated theoretically onto the specific atoms and refined isotropically. However, hydrogen atoms of water
molecules in 1 could not be theoretically generated, and they also have not been added by difference
Fourier maps due to the lack of large enough Q peaks around the water oxygen atom. The free acetonitrile
molecule in 1 is highly disordered, and the contribution of solvent electron density was removed by the
SQUEEZE routine in PLATON.13 Further details for structural analysis are summarized in Table 1, and
selected bond lengths and angles for 1ꢀ3 are listed in Table S1.
(Insert Table 1)
Electrochemical Performance Measurements. All electrochemical measurements were performed
using a CHI660D potentiostat (Chenhua Instrument Co. Ltd., Shanghai, China) coupled with a
threeꢀelectrode system at room temperature. CoO and Co3O4 particles were coated in a nickel foam (size: 1
cm2, thickness: 2 mm) as the work electrode, respectively. The detailed electrode fabrication was descripted
as blow. Ag/AgCl (saturated KCl) electrode and platinum sheet were used as the reference and counter
electrode, respectively. A 2 M KOH solution was used as the electrolyte. Cyclic voltammograms (CV) were
measured within the potential range from 0.0 to 0.5 V at scan rates of 5, 10, 20, 50 and 100 mV sꢀ1,
respectively. The galvanostatic chargeꢀdischarge test was carried out within a potential window from 0.10
to 0.45 V at different current densities of 0.5, 1.0, 2.0, 3.0, 4.0 and 5.0 A gꢀ1, respectively. The longꢀterm
cycle stability of cobalt oxide electrode was evaluated by the galvanostatic chargeꢀdischarge measurement
at the current density of 1 A·gꢀ1 in 2 M KOH aqueous solution.
Fabrication of Electrodes. Typically, 8 mg CoO (or Co3O4) powder, 1 mg acetylene black as the
conducting agent and 50 ml polyvinylidone fluoride (PVDF) as a binder were homogeneously mixed with a
few drops of ethanol in an agate mortar until a homogeneous black slurry was obtained. The resulting
slurry was brushed into the nickel foam. The coated nickel foam was dried and pressed under a pressure of
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10 MPa.
Results and Discussion
Synthesis Consideration. In this work, three isomeric benzenedicarboxylates, oꢀbdc, mꢀbdc, and pꢀbdc,
were employed as ancillary ligands to perform a systematic study on the structure diversity of CPs based on
the cobalt–bibp matrix. Evidently, three carboxylic ligands all possess two carboxylate groups, but exhibit
various distributing positions on the phenyl ring. The rigid ligand bibp was chosen as the primary ligand
owing to the fixed bidentate bridging mode, which makes it possible to focus on the effect of aromatic acid.
Moreover, the linear bibp ligand usually leads to high interpenetration because of the long spacer.14 With
the concept of mixedꢀligand frameworks in mind, we employed three above benzenedicarboxylates as
ancillary ligands to construct CPs 1ꢀ3 with different dimensional architectures.
CPs 1ꢀ3 were all synthesized from the solvothermal methods. It is worthy to note that the reaction
conditions have great differences, especially the solvent agents and temperatures. 1 was prepared under a
CH3CNꢀH2O system at 125 °C. Under the similar conditions but using mꢀH2bdc to replace oꢀH2bdc, only a
muddy substance was obtained. So we continued to change the organic solvent at the fixed ratio of 4:3 to
water, and the temperature was kept rising at the step of 10 °C. However, when the temperature up to
185 °C, there is still no suitable crystals for single crystal Xꢀray diffraction by employing a wide range of
organic solvents, such as CH3OH, C2H5OH and (CH3)2NCHO. Fortunately, when a little ammonia water
was added into the C2H5OHꢀH2O system at 185 °C, high quality block crystals of 2 were isolated. 3 was
also successfully synthesized through plenty of tentative experiments, which was well crystallized in the
C2H5OHꢀH2O system at 165 °C. The above statement shows how a variety of synthetic strategies for the
generation of a series of CPs, and could be adapted to each particular reaction system.
For the synthesis of cobalt oxide particles, a facile and controllable approach was employed in our
experiment. Firstly, CoO particles were prepared by the simple thermolysis of crystals of 1 under N2 gas
flow via a calcination treatment. However, when the air including O2 gas was introduced into the
calcination system, another cobalt oxide Co3O4 was successfully prepared. The result shows that the gas
environment plays an important role in the formation of oxide particles.
Crystal Structure of 1. CP 1 crystallizes in the monoclinic space group P21/c with six
crystallographically independent CoII ions in the asymmetric unit (Figure 1). There are two coordination
symmetries for the metal center, i.e. the tetrahedron and octahedron. The Co1, Co3 and Co5 ions lie in a
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tetrahedral coordination environment, which are coordinated by three carboxylic O atoms of distinct
oꢀbdc2ꢀ ions and one bibp N atom. However, the Co2, Co4 and Co6 ions lie in an octahedral coordination
environment, which are coordinated by five carboxylic O atoms from three oꢀbdc2ꢀ ions (Particularly for
the Co6 ion, one oꢀbdc2ꢀ O atom is displaced by one water molecule) and one bibp N atom. The
coordination bond lengths that involve the metal center widely range from 1.933(4) to 2.193(4) Å for the
Co–O bonds, whereas a narrow range [2.000(4)ꢀ2.088(4) Å] for the Co–N bonds (Table S1), which are all
comparable to those typically observed values.15 The coordination angles of tetrahedral or octahedral node
span a wide range ([94.64(16)ꢀ130.10(16)º] and [60.00(12)ꢀ177.74(14)º], respectively), which illustrates
that the CoII ion has a distorted tetrahedral or octahedral coordination geometry.
(Insert Figure 1)
The carboxylate group adopts four different coordination modes, i.e. the synꢀsyn, synꢀanti, bidentate
chelate and monatomic modes. Then the oꢀbdc2ꢀ ion takes three bridging patterns ((κ1ꢀκ0)ꢀ(κ1ꢀκ1)ꢀꢁ2,
(κ2)ꢀ(κ1ꢀκ1)ꢀꢁ3 and (κ1ꢀκ1)ꢀ(κ1ꢀκ1)ꢀꢁ3) to connect the cobalt ion into a 1D chain (Figure 2a).16 Owing to
various bridging modes of oꢀbdc2ꢀ ion, there are five kinds of magnetic coupling pathways mediated by the
carboxylate group, with the Co–Co distances being about 3.81, 3.96, 4.40, 4.46 and 4.90 Å, respectively.
Each bibp ligand, adopting a bidentate transꢀcoordinated conformation, connects two metal centers from
diverse carboxylate∩cobalt chains. As a result, two neighboring 1D chains are bridged by the bibp ligand
to give a 1D belt (Figure 2b), which are further packed together to afford a 3D supermolecular architecture
(Figure S2). The remaining space within this 3D supermolecular framework is occupied by the disordered
CH3CN molecule.
(Insert Figure 2)
Crystal Structure of 2. Singleꢀcrystal Xꢀray diffraction analysis reveals that 2 crystallizes in the triclinic
space group Pꢀ1. The asymmetric unit consists of one crystallographically independent CoII ion, one
mꢀbdc2ꢀ ion and one bibp ligand. As shown in Figure 3a, the CoII ion lies in a distorted octahedral
coordination environment, which is surrounded by two monodentate carboxylic groups from distinct
mꢀbdc2ꢀ ions, one bidentate chelate carboxylic group, and two imidazole rings of individual bibp ligands.
The coordination bond lengths span from 2.0342(15) to 2.1973(16) Å for Co–O bonds, whereas 2.1405(18)
and 2.1462(17) Å for Co–N bonds, which are all comparable to the above corresponding values.
(Insert Figure 3)
The fullyꢀdeprotonated mꢀbdc2ꢀ ion bridges the CoII ion to form a 1D doubleꢀtrack chain by employing
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two carboxylic groups with different coordination modes, i.e. the bidentate chelate and synꢀanti modes
(Figure 3b). Obviously, the 1D chain contains a binuclear cluster, which is bridged by two synꢀanti
carboxylate groups. It is worth mentioning that the Co1–O1–C25–O2–Co1# (symmetry mode: ꢀx+1, ꢀy+1,
ꢀz+1) skeleton appreciably deviates from planarity [[dihedral angle between the planes Co1–O1–C25 and
Co1#–O2–C25: 82.82(28)°], which plays a relevant role in the magnetic behavior. The bidentate ligand
bibp, adopting a transꢀcoordinated conformation, connects two nodes from two neighboring 1D chains. As
a result, the 1D chain is bridged by the bibp ligand to form a 2D extended layer (Figure 4). If the binuclear
unit was considered as a node, and both mꢀbdc2ꢀ and bibp ligands were viewed as linear linkers, then the
topological simplification affords a 2D doubleꢀbridging (4,4) lattice.
(Insert Figure 4)
Crystal Structure of 3. CP 3 crystallizes in the orthorhombic space group Pnna with one
crystallographically independent CoII ion located on the inversion center. As shown in Figure 5a, the CoII
ion holds the CoO2N2 distorted tetrahedral coordination geometry, which is provided by two pꢀbdc2ꢀ
oxygen atoms, and two bibp nitrogen atoms. The CoꢀO/N bond lengths are 1.9781(15) and 2.0355(19) Å
respectively, in accord with the above reported values.
(Insert Figure 5)
The pꢀbdc2ꢀ ion connects the metal ion into a 1D Wꢀtype chain with two monodentate carboxylate
groups. The 1D chain is further connected by the cisꢀcoordinated bibp ligand to form a 3D framework in a
crissꢀcross mode (Figure 5b & S3). The topological method was applied to explore the nature of this
fascinating structure. The pꢀbdc2ꢀ and bibp ligands were viewed as linear linkers, and thus the CoII ion
could be simplified as a 4ꢀconnected node. Topological analysis performed on TOPOS Software gives a
uninodal 4ꢀconnected network with the point symbol of 65·8 topology and long vertex symbol of
[6·6·6·6·62·83],17 which is topologically identical to the unusual dmp net (Figure 6). Obviously, the dmp
net is different from other 4ꢀconnected types: the wellꢀknown dia (66),18 NbO (64·82),19 PtS (42·84)20 and so
on. It is worth noting that the present dmp net is closely related to the cds net.21 Both of them have the same
Schläfli symbol of 65·8, but with different vertex symbols (6·6·6·6·62·83 for the dmp net, and 6·6·6·6·62·82
for the cds net). Notably, the cds net is based on the square node, and two neighboring layers are orthogonal.
However, the dmp net is based on the tetrahedral node, which is a little similar to the 4ꢀconnected node of
dia net, but the 8ꢀmembered circuit exists in the dmp net (being absent for the dia net). To the best of our
knowledge, 3 shows the first example of dmp topology exclusively based upon the coordination bonds.
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(Insert Figure 6)
Because of large scale edges provided by the bulky building blocks pꢀbdc2ꢀ and bibp, the dmp
framework illustrates a quadrangular chiral channel with the dimension of approximately 11.0 × 17.2 Ǻ2
along the c axis (Figure S4). However, these 1D channels are packed in an antiparallel manner with
different chirality to cause an achiral CP, which is also indicated by the centrosymmetric space group Pnna.
The potential void space provided by these 1D channels is destroyed by the interpenetration of three
independent equivalent frameworks interlocked with each other. Thus, as we are aware, 3 represents an
unprecedented threeꢀfold interpenetrating dmp network (Figure 7).
(Insert Figure 7)
Effects of Orientation of Carboxylate Groups. CPs 1ꢀ3, constructed from three structurally related
aromatic dicarboxylic acids and the bibp ligand, illustrate 1D, 2D and 3D architectures based on
heteroleptic CoII nodes, respectively. In these mixedꢀligand structures, the bibp ligand shows an invariable
bidentate bridging mode, which facilitates us to perform a systematic study on the effect of benzoate.
Interestingly, all three aromatic acids possess two carboxylate groups, which are deprotonated to participate
in the construction of CPs. Thus, the title research could be focused on the orientation of carboxylate group,
which is close related to the shape of aromatic acid. To elucidate the effect of benzenedicarboxylate on the
topological structure, we assume the parameter φ as the angle between two carboxylate vectors.
The oꢀbdc2ꢀ ion with φ = 60º is employed in 1. The CoII ion illustrates two coordination symmetries
(tetrahedron and octahedron), and the oꢀbdc2ꢀ ion adopts three bridging patterns ((κ1)ꢀ(κ1ꢀκ1)ꢀꢁ2,
(κ2)ꢀ(κ1ꢀκ1)ꢀꢁ3 and (κ1ꢀκ1)ꢀ(κ1ꢀκ1)ꢀꢁ3) via two oꢀCOOꢀ groups. In such a way, the CoII ion and oꢀbdc2ꢀ
ligands are assembled into a 1D carboxylate∩cobalt chain with short distances in the range of 3.81ꢀ4.90 Å
for two neighboring CoII centers (Chart 1). Furthermore, only one free coordination site is available for
each metal ion in the 1D chain. The bibp ligand occupies the free site with a terminal N atom, and thus
each bidentate bibp ligand bridegs two carboxylate∩cobalt chains to form a 1D belt. In 2, the aromatic acid
have two mꢀCOOꢀ groups with φ = 120º, which adopts a bidentate chelate and synꢀanti mode, respectively.
The octahedral CoII ion is bridged by the mꢀbdc2ꢀ ion to form a 1D doubleꢀtrack chain. The
carboxylate∩cobalt chain contains a binuclear cluster with the Co···Co distance of 4.34 Å, while other
intrachain neighboring metal centers are 7.09 or 10.05 Å apart. Moreover, two free coordination sites of
each CoII ion are available for the bibp ligand, and therefore the 1D chain is extended by the bidentate
ligand into a 2D doubleꢀbridging (4,4) lattice based on the binuclear unit. Referring to 3, four coordination
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sites of the tetrahedral CoII node are shared by the pꢀbdc2ꢀ and bibp ligands. The aromatic acid possesses
two pꢀCOOꢀ groups with φ = 180º, which use the monatomic mode to connect the CoII ion into a 1D
Wꢀtype chain [Co(pꢀbdc2ꢀ)]∞ with the shortest Co···Co distance of 10.98 Å. Meanwhile, another 1D chain
[Co(bibp)]∞ is also formed. Then the assembly of above two kinds of 1D chains affords an unusual dmp
net. Further inspection shows that the dmp framework presents a quadrangular channel, which is large
enough to allow such three equivalent frameworks entangled with each other to form an unprecedented
threeꢀfold interpenetrating dmp network.
(Insert Chart 1)
The results indicate that the orientation of carboxylate group plays an important role in the assembly of
mixedꢀligand frameworks. The angle φ directly determines the distance of two carboxylate groups. The
smaller of φ value, two COOꢀ groups get closer, which is contributed to the formation of polynuclear units.
In 1, two COOꢀ groups of oꢀbdc2ꢀ ion are neck to neck, and the angle φ = 60º is the smallest among those of
three aromatic acids. Two COOꢀ groups could chelate one CoII ion to yield a sevenꢀatom ring, which is
impossible for other two benzenedicarboxylates. As a result, the assembly of oꢀbdc2ꢀ and CoII ions gives a
1D magnetic chain, which could be considered as an infiniteꢀnuclear unit. For 2, two COOꢀ groups of
mꢀbdc2ꢀ ion intersect with the angle of 120º, and the 1D carboxylate∩cobalt chain contains a magnetic
binuclear cluster. The pꢀbdc2ꢀ ion has the angle of 180º, and two carboxylate groups exhibit the longest
separation, which leads to the isolated CoII ion in the carboxylate∩cobalt chain. Evidently, the rodꢀlike
building block pꢀbdc2ꢀ serves as the longest CoꢁꢁꢁCo edge comparing to oꢀbdc2ꢀ and mꢀbdc2ꢀ, and thereby
the dmp framework in 3 presents a 1D quadrangular channel. The potential void space is large enough for
three equivalent dmp frameworks interlocked with each other. Therefore, the φ angle with a big value is apt
to cause interpenetration.
Thermogravimetric Analysis. The thermal stability of CPs 1ꢀ3 has been determined by the TG analysis
(Figure S5). The TG plot of 1 illustrates two steps of weight loss. In the first region of 71ꢀ197 ºC, it lost all
solvent molecules (the free acetonitrile and coordinated water molecules: 3.6% obsd; 3.5% calcd). Upon
further heating, the material lost weight continuously. From 338ꢀ476 ºC, the TG trace showed a sharp
decline to give a weight decrease of 75.4 %, corresponding to the complete loss of organic ligand (76.7%
calcd). The final mass remnant of 21.0% was consistent with decomposition of CoO (19.8% predicted).
Comparing to 1, CPs 2 and 3 contain no solvent molecules, and show only one step of weight loss. The
mass of 2 remained largely unchanged until the decomposition onset temperature of 316 °C. All removal of
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organic ligand was completed at 492 °C, indicated by a further weight loss of 86.4% (85.3% calcd). The
final mass remnant of 13.6% was very close to that predicted for CoO (14.7%). The thermal stability of 3
was slightly greater than 2, with an onset temperature for degradation of 323 °C. The quick weight loss of
86.2% was complete at 479 °C, corresponding to the full decomposition of 3. The final mass remnant of
13.8% was indicative of CoO (14.7% calcd).
Magnetic Properties. In order to confirm the phase purity of bulk material, XRD experiments have been
carried out. The XRD experimental and computerꢀsimulated patterns are shown in Figure S6. The
experimental patterns of 1ꢀ3 are in good agreement with the corresponding simulated ones, indicating that
asꢀgrown crystals are homogeneous. Then the variable temperature magnetic susceptibility data (χ) of 1 and
2 were measured on the crushed crystals under 1 kOe in the range of 2ꢀ300 K.
The molar directꢀcurrent magnetic susceptibilities of 1 are shown in Figure 8a. At room temperature, the
χmT value per formula is 17.18 cm3·K·molꢀ1, significantly higher than the spinꢀonly value expected for six
isolated CoII ions with S = 3/2 (11.25 cm3·K·molꢀ1; g = 2.0). It gives a realistic gCo value of 2.47 if we
assume that the ions are uncoupled, which is attributed to the significant contribution from the unquenched
4
orbital momentum in the octahedral field (the T1g state). Upon temperature cooling, the χmT curve
monotonously decreases to a value of 5.04 cm3·K·molꢀ1 at 2.0 K, which infers dominant antiferromagnetic
interactions between CoII ions; meanwhile, spinꢀorbital coupling effects may also influence the overall
profile.22 The data above 50 K follow the CurieꢀWeiss law with C = 18.55 cm3·K·molꢀ1 and θ = ꢀ24.67 K.
The large C value compared with the spinꢀonly value indicates significant contribution from orbital
momentum, and the medium negative θ value can be indicative of an overall antiferromagnetic coupling.
According to the structure of 1, the system can magnetically be treated as a 1D carboxylate∩cobalt chain
in which magnetic coupling is mediated through the carboxylate group. The interꢀchain magnetic
interaction through the rodꢀlike ligand bibp is so weak that the coupling can be negligible. In the 1D chain,
there are five kinds of magnetic coupling pathways with different Co···Co distances mediated by the
carboxylate groups. Consequently, at least five J parameters are needed to fully mode the magnetic
behavior. Besides, several parameters, such as λ (spinꢀorbital coupling parameter), α (orbital reduction
factor) and ꢂ (axial distortion factor), are needed to account for the influences of singleꢀsite factors on the
octahedral CoII ion.23 More terribly, the title 1D chain contains two different CoII ions, one octahedron and
the other tetrahedron. For the tetrahedral CoII ion, the orbitally nonꢀdegenerate 4A2 ground state experiences
no firstꢀorder spinꢀorbital coupling. Instead, a true spin of 3/2 with g significantly deviating from 2.0 should
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be considered.24 Thus, there are no reliable models to quantitatively analyze the strength of magnetic
exchange interactions in the complicated 1D system.
(Insert Figure 8)
The magnetic susceptibility data of 2 are shown in Figures 8b in the forms of χM and χMT vs T plot. The
value of χMT at room temperature (6.13 cm3·K·molꢀ1) is larger than the expected value for two isolated CoII
ions with S = 3/2 (3.75 cm3·K·molꢀ1; g = 2.0). It gives a realistic gCo value of 2.56 if we assume that the
ions are uncoupled. As the temperature is lowered, the χMT value continues to decrease slowly, and reaches
a minimum value of 4.59 cm3·K·molꢀ1 at 12 K, after which a slight rise is observed. The variation of χMꢀ1 vs
T obeys the CurieꢀWeiss law in a rather high temperature range (300ꢀ60 K), giving a negative Weiss
constant θ = ꢀ8.74 K and Curie constant C = 6.14 emu·K·molꢀ1. The C value is as expected for the
binuclear cluster.25 It must be noted that the small negative θ constant is not necessarily indicative of
antiferromagnetic interactions because the thermal magnetic behavior is strongly influenced by the
4
spinꢀorbital coupling intrinsic to an octahedral CoII ion. The spinꢀorbital coupling splits the T1g ground
state into three Kramers levels: one sextet, quartet and ground doublet, respectively. Cooling the
depopulation of above levels could cause a significant decrease for the χMT variation, which is associated
with a negative θ constant (up to 20 K).26 Therefore, the small θ constant may imply weak ferromagnetic
coupling between metal ions, which is overcompensated by the effect of spinꢀorbital coupling above 12 K.
The ferromagnetic coupling is also supported by the final increase at low temperatures (< 12 K).
According to the structure of 2, the magnetic behavior is attributed to the binuclear cobalt cluster
wellꢀseparated by long organic ligands, in which the magnetic coupling is mediated through two
synꢀanti−COO‾ bridges. Consequently, to estimate the strength of ferromagnetic interactions (J) within the
binuclear unit, an empirical approach proposed recently by Lloret et al was used to fit the experimental
data.27 In this approach, the CoII ion is assumed to have an effective spin of Seff = 1/2, which corresponds to
the Kramers doublet and is related to the real spin (S = 3/2) by a factor of 5/3 (eqn (1)). The effectiveꢀspin
Hamiltonian for the dinuclear CoII system is described by eqn (2), and the magnetic susceptibility is
expressed by eqn (3):
ˆ
ˆ
S = (5/3)Seff
eqn (1)
eqn (2)
A
B
A
B
ˆ
ˆ
ˆ
ˆ
ˆ
H =
−
(25/9)JSeff ⋅Seff −G(T, J)βH(Seff + Seff )
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2
2N
β
[G(T,J)]
KT
1
χbinuclear
=
eqn (3)
3+exp[(25/9)J /(KT)]
where, G(T, J) is a fictitious Landé factor dependent upon λ, α, ꢂ and J.
The best fit of experimental data leads to the following set of parameters: λ = ꢀ130.55 cmꢀ1, α = 1.35, ꢂ =
ꢀ711.46 cmꢀ1, J = 1.28 cmꢀ1 and R = 1.03×10ꢀ4 (R value is defined as ∑[(χM calc]2/∑[(χM obs]2). The
)
obsꢀ(χM
)
)
values of parameters λ, α and ꢂ fall within the usual ranges expected for the sixꢀcoordinated CoII
complexes.28 The low value of |J/λ| ratio (0.01) supports the use of above empirical approach (this method
is valid when |J/λ| < 0.1).29 The obtained ferromagnetic coupling is in agreement with the wellꢀknown
ability of the synꢀanti carboxylate bridge to mediate weak ferromagnetic interactions.30 For the synꢀanti
carboxylate mediator, magnetic orbitals are unfavorably oriented to produce a significant overlap and thus
the ferromagnetic interaction is observed. In addition, the Co1–O1–C25–O2–Co1# (symmetry mode: ꢀx+1,
ꢀy+1, ꢀz+1) skeleton appreciably deviates from planarity [the dihedral angle between planes Co1–O1–C25
and Co1#–O2–C25: 82.82(28)°]. The nonꢀplanarity of Co1–O1–C25–O2–Co1# skeleton also reduces the
overlap of magnetic orbitals and increases the ferromagnetic coupling.31
Electrochemistry Properties. Based on the TGA results, CP 1 exhibits good stability, but begins to
decompose if heated up to about 330 °C. Fortunately, CoO and Co3O4 particles were prepared by the
controllable calcinationꢀthermolysis of crystals of 1, and the XRD pattern tests were then performed
(Figure S7). Two samples can be assigned to the faceꢀcentered cubic phase CoO and Co3O4, according to
standard JCPDS cards no. 43ꢀ1004 and 65ꢀ3103, respectively. No impurity diffraction peaks were detected,
indicating very high purity. The morphologies and microstructures of CoO and Co3O4 were investigated by
SEM (Figure S8). Obviously, irregular particle agglomerates of CoO are observed, revealing that the
asꢀsynthesized CoO is actually composed of numerous microparticles, and these particles undergo further
aggregation to form a porous block structure. However, the asꢀsynthesized Co3O4 looks like much smaller,
and presents nearly spherical nanoparticles with the sizes of 10ꢀ50 nm, which undergo further aggregation
to form an agglomerate structure.
Figure 9 shows CV curves of CoO and Co3O4 electrodes at various scan rates. Both electrodes exhibit
two pairs of wellꢀdefined redox peaks within the potential range of 0.0ꢀ0.5 V. Each pair of redox peaks
corresponds to the reversible conversion between two different cobalt oxidation states. Thus, for both
electrodes, the electrochemical process can be expressed as two sequential reactions:32
CoO + OHꢂ ↔ CoOOH + eꢂ;
CoOOH + OHꢂ ↔ CoO2 + H2O + eꢂ
(1)
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Co3O4 + H2O + OHꢂ ↔ 3CoOOH + eꢂ; CoOOH + OHꢂ ↔ CoO2 + H2O + eꢂ
(2)
Also seen from Figure 9, the oxidation and reduction peaks shift continuously to higher and lower
potentials, respectively. With increasing scanning rate, the potential separation between the oxidation and
reduction peaks become wider due to the polarization of the electrode.33 Figure S9 shows the representative
CV curves of CoO and Co3O4 electrodes at the scan rate of 5 mV·sꢀ1, which suggest that the Co3O4
electrode storages higher charge than the CoO electrode.
Galvanostatic chargeꢀdischarge measurements of cobalt oxide electrodes were carried out at various
current densities in a potential window of 0.10ꢀ0.45 V. The specific capacitance was calculated by the eqn
(4):34
C = Iꢃt / ꢃVm
eqn (4)
where C represents the mass specific capacitance, I the constant discharge current, ꢃt the discharging time,
ꢃV the voltage drop upon discharging, m the total mass of cobalt oxide powder. The capacitance values of
CoO electrode are 45.3, 42.6, 37.1, 33.4, 29.7 and 25.7 F·gꢀ1 at specific current densities of 0.5, 1.0, 2.0, 3.0,
4.0 and 5.0 A·gꢀ1, respectively (Figure 10a). The capacitance values of Co3O4 electrode are 104.7, 99.4,
89.1, 78.9, 69.7 and 57.1 F·gꢀ1 at corresponding current densities, respectively (Figure 10b). It is noted that
the capacitance values decreased with increasing current density, which is mainly attributed to the
insufficient faradic redox reaction arising from the surface reduction by the OHꢀ ion under high discharge
current density.35 The specific capacitance at these current densities for two electrodes was calculated and
compared (Figure 11). The Co3O4 electrode always exhibits higher specific capacitance; its discharge
specific capacitance is measured to be 104.7 F·gꢀ1 at 0.5 A·gꢀ1, more than two times of that of the CoO
electrode (45.3 F·gꢀ1).
Both the voltammogram and chargeꢀdischarge measurements show that the Co3O4 electrode storages
higher charge than the CoO electrode, which has more potential to be used as the electrode material of
electrochemical capacitor. At the same time, the Co3O4 electrode exhibits excellent longꢀterm cycle life
and high reversibility over entire cycle numbers (Figure 12), and the capacitance decreased only 11.7 % of
initial capacitance after 2500 cycles. The excellent cycle stability of Co3O4 electrode is ascribed to the large
surface area and dense reaction site, which can significantly improve the utilization and capacitance of
active material, provided by the effective mesoporous structure of the Co3O4 electrode.36
Conclusion
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Three novel CPs 1ꢀ3 based on the cobalt–bibp matrix have been successfully synthesized via
incorporation of three benzenedicarboxylates. Owing to the different orientations (φ) of carboxylate groups,
the benzoates adopt various bridging modes to connect metal centers into diverse carboxylate∩cobalt
chainꢀlike architectures with 1D chain, binuclear and singleꢀion magnetic units, respectively. The metal
nodes lying on these 1D architectures are further decorated by the bibp ligand to afford 1D, 2D and 3D
mixedꢀligand frameworks, respectively. Interestingly,
3 represents an unprecedented threeꢀfold
interpenetrating dmp network. Thereby the orientation of carboxylate group controls over the structure of
mixedꢀligand framework: the small value of φ is liable to form the polynuclear unit, while the large is
beneficial to the highly dimensional and interpenetrating framework. Furthermore, the research on the
magnetic property shows the antiferromagnetic coupling in 1, and the ferromagnetic coupling constant of 2
is estimated by fitting the experimental data. Significantly, CoO and Co3O4 particles were obtained from
the gasꢀcontrollable calcinationꢀthermolysis of crystals of 1, which appear as microꢀ or nanoparticles and
have the potential as the electrode material of supercapacitor.
Acknowledgment. This work was supported by the National Natural Science Foundation of China
(21103138), New teacher fund of doctorial program sponsored by Ministry of Education of China
(20116101120023), and Natural Science Foundation of Shannxi (2014JQ2057, 2013JK0672, 14JS007).
Supporting Information Available: Xꢀray crystallographic data for complexes 1ꢀ3 in CIF format, selected
bond lengths and angles, additional plots in WORD format, IR figures, TG figures, XRD figures, SEM
images and representative CV curves. This material is available free of charge via the Internet at http://pubs.
rsc.org.
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Captions to Figures
Chart 1. A series of 1D carboxylate∩cobalt chains constructed from three structurally related aromatic
dicarboxylic acids, which take different bridging modes.
Figure 1. View of the asymmetric unit, and coordination environments of CoII ions in 1 (the O atoms
cycled by the black ring are equivalent).
Figure 2. View of the (a) 1D carboxylate∩cobalt chain; (b) 1D belt in 1.
Figure 3. View of the (a) coordination environment of CoII ion; (b) 1D carboxylate∩cobalt chain
containing a binuclear cluster in 2.
Figure 4. The 2D doubleꢀbridging (4,4) layer based on the binuclear cluster in 2.
Figure 5. View of the (a) coordination environment of CoII ion; (b) 1D carboxylate∩cobalt chain, further
connected by the bibp ligand into a 3D framework in 3.
Figure 6. View of the 65·8 topology in 3 (the green and red rings denote the 6ꢀmembered and
8ꢀmembered circuits, respectively).
Figure 7. The threeꢀfold interpenetrating dmp network.
Figure 8. Thermal dependence of χMꢀ1, χM, χMT, and corresponding theoretical curves: (a) 1; (b) 2.
Figure 9. The CV curves at scan rates of 5, 10, 20, 50 and 100 mV·sꢀ1: (a) CoO; (b) Co3O4.
Figure 10. Galvanostatic chargeꢀdischarge curves at different current densities: (a) CoO; (b) Co3O4.
Figure 11. The current density dependence of specific capacitance for the CoO and Co3O4 electrodes.
Figure 12. Average specific capacitance versus cycle number of the Co3O4 electrode at a galvanostatic
chargeꢀdischarge current density of 1 A·gꢀ1. Inset: first ten charge/ discharge curves of the
Co3O4 electrode.
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Table 1. Crystallographic Data and Structure Refinement Parameters for 1ꢀ3.
Complex
1
2
3
Chemical formula
formula weight
crystal system
space group
a (Å)
C105H72.5Co6N13.5O25
2276.84
Monoclinic
P21/c
C26H18CoN4O4
509.37
C26H18CoN4O4
509.37
Orthorhombic
Pnna
Triclinic
Pꢀ1
23.577(4)
21.515(4)
20.120(4)
90
9.990(4)
10.046(4)
11.176(5)
92.603(6)
92.028(6)
97.537(7)
1109.9(8)
2
13.152(3)
17.365(4)
9.3149(19)
90
b (Å)
c (Å)
α
β
γ
(deg)
112.540(3)
90
90
90
V (Å3)
9426(3)
4
2127.3(7)
4
Z
Dcalcd. (g cm–3)
µ (mm–1)
F(000)
1.604
1.524
1.590
1.120
0.816
0.852
4636
522
1044
reflns collected/unique
R(int)
68562/23509
0.0461
8467/5639
0.0186
21021/2444
0.0527
0.0416
0.0991
1.099
R1a [I > 2σ (I)]
0.0682
0.0345
b
wR2
0.2040
0.0939
GOF
1.044
1.088
ꢃρmax/ꢃρmin (e Ǻꢀ3)
1.041/ꢀ0.734
0.331/ꢀ0.334
0.318/ꢀ0.284
2 2 1/2
aR = Σ||Fo| − |Fc|| / Σ|Fo|; bRw = [Σ[w(Fo2 − Fc2)2] / Σw(Fo ) ] .
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Chart 1
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Figure 1
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(a)
(b)
Figure 2
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(a)
(b)
Figure 3
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Figure 4
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(a)
(b)
Figure 5
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Figure 6
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Figure 7
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