Structures and properties of coordination polymers involving asymmetric biphenyl-3,2′,5′-tricarboxylate

Article abstract of DOI:10.1039/c4ce01225a

An asymmetric polycarboxylate ligand, biphenyl-3,2′,5′-tricarboxylic acid (H₃bptc) with a rotatable coordination vertex, was employed to react with metal ions under hydrothermal conditions. Four new coordination polymers {[Mn₃(bptc)₂(4,4′-bpy)₃(H₂O)₂]}_n (1), {[Ni(Hbptc)(4,4′-bpy)_1.5(H₂O)₂]·H₂O}_n (2), {[Cd₂(Hbptc)₂(phen)₄]·3H₂O}_n (3) and {[Cu₂(Hbptc)₂(2,2′-bpy)₂(H₂O)₂]·2H₂O}_n (4) were successfully synthesized and characterized. Single crystal X-ray analysis revealed that compound 1 has a 3D coordination network with linear Mn(ii)₃-based subunits, while 2 is a 1D ladder chain-based structure further interconnected by hydrogen bonds to a 3D supramolecular network. Compounds 3 and 4 are each composed of a supramolecular network, which is supported by hydrogen bonds and π...π interactions and consists of dimers, [Cd₂(Hbptc)₂(phen)₄] and [Cu₂(Hbptc)₂(2,2′-bpy)₂], respectively. The conformation stability and coordination modes of the H₃bptc ligand have been discussed. The magnetic and fluorescent properties of the complexes have been investigated.

Full text of DOI:10.1039/c4ce01225a

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Structures and properties of coordination
polymers involving asymmetric
Cite this: CrystEngComm, 2014, 16,
biphenyl-3,2′,5′-tricarboxylate†
10006
a
a
a
a
a
a
*
Jie Zhao, Li-Qiong Xie, Ying-Ming Ma, Ai-Ju Zhou, Wen Dong, Jing Wang,
Yan-Cong Chenb and Ming-Liang Tongb
An asymmetric polycarboxylate ligand, biphenyl-3,2′,5′-tricarboxylic acid (H3bptc) with a rotatable
coordination vertex, was employed to react with metal ions under hydrothermal conditions. Four new
coordination polymers {ijMn3(bptc)2IJ4,4′-bpy)3IJH2O)2]}n IJ1), {ijNi(Hbptc)IJ4,4′-bpy)1.5IJH2O)2]·H2O}n IJ2),
{ijCd2(Hbptc)2IJphen)4]·3H2O}n IJ3) and {ijCu2(Hbptc)2IJ2,2′-bpy)2IJH2O)2]·2H2O}n IJ4) were successfully
synthesized and characterized. Single crystal X-ray analysis revealed that compound 1 has a 3D coordination
network with linear MnIJII)3-based subunits, while 2 is a 1D ladder chain-based structure further inter-
connected by hydrogen bonds to a 3D supramolecular network. Compounds 3 and 4 are each composed
of a supramolecular network, which is supported by hydrogen bonds and π⋯π interactions and consists
of dimers, ijCd2(Hbptc)2IJphen)4] and ijCu2(Hbptc)2IJ2,2′-bpy)2], respectively. The conformation stability and
coordination modes of the H3bptc ligand have been discussed. The magnetic and fluorescent properties
of the complexes have been investigated.
Received 15th June 2014,
Accepted 28th August 2014
DOI: 10.1039/c4ce01225a
www.rsc.org/crystengcomm
interest and challenge.2 Many construction features of ligands
could affect the structural assembly process of these materials,
Introduction
In the field of materials science, much research has been
conducted on coordination polymers, based on metal centres
and multifunctional bridging ligands, due to their fascinating
structures and new topologies, as well as their tremendous
potential applications as functional materials including
catalysis, gas storage, fluorescent sensing, electronic and
magnetic devices.1 More and more efforts have mainly been
focused on the construction and preparation of versatile coor-
dination polymers and their structure–property relationships.
The structural information stored in the organic ligands along
with the coordination geometry of the metal ions should be
taken into account. Therefore, the judicious choice and design
of organic moieties is very important for constructing novel
structures, and searching new bridging ligands is of great
such as their size and shape, the flexibility and the functional
groups within the ligands.3
Polycarboxylate ligands have been proven to be good
candidates for constructing novel structures, because they
can be regarded not only as hydrogen bond acceptors but
also as donors, depending on the number of deprotonated
carboxylic groups. Of the aromatic carboxylates, the
rigid 1,4-benzenedicarboxylate,4 1,3,5-benzenetricarboxylate,5
1,2,4,5-benzenetetracarboxylate,6 1,2,3,4,5-benzenepenta-
carboxylate7 and 1,2,3,4,5,6-benzenehexacarboxylate8 were
studied extensively to construct numerous coordination poly-
mers. Compared to those rigid aromatic ligands, semi-rigid
biphenyl ligands possess more flexibility, because of the free
rotation around the C–C bond between the phenyl rings, and
therefore may build coordination polymers with more diverse
structures. More recently, versatile coordination polymers,
assembled from biphenyl carboxylate ligands, such as
biphenyldicarboxylate,9 biphenyl-3,4′,5-tricarboxylate,10 and
biphenyl-3,5,3′,5′/3,4,3′,4′/2,4,2′,4′-tetracarboxylate11 com-
pounds, have been reported. Most of these polycarboxylate
ligands possess a highly symmetric geometry which results
in symmetric networks. In contrast, coordination polymers
based on asymmetric polycarboxylate ligands are far less
prevalent, probably due to their asymmetric geometry,
making it difficult to predict and control the final coordina-
tion networks.12
a Guangzhou Key Laboratory for Environmentally Functional Materials and
Technology, School of Chemistry and Chemical Engineering, Guangzhou University,
Guangzhou 510006, PR China. E-mail: wangjgzhu@163.com; Fax: +86-02039366908;
Tel: +86-02039366908
b State Key Laboratory of Optoelectronic Materials and Technologies,
School of Chemistry and Chemical Engineering, Sun Yat-Sen University,
Guangzhou 510275, PR China
† Electronic supplementary information (ESI) available: Crystallographic data in
CIF format for 1–4, computational details for free H3bptc, XRPD patterns for
1–4, TG curves for 1–4, excitation and emission spectra for 1, 2 and 4, selected
bond lengths and angles for 1–4, hydrogen bond lengths and angles for 2 and 4.
CCDC 996435–996438. For ESI and crystallographic data in CIF or other
electronic format see DOI: 10.1039/c4ce01225a
10006 | CrystEngComm, 2014, 16, 10006–10016
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To the best of our knowledge, coordination polymers
based on biphenyl-3,2′,5′-tricarboxylic acid (H3bptc) have
been rarely documented.13 H3bptc, as one type of bridging
biphenyl polycarboxylate ligand, has a rotatable coordination
vertex and an asymmetric geometry with three carboxylate
groups. Learning from our previous study on conformational
flexible cyclohexane-polycarboxylate coordination polymers,14
the H3bptc with a rotatable coordination vertex and asymmet-
ric geometry may be a good candidate for constructing versa-
tile coordination networks with functional properties. With
the introduction of an auxiliary ligand, such as 4,4′-bipyridine
(4,4′-bpy), 1,10-phenanthroline (phen) and 2,2′-bipyridine
(2,2′-bpy) into the reaction system, four new coordination
polymers have been obtained. Herein, we report the syn-
theses and structural characterizations of the four new
coordination polymers, {ijMn3(bptc)2IJ4,4′-bpy)3IJH2O)2]}n IJ1),
{ijNi(Hbptc)IJ4,4′-bpy)1.5IJH2O)2]·H2O}n IJ2), {ijCd2(Hbptc)2IJphen)4]·3H2O}n
IJ3), {ijCu2(Hbptc)2IJ2,2′-bpy)2IJH2O)2]·2H2O}n IJ4), which exhibit
a systematic variation of architectures from an 0D dimer to a
3D framework constructed from deprotonated H3bptc and the
auxiliary ligands. The conformation stability and coordination
modes of the H3bptc ligand have been discussed. In addition,
the magnetic and fluorescent properties of the complexes have
been investigated.
with water. IR (KBr, cm−1): 3626w, 3072w, 2367w, 1582s, 1385s,
1224w, 1073w, 819m, 784m, 714w, 645m, 518w. Elemental
analysis calcd for C30H21N3O7Mn1.50: C, 58.31; H, 3.43; N, 6.80.
Found: C, 58.99; H, 3.10; N, 6.95.
Synthesis of {ijNi(Hbptc)IJ4,4′-bpy)1.5IJH2O)2]·H2O}n IJ2). A
mixture of H3bptc (0.053 g, 0.20 mmol) and 4,4′-bpy (0.156 g,
1.00 mmol) in H2O (5.0 mL) was added to an aqueous
solution (12.0 mL) of NiIJNO3)2·6H2O (0.114 g, 0.50 mmol)
and stirred. The resultant solution was heated in a stainless
steel reactor with Teflon liner at 120 °C for 168 h. After a
period of approximately 12 h cooling to room temperature,
green block crystals of 2 (yield ca. 80% based on Ni)
were obtained, isolated by filtration and washed with water.
IR (KBr, cm−1): 3615m, 3442m, 3118m, 2587w, 2367w,
1709s, 1628s, 1570vs, 1408vs, 1373vs, 1224s, 1073m, 923w,
796s, 680m, 623m, 507w. Elemental analysis calcd for
C
30H26N3O9Ni: C, 57.08; H, 4.15; N, 6.66. Found: C, 58.21; H,
4.10; N, 6.85.
Synthesis of {ijCd2(Hbptc)2IJphen)4]·3H2O}n IJ3). A mixture of
H3bptc (0.053 g, 0.20 mmol) and phen (0.180 g, 1.00 mmol) in
H2O (5.0 mL) was added to an aqueous solution (12.0 mL) of
CdIJNO3)2·4H2O (0.122 g, 0.50 mmol) and stirred. The resultant
solution was heated in a stainless steel reactor with Teflon liner
at 120 °C for 120 h. After a period of approximately 12 h cooling
to room temperature, colorless block crystals of 3 (yield ca. 70%
based on Cd) were obtained, isolated by filtration and washed
with water. IR (KBr, cm−1): 3049w, 2367w, 1720s, 1547vs,
1407vs, 1373s, 1224s, 1097m, 853s, 726s, 645w, 506w. Elemental
analysis calcd for C78H54N8O15Cd2 (%): C, 59.74; H, 3.47; N,
7.15. Found: C, 62.12; H, 3.25; N, 7.45.
Synthesis of {ijCu2(Hbptc)2IJ2,2′-bpy)2IJH2O)2]·2H2O}n IJ4). A
mixture of H3bptc (0.027 g, 0.10 mmol) and 2,2′-bpy (0.078 g,
0.50 mmol) in H2O (5.0 mL) was added to an aqueous
solution (12.0 mL) of CuIJNO3)2·3H2O (0.089 g, 0.50 mmol)
and stirred. The resultant solution was heated in a stainless
steel reactor with Teflon liner at 120 °C for 72 h. After a
period of approximately 12 h cooling to room temperature,
blue needle-like crystals of 4 (yield ca. 75% based on Cu)
were obtained, isolated by filtration and washed with water.
IR (KBr, cm−1): 3049w, 2367w, 1709m, 1604vs, 1582vs, 1397s,
1362vs, 1246m, 1038w, 761m, 715w. Elemental analysis calcd
for C25H20N2O8Cu: C, 55.61; H, 3.73; N, 5.19. Found: C,
55.44; H, 3.68; N, 5.25.
Experimental section
Materials and methods
The reagents and solvents employed were commercially
available and used as received without further purification.
The C, H, and N microanalyses for the four compounds were
performed with fresh samples using an Elementar Vario-EL
CHN elemental analyzer. FT-IR spectra were recorded with
KBr pellets in the range of 4000–400 cm−1 using a Bio-Rad
FTS-7 spectrometer. X-ray powder diffraction (XRPD) intensi-
ties for the four compounds were measured at 293 K using a
Bruker D8 X-ray diffractometer (Cu-Kα, λ = 1.54056 Å). The
crushed poly-crystalline powder samples were prepared by
crushing the crystals and scanned from 5–60° with a step rate
of 0.1° s−1, and the calculated patterns were generated with
PowderCell. Thermogravimetric (TG) analyses were carried
out using the NETZSCH TG209F3 thermogravimetric analyzer.
Variable-temperature magnetic susceptibility measurements
were performed using
a SQUID magnetometer MPMS
(Quantum Design) at 1.0 kOe for 1, 2, 4, and the diamagnetic
correction was applied using Pascal's constants. The emission/
excitation spectra for 1–4 were measured using an F-4500
Fluorescence Spectrophotometer.
Synthesis of {ijMn3(bptc)2IJ4,4′-bpy)3IJH2O)2]}n IJ1). A mixture of
H3bptc (0.053 g, 0.20 mmol) and 4,4′-bpy (0.156 g, 1.00 mmol)
in H2O (5.0 mL) was added to an aqueous solution (12.0 mL) of
MnIJOAc)2·4H2O (0.123 g, 0.50 mmol) and stirred. The resultant
solution was heated in a stainless steel reactor with Teflon liner
at 120 °C for 120 h. After a period of approximately 12 h cooling
to room temperature, colorless block crystals of 1 (yield ca. 78%
based on Mn) were obtained, isolated by filtration and washed
Computational details
All of the structures were optimized using the Becke three-
parameter Lee–Yang–Parr (B3-LYP) exchange–correlation
functional method15 with the 6-31G(d,p) basis sets16 (Fig. S1†).
The stable configurations of the structures were confirmed
using vibrational frequency analysis, in which no imaginary
frequency was found for the configuration at the energy
minima. The sum of the electronic and thermal free ener-
gies was used to compare their stabilities (Table S1†). All
calculations were performed using the Gaussian 09 compu-
tational package.17
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Single-crystal structure determination
The data collection and structural analysis of the crystals, 1–4,
were performed using a SMART (Bruker, 2002) diffractometer
equipped with Mo-Kα radiation (λ = 0.71073 Å) at 296(2) K.
Absorption corrections were applied using multi-scan program
SADABS. The structures were solved using direct methods and
refined with a full-matrix least-squares technique using the
SHELXTL program package.18 Anisotropic thermal parameters
were applied to all non-hydrogen atoms. The organic hydrogen
atoms were generated geometrically (C–H 0.96 Å), the water
hydrogen atoms were located using difference maps and
refined using isotropic temperature factors. The crystal data as
well as details of the data collection and refinements are sum-
marized in Table 1. Selected bond lengths and bond angles are
listed in Table S2.† Crystallographic data for the structures
reported in this paper have been deposited in the Cambridge
Crystallographic Data Center with the CCDC reference num-
bers 996435–996438 for compounds 1–4.
Scheme 1 Biphenyl-3,2′,5′-tricarboxylic acid (H3bptc).
changing dihedral angle and reaches a maximum when the
phenyl rings are almost co-planar (Fig. S1†).
According to our previous studies on coordination polymers
with flexible ligands, the size and the versatile coordination envi-
ronments of the metal ion may play an important role in con-
trolling the conformation of the flexible ligand.11 In the present
H3bptc system, four metal ions were used, MnIJII), NiIJII), CdIJII)
and CuIJII), resulting in different conformations and coordination
modes of the ligand (Fig. 1). The dihedral angles between the
two phenyl rings in compounds 1–4 are 47.3°, 56.6°, 42.2° and
35.4°, respectively (Table S1†). The fully deprotonated bptc3−
ligand in compound 1 adopts a μ5:η2,η2,η1 bridging mode,
while the partly deprotonated Hbptc2− ligands adopt a μ2:η1,η1
bridging mode in compounds 2 and 4, and a μ2:η2 bridging
mode in compound 3. It should be noted that the different
conformations and coordination modes of the ligand not only
relate to the metal ions, but also to the auxiliary ligands.
Results and discussion
Theoretical calculations on free H3bptc and coordination
modes of the deprotonated bptc3−/Hbptc2−
The biphenyl-3,2′,5′-tricarboxylic acid (H3bptc) has a rotatable
coordination vertex and an asymmetric geometry with three
carboxylate groups (Scheme 1). The main conformational
degrees of the free H3bptc involve the rotation around the C–C
bond between the two phenyl rings. Theoretical calculations
show that the thermodynamically most stable conformation in
the gas phase is the one with a dihedral angle of 53.5° between
the two phenyl rings. The energy of the molecule increases with
Structure description
{ijMn3(bptc)2IJ4,4′-bpy)3IJH2O)2]}n IJ1). The X-ray crystallo-
graphic study of 1 revealed an infinite 3D coordination
Table 1 Crystallographic data and refinement parameters for compounds 1–4
Compound
1
2
3
4
Empirical formula
Fw
Crystal system
Space group
a (Å)
b (Å)
c (Å)
α (°)
β (°)
C60H42N6O14Mn3
1235.82
Monoclinic
P21/n
9.3553(2)
24.7637(4)
11.6174(2)
90.00
113.4110IJ10)
90.00
2469.86(8)
2
C30H26N3O9Ni
631.25
C78H54N8O15Cd2
1568.09
Monoclinic
C2/c
25.6565(6)
12.4636(3)
21.2198(6)
90.00
105.910(2)
90.00
6525.6(3)
4
C50H40N4O16Cu2
1079.94
Triclinic
Triclinic
¯
¯
P1
P1
9.5251(6)
9.8837(6)
16.3942IJ11)
72.834(4)
87.431(4)
71.093(4)
1392.88IJ15)
2
7.4683(10)
9.9602(2)
15.8627IJ3)
81.110(10)
88.078(10)
69.099(10)
1088.78IJ3)
1
γ (°)
V (Å3)
Z
Dc (g cm−3
)
1.662
0.838
1262
1.505
0.759
654
1.596
0.732
3176
1.647
1.062
554
μ (mm−1
FIJ000)
)
Reflections collected
Rint
39 660
0.0465
22 494
0.0385
24 648
0.0702
17 461
0.0286
Gof
1.047
1.062
0.985
1.034
a
a
a
a
Final R indices
[I > 2σIJI)]
R indices (all data)
R1 = 0.0344
R1 = 0.0388
R1 = 0.0440
R1 = 0.0319
b
b
b
b
wR2 = 0.0814
wR2 = 0.0936
wR2 = 0.0867
wR2 = 0.0835
a
a
a
a
R1 = 0.0513
R1 = 0.0566
R1 = 0.0785
R1 = 0.0395
b
b
b
b
wR2 = 0.0881
wR2 = 0.1062
wR2 = 0.0964
wR2 = 0.0876
CCDC number
996438
996437
996435
996436
P
P
P
P
a
2
R1 = ‖Fo| − |Fc‖/ |Fo|. b wR2 = ij w(Fo − Fc2)2/ wIJFo2)2]1/2
.
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Fig. 1 The coordination modes of bptc3−/Hbptc2− in the compounds 1 (a), 2 (b), 3 (c) and 4 (d).
network based on a linear MnIJII)3-subunit that crystallizes
in the monoclinic space group P21/n. The asymmetric unit
contains two crystallographically different MnIJII) atoms
(one of which lies in an inversion centre), one deprotonated
bptc3− ligand, two 4,4′-bpy ligands (one of which lies in an
inversion centre), and one coordinated water molecule
(Fig. 2a). Mn1 adopts a slightly distorted octahedral geometry,
coordinating to three carboxylate oxygen atoms (O1, O4A, O6B)
from three bptc3− ligands, to one water oxygen atom (O1W),
and two nitrogen atoms (N1, N2C) from two different 4,4′-bpy
ligands. Mn2 lies in an inversion centre and also adopts an
octahedral geometry, coordinating to four carboxylate oxygen
atoms (O2, O5B, O2D, O5E) from four bptc3− and two nitrogen
atoms (N3, N3D) from two symmetric 4,4′-bpy ligands.
The bond lengths of Mn–OIJcarboxylate) fall in the range of
2.1383IJ15)–2.2055IJ13) Å, the axial Mn–OIJwater) bond length is
2.2767(17) Å, and the bond lengths of Mn–N are in the range of
2.2691IJ15)–2.2740IJ15) Å (Table S2†). The bptc3− ligand with a
dihedral angle of 47.3° between the two phenyl rings is fully
depronated. The three depronated carboxylate groups have
different coordination modes, including monodentate and
bridging μ2-η1:η1 modes, to link five MnIJII) atoms (Fig. 1a). The
bridging carboxylate groups from four different ligands bind
three MnIJII) atoms, forming a linear MnIJII)3-building unit
(Fig. 2b). Each trinuclear unit connects six bptc3− ligands
(Fig. 2b), while each of the two bptc3− ligands connect four
trinuclear units (Fig. 2c). The trinuclear subunits are linked by
μ5-bridging bptc3− ligands to form a 3D coordination network
(Fig. 2d). Meanwhile, there are two kinds of bridging 4,4′-bpy
ligands, one of which lies across a 2-fold axis and the other is
asymmetric with a dihedral angle of 29.2° between the two
pyridyl rings. A 3D condensed framework is thus generated
by the connection of the bptc3− and 4,4′-bpy ligands with no
lattice water molecules (Fig. 2e). There are intramolecular
hydrogen bonds between the coordinated water and the
uncoordinated carboxylate groups (O1W⋯O3 a = 2.698(2) Å,
O1W–H1WA⋯O3 a = 170IJ4)°, a: x, y, z + 1).
crystallographic type of NiIJII) atom, one partly deprotonated
Hbptc2− ligand, two 4,4′-bpy ligands (one of which lies in
an inversion centre), two coordinated and one lattice water
molecule (Fig. 3a). Ni1 adopts a slightly distorted octahedral
geometry, coordinating to two carboxylate oxygen atoms
(O2, O3A) from two Hbptc2− ligands, to two water oxygen atoms
(O1W, O2W), and two nitrogen atoms (N1, N3) from two differ-
ent 4,4′-bpy ligands. The Ni–N bond lengths are 2.0596(18) and
2.0943(19) Å, Ni–OIJwater) 2.0908(17) and 2.0921(18) Å, and
Ni–OIJcarboxylate) 2.0597(15) and 2.0634(15) Å (Table S2†). The
Hbptc2− ligand with a dihedral angle of 56.6° between the two
phenyl rings is partly depronated at the 2′,5′-carboxylate groups
(Fig. 1b). The partly deprotonated Hbptc2− adopts a μ2-bridging
coordination mode through two deprotonated carboxylate
groups on the same phenyl ring, connecting the NiIJII) atoms
to form a 1D Ni-Hbptc chain (Fig. 3b). There are two kinds of
4,4′-bpy ligands adopting a monodentate and a μ2-bridging
coordination mode, respectively. The μ2-bridging 4,4′-bpy
ligand, lying in an inversion centre, connects two Ni-Hbptc
chains to form a 1D ladder chain, and the monodentate
4,4′-bpy ligands, with a dihedral angle of 20.6° between the two
pyridyl rings, are arrayed along both sides of the chain
(Fig. 3b). The ladder chains are zipped together through their
uncoordinated carboxylate groups and the monodentate
4,4′-bpy via hydrogen bonds (Fig. 3c). Thus, a 3D supramolecu-
lar network, with 1D channels occupied by water molecules, is
generated (Fig. 3d) and further strengthened by rich hydrogen
bonds between the carboxylate groups, 4,4′-bpy and the water
molecules. (O⋯O = 2.664IJ3)–3.137IJ3) Å, O–H⋯O = 151–172°)
(Table S3†).
{ijCd2(Hbptc)2IJphen)4]·3H2O}n IJ3). The X-ray crystallo-
graphic study revealed that 3 crystallizes in the monoclinic
C2/c space group. The asymmetric unit contains one crystal-
lographic type of CdIJII) atom, one partly deprotonated Hbptc2−
ligand, two phen ligands and three disordered lattice water
molecules. Cd1 lies on a general position and has a six-
coordinated distorted octahedral geometry, coordinating to
two carboxylate oxygen atoms (O1, O2A) from two Hbptc2−
ligands and to four nitrogen atoms (N1, N2, N3, N4) from two
phen ligands (Fig. 4a). The bond lengths of Cd–O are 2.246(2)
and 2.270(3) Å and those of Cd–N fall between 2.298(3) and
{ijNi(Hbptc)IJ4,4′-bpy)1.5IJH2O)2]·H2O}n IJ2). The X-ray crystal-
lographic study of 2 revealed a supramolecular network, based
on 1D coordinated ladder chains, that crystallizes in the
¯
triclinic space group P1. The asymmetric unit contains one
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Fig. 2 (a) View of the coordination environments around the MnIJII) atoms in 1 (A: x − 1/2, −y + 1/2, z + 1/2; B: x − 1, y, z; C: x, y, z − 1; D: −x, −y, −z;
E: −x + 1, −y, −z); (b) the linear MnIJII)3-building unit; (c) the coordination of each of the two bptc3− ligands surrounded by four linear
MnIJII)3-subunits; (d) the 3D coordination network with bridging bptc3− ligands along the c-axis (4,4′-bpy ligands are omitted for clarity); (e) the 3D
coordination framework with bridging bptc3− and 4,4′-bpy ligands (the coordinated water molecules in the framework are highlighted as sky
blue spheres).
2.432(3) Å (Table S2†). The Hbptc2− ligand, with a dihedral
angle of 42.2° between the two phenyl rings, is partly
deprotonated at the 3,2′-carboxylate groups (Fig. 1c). The
partly deprotonated Hbptc2− connects two CdIJII) atoms in
a μ2-bridging coordination mode through its deprotonated
2′-carboxylate group. Two Hbptc2− ligands and four phen
ligands connect two CdIJII) atoms to form a dimer building
block ijCd2(Hbptc)2IJphen)4], which lies in an inversion centre
IJFig. 4b). The dimers are connected with each other to form a
1D chain via hydrogen bonds between the deprotonated and
protonated carboxylate groups. Furthermore, the adjacent
chains are held together via π⋯π interactions between the
phen ligands, forming a 3D supramolecular network (Fig. 4c).
The shortest distance between the two parallel phen planes is
3.7 Å, which is within the common range of π⋯π interactions
between the two phenyl rings. There are hydrogen bonds
between the partly deprotonated carboxylate groups with
shared hydrogen atoms (O4⋯O4 a = 2.430(5) Å, O4–H⋯O4
a = 115°, a: −x, y, −z + 3/2). The three disordered lattice water
molecules are removed using the SQUEEZE procedure and
can be easily lost above room temperature because of the
hydrophobic effect of the molecules.
{ijCu2(Hbptc)2IJ2,2′-bpy)2IJH2O)2]·2H2O}n IJ4). The X-ray
crystallographic study of 4 revealed a supramolecular network,
based on ijCu2(Hbptc)2IJ2,2′-bpy)2] dimers, that crystallizes in
¯
the triclinic space group P1. The asymmetric unit contains one
crystallographic type of CuIJII) atom, one partly deprotonated
Hbptc2− ligand, one 2,2′-bpy ligand, one coordinated and one
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Fig. 3 (a) View of the coordination environment around the NiIJII) atom in 2 (A: x + 1, y − 1, z); (b) the 1D ladder chain with bridging Hbptc2− and
4,4′-bpy ligands; (c) the adjacent ladder chains zipped together through their uncoordinated carboxylate groups and monodentate 4,4′-bpy by
hydrogen bonds viewed along the b-axis (the lattice water molecules are highlighted in sky blue spheres ); (d) the 3D supramolecular framework
with 1D channels, viewed along the c-axis (the lattice water molecules in the channels are highlighted by sky blue spheres).
lattice water molecule. Cu1 adopts a slightly distorted tetra-
gonal pyramidal geometry, coordinating to two carboxylate
oxygen atoms (O2, O4A) from two Hbptc2− ligands, to one
water oxygen atom (O1W), and to two nitrogen atoms
(N1, N2) from one 2,2′-bpy ligand (Fig. 5a). The bond lengths
of Cu–N are 1.9929(16) and 2.0089(16) Å. The axial Cu–OIJwater)
bond length IJ2.2187IJ15) Å) is much longer than the equato-
rial Cu–OIJcarboxylate) ones IJ1.9115IJ13) and 1.9904(13) Å)
(Table S2†). The Hbptc2− ligand, with a dihedral angle of
35.4° between the two phenyl rings, is partly deprotonated at
the 2′,5′-carboxylate groups (Fig. 1d). The partly deprotonated
Hbptc2− ligand connects two CuIJII) atoms in a μ2-bridging
coordination mode through two deprotonated carboxylate
groups. Two Hbptc2− ligands and two 2,2′-bpy ligands
connect two CuIJII) atoms to form a dimer building block,
which lies in an inversion centre (Fig. 5b). Due to the differ-
ent bridging modes of the Hbptc2− ligand in 3 and 4, the
Cu⋯Cu distance of 9.4 Å in the ijCu2(Hbptc)2IJ2,2′-bpy)2]
dimer is much longer than the Cd⋯Cd distance of 4.6 Å in
the ijCd2(Hbptc)2IJphen)4] dimer. The adjacent dimers are
held together via π⋯π interactions to form a 1D chain
with a distance of 3.3 Å between the phenyl ring of the
Hbptc2− ligand and the pyridyl ring of 2,2′-bpy (Fig. 5c).
Furthermore, rich hydrogen bonds between the water mole-
cules and the carboxylate groups connect the chains, forming
a 3D supramolecular network (O⋯O = 2.638IJ3)–3.023IJ2) Å,
O–H⋯O = 157IJ4)–175IJ2)°) (Table S3†).
Thermal stability analysis
The XRPD measurements were carried out at room tempera-
ture to confirm the phase purity of compounds 1–4. As
shown in Fig. S2,† the peak positions of the simulated
and experimental XRPD patterns are in agreement with
each other, demonstrating the good phase purity of the four
compounds.
In order to examine the thermal stabilities of the four
compounds, the thermal gravimetric (TG) analyses were car-
ried out under a dry nitrogen atmosphere from 30 to 650 °C
(Fig. S3†). For compound 1, the TGA curve shows that the
first weight loss of 3.0% at the beginning from 80 °C to
140 °C corresponds to the loss of the coordinated water mole-
cule (calcd 2.9%) of one unit cell. The network is decomposed
at 300 °C. The remaining weight of 17.2% indicates that the
final product is MnO (calcd 17.2%). Compound 2 loses the
lattice and coordinated water molecules (calcd 8.7% and expt
8.8% ) from the beginning to 150 °C, and then the supra-
molecular network is decomposed quickly, resulting in residual
NiO (calcd 11.8% and expt 10.0%). For compound 3, two disor-
dered water molecules have been lost at room temperature
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Fig. 4 (a) View of the coordination environment around the CdIJII) atom in 3 (A: −x + 1/2, −y + 3/2, −z + 1); (b) the dimer building block
ijCd2(Hbptc)2IJphen)4]; IJc) the 3D packed supramolecular network, held together via π⋯π interactions, viewed along the a-axis.
Fig. 5 (a) View of the coordination environment around the CuIJII) atom in 4 (A: −x + 2, −y + 1, −z + 1); (b) the dimer building block
ijCu2(Hbptc)2IJ2,2′-bpy)2]; IJc) the π⋯π interactions between the phenyl and the pyridyl rings from adjacent dimers (the hydrogen atoms are omitted
for clarity); (d) the stacked supramolecular network with lattice water molecules, viewed along the a-axis (the lattice water molecules in the channels
are highlighted by sky blue spheres).
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because of the hydrophobic effect of the molecules. The third
water molecule (calcd 1.1% and expt 1.1%) was lost from the
beginning to 80 °C and the network collapsed in the tempera-
ture range 280–620 °C, which indicates that it is thermally
stable over a wide temperature range. The observed weight
losses include three steps between 280 °C and 620 °C before
the final formation of CdO, which can be assigned to the elim-
ination of the phen and Hbpt2− ligands. For compound 4, the
TGA curve shows that the first weight loss of 3.2% from the
beginning to 90 °C corresponds to the loss of the lattice water
molecule (calcd 3.3%) of one unit cell, the coordinated water
molecule is lost at temperatures up to 150 °C. The network is
stable between 150 °C to 265 °C and then collapses quickly
before the final formation of the metal oxide.
Magnetic properties
The variable temperature magnetic susceptibility was mea-
sured for compound 1 in a 1 kOe dc field. At 300 K, the
experimental χMT value is 13.0 cm3 mol−1 K, which is consis-
tent with the expected value (13.125 cm3 mol−1 K) for three
MnIJII) ions (S = 5/2, g = 2) (Fig. 6a). Upon cooling, the χMT
value steadily decreases to 3.89 cm3 mol−1 K at 1.8 K. Fitting
the experimental data to the Curie–Weiss law in the tempera-
ture range of 10–300 K leads to an antiferromagnetic Weiss
constant of θ = −7.1 K (Fig. 6b). The dominating antiferro-
magnetic interactions in 1 are also implied by the unsaturated
magnetization values of 11.6 Nβ at 2 K and 70 kOe (compared
with the expected 15 Nβ) (Fig. 6c). Moreover, due to the
symmetrical linear MnIJII) trimer sub-structure of 1, the
magnetic susceptibility can also be satisfactorily fitted to the
exchange model using the spin Hamiltonian:19
n
n
ꢁ ꢁ
J S •S
j
ꢀ
H  2

ij
i
i1 ji
The results reveal an intra-trimer J = −0.475IJ4) cm−1 and
an inter-trimer zJ = −0.027IJ2) cm−1 with g = 2.007(1), indicat-
ing weak antiferromagnetic interactions both inside
(Mn⋯Mn = 5.0 Å) and among (Mn⋯Mn = 9.4 Å) the MnIJII)3
sub-structures, compared to the reported linear MnIJII)3-based
coordination polymers.20
On the other hand, the magnetic properties of 2 and 4
mainly come from the single-ion behavior of NiIJII) and CuIJII),
respectively, owing to the lack of strong magnetic inter-
Fig. 6 (a) The temperature-dependence of χMT in the range of
1.8–300 K (the solid line is fitted to the exchange model, the insert is the
scheme of the MnIJII)3-subunit); (b) the temperature-dependence of χM
(the solid line is fitted to the Curie–Weiss law); and (c) the field-dependent
magnetization at 2 K (the solid line is a guide to the eye), for 1.
−1
actions as in 1. At 300 K, the χMT values are 1.31 cm3 mol−1
K
for 2 (Fig. 7a) and 0.43 cm3 mol−1 K for 4 (Fig. 7b). Both
of them are larger than the spin-only values (1.0 for 2 and
0.375 for 4), indicating the presence of spin–orbit coupling
which is commonly observed for NiIJII) and CuIJII).19,21 Upon
cooling, the χMT values decrease slowly, mainly owing to the
weak antiferromagnetic interactions and/or zero-field split-
ting. Fitting the experimental data to the Curie–Weiss law in
the range of 1.8–300 K leads to the negative Weiss constants
of θ = −0.83 K for 2 and −0.54 K for 4, both of which are
much smaller in their absolute value than that of 1. The mag-
netization curves at 2 K also show a typical paramagnetic
shape as expected, reaching a saturation of 2.29 Nβ for 2 and
1.00 Nβ for 4 (Fig. 7c).
Luminescent properties
The solid-state luminescent properties of the free H3bptc
ligand and compound 3 were investigated in the solid state
at room temperature. The emission band, with a maximum
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Fig. 8 The excitation (insert) and emission spectra of (a) the free H3bptc
ligand and (b) compound 3 in the solid state at room temperature.
(compared to free phen), can be assigned to the cooperative
effect of the metal-perturbed intra-ligand emission between
H3bptc and phen (Fig. 8b).24 Notably, the chelation of
the H3bptc ligand to metal ions can effectively change the
dihedral angle between the two phenyl rings, and reduce the
loss of energy produced by the radiationless decay.24 Thus,
the enhancement and blue-shift in compound 3 may be
attributed to the altered energy level of the π*→n or π*→π
transition. Different from compound 3 with a d10 configuration,
the emission bands of compound 1, 2, and 4 with unpaired
electrons are observed around 498 nm and 520 nm with weak
intensities (Fig. S4†), which may be due to the ligand-field
transitions (d–d) via electrons, or the energy transfer via the
partially filled d-orbitals.25
−1
Fig. 7 (a) The temperature-dependence of χMT (left) and χM (right) in
the range of 1.8–300 K (the solid line is fitted to the Curie–Weiss law)
−1
for 2; (b) the temperature-dependence of χMT (left) and χM (right) in
the range of 2–300 K (the solid line is fitted to the Curie–Weiss law) for
4; and (c) the field-dependent magnetization at 2 K for 2 (red) and 4
(blue) (the solid lines are a guide to the eye).
observed at 484 nm upon excitation at 296 nm, for H3bptc
can be attributed to the π*→n or π*→π transition
(Fig. 8a).1c,22 According to the reported literature, the free
phen molecule displays strong emission around 390 nm in
the solid state at room temperature, attributed to the π*→π
transition.7a,23 Comparably, the emission of compound 3,
observed at 453 nm (λex = 365 nm) with a blue-shift of 31 nm
(compared to free H3bptc) and a red shift of ca. 40 nm
Conclusions
Four new coordination polymers, involving an asymmetric
ligand with a rotatable coordination vertex and N-donor
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ligands, have been successfully synthesized and character-
ized. According to single crystal X-ray analysis, compound 1
forms a 3D coordination network, 2 forms a 1D ladder chain-
based structure, and 3 and 4 form 0D dimer-based structures.
The asymmetric H3bptc ligand has been partly or fully
deprotonated and adopts different coordination modes in
these compounds. Theoretical calculations on the free H3bptc
ligand show that the thermodynamically most stable confor-
mation is the one with a dihedral angle of 53.5°, while the
dihedral angles are 47.3°, 56.6°, 42.2° and 35.4° in com-
pounds 1–4, respectively. The observed weight losses include
three steps between 280 °C and 620 °C before the final for-
mation of CdO, which can be assigned to the elimination of
the phen and Hbpt2− ligands. Magnetic property studies indi-
cated the existence of antiferromagnetic interactions in com-
pounds 1, 2 and 4. Luminescent property studies revealed
that compound 3 displays an intense blue emission in the
solid state at room temperature, which may be a good candi-
date for potential photoactive materials.
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Acknowledgements
This work was supported by the National Science Foundation
of China (no. 20901018) and the Yang-cheng Scholars Foun-
dation of Guangzhou (no. 12A002D).
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