A comparison of the coordination preference of Cd, Zn, Cu(II) with flexible homophthalic acid and rigid bipyridine ligands

Article abstract of DOI:10.1016/j.poly.2011.02.053

The mild hydrothermal reaction of Cd, Zn, or Cu(II) acetate salts with the deprotonated flexible linker homophthalic acid (H₂hmph) and the rigid molecule 4,4′-bipyridine (bpy) produced the following complexes, featured structurally by the assem

Full text of DOI:10.1016/j.poly.2011.02.053

Polyhedron 30 (2011) 1487–1493
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Polyhedron
journal homepage: www.elsevier.com/locate/poly
A comparison of the coordination preference of Cd, Zn, Cu(II) with flexible
homophthalic acid and rigid bipyridine ligands
⇑
Guang-Zhen Liu, Jun Zhang, Li-Ya Wang
College of Chemistry and Chemical Engineering, Luoyang Normal University, Luoyang 471022, PR China
a r t i c l e i n f o
a b s t r a c t
Article history:
The mild hydrothermal reaction of Cd, Zn, or Cu(II) acetate salts with the deprotonated flexible linker
homophthalic acid (H2hmph) and the rigid molecule 4,40-bipyridine (bpy) produced the following com-
plexes, featured structurally by the assembly of various metal carboxylate subunits cohered further by
the bpy ancillary ligand: {[Cd2(hmph)2(bpy)(H2O)2]ꢀH2O}n (1) manifests cadmium carboxylate zigzag
chain motifs, containing the alternation of a hmph-bridged Cd2(hmph)2 dimer and an edge-shared
Cd2N4O10 dimer, connected further by the bpy ligands to form a layered structure with a large 50-mem-
bered ring; [Zn(hmph)(bpy)]n (2) processes hmph-bridged Cd2(hmph)2 dimer motifs inter-linked further
by the bpy ligands to generate mutually embedded layered structures with square (4,4) grids; and
{[Cu(hmph)(bpy)]ꢀH2O}n (3) consists of copper carboxylate helix motifs, featuring hmph-bridged CuN2O4
octahedra cross-linked further by the bpy ligands into a twofold-interpenetrating 3D chiral framework
with a homochiral topology of a quartz dual net. This well-defined synthetic system was proposed to
highlight the fact that organic linkers display markedly different coordination preferences at specific
metal ions. In addition, the fluorescence properties of complexes 1 and 2 were rationalized in terms of
the local ligand environments in the crystal structures, and magnetic properties of complex 3 are also
given.
Received 13 February 2011
Accepted 22 February 2011
Available online 15 March 2011
Keywords:
Crystal engineering
Coordination preference
Flexible dicarboxylate
Fluorescence
Magnetic properties
Ó 2011 Elsevier Ltd. All rights reserved.
1. Introduction
Rigid aromatic poly-carboxylates are among the most widely
studied class of linkers in this chemistry due to their structural
Coordination polymers have become universally popular and
recognized as one of the most promising class of functional mate-
rials, the structure of which can be conceptually simplified in such
a way that a metal-containing inorganic cluster acting as the nodes
is linked throughout space by a polydentate organic ligand as the
linker to form an organic–inorganic hybrid architecture [1]. The
use of combinatorial chemistry and designed synthetic strategies
can provide innovative coordination polymers materials for many
advanced technological applications such as catalysis [2], hydrogen
storage [3], ion exchange [4], molecular separations [5], non-linear
optics [6] and photoactive materials [7]. Considering the fact that
coordination polymers are built on the inherent strong coordina-
tion of the linker to a metal center, the preparation of such mate-
rials is strongly influenced by the nature of the organic ligands,
which have different molecule sizes, backbone flexibility, confor-
mational preferences, symmetry and substituent groups, and by
the coordination preference of the central metal ions, in addition
to some other factors such as the crystallization conditions, the
metal/ligand ratio, the reaction solvent system, etc.
rigidity and multi-functional coordination capability, which leads
to a rich structural diversity of coordination networks [8]. Recently,
work was geared toward syntheses using flexible linkers with
long-spanning carboxyl groups [9,10], whereas aromatic poly-car-
boxylate linkers combining rigid and flexible carboxyl groups re-
main somewhat of a mystery in this field [11]. Compared to rigid
ligands, using flexible ones to construct coordination polymers is
prone to promote the formation of some particular structures, for
example, helical or looped structural motifs through which other
organic linkers can penetrate, because flexible molecules can easily
adjust their conformations to meet various coordination require-
ments of the metal ion by their ‘‘breathing’’ ability in the solid state
and their adaptive recognition property for coexisting guests or
counterions [9a]. Thus, developing systematic methodologies for
synthesizing such materials by using flexible ligands can afford a
good opportunity to further enrich the structures and properties
of coordination polymers and to provide more understanding for
the directional synthesis of target complexes. On the other hand,
one of the more recent strategies is cooperatively coordinating to
metal centers the combination of a carboxylate linker and a N-do-
nor ancillary ligand, affording some novel frameworks with attrac-
tive features [12].
⇑
Corresponding author. Tel.: +86 0379 65523593; fax: +86 0379 65511205.
E-mail address: wlya@lynu.edu.cn (L.-Y. Wang).
0277-5387/$ - see front matter Ó 2011 Elsevier Ltd. All rights reserved.
doi:10.1016/j.poly.2011.02.053

1488
G.-Z. Liu et al. / Polyhedron 30 (2011) 1487–1493
Our recent work presented a well-defined synthetic system
63%. Anal. Calc. for C19H14N2O4Zn: C, 57.38; H, 3.55; N, 7.04. Found:
C, 57.33; H, 3.71; N, 7.09%. Selected IR (KBr, cmꢁ1): 3456(w, br),
1611(vs), 1603(vs), 1580(s), 1526(s), 1499(s), 1451(s), 1422(vs),
1378(vs), 1271(m), 1224(m), 1146(m), 1081(s), 1049(m), 827(s),
741(m), 714(m), 645(s), 558(m), 502(w). {[Cu(hmph)(bpy)]ꢀH2O}n
(3): Yield (based on Cu): 57%. Anal. Calc. for C19H16CuN2O5: C,
54.87; H, 3.88; N, 6.74. Found: C, 54.83; H, 3.94; N, 6.81%. Selected
IR (KBr, cmꢁ1): 3402(s, br), 1605(vs), 1587(vs), 1526(vs), 1492(s),
1478(s), 1380(vs), 1172(m), 1016(s), 857(m), 760(s), 726(s),
701(s), 638(m), 620(m), 602(w).
based on a homophthalic acid linker (H2hmph) with rigid –COOH
and flexible –CH2COOH carboxyl groups to investigate the sensitiv-
ity of the structures and properties of the formed coordination
polymers to various N-donor ancillary ligands with differing flexi-
bility [13]. To extend the above study, the aim of the present paper
turned to illustrate the effect of different metal centers on the reac-
tion outcome. Our strategy is to perform a series of reactions under
a similar conditions, with only the metal cations varied. Thus, the
widely used organic molecule 4,40-bipyridine (bpy) was adopted
as an ancillary ligand for each synthesis, because this pillared mol-
ecule is known to be an ideal linker between metal atoms for the
propagation of coordination networks, resulting in the formation
of a variety of structural types.
2.3. X-ray crystallography
The crystallographic data collections for complexes 1–3 were
In this contribution we present the preparation and structural
characterization of {[Cd2(hmph)2(bpy)(H2O)2]ꢀH2O}n (1), with a
layered structure containing the larger 50 membered ring,
[Zn(hmph)(bpy)]n (2), featuring a mutually embedded layered
structure with square (4,4) grids, and {[Cu(hmph)(bpy)]ꢀH2O}n
(3), which reveals a twofold-interpenetrating 3D chiral framework
with quartz-dual (qzd) topology. In addition to highlighting their
crystal chemistry, we also provide some insight into the influence
of the local ligand environments in the crystal structures upon the
fluorescence properties of complexes 1 and 2, and magnetic prop-
erties of complex 3 are also given. It is shown that the structures
and physical properties of all these complexes are strikingly sensi-
tive to the nature of the metal center.
carried out on a Bruker SMART APEX II CCD diffractometer
equipped with graphite-monochromated Mo
Ka radiation
(k = 0.71073 Å) by using the // scan technique at room tempera-
x
ture. Absorption corrections were based on symmetry equivalent
reflections using the SADABS program [14]. The structures were
solved by direct methods followed by successive difference Fourier
syntheses, and a full-matrix least-squares refinement on F2 was
carried out using the SHELX 97 program package [15] with aniso-
tropic thermal parameters for all non-hydrogen atoms. The hydro-
gen atoms were placed in calculated positions and refined
isotropically with a riding model, except for those bound to water
molecules of crystallization in 1 and 3 which were assigned using a
Fourier map with common isotropic displacement factors and were
included in the final refinement by use of geometrical restraints.
Relevant crystallographic data for 1–3 are listed in Table 1.
2. Experimental
2.1. Materials and physical measurements
3. Results and discussion
All commercially available solvents and reagents for the synthe-
ses were of reagent grade and were used without further purifica-
tion. Elemental analysis for C, H and N were performed on a Vario
EL III elemental analyzer. Infrared spectra (IR) were obtained from
sample powders pelletized with KBr on an Avatar(tm) 360 E.S.P.IR
spectrophotometer over the range 4000–400 cmꢁ1. The lumines-
cent spectra were recorded at room temperature on an Aminco
Bowman Series 2 spectrophotometer with a xenon arc lamp as
the light source. Variable-temperature magnetic susceptibilities
were measured using a MPMS-7 SQUID magnetometer. Diamag-
netic corrections were made with Pascal’s constants for all constit-
uent atoms.
3.1. Synthesis and IR spectra
Complexes 1–3 were prepared by following almost uniform
hydrothermal procedures, including various synthetic parameters,
except using different metal (Cd, Zn or Cu(II)) acetate salts for each
run. In each synthetic process, NaOH was added as a mineralizer.
Since the only difference in these syntheses is that of different metal
centers, it is clear that the nature of the metal plays a key role in con-
trolling the assembly of the complexes. The IR spectral data show
features attributable to the carboxylate stretching vibrations of
the complexes. The asymmetric stretching and the symmetric
stretching bands for the carboxyl groups are in the range 1488–
1602 and 1384–1460 cmꢁ1 for 1, 1526–1611 and 1377–1499 cmꢁ1
for 2, and 1524–1605 and 1380–1492 cmꢁ1 for 3, respectively. The
strong broad band at about 3400 cmꢁ1 corresponds to the vibration
of water in the complexes 1 and 3. The absence of bands in the range
1760–1630 cmꢁ1 indicates the complete deprotonation of the
H2hmph ligand, which is also evidenced by the results of the struc-
ture analyses. The various coordination modes of H2hmph observed
in complexes 1–3 are shown in Scheme 1.
2.2. Syntheses of the complexes
Complexes 1–3 were hydrothermally synthesized by the
reaction of the metal (Cd, Zn or Cu) acetate, H2hmph ligand and
bpy molecule. Typically, the metal acetate hydrate (0.10 mmol,
0.0266 g for Cd(OAc)2ꢀ2H2O, 0.10 mmol, 0.0219 g for Zn(OAc)2ꢀ
2H2O or 0.10 mmol, 0.0199 g for Cu(OAc)2ꢀH2O), H2hmph
(0.10 mmol, 0.0180 g) and bpy (0.10 mmol, 0.0156 g) were placed
into deionized water (8 mL) in a 25 mL Teflon-lined stainless steel
vessel. Subsequently, 0.5 M NaOH solution (0.15 mmol) was added
while constantly stirring. The vessel was then sealed and heated at
120 °C for 4 days under autogenous pressure, whereupon it was
cooled slowly to room temperature. The resulting crystals were iso-
lated after washing with distilled water, ethanol and acetone, and
were then allowed to dry in air. {[Cd2(hmph)2(bpy)(H2O)2]ꢀH2O}n
(1): Yield (based on Cd): 45%. Anal. Calc. for C28H20Cd2N2O11: C,
42.83; H, 2.57; N, 3.57. Found: C, 42.67; H, 2.69; N, 3.51%. Selected
IR (KBr, cmꢁ1): 3413(s, br), 1602(vs), 1548(vs), 1532(vs), 1488(s),
1460(s), 1413(vs), 1384(vs), 1219(m), 1065(m), 813(m), 807(s),
733(s), 633(s), 507(w). [Zn(hmph)(bpy)]n (2): Yield (based on Zn):
3.2. Structural description of {[Cd2(hmph)2(bpy)(H2O)2]ꢀH2O}n (1)
Single-crystal X-ray analysis revealed a layered structure for 1
with a large 50 membered ring. The asymmetric unit contains
two crystallographically distinct Cd(II) cations, two completely
deprotonated hmph anions, one bpy molecule, two coordinated
water molecules and one guest water molecule, as shown in
Fig. 1a.
The two Cd centers display a relatively rare combination of
coordination geometries: the Cd1 atom is heptacoordinated with
a distorted CdN2O5 pentagonal bipyramidal coordination sphere,
which is defined by two oxygen atoms from the –COOꢁ and

G.-Z. Liu et al. / Polyhedron 30 (2011) 1487–1493
1489
Table 1
Crystal and structure refinement data for complexes 1–3.
1
2
3
Empirical formula
Formula weight
Crystal system
Space group
a (Å)
C
28H20Cd2N2O11
C19H14N2O4Zn
399.69
monoclinic
P21/c
8.419(5)
20.234(11)
10.666(6)
90
C19H16CuN2O5
415.88
hexagonal
P61
11.1755(8)
11.1755(8)
24.825(3)
90
785.26
monoclinic
C2/c
28.054(2)
11.8098(10)
16.9995(14)
90
b (Å)
c (Å)
a
(°)
b (°)
96.1840(10)
103.340(7)
90
c
(°)
90
5599.3(8)
8
1.863
1.585
90
120
2685.1(4)
6
1.543
1.254
V (Å3)
1767.9(17)
4
1.502
1.416
Z
Dcalc (g cmꢁ3
)
l
(mmꢁ1
)
F(0 0 0)
3088
816
1278
Crystal size (mm)
h Range (°)
0.42 ꢂ 0.35 ꢂ 0.31
0.47 ꢂ 0.15 ꢂ 0.10
0.33 ꢂ 0.19 ꢂ 0.08
2.41–25.49
2.21–25.50
2.26–25.49
Reflections collected/unique
Completeness
17933/5205, [Rint = 0.0130]
99.7%
13145/3292 [Rint = 0.1106]
99.9%
13489/2992 [Rint = 0.0686]
99.9%
Data/restraints/parameters
Goodness-of-fit
5205/0/388
1.065
3292/0/235
0.992
3891/3/245
1.016
Absolute structure parameter
none
none
0.185(18)
Final R indices [I > 2
R indices (all data)
r
(I)]a
R1 = 0.0200, wR2 = 0.0577
R1 = 0.0217, wR2 = 0.0587
0.725 and ꢁ0.307
R1 = 0.0631, wR2 = 0.1232
R1 = 0.1381, wR2 = 0.1454
0.676 and ꢁ0.523
R1 = 0.0437, wR2 = 0.0740
R1 = 0.0776, wR2 = 0.0842
0.270 and ꢁ0.258
Largest differences in peak and hole (e Åꢁ3
)
a
R1
=
R
(|Fo| ꢁ |Fc|)/
R
|Fo|; wR2 = {
R
[w(|Fo|2 ꢁ |Fc|2)2]/
R .
[w(|Fo|2)2]}12
tagonal bipyramid with a larger size. Another dimer concerns two
Cd(2)O6 octahedra bridged by a pair of hmph anions via a l1 :
g1 g1
-
chelating coordination mode (Scheme 1a). The larger Cdꢀ ꢀ ꢀCd sep-
aration of 5.8370(4) Å is constrained by the bridging geometry of
the hmph groups. The two types of dimers are linked alternatively
by another type of hmph anion to form cadmium carboxylate zig-
zag chains, wherein the rigid –COOH group of the hmph anion
adopts a l2
ferent type of dimers, while the flexible –CH2COOꢁ group grafts to
the edge-shared dimer by a l2
g2 coordination mode (Scheme
- :
g1 g1 bridging coordination mode to link the two dif-
-
g1
:
1b). The edge-shared dimers in adjacent zigzag chains are further
linked by bpy ligands to form a layered structure with the large
50 membered ring, as shown in Fig. 1b. In each layer, there exists
weaker
p–p interactions between the phenyl ring and pyridine
ring, with a centroid–centroid distance ranging from 4.104 to
4.564 Å, and a dihedral angle range from 10.113° to 34.663°; all
the pyridine rings are almost paralleled to each other, with a
dihedral angle range from 0.260° to 8.572°, and centroid–centroid
distance range from 4.048 to 4.414 Å.
The adjacent layers are stacked in an –ABAB– sequence and are
entirely cohered together by relatively weak van der Waals
interactions to form a 3D supramolecular network (Fig. S1); no
Scheme 1. Coordination modes of H2hmph observed in complexes 1–3.
–CH2COOꢁ groups of one hmph anion, two oxygen atoms belong-
ing to one chelating didentate –CH2COOꢁ group of the symmetry-
related hmph moiety and one water molecule coordinate to the
cadmium center in the equatorial plane, with two nitrogen atoms
from two bpy ligands located at the axial positions of the coordina-
tion sphere; while the Cd2 atom is octahedrally coordinated by
four oxygen atoms belonging to chelating didentate –COOꢁ and
–CH2COOꢁ groups of two symmetry-related hmph moieties, and
the remaining two oxygen atoms belonging to the –COOꢁ group
of another hmph anion and a coordinated water molecule. The
Cd–O distances range from 2.1836(19) to 2.4946(18) Å, and the
Cd–N distances are 2.3428(18) and 2.3448(18) Å.
Two categories of cadmium-based dimers as the fundamental
building motifs are observed in complex 1. One dimer is an edge-
shared Cd(1)N2O5 pentagonal bipyramid. The Cdꢀ ꢀ ꢀCd separation
of 4.0795(4) Å does not indicate any significant direct interaction
between the metal atoms due to the coordination geometry of pen-
significant
p–p interaction is observed between adjacent layers
(phenyl–phenyl distance = 6.331 Å).
3.3. Structural description of [Zn(hmph)(bpy)]n (2)
Complex 2 contains mutually embedded layered structures. The
asymmetric unit contains one crystallographically distinct Zn(II)
cation, one completely deprotonated hmph anion and one bpy
molecule, as shown in Fig. 2a. The Zn atom is tetrahedrally coordi-
nated by two N atoms belonging to two symmetry related bpy
molecules, with Zn–N bond lengths of 2.036(5) and 2.044(5) Å,
and by two O atoms belonging respectively to the rigid –COOH
group and flexible –CH2COOꢁ group of two symmetry-related
hmph anions, with a l1-
g1 monodentate coordination mode
(Scheme 1c). All the Zn–O bond separations are in the range
1.937(4)–1.945(4) Å.

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G.-Z. Liu et al. / Polyhedron 30 (2011) 1487–1493
Fig. 1. (a) ORTEP plot of the asymmetric unit showing the local coordination
environments of the Cd atoms in complex 1, with the symmetry related part drawn
as open circles. Thermal ellipsoids are given at 30% probability. All the H atoms and
an isolated water molecule are omitted for clarity. Symmetry codes: A = 0.5 ꢁ x,
ꢁ0.5 ꢁ y, 1 ꢁ z; B = ꢁx, y, 0.5 ꢁ z; C = x, ꢁ1 + y, z. (b) View of a layer extended by the
bpy ligands. CdO6, grey, octahedra; CdN2O5, white, pentagonal bipyramids.
The fundamental building unit of 2 can be considered as a hmph
anion-bridged Zn dimer with a Znꢀ ꢀ ꢀZn distance of 4.4001(20) Å.
The adjacent dimers are inter-linked by bpy ligands along two dif-
ferent directions to generate a square (4,4) grid layer, with hmph
anions above and below the layer as dangling lateral arms. Each
layer contains a large 52 membered ring, as shown in Fig. 2b.
Individual layers stack together in an –ABCABC– motif, with the
dangling lateral arms in each layer deeply penetrating into the 52
membered rings of two adjacent layers (i.e., the 52 membered ring
pores visible in Fig. 2b are, in fact, entirely occupied by the dan-
gling lateral arms of two additional adjacent layers), giving a dense
structure without any significant overall porosity. As a result, each
phenyl ring is almost parallel to a pyridine ring in the adjacent
layer, with a dihedral angle of 15.018° and a centroid–centroid dis-
Fig. 2. (a) ORTEP plot of the asymmetric unit showing the local coordination
environments of Zn atoms in complex 2, with the symmetry related part drawn as
open circles. Thermal ellipsoids are given at 30% probability. All the H atoms are
omitted for clarity. Symmetry codes: A = ꢁx, 1 ꢁ y, 2 ꢁ z; B = ꢁ1 + x, 0.5 ꢁ y, 0.5 + z.
(b) View of the layered structure in 2. ZnO2N2, grey, tetrahedra. (c) View of the 3D
packing of layers showing three adjacent layers (grey, black and grey).
hmph anion, one bpy molecule and one coordination water, as
shown in Fig. 3a. The Cu atom is six-coordinated with a distorted
CuN2O4 octahedral coordination sphere, which is defined by two
oxygen atoms belonging to one l1
COOꢁ group of one hmph anion (Scheme 1d), two oxygen atoms
belonging to one l1
g1 monodentate –CH2COOꢁ group of the
- :
g1 g1 chelating didentate –
tance of 3.826 Å. Thus there exists strong
p–p interactions be-
-
tween these phenyl and pyridine rings to stabilize the 3D
supramolecular architecture of 2 (Fig. 2c).
symmetry-related hmph moiety and one water molecule in
the equatorial plane, with two nitrogen atoms from two bpy
ligands located at the axial positions of the coordination sphere.
The Cu–N bond lengths are in the range 2.042(3)–2.045(3) Å, and
the Cu–O bond distances are in the range 1.950(3)–2.510(3) Å.
3.4. Structural description of {[Cu(hmph)(bpy)]ꢀH2O}n (3)
Complex 3 crystallizes in the hexagonal chiral space group P61
and reveals a remarkable extended twofold-interpenetrating 3D
chiral framework. The asymmetric unit contains one crystallo-
graphically distinct Cu(II) atom, one completely deprotonated
The Cu atoms are connected by l2-bridging hmph anions to
generate some helixes with homochirality propagating along the
c-direction (Fig. 3b). The homochiral helixes are further cross-
linked by bpy molecules affording a 3D chiral network with large

G.-Z. Liu et al. / Polyhedron 30 (2011) 1487–1493
1491
another independent net (Fig. 3e), resulting in a dense structure
without any significant overall porosity, that is two interpenetrat-
ing chiral nets coexist within the 3D chiral framework. Topological
analysis reveals that the framework of 3 represents a commonly
observed uninodal 4-connected net with a quartz-dual (qzd) topol-
ogy (Fig. S2), featuring two chiral interpenetrating nets of quartz
topology, which can be characterized by a short vertex symbol
(75ꢀ9) with each hmph and bpy ligand as 2-connected nodes and
each Cu center as a 4-connected node. Moreover, in the overall
crystal structure, each pyridine ring in one quartz net is almost par-
alleled to a pyridine ring in another quartz net with a dihedral an-
gle of 8.314° and a centroid–centroid distance range of 4.586–
6.212 Å. Thus there exist significantly weak
p–p interactions be-
tween adjacent pyridine rings belonging to the two interpenetrat-
ing quartz nets that stabilize the entire 3D supramolecular
framework of 3.
From the above structural results, it is demonstrated that the
structures of all these complexes are strongly dependent on the
nature of metal cations. The Cd(II) atoms in 1 exhibit octahedral
and pentagonal bipyramidal coordination environments affording
a layered structure, which are consistent with the tendency of
Cd(II) to have a high coordination number, the Zn(II) centers in 2
have common tetrahedral coordination geometries producing
mutually embedded layered coordination networks, whereas the
Cu(II) cations display octahedral configurations, forming a 3D chi-
ral framework with a quartz-dual (qzd) topology in complex 3.
3.5. Photoluminescent properties of 1 and 2
The solid-state electronic emission spectra of compounds 1 and
2 are shown in Fig. S3. It can be seen that complex 1 has a strong
emission with a maximum at 430 nm (kex = 354 nm), while 2
exhibits a rather weak emission with a maximum at 392 nm
(kex = 342 nm). To more thoroughly understand the nature of the
emission band, we also investigated the luminescence of the free
powdered H2hmph and bpy ligands. The H2hmph ligand displays
a strong fluorescence emission with a maximum at 452 nm
(kex = 390 nm), and a very weak photoluminescence emission is
observed for the bpy molecule (Fig. S3).
It is a great challenge to understand the luminescence spectra of
the cooperatively coordinated complexes mixing two types of link-
ers due to their unpredictable luminescence features. Fluorescence
emission of both complexes can be tentatively attributed to the
intraligand
p
–
p
⁄ or n– ⁄ transition [16] originating from different
p
chromophores in the crystal structures. The strong emission of
complex 1 is assigned to the hmph-based intraligand fluorescence
emission, since this ligand also has a similar emission characteris-
tic, and the rather weak emission of 2 has a total spectrum charac-
teristic similar to that of the free powdered bpy ligand.
We consider that the different fluorescence emissions between
1 and 2 are mainly associated with the interchromophore interac-
tions observed in the crystal structures, in addition to a small con-
tribution from other uncertain factors, such as deprotonation of the
ligands, inorganic architecture influences and crystallite size ef-
fects, etc. The neighbor distances between ring centroids and the
dihedral angle between ring planes are 4.104–4.564 Å, 10.113–
34.663° (for 1, between a phenyl ring and a pyridine ring in an
individual layer), 4.048–4.414 Å, 0.260–8.572° (for 1, between
paired pyridine rings in an individual layer) and 3.826 Å, 15.018°
(for 2, between phenyl ring and pyridine ring in adjacent layers).
The longer cofacial chromophore distances in 1 lead to its fluores-
cence being similar to the free H2hmph ligand, but with stronger
emission. The intense interchromophore interactions in 2 may re-
sult in the rearrangement of energy levels, thus making the highest
occupied molecular orbitals (HOMOs) and the lower unoccupied
Fig. 3. (a) ORTEP plot of the asymmetric unit showing the local coordination
environments of the Cu cations in complex 3, with the symmetry related part
drawn as open circles. Thermal ellipsoids are given at 30% probability. All the H
atoms are omitted for clarity. Symmetry codes: A = ꢁy, ꢁ2 + x ꢁ y, 0.3333 + z; B = x,
ꢁ1 + y, z. (b) View of a helix motif running along the [0 0 1] direction. CuN2O4,
hatched octahedra. (c) View of one set of two interpenetrating networks showing
large propeller-like channels. CuN2O4, hatched octahedra. (d) Description of the
helical tube in detail. Light grey, deep grey and black representing the three strands
of the chiral-stranded helix; black, bpy ligands. (e) View of the 3D packing
structure, showing twofold-interpenetrating networks (grey and black).
propeller-like channel pores (Fig. 3c). In fact, each channel is
enclosed by three homochiral helixes cohered together by bpy
molecules between them to form a helical tubular (Fig. 3d). Unfor-
tunately, these large channels allows each net to be penetrated by
molecular orbitals (LUMOs) probably associated with the
p and

1492
G.-Z. Liu et al. / Polyhedron 30 (2011) 1487–1493
p⁄ orbitals from the bpy molecules [17]. This may be the origin
why the fluorescence spectrum of 2 is similar to that of the free
bpy molecule, with a weak emission. Certainly, some theoretical
calculations on the experimental geometries are expected to fur-
ther support the assignment of these emissions. Furthermore, the
enhanced rigidity on the ligand moieties upon metal coordination
in both complexes is responsible for the strength in fluorescence
emission intensity of the linker, resulting from the reduced effi-
ciency of non-radiative pathways [18]. Thus, the high lumines-
long Cuꢀ ꢀ ꢀCu separation and the complicated magnetic pathway
by the ligand hmph.
4. Conclusions
Similar mild hydrothermal methods produce three metal (II)
complexes, cooperatively coordinated by the flexible hmph anion
and rigid bpy molecule. In these polymers, the metal centers adopt
various coordination geometries cohered by hmph anions to form
diversified metal carboxylate motifs manifesting a marked struc-
tural dependence on the nature of the metal ion. Complex 1 con-
tains a cadmium carboxylate zigzag chain featuring alternatively
edge-shared Cd pentagonal bipyramids and hmph-bridged dinucle-
ar Cd octahedra, complex 2 possesses hmph-bridged dinuclear Zn
tetrahedra, and complex 3 is composed of a copper carboxylate
helix featuring hmph-bridged mononuclear Cu octahedra. In addi-
tion, complexes 1 and 2 exhibit intraligand fluorescence emissions
originating from different chromophores, which can be rationalized
in terms of the local ligand environments in the crystal structures,
and complex 3 displays an extremely weak antiferromagnetic inter-
action between the Cu(II) ions due to the long Cuꢀ ꢀ ꢀCu separation in
its structure. This work demonstrates the continued utility of the
nature of metal ion in determining accessible coordination polymer
structures, and will expand the coordination chemistry of benzene
carboxylate ligands. Further systematic studies for the design and
synthesis such crystalline materials with H2hmph and other ancil-
lary ligands are underway in our laboratory.
cence efficiency indicates that
luminescent materials.
1 is a good candidate for
3.6. Magnetic properties of 3
The variable-temperature magnetic susceptibility of 3 has been
measured from 2 to 300 K in a 2000 Oe applied field to investigate
the spin communication between the Cu(II) cations. As illustrated
in Fig. S4, the v
mT value of 0.372 cm3 K molꢁ1 at 300 K is basically
in accordance with the spin-only value of 0.375 cm3 K molꢁ1 ex-
pected for one uncoupled Cu(II) ion with S = 1/2. Upon cooling,
the
vmT product remains almost constant until 50 K, and subse-
quently decreases rapidly to 0.295 cm3 K molꢁ1 at 2 K. Such behav-
ior can be referred to the presence of an extremely weak
antiferromagnetic exchange behavior between the neighboring
Cu(II) ions, which allowed the magnetism to obey the Curie–Weiss
law
v
m = C/(T ꢁ h) over the full temperature range, giving a Curie
constant C = 0.37 cm3 K molꢁ1 and a Weiss constant h = ꢁ0.56 K.
As is shown in the crystallographic part, the 3D framework of
complex 3 is made up of isolated Cu(II) entities connected by the
ligand hmph to afford carboxylate helix motifs, with the bpy mol-
ecules acting as bridges between the interchains. Until now, no
appropriate theory model has been established to determine the
magnetic coupling constant between metal ions for a 3D polymeric
framework. In order to evaluate the magnetic interactions in 3, we
expect that the magnetic pathways between the neighboring Cu(II)
ions are through an intrachain exchange mode. Thus, the magnetic
susceptibility of the complex has been fitted by an isotropic anti-
ferromagnetic chain model of S = 1/2 spin. The following Eq. (1)
is induced from the Hamiltonian [19]
Acknowledgments
This work was supported by the National Natural Science Foun-
dation of China (Nos. 20971064 and 21071074) and the Foundation
of Education Committee of Henan Province (No. 092102210315).
Appendix A. Supplementary data
CCDC 804745, 804746 and 804747 contains the supplementary
crystallographic data for 1, 2 and 3. These data can be obtained free
of charge via http://www.ccdc.cam.ac.uk/conts/retrieving.html, or
from the Cambridge Crystallographic Data Centre, 12 Union Road,
Cambridge CB2 1EZ, UK; fax: (+44) 1223-336-033; or e-mail: de-
posit@ccdc.cam.ac.uk. Supplementary data associated with this
article can be found, in the online version, at doi:10.1016/
j.poly.2011.02.053.
i¼nꢁ1
X
^
H ¼ ꢁJ
SiSiþ1
i¼1
Ng2b2
0:25 þ 0:074975x þ 0:075235x2
vm0
¼
kT 1:0 þ 0:9931x þ 0:172135x2 þ 0:757825x3
jJj
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