Zinc(ii) and cadmium(ii) carboxyphosphonates with a 3D pillared-layered structure: Synthesis, crystal structures, high thermal stabilities and luminescent properties

Article abstract of DOI:10.1039/c2ra22571a

Two novel divalent metal carboxyphosphonates with a 3D pillared-layered structure, [Zn(HL)] (1) and [CdCl(H₂L)] (2) (H₃L = 4-HOOCC₆H₄CH₂NHCH₂PO ₃H₂), have been synthesized under hydrothermal conditions. Compounds 1 and 2 both feature three-dimensional (3D) framework structures with two-dimensional (2D) inorganic layers pillared by H₃L. In compound 1, each {ZnO₄} tetrahedron is linked via corner-sharing by three {CPO₃} tetrahedra to form a two-dimensional (2D) inorganic layer in the ac-plane. The adjacent layers are further pillared by the organic backbone {-C₆H₄CH₂NHCH₂-} of the carboxyphosphonate ligand, generating a 3D pillared-layered structure. For compound 2, each Cd(ii) cation is six-coordinated, and the interconnection of two {CdCl₂O₄} octahedra forms a dimer via edge-sharing. The dimers are interconnected by {CPO₃} tetrahedra via corner-sharing to form a 2D inorganic layer in the ab-plane. Such neighboring 2D inorganic layers are further cross-linked through the organic pillars of the carboxyphosphonate ligand, thus resulting in a 3D pillared-layered structure. An interesting feature of compounds 1 and 2 is the presence of alternate left- and right-handed helical chains in their structures. It is of note that two title compounds are both stable up to high temperature. The luminescent properties of compounds 1 and 2 have also been studied.

Full text of DOI:10.1039/c2ra22571a

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Zinc(II) and cadmium(II) carboxyphosphonates with a
3D pillared-layered structure: synthesis, crystal
structures, high thermal stabilities and luminescent
properties3
Cite this: RSC Advances, 2013, 3, 623
Wei Chu, Yan-Yu Zhu, Zhen-Gang Sun,* Cheng-Qi Jiao, Jing Li, Shou-Hui Sun,
Hui Tian and Ming-Jing Zheng
Two novel divalent metal carboxyphosphonates with a 3D pillared-layered structure, [Zn(HL)] (1) and
[CdCl(H2L)] (2) (H3L
= 4-HOOCC6H4CH2NHCH2PO3H2), have been synthesized under hydrothermal
conditions. Compounds 1 and 2 both feature three-dimensional (3D) framework structures with two-
dimensional (2D) inorganic layers pillared by H3L. In compound 1, each {ZnO4} tetrahedron is linked via
corner-sharing by three {CPO3} tetrahedra to form a two-dimensional (2D) inorganic layer in the ac-plane.
The adjacent layers are further pillared by the organic backbone {–C6H4CH2NHCH2–} of the carboxypho-
sphonate ligand, generating a 3D pillared-layered structure. For compound 2, each Cd(II) cation is six-
coordinated, and the interconnection of two {CdCl2O4} octahedra forms a dimer via edge-sharing. The
dimers are interconnected by {CPO3} tetrahedra via corner-sharing to form a 2D inorganic layer in the ab-
plane. Such neighboring 2D inorganic layers are further cross-linked through the organic pillars of the
carboxyphosphonate ligand, thus resulting in a 3D pillared-layered structure. An interesting feature of
compounds 1 and 2 is the presence of alternate left- and right-handed helical chains in their structures. It is
of note that two title compounds are both stable up to high temperature. The luminescent properties of
compounds 1 and 2 have also been studied.
Received 18th October 2012,
Accepted 12th November 2012
DOI: 10.1039/c2ra22571a
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new structural types of metal phosphonates but also lead to
interesting properties.
Introduction
Design and assembly of metal phosphonate materials with
intriguing diversity of architectures and properties have grown
rapidly in recent decades due to their potential applications in
catalysis, photochemistry, magnetism, ion exchange, molecu-
lar recognition and materials chemistry.1 In order to obtain
metal phosphonate compounds with a variety of architectures
and interesting physical or chemical properties, an encoura-
ging research direction is the synthesis of metal phosphonates
functionalized with amine, hydroxyl and carboxylate groups.
Much work of metal phosphonates has shown that the use of
bi- and multifunctional phosphonic acids containing –NH2,
–OH, and –COOH sub-functional groups may not only result in
In recent years, a number of metal phosphonate frame-
works with additional functionalities have been reported.2–4 By
attaching functional groups such as amine, hydroxyl and
carboxylate groups to the phosphonic acid, a series of metal
phosphonates with framework structures have been isolated in
our laboratory.5 Results from ours and other groups indicate
that carboxyphosphonic acids are good candidates for the
synthesis of metal phosphonates with framework structures,
in which the organic part has a controllable spacer role and
the –COOH and –PO3 groups chelate metal ions to form novel
structure types.6 Recently, increased attention has been paid to
rigid aromatic carboxyphosphonate ligands, which play an
important role to design metal phosphonates with novel
architectures and properties.7 Three zinc carboxyphospho-
nates based on 4-phosphonobenzoic acid reported by Zhao
and co-workers feature 3D zeolite-like structure, layer structure
and 3D pillar-layered structure, respectively.7a Zheng’s group
newly reported the synthesis of metal phosphonates with
2-carboxyphenylphosphonic acid as ligand, which show
reversible dehydration–rehydration behavior and ferrimagne-
tism.7b In a recent article, a carboxyphosphonic acid with rigid
School of Chemistry and Chemical Engineering, Liaoning Normal University, Dalian
116029, P. R. China. E-mail: szg188@163.com
Electronic supplementary information (ESI) available: X-ray crystallographic
3
files in CIF format for compounds 1 and 2. IR spectra of compounds 1 and 2.
XRD pattern of the intermediate products in the thermal decomposition for
compounds 1 and 2 as well as XRD patterns of the experiments compared to
those simulated from X-ray single-crystal data for compounds 1 and 2. CCDC
reference numbers 874503 (1) and 874504 (2). For ESI and crystallographic data
in CIF or other electronic format see DOI: 10.1039/c2ra22571A
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Found: C, 35.09; H, 3.23; N, 4.58; P, 10.07; Zn, 21.15%. IR (KBr,
cm21): 3448(m), 3014(s), 2865(w), 2807(m), 1603(s), 1557(s),
1473(m), 1428(m), 1395(s), 1136(s), 1078(s), 1026(s), 973(m),
754(m), 598(m), 521(w), 430 (w).
structure, 4-HOOCC6H4CH2NHCH2PO3H2 (H3L) was used as a
ligand in the synthesis of metal phosphonates with high
dimensionality. To the best of our knowledge, only one
investigation on the synthesis of lanthanide carboxypho-
sphonates based on 4-HOOCC6H4CH2NHCH2PO3H2 has
recently been reported by Mao et al.8 However, the correspond-
ing transition-metal carboxyphosphonates have not been
reported up to now. With the aim of exploring novel metal
phosphonates with interesting structures and properties, we
further investigated the rigid and multifunctional carboxypho-
sphonic acid ligand 4-HOOCC6H4CH2NHCH2PO3H2 (H3L),
since it is expected to adopt various coordination modes
under different reaction conditions which may result in a
variety of interesting structures. More importantly, its rela-
tively rigid structure and p-conjugated character is useful to
produce coordination polymers with active photoluminescent
properties. In the present paper, by employing carboxypho-
sphonic acid, 4-HOOCC6H4CH2NHCH2PO3H2 (H3L) as ligand,
we have successfully obtained two novel divalent metal
carboxyphosphonates with a 3D pillared-layered structure,
namely, [Zn(HL)] (1) and [CdCl(H2L)] (2). Herein, we report
their syntheses, crystal structures, and thermal stabilities. The
luminescence properties of compounds 1 and 2 have also been
studied.
SYNTHESIS OF [CDCL(H2L)] (2). A mixture of H3L (0.24 g, 1 mmol)
and Cd(Ac)2?2H2O (0.27 g, 1 mmol) was dissolved in 10 mL
distilled water. The pH value was adjusted to 4.0 by adding 3 M
HCl solution dropwise. The resulting solution was stirred for
about 1 h at room temperature, sealed in a 20 mL Teflon-lined
stainless-steel autoclave, and then heated at 160 uC for 4 days
under autogenous pressure. After the mixture was cooled
slowly to room temperature, colorless plate crystals of
compound 2 were obtained. The pH value of the resultant
solution was 3.5. Yield: 67.4% (based on Cd). Anal. Calc. for
C9H11NO5ClPCd: C, 27.57; H, 2.83; N, 3.57; P, 7.90; Cd, 28.67.
Found: C, 27.52; H, 2.88; N, 3.53; P, 7.95; Cd, 28.61%. IR (KBr,
cm21): 3445(m), 3033(s), 2807(w), 2535(w), 1661(s), 1609(s),
1578(m), 1460(m), 1414(m), 1324(s), 1142(s), 1084(s), 1026(m),
974(s), 851(w), 767(m), 702(w), 579(m), 527(m), 495(w).
Crystallographic studies
Data collections for compounds 1 and 2 were performed on a
Bruker AXS Smart APEX II CCD X-diffractometer equipped
with graphite-monochromated Mo-Ka radiation (l = 0.71073 Å)
at 293 ¡ 2 K. An empirical absorption correction was applied
using the SADABS program. All structures were solved by direct
methods and refined by full-matrix least-squares fitting on F2
by SHELXS-97.10 All non-hydrogen atoms were refined with
anisotropic thermal parameters. Hydrogen atoms of organic
ligands were generated geometrically with fixed isotropic
thermal parameters, and included in the structure factor
calculations. Details of crystallographic data and structural
refinements of compounds 1 and 2 are summarized in Table 1.
Selected bond lengths and angles of compounds 1 and 2 are
listed in Table 2. Hydrogen bonds for compounds 1 and 2 are
given in Table 3.
Experimental
Materials and measurements
The ligand 4-HOOCC6H4CH2NHCH2PO3H2 (H3L) was synthe-
sized according to procedures described previously.8,9 All other
chemicals were obtained from commercial sources and used
without further purification. C, H and N analyses were
determined by using a PE-2400 elemental analyzer. Zn, Cd
and P were determined by using an inductively coupled
plasma (ICP) atomic absorption spectrometer. IR spectra were
recorded on a Bruker AXS TENSOR-27 FT-IR spectrometer with
KBr pellets in the range 4000–400 cm21. X-Ray powder
diffraction data was collected on a Bruker AXS D8 Advance
diffractometer using Cu-Ka radiation (l = 1.5418 Å) in the 2h
range of 5–60u with a step size of 0.02u and a scanning rate of
3u min21. Thermogravimetric (TG) analyses were performed
on a NETZSCH STA-449-F3 at a heating rate of 10 uC min21
under an air flow from 50 to 1200 uC. The luminescence
spectra were reported on a HITACHI F-7000 spectrofluorimeter
(solid).
SYNTHESIS OF [ZN(HL)] (1). A mixture of H3L (0.12 g, 0.5 mmol)
and ZnSO4?7H2O (0.14 g, 0.5 mmol) was dissolved in 10 mL
distilled water. The pH value was adjusted to 4.0 by adding an
ethylenediamine solution dropwise. The resulting solution was
stirred for about 1 h at room temperature, sealed in a 20 mL
Teflon-lined stainless steel autoclave, and then heated at 160
uC for 4 days under autogenous pressure. After the mixture was
cooled slowly to room temperature, colorless plate crystals of
compound 1 were obtained. The pH value of the resultant
solution was 4.0. Yield: 64.2% (based on Zn). Anal. Calc. for
C9H10NO5PZn: C, 35.03; H, 3.27; N, 4.54; P, 10.04; Zn, 21.20.
Table 1 Crystal data and structure refinements for compounds 1 and 2
Compound
1
2
Empirical formula
Mr
Crystal system
Space group
a/Å
C9H10NO5PZn
308.52
Orthorhombic
Pca21
9.9240(11)
12.4951(14)
8.6343(9)
1070.7(2)
4
C9H11ClNO5PCd
392.01
Orthorhombic
Pbca
7.6418(11)
12.7390(18)
24.237(4)
2359.4(6)
8
2.207
1536
b/Å
c/Å
V/Å3
Z
Dc/g cm23
F(000)
1.914
624
Crystal size/mm
h Range/u
Reflections
collected/unique
Rint
0.05 6 0.03 6 0.01
2.62–26.49
5792, 2075
0.08 6 0.07 6 0.02
3.15–26.49
11 804, 2442
0.0616
1.034
0.0542, 0.1076
0.0782, 0.1184
0.611, 20.473
0.0779
1.013
0.0382, 0.0749
0.0663, 0.0862
0.570, 20.667
GOF on F2
a
R1, wR2 [I . 2s(I)]
a
R1, wR2 (all data)
Drmax, min/e Å23
a
b
2 1/2
R1 = g(|Fo| 2 |Fc|)/g|Fo|. wR2 = [gw(|Fo| 2 |Fc|)2/gwFo
]
.
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Table 3 Hydrogen bonds for compounds 1 and 2a
Results and discussion
…
… … …
d(D–H)/Å d(H A)/Å h(D–H A)/u d(D A)/Å
Syntheses
D–H
A
Hydrothermal synthesis has been extensively explored for the
preparation of metal phosphonate materials. The product
composition depends on a number of critical conditions,
including pH of the medium, temperature, different metal
salts and so on. With the aim to explore the optimum method
for obtaining pure phase materials, two systematic experi-
1
…
…
N(1)–H(1C) O(2)#3 0.90
N(1)–H(1D) O(5)#7 0.90
1.94
1.97
178
164
2.840(9)
2.843(10)
2
…
…
…
N(1)–H(1B) Cl(1)#7 0.90
2.32
1.84
1.68
175
166
170
3.216(5)
2.726(5)
2.524(5)
N(1)–H(1E) O(1)#7 0.90
O(4)–H(4B) O(2)#6 0.85
mental investigations have been designed for the system Zn2+
/
a
H3L of compound 1. The first experiment was designed to
investigate the influence of the anions of zinc salts on the
reaction products. Thus, four different zinc salts (ZnCl2,
Zn(Ac)2?2H2O, Zn(NO3)2?6H2O and ZnSO4?7H2O) were reacted
keeping a constant Zn2+ : H3L = 1 : 1 ratio at their original pH
(T = 160 uC, 96 h). Our experiments demonstrated that the final
reaction products synthesized from different zinc salts exhibit
different phases. ZnCl2 (original pH = 2.5) and Zn(NO3)2?6H2O
(original pH = 2) led to amorphous powders. However, mixed
phases (tiny crystals and powder) for compound 1 were
Symmetry transformations used to generate equivalent atoms. For
1: #3 x + 1/2, 2y + 2, z; #7 2x, 2y + 1, z + 1/2; for 2: #6 2x + 1, y +
1/2, 2z + 1/2; #7 x + 1/2, 2y + 3/2, 2z.
constant Zn2+ : H3L = 1 : 1 ratio (T = 160 uC, 96 h). This second
experiment showed that mixed phases (tiny crystals and
powder) of compound 1 were formed from Zn(Ac)2?2H2O as
zinc salt at pH = 4, with amorphous powders forming at all
other pH values under the same synthesis conditions.
Unexpectedly, a pure phase of large-plate single crystals of
compound 1 were obtained at pH = 4 with ZnSO4?7H2O as zinc
salt. However, formation of amorphous powders or mixed
phases (tiny crystals and powder) for compound 1 was
observed at all other pH values. The results of the two
experimental investigations indicate that ZnSO4?7H2O is the
optimal zinc salt to synthesize the pure phase of single crystals
for compound 1 at pH = 4 at a constant Zn2+ : H3L = 1 : 1 ratio
(T = 160 uC, 96 h). Analogous experimental investigations were
also designed to obtain the optimum method for synthesizing
compound 2. Also, pure crystal phase production of com-
obtained from Zn(Ac)2?2H2O (original pH
=
4) and
ZnSO4?7H2O (original pH 2). We thus realized that
=
ZnSO4?7H2O and Zn(Ac)2?2H2O may be more adaptable zinc
salts as reactants to synthesize compound 1 keeping a
constant Zn2+ : H3L = 1 : 1 ratio. One other variable that has
a profound impact on the product formation of hydrothermal
syntheses is the pH value of the reaction mixture. To gain a
better understanding of the influence of the pH value within
the system Zn2+/H3L, a second experiment was designed. The
system Zn2+/H3L was investigated using the two more
adaptable zinc salts at different pH, namely, ZnSO4?7H2O
and Zn(Ac)2?2H2O, under hydrothermal conditions keeping a
pound
2
was achievable by appropriate adjustment:
Cd(Ac)2?2H2O as the optimal cadmium salt at pH = 4 keeping
a constant Cd2+ : H3L = 1 : 1 ratio (T = 160 uC, 96 h).
Table 2 Selected bond lengths (Å) and angles (u) for compounds 1 and 2a
Description of the crystal structures
1
2
X-Ray single-crystal diffraction revealed that compound 1
crystallizes in the orthorhombic space group Pca21 (Table 1).
The asymmetric unit of the structure for compound 1 is
comprised of one Zn(II) cation and one HL22 anion. As shown
in Fig. 1, the Zn(II) cation is four-coordinated by one
carboxylate oxygen atom (O4C) from one HL22 anion and
three phosphonate oxygen atoms (O1, O2A and O3B) from
three separate HL22 anions. On the other hand, the
carboxyphosphonate ligand acts as a tetradentate metal linker,
bridging with four Zn(II) cations through three phosphonate
oxygen atoms and one carboxylate oxygen atom. The phos-
phonate oxygen atoms (O1, O2 and O3) and one carboxylate
oxygen atom (O4) are all monodentate. The Zn–O distances
range from 1.920(4) to 1.985(5) Å (Table 2), which are similar to
those reported for other zinc(II) carboxyphosphonates.11 Both
phosphonate and carboxylate groups of the ligand are
deprotonated, but the amine group is protonated based on
the requirement of charge balance.
Zn(1)–O(3)#1
Zn(1)–O(1)
Zn(1)–O(4)#2
Zn(1)–O(2)#3
P(1)–O(3)
P(1)–O(2)
P(1)–O(1)
1.920(4) Cd(1)–O(3)#1
1.933(4) Cd(1)–O(1)#2
1.936(4) Cd(1)–O(2)
1.985(5) Cd(1)–O(5)#3
1.495(5) Cd(1)–Cl(1)
1.512(4) P(1)–O(1)
1.517(5) P(1)–O(2)
1.831(7) P(1)–O(3)
2.197(3)
2.276(3)
2.288(4)
2.3874(14)
2.6118(14)
1.507(4)
1.535(4)
1.498(4)
P(1)–C(1)
O(3)#1–Zn(1)–O(1)
104.7(2)
O(3)#1–Cd(1)–O(1)#2
O(3)#1–Cd(1)–O(2)
O(1)#2–Cd(1)–O(2)
82.58(13)
100.39(13)
161.94(12)
O(3)#1–Zn(1)–O(4)#2 121.1(2)
O(1)–Zn(1)–O(4)#2 118.9(2)
O(3)#1–Zn(1)–O(2)#3 100.9(2)
O(1)–Zn(1)–O(2)#3
O(4)#2–Zn(1)–O(2)#3 105.9(2)
O(3)–P(1)–O(2)
O(3)–P(1)–O(1)
O(2)–P(1)–O(1)
O(3)–P(1)–C(1)
O(2)–P(1)–C(1)
O(1)–P(1)–C(1)
O(3)#1–Cd(1)–O(5)#3 102.14(10)
102.13(19) O(1)#2–Cd(1)–O(5)#3 81.15(9)
O(2)–Cd(1)–O(5)#3
O(1)#2–Cd(1)–Cl(1)
O(5)#3–Cd(1)–Cl(1)
O(3)#1–Cd(1)–Cl(1)#4 170.56(10)
O(5)#3–Cd(1)–Cl(1)#4 81.13(5)
Cl(1)–Cd(1)–Cl(1)#4
Cd(1)–Cl(1)–Cd(1)#4
80.81(10)
95.03(9)
163.68(5)
114.1(3)
110.2(3)
110.4(2)
106.9(3)
106.3(3)
108.7(3)
82.97(4)
97.03(4)
a
Symmetry transformations used to generate equivalent atoms. For
1: #1 2x, 2y + 2, z 2 1/2; #2 x, y + 1, z; #3 x + 1/2, 2y + 2, z; #4 x, y
2 1, z; #5 x 2 1/2, 2y + 2, z; for 2: #1 2x + 1, 2y + 1, 2z; #2 2x +
1/2, y 2 1/2, z; #3 2x + 1, y 2 1/2, 2z + 1/2; #4 2x, 2y + 1, 2z; #5
2x + 1/2, y + 1/2, z.
Compound 1 exhibits a three-dimensional framework with
pillared-layered structure. Each {CPO3} tetrahedron connects
three {ZnO4} tetrahedra through three phosphonate oxygen
atoms, and the {ZnO4} tetrahedra are interconnected by
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Fig. 1 Structure unit of compound 1 showing the atom labeling. Thermal
ellipsoids are shown at the 50% probability level. Symmetry code for the
generated atoms: (A) x + 1/2, 2y + 2, z; (B) 2x, 2y + 2, z 2 1/2; (C) x, y + 1, z.
{CPO3} tetrahedra via corner-sharing to form a two-dimen-
sional (2D) inorganic layer in the ac-plane (Fig. 2a). The result
of connections in this manner is formation of regular windows
made up of 12 atoms, which consist of three Zn, three P and
six O atoms with the sequences Zn–O–P–O–Zn–O–P–O–Zn–O–
P–O in the inorganic layer. Such neighboring 2D inorganic
layers are further cross-linked via the organic backbone
{–C6H4CH2NHCH2–} of the carboxyphosphonate ligands, gen-
erating a 3D pillared-layered structure with a 1D channel
system along the a-axis (Fig. 2b). The channels running along
the a-axis are formed by 28-membered rings composed of
three Zn(II) cations, two HL22 ligands and one phosphonate
group of the ligand (Fig. 2c). The size of the channel is
estimated to be 12.3 Å (P1–O2) 6 3.2 Å (C4–C7) based on
structure data. The adjacent benzyl rings of the carboxypho-
sphonate ligands in the pillared-layered structure of com-
pound 1 are almost parallel to each other. The face-to-face
distance (3.5 Å) between adjacent benzyl rings is in the normal
range (3.3–3.8 Å) for such interactions, hence there are p–p
stacking interactions in the 3D framework structure of
compound 1 (Fig. 2d). In addition hydrogen bonds occur in
compound 1. As shown in Fig. 3, there are intermolecular
hydrogen bonds among the protonated nitrogen atom (N1),
the carboxylate oxygen atom (O5) and the phosphonate oxygen
Fig. 3 The connectivity of hydrogen bonds for compound 1 (all H atoms
attached to C atoms are omitted for clarity).
A notable feature for compound 1 is the presence of left- and
right-handed helical chains in the inorganic layer structure of
the ac-plane (Fig. 4). The interconnection of one Zn(II) cation
and a pair of HL22 ligands by phosphonate oxygen atoms (O1,
O2 and O3) leads to infinite 1D left- and right-handed helical
chains along the c-axis. However, compound 1 exhibits an
achiral 3D framework structure because of the presence of the
mirror symmetry.
Compound 2 crystallizes in the orthorhombic space group
Pbca (see Table 1). Each asymmetric unit contains one Cd(II)
cation, one H2L2 anion and one chloride anion. As shown in
Fig. 5, the Cd(II) cation exhibits a six-coordinated environ-
ment, bonding to two Cl2 anions, one carboxylate oxygen atom
(O5D) from one H2L2 anion and three phosphonate oxygen
atoms (O1B, O2 and O3C) from three separate H2L2 anions.
The carboxyphosphonate ligand functions as a tetradentate
ligand, binding four Cd(II) cations through three phosphonate
oxygen atoms and one carboxylate oxygen atom. The phos-
phonate oxygen atoms (O1, O2 and O3) and one carboxylate
oxygen atom (O5) are all monodentate. The Cd–O distances
range from 2.197(3) to 2.3874(14) Å. The Cd–Cl bond lengths
are 2.6118(14) and 2.6804(14) Å, which are much larger than
the Cd–O bonds (see Table 2). These values are in agreement
…
atom (O2) with distances of 2.843(10) Å (N1–H1C O5#7) and
…
2.840(9) Å (N1–H1D O2#3) and corresponding angles of 164
and 178u. These hydrogen bonds give rise to an infinite three-
dimensional network and enhance the stability of this
network.
Fig. 2 (a) View of the layer structure for compound 1 in the ac-plane; (b) the 3D framework structure of compound 1 along the a-axis; (c) a 28-atom ring in
compound 1 generated from equivalent positions (x, 21 + y, z), (21/2 + x, 2 2 y, z) and (21/2 + x, 1 2 y, z); (d) the p–p stacking interaction in compound 1 (a = 3.5 Å).
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H2L2 anion are monoprotonated, respectively. The carbox-
yphosphonate ligand is a univalent anion.
The structure of compound 2 also features a 3D pillared-
layered framework. The two {CdCl2O4} octahedra are inter-
connected into a dimer via edge-sharing, and the so-built
dimer {Cd2Cl2O8} polyhedra and {CPO3} tetrahedra connect
with each other by corner-sharing to form a 2D inorganic layer
in the ab-plane (Fig. 6a). Adjacent 2D inorganic layers are
further cross-linked through the carboxyphosphonate ligands
from one layer to Cd(II) cations from adjacent layers into a 3D
pillared-layered structure with a 1D channel system along the
a-axis (Fig. 6b). The channel system running along the a-axis is
assembled by 40-atom rings, and the size of the channel is
estimated to be 8.9 Å (O2–O2) 6 3.4 Å (C4–C7) based on
structure data. The diameter of the pore (as shown by the
yellow ball) inside the channel is estimated to be 3.4 Å
(Fig. 6c). Hydrogen-bonding interactions are also observed in
the structure of compound 2. The nitrogen atom (N1) forms a
Fig. 4 (a) The left-handed helical chain; (b) the layer structure of compound 1
without the organic moieties of the HL22 ligands viewed in the ac-plane; (c) the
right-handed helical chain.
…
hydrogen bond with the chloride atom (Cl1): N1–H1B Cl1#7
and the bond distance and the angle are 3.216(5) Å and 175u,
respectively. Meanwhile, the nitrogen atom (N1) and the
phosphonate oxygen atom (O1) also form a hydrogen bond
…
with a distance of 2.726(5) Å for N1–H1E O1#7. Another
important hydrogen bonding interaction is between the
carboxylate oxygen atom (O4) and phosphonate oxygen atom
…
(O2): O4–H4B O2#6, and the bond distance and the angle are
2.524(5) Å and 170u, respectively. These strong hydrogen bonds
enhance the stability of the network and result in a two-
dimensional network in the (0, 0, 1) plane (Fig. 6d). Differently
from compound 1, the close contact distance between adjacent
benzyl rings, which are almost parallel to each other, is not in
the normal range (3.3–3.8 Å) for such interactions and so p–p
stacking interactions are not observed in the framework of
compound 2. It is of note that in the Cd-phosphonate
inorganic layer of the ab-plane, alternate left- and right-
handed helical chains occur in this structure, which is similar
to compound 1 (Fig. 7). The interconnection of one Cd(II)
cation and a pair of HL22 ligands by phosphonate oxygen
atoms (O1, O2 and O3) leads to infinite 1D left- and right-
handed helical chains along the a-axis. Compound 2 also
features an achiral 3D framework structure.
Fig. 5 Structure unit of compound 2 showing the atom labeling. Thermal
ellipsoids are shown at the 50% probability level. Symmetry code for the
generated atoms: (A) 2x, 2y + 1, 2z; (B) 2x + 1/2, y 2 1/2, z; (C) 2x + 1, 2y + 1,
2z; (D) 2x + 1, y 2 1/2, 2z + 1/2.
with those reported for other cadmium(II) carboxyphospho-
nates.12 The C9–O4 bond length (1.3109 Å) is significantly
longer than that of the C9–O5 bond (1.2221 Å) in the
carboxylate group of the carboxyphosphonate ligand, which
is also reflected from its IR data showing a strong absorption
band around 1661 cm21 associated with COOH.13 Based on
charge balance considerations, the nitrogen atom of the amine
group and the oxygen atom of the carboxylate group in the
Fig. 6 (a) The 2D inorganic layer structure of compound 2 in the ab-plane. (b) View of the 3D framework for compound 2 along the a-axis. (c) The 40-atom rings in
compound 2; the yellow sphere represents the pore defined within the frameworks, the diameter of the yellow ball inside the channels is 3.4 Å. (d) The connectivity of
hydrogen bonds for compound 2 (all H atoms attached to C atoms are omitted for clarity).
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Fig. 7 (a) The left-handed helical chain, (b) the layer structure of compound 2 without the organic moieties of the H2L2 ligands viewed in the ab-plane, and (c) the
right-handed helical chain. (d) Space-filling model of the left- and right-handed helical chains in compound 2.
weak bands at low energy for the title compounds are found.
These bands are probably due to bending vibrations of the
tetrahedral CPO3 groups.
IR spectroscopy
The IR spectra for compounds 1 and 2 were recorded in the
region from 4000–400 cm21 (Fig. S1 and S2, ESI ). The
3
absorption bands at 3448 cm21 for 1 and 3445 cm21 for 2
correspond to the stretching vibration of N–H.14a The C–H
stretching vibrations are observed as sharp, weak bands close
to 3000 cm21 for compounds 1 and 2. The strong absorption
bands around 1395 cm21 for 1 and 1324 cm21 for 2 are
assigned to the bending vibrations of C–H. For compound 1,
there are two strong bands (1603 and 1557 cm21) and two
medium bands (1473 and 1428 cm21), which are assigned to
the antisymmetrical and symmetrical stretching vibrations of
C–O bonds of the carboxylate groups. They are attributed to
the carboxylate groups coordinated to the metal, thus showing
that also the carboxylate groups are deprotonated and in
agreement with structural results. Bands at 1609, 1578, 1460
and 1414 cm21 for compound 2 are also observed, correspond-
ing to the asymmetric and symmetric stretching vibrations of
C–O bonds of the carboxylate groups.14b At the same time, the
bands at around 1661 cm21 can be attributed to the CLO
vibration of the carboxylic acid group, suggesting that the
COOH group is protonated. The set of bands between 1200
and 900 cm21 are assigned to stretching vibrations of the
tetrahedral CPO3 groups for compounds 1 and 2.14c Additional
Thermal analyses
In order to examine the thermal stabilities of compounds 1
and 2, thermogravimetric analyses were performed (Fig. 8).
The TG curve shows two steps of weight losses for compound
1, which is stable up to a high temperature of 427 uC. At
approximately 545 uC, compound 1 completes its first step of
weight loss (36.0%), which corresponds to partial decomposi-
tion of organic groups. During the thermal decomposition of 1
intermediate compounds are formed between 545 and 870 uC.
The intermediate phases contain Zn2P2O7 (JCPDS 00-007-0153)
and black glassy carbon, as identified by XRD powder studies
and the color (Fig. S3, ESI ).15 The theoretical weight loss is
3
50.6% assuming an intermediate compound Zn2P2O7 and is
larger than the observed weight loss because of the presence of
amorphous product.16,1e A second weight loss occurs between
870 and 1068 uC, which can be attributed to the decomposition
of the inorganic salt Zn2P2O7, releasing P2O5 as a gas, forming
ZnO,14c and also decomposition of the amorphous product.
The observed weight loss of 34.3% is larger than the calculated
value of 23.0%, in accord with the loss of the amorphous
Fig. 8 TG curves of (a) 1 and (b) 2.
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Fig. 9 PXRD patterns for (a) 1 on heating from 25 to 400 uC and (b) 2 on heating from 25 to 350 uC.
product. The total weight loss of 70.3% is close to the
calculated value (73.6%) if the final product is assumed to be
ZnO.17 For compound 2, the TG curve also shows a two step
weight losses. The first step starts at 325 uC and was completed
at 546 uC (28.3%), which corresponds to the removal of
hydrogen chloride and partial decomposition of the organic
ligand. During the thermal decomposition, intermediate
compounds are formed between 546 and 803 uC, which are
Cd2P2O7 (JCPDS 00-031-0233) and black glassy carbon, as
uC, the pattern is still similar to that at 25 uC, which indicates
that the structure of compound 1 was thermally stable below
380 uC. For 2 the pattern at 25–280 uC is consistent with the
simulated for 2 and changes at 300 uC. The PXRD experiment
thus shows 2 is thermally stable up to 280 uC.
Luminescent properties
The solid-state luminescent properties of the free H3L ligand
as well as compounds 1 and 2 were investigated at room
temperature and are shown in Fig. 10.
shown by XRD powder studies and the color (Fig. S4, ESI ). The
3
The free carboxyphosphonate ligand H3L displays a fluor-
escent emission band at 336 nm upon excitation at 307 nm. In
contrast, compounds 1 and 2 give broad fluorescent emissions
under the same experimental conditions. Compound 1 dis-
plays a broad blue fluorescent emission band at lmax = 361 nm
upon excitation at 307 nm. Compared to the free carboxypho-
sphonate ligand, compound 1 shows a pronounced red-shifted
emission band with slightly weakened intensities, which is
probably originate from the ligand-to-metal charge transfer
(LMCT).18 Compound 2 exhibits a broad emission band
between 320 and 500 nm with one strong blue fluorescent
calculated value of 49.1% for the weight loss is much larger
than the observed weight loss (28.3%), similar to compound 1,
again attributable to the existence of amorphous product. The
second weight loss step from 803 to 1000 uC (44.6%),
corresponds to the decomposition of the inorganic salt
Cd2P2O7 to CdO with release of P2O5 and loss of the
amorphous product. The total weight loss of 72.9% is close
to the calculated value (67.2%) if the final product is assumed
to be CdO. To further understand the thermal stability of
compounds 1 and 2, powder X-ray diffraction studies (PXRD)
were performed from 25 to 400 uC. As shown in Fig. 9a, at 380
Fig. 10 (a) Solid-state emission spectra of H3L (red line) and compound 1 (black line) at room temperature. (b) Solid-state emission spectra of H3L (red line) and
compound 2 (black line) at room temperature.
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C. Mellot-Draznieks, P. Lightfoot and P.-A. Wright, Chem.
Commun., 2006, 3305; (c) P.-O. Adelani and T.-E. Albrecht-
Schmitt, Inorg. Chem., 2010, 49, 5701; (d) W. Ouellette, M.-
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emission peak at 422 nm (lex = 307 nm). The fluorescent
spectrum of compound 2 shows a significant enhancement in
intensity and a large shift to higher wavelength in emission
band compared with that of the free carboxyphosphonate
ligand (H3L), which is attributed to metal-to-ligand charge
transfer (MLCT).19 Unfortunately, the luminescent lifetimes of
compounds 1 and 2 were too short to be measured. The
luminescent intensity of compound 2 is stronger than that of
compound 1, which is probably due to the differences in the
metal ion and the structure of the complexes since the
luminescence behavior is closely associated with the metal
ions and the ligands coordinated around them.20 The
investigation of luminescent properties indicates that com-
pounds 1 and 2 may be good blue-light luminescent materials.
´
2 (a) M. Purgel, Z. Baranyai, A. Blas, T. Rodrıguez-Blas,
´
´
I. Banyai, C. Platas-Iglesias and I. Toth, Inorg. Chem., 2010,
49, 4370; (b) A.-N. Alsobrook, B.-G. Hauser, J.-T. Hupp, E.-
V. Alekseev, W. Depmeierc and T.-E. Albrecht-Schmitt,
Chem. Commun., 2010, 46, 9167; (c) D.-P. Dong, L. Liu, Z.-
G. Sun, C.-Q. Jiao, Z.-M. Liu, C. Li, Y.-Y. Zhu, K. Chen and
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Conclusions
In summary, two novel divalent metal phosphonates with
4-HOOCC6H4CH2NHCH2PO3H2 (H3L) as ligand, namely,
[Zn(HL)] (1) and [CdCl(H2L)] (2) have been synthesized under
hydrothermal conditions. To the best of our knowledge, this is
the first time that transition-metal carboxyphosphonates are
reported with this ligand. Compounds 1 and 2 both feature
three-dimensional (3D) framework structures with two-dimen-
sional (2D) inorganic layers pillared by H3L. In compound 1,
{ZnO4} tetrahedra and {CPO3} tetrahedra connect with each
other to form a 2D inorganic layer by corner-sharing. Such
neighboring 2D inorganic layers are further supported by
organic pillars {–C6H4CH2NHCH2–} of the carboxyphosphonate
ligand to form a 3D pillared-layered structure. For compound 2,
the interconnection of the {Cd2Cl2O8} polyhedra and {CPO3}
tetrahedra is in turn linked by corner-sharing to form a 2D
inorganic layer in the ab-plane. The adjacent 2D inorganic
layers are further cross-linked through the organic pillars of the
carboxyphosphonate ligand, generating a 3D pillared-layered
structure. A remarkable characteristic of compounds 1 and 2 is
the presence of alternate left- and right-handed helical chains in
the structures. We have systematically investigated the influ-
ence of different metal salts and pH value on the synthesis of
two title compounds. Thermal stabilities and the PXRD
experiment reveal that compounds 1 and 2 are thermally stable
up to 380 and 280 uC, respectively. The luminescence analysis
indicates that compounds 1 and 2 may be good candidates for
blue-light luminescent materials.
¨
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Growth Des., 2011, 11, 4623; (d) D.-P. Dong, Z.-G. Sun,
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G. Ferey, Inorg. Chem., 2004, 43, 3159; (c) D. Cave, F.-
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Acknowledgements
`
P.-A. Jaffres, Cryst. Growth Des., 2009, 9, 4262; (d) J.-L. Song,
This work is supported by the National Natural Science
Foundation of China (Grant No. 21071072) and the Program for
Liaoning Excellent Talents in University (Grant No. LR2011030).
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