Two luminescent frameworks constructed from lead(II) salts with carboxylate ligands containing dinuclear lead(II) units

Article abstract of DOI:10.1016/j.jssc.2007.06.010

Two luminescent Pb(II) coordination frameworks containing dinuclear lead(II) units, [Pb(PYDC)(H₂O)]_n (1) and [Pb(HPHT)]_n (2) have been prepared by the self-assembly of lead(II) salts with pyridinecarboxylate and benzenecarboxylate. Single-crystal X-ray diffraction analyses reveal that compound 1 is a three-dimensional architecture consisting of Pb₂O₂ dimeric building units, whereas compound 2 is a two-dimensional layer structure containing one-dimensional lead-oxide chains. The luminescent properties of 1 and 2 have been investigated in the solid state at room temperature, indicating structure-dependent photoluminescent properties of the coordination frameworks.

Full text of DOI:10.1016/j.jssc.2007.06.010

ARTICLE IN PRESS
Journal of Solid State Chemistry 180 (2007) 2386–2392
www.elsevier.com/locate/jssc
Two luminescent frameworks constructed from lead(II) salts with
carboxylate ligands containing dinuclear lead(II) units
Xiandong Zhu, Xiaoju Li, Qingyan Liu, Jian Lu, Zhengang Guo, Jinrun He,
¨
Ã
Yafeng Li, Rong Cao
State Key Laboratory of Structural Chemistry, Fujian Institute of Research on the Structure of Matter, Chinese Academy of Sciences, Fujian,
Fuzhou 350002, PR China
Received 14 March 2007; received in revised form 3 June 2007; accepted 4 June 2007
Available online 20 June 2007
Abstract
Two luminescent Pb(II) coordination frameworks containing dinuclear lead(II) units, [Pb(PYDC)(H2O)]n (1) and [Pb(HPHT)]n (2)
have been prepared by the self-assembly of lead(II) salts with pyridinecarboxylate and benzenecarboxylate. Single-crystal X-ray
diffraction analyses reveal that compound 1 is a three-dimensional architecture consisting of Pb2O2 dimeric building units, whereas
compound 2 is a two-dimensional layer structure containing one-dimensional lead-oxide chains. The luminescent properties of 1 and 2
have been investigated in the solid state at room temperature, indicating structure-dependent photoluminescent properties of the
coordination frameworks.
r 2007 Elsevier Inc. All rights reserved.
Keywords: Lead(II); Coordination frameworks; Luminescent properties
1. Introduction
our present work. On the other hand, the ligands play an
important role in exploring the coordination behavior of
metal ions. Thus, two ligands have been chosen to explore
the coordination chemistry of lead(II). First, 3,4-pyridine-
dicarboxylic acid (H2PYDC), possessing rigid ring and
multiple coordination modes, was coming into our sight. It
is well known that lead(II) is affinitive to oxygen; thus, a
large number of lead-carboxylate or -phosphate coordina-
tion compounds have been synthesized [6,7]. Exploring the
coordination chemistry of lead(II) to ligands possessing
oxygen and nitrogen donors would be a fascinating task
[8]. Second, an asymmetric and flexible ligand of homo-
phthalic acid (H2HPHT) was selected. Symmetrical ben-
zene dicarboxylic acids, such as terephthalic acid and 1,3,5-
benezetricarboxylic acid, as O-donor ligands, have been
widely used to synthesize coordination polymers contain-
ing transition-metal ions [9]. However, many asymmetric
and flexible benzene carboxylic acids such as homophthalic
acid have not been investigated extensively, and the study
on the structures constructed from these ligands remains
undeveloped [10]. Our aim is to use such ligands to
construct lead-organic complexes with interesting topology
The rational design and synthesis of inorganic–organic
hybrid coordination frameworks is one of the most active
areas of materials research [1,2]. The increasing interest is
justified not only by their unique application as functional
materials, but also by their particular structural diversity of
the architectures [3]. The resulting architectures and
properties mainly depend on the nature of metal centers
and the structural characterization of organic ligands used.
Until now, much of the effort has been focused on the
coordination of transition- and lanthanide-metal ions [4].
However, the analogous chemistry of the heavy metal ions
of main group is not profoundly developed despite their
important applications in electroluminescent devices or
organic light-emitting diode (OELD) technology [5].
Considering the large radius, rich coordination numbers
and geometries, the possible occurrence of stereochemically
active lone pair of electrons, Pb(II) cation is employed in
Ã
Corresponding author. Fax: +86 591 83714946.
E-mail address: rcao@ms.fjirsm.ac.cn (R. Cao).
0022-4596/$ - see front matter r 2007 Elsevier Inc. All rights reserved.
doi:10.1016/j.jssc.2007.06.010

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X. Zhu et al. / Journal of Solid State Chemistry 180 (2007) 2386–2392
2387
or properties for potential application, and to investigate
Table 1
Crystal data and structure refinement for compounds 1 and 2
the different coordination geometries which Pb(II) cations
may adopt. Recently, we have reported a heterometallic
polymer containing heptanuclear units and one-dimen-
sional lead-oxide chains producing by the hydrothermal
reaction of 5-hydroxyisophthalic acid with lead and nickel
nitrates [11]. In this paper, the hydrothermal reactions of
lead(II) salts with H2PYDC/H2HPHT and the character-
izations of two novel polymeric complexes, [Pb(PYDC)
(H2O)]n (1) and [Pb(HPHT)]n (2), will be reported.
1
2
Empirical formula
Formula weight
Crystal system
Space group
C7H3NO5Pb
388.29
C9H6O4Pb
385.33
Monoclinic
P2(1)/c
13.602(1)
6.6961(5)
9.6654(8)
90
Triclinic
P-1
˚
a (A)
8.336(2)
8.705(2)
11.750(3)
96.171(8)
106.50(1)
90.650(1)
812.0(3)
4
˚
b (A)
˚
c (A)
a (deg)
b (deg)
g (deg)
90.999(5)
90
2. Experimental section
3
˚
V (A )
Z
880.2(1)
4
2.908
2.1. Materials and general methods
rcalcd. (Mg/m3)
3.176
20.770
m (MoKa) (mmꢀ1
No. of data collected
No. of unique data (all)
)
19.149
5290
1542
All chemicals were of reagent-grade quality obtained
commercially and used without further purification. Ele-
mental analyses were carried out on an Elementar Vario
EL III analyzer. Infrared (IR) spectra were recorded on
PerkinElmer Spectrum One as KBr pellets in the range
4000–400 cmꢀ1. Fluorescent spectra were measured at
room temperature with an Edinburgh FL-FS90 TCSPC
system.
6321
3669
(R(int) ¼ 0.0589)
3669
(R(int) ¼ 0.0773)
No. of observed data
[F42(sF)]
Parameters
1542
253
128
R1,a wR2 (I42s (I))
0.0654, 0.1618
0.0841, 0.1718
1.061
0.0487, 0.1136
0.0560, 0.1156
1.037
b
R1,a wR2 (all data)
b
GOF
P
P
aR ¼ ðjjFoj ꢀ jFcjjÞ= jFoj:
2.2. Syntheses of the compounds
P
P
b
2
2
1=2
wRðF2Þ ¼ ½ wðFo2 ꢀ Fc2Þ = wðFo2Þ ꢁ
:
2.2.1. Preparation of [Pb(PYDC)(H2O)]n (1)
A mixture of Pb(NO3)2 (0.065 g, 0.20 mmol) and
H2PYDC (0.032 g, 0.20 mmol) in a 1:1 molar ratio in
H2O (10 mL) was transferred to a 25 mL stainless steel
reactor with Teflon liner and heated to 160 1C for 72 h, then
the reaction system was cooled to room temperature over
48 h. A large amount of yellow prism-shaped crystals of
compound 1 were obtained. Yield: 72% (based on Pb).
Elemental analysis (%): calcd. for 1 C7H5NO5Pb (390.32):
C 21.54, H 1.29, N 3.59; found: C 21.49, H 1.25, N 3.55. IR
(KBr, cm–1): 3438(s), 1606(m), 1567(s), 1539(s), 1486(m),
1381(s), 840(s), 719(s), 676(s).
direct methods and refined by the full-matrix least-squares
on F2 using the SHELXTL-97 program [13]. All of the
non-hydrogen atoms were refined with anisotropic thermal
parameters. The positions of H atoms were generated
˚
geometrically (C–H bond fixed at 0.96 A), assigned
isotropic thermal parameters, and allowed to ride on their
parent carbon atoms before the final cycle of refinement.
Crystallographic data and structure determination sum-
maries for compounds 1 and 2 are listed in Table 1.
Selected bond lengths and bond angles are listed in Table 2.
3. Results and discussions
2.2.2. Preparation of [Pb(HPHT)]n (2)
A mixture of H2HPHT (0.09 g, 0.5 mmol) and Pb(NO3)2
(0.166 g, 0.5 mmol) were dissolved in 10 mL water. Then,
ethanol was allowed to diffuse into the resultant solution
slowly. After two weeks, colorless block-shaped crystals of
compound 2 were collected. Yield: 53% (based on Pb).
Elemental analysis (%): calcd. for 2 C9H6O4Pb (385.33): C
28.05, H 1.57; found: C 27.89, H 1.62. IR (KBr, cmꢀ1):
3392(m), 3058(m), 1623(m), 1589(m), 1516(s), 1385(s),
1284(s), 854(s), 723(s).
3.1. Structural description of [Pb(PYDC)(H2O)]n (1)
Single-crystal X-ray diffraction analysis reveals that
compound 1 is a three-dimensional architecture consisting
of Pb2O2 dimeric building units. As shown in Fig. 1, the
asymmetric unit of compound 1 consists of two crystal-
lographically independent Pb(II) motifs. Pb(1) is five-
coordinated by four carboxylate oxygen atoms from three
PYDC ligands and one oxygen atom from water molecule.
It adopts a highly distorted triangle bipyramid geometry,
which is unusual compared to coordination geometry of
transition metals. One monodentate carboxylate oxygen
and water oxygen atoms occupy the apexes of the pyramid,
while the other oxygen atoms comprise the equatorial
plane. We presume that the electron geometry can be
described as a c-octahedral PbO5 in which the lone pair of
electrons on the Pb2+ ion occupies the sixth coordination
2.2.3. X-ray crystallographic study
X-ray diffraction data of compounds 1 and 2 were
collected on a Rigaku diffractometer with a Mercury CCD
area detector with graphite-monochromated MoKa
˚
(l ¼ 0.71073 A) radiation at room temperature. Empirical
absorption corrections were applied to the data using the
CrystalClear program [12]. The structures were solved by

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2388
Table 2
a
Selected bond lengths (A) and angles (deg) for compounds 1 and 2a
˚
Compound 1
Pb(1)–O(1)
2.513(14)
2.624(10)
2.730(13)
2.586(10)
2.738(12)
2.796(14)
78.5(4)
Pb(1)–O(7A)
Pb(1)–OW1
Pb(2)–O(5)
Pb(2)–O(3D)
Pb(2)–OW2
2.606(11)
2.646(14)
2.441(13)
2.621(11)
2.757(13)
Pb(1)–O(3B)
Pb(1)–O(8A)
Pb(2)–O(7C)
Pb(2)–O(2)
Pb(2)–N(2E)
O(1)–Pb(1)–O(7A)
O(7A)–Pb(1)–O(3B)
O(7A)–Pb(1)–OW1
O(1)–Pb(1)–O(8A)
O(3B)–Pb(1)–O(8A)
O(5)–Pb(2)–O(7C)
O(7C)–Pb(2)–O(3D)
O(7C)–Pb(2)–O(2)
O(5)–Pb(2)–OW2
O(3D)–Pb(2)–OW2
O(7C)–Pb(2)–N(2E)
O(2)–Pb(2)–N(2E)
O(1)–Pb(1)–O(3B)
O(1)–Pb(1)–OW1
O(3B)–Pb(1)–OW1
O(7A)–Pb(1)–O(8A)
OW1–Pb(1)–O(8A)
O(5)–Pb(2)–O(3D)
O(5)–Pb(2)–O(2)
71.4(4)
141.4(4)
71.5(4)
49.0(3)
105.9(4)
77.7(4)
97.8(4)
129.5(3)
72.4(4)
121.2(4)
147.9(4)
78.0(4)
65.9(3)
77.3(4)
79.3(4)
112.4(3)
74.1(4)
66.2(3)
161.2(4)
142.0(4)
72.8(4)
O(3D)–Pb(2)–O(2)
O(7C)–Pb(2)–OW2
O(5)–Pb(2)–N(2E)
O(3D)–Pb(2)–N(2E)
OW2–Pb(2)–N(2E)
92.6(4)
76.9(4)
Compound 2
Pb(1)–O(2A)
2.410(11)
2.625(8)
2.630(8)
115.1(3)
120.4(2)
71.7(2)
Pb(1)–O(3B)
Pb(1)–O(4D)
Pb(1)–O(1)
2.610(8)
2.629(8)
2.752(9)
77.3(3)
Pb(1)–O(3C)
Pb(1)–O(4B)
Fig. 1. The coordination environment of the two Pb(II) ions in compound
1 with the thermal ellipsoids at 30% probability level. All the H atoms are
omitted for clarity.
O(2A)–Pb(1)–O(3B)
O(3B)–Pb(1)–O(3C)
O(3B)–Pb(1)–O(4D)
O(2A)–Pb(1)–O(4B)
O(3C)–Pb(1)–O(4B)
O(2A)–Pb(1)–O(1)
O(3C)–Pb(1)–O(1)
O(4B)–Pb(1)–O(1)
O(2A)–Pb(1)–O(3C)
O(2A)–Pb(1)–O(4D)
O(3C)–Pb(1)–O(4D)
O(3B)–Pb(1)–O(4B)
O(4D)–Pb(1)–O(4B)
O(3B)–Pb(1)–O(1)
O(4D)–Pb(1)–O(1)
79.6(3)
156.8(3)
50.2(2)
95.8(3)
71.4(2)
angles of 65.901 and 66.241 and Pb–O–Pb bond angles of
112.91 and 114.71, respectively. This Pb2O2 unit is
essentially symmetric with the Pb–O distances 2.62(1) and
112.9(2)
76.3(3)
128.4(2)
151.8(3)
74.9(3)
71.1(3)
˚
2.62(1) for O(3) and 2.61(1) and 2.59(1) A for O(7). The
Pb–(m-O) bond lengths are comparable to those found in
other Pb2(m-OR)2 systems [15]. The associated nonbonded
aSymmetry codes: for 1 (A) ꢀx, ꢀy+2, ꢀz; (B) ꢀx, ꢀy+2, ꢀz+1; (C)
ꢀx, ꢀy+1, ꢀz; (D) ꢀx, ꢀy+1, ꢀz+1; (E) xꢀ1, y, z. 2 (A) ꢀx+2, yꢀ1/2,
ꢀz+3/2; (B) x, ꢀy+1/2, zꢀ1/2; (C) ꢀx+2, ꢀy+1, ꢀz+2; (D) ꢀx+2,
ꢀy, ꢀz+2.
˚
Pb(1)?Pb(2) distance is about 4.373 A. The dinuclear
Pb2O2 units are bridged by PYDC ligands through
carboxylate oxygen atoms, leading to a two-dimensional
layered coordination network, as illustrated in Fig. 2. It is
noted that the PYDC ligands adopt the coordination mode
like V style just using carboxylate groups. Such two-
dimensional layers are further interconnected through the
remaining nitrogen atoms of pyridine rings to form a three-
dimensional infinite network. Within the two-dimensional
layer, there are two types of ring. One is the four-
membered ring formed by Pb2O2 unit, and the other is
the 14-membered ring formed by two lead(II) ions and two
bridging PYDC ligands (Pb–(O–C–C–C–C–O)2–Pb). The
latter rings form open channels along the a-axis (Fig. 3).
The pyridine rings between the channels are orientated
absolutely parallel to each other. The shortest distance
site on the equatorial plane [7a,8b]. The Pb(1)–O distances
˚
range from 2.51(2) to 2.73(2) A and the Pb–OW1 distance
˚
is 2.65(1) A, similar to those reported for other lead(II)
carboxylates and phosphonates [6,7,14]. However, the six-
coordinate Pb(2) is in a distorted octahedron geometry
with the two apical positions occupied by monodentate
carboxylate oxygen atoms and the four basal positions
occupied by two monodentate carboxylate oxygen atoms,
one pyridyl nitrogen atom and one oxygen atom from
water molecule. The average Pb–N and Pb–O distances are
˚
2.80(1) and 2.63(1) A, respectively. Similar to that of Pb(1),
the electron geometry around Pb(2) can be described as c-
pentagonal bipyramidal PbO6 in which the seventh
coordination site is occupied by the lone pair electrons of
the Pb(II) ion. Although the two independent PYDC
ligands both act as tetradentate ligands, they exhibit
different coordination modes as illustrated in Scheme 1a
and Scheme 1b. The nitrogen atoms are coordinated to
Pb(II) centers in Scheme 1b, whereas left uncoordinated in
Scheme 1a.
˚
between two neighboring pyridine ring centers is 3.65 A;
therefore it is expected that there is p–p interaction between
these pyridine rings.
3.2. Structural description of [Pb(HPHT)]n (2)
The X-ray analysis of compound 2 reveals a two-
dimensional layer structure containing one-dimensional
lead-oxide chains. As shown in Fig. 4, the asymmetric unit
of compound 2 consists of one unique Pb(II) ion compared
As shown in Fig. 1, Pb(1) atoms are linked to Pb(2) by
the O(3A) and O(7A) atoms of the PYDC ligands, forming
a nearly planar four-membered ring with O–Pb–O bond

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2389
Scheme 1. The coordination modes of PYDC and HPHT ligands in compounds 1 and 2.
Fig. 2. View of the two-dimensional network of compound 1, in which the
dinuclear Pb2O2 units have been shown.
Fig. 4. The coordination environment around Pb(II) ion in compound 2
with the thermal ellipsoids at 30% probability level. All the H atoms are
omitted for clarity.
depicted above. Obviously, the electron geometry can be
described as a c-pentagonal bipyramidal {PbO6} with the
lone pair of electrons on the Pb2+ ion completing the
seventh coordination site at the apical position [7a,8b]. The
HPHT ligand links two Pb(II) ions, in which the formate
group of HPHT anion bridges two Pb(II) ions, while
acetate group chelates one Pb(II) ion and also bridges two
Pb(II) ions. Thus the HPHT ligand behaves as a m5-bridge
connecting five Pb(II) atoms through one bidentate
formate group and m3–Z2:Z2 acetate group as illustrated in
Scheme 1c. To the best of our knowledge, this type of
coordinating mode is not observed in the previously
reported HPHT complexes. Two Pb(II) ions connected
by two bridging ethylic carboxylate groups with intera-
Fig. 3. Perspective view of the three-dimensional packing structure of
compound 1 along the a-axis.
˚
tomic distance of 4.019 A form binuclear Pb2O2 basic
building unit of the whole structure. The associated
to compound 1 and five HPHT ligands. Pb(II) is six-
coordinated by six carboxylate oxygen atoms from five
HPHT ligands. The Pb–O distances are in the range
˚
nonbonded Pb(1)?Pb(1A) distance (4.019 A) is closer to
˚
the sum of van der Walls radii (3.20 A) compared to that of
˚
˚
2.411–2.752 A, with an average value of 2.610 A. It adopts
a highly distorted pentagonal pyramid geometry, which is
unusual compared to ordinary transition metals as we
compound 1 and other reported lead(II) compounds [16].
With significant difference from compound 1, the Pb2O2
ring inclined to fold with the torsion angle of

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Fig. 5. View of the one-dimensional lead-oxide chain in compound 2
along the c-axis.
Fig. 7. View of the interlayer space between two-dimensional layers in
compound 2 along the b-axis.
1
2
Fig. 6. View of the two-dimensional layer structure in compound 2.
(Pb1–O3A?O4A–Pb1A) 141.51, which mainly depended
on the flexible arm of HPHT ligand. {PbO6} polyhedrons
are connected together via edge-sharing fashion giving rise
to a zigzag one-dimensional lead-oxide chain (Fig. 5). The
HPHT spacers stretch the one-dimensional chain into the
final two-dimensional layer structure completing the
coordination sphere of Pb(II) ions, as shown in Fig. 6.
The benzene rings of the ligand are orientated toward the
interlayer space. The shortest distance between two
350
400
450
500
550
600
650
Wavelength (nm)
Fig. 8. The solid-state emission spectrum of compound 1 (lex ¼ 303 nm)
and 2 (lex ¼ 364 nm) at room temperature.
˚
neighboring benzene ring centers is 4.34 A; therefore it is
expected that there is no p–p interaction between these
benzene rings (Fig. 7).
coordination geometry, which is consistent with the crystal
structures of 2.
3.4. Luminescent properties
3.3. Vibrational spectroscopic study
Although the photoluminescence properties of transi-
tion-metal complex was of interest in numerous investiga-
tions for several decades, s2-metal complexes are not well
studied [18]. The luminescent properties of 1 and 2 have
been investigated in solid state at room temperature. The
emission spectra of both compounds are dominated by an
intense and broad emission band. Compound 1 exhibits an
intense emission with a maximum at ca. 415 nm upon
excitation at 303 nm. In the case of 2, photoluminescence
with a maximum emission at ca. 602 nm upon excitation at
364 nm is observed (Fig. 8). In order to assign the two
emission bands, luminescent measurements were taken for
The FT-IR spectrum of compound 1 exhibits a strong
and broad absorption centered at 3438 cmꢀ1, which can be
ascribed to the presence of water molecules in the structure.
Characteristic bands of carboxyl groups at 1567 cmꢀ1 for
the asymmetric stretching and at 1539 and 1381 cmꢀ1 for
symmetric stretching, indicate the presence of chelating and
bridging coordination modes in compound 1 [17]. The
asymmetric and symmetric stretching vibrations of com-
pound 2 at 1589, 1516 and 1385 cmꢀ1 show that the
carboxylate groups employ the chelating and monodentate

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2391
both free ligands and it was found that the strongest
emission peak for H2PYDC is located at ca. 370 nm
(lex ¼ 276 nm), whereas for H2HPHT it is at ca. 455 nm
(lex ¼ 370 nm) [19]. The strongest emission peak of
compound 1 (415 nm) is very close to the free ligand
(370 nm). In addition, a series of lead(II) b-diketonates
compounds showing typical intraligand (IL) phosphores-
cence has been reported [20a]. Therefore, this emission
band might be attributed to the intraligand (IL) emission
from the PYDC ligand [20]. The bathochromic shift and
enhancement of PYDC ligand in compound 1 compared to
free ligand may be attributed to the coordination bond
between the ligand and Pb(II), which increases the rigidity
of the ligand and reduces the loss of energy by radiationless
decay of the intraligand emission excited state [21]. The
emission wavelength of compound 2 occurs at a much
lower energy of 602 nm, which is much longer than that of
the free ligand (455 nm). The low-energy emissions
associated with large Stokes shifts have been commonly
observed for other s2–metal complexes, which can be
assigned to a metal-centered transition involving the s and
p metal orbital as proposed by Vogler [18b,c]. Thus the
emission band of 2 can be assigned to a metal-centered s-
p transition. In addition, the dinuclear Pb2O2 cluster in the
[Pb(HPHT)]n. These data can be obtained free of charge
via www.ccdc.cam.ac.uk/conts/retriving.html, or from the
Cambridge Crystallographic Data Center, 12 Union Road,
Cambridge CB2 1EZ, UK; fax: (+44) 1223-336-033; or
e-mail: deposit@ccdc.cam.ac.uk.
Acknowledgments
This work was financially supported by NSFC
(90206040, 20325106, 20521101 and 20333070), NSF of
Fujian Province (2005HZ01-1, E0520003), The Distin-
guished Oversea Scholar Project and One Hundred Talent
Project from CAS.
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In summary, the self-assembly of lead(II) salts with
pyridinecarboxylate and benzenecarboxylate affords two
luminescent Pb(II) coordination frameworks containing
dinuclear lead(II) units, [Pb(PYDC)(H2O)]n (1) and
[Pb(HPHT)]n (2). This paper investigated the microscopic
features of molecular packing and the luminescent proper-
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Compound 1 is a three-dimensional architecture consisting
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two-dimensional layer structure containing one-dimen-
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stereochemically active lone pair of electrons usually leads
to the rich coordination numbers and asymmetric coordi-
nation geometry of Pb(II). Compounds 1 and 2 display
different fluorescent emissions at room temperature,
indicating structure-dependent photoluminescent proper-
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