Two lead(II) 2,4-dioxo-1,2,3,4-tetrahydropyrimidine-5-carboxylate complexes exhibiting different topologies and fluorescent properties

Article abstract of DOI:10.1016/j.jssc.2011.03.031

The reactions of PbCl₂ with 2,4-dioxo-1,2,3,4- tetrahydropyrimidine-5-carboxylic acid (H₃iso) gave two complexes [Pb(H₂iso)₂(H₂O)]_n (1) and [Pb(Hiso)(H₂O)]_n (2), which were characterized by IR spectroscopy, elemental analysis, thermogravimetric analysis, powder X-ray diffraction and single crystal X-ray diffraction analysis. The two complexes display different topologies. 1 shows a three-dimensional framework with the Schlaefli symbol (4.8⁵)(4.8²) no matter if the weak PbO bonds are included or not. However, 2 presents a 3,3-connected two-dimensional sheet with the Schlaefli symbol (4.8²)(4.8 ²) based on the calculation of only the normal PbO bonds and a 5,5-connected 3D network with the Schlaefli symbol (4¹⁵.6 ⁴)(4⁴.6⁸.8²) when the weak PbO bonds are also included. The fluorescent studies reveal an emission attributed to intraligand emission for 1 and an emission assigned to LMCT for 2.

Full text of DOI:10.1016/j.jssc.2011.03.031

Journal of Solid State Chemistry 184 (2011) 1063–1069
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Journal of Solid State Chemistry
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Two lead(II) 2,4-dioxo-1,2,3,4-tetrahydropyrimidine-5-carboxylate
complexes exhibiting different topologies and fluorescent properties
Zilu Chen n, Jiehua Yan, Huihui Xing, Zhong Zhang, Fupei Liang n
Key Laboratory for the Chemistry and Molecular Engineering of Medicinal Resources (Ministry of Education of China), School of Chemistry &
Chemical Engineering of Guangxi Normal University, Yucai Road 15, Guilin 541004, PR China
a r t i c l e i n f o
a b s t r a c t
Article history:
The reactions of PbCl2 with 2,4-dioxo-1,2,3,4-tetrahydropyrimidine-5-carboxylic acid (H3iso) gave two
complexes [Pb(H2iso)2(H2O)]n (1) and [Pb(Hiso)(H2O)]n (2), which were characterized by IR spectro-
scopy, elemental analysis, thermogravimetric analysis, powder X-ray diffraction and single crystal
X-ray diffraction analysis. The two complexes display different topologies. 1 shows a three-dimensional
framework with the Schla¨fli symbol (4.85)(4.82) no matter if the weak Pb–O bonds are included or not.
However, 2 presents a 3,3-connected two-dimensional sheet with the Schla¨fli symbol (4.82)(4.82) based
on the calculation of only the normal Pb–O bonds and a 5,5-connected 3D network with the Schla¨fli
symbol (415.64)(44.68.82) when the weak Pb–O bonds are also included. The fluorescent studies reveal
an emission attributed to intraligand emission for 1 and an emission assigned to LMCT for 2.
& 2011 Elsevier Inc. All rights reserved.
Received 10 August 2010
Received in revised form
9 March 2011
Accepted 14 March 2011
Available online 21 March 2011
Keywords:
Lead(II)
2,4-Dioxo-1,2,3,4-tetrahydropyrimidine-5-
carboxylate
Crystal structure
Topology
Fluorescent properties
1. Introduction
select 2,4-dioxo-1,2,3,4-tetrahydropyrimidine-5-carboxylic acid
(H3iso) as the ligand to investigate the coordination chemistry
The chemistry of lead has drawn a lot of interests not only for
its wide applications in fields such as fuel additives, batteries, oil
refining and paint manufacturing, but also for its contamination
in the environment [1–3]. For environmental protection, it is very
urgent to conduct further studies about the coordination chem-
istry of lead to solve the problem of lead contamination. As well
known, lead(II) ion presents a variable stereochemical activity
and a flexible coordination environment as a result of possessing
a 6s2 lone electron pair and a large radius [4,5]. Interesting, Pb(II)
ion presents hemidirected and holodirected coordination geome-
tries with the coordination number ranging from 2 to 10 [6–8].
These intrinsic features of lead(II) ion may help to construct some
networks with novel topologies or interesting properties [5,9–12],
which may exhibit some advantages over those transition metal-
organic frameworks in the corresponding aspects [6,12,13]. To
prepare targeted Pb-organic compounds, the selection of appro-
priate linkers is of importance [12–18]. Hydroxyl and carboxylato
groups are two usually used linking groups for their versatile
coordination modes in bridging metal ions. With the mind of
combining hydroxyl and carboxylato groups in one linker, we
of Pb(II) ion. Therefore, we report here the structure and fluor-
escent properties of two complexes [Pb(H2iso)2(H2O)]n (1) and
[Pb(Hiso)(H2O)]n (2) obtained from the reactions of PbCl2 with
2,4-dioxo-1,2,3,4-tetrahydropyrimidine-5-carboxylic acid (H3iso).
2. Experimental section
2.1. Materials and measurements
All reagents were purchased commercially and used without
further purification. Hydrothermal reactions were performed in
23 mL Teflon-lined reactors. Elemental analyses (C, H, and N) were
performed on an American PE2400II analyzer. The infrared spectra
were recorded from KBr pellets in the range 4000–400 cmꢀ1 on a
Perkin-Elmer spectrum one FT-IR spectrometer. Thermogravi-
metric analyses were recorded with a Perkin-Elmer Pyris Diamond
TG/DTA analyzer at a rate of 10 1C/min from room temperature to
940 1C under a nitrogen atmosphere. The powder X-ray diffraction
(PXRD) data were collected with a Rigaku D/max 2500v/pc
˚
diffractometer with CuK
a radiation (l¼1.5418 A). Fluorescent
data were collected on a FL3-TCSPC luminescent spectrometer
with a Xe-CW-source (450 W) and a RR928P photomultiplier for
signal detection.
n Corresponding authors. Fax: þ86 773/5832294.
E-mail address: chenziluczl@yahoo.co.uk (Z. Chen).
0022-4596/$ - see front matter & 2011 Elsevier Inc. All rights reserved.
doi:10.1016/j.jssc.2011.03.031

1064
Z. Chen et al. / Journal of Solid State Chemistry 184 (2011) 1063–1069
2.2. Synthesis of [Pb(H2iso)2(H2O)]n (1)
3. Results and discussion
A mixture of PbCl2 (0.25 mmol, 0.0695 g), 2,4-dioxo-1,2,3,4-
tetrahydropyrimidine-5-carboxylic acid (H3iso) (0.5 mmol, 0.087 g),
NaOH (0.25 mmol, 0.01 g) and water (10 mL) was placed in a Teflon-
lined reactor (23 mL) and heated at 110 1C for 6 days. Then, it was
slowly cooled down to room temperature, giving colorless crystals of
1 in 70% yield after being filtered, washed with water for three times
and dried. C10H8N4O9Pb (535.39): calcd. C 22.43, H 1.50, N 10.46;
found C 22.13, H 1.73, N 10.21%. IR (KBr pellet): 3478(w), 3170(m),
3052(m), 2813(m), 1759(m), 1728(m), 1706(s), 1693(s), 1660(m),
1613(w), 1563(m), 1496(m), 1443(w), 1421(m), 1389(m), 1360(m),
1336(w), 1325(w), 1218(m), 1190(m), 1001(w), 845(w), 807(w),
3.1. Synthesis and characterization
Complexes 1 and 2 were synthesized by direct reactions of PbCl2
with H3iso in aqueous solution with appropriate amount of NaOH.
The dependence of the product on the ratio of starting materials
was investigated. It was shown that the reactions of PbCl2 with
H3iso in the presence of NaOH gave 1 when their ratio was set at
1:1:1, 1:2:1 or 1:3:1 and 2 when the their ratio was set at 3:2:4,
1:1:2, 1:2:4, 1:2:3 or 1:2:2. This reveals that the formation of
products has a strong dependence on the ratio of H3iso:NaOH, but
little dependence on the ratio of PbCl2:H3iso.
782(w), 648(m), 639(m), 566(w), 557(w) cm–1
.
The IR spectra display several bands in the range of
1759–1660 cmꢀ1 for the carboxylato groups in 1 and only one
band at 1622 cmꢀ1 for the carboxylato group in 2, which agrees
well with the existence of two kinds of carboxylato groups in 1
and only one kind of carboxylato group in 2 as demonstrated by
single crystal X-ray diffraction analysis. The PXRD patterns (Fig.
S1) of the microcrystals of 1 and 2 agree well with the simulated
ones based on the crystal structures of 1 and 2, respectively,
confirming their phase purity. The TG curve of 1 is shown in Fig.
S2. It reveals a sharp weight loss (18.14%) between 220 and
285 1C, corresponding to the loss of one coordinated water
molecule and a dioxopyrimidine part of one H2isoꢀ ligand in 1
(calculated: 17.43%). When the temperature rises further, the
weight loss becomes much slower. The weight loss is complete at
808 1C, giving a residue of PbO (observed: 40.20%; calculated:
41.67%). The TG curve of 2 (Fig. S3) reveals that 2 is stable up to
200 1C. The first weight loss (25.52%) between 200 and 580 1C
corresponds to the loss of one coordinated water molecule and
one pyrimidine moiety of one Hiso2ꢀ ligand (calculated: 25.33%).
When the temperature rises further, the decomposition of 2
becomes very slow, then a little faster again above 800 1C. But
the decomposition is not complete even when the temperature
rises to 940 1C.
2.3. Synthesis of [Pb(Hiso)(H2O)]n (2)
C5H4N2O5Pb (2) was synthesized in a way similar to that of 1 by
changing the amount of NaOH into 0.5 mmol (0.02 g). The yield is 70%.
C5H4N2O5Pb (379.29): calcd. C 15.83, H 1.06, N 7.39; found C 16.20,
H 1.26, N 7.23%. IR (KBr pellet): 3408(s), 3127(w), 2969(m), 2750(w),
2376(w), 1622(s), 1551(m), 1483(m), 1400(m), 1388(m), 1308(m),
1300(m), 1191(w), 1140(w), 1109(w), 1002(w), 992(w), 880(w),
852(w), 816(w), 795(w), 660(m), 642(m), 586(w), 471(w) cm–1
.
2.4. Single-crystal structure determination
The diffraction data for 1 and 2 were collected on a Bruker Smart
Apex-II CCD diffractometer using graphite-monochromated MoK
a
˚
radiation (
l
¼0.71073 A). Absorption corrections were applied by
using the multi-scan program SADABS [19]. The structure was
solved by direct methods and expanded with difference Fourier
techniques. All calculations in the structural solution and refinement
were performed using the SHELXTL program [20]. All non-hydrogen
atoms were refined anisotropically. All hydrogen atoms on C and N
atoms were added geometrically and allowed to ride on their
respective parent atoms. H atoms of water molecules were located
in a difference Fourier map and allowed to ride on their parent
atoms. Details of crystal data, collection and refinement are listed in
Table 1. The selected bond lengths and bond angles are listed in
Table 2.
3.2. Crystal structure of 1
The single crystal X-ray diffraction analysis revealed that 1
has one Pb(II) ion, two crystallographically independent
H2isoꢀ ligands and one coordinated water molecule in the asym-
metric unit as shown in Fig. 1. The Pb(II) ion is coordinated by four
carboxylato O atoms and two carbonyl O atoms from five
H2isoꢀ ligands and one water molecule, forming a holodirected
geometry. The Pb–O bond lengths are in the range of 2.491(3)–
Table 1
Crystallographic data and structure refinement for 1 and 2.
Complex
1
2
˚
2.843(3) A, which are much shorter than the commonly accepted
Formula
C10H8N4O9Pb
535.39
C5H4N2O5Pb
379.29
˚
sum of van der Waals radii (3.44 A) of lead(II) and oxygen [16] and
fw
comparable to the related Pb–O bond lengths reported [12,16,21].
The valence of Pb calculated from the Pb–O bonds described above
is 1.851. Based on the analysis of the Pb–O bonds mentioned
above, one kind of H2isoꢀ ligand bridges three Pb(II) ions with its
carboxylato group linking two Pb(II) ions in a syn–anti bridging
mode and its 2-carbonyl oxygen atom coordinating to the third
Pb(II) ion as shown in type I of Scheme 1. However, the other kind
of H2isoꢀ bridges only two Pb(II) ions using one carboxylato
oxygen atom with its 4-carbonyl oxygen atom also coordinating
to one of the two Pb(II) ions (type II of Scheme 1). 1 presents
another three weak Pb–O bonds of Pb1–O8B, Pb1–O7C and
Pb1–O4A with the bond distances of 3.134(3), 3.179(3) and
T (K)
223(2)
273(2)
0.71073
0.71073
˚
l
(A)
Cryst syst.
Monoclinic
P21/n
Monoclinic
P21/c
Space group
6.6443(13)
6.9995(13)
˚
a (A)
15.648(3)
12.499(3)
16.468(3)
˚
b (A)
6.5392(13)
˚
c (A)
a
b
g
(deg)
90
90
103.38(3)
90
111.809(2)
90
(deg)
(deg)
3
1264.3(4)
699.8(2)
˚
V (A )
Z
4
4
Dc (g cm–3
)
2.813
13.412
1000
3.600
24.099
680
m
(mm–1
)
˚
3.241(3) A, respectively, which increase the valence of the Pb to
F(0 0 0)
GOF on F2
R1 (I42 (I))
wR2 (I42 (I))
˚
2.017 A. Taking into account of these weak Pb–O bonds, the two
1.053
0.0232
0.0434
0.0311
0.0454
1.039
0.0267
0.0615
0.0336
0.0645
kinds of H2isoꢀ ligands in 1 present the coordination modes as
shown in Types IV and V of Scheme 1. The change of the
coordination modes from I and II into IV and V, respectively, does
not change the linking modes of Pb(II) ion and H2isoꢀ ligand. This
s
s
R1 (all data)
wR2 (all data)

Z. Chen et al. / Journal of Solid State Chemistry 184 (2011) 1063–1069
1065
Table 2
˚
Selected bond lengths (A) and angles (deg) for 1 and 2.
Complex 1
Pb1–O3
2.491(3)
2.496(3)
2.498(3)
2.651(3)
89.02(9)
69.18(10)
80.51(10)
117.75(9)
131.67(9)
73.95(8)
65.62(8)
69.53(9)
125.18(8)
156.61(9)
164.42(9)
75.51(9)
109.17(9)
75.23(8)
106.27(8)
120.94(8)
Pb1–O2
2.667(3)
2.725(3)
2.843(3)
115.98(9)
159.21(9)
85.35(8)
74.50(8)
66.20(9)
75.97(8)
148.62(8)
68.46(9)
44.01(8)
125.18(8)
118.46(8)
95.31(8)
80.35(8)
132.76(8)
134.79(8)
93.00(8)
Pb1–O8B
3.134(3)
3.179(3)
3.241(3)
64.13(8)
108.75(8)
42.67(7)
72.35(7)
107.49(8)
62.83(9)
42.67(8)
70.64(10)
132.07(9)
67.23(8)
131.56(8)
95.23(8)
163.63(8)
110.82(10)
87.00(8)
84.69(8)
Pb1–O9
Pb1–O5
Pb1–O7C
Pb1–O3A
Pb1–O8C
Pb1–O4A
Pb1–O7B
O9–Pb1–O8C
O3A–Pb1–O8C
O7B–Pb1–O8C
O2–Pb1–O8C
O5–Pb1–O8C
O3–Pb1–O8B
O9–Pb1–O8B
O3A–Pb1–O8B
O7B–Pb1–O8B
O2–Pb1–O8B
O5–Pb1–O8B
O8C–Pb1–O8B
O3–Pb1–O7C
O9–Pb1–O7C
O3A–Pb1–O7C
O7B–Pb1–O7C
O2–Pb1–O7C
O5–Pb1–O7C
O8C–Pb1–O7C
O8B–Pb1–O7C
O3–Pb1–O4A
O9–Pb1–O4A
O3A–Pb1–O4A
O7B–Pb1–O4A
O2–Pb1–O4A
O5–Pb1–O4A
O8C–Pb1–O4A
O8B–Pb1–O4A
O7C–Pb1–O4A
Pb1–O3–Pb1A
Pb1D–O7–Pb1E
Pb1E–O8–Pb1D
O3–Pb1–O9
O3–Pb1–O3A
O9–Pb1–O3A
O3–Pb1–O7B
O9–Pb1–O7B
O3A–Pb1–O7B
O3–Pb1–O2
O9–Pb1–O2
O3A–Pb1–O2
O7B–Pb1–O2
O3–Pb1–O5
O9–Pb1–O5
O3A–Pb1–O5
O7B–Pb1–O5
O2–Pb1–O5
O3–Pb1–O8C
Symmetry codes: (A) –xþ2, –yþ1, –zþ1; (B) xþ1/2, –yþ3/2, zþ1/2; (C) –xþ1/2, yꢀ1/2, –zþ1/2; (D) xꢀ1/2, –yþ3/2, zꢀ1/2; (E) –xþ1/2, yþ1/2, –zþ1/2.
Complex 2
Pb1–O5
2.455(6)
2.508(6)
2.560(6)
83.6(2)
Pb1–N2A
2.646(6)
Pb1–O2C
2.915(6)
Pb1–O3
Pb1–O3B
2.650(5)
Pb1–O4D
3.066(6)
Pb1–O1
Pb1–O2B
2.787(5)
Pb1–O4A
3.215(7)
O5–Pb1–O3
O5–Pb1–O1
O3–Pb1–O1
O5–Pb1–N2A
O3–Pb1–N2A
O1–Pb1–N2A
O5–Pb1–O3B
O3–Pb1–O3B
O1–Pb1–O3B
N2A–Pb1–O3B
O5–Pb1–O2B
O3–Pb1–O2B
Pb1B–O2–Pb1F
O1–Pb1–O2B
N2A–Pb1–O2B
O3B–Pb1–O2B
O5–Pb1–O2C
O3–Pb1–O2C
O1–Pb1–O2C
N2A–Pb1–O2C
O3B–Pb1–O2C
O2B–Pb1–O2C
O5–Pb1–O4D
O3–Pb1–O4D
O1–Pb1–O4D
Pb1–O3–Pb1B
167.42(19)
102.39(19)
47.67(17)
144.21(18)
131.9(2)
N2A–Pb1–O4D
O3B–Pb1–O4D
O2B–Pb1–O4D
O2C–Pb1–O4D
O5–Pb1–O4A
O3–Pb1–O4A
O1–Pb1–O4A
N2A–Pb1–O4A
O3B–Pb1–O4A
O2B–Pb1–O4A
O2C–Pb1–O4A
O4D–Pb1–O4A
Pb1G–O4–Pb1E
134.2(2)
89.21(18)
67.42(16)
74.9(2)
67.95(19)
66.81(17)
61.50(16)
81.60(18)
164.3(2)
135.3(2)
73.52(19)
70.95(19)
63.4(2)
99.90(17)
74.74(19)
124.27(17)
90.25(16)
137.62(16)
87.19(19)
124.69(19)
116.6(2)
117.34(16)
44.16(18)
106.54(16)
60.82(15)
63.40(16)
100.36(15)
79.64(15)
128.34(18)
138.2(2)
78.22(17)
110.97(16)
89.75(16)
Symmetry codes: (A) xþ1, –yþ1/2, zþ1/2 ; (B) –xþ1, –y, –zþ1; (C) xþ1, y, z; (D) –xþ1, yꢀ1/2, –zþ1/2; (E) xꢀ1, –yþ1/2, zꢀ1/2; (F) xꢀ1, y, z; (G) –xþ1, yþ1/2, –zþ1/2.
Fig. 1. Molecular structure of 1 showing the coordination modes of Pb(II) ion and
H2isoꢀ ligand with displacement ellipsoids drawn at the 30% probability level.
Hydrogen atoms are omitted for clarity. Symmetry codes: (A) –xþ2, –yþ1, –zþ1;
(B) xþ1/2, –yþ3/2, zþ1/2; (C) –xþ1/2, yꢀ1/2, –zþ1/2; (D) xꢀ1/2, –yþ3/2, zꢀ
1/2; and (E) –xþ1/2, yþ1/2, –zþ1/2.
means that these weak Pb–O bonds do not change the topology of
1, thus they are not discussed further in the text.
Two adjacent Pb(II) ions in 1 are bridged by two carboxylato
oxygen atoms from two
dinuclear unit with a PbyPb separation of 4.107(1) A. The
m
2-bridging H2isoꢀ ligands to build a
˚
Scheme 1. Coordination modes of H2isoꢀ in 1 (I, II, IV and V) and Hiso2ꢀ in 2 (III
and VI). The weak Pb–O bonds are not included in I–III and included in IV–VI.

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Z. Chen et al. / Journal of Solid State Chemistry 184 (2011) 1063–1069
nearest Pb(II) ions from the neighboring dinuclear units are
connected by two syn–anti carboxylato groups from two
ligands leads to the construction of a three-dimensional frame-
work with the Schla¨fli symbol (4.85)(4.82) as shown in Fig. 4.
m
3-bridging H2isoꢀ ligands to build an one-dimensional chain
˚
(Fig. 2) along the a-axis with a PbyPb separation of 4.031(1) A.
3.3. Crystal structure of 2
The 2-carbonyl oxygen atoms from the
m
3-bridging H2isoꢀ
ligands on the two sides of each one-dimensional chain coordi-
nate to the Pb(II) ions from another two adjacent one-dimen-
sional chains, leading to the connection of each one-dimensional
chain with another four ones. This results in the construction of a
three-dimensional framework as shown in Fig. 3.
The single crystal X-ray diffraction analysis revealed that 2 has
one Pb(II) ion, one Hiso2ꢀ and one coordinated water molecule in
the asymmetric unit as shown in Fig. 5. Ignoring the Pb–O
As described above, each Pb(II) ion is coordinated to two
m
2-bridging H2isoꢀ ligands and three 3-bridging H2isoꢀ ligands.
m
Each m m
2-bridging H2isoꢀ ligand and 3-bridging H2isoꢀ ligand in
return connects two and three Pb(II) ions, respectively. The
interaction of the unique Pb(II) ion with the two types of H2isoꢀ
Fig. 2. A view of one-dimensional chain in 1.
Fig. 4. The overall network topology of 1 seen down the a-axis. Green (big) and
red (small) spheres represent Pb(II) and
two Pb(II) ions represent two
2-H2isoꢀ ligands. (For interpretation of the
m
3-H2isoꢀ, respectively. The line between
m
references to color in this figure legend, the reader is referred to the web version
of this article.)
Fig. 5. Molecular structure of 2 showing the coordination modes of Pb(II) ion and
Hiso2ꢀ ligand with displacement ellipsoids drawn at the 30% probability level.
Hydrogen atoms are omitted for clarity. Symmetry codes: (A) xþ1, –yþ1/2, zþ
1/2; (B) –xþ1, –y, –zþ1; (C) xþ1, y, z; (D) –xþ1, yꢀ1/2, –zþ1/2; (E) xꢀ1, –yþ
1/2, zꢀ1/2; (F) xꢀ1, y, z; and (G) –xþ1, yþ1/2, –zþ1/2.
Fig. 3. A view of three-dimensional framework of 1. Hydrogen atoms are omitted
for clarity.

Z. Chen et al. / Journal of Solid State Chemistry 184 (2011) 1063–1069
1067
˚
distances larger than 2.90 A, each Pb(II) ion is coordinated in
hemidirected pentagonal pyramidal geometry with three carbox-
ylato oxygen atoms, one nitrogen atom and one carbonyl oxygen
atom at the basal positions and one water molecule at the apical
position. The vacant site opposite to the apical site is left for
stereochemically active lone electron pair. The Pb–O bond lengths
˚
are in the range of 2.455(6)–2.787(5) A, which are comparable to
the normal Pb–O bond lengths. These Pb–O bond lengths gave a
valence value of 1.748 for Pb. It is noteworthy that two atoms
(O2C and O4D) are found in the place left for the lone electron
pair around the Pb(II) ion with the Pb1–O2C and Pb1–O4D bond
˚
lengths of 2.915(6) and 3.066(6) A, respectively. Furthermore,
another oxygen atom (O4A) was located around the Pb(II) ion
˚
with the Pb1–O4A bond length of 3.215(7) A. The three Pb–O
bond lengths are longer than the normal ones, but still shorter
than the commonly accepted sum of van der Waals radii of
˚
lead(II) and oxygen (3.44 A), suggesting the presence of weak
Pb–O bonds. The valence of Pb calculated from all aforementioned
Pb–O bond lengths increases to 1.989.
Ignoring the three weak Pb–O bonds, the Hiso2ꢀ ligand behaves
as a m3, Z
5-bridge (Type III in Scheme 1) as shown in Figs. 5 and 6. It
uses its carboxylato group to chelate one Pb(II) ion and uses one of
the carboxylato oxygen atoms and 4-carbonyl oxygen atom to
chelate another Pb(II) ion. This results in the connection of two
Pb(II) ions by two carboxylato groups from two Hiso2ꢀ ligands,
˚
forming a dinuclear unit with a PbyPb distance of 4.3901(7) A and
a Pb–O–Pb bond angle of 116.618(5)1. Each dinuclear unit is linked
to another four ones by the coordination of 1-nitrogen atoms of
Hiso2ꢀ ligands to the Pb(II) ions from the neighboring dinuclear
units, constructing a two-dimensional sheet (Fig. 6). Each Pb(II) ion
was coordinated by three Hiso2ꢀ ligands and each Hiso2ꢀ ligand in
return bridges three Pb(II) ions. Topologically, both Pb(II) and
Hiso2ꢀ act as three-connected nodes. Thus a 3,3-connected sheet
(Fig. 7) with the Schla¨fli symbol (4.82)(4.82) was built.
Fig. 7. The 2D topology of 2 viewed down the c-axis. Green (big) and red (small)
spheres represent three-connected Pb(II) ions and Hiso2ꢀ ligands, respectively.
(For interpretation of the references to color in this figure legend, the reader is
referred to the web version of this article.)
Taking into account of the three weak Pb–O bonds together with
, -
the six normal Pb–O bonds, each Hiso2ꢀ behaves as a m5 Z8
bridging ligand (Type VI in Scheme 1) to connect five Pb(II) ions
with the carboxylato group acting as a m3
4-bridge. The interaction
of Pb(II) ions with m3
4-bridging carboxylato groups leads to the
,Z
,Z
formation of one-dimensional Pb-carboxylato chain along the
a-axis as shown in Fig. 8. One kind of two neighboring Pb(II) ions
in the chain are connected by two carboxylato oxygen atoms from
two Hiso2ꢀ ligands with a PbyPb separation of 4.0242(5) A and a
˚
Pb–O–Pb bond angle of 89.754(4)1. The other kind of two neighbor-
ing Pb(II) ions in the chain are also linked by two carboxylato
oxygen atoms from another two Hiso2ꢀ ligands with a PbyPb
Fig. 8. A view of one-dimensional chain in 2.
˚
separation of 4.3901(7) A and a Pb–O–Pb bond angle of 116.618(5)1.
The former two neighboring Pb(II) ions are further linked by two
2-carbonyl oxygen atoms from another two Hiso2ꢀ ligands with a
Pb–O–Pb bond angle of 79.643(3)1. Each one-dimensional chain is
linked to another four ones by Hiso2ꢀ ligands, constructing a three-
dimensional framework as depicted in Fig. 9.
For further understanding the three-dimensional framework
of 2 with a consideration of the three weak Pb–O bonds together
with the six normal Pb–O bonds, its topology is discussed here. As
discussed above, each Pb(II) ion is coordinated by five Hiso2ꢀ
ligands, and each Hiso2ꢀ ligand in return coordinates to five Pb(II)
ions. From a topological view, both Pb(II) and Hiso2ꢀ behaves as
five-connected nodes. The overall 5,5-connected 3D network has
the Schla¨fli symbol (415.64)(44.68.82) as shown in Figs. 10 and S4.
Fig. 6. A view of the two-dimensional sheet of 2.

1068
Z. Chen et al. / Journal of Solid State Chemistry 184 (2011) 1063–1069
Fig. 9. A view of the three-dimensional framework of 2. Hydrogen atoms are
omitted for clarity.
Fig. 11. Solid-state emission spectra of H3iso, 1 and 2 in the solid state at room
temperature.
(4.85)(4.82) no matter if the three weak Pb–O bonds are taking into
account or not. Topologically, 2 shows a 3,3-connected sheet with
the Schla¨fli symbol (4.82)(4.82) by ignoring the three weak Pb–O
bonds and a (6,6)-connected (48, 66, 8)(413, 62) network by taking
into account of the three weak Pb–O bonds together with the six
normal Pb–O bonds. As revealed in the documents that the photo-
luminescence behavior is closely associated with the metal ions and
the ligands coordinated around them, the different fluorescent
properties of 2 from 1 can presumably be caused fractionally by
different degree of deprotonation of the H3iso ligand and largely by
the different coordination environments around Pb(II) ions and even
the different structural topologies of the two compounds as men-
tioned above [23–28].
Fig. 10. The overall network topology of 2 viewed down the c-axis. Green (big)
and red (small) spheres represent five-connected Pb(II) ions and Hiso2ꢀ ligands,
respectively. (For interpretation of the references to color in this figure legend, the
reader is referred to the web version of this article.)
4. Conclusion
In summary, two lead(II) complexes based on the H3iso ligand
was prepared and characterized. The H3iso ligand in compounds 1
and 2 exists as monovalent anion form of H2isoꢀ and divalent anion
form of Hiso2ꢀ, respectively. Whether the weak Pb–O bonds are
included or not, each Pb(II) ion in 1 is coordinated by two
This is much different from the reported Pb(II) orotate (2,6-dioxo-
1,2,3,6-tetrahydropyrimidine-4-carboxylate) compound, in which
both Pb(II) ion and orotate ligand act as 6-connected nodes to
construct a (6,6)-connected (48, 66, 8)(413, 62) network [22].
m
2-bridging H2isoꢀ ligands and three
m
3-bridging H2isoꢀ ligands.
m
3-bridging H2isoꢀ ligand in
Each
m
2-bridging H2isoꢀ ligand and
return connects two and three Pb(II) ions, respectively. This leads to
the construction of a three-dimensional framework of 1 with the
Schla¨fli symbol (4.85)(4.82). Ignoring the weak Pb–O bonds, both
Pb(II) and Hiso2ꢀ in 2 act as three-connected nodes, giving a 3,3-
connected sheet with the Schla¨fli symbol (4.82)(4.82). Taking into
account of the weak Pb–O bonds, both Pb(II) and Hiso2ꢀ behaves as
five-connected nodes, giving a 5,5-connected 3D network with the
Schla¨fli symbol (415.64)(44.68.82). The fluorescent measurements
reveal an intraligand emission in 1 and a LMCT emission in 2.
3.4. Luminescent properties
The luminescent properties of free H3iso ligand, 1 and 2 in the
solid state at room temperature were investigated as shown in
Fig. 11 and Figs. S5–S7. The free H3iso ligand displays emission at
341 nm when excited at 303 nm, which can probably be assigned to
the
p-pn transition. 1 shows an emission band at 334 nm upon
excitation at 300 nm, which is very close to that of the free H3iso
ligand and attributed to the intraligand emission for the H2isoꢀ
ligand. However, 2 shows an emission band at 459 nm with an
excitation at 376 nm, which has a bathochromic shift of 118 nm in
contrast to that for free H3iso ligand. The emission band of 2 might
5. Supplementary material
be assigned to LMCT between delocalized
p
bonds of the Hiso2ꢀ
ligand and p orbitals of Pb2þ ion [6,12]. Based on the structural
analyses, the Pb(II) ions in 1 and 2 are seven-coordinated in
holodirected geometries and six-coordinated in hemidirected pen-
tagonal pyramidal geometries, respectively. Both 1 and 2 have
another three weak Pb–O bonds. Furthermore, 1 shows a three-
dimensional topological framework with the Schla¨fli symbol
Crystallographic data for the structures reported in this paper have
been deposited with the Cambridge Crystallographic Data Centre as
supplementary publication no. CCDC 786357 and 786358 for com-
plexes 1 and 2, respectively. Copies of the data can be obtained free of
charge on application to CCDC, 12 Union Road, Cambridge CB2 1EZ,
UK (fax: (44) 1223 336-033; e-mail: deposit@ccdc.cam.ac.uk).

Z. Chen et al. / Journal of Solid State Chemistry 184 (2011) 1063–1069
1069
Acknowledgments
[8] A. Thirumurugan, C.N.R. Rao, J. Solid State Chem. 181 (2008) 1184–1194.
[9] Y.-H. Zhao, H.-B. Xu, K.-Z. Shao, Y. Xing, Z.-M. Su, J.-F. Ma, Cryst. Growth Des.
7 (2007) 513–520.
The authors thank the financial support by National Natural
Foundation of China (Grant no. 20962003), Guangxi Natural Science
Foundation of China (Grant nos. 0991008 and 2010GXNSFF013001)
and Program for Excellent Talents in Guangxi Higher Education
Institutions.
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Appendix A. Supporting information
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