Hybrid structures formed by homo- and heteroleptic aliphatic dicarboxylates of lead with 2-D inorganic connectivity

Article abstract of DOI:10.1016/j.jssc.2008.02.021

Three-dimensional homoleptic (single type of ligand) lead dicarboxylates with hybrid structures involving Pb-O-Pb linkages of the compositions, Pb(C₅H₆O₄), I, and Pb(C₆H₈O₄), II and III, have been synthesized and characterized. Three-dimensional heteroleptic (mixed ligands) lead dicarboxylates of the formulae, Pb₂(C₂O₄)(C₄H₄O₄), IV and Pb₂(C₂O₄)(C₆H₈O₄), V, with hybrid structures involving Pb-O-Pb linkages have also been prepared and characterized along with a novel two-dimensional lead nitrate-oxalate of the composition, (OPb₂)₂(C₂O₄)(NO₃)₂, VI. In all these dicarboxylates, there is two-dimensional inorganic connectivity and the lead (II) cation has hemi- or holo-directed coordination geometry. Depending upon the torsional angle and the coordination mode of the dicarboxylate anions as well as the geometry of the lead (II) cations, these hybrid compounds exhibit two types of two-dimensional inorganic connectivities.

Full text of DOI:10.1016/j.jssc.2008.02.021

ARTICLE IN PRESS
Journal of Solid State Chemistry 181 (2008) 1184–1194
www.elsevier.com/locate/jssc
Hybrid structures formed by homo- and heteroleptic aliphatic
dicarboxylates of lead with 2-D inorganic connectivity
Ã
A. Thirumurugan, C.N.R. Rao
Chemistry and Physics of Materials Unit, Jawaharlal Nehru Centre for Advanced Scientific Research, Jakkur P.O., Bangalore 560064, India
Received 17 December 2007; received in revised form 12 February 2008; accepted 18 February 2008
Available online 29 February 2008
Abstract
Three-dimensional homoleptic (single type of ligand) lead dicarboxylates with hybrid structures involving Pb–O–Pb linkages of the
compositions, Pb(C5H6O4), I, and Pb(C6H8O4), II and III, have been synthesized and characterized. Three-dimensional heteroleptic
(mixed ligands) lead dicarboxylates of the formulae, Pb2(C2O4)(C4H4O4), IV and Pb2(C2O4)(C6H8O4), V, with hybrid structures
involving Pb–O–Pb linkages have also been prepared and characterized along with a novel two-dimensional lead nitrate-oxalate of the
composition, (OPb2)2(C2O4)(NO3)2, VI. In all these dicarboxylates, there is two-dimensional inorganic connectivity and the lead (II)
cation has hemi- or holo-directed coordination geometry. Depending upon the torsional angle and the coordination mode of the
dicarboxylate anions as well as the geometry of the lead (II) cations, these hybrid compounds exhibit two types of two-dimensional
inorganic connectivities.
r 2008 Elsevier Inc. All rights reserved.
Keywords: Inorganic–organic hybrid frameworks; Aliphatic dicarboxylates; Mixed ligands; Nitrate; Lead
1. Introduction
(I) and organic (O) connectivities, to define a character-
istic InOm. Hybrid structures with two-dimensional
(2-D) inorganic connectivity (I2) are known in the literature
[8,9,18–24]. There are very few mixed aliphatic dicar-
boxylates known today, the only examples, to our knowl-
edge being the neodymium oxalate-glutarate [25],
Nd4(H2O)2(C5H6O4)4(C2O4)2, lanthanum oxalate-succi-
nate [26], [La2(C2O4)(C4H4O4)2(H2O)4] ꢀ 4H2O, lanthanum
oxalate-adipate [26], La2(C2O4)2(C6H8O4)(H2O)2, lantha-
nide oxalate-fumarate [27], [La2(C2O4)(C4H2O4)2 (H2O)4] ꢀ
4H2O (Ln ¼ Eu, Tb). In this article, we describe the
synthesis and structure of hybrid aliphatic dicarboxylates
of lead with 2-D inorganic connectivities of the type
I2Om where m ¼ 0 or 1. The compounds synthesized
and characterized by us include a glutarate, Pb(C5H6O4),
I, and two adipates of the same formula, Pb(C6H8O4),
II, and III, in addition to two mixed carboxylates,
an oxalate-succinate, Pb2(C2O4)(C4H4O4), IV, and an
oxalate-adipate, Pb2(C2O4)(C6H8O4), V. We also report
Metal carboxylates with different framework structures
are being investigated in-depth in the last few years [1–4].
Some of the carboxylates possess interesting properties as
exemplified by the metal-organic frameworks synthesized
by Yaghi et al. [5], which show excellent hydrogen sorption
properties. Then, there are trinuclear chromium cluster-
based benzene di- and tri-carboxylates synthesized by
Fe
´
rey et al. [6,7] which possess very large surface areas
3
˚
and unit cell volumes going up to 700,000 A . Of particular
interest to us are the inorganic organic hybrid frame-
works with infinite metal–oxygen–metal bonds. Hybrid
metal carboxylates with interesting properties have been
reported in the literature. Aliphatic dicarboxylic acids
have been used as good linkers in the construction of
hybrid compounds with interesting structures [8–16].
Recently [17], hybrid frameworks have been classified on
the basis of the dimensionality (n, m) of the inorganic
a
nitrate-oxalate, (OPb2)2(C2O4)(NO3)2, VI, where
Ã
the nitrate and oxalate anions coordinate with the metal
ion.
Corresponding author. Fax: +91 80 2208 2760.
E-mail address: cnrrao@jncasr.ac.in (C.N.R. Rao).
0022-4596/$ - see front matter r 2008 Elsevier Inc. All rights reserved.
doi:10.1016/j.jssc.2008.02.021

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A. Thirumurugan, C.N.R. Rao / Journal of Solid State Chemistry 181 (2008) 1184–1194
1185
2. Experimental
Powder XRD patterns of the products were recorded
using CuKa radiation (Rich-Seifert, 3000TT). The patterns
agreed with those calculated for single crystal structure
determination. Thermogravimetric analysis (TGA) was
carried out (Metler-Toledo) in oxygen atmosphere (flow
rate ¼ 50 mL/min) in the temperature range 25–800 1C
(heating rate ¼ 5 1C/min).
2.1. Synthesis and characterization
2.1.1. Materials and methods
Pb(NO3)2 (Qualigens, India, 99%), oxalic acid dihydrate
(C2H2O4 ꢀ 2H2O) (S.D. Fine, India, 99.5%), succinic acid
(C4H6O4) (Qualigens, India, 99%), glutaric acid (C5H8O4)
(Spectrochem, India, 99%), adipic acid (C6H10O4) (Merck,
India, 99%), NaOH (Merck, India, 99%) of high purity
and double distilled water were used for the synthesis.
The Pb aliphatic dicarboxylates I–VI were synthesized
under hydrothermal conditions by heating the homoge-
nized reaction mixtures in a 23 mL PTFE-lined bomb at the
180 1C temperature for 72 h under autogeneous pressure.
The pH of the starting reaction mixture was generally in
the range 5–6. The pH after the reaction did not show
appreciable change. The products of the hydrothermal
reactions were vacuum-filtered and dried under ambient
conditions. The starting compositions for the different
compounds synthesized by us are given in Table 1. All the
compounds were obtained as single phase materials, except
III, whose crystals were admixed with small quantities of
lead succinate, Pb(C4H4O4) [28]. Attempts were made
unsuccessfully to prepare III in pure form without the
addition of succinic acid. The crystals of I, II, IV, V and VI
were separated under a polarizing microscope and used for
all the characterization. Other than the single-crystal X-ray
diffraction (XRD), III could not be subjected to other
methods of characterization due to the difficulty in
separating the lead succinate impurity.
Infrared (IR) spectra of KBr pellets of the compounds
were recorded in the mid IR region (Bruker IFS-66v). All
the compounds show characteristic bands of the functional
groups [29–31]. The bands around 1600 and 1400 cmꢁ1 are
assigned to the asymmetric (nas CꢁO) and symmetric
(ns C=O) stretching of the carboxylate anion. The bands
at 1285 (nas NꢁO), 1017 (ns NꢁO), 752 ðdNO
Þ
and
in-plane
3
817 cmꢁ1 ðdNO Þout-of-plane indicate the presence of the
3
nitrate groups in VI.
Thermogravimetric analyses of the Pb dicarboxylates are
as follows. All the compounds show single-step weight loss.
For I, the weight loss of 34.36% (calcd. 33.80%) occurred
in the 310–450 1C range. For II, the weight loss of 36.82%
(calcd. 36.44%) occurred in the 295–440 1C range. For IV,
the weight loss of 28.17% (calcd. 27.79%) occurred in the
300–450 1C range. For V, the weight loss of 31.33% (calcd.
30.92%) occurred in the 295–450 1C range. The total
weight loss matches very well with the loss of CO2 and H2O
and the formation of PbO (PDF no. 00-004-0561) in all the
cases. TGA could not be performed in VI due to the highly
exothermic nature of the decomposition.
A suitable single crystal of each compound was carefully
selected under a polarizing microscope and glued to a thin
glass fiber. Crystal structure determination by XRD was
performed on a Bruker–Nonius diffractometer with Kappa
geometry attached with an APEX-II-CCD detector and a
graphite monochromator for the X-ray source (MoKa
Elemental analyses were satisfactory. For I, (C5H6PbO4)
calcd.: C, 17.79%; H, 1.78%. Found: C, 18.09%; H,
1.63%; For II, (C6H8PbO4) calcd.: C, 20.50%; H, 2.28%.
Found: C, 21.06%; H, 2.15%. For IV, (C3H2PbO4) calcd.:
C, 11.64%; H, 0.65%. Found: C, 11.92%; H, 0.87%. For
V, (C4H4PbO4) calcd.: C, 14.85%; H, 1.24%. Found: C,
15.12%; H, 1.36%. For VI, (CNPb2O6) calcd.: C, 2.24%;
N, 2.61%. Found: C, 2.36%; N, 2.84%.
˚
radiation, l=0.71073 A) operating at 50 kV and 30 mA.
An empirical absorption correction based on symmetry
equivalent reflections was applied using the SADABS
program [32]. The structure was solved and refined using
SHELXTL-PLUS suite of programs [33]. For the final
Table 1
Synthetic conditions for compounds I–VI
Compound
no.
Formula
Composition (mole ratio)
Yield based
on lead (%)
Pb(NO3)2
Dicarboxylic acid
NaOH (5 M
soln.)
Water
I
Pb(C5H6O4)
Pb(C6H8O4)
Pb(C6H8O4)
(0.333 g)1
(0.333 g)1
(0.333 g)1
Glutaric acid (0.267 g) 2
Adipic acid (0.295 g) 2
(0.5 mL) 2.5
(0.6 mL) 3
(0.5 mL) 2.5
(5 mL) 278
(5 mL) 278
(5 mL) 278
54
61
–
II
III
Succinic acid
(0.0596 g) 0.5
Oxalic acid ꢀ 2H2O
(0.127 g)1
Adipic acid
(0.295 g) 2
Succinic acid
(0.238 g) 2
Adipic acid
(0.295 g) 2
IV
V
Pb2(C2O4)(C4H4O4)
Pb2(C2O4)(C6H8O4)
(OPb2)2(C2O4)(NO3)2
(0.333 g)1
(0.333 g)1
(0.333 g)1
(1.0 mL) 5
(0.8 mL) 4
(0.6 mL) 3
(5 mL) 278
(5 mL) 278
(5 mL) 278
67
64
56
Oxalic acid ꢀ 2H2O
(0.127 g)1
VI
Oxalic acid ꢀ 2H2O
(0.19 g ) 1.5

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1186
refinement the hydrogen atoms were placed geometrically
and held in the riding mode. Final refinement included
atomic positions for all the atoms, anisotropic thermal
parameters for all the non-hydrogen atoms and isotropic
thermal parameters for the hydrogen atoms. All the
hydrogen atoms were included in the final refinement.
Details of the structure solution and final refinements for
compounds I–VI are given in Tables 2 and 3.
Table 3
Crystal data and structure refinement parameters for IV–VI
Structure parameter IV
V
VI
Empirical formula
Formula weight
Crystal system
C3H2PbO4
309.24
C4H4PbO4
323.27
Monoclinic
CNPb2O6
536.40
Monoclinic
Monoclinic
P21/c (no. 14)
12.6902(5)
5.2086(2)
6.9389(2)
100.507(2)
450.96(3)
4
Spacꢁe1group
P21/m (no. 11) P21/c (no. 14)
˚
a (A
)
)
15.1995(8)
5.1505(3)
7.0289(4)
98.072(3)
544.81(5)
4
11.8682(3)
5.2501(1)
9.0989(2)
96.741(2)
563.03(2)
4
˚
b (Aꢁ) 1
˚
c (A
3. Results and discussion
b (degꢁ1
)
ꢁ3
˚
v (A
)
We have synthesized three homoleptic (single type of
ligand) hybrid metal dicarboxylates of the composition,
Pb(C5H6O4), I, and two polymorphs of Pb(C6H8O4), II
and III, involving the coordination of glutarate and adipate
anions to the Pb(II) cation, in addition to two heteroleptic
(mixed) carboxylates of lead, Pb2(C2O4)(C4H4O4), IV, and
Pb2(C2O4)(C6H8O4), V, formed by oxalate and succinate or
adipate. We have also prepared an unusual dicarboxylate,
(OPb2)2(C2O4)(NO3)2, VI, where the nitrate ion coordi-
nates with the metal ion along with the oxalate anion. In
what follows, we discuss the structures of I–VI.
The glutarate, Pb(C5H6O4), I, has a three-dimensional
(3-D) structure with an asymmetric unit of nine non-
hydrogen atoms (Fig. 1a). The asymmetric unit contains a
crystallographically distinct Pb2+ ion and half of the
glutarate anion where all the atoms have a 0.5 occupancy
except O(3) which sits at the 4f position. The glutarate
anion with a molecular mirror plane exhibits a coordina-
Z
D(calcd.) (g/cm3)
4.555
37.312
3.941
6.328
m (mmꢁ1
Total data collected 7700
)
30.894
11864
965
59.696
3355
1007
Unique data
Observed data
[I42s (I)]
Rmerg
720
710
0.0236
1.136
939
0.0183
1.214
R1 ¼ 0.0305a
952
0.0915
1.058
R1 ¼ 0.0528a
Goodness of fit
R indexes
R1 ¼ 0.0428a
[I42s(I)]
R indexes
[All data]
wR2 ¼ 0.1012b wR2 ¼ 0.0764b wR2 ¼ 0.1187b
R1 ¼ 0.0430a
R1 ¼ 0.0310a
R1 ¼ 0.0548a
wR2 ¼ 0.1014b wR2 ¼ 0.0768b wR2 ¼ 0.1211b
aR1 ¼ S||F0|ꢁ|Fc||/S |F0|.
bwR2 ¼ {S[w(F02–F2c)2]/S[w(F02)2]}1/2. w ¼ 1/[s2(F0)2+(aP)2+bP], P ¼
[max. (F20,0)+2(Fc)2]/3, where a ¼ 0.0838, b ¼ 0.3233 for IV, a ¼ 0.00467,
b ¼ 1.7204 for V and a ¼ 0.0764, b ¼ 10.0162 for VI.
tion mode with (1222) connectivity (for description of
connectivity see [34]) with the torsional angle of 95.01(2)1
(Fig. 1b). The Pb atom is hemi-directed (where the ligating
atoms bind only from one side of the polyhedra and form a
hemisphere like geometry, due to the stereochemically
active lone pair of electrons in the cation) and seven-
coordinated by oxygen atoms (PbO7) from five different
glutarate anions. Six of the oxygens have m3 connections
linking each Pb with four other Pb atoms. Thus, a PbO7
polyhedron shares its two corners with two different PbO7
polyhedra and two edges with two other PbO7 forming an
infinite 2-D Pb–O–Pb layer of the I2O0 type, with a (4,4)
net topology (Fig. 1c). This layer can be viewed as chains of
edge shared PbO7 polyhedra connected through the corners
of PbO7 polyhedra of the adjacent chains. The layers get
further connected to each other through glutarate anions
into a 3-D structure of the I2O1 type (Fig. 1d). The Pb–O
Table 2
Crystal data and structure refinement parameters for I–III
Structure parameter
I
II
III
Empirical formula
Formula weight
Crystal system
C5H6PbO4
337.29
Monoclinic
C6H8PbO4
351.31
Monoclinic
C6H8PbO4
351.31
Monoclinic
P21/c (no. 14)
20.5361(7)
5.0737(2)
7.0706(2)
92.147(1)
736.20(4)
4
Spacꢁe1group
P21/m (no. 11) P21/c (no. 14)
˚
a (Aꢁ1
)
4.7454(1)
7.2032(1)
9.6750(1)
93.164(1)
330.21(1)
2
7.2156(1)
4.7367(1)
21.9305(4)
90.417(1)
749.52(2)
4
˚
b (Aꢁ1
)
˚
c (A
)
b (deꢁgꢁ3 1
)
˚
V (A
)
Z
D(calcd.) (g/cm3)
3.392
25.494
3.113
3.170
22.876
m (mmꢁ1
Total data collected 2170
)
22.470
4825
1387
7160
1269
˚
bond lengths are in the 2.399–2.853 A range.
Unique data
Observed data
[I42s(I)]
607
591
The adipate, Pb(C6H8O4), II, has a 3-D structure similar
to I, with an asymmetric unit of 11 non-hydrogen atoms
(Fig. 2a). The asymmetric unit contains a crystallographi-
cally distinct Pb2+ ion and one adipate anion. The adipate
anion exhibits a coordination mode with (1222) connectiv-
ity and a torsional angle of 108.19(3)1. The Pb atom is
hemi-directed and seven-coordinated by oxygen atoms
from five different adipate anions. Six of the oxygens have
m3 connections linking each Pb with four other Pb atoms
giving rise to an infinite 2-D Pb–O–Pb layer of the I2O0
type, with a (4,4) net topology just as in I (Fig. 1c) and a
1307
1221
Rmerg
Goodness of fit
R indexes
0.0277
1.082
0.0349
1.037
0.0276
1.133
R1 ¼ 0.0396a
R1 ¼ 0.0182a
R1 ¼ 0.0450a
[I 42s (I)]
R indexes
[All data]
wR2 ¼ 0.0897b wR2 ¼ 0.0416b wR2 ¼ 0.1080b
R1 ¼ 0.0401a
R1 ¼ 0.0200a
R1 ¼ 0.00459a
wR2 ¼ 0.0906b wR2 ¼ 0.0424b wR2 ¼ 0.1106b
aR1 ¼ S||F0|–|Fc||/S|F0|.
bwR2 ¼ {S[w(F02–F2c)2]/S[w(F02)2]}1/2. w ¼ 1/[s2(F0)2+(aP)2+bP], P ¼
[max.(F20,0)+2(Fc)2]/3, where a ¼ 0.0773, b ¼ 0 for I, a ¼ 0.0183, b ¼
0.3362 for II and a ¼ 0.0828, b ¼ 0.7243 for III.

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1187
Fig. 1. (a) ORTEP plot of Pb(C5H6O4), I (thermal ellipsoids are shown at 50% probability), (b) coordination mode of the glutarate moiety in I, (c) view of
the inorganic layer with the infinite Pb–O–Pb linkages of the I2O0 type in I and (d) the 3-D structure of I viewed along the a-axis.
3-D structure of the I2O1 type through adipate connectivity
(Fig. 2b). The Pb–O bond lengths in II are in the
˚
2.398–2.864 A range.
with a (4,4) net topology different from compounds I and
II (Fig. 3c). This layer contains of PbO7 polyhedra
connected to each other through edges and corners. The
layers are further connected to each other through the
adipate anions into a 3-D structure of the I2O1 type
Another adipate, Pb(C6H8O4), III, is a polymorph of II,
also has a 3-D structure with an asymmetric unit of 11 non-
hydrogen atoms (Fig. 3a). The asymmetric unit contains a
crystallographically distinct Pb2+ ion and an adipate
anion. The adipate anion exhibits a coordination mode
with (1212) connectivity and a torsional angle of 01
(Fig. 3b). The Pb atom is holo-directed (where the ligating
atoms bind from all the sides of the polyhedra) and seven-
coordinated by oxygen atoms from six different adipate
anions. Six of the oxygens have m3 connections linking each
Pb atom with four other Pb atoms. Thus, a PbO7
polyhedron shares its two corners with two PbO7
polyhedra and two of its edges with other two PbO7
forming an infinite 2-D Pb–O–Pb layer of the I2O0 type,
˚
(Fig. 3d). The Pb–O bond lengths are in the 2.386–2.925 A
range. Compounds I and II provide a nice example for the
structural polymorphism with same chemical formula but
with different 2-D inorganic connectivity.
The oxalate-succinate, Pb2(C2O4)(C4H4O4), IV, has a
3-D structure with an asymmetric unit of 8 non-hydrogen
atoms (Fig. 4a). The asymmetric unit contains a crystal-
lographically distinct Pb2+ ion, one half of the oxalate and
one half of the succinate anions. The oxalate and succinate
anions exhibit coordination modes with (2222) and
(1121) connectivities, respectively, both with 01 torsional
angle (Figs. 4b and c). The Pb atom is holo-directed and

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1188
Fig. 2. (a) ORTEP plot of Pb(C6H8O4), II (thermal ellipsoids are shown at 50% probability), (b) the 3-D structure of II viewed along the b-axis.
seven-coordinated by oxygen atoms from two oxalate and
three succinate anions. Six of the oxygens have m3
connections linking two different Pb atoms. This leads to
a connectivity wherein each Pb atom is connected with four
other Pb atoms. Thus, each PbO7 polyhedron shares four
of its corners with four other PbO7 polyhedra forming an
infinite 2-D Pb–O–Pb layer of the I2O0 type, with a (4,4)
net topology very similar to that in III (Fig. 4d). This layer
can be viewed as chains of only corner shared PbO7
polyhedra and connected through the corners of PbO7
polyhedra in the adjacent chains. Two of these layers are
connected to each other through the oxalate anions into a
bilayer structure (Fig. 4e). These bilayers are further
connected to each other through succinate anions into a
3-D structure of the I2O1 type (Fig. 4e). The Pb–O bond
different Pb atoms, leading to a connectivity just as in IV,
giving rise to an infinite 2-D Pb–O–Pb layer of the I2O0
type, with a (4,4) net topology. The layers are connected to
each other through the oxalate anions to form bilayers
(Fig. 5c). The bilayers further get connected through
adipate anions into a 3-D structure of the I2O1 type
˚
(Fig. 5c). The Pb–O bond lengths are in the 2.402–2.867 A
range.
The 3-D lead aliphatic dicarboxylates, I–III (homoleptic)
and IV and V (heteroleptic or mixed ligands) all possess
infinite 2-D layers of the I2O0 type which are connected by
the dicarboxylate moiety. The inorganic layers can be
further categorized in to two types: Type A: compounds
I–II (Fig. 1c) and Type B: III–V (Figs. 3c and 4d). The
difference between A and B arises from the different
polyhedral connectivities in the inorganic layers. The origin
of the difference can be attributed to the torsional angles of
the dicarboxylate moieties. In type A, the torsional angles
are close to 1001, whereas it is zero in type B. This
observation gains support from the structures of other lead
aliphatic dicarboxylates. Thus, lead azelate [35],
Pb(C9H14O4), with a torsional angle of 93.111, possesses
A type inorganic layers connected by the azelate moieties.
Lead succinate, Pb(C4H4O4) [28] , with a torsional angle of
90.411, possesses M–O–M inorganic chains, closely related
to the connectivity in A type layers. In lead succinates
[28,36], the conformation of the acid plays a role in
˚
lengths are in the 2.394–2.890 A range.
The oxalate-adipate, Pb2(C2O4)(C6H8O4), V, has a 3-D
structure similar to IV, with an asymmetric unit of nine
non-hydrogen atoms (Fig. 5a). The asymmetric unit
contains a crystallographically distinct Pb2+ ion, one half
of the oxalate and one half of the adipate anions. The
oxalate and adipate anions exhibit coordination modes
with (2222) and (1212) connectivities, respectively, both
with 01 torsional angle (Figs. 4b and 5b). The Pb atom is
holo-directed and seven-coordinated by oxygen atoms
from two different oxalate and three different succinate
anions. Six of the oxygens have m3 connections linking two

ARTICLE IN PRESS
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1189
Fig. 3. (a) ORTEP plot of Pb(C6H8O4), III (thermal ellipsoids are shown at 50% probability), (b) coordination mode of the adipate moiety in III, (c) view
of the inorganic layer with the infinite Pb–O–Pb linkages of the I2O0 type in III and (d) the 3-D structure of III viewed along the b-axis.
determining the structure. In the lower dicarboxylates, such
as malonates [37,38] and oxalates [39–41], the main factor
is the coordination mode of the acid, coordination number
(CN) and the geometry of lead cation, the torsional angle
being always close to zero.
The heteroleptic 3-D compounds, IV (oxalate-succinate)
and V (oxalate-adipate), possess a unique bilayer oxalate
with the inorganic layer of the I2O1 type (Fig. 4d) con-
nected by the succinate or adipate anions into 3-D
structures. The other known hybrid heteroleptic aliphatic
dicarboxylates are neodymium oxalate-glutarate [25],
Nd4(H2O)2(C5H63O4)4(C2O4)2, lanthanum oxalate-succi-
nate [26], [La2(C2O4)(C4H4O4)2(H2O)4] ꢀ 4H2O, lanthanum
oxalate-adipate [26], La2(C2O4)2(C6H8O4) (H2O)2, and
lanthanide oxalate-fumarate [27], [La2(C2O4)(C4H2O4)2
(H2O)4] ꢀ 4H2O, (Ln ¼ Eu, Tb). The first three are the
3-D compounds possessing one-dimensional (1-D) I1O0
type M–O–M infinite chains. The oxalate-fumarate is a
coordination polymer of the I0O3 type where four member
Ln clusters are connected by carboxylate moieties.
The oxalate-nitrate, (OPb2)2(C2O4)(NO3)2, VI, has a 2-D
structure with an asymmetric unit of 10 atoms (Fig. 6a).
There are two crystallographically distinct Pb2+ ions Pb(1)
and Pb(2) in the asymmetric unit. One nitrate anion, one

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Fig. 4. (a) ORTEP plot of Pb2(C2O4)(C4H4O4), IV (thermal ellipsoids are shown at 50% probability) (b) coordination mode of the oxalate moiety in IV
(c) coordination mode of the succinate moiety in IV (d) view of the inorganic layer with the infinite Pb–O–Pb linkages of the I2O0 type in IV and (e) the
3-D structure of IV viewed along the b-axis.
half of the oxalate anion and one independent oxo dianion
are also in the asymmetric unit. The oxalate anion exhibits
a coordination mode with (1221) connectivity with zero
torsional angle and the planar nitrate anion is having (233)
connectivity (Fig. 6b). Pb(1) is hemi-directed and seven-
coordinated by oxygen atoms (PbO7) from three different
nitrate anions and an apical oxo dianion. Six of the
oxygens have m4 connections linking Pb(1) with two other

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Fig. 5. (a) ORTEP plot of Pb2(C2O4)(C6H8O4), V (thermal ellipsoids are shown at 50% probability), (b) coordination mode of the succinate moiety in V
and (c) the 3D structure of V viewed along the b-axis.
Pb(1) atoms and one Pb(2) atom. Thus, a Pb(1)O7
polyhedron shares its six corners with other Pb(1)O7
polyhedra forming an infinite 2-D Pb–O–Pb layer of the
I2O0 type, with a (6,3) 2-D net topology (Fig. 6c). The oxo
dianion has m2 connectivity and links each Pb(1) with a
Pb(2) atom. Pb(2) is hemi-directed and six-coordinated by
oxygen atoms (PbO6) from an oxo dianion, two different
nitrate anions and two different oxalate anions. These
oxgens have m2, m4 and m3 connections, respectively. Thus,
each Pb(2)O6 polyhedron is linked by the oxalate anion to
two of the polyhedron by corner-sharing to give rise an
infinite 1-D chain. The chains of Pb(2)O6 polyhedra are
further connected to the layers formed by Pb(1)O7 units
through oxo and nitrate anions, forming an infinite 2-D
layer with Pb–O–Pb linkages (Fig. 7a). While the active
lone pair electron of the Pb(1) projects out on one side of
the layer, the other side is decorated with chains of hemi-
directed Pb(2)O6 polyhedra. The oxalate anions connecting
these layers through the Pb(2) atom, form a double layer
structure with the lone pair of electrons projecting out from
both the sides of the double layer (Fig. 7b). These layers
are packed in a yAAAy manner without any H-bonding
interactions. The Pb–O bond lengths are in the
˚
2.268–2.830 A range.
The oxalate-nitrate, VI is a unique hybrid framework
wherein infinite 2-D layers of lead nitrate of the I2O0 type
are connected by the oxalate moiety into a bilayer
structure. Unlike the nitrate anion, oxy-anions such as
phosphate, sulfate, sulfite and selenate are known to form
organically templated inorganic framework compounds
[42,43]. There are, however few organic inorganic hybrid
compounds, wherein the nitrate anion is involved in the
network formation [44–54]. Most of these compounds are
with low-dimensional inorganic connectivity, Im (mo2).
The value of m depends on the nature of chelation, ligation
number, coordination mode and connectivity of the nitrate

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1192
Fig. 6. (a) ORTEP plot of (OPb2)2(C2O4)(NO3)2, VI (thermal ellipsoids are shown at 50% probability), (b) coordination mode of the nitrate moiety in VI
and (c) view of the inorganic layer with the infinite Pb–O–Pb linkages of the I2O0 type in VI.
ions. The commonly observed connectivities in the low-
dimensional hybrids are (110), (111), (112) and (122) with
zero to two chelations per single nitrate moiety. The hybrid
compound VI is appears to be only the second example
after Ag(C5H5NO)NO3 [54], possessing a 2-D inorganic
metal nitrate layer with infinite M–O–M linkages.
The connectivities of the nitrate in VI are unique with
(233) connectivity with three chelations (Fig. 6b). In
Ag(C5H5NO)NO3, the connectivity is (222) with three
chelations.
The Pb(II) cations in I–V exhibit either hemi- and holo-
directed geometry with a CN of 7. I and II, with type A
inorganic layers possess Pb(II) cations which in hemi-
directed geometry with a CN of 7, whereas III–V, with type
B inorganic layers possess Pb(II) cations in holo-directed
geometry with a CN of 7. The 2-D compound, VI, with
a inorganic bilayer structure has Pb(II) cations in
hemi-directed geometries with CN of 6 and 7 respectively.
A situation has been reported in another Pb carboxylate
where the Pb(II) cations are present with both hemi- and
holo-directed geometries [24].
4. Conclusions
In conclusion, we have synthesized and characterized five
3-D (I–V) and one 2-D (VI) hybrid lead dicarboxylates, all
possessing 2-D inorganic connectivity. Of these two of
them, the Pb glutarate, I, and adipate, II, possess one type
of 2-D inorganic connectivity with near 901 torsional angle
of the dicarboxylate anions and hemi-directed Pb(II)
cations. The Pb adipate, III, as well as the oxalate-
succinate, IV, and oxalate-adipate, V, possess another type

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Fig. 7. (a) View of the inorganic layer with the infinite Pb–O–Pb linkages of the I2O0 type in (OPb2)2(C2O4)(NO3)2, VI showing the polyhedral
connectivity between Pb(1)O7 (orange), Pb(2)O7 (cyan) and NO3 (pink) moieties and (b) the packing arrangement in VI.
of 2-D inorganic connectivity with the dicarboxylate
anions in the zero torsional angle and holo-directed
Pb(II) cations. The bilayer nitrate-oxalate, VI, is unusual
in that it possesses a (6,3) net topology of the 2-D in-
organic metal nitrate layer, with infinite M–O–M
linkages. The Pb(II) cations in VI are in hemi-directed
geometry.
Data Centre as supplementary publication numbers.
CCDC 670598–670603. 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).
Acknowledgment
Supporting information available
AT thanks the Council of Scientific and Industrial
Research (CSIR), Government of India, for the award of
the Senior Research Fellowship and Dr. Sukhendu Mandal
for his help in solving a crystal structure.
Crystallographic data (excluding structure factors) for
the structures (compounds I–VI) reported in this paper
have been deposited with the Cambridge Crystallographic

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Appendix A. Supplementary materials
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found in the online version at doi:10.1016/j.jssc.2008.
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