Tribasic lead maleate and lead maleate: Synthesis and structural and spectroscopic characterizations

Article abstract of DOI:10.1021/ic050611y

We report on the synthesis and structure of tribasic lead maleate hemihydrate ([Pb₄O₃]C₂H₂(CO ₂)₂·1/2H₂O, TRIMAL) and lead maleate (PbC₂H₂(CO₂)₂, PBMAL). The structure of [Pb₄O₃]C₂H₂(CO₂) ₂·1/2H₂O, solved ab initio from X-ray powder diffraction data, consists of infinite slabs of edge-sharing OPb₄ tetrahedra, of composition [Pb₄O₃], running along the c axis and linked together into a three-dimensional network by tetradentate maleate anionic ligands. The structure of PbC₂H₂(CO ₂)₂, solved from single crystal diffraction data, is lamellar and contains double layers of heptacoordinated lead atoms, bonded only to the oxygen atoms of the maleate ligands. In both compounds, lead is in the oxidation state 2+ and the coordination polyhedra around the Pb²⁺ exhibit a hemidirected geometry and are strongly distorted as a result of the lone pair of electrons. The absence of protons on the acidic portion of the maleate moieties was confirmed by Raman spectroscopy and by ¹H MAS and ¹H-¹³C CP MAS NMR experiments. The two compounds were further characterized using chemical and thermogravimetric analyses.

Full text of DOI:10.1021/ic050611y

Inorg. Chem. 2005, 44, 7394−7402
Tribasic Lead Maleate and Lead Maleate: Synthesis and Structural and
Spectroscopic Characterizations
Franc¸
ois Bonhomme,† Todd M. Alam,‡ Aaron J. Celestian, David R. Tallant,§ Timothy J. Boyle,|
Brian R. Cherry,‡,# Ralph G. Tissot,*,§ Mark A. Rodriguez,§ John B. Parise, and May Nyman†
Geochemistry Department, Sandia National Laboratories, Albuquerque, New Mexico 87185-0754,
Department of Biomolecular and Chemical Analysis, Sandia National Laboratories,
Albuquerque, New Mexico 87185-0886, Materials Characterization Department, Sandia National
Laboratories, Albuquerque, New Mexico 87185-1411, Inorganic Chemistry and Nanomaterials,
AdVanced Materials Laboratory, Sandia National Laboratories, 1001 UniVersity BouleVard SE,
Albuquerque, New Mexico 87106, and Center for EnVironmental and Molecular Sciences,
Stony Brook UniVersity, Stony Brook, New York 11794-2100
Received April 20, 2005
We report on the synthesis and structure of tribasic lead maleate hemihydrate ([Pb4O3]C2H2(CO2)2
and lead maleate (PbC2H2(CO2)2, PBMAL). The structure of [Pb4O3]C2H2(CO2)2
1/2H2O, solved ab initio from X-ray
‚
1/2H2O, TRIMAL)
‚
powder diffraction data, consists of infinite slabs of edge-sharing OPb4 tetrahedra, of composition [Pb4O3], running
along the c axis and linked together into a three-dimensional network by tetradentate maleate anionic ligands. The
structure of PbC2H2(CO2)2, solved from single crystal diffraction data, is lamellar and contains double layers of
heptacoordinated lead atoms, bonded only to the oxygen atoms of the maleate ligands. In both compounds, lead
+
is in the oxidation state 2
+
and the coordination polyhedra around the Pb2 exhibit a hemidirected geometry and
are strongly distorted as a result of the lone pair of electrons. The absence of protons on the acidic portion of the
1
1
maleate moieties was confirmed by Raman spectroscopy and by H MAS and H
The two compounds were further characterized using chemical and thermogravimetric analyses.
−
13C CP MAS NMR experiments.
Introduction
state-of-the-art organotin mercaptide compounds.1 These
lead phases, based on their assigned empirical formulas,
are simply described as small insoluble filler pieces of
litharge to which various organic groups are attached to
improve molecular level mixing with the polymer matrix.
Tribasic lead maleate (given in the literature as 3(PbO)‚
PbC2H2(CO2)2‚1/2H2O, hereafter TRIMAL) is one of several
basic lead carboxylates used as a heat stabilizer for haloge-
nated polymers.1,2 The mechanisms by which these phases
function as stabilizers and modifiers are not fully understood.
Therefore, the knowledge of the structural details of these
industrially important phases is pertinent to understanding
the reactions responsible for their heat-stabilizing properties
and to guiding the development of optimized lead-based
additives.
Basic lead carboxylates are extremely active heat stabiliz-
ers for halogenated polymers such as poly(vinyl chloride)
and act as scavengers for the HCl evolved during the
alteration of the polymer, thereby slowing down its degrada-
tion. In these phases, the lead atoms are bonded to both
polydentate-deprotonated carboxylate moieties and to oxygen
atoms that do not belong to these ligands. Mono-, di-, tri-,
and tetrabasic compounds are known, and this commonly
used nomenclature describes the number of formal (PbO)
units in their empirical formulas. Their performance at
stabilizing poly(vinyl chloride) is comparable to that of the
* To whom correspondence should be addressed. E-mail: rgtisso@
sandia.gov. Fax: (505) 844-9781.
† Geochemistry Department, Sandia National Laboratories.
‡ Department of Biomolecular and Chemical Analysis, Sandia National
Laboratories.
To our knowledge, however, no basic lead carboxylate
phase commercially used as a stabilizer has yet been
structurally characterized. These compounds are extremely
§ Materials Characterization Department, Sandia National Laboratories.
| Inorganic Chemistry and Nanomaterials, Advanced Materials Labora-
tory, Sandia National Laboratories.
Stony Brook University.
(1) Grossman, R. F.; Krausnick, D. J. Vinyl Addit. Technol. 1997, 3, 7.
(2) Grossman, R. F. J. Vinyl Addit. Technol. 1999, 5, 148.
# Present address: Bruker Biospin Inc., Billerica, MA.
7394 Inorganic Chemistry, Vol. 44, No. 21, 2005
10.1021/ic050611y CCC: $30.25
© 2005 American Chemical Society
Published on Web 09/10/2005

Tribasic Lead Maleate and Lead Maleate
resonance of glycine (δ ) 176), with respect to TMS (δ ) 0). The
solid-state double-quantum (DQ) correlation experiments utilized
the offset-compensated back-to-back multiple-pulse train for the
excitation and reconversion of multiple-quantum coherences. The
sequence used a 64-step phase cycle, with 128 rotor-synchronized
t1 increments, and a excitation/reconversion length of 133 µs (4τR),
with States-TPPI (time proportional phase increment) phase detec-
tion in the F1 dimension.
Raman spectra were obtained using for excitation the 514-nm
line of an Ar laser operating at 1 mW. A microscope accessory
with a magnification of 80×, a triple spectrograph, and a charge-
coupled-device detector were used to collect the spectra. Samples
were examined with a JEOL 6300V scanning electron microscope.
Thermal analysis was performed with a TA Instruments SDT
Q600 simultaneous TGA-DSC under an argon or air flow of 100
cm3/min with a heating rate of 5 °C/min. Lead elemental analysis
was executed on a Perkin-Elmer Sciex Elan 6100 ICP-MS using
an argon plasma flame, with the samples having been dissolved in
hot nitric acid. The carbon and hydrogen contents were determined
by the combustion method using a Leco CHN-900 analyzer. The
sample density was determined using a Micromeritics AccuPyc
1330 helium displacement pycnometer.
Structure Determination. The structure of TRIMAL was solved
ab initio from powder diffraction data. The small size of the blade-
shaped crystallites (less than 700 nm, with an aspect ratio of larger
than 10, as determined from scanning electron microscopy micro-
graphs) led to significant peak broadening, with the minimum full
width at half-maximum (fwhm) being 0.16°, about 3 times the
instrumental resolution. The powder pattern was indexed by the
program TREOR903 with a monoclinic C-centered cell of ap-
proximately 1021 Å3. The refined lattice constants are given in
Table 1. The extinction laws are consistent with the space group
C2/c and, of course, Cc. A whole pattern profile refinement by the
Le Bail method4 (program FULLPROF2K5) with the space group
C2/c converged to ø2 ) 7.27 and confirmed the adequacy of this
cell and systematic absences. The structure was solved in C2/c using
the direct-space reverse Monte Carlo optimization method as
implemented in the program FOX6 coupled with manual model
buildup. No structure solution was found using the space group
C2/m or Cm, which are also allowed by the extinction laws. The
chemical analyses and observed density indicated that 16 lead atoms
were present per unit cell. The coordinates of two independent lead
sites in general positions were immediately revealed. The wide
variability generally observed of the oxygen coordination of Pb2+
precluded the use of regular polyhedral building units in the
optimization process and complicated further the completion of the
structure.7 At that stage, difference Fourier synthesis did not reveal
any chemically sensible site. However, the examination of the
Pb-Pb distances allowed us to determine the likely position of
three oxygen sites tetrahedrally bonded only to lead, with bond
distances ranging from 2.3 to 2.7 Å. A Rietveld refinement of this
partial model confirmed its validity and allowed us to proceed with
the location of the organic acid. We kept fixed these two lead and
three oxygen sites previously found, introduced a maleic acid
molecule in the model, and optimized its position and orientation
with FOX. This so-obtained model was then fully refined by the
insoluble in their synthesis media and so far obtaining
crystals suitable for single-crystal X-ray diffraction has not
been realized. Previously, a series of basic lead stabilizers
had been studied by NMR and IR spectroscopies;1,2 however,
no definitive structural information was forthcoming.
We report here the structure determination of TRIMAL
from X-ray powder diffraction data, 1H magic angle spinning
1
(MAS) and H-13C cross-polarized (CP) MAS NMR, and
Raman spectroscopy. To establish a baseline for the inter-
pretation of the TRIMAL data, we also synthesized and
characterized in parallel a related coordination compound,
lead maleate (PbC2H2(CO2)2, PBMAL), for which a single-
crystal X-ray structure was solved.
Experimental Section
Synthesis. All of the following starting materials were used as
received: acetic acid (Aldrich), maleic anhydride (Aldrich), maleic
acid (Fisher), and litharge/massicot (PbO; Baker Chemical Co.).
(a) [Pb4O3]C2H2(CO2)2‚1/2H2O (TRIMAL). The following
preparation was undertaken based on minor modifications to the
manufacturing specifications supplied by Halstab, a Division of
Hammond Group, Inc., Hammond, IN. The synthesis was carried
out using standard Schlenk line techniques. PbO (10.80 g, 48.3
mmol) was slurried in 250 mL of deionized (DI) water. To the
stirring mixture was added via syringe acetic acid (0.20 g, 3.33
mmol). The reaction was warmed to about 50 °C, and maleic
anhydride (6.00 g, 61.2 mmol) was slowly added via syringe. After
the reaction mixture was stirred for about 4 h, the volatile portion
was removed by vacuum distillation. After drying, the powder was
washed repeatedly with water and used without further purification.
A quantitative conversion of the PbO precursor to TRIMAL was
recorded.
(b) PbC2H2(CO2)2 (PBMAL). PbO (5.00 g, 22.4 mmol) was
mixed in 300 mL of DI water; maleic acid (2.60 g, 22.4 mmol)
was dissolved at 25 °C in 100 mL of DI water, and the resulting
solution was added under agitation to the PbO/water mixture. The
reaction mixture was then heated in an open beaker to 70 °C and
stirred continuously for 2 h. After cooling to room temperature, an
off-white crystalline powder was recovered by filtration, washed
repeatedly with water, and dried in air overnight at room temper-
ature. The overall yield of the reaction is 85% (6.21 g of product
recovered).
Characterization. X-ray powder diffraction was performed with
a Bruker D8 Advance diffractometer in Bragg-Brentano geometry
with Ni-filtered Cu KR radiation. Single-crystal diffraction data
for PBMAL were collected at room temperature using a Bruker
AXS P4 diffractometer equipped with a SMART 1000 CCD
camera, with Mo KR radiation.
The solid-state 1H MAS NMR spectra were measured on a
Bruker Avance 600 operating at a proton frequency of 600.1 MHz
using a 2.5-mm broad-band double-resonance probe spinning at
30 kHz. The 1H chemical shift is referenced to the secondary
standard adamantane (δ ) 1.63) with respect to tetramethylsilane
(TMS; δ ) 0.0). The solid-state 13C CP MAS NMR spectra were
obtained on a Bruker Avance 400 operating at a carbon frequency
of 100.63 MHz using a 4-mm broad-band double-resonance probe
spinning at 10 kHz. A variable-amplitude cross-polarization
(3) Werner, P. E.; Eriksson L.; Westdahl, M. J. Appl. Crystallogr. 1985,
18, 367.
(4) Le Bail, A.; Duroy H.; Fourquet, J. Mater. Res. Bull. 1988, 23, 447.
(5) Rodriguez-Carvajal, J. A. Collected Abstracts of Powder Diffraction
Meeting, Toulouse, France, 1990; p 127.
(6) Favre-Nicolin, V.; Cerny, R. J. Appl. Crystallogr. 2002, 35, 734.
(7) Shimoni-Livny, L.; Glusker, J. P.; Bock, C. W. Inorg. Chem. 1998,
37, 1853.
1
sequence with a 3-µs H π/2 pulse, a 2-ms contact pulse, 1024
scan averages, a 10-s recycle delay, and a two-pulse phase-
1
modulated H decoupling was utilized. The 13C NMR chemical
shifts were referenced to the secondary standard of the carbonyl
Inorganic Chemistry, Vol. 44, No. 21, 2005 7395

Bonhomme et al.
Table 1. Crystal Data and Structure Refinement Parameters for
TRIMAL
compound
[Pb4O3]C2H2(CO2)2‚1/2H2O
formula weight
crystal system
space group
999.8 g
monoclinic
C2/c
unit cell dimensions
a ) 11.3720(9) Å
b ) 17.272(1) Å
c ) 5.7028(4) Å
â ) 114.250(4)°
volume
1021.3(1) Å3
Z
4
density (calcd, obsd)
temperature
wavelength
2θ range
step size 2θ
6.50, 6.69(4) g/cm3
298(2) K
Cu KR
8.0-80.0°
0.04°
40 s
0.16°
10
63
time per step
min fwhm
no. of independent atoms
no. of refined parama
no. of structural parama
no. of “independent”
reflections
Figure 1. Observed (red dots), calculated (black line), and difference (blue
line) Rietveld patterns for TRIMAL.
23
426
Table 3. Crystal Data and Structure Refinement for PBMAL
final R indices
Rp) 6.21%; Rwp) 8.37%
compound
Pb[C2H2(CO2)2]
321.2 g
monoclinic
Rexp) 2.20%; ø2) 14.4
formula weight
crystal system
space group
RBragg) 3.23%
soft constraints
CdC ) 1.33(1) Å; C-C ) 1.47(1) Å
C-O ) 1.25(2) Å; CdCsC ) 130(1)°
C-C-O ) 120(1)°; O-C-O ) 120(1)°
CsCdCsC ) 0(1)°
P21/c
unit cell dimensions
a ) 9.920(3) Å
b ) 6.9793(18) Å
c ) 8.293(2) Å
â ) 111.094(6)°
535.7(2) Å3
a 21 positional parameters and 2 isotropic atomic displacement param-
eters; 13 profile parameters and 27 linearly interpolated background points.
volume
Z
density (calcd, obsd)
temperature
wavelength
absorption coefficient
F(000)
crystal size
θ range for data collection
index ranges
4
3.98, 3.96(2) g/cm3
293(2) K
Table 2. Selected Interatomic Distances for TRIMAL
Pb1-O3
Pb1-O5
Pb1-O4
Pb1-O3
Pb1-O1
Pb1-O1
Pb1-Ow
2.24(4)
2.25(1)
2.34(5)
2.37(5)
3.18(3)
3.27(4)
Pb2-O4
Pb2-O5
Pb2-O1
Pb2-O2
Pb2-O2
Pb2-O2
Pb2-O2
2.26(5)
2.39(2)
2.43(4)
2.48(4)
3.15(4)
3.17(4)
3.21(5)
0.71073 Å
31.4 mm-1
560
0.050 × 0.030 × 0.009 mm
2.20-8.23°
-12 e h e 12, -8 e k e 8,
-10 e l e 10
3739
2.30 average
reflections collected
independent reflections
completeness to θ ) 28.23°
refinement method
data/restraints/parameters
goodness of fit on F 2
final R indices [I > 2σ(I)]
R indices (all data)
1242 [R(int) ) 0.0530]
Rietveld method, using bond distance and angle restraints for the
maleate moiety, corresponding to the geometric values found in
the literature.8,9 The refinement eventually converged to a satisfac-
tory agreement factor RBragg ) 3.29% and ø2 ) 14.78. With the
help of the H MAS NMR analysis, a reasonable position for a
water molecule with 25% occupancy was determined and its
93.9%
full-matrix least-squares on F 2
1242/0/83
1.038
R1 ) 0.0338, wR2 ) 0.0785
R1 ) 0.0428, wR2 ) 0.0853
0.0040(5)
1
extinction coefficient
largest diff. peak and hole
2.437 and -1.703 e Å-3
inclusion in the model improved the agreement factors to RBragg
)
3.22% and ø2 ) 14.42. The relatively high ø2 is due to difficulties
in modeling the peak profiles, which are slightly affected by
anisotropic peak broadening because of the anisotropic size of the
crystallites. The Le Bail fit, where the profile intensities are not
constrained by a structural model, did converge only down to ø2 )
7.27, confirming that the high residuals are mostly due to an
inadequate profile description. Attempts to model an hkl-dependent
peak width failed, likely because many extra profile parameters
are necessary with a crystal structure of low symmetry. A summary
of crystallographic data, including the values of the geometrical
constraints and measurement conditions, is given in Table 1.
Selected interatomic distances are given in Table 2. A CIF file
containing the atomic coordinates is deposited as Supporting
Information. The observed, calculated, and difference Rietveld plots
are given in Figure 1.
The crystal structure of PBMAL was solved from single-
crystal data by direct methods (program SIR9710), completed by
difference Fourier syntheses, and refined by full matrix least
squares (program SHELXL9711). All non-hydrogen atoms were
refined anisotropically; the hydrogen atoms were placed geo-
metrically and refined using the riding atom model. A summary of
crystallographic data is given in Table 3, selected interatomic
distances are given in Table 4, and a CIF file is deposited as
Supporting Information. All of the peaks present in the powder
diffraction pattern of PBMAL could be indexed by the single-crystal
unit cell, thereby confirming the absence of crystalline impurities
in the bulk sample.
(10) Altomare, A.; Burla, M. C.; Camalli, M.; Cascarano, G. L.; Giaco-
vazzo, C.; Guagliardi, A.; Moliterni, A. G. G.; Polidori, G.; Spagna,
R. J. Appl. Crystallogr. 1999, 32, 115.
(8) Cernak, J.; Chomic, J.; Kappenstein, C.; Robert, F. J. Chem. Soc.,
Dalton Trans. 1997, 2981.
(9) James, M. N. G.; Williams, G. J. B. Acta Crystallogr. 1974, B30,
(11) Sheldrick, G. M. Programs for Crystal Structure Analysis (Release
97-2); Institu¨t fu¨r Anorganische Chemie der Universita¨t: Go¨ttingen,
Germany, 1998.
1249.
7396 Inorganic Chemistry, Vol. 44, No. 21, 2005

Tribasic Lead Maleate and Lead Maleate
Table 4. Selected Interatomic Distances for PBMAL
Pb-O4
Pb-O1
Pb-O2
Pb-O3
2.423(6)
2.453(6)
2.524(7)
2.619(7)
Pb-O4
Pb-O1
Pb-O3
2.688(6)
2.741(7)
2.860(8)
O4-C4
O3-C1
1.286(11)
1.243(12)
O2-C4
O1-C1
1.254(11)
1.282(11)
C3-C2
C4-C3
1.342(14)
1.462(13)
C2-C1
1.496(14)
C2-H2
0.93
C3-H3
0.93
Results and Discussion
Synthesis. The synthesis of both of these maleate lead
compounds use maleic acid to complex and dissolve the PbO
precursor; however, TRIMAL uses a maleic anhydride/Pb
ratio of 1.27:1 with a catalytic amount of acetic acid (∼5%
of the anhydride concentration). This reaction is designed
for slow conversion of the maleic anhydride to maleic acid
by the weak acetic acid. PBMAL, on the other hand, is
synthesized under more harsh conditions using a 1:1 mixture
of PbO/maleic acid, which results in a faster digestion of
PbO. This is easily observed by the rapid change of the
original yellow-orange slurry of litharge in water to creamy
off-white upon the addition of maleic acid for PBMAL in
comparison to the slow change observed for TRIMAL.
These different reaction conditions led to different types
of structures (see below). TRIMAL is essentially a network
of infinite Pb-O slabs, related to those found in the litharge
precursor, that are linked together by maleate ligands. On
the other hand, PBMAL is composed of polymeric lead/
maleate coordination complexes, and all of the oxygens
bonded to the lead originate from the maleate species.
Structure Description. For TRIMAL, the lone pair of
electrons on the two independent Pb2+ atoms causes their
coordination geometry to be distorted and hemidirected.7
There are two groups of Pb-O distances present in the
structure, one ranging from 2.24(4) to 2.48(4) Å and the other
from 3.15(4) to 3.27(4) Å. Using 3.30 Å as the cutoff, the
Pb1 and Pb2 coordination numbers are six and seven,
respectively. Each lead atom has four short Pb-O bonds
and two or three long Pb-O bonds. The structural environ-
ments of the lead atoms and Pb-O distances are shown in
Figure 2, with the lone pair approximately directed opposite
to the short Pb-O bonds. The discussion of the extended
structure based on the linkage of these PbO6 and PbO7
distorted polyhedra would be quite complex however. The
crystal structure is better understood by considering the
coordination polyhedra of some of the oxygen atoms.12 The
three independent oxygen atoms that do not belong to the
maleate ligand (O3, O4, and O5) are tetrahedrally coordi-
nated by lead atoms, with Pb-O distances ranging from
2.24(4) to 2.39(2) Å. The average Pb-O and Pb-Pb
distances for these three independent tetrahedra are 2.31 and
3.76 Å, respectively, and correspond closely to the values
observed in litharge13 (2.31 and 3.77 Å). The three inde-
pendent OPb4 tetrahedra share edges with each other and
Figure 2. Coordination and Pb-O distances (Å) around Pb1 and Pb2 in
TRIMAL.
Figure 3. Infinite [Pb4O3] slab in TRIMAL, built up by (OPb4) tetrahedra,
viewed approximately along the a axis (lead, small black spheres; oxygen,
big blue spheres).
form infinite slabs running along c, with a height of three
tetrahedra along b and a thickness of one tetrahedron along
a (see Figure 3), giving a composition of [Pb4O3]. The
translation period of the [Pb4O3] slab is 5.70 Å (i.e., the c
cell parameter of TRIMAL), which is similar to 5.71 and
5.81 Å observed for the related [Pb3O2] slabs found in lead
oxide hydroxide nitrate phases.14,15 Pb2 belongs to the
peripheral tetrahedra and is located on the top and bottom
of the slabs, whereas Pb1 occupies the middle section. Each
tetradentate ligand is bonded to three adjacent slabs through
four O-Pb2 bonds of 2.43(4) and 2.48(4) Å and link these
slabs together in the ab plane, thereby forming a three-
dimensional structure. These Pb-O distances are similar to
those encountered in lead diphosphonate,16 for example. Each
[Pb4O3] slab is thus bonded to six neighboring slabs (see
Figure 4). The anisotropy of the structure is reflected in the
morphology of the crystallites, which are thin elongated
plates. Each oxygen atom of the maleate makes only one
bond with a lead atom, Pb2 (see Figure 5). Although many
lead compounds containing related infinite slabs built up by
edge-sharing oxo-centered OPb4 tetrahedra are known,
especially with slab compositions [Pb2O] or [Pb3O2],12 this
is to our knowledge the first observation of a [Pb4O3] unit.
PBMAL adopts a two-dimensional (2D) structure wherein
lead maleate double layers are arranged parallel to the bc
(13) Boher, P.; Garnier, P.; Gavarri, J. R.; Hewat, A. W. J. Solid State
Chem. 1985, 57, 343.
(14) Li, Y.; Krivovichev, S. V.; Burns, P. C. J. Solid State Chem. 2000,
153, 365.
(15) Krivovichev, S. V.; Li, Y.; Burns, P. C. J. Solid State Chem. 2001,
158, 78.
(16) Irran, E.; Bein, T.; Stock, N. J. Solid State Chem. 2003, 173, 293.
(12) Krivovichev, S. V.; Armbruster, T.; Depmeier, W. J. Solid State Chem.
2004, 177, 1321.
Inorganic Chemistry, Vol. 44, No. 21, 2005 7397

Bonhomme et al.
Figure 4. View of TRIMAL, approximately along the c axis (yellow,
OPb4 tetrahedra; black spheres, carbon; red spheres, oxygen; white spheres,
hydrogen; blue spheres, lead).
Figure 7. Heptacoordination of Pb by the maleate ligands and relevant
distances (Å) in PBMAL.
Figure 5. Tetradentate maleate ligand linking three slabs together and
relevant distances (Å) in TRIMAL.
Figure 8. Tetradentate maleate ligand and relevant distances (Å) in
PBMAL.
layer at 2.59 and 2.65 Å, respectively. This weak interlayer
bonding imparts stability to the overall structure packing.
The bond valence sum (BVS)17,18 for the heptacoordinated
lead atom, using a bond cutoff of 2.9 Å, is 1.86, thereby
confirming the Pb2+ assignment. The four oxygen atoms have
a BVS ranging from 1.89 to 1.94, showing that none of the
oxygen atoms are protonated. The oxygen O2, which is
bonded to only one Pb2+ atom, has a BVS of 1.76, which
reaches 1.89 when the two hydrogen bonds with H2 and H3
are taken into account. The interlayer volume is too small
to accommodate any water molecules.
Thermogravimetric and Elemental Analyses. The ther-
mogravimetric analysis (TGA) of TRIMAL, performed under
argon, revealed two distinct thermal events. The first step is
ascribed to the gradual loss of the bound lattice water, starting
around 50 °C and leading to a plateau in the differential
thermal analysis (DTA) signal at 296 °C and a corresponding
weight loss of 1.1 wt % (calculated 0.9 wt %). This
dehydration step is followed by the decomposition of the
compound, giving a sharp loss of 9.5 wt % that peaks in the
DTA signal at 358 °C. The decomposition is achieved at
about 425°C, and no further thermal events are observed.
The final product is PbO, and the corresponding total weight
Figure 6. Layered structure of PBMAL, viewed approximately along the
c axis (black spheres, carbon; red spheres, oxygen; white spheres, hydrogen;
blue spheres, lead).
plane (see Figure 6). The single independent lead atom is
bonded to seven oxygen atoms (with distances ranging from
2.423(6) to 2.860(8) Å), belonging to four symmetry-
equivalent deprotonated maleate ligands (see Figure 7). As
in TRIMAL, the coordination sphere around the Pb2+ is
asymmetric and hemidirected, showing the expression of the
lone pair directed along the shortest intralayer Pb-Pb contact
of 3.89 Å. Each maleate ligand is bonded to four lead
atoms: O1, O3, and O4 are each bonded to two lead atoms,
whereas O2 is only bonded to one (see Figure 8). The O2 is
situated close to the surface of the layers and participates in
two weak hydrogen bonds with H2 and H3 of an adjacent
(17) Brown, I. D.; Altermatt, D. Acta Crystallogr. 1985, B41, 244.
(18) Brese, N. E.; O’Keeffe, M. Acta Crystallogr. 1991, B47, 192.
7398 Inorganic Chemistry, Vol. 44, No. 21, 2005

Tribasic Lead Maleate and Lead Maleate
Table 5. Solid-State NMR Results: Chemical Shifts of the Main 13C
1
and H Resonances
13C chemical shifts (ppm)
1H chemical shifts (ppm)
carboxylic
olefinic
acid protons alkene protons
maleic acid
TRIMAL
PBMAL
172; 168
174; 172
182; 175
139; 132
136; 131
144; 126
15.7; 13.0
none
none
6.87
5.91
6.97; 5.65
cm-1) has a small shoulder on its lower frequency side. The
possibility that this feature is due to carboxylic acid groups
(-COOH) has been ruled out because, while the frequency
is consistent with O-H on carboxylic acid groups, the feature
is too narrow. Further, if carboxylic acid groups were present,
there would be a more intense band, due to CdO, peaking
near 1700 cm-1. This feature is probably an overtone or com-
bination of bands peaking at lower frequency. The Raman
spectrum of PBMAL contains essentially the same features
as those found in the spectrum of TRIMAL, but some
significant differences exist. The spectrum of PBMAL does
not include the bands due to hydroxyl species, which we
associate with lattice waters in TRIMAL. Other bands, pri-
marily those associated with carboxylate groups (CdO
stretch band peaking near 1400 cm-1), are shifted in fre-
quency between the two spectra. These shifts are most likely
associated with different structural arrangements of the male-
ate ligands and their mode of bonding to the lead atoms. To
summarize, neither TRIMAL nor PBMAL contain acidic
protons (in both cases the maleic acid is deprotonated), and
TRIMAL, contrary to PBMAL, does contain water of
crystallization.
Figure 9. Raman spectra of TRIMAL and PBMAL.
loss is 10.7 wt % at 780 °C, in agreement with the 10.7 wt
% calculated for the composition (Pb4O3)[C2H2(CO2)2]‚
1/2H2O. One water molecule per formula unit would lead to
a calculated loss of 11.5 wt %, whereas an anhydrous phase
would lose 9.9 wt %. The elemental analyses are in
agreement with the composition derived from the crystal
structure. Measured: Pb ) 83.9 wt %; C ) 4.77 wt %; H )
0.21 wt %. Calculated: Pb ) 82.9 wt %; C ) 4.80 wt %; H
) 0.20 wt %. It should be noted that, whereas water
hydrogen cannot be detected by combustion analysis, the
hydroxyl hydrogen would be detected. Moreover, no weight
loss in the temperature range usually corresponding to the
disassociation of a hydroxyl group (400-600 °C) was
observed by TGA.
One main weight loss event was noted in the TGA of
PBMAL, corresponding to the decomposition of the com-
pound with an onset temperature of 280 °C. No weight loss
is observed below 250 °C, confirming the absence of lattice
water in the phase. The total weight loss is 30.9 wt %, in
agreement with the expected 30.5 wt %. Elemental analyses
also confirm the stoichiometry derived from the crystal
structure. Measured: Pb ) 65.53 wt %; C ) 14.83 wt %; H
) 0.74 wt %. Calculated: Pb ) 64.51 wt %; C ) 14.94 wt
%; H ) 0.62 wt %.
Raman Spectroscopy. The Raman spectra of PBMAL and
TRIMAL are given Figure 9. The Raman bands due to
distinctive functional groups are labeled: carboxylate (COO-),
alkene (-CdC- and C-H), and hydroxyls. For TRIMAL,
there are two Raman bands in the O-H region of the
spectrum, peaking near 3229 and 3477 cm-1. Because these
two bands have relatively narrow bandwidths, they are
probably not hydrogen bonded, i.e., associated with other
hydroxyl groups or water molecules. The band peaking near
3229 cm-1 is in the region associated with alcohols, but it is
significantly narrower than the band commonly observed
from alcoholic O-H vibrations. The most likely source of
the two O-H bands is water of crystallization in two well-
defined crystallographic sites. Both the frequencies of the
bands and their narrow bandwidths are consistent with this
interpretation. It is of note that the alkene C-H band (3030
Solid-State MAS NMR Spectroscopy. A solid-state
NMR study has been undertaken to provide further insight
into the C and H environments in these compounds as well
as additional structure confirmation for TRIMAL. To aid in
the interpretation of solid-state NMR data for TRIMAL,
maleic acid and PBMAL were also investigated. The
1
chemical shifts of the main 13C and H resonances for the
three phases are summarized in Table 5.
Figure 10 shows the 13C CP MAS NMR spectra of maleic
acid, TRIMAL, and PBMAL. The spectrum of each com-
pound is comprised of four resonances: two in the carboxylic
region and two in the olefinic region. These 13C MAS NMR
results reveal that there are several inequivalent carbon
environments present within all three compounds. The NMR
spectrum of TRIMAL is different from that of maleic acid
or PBMAL. The olefinic carbon line widths in TRIMAL are
much broader than those observed for maleic acid or
PBMAL, indicating an increase in the disorder of the organic
species for TRIMAL. In maleic acid, the presence of protons
on the carboxylic acid group shifts the observed 13C NMR
resonances to about 170 ppm, while the deprotonated
carboxylic acid carbons of PBMAL are observed between
175 and 184 ppm. The observed 13C chemical shifts of
TRIMAL are intermediate between those of PBMAL and
maleic acid, supporting the argument that the TRIMAL
carboxylic acids are involved in hydrogen bonding through
water instead of acidic protons or direct hydrogen bonding
via the methane groups. In addition, the observed splittings
of the carboxylic acid and olefinic resonances in TRIMAL
Inorganic Chemistry, Vol. 44, No. 21, 2005 7399

Bonhomme et al.
Figure 10. 13C CP MAS NMR spectra.
Figure 11. 1H MAS NMR spectra.
(∼2 and 5 ppm, respectively) are smaller than those of maleic
acid (∼4 and 7 ppm) or PBMAL (∼7 and 18 ppm). This
decrease in chemical shift differences for the carboxylic and
olefinic carbons reflects the increased similarity in the local
carbon environment from one end of the maleate group to
another, consistent with the proposed structure. In fact, our
structural model for TRIMAL, described in C2/c, contains
only two independent carbon sites, one olefinic and one
carboxylic, with the two sides of the maleate being symmetry
related. Although the NMR results (four independent carbon
sites) are not fully consistent with this model, the similarity
of the chemical shifts for the doublets and the likely presence
of disorder of the maleates, evidenced by the line widths of
the resonances, show that this more symmetrical model
provides, nevertheless, a good description of the compound.
When described in Cc, a noncentrosymmetric axially polar
subgroup of C2/c, the structural model of TRIMAL does
contain four independent carbon sites and is thus consistent
with the NMR results. However, the Rietveld refinement
using the Cc symmetry did not converge, probably because
of the high correlation between parameters and the fact that
the true distribution of the lead atoms is very close to being
centrosymmetric. The positions of the lead atoms refined in
Cc did converge and are still, within 0.07 Å, related by a
center of symmetry. The fact that the majority of the
scattering power in the cell possesses pseudosymmetry likely
prevents the refinement of the organic part of the structure,
which may exhibit a lower symmetry. The large differences
in the olefinic carbon 13C NMR chemical shifts in PBMAL
reveal that these carbons are inequivalent. This large differ-
ence in the local environment is also supported by the distinct
alkene proton resonances observed in the H MAS NMR
spectra (discussed below).
1
The high-speed 1H MAS NMR spectra (Figure 11) further
highlight the deviation of the structure of the organic content
1
in TRIMAL and PBMAL from that of maleic acid. The H
MAS NMR spectrum of maleic acid shows three resonances
at 15.7, 13.0, and 6.87 ppm. The assignment of the two high-
frequency resonances to acid protons is consistent with the
previously reported literature of 16 and 13.3 ppm.19 A linear
1
correlation between the H NMR chemical shift and the
H‚‚‚O hydrogen bond length in crystals has been given:20,21
1
δ H ) 44.68-19.3 [dOH‚‚‚H (Å)], predicting 1.50 and 1.64 Å
O‚‚‚H bond distances for the 15.7 and 13.0 ppm protons,
respectively. This leads to the assignment of 15.7 ppm to
the intramolecular H bond (O-H )1.59 Å) and of 13 ppm
to the intermolecular H bond (O-H ) 1.66 Å).9 The other
1H MAS NMR resonance for maleic acid at 6.87 ppm is
assigned to the alkene protons. This assignment is consistent
with 6.2 ppm observed for the alkene protons of maleic acid
dissolved in CDCl3. The lack of any resonances at >8 ppm
in either TRIMAL or PBMAL clearly demonstrates that there
are no acidic protons present within these phases, consistent
with the nonprotonated carboxylic acid obtained through
structure refinement and with the chemical analysis results.
(19) Harris, R. K.; Jackson, P.; Merwin, L. H.; Say, B. J.; Hagele, G. J.
Chem. Soc., Faraday Trans. 1988, 84, 3649.
(20) Jeffery, A.; Yeon, Y. Acta Crystallogr., Sect. B 1986, 42, 410.
(21) Alam, T. M.; Fan, H. Macromol. Chem. Phys. 2003, 204, 2023.
7400 Inorganic Chemistry, Vol. 44, No. 21, 2005

Tribasic Lead Maleate and Lead Maleate
This is also consistent with the lack of carboxylic acid
stretches in their Raman spectra.
1
The H MAS NMR spectrum of TRIMAL is dominated
(78%) by the single resonance at 5.91 ppm, which is assigned
to the olefinic protons in this sample. There are also
numerous resonances between 4.5 and 0.7 ppm, which are
assigned to either OH or H2O proton species. This range of
chemical shifts supports the idea that we have different types
of water environments present within the material. The
observation of only a single resonance for the two alkene
protons indicates that they have chemically similar environ-
1
ments. On the other hand, the H MAS NMR spectrum of
PBMAL shows two nonequivalent alkene proton resonances
at 6.97 and 5.65 ppm (each 47.5%). This large inequivalency
is consistent with the large splitting of the alkene resonance
observed in the 13C CP MAS spectrum of this compound.
Also observed are two minor resonances at 3.3 and 1.02 ppm
that can be assigned to either H2O or OH. These species
1
constitute only 5% of the total H density. Heating at 100
°C did not reduce or change the intensity of these signals
that could originate from a minor amorphous impurity not
detected by X-ray diffraction.
To investigate spatial relationships and to further support
the assignment of the different hydrogen environments in
these materials, 2D DQ correlation NMR spectra were
obtained. Figure 12 shows the 2D DQ NMR spectra for
maleic acid, TRIMAL, and PBMAL. These types of 1H DQ
MAS NMR exchange experiments consist of a single-
quantum dimension (horizontal) plotted versus the DQ
dimension (vertical). The observation of correlations, or
cross-peaks, arises because of the dipolar coupling between
different protons. For the mixing times used in the present
DQ exchange experiments, the presence of a correlation
implies that the dipolar-coupled protons are separated by
<5Å. The lack of a DQ correlation may be the result of too
long 1H-1H distances (weak dipolar couplings) or a reduc-
tion (averaging) of the dipolar coupling due to molecular
motions. Resonances that occur along the diagonal (ω, 2ω)
are referred to as autocorrelation peaks and result from
dipolar interactions between protons with the same chemical
shift. Off-diagonal resonances (ωa, ωa + ωb) and (ωb, ωb +
ωa) are produced by dipolar interactions with different
chemical shifts.
1
The H DQ MAS NMR exchange spectrum for maleic
acid (Figure 12a) shows many different correlation reso-
nances. The acidic protons at 15.7 and 13.0 ppm show a
strong correlation in between (resonance AB), revealing that
there are close intermolecular contacts between the acid
protons of the carboxylic groups. The lack of or small
intensity of autocorrelation peaks for the acid resonances (AA
and BB) reveals that protons of similar environments are
not spatially near each other in the material; they are actually
more than 4 Å apart. The acid protons also show correlations
with the olefinic protons (AC and BC) again consistent with
the spatial closeness in the maleate group (d ) 2.70 Å). The
unresolved methane protons show a strong autocorrelation
peak (CC) also consistent with the proposed structure where
the methane protons are 2.24 Å apart. A weak autocorrelation
Figure 12. 2D 1H DQ MAS NMR spectra of (a) maleic acid, (b) TRIMAL,
and (c) PBMAL.
peak is observed at ∼1 ppm (DD), resulting from trace water
within the maleic acid structure. The 1H DQ MAS spectrum
for TRIMAL (Figure 12b) is different in appearance. In this
Inorganic Chemistry, Vol. 44, No. 21, 2005 7401

Bonhomme et al.
spectrum, the dominant peak is the autocorrelation resonance
(AA) for the nonresolved methane protons. There are also a
series of autocorrelation resonances (BB and CC) for the
different water species, as well as spatial correlation between
these two different water species (BC). Finally, there is a
weak but significant correlation between the water resonance
at 1.9 ppm and the methane resonance at 5.9 ppm, revealing
that in TRIMAL there are water species spatially near the
olefinic protons of the maleate group. The 2D 1H DQ MAS
NMR spectrum of PBMAL (Figure 12c) clearly shows the
correlation between the resolved nonequivalent olefinic
protons of the maleate group (AB), with no autocorrelation
peaks (AA or BB) observed. This is consistent with the short
1H-1H distance of 2.21 Å being the intramolecular con-
nectivity of these protons. Weak autocorrelation peaks
between the residual water resonances (CC and DD) were
also observed, but no correlations between the alkene protons
and the residual water were observed. This confirms that
water is not spatially located near the maleate group in
PBMAL, consistent with the lack of water of crystallization
found by the structural analysis and TGA measurement. It
is likely that this small amount of residual water belongs to
a minor impurity phase not detected by X-ray powder
diffraction or to surface-adsorbed water.
slabs of composition [Pb4O3], linked together into a three-
dimensional network by tetradentate deprotonated maleate
ligands. We have also synthesized and characterized a related
phase, lead maleate (PBMAL), which provided a reference
point for the interpretation of the spectroscopic data collected
on the tribasic compound. The Raman and NMR data
confirmed that the maleate ligands are deprotonated in both
TRIMAL and PBMAL and that the structure of TRIMAL
contains water of crystallization. The structure determination
of TRIMAL represents a first step toward the understanding
of the structure-property relationships of basic lead car-
boxylates and thus the optimization of their performance as
thermal stabilizers of halogenated polymers.
Acknowledgment. This research was supported by San-
dia National Laboratories’ Laboratory-Directed Research and
Development program (Grant DE-FG07-1ER63282). Sandia
National Laboratories is a multiprogram laboratory operated
by Sandia Corp., a Lockheed Martin Co. for the U.S.
Department of Energy’s National Nuclear Security Admin-
istration under Contract DE-AC04-94AL85000. The authors
thank Dr. Dick Grossman, formerly with Halstab, for his
comments and help on the TRIMAL synthesis. J.B.P. and
A.J.C. acknowledge the Center for Environmental and
Molecular Sciences NSF-CHE-0221934 and NSF-DMR-
0452444.
Conclusions
We have determined from X-ray powder diffraction the
structure of tribasic lead maleate (TRIMAL) and confirmed
the adequacy of the derived structural model using Raman
Supporting Information Available: X-ray crystallographic data
for TRIMAL and PBMAL, in CIF format. This material is available
free of charge via the Internet at http://pubs.acs.org.
1
1
spectroscopy and H MAS and H-13C CP MAS NMR
experiments. TRIMAL consists of infinite one-dimensional
IC050611Y
7402 Inorganic Chemistry, Vol. 44, No. 21, 2005