Coordination geometry of lead carboxylates - Spectroscopic and crystallographic evidence

Article abstract of DOI:10.1039/c4dt03075c

Despite their versatility, only a few single-crystal X-ray structures of lead carboxylates exist, due to difficulties with solubility. In particular, the structures of long-chain metal carboxylates have not been reported. The lone electron pair in Pb(ii) can be stereochemically active or inactive, leading to two types of coordination geometries commonly referred to as hemidirected and holodirected structures, respectively. We report ¹³C and ²⁰⁷Pb solid-state NMR and infrared spectra for a series of lead carboxylates, ranging from lead hexanoate (C6) to lead hexadecanoate (C18). The lead carboxylates based on consistent NMR parameters can be divided in two groups, shorter-chain (C6, C7, and C8) and longer-chain (C9, C10, C11, C12, C14, C16, and C18) carboxylates. This dichotomy suggests two modes of packing in these solids, one for the short-chain lead carboxylates and one for long-chain lead carboxylates. The consistency of the ¹³C and ²⁰⁷Pb NMR parameters, as well as the IR data, in each group suggests that each motif represents a structure characteristic of each subgroup. We also report the single-crystal X-ray diffraction structure of lead nonanoate (C9), the first single-crystal structure to have been reported for the longer-chain subgroup. Taken together the evidence suggests that the coordination geometry of C6-C8 lead carboxylates is hemidirected, and that of C9-C14, C16 and C18 lead carboxylates is holodirected.

Full text of DOI:10.1039/c4dt03075c

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Coordination Geometry of Lead Carboxylates – Spectroscopic and
Crystallographic Evidence †
Jaclyn Catalanoa,b, Anna Murphyb, Yao Yaob, Glenn P. A. Yapb, Nicholas Zumbulyadisc, Silvia A.
Centenoa, Cecil Dybowskib*
aDepartment of Scientific Research, The Metropolitan Museum of Art, New York, NY 10028,
U.S.A.
bDepartment of Chemistry and Biochemistry, University of Delaware, Newark, DE 19716, U.S.A.
cIndependent Researcher, Rochester, NY 14613, U.S.A.
*email: dybowski@udel.edu
†Electronic supplementary information (ESI) available: Detailed xꢀray crystallographic
data collection and fitting.
Abstract
Despite their versatility, only a few singleꢀcrystal Xꢀray structures of lead carboxylates
exist, due to difficulties with solubility. In particular, the structures of longꢀchain metal
carboxylates have not been reported. The lone electron pair in Pb(II) can be stereochemically
active or inactive, leading to two types of coordination geometries commonly referred to as
13
hemidirected and holodirected structures, respectively. We report C and 207Pb solidꢀstate NMR
and infrared spectra for a series of lead carboxylates, ranging from lead hexanoate (C6) to lead
hexadecanoate (C18). The lead carboxylates based on consistent NMR parameters can be divided
in two groups, shorterꢀchain (C6, C7, and C8) and longerꢀchain (C9, C10, C11, C12, C14, C16,
and C18) carboxylates. This dichotomy suggests two modes of packing in these solids, one for
the shortꢀchain lead carboxylates and one for longꢀchain lead carboxylates. The consistency of the
13C and 207Pb NMR parameters, as well as the IR data, in each group suggests that each motif
represents a structure characteristic of each subgroup. We also report the singleꢀcrystal Xꢀray
diffraction structure of lead nonanoate (C9), the first singleꢀcrystal structure to have been reported
for the longerꢀchain subgroup. Taken together the evidence suggests that the coordination
geometry of C6ꢀC8 lead carboxylates is hemidirected, and that of C9ꢀC14, C16 and C18 lead
carboxylates is holodirected.

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Introduction
The longꢀchain carboxylates of divalent metal ions are of considerable technical and
scientific interest. They have been used as secondary driers in linseedꢀoilꢀ and alkydꢀresinꢀbased
paints,1 as corrosion inhibitors for passivating lead metal surfaces,2 emulsifiers, grease thickeners,
and dispersant agents.3, 4 Lead carboxylates have also been used in metal organic frameworks
based on aromatic5 and aliphatic6 lead(II) polycarboxylates because of their gas sorption
properties, in the synthesis of monodispersed superconducting Pb(0) nanocrystals,7 and in
octahedral clusters of PbSe nanocrystals for highꢀefficiency solar cells.8
Lead carboxylates are a major concern in the conservation and preservation of oil
paintings.9,10 Lead hexadecanote, lead octadecanoate and, in some cases, lead azelate have been
found in inclusions by a variety of techniques.11, 12 Soaps (carboxylates of metal ions) form when
saturated fatty acids, resulting from the hydrolysis of glycerides in the oil binder, react with
metals to form inclusions that grow over time, compromising the condition of the artwork. Soaps
may also remain evenly distributed in the paint layers, modifying their optical properties and
leading to an increased transparency.10 Soaps formed in the paint layers may migrate to the
surface of a painting where they react with gases in the environment, resulting in carbonates,
sulfates, possibly oxalates, and other more stable carboxylates in the form of highly disruptive
whitish crusts.13 Knowledge of the coordination environment and the crystal packing of these lead
soaps may provide insights into the migration of these soaps and how to arrest these processes.
There has recently been a resurgence of interest in the coordination chemistry of divalent
lead, motivated by the toxicity of lead and its widespread occurrence in the environment. A
knowledge of the coordination properties leading to preferential binding of lead over other
essential metal ions is crucial for understanding the toxicological properties of lead, the design of
selective chelationꢀtherapy agents,14 and the development of efficient chelating agents for the
remediation of polluted water and soil.15, 16 Considerable effort has been devoted to controlling
the stereochemistry around Pb(II) synthetically.16ꢀ19 Surprisingly, there is a paucity of singleꢀ
crystal Xꢀray diffraction data for Pb(II) carboxylates. Only a handful of structures have been
reported, notably lead formate,20 lead acetate,20 lead crotonate,21 lead heptanoate,2, 22 and lead
azelate.23 The structures of aliphatic lead dicarboxylate metalꢀorganic frameworks have also been
reported.6
The ligand coordination number and geometry around Pb(II) are quite diverse, with at
least five different coordination polyhedra reported in the literature where the coordination
numbers vary anywhere between 2 and 10.24 The electron configuration of Pb(II) entails a 6s2
lone pair. The diversity of coordination geometries depends on whether the lone pair is
stereochemically active or inactive. ShimonyꢀLivni et al. have identified two limiting cases for
the diverse geometries.14 Using their terminology the two cases are named “holodirected, in
which the bonds to ligand atoms are directed throughout the surface of an encompassing globe”
and “hemidirected, in which the bonds to ligand atoms are directed throughout only part of the
globe, that is, there is an identifiable void (or gap) in the distribution of bonds to the ligands”.14
Pb(II) compounds are hemidirected for low coordination numbers (2ꢀ5) and holodirected for high
coordination numbers (9 and 10), but for intermediate coordination numbers (6ꢀ8), examples of
both types of stereochemistry have been reported.6, 20, 25
Ab initio molecular orbital calculations and a survey of published structures have
identified the primary factors determining coordination geometry.14, 26 Hemidirected geometry is
favored if the ligand coordination number is low, the ligands are hard, and there are attractive
interactions between the ligands. In such complexes, the loneꢀpair orbital has p character and
fewer electrons are transferred from the ligands to the bonding orbitals of Pb(II), resulting in
bonds that are more strongly ionic. Holodirected geometry is favored when the coordination
number is high and the ligands are soft and bulky, or when there are strong interligand repulsions.
In holodirected structures, the lone pair has essentially no p character, more ligand electrons are
transferred to Pb(II), and the bonds are more covalent. The energy cost of converting a

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hemidirected to a holodirected structure is on the order 8ꢀ12 kcal/mol in the absence of strong
interligand interactions.
For most lead salts of saturated monocarboxylic acids, particularly those of longer chain
length, information about coordination geometry is accessible only by ssNMR because of their
insolubility and the difficulties encountered in attempting to grow sufficiently large single
crystals for Xꢀray diffraction studies. Several solutionꢀ and solidꢀstate NMR (ssNMR) studies
have established correlations between the geometry around the lead coordination site and the
observed values of 207Pb isotropic chemical shifts or principal components of 207Pb chemicalꢀshift
tensors in lead compounds and coordination polymers. Harrison et al. reported solutionꢀstate
207Pb chemical shifts for several Pb(II) complexes and their dissociation products in various
solvents.27 They observed that species in which the lone pair is stereochemically inactive are
characterized by strongly isotropically shielded 207Pb resonances. When the lone pair is
stereochemically active, the chemical shifts are comparatively isotropically deshielded.27,28 Reger
et al. observed a similar trend in a series of lead complexes with various poly(pyrazolyl) borate
ligands.18
In this paper, we report the 207Pb and 13C ssNMR characterization of lead carboxylates of
chain length C6, C7, C8, C9, C10, C11, C16 and C18. A clear division between two subgroups is
observed from the NMR parameters based on carboxylate chain length and is supported by FTIR
spectra. We have also determined the structure of lead nonanoate (C9) by Xꢀray diffraction,
which demonstrates the relevance of the NMR properties to the local electronic structure.
Experimental Section
Synthesis of Materials
Lead carboxylates of hexanoic (caproic) (C6), heptanoic (enanthic) (C7), octanoic
(caprylic) (C8), nonanoic (pelargonic) (C9), decanoic (capric) (C10), undecanoic (undecylic)
(C11), hexadecanoic (palmitic) (C16), and octadecanoic (stearic) (C18) acids were synthesized by
methods adapted from previously published protocols.29, 30 Specifically, the lead carboxylates
were synthesized from equimolar amounts (3.25 mmoles) of lead nitrate dissolved in 20 mL of
water and the respective acid dissolved in 50 mL of ethanol, except that hexanoic acid was
dissolved in 20 mL of ethanol. The two solutions were mixed with an equimolar amount of
o
aqueous potassium hydroxide (5 M solution) and reacted for 20 minutes at 80 C. The reaction
mixture was cooled to room temperature, filtered, and the residue was washed with several
portions of water, methanol, ethanol, and acetone. The resulting material was dried and
recrystallized from ethanol.
Analysis of Materials
The purity of each compound was verified by infrared spectroscopy (FTIR) and powder
Xꢀray diffraction (PXRD) analysis. The FTIR measurements were carried out with a Bruker
Vertex70® spectrometer coupled to a Hyperion® microscope. The samples were mounted between
the windows of a diamond anvil cell and observed in the transmission mode, with a spectral
resolution of 4 cmꢀ1. The PXRD measurements were performed using a Philips PW1835
diffractometer with Cu Kα (λ = 1.542 Å) radiation (45 kV acceleration voltage, and 35 mA
current).
NMR Spectroscopy
Solidꢀstate 13C and 207Pb ssNMR spectra of the lead carboxylates were recorded at 11.75
tesla (500.13 MHz proton frequency; 125.76 MHz carbonꢀ13 frequency; 104.63 MHz leadꢀ207
frequency) with a standard Bruker 4ꢀmm probe. The protocols have been previously published.30
The spectra were obtained from approximately 100 mg of sample packed in a 4 mm rotor. Solid

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lead nitrate was used as a secondary external reference for the 207Pb spectra, the isotropic
chemical shift being ꢀ3491 ppm relative to tetramethyllead (TML) at 298 K.31 A plot of spinning
speed versus isotropic chemical shift of lead nitrate was obtained to compensate for the
temperature increase due to spinning. Glycine was used as a secondary external reference for the
13C spectra, with an isotropic chemical shift of the carboxyl peak of 176 ppm relative to
tetramethylsilane.
13C NMR spectra were acquired using cross polarization and magicꢀangle spinning at 12
kHz (CPMAS). Acquisition parameters for these experiments included a contact time of 1 ms, 13C
pulse width of 4 ꢀs, and a recycle delay of 5 s. During acquisition, proton decoupling (SPINALꢀ
64 at 100 kHz) was used to suppress dipolar couplings. The number of scans per spectrum was
1024.
207Pb spectra of the carboxylates were acquired by direct excitation using spinꢀ
temperature alteration with magic angle spinning (STA/MAS) (νrot = 11 or 12 kHz) to minimize
the effects of ringdown of the probe circuits.32 General conditions for these experiments included
a π pulse width of 8.5 ꢀs, a delay of 1 ms, and a π/2 pulse width of 4.25 ꢀs. Highꢀpower proton
decoupling (SPINALꢀ64 at 100 kHz) was used to suppress dipolar couplings. The recycle delay
in most experiments was 5 s and approximately 13,000 scans per spectrum were acquired.
207Pb WURSTꢀCPMG spectra of static samples were recorded using the parameters of
MacGregor et al.33 WURST pulse widths were 50 ꢀs, with pulse shapes created via the shape tool
in Topspin 3.1.30 75 Meiboom–Gill loops were acquired for the WURST–CPMG experiments,
with a 200ꢀꢀs echo, and a sweep range of 0.5 MHz in all cases. The recycle delay was 7 s. Highꢀ
power proton decoupling (CW at 100 kHz) was used to suppress dipolar couplings. For lead
hexanoate, lead heptanoate and lead octanoate, multiple WURSTꢀCPMG subspectra were
collected at different carrier frequencies by shifting the carrier frequency a multiple of the
spikelet separation (981.934 ppm) from spectrum to spectrum. Each spectrum contained the
signal accumulated in 2048 scans. The collected subspectra were superimposed to form the final
spectrum.
The analysis of the 207Pb chemicalꢀshift tensors was performed by simultaneously fitting
the envelope of the WURSTꢀCPMG spectrum and the MAS sideband pattern. The isotropic
chemical shift acquired in the MAS experiment was fixed in the fitting procedure, because it
could be determined accurately. For compounds with smaller spans, an initial estimate of the
principal components of the chemicalꢀshift tensor was made with the HerzfeldꢀBerger method,34
as implemented in the program HBA 3.1 developed by Eichele.35 The final fit was refined by
performing a simultaneous fit to the sideband patterns and the WURSTꢀCPMG envelope. Fits to
the WURSTꢀCPMG envelopes were aided by simulation of the powder pattern with the program
WSOLIDS.35
Single-Crystal X-ray Diffraction
Crystals, obtained by slow evaporation of a saturated solution prepared from 38 mg of
lead nonanoate (C9) in 30 mL of toluene, were sectioned, mounted using viscous oil onto a
plastic mesh, and cooled to the data collection temperature. Data were collected on a BrukerꢀAXS
APEX CCD diffractometer with graphiteꢀmonochromated MoꢀKα radiation (λ=0.71073 Å). Unit
cell parameters were obtained from 36 data frames, 0.3º ω, from three different sections of the
Ewald sphere. The systematic absences in the diffraction data are consistent with P2/c (No. 13)
and Pc (No. 7). Detailed information on data collection and fitting is included in the
supplementary information. The CIF has been deposited under CCDC 1023395.

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Results and discussion
NMR Spectroscopy
Carbon ssNMR provides information on the crystal packing of the lead carboxylates.
Carbon ssNMR spectra (Figure 1) show resonance doubling of the carbons closest to the lead site
for all lead carboxylates studied, indicating two carboxylate conformations in the asymmetric
unit. This result is in agreement with previous NMR reports for lead decanoate and
octadecanoate,38 electron diffraction measurements of LangmuirꢀBlodgett films of lead
octadecanoate,39 and the published crystal structure of lead heptanoate,22 as well as the crystal
structure of lead nonanoate discussed below. There are two resonances in the carboxylate region
of the spectrum with relative intensities of 1:1 for all the carboxylates. However, the series parses
into two subseries based on the chemicalꢀshift difference between these two resonances. For the
longꢀchain carboxylates (C9, C10, C11, C16, and C18), the difference is in the range of 1.13ꢀ1.25
ppm. For the shorterꢀchain carboxylates (C6, C7, and C8), the chemicalꢀshift difference of the
two carboxylate forms is in the range of 0.50 – 0.69 ppm. A similar trend is seen for the αꢀ
carbon, with chemicalꢀshift differences of 2.0ꢀ2.1 ppm for longꢀchain carboxylates, and 0.50 –
0.69 ppm for shortꢀchain carboxylates.
Carbon atoms further from the carboxylate center also show doubling of resonances but,
as proposed by Burrows et al.,28 the doubling in these regions is not necessarily indicative of
multiple carboxylate coordination geometries but rather it results from a distortion of the usual
allꢀtrans arrangement of such chains. Especially noteworthy is the observation that the methyl
resonances of the longꢀchain carboxylates are singlets, whereas the shortꢀchain carboxylates show
doubling (1:1) of the methyl resonance, a fact demonstrating that there are two chainꢀend
conformations for these shortꢀchain members of the series.
Lead WURSTꢀCPMG and STA/MAS (not shown) spectra demonstrate that the local lead
environments in the longꢀchain lead carboxylates are different from those of the shortꢀchain
carboxylates. Figure 2 shows the 207Pb WURST CPMG spectra of the C6 – C11, C16 and C18
lead carboxylates, and the principal components of the chemicalꢀshift tensors for the various
carboxylates are reported in Table 1. The 207Pb chemicalꢀshift tensors for lead hexadecanoate and
lead octadecanoate in Table 1 have been previously reported and included for comparison.30 In
every case, the 207Pb spectrum is fit as a single pattern implying the presence of a single lead
center per unit cell. Due to the similarity between C11, C16, and C18, it is expected that similar
spectra would be obtained for lead carboxylates of chain length C12ꢀC15 and C17.
Based on the 207Pb parameters, the lead carboxylates fall into the same two subgroups as
determined by the 13C NMR parameters. The C6, C7, and C8 carboxylates have spans of
approximately 2700 ppm, which suggest that they are hemidirected. The longerꢀchain
carboxylates have much smaller spans (on the order of 750 ppm), which suggests that these
carboxylates are holodirected. The spans are correlated with the isotropic shifts, the hemidirected
carboxylates having isotropic shifts around ꢀ1100 ppm and the holodirected carboxylates having
isotropic shifts around ꢀ2125 ppm. In addition, the hemidirected carboxylates show skews in the
range of 0.65 – 0.70, whereas the holodirected carboxylates have skews in the range of 0.42 –
0.48. Katz et al. found a similar dependence of the lead isotropic shifts, spans, and skews in two
holodirected birefringent leadꢀgold cyanide complexes on the stereochemical activity.40,41 In
another study, Briand et al. demonstrated that the very large spans and large skews of 207Pb
chemicalꢀshift tensors in 4ꢀsubstituted pyridine adducts of lead thiolates are attributable to the
presence of the stereochemically active lone pair.42 Earlier work has shown that the holodirected
43
geometry around nineꢀcoordinate Pb in PbCO3 results in a much narrower span for the 207Pb
chemicalꢀshift tensor.32 Similar studies of the thiourea and phenanthroline complexes of the lead
halides show spans of the order of 600 – 700 ppm, suggesting a holodirected geometry in those
cases.44, 45

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The difference between the 207Pb NMR properties of the two types of lead carboxylate
structures (Table 1), whether in the span or the isotropic shift, is primarily the result of the values
of differences in δ11 and δ22, the two least shielded components. The values of δ33, the most
shielded component, are similar for all of the lead carboxylates. Both δ11 and δ22 are more
shielded by approximately 1500 ppm in the longerꢀchain carboxylates than in the shorterꢀchain
carboxylates. This difference in these two components is an indication of a difference in lead
coordination geometry that is also reflected by the NMR properties of the carboxyl carbons. A
similar change in coordination geometry around the metal ion when comparing the octanoate and
nonanoate ligands is also encountered when comparing the Raman spectra of copper octanoate
and copper decanoate.46, 47
The precise point in the series at which a transition from shortꢀchain to longꢀchain
behavior is manifested depends on the manner in which one examines the material. The present
ssNMR experiments are consistent with a change in conformational behavior or packing between
C8 and C9. On the other hand, from infrared and Xꢀray studies at room temperature, Ellis et al.
conclude that this transition occurs between C12 and C14.48 Based on the number of intermediate
liquid crystalline phases detected by differential thermal analysis (DTA), Adeosun and Sime
place the transition between C8 and C12, with C10 exhibiting an intermediate behavior.49 In these
49
experiments,48,
only the members of the series having even numbers of carbons were
investigated. It may be that, as suggested by Arenas et al., the packing of the chains of lead
carboxylates of C9ꢀC12 is affected by the thermal history of the sample.50 This is not the case for
lead carboxylates of chain length greater than 13 or less than 8, where no differences between the
Raman spectra of the recrystallized and premelted samples are obtained.50
Table 1. The Principal Components of the 207Pb Chemical-Shift Tensors for the Lead
Carboxylates.a
Ω
(ppm)
δ11
(ppm)
δ22
(ppm)
δ33
(ppm)
δiso
(ppm)
Compound
κ
Lead hexanoate
Lead heptanoate
Lead octanoate
ꢀ100±3
ꢀ40±8
0±4
ꢀ533±8 ꢀ2745±5 ꢀ1126±3 2645±6
ꢀ480±10 ꢀ2750±5 ꢀ1090±3 2710±9
ꢀ469±8 ꢀ2740±5 ꢀ1070±3 2740±6
0.67 ±0.01
0.68 ±0.01
0.66 ±0.01
0.42 ±0.02
0.43 ±0.03
0.42 ±0.02
0.48±0.02
0.47±0.03
Lead nonanoate
Lead decanoate
Lead undecanoate
Lead hexadecanoate30
ꢀ1819±3 ꢀ2035±5 ꢀ2565±7 ꢀ2140±3
ꢀ1836±5 ꢀ2047±6 ꢀ2575±8 ꢀ2153±4
ꢀ1818±3 ꢀ2036±5 ꢀ2570±7 ꢀ2141±3
ꢀ1820±3 ꢀ2013±5 ꢀ2560±3 ꢀ2131±3
ꢀ1810±3 ꢀ2007±6 ꢀ2555±3 ꢀ2124±4
746±8
739±9
752±8
740±4
745±4
Lead octadecanoate30
a
The fitting procedure used the isotropic shift, as determined from a MAS spectrum of the
material, with simulation of the values of δ11, δ22, and δ33 determined by the edges of the
WURSTꢀCPMG spectrum. Span (ꢁ) and skew (κ) were calculated. ꢁ = |δ33 ꢀ δ11| and κ = 3*(δiso
ꢀ δ22)/ꢁ.

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Figure 1. 13C NMR spectra of lead carboxylates obtained with cross polarization and proton
decoupling spinning at 12 kHz. (a) Lead octadecanoate, (b) lead hexadecanoate, (c) lead
undecanoate, (d) lead decanoate, (e) lead nonanoate, (f) lead octanoate, (g) lead heptanoate, and
(h) lead hexanoate. For carbons closest to the lead, the resonances are doubled, indicating two
different carboxylate chain conformations in the unit cell.

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Figure 2. 207Pb NMR spectra of lead carboxylates obtained with WURSTꢀCPMG. (a) Lead
octadecanoate, (b) lead hexadecanoate, (c) lead undecanoate, (d) lead decanoate, (e) lead
nonanoate, (f) lead octanoate, (g) lead heptanoate, and (h) lead hexanoate. The spectra show that
the lead carboxylates can be divided into two subgroups, shorter chains (C6ꢀC8) having a larger
span than the longꢀchain carboxylates (C9ꢀC11, C16, and C18).

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FTIR Spectroscopy
Figure 3 shows FTIR spectra of the lead carboxylates taken in our laboratory. The
interpretation of the IR results is consistent with the NMR spectra showing that the lead
carboxylates are divided into two subgroups. The absorption assigned to the antisymmetric
carboxylate stretch consists of a distinct doublet for lead carboxylates C9, C10, C11, C16, and
C18 with peaks at ~1515 and 1540 cmꢀ1 and for C6, C7, and C8 there is only a broad peak
centered around 1525 cmꢀ1. There are differences in the lowꢀfrequency region, as well, where
bands are quite sensitive to the alkyl chain packing.48 In the C6, C7, and C8 carboxylates, a peak
assigned to a CH2 rocking mode appears around 720 cmꢀ1.51 In C9, C10, C11, C16 and C18
carboxylates, this region contains multiple features, consistent with the IR results of Mesubi et al.
who found a singlet for lead carboxylates of C6 and C8 but a doublet for C10, C12, C14, and C16
carboxylates.51 In the range between 1150 and 1350 cmꢀ1, C9, C10, C11, C16 and C18
carboxylates show the regular progression of bands associated with the wagging and twisting of
the hydrocarbon chains.51 In this region, the smaller carboxylates do not show a particularly
regular set of bands, demonstrating that the chain motions are different from those in the larger
carboxylates.
Figure 3. FTIR spectra of (a) lead octadecanoate, (b) lead hexadecanoate, (c) lead undecanoate,
(d) lead decanoate, (e) lead nonanoate, (f) lead octanoate, (g) lead heptanoate, and (h) lead
hexanoate. The FTIR spectra show differences in the two subgroups (shortꢀ and longꢀchain lead
carboxylates) in the ranges 1515ꢀ1540cmꢀ1 and 1150ꢀ1350 cmꢀ1, and around 720 cmꢀ1.
Crystal Structure of Lead Nonanoate
A consistent theme in studies of the spectroscopy of lead is the existence of two
structural motifs around the lead center.14 The 207Pb NMR parameters and the FTIR data suggest

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that the carboxylates fall into two subgroups, one being C8 and shorterꢀchain Pbꢀcarboxylates and
the other being C9 and longerꢀchain salts. The structure of lead heptanoate by singleꢀcrystal Xꢀ
ray diffraction has been reported.22 Of the chains greater than C7, only lead azelate has been
previously reported,23 however it is a dicarboxylate, which may influence the packing. Thus, there
is no representative singleꢀcrystal structure of a longꢀchain lead monocarboxylate.
Lead nonanoate displays poor solubility in many common solvents and, as a result, is
often deposited as powders. After several trials, thin plateꢀlike crystals were afforded from a slow
evaporation of a saturated toluene solution. Because of the large aꢀaxis dimension, initial unitꢀcell
determinations using Moꢀkα (λ = 0.71073 Å) failed because of poor resolution of the virtually
overlapping diffraction spots. Data collected using Cuꢀkα (λ = 1.54178 Å) allowed proper
resolution of the diffraction pattern and thereby allowed successful unit cell measurement and
subsequently successful integration of the twoꢀdimensional frame data. The crystal data and
refinement details of lead nonanoate are given in Table 2.
Table 2. Crystal Data and Refinement Details of Lead Nonanoate.
Crystal System
Space Group
Monoclinic
P 2/c
Empirical Formula
Temperature (K)
Wavelength (Å)
PbC18H34O4
200(2)
0.71073
a = 54.872(6)
b = 4.9603(5)
c = 7.1628(7)
β = 90.368(2)
1949.54(3)
1.777
Unit cell Dimensions (Å, o)
Cell Volume (Å3)
Density (g/cm3)
Z
4
Absorption coefficient (mmꢀ1)
Min./Max. Transmission
Reflections collected / unique
Completeness to θ = 24.242 (%)
Data / restraints / parameters
R1, wR2 (%)
8.670
0.4056 / 0.7456
25918 / 4459
99.8
4459 / 261 / 198
8.39, 16.48
1.311
Goodnessꢀofꢀfit on F2
Figure 4. Solidꢀstate molecular diagram and numbering scheme of lead nonanoate monomer, and
coordination sphere of the lead atom, down the c axis showing the monodentate binding to each
carboxylate group. The alkyl groups are aligned in an allꢀtrans structure. Thermal ellipsoids are at
50% probability. Hꢀatoms are depicted at an arbitrary radius. Symmetry operations: i = x, ꢀy,
z+1/2, ii = x, y+1, z.

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Table 3. Selected bond distances (Å) and angles (o)
Pb1ꢀO1
Pb1ꢀO3
Pb1ꢀO1i
Pb1ꢀO2i
Pb1ꢀO4i
Pb1ꢀO4ii
2.660(10)
2.403(10)
2.437(10)
2.856(10)
2.583(10)
2.661(10)
O1ꢀPb1ꢀO3
O1ꢀPb1ꢀO1i
O1ꢀPb1ꢀO2i
O1ꢀPb1ꢀO4ii
O3ꢀPb1ꢀO1i
O3ꢀPb1ꢀO4i
90.7(3)
95.3(2)
114.0(3)
86.0(3)
91.1(4)
78.5(3)
O3ꢀPb1ꢀO4ii
84.9(3)
O4iꢀPb1ꢀO4ii 99.9(3)
O1iꢀPb1ꢀO4i
O1iꢀPb1ꢀO2i
O4iꢀPb1ꢀO2i
78.1(3)
78.3(3)
75.8(3)
O4iiꢀPb1ꢀO2i 104.8 (3)
Symmetry operations: i = x, ꢀy, z+1/2, ii = x, y+1, z.
The structure of lead nonanoate consists of lead sites coordinated with the oxygen of the
carboxylate groups to form extended sheets, which are packed along the c direction to form the
threeꢀdimensional structure. Each lead center is apparently surrounded by six oxygen atoms in a
distorted octahedral array (Figure 4). A closer inspection of the octahedron and reported bond
distances and angles in Table 3, however, reveals a remarkably large O1ꢀPb1ꢀO2i angle, 114.0(3)o
(ideally 90o) and O2 that is roughly in the same plane as Pb1, O1 and O2i. Furthermore the Pb1ꢀ
O2 distance at 2.977(10) Å is comparable to Pb1ꢀO2i while, in contrast, Pb1ꢀO4 is much longer at
3.383(10) Å. This observation suggests that the lead coordination sphere could be properly
viewed as seven coordinate.
In Figure 5, we compare the local structures of lead heptanoate and lead nonanoate
around the Pb site, including atoms within 3.1 Angstroms of the lead center. In the heptanoate
(Figure 5b), the structure is best described as lead coordinated to seven nearby oxygen atoms. The
authors claim that the lead atom is coordinated to six oxygen atoms in a “very distorted
polyhedron”.22 From Figure 5b, one sees that the structure resembles a pentagonal pyramid with
the apex of the pyramid being capped with two oxygen atoms, rather than one. The lead site in the
nonanoate (Figure 5a) is also 7ꢀcoordinate. It resembles a strained octahedral arrangement of
oxygen atoms with an added oxygen atom or a distorted pentagonal bipyramid. Although neither
structure has exact spherical or cubic point symmetry about the lead nucleus, from the
comparison of the structures in Figure 5, it is obvious that the local symmetry around the lead site
in lead nonanoate is less asymmetric than that in lead heptanoate. A qualitative argument implies
that the 207Pb NMR span in lead nonanoate should be smaller than the span for lead heptanoate,
which is in good agreement with the spectroscopic results (Table 1). Thus, one sees that a large
207Pb span (~2600 – 2700 ppm) is consistent with a hemidirected structure, whereas a “small”
207Pb span (~730 – 750 ppm) is consistent with a holodirected structure. This suggests the C6, C7,
and C8 lead carboxylates crystallize in a hemidirected structure, whereas C9, C10, C11, C16, and
C18 crystallize in a holodirected structure. By analogy, the local structure of lead azelate, as
demonstrated by the 207Pb NMR parameters,30 has a local coordination similar to that of the shortꢀ
chain carboxylates, which is also supported by the crystal structure.23

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Figure 5. (a) The local structure around the lead atom in lead nonanoate obtained in the present
study. (b) The local structure around the lead atom in lead heptanoate, as reported by Lacouture et
al.22
Conclusions
Spectroscopic and Xꢀray diffraction analyses of a series of lead carboxylates have
provided insight into the relationship between structure and spectroscopic parameters. The C6 –
C18 lead carboxylates can be classified into two subgroups based on the spectroscopic and
diffraction parameters. In the shortꢀchain group (i.e. C6 – C8), the structure around the lead site is
13
hemidirecting. The 207Pb NMR span is rather large (~2600 – 2700 ppm) and the C NMR data
show that there are two unique carboxylate and methyl groups per unit cell. The FTIR spectra
show a broad peak for the asymmetric carboxylate stretch at 1525 cmꢀ1, a sharp single CH2
rocking mode around 720 cmꢀ1, and the twisting and bending modes of the chain (1150 ꢀ1350 cmꢀ
1) indicate a complex motion of the alkyl chains.
For the longꢀchain group (C9 – C18), the 207Pb NMR span is relatively small (~730 – 750
ppm), indicating a more nearly spherical local structure at the lead atom site, which would be
termed holodirecting. The 13C NMR spectra show that there are two distinct carboxylates and the
single resonance for the methyl groups at the ends of the aliphatic chains show that the methyl
groups are in similar environments, not affected by the structure at the carboxylate carbons/lead
ions. The FTIR spectra show a doublet in the carboxylate asymmetric stretch region, and the
bands due to the CH2 rocking around 720 cmꢀ1 suggest that there are multiple slightly different
modes. The positions of features assigned to twisting modes (1150 – 1350 cmꢀ1) for these longꢀ
chain molecules show a regular progression, suggesting a systematic set of vibrational motions of
the chains when the carboxylates are nine carbons or longer. The dicarboxylate, lead azelate, has
NMR parameters like the shortꢀchain carboxylates, indicating that the presence of two
carboxylates on the change is sufficient to move the local structure at the lead site into the
hemidirected structure.
Due to the similarity in the NMR tensors of lead nonanoate and lead hexadecanoate and
octadecanoate, we propose that the crystal structure of lead nonanoate serves as a model for the
structure of lead hexadecanoate and lead octadecanoate. In addition, the NMR results directly
correlate with features of the IR data. That connection to structure should allow the use of ATR
(attenuated total reflectance) ꢀFTIR as a nonꢀinvasive IR technique to study soap formation in
paintings. This information may assist in future studies of the migration of lead soaps in paint
films and in the development of treatments to arrest the soap deterioration process in oil
paintings.
Acknowledgements
CD acknowledges the support of the National Science Foundation (NSF) under Grants
CHEMꢀ0956006 and CHEꢀ1139192. SC acknowledges the support of the NSF under Grant
CHEMꢀ1139190. GPAY thanks the NSF for CRIF 1048367. We acknowledge helpful
discussions with Shi Bai and Guangjin Hou about the development of wideꢀband inversion and
the spectroscopy of 207Pb. We also acknowledge Joen Hermans (University of Amsterdam),
Katrien Keune (University of Amsterdam and TU Delft), and Ronald Birke (City College of New
York), for helpful initial discussions regarding this topic.
References
1.
2.
R. Vangorkum and E. Bouwman, Coord. Chem. Rev., 2005, 249, 1709ꢀ1728.
E. Rocca and J. Steinmetz, Corr. Sci., 2001, 43, 891ꢀ902.

Page 13 of 15
Dalton Transactions
View Article Online
DOI: 10.1039/C4DT03075C
3.
4.
R. G. Bossert, J. Chem. Educ., 1950, 27, 10.
M. S. Akanni, E. K. Okoh, H. D. Burrows and H. A. Ellis, Thermochim. Acta,
1992, 208, 1ꢀ41.
5.
Y. Zhang, J. Wang, X. Yan, X. Liu, H. Zhou and A. Yuan, Micropor. Mesopor.
Mat., 2014, 184, 15ꢀ20.
6.
7.
A. Thirumurugan and C. N. R. Rao, J. Solid State Chem., 2008, 181, 1184ꢀ1194.
I. Resa, H. Moreira, B. Bresson, B. Mahler, B. Dubertret and H. Aubin, J. Phys.
Chem. C, 2009, 113, 7120ꢀ7122.
8.
D. K. Britt, Y. Yoon, P. Ercius, T. D. Ewers and A. P. Alivisatos, Chem. Mater.,
2013, 25, 2544ꢀ2548.
9.
10.
K. Keune, A. van Loon and J. J. Boon, Microsc. Microanal., 2011, 17, 696ꢀ701.
P. Noble, A. van Loon and J. J. Boon, ICOM Committee for Conservation 14th
Triennnial Meeting, The Hague, 2005.
11.
C. Higgitt, M. Spring and D. Saunders, National Gallery Technical Bulletin,
2003, 24, 75ꢀ91.
12.
13.
14.
K. Keune, PhD Thesis, University of Amsterdam, 2005.
A. van Loon, PhD Thesis, University of Amsterdam, 2008.
L. ShimoniꢀLivny, J. P. Glusker and C. W. Bock, Inorg. Chem., 1998, 37, 1853ꢀ
1867.
15.
16.
17.
18.
19.
D. EstebanꢀGómez, C. PlatasꢀIglesias, T. EnríquezꢀPérez, F. Avecilla, A. de Blas
and T. RodríguezꢀBlas, Inorg. Chem., 2006, 45, 5407ꢀ5416.
A. Pellissier, Y. Bretonnière, N. Chatterton, J. Pécaut, P. Delangle and M.
Mazzanti, Inorg. Chem., 2007, 46, 3714ꢀ3725.
D. L. Reger, J. E. Collins, A. L. Rheingold, L. M. LiableꢀSands and G. P. A. Yap,
Inorg. Chem., 1997, 36, 345ꢀ351.
D. L. Reger, M. F. Huff, A. L. Rheingold and B. S. Haggerty, J. Am. Chem. Soc.,
1992, 114, 579ꢀ584.
D. L. Reger, T. D. Wright, C. A. Little, J. J. S. Lamba and M. D. Smith, Inorg.
Chem., 2001, 40, 3810ꢀ3814.
20.
21.
P. G. Harrison and A. T. Steel, J. Organomet. Chem., 1982, 239, 105ꢀ113.
W. Clegg, I. R. Little and B. P. Straughan, Acta Crystallogr. C, 1986, 42, 1319ꢀ
1322.
22.
23.
24.
25.
F. Lacouture, M. François, C. Didierjean, J.ꢀP. Rivera, E. Rocca and J. Steinmetz,
Acta Crystallogr. C, 2001, 57, 530ꢀ531.
M. J. Plater, B. De Silva, T. Gelbrich, M. B. Hursthouse, C. L. Higgitt and D. R.
Saunders, Polyhedron, 2003, 22, 3171ꢀ3179.
R. L. Davidovich, V. Stavila, D. V. Marinin, E. I. Voit and K. H. Whitmire,
Coord. Chem. Rev., 2009, 253, 1316ꢀ1352.
M. L. Hu, A. Morsali and L. Aboutorabi, Coord. Chem. Rev., 2011, 255, 2821ꢀ
2859.
26.
27.
A. Walsh and G. W. Watson, J. Solid State Chem., 2005, 178, 1422ꢀ1428.
P. G. Harrison, M. A. Healy and A. T. Steel, J. Chem. Soc. Dalton Trans., 1983,
492, 1845ꢀ1848.
28.
29.
H. D. Burrows, C. Geraldes, T. J. T. Pinheiro, R. K. Harris and A. Sebald, Liq.
Cryst., 1988, 3, 853ꢀ860.
L. Robinet and M. C. Corbeil, Stud. in Conserv., 2003, 48, 23ꢀ40.

Dalton Transactions
Page 14 of 15
View Article Online
DOI: 10.1039/C4DT03075C
30.
31.
32.
33.
J. Catalano, Y. Yao, A. Murphy, N. Zumbulyadis, S. A. Centeno and C.
Dybowski, Appl. Spectrosc., 2014, 68, 280ꢀ286.
G. Neue and C. Dybowski, Solid State Nuclear Magnetic Resonance, 1997, 7,
333ꢀ336.
G. Neue, C. Dybowski, M. L. Smith, M. A. Hepp and D. L. Perry, Solid State
Nucl. Magn. Reson., 1996, 6, 241ꢀ250.
A. W. MacGregor, L. A. O'Dell and R. W. Schurko, J. Magn. Reson., 2011, 208,
103ꢀ113.
34.
35.
36.
37.
38.
J. Herzfeld and A. E. Berger, J. Chem. Phys., 1980, 73, 6021ꢀ6030.
K. Eichele, HBA 3.1 and WSOLIDS, University of Tubingen, Tubingen, 2013.
Check.def File Version 5/2/14, http://checkcif.iucr.org.
G. M. Sheldrick, Acta Crystallogr. A, 2008, 64, 112 ꢀ 122.
G. Feio, H. D. Burrows, C. F. G. C. Geraldes and T. J. T. Pinheiro, Liq. Cryst.,
1991, 9, 417ꢀ432.
39.
40.
J. F. Stephens and C. TuckꢀLee, J. Appl. Crystallogr., 1969, 2, 1ꢀ10.
M. J. Katz, P. M. Aguiar, R. J. Batchelor, A. A. Bokov, Z. G. Ye, S. Kroeker and
D. B. Leznoff, J. Am. Chem. Soc., 2006, 128, 3669ꢀ3676.
M. J. Katz, V. K. Michaelis, P. M. Aguiar, R. Yson, H. Lu, H. Kaluarachchi, R. J.
Batchelor, G. Schreckenbach, S. Kroeker, H. H. Patterson and D. B. Leznoff,
Inorg. Chem., 2008, 47, 6353ꢀ6363.
41.
42.
43.
44.
45.
G. G. Briand, A. D. Smith, G. Schatte, A. J. Rossini and R. W. Schurko, Inorg.
Chem., 2007, 46, 8625ꢀ8637.
G. Chevrier, G. Giester, G. Heger, D. Jarosch, M. Wildner and J. Zemann, Z.
Kristall., 1992, 199, 67ꢀ74.
C. Dybowski, A. Glatfelter, S. Bai, D. Martinez, S. Segarra and D. L. Perry,
Mater. Res. Soc. Symp. Proc., 2007, 984.
A. Glatfelter, S. Bai, O. Dmitrenko, D. L. Perry, S. E. Van Bramer and C.
Dybowski, Canad. J. Chem., 2011, 89, 863ꢀ869.
46.
47.
48.
T. R. Lomer and K. Perera, Acta Crystallogr. B, 1974, 30, 2912ꢀ2913.
T. R. Lomer and K. Perera, Acta Crystallogr. B, 1974, 30, 2913ꢀ2915.
H. A. Ellis, N. A. S. White, R. A. Taylor and P. T. Maragh, J. Mol. Struct., 2005,
738, 205ꢀ210.
49.
50.
S. O. Adeosun and S. Sime, Thermochim. Acta, 1976, 17, 351.
A. S. Arenas, M. V. Garcia, M. I. Redondo, J. A. R. Cheda, M. V. Roux and C.
Turrion, Liq. Cryst., 1995, 18, 431ꢀ441.
51.
M. A. Mesubi, J. Mol. Struct., 1982, 81, 61ꢀ71.

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Spectroscopic and crystallographic data show lead carboxylate coordination is chain length
dependent, shorterꢀchains (C6ꢀC8) are hemidirected and longerꢀchains (C9ꢀC18) are
holodirected..