Lead(ii) soaps: Crystal structures, polymorphism, and solid and liquid mesophases

Article abstract of DOI:10.1039/c7cp02351k

The long-chain members of the lead(ii) alkanoate series or soaps, from octanoate to octadecanoate, have been thoroughly characterized by means of XRD, PDF analysis, DSC, FTIR, ssNMR and other techniques, in all their phases and mesophases. The crystal structures at room temperature of all of the members of the series are now solved, showing the existence of two polymorphic forms in the room temperature crystal phase, different to short and long-chain members. Only nonanoate and decanoate present both forms, and this polymorphism is proven to be monotropic. At higher temperature, these compounds present a solid mesophase, defined as rotator, a liquid crystal phase and a liquid phase, all of which have a similar local arrangement. Since some lead(ii) soaps appear as degradation compounds in oil paintings, the solved crystal structures of lead(ii) soaps can now be used as fingerprints for their detection using X-ray diffraction. Pair distribution function analysis on these compounds is very similar in the same phases and mesophases for the different members, showing the same short range order. This observation suggests that this technique could also be used in the detection of these compounds in disordered phases or in the initial stages of formation in paintings.

Full text of DOI:10.1039/c7cp02351k

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Casado, M. Ramos Riesco, J. A. Cheda, M. I. Redondo Yélamos, L. Garrido, A. Fernandez-Martinez, J.
García-Barriocanal, I. da Silva, M. A. Duran-Olivencia and A. Poulain, Phys. Chem. Chem. Phys., 2017,
DOI: 10.1039/C7CP02351K.
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Lead(II) Soaps: Crystal Structures, Polymorphism, Solid and Liquid
Mesophases
F. J. Martínez-Casado,*a M. Ramos-Riesco,b J. A. Rodríguez-Cheda,b M. I. Redondo-Yélamos,b L.
Garrido,c A. Fernández-Martínez,d J. García-Barriocanal,e I. da Silva,f M. Durán-Olivencia,g A.
Poulain.h
Received 00th January 20xx,
Accepted 00th January 20xx
DOI: 10.1039/x0xx00000x
www.rsc.org/
The long-chain members of lead(II) alkanoates series or soaps, from octanoate to octadecanoate, have been thoroughly
characterized by means of XRD, PDF-Analysis, DSC, FTIR, ssNMR and other techniques, in all their phases and mesophases.
The crystal structures at room temperature of all the members of the series are now solved, showing the existence of two
polymorphic forms in the room temperature crystal phase, different for short and long-chain members. Only nonanoate
and decanoate present both forms, and this polymorphism is proven to be monotropic. At higher temperature, these
compounds present a solid mesophase, defined as rotator, a liquid crystal phase and a liquid phase, all of them with a
similar local arrangement. Since some lead(II) soaps appear as degradation compounds in oil paintings, the solved crystal
structures of lead(II) soaps can now be used as fingerprints for their detection by X-ray diffraction. Pair Distribution
Function Analysis on these compounds are very similar in the same phases and mesophases for the different members,
showing the same short range order. This observation suggests that this technique could also be used in the detection of
these compounds in disordered phases or in the initial stages of formation in paintings.
.
intensively analysed mainly due to their low temperature
thermotropic ionic liquid crystal phase, as well as the short
chain members, which are not mesogens but easily quench
Introduction
Pure metal n-alkanoates or soaps have been largely studied for
their physicochemical properties,1-5 derived from their
structure, which makes them present tendency to form ionic
liquid crystal phases and other mesophases. In particular,
lead(II) soaps with medium and large chain length,6-13 were
into different glass states,14-16 or decompose as in the case of
lead(II) acetate.17
The main physicochemical feature of this series (Pb(Cn)2 from
now on, where n is the total number of carbons of the
alkanoate group) is the presence of polymesomorphism:18
there is a first transition from a completely ordered crystal at
RT (SII) to a solid mesophase (SI), that we identified as a
rotator phase,13 and secondly followed by a fusion to a liquid
crystal (neat phase or smectic A-like), which thirdly melts into a
stable isotropic liquid (ionic) at the clearing point. Besides the
polymesomorphism (rotator and smectic A-like), we prove in
this work, that some members of this series present as well
monotropic polymorphism. This property has been recently
discovered in other metal soaps, like copper(II) alkanoates19 or
lithium acetate.20 Apart from this, the crystal structure of long-
chain lead(II) soaps is also given here for the first time, since,
until now, only the ones of short and medium members were
solved.16,17,21-23
† Electronic Supplementary Information (ESI) available:. Crystallographic information
files (cif, 16 in total) of Pb(C8)2 (CCDC-1473674, at 100 K), Pb(C9)2 polymorph A (CCDC-
1473675, at 298 K), Pb(C10)2 polymorph A (CCDC-1473676, at 298 K), solved by SCXRD,
and Pb(C8)2 (CCDC-1473677, at 298 K), Pb(C9)2 polymorph A (CCDC-1473795, at 298 K),
Pb(C9)2 polymorph B (CCDC-1473796, at 298 K), Pb(C10)2 polymorph A (CCDC-1473684,
at 298 K), Pb(C10)2 polymorph B (CCDC-1473685, at 298 K), Pb(C11)2 (CCDC-1473797, at
298 K), Pb(C12)2 (CCDC-1473683, at 298 K), Pb(C13)2 (CCDC-1473686, at 298 K),
Pb(C14)2 (CCDC-1473687, at 298 K), Pb(C15)2 (CCDC-1473688, at 298 K), Pb(C16)2
(CCDC-1473689, at 298 K), Pb(C17)2 (CCDC-1473690, at 298 K), and Pb(C18)2 (CCDC-
1473691, at 298 K), solved by HRPD. For ESI and crystallographic data in CIF or other
electronic format see DOI: 10.1039/x0xx00000x
Lead(II) exhibits intense optical properties, showing
luminescence in 3D-coordination polymers,24 including some
lead(II) butyrate-based compounds,25 which show potential
application as X-ray phosphors and luminescent materials in
light-emitting devices.17 More surprisingly, pure lead(II) soaps
(2D coordination polymers) present weak fluorescence in the
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crystal phase at low temperature,26 but strong experiments are given in Table 1, for all of the samples. In the
phosphorescence in the glass states (from lead(II) propionate case of the Pb(C10)2, crystals of polymorph B were also found
to heptanoate) even at room temperature.16 On the other in the U tube mixed with Pb(C10)2-A, with poor quality,
hand, lead(II) soaps might as well be potential candidates to sufficient enough to solve the structure and obtain a model
prepare glassy composites, which can be prepared using but yielding poor statistics. Thus, the structure of Pb(C10)2-B
mesomorphic metal alkanoates and that have been proven to was solved by High Resolution Powder Diffraction (HRPD)
behave as non-linear optics materials.27,28
using the aforementioned model.
Besides all these remarkable properties, the relevance of
lead(II) soaps has grown hugely in the last two decades, Single Crystal X-ray Diffraction (SCXRD)
because they were found as degradation products in historical
Measurements for Pb(C8)2 were conducted using synchrotron
oil paintings (detected for the first time in Rembrandt’s
masterpiece The Anatomy Lesson of Dr. Nicolaes Tulp)29 in the
form of protrusions which deteriorate the painting.30 Since
then, not only lead soaps, but also of other metals (e.g., Zn,
Cu(II), K, etc.)31-33 were found, creating several and diverse
problems in the conservation of artworks. Metal soaps appear
due to the reaction between the pigments or drier agents (for
instance, white lead or litharge, respectively) with fatty acids
formed from the hydrolysis of the binding linseed oil
(consisting in a mixture of triglycerides esters of large fatty
acids and glycerin).31,34-37 The lead(II) soaps that are mainly
detected are lead(II) hexadecanoate or palmitate,
octadecanoate or stearate, or others from unsaturated acids
(oleate, linoleate, linolenate). The detection of lead and other
radiation (with λ = 0.7379 Å) at the BM16 Spanish beamline of
ESRF with a CCD detector (ADSCq210rCCD), making phi scans
when collecting the data. The oscillation range (Δ ϕ) used for
each image was one degree. The compound was measured at
100 K. Diffraction data for anhydrous Pb(C9)2-A and Pb(C10)2-A
were collected on
a Bruker APEXII CCD diffractometer
equipped with graphite-monochromated Cu-Kα radiation with
radiation wavelength 1.54178 Å, by using the phi-omega scan
technique at 296 K.
The experimental parameters, crystal size, and main
crystallographic data for Pb(C8)2, Pb(C9)2-A and Pb(C10)2-A are
shown in Table 1.
Table 1. Experimental parameters and main crystallographic data for the
metal soaps in paintings is carried out typically by infrared compounds studied by single crystal XRD.
spectroscopy or diffraction. But, since they are compared to
Pb(C9)2
Pb(C10)2
Data
Pb(C8)2
polymorph A polymorph A
standards for frequencies or d-spacings, respectively, it is
difficult to detect them in the shape of nanocrystals, non-
crystalline or partially disordered phases, or in the early stages
of formation, although amorphous lead carboxylates have
been detected by FTIR recently.32,38 For that, we propose the
use of techniques which focus in the local ordering, such as
Pair Distribution Function (PDF) Analysis or X-ray Absorption.
Thus, the crystal structures and the PDF curves (of the
different phases and mesophases) given in this work may be
used as fingerprints in the detection of lead(II) soaps in the
crystal form and in any other phase or mesophase,
respectively.
Empirical Formula
Mr (g·mol-1)
PbC16H30O4
493.59
PbC18H34O4
521.64
PbC20H38O4
549.69
Crystal system
Space group (No.)
Triclinic
P-1 (2)
Triclinic
P-1 (2)
Triclinic
P-1 (2)
0.080 × 0.030 0.120 × 0.100 0.150 × 0.100
Crystal size (mm)
× 0.010
0.73790
100(2)
× 0.050
1.54178
298(2)
× 0.040
1.54178
298(2)
λ (Å)
Temperature (K)
a (Å)
4.788(1)
7.1940(14)
25.648(5)
87.63(3)
85.55 (3)
89.31 (3)
880.0(3)
2
4.8937(2)
7.3246(3)
28.2872(13)
91.643(2)
94.767(2)
90.710(2)
1009.90(7)
2
4.910(5)
7.327(5)
30.806(5)
88.565(5)
85.855(5)
89.391(5)
1105.0(14)
2
b (Å)
c (Å)
α (°)
β (°)
γ (°)
V (Å3)
Experimental
Z
Sample Preparation
Dc (g·cm-3)
1.863
1.715
1.652
μ (mm-1)
9.60
16.369
14.994
The method of synthesis consists in the metathesis of the
potassium ion from the corresponding alkanoate in absolute
ethanol solution, by the lead(II) ion, added as nitrate, dissolved
in a small amount of water, followed by at least two
recrystallizations in benzene.10 The chemicals used for the
synthesis are specified in the Supplementary Information
(Table S1). The purity of the synthesized salts was assessed by
differential scanning calorimetry (DSC), with values higher than
99.5 %.
Absorption correction
Reflection collected
Reflections with I > 2σ(I).
multi-scan
3775
multi-scan
3370
multi-scan
3734
3633
3179
3420
Parameters refined
/restrains
191 / 0
208 / 0
227 / 0
Hydrogen treatment
R-factor
not refined
0.0738 / 0.1778
0.0746 / 0.1808
1.052
not refined
0.0652
not refined
0.0465
wR2-factor
0.1769
0.1086
Goodness of fit
CCDC deposition
numbers
1.093
1.100
Crystals of Pb(C8)2, Pb(C9)2-A and Pb(C10)2-A, suitable for X-
ray studies were grown by counter diffusion in agarose gel in U
tubes, from aqueous solutions of 50 mM of Pb(NO3)2 and 100
mM of the corresponding sodium alkanoate (alkanoic acid +
NaOH). The sizes of the single crystals used in the SCXRD
1473674
1473675
1473676
The structures were solved by direct methods and subsequent
Fourier syntheses using the SHELXS-97 program,39 and were
refined by the full-matrix least-squares technique against F2,
2 | J. Name., 2012, 00, 1-3
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using the SHELXL-97 program. Anisotropic thermal parameters compounds were also carried out using this set-up and
were used to refine all non-H atoms. Hydrogen atoms were controlling the temperature (from 285 to 410 K) with an
placed in idealized positions, and their parameters were not Oxford Cryojet5.
refined. A semi-empirical absorption correction (‘multi-scan’) Measurement for Pb(C12)2 was conducted at RT using
was done for these compounds, due to the presence of heavy synchrotron radiation (with λ = 0.8266 Å) at the SpLine
atoms.
beamline (BM25) at the European Synchrotron Radiation
Facility (ESRF, Grenoble), using a 0.7 mm spinning capillary,
from 0.5° to 50° and in a 2θ-step scan mode with 0.015° step
and 3-5 sec. acquisition time per point.
Powder X-ray Diffraction
HRPD measurements were performed on all the compounds,
from Pb(C8)2 to Pb(C18)2. In the case of Pb(C8)2, it was
measured at room temperature to confirm the structure
solved by SCXRD (at 100 K). The structures of Pb(C9)2-B and
Pb(C10)2-B were solved from mixtures of polymorphs A and B,
where polymorphs A were solved by SCXRD in both cases.
Pb(C8)2 and the mixture of Pb(C10)2-A and B were measured at
RT in transmission mode in 0.5 mm spinning capillaries at the
I711 beamline of Max II synchrotron (MAX IV Laboratory, Lund,
Sweden), with a wavelength of 0.9941 Å, and using a Newport
diffractometer equipped with a Pilatus 100K area detector
mounted 76.5 cm from the sample. The detector was scanned
continuously, from 2 to 70° (for Pb(C8)2) and from 0 to 60° (for
the mixture of Pb(C10)2), in approx. 6-10 min, recording 125
images/° (step size 0.008°) for each measurement. The true 2-
theta position of each pixel was recalculated, yielding an
average number of 100000 pixels contributing to each 2Θ
value. Integration, applying no corrections for the tilt of the
detector, provided FWHM values of 0.03-0.08° from 0 to 120°.
Measurements in the SI phase (high temperature) for all the
The mixture of polymorphs of Pb(C9)2-A and B and the rest of
the compounds, Pb(C11)2 and Pb(C13)2-Pb(C18)2, were
measured at RT in transmission mode in a Panalytical X’Pert
PRO diffractometer equipped with a focusing mirror and a fast
detector X’Celerator (Cu Kα1 radiation, 40 kV, 45 mA). The
measurement ranges of 2θ were from 2° to 90°, for Pb(C9)2
(step size 0.008°) and from 1° to 90° for the rest of the
compounds (step size 0.017°). The samples were prepared in
0.5 mm-diameter glass capillaries, and rotated during
exposure.
The Rietveld refinement was performed with the FullProf40,41
program, introducing the atomic coordinates previously
obtained from the single crystal data in each case. The main
experimental parameters and crystallographic data are shown
in Tables 2 (from Pb(C8)2 to Pb(C10)2) and 3 (from Pb(C11)2 to
Pb(C18)2). The fitting curves for all the compounds are given in
the Supporting information (Figures S3 and S4).
Table 2 Experimental parameters and main crystallographic data for Pb(C8)2, and the mixtures of polymorphs (A and B) of Pb(C9)2 and Pb(C10)2.
Pb(C9)2
Pb(C10)2
Pb(C8)2
Mixture of polymorphs A + B
Mixture of polymorphs A + B
Crystal data
polymorph A
polymorph B
PbC18H34O4
521.64
polymorph A
polymorph B
Empirical Formula
Mr (g·mol-1)
PbC16H30O4
493.59
PbC20H38O4
549.69
Triclinic, P-1 (2)
4.86902(3)
7.30619(5)
25.6951(2)
88.1812(8)
85.2246(7)
89.1022(7)
910.369(12)
2, 1.785
Triclinic, P-1 (2)
4.89365
Cell setting, space group (no.)
a (Å),
Monoclinic, P2/c (13)
Triclinic, P-1 (2)
Monoclinic, P2/c (13)
60.021(2)
54.9422(3)
4.98011(10)
7.25685(16)
90.00000
4.8865(4)
7.2966(6)
b (Å)
7.32459
4.9611(2)
c (Å)
28.2414(12)
91.64034
94.75932
90.71046
1008.27(4)
2, 1.718
30.7267(14)
88.630(11)
85.856(10)
89.552(16)
1092.36(14)
2, 1.672
7.2496(4)
α (°)
90.00000
β (°)
90.5625(15)
90.00000
90.272(8)
γ (°)
90.00000
Volume (Å3)
1985.51(6)
4, 1.745
2158.68(18)
4, 1.692
Z, Dc (g·cm-3)
Diffractometer
Wavelength (Å)
Temperature (K)
2θ range (°), step size (° 2θ)
Rp, Rwp, Rexp
I711 (MAX IV Lab.)
0.9941
PANalytical XPERT-PRO
I711 (MAX IV Laboratory)
Cu Kα1
298(2)
0.9941
298(2)
298(2)
2-70, 0.008
0.0681, 0.1032, 0.0761
1.39
2-90, 0.008
0.0827, 0.1162, 0.0268
4.33
0.015-60, 0.008
0.0636, 0.0913, 0.022
3.52
Goodness-of-fit
No. of parameters
No. of contributing reflections
RF, RBRAGG
98
115
19
3494
3543
3602
0.0304, 0.0434
92.5
4549
2707
0.0290, 0.0636
39.0
0.0185, 0.0400
100
0.0292, 0.0895
7.5
0.0228, 0.0597
61.0
Mass fraction (%)
CCDC deposition number
1473677
1473795
1473796
1473684
1473685
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Routine XRD measurements as a function of temperature were scattering structure factors and PDFs were obtained using the
carried out in reflection mode with a Panalytical X’Pert PRO PDFGetX3 software.44
MPD X-ray diffractometer with vertical goniometer θ/θ and
RTMS X’Celerator detector equipped with a high-temperature Differential Scanning Calorimetry (DSC)
camera Anton Paar HTK1200 (Cu Kα1 radiation, 1.54056 Å, 45
A TA Instruments DSC Model Q10 was used in this work.
kV, 40 mA, Ni filter). In-situ XRPD scans were measured in inert
atmosphere (N2(g)) from 2 = 2-40° at several temperatures in
Tightly sealed aluminum volatile pans (in N2 atmosphere) were
used to scan at different heating rates (in dry nitrogen
atmosphere at a gas flow of about 50 mL·min-1). An MT5
Mettler microbalance was used to weigh the about 10 mg
θ
the range from 298 to 393 K during heating and cooling, after
programming every temperature change at a rate of 3 K·min-1.
samples (error:
± 0.001 mg). The calorimeter was calibrated
High-energy Total X-ray Scattering and PDF Analysis.
for temperature using standard samples of In and Sn, supplied
by TA (purity > 99.999 % and > 99.9 %, respectively), and of
benzoic acid (purity > 99.97 %), supplied by the former NBS
(lot 39i), and for enthalpy using the standards of In and Sn.
Finely powdered dry samples were loaded into 1.0 mm
diameter kapton capillaries. High-energy X-ray total scattering
data acquisition was performed at beamline ID31, at the ESRF.
Scattering data were collected at different temperatures with
a flat panel Perkin Elmer using the Rapid-Acquisition Pair
FTIR spectroscopy.
Distribution Function technique.42 Samples, an empty capillary Infrared spectra of samples in KBr pellets were recorded using a
and the background were measured at RT from 1–25.0 Å−1. Nicolet Magna 750 FTIR spectrometer at a resolution of 2 cm-1. A
The X-ray wavelength was refined using a CeO2 standard (λ = commercial variable temperature cell, SPECAC VTL-2, adapted for
0.17844 Å). Corrections for sample-detector distance, tilt angle solid samples, was employed to obtain IR spectra of the heated
of the detector with respect to the direction of the incident samples.
radiation and polarization were performed using Fit2D.43 Total
Table 3 Experimental parameters and main crystallographic data for compounds from Pb(C11)2 to Pb(C18)2 (all of them with the polymorph B structure)
Crystal data
Pb(C11)2
Pb(C12)2
Pb(C13)2
Pb(C14)2
Pb(C15)2
Pb(C16)2
Pb(C17)2
Pb(C18)2
Empirical Formula
Mr (g·mol-1)
PbC22H42O4
577.76
PbC24H46O4
605.81
PbC26H50O4
633.86
PbC28H54O4
661.91
PbC30H58O4
689.98
PbC32H62O4
718.02
PbC34H66O4
746.07
PbC36H70O4
774.14
Cell setting, space group
(no.)
Monoclinic,
P2/C (13)
Monoclinic,
P2/C (13)
Monoclinic,
P2/C (13)
Monoclinic,
P2/C (13)
Monoclinic,
P2/C (13)
Monoclinic,
P2/C (13)
Monoclinic,
P2/C (13)
Monoclinic,
P2/C (13)
a (Å),
b (Å)
65.1516(9)
4.9710(7)
7.2708(1)
90.498(3)
2354.73(3)
4, 1.630
70.263(2)
75.2787(17) 80.5448(19) 85.4764(13) 90.6159(13)
95.649(2)
4.9544(2)
7.2989(4)
90.492(6)
3458.7(3)
4, 1.433
Cu Kα1
100.847(2)
4.9589(2)
7.3030(4)
90.488(4)
3652.0(2)
4, 1.408
Cu Kα1
4.96672(15) 4.96255(12) 4.96393(16) 4.96236(12) 4.96125(11)
c (Å)
7.2705(2)
90.564(3)
7.2836(2)
90.581(3)
7.2966(3)
90.514(3)
7.2959(2)
90.590(3)
7.30027(18)
90.463(3)
β (°)
Volume (Å3)
Z, Dc (g·cm-3)
Wavelength (Å)
Temperature (K)
2537.09(1)4) 2720.83(1)2) 2917.22(1)6) 3094.50(1)3) 3281.86(1)2)
4, 1.586
0.8266
298(2)
4, 1.547
Cu Kα1
298(2)
4, 1.507
Cu Kα1
298(2)
4, 1.481
Cu Kα1
298(2)
4, 1.453
Cu Kα1
298(2)
Cu Kα1
298(2)
298(2)
298(2)
2θ range (°), step size (°
2θ)
0.5-50,
0.015
1-90, 0.017
1-90, 0.017
1-90, 0.017
1-90, 0.017
1-90, 0.017
1-90, 0.017
1-90, 0.017
0.0773,
0.1133,
0.0287
0.0271,
0.0671
4.04
0.0451,
0.0671,
0.0279
0.0414,
0.0654
2.40
0.0821,
0.1272,
0.0214
0.0345,
0.0767
0.1062,
0.1563,
0.0213
0.0628,
0.0725
0.0692,
0.1106,
0.0568
0.0592,
0.0524
0.0469,
0.0790,
0.0196
0.0302,
0.0324
0.0925,
0.1643,
0.0194
0.0514,
0.0611
0.0888,
0.1645,
0.0139
0.0232,
0.0448
Rp, Rwp, Rexp
RF, RBRAGG
Goodness-of-fit
5.94
7.35
1.95
3.88
8.47
8.07
No. contributing
reflections
4524
3488
5145
5268
5523
6202
6270
6618
No. parameters
82
69
13
16
10
14
13
86
CCDC deposition number
1473797
1473683
1473686
1473687
1473688
1473689
1473690
1473691
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C-13 and Pb-207 CP/MAS NMR.
However, the main difference between them lies in the
coordination of the Pb atoms: hemidirected (with the O atoms
directed throughout one hemisphere of the coordination
sphere) and holodirected (with the O atoms more regularly
situated around the Pb atom).
Polymorph A shows two types of carboxylates: (a) a µ2-η2η1
carboxylate (with both O atoms chelating one Pb atom and
one of them also coordinating another Pb) and (b) a µ3-η2η2
carboxylate (with both oxygens bridging and chelating). The
coordination number of lead(II) is seven, with the O atoms
Solid state C-13 and Pb-207 NMR measurements were
performed in a Bruker AvanceTM 400 spectrometer (Bruker
BioSpin GmbH, Rheinstetten, Germany) equipped with a 89
mm wide bore, 9.4 T superconducting magnet (C-13 and Pb-
207 Larmor frequencies at 100.61 and 83.71 MHz,
respectively). Powdered samples were placed in 4 mm zirconia
rotors (about 60 mg of sample). All reported data were
acquired at two temperatures, first at (296 ± 0.1) K and then
after thermal equilibrium was reached at a temperature
corresponding to the SI phase (ranging aprox. 370-380 K, with
a precision of ± 0.1 K). We used a standard Bruker double
resonance 4 mm cross-polarization (CP)/magic angle spinning
(MAS) NMR probe head using a 90° C-13 pulse length of 4.2 µs.
The C-13 spectra were acquired with 1 ms CP contact time, 5 s
recycle delay, MAS spinning rates of 6.5 kHz and 1000
transients. High-power proton decoupling of 75 kHz was used.
The NMR spectra were evaluated with the spectrometer
manufacture’s software package TopspinTM. All free-induction
decays were subjected to standard Fourier transformation
with 5 Hz line broadening and phasing. The chemical shifts
were externally referenced to adamantane (29.5 ppm)
secondary to TMS (0.0 ppm).
forming a distorted monocapped trigonal prism, with
a
hemidirected geometry. This polymorph appears in the short
lead(II) soaps (Pb(C2)2-Pb(C7)2 already reported),16,17,21,22 and
present a monoclinic unit cell (P21/m) for Pb(C2)2-Pb(C4)2, and
triclinic (P-1) for Pb(C5)2-Pb(C10)2 (see Figure 1a).
The Pb-207 NMR spectra were acquired with 4 ms CP contact
time, 10 s recycle delay, MAS spinning rates of between 4.0
and 9.0 kHz and 3000 to 12000 transients. High-power proton
decoupling of 75 kHz was used and the NMR spectra were also
evaluated with TopspinTM. All free-induction decays were
subjected to standard Fourier transformation with 400 Hz line
broadening and phasing. The chemical shifts were externally
referenced to tetra-phenyl-lead (IV) (-135 ppm).45
a)
Polymorph A
Results and discussion
Crystal structures and polymorphism (solid phase SII)
The SII phase in lead(II) soaps is the crystalline phase at room
temperature. However, this generically called SII phase
consists actually in two different polymorphic forms from the
short to the long members of the series. This polymorphism
was already suspected in previous studies (see Supp. Info.,
Figure S24)46 and it is confirmed here.
b)
Polymorph B
The crystal structures in the SII phase of all the compounds
from Pb(C8)2 to Pb(C18)2 have been solved and refined here,
being only Pb(C9)2-B reported recently,47 from these salts. The
main aspect of this crystallographic study, as mentioned
before, is the existence of two different polymorphs in the
lead(II) alkanoates series in their crystal phase (SII): A, from
Pb(C2)2 to Pb(C10)2, and B, from Pb(C9)2 to Pb(C18)2. Only
Pb(C9)2 and Pb(C10)2 present both polymorphic phases.
Fig. 1 Crystal structures of polymorphs A (a) and B (b) of Pb(C10)2,
represented in the indicated directions and showing the coordination of the
Pb atoms in both cases (hemidirected and holodirected, respectively).
On the other hand, polymorph B presents a monoclinic cell
(P2/c) with a doubled long cell parameter (two bilayers per
cell), with respect to polymorph A. The two carboxylates are
(a) a µ3-η2η1 carboxylate (with both O atoms bridging) and (b)
Both polymorphs, A and B, show a lamellar structure forming a
2D coordination polymer, with one Pb atom and two alkyl
chains in the asymmetric unit. For both as well, the alkyl chains
present an all-trans conformation, and the planes formed by
the C atoms in the hydrocarbon chains (all-trans) are arranged
with a herring-bone structure (internal orientational order).18
a
µ3-η2η2 carboxylate (with both oxygens bridging and
chelating). The coordination is seven as well but, in this case,
with a pentagonal bipyramidal geometry around the Pb atom
(holodirected) (see Figure 1b). This new polymorphic phase
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was first reported by Catalano et al. for Pb(C9)2-B, studying as pure or in mixtures with high ratio of B (A:B ratio of 7:93 or
well the different coordination of the Pb atoms by ssNMR.23,47
higher in B, with this route). In the case of Pb(C10)2, a mixture
C-13 and Pb-207 ssNMR measurements were also carried out of an approx. 1:9 ratio is obtained, using the aforementioned
here for Pb(C10)2, Pb(C11)2, Pb(C14)2, and Pb(C15)2 (see Supp. synthesis. On the other hand, cooling from the melt, the ratio
Info., Figures S17 to S23, and Tables S6 and S7). The C-13 of polymorphs A increases (approx. 4:6 and 6:4 for Pb(C9)2 and
results show that in the SII phase, two carboxylates and only Pb(C10)2, respectively), being almost the only crystal phase
one signal from the methyl groups are observed (there are two when the quenching process is fast enough (9:1 and almost
kinds from the crystallographic point of view but with similar 1:0, respectively, at cooling rates higher that 50 K·min-1). The
environment and indistinguishable by NMR), as it could be effect of a heating-cooling cycle 296-365-296 K on the
expected for polymorph B,23 with the exception of Pb(C10)2 appearance of C-13 ssNMR spectra of Pb(C10)2 is shown in Fig.
showing a more complex (higher multiplicity of resonances) C- S17 (Supp. Info.). The variation in intensity of the carboxylate
13 spectrum. It is noteworthy highlighting that three and methyl peaks between the spectra collected at 296 K,
carboxylate (C1) peaks at 184.9, 184.2 and 183.8 are observed before and after heating, suggests a change in the sample
and, also, the methyl group (C10) shows two peaks at 15.5 and polymorphs composition. Finally, samples prepared by counter
15.1 ppm (see Supp. Info., Figure S17), implying a mixture of diffusion in gel favour the formation of polymorph A in both
polymorphs A and B may be present this sample.23
Pb-207 ssNMR spectra of these alkanoates show isotropic quality, were found in the case of Pb(C10)2.
chemical shifts, iso, between -2136 and -2114 ppm (Figs. S18, For both Pb(C9)2 and Pb(C10)2, the SII-SI transition for
cases, although some crystals of B, smaller and of worse
δ
S21, S23 and Table S7, in Supp. Info.), lower values than those polymorph B seems to be a complex process and it occurs in
observed in polymorphs A and suggesting longer Pb-O bond two steps, while it happens in a single step for polymorph A.
lengths in polymorphs B, as indicated earlier. Also, a small Polymorph B is denser than A for these two compounds, and
span, Ω = 702-675 ppm, that is characteristic of a holodirected the final temperatures and enthalpies of the SII-SI transitions
coordination geometry is observed.23,47
are higher for B, confirming that this is the stable polymorph
The PDFs of both kinds of polymorphs show clear differences, for Pb(C9)2 and Pb(C10)2. The thermal data (temperatures,
especially in the Pb-O and Pb-Pb distances, as expected from enthalpies and entropies) for all the members of the lead
the different coordination environments and arrangement of soaps series are given in the Supp. Info. (Tables S1 and S2 and
Pb atoms. On the other hand, the PDFs of the same polymorph Figures S1 and S2).
in different members are similar in the short-medium range
(for distances smaller than the stacking of layers; that is, the Intermediate solid phase (S
long d-spacing). This is shown in Figure 2 for polymorph A
I) and fluid phases (LC and IL)
Three different phases exist at high temperature in the lead(II)
alkanoates series, besides the crystal phases: the intermediate
solid phase (solid I or SI), the liquid crystal mesophase (LC) and
the isotropic liquid phase (IL). SI and IL are present for all the
members, while the LC phase exist only from Pb(C6)2 to
Pb(C12)2, both included.
(Pb(C7)2 and Pb(C8)2) and B (Pb(C9)2, Pb(C10)2 and Pb(C11)2).
15
Pb(C7) polymorph A
2
Pb(C8) polymorph A
2
Pb-Pb
Pb(C9) polymorph B
2
10
Pb(C10) polymorph B
2
Pb(C11) polymorph B
Pb-O
2
SII (RT)
Pb-Pb
SI
10
C-C
5
LC
IL
C-O
Pb-O
5
0
C-C
C-O
-5
0
0
5
10
15
r / Å
-5
Fig. 2 Pair Distribution Function (PDF) analysis of high-energy X-ray total
scattering data for Pb(C7)2, Pb(C8)2, Pb(C9)2, Pb(C10)2 and Pb(C11)2 on the
SII phase (RT) in the indicated polymorphic phase. The data for Pb(C7)2 is
taken from reference 16).
0
5
10
15
r / Å
Fig. 3. PDF analysis for the different phases of Pb(C11)2: crystal phase at RT
(SII, polymorph B), intermediate solid phase (SI), liquid crystal (LC) and liquid
phase (IL).
Pb(C9)2 and Pb(C10)2 present both polymorphic forms, A and
B, and in both cases, the latter is the stable one. Depending on
the route used, the different polymorphs or mixtures of them
can be obtained. Thus, for Pb(C9)2, polymorph B is favoured in
the synthesis explained above, sometimes obtained almost
The use of complementary techniques, such as diffraction, PDF
Analysis, FTIR, ssNMR and optical microscopy allow pointing
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ARTICLE
out the differences and similarities of these phases. Thus, for to SI), which, obviously, does not occur, as it was proven by
instance, diffraction clearly shows the solid and crystalline Bazuin et al. by densitometry measurements in Pb(C10)2,6 and
nature of the SI phase while the other two are fluid (few peaks by our group by dilatometry in Pb(C5)2 and Pb(C9)2.13
and none for the LC and IL phases, respectively). PDF-Analysis, However, this apparent paradox could be explained with the
on the other hand, reveals that the same short range order is superstructures studied by Skoulios et al. for the high
common for these three phases (see Figure 3, and Supp. Info., temperature phases in alkaline soaps.50,51 Thus, these
Figures S12 and S13).
superstructures are formed by the packing of domains (ribbon-
like), with a dense arrangement of the polar heads in the
central layers of the ribbons and a much less dense packing
1- SI phase (rotator)
This is an crystalline and semi-ordered phase (with positional, between ribbons, explaining the overall decrease of density
external orientation, conformational order)18, considered a with respect to the crystal phase (see Supp. Info., Figure S25).
rotator phase,13,15,16 characterized by a high crystallinity, a
rotational motion of the alkyl chain (internal orientational
disorder),18 and considerably higher conductivity than the
crystal phase (see Supp. Info., Figure S16).13
SI phase
Some of the diffraction patterns in this phase were indexed
with the DICVOL06 program48 (included in the Fullprof Suite)
for different members of the series (see Supp. Info. Table S5
and Figure S7), giving possible unit cells with common values
for the a and b parameters: approx. 8.52 and 14.90 Å,
respectively. These parameters would correspond to the
structure in the layers, as it occurs in the two polymorphs at
room temperature with common parameters: approx. 4.9 and
7.3 Å. If we compare the results and consider the unit cells in
the SI phase doubled in both axes, the values would be 4.26
and 7.45 Å (×2 in a and b).
LC phase
200µm
Fig. 4. Formation of hexagonal domains (highlighted) observed by optical
microscopy (with crossed polars), corresponding to the formation of the SI
phase on cooling from the liquid crystal phase.
On the other hand, the comparison of the PDFs between the
SII and SI phases (see Figure 3) shows similar distances for C-C,
C-O and Pb-O distances at short range, and a clear decrease in
the Pb-Pb distances: from 4.45(1) to 4.18(1) Å in the first one
(from SII to SI). Since the lead(II) soaps contain only Pb, O, C
and H atoms, the signal coming from the Pb atoms (much
higher Z), especially after the first Pb-Pb peaks appear. To
prove this point, a rough fitting was carried out (using the
PDFgui program49) in the SII phase using only Pb atoms and the
crystallographic data obtained by XRD, showing a good
agreement. Then, a simulation was performed in the SI phase,
refining the cell parameters and the position of the Pb atoms,
obtaining values for a and b of approx. 4.33 and 7.33 Å,
respectively. These values are similar to the aforementioned
ones calculated by XRD. At the same time, the refined
positions of the Pb atoms in the plane (pseudo-hexagonal) fit
the Pb-Pb distances observed in the PDF of the SI phase. The
fittings for both phases and the calculated arrangement of Pb
atoms (in the SI phase) are given in the Supporting Information
(Figures S11 and S12).
FTIR and ssNMR reveal interesting and distinctive features of
this phase, especially about the methyl and methylene groups
and carboxylates of the alkyl chains and the coordination of
the heavy atoms. On one hand, infrared spectra show that at
the SII to SI transition temperature only one band at 2960 cm-1
is observed for this mode indicating that from this point free
rotation of the methyl groups takes place (see Figure 5a). The
CH2 symmetric (υsymm) and asymmetric (υasymm) stretching
bands at 2849 cm-1 and 2918 cm-1 respectively shift to 2855
and 2826 cm-1 in the rotator solid (SI). This shifting to higher
frequencies is characteristic of the presence of gauche
disorder in the alkyl chain. A change from a doublet to a single
band is observed in the carboxylate asymmetric stretching
band from the SII to SI phase, proving that changes in the
coordination of the carboxylate group to the Pb(II) have taken
place at this transition (Figure 5b). Moreover, the crystal field
splitting observed when n > 9 in the CH2 scissoring (around
1465 cm-1) and rocking (around 720 cm-1) modes also
disappears at this temperature (Figure 5c). These facts, point
to a polymorphic change to a more disordered solid (the all-
trans carbon planes start to rotate) taking place at this
temperature. In addition, very weak bands ascribed to
localized gtg (kink) sequences at 1306 cm-1 and end gauche
defects at about 1340 cm-1 can be observed. These bands
correspond to kink defects appearing in the alkyl chain as
consequence of methylene rotational movements taking place
at the solid transition temperature.
The pseudo-hexagonal arrangement of the Pb atoms would
also explain the formation of hexagons observed by optical
microscopy when cooling down from the LC to the SI phase
(see Figure 4).
This hypothetical structure in the SI phase explains the
shortening of the Pb-Pb distances observed by PDF analysis,
with a reduction of the area in the layers (a and b parameters).
On the other hand, the interlayer d-spacing observed by XRD
decreases as well from the SII to the SI phase (see Figure S5 in
the Supp. Info.). Both results would result in an increase in
density from the low to the high temperature solid phase (SII
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not significant differences between the PDFs of the LC and the
IL phases, indicating the same local order up to 10 Å, which
rules out the existence of isolated Pb(Cn)2 units. Therefore, the
IL phase cannot be considered completely disordered and the
difference between the LC and the IL phases could be due to
the domain size, bigger in the case of the liquid crystal, so that
the birefringence and also diffraction can be detected.
IL
SI
SII
T
a)
2950 2900 2850
b)
c)
700
Conclusions
750
1600
1500
1400
1300
1200
Wavenumbers / cm-1
The family of lead(II) soaps, including in this work members
from acetate to stearate (octadecanoate), has been fully
analysed by means of calorimetry, diffraction and scattering,
spectroscopy and other techniques. Four phases are present in
these compounds: the crystal phase at room temperature, a
solid mesophase, a liquid crystal and a liquid phase.
Fig. 5. FTIR spectra of Pb(C14)2, at 298, 313, 333, 353, 368, 373, 378 and
393 K, showing the different phases (SII, SI and IL).
ssNMR confirms the results observed by FTIR. Overall, the C-13
spectra exhibit a line narrowing effect in all peaks due to an
increase in chains mobility at the SI phase (see Figs S17, S19,
S20 and S22 in Supp. Info.). One methyl (14.7 ppm) and
carboxylate (184.7 ppm) are shown in the C-13 spectra in this
phase for long-chain lead soaps, supporting the idea of the
rotational motion of the alkyl chains. In addition, a high field
The crystal structure of the room temperature solid phase
from octanoate to octadecanoate has been solved for the first
time, finding two polymorphic forms for short and long-chain
members and monotropic polymorphism for nonanoate and
decanoate.
shift (∼0.4 ppm), from SII all trans chain conformations, of the
The solid intermediate phase, defined as rotator, is crystalline
and shows some cooperative mobility in the alkyl chains. PDF-
Analysis shows that the first Pb-Pb distance shrinks with
respect to the crystal phase. A model for the structure of the
Pb atoms is also given for this phase. The PDF curves show that
this solid mesophase present a similar local structure in the
very short range as the liquid crystal and the liquid,
independently of the alkyl chain length.
methyl peak due to the gauche effect is observed, and in other
carbons in the chain as well. The increased molecular mobility
in the SI phase is also seen in the corresponding Pb-207 ssNMR
spectra of Pb(C10)2 and Pb(C14)2 acquired at 364.9 and 379.5
K, respectively (see Figs S18 and S21, and Table S7 in Supp.
Info.). In both cases, the spectral span, Ω, is reduced from
about 700-677 ppm to 490-462 ppm. In addition, the increase
in the value of δiso from
∼-2130 ppm to ∼-1550 could be
attributed partly to a decrease in the mean Pb-O bond
length.52 The higher disorder and possible motion might affect
to the ionic parts, and as it happens in shorter members of the
lead soaps family, the conductivity increases considerably in
the SI phase, in contrast with the crystal phase. FTIR, ssNMR
and conductivity data for some members of the series are
shown in the Supporting Information.
PDF Analysis: a new technique to study lead soaps in Art.
The detection of lead soaps in artworks, as degradation
products (e.g. Pb(C16)2 and Pb(C18)2), is mainly carried out by
FTIR and other techniques like diffraction or electron
microscopy. The vibrational frequencies or diffraction patterns
used to identify lead soaps are those of the crystal phase. This
poses an important limitation to the early detection of these
salts during their initial formation, where nanocrystals or
mesophases can be formed lacking long-range ordering.
2- LC phase
This fluid phase is birefringent and shows a coherent order of
stacking of layers (few peaks in diffraction). The liquid crystal
phase has been well described,9-13,16 and it corresponds to a
smectic-A like (or neat) liquid crystal. This structure can be
deduced from the fan-shaped domains studied by optical
microscopy with crossed polars (see Figure 4).10,16,53
The molecules are grouped in layers and, then, in domains.
The alkyl chains are “melted” (dynamical disorder of gauche
defects), with the director vector perpendicular to the layers.
Although the LC phase is fluid, PDF shows a similar order in the
short range (up to 5-6 Å), revealing that the atoms (especially
the heavy ones) are in the same environment and coordination
as in the SI phase (Figure 3).
The experiments carried out here on the members of the
series in all their phases show that the PDF offers a unique
signature of the local structure, very similar for all the lead
soaps phases. This means that any lead soap can be identified
using the PDF pattern as a fingerprint. In the particular case of
SII, the PDF in the short range is obviously different for
polymorphs
A and B, but again similar for the same
polymorphic form in different members of the series. The use
of PDF analysis can therefore be of significant help significantly
in the detection and the structural study of disordered as well
as ordered phases of these systems. The sensitivity of the
technique using X-rays is higher for high-Z atoms, which would
favour the detection of lead soaps. In this sense, PDF Analysis
or X-ray Absorption (XAS) using micro-sized beams are
potential tools, complementary to the existing ones, that could
be used on the detection of metal soaps in paintings, even in
the initial stages of formation.
3- IL phase
The so-called “isotropic" liquid phase shows no birefringence
and no peaks in diffraction. The phase is fluid and the alkyl
chains are completely disordered (FTIR). However, there are
8 | J. Name., 2012, 00, 1-3
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ARTICLE
17 F. J. Martínez-Casado, M. Ramos-Riesco, J. A.
Rodríguez-Cheda, F. Cucinotta, E. Matesanz, I. Miletto,
Acknowledgements
Partial support of this research by the DGICYT of the Spanish
"Ministerio de Educación y Ciencia" (Grants in aid for Scientific
Research CTQ2008-06328, CTQ2013-41781-P and CTQ2015-
67755-C2-1-R) is gratefully acknowledged. LG acknowledges
financial support provided by CSIC. The authors wish to thank
the beamlines BM25-Spline and ID31 (ESRF, Grenoble, France),
I711 (Max II ring, MAX IV Laboratory, Lund, Sweden), the
Center of Scientific Instrumentation at the University
Complutense of Madrid (CAI of XRD, Centro de Asistencia a la
Investigación), and the Laboratory for Crystallographic Studies
(IACT, CSIC-UGR), and their staff, for the use of their technical
facilities and help.
E. Gianotti and L. Marchese, Inorg. Chem. 2016, 55
8576.
18 F. J. Martínez-Casado, M. Ramos-Riesco, M. I. Redondo-
Yélamos, A. Sánchez Arenas and J. A. R. Cheda, J.
Therm. Anal. Calorim., 2012, 108, 399.
19 a) M. Ramos Riesco, F. J. Martínez-Casado, S. López-
Andrés, M. V. García Pérez, M. I. Redondo-Yélamos, M.
R. Torres, L. Garrido and J. A. R. Cheda, Cryst. Growth
,
Des. 2008, 8, 2547; b) M. Ramos Riesco, F. J. Martínez-
Casado, J. A. Rodríguez Cheda, M. I. Redondo-Yélamos,
A. Fernández-Martínez and S. López de Andrés, Cryst.
Growth Des., 2015, 15, 497; c) M. Ramos Riesco, F. J.
Martínez-Casado, J. A. Rodríguez-Cheda, M. I.
Redondo-Yélamos, I. da Silva, T. Plivelic, S. López de
Andrés and P. Ferloni, Cryst. Growth Des., 2015, 15
2005.
,
20 F. J. Martínez-Casado, M. Ramos-Riesco, M. I. Redondo-
Yélamos, D. Choquesillo-Lazarte, S. López-Andrés, J. A.
Rodríguez -Cheda, Cryst. Growth Des., 2011, 11, 1021.
21 F. Lacouture, M. Francois, C. Didierjean, J. P. Rivera, E.
Rocca and J. Steinmetza, Acta Cryst., 2001. C57, 530.
22 F. J. Martínez-Casado, M. Ramos-Riesco, I. da Silva, M.
I. Redondo-Yélamos, A. Labrador and J. A. R. Cheda,
Cryst. Growth Des., 2011, 11, 759.
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