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Unraveling the Decomposition Process of Lead(II) Acetate: Anhydrous Polymorphs, Hydrates, and Byproducts and Room Temperature Phosphorescence Francisco J. Martínez-Casado,*,† Miguel Ramos-Riesco,‡ José A. Rodríguez-Cheda,‡ Fabio Cucinotta,§ Emilio Matesanz,⊥ Ivana Miletto,§ Enrica Gianotti,§ Leonardo Marchese,§ and Zdeněk Matěj† †

MAX IV Laboratory, Lund University, SE-221 00 Lund, Sweden Departamento de Química Física I, Facultad Ciencias Químicas, and ⊥Centro de Asistencia a la Investigación, Difracción de Rayos X, Facultad de Ciencias Químicas, Universidad Complutense, 28040 Madrid, Spain § Dipartimento di Scienze e Innovazione Tecnologica and Nano-SISTeMI Interdisciplinary Centre, Universitá del Piemonte Orientale “A. Avogadro”, via Teresa Michel 11, I-15121 Alessandria, Italy ‡

S Supporting Information *

ABSTRACT: Lead(II) acetate [Pb(Ac)2, where Ac = acetate group (CH3−COO−)2] is a very common salt with many and varied uses throughout history. However, only lead(II) acetate trihydrate [Pb(Ac)2·3H2O] has been characterized to date. In this paper, two enantiotropic polymorphs of the anhydrous salt, a novel hydrate [lead(II) acetate hemihydrate: Pb(Ac)2·1/2H2O], and two decomposition products [corresponding to two different basic lead(II) acetates: Pb4O(Ac)6 and Pb2O(Ac)2] are reported, with their structures being solved for the first time. The compounds present a variety of molecular arrangements, being 2D or 1D coordination polymers. A thorough thermal analysis, by differential scanning calorimetry (DSC) and thermogravimetric analysis (TGA), was also carried out to study the behavior and thermal data of the salt and its decomposition process, in inert and oxygenated atmospheres, identifying the phases and byproducts that appear. The complex thermal behavior of lead(II) acetate is now solved, finding the existence of another hydrate, two anhydrous enantiotropic polymorphs, and some byproducts. Moreover, some of them are phosphorescent at room temperature. The compounds were studied by TGA, DSC, X-ray diffraction, and UV−vis spectroscopy. well.17−20 However, the behavior of this salt, studied for over a century, and the structures of the anhydrous polymorphs, hydrates, and some of the basic salts (some appearing as decomposition byproducts) have not been completely unraveled yet. Pb(Ac)2 was also analyzed by this research group some years ago,21 together with two more members of the lead(II) alkanoate series. In this sense, the data that we present here confirm that the ones shown in that study did not correspond to anhydrous lead(II) acetate but to a basic lead acetate, as will be explained in detail. Anhydrous lead(II) acetate belongs to the family of metal alkanoates,22 which have shown their ability to present polymorphism23−25 and/or polymesomorphism.21,26−30 In particular, the lead(II) alkanoate series has been studied in depth for some decades.31−38 These compounds present two mesophases, rotator (members with n ≥ 4, with n being the number of C atoms)37,39 and a liquid-crystal phase (6 ≤ n ≤

1. INTRODUCTION Lead(II) acetate [hereafter referred to as Pb(Ac)2, where Ac = acetate group (CH3−COO−)2] is a very well-known compound with many and varied uses throughout the centuries:1 as a sweetener and sugar substitute (known as “sugar of lead”), despite its toxicity;2 in cosmetics, as a hair-coloring product3 and in skin-whitening lotions;4 in medical uses, as an astringent and as a remedy for sore nipples, in solution with lead(II) oxide (Goulard’s extract);5,6 in industrial uses, as a hydrogen sulfide detector, as a mordant in dyeing and textile printing, and as a drier in paints and varnishes;7 etc. Related compounds, different hydrates of a basic lead(II) acetate [Pb3O2(Ac)2· H2O8,9 and Pb3O2(Ac)2·1/2H2O,10], also appear in the corrosion of lead. Recently, lead(II) acetate trihydrate was obtained from spent lead acid battery pastes and used to synthesize ultrafine lead oxide.11 Several studies on the decomposition of Pb(Ac)2 were carried out,10−15 and some of the byproducts, such as basic lead(II) acetates, had already been described a century ago.16 The crystal structure of lead(II) acetate trihydrate was solved as © XXXX American Chemical Society

Received: May 6, 2016

A

DOI: 10.1021/acs.inorgchem.6b01116 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry Table 1. Experimental Parameters and Main Crystallographic Data for the Compounds Studied by SCXRD α-Pb(Ac)2 empirical formula Mr (g·mol−1) cryst syst space group cryst size (mm) coordination dimensionality morphology λ (Å) temperature (K) a (Å) b (Å) c (Å) α (deg) β (deg) γ (deg) V (Å3) Z Dc (g·cm−3) μ (mm−1) abs corrn refln collected reflns with I > 2σ(I) param refined/restraints hydrogen treatment R factor wR factor GOF CCDC deposition numbers a

β-Pb(Ac)2

Pb10C40H60O40 3252.78 monoclinic P21 (No. 4) 0.18 × 0.161 × 0.028 2D

PbC4H6O4 325.28 monoclinic P21/m (No. 11) 0.20 × 0.10 × 0.020 2D

plate 1.54178 (Cu Kα) 296(2) 12.6171(5) 7.1137(3) 38.2093(12) 90 99.017(2) 90 3387.1(2) 2 3.189 48.053 analytical 10377 8733 720/1 not refined 0.0525 0.1300 1.028 1473436

plate 0.7513 348(2) 4.7920(10) 7.1480(14) 10.058(2) 90 99.48(3) 90 339.81(12) 4 3.179 27.64 cylindrical 422 391 47/0 not refined 0.0841 0.2297 1.188 1473437

Pb(Ac)2·1/2H2O Pb2C8H14O9 668.57 orthorhombic Pna21 (No. 33) 0.050 × 0.025 × 0.010 1D bonded by hydrogen bonds needle 0.8220 100(2) 14.646(3) 17.632(4) 5.3720(11) 90 90 90 1387.3(5) 4 3.201 27.59 spherical 3090 2831 167/4 mixed 0.0319 0.0810 1.072 1473438

Pb2O(Ac)2

Pb4O(Ac)6

Pb4O(Ac)6·xH2Oa

Pb4C8H12O10 1096.94 monoclinic P2/n (No. 13) 0.29 × 0.100 × 0.050 2D-corrugated

Pb4C12H18O13 1199.02 monoclinic C2/c (No. 15) 0.150 × 0.100 × 0.030 2D

Pb4C12H23O15.5 1244.06 orthorhombic Pban (No. 50) 0.12 × 0.080 × 0.030 2D

needle 1.54178 (Cu Kα) 296(2) 10.9810(6) 10.0872(4) 15.4305(7) 90 97.780(2) 90 1693.47(14) 4 4.302 76.285 analytical 2912 2669 197/0 not refined 0.0477 0.1206 1.035 1473439

plate 0.7514 100(2) 24.394(6) 6.7920(14) 13.872(3) 90 106.28(3) 90 2206.2(8) 4 3.357 34.03 cylindrical 2236 1983 140/12 not refined 0.0771 0.2312 1.139 1473440

plate 0.8000 100(2) 6.783(5) 13.823(5) 13.937(5) 90.00 90.00 90.00 1306.8(12) 2 3.162* 29.26 refdelf 1102 1083 79/9 not refined 0.0690 0.1732 1.147 1473441

Calculated for five water molecules per unit cell in the voids of the structure of Pb4O(Ac)6·xH2O, so x = 2.5.

12). The shortest members of the series (n ≤ 7)20,38,40 show also a great tendency to form different kinds of glass states: regular, rotator, and liquid-crystal glasses.41 On the other hand, the first crystal structure of this series was solved for lead(II) heptanoate42 and, more recently, for members of the series between propionate and hexanoate.39,43 The high-temperature polymorph of anhydrous Pb(Ac)2 is isostructural with those showing the same bilayered structure, which is not usual for metal acetates. In fact, acetates have been proven to be very special members in the metal alkanoate series, analogous to lithium acetate,44 with higher “inorganic” nature and presenting different structures (3D coordination polymers) and thermal behavior than longer members and with the existence of hydrates and also showing a strong tendency to form glasses (pure or in binary mixtures).45−47 On the other hand, lead is still a very interesting metal within the heavy p-block elements despite its toxicity because of the variety of structures that lead(II) compounds show. It exhibits a variable coordination number and geometry with or without a stereochemically active lone pair of electrons and could therefore show interesting topological arrangements with redox as well as catalytic48−51 and intense optical properties. In this sense, lead(II) shows luminescence in inorganic and metallorganic complexes.52,53 Lead(II) compounds have been receiving increasing attention in the last decades for their potential application as X-ray phosphors and luminescent materials in light-emitting devices.54−57 The 6s2 outer-electron configuration of lead(II) and its large radius, along with the

absence of crystal-field stabilization energy effects, can give rise to several coordination geometries and, in turn, supramolecular networks. The particular photophysical properties of lead(II) compounds are mainly related to the intrinsic emission that originates from metal-centered (MC) sp triplet states and that depends strongly on the temperature and lattice interactions.54,56,58 In particular, lead(II) alkanoates have shown luminescent properties, from weak fluorescence in the crystal phase at low temperature59 to strong phosphorescence in the glass states in the shortest members even at room temperature.39 Mixed-ligand lead(II) complexes, based on alkanoate anions, also present interesting luminescent properties.56 In this paper, we present for the first time a thorough study of the behavior of Pb(Ac)2, the two enantiotropic polymorphs of the anhydrous salt α and β (low- and high-temperature polymorphs respectively α-Pb(Ac)2 and β-Pb(Ac)2 from now on), a new hydrate species, and their two decomposition byproducts, reporting for the first time the interesting photophysical properties of these compounds that arise from their unique structures.

2. EXPERIMENTAL SECTION 2.1. Sample Preparation. Three methods were performed to obtain anhydrous Pb(Ac)2 from commercial lead(II) acetate trihydrate (Fluka, >99.5%): (a) by heating at 415 K under vacuum for 3 h and recrystallizing in ethanol (already reported),20 (b) by removing the solvent completely in a rotavapor from a solution of the hydrated sample in methanol, and (c) by freeze-drying (at 223 K). When the B


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Inorganic Chemistry Table 2. Experimental Parameters and Main Crystallographic Data for the Compounds Studied by Powder XRD Pb(Ac)2· 1/2H2O empirical formula Mr (g·mol−1) cell setting, space group temperature (K) a (Å) b (Å) c (Å) α (deg) β (deg) γ (deg) volume (Å3) Z, Dc (g·cm−3) wavelength (Å) μ (mm−1)

Pb2C8H14O9 668.57 orthorhombic, Pna21 (No. 33) 298(2) 14.8557(6) 17.6453(7) 5.3968(2) 90 90 90 1414.69(10) 4, 3.139 0.9938 7.30

diffractometer specimen mounting data collection mode scan mode 2θ range (deg), step size (deg 2θ)

0−120, 0.016

refinement method profile function Rp, Rwp, Rexp RF, RBragg GOF no. of contributing reflns no. of param CCDC deposition number

0.0346, 0.0495, 0.0195 0.0184, 0.0347 2.53 4425 40 1473456

β-Pb(Ac)2

α-Pb(Ac)2

PbC4H6O4 Pb10C40H60O40 325.28 3252.78 monoclinic, P21/m (No. 11) monoclinic, P21 (No. 4) 410(2) 285(2) 4.7983(5) 12.5687(2) 7.1876(7) 7.08687(12) 10.2694(10) 38.0318(6) 90 90 99.4998(4) 99.0918(6) 90 90 349.31(6) 3345.03(10) 2, 3.093 2, 3.230 0.9938 0.9938 8.01 8.01 Data Collection I711 (MAX IV Laboratory) borosilicate glass capillary transmission 2θ continious scan 0−90, 0.016 0−90, 0.016 Refinement full-matrix least squares on Inet pseudo-Voigt with axial divergence asymmetry 0.0366, 0.0593, 0.0850 0.0425, 0.0681, 0.0849 0.0517, 0.0634 0.0550, 0.0292 0.70 0.80 1278 12313 114 89 1473467 1473470

first two methods were employed, a decomposition byproduct was crystallized, later identified as Pb4O(Ac)6. If some water was present during recrystallization in ethanol or methanol, the structure of Pb4O(Ac)6 swelled and solvent molecules penetrated between the layers, forming crystals of Pb4O(Ac)6·xH2O from the solution. By means of the third method, anhydrous Pb(Ac)2 (α polymorph) was obtained [with around 0.5% impurity of Pb4O(Ac)6, as will be explained in sections 2.3 and 3.2]. Thus, it was proven that the obtaining methods previously reported failed and possibly yielded different compounds. Despite the great difficulty of growing crystals as a result of the obtaining method, very tiny ones of anhydrous αPb(Ac)2 (low-temperature phase) were obtained by freeze-drying, suitable for single-crystal X-ray diffraction (SCXRD). One crystal of this phase was analyzed above the solid-to-solid transition temperature (polymorph α to β). The structures of both polymorphs are bilayered (ionic and lipidic layers). The joint between these latter layers is very weak, making it difficult to obtain good crystals because of the ease of exfoliation. This feature is shared by most of the metal alkanoates,42,60 so they grow into flake-shaped crystals, as was also found recently for the common family of lead(II) alkanoates.39,41,42 Anhydrous Pb(Ac)2 was found to be highly hygroscopic, and, in fact, Pb(Ac)2·1/2H2O is formed completely by hydration of Pb(Ac)2, in the shape of tiny crystals, after around 5 days at room temperature and around 40% humidity. This sample remains stable in time and does not get hydrated at those conditions. Pb2O(Ac)2 was obtained by heating commercial Pb(Ac)2·3H2O, in a hot stage (in an air atmosphere) above the dehydration temperature and up to the melting point of the anhydrous salt (480 K). The sample started to decompose immediately after melting (see section 3.2), so it was then cooled very slowly (0.1 K·min−1) to 460 K, and needleshaped crystals of Pb2O(Ac)2 formed. The sizes of the single crystals used in the SCXRD experiments are given in Table 1 for all of the samples.

Pb4O(Ac)6 Pb4C8H14O11 1199.02 monoclinic, C2/c (No. 15) 298(2) 24.5037(5) 6.82814(17) 13.9738(4) 90 106.283(2) 90 2244.48(9) 4, 3.549 Cu Kα1/Kα2 57.61 PANalytical XPERT-PRO

2θ step scan 2.5−85, 0.017

0.0598, 0.0856, 0.0544 0.0536, 0.0769 1.574 1059 77 1473471

2.2. SCXRD. Diffraction data for anhydrous α-Pb(Ac)2 and Pb2O(Ac)2 were collected on a Bruker APEXII CCD diffractometer equipped with graphite-monochromated Cu Kα radiation with a radiation wavelength of 1.54178 Å, by using the ϕ−ω scan technique at 296 K. The measurements for anhydrous β-Pb(Ac)2, anhydrous Pb4O(Ac)6, Pb4O(Ac)6·xH2O, and Pb(Ac)2·1/2H2O were conducted in different experiments using synchrotron radiation (with λ = 0.7514, 0.9779, 0.8000, and 0.8220 Å, respectively). The first two compounds were measured at the beamline BM16-LLS of the European Synchrotron Radiation Facility (ESRF; Grenoble, France) with a CCD detector (ADSCq210rCCD), while data for the second two were collected at the beamline I911-361 of MAX IV Laboratory (Lund, Sweden) with a MAR225 CCD detector, making ϕ scans when collecting the data. Polymorph β of Pb(Ac)2 was measured at 348 K, above the solid-to-solid transition (polymorph α to β), while data for Pb4O(Ac)6, Pb4O(Ac)6·xH2O, and Pb(Ac)2·1/2H2O were collected at 100 K. Because of the high absorption of lead and the great intensity of the beam using synchrotron radiation, the first three crystals suffered severe radiation damage, with it only being possible to collect around 180 images before their decomposition. However, the data are definitively reliable, and the structures were confirmed by the Rietveld method62 from high-resolution powder X-ray diffraction (HRPD) data at room temperature. Pb4O(Ac)6·xH2O could not be measured at room temperature by SCXRD or powder X-ray diffraction (XRD) because the crystals decompose immediately, losing water molecules. The structures were solved by direct methods and subsequent Fourier syntheses using the SHELXS-97 program and were refined by the full-matrix least-squares technique against F2 using the SHELXL-97 program.63 Anisotropic thermal parameters were used to refine all non-H atoms. H atoms of water [for Pb(Ac)2·1/2H2O] were located in the Fourier difference map. The other H atoms were placed in idealized positions, in every case, and their parameters were not refined. An absorption correction was done for all of the compounds C


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Inorganic Chemistry

Figure 1. Crystal structures of anhydrous β-Pb(Ac)2 (a and b) and α-Pb(Ac)2 (c and d). The structures are presented along the a (a) and b (b) axes, parallel to the plane (102) (c), and along the b axis (d), respectively. because of the presence of lead. In the case of Pb4O(Ac)6·xH2O, voids were found in the structure, and the data (.hkl file) were treated with the SQUEEZE routine within PLATON.64 A void volume of 299 Å3 per unit cell was found, and the calculated electron density was 52 electrons. The experimental parameters, crystal sizes, and main crystallographic data for the compounds studied are shown in Table 1. 2.3. Powder XRD. HRPD measurements were performed on Pb(Ac)2·1/2H2O, and Pb4O(Ac)6 at room temperature, to confirm their structures solved by SCXRD (at 100 K). Pb(Ac)2·1/2H2O was measured as a function of the temperature, so β-Pb(Ac)2 (410 K) was obtained by dehydration and then α-Pb(Ac)2 after cooling to 285 K. All of the structures were solved and refined by the Rietveld method. Pb(Ac)2·1/2H2O and Pb(Ac)2 (polymorphs β and α) were measured in transmission mode in 0.5 mm spinning capillaries at the beamline I711 of Max II (MAX IV Laboratory, Lund, Sweden) using a Newport diffractometer equipped with a Pilatus 100 K area detector mounted 76.5 cm from the sample. The detector was scanned continuously, from 0° to 120° [for Pb(Ac)2·1/2H2O] and to 90° (for both anhydrous polymorphs), in approximately 6−10 min, recording 62.5 images·deg−1 (step size 0.016°) for each measurement. The true 2θ 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 full width at halfmaximum (fwhm) values of 0.03−0.08° from 0 to 120°. The temperature (from 285 to 410 K) was controlled using the Oxford cryojet5. A very small amount of Pb4O(Ac)6 (∼0.5% calculated by the Rietveld method) was found in the three phases as a contaminant. Pb4O(Ac)6 was measured at room temperature in transmission mode in a Panalytical X’Pert PRO diffractometer equipped with a focusing mirror and an X’Celerator fast detector (Cu Kα1 radiation, 40 kV, 45 mA). The measurement range of 2θ was from 2.5° to 85° and

the step size 0.017°. In this case, the sample was placed in a 0.3-mmdiameter glass capillary and rotated during exposure. The Rietveld refinement was performed with the FullProf65,66 program, introducing the atomic coordinates previously obtained from the single-crystal data in each case. The main experimental parameters and crystallographic data for the compounds studied by powder XRD are shown in Table 2. The fitting in the four cases is given in Figure S1. 2.4. Differential Scanning Calorimetry (DSC). A TA Instruments DSC model Q10 was used in this work. Tightly sealed aluminum volatile pans (in a dinitrogen atmosphere) were used to scan at different heating rates (in a dry dinitrogen atmosphere at a gas flow of 50 mL·min−1). An MT5 Mettler microbalance was used to weigh the approximate 10 mg samples (error: ±0.001 mg). The calorimeter was calibrated for temperature using standard samples of indium and tin, supplied by TA Instruments (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 indium and tin. 2.5. Thermogravimetric Analysis (TGA). TGA was carried out on a TA Instruments SDT Q600 in a dinitrogen atmosphere (flowing at 100.0 mL·min−1) from room temperature to 600 K using aluminum crucibles, at a heating rate of 5 K·min−1. 2.6. UV−Vis Spectroscopy. UV−vis diffuse-reflectance (DR) spectra were recorded using a PerkinElmer model Lambda 900 spectrophotometer, equipped with a DR sphere accessory (DR-UV− vis). Prior to analysis, the solid compounds were dispersed in an anhydrous BaSO4 matrix (10% in weight). The samples Pb(Ac)2·1/2H2O and Pb2O(Ac)2 were measured at room temperature. Steady-state emission spectra were recorded on a Horiba Jobin Yvon model IBH FL-322 Fluorolog 3 spectrometer equipped with a 450-W xenon arc lamp, double-grating excitation and emission monochromators (2.1 nm/mm dispersion; 1200 grooves/mm), and a Hamamatsu model R928 photomultiplier tube. Emission and D


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Inorganic Chemistry

Figure 2. Crystal structure of Pb(Ac)2·1/2H2O: (a) asymmetric unit showing the coordination polyhedron (hemidirected geometry) of the PbII atoms (atoms generated by symmetry are drawn semitransparent); (b) representation along the c axis highlighting the O atoms of the water molecules and showing the 1D catena growing along this axis (in blue) 1D and the hydrogen bonds.

Figure 3. Crystal structure of Pb2O(Ac)2: (a) asymmetric unit (atoms generated by symmetry are drawn semitransparent); (b) representation along the a axis, showing the corrugated 2D layers (one highlighted in blue). excitation spectra were corrected for source intensity (lamp and grating) and emission spectral response (detector and grating) by standard correction curves. Time-resolved measurements up to ∼5 μs were performed using the time-correlated single-photon-counting (TCSPC) option on the Fluorolog 3 spectrometer. A NanoLED (370 nm; fwhm = 1.2 ns) with repetition rates between 10 kHz and 1 MHz was used to excite the sample. The excitation source was mounted directly on the sample chamber at 90° to a double-grating emission monochromator (2.1 nm·mm−1 dispersion; 1200 grooves·mm−1), and the signals were collected using an IBH Data Station Hub photoncounting module. Data analysis was performed using the commercially available DAS6 software (Horiba Jobin Yvon IBH). For excited-state lifetimes >5 μs, a SpectraLED was used as the excitation source (460 nm; fwhm = 1.2 ns), and data collection and analysis was made as described above.

structure consists of 1D catenae, formed by acetate anions (μ4) and Pb atoms. Each Pb atom is coordinated by a chelating acetate (μ2) and two water molecules (which present hydrogen bonds with other water molecules and one O atom of the chelating acetate of the same catena). The other water molecule is located at hydrogen-bond distances with the other two water molecules. Thus, the whole structure is composed of 1D catenae joined between each other by hydrogen bonds. The unit cell is monoclinic (space group C/2m). The structure of β-Pb(Ac)2 [the high-temperature polymorph of lead(II) acetate] is related to the one of Pb(Ac)2· 3H2O. In this case, the absence of water molecules causes the shortening of the distance between two contiguous catenae, and the chelating acetate turns into chelating−bridging (μ3), coordinating to another Pb atom of another catena and forming a 2D arrangement (Figure 1a,b) in a small monoclinic unit cell (space group P/21m). Despite presenting rather different cell parameters, the lowtemperature polymorph of anhydrous lead(II) acetate, αPb(Ac)2, shows a molecular arrangement very similar to that of β-Pb(Ac)2, in a bilayered structure (2D), but showing distortions in the positions of Pb atoms and acetate groups (Figure 1c,d), which also show μ3 and μ4 coordination. The

3. RESULTS AND DISCUSSION 3.1. Crystal Structures and Polymorphism of Anhydrous Pb(Ac)2. Crystal Structures. The structures of lead(II) acetates, hydrates, and byproducts have been solved by SCXRD and HRPD and present different crystal arrangements. The main structure of lead(II) acetate derivatives reported was Pb(Ac)2·3H2O.16−19 In this compound, two different types of acetate anions and water molecules are observed. The E


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Inorganic Chemistry unit cell is monoclinic (space group P21), and 10 Pb atoms and 20 acetate anions are present in the asymmetric unit. The coordination number of Pb is 7 for both α-Pb(Ac)2 and A in a hemidirected geometry. Pb(Ac)2·1/2H2O shows a 3D structure formed by 1D catenae joined by hydrogen bonds, as in the case of lead(II) acetate trihydrate. The unit cell is orthorhombic (space group Pna21). The asymmetric unit presents two Pb atoms (one coordinated by eight O atoms and another by seven), four acetate groups (one μ2-chelating, two μ3-bridging−chelating, and one μ6bridging−chelating), and one water molecule (Figure 2a). This water molecule coordinates one Pb atom and forms two hydrogen bonds: one internal (to an O atom of an acetate group in the same catena) and one bonding to a contiguous catena (Figure 2b). Considering each 1D catena as an entity, the arrangement shows a herringbone pattern along the c axis. The structure of this compound at 150 K was previously reported by Kooijman et al. in a private communication.67 Pb2O(Ac)2 presents a 2D structure with corrugated layers, joined by van de Waals forces (Figure 3). The unit cell is monoclinic (space group P2/n). The asymmetric unit is formed by four Pb atoms (three coordinated by six O atoms and one by seven, all of them with hemidirected coordination), four acetate groups (three of them μ4-bridging and one μ5-bridging− chelating), and two O atoms (μ4-oxo ligands, which are in the center of a tetrahedral with four Pb atoms in the vertexes). Finally, the structure of Pb4O(Ac)6 is layered (2D) in a monoclinic unit cell (space group C2/c; Figure 4). There are

Figure 5. Crystal structure of Pb4O(Ac)6·xH2O: (a) asymmetric (atoms generated by symmetry are drawn semitransparent); scheme of the corrugated layers showing the unit cell; representation along the b axis, showing the stacking of layers the large voids between them.

unit (b) (c) and

molecules can enter. A disorder acetate is also present in this structure, pivoting around O3 and leaving O4 exposed for possible hydrogen bonds with the interstitial water. The void volume (299 Å3 per unit cell) allows the presence of several water molecules, and 52 electrons were calculated to be in the voids, which would approximately coincide with five water molecules. A TGA experiment was carried out on Pb4O(Ac)6· xH2O, and five water molecules were detected per unit cell (so x corresponds to 2.5). However, the experiment showed that water can be lost in this compound already at room temperature, so a higher amount of water can fit in the structure. In fact, the corresponding compound with x = 3 was described by Jackson:15 3Pb(Ac)2·PbO·3H2O. Another compound with stoichiometry 3Pb(Ac)2·PbO·H2O was reported by Kwestroo et al.,12 so it can be assumed that different amounts of water molecules can be found in this structure. It is worth noting that coordination of the Pb atoms is not completely hemidirected in the cases of Pb(Ac)2·1/2H2O and Pb4O(Ac)6, while anhydrous Pb(Ac)2 presents more exposed Pb atoms, and this could explain the ease for hydration in this compound, where water molecules could interact with lead and transform the structure. On the other hand, the Pb−Pb distances are important to explain the luminescent properties because they affect the extent of interaction between the metal orbitals and their overlap. Thus, Pb(Ac)2·1/2H2O shows slightly shorter distances than the anhydrous Pb(Ac)2 (in the ranges of 4.17−4.64 and 4.20−4.99 Å, respectively), but the shortest ones are found for Pb2O(Ac)2 and Pb4O(Ac)6 (3.43−4.77 and 3.55−4.86 Å, respectively). Polymorphism of Anhydrous Pb(Ac)2: from α to β. Two polymorphs of anhydrous Pb(Ac)2, α and β, were discovered in this work. These phases are enantiotropic, so the conversion between them is reversible with temperature. As has been explained, the structures of both polymorphs are very similar, and the difference between them lies in the distortion observed in the acetate groups and the Pb atoms in the low-temperature phase, α-Pb(Ac)2, with respect to β-Pb(Ac)2. Thus, the transition can be explained as an ordering in the latter phase due to thermal agitation of the atoms, which makes them be in average positions to form a simpler structure with higher

Figure 4. Crystal structure of Pb4O(Ac)6: (a) asymmetric unit (atoms generated by symmetry are drawn semitransparent); (b) scheme of the corrugated layers showing the unit cell; (c) representation along the b axis, showing the stacking of layers and the large voids between them.

two Pb atoms (coordinated by seven O atoms), two acetates (μ5-bridging−chelating), one μ4-oxo ligand [in a tetrahedral, like in Pb2O(Ac)2], and one μ3-acetate group (disordered in two positions, with one O atom as a pivot, O5) in the asymmetric unit. The occupancy for all of the atoms is 1, except for the O atom of the oxo group, which is in a special position (occupancy 0.5). The d spacing between these layers is 11.76 Å. This value was given erroneously for anhydrous Pb(Ac)2 in ref 42. The corrugated layers of Pb4O(Ac) 6 (represented in Figure 4b) can be swelled and filled with water molecules, and the structure of Pb4O(Ac)6·xH2O is obtained (Figure 5). The molecular units are the same as those for Pb4O(Ac)6, but the difference lies in the separation and different stacking of the layers, which creates a gallery of connected pores where water F


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Inorganic Chemistry

Pb(Ac)2 has been studied in the last years,13,14,20 a thorough analysis has revealed large discrepancies with respect to what was already published, like the solid-to-solid transition found or the instability of Pb(Ac)2 when melted. In spite of the apparent ease of preparation from Pb(Ac)2·3H2O, dehydration through temperature or the use of a slightly oxygenated atmosphere may produce byproducts. Although several decomposition mechanisms of Pb(Ac)2 and many thermal studies about it have been reported,10−14 there is a lack of reliable data about its thermal behavior. Even in this study, using three complementary techniques (DSC, TGA, and XRD), almost pure Pb(Ac)2 showed a small amount of impurities (less than 1%), identified as Pb4O(Ac)6. Anhydrous Pb(Ac)2 [obtained by freeze-drying of Pb(Ac)2· 3H2O] presents a solid-to-solid transition (polymorph α to β) at 335.1 K with a small enthalpy, and a fusion, immediately after which it starts to decompose, as was observed when consecutive thermograms were registered (Figure 7). A small

symmetry. As will be explained in section 3.2, the enthalpy of the transition is relatively small, as expected from the small changes between one phase and the other. A diffraction experiment was carried out to study the solid− solid transition, starting from the hemihydrated compound. This compound (Figure 6, 1) was heated until dehydration, and

Figure 6. Diffraction patterns of Pb(Ac)2·1/2H2O (1, at room temperature), β-Pb(Ac)2 (2, heating after dehydration), and αPb(Ac)2 (3, cooling to 285 K). These three compounds show the presence of Pb4O(Ac)6 (4, shown for comparison) as an initial impurity (∼0.5%, calculated by Rietveld refinement).

β-Pb(Ac)2 was obtained (Figure 6, 2). α-Pb(Ac)2 was later obtained by cooling to 285 K (Figure 6, 3). On the other hand, a single crystal of α-Pb(Ac)2 was heated until β-Pb(Ac)2 was formed and measured by SCXRD. This proved not only the reversibility of the transition but also the small differences between both polymorphs because the single crystal remained from one phase to the other. Another important fact is that β-Pb(Ac)2 presents a cell and molecular arrangement similar to those of the other short members of the lead(II) alkanoate series (from propionate to heptanoate),39,41,42 and it can be considered isostructural with them. These compounds present a bilayered structure, forming a 2D coordination polymer in each case. The cell parameters of short lead(II) alkanoates show very similar values for the a and b axes, slight variations in the angles, and a logical increase in the c axis (due to the different lengths of the alkyl chain). For all cases, there exists a very compact packing in the alkyl chains, according the values of the cross-sectional area (S′).68−70 This can be calculated in bilayered compounds from the area per polar head (S, which is calculated by multiplying a and b and dividing by the number of Pb atoms, 2), and the torsion (tilt) of the plane of the chains with respect to the ionic plane (α), with the following equation: S′ = S cos(α). The values of the torsion angles for the alkyl chains are almost negligible [or nonexistent, 0°, for Pb(Ac)2], in each case. Thus, the values of S and S′ are practically the same in every case and show very little increase from β-Pb(Ac)2 (17.24 Å2) to Pb(C7)2 (17.74 Å2) (Table S1). These values of S′ indicate the dense packing of the chains, showing that there is no possibility for them to present gauche defects in the crystal phase (from propionate on). 3.2. Thermal Behavior and Decomposition Mechanism. DSC. Although the thermal behavior of anhydrous

Figure 7. Consecutive DSC thermograms of Pb(Ac)2 [obtained by freeze-drying of Pb(Ac)2·3H2O] in the indicated heatings, at 10 K· min−1. The transition temperature of the solid-to-solid transition and fusion and the presence of the initial impurity (in less than 1%) are indicated in the plot (*).

peak between these two transitions was detected in the first heating, which could represent another process of anhydrous Pb(Ac)2, but this hypothesis is ruled out because the peak remains and grows when the compound clearly decomposes. The final thermogram (5th, in Figure 7) remains constant in subsequent heatings and presents values similar to those reported in ref 20, which correspond to a decomposition byproduct, Pb4O(Ac)6, and not to real anhydrous Pb(Ac)2, as the XRD and DSC data revealed. On the other hand, Pb4O(Ac)6 presents a fusion in the first heating, and a glass phase is obtained by quenching from the melt, which shows a glass transition, a subsequent crystallization, and again the fusion (Figure S5). The thermal data of anhydrous Pb(Ac)2 and Pb4O(Ac)6 are given in Table 3. Decomposition of Lead(II) Acetate. TGA Experiments. The decomposition of lead(II) acetate has been studied for the last 50 years by many authors,10−14 with the aim of studying the intermediate compounds formed in each stage and the final residues. These intermediates, called basic lead acetates, were found to be composed of stoichiometric amounts of lead(II) G


Article

Inorganic Chemistry Table 3. Thermal Data of the Phase Transitions for Anhydrous Lead(II) Acetate Pb(Cn)2

transition solid-to-solid (α to β) fusion fusion Tgb

Pb(Ac)2a Pb4O(Ac)6 a

T/K 335.1 480.3 466.9 327.7

± ± ± ±

0.3 0.3 0.2 0.3

ΔtrsH/kJ·mol−1

ΔtrsS/J·K−1·mol−1

1.6 ± 0.2 12.9 ± 0.2 65.0 ± 0.3

4.8 ± 0.6 26.9 ± 0.4 139.2 ± 0.7

Impurities present in the sample: ∼0.5%. bMeasureat at 5 K·min−1.

Pb3O2(Ac)2 (obsd, 32.37%; calcd, 32.19%), between 555 and 600 K. According to the literature, the volatile products formed are acetone and CO 2.13,14 DSC data show the same temperature for decomposition of anhydrous Pb(Ac)2 immediately after melting (II). Finally, Pb3O2(Ac)2 decomposes between 600 and 660 K (IV), and the final product detected in the last decomposition process is metallic lead (in agreement with Manabe et al. in a dinitrogen atmosphere),11 instead of PbO, which was the main residue found in those mentioned studies.13,14 The observed value of the final weight loss of Pb(Ac)2·3H2O was 45.20%, which matches exactly with the calculated one for lead (45.37%) and differs for PbO (41.16%). Moreover, a DSC thermogram in the residue proved the presence of metallic lead, showing its melting point (600.6 K). A similar behavior was found for lead(II) acetate hemihydrate and for anhydrous Pb(Ac)2, with the corresponding water loss or absence of it, respectively. In the case of Pb(Ac)2·1/2H2O, the water loss ends completely at 360 K. Their TGA thermograms are shown in Figure S3. Decomposition of Pb(Ac)2·3H2O was also carried out in air (Figure 8b). In this case, only three steps are observed. The weight losses match with the dehydration (I), formation of Pb2O(Ac)2 (II), and decomposition to PbO (III). The scheme of this process is as follows:

acetate, lead(II) oxide, and water (in some cases). However, there are big differences in the behavior of lead(II) acetate, depending on the atmosphere: inert, with dinitrogen, or oxygenated. TGA was carried out in a dinitrogen atmosphere for Pb(Ac)2·3H2O (Figure 8a). According to the values of weight

I

II

III

2Pb(Ac)2 ·3H 2O → 2Pb(Ac)2 → Pb2 O(Ac)2 → 2PbO

It is worth noting that no direct evidence of the formation of the byproduct Pb4O(Ac)6 was found in these TGA experiments, in agreement with the literature.11−14 However, this compound has clearly been observed, upon heating of the compound just above the melting point, by XRD and DSC. Moreover, there are indirect signs of the previous detection of the existence of Pb4O(Ac)6 in past studies. Thus, the diffraction peaks of anhydrous lead(II) acetate (dehydrated at 313 K in a vacuum desiccator) reported by Kwestroo et al.11 correspond, in fact, to the sum of the peaks of Pb(Ac)2 and Pb4O(Ac)6. In that case, the sum of the diffraction patterns (from both species) has been demonstrated, with partial decomposition carried out in air at 473 K for 2 h (Figure S2). Although the vacuum was used in the three methods used in this work (see section 2.1), the main difference lies in the working temperature: 298 K (for methods a and b) and 223 K (for method c). Pure Pb4O(Ac)6 can be isolated by methods a and b (employed for the HRPD analysis), while almost pure anhydrous Pb(Ac)2 is formed by method c [although some traces of Pb4O(Ac)6 were detected by powder XRD and DSC]. This points out that higher temperatures favor partial decomposition of lead(II) acetate trihydrate involving the formation of acetic acid and water, as happens in the decomposition of silver and copper(II) acetates.71 Finally, decomposition of Pb4O(Ac)6 in an inert atmosphere was also analyzed, showing the formation of Pb2O(Ac)2 and Pb3O2(Ac)2 in two steps. Pb4O(Ac)6·xH2O shows the

Figure 8. TGA thermograms of Pb(Ac)2·3H2O in a dinitrogen atmosphere (a) and in air (b), showing the different steps of decomposition and the observed and calculated weight losses (in black and red, respectively).

loss and the previous studies, the decomposition process can be summarized with the next scheme (showing the solid residues): I

II

6[Pb(Ac)2 · 3H 2O] → 6Pb(Ac)2 → 3[Pb2 O(Ac)2 ] III

IV

→ 2[Pb3O2 (Ac)2 ] → 6Pb

The first step (I), up to approximately 450 K, corresponds to the loss of water molecules (obsd, 14.16%; calcd, 14.25%). In processes II and III, where the partial decompositions of the acetate ligand take place, two basic salts form as solid intermediates: first, Pb 2 O(Ac) 2 (obsd, 27.54%; calcd, 27.70%), between 500 and 550 K, and, subsequently, H


Article

Inorganic Chemistry

Stokes shifts, typical of MC excited states with a high degree of geometrical distortion compared with the corresponding ground states.73−76 It is worth noting that, apart for a spectral broadening in the emission band of Pb4O(Ac)6 compared with Pb(Ac)2·1/2H2O, the two compounds luminesce in the same energy range. More interestingly, phosphorescence is found for these compounds to occur at room temperature, similar to that reported for PbO, which emits around 510 nm with a lifetime of 539 ms.72,77 The lifetimes of the MC excited states remain generally long, in the range of hundreds of microseconds, to demonstrate the nature of the spin-forbidden emission transitions. This trend differs from what was observed by the authors in a series of lead(II) alkanoates, which display phosphorescence only from frozen glass phases.38 The phenomenon can well be a consequence of an enhanced structural rigidity in these mixed oxides/acetates, which was lacking in systems only featuring purely acetate chains that may introduce additional deactivation pathways to the excited states, facilitated by vibrational motions in their “soft” lattices. Surprisingly, a short-lived emission is visible for Pb4O(Ac)6 at 435 nm, recalling that of other lead(II) clusters and coordination polymers whereby charge-transfer excited states of oxygen-bridged lead ions prevail over the isolated MC centers that show phosphorescence.57,72,73

formation of the same byproducts after the loss of the water molecules (Figure S4). 3.3. Photoluminescence (PL) Properties. The photophysical properties of Pb(Ac)2·1/2H2O and Pb4O(Ac)6 have been investigated and are reported here (Figure 9). The two

4. CONCLUSIONS The intricate thermal behavior and structures of lead(II) acetate, its hydrates, and decomposition byproducts have been fully unraveled for the first time. The anhydrous salt, obtained by freeze-drying lead(II) acetate trihydrate, presents enantiotropic polymorphism. The high-temperature polymorph (β) is isostructural with longer lead(II) alkanoates. Different from those, lead(II) acetate is hygroscopic and yields the hemihydrated salt, stable at normal conditions and observed in this work for the first time. Some other basic lead(II) acetates, which are stoichiometric mixtures of lead(II) oxide and lead(II) acetate, are formed as byproducts in the decomposition process and have been also analyzed and solved. The structures of all of these compounds have been solved. They present a different variety of dimensionalities, from 1D to planar or corrugated 2D coordination polymers. The PbII atom appears to be more exposed in the cases of anhydrous Pb(Ac)2 and Pb2O(Ac)2, explaining also their ease of hydration. On the other hand, Pb(Ac) 2 · 1 / 2 H 2 O and Pb 4 O(Ac) 6 are less susceptible to humidity and are stable over time in normal conditions, like the well-known lead(II) trihydrate. Finally, the structural properties of these compounds are the base of the observed room temperature phosphorescence for Pb(Ac)2·1/2H2O and Pb4O(Ac)6, which is affected by the rigidity imposed from the formation of Pb−O clusters in the lattice. This effect shows the vaste potential that lead(II) metal−organic compounds have as luminescent materials. In particular, the possibility of controlling the light emission properties by imposing certain structural constraints is very attractive, and further studies are currently ongoing in order to develop simple synthetic routes to obtain tunable luminescence

Figure 9. DR-UV−vis (black solid lines), PL excitation (black dashed lines), and PL emission (red solid lines) spectra of (a) Pb(Ac)2·1/2H2O (λexc = 300 nm for the emission spectra, while λem = 470 nm for the excitation spectra) and (b) Pb4O(Ac)6 (λexc = 334 nm for the emission spectra, while λem = 455 nm for the excitation spectra).

compounds display some common features, and the optical data are summarized in Table 4. DR-UV−vis spectra are characterized by the presence of bands in the UV region, between 200 and 300 nm, where 6s6p ← 6s2 electronic transitions occur.58,72 The two acetates also show broad emission bands in the 400−600 nm visible range with large Table 4. Photophysical Properties of Pb(Ac)2·1/2H2O and Pb4O(Ac)6a compound Pb(Ac)2· /2 H2O 1

absorption/ nm

excitation/ nm

emission/ nm

lifetime

256

288

428

147 μs (83%)*

410 448 563

1428 μs (17%)* 1.04 ns (67%)§ 4.71 ns (33%)§ 290 μs*

360 Pb4O(Ac)6

a

262

300 334

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.6b01116.

Lifetimes recorded at (*) 550 nm and (§) 435 nm. I


Article

Inorganic Chemistry

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(11) Sun, X.; Yang, J.; Zhang, W.; Zhu, X.; Hu, Y.; Yang, D.; Yuan, X.; Yu, W.; Dong, J.; Wang, H.; Li, L.; Vasant Kumar, R.; Liang, S. J. Power Sources 2014, 269, 565−576. (12) Kwestroo, W.; Langereis, C. J. Inorg. Nucl. Chem. 1965, 27, 2533−2536. (13) Manabe, K.; Kubo, T. Kogyo Kagaku Zasshi 1966, 69, 1733. (14) Leibold, R.; Huber, F. J. Therm. Anal. 1980, 18, 493−500. (15) Mohamed, M. A.; Halawy, S. A.; Ebrahim, M. M. Thermochim. Acta 1994, 236, 249−262. (16) Jackson, R. F. Bur. Stand., Bull. 1915, 11, 331−345. (17) Rajaram, R. K.; Rao, J. K. M. Curr. Sci. 1975, 44, 184−185. (18) Rajaram, R. K.; Rao, J. K. M. Z. Kristallogr. 1982, 160, 225−233. (19) Harrison, P. G.; Steel, A. T. J. Organomet. Chem. 1982, 239, 105−113. (20) Bryant, R. G.; Chacko, V. P.; Etter, M. C. Inorg. Chem. 1984, 23, 3580−3584. (21) Martínez-Casado, F. J.; Sánchez Arenas, A.; García Pérez, M. V.; Redondo Yélamos, M. I.; López de Andrés, S.; Cheda, J. A. R. J. Chem. Thermodyn. 2007, 39, 455−461. (22) Franzosini, P.; Sanesi, M. Thermodynamic and transport properties of organic salts; Pergamon Press: Oxford, U.K., 1980. (23) Ramos Riesco, M.; Martínez-Casado, F. J.; López-Andrés, S.; García Pérez, M. V.; Yélamos, M. I. R.; Torres, M. R.; Garrido, L.; Cheda, J. A. R Cryst. Growth Des. 2008, 8, 2547−2554. (24) Ramos-Riesco, M.; Martínez-Casado, F. J.; Rodríguez-Cheda, J. A.; Redondo-Yélamos, M. I.; Fernández-Martínez, A.; López-Andrés, S. Cryst. Growth Des. 2015, 15, 497−509. (25) Ramos-Riesco, M.; Martínez-Casado, F. J.; Rodríguez-Cheda, J. A.; Redondo-Yélamos, M. I.; da Silva, I.; Plivelic, T.; López-Andrés, S.; Ferloni, P. Cryst. Growth Des. 2015, 15, 2005−2016. (26) Giroud-Godquin, A. M. Handbook of Liquid Crystals; Demus, D., Goodby, J., Gray, G. W., Spiess, K. W., Vill, V., Eds.; Wiley-VCH: Weinheim, Germany, 1998; Vol. 2B, Chapter XIV, p 901. (27) Polishchuk, A. P.; Timofeeva, T. V. Russ. Chem. Rev. 1993, 62, 291−321. (28) Mirnaya, T. A.; Prisyazhnyi, V. D.; Shcherbakov, V. A. Russ. Chem. Rev. 1989, 58, 821−834. (29) Binnemans, K. Chem. Rev. 2005, 105, 4148−4204. (30) Martínez-Casado, F. J.; Ramos Riesco, M.; Cheda, J. A. R. J. Therm. Anal. Calorim. 2007, 87 (1), 73−77. (31) Bazuin, C. G.; Guillon, D.; Skoulios, A.; Amorim da Costa, A. M.; Burrows, H. D.; Geraldes, C. F. G. C.; Teixeira-Dias, J. J. C.; Blackmore, E.; Tiddy, G. J. T. Liq. Cryst. 1988, 3 (12), 1655−1657. (32) Schwede, J.; Koehler, L.; Grossmann, H. P.; Pietralla; Burrows, H. D. Liq. Cryst. 1994, 16 (2), 267−276. (33) Ellis, H. A.; de Vries, J. W. C. Mol. Cryst. Liq. Cryst. 1988, 163, 133−138. (34) Amorim da Costa, A. M.; Burrows, H. D.; Geraldes, C. F. C. G.; Teixeira-Dias, J. J. C.; Bazuin, C. G.; Guillon, D.; Skoulios, A.; Blackmore, E.; Tiddy, G. J. T.; Turner, D. L. Liq. Cryst. 1986, 1 (3), 215−226. (35) Sánchez Arenas, A.; García, M. V.; Redondo, M. I.; Cheda, J. A. R.; Roux, M. V.; Turrión, C. Liq. Cryst. 1995, 18, 431−441. (36) Adeosun, S. A.; Sime, S. J. Thermochim. Acta 1978, 27, 319−327. (37) Ekwunife, M. E.; Nwachukwu, M. U.; Rinehart, F. P.; Sime, S. J. Chem. Soc., Faraday Trans. 1 1975, 71 (7), 1432−1446. (38) Martínez Casado, F. J.; García Pérez, M. V.; Redondo Yélamos, M. I.; Rodrı ́guez Cheda, J. A.; Sánchez Arenas, A.; López-Andrés, S.; García-Barriocanal, J.; Rivera, A.; León, C.; Santamaría, J. J. Phys. Chem. C 2007, 111 (18), 6826−6831. (39) Martínez Casado, F. J.; Ramos Riesco, M.; Sánchez Arenas, A.; García Pérez, M. V.; Redondo, M. I.; López-Andrés, S.; Garrido, L.; Cheda, J. A. R. J. Phys. Chem. B 2008, 112 (51), 16601−16609. (40) Martínez Casado, F. J.; Ramos Riesco, M.; Rodríguez Cheda, J. A.; Cucinotta, F.; Fernández-Martínez, A.; Garrido, L.; Matesanz, E.; Marchese, L. J. Mater. Chem. C 2014, 2 (44), 9489−9496. (41) Martínez Casado, F. J.; Ramos Riesco, M.; Redondo Yélamos, M. I.; Sánchez Arenas, A.; Rodrı ́guez Cheda, J. A. J. Therm. Anal. Calorim. 2012, 108 (2), 399−413.

HRPD and Rietveld refinements (Figure S1), partial decomposition of lead(II) acetate in air (powder XRD) (Figure S2), values of the area per polar head in the short lead(II) alkanoates, from acetate to heptanoate (Table S1), TGA thermograms of anhydrous Pb(Ac)2, Pb(Ac)2·1/2H2O, Pb4O(Ac)6, and Pb4O(Ac)6·xH2O in an inert atmosphere (Figures S3 and S4), and DSC thermogram (first and second heatings) of Pb4O(Ac)6 (Figure S5) (PDF) CIF file of α-Pb(Ac)2 (CCDC 1473436, at 296 K), βPb(Ac)2 (CCDC 1473437, at 348 K), Pb(Ac)2·1/2H2O (CCDC 1473438, at 100 K), Pb2O(Ac)2 (CCDC 1473439, at 296 K), Pb4O(Ac)6 (CCDC 1473440, at 100 K), and Pb4O(Ac)6·xH2O (CCDC 1473441, at 100 K) solved by SCXRD (CIF) CIF file of Pb(Ac)2·1/2H2O (CCDC 1473456, at 298 K), β-Pb(Ac)2 (CCDC 1473467, at 410 K), α-Pb(Ac)2 (CCDC 1473470, at 285 K), and Pb4O(Ac)6 (CCDC 1473471, at 298 K) solved by HRPD (CIF)

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Author Contributions

The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS 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. F.J.M.C. acknowledges a JAE-DOC postdoctoral contract (by CSIC). F.C. acknowledges Regione Piemonte for a postdoctoral fellowship. The authors thank beamlines I711 and I911-3 (Max II ring, MAX IV Laboratory, Lund, Sweden) and BM16LLS (ESRF, Grenoble, France) and the Centers of Scientific Instrumentation at the University Complutense of Madrid (CAI of XRD, Centro de Asistencia a la Investigación) and at the University of Granada, and their staff, for the use of their technical facilities and help.

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