Monomers, Dimers, and Helices: Complexities of Cerium and

Apr 12, 2016 - This monomer comprises a CeIII cation bound by one PDA2− anion and one PDAH1− ligand. Bridging by the carboxylates leads to both di...
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Monomers, Dimers, and Helices: Complexities of Cerium and Plutonium Phenanthrolinecarboxylates Samantha K. Cary,† Maryline G. Ferrier,‡ Ryan E. Baumbach,§ Mark A. Silver,† Juan Lezama Pacheco,⊥ Stosh A. Kozimor,‡ Henry S. La Pierre,‡ Benjamin W. Stein,‡ Alexandra A. Arico,† Danielle L. Gray,∥ and Thomas E. Albrecht-Schmitt*,† †

Department of Chemistry and Biochemistry, Florida State University, 95 Chieftan Way, Tallahassee, Florida 32306, United States Chemistry Divisions, Los Alamos National Laboratory, Los Alamos, New Mexico 87545, United States § National High Magnetic Field Laboratory, Tallahassee, Florida 32310, United States ∥ Department of Chemistry, George L. Clark X-ray Facility & 3M Materials Laboratory, University of Illinois, 505 South Mathews Avenue, Urbana, Illinois 61801, United States ⊥ School of Earth, Energy, and Environmental Science, Stanford University, Stanford, California 94305, United States ‡

S Supporting Information *

ABSTRACT: The reaction of CeIII or PuIII with 1,10-phenanthroline-2,9dicarboxylic acid (PDAH2) results in the formation of new f-element coordination complexes. In the case of cerium, Ce(PDA)(H2O)2Cl·H2O (1) or [Ce(PDAH)(PDA)]2[Ce(PDAH)(PDA)] (2) was isolated depending on the Ce/ligand ratio in the reaction. The structure of 2 is composed of two distinct substructures that are constructed from the same monomer. This monomer is composed of a CeIII cation bound by one PDA2− dianionic ligand and one PDAH− monoanionic ligand, both of which are tetradentate. Bridging by the carboxylate moieties leads to either [Ce(PDAH)(PDA)]2 dimers or [Ce(PDAH)(PDA)]1∞ helical chains. For plutonium, Pu(PDA)2 (3) was the only product isolated regardless of the Pu/ligand ratio employed in the reaction. During the reaction of plutonium with PDAH2, PuIII is oxidized to PuIV, generating 3. This assignment is consistent with structural metrics and the optical absorption spectrum. Ambiguity in the assignment of the oxidation state of cerium in 1 and 2 from UV−vis−near-IR spectra invoked the use of Ce L3,2-edge X-ray absorption near-edge spectroscopy, magnetic susceptibility, and heat capacity measurements. These experiments support the assignment of CeIII in both compounds. The bond distances and coordination numbers are also consistent with these assignments. 3 contains 8-coordinate PuIV, whereas the cerium centers in 1 and 2 are 9- and/or 10-coordinate, which correlates with the increased size of CeIII versus PuIV. Taken together, these data provide an example of a system where the differences in the redox behavior between these f elements creates more complex chemistry with cerium than with plutonium.

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INTRODUCTION Cerium provides a useful, nonradioactive analogue of plutonium owing to their similar ionic radii when in the 4+ oxidation state, and as a result, several families of coordination complexes and materials form isomorphous series.1−4 Examples of this include a variety of phosphonates such as M[C6H4(PO3H)(PO3H2)][C6H4(PO3H)(PO3)]3·2H2O (M = Ce, Pu),5,6 the cationic framework tellurites, [M2Te4O11]Cl2 (M = Ce, Pu),7 and a large collection of sulfates.8 However, there are notable deviations in the oxidation state, reactivity, and coordination chemistry between cerium and plutonium, as documented in the PuIV maltol complex, Pu(C6H5O3)4,9 the mixed-valent molybdate, CsPu3Mo6O24(H2O),10 and the hydroxypyridonate, Pu{5LIO(Me-3,2-HOPO)}2.2 In Pu(C6H5O3)4 and Pu{5LIO(Me-3,2-HOPO)}2, the differences are subtle and lie in divergence in the point symmetry of the local coordination environments without a change in the coordination number.2,9 For CsPu3Mo6O24(H2O), similar © XXXX American Chemical Society

reactions with cerium yield Ce3Mo6O24(H2O)2, which does have a 3D framework similar to that found for the plutonium compound, but CsPu3Mo6O24(H2O) is a black-colored, semiconductor containing mixed-valent molybdenum, whereas Ce3Mo6O24(H2O)2 is a straightforward, electron-precise insulator.10 In many cases, these variations are likely related to the differences in the reduction potentials between the strongly oxidizing CeIV versus the less reactive PuIV. This is evidenced by their standard reduction potentials differing by ∼600 mV, albeit these values will significantly vary with different complexants.11−13 Additional insight into CeIV and PuIV reactivity differences has been provided by previous ligand K-edge X-ray absorption near-edge spectroscopy (XANES) measurements that suggested that there are changes in the Received: January 15, 2016

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DOI: 10.1021/acs.inorgchem.6b00077 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry

temperature at a rate of 5 °C/h. The reaction resulted in yellow octahedral crystals (Figure 1a) with a yield of 41%.

involvement of the 4f and 5f orbitals in bonding in CeIV versus PuIV, respectively.14,15 To further understand the convergence and divergence in the reaction chemistry between cerium and plutonium complexes and better characterize the viability of using CeIV as a nonreactive analogue of PuIV, the mixed N- and O-donor 1,10-phenanthroline-2,9-dicarboxylic acid (PDA) was chosen as a complexant. The tetradentate PDA ligand is exceptionally suited for comparative studies with f elements. For example, many lanthanide- and actinide-containing PDA complexes have been prepared that demonstrate the ability of PDA to provide a suitable coordination environment for large, trivalent, oxophilic ions.16−22 PDA has also provided a platform to interrogate felement electronic structure and bonding in EuIII and TbIII complexes through sensitization studies of EuIII luminescence18 and to evaluate the differences in the thermodynamics of complexation with the early actinides ThIV,19 UVI,19 and NpV.20 This ligand is additionally attractive given that it, as well as its derivatives, are being investigated for use in the separation of americium and curium from lanthanides in advanced nuclear fuel cycles.16,17 Herein, we report on the synthesis and characterization of cerium and plutonium PDA complexes that further illustrate the subtle differences that drive the divergent chemistries of cerium and plutonium.

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Figure 1. Photographs of single crystals of (a) 1, (b) 2, and (c) 3. [Ce(PDAH)(PDA)]2[Ce(PDAH)(PDA)] (2). CeCl3·7H2O (0.0303 mmol, 0.0113 g), PDAH2·H2O (0.0674 mmol, 0.0193 g), and water (200 μL) were added to a 10 mL PTFE liner. The liner was then sealed into a Parr 4749 autoclave and heated to 180 °C for 1 day. The reaction was allowed to cool to room temperature at a rate of 5 °C/h. The reaction resulted in bright-orange octahedral crystals (Figure 1b) with a yield of 47%. Pu(PDA)2 (3). PuO2 (0.0173 mmol, 4.7 mg) and concentrated HBr (50 μL) were added to a 10 mL PTFE liner. The open liner was placed inside a box furnace preheated to 130 °C and dried to a dark-purple residue. PDAH2·H2O (0.0436 mmol, 0.0117 g) and water (200 μL) were added to the 10 mL PTFE liner and then sealed into a Parr 4749 autoclave. The reaction was heated to 180 °C for 1 day and then allowed to cool at a rate of 5 °C/h, yielding golden crystals with blocklike habit (Figure 1c). This reaction was run in an inertatmosphere glovebox. Crystallographic Studies. Single crystals of each compound were mounted on Mitogen mounts with immersion oil and optically aligned on a Bruker D8 Quest X-ray diffractometer using a built-in digital camera. Initial intensity measurements were performed using an IμS Xray source, a 50 W microfocused sealed tube (Mo Kα, λ = 0.71073 Å) with high-brilliance and high-performance focusing Quest multilayer optics. Standard APEXII software was used for determination of the unit cells and data collection optimization. The intensities of the reflections of a sphere were collected by a combination of four sets of exposures (frames). Each set had a different φ angle for the crystal, and each exposure covered a range of 0.5° in ω. A total of 1464 frames were collected with an exposure time per frame of 30−100 s, depending on the crystal. SAINT software was used for data integration including Lorentz and polarization corrections. PLATON was used to check each structure for missed symmetry and other issues.23 CIFs are available from the Cambridge Crystallographic Data Centre (CCDC) and are given in the Supporting Information. UV−Vis−Near-IR (NIR) Spectroscopy. UV−vis−NIR data were collected for each compound using a Craic Technologies microspectrophotometer. Single crystals were placed on quartz slides under immersion oil, and the data were collected from 350 to 1350 nm. XANES. The cerium compounds were prepared for XANES analysis on the benchtop and analyzed at the Stanford Synchrotron Radiation Lightsource (SSRL) on beamline 11-2. The samples were diluted with boron nitride (BN), which was obtained commercially (Fisher) and used as received. A mixture of the analyte and BN was weighed out, such that the edge jump for the absorbing atom was calculated to be at 1 absorption length in transmission (between 5 and 20 mg of the sample and ∼30 mg of BN). Samples were loaded into a slotted aluminum plate that was equipped with polypropylene windows (4 μm thick). The samples were placed in a polycarbonate box, through which helium gas was continuously flowed. The box was separated from the beam pipe by a continuously flowing helium flight path sealed with Kapton windows (1 mil thick). The measurements were made using three ionization chambers, through which nitrogen gas was continually flowed. One ionization chamber was positioned before the polycarbonate box, to monitor the incident radiation (I0). The second chamber was positioned after the polycarbonate box, such that the sample transmission (I1) could be evaluated against I0, while a third

EXPERIMENTAL SECTION

Synthesis. CeCl3·7H2O (99.5%, Alfa-Aesar), 1,10-phenanthroline2,9-dicarboxylic acid hydrate (98%, Alfa-Aesar), HBr (48%, SigmaAldrich), and deionized water were all used without further purification. Plutonium (94% 239Pu, 6% 240Pu; Material Type 52) in the form of PuO2 was used as received from Los Alamos National Laboratory. Reactions were run in poly(tetrafluoroethylene) (PTFE)lined Parr 4749 autoclaves with a 10 mL internal volume. Caution! 239 Pu (t1/2 = 24065 years) and 240Pu (t1/2 = 6537 years) represent serious health risks, owing to their α emission as well as the radiotoxicity associated with their α-, β-, and γ-emitting daughters. All studies with plutonium were conducted in a laboratory dedicated to studies on transuranium elements. This laboratory is located in a nuclear science facility and is equipped with HEPA-f iltered hoods and gloveboxes. A series of instruments continually monitor radiation levels in the laboratory. All experiments were carried out with approved safety operating procedures. All f ree-f lowing plutonium solids are handled in negative pressure gloveboxes, and products are only examined when coated with either water or immersion oil. There are signif icant limitations in accurately determining yields with plutonium compounds because this requires drying, isolating, and weighing a solid, which poses certain risks, as well as manipulation dif f iculties given the small quantities employed in the reactions. PuBr3·nH2O was selected as the source of PuIII instead of anhydrous PuCl3 for three reasons. First, concentrated HBr(aq) is a strong reducing agent for accomplishing the complete reduction of higher oxidation states of plutonium to PuIII. The same product is achieved whether metallic plutonium or PuO2 is used as the initial plutonium source. The oxidation of bromide leads to the formation of Br2, which then leaves as a gas. It is a convenient route to a reactive PuIII starting material that can be dehydrated through the addition of trimethylsilyl bromide. Higher oxidation states of plutonium have not been detected in this starting material. Second, anhydrous PuCl3 is only produced at Los Alamos National Laboratory and is currently unavailable. Third, chloride has a tendency to stabilize PuIV, especially in the form of [PuCl6]2−, and is therefore not an ideal counterion in some cases. Single synthetic reactions are provided for the sake of brevity even though many different reaction ratios were explored. Ce(PDA)(H2O)2Cl·H2O (1). CeCl3·7H2O (0.0314 0244 mmol, 0.0117 0091 g), PDAH2·H2O (0.02459 mmol, 0.007067 g), and water (200 μL) were added to a PTFE liner with an internal volume of 10 mL. The liner was then sealed into a Parr 4749 autoclave and heated to 180 °C for 1 day. The reaction was allowed to cool to room B

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orange color that one normally associates with CeIV. However, we will show that these cerium compounds contain CeIII only. Structure Descriptions. Elucidation of the structures of these compounds via single-crystal X-ray diffraction reveals that both cerium compounds have significant complexity and contain polymeric features, whereas 3 is monomeric (Table 1). In 1, the cerium center is found within a 9-coordinate

chamber (I2) was positioned downstream from I1 so that the XANES of a chromium calibration foil could be measured in situ during the Xray absorption fine structure (XAFS) experiments against I1. The XANES spectra were measured under dedicated operating conditions (3.0 GeV, 5%, 500 mA) on end station 11-2. This beamline was equipped with a 26-pole, 2.0 T wiggler, utilized a liquid-nitrogencooled double-crystal Si[220] monochromator, and employed collimating and focusing mirrors. A single energy was selected from the white beam with a liquid-nitrogen-cooled double-crystal monochromator utilizing Si[220] (φ = 0) crystals. The crystals were run detuned by 30% at approximately 500 eV above the L2-absorption edge (6600 eV). Samples were calibrated to the energy of the first inflection point of a chromium calibration foil (5989 eV). All spectra were measured in transmission mode. Magnetic Susceptibility Measurements. Magnetization M(T,H) measurements were carried out for a collection of randomly aligned single crystals for temperatures T = 1.8−300 K under an applied magnetic field of H = 5 kOe using a Quantum Design VSM Magnetic Property Measurement System. The magnetic susceptibility χ is defined as the ratio M/H. The heat capacity measurements were performed for a pressed pellet composed of randomly aligned single crystals using a Quantum Design Physical Properties Measurement System for temperatures of 2 K < T < 100 K.

Table 1. Selected Crystallographic Information for Compounds 1−3 formula mass color and habit space group a (Å) b (Å) c (Å) α (deg) β (deg) γ (deg) V (Å3) Z T (K) λ (Å) maximum 2θ (deg) ρcalc (g/cm3) μ(Mo Kα) (cm−1) R(F)a Rw(Fo2)b

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RESULTS AND DISCUSSION Synthesis. The complexity of the redox chemistry of plutonium is unparalleled by any other element. This makes the oxidation state assignment challenging, particularly from visual coloration alone. There are, for instance, blue compounds containing PuIV,24,25 although this color is normally indicative of PuIII. Likewise, PuIV complexes yield a variety of colors, with red and green being most common.4−6 Mixtures of oxidation states are more common than not for plutonium in solution but quite rare in the solid state because crystallization is always under solubility control and may or may not reflect the dominant species in solution.26,27 Fortunately, the fingerprint spectra of intra-f transitions for plutonium in different oxidation states have been well established for decades, and identification of the formal charge from electronic absorption spectra is relatively straightforward, particularly in solids.4−6 The reaction of PuIII with PDA results in the formation of a solid with a golden color that is not clearly indicative of any particular oxidation state. However, both the absorption spectrum and structural data are consistent with PuIV (vide infra), and the compound has the straightforward formulation of 3. Atypical coloration in these and other systems with highly charged metal centers and noninnocent or polarizable ligands is often caused by metal-to-ligand (MLCT) or ligand-to-metal (LMCT) charge-transfer bands, where the band edges trail from the near-UV into visible wavelengths that induce a substantial alteration in visual color from what is typically perceived. Although the solution and solid-state chemistries of cerium are ostensibly simpler than those of plutonium, cerium’s ability to oxidize from CeIII to CeIV when bound by strong complexants creates significant hurdles in the oxidation state assignment. Unlike plutonium, these challenges are not diminished in the solid state, and mixed- and intermediatevalent cerium compounds are common.28−30 These issues are further compounded in two ways. First, the absorption spectra of the CeIII and CeIV compounds can be quite similar to broad and allowed 4f−5d transitions for CeIII in the near-UV, occurring in the same range as that of LMCT bands for CeIV. Second, MLCT in CeIII compounds can give rise to the same coloration as LMCT in CeIV compounds. This is exactly what occurs in the cerium PDA system, with 2 being the same deep-

a

1

2

3

489.78 yellow, octahedral P21/c 10.8880(10) 18.3035(17) 7.9458(7) 90 93.6093(2) 90 1580.4(2) 4 100(2) 0.71073 27.496 2.058 30.09 0.0226 0.0523

2020.63 orange, octahedral Pca21 34.693(3) 15.4852(13) 12.4208(10) 90 90 90 6672.7(10) 4 296(2) 0.71073 27.583 2.011 21.16 0.0475 0.1099

1542.93 golden, prism P1̅ 7.7651(7) 11.2347(10) 14.3242(13) 95.438(2) 99.372(2) 109.915(10) 1143.91(18) 1 100(2) 0.71073 27.451 2.240 29.49 0.0356 0.0691

R(F) = ∑||Fo|Fc||/∑|Fo|. bRw(Fo2) = [∑[w(Fo2 − Fc2)]/∑wFo4]1/2.

tricapped trigonal prism formed from a tetradentate PDA2− ligand, a chloride anion, and two water molecules (Figure 2). The ninth coordination site is created via bridging by two

Figure 2. View of a portion of the 1D chain in 1 formed via linking of the CeIII centers by carboxylate moieties. The local coordination environment around the cerium centers is formed via chelation by the tetradentate PDA2− anions, two water molecules, and a chloride anion. The ninth site is occupied by the oxygen atom from a carboxylate group of a crystallographically equivalent [Ce(PDA)(H2O)2Cl] structural building unit. C

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Inorganic Chemistry Table 2. Selected Bond Lengths (Å) for 1−3 structure (metal) 1 (Ce) O3 O4 O5 O2 O1 O5 N2 N1 Cl1

2.511(2) 2.528(2) 2.536(2) 2.556(2) 2.558(2) 2.621(2) 2.689(2) 2.693(2) 2.8717(8)

2 (Ce1) O2 O4 O5 O6 O8 N1 N2 N3 N4

2.596(8) 2.425(8) 2.670(8) 2.531(8) 2.556(8) 2.669(8) 2.604(8) 2.603(8) 2.610(8)

2 (Ce2) O10 O12 O14 O16 O21 N5 N6 N7 N8

2.506(7) 2.592(7) 2.588(7) 2.417(7) 2.587(8) 2.629(8) 2.646(8) 2.635(7) 2.614 (8)

2 (Ce3) O9 O18 O20 O22 O24 N9 N10 N11 N12

2.601(8) 2.561(7) 2.415(8) 2.500(7) 2.616(8) 2.636(8) 2.583(8) 2.654(8) 2.658(7)

3 (Pu) O3 O5 O7 O8 N4 N1 N2 N3

2.243(4) 2.259(4) 2.259(4) 2.252(3) 2.496(4) 2.497(4) 2.504(4) 2.507(4)

Figure 3. Depictions of the two distinct substructures in 2. (a) View of helical [Ce(PDAH)(PDA)]1∞ chains formed via the bridging of [Ce(PDAH)(PDA)] units by carboxylate groups. (b) Illustration of dimers created from [Ce(PDAH)(PDA)] monomers that are again linked by carboxylate moieties. The CeIII centers are 9-coordinate within the dimers and 10-coordinate within the chains.

for several reasons because the c axis is polar in this space group, which means that the origin with respect to z is arbitrarily defined and a center of inversion is absent. This is also the axis that the 21 screw propagates along, and a helical structure should exist. In fact, this is exactly what is observed, and the [Ce(PDAH)(PDA)]1∞ chains are helical. The structure refinement shows that it is an enantiomorphic twin with a Flack parameter of 0.41(2), suggesting a small enantiomeric excess. While differences exist between the Ce−O and Ce−N bond lengths found in 1 versus 2 (Table 2), these variations are, at best, only marginally statistically significant and are all consistent with CeIII.10 The minor variations are mostly likely induced by the different states of protonation of the PDA ligands in the two compounds that will certainly affect the observed Ce−O and Ce−N bond lengths. In 2, the Ce−O bond lengths for the helices average 2.541(8) Å, whereas the average Ce−O bond lengths for the dimer are 2.536(7) and 2.530(8) Å for Ce(2) and Ce(3), respectively. Unlike the cerium systems, the crystal structure of the plutonium complex is much straightforward and is formed via

carboxylate moieties on opposite edges of the [Ce(PDA)(H2O)2Cl] units. This edge-sharing creates 1D chains. The average Ce−O and Ce−N bond lengths are 2.552(2) and 2.691(2) Å, respectively. Selected bond lengths are provided in Table 2. These distances are consistent with that of CeIII and are approximately 0.1 Å longer than that expected for CeIV.13,28 There are three crystallographically unique cerium atoms in 2, as depicted in Figure 3. Two cerium atoms (Ce2 and Ce3) are linked together by bridging carboxylate anions, creating a dimer formed from two [Ce(PDAH)(PDA)] monomers. Chelation of the cerium centers by two PDAH− anions creates eight binding sites, with a ninth being created by the bridging carboxylate. The third cerium center, Ce1, is found within a helical chain that is constructed from the same monomer as that found in the dimer. As found in 1, carboxylate bridges are used to create the chains that extend along the c axis. Within this chain, there are two bridging sites around each cerium center and the coordination number expands from 9 in the dimers to 10 in the chains. In addition, 2 crystallizes in the orthorhombic space group Pca21. This space group is significant D

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Inorganic Chemistry the chelation of PuIV by two PDA2− anions. The intermolecular forces that govern the packing of these molecules are created by hydrogen bonds and π−π stacking of the PDA ligands. Chelation by two tetradentate PDA2− anions creates an 8coordinate environment that, while expected with these large, planar ligands, is not expected for either tri- or tetravalent plutonium in general (vide infra). The lower oxidation states of f elements are normally found within relatively isotropic coordination environments, but this is simply not possible with these large, rigid ligands. The ligands are approximately orthogonal to one another, creating a crosslike geometry, as shown in Figure 4. In this coordination geometry, the apparent

Figure 5. Absorption spectrum acquired from a single crystal of 3 showing characteristic f−f transitions indicative of PuIV.

Figure 4. View of the molecular structure 3 formed via the chelation of a PuIV cation by two tetradentate PDA2− anions, creating an 8coordinate, cross-shaped geometry where the PDA2− ligands are roughly orthogonal to each other. These molecule form short π···π contacts with an intermolecular distance of 3.285(4) Å.

Figure 6. Absorption spectrum acquired from a single crystal of 2. The intense transition at ca. 400 nm is a combination MLCT and IVCT transitions (i.e., π−π*) of the PDAHx ligands.

open space around the PuIV inner sphere suggests significant steric unsaturation. This is apparently tolerated with small PuIV ions but, as observed in 2, results in additional ligation with the larger CeIII ion. In the cerium scenario, the CeIII ion reaches steric saturation by allowing the carboxylate ligands to bridge, thereby filling the open spaces and expanding the CeIII versus PuIV coordination number.31 The average Pu−N and Pu−O bond distances, which are provided in Table 2, are 2.501(4) and 2.253(4) Å, respectively. Both of these distances are consistent with previously reported PuIV bond distances and are significantly shorter than those found in 1 and 2.2,5 UV−Vis−NIR Absorption Spectroscopy. The solid-state absorption spectra of single crystals of 1−3 were acquired using a microspectrophotometer. The characteristic Laporte-forbidden f → f transitions characteristic of PuIV are exhibited by 3.32 These include the 5F2 and 5I6 transitions near 1100 nm and the multiple J state contributions near 700 nm.32 Features consistent with PuIII are absent. A broad LMCT is also present that starts in the near-UV and continues to higher energies (Figure 5). The characteristic f → d transitions for CeIII are observed starting at approximately 400 nm for both 1 and 2. However, in 2 the transition starting at 400 nm is much broader than that observed for 1, as shown in Figures 6 and 7, respectively. In addition, there is a broad band centered near 1200 nm for 2. This feature was the cause of some concern given the possibility of oxidation state ambiguity with cerium, and there was some initial speculation that it might correspond to either an intravalent charge-transfer (IVCT) band for a mixed- or intermediate-valent CeIII,IV compound or perhaps the radical anion form of a partially oxidized form of PDA. However, the structural evidence, heat capacity, and magnetic susceptibility measurements are all inconsistent with this interpretation, and as we will see shortly, X-ray absorption spectroscopic measurements are also inconsistent with anything

Figure 7. Absorption spectrum measured from a single crystal of 1.

other than CeIII. This feature in the NIR most likely corresponds to low-energy charge-transfer transitions between different PDA units in the structure that are enhanced by the permanent dipole of the structure. In addition, in compounds with small pKa differences between various protonation states of bases, the protons are typically delocalized and low-energy charge-transfer features are common.33 XAFS Measurements. The background-subtracted and normalized X-ray absorption near edge spectroscopy (XANES) results are shown in Figure 8 from 1 and 2. The spectra were dominated by large edge features superimposed on a steplike absorption threshold. Superficially, from the perspective of a free ion, the edge features in these spectra originated from electric-dipole-allowed transitions from Ce 2p orbitals to unoccupied states that contain Ce 5d character; e.g., for CeIII, there would be 2p6...4f15d0 → 2p5...4f15d1 transitions. These final states were further split into two primary L3- and L2-edges, owing to spin−orbit coupling of the 2p5 core hole. The L3,2E

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Figure 10. Summary of the magnetic properties and heat capacity for 2 showing behavior that is consistent with that of an insulating trivalent cerium compound. (a) Inverse magnet susceptibility χ−1 = (M/H)−1 versus temperature T collected in a magnetic field H = 5 kOe. (b) Magnetization M versus H at T = 2 K. (c) Heat capacity divided by temperature C/T versus T. (d) C/T versus T2 for low T.

Figure 8. Comparison of Ce L3-edge XANES spectra from 1, 2, (PPh4)3CeCl6, and (NEt3)2CeCl6 (gold, crimson, cyan, and pink traces). Measurements made on the PDA compounds were obtained at room temperature, while the CeCl6x− spectra were collected at 100 K.

susceptibility χ−1 versus temperature T data for 1.8 K < T < 300 K in an applied magnetic field H = 5 kOe for 2. The Curie−Weiss behavior is observed over nearly the entire temperature range with no evidence for magnetic ordering. A fit to the data using the expression χ(T) = C/(T − Θ) yields an effective magnetic moment μeff = 2.5 μB/Ce and a Curie−Weiss temperature Θ = 1.2 K. This value for μeff is consistent with the cerium ions being in the trivalent state, with the calculated freeion moment for CeIII being 2.54 μB. The small but positive Θ possibly indicates weak ferromagnetic correlations, but we note that this is a small value and its sign is sensitive to the fit range. In Figure 10b, we show magnetization M versus H at T = 2 K. Typical Brillouin-like behavior is observed, with a saturation moment Msat = 1.5 μB/Ce, again showing that the cerium ions are localized in the 3+ valence state and that no magnetic ordering occurs down to this temperature. The heat capacity C divided by T versus T data are shown in Figure 10c, where typical phonon-dominated behavior is seen. A fit to the data at low temperatures (Figure 10d) using the expression C/T = γ + βT2 yields γ ≈ 0 and β = 15.1 mJ/mol·K4, giving a Debye temperature ΘD = 175 K, where there are 42 atoms per monomer.

edge peak positions and line shape in both spectra were similar to those of previously analyzed CeIII compounds. The edge positions were defined by pronounced absorption edges with inflection points of 5724.2 and 6165.3 eV for 1 and 5724.4 and 6165.2 eV for 2. The L3,2-edge peak intensity ratio was determined to be 0.58 for 1 and 0.63 for 2 using a graphical approach based on integration of the second-derivative spectrum, which was consistent with previous analyses from other compounds.14 This ratio was defined as A3/(A3 + A2), where A3 and A2 represent the total areas under the L3- and L2edge second derivatives. For comparison, previous analyses showed the inflection points for (PPh4)3CeCl6 to be 5723.1 eV (L3-edge) and 6164.1 eV (L2-edge) with an intensity ratio of 0.47(2), which is similar to the spectral metrics obtained from 1 and 2.14 Figure 9 highlights the similarities between 1, 2, and the trivalent (PPh4)3CeCl6. It also accentuates how different the spectra of 1 and 2 are from that of tetravalent (NEt4)2CeCl6, which clearly indicates that 1 and 2 were in the trivalent oxidation state. Magnetic Susceptibility and Heat Capacity Measurements. Shown in Figure 10a are the inverse magnetic

Figure 9. Ce L3,2-edge XANES spectra from 1 (gold trace) and 2 (crimson trace) obtained at room temperature. F

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CONCLUSIONS The challenges, both real and perceived, in working with radioactive elements like plutonium necessitate the use of less nonradioactive analogues for a variety of reasons. Among the acceptable reasons for conducting chemistry with analogues of radionuclides is actually to discern differences between elements that share common physicochemical features. Cerium and plutonium both possess stable 3+ oxidation states, and both can be oxidized to 4+. CeIV and PuIV have negligibly different ionic radii, and their structural chemistry can be quite similar (vide supra). Normally, if differences occur, the plutonium system is more complex because plutonium undergoes more facile redox chemistry than cerium and readily oxidizes to states well beyond 4+. While PuIII does oxidize to PuIV when complexed by PDA, this results in a simple, molecular bis-chelate, 3, where both the structure and oxidation state assignment are straightforward. In contrast, CeIII does not undergo oxidation under these conditions, but the large size of the CeIII ion relative to PuIV results in increased coordination numbers that yield complex dimeric and polymeric structures. In addition, MLCT in the cerium compounds creates intricacies that required both extensive spectroscopic and physical property interrogation to remove ambiguity in the oxidation state assignment. These results add to the growing body of knowledge that indicates that, while analogues of radionuclides are useful guides, they should not be treated as true surrogates, particularly with plutonium.

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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.6b00077. X-ray crystallographic file in CIF format, including photographs of the crystals (CIF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This research was supported by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences, Heavy Elements Chemistry Program, under Award DE-FG0213ER16414. Synchrotron studies were supported under the Heavy Element Chemistry Program at the LANL by the Division of Chemical Sciences, Geosciences, and Biosciences, Office of Basic Energy Sciences, U.S. Department of Energy (Kozimor, Lezama, Trujillo). We are grateful for postdoctoral and graduate fellowships from the Glenn T. Seaborg Institute (Ferrier, Stein) and the LANL Director’s Postdoctoral Fellowship (La Pierre). Los Alamos National Laboratory is operated by Los Alamos National Security, LLC, for the National Nuclear Security Administration of U.S. Department of Energy (Contract DE-AC52-06NA25396). The synchrotron studies were carried out at the SSRL, a Directorate of the SLAC National Accelerator Laboratory and an Office of Science User Facility operated for the U.S. Department of Energy Office of Science by Stanford University. G

DOI: 10.1021/acs.inorgchem.6b00077 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry (33) Cruz-Cabeza, A. J. CrystEngComm 2012, 14, 6362−6365.

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DOI: 10.1021/acs.inorgchem.6b00077 Inorg. Chem. XXXX, XXX, XXX−XXX