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)...

...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 f-element electronic structure and bonding in EuIII and TbIII complexes through sensitization studies of EuIII luminescence(18) 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.

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.

Figure 1. Photographs of single crystals of (a) 1, (b) 2, and (c) 3.

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.

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.

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

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

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., ?–?*) of the PDAHx ligands.

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

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 ?–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.

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.