Spontaneous Partitioning of Californium from Curium: Curious Cases from the Crystallization of Curium Coordination Complexes

The reaction of ²⁴⁸CmCl₃ with excess 2,6-pyridinedicarboxylic acid (DPA) under mild solvothermal conditions results in crystallization of the tris-chelate complex Cm­(HDPA)₃·H₂O. Approximately half of the curium remains in solution at the end of this process, and evaporation of the mother liquor results in crystallization of the bis-chelate complex [Cm­(HDPA)­(H₂DPA)­(H₂O)₂Cl]­Cl·2H₂O. ²⁴⁸Cm is the daughter of the α decay of ²⁵²Cf and is extracted in high purity from this parent. However, trace amounts of ^249,250,251Cf are still present in all samples of ²⁴⁸Cm. During the crystallization of Cm­(HDPA)₃·H₂O and [Cm­(HDPA)­(H₂DPA)­(H₂O)₂Cl]­Cl·2H₂O, californium­(III) spontaneously separates itself from the curium complexes and is found doped within crystals of DPA in the form of Cf­(HDPA)₃. These results add to the growing body of evidence that the chemistry of californium is fundamentally different from that of earlier actinides

Monomers, Dimers, and Helices: Complexities of Cerium and Plutonium Phenanthrolinecarboxylates

The reaction of Ce^III or Pu^III with 1,10-phenanthroline-2,9-dicarboxylic acid (PDAH₂) results in the formation of new f-element coordination complexes. In the case of cerium, Ce­(PDA)­(H₂O)₂Cl·H₂O (<b>1</b>) or [Ce­(PDAH)­(PDA)]₂[Ce­(PDAH)­(PDA)] (<b>2</b>) was isolated depending on the Ce/ligand ratio in the reaction. The structure of <b>2</b> is composed of two distinct substructures that are constructed from the same monomer. This monomer is composed of a Ce^III cation bound by one PDA^2– 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 (<b>3</b>) was the only product isolated regardless of the Pu/ligand ratio employed in the reaction. During the reaction of plutonium with PDAH₂, Pu^III is oxidized to Pu^IV, generating <b>3</b>. This assignment is consistent with structural metrics and the optical absorption spectrum. Ambiguity in the assignment of the oxidation state of cerium in <b>1</b> and <b>2</b> from UV–vis–near-IR spectra invoked the use of Ce L_3,2-edge X-ray absorption near-edge spectroscopy, magnetic susceptibility, and heat capacity measurements. These experiments support the assignment of Ce^III in both compounds. The bond distances and coordination numbers are also consistent with these assignments. <b>3</b> contains 8-coordinate Pu^IV, whereas the cerium centers in <b>1</b> and <b>2</b> are 9- and/or 10-coordinate, which correlates with the increased size of Ce^III versus Pu^IV. 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