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 (PDAH(2)) results in the formation of new f-element coordination complexes. In the case of cerium, Ce(PDA)(H2O)(2)Cl center dot 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 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 infinity) 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 PDAH(2), Pu-III is oxidized to Pu-IV, 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 L-3,L-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. 3 contains 8-coordinate Pu-IV, whereas the cerium centers in 1 and 2 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.