Much of what is currently known about the f-elements consists of studying their behavior under ambient conditions, which restricts most of these elements to the +3 state (excluding some of the early actinides). While the divalent states of the lanthanides and actinides were studied as early as the late 1900’s, the last few decades have introduced more in-depth characterization of their low-valent states including the isolation of single crystals of divalent lanthanides and a few divalent actinides for greater structural knowledge. For the lanthanides, this has led to a better understanding of whether they adopt a 4fⁿ⁺¹ or 4fⁿ5d¹ electron configuration in the divalent state as well as their usefulness in material applications such as medical imaging, sensing, catalysis, and lighting. Less is known about the divalent actinides, especially the later actinides, where a stabilization of the divalent oxidation state is observed from californium through nobelium; the latter of which is most stable in the +2 state at normal atmospheric and aqueous conditions. Macrocyclic organic ligands that are well known for exhibiting a high affinity for low valent metal ions have recently become popular within the f-element community for their propensity to preserve the divalent state of the lanthanides. Compounds synthesized using divalent lanthanide analogues such as Eu(II) and Sm(II) display an intriguing level of tunability in spectroscopic properties based on subtle changes in the metal ion coordination environment and substitution of functional groups to the backbone of the macrocycle. Thus far, investigation of these macrocycles as well as some of their unique derivatives demonstrate notable interactions with divalent lanthanides that make them suitable candidates for the stabilization of divalent californium in a single-crystal structure.

Thomas Albrecht-Schoenzart

Appearances