Isolation of a californium(II) crown–ether complex

The actinides, from californium to nobelium (Z = 98–102), are known to have an accessible +2 oxidation state. Understanding the origin of this chemical behaviour requires characterizing Cf II materials, but investigations are hampered by the fact that they have remained difficult to isolate. This partly arises from the intrinsic challenges of manipulating this unstable element, as well as a lack of suitable reductants that do not reduce Cf III to Cf°. Here we show that a Cf II crown–ether complex, Cf(18-crown-6)I 2 , can be prepared using an Al/Hg amalgam as a reductant. Spectroscopic evidence shows that Cf III can be quantitatively reduced to Cf II , and rapid radiolytic re-oxidation in solution yields co-crystallized mixtures of Cf II and Cf III complexes without the Al/Hg amalgam. Quantum-chemical calculations show that the Cf‒ligand interactions are highly ionic and that 5 f /6 d mixing is absent, resulting in weak 5 f →5 f transitions and an absorption spectrum dominated by 5 f →6 d transitions. Californium is difficult to prepare in its divalent state. Now, crystals of a Cf(II) crown–ether complex have been synthesized by reduction of a Cf(III) precursor with an Al/Hg amalgam. They exhibit 5 f →6 d transitions in the visible region and near-infrared emission that are highly sensitive to changes in the coordination environment.