Isolation of a Californium(II) Crown-Ether Complex

Abstract Californium (Z = 98) is the first member of the actinide series displaying metastability of the 2 + oxidation state. Understanding the origin of this chemical behavior requires characterizing CfII materials, but isolating a complex with this state has remained elusive. The source of its inaccessibility arises from the intrinsic challenges of manipulating this unstable element as well as a lack of suitable reductants that do not reduce CfIII to Cf0. Herein we show that a CfII crown-ether complex, Cf(18-crown-6)I2, can be prepared using an Al/Hg amalgam as a reductant. While spectroscopic evidence shows that CfIII can be quantitatively reduced to CfII, rapid radiolytic re-oxidation back to the CfIII parent occurs and co-crystallized mixtures of CfII and CfIII complexes are isolated if the crystallization is not conducted over the Al/Hg amalgam. Quantum chemical calculations show that the Cf‒ligand interactions are highly ionic and that 5f/6d mixing is absent, resulting in remarkably weak 5f→5f transitions and an absorption spectrum dominated by 5f→6d transitions.