Towards a Cis-Uranyl Complex

The uranyl (UO22+) subunit is the major form of uranium in spent nuclear fuel. The trans- moiety is very stable and characterized by short U=O bond lengths (ca. 1.78 Å) and a linear O=U=O bond angle.1 There have been many attempts to synthesize a cis-uranyl complex, however, none have provided substantial evidence for its formation. In our attemps to synthesize a cis-uranyl complex, small, constraining macrocyclic ligands such as cyclam (1) (cyclam = (NHCH2CH2NHCH2CH2CH2)2) and tmtaaH2 (2) (tmtaa = dibenzotetramethyltetraaza[14]annulene) have been utilized. Importantly, the small ring size of these macrocycles forces the ligand to occupy only one hemisphere of the metal ion, leaving two mutually cis- sites vacant.