Uranium Tris-aryloxide Derivatives Supported by Triazacyclononane: Engendering a Reactive Uranium(III) Center with a Single Pocket for Reactivity

The synthesis and spectroscopic characterization of the mononuclear uranium complex
[((ArO)3tacn)UIII(NCCH3)] is reported. The uranium(III) complex reacts with organic azides to yield uranium(IV) azido as well as uranium(V) imido complexes, [((ArO)3tacn)UIV(N3)] and [((ArO)3tacn)UV(NSi(CH3)3)].
Single-crystal X-ray diffraction, spectroscopic, and computational studies of this analogous series of uranium
tris-aryloxide complexes supported by triazacyclononane are described. The hexadentate, tris-anionic ligand
coordinates to the large uranium ion in unprecedented fashion, engendering coordinatively unsaturated
and highly reactive uranium centers. The macrocyclic triazacyclononane tris-aryloxide derivative occupies
six coordination sites, with the three aryloxide pendant arms forming a trigonal plane at the metal center.
DFT quantum mechanic methods were applied to rationalize the reactivity and to elucidate the electronic
structure of the newly synthesized compounds. It is shown that the deeply colored uranium(III) and uranium(V) species are stabilized via π-bonding interaction, involving uranium f-orbitals and the axial acetonitrile
and imido ligand, respectively. In contrast, the bonding in the colorless uranium(IV) azido complex is purely
ionic in nature. The magnetism of the series of complexes with an [N3O3-Nax] core structure and oxidation
states +III, +IV, and +V is discussed in context of the electronic structures.