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

Abstract

The synthesis and spectroscopic characterization of the mononuclear uranium complex [((ArO)(3)tacn)U(III)(NCCH(3))] is reported. The uranium(III) complex reacts with organic azides to yield uranium(IV) azido as well as uranium(V) imido complexes, [((ArO)(3)tacn)U(IV)(N(3))] and [((ArO)(3)tacn)U(V)(NSi(CH(3))(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 pi-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-N(ax)] core structure and oxidation states +III, +IV, and +V is discussed in context of the electronic structures.

Pentavalent uranium chemistry—synthetic pursuit of a rare oxidation state

“…10). 55 The molecular structure for one of the two independent molecules in the unit cell of 50 is presented in Figure 10. Along with the short U=Nimido distance (U ( 1) N (4) …”

“…Figure II Molecular structure of [(Ad-ArD)3tacnJU(=O) (55 (57) was characterized crystallographically, and the molecular structure of the U V anion is shown in Figure 12 The molecular structure of complex 63 is provided in Figure IS.…”

“…12 These single-electron systems provide fertile ground for not only advancing theoretical capabil ities but also our fun damental understanding of actinide electronic structure, reactivity and bonding. For example, because electron 55 repulsion is absent and at most six transitions are allowed in the optical spectrum, the fl electronic configuration fac ilitates the interpretation of spectrosopic 13 and magnetic l4 data. This enables electron paramagnetic resonance (EPR) studies and magnetic susceptibil ity measurements on systems that 2 60 promote the formation of Kramers doublet ground states.…”

Evidence for Alkane Coordination to an Electron-Rich Uranium Center

“…In contrast, d (Uo-o-p) in 2 was determined to be 0.44 Å, which is indicative of a pronounced covalent U(III)-NCCH 3 interaction. 9 The uranium-carbon bond distances, d(U-C1S), in 1c and 1d were determined to be 3.864 and 3.798 Å, with the shortest U-C bond distance of 3.731 Å found in the solid-state structure of complex 1e. Additionally, structures 1a-e exhibit short contacts between the peripheral tert-butyl groups and the axial alkane ligand.…”

“…The structure of the ((ArO) 3 tacn)U core fragment of all alkane adducts is similar to that found for the previously reported seven-coordinate complex [((ArO) 3 tacn)U(NCCH 3 )] (2). 9 The average U-N(tacn) and U-O(ArO) bond distances in 1c, for instance, were determined to be 2.676-(4) and 2.244(3) Å, which are comparable to 2.699(6) and 2.265(5) Å found in 2.A significant difference between the solid-state molecular structures of 1c-e and 2 is the out-of-plane shift, d (Uo-o-p), of the uranium center from an idealized trigonal plane formed by the three aryloxide oxygen atoms toward the polyamine chelator. The uranium displacements in 1c-e were determined to be 0.66 Å below the aryloxide plane.…”