A Series of Uranium (IV, V, VI) Tritylimido Complexes, Their Molecular and Electronic Structures and Reactivity with CO₂

A series of uranium tritylimido complexes with structural continuity across complexes in different oxidation states, namely U^IV, U^V, and U^VI, is reported. This series was successfully synthesized by employing the trivalent uranium precursor, [((^nP,MeArO)₃tacn)­U^III] (1) (where (^nP,MeArO)₃tacn^3– = trianion of 1,4,7-tris­(2-hydroxy-5-methyl-3-neopentylbenzyl)-1,4,7-triazacyclononane), with the organic azides Me₃SiN₃, Me₃SnN₃, and Ph₃CN₃ (tritylazide). While the reaction with Me₃SiN₃ yields an inseparable mixture of both the azido and imido uranium complexes, applying the heavier Sn homologue yields the bis-μ-azido complex [{((^nP,MeArO)₃tacn)­U^IV}₂(μ-N₃)₂] (2) exclusively. In contrast to this one-electron redox chemistry, the reaction of precursor 1 with tritylazide solely leads to the two-electron oxidized U^V imido [((^nP,MeArO)₃tacn)­U^V(N-CPh₃)] (3). Oxidation and reduction of 3 yield the corresponding U^VI and U^IV complexes [((^nP,MeArO)₃tacn)­U^VI(N-CPh₃)]­[B­(C₆F₅)₄] (4) and K­[((^nP,MeArO)₃tacn)­U^IV(N-CPh₃)] (5), respectively. In addition, the U^V imido 3 engages in a H atom abstraction reaction with toluene to yield the closely related amido complex [((^nP,MeArO)₃tacn)­U^IV(N­(H)-CPh₃)] (6). Complex 6 and the three tritylimido complexes 3, 4, and 5, with oxidation states ranging from +IV to +VI and homologous core structures, were investigated by X-ray diffraction analyses and magnetochemical and spectroscopic studies as well as density functional theory (DFT) computational analysis. The series of structurally very similar imido complexes provides a unique opportunity to study electronic properties and to probe the uranium imido reactivity solely as a function of electron count of the metal–imido entity. Evidence for the U–N bond covalency and f-orbital participation in complexes 3–6 was drawn from the in-depth and comparative DFT study. The reactivity of the imido and amido complexes with CO₂ was probed, and conclusions about the influence of the formal oxidation state are reported.