Examining the Effects of Ligand Variation on the Electronic Structure of Uranium Bis(imido) Species

Abstract

Arylazide and diazene activation by highly reduced uranium(IV) complexes bearing trianionic redox-active pyridine(diimine) ligands, [Cp^PU(^MesPDI^Me)]₂ (1-Cp^P), Cp*U(^MesPDI^Me)(THF) (1-Cp*) (Cp^P = 1-(7,7-dimethylbenzyl)cyclopentadienide; Cp* = η⁵-1,2,3,4,5-pentamethylcyclopentadienide), and Cp*U(^tBu-^MesPDI^Me) (THF) (1-^tBu) (2,6-((Mes)N═CMe)2-p-R-C₅H₂N, Mes = 2,4,6-trimethylphenyl; R = H, ^MesPDI^Me; R = C(CH₃)₃, ^tBu-^MesPDI^Me), has been investigated. While 1-Cp* and 1-Cp^P readily reduce N₃R (R = Ph, p-tolyl) to form trans-bis(imido) species, Cp^PU(NAr)₂(^MesPDI^Me) (Ar = Ph, 2-Cp^P; Ar = p-Tol, 3-Cp^P) and Cp*U(NPh)₂(^MesPDI^Me) (2-Cp*), only 1-Cp* can cleave diazene N═N double bonds to form the same product. Complexes 2-Cp*, 2-Cp^P, and 3-Cp^P are uranium(V) trans-bis(imido) species supported by neutral [^MesPDI^Me]⁰ ligands formed by complete oxidation of [^MesPDI^Me]^3- ligands of 1-Cp^P and 1-Cp*. Variation of the arylimido substituent in 2-Cp* from phenyl to p-tolyl, forming Cp*U(NTol)₂(^MesPDI^Me) (3-Cp*), changes the electronic structure, generating a uranium(VI) ion with a monoanionic pyridine(diimine) radical. The tert-butyl-substituted analogue, Cp*U(NTol)₂(^tBu-^MesPDI^Me) (3-^tBu), displays the same electronic structure. Oxidation of the ligand radical in 3-Cp* and 3-^tBu by Ag(I) forms cationic uranium(VI) [Cp*U(NTol)₂(^MesPDI^Me)][SbF₆] (4-Cp*) and [Cp*U(NTol)₂(^tBu-^MesPDI^Me)][SbF₆] (4-^tBu), respectively, as confirmed by metrical parameters. Conversely, oxidation of pentavalent 2-Cp* with AgSbF₆ affords cationic [Cp*U(NPh)₂(^MesPDI^Me)][SbF₆] (5-Cp*) from a metal-based U(V)/U(VI) oxidation. All complexes have been characterized by multidimensional NMR spectroscopy with assignments confirmed by electronic absorption spectroscopy. The effective nuclear charge at uranium has been probed using X-ray absorption spectroscopy, while structural parameters of 1-Cp^P, 3-Cp*, 3-^tBu, 4-Cp*, 4-^tBu, and 5-Cp* have been elucidated by X-ray crystallography.