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

Arylazide and diazene activation by highly reduced uranium(IV) complexes bearing trianionic redox-active pyridine(diimine) ligands, [Cp ^P U( ^Mes PDI ^Me )] ₂ (1-Cp ^P ), Cp*U( ^Mes PDI ^Me )(THF) (1-Cp*) (Cp ^P = 1-(7,7-dimethylbenzyl)cyclopentadienide; Cp* = η ⁵ -1,2,3,4,5-pentamethylcyclopentadienide), and Cp*U( ^t Bu- ^Mes PDI ^Me ) (THF) (1- ^t Bu) (2,6-((Mes)N═CMe)2-p-R-C ₅ H ₂ N, Mes = 2,4,6-trimethylphenyl; R = H, ^Mes PDI ^Me ; R = C(CH ₃ ) ₃ , ^t Bu- ^Mes PDI ^Me ), has been investigated. While 1-Cp* and 1-Cp ^P readily reduce N ₃ R (R = Ph, p-tolyl) to form trans-bis(imido) species, Cp ^P U(NAr) ₂ ( ^Mes PDI ^Me ) (Ar = Ph, 2-Cp ^P ; Ar = p-Tol, 3-Cp ^P ) and Cp*U(NPh) ₂ ( ^Mes PDI ^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 [ ^Mes PDI ^Me ] ⁰ ligands formed by complete oxidation of [ ^Mes PDI ^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 ( ^Mes PDI ^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 ( ^t Bu- ^Mes PDI ^Me ) (3- ^t Bu), displays the same electronic structure. Oxidation of the ligand radical in 3-Cp* and 3- ^t Bu by Ag(I) forms cationic uranium(VI) [Cp*U(NTol) ₂ ( ^Mes PDI ^Me )][SbF ₆ ] (4-Cp*) and [Cp*U(NTol) ₂ ( ^t Bu- ^Mes PDI ^Me )][SbF ₆ ] (4- ^t Bu), respectively, as confirmed by metrical parameters. Conversely, oxidation of pentavalent 2-Cp* with AgSbF ₆ affords cationic [Cp*U(NPh) ₂ ( ^Mes PDI ^Me )][SbF ₆ ] (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- ^t Bu, 4-Cp*, 4- ^t Bu, and 5-Cp* have been elucidated by X-ray crystallography.