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^PU­(^MesPDI^Me)]₂ (<b>1-Cp</b>^<b>P</b>), Cp*U­(^MesPDI^Me)­(THF) (<b>1-Cp*</b>) (Cp^P = 1-(7,7-dimethylbenzyl)­cyclopentadienide; Cp* = η⁵-1,2,3,4,5-pentamethylcyclopentadienide), and Cp*U­(^<i>t</i>Bu-^MesPDI^Me) (THF) (<b>1-</b>^{<i><b>t</b></i>}<b>Bu</b>) (2,6-((Mes)­NCMe)­2-<i>p</i>-R-C₅H₂N, Mes = 2,4,6-trimethylphenyl; R = H, ^MesPDI^Me; R = C­(CH₃)₃, ^<i>t</i>Bu-^MesPDI^Me), has been investigated. While <b>1-Cp*</b> and <b>1-Cp</b>^<b>P</b> readily reduce N₃R (R = Ph, <i>p</i>-tolyl) to form <i>trans</i>-bis­(imido) species, Cp^PU­(NAr)₂(^MesPDI^Me) (Ar = Ph, <b>2-Cp</b>^<b>P</b>; Ar = <i>p</i>-Tol, <b>3-Cp</b>^<b>P</b>) and Cp*U­(NPh)₂(^MesPDI^Me) (<b>2-Cp*</b>), only <b>1-Cp*</b> can cleave diazene NN double bonds to form the same product. Complexes <b>2-Cp*</b>, <b>2-Cp</b>^<b>P</b>, and <b>3-Cp</b>^<b>P</b> are uranium­(V) <i>trans</i>-bis­(imido) species supported by neutral [^MesPDI^Me]⁰ ligands formed by complete oxidation of [^MesPDI^Me]^3– ligands of <b>1-Cp</b>^<b>P</b> and <b>1-Cp*</b>. Variation of the arylimido substituent in <b>2-Cp*</b> from phenyl to <i>p</i>-tolyl, forming Cp*U­(NTol)₂(^MesPDI^Me) (<b>3-Cp*</b>), changes the electronic structure, generating a uranium­(VI) ion with a monoanionic pyridine­(diimine) radical. The <i>tert</i>-butyl-substituted analogue, Cp*U­(NTol)₂(^<i>t</i>Bu-^MesPDI^Me) (<b>3-</b>^{<i><b>t</b></i>}<b>Bu</b>), displays the same electronic structure. Oxidation of the ligand radical in <b>3-Cp*</b> and <b>3-</b>^{<i><b>t</b></i>}<b>Bu</b> by Ag­(I) forms cationic uranium­(VI) [Cp*U­(NTol)₂(^MesPDI^Me)]­[SbF₆] (<b>4-Cp*</b>) and [Cp*U­(NTol)₂(^<i>t</i>Bu-^MesPDI^Me)]­[SbF₆] (<b>4-</b>^{<i><b>t</b></i>}<b>Bu</b>), respectively, as confirmed by metrical parameters. Conversely, oxidation of pentavalent <b>2-Cp*</b> with AgSbF₆ affords cationic [Cp*U­(NPh)₂(^MesPDI^Me)]­[SbF₆] (<b>5-Cp*</b>) 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 <b>1-Cp</b>^<b>P</b>, <b>3-Cp*</b>, <b>3-</b>^{<i><b>t</b></i>}<b>Bu</b>, <b>4-Cp*</b>, <b>4-</b>^{<i><b>t</b></i>}<b>Bu</b>, and <b>5-Cp*</b> have been elucidated by X-ray crystallography