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, [(CpU)-U-P((PDIMe)-P-Mes)]2 (1-Cp-P), Cp*U((PDIMe)-P-Mes)(THF) (1-Cp*) (Cp-P = 1-(7,7-dimethylbenzyl)cyclopentadienide; Cp* = eta(5)-1,2,3,4,5-pentamethylcyclopentadienide), and Cp*U(Bu-t-(PDIMe)-P-Mes) (THF) (1-Bu-t) (2,6-((Mes)N-CMe)2-p-R-C5H2N, Mes = 2,4,6-trimethylphenyl; R = H, MesPDIMe; R = C(CH3)(3), Bu-t-(PDIMe)-P-Mes), has been investigated. While 1-Cp* and 1-Cp-P readily reduce N3R (R = Ph, p-tolyl) to form trans-bis(imido) species, CpPU(NAr)(2)((PDIMe)-P-Mes) (Ar = Ph, 2-Cp-P; Ar = p-Tol, 3-Cp-P) and Cp*U(NPh)(2)((PDIMe)-P-Mes) (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 [(PDIMe)-P-Mes](0) ligands formed by complete oxidation of [(PDIMe)-P-Mes](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)((PDIMe)-P-Mes) (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)(Bu-t-(PDIMe)-P-Mes) (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)((PDIMe)-P-Mes)][SbF6] (4-Cp*) and [Cp*U(NTol)(2)(Bu-t-(PDIMe)-P-Mes)][SbF6] (4-Bu-t), respectively, as confirmed by metrical parameters. Conversely, oxidation of pentavalent 2-Cp* with AgSbF6 affords cationic [Cp*U(NPh)(2)((PDIMe)-P-Mes)][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-Bu-t, 4-Cp*, 4-Bu-t, and 5-Cp* have been elucidated by X-ray crystallography.