Tailoring the Electronic Structure of Uranium Mono(imido) Species through Ligand Variation

Uranium mono(imido) species have been prepared via the oxidation of Cp*U(^MesPDI^Me)(THF) (1-Cp*) and [Cp^PU(^MesPDI^Me)]₂ (1-Cp^P), where Cp* = η⁵-1,2,3,4,5-pentamethylcyclopentadienide, Cp^P = 1-(7,7-dimethylbenzyl)cyclopentadienide, ^MesPDI^Me = 2,6-[(Mes)N═CMe]₂C₅H₃N, and Mes = 2,4,6-trimethylphenyl, with organoazides. Treating either with N₃DIPP (DIPP = 2,6-diisopropylphenyl) formed uranium(IV) mono(imido) complexes, Cp^PU(NDIPP)(^MesPDI^Me) (2-Cp^P) and Cp*U(NDIPP)(^MesPDI^Me) (2-Cp*), featuring reduced [^MesPDI^Me]^-. The addition of electron-donating 1-azidoadamantane (N₃Ad) to 1-Cp* generated a dimeric product, [Cp*U(NAd)(^MesHPDI^Me)]₂ (3), from radical coupling at the p-pyridine position of the pyridine(diimine) ligand and H-atom abstraction, formed through a monomeric intermediate that was observed in solution but could not be isolated. To support this, Cp*U(^tBu-^MesPDI^Me)(THF) (1-^tBu), which has a tert-butyl group protecting the para position, was also treated with N₃Ad, and the monomeric product, Cp*U(NAd)(^tBu-^MesPDI^Me) (2-^tBu), was isolated. All isolated complexes were analyzed spectroscopically and structurally, and the dynamic solution behavior was examined using electronic absorption spectroscopy.