Origin of Bond Elongation in a Uranium(IV) cis-Bis(imido) Complex.

The activation of U-N multiple bonds in an imido analogue of the uranyl ion is accomplished by using a system that is very electron-rich with sterically encumbering ligands. Treating the uranium(VI) trans-bis(imido) UI2(NDIPP)2(THF)3 (DIPP = 2,6-diisopropylphenyl and THF = tetrahydrofuran) with tert-butyl(dimethylsilyl)amide (NTSA) results in a reduction and rearrangement to form the uranium(IV) cis-bis(imido) [U(NDIPP)2(NTSA)2]K2 (1). Compound 1 features long U-N bonds, pointing toward substantial activation of the N═U═N unit, as determined by X-ray crystallography and 1H NMR, IR, and electronic absorption spectroscopies. Computational analyses show that uranium(IV)-imido bonds in 1 are significantly weakened multiple bonds due to polarization toward antibonding and nonbonding orbitals. Such geometric control has important effects on the electronic structures of these species, which could be useful in the recycling of spent nuclear fuels.