Divergent uranium- versus phosphorus-based reduction of Me3SiN3 with steric modification of phosphido ligands

We describe an example of a two-electron metal- and ligand-based reduction of Me3SiN3 using uranium(IV) complexes with varying steric properties. Reaction of (C5Me5)2U(CH3)[P(SiMe3)(Ph)] with Me3SiN3 produces the imidophosphorane complex, (C5Me5)2U(CH3)[NP(SiMe3)2(Ph)] through oxidation of phosphorus. However, a similar reaction with a more sterically encumbering phosphido ligand, (C5Me5)2U(CH3)[P(SiMe3)(Mes)] forms the U(IV) complex, (C5Me5)2U[κ2-(N,N)–N(SiMe3)P(Mes)N(SiMe3)]. In probing the mechanism of this reaction, a U(VI) bis(imido) complex, (C5Me5)2U(NSiMe3){N[P(SiMe3)(Mes)]} was isolated. DFT calculations show an intramolecular reductive cycloaddition reaction leads to the formation of the U(IV) bis(amido)phosphane from the U(VI) bis(imido) complex. This is a rare example of the isolation of a reaction intermediate in f element chemistry.