Divergent uranium- versus phosphorus-based reduction of Me(3)SiN(3) with steric modification of phosphido ligands

We describe an example of a two-electron metal- and ligand-based reduction of Me(3)SiN(3) using uranium(iv) complexes with varying steric properties. Reaction of (C(5)Me(5))(2)U(CH(3))[P(SiMe(3))(Ph)] with Me(3)SiN(3) produces the imidophosphorane complex, (C(5)Me(5))(2)U(CH(3))[N[double bond, length as m-dash]P(SiMe(3))(2)(Ph)] through oxidation of phosphorus. However, a similar reaction with a more sterically encumbering phosphido ligand, (C(5)Me(5))(2)U(CH(3))[P(SiMe(3))(Mes)] forms the U(iv) complex, (C(5)Me(5))(2)U[κ(2)-(N,N)–N(SiMe(3))P(Mes)N(SiMe(3))]. In probing the mechanism of this reaction, a U(vi) bis(imido) complex, (C(5)Me(5))(2)U([double bond, length as m-dash]NSiMe(3)){[double bond, length as m-dash]N[P(SiMe(3))(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.