Ligand non-innocence allows isolation of a neutral and terminal niobium nitride

Abstract

Complex (PNP)NbCl₂(N[^tBu]Ar) (1) (PNP⁻ = N[2-PⁱPr₂-4-methylphenyl]₂; Ar = 3,5-Me₂C₆H₃) reacts with one equiv. of NaN₃ to form a mixture of (PNPN)NbCl₂(N[^tBu]Ar) (2) and (PNP)NbN(N[^tBu]Ar) (3), both of which have been spectroscopically and crystallographically characterized, including ¹⁵N isotopic labelling studies. Complex 3 represents the first structurally characterized example of a neutral and mononuclear Nb nitride. Independent studies established 3 to form via two-electron reduction of 2, whereas oxidation of 3 by two-electrons reversed the process. Computational studies suggest the transmetallation step to produce the intermediate [(PNP)NbCl(N₃)(N[^tBu]Ar)] (A) which extrudes N₂ to form the phosphinimide [(PNPN)NbCl(N[^tBu]Ar)] (B) followed by disproportionation to 2 and low-valent [(PNPN)Nb(N[^tBu]Ar)] (C). The latter then undergoes intramolecular N-atom transfer to form the nitride moiety in 3.