Niobaziridine Hydrides

Presented herein are synthetic, structural, and reactivity studies
delineating the characteristics of the niobaziridine hydride functional
group as it pertains to the stabilization of trisanilide niobium complexes
of the type Nb(N[R]Ar)3 (1R, Ar
= 3,5-Me2C6H3). Utilization of the N-isopropyl anilide ligand, N[i-Pr]Ar,
results in the niobaziridine hydride dimer [Nb(H)(η2-Me2CNAr)(N[i-Pr]Ar)2]2 ([2i-Pr-H]2). Dimer [2i-Pr-H]2 is thermally unstable at room temperature and decomposes
via ortho-metalation and i-Pr radical
ejection to a species containing a Nb−Nb bond. The ligand variant
N[Np]Ar (Np = neopentyl) provides the room-temperature-stable niobaziridine
hydride monomer Nb(H)(η2-t-Bu(H)CNAr)(N[Np]Ar)2 (2Np-H). Thermal decomposition of 2Np-H at elevated temperature (75 °C) provides
the neopentyl imido complex Nb(NNp)(Ar)(N[Np]Ar)2 (5Np). H/D isotopic labeling studies provide evidence
for reversible β-H elimination interconverting 2Np-H and its trisanilide tautomer [Nb(N[Np]Ar)3] (1Np), with the latter thereby implicated
as an intermediate during the 2Np-H → 5Np conversion. Reactivity studies between 2Np-H and certain small-molecule substrates confirm
that the niobaziridine hydride group can effectively mask a reactive d2 Nb(III) trisanilide center. However, 2Np-H exhibits insertion chemistry when treated
with a variety of unsaturated organic substrates, thus demonstrating
a pronounced tendency to additionally function as a Lewis acidic,
early transition metal hydride species. A general mechanism accounting
for the divergent reactivity of 2Np-H is proposed.
Niobaziridine hydride complexes derived from the amido ligands N[CH2Ad]Ar, N[Cy]Ar, and NCy2 (Ad = 1-adamantyl, Cy
= cyclohexyl) are also presented, and their thermal behavior and reaction
chemistry are compared with those of 2Np-H.
In addition, the radical anion of 2Np-H is
reported and compared with the neutral d1 molybdaziridine hydride complex Mo(H)(η2-Me2CNAr)(N[i-Pr]Ar)2 (3i-Pr-H), to which it is isoelectronic.