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) (1(R), Ar = 3,5-Me2C6H3). Utilization of the N-isopropyl anilide ligand, N[i-Pr]Ar, results in the niobaziridine hydride dimer [Nb(H)(eta(2)-Me2C=NAr)(N[i-Pr]Ar)(2)](2) ([2(i-Pr)-H](2)). Dimer [2(i-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)(eta(2)-t-Bu(H)C=NAr)(N[Np]Ar)(2) (2(Np)-H). Thermal decomposition of 2(Np)-H at elevated temperature (75 degrees C) provides the neopentyl imido complex Nb(NNp)(Ar)(N[Np]Ar)(2) (5(N)P). H/D isotopic labeling studies provide evidence for reversible beta-H elimination interconverting 2(Np)-H and its trisanilide tautomer [Nb(N[Np]Ar)(3)] (1(Np)), with the latter thereby implicated as an intermediate during the 2(Np)-H -> 5(Np) conversion. Reactivity studies between 2(Np)- H and certain small-molecule substrates confirm that the niobaziridine hydride group can effectively mask a reactive d(2) Nb(III) trisanilide center. However, 2(Np)-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 2(Np)-H is proposed. Niobaziridine hydride complexes derived from the amido ligands N[CH(2)Ad]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 2(Np)-H. In addition, the radical anion of 2(Np)-H is reported and compared with the neutral d(1) molybdaziridine hydride complex Mo(H)(eta(2)-Me2C=NAr)(N[i-Pr]Ar)(2) (3(i-Pr)-H), to which it is isoelectronic.