Niobaziridine Hydrides

Abstract

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)(η2-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)(η2-t-Bu(H)CNAr)(N[Np]Ar)2 (2 Np-H). Thermal decomposition of 2 Np-H at elevated temperature (75 °C) provides the neopentyl imido complex Nb(NNp)(Ar)(N[Np]Ar)2 (5 Np). H/D isotopic labeling studies provide evidence for reversible β-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[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 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)(η2-Me2CNAr)(N[i-Pr]Ar)2 (3 i-Pr-H), to which it is isoelectronic.

Theoretical study of the ground-state structures and properties of niobium hydrides under pressure

“…15 The Nb-H elongation in the extended 18 while a bridging (Nb-H-Nb) separation is in the range of 1.8-2.12Å. 19,20 Of course, the higher n is in NbH n , the larger will be in general the number of near Nb-H separations. The histograms show a clear gap between a first group of Nb-H separations of 1.70-1.94Å (but for NbH 6 there is a single Nb-H separation at 2.1Å), and the next longer group, of 2.60-3.56Å.…”

“…In Fig. S1 in Supplementary Material, 17 we show a selection of these [18][19][20] and we note that in these systems, hydrogen is terminally bonded to niobium, and also bridging two niobiums. X-ray determined crystal structures do not locate hydrogens reliably, but neutron diffraction experiments (less plentiful, of course) do so; we then see Nb-H separations of ∼1.80Å for terminal Nb-H, and 1.9-2.0Å for bridging hydrogens.…”

Unusual Structure, Fluxionality, and Reaction Mechanism of Carbonyl Hydrosilylation by Silyl Hydride Complex [(ArN)Mo(H)(SiH2Ph)(PMe3)3]

“…[14] Rather, the phosphine addition merely shifts the equilibrium towards complex 3, thus establishing the b-CH activation in the OiPr ligand. Such a reversible carbonyl insertion into an earlytransition-metal À hydride bond is very rare, [46] because alkoxides that are ligated to electropositive early metals usually defy CH activation. b-CH activation in the alkoxide ligand was further confirmed by the treatment of complex 18 with one equivalent of benzonitrile, which resulted in the fast transfer hydrogenation of the nitrile and the quantitative formation of the benzylidenamide derivative, Even more surprisingly, the addition of a different silane, m-TolSiH 3 , to complex 18 results in the quantitative regeneration of complex 3, with the complete retention of the SiH 2 Ph group (Scheme 9).…”

Reversible Oxidative-Addition and Reductive-Elimination of Thiophene from a Titanium Complex and Its Thermally-Induced Hydrodesulfurization Chemistry

“…6 In this regard, much attention has been devoted to the development of late 3d-metal OA/RE catalysts; [7][8][9][10][11] yet, the abundance of early-metals (Groups 3-5) and their relatively low toxicity offer viable alternatives. While it is well-established that low valent early-metals (electron counts ≥ d 2 ) can undergo two-electron OA reactions, including across C-H bonds, [12][13][14][15][16][17] the reverse RE is more challenging. 18 At issue, in these highly electropositive metals, is their thermodynamic preference for higher oxidation states, which provides a significant barrier for reversible two-electron chemistries.…”