Molecular Rearrangements of a Low-Valent Niobium Amide:  Ligand C−H Bond Oxidative Addition and Reductive Elimination

Tayebani, Maryam; Feghali, Khalil; Gambarotta, Sandro; Yap, Glenn P. A.

doi:10.1021/om980302y

Cited by 22 publications

(11 citation statements)

References 52 publications

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“…47 Complex 4 i-Pr is remarkable in that it is the product of not only Nb-Nb bond formation and ortho-metalation with formal H atom loss but also of i-Pr radical ejection from the N[i-Pr]Ar ligand. Notably, the two former ligand degradation modes have been observed by both Rothwell 48,49 and Gambarotta [50][51][52][53] in their investigations of low-valent Nb complexes supported by monoanionic aryloxide and amido ligands, respectively.…”

Section: Resultsmentioning

confidence: 99%

“…Furthermore, the marked ability of putative lowvalent Nb species to activate C-N bonds in organoamide ligands has been documented by Gambarotta for several NR 2 derivatives. 50,52,53 Accordingly, such rampant C-N activation capability further punctuates the ability of the niobaziridine hydride unit, as derived from the N[Np]Ar ligand, to impart kinetic stabilization to the reactive 1 Np fragment in solution.…”

Section: Resultsmentioning

confidence: 99%

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Niobaziridine Hydrides

et al. 2010

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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.

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Section: Resultsmentioning

confidence: 99%

Section: Resultsmentioning

confidence: 99%

Niobaziridine Hydrides

et al. 2010

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“…There are several examples of C-H oxidative addition to low-valent early transition metals 160,180–194,197,198 , including several that lead to stoichiometric dehydrogenation 188–190 . The majority of dehydrogenation reactions promoted by early transition metals are, however, predicated on high-valent, electrophilic metal reactivity, such as 1,2-C-H addition across metal-ligand multiple bonds 199–201 .…”

Section: Functionalization and Defunctionalizationmentioning

confidence: 99%

Modern applications of low-valent early transition metals in synthesis and catalysis

et al. 2018

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Low-valent early transition metals are often intrinsically highly reactive as a result of their strong propensity toward oxidation to more stable high-valent states. Harnessing these highly reducing complexes for productive reactivity is potentially powerful for C-C bond construction, organic reductions, small-molecule activation and many other reactions that offer orthogonal chemoselectivity and/or regioselectivity patterns to processes promoted by late transition metals. Recent years have seen many exciting new applications of low-valent metals through building new catalytic and/or multicomponent reaction manifolds out of classical reactivity patterns. In this Review, we survey new methods that employ early transition metals and invoke low-valent precursors or intermediates in order to identify common themes and strategies in synthesis and catalysis.

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“…To a smaller extent, the reductive prowess of LVEMs has been utilized to perform challenging chemical transformations such as the reductive cleavage of CO, − C–H bond activation via oxidative addition, − and the reduction and functionalization of N 2 . ,− In fact, low-valent titanium complexes have been shown to be sufficiently reducing to effect the catalytic activation of N 2 for nitrogen incorporation into organic compounds . For instance, treatment of substoichiometric amounts of TiCl 4 with an excess of Li 0 and Me 3 SiCl under N 2 can nitrogenate ketoalkynes to indoles in modest yields .…”

Section: Introductionmentioning

confidence: 99%

Redox Character and Small Molecule Reactivity of a Masked Titanium(II) Synthon

et al. 2019

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The two-electron reduction of the Ti(IV) guanidinate (Ketguan)(ImDippN)Ti(OTf)2 (2 OTf ) (Ketguan = [( t Bu2CN)C(NDipp)2]−; ImDippN– = 1,3-bis(Dipp)imidazolin-2-iminato; Dipp = 2,6-diisopropylphenyl) with an excess of KC8 generates the masked complex (Ketguan)(η6-ImDippN)Ti (1). Conversely, reduction of the chloride analogue (Ketguan)(ImDippN)TiCl2 (2 Cl ) with an excess of Na/Hg amalgam produces the Ti(III) compound (Ketguan)(ImDippN)TiCl (3), while treatment of 2 Cl with 3.0 equiv of KC8 affords a complicated mixture from which (ImDippN)(DippN)[η2-( t Bu2C)NC(NDipp)](THF)Ti (4) is isolated as the product of reductive ligand cleavage. These results clearly indicate that the success of early metal reduction chemistry is highly sensitive to the halide coligands and reaction conditions. Complex 1, despite possessing a Ti(IV) canonical form, behaves as a Ti(II) synthon and appreciable reducing agent. For instance, 1 effects the one-electron reduction of benzophenone and pyridine to give the Ti(III) products (Ketguan)(ImDippN)Ti(η1-OC·Ph2) (6) and [(Ketguan)(ImDippN)Ti]2[μ2-(NC5H5–H5C5N)] (7), providing an approximate chemical redox potential range for 1 between ca. −2.3 to −3.1 V (vs [Cp2Fe]0/+). Additionally, treatment of 1 with π-acids such as CNCy (Cy = cyclohexyl) or NC t Bu leads to the formation of the Ti(III) and Ti(IV) products (Ketguan)(ImDippN)Ti(CN)(CNCy) (9) and (ImDippN)[(DippN)(2- i PrC6H3-6-(η1-CH3CHCH2)N)C(NC t Bu2)]Ti[NC(H) t Bu] (10), respectively, via reduction of the π-acid substrate. The two-electron reduction proclivity of 1 is demonstrated by its reactivity with chalcogen sources (e.g., N2O) and organoazides to give the Ti(IV) products (Ketguan)(ImDippN)Ti(E) (E = O (13), S (14), Se (15), S2 (16), NSiMe3 (17), NAd (18)). In addition to illustrating the versatile Ti(II) synthon character of 1, the synthesis of these compounds shows that the 3N-coordinated [(Ketguan)(ImDippN)Ti] n+ manifold can readily accommodate metal–ligand multiple bonds, including relatively rare examples of terminally bound TiS and TiSe bonds. Taken altogether, the redox chemistry of 1, as a Ti(II) synthon, clearly shows the chemical diversity of low-valent early metals (LVEMs) and their ability to reductively activate a wide range of substrates.

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Molecular Rearrangements of a Low-Valent Niobium Amide: Ligand C−H Bond Oxidative Addition and Reductive Elimination

Cited by 22 publications

References 52 publications

Niobaziridine Hydrides

Niobaziridine Hydrides

Modern applications of low-valent early transition metals in synthesis and catalysis

Redox Character and Small Molecule Reactivity of a Masked Titanium(II) Synthon

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