Mechanistic Investigation of Enantioconvergent Kumada Reactions of Racemic α‑Bromoketones Catalyzed by a Nickel/Bis(oxazoline) Complex

In recent years, a wide array of methods for achieving nickel-catalyzed substitution reactions of alkyl electrophiles by organometallic nucleophiles, including enantioconvergent processes, have been described; however, experiment-focused mechanistic studies of such couplings have been comparatively scarce. The most detailed mechanistic investigations to date have examined catalysts that bear tridentate ligands and, with one exception, processes that are not enantioselective; studies of catalysts based on bidentate ligands could be anticipated to be more challenging, due to difficulty in isolating proposed intermediates as a result of instability arising from coordinative unsaturation. In this investigation, we explore the mechanism of enantioconvergent Kumada reactions of racemic α-bromoketones catalyzed by a nickel complex that bears a bidentate chiral bis­(oxazoline) ligand. Utilizing an array of mechanistic tools (including isolation and reactivity studies of three of the four proposed nickel-containing intermediates, as well as interrogation via EPR spectroscopy, UV–vis spectroscopy, radical probes, and DFT calculations), we provide support for a pathway in which carbon–carbon bond formation proceeds via a radical-chain process wherein a nickel­(I) complex serves as the chain-carrying radical and an organonickel­(II) complex is the predominant resting state of the catalyst. Computations indicate that the coupling of this organonickel­(II) complex with an organic radical is the stereochemistry-determining step of the reaction

Mechanistic Investigation of Enantioconvergent Kumada Reactions of Racemic α‑Bromoketones Catalyzed by a Nickel/Bis(oxazoline) Complex

In recent years, a wide array of methods for achieving nickel-catalyzed substitution reactions of alkyl electrophiles by organometallic nucleophiles, including enantioconvergent processes, have been described; however, experiment-focused mechanistic studies of such couplings have been comparatively scarce. The most detailed mechanistic investigations to date have examined catalysts that bear tridentate ligands and, with one exception, processes that are not enantioselective; studies of catalysts based on bidentate ligands could be anticipated to be more challenging, due to difficulty in isolating proposed intermediates as a result of instability arising from coordinative unsaturation. In this investigation, we explore the mechanism of enantioconvergent Kumada reactions of racemic α-bromoketones catalyzed by a nickel complex that bears a bidentate chiral bis­(oxazoline) ligand. Utilizing an array of mechanistic tools (including isolation and reactivity studies of three of the four proposed nickel-containing intermediates, as well as interrogation via EPR spectroscopy, UV–vis spectroscopy, radical probes, and DFT calculations), we provide support for a pathway in which carbon–carbon bond formation proceeds via a radical-chain process wherein a nickel­(I) complex serves as the chain-carrying radical and an organonickel­(II) complex is the predominant resting state of the catalyst. Computations indicate that the coupling of this organonickel­(II) complex with an organic radical is the stereochemistry-determining step of the reaction

Cerium(III) and Uranium(IV) Complexes of the 2‑Fluorophenyl Trimethylsilyl Amide Ligand: C–F → Ln/An Interactions that Modulate the Coordination Spheres of f‑Block Elements

2-fluorophenyl trimethylsilyl amide, N­(SiMe₃)­(C₆H₄F)⁻ was shown to engage in stronger C–F → Ce^III interactions than pentafluorophenyl trimethylsilyl amide, N­(SiMe₃)­(C₆F₅)⁻, through a comparative study of the Ce^III model complexes Ce­[N­(SiMe₃)­(C₆H₄F)]₃ (<b>1-F</b>_<b>1</b>) and Ce­[N­(SiMe₃)­(C₆F₅)]₃ (<b>1-F</b>_<b>5</b>). The presence of multiple C–F → U^IV interactions led to complexes <b>2-X</b> (X = Cl, CCPh, OMe) with threefold geometries, featuring a trigonal pyramidal UN₃Cl core in the solid-state structures. Density functional theory calculations were applied to <b>2-Cl</b> to investigate the strength of the C–F → U^IV interactions and the influence of such interactions on resulting geometries

Cerium(III) and Uranium(IV) Complexes of the 2‑Fluorophenyl Trimethylsilyl Amide Ligand: C–F → Ln/An Interactions that Modulate the Coordination Spheres of f‑Block Elements

2-fluorophenyl trimethylsilyl amide, N­(SiMe₃)­(C₆H₄F)⁻ was shown to engage in stronger C–F → Ce^III interactions than pentafluorophenyl trimethylsilyl amide, N­(SiMe₃)­(C₆F₅)⁻, through a comparative study of the Ce^III model complexes Ce­[N­(SiMe₃)­(C₆H₄F)]₃ (<b>1-F</b>_<b>1</b>) and Ce­[N­(SiMe₃)­(C₆F₅)]₃ (<b>1-F</b>_<b>5</b>). The presence of multiple C–F → U^IV interactions led to complexes <b>2-X</b> (X = Cl, CCPh, OMe) with threefold geometries, featuring a trigonal pyramidal UN₃Cl core in the solid-state structures. Density functional theory calculations were applied to <b>2-Cl</b> to investigate the strength of the C–F → U^IV interactions and the influence of such interactions on resulting geometries

Electrophilic Ln(III) Cations Protected by C–F → Ln Interactions and Their Coordination Chemistry with Weak σ- and π‑Donors

A homoleptic cerium­(III) amide complex, Ce­(NPhF2)3 (1-Ce) (PhF = pentafluorophenyl), in an unusual pseudo-trigonal planar geometry featuring six C–F → Ce interactions was prepared. The C–F → Ln interactions in solution were evident by comparison of the 19F NMR shifts for the paramagnetic 1-Ce with those of the 4f0 lanthanum­(III) analogue. Coordination of weak σ- and π-donors, including ethers and neutral arene molecules, was achieved by the reversible displacement of the weak C–F → Ce interactions. Computational studies on Ce­(NPhF2)3 and Ce­(NPhF2)3(η6-C6H3Me3) provide information on the F → Ce interactions and Ce−η6-arene bonding

Luminescent Ce(III) Complexes as Stoichiometric and Catalytic Photoreductants for Halogen Atom Abstraction Reactions

Luminescent Ce­(III) complexes, Ce­[N­(SiMe₃)₂]₃ (<b>1</b>) and [(Me₃Si)₂NC­(RN)₂]­Ce­[N­(SiMe₃)₂]₂ (R = ^<i>i</i>Pr, <b>1-</b>^{<i><b>i</b></i>}<b>Pr</b>; R = Cy, <b>1-Cy</b>), with <i>C</i>_3<i>v</i> and <i>C</i>_2<i>v</i> solution symmetries display absorptive 4f → 5d electronic transitions in the visible region. Emission bands are observed at 553, 518, and 523 nm for <b>1</b>, <b>1-</b>^{<i><b>i</b></i>}<b>Pr</b>, and <b>1-Cy</b> with lifetimes of 24, 67, and 61 ns, respectively. Time-dependent density functional theory (TD-DFT) studies on <b>1</b> and <b>1-</b>^{<i><b>i</b></i>}<b>Pr</b> revealed the ²A₁ excited states corresponded to singly occupied 5d_<i>z</i>² orbitals. The strongly reducing metalloradical character of <b>1</b>, <b>1-</b>^{<i><b>i</b></i>}<b>Pr</b>, and <b>1-Cy</b> in their ²A₁ excited states afforded photochemical halogen atom abstraction reactions from sp³ and sp² C–X (X = Cl, Br, I) bonds for the first time with a lanthanide cation. The dehalogenation reactions could be turned over with catalytic amounts of photosensitizers by coupling salt metathesis and reduction to the photopromoted atom abstraction reactions

Luminescent Ce(III) Complexes as Stoichiometric and Catalytic Photoreductants for Halogen Atom Abstraction Reactions

Luminescent Ce­(III) complexes, Ce­[N­(SiMe₃)₂]₃ (<b>1</b>) and [(Me₃Si)₂NC­(RN)₂]­Ce­[N­(SiMe₃)₂]₂ (R = ^<i>i</i>Pr, <b>1-</b>^{<i><b>i</b></i>}<b>Pr</b>; R = Cy, <b>1-Cy</b>), with <i>C</i>_3<i>v</i> and <i>C</i>_2<i>v</i> solution symmetries display absorptive 4f → 5d electronic transitions in the visible region. Emission bands are observed at 553, 518, and 523 nm for <b>1</b>, <b>1-</b>^{<i><b>i</b></i>}<b>Pr</b>, and <b>1-Cy</b> with lifetimes of 24, 67, and 61 ns, respectively. Time-dependent density functional theory (TD-DFT) studies on <b>1</b> and <b>1-</b>^{<i><b>i</b></i>}<b>Pr</b> revealed the ²A₁ excited states corresponded to singly occupied 5d_<i>z</i>² orbitals. The strongly reducing metalloradical character of <b>1</b>, <b>1-</b>^{<i><b>i</b></i>}<b>Pr</b>, and <b>1-Cy</b> in their ²A₁ excited states afforded photochemical halogen atom abstraction reactions from sp³ and sp² C–X (X = Cl, Br, I) bonds for the first time with a lanthanide cation. The dehalogenation reactions could be turned over with catalytic amounts of photosensitizers by coupling salt metathesis and reduction to the photopromoted atom abstraction reactions

Electrophilic Ln(III) Cations Protected by C–F → Ln Interactions and Their Coordination Chemistry with Weak σ- and π‑Donors

A homoleptic cerium­(III) amide complex, Ce­(NPh^F₂)₃ (<b>1-Ce</b>) (Ph^F = pentafluorophenyl), in an unusual pseudo-trigonal planar geometry featuring six C–F → Ce interactions was prepared. The C–F → Ln interactions in solution were evident by comparison of the ¹⁹F NMR shifts for the paramagnetic <b>1-Ce</b> with those of the 4f⁰ lanthanum­(III) analogue. Coordination of weak σ- and π-donors, including ethers and neutral arene molecules, was achieved by the reversible displacement of the weak C–F → Ce interactions. Computational studies on Ce­(NPh^F₂)₃ and Ce­(NPh^F₂)₃(η⁶-C₆H₃Me₃) provide information on the F → Ce interactions and Ce−η⁶-arene bonding

Anomalous One-Electron Processes in the Chemistry of Uranium Nitrogen Multiple Bonds

Novel reaction pathways are illustrated in the synthesis of uranium­(IV), uranium­(V), and uranium­(VI) monoimido complexes. In contrast to the straightforward preparation of UV(NSiMe3)­[N­(SiMe3)2]3 (1), the synthesis of a uranium­(V) tritylimido complex, UV(NCPh3)­[N­(SiMe3)2]3 (4), from UIII[N­(SiMe3)2]3 and Ph3CN3 was found to proceed through multiple one-electron steps. Whereas the oxidation of 1 with copper­(II) salts produced the uranium­(VI) monoimido complexes UVI(NSiMe3)­X­[N­(SiMe3)2]3 (X = Cl, Br), the reaction of 4 with CuBr2 undergoes sterically induced reduction to form the uranium­(VI) monoimido complex UVI(NCPh3)­Br2[N­(SiMe3)2]2, demonstrating a striking difference in reactivity based on imido substituent. The facile reduction of compounds 1 and 4 with KC8 allowed for the synthesis of the uranium­(IV) monoimido derivatives, K­[UIV(NSiMe3)­[N­(SiMe3)2]3] (1-K) and K­[UIV(NCPh3)­[N­(SiMe3)2]3] (4-K), respectively. In contrast, an analogous uranium­(IV) monoimido complex, K­[UIV(NPhF)­[N­(SiMe3)­PhF]], PhF = -pentafluorophenyl (6), was prepared through a loss of N­(SiMe3)2PhF concomitant with one-electron oxidation of a uranium­(III) center. The uranium­(IV) monoimido complexes were found to be reactive toward electrophiles, demonstrating N–C and N–Si single bond formation. One-electron reduction of nitrite provided a route to the uranium­(VI) oxo/imido complex, [Ph4P]­[UVIO­(NSiMe3)­[N­(SiMe3)2]3]. The energetics and electrochemical processes involved in the various oxidation reactions are discussed. Finally, comparison of the UVI(NSiMe3)­X­[N­(SiMe3)2]3, X = Cl, Br, complexes with the previously reported UVIOX­[N­(SiMe3)2]3, X = Cl, Br, complexes suggested that the donor strength of the trimethylsilylimido ligand is comparable to the oxo ligand

Anomalous One-Electron Processes in the Chemistry of Uranium Nitrogen Multiple Bonds

Novel reaction pathways are illustrated in the synthesis of uranium­(IV), uranium­(V), and uranium­(VI) monoimido complexes. In contrast to the straightforward preparation of U^V(NSiMe₃)­[N­(SiMe₃)₂]₃ (<b>1</b>), the synthesis of a uranium­(V) tritylimido complex, U^V(NCPh₃)­[N­(SiMe₃)₂]₃ (<b>4</b>), from U^III[N­(SiMe₃)₂]₃ and Ph₃CN₃ was found to proceed through multiple one-electron steps. Whereas the oxidation of <b>1</b> with copper­(II) salts produced the uranium­(VI) monoimido complexes U^VI(NSiMe₃)­X­[N­(SiMe₃)₂]₃ (X = Cl, Br), the reaction of <b>4</b> with CuBr₂ undergoes sterically induced reduction to form the uranium­(VI) monoimido complex U^VI(NCPh₃)­Br₂[N­(SiMe₃)₂]₂, demonstrating a striking difference in reactivity based on imido substituent. The facile reduction of compounds <b>1</b> and <b>4</b> with KC₈ allowed for the synthesis of the uranium­(IV) monoimido derivatives, K­[U^IV(NSiMe₃)­[N­(SiMe₃)₂]₃] (<b>1-K</b>) and K­[U^IV(NCPh₃)­[N­(SiMe₃)₂]₃] (<b>4-K</b>), respectively. In contrast, an analogous uranium­(IV) monoimido complex, K­[U^IV(NPh^F)­[N­(SiMe₃)­Ph^F]], Ph^F = -pentafluorophenyl (<b>6</b>), was prepared through a loss of N­(SiMe₃)₂Ph^F concomitant with one-electron oxidation of a uranium­(III) center. The uranium­(IV) monoimido complexes were found to be reactive toward electrophiles, demonstrating N–C and N–Si single bond formation. One-electron reduction of nitrite provided a route to the uranium­(VI) oxo/imido complex, [Ph₄P]­[U^VIO­(NSiMe₃)­[N­(SiMe₃)₂]₃]. The energetics and electrochemical processes involved in the various oxidation reactions are discussed. Finally, comparison of the U^VI(NSiMe₃)­X­[N­(SiMe₃)₂]₃, X = Cl, Br, complexes with the previously reported U^VIOX­[N­(SiMe₃)₂]₃, X = Cl, Br, complexes suggested that the donor strength of the trimethylsilylimido ligand is comparable to the oxo ligand