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Control of structure and reactivity by ligand design : applications to small molecule activation by low-valent uranium complexes


The ability of uranium to support multiple oxidation states, achieve various coordination numbers combined with the highly reducing nature of the U(III) ion makes this metal a great candidate for small molecule activation. The challenge lies in designing the appropriate ligand that can stabilize a highly reactive uranium center while simultaneously promoting activation and functionalization of small molecules in a controlled manner. Presented herein are the syntheses of two new ligand systems for uranium coordination chemistry. The effects of different ligand environments on the reactivity of trivalent uranium complexes toward small molecules are investigated. The U(III) complexes containing triazacylononane tris- aryloxide ligands [((R/ArO)₃/tacn)U] (R = t-Bu, Ad) were found to stabilize charge-separated species with radical anionic ligands. Isolation of uranium ketyl radical complex, [((t-Bu/ArO₃/tacn)UIV(OC*t-Bu/Ph₂)], was achieved through a one-electron reduction of di-tert-butyl benzophenone with [((t-Bu/ArO₃/tacn)U]. One-electron reduction of diphenyldiazomethane by [((Ad/ArO)₃/tacn)U] generated a highly reactive charge-separated intermediate species [(([((Ad/ArO)₃/tacn)U([Eta]²-NNCPh₂)]⁺+С that underwent C--H activation, N-insertion, and H₂ elimination to yield the uranium indazole complex [((Ad/ArO)₃/ tacn)U([Eta]²-3-phen(Ind))]. A comparatively bulkier diamantyl functionalized ligand system was developed and the reactivity of the corresponding U(III) precursor complex was investigated. The molecular structures obtained for [((Dia/ArO₃/tacn)U(Cl)] and [((Dia/ArO₃/ tacn)U(NTMS)] revealed a much deeper coordination cavity than in those of corresponding uranium complexes supported by the tert-butyl and adamantyl ligand systems. The deeper hydrophobic pocket is potentially advantageous for activation and functionalization of alkanes. A highly reactive trivalent uranium complex was also prepared supported by a more flexible single N-anchored ligand, [((Ad/ArO)₃/N)U]. This U(III) complex is reactive towards a wide range of substrates. For instance, reductive cleavage of CO₂ was observed to form bridging carbonate complex [{((Ad/ArO)₃/N)U}₂/[Mu]-[Eta]¹:[Kappa]²/-CO₃)]. A rare transformation involving reductive coupling of CS₂ was achieved generating uranium thiooxalate complex [{((Ad /ArO)₃/N)U}₂/[Mu-[Kappa]²:[Kappa]²-C₂S₄)]. Activation of organic azides such as azidotrimethylsilane yielded [((Ad/ ArO)₃/N)U(N₃)] and [((Ad/ArO)₃/N)U(NTMS)]. Uranium mesitylimide complex [((Ad/ArO)₃/N)U(NMes)] can also be synthesized from activation of mesityl azide. The trivalent uranium complex [((Ad/ArO)₃/N)U] can also activate chalcogenides such as elemental sulfur and selenium to form bridging chalcogenide complexes of the type [{((Ad/ArO)₃/N)U}₂/[Mu]-E)], E = S, Se. In the presence of Na/Hg, [Na₂][{((Ad/ArO)₃/N)U}₂/[Mu]-E)₂]-type complexes, E = S, Se, Te were also obtained

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