Synthetic studies on the redox chemistry of trivalent uranium monoarene complexes were undertaken with a complex derived from the chelating tris(aryloxide)arene ligand (^Ad,MeArO)₃mes³⁻. Cyclic voltammetry of [{(^Ad,MeArO)₃mes}U^III] (1) revealed a nearly reversible and chemically accessible reduction at −2.495 V vs. Fc/Fc⁺—the first electrochemical evidence for a formally divalent uranium complex. Chemical reduction of 1 indicates that reduction induces coordination and redox isomerization to form a uranium(IV) hydride, and addition of a crown ether results in hydride insertion into the coordinated arene to afford uranium(IV) complexes. This stoichiometric reaction sequence provides structural insight into the mechanism of arene functionalization at diuranium inverted sandwich complexes.

The low-temperature (<−35 °C) reduction of the trivalent uranium monoarene complex [{(^Ad,MeArO)₃mes}U] (1), with potassium spheres in the presence of a slight excess of 2.2.2-cryptand, affords the quantitative conversion of 1 into the uranium(II) monoarene complex [K(2.2.2-crypt)][((^Ad,MeArO)₃mes)U] (1-K). The molecular and electronic structure of 1-K was established experimentally by single-crystal X-ray diffraction, variable-temperature ¹H NMR and X-band EPR spectroscopy, solution-state and solid-state magnetism studies, and optical absorption spectroscopy. The electronic structure of the complex was further investigated by DFT calculations. The complete body of evidence confirms that 1-K is a uranium(II) monoarene complex with a 5f ⁴ electronic configuration supported by δ backbonding and that the nearly reversible, room-temperature reduction observed for 1 at −2.495 V vs. Fc/Fc⁺ is principally metal-centered.

A rare, low-spin Fe^IV imide complex [(pyrr₂py)FeNAd] (pyrr₂py²⁻=bis(pyrrolyl)pyridine; Ad=1-adamantyl) confined to a cis-divacant octahedral geometry, was prepared by reduction of N₃Ad by the Fe^II precursor [(pyrr₂py)Fe(OEt₂)]. The imide complex is low-spin with temperature-independent paramagnetism. In comparison to an authentic Fe^III complex, such as [(pyrr₂py)FeCl], the pyrr₂py²⁻ ligand is virtually redox innocent.

Synthetic studies on the redox chemistry of trivalent uranium monoarene complexes were undertaken with a complex derived from the chelating tris(aryloxide)arene ligand (^Ad,MeArO)₃mes³⁻. Cyclic voltammetry of [{(^Ad,MeArO)₃mes}U^III] (1) revealed a nearly reversible and chemically accessible reduction at −2.495 V vs. Fc/Fc⁺—the first electrochemical evidence for a formally divalent uranium complex. Chemical reduction of 1 indicates that reduction induces coordination and redox isomerization to form a uranium(IV) hydride, and addition of a crown ether results in hydride insertion into the coordinated arene to afford uranium(IV) complexes. This stoichiometric reaction sequence provides structural insight into the mechanism of arene functionalization at diuranium inverted sandwich complexes.

The low-temperature (<−35 °C) reduction of the trivalent uranium monoarene complex [{(^Ad,MeArO)₃mes}U] (1), with potassium spheres in the presence of a slight excess of 2.2.2-cryptand, affords the quantitative conversion of 1 into the uranium(II) monoarene complex [K(2.2.2-crypt)][((^Ad,MeArO)₃mes)U] (1-K). The molecular and electronic structure of 1-K was established experimentally by single-crystal X-ray diffraction, variable-temperature ¹H NMR and X-band EPR spectroscopy, solution-state and solid-state magnetism studies, and optical absorption spectroscopy. The electronic structure of the complex was further investigated by DFT calculations. The complete body of evidence confirms that 1-K is a uranium(II) monoarene complex with a 5f ⁴ electronic configuration supported by δ backbonding and that the nearly reversible, room-temperature reduction observed for 1 at −2.495 V vs. Fc/Fc⁺ is principally metal-centered.

The first sulfite [{((^nP,MeArO)₃tacn)U^IV}₂(μ-κ¹:κ²-SO₃)] (tacn=triazacyclononane) and dithionite [{((^nP,MeArO)₃tacn)U^IV}₂(μ-κ²:κ²-S₂O₄)] complexes of uranium from reaction with gaseous SO₂ have been prepared. Additionally, the reductive activation of CO₂ was investigated with respect to the rare oxalate [{((^nP,MeArO)₃tacn)U^IV}₂(μ-κ²:κ²-C₂O₄)] formation. This ultimately provides the unique S₂O₄²⁻/C₂O₄²⁻ and SO₃²⁻/CO₃²⁻ complex pairs. All new complexes were characterized by a combination of single-crystal X-ray diffraction, elemental analysis, UV/Vis/NIR electronic absorption, IR vibrational, and ¹H NMR spectroscopy, as well as magnetization (VT SQUID) studies. Moreover, density functional theory (DFT) calculations were carried out to gain further insight into the reaction mechanisms. All observations, together with DFT, support the assumption that SO₂ and CO₂ show similar (dithionite/oxalate) to analogous (sulfite/carbonate) activation behavior with uranium complexes.

Starting from phenols ^R1,R2ArOH (5) and the anisole derivative 3,5-di-tert-butyl-2-methoxybenzyl bromide (13), a series of new tacn-based ligands (^R1,R2ArOR³)₃tacn (2) have been synthesized with substituents of varying bulkiness and electronic nature at the ortho and para positions with respect to the oxygen coordination site. It was observed that these groups not only determine the steric shielding and solubility properties of 2, but also deactivate the reactivity of the phenols in the modified Mannich reaction when electron-withdrawing groups are introduced at the para position in 5. Treatment of the ligands with UCl₄ in thf led to the isolation of four uranium(IV) chloro complexes [{(^R1,R2ArO)₃tacn}U^IVCl] (14), which were characterized by different spectroscopic and physical methods (e.g., ¹H NMR, UV/Vis, SQUID), corroborating the +4 oxidation state in 14. Single-crystal X-ray structure analyses revealed that 14a·CH₂Cl₂·CH₃CN and 14b·1.25CH₂Cl₂ crystallize in the chiral, orthorhombic space group P2₁2₁2₁ [a = 24.934(3), b = 27.941(3), c = 9.045(1) Å, V = 6302(2) ų, Z = 4] and the chiral, hexagonal space group P6₃ [a = 22.097(3), c = 17.941(2) Å, V = 7587(2) ų, Z = 4], respectively. Interestingly, complexes 14a,b self-organize in the solid state into homochiral 1D polymeric superstructures as a result of weak intermolecular C–H···Cl contacts.