Homoleptic Acetylacetonate (acac) and β-Ketoiminate (acnac) Complexes of Uranium

Transmetalation of potassium salts of differently substituted acetylacetonate (acac) and β-ketoiminate (acnac) with [U(I)₃(dioxane)_1.5] and [U(I)₄(dioxane)₂] resulted in the formation of homoleptic, octahedral complexes [U(acnac^Ph)₃] (with acnac^Ph = 2,2,6,6-tetramethyl-5-(phenylimino)heptan-3-onate) in the oxidation states +III and +IV and the homoleptic, square prismatic complexes [U^IV(^Meacnac^Ph)₄] (with ^Meacnac^Ph = 4-(phenylimino)pentan-2-onate) and the homoleptic, square antiprismatic complexes [U(acac)₄] [with acac = 2,2,6,6-tetramethyl-3,5-heptanedionate (acac), 2,2,6,6-tetramethyl,4-methyl-3,5-heptanedionate (ac^Meac), and 2,2,6,6-tetramethyl-4-phenyl-3,5-heptanedionate (ac^Phac)] in oxidation states +III, +IV, and +V. Oxidation of [U^III(acnac^Ph)₃] (1) with AgOTf yielded [U^IV(acnac^Ph)₃][OTf] (2), which was fully characterized by single-crystal X-ray diffraction analysis, a combination of ultraviolet/visible/near-infrared, nuclear magnetic resonance, and infrared spectroscopies, and solid-state superconducting quantum interference device magnetization studies. Complexation of the sterically less encumbering ligand derivative ^Meacnac^Ph provided access to the tetravalent, square antiprismatic complex [U^IV(^Meacnac^Ph)₄] (3). Cyclovoltammetric analysis of the square antiprismatic [U^IV(acac)₄] (4), [U^IV(ac^Meac)₄] (5), and [U^IV(ac^Phac)₄] (6) revealed reversible anodic and cathodic waves, attributable to the U(III/IV) and U(IV/V) redox couples, both being chemically accessible, as tested in the case of 5. The corresponding U(III) and U(V) compounds, [K(2.2.2-cryptand)][U^III(ac^Meac)₄] (7) and [U^V(ac^Meac)₄][SbF₆] (8), were synthesized accordingly. Unfortunately, reduced 7 proved to be too reactive for isolation and could only be detected by electron paramagnetic resonance spectroscopy. Notably, electrochemical studies on homoleptic uranium(IV) complexes with differently derivatized (R) ac^Rac ligands (R = H, Me, or Ph) feature large electrochemical windows of up to 2.91 V, measured between the uranium(III) and the uranium(V) species, in addition to high stability toward repeated potential scans.