A closed synthetic cycle for the transformation of carbon dioxide to carbonate with a mid-valent tris(aryloxide)ligated uranium(IV) terminal oxo complex is presented herein. Starting from the previously reported uranium(III) complex, [U III (OAr Ad,Ad,Me ) 3 ], the uranium(V) terminal oxo complex, obtained by oxidation with N 2 O or NO, can be reduced with KC 8 to yield CO 2 activating anionic uranium(IV) oxo complexes [( Ad,Ad,Me ArO) 3 U IV (O)] − . The cycle proceeds with the formation of [( Ad,Ad,Me ArO) 2 (κ 2 -Ad,Ad,Me ArOCO 2 )U IV (μ-κ 1 :κ 2 -CO 3 )] − upon reaction of the oxo anion with excess CO 2 that adds to the terminal oxido ligand and inserts into the uranium−aryloxide bond. Treating this complex with trimethylsilyl halide eliminates both carbonates and generates, for instance, [K(2.2.2-crypt)]-[( Ad,Ad,Me ArO) 3 U IV (I) 2 ], which can ultimately be reduced with KC 8 to recover the trivalent starting material, thus closing the cycle. In contrast, reaction of the 18-crown-6 stabilized U(IV) oxo analogue with CO 2 yields a rare and isolable example of a uranium complex with a μ-κ 1 :κ 2 -CO 3 -bound carbonate weakly interacting with the [K(18-crown-6)(THF)] + moiety; here, CO 2 insertion and formation of an aryl carbonate ligand are not observed.

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( tBu acnac Ph ) 3 ] (with tBu 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 ( Me acnac Ph ) 4 ] (with Me acnac Ph = 4-(phenylimino)pentan-2-onate) and the homoleptic, square antiprismatic complexes [U( tBu acac) 4 ] [with acac = 2,2,6,6-tetramethyl-3,5-heptanedionate ( tBu acac), 2,2,6,6-tetramethyl,4-methyl-3,5-heptanedionate ( tBu ac Me ac), and 2,2,6,6-tetramethyl-4-phenyl-3,5-heptanedionate ( tBu ac Ph ac)] in oxidation states +III, +IV, and +V. Oxidation of [U III ( tBu acnac Ph ) 3 ] (1) with AgOTf yielded [U IV ( tBu 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 Me acnac Ph provided access to the tetravalent, square antiprismatic complex [U IV ( Me acnac Ph ) 4 ] (3). Cyclovoltammetric analysis of the square antiprismatic [U IV ( tBu acac) 4 ] (4), [U IV ( tBu ac Me ac) 4 ] (5), and [U IV ( tBu ac Ph ac) 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 ( tBu ac Me ac) 4 ] (7) and [U V ( tBu ac Me ac) 4 ][SbF 6 ] (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 R ac 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.