Homoleptic Acetylacetonate (acac) and β-Ketoiminate (acnac) Complexes of Uranium
Pablo Waldschmidt, Judith Riedhammer, Douglas R. Hartline, Frank W. Heinemann, Karsten Meyer
Abstract
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( t Bu acnac Ph ) 3 ] (with t Bu 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( t Bu acac) 4 ] [with acac = 2,2,6,6-tetramethyl-3,5-heptanedionate ( t Bu acac), 2,2,6,6-tetramethyl,4-methyl-3,5-heptanedionate ( t Bu ac Me ac), and 2,2,6,6-tetramethyl-4-phenyl-3,5-heptanedionate ( t Bu ac Ph ac)] in oxidation states +III, +IV, and +V. Oxidation of [U III ( t Bu acnac Ph ) 3 ] ( 1 ) with AgOTf yielded [U IV ( t Bu 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 ( t Bu acac) 4 ] ( 4 ), [U IV ( t Bu ac Me ac) 4 ] ( 5 ), and [U IV ( t Bu 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 ( t Bu ac Me ac) 4 ] ( 7 ) and [U V ( t Bu 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.