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

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( 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.

How does chemistry contribute to circular economy in nuclear energy systems to make them more sustainable and ecological?

“…Very recently, Meyer's group reported synthesis and characterization of homoleptic β-diketonato and β-ketoiminato complexes of U ions in different oxidation states to develop reversible redox couples for URFB. 97 In most cases, U 4+ compounds such as [U IV ( t Bu acnac Ph ) 3 ] + , [U IV ( Me acnac Ph ) 4 ], and [U IV ( t Bu ac Me ac) 4 ] (Fig. 12a) are formed, but the corresponding U 3+ and U 5+ species may also be accessible through chemical reactions under appropriately selected conditions.…”

“…Fig. 12 Schematic structures of [U IV ( tBu acnac Ph ) 3 ] + , [U IV ( Me acnac Ph ) 4 ], and [U IV ( tBu ac Me ac) 4 ] complexes studied by Waldschmidt et al (a) and cyclic voltammograms of [U IV ( tBu ac Me ac) 4 ] in benzene/MeCN 1 : 1 solution with ∼0.1 M [N(nBu) 4 ][PF 6 ] at 200 mV s −1 potential sweep rate (b) 97. Reprinted with permission from American Chemical Society (Copyright 2023).…”

Bis(acyl)phosphide Complexes of U(III)/U(IV): A Case of a Hidden Redox-Active Ligand

“…25,26 Nevertheless, the exploration of the reactivity between U(III) or U(IV) and substituted acetylacetonate (acac)/β-ketoiminate ligands led to the isolation of trivalent, tetravalent, and pentavalent homoleptic uranium complexes, wherein the ligands were found to be redox-innocent. 27 Recently, Carpenter et al reported two homoleptic complexes of uranium with bis(mesitoyl)phosphide ( mes BAP) and bis-(2,4,6-triisopropylbenzoyl)phosphide ( tripp BAP) ligands. 28 The bis(acyl)phosphide (BAP) ligand is analogous to the acac ligand (see Figure 1a).…”

“…Interestingly, when a redox-active benzophenone ligand reacted with U 3+ , it resulted in the reduction of the carbonyl bonds, yielding ketyl radicals. − This led to elongation of the CO bonds. β-diketonate ligands possessing CO bonds were also found to be redox-active in chromium complexes. , Nevertheless, the exploration of the reactivity between U­(III) or U­(IV) and substituted acetylacetonate (acac)/β-ketoiminate ligands led to the isolation of trivalent, tetravalent, and pentavalent homoleptic uranium complexes, wherein the ligands were found to be redox-innocent . Recently, Carpenter et al reported two homoleptic complexes of uranium with bis­(mesitoyl)­phosphide ( mes BAP) and bis­(2,4,6-triisopropylbenzoyl)­phosphide ( tripp BAP) ligands .…”