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

Transmetalation of potassium salts of differently substituted acetylacetonate (acac) and ß-ketoiminate (acnac) with [U(I)3(dioxane)1.5] and [U(I)4(dioxane)2] resulted in the formation of homoleptic, octahedral complexes [U(tBuacnacPh)3] (with tBuacnacPh = 2,2,6,6-tetramethyl-5-(phenylimino)heptan-3-onate) in the oxidation states +III and +IV and the homoleptic, square prismatic complexes [UIV(MeacnacPh)4] (with MeacnacPh = 4-(phenylimino)pentan-2-onate) and the homoleptic, square antiprismatic complexes [U(tBuacac)4] [with acac = 2,2,6,6-tetramethyl-3,5-heptanedionate (tBuacac), 2,2,6,6-tetramethyl,4-methyl-3,5-heptanedionate (tBuacMeac), and 2,2,6,6-tetramethyl-4-phenyl-3,5-heptanedionate (tBuacPhac)] in oxidation states +III, +IV, and +V. Oxidation of [UIII(tBuacnacPh)3] (1) with AgOTf yielded [UIV(tBuacnacPh)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 MeacnacPh provided access to the tetravalent, square antiprismatic complex [UIV(MeacnacPh)4] (3). Cyclovoltammetric analysis of the square antiprismatic [UIV(tBuacac)4] (4), [UIV(tBuacMeac)4] (5), and [UIV(tBuacPhac)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)][UIII(tBuacMeac)4] (7) and [UV(tBuacMeac)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) acRac 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.

Evaluation of Manganese Cubanoid Clusters for Water Oxidation Catalysis: From Well-Defined Molecular Coordination Complexes to Catalytically Active Amorphous Films.

With a view to developing multimetallic molecular catalysts that mimic the oxygen-evolving catalyst (OEC) in Nature's photosystem II, the synthesis of various dicubanoid manganese clusters is described and their catalytic activity investigated for water oxidation in basic, aqueous solution. Pyridinemethanol-based ligands are known to support polynuclear and cubanoid structures in manganese coordination chemistry. The chelators 2,6-pyridinedimethanol (H2 L1 ) and 6-methyl-2-pyridinemethanol (HL2 ) were chosen to yield polynuclear manganese complexes; namely, the tetranuclear defective dicubanes [MnII 2 MnIII 2 (HL1 )4 (OAc)4 (OMe)2 ] and [MnII 2 MnIII 2 (HL1 )6 (OAc)2 ] (OAc)2 ⋅2 H2 O, as well as the octanuclear-dicubanoid [MnII 6 MnIII 2 (L2 )4 (O)2 (OAc)10 (HOMe/OH2 )2 ]⋅3MeOH⋅MeCN. In freshly prepared solutions, polynuclear species were detected by electrospray ionization mass spectrometry, whereas X-band electron paramagnetic resonance studies in dilute, liquid solution suggested the presence of divalent mononuclear Mn species with g values of 2. However, the magnetochemical investigation of the complexes' solutions by the Evans technique confirmed a haphazard combination of manganese coordination complexes, from mononuclear to polynuclear species. Subsequently, the newly synthesized and characterized manganese molecular complexes were employed as precursors to prepare electrode-deposited films in a buffer-free solution to evaluate and compare their stability and catalytic activity for water oxidation electrocatalysis.