title: Polynuclear cation-cation complexes of pentavalent uranyl: relating stability and magnetic properties to structure.
abstract: Reaction of {[UO2Pys][mu-KI2Py2]}n (1) with 2 equiv of potassium dibenzoylmethanate (Kdbm) in pyridine or acetonitrile affords, respectively, the corresponding tetranuclear complexes of pentavalent uranyl ([UO2(dbm),2]2[mu-K(Py)2]2[mu8-K(Py)]}2I2 x Py2 (2) (in 70% yield) and {[UO2(dbm)2]2[mu-K(MeCN)2][mu8-K]}2 (3) (in 40% yield) in which four UO2+ are mutually coordinated (T-shaped "cation-cation" interaction). The X-ray structures of 2 and 3 show also the presence of, respectively, six and four potassium cations involved in UO2+...K+ interactions. Reaction of 2 with an excess of 18-crown-6 (18C6) affords the dimeric complex [UO2(dbm)2K(18C6)]2 (4) presenting a diamond-shaped interaction between two UO2+ groups, in 45% yield. 1H and PFGSTE diffusion NMR spectroscopy of 2 and 3 in pyridine show unambiguously the presence of UO2+...UO2+ and UO2+...K+ interactions (tetrametallic species) in solution, which leads to a rapid (7 days) disproportionation of pentavalent uranyl to afford [U(dbm)4] and [UO2(dbm)2] species. The UO2+...K+ interaction plays an important synergistic role in the stabilization of the UO2+...UO2+ interactions. Accordingly, the lower affinity of (K(18C6))+ for the uranyl(V) oxygen in complex 4 results in a lower number of coordinated K+ and therefore in a weakened UP2+...UO2+ interaction. The UO24+...UO2+ interactions is completely disrupted in dmso or in the presence of Kdbm, preventing disproportionation of pentavalent uranyl. Solid-state variable-temperature magnetic susceptibility studies showed the unambiguous presence of antiferromagnetic coupling between the two oxo-bridged uranium centers of complex 4, with the appearance of a maximum in chi versus T at approximately 5 K. The different behavior of the tetrameric complex 3, which probably involves a magnetic coupling occurring at lower temperature, can be ascribed to the different geometric arrangement of the interacting uranyl(V) groups.
authors.affiliation: Laboratoire de Reconnaissance Ionique et Chimie de Coordination, Service de Chimie Inorganique et Biologique (UMR-E 3 CEA-UJF), INAC, CEA-Grenoble, 38054 Grenoble Cedex 09, France.
authors.forename: Grégory
authors.initials: G
authors.lastname: Nocton
authors.forename: Pawel
authors.initials: P
authors.lastname: Horeglad
authors.forename: Jacques
authors.initials: J
authors.lastname: Pécaut
authors.forename: Marinella
authors.initials: M
authors.lastname: Mazzanti
mesh_terms: D004058:Diffusion; D009682:Magnetic Resonance Spectroscopy; D008280:Magnetics; D008958:Models, Molecular; D008968:Molecular Conformation; D009942:Organometallic Compounds / Q000737:chemistry*; D012996:Solutions; D013696:Temperature; D014501:Uranium / Q000737:chemistry*
chemical_list: D009942:Organometallic Compounds; D012996:Solutions; D014501:Uranium