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Polynuclear Cation−Cation Complexes of Pentavalent Uranyl: Relating Stability and Magnetic Properties to Structure

2008; American Chemical Society; Volume: 130; Issue: 49 Linguagem: Inglês

10.1021/ja804766r

1943-2984

Grégory Nocton, Paweł Horeglad, Jacques Pécaut, Marinella Mazzanti,

Crystal Structures and Properties

Reaction of {[UO2Py5][μ-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[μ-K(Py)2]2[μ8-K(Py)]}2I2·Py2 (2) (in 70% yield) and {[UO2(dbm)2]2[μ-K(MeCN)2][μ8-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 UO2+···UO2+ interaction. The UO2+···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 χ versus T at ∼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.