Uranyl–copper(ii) heterometallic oxalate complexes: coordination polymers and frameworks

Abstract

Five oxalate complexes containing both uranyl and copper(II) ions and 2,2′-bipyridine (bipy), 2,2′-bipyrimidine (bipym) or 1,10-phenanthroline (phen) as co-ligands have been synthesized under hydrothermal conditions and their crystal structures were determined. [UO₂Cu(C₂O₄)(NO₃)₂(bipy)(H₂O)] (1) is a molecular complex in which the two cations are connected by the bis-chelating oxalate. A one-dimensional uranyl oxalate ribbon is formed in [UO₂Cu(C₂O₄)₂(bipym)(H₂O)] (2), with uranyl ions bound to two chelating anions and to a third, monodentate oxalate which holds the decorating [Cu(bipym)(H₂O)]²⁺ groups. Complex [(UO₂)₂Cu₂(C₂O₄)₂(bipym)₃(OH)₂]·2NO₃ (3) is a two-dimensional assembly in which uranyl oxalate chains are linked to one another by [Cu₂(bipym)₃]⁴⁺ dinuclear units through bridging hydroxide ions. Finally, [UO₂Cu(C₂O₄)₂(bipy)] (4) and [UO₂Cu(C₂O₄)₂(phen)] (5) display identical three-dimensional arrangements containing uranyl oxalate sheets connected to one another by copper oxalate dinuclear units. The magnetic properties of compounds 2 and 4 have been investigated. Weak inter-chain antiferromagnetic interactions between copper(II) atoms are present in 2, while 4 displays strong oxalate-mediated antiferromagnetic exchange (J = −335 cm⁻¹). These complexes are the first uranyl–d block metal oxalate complexes to be reported.