Uranyl–3d block metal ion heterometallic carboxylate complexes including additional chelating nitrogen donors

Abstract

Eight uranyl–3d block metal ion heterometallic carboxylate complexes including additional 2,2′-bipyridine (bipy) or 1,10-phenanthroline (phen) ligands were obtained, mainly under hydrothermal conditions, and their crystal structures were determined. The carboxylate ligands were chosen so as to provide a large range of nature and geometry of the carbon skeleton, and number of donor atoms. In the three complexes [Ni(bipy)₃]₂[UO₂(NO₃)₃]₂[UO₂(HCO₂)(NO₃)₂]₂ (1), [Mn(bipy)₃][(UO₂)₃(CH₃CO₂)(NO₃)O₂(OH)(bipy)]₂ (2) and [Ni(bipy)₃][UO₂(Hcbtc)]₂·5H₂O (3, H₄cbtc = trans,trans,trans isomer of 1,2,3,4-cyclobutanetetracarboxylic acid), the [M(bipy)₃]²⁺ cation acts as a counter-ion and the uranyl-containing species are zero-, one- or two-dimensional, respectively. The chains in 2 are notable for the presence of cation–cation interactions connecting di- and tetranuclear subunits. Genuine heterometallic complexes are formed in all the other cases, in which the 3d metal ions are chelated by either one or two bipy or phen molecules and are also bound to additional carboxylate donors. These five complexes, [UO₂Cu(thftc)(bipy)(H₂O)]·2H₂O (4, H₄thftc = (2R,3R,4S,5S)-tetrahydrofurantetracarboxylic acid), [(UO₂)₂Cu(pydc)₃(bipy)(H₂O)₂]·3H₂O (5, H₂pydc = 2,6-pyridinedicarboxylic acid), [(UO₂)₂Ni(Hcit)₂(bipy)₂(H₂O)] (6, H₄cit = citric acid), [UO₂Cu(citml)(NO₃)(phen)(H₂O)]₂·H₂O (7, H₃citml = D(−)-citramalic acid), and [(UO₂)₂Mn(ml)₂(bipy)₂]·H₂O (8, H₃ml = DL-malic acid) are all polymeric, with a dimensionality of one (5–7), two (4) or three (8). The 3d metal cation is either terminal (4, 6 and 7) or involved in the building of the polymer (5 and 8). The complexes of the closely related ligands Hcit³⁻, citml³⁻ and ml³⁻ all contain {[UO₂(L)]₂}²⁻ dimeric subunits, which are assembled into chains with decorating 3d metal ions in 6 and 7, while bridging manganese(II) ions in 8 connect chains running along two different directions to generate a three-dimensional framework. This series of complexes illustrates the different roles (counter-ion, decorating species, network node) that can be played by the 3d cations in uranyl–organic assemblies, with complex 8 in particular exemplifying neatly the expected dimensionality increase.