Mono- and dinuclear uranyl(VI) complexes with chiral Schiff base ligand

Abstract

Slow evaporation of an equimolar solution containing chiral uranyl–salen–pyrene complex and (L)-phenylalanine tetramethylammonium salt ((L)-Phe-TMA) in toluene/EtOH mixture produced crystals built by anionic mononuclear [(UO2)(L)(L′)]− and dinuclear [(UO2)2(L)2(μ2-OH)]− (L = salen–pyrene, L′ = pyrenolate) complexes, counterbalanced by tetramethylammonium (TMA) cations. The single-crystal X-ray diffraction analysis shows that in both complexes the salen–pyrene ligand acts as a tetradentate ligand through its nitrogen and oxygen atoms chelating the UO22+ ion at the equatorial plane. Uranium centers show a seven-coordinated pentagonal–bipyramidal environment, where the fifth equatorial site is occupied by an oxygen atom from the pyrenolate ligand in the monomer and a μ2-OH bridging group in the dimer. In the dinuclear complex the central μ2-OH bridges two UO22+ ions forming a double-stranded helical structure. TMA cations are sandwiched between the anionic uranyl–salen–pyrene complexes through weak hydrogen bonds and cation⋯π interactions. This produces alternating monomer/TMA/dimer/TMA pillars assembled by van der Waals interactions in the three-dimensional crystal lattice. The chiral uranyl–salen–pyrene receptor and the mixture of mono- and dimeric complexes have been also characterized by ESI-MS, UV–Vis and fluorescence spectroscopy.