Synthesis, Structures, and Properties of Uranyl Hybrids Constructed by a Variety of Mono- and Polycarboxylic Acids

Abstract

A series of uranyl-organic coordination polymers have been hydrothermally synthesized by using a variety of carboxylic ligands, 3,3'-((2-((3-carboxyphenoxy)methyl)-2-methylpropane-1,3-diyl)bis(oxy))dibenzoic acid (H3L(1)), 4,4'-(3-(4-carboxyphenethyl)-3-hydroxypentane-1,5-diyl)dibenzoic acid (H3L(2)), chelidamic acid (H2L(3)), and benzoic acid (HL(4)) in the presence of N-bearing coligands, including 2,2'-bipyridine (bipy), 1,10-phenanthroline (phen), 1-([1,1'-biphenyl]-4-yl)-1H-imidazole (bpi), and 1,4-di(1H-imidazol-1-yl)benzene (dib). Compounds (UO2)(HL(1)) (1) and Zn(H2O)3(UO2)2(O)(OH)(L(2))·H2O (2) are constructed by semirigid ligands. The former is a one-dimensional ribbon-like structure with UO7 pentagonal bipyramids as the building unit, while the latter adopts a tetramer of UO7 pentagonal bipyramids to build a layered structure. Mononuclear UO7 pentagonal bipyramids are connected by L(3) groups to generate a two-dimensional arrangement of UO2(L(3))(H2dib)0.5 (3), in which the protonated dib molecules provide space filling and form π···π interactions with the layers. Compounds UO2(L(3))(phen) (4), UO2(L(3))2(Hbpi)2 (5), and UO2(L(4))2(bipy) (6) are molecular complexes, in which 4 and 6 are neutral, and 5 comprises protonated bpi as the counterion. The uranyl center in compound 4 is chelated by one phen and one L(3) group to form a UO5N2 pentagonal bipyramid, while in compound 5, two L(3) groups are coordinated to an uranyl center, producing a UO8 polyhedron. Compound 6 consists of a UO6N2 polyhedron of uranyl unit coordinated by one bipy and two benzoate groups. Compounds Zn(phen)3[(UO2)(C2O4)(L(4))]2 (7) and Zn(bpi)2(UO2)(O)(C2O4)0.5(L(4))·H2O (8) feature one-dimensional structures. In 7, UO7 pentagonal bipyramids are alternatively connected by oxalate groups to form the chain, in which unidentate benzoate groups are coordinated to the uranium atoms. Zn(phen)3 cations fill the void space of the chains to compensate the negative charge. Differently, the chain of 8 can be seen as the heterometallic tetramer of UO7 and ZnO2N2 polyhedra connected by oxalate groups, and then bpi and benzoate groups are coordinated to the chain. All of the compounds have been characterized by IR and photoluminescent spectroscopy, and compounds 2, 3, 5, 6, and 8 exhibit characteristic emissions of uranyl cations.