Structural Consequences of 1,4-Cyclohexanedicarboxylate Cis/Trans Isomerism in Uranyl Ion Complexes: From Molecular Species to 2D and 3D Entangled Nets.

Abstract

trans-1,4-Cyclohexanedicarboxylic acid (t-1,4-chdcH2) or the commercially available mixture of the cis and trans isomers (c,t-1,4-chdcH2) has been used in the synthesis of a series of 14 uranyl ion complexes, all obtained under solvohydrothermal conditions, some in the presence of additional metal cations and/or 2,2'-bipyridine (bipy). With its two isomeric forms having very different shapes and its great sensitivity to the experimental conditions, 1,4-chdc2- appears to be suitable for the synthesis of uranyl ion complexes displaying a wide range of architectures. Under the conditions used, the pure trans isomer gives only the complexes [UO2(t-1,4-chdc)(H2O)2] (1) and [UO2(t-1,4-chdc)] (2), which crystallize as one- and two-dimensional (1D and 2D) species, respectively. Complexes containing either the cis isomer alone or mixtures of the two isomers in varying proportion were obtained from the isomer mixture. The neutral complexes [UO2(c-1,4-chdc)(DMF)] (3) and [UO2(c-1,4-chdc)(bipy)] (4) are 2D and 1D assemblies, respectively, while all the other complexes are anionic and include various counterions. [C(NH2)3]3[H2NMe2][(UO2)4(c-1,4-chdc)6]·H2O (5) crystallizes as a three-dimensional (3D) framework with {103} topology. While [H2NMe2]2[(UO2)2(c-1,4-chdc)2(t-1,4-chdc)]·DMF·2H2O (6) is a 1D ladderlike polymer, [H2NMe2]2[(UO2)2(c-1,4-chdc)(t-1,4-chdc)2]·2H2O (7), which differs in the cis/trans ratio, is a 3-fold 2D interpenetrated network with {63} honeycomb topology. The related [H2NMe2]2[(UO2)2(c,t-1,4-chdc)3]·2.5H2O (8), with one disordered ligand of uncertain geometry, is a 3-fold 3D interpenetrated system. The two isomorphous complexes [Co(bipy)3][(UO2)2(c-1,4-chdc)3]·1.5H2O (9) and [Cd(bipy)3][(UO2)2(c-1,4-chdc)3]·1.5H2O (10) form 3D frameworks with the {103} srs topological type. In contrast, [Ni(bipy)3]2[(UO2)4(c-1,4-chdc)2(t-1,4-chdc)(NO3)6]·2H2O (11) is a molecular, tetranuclear complex due to the presence of terminal nitrate ligands. A 2-fold 3D interpenetration of frameworks with {103} ths topology is observed in [Cu(bipy)2]2[(UO2)2(c-1,4-chdc)2(t-1,4-chdc)]·2H2O (12), while [Zn(bipy)3][(UO2)2(c-1,4-chdc)3]·4H2O (13) crystallizes as a 2D net with the common {4.82} fes topological type. The additional PbII cation is an essential part of the 3D framework formed in [UO2Pb2(c-1,4-chdc)(t-1,4-chdc)2(bipy)2] (14), in which uranyl and its ligands alone form 1D subunits. Together with previous results, the solid-state uranyl emission properties of seven of the present complexes evidence a general trend, with the maxima for the complexes with O6 equatorial environments being blue-shifted with respect to those for complexes with O5 environments.