Abstract
Uranyl nitrate was treated with racemic or enantiopure (1R,2R) forms of trans-1,2-cyclohexanedicarboxylic acid (H2chdc and R-H2chdc, respectively) in the presence of additional cations, mostly alkali or alkaline-earth metal cations, under solvohydrothermal conditions to generate a series of one homo- and seven heterometallic complexes which all contain the pseudotetrahedral [(UO2)4((R-)chdc)6]4– cluster previously found in sodium(I)-, silver(I)-, and lead(II)-containing derivatives. These clusters are for the first time obtained as isolated species in [NH4]4[(UO2)4(chdc)6] (1), in which the ammonium cations are held by hydrogen bonds close to the faces of the tetrahedron. In both compounds [(UO2)4K4(R-chdc)6(H2O)6] (2) and [(UO2)4Ba2(R-chdc)6(H2O)8] (8), the uranyl tetrahedra are assembled into three-dimensional frameworks by bridging potassium(I) or barium(II) cations, these being bound to carboxylate groups from different clusters. A closer association of uranyl tetrahedra and countercations is found wi...
Highlights
During an investigation of complexes formed by uranyl ions with racemic or (1R,2R) enantiopure trans-1,2cyclohexanedicarboxylic acid (H2chdc and ring in (R-)H2chdc, respectively), we have recently found that, in the presence of NaI, AgI or PbII cations, anionic uranyl tetrahedral clusters [(UO2)4((R-)chdc)6]4– (in which (R-)chdc2– denotes either of the two forms, racemic or enantiopure, of the dianionic ligand) are readily formed, which are assembled into three-dimensional (3D) frameworks by bridging counterions.[27]
The organic cosolvent is retained as a coligand in complex 4 only, but its importance, even in cases in which it is absent in the final compound, is well illustrated by complex 7: when the same experiment is performed in pure water, the previously reported complex [UO2(R-chdc)] (9), which crystallizes as a two-dimensional assembly,[27] is obtained instead
The chirality of the ligand appears to have little effect on the ease of formation of the tetranuclear cluster, this is not true in relation to the exact symmetry of the cluster or more generally of the reaction of (R-)chdc2– with uranyl ions,[27] but it is notable that most complexes in the present series contain the enantiopure Rchdc2– ligand, possibly indicating readier crystallization in this case
Summary
Polynuclear uranyl-containing closed species, whether they be tubules,[1,2,3,4,5,6,7,8,9,10,11] cages[9,12,13,14,15,16,17,18,19,20,21] or large rings,[14,22,23] are appealing for esthetic reasons, and because they go against the general propensity of the uranyl cation to generate quasi-planar assemblies, as most often observed in uranyl–organic coordination polymers.[24,25,26] To ensure closure around a central cavity, the equatorial ligands on the uranyl centres have to provide the required curvature, and this can be achieved with a variety of anions, from the simplest such as peroxides[12,17,18,19,20,21] to more complicated ones such as polytopic organic ligands. An excitation wavelength of 420 nm, a commonly used point only part of a broad manifold, was used in all cases and the emission was monitored between 450 and 650 nm
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