Crown Ethers and Their Alkali Metal Ion Complexes as Assembler Groups in Uranyl–Organic Coordination Polymers with cis-1,3‑, cis-1,2‑, and trans-1,2-Cyclohexanedicarboxylates

Alkali metal cations (Na⁺, K⁺) and crown ether molecules (12C4, 15C5, 18C6) were used as additional reactants during the hydrothermal synthesis of uranyl ion complexes with cis/trans-1,3-, cis-1,2- and trans-1,2-cyclohexanedicarboxylic acids (c/t-1,3-chdcH₂, c-1,2-chdcH₂, and t-1,2-chdcH₂, respectively, the latter as racemic or pure (1R,2R) enantiomer). Oxalate anions generated in situ are present in all the six complexes isolated and crystallographically characterized, [(UO₂)₂(c-1,3-chdc)₂(C₂O₄)]­[UO₂(H₂O)₅]·(12C4)·2H₂O (1), [(UO₂)₄Na₂(c-1,2-chdc)₂(C₂O₄)₃(15C5)₂] (2), [(UO₂)₄K₂(c-1,2-chdc)₂(C₂O₄)₃(18C6)_1.5(H₂O)_1.5] (3), [(UO₂)₁₂K₅(R-t-1,2-chdc)₄(C₂O₄)₁₀(18C6)₅(OH)­(H₂O)₃]·4H₂O (4), [(UO₂)₁₂K₅(rac-t-1,2-chdc)₄(C₂O₄)₁₀(18C6)₅(OH)­(H₂O)₃]·4H₂O (5), and [(UO₂)₈K₄(rac-t-1,2-chdc)₄(C₂O₄)₆(18C6)₃(H₂O)₂] (6). In complex 1, the [UO₂(H₂O)₅]²⁺ counterions link the ladderlike uranyl-containing one-dimensional polymers and the uncomplexed crown ether molecules through hydrogen bonds. In all the other complexes, two-dimensional uranyl/chdc/oxalate subunits are formed, with topologies depending on the stoichiometry, in which the 1,2-chdc^2– ligands are bound to three uranium atoms, one of them chelated by the two carboxylate groups, and the oxalate ligands are bis-chelating. In complex 2, the Na­(15C5)⁺ cations are bound to one layer through double Na-carboxylate or Na-oxo cis-bonding and they are thus mere decorating groups. In contrast, the quasi-planar K­(18C6)⁺ groups in 3–6, partially affected by disorder, are generally trans-coordinated to two uranyl oxo groups pertaining to different layers, thus uniting the latter into a three-dimensional framework.