Synthesis, crystal and molecular structure of the 3:2 complex between europium nitrate and the A‐isomer of dicyclohexyln‐18‐crown‐6: Conformational study of the ligand

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Abstract
The crystal and molecular Structure of bis[dinitrato‐(2,5,8,15,18,21‐hexaoxatricyclo[20.4.0.09,14]hexa‐consane)europium(III)]pentakis(nitrato) europiate(III) ([Eu(NO3)2·LA]2[Eu(NO3)5]) has been determined from single‐crystal X‐ray diffraction. The complex crystallizes in the monoclinic space group P21/c (ITC No. 14): a = 13.614(3)Å, b = 21.697(4)Å, c = 22.591(5)Å, β = 107.15(2)°, Z = 4. The structure was refined to a final R value of 0.055 (Rw = 0.055). The asymmetric unit contains three independent ions with approximate C2 symmetry: [Eu(NO3)5]2− and two distinct [Eu(NO3)2.La]+ cations with the macrocyclic ligand in the cis‐syn‐cis‐conformation (A‐isomer). The Eu(III) ions are 10‐coordinated with the following mean bond lengths: EuO(nitrate) = 2.46(3)Å in the anion and the two cations, EuO(ether) = 2.55(9)Å in both cations. For the uncomplexed A‐and B‐isomers, as well as for their complexes with various metal ions, a conformational analysis has been made on the six O‐atoms of the ligand which can be considererd as a fluxional ring. In the presently reported europium complex cations, the oxygen‐ring conformation is almost a perfect boat with the metal ions lying in the least‐sqiares plane of the O‐atoms (deviation: 0.02–0.05Å). The same conformation prevails in all the complexes containing the A‐isomer(exception: dimethylthallium complex) and in most of the complexes with the B‐isomer. For this isomer, a chair conformation is found in the uncomplexed ligand, in the sodium complex, and in the complex with dimethylthallium. The occurrence of these conformations is discussed with respect to the crystallographic symmetry of the complexes and the relative mean MO and OO distances.

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