Structural and Luminescence Study of the 3:2 Complex between Europium Nitrate and the B Isomer of Dicyclohexyl-18-crown-6

The crystal and molecular structure of bisdinitrato-(2,4,8,15,18,21-hexaoxatricyclo20.4.0.0^9,14]hexacosane)europium(III)]pentakis(nitrato)europiate(III) Eu(NO₃)₂L_B]₂(Eu(NO₃)₅]), has been determined at 170 K from single-crystal X-ray diffraction. The complex crystallizes in the monoclinic space group P2₁/n (ITC No. 14): a = 16.338(3) Å, b = 15.704(3) Å, c = 24.474(4) Å, β = 97.73(1)°, Z = 4. The structure was refined to a final R value of 0.058 (R_w = 0.060). The asymmetric unit contains three independent ions lying on general positions: Eu(NO₃)₅]^2? and two distinct Eu(NO₃)₂L_B]⁺ cations with the macrocyclic ligand in the cis-anti-cis conformation (B-isomer). The Eu^IIIions are 10-coordinate with the following mean bond lengths: Eu–O(nitrate) = 2.48(2) Å in the anion and 2.45(2) Å in the two cations, Eu–O(ether) = 2.56(8) and 2.55(5) Å. Small but significant differences are observed between the two complex cations, especially with respect to the positions of the cyclohexyl substituent. A conformational analysis performed on the six O-atoms of the complex cations confirms the predictions of a simple model. The metal ion sites of the complex have been probed by high-resolution excitation and emission spectra at 296 and 77 K. The ⁵D₀←⁷F₀ excitation spectrum displays two main bands along with several other minor components. A detailed analysis of the corresponding and selectively excited emission spectra leads to the observation of three types of spectra corresponding to the three crystallographically different Eu^III ions. Moreover, three minor sites are identified, one anionic and two cationic, with a population equal to ca. 10% of the population of the main sites. We interpret this finding as reflecting the presence of molecules with slightly different conformations.