Atypical Spectroscopic Behavior in Divalent Lanthanide Dibenzyldiaza-18-crown-6 Complexes (Ln = Sm, Eu, Yb)

Direct synthesis of LnI(2) with 1,10-dibenzyl-1,10-diaza-18-crown-6 in organic media under air-free conditions yields several new Ln(2+) diaza-crown complexes, where Ln = Sm, Eu, and Yb. Increased lability because of the nitrogen donors in the backbone of the ligand leads to distortion of the diaza-crown upon coordination such that the oxygen donors in the crown ether share closer interactions with the Ln(2+) center. This results in three isostructural 8-coordinate complexes of the formula Ln(1,10-dibenzyl-1,10-diaza-18-crown-6)I-2 that exhibit a distorted hexagonal bipyramidal geometry and crystallize in the monoclinic space group P2(1)/c (beta). Alternatively, the larger ionic radii of Sm2+ and Eu2+ support longer Ln-O-bonding interactions leading to structural rearrangement of the crown ether molecule, forming two separate complexes of the same chemical formula that exhibit a less distorted hexagonal bipyramidal geometry as a result. This structural rearrangement alters crystal packing, yielding a separate polymorph that crystallizes in the orthorhombic space group Pbcn (alpha). While alpha and beta do not exhibit differences in spectroscopic behavior, variable-temperature solid-state absorption and photoluminescence spectroscopy reveal unique behavior for each compound in comparison with other Ln(2+) crown ether complexes reported in the literature. While Sm2+ luminescence was effectively quenched in the solid state by the 1,10-dibenzyl-1,10-diaza-18-crown-6 ligand, the 5d. 4f emission of the Eu2+ analogue was significantly enhanced, yielding visible blue emission at 20 degrees C and a more intense emission at -180 degrees C. Interestingly, excitation from the Xe lamp of the solid-state spectrophotometer caused a photooxidation event in beta-Yb characterized by a noticeable color change in the single crystals and an accompanying broad band in the visible region. This was identified as a short-lived organic radical resulting in the irreversible photooxidation of the metal center indicated by the in-growth of Yb3+ 4f -> 4f transitions in the near-infrared (NIR) region of the absorption spectrum at 20 degrees C. Implications of these findings are discussed along with comparisons to relevant literature data.