Structural and Spectroscopic Analysis of Ln(II) 18-crown-6 and Benzo-18-crown-6 Complexes (Ln = Sm, Eu, Yb)
Todd N. Poe, Sarah Molinari, Samantha Justiniano, Grace M. McLeod, Thomas E. Albrecht‐Schmitt
Abstract
Three Ln2+ 18-crown-6 complexes of the formula Ln(18-crown-6)I2 (Ln = Sm, Eu, Yb) were isolated from the explicit synthesis of the corresponding LnI2 salts with 18-crown-6 and tetrabutylammonium tetraphenylborate in organic media under air-free conditions. Each metal complex forms a distorted hexagonal bipyramidal geometry and crystallizes in the monoclinic space group P21/n. Comparatively, crystallization of Ln(benzo-18-crown-6)I2 (Ln = Sm, Eu, Yb) from the reaction of LnI2 with tetrabutylammonium tetraphenylborate and benzo-18-crown-6 in THF/ethanol under similarly air-free conditions yields two polymorphs. The first form, α, crystallizes in the monoclinic space group P21/c (or the nonstandard setting P21/n for α-Yb); whereas the second polymorph, β, crystallizes in P1. While the geometries of the molecules only vary slightly, the molecular packing and intramolecular contacts are quite different. In the structure of β, π–π interactions between the benzo- moieties of adjacent molecules are observed, whereas these interactions are absent in α. Despite the similarities in these classically 4fn+1 lanthanide systems, the complexes display distinct spectroscopic features in their respective absorption and photoluminescence spectra. Broadband 5d → 4f photoluminescence was observed for the Sm and Eu compounds in the NIR region and UV–visible region, respectively. None of the three Yb compounds exhibit photoluminescence UV–visible-NIR region; however, a unique photooxidation event was observed resulting in characteristic Yb(III) 4f → 4f transitions in the NIR region of the absorption spectra of these compounds. These findings are discussed along with structural comparisons of the 18-crown-6 and benzo-18-crown-6 compounds as well as other reported Ln(II) crown complexes in the literature.