Coordination Chemistry and Photoluminescence of Sm(II) Dibenzo-24-crown-8 Complexes

Three Sm(II) dibenzo-24-crown-8 (db24c8) complexes were synthesized in anhydrous, air-free conditions via the reaction of SmI₂ with db24c8 and tetrabutylammonium tetraphenylborate ([TBA][BPh₄]; where needed) in acetonitrile (CH₃CN), dimethoxyethane (DME), and tetrahydrofuran (THF) to yield [Sm(db24c8)(CH₃CN)₂][BPh₄][I]·CH₃CN, [Sm(db24c8)(DME)]I₂, and [Sm(db24c8)(THF)₂]I₂, respectively. In each case, a 10-coordinate, staggered dodecahedral (2:6:2) environment is formed around the Sm²⁺ center that is completed by either two solvent molecules (CH₃CN or THF) or one bidentate solvent molecule (DME). Inner-sphere solvent molecules can be excluded by reacting SmI₂ with db24c8 in 1:3 THF:toluene to yield Sm(db24c8)I₂. This molecule features a distorted, eight-coordinate, hexagonal pyramidal Sm²⁺ metal center, where the coordinated db24c8 molecule shows a torsion angle unexpectedly close to the 180° antiperiplanar arrangement and two uncoordinated db24c8 oxygen atoms. Solution UV-vis-NIR measurements demonstrate that Sm²⁺ is a good size match for the cavity of various db24c8 conformations and that Eu²⁺ and Yb²⁺ exhibit competition between acetonitrile solvation and the Eu²⁺ and Yb²⁺/db24c8 complexes in solution. During excitation by 546 nm light, both [Sm(db24c8)(DME)]I₂ and [Sm(db24c8)(THF)₂]I₂ exhibit mixed 5d → 4f and 4f → 4f emission at 20 °C and exclusively 4f → 4f at -180 °C, whereas Sm(db24c8)I₂ only shows 5d → 4f emission regardless of temperature. Photoluminescence from [Sm(db24c8)(CH₃CN)₂][BPh₄][I]·CH₃CN is quenched.