Molecular and Electronic Structure, and Hydrolytic Reactivity of a Samarium(II) Crown Ether Complex

The reaction of SmI₂ with dibenzo-30-crown-10 (DB30C10), followed by metathesis with [Bu₄N][BPh₄], allows for the isolation of [Sm^II(DB30C10)][BPh₄]₂ as bright-red crystals in good yield. Exposure of [Sm(DB30C10)]²⁺ to solvents containing trace water results in the conversion to the dinuclear Sm^III complex, Sm₂(DB30C10)(OH)₂I₄. Structural analysis of both complexes shows substantial rearrangement of the crown ether from a folded, Pac-Man form with Sm^II to a twisted conformation with Sm^III. The optical properties of [Sm^II(DB30C10)][BPh₄]₂ exhibit a strong temperature dependence and change from broad-band absorption features indicative of domination by 5d states to fine features characteristic of 4f → 4f transitions at low temperatures. Examination of the electronic structure of these complexes via ab initio wave function calculations (SO-CASSCF) shows that the ground state of Sm^II in [Sm^II(DB30C10)]²⁺ is a 4f⁶ state with low-lying 4f⁵5d¹ states, where the latter states have been lowered in energy by ∼12 000 cm^-1 with respect to the free ion. The decacoordination of the Sm^II cation by the crown ether is responsible for this alteration in the energies of the excited state and demonstrates the ability to tune the electronic structure of Sm^II.