Isolation of Inner-Sphere Aquo Complexes of Samarium(II)

Abstract

The cis-anti-cis and cis-syn-cis isomers of [Sm(dicyclohexano-18-crown-6)(H₂O)₂]I₂ exhibiting trans water molecules bound to the Sm²⁺ ion have been isolated and characterized. Sm²⁺ possesses an electrochemical potential sufficient for water reduction, and thus these complexes add to the recent body of evidence that the oxidation of Sm²⁺ by water can operate by a mechanism that is not straightforward. These complexes are obtained by the direct addition of stoichiometric amounts of water to solutions of the respective Sm(dicyclohexano-18-crown-6)I₂ isomers under an inert atmosphere. The parent complex, Sm(dicyclohexano-18-crown-6)I₂, lacking coordinating water molecules can be obtained through rigorous exclusion of water. It was determined that the bulky cyclohexano-substituents deter intramolecular interactions between [Sm(dicyclohexano-18-crown-6)(H₂O)₂]I₂ complexes and slow the oxidization of the metal centers. The extent of the stability of these complexes to the presence of water has been further probed through cyclic voltammetry, where it was found that the redox potential of both isomers of [Sm(dicyclohexano-18-crown-6)(H₂O)₂]I₂ maintains quasi-reversible behavior with a 50,000-fold excess of water to Sm²⁺ in solution with the cis-syn-cis complex being quasi-reversible at even higher concentrations of water. Solution-phase spectroscopy of these complexes in acetonitrile shows a corresponding hypsochromic shift of the Sm²⁺ 4f → 5d transition typically observed in the visible region from Sm²⁺ complexes. The crystalline compounds obtained in this study support solid-state spectroscopic trends observed from other Sm²⁺ crown-ether complexes containing iodide counterions, wherein the proximity of the iodide ions to the metal center determines whether the complex can exhibit 4f → 4f photoluminescence.