Effect of Crown Ethers on the Ground and Excited State Reactivity of Samarium Diiodide in Acetonitrile

Electron transfer from the ground and excited states of Sm­[15-crown-5]₂I₂ complex to a series of electron acceptors (benzaldehyde, acetophenone, benzophenone, nitrobenzene, benzyl bromide, benzyl chloride, 1-iodohexane, and 1,4-dinitrobenzene) was investigated in acetonitrile. Electron transfer from the ground state of the Sm­(II)-crown system to aldehydes and ketones has a significant inner sphere component indicating that the oxophilic nature of Sm­(II) prevails in the system even in the presence of bulky ligands such as 15-crown-5 ether. Activation parameters for the ground state electron transfer were determined, and the values were consistent with the proposed mechanistic models. Since crown ethers stabilize the photoexcited states of Sm­(II), the photochemistry of Sm­[15-crown-5]₂I₂ system in solution state has been investigated in detail. The results suggest that photoinduced electron transfer from Sm­(II)-crown systems to a wide variety of substrates is feasible with rate constant values as high as 10⁷ M^–1 s^–1. The results described herein imply that the present difficulty of manipulating the extremely reactive excited state of Sm­(II) in solution phase can be overcome through stabilizing the excited state of the divalent metal ion by careful design of the ligand systems.