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.