Determination of the Effective Redox Potentials of SmI₂, SmBr₂, SmCl₂, and their Complexes with Water by Reduction of Aromatic Hydrocarbons. Reduction of Anthracene and Stilbene by Samarium(II) Iodide–Water Complex

Samarium­(II) iodide–water complexes are ideally suited to mediate challenging electron transfer reactions, yet the effective redox potential of these powerful reductants has not been determined. Herein, we report an examination of the reactivity of SmI₂(H₂O)_n with a series of unsaturated hydrocarbons and alkyl halides with reduction potentials ranging from −1.6 to −3.4 V vs SCE. We found that SmI₂(H₂O)_n reacts with substrates that have reduction potentials more positive than −2.21 V vs SCE, which is much higher than the thermodynamic redox potential of SmI₂(H₂O)_n determined by electrochemical methods (up to −1.3 V vs SCE). Determination of the effective redox potential demonstrates that coordination of water to SmI₂ increases the effective reducing power of Sm­(II) by more than 0.4 V. We demonstrate that complexes of SmI₂(H₂O)_n arising from the addition of large amounts of H₂O (500 equiv) are much less reactive toward reduction of aromatic hydrocarbons than complexes of SmI₂(H₂O)_n prepared using 50 equiv of H₂O. We also report that SmI₂(H₂O)_n cleanly mediates Birch reductions of substrates bearing at least two aromatic rings in excellent yields, at room temperature, under very mild reaction conditions, and with selectivity that is not attainable by other single electron transfer reductants.