Determination of the Effective Redox Potentials of SmI<sub>2</sub>, SmBr<sub>2</sub>, SmCl<sub>2</sub>, 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 SmI<sub>2</sub>(H<sub>2</sub>O)<sub><i>n</i></sub> 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 SmI<sub>2</sub>(H<sub>2</sub>O)<sub><i>n</i></sub> 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 SmI<sub>2</sub>(H<sub>2</sub>O)<sub><i>n</i></sub> determined by electrochemical methods (up to −1.3 V vs SCE). Determination of the effective redox potential demonstrates that coordination of water to SmI<sub>2</sub> increases the effective reducing power of Sm­(II) by more than 0.4 V. We demonstrate that complexes of SmI<sub>2</sub>(H<sub>2</sub>O)<sub><i>n</i></sub> arising from the addition of large amounts of H<sub>2</sub>O (500 equiv) are much less reactive toward reduction of aromatic hydrocarbons than complexes of SmI<sub>2</sub>(H<sub>2</sub>O)<sub><i>n</i></sub> prepared using 50 equiv of H<sub>2</sub>O. We also report that SmI<sub>2</sub>(H<sub>2</sub>O)<sub><i>n</i></sub> 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.