Proton-Coupled Electron Transfer in the Reduction of Arenes by SmI2-Water Complexes

The presence of water has a significant impact on the reduction of substrates by SmI2. The reactivity of the Sm(II)-water reducing system and the relationship between sequential or concerted electron-transfer, proton-transfer is not well understood. In this work, we demonstrate that the reduction of an arene by SmI2-water proceeds through an initial proton-coupled electron transfer. The use of thermochemical data available in the literature shows that upon coordination of water to Sm(II) in THF, significant weakening of the O-H bond occurs. The derived value of nearly 73 kcal/mol for the decrease in the bond dissociation energy of the O-H bond in the Sm(II)-water complex is the largest reported to date for low-valent reductants containing bound water.