Experimental and Theoretical Studies on the Implications of Halide-Dependent Aqueous Solvation of Sm(II)

The addition of water to samarium(II) has been demonstrated to have a significant impact on the reduction of organic substrates, with the majority of research dedicated to the most widely used reagent, samarium diiodide (SmI₂). The work presented herein focuses on the reducing capabilities of samarium dibromide (SmBr₂) and demonstrates how the modest change in halide ligand results in observable mechanistic differences between the SmBr₂-water and the SmI₂-water systems that have considerable implications in terms of reactivity between the two reagents. Quantum chemical results from Born-Oppenheimer molecular dynamics simulations show significant differences between SmI₂-water and SmBr₂-water, with the latter displaying less dissociation of the halide, which results in a lower coordination number for water. Experimental results are consistent with computational results and demonstrate that the coordination sphere of SmBr₂ is saturated at lower concentrations of water. In addition, coordination-induced bond-weakening of the O-H bond is demonstrably different for water bound to SmBr₂, leading to an estimated O-H bond-weakening of at least 83 kcal/mol, nearly 10 kcal/mol larger than the bond-weakening observed in SmI₂-H₂O. Experimental results also demonstrate that the use of alcohols in place of water with SmBr₂ leads to substrate reduction, albeit several orders of magnitude slower than for SmBr₂-water. The difference in rates resulting from the change in proton donor is attributed to a rate-limiting proton-coupled electron transfer in SmBr₂-water and a sequential electron transfer then proton transfer in SmBr₂-alcohol systems, where electron transfer is rate-limiting.