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 (SmI 2 ). The work presented herein focuses on the reducing capabilities of samarium dibromide (SmBr 2 ) and demonstrates how the modest change in halide ligand results in observable mechanistic differences between the SmBr 2 −water and the SmI 2 −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 SmI 2 −water and SmBr 2 −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 SmBr 2 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 SmBr 2 , 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 SmI 2 −H 2 O. Experimental results also demonstrate that the use of alcohols in place of water with SmBr 2 leads to substrate reduction, albeit several orders of magnitude slower than for SmBr 2 − water. The difference in rates resulting from the change in proton donor is attributed to a rate-limiting proton-coupled electron transfer in SmBr 2 −water and a sequential electron transfer then proton transfer in SmBr 2 −alcohol systems, where electron transfer is rate-limiting.

Coordination-induced desolvation or ligand displacement by cosolvents and additives is a key feature responsible for the reactivity of Sm­(II)-based reagent systems. High-affinity proton donor cosolvents such as water and glycols also demonstrate coordination-induced bond weakening of the O–H bond, facilitating reduction of a broad range of substrates. In the present work, the coordination of ammonia to SmI2 was examined using Born–Oppenheimer molecular dynamics simulations and mechanistic studies, and the SmI2-ammonia system is compared to the SmI2-water system. The coordination number and reactivity of the SmI2-ammonia solvent system were found to be similar to those of SmI2-water but exhibited an order of magnitude greater rate of arene reduction by SmI2-ammonia than by SmI2-water at the same concentrations of cosolvent. In addition, upon coordination of ammonia to SmI2, the Sm­(II)-ammonia solvate demonstrates one of the largest degrees of N–H bond weakening reported in the literature compared to known low-valent transition metal ammonia complexes.

Coordination of water to low-valent Sm leads to O–H bond-weakening that enables PCET to substrates.