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

Abstract

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.

Tools and techniques for solution‐phase structural understanding of SmI2–additive complexes

“…[ 21 ] The data plotted in Figure 10 demonstrate that iodides are relatively easily displaced compared with bromides, which is in line with the leaving group ability of these two anions. [ 21 ]…”

“…The presence of bromide ions at a close proximity of Sm(II) reduces the CN to 7.5 for water coordination to SmBr 2 . [ 21 ] These findings were supported by absorption spectroscopy and conductivity measurements. [ 21 ]…”

“…In a relatively recent study, water coordination to SmBr 2 was studied with conductivity measurements. [ 21 ] In this study, conductivity enhancement in case of SmBr 2 /water system was compared with that of SmI 2 /water system. [ 21 ] The data plotted in Figure 10 demonstrate that iodides are relatively easily displaced compared with bromides, which is in line with the leaving group ability of these two anions.…”

“…The plot of conductivity of 2.5 mM of SmI 2 (red) and SmBr 2 (blue) with increasing concentration of water. Reproduced from ref., [ 21 ] Copyright (2018), with permission from ACS…”

“…In a separate study, Ramírez‐Solís et al compared the water complex formation with SmI 2 and SmBr 2 . [ 21 ] Addition of water leads to rapid dissociation of SmI bonds, where one iodide remains at relatively short distance (approximately 3.6 Å) and other one is ejected from the first solvation shell and remains at an average distance of approximately 5.5 Å (Figure 11, left). This scenario leads to coordination number (CN) of SmI 2 8.4 for water molecule.…”