Is samarium diiodide an inner- or outer-sphere electron donating agent?

Evidence for Ionic Samarium(II) Species in THF/HMPA Solution and Investigation of Their Electron-Donating Properties

“…The presence of 20 mm Bu 4 NI in the THF/0.2 m Bu 4 NPF 6 solution had no effect on the position of the peaks for the HMPA-coordinated complexes, contrary to the situation previously found for the SmI 2 /SmI 2 redox system with a shift of more than 100 mV in a negative direction. [3] Conductivity measurements: To identify more exactly the samarium complexes in the THF/HMPA solutions, the conductivity of SmI 2 , SmI 3 , and Bu 4 NI was plotted against the number of equivalents of HMPA added ( Figure 2). Whereas the conductivity of a solution of SmI 2 in pure THF was near zero as expected, it increased substantially upon addition of HMPA, surpassing even that of the corresponding Bu 4 NI solution.…”

“…Recently, we addressed the mechanistic aspects of SmI 2 -induced ET reactions involving two organic substrates, benzyl bromide and acetophenone. [3] Inner-sphere/outer-sphere concepts were invoked in the description of the transition state structures: whereas the ET to acetophenone was clearly shown to involve an inner-sphere process, the ET to benzyl bromide was closer to an outer-sphere process. A prerequisite for the above description was the elucidation of the fundamental redox behavior of the SmI 2 /SmI 2 couple by cyclic voltammetry, which provided the formal potential E8 À 0.89 V versus SCE (À 1.41 V versus ferrocenium/ferrocene (Fc /Fc)) in tetrahydrofuran (THF).…”

“…A prerequisite for the above description was the elucidation of the fundamental redox behavior of the SmI 2 /SmI 2 couple by cyclic voltammetry, which provided the formal potential E8 À 0.89 V versus SCE (À 1.41 V versus ferrocenium/ferrocene (Fc /Fc)) in tetrahydrofuran (THF). [3] The addition of hexamethylphosphoramide (HMPA) to a THF solution of the divalent lanthanide reagent leads to a substantial increase in the electron-donating abilities of SmI 2 . [2,4] Other effects observed when this cosolvent is used include an increase in the stability of the organosamarium species formed, allowing reactions to be performed under Grignard conditions, [5] enhancement of stereoselectivities, [5c, 6] and promotion of certain radical cyclizations.…”

“…[3] For this purpose clarification of the fundamental structural and redox features of the samarium complexes in THF/HMPA solutions was desirable. [10] Four different experimental approaches were used: a) cyclic voltammetry; b) conductivity measurements; c) UV spectroscopy; d) kinetic studies of ET processes involving the electron acceptors 1-iodobutane and benzyl chloride.…”

Samarium(II) Iodide Mediated Reductions − Influence of Various Additives

“…Skrydstrup and co-workers also determined the standard potential E°of SmI 2 ϩ /SmI 2 by cyclic voltammetry, which was Ϫ1.41 Ϯ 0.08 V vs. Fe ϩ / Fe. [34] Further work on the SmI 2 -HMPA reagent proved, by vapour pressure osmometry (VPO), that SmI 2 is a monomer in HMPA-THF. [35] Based on UV/Vis studies it was also concluded that there are four HMPA molecules coordinated to SmI 2 in solution.…”