Samarium(II) iodide (SmI₂) is a green solid composed of samarium and iodine, with a melting point of 520 °C ^[1]. It can be formed by high temperature decomposition of SmI₃ (the more stable iodide), but a convenient lab preparation is to react Sm powder with 1,2-diiodoethane in anhydrous THF ^[2], or CH₂I₂ may also be used ^[4]. Samarium(II) iodide is a powerful reducing agent- for example it rapidly reduces water to hydrogen. It is available commercially as a 0.1 M solution in THF.

Samarium(II)iodide has become a popular reagent for carbon-carbon bond formation, for example in a Barbier reaction (similar to the Grignard reaction) between a ketone and an alkyl iodide to form a tertiary alcohol ^[3]:

RI + R'COR ----> R(R')C(OH)R

Uses SmI₂ in THF in the presence of catalytic NiI₂.

Esters react similarly (adding two R groups), but aldehydes give by-products. The beauty of the reaction is that it is often very rapid (5 minutes or less in the cold). Also, it can be monitored by the color change that occurs as the dark blue color of SmI₂ in THF discharges to a light yellow once the reaction has occurred. The picture shows the dark colour disappearing immediately upon contact with the Barbier reaction mixture.

Work-up is with dilute HCl, and the samarium is removed as aqueous SmI₃.

The applications of SmI₂ have been reviewed ^[4].

References

1. Chemistry of the Elements, NN Greenwood & A Earnshaw, Pergamon Press.
2. J. Am. Chem. Soc. 1980, 102, 2693-8.
3. Synlett, 1996, 633-4.
4. J. Chem. Soc., Perkin Trans. I, 2001, 2727-2751.