The perfect samarium iodide recipe

01 April 2012 - Mythbusters

Szostak, Spain & Proctor have seriously investigated (11 JORG pages!) several procedures for the synthesis of samarium iodide, an important reagent in organic chemistry (DOI). A traditional method is reaction of 2-fold excess samarium metal with diiodoethane or iodine in THF with stringent exclusion of water and air in a Schlenk line. Prepare for some convictions smashed.
The onset of a blue colour is not really indicative of the completion of the reaction: it is not possible to distinguish a 0.005 from a 0.1 molar solution. The presence of water does not affect the overall yield of the reaction although it correlates with the induction time ranging from 10 minutes to up to 2 hours during which water eats away at the samarium metal. This is a time-saver for the front-line bench chemist. As surprise number two, against all conventional lore the reaction does not even suffer from the presence of oxygen. So get rid of that Schlenk line and quit degassing all the time! The only precaution that still makes sense is excluding oxygen from the finished product in storage because Sm(II) is easily oxidized to Sm(III). Ditto the peroxide content of THF was found to be irrelevant.
And then there is unusual case of the inactive batches of samarium metal that have plagued chemists for the past 30 years. Typically attributed to the formation of an oxide layer Szostak, Spain & Proctor did not find the corresponding evidence. The true explanation remains shrouded in mystery but at least an inactive batch can now be identified by measurement of its density. Again in the true spirit of alchemy an inactive batch can be activated by dry-rolling samarium in an inert atmosphere followed by oxidation with iodine but not with diiodoethane. Equally intriguing: the shelf-live of SmI2 derived from iodine is much shorter than that derived from diiodoethane.