SamariumII

Samarium(II) refers to compounds in which samarium is in the +2 oxidation state. Sm2+ is a strong one-electron reductant and is comparatively less stable than Sm3+ in water but can be stabilized in nonaqueous solvents and with suitable ligands. The most important member of this family is samarium(II) iodide (SmI2), a deep-blue solution in tetrahydrofuran (THF) that is widely used in organic synthesis. SmI2 is typically prepared in situ by reducing samarium(III) iodide (SmI3) with zinc or by reducing samarium metal with iodine in THF. Other divalent samarium reagents such as SmCl2 and SmBr2 are also employed, often in coordinating solvents like THF, glyme, or diglyme, where the Sm2+ center is stabilized.

Sm2+-reagents operate via single-electron transfer to substrates, enabling a range of reductive and radical processes. The best-known applications include reductive dehalogenation of alkyl halides, reductions of carbonyl compounds to alcohols, and, in the presence of proton donors, the pinacol coupling of carbonyl compounds to give 1,2-diols. SmI2 also promotes various cyclizations and fragmentations that are difficult with other reductants. Reaction outcomes are highly tunable through solvent choice, co-solvents, and additives such as water or alcohols, which influence reactivity and selectivity. Ligand environments such as HMPA can dramatically enhance reactivity, though toxic alternatives like glyme/diglyme are often preferred for safety.

Handling and safety are essential: SmI2 and related reagents are air- and moisture-sensitive, requiring inert-atmosphere techniques and storage under nitrogen or argon. Proper laboratory safety practices should be followed, given the reagents’ reducing power and potential hazards.