The reactivity of samarium(II) complexes

Since the introduction of samarium diiodide (SmI2) as a single electron reductant by Kagan in the late 1970's, it has become a powerful tool of the organic chemist. This is evident from the over 100 publications a year on its use. SmI2 based reductants have been found to selectively reduce many common functional groups encountered in organic synthesis and is also useful in several C-C bond forming reactions.;In recent years, the driving force that has enhanced utility of SmI 2 has been the use of additives or co-solvents. Through the judicious choice of solvent and ligand environment the properties of SmI2 can be significantly altered. Certain combinations can lead to the production of a highly reactive single electron reductant while other combinations can lead to the production of a very stable, highly luminescent complex.;The most utilized and most studied ligand used to increase the reducing ability of SmI2 is hexamethylphosphoramide (HMPA). While HMPA remains as a useful additive in SmI2 mediated reactions its carcinogenic potential outweighs its utility as a co-solvent. As a result, the search for a non-carcinogenic replacement of HMPA, that offers the same benefits, is a worthwhile endeavor. This search led us to the utilization of glycols and their derivatives as replacements for HMPA. Glycols are high affinity ligands for SmI2, which will provide steric bulk around the metal center as well as a proton donor source.;At the opposite end of the spectrum, is developing highly stable and luminescent SmI2 species through the correct solvent/ligand combination. The addition of 15-crown-5 ether derivatives to SmI2 in acetonitrile produces this effect. These complexes demonstrate emission maxima greater than 700nm, which can potentially be used as long wavelength light emitting dyes (fluorescent probes) for biological imaging and assays. These complexes also exhibit long luminescence lifetimes, which is due to the encapsulation of the metal through a strong metal-ligand interaction and decreasing the frequency of solvent collision.;Sonication was also used as a rapid means to generate Sm(II) species in solution. Typical preparation of Sm(II) species takes a few hours by stirring the Sm metal with ligand in the desired solvent. Sonication can accomplish this task in a matter of minutes and also allows the synthesis of unique Sm(II) species which are not commonly accessible through standard protocol.