Molecular structures of HMPA-coordinated samarium(II) and ytterbium(II) iodide complexes. A structural basis for the HMPA effects in SmI2-promoted reactions

The reaction of ytterbium(II) iodide (YbI2) with 4.5 equiv of hexamethylphosphoric triamide (hmpa) in tetrahydrofuran (thf) gave [Yb(hmpa)(4)(thf)(2)]I-2 (1) as yellow crystals. A similar reaction of SmI2 with hmpa afforded black-purple [SmI2(hmpa)(4)] (2) after recrystallization from toluene. When 10 equiv of hmpa were used in the reaction with SmI2, [Sm-(hmpa)(6)]I-2 (3) was obtained as purple blocks. Complexes 1-3 have been structurally characterized by crystallographic studies. The structural data of these complexes satisfactorily explain the hmpa effects observed in SmI2-promoted reactions, and also provide a guideline for the use of HMPA in these reactions.