Stepwise reduction of samarium cryptates in propylene carbonate: anions and water concentration effects on the redox behavior of the Sm(III)/Sm(II) couple

The polarographic reduction of samarium(III) cryptates with cryptands 222, 221, and 22 was investigated in propylene carbonate. The samarium(III) cryptates are reduced in two consecutive steps, the first step was reversible and corresponded to the reduction of the Sm(III) to the Sm(II) cryptate. The Sm(III)/Sm(II) redox potential of the cryptates depended on the anion used in the complex and, in the case of the cryptate with chloride anions, a stable mixed complex was observed in propylene carbonate. The Sm(III)/Sm(II) redox potentials of the cryptates were more cathodic than the redox potential of the uncomplexed Sm(III)/Sm(II) couple, which is typical of a lower stability of the reduced cryptate. Propylene carbonate does not stabilize low oxidation states of lanthanides by cryptation. This is at variance with behavior observed previously in other media like water and methanol. Variations of redox potentials as a function of increasing amounts of water were accounted for by solvent shielding of samarium(...