Evaluating Electron Transfer Reactivity of Rare-Earth Metal(II) Complexes Using EPR Spectroscopy

To evaluate the relative reducing capacities of rare-earth metal complexes of Sc(II), Y(II), and complexes of the lanthanide metals in their +2 oxidation state, a series of reactions of trivalent LnIIIA3 compounds with divalent [Ln′IIA′3]1– complexes has been examined, where Ln = Sc, Y, or a lanthanide and A is C5H4SiMe3 (Cp′), C5H3(SiMe3)2 (Cp″), C5Me4H (Cptet), N(SiMe3)2 (NR2), 2,6-tBu2-C6H3O (OAr), or 2,6-tBu2-4-Me-C6H2O (OAr′). The specific combinations were chosen to allow evaluation by EPR spectroscopy of the Ln(II) complex. The [LnIICp′3]1– complexes of Y(II), La(II), and Lu(II) have similar reducing abilities in that they all reduce LnIIICp′3 complexes of the other metals in this group. However, these Y(II), La(II), and Lu(II) complexes all are stronger reductants than [GdIICp′3]1–, which cannot reduce LnIIICp′3 complexes of Y, La, and Lu. These results do not apply to all ligand sets, since [GdII(NR2)3]1– can reduce YIII(NR2)3 to [YII(NR2)3]1–. The amide and aryloxide complexes of Y and Sc are similar in that [YII(NR2)3]1– reduces ScIII(NR2)3 and [YII(OAr′)3]1– reduces ScIII(OAr′)3. Both [YII(NR2)3]1– and [YII(OAr′)3]1– reduce YIIICp′3. [LaIICptet3]1– has reductive capacity similar to that of [LaIICp′3]1–, and both are stronger reductants than [LaIICp″3]1–. None of the LnIII2 complexes of Sm, Tm, Dy, and Nd can reduce LnIIIA3 complexes of Y and La to [LnIIA3]1–. In the "same-metal-different-ligands" reactions, multiple EPR signals are found, suggesting that ligand exchange occurs alongside the electron transfer reactivity.