Evaluating Electron-Transfer Reactivity of Complexes of Actinides in +2 and +3 Oxidation States by using EPR Spectroscopy

The possibility that the relative reactivity of complexes of actinide metals in the +2 and +3 oxidation states could be investigated by examining reactions between An^III and An^II species of Th and U with rare-earth metal reagents that provide EPR confirmation of electron transfer reactivity has been explored. Neither Cp’’₃Th^III nor Cp’’₃U^III will reduce Cp’’₃La^III or Cp’₃Y^III (Cp’=C₅H₄SiMe₃, Cp’’=C₅H₃(SiMe₃)₂). However, both [K(2.2.2-cryptand)][Cp’’₃Th^II] and [K(2.2.2-cryptand)][Cp’’₃U^II] reduce Cp’’₃La^III and Cp’₃Y^III to form [Cp’’₃La^II]¹⁻ and [Cp’₃Y^II]¹⁻, respectively, which were identified by EPR spectroscopy. The reverse reactions also occur which indicates that the reduction potentials are similar. [Cp’’₃La^II]¹⁻ reduces Cp’₃Y^III and the reverse Y^II/La^III combination also occurs. In both cases, the reactions generate EPR spectra indicative of multiple species in the mixtures of La^II and Y^II, which is consistent with ligand exchange and demonstrates that numerous heteroleptic complexes of these Ln^II ions exist.