Actinide redox-active ligand complexes: reversible intramolecular electron-transfer in U(dpp-BIAN)2/U(dpp-BIAN)2(THF)

Actinide complexes of the redox-active ligand (dpp-BIAN)(2-) (dpp-BIAN = 1,2-bis(2,6-diisopropylphenylimino)acenaphthylene), U(dpp-BIAN)(2) (1), U(dpp-BIAN)(2)(THF) (1-THF), and Th(dpp-BIAN)(2)(THF) (2-THF), have been prepared. Solid-state magnetic and single-crystal X-ray data for complex 1 indicate a ground-state U(IV)-pi*(4) configuration, whereas a (dpp-BIAN)(2-)-to-uranium electron transfer occurs for 1-THF, resulting in a U(III)-pi*(3) ground configuration. The solid-state magnetic data also indicate that interconversion between the two forms of the complex is possible, limited only by the ability of tetrahydrofuran (THF) vapor to penetrate the solid upon cooling of the sample. In contrast to those in the solid state, spectroscopic data acquired in THF indicate only the presence of the U(IV)-pi*(4) form for 1-THF in solution, evidenced by electronic absorption spectra and by measurement of the solution magnetic moment in THF-d(8) using the Evans method. Also reported is the electrochemistry of the complexes collected in CH(2)Cl(2), CF(3)C(6)H(5), and THF. As expected from the solution spectroscopic data, only small differences are observed in half-wave potentials of ligand-based processes in the presence of THF, consistent with the solution U(IV)-pi*(4) configuration of the complexes in all cases. Density functional theory calculations were undertaken for complexes 1 and 1-THF to determine if intrinsic energetic or structural factors underlie the observed charge-transfer process. While the calculated optimized geometries agree well with experimental results, it was not possible to arrive at a convergent solution for 1-THF in the U(III)-pi*(3) configuration. However, perturbations in the orbital energies in 1 versus 1-THF for the U(IV)-pi*(4) configuration do point to a diminished highest occupied molecular orbital-lowest unoccupied molecular orbital energy gap in 1-THF, consistent with the solid-state magnetic data. These results represent the first example of a stable and well-defined, reversible intramolecular electron transfer in an actinide complex with redox-active ligands.