Synthesis, characterization, and multielectron reduction chemistry of uranium supported by redox-active α-diimine ligands

摘要

Uranium compounds supported by redox-active α-diimine ligands, which have methyl groups on the ligand backbone and bulky mesityl substituents on the nitrogen atoms {~(Mes)DAB~(Me) = [ArN=C(Me)C(Me)=NAr], where Ar = 2,4,6-trimethylphenyl (Mes)}, are reported. The addition of 2 equiv of ~(Mes)DAB~(Me), 3 equiv of KC_8, and 1 equiv of UI_3(THF)_4 produced the bis(ligand) species (~(Mes)DAB~(Me))_2U(THF) (1). The metallocene derivative, Cp_2U(~(Mes)DAB~(Me)) (2), was generated by the addition of an equimolar ratio of ~(Mes)DAB~(Me) and KC_8 to Cp_3U. The bond lengths in the molecular structure of both species confirm that the α-diimine ligands have been doubly reduced to form ene-diamide ligands. Characterization by electronic absorption spectroscopy shows weak, sharp transitions in the near-IR region of the spectrum and, in combination with the crystallographic data, is consistent with the formulation that tetravalent uranium ions are present and supported by ene-diamide ligands. This interpretation was verified by U L_(III)-edge X-ray absorption near-edge structure (XANES) spectroscopy and by variable-temperature magnetic measurements. The magnetic data are consistent with singlet ground states at low temperature and variable-temperature dependencies that would be expected for uranium(IV) species. However, both complexes exhibit low magnetic moments at room temperature, with values of 1.91 and 1.79 μ_B for 1 and 2, respectively. Iodomethane was used to test the reactivity of 1 and 2 for multielectron transfer. While 2 showed no reactivity with CH_3I, the addition of 2 equiv of iodomethane to 1 resulted in the formation of a uranium(IV) monoiodide species, (~(Mes)DAB~(Me))(~(Mes)DAB~(Me2))UI {3; ~(Mes)DAB~(Me2) = [ArN=C(Me)C(Me_2)NAr]}, which was characterized by single-crystal X-ray diffraction and U M_4- and M_5-edge XANES. Confirmation of the structure was also attained by deuterium labeling studies, which showed that a methyl group was added to the ene-diamide ligand carbon backbone.