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Synthesis, Characterization, and Multielectron Reduction Chemistry of Uranium Supported by Redox-Active α-Diimine Ligands

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posted on 17.10.2011, 00:00 by Steven J. Kraft, Ursula J. Williams, Scott R. Daly, Eric J. Schelter, Stosh A. Kozimor, Kevin S. Boland, James M. Kikkawa, William P. Forrest, Christin N. Christensen, Daniel E. Schwarz, Phillip E. Fanwick, David L. Clark, Steve D. Conradson, Suzanne C. Bart
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 {MesDABMe = [ArNC(Me)C(Me)NAr], where Ar = 2,4,6-trimethylphenyl (Mes)}, are reported. The addition of 2 equiv of MesDABMe, 3 equiv of KC8, and 1 equiv of UI3(THF)4 produced the bis(ligand) species (MesDABMe)2U(THF) (1). The metallocene derivative, Cp2U(MesDABMe) (2), was generated by the addition of an equimolar ratio of MesDABMe and KC8 to Cp3U. 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 LIII-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 μB 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 CH3I, the addition of 2 equiv of iodomethane to 1 resulted in the formation of a uranium(IV) monoiodide species, (MesDABMe)(MesDABMe2)UI {3; MesDABMe2 = [ArNC(Me)C(Me2)NAr]}, which was characterized by single-crystal X-ray diffraction and U M4- and M5-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.