The Synthesis, Structural, and Spectroscopic Characterization of Uranium(IV) Perrhenate Complexes

We report the synthesis, structural, and spectroscopic characterization of a series of uranium(IV)−perrhenato complexes. Three isostructural complexes with general formula [U(ReO₄)₄(L)₄] (where L = tri-n-butylphosphine oxide/TBPO (2), triethyl phosphate/TEP (3), or tri-iso-butyl phosphate/TⁱBP (4)), have been synthesized, both through the photoreduction of ethanolic {UO₂}²⁺ solutions and also via a novel U^IV starting material, U(ReO₄)₄·5H₂O (1). Compound 1 has also been used in the preparation of [U(ReO₄)₄(TPPO)₃(CH₃CN)]·2CH₃CN (5) and [U(ReO₄)(DPPMO₂)₃(OH)][ReO₄]₂·2CH₃CN (6), where TPPO represents triphenylphosphine oxide and DPPMO₂ represents bis(diphenylphosphino)methane dioxide. All six complexes have been spectroscopically characterized using NMR, UV−vis−NIR, and IR techniques, with 2, 3, 5, and 6 also fully structurally characterized. The U atoms in compounds 2-6 all exhibit eight-coordinate geometry with up to four perrhenate groups in addition to three (DPPMO₂ and TPPO) or four (TEP, TⁱBP, TBPO) coordinated organic ligands. In the case of compounds 5 and 6, the coordination of eight ligands to the U^IV center is completed by the binding of a solvent molecule (CH₃CN) and OH^-, respectively. Solid-state physical analysis (elemental and thermogravimetric) and infrared spectroscopy are in agreement with the structural studies. The crystallographic data suggest that the strength of the U^IV−O-donor ligand bonds decreases across the series R₃PO > [ReO₄]^-> (RO)₃PO. Solution-state IR and ³¹P NMR spectroscopy appear to be in agreement with these solid-state results.