Multielectron Redox Chemistry of Uranium by Accessing the plus II Oxidation State and Enabling Reduction to a U(I) Synthon

Journal of the American Chemical Society(2023)

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Thesynthesis of molecular uranium complexes in oxidation stateslower than +3 remains a challenge despite the interest for their multielectrontransfer reactivity and electronic structures. Herein, we report theone- and two-electron reduction of a U(III) complex supported by anarene-tethered tris(siloxide) tripodal ligand leading to the mono-reducedcomplexes, [K(THF)U((OSi((OBu)-Bu- t )(2)Ar)(3)-arene)(THF)] (2) and [K(2.2.2-cryptand)][U((OSi((OBu)-Bu- t )(2)Ar)(3)-arene)(THF)](2-crypt), and to the di-reduced U(I) synthons, [K-2(THF)(3)U((OSi((OBu)-Bu- t )(2)Ar)(3)-arene)](& INFIN;) (3) and [(K(2.2.2-cryptand))](2)[U((OSi((OBu)-Bu- t )(2)Ar)(3)-arene)] (3-crypt). EPR and UV/vis/NIR spectroscopies, magnetic, cyclic voltammetry,and computational studies provide strong evidence that complex 2-crypt is best described as a U(II), where the U(II) is stabilizedby & delta;-bonding interactions between the arene anchor and the uraniumfrontier orbitals, whereas complexes 3 and 3-crypt are best described as having a U(III) ion supported by the di-reducedarene anchor. Three quasi-reversible redox waves at E (1/2) = -3.27, -2.45, and -1.71 V wereidentified by cyclic voltammetry studies and were assigned to theU(IV)/U(III), U(III)/U(II), and U(II)/U(III)-(arene)(2-) redox couples. The ability of complexes 2 and 3 in transferring two- and three-electrons, respectively,to oxidizing substrates was confirmed by the reaction of 2 with azobenzene (PhNNPh), leading to the U(IV) complex, [K(Et2O)U((OSi((OBu)-Bu- t )(2)Ar)(3)-arene)(PhNNPh)(THF)] (4), and of complex 3 with cycloheptatriene, yielding the U(IV) complex, [(K(Et2O)(2))U((OSi((OBu)-Bu- t )(2)Ar)(3)-arene)(& eta;(7)-C7H7)](& INFIN;) (6). These results demonstratethat the arene-tethered tris(siloxide) tripodal ligand provides anexcellent platform for accessing low-valent uranium chemistry whileimplementing multielectron transfer pathways as shown by the reactivityof complex 3, which provides the third example of a U(I)synthon.
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