Journal
JOURNAL OF THE AMERICAN CHEMICAL SOCIETY
Volume 145, Issue 29, Pages 16271-16283Publisher
AMER CHEMICAL SOC
DOI: 10.1021/jacs.3c05626
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The synthesis of molecular uranium complexes in oxidation states lower than +3 remains challenging despite their reactivity and electronic structures. In this study, one- and two-electron reductions of a U(III) complex produced mono-reduced complexes and di-reduced U(I) synthons. EPR, UV/vis/NIR spectroscopy, magnetic, cyclic voltammetry, and computational studies revealed the electronic structures and redox couples of the complexes. The complexes demonstrated the ability to transfer multiple electrons to oxidizing substrates.
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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