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

The synthesis of molecular uranium complexes in oxidation states lower than +3 remains a challenge despite the interest for their multielectron transfer reactivity and electronic structures. Herein, we report the one- and two-electron reduction of a U(III) complex supported by an arene-tethered tris(siloxide) tripodal ligand leading to the mono-reduced complexes, [K(THF)U((OSi(O t Bu) 2 Ar) 3 -arene)(THF)] ( 2 ) and [K(2.2.2-cryptand)][U((OSi(O t Bu) 2 Ar) 3 -arene)(THF)] ( 2-crypt ), and to the di-reduced U(I) synthons, [K 2 (THF) 3 U((OSi(O t Bu) 2 Ar) 3 -arene)] ∞ ( 3 ) and [(K(2.2.2-cryptand))] 2 [U((OSi(O t Bu) 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 stabilized by δ-bonding interactions between the arene anchor and the uranium frontier orbitals, whereas complexes 3 and 3-crypt are best described as having a U(III) ion supported by the di-reduced arene anchor. Three quasi-reversible redox waves at E 1/2 = -3.27, -2.45, and -1.71 V were identified by cyclic voltammetry studies and were assigned to the U(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(Et 2 O)U((OSi(O t Bu) 2 Ar) 3 -arene)(PhNNPh)(THF)] ( 4 ), and of complex 3 with cycloheptatriene, yielding the U(IV) complex, [(K(Et 2 O) 2 )U((OSi(O t Bu) 2 Ar) 3 -arene)(η 7 -C 7 H 7 )] ∞ ( 6 ). These results demonstrate that the arene-tethered tris(siloxide) tripodal ligand provides an excellent platform for accessing low-valent uranium chemistry while implementing multielectron transfer pathways as shown by the reactivity of complex 3 , which provides the third example of a U(I) synthon.