Multielectron Redox Chemistry of Uranium by Accessing the +II Oxidation State and Enabling Reduction to a U(I) Synthon
Megan Keener, R. A. Keerthi Shivaraam, Thayalan Rajeshkumar, Maxime Tricoire, Rosario Scopelliti, Ivica Živković, Anne‐Sophie Chauvin, Laurent Maron, Marinella Mazzanti
Abstract
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.