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)2Ar)3-arene)(THF) (2) and K(2.2.2-cryptand)U((OSi(O t Bu)2Ar)3-arene)(THF) (2-crypt), and to the di-reduced U(I) synthons, K2(THF)3U((OSi(O t Bu)2Ar)3-arene)∞ (3) and (K(2.2.2-cryptand))2U((OSi(O t Bu)2Ar)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(Et2O)U((OSi(O t Bu)2Ar)3-arene)(PhNNPh)(THF) (4), and of complex 3 with cycloheptatriene, yielding the U(IV) complex, (K(Et2O)2)U((OSi(O t Bu)2Ar)3-arene)(η7-C7H7)∞ (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.
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