Isolation and redox reactivity of cerium complexes in four redox states

Abstract

The chemistry of lanthanides is limited to one electron transfer reactions due to the difficulty of accessing multiple oxidation states. Here we report that a redox-active ligand combining three siloxides...

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

“…It should be noted that the cyclic voltammogram of complex 1 is very different from that previously reported for the analogous Ce­(III) complex, [Ce­((OSi­(O t Bu) 2 Ar) 3 -arene)­(THF)], which displayed only one chemically reversible reduction wave at E 1/2 = −2.90 V . Since no reduction events were observed within the window permitted by THF in the voltammogram of the ligand salt, ((OSi­(O t Bu) 2 Ar) 3 -arene)­K 3 , the redox event was tentatively assigned to a metal-centered reduction (Ce­(III)/Ce­(II)).…”

“…It should be noted that the cyclic voltammogram of complex 1 is very different from that previously reported for the analogous Ce(III) complex, [Ce((OSi(O t Bu) 2 Ar) 3 -arene)-(THF)], which displayed only one chemically reversible reduction wave at E 1/2 = −2.90 V. 50 Since no reduction events were observed within the window permitted by THF in the voltammogram of the ligand salt, ((OSi(O t Bu) 2 Ar) 3 -arene)K 3 , the redox event was tentatively assigned to a metal-centered reduction (Ce(III)/Ce(II)). The important differences observed in the voltammograms of the Ce(III) and U(III) complexes, in which the ions are of similar charge and ionic radii, confirms the assignment of wave II as a U(III)/U(II) couple, with formulations of 2 and 2-crypt in solution as U(II) complexes (Table S5).…”

“…At first, we investigated the synthesis of the neutral tris­( tert -butoxy)­siloxide arene U­(III) complex, [U­((OSi­(O t Bu) 2 Ar) 3 -arene)­(THF)] ( 1 ), by protonolysis (Scheme ). Addition of 1.0 equiv of [U­(N­(SiMe 3 ) 2 ) 3 ] to 1.0 equiv of the (HOSi­(O t Bu) 2 Ar) 3 –arene ligand in THF at room temperature resulted in a color change from purple to dark brown-red. Analysis of the reaction mixture by 1 H NMR spectroscopy indicated the disappearance of the precursor [U­(N­(SiMe 3 ) 2 ) 3 ] and the concomitant formation of HN­(SiMe 3 ) 2 .…”

“…Considering the ability of siloxide ligands to stabilize uranium in low oxidation states, ,, their high stability both in reducing and oxidizing conditions, ,, and their ability to support original reactivity with small molecules, ,, we reasoned that a tris(siloxide) arene-anchored tripodal ligand may allow access to lower oxidation states while promoting unprecedented redox-reactivity. Indeed, we very recently showed that the tris( tert -butoxy)siloxide arene can stabilize cerium complexes in four states of charge . Herein, we report the synthesis of the neutral tris( tert -butoxy)siloxide arene U(III) complex, [U((OSi(O t Bu) 2 Ar) 3 -arene)(THF)] ( 1 ), and demonstrate that this complex undergoes two quasi-reversible redox processes.…”

“…Indeed, we very recently showed that the tris(tert-butoxy)siloxide arene can stabilize cerium complexes in four states of charge. 50 Herein, we report the synthesis of the neutral tris(tert-butoxy)siloxide arene U(III) complex, [U((OSi(O t Bu) 2 Ar) 3 -arene)(THF)] (1), and demonstrate that this complex undergoes two quasireversible redox processes. Chemical reduction by one-electron afforded a new U(II) complex, supported by δ-bonding interactions with the arene anchor, which can be further reduced to yield a complex that is best described as a U(III)− (arene) 2− species but acts as a U(I) synthon by transferring three-electrons to cycloheptatriene (CHT).…”