Multimetallic
Cooperativity in Uranium-Mediated CO<sub>2</sub> Activation
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Abstract
The metal-mediated
redox transformation of CO<sub>2</sub> in mild
conditions is an area of great current interest. The role of cooperativity
between a reduced metal center and a Lewis acid center in small-molecule
activation is increasingly recognized, but has not so far been investigated
for f-elements. Here we show that the presence of potassium at a U,
K site supported by sterically demanding tris(<i>tert</i>-butoxy)siloxide ligands induces a large cooperative effect in the
reduction of CO<sub>2</sub>. Specifically, the ion pair complex [K(18c6)][U(OSi(O<sup>t</sup>Bu)<sub>3</sub>)<sub>4</sub>], <b>1</b>, promotes the
selective reductive disproportionation of CO<sub>2</sub> to yield
CO and the mononuclear uranium(IV) carbonate complex [U(OSi(O<sup>t</sup>Bu)<sub>3</sub>)<sub>4</sub>(μ-κ<sup>2</sup>:κ<sup>1</sup>-CO<sub>3</sub>)K<sub>2</sub>(18c6)], <b>4</b>. In contrast,
the heterobimetallic complex [U(OSi(O<sup>t</sup>Bu)<sub>3</sub>)<sub>4</sub>K], <b>2</b>, promotes the potassium-assisted two-electron
reductive cleavage of CO<sub>2</sub>, yielding CO and the U(V) terminal
oxo complex [UO(OSi(O<sup>t</sup>Bu)<sub>3</sub>)<sub>4</sub>K], <b>3</b>, thus providing a remarkable example of two-electron transfer
in U(III) chemistry. DFT studies support the presence of a cooperative
effect of the two metal centers in the transformation of CO<sub>2</sub>