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>

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