Stepwise Reduction of Dinitrogen by a Uranium–Potassium Complex Yielding a U(VI)/U(IV) Tetranitride Cluster

Multimetallic cooperativity is believed to play a key role in the cleavage of dinitrogen to nitrides (N3–), but the mechanism remains ambiguous due to the lack of isolated intermediates. Herein, we report the reduction of the complex [K2{[UV(OSi­(OtBu)3)3]2(μ-O)­(μ-η2:η2-N2)}], B, with KC8, yielding the tetranuclear tetranitride cluster [K6{(OSi­(OtBu)3)2UIV}3­{(OSi­(OtBu)3)2UVI}­(μ4-N)3­(μ3-N)­(μ3-O)2], 1, a novel example of N2 cleavage to nitride by a diuranium complex. The structure of complex 1 is remarkable, as it contains a unique uranium center bound by four nitrides and provides the second example of a trans-NUVIN core analogue of UO2 2+. Experimental and computational studies indicate that the formation of the U­(IV)/U­(VI) tetrauranium cluster occurs via successive one-electron transfers from potassium to the bound N2 4– ligand in complex B, resulting in N2 cleavage and the formation of the putative diuranium­(V) bis-nitride [K4{[UV(OSi­(OtBu)3)3]2­(μ-O)­(μ-N)2}], X. Additionally, cooperative potassium binding to the U-bound N2 4– ligand facilitates dinitrogen cleavage during electron transfer. The nucleophilic nitrides in both complexes are easily functionalized by protons to yield ammonia in 93–97% yield and with excess 13CO to yield K13CN and KN13CO. The structures of two tetranuclear U­(IV)/U­(V) bis- and mononitride clusters isolated from the reaction with CO demonstrate that the nitride moieties are replaced by oxides without disrupting the tetranuclear structure, but ultimately leading to valence redistribution.