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 [K-2{[U-V(OSi((OBu)-Bu-t)(3))(3)](2)(mu-O)(mu-eta 2(:)eta(2)-N-2)}], B, with KC8, yielding the tetranuclear tetranitride cluster [K-6{(OSi((OBu)-Bu-t)(3))(2)UIV}(3){(OSi((OBu)-Bu-t)(3))(2)UVI}(mu(4)-N)(3)(mu(3)-N)(mu(3)-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=U-VI=N core analogue of UO22+. 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 N-2(4-) ligand in complex B, resulting in N2 cleavage and the formation of the putative diuranium(V) bis-nitride [K-4{[UV(OSi-((OBu)-Bu-t)(3))(3)](2)(mu-O)(mu-N)(2)}], X. Additionally, cooperative potassium binding to the U-bound N24- 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 (CO)-C-13 to yield (KCN)-C-13 and (KNCO)-C-13. 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.