A trinuclear metallasilsesquioxane of uranium(III).

The silsesquioxane ligand (iBu)7Si7O9(OH)3 (iBuPOSSH3) is revealed as an attractive system for the assembly of robust polynuclear complexes of uranium(III) and allowed the isolation of the first example of a trinuclear U(III) complex ([U3(iBuPOSS)3]) that exhibits magnetic communication and promotes dinitrogen reduction in the presence of reducing agent.

Cation assisted binding and cleavage of dinitrogen by uranium complexes.

The role of alkali promoters in N2 cleavage by metal complexes remains poorly understood despite its relevance to the industrial production of ammonia from N2. Here we report a series of alkali bound-oxo-bridged diuranium(iii) complexes that provide a unique example of decreasing N2 binding affinity with increasing cation size (from K to Cs). N2 binding was found to be irreversible in the presence of K. A N2 complex could be isolated in the solid state in the presence of the Rb cation and crystallographically characterized, but N2 binding was found to be reversible under vacuum. In the presence of the Cs cation N2 binding could not be detected at 1 atm. Electrochemical and Computational studies suggest that the decrease in N2 binding affinity is due to steric rather than electronic effects. We also find that weak N2 binding in ambient conditions does not prevent alkali assisted N2 cleavage to nitride from occurring. More importantly, we present the first example of cesium assisted N2 cleavage leading to the isolation of a N2 derived multimetallic U/Cs bis-nitride. The nitrides readily react with protons and CO to yield ammonia, cyanate and cyanide.

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 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 N24- 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 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 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.