Thermally Stable Uranium Dinitrogen Complex with Siloxide Supporting Ligands

A new dinitrogen adduct of a homoleptic uranium tris­(siloxide) complex, [U­{OSi­(Mes)3}3]2(μ-η2:η2-N2), is reported. Synthesis of the 15N-labeled isotopomer and Raman spectroscopy confirm the reductive activation of N2 to a (N2)2– dianion. The 15N NMR shift of the 15N2-labeled isotopomer is also reported. Crystallographic characterization shows a side-on (N2)2– coordinated in either an eclipsed or staggered conformation in different crystals. The U–N2–U complex is stable to vacuum and shows high thermal stability, retaining the formally reduced dinitrogen at 100 °C. The parent three-coordinate uranium­(III) [U­{OSi­(Mes)3}3] could not be isolated in our hands, with N2-free syntheses affording only uranium­(IV) compounds. The rational synthesis and full characterization of two such U­(IV) byproducts, [U­{OSi­(Mes)3}­{N­(SiMe3)2}3] and [U­{OSi­(Mes)3}4], is also reported.