Molecular nitrogen (N) is cheap and widely available, but its unreactive nature is a challenge when attempting to functionalize it under mild conditions with other widely available substrates (such as carbon monoxide, CO) to produce value-added compounds. Biological N fixation can do this, but the industrial Haber-Bosch process for ammonia production operates under harsh conditions (450 degrees Celsius and 300 bar), even though both processes are thought to involve multimetallic catalytic sites. And although molecular complexes capable of binding and even reducing N under mild conditions are known, with co-operativity between metal centres considered crucial for the N reduction step, the multimetallic species involved are usually not well defined, and further transformation of N-binding complexes to achieve N-H or N-C bond formation is rare. Haber noted, before an iron-based catalyst was adopted for the industrial Haber-Bosch process, that uranium and uranium nitride materials are very effective heterogeneous catalysts for ammonia production from N. However, few examples of uranium complexes binding N are known, and soluble uranium complexes capable of transforming N into ammonia or organonitrogen compounds have not yet been identified. Here we report the four-electron reduction of N under ambient conditions by a fully characterized complex with two U ions and three K centres held together by a nitride group and a flexible metalloligand framework. The addition of H and/or protons, or CO to the resulting complex results in the complete cleavage of N with concomitant N functionalization through N-H or N-C bond-forming reactions. These observations establish that a molecular uranium complex can promote the stoichiometric transformation of N into NH or cyanate, and that a flexible, electron-rich, multimetallic, nitride-bridged core unit is a promising starting point for the design of molecular complexes capable of cleaving and functionalizing N under mild conditions.

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Thermolysis of the nitride-bridged diuranium(IV) complex Cs{(μ-N)[U(OSi(O(t) Bu)3)3]2} (1) showed that the bridging nitride behaves as a strong nucleophile, promoting N-C bond formation by siloxide ligand fragmentation to yield an imido-bridged siloxide/silanediolate diuranium(IV) complex, Cs{(μ-N(t) Bu)(μ-O2 Si(O(t) Bu)2)U2 (OSi(O(t) Bu)3)5}. Complex 1 displayed reactivity towards CS2 and CO2 at room temperature that is unprecedented in f-element chemistry, affording diverse N-functionalized products depending on the reaction stoichiometry. The reaction of 1 with two equivalents of CS2 yielded the thiocyanate/thiocarbonate complex Cs{(μ-NCS)(μ-CS3 )[U(OSi(O(t)Bu)3)3]2} via a putative NCS(-)/S(2-) intermediate. The reaction of 1 with one equivalent of CO2 resulted in deoxygenation and N-C bond formation, yielding the cyanate/oxo complex Cs{(μ-NCO)(μ-O)[U(OSi(O(t) Bu)3 )3]2}. Addition of excess CO2 to 1 led to the unprecedented dicarbamate product Cs{(μ-NC2O4)[U(OSi(O(t) Bu)3)3]2}.

Cleavage of dihydrogen is an important step in the industrial and enzymatic transformation of N into ammonia. The reversible cleavage of dihydrogen was achieved under mild conditions (room temperature and 1 atmosphere of H ) by the molecular uranium nitride complex, [Cs{U(OSi(O Bu) ) } (μ-N)] 1, leading to a rare hydride-imide bridged diuranium(IV) complex, [Cs{U(OSi(O Bu) ) } (μ-H)(μ-NH)], 2 that slowly releases H under vacuum. This complex is highly reactive and quickly transfers hydride to acetonitrile and carbon dioxide at room temperature, affording the ketimide- and formate-bridged U species [Cs{U(OSi(O Bu) ) } (μ-NH)(μ-CH CHN)], 3 and [Cs{U(OSi(O Bu) ) } (μ-HCOO)(μ-NHCOO)], 4.

Uranium nitrides are important materials with potential for application as fuels for nuclear power generation, and as highly active catalysts. Molecular nitride compounds could provide important insight into the nature of the uranium-nitride bond, but currently little is known about their reactivity. In this study, we found that a complex containing a nitride bridging two uranium centers and a cesium cation readily cleaved the C≡O bond (one of the strongest bonds in nature) under ambient conditions. The product formed has a [CsU2 (μ-CN)(μ-O)] core, thus indicating that the three cations cooperate to cleave CO. Moreover, the addition of MeOTf to the nitride complex led to an exceptional valence disproportionation of the CsU(IV) -N-U(IV) core to yield CsU(III) (OTf) and [MeN=U(V) ] fragments. The important role of multimetallic cooperativity in both reactions is illustrated by the computed reaction mechanisms.