Nitrogen reduction and functionalization by a multimetallic uranium nitride complex

Molecular nitrogen (N-2) 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-2 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 (1, 2). And although molecular complexes capable of binding and even reducing N-2 under mild conditions are known, with co-operativity between metal centres considered crucial for the N-2 reduction step (1-14), the multimetallic species involved are usually not well defined, and further transformation of N-2-binding complexes to achieve N–H or N–C bond formation is rare (2, 6, 8, 10, 15, 16). Haber noted (17), 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-2. However, few examples of uranium complexes binding N-2 are known (18-22), and soluble uranium complexes capable of transforming N-2 into ammonia or organonitrogen compounds have not yet been identified. Here we report the four-electron reduction of N-2 under ambient conditions by a fully characterized complex with two U-iii ions and three K+ centres held together by a nitride group and a flexible metalloligand framework. The addition of H-2 and/or protons, or CO to the resulting N-2(4-)complex results in the complete cleavage of N-2 with concomitant N-2 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-2 into NH3 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-2 under mild conditions.