Nitrogen Reduction by Multimetallic trans-Uranium Actinide Complexes: A Theoretical Comparison of Np and Pu to U

There is recent interest in organometallic complexes of the trans-uranium elements. However, preparation and characterization of such complexes are hampered by radioactivity and chemotoxicity issues as well as the air-sensitive and poorly understood behavior of existing compounds. As such, there are no examples of small-molecule activation via redox reactivity of organometallic trans-uranium complexes. This contrasts with the situation for uranium. Indeed, a multimetallic uranium­(III) nitride complex was recently synthesized, characterized, and shown to be able to capture and functionalize molecular nitrogen (N2) through a four-electron reduction process, N2 → N2 4–. The bis-uranium nitride, U–N–U core of this complex is held in a potassium siloxide framework. Importantly, the N2 4– product could be further functionalized to yield ammonia (NH3) and other desirable species. Using the U–N–U potassium siloxide complex, K3U–N–U, and its cesium analogue, Cs3U–N–U, as starting points, we use scalar-relativistic and spin–orbit coupled density functional theory calculations to shed light on the energetics and mechanism for N2 capture and functionalization. The N2 → N2 4– reactivity depends on the redox potentials of the U­(III) centers and crucially on the stability of the starting complex with respect to decomposition into the mixed oxidation U­(IV)/U­(III) K2U–N–U or Cs2U–N–U species. For the trans-uranium, Np and Pu analogues of K3U–N–U, the N2 → N2 4– process is endoergic and would not occur. Interestingly, modification of the Np–O and Pu–O bonds between the actinide cores and the coordinated siloxide framework to Np–NH, Pu–NH, Np–CH2, and Pu–CH2 bonds drastically improves the reaction free energies. The Np–NH species are stable and can reductively capture and reduce N2 to N2 4–. This is supported by analysis of the spin densities, molecular structure, long-range dispersion effects, as well as spin–orbit coupling effects. These findings chart a path for achieving small-molecule activation with organometallic neptunium analogues of existing uranium complexes.