Uranium(III)-carbon multiple bonding supported by arene delta-bonding in mixed-valence hexauranium nanometre-scale rings

Despite the fact that non-aqueous uranium chemistry is over 60 years old, most polarised-covalent uranium-element multiple bonds involve formal uranium oxidation states IV, V, and VI. The paucity of uranium(III) congeners is because, in common with metal-ligand multiple bonding generally, such linkages involve strongly donating, charge-loaded ligands that bind best to electron-poor metals and inherently promote disproportionation of uranium(III). Here, we report the synthesis of hexauranium-methanediide nanometre-scale rings. Combined experimental and computational studies suggest overall the presence of formal uranium(III) and (IV) ions, though electron delocalisation in this Kramers system cannot be definitively ruled out, and the resulting polarised-covalent U = C bonds are supported by iodide and delta-bonded arene bridges. The arenes provide reservoirs that accommodate charge, thus avoiding inter-electronic repulsion that would destabilise these low oxidation state metalligand multiple bonds. Using arenes as electronic buffers could constitute a general synthetic strategy by which to stabilise otherwise inherently unstable metal-ligand linkages.We thank the Royal Society (grant UF110005), European Research Council (grants StG239621 and CoG612724), Engineering and Physical Sciences Research Council (grants EP/F030517/1, EP/M027015/1, and EP/P001386/1), Natural Environment Research Council (NE/M014088/1), UK EPSRC National EPR Service, The University of Manchester, and the UK National Nuclear Laboratory for generously supporting this work. Diamond Light Source and the Canberra Australian Synchrotron are thanked for the allocation of XANES beam-time (awards SP9621, SP13559, and M7964). Computational work was supported by the U.S. Department of Energy Director, Office of Basic Energy Sciences (award DE-SC002183) and the High Performance Computing systems of the University of South Dakota. Dr Shu Hayama (Diamond Light Source) and Prof