Synthesis, structure, and reactivity of uranium(VI) nitrides

Uranium nitride compounds are important molecular analogues of uranium nitride materials such as UN and UN2 which are effective catalysts in the Haber-Bosch synthesis of ammonia, but the synthesis of molecular nitrides remains a challenge and studies of the reactivity and of the nature of the bonding are poorly developed. Here we report the synthesis of the first nitride bridged uranium complexes containing U(vi) and provide a unique comparison of reactivity and bonding in U(vi)/U(vi), U(vi)/U(v) and U(v)/U(v) systems. Oxidation of the U(v)/U(v) bis-nitride [K-2{U(OSi((OBu)-Bu-t)(3))(3)(mu-N)}(2)], 1, with mild oxidants yields the U(v)/U(vi) complexes [K{U(OSi((OBu)-Bu-t)(3))(3)(mu-N)}(2)], 2 and [K-2{U(OSi((OBu)-Bu-t)(3))(3)}(2)(mu-N)(2)(mu-I)], 3 while oxidation with a stronger oxidant ("magic blue") yields the U(vi)/U(vi) complex [{U(OSi((OBu)-Bu-t)(3))(3)}(2)(mu-N)(2)(mu-thf)], 4. The three complexes show very different stability and reactivity, with N-2 release observed for complex 4. Complex 2 undergoes hydrogenolysis to yield imido bridged [K-2{U(OSi((OBu)-Bu-t)(3))(3)(mu-NH)}(2)], 6 and rare amido bridged U(iv)/U(iv) complexes [{U(OSi((OBu)-Bu-t)(3))(3)}(2)(mu-NH2)(2)(mu-thf)], 7 while no hydrogenolysis could be observed for 4. Both complexes 2 and 4 react with H+ to yield quantitatively NH4Cl, but only complex 2 reacts with CO and H-2. Differences in reactivity can be related to significant differences in the U-N bonding. Computational studies show a delocalised bond across the U-N-U for 1 and 2, but an asymmetric bonding scheme is found for the U(vi)/U(vi) complex 4 which shows a U-N sigma orbital well localised to U=N and pi orbitals which partially delocalise to form the U-N single bond with the other uranium.