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

Abstract

Uranium nitride compounds are important molecular analogues of uranium nitride materials such as UN and UN2which 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 [K2{U(OSi(OtBu)3)3(μ-N)}2],1, with mild oxidants yields the U(v)/U(vi) complexes [K{U(OSi(OtBu)3)3(μ-N)}2],2and [K2{U(OSi(OtBu)3)3}2(μ-N)2(μ-I)],3while oxidation with a stronger oxidant (“magic blue”) yields the U(vi)/U(vi) complex [{U(OSi(OtBu)3)3}2(μ-N)2(μ-thf)],4. The three complexes show very different stability and reactivity, with N2release observed for complex4. Complex2undergoes hydrogenolysis to yield imido bridged [K2{U(OSi(OtBu)3)3(μ-NH)}2],6and rare amido bridged U(iv)/U(iv) complexes [{U(OSi(OtBu)3)3}2(μ-NH2)2(μ-thf)],7while no hydrogenolysis could be observed for4. Both complexes2and4react with H+to yield quantitatively NH4Cl, but only complex2reacts with CO and H2. 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 for1and2, but an asymmetric bonding scheme is found for the U(vi)/U(vi) complex4which shows a U-N σ orbital well localised to UN and π orbitals which partially delocalise to form the U-N single bond with the other uranium.