Insititut des Sciences et Ingénierie Chimiques, Ecole Polytechnique Fédérale de Lausanne (EPFL), 1015 Lausanne Switzerland.; Department of Chemistry, University of Manchester, Oxford Road, Manchester M13 9PL UK
Abstract:
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 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], 2 and K2{U(OSi(OtBu)3)3}2(μ-N)2(μ-I)], 3 while 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 N2 release observed for complex 4. Complex 2 undergoes hydrogenolysis to yield imido bridged K2{U(OSi(OtBu)3)3(μ-NH)}2], 6 and rare amido bridged U(iv)/U(iv) complexes {U(OSi(OtBu)3)3}2(μ-NH2)2(μ-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 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 for 1 and 2, but an asymmetric bonding scheme is found for the U(vi)/U(vi) complex 4 which 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.The first examples of molecular compounds containing the cyclic (U(vi)N)2 and (U(v)U(vi)N)2 cores were obtained by oxidation of the (U(v)U(v)N)2 analogue. Different bonding within these complexes yields different stability and reactivity with CO and H2.
