Applications of boroxide ligands in supporting small molecule activation by U(iii) and U(iv) complexes

The boroxide ligand [OBAr2](-) (Ar = Mes, Trip) is shown to be able to support both U-III and U-IV centres for the first time. The synthesis and structures of homoleptic and heteroleptic U-III and U-IV complexes are reported. The UX3 complex with larger substituents, [U(OBTrip(2))(3)](2), exhibits greater thermal stability compared to less encumbered [U(OBMes(2))(3)](2) but reacts with a smaller range of the small molecules tested to date. Initial studies on their capacity to participate in small molecule chemistry show that dark purple [U(OBMes(2))(3)](2) binds and/or reductively activates a variety of small molecules such as pyridine-oxide, triphenylphosphineoxide, sulfur, and dicyclohexylcarbodiimide. While [U(OBMes(2))(3)](2) shows no reaction with CO or CO2, [U(OBTrip(2))(3)](2) is oxidised by both, in the former case forming [U(OBTrip(2))(4)], and in the latter case forming a small quantity of the structurally characterised -carbonate product [(-CO3){U(OBTrip(2))(3)}(2)].