Dinitrogen binding and activation at a molybdenum–iron–sulfur cluster

The Fe–S clusters of nitrogenases carry out the life-sustaining conversion of N2 to NH3. Although progress continues to be made in modelling the structural features of nitrogenase cofactors, no synthetic Fe–S cluster has been shown to form a well-defined coordination complex with N2. Here we report that embedding an [MoFe3S4] cluster in a protective ligand environment enables N2 binding at Fe. The bridging [MoFe3S4]2(μ-η1:η1-N2) complex thus prepared features a substantially weakened N–N bond despite the relatively high formal oxidation states of the metal centres. Substitution of one of the [MoFe3S4] cubanes with an electropositive Ti metalloradical induces additional charge transfer to the N2 ligand with generation of Fe–N multiple-bond character. Structural and spectroscopic analyses demonstrate that N2 activation is accompanied by shortened Fe–S distances and charge transfer from each Fe site, including those not directly bound to N2. These findings indicate that covalent interactions within the cluster play a critical role in N2 binding and activation.