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

The Fe-S clusters of nitrogenases carry out the life-sustaining conversion of N-2 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 N-2. Here we report that embedding an [MoFe3S4] cluster in a protective ligand environment enables N-2 binding at Fe. The bridging [MoFe3S4](2)(mu-eta(1):eta(1)-N-2) 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 N-2 ligand with generation of Fe-N multiple-bond character. Structural and spectroscopic analyses demonstrate that N-2 activation is accompanied by shortened Fe-S distances and charge transfer from each Fe site, including those not directly bound to N-2. These findings indicate that covalent interactions within the cluster play a critical role in N-2 binding and activation.