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 NH 3 . 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 [MoFe 3 S 4 ] cluster in a protective ligand environment enables N 2 binding at Fe. The bridging [MoFe 3 S 4 ] 2 (μ-η 1 :η 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 [MoFe 3 S 4 ] 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. Although iron–sulfur cofactors are known to carry out biological nitrogen fixation, how these clusters bind dinitrogen remains poorly understood. Now, a dinitrogen complex of a synthetic iron–sulfur cluster has been characterized, and electronic cooperation in the cluster has been shown to result in strong N–N bond activation.