The active centers of the nitrogen-fixing enzymes nitrogenases comprise iron−sulfur clusters. The binding of the substrate N 2 to these clusters plays a fundamental role for the subsequent ammonia synthesis. However, due to the complexity of the natural system and the lack of suitable synthetic models, the interaction of N 2 with the iron−sulfur clusters remains largely elusive. In this contribution, we demonstrate the gas-phase preparation and investigation of the cationic Fe 2 S 2 + , Fe 3 S 3 + , and Fe 4 S 4 + clusters. These clusters represent the first cluster model systems containing more than one iron and sulfur atom, respectively, which are found to bind N 2 . Temperature-dependent kinetic measurements allow, for the first time, for the determination of experimental binding energies of N 2 to iron−sulfur clusters. In addition, concurrent firstprinciples simulations reveal the ground state and isomeric structures of the iron−sulfur clusters with and without adsorbed N 2 and provide a conceptual understanding of the interaction between N 2 and iron−sulfur clusters. In particular, we present molecular level details of N 2 bonding such as the identification of the adsorption sites, the bond geometry, the bond strength, and the nature of the cluster-N 2 binding interaction, which is found to be promoted by occupation of the empty antibonding orbitals of the free N 2 molecule through interaction with the frontier orbitals of the iron−sulfur complex involving d-orbitals of iron hybridized with sulfur porbitals.

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