Car-Parrinello Molecular Dynamics Study of the Initial Dinitrogen Reduction Step in Sellmann-Type Nitrogenase Model Complexes

We have studied reduction reactions for nitrogen fixation at Sellmann‐type model complexes with Car–Parrinello simulation techniques. These dinuclear complexes are especially designed to emulate the so‐called open‐side FeMoco model. The main result of this work shows that in order to obtain the reduced species several side reactions have to be suppressed. These involve partial dissociation of the chelate ligands and hydrogen atom transfer to the metal center. Working at low temperature turns out to be one necessary pre‐requisite in carrying out successful events. The successful events cannot be described by simple reaction coordinates. Complicated processes are involved during the initiation of the reaction. Our theoretical study emphasizes two experimental strategies which are likely to inhibit the side reactions. Clamping of the two metal fragments by a chelating phosphane ligand should prevent dissociation of the complex. Furthermore, introduction of tert‐butyl substituents could improve the solubility and should thus allow usage of a wider range of (mild) acids, reductants, and reaction conditions. Antibonding occupation does the trick: CPMD simulations of the dinitrogen reduction at protonated FeII–and RuII–sulfur complexes demonstrate that the occupation of an antibonding π* orbital activates N2 to give diazene (see scheme), although side reactions have to be suppressed.