QUANTUM CHEMICAL INVESTIGATIONS INTO THE PROBLEM OF BIOLOGICAL NITROGEN FIXATION: SELLMANN-TYPE METAL–SULFUR MODEL COMPLEXES. Elsevier; 2004. pp. 55-100. [DOI: 10.1016/s0898-8838(04)56003-8]

Evaluating Diazene to N2 Interconversion at Iron-Sulfur Complexes

Biological N2 reduction occurs at sulfur-rich multiiron sites, and an interesting potential pathway is concerted double reduction/ protonation of bridging N2 through PCET. Here, we test the feasibility of using synthetic sulfur-supported diiron complexes to mimic this pathway. Oxidative proton transfer from μ-η1 : η1-diazene (HN=NH) is the microscopic reverse of the proposed N2 fixation pathway, revealing the energetics of the process. Previously, Sellmann assigned the purple metastable product from two-electron oxidation of [{Fe2+(PPr3)L1}2(μ-η1 : η1-N2H2)] (L1=tetradentate SSSS ligand) at -78 °C as [{Fe2+(PPr3)L1}2(μ-η1 : η1-N2)]2+, which would come from double PCET from diazene to sulfur atoms of the supporting ligands. Using resonance Raman, Mössbauer, NMR, and EPR spectroscopies in conjunction with DFT calculations, we show that the product is not an N2 complex. Instead, the data are most consistent with the spectroscopically observed species being the mononuclear iron(III) diazene complex [{Fe(PPr3)L1}(η2-N2H2)]+. Calculations indicate that the proposed double PCET has a barrier that is too high for proton transfer at the reaction temperature. Also, PCET from the bridging diazene is highly exergonic as a result of the high Fe3+/2+ redox potential, indicating that the reverse N2 protonation would be too endergonic to proceed. This system establishes the "ground rules" for designing reversible N2/N2H2 interconversion through PCET, such as tuning the redox potentials of the metal sites.

Intrinsic Dinitrogen Activation at Bare Metal Atoms

There is ongoing interest in metal complexes which bind dinitrogen and facilitate either its reduction or oxidation under mild conditions. In nature, the enzyme nitrogenase catalyzes this process, and dinitrogen fixation occurs under mild and ambient conditions at a metal-sulfur cluster in the center of the MoFe protein, but the mechanism of this process remains largely unknown. In the last few years, new important discoveries have been made in this field. In this review are discussed recent findings on the interaction of N(2) with metal atoms and metal-atom dimers from all groups of the periodic table as provided by gas-phase as well as matrix-isolation experiments. Intrinsic dinitrogen activation at such bare metal atoms is then related to corresponding processes at complexes, clusters, and surfaces.

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.