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

Abstract

Car-Parrinello molecular dynamics study of the initial dinitrogen reduction step in Sellmann-type nitrogenase model complexes AbstractWe 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.

Real-World Predictions from Ab Initio Molecular Dynamics Simulations

“…A dinuclear complex -designed to emulate the open-side FeMoco modelwas simulated. Several side reactions were observed which have to be suppressed in order to arrive at the reduced species [194]. Chelate effects and their partial dissociation as well as low temperatures led to successful events.…”

“…Chelate effects and their partial dissociation as well as low temperatures led to successful events. An optimized design of the complexes to inhibit side reactions was suggested [194].…”

“…In 2004 and 2005 the photochemical activation of dinitrogen with transition metal model complexes of the Sellmann type nitrogenase was studied using CPMD [193,194]. A dinuclear complex -designed to emulate the open-side FeMoco modelwas simulated.…”

Toward an Inverse Approach for the Design of Small-Molecule Fixating Catalysts

“…However, it may also simply bind dinitrogen without bond activation such that the N-N bond length is increased by less than one or two pm. Then, a subsequent activation by electron transfer onto the ligand will be necessary to start the chemical reduction process [16,17].…”

“…We have studied this problem with quantum chemical methods [10][11][12][13][14][15][16][17][18][19][20][21][22] and identified the most crucial steps in the whole catalytic cycle. These reaction steps are the feasibility of dinitrogen binding, the transfer of the first proton-electron pair onto the bound dinitrogen ligand, the exchange of the finally produced ammonia molecule by the next incoming dinitrogen ligand, and finally the prevention of side reactions such as ligand reduction or hydride formation at the transition metal center which would lead to fast inactivation of the catalyst.…”