Kinetics and Mechanism of Acetylene Reduction with Europium Amalgam Catalyzed by Isolated Active Center of Nitrogenase

The reaction kinetics of C₂H₂ reduction with europium amalgam (Eu/Hg) catalyzed by nitrogenase active center separated from the enzyme, the molybdenum–iron–sulfur cluster MoFe₇S₉ · homocitrate] (FeMoco), was studied. The dependence of the rates of ethylene and ethane formation on the concentrations of catalyst, substrate, protonating agent, and amalgam was studied. The stereospecificity of the reaction was studied by Fourier transform IR spectroscopy. It was found that the reaction occurred at the amalgam surface via the adsorption of the compound FeMoco · PhSH]. Upon reduction, this compound can simultaneously coordinate several substrate molecules to activate them for the subsequent reactions. A study of the IR spectra of the gas phase of the reaction demonstrated that cis-1,2-dideuterioethylene is the main product of C₂D₂ reduction. Taking into account this fact and the dependence of the reaction rate on the concentration of a protonating agent, we concluded that substrate molecules bound to the cofactor underwent protonation by intramolecular hydrogen transfer from the iron or sulfur atoms of FeMoco to coordinated C₂H₂.