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In order to identify common and distinctive features in the catalytic behavior of natural and artificial nitrogen fixation clusters, the kinetics of the catalytic reduction of C 2 H 2 in the presence of Mg-Mo cluster (1) was investigated and compared with the kinetics of acetylene reduction catalyzed by the cluster FeMoco (2) isolated from the enzyme nitrogenase we studied previously. The reactions were conducted in the presence of Zn/Hg and Eu/Hg as reducing agents and PhSH and C 6 F 5 SH as proton donors, i.e., under the same conditions as had been used in the case of 2. Both polynuclear Mg-Mo complex and the europium amalgam reduced FeMoco have multiple interdependent binding sites for substrates and/or inhibitors. Carbon monoxide inhibits the acetylene reduction much less efficiently in systems with clus ter 1 than in systems with cluster 2, although the type of inhibition is mixed in both systems: CO binds to multiple sites of the cluster and affects both C 2 H 2 complexation to the reduced cluster and decomposition of the catalyst-substrate complex to give the products. Unlike isolated FeMoco, the Mg-Mo cluster efficiently catalyzes the reduction of molecular nitro gen. The reaction is greatly inhibited by acetylene, while no inhibiting effect of N 2 is observed in acetylene reduction, as was found earlier for a system with the natural cluster as the catalyst.Previously, 1 a nitrogen reducing system based on poly nuclear molybdenum complexes was discovered. Cur rently, this is the only known non enzyme system capable of catalytic reduction of N 2 at atmospheric pressure and room temperature at rates comparable with nitro genase. The reaction is carried out in methanol with a minor water additive, which apparently serves as the pro ton donor in ammonia and/or hydrazine formation. Sodium amalgam, europium amalgam or a cathode with a specified potential equal to the Na/Hg potential can serve as reducing agents. 2 The molecular structure of the Mg containing molybdenum anionic cluster {[Mg 2 Mo 8 O 22 (MeO) 6 (MeOH) 4 ] 2-[Mg(MeOH) 6 ] 2+ }• •6MeOH (1), which forms the active site of the system upon reduction, has been determined 3 by X ray diffrac tion (Fig. 1).The available data 1-4 indicate that this compound is reduced with retention of the cluster core, although, strictly speaking, the structure of the complex active with respect to N 2 is unknown. The molecular mechanism of N 2 catalytic reduction with participation of this cluster is also unknown.The active site of the natural nitrogen fixing enzyme nitrogenase incorporates an octanuclear heterobimetallic cluster FeMoco ((µ 6 N)MoFe 7 S 9 •homocitrate (2)) in which nitrogen is bound and then reduced to ammonia. 5 The chemical mechanism of the multielectron reduction of nitrogenase substrates catalyzed by the cluster is un known.In order to elucidate the function of the cofactor and the contribution made by the whole protein matrix and by the amino acids located most closely to FeMoco to the nitrogen reduction under mild conditions, we studied the catalyt...

The effect of potential value and chemical properties of an external electron donor on C 2 H 2 reduction catalyzed by nitrogenase active center (cluster [(µ 6 N)Fe 7 MoS 9 •homocitrate] FeMoco isolated from the enzyme) has been investigated in the presence of proton donors of different acidity. The temperature-reaction rate dependences of these reactions have been studied. It has been shown that the rate limiting steps of the reactions differ depending on the proton donor used. When thiophenol or water are used as proton donors, and electrochemical step -the electron transfer from cathode to adsorbed catalytic cluster -has been found to be a rate limiting one. The effective activation energy of ethane formation as a product of four electron C 2 H 2 reduction is found to be 1.5 times lower than that of ethylene, namely, 13 kcal mol -1 . When stronger acid, pentafluorothiophenol, is used as a proton donor, the chemical step of intramolecular rearrangement of the catalyst-substrate complex taking place in solution becomes a rate limiting one. The effective activation energies of both ethylene and ethane become equal to 32 kcal mol -1 .The main function of the enzyme nitrogenase is the catalysis of the mild reduction of atmospheric nitrogen to ammonia which is the first step in the global "nitrogen" cycle. The enzyme has already been studied for a long time, and the last decade was the most productive in terms of obtaining information on the enzyme structure. The three dimensional X ray crystal structures of both protein components of nitrogenase, Fe protein and MoFe protein, were determined for several nitrogenases from different bacteria. 1-3 The structures of the metal clusters included in the protein components of nitrogenases were established. The Fe protein contains a [4Fe-4S] cluster, while the MoFe protein contains two types of metal clus ters unique in composition and structure: the so called P cluster [8Fe-7S], 2 and M center: iron molybdenum cofactor [(µ 6 N)Fe 7 MoS 9 •homocitrate] or FeMoco. 2,3 Generally, the functions of both protein components of the enzyme and all three types of metal clusters are clear. From the experimental data available up to date, it is commonly accepted that FeMoco acts as catalytic center of enzyme. 4 Although the composition and molecular structure of the cluster are known, the detailed mecha nism of catalysis of N 2 reduction involving FeMoco is to be elucidated. Where and how a substrate binds to the reduced cofactor? What is the mechanism of nitrogen protonation to form ammonia? What intermediate states are formed during this process?Unlike for P cluster, it is possible to probe the reactiv ity of FeMoco not only as a part of enzyme but also as an isolated cluster. 5 FeMoco as the M center of the MoFe protein and the cofactor isolated from the protein are not identical species but very similar. 6,7 We study the reactiv ity of FeMoco extracted from the protein as a catalyst of reactions of nitrogenase substrate reduction when elec trons, protons, and the substrate are provi...

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