G. N. Petrova

Catalytic reduction of acetylene and dinitrogen with the participation of the iron-molybdenum cofactor of nitrogenase and synthetic polynuclear molybdenum(iii) complexes

Catalytic reduction of acetylene and dinitrogen was carried out by sodium, zinc, and europium amalgams in the presence of polymolybdenum clusters and the iron-molybdenum cofactor of nitrogenase isolated from the enzyme. The activity of both catalysts toward acetylene changes in the sequence Zn(Hg)<Eu(Hg)<Na(Hg), increasing as the redox potential of the reducing agent is shifted to the negative region. The catalytic reduction of N2 occurs only by the action of sodium and europium amalgams and only in the presence of synthetic polymolybdenum complexes; in the case of Na(Hg), the main product is hydrazine; in the case of Eu(Hg), it is ammonia.

Catalytic behavior of the nitrogenase iron-molybdenum cofactor extracted from the enzyme in the reduction of C2H2 under nonenzymatic conditions

To compare the catalytic effect of the active center of nitrogenase (iron-molybdenum cofactor (FeMoco)) under nonenzymatic conditions with the behavior of FeMoco incorporated in a protein, the kinetics of C2H2 reduction with Zn and Eu amalgams was examined in the presence of the cofactor extracted from the MoFe protein of nitrogenase (the specific activity of the extracted FeMoco after its integration into the cofactordeficient MoFe protein ofKp 5058 was 200 ± 20 mol of C2H4 (mol of Mo)-1 min-1. It was found that under exposure to reducing agents of different strength—Zn amalgam (I) (−0.84 V with respect to a normal hydrogen electrode (NHE)) and Eu amalgam (II) (−1.4 V with respect to NHE)—different reduction states of FeMoco were produced. They differed in the number and properties of substrateand inhibitor-coordinating active sites. For I, the rate of ethylene formation was described by a hyperbolic function of substrate concentration (KM = 0.045 atm). Carbon monoxide reversibly inhibited the reduction of acetylene(Ki- 0.05). For II, a sigmoid relationship between the rate of accumulation of C2H4 or C2H6 and substrate concentration was found. This relationship was explained by the occurrence of three interrelated sites of acetylene coordination and reduction with the apparent constantKM = 0.08 atm in the FeMoco cluster reduced by europium amalgam. In this case, the specific activity was 40–60 mol of C2H4 (mol of Mo)−1 min−1. For the system with Eu (Hg), the CO inhibition constants were 0.004 and 0.009 atm for the formation of ethylene and ethane, respectively. The behavior of FeMoco as a catalyst for acetylene reduction and the inhibition of this reaction by carbon monoxide in various reducing protein and nonprotein media were compared. This comparison demonstrated that typical features of the catalytic behavior of FeMoco depend primarily on its composition and structure and only secondarily on the type of the reducing agent and on the reaction medium.

Mutual effects of substrates and inhibitors in reactions catalyzed by isolated iron-molybdenum cofactor of nitrogenase

The inhibiting effects of CO and N2 on the ability of the nitrogenase iron–molybdenum cofactor (FeMoco) to catalyze acetylene reduction outside the protein were studied to obtain data on the mechanism of substrate reduction at the active center of the enzyme nitrogenase. It was found that CO and N2 reacted with FeMoco that was separated from the enzyme and reduced by zinc amalgam (E = –0.84 V relative to a normal hydrogen electrode (NHE)) (I) or europium amalgam (E = –1.4 V relative to NHE) (II). In system I, CO reversibly inhibited the reaction of acetylene reduction to ethylene with Ki = 0.05 atm CO. In system II, CO inhibited the formation of the two products of C2H2 reduction in different manners: the mixed-type or competitive inhibition was found for ethylene formation with Ki = 0.003 atm CO and the incomplete competitive inhibition was found for ethane formation with Ki = 0.006 atm CO. The fraction of C2H6 in the reaction products was greater than 50% at a CO pressure of 0.05 atm because of the stronger inhibiting effect of CO on the formation of C2H4. The change in the product specificity of acetylene-reduction centers under influence of CO was explained by some stabilization of the intermediate complex [FeMoco · C2H2] upon the simultaneous coordination of CO to the catalytic cluster. Because of this, the fraction value of ethane as a multielectron reduction product increased. The experimental results suggest that several active sites at the FeMoco cluster reduced outside the protein can be simultaneously occupied by substrates and (or) inhibitors. The inhibition of both ethane and ethylene formation by molecular nitrogen in system II is competitive with Ki = 0.5 atm N2 for either product. That is, N2 and C2H2 as ligands compete for the same coordination site at the reduced FeMoco cluster. The inhibiting effects of CO and N2 on the catalytic behaviors of both isolated FeMoco and that in the enzyme were compared.

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.

Copolymerization of acetylene with ethylene in the presence of dibenzenetitanium(0)

The interaction between acetylene and dibenzenetitanium(0) at a room temperature results in the acetylene polymerization and its reduction to ethylene, ethane, and methane at the expense of H atoms of the acetylene molecule. The catalytically active species capable of copolymerizing acetylene with ethylene that are formed during the reaction or are added into the system originate from the interaction of dibenzenetitanium(0) with acetylene.

Mutual effects of substrates and inhibitors in reactions catalyzed by the nitrogenase iron-molybdenum cofactor outside the enzyme

The inhibiting effects of CO and N2 on the ability of the nitrogenase iron–molybdenum cofactor (FeMoco) to catalyze acetylene reduction outside the protein were studied to obtain data on the mechanism of substrate reduction at the active center of the enzyme nitrogenase. It was found that CO and N2 reacted with FeMoco that was separated from the enzyme and reduced by zinc amalgam (E = –0.84 V with reference to a normal hydrogen electrode (NHE)) (I) or europium amalgam (E = –1.4 V with reference to NHE) (II). In system I, CO reversibly inhibited the reaction of acetylene reduction to ethylene with Ki = 0.05 atm CO. In system II, CO inhibited the formation of the two products of C2H2 reduction in different manners: the mixed-type or competitive inhibition of ethylene formation with Ki = 0.003 atm CO and the incomplete competitive inhibition of ethane formation with Ki = 0.006 atm CO. The fraction of C2H6 in the reaction products was higher than 50% at a CO pressure of 0.05 atm because of the stronger inhibiting effect of CO on the formation of C2H4. A change in the product specificity of acetylene-reduction centers under exposure to CO was explained by some stabilization of the intermediate complex [FeMoco · C2H2] upon the simultaneous coordination of CO to the catalytic cluster. Because of this, the fraction of the many-electron reduction product (ethane) increased. The experimental results suggest that several active sites in the FeMoco cluster reduced outside the protein can be simultaneously occupied by substrates and (or) inhibitors. The inhibition of both ethane and ethylene formation by molecular nitrogen in system II is competitive with Ki = 0.5 atm N2 for either product. That is, N2 and C2H2 as ligands compete for the same coordination site in the reduced FeMoco cluster. The inhibiting effects of CO and N2 on the catalytic behaviors of FeMoco outside the protein and as an enzyme constituent were compared.

Electroreduction of carbon monoxide in aqueous solutions of molybdenum (III)-pyrocatechol complex

Conclusions1.Molybdenum (III) complexes with pyrocatechol catalyze the electrochemical reduction of CO on a mercury cathode in aqueous solution to give C1-C4 hydrocarbons.2.The reaction proceeds in a thin preelectrode layer and is accompanied by the parallel liberation of hydrogen.

Electrochemical reduction of carbon monoxide in the titanium(III) - molybdenum(III) - pyrocatechol system in aqueous alkaline solutions

ConclusionsElectrochemical reduction of carbon monoxide to C1-C4 hydrocarbons takes place in the Ti(III)-Mo(III)-pyrocatechol catalytic system in aqueous solutions at pH 6–13. The catalytically active particle in this system is a cartaonyl complex of Mo(III), while the pyrocatechol complex of Ti(III) plays the role of agent for transfer of electrons from the cathode to the active particle in the volume of the solution.

Catalytic Reduction of Acetylene with Participation of Isolated FeMo-co from Azotobacter vinelandii in Non-Enzymatic Conditions

We have been studying the reactions of the isolated iron-molybdenum cofactor nitrogenase with acetylene. FeMo-co from Azotobacter vinelandii has been isolated from the MoFe protein as published (McLean, et al. 1989). The reaction with C2H2 was carried out in DMF because FeMo-co survives in this solvent without destruction for a long time in the absence of water and oxygen. We found that it was an effective catalyst for acetylene reduction by zinc amalgam (Bazhenova et al., 1997). Cyclic voltammetry of FeMo-co in DMF solution showed a quasi-reversible reduction apparently to the substrate reducing state with a cathodic wave potential of − 0.73 V vs. NHE (Strelets, V.V., personal communication). For zinc amalgam, an E o (Zn2+/Zn(Hg)) in DMF in the presence of PhSH of − 0.84 V vs. NHE was measured. Thiophenol was a source of protons and an important cocatalyst. It stabilizes the system and the reaction rate does not change for several hours. We found that the reaction proceeds on the amalgam surface. Apparently thiophenol increases the adsorption ability of FeMo-co and acts as an electron conductive bridge between the multi-electron reductant and the catalyst (Bazhenova, et al., unpublished).