A. E. Shilov

Catalytic reduction of molecular nitrogen in solutions

Reports on nitrogen fixation in solution are reviewed. The optimum catalyst is the polynuclear complex. The reaction proceeds as a multielectron process, and the limiting step involves the electron transfer from a reducing agent.

Gold helps bacteria to oxidize methane.

With the use of labeled methane-14C and by chromatographic analysis it was shown that gold-containing protein (Au-protein), isolated from goldphilic Micrococcus luteus bacteria, catalyzes the oxidation of methane to methanol in the system also containing NADH, air, K(3)Fe(CN)(6) and Tris-HCl buffer. Presumably Au-protein helps bacteria to survive when usual sources of carbon and energy are scarce.

Mono- and binuclear σ-aryl iron-lithium hydrides; synthesis and molecular structure

Abstract Phenyl- and p-tolyl-iron-lithium trans-dihydrides have been synthesised in the reaction of dihydrogen with the corresponding at-complexes of Fe0 and FeII. The molecular structures and IR spectra of the complexes have been investigated as well as those of the binuclear iron(II)-lithium complexes produced from iron trans-dihydrides under the action of polar solvents (THF, diethyl ether). Unlike mononuclear dihydrides which are inert to dinitrogen, binuclear complexes react with N2 producing new complexes containing reduced dinitrogen.

Methane Oxidation Catalyzed by the Au-Protein from Micrococcus luteus

Earlier, we showed that the Au-protein from the aurophilic bacteria Micrococcus luteus displays activity towards methane [1]. With the use of gas chromatography, it was shown that the amount of methane decreases in Au-protein-containing incubation medium in the presence of NADH, air, K 3 Fe ( CN ) 6 , and TrisHCl buffer. In addition, using the ESR method, we demonstrated a change in the redox state of the Au-protein induced by methane. Based on the results obtained, we assumed that gold incorporated in the active site of the enzyme involves methane into its oxidative cycle. Presumably, the first oxidation product could be methanol. In the present study, we sought to verify the above assumption using the isotopic-dilution approach for quantitative analysis of methanol with the use of 14 C-labeled methane as a substrate for the Au-protein.

Catalytic reduction of acetylene in the presence of molybdenum and iron clusters, including FeMo cofactor of nitrogenase

Abstract Acetylene was reduced by zinc amalgam in the presence of three synthetic polynuclear complexes: {[Mg2Mo8O22(OMe)6(MeOH)4]−2·[Mg(MeOH)6]2+}6MeOH (I), (Bu4N)2[Fe4S4(SPh)4] (II), [Me4N][VFe3S4Cl3(DMF)3]·2DMF (III) and the iron-molybdenum cofactor of nitrogenase Azotobacter vinelandii MoFe7(S2−)9·homocitrate, FeMo-co (IV). Thiophenol was found to greatly facilitate the reaction in the presence of complexes I, II, IV. The reaction is catalytic and for I and IV proceeds at the amalgam surface. Thiophenol seems to increase the adsorption of the complexes, serving as an electron bridge to transfer electrons to the catalyst. In the case of II a homogeneous reduction of the substrate occurs presumably after the cluster reduction at the surface and with III the catalytic reduction proceeds only under the action of sodium amalgam; no thiophenol cocatalytic action is observed. Relevance to N2 enzymatic reduction is discussed.

On the coupled oxidation-reduction mechanism of molecular nitrogen fixation

A new mechanism for the catalytic reduction of N2 was proposed. According to the mechanism, reduction is preceded by the oxidation step with the formation of N2O. The mechanism allows the participation of weaker reducing agents than those in purely reductive processes. Probable individual steps are considered, in particular, the oxygen atom transfer from the superoxide radical anion O2–· in a cyclic complex containing the N2 molecule in the coordination sphere of a metal. The proposed mechanism can explain N2 reduction involving recently discovered nitrogenase in which O2–· acts as an electron donor and N2 reduction in purely chemical systems including the air nitrogen and relatively weak reducing agents.

Electrochemical and ESR studies of au-protein from Micrococcus luteus

Au-protein from Micrococcus luteus, with and without Au inactive center, and chloroauric acid (HAu IIICl4·4H2O) with the addition of rutin, catechol, and riboflavin have been studied by means of electrochemistry and ESR. The redox potentials for Au-protein, as well as for the complexes Au-rutin and Au-catechol, have been measured, and ESR spectra of complexes Au-rutin and Au-catechol have been recorded. It has been shown that the Au atom binds to Au-protein via OH-groups of rutin. Flavin does not participatein gold binding. Au-protein is characterized by two peaks of cyclic voltammogram, −0.37 and −0.54 V. Au-protein with these potentials is able to function in the electron-transport chain of membranes between flavoproteins and quinones.

Molecular structure and reactions of azobenzene complexes with iron-lithium compounds

Abstract Reactions of azobenzene have been studied with heteronuclear iron-lithium compounds formed in the reaction of FeCl3 with LiPh, one of the dinitrogen reducing systems of the Volpin type: Ph4FeLi4(OEt2)4 (1) and (H2)FePh4Li4(OEt2)4 (2). The structures of the azobenzene complexes formed, (N2Ph2)3FeLi3(OEt2)3 (3) and (N2Ph2)3FeLi2(THF)2 (4), as well as an ether-containing analog of the latter, (N2Ph2)3FeLi2(OEt2)2 (5), were determined by X-ray analysis of single crystals. Coordination of azobenzene at FeLi3 and FeLi2 clusters was shown to result in a sigificant elongation of the NN bond; partial cleavage of this bond on protolysis of the complexes resulted in the formation of hydrazobenzene and aniline. Magnetic susceptibility measurements and theoretic analysis of a similar model complex leads to the conclusion that the iron oxidation state in 3 may be considered between iron (I) and iron(III) (close to iron(I)), whereas in 4 and 5 it is close to iron(II).

Kinetics of water oxidation with cerium(IV) compounds catalyzed by a tetranuclear ruthenium complex

The kinetics and mechanism of water oxidation with cerium(IV) compounds catalyzed by a tetranuclear ruthenium complex containing two polyoxotungstate ligands are reported. Four water molecules are oxidized via an eight-electron process to form two oxygen molecules.

On the Nuclearity of the Vanadium(II)–Pyrocatechol Complex Active in the Reaction of Molecular Nitrogen Reduction

A detailed analysis of the previously obtained EPR spectra of the V(II)–pyrocatechol complex active in the reduction reaction of molecular nitrogen was performed. On the basis of structural data for related systems, a previous interpretation based on the complete disappearance of exchange interactions was revised. The hyperfine structure of the EPR spectrum of the test complex was explained as a consequence of strong exchange interactions, which effectively withdraw a portion of V atoms. A conclusion on the tetranuclear character of the active complex was made.