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

Abstract

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 cot'actor 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 N;~ 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 ma/.n product is hydrazine; in the case of Eu(Hg), it is ammonia.In 1964, M. E. Volpin and V. B. Shur published pioneering work I demonstrating the possibility of reducing molecular nitrogen in solutions of metal complexes. This work laid the foundation for a new area in chemistry: the low-temperature chemistry of molecular nitrogen. The first systems reducing N 2 under mild conditions contained transition metal (titanium, iron, molybdenum, and others) compounds in combillation with strong reducing agents, such as organomagnesium and organolithium compounds, metal hydrides, and free metals. The further study of these systems showed that the reaction of the reducing agent and a transition metal compound results in the reduction of the latter, and the derivative of the transition metal in the low oxidation state that formed reacts with dinitrogen to give complex nitrides. The subsequent hydrolysis of the reaction mixture gives ammonia, z Of course, the reaction with dinitrogen can occur only in an aprotic medium, because the strong reducing agents used are unstable in protic solvents (water, alcohols) and react with the solvent. The amount of ammonia (after hydrolysis of the products) does not exceed the stoichiometric amount calculated with respect to the transition metal taken; however, catalytic systems were also observed, z For example, in the reaction of N 2 with the TiCIa--AI--AIBr 3 mixture (with an excess of the reducing agent AI--A1Br3), the amount of ammonia formed after the hydrolysis of the products is hundreds of times greater than that of titanium taken.However, no catalytic reduction of N 2 involving protons was observed in the first works.Nevertheless, in the well-known process of biological nitrogen fixation, which also involves the transition metals iron and molybdenum, the N 2 reduction involves protons of water. The stoiehiometric equation of this complicated reaction catalyzed by the nitrogenase enzyme includes the biological reducing agent ferredoxin or flavodoxin (in vivo) or dithionite in experiments in vitro and takes into account the conjugated hydrolysis of adenosine triphosphate (ATP). N 2 + 8 H "~ + 8 e-($2042-) + 16 MgATP NH 3 + H + 16 MgAC'P + 16 1~O42-The enzymatic process occurs under the action of considerably weaker reducing agents than those in the first nitrogen-reducing Volpin--Shur systems. Protons of the medium most likely participate in the early stages of the reaction without preliminary formati...

Ammonia synthesis at low temperatures

“…So far no one has been able to do that, but a first step in this direction has been taken by Pickett et al 25 who have have reported that for large enough negative bias, H 2 can be produced, showing that the chemical potential of (H ϩ ϩe Ϫ ) in the active site of the enzyme can be raised electrochemically above that of hydrogen in H 2 . Shilov et al 40 have catalytically reduced acetylene by means of the FeMoco and various amalgams as reductants. In this connection it is also relevant to note that there has been several reports on the protolysis of N 2 containing transition metal complexes under strongly reducing conditions.…”

“…It cannot be excluded that the enzyme has a structure that facilitates the proton transfer particularly well, and that it is difficult to obtain equally good conditions at a metal surface. On the other hand, it is known that even the isolated cofactor can produce H 2 , 25 can protonate acetylene, 40 and binds CO in much the same way as in the enzyme. 26 Recently it has been reported that ammonia was produced electrochemically by a Pd catalyst on a proton conducting oxide.…”

Effect of the potential of an external electron donor on C2H2 reduction catalyzed by the nitrogenase active center (FeMoco) isolated from the enzyme

“…For example, the system with Mg-Mo clusters as catalysts and Na/Hg as electron donor reduces actively N 2 to N 2 H 4 and NH 3 , and even a weaker reducing agent (Eu/Hg) can be used for this reaction in the presence of a more acidic proton source (thiophenol). 25, 26 For this sys tem, in particular, it has been studied the dependence of the rate of catalytic С 2 Н 2 reduction on the specified po tential of the zinc amalgam cathode. The rate showed an exponential dependence plotted on the electrode poten tial.…”

Catalytic behavior of a polynuclear Mg-Mo complex and nitrogenase active site (FeMoco) isolated from the enzyme in reactions with C2H2, N2, and CO: A comparative study

“…The Mg-Mo complex, unlike FeMoco extracted from the protein, effi ciently catalyzes the reduction of N 2 to hydrazine and ammonia (see, for example, Ref. 13 and references cited therein). It was of interest to determine the characteristics of the dinitrogen complex formed at the first step of N 2 activation to compare them with the results obtained for the FeMoco catalyst.…”

“…12 The con centration of ammonia was determined by the indophenol method modified as described previously. 13 The gaseous reaction products, viz., ethylene, ethane, and methane, were analyzed by gas chromatography on a Biochrom chromatograph (column filled with activated alumina (0.25-0.5 mm), column temperature 90 °C, argon as the carrier gas, flame ionization detector).…”