Dinitrogen reduction catalytic systems

Since the first compound of this type, [Ru(NH2)5(N2)]Bt2 [15246-25-0] was synthesized (178), most transition metals have been found to form similar compounds (179,180). Many dinitrogen compounds are so stable that they ate unreactive toward reduction and so have Htde chance to form the basis of a catalytic system. 

Studies on model polynuclear catalytic systems have confirmed that for the catalytic reduction of dinitrogen under mild conditions, it is necessary to use a polynuclear transition metal complex capable of donating four electrons to form the hydrazine derivative. 

The yields were found also to increase in the presence of phosphines, particularly trimethyl or tributyl phosphine. After all the improvements of the catalyst and reaction conditions the system became by far the most active of known non-biological catalytic systems for the reduction of dinitrogen at ambient temperature and pressure. The specific activity (the rate of N2 reduction per mole of the complex) reached and even exceeded that of nitrogenase. Up to 1000 turnovers relative to the molybdenum complex can be observed at atmospheric pressure and more than 10 000 turnovers at elevated N2 pressures. 

A range of metals and metal oxides catalyze the reduction of NO. The most successful reducing agent is synthesis gas since the catalytic process is relatively fast 184), unlike the catalyzed decomposition of NO to N2 and O2. The observed nitrogen-containing products depend on the catalytic system used Pd- and Pt-based catalysts convert NO to NH3 184) whereas iridium and ruthenium systems minimize ammonia production and convert nitric oxide to dinitrogen. 

The research of Shilov also seems to have real commercial potential. He has discovered several new systems, some of which are catalytic and function in protic solvents such as methanol. His most recent system uses sodium amalgam or electrochemical reduction of dinitrogen mediated by molybdenum, probably in the form of a molybdenum(III) complex. Phosphines enhance the reaction, and so does lethicin, which coats the amalgam surface. The reaction rate at room temperature and atmospheric pressure is greater than 0.3 mol N2 reduced per catalytic center per second, and the turnover number is several hundreds per molybdenum. " No firm conclusions have yet been drawn concerning the mechanism of dinitrogen reduction, though it is believed that polynuclear species are... 

The catalytic reduction of dinitrogen to ammonia in biological systems is thermodynamically favorable overall and takes place according to equation 6.2 ... 

Thus, a number of systems of the catalytic and noncatalytic reduction of dinitrogen to hydrazin and ammonia and the successful synthesis of model iron- and iron-molibdenum (vanadium) clusters have been reported. These investigations have formed a basis for subsequent progress in mimicking the nitrogenase reaction. 

In this article, the reasons for this mechanism in catalytic reduction of N2 will be considered, and results for electron transfer in biological N2 reduction and model synthetic systems will be compared. Hopefully, this consideration will lead to understanding how inert dinitrogen can be turned into a very active substrate readily reacting in solution in the presence of comparatively mild reducing agents. 

If the complex is reduced to the Mo state and there is no external electron donor in the system (the amalgam is removed) there is no reduction of dinitrogen, even stoichiometric, confirming the mechanism in which the reducing agent participates directly, in a coupled manner, in the reduction of the substrate which is coordinated by the catalytic complex. 

Iron can be said to be the most chemically versatile of all the elements used by nature. It is essential for dioxygen uptake and transport in the vast majority of living systems, is ubiquitous in electron transfer relays and oxygen metabolism, is used widely as the catalytic center in enzymes catalyzing chemical changes as diverse as dinitrogen fixation, nitrate reduction, and isopenicillin-N-synthase, and is vital for DNA... 

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