Molybdenum complexes dinitrogen activation

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. 

Tian Y-H, Pierpont AW, Batista ER. How does Nishibayashi s molybdenum complex catalyze dinitrogen reduction to ammonia Inorg Chem. 2014 53 4177-4183. Kinoshita E, Arashiba K, Kuriyama S, et al. Synthesis and catalytic activity of molybdenum-dinitrogen complexes bearing unsymmetric PNP-type pincer ligands. Organometallks. 2012 31 8437—8443. 

It seems likely that the active site for dinitrogen binding involves the molybdenum atom. It has been established by EXAFS that the coordination sphere consists of several sulfur atoms at distances of about 235 pm. An Mo=0 double bond, so common in complexes of Mo(IV) and Mo(Vl), is not present. There are other heavy atoms, perhaps iron, nearby (—270 pm). The ultimate source of reductive capacity is pyruvate, and the electrons are transferred via ferredoxin (see page 911) to nitro-... 

Increasing the number of electrons reduces the activation of N2, because the electrons occupy the orbitals which are bonding with respect to the NN bond, and actually stabilize it. In agreement with this prediction dinitrogen is sufficiently activated to be reduced by protonation by dinuclear complexes of titanium(II), zirco-nium(Il), niobium(III), tantalum(III), molybdenum(IV), and tungsten(IV), whereas it is not reduced by protonation by certain d -d complexes, such as those of molybdenum(O), ruthenium(II), or rhodium(I). Apparently dinuclear complexes M-N=N-M in which M has the d electronic configuration can be intermediates in dinitrogen reduction in protic media, particularly if they represent part of polynuclear complexes (vide infra). 

We found these conditions by introducing surface-active materials to a solution of a specially prepared catalytic complex containing molybdenum and stabilized by magnesium ions [22]. The structure of the complex which was isolated from the solution in its oxidized form is presented on Figure 5 [23]. When it is reduced to the Mo state the complex becomes an active catalyst of dinitrogen reduction to hydrazine and ammonia by sodium amalgam. Phospholipid (phosphatidylcholine)... 

However, this work pre-dated the discovery of protonation of dinitrogen in the molybdenum and tungsten complexes and, once this had come to light, we turned our attention to the behaviour of isocyanides and alkynes at those sites that activated dinitrogen to reduction. For isocyanides, the first step was to bind these ligands in place of dinitrogen in the complexes trflns-[M(N2)2(dppe)2], which was done by a displacement reaction [Equation (23)]. ... 

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