Polynuclear Dinitrogen Complexes

Few dinitrogen complexes of chromium are known. Complex 8 decomposes at 20 C with loss of both N2 and PMes. Treatment of complex 9 with acid produces small quantities of ammonia (7%) and hydrazine (1%). Reaction of the polynuclear complex 10 with water regenerates the starting rhenium complex tran5-[ReCl(N2)(PMe2Ph)4], and with dioxygen produces the cation [ReCl(N2)(PMe2Ph)4]. ... 

No reactions of complexes 62 and 63 have been reported. Palladium and platinum form no well-defined dinitrogen complexes. However, until relatively recently there were few carbonyl complexes of palladium and platinum. This has changed rapidly with the preparation of a wide variety of compounds such as dinuclear complexes, e.g., [M2Cl2(ju,-CO)-(ju. -dppm)2] where M = Pd, Pt, and neutral and anionic polynuclear complexes such as [Os2(CO)6 /t-Pt-(CO)(PPh3) 2] and [Pt9(CO)i8]. The absence of simple, mononuclear palladium and platinum dinitrogen complexes should not be construed as evidence that this is a barren area for research. 

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. 

Peculiarities of the N2 molecule make it necessary to use special means of electron transfer to and inside the active center containing the substrate. The mechanism of the catalysis in protic surroundings, at least for dinitrogen reduction, presumably necessarily includes coupled one-electron transfer from an external electron donor and multi-electron transfer to the substrate coordinated in the polynuclear complex. The coupled electron transfer helps to activate and reduce the difficult substrate dinitrogen at ambient temperatures. 

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). 

As we have already seen, FeMo cofactor of nitrogenase is a polynuclear complex of composition Fc7MoS9 (homocitrate), and all the available evidence implies it is the active site at which dinitrogen and other nitrogenase substrates are activated and reduced. Yet despite its first isolation in 1977 the catalytic activity of FeMoco was detected only in 1997, i.e. 20 years later. 

Reduction of dinitrogen molecule by divalent lanthanide complexes has been a popular subject in coordination and organometallic chemistry of the lanthanide elements. In the course of these studies, a large number of polynuclear lanthanide clusters featuring a core of lanthanide atoms organized around a dinitrogen unit have been obtained and structurally characterized. 

Starting from the nitrido complex TpOs( = N)Cl2, several unusual polynuclear compounds have been obtained. For example, TpOs( = N)Cl2 reacts readily with cobaltocene (CP2C0) to give the mixed-valence dinitrogen-bridged anionic complex [Tp20s2(ia cr(iV),(A )-N2)Cl4][Cp2Co]. Upon reaction of TpOs( = N)Cl2 with the... 

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... 

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