Chatt cycle

The alternative mechanistic scenario for the protonation and reduction of end-on terminally coordinated N2 through the Schrock cycle is represented by the Chatt cycle which has been developed many years earlier (5). This system is based on Mo(0) and W(0) dinitrogen complexes with phosphine coligands (Fig. 3). As expected, the intermediates of the dinitrogen reduction scheme are very similar to those of the Schrock cycle. Moreover, a cyclic generation of NH3 from N2 has been demonstrated on the basis of this system, however, with very small yields (3,4a). In order to obtain general insight into the mechanism of the Chatt cycle we have studied most of the intermediates of Fig. 3 with... 

Fig. 3. The Chatt cycle with its intermediates. The cycle is divided in three stages (a) protonation of bound N2, (b) cleavage of the N-N bond, releasing one molecule of NH3, and (c) reduction and protonation of nitrido complexes generating the second molecule of NH3.

Fig. 4. Force constants f/mdynA 1 of the MN and the NN bonds in the intermediates of the Chatt cycle l pe, 2 ppeF, 3dppeF, 4dePe,F> dppe.Nn> and 6 peC1 (15,17).

Further reduction and protonation of the imido group leads to the formation of a Mo(I) ammine complex (8b) in a mechanism similar to the fluoro system in the classic Chatt cycle. The Mo(I) ammine complex 8b is first reduced to the corresponding Mo(0) complex 8c at which stage the ammine ligand is exchanged with... 

Fig. 8. Mo/W dinitrogen complex with the P/N ligand N,N,N, N -tetrakis-(diphenylphosphinomethyl)-2,6-diaminopyridine (pyN2P

In the preceding sections, experimental and theoretical studies on the mechanism and various modifications of the Chatt cycle have been presented. Our investigations have started with the detailed spectroscopic and theoretical characterization of the intermediates of the classic Chatt cycle. These studies are not yet... 

One possible reaction scheme, an elaboration of the so-called Chatt cycle, results in the nitrogenase cycle shown in Figure 6.16.51 The cycle shown operates between molybdenum oxidation states Mo(0) and Mo(IV), but others have been proposed that operate between Mo(II) and Mo(VI) levels. 

A series of model studies initiated in the early 1960s by the groups of Chatt and Hidai demonstrated that dinitrogen could be bound and reduced to ammonia at a single metal centre by Mo and W complexes (Chatt et al., 1978 Hidai, 1999). However, although examples of virtually all the proposed intermediates in a Chatt cycle were isolated, no catalytic reduction of N2 to NH3 was ever achieved. Catalytic reduction of dinitrogen to ammonia... 

Figure 6.8 Chatt cycle with intermediates and ligand systems.

This includes for example reactivity studies on tra s-[M(N2)2(dppe)2] (98-M M = Mo (98-Mo) and W (98-W)) toward electrophiles such as organic acid chlorides, alkyl hahdes, and inorganic acids.These investigations formed the basis of the so-called Chatt cycle (Scheme 31)" and the intermediates 99-106 involved in the formation of NH3 from N2 have been investigated in detail by computational and spectroscopic methods.However, it is of note that a catalytic turnover has yet to be achieved with these systems. 

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