Rare dinitrogen activation

So far, studies on the reactivity of the divalent complexes mentioned above in Section 2.3 have been very limited. Dinitrogen activation has been observed not only for isolated divalent complexes but also for systems containing divalent species or trivalent rare earths in the presence of reducing agents, thus displaying divalent-like behaviour this important topic will be developed later in Section 2.3.4. 

There are no mechanistic details known from intermediates of copper, like we have seen in the studies on metathesis, where both metal alkylidene complexes and metallacyclobutanes that are active catalysts have been isolated and characterised. The copper catalyst must fulfil two roles, first it must decompose the diazo compound in the carbene and dinitrogen and secondly it must transfer the carbene fragment to an alkene. Copper carbene species, if involved, must be rather unstable, but yet in view of the enantioselective effect of the ligands on copper, clearly the carbene fragment must be coordinated to copper. It is generally believed that the copper carbene complex is rather a copper carbenoid complex, as the highly reactive species has reactivities very similar to free carbenes. It has not the character of a metal-alkylidene complex that we have encountered on the left-hand-side of the periodic table in metathesis (Chapter 16). Carbene-copper species have been observed in situ (in a neutral copper species containing an iminophosphanamide as the anion), but they are still very rare [9],... 

There are several rare-earth-based systems that have been shown to activate dinitrogen this topic has been reviewed fairly recently (Evans and Lee, 2005). Some of these systems include Sm" compounds and thus they fall outside the topic of this chapter. Other utilise Tm", Dy" and Nd" compounds, and dinitrogen is eventually also activated in reactions in which a trivalent rare-earth precursor is present together with a strong... 

FIGURE 41 The structure of [ [(SiMe3)2N]2Y(THF) 2(/i-7j 77 -N2)], obtained in the reaction of [Y N(SiMe3)2 3] with potassium under dinitrogen in THF dinitrogen can be activated by systems involving non-reducible trivalent rare-earth precursors. 

The low-valent molecular chemistry of rare earths was once thought to be restricted to divalent samarium, europium and ytterbium. This chemistry has now been extended to many of the rare earths in the zero-valent state, to mono- and divalent scandium, to divalent lanthanum, cerium, neodymium, dysprosium and thulium, and to systems in which dinitrogen is activated and that may contain yet other highly reactive divalent rare earths. It is the opinion of the author that this research area is likely to find fascinating developments in the near future. 

Fixation of carbon dioxide and formaldehyde in their intact form on a metal centre is a primary goal in metal-promoted transformations of a C molecule, provided it forms metal-carbon bonds. Formation of formaldehyde and carbon dioxide complexes is, however, a quite rare reaction, in spite of the various strategies applied so far. On the contrary, coordination of dinitrogen has been found in a number of complexes all the mononuclear compounds so far identified prefer the end-on bonding mode. Activation of dinitrogen, however, can be much more pronounced in ca.se N2 binds the metal in a side-on fashion. 

The roles of alkali, alkali earth and rare earth metal oxides seem different from the structural promoters. These oxides are able to increase the specific activity per unit surface area, while decrease the heat-resisting and anti-toxic ability. Thus, they are called as electronic promoters. Because the diameter of K+ ions is quite large, it is not possibly for K to enter into the lattice of magnetite. After reduction, K2O diffuses to the surface of crystallite. The surface potassium is able to accumulate with various forms during reduction and operations, to accelerate the recrystallization effect, but due to the electron, negative alkali metals decrease the effusion work of iron atoms, and accelerate the adsorption of dinitrogen or desorption of ammonia and finally are able to increase the specific activity per unit surface area. 

Several different coordination modes have been found in dinitrogen metal compounds (Fig. 2) and they are strongly influenced by the identity of the metal atom(s), their oxidation state(s), and the ligand environment. This results in different degrees of reduction of the N=N bond, which is often also referred to as activation . For monomeric complexes, the end-on coordination mode A is most frequently encountered. However, in very rare cases side-on coordination B has also been observed, eg, a metastable Tj -bound Nz hgand can be trapped and structurally characterized upon photochemical activation of [Os(NH3)5(r -N2)][PF6]2. The large overlap between the osmium d-orbitals with the 2p orbitals of the bound Nz confers stability to this species. 

Tellers DM, Bergman RG. C—H bond activation by a rare cationic iridium dinitrogen complex. An important electronic effect in the chemistry of the hydridotris(pyrazolyl) borate ligand. /Am Chem Soc. 2000 122 954-955. 

Fieser ME, Bates JE, ZiUerJW, Furche F, Evans WJ. Dinitrogen reduction via photochemical activation of heteroleptic tris(cyclopentadienyl) rare-earth complexes. 

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