Metal-nitrogen-carbon complexes

As a solution to provide a long-term solution to Pt cost and scarcity, a variety of non-noble metal-based catalysts has been explored as promising cathode catalysts for fuel cells. These ORR catalysts include heat-treated metal-nitrogen-carbon complexes (M-Nx/C, M = Fe or Co), carbon-supported chalcogen-ides, and carbon-supported metal oxides. These catalysts have been synthesized and showed considerable ORR activity and stability when compared to those of Pt/C catalyst. In the exploration, RDE/RRDE techniques are the most commonly employed tools in evaluating the catalysts activity and stability toward ORR and its associated mechanism. 

One-electron oxidation of the vinylidene complex transforms it from an Fe=C axially symmetric Fe(ll) carbene to an Fe(lll) complex where the vinylidene carbon bridges between iron and a pyrrole nitrogen. Cobalt and nickel porphyrin carbene complexes adopt this latter structure, with the carbene fragment formally inserted into the metal-nitrogen bond. The difference between the two types of metalloporphyrin carbene, and the conversion of one type to the other by oxidation in the case of iron, has been considered in a theoretical study. The comparison is especially interesting for the iron(ll) and cobalt(lll) carbene complexes Fe(Por)CR2 and Co(Por)(CR2) which both contain metal centers yet adopt... 

H-NMR studies of oligocarbene Ru(II) complexes indicate a substantial barrier to rotation about the metal-carbene carbon and nitrogen-R bonds. This restricted rotation is thought to arise as a consequence of intramolecular non-bonding cis interactions of the carbene nitrogen-R substituents, and not because of any significant double bond character in ruthenium-carbene carbon (76). 

This section deals with alkylidene complexes L M=CR2 and vinylidene complexes LnM=(C)n,=CR2 in which the metal-bound carbon atom bears only hydrogen, alkyl, or aryl groups, but neither heteroatoms (halogen, nitrogen, oxygen, or sulfur) nor electron-withdrawing groups. Dimetallacyclopropanes and ketene complexes will not be discussed. 

Carbon dioxide does not insert into all metal-carbon bonds, as demonstrated by the lack of reaction with [HIr(PMe3)4CH2CN]+ 137). Similarly, benzylchromium complexes failed to insert C02 142), and in the reaction of Me(Me2N)2W=W(NMe2)2Me with C02, the molecule inserts into the metal-nitrogen bonds, but not into the metal-carbon bonds 143). 

The cobalt-nitrogen double bond in VI may be assumed to arise from interaction between the 2p orbital of nitrogen and one of the 3d orbitals of cobalt. Such a double bond may be compared with the metal-carbon and metal-nitrogen double bonds proposed for cyano and nitro complexes (p. 359), but in a structure such as VI, the ligand atom, rather than the metal ion, assumes the role of r-electron donor. 

Metal rf-inline complexes with various transition metals [1-10] and lanthanides [11,12] are well known in the literature. Early transition metal if-imine complexes have attracted attention as a-amino carbanion equivalents. Zirconium rf-imine complexes, or zirconaaziridines (the names describe different resonance structures), are readily accessible and have been applied in organic synthesis in view of the umpolung [13] of their carbons whereas imines readily react with nucleophiles, zirconaaziridines undergo the insertion of electrophilic reagents. Accessible compounds include heterocycles and nitrogen-containing products such as allylic amines, diamines, amino alcohols, amino amides, amino am-idines, and amino acid esters. Asymmetric syntheses of allylic amines and a-amino acid esters have even been carried out. The mechanism of such transformations has implications not only for imine complexes, but also for the related aldehyde and ketone complexes [14-16]. The synthesis and properties of zirconaaziridines and their applications toward organic transformations will be discussed in this chapter. 

Complex carbides containing boron, occurring frequently in boron-alloyed steels and superalloys, are also named carboborides. Metal borocarbides (see Table 1) are synthesized by powder metallurgical methods or are extracted from a metal matrix. There are pseudoternary or -quaternary borocarbides, such as Mn23(B, C) or (Cr, Mn, Fe)23 (B, C)g (t phases) although boron-carbon substitution in borocarbides is less pronounced than nitrogen-carbon substitution in metal carbonitrides. 

N4-M catalysts can also be made from nonmacrocyclic precursors. - In the presence of a nitrogen source, they can be prepared by pyrolyzing transition metal salts or complexes adsorbed on a carbon support. Examples of salts/complexes include acetate,Ee(OH)2 (derived from EeS04), " and phenanthroline complexes. The nitrogen sources may come from an external supply such as or... 

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