Metal clusters, carbon-centered

The objective of most research in the area of pyrolyzed metal/N/C materials has centered around understanding the nature of the active site for the ORR. Similar to heat-treated macrocycles, there has been a parallel controversy over the nature of the active sites and the role of Fe or Co in these metal-nitrogen-carbon catalysts. Based on the activity attainable from a wide-range of precursors, it seems safe to assume that above a certain temperature, the active site formed is the same regardless of the metal-nitrogen-carbon starting material (macrocycle or otherwise). Initially, some researchers believed that the metal clusters protected by a layer of carbon (which prevented leaching of the metal in the acidic electrolyte) were the source of catalytic... 

Carbon itself may play quite a different role than commonly encountered in organic chemistry. Certain metal clusters encapsulate carbon atoms the resulting carbon-centered clusters, frequently called carbide clusters, in some cases contain carbon bonded to five, six, or more surrounding metals. The traditional notion of carbon forming bonds to, at most, four additional atoms, must be reconsidered. Two examples of carbide clusters are included in Figure 13-2. 

Carbon plays an unusual role in a number of metal cluster compounds in which a carbon atom is at the center of a polyhedron of metal atoms. Examples of carbon-centered clusters with five, six, or more surrounding metals are known (Figure 1.7). The striking role that carbon plays in these clusters has provided a challenge to theoretical inorganic chemists. 

Many compounds have been synthesized, often fortuitously, in which one or more atoms have been partially or completely encapsulated within metal clusters. The most common have been the carbon-centered clusters, also called carbide or carbido clusters, with carbon exhibiting coordination numbers and geometries not found in classic organic molecules. Examples of these unusual coordination geometries are shown in Figure 15.23. 

It is thus clear that a rigorous study of the chemical shift anisotropy, combining theory and experiment, is essential to understand at the molecular level the variation of coordination of a ligand on a metal center. This can in fact be used to understand the mode of absorption of carbon monoxide on metallic particles and metallic clusters. ... 

---