Encaged metal carbonyls

The encaged clusters considered in this chapter are almost exclusively metal carbonyls and metals (including bimetallics). llte encaged metal carbonyls that have been most investigated include [Rh (CO)i6], [Ir4(CO)i2], and the isomers of [Ir (CO),d the crystal structures of the iridium clusters are shown in Figure 4-6. Some of the most thoroughly characterized encaged metal dusters have been made from these metal carbonyls. Brief mention is made of metal oxide and also nonmetal clusters ionic dusters are scarcely considered. Synthesis, characterization, reactivity, and catalytic and other properties are considered for these materials. 

An important consequence of the effects of the adduct formation on the Vco infrared spectra of encaged metal carbonyl clusters is that the spectra of dusters with bridging carbonyl groups, which are rather strongly basic, are significantly shifted from those of the clusters in neutral solvents, and identification by comparison with the spectra of the same dusters in solution is usually not straightforward. This point has not always been appredated in the literature. 

A different effect of CO on zeolite-encaged metal has been observed for Rh particles. In the presence of protons CO causes complete disintegration of small Rh particles, forming Rh(CO)2 (104). This effect was observed earlier for Rh supported on amorphous supports such as AI2O3 (195-197). It is due to oxidation of Rh by protons in conjunction with formation of the stable carbonyl ... 

Extraction metal carbonyl clusters internal or external location of metal carbonyl cluster in zeolite Carbonyl cluster encaged in the zeolite cages cannot diffuse through the zeolite aperture and cannot be extracted out effective for anionic dusters but not effective for some neutral carbonyl clusters that are difficult to dissolve in solvent. 

Binary metal carbonyls can also react with the hydroxyl groups of zeolites to form zeolite encaged anions, the reactivity being analogous to that of metal carbonyls with surface hydroxyl groups on metal oxide surfaces or with hydroxide ions in solution (Eq. 4.6). [98, 99]... 

Characterization is relatively simple since the dusters contain only metal atoms, and usually of only a single element. However, there is one problem asso-dated with the characterization of metal clusters in cages. In contrast to the situation for metal carbonyl dusters, there is no base data set for these metal clusters themselves in a pure state to be used for comparison. This means that spectra of encaged metal dusters cannot be compared with those of their analogues in the liquid or solid state because they simply are not known. Thus the basis for structure determination is in a sense weaker than that for metal carbonyl dusters. 

In the presence of carbon monoxide, CO, metals such as Mo, Re or Ru frequently form carbonyls, which subsequently may be encaged in zeolites and, after decomposition, transformed into small metal aggregates. However, in several cases, the encapsulated carbonyls are transferred into cationic species and anchored as such in the zeolite matrix. For example, Co2(CO)g was oxidized by the protons of H-Y to Co + and held at sites in the supercages, as suggested by IR investigations [251]. Formation of the cations may be due to a reaction as described by Eq. (23). 

Gehn et al. [194] studied rhodium loading by sublimation of Rhg(CO)ig at 353 K onto HY zeolite followed by thermal decarbonylation in Hj or imder vacuum at 373 K. Partially stripped rhodium carbonyl species entered the zeolite pores and were re-carbonylated in situ by CO at 373 K to give encaged polynuclear rhodium carbonyls. This loading process was effective only for low metal loadings (0.5 wt.%), otherwise rhodium was mainly loaded on the external surface. 

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