Encaged metal

For the preparation of CoSalophen Y the Co—Y was impregnated by salicy-laldehyde, and 1,2-phenylenediamine in methanol was added slowly to the mixture.107 This was a successful encapsulation of a salen-type complex with larger diamine than the ethylenediamine, a successful preparation of an encaged metal-salen complex by the intrazeolite ligand synthesis method, and a successful intrazeolite synthesis using two different precursor molecules. 

CO and particle size, 39 159 CO on zeolite-encaged metal, 39 158 EXAFS functions, 39 155-157 Pd particle location and size, 39 155-158... 

As regards the interactions between the encaged metal ions and the host cage, the Sc2 C84 system presents a very different picture to the Ce2 C72 case. In Sc2 C84 the fine structure and branching ratio in the Sc-L2 3 x-ray absorption spectra require a strong hybridisation between the 3d levels of the Sc ions and carbon-derived MO s of the C84 molecules. 

Rare earth endohedral metallofullerenes are an interesting class of full-erenes because electron transfer from the encaged metal atom to the carbon cage has been known to occur and this dramatically alters electronic and magnetic properties of the fullerenes. 

Absorption spectra of endohedral mefallofullerenes in fhe ulfraviolet-visible-near IR (UV-Vis-NIR) region are unique as compared with those of empty fullerenes. Normally, the absorption spectra of mefallofullerenes have long fails fo fhe red down fo 1,500 nm or more. The absorption spectra of fhe major isomers of mono-mefallofullerenes M Cs2 (M = Y, La, Ce, Pr, Nd, Gd, Tb, Dy, Ho, Er, Lu) are similar to each other and well represented by a sharp peak around 1,000 nm and a broad peak around 1,400 nm. These absorption peaks may be related to the intrafullerene electron transfers from the encaged metal atom to the carbon cage. 

As clarified later, such a classification is reasonable and useful because electron transfers exist from the encaged metallic species to the fullerene cages so that the structures and properties depend strongly on the encapsulated atom(s). Particularly, cluster metallofullerenes show different properties from those containing only metals (mono-metallofullerenes and di-metallofullerenes), which, in return, strongly affects the synthesis and extraction processes, structures, chemical reactivities, and their applications. Consequently, we must, to a certain degree, address cluster metallofullerenes separately in the following text. 

Both the structures of fullerene cages and the positions and motions of the encaged metallic species are characteristic of EMFs. 

The most unique feature of EMFs, as distinguished from nonmetallic endofullerenes (such as N C6o) and empty fullerenes, is the strong interaction between the encaged metallic species and the fullerene cage, as represented by the electron transfer from the inner metallic species to the outer fullerene cage intramolecular charge transfer. Consequently, the EMF molecules are a type of superatom, or a type of salt, but remain undissociated in any solvent (see Figure 7.6). 

For mono-metallofullerenes with rare earth metals, three electrons are donated from the encaged metal to the fullerene cage, such as Y and most lanthanides (La, Ce, Pr, Nd, Gd, Tb, Dy, Ho, Er, andLu) [2, 108, 109]. However, Sc, Sm [110], Eu [111], Tm [112], and Yb [113] are found to donate only two electrons and prefer to take the valence of +2. Consequently, the EMEs containing rare earth metals are classifiable into two categories according to the electronic state of the encaged metal(s) trivalent EMFs and divalent EMFs. Eor example, La Cs2 should be expressed as La + Cg2. whereas Yb Cs2 has the formula of Yb + CgJ. These two types of EME differ markedly from each other in their structures and properties, (i) When three electrons are transferred to the carbon cage, the resultant -type EMFs... 

It is significant that the electronic state of Sc is variable, and sometimes controversial. Both experimental and theoretical results recommend the divalent state for Sc in mono-metallofullerenes and di-metallofullerenes [ 119], but it was determined that the three Sc atoms in Sc3N Cso take the - -3 valency. Variable electronic states of Tm in different EMFs were also found. For example, Tm was found to be -1-2 in Tm Cs2, but -1-3 in Tm3N Cso [120]. It is worth noting that a purely ionic picture is not valid to describe the electronic structure of the encaged metals because the charges are not observable quantities [2]. 

Since the first proposal of the endohedral structure of EMFs, there has been a desire to control the positions or motions of the encaged metals. Recent results show that the positions or movements of untouchable metals in EMFs can be controlled certainly by exohedral modifications. 

The chemical reactivity of EMFs strongly depends on the nature of the encaged metallic species because charge transfer takes place between them. A clear example is the different reactivity of Sc3N Cso and La2 C8o toward disilirane (1) [126]. In fact, La2 C8o is reactive both... 

Use of metallofullerenes as radiopharmaceuticals is a promising application of EMFs because the encaged metal(s) is useful as a radiotracing element, whereas the fullerene cage serves as both a protector for the toxic metal ions, and a carrier with functional groups of various types. 

With the confinement of CNTs, direct observations of the dynamic motions of EMFs and the encaged metallic species were also achieved using high-resolution transmission electronic microscopy (HRTEM). Figure 7.18 shows that the motions of the fullerene cage and the encaged ErsN cluster are clearly resolved [185],... 

The formation mechanism of EMFs remains unclear. Informative results concerning the interactions between the encaged metals and the fullerene cage in isolated EMFs have been obtained, but many mysteries remain in the early stage of the arc-discharge process. Disclosure of these mysteries is helpful to increase the production yield and broaden the applications of EMFs. 

Lu, X., Nikawa, H., Nakahodo, T. et al. (2008) Chemical understanding of a non-IPR metallofullerene stabilization of encaged metals on fused-pentagon bonds in La2 Cv2. Journal of the American Chemical Society, 130, 9129-9136. 

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 ... 

Volatile transition metal complexes have been widely used for the production of zeolite-encaged metal particles or organometallic compounds (222,223). The resulting catalysts are active for a wide range of reactions (224) and in some cases are superior to other preparations. For those metals... 

These zeolite-encaged metal complexes are of importance as catalysts, too. From this point of application they possess two particular features each catalytic centre are separated, and the stability of the complex is enhanced (since the zeolite- cage protects the molecule from decomposition). Due to these features the zeolite encaged metal complexes resemble in a certain extent to enzymes, as well, where the catalytic centre might be a transition metal ion, and the stability and steric constraints are provided by the protein. In both systems the complexes of multivalent transition metal ions can catalyze the process of oxygen transfer for mild oxidations. 

Saito, Y., Synthesis and characterization of carbon nanocapsules encaging metal and carbide crystallites, in Fullerenes Recent Advances in the Chemistry and Physics of Fullerenes and Related Materials, Kadish, K. and Ruoff, R. Eds., The Electrochemical lYoceedings Series, Pennington, NJ, 1994, pp. 1419-1447, (PV 94-24). 

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. 

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. 

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