Endohedral chemistry

One of the most interesting new aspects of synthetic chemistry is endohedral chemistry, that is the chemistry inside of cages [85], Consequently, it is necessary to be able to build intramolecular cavities with variable diameters. 

For hollow objects like the fullerenes, a general distinction has to be made between outside and inside reactivity. Modifications to the outside are termed exohedral functionalization, and those to the inside are endohedral. Both variants are observed for the fullerenes. Classical fullerene chemistry deals with exohedral functionalization by one or more groups attached to the carbon atoms. Endohedral chemistry, on the other hand, studies compounds consisting of atoms or small molecules included in the cavity within the fullerene cage. The exohedral processes may further be divided into covalent and noncovalent interactions with the reaction partner. 

J. Cioslowski, /. Am. Chem. Soc., 113, 4139 (1991). Endohedral Chemistry Electronic Structures of Molecules Trapped Inside the Cjo Cage. 

Walter, M. and Hakkinen, H. (2006) A hollow tetrahedral cage of hexadecagold dianion provides a robust backbone for a tuneable sub-nanometer oxidation and reduction agent via endohedral doping. Physical Chemistry Chemical Physics, 8, 5407-5411. 

Endohedral fullerenes, 12 230-231 chemistry of, 12 253 Endonucleases, artificial, 17 636 Endoperoxides, 18 442—443 Endopolygalacturonase, 11 598 Endo-receptors, convergent, 16 774 Endosseous dental implants, 8 344 Endothall (Aquathol Granular, Aquathol K), 13 315... 

Nagase S, Kobayashi K, Akasaka T, Wakahara T (2000) Endohedral metallofullerenes theory, electrochemistry, and chemical reactions. In Kadish KM, Ruoff RS (eds.) Fullerenes chemistry, physics, and technology. Wiley, New York, pp. 395 136. 

A. L. Balch, FuUerenes as encapsulating hosts preparation, detection, and structures of endohedral fullerenes, in Encyclopedia of Supramolecular Chemistry (eds J. L. Atwood and J. W. Steed), Marcel Dekker, New York, 2004, p. 579. 

It is therefore the right time to give a first comprehensive overview of fullerene chemistry, which is the aim of this book. This summary addresses chemists, material scientists and a broad readership in industry and the scientific community. The number of publications in this field meanwhile gains such dimensions that for nonspecialists it is very difficult to obtain a facile access to the topics of interest. In this book, which contains the complete important literature, the reader will find all aspects of fullerene chemistry as well as the properties of fullerene derivatives. After a short description of the discovery of the fullerenes all methods of the production and isolation of the parent fullerenes and endohedrals are discussed in detail (Chapter 1). In this first chapter the mechanism of the fullerene formation, the physical properties, for example the molecular structure, the thermodynamic, electronic and spectroscopic properties as well as solubilities are also summarized. This knowledge is necessary to understand the chemical behavior of the fullerenes. 

The discovery of fullerenes in 1985 led to the era of nanomaterials.1 The three-dimensional geometry of these molecules as well as their unique properties distinguishes them from conventional molecules encountered in organic chemistry. Due to recent discoveries in this field, the horizons of this area have broadened to encompass various new molecules such as endohedral fullerenes, nanotubes, carbon nanohorns, and carbon nano-onions. This chapter discusses the electrochemical behavior of some of these carbon nanoparticles with special emphasis on endohedral fullerenes. Since a large number of fullerene derivatives have been prepared and their various electrochemical studies in different solvents and electrolytes have been reported, the electrochemistry of these derivatives is beyond the scope of this text.2 3 Among the other carbon nanoparticles, the electrochemistry of derivatives of carbon nanotubes has been reported. These studies have been highlighted in the final part of the chapter. 

After completing a Ph.D. degree and postdoctoral research in analytical chemistry, Saul Goldman joined the University of Guelph in 1972. As a reformed experimentalist, he developed a research program based on statistical mechanics,277 with special interests in supercritical fluid extraction, spectra of endohedral fullerenes,278 the transport of ions through biological channels, and the molecular basis for the properties of liquids and solutions.279... 

The incorporation of guest molecules can be achieved during their growth or is executed at defect sites and holes via wet chemistry, by surface diffusion and gas-phase transport. Encapsulated fullerenes tend to form chains that are coupled by van der Waals forces. Upon annealing, the encapsulated fullerenes coalesce in the interior of the SWCNTs, resulting in pill-shaped, concentric, endohedral capsules a few nanometers in length [265], The progress of such reactions inside the tubes could be monitored in real time by use of HR-TEM [266],... 

Fullerenes can encapsulate various atoms within the cages, and these compounds have been referred to as endohedral fullerenes. For example, the symbolic representations La C6o and La2 Cso indicate that the fullerene cage encapsulates one and two lanthanum atom(s), respectively. The IUPAC description refers to these fullerenes species as incar-fullerenes, and the formulas are written as t LaCeo and tl Cso, (i is derived from incarcerane). Some metal endohedral fullerenes are listed in Table 14.2.1. The endohedral fullerenes are expected to have interesting and potentially very useful bulk properties as well as a fascinating chemistry. Some non-metallic elements, such as N, P, and noble gases, can be incarcerated into fullerenes to form N 0,0, P C6o, N C o, Sc3N C80, Ar Oo, etc. 

New carbon compounds, namely, endohedral metallofullerenes (EMF.v) are promising building blocks to be used in the design of nanosized materials of a new generation. They exhibit unique electrical, magnetic and chemical properties. Now fullerene chemistry is most developed due to that fullerenes are accessible to a wide range of researchers. However, chemical properties of EMFs are very weakly studied because of problems of the synthesis and isolation of pure EMF.v in preparative quantities. 

Intercalation of fullerenes into nanotubes ("peapods") was discovered [93], and even endohedral fullerenes (e.g., Gd C82 inside SWCNT) [94,95]. A very rich chemistry was soon explored to bond C(,o covalently to disparate organic ligands (an early smorgasbord is shown in Fig. 12.13) [96]. 

As described in the preceding section, physiological and medical applications of the endohedral metallofullerenes will become extremely important in relation to tracer chemistry in biological systems, and await further studies. In any case, the endohedral metallofullerenes will continue to tantalize physicists, chemists and materials scientists for years to come. 

Although macroscopic quantities of the fullerenes have only been available since late 1990, huge advances have been made in their chemistry. Fullerene chemistry may be constructively divided into three broad areas materials containing discrete fullerene molecnles, chemical fimctionahzationofthe fullerene cage, and endohedral fullerenes. 

Figure 7.5 Separation of La-EMFs from the TCB extract by chemical reduction mass spectra of (a) TCB extract, (b) the fraction not reduced, and (c) the reduced fraction [74], (Reprinted with permission from T. Tsuchiya et aL, Reduction of endohedral metallofullerenes a convenient method for isolation, Chemistry of Materials, 16, 4343 346, 2004. 2004 American Chemical Society.)...

Huang, H.J. and Yang, S.H. (2000) Toward efficient synthesis of endohedral metallofullerenes by arc discharge of carbon rods containing encapsulated rare earth carbides and ultrasonic Soxhlet extraction. Chemistry of... 

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