Dynamic covalent chemistry relates to chemical reactions carried out reversibly under conditions of equilibrium control. The reversible nature of the reactions introduces the prospects of "error checking" and "proof-reading" into synthetic processes where dynamic covalent chemistry operates. Since the formation of products occurs under thermodynamic control, product distributions depend only on the relative stabilities of the final products. In kinetically controlled reactions, however, it is the free energy differences between the transition states leading to the products that determines their relative proportions. Supramolecular chemistry has had a huge impact on synthesis at two levels: one is noncovalent synthesis, or strict self-assembly, and the other is supramolecular assistance to molecular synthesis, also referred to as self-assembly followed by covalent modification. Noncovalent synthesis has given us access to finite supermolecules and infinite supramolecular arrays. Supramolecular assistance to covalent synthesis has been exploited in the construction of more-complex systems, such as interlocked molecular compounds (for example, catenanes and rotaxanes) as well as container molecules (molecular capsules). The appealing prospect of also synthesizing these types of compounds with complex molecular architectures using reversible covalent bond forming chemistry has led to the development of dynamic covalent chemistry. Historically, dynamic covalent chemistry has played a central role in the development of conformational analysis by opening up the possibility to be able to equilibrate configurational isomers, sometimes with base (for example, esters) and sometimes with acid (for example, acetals). These stereochemical "balancing acts" revealed another major advantage that dynamic covalent chemistry offers the chemist, which is not so easily accessible in the kinetically controlled regime: the ability to re-adjust the product distribution of a reaction, even once the initial products have been formed, by changing the reaction's environment (for example, concentration, temperature, presence or absence of a template). This highly transparent, yet tremendously subtle, characteristic of dynamic covalent chemistry has led to key discoveries in polymer chemistry. In this review, some recent examples where dynamic covalent chemistry has been demonstrated are shown to emphasise the basic concepts of this area of science.

https://onlinelibrary.wiley.com/doi/pdf/10.1002/1521-3773%2820020503%2941%3A9%3C1460%3A%3AAID-ANIE11111460%3E3.0.CO%3B2-N

Dynamic covalent chemistry.

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Spherical aromaticity of fullerenes.

Structural aspects of fullerene chemistry--a journey through fullerene chirality.

This Account focuses on the most recent and systematic efforts in the area of functionalization chemistry of the single-wall carbon nanotubes (SWNTs) which utilizes direct fluorination for the preparation of "fluoronanonotubes" and their subsequent derivatization. The results obtained prove that the addition of fluorine drastically enhances the reactivity of the nanotube side walls. The use of this strategy as a versatile tool for preparation and manipulation of SWNTs with variable side-wall functionalities has been demonstrated. The functionalized SWNTs have shown an improved solubility in selected solvents and significantly altered electrical, mechanical, and optical properties. An overview of new synthetic methods for preparation and a discussion of characterization data for the functionalized SWNTs are provided.

Fluorination of single-wall carbon nanotubes and subsequent derivatization reactions.

Endohedral metallofullerenes (EMFs), a new class of hybrid molecules formed by encapsulation of metallic species inside fullerene cages, exhibit unique properties that differ distinctly from those of empty fullerenes because of the presence of metals and their hybridization effects via electron transfer. This critical review provides a balanced but not an exhaustive summary regarding almost all aspects of EMFs, including the history, the classification, current progress in the synthesis, extraction, isolation, and characterization of EMFs, as well as their physiochemical properties and applications in fields such as electronics, photovoltaics, biomedicine, and materials science. Emphasis is assigned to experimentally obtained results, especially the X-ray crystallographic characterizations of EMFs and their derivatives, rather than theoretical calculations, although the latter has indeed enhanced our knowledge of metal–cage interactions. Finally, perspectives related to future developments and challenges in the research of EMFs are proposed. (381 references)

Current status and future developments of endohedral metallofullerenes

Buckminsterfullerene C60: a chemical Faraday cage for atomic nitrogen

(atomic nitrogen inside C60) is produced by ion implantation. Two different production methods are employed: Kaufman ion source and glow discharge. After the bombarded material is dissolved in toluene or CS2 and is filtered, several milligrams of C60 containing N@C60 in a concentration of 10-4 to 10-5 are obtained. N@C60 gives a very clear hyperfine-split electron paramagnetic resonance signal. The most prominent features of N@C60 are: (i) Nitrogen in C60 keeps its atomic electronic configuration and occupies the on-center position. (ii) N@C60 is stable at ambient conditions, the thermal instability starts at 260 °C. (iii) The complex survives exohedral addition reactions and is a sensitive detector of cage distortions caused by addends. (iv) C60 and N@C60 exhibit slightly different retention times in column chromatography, thus permitting an enrichment of N@C60 by this method.

Atomic nitrogen in C60:N@C60

Fourier transform EPR study of N@C60 in solution

The synthesis and EPR spectroscopic investigations of a family of six endohedral fullerenes, namely, N@C60 (1), N@C61(COOC2H5)2 (2), N@C66(COOC2H5)12 (3), N@C66(COOC2D5)12 (4), N@C61(COOC2D5)2 (5), and N@C70 (6), containing atomic nitrogen in the 4S3/2 ground state is described. The parent systems N@C60 (1) and N@C70 (6) were synthesized by nitrogen ion implantation. The syntheses of the C2v-symmetric monoadducts 2 and 5 and the Th symmetric hexaadducts 3 and 4 exhibiting well-defined cage distortions were accomplished via cyclopropanation with the corresponding malonates. With respect to these additions the reactivity of 1 is indistinguishable from that of empty C60. The quartet electronic spin of the encapsulated N-atoms is a very sensitive probe for cage modifications. In the monoadducts 2 and 5 a permanent zero field splitting (ZFS) tensor with rigidly aligned axes was revealed reflecting the intrinsic droplet like cage distortion. In contrast, no fine structure due to intrinsic distortions was monito...

Atomic Nitrogen Encapsulated in Fullerenes: Effects of Cage Variations

Well resolved EPR spectra of P@C60 in solution have been recorded, proving that the encased phosphorus atoms are in their quartet spin ground state. The isotropic hyperfine interaction is increased by a factor of 2.5 compared with the values measured for free atoms. An analysis of spin relaxation data reveals that fluctuating zero field splitting (ZFS) interaction induced by collision-induced deformations of the carbon shell constitutes the dominant relaxation mechanism. The variance of the time fluctuating ZFS interaction is about a factor of 10 larger than that observed for N@C60 under identical conditions. Values for the correlation time of the deformation of the fullerene cage range from 5ps to 32 ps in the temperature interval 190-300 K in toluene.

M. Waiblinger

Papers

Buckminsterfullerene C60: a chemical Faraday cage for atomic nitrogen

Atomic nitrogen in C60:N@C60

Fourier transform EPR study of N@C60 in solution

Atomic Nitrogen Encapsulated in Fullerenes: Effects of Cage Variations

Electron paramagnetic resonance study of atomic phosphorus encapsulated in [60]fullerene