Structural Aspects of Fullerene ChemistryA Journey through Fullerene Chirality

One-pot enantioselective extraction of chiral fullerene C76 using a cyclic host carrying an asymmetrically distorted, highly pi-basic porphyrin module

Optical resolution of nonsubstituted chiral fullerenes such as C(76) is one of the most challenging subjects in host-guest chemistry. The novel cyclic host 1(2H), which bears a meso-diaryl-beta-octaethylporphyrin (P(2H)) unit on one side and its chiral N-2-acetoxyethyl derivative (P(N-EtOAc)) on the other, traps C(76) enantioselectively in its cavity and furnishes 7% enantiomeric excess in a single extraction. Control experiments using reference hosts indicated the importance of the high pi-basicity and large asymmetric distortion of the chiral N-substituted porphyrin unit (P(N-EtOAc)) in 1(2H) for the enantioselection of C(76).

All-carbon scaffolds by rational design

The search for new molecular and regular polymeric allotropes of carbon has greatly stimulated the preparation and investigation of pi-conjugated acetylenic macrocycles, which often represent substructures of proposed 2D carbon networks. Perethynylated dehydroannulenes, expanded radialenes, and radiaannulenes with large, multi-nanometer-sized all-carbon cores are potent electron acceptors, and their optoelectronic as well as stability and solubility properties are greatly enhanced by peripheral donor substitution. Acetylenic scaffolding into three dimensions has generated an expanded cubane with a C(56) core, the first representative of a new class of "platonic" objects. Exceptional chiroptical properties displayed by enantiomerically pure alleno-acetylenic, shape-persistent macrocycles promise fascinating perspectives for the development of molecular and supramolecular chiroptical materials.

trans-2 addition pattern to power charge transfer in dendronized metalloporphyrin C60 conjugates

Coordinating different transition metals--manganese(III), iron(III), nickel(II), and copper(II)--by a dendronized porphyrin afforded a new family of redox-active metalloporphyrins to which C(60) was attached as a ground-state electron acceptor. Such a strategy introduced an additional center of redoxactivity, that is, a change of the oxidation state of the metal. Cyclic voltammetry and absorption/fluorescence measurements provided support for mutual interactions between the redox-active constituents in the ground state. In particular, slightly anodic shifted reduction potentials/cathodic shifted oxidation potentials and the occurrence of new charge transfer features in the 700-900 nm range prompt to sizable electronic coupling in the range of 300 cm(-1). Photophysical means--steady-state/time-resolved fluorescence and transient absorption measurements--shed light on the excited-state interactions. To this end, we have added pulse radiolytic investigations to characterize the radical cation (i.e., metalloporphyrins) and radical anion (i.e., fullerene) characteristics. Pi-pi stacking of the excited state electron donor and the electron acceptor is key to overcome the intrinsically fast deactivation of the excited states in these metalloporphyrins and to power an exothermic charge transfer. The lifetimes of the rapidly and efficiently generated radical ion pair states, which range from 15 to >3000 ps, revealed several important trends. First, they were found to depend on the solvent polarity. Second, the nature of the transition metal plays a similarly decisive role. It is important that the product of charge recombination, namely tripmultiplet excited states versus ground state, had a great impact. Finally, a correlation between the charge transfer rate (i.e., charge separation and charge recombination) and the free energy change for the underlying reaction reveals a parabolic dependence with parameters of the reorganization energy (0.84 eV) and electronic coupling (70 cm(-1)) closely resembling that seen for the zinc(II) and free base analogues.

Acyl radical reactions in fullerene chemistry: direct acylation of [60]fullerene through an efficient decatungstate-photomediated approach

A versatile and highly efficient photochemical methodology for the direct acylation of C(60) has been developed. This approach utilizes a wide variety of acyl radicals derived from aldehydes through a hydrogen atom abstraction process mediated by tetrabutylammonium decatungstate [(n-Bu(4)N)(4)W(10)O(32)]. The single addition reaction of these acyl radicals to [60]fullerene proceeded selectively to afford a novel class of previously unexplored fullerene-based materials. Product analysis of this reaction showed that decarbonylation and acylation pathways compete when a tertiary or phenylacetyl aldehyde is the starting material. However, a decrease of the reaction temperature was found to be effective in overcoming the decarbonylation encountered in certain acyl radical additions to C(60); the carbonyl radical addition precedes decarbonylation even in the cases where the decarbonylation rate constant exceeds 10(6) s(-1) (i.e., phenylacetaldehyde). The regiochemistry of the t-butyl radical addition was also found to be thermally controlled. The present methodology is directly applicable even in the cases of the cyclopropyl-substituted aldehydes, where rapid rearrangement of the cyclopropyl acyl radical intermediate can potentially occur. A mechanistic approach for this new reactivity of C(60) has been also provided, based mainly on intra- and intermolecular deuterium isotope effect studies.

The Diels-Alder reaction on endohedral Y3N@C78: the importance of the fullerene strain energy

We have studied the Diels-Alder reaction of 1,3-butadiene with all nonequivalent bonds of Y3N@D3h-C78 at the BP86/TZP//BP86/DZP level of theory. The results obtained are compared with those extracted from a previous study on the free and Sc3N-endohedral C78 fullerene (J. Am. Chem. Soc. 2008, 130, 6206-6214). Our study shows that the most stable regioisomer for the Y3N compound is obtained for the reaction over a corannulene-type [5,6] bond (d), which exhibits the longest bond distance (1.47 A) and a large pyramidalization angle. As far as we know, this is the first case of a cycloaddition reaction where the most stable addition is obtained over one of the longest C-C bonds in the cage. In contrast to Sc3N@D3h-C78, where bonds close to the scandium atoms were destabilized, this bond d has one of the yttrium atoms in close contact. This preference for reacting with those bonds situated close to the yttrium atoms is due to two different factors: first, the D3h cage is extremely deformed, especially in the areas situated close to the yttrium atoms (which contain the most reactive bond d), so the attack reduces the strain energy of the cage; second, in the final adduct, the Y3N cluster gets additional space to adopt a more planar configuration. Since it has been shown (J. Phys. Chem. B 2007, 111, 3363-3369) that the D3h isomer is not the most favorable isomer for endohedral Y3N@C78 (at variance with Sc3N@C78), we also studied the more favorable C2 isomer. The latter contains [5,5] bonds, which are shown to be the most reactive bonds for cycloaddition, in contrast to previous theoretical predictions (J. Org. Chem. 2006, 71, 46-54). This preference for [5,5] bonds is observed for the C2 isomers of both endohedral (Sc3N, Y3N) and free C78 fullerene and is dictated by the fullerene strain energy. We therefore expect that the Diels-Alder reaction on other endohedral metallofullerenes that have already been synthesized (e.g., Tm3N@C2-C78, Dy3N@C2-C78) might lead to the same [5,5] adduct.

Functionalization of hexakis methanofullerene malonate crown-ethers: promising octahedral building blocks for molecular networks

Hexaazido and hexaiodo macrocyclic methanofullerenes undergo high-yielding sixfold click reactions or sixfold Heck, Sonogashira and Suzuki cross-coupling reactions, respectively, to yield all organic building blocks for higher molecular architectures.

Architecture of clathrin fullerene cages reflects a geometric constraint--the head-to-tail exclusion rule--and a preference for asymmetry

Fullerene cages have n trivalent vertices, 12 pentagonal faces, and (n-20)/2 hexagonal faces. The smallest cage in which all of the pentagons are surrounded by hexagons and thus isolated from each other has 60 vertices and is shaped like a soccer ball. The protein clathrin self-assembles into fullerene cages of a variety of sizes and shapes, including smaller ones with adjacent pentagons as well as larger ones, but the variety is limited. To explain the range of clathrin architecture and how these fullerene cages self-assemble, we proposed a hypothesis, the "head-to-tail exclusion rule" (the "Rule"). Of the 5769 small clathrin cage isomers with n< or =60 vertices and adjacent pentagons, the Rule permits just 15, three identified in 1976 and 12 others. A "weak version" of the Rule permits another 99. Based on cryo-electron tomography, Cheng et al. reported six raw clathrin fullerene cages. One was among the three identified in 1976. Here, (1) we identify the remaining five. (2) Four are new and are among the 12 others permitted by the Rule. (3) One, also new, is among the 99 weak version cages. (4) Of particular note, none of the remaining 5565 excluded cages has been identified. These findings provide powerful experimental confirmation of the Rule and the principle on which it is based. (5) Surprisingly, the newly identified clathrin cages are among the least symmetric of those permitted. (6) By devising a method for counting assembly paths, (7) we show that asymmetric cages can be assembled by larger numbers of paths, thus providing a kinetic explanation for the prevalence of asymmetric cages. (8) Finally, we show that operation during cage growth of the Rule greatly increases the likelihood of producing a closed fullerene cage, specifically one of those permitted, but efficient assembly still appears to require internal remodeling.