Open-Cage Fullerene as a Macrocyclic Ligand for Na, Pt, and Rh Metal Complexes.

An open-cage [60]fullerene derivative was prepared through Malaprade oxidation of a vicinal triol moiety as the key step. Above the 17-membered orifice, there is one carboxyl group. Three ketone carbonyl groups and one lactone carbonyl group are located on the rim of the orifice. The carboxylic and carbonyl oxygen atoms around the orifice act as strong polydentate ligands for a sodium ion. These oxygen atoms also react with [Rh(CO)2Cl]2 to form various isomeric rhodium complexes with comparable stability. The fullerene C═C bond on the rim of the orifice forms a stable platinum complex when treated with Pt(PPh3)4. Single crystal X-ray diffraction data reveal that one of the carboxylic oxygen atoms above the orifice forms a H-bond with the water molecule trapped in the cage.

Open-Cage Fullerene as a Selective Molecular Trap for LiF/[BeF].

The insertion of ionic compounds into open-cage fullerenes is a challenging task due to the electropositive nature of the cavity. The present work reports the preparation of an open-cage C60 derivative with a hydroxy group pointing towards the centre of the cavity, which can coordinate to a metal cation, thus acting as a bait/hook to trap the metal cation such as the lithium cation in neutral LiF and the beryllium cation in the cationic [BeF]+ species. Other metal salts could not be inserted under similar conditions. The structure of MF in the cage was unambiguously determined by single-crystal X-ray diffraction. Owing to its tendency to undergo polycoordination, Li+ monomer salts have not been isolated before, despite extensive research on Li bonds. The present results provide a unique example of a Li bond.

Open-Cage Fullerene as Molecular Container for F- , Cl- , Br- and I.

A 19-membered open-cage fullerene derivative was prepared from C60 in 7 steps and 5.5 % yield through the peroxide-mediate pathway. There are four carbonyl groups, an ether oxygen and a quinoxaline moiety on the rim of the orifice. A chloride anion could be inserted into its cavity by heating with hydrochloric acid at 60 °C for 4 h. Encapsulation of fluoride, bromide and iodide anions was also achieved at slightly more forcing conditions, 90 °C for 14 h. Single crystal X-ray structures of the sodium salt of the chloride and the bromide encapsulated derivatives were obtained, which showed the halide anion in the center of the cavity and two sodium cations connecting two cages through coordination to the oxygen atoms on the rim of the orifices. The halide encapsulation ratio is quantitative in the isolated products.

Molecular Containers Derived from [60]Fullerene through Peroxide Chemistry.

Molecular containers can keep guest molecules in a confined space that is completely separated from the solution. They have wide potential applications, including selective trapping of reactive intermediates, catalysis within the cavity, and molecular delivery. Numerous molecular containers have been prepared through covalent bonds, metal-ligand interactions and H-bonding or hydrophobic interactions. Fullerenes are all-carbon molecules with a spherical structure. Partial opening of the cage structure results in open-cage fullerenes, which can serve as molecular containers for various small molecules and atoms. Compared with classical molecular containers, open-cage fullerenes exhibit some unusual phenomena because of the unique structure of the fullerene cage. The synthesis of an open-cage fullerene with a large enough orifice as a molecular container requires consecutive cleavage of multiple fullerene skeleton bonds within a local area on the cage surface. In spite of the difficulty, remarkable progress has been achieved. Several reactions have been reported to cleave fullerene C-C bonds selectively to form open-cage fullerenes, some of which have been successfully used as molecular containers for molecules such as H2O. The size and shape of the orifice play a key role in the encapsulation of the guest molecule. To date the focus in this area has been the preparation of open-cage fullerenes and encapsulation of small molecules. Little information has been reported about the functional properties of these host-guest systems. Potential applications of these systems need to be explored. This Account mainly presents our results on the encapsulation of small molecules in open-cage fullerenes prepared in my group. The preparation of our open-cage fullerenes is based on fullerene-mixed peroxides, which are briefly mentioned herein. The encapsulation and release of a single molecule of water is discussed in detail. Quantitative water encapsulation was achieved by heating the open-cage fullerene in a homogeneous CDCl3/H2O/EtOH mixture at 80 °C for 18 h. The kinetics of the water release process was studied by blackbody IR radiation-induced dissociation (BIRD) and theoretical calculations. The trapped water could also be released by H-bonding with HF. To control the encapsulation and release processes, we prepared open-cage fullerenes with a switchable stopper on the rim of the orifice. Besides H2O, encapsulations of H2, HF, CO, O2, and H2O2 were also achieved by using different open-cage fullerenes. The encapsulation of CO is quite unusual in that the trapped CO is derived from a fullerene skeleton carbon that was pushed into the cavity by oxidation under ambient conditions at room temperature. The trapped O2/H2O2 could be released slowly under mild conditions, and these systems are now being studied as a new type of oxygen-releasing materials for biomedical research. The present results demonstrate that open-cage fullerenes are suitable molecular containers for small molecules. Our future work will focus on optimizing the conditions for the preparation of open-cage fullerenes and applications of open-cage fullerenes in areas such as oxygen delivery for photodynamic therapy.

Concise Synthesis of Open-Cage Fullerenes for Oxygen Delivery.

Open-cage fullerenes with a 19-membered orifice were prepared in three steps from C60 . The key step for cage-opening is aniline mediated ring expansion of a fullerene-mixed peroxide with a ketolactone moiety on the orifice. Release of ring strain on the spherical fullerene cage served as the main driving force for the efficient cage-opening sequence. Encapsulation of oxygen could be achieved at room temperature under moderate pressure (50 atm) and the encapsulated oxygen could be released slowly under ambient conditions. The activation energy of the oxygen-releasing process is 18.8 kcal mol-1 and the half-life at 37 °C was 73 min, which makes this open-cage fullerene derivative a potential oxygen-delivery material.

Synthesis of Metal Complexes with an Open-Cage Fullerene as the Ligand.

An open-cage C60 derivative 9 with anilino, hemiketal, and lactone moieties on the edge of the opening was prepared through a fullerene-mixed peroxide procedure. Key steps include decarbonylation, intramolecular Friedel-Crafts reaction and oxetane ring opening rearrangement. The open-cage compound 9 readily reacts with various metal salts. A nickel complex was obtained by treating 9 with NiCl2 at r.t. followed by purification on a silica gel column. Single-crystal X-ray diffraction analysis showed that the nickel ion is coordinated to two ligands with an octahedral geometry.

Controlled Synthesis of Nitrogen-Doped Graphene on Ruthenium from Azafullerene.

The controlled synthesis of high-quality nitrogen (N) doped single layer graphene on the Ru(0001) surface has been achieved using the N-containing sole precursor azafullerence (C59NH). The synthesis process and doping properties have been investigated on the atomic scale by combining scanning tunneling microscopy and X-ray photoelectron spectroscopy measurements. We find for the first time that the concentration of N-related defects on the N-doped graphene/Ru(0001) surface is tunable by adjusting the dosage of sole precursor and the number of growth cycles. Two primary types of N-related defects have been observed. The predominant bonding configuration of N atoms in the obtained graphene layer is pyridinic N. Our findings indicate that the synthesis from heteroatom-containing sole precursors is a very promising approach for the preparation of doped graphene materials with controlled doping properties.

Oxygen-Delivery Materials: Synthesis of an Open-Cage Fullerene Derivative Suitable for Encapsulation of H2 O2 and O2.

An open-cage [60]fullerene was prepared through a multiple-step sequence based on peroxide-mediated cage-opening reactions. Key steps include repeated C60 -sensitized singlet-oxygen oxidation of electron-rich amino enol double bonds to form two lactone and two lactam moieties on the rim of the orifice. Single-crystal X-ray analysis shows that the 22-membered orifice has an ellipsoid shape with the major axis at 6.7 Šand the minor axis at 3.5 Å. Encapsulation of H2 O2 was observed under atmospheric pressure at room temperature. Oxygen is also effectively trapped during the process.

[60]Fullerene-Based Macrocycle Ligands.

Macrocycle ligands have three or more donor sites. Selective replacement of skeleton carbon atoms by heteroatoms and vacancies in C60 could lead to various macrocycle ligands with a cage-shaped backbone. Theoretical calculations indicate that such C60 -based macrocycle ligands are as stable as C60 thermodynamically according to their similar HOMO-LUMO gaps. The synthesis of these ligands is a challenging task. Nevertheless important progresses have been reported. This concept article focuses on the structures of possible C60 -based macrocycle ligands and related synthetic results.

Synthesis of C70 -Based Fluorophores through Sequential Functionalization to Form Isomerically Pure Multiadducts.

Selective addition to the C70 cage divides its π-conjugated system into various smaller π-conjugated systems with enhanced fluorescent properties. Key reactions include chlorination, methoxylation, ozonation, and Bingel or Bingel-Hirsch reactions. The maximum emission wavelength of the C70 multiadducts ranges from 450 to 655 nm. Among the C70 multiadducts, C70 (OMe)8 (C(COOEt)2 )3 showed the highest quantum yield (ΦF =0.18) and the largest Δ[λmax (emission)- λmax (absorption)] (402 nm), with maximum emission at 655 nm.

Selective Multiamination of C70 Leading to Curved π†Systems with 60, 58, 56, and 50 π†Electrons.

Secondary aliphatic amines add to a pole pentagon of [70]fullerene in the presence of N-fluorobenzenesulfonimide to form cyclopentadienyl-type adducts, C70(NSO2 Ph)(NR(1)R(2))4 (1), which can be converted into analogous C70 derivatives such as C70 (NHSO2 Ph)(NHTol)5 (2). Further addition reactions of either 1 or 2 take place selectively at the opposite pole pentagon of the C70 cage, thus forming curved π systems with a reduced number of π electrons, and the products include a dodecakis-adduct with a Vögtle belt motif.

Fullerene-Based Macro-Heterocycle Prepared through Selective Incorporation of Three†N and Two O†Atoms into C60.

A 14-membered heterocycle is created on the C60 cage skeleton through a multistep procedure. Key steps involve repeated PCl5 -induced hydroxylamino N-O bond cleavage leading to insertion of nitrogen atoms, and also piperidine-induced peroxo O-O bond cleavage leading to insertion of oxygen atoms. The hetero atoms form one pyrrole, two pyran, and one diazepine rings in conjunction with the C60 skeleton carbon atoms. The fullerene-based macrocycle showed unique reactivities towards fluoride ion and copper salts.

Release of the Water Molecule Encapsulated Inside an Open-Cage Fullerene through Hydrogen Bonding Mediated by Hydrogen Fluoride.

A reversible wetting/dewetting procedure is reported for an open-cage fullerene with an 18-membered orifice. In a homogeneous mixture of H2O/EtOH/CHCl3, water was encapsulated into the cavity of the open-cage compound quantitatively at 80 °C. Addition of aqueous hydrogen fluoride into the water-encapsulated complex removed the encapsulated water completely at room temperature. H-bonding between the trapped water and fluoride is shown to play a key role for the water release process.

Peroxide-mediated selective cleavage of [60]fullerene skeleton bonds: towards the synthesis of open-cage fulleroid C55O5.

Replacement of a pentagon in [60]fullerene with five oxygen atoms yields the open-cage compound C55O5 with five carbonyl groups on the rim of the orifice. Our attempts to synthesize such a target molecule starting from C60 have led us to prepare the fullerene-mixed peroxides such as C60(OO-t-Bu)6 with all the peroxo addends surrounding the same pentagon. Further investigations of the peroxide chemistry have generated various open-cage fullerene derivatives, including the carbon monoxide encapsulated endohedral compound CO@C59O6. This Personal Account mainly discusses peroxide-based processes resulting in selective cleavage of the fullerene skeleton bonds.

Synthesis and chemical reactivity of tetrahydro[60]fullerene epoxides with both amino and aryl addends.

Tetrahydro[60]fullerene epoxides C60(O)Ar(n)(NR2)(4-n), n = 1, 2, have been prepared by treating 1,4-adducts C60(OH)Ph and C60(Tol)2 with cyclic secondary amines. The epoxy moieties in these mixed tetrahydro[60]fullerene epoxides were hydrolyzed into the corresponding diol derivatives, which were further oxidized into diketone open-cage fullerenes with a 10-membered orifice. A few other reactions also showed that the present tetrahydro[60]fullerene epoxides with both amino and aryl addends exhibit improved chemical reactivity over the tetraamino[60]fullerene epoxide without any aryl group.

Selective addition of secondary amines to C60: formation of penta- and hexaamino[60]fullerenes.

Secondary amines are well-known to add to [60]fullerene to form the tetraamino epoxy adduct C60(O)(NR1R2)4 under both photolysis and thermal conditions in the presence of oxygen. We have now found that pentaamino hydroxyl adduct C60(OH)(NR1R2)5 and hexaamino adduct C60(NR1R2)6 can be formed as the major products in the dark in the presence of oxygen. Key steps of the reaction mechanism probably involve repeated oxygen oxidation of the radical ion pair between fullerene and amines.

Near-infrared absorbing compounds based on π-extended tetrathiafulvalene open-cage fullerenes.

Tetrathiafulvalene (TTF) is attached to open-cage fullerenes through a quinoxaline junction. The resulting linear π-conjugation system shows intense absorption in the near-infrared region. A unique o-diaminobenzene-induced furan ring formation process from a conjugated 1,4-dione moiety was observed on the rim of a 18-membered orifice.

Pentafluorophenyl transfer reaction: preparation of pentafluorophenyl [60]fullerene adducts through opening of fullerene epoxide moiety with trispentafluorophenylborane.

Unlike the extensively studied perfluoroalkyl fullerene adducts, perfluorophenyl fullerene adducts are quite difficult to prepare by known methods. Trispentafluorophenylborane was found to react with fullerene epoxide to form the 1,2-perfluorophenylfullerenol. The method can be applied to both the simple epoxide C60(O) and fullerene multiadducts containing an epoxide moiety. Single crystal X-ray structure analysis confirmed the addition of the pentafluorophenyl group.

Synthesis of C60(O)3: an open-cage fullerene with a ketolactone moiety on the orifice.

Four isomers are currently known for the trioxygenated fullerene derivative C(60)(O)(3), three regioisomers with all of the oxygen addends as epoxy groups and the unstable ozonide isomer with a 1,2,3-trioxlane ring. Here we report the synthesis of an open-cage isomer for C(60)(O)(3) with a ketolactone moiety embedded into the fullerene skeleton through a three-step procedure mediated by fullerene peroxide chemistry. Two fullerene skeleton carbon-carbon bonds are cleaved in the process. The open-cage derivative C(60)(O)(3) can be converted back to C(60) through deoxygenation with PPh(3). Single crystal X-ray structure confirmed the open-cage structure.

Selective synthesis of fullerenol derivatives with terminal alkyne and crown ether addends.

A series of isomerically pure alkynyl-substituted fullerenol derivatives such as C(60)(OH)(6)(O(CH(2))(3)CCH)(2) were synthesized through Lewis acid catalyzed epoxy ring opening and/or S(N)1 replacement reactions starting from the fullerene-mixed peroxide C(60)(O)(t-BuOO)(4). Copper-catalyzed azide-alkyne cycloaddition readily converted the terminal alkynyl groups into triazole groups. Intramolecular oxidative alkyne coupling afforded a fullerenyl crown ether derivative.