Open-cage Fullerene Research Articles

(Invited) Interactions of Open Cage Fullerenes with Metal Complexes

Open-cage fullerenes have complex structures with an the outer surface that generally contains Lewis basic sites produced by the cage opening reactions as well as an inner void that could accommodate a metal ion. This talk will discuss the reactions of open-cage fullerenes with metal complexes and highlight instances where unanticipated chemical reactions accompany the addition of metal complexes to open-cage fullerenes. Figure 1

Selective preparation of 18-membered open-cage fullerene with one imino and five carbonyl groups on the rim of the orifice

Selective preparation of 18-membered open-cage fullerene with one imino and five carbonyl groups on the rim of the orifice

Open-cage fullerenes as ligands for metals.

The remarkable structures of open-cage fullerenes with functionalization on the outer surface and an accessible inner void make them interesting ligands for reactions with metal complexes. The behaviors of open-cage fullerenes in reactions with various metal complexes are examined and compared to the corresponding reactions with intact fullerenes. The structural results from X-ray diffraction are emphasized. Open-cage fullerenes frequently undergo unanticipated structural changes such as carbon-carbon bond cleavage upon reactions with metal complexes. Much more remains to be learned about the possibility of inserting metal ions larger than Li+ into the interior void of these open-cage fullerenes and about the effects of redox reactions on metal complexes of open-cage fullerenes.

Synthesis of η2-platinum complexes on the rim of open-cage fullerenes

Three new open-cage fullerene derivatives with a 19-membered orifice have been prepared, two of which readily formed Pt complexes when treated with Pt(PPh3)4. Several related open-cage fullerene derivatives were also found to form very stable platinum complexes, which could be purified by chromatography on silica gel under ambient conditions. Single crystal X-ray diffraction data revealed that the Pt metal formed η2-type coordination pattern with a fullerene double bond on the rim of the orifice. Regio-selectivity of the platinum coordination is determined by the electron density of double bonds on the cage. The most electron deficient double bond shows the highest reactivity.

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.

Alkali metals doping into open-cage fullerenes: Novel alkalides (M+@C50N5H5)Mʹ− with large nonlinear optical responses

Based on open-cage fullerenes C50N5H5, a novel type of compounds (M+@C50N5H5)Mʹ− (M = Li, Na; Mʹ = Li, Na and K) was designed and studied. Firstly, their alkalides’ properties are confirmed by their FMO and NPA charges analysis. In particular, the proposed alkalides exhibited considerable first hyperpolarizability (β0) up to 1.91 × 105 a.u., which indicated that they could be regarded as high-performance novel NLO molecules. These alkalides will pave the way for further exploration of electronic-stable, high-performance NLO materials.

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.

Probing the Structure-Property Relationships of Na+···Cl-@C50N5H5 under the External Electric Field.

Endohedral open-cage fullerenes are one of the attractive types of fullerenes due to their interesting electronic properties which make it potentially promising for applications in molecular electronics. In this paper, a novel structure of Na+···Cl-@C50N5H5 with Na+ inside the cavity and Cl- at the opening of the open-cage fullerene is designed to explore the chemical bonding and interaction between the open-cage fullerene system C50N5H5 and NaCl salt. Further, the directional migration of Na+ was achieved by applying an external electric field (EEF) along the X-axis. Notably, when the EEF ranges from -85 to -86 × 10-4 au, Na+ sharply approaches Cl- to regain its ionic bonding character. In addition, the Wiberg bond index, electron localization function topological analysis, and interaction energy (Eint) of the structure under the effect of the EEF also experienced a series of interesting changes. We hope that this work will provide a new strategy for the design of innovative materials for molecular electronics.

Fabrication of three-dimensional nanocomposite with open cage fullerene polymer and MoS2 for enhanced electrochemical hydrogen evolution activity

Development of advanced materials for electrocatalytic water splitting to generate hydrogen is the key in utilization of renewable energy. In the present work, we constructed three-dimensional (3D) nanoparticles (P-PC60@MoS2) based on an open-cage fullerene polymer (P-PC60), on the surface of which two-dimensional (2D) molybdenum disulfide (MoS2) are grafted vertically. The rational design endows the nanocomposite materials with combined advantages of open-cage fullerenes and MoS2. Especially, compared with traditional fullerenes, polymeric fullerene derivatives are expected to have remarkable electronic, optical, or catalytic properties due to the formation of an interconnected continuous network. The results indicate that the few-layer MoS2 slices arranged vertically on the surface of the fullerene, showing an expanded layer spacing of 6.4 Å, which is beneficial for maximally exposing active edge sites. The hybrid P-PC60@MoS2 with proper synthetic engineering possesses superior catalytic kinetics with surpassing overpotential of 59 mV, Tafel slope of 37 mV dec−1, which are much lower than those of pure MoS2 and P-PC60. The small values indicate fast electrochemical hydrogen evolution reaction (HER) kinetics.

Preparation of π-extended fullerene derivatives through addition of phenylenediamine to open-cage fullerene derivatives

Open-cage fullerenes with a quinoxaline moiety on the rim of the orifice showed evident π-system extension effect on the NMR and UV-Vis spectra.

An H2 O2 Molecule Stabilized inside Open-Cage C60 Derivatives by a Hydroxy Stopper.

An H2 O2 molecule was isolated inside hydroxylated open-cage fullerene derivatives by mixing an H2 O2 solution with a precursor molecule followed by reduction of one of carbonyl groups on its orifice. Depending on the reduction site, two structural isomers for H2 O2 @open-fullerenes were obtained. A high encapsulation ratio of 81 % was attained at low temperature. The structures of the peroxosolvate complexes thus obtained were studied by 1 H NMR spectroscopy, X-ray analysis, and DFT calculations, showing strong hydrogen bonding between the encapsulated H2 O2 and the hydroxy group located at the center of the orifice. This OH group was found to act as a kinetic stopper, and the formation of the hydrogen bonding caused thermodynamic stabilization of the H2 O2 molecule, both of which prevent its escape from the cage. One of the peroxosolvates was isolated by HPLC, affording H2 O2 @open-fullerene with 100 % encapsulation ratio, likely due to the intramolecular hydrogen-bonding interaction.

Open-cage Fullerenes as Ligands

Open-cage Fullerenes as Ligands

(Invited) Structural Studies of Open-Cage Fullerenes

Open-cage fullerenes, fullerenes with orifices created by breaking some of the C-C bonds in a fullerene such as C60, are molecules that allow chemical access to both the external and the internal surfaces of the carbon cage. We have found such molecules to be challenging to crystallize and thereby fully structurally characterize through single crystal X-ray diffraction studies. We have also found that the reactivity of these open cage fullerenes can differ significantly from that of the parent fullerene (see A. Aghabali, S. Jun, M. M. Olmstead, A. L. Balch, Silver(I)-Mediated Modification, Dimerization, and Polymerization of an Open-Cage Fullerene. J. Am. Chem. Soc. 2016, 138, 16459-16465.) This talk will report recent studies of the crystallographically determined structures and reactivities of several open-cage fullerenes with particular attention given to the accessibility of the cage opening to chemical reactions.

The Synthesis of Conical Carbon.

Conical carbon, specifically multi-walled carbon nanocones (CNCs) and single-walled carboncones, is a new class of sp2 -hybridized carbon allotrope, in addition to fullerene, carbon nanotubes (CNTs), and graphene. Characterized by a conical and delocalized aromatic configuration, the conical carbon structure is considered the intermediate structure between planar graphene and open-cage fullerene. CNCs can be stiffer than CNTs and exhibit intriguing physical and chemical properties owing to their unique hollow conical structure, which make these materials promising for application as field emission sources and scanning probes. The research on conical carbon structures is in its nascent stage, mainly because of the limitations in the synthesis and purification of conical carbons. This review summarizes the significant progress in the synthesis of CNCs and carboncones. Particularly, the synthetic methods, which can be divided into traditional physical-chemical synthesis methods for multi-walled CNCs and emerging bottom-up organic synthesis methods for single-walled carboncones, are comprehensively discussed. In addition, the advantages and disadvantages of the various synthetic methods as well as the possible formation and growth mechanisms of CNCs and carboncones are discussed. Finally, some outlooks on the potential solutions to the synthesis of single-walled carboncones with uniform apex angles are presented.

Introduction of a (Ph3P)2Pt group into the rim of an open-cage fullerene by breaking a carbon-carbon bond.

Treatment of an open-cage fullerene, designated as MMK-9, with (Ph3P)4Pt in toluene solution at room temperature allows a (PPh3)2Pt unit to be incorporated into the rim of the cage so that it becomes an integral part of the carbon cage skeleton. The structure of the adduct has been determined by single crystal X-ray diffraction and reveals that the platinum atom has planar PtC2P2 coordination, rather than the usual η2-bonding to an intact C-C double bond of the fullerene.

Dynamics and magnetic properties of NO molecules encapsulated in open-cage fullerene derivatives evidenced by low temperature heat capacity

Low-temperature heat capacity analyses for an NO-encapsulated fullerene derivative revealed (i) low-energy motion and (ii) strong magnetic anisotropy of the NO molecule due to its orbital angular momentum. The low-energy motion was attributed to reorientational motions of the NO molecules, in which only a small number (n ∼ 0.04) of NO molecules were found to participate. The NO molecules were confirmed to be paramagnetic even at 1 K. Ab-initio calculation indicated that the magnetic properties of the NO unit strongly depended on its surroundings, allowing the conformation of the fullerene cage to be estimated.

Synthesis of Open-Cage Fullerenes with a Long Tail

To explore potential applications for open-cage fullerenes, we employed 4-((6-bromohexyl)oxy)aniline to react with an open-cage fullerene precursor which has an 11-membered orifice and prepared open-cage fullerenes with an 18-membered orifice. The bromo atom at the end of the hexyl chain in these open-cage compounds could be easily replaced by alkoxyl groups to further extend the linear chain. The results also show that the presence of the alkyl chain slightly changes the reactivity of the orifice-expansion reaction.

(Invited) Synthesis and Properties of Endohedral Open-Cage Fullerenes

Open-cage fullerene derivatives are fascinating molecules because a small molecule can be introduced inside the fullerene cage through the opening. When the NMR active molecules are introduced inside the open-cage fullerene derivatives, the resulting endohedral species are used for the study with the encapsulated molecule as a magnetic probe. First, the regioselectivity in the Diels.Alder reaction of an unsymmetrical open-cage C60 derivative with anthracene was studied. By using an encapsulated H2 molecule as a magnetic probe, the product population was successfully evaluated in detail, indicating the formation of approximately ten compounds as major components. The NICS calculations showed a close resemblance to the observed 1H NMR spectrum, which allowed for a facile characterization of the products. Theoretical studies on the formation of all 29 possible anthracene adducts were also performed. The results indicated that the regioselectivity is strongly governed by steric factors, values of the frontier orbital coefficients, and thermodynamic stabilities. Single-crystal X-ray analysis of the dominant compound revealed the supramolecular architecture between the anthracene moiety and the Π-sphere of a neighboring molecule.[1] Second, The H2O/CH2=CH2-type hydrogen-bonding model was experimentally constructed using a water complex of an open-cage C60 derivative, in which an olefinic double bond and a single molecule of H2O are geometrically confined. To investigate OH/Π-type H-bonding, we performed 1H NMR spectroscopic studies that demonstrated the monotonic downfield shift of the proton signal corresponding to H2O with remarkable rotational perturbation by lowering the temperature. From the temperature dependence of the angular momentum correlation time, the interaction energy was quantitatively estimated to be ca. 0.3 kcal/mol. The computational studies were thoroughly conducted to clarify its inherent nature. As a consequence, the orientation of H2O was found to play a prominent role in varying the bonding strength as well as contribution from the electrostatic attraction and orbital-orbital interaction significantly driven by the favorable orbital overlap.[2][1] Probing the Regioselectivity by Encapsulated H2: Diels-Alder Reaction of a Cage-Opened C60 Derivative with Anthracene, Hashikawa, Y.; Murata, Y. Chem. Eur. J. 2019, 25, 2482-2485.[2] H2O/Olefinic-Π Interaction inside a Carbon Nanocage, Hashikawa, Y.; Murata, Y. J. Am. Chem. Soc. 2019, 141, 12928-12938.

(Invited) Preparation of Open-Cage Fullerene Derivatives By Rhodium(I)-Catalyzed [2+2+2] Cycloaddition of Diynes and C60: Synthesis, Computational Studies and Application in Perovskite Solar Cells

Since the discovery of C60 in 1985 [1], fullerenes have attracted the attention of chemists due to their unique structure, properties and reactivity, along with their potential applications in a variety of fields ranging from materials science to biomedicine [2–3]. The preparation of cyclohexadiene-fused C60 derivatives can be achieved by [2+2+2] cycloaddition reactions of C60 with alkynes [4–6]. This particular class of fullerene derivatives is interesting from the synthetic point of view because they can be converted into the corresponding open-cage bis(fulleroids) when subjected to photochemical conditions [7]. Based on our previous theoretical DFT study that showed the viability of a rhodium-catalyzed [2+2+2] cycloaddition reaction of two alkynes and C60 [8], here we report the first example of [2+2+2] cycloaddition involving C60 and diynes under catalytic conditions promoted by a rhodium(I) catalyst, which leads to the formation of a series of bis(fulleroids) [9]. The process takes place in a single step and no photoexcitation is required. The process mechanism has been studied by means of DFT calculations, proving that the cyclohexadiene-fused intermediate resulting from a [2+2+2] cycloaddition evolves into the final product through a Rh-catalyzed di-π-methane rearrangement. Finally, the electrochemical behavior of the synthesized products has also been studied, as well as their performance as electron transporting materials (ETMs) in perovskite solar cells (PSCs) [10].