Isomeric Sc2O@C78 Related by a Single-Step Stone–Wales

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Isomeric Sc2O@C78 Related by a Single-Step Stone−Wales Transformation: Key Links in an Unprecedented Fullerene Formation Pathway Yajuan Hao,†,⊥ Qiangqiang Tang,‡,⊥ Xiaohong Li,‡ Meirong Zhang,† Yingbo Wan,‡ Lai Feng,*,† Ning Chen,*,‡ Zdeněk Slanina,*,# Ludwik Adamowicz,# and Filip Uhlík§ †

College of Physics, Optoelectronics and Energy & Collaborative Innovation Center of Suzhou Nano Science and Technology, Soochow University, Suzhou 215006, China ‡ College of Chemistry, Chemical Engineering and Materials Science, Soochow University, Suzhou 215163, China # Department of Chemistry and Biochemistry, University of Arizona, Tucson, Arizona 85721-0041, United States § Department of Physical and Macromolecular Chemistry, Faculty of Science, Charles University in Prague, Albertov 6, 128 43 Praha 2, Czech Republic S Supporting Information *

ABSTRACT: It has been proposed that the fullerene formation mechanism involves either a top-down or bottomup pathway. Despite different starting points, both mechanisms approve that particular fullerenes or metallofullerenes are formed through a consecutive stepwise process involving Stone−Wales transformations (SWTs) and C2 losses or additions. However, the formation pathway has seldomly been defined at the atomic level due to the missing-link fullerenes. Herein, we present the isolation and crystallographic characterization of two isomeric clusterfullerenes Sc2O@C2v(3)-C78 and Sc2O@D3h(5)-C78, which are closely related via a single-step Stone−Wales (SW) transformation. More importantly, these novel Sc2O@C78 isomers represent the key links in a well-defined formation pathway for the majority of solvent-extractable clusterfullerenes Sc2O@C2n (n = 38−41), providing molecular structural evidence for the less confirmed fullerene formation mechanism. Furthermore, DFT calculations reveal a SWT with a notably low activation barrier for these Sc2O@C78 isomers, which may rationalize the established fullerene formation pathway. Additional characterizations demonstrate that these Sc2O@C78 isomers feature different energy bandgaps and electrochemical behaviors, indicating the impact of SW defects on the energetic and electrochemical characteristics of metallofullerenes.

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INTRODUCTION

interconvertible via multiple C2 losses or additions as well as consecutive Stone−Wales (SW) or “pyracylene” rearrangements.14,15 However, it is still difficult to capture any of the key links or intermediates that would provide direct evidence of the formation mechanisms mentioed above, probably due to their ultralow stabilities. Endohedral metallofullerenes (EMF) are special members of the fullerene family that encapsulate metal atoms or metallic clusters inside a fullerene cage.16,17 As revealed by a number of experiments, pentalene- or heptagon-containing cages, which are otherwise unstable in the form of empty fullerenes, can be stabilized by an endohedral metal or cluster.18−21 Thus, it is reasonable to propose that these unconventional fullerene cages are capable of undergoing C2 extrusion or addition to form smaller or larger species under appropriate conditions. On the

In the past decade, fullerenes have been extensively studied owing to their potential applications in material and life sciences.1−6 It is noteworthy that most fullerene species have a large number of isomers. For selective synthesis of specific a fullerene with effective yield, a thorough understanding of the fullerene formation mechanism is essential. Nevertheless, though many efforts have been devoted to this topic,7−12 the fullerene formation mechanism is still unclear and under debate. in particular, a “top-down” formation mechanism11 has been suggested according to a transmission electron microscopy observation that graphene sheets can roll and wrap to form fullerenes under electron beam irradiation.13 Meanwhile, a classic “bottom-up” mechanism has been well developed, providing many deep insights into fullerene formation.12 Despite their different starting points, both mechanisms commonly suggest that various fullerene species synthesized under particular conditions shall be highly correlated and © XXXX American Chemical Society

Received: August 6, 2016

A

DOI: 10.1021/acs.inorgchem.6b01894 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry

of a platinum working electrode, a platinum counter-electrode, and a saturated calomel reference electrode (SCE) was used for both cyclic voltammetry (CV) and differential pulse voltammetry (DPV). All potentials were recorded against an SCE reference electrode and corrected against Fc0/+. CV and DPV were measured at scan rates of 100 and 20 mV s−1, respectively. Single-Crystal X-ray Diffraction Analysis. Black cocrystals of Sc2O@C2v(3)-C78·[NiII(OEP)]·0.95C6H6·0.05CHCl3 and Sc2O@ D3h(5)-C78·[NiII(OEP)]·0.20C6H6·0.30CHCl3 were obtained by slowly diffusing the benzene solution of fullerene and the chloroform solution of [NiII(OEP)]. X-ray data were collected at 90 or 123 K using a diffractometer (APEX II; Bruker Analytik GmbH) equipped with a CCD collector. The multiscan method was used for absorption correction. The structure was resolved using direct methods (SIR2004)32 and refined on F2 using full-matrix least-squares using SHELXL201333 within the WinGX package.34 Hydrogen atoms were inserted at calculated positions and constrained with isotropic thermal parameters. Crystal Data for Sc 2 O@C 2v (3)-C 78 ·[Ni II (OEP)]·0.95C 6 H 6 · 0.05CHCl3. C119.75H49.75Cl10.25N4NiOSc2, Mr = 1717.88, 0.15 × 0.12 × 0.10 mm, monoclinic, C 2/m (No.12), a = 25.2588(8), b = 14.9157(5), c = 19.5163(6), α = 90, β = 92.305(1), γ = 90, V = 7346.9 (4) Å3, Z = 4, ρcalcd = 1.553 g cm−3, μ (Mo Kα) = 0.511 mm−1, θ = 2.731−28.682, T = 90 K, R1 = 0.1430, wR2 = 0.2173 for all data; R1 = 0.0905, wR1 = 0.1912 for 6038 reflections (I > 2.0σ(I)) with 1002 parameters. Goodness of fit indicator, 1.085. Maximum residual electron density, 1.127 e Å−3. Table S1 and CCDC 1496949 contain the supplementary crystallographic data for this paper. Crystal Data for Sc 2 O@D 3h (5)-C 78 ·[Ni II (OEP)]·0.20C 6 H 6 · 0.30CHCl3. C115.50H45.50Cl0.9N4NiOSc2, Mr = 1711.02, 0.20 × 0.15 × 0.12 mm, monoclinic, C 2/m (No.12), a = 25.193(5), b = 15.091(3), c = 19.402(4), α = 90, β = 93.521(5), γ = 90, V = 7362 (4) Å3, Z = 4, ρcalcd = 1.544 gcm−3, μ (Mo Kα) = 0.544 mm−1, θ = 2.575− 27.563, T = 123 K, R1 = 0.0772, wR2 = 0.2060 for all data; R1 = 0.0730, wR1 = 0.2016 for 7946 reflections (I > 2.0σ(I)) with 1001 parameters. Goodness of fit indicator, 0.955. Maximum residual electron density, 1.039 e Å−3. Table S2 and CCDC 1497033 contain the supplementary crystallographic data for this paper. The unit in each cocrystal consists of a Sc2O@C78 molecule, a [NiII(OEP)] molecule, and disordered solvent molecules. The [NiII(OEP)] molecule and the fullerene cage are fully ordered. All cage carbon atoms were anisotropic, and SIMU constraint was applied. Distance restraints corresponding to the C2v(3)-C78 or D3h(5)-C78 symmetry were applied with SADI and DFIX commands. Inside the fullerene cage, the O atom is fully ordered with an occupancy of 0.5, whereas Sc atoms are disordered with the summed occupancy of 1.0. Thus, the ratio between cage, O, and Sc atoms is 1:1:2, which is fully consistent with the composition of Sc2O@C78. In the cocrystal, besides the identified solvent, there is another severely disordered lattice of C6H6 and CHCl3 molecules that could not be modeled properly; program SQUEEZE, a part of the PLATON package of crystallographic software,35,36 was used to calculate the solvent disorder area and remove its contribution from the intensity data. More refinement details can be found in the crystallographic information file (CIF).

other hand, theoretical computations have predicted that the isomerization of EMF via SW transformation can occur more easily than that of empty fullerene owing to the charge transfer.22 In recent years, a couple of SW-related isomeric EMFs such as Sm@C2n (2n = 82, 84, 90)23−25 and Sc2S@C8226 have been isolated and unambiguously characterized, which are indicative of the popularity of SW transformation in EMF chemistry, though the SW transformation barrier in these studies has not been discussed in detail. Summarizing all of these results, it appears that the presence of endohedral metallic species may facilitate the key steps (i.e., C2 extrusion or addition and SW transformation) of fullerene formation. In this respect, EMF might be considered as a better model for mechanism investigation. Typically, Dorn and co-workers reported unprecedented formation pathways from a lowsymmetric C1(51383)-C84 cage (observed in the form of M2C2@C1(51383)-C84 with M = Gd, Y) to a variety of wellcharacterized metallofullerene cages (i.e., C82 and C80 cages observed in the form of EMFs).11 However, other studies also suggested that the variety of encapsulated metallic species and the charge-transfer between endohedral and cage shall be determining factors that govern fullerene formation.12 Herein, we report a pair of novel EMF isomers, Sc2O@ C2v(3)-C78 and Sc2O@D3h(5)-C78, which are closely related via a single-step SW transformation as revealed by crystallographic characterization. Interestingly, we discover that the isomeric Sc2O@C78 can convert to either larger or smaller species, such as Sc2O@C2v(5)-C80,27 Sc2O@Cs(6)-C82,28 Sc2O@C3v(8)-C8229 and Sc2O@Td(1)-C76,30 via simple mechanism steps involving SW transformation and C2 extrusion or addition. Thus, a welldefined formation pathway can be experimentally established, for the first time, for the specific EMF series Sc2O@C2n. In addition, the low-barrier SW transformations between these Sc2O@C78 isomers are proposed by means of density functional theory (DFT) calculations. Further characterization reveals that these two isomers exhibit very different energy bandgaps as well as electrochemical behaviors, indicative of the remarkable effect of SW defects on the energetic and electrochemical characteristics of fullerenes.

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EXPERIMENTAL SECTION

Synthesis, Isolation, and Characterization. Isomeric Sc2O@C78 species were prepared using a modified arc-discharge method.31 In a typical preparation, the graphite anode was filled with a mixture of Sc2O3 and graphite powder with a molar ratio of 1:1 and burned under DC-arc discharging conditions in a He/CO2 atmosphere (9:1, v/v). The as-produced soot was Soxhlet extracted with chlorobenzene to obtain the fullerene mixture. Then, the fullerene mixture was subjected to multistep HPLC separations, which were subsequently performed on the Buckyprep M column (25 mm × 250 mm, Nacalai Tesque, Japan), Buckyprep column (10 mm × 250 mm, Nacalai Tesque, Japan), and 5PBB column (10 mm × 250 mm, Nacalai Tesque, Japan). Mass characterizations were performed by MALDI-TOF mass spectrometry using the Biflex III spectrometer (Bruker, Germany). UV−vis−NIR spectra of the purified Sc2O@C78 samples were measured in toluene solution with a UV 3600 spectrometer (Shimadzu, Japan) using a quartz cell of 1 mm layer thickness and 1 nm resolution. The 45Sc NMR spectroscopic study was performed at 145 MHz with the Agilent 600 MHz NMR spectrometer at 298 K in CS2 solution with D2O as the lock and a 0.4 M Sc(NO3)3 solution in D2O as the reference. Electrochemical measurements were conducted in o-dichlorobenzene (o-DCB) containing 0.05 M tetra-(n-butyl)-ammonium hexafluoro-phosphate ((n-Bu)4NPF6) as supporting electrolyte using a CHI-660E instrument. A conventional three-electrode cell consisting

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COMPUTATIONAL METHODS

All calculations were carried out using the Gaussian 09 program package.37 Geometry optimizations and population analysis were performed using the density functional theory with the recently introduced M06-2X functional, 3-21G basis set for the C and O atoms, and the SDD basis set (with the SDD effective core potential) for the Sc atom (the M06-2X/3-21G∼SDD level).38−40 The orbital and interisomeric energies were also calculated at a higher level (the M062X/6-311G*∼SDD level).38,39,41

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RESULTS AND DISCUSSION Synthesis and Isolation of Isomeric Sc2O@C78. Carbon soot containing clusterfullerenes Sc2O@C2n (n = 35−47) was produced under a He/CO2 atmosphere as reported earlier.31 B

DOI: 10.1021/acs.inorgchem.6b01894 Inorg. Chem. XXXX, XXX, XXX−XXX

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smaller relative to those of the larger species (i.e., 160.79° for Sc2O@C2v(5)-C80 and 156.6° for Sc2O@Cs(6)-C82).27,28 These comparisons demonstrate that the Sc2O cluster is more compressed in comparison to that in a larger fullerene cage. Moreover, inside the C2v(3)-C78 cage, only three additional minor Sc sites (Sc3, Sc4, Sc5) with occupancies ranging from 0.054 to 0.094 were identified around the major Sc sites (Sc1, Sc2), indicating a slight vibration of the Sc2O cluster. Nevertheless, because the crystallographic mirror plane is mismatching with the fullerene cage symmetry plane, the cluster position relative to the cage cannot be solely determined. Besides the structure shown in Figure 1, another possible structure is given in Figure S7. Computations will be utilized to determine the absolute structure of Sc2O@C2v(3)C78 (vide infra). As for other isomers, a fully ordered cage of D3h(5)-C78 can be identified, whereas the endohedral Sc2 unit is disordered over two sites with respect to the ordered O atom. The major Sc sites have fractional occupancies of 0.313 for Sc 1 and 0.277 for Sc2 (Figure 2). The structural factors of the Sc2O cluster in

The fullerene components were extracted with chlorobenzene and subjected to multistage high-pressure liquid chromatographic (HPLC) isolation processes (see Figures S1−4 for isolation details). Two Sc2O@C78 isomers (i.e., Sc2O@C78-i and Sc2O@C78-ii, where i and ii denote their retention time on the Buckyprep column) were separated and purified with relative abundance of approximately 3:1 (i:ii). Both isomers were characterized using a MALDI-TOF mass spectrometer (see Figures S5 and S6). Each shows a single peak at 1042 m/z in the mass spectrum corresponding to the calculated mass of Sc2OC78 (Sc2OC78: 1041.9 m/z). The isotopic distribution also agrees well with the theoretical prediction, confirming the composition of Sc2OC78 (see insets in Figures S5 and S6). Crystallographic Studies. The structures of these Sc2O@ C78 isomers (i and ii) were unambiguously determined by crystallographic studies. The black cocrystals of Sc2O@C2v(3)C78·[NiII(OEP)]·0.95C6H6·0.05CHCl3 were obtained by slow diffusion of a benzene solution of Sc2O@C78-i into a CHCl3 solution of [NiII(OEP)]. Likewise, the cocrystals of Sc2O@ D3h(5)-C78·[NiII(OEP)]·0.20C6H6·0.30CHCl3 were prepared from the sample of Sc2O@C78-ii. In particular, Figure 1

Figure 2. ORTEP drawing of Sc2O@D3h(5)-C78·[NiII(OEP)] with 20% thermal ellipsoids, showing the relationship between the fullerene cage and [NiII(OEP)]. Only the major Sc sites (Sc1 with 0.313 occupancy and Sc2 with 0.277 occupancy) are shown. For clarity, the solvent molecules and minor metal sites are omitted.

Figure 1. ORTEP drawing of Sc2O@C2v(3)-C78·[NiII(OEP)] with 20% thermal ellipsoids, showing the relationship between the fullerene cage and [NiII(OEP)]. Only the major Sc sites (Sc1 with 0.427 occupancy and Sc2 with 0.351 occupancy) are shown. For clarity, the solvent molecules and minor metal sites are omitted.

Sc2O@D3h(5)-C78, including the Sc−O bond lengths (i.e., 1.903(4) and 1.979(4) Å) and the cluster angle (i.e., Sc1−O− Sc2: 135.21(12)o), are all very similar to those of Sc2O@ C2v(3)-C78, indicating their similar cluster-cage interactions despite their different SW patterns or defects.

shows the X-ray structure of Sc2O@C2v(3)-C78 and its relationship to the adjacent porphyrin moiety. The C2v(3)-C78 cage satisfies the isolated pentagon rule (IPR) and is fully ordered with occupancy of 0.5. The adjacent porphyrin is approaching a flat region of the C2v(3)-C78 cage with a distance of 2.902 Å between the Ni ion and the nearest cage carbon, suggesting significant stacking interaction between them. Inside the cage, the Sc atoms are slightly disordered with respect to the fully ordered O atom. Only the major Sc sites with occupancies of 0.44 for Sc1 and 0.35 for Sc2 are shown in Figure 1. The Sc−O bond length in Sc2O@C2v(3)-C78 is determined to be 1.868(4) Å for Sc1 and 1.905(4) Å for Sc2, similar to those reported for other Sc2O@C2n species (i.e., 1.972−1.825 Å for Sc2O@Td(1)-C76,30 2.017−1.861 Å for Sc2O@C2v(5)-C80,27 and 1.937−1.888 Å for Sc2O@Cs(6)C82).28 The Sc1−O−Sc2 angle is 134.38(14)o, a value close to that observed in Sc2O@Td(1)-C76 (133.9°)30 but much

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COMPUTATIONAL STUDIES To confirm and better understand the crystallographic results, we optimized the two Sc2O@C78 isomers in the form of complexes (i.e., Sc2O@C78/[NiII(OEP)]) at the DFT-M062X/3-21G*∼SDD level of theory. As shown in Figures S9 and S10, different orientations of the Sc2O cluster were taken into account, giving rise to the conformers with significant energy differences (up to 15.5 kcal mol−1). For Sc2O@C2v(3)-C78, the optimization process yielded a geometry with the Sc2O cluster symmetrically aligned around the C2 axis of the C2v(3)-C78 cage (Figure S9). The optimized Sc−O distances of 1.883−1.891 Å and Sc−O−Sc angle of 135.6° are very close to those of the Xray results. Particularly, the two Sc atoms are bonded to [6:6] C

DOI: 10.1021/acs.inorgchem.6b01894 Inorg. Chem. XXXX, XXX, XXX−XXX

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from the Sc2O cluster to the C78 cage.44 Thus, the electronic structures of these Sc2O@C78 isomers are commonly described as (Sc2O)4+@(C78)4−. Such an electronic feature is similar to those of other Sc2O@C2n clusterfullerenes but different from previously reported C78 EMFs (i.e., M2@C78, M3N@C78, Ti2C2@C78, and Ti2S@C78) with a formal transfer of six electrons from the cluster to the cage.16 In addition, as shown in Figure 3, the two isomeric Sc2O@C78 show similar MO distributions: not only the HOMO but also the LUMO are mainly localized on the C78 cage with a small fraction on the Sc2O cluster. The HOMO−LUMO gap is computed to be 3.08 eV for Sc2O@C2v(3)-C78 and 2.51 eV for Sc2O@D3h(5)-C78. In comparison, the isomeric empty C78 fullerenes exhibit very similar bandgaps (3.30 eV for C2v(3)-C78 and 3.24 eV for D3h(5)-C78 calculated at the same computational level). These results indicate that, after the encapsulation of the Sc2O cluster, the SW defects present much more impact on the energy gap of the clusterfullerene. It is noteworthy that the calculated Sc−O distances in pristine Sc2O@C2v(3)-C78 or Sc2O@D3h(5)-C78 are equal (1.887 Å for the C2v isomer and 1.897 Å for the D3h isomer), whereas they are unequal in the case of the complex. Closer investigation based on the population analysis reveals that the presence of [NiII(OEP)] induces a Mulliken charge redistribution in Sc2O@C78, which might account for the unequal Sc−O distances in the complex. In particular, in the complex of Sc2O@C2v(3)-C78/[NiII(OEP)], the endohedral Sc ion close to the [NiII(OEP)] moiety bears considerably more positive Mulliken charges (2.81 e) relative to the other (2.17 e), whereas in the pristine clusterfullerene, each Sc ion has a charge of 2.18 e. On the other hand, the Mulliken charge on the O ion remains constant (i.e., −0.53 e) irrespective of the presence of [NiII(OEP)]. Thus, this comparative study reveals for the first time that the complexing interaction between the clusterfullerene and [NiII(OEP)] moiety might slightly influence the geometric and electronic structure of the endohedral cluster. Interestingly, it is clearly seen that Sc2O@C2v(3)-C78 and Sc2O@D3h(5)-C78 can be interconverted via a single-step Stone−Wales transformation (Figure 4). As shown in Figure 4a, apart from the critical pyracylene patch undergoing the SW transition (SW defect in blue), the remaining cage fragments of the isomeric C78 have essentially the same atom locations and connectivities. For the SW rearrangement to be assessed, DFT activated-complex searches were conducted to identify the possible transition state (TS) as well as the activation energy barrier for the Stone−Wales transformation of Sc2O@C78 isomers. The atomic configuration of the possible TS and schematic energy diagram along the reaction coordinate of the SW transformation are provided in Figure 4a and b. In TS-I, the rotating C2 unit (shown in blue) has a bond length of 1.50 Å with 1.57−1.58 Å bonds to each of the two nearest carbons. The calculated activation barrier is 145.5 kcal mol−1 (or 6.31 eV), which is slightly lower than that computed for the empty C78 isomers (153.7 kcal mol−1 or 6.67 eV). This result is in agreement with previous studies, which demonstrated that the energy barrier for SW transformation in fullerenes is reduced in the presence of endohedral dopant due to the charge transfer.22 In TS-II, the rotating C2 unit is complexing with an exohedral Sc atom with Sc−C distances of 1.914−2.196 Å. The corresponding activation barrier is calculated to be 73.1 kcal mol−1 (or 3.17 eV), significantly reduced relative to that involving TS-I and very compatible with the fullerene annealing temperature (∼1000 K).8 Considering the high-density metal

Figure 3. Molecular orbital diagrams for Sc2O@C2v(3)-C78 and Sc2O@D3h(5)-C78.

Figure 4. (a) Schematic energy profile for the possible transformation from Sc2O@D3h(5)-C78 to Sc2O@C2v(3)-C78, which involves TS-I and TS-II, respectively. The pyracyclene patch undergoing SW transformation is highlighted (in blue). (b) DFT calculated TS-I and TS-II. The rotating C2 unit is highlighted (in blue).

ring junctions of pyracylene patches with the shortest Sc−C distances falling in a narrow range of 2.233−2.299 Å, in good agreement with the X-ray data (2.179−2.285 Å). As for the complex of Sc2O@D3h(5)-C78/[NiII(OEP)], the energy-lowest geometry shows that the Sc2O cluster lies close to the σh plane of the D3h(5)-C78 cage (the plane crossing three pyracylene patches that are linked by hexagons; see Figure S10). The Sc-pyracylene contact ranges from 2.229 to 2.304 Å, consistent with the X-ray data (2.125−2.287 Å). These values are also similar to those observed in the complex containing Sc2O@C2v(3)-C78 and within the range of Sc−C bond lengths (2.204−2.430 Å) found in many Sc complexes in the Cambridge Structural Data,42,43 which indicate a remarkable bonding-interaction between Sc and pyracylene patches. Inside the D3h(5)-C78 cage, the Sc−O distances are calculated to be 1.896−1.903 Å, which are close to those of the X-ray data. The optimized Sc−O−Sc cluster angle is 144.6°, which is slightly larger than that found in the X-ray structure. Furthermore, the pristine Sc2O@C2v(3)-C78 and Sc2O@ D3h(5)-C78 were optimized at the same computational level. It is suggested that there is a formal transfer of four electrons D

DOI: 10.1021/acs.inorgchem.6b01894 Inorg. Chem. XXXX, XXX, XXX−XXX

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Figure 5. Transformation map for the cages of C2v(5)-C80, C2v(3)-C78, D3h(5)-C78, and Td(1)-C76. Colors are used to visualize the motifs involved in the proposed mechanism steps, such as SW transformation and C2 extrusion or addition.

calculations also check the influence of the endohedral cluster on the SW transformation. In particular, the TS with the SW defect bonding with an endohedral Sc ion (see Figure S12) corresponds to an activation barrier of 164.9 kcal mol−1, which is even higher than that of empty fullerene. This result might suggest that the chemical bonding between the endohedral metal ion and SW defect does not promote SW transformation, which agrees well with the X-ray result that, in the Sc2O@C78 isomer, the SW defect is free from bonding with the endohedral Sc ions. Formation Pathway of Sc2O@C2n (n = 38−41). The mass spectrum of the fullerene mixture obtained from the production of Sc2O@C2n suggested that the endohedral Sc2O cluster could guide the formation of fullerene cages ranging from C70 to C94.31 Nevertheless, the formation pathway cannot be well-defined until all links have been structurally characterized via X-ray analysis. The fullerene cages described in Figure 5 and Figure S13 involve all well-characterized species, which account for the majority of the solventextractable yield of Sc2O@C2n. In particular, the C2v(3)-C78 and D3h(5)-C78 cages can enlarge to form larger cages such as C2v(5)-C80, Cs(6)-C82, and C3v(8)-C82, all available in the form of Sc2O@C2n via simple mechanistic steps involving SW transformations and C2 additions (see Figure 5 and Figure S13). Furthermore, these isomeric C78 cages can also shrink to form a previously reported small cage of Td(1)-C76 via a similar process (see Figure 5). Thus, the discovery and structural characterizations of these isomeric Sc2O@C78 represent key

Figure 6. UV−vis−NIR absorption spectra of Sc2O@C2v(3)-C78 and Sc2O@D3h(5)-C78 in toluene solution. (inset) 45Sc NMR spectrum of isomeric Sc2O@C78 measured in CS2 with D2O as the lock and a 0.4 M Sc(NO3)3 solution in D2O as the reference.

vapor in the gas phase of EMF synthesis by evaporation of metal-doped graphite, this Sc-catalyzed SW transformation should be quite possible. A close investigation revealed that the complexed Sc atom donates approximately 1 electron (0.956 e calculated by population analysis) to the C78 cage (mainly on the SW defect), thus further facilitating the SW transformation in Sc2O@C78 by additional charge transfer. Moreover, DFT

Table 1. Redox Potentials (V vs Fc0/+) of Sc2O@C2v(3)-C78 and Sc2O@D3h(5)-C78 Relative to Those of La2@D3h(5)-C78 and Sc3N@D3h(5)-C78 ox

Sc2O@C2v(3)-C78 Sc2O@D3h(5)-C78 La2@D3h(5)-C78d Sc3N@D3h(5)-C78e a

E2 a

0.64 (0.64c) 0.62c 0.62 0.68

ox

red

E1

red

E1

−1.17 (−1.12c) −0.67a −0.40 −1.56b

a

b

0.16 (0.16c) 0.18a 0.26 0.21

red

E2

−1.66 (−1.62c) −0.86a −1.84 −1.91b b

E3

−1.93b (−1.85c)

EC gap 1.33 1.28 0.85 0.66 1.77

Half-wave potentials. bPeak potentials. cDPV potentials. dRef 48. eRef 49. E

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Figure 7. Cyclic voltammograms (up) and differential pulse voltammograms (down) of (a) Sc2O@C2v(3)-C78 and (b) Sc2O@D3h(5)-C78 in odichlorobenzene (0.05 M (n-Bu)4NPF6; scan rates of 100 and 20 mV s−1 for CV and DPV, respectively).

The electrochemical properties of Sc2O@C78 isomers were investigated by cyclic voltammetry (CV) and differential pulse voltammetry (DPV). All of the obtained redox potentials are summarized in Table 1 and compared with those of previously reported C78 EMFs. As shown in Figure 7, in the anodic region, both Sc2O@C2v(3)-C78 and Sc2O@D3h(5)-C78 exhibit reversible first oxidation wave with half-wave potentials of 0.16 and 0.18 V (oxE1 vs Fc0/+), respectively. These potential values are 30−100 mV lower than those of C78 endohedrals reported earlier (i.e., La2@D3h(5)-C78 and Sc3N@D3h(5)-C78),48,49 indicating the better electron-donating abilities of these Sc2O@C78 isomers. On the other hand, in the cathodic region, two isomers display very different reductive behaviors: The reduction processes of Sc2O@C2v(3)-C78 are irreversible, whereas Sc2O@D3h(5)-C78 exhibits fully reversible reduction waves. The first and second reduction potentials of the C2v isomer notably shift by −460 mV and −760 mV compared to those of the D3h isomer. These results thus indicate the considerable impact of the SW defects on the reduction behavior of Sc2O@C78, fully consistent with the remarkable energy difference between their DFT-calculated LUMOs. The electrochemical gaps of Sc2O@C2v(3)-C78 and Sc2O@D3h(5)C78 are calculated as 1.28−1.33 and 0.85 V, respectively. This trend is again consistent with what we found in the comparison between their optical bandgaps as well as their HOMO− LUMO gaps. Moreover, it is also interesting to find that the electrochemical gaps of various D3h(5)-C78-based EMFs are increasing in the following order: La2@C78 (0.66 V) < Sc2O@ C78 (0.85 V) < Sc3N@C78 (1.77 V). This result thus suggests that the bandgap of clusterfullerenes is highly dependent on not only the fullerene cage but also the encaged cluster.

links in a formation pathway of clusterfullerenes Sc2O@C2n (n = 38−41). It is noteworthy that this formation pathway of Sc2O@C2n matches well with either the “top-down” or “bottom-up” formation mechanism. It is almost impossible to make further assignment that will require additional experimental evidence. Nevertheless, the present study establishes, for the first time, a complete and well-defined fullerene formation pathway with all key links being unambiguously characterized. The discovery of the isomeric C78 species as well as the proposed low-barrier SW transformation between them may rationalize the mechanistic pathway of Sc2O@C2n as well as aid in structural prediction of other unidentified species. Moreover, it is evident that the formation pathway of Sc2O@C2n is different from that of wellstudied nitride clusterfullerenes M3N@C2n (M = Sc, Gd). The latter involves the cage of D3h(5)-C78 or C2(22010)-C78 rather than C2v(3)-C78 that is only stabilized by endohedral Sc2O cluster or the transfer of four electrons from the cluster to the cage. This confirms the previous proposal that EMFs have different specific pathways and intermediates due to the influence of metal cluster geometry, size, or charge.11 Spectroscopic and Electrochemical Studies. Figure 6 presents the UV−vis−NIR absorption spectra of Sc2O@C2v(3)C78 and Sc2O@D3h(5)-C78. In particular, the C2v isomer shows feature absorptions at 434, 534, 610, 721, and 801 nm and an onset of 903 nm, which is unlike any of the previously reported endohedral C78 species. For the D3h isomer, absorptions at 450, 673, 774, and 956 nm are observed with an onset around 1095 nm. Thus, the optical band gap of the C2v isomer is estimated to be 1.37 eV larger than that of the D3h isomer (1.13 eV). These values are in good agreement with their HOMO−LUMO gaps in magnitude, confirming the effect of the SW defect on the bandgap of Sc2O@C78 isomers. In addition, it is also noteworthy that the absorption spectrum of Sc2O@D3h(5)C78 partially resembles those of Sc3N@D3h(5)-C7845 as well as Ti2C2@D3h(5)-C7846 and Ti2S@D3h(5)-C78,47 although they have different electronic structures. The 45Sc NMR spectra of Sc2O@C2v(3)-C78 and Sc2O@D3h(5)-C78 recorded at 298 K are provided in Figure 6 (inset). Each isomer shows a single 45Sc signal, indicating the free rotation of the Sc2O cluster at this temperature. The 45Sc signal shifts from 115.7 ppm for Sc2O@ C2v(3)-C78 to 90.6 ppm for Sc2O@D3h(5)-C78, suggesting that the chemical shift of the endohedral Sc atom is very sensitive to the cage structure or SW defect.

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CONCLUSIONS In summary, crystallographic characterizations of two isolated Sc2O@C78 isomers reveal their cage structures as Sc2O@ C2v(3)-C78 and Sc2O@D3h(5)-C78, respectively, which are closely related via a single-step SWT. In each Sc2O@C78 isomer, the endohedral Sc ions are found to be strongly bonded to the [6,6]-junctions of two different pyracylene patches at the midsection of the fullerene cage, whereas the pyracylene unit undergoing SWT (SW defect) is free from the bonding with the endohedral Sc ion. Moreover, DFT calculations show that the presence of the endohedral Sc2O cluster as well as the exohedrally complexed Sc atom is effective in the reduction of the SW activation barrier. Further F

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(4) Kirner, S.; Sekita, M.; Guldi, D. M. 25th Anniversary Article: 25 Years of Fullerene Research in Electron Transfer Chemistry. Adv. Mater. 2014, 26, 1482−1493. (5) Kiguchi, M.; Tal, O.; Wohlthat, S.; Pauly, F.; Krieger, M.; Djukic, D.; Cuevas, J. C.; van Ruitenbeek, J. M. Highly Conductive Molecular Junctions Based on Direct Binding of Benzene to Platinum Electrodes. Phys. Rev. Lett. 2008, 101, 046801. (6) Kiguchi, M.; Takahashi, T.; Takahashi, T.; Yamauchi, T.; Murase, T.; Fujita, M.; Tada, T.; Watanabe, S. Electron Transport through Single Molecules Comprising Aromatic Stacks Enclosed in SelfAssembled Cages. Angew. Chem., Int. Ed. 2011, 50, 5708−5711. (7) Curl, R. F.; Haddon, R. C. On the Formation of the Fullerenes [and Discussion]. Philos. Trans. R. Soc., A 1993, 343, 19−32. (8) Cross, R. J.; Saunders, M. Transmutation of Fullerenes. J. Am. Chem. Soc. 2005, 127, 3044−3047. (9) Dunk, P. W.; Kaiser, N. K.; Hendrickson, C. L.; Quinn, J. P.; Ewels, C. P.; Nakanishi, Y.; Sasaki, Y.; Shinohara, H.; Marshall, A. G.; Kroto, H. W. Closed Network Growth of Fullerenes. Nat. Commun. 2012, 3, 855. (10) Berné, O.; Tielens, A. G. G. M. F. Formation of Buckminsterfullerene (C60) in Interstellar Space. Proc. Natl. Acad. Sci. U. S. A. 2012, 109, 401−406. (11) Zhang, J.; Bowles, F. L.; Bearden, D. W.; Ray, W. K.; Fuhrer, T.; Ye, Y.; Dixon, C.; Harich, K.; Helm, R. F.; Olmstead, M. M.; Balch, A. L.; Dorn, H. C. A Missing Link in the Transformation from Asymmetric to Symmetric Metallofullerene Cages Implies a Topdown Fullerene Formation Mechanism. Nat. Chem. 2013, 5, 880−885. (12) Dunk, P. W.; Mulet-Gas, M.; Nakanishi, Y.; Kaiser, N. K.; Rodríguez-Fortea, A.; Shinohara, H.; Poblet, J. M.; Marshall, A. G.; Kroto, H. W. Bottom-up Formation of Endohedral Mono-Metallofullerenes is Directed by Charge Transfer. Nat. Commun. 2014, 5, 5844. (13) Chuvilin, A.; Kaiser, U.; Bichoutskaia, E.; Besley, N. A.; Khlobystov, A. N. Direct Transformation of Graphene to Fullerene. Nat. Chem. 2010, 2, 450−453. (14) Stone, A.; Wales, D. Theoretical Studies of Icosahedral C60 and Some Related Species. Chem. Phys. Lett. 1986, 128, 501. (15) Wales, D. J.; Miller, M. A.; Walsh, T. R. Archetypal Energy Landscapes. Nature (London, U. K.) 1998, 394, 758. (16) Popov, A. A.; Yang, S.; Dunsch, L. Endohedral Fullerenes. Chem. Rev. 2013, 113, 5989−6113. (17) Lu, X.; Feng, L.; Akasaka, T.; Nagase, S. Current Status and Future Developments of Endohedral Metallofullerenes. Chem. Soc. Rev. 2012, 41, 7723−7760. (18) Tan, Y. Z.; Xie, S. Y.; Huang, R. B.; Zheng, L. S. The Stabilization of Fused-Pentagon Fullerene Molecules. Nat. Chem. 2009, 1, 450. (19) Yamada, M.; Kurihara, H.; Suzuki, M.; Guo, J. D.; Waelchli, M.; Olmstead, M. M.; Balch, A. L.; Nagase, S.; Maeda, Y.; Hasegawa, T.; Lu, X.; Akasaka, T. Sc2@C66 Revisited: An Endohedral Fullerene with Scandium Ions Nestled within Two Unsaturated Linear Triquinanes. J. Am. Chem. Soc. 2014, 136, 7611−7614. (20) Hao, Y.; Feng, L.; Xu, W.; Gu, Z.; Hu, Z.; Shi, Z.; Slanina, Z.; Uhlík, F. Sm@C2v(19138)-C76: A Non-IPR Cage Stabilized by a Divalent Metal Ion. Inorg. Chem. 2015, 54, 4243−4248. (21) Zhang, Y.; Ghiassi, K. B.; Deng, Q.; Samoylova, N. A.; Olmstead, M. M.; Balch, A. L.; Popov, A. A. Synthesis and Structure of LaSc2N@Cs(hept)-C80 with One Heptagon and Thirteen Pentagons. Angew. Chem., Int. Ed. 2015, 54, 495−499. (22) Choi, W. I.; Kim, G.; Han, S.; Ihm, J. Reduction of Activation Energy Barrier of Stone-Wales Transformation in Endohedral Metallofullerenes. Phys. Rev. B: Condens. Matter Mater. Phys. 2006, 73, 113406. (23) Yang, H.; Jin, H.; Wang, X.; Liu, Z.; Yu, M.; Zhao, F.; Mercado, B. Q.; Olmstead, M. M.; Balch, A. L. X-ray Crystallographic Characterization of New Soluble Endohedral Fullerenes Utilizing the Popular C82 Bucky Cage. Isolation and Structural Characterization of Sm@C3v(7)-C82, Sm@Cs(6)-C82, and Sm@C2(5)-C82. J. Am. Chem. Soc. 2012, 134, 14127−14136.

characterizations demonstrate that these Sc2O@C78 isomers feature different energy bandgaps and electrochemical behaviors, indicating the impact of SW defect on the energetic and electrochemical characteristics of EMFs. More importantly, a well-defined formation pathway is established for the EMF series Sc2O@C2n (n = 38−41) for the first time, which involves simple mechanism steps, such as SWT and C2 loss or addition. The unambiguously characterized Sc2O@C78 isomers represent the key links in this unprecedented formation pathway. The proposed low-barrier SWT between these Sc2O@C78 isomers may further rationalize the established pathway. This study thus not only contributes to the better understanding of fullerene formation but also may stimulate further interest in the exploration of the growth or formation of other fullerene-related carbon materials.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.6b01894. HPLC profiles for the separation of Sc2O@C2v(3)-C78 and Sc2O@D3h(5)-C78; mass spectra, selected X-ray results, and calculation results of Sc2O@C2v(3)-C78 and Sc2O@D3h(5)-C78 (PDF) Crystal data for Sc 2 O@C 2v (3)-C 78 ·[Ni II (OEP)]· 0.95C6H6·0.05CHCl3 (CIF) Crystal data for Sc 2 O@D 3h (5)-C 78 ·[Ni II (OEP)]· 0.20C6H6·0.30CHCl3 (CIF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. *E-mail: [email protected]. Author Contributions ⊥

Y.H. and Q.T. contributed equally to this work.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We cordially thank Shuao Wang, Daopeng Sheng (Soochow University), and Xing Lu (Huazhong University of Science and Technology) for the kind help in the single-crystal X-ray diffraction measurements. This work is supported in part by the NSFC (51372158, 21305098, 51302178), SRFDP (20123201120014), Jiangsu Specially Appointed Professor Program (SR10800113), the Project for Jiangsu Scientific and Technological Innovation Team (2013), and the Czech Science Foundation/GACR (P208/10/1724).

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REFERENCES

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DOI: 10.1021/acs.inorgchem.6b01894 Inorg. Chem. XXXX, XXX, XXX−XXX