Article pubs.acs.org/IC
Sc2C2@D3h(14246)‑C74: A Missing Piece of the Clusterfullerene Puzzle Yaofeng Wang,† Qiangqiang Tang,† Lai Feng,*,‡ and Ning Chen*,† †
Laboratory of Advanced Optoelectronic Materials, College of Chemistry, Chemical Engineering and Materials Science, Soochow University, Suzhou, Jiangsu 215123, China ‡ College of Physics, Optoelectronics and Energy & Collaborative Innovation Center of Suzhou Nano Science and Technology, Soochow University, Suzhou, Jiangsu 215006, China S Supporting Information *
ABSTRACT: Clusterfullerenes with variable carbon cages have been extensively studied in recent years. However, despite all these efforts, C74 cage-based clusterfullerene remains a missing piece of the puzzle. Herein, we show that singlecrystal X-ray crystallographic analysis unambiguously assigns the previously reported dimetallofullerene Sc2@C76 to a novel carbide clusterfullerene, Sc2C2@D3h(14246)-C74, the first experimentally proven clusterfullerene with a C74 cage. In addition, Sc2C2@ D3h(14246)-C74 was charaterized by mass spectrometry, ultraviolet−visible−nearinfrared absorption spectroscopy, 45Sc nuclear magnetic resonance, and cyclic voltammetry. Comparative studies of the motion of the carbide cluster in Sc2C2@ D3h(14246)-C74 and Sc2C2@C2n (n = 40−44) revealed that a combination of factors, involving both the shape and size of the cage, is crucial in dictating the cluster motion. Moreover, structural studies of D3h(14246)-C74 revealed that it can be easily converted to Cs(10528)-C72 and Td(19151)-C76 cages via C2 desertion/insertion and Stone−Wales transformation. This suggests that D3h(14246)-C74 might play an important role in the growth pathway of clusterfullerenes.
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INTRODUCTION Endohedral fullerenes have attracted much interest in the fullerene field because of their unique structures.1−4 The most fascinating features of endohedral fullerenes are their capacity to encapsulate the variable atoms and small molecules inside the carbon cage. The endohedral clusters are versatile, including nitrides,5−7 carbides,8,9 hydrocarbides,10,11 oxides,12−16 sulfides,17,18 and carbonitrides.19 These clusters often endow the fullerenes with remarkable properties, which give rise to their great potential application in the fields of MRI contrasting agents,3,20,21 molecular electronic devices,22−24 solar cells,25,26 and biomedicine.27 Though numerous clusterfullerenes have been synthesized and characterized, clusterfullerenes with a C74 cage have never been reported experimentally and remain a missing piece of the clusterfullerene puzzle.28−30 Sc2S@C74 is the only species that has been theoretically investigated and demonstrated a linear Sc−S−Sc cluster inside the cage. However, the absolute structure has never been experimentally proven.31 Metallic carbide clusterfullerenes (MCCFs) were discovered in 2001. In that study, the molecular structure of Sc2@C86 was proven to be
[email protected] Since then, a series of scandium carbide fullerenes have been reported, including Sc2C2@ C2v(6073)-C68,33 Sc2C2@Cs(10528)-C72,34 Sc2C2@C2v(5)C80,35 Sc2C2@C3v(8)-C82,35,36 Sc2C2@D2d(23)-C84,35 Sc2C2@ C2v(9)-C86,37 and Sc2C2@Cs(hept)-C88.38 Interestingly, some of these studies have shown that several previously assigned Sc2@ C2n dimetallofullerenes have turned out to be carbide clusterfullerenes.39,40 Recently, we performed a series of studies of the oxide clusterfullerenes, Sc2O@C2n (n = 35−41).14−16,41 © XXXX American Chemical Society
In this work, we found a certain amount of Sc2@C76 was generated during the synthesis of these Sc2O@C2n fullerenes. Sc2C76 was reported as a dimetallofullerene in 2001, but its absolute structure has never been crystallographically elucidated.42 Thus, in this work, we performed a crystallographic study of Sc2C76 along with spectroscopic and electrochemical studies. Instead of a dimetallofullerene, crystallographic analysis unambiguously assigned this structure to a novel carbide clusterfullerene, Sc2C2@D3h(14246)-C74, which is also the first clusterfullerene with a C74 cage.43−47
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RESULTS AND DISCUSSION Preparation and Purification. The MCCFs were synthesized in a conventional Krätschmer−Huffman arcdischarge reactor under a He and CO2 atmosphere (200 Torr of helium with 20 Torr of CO2). The soot was collected together, refluxed in chlorobenzene under an argon atmosphere for 24 h, and filtered through filter paper. After the solvent had been removed, the extracted fullerenes were then dissolved in toluene and filtered through a polytetrafluoroethylene (PTFE) membrane. A multistage high-performance liquid chromatography (HPLC) procedure was employed to obtain the purified Sc2C76 (see Figure S1). After three-stage separation and purification, approximately 1 mg of Sc2C76 was obtained. The purified sample of Sc2C76 was characterized by positive ion mode matrix-assisted laser desorption ionization time-ofReceived: October 20, 2016
A
DOI: 10.1021/acs.inorgchem.6b02512 Inorg. Chem. XXXX, XXX, XXX−XXX
Article
Inorganic Chemistry flight mass spectrometry (MALDI-TOF-MS). The mass spectrum shows a major peak at m/z 1001.925, corresponding to the mass of Sc2C76, and several minor peaks [m/z 977.923 (Sc2@C74), m/z 1090.040 (Sc2O@C82), and m/z 1110.881 (Sc3O@C80)]. The experimentally observed isotopic distribution also agrees well with the theoretical prediction (see Figure 1).
Figure 2. X-ray structure of Sc2C2@D3h(14246)-C74, showing the major cage orientation along with the major C2 unit and the Sc sites with occupancies of >0.10 (left). X-ray structure of Sc2C2@ D3h(14246)-C74, showing the minor cage orientation along with the minor C2 unit and the Sc sites with occupancies of >0.10 (right).
(i.e., Sc2 and Sc5) and the major C2 unit are shown in Figure 3. Like most previously reported cases, the Sc2C2 cluster exhibits a
Figure 1. Positive ion mode MALDI-TOF mass spectra of purified Sc2C76. The inset shows the experimental and theoretical isotopic distribution of Sc2C76.
X-ray Structure of Sc2C2@D3h(14246)-C74. The structure of Sc2C76 was unambiguously characterized by single-crystal Xray diffraction (XRD). Black cocrystals of Sc2C76 and nickel(II) octaethylporphyrin [NiII(OEP)] were obtained by diffusing a benzene solution of Sc2C76 into a CHCl3 solution of NiII(OEP). After refinement, a disordered D3h(14246)-C74 cage containing a disordered Sc2C2 cluster was clearly identified. In particular, the D3h(14246)-C74 cage is disordered over two orientations with fractional occupancies of 0.53 and 0.47. The relative flat cage region is close to the adjacent NiII(OEP) moiety with the shortest Ni−cage distance ranging from 2.790(11) to 2.817(15) Å, very close to that found in Sm@D3h-C74/NiII(OEP) (2.606−2.867 Å).48 Inside of the D3h(14246)-C74 cage, 12 Sc sites together with two C2 unit sites are distinguishable, indicating a rotating carbide cluster. Previous studies of the motional behavior of Sc2C2@C2n (n = 40−42) suggested that the size of the cage is an important factor dictating cluster motion.35 In particular, rotational movement was also observed in Sc2C2@D2d(23)-C84 and Sc2C2@C3v(8)-C82, whose cages are much larger and are rather round.35 In contrast, the restricted Sc2C2 cluster was found in Sc2C2@C2v(5)-C80 and Sc2C2@C2v(9)-C86, which possess elliptic cages.35,37 Thus, it is more likely to be a combination of factors playing an important role in dictating the cluster motion, rather than just size or just shape. Moreover, the disordered C2 units can be safely assigned to each cage orientation according to their consistent occupancies. As for the multiple Sc sites, at least six of them can be paired into three sets (i.e., Sc2 and Sc5, Sc7 and Sc8, and Sc3 and Sc4) according to their stereolocations and similar occupancies (see Figure 2). The Sc−Sc distances range from 3.888(16) to 4.474(9) Å, comparable to those reported for Sc2C2@ Cs (10528)-C 72 (4.2 Å), 34 Sc 2C 2 @C 2v(5)-C 80 (4.31 Å), 35 Sc2C2@C3v(8)-C82 (3.86−4.09 Å),35 Sc2C2@D2d(23)-C84 (4.46 Å),35 Sc2C2@C2v(9)-C86 (4.58 Å),37 and Sc2C2@ Cs(hept)-C88 (4.81 and 4.83 Å).38 Only the major Sc pair
Figure 3. Drawing of Sc2C2@D3h(14246)-C74/[NiII(OEP)] with 15% thermal ellipsoids. Only the major Sc sites (Sc7 and Sc8) are shown. For the sake of clarity, the minor cage orientation, solvent molecules, and minor metal sites have been omitted.
butterfly shape with a Sc−C2−Sc dihedral angle of 120.8° (see Figure S3). This dihedral angle is smaller than those (127.0− 175.3°) reported for Sc2C2@C2n (n = 40−44) with a larger cage size, indicating a very bent configuration of the carbide cluster. It appears that the Sc2C2 cluster is more compressed inside a smaller fullerene cage, which agrees well with the previous proposal that fullerene cage sizes may influence the shape of the M2C2 unit in endohedrals by compressing it.49 Possible Role of D3h-C74 in the Formation Pathway for Clusterfullerenes. D3h(14246)-C74 is the only C74 isomer that obeys the isolated pentagon rule (IPR). Empty D3h(14246)-C74 has been seldom studied experimentally because of its low stability and insolubility.50 However, the D3h(14246)-C74 cage has often been observed in the form of monometallofullerenes M@C74 (M = Sm, Yb, Ca, Ba, or La),43−45,47,51 indicating its popularity in endohedral fullerene chemistry. Herein, it is surprising to see the D3h(14246)-C74 cage in the form of a carbide clusterfullerene [i.e., Sc2C2@D3h(14246)C74], which is inconsistent with the theoretical prediction that isoelectronic species Sc2S@C74 would have a non-IPR Cs(13333)-C74 cage.31 Reaching a conclusion that the C74 cage always prefers D3h symmetry is also unusual, regardless of whether it is in an empty form or with endohedral species inside accompanied by two-, three-, or four-electron transfer. B
DOI: 10.1021/acs.inorgchem.6b02512 Inorg. Chem. XXXX, XXX, XXX−XXX
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Inorganic Chemistry
840 nm. Interestingly, we found that these features are identical to those of the previously reported
[email protected] Thus, it adds to the confirmation that the previously assigned Sc2@C76 is actually a carbide clusterfullerene, Sc2C2@D3h(14246)-C74. In the recent studies by Akasaka et al., the previously reported Sc2@C84 and Sc2@C82 have been identified as being carbide endohedral fullerenes [i.e., Sc2C2@C2v(9)-C82 and Sc2C2@ C2v(5)-C80] instead of dimetallofullerenes.39,40 This study provides another sample showing that when two scandium atoms are encapsulated, in many cases, they intend to form carbide cluster that can be stabilized inside the carbon cage. On the other hand, this feature is dramatically different from that observed for La@D3h(14246)-C74,44 Ca@D3h(14246)-C74,45 Sm@D3h(14246)-C74,48 and Eu@D3h(14246)-C74,52 though they share the same carbon cage. This may be related to their different electronic structures, in which the metallic endohedral donates either two, three, or four electrons to the cage. The onset of absorption of Sc2C2@D3h(14246)-C74 is around 1515 nm, translating to an optical gap of 0.82 eV. This band gap is larger than that of Eu@D3h(14246)-C74 (0.48 eV),52 which agrees well with the previous proposal that the band gap of clusterfullerenes is generally larger than those of the monometallofullerenes. 45 Sc NMR studies were also used to study Sc2C2@ D3h(14246)-C74. As shown in Figure 6, a single peak at 144
The discovery of Sc2C2@D3h(14246)-C74 also alowled us to study the possible growth path of those clusterfullerenes featured by four-electron transfer, which had been disconnected because of the lack of characterization of C74 species. Figure 4
Figure 4. Assumed conversion route among Cs (10528)-C72, D3h(14246)-C74, and Td(19151)-C76 cages, involving a C2 insertion and SW transformation.
shows that Cs(10528)-C72, a non-IPR cage available for Sc2C2@ C72 and Sc2O@C72, can be enlarged to D3h(14246)-C74 via simple mechanistic steps involving a C2 insertion and Stone− Wales transformation. On the other hand, though there has been no detailed study of Sc2C2@C76, the cage of Td(19151)C76 observed in an isoelectronic species Sc2O@Td(19151)-C76 is shown to be closely related to the D3h(14246)-C74 cage via a C2 insertion and SW transformation. Such a close structural relationship between D3h(14246)-C74, Cs(10528)-C72, and Td(19151)-C76 suggests that D3h(14246)-C74 might play a key role in the formation pathway for Sc2O@C2n (n = 35−47), Sc2S@C2n (n = 40−50), and Sc2C2@C2n (n = 34, 36, and 40− 47), all of which possess isoelectronic structures and feature four-electron transfer from the cluster to the cage. Spectroscopic Characterization. Ultraviolet−Visible− Near-Infrared (UV−vis−NIR) and 45Sc Nuclear Magnetic Resonance (NMR). The purified sample of Sc2C2@D3h(14246)C74 presents a brown color in CS2 solution. Figure 5 shows that the UV−vis−NIR spectrum of Sc 2 C 2 @D 3h (14246)-C 74 presents four characteristic absorption peaks at 569, 678, 1052, and 1222 nm along with an absorption band from 760 to
Figure 6. 45Sc NMR spectrum of Sc2C2@D3h(14246)-C74 in CS2 at room temperature.
ppm was recorded for Sc2C2@D3h(14246)-C74 at 298 K, which is similar to that observed for Sc2C2@C3v(8)-C82.53 However, two broad 45Sc NMR signals were observed for Sc2C2@C2v(5)C8040 and Sc2C2@Cs(6)-C82.54 All these results might indicate different cluster−cage interactions in these carbide clusterfullerenes. Moreover, it is noteworthy that, as summarized in Table 1, the 45Sc NMR signal for Sc2C2@D3h(14246)-C74 is Table 1. 45Sc NMR Chemical Shifts of Sc2C2@C2n Sc2C2@D3h(14246)-C74 Sc2C2@Cs-C72 Sc2C2@C2v(5)-C80 Sc2C2@Cs(6)-C82 Sc2C2@C3v(8)-C82
δ(Sc)
ref
144 130 130/170a 200/245a 225a
34 40 54 53
a Exact values were not given and are estimated from the figures in the original papers.
dramatically shifted from those of Sc2C2@C2v(5)-C80 and Sc2C2@Cs(6)-C82, suggesting that the fullerene cage has a significant impact on the chemical shifts of 45Sc of the encapsulated metallic carbide cluster. Electrochemical Studies. The electrochemical properties of Sc2C2@D3h(14246)-C74 were investigated by cyclic voltammetry (CV). The overall redox pattern of Sc2C2@D3h(14246)C74 shows a typical redox behavior of clusterfullerenes, with
Figure 5. UV−vis−NIR absorption spectrum of Sc2C2@D3h(14246)C74 in a CS2 solution. The inset shows a photograph of Sc2C2@ D3h(14246)-C74 dissolved in CS2. C
DOI: 10.1021/acs.inorgchem.6b02512 Inorg. Chem. XXXX, XXX, XXX−XXX
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Inorganic Chemistry
assigned Sc2C76 to a novel carbide clusterfullerene, Sc2C2@ D3h(14246)-C74, which is the first clusterfullerene based on the C74 cage reported thus far. It was also revealed that the Sc−C2− Sc dihedral angle of the carbide cluster is 120.8°, smaller than those reported for Sc2C2@C2n (n = 40−44) with a larger cage size, probably indicating that the cluster is more compressed inside a smaller cage. Comparative studies of the motion of the carbide cluster in Sc2C2@D3h(14246)-C74 and Sc2C2@C2n (n = 40−44) revealed that a combination of factors, involving both the shape and size of the cage, is crucial in dictating the cluster motion. Moreover, structural studies revealed that D3h(14246)C74 can be converted to Cs(10528)-C72 or Td(19151)-C76 via a simple route involving a C2 desertion/insertion and SW transformation. This suggests that D3h(14246)-C74 might play an important role in the growth pathway of related clusterfullerenes.
multiple overlapping reduction processes and relatively separated oxidation processes (see Figure 7). The first
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Figure 7. Cyclic voltammograms of Sc2C2@D3h(14246)-C74 in (nBu4N)(PF6)/o-DCB with ferrocene as the internal standard. The scan rate was 100 mV s−1.
EXPERIMENTAL SECTION
Preparation and Purification of Sc2C2@D3h(14246)-C74. MCCFs were synthesized using a modified arc-discharge method.56−59 Graphite rods were packed with a 20:1 molar ratio of graphite powders and Sc2O3 powders. The graphite rods were first annealed in a tube furnace at 1000 °C for 10 h and then burned in the arcing chamber under 200 Torr of helium with 20 Torr of CO2 added. The soot was collected and refluxed in chlorobenzene under an argon atmosphere for 24 h and filtered through filter paper. After the solvent had been removed, the extracted fullerene was dissolved in toluene and filtered through a PTFE membrane. A multistage HPLC (LC-9230 II NEXT, Japan Analytical Industry Co., Ltd.) procedure was used to isolate and purify Sc2C2@D3h(14246)-C74. A combination of a 20 mm × 250 mm Buckyprep-M column (Cosmosil, Nacalai Tesque), a 10 mm × 250 mm Buckyprep column (Cosmosil, Nacalai Tesque), and a 10 mm × 250 mm 5PBB column (Cosmosil, Nacalai Tesque) was used for these processes. Toluene was used as the mobile phase at a flow rate of 4.0 mL/min. The UV detector was set to 310 nm for fullerene detection. The corresponding positive ion mode MALDITOF-MS (Ultraflextreme, Bruker) data for the isolated product are shown in Figure 1. Spectroscopic Studies of Sc2C2@D3h(14246)-C74. The UV− vis−NIR spectrum of Sc2C2@D3h(14246)-C74 in a CS2 solution was recorded on a Cary 5000 UV−vis−NIR spectrophotometer (Agilent). The 45Sc NMR spectroscopic measurements were taken at 145 MHz with an Agilent Direct-Drive II 600 MHz spectrometer at room temperature in CS2 with D2O as the lock and a 0.2 M Sc(NO3)3 solution in D2O as the reference. Electrochemical Studies of Sc2C2@D3h(14246)-C74. Cyclic voltammograms were measured in o-dichlorobenzene containing 0.05 M (n-Bu4N)(PF6) as the supporting electrolyte, using a glassy carbon disk as the working electrode, a silver wire as the reference electrode, and a platinum wire as the counter electrode with an electrochemical workstation (CHI-760E). The CV was measured at a scan rate of 100 mV/s. Black Cocrystals of Sc 2 C 2 @D 3h (14246)-C 74 ·[Ni II (OEP)]· 1.69C6H6·0.31CHCl3. These crystals were obtained by allowing the benzene solution of fullerene and the chloroform solution of
oxidation step is fully reversible. When sweeping to the second oxidation step, we found both oxidation processes turned out to be irreversible, suggesting an electrochemical−chemical (EC) process after the first oxidation step. The half-wave potentials of the first reduction and the first oxidation are −0.80 and +0.50 V, respectively, which result in an electrochemical gap of 1.30 eV. We compared the electrochemical behavior of Sc2C2@ D3h(14246)-C74 to those of Sc2C2@C2v(5)-C80, Sc2C2@Cs(6)C82, and Sc2C2@C3v(8)-C82. It is noteworthy that the reduction of Sc2C2@D3h(14246)-C74 comprises as many as six steps, much more complex than the cases of other reported analogous scandium carbide clusterfullerenes. Moreover, the first oxidation potential of Sc2C2@D3h(14246)-C74 (+0.50 V) is far more positive than those of others, suggesting a poorer electron donating property of Sc2C2@D3h(14246)-C74. On the other hand, the first reduction potential (−0.80 V) of Sc2C2@ D3h(14246)-C74 is more positive than those of Sc2C2@Cs(6)C82 and Sc2C2@C3v(8)-C82 and more negative than that of Sc2C2@C2v(5)-C80 (see Table 2). These results might indicate that the cage size and symmetry of scandium carbide clusterfullerenes demonstrated a dramatic influence on their electronic structures.
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CONCLUSIONS A systematic investigation of Sc2C76 was performed using mass spectrometry, UV−vis−NIR absorption spectroscopy, 45Sc NMR spectrometry, cyclic voltammetry, and single-crystal Xray diffraction. The crystallographic analysis unambiguously
Table 2. Redox Potentials (volts vs Fc/Fc+) of Sc2C2@D3h(14246)-C74, Sc2C2@C2v(5)-C80,40 Sc2C2@Cs(6)-C82,54 and Sc2C2@ C3v(8)-C8255 Obtained in (n-Bu4N)(PF6)/o-DCB with Ferrocene (Fc) as the Internal Standard compound Sc2C2@D3h(14246)-C74 Sc2C2@C2v(5)-C80 Sc2C2@Cs(6)-C82 Sc2C2@C3v(8)-C82
E2+/+ +1.00
b
+0.64a +0.93b
E+/0
E0/−
E−/2−
E2−/3−
E3−/4−
E4−/5−
E5−/6−
Egap,ec (V)
a
−0.80 (49) −0.74b −0.93a −0.94b
−1.19 (61) −1.33b −1.30a −1.15a
−1.58
−1.84
−2.18 (56)
−2.41
1.30 1.15 1.35 1.41
+0.50 (55) +0.41b +0.42a +0.47a
a
a
b
−1.60a
b
a
b
a Half-wave potential in volts (reversible redox process). The values in parentheses are the differences between the peak potentials and the half-wave potentials in millivolts. bPeak potential in volts (irreversible redox process).
D
DOI: 10.1021/acs.inorgchem.6b02512 Inorg. Chem. XXXX, XXX, XXX−XXX
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Inorganic Chemistry [NiII(OEP)] to diffuse together. X-ray data were collected at 120 K using a diffractometer (APEX II, Bruker Analytik GmbH) equipped with a CCD collector. A multiscan method was used for absorption correction. The structure was resolved using direct methods (SIR2004)60 and refined on F2 using full-matrix least squares using SHELXL201361 within the WinGX package.62 Hydrogen atoms were inserted at calculated positions and constrained with isotropic thermal parameters. The asymmetric unit consists of a Sc 2 C 2 @D 3h (14246)-C 74 molecule, a [NiII(OEP)] molecule, and disordered solvent molecules. The [NiII(OEP)] molecule is fully ordered, while disorder is observed in both the fullerene cage and the endohedral carbide cluster. All cage carbon atoms were anisotropic, and the SIMU constraint was applied. Distance restraints corresponding to D3h(14246)-C74 symmetry were applied with SADI and DFIX commands. The sum of the cage occupancies is fixed to 1.0. However, the occupancies of disordered Sc sites are freely refined. The sum of the occupancies of these Sc sites is
