Sc₂S@C₂(7892)–C₇₀: a metallic sulfide cluster inside a non-IPR C₇₀ cage

Abstract

A new cage isomer of C₇₀, Sc₂S@C₂(7892)–C₇₀, has been isolated and characterized by mass spectrometry, UV-Vis-NIR absorption spectroscopy, cyclic voltammetry and DFT calculations. The combined experimental and computational studies lead to the unambiguous assignment of the cage symmetry to C₂(7892)–C₇₀. The comparison between Sc₂S@C₂(7892)–C₇₀ and related endohedral structures has been discussed. A close structural resemblance between Sc₂S@C₂(7892)–C₇₀ and Sc₂S@C_s(10 528)–C₇₂ suggests that the conversion of these two molecules may be the result of a simple insertion of C₂ and the structural difference between Sc₂S@C₂(7892)–C₇₀ and Sc₃N@C_2v(7854)–C₇₀ shows that the nature and geometry of the encaged cluster plays an important role on the selection of the non-IPR cage.

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Introduction

Fullerenes are spherical molecules composed of pentagons and hexagons.^1,2 Endohedral fullerenes, which possess outer surface available for chemical modification and inner space capable of encapsulation of variable clusters and molecules, have generated considerable recent research interest.^3,4 As a special class of endohedral fullerenes, clusterfullerenes (CFs) are known for their exceptionally high yield and fascinating structural versatility of the encapsulated clusters and carbon cages.^5–7 After the discovery of the first clusterfullerene in 1999, Sc₃N@I_h–C₈₀,⁸ nitride cluster fullerenes (NCFs),⁹ carbide cluster fullerenes (CCFs),^10,11 oxide cluster fullerenes (OCFs)^12–14 and sulfide cluster fullerenes (SCFs)^15–17 were discovered and the encapsulated clusters were found to have a significant influence on the cage structure and their chemical reactivities.¹⁸ With the remarkable versatility to alter their physical and chemical properties based on the structural variety of both cages and clusters, these molecules are attracting widespread interest in applications such as MRI agents and in molecular electronic devices and solar cells.^19–27

Sulfide cluster fullerenes (SCFs) are the newest member in the CF families.^15,16 Different methods have been utilized to synthesize these compounds. Dunsch and co-workers reported the formation of one isomer of M₂S@C₈₂ (M = Sc, Lu, Dy) by introducing CH₅N₃·HSCN as a solid sulfur source.¹⁵ We demonstrated that an extensive family of novel scandium sulfide cluster fullerenes with cages ranging from C₆₈ to C₁₀₀ can be obtained in macroscopic quantities by introducing SO₂ into the arc reactor.¹⁶ Further isolation and characterization of some of these new species revealed novel structures and properties which are unique to this family. Two isomers of Sc₂S@C₈₂, Sc₂S@C_s(6)–C₈₂ and Sc₂S@C_3v(8)–C₈₂, were identified as the most abundant products in this family and were recently characterized by single crystal X-ray crystallography.¹⁷ Sc₂S@C_s(6)–C₈₂, in particular, was found to possess completely ordered cage and cluster in its single crystal structure, the first in the clusterfullerene series.¹⁷ Very recently, we reported the isolation of a metallic clusterfullerene in a non-IPR (Isolated Pentagon Rule) C₇₂ cage, Sc₂S@C₇₂. Crystallography clearly established that the C₇₂ cage has an unusual C_s(10 528) symmetry.²⁸ A combined computational and experimental study revealed that the unique geometry of the Sc₂S cluster together with a formal charge transfer of four electrons between the cluster and the cage play an important role in the stabilization of this new cage which had never been detected experimentally before in any of the CF families. These results indicate that more novel endohedral structures are likely to be found in the SCF family and systematic studies of these structures might provide a better understanding of the fundamental aspects of SCFs, and useful design principles to guide the preparation of high yielding endohedral compounds with unique and useful optoelectronic properties.

Herein we report a new sulfide cluster fullerene, Sc₂S@C₂(7892)–C₇₀, in which the sulfide cluster is encapsulated inside a non-IPR C₇₀ cage. This represents the third isomer of a C₇₀ cage discovered to date. Combined computational and experimental studies lead to the unambiguous assignment of the cage symmetry to C₂(7892)–C₇₀.

Results and discussion

Preparation and characterization of Sc₂S@C₇₀

The SCFs were synthesized in a conventional Krätschmer–Huffman reactor using an atmosphere of helium and SO₂.^16,29 The as-produced soot was Soxhlet-extracted with CS₂ and a multi stage high-performance-liquid chromatography (HPLC) procedure was utilized to isolate and purify Sc₂S@C₇₀.

Sc₂S@C₇₀ is the second smallest fullerene in the SCF family after Sc₂S@C₆₈. The HPLC-MALDI TOF analyses show that on a 5PYE column, the Sc₂S@C₇₀ fraction overlaps with those of C₇₆, C₇₈ and Sc₂S@C₇₂ (see Fig. 1). The retention time of Sc₂S@C₇₀ is shorter than that of Sc₂S@C₇₂, which agrees with the relatively smaller size of this compound. This fraction was further separated by a two-stage recycling HPLC procedure using a Buckyprep column, that resulted in the isolation of pure Sc₂S@C₇₀ (see ESI†). However, compared to the previously reported Sc₂S@C_s(10 528)–C₇₂, a very similar non-IPR SCF in a small cage, the yield of this compound is much lower than that of Sc₂S@C_s(10 528)–C₇₂. Out of the arcing processes of 60 packed graphite rods, along with 2.0 mg of Sc₂S@C_s(10 528)–C₇₂, only 0.4 mg of Sc₂S@C₇₀ were obtained after HPLC purification.

Fig. 1 HPLC Chromatograms of the fullerene extract obtained on a 10 mm × 250 mm 5PYE column using λ = 320 nm, a flow rate of 4 mL min⁻¹, and toluene as the eluent at 25 °C. Inset: HPLC of purified Sc₂S@C₇₀ obtained on a 10 mm × 250 mm 5PYE column using λ = 320 nm, a flow rate of 4 mL min⁻¹, and toluene as the eluent at 25 °C.

The MALDI-TOF spectrum (Fig. 2) of the isolated fraction shows a single peak at 961.904. The isotopic distribution of the experimental MALDI spectrum shows excellent agreement with the corresponding theoretical spectrum (see Fig. 2). The purity of this sample was further checked by HPLC as shown in Fig. 1.

Fig. 2 Mass spectrum of the purified Sc₂S@C₇₀. Inset: the experimental and theoretical isotopic distribution for Sc₂S@C₇₀.

The purified Sc₂S@C₇₀ has a yellow-brown color in toluene solution. The UV-Vis-NIR absorption of Sc₂S@C₇₀ is shown in Fig. 3. A relatively strong absorption occurs at 1102 nm along with other absorptions at 461, 634, 699 and 952 nm. The UV-Vis-NIR absorption spectrum of Sc₂S@C₇₀ shows some similarities to that of Sc₂S@C_s(10 528)–C₇₂, which also features a strong absorption at 1076 nm around the absorption onset region, but is rather featureless in other spectral regions. The characteristic features of this spectrum are also substantially different from those of empty C₇₀ and from those of the previously reported Sc₃N@C_2v(7854)–C₇₀, which presents the strongest absorption at 696 nm along with several shoulder peaks at 468, 558, 807 and 894 nm.³⁰ Since the absorption spectra of fullerenes in the visible and NIR region are dominated by the π–π* transitions of the carbon cages and the spectra are very sensitive to the carbon cage symmetries,³ these differences clearly indicate that Sc₂S@C₇₀ has a non-IPR C₇₀ cage and the cage symmetry is likely different from that of Sc₃N@C_2v(7854)–C₇₀.

Fig. 3 UV-Vis-NIR absorption of Sc₂S@C₇₀ in CS₂ solution.

The cyclic voltammogram (CV) of Sc₂S@C₂(7892)–C₇₀ was recorded in o-dichlorobenzene (o-DCB) containing 0.05 M tetra(n-butyl)ammonium-hexafluorophosphate, (n-Bu₄NPF₆), as the supporting electrolyte using a scan rate of 100 mV s⁻¹ (Fig. 4). The CV of Sc₂S@C₂(7892)–C₇₀ shows some similarities and differences when compared to those of the other reported SCFs. The CV shows a reversible first oxidation followed by an irreversible second oxidation step, which is very similar to the oxidative behavior of Sc₂S@C_s(10 528)–C₇₂,²³ but different from those of the two isomers of Sc₂S@C₈₂, which exhibit two reversible oxidative steps.¹⁶ On the other hand, while Sc₂S@C_s(10 528)–C₇₂ shows all-reversible reductive processes, the reductive behavior of Sc₂S@C₂(7892)–C₇₀ shows similarities to those of the two isomers of Sc₂S@C₈₂ as well as to most of the cluster fullerenes, which typically exhibit irreversible reductive processes.³¹

Fig. 4 Cyclic voltammograms of Sc₂S@C_s(7892)–C₇₀ in n-Bu₄NPF₆/o-DCB with ferrocene as the internal standard; scan rate 100 mV s⁻¹.

The redox potentials of Sc₂S@C₂(7892)–C₇₀ also show major differences from those of other reported SCFs. The first oxidation potential is shifted dramatically from 0.64 V for Sc₂S@C_s(10 528)–C₇₂, to 0.14 V for Sc₂S@C₂(7892)–C₇₀ (see Table 2).²⁸ This shows that Sc₂S@C₂(7892)–C₇₀ is much easier to oxidize even though the cage sizes and topologies are very similar (see below). The electrochemical gap of Sc₂S@C₂(7892)–C₇₀ is 1.57 V, which is smaller than that of Sc₂S@C_s(10 528)–C₇₂ (1.78 V) but somewhat larger than those of the corresponding Sc₂S@C₈₂ isomers (1.56 eV for Sc₂S@C_3v(8)–C₈₂ and 1.47 eV for Sc₂S@C_s(6)–C₈₂).

Computational studies

Besides the experimental characterization of Sc₂S@C₇₀, we also performed an exhaustive computational study to assign the C₇₀ cage symmetry among the 8149 possible isomers.§ From the electronic structure of the different computed isomers of Sc₂S@C₇₀, we verified that there is a formal transfer of four electrons, (Sc₂S)⁴⁺@(C₇₀)⁴⁻, as for the other Sc₂S@C_2n (2n = 72 and 82) known so far (see orbital interaction diagram, Fig. 5).^32,33 We have computed the energies of the tetraanions using DFT at BP86/TZP level for all 111 C₇₀ cages with three or less adjacent pentagon pairs (APP): 1 IPR structure, 1 APP1, 18 APP2 and 91 APP3 (see ESI†). The SCF corresponding to the lowest-energy tetraanionic cages were also computed (a total of 17 isomers, Table 1).

Fig. 5 Orbital interaction diagram for Sc₂S@C₂(7892)–C₇₀. The fragments, Sc₂S and C₂(7892)–C₇₀, were calculated with the same geometry they have in the clusterfullerene.

Table 1 Relative energies (in kcal mol⁻¹) for several isomers of C₇₀ in the tetraanion and endohedral forms^a,b

Isomer	APP	C70^4−	Sc2S@C70
a Isomer number according to the spiral algorithm of Fowler and Manolopoulos.^35b APP: number of adjacent pentagon pairs.
8149	0	0.0	20.6
7892	2	3.9	0.0
7957	2	4.7	19.1
7851	3	6.2	20.3
7852	3	6.3	23.5
7887	3	7.0	21.6
7854	3	8.5	26.3
7893	3	9.4	19.7
7886	3	10.8	21.5
7924	2	11.6	18.6
7846	3	13.2	29.3
7960	2	14.1	21.9
7922	3	14.2	27.7
7921	3	14.7	29.1
7850	3	14.7	33.2
7847	3	15.6	28.8
8094	1	16.7	26.5

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Interestingly, the IPR cage, D_5h–C₇₀(8149), was found to be the lowest-energy tetraanion. Other cage isomers with two and three APPs show relative energies within 10 kcal mol⁻¹. In particular, cage C₂(7892)–C₇₀ exhibits the second most-stable tetraanion and is only 4 kcal mol⁻¹ higher in energy than the IPR cage tetraanion (see Table 1). Among the 18 isomers with two pairs of fused pentagons, cage C₂(7892)–C₇₀ shows the lowest number of pyracylene motifs and maximally separated pentagonal units (see ESI†).³⁴

To predict the stability of a given clusterfullerene, it is necessary to consider electron transfer from the cluster to the cage but also the stability provided by the interaction between the metal atoms of the trapped cluster and the carbon cage. These interactions are not so critical for IPR cages; however, the cluster–cage interactions are crucial for those CFs that have one or more APPs. The systems with non-IPR cages are stabilized by the proximity of the metal cations of the cluster to the pentalene motifs, as in, for example, Sc₃N@D₃(6140)–C₆₈,³⁶ Gd₃N@C_s(39 663)–C₈₂,³⁷ M₃N@C_s(51 365)–C₈₄ (M = Gd, Tm)³⁸ and the recently reported Sc₂S@C_s(10 528)–C₇₂.²⁸ Thus, appropriate localization of the APPs on the fullerene cage that provide optimized interactions between the metal cations and the pentalene motifs is a key factor in the stabilization of non-IPR clusterfullerenes. In the present case, which resembles that of Sc₂S@C_s(10 528)–C₇₂,²⁸ it is only after the encapsulation of Sc₂S that the 7892–C₇₀ cage becomes by far (∼20 kcal mol⁻¹) the most stable endohedral isomer (Table 1). The most characteristic structural parameters, 2.352 Å for the Sc–S distance and 97.8° for the Sc–S–Sc angle, resemble those previously found for the two Sc₂S@C₈₂ isomers¹⁹ and for Sc₂S@C_s(10 528)–C₇₂.²⁸ In particular, the value of the Sc–S–Sc angle is smaller than those for Sc₂S@C_s(6)–C₈₂ (114°) and Sc₂S@C_s(10 528)–C₇₂ (124°), but similar to that found for Sc₂S@C_3v(8)–C₈₂ (97°). The IPR Sc₂S@D_5h(8149)–C₇₀ is destabilized due to the lack of the metal cation–pentalene interactions. The other non-IPR cages do not possess optimal positioning of the APPs to maximize the metal–pentalene interactions as observed for Sc₂S@C₂(7892)–C₇₀ (see Fig. 6).

Fig. 6 DFT-optimized structure for Sc₂S@C₂(7892)–C₇₀.

The molar fractions of the lowest-energy Sc₂S@C₇₀ isomers as a function of the temperature were also computed using the rigid rotor and harmonic oscillator (RRHO) approximation and the related free-encapsulating model (FEM) proposed by Slanina (see Fig. 7).^39,40 Somewhat different results are obtained for the FEM and RRHO approximations. For the FEM, the SCF with cage 7892 is the most abundant isomer in the whole temperature range (to 4000 K). Within the RRHO approximation, there is an isomer preference crossing at around 1700 K, with the IPR isomer predominating at higher temperatures. So far, the predictions derived from the FEM model agree better with experiments than the ones made using the RRHO approximation. In the FEM model we consider that if at high temperature the cluster is rotating freely inside the carbon cages, its contribution to the partition function will be similar for the different cages and will cancel out. In the present case, the Sc₂S will probably rotate inside the spherical IPR cage at sufficiently high temperatures, but it is not so clear that the Sc₂S cluster will freely rotate inside the non-IPR APP2 carbon cage 7892 since the interaction of the Sc ions with the pentalene motifs is significant and there is not a lot of space to rotate inside this cage. The FEM approximation would be the most suitable for the IPR clusterfullerene, but it is not so clear that it would also be the best for the non-IPR 7892 cage, so reality probably lies somewhere between these two approximations. Consequently, we predict Sc₂S@C₂(7892)–C₇₀ to be the most abundant isomer produced within the temperature range present in the arc.

Fig. 7 Predicted molar fractions within the RRHO (top) and FEM (bottom) models as a function of temperature for ten different isomers of Sc₂S@C₇₀.

Good agreement of the computed first and second oxidation and reduction potentials of Sc₂S@C₂(7892)–C₇₀ with the peaks measured experimentally by cyclic voltammograms reaffirms the assignment, see Table 2. The first anodic potential is predicted to appear at +0.04 V, in good agreement with the experimental half-wave potential (+0.14 V).⁴¹ The predicted first cathodic potential is also close to the experimental value with a difference of around 100 mV (Table 2). The computed electrochemical (EC) gap, 1.37 V, compares reasonably well with experiment, 1.58 V, although the error (210 mV) is somewhat larger than for nitride clusterfullerenes.⁴¹ Inclusion of thermal and entropic effects does not improve the computed gap, as for the family of nitrides.⁴¹ It is worth noting that, for irreversible processes, the comparison between experimental and theoretical electrochemical gaps is difficult, because computations predict standard potentials, and not experimental peak potentials. The predictions for the second anodic and cathodic potentials are in excellent agreement with experiment (with an error of only 50 mV). The HOMO–LUMO gap for the IPR clusterfullerene (0.56 eV) is much smaller than for the 7892 cage (0.95 eV). We have computed the first oxidation potential for Sc₂S@D_5h(8149)–C₇₀ and have found that it is more easily oxidized than Sc₂S@C₂(7892)–C₇₀ (−0.16 V vs. +0.04 V, respectively). We have had problems computing the first reduction potential for Sc₂S@D_5h(8149)–C₇₀ because the LUMO and LUMO+1 orbitals are almost degenerate and the DFT mono-determinant approach is not suitable to describe such a reduced state (multi-determinant wavefunctions would be needed, but the computational cost is too high for such large systems). According to the energy of the LUMOs (Sc₂S@D_5h(8149)–C₇₀: −4.04 eV; Sc₂S@C₂(7892)–C₇₀: −3.84 eV) the first reduction potential for Sc₂S@D_5h(8149)–C₇₀ should be lower (around 200 mV) than for Sc₂S@C₂(7892)–C₇₀, leading to a much smaller EC gap (around 1 eV or even smaller). So, the comparison between computed and experimental electrochemical properties discards Sc₂S@D_5h(8149)–C₇₀ and suggests Sc₂S@C₂(7892)–C₇₀ as the isomer formed in the arc.

Table 2 Experimental oxidation and reduction potentials (in V versus Fc⁺/Fc) and computed oxidation and reduction potentials (in V) for Sc₂S@C₂(7892)–C₇₀

	O2	O1	R1	R2	R3	R4	EC gap
a Half wave potential. The values in parentheses are the differences between the peak potentials and the half-wave potentials in millivolts. Scan rate, 100 mV s^−1.b Peak potentials.
Exp.	0.65^b	0.14(70)^a	−1.44^b	−1.87^b	−1.99^b	−2.45^b	1.58
Comp.	0.60	0.04	−1.33	−1.82			1.37

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The definitive experiment that confirms isomer 7892 as the carbon cage that encapsulates Sc₂S is the UV-Vis-NIR spectrum. We have computed the spectra using time-dependent (TD) DFT for the two possible SCFs, Sc₂S@D_5h(8149)–C₇₀ and Sc₂S@C₂(7892)–C₇₀, in the Vis-NIR region, for wavelengths larger than 500 nm. Despite the systematic underestimation of excitation energies by TD-DFT, this methodology provides a reasonable agreement with experiments for the family of SCFs.²⁸ From the computations we can clearly discard Sc₂S@D_5h(8149)–C₇₀ because its spectrum shows transitions at large wavelengths (around 1800 and 1600 nm, see Table 3 and ESI†), in total contrast with experimental observations. The spectral onset appears at 1224 nm, see Fig. 3. In addition, the spectrum predicted for Sc₂S@C₂(7892)–C₇₀ agrees rather well with the experimental one, with the first transition (corresponding to the HOMO–LUMO transition) at 1158 nm (to be compared with the experimental 1102 nm). Therefore, we feel confident that the isolated Sc₂S@C₇₀ clusterfullerene corresponds to Sc₂S@C₂(7892)–C₇₀.

Table 3 TDDFT predictions for the most intense lowest-energy excitations in the absorption spectrum of Sc₂S@C₂(7892)–C₇₀ and Sc₂S@D_5h(8149)–C₇₀

E (eV)	λ (nm)	f^a	Leading configurations^b (%)
a Only excitations with f (oscillator strength) > 0.001 are listed.b Contributions less than 10% are omitted.
Sc2S@C2(7892)–C70
1.071	1158	0.00650	HOMO → LUMO (98)
1.212	1023	0.00356	HOMO → LUMO+1 (98)
1.664	745	0.00104	HOMO → LUMO+3 (99)
1.681	738	0.00519	HOMO → LUMO+4 (96)
1.853	669	0.00721	HOMO-1 → LUMO (89)
Sc2S@D5h(8149)–C70
0.688	1802	0.00365	HOMO → LUMO (63); HOMO → LUMO+1 (36)
0.763	1625	0.00103	HOMO-1 → LUMO+1 (63); HOMO-1 → LUMO (26)
0.802	1546	0.00715	HOMO-1 → LUMO (71); HOMO-1 → LUMO+1 (28)
0.980	1265	0.00138	HOMO → LUMO+2 (50); HOMO-1 → LUMO+3 (47)
1.295	958	0.05558	HOMO-1 → LUMO+3 (46); HOMO → LUMO+2 (35)

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At this point, it is worth remarking that C₂(7892)–C₇₀, the proposed cage for Sc₂S@C₇₀, and C_s(10 528)–C₇₂, the one found for Sc₂S@C₇₂, are intimately related as the Schlegel diagrams in Fig. 8 show. In fact, cage C_s(10 528)–C₇₂ can be obtained by a single C₂ addition to a hexagon of C₂(7892)–C₇₀ (see ESI†). Therefore, conversion of these two SFCs may happen by single addition/extrusion of a C₂ molecule without further atomic rearrangements. Moreover, it is possible that Sc₂S@C_s(10 528)–C₇₂ could form by C₂ addition to Sc₂S@C₂(7892)–C₇₀ through the recently proposed closed network growth (CNG) mechanism.^42,43

Fig. 8 Schlegel diagrams for cages C₂(7892)–C₇₀ and C_s(10 528)–C₇₂. The difference between the two cages is highlighted in black with the added C₂ unit in red. The larger distance between the two APPs in cage C_s(10 528)–C₇₂ compared to that for C₂(7892)–C₇₀ makes the Sc–S–Sc angle in Sc₂S@C_s(10 528)–C₇₂ (124°) larger than that found for Sc₂S@C₂(7892)–C₇₀ (98°).

Beside Sc₂S@C₂(7892)–C₇₀, only one non-IPR C₇₀ endohedral fullerene, Sc₃N@C_2v(7854)–C₇₀, has been reported so far. Very recently, Nagase, Zhao and co-workers proposed, using density functional theory, that Sc₂@C_2v(7854)–C₇₀ and not the corresponding carbide, Sc₂C₂@C_2v(6073)–C₆₈, is the metallofullerene detected as Sc₂C₇₀.⁴⁴ These two isomers, C₂(7892)–C₇₀ and C_2v(7854)–C₇₀, are rather close in energy when computed as tetraanions (Table 4). On the other hand, the hexaanionic state is much more favored for C_2v(7854)–C₇₀. If the trapped cluster is Sc₃N, a six electron transfer combined with the optimal interaction between the three Sc atoms and the three pentalenes on cage C_2v(7854)–C₇₀ (Fig. 9) lead to the preferential stabilization of Sc₃N@C_2v(7854)–C₇₀, as proposed by Yang et al.³⁰ (Table 4). However, with the Sc₂S cluster inside the Sc–pentalene interactions are optimized for Sc₂S@C₂(7892)–C₇₀, which has, by far, the lowest energy (Tables 1 and 4). Although sharing the same cage size, the number and the position of the pentalene motifs are significantly different in these two isomers. This shows that the nature and geometry of the encaged cluster plays an important role on the selection of the cage isomer, along with the electronic stabilization resulting from cluster–cage electron transfer.

Table 4 Relative energies (in kcal mol⁻¹) for cages C₂(7892)–C₇₀ and C_2v(7854)–C₇₀ in different anionic states and encapsulating different clusters

	C2(7892)–C70	C2v(7854)–C70
a The stabilization of cage C2v(7854)–C70vs. C2(7892)–C70 (10.5 kcal mol^−1) is smaller than for the corresponding hexaanions (16.2 kcal mol^−1) due to larger structural distortions of the Sc3N cluster inside the C2v(7854)–C70 cage (see Fig. 9 and ESI†).
C70^4−	0.0	4.6
C70^6−	16.2	0.0
Sc2S@C70	0.0	26.3
Sc3N@C70^a	10.5	0.0

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Fig. 9 DFT-optimized structures for Sc₃N@C_2v(7854)–C₇₀ and Sc₂S@C_2v(7854)–C₇₀.

Conclusions

In summary, a new isomer of C₇₀, Sc₂S@C₂(7892)–C₇₀, has been isolated and characterized by mass spectrometry, UV-Vis-NIR absorption spectroscopy, cyclic voltammetry and DFT calculations. The combined experimental and computational studies led to the unambiguous assignment of the cage symmetry to C₂(7892)–C₇₀, the second non-IPR isomer of C₇₀ to be detected experimentally. Sc₂S@C₂(7892)–C₇₀ is by far the lowest-energy isomer and the major candidate to be the experimentally detected Sc₂S@C₇₀. At high temperatures, isomer Sc₂S@D_5h(8149)–C₇₀ could also be formed, but comparison of the experimental and theoretical electrochemical properties and UV-Vis-NIR spectra clearly discards the IPR Sc₂S@D_5h(8149)–C₇₀. Cage C₂(7892)–C₇₀ is structurally intimately related to cage C_s(10 528)–C₇₂, the one that has been recently detected by single crystal X-ray crystallography for Sc₂S@C₇₂. Thus, besides Sc₂S@C_s(10 528)–C₇₂, Sc₂S@C₂(7892)–C₇₀ provides an additional example of stabilization of a new non-IPR cage resulting from the combined effect of a four-electron transfer and the unique geometrical fit between the Sc₂S cluster and the cage. Many more structures and interesting properties are yet to be explored in the SCF family.

Acknowledgements

We thank the Robert A. Welch Foundation for an endowed chair, grant #AH-0033 and the US NSF for grant CHE-1110967, which provided generous support for this work. This work is also supported by the Spanish Ministry of Science and Innovation (Project no. CTQ2011-29054-C02-01) and by the Generalitat de Catalunya (2009SGR462 and XRQTC). Additional support from the US NSF, grant CHE-1124075 and from the Spanish Ministry of Science and Innovation (PRI-PIBUS-2011-0995) for a joint US-Spain collaboration is also acknowledged.

Notes and references

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Footnotes

† Electronic supplementary information (ESI) available: HPLC chromatograms and computational results (relative energies and plots of the structures for all the isomers computed in this work, schematic representation showing the relationship between C₂(7892)–C₇₀ and C_s(10 528)–C₇₂ cages, and xyz coordinates for the lowest-energy computed isomers). See DOI: 10.1039/c2sc21101g

‡ Current address: College of Chemistry, Chemical Engineering and Materials Science, Soochow University, Suzhou, Jiangsu, 212163, China. Email : E-mail: chenning@suda.edu.cn

§ The calculations were carried out by using DFT methodology with the ADF 2010 package.^45,46 The exchange-correlation functionals of Becke and Perdew were used.^47,48 Relativistic corrections were included by means of the ZORA formalism. Slater triple-zeta polarization basis sets were employed to describe the valence electrons of C, S and Sc. Frozen cores consisting of the 1s shell for C and the 1s and 2p shells for S and Sc were described by means of single Slater functions. The computational study of the UV-Vis-NIR spectra has been performed using TDDFT methodology (also at BP86/TZP level).

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