Unexpected and divergent mechanosynthesis of furanoid-bridged fullerene dimers C₁₂₀O and C₁₂₀O₂

Abstract

An unexpected, divergent and efficient approach toward furanoid-bridged fullerene dimers C₁₂₀O and C₁₂₀O₂ was established under different solvent-free ball-milling conditions by simply using pristine C₆₀ as the starting material, water as the oxygen source and FeCl₃ as the mediator. The structures of C₁₂₀O and C₁₂₀O₂ were unambiguously established by single-crystal X-ray crystallography. A plausible reaction mechanism is proposed on the basis of control experiments. Furthermore, C₁₂₀O₂ has been applied in organic solar cells as the third component and exhibits good performance.

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Over the past three decades, [60]fullerene (C₆₀) and its derivatives have attracted significant attention due to their remarkable applications in numerous fields of materials science, biological applications and nanotechnology.¹ Fullerene dimers, including C₁₂₀, C₁₂₀O and C₁₂₀O₂, are molecular structures linking two fullerene skeletons directly or via bridge(s) and are the fundamental subunits of fullerene polymers. Fullerene dimers exhibit many unique properties and thus have enormous potential for various applications.²

The selective formation of the dumbbell-shaped dimer C₁₂₀ under solvent-free and mechanical milling conditions was first disclosed in 1997 by Wang et al.^3a Since then, more solvent-free mechanochemical methods have been reported to generate fullerene dimer C₁₂₀via different promoters, such as inorganic salts, organic compounds and alkali metals.^3b,c Later, the FeCl₃-mediated solution-phase synthesis of C₁₂₀ was reported, providing a new approach for the synthesis of fullerene dimers (Scheme 1a).⁴ To date, there are few methods for synthesizing furanoid-bridged dimers C₁₂₀O and C₁₂₀O₂, and their syntheses are generally limited by the use of preprepared fullerene derivatives as the starting material, high temperatures, long reaction times and inert atmospheres.^5,6 C₁₂₀O with one furan bridge was previously prepared by heating a mixture of C₆₀O and 5–6 equiv. of C₆₀ in the solid state at 200 °C for 1 h (19–20% yield)^5a or in 1,2-dichlorobenzene (1,2-C₆H₄Cl₂) at 180 °C for 3 days (26% yield).^5b C₁₂₀O₂ with cages bis-linked by adjacent furanoid bridges was formed in ca. 15% yield by heating solid C₁₂₀O to 400 °C for 1 h in an argon atmosphere.⁶ C₆₀O was alternatively prepared by photooxygenation,^7a oxidation with dimethyldioxirane,^7bm-chloroperoxybenzoic acid,^7c cytochrome P450 chemical models,^7d methyltrioxorhenium–hydrogen peroxide^7e or methyl (trifluoromethyl)dioxirane^7f in yields ranging from 4% to 35%. Therefore, the overall yields of furanoid-bridged C₁₂₀O and C₁₂₀O₂ starting from C₆₀ are estimated to be only 0.8–9% and 0.1–1%, respectively. The identities of both C₁₂₀O and C₁₂₀O₂ were deduced from their spectral data and theoretical calculations,^5,6,8 but confirmation by single-crystal X-ray structures is lacking. The harsh reaction conditions and use of preprepared C₆₀O and C₁₂₀O as the starting materials in these methodologies encounter obvious limitations and prevent their practical applications. Therefore, a more straightforward and efficient approach for accessing C₁₂₀O and C₁₂₀O₂, particularly from pristine C₆₀, is highly desirable.

Scheme 1 Strategies for the synthesis of fullerene dimers.

Mechanochemistry has gained increasing interest in recent years. Apart from making various chemical reactions possible under solvent-free conditions, mechanochemical reactions feature many unique advantages, such as shorter reaction times, cleaner and safer reaction conditions, lower energy consumption, higher product yields and even different product selectivities.⁹ C₆₀ is barely soluble in common organic solvents, and its poor solubility somewhat restricts the exploration of its chemical reactions. Thus, mechanochemistry has emerged as an attractive alternative to conventional solution-based reactions in the field of fullerene chemistry.¹⁰

Due to the potential application of fullerene dimers and our continuous interest in fullerene mechanochemistry,^10,11 herein we disclose the unexpected, divergent and straightforward mechanosynthesis of furanoid-bridged fullerene dimers C₁₂₀O and C₁₂₀O₂ directly from easily available pristine C₆₀ (Scheme 1b). This highly efficient mechanochemical method will provide more possibilities for practical application of these fullerene dimers and new avenues for other bridged fullerene dimers.

In attempts to mechanochemically synthesize C₁₂₀ from C₆₀ and FeCl₃, it was intriguing to discover that the addition of H₂O could alter the reaction product from C₁₂₀ to furanoid-bridged fullerene dimers. Therefore, the mechanochemical reaction of C₆₀ with FeCl₃ and H₂O was systematically investigated. The results of the reaction optimizations for the furanoid-bridged dimer C₁₂₀O are shown in Table 1. Initially, C₆₀ (0.05 mmol), 6 equiv. of FeCl₃ and 15 equiv. of H₂O were added into a stainless steel jar (5 mL) together with 4 stainless steel balls (5 mm in diameter) under solvent-free and ambient conditions and milled vigorously at 1800 cycles per minute (30 Hz) in a GT 300 mixer mill at room temperature for 90 min. The reaction mixture was dissolved in 1,2-C₆H₄Cl₂ and monitored by high-performance liquid chromatography (HPLC) on a Cosmosil Buckyprep-D column with toluene as the mobile phase. It was found that the reaction mixture consisted of the furanoid-bridged dimer C₁₂₀O, dumbbell-shaped dimer C₁₂₀ and unreacted C₆₀. The percentage of each component was calculated by quantitative analysis based on HPLC peak areas. The yield of C₁₂₀O was thus determined to be 30%, in addition to the 33% yield of C₁₂₀ (Table 1, entry 1). By increasing the amount of FeCl₃ to 7 equiv., the yield of C₁₂₀O was enhanced to 33% (Table 1, entry 2), yet further increasing the amount of FeCl₃ to 8 equiv. resulted in a substantially decreased yield of 20% due to the formation of C₁₂₀O₂ (Table 1, entry 3). In addition, no benefit to the product yield could be achieved by shortening or prolonging the reaction time (Table 1, entries 4 and 5). When the reaction was attempted in the absence of H₂O, only trace amounts of C₁₂₀ and C₁₂₀O were detected, highlighting the pivotal role of H₂O in this mechanochemical reaction (Table 1, entry 6). Increasing or reducing the amount of H₂O resulted in a slightly lower yield (Table 1, entries 7 and 8 vs. entry 2). Furthermore, the product yield of C₁₂₀O was lowered as the milling frequency was increased or decreased (Table 1, entries 9 and 10). On the basis of the results presented above and the isolated yield of C₁₂₀O for each entry, the optimized conditions to afford C₁₂₀O were determined as shown in entry 2 of Table 1: C₆₀ (0.05 mmol), FeCl₃ (0.35 mmol), H₂O (0.75 mmol), a milling frequency of 30 Hz and a milling time of 90 min. The HPLC chromatogram on a Cosmosil Buckyprep-D (10 × 250 mm) column with toluene as the eluent at a flow rate of 1 mL min⁻¹ and the detector wavelength at 326 nm is shown in Fig. 1. The retention times of C₆₀, C₁₂₀, C₁₂₀O and C₁₂₀O₂ were 10.6 min, 16.1 min, 17.3 min and 20.0 min, respectively. The isolated yield of C₁₂₀O by recycling HPLC on a Cosmosil Buckyprep-D (10 × 250 mm) column was 25%, which was significantly higher than the 0.8–9% for the two-step procedure starting from C₆₀ (vide supra).

Table 1 Optimization of the reaction conditions to afford C₁₂₀O^a

Entry	Ratio^b	Yield of C120O^c (%)	Yield of C120^c (%)	Recovered C60^c (%)
a Unless otherwise noted, all reactions were performed with 0.05 mmol of C60, FeCl3, and H2O together with 4 stainless steel balls (5 mm in diameter) in a stainless steel jar (5 mL) and milled vigorously (30 Hz) at room temperature for 90 min. b The molar ratio refers to C60/FeCl3/H2O. c Based on HPLC area ratios on a Cosmosil Buckyprep-D (10 × 250 mm) column. d The reaction time was 60 min. e The reaction time was 120 min. f The reaction frequency was 25 Hz. g The reaction frequency was 35 Hz.
1	1 : 6 : 15	30	33	37
2	1 : 7 : 15	33	27	33
3	1 : 8 : 15	20	11	28
4^d	1 : 7 : 15	19	33	48
5^e	1 : 7 : 15	29	16	32
6	1 : 7 : 0	Trace	Trace	36
7	1 : 7 : 20	28	34	34
8	1 : 7 : 10	30	26	43
9^f	1 : 7 : 15	20	30	50
10^g	1 : 7 : 15	26	38	29

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Fig. 1 HPLC chromatogram for the synthesis of C₁₂₀O from C₆₀, H₂O and FeCl₃ under optimal ball-milling conditions.

During the optimization process for the synthesis of the furanoid-bridged C₁₂₀O, it was found that increasing the amount of FeCl₃ as well as the milling frequency would favour the formation of the fullerene dimer C₁₂₀O₂ with bis-linked furanoid bridges. Therefore, we further modified our reaction conditions with the expectation of obtaining C₁₂₀O₂ dominantly or even selectively. The results from HPLC analyses are summarized in Table 2. A reaction mixture of C₆₀ (0.05 mmol), 9 equiv. of FeCl₃ and 15 equiv. of H₂O was milled at 2500 cycles per minute (41.7 Hz) in a GT 600 mixer mill at room temperature for 90 min. To our delight, C₁₂₀O₂ was obtained in 57% yield along with a small amount of C₁₂₀O (7%) and a trace amount of C₁₂₀ (Table 2, entry 1). By increasing the amount of FeCl₃ to 10 equiv., the yield was enhanced to 69% (Table 2, entry 2). Further increasing the amount of FeCl₃ could improve the selectivity and relative yield of C₁₂₀O₂ (Table 2, entries 3 and 4), but led to a decrease in the HPLC intensity, indicating that the amount of C₁₂₀O₂ was actually decreased and would lead to a lower isolated yield. It is believed that more insoluble oligomers were generated in the presence of more FeCl₃. Reducing the reaction time led to a lower yield, while increasing the reaction time had no beneficial effect (Table 2, entries 5 and 6). When this reaction was performed without H₂O, only a trace amount of C₁₂₀O₂ was observed, indicating that H₂O was crucial for promoting this reaction (Table 2, entry 7). In addition, no benefit to this reaction could be achieved by varying the amount of H₂O (Table 2, entries 8 and 9). Decreasing the milling frequency to 35 Hz resulted in more C₁₂₀O and more recovered C₆₀, and increasing the milling frequency to 45 Hz did not improve the yield of C₁₂₀O₂ (Table 2, entries 10 and 11). In efforts to achieve more efficient synthesis of the peculiar C₁₂₀O₂, various Lewis acids, including AlCl₃, FeCl₂ and NiCl₂, and oxidizing agents, such as Oxone, m-chloroperbenzoic acid (m-CPBA) and H₂O₂, were explored. However, all of these additives were detrimental to the formation of C₁₂₀O₂ and afforded C₁₂₀ exclusively (Table 2, entries 12–17). To compare the present solvent-free reaction with its liquid-phase counterpart, the reaction of C₆₀ (0.05 mmol) with FeCl₃ (0.50 mmol) and H₂O (0.75 mmol) was performed in 0.5 mL of 1,1,2,2-tetrachloroethane at 150 °C for 48 h. Neither the desired product C₁₂₀O₂ nor C₁₂₀O could be isolated, and C₁₂₀ was instead formed in 15% yield (Table 2, entry 18). Other solvents, including toluene, chlorobenzene or 1,2-C₆H₄Cl₂, were also examined. However, the generation of C₁₂₀O₂ could not be observed in any of these solvents. Thus, it is obvious that the present mechanochemical solvent-free protocol shows advantages and uniqueness compared to the corresponding liquid-phase reaction. On the basis of the results presented above, the optimized conditions to afford the bisfuranoid-bridged dimer C₁₂₀O₂ were determined as shown in entry 2 of Table 2: C₆₀ (0.05 mmol), FeCl₃ (0.50 mmol), H₂O (0.75 mmol), a milling frequency of 41.7 Hz and a milling time of 90 min. The HPLC chromatogram on a Cosmosil Buckyprep (4.6 × 250 mm) column with toluene as the eluent at a flow rate of 1 mL min⁻¹ and the detector wavelength at 326 nm is shown in Fig. 2. The retention times of C₆₀, C₁₂₀O and C₁₂₀O₂ were 7.5 min, 18.1 min and 20.7 min, respectively. The isolated yield of C₁₂₀O₂ by HPLC on a Cosmosil Buckyprep-D (10 × 250 mm) column was 45%, which was dramatically higher than the overall yield of 0.1–1% for the three-step process starting from C₆₀ (vide supra).

Table 2 Optimization of the reaction conditions to afford C₁₂₀O₂^a

Entry	Additive	Ratio^b	Yield of C120O2^c (%)	Yield of C120O^c (%)	Yield of C120^c (%)	Recovered C60^c (%)
a Unless otherwise noted, all reactions were performed with 0.05 mmol of C60, FeCl3, and H2O together with 4 stainless steel balls (5 mm in diameter) in a stainless steel jar (5 mL) and milled vigorously (41.7 Hz) at room temperature for 90 min. b The molar ratio refers to C60/FeCl3/H2O. c Based on HPLC area ratios on a Cosmosil Buckyprep (4.6 × 250 mm) and/or Buckyprep-D (10 × 250 mm) column. d The reaction time was 60 min. e The reaction time was 120 min. f The reaction frequency was 35 Hz. g The reaction frequency was 45 Hz. h The reaction was performed in 1,1,2,2-tetrachloroethane (0.5 mL) at 150 °C for 48 h. i Under a N2 atmosphere. j C120O was used instead of C60.
1	FeCl3	1 : 9 : 15	57	7	Trace	36
2	FeCl 3	1 : 10 : 15	69	9	Trace	22
3	FeCl3	1 : 11 : 15	71	Trace	Trace	26
4	FeCl3	1 : 12 : 15	80	0	0	20
5^d	FeCl3	1 : 10 : 15	52	10	Trace	32
6^e	FeCl3	1 : 10 : 15	67	7	Trace	26
7	FeCl3	1 : 10 : 0	Trace	Trace	Trace	50
8	FeCl3	1 : 10 : 10	57	7	Trace	36
9	FeCl3	1 : 10 : 20	63	8	Trace	29
10^f	FeCl3	1 : 10 : 15	50	13	Trace	36
11^g	FeCl3	1 : 10 : 15	60	0	0	40
12	AlCl3	1 : 10 : 15	0	Trace	50	50
13	FeCl2	1 : 10 : 15	0	Trace	54	45
14	NiCl2	1 : 10 : 15	0	Trace	44	55
15	Oxone	1 : 10 : 15	Trace	Trace	38	42
16	m-CPBA	1 : 10 : 15	Trace	Trace	41	45
17	H2O2	1 : 10 : 15	Trace	Trace	39	56
18^h	FeCl3	1 : 10 : 15	0	Trace	15	83
19^i	FeCl3	1 : 10 : 15	54	17	Trace	27
20^j	FeCl3	1 : 10 : 15	15	49	16	20

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Fig. 2 HPLC chromatogram for the synthesis of C₁₂₀O₂ using FeCl₃ under optimal ball-milling conditions.

The molecular structures of C₁₂₀O and C₁₂₀O₂ were unambiguously established by single-crystal X-ray crystallography and are shown in Fig. 3. The single-crystal structure of C₁₂₀O reveals that it has a furanoid ring linking two fullerene cages via [6,6]-ring junctions, clarifying the uncertainty of the oxygen connectivity to fullerenes.¹² The single-crystal structure of C₁₂₀O₂ shows that the two fullerene cages are connected by adjacent furanoid rings via [6,6]-ring junctions as in the case of C₁₂₀O, resulting in a central four-membered-ring bridge via [5,6]-ring junctions. The obtained single-crystal structure of our C₁₂₀O₂ confirms that it has C_2v symmetry, is the most stable and plausible structure predicted by theoretical calculations⁸ and is neither the minor byproduct C₁₂₀O₂ with C₁ symmetry accompanying C₁₂₀O from the solid–state reaction of C₆₀ and C₆₀O at 200 °C¹³ nor the C₁₂₀O₂ isomer with C₂ symmetry from the dimerization of [5,6]-C₆₀O.¹⁴ C₁₂₀O and C₁₂₀O₂ were stable, and their thermogravimetric analyses (TGA) showed obvious weight loss above 370 °C (Fig. S5 and S6†).

Fig. 3 ORTEP diagrams of (a) C₁₂₀O, (b) C₁₂₀O₂ and (c) top and side views around the two oxygen atoms in C₁₂₀O₂ with thermal ellipsoids shown at 10% probability. The solvent molecules were omitted for clarity.

To explore whether O₂ in the air atmosphere played a role during the milling process, the milling jar containing the reaction mixture was filled with nitrogen (N₂) in a glovebox and milled under the optimal conditions, which did not inhibit the production of C₁₂₀O₂ (Table 2, entry 19). When C₁₂₀O was used to replace C₆₀ under the optimal conditions, the yield of C₁₂₀O₂ decreased dramatically, and a considerable amount of C₆₀ was produced (Table 2, entry 20). This result suggested that C₁₂₀O₂ was more likely to be generated directly from C₆₀ rather than from C₁₂₀O as a precursor. Therefore, C₁₂₀O and C₁₂₀O₂ should be formed independently from C₆₀via different reaction pathways.

The mechanochemical ¹⁷O/¹⁸O labelling using H₂¹⁷O/H₂¹⁸O has been reported.¹⁵ To further identify the source of the bridging oxygen atom in C₁₂₀O₂, H₂¹⁸O was introduced to the reaction system, and the reaction process was monitored by HRMS. When H₂¹⁸O (10 atom % ¹⁸O) was used, both C₁₂₀¹⁶O₂ and C₁₂₀¹⁶O¹⁸O were observed (Scheme 2a and Fig. S7†). Next, the addition of H₂¹⁸O (95 atom % ¹⁸O) under the optimal reaction conditions provided C₁₂₀¹⁶O¹⁸O and C₁₂₀¹⁸O₂ (Scheme 2b and Fig. S8†). These results reinforced the conclusion that the bridging oxygen atom in C₁₂₀O₂ originated from H₂O. Furthermore, when potassium ferricyanide was added to the reaction mixture, a dark blue precipitate was generated (Fig. S9†), indicating the presence of ferrous ions. More importantly, the X-ray photoelectron spectroscopy (XPS) measurement of the reaction mixture revealed that an Fe(II) species was generated after the reaction was completed (Fig. S10†).

Scheme 2 ¹⁸O labelling experiments with (a) H₂¹⁸O (10 atom % ¹⁸O) and (b) H₂¹⁸O (95 atom % ¹⁸O).

On the basis of the above experimental results and the literature,¹⁶ a plausible reaction mechanism is outlined in Scheme 3. First, the initial coordination of C₆₀ with FeCl₃ provides the complex FeCl₂(η²-C₆₀) (A),¹⁶ which is followed by the nucleophilic addition of H₂O to afford intermediate B in a 1,4-addition pattern. Loss of H⁺ from intermediate B gives intermediate C. The hydroxy group in C directly attacks another molecule of pristine C₆₀ to form an oxonium-bridged zwitterionic dimer D. Then, intramolecular cyclization with the removal of FeCl₂ and H⁺ generates the furanoid-bridged dimer C₁₂₀O. With an increased amount of FeCl₃ and a higher milling frequency, the amount of the generated intermediate C can surpass that of the pristine C₆₀, and self-dimerization of C would dominate to give doubly oxonium-bridged intermediate E. Finally, E undergoes dual intramolecular cyclization with elimination of FeCl₂ and H⁺ to provide the bisfuranoid-bridged dimer C₁₂₀O₂.

Scheme 3 Proposed reaction mechanism.

Given that fullerene derivatives have been applied in organic solar cells (OSCs) as the third component,¹⁷ preliminary results showed that C₁₂₀O₂ could be employed in OSCs with the configuration of ITO/PEDOT:PSS/D18-Cl:N3:fullerene (1:1.4:0.12)/PDIN/Ag (Fig. 4). The device with C₁₂₀O₂ as the third component showed a PCE of 17.94% with a J_SC of 28.05 mA cm⁻², a V_OC of 0.86 V and an FF of 74.08%. The control device without a fullerene additive showed a lower PCE of 17.27% with a J_SC of 26.41 mA cm⁻², a V_OC of 0.87 V and an FF of 75.03%. The device with pristine C₆₀ as the third component showed an inferior PCE of 15.77% with a J_SC of 26.68 mA cm⁻², a V_OC of 0.85 V and an FF of 69.24%. These results revealed that pristine C₆₀ had an adverse effect, while the bisfuranoid-bridged dimer C₁₂₀O₂ was a promising third-component material in the active layer of the OSCs. Unfortunately, the synthesized C₁₂₀O has limited solubility and could not be used as a third component in the current OSCs.

Fig. 4 (a) Schematic illustration of the OSC structure used in this work. (b) J–V curves of D18-Cl:N3 (blue line), D18-Cl:N3:C₁₂₀O₂ (red line) and D18-Cl:N3:C₆₀ (brown line)-based OSCs.

Conclusions

In summary, we have disclosed the unexpected, divergent and highly efficient synthesis of furanoid-linked C₁₂₀O and C₁₂₀O₂ from the FeCl₃-mediated mechanochemical reaction of C₆₀ with H₂O as the bridging oxygen source at room temperature under an air atmosphere. The present protocol avoids the usage of preprepared C₆₀O and C₁₂₀O. The selective synthesis of C₁₂₀O with one furanoid bridge and C₁₂₀O₂ with two furanoid bridges directly from pristine C₆₀ can be achieved by altering the amount of FeCl₃ and the milling frequency. The excellent yields, simplicity of this synthetic process and solvent-free and ambient conditions compared to those of previous multistep processes make this mechanochemical protocol an efficient method for the divergent synthesis of C₁₂₀O and C₁₂₀O₂. The specific structures of C₁₂₀O and C₁₂₀O₂ have been unequivocally established by single-crystal X-ray crystallography. A plausible reaction mechanism has been proposed based on the control experiments. The bisfuranoid-bridged dimer C₁₂₀O₂ has also been utilized as the third component in organic solar cells and has shown improved performance in OSC devices.

Data availability

The data supporting the findings of this study are available within the article and its ESI.†

Author contributions

G.-W. W. and S. Y. supervised the project. G. S. and J.-S. C. performed experiments and data analysis and wrote original manuscript, Y.-Y. L. fabricated the organic solar cell devices and analyzed data. G. S., C. N., Z.-C. Y., Y.-R. Y. and M. C. characterized the X-ray structures. S.-Q. Y. performed XPS test and data analysis. X.-L. X. assisted in data analysis and original manuscript preparation. G.-W. W. acquired funding and wrote the manuscript. All the authors contributed with comments and revisions to the manuscript and gave approval to the final version.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

We are grateful for financial support from the National Natural Science Foundation of China (22071231, 21372211).

Notes and references

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Footnotes

† Electronic supplementary information (ESI) available: Detailed experimental procedures and characterization data, NMR spectra of C₁₂₀O and C₁₂₀O₂, X-ray crystallographic data for C₁₂₀O and C₁₂₀O₂. CCDC 2289163, 2289161. For ESI and crystallographic data in CIF or other electronic format see DOI: https://doi.org/10.1039/d4sc04167d

‡ Authors with equal contribution.

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