Solution-Phase Perfluoroalkylation of C₆₀ Leads to Efficient and Selective Synthesis of Bis-Perfluoroalkylated Fullerenes

Abstract

A solution-phase perfluoroalkylation of C₆₀ with a series of R_FI reagents was studied. The effects of molar ratio of the reagents, reaction time, and presence of copper metal promoter on fullerene conversion and product composition were evaluated. Ten aliphatic and aromatic R_FI reagents were investigated (CF₃I, C₂F₅I, n-C₃F₇I, i-C₃F₇I, n-C₄F₉I, (CF₃)(C₂F₅)CFI, n-C₈F₁₇I, C₆F₅CF₂I, C₆F₅I, and 1,3-(CF₃)₂C₆F₃I) and eight of them (except for C₆F₅I and 1,3-(CF₃)₂C₆F₃I) were found to add the respective R_F groups to C₆₀ in solution. Efficient and selective synthesis of C₆₀(R_F)₂ derivatives was developed.

1. Introduction

The chemistry of perfluoroalkylfullerenes (PFAFs) has been rapidly evolving in recent years. Many isomerically-pure derivatives of C₆₀,[1–5] C₇₀,[6–9] hollow higher fullerenes,[10–13] and endometallofullerenes[14–16] have been synthesized, isolated, and studied by various techniques. Perfluoroalkylfullerenes were shown to be attractive electron acceptors due to their high thermal and chemical stability, reversible electrochemical behavior in solution, and a broad range of their first reduction potentials.[1,8,17] It was also shown that PFAFs can be further derivatized chemically to form donor-acceptor diads and potentially other complex molecular systems.[18,19] The vast majority of PFAF syntheses described in the literature relied on heterogeneous high-temperature reactions between fullerenes and R_F· precursors (e.g., R_FI or AgOOCCF₃, see refs. above). Such conditions typically favor the formation of PFAFs with a large number of R_F substituents, with simple bis-derivatives fullerene(R_F)₂ either absent from crude reaction mixtures or present in trace amounts.[4,5,20,21] Only 1,7-C₆₀(CF₃)₂,[20,22] 1,7-C₆₀(C₂F₅)₂,[23] and 1,7-C₆₀(i-C₃F₇)₂[24] have been isolated and characterized using high-temperature heterogeneous synthetic methods. It is notable that previous studies of C₆₀ halogenation demonstrated that selective synthesis of simple bis-derivatives C₆₀Hal₂ (Hal = F, Cl, and Br; iodofullerenes have not been reported) is difficult to achieve while many multisubstituted C₆₀Hal_n have been prepared (e.g., C₆₀F₁₈, C₆₀F₃₆, C₆₀F₄₈, C₆₀Cl₆, C₆₀Cl₂₄, and C₆₀Br₂₄). For example, single isomers of C₆₀F₂[25] and C₆₀Cl₂[26] were only prepared as minor products; C₆₀Br₂ has never been observed experimentally.

Recently we reported a fundamental study of substituent effects in a series of 1,7-C₆₀(R_F)₂ compounds with R_F = CF₃, C₂F₅, n-C₃F₇, i-C₃F₇, n-C₄F₉, s-C₄F₉ (secondary-CF(CF₃)(C₂F₅)), and n-C₈F₁₇ using cyclic voltammetry, low-temperature anion photoelectron spectroscopy, and DFT calculations.[23] The similarity of the first reduction potentials of these compounds, their largely intact π-systems, and differences in their physical properties (e.g., different solubility[23] and crystal packing[23,24,27]) makes them an intriguing series of compounds that may be used to study the effects of morphology of the active polymer-fullerene blend layers on the function of microelectronic devices such as solar cells. Except for 1,7-C₆₀(R_F)₂ compounds with R_F = CF₃ and C₂F₅, all other 1,7-C₆₀(R_F)₂ compounds in that study were prepared using a solution-phase perfluoroalkylation technique.[23] In this paper we report the development and optimization of this synthetic method.

Krusic and co-workers were the first to explore a solution-phase fullerene perfluoroalkylation in 1991.[28] They observed the formation of C₆₀(CF₃)· radical by ESR spectroscopy in a reaction of C₆₀ with (CF₃CO₂)₂ under UV irradiation in Freon-113.[28] In 1993 Fagan, Krusic, and co-workers described fullerene perfluoroalkylation with several R_F radical sources (CF₃I, C₂F₅I, n-C₃F₇I, n-C₆F₁₃I, (CF₃CO₂)₂, (C₂F₅CO₂)₂, and (n-C₃F₇CO₂)₂) in various solvents (1,2,4-trichlorobenzene, benzene, hexafluorobenzene, tert-butylbenzene, and chlorobenzene) at elevated temperatures or under UV irradiation.[29] Complex mixtures of C_60,70(R_F)_nH_m products with n up to 16 and m up to 30 have been prepared but no single isomers were isolated.[29] Several papers described ESR studies of various C₆₀(R_F)· and C₇₀(R_F)· species formed in solutions of aromatic and aliphatic solvents (benzene, toluene, tert-butylbenzene, and methylcyclohexane) from corresponding bare-cage fullerenes and various R_F radical precursors (R_FI, R_FBr, and (R_FCO₂)₂) at elevated temperatures or under UV irradiation.[30–33] In 1993 Yoshida and co-workers reported that a reaction of C₆₀ with (R_FCO₂)₂ (R_F = CF₃ and n-C₃F₇) in deoxygenated chlorobenzene solution at 40 °C resulted in formation of 1,7-C₆₀R_F(OH) alcohol and 1,9-C₆₀O₂CR_F(OH) “orthoester” (CR_F(OH) moiety connected to C₆₀ via two oxygen atoms forming a 1,3-dioxolane cycle).[34] Interestingly, in 1994 the same group reported a reduction of (C₆₀R_F)₂ dimer by Bu₃SnH to form C₆₀R_FH;[35] they claimed that the dimer was prepared and reported in their 1993 paper using the solution-phase reaction of C₆₀ with [R_FCO₂]₂ (the dimer formation or isolation was not mentioned in that paper).[34] In 1999 Yoshida and co-workers reported an optimized procedure for (C₆₀R_F)₂ synthesis based on a reaction of C₆₀ with R_FI (R_F = n-C₃F₇, n-C₄F₉, n-C₆F₁₃) in 1,2-dichlorobenzene (ODCB) solution in the presence of hexabutylditin or hexamethylditin under irradiation with metal-halide lamp.[36] A solution-phase synthesis of a mixture of perfluorohexylated species with an average composition C₆₀(C₆F₁₃)_~5 in a reaction of C₆₀ with n-C₆F₁₃I in 1,2,4-trichlorobenzene at 200 °C was reported in 1999, but the elemental analysis was the only characterization method used in that study.[37]

In this work we studied the solution-phase perfluoroalkylation of C₆₀ in detail using HPLC analysis and mass spectrometry with the aim of selective and efficient preparation of C₆₀(R_F)₂ derivatives and other PFAFs. Multiple small-scale experiments were carried out in order to ascertain the effects of the molar ratio of the reagents, reaction time, and copper metal promoter on the product composition and C₆₀ conversion.

2. Materials and methods

2.1. Solvents and Reagents

The solvents toluene (Fisher Scientific; HPLC grade), n-heptane (Fisher Scientific; HPLC grade), acetonitrile (Fisher Scientific; HPLC grade), dimethyl sulfoxide (Fisher Scientific; ACS grade), 1,2-dichlorobenzene (ODCB; Acros Organics; ACS grade), chloroform-d (99.8% D, Cambridge Isotope Laboratories, Inc.) were used as received. The compounds C₆₀ (99.9%, Term-USA), R_FI (R_F = CF₃, C₂F₅, n-C₃F₇, i-C₃F₇, n-C₄F₉, s-C₄F₉, n-C₈F₁₇, CF₂C₆F₅, C₆F₅, 3,5-(CF₃)₂C₆F₃) (SynQuest Labs), and copper powder (Fisher Scientific; 325 mesh; electrolytic grade) were used as received.

2.2. Instruments

HPLC analysis and separation was accomplished using Shimadzu HPLC instrumentation (CBM-20A control module, SPD-20A UV-detector set to 300 nm detection, LC-6AD pump, manual injector valve) equipped with a 10-mm I.D. × 250 mm semipreparative Cosmosil Buckyprep column (Nacalai Tesque, Inc.) Atmospheric-pressure chemical ionization (APCI) mass spectra were recorded using Agilent Technologies Model 6210 TOF spectrometer with APCI source. The carrier solvent was acetonitrile with 1% toluene; PFAF samples were injected as toluene solutions. Fluorine-19 NMR spectra were recorded on Varian 400 MR spectrometer at 376 MHz frequency (CDCl₃ solvent was used).

2.3. Perfluoroalkylation experiments

Aliquots of 500 μL of a stock solution of C₆₀ in ODCB (13.8 mM) was transferred into a Pyrex glass ampoule (O.D. = 8.0 mm; I.D. = 5.0 mm; L = 140 mm; V = 2.7 mL) containing ca. 500 mg of copper powder. An R_FI reagent was then added via a gas-tight syringe (for small excesses of 6 equivalents a diluted stock solution of i-C₃F₇I in ODCB was used), and the solution was diluted to 1.0 mL with pure ODCB (in several experiments 250 μL of DMSO were also added prior to the ODCB dilution). The gaseous reagents CF₃I (b.p. = −21.85 °C[38]) and C₂F₅I (b.p. = 12.5 °C, vendor data) were measured volumetrically and transferred on a vacuum line equipped with a calibrated volume and an electronic manometer (MKS Instruments Baratron vacuum gauge, 0–1.3 bar). In these two cases the 500 μL stock solution of C₆₀ was diluted to 1.0 mL volume with ODCB prior to the addition of CF₃I and C₂F₅I. In all cases the ampoules were degassed three times using a freeze-pump-thaw technique, frozen in liquid nitrogen, and flame-sealed under vacuum. The ampoules were heated at 180 °C in a tube furnace, then cooled down, frozen in liquid nitrogen, and opened. The crude reaction mixtures were evaporated under vacuum to dryness (reaction mixtures containing DMSO were washed with water three times prior to drying). The dry residues were dissolved in 10.0 mL of toluene, filtered, and analyzed using HPLC (100 μL sample injections) and APCI mass spectrometry.

3. Results

The effects of the reaction time and the molar ratio of the reagents on C₆₀ conversion and product distribution were studied using reactions between C₆₀ and i-C₃F₇I. The reaction conditions were kept the same except for varying amounts of i-C₃F₇I (6, 18, 54, and 162 equivalents) and reaction time (1 and 3 days). In all cases reactions were carried out in degassed 1,2-dichlorobenzene (ODCB) solution at 180 °C in sealed glass ampoules of the same volume (2.7 mL). A large excess of copper metal powder (500 mg; 7.9 mmol) was added to each of the reaction mixtures prior to degassing and heating. In all solution-phase experiments described in this work the total volume of the reaction mixture was 1.0 mL and the initial concentration of C₆₀ was 6.9 mM. The crude reaction mixtures were filtered, evaporated to dryness, and dissolved in 10.0 mL of toluene. These solutions were analyzed by HPLC using pure toluene as the primary eluent and the HPLC traces were stacked for direct comparison, see Figure 1 (injection volume was 100 μL; retention time t_R(C₆₀) = 8.7 min; t_R(1,7-C₆₀(i-C₃F₇)₂) = 5.7 min[23]). Pure toluene was not effective for the analysis of C₆₀(i-C₃F₇)_n>4 products due to their short retention times; for such mixtures pure heptane was used instead to provide adequate peak separation (see inserts in Figure 1). The crude products were also analyzed by negative-ion APCI mass spectrometry which showed distributions of peaks corresponding to the C₆₀(C₃F₇)_n⁻ anions with n = 1–8. The numbers shown to the immediate right of the HPLC traces on Figure 1 represent the distributions of the C₆₀(C₃F₇)_n⁻ peaks observed in the NI-APCI mass spectra of the corresponding products (the large-font numbers represent the most intense peaks in the spectra). It is notable that no evidence of the formation of dimeric species (C₆₀R_F)₂ were observed throughout our study (neither by mass spectrometry nor by HPLC analysis).[34–36]

Figure 1.

The HPLC analysis of several C₆₀ + N(i-C₃F₇I) reactions carried out in o-DCB solution in the presence of copper powder at 180 °C. The peaks at t_R = 5.7 minutes marked with daggers corresponds to 1,7-C₆₀(i-C₃F₇)₂; the peaks at t_R = 8.7 minutes marked with stars correspond to unreacted C₆₀. The numbers on the right-hand side of the HPLC traces show the distribution of C₆₀(C₃F₇)_n⁻ anions observed in the NI-APCI MS analysis of the corresponding samples (the numbers given with a large font correspond to the most intense ion in the spectra). The inserts show the HPLC traces of four samples acquired using 100% heptane eluent.

The effect of the copper metal was studied in a separate experiment. A sample of C₆₀ was allowed to react with 162 equivalents of i-C₃F₇I in ODCB solution at 180 °C for 3 days without any copper present. Upon the workup the reaction mixture was found to contain a significant amount of iodine (no iodine was observed in any of the reactions carried out in the presence of copper). The product was analyzed by HPLC and NI-APCI mass spectrometry; the composition of the PFAF products formed in a three-day reaction between C₆₀ and 162 equivalents of i-C₃F₇I without copper is similar to the products formed in one-day reaction of C₆₀ and 54 equivalents of i-C₃F₇I with copper (see Figures 1 and 2).

Figure 2.

HPLC and NI-APCI mass spectrometry analysis of the products of the reaction of C₆₀ with 162 equivalents of i-C₃F₇I reagent in ODCB solution at 180 °C for three days with copper metal powder (A) and without copper metal powder (B). The n values represent the distributions of C₆₀(C₃F₇)_n⁻ species observed in the NI-APCI mass spectra.

Several reactions between C₆₀ and 54 and 18 equivalents of CF₃I and 18 equivalents of i-C₃F₇I were also carried in 25/75 v/v DMSO/ODCB mixture at 180 °C, both with and without copper metal. The crude reaction mixtures were washed with water several times to remove DMSO; then they were treated analogously to the crude products prepared in pure ODCB. The resulting products quickly decomposed upon their exposure to air (the freshly filtered solutions quickly changed color and became murky with white insoluble precipitates) which made them impossible to analyze using conventional HPLC or mass spectrometry.

The reactivity of different R_FI reagents (R_F = CF₃, C₂F₅, n-C₃F₇, i-C₃F₇, n-C₄F₉, s-C₄F₉, n-C₈F₁₇, CF₂C₆F₅, C₆F₅, and 3,5-(CF₃)₂C₆F₃) toward C₆₀ in ODCB solution was also studied; 18 equivalents of the R_FI reagents were used in all cases and the reaction time was one day. The resulting reaction mixtures were studied by HPLC using pure toluene as the eluent, see Figure 3 (50/50 v/v toluene/heptane mixture was also used for the perfluorobenzylated products, see the insert in Figure 3). The aliphatic R_FIs and perfluorobenzyl iodide yielded the corresponding C₆₀(R_F)_n products (this was confirmed by NI-APCI mass spectrometry, figures not shown). No reaction was observed for the aromatic R_FI reagents C₆F₅I and 3,5-(CF₃)₂C₆F₃I.

Figure 3.

HPLC analysis of the products of C₆₀ reactions with 18 equivalents of various R_FI reagents. Reactions were carried out in ODCB solution in the presence of copper powder at 180 °C for one day.

It is notable that most of the C₆₀ reagent precipitated out of the hot ODCB after several hours of heating when CF₃I, 3,5-(CF₃)₂C₆F₃I, or C₆F₅I were used. In these cases the hot ODCB solutions changed from a deep magenta to a pale-pink color and black precipitates were formed. After cooling the black precipitates were redissolved forming the deep magenta-colored solutions typical of C₆₀ (the HPLC analysis also confirmed that the C₆₀ reagent was unchanged, see Figure 3). These observed precipitations are consistent with the experimentally studied anomalous temperature dependence of C₆₀ solubility in organic solvents including ODCB.[39,40]

Finally, we performed several experiments with ca. 0.027 bar of gaseous C₂F₅I, n-C₃F₇I, and i-C₃F₇I and C₆₀ in a GTGS reactor.[22] The reaction between C₂F₅I and C₆₀ without copper metal at ca. 480 °C was largely unsuccessful due to a very low C₆₀ conversion (less than 5%).[23] In order to improve the conversion of C₆₀ we performed an experiment with the use of Cu powder as a promoter but it led to a high abundance of C₆₀(C₂F₅)_n products with n > 2 (P(C₂F₅I) = 0.021–0.016 bar; T = 430 °C; the yield of 1,7-C₆₀(C₂F₅)₂ was ca. 1–2 mol%; see Supporting Information of ref. [23]). Attempts to use a GTGS reactor to carry out reactions with n-C₃F₇I and i-C₃F₇I did not yield any PFAF products, and all starting C₆₀ was recovered unchanged both with and without copper metal powder; a range of the reaction temperatures up to 500 °C was explored.

4. Discussion

Reactions involving perfluoroalkyl radicals are commonly used for a preparation of a variety of organofluorine compounds.[41] Substitution of aromatic hydrogen atoms by R_F· radicals has been used to prepare aromatic compounds bearing various R_F groups[42–46] (see also [47,48] for similar catalytic processes); addition of R_F· radicals to alkenes has also been well-studied.[49,50] Thermolysis of R_FI or (R_FCO₂)₂ precursors has been commonly used to generate R_F· radicals (see for example [42,43,46]). The other common method of R_F· generation relies on a rapid reaction of R_FI with CH₃· or C₆H₅· radicals formed in situ (in the reaction of acetone with hydrogen peroxide[44] or by photolysis of (PhCO₂)₂[45,51], correspondingly).

The original work by Fagan and co-workers[29] was the first to employ the addition of perfluoroalkyl radicals to fullerenes leading to a successful synthesis of mixtures of various perfluoroalkylated fullerenes (PFAFs). In that work, R_F· radicals were formed in situ by thermolysis or UV irradiation of solutions of R_FI or (R_FCO₂)₂ radical precursors; R_F· then reacted with C₆₀ (or C₇₀) which acted as a radical trap.[29,37] The formation of radical intermediates C₆₀(R_F)· was confirmed by ESR spectroscopy.[29–33] Large excess of R_F· radical precursors were used, resulting in mixtures of highly perfluoroalkylated PFAFs. It is notable that the use of C₆H₆ reaction medium resulted in the formation of some hydrogenated PFAFs.[29] This process was eliminated by the use of fluorinated solvents such as C₆F₆ or Freon-113,[29] but these reactions must have been heterogeneous since fullerenes are practically insoluble in fluorinated solvents.[52]

In this study, thermolysis of R_FI reagents was chosen for the generation of R_F· radicals in order to avoid possible side-reactions of radicals other than R_F· with C₆₀ (e.g., CH₃· or C₆H₅· radicals[44,45,51]). Note that C₆₀I_n compounds with covalent C₆₀-I bonds have not been reported; this is believed to be due to the low energy of C₆₀-I bonds.[53] In this work we have not observed any evidence for the formation of persistent C₆₀(R_F)_nI_m species.

The reaction medium was chosen to be 1,2-dichlorobenzene (ODCB) due to several factors. First, C₆₀ and other fullerenes and fullerene derivatives are known to be very soluble in ODCB[52] (anomalous temperature dependence of fullerene solubility notwithstanding[39,40]). Second, ODCB has a high boiling point (180 °C) and a relatively low vapor pressure which eliminates the need for high-pressure reactors. Third, it is rather inert toward radical reactions; we have not observed appreciable amounts of fullerene products with substituents other than R_Fs (e.g., hydrogen atoms[29] or aryl groups) or products resulting from a perfluoroalkylation of ODCB. The higher-boiling 1,2,4-trichlorobenzene was also considered, but it is harder to remove under vacuum which complicates workup of the crude products.

Copper metal powder was used to promote the decomposition of R_FI reagents and accelerate fullerene perfluoroalkylation in solution. It was found that C₆₀ perfluoroalkylation also takes place without copper metal but the reaction rate was several times slower. Copper metal serves as an iodine scavenger,[22] thereby shifting the equilibrium of R_FI dissociation toward the formation of the R_F radicals, see Scheme 1. Even with copper metal present, the decomposition of the R_FI reagents at 180 ·C is relatively slow, which necessitated the use of high stoichoimetric ratios of the reagents (n(R_FI)/n(C₆₀) up to 162 equivalents for some reactions with i-C₃F₇I). It is likely that other metals like Ag and Tl can also be used as iodine scavengers. We have not tested these metals in this work due to their higher price and high toxicity.

Scheme 1.

Fullerene perfluoroalkylation in solution (possible chain termination processes and are not shown).

It is notable that high-boiling polar solvents like DMSO, DMF, pyridine, and HMPA have been used extensively for solution-phase coupling reactions between aliphatic and aromatic halogenated substrates and R_FI reagents in the presence of copper metal. In such reactions a halogen atom (typically iodine or bromine) is substituted by an R_F group.[54–56] These reactions are likely to involve the initial formation of perfluoroorganocopper intermediates [R_FCu] (formation of such intermediates was shown in several cases, see refs. above). The use of polar solvents (donor number > 19) is believed to be necessary to minimize the unwanted formation of R_F radicals which decreases the selectivity and yield of the coupling reactions.[55] Fullerenes are practically insoluble in polar solvents,[52] so we used 25/75 v/v mixture of DMSO and ODCB to carry out fullerene perfluoroalkylation. Air-sensitive products of unknown compositions were formed, therefore this direction was not pursued further. The fact that C₆₀ perfluoroalkylation by R_FI in hot ODCB occurs even without any copper metal present (albeit at a slower rate, see above) suggests that the formation of organocuprate reagents is probably not involved in this reaction which is likely to be a simple radical addition.

The solution-phase reaction of i-C₃F₇I with C₆₀ in ODCB in the presence of copper showed that molar ratios of these reagents and reaction time have a strong effect on the composition of products, see Figure 1. A low excess of i-C₃F₇I (6 equivalents) led to a very low C₆₀ conversion after a one-day reaction. Most of the starting C₆₀ was left unchanged but the HPLC analysis and the NI-APCI mass spectrometry showed that a small amount of 1,7-C₆₀(i-C₃F₇)₂ was formed (the single isomer of this compound was isolated earlier from the products of heterogeneous C₆₀ perfluoroalkylation, see ref. [23]). A longer reaction time (three days) led to a better C₆₀ conversion and a relatively selective formation of 1,7-C₆₀(i-C₃F₇)₂ with a ca. 25% yield (based on the HPLC analysis). The reaction with 18 equivalents of i-C₃F₇I over a period of one day led to a lower conversion of C₆₀ and a selective formation of 1,7-C₆₀(i-C₃F₇)₂ with a ca. 40% yield. The HPLC analysis and the mass spectrometry also showed that only small amounts of C₆₀(i-C₃F₇)_4,6 compounds were formed (highly-substituted C₆₀(R_F)_n compounds typically have shorter retention times[21,22]). Higher molar ratios of i-C₃F₇I to C₆₀ and longer reaction time resulted in formation of more highly substituted products. A high excess of i-C₃F₇I (162 equivalents) used in a three-day reaction, resulted in the formation of a major C₆₀(i-C₃F₇)₈ product as shown by the mass spectrometry. Fluorine-19 NMR spectroscopy of this product suggested it to be a mixture of isomers (figure not shown). This shows that the maximum number of i-C₃F₇ groups that can be added to C₆₀ under these conditions is limited to eight (Max(i-C₃F₇/C₆₀) = 8). This lies in agreement with the results of high-temperature heterogeneous reactions between C₆₀ and high-pressure gaseous i-C₃F₇I that also did not produce any products with more than eight i-C₃F₇ substituents (but multiple isomers of C₆₀(i-C₃F₇)_4,6,8 were isolated and characterized).[24] The maximum number of R_F additions to the C₆₀ cage depends on the steric demand of the R_F groups. For the smallest R_F group, CF₃, C₆₀(CF₃)₂₂ species have been observed experimentally (Max(CF₃/C₆₀) = 22).[57] As the size of the R_F substituents increases, the highest experimentally observed degree of C₆₀ perfluoroalkylation decreases (Max(C₂F₅/C₆₀) = 16; Max(n-C₃F₇/C₆₀) = 12; see also ref. [21] for other experimental Max(R_F/C₆₀) values).

Reaction conditions that led to a low conversion of C₆₀ produced 1,7-C₆₀(i-C₃F₇)₂ with a high degree of selectivity, see Figure 1. In our experiments it corresponded to 6 and 18 equivalents of i-C₃F₇I (the high excess implies that not all of this reagent is used up productively during the reaction). The mass spectrometry analysis of the corresponding products showed a wide range of C₆₀(i-C₃F₇)_n with up to eight substituents were formed. It is notable that no R_F fragmentation was observed in any of the solution-phase reactions described in this work; such fragmentation was observed above 300 °C when longer-chain R_FI reagents were used for heterogeneous perfluoroalkylation of fullerenes[21].

Based on this work we developed several efficient solution-phase syntheses of new 1,7-C₆₀(R_F)₂ compounds with R_F = n-C₃F₇, i-C₃F₇, n-C₄F₇, s-C₄F₇, and n-C₈F₁₇ which are described in a separate paper.[23] After some optimization of the reaction conditions and HPLC-based separation procedures these compounds were prepared with 10–20 mol% yields, see ref. [23] for synthetic details and analytical data of the isolated compounds. Higher yields may be achievable; e.g., ca. 40% yield (based on the HPLC trace integration) was achieved in a small-scale synthesis of 1,7-C₆₀(i-C₃F₇)₂, see Figure 1 (N(i-C₃F₇I) = 18; reaction time = 1 day). In the corresponding large-scale experiments the yields varied between 10–20% depending on small variations of the heating regime and other reaction parameters such as reactor size and geometry;[23] the yields and conversion were kept intentionally low to suppress the formation of C₆₀(R_F)_n>2 which, unlike unreacted C₆₀, cannot be recycled.

5. Conclusions

The results presented in this work indicate that homogeneous perfluoroalkylation of C₆₀ is currently the most efficient method for preparation of bis-C₆₀(R_F)₂ derivatives with a variety of different R_F groups. Currently it is the only method of synthesis for 1,7-C₆₀(R_F)₂ with R_F = n-C₃F₇, n-C₄F₉, s-C₄F₉, and n-C₈F₁₇. Lower reaction temperatures used for solution-phase perfluoroalkylation were found to completely suppress the temperature-induced fragmentation of the longer R_F chains (which leads to the unfavorable formation of “mixed” PFAFs with different R_F substituents above 300 °C in heterogeneous processes[21]). Solution-phase C₆₀ perfluoroalkylation is also found to be a good method for preparation of fullerene derivatives with the highest sterically possible numbers of R_F substituents (such reactions should use a high excess of the R_FI reagent and long reaction times). Experimental results showed that many different C₆₀(R_F)_n products with an “intermediate” number of substituents (2 < n < Max(R_F/C₆₀)) are formed using the solution-phase perfluoroalkylation process. HPLC analysis of such mixtures suggests that many of these compounds can be isolated in pure state (although with a low yield). These results show that solution-phase fullerene perfluoroalkylation is a versatile technique suitable for preparation of many PFAFs with varying number of perfluorinated aliphatic or benzylic R_F groups.

Highlights.

Efficient perfluoroalkylation of C₆₀ with R_FI reagents under homogeneous conditions.

• Strong effect of reagent ratio, reaction time, and copper promoter on product composition

• Selective synthesis of bis-adducts is based on “low-conversion” reaction regime