Preferential precipitation of C₇₀ over C₆₀ with p-halohomooxacalix[3]arenes

Abstract

Preferential precipitation of C₇₀ from a toluene solution of C₆₀ and C₇₀ was accomplished with p-trihalohomooxacalix[3]arenes (1₃·X·X·X) prepared by the reductive coupling of diformylphenols. Heavy halogens, Br and/or I, are essential at the para position of 1₃·X·X·X for obtaining good yields and selectivities. C₇₀ with up to 92% purity was obtained after the preferential precipitation.

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Introduction

Encapsulation of fullerenes is a subject of current interest in the fields of supramolecular and fullerene chemistry.^1–8 A practical goal in this area of chemistry is to develop an efficient method for the separation of specific size and/or isomers of fullerenes from mixtures.⁹ Actually, selective complexation of fullerenes has so far been reported with calixarenes,^10–22 cyclodextrins,^15,23–25 cyclotriveratrylenes^26–28 and metalloporphyrins.²⁹ In 1992, selective extraction of C₆₀ into a water layer was accomplished using γ-cyclodextrin²⁵ and water soluble calix[8]arene²² by Wennerström and Verhoeven and their co-workers, respectively. Selective precipitation from an organic solution of a fullerene mixture, which is a more practical method by which to separate fullerenes, was reported in 1994 using p-tert-butylcalix[8]arene^19,20 and cyclotriveratrylene²⁸ by Shinkai and Atwood and their co-workers. More recently, a few bridged or cyclic dimers were found to encapsulate C₇₀ preferentially to C₆₀ in solution.^14,17,26,29 However, no host molecules preferentially precipitating C₇₀ have been reported except for one example using p-tert-butylcalix[6]arene.^16,20 Herein is described a selective precipitation of C₇₀ over C₆₀ with p-halohomooxacalix[3]arenes.

Results

Synthesis of homooxacalix[n]arenes (1_n·R)

The homooxacalix[n]arenes (1_n·R) were prepared according to the recent report based on the reductive coupling of 4-substituted-2,6-diformylphenols (2·R).³⁰p-Triiodohomooxacalix[3]arene (1₃·I·I·I) was prepared stepwise from 2,6-diformylphenol (2·H)³¹ through a combination of iodination^32,33 and reductive coupling³⁰ (Scheme 1) because diformylation of p-iodophenol³⁴ gave 2·I in very low yield. p-Trihalohomooxacalix[3]arenes with different halogens, 1₃·I·I·Br and 1₃·I·Br·Br, were prepared through the iodination³³ of the corresponding 1₃·H·H·Br and 1₃·H·Br·Br as shown in Scheme 2. Other p-halohomooxacalix[3]arenes with different substituents, 1₃·I·I·Bz (Bz = CH₂Ph), 1₃·I·Bz·Bz, 1₃·I·I·Oct^t (Oct^t = 1,1,3,3-tetramethylbutyl) and 1₄·I·I·I·Oct^t, were prepared by reductive heterocoupling between 4-substituted-2,6-diformylphenol (2·R) and the tris(trimethylsilyl) ether of 4-substituted-2,6-bis(hydroxymethyl)phenol (4·R)³⁰ which was prepared via3·R as shown in Scheme 3. The 2,6-bis(hydroxymethyl)phenols with benzyl and tert-octyl substituents (3·Bz and 3·Oct^t) were prepared according to the reported method.³⁵ Since the direct hydroxymethylation of p-iodophenol was reported to fail, 3·I was prepared in a similar indirect route starting from the commercial compound to that described in ref. 36. The trimethylsilylation of 3 was carried out with N,O-bis(trimethylsilyl)trifluoroacetamide (BTSTA) in acetonitrile to give 4 in high yields after rapid column chromatography with hexane and careful evaporation of the solvents.^37,38

Scheme 1

Scheme 2

Scheme 3

Precipitation of C₇₀ with homooxacalix[n]arenes

Thus prepared homooxacalix[n]arenes with a variety of para-substituents were first examined for precipitation of C₇₀ (Table 1). About 70% of C₇₀ was precipitated by 1₃·I·I·I from a toluene solution of C₇₀ (ca. 1 mg cm⁻³) as a complex (runs 1 and 2). Under similar conditions, however, no precipitate was observed for C₆₀ with 1₃·I·I·I (run 3). These contrasting results clearly show a potential of 1₃·I·I·I for separation of C₇₀ from a mixture of C₆₀ and C₇₀.

Table 1 Precipitation of C₇₀ with p-halohomooxacalix[n]arenes with various substituents^a

Run	Host (1n·R)			C70/mg (µmol)	Toluene/cm^3	Precipitate
	n	R	Weight/mg (µmol)			Weight/mg	Yield (%) of C70^b
a Mixtures were stirred at room temperature overnight unless otherwise noted. b Calculated from recovery (%) of C70 from the filtrate. c C60. d Bz = CH2Ph. e Oct^t = 1,1,3,3-tetramethylbutyl. f At −30 °C.
1	3	I·I·I	24.1 (31)	51.3 (61)	48	52.5	71
2	3	I·I·I	52.8 (67)	111.6 (130)	96	109.2	67
3	3	I·I·I	4.3 (5.5)	3.8 (5.3)^c	4.5	0	0^c
4	3	I·I·Bz^d	12.7 (17)	30.2 (36)	21.5	9.6	22
5	3	I·I·Oct^t^e	24.0 (31)	54.8 (65)	43	16.1	20
6	4	I·I·I·Oct^t^e	15.8 (15)	31.5 (37)	21.5	9.2	20
7	3	I·Bz·Bz	5.1 (7.1)	4.7 (5.6)	3.5	trace	∼0
8^f	3	I·Bz·Bz^d	5.6 (7.8)	4.7 (5.6)	5.5	trace	∼0
9	3	Br·Br·Br	45.9 (71)	111.1 (130)	96	52.5	33

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The number of alkyl groups and the nature of the halogens at the para position of p-halohomooxacalix[n]arenes (n = 3 or 4) remarkably affected the yield of C₇₀ (runs 4–9). p-Iodohomooxacalix[n]arenes (n = 3 or 4) with one alkyl group afforded C₇₀ precipitates in low yields (runs 4–6), while only a trace amount of precipitate was obtained using p-iodohomooxacalix[3]arene with two alkyl groups (runs 7–8). p-Tribromohomooxacalix[3]arene gave C₇₀ complex in 33% yield (run 9), which is less than half of the yield with p-triiodohomooxacalix[3]arene (runs 1 and 2). A similar precipitation experiment with a more concentrated toluene solution of C₇₀ (2.5 mg cm⁻³) has been carried out with p-tert-butylcalix[6]arene which gives C₇₀ complex in 31% yield.²⁰

Preferential precipitation of C₇₀ over C₆₀ with homooxacalix[3]arenes

Selective precipitation of C₇₀ from fullerene mixtures was carried out under more concentrated conditions than those in Table 1.³⁹ The results are summarized in Table 2. The selectivity and the yield of C₇₀ strongly depend on the nature of the halogens at the para position of the host molecules. The order of efficiency in the purification of C₇₀ followed the size of the halogens, I > Br > Cl > F (runs 2, 7, 13 and 14). As compared to 1₃·I·I·I and 1₃·Br·Br·Br, 1₃·Cl·Cl·Cl gave low selectivity (run 12) or low yield of C₇₀ (run 13), and 1₃·F·F·F afforded low yield of C₇₀ with the highest purity (run 14). When 1₃·I·I·I or 1₃·Br·Br·Br was added to the toluene solution of a fullerene mixture (C₆₀ ∶ C₇₀ = 1 ∶ 1, w/w), the ratios of C₆₀ ∶ C₇₀ and the yields of C₇₀ of the precipitates were 21 ∶ 79–38 ∶ 62 and 75–91%, respectively (runs 2, 7 and 9–11). The purity of C₇₀ in the precipitates increased to 92% and 88% from 83% and 80%, respectively, after the selective precipitation (runs 1 and 8). The attempt to obtain pure C₇₀ by this method was not successful; 92% purity is the upper limit of this purification. p-Iodohomooxacalix[3]arenes with one or two Br groups, 1₃·I·I·Br and 1₃·I·Br·Br (runs 3 and 6), afforded almost the same results as that of 1₃·I·I·I (run 2), while the ones with one alkyl group, 1₃·I·I·Bz and 1₃·I·I·Oct^t (Oct^t = 1,1,3,3-tetramethylbutyl), gave much lower yields of C₇₀ (runs 4 and 5). These results show that three heavy halogens are essential for obtaining both good yield and selectivity. The selectivity seems to be similar to the reported ones with tert-butylcalix[6]arene.^16,20 When other solvent systems were used instead of toluene, similar results were obtained (runs 10 and 11). The precipitate from a toluene solution of the fullerene mixture (C₆₀ ∶ C₇₀ ∶ C_>70 = 2 ∶ 6 ∶ 2) with 1₃·I·I·I included a considerable amount of higher fullerenes (C_>70) along with C₇₀; that is, 1₃·I·I·I cannot discriminate C₇₀ and C_>70, which is another limitation of this method for the purification of fullerenes. When a toluene solution of 1₃·I·I·I was added to a toluene solution of raw fullerene mixture (fullerene extract, C₆₀ ∶ C₇₀ = 8 ∶ 2), precipitation was not observed to give a dark brown homogeneous solution.

Table 2 Selective precipitation of C₇₀ with p-halohomooxacalix[3]arenes with various substituents^a

Run	Host (13·R)		Fullerene mixture	Toluene/cm^3	Precipitate		Filtrate
	R	Weight/mg (µmol)	C60(mg)/C70(mg) (ratio)		C60 ∶ C70^b	Yield (%)^c	C60 ∶ C70^b
a Mixtures were stirred at room temperature overnight unless otherwise noted. b Weight ratio based on area ratio in HPLC. c Yield of C70 in the precipitates was calculated from the weights of C60 and C70, and the C60 ∶ C70 ratios of the precipitates and filtrates. d At −30 °C for 2 days. e Bz = CH2Ph. f Oct^t = 1,1,3,3-tetramethylbutyl. g Toluene (10 cm^3) + hexane (5 cm^3). h 1,1,2,2-Tetrachloroethane (5 cm^3) + hexane (5 cm^3).
1	I·I·I	6.9 (8.8)	2.6/13.0 (17 ∶ 83)	7	8 ∶ 92	93	63 ∶ 37
2	I·I·I	12.8 (16)	20.3/20.4 (50 ∶ 50)	11	24 ∶ 76	88	86 ∶ 14
3	I·I·Br	17.0 (23)	20.2/20.2 (50 ∶ 50)	11	24 ∶ 76	93	91 ∶ 9
4^d	I·I·Bz^e	11.6 (15)	10.2/9.9 (51 ∶ 49)	11	14 ∶ 86	18	55 ∶ 45
5^d	I·I·Oct^t^f	41.3 (53)	20.9/21.7 (49 ∶ 51)	17	11 ∶ 89	20	55 ∶ 45
6	I·Br·Br	15.3 (22)	19.6/20.1 (49 ∶ 51)	11	24 ∶ 76	90	87 ∶ 13
7	Br·Br·Br	15.3 (24)	20.1/19.8 (50 ∶ 50)	11	38 ∶ 62	91	84 ∶ 16
8	Br·Br·Br	11.5 (18)	4.0/16.2 (20 ∶ 80)	7	12 ∶ 88	84	45 ∶ 55
9	Br·Br·Br	8.0 (12)	10.0/9.8 (50 ∶ 50)	6	23 ∶ 77	75	76 ∶ 24
10	Br·Br·Br	7.4 (11)	9.8/10.0 (50 ∶ 50)	10 + 5^g	22 ∶ 78	79	78 ∶ 22
11	Br·Br·Br	7.3 (11)	10.2/9.8 (51 ∶ 49)	5 + 5^h	21 ∶ 79	76	78 ∶ 22
12	Cl·Cl·Cl	11.6 (23)	9.7/9.7 (50 ∶ 50)	5	48 ∶ 52	93	67 ∶ 33
13	Cl·Cl·Cl	12.1 (24)	20.2/20.1 (50 ∶ 50)	11	37 ∶ 63	63	63 ∶ 37
14	F·F·F	10.9 (24)	9.8/9.8 (50 ∶ 50)	5	6 ∶ 94	19	55 ∶ 45

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In order to recover the fullerene and reuse the host molecules, liberation of the complexes was carried out. Since the binding of the C₇₀ complexes with 1₃·I·I·I, 1₃·I·I·Br and 1₃·I·Br·Br was very tight, it was very difficult to liberate the fullerene by a simple way like that used for the tert-butylcalix[8]arene-C₆₀ complex.^19,20 Eventually, C₇₀-1₃·I·I·I was dissolved in o-dichlorobenzene (ODCB), and the host molecules were extracted with basic aqueous solution.

Discussion

The host ∶ guest ratio of the 1₃·I·I·I–C₇₀ and 1₃·Br·Br·Br–C₇₀ complexes is calculated to be about 2 ∶ 5 and 1 ∶ 3 from the results of the elemental analyses.† The ratio was reported to be 1 ∶ 2 in p-tert-butylcalix[6]arene–C₇₀,²⁰p-tert-butylcalix[6]arene–C₆₀ and calix[6]arene–C₇₀ complexes,¹⁶ 1 ∶ 1 in 1₃·Br·Br·Br–C₆₀,⁴⁰1₃·Bu^t·Bu^t·Bu^t–C₆₀,⁴¹p-tert-butylcalix[5]arene–C₆₀⁴¹ and p-methylcalix[5]arene–C₆₀ complexes,⁴² and 2 ∶ 1 in 1₃·Bz·Bz·Bz–C₆₀ (Bz = CH₂Ph),⁴³p-phenylcalix[5]arene–C₆₀,¹¹ hexahomotrioxacalix[3]naphthalene–C₆₀,¹³ and p-diiodotrimethylcalix[5]arene–C₆₀ complexes.^42,44 Such an unprecedented large capacity of p-trihalohomooxacalix[3]arenes for C₇₀ may be attributed to its shallow cavity, intermolecular π–π interactions among the guest molecules,⁴⁵ the large van der Waals radii of the heavy halogens, and strong van der Waals interactions between the heavy halogens and the guests.^44,46 Unfortunately, a crystal structure of the complexes has not been determined yet.

The time-course of the change in absorption spectra of C₇₀ was measured in the presence of 1₃·I·I·I in toluene at 20 °C as shown in Fig. 1. The brown color of C₇₀ discharged gradually as the precipitation of C₇₀-1₃·I·I·I proceeded with time. Although a monotonous decrease in the absorption spectra was observed, there was no significant change in the shape of the spectra, indicating that no stable complex existed in solution. On the other hand, no spectroscopic change occurred in the solution of C₆₀ and 1₃·I·I·I, indicating that no complex or precipitate formed under the conditions.

Fig. 1 Time-course of the absorption spectra of C₇₀ (2.0 × 10⁻⁴ M)–p-triiodohomooxacalix[3]arene (1₃·I·I·I, 8.4 × 10⁻³M) in toluene at 20 °C.

The results mentioned above imply that most of the starting compounds, 1₃·I·I·I and the fullerenes, exist separately in solution, and that the complexes with two or three C₇₀, which should exist in a small amount in solution, form precipitates (Scheme 4b), while the C₆₀ complexes do not precipitate at all (Scheme 4a). On the preferential precipitation of the fullerene mixture, at least one of the complexes, 1₃·I·I·I–C₆₀C₇₀, 1₃·I·I·I–(C₇₀)₂C₆₀, and 1₃·I·I·I–(C₆₀)₂C₇₀, is considered to precipitate along with the C₇₀ complexes, because there is always some C₆₀ in the precipitate (Scheme 4c).

Scheme 4

Experimental

¹H (400 MHz) and ¹³C (100 MHz) NMR spectra were obtained on a JEOL JNM-FX400 in CDCl₃ with Me₄Si as internal standard. Chemical shifts are reported in δ and the coupling constants (J) are in Hz. MALDI-TOF-MS analyses were performed by a Shimadzu/Kratos KOMPACT MALDI II using an ethanol–water solution of 2,5-dihydroxybenzoic acid or α-cyano-4-hydroxycinnamic acid as a matrix. High resolution mass spectra were recorded on an Applied Biosystems, Marier (ESI-MS). Absorption spectra were measured in toluene on a Hitachi UV2001. HPLC analyses were performed at 35 °C on a Shimadzu LC-10 equipped with an Imtakt Cadenza CD-C18 column (100 × 4.6 mm) with hexane–propan-2-ol = 4 ∶ 6 (v/v) as an eluent at 0.8 cm³ min⁻¹ using UV detection at 285 nm. Melting points were determined with a Yanaco MP-J3 apparatus and are uncorrected. Flash chromatography was performed with a Wakogel C-300. CH₂Cl₂ was freshly distilled from CaH₂ under argon.

Synthesis of 4-substituted-2,6-diformylphenols (2·R)

Compounds 2·R were prepared according to the literature method³⁴ except for 4-iodo-2,6-diformylphenol.

2,6-Diformyl-4-fluorophenol (2·F). Yield 20 %; mp 108–109 °C (Found: C, 57.05; H, 2.97. Calc. for C₈H₅FO₃: C, 57.15; H, 3.00%); δ_H 7.69 (2H, d, J 7.2, Ar–H), 10.22 (2H, s, CHO), 11.39 (1H, s, OH); δ_C 123.3, 123.8, 153.1, 157.9, 159.8, 191.0.

4-Benzyl-2,6-diformylphenol (2·Bz). Yield 47 %; mp 105–107 °C (Found: C, 75.04; H, 4.94. Calc. for C₁₅H₁₂O₃: C, 74.99; H, 5.03%); δ_H 4.00 (2H, s, CH₂), 7.16–7.34 (5H, m, Ph–H), 7.78 (2H, s, Ar–H), 10.19 (2H, s, CHO), 11.50 (1H, s, OH); δ_C 40.4, 123.0, 126.7, 128.7, 128.8, 133.1, 137.7, 139.5, 162.1, 192.1.

2,6-Diformyl-4-iodophenol (2·I). Since the diformylation of 4-iodophenol gave the desired product only in 3% yield, 2,6-diformylphenol (2·H)³¹ was iodinated with benzyltrimethylammonium dichloroiodate (BTMA·ICl₂) in the presence of NaHCO₃.³³

To a CH₂Cl₂–methanol (40 cm³ ∶ 16 cm³) solution of 2·H (0.60 g, 4.0 mmol) was added BTMA·ICl₂ (1.6 g, 4.6 mmol) and NaHCO₃ (2.3 g, 27 mmol), and the resulting yellow suspension was stirred at room temperature for 9 hours. After filtration through a Celite-SiO₂ bed, the filtrate was concentrated and submitted to flash column chromatography on silica gel to give 2·I (0.77 g, 68%). Mp 145–148 °C (Found: C, 35.09; H, 1.73. Calc. for C₈H₅IO₃: C, 34.81; H, 1.83%); δ_H 8.22 (2H, s, Ar–H), 10.16 (2H, s, CHO), 11.55 (1H, s, OH); δ_C 80.8, 125.0, 154.6, 162.9, 190.8.

Synthesis of 4-substituted-2,6-bis(hydroxymethyl)phenols (3·R)

Compounds 3·R were prepared according to the literature method³⁵ except for 4-iodo-2,6-bis(hydroxymethyl)phenol.

4-Benzyl-2,6-bis(hydroxymethyl)phenol (3·Bz). Mp 92–93 °C (ethyl acetate) (Found: C, 73.76; H, 6.57. Calc. for C₁₅H₁₆O₃: C, 73.74; H, 6.61%); δ_H 2.47 (2H, s, OH), 3.87 (2H, s, CH₂Ph), 4.75 (4H, d, J 2.4, CH₂O), 6.89 (2H, s, Ar–H), 7.14–7.29 (5H, m, CH₂Ph), 7.92 (1H, s, ArOH).

4-tert-Octyl-2,6-bis(hydroxymethyl)phenol (3·Oct^t). Mp 63–66 °C (ethyl acetate) (Found: C, 69.76; H, 9.74. Calc. for C₁₆H₃₀O₄(C₁₆H₂₆O₃ + ½ethyl acetate): C, 69.64; H, 9.74%); δ_H 0.72 (9H, s, C(CH₃)₃), 1.32 (6H, s, C(CH₃)₂), 1.68 (2H, s, CH₂), 4.81 (4H, s, CH₂O), 7.05 (2H, s, Ar–H).

4-Iodo-2,6-bis(hydroxymethyl)phenol (3·I)³⁶. A commercial 2-methoxyisophthalic acid was demethylated in aqueous HI solution at reflux temperature,⁴⁷ reduced to dimethanol with lithium aluminum hydride,⁴⁸ and iodinated with BTMA·ICl₂ in the presence of NaHCO₃.³⁶

Typical procedure for the synthesis of 4-substituted-2,6-bis(trimethylsilyloxymethyl)phenol trimethylsilyl ethers (4·R)

To an acetonitrile solution (30 cm³) of 3·Oct^t was added BTSTA (4.5 cm³, 17 mmol) dropwise over 5 minutes at room temperature under Ar. The resulting pale yellow solution was stirred for 22 hours, the mixture was concentrated on a rotary evaporator and rapidly passed through a short column using hexane as eluent. After careful concentration on a rotary evaporator, the desired compound, 4·Oct^t, was obtained as a colourless oil containing a small amount of hexane in almost quantitative yield, and used immediately in the next coupling reaction.

4-tert-Octyl-2,6-bis(trimethylsilyloxymethyl)phenol trimethylsilyl ether (4·Oct^t). δ _H 0.14 (18H, s, TMS), 0.23 (9H, s, TMS), 0.69 (9H, s, C(CH₃)₃), 1.36 (6H, s, C(CH₃)₂), 1.71 (2H, s, CH₂), 4.65 (4H, s, CH₂O), 7.27 (2H, s, Ar–H).

4-Benzyl-2,6-bis(trimethylsilyloxymethyl)phenol trimethylsilyl ether (4·Bz). δ _H 0.11 (18H, s, TMS), 0.22 (9H, s, TMS), 3.94 (2H, s, CH₂Ph), 4.61 (4H, s, CH₂O), 7.09 (2H, s, Ar–H), 7.17–7.29 (5H, m, Ph).

4-Iodo-2,6-bis(trimethylsilyloxymethyl)phenol trimethylsilyl ether (4·I). δ _H 0.16 (18H, s, TMS), 0.24 (9H, s, TMS), 4.57 (4H, s, CH₂O), 7.60 (2H, s, Ar–H).

Synthesis of homooxacalix[n]arenes via reductive coupling

7,15,23-Triiodo-2,3,10,11,18,19-hexahomo-3,11,19-trioxacalix[3]arene-25,26,27-triol (p-triiodohomooxacalix[3]arene, 1₃·I·I·I). As shown in Scheme 1, two synthetic routes were followed from 2·H. The reductive homocoupling of 2·H and 2·I afforded 1₃·H·H·H and 1₃·I·I·I in 14% and 11% yields, respectively, following the reported procedure.³⁰ The 1₃·H·H·H obtained was iodinated according to the following procedure: a yellow solution of 1₃·H·H·H (0.13 g, 0.32 mmol) and BTMA·ICl₂ in dichloromethane (30 cm³)–methanol (12 cm³) was stirred at room temperature for 0.5 hour. Then, CaCO₃ (0.20 g, 2.0 mmol) was added to the mixture. After being stirred for 19 hours, the suspension was filtered through Celite, and the filtrate was concentrated to about a half in volume and washed with 5% NaHSO₃ (30 cm³) twice. The combined water layer was extracted with dichloromethane (30 cm³) once, and the dichloromethane layers were combined, concentrated and chromatographed on silica gel to give 1₃·I·I·I (0.19 g, 76%) as a white solid. Mp >300 °C (Found: C, 36.72; H, 2.64. Calc. for C₂₄H₂₁I₃O₆: C, 36.67; H 2.69%); δ_H 4.62 (12H, s, CH₂), 7.42 (6H, s, Ar–H), 8.67 (3H, s, OH); δ_C 70.3, 81.1, 126.4, 138.3, 155.6; MALDI-TOF-MS (pos): calcd. for C₂₄H₂₁I₃NaO₆ 809.12, found 809.06 (M + Na⁺).

7,15,23-Trifluoro-2,3,10,11,18,19-hexahomo-3,11,19-trioxacalix[3]arene-25,26,27-triol (p-trifluorohomooxacalix[3]arene, 1₃·F·F·F). 1 ₃ ·F·F·F was prepared according to the reported procedure.³⁰ Yield 29%; mp 218–220 °C (Found: C, 62.06; H, 4.63. Calc. for C₂₄H₂₁F₃O₆: C, 62.34; H, 4.58%); δ_H 4.66 (12H, s, CH₂), 6.86 (6H, d, Ar–H), 8.55 (3H, s, OH); δ_C 70.8, 115.9, 116.2 (d, J 23), 125.1 (d, J 7.1), 151.7 (d, J 2.3), 155.6 (d, J 240); MALDI-TOF-MS (pos): calcd. for C₂₄H₂₁F₃NaO₆ 485.41, found 485.40 (M + Na⁺).

7,15-Diiodo-23-benzyl-2,3,10,11,18,19-hexahomo-3,11,19-trioxacalix[3]arene-25,26,27-triol (1₃·I·I·Bz). 1 ₃ ·I·I·Bz was prepared according to the reported procedure³⁰ using 2,6-diformyl-4-iodophenol (0.19 g, 0.72 mmol), 2,6-bis(trimethylsilyloxymethyl)-4-benzylphenol trimethylsilyl ether (0.17 g, 0.31 mmol), trimethylsilyl trifluoromethanesulfonate (0.26 cm³, 1.4 mmol), triethylsilane (0.24 cm³, 1.5 mmol) and dichloromethane (20 cm³). Yield 6%; δ_H 3.86 (2H, s, CH₂Ph), 4.61 (4H, s, CH₂), 4.62 (4H, s, CH₂), 4.65 (4H, s, CH₂), 6.94 (2H, s, Bz–Ar–H), 7.11–7.26 (5H, m, Bz), 7.41 (2H, s, I–Ar–H), 8.45 (1H, s, Bz–Ar–OH), 8.76 (2H, s, I–Ar–OH); δ_C 40.82, 70.23, 70.30, 71.27, 80.97, 123.84, 126.04, 126.42, 126.65, 128.41, 128.77, 130.40, 132.39, 138.18, 138.26, 141.09, 153.85, 155.69; MALDI-TOF-MS (pos): calcd. for C₃₁H₂₈I₂NaO₆ 772.81, found 773.14 (M + Na⁺).

15,23-Dibenzyl-7-iodo-2,3,10,11,18,19-hexahomo-3,11,19-trioxacalix[3]arene-25,26,27-triol (1₃·I·Bz·Bz). 1 ₃ ·I·Bz·Bz was prepared according to the reported procedure³⁰ using 4-benzyl-2,6-diformylphenol (0.10 g, 0.42 mmol), 2,6-bis(trimethylsilyloxymethyl)-4-iodophenol trimethylsilyl ether (0.11 g, 0.22 mmol), trimethylsilyl trifluoromethanesulfonate (0.14 cm³, 0.80 mmol), triethylsilane (0.14 cm³, 0.88 mmol) and dichloromethane (12 cm³). Yield 26%; δ_H 3.84 (4H, s, CH₂Ph), 4.59 (2H, s, CH₂), 4.63 (2H, s, CH₂), 4.63 (2H, s, CH₂), 6.92 (4H, s, Bn–Ar–H), 7.07–7.26 (10H, m, Ph), 7.38 (2H, s, I–Ar–H), 8.54 (2H, s, Bn–Ar–OH), 8.85 (1H, s, I–Ar–OH); δ_C 40.82, 70.24, 71.23, 80.86, 123.90, 124.13, 126.01, 126.72, 128.39, 128.77, 130.30, 130.37, 132.26, 138.14, 141.16, 153.95, 155.80.

7,15-Diiodo-23-tert-octyl-2,3,10,11,18,19-hexahomo-3,11,19-trioxacalix[3]arene-25,26,27-triol (1₃·I·I·Oct^t). 1 ₃ ·I·I·Oct^t was prepared according to the reported procedure³⁰ using 2,6-diformyl-4-iodophenol (0.20 g, 0.73 mmol), 2,6-bis(trimethylsilyloxymethyl)-4-tert-octylphenol trimethylsilyl ether (0.15 g, 0.31 mmol), trimethylsilyl trifluoromethanesulfonate (0.26 cm³, 1.4 mmol), triethylsilane (0.24 cm³, 1.5 mmol) and dichloromethane (20 cm³). Yield 17%; δ_H 0.71 (9H, s, C(CH₃)₃), 1.30 (6H, s, C(CH₃)₂), 1.67 (2H, s, –CH₂–Bu^t), 4.62 (4H, s, CH₂), 4.63 (4H, s, CH₂), 4.70 (4H, s, CH₂), 7.09 (2H, s, Oct^t–Ar–H), 7.43 (4H, s, I–Ar–H), 8.40 (1H, s, Oct^t–Ar–OH), 8.79 (2H, s, I–Ar–OH); δ_C 31.54, 31.84, 32.32, 37.83, 56.80, 70.16, 70.31, 71.77, 80.95, 122.99, 126.44, 126.75, 127.56, 138.12, 138.26, 141.67, 152.99, 155.70; MALDI-TOF-MS (pos): calcd. for C₃₂H₃₈I₂NaO₆ 795.49, found 795.48 (M + Na⁺).

31-tert-Octyl-7,15,23-triiodo-2,3,10,11,18,19,26,27-octahomo-3,11,19,27-tetraoxacalix[4]arene-33,34,35,36-tetraol (1₄·I·I·I·Oct^t). This compound was obtained in a small amount (3.8 mg) as a byproduct in the preparation of 1₃·I·I·Oct^t; δ_H 0.71 (9H, s, C(CH₃)₃), 1.29 (6H, s, C(CH₃)₂), 1.67 (2H, s, –CH₂–Bu^t), 4.63 (8H, s, CH₂), 4.64 (4H, s, CH₂), 4.69 (4H, s, CH₂), 7.09 (2H, s, Oct^t–Ar–H), 7.41–7.44 (6H, m, I–Ar–H), 8.01 (1H, s, Oct^t–Ar–OH), 8.30 (1H, s, I–Ar–OH), 8.37 (2H, s, I–Ar–OH); δ_C 31.56, 31.85, 32.33, 37.80, 56.73, 70.27, 70.38, 70.38, 71.87, 80.90, 80.90, 123.02, 126.41, 126.46, 126.82, 127.72, 138.33, 138.43, 138.54, 141.60, 153.17, 155.76, 155.88; MALDI-TOF-MS (pos): calcd. for C₄₀H₄₅I₃NaO₈ 1057.55, found 1057.79 (M + Na⁺).

Synthesis of p-trihalohomooxacalix[3]arenes (1₃·I·I·Br and 1₃·I·Br·Br, Scheme 2)

p-Bromohomooxacalix[3]arenes (1₃·H·H·Br and 1₃·H·Br·Br) were obtained in 3% yield after the purification of the mixture from the reductive coupling of 2,6-diformylphenol (2·H) and 4-bromo-2,6-diformylphenol (2·Br).³⁰ The 1₃·I·I·Br and 1₃·I·Br·Br were obtained by iodination of 1₃·H·H·Br and 1₃·H·Br·Br with BTMA·ICl₂ (2.2 and 1.1 equiv.) in the presence of NaHCO₃ (4.6 and 2.3 equiv.), respectively, in dichloromethane–methanol (5 ∶ 2 v/v) followed by a similar work-up to that in the synthesis of 1₃·I·I·I.

23-Bromo-7,15-diiodo-2,3,10,11,18,19-hexahomo-3,11,19-trioxacalix[3]arene-25,26,27-triol (1₃·I·I·Br). δ _H 4.62 (2H, s, CH₂), 4.63 (2H, s, CH₂), 4.64 (2H, s, CH₂), 7.25 (2H, s, Ar–H), 7.42 (4H, s, Ar–H), 8.65 (1H, s, OH), 8.67 (2H, s, OH); δ_C 70.33, 70.34, 70.51, 81.07, 111.26, 125.77, 126.27, 132.27, 138.18, 154.62, 155.45; HRMS (EI): calcd. for C₂₄H₂₀BrI₂O₆ 736.8527, found 736.8541.

15,23-Dibromo-7-iodo-2,3,10,11,18,19-hexahomo-3,11,19-trioxacalix[3]arene-25,26,27-triol (1₃·I·Br·Br). δ _H 4.63 (2H, s, CH₂), 4.64 (2H, s, CH₂), 4.64 (2H, s, CH₂), 7.25 (4H, s, Ar–H), 7.43 (2H, s, Ar–H), 8.66 (2H, s, OH), 8.68 (1H, s, OH); δ_C 70.34, 70.53, 81.06, 111.27, 125.79, 126.29, 132.22, 132.28, 132.33, 138.14, 138.19, 138.25, 154.64, 155.46; HRMS (EI): calcd. for C₂₄H₂₀Br₂IO₆ 688.8677, found 688.8683.

General procedure for the precipitation of the complex of fullerene with homooxacalix[n]arene (Tables 1 and 2)

To a toluene solution of fullerene was added a toluene solution of homooxacalix[n]arene, and the mixture was stirred at room temperature. After filtration of the formed precipitate, the filtrate was concentrated and washed with acetone thoroughly to give the recovered fullerene. The ratios of C₆₀ ∶ C₇₀ in both precipitate and filtrate were determined by HPLC.

Recovery of fullerene from the C₆₀/C₇₀–1₃·I·I·I complex

To an o-dichlorobenzene solution (12 cm³) of the complex (12.4 mg) was added 1 M NaOH (10 cm³), and the resulting heterogeneous solution was vigorously stirred at room temperature for an hour. After separation of the two layers, the organic layer was washed with 1 M NaOH (15 cm³) twice. The combined water extracts were made acidic with conc. HCl and extracted with dichloromethane (20 cm³) three times and toluene (30 cm³) once. Concentration of the combined dichloromethane extracts gave pure 1₃·I·I·I (3.7 mg). On the other hand, the o-dichlorobenzene layer was combined with toluene extract, dried over MgSO₄ and concentrated in vacuo to give liberated fullerenes (9.0 mg).

Acknowledgements

The author is grateful to Professors Keiji Maruoka, Atsuhiro Osuka and Kazumi Matsushige (Kyoto University) for their encouragement, Dr Tomonari Wakabayashi (Kyoto University) for his helpful suggestions, Mr Itaru Yazawa (Imtakt Co.) for his kind offering of a HPLC column (Cadenza CD-C18), Mr Koji Sasaki for his assistance of measuring HRMS, and Messrs Takashi Onozawa and Susumu Kosugiyama (Tokyo Chemical Industry Co., Ltd.) for their assistance with experiments and their kind donation of some reagents. A part of this work was supported by a Grant-in-Aid for Research for Young Researchers from Kyoto University-Venture Business Laboratory (KU-VBL).

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Footnote

† Anal. Calcd for C₃₉₈H₄₂I₆O₁₂ (2 ∶ 5 complex): C 82.76, H 0.73, I 13.18; found: C 83.14, H 0.84, I 12.61%. Anal. Calcd for C₂₃₄H₂₁Br₃O₆ (3 ∶ 1 complex): C 88.71, H 0.67, Br 7.57; found: C 88.66, H 0.65, Br 7.67%.

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