A Solid‐State Intramolecular Wittig Reaction Enables Efficient Synthesis of Endofullerenes Including Ne@C₆₀, ³He@C₆₀, and HD@C₆₀

Abstract

An open‐cage fullerene incorporating phosphorous ylid and carbonyl group moieties on the rim of the orifice can be filled with gases (H₂, He, Ne) in the solid state, and the cage opening then contracted in situ by raising the temperature to complete an intramolecular Wittig reaction, trapping the atom or molecule inside. Known transformations complete conversion of the product fullerene to C₆₀ containing the endohedral species. As well as providing an improved synthesis of large quantities of ⁴He@C₆₀, H₂@C₆₀, and D₂@C₆₀, the method allows the efficient incorporation of expensive gases such as HD and ³He, to prepare HD@C₆₀ and ³He@C₆₀. The method also enables the first synthesis of Ne@C₆₀ by molecular surgery, and its characterization by crystallography and ¹³C NMR spectroscopy.

Solid‐state filling of a stable phosphorus ylid open‐cage fullerene is followed by heating, inducing an in situ intramolecular Wittig reaction to reduce the size of the cage opening and trapping a single gas atom or molecule inside. Expensive gases can be compressed to high pressure, leading to high incorporation of the endohedral species. Cage closure steps allow preparation of gram‐scale endohedral fullerenes including ³He@C₆₀, HD@C₆₀, and Ne@C₆₀.

Introduction

Compounds in which atoms or small molecules (A) are trapped in the cavity of cage fullerenes such as C₆₀ are known as endofullerenes and denoted A@C₆₀. They are of interest for study of their material properties and the properties of the isolated endohedral species.[ ^1a , ^1b , ^1c ]

The noble gas endofullerenes of C₆₀ have been the object of sustained theoretical studies[ ^2a , ^2b , ^2c , ^2d , ^2e , ^2f , ^2g , ^2h , ²ⁱ , ^2j , ^2k ] of their geometry, reactivity and electronic structure, and subject to a recent review. ^[3] Encapsulation of a noble gas in C₆₀ was first detected by mass spectrometry in the formation of ⁴He@C₆₀ from collision of accelerated C₆₀ ^+. with helium gas, ^[4] and ⁴He@C₆₀ was also observed at the part‐per‐million level in C₆₀ formed by the carbon arc discharge method using He as the buffer gas. ^[5] High temperature and pressure exposure of C₆₀ to the noble gases allows direct incorporation, and isolation of samples containing approximately 0.1 % of the endohedral atom He, Ne, Ar, or Kr, or 0.03 % of Xe. ^[6] Incorporation is improved by addition of KCN, to levels of 1 % for He and circa 0.3 % for Ar, Kr, or Xe, albeit at the cost of lower recovery.[ ^7a , ^7b , ^7c ] High‐energy helium bombardment of C₆₀ under explosive conditions has also been used for direct encapsulation of the noble gas. ^[8] Removal of empty C₆₀ using many cycles of preparative HPLC has been reported for the heavier noble gas endofullerenes (Ar@C₆₀, 1.3 mg, 98 % filled;[ ^7c , ⁹ ] Kr@C₆₀, 0.14 mg, 90 % filled; ^[10] Xe@C₆₀ 0.32 mg, 50 % filled ^[7b] ), but direct encapsulation methods are not practical for synthesis of larger amounts of a noble gas endofullerene, and cannot be used to prepare molecular endofullerenes such as H₂@C₆₀.

Yet, many applications exist to benefit from the availability of larger‐scale synthetic methods. Noble gas endofullerenes encapsulating the ³He or ¹²⁹Xe isotopes, with nuclear spin=$1/2$ , are potentially valuable as biosensors for detection by magnetic resonance, or as tools to monitor the course of fullerene reactions by NMR.[ ^11a , ^11b , ^11c , ^11d ] Determining the quantised rotational and translational energy levels of an endohedral species, using inelastic neutron scattering and IR/THz spectroscopy, provides a powerful test of current models of non‐bonding interactions.[ ^12a , ^12b , ^12c ] Endofullerenes in which a trapped molecule exhibits nuclear spin isomerism are of importance for the study of spin isomer interconversion, allotrope enrichment, and have potential applications to chemical and clinical magnetic resonance. ^[13] Here, nuclear spin conversion in H₂@C₆₀[ ^14a , ^14b , ^14c , ^14d ] and H₂O@C₆₀[ ^15a , ^15b ] has been studied to date.

To address these needs, a great deal of progress has been made in the synthesis of endofullerenes by multi‐step routes (termed “molecular surgery”) in which a hole is chemically opened in the fullerene, an atom or small molecule enters the cavity, and the opening is then repaired to restore the original carbon cage with the atom or small molecule entrapped. Following the first insertion of He and H₂ into an open fullerene by Rubin et al., ^[16] cage closure was pioneered by Komatsu and Murata who reported syntheses of H₂@C₆₀ and ⁴He@C₆₀ from open fullerene 1, by insertion of H₂ directly into 1 (Figure 1), and of ⁴He into the sulfoxide derivative, under high pressure (⁴He at 1230 atm, H₂ at 800 atm) followed by a series of chemical reactions to re‐form the C₆₀ cage.[ ^17a , ^17b , ^17c ] The method allows larger‐scale synthesis of the noble gas endofullerene than is possible using direct encapsulation, 38 mg of ⁴He@C₆₀ with 30 % ⁴He filling was obtained. Similarly, the molecular endofullerene H₂@C₆₀ was prepared with 118 mg mass recovery and >90 % H₂ filling. Murata and co‐workers also developed the synthesis and orifice‐suture of open‐cage fullerene 2 in their preparation of H₂O@C₆₀, ^[18] and we have since reported an optimised procedure to obtain H₂O@C₆₀, and syntheses of H₂@C₆₀ and HF@C₆₀ that also rely upon encapsulation of the endohedral molecule by 2.[ ^19a , ^19b ] Recently, we used an open‐cage fullerene with a larger opening to prepare Ar@C₆₀ and CH₄@C₆₀.[ ^20a , ^20b ]

Figure 1.

Open‐cage fullerenes used in the reported syntheses of ⁴He@C₆₀, H₂@C₆₀, H₂O@C₆₀, and HF@C₆₀.

We now report a phosphorus ylid derivative of 2 that can be filled in the solid state and the orifice closed in situ by raising the temperature, enabling efficient synthesis of H₂@C₆₀ and ⁴He@C₆₀, and their expensive isotopologues HD@C₆₀, D₂@C₆₀, and ³He@C₆₀, as well as the first “molecular surgery” synthesis of Ne@C₆₀.

Results and Discussion

Our reported synthesis of H₂@C₆₀ involved trapping of H₂ inside open‐cage fullerene 2. ^[19b] In this key step, 2 was formed in situ upon heating its hydrate, bis(hemiketal) 3, in solution with 3 Å molecular sieves under a high‐pressure atmosphere of H₂, to give H₂@2 with 60 % H₂ incorporation. Then, heating H₂@2 with Ph₃P induced the first stage of ring closure, giving H₂@5, but also had to be conducted under a high pressure of H₂ to avoid loss of the endohedral molecule (Scheme 1). We later showed that HF@3 undergoes dehydration and slow reaction with Ph₃P at room temperature, to form a stable phosphorus ylid that is an intermediate in the ring closure leading to 5 but, unfortunately, only undergoes the necessary intramolecular Wittig reaction upon heating—leading to complete loss of HF. The structure of the phosphorus ylid, HF@4, was suggested by close agreement of a calculated ¹³C NMR spectrum of 4 with the experimental spectrum of the regioisomer shown in Scheme 1, ^[19a] and we have now confirmed the structure of 4 by X‐ray crystallography (Figure 2). The crystal structure reveals that the C=O bond (1.223 Å) of the carbonyl group adjacent to the phosphorus ylid moiety is longer than those of the remote carbonyl groups (1.209 Å and 1.202 Å), suggesting that the ylid has substantial enolate character which may explain its good stability. ^[21]

Scheme 1.

Phosphorus ylid 4 is a precursor in reported syntheses of endofullerenes, including H₂@C₆₀, and may be prepared from known (bis)hemiketal 3 under mild conditions. Stable, and isolable, 4 is a suitable intermediate for encapsulation and in situ entrapment of an endohedral species A.

Figure 2.

Crystal structure of phosphorus ylid 4. Thermal ellipsoids are shown at the 50 % probability level and hydrogen atoms are omitted for clarity. Deposition Number 1953259 contains the supplementary crystallographic data for this paper. These data are provided free of charge by the joint Cambridge Crystallographic Data Centre and Fachinformationszentrum Karlsruhe Access Structures service www.ccdc.cam.ac.uk/structures. Structure details are reported in Section S3.1 of the Supporting Information.

If entry of H₂ or another species into 4 occurred at a temperature lower than that required for the intramolecular Wittig reaction, it would be possible to fill the ylid 4 then induce the Wittig reaction that traps the endohedral species simply by raising the temperature. This would avoid the need to vent gas, add Ph₃P reagent, and re‐pressurise as in our synthesis of H₂@C₆₀. Comparison of the activation enthalpies for entry of some small atoms and molecules (He, Ne and H₂) through the 16‐membered openings of 2 and 4 was made by density functional theory calculations and, in each case, the barrier to entry into 4 was only around 10 kJ mol⁻¹ higher than that for entry into 2 (Table 1).

Table 1.

Binding and activation energies for entry and exit of He, Ne, and H₂ into open fullerenes 2 and 4.^[a]

	ΔH (binding) [kJ mol−1]	ΔH ≠ (entry) [kJ mol−1]	ΔH ≠ (exit) [kJ mol−1]
H2+2 a⇌H2@2 a	−22.3	50.6	72.9
He+2 a⇌He@2 a	−10.4	31.5	41.9
Ne+2 a⇌Ne@2 a	−20.1	52.8	72.9
H2+4 a⇌H2@4 a	−21.0	60.6	81.6
He+4 a⇌He@4 a	−10.3	39.0	49.4
Ne+4 a⇌Ne@4 a	−19.2	64.6	83.8

[a] Energies were calculated with density functional theory using the M06‐2X functional and cc‐pVTZ basis set at M06‐2X/cc‐pVDZ geometries. Model structures 2 a and 4 a, in which the 6‐tert‐butylpyridyl groups are replaced by methyl substituents, were used to represent 2 and 4. For full details and references see the Supporting Information.

So, under the same conditions of our solution‐phase H₂ filling of 2 (120 atm of H₂, 120 °C in 1,2‐dichlorobenzene) ^[19b] we obtained comparable 62 % H₂ encapsulation by 4, implying that closure of 4 does not occur before equilibration of H₂ between the cavity and outside. Since the Wittig closure reaction is now unimolecular, a solvent should not be necessary and, indeed, heating solid 4 gave 5 in excellent yield. The closure was significantly slower than in solution, for example, heating 4 for 3 h at 120 °C gave 55 % conversion to 5 in 1,2‐dichlorobenzene‐d ₄ solution, but only 4 % conversion in the solid state.

At 160 °C closure of solid 4 was complete in less than 1 h, and heating the solid phosphorus ylid 4 under approximately 500 atm H₂ at 160 °C gave H₂@5 in 88 % yield with 80 % encapsulation, demonstrating that endohedral incorporation of the gas is faster than ring closure in the solid state.

Open fullerenes have been filled in the solid state before,[ ^17b , ^17c , ^22a , ^22b , ^22c ] but this is the first example where contraction of the cage opening can be carried out in situ, and there are huge practical advantages. The pressure reactor can be small in volume, allowing higher pressures to be used safely, and the volume of gas needed to achieve the high pressure is much smaller—essential if the gas is expensive and/or rare (such as for encapsulation of ³He, see below). To apply this method of solid‐state filling and in situ closure for large‐scale synthesis, stainless steel pressure reactors with volumes between 1.2–5.0 mL and with pressure ratings of 2400–4000 atm were constructed as part of a bespoke apparatus for compression of gas using a manual intensifier (see Supporting Information). Results reporting the preparation of A@5, where A=Ne and isotopologues of H₂ and He, are summarised in Table 2, and described in detail below.

Table 2.

Solid‐state filling of 4 and in situ ring contraction.^[a]

Entry	Gas A	Pressure [atm]	A@5 filling factor [%][b]	Yield of isolated A@5 [%][c]
1	HD	520	83	75
2	D2	423	73	72
3	H2	1806	95	79
4	4He	2374	50	84
5	3He	2315	52	79
6	Ne	1742	63	82

[a] All reactions were performed under the stated pressure of gas A, at temperatures in the range 140–186 °C and for 0.75–14 h. For full details see the Supporting Information. [b] Filling factors were calculated by comparison of integrals in the ¹H NMR spectrum (A=H₂) or by comparison of peak intensities for the filled and empty species in the ¹³C NMR spectrum (A=Ne, He, HD, or D₂). [c] Yield of isolated product, following purification by column chromatography.

H₂@C₆₀, HD@C₆₀, and D₂@C₆₀

HD@C₆₀ is an interesting isotopologue of H₂@C₆₀ as it lacks the nuclear symmetry, and thus selection rules in the rotational energy levels. HD@C₆₀ has been prepared before, and its IR and inelastic neutron scattering (INS) spectra acquired, but as a mixture with H₂@C₆₀ and D₂@C₆₀.[ ^23a , ^23b ] Detailed predictions of the variable temperature INS spectra of HD@C₆₀ have been made, ^[24] but to test them experimentally requires a large sample of pure material. Although the use of solid‐state filling helps overcome the problem of the high cost of the gas, the low pressure (10 atm) at which HD is available limits the pressure our intensifier could generate to 80–85 atm. With an initial pressure of 82 atm HD gas at room temperature, heating with 4 at 160 °C for 2 h gave HD@5 with only 44 % incorporation. The problem was alleviated by cooling the pressure reactor in liquid nitrogen before charging to 119 atm HD. Upon warming to room temperature, a pressure of 420 atm was achieved and at 140 °C, 520 atm. HD@5 was obtained in 75 % yield and with an 83 % filling factor of HD (Table 2, entry 1). Of concern was the known disproportionation of HD to H₂+D₂ in contact with iron. ^[25] Under the described conditions, disproportionation was limited to <1 %, but when higher temperatures and pressures were used for the filling/closure it became significant (after overnight exposure of 4 to 800 atm of HD at 180 °C, 35 % disproportion, as measured inside the cage, had occurred).

Similarly, cryogenic charging of the pressure reactor enabled heating of phosphorus ylid 4 under 423 atm of D₂ gas, or circa 1800 atm of H₂. Respectively, D₂@5 was obtained with 73 % filling and H₂@5 with 95 % filling, both in good yield (Table 2, entries 2 and 3).

Closure of the cage opening of each compound, HD@5, D₂@5, and H₂@5, was carried out according to the two‐step procedure previously reported (Scheme 2). ^[19b] From a single high‐pressure filling experiment we were easily able to prepare 100–150 mg of HD@C₆₀ (83 % filling), D₂@C₆₀ (73 % filling), or H₂@C₆₀ (approx. 95 % filling), and obtain these endofullerenes on gram scale over several batches.

Scheme 2.

Cage closure of A@5 to prepare A@C60, where A=Ne, ³He, ⁴He H₂, D₂, or HD. After reduction to A@6, the second step involves sequential [4+2], retro[4+2], and [2+2+2] cycloaddition. ^[19b]

⁴He@C₆₀ and ³He@C₆₀

Helium has much less favourable enthalpy of binding into open fullerene 4 than H₂ (Table 1) so a higher pressure is required for good incorporation. Our pressure intensifier has a limit of 1000 atm, but the cryogenic method described above enabled pressures well above 2000 atm to be generated in the reactor at a temperature of 180 °C. Filling and ring contraction of 4 under almost 2400 atm of ⁴He lead to 50 % helium incorporation in ⁴He@5 (Table 2, entry 4) while, for comparison, 600 atm gave a filling factor of only 21 %. Synthesis of ⁴He@C₆₀ with 30 % ⁴He encapsulation has been reported by Komatsu and Murata, ^[17b] although the method we describe herein is more easily scalable for preparation of the endofullerene in larger (gram) quantities. The rarity and expense of ³He has previously been an obstacle to the preparation of ³He@C₆₀—only 10 L at STP was available to us. So, a compressor was designed and built to minimise the dead volumes in our existing apparatus, and enabled ³He compression into a 2.4 mL capacity pressure reactor using the cryogenic charging method. This gave a pressure in excess of 2300 atm under an elevated temperature of >170 °C for the intramolecular Wittig reaction, and ³He@5 was obtained with 52 % ³He filling (Table 2, entry 5). Importantly, we were able to recover >99.8 % of the ³He gas back into the apparatus and source cylinder after each experiment. Around 40 % of the unrecovered gas had been encapsulated by the fullerene. Once again, conversion of He@5 to He@C₆₀ was conducted as described in Scheme 2, and >1 g material with circa 50 % filling was prepared for each isotope.

Ne@C₆₀

The DFT calculations given in Table 1 suggested that incorporation of Ne into 2 or 4 should be possible. The synthesis of Ne@C₆₀ was first achieved by filling solid fullerene 2 (under Ne gas at 380 atm, 150 °C, 17 h), and isolation of the product as its hydrate, Ne@3. The filling factor of Ne@3 was estimated to be 15–20 % from the ¹H NMR spectrum. Although an attempt to convert Ne@3 to Ne@5 was made using the established method of heating with Ph₃P at 120 °C, complete loss of neon occurred. Changing the conditions to PhP(2‐furyl)₂ at 60 °C led to isolation of Ne@5 without loss of endohedral neon, and Ne@C₆₀ was subsequently obtained with 16 % filling following the two‐step procedure of Scheme 2. Next, we examined the solid‐state filling and in situ ring contraction of ylid 4 for preparation of Ne@5 with higher incorporation of neon. From exposure of 4 to 600 atm Ne at 160 °C for 2 h, Ne@5 was recovered with approximately 40 % filling estimated from the high resolution ESI+ mass spectrum by comparison of peak intensities for the filled and empty species. Furthermore, using the cryogenic method described earlier a pressure of >1700 atm could be attained, and a filling of 63 % in Ne@5 was achieved (Table 2, entry 6). As expected, Ne@C₆₀ with the same filling factor of 63 % was ultimately obtained, in 43 % yield from Ne@5.

Ne@C₆₀ has been previously prepared only using the direct insertion method, by heating C₆₀ at high temperature and pressure with the gas, and resulting in unreported (small) quantities of material with 0.1–0.3 % Ne incorporation.[ ⁶ , ^7a ] With >0.4 g in hand, we carried out enrichment of Ne@C₆₀ (63 % filled) to a sample with >99.5 % incorporation of neon, by recycling preparative HPLC, and obtained a crystal structure of the nickel(II) octaethylporphyrin/benzene solvate ^[26] in which the C₆₀ cage is indistinguishable from that of empty C₆₀ (Figure 3). ^[27] The structure shows the neon atom at the centre of the cage.

Figure 3.

Crystal structure for the nickel(II) octaethylporphyrin/benzene solvate of Ne@C₆₀. Thermal ellipsoids are shown at the 50 % probability level. Hydrogen atoms and benzene molecules are omitted for clarity. Deposition Number 1953465 contains the supplementary crystallographic data for this paper. These data are provided free of charge by the joint Cambridge Crystallographic Data Centre and Fachinformationszentrum Karlsruhe Access Structures service www.ccdc.cam.ac.uk/structures. Structure details are reported in Section S3.2 of the Supporting Information.

In the noble gas@C₆₀ series studied by ¹³C NMR spectroscopy to date, deshielding of the cage resonance with respect to empty C₆₀ increases with the van der Waals radius of the entrapped atom: He@C₆₀, Δδ=+0.02 ppm; ^[17b] Ar@C₆₀, Δδ=+0.18 ppm; ^[20a] Kr@C₆₀, Δδ=+0.39 ppm; ^[10] and Xe@C₆₀, Δδ=+0.96 ppm. ^[7b] Shown in Figure 4 a, the ¹³C NMR resonance of Ne@C₆₀ was measured with a chemical shift of δ _C=142.83 ppm in 1,2‐dichlorobenzene‐d ₄ at 298 K, deshielded by Δδ=+0.024 ppm relative to empty C₆₀. The absence of a visible peak for empty C₆₀ attests to the high purity of the Ne@C₆₀ sample, and displacement of the ¹³C peak of empty C₆₀ by 23.6 ppb in the shielding (negative δ) direction, relative to the ¹³C peak of Ne@C₆₀, was measured from a mixed sample of Ne@C₆₀ and C₆₀. The slight deshielding of the cage ¹³C resonance in Ne@C₆₀ is within 1.2 ppb of that measured for He@C₆₀ (Δδ=+0.025 ppm relative to C₆₀), and is much less pronounced than in Ar@C₆₀ and the heavier noble gas endofullerenes.

Figure 4.

a) ¹³C NMR spectrum of Ne@C₆₀ (>99.5 % neon filling), 31–32 mm solution in degassed 1,2‐dichlorobenzene‐d ₄ at a field of 176 MHz and 298 K, acquired with 152 transients. There is no visible peak from unfilled C₆₀. b) Expanded view of the base of the Ne@C₆₀ resonance, acquired at 298 K with 680 transients, to show side peaks arising from minor isotopomers with adjacent ¹³C nuclei that share either a hexagon–pentagon (HP) or hexagon–hexagon (HH) edge.

Figure 4 b shows two side peaks with an intensity ratio of 2:1 that are assigned to minor isotopomers of Ne@C₆₀ that each contain a pair of neighbouring ¹³C nuclei separated by one bond. ^[28] Two peaks are observed since there are two types of carbon−carbon bond in C₆₀, either a hexagon–pentagon (HP) or shorter hexagon–hexagon (HH) shared edge, present in a 2:1 ratio, respectively. The shifts of the side peaks relative to the main Ne@C₆₀ resonance correspond to one‐bond isotope shifts of ¹Δ_HP=12.53±0.01 ppb for the inner peak, and ¹Δ_HH=19.94±0.03 ppb for the outer peak. Side peaks of this kind were also observed for the helium endofullerenes, ³He@C₆₀ and ⁴He@C₆₀. ^[29]

Large‐scale synthesis of phosphorus ylid 4

The methods described above allow the filling of large amounts of open‐cage fullerene 4 with the various endohedral species described. Synthesis of bis(hemiketal) 3, the precursor to 4, is well described by Murata[ ¹⁸ , ³⁰ ] but optimisation of the early cage‐opening steps was necessary to more efficiently supply material for our scaled‐up filling procedure (Scheme 3). After opening the C₆₀ cage by reaction with 3,6‐bis(6‐(tert‐butyl)pyridin‐2‐yl)pyridazine, ^[18] the existing method for expanding the orifice by photo‐oxygenation with singlet oxygen required irradiation of 6 in a mixture of CS₂ (flashpoint circa 30 °C, auto‐ignition temperature 100 °C) and 1‐chloronaphthalene, using a 500 W Xenon lamp as O₂ is passed through the mixture over 23 h. Although recent improvements in both the reaction yield and safety have resulted from use of an LED light source and replacement of CS₂ with CCl₄, a reaction time of 48 h is required (from 3 g C₆₀). ^[30]

Scheme 3.

Optimised cage opening of C₆₀.

We found that use of 2 molar equiv of C₆₀ (relative to the pyridazine reagent) in the first (cycloaddition) step gave a cleaner reaction—presumably by suppressing the formation of multiple addition products. Simple dilution of the product reaction mixture with toluene and direct transfer to the photoreaction vessel enabled the excess remaining C₆₀ to serve as an efficient photosensitizer for the formation of ¹O₂, and the reaction time of photo‐oxygenation was correspondingly reduced. Indeed, use of a 400 W high‐pressure sodium lamp in a water‐cooled immersion well led to completion of the photochemical reaction in just 1 h (on a scale using 10 g C₆₀). Excess C₆₀ was readily recovered during purification of 5 by column chromatography and the overall yield was 70 %, based on the portion of C₆₀ that was consumed in the reaction (see Supporting Information). Conversion of 5 to bis(hemiketal) 3 is carried out by oxidative cleavage using 4‐methylmorpholine 4‐oxide following the published procedure, ^[18] and 3 is converted to phosphorus ylid 4 upon treatment with 16 molar equiv Ph₃P at 60 °C. ^[19a] For this final step, an excellent and reproducible yield (85–90 %) of 4 is achieved at multi‐gram scale. Overall, the improved methods allow a 10 g batch of ylid 4 to be prepared in a few days.

Conclusion

Optimised synthesis and solid‐state filling of a phosphorus ylid 4 with H₂, He, or Ne, with in situ contraction of the cage opening via an intramolecular Wittig reaction, traps the gas inside. Large amounts of material can be processed in a small pressure reactor, allowing the use of high‐pressure conditions and expensive gases. Further transformations gave up to 1 g each of endofullerenes ³He@C₆₀, ⁴He@C₆₀, Ne@C₆₀, H₂@C₆₀, D₂@C₆₀, and HD@C₆₀, which will enable experimental studies of these interesting species, for example using NMR, IR, and inelastic neutron‐scattering spectroscopy. These studies are underway.

The new availability of a large quantity of ³He@C₆₀ offers the potential to extend its use in magnetic resonance imaging, where ³He gas is already used to visualise air passages in the lung by attachment to other species—although the development of methods for hyperpolarisation of the ³He nucleus will be needed to achieve this practical application. The first synthesis of pure HD@C₆₀ will permit the rotational and translational energy levels to be determined by INS, and compared with the established predictions as a test of these theoretical methods.

Synthesis of pure Ne@C₆₀ completes the series of noble gas endofullerenes to be prepared in macroscopic quantities (up to Xe@C₆₀). The absolute chemical shift, and relative endohedral shift, of the cage ¹³C NMR resonance of Ne@C₆₀ indicates very similar interactions between the noble gas atom and the fullerene cage to that in He@C₆₀.

Conflict of interest

The authors declare no conflict of interest.

Supporting information

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Supplementary

G. Hoffman, M. C. Walkey, J. Gräsvik, G. R. Bacanu, S. Alom, S. Bloodworth, M. E. Light, M. H. Levitt, R. J. Whitby, Angew. Chem. Int. Ed. 2021, 60, 8960.