Abstract
AbstractAn 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.
Highlights
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
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-d]. 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-c]. 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]
Murata and co-workers 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 rely upon encapsulation of the endohedral molecule by 2.[19a-b]. Recently, we used an open-cage fullerene with a larger opening to prepare Ar@C60 and CH4@C60.[20a-b]. We 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
Summary
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. 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-c] 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.
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