Distance-Dependent Paramagnet-Enhanced Nuclear Spin Relaxation of H2@C60 Derivatives Covalently Linked to a Nitroxide Radical

A series of H2@C60 derivatives covalently linked to a nitroxide radical has been synthesized. We report distance-dependent nuclear spin relaxivity of H2 in these derivatives. The results clearly indicate that the relaxivity of H2 is distancedependent and in good agreement with the Solomon-Bloembergen equation, which predicts a 1/r dependence. SECTION Kinetics, Spectroscopy N uclear spin relaxation of H2 and its isotopomers has been extensively studied in the gas, liquid, and solid phases. Recently, a few reports of nuclear spin relaxation of H2 in ordinary organic solvents have appeared. The discovery that the hydrogen molecule can be encapsulated in the fullerene (C60) cage leads to intriguing investigations to explore the interactions of the endohedral H2with the outside world. Nuclear spin relaxation is a quantitative approach toprobe these interactions.According to recent reports, the relaxation times (T1) are 10-20 times shorter for H2@C60 than those for H2 in organic solvents. However, in the presence of paramagnetic relaxants, such as nitroxide radicals, the relaxationeffectof theparamagnet is enhanced five-fold inH2@C60 compared to that for H2 under the same conditions. 8 These results indicate that the hydrogen molecule in H2@C60 is not insulated from magnetic contact with the outside world. Encapsulation of H2 inside of C60 also enables us to take advantage of C60 chemical reactivity to covalently attach a paramagnet to theC60 cage, which provides a platform for studying intramolecular spin relaxation of H2 (so-called inner sphere proton relaxivity). According to the Solomon-Bloembergen equation, the inner sphere relaxivity (1/T1) is proportional to the reciprocal of the sixth power of the distance between the proton and the paramagnet centers.Most reports on inner-sphere proton relaxivity are related to the first coordination sphere of complexes formed by water and paramagneticmetal ions. Due to the difficulty in obtaining a large range of accurate distance variations between the proton and the paramagnetic ion centers, there seems to have been no previous report of distance-dependent proton spin relaxation by the same kind of relaxant. In the present investigation, we have synthesized a series of H2@C60 derivatives covalently linked to a nitroxide radical, as shown in Chart 1. By varying the spacers between C60 and the TEMPO functional groups, the calculated distance (by molecular modeling) between the encapsulated H2 and the nitroxide radical centers increases in the order from 1 to 6. The derivative 7 consists of a nitroxide biradical substituent as a counterpart of the monoradical 4. We report distancedependent nuclear spin relaxation results for these H2@C60 nitroxide derivatives. Derivatives 1-3 were synthesized using the well-known Prato reaction following literature procedures. Synthesis of derivatives 4 and 7 was completed by the Bingle-Hirsh reaction following literature reports. Details of the synthesis ofderivatives5 and6 are given in the Supporting Information. Nuclear spin relaxation times (T1) were measured by the inversion-recovery method in argon-bubbled 1,2-dichloroChart 1. Structures of H2@C60 Derivatives 1-7 a Blue balls indicate incarcerated H2 or HD. Received Date: May 17, 2010 Accepted Date: June 17, 2010