Distance - Dependent para - H 2 f ortho - H 2 Conversion in H 2 @ C 60 Derivatives Covalently Linked to a Nitroxide Radical

W recently reported 1 distance-dependent nuclear spin relaxation (1/T1) of a series ofH2@C60 derivatives covalently linked to a nitroxide radical. The results show that T1 increases with the distance between the encapsulated H2 and the radical centers (r), and the relaxivity rate constant is proportional to r . Another aspect of interest for such H2@C60 derivatives is the distance dependence of the nitroxide spin catalyzed para/orthoH2 conversion. 2,3 The H2 molecule exists as two allotropes (para-H2 and orthoH2). 4 At room temperature normal (equilibrium) H2 consists of 75% of ortho-H2 (oH2) and 25% of para-H2 (pH2). When the temperature decreases to 77 K, the composition at equilibrium of pH2 and oH2 is 50%/50%. With adventitious spin catalysts, interconversion of pH2 and oH2 is very slow, typically months or longer. In the presence of a well-defined and selected paramagnetic spin catalyst, the interconversion rate of pH2 and oH2 can be greatly increased. The availability of a host guest system, H2@C60, provides an opportunity to explore the ortho/para conversion of H2 incarcerated in a C60 under welldefined conditions. Oxygen has been used as a spin catalyst for para/ortho-H2 interconversion for the H2@C60 system. The lifetime of pH2 f oH2 conversion at room temperature is ∼100 h in an oxygen-saturated solution. The latter time scale is comparable to that for a small molecule of nitroxide, 2,2,6,6tetramethylpiperidine-1-oxyl (TEMPO), used as a spin catalyst at a concentration of 50 mM. The spin conversions of oH2 and pH2 with both oxygen and TEMPO as spin catalysts in H2@C60 are bimolecular processes, that is, the paramagnetic species diffuses, and encounters the H2@C60 molecule in order to catalyze the conversion. Compared with H2 gas, an advantage of H2 encapsulated inside C60 is that one can synthesize a range of H2@C60 derivatives in which a paramagnetic spin catalyst is covalently linked to the C60 surface. Once the nitroxide radical is attached to the H2@C60 surface, a spin catalyst with a predetermined and relative fixed distance for the intramolecular para/ortho-H2 conversion is realized. Through available synthetic methods, one can readily systematically vary the distance between encapsulatedH2 and the attached nitroxide radical centers. With these nitroxide derivatives of C60, an investigation of distance-dependent conversion of para/ortho-H2 is possible and is the topic of this report. Chart 1 shows a series of H2@C60 nitroxide derivatives (1 7) with a 2-fold variation in the distance between the encapsulated H2 and the nitroxide radical centers. 1 A structurally similar diamagnetic H2@C60 derivative (D1) was used as a control compound to provide benchmark data. The pH2 f oH2 conversion at room temperature was examined by monitoring the change of the H NMR spectrum and the resulting conversion rates are analyzed by a modification of Wigner’s theory. The synthesis of 1 7 andD1 has been previously reported. From molecular modeling calculations, the distance between encapsulated H2 and the nitroxide radical centers increases monotonically from 1 to 6. The derivative 7 is similar in structure to 4, but has a biradical attached to the C60 surface. In this report, a mixture of isotopomers of H2 and HD were encapsulated in the C60 derivatives, so that the HNMR signal of the HD is an internal standard. The conversion of oH2f pH2 of 1 7 andD1 was performed at 77 K with liquid oxygen as the spin catalyst following the procedures described before. Once the 50%/50% ratio of