N@C₆₀ is a paramagnetic endohedral fullerene species comprised of a single nitrogen atom incarcerated within a C₆₀ carbon cage. Shielding of the nitrogen from the external environment results in extremely long relaxation times of its electron spin (T₁ = 0.375 ms, T₂ = 0.25 ms). This provides the molecule with a very sharp EPR signal,¹ which undergoes observable dipolar broadening in the presence of other paramagnetic species, suggesting that N@C₆₀ derivatives would be ideal candidates as spin probes for biological applications.^2–4 In order for this to be realised functionalisation of the inherently hydrophobic fullerene cage must be undertaken to render it compatible with aqueous environments.

Several methodologies for the solubilisation of empty cage and other endohedral fullerene species have been demonstrated,^5–7 however many of these are unsuitable for use with N@C₆₀ due to its instability at elevated temperatures.⁸ Herein we report the first examples of water-soluble derivatives of N@C₆₀, prepared using an adaptation of the widely used Bingel reaction.⁹ Preliminary investigations towards the use of these materials as electron paramagnetic resonance imaging (EPRI) agents is also discussed.¹⁰

1 J. J. L. Morton, A. M. Tyryshkin, A. Ardavan, K. Porfyrakis, S. A. Lyon and G. Andrew D. Briggs, J. Chem. Phys., 2006, 124, 014508.

2 K.-P. Dinse, H. Käß, C. Knapp and N. Weiden, Carbon, 2000, 38, 1635–1640.

3 G. Liu, A. N. Khlobystov, G. Charalambidis, A. G. Coutsolelos, G. A. D. Briggs and K. Porfyrakis, J. Am. Chem. Soc., 2012, 134, 1938–1941.

4 S. Zhou, M. Yamamoto, G. A. D. Briggs, H. Imahori and K. Porfyrakis, J. Am. Chem. Soc., 2016, 138, 1313–1319.

5 J. Li, A. Takeuchi, M. Ozawa, X. Li, K. Saigo and K. Kitazawa, J. Chem. Soc. Chem. Commun., 1993, 0, 1784–1785.

6 M. Brettreich and A. Hirsch, Tetrahedron Lett., 1998, 39, 2731–2734.

7 I. Rašović, Mater. Sci. Technol., 2016, 0, 1–18.

8 A. Iwasiewicz-Wabnig, K. Porfyrakis, G. A. D. Briggs and B. Sundqvist, Phys. Status Solidi B, 2009, 246, 2767–2770.

9 S. Zhou, I. Rašović, G. A. D. Briggs and K. Porfyrakis, Chem. Commun., 2015, 51, 7096–7099.

10 S. P. Cornes, S. Zhou and K. Porfyrakis, Chem. Commun., 2017, DOI:10.1039/C7CC07106J.