Long-Lived C-60 Radical Anion Stabilized Inside an Electron-Deficient Coordination Cage

Fullerene C-60 and its derivatives are widely used in molecular electronics, photovoltaics, and battery materials, because of their exceptional suitability as electron acceptors. In this context, single-electron transfer on C60 generates the C-60(center dot-) radical anion. However, the short lifetime of free C-60(center dot-) hampers its investigation and application. In this work, we dramatically stabilize the usually short-lived C-60(center dot-) species within a self-assembled M2L4 coordination cage consisting of a triptycene-based ligand and Pd(II) cations. The electron-deficient cage strongly binds C-60 by providing a curved inner p-surface complementary to the fullerene's globular shape. Cyclic voltammetry revealed a positive potential shift for the first reduction of encapsulated C-60, which is indicative of a strong interaction between confined C-60(center dot-) and the cationic cage. Photochemical one-electron reduction with 1benzyl-1,4-dihydronicotinamide allows selective and quantitative conversion of the confined C-60 molecule in millimolar acetonitrile solution at room temperature. Radical generation was confirmed by nuclear magnetic resonance, electron paramagnetic resonance, ultraviolet-visible-near-infrared spectroscopy and electrospray ionization mass spectrometry. The lifetime of C-60(center dot-) within the cage was determined to be so large that it could still be detected after one month under an inert atmosphere.