Supramolecular Fullerene-Porphyrin Chemistry. Fullerene\nComplexation by Metalated “Jaws Porphyrin” Hosts

Abstract

Porphyrins and fullerenes are spontaneously attracted to each other. This new supramolecular\nrecognition element is explored in discrete, soluble, coordinatively linked porphyrin and metalloporphyrin\ndimers. Jawlike clefts in these bis-porphyrins are effective hosts for fullerene guests. X-ray structures of\nthe Cu complex with C₆₀ and free-base complexes with C₇₀ and a pyrrolidine-derivatized C₆₀ have been\nobtained. The electron-rich 6:6 ring-juncture bonds of C₆₀ show unusually close approach to the porphyrin\nor metalloporphyrin plane. Binding constants in toluene solution increase in the order Fe(II) < Pd(II) <\nZn(II) < Mn(II) < Co(II) < Cu(II) < 2H and span the range 490−5200 M^-1. Unexpectedly, the free-base\nporphyrin binds C₆₀ more strongly than the metalated porphyrins. This is ascribed to electrostatic forces,\nenhancing the largely van der Waals forces of the π−π interaction. The ordering with metals is ascribed to\na subtle interplay of solvation and weak interaction forces. Conflicting opinions on the relative importance\nof van der Waals forces, charge transfer, electrostatic attraction, and coordinate bonding are addressed.\nThe supramolecular design principles arising from these studies have potential applications in the preparation\nof photophysical devices, molecular magnets, molecular conductors, and porous metal-organic frameworks.