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

Abstract

Porphyrins and fullerenes are spontaneously attracted to each other. This new supramolecular recognition element is explored in discrete, soluble, coordinatively linked porphyrin and metalloporphyrin dimers. Jawlike clefts in these bis-porphyrins are effective hosts for fullerene guests. X-ray structures of the Cu complex with C60 and free-base complexes with C70 and a pyrrolidine-derivatized C60 have been obtained. The electron-rich 6:6 ring-juncture bonds of C60 show unusually close approach to the porphyrin or metalloporphyrin plane. Binding constants in toluene solution increase in the order Fe(II) < Pd(II) < Zn(II) < Mn(II) < Co(II) < Cu(II) < 2H and span the range 490−5200 M-1. Unexpectedly, the free-base porphyrin binds C60 more strongly than the metalated porphyrins. This is ascribed to electrostatic forces, enhancing the largely van der Waals forces of the π−π interaction. The ordering with metals is ascribed to a subtle interplay of solvation and weak interaction forces. Conflicting opinions on the relative importance of van der Waals forces, charge transfer, electrostatic attraction, and coordinate bonding are addressed. The supramolecular design principles arising from these studies have potential applications in the preparation of photophysical devices, molecular magnets, molecular conductors, and porous metal-organic frameworks.

Extending supramolecular fullerene-porphyrin chemistry to pillared metal-organic frameworks

“…This is the expected result of maximizing van der Waals contact. It is observed in the discrete supramolecular chemistry of jaws porphyrin (27,28) and cocrystallate structures (24,39).…”

“…Past work has relied almost exclusively on hydrogen bonding or coordinate bonding to construct the scaffolding of framework structures. It should be possible to fine-tune the application with metals in the porphyrins because most metalloporphyrins bind to fullerenes about as well as do free-base porphyrins (28,40).…”

“…The unexpectedly close approach of a curved surface to the center of a flat surface was originally recognized in the x-ray crystal structure of a fullerene-porphyrin conjugate (23). Subsequent cocrystallization studies of fullerenes with porphyrins (or metalloporphyrins) established the generality of the phenomenon (24,25), and discrete complexes of fullerenes in bisporphyrin (26) or ''jaws porphyrin'' hosts (27,28) have illustrated the supramolecular chemistry principles involved. The interaction is primarily van der Waals in nature (24) but is influenced by a summation of other weak forces.…”

“…The interaction is primarily van der Waals in nature (24) but is influenced by a summation of other weak forces. These include electrostatics, coordinate bond formation, charge transfer, and solvation effects (28). Notably, the traditional paradigm of supramolecular chemistry is not followed.…”

Fullerene−Porphyrin Constructs

“…A parallel is seen in the selectivity for C 70 over C 60 observed in static solution with "Jaws" porphyrins. 14 Columns of porphyrin-appended silica are also superior for the separation of endohedral metallofullerenes such as Ln@C 82 . 41 Their longer retention times relative to isostructural metal-free fullerenes are consistent with the idea that endohedral fullerenes have external fulleride character, arising from the positive oxidation state of the metal and are therefore better donors to a metalloporphyrin.…”