π-Arene/Cation Structure and Bonding. Solvation versus Ligand Binding in Iron(III) Tetraphenylporphyrin Complexes of Benzene, Toluene, p-Xylene, and [60]Fullerene

Abstract

Benzene, toluene, p-xylene, and [60]fullerene are shown to be weak ligands to a hard metal such as iron(III) in Fe(TPP) + cation (TPP) tetraphenylporphyrinate). X-ray crystal structures of [Fe(TPP)-(C 6 H 6)][CB 11 H 6 Br 6 ]‚3.5C 6 H 6 (1), [Fe(TPP)(C 7 H 8)][CB 11 H 6 Cl 6 ]‚2C 7 H 8 (2), [Fe(TPP)(C 8 H 10)][Ag(CB 11 H 6 Br 6) 2 ]‚ arene (3), and [Fe(TPP)(C 60)][F 20-BPh 4 ]‚2.5dichlorobenzene (4) show distinctively short Fe‚‚‚C contacts, in the range 2.65-2.95 Å, which distinguish ligation from π-π cocrystallized solvation. Dihedral angles between the arene and metalloporphyrin planes are also diagnostic of ligand versus solvate roles. The essential features of the arene coordination can be faithfully reproduced using density functional theory. In contrast to alkali metal cation/π-arene interactions, a component of the covalent bonding can be recognized. In a broader context, this study suggests that solvents such as benzene should always be viewed as potential ligands in the presence of coordinatively unsaturated cations. The common use in supramolecular chemistry of the term "noncovalent" to describe entities with metal-ligand bonds is viewed as misleading.

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

“…The electron-rich 6:6 ring juncture is attracted to the protic region at the center of the porphyrin. The closest approach of a fullerene carbon atom to the mean plane of the 24-atom porphyrin core is 2.59 Å, the closest yet observed in a free-base porphyrin (24,38). This may be the result of a compression, related to sandwiching the fullerenes between layers and minimizing crystal void space.…”

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

“…More examples followed, [19][20][21][22][23][24][25] and fullerenes are attracted to other flat π surfaces. 13,[26][27][28] The curved to flat π-π attraction has been analyzed by molecular mechanics modeling and shown to be largely the result of van der Waals dispersion forces.…”

“…19 This implies charge transfer via coordinate bonding. The Fe atom is slightly out-of-plane toward the C 60 indicating the presence of a weak axial coordinate bond with at least some degree of covalence.…”

“…The orbitals involved were identified by density functional theory in closely related complexes of Fe(TPP) + with η 2 -bonded arenes. 19 Because the Fe(III) center is cationic and its d z 2 orbital is only half occupied, it would be difficult to argue that the direction of charge transfer is not with the fullerene as the donor. On the other hand, many fullerene-M(OEP) structures have been interpreted as indicating no covalent interaction, 18 and some structures apparently have the less electron-rich 5:6 (rather than a 6:6) ring-juncture bond closest to the metalloporphyrin.…”

“…23 Evidence that C 60 has sufficient ligand field strength to cause a high to low spin-state change in Mn(TPP) has also been forwarded, 23 but this contradicts field strength deductions based on other Mn(TPP) chemistry 40 and isoelectronic Fe(III) chemistry. 19 A recent communication reports that binding constants for C 60 increase as a function of the metal in a bis-porphyrin host in the order Ag(I) < Ni(II) < Cu(II) < Zn(II) < free base < Co(II) < Rh(III). 41 With the exception of rhodium(III), the differences are quite small, and the ordering has not been interpreted.…”